Performance Evaluation of Polybenzimidazole for Potential ...

Performance Evaluation of

Polybenzimidazole for Potential

Aerospace Applications

Performance Evaluation of

Polybenzimidazole for Potential Aerospace

Applications

Proefschrift

ter verkrijging van de graad van doctor aan de

Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen op woensdag 15th of Januari 2014 om 15:00 uur

door

Hafiz Muhammad Saleem, IQBAL

Hafiz Muhammad Saleem Iqbal

geboren te Lahore, Pakistan

Dit proefschrift is goedgekeurd door de promotor:

Prof. Dr. ir. R. Benedictus

Prof. Dr. S. Bhowmik

Samenstelling promotiecommissie:

Rector Magnificus, voorzitter

Prof. R. Benedictus, Technische Universiteit Delft, promotor

Prof. Dr. S. Bhowmik, Amrita University, copromotor

Prof. dr. S.J. Picken, Technische Universiteit Delft

Prof. dr. ir. R. Marissen, Technische Universiteit Delft

Prof. dr.ir. H.E.J.G. Schlangen, Technische Universiteit Delft

Dr. ir. K.M.B. Jansen, Technische Universiteit Delft

Prof. Dr. Ir. R. Akkerman, Technische Universiteit Twente

ISBN: 978-90-8891-781-3

Keywords: Polybenzimidazole, Carbon Nano-fibers, Fire Testing, Radiation

testing, Adhesive bonding, PBI coating, Environmental Testing

Copyrights @ 2014 by Hafiz M. Saleem Iqbal

All rights reserved. No part of this publication may be reproduced, stored in a

retrieval system or transmitted in any form or by any means, electronic,

mechanical, photocopying, recording or otherwise, without the prior written

permission of the author.

Printed in Netherlands by Uitgeverij BOXPress, Oisterwijk

Dedicated To

My caring Parents My loving wife

& My sweet Ali and Inaayah

Summary

vii

SUMMARY

Performance Evaluation of Polybenzimidazole for Potential Aerospace Applications

With the increasing use of polymer based composite materials, there is

an increasing demand of polymeric resins with high glass transition

temperature (Tg), high thermal stability and excellent mechanical

properties at high temperature. Polybenzimidazole (PBI) is a recently

emerged high performance polymer. It has the highest glass transition

temperature (425°C) of any commercially available organic polymer,

high decomposition temperature (500°C – 600°C), good oxidation

resistance and it maintains excellent strength at cryogenic

temperatures. Due to its excellent thermal and mechanical properties,

PBI has great potential to be used for many high temperature

applications. However, at the same time, it has very high melt viscosity

which is too high to allow its processing by conventional manufacturing

techniques.

The processing of PBI itself is a real challenge due to high processing

temperature and pressure requirement. This is the main reason that

compression molded PBI has found very few industrial applications.

Recently, PBI is also available in solution form but its performance has

not been explored for potential applications. In solution form, a great

potential lies in exploring PBI as a film and coating material. Therefore,

the objective of present work is to investigate the potential of PBI film

and coating for various aerospace applications. The possible

applications can be the thermal control films in spacecraft, protective

coating for polymer composites in low earth orbit (LEO) and the

radiation shielding material in geosynchronous earth orbit (GEO). The

polymer also has the great prospective to be used as protective coating

for composite aircraft. Due to its superior non-flammability, the

material has a potential to be used as a fire resistant coating for aircraft

application.

Summary

viii

In present work, unfilled and carbon nano-fibers (CNFs) reinforced PBI

films were manufactured and their thermal and mechanical properties

were investigated. Unfilled PBI film has exhibited thermal properties

comparable to the thermal properties of molded PBI whereas it has

demonstrated higher mechanical properties than molded PBI. Addition

of 2 weight percent of CNFs has further improved the thermal and

mechanical properties of PBI film.

In present work, performance of PBI is also evaluated as a fire resistant

coating for aircraft. Unfilled PBI could not improve the fire resistant

properties of aerospace grade unidirectional epoxy/carbon composite.

However, addition of 2 weight percent of CNFs to PBI has improved the

coating performance remarkably. Nano-filled PBI coating has reduced

the average heat release rate (HRR) from 27 KW/m2 (for uncoated

epoxy/carbon composite) to 1 kW/m2. Also, nano-filled PBI coating has

reduced the peak heat release rate (PHHR) from 87 KW/m2 (for

uncoated epoxy/carbon composite) to 53 KW/m2. Furthermore, ignition

time of epoxy/carbon composite has increased from 165 seconds to

730 seconds. These results demonstrate the effectiveness and potential

of PBI for fire resistant applications in aircraft.

Performance of both unfilled and nano-filled PBI films is evaluated after

exposure to simulated LEO environment. Both unfilled and nano-filled

PBI have retained most of thermal and mechanical properties after

exposure to simulated LEO environmental conditions. Though, PBI has

exhibited high erosion yield value which is a mass dependent

parameter. However, a detailed investigation has shown that value of

high erosion yield is due to the loss of water (absorbed by PBI) rather

than the degradation of PBI itself.

Both unfilled PBI and nano-filled PBI were also exposed to different

high energy radiations and they have retained most of their thermal

and mechanical properties after exposure to these high energy

radiations. Though, unfilled PBI has shown about 40% decrease in

tensile strain after exposure to mixed radiations. However, addition of

CNFs to PBI has improved its performance by retaining 80% tensile

strain after exposure to mixed radiations. PBI has retained most of its

properties after exposure to even higher doses of gamma radiations

and electron radiations in present study. These results are encouraging

and research can be further extended to evaluate the performance of

Summary

ix

PBI after exposure to different radiations under various temperature

conditions.

PBI is also evaluated as an adhesive for high temperature applications.

Due to high solvent contents, the process optimization required lot of

efforts to form PBI bonded joints with considerable lap shear strength.

Therefore, in present work, efforts are devoted to optimize the

adhesive bonding process of PBI. PBI adhesive bonded joints were

formed successfully with single lap shear strength of 30 MPa using

epoxy based unidirectional carbon fiber composite substrate. These

results are quite encouraging and efforts can be made to further

optimize the bonding process in order to improve the lap shear

strength. PBI adhesive bonded joints were also exposed to hot/wet

environment at 80oC and 95% RH for 1000 hours. Exposed joints have

retained about 80% of the joint strength even after exposure to

hot/wet environment for 1000 hours. Furthermore, PBI adhesive

bonded joints were tested at 80oC to evaluate the high temperature

performance of PBI. Bonded joints have retained about 50% of the

original joint strength.

The present work has contributed to the knowledge and understanding

of several aspects of the PBI polymer. Different problems related to the

processing of PBI are highlighted and efforts are made to evaluate the

performance of PBI as a film, coating and adhesive. PBI has not been

manufactured and characterized in such detail in the past. Also the

potential of nano-filler based PBI has never been explored in the past.

Present work has produced some encouraging results to perform

further research on PBI as a coating and adhesive material for different

aerospace applications.

H.M.S.IQBAL

Delft, Netherlands

Contents

xi

Contents

SUMMARY………………………………………………………………………………………………..vii

ABBREVIATIONS………………………………………………………………..………………..xix

Chapter 1 Introduction…………………………………………………….1

1.1. General Background………………………………………………………...………… 1

1.2. High Performance Polymers .......................................................... 2

1.2.1. Definition of High Performance Polymers ...................................... 2

1.2.2. History of High Performance Polymers .......................................... 2

1.3. Polybenzimidazole – A high Performance Polymer ........................ 3

1.3.1. Historical background of Polybenzimidazole (PBI) ......................... 4

1.3.2. Applications of Polybenzimidazole (PBI) ........................................ 5

1.4. Research Goals .............................................................................. 5

1.5. Outlines of Thesis .......................................................................... 6

1.6. References .................................................................................... 8

Chapter 2 Processing and Characterization of Unfilled and

Nano-fibers Reinforced Polybenzimidazole……………………………11

2.1. Introduction ................................................................................ 11

2.2. Polymer Based Nano-composites ................................................ 12

2.2.1. Processing issues of Carbon Nano-fibers ..................................... 13

2.3. Experimental............................................................................... 13

2.3.1. Materials ..................................................................................... 13

2.3.2. Processing of Polybenzimidazole by Compression Molding ......... 13

Contents

xii

2.3.3. Solution casting of Basic PBI Film ................................................ 14

2.3.4. Solution casting of PBI nano-composite film ................................ 15

2.4. Thermal and Mechanical Characterization .................................. 15

2.4.1. Thermal Gravimetric Analysis (TGA) ............................................ 15

2.4.2. Dynamic Mechanical Analysis (DMA) .......................................... 16

2.4.3. Tensile Testing ............................................................................. 16

2.4.4. Scanning Electron Microscopy (SEM Analysis) ............................. 16

2.5. Results and Discussion ................................................................ 16

2.5.1. Compression Molding of PBI ........................................................ 16

2.5.2. Processing of PBI neat film and nano-composite film .................. 18



2.5.5. Tensile testing ............................................................................. 24

2.5.6. Scanning Electron Microscope (SEM) Analysis ............................. 26

2.5.7. Fracture morphology ................................................................... 27

2.6. Conclusions ................................................................................. 28

2.7. References .................................................................................. 29

Chapter 3 Fire Testing of Nano-fibers Reinforced

Polybenzimidazole Coating for Aircraft Application…………..31

3.1. Introduction ................................................................................ 31

3.2. Combustion Process of Polymers ................................................ 33

3.3. Classification of Polymers Based on Thermal Decomposition

Mechanism ........................................................................................... 35

3.4. Effect of Char Formation on Fire Resistant Properties of Polymers

36

3.5. Elements of Material Flammability ............................................. 37

3.5.1. Time-to-Ignition (TTI) .................................................................. 37

3.5.2. Heat Release Rate (HRR) ............................................................. 38

Contents

xiii

3.5.3. Flame Propagation Index (FPI) .................................................... 38

3.5.4. Smoke generation ....................................................................... 38

3.6. Methods to Improve the Fire Resistance of Polymers and Polymer

Based Composites................................................................................. 39

3.6.1. Addition of flame retardants to Polymers .................................... 39

3.6.2. Limitation of Using Fire retardants .............................................. 40

3.6.3. Nano-filler based flame retardant nano-composites ................... 40

3.6.4. Application of fire protective coatings ......................................... 41

3.7. Experimental............................................................................... 43

3.7.1. Materials ..................................................................................... 43

3.7.2. Preparation of Composite Laminates .......................................... 43

3.7.3. Plasma Treatment of Composite Laminates ................................ 43

3.7.4. Contact Angle Measurement ....................................................... 43

3.7.5. Adhesion Testing ......................................................................... 43

3.7.6. Preparation of Unfilled PBI Coated Samples ................................ 44

3.7.7. Preparation of Nano-filled PBI Coated Samples ........................... 44

3.7.8. Thermal gravemetric Analysis (TGA) ........................................... 44

3.7.9. Cone Calorimeter and fire testing ................................................ 45



3.8.2. Contact Angle measurement of Plasma treated Composite

Substrate ............................................................................................... 47

3.8.3. Adhesion Testing ......................................................................... 48

3.8.4. Cone Calorimeter test results ...................................................... 50

3.9. Conclusions ................................................................................. 58

3.10. References .................................................................................. 60

Chapter 4 Performance Evaluation of Polybenzimidazole

in Simulated Low Earth Orbit Environment…………………………....63

4.1. Introduction ................................................................................ 63

Contents

xiv

4.2. LEO Space Environment .............................................................. 64

4.2.1. Ultra High Vacuum ...................................................................... 64

4.2.2. Ultraviolet radiations (UV radiations) .......................................... 64

4.2.3. Thermal Cycling ........................................................................... 65

4.2.4. Atomic Oxygen (AO) .................................................................... 66

4.3. Experimental............................................................................... 67

4.3.1. Materials ..................................................................................... 67

4.3.2. Solution casting of Unfilled PBI Film ............................................ 67

4.3.3. Solution casting of nano-filled PBI films ....................................... 67

4.3.4. Exposure to LEO Environment ..................................................... 67

4.3.5. Tensile Testing ............................................................................. 70

4.3.6. Scanning Electron Microscopy (SEM) ........................................... 70

4.3.7. Atomic Force Microscopy (AFM) .................................................. 70


4.4.1. Mass Loss and Erosion Yield Measurements of exposed samples 70

4.4.2. Tensile Test Results ..................................................................... 73

4.4.3. Scanning Electron Microscopy (SEM) ........................................... 77

4.4.4. Atomic Force Microscopy (AFM) .................................................. 80

4.5. Conclusion .................................................................................. 82

4.6. References .................................................................................. 82


under High Energy Radiations Environment……………………….... 85

5.1. Introduction ................................................................................ 85

5.2. Effect of Ionizing Radiations on Polymeric Materials ................... 86

5.3. Experimental............................................................................... 87

5.3.1. Materials ..................................................................................... 87

5.3.2. Preparation of Unfilled PBI and Nano-filled PBI Film ................... 87

5.3.3. Exposure to Electron radiations and Gamma radiations .............. 87

5.3.4. Exposure to Mixed radiations ...................................................... 87

Contents

xv

5.4. Testing and Characterization ....................................................... 88



5.4.3. Tensile Testing ............................................................................. 89

5.4.4. UV/VIS Spectroscopy ................................................................... 89

5.4.5. Surface morphology .................................................................... 89


5.5.1. Thermal gravimetric analysis (TGA) ............................................. 89


5.5.3. Tensile Test Results ..................................................................... 96

5.5.4. UV/VIS photospectroscopy .......................................................... 98

5.5.5. Surface morphology .................................................................. 101

5.6. Conclusion ................................................................................ 103

5.7. References ................................................................................ 104


as an Adhesive for Aerospace Applications…………………………. 107

6.1. Introduction .............................................................................. 107

6.2. Selection of the surface treatment ............................................ 109

6.2.1. Atmospheric Pressure Plasma Treatment (APPT) ...................... 110

6.3. Experimental............................................................................. 110

6.3.1. Materials ................................................................................... 110

6.3.2. Atmospheric Pressure Plasma Treatment of Composite Specimen

………………………………………………………………………………………………..111

6.3.3. Contact Angle Measurements ................................................. 111

6.3.4. Specimen Preparation for Lap Shear Testing ............................. 112

6.3.5. Environmental Conditioning of Specimens ................................ 112

6.3.6. Specimen Preparation for Lap Shear Testing ............................. 112

6.3.7. Scanning Electron Microscopy (SEM) ......................................... 113

6.4. Results and Discussion .............................................................. 113

Contents

xvi

6.4.1. Atmospheric Pressure Plasma Treatment .................................. 113

6.4.2. Surface Treatment for Lap Shear Testing................................... 120

6.4.3. PBI adhesive bonded joints Preparation .................................... 121

6.4.4. Process Optimization of PBI Adhesive to form bonded joints ..... 121

6.4.5. Environmental Conditioning of PBI adhesive bonded joints ....... 128

6.4.6. Testing of PBI adhesive bonded Joints at High Temperature ..... 131

6.4.7. Study of Failure Modes .............................................................. 133

6.5. Conclusions ............................................................................... 135

6.6. References ................................................................................ 136


coating for Aircraft Application…………………………….……………………139

7.1. Introduction .............................................................................. 139

7.2. Experimental............................................................................. 140

7.2.1. Materials ................................................................................... 140

7.3. Testing and Characterization ..................................................... 141

7.3.1. Atmospheric pressure Plasma Treatment of Composite Substrate

………………………………………………………………………………………………. 141

7.3.2. Contact Angle Measurements ................................................... 141

7.3.3. Application of Coating ............................................................... 142

7.3.4. Adhesion Testing of PBI coating ................................................ 142

7.4. Environmental Resistance of PBI coating .................................. 142

7.4.1. Conditioning of PBI coated Panels at high Temperature and

Humidity.............................................................................................. 142

7.4.2. Liquid Immersion of PBI Coated Panels ...................................... 142

7.4.3. Adhesion testing of Conditioned Samples .................................. 143

7.4.4. Scratch testing of PBI Coated panels after exposure to Various

Environmental Conditions ................................................................... 143

Contents

xvii

7.4.5. Nano-indentation testing of PBI Coated panels after exposure to

Various Environmental Conditions ....................................................... 143

7.5. Results and Discussion .............................................................. 144

7.5.1. Contact angle measurement of Atmospheric Pressure Plasma

Treated Composite Substrate .............................................................. 144

7.5.2. Adhesion Testing of PBI coating ................................................ 145

7.5.3. Adhesion testing of Conditioned Specimens .............................. 147

7.5.4. Scratch Testing of PBI coated Panels after exposing to Various

Environmental Conditions ................................................................... 148

7.5.5. Nano-indentation testing of PBI coated Panels after exposing to

Various Environmental Conditions ....................................................... 153

7.6. Conclusions ............................................................................... 156

7.7. References ................................................................................ 157

Conclusions and Recommendations for Future Work……..159

8.1. Conclusions ............................................................................... 159

8.1.1. Compression Molding of PBI ...................................................... 159

8.1.2. Solution Casting of PBI film ....................................................... 160

8.1.3. Thermal and Mechanical Properties of PBI ................................ 161

8.1.4. Testing in LEO Simulated Environment ...................................... 161

8.1.5. Radiation Testing ...................................................................... 162

8.1.6. Fire Testing of PBI Coating ........................................................ 163

8.1.7. Lap Shear Testing of PBI ............................................................ 163

8.1.8. Testing for Aircraft Application ................................................. 164

8.2. Recommendations for future work ........................................... 165

CURRICULUM VITAE……………………………………………………………….…………….170

LIST OF PUBLICATIONS………………………………………………………………………...171

ACKNOWLEDGEMENT…………………………………………………………………………..174

Abbreviations

xix

ABBREVIATIONS

PBI Polybenzimidazole

DMAc Dimethyl-acetamide

CNFs Carbon Nano-fibers

APPT Atmospheric Pressure Plasma Treatment

TGA Thermal Gravimetric Analysis

DMA Dynamic Mechanical Analysis

SEM Scanning Electron Microscope

HRR Heat Release Rate

PHRR Peak Heat Release Rate

TTI Time-to-Ignition

SEA Smoke Extinction Area

MLR Mass loss rate

LEO Low Earth Orbit

GEO Geosynchronous Earth Orbit

UV Ultraviolet

AO Atomic Oxygen

AFM Atomic force microscopy

CA Contact Angle

CSM Continuous Stiffness Measurement

1

CHAPTER 1

Introduction

In this chapter, a brief introduction about Polybenzimidazole

and its historical background is presented. Motivations behind

this thesis and its research objectives are described. The

structure of this dissertation is also outlined briefly.

1.1. General Background

Polymers and polymer based composite materials have been gaining

significant attention since last decade. With wide range of possible

fillers and resins, composite materials have opened the way to an

enormous range of materials with diverse chemical, physical, and

mechanical properties. These materials have an impressive and diverse

range of applications in automotive, aviation, spacecrafts, civil

infrastructure and sports industry [1-5]. In recent times, there has

been a steady increase in the use of polymer based composite

materials in both military and commercial aircraft [3, 6-7]. Polymeric

materials are also being extensively used in space systems as multi-

layer insulations, matrix in the substrate for solar panels, adhesives,

thermal control coatings for spacecraft and insulation for electrical

wiring [8, 9]. Despite having many advantages, polymeric based

materials are facing challenges for demanding aerospace applications.

As an example, at a temperature of above 200oC, many of the

polymeric materials cannot maintain their thermal stability and

mechanical properties which ultimately leads to collapse of the

composite structures [10]. Also, the exposure of these materials to

harsh space environment, affects the thermal, mechanical and optical

properties of these materials [9, 11-12]. These demanding conditions

are limiting the use of polymer based materials for aerospace industry.

Introduction

2

Therefore, in order to meet the requirement of aerospace industry,

research is continued to develop new high performance polymers which

can maintain the desired properties under working environmental

conditions for longer duration of time and serve their purpose properly.

1.2. High Performance Polymers

1.2.1. Definition of High Performance Polymers

High performance polymers can be defined in many ways depending on

the type of application. High performance is a general term which can

be used to describe many polymers. In term of high temperature

applications, the most popular definition of high performance polymer is

the polymer which can retain the useable properties at a temperature

greater than 177oC [13]. These polymers have a glass transition (Tg)

greater than 200oC and they can maintain their thermal stability (5%

weight loss) up to a temperature of 450oC [13].

1.2.2. History of High Performance Polymers

High performance polymers have long historical background. The

development of high performance polymers started in late 1950s

mainly to meet the requirement of aerospace industry. The most

productive period for the development of high performance polymers

was between 1960 and 1970. During this period, many high

performance polymers were developed with high glass transition

temperature (Tg) and high thermal stability. Despite their high thermal

stability, cost and processing of these polymers was the major issue.

The chemical structure of these high performance polymers resulted in

difficult processing and poor solubility in organic solvents. Their

softening points or melt viscosities were too high to process using

conventional manufacturing techniques. Polybenzimidazole, polyimide

and polysulfone are the main examples of initially developed high

performance polymers. Polybenzimidazole could not get the attraction

to a sizeable market due to its high cost and difficult processability [13,

14]. Polyimide (PI) film designated as Kapton was the first material

among high performance polymers which was fabricated by DuPont.

Kapton attracted a big market and since then it has been the largest

selling high performance polymer in the world. In aerospace industry,

it is used primarily as insulation for aircraft and spacecraft wiring. Other

applications include high performance flexible films for photovoltaic

substrates and printed circuit boards. After the commercialization of

Chapter 1

3

Kapton in 1970s, efforts were made to develop other high performance

polymers with low cost, easy processing and improved mechanical

properties. One example of such polymer is thermoplastic

Polyetherimide (PEI) designated as Ultem produced by General Electric

[13]. Kapton is also being used in satellites for thermal management

[15]. Other important developments during this period were polyamide

imide (PAI) and polybenzoxazole (PBO) but they never reached the

success of PI.

1.3. Polybenzimidazole – A high Performance Polymer

One of the families of high performance polymers is the imidized

family. Imidized polymers are said to offer the best combination of

thermal and mechanical properties of any commercially available

polymer. The family of imidized polymers includes polyimide (PI),

polyamideimide (PAI), polybenzimidazole (PBI), and polyetherimide

(PEI). Other than PI from imidized family, PBI has gained wide

recognition in recent years as high-temperature polymer. PBI is a

thermoplastic polymer and, in molded form, it has the highest

compressive and tensile strength of any unfilled polymeric resin [16-

18]. PBI has the highest glass transition temperature (425°C) of any

commercial available organic polymer [19]. It has high decomposition

temperatures (500°C-600°C), good oxidation resistance and it

maintains excellent strength at cryogenic temperatures [20]. The

chemical structure of PBI is shown in figure 1.1

Figure 1.1 Chemical Structure of PBI

PBI has both aromatic and heterocyclic rings in its backbone structure.

Aromatic rings are hydrocarbons which contain benzene (C6H6).

Benzene is often drawn as a ring of six carbon atoms, with alternating

double bonds and single bonds. Chemical structure of benzene is shown

in figure 1.2.

Introduction

4

Figure 1.2 Chemical Structure of Benzene (Aromatic Ring)

Aromatic rings which contain non-carbon atoms (Oxygen, Nitrogen or

Sulfur) in the ring are called heterocyclic rings. The examples of

heterocyclic rings are shown in figure 1.3.

Imidazole Benzimidazole

Figure 1.3 Examples of Heterocyclic Rings

Because of these aromatic and heterocyclic rings in the backbone

structure of PBI, it exhibits high thermal stability and resistance to

oxidative decomposition [21]. Due to its excellent thermal and

mechanical properties, PBI can be one of the leading candidates for

high temperature aerospace applications.

1.3.1. Historical background of Polybenzimidazole (PBI)

Polybenzimidazole is regarded as the first high performance polymer

which was developed in early 1960s. But this polymer could not get the

attraction of sizeable market due to high cost and difficult processing

[13, 14]. Due to the unique characteristics of PBI polymer, efforts were

continued to develop some techniques for the processing of this

polymer. It was not until mid 1980s that compression molded PBI

parts were manufactured successfully by Hoechst Celanese and Alpha

Precision Plastics [14]. The marketing of molded PBI was started with

the trade name Celazole. Alpha took over the technology of Celazole

completely in 1995 and continued marketing of molded PBI parts for

sealing elements in high-temperature corrosive environments.

Currently, PBI is finding a lot more applications in many diverse fields

in the form of fabric, molded parts and thin films.

Chapter 1

5

1.3.2. Applications of Polybenzimidazole (PBI)

PBI is claimed to have better strength than PI and PAI at higher

temperature and superior in terms of chemical and radiation resistance

[17, 22]. It has shown considerable promise, not only as a matrix for

high-temperature structural composite materials, but also as fibers

[23]. Automotive and aerospace applications of PBI include thermal

insulators, high-performance bearings, electrical connectors, nose cone

of aircraft and ablative structures [17, 24]. Due to its superior non-

flammability, PBI has been used for firefighters’ protective clothing,

high-temperature gloves, and astronaut flight suits [25-28]. In recent

years, acid doped PBI has emerged as a promising material for the

application in the membrane of the fuel cell [28-29].

1.4. Research Goals

Inspired by the excellent thermal and mechanical properties of PBI, the

aim of this work is to explore the potential of PBI as a film, coating and

adhesive material for different aerospace applications. Another direction

which has not been explored yet is the manufacturing of nano-

composite PBI. Therefore, another objective of this work is to

manufacture the carbon nano-fibers (CNFs) reinforced PBI film and

coating and to study the effect of CNFs on thermal and mechanical

properties of PBI. Considering the high thermal and mechanical

properties of PBI, it has great potential to be used for various

aerospace applications. The possible applications are the film for

thermal management in spacecraft, protective coating for polymer

composite in LEO environment and the radiation shielding material in

GEO environment. The polymer also has great perspective to be used

as protective coating for composite aircraft. Due to its superior non-

flammability, this material must be evaluated as a fire resistant coating

for aircraft application. Based on above mentioned objectives, the most

important questions to be addressed in this work are:

a. What are the issues that may be encountered during the

processing of PBI molded parts, unfilled PBI film and CNFs

reinforced PBI film?

b. In solution form, PBI can be used as coating material. However, it

contains about 75% of the solvent in the solution and it requires

a temperature of around 200oC to extract the maximum solvent

out of it. But this temperature may not be achieved while

Introduction

6

applying PBI coating on composite substrate due to the fact that

properties of most of the composites degrade at high

temperature. This fact will make it challenging to apply PBI

coating on composite substrate. Therefore, the question need to

be addressed that if PBI coating can be applied on the composite

substrate at low curing temperature? And if the application of PBI

coating is possible at low temperature then to what extent it will

affect the performance of PBI?

c. PBI has superior non-flammability in molded form. However, it

has never been tested as a fire resistant coating. Inspired by its

thermal properties, one of the objectives of this work is to

evaluate that if PBI has a real potential to be used as a fire

resistant coating for epoxy based carbon fiber composite?

d. Space environment consist of ultraviolet radiations, ionizing

radiations, high vacuum, thermal cycling. These radiations and

thermal cycling degrade the thermo-mechanical and optical

properties of polymers. In this context, the question need to be

addressed is; how and to what extent, the exposure of PBI to

simulated Low Earth Orbit environment and under high energy

radiations, will affect the thermo-mechanical and optical

properties of PBI.

This work only includes the initial evaluation of PBI for different

aerospace applications. Detail evaluation of the material for any specific

application is out of the scope of this work. However, recommendations

for the future work will be given on the basis of the results obtained in

this work. On the basis of these recommendations, PBI can be further

evaluated in detail for a specific application.

1.5. Outlines of Thesis

This thesis mainly divided into three parts. Part 1 includes the process

optimization, thermal and mechanical characterization of unfilled and

nano-fibers reinforced PBI. Part 2 mainly relates to the evaluation of

unfilled and nano-filler based PBI film and coating materials for

potential aerospace applications. Part 3 is related to the evaluation of

PBI as an adhesive for high temperature applications. A separate

chapter is included for the conclusions and recommendation for future

work.

Chapter 1

7

A brief history of high performance polymers and their development

with the course of time is presented in the introductory chapter. The

development history of Polybenzimidazole (PBI) and its thermal and

mechanical properties are presented in this chapter. Current state of

art applications and potential future applications of PBI are highlighted

in this chapter.

Chapter 2 explains the manufacturing of both compression molded PBI

and PBI film. Different techniques are illustrated to manufacture the

CNFs based PBI nano-composite. Thermal and mechanical properties of

PBI in different forms are presented in this chapter. SEM analysis of

fractured surfaces of tested specimens is also presented in this chapter.

Chapter 3 deals with the study and evaluation of PBI as a fire resistant

coating for epoxy based unidirectional carbon fiber composite. A

comparison of fire resistant properties of uncoated carbon/epoxy

composite and composite coated with unfilled and nano-filled PBI

coating is presented in this chapter.

Chapter 4 and chapter 5 mainly deals with the exposure of basic PBI

and PBI nano-composite to different space environmental conditions

and its evaluation after having exposed to these conditions. The effect

of simulated LEO environment and high energy radiations on

characteristic behavior of unfilled PBI and PBI nano-composite is

assessed and results are presented to evaluate its potential for future

space applications.

Chapter 6 shed light whether or not it is feasible to fabricate the

bonded joints of polymer based composite material for aerospace

application using PBI as an adhesive. On the basis of the results

obtained, the potential of PBI as an adhesive for high temperature

applications is also discussed.

Chapter 7 studies the behavior of PBI as a protective coating for epoxy

based carbon fiber composite for aircraft application. Response of PBI is

described after exposure to hot wet environment and liquid immersion.

Critical properties including hardness, scratch resistance and adhesion

of PBI coating are assessed and the results are presented.

Introduction

8

Chapter 8 summarizes the undertaken research and highlights the main

findings of this work. On the basis of these finding, some conclusions

are made and recommendations are given for future work.

1.6. References

1- A. Baker, S. Dutton, K. Donald, Composite materials for aircraft structures, 2nd edition, 2004

2- A.P. Mouritz, E. Gellert, P. Burchill, K. Challis, Review of advanced composite structures for naval ships and submarines, Composite structures 53 (2001)

3- A.P.Mouritz and A.G.Gibson, Fire Properties of polymer composite materials,

Solid Mechanics and its applications, Volume 143, 2006, ISBN-13 978-1-4020-5356-6

4- A.P.Mouritz, Fire safety of advanced composites for aircraft, B2004/0046,

April 2006

5- E. Grossman, I. Gouzman, Space Environment Effects on Polymers in Low Earth Orbit, Nuclear Instruments and Methods in Physics Research B 208, 2003

6- A.P. Mouritz, Fire resistance of aircraft composite laminates, Journal of

Material Science Letters, 22, (2203), pp. 1507-1509

7- S. Gandhi, Postcrash Health Hazards from Burning Aircraft Composites, Galaxy Scientific Corporation, Fire Safety Section, AAR-422, Federal Aviation Administration

8- L. R. Kiefer and A. R. Orwoll, space environmental effects on polymeric

materials, final technical report n88- 168 79, 1987

9- E. M. Silverman, Space environmental effects on spacecraft: LEO materials selection guide, NASA Contractor Report 4661, Part 2

10- S. Feih, A. P. Mouritz, Z. Mathys and A.G. Gibson, Fire Structural Modeling of

Polymer Composites with Passive Thermal Barrier, Journal of Fire Sciences, Vol. 28 – March 2010

11- Joo-Hyun Han, Chun-Gon Kim, Low Earth Orbit Space Environment Simulation

and its Effects On Graphite/Epoxy Composites, Composite Structures vol. 72, 2006

12- Kwang-Bok Shina, Chun-Gon Kima, Chang-Sun Honga, Ho-Hyung Leeb,

Prediction of Failure Thermal Cycles in Graphite/Epoxy Composite Materials Under Simulated Low Earth Orbit Environments, Composites: Part B, Vol. 31, 2000

Chapter 1

9

13- Paul M. Hergenrother, The Use, Design, Synthesis, and Properties of High

Performance/High Temperature Polymers: An Overview, 2003

14- Tai-Shung Chung, A Critical Review of Polybenzimidazoles , Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Republic of Singapore, 119260, Available online: 23 Sep 2006

15- A. N. Hammoud, E. D. Baumann, E. Overton, I. T. Myers, J. L. Suthar, W.

Khachen and J. R. Laghari, High Temperature Dielectric Properties of Apical, Kapton, Peek, Teflon AF, And Upilex Polymers, Conference on Electrical Insulation and Dielectric Phenomenon, USA, 1992, 549-554

16- Tai-Shung Chung, A Critical Review of Polybenzimidazoles , Department of

Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Republic of Singapore, 119260, Available online: 23 Sep 2006

17- Kemmish, D. J., “High Performance Engineering Plastics,” Rapra Review

Reports, Vol. 8, No. 2, 1995, Rept. 86.

18- Beland, S., High Performance Thermoplastic Resins and Their Composites, 1st ed., Noyes Data Corp., Park Ridge, NJ, 1990, pp. 47–50

19- Steinerand, P. A., and Sandor, R., “Polybenzimidazole Prepreg: Improved

Elevated Temperature Properties with Autoclave Processability,” High Performance Polymers,Vol. 3, No. 3, 1991, pp. 139–150.

20- Bhowmik, S., Bonin, H.W., Bui, V. T., and Weir, R. D., “Modification of High-Performance Polymer Composite Through High-Energy Radiation and Low-Pressure Plasma for Aerospace and Space Applications,” Journal of Applied Polymer Science, Vol. 102, Jan. 2006, pp. 1959–1967.

21- O. R. Hughes, O. N. Chen, W.M. Cooper, L. P. Disano, E. Alvarez and T.E. Andres, PBI powder processing to performance parts, Journal of Applied Polymer Science, Published online 10 March 2003, Vol. 53, No. 5,1994, pp. 485- 496

22- Yasuhiko Onishi, Atsushi Maeno, Hideyuki Tanigawa, Kazuo bra,Takashi Ito,

Masayuki Nagata and Tsuneo Sasuga, Poly( benzimidazole) as insulator for superconductors, Cryogenics 35 ( 1995) 795-798

23- “Fibrous Reinforcement for Space Applications,” NASA CR-796, 1967.

24- Robert R. Dickey a, John H. Lundell a & John A. Parker, The Development of

Polybenzimidazole Composites as Ablative Shields, NASA-Ames Research Center, Moffett Field

25- Zhang, L., Ni, Q.-Q., Shiga, A., Fu, Y., and Natsuki, T., “Synthesis and Mechanical

Properties of Polybenzimidazole Nanocomposites Reinforced byVapor Grown Carbon Nanofibers,” Polymer Composites, Vol. 31, No. 3, 2009, pp. 491–496.

Introduction

10

26- Okamoto, M., Fujigaya, T., and Nakashima, N., “Individual Dissolution of Single-

Walled Carbon Nanotubes by Using Polybenzimidazole, and Highly Effective Reinforcement of Their Composite Films,” Advanced Functional Materials, Vol. 18, No. 12, 2008, pp. 1776–1782.

27- Sidman, K. R., and Gregory, J. B., “Development of Thermally Stable

Polybenzimidazole (PBI) Fiber,” Aeronautical Systems Div., TR 72-50, Nov. 1971.

28- Sannigrahi, A., Arunbabu, D., Sankar, M., and Jana, T., “Tuning the Molecular

Properties of Polybenzimidazole by Copolymerization,” Journal of Physical Chemistry B, Vol. 111, No. 42, 2007, pp. 12124–12132.

29- Hel, R., Li, Q., Bach, A., Jensen, J. O., and Bjerrum, N. J., “Physicochemical

Properties of Phosphoric Acid Doped Polybenzimidazole Membranes for Fuel Cells,” Journal of Membrane Science, Vol. 277, 2006, pp. 38–45.

11

CHAPTER 2

Processing and Characterization of Unfilled and Nano-fibers Reinforced Polybenzimidazole

In this chapter, processing of Polybenzimidazole is presented

both in powder and solution form. Processing of nano-fibers

reinforced PBI films is also discussed. Thermal and mechanical

properties of compression molded PBI, unfilled PBI film and

carbon nano-fibers reinforced PBI films were evaluated using

Thermal-Gravimetric Analysis, Dynamic Mechanical Analysis

and tensile testing and results are presented.

2.1. Introduction

Polybenzimidazole (PBI) is a thermoplastic polymer having excellent

thermo-mechanical and chemical properties [1]. In molded form, PBI

has the highest compressive and tensile strength of any unfilled

polymeric resin [2, 3]. It has the highest glass transition temperature

(425°C) of any commercial available organic polymer [1]. It exhibits

high decomposition temperatures (500°C-600°C), good oxidation

resistance and it maintains excellent strength at cryogenic

temperatures [4]. Despite having better thermal and mechanical

properties, the real challenge is the processing of PBI as such, because

this particular polymer is not melt processable. Moreover, compression

molded PBI requires very high processing temperature (440°C) and

pressure (50 MPa). Therefore, very few research articles are available

till today regarding processing of PBI. Some researchers have

demonstrated compression molding processing of PBI using PBI powder

whereas only two articles have demonstrated the processing of PBI in

solution form to produce PBI nano-composite films [5-7]. However, a

Processing and Characterization of Unfilled and Nano-fibers Reinforced PBI

12

detailed study about thermal and mechanical behavior of unfilled PBI

and carbon nano-fibers reinforced PBI nano-composite under different

environmental conditions is not conducted till date. In this context,

present chapter will cover the processing of PBI using PBI powder and

PBI solution. A detailed study about thermal ad mechanical behavior of

unfilled and nano-filled PBI will also be the part of this chapter.

2.2. Polymer Based Nano-composites

Nano-composites are a class of composite materials where one of the

constituents has dimensions in the range of nanometers [8, 9]. In the

last decade, there has been a strong emphasis on the development of

polymeric nano-composites. The increasing interest in the development

of polymeric nano-composites exists not only because of their high

specific strength but also for the possibility of making products with

unique mechanical, electrical and thermal properties [10, 11]. However

these properties depend on several factors such as type of nano-filler,

surface treatments, type of polymer matrix and synthesis methods

[12]. At present, there are many types of nano-fillers available to

produce polymer nano-composites. Some of the most commonly used

nano-fillers are carbon nano-fibers (CNFs), carbon nano-tubes (CNTs)

and nano-clay. CNFs and CNTs have been used to modify several

polymers including polypropylene, polymethyl-methacrylate,

polyethylene-terepthalate, polystyrene and polycarbonate [13]. Both

CNFs and CNTs have similar rope-like structures, but CNTs exhibits

much smaller diameter and better mechanical, thermal and electrical

properties compared to CNFs [11, 14]. However, addition of CNTs to

polymeric resin significantly increases the viscosity of polymers. In

addition, it is difficult to uniformly disperse CNTs into polymer due to

strong Vander Waals forces between them. Contrary to CNTs, CNFs are

drawing significant attention to form nano-composite because of

relatively better dispersion and low cost [10-11, 15]. The potential of

nano-filler reinforced PBI nano-composite has not been explored yet.

Considering the high thermal and mechanical properties of PBI, CNFs

based PBI nano-composite has great potential to be used for various

aerospace applications. Therefore, the objective of this work is to

manufacture CNFs reinforced PBI film and coating in order to study the

effect of nano-fibers on thermal and mechanical properties of PBI. CNFs

were selected as nano-filler due to the ease of dispersion and relatively

low cost as compare to CNTs.

Chapter 2

13

2.2.1. Processing issues of Carbon Nano-fibers

Dispersion of CNFs into polymer matrix is not an easy task. There are

numbers of challenges arise from the small size of CNFs during the

manufacturing process. Although significant advances are made in

recent years to overcome the problems of dispersing CNFs, processing

still remains a key challenge in fully utilizing the properties of these

fibers. A primary difficulty while adding the nano-filler to the polymer is

to attain a good dispersion of the nano-filler, independent of filler shape

and aspect ratio. Without proper dispersion, filler agglomerations tend

to act as defect sites which limit the mechanical performance of the

nano-composite. Therefore, it is very important to get the uniform

dispersion of CNFs in the polymer matrix to take the full advantage of

nano-fibers reinforcement. Different dispersion methods are being used

including the dilution method, mechanical stirring, bath sonication,

melt spinning, extrusion processing and high shear mixing [16, 17].

Researchers have obtained well dispersed carbon nano-fillers in a

polymer by high energy sonication of polymer solution containing

dispersed nano-fillers followed by a solvent-evaporation method which

successfully achieved homogeneous composites [17, 18].

2.3. Experimental

2.3.1. Materials

PBI powder having molecular weight of 20,000 g/mole was supplied by

PBI Performance Products. 26% concentrated solution of

Polybenzimidazole (PBI) in Dimethyl-acetamide (DMAc) was supplied by

PBI Performance Products. This solution was used for casting the film of

unfilled PBI and CNFs reinforced PBI. CNFs with diameter ranging from

70 nm to 200 nm and length 50 μm to 200 μm were supplied by

Pyrograf Products, Inc. with trade name PR-19-XT-LHT.

2.3.2. Processing of Polybenzimidazole by Compression Molding

Processing of PBI powder was carried out using compression molding.

As PBI is not melt processable, therefore, during compression molding

process, the polymer is heated above its glass transition temperature.

The pressure was varied during compression molding in order to get

specimens with high tensile properties. Following are the three different

methods that were used to make the specimens using PBI powder.


14

Method 1

In first method, mold was filled with required amount of PBI powder

and placed in the press. Mold was heated to 440°C at a rate of

5°C/min. It was kept at 440°C for 30 minutes to get a uniform heat

distribution followed by applying a pressure of 10 MPa. The mold was

kept at this pressure and temperature for 3 hours. After 3 hours, the

mold was allowed to cool down to temperature of 50°C. Afterwards,

pressure was released and the molded part was removed.

Method 2

In second method, mold was filled with required amount of PBI powder

and placed in the press. Mold was heated to 440°C at a rate of

5°C/min. It was kept at 440°C for 30 minutes to get a uniform heat

distribution followed by applying a pressure of 50 MPa. The mold was

kept at this pressure and temperature for 3 hours. After 3 hours, the

mold was allowed to cool down to a temperature of 50°C. Afterwards,

pressure was released and the molded part was removed. The

difference between first and second method is the change of pressure

during compression molding. The pressure has increased from 10 MPa

to 50 MPa in the second method.

Method 3

In this method, PBI powder was dried at 200°C for 12 hours in forced

air convection oven. After drying the PBI powder, mold was filled and

placed in the press. Mold was heated to 440°C at a rate of 5°C/min. It

was kept at 440°C for 30 minutes to get a uniform heat distribution.

Then a pressure of 50 MPa was applied on the mold with same

temperature for 3 hours. During this period, the pressure was released

for 30 seconds in order to escape any gas evolved during heating

process. After 3 hours, the mold was allowed to cool down to a

temperature of 50°C. At this point, the pressure was released and the

molded part was removed.

2.3.3. Solution casting of Basic PBI Film

In present work, efforts were made to fabricate PBI films using PBI

solution in DMAc. The as-received solution of PBI in DMAc was highly

viscous and therefore, it was diluted to 17% by adding DMAc followed

by mechanical stirring at 60°C for 15 minutes. The mixture was then

used to produce 60 to 80 µm thick film. The film was prepared by

spreading the mixture over the glass plate with the help of adjustable

Chapter 2

15

doctor blade. The film was allowed to dry in the vacuum oven at 80°C

for 2 hours and then it was further heated at 200°C for overnight. Then

the film was peeled off from the glass plate by immersing in the hot

distilled water. After removing the film from hot distilled water,

wrinkles were formed in the film more likely due the presence of some

DMAc and moisture. Therefore, in order to remove the wrinkles before

mechanical testing, the film was pressed in the hot press at 250°C and

at a pressure of 0.5 MPa.

2.3.4. Solution casting of PBI nano-composite film

In present work, PBI nano-composite film was prepared using

mechanical stirring and bath sonication methods. In first method, pre-

calculated amount of CNFs was carefully weighed and directly mixed in

the diluted solution of PBI. The mixture was then stirred mechanically

using IKA stirrer at 300 rpm for 30 minutes. In another method,

calculated amount of CNFs was weighed and added to the DMAc

solvent. CNFs were dispersed in DMAc solvent using bath sonication for

30 minutes at 60°C. After bath sonication of CNFs in DMAc, they were

added to PBI solution. The sonication process in combination with

mechanical stirring was continued for next 15 minutes. The mixture

was then used to cast the film on the glass plate as described in section

2.3.3. The nano-composite films were prepared with CNFs contents of

0.5 wt%, 1 wt% and 2 wt%.

2.4. Thermal and Mechanical Characterization

2.4.1. Thermal Gravimetric Analysis (TGA)

Thermal gravimetric analysis (TGA) was conducted to determine the

thermal stability of PBI powder, compression molded PBI, PBI neat film

and PBI nano-composite film. Tests were performed using Perkins

Elmer Thermal Analysis Instrument (Pyris Diamond Thermal

Gravimetric Analyzer). Compression molded samples were cut into

small pieces using a cutter to get the small size specimens for TGA test.

The weight of all the samples was maintained between 5 to 10 mg. The

samples were heated from a temperature range of 25°C to 550°C at a

heating rate of 10°C/min. The furnace was purged with nitrogen gas to

prevent oxidation at a flow rate of 25ml/min.


16

2.4.2. Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis (DMA) was performed in tensile mode at

an oscillation frequency of 1 Hz using Perkin-Elmer dynamic mechanical

analyzer (Pyris Dynamic Mechanical Analyzer). Samples of compression

molded PBI were cut into rectangular shape to final dimensions of

(40x8x1.2) mm3. PBI neat film and nano-composite films were cut into

rectangular shape having dimension of (40x8x0.06) mm3. Data was

collected from 25oC to 500oC at a scanning rate of 3oC/min. Elastic

modulus and loss factor of the all the samples were measured.

2.4.3. Tensile Testing

Tensile testing of compression molded PBI, unfilled PBI film and nano-

composite films was carried out using Zwick Tensile machine at a test

speed of 2 mm/min at 20°C. Rectangular specimens of 100x10x0.06

mm3 were cut from the casted films. Compression molded specimens

were machined to Dumbbell-shape according to ISO-527 for tensile

testing. Five specimens for each material were tested for the

reproducibility of the results. Force-displacement curves were recorded

from which Young’s Modulus and tensile strength was evaluated. An

extensometer was also used to determine the exact value of Young’s

Modulus.

2.4.4. Scanning Electron Microscopy (SEM Analysis)

Scanning electron microscopy of gold coated films was carried out to

examine the dispersion of CNFs in the polymer matrix and to study the

fracture surfaces after mechanical testing. Images were obtained using

JOEL-7500-FE scanning electron microscope.

2.5. Results and Discussion

2.5.1. Compression Molding of PBI

Compression molding process was used to fabricate molded parts of

PBI using PBI powder. The aim was to manufacture the specimens with

high tensile strength. Three different methods were adapted to

manufacture the specimen using compression molding process. In first

method, mold was heated to 440°C followed by applying a pressure of

10 MPa. The mold was kept at this pressure and temperature for 3

hours. The specimen obtained after compression molding is shown in

figure 2.1a. The specimen was not fully compacted and ultimately

Chapter 2

17

broken while removing from the mold. Failure in getting proper

specimen is more likely due to the low pressure during compression

molding. This low pressure resulted in incomplete compactness of the

material. Required temperature and pressure was not applied to get

enough compaction of the material so that it can get desired strength.

Figure 2.1 (a) Molded specimen compressed at 10MPa and 440°C (b) Molded specimen compressed at 50 MPa and 440°C (c) Molded specimen compressed at 50

MPa and 440°C using pre-dried PBI powder

In the second method, molding pressure was increased from 10 MPa to

50 MPa. The mold was heated to 440°C followed by applying a pressure

of 50 MPa at 440°C. The mold was kept at this pressure and

temperature for 3 hours. The specimen obtained after compression

molding is shown in figure 2.1b. This time the specimen got enough

compaction and specimen with desired shape is obtained. However, C-

Scan of this specimen depicted some voids in the specimen which was

more likely due to the fact that PBI powder was not dried before

performing the compression molding process. Due to the presence of

moisture, the molded specimen got voids inside which ultimately

resulted in decreased tensile strength. The results of tensile tests are

presented in coming section.

(a)

(b)

(c)


18

In order to prevent voids in the molded specimen, PBI powder was

dried at 200°C for 12 hours in forced air convection oven. After drying

the PBI powder, compression molding process was used to form PBI

molded parts using a temperature of 440°C and molding pressure of 50

MPa. The specimen obtained with this method is shown in figure 2.1c.

Using this method, proper specimen was obtained having no voids.

2.5.2. Processing of PBI neat film and nano-composite film

PBI films were produced to evaluate thermal and mechanical properties

of the polymer film. Unfilled and CNFs reinforced PBI films obtained

after solution casting is shown in figure 2.2.

Figure 2.2 (a) PBI neat film (b) PBI with 2 wt. % of CNF obtained after direct mechanical mixing (c) PBI with 2 wt. % of CNF obtained after bath sonication of CNFs into DMAc solvent followed by mechanical mixing (d) Compression molded PBI sample

Unfilled PBI film is shown in figure 2.2a whereas, PBI nano-composite

films obtained after performing two different dispersion techniques are

shown in figure 2.2b and 2.2c. The nano-composite film obtained with

separate dispersion of CNFs in DMAc solvent using bath sonication is

shown in figure 2.2c. After dispersion of CNFs in the polymer solution,

the color of the film was totally changed from brown, as in the case of

unfilled PBI film, to blackish which could be the first test of examining

the uniform dispersion of CNFs in the polymer matrix. PBI nano-

composite film obtained by directing mixing of CNFs into the PBI resin

has shown agglomeration which is clearly evident from figure 2.2b.

(a)

(d) (c)

(b)

Chapter 2

19

These results indicate that direct mixing of CNFs into PBI resin is not an

effective way to prepare nano-fibers reinforced PBI. Therefore, it can be

concluded that bath sonication of CNFs before adding to PBI solution is

more effective way to get PBI nano-composite films. Compression

molded specimen before mechanical testing is shown in figure 2.4d.


Thermal Gravimetric Analysis (TGA) of PBI powder, compression

molded PBI, PBI neat film and PBI nano-composite film was carried out

to determine the thermal stability of PBI. Figure 2.3a represents a

comparison of thermal stability of PBI in different forms.

Figure 2.3 Comparison of thermal stability of (a) PBI in different forms (b) Comparison of thermal stability of unfilled PBI and nano-filled PBI film with 0.5 wt%, 1wt% and 2wt% of CNFs

TGA curve of PBI powder has shown two steps decomposition. PBI has

shown initial weight loss of 2.35% which occurred between 40°C and

130°C. This initial weight loss is more likely due to the evaporation of

water. The weight loss at the initial stage can be minimized by drying

the powder in the oven. After first degradation step, weight loss starts

very slowly but continuously until the temperature reached to 550°C.

PBI has depicted only 5% weight loss up to this temperature. The high

thermal stability of PBI is due to the presence of aromatic and

heterocyclic ring in the polymer chain. The second degradation step

started around 500°C. The weight loss around 500°C is more likely due

to the loss of phenol and other gaseous products which is confirmed by

experiments [19]. Due to high thermal stability, PBI has real potential

to be used for high temperature applications. Comparing the thermal

decomposition behavior of compression molded PBI with PBI powder, it

52

62

72

82

92

102

0 100 200 300 400 500 600

Wei

ght

(%)

Temperature (°C)

PBI-PowderPBI-MoldedPBI Film

a b c

a

b

c (a)

52

62

72

82

92

102

0 100 200 300 400 500 600

Wei

ght

(%)

Temperature (°C)

Unfilled PBI

PBI+0.5%CNF

PBI+1%CNF

PBI+2%CNF

a

b d

c

a b c d

(b)


20

is observed that compression molded PBI has high thermal stability at

initial stage up to a temperature range of 200°C. The high thermal

stability of compression molded specimen is more likely because of its

exposure to the high temperature during compression molding process.

High temperature has dried the material and removes moisture which

was present in PBI powder. On the other hand, for compression molded

PBI, the weight loss occurred continuously up to a temperature of

325°C without any stable plateau. After this temperature, the polymer

exhibits high thermal stability and only 0.2% weight loss occurred from

a temperature of 400°C to a temperature of 550°C. Compression

molded PBI has depicted a total weight loss of 6.5% up to the

temperature of 550°C which is the indication of high thermal stability of

any polymer at this temperature.

TGA curve for unfilled PBI film and carbon nano-fibers reinforced PBI

film is shown in figure 2.3b. TGA curve for unfilled PBI film shows two

steps degradation. The first degradation step started around 50°C and

continued up to 150°C with a total weight loss of 9%. The weight loss

is more likely due to absorption of moisture by the polymer and any

DMAc remained inside the polymer film. After first degradation step,

the plateau of TGA curve remained stable and polymer has shown only

10% total weight loss up to the temperature of 400°C. Afterwards,

second degradation step started and unfilled PBI has depicted a total

weight loss of 13% up to the temperature of 550°C. Addition of 0.5 wt.

% and 1wt % of CNFs to PBI has improved the thermal stability of PBI.

Addition of CNFs to PBI has reduced the first degradation step and

improved the overall thermal stability of polymer. After addition of

CNFs to PBI, it has depicted a total weight loss of 8% up to the

temperature of 550°C. Furthermore, addition of 2 weight % of CNFs to

PBI has almost eliminated the first degradation step. These results

indicate that addition of CNFs to PBI has reduced the moisture

absorption ability of PBI.


Dynamical Mechanical Analysis (DMA) is performed in tensile mode at a

fixed frequency of 1Hz to study the storage modulus (E’) as a function

of temperature and to determine the glass transition temperature (Tg).

A comparison of storage modulus of compression molded PBI and PBI

film is shown in figure 2.4a.

Chapter 2

21

Figure 2.4 Comparison of (a) storage modulus and (b) loss factor curve of molded PBI and PBI film

Figure 2.4a reveals that compression molded specimen has higher

value of storage modulus throughout the temperature range when

compared to the storage modulus of PBI film. One of the possible

explanations of low storage modulus of PBI film is the DMAc solvent

which could be present in the film. The presence of DMAc solvent could

have an effect on the stiffness of PBI film. The decrease in storage

modulus after the first plateau is typically related to the small

movement of polymeric molecules in the side chains [20]. This is the

typical characteristic of an amorphous polymer which shows two

plateaus in the storage modulus and then there is a sharp decrease in

storage modulus. The sharp decrease in storage modulus after second

plateau is due to the large movements of the molecules in the polymer

backbone which cause the polymer to get softens and thus a decrease

in stiffness of the polymer is observed [20].

When compared the loss factor curves of both compression molded PBI

and PBI film, it is observed that compression molded PBI has a glass

transition temperature (Tg) around 430°C. This Tg is higher than the Tg

of unfilled PBI film as shown in figure 2.4b. On the other hand, PBI film

has a wider loss factor curve which indicates the better cross linking of

PBI film.

A comparison of storage modulus of unfilled PBI film and CNFs

reinforced PBI films is shown in figure 2.5a. The figure demonstrates

that addition of 0.5 wt. % of CNFs has increased the storage modulus

up to 88% (from 4.2 to 8 GPa). By further adding CNFs in polymer up

to 1 wt. %, a small decrease in storage modulus is observed but this

value is still higher than storage modulus of unfilled PBI film. An

1E+06

1E+07

1E+08

1E+09

1E+10

0 100 200 300 400 500 600

Sto

rage

Mo

du

lus

(E')

Temperature (°C)

PBI-MoldedPBI Film

a

b

a b

(a)

0

0,2

0,4

0,6

0,8

1

1,2

0 100 200 300 400 500 600

Loss

Fac

tor

(tan

δ)

Temperature (°C )

PBI-Molded

PBI Film

b

a

a b

(b)


22

improvement of about 50% in storage modulus (from 4.2 to 6.4 GPa) is

observed with 2 wt. % of CNFs.

Figure 2.5 Comparison of (a) storage modulus and (b) loss factor curve of unfilled PBI and nano-filled PBI film with 0.5 wt%, 1wt% and 2wt% of CNFs

The loss factor curve of the unfilled PBI film and PBI nano-composite

measured by DMA is shown in figure 2.5b. The curve represents two

transition peaks at different temperatures. The transition peak at

higher temperature is normally referred as glass transition (Tg) which

occurs when the main polymeric chains rotate freely and this peak is

associated with a substantial fall in material stiffness. The peak height

of loss factor increased and became narrower by adding 0.5 wt. of

CNFs. By further adding CNFs, a decrease in peak height of loss factor

curve is observed. However, there is a broadening of the peak in the

loss factor curve is observed which is due to the unconstrained segment

of polymeric molecules.

A secondary transition called β transition is also observed in the loss

factor curve of neat PBI and PBI nano-composite. Addition of 2 weight

% of CNFs has significantly reduced the peak height of tanδ curve

which is an indication of increased elasticity and reduced damping

properties of the material [15, 16]. Also, by adding CNFs to polymer,

the value of secondary transition temperature has increased up to 30oC

which indicate that CNFs have restricted the motion of the side chain

molecules very effectively. All these results indicate that addition of

CNFs has improved the performance of PBI to such an extent that it can

maintain high storage modulus even at a temperature of 350oC which

shows the potential of nano-filled PBI for high temperature applications.

A summary of storage modulus of unfilled and nano-filled PBI at

different temperatures is presented in table 2.1.

1E+06

1E+07

1E+08

1E+09

1E+10

0 100 200 300 400 500 600

Sto

rage

Mo

du

lus

(E')

Temperature (oC)

Unfilled PBI

PBI+0.5%CNF

PBI+1%CNF

PBI+2%CNF

a b c d

a b

c d (a)

0

0,2

0,4

0,6

0,8

1

1,2

0 100 200 300 400 500 600

Loss

Fac

tor

(tan

δ)

Temperature (oC)

Unfilled PBIPBI+0.5%CNFPBI+1%CNFPBI+2%CNF

a b c d

a

b

c

d

(b)

Chapter 2

23

Table 2.1 Storage Modulus of compression molded PBI, PBI neat film and nano-composite films as a function of temperature

Materials

E’30°C

(GPa)1

E’100°C

(GPa)

E’200°C

(GPa)

E’250°C

(GPa)

E’300°C

(GPa)

E’350°C

(GPa)

E’400°C

(GPa)

Glass Transition

Temperature (Tg)2

(oC)

Compression molded PBI 6.9 6.8 4.9 3.8 3.2 2.8 2.1 430

Unfilled PBI Film 4.2 3.9 3.7 3.1 2.1 1.2 0.2 416

PBI-0.5% CNF nano-composite 8.0 7.4 6.5 5.6 3.9 3.0 0.7 421

PBI-1% CNF nano-composite 5.8 5.7 4.9 3.8 2.4 1.1 0.1 418

PBI-2% CNF nano-composite 6.4 6.3 5.6 4.6 2.3 1.6 0.8 419

1 = Value of storage modulus with an error of ±0.05GPa 2 = Glass Transition onset of peak of loss factor curve

Processing and Characterization of Unfilled and nano-filled PBI

24

Table 2.1 shows that both compression molded PBI and unfilled PBI film

have retained high storage modulus even at a temperature of 350oC.

Furthermore, addition of CNFs to PBI has further improved the

performance of PBI film and film has retained even higher storage

modulus at 350oC. Table 2.1 also represents the glass transition

temperature measurement as an onset of maximum peak height of loss

factor curve. Both unfilled PBI film and nano-filled PBI films have

depicted very high glass transition temperature as reported in the table

2.1.

2.5.5. Tensile testing

Tensile tests were performed on molded PBI, unfilled PBI films and PBI

nano-composite films. A comparison of tensile test results for each

material is presented in table 2. Table 2.2 Tensile test results for compression molded PBI, PBI neat film and PBI nano-composite film

Materials

Tensile

Strengthc

(MPa)

Std. Dv.

(MPa)

Elastic Modulus

(GPa) 0.05 GPa

Strain

(%)

Compression molded

PBI-1a 112 6.3 5.6 2.3

Compression molded

PBI-1b 145 4.4 6.7 3.1

unfilled PBI Film 157 3.1 5.5 13.6

PBI+0.5% CNF nano-

composite 165 4.1 6.0 11.9

PBI+1% CNF nano-

composite 171 2.2 6.4 10.2

PBI+2% CNF nano-

composite 181 4.9 7.1 7.5

a: PBI powder was not dried before performing the compression molding b: PBI powder was dried in oven at 200OC for 1hours before performing the compression molding

c: Results are the average of five replicate specimens

Compression molded PBI processed at a temperature of 440oC and at a

pressure of 50 MPa has exhibited a tensile strength of 112 MPa whereas

pre-dried PBI powder molded at a temperature of 440oC and at a

pressure of 50 MPa has depicted a tensile strength of 145 MPa which is

the highest tensile strength of any unfilled polymer. In the first case,

PBI powder was not dried and it contained moisture. Also, the pressure

is applied continuously on the PBI powder during heating, the moisture

Chapter 2

25

and decomposed gases resulting from the heating of PBI could not

escape from the molded PBI and therefore, remains therein as voids.

These voids caused cracking of the specimen which resulted in

decreased tensile strength. The decomposed gases comprise the vapor

resulting from the reaction of lithium chloride (this is added to the PBI

resin as a stabilizer) with the PBI resin at high temperatures [19]. The

decomposed gases include CO, CO2, CH4, chloroform and phenol [19].

These voids are the main cause of low tensile strength of PBI specimen

when molded at low pressure.

A comparison of tensile strength and Young’s Modulus of PBI neat film

and PBI nano-composite film is illustrated in figure 2.6. A tensile

strength of 157 MPa is achieved for unfilled PBI film. Further increase in

strength is observed by adding 0.5% and 2% CNFs in the polymer as

shown in figure 2.6. A 9% increase in tensile strength is achieved by

dispersing 1% CNFs in the PBI. Addition of 2 wt. % of CNFs has

increased the tensile strength of PBI from 157 MPa to 181 MPa; an

improvement of about 15%. The value of Young’s modulus has also

increased from 5.5 GPa to 7.1 GPa; an improvement of about 30%.

However, addition of 2 weight % CNFs to PBI has decreased the tensile

strain from 13.6% to 7.5%; a decrease of about 45%.

Figure 2.6 Comparison of tensile strength and Young’s Modulus of PBI neat film and nano-composite film with different CNFs loadings

5,5

6

6,5

7

7,5

8

140

150

160

170

180

190

0 0,5 1 1,5 2

You

ng'

s M

od

ulu

s (G

Pa)

Ten

sile

Str

engt

h (

MP

a)

CNFs Weight percent (%)

Tensile Strength

Young's Modulus


26

2.5.6. Scanning Electron Microscope (SEM) Analysis

As mentioned earlier that primary difficulty while adding the nano-filler

to the polymer is to attain good dispersion of the nano-fillers. Without

proper dispersion, filler agglomerations tend to act as defect sites which

limit the mechanical performance of the nano-composite. Therefore,

dispersion of CNFs into polymer resin is a key challenge in fully utilizing

the properties of these fibers. Scanning electron microscopy of neat PBI

and PBI nano-composite was carried out to examine the dispersion of

CNFs. CNFs were dispersed by two different methods. First method was

the direct mixing of CNFs into PBI and in second method; CNFs were

separately dispersed in DMAc solvent followed by mechanical mixing of

CNFs in to PBI resin. SEM analysis was carried out on the samples

obtained by both these methods and SEM micrographs are shown in

figure 2.7.

Figure 2.7 SEM analyses of PBI neat film and nano-composite film with different

nano-fibers loadings (a) Unfilled PBI film (b) PBI-0.5 wt. % CNFs (c) PBI-2 wt. %

CNFs produced by separate mixing of CNFs into DMAc and then added to PBI (d) PBI-

2 wt. % CNFs, produced by direct mixing of CNFs in to PBI (3000x magnification)

Figure 2.7 (c) shows that PBI film reinforced with 2 wt% of CNFs and

obtained by second method has depicted good dispersion of CNFs into

100 μm

100 μm

1 μm

(a) (b)

(d) (c)

Chapter 2

27

PBI resin Whereas samples prepared by direct mixing of 2 wt% of CNFs

to PBI has shown large agglomeration area as shown in figure 2.7 (d).

2.5.7. Fracture morphology

Scanning electron microscopy was also carried out to examine the

fracture morphology after tensile testing of unfilled PBI film, PBI nano-

composite film and compression molded PBI. A comparison of fracture

morphology of unfilled PBI, nano-filled PBI and compression molded PBI

is shown in figure 2.8.

Figure 2. 8 Comparison of SEM micrographs of fractured surfaces of compression

molded PBI, PBI unfilled film and nano-composite film with different nano-fibers loadings (a) Unfilled PBI film (b) PBI+0.5wt.% CNFs (c) PBI+2 wt.% CNFs (d) Compression molded PBI (3000x magnification)

A ductile failure is observed for unfilled polymer film as shown in figure

2.8 (a). The fracture surface has shown a wave like pattern which is

more likely due to the plastic yielding occurred locally when the load

was applied. By adding 0.5 wt % of CNFs to PBI, a mixed kind of failure

is observed and a phase transition from ductile to brittle failure

appeared. Analyzing the fracture surfaces of nano-composite film

reinforced with 2 weight percent of CNFs, it is observed that

100 μm

1 μm

(a) (b)

(c) (d)


28

deformation occurred in different directions. This is more likely due to

the fact that crack growth is diverged from one direction to another as

a result of interaction with the CNFs in PBI matrix. A much rougher

fracture surface is observed upon adding 2 wt. % of CNF to PBI as

shown in figure 2.8 (c). The increased surface roughness implies that

the path of the crack tip is distorted because of the CNFs, making crack

propagation more difficult. Fracture surface of compression molded PBI

is shown in figure 2.8 (d). Compression molded specimen failed in a

brittle fashion both on microscopic and macroscopic level. Cracks are

formed, and there is no sign of large-scale plastic deformation.

2.6. Conclusions

In this study, efforts were made to develop CNFs reinforced PBI nano-

composite. Processing of PBI was carried out using PBI both in powder

and solution form. Thermo-mechanical properties of compression

molded PBI, unfilled PBI film and nano-composite films were

investigated using TGA, DMA and tensile testing. TGA analysis shows

that both compression molded PBI and PBI film have depicted only 5%

and 9% weight loss respectively up to a temperature of 550oC which

indicate that PBI has high thermal stability. DMA studies indicate that

both compression molded PBI and unfilled PBI film exhibit very high

storage modulus even at a temperature of 300oC. PBI nano-composite

films were casted with different loading of CNFs. Addition of CNFs has

improved the thermal stability and storage modulus of PBI film.

Mechanical testing shows that both compression molded PBI and PBI

film have the highest ultimate tensile strength compared to any unfilled

polymer. Scanning electron microscopy has confirmed the uniform

dispersion of CNFs in polymer solution. Analysis of fractured surfaces

revealed that a ductile failure was occurred for unfilled PBI film.

Contrary to the unfilled PBI, a much rough fracture surface is observed

in case of PBI nano-composite which implies that the path of the crack

tip is distorted due to the presence of CNFs. Addition of CNFs to PBI has

made the crack propagation more difficult. These results end up with

the conclusion that both unfilled PBI and nano-filled PBI have excellent

thermal and mechanical properties and these properties make PBI a

potential candidate for future aerospace applications. Furthermore,

addition of CNFs to PBI polymer has improved both thermal and

mechanical properties of PBI which will be beneficial for different high

temperature applications of polymer.

Chapter 2

29

2.7. References

1- Paul A. Steinerand Robert Sandor, “Polybenzimidazole Prepreg: Improved Elevated Temperature Properties with Autoclave Processability, “High performance polymers, Vol. 3, No. 3, 1991, pp. 139-150

2- D.J.Kemmish, High performance engineering plastics, Rapra review reports,

Vol. 8, No. 2, Report 86, 1995

3- Sylvie beland, High performance thermoplastic resins and their composites, 1st ed., ISBN-08155 1278-3, Noyes, New Jersy, 1990, pp. 47-50

4- S. Bhowmik, H. W. Bonin, V. T. Bui, R. D. Weir, Modification of high-

performance polymer composite through high-energy radiation and low-pressure plasma for aerospace and space applications, “Jouranl of Applied Polymer Science, Jan 2006, Vol. 102, 2006, pp. 1959-1967

5- O. R. Hughes, O. N. Chen, W.M. Cooper, L. P. Disano, E. Alvarez and T.E. Andres,

PBI powder processing to performance parts, Journal of Applied Polymer Science, Published online 10 March 2003, Vol. 53, No. 5,1994, pp. 485- 496

6- Li Zhang, Qing-Qing Ni, Akihiko Shiga, Yaqin Fu, Toshiaki Natsuki, “Polymer

Composites, “Synthesis and Mechanical Properties of Polybenzimidazole Nanocomposites Reinforced by Vapor Grown Carbon Nanofibers, 2009, Vol. 31, pp. 491-496

7- Minoru Okamoto, Tsuyohiko Fujigaya, and Naotoshi Nakashima, “Individual

Dissolution of Single-Walled Carbon Nanotubes by Using Polybenzimidazole, and Highly Effective Reinforcement of Their Composite Films, “Advanced Functional Materials, 2008, Vol. 18, pp.1776–1782

8- Milo S.P. Shaffer and Jan K.W. Sandler, Carbon Nanotube/Nanofibre Polymer

Composites, Department of Chemistry, Imperial College London

9- Farzana Hussain, M.H., Masami Okamoto and Russell E. Gorga, Polymer-matrix Nanocomposites, Processing, Manufacturing, and Application: An Overview, 1511,Journal of Composite Materials, 2006. 40(17): p. 65.

10- Aruna Kumar Barick, Deba Kumar Tripathy, Effect of nanofiber on material properties of vapor-grown carbon nanofiber reinforced thermoplastic polyurethane (TPU/CNF) nanocomposites prepared by melt compounding, composite Part A, 2010, Vol. 41, pp. 1471-1482

11- Zhongfu Zhao and Jan Gou, Improved fire retardancy of thermoset composites

modified with carbon nanofibers, Sci. Technol. Adv. Mater. 10 (2009) 015005 (6pp)


30

12- Joseph H. Koo, Polymer Nanocomposites, processing characteristics and applications, Department of Mechanical Engineering The University of Texas at Austin Austin, Texas

13- Bernadette A. Higgins, William J. Brittain, Polycarbonate carbon nanofiber composites, European Polymer Journal 41 (2005) 889–893

14- Shuying Yang, Jaime Taha-Tijerina, Vero´nica Serrato-Diaz, Krystal Hernandez, Karen LozanoDynamic mechanical and thermal analysis of aligned vapor grown carbon nanofiber reinforced polyethylene, Composites: Part B 38 (2007) 228–235

15- M.C. Saha, Md.E. Kabir, S. Jeelani , Enhancement in thermal and mechanical

properties of polyurethane foam infused with nanoparticles, Materials Science and Engineering A 479 (2008) 213–222,

16- S.G. Prolongo, M.B., M.R. Gude, R. Chaos-Morán, M. Campo, A. Urena, Effects of dispersion techniques of carbon nanofibers on the thermo-physical properties of epoxy nanocomposites. Composites Science and Technology, 2008. 68: p. 9.

17- Young-Kuk Choi, K.-i.S., Sung-Moo Song, Yasuo Gotoh, Yutaka Ohkoshi,

Morinobu Endo, Mechanical and physical properties of epoxy composites reinforced by vapor grown carbon nanofibers Carbon 2005. 43: p. 10.

18- Peng He, Y.G., Jie Lian, Lumin Wang, Dong Qian, Jian Zhao, Wei Wang, Mark J.

Schulz, Xing Ping Zhou, Donglu Shi, Surface modification and ultrasonication effect on the mechanical properties of carbon nanofiber/polycarbonate composites Composites: Part A, 2006. 37: p. 6.

19- M. Kurisaki, Y.Sasaki, U.S. Patenet 5770142, “Process for manufacturing

sintered polybenzimidazole article, filed 13 June 1997

20- P.M.Kevin, Dynamic Mechanical Analysis: A practical Introduction, 2nd edition, Florida (1999)

31

CHAPTER 3

Fire Testing of Nano-fibers Reinforced Polybenzimidazole Coating for Aircraft Application

In this chapter, a brief overview of combustion process and

decomposition mechanism of polymeric materials is

presented. Fire reaction properties of uncoated carbon/epoxy

composite and composite with unfilled and carbon nano-fibers

reinforced PBI coating are studied using thermal gravimetric

analysis and cone calorimeter test and results are presented.

3.1. Introduction

In recent times, the use of polymer based composite materials is

becoming significantly popular. Composites materials, with their wide

range of possible fillers and resins, have opened the way to an

enormous range of materials with diverse chemical, physical, and

mechanical properties. These materials have an impressive and diverse

range of applications in automotive, aviation, spacecrafts, civil

infrastructure and sports industry [1-4]. In recent years, there has

been a steady increase in the use of composite materials in both

military and commercial aircraft. The body of Boeing 777 aircraft had

consumed about 12% of polymer composite and 50% of Aluminum by

weight. The weight percent of composite has increased from 12 percent

to 50 percent in the newly developed Boeing 787 and A380 passenger

aircraft [3, 5-7]. The increasing interest in the use of composite

materials is due to some advantages of composites materials over

many metallic materials. The key advantages are low weight, high

strength to weight ratio, excellent corrosion resistance, outstanding

Fire Testing of Nano-fibers Reinforced PBI coating for Aircraft Application

32

thermal insulation and low thermal expansion. However, there are

some disadvantages of composite materials and these include low

through thickness mechanical properties, poor impact damage and poor

performance under fire [3].

An important consideration while using composites in load-bearing

structures is their mechanical integrity in the event of fire. When

composite materials are exposed to high temperature of 300oC to

400oC, organic matrix decomposes with the release of heat, smoke and

toxic volatiles. Softening and decomposition of the polymer matrix

caused by fire rapidly reduces the properties of composite material

which ultimately lead to collapse of the composite structure [8].

Although polymer based composites are flammable at lower

temperature than Aluminum, yet these materials possess some

inherently useful properties which are not the characteristics of metals.

The rate of heat conduction of composite material is much lower than

metals [3]. The low heat conduction rate is significantly beneficial in

slowing the fire spread rate. Other important characteristic of many

polymer composites is that they can provide protective barrier in the

form of char (carbonaceous layer) against flame, heat smoke and toxic

fumes.

Due to their high flammability, epoxy composite cannot be used in the

aircraft cabin. Phenolic based composites have better fire resistant

properties. Therefore, phenolic based composites are most commonly

used material in the aircraft cabin and they account for 80 to 90

percent of the interior furnishings of modern passenger aircraft [4].

However, these materials have low mechanical properties [9].

Therefore, phenolic based composites are not being used as structural

components in the airframe of the aircraft. It is very important that

composite materials used in the structural parts of aircraft should retain

their load bearing properties during and after a fire. One of the worst

accidents due to fire in the aircraft occurred in May 1996 when a

ValuJet DC-9 crashed in Florida. The fire developed in the cargo after

the take-off and soon after, the aircraft crashed resulted in the death of

all 105 passengers and crew members. The investigation about the

accident has shown that failure of flight control systems due to extreme

heat and structural collapse resulted in the loss of control of the aircraft

[3]. This incident demonstrates the importance of fire resistant

properties of structural components.

Chapter 3

33

Fire in the aircraft is extremely hazardous because of little time for

response. During a fire in an aircraft, the flight crew has two minutes to

escape and the pilots have about 14 minutes to land and evacuate

safely [3]. The growing use of composite material in aircraft highlights

the importance of understanding fire reaction and fire resistant

properties of these materials. In the coming sections, an understanding

about the decomposition mechanism of polymeric based materials will

be presented. After having an understanding of fire reaction properties

of these materials, efforts will be devoted to develop fire resistant PBI

coating for aircraft application.

3.2. Combustion Process of Polymers

Polymeric materials undergo both physical and chemical changes when

exposed to heat. The chemical processes are responsible for the

generation of flammable volatiles whereas physical changes are

responsible for melting and charring which can markedly alter the

decomposition and burning characteristics of materials [10, 11]. The

behavior of polymer based composite material in fire is governed

mainly by the chemical processes which are responsible for thermal

decomposition of the polymer matrix and fibers. In case of fire, polymer

combustion occurred as a cycle of coupled events namely, heating of

the polymer, decomposition, ignition and combustion. Heat transferred

to the polymer results in increased temperature of the polymer to a

point where it starts to decompose and gives out non-combustible

(CO2, H2O) and combustible volatiles (hydrocarbons, carbon mono

oxide and monomers). These combustible volatiles diffuse into the

flame zone above the burning polymer and undergo the combustion

process. As a result, large amount of heat is evolved and under steady

state burning conditions, a part of heat is transferred back to the

polymer surface to sustain the combustion cycle [3, 12]. The overall

burning mechanism of polymer based composite material during a fire


The decomposition products, rate of decomposition and mechanisms of

thermal decomposition not only depends on the chemical composition

of the polymer but also on its physical properties such as glass

transition, melting, and decomposition temperatures. All these

temperatures influence the thermal decomposition processes due to the

fact that polymer undergoes phase transitions at these temperatures.

These phase transitions are responsible for the changes in physical


34

properties of polymer such as thermal conductivity, viscosity, density,

and modulus [13].

Figure 3.1 Thermal decomposition cycle of polymer composite during a fire

The phenomenon of polymer combustion is a combination of gas and

condensed phase [11, 12 and 14]. When polymer is exposed to fire,

large polymeric molecules break into smaller molecules that can

vaporize immediately upon their creation while heavier molecules

remained in the condensed phase. These heavier molecules undergo

further decomposition to lighter fragments which are more easily

vaporized. However it is not always a case that all solid material

becomes fuel vapors. More often, solid residue left behind in the form

of carbonaceous char or inorganic residue [14]. When char forming

materials are thermally decomposed, the volatiles produced due to the

burning of underlying layers must pass through the char above them to

reach the surface. If a uniform layer of carbonaceous char is formed

then it can significantly slow down further decomposition. More often,

the physical characteristics of char dictate the rate of thermal

decomposition of underlying polymeric layers. A continuous low density

char acts as better thermal insulator and it can significantly reduce the

flow of heat from the combustion zone back to the condensed phase

[14].

Chapter 3

35

3.3. Classification of Polymers Based on Thermal Decomposition Mechanism

Based on decomposition mechanism, polymers can be classified in

three categories [3]. In first class of polymers, degradation process

takes place by random chain scission reaction in which all of the

molecular structure is converted into volatile gases on heating and no

char left behind them. The examples of such polymers include some

thermosets and thermoplastics such as polypropylene, polyethylene

Polystyrene and poly methyl methacrylate [3].

The second group undergoes random chain scission, end-chain scission

and chain stripping reactions which leads to the loss of hydrogen

atoms, pendant groups (atoms attached to backbone of polymer) and

other low molecular weight organic groups from the main chain. These

polymers yield a small amount of char of about 5-20% of the original

mass [3]. Examples of such polymers are polyesters, vinyl esters,

epoxies and polyvinyl chlorides.

The third group of polymers is characterized by a high aromatic ring

contents that decomposes into aromatic fragments which results in the

formation of char. The examples of such polymers are highly aromatic

thermosets (Phenolics, polyimides, cyanate esters) and some

thermoplastics (PPS, PI, PEEK, PBI). These polymers tend to form a

carbonaceous char residue through cross-linking reaction. The char

yield is the mass fraction of carbonaceous char that remains after

flaming combustion of the polymer.

Study shows that polymers with aliphatic hydrocarbon such as PE, PP,

and PS are much more flammable than the polymers that contain

aromatic, hetero-aromatic rings with hetero atoms such as N, S and O

[12]. As the decomposition of aliphatic polymers starts by bond

scission, low molecular fragments are generated which can easily

evaporated and provide additional fuel to fire. However, aromatic

polymers usually form relatively large aromatic fragments that can be

kept in solid state for a longer time. Also, aromatic polymers usually

produce relatively high char yields. The char can reduce the amount

and release rate of volatile fuels and act as a barrier for heat and mass

transfer. Therefore, aromatic polymers generally have lower

flammability [12]. Based on above discussion it can be summarized


36

that, in general, the ideal fire-resistant polymers should have the

following characteristics:

a. high decomposition temperature

b. Ability to form high char yield

c. low amount and release rate of volatile fuels

d. release of chemical flame-retardant molecules such as halogen

and water

3.4. Effect of Char Formation on Fire Resistant Properties of Polymers

When polymeric material is exposed to a sufficiently large heat flux

radiated from a fire, the polymer matrix decomposes to yield volatile

gases, solid carbonaceous char and airborne soot particles (smoke).

Char formation at the surface of the polymer during the decomposition

slows down further decomposition of underlying polymeric layers. The

physical structure of char is very important in slowing down the

decomposition of a polymer [14]. The important characteristics of char

are density, continuity, adherence, oxidation resistance and thermal

insulation properties. A dense char layer with low thermal conductivity

over the exposed surface acts a thermal insulator for underneath

polymer and at the same time it acts as physical barrier to combustible

gaseous products [11].

Aromatic rings are the basic building blocks from which char is formed.

The higher the aromatic content of the polymer, the higher the char

yield is obtained. Studies on different polymers with varying amount of

aromatic contents in their backbone reveal that the char yield increases

linearly with the concentration of multiple-bonded aromatic ring groups

in the polymer system. Figure 3.2 shows the relationship between the

density of aromatic groups in the back-bone of polymers against their

percentage yields of volatile gas and char [3]. There is a linear

correlation between the density of aromatic groups and char yield, with

a corresponding decrease in volatiles. Hence, increasing the aromatic

contents in polymer backbone generally results in better flame

retardant properties of polymer. Figure 3.2 shows that phenolic resins

which are well known for their fire resistant properties, form a char

residue of about 65% with a volatile yield of about 35%. On the other

hand, polybenzimidazole (PBI) which is the candidate coating for this

study, form a residue of about 85% with a volatile yield of about 15%.

Chapter 3

37

This is one of rationale which encouraged the author to evaluate the

fire reaction properties of PBI.

Figure 3.2 Relation between Aromatic contents, char yield and volatile yield for different polymers [adapted from ref. 3]

3.5. Elements of Material Flammability

3.5.1. Time-to-Ignition (TTI)

Ignition is an important fire reaction property of polymeric materials

which describes the fire hazard of these materials. When a polymeric

material is exposed to an external heat flux, its surface temperature

starts to rise and after a certain time, the surface temperature reaches

to a point where pyrolysis of material begins. During this process,

flammable volatiles are generated which flow back from polymer into

the fire. When the concentration of these volatiles reaches to a critical

value, the polymer gets ignited and flaming combustion starts. The

ease of ignition of a polymer is characterized by the time-to-ignition,

which is the minimum time that a material can withstand external heat

flux before it gets into sustained flaming combustion. The ignition time

depends on various factors such as oxygen availability, temperature

and thermo-physical properties of the polymer matrix [3, 13].


38

3.5.2. Heat Release Rate (HRR)

Heat release is the thermal energy produced per unit area of surface

when exposed to external heat flux. It is the single most important fire

reaction property of polymers and polymer based composite materials

which provide the information about the material response during a

fire. Higher heat release rate (HRR) from a material could result in

rapid fire growth because the heat released by a burning material can

provide the additional thermal energy required for the growth and

spread of fire [3, 15]. Several other flame resistant properties of

materials, such as flame spread rate, smoke generation and CO

emission are related to the HRR [16].

3.5.3. Flame Propagation Index (FPI)

As mentioned earlier that heat release rate of a material during a fire is

the single most important parameter in characterizing the hazard of a

material in fire. Recently, it has been established that fire propagation

index (FPI) which is the ratio of the peak heat release rate (PHRR) to

TTI is a more accurate predictor of the time to flashover in both room

and aircraft compartment fires because it also takes into accounts the

thickness effect of the materials [15].

PHRR (KW/m2) FPI =

TTI (Seconds)

In a compartment fire, the time to flashover is the time available for

escape which is one of the most important factors in determining the

fire hazards of a material in a compartment fire. The time to flashover

can be derived using the following relation [9, 15].

Time to Flashover (Sec) = 991- 631log10FPI

3.5.4. Smoke generation

Smoke production and emission of toxic gases are the most important

fire hazards other than HRR. Polymers and polymer based composites

release dense smoke in a fire. The smoke produced by the burning of

these materials consists of small fiber fregments and soot particles. The

smoke can be extremely dense and limit the visibility which cause the

hindrance of the people trying to escape from fire. Both aliphatic and

aromatic polymers can emit large quantities of smoke during

Chapter 3

39

combustion. However, highly aromatic polymers such as aromatic

polyimides, phenolics and PBI give little smoke [3]. With these

polymers, effective char formation leads to little aromatic fuel release.

Several factors such as high thermal stability of polymers, their char

forming capability and small release of volatile fuels are important

factors which can reduce the smoke generated during a fire [10].

3.6. Methods to Improve the Fire Resistance of Polymers and Polymer Based Composites

Fire resistance of organic polymers can be improved by the following

ways.

a. Addition of Flame retardants to Polymers

b. Addition of nano-fillers to Polymers

c. Application of fire protective coatings

3.6.1. Addition of flame retardants to Polymers

One of the most commonly used approach to get the fire resistant

polymer is the use of fire retardant additives to the inexpensive

polymers such as PP, PE and PS. These polymers can be made fire

retardant by adding some halogen (chlorine, bromine) or phosphorus

additives which enhance the ability of cross linking of these polymers

and thus increase the char formation during the fire [3, 10, 11 and 17].

The action of the flame retardant depends on the structure of the

additive and polymer. Generally, the main flame retardant action of

halogenated polymers is the disruption of the gas phase reactions that

control the flame temperature of a fire. Reactive halogen species are

released from a decomposing brominated or chlorinated polymer into

the flame where they terminate the exothermic decomposition

reactions of organic volatiles, and thereby lower the temperature [12].

The flammability resistance of polymers can also be greatly improved

by the addition of phosphorus. The most commonly used phosphorus

based flame retardants are ammonium polyphosphates and

trialylphosphates. The flame-retardant mechanisms of these

phosphorus compounds include the formation of a glassy layer on the

surface to protect the underlying substrate from flame and enhanced

capability of the material to form char [17].


40

3.6.2. Limitation of Using Fire retardants

Although fire retardants are widely used in polymers but they have

some limitations as well. A major concern with halogenated polymers is

the release of smoke which contains acidic and toxic gases. These

gases are serious hazards for health and environment [12, 17].

Chlorinated polymers release plentiful HCl gas which attacks the

respiratory system and eyes and ultimately hinder the people to escape

from fire. Brominated polymers, on decomposition, produce a variety of

toxic bromine-containing volatiles which also affect breathing. Of

particular concern is the formation of brominated dibenzodioxine

compound [12, 17]. Exposure to a high concentration of dioxins can

cause a variety of health problems, including cancers, skin discoloration

and skin rashes. For these reasons, environmental groups have

pressurized the Governments especially in Europe to ban or severely

restrict the use of halogens [11].

3.6.3. Nano-filler based flame retardant nano-composites

Nano-fillers have been increasingly used to fabricate polymer nano-

composites. Common nano-fillers include nano-clay, carbon nano-tubes

(CNTs) and carbon nano-fibers (CNFs). Recent studies have confirmed

that the addition of these nano-fillers to polymers could reduce the

materials flammability, while improving their mechanical properties

[18]. The interest in polymer-clay nano-composites has developed due

to the fact that the presence of layered silicate materials at relatively

low loading levels of about 3 to 5 weight percent can greatly improve

the mechanical properties, enhance the barrier properties and improve

the fire retardancy of polymers [3]. Layered silicates are widely used

for an improvement in flame retardancy, but carbon nano-tubes (CNTs)

have been proved more efficient [19]. A Better fire resistant

performance of polymer reinforced with CNTs is observed due to the

fact that polymers with CNTs formed a relatively uniform carbonaceous

layer covering the entire sample surface without any cracks or gaps

[20]. The network layer re-emits much of the incident radiations back

into the gas phase from its hot surface. As a result, it reduces the

transmitted flux to the underlying polymer layer and thus slows down

the polymer pyrolysis rate [20]. Just like CNTs, CNFs form continuous

protective barrier layer through the accumulation of nano-fillers on the

combusting surfaces [19]. However, one of the previous studies shows

that the flame-retardant performance of the CNFs reinforced polymers,

containing cracks in the protective char layer, was much poorer than

Chapter 3

41

that of the nano-composites forming a continuous protective network

layer [21]. Thus, the formation of a continuous protective network layer

without any opening or crack is an essential requirement for the large

reduction in heat release rate. Many flammability studies have been

performed on nano-filler based polymers [19, 20, 22-24]. All these

studies have shown improvement in the fire resistant properties of

polymers. Many of these studies were performed using commodity

polymers to study the effect of nano-fillers on polymer flammability.

However, effect of these nano-fillers on flammability properties of

inherently flame retardant polymers has not been studied till date.

Therefore, in this context, one of the objectives of this work is to study

the effect of CNFs on fire resistant properties of inherently flame

retardant Polybenzimidazole (PBI).

3.6.4. Application of fire protective coatings

One of the methods to improve the fire resistant performance of

composite material is to insulate the composite structure using a

thermal barrier protective coating to extend the survival time in high

temperature fires. The purpose of applying a coating on composite

materials is to slow down the heat conduction into the composite from

the fire and thus improve the fire endurance and survivability of

composite materials. The coatings can extend the time to flashover by

slowing the heat-up rate and decomposition rate of the composite

material. Research on composite materials with thermal barrier

coatings have shown an improvement in fire reaction properties of

composites such as increased time-to-ignition and reduced heat release

rate [8]. For effective protection of composite material, it is desirable to

have a coating with low flammability, low thermal conductivity and

strong adhesion.

Coating can be classified into the following three groups.

a. flame retardant paints

b. thermal barriers

c. Intumescent

Flame retardant paints are inherently fire resistant resins that are

applied over the composite substrate. These paints delay ignition and

flaming combustion of the substrate due to their high thermal stability.

The most commonly used fire retardant paints are phenolics,

brominated paints and alkyde resins. These coatings include moderate

cost, light-weight and good chemical compatibility with the composite


42

substrate that ensures good adhesion [3]. Phenolic coatings are an

effective and low cost method to improve the flammability resistance of

composites materials. Phenolic resins improve the fire performance due

to their low yield of flammable volatiles that delays ignition and forms

an insulating char layer. However, a thick layer is required to create an

effective insulation while using phenolic paint. Some commercial

organic coatings do not provide much protection with a coating

thickness of about 0.5 mm [3].

Thermal barrier coatings are usually ceramic-based coatings that are

non-flammable and have low heat of conduction. Examples of these

coatings include silica, rockwool fibrous mats and zirconia (a plasma

spray coating). Thermal barrier coatings provide fire protection due to

the fact that these coatings having excellent insulating properties and

some ceramic based coatings have heat reflective properties which

ultimately help to direct the heat back towards the fire. Despite the

improved fire resistance, ceramic coatings are used carefully on

composites due to their high cost. Furthermore, these coatings are

prone to cracking due to the mismatch in their thermal expansion

properties with the substrate.

Intumescent coatings are excellent heat insulators which slow down the

rate of heat transfer into composite laminates during fire. Intumescent

materials provide fire protection by undergoing a chemical reaction at

elevated temperature that causes the coating to foam and swell. This

reaction produces a thick char layer which has very low thermal

conductivity [3]. A major problem with many commercial instumecent

coating is that they do not bond strongly with the substrate, and often

fall off during swelling, exposing the underlying composite directly to

the flame [3]. Therefore, it is essential that the coating is bonded

strongly to the substrate and has mechanical strength to ensure

adequate fire protection. Another drawback of these coatings is their

incompatibility with certain manufacturing processes, poor durability,

and low resistance to wear and erosion [3].

Based on the discussion in the previous section, in present work, efforts

are devoted to improve the fire resistance performance of aerospace

grade epoxy based carbon fiber composite by applying an inherently

flame retardant coating reinforced with CNFs.

Chapter 3

43

3.7. Experimental

3.7.1. Materials

Solution of PBI in DMAc (with 26% concentration of PBI) supplied by

PBI Performance Products. 99% concentrated N, N-Dimethyl-acetamide

(DMAc) is purchased from Aldrich chemicals to dilute the PBI solution.

M21 epoxy based unidirectional (UD) carbon fiber prepregs were

supplied by Hexcel. CNFs with diameter ranging from 70 nm to 200 nm

and length 50 μm to 200 μm were supplied by Pyrograf Products, Inc.

with trade name PR-19-XT-LHT.

3.7.2. Preparation of Composite Laminates

Composite laminates were prepared by stacking up number of prepreg

layers followed by curing in the autoclave at a pressure of 7 bars and at

a temperature of 180°C for 2 hours. Laminates with a thickness of 3.4

mm was prepared by using a cure cycle provided by the supplier of the

material.

3.7.3. Plasma Treatment of Composite Laminates

Composite samples were plasma treated using TIGRES Plasma-

BLASTER MEF equipment. For this particular study, the gas used for

treatment was air at a pressure of 4.5 bars. Before performing the

plasma treatment, the samples were first cleaned with methanol using

ultrasonic cleaning to remove any contamination on the surface. After

cleaning, the specimens were heated at 80°C under vacuum for four

hours. Then the surfaces of these specimens were treated using

atmospheric pressure plasma.

3.7.4. Contact Angle Measurement

Change in the surface energy after atmospheric pressure plasma

treatment was determined in term of contact angle value. A reduced

value of contact angle indicates an improvement in surface energy of

material which in turn results in an improved adhesion of the coating.

Water was used as a liquid to determine the contact angle on

composite surface before and after the plasma treatment.

3.7.5. Adhesion Testing

Lap shear tests were performed to study the effect of atmospheric

pressure plasma treatment on the adhesion properties of PBI.


44

Specimens for lap shear testing were cut to the dimensions of (100 x

25 x 3.4) mm3 and they were adhesively bonded for single lap shear

tensile tests. Lap shear tests were performed at Zwick tensile testing

machine using a test speed of 5 mm/min.

3.7.6. Preparation of Unfilled PBI Coated Samples

The as-received 26% solution of PBI in DMAc was highly viscous and

therefore, it was essential to dilute the solution for proper processing.

DMAc was added to the PBI solution to dilute the solution down to the

17%. The solution was stirred mechanically at 60°C for 15 minutes to

get a uniform mixture of PBI in DMAc. The mixture was then used as a

coating material for composite laminates to produce a coating thickness

of about 750 - 800 µm. The coating was applied on cured composite

laminates with the help of adjustable doctor blade which also allows

controlling the thickness of the coating. The coating was allowed to dry

in the vacuum oven at 80°C for 2 hours. Afterwards, the coated

samples were further dried in vacuum oven at 125°C for overnight.

3.7.7. Preparation of Nano-filled PBI Coated Samples

CNFs reinforced PBI solution was prepared as described in section

2.3.4. CNFs reinforced PBI solution was used as a coating material for

cured composite laminates to produce a coating thickness of about

750-800 µm. The nano-filled coating was prepared with 2 weight

percent of CNFs. The coating was applied on composite laminates in the

same way as the unfilled PBI coating.

3.7.8. Thermal gravemetric Analysis (TGA)

TGA is used to measure the change in mass of a sample as a function

of temperature, time or both. This technique can provide quantitative

information about the thermal decompositions of the polymeric

materials from which thermal stability of the polymer can be evaluated.

TGA was conducted to determine the thermal stability of unfilled PBI

film, CNFs reinforced PBI film and carbon/epoxy composite. Tests were

performed using Perkins Elmer Thermal Analysis Instrument (Pyris

Diamond Thermal gravimetric Analyzer). The weight of all the samples

was maintained between 7 to 10 mg. The samples were heated in air

from a temperature range of 25°C to 575°C at a heating rate of

10°C/min.

Chapter 3

45

3.7.9. Cone Calorimeter and fire testing

Cone calorimeter is the most versatile bench-scale instrument for

measuring the fire reaction properties of combustible materials. The

popularity of the cone calorimeter is largely due to its ability to

determine a large number of fire reaction properties in a single test

using a small specimen. Another reason for the success of the cone

calorimeter is that the burning environment is considered a good

representation of the majority of actual fire conditions [3]. In cone

calorimeter test, specimen burns in ambient air conditions subjected to

a pre-determined external heat flux (typically 25 KW/m2, 35 KW/m2, 50

KW/m2, and 100 KW/m2). Test results are expressed in terms of time

to ignition (TTI); peak heat release rate (PHRR), average HRR, and

total HRR; mass loss and mass loss rate (MLR); visible smoke

development; and release rates of carbon monoxide (CO) and carbon

dioxide (CO2).

Cone calorimetric measurements were carried out on uncoated and PBI

coated unidirectional carbon/epoxy composite laminates. Three samples

for each material were cut to the dimensions of (100x100x3.4) mm3.

Tests were performed using the standardized cone calorimeter

procedure (ISO 5660). An external heat flux of 35 KW/m2 was used to

represent a well-ventilated, developing fire condition. Different fire

properties including heat release rate (HRR), time to ignition (TTI),

mass loss rate (MLR), CO and CO2 yield were measured.



TGA of epoxy resin reinforced with UD-carbon fibers, unfilled PBI and

CNFs reinforced PBI was carried out to determine the thermal stability

of three materials in air environment. Figure 3.3 represents a

comparison of thermal stability of three materials. Epoxy resin

reinforced with UD-carbon fibers has initially shown high thermal

stability and it has shown only 2 % weight loss until a temperature of

350oC. But after this temperature, Carbon/Epoxy composite started to

show weight loss and it has shown a sharp decline in the weight around

a temperature of 400oC. Carbon/Epoxy composite has shown a weight

loss of about 27% up to the temperature of 575oC. Almost all the epoxy

resin has decomposed up to this temperature and carbon fibers are

only left in the residue. A sharp decline in the mass of carbon/epoxy


46

composite around 400oC shows that epoxy resin starts to degrade

quickly after this temperature. Therefore, the objective of this work is

to provide a way which can prevent or delay the thermal degradation of

epoxy resin in carbon/epoxy composite for longer time even at higher

temperature so that composite can maintain its structural properties for

longer time.

Figure 3.3 Comparison of thermal stability of carbon/epoxy composite, unfilled and

Nano-filled PBI

Mass loss curve for unfilled PBI film and CNFs reinforced PBI film is

shown in figure 3.3. Both unfilled and nano-filled PBI has shown initial

weight loss from a temperature of 70°C up to a temperature of 200°C.

The weight loss up to this temperature for both unfilled and nano-filled

PBI is about 7.5%. This weight loss is due to evaporation of water

present in the film. This fact is confirmed by drying the PBI film in the

vacuum oven and then exposed it back to the ambient conditions. The

film again has shown similar weight gain after exposure to the ambient

conditions. After first degradation step, both unfilled PBI and nano-filled

PBI films have shown a stable plateau and they have only shown a total

of 11% weight loss up to a temperature of 575oC which is an indication

of high thermal stability of PBI in air environment. PBI reinforced with

CNFs have shown similar mass loss curve. However, CNFs could help to

improve the fire resistant properties of PBI by forming a stable char

layer as studied by other researchers [19, 20, 22-24]. Both unfilled and

nano-filled PBI have shown high thermal stability and char yield

compared to epoxy resin and PBI has not shown a sharp decline in

weight. The high thermal stability of PBI is due to the presence of

aromatic and heterocyclic ring in the polymer chain [25].

55

65

75

85

95

105

0 100 200 300 400 500 600

Wei

ght

(%)

Temperature (°C)

Epoxy Composite

Unfilled PBI film

Nano-filled PBI film

a a b c

b c

Chapter 3

47

Mass loss rate (MLR) is another important parameter to evaluate the

material response during fire. MLR of a material gives an indication

about the rate at which fuel is supplied to the fire. The temperature at

which significant mass loss occurs during decomposition in air provides

information about the ignition temperature of the polymer [26]. This is

the point when significant fuel is supplied by the polymer to the fire. A

comparison of MLR of three materials is shown in figure 3.4.

Figure 3.4 comparison of Mass Loss Rate of carbon/epoxy composite, unfilled and

Nano-filled PBI

Figure 3.4 shows that carbon/epoxy composite has shown very low MLR

up to a temperature 300oC and afterwards, it started to show an

increase in MLR. Carbon/epoxy composite has shown a peak in the MLR

around a temperature of 400oC which is an indication that

carbon/epoxy composite reached to its ignition temperature. This is the

same temperature where carbon/epoxy composite has shown a large

decrease in the weight during TGA. On the other hand, PBI has shown a

small initial peak in the MLR which is due to the evaporation of water as

mentioned earlier and then it remained stable and has not shown any

further peak in MLR up to a temperature of 550oC. After this

temperature PBI has started to show a rise in MLR but this high

temperature is not expected during the fire testing under an external

heat flux of 35 KW/m2 [27].

3.8.2. Contact Angle measurement of Plasma treated Composite Substrate

Very often, composite materials do not possess the surface properties

needed to achieve better adhesion of the coating. Atmospheric pressure

0

1

2

3

4

5

6

7

0 100 200 300 400 500 600

DTG

(%

/min

)

Temperature (°C)

Epoxy Composite

Unfilled PBI film

Nano-filled PBI film

b

a b c

a

c


48

plasma treatment (APPT) is an efficient dry surface treatment method

which offers a way to improve the surface energy of polymer based

composite [28]. APPT induces chemical changes at the surface of these

materials which result in increased concentration of polar groups on the

surface and thus increase the polar component of the surface energy

[29]. Motivated by these facts, APPT was used in the current study to

improve the surface energy of carbon/epoxy composite. Change in

surface energy was determined in term of contact angle. Contact angle

of water on composite surface was taken before and after the plasma

treatment. A graphic demonstration of contact angle before and after

APPT is shown in figure 3.5.

Figure 3.5 Comparison of Contact angle of water on carbon epoxy composite (a)

Untreated Sample (b) Atmospheric Pressure Plasma Treated samples

Figure 3.5 shows that atmospheric pressure plasma treatment has

reduced the contact angle of water on the carbon/epoxy composite

surface. The value of contact angle decreased from 72o to 17o after

performing the atmospheric pressure plasma treatment. This decrease

in contact angle has ultimately increased the surface energy of the

material and hence increased the surface wetting.

3.8.3. Adhesion Testing

Single lap shear tests were performed to study the effect of APPT on

the adhesion properties of PBI. Lap shear tests for untreated and

atmospheric pressure plasma treated PBI adhesive bonded joints were

performed. A comparison of lap shear strength of bonded joints of

untreated and plasma treated composite substrate is shown in figure

3.6.

(a) (b)

Chapter 3

49

Figure 3.6 Lap shear strength of untreated (UT) and atmospheric pressure plasma

treated (APPT) composite bonded joints

Results in figure 3.6 indicate that untreated carbon/composite has

shown lap shear strength of about 5 MPa. After performing the

atmospheric pressure plasma treatment on carbon/epoxy composite,

lap shear strength increased from 5 MPa to 18 MPa; an improvement of

about 250%. This increase in joint strength shows the effectiveness of

performing APPT prior to the application of the coating material on

composite surface.

SEM micrographs of fractured joints of untreated and plasma treated

composite are shown in figure 3.7. The micrographs in figure 3.7 reveal

that APPT has changed the mode of failure from adhesive to cohesive

which further strengthen the effectiveness of using APPT.

Figure 3.7 SEM micrographs of fractured surfaces of bonded joints of M21/carbon composite (a) Untreated (b) Atmospheric pressure plasma treated (at 200x magnification)

UT

AP

PT

0

5

10

15

20

M21/Carbon epoxy composite

Lap

Sh

ear

Str

engt

h (

MP

a)


50

3.8.4. Cone Calorimeter test results

(a) Heat release rates

A comparison of heat release rates (HRR) of uncoated carbon/epoxy

composite and composite samples with unfilled and nano-filled PBI

coating is shown in figure 3.8. The samples were exposed to an

external heat flux of 35 KW/m2. There is an initial delay in the ignition

of uncoated and PBI coated epoxy composite. During this period, the

material did not release any heat because the temperature of the

material was below the pyrolysis temperature of organic resin. This

delay in the ignition of material is called time to ignition (TTI). For a

particular fire scenario, TTI of a material depends on the amount of

heat absorbed by the material from external heat flux, the penetration

of heat into the bulk of the polymer and volatiles formed by the fuel

[27].

Figure 3.8 HRR of uncoated epoxy composite and composite with unfilled and nano-filled coating

Figure 3.8 shows that uncoated epoxy composite has demonstrated a

TTI of 165 seconds and it has depicted an average HRR of 27 KW/m2.

During the fire scenario, once the material gets sustained ignition after

initial delay, there is a rapid release of heat due to the combustion of

organic material. At certain point, the material releases maximum heat

and then HRR starts to decrease. This maximum heat release rate is

the Peak heat release rate (PHRR) of the material during ignition. The

uncoated composite has shown maximum HRR rate at 285 sec and this

maximum heat release rate is the PHRR of uncoated epoxy composite.

After PHRR, the HRR starts to decrease and reaches to the lowest level

after 555 sec. The decrease in the HRR is due to the fact that there is

0

20

40

60

80

100

0 200 400 600 800 1000 1200 1400

HR

R (

KW

/m2)

Time (Sec)

UncoatedUnfilled CoatingNano-filled Coating

a

b

c

a b c

Chapter 3

51

no fuel available to the fire as the resin content in the carbon/epoxy

composite is totally consumed.

Unlike carbon/epoxy samples without a coating which has shown only

one clearly defined peak heat release rate, samples with unfilled PBI

coating has shown two peaks in HRR curve. First peak appeared at 50

sec immediately after the start of ignition and then there is a decrease

in HRR followed by another peak at 405 sec just before the end of

burning. It is the characteristic of some thermally thick charring

materials that they show a HRR peak at the beginning until a thick char

layer is formed which resulted in a decrease of HRR [27]. The

occurrence of second peak could be attributed to the cracking of the

char layer near the end of combustion process. Having a look onto the

HRR curve of composite sample with unfilled PBI coating, it appears

that unfilled PBI coating which is much lower in thickness as compared

to the epoxy/composite, absorbed most of the heat at the initial stages

and quickly reaches to the ignition temperature. Therefore at the early

stages of the test, carbon/epoxy composite with unfilled PBI coating

has shown a peak in the HRR curve until char is formed which resulted

in decrease of HRR. The char formation has also delayed the

decomposition of underlying epoxy/composite. Once the underlying

epoxy/composite reached to its ignition temperature, it started to burn

and generated exactly the similar kind of HRR curve as generated by

uncoated carbon/epoxy composite. But still the exact burning

mechanism of carbon/epoxy composite with PBI coating can only be

understandable if cone calorimeter test is stopped at the point of

interest and then a visual inspection followed by mechanical testing

should be performed. A comparison of different fire resistant properties

of uncoated and PBI coated carbon/epoxy composite is shown in table

3.1.

Comparison of different parameters in table 3.1 shows that unfilled PBI

coating could not improve the fire resistance performance of

carbon/epoxy composite in term of HRR, PHRR, THR and TTI. The

reason of high values of HRR and PHRR with PBI coating is the low

thickness of PBI coating. The thickness of PBI coating for this particular

study was only 0.8 mm which is considered a thin coating to protect

the composite from fire [3]. Also, the coating has a low thickness

compared to many fire resistant coatings which are commercially

available [30]. It is also worth mentioning here that even the phenolic

resins which are well known for their outstanding fire resistant


52

properties cannot perform well when applied as thin coating [3]. This

explains that even the inherently flame retardant materials cannot

perform well with low thickness. Previous research shows that both

thickness and thermal conductivity of the material has a great effect on

fire resistant properties of materials [31, 32].

Despite having higher values of HRR, PHRR and THR, one of the

benefits of using PBI coating is the delayed ignition of underneath

epoxy composite. It is one of the main functions of fire resistant coating

that it slows down the rate of heat flow deeper into the underlying

epoxy composite so that it takes more time for the epoxy resin to reach

to its glass transition (Tg) temperature. More the time that epoxy resin

will take to reach its Tg, more the time the composite will have before

losing its load bearing capabilities. The performance of PBI coating

could be improved either by increasing the thickness of PBI coating or

by adding some fire retardant nano-filler.

It is also important to mention here that one of the main objectives of

this study is to reduce the thickness of load bearing structure but still

to protect its load bearing capabilities for longer time. Therefore,

adding the thickness of coating would increase the structural weight.

Keeping this fact in mind, it was decided to add CNFs in the PBI coating

to improve its performance. Also, the improved fire resistant properties

of CNFs reinforced polymers in previous work [19, 20, 22-24]

motivated the author to evaluate the potential of CNFs with inherently

flame retardant PBI polymer.

As expected, addition of 2 weight percent of CNF to PBI coating has

shown a remarkable improvement in the fire resistant properties of

underlying carbon/epoxy composite both in term of TTI, HRR and

PHRR. Nano-filled PBI coating has increased the TTI of the coated

composite from 195 sec to 730 sec; about 3 times increase in TTI of

the material. Dispersion of CNF into PBI coating has also reduced the

HRR of material which demonstrates the effectiveness of adding CNFs

to PBI. Average HRR of composite with nano-filled PBI coating for first

300 sec is only 1 KW/m2 which is almost negligible when compare to

the Average HRR300 of 27 KW/m2 for uncoated epoxy composite.

Chapter 3

53

Table 3.1 Cone calorimeter results for uncoated and coated carbon/epoxy composite at 35 KW/m2 flux

*TTI: Time to ignition, HHRavg: Average Heat release rate, PHRR: Peak heat release rate, TTPHRR: Time to peak heat release rate, THRR: Total heat release rate, FPI: Flame propagation index ** Average for first 300 sec

Material TTI*

(sec)

HRR*300

**

(KW/m2)

HRR*avg

(KW/m2)

PHRR*

(KW/m2)

TTPHRR*

(Sec)

THR*300

(MJ/m2)

THR*avg

(MJ/m2) FPI

Uncoated

Composite 165 27 29 87 285 8.0 15.9 0.527

Composite with

Unfilled Coating 35 32 34. 88 405 9.8 30.4 2.523

Composite with

Nano-filled Coating 730 01 12 53 820 0.2 15.5 0.0725


54

Composite with nano-filled PBI coating has also the lowest PHRR of the

three materials and it takes maximum time before it reached the PHRR

value. The main reason of this improvement in fire resistant

performance of nano-filled PBI coating is the formation of an effective

char layer which acts as a barrier for underneath epoxy composite. This

nano-fibers reinforced carbonaceous char is responsible for the reduced

mass loss rate and hence the lower HRR. The MLR data obtained from

TGA and cone calorimeter test indicates that the char layer formed with

nano-filled PBI coating acts as a barrier to fuel release, thus

lengthening the burn time of the coating /composite system and

lowering the HRR.

Digital photographs of the samples after cone calorimeter test are

shown in figure 3.9.

Figure 3.9 Digital photographs of three samples after fire testing (a) Uncoated composite (b) composite with unfilled coating (c) Composite with nano-filled coating

It is notable from the figure that there is no char formed during the

combustion of uncoated epoxy composite meaning the combustion

process takes place in the form of boiling and vaporization of the resin

present in epoxy composite. On the other hand, thick char layer is

formed with both unfilled and nano-filled PBI coating. However, the

char layer formed with unfilled PBI coating was not as effective as with

nano-filled PBI coating. The char layer formed with unfilled PBI coating

did not act as a shield for underlying polymer for longer time and heat

passed through quickly and decomposed the epoxy resin in short period

of time. Contrary with unfilled PBI coating, a thick continuous char

layer is formed with nano-filled PBI coating which worked as a barrier

to heat for longer time and thus maintained the heat release rate to low

value for longer time. Therefore, it is very important to get the full

(a) (c) (b)

Chapter 3

55

advantage from any char forming material that a continuous network

structured layer formed without the formation of cracks. Formation of

the cracks can compromise the effectiveness of char layer.

Nano-filled PBI coating has also improved the material performance in

term fire propagation index (FPI). FPI is an index to rank the fire

hazard of different materials. It is the ratio of the PHRR and TTI. The

higher value of FPI suggests a faster fire spread rate of the material.

The values of FPI in Table 3.1 shows that nano-filled PBI coating has

reduced the FPI about 7 times as compare to the FPI of uncoated epoxy

composite which is a significant improvement in the material

performance.

Another important parameter which is derived from FPI is the flashover

time. Time to flashover is the time available for escape during a fire

and this is one of the most important factors in determining the fire

hazards of a material in a compartment fire. A comparison of flashover

time of uncoated and coated composite is shown in figure 3.10. Figure

shows that nano-filled PBI coating has increased the flashover time of

composite from 1166 sec to 1710 sec which shows that with nano-filled

PBI coating, more time will be available to escape during fire.

Figure 3.10 Comparison of Flashover time of uncoated and coated composite

(b) Mass Loss Rate in Cone Calorimeter Test

Mass loss rate (MLR) of a polymer during fire shows the rate at which a

polymer decomposed and subsequent combustible volatiles added to

1166

725

1710

0 500 1000 1500 2000

Uncoated

Composite

Unfilled Coating

Nano-filled

Coating

Time (Sec)

Time to Flashover


56

the fire. A comparison of average MLR for uncoated and coated samples

is shown in table 3.2. Table 3. 2 Comparison of Average Mass Loss Rate of uncoated and coated samples

Materials Average MLR

(g/sec)

Uncoated Carbon/Epoxy

composite 0.02

Composite with unfilled coating 0.02

Composite with Nano-filled

coating 0.01

Table 3.2 shows that both uncoated composite and composite with

unfilled PBI coating have shown the same average MLR which indicates

that unfilled PBI coating has not improved the performance of

carbon/epoxy composite. Previous research shows that average MLR

has a direct correlation with average HRR [16]. A higher average MLR

mean higher average HRR value. On the other hand, nano-filled PBI

coating has reduced the average MLR to about 50 percent. The reason

of having reduced average MLR with nano-filled coating is the formation

of a continuous char layer on the surface which has reduced the flow of

heat deep into the specimen and ultimately slowed down the MLR.

Hence, a continuous thick char layer with nano-filled PBI coating

worked well as compare to char layer formed by unfilled PBI with many

cracks as shown in figure 3.6. Formation of cracks can reduce or

diminish the effectiveness of char and this is what we have observed in

this study.

(c) Smoke Emission

Smoke production and emission of toxic gases are the most important

fire hazards other than HRR. Polymers and polymer based composites

release dense smoke in a fire. A useful measurement of the amount of

smoke produced by a burning material is the total effective cross-

sectional area of the smoke particle. This area is known as extinction

area of smoke and the measure of extinction area of smoke per unit

mass of fuel burnt is known as specific extinction area [16]. Specific

extinction area (SEA) of a material can be determined using cone

calorimeter test. Measurement of SEA during cone calorimeter test is a

convenient way to measure the smoke produced by a given material. A

comparison of SEA of uncoated and PBI coated samples is give in figure

3.11.

Chapter 3

57

Figure 3.11 Comparison of Specific Extinction Area for uncoated and coated epoxy composite

Figure 3.11 shows that uncoated carbon/epoxy composite has depicted

higher SEA as compare to the coated samples. Sample with unfilled PBI

coating has shown a 10% decrease in smoke emission which indicates

that unfilled PBI coating with a surface char layer has reduced the

smoke emission to some extent. On the other hand, much better

results are obtained with a nano-filled PBI coating. Carbon/epoxy

composite with nano-filled PBI coating has shown a 70% reduction in

smoke emission. Nano-filled PBI coating with a continuous char layer

without having any cracks has reduced the smoke emission to great

extent. Carbon/epoxy composite with nano-filled PBI coating has shown

a significant improvement in the material performance in term of

smoke emission.

(d) Gas Emission

The gas products released by the decomposition of polymer based

composite depends on the nature of chemical structure of organic

material, availability of oxygen and temperature during fire [16]. The

type and amount of gas products for different materials can vary but all

materials release CO and CO2 on decomposition. CO is a major threat

because inhalation of CO can be the cause of human death [16]. Values

of average CO yield for uncoated and coated samples are shown in

figure 3.12.

Uncoated Composite

Composite with Unfilled

Coating

Composite with Nano-

filled Coating

0

100

200

300

400

500

SEA

(m

2/K

g)


58

Figure 3. 12 Comparison of CO yield for uncoated and coated epoxy composite

Figure 3.12 shows that unfilled PBI coating has increased the average

yield of CO about 60% which is one of the crucial parameter while

selecting a material for fire application. On the other hand, nano-filled

PBI coating has reduced about 73% of CO yield. This is about the

same reduction in average yield of CO as we have seen a reduction in

smoke with nano-filled PBI coating. The strong and continues char layer

formed with nano-filled PBI coating has suppressed the smoke emission

thus reducing the yield of CO. All these results indicate that addition of

CNFs to PBI coating has increased the performance of PBI coating

which ultimately has increased the fire resistance of carbon/epoxy

composite.

3.9. Conclusions

In present study, fire resistant performance of PBI is evaluated using

cone calorimeter test. PBI coating was applied on epoxy based carbon

fiber composite. The objective was to improve the fire resistant

performance of currently used carbon/epoxy based composite materials

for aircraft. Three kinds of samples were tested in this study; uncoated

carbon/epoxy composite, composite with unfilled PBI coating and

composite with CNFs reinforced PBI coating. Thermal stability, char

yield, MLR, average HRR, PHRR, TTI, smoke and yield of CO were

determined by TGA and cone calorimeter test.

Uncoated Composite

Composite with Unfilled

Coating

Composite with Nano-

filled Coating

0

0,002

0,004

0,006

0,008

0,01

0,012

0,014

CO

Yie

ld (

Kg/

Kg)

Chapter 3

59

TGA results show that both unfilled PBI and nano-filled PBI have shown

better thermal stability as compare to the thermal stability of

carbon/epoxy composite. Epoxy based carbon fiber composite has

shown a large decrease in weight around a temperature of 400oC and

epoxy resin is totally decomposed around a temperature of 550oC. On

the other hand, both unfilled and nano-filled PBI have shown thermal

stability even up to a temperature of 575oC and only 10% weight loss

occurred up to this temperature. A high thermal stability is an

important requirement for fire resistant materials.

Results obtained from cone calorimeter test show that unfilled PBI

coating could not improve the fire resistant performance of

carbon/epoxy composite in term of Average HRR, peak HRR, gas

emission. Contrary to the unfilled PBI coating; CNFs reinforced PBI

coating has shown a significant improvement in the fire resistant

properties of carbon/epoxy composite. Nano-filled PBI coating has

reduced the average HRR300 of carbon/epoxy composite from 27 KW/m2

to 1 KW/m2; a remarkable reduction in average HRR. Nano-filled PBI

coating has also reduced the peak HRR from 87 KW/m2 to 53 KW/m2; a

reduction of about 40% in peak HRR. Average HRR and peak HRR are

two important parameters which are used to evaluate the fire resistant

of a material. These two parameters give an indication about fire

growth rate. Hence, the performance of carbon/epoxy composite with

PBI nano-filled coating is improved significantly. Smoke and CO are of

concern too during a fire in the aircraft as most of the deaths during a

fire in aircraft occurred due to the reduced visibility and inhalation of

toxic gases. Nano-filled PBI coating has also reduced the smoke and CO

emission up to 73% as compare to the uncoated carbon/epoxy

composite and this is another important outcome of current study. It is

also worth mentioning here that such kind of improvement in the fire

resistant properties of coating with CNFs is never achieved in the past.

One of the reason of not having an improvement to such an extent that

most of the fire resistant studies with CNFs or CNTs are performed

using commodity plastics which are not inherently flame retardant. A

strong outcome of this work is that CNFs even in very small quantity

can be more effective to improve the fire resistant performance of

inherently flame retardant material. Another important outcome is that

with this low coating thickness such an improvement in fire resistant

properties is quite remarkable.


60

3.10. References

1- Baker, Alan; Dutton, Stuart; Kelly, Donald, Composite materials for aircraft structures, 2nd edition, 2004

2- A.P. Mouritz, E. Gellert, P. Burchill, K. Challis, Review of advanced composite structures for naval ships and submarines, Composite structures 53 (2001)

3- A.P.Mouritz and A.G.Gibson, Fire Properties of polymer composite materials,

Solid Mechanics and its applications, Volume 143, 2006, ISBN-13 978-1-4020-5356-6

4- Mouritz, AP, Fire safety of advanced composites for aircraft, B2004/0046,

April 2006

5- AP Mouritz, Fire resistance of aircraft composite laminates, Journal of Material Science Letters, 22, (2203), pp. 1507-1509

6- Sanjeev Gandhi, Postcrash Health Hazards from Burning Aircraft Composites,

Galaxy Scientific Corporation, Fire Safety Section, AAR-422, Federal Aviation Administration

7- Jinfeng Zhuge, Jihua Gou, Ruey-Hung Chen, Ali Gordon, Jayanta Kapat, Dustin

Hart, Christopher Ibeh, Fire Retardant Evaluation of Carbon Nanofiber/Graphite Nanoplatelets Nanopaper-Based Coating under Different Heat Fluxes, Composites: Part B (2012)

8- S. Feih and A. P. Mouritz, Z. Mathys and A.G. Gibson, Fire Structural Modeling of Polymer Composites with Passive Thermal Barrier, Journal of Fire Sciences, Vol. 28 – March 2010

9- Usman sorathia, Materials and fire threat, SAMPE journal, Vol.32, No.3, 8-15,

1996

10- Final report 2, DOT/FAA/AR-05/14, Polymer flammability, may 2005

11- Takashi Kashiwagi, Polymer Combustion and Flammability-Role of Condensed Phase, 25th Symposium On Combustion, 1994, Pp 1423-1437

12- Final report, September 2004, Fire safe polymer and polymer composite,

DOT/FAA/AR-04/11

13- S. Chapple and R. Anandjiwala, Flammability of Natural Fiber-reinforced Composites and Strategies for Fire Retardancy: A Review, Journal of Thermoplastic Composite Materials, Vol. 23 (2010)

Chapter 3

61

14- Craig L. Beyler and Marcelo M. Hirschler, The SFPE handbook of Fire protection Engineering, 3rd edition, Thermal decomposition of polymers (Chapter 7)

15- Richard E. Loyn, Fire response of Geopolymer Structural Composite, DOT/FAA/AR-TN95/22, jan 1996

16- A.P. Mouritz, Z. Mathys, A.G. Gibson, Heat release of polymer composites in fire, Composites: Part A 37 (2006) 1040–1054

17- G. L. Nelson, 1 P. L. Kinson, and C. B. Quinn, Fire Retardant Polymers, Annual Review of Materials Science, 1974, pp. 391-414

18- Yong Tanga, Jinfeng Zhugea, Jihua Goua, Ruey-Hung Chena, Christopher Ibehb and Yuan Huc Morphology, thermal stability, and flammability of polymer matrix composites coated with hybrid nanopapers, polymers for advanced technologies, Vol. 22, 2009

19- Zhongfu Zhao and Jan Gou, Improved fire retardancy of thermoset composites modified with carbon nanofibers, Sci. Technol. Adv. Mater. 10 (2009)

20- Takashi Kashiwagi, Eric Grulke, Jenny Hilding, Richard Harris, Walid Awad,Jack Douglas Thermal Degradation and Flammability Properties of Poly(propylene)/Carbon Nanotube Composites, Macromolecular Rapid Communications, ( 2002)

21- Takashi Kashiwagi1, Fangming Du, Jack F. Douglas3, Karen I. Winey, Richard H. Harris Jr and John R. Shields, Nanoparticle networks reduce the flammability of polymer nanocomposites, nature materials, (2005)

22- Alexander B. Morgan, and Weidong Liu, Flammability of thermoplastic carbon nanofiber nanocomposites, Fire and Materials, Fire Mater. 2011

23- Zhongfu Zhao and Jan Gou, Improved fire retardancy of thermoset composites modified with carbon nanofibers, Sci. Technol. Adv. Mater. 10 (2009)

24- S. C. Lao, C. Wu,1 T. J. Moon, J. H. Koo, A. Morgan, L. Pilato, And G. Wissler, Flame-retardant Polyamide 11 and 12, Nanocomposites: Thermal and Flammability Properties, Journal of Composite Materials, 2009

25- A.H. Frazer, High temperature resistant polymers, SBN 470-276509

26- Xiang Fu, Chuck Zhang, Tao Liu, Richard Liang and BenWang, Carbon nanotube buckypaper to improve fire retardancy of high-temperature/high-performance polymer composites, Nanotechnology 21 (2010)

27- T. Richard Hull, Anna A. Stec, and Shonali Nazare, Fire Retardant Effects of, Polymer Nanocomposites, Journal of Nanoscience and Nanotechnology, Vol.9, (2009)


62

28- J. Rotheiser, Joining of Plastics; handbook for designers and engineers, 3rd Ed. Hanser publisher, (1999).

29- J. W. Chin and J. P. Wightman, Surface characterization and adhesive bonding

of toughened bismaleimide composite, Composites Part A, (1996);27(06):419-428

30- Thomas J. Ohlemiller, John R. Shields, The effect of surface coating on fire growth over composite material in a corner configuration, Fire safety Journal (32), (1999)

31- M. J. Scudamore, Fire Performance Studies on Glass-reinforced Plastic Laminates, Fire and Materials, (1994)

32- M. J. Scudamore, P. J. Briggs and F. H. Prager, Cone Calorimetry- A Review of Tests Carried out on Plastics for the Association of Plastic Manufacturers in Europe, Fire and Materials, (1991)

63

CHAPTER 4

Performance Evaluation of Polybenzimidazole in Simulated Low Earth Orbit Environment

This chapter mainly deals with the exposure of unfilled PBI

and PBI nano-composite to simulated Low Earth Orbit

environmental conditions. Performance of unfilled and nano-

filled PBI is presented in term of erosion yield and mechanical

properties. The surfaces of exposed samples are also analyzed

using scanning electron microscope and atomic force

microscope and the results are presented.

4.1. Introduction

Polymers are extensively used as construction materials in space

systems due to their high strength-to-weight ratio and a variety of

mechanical, thermal, electrical and optical properties [1]. Typically,

polymers are used as multi-layer insulations, matrix in the substrate for

solar panels, adhesives, thermal control coatings for spacecraft and

insulation for electrical wiring [2, 3]. The commonly used polymers for

space applications are silicones, epoxies, polyurethanes, polyesters,

polyamides, polyimide, and Teflon FEP (fluorinated ethylene

propylene). Polyimide (Kapton) and Teflon FEP are the most commonly

used spacecraft materials because of their desirable properties such as

flexibility, low density, thermal and optical properties [1, 4]. However,

these materials when exposed to low earth space orbit (LEO) are

affected by various environmental conditions. The LEO space

environment consists of ultra high vacuum, thermal cycling, UV

radiations and atomic oxygen. These LEO environmental conditions

significantly degrade the properties of polymers [1, 3 and 4].

Therefore, research is continued to develop new polymers which can

survive under these space environmental conditions for longer duration

Performance Evaluation of PBI in Simulated Low Earth Orbit Environment

64

of time and serve their purpose properly. Polybenzimidazole (PBI) is

one such recently emerged high performance polymer which has great

potential to be used for different space applications. PBI is an aromatic

thermoplastic polymer which exhibits excellent thermo-mechanical

properties and outstanding mechanical properties [5]. PBI has the

highest glass transition temperature (425°C) of any commercial available

organic polymer [6] and it maintains excellent strength at cryogenic

temperatures [7]. These properties make PBI a potential candidate for

space applications.

4.2. LEO Space Environment

Prior to the application of PBI in space, it is important to understand

the influence of individual components of the space environment, as

well as their synergistic effects on the properties of PBI. In this context,

current study deals with the exposure of PBI to simulated LEO

environmental conditions and its effect on thermo-mechanical

properties of PBI. Before going into the experimental detail, it is

important to understand the effect of individual component of LEO

space environment and their effect on material properties.

4.2.1. Ultra High Vacuum

The value of vacuum in space ranges from 10-5 Torr to 10-12 Torr

depending on the altitude and solar effect. This value of vacuum varies

from an altitude of 200 km to an altitude of 6500 km [8]. The ultra high

vacuum surrounding the spacecraft leads to materials out-gassing. The

out-gassed products such as moisture and other volatiles can

consequently induces loss of dimensional stability of materials,

distortion of structures, contamination of the adjacent spacecraft

components and unfavorable effects on material properties [8, 9].

Specifically, these out-gassed products severely deteriorate the

performance of adjacent optical mirrors or thermal control surfaces [3,

8]. Therefore, it is important to select the materials with low out-

gassing for spacecraft applications.

4.2.2. Ultraviolet radiations (UV radiations)

UV radiations in the range of 100-400 nm are of particular importance

in determining the effect of solar radiations on material properties. The

total energy provided by the sun in all wavelengths up to 1000 µm is

called solar constant and its value is 1366 W/m2. UV radiations in the

Chapter 4

65

range of 100 to 400 nm are about 8% of the solar constant [1, 3].

While orbiting the Earth’s atmosphere, spacecraft is exposed to these

UV radiations which have enough energy to break different molecular

bonds in polymer (e.g. C–C, C–O) and other functional groups [1, 3 and

8]. Figure 4.1 gives an idea about the energy required to break

different bonds that may be present in the chemical structure of

different polymers [3].

4. 1 Wavelength requirement to break various polymeric bonds [adapted from 3]

Solar UV radiations can induce the cross-linking or chain scissioning in

the polymer. Cross-linking results in the embitterment which ultimately

leads towards surface cracking of polymer films [3, 8]. Chain scission

reactions leads towards the creation of volatiles (mass loss take place)

which ultimately decrease the mechanical properties of polymers [1, 3

and 8].

4.2.3. Thermal Cycling

During the operation, spacecraft orbiting the Earth experiences severe

thermal difference between the surface exposed to solar rays and the

surface remains unexposed. Thermal cycling due to the period of solar

eclipse and sun illumination can induce micro-cracking in resin and

degrade the mechanical properties through thermal stresses [8, 9].

Also, the thermal cycling generates a mismatch in the coefficients of

thermal expansion (CTE) of different materials intact with each other

hence initiates micro cracks in the materials [3, 8].


66

4.2.4. Atomic Oxygen (AO)

Atomic oxygen (AO) is formed by photo dissociation of molecular

oxygen caused by UV radiations in the upper atmosphere. AO is the

most hazardous constituent for polymeric materials in LEO

environment. Atomic oxygen provides an aggressive environment for

materials due to its high chemical activity. The ability of AO to react

with spacecraft materials is further enhanced in the presence of solar

UV radiations which energize molecular bonds and makes AO reaction

easier [3]. The AO flux is determined by different parameters such as

altitude, orbital inclination, solar activity and time of day [1]. At an

altitude of about 300 km, the densities of AO during minimum and

maximum solar activities are approximately 2x109 and 8x109

atoms/cm3, respectively. The front surface of spacecraft interacts with

the atomic oxygen at a relative velocity of 8 km/sec which corresponds

to a kinetic energy of 5ev [8]. Therefore, AO interacts with spacecraft

materials with a kinetic energy of about 5 eV and the nominal AO flux

approximately ranging from 1014 to 1015 atoms/cm2-sec [3, 8]. The

Collision of AO with materials results in surface erosion, change in

surface morphology, mass loss, degradation of mechanical, thermal and

optical properties, and changes in chemical compositions of polymers

[8, 10]. One of effects of atomic oxygen in LEO environment is the

occurrence of glow phenomenon. Glow phenomenon occurs when

atomic oxygen reacts with nitrogen atoms on the spacecraft surfaces to

form nitrous oxide. This nitrous oxide in the excited state emits visible

radiations near the surfaces of spacecraft [11, 12]. As many spacecraft

sensing systems are light based, spacecraft glow can be a serious

problem for optical sensor systems [3, 8 and 13]. Therefore, it is

important to study the influence of AO on the properties of PBI

beforehand in order to determine its potential for space applications in

LEO environment. In present study, the performance of both unfilled

PBI and PBI nano-composite is evaluated under simulated LEO

environmental conditions. Carbon nano-fibers (CNFs) and Titanium

Dioxide (TiO2) are the two nano-fillers used in this study to

manufacture PBI nano-composite. TiO2 was added to PBI due to its

capability to stop UV radiations. TiO2 provides good UV protection by

reflecting or scattering most of the UV-rays below the wavelength of

400 nm through its high refractive index [14]. UV radiation blocking

ability of TiO2 can be effectively used for the protection of coated

polymeric substrate.

Chapter 4

67

4.3. Experimental

4.3.1. Materials

Solution of PBI in DMAc (with 26% concentration of PBI) was supplied

by CELAZOLE. 99% concentrated DMAc is purchased from Aldrich

chemicals. CNFs were supplied by Pyrograf Products, Inc. with diameter

ranging from 70 nm to 200 nm and length 50 μm to 200 μm. Titanium

dioxide (TiO2) was supplied by DuPont with a diameter of around 200

nm.

4.3.2. Solution casting of Unfilled PBI Film

The as-received solution of PBI in DMAc was highly viscous and

therefore, DMAc was added to the solution to dilute it. The solution of

PBI in DMAc was then stirred mechanically at 60°C for 15 minutes to

get a uniform mixture of PBI in DMAc. The mixture was then used to

produce 80 µm thick films of PBI. The complete detail of PBI film

manufacturing is presented in chapter 2.

4.3.3. Solution casting of nano-filled PBI films

To fabricate the CNFs reinforced film, pre-calculated amount of CNFs

was carefully weighed and then added to the DMAc solvent. These

nano-fibers are dispersed in DMAc by ultrasonic mixing for 30 minutes

at 60°C. After ultrasonic mixing of CNFs, they were added to PBI

solution. The ultrasonic process in combination with mechanical stirring

was continued for next 15 minutes. The mixture was then used to cast

the film on the glass plate. The nano-composite films were prepared

with 2 wt% of CNFs. Film reinforced with TiO2 (2 and 5 weight percent)

nano-fillers were prepared by making a paste of TiO2 in DMAc followed

by the mixing of TiO2 paste into the diluted PBI solution. Afterwards,

films were manufactured in the same way as the film manufactured

with CNFs.

4.3.4. Exposure to LEO Environment

Unfilled PBI and nano-filled PBI films were exposed to AO alone and in

combination with other LEO components (UV, thermal cycling and

Vacuum) in order to evaluate the performance of PBI in simulated LEO

environment.


68

(a) Vacuum

In this study, a pumping system composed of rotary pump and

diffusion pump for low and high vacuum was used to produce a

chamber pressure of the order of 10-6 Torr. A vacuum gauge controller

was used to control the vacuum.

(b) UV radiation

UV radiations with a wavelength less than 200 nm in the LEO

simulation facility were produced using UV lamp. This lamp was turned

on during heating time (sun facing) because UV radiation and heating

occur simultaneously in orbit due to radiations from the sun. The lamp

was turned off during cooling (shadow facing). The UV lamp was placed

on exterior of the chamber to avoid excessive out-gassing phenomenon

on the source when placed inside the chamber. A schematic diagram of

LEO simulation facility is shown in figure 4.2.

4.2 Schematic diagram of LEO space environment simulation facility

Chapter 4

69

(c) Thermal Cycling

In this study, thermal cycling of the materials is performed between

+100oC (sun facing) and -70oC (shadow). The sun facing temperature

was simulated using a halogen lamp, set inside the chamber, while the

shadow facing temperature was simulated using a refrigerator coolant

through a pipe placed inside a copper cooling plate. The copper plate on

which the samples were placed inside the main chamber is shown in

figure 4.2. A temperature sensor was set on the copper plate to read

the temperature of specimen. The heating rate of the samples was

approximately 5oC/min and cooling rate was approximately 3 to

4oC/min and it took about 64 minutes to complete a thermal cycle

(+100oC to -70oC and back to 100oC) as shown in figure 4.3. The

samples were exposed for 14 thermal cycles.

4.3 Temperature history of a specimen during 20 h LEO simulated environment experiment

(d) Simulation of Atomic Oxygen

Atomic oxygen generation system equipped in LEO space environment

simulation facility generates AO flux through weakly ionized remote

oxygen plasma with a radio-frequency (RF) plasma source. The system

is mainly operated through O2 and Ar gases with a power supply source

of 600 W at 13.56 MHz RF and a pumping system to generate the

vacuum condition in plasma chamber.


70


Tensile testing of unexposed and exposed samples of unfilled and nano-

filled PBI films was carried out using Zwick Tensile machine at a test

speed of 2 mm/min. Rectangular specimens of (80x6x0.06) mm3 were

cut from the films. An extensometer was also used to determine the

Young’s Modulus and strain values. Five specimens for each material

were tested for reproducibility of the results.

4.3.6. Scanning Electron Microscopy (SEM)

Scanning electron microscopy was carried out to study the surface

morphology of unexposed samples and samples exposed to simulated

LEO environment. Images were obtained using JOEL JSM-7500 field

emission scanning electron microscope (FE-SEM) which operates at

30kV accelerating voltage. Surface of each sample was coated with a

thin layer of gold to minimize sample charging effects.

4.3.7. Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) was performed to study the change in

surface roughness of PBI samples after exposure to AO. Analysis was

performed using scanning probe microscope supplied by NT-MDT

Corporation. The AFM was used in the "tapping" mode and all

measurements were performed in air.


4.4.1. Mass Loss and Erosion Yield Measurements of exposed samples

Erosion of polymers by AO in LEO environment is a serious threat to

spacecraft performance and durability. It is therefore essential to

determine the AO erosion yield (the volume loss of material per

incident oxygen atom) for spacecraft applications. A common technique

for determining the erosion yield (Ey) of materials is based on mass loss

and is calculated by taking the mass of material before and after

exposure to AO. Therefore, in this context, mass loss of the samples

exposed to AO alone and in combination with UV radiations and thermal

cycling was measured. A comparison of mass loss for different

materials is shown in figure 4.4. Figure 4.4 shows that under the

influence of AO alone, unfilled PBI and PBI filled with 5 wt. % of TiO2

have shown higher mass loss per unit area as compare to the PBI

reinforced with 2 wt. % of CNFs and PBI reinforced with 2 wt. % of

Chapter 4

71

TiO2. Both have shown almost equal mass loss per unit area. However,

a different trend in the mass loss of four materials is observed under

the synergistic effect of AO, UV radiations and thermal cycling. PBI

reinforced with CNFs has shown higher mass loss as compare to the

unfilled PBI. Contrary to the CNFs, addition of 2 wt. % of TiO2 in PBI

has shown the least mass loss of all four materials. PBI filled with 2 wt

% of TiO2 has shown better resistance to erosion under the synergic

effect of AO, UV radiations and thermal cycling. By adding 5 wt % of

TiO2, it has shown a higher mass loss which means that higher weight

percent of TiO2 is not effective in reducing the mass loss of PBI.

4.4 Comparison of mass loss of unfilled and nano-filled PBI after exposure to simulated LEO environment (AO = Atomic Oxygen, TC = Thermal Cycling, UV = Ultraviolet)

To compare the performance of different materials under simulated LEO

environment, erosion yield is a more comprehensive parameter as

compare to the mass loss of material. Erosion yield can give a better

indication of material performance due to the fact that it also takes into

account the density and exposed area of the samples. The erosion yield

of the sample is determined through the following equation [4].

M

sEy A F

s s

----------------------- (4.1)

Where

Ey = Erosion yield of exposed sample (cm3/atom)

ΔMS = Mass loss of the exposed sample (g)

AS = Surface area of the sample exposed to atomic oxygen (cm2)

s = Density of flight sample (g/cm3)

F = Fluence of atomic oxygen (atoms/cm2)

The AO fluence (F) can be determined through the mass loss of a

reference Kapton sample. Kapton is one of the most studied materials

PBI

PBI+2CNF PBI+2TiO2

PBI+5TiO2

0

0,1

0,2

0,3

0,4

0,5

0,6

Vacuum+AO

Mas

s Lo

ss (

mg/

Cm

2)

(a) PBI

PBI+2CNF

PBI+2TiO2

PBI+5TiO2

0

0,2

0,4

0,6

0,8

1

1,2

Vacuum+AO+TC+UV

Mas

s Lo

ss (

mg/

Cm

2)

(b)


72

susceptible to AO degradation in the LEO environment. Therefore,

Kapton is considered as a reference standard for comparison between

ground simulation laboratory tests and flight experiments. It has a well

characterized erosion yield (3.0×10-24 cm3/atom) in the LEO

environment. Therefore, the AO fluence (AO flow rate) can be

calculated using the following equation:

k

k k k

MF

A E

------------------------- (4.2)

Where

F = Low earth orbit atomic oxygen fluence (atoms/cm3)

kM = Mass loss of kapton H witness sample (g)

kA = Surface area of kapton H witness sample exposed to atomic oxygen (cm2)

k = Density of kapton H witness sample (1.42 g/cm3)

kE = Erosion yield of kapton H witness sample (3 x 10-24 cm3/atom)

Thus erosion yield of the exposed sample can be calculated by using

the following equation.

s k ky K

k s s

M AE E

M A

---------------------- (4.3)

In this study, 3 cm x 3 cm Kapton films were used as reference

samples. These kapton films were aged in the testing facility for 8

hours under simulated LEO conditions. High vacuum, AO, UV radiations

and thermal cycling were simultaneously applied. After aging, the mass

loss of the Kapton films was measured and the equivalent AO flow rate

was calculated by using equation 4.2. Based on mass loss

measurements for reference kapton samples, the calculated equivalent

AO flow rate was 9.15x1014 atoms/cm2-sec. This value was used to

calculate the experimental duration in order to achieve a total dose

value during the ground experiment which should be equivalent to total

AO dose during space shuttle mission (STS-4). The duration of

experiment was determined based on total exposed AO quantity

because AO is the most severe condition among the LEO environmental

factors. During an STS-4 mission, the total AO quantity directed at the

space shuttle during operation was 0.65x1020 atoms/cm2 [4]. By using

the calculated AO flow rate, the total AO dose was achieved in the

duration of 20 hours.

Chapter 4

73

Erosion efficiency of unfilled and nano-filled PBI samples was calculated

by using the values of mass loss, density and exposed area of these

samples in equation 4.1 and results are shown in figure 4.5. Figure

shows the erosion yield of unfilled and nano-filled PBI samples exposed

to atomic oxygen alone and in combination with UV radiations and

thermal cycling. Results shows that under the influence of AO alone,

unfilled PBI and CNF reinforced PBI have shown almost the same

erosion yield. CNFs have not improved the performance of PBI in term

of erosion yield. PBI reinforced with 2 and 5 weight percent of TiO2 has

shown even higher erosion yield than unfilled PBI.

4.5 Comparison of Erosion Yield of unfilled and nano-filled PBI after exposure to

simulated LEO environment (AO = Atomic Oxygen, TC = Thermal Cycling, UV = Ultraviolet)

Samples exposed to synergistic effect of AO, UV radiations and thermal

cycling have shown higher erosion yield as compare to the erosion yield

of these samples under the influence of AO alone. Only PBI samples

with 2 weight percent of TiO2 has shown more or less the similar

erosion yield both under the exposure of AO alone and under the

synergistic effect of AO, UV radiations and thermal cycling. High

erosion yield of these materials under the combined influence of AO, UV

radiations and thermal cycling indicates that UV radiations and thermal

cycling have increased the reaction efficiency of both unfilled and nano-

filled PBI which ultimately increased the mass loss of the material and

thus the erosion yield.

4.4.2. Tensile Test Results

Tensile testing of unfilled and nano-filled PBI before and after the

exposure to various LEO environmental conditions is performed. A

comparison of tensile strength of unfilled and nano-filled PBI before

exposure to LEO environment is shown in figure 4.6. Unfilled PBI has


74

shown a tensile strength of 170 MPa with a failure strain of about 17%.

After addition of 2 weight percent of CNFs, the tensile strength of PBI

increased from 170 MPa to 184 MPa; an increase of about 8%.

However, a decrease of about 47% in failure strain is observed after

the addition of 2 weight percent of CNFs. Therefore, addition of CNFs

has improved the tensile strength at the expense of failure strain.

Contrary to the effect of CNFs on mechanical properties of PBI, addition

of TiO2 to PBI has shown a reduction in both strength and strain of PBI.

4.6 Comparison of tensile properties of Unfilled and nano-filled PBI before exposure to simulated LEO environment

A comparison of tensile strength and failure strain of unfilled and nano-

filled PBI after exposure to different LEO environmental conditions is

shown in figure 4.7. Figure 4.7a shows a comparison of tensile

properties of unfilled PBI when expose to AO alone and in combination

with UV radiations and thermal cycling. The results show that exposure

of unfilled PBI to simulated LEO environment has not affected the

tensile strength to greater extent. The tensile strength of PBI is

decreased from 170 MPa to 158 MPa (about 9% decrease) when

exposed to combined effect of AO, UV radiation and thermal cycling.

However, LEO simulated environment has more severe effect on failure

strain of unfilled PBI. The exposure to simulated LEO environment has

reduced the failure strain from 17% to about 4.6%; a decrease of

about 73%. Addition of nano-fillers to PBI has improved the

performance of PBI under simulated LEO environmental conditions.

CNFs reinforced PBI has shown a decrease in tensile strength of about

3% and decrease in tensile strain of about 35% after exposure to

combined AO, UV radiations and thermal cycling. The decrease in

0

40

80

120

160

200

0 5 10 15 20

Ten

sile

Str

engt

h (

MP

a)

Strain (%)

Unfilled PBIPBI-2CNFPBI-2TiO2PBI-5TiO2

a

c

b

d

a b c d

Chapter 4

75

tensile strength is 3 times less and decrease in failure strain is 2 times

less with CNFs as compare to unfilled PBI.

4.7 Comparison of tensile properties of unfilled and nano-filled PBI after exposure to simulated LEO Environment (C = Controlled AO = Atomic Oxygen, TC = Thermal

Cycling, UV = Ultraviolet)

Tensile test results demonstrate that addition of CNFs to PBI has

improved the performance of PBI. Figure 4.7c and 4.7d represents the

tensile properties of PBI reinforced with 2 weight percent and 5 weight

percent TiO2. Contrary to the fact that addition of TiO2 to PBI has

reduced failure strength and strain of unexposed PBI, addition of TiO2

has shown a 15% increase in tensile strength after exposure to

simulated LEO environment while a decrease of 40% in failure strain is

observed. This decrease in failure strain is almost the same as the

decrease in strain with 2 weight percent of CNFs. A comparison of

tensile strength and strain for unfilled and nano-filled PBI is shown in

figure 4.8. Results in figure 4.8 shows that exposure of unfilled and

nano-filled PBI to AO, UV and thermal cycling has very little effect on

tensile strength of the PBI. However, failure strain of unfilled and nano-

0

40

80

120

160

200

0 4 8 12 16 20

Ten

sile

Str

engt

h (

MP

a)

Strain (%)

PBI-C

PBI-AO

PBI-AO+UV+TC

a b

c a b c

(a)

0

50

100

150

200

0 4 8 12 16 20

Ten

sile

Str

engt

h (

MP

a)

Strain (%)

PBI-2CNF-C

PBI-2CNF-AO

PB-2CNF-AO+UV+TC

c b

a

a b c

(b)

0

50

100

150

200

0 4 8 12 16 20

Ten

sile

Str

engt

h (

MP

a)

Strain (%)

PBI-2TiO2-C

PBI-2TiO2-AO

PBI-2TiO2-AO+UV+TC

c b

a

a b c

(c)

0

50

100

150

200

0 4 8 12 16 20

Ten

sile

Str

engt

h (

MP

a)

Strain (%)

PBI-5TiO2-C

PBI-5TiO2-AO

PBI-5TiO2-AO+UV+TC

c b a

a b c

(d)


76

filled PBI has decreased. The strain of unfilled PBI is more affected as

compare to nano-filled PBI which indicates that nano-fillers have

improved the performance of PBI. The decrease in failure strain of

unfilled and nano-filled PBI is more likely due to the cross-linking

induced by thermal cycling and UV radiations. This explanation can be

supported with the fact that tensile modulus of all four materials has

increased after exposure to simulated LEO environment.

4.8 Comparison of (a) Tensile strength and (b) strain of unfilled and nano-filled PBI before and after

exposure to simulated LEO environment (V = Vacuum, AO = Atomic Oxygen, TC = Thermal Cycling,

UV = Ultraviolet)

A comparison of tensile modulus of unfilled and nano-filled PBI before

and after exposure to simulated LEO environment is shown in figure

4.9. Figure 4.9 clearly demonstrate that exposure of unfilled and nano-

filled PBI to LEO environment has increased the value of tensile

modulus. The increase in tensile modulus is more pronounced in case of

PBI reinforced with 2 weight percent of TiO2.

4.9 Comparison of Elastic Modulus of unfilled and nano-filled PBI before and after exposure to simulated LEO environment (V = Vacuum, AO = Atomic Oxygen, TC = Thermal Cycling, UV = Ultraviolet)

PB

I P

BI+

2C

NF

PB

I+2

TiO

2

PB

I+5

TiO

2

0

40

80

120

160

200

Unexposed V+AO V+AO+TC+UV

Ten

sile

Str

engt

h (

MP

a) (a)

PB

I P

BI+

2C

NF

PB

I+2

TiO

2

PB

I+5

TiO

2

0

5

10

15

20


Stra

in (

%)

(b) P

BI

PB

I+2

CN

F P

BI+

2Ti

O2

P

BI+

5Ti

O2

0

2

4

6

8


Elas

tic

Mo

du

lus

(GP

a)

Chapter 4

77

The tensile test results obtained after exposure to simulated LEO

environment and erosion yield data for unfilled and nano-filled PBI are

not supporting each other. High erosion yield values show that both

unfilled and nano-filled PBI are severely affected by AO which

ultimately should have an effect on tensile strength of material as

observed by previous study on polyimide [15]. Therefore, it was

important to find the reason why on one hand, both unfilled and nano-

filled PBI has shown high erosion yield and on other hand, it has shown

small decrease in mechanical properties. For this purpose, a sample of

unfilled PBI is exposed to thermal cycling and vacuum without exposure

to atomic oxygen and UV radiations for the same duration as it was

exposed to AO in combination with UV radiations and thermal cycling.

Weight measurement of this sample was performed before and after

exposure to thermal cycling under vacuum. This sample has shown a

weight loss of 8.07% while PBI has shown a weight loss of 9.06% when

it was exposed to combined effect of AO, thermal cycling and UV

radiations. These results indicate that PBI has shown most of the

weight loss due to the heating under vacuum. This weight loss is more

likely due to the removal of moisture that PBI contained. To further

support this conclusion, a sample of PBI was placed in vacuum oven at

125oC for overnight. Weight measurement of this sample was

performed before and after removing the sample from the oven. PBI

has shown a weight loss of about 8.4%. Again this is the same weight

loss as the weight loss of PBI during the thermal cycling in vacuum.

More interestingly, the same sample when kept at ambient conditions

of temperature and humidity, it has shown about the same weight gain.

These results explain the reason why PBI has shown higher erosion

yield without showing a considerable decrease in tensile strength. As

the erosion yield of the material is based on mass loss of the material

during the exposure to simulated LEO environment, therefore, it is

important to determine the accurate mass loss of material in order to

calculate the precise erosion yield. In present study, almost all the

mass loss of PBI occurred due to moisture removal rather due to the

AO attack. This is the reason that both unfilled PBI and nano-filled PBI

have shown very small decrease in tensile strength despite of having

high erosion yield.


SEM analysis of unfilled PBI and nano-filled PBI was carried out to

examine the dispersion of nano-fillers and to study the surface

morphology after exposure to simulated LEO environment. Though


78

CNFs are difficult to disperse in the polymer matrix, but SEM

micrograph of the PBI reinforced with CNFs in figure 4.10a has shown

good dispersion of CNFs. As a result of better dispersion, CNFs have

improved the strength of PBI.

4.10 SEM micrographs of PBI reinforced with (a) CNFs (b) TiO2 at 3000x

Contrary to the uniform dispersion of CNFs, TiO2 has formed pockets in

the PBI resin and these pockets contain lot of TiO2 particles which can

be seen in figure 4.10b. This could be one of the reasons that

unexposed PBI reinforced with TiO2 has shown a decrease in failure

strength and failure strain values. Therefore, the dispersion process of

TiO2 in PBI needs to be improved in order to achieve better mechanical

properties.

A comparison of surface topography of unexposed and exposed

samples using SEM is shown in figure 4.11. Visible inspection of

exposed unfilled PBI has shown no change in color or any sign of

surface roughness. However, SEM analysis of exposed unfilled PBI has

shown a small increase in surface roughness which can be seen in

figure 4.11b. Contrary to the unfilled PBI, CNFs reinforced PBI has

exhibited a surface roughness which could be observed with visual

inspection. SEM analysis has further explained the reason of increased

surface roughness. After exposure of CNFs reinforced PBI to simulated

LEO environment, PBI resin from the surface is eroded while embedded

nano-fibers came onto the surface as highlighted in figure 4.11d. These

fibers onto surface have increased the surface roughness of the

samples.

100 μm

1 μm

TiO2 (a) (b)

Chapter 4

79

Unexposed PBI Exposed PBI

Unexposed PBI+2CNFs Exposed PBI+2CNFs

Unexposed PBI+2TiO2 Exposed PBI+2TiO2


4.11 SEM micrographs of unexposed and exposed samples (5000x)

CNFs

TiO2

(a) (b)

(c) (d)

(e) (f)

(g) (h)


80

Visual inspection of PBI samples reinforced with TiO2 has exhibited

some powder like particles on the surface after exposure to simulated

LEO environment. The surfaces of these samples were still looked

smooth. Further analysis using SEM has revealed that after exposure to

simulated LEO environment, TiO2 embedded into the PBI resin

appeared on to the surface after the erosion of resin from the surface

which can be easily observed in figure 4.11f and 4.11h.

4.4.4. Atomic Force Microscopy (AFM)

AFM analysis was performed to investigate the changes in surface

topography and to measure the surface roughness after exposure to

different LEO environmental conditions. All samples have shown an

increase in surface roughness upon exposure to LEO simulated

environment. A comparison of surface roughness of unfilled and nano-

filled PBI is given in table 4.1. Table 4.1 shows that surface roughness

of all four materials exposed to simulated LEO environment has

increased. However, unfilled PBI has shown a small increase in surface

roughness as compare to nano-filled PBI. Table 4. 1 Comparison of surface roughness of unexposed and exposed samples measured by AFM analysis

Materials Average Roughness(Ra)

(nm)

Root Mean Square

Roughness (Rr.m.s)

(nm)

PBI unexposed 24.8 34.9

PBI exposed 30.0 44.7

PBI+2CNF unexposed 24.4 31.3

PBI+2CNF exposed 54.9 77.6

PBI+2TiO2 unexposed 10.9 16.8

PBI+2TiO2 exposed 52.5 66.6

PBI+5TiO-unexposed 9.8 7.2

PBI+5TiO-exposed 28.2 36.7

Atomic force microscopy was also performed to analyze the surface

topography of unexposed and exposed samples. AFM images in figure

4.12 reveal that exposure of PBI to LEO simulated environment has

generated two glowing spots on the surface of unfilled PBI. These spots

can be observed in AFM image shown in figure 4.12b. The optical glow

may hinder the visibility of the optical systems of any given space

structure. Figures 4.12c and 4.12d show the AFM images of PBI

reinforced with CNFs. Few holes can be seen in figure 4.12c which are

due to the presence of CNFs in PBI resin. These holes have grown

bigger due to the AO attack. As a result of AO attack, PBI resin in the

surrounding of the hole is eroded.

Chapter 4

81

Unexposed PBI Exposed PBI

Unexposed PBI+2CNFs Exposed PBI+2CNFs 4.12 AFM of unexposed and LEO exposed samples of unfilled PBI and PBI with 2

wt. % CNF using a 10 µm x 10 µm sample area


Unexposed PBI+5TiO2 Exposed PBI+5TiO2 4.13 AFM of unexposed and LEO exposed samples of PBI reinforced with 2 and 5 wt. % of TiO2 using a 10 µm x 10 µm sample area

100 μm

5 μm

100 μm

5 μm

(a) (b)

(c) (d)

(a) (b)

(c) (d)


82

Figure 4.13 shows the AFM images of unexposed and exposed PBI films

reinforced with 2 and 5 weight percent of TiO2. PBI reinforced with 2

weight percent of TiO2 has also shown big glowing spots which can be

seen in figure 4.13b. AFM images also reveal that PBI reinforced with 5

weight % of TiO2 has shown a carpet like surface after exposure to LEO

simulated environment.

4.5. Conclusion

The objective of present study was to evaluate the performance of PBI

under the simulated LEO environmental conditions. This is the first

detailed study which has given an impression about the response of PBI

when exposed to simulated LEO environment. Effect of nano-fillers on

the erosion yield and mechanical properties of PBI was also studied in

present work. Results show that both unfilled PBI and nano-filled PBI

have shown high erosion yield values. Contrary to the high erosion

yield values, both unfilled and nano-filled PBI have shown a small

decrease in mechanical properties. Unfilled PBI has shown a decrease in

strength of about 9% whereas CNFs reinforced PBI exhibited a

decrease in strength of about 3%. More interestingly, PBI reinforced

with 2 weight percent of TiO2 has shown an increase in strength of

about 15% after exposure to LEO simulated conditions. Results

obtained from mechanical testing indicate that mass loss obtained to

calculate the erosion yield was not the true mass loss due to AO attack.

Keeping the samples in the vacuum oven at 125oC has proved this fact

that almost all the mass loss of PBI was due to removal of the moisture

rather than the erosion due to AO attach. Therefore, it can be

concluded that mechanical properties of unfilled and nano-filled PBI are

not very much affected under the influence of simulated LEO

environmental conditions with an AO dose used in this study. However,

further study is required to accurately determine the erosion yield data

for unfilled and nano-filled PBI.

4.6. References

1. E. Grossman, I. Gouzman, Space Environment Effects on Polymers in Low Earth Orbit, Nuclear Instruments and Methods in Physics Research B 208, 2003

2. Richard L. Kiefer and Robert A. Orwoll ,space environmental effects on

polymeric materials, final technical report n88- 168 79, 1987

Chapter 4

83

3. Edward M. Silverman, Space Environmental Effects on Spacecraft: LEO Materials Selection Guide, NASA Contractor Report 4661, Part 2

4. Kim K. de Groh and Bruce A. Banks, MISSE PEACE Polymers Atomic Oxygen

Erosion Results, NASA/TM—2006-214482

5. Paul A. Steinerand Robert Sandor, “Polybenzimidazole Prepreg: Improved Elevated Temperature Properties with Autoclave Processability, “High performance polymers, Vol. 3, 1991

6. D.J.Kemmish, High performance engineering plastics, Rapra review reports,

Vol. 8, Report 86, 1995

7. S. Bhowmik, H. W. Bonin, V. T. Bui, R. D. Weir, Modification of High-Performance Polymer Composite Through High-Energy Radiation and Low-Pressure Plasma for Aerospace and Space Applications, Jouranl of Applied Polymer Science, Vol. 102, 2006

8. Joo-Hyun Han, Chun-Gon Kim, Low Earth Orbit Space Environment Simulation

and its Effects On Graphite/Epoxy Composites, Composite Structures vol. 72, 2006

9. Kwang-Bok Shina, Chun-Gon Kima, Chang-Sun Honga, Ho-Hyung Leeb,

Prediction of Failure Thermal Cycles in Graphite/Epoxy Composite Materials Under Simulated Low Earth Orbit Environments, Composites: Part B, Vol. 31, 2000

10. Zhao Wei, Li Weiping, Liu Huicong, Zhu Liqun, Erosion of a Polyimide Material

Exposed to Simulated Atomic Oxygen Environment, Chinese Journal of Aeronautics, Vol. 23, 2010

11. M. Raja Reddy, Review effect of low earth orbit atomic oxygen on spacecraft materials, J. Mat. Sci. 30 (1995) 281-307

12. Myer Kutz, Handbook of Environmental Degradation of Materials

13. Alexander Barrie, A report on Spacecraft Interactions in a LEO Environment, , December 2004

14. Hongying Yang, Sukang Zhu, Ning Pan, Studying the Mechanisms of Titanium Dioxide as Ultraviolet-Blocking Additive for Films and Fabrics by an Improved Scheme, J. App. Poly. Sci. Vol. 92, 2004, 3201-3210

15. Hiroyuki Shimamura and Takashi Nakamura, Effects of Atomic Oxygen on

Mechanical Properties of Polyimide Films, AIP conf. proceeding, 2009

85

CHAPTER 5

Performance Evaluation of Polybenzimidazole under High Energy Radiations Environment

This chapter mainly deals with the exposure of unfilled PBI

and PBI nano-composite to different kind of high energy

radiations. Characteristic behavior of unfilled and nano-filled

PBI after exposure to various radiations is presented in term

of thermal, mechanical and optical properties.

5.1. Introduction

Polymeric materials are widely used in spacecraft for load bearing

structures, antennas, fuel tank, electrical insulation, sealant, thermal

control coating and many other applications. These materials are ideally

suited for space applications due to their light weight, flexibility in use,

good thermal and electrical insulation properties, and ease in

manufacturing [1-5]. Depending on the applications, materials are

exposed to high vacuum, thermal cycling, high energy UV radiations,

atomic oxygen and high-energy electrons, protons and heavy ions.

Over a design lifetime of 20 years, a satellite in geosynchronous earth

orbit (GEO) could be exposed to a high-energy radiation dose of about

15–20 MGy, or possibly more in some circumstances [4, 6]. These high

energy radiations in GEO environment is one of the main concerns

regarding the thermal, mechanical and optical properties of these

materials. Therefore, it is important to understand the effect of these

high energy radiations on polymeric materials.

Performance Evaluation of PBI under High Energy Radiations Environment

86

5.2. Effect of Ionizing Radiations on Polymeric Materials

Polymeric materials exhibit a wide variety of radiation effects. Physical

mechanisms of radiation effects on polymeric materials depend on the

type and energy of radiation, the type of irradiated material and

radiation conditions including the total radiation dose, dose rate,

temperature of the material and some other factors as well [7]. The

formation of new chemical bonds after irradiation usually results in

irreversible effects. Generally, these effects can be visible as change in

appearance and change in mechanical, electrical, and thermal

properties [4, 5, and 8]. Irradiated polymers generally undergo cross

linking and chain scission reactions. Cross-linking reaction results in

formation of chemical bonds between two adjacent polymer molecules.

This reaction increases the molecular weight of the polymer until the

material is eventually bound into an insoluble three-dimensional

network. Chain scission decreases the molecular weight and increases

solubility. Both reactions can significantly alter the physical and

chemical properties of a polymer. In general, chain scission leads

towards a decreased Young’s modulus, increased elongation, decreased

hardness, and decreased elasticity. Cross linking generally has the

opposite effect on polymer properties [9].

The radiation stability of a polymer depends on the chemical structure

of the material. Addition of energy-absorbing aromatic rings to the

chemical structure significantly increases the radiation stability of some

polymers by aiding in the redistribution of the excitation energy

throughout the material. It is established in previous studies that

aromatic compounds are more resistant to radiations compared to

aliphatic compounds [10, 11]. The increased radiation resistance of

aromatic compounds is due to the fact that the phenyl ring in the

aromatic compounds can absorb energy by going to an excited state

and can dissipate this energy without disrupting the molecule [6, 10].

Kapton is the best known of the polyimides which displays an excellent

resistance towards degradation and loss of material properties when it

is exposed to high energy radiations. This resistance of kapton towards

high-energy radiation arises because the polymer contains a high

concentration of aromatic groups which can dissipate the absorbed

energy to heat through the vibration states associated with these

groups. For this reason, Kapton has found many space applications

Chapter 5

87

where resistance to high-energy radiation is important.

Polybenzimidazole (PBI) is another high performance polymer with high

concentration of aromatic groups. PBI with high concentration of

aromatic groups combined with high thermal and mechanical properties

has the real potential for different space applications under high energy

radiation environment. However, performance of PBI under high energy

radiation environment has not been evaluated till date. In this context,

efforts are made in present work to study the effects of different

radiations on thermal, mechanical and optical properties of PBI.

5.3. Experimental

5.3.1. Materials

26% concentrated solution of Polybenzimidazole (PBI) in

Dimethyleacetamide (DMAc) was supplied by CELAZOLE, PBI

performance products. 99% concentrated DMAc is purchased from

Aldrich chemicals. Carbon nano-fibres (CNFs) were supplied by Pyrograf

Products, Inc. with diameter ranging from 70 nm to 200 nm and length

ranging from 50 μm to 200 μm.

5.3.2. Preparation of Unfilled PBI and Nano-filled PBI Film

Unfilled and nano-filled PBI films were obtained using solution casting

method. The detail of preparation of these films is already given in

section 2.3.3 and 2.3.4. These films were used for the exposure to

different high energy radiations.

5.3.3. Exposure to Electron radiations and Gamma radiations

Unfilled and nano-filled PBI samples were irradiated at LEONI Studer

Hard AG, Switzelands. The samples were exposed to electron and

gamma radiations with energy of 1 MeV. The samples were irradiated

with gamma radiations and electron radiations for a total dose of 300

KGy and 1000 KGy respectively. The dose rate at the sample positions

was 4 kGy/sec.

5.3.4. Exposure to Mixed radiations

Unfilled and nano-filled PBI samples were irradiated with mixed

radiations in the pool of SLOWPOKE-2 nuclear reactor located at Royal

Military College of Canada. The pool is fixed in this reactor and

produces a mixed field of thermal and epithermal neutrons, energetic


88

electrons, protons and gamma rays. The samples were held in position

in the reactor pool by a device called ‘the elevator’, designed to position

and maintain samples at irradiation sites as shown in figure 5.1.

Samples were exposed to mixed radiations for 6 hours with a dose rate

of a 37 kGy/h.

5.1 Sample holder mounted on the radial arm of the SLOWPOKE-2 nuclear reactor pool elevator

5.4. Testing and Characterization


Thermal gravimetric analysis (TGA) was conducted to study the effect

of radiations on thermal stability of unfilled and nao-filled PBI. Tests

were performed using Perkins Elmer Thermal Analysis Instrument (Pyris

Diamond Thermogravimetric Analyzer). The weight of all the samples

was maintained between 4 to 5 mg. Samples were heated from a

temperature range of 25°C to 550°C at a heating rate of 10°C/min. The

furnace was purged with nitrogen gas at a flow rate of 25ml/min to

prevent oxidation.


Dynamic mechanical analysis (DMA) of irradiated samples was

performed in tensile mode at an oscillation frequency of 1 Hz using

Perkin-Elmer dynamic mechanical analyzer (Pyris Dynamic Mechanical

Analyzer). Samples of unfilled and nano-filled PBI were cut into

rectangular shape to the final dimensions of (40 x 6 x 0.08) mm3. Data

is collected from 25oC to 450oC at a scanning rate of 3oC/min.

Chapter 5

89


Tensile testing of irradiated unfilled and nano-filled PBI films was

carried out using Zwick tensile machine at a test speed of 2 mm/min.

Rectangular specimens of (80x6x0.08) mm3 were cut from unexposed

and irradiated samples. An extensometer was also used to determine

the Young’s Modulus and strain values. Five specimens for each

material were tested for the reproducibility of the results.

5.4.4. UV/VIS Spectroscopy

UV/VIS Spectroscopy was performed to study the effect of radiation on

transmittance and absorbance behavior of unfilled and nano-filler PBI.

The spectra were recorded on a Perkin Elmer Lambda 2 spectrometer

using a scan speed of 120 nm min-1 over the range of 200 to 800 nm

at room temperature. A baseline was recorded on an air reference prior

to measurement of each film. The thicknesses of the film samples were

measured using a micrometer screw and were taken as the average of

three measurements.

5.4.5. Surface morphology


morphology of controlled and irradiated samples. Images were obtained

using JOEL-7500 field emission scanning electron microscope (FE-SEM)

which operates at 30kV accelerating voltage. Surface of each sample

was coated with a thin layer of gold to minimize sample charging

effects.


5.5.1. Thermal gravimetric analysis (TGA)

TGA of unexposed samples and samples exposed to electron, gamma

and mixed radiations was performed to determine the effect of these

radiations on thermal stability of PBI. Thermal stability of material can

be considered in two ways; physical thermal stability and chemical

thermal stability [12]. Physical thermal stability is the mechanical

characteristics which can be represented as a function of temperature

such as storage modulus and glass transition temperature. The

chemical thermal stability is the change in the characteristics which can

be represented as a function of temperature and time, such as thermal

decomposition and thermal degradation [12]. TGA is an important tool


90

to determine the chemical thermal stability of the material. Thermal

stability of materials exposed to space environment is important factor

particularly when these materials are exposed to a temperature of

around 300oC. In this context, TGA of both unfilled PBI and CNFs

reinforced PBI is carried out. Figure 5.2 represents a comparison of

thermal stability of unexposed and irradiated samples of unfilled PBI.

TGA results for unexposed PBI are presented in chapter 2 in more

detail. In summary, it is important to mention here that unexposed PBI

has shown two steps decomposition with a total weight loss of about

13% even up to up to a temperature of 550°C. It is also important to

highlight that most of the weight loss occurred due to the evaporation

of water which PBI has absorbed due to its hygroscopic nature [13].

Comparison of thermal stability of unfilled PBI film exposed to different

radiations is shown in figure 5.2.

5.2 Comparison of thermal stability of unexposed PBI samples and samples exposed

to different radiations (UE = Unexposed, E = Electron Radiations, G = Gamma Radiations, M = Mixed Radiations)

TGA curves in figure 5.2 reveal that exposure of unfilled PBI to different

radiations has improved the thermal stability of PBI to some extent.

The improvement in thermal stability is more pronounced for the

samples exposed to gamma radiations and mixed radiations. After

exposing the samples to gamma radiations and mixed radiations, the

second degradation step has almost eliminated. Polymer has

maintained a stable plateau until a temperature of 550°C. Samples

irradiated by electron radiations have also shown a slight increase in

thermal stability at high temperature. The total dose value of electron

52

62

72

82

92

102

0 100 200 300 400 500 600

Mas

s (%

)

Temperature (°C)

PBI-UEPBI-EPBI-GPBI-M

a b c d

a b

d c

Chapter 5

91

irradiated PBI samples was high as compare to the dose value of

gamma radiations and mixed radiations. At low dose values, it is

expected that radiations has induced cross-linking effect in the

polymer. As the dose value has increased, chain scission reactions also

started. But the net effect of electron radiations in this study is

radiation induced cross-linking which resulted in slight increase in

thermal stability of unfilled PBI.

Samples of PBI reinforced with CNFs were also exposed to various

radiations. Figure 5.3 represents a comparison of thermal stability of

unexposed and irradiated samples of nano-filled PBI.

5.3 Comparison of thermal stability of unexposed Nano-filled PBI samples and samples exposed to different radiations (UE = Unexposed, E = Electron Radiations, G = Gamma Radiations, M = Mixed Radiations)

TGA curve of unexposed nano-filled PBI sample has shown two steps

decomposition and it has shown a total weight loss of about 10%.

Addition of CNFs to PBI has improved its thermal stability after

exposure to different radiations. Nano-filled PBI has depicted similar

kind of improvement in thermal stability after exposure to all three

radiations while unfilled PBI has shown a small improvement in thermal

stability after exposure to electron radiations. These results indicate

that addition of CNFs to PBI has improved the resistance of PBI to

electron radiations for higher dose values. The improvement in the

thermal stability of nano-filled PBI is more likely due the increased

cross-linking in the presence of CNFs.

52

62

72

82

92

102

0 100 200 300 400 500 600

Mas

s (%

)

Temperature (°C)

PBI+2CNF-UE

PBI+2CNF-E

PBI+2CNF-G

PBI+2CNF-M

a b c d

a

b

c d


92


Dynamic mechanical analysis (DMA) is an important tool to determine

the physical thermal stability of polymers. Dynamic mechanical analysis

is performed in tensile mode at a fixed frequency of 1Hz. The storage

modulus (E’) and loss factor (tanδ) for both unfilled and nano-filled PBI

were determined as a function of temperature. Glass transition

temperature (Tg) for these materials was also determined using loss

factor curve. Storage modulus represents the stiffness of visco-elastic

material while loss factor represents the damping behavior of the

material. A comparison of storage modulus and loss factor of

unexposed and irradiated samples of unfilled PBI is shown in figure 5.4.

5.4 Comparison of (a) storage modulus and (b) loss factor curve of unexposed PBI samples and samples exposed to different radiations (UE = Unexposed, E = Electron Radiations, G = Gamma Radiations, M = Mixed Radiations)

Storage modulus curve in figure 5.4a reveals that unexposed sample of

unfilled PBI has maintained a stable plateau up to a temperature of

250oC. Afterwards, storage modulus started to decrease followed by a

sharp decline around a temperature of 350oC. These results indicate

that unfilled PBI can maintain the structural stiffness up to a

temperature of 350oC and afterwards its stiffness started to decrease

sharply. The storage modulus curve of unfilled PBI typically represents

an amorphous polymer. It is the characteristic of an amorphous

polymer that it shows two plateaus in the storage modulus. The

decrease in storage modulus after first plateau is typically related to the

small movement of molecules in the side chains [14]. The sharp

decrease in storage modulus after second plateau is due to the large

movement of the molecules in the polymer backbone which results in

the softening of polymer and thus a decrease in stiffness of the polymer

[14]. Figure 5.4a shows that radiated samples of unfilled PBI have

1E+07

1E+08

1E+09

1E+10

0 100 200 300 400 500

Sto

rage

Mo

du

lus

(GP

a)

Temperature (°C)

PBI-UE

PBI-E

PBI-G

PBI-M

a b c d

a

b

c

d (a)

0

0,4

0,8

1,2

1,6

2

0 100 200 300 400 500

Loss

Fac

tor

(tan

δ)

Temperature (°C)

PBI-C

PBI-E

PBI-G

PBI-M

a b c d

a

b

c

d (b)

Chapter 5

93

exhibited an improvement in storage modulus. Unexposed PBI sample

has shown a sharp decrease in storage modulus around 350oC whereas

irradiated samples have maintained a stable storage modulus until a

temperature of 390oC. These results indicate that exposure of unfilled

PBI to different radiations has no detrimental effect on thermo-

mechanical properties of PBI. Instead, these radiations have improved

the storage modulus of unfilled PBI. The increase in storage modulus is

more likely due to the increase in cross-link densities of the exposed

samples.

A comparison of loss factor of unexposed and radiated samples of

unfilled PBI is presented in figure 5.4b. Two peaks at different

temperatures can be observed in the loss factor curve of unfilled PBI.

Essentially, it is the characteristic of all amorphous polymers that they

possess at least two relaxation processes namely “α” relaxation and “β”

relaxation. “α” relaxation occurred due to the movement of molecules

in the backbone of the polymeric structure and is associated with a

substantial fall in material stiffness [14]. The temperature corresponds

to this relaxation in the loss factor curve represents the glass transition

temperature (Tg) of the polymer. Tg of unexposed sample of unfilled

PBI as determined from the peak of tanδ curve is 417oC. Before

reaching to the Tg, amorphous polymers exhibit at least one additional

relaxation process “β” relaxation. This relaxation arises due to the

rotation or oscillation of side groups in the polymeric structure [14].

PBI has also exhibited this peak which can be observed in the tanδ

curve around a temperature of 280oC. In the present study, PBI has

depicted the highest glass transition temperature of any commercially

available polymer. Glass transition temperature gives an idea about the

operational temperature of a polymer used in structural components.

Figure 5.4b shows that the irradiated samples of unfilled PBI have

shown high tanδ peaks as compare to unexposed sample. The peaks

are also narrowed and shifted towards higher temperature. The

narrowing of tanδ curve indicates that after exposure of PBI to different

radiations, cross-lnking among the polymeric chains has increased

which restricted the molecular motions. The increased resistance to

molecular motion results in increased Tg of polymer. Tg has increased

from 417oC to 432oC after exposure of PBI to different radiations.

Irradiation of PBI samples has also shifted the β-transition towards high

temperature. The β-transition temperature has increased from 275oC to

297oC which indicates that radiation induced cross-linking in unfilled


94

PBI has also restricted the motion of the side chains until certain higher

temperature.

A comparison of storage modulus and loss factor of unexposed and

irradiated samples of nano-filled PBI is shown in figure 5.5. Figure 5.5a

shows that after adding 2 weight percent of CNFs to PBI, an

improvement in storage modulus is observed throughout temperature

range. There is no sharp decrease in storage modulus during the whole

temperature range. The storage modulus has decreased gradually to

1.9 GPa up to the temperature of 430oC and afterwards, it again

started to increase. This is about the same temperature where nano-

filled PBI has shown a peak in tanδ curve. The peak in tanδ curve

represents the glass transition temperature of nano-filled PBI. The

increase in storage modulus around glass transition temperature could

be explained with the fact that molecular relaxation takes place at this

temperature which allowed CNFs to align themselves and ultimately

they tend to reinforce the polymer and resist the viscous flow of

polymer. This reinforcement affect helped to regain the storage

modulus at high temperature.

5.5 Comparison of (a) storage modulus and (b) loss factor curve of unexposed Nano-filled PBI samples and samples exposed to different radiations (UE = Unexposed, E = Electron Radiations, G = Gamma Radiations, M = Mixed Radiations)

Comparison of storage modulus of unexposed and irradiated samples of

nano-filled PBI shows that samples exposed to gamma radiations and

mixed radiations have shown little improvement in storage modulus

throughout the temperature range. However, samples exposed to

electron radiations have exhibited a decrease in storage modulus at

high temperature. These results indicate that electron irradiation of

nano-filled PBI has not affected the room temperature stiffness of

material. However, high temperature properties of the material are

greatly affected by these radiations but still the storage modulus of

1E+07

1E+08

1E+09

1E+10

0 100 200 300 400 500

Elas

tic

Mo

du

lus

(GP

a)

Temperature (oC)

PBI+2CNF-UE

PBI+2CNF-E

PBI+2CNF-G

PBI+2CNF-M

(a)

a b c d

a

d

c

b

0

0,5

1

1,5

2

0 100 200 300 400 500

Loss

Fac

tor

(tan

δ)

Temperature (oC)

PBI+2CNF-UE

PBI+2CNF-E

PBI+2CNF-G

PBI+2CNF-M

(b)

a b c d

a d

b

c

Chapter 5

95

nano-filled PBI is 1.3 GPa at 400oC which is almost double than the

storage modulus of unfilled PBI at the same temperature after electron

irradiation.

Figure 5.5b shows that addition of 2 weight percent of CNFs to PBI has

significantly reduced the peak height of tanδ curve when compare to

the peak height of unfilled PBI. The reduction in peak height is an

indication of increased elasticity and reduced damping properties of the

material [14, 15]. Also, by adding CNFs to the polymer, the value of

secondary transition temperature has increased up to 30oC which

indicate that CNFs have restricted the motion of the side chain

molecules very effectively. All these results indicate that addition of

CNFs has improved the performance of PBI to such an extent that it can

maintain high storage modulus even at a temperature of 400oC which

shows the potential of nano-filled PBI for high temperature applications.

A comparison of loss factor of unexposed and irradiated samples of

nano-filled PBI shows that position of glass transition and β-transition is

not changed after exposure to gamma radiation and mixed radiation.

However, for electron irradiated samples, the peak height of tanδ curve

has increased.

A comparison of storage modulus and glass transition temperature of

unfilled PBI and nano-filled PBI after exposure to different radiations is

presented in table 5.1.

Table 5. 1 Storage Modulus and glass transition temperature of unexposed and exposed samples of unfilled PBI and nano-filled PBI

Materials

1E50°C

(GPa)

E100°C

(GPa)

E150°C

(GPa)

E200°C

(GPa)

E300°C

(GPa)

E350°C

(GPa)

E400°C

(GPa)

2Tg

( oC )

PBI-UE 4.1 3.6 3.9 3.7 2.1 1.2 0.2 417

PBI-E 5.0 4.7 4.5 4.2 2.8 2.1 0.6 433

PBI-G 4.8 4.6 4.4 4.1 2.8 2.2 0.8 429

PBI-M 3.8 3.5 3.4 3.3 2.6 2.1 0.7 432

PBI-2CNF-UE 5.2 4.5 4.4 4.4 3.3 2.6 2.0 406

PBI-2CNF-E 5.6 5.0 4.74 4.5 3.4 2.5 1.3 430

PBI-2CNF-G 6.6 5.0 4.4 4.3 3.8 3.2 2.3 407

PBI-2CNF-M 5.7 4.6 4.3 4.3 3.6 3.1 2.2 406

1 = Values of E with an error of 0.05GPa 2 = Glass Transition onset of peak of loss factor curve

(UE = Unexposed, E = Electron Radiations, G = Gamma Radiations, M = Mixed Radiations)


96

5.5.3. Tensile Test Results

Retention of mechanical properties of a material after irradiation is an

important factor which determines the sustainability of a polymer for

use in radiation environment. Therefore, to evaluate the performance of

PBI under radiation environment, tensile testing of unfilled and nano-

filled PBI is performed before and after exposure to different radiations.

As in present study, PBI is exposed to different radiations with different

dose levels. Therefore, a direct comparison of tensile properties of PBI

exposed to different radiations is not possible. The ultimate objective of

this study was to evaluate the performance of PBI when exposed to

different radiations. A comparison of stress-strain curve of unexposed

and irradiated samples of unfilled PBI is shown in figure 5.6.

5.6 Comparison of tensile properties of unexposed PBI samples and samples exposed to different radiations (UE = Unexposed, E = Electron Radiations, G = Gamma

Radiations, M = Mixed Radiations)

Figure 5.6 shows that samples of unfilled PBI exposed to electron

radiations and gamma radiations have depicted a slight decrease in

tensile strength. However, samples exposed to mixed radiations have

more detrimental effect on tensile strength of unfilled PBI. Samples

exposed to gamma radiations exhibited a minor increase in both tensile

strength and tensile modulus. At the same time, these samples have

shown a decrease in tensile strain of about 18%. The small increase in

tensile strength and decrease in tensile strain after exposure to gamma

radiation is more likely due to the radiation induced cross-linking in

PBI. These results indicate that gamma radiations with a total dose

0

40

80

120

160

200

0 5 10 15 20

Ten

sile

Str

engt

h (

MP

a)

Strain (%)

PBI-UEPBI-GPBI-EPBI-M

a b c d

a

b

c

d

Chapter 5

97

value of 300 KGy have not greatly influenced the tensile strength

properties of unfilled PBI but it has an influence on failure strain.

Tensile testing of electron irradiated PBI samples reveals that tensile

strength of unfilled PBI is very slightly decreased. However, these

radiations have reduced the strain of unfilled PBI to about 41%. These

results demonstrate that PBI can withstand high electron radiation dose

values without being affected in term of tensile strength. Unfilled PBI

samples irradiated with mixed radiations have depicted a reduction of

about 16% in the tensile strength which indicate that a more

pronounced change in tensile properties of PBI is observed after

exposure to mixed radiation.

A comparison of stress-strain curve of unexposed and irradiated

samples of nano-filled PBI is shown in figure 5.7.

5.7 Comparison of tensile properties of unexposed samples of Nano-filled PBI and

samples exposed to different radiations (UE = Unexposed, E = Electron Radiations, G

= Gamma Radiations, M = Mixed Radiations)

Figure 5.7 shows that exposure of nano-filled PBI to gamma radiations

has increased the tensile strength from 181 MPa to 185 MPa. However,

a reduction of about 19% in the tensile strain is observed for the

samples exposed to gamma radiations. Tensile testing of electron

irradiated nano-filled PBI samples reveals that exposure to electron

radiations has decreased the tensile strength about 3%. A reduction of

about 6% in tensile strain of nano-filled PBI is also observed after

exposure to electron radiations. Exposure of nano-filled PBI to mixed

0

40

80

120

160

200

0 2 4 6 8

Ten

sile

Str

engt

h (

MP

a)

Strain (%)

PBI+2CNF-UE

PBI+2CNF-G

PBI+2CNF-E

PBI+2CNF-M

a b c d

a

c

d

b


98

radiations has reduced the tensile strength from 181 MPa to 163 MPa; a

decrease of about 22% in tensile strength while a reduction of 10% in

tensile strain is also observed. The results obtained from tensile testing

of irradiated nano-filled PBI reveal that all three kinds of radiations has

induced less damage to nano-filled PBI than unfilled PBI.

A comparison of tensile strength and tensile strain of irradiated unfilled

and nano-filled PBI is shown in figure 5.8.

5.8 Comparison of (a) Tensile strength and (b) Tensile strain of unfilled and nano-filled PBI after exposure to different radiations (UE = Unexposed, E = Electron

Radiations, G = Gamma Radiations, M = Mixed Radiations)

Figure 5.8 demonstrates that nano-filled PBI has depicted better

performance when exposed to different radiation environment. Both

unfilled and nano-filled PBI has exhibited an improvement in tensile

strength after exposure to gamma radiations while these materials

have shown a decrease in tensile strength after exposure to electron

radiations and mixed radiations. However, decrease in tensile strength

is more pronounced with unfilled PBI which indicate that nano-filled PBI

has shown more resistance to radiations. More importantly, nano-filled

PBI has demonstrated less decrease in tensile strain as compare to the

decrease in tensile strain of unfilled PBI. Hence, it will be beneficial to

disperse CNFs into PBI before exposure to radiation environment.

5.5.4. UV/VIS photospectroscopy

Irradiation of polymeric films involves variety of factors which

contribute towards change in physical and chemical properties.

Chemical reactions initiated by high energy radiations are major

sources of film property alteration. The light absorption and

transmittance characteristics of a given film are the determining factors

PB

I P

BI+

2C

NF

0

40

80

120

160

200

UE G E M

Ten

sile

Str

engt

h (

MP

a) (a)

PB

I

PB

I+2

CN

F 0

4

8

12

16

20

UE G E M

Ten

sile

Str

ain

(%

)

(b)

Chapter 5

99

for photo-chemical changes in the film properties. Irradiation can

induce discoloration of polymeric film as a result of chemical

degradation. The discoloration results in change of optical

characteristics of polymers. The radiation induced discoloration can be

observed visually. In present study, samples are visually inspected

after irradiation, however, no signs of discoloration are observed. To

further verify that radiations have not induced any chemical change in

the structure of PBI, optical properties of PBI were studied using

UV/VIS photo-spectroscopy. Transmittance and reflectance curves of

unfilled and nano-filled PBI are obtained from experiments. Using

experimental results, absorbance values are derived. A comparison of

transmittance of unexposed and irradiated samples of unfilled PBI is


5.9 Comparison of transmittance of unexposed and irradiated samples of unfilled PBI (UE = Unexposed, E = Electron Radiations, G = Gamma Radiations, M = Mixed Radiations)

Figure 5.9 shows the transmittance curves of unexposed and irradiated

PBI sample as a function of wavelength. Unexposed PBI has shown

more or less similar kind of transmittance curve as depicted by kapton

which is a high performance polymer and is being used for many space

applications. There is a strong absorption of light by PBI in visible

region which is more likely due to the presence of chromophores. The

chromophore is a region in the molecule where the energy difference

between two different molecular orbital falls within the range of the

visible spectrum. Visible light that hits the chromophores can thus be

absorbed by exciting an electron from its ground state into an excited

0

20

40

60

80

200 300 400 500 600 700 800

Tran

smit

tan

ce (

%)

Wavelength (nm)

PBI-UE

PBI-E

PBI-G

PBI-M

a b c d

a b

c

d


100

state. The examples of such chromophore in PBI are pi-electron and

hetero atoms having non-bonding valence-shell electron pairs.

As mentioned earlier that no color change of irradiated PBI was

observed during visual inspection. Photo-spectroscopic results of

irradiated PBI are in line with visual results. Exposure of PBI to different

radiations has not changed its light transmitting characteristic. PBI

exposed to mixed radiations has exhibited a slight decrease in

transmittance in the visible region. However, in near ultraviolet region,

the optical behavior of all irradiated samples remained unaffected.

The transmittance and absorbance curves of unexposed and irradiated

samples of nano-filled PBI are shown in figure 5.10.

5.10 Comparison of transmittance and absorbance of controlled and irradiated samples of nano-filled PBI

It can be observed in figure 5.10a that nano-filled PBI has shown high

absorbance values which remained unaffected in the whole wavelength

spectrum. The absorption behavior of nano-filled PBI has not changed

after exposure to various radiations. For some space applications, it is

the requirement of surface materials to absorb maximum light. The

examples of such materials are black coatings which are used as solar

absorber [16]. Therefore, nano-filled PBI with better thermo-

mechanical properties, improved radiation resistance and high

absorption values can be used as black coating for specific space

applications. Figure 5.10b shows that after dispersion of 2 weight

percent of CNFs in PBI, almost no light is transmitted through the nano-

filled PBI film. Optical transparency is very important for polymer films

in certain space applications. Therefore, nano-filled PBI cannot be used

for such specific applications.

0

20

40

60

80

100

400 500 600 700 800

Ab

sorb

ance

(%

)

Wavelength (nm)

PBI-2CNF-UE

PBI-2CNF-E

PBI-2CNF-G

PBI-2CNF-M

(a)

a b c d

0

1

2

3

4

5

200 300 400 500 600 700 800

Tran

smit

tan

ce (

%)

Wavelength (nm)

PBI-2CNF-UE

PBI-2CNF-E

PBI-2CNF-G

PBI-2CNF-M

a b c d

(b)

a d

c b

Chapter 5

101

5.5.5. Surface morphology

SEM analysis was performed to study the surface morphology of

unfilled and nano-filled PBI after exposure to different radiations. The

purpose of studying surface morphology of radiated samples was to

determine if there is any blistering effect induced on the surface.

Blistering is the plastic deformation of irradiated surface layer due to

the evolution of different gases near the surface region. Research

conducted on gamma irradiated and electron irradiated aromatic

polymers reveal that production of different volatile gases including CO,

CO2, N2 and H2 induce undesirable effects on material properties [11,

17 and 18]. This ultimately leads to premature failure of the material

due to degradation in mechanical properties and thus limiting the

service life. Although aromaticity in polymeric structures generally

promotes high stability in term of thermal and mechanical properties

under the influence of radiation, yet the properties of these materials

are affected by the production of different gases upon irradiation. A

recent study performed on kapton under the influence of high energy

gamma radiation shows that exposure of kapton to high energy gamma

radiations resulted in evolution of different gases [18].The gas

evolution caused the bubbles nucleation which continued to grow on

further exposure to these radiations. Ultimately at certain higher

pressure of the evolved gases, these bubbles collapse and plastically

deformed [18]. Therefore, present work has also included the study of

surface morphology of irradiated unfilled and nano-filled PBI. Figure

5.11 and 5.12 shows the surface morphology of unexposed and

irradiated samples of unfilled PBI and nano-filled PBI respectively.

Figure 5.11 and 5.12 shows that irradiation of both unfilled and nano-

filled PBI has changed the surface topography of exposed samples. All

three kinds of radiations have increased the surface roughness of the

exposed samples. However, no sign of bubbling or blistering is

observed. There is also possibility that during the irradiation process,

the bubbles due to gas evolution are nucleated and collapsed which

ultimately induced a change in surface topography of exposed samples

as shown in figure 5.11 and 5.12. However, no conclusive statement

can be made about any gas evolution or bubble formation during the

irradiation of the PBI exposed sample.


102

5.11 Comparison of surface topography of unfilled PBI (a) Controlled (b) Gamma irradiated (c) Mixed field irradiated (c) Electron irradiated (5000x magnification)

5.12 Comparison of surface topography of nano-filled PBI (a) Controlled (b) Gamma irradiated (c) Mixed field irradiated (c) Electron irradiated (5000x magnification)

(a) (b)

(d)

100 μm

1 μm

(c) (d)

(b)

(c) (d)

100 μm

1 μm

Chapter 5

103

5.6. Conclusion

In present work, efforts are made to study the effects of different

radiations on thermal, mechanical and optical properties of PBI. Unfilled

and nano-filled PBI samples were exposed to gamma radiations,

electron radiations and mixed radiations. TGA curves of unexposed and

irradiated samples show that all three kinds of radiations have induced

the cross-linking in PBI which ultimately has increased the thermal

stability particularly at high temperature. DMA experiments on unfilled

and nano-filled PBI sample shows that the addition of CNFs has

improved the thermo-mechanical properties of PBI. Unfilled PBI has

shown a sharp decrease in storage modulus around a temperature of

350oC while nano-filled PBI has maintained storage modulus up to a

temperature of 400oC. Also, no sharp decrease in storage modulus is

observed with nano-filled PBI which demonstrates the effectiveness of

dispersing CNFs into PBI. DMA experiments of irradiated unfilled PBI

revealed that all three kinds of radiations have induced cross-linking in

PBI films. As a result, an improvement in storage modulus and glass

transition is observed. Unexposed sample of unfilled PBI has shown a

sharp decline in storage modulus around 350oC whereas radiated

samples have depicted a decline in storage modulus around a

temperature of 390oC. Also, irradiation of unfilled PBI has increased the

glass transition temperature from 417oC to 432oC. These results

indicate that irradiation of unfilled PBI has enhanced the thermo-

mechanical properties of PBI. DMA experiments on nano-filled PBI

demonstrate that irradiation has slightly increased the storage modulus

through the whole temperature range while the glass transition

temperature remained unaffected. Another important observation is

that nano-filled PBI has not shown a sharp decline in storage modulus

except the sample irradiated with 1000 KGy electron radiations. These

results demonstrate that addition of CNFs to PBI is helpful in

maintaining the storage modulus even at a temperature of around

400oC. Tensile testing of unexposed and irradiated samples of unfilled

and nano-filled PBI shows that effect of gamma radiations and electron

radiations has very little effect on tensile strength of PBI. However,

these radiations have affected the failure strain to some extent.

Addition of CNFs to PBI has increased the resistance of PBI against

radiations. Irradiated nano-filled PBI samples have depicted less

decrease in failure strain as compare to irradiated unfilled PBI which is

another benefit of adding CNFs to PBI. Visual inspection of both unfilled

and nano-filled PBI and results obtained from photo-spectroscopy of


104

these samples has not revealed any kind of degradation in chemical

structure. Results obtained during this study are quite encouraging and

further studies can be performed on PBI using higher dose values to

evaluate its performance. Also, irradiation under vacuum and thermal

cycling environment will give more realistic idea about the performance

of PBI.

5.7. References

1. Kresten L.C. Nielsen, David J.T. Hill, Kent A. Watson, John W. Connell, Shigetoshi Ikeda, Hisaaki Kudo, Andrew K. Whittaker The radiation degradation of a nano tube polyimide nanocomposite, Polymer Degradation and Stability 93 (2008)

2. T. Rohr, M. van Eesbeek, Polymer Materials in the Space Environment, 8th

Proceeding of Polymers for Advanced Technologies International Symposium, Budapest, Hungary, (2005)

3. Xianqiang Pei, Yan Li, Qihua Wang, Xiaojun Sun, Effects of atomic oxygen

irradiation on the surface properties of phenolphthalein poly(ether sulfone), Applied Surface Science 255 (2009)

4. Sheila Devasahayam, David J. T. Hill and John W. Connell, Effect of Electron

Beam Radiolysis on Mechanical Properties of High Performance Polyimides. A Comparative Study of Transparent Polymer Films, High Performance Polymers (2005)

5. Phil-Hyun Kang , Young-Kyou Jeon, Joon-Pyo Jeun, Jin-Wook Shin, Young-

Chang Nho, Effect of electron beam irradiation on polyimide film Journal of Industrial and Engineering Chemistry 14 (2008)

6. A.N. Netravali And A. Manji, Effect of Gamma Radiation on the Mechanical

Properties of Epoxy Blends and Epoxy-Graphite Fiber Interface, Polymer Composites, Vol. 12, (1997)

7. B.A. Briskman, E.R. Klinshpont, V.F. Stepanov, Dose Rate Effects in Polymer

Materials Irradiated in Vacuum, Protection Of Materials And Structures From Space Environment, Space Technology Proceeding, vol. 5, (2004)

8. J. Chen, M. Czayka, Roberto M. Uribe, Effects of electron beam irradiations on

the structure and mechanical properties of polycarbonate, Radiation Physics and Chemistry 74 (2005)

9. Nuclear and Space Radiation Effects on Materials, NASA Report SP -8053,

(1970)

Chapter 5

105

10. J.D. Memory, R.E. Fornes and R.D. Gilbert,Radiation Effects on Graphite Fiber Reinforced Composites Journal of Reinforced Plastics and Composites, Vol. 7 (1988)

11. El-Sayed A. Hegazy, T. Sasuga, M. Nishii and T. Seguchi, Irradiation affects on

aromatic polymers: Gas evolution during electron beam irradiation, Polymer, Volume 33, (1992)

12. Hiroshi Inoue, Hidemasa Okamoto, Yukio Hiraoka, Effect of the chemical

structure of acid dianhydride in the skeleton on the thermal property and radiation resistance of polyimide, Int. J Radiat Appli & Inst. Par C, Vol. 29 (1987) pp. 282-288

13. Tai-Shung Chung, A Critical Review of Polybenzimidazoles, Journal of

Macromolecular Science, Part C: Polymer Reviews, (1997)

14. Kevin P. Menard, Dynamic Mechanical Analysis: A practical Introduction, ISBN 0-8493-8688-8, (1999)

15. Elsa Reichmanis, James H. O’Donnell, The effect of radiations on high

technology polymers, ACS Symposium Series 381, (1989)

16. K. Ramaseshan, M. Viswanathan and G. K. M. Thutupalli, Optical Black Coating for Space Application, Bull Mater. Sci. Vol. 8, (1986)

17. El-Sayed A. Hegazy, T. Sasuga, M. Nishii and T. Seguchi, Irradiation affects on

aromatic polymers: Gas evolution during gamma irradiation, Polymer, Volume 33, (1992)

18. Siddhartha, Suveda Aarya, Monika Mishra, A. K. Srivastava,M. A. Wahab,

Formation of Blisters in Kapton Polymer by the Effect of 1.25 MeV Gamma Irradiation, Journal of Applied Polymer Science, Vol. 120, (2011)

107

CHAPTER 6

Performance Evaluation of Polybenzimidazole as an Adhesive for Aerospace Applications

This chapter highlights the important aspects of adhesive

bonding process. In the first part, process optimization of PBI

as an adhesive is presented. Characteristics behaviour of PBI

adhesive after exposure to various environmental conditions

is discussed in the second part of this chapter. Scanning

electron microscopic analysis of failed joints is also presented

in detail.

6.1. Introduction

The use of polymer-based composite materials is becoming significantly

popular due to their high strength to weight ratio, excellent corrosion

resistance, outstanding thermal insulation and low thermal expansion.

They have an impressive and diverse range of applications in

automotive, aviation, spacecraft, civil infrastructure and sports

industries [1-4]. Metallic materials are joined by riveting, bolting,

welding, soldering and other methods. These are mature technologies

for joining metallic materials. In contrast to the metallic materials,

composite materials are often joined by adhesive bonding to form

structural components. Though mechanical fastening is also being used

for composite materials to some extent but adhesive bonding is given

more preference over mechanical fastening because of some

advantages. Adhesive bonding provides uniform stress distribution over

the entire bond line while mechanical fastening create large stress

concentration around the drilled holes [5, 6]. Also, adhesive bonding

Performance Evaluation of PBI as an Adhesive for Aerospace Applications

108

technique provides more design flexibility compared to the mechanical

fastening [7-9].

Despite all the advantages of adhesive bonding, the use of adhesive

bonding for aerospace applications is becoming more challenging. The

main reason is that adhesive bonding has limitation of being used at

elevated temperature and under thermal cycling conditions. For some

adhesives, oxidative degradation occurs at high temperatures whereas

some adhesives show brittleness at low temperatures [10]. In contrast

to the adhesive bonding, mechanical fasteners rarely limit the end use

temperature of the composite materials and perhaps this is one of the

primary reasons that mechanical joining is still being used extensively

for composite joining.

The increased usage of high-temperature resistant resins for composite

materials has necessitated the development of compatible and equally

thermally stable adhesive systems. Epoxy adhesives are commonly

used to bond epoxy based composites because of the compatibility

between resin and adhesive. However, prolonged exposure or even

short-term exposure of these adhesive to elevated temperatures often

produces irreversible chemical and physical changes within adhesives.

Thermal and mechanical properties of these adhesives are degraded

which ultimately degrade the performance of bonded joints.

Consequently, these adhesives cannot be used at very high

temperature [11-12]. Therefore, for high temperature applications, it is

important to select an adhesive which can maintain its thermal and

mechanical properties at high temperature. In order to meet the

demands of aerospace industry, emphasis is being given on the

development of high performance adhesives which can perform well

both at elevated temperature and under thermal cycling conditions.

Polybenizimidazole (PBI) is one such high performance polymer which

has gained great attraction in recent years. Polybenzimidazole (PBI) is

a thermoplastic polymer which has the highest glass transition temperature

Tg (425°C) of any commercial available organic polymer [13]. It has high

decomposition temperatures (500°C-600°C), good oxidation resistance and

it maintains excellent strength at cryogenic temperatures [14]. Due to its

high thermo-mechanical properties, it has great potential to be used as

an adhesive for high temperature applications. Therefore, the objective

of present work is to optimize the bonding process of PBI and to

evaluate its performance under various environmental conditions and at

elevated temperature.

Chapter 6

109

Another important aspect of adhesive bonding of composite materials is

the surface preparation of these materials prior to bonding. For

successful application of composite materials to form structural parts

using adhesive bonding, they need to have special surface properties

such as hydrophilicity and roughness [15, 16]. Very often, these

materials do not possess the surface properties needed to form bonded

parts. For these reasons, surface modification techniques are used

which can improve the adhesion properties of composite materials to

such an extent that failure should be cohesive (failure within the

adhesive or substrate) rather than adhesive (failure at the interface).

Surface modification improves the bond performance by introducing the

polar functional groups onto the surface which ultimately react with the

adhesive to form strong bonds [17]. Also, surface modification imparts

roughness which resulted in increased mechanical interlocking and

hence increased bond strength [17].

6.2. Selection of the surface treatment

Various surface treatment methods are used to improve the surface

energy of polymer based composite materials. Typical composite

surface treatments include traditional abrasion and solvent cleaning,

acid etching, grit blasting, peel-ply, corona discharge treatment,

Vacuum plasma and atmospheric pressure plasma treatment (APPT).

These treatments increased the adhesion properties by changing the

surface chemistry or surface morphology of composite materials. Each

of these treatment methods has its own advantages and drawbacks.

Detail about these surface treatment methods is given in the previous

studies [17, 18]. Due to the environmental hazards of chemical

treatment methods, dry surface treatments like corona and plasma

treatment are the most commonly used methods. Plasma treatment not

only transforms the inherent surface chemistry to improve wetting

characteristic of polymer but also increases the surface roughness at

the same time [19, 20]. The objective of present work is to study the

affect of plasma treatment on the adhesion properties of two aerospace

grade epoxy based carbon fiber composites. Other objective of this

work is to optimize the bonding process of high performance

polybenzimidazole (PBI) and to evaluate its performance under

different environmental conditions. A detail of plasma treatment is

presented in the following section followed by the experimental detail of

adhesive bonding process of PBI.


110

6.2.1. Atmospheric Pressure Plasma Treatment (APPT)

Atmospheric pressure Plasma treatment (APPT) has become a very

popular treatment for small to medium size parts. APPT introduce polar

chemical groups onto the surface which further react with the adhesive

to form strong bonds [18, 20]. Another important aspect of using APPT

is the increased surface roughness of the treated samples. In one of

our previous studies on PPS/glass fiber bonded joints, it was

highlighted that plasma treatment increased the surface roughness of

these materials [21]. However, the reason of this increase in surface

roughness was not known. According to author’s knowledge, the only

recent study about the plasma treatment of Poly(methyl methacrylate)

has pointed out that increased surface roughness and formation of the

cracks on the surface of plasma treated samples was due to the

increase in surface temperature to above 200oC [22]. This high

temperature could result in local melting of the exposed resin on the

surface. The rapid heating and cooling in the air could generate the

residual stresses in the treated samples which ultimately lead towards

crack formation and increased surface roughness. The high

temperature can also degrade the mechanical properties of the

composite material itself [23]. Despite this fact, still the APPT can be an

important tool to treat different composite surfaces. In the context of

above discussion, it was decided to study the effect of APPT on surface

energy and surface morphology of two aerospace grade epoxy based

composite. Surfaces of these materials were also prepared using hand

sanding. The objective was to compare the surface morphology and

adhesion properties of these materials after having different surface

treatments. Bonded joints of these materials were formed using PBI

adhesive.

6.3. Experimental

6.3.1. Materials

26% concentrated solution of PBI in Dimethyleacetamide (DMAc) was

supplied by CELAZOLE, PBI performance products. M21 and DT120

epoxy based Unidirectional (UD) carbon fiber prepregs were supplied by

Hexcel and Delta Tech respectively. In the following sections, these

composites will be named as M21/carbon composite and DT120/carbon

composite. Composite laminates were prepared by stacking up a

number of pre-impregnated layers to achieve a cured laminate

Chapter 6

111

thickness of 4 mm. Laminates were manufactured in an autoclave by

using curing cycle specified by suppliers of the materials.

6.3.2. Atmospheric Pressure Plasma Treatment of Composite Specimen

Composite surfaces were treated with atmospheric pressure plasma

using TIGRES Plasma-BLASTER MEF equipment shown in figure 6.1. For

this particular study, air was used for treatment at a pressure of 4.5

bars. Before performing the plasma treatment, the samples were first

cleaned with methanol using an ultrasonic bath to remove any

contamination on the surface. After cleaning, the specimens were dried

in a vacuum oven at 80oC for 4 hours. Distance of the nozzle tip from

the substrate surface is an important parameter in determining the

treatment time required to achieve the lowest contact angle for any

particular surface. As the equipment can only perform APPT by varying

the distance between 5 mm and 20 mm. Therefore, it was decided to

perform the APPT using two different distances of 10 mm and 20 mm

from the surface. Plasma treatment on these samples was performed

for 15 sec, 30 sec, 45 sec, 60 sec, 75 sec and 90 sec. The goal was to

study the response of two different materials in term of contact angle,

surface temperature and surface morphology with increasing treatment

time and by varying the height of nozzle. A thermocouple was used to

determine the surface temperature of samples during plasma

treatment.

6.1 Apparatus for Atmospheric Pressure Plasma Treatment

6.3.3. Contact Angle Measurements

The change in the surface energy after plasma treatment was

determined in term of contact angle using water as a liquid. A reduced

value of contact angle indicates an improvement in surface energy of

material which in turn improves the adhesion properties of materials.

Contact angle measurements were carried out by the Modular CAM


112

200–Optical Contact Angle and Surface Tension Meter from KSV

Instruments.

6.3.4. Specimen Preparation for Lap Shear Testing

PBI adhesive bonded joints of M21/carbon composite and

DT120/carbon composite were prepared after performing APPT. Bonded

joints of these materials were also prepared after treating the surface

using hand sanding. The purpose was to compare the effect of two

different treatment methods on lap shear strength of bonded joints.

Specimens for lap shear testing were cut to the dimensions of (100 x

25 x 4) mm3 and they were adhesively bonded for single lap shear

tensile tests. Prior to the preparation of an adhesive joint, degassing of

the adhesive was carried out for 10 min. The lap shear tensile

specimens were prepared by applying PBI adhesive. Pressure was

applied to the lap joint during the curing cycle by two standard clips.

Bonded joints were formed using different curing temperature for PBI

adhesive in order to attain an optimum joint strength.

6.3.5. Environmental Conditioning of Specimens

Bonded joints of both types of composite materials were prepared for

environmental conditioning. The purpose was to evaluate the influence

of temperature and humidity on the bonded joint strength. Joints were

conditioned for 1000 hours at 80oC and 95% relative humidity. The

bonded joints were removed from the climate chamber after 1000

hours and immediately tested for lap shear strength. PBI adhesive

bonded joints of M21/carbon composite and DT120/carbon composite

were also formed to perform the lap shear testing at 80oC. The purpose

was to evaluate the performance of PBI at high temperature.

6.3.6. Specimen Preparation for Lap Shear Testing

A summary of the specimens prepared for lap shear testing during this

study is given below. Lap shear testing was carried out at ambient

conditions unless mentioned otherwise.

a. Bonded joints of M21 and DT120 epoxy based carbon fiber

composite specimens with untreated surfaces

b. Composite bonded joints with a surface preparation using

atmospheric pressure plasma treatment (APPT) prior to bonding

Chapter 6

113

c. Composite bonded joints with a surface preparation using hand

sanding prior to bonding

d. Composite bonded joints with a surface preparation using both

atmospheric pressure plasma treatment and hand sanding prior

to bonding (hand sanding followed by APPT)

e. Composite bonded joints with a surface preparation using

atmospheric pressure plasma treatment prior to bonding followed

by environmental conditioning for 1000 hours at 80oC and 95%

relative humidity

f. Composite bonded joints with a surface preparation using

atmospheric pressure plasma treatment prior to bonding followed

by lap shear testing at 80oC

Lap shear tests were carried out using a computer-controlled ZWICK

tensile testing machine using a load cell of 50 KN. The samples were

loaded in tension at a test speed of 5 mm/min at room temperature.

Five specimens were used for each condition for reproducibility of the

results.



topography of samples prior to and after surface treatment. SEM

analysis was also performed to study the fractured surfaces in order to

determine the failure modes after lap shear testing. Images were

obtained using JOEL 7500 field emission scanning electron microscope

(FE-SEM). Surface of each sample was gold sputtered to minimize

sample charging effects.


6.4.1. Atmospheric Pressure Plasma Treatment

Atmospheric pressure plasma treatment (APPT) was performed on

M21/carbon fiber composite and DT120/carbon fiber composite. A

recent study conducted on plasma treatment of polymer based

composite have revealed that during APPT, surface temperature raised

to above 200oC [22]. However, the effect of this increase in surface

temperature on surface topography and surface energy of polymeric

based materials has not been included. Therefore, one of the objectives

of this work is to perform a sequential study to determine the thermal


114

effect of APPT on surface topography of polymer based composite

materials. For this purpose, M21/carbon fiber composite, DT120/carbon

fiber composite and unfilled polyetheretherketone (PEEK) was included

in this study. The purpose of including PEEK in this study was to

analyze the effect of APPT on surface energy and surface topography of

both thermoset and thermoplastic based materials. The glass transition

temperature and melting/decomposition temperature of three materials

is shown in table 6.1.

Table 6.1 Glass transition temperature and melting/decomposition temperature of three materials

Materials Glass Transition

(Tg) oC

Melting/Decomposition

Tm/Tdeco (oC)

M21/carbon fiber composite 195 395

DT120/carbon fiber composite 110 -

PEEK 143 343

Effort is made to establish a correlation between treatment time,

surface temperature, contact angle and surface roughness of plasma

treated samples. A comparison of temperature profile at the surface of

three materials during APPT is shown in figure 6.2. The plasma

treatment was performed using a nozzle height of 20mm from the

surface of the sample.

6.2 Comparison of surface temperature of different materials during APPT as a function of time

Figure 6.2 shows that during APPT of three materials, the surface

temperature have risen to above 200oC. Initially, all three materials

0

50

100

150

200

250

0 15 30 45 60 75 90 105 120

Tem

per

atu

re (

oC

)

Time (Sec)

M21/Carbon Composite

DT120/Carbon Composite

PEEK

Chapter 6

115

have depicted a sharp increase in surface temperature. With increasing

treatment time, the increase in temperature slowed down and after a

certain time during APPT, the increase in temperature was almost

leveled off. During this study, the temperature of the gases was

measured using a thermocouple. All three materials have shown high

surface temperature due to the fact that the gases coming out through

the plasma equipment used to have a temperature around 400oC [24].

This fact is also proved from present study. Due to high temperature of

gases coming out from plasma nozzle, the temperature on surface of all

the substrates has raised to above 200oC just in few seconds. It is also

expected that the local temperature can be even higher than the

temperature measured at the surface of these samples because the

thermocouple was few mm away from the point of treatment. This

could be the reason that after certain time during the APPT, top layer of

the resin is either melted or decomposed.

The change in surface topography of these materials with increasing

treatment time is shown in figure 6.3 and figure 6.4.

6.3 Comparison of surface topography of DT120/carbon composite with increasing plasma treatment time with an interval of 15 sec (500x magnification)

100 μm 10 μm


116

6.4 Comparison of surface topography of M21/carbon composite with increasing plasma treatment time with an interval of 15 sec (500x magnification)

A change in surface topography of PEEK with increasing treatment time


6.5 Comparison of surface topography of PEEK with increasing plasma treatment time with an interval of 15 sec (500x magnification)

100 μm 10 μm

10 μm

100 μm

Chapter 6

117

Figure 6.5 shows that with increasing treatment time, surface

roughness of PEEK has increased initially and after a certain time it

started to decrease. The initial increase in surface roughness followed

by a decrease can be correlated to the increase in surface temperature

during plasma treatment. With increasing treatment time during APPT,

surface temperature of PEEK has raised as shown earlier in figure 6.2.

After certain time during the APPT, surface temperature is expected to

reach above glass transition temperature of PEEK which caused the

local rearrangement of the molecules. With increasing treatment time,

surface temperature raised to a point where it is more likely that local

melting occurred which increased the surface roughness of polymer.

After certain time during APPT, it is more likely that the whole surface

melted and a smooth surface layer is formed.

Further efforts are made to correlate the treatment time with contact

angle of these materials. The change in contact angle of M21/carbon

fiber composite with treatment time is shown in figure 6.6.

6.6 Change of contact angle of M21/carbon composite with increasing plasma treatment time with an interval of 15 sec

Figure 6.6 shows that with increasing treatment time, contact angle has

initially decreased and then increased. The possible explanation of

initial decrease in contact angle followed by an increase in contact

angle is that with increasing plasma treatment time, surface roughness


118

has initially increased due to the increase in temperature. Also, new

functional groups are formed during APPT [16]. Both these factors are

responsible for the reduction in the contact angle of composite. After

certain time, fibers are exposed to the surface having low surface

energy which ultimately resulted in increased contact angle.

A comparison of surface temperature and contact angle of three

materials with increasing plasma treatment time is given in table 6.2.

Table shows that all three materials have shown similar kind of trend.

However, DT120/carbon composite has shown less improvement in the

surface energy even with higher treatment time. Contrary to

DT120/carbon epoxy composite, other two materials have depicted

significant improvement in surface energy.

Table 6. 2 Surface temperature and contact angle measurements of three materials with increasing plasma treatment time.

Time

(Sec)

M21/carbon fiber

composite

DT120/carbon

fiber composite PEEK

T (oC)

C.A (θo)

T (oC)

C.A (θo)

T (oC)

C.A (θo)

0 23 74 23 64 23 72

15 140 36 150 43 132 47

30 174 22 187 34 166 21

45 192 17 206 47 180 13

60 203 19 219 59 190 63

75 212 26 232 62 202 36

90 222 35 237 57 211 21

A comparison of improvement in contact angle for three materials after

APPT is shown in figure 6.7.

6.7 Comparison of contact angle measurements of untreated and atmospheric

pressure plasma treated materials (a) absolute contact angle (b) Normalized values

Un

trea

ted

AP

PT

0

10

20

30

40

50

60

70

80

M21 DT120 PEEK

Co

nta

ct A

ngl

e (θ

o)

(a)

Un

trea

ted

AP

PT

0

0,2

0,4

0,6

0,8

1

1,2

M21 DT120 PEEK

No

rmal

ized

C.A

(b)

Chapter 6

119

A further study was performed to determine the effect of nozzle height

of plasma equipment on the surface temperature and surface energy of

M21/carbon epoxy composite. A comparison of surface temperature as

a function of nozzle height during plasma treatment is shown in figure

6.8.

6.8 Comparison of surface temperature and contact angle of M21/Carbon composite as a function of nozzle height

Results in figure 6.8 reveal that surface temperature increased sharply

by decreasing the nozzle height of plasma equipment. By decreasing

the height of plasma nozzle from 20 mm to 10 mm, the surface

temperature of composite reached to 203oC just within 15 seconds.

While using 20 mm nozzle height, the surface temperature reached to

the same temperature in about 90 seconds of plasma treatment. Also,

peak surface temperature was much higher with 10 mm nozzle height.

With decreased nozzle height, rapid heating of composite followed by

cooling in air could induce the thermal stresses in the composite which

can adversely affect the mechanical properties of material. Also, rapid

heating can induce warpage in the materials which can affect the

balance of the part. Therefore, it is important to take into account the

thermal properties of the material while selecting the plasma nozzle

height.

A comparison of contact angle as a function of temperature using two

different treatment heights is shown in figure 6.9. Figure 6.9 shows

that with 20 mm nozzle height, the increase in temperature is slow as

compare to the temperature with 10 mm nozzle height but it took more

time to attain the same contact angle value. The benefit of using 20mm

nozzle height is the reduced surface temperature value of composite

substrate.

0

100

200

300

400

0 15 30 45 60 75 90 105

Tem

per

atu

re (

oC

)

Time (Sec)

M21-20 mm

M21-10 mm

(a)

0

20

40

60

80

100

0 15 30 45 60 75 90 105C

on

tact

An

gle

(θo)

Time (Sec)

M21-20 mm

M21-10 mm

(b)


120

6.9 Comparison of Contact angle of M21/Carbon epoxy composite as a function of temperature after performing the Atmospheric Pressure Plasma Treatment using two different heights

6.4.2. Surface Treatment for Lap Shear Testing

In present study, lap shear testing of M21/carbon composite and

DT120/carbon of composite was carried out. In addition to APPT, hand

sanding was also used to evaluate the effect of these treatments on

surface energy and hence on bonded joint strength. A comparison of

both these treatments was made in term of change in contact angle

and surface morphology of two epoxy based carbon fiber composites.

Comparison of contact angles after performing the APPT and hand

sanding of these materials is shown in figure 6.10. Hand sanding of

these materials followed by APPT was also performed to study the

combined effect of these treatments on contact angle values.

6.10 Comparison of contact angles of M21 and DT120 epoxy based carbon fiber

composites after performing the different surface treatments (UT= untreated, APPT= Atmospheric pressure plasma treatment, HS= Hand Sanding)

0

20

40

60

80

100

0 50 100 150 200 250 300

Co

nta

ct A

ngl

e (θ

o)

Temperature (oC)

M21-20mm

M21-10 mm

UT

AP

PT

HS

HS+

AP

PT

0

10

20

30

40

50

60

70

80

90

100

M21 DT-120

Co

nta

ct a

ngl

e (θ

o)

Chapter 6

121

Figure 6.10 demonstrates that different surface treatments have

induced different level of improvement in surface energy of two

materials. APPT alone or in combination with hand sanding have

demonstrated better results as compare to hand sanding treatment. As

mentioned earlier, APPT induced two different effects simultaneously;

formation of polar functional groups and increased surface roughness

while on the other hand, sanding or any other abrasion process cannot

induce both these affects at the same time. Hence, simultaneous

affects of polar group formation and increased surface roughness

during APPT resulted in much higher improvement in surface energy as

compare to any hand sanding treatment.

6.4.3. PBI adhesive bonded joints Preparation

Bonded joints of DT120/Carbon epoxy composite were formed using

PBI solution in order to evaluate its performance as an adhesive. This is

the first ever study about the lap shear testing of PBI adhesive in

solution form. In the past, PBI has been used as adhesive and was

available in the form of film on glass cloth [25]. However, it required

high processing temperature and pressure to form the bonded joints.

Processing of PBI was being carried out in a pre-heated press at 370°C

with a pressure between 0.6 to 1.4 MPa and a temperature of 370°C for

3 hours. After removing from the press, post-curing in an inert

atmosphere (nitrogen, helium, or vacuum oven) was being carried out

in order to improve the joint properties. The recommended post curing

conditions were to heat the bonded joints for 24 hours each at 316°C,

345°C, 370°C, and 400°C followed by 8 hours at 427°C in air to

achieve maximum properties. These were quite demanding conditions

to form the bonded joints of PBI with high bond strength. Therefore,

one of the objectives of present work was to apply PBI solution as an

adhesive with low processing temperature and pressure. Efforts are

made to form the composite bonded joints using PBI solution. Due to

high solvent contents, the process optimization required lot of efforts to

form PBI bonded joints with considerable lap shear strength. Successful

bonding process using PBI adhesive will make PBI a potential candidate

for high temperature applications.

6.4.4. Process Optimization of PBI Adhesive to form bonded joints

The process optimization of PBI film using PBI solution was described in

chapter 2 in detail. Using the same manufacturing process, PBI bonded

joints were formed using a cured temperature of 95oC. Standard


122

clamps were used to apply the pressure during the curing process.

Bonded joints were heated in the oven at 95oC for 24 hours followed by

lap shear testing. The bonded joints failed at lap shear strength of 4

MPa which was much lower lap shear strength than the expected value.

On visual inspection of tested joints, it was observed that PBI adhesive

was not fully cured at this temperature. As a result of incomplete

curing, bonded joints have exhibited very low lap shear strength. In the

second attempt, the curing temperature was increased from 95oC to

125oC while using the same curing pressure and curing time. Again, the

results obtained from lap shear testing were not promising. Joints

again failed without demonstrating high values of lap shear strength.

The inspection of failed joints has revealed the fact that adhesive was

fully cured but heating of the bonded joints at 125oC resulted in the

fast evaporation of the solvent present in PBI adhesive. As a result of

fast evaporation of solvent, voids were formed. These voids were the

main cause of early failure of the bonded joints. On the basis of the

results obtained from first two attempts, it was decided to cure the PBI

adhesive bonded joints using the same curing temperature and

pressure as used in the second attempt. However, this time heating

was performed in steps. Bonded joints were heated to a temperature of

60oC and kept at this temperature for two hours. After two hours, the

curing temperature was raised to 100oC. The bonded joints were

heated at this temperature for 2 hours followed by heating of the

bonded joints at 125oC for 20 hours. This attempt was successful and

much higher lap shear strength was achieved. A comparison of lap

shear strength of PBI adhesive bonded joints with partially cured and

fully cured adhesive is shown in figure 6.11.

6.11 Comparison of lap shear strength of PBI adhesive bonded joints cured at different temperature

Cured @ 95oC

Cured @ 125oC

0

5

10

15

20

25

DT120/Carbon composite

Lap

Sh

ear

Stre

ngt

h (

MP

a)

Chapter 6

123

Figure 6.11 shows that curing of PBI adhesive bonded joints at 125oC

have significantly improved the lap shear strength. Curing of PBI

adhesive bonded joints at 125oC has increased the joint strength from 4

MPa to 21 MPa.

SEM micrographs of failed joints cured at different temperatures are

shown in figure 6.12. SEM micrographs in figure 6.12a shows that

adhesive layer remained on the surface after lap shear testing of

bonded joints cured at 95oC while the joints cured at 125oC was failed

cohesively. Failure in the substrate demonstrates the strong interface

between adhesive and composite substrate. As a result of strong

adhesion, the adhesive has torn away the fibers from the surface as

shown in figure 6.12b. These results indicate that processing of PBI as

an adhesive has been optimized to greater extent. However, further

tests can be performed by curing the joints even at temperature higher

than 125oC. This will help to determine if there is further margin of

improvement in the lap shear strength of PBI adhesive bonded joints.

6.12 SEM micrographs of tested PBI adhesive bonded joints after curing at different temperatures (a) Joint cured at 95oC failed adhesively (b) Joint cured at 125oC failed cohesively both within adhesive and substrate (200x magnification)

After optimizing the processing of PBI as an adhesive, bonded joints of

M21/Carbon composite and DT120/carbon composite were formed. A

comparison of lap shear strength of bonded joints of two composites is

shown in figure 6.13. These joints were formed without performing any

surface treatment. Results in figure 6.13 indicate that bonded joints of

M21/Carobn composite and DT120/carbon composite have depicted

different lap shear strength. M21/carbon composite has shown lap

shear strength of about 5 MPa while DT120/carbon composite has

depicted lap shear strength of 21 MPa. The lap shear strength of

DT120/carbon composite is about four times higher as compare to the

(b) (a)

Uncured

Adhesive

on surface

Cohesive

failure of

adhesive

Fibers

failure

Smooth

epoxy

layer

100 μm

50 μm


124

lap shear strength of M21/Carbon composite bonded joints. One of the

possible explanations of exhibiting different lap shear strength by the

bonded joints of two composites is that two different epoxy based

composite materials can have different chemical interaction with PBI

adhesive. As a result, bonded joints of these composite materials have

exhibited different lap shear strength. Another reason of exhibiting

different lap shear strength by two composite materials is the different

surface topography of these materials. This fact was confirmed by

studying the surface of two composite materials using confocal

microscope.

6.13 Comparison of lap shear strength of PBI adhesive bonded joints of untreated DT120/Carbon composite and M21/Carbon composite

A comparison of the surface topography of these two materials is


6.14 Surface topography of (a) M21/Carbon composite (b) DT120/Carbon composite at 20x magnification using confocal microscope

M21

DT120

0

5

10

15

20

25

30

Untreated

Lap

Sh

ear

Str

engt

h (

MP

a)

Chapter 6

125

Figure 6.14 shows that M21/Carbon composite has an epoxy layer on

its outer surface whereas fiber texture and fiber direction can easily be

observed in case of DT120/Carbon composite. There is a clear

difference of surface topography of these two materials. As a result of

different surface topography, two epoxy based composites have

exhibited different lap shear strength. DT120/Carbon has very thin

layer of epoxy resin on the surface with prominent fiber texture which

is nearly an ideal surface for adhesive bonding. Consequently, bonded

joints of DT120/Carbon composite has exhibited much higher lap shear

strength than the lap shear strength of M21/Carbon composite bonded

joints.

A comparison of SEM micrographs of fractured surfaces of two

composites bonded joints is shown in figure 6.15.

6.15 SEM micrographs of untreated PBI adhesive bonded joints cured at 125oC (a) M21/Carbon epoxy composite bonded joints (b) DT120/Carbon epoxy composite

bonded joints (200x magnification)

SEM micrographs in figure 6.15 show that untreated bonded joints of

M21/carbon composite have demonstrated adhesive failure which

indicates that untreated M21/carbon composite has very low surface

energy which ultimately leads towards early failure of the joints.

Smooth adhesive layer and epoxy resin layer on the fractured surfaces

of untreated M21/carbon composite are highlighted in figure 6.15a. No

fibers are exposed to the surface after lap shear testing. Contrary to

M21/Carbon composite bonded joints, bonded joints of DT120/carbon

composite have depicted three different failure modes which are

highlighted in figure 6.15b. The highlighted areas show the fibers at the

surface, fractured surface of the adhesive and smooth epoxy layer.

These results indicate that cohesive failure is dominant in

(b) (a)

Smooth

epoxy layer

Cohesive

failure of

adhesive

Fibers

failure

Smooth

epoxy

layer

Adhesive

layer

100 μm

50 μm


126

DT120/carbon composite. A very small portion of the bond failed

adhesively as well.

In present study, atmospheric pressure plasma (APP) and hand sanding

were used to compare the effect of these two treatments on lap shear

strength of composite bonded joints. APP and hand sanding was used

individually and in combination in order to prepare the surfaces prior to

bonding. A comparison of lap shear strength of PBI adhesive bonded

joints of two epoxy based composites is shown in figure 6.16.

6.16 Comparison of (a) absolute and (b) normalized lap shear strength of PBI adhesive bonded joints after using different surface treatment methods (UT= untreated, APPT= Atmospheric pressure plasma treatment, HS= Hand Sanding)

Figure 6.16 shows that lap shear strength of untreated samples of

M21/carbon composite and DT120/carbon composite was 5 MPa and 21

MPa respectively. APP has improved the lap shear strength of both

kinds of epoxy based composites. However, the improvement in lap

shear strength is more significant with M21/Carbon composite bonded

joints as compare to the improvement in the joint strength of

DT120/carbon composite. The improvement in lap shear strength of

these materials can be correlated with the improvement in surface

energy of these materials after atmospheric pressure plasma treatment

(APPT). Results on contact angles measurements illustrate that APPT

have induced higher improvement in surface energy of M21/Carbon

epoxy composite as compare to the improvement in surface energy of

DT120/Carbon epoxy composite. As a result, M21/Carbon composite

revealed higher improvement in lap shear strength than the

improvement in the lap shear strength of APPT DT120/Carbon epoxy

composite.

UT

AP

PT

HS

HS+

AP

PT

0

5

10

15

20

25

30

35

M21 DT120

Lap

Sh

ear

Str

engt

h (

MP

a)

(a)

UT

AP

PT

HS

HS+

AP

PT

0

1

2

3

4

M21 DT120

No

rmal

ized

Lap

Sh

ear

Stre

ngt

h

(b)

Chapter 6

127

A comparison of lap shear strength of M21/Carbon composite bonded

joints and DT120/carbon composite bonded joints with hand sanding

treatment is also shown in figure 6.16. Results show that hand sanding

treatment has improved the strength of M21/Carbon composite joints

to about 170% as compare to the joint strength of untreated surfaces.

While DT120/carbon composite bonded joints have depicted only 15%

increase in joint strength with hand sanding treatment.

A comparison of SEM micrographs of hand sanded specimens of

M21/Carbon composite and DT120/carbon composite is shown in figure

6.17.

6.17 SEM micrographs of hand sanded (a) M21/carbon composite (b) DT120/carbon composite (200x magnification)

SEM micrographs in figure 6.17 revealed the fact that hand sanding

treatment has not only removed the resin from the surface of

DT120/carbon composite but also damaged the fibers at the surface

which ultimately reduced the adhesion between fiber/adhesive

interfaces. On the other hand, M21/carbon epoxy composite has a thick

epoxy resin on the surface. Therefore, during hand sanding treatment

of M21/carbon epoxy composite, fibers are not exposed to the surface.

Hand sanding has only increased the surface roughness of M21/carbon

epoxy composite which allowed mechanical interlocking of adhesive and

ultimately a much higher joint strength is achieved as compare to the

untreated surfaces. Due to the different effect of hand sanding on

surface morphology of two different epoxy composites, these materials

have demonstrated different improvement in lap shear strength.

Therefore, it is important to take into account the surface topography

of the material before deciding the method of surface treatment.

Damaged

fibers

(a) (b)

100 μm

50 μm


128

A summary of lap shear strength of M21/Carbon and DT120/Carbon

composite bonded joints with different surface treatment methods is

presented in table 6.3. Results have also included the standard

deviation values.

Table 6. 3 Lap shear strength of M21/Carbon and DT120/Carbon composite bonded joints with different surface treatment methods

Substrate

Material Treatment

Failure

Mode

Joint Strength

(MPa)

SD

(MPa)

DT120/CF

composite Untreated

Mixed

failure 21 2.6

DT120/CF

composite APPT Cohesive 30 1.9

DT120/CF

composite Hand Sanded Cohesive 24 1.2

DT120/CF

composite

Hand Sanded+

APPT Cohesive 24 2.6

M21/CF

composite Untreated Adhesive 5 1.3

M21/CF

composite APPT Cohesive 18 1.5

M21/CF

composite Hand Sanded Cohesive 14 1.0

M21/CF

composite

Hand Sanded+

APPT Cohesive 15 2.8

6.4.5. Environmental Conditioning of PBI adhesive bonded joints

Adhesive bonded joints are exposed to various environmental

conditions during service life. These environmental conditions can

deteriorate the properties of adhesive which ultimately affect the

performance of bonded joints. Temperature and humidity are the two

main factors which should be considered while determining the long

term durability of adhesive bonded joints. Prolong exposure to high

temperature and humidity can induce irreversible changes in the

adhesive. Both chemical and physical changes can occur in the

adhesive which leads towards earlier failure of bonded joints. The

presence of moisture in adhesive joints may not only weaken the

physical and chemical properties of the adhesive itself but it can also

affect the interface between the adhesive and the substrate. In this

context, bonded joints of M21/Carbon composite and DT120/Carbon

composite were prepared and conditioned in a climate chamber for

1000 hours at 80oC and 95% relative humidity. The purpose was to

evaluate the performance of PBI adhesive under hot-wet environment.

Chapter 6

129

A comparison of lap shear strength of bonded joints of M21/Carbon

composite and DT120/Carbon composite before and after exposure to

hot-wet environment is shown in figure 6.18.

6.18 Comparison of (a) absolute and (b) normalized lap shear strength of PBI adhesive bonded joints conditioned at 80oC and 95% relative humidity for 1000 hours

6.18 shows that exposure of adhesive bonded joints to hot-wet

environment has affected the joint strength to some extent. Lap shear

strength of M21/Carbon composite bonded joints decreased from 18

MPa to 15 MPa whereas the lap shear strength of DT120/Carbon

composite bonded joints decreased from 30 MPa to 23 MPa. Both

M21/Carbon composite and DT120/Carbon composite have exhibited a

reduction in joint strength of about 17% and 27% respectively. The

decrease in lap shear strength is different for two composites which

indicate that lap shear strength not only depends on the adhesive

properties but also depends on substrate properties. DT120/Carbon

composite bonded joints have exhibited more reduction in lap shear

strength than the reduction in the lap shear strength of M21/Carbon

composite bonded joints. This is more likely because of low Tg of

DT120/Carbon composite. The composite has a Tg around 110oC and

bonded joints were conditioned at 80oC and at 95% humidity. The

physical and mechanical properties of composite itself are affected

close to the Tg of composite [26]. As the bonded joints of

DT120/Carbon composite were conditioned close to the Tg, therefore, it

has higher tendency to absorb more water in hot-wet environment as

compare to M21/Carbon composite which has much higher Tg.

Therefore, it is more likely that hot-wet environment has either

weakened the fiber-matrix interface in the epoxy composite or it has

weakened the adhesive/composite interface.

0

5

10

15

20

25

30

35

M21 DT120

Lap

Sh

ear

Stre

ngt

h (

MP

a)

Unexposed

Conditioned(a)

0

0,2

0,4

0,6

0,8

1

1,2

M21 DT120

No

rmal

ized

Lap

Sh

ear

Stre

ngt

h

Unexposed Conditioned

(b)


130

To investigate the effect of hot-wet environment on

composite/adhesive interface, SEM analysis of fractured surfaces of

DT120/carbon composite and M21/carbon composite bonded joint was

performed. A comparison of fractured surface of unexposed bonded

joints and bonded joints failed after environmental conditioning is

shown in figure 6.19 and figure 6.20.

6.19 SEM micrographs of fractured surface of plasma treated bonded joints of DT120/Carbon composite (a) Controlled sample (c) Sample tested after conditioning

at 80oC and 95% humidity for 1000 hours (200x magnification)

6.20 SEM micrographs of fractured surface of plasma treated bonded joints of M21/Carbon composite (a) Controlled sample (c) Sample tested after conditioning at

80oC and 95% humidity for 1000 hours (200x magnification)

Figure 6.19 and 6.20 shows that mode of failure of bonded joint has

not changed after exposure to hot wet environment. Bonded joint of

controlled and conditioned samples have failed cohesively through the

substrate. These results indicate that high temperature and moisture

has not affected the composite/adhesive interface and hence cohesive

failure occurred.

(a) (b)

100 μm

100 μm

50 μm

50 μm

Chapter 6

131

6.4.6. Testing of PBI adhesive bonded Joints at High Temperature

The increased usage of high-temperature resistant resins for composite

materials has necessitated the development of compatible and equally

thermally stable adhesive systems. PBI is well known for its high

thermal stability and it can maintain its thermo-mechanical properties

even at a temperature of above 300oC as studied in chapter 2. In this

context, bonded joints of DT120/Carbon composite and M21/Carbon

composite were prepared and tested at 80oC. A comparison of lap shear

strength of bonded joints of DT120/Carbon composite and M21/Carbon

composite is shown in figure 6.21.

6.21 Comparison of (a) absolute and (b) normalized lap shear strength of PBI adhesive bonded joints tested at 80oC

Figure 6.21 shows that high temperature has significantly affected the

performance of bonded joint strength. Lap Shear strength of

M21/carbon composite bonded joints has decreased from 18 MPa to 8.5

MPa while the Lap Shear strength of M21/carbon composite bonded

joints has decreased from 30 MPa 14 MPa. Both M21/Carbon

composite and DT120/Carbon composite have exhibited a reduction in

joint strength of about 50%. Despite the fact that PBI has very high

thermal stability and can maintain thermo-mechanical properties even

at a temperature of 300oC, still PBI adhesive bonded joints have

revealed a significant decrease in lap shear strength. These results

indicate that performance of bonded joints not only depends on the

properties of the adhesive but it also depends on thermo-mechanical

properties of substrates. Without performing the detail testing of PBI

adhesive by using different substrate materials, a conclusive statement

cannot be made about the high temperature performance of PBI

adhesive.

25oC

25oC

80oC

80oC

0

5

10

15

20

25

30

35

M21 DT120

Lap

Sh

ear

Str

engt

h (

MP

a)

(a) 25oC 25oC

80oC 80oC

0

0,2

0,4

0,6

0,8

1

1,2

M21 DT120

No

rmal

ized

Lap

Sh

ear

Stre

ngt

h

(b)


132

To investigate the effect of temperature on adhesive composite

interface, SEM analysis of fractured surfaces of DT120/carbon

composite and M21/carbon composite bonded joints was performed. A

comparison of fractured surface of sample tested at ambient

temperature and sample tested at 80oC is shown in figure 6.22 and

figure 6.23.

6.22 SEM micrographs of fractured surface of plasma treated bonded joints of DT120/Carbon composite (a) unexposed sample (c) Sample tested at 80oC (200x magnification)

Figures 6.22 and 6.23 show that failure mode of DT120/carbon

composite and M21/carbon composite bonded joint has not changed

when tested at high temperature. Cohesive failure of the substrates

occurred in both conditions. However, in case of DT120/carbon

composite bonded joints, a brittle failure is observed. As lap shear

testing is carried out close to the glass transition temperature of

DT120/carbon composite which ultimately has an effect on thermo-

mechanical properties of carbon/epoxy composite.

6.23 SEM micrographs of fractured surface of plasma treated bonded joints of M21/Carbon composite (a) unexposed sample (c) Sample tested at 80oC and (200x

magnification)

(a) (b)

(b)

100 μm

100 μm

50 μm

50 μm

Chapter 6

133

A summary of the results obtained after the lap shear testing of two

kind of composite bonded joints tested in various environmental

conditions is presented in table 6.4.

Table 6. 4 Lap shear strength of M21/Carbon and DT120/Carbon composite bonded joints tested in various environmental conditions

Substrate

Material

Testing

Environment

Failure

Mode

Joint Strength

(MPa)

SD

(MPa)

DT120/CF

composite ambient

Mixed

failure 30 1.9

DT120/CF

composite Testing at 80oC Cohesive 14 0.7

DT120/CF

composite

Conditioning @ 80oC

and 95% humidity Cohesive 23 0.3

M21/CF

composite Ambient Adhesive 18 1.5

M21/CF

composite Testing at 80oC Cohesive 8.5 1.0

M21/CF

composite

Conditioning @ 80oC

and 95% humidity Cohesive 15 1.1

6.4.7. Study of Failure Modes

Different kinds of failure modes are expected during lap shear testing of

composite bonded joints. The most common failure modes of composite

bonded joints are adhesive failure, cohesive failure, fiber tear failure

and substrate failure [8]. Substrate failure occurred rarely during lap

shear testing. SEM of fractured surfaces was performed to study the

failure mode of composite bonded joints.

A comparison of failure modes of untreated and treated DT120/carbon

composite bonded joints is shown in figure 6.24. SEM micrographs of

fractured surfaces revealed that composite bonded joints have

exhibited different kinds of failure during lap shear testing. Untreated

bonded joints of DT120/carbon composite have demonstrated mixed

mode failure as shown in figure 6.24a. The highlighted areas show the

fibers at the surface, the top layer of the epoxy resin and a smooth

adhesive layer. Therefore, untreated DT120/carbon composite have

exhibited cohesive and adhesive failure modes. SEM micrograph in

figure 6.24b shows the fracture surface of APPT bonded joints.

Atmospheric pressure plasma treated composite joints have exhibited

cohesive failure in the substrate which indicates that APPT has

increased the adhesion between composite/adhesive interfaces. Due to

the strong adhesion, tearing of the fiber from the surface of composite

has occurred. Also, bonded joints have exhibited improved lap shear

strength after atmospheric pressure plasma treatment. SEM micrograph


134

in figure 6.24c represents the fracture surface of bonded joints which

were formed after hand sanding surface treatment. Again, the bonded

joints have depicted cohesive failure within the substrate.

6.24 SEM micrographs of fractured surfaces of bonded joints of DT120/carbon composite (a) Untreated (b) plasma treated (c) hand sanded (200x magnification)

A comparison of failure modes of M21/carbon composite bonded joints

is shown in figure 6.25. SEM micrographs of fractured surfaces of

M21/carbon composite bonded joints reveal that M21/carbon composite

bonded joints have exhibited different kinds of failure during lap shear

testing. Untreated bonded joints of M21/carbon composite have

demonstrated adhesive failure which indicates that untreated

M21/carbon composite has very low surface energy which ultimately

leads towards early failure of the joints. Smooth adhesive layer and

epoxy resin layer on the fractured surfaces of untreated M21/carbon

composite are highlighted in figure 6.25a. No fibers are exposed to the

surface after lap shear testing.

6.25 SEM micrographs of fractured surfaces of bonded joints of M21/carbon composite (a) Untreated (b) plasma treated (c) hand sanded (200x magnification)

SEM micrograph in figure 6.25b shows the fracture surface of

atmospheric pressure plasma treated bonded joints. These joints have

exhibited cohesive failure in the substrate which indicates that

atmospheric pressure plasma treatment has increased the adhesion

between composite/adhesive interfaces. Due to the strong adhesion,

(a) (b) (c)

(c)

100 μm

100 μm

50 μm

50 μm

Chapter 6

135

tearing of the fiber from the surface of composite has occurred. Also,

bonded joints have exhibited improved lap shear strength after APPT.

SEM micrograph in figure 6.25c represents the fracture surface of

bonded joints which were formed after hand sanding surface treatment.

Again, the bonded joints have depicted cohesive failure in the

substrate.

6.5. Conclusions

In present study, PBI has been evaluated as an adhesive after exposure

to various environmental conditions. The main objective of this study

was to optimize the adhesive bonding process of PBI in order to

evaluate its performance as an adhesive for aerospace applications.

Bonded joints of DT120/Carbon composite and M21/Carbon composite

were formed using PBI adhesive. Atmospheric pressure plasma

treatment (APPT) was performed to improve the adhesion properties of

composite substrates prior to the formation of bonded joints. Though,

APPT improves the surface energy of these materials in short duration

of time. However, it can induce the thermal stresses in the composite

materials because of the high temperature generated during the

surface treatment. Current work shows that during APPT, surface

temperature of composite materials have raised to above 200oC in very

short duration of time and this temperature was well above glass

transition temperature of composites used in this study. This high

temperature can induce residual thermal stresses and also have an

impact on mechanical properties. Therefore, one of the important

conclusions of this study is that thermal properties of polymer based

composite materials should always be considered before performing

APPT. APPT can significantly damage the surface as well as bulk

properties of the materials. Therefore, it is important to optimize the

height of the plasma nozzle from composite surface and plasma

treatment time. By increasing the height of the plasma nozzle from

composite, the temperature generated at the surface can be reduced

which ultimately would have less affect on bulk properties of the

materials.

Results obtained from contact angle measurement after APPT shows

that APPT have improved the wetting properties of two kinds of epoxy

composite. A higher improvement in the surface energy of M21/Carbon

composite is observed as compare to the improvement in the surface

energy of DT120/Carbon composite. However, DT120/Carbon


136

composite bonded joints have exhibited 75% higher lap shear strength

than the lap shear strength of M21/Carbon composite. Therefore,

another important conclusion of this study is that there are factors

other than the surface energy which can affect the joint strength.

Surface topography of composite is very crucial factor to achieve high

bond strength. Surface topography of composite materials was studied

sequentially with increasing plasma treatment time. With increasing

treatment time, surface roughness of composites has increased. This

increase in surface roughness is correlated with the increase in surface

temperature during APPT. Surface temperature during APPT reached

above glass transition temperature of polymer and polymer based

composite. This increase in surface temperature caused local

rearrangement of molecules and in case of unfilled PEEK, local melting

is observed which ultimately led towards increased surface roughness.

Bonded joints of DT120/Carbon composite and M21/Carbon composite

were conditioned at 80oC and 95% humidity. Exposure of both kinds of

composite bonded joints to hot-wet environment for 1000 hours

resulted in a reduction of lap shear strength of 17% and 27% for

DT120/Carbon composite and M21/Carbon composite respectively.

These results indicate that hot-wet environment has not significantly

deteriorated the properties of PBI adhesive even after 1000 hours of

conditioning. However, testing of PBI adhesive bonded composite joints

at 80oC has reduced the lap shear strength to about 50%. Despite the

fact that PBI has very high thermal stability and can maintain thermo-

mechanical properties even at a temperature of 300oC, still PBI

adhesive bonded joints have revealed about 50% decrease in lap shear

strength. These results indicate that performance of bonded joints not

only depends on the properties of the adhesive but it also depends on

thermo-mechanical properties of substrates. Without performing the

detail testing of PBI adhesive using different substrate materials, a

conclusive statement cannot be made about the high temperature

performance of PBI adhesive.

6.6. References

1- Baker, Alan; Dutton, Stuart; Kelly, Donald, Composite materials for aircraft structures, 2nd edition, 2004

2- A.P. Mouritz, E. Gellert, P. Burchill, K. Challis, Review of advanced composite structures for naval ships and submarines, Composite structures, (2001)

Chapter 6

137

3- A.P.Mouritz and A.G.Gibson, Fire Properties of polymer composite materials, Solid Mechanics and its applications, Volume 143, 2006, ISBN-13 978-1-4020-5356-6

4- Mouritz, AP, Fire safety of advanced composites for aircraft, B2004/0046,

April 2006

5- Jack R. Vinson, Adhesive Bonding of Polymer Composites, Polymer Engineering and Science, Vol. 29, No. 19 (1989)

6- Urena A, Gude R. M, Prolongo S. G. , Carbon nanofibers reinforced adhesives for joining carbon fiber epoxy composites.13th European conference on composite materials, (2008)

7- M. D. Banea and L. F M da Silva, Adhesively bonded joints in composite materials: An overview, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials Design and Applications, (2009)

8- Bowditch M. R., Shaw S. J., Adhesive bonding for high performance materials

Adv. Perform Mater, Vol. 3, (1996)

9- Shenton M.J, Lovell-Hoare M. C, Stevens G. C. Adhesion enhancement of polymer surfaces by atmospheric plasma treatment, J. Phys D:Appl. Phys Vol. 34, (2001)

10- Handbook of Plastics Joining (Second Edition), A Practical Guide

11- Shields J. , Adhesive bonding, London:Oxford Universiy Press, (1974)

12- Leahy W,Barron V,Buggy M,Young T,Mas A,Schue F,.Plasma surface treatment of aerospace materials for enhanced adhesive bonding.J Adhes, vol. 77, (2001)

13- Steinerand, P. A., and Sandor, R., “Polybenzimidazole Prepreg: Improved Elevated Temperature Properties with Autoclave Processability,” High Performance Polymers,Vol. 3, No. 3, 1991, pp. 139–150.

14- Bhowmik, S., Bonin, H.W., Bui, V. T., and Weir, R. D., “Modification of High-Performance Polymer Composite through High-Energy Radiation and Low-Pressure Plasma for Aerospace and Space Applications,” Journal of Applied Polymer Science, Vol. 102, Jan. 2006, pp. 1959–1967.

15- Chan C-M, KoT-M, Hiraoka H., Polymer surface modification by plasma and

photons Surf Sci Rep, Vol. 24, (1996)

16- Michael Noeske, Jost Degenhardt, Silke Strudthoff, Uwe Lommatzsch, Plasma jet treatment of five polymers at atmospheric pressure: surface modifications and the relevance for adhesion, Int. J. Adhes. Adhes. Vol. 24, (2004)

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138

17- J.R.J Wingfield Treatment of composite surfaces for adhesive bonding, int. J. Adh. & Adh. Vol. 13 (1993)

18- P. Molitor, V. Barron_, T. Young, Surface treatment of titanium for adhesive

bonding to polymer composites: a review, International Journal of Adhesion & Adhesives, vol. 21 (2001)

19- Petrie E.M.,The fundamentals of adhesive joint design and construction, Metal Finishing, Vol. 106, (2008)

20- Rotheiser J. Joining of plastics: handbook for designers and engineers. Munich:

Hanser; 1999.

21- H.M.S. Iqbal,S.Bhowmik n, R.Benedictus, Surface modification of high performance polymers by atmospheric pressure plasma and failure mechanism of adhesive bonded joints, International Journal of Adhesion & Adhesives, Vol. 30 (2010)

22- Shen Tang and Ho Suk Choi, Comparison of Low- and Atmospheric-Pressure

Radio Frequency Plasma Treatments on the Surface Modification of Poly(methyl methacrylate) Plates, J. Phys. Chem. C, Vol. 112, (2008)

23- Jin Kook Kim, Hak Sung Kim And Dai Gil Lee, Investigation of optimal surface

treatments for carbon/epoxy composite adhesive joints, J. Adhesion Sci. Technol., Vol. 17, No. 3, pp. 329–352 (2003)

24- Vijay Nehra, Ashok Kumar and H K Dwivedi, Atmospheric non-thermal plasma sources, Int. J. of Engineering, vol. 2

25- Sina Ebnesajjad , Handbook of Adhesives and Surface

Preparation: Technology, Applications and Manufacturing, ISBN-10-1437744613, (2010)

26- Roger Vodicka, Accelerated Environmental Testing of Composite Materials,

Report DSTO-TR-0657

139

CHAPTER 7

Performance Evaluation of Polybenzimidazole coating for Aircraft Application

In this chapter, characteristic behavior of PBI as a protective

coating for epoxy based carbon fiber composite for aircraft

application is presented. Response of PBI is described after

exposure to hot wet environment and liquid immersion.

Critical properties including hardness, scratch resistance and

adhesion of PBI coating are assessed and results are

presented.

7.1. Introduction

Polymer based composite materials are gaining wide attraction for

aerospace applications due to their high strength to weight ratio,

excellent corrosion resistance, outstanding thermal insulation and low

thermal expansion. However, there are some disadvantages of

composite materials and these include low through thickness

mechanical properties and poor impact damage [1]. Under flight service

environment of aircraft, composite materials are exposed to different

environmental conditions of temperature, humidity, particle erosion and

ultraviolet radiations [2]. All these conditions play a part in minimizing

the integrity of composites. Most of properties of composite materials

are matrix dependent and matrix resins are more sensitive to above

mentioned environmental conditions. One of the main concerns

regarding the durability of composite material is their poor performance

against particle erosion [3]. Composite materials used for aircraft are

very susceptible to erosion due to the brittle nature of the matrix

material. Durability of composite materials can be improved either by

using a high performance matrix resin which have better resistance to

Performance Evaluation of PBI Coating for Aerospace Applications

140

erosion and other environmental conditions or by applying a high

performance coating material which can protect these materials from

flight service environment. Currently, different multi-component

protective coatings are being used for the protection of composite

aircraft. There are also some other questions which are raised about

the performance of these coatings in terms of their application process,

cost and durability. The coating systems currently used for aircraft

protection consist of an epoxy primer and a polyurethane top coat [4].

Most of the times, an intermediate coat is also applied to get some

desired function. More than one component is required to achieve the

desired functions from a coating system. The total dry thickness of the

multi-coat system ranges between 200 μm to 380 μm [4]. Application

of using two to three different coats to get the desired performance,

ultimately, increase the time of application and add considerable weight

to the aircraft. Also, some pigments and additives are added to these

coatings to improve their performance against environmental service

conditions. Addition of pigments in the coating material causes the

problem of chalking which ultimately affect the performance of the

coating material [5].

Based on the above discussion, the objective of this work is to develop

a coating material which can be applied directly to the composite

substrate without using any primer and it should also perform the

desired functionality. Elimination of the primer coat will help to reduce

the application time and weight which in turn can save the cost of

material, manpower and in flight service cost. In this context,

Polybenzimidazole (PBI) coating will be tested and evaluated for its

performance after exposure to various environmental conditions. Based

on the properties of PBI, it is expected that the material will

demonstrate properties which will be helpful to retain the properties of

composite substrate against particle erosion.

7.2. Experimental

7.2.1. Materials

26% concentrated solution of Polybenzimidazole (PBI) in

Dimethyleacetamide (DMAc) was supplied by CELAZOLE, PBI

performance products. M21 epoxy based unidirectional (UD) carbon

fiber prepregs were supplied by Hexcel. Composite laminates were

prepared by stacking up a number of pre-impregnated layers to achieve

Chapter 7

141

a cured laminate thickness of 4 mm. Laminates were manufactured in

an autoclave using curing cycle specified by supplier of the material.

7.3. Testing and Characterization

7.3.1. Atmospheric pressure Plasma Treatment of Composite Substrate

The composite surface was treated with atmospheric pressure plasma

for better adhesion of coating with the composite laminate. Samples

were plasma treated using TIGRES Plasma-BLASTER MEF equipment


7.1 Apparatus for Atmospheric Pressure Plasma Treatment

For this particular study, the treatment distance of composite surface

from nozzle head of plasma equipment was 20 mm and the gas used

for treatment was air at a pressure of 4.5 bars. Before performing the

plasma treatment, the samples were first cleaned with methanol using

an ultrasonic method to remove any contamination on the surface.

After cleaning, the specimens were dried at 80oC under vacuum for four

hours. Samples of (3 x 3) cm2 were treated for 60 sec. Surface

treatment time for this study was decided on the basis of the results

obtained in chapter 6.

7.3.2. Contact Angle Measurements

The change in the adhesion properties followed by plasma treatment

was determined in term of contact angle value. Water was used as a

liquid to measure the contact angle before and after plasma treatment.

A reduced value of contact angle indicates an improvement in surface

energy of material which in turn improves the adhesion properties of

materials. The detail of contact angle measuring equipment is given in

chapter 6.


142

7.3.3. Application of Coating

The as-received 26% concentrated solution of PBI in DMAc was highly

viscous and therefore it was essential to dilute the solution for proper

processing. DMAc was added to the PBI solution to dilute the solution

down to the 17%. The solution was stirred mechanically at 60°C for 15

minutes to get a uniform mixture of PBI in DMAc. The mixture was then

used as a coating for composite laminate to produce a coating

thickness of about 100-120 µm. The coating was applied on composite

laminates using an adjustable doctor blade which allows controlling the

thickness of the coating. The coating was allowed to dry in the vacuum

oven at 80°C for 2 hours. Afterwards, the coated samples were further

dried in vacuum oven at 125°C for overnight.

7.3.4. Adhesion Testing of PBI coating

For this particular study, lap shear test is used in order to determine

the coating adhesion. Lap shear test provides both quantitative as well

as qualitative information about coating adhesion. Single lap shear

joints of carbon/epoxy composite were prepared with an overlap length

of 12 mm x 25 mm. Joints with both untreated surfaces and

atmospheric pressure plasma treated surfaces were prepared in order

to study the effect of surface treatment on adhesion of PBI coating. The

detail of joint preparation for lap shear testing is given in chapter 6.

7.4. Environmental Resistance of PBI coating

7.4.1. Conditioning of PBI coated Panels at high Temperature and Humidity

Coated panels were prepared and exposed to high temperature and

humidity environment. The purpose was to evaluate the influence of

temperature and humidity on the coating performance. Coated coupons

of 30 mm by 30 mm were prepared and conditioned at 80°C with a

95% relative humidity (RH) for 1000 hours. Resistance of PBI coating

to hot-wet environment was evaluated through scratch test and nano-

indentation test.

7.4.2. Liquid Immersion of PBI Coated Panels

Resistance of the coating to water and Skydrol is also examined by full

immersion test. Skydrol is a harsh solvent used in aerospace industry.

Therefore coating resistance is examined against this solvent. Coated

Chapter 7

143

coupons of 30 mm by 30 mm were immersed in water and Skydrol for

1000 hours at ambient conditions. After 1000 hours, the panels were

visually inspected for any sign of blistering or delamination. Resistance

of the coating was evaluated through scratch test and nano-indentation

test.

7.4.3. Adhesion testing of Conditioned Samples

Coating adhesion was evaluated both before and after exposure to hot-

wet environment using lap shear test. The purpose was to evaluate the

influence of temperature and humidity on the bonded joint strength.

Joints were conditioned in a climate chamber for 1000 hours at 80oC

and 95% relative humidity.

7.4.4. Scratch testing of PBI Coated panels after exposure to Various Environmental Conditions

Scratch tests on unexposed PBI coated panels and the panels exposed

to various environmental conditions were performed. The purpose was

to evaluate the scratch resistance of PBI coating after having exposed

to different environmental conditions. Scratch test is a quantitative

technique in which critical loads are used to compare the cohesive or

adhesive properties of coating material. Critical loads are the load at

which different failures appears in the coating. Scratch test can be

performed by applying a progressive load or constant load. In a

progressive load scratch test, load increases linearly over the length of

the scratch. In present study, scratch tests were conducted using a

Micro scratch tester by CSM instruments. Rockwell diamond tip with

radius of 100 μm was used to generate the scratch. A progressive load

from 0.03N to 15N was used over a scratch length of 10 mm. Scratch

tests on unexposed PBI coated panels were performed using loading

rate of 15N/min, 30 N/min and 60 N/min. The purpose was to study the

effect of loading rate on failure mechanism of PBI coating. Different

parameters including depth of penetration, residual depth and acoustic

emission (AE) are measured during the scratch test to evaluate the

performance of coating.

7.4.5. Nano-indentation testing of PBI Coated panels after exposure to Various Environmental Conditions

Nano-indentation test was performed to study the hardness of PBI

coating before and after exposure to various environmental conditions.

Nano-indentation is getting a popular method for measuring the elastic


144

modulus and hardness of materials with smaller volumes. In nano-

indentation, a load is continuously applied to the indenter and the

displacement of the indenter is measured as a function of load. From

the load-displacement data and the unloading slope of the load-

displacement curve, hardness and contact stiffness are calculated using

well-established models [6, 7]. In present study, nano-indentation tests

were conducted under the continuous stiffness measurement (CSM)

model. A CSM technique allows the contact stiffness to be measured at

any point along the loading curve and not just at the point of unloading

[6, 7]. Thus, this technique is ideal for mechanical property

measurements of nanometer thick films. The nano-indentation

measurements were performed using Agilent nano-indenter with a

Berkovich diamond tip. 25 indentations were made in each sample and

the results shown in the present study are the average of these

measurements. All the specimens were indented to a depth of about

1000 nm.


7.5.1. Contact angle measurement of Atmospheric Pressure Plasma Treated Composite Substrate

Atmospheric pressure plasma treatment (APPT) was performed in the

current study before applying PBI coating in order to improve the

adhesion properties of epoxy based carbon fiber composite. Contact

angle of water on composite surface was taken before and after the

plasma treatment. A graphic demonstration of contact angle before and

after APPT is shown in figure 7.2.

7.2 Comparison of Contact angle of water on carbon epoxy composite (a) Untreated Sample (b) Sample after 60 Sec of Atmospheric Pressure Plasma Treated sample

Chapter 7

145

Figure 7.2 shows that APPT has reduced the contact angle of water on

the carbon/epoxy composite surface. This decrease in contact angle has ultimately increased the surface energy of the material and hence

increased the surface wetting.

7.5.2. Adhesion Testing of PBI coating

Lap Shear Testing

Lap shear testing of carbon/epoxy composite substrate was performed

to evaluate the effect of APPT on the adhesion properties of PBI

coating. The detail of the APPT effect on bonded joint strength is given

in chapter 6. Summarizing the results, with APPT treatment, an

improvement of about 250% in composite bonded joint strengths is

observed. The bonded joint strength has increased from 5 MPa to 18

MPa after APPT. A comparison of single lap shear strength of untreated

and APPT composite bonded joints is shown in figure 7.3.

7.3 Comparison of lap shear strength of PBI adhesive bonded joints of untreated and Atmospheric pressure plasma treated (APPT) M21/Carbon composite

Figure 7.3 shows that APPT has improved the lap shear strength by

introducing new functional groups on the surface of substrate material

and this has been proved in previous study [8,9]. As a result of these

functional groups, wetting properties of composite have improved

which ultimately has an effect on lap shear joint strength. The other

aspect of APPT which has not been emphasized in previous studies is

the change in surface topography. SEM analysis conducted in the

present study has revealed that APPT has also changed the surface

topography of carbon/epoxy composite and exhibited increased surface

roughness. A comparison of surface topography of untreated and APPT

M21 epoxy/composite is shown figure 7.4.

UT

AP

PT

0

5

10

15

20

M21

Lap

Sh

ear

Stre

ngt

h (

MP

a)


146

7.4 Surface topography of M21/carbon composite (a) Untreated (b) plasma treated (200x magnification)

SEM micrographs in figure 7.4 show that APPT has changed the surface

topography of composite material. After APPT, epoxy resin from the

surface is removed and fibers are exposed to the surface. Also, surface

roughness of composite has increased after plasma treatment. APPT

has also changed the failure mode of bonded joints from adhesive

failure to cohesive failure.

A comparison of failure modes of M21/carbon composite bonded joints


7.5 SEM micrographs of fractured surfaces of bonded joints of M21/carbon composite (a) Untreated (b) plasma treated (200x magnification)

SEM micrographs of fractured surfaces revealed that M21/carbon

composite bonded joints have exhibited different kinds of failure during

lap shear testing. Untreated bonded joints of M21/carbon composite

have demonstrated adhesive failure which indicates that untreated

M21/carbon composite has very low surface energy which ultimately

100 μm

100 μm 10 μm

10 μm

Chapter 7

147

resulted in early failure of the joints. Smooth adhesive layer and epoxy

resin layer on the fractured surfaces of untreated M21/carbon

composite are highlighted in figure 7.5a. No fibers are exposed to the

surface after lap shear testing. SEM micrograph in figure 7.5b shows

the fractured surface of APPT bonded joints. APPT composite joints

have exhibited cohesive failure in the substrate which indicates that

APPT has increased the adhesion between composite/adhesive

interfaces. Due to the strong adhesion, tearing of the fiber from the

surface of composite has occurred. Also, bonded joints have exhibited

improved lap shear strength after APPT.

7.5.3. Adhesion testing of Conditioned Specimens

Temperature and humidity are the two main factors which should be

considered while determining the long term durability of coating

adhesion. Prolong exposure to high temperature and humidity can

induce irreversible changes in the coating materials. In this context,

bonded joints of M21/Carbon composite were prepared and conditioned

in a climate chamber for 1000 hours at 80oC and 95% relative

humidity. The results about the lap shear testing of conditioned

specimens are presented in more detail in chapter 6. Summarizing the

results, conditioning of the bonded joints at 80oC and 95% relative

humidity for 1000 hours has reduced about 17% joint strength. The lap

shear strength decreased from 18 MPa to 15 MPa. A comparison of lap

shear strength of bonded joints of M21/Carbon composite before and

after exposure to hot-wet environment is shown in figure 7.6.

7.6 Comparison of Lap shear strength of controlled samples and samples conditioned at 80oC and 95% humidity for 1000 hours

To investigate the effect of hot-wet environment on

composite/adhesive interface, SEM analysis of fractured surface of

Co

ntr

olle

d

Co

nd

itio

ned

0

5

10

15

20

M21/Carbon Epoxy Composite

Lap

Sh

ear

Stre

ngt

h (

MP

a)


148

M21/carbon composite bonded joint was performed. A comparison of

fractured surface of unexposed sample and conditioned sample is

shown in figure 7.7. Figure 7.7 shows that mode of failure of bonded

joint has not changed after exposure to hot wet environment. Bonded

joint of unexposed and conditioned samples have failed cohesively

through the substrate. These results indicate that high temperature and

moisture has not affected the composite/adhesive interface and hence

cohesive failure occurred.

7.7 SEM micrographs of fractured surface of plasma treated bonded joints of

M21/Carbon composite (a) Controlled sample (b) Sample tested after conditioning at 80oC and 95% humidity for 1000 hours (200x magnification)

7.5.4. Scratch Testing of PBI coated Panels after exposing to Various Environmental Conditions

Scratch tests of unexposed PBI coated panels and the panels exposed

to various environmental conditions were performed. The purpose was

to evaluate the scratch resistance of PBI coating after having exposed

to different environmental conditions. Scratch resistance is a desirable

characteristic for coating material and is considered a key performance

property in evaluating the durability of coating. In present study, a

progressive load from 0.03N to 15N was used over a scratch length of

10 mm. Scratch test on unexposed PBI coated samples was performed

using loading rate of 15N/min, 30 N/min and 60 N/min in order to

determine the effect of loading rate on scratch resistant performance of

PBI coating. Different parameters are measured during the scratch test

to evaluate the performance of coating. These parameters include

depth of penetration, residual depth and acoustic emission (AE).

100 μm 10 μm

Chapter 7

149

A comparison of penetration depth (Pd) and residual depth (Rd) during

scratch test performed on unexposed PBI coated panels with different

loading rates is shown in figure 7.8. The pre-scan and post scan modes

were used to determine the penetration depth and residual depth

during scratch test. The residual depth indicates the ability of the

coating material for visco-elastic recovery after the scratch test.

7.8 Comparison of penetration depth and residual depth obtained by performing the scratch test on unexposed PBI coated panels at loading rate of 15N/min, 30N/min

and 60 N/min

Figure 7.8 shows that increased loading rate has a minute effect on

depth of penetration and residual depth of PBI coating. These results

indicate that loading rate has little effect on scratch characteristics of

PBI. The coating has depicted about 98% and 90% visco-elastic

recovery up to a load of 1 N and 1.7 N respectively. Almost a linear

decrease in visco-elastic recovery is observed with increasing load. At

maximum load of 15 N/min, PBI coating has depicted about 58% elastic

recovery.

A comparison of AE as a function of the force with different loading

rates is shown in figure 7.9. Fluctuation in AE during scratch test

indicates coating failure. Figure 7.9 demonstrate that there is no

fluctuation in AE at any point during the scratch testing of unexposed

PBI coated panels which indicate that there is no coating failure

occurred during scratch test. To further validate these results, SEM

analysis was carried out on tested samples.

0

10

20

30

40

50

60

0 5 10 15 20

Dep

th o

f P

enet

rati

on

'Pd

' (u

m)

Force (N)

15N/min

30N/min

60N/min

a b c

a b

c

(a)

0

5

10

15

20

25

30

0 5 10 15 20

Res

idu

al D

epth

'Rd

' (u

m)

Force (N)

15N/min

30N/min

60N/min

a b c

a

b

c

(b)


150

7.9 Comparison of depth of penetration and acoustic emission obtained by performing the scratch test on unexposed PBI coated panel with different loading rate

SEM micrographs of unexposed PBI coated panels tested at different

loading rates are shown in figure 7.10. SEM images of PBI coated

panels after scratch testing at different loading rates have not exhibited

any visible change in coating response. With all three loading rates, the

coating has not depicted any damage during the scratch test even at

maximum load of 15N. However, some plastic deformation can be

observed along the edges of the scratch path but no chipping or crack

formation is observed at all loading rates.

7.10 SEM micrographs of Unexposed PBI coated panel after scratch testing at different loading rates (a) 15N/min (b) 30 N/min (c) 60 N/min (100x magnification)

0

1

2

3

4

0 5 10 15 20

Aco

ust

ic E

mis

sio

n A

E (%

)

Force (N)

15N/min30N/min60N/min

a b c

100 μm 100 μm μm

(a) (b) (c)

Chapter 7

151

To study the effect of different environmental conditions on the

performance of PBI coating, PBI coated panels were immersed in water

and skydrol solution followed by scratch testing. Scratch tests were also

performed on the samples conditioned in hot wet environment at 80oC

and 95% RH for 1000 hours. A comparison of scratch test result

performed on PBI coated panels exposed to various environmental

conditions at a loading rate of 30 N/min is given in figure 7.11.

7.11 Comparison of penetration depth and residual depth obtained by performing the scratch test on PBI coated panels exposed to various environmental conditions (UE= Unexposed S=Skydrol W= Water C= Climate Chamber Conditioning)

Figure 7.11 shows a comparison of penetration depth and residual

depth during scratch testing of PBI coated panels after exposure to

various environmental conditions. Figure shows that PBI coated panels

exposed to various conditions have exhibited different penetration

depths. Analysis on penetration depth and residual depth of different

samples during scratch test shows that the visco-elastic recovery has

increased from 58% (unexposed sample) to 71% with the PBI coated

panel immersed in skydrol. However, no major effect on visco-elastic

recovery is observed with the sample immersed in water and the

sample conditioned in hot-wet environment. These results indicate that

immersion of PBI coated panel in skydrol has induced the cross-linking

which ultimately has increased the stiffness of PBI coating. As a result,

PBI coating has exhibited an increase in elastic recovery.

A comparison of elastic recovery of the unexposed PBI coated panel

and panels exposed to various conditions is presented in table 7.1.

0

10

20

30

40

50

60

70

80

0 5 10 15 20

Pen

etra

tio

n D

epth

'Pd

' (u

m)

Normal Force (Fn)

PBI-UE

PBI-S

PBI-W

PBI-C

a b c d

(a)

a

b

c

d

0

5

10

15

20

25

30

0 5 10 15 20

Res

idu

al D

epth

'Rd

' (u

m)

Normal Force (Fn)

PBI-UE

PBI-S

PBI-W

PBI-C

a b c d

(b)

a b

d

c


152

Table 7. 1 Comparison of elastic recovery of the controlled PBI coated panel and

panels exposed to various conditions

Specimen

Condition

100%

elastic

recovery at

load

(N)

Elastic

recovery

at force of

5N

Elastic

recovery

at force of

10N

Elastic

recovery

at force of

15N

unexposed PBI

coated panel 1.27 70% 64% 58%

Immersion in

water 0.40 68% 63% 54%

Immersion in

Skydrol 1.17 68% 66% 71%

Aging @ 80oC

and 90%RH 0.32 62% 60% 57%

A comparison of AE as a function of the force for the PBI coated panels

exposed to various environmental conditions is shown in figure 7.12.

7.12 Comparison of depth of penetration and acoustic emission obtained by performing the scratch test on PBI coated panel exposed to various environmental conditions (UE= Unexposed S=Skydrol W= Water C= Climate Chamber Conditioning)

Figure 7.12 shows no change in AE signal for the samples exposed to

various conditions except the sample which was immersed in skydrol

solution. Sample immersed in skydrol solution has exhibited an

increase in AE signal when applied force was 13N. These results

indicate that the scratch performance of PBI coated panel immersed in

water and the panels conditioned in hot-wet environment for 1000

hours are not affected. However, immersion of PBI coated panels in

skydrol resulted in a damage of coating at higher scratch load. SEM

analysis was carried out in order to validate the results obtained from

scratch tests.

2

2,5

3

3,5

4

0 5 10 15 20

Aco

ust

ic E

mis

sio

n 'A

E' (

%)

Normal Force (Fn)

PBI-UE

PBI-S

PBI-W

PBI-C

a b c d

b

Chapter 7

153

Figure 7.13 shows SEM micrographs after scratch testing of unexposed

PBI coated panels and panels exposed to various environmental

conditions.

7.13 SEM micrographs after scratch testing of PBI coated panel after exposing to various environmental conditions (a) Unexposed (b) Conditioned in climate chamber

(c) immersed in water (d) Immersed in Skydrol (100x magnification)

SEM micrographs in figure 7.13 shows that coated panels exposed to

various environmental conditions have not depicted any sign of damage

or failure along the edges of scratch path. However, sample immersed

in skydrol has exhibited a brittle kind of failure under the scratch tip

which indicates that skydrol solution has induced changes in PBI

coating which ultimately resulted in the failure of coating at higher load.

7.5.5. Nano-indentation testing of PBI coated Panels after exposing to Various Environmental Conditions

The nano-indentation measurements were performed using Agilent

nano-indenter with a Berkovich diamond tip. The tests were conducted

from top-down direction of the coating surface. Nano-indentation

method can be used in force controlled mode or displacement

controlled mode. In present study, displacement controlled mode was

used and the force on the indenter is applied until a displacement of

1000 nm is achieved. Mechanical properties measured using nano-

indentation is hardness (H), and elastic modulus (E). As the indenter is

pressed into the sample, both elastic and plastic deformation occurred

100 μm

100 μm μm

100 μm 100 μm μm

(a) (b) (c) (d)


154

which resulted in the formation of a hardness impression conforming to

the shape of the indenter. During indenter withdrawal, only the elastic

portion of the displacement is recovered.

Representative load–displacement curves obtained from the nano-

indentation tests for unexposed and exposed samples are shown in

figure 7.14.

7.14 Load-displacement curve (obtained from nano-indentation) of unexposed PBI coated panel and panels immersed in water and skydrol for 1000 hours at ambient conditions

Twenty five indentations are made on each sample in order to

determine the reproducibility of the results. The loading and unloading

curves for all twenty five indentations on each sample repeat very well

and the scatter in each case was very small. The force during the test is

applied on the coated sample until the desired displacement is

achieved. At this point, there was a hold for several seconds to reduce

the creep effects followed by unloading segment. Figure 7.14 shows

that at the same indentation depth, the load on sample immersed in

skydrol is little higher than the load on the unexposed sample which

means that immersion of PBI coated panels in skydrol has increased

the hardness of the coating material. Similar kind of trend is observed

after the scratch testing of the sample immersed in skydrol. The

unexposed sample and sample immersed in skydrol has almost same

shape of load displacement curve whereas the sample immersed in

water has a little flatter curve which indicates a decrease in harness

and modulus of PBI coating. Immersion of PBI coated panel in water

has decreased the modulus from 6.5 GPa to 5.7 GPa; a decrease of

0

2

4

6

8

10

0 200 400 600 800 1000 1200

Forc

e (

mN

)

Displacement (nm)

PBI-Unexposed

PBI-Water

PBI-Skydrol

a b c

a b

c

Chapter 7

155

about 12% whereas, these samples have not shown any degradation in

hardness.

A comparison of elastic modulus and hardness of PBI samples after

exposure to various environmental conditions is shown in figure 7.15.

Figure 7.15 shows that PBI coated panel immersed in skydrol has not

depicted any decrease in elastic modulus ad hardness whereas PBI

coated panel has shown a decrease in both modulus and hardness after

immersion in water for 1000 hours. It is more likely that the absorption

of water has induced the effect of plasticization in PBI coating which

ultimately has degraded its modulus and hardness values.

7.15 Comparison of elastic modulus and Hardness (obtained from nano-indentation) of unexposed PBI coated panel and panels exposed to various environmental conditions (UE = Unexposed coated panel, W = immersed in water for 1000 hours, S = Immersed in Skydrol

The average values of elastic modulus and hardness measured for

unexposed and exposed samples is presented in table 7.2.

Table 7. 2 Comparison of elastic modulus and hardness of unexposed and conditioned PBI coated panels

Specimen

Elastic

Modulus

(GPa)

Std. Dv.

(GPa) Hardness

(MPa)

Std. Dv.

(MPa)

Unexposed PBI coated

Panel 6.5 0.05 547 9

PBI coated Panel immersed

in Water for 1000 hours 5.7 0.01 496 4

PBI coated Panel immersed

Skydrol for 1000 hours 6.4 0.04 548 6

PBI coated Panel

Conditioned @ 80oC and

95%RH

Not

Completed

Not

Completed

PB

I-U

E

PB

I-W

PB

I-S

0

1

2

3

4

5

6

7

Elas

tic

Mo

du

lus

(GP

a)

(a)

PB

I-U

E

PB

I-W

PB

I-S

0

100

200

300

400

500

600

Har

dn

ess

(GP

a)

(b)


156

7.6. Conclusions

The purpose of this study was to improve the performance of epoxy

based carbon fiber composite under the flight service environment.

Particle erosion is one of the major factors which degrade the

mechanical properties of carbon/epoxy composite. PBI coating was

applied on M21/Carbon epoxy composite and its performance was

evaluated after exposing the coated panels to various environmental

conditions. Adhesion of the coating to the substrate is evaluated by

using single lap shear test. Results obtained from lap shear testing

indicate that composite bonded joints have retained about 83% of the

joint strength even after 1000 hours of conditioning in hot-wet

environment. These results indicate that PBI adhesive has

demonstrated better resistance against moisture and humidity which

ultimately helped in maintaining the higher joints strength.

Performance of the PBI coating was evaluated by exposing the panels

to different environmental conditions. PBI coated panels were

immersed in water and skydrol for 1000 hours. Coated panels were also

conditioned in hot-wet environment at 80oC and 95% for 1000 hours.

The performance of PBI coated panels was evaluated by using scratch

test and nano-indentation test. Scratch test on unexposed PBI coated

panels show that response of the PBI coating remains unchanged when

tested at different scratch velocities. No damage is observed along the

scratch path during the scratch testing of PBI coated panels at different

scratch velocities. PBI coated panels have exhibited similar depth of

penetration and residual depth at three different scratch velocities.

Residual depth gives an indication about elastic recovery of PBI coating.

At maximum load of 15 N, PBI coating has depicted about 58% elastic

recovery.

Scratch test performed on the conditioned panels show that PBI coated

panel immersed in water and PBI coated panel exposed to hot-wet

environment has increased the depth of penetration during the scratch

test which indicates the decreased hardness of PBI coating. However,

these panels have exhibited same elastic recovery as depicted by

unexposed PBI coated panel at a maximum load of 15 N. Contrary, PBI

coated panel immersed in skydrol has depicted an increase in elastic

recovery at maximum load which indicates that immersion of PBI

coated panel in skydrol has increased the cross-linking of PBI which

ultimately has increased the elastic modulus of PBI coating. However,

Chapter 7

157

PBI coated panel immersed in skydrol has exhibited a brittle kind of

failure along the scratch path at higher load of 13 N.

Nano-indentation tests on PBI coated panels were performed to study

the effect of different environmental conditions on the hardness and

modulus of PBI coating. Results obtained from nano-indentation

indicate that immersion of PBI coated panels in water for 1000 hours

resulted in a decrease of hardness and elastic modulus about 12% and

10% respectively. However, immersion of PBI coated panel in skydrol

has exhibited a very slight increase in hardness of the coating while the

elastic modulus of the coating after immersion in the skydrol for 1000

hours remained unchanged. Results obtained from different tests

conducted on PBI coating indicate that PBI coating has retained most of

its properties even after exposure to various environmental conditions

for 1000 hours. However, further testing is required for complete

evaluation of coating.

7.7. References

1. A.P. Mouritz and A.G.Gibson, Fire Properties of polymer composite materials, Solid Mechanics and its applications, Volume 143, (2006)

2. Roger Vodika, “Accelerated Environmental Testing of Composite Materials”

DSTO-TR-0657

3. Jozeph Zahavi, “Solid Particle Erosion of Reinforced Composite Materials” Wear, 71 (1981)

4. Hegedus, Charles R., Green, William J. (Clementon, NJ), “US Patent Patent

488532” (1989)

5. Juergen H. Braun, “Titanium Dioxide’s Contribution to the Durability of Paint Films” Progress in Organic Coatings, 15 (1987)

6. Xiaodong Li, Bharat Bhushan, A review of nano-indentation continuous stiffness measurement technique and its applications, Materials Characterization, 48 (2002)

7. Tao Wen, Jianghong Gong, Zhijian Peng, Danyu Jiang, Chengbiao Wang, Zhiqiang Fu, Hezhuo Miao, Analysis of continuous stiffness data measured during nanoindentation of titanium films on glass substrate, Materials Chemistry and Physics 125 (2011)


158

8. Michael Noeske, Jost Degenhardt, Silke Strudthoff, Uwe Lommatzsch, Plasma jet treatment of five polymers at atmospheric pressure: surface modifications and the relevance for adhesion, Int. J. Adhes. Adhes. Vol. 24, (2004)

9. J.R.J Wingfield Treatment of composite surfaces for adhesive bonding, int. J. Adh. & Adh. Vol. 13 (1993)

159

CHAPTER 8

Conclusions and Recommendations for Future Work

With the increasing use of polymer based composite materials, there is

an increasing demand of polymer resins with high glass transition

temperature (Tg), high thermal stability and excellent mechanical

properties at high temperature. Polybenzimidazole (PBI) is a recently

emerged high performance polymer. Due to its excellent thermal and

mechanical properties, PBI has great potential to be used for many high

temperature applications. However, in powder form, it has very high

melt viscosity which is too high to allow its processing by conventional

manufacturing techniques. Therefore, the processing of PBI itself is a

real challenge. Most of the PBI applications are found in the form of

molded parts. Due to high processing temperature and pressure

requirement, PBI has not won the attraction for many applications.

However, recently, PBI is also available in solution form but its

performance has not been explored for potential applications. Due to its

high thermal stability and mechanical properties, PBI has great

potential to be used for many high temperature applications. Therefore,

objective of this work was to optimize the processing of PBI using PBI

solution and to evaluate its performance for potential aerospace

applications. Based on the results obtained I this work, following

conclusions are made.

8.1. Conclusions

8.1.1. Compression Molding of PBI

In present study, compression molding process was used to form the

molded parts of PBI. Efforts were made to enhance the processability of

the polymer by varying different parameters during processing.

Compression molding process was performed by heating and pressing


160

the PBI powder by heating the polymer at 440°C and by varying the

pressure between 10 MPa and 50 MPa. Heating of pre-dried PBI resin

close to its glass transition temperature for 30 minutes followed by

pressing the PBI resin at a pressure of 50 MPa was the most optimum

method which has formed compression molded parts of PBI with

desired tensile properties. Due to high processing temperature and

pressure requirement, the initial idea of manufacturing the CNFs

reinforced PBI molded parts was postponed. Due to amorphous nature

of PBI, it was not possible to melt PBI powder in order to disperse

nano-fibers. Therefore, dry mixing was the only way to mix the nano-

fibers into the PBI resin. It was expected that dry mixing of CNFs would

result in poor interfacial adhesion which would ultimately led towards

poor mechanical properties of PBI nano-composite. At this point, it was

concluded that manufacturing of PBI nano-composite using

compression molding process will not be a feasible and economical way

to form PBI parts for aerospace application.

8.1.2. Solution Casting of PBI film

Due to high processing temperature and pressure requirement for

compression molding PBI, we started to rethink about other methods to

manufacture PBI nano-composite and to explore its potential for

different aerospace applications. Due to its excellent thermal properties

and superior non-flammability, PBI has been used for firefighters’

protective clothing and high-temperature gloves. However,

performance of PBI in the form of film, coating or adhesive has not

been explored for potential aerospace applications. Also, nano-

composite of PBI has not been manufactured till date. Therefore, the

main objective of this study was to optimize the processing of unfilled

and CNFs reinforced PBI film with high thermal and mechanical

properties. Furthermore, the potential of PBI was explored for different

aerospace applications as a film and coating material. The nano-

composite PBI films using PBI solution were prepared with CNFs

contents of 0.5 wt%, 1 wt% and 2 wt%. SEM was carried out to

examine the dispersion of CNFs in PBI films. SEM analysis has revealed

that sample prepared by direct mixing of CNFs into PBI has shown large

agglomeration Whereas, samples manufactured using bath sonication

followed by mechanical mixing have revealed uniform dispersion of

CNFs with all fiber loadings. Therefore, in conclusion, bath sonication

followed by mechanical stirring seems to be an effective technique for

the dispersion of CNFs into PBI.

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161

8.1.3. Thermal and Mechanical Properties of PBI

Thermal and mechanical properties of compression molded PBI, unfilled

PBI film and nano-composite films were investigated using TGA, DMA

and tensile testing. TGA analysis shows that both compression molded

PBI and PBI film has depicted high thermal stability even up to a

temperature of 550oC. TGA curve of PBI has shown two steps

decomposition both in the molded form and in the form of film.

Compression molded PBI has shown only 5% weight loss up to the

temperature of 545°C. This weight loss is significantly low when

compared to other polymers. However, PBI film has revealed a higher

weight loss as compare to the weight loss of compression molded PBI.

It has shown a weight loss of about 8% up to a temperature of 150°C.

The higher weight loss of PBI film up to this temperature is due to the

fact that PBI film exhibits higher tendency to absorb moisture from

surrounding. Due to higher moisture absorption, PBI film has depicted

higher weight loss in the first degradation step. After the first

degradation step, PBI film has shown thermal stability and it only

exhibited 4% weight loss from a temperature of 150°C to a

temperature of 550°C. PBI film has shown a total weight loss of about

12% up to a temperature of 550°C.

DMA studies indicate that both compression molded PBI and PBI neat

film exhibited a storage modulus of 3.81 GPa and 3.12 GPa even at a

temperature of 250oC. Addition of 2 weight percent CNFs has improved

storage modulus of PBI film about 48% at a temperature of 250oC.

Also, no sharp decrease in storage modulus is observed with nano-filled

PBI which demonstrates the effectiveness of dispersing CNFs in PBI.

Mechanical testing shows that both compression molded PBI and PBI

film exhibited highest ultimate tensile strength compared to any unfilled

polymer. These results end up with the conclusion that PBI with high

thermal and mechanical properties can be an excellent candidate for

different high temperature applications. Furthermore, addition of CNFs

in the PBI polymer has improved both thermal and mechanical

properties of PBI which will be beneficial for different high temperature

applications of polymer.

8.1.4. Testing in LEO Simulated Environment

The performance of PBI under the simulated LEO environmental

conditions was evaluated. Effect of CNFs on the erosion yield and


162

mechanical properties of PBI was also studied in present work. Results

show that both unfilled PBI and nano-filled PBI have shown high

erosion yield values. Contrary to the high erosion yield values, both

unfilled and nano-filled PBI have shown very slight decrease in

mechanical properties. Unfilled PBI has shown a decrease in tensile

strength of about 9% whereas, CNFs reinforced PBI exhibited a

decrease in tensile strength of about 3% when exposed to total AO

dose values of 0.65x1020 atoms/cm2 with energy of 0.04eV. Results

obtained from mechanical testing of LEO exposed samples indicate that

mass loss obtained during the exposure to simulated LEO

environmental conditions could be a misleading value. Keeping the

samples in the vacuum oven at 125oC has proved this fact that almost

all the mass loss of PBI was due to the removal of the moisture rather

than the erosion due to AO attack. Therefore, it is concluded that

mechanical properties of unfilled and nano-filled PBI are not very much

affected under the influence of simulated LEO environmental conditions

with an AO dose used in this study. Furthermore, nano-fillers have

improved the performance of PBI as compared to the unfilled PBI.

However, further study is required to accurately determine the erosion

yield data for unfilled and nano-filled PBI.

8.1.5. Radiation Testing

Unfilled and nano-filled PBI samples were exposed to gamma

radiations, electron radiations and mixed radiations. TGA shows that all

three kinds of radiations have induced cross-linking in PBI which

ultimately has increased the thermal stability particularly at high

temperature. DMA experiments performed on unfilled PBI after

irradiation revealed an improvement in storage modulus and glass

transition temperature. These results indicate that irradiation of unfilled

PBI has enhanced the thermo-mechanical properties of PBI. DMA

experiments performed on nano-filled PBI demonstrate that irradiation

has slightly increased the storage modulus through the whole

temperature range whereas, glass transition temperature remained

unaffected. These results demonstrate that addition of CNFs to PBI is

helpful in maintaining the storage modulus even at a temperature of

around 300oC. Tensile testing of controlled and irradiated samples of

unfilled and nano-filled PBI shows that gamma radiations and electron

radiations has very slightly degraded the tensile strength of unfilled

PBI. However, these radiations have affected the failure strain to some

extent. Results obtained during this study are quite encouraging and

Chapter 8

163

further studies could be performed by using higher radiation dose and

higher temperature values to evaluate the performance of PBI.

8.1.6. Fire Testing of PBI Coating

The purpose of this study was to improve the fire performance of epoxy

based carbon fiber composite with unfilled and nano-filled PBI coating.

Cone calorimeter results show that the unfilled PBI coating did not

improve the fire retardancy of the carbon/epoxy composite in terms of

average HRR, peak HRR and CO yield. Contrary to the unfilled PBI

coating; CNFs PBI coating has shown a significant improvement in the

fire retardant properties of the carbon/epoxy composite. About 60%

reduction in average HRR and 40% reduction in PHRR of carbon/epoxy

composite is observed with nano-filled PBI coating. Smoke and CO are

also of concern during a fire in an aircraft as most deaths during a fire

occur due to reduced visibility and the inhalation of toxic gases. Nano-

filled PBI coating has reduced the smoke and CO emissions up to 73%

when compared to the uncoated carbon/epoxy composite. A significant

outcome of this work is that CNFs, even when present in very small

quantities, can be more effective in improving the fire performance of

an inherently flame retardant material. With a low coating thickness,

such an improvement in fire properties is quite remarkable.

8.1.7. Lap Shear Testing of PBI

In present study, performance of PBI as an adhesive was evaluated for

aerospace applications. Bonded joints of epoxy based carbon fiber

composite were formed using PBI adhesive. Atmospheric pressure

plasma treatment (APPT) was used to improve the adhesion properties

of composite materials. Results obtained from contact angle

measurement after APPT shows that APPT has improved the wetting

properties of epoxy based carbon fiber composite. Current work shows

that during APPT, surface temperature of composite materials have

raised to above 200oC in very short duration of time and this

temperature was well above glass transition temperature of composites

used in this study. This high temperature can induce residual thermal

stresses and also have an impact on mechanical properties. Therefore,

one of the important conclusions of this study is that thermal properties

of polymer based composite materials should always be considered

before performing APPT. APPT can significantly damage the surface as

well as bulk properties of the materials. Therefore, it is important to

optimize the height of the plasma nozzle from composite surface and


164

plasma treatment time. By increasing the height of the plasma nozzle

from composite, the temperature generated at the surface can be

reduced which would have less affect on bulk properties of the

materials. Another important observation of this study is that surface

topography of composite is another crucial factor to achieve high bond

strength. Surface topography of composite material was studied

sequentially with increasing plasma treatment time. With increasing

treatment time, surface roughness of composites has increased. The

increase in surface roughness is correlated with the increase in surface

temperature during APPT. Surface temperature during APPT reached

above glass transition temperature of polymer and polymer based

composite. This increase in surface temperature caused local

rearrangement of molecules and in case of unfilled PEEK, local melting

is observed which ultimately resulted in increased surface roughness.

Bonded joints of epoxy based composite were conditioned at 80oC and

95% humidity. Exposure of bonded joints to hot-wet environment for

1000 hours has reduced the lap shear strength of about 17%. These

results indicate that hot-wet environment has not significantly

deteriorated the performance of PBI adhesive even after 1000 hours of

conditioning. However, testing of PBI adhesive bonded composite joints

at 80oC has reduced the lap shear strength to about 50%. Despite of

the fact that PBI has very high thermal stability and can maintain

thermo-mechanical properties even at a temperature of 300oC, still PBI

adhesive bonded joints have revealed a significant decrease in lap

shear strength. These results indicate that performance of bonded

joints not only depends on the properties of the adhesive but it also

depends on thermo-mechanical properties of substrates. Without

performing the detail testing of PBI adhesive using different substrate

materials, a conclusive statement cannot be made about the high

temperature performance of PBI adhesive.

8.1.8. Testing for Aircraft Application

Performance of PBI was also evaluated as coating material to protect

the epoxy based carbon fiber composite under the aircraft flight service

environment. PBI coating was applied on M21/Carbon epoxy composite

and its performance was evaluated after exposing the coated panels to

various environmental conditions. PBI coated panels were immersed in

water and skydrol for 1000 hours. Coated panels were also conditioned

in hot-wet environment at 80oC and 95% for 1000 hours. The

performance of PBI coated panels was evaluated by using scratch test

Chapter 8

165

and nano-indentation test. Scratch test performed on the conditioned

panels revealed that depth of penetration during the scratch test has

increased after immersion of PBI coated panel in water and also after

exposure of coated panels to hot-wet environment. These results

indicate that exposure of PBI coating to these environmental conditions

has reduced its hardness. However, these panels have not exhibited

any change in elastic recovery up to a maximum load of 15 N. On the

other hand, PBI coated panel immersed in skydrol has depicted an

increase in elastic recovery at maximum load which indicates that

immersion of PBI coated panel in skydrol has increased the cross-

linking of PBI which ultimately has increased the elastic modulus of PBI

coating. However, PBI coated panel immersed in skydrol has exhibited

a brittle kind of failure along the scratch path at a load of 13 N.

Nano-indentation tests on PBI coated panels were performed to study

the effect of different environmental conditions on the hardness and

modulus of PBI coating. Results obtained from nano-indentation

indicate that immersion of PBI coated panels in water for 1000 hours

resulted in a decrease of hardness and elastic modulus of about 12%

and 10% respectively. However, immersion of PBI coated panel in

skydrol has exhibited a very slight increase in hardness of the coating

whereas the elastic modulus of the coating after immersion in the

skydrol for 1000 hours remained unchanged. Results obtained from

different tests conducted on PBI coating indicate that PBI coating has

retained most of its properties even after 1000 hours exposure to

various environmental conditions.

8.2. Recommendations for future work

1. The manufacturing process of unfilled and CNFs reinforced PBI

film was optimized and material has exhibited high thermal and

mechanical properties. However, PBI film has depicted higher

mass loss between the temperature of 50oC and 150oC as

compare to the mass loss of compression molded PBI. It was

proposed that this loss in weight was due to the loss of water or

DMAc present in the film. However, the exact reason of this

weight loss could not be determined. Therefore, it is

recommended that thermo-gravimetric analysis along with mass

spectroscopy should be performed. A more realistic reason of

higher mass loss between the temperature of 50oC and 150oC can


166

be determined when TGA will be performed along with mass

spectroscopy.

2. PBI film was manufactured by solution casting followed by

immersion of film in hot water for about an hour in order to

remove the DMAc solvent. Afterwards, the film was hot pressed

at a temperature of 225oC in order to further remove the water

and solvent contents. It is more likely that due to the immersion

of film in hot water after removing from the glass plate,

mechanical properties of the film are affected. It is expected that

immersion of film in hot water resulted in decreased tensile

strength and increased tensile strain due to the effect of

plasticization. The purpose of keeping the film in hot water was to

remove the maximum solvent contained by the film. It is

recommended for future work that mechanical testing of PBI

should be performed without keeping the film in hot water for

longer time. This will help in determining the effect of long term

water exposure on thermal and mechanical properties of PBI.

3. The performance of PBI under the simulated LEO environmental

conditions was evaluated. Both unfilled PBI and nano-filled PBI

have shown high erosion yield values when exposed to atomic

oxygen. Contrary to the high erosion yield values, mechanical

properties of both unfilled and nano-filled PBI are very little

affected. Unfilled PBI has shown a decrease in strength of about

9% while CNFs reinforced PBI exhibited a decrease in strength of

about 3% when exposed to total dose values 0.65x1020

atoms/cm2. High erosion yield and low affect on mechanical

properties after exposure of PBI film to atomic oxygen are two

contradictory results. Therefore, it is recommended that before

exposure of PBI film to the atomic oxygen, it should be dried in a

vacuum oven at 150oC. Afterwards, it should be immediately

transferred to LEO simulated facility to expose the film to atomic

oxygen. After removing from the LEO simulation facility, the

weight of the film should be taken immediately. This procedure

will give a better idea about the erosion yield of PBI and it will

also give a more accurate indication about the potential of PBI to

be used in LEO environment.

Chapter 8

167

4. PBI film was exposed to atomic oxygen for a dose value of

0.65x1020 atoms/cm2 with energy of 0.04eV. In real space

environment, AO strikes the material with energy of 5eV which is

much higher than the energy used in this study. Therefore, it is

recommended that performance of PBI should also be evaluated

by using high energy atomic oxygen with higher dose values.

5. Unfilled and nano-filled PBI films were exposed to gamma

radiations, electron radiations and mixed radiations. PBI was

exposed to different radiations in air environment at ambient

temperature. Results obtained under these conditions are quite

encouraging. However, space environment consist of ultra high

vacuum and thermal cycling. Therefore, further studies should be

performed on PBI by irradiating the material under vacuum and

thermal cycling environment. Material should also be tested at

higher dose values. These steps will give more realistic idea about

the potential of PBI for space application.

6. PBI was used as a coating to improve the fire resistance

performance of epoxy based carbon fiber composite used for

structural application in aircraft. Results obtained from cone

calorimeter test indicated that unfilled PBI coating could not

improve the fire retardant performance of carbon/epoxy

composite. However, CNFs reinforced PBI coating have shown a

significant improvement in the fire retardant properties of the

carbon/epoxy composite. In present study, only two weight

percent of CNFs was dispersed in PBI coating which has

significantly improved the performance of PBI. Further study can

be performed by increasing the contents of CNFs. Increased

quantity of CNFs can help in further improving the quality of the

carbonaceous layer which will ultimately reduce the ingress of the

heat down towards the structural part.

7. In present study, the cone calorimeter test was run for about 15

minutes while using nano-filled PBI coating. There are some

specific requirement from FAA for structural materials after 2

minutes and 5 minutes fire testing. Therefore, while performing

the cone calorimeter test, test should be stop at 2 minutes and 5

minutes and then flexure properties of carbon/epoxy composite


168

8. should be determined in order to evaluate the effect of fire on

mechanical properties of structural composite.

9. Testing of PBI adhesive bonded joints of carbon/epoxy composite

at 80oC has reduced the lap shear strength to about 50%.

Despite of the fact that PBI has very high thermal stability and

can maintain thermo-mechanical properties even above 300oC,

still PBI adhesive bonded joints have revealed a significant

decrease in lap shear strength. These results indicate that

performance of bonded joints not only depends on the properties

of the adhesive but it also depends on thermo-mechanical

properties of substrates. Therefore, it is important to evaluate the

performance of PBI adhesive by using substrate materials with

different glass transition temperatures and different coefficient of

thermal expansions. Testing of PBI adhesive with different

substrate materials will give a clear indication about the high

temperature performance of PBI adhesive.

10. PBI was also used as a coating to improve the performance

of epoxy based carbon fiber composite under the flight service

environment of aircraft. Performance of the PBI coating was

evaluated by exposing the PBI coated panels to different

environmental conditions. PBI coated panels were immersed in

water and skydrol for 1000 hours. Coated panels were also

conditioned in hot-wet environment at 80oC and 95% for 1000

hours. PBI coating has depicted better performance under these

conditions. However, during the actual flight service environment,

the coating is also exposed to UV radiations which could severely

degrade the coating properties in the presence of temperature

and humidity. Therefore, it is important to evaluate the

performance of the coating after exposing the coating panel to UV

radiations along with high temperature and humidity. This will

give a real indication about potential of PBI coating for aircraft

application.

CURRICULUM VITAE

170

CURRICULUM VITAE

The first day of the New Year bring the happiness as the author was

born on January 01, 1980. The author has completed his B.Sc

Mechanical Engineering from University of Engineering and Technology

Lahore in 2005. After graduating, he joined Dynamic Equipment &

Control and worked there for one year as a project engineer. In 2005,

he joined institute of space technology (IST) and worked there as a

researcher for one year. The Author was awarded a scholarship for

M.Sc studies by Institute of Space Technology. After searching different

universities, the author’s ultimate destination was Delft University of

Technology. He obtained his Master of Science degree in Aerospace

Engineering in 2008.

After completing his M.Sc, the author was given an opportunity to

pursue his PhD at Aerospace Department in the group of Aerospace

Materials. He started his PhD in September 2008 under the supervision

of Prof. Dr. Rinze Benedictus and Dr. Shantanu Bhowmik. He worked on

the thesis entitled “Performance Evaluation of Polybenzimidazole for

Potential Aerospace Applications”. During his PhD research, he has

communicated his work in various reputed international journals and

conferences.

LIST OF PUBLICATIONS

171


Journal Publications:

1. H.M.S.Iqbal, S.Bhowmik and R.Benedictus, Process Optimization

of Polybenzimidazole Adhesive for High Temperature Applications,

International Journal of Adhesion and Adhesives, Vol. 48 (2014).

2. H.M.S.Iqbal, S.Bhowmik and R.Benedictus, Study of the fire

resistant behavior of unfilled and carbon nanofibers reinforced

polybenzimidazole coating for structural applications, Polymers

for Advanced Technologies, (2013)

3. Adhesion Characteristics of High Temperature Resistant Polymer,

Journal of Adhesion Science and Technology, Vol. (2012)

4. H.M.S.Iqbal, S.Bhowmik and R.Benedictus, Processing and

Characterization of Space durable High Performance Polymeric

Nano-composite, AIAA journal of Thermo-Physics and Heat

Transfer, Vol. (2011)

5. H.M.S. Iqbal, S.Bhowmik and R. Benedictus, Surface Modification

of PEEK by Atmospheric Pressure Plasma and failure mechanism

of Adhesive bonded Joint, International Journal of Adhesion and

Adhesives, Vol. (2010)

6. H.M.S. Iqbal, S.Bhowmik and R. Benedictus, Modification of High

Performance Nano Adhesive Bonding of Glass fiber reinforced and

Carbon Fiber reinforced Polyphenylene Sulfide using Plasma

treatment and high energy Radiations, Journal of Polymer

Engineering and Science, Vol. (2010)


172

7. Experimental Investigation into The Effect of Adhesion Properties

of PEEK modified by Atmosheric Pressure Plasma and Low

Pressure Plasma, Vol. (2010)

List of Conference Publications:

1. H.M.S. Iqbal, S.Bhowmik and R. Benedictus, Development of

Nanofibers Reinforced polymer composite for space application,

17TH International Conference on Composite Materials, 27th –

31st July 2009

2. H.M.S. Iqbal, M.I.Faraz, S.Bhowmik and R. Benedictus,

Comparative Study of Adhesion Properties of High Performance

Polymer Modified by Atmospheric Pressure Plasma and Low

Pressure Plasma, 25th International Symposium SWISS

BONDING, 11th-13th May, 2009, Switzerland.

3. H.M.S. Iqbal, S.Bhowmik and R. Benedictus, Thermo-mechanical

Characteristics of Space Durable Adhesive Joint of High

Performance Polymer, 50th AIAA/ASME/ASCE/ASC Structural

Dynamics, and Materials Conference, 4th – 7th May, 2009, Palm

spring, California.

4. S. Bhowmik, R. Benedictus, H.M.S.Iqbal and M.I.Faraz,

Application of Polymeric Nano-Composites at Low Earth Orbit and

Geosynchronous Earth Orbit, 13th European Conference on

Composite Materials, June 2-5 2008 Stockholm, Sweden

Articles to be communicated:

1. Study the thermal Effects of Atmospheric Pressure Plasma

Treatment on surface topography and surface energy of

Unidirectional Epoxy based Carbon Fiber Composite,

communicated to International Journal of Adhesion and Adhesives

2. H.M.S. Iqbal, S.Bhowmik and R. Benedictus, Study the Effect of

Simulated Low Earth Orbit on thermal and mechanical properties

of Polybenzimidazole, to be communicated to AIAA journal


173

3. H.M.S. Iqbal, S.Bhowmik and R. Benedictus, Comparitive study

on Thermo-Mechanical Properties of Unfilled and Nano-filler

Reinforced Polybenzimidazole in Simulated Low Earth Orbit

conditions, to be communicated to AIAA journal

4. H.M.S. Iqbal, S.Bhowmik and R. Benedictus, Study the Effect of

High Energy Radiations on thermal and mechanical properties of

Polybenzimidazole, to be communicated to AIAA journal

5. Performance Evaluation of Polybenzimidazole Adhesive under

Various Environmental Conditions, to be communicated to

International Journal of Adhesion and Adhesives

6. Performance Evaluation of Polybenzimidazole Coating for Aircraft

Application, to be communicated to Journal “Progress in Organic

Coatings”

ACKNOWLEDGEMENT

174

ACKNOWLEDGEMENT

Many people have made contribution to this thesis in a direct or indirect

way. First of all, I would like to thank my promoter Rinze Benedictus

and my daily supervisor Shantanu Bhowmik for their guidance and

supervision during my PhD work.

Financial resources are very important for any project. Doing research

on any idea sometime becomes impossible due to financial limitations

and same was the case with me. I had an idea to work on but no

financial resources. Under these circumstances, my promoter Rinze

Benedictus did his efforts to get some money from first money stream

in order to start working on proposed project. I am especially thankful

to Rinze for his efforts under difficult financial conditions. Also special

thanks to my daily supervisor Shantanu Bhowmik who convinced Rinze

to generate the financial resources for my PhD research work.

I would like to thank my group secretary Gemma for her help and

support to handle all kind of administrative matters during my PhD. I

would like to extend my thanks to my friends and colleagues Amir,

Gustavo, Sharif, Ricaardo, Shafqat, Ping Liu, Rafi, Chris, Akram,

Iftikhar, Greg , Milan, Mina and Jesus. Thank you all of you guys for

having good time with me. I would like to thank Santiago and Maruti

for useful technical discussions and suggestions. I especially appreciate

the support of Maruti for performing my experiments on TGA and DMA

equipments.

I would like to thank to Dr. Jansen from mechanical engineering

department for performing moisture absorption study and Dr. Erik from

Civil engineering department for performing nano-indentation tests.

I would like to thank Bob, Frans, serge and Fred from Delft Aerospace

Laboratory for their help and support to perform the experimental

ACKNOWLEDGEMENT

175

work. I would also like to thank to Bus from Chemical engineering

department for his help to perform AFM equipment.

I owe my special thanks to my parents for their support and

encouragement throughout my career. Their prayers have been a

source of motivation and strength for me. I am also greatly thankful to

my younger brother Naeem for taking care of all the family members

back to home in my absence. Without his support, it would almost

impossible for me to accomplish my PhD. I would like to extend my

thanks to my parents-in-law for their prayers and support during my

PhD.

Last but not least, a special thanks to my wife for her support during

my PhD. I appreciate her patience during my long working hours. Her

patience during the days of her serious illness is admirable. Without her

understanding and patience, I would not be able to complete this

research work.

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