Unsaturated polyester‐toughened epoxy composites: Effect of sisal fiber on thermal and dynamic...

Unsaturated Polyester-Toughened Epoxy Composites: Effectof Sisal Fiber on Thermal and Dynamic MechanicalProperties

Nagarjuna Reddy Paluvai,1,2 Smita Mohanty,1,2 S.K. Nayak1,2

1Advanced Research School for Technology and Product Simulation (ARSTPS), Central Institute of PlasticsEngineering and Technology (CIPET), Chennai, India

2Laboratory for Advanced Research in Polymeric Materials (LARPM), Central Institute of Plastics Engineeringand Technology (CIPET), Bhubaneswar, India

In the present study, the mechanical and thermal prop-erties of sisal fiber-reinforced unsaturated polyester(UP)-toughened epoxy composites were investigated.The sisal fibers were chemically treated with alkali(NaOH) and silane solutions in order to improve theinterfacial interaction between fibers and matrix. Thechemical composition of resins and fibers was identi-fied by using Fourier-transform infrared spectroscopy.The UP-toughened epoxy blends were obtained by mix-ing UP (5, 10, and 15 wt%) into the epoxy resin. Thefiber-reinforced composites were prepared by incorpo-rating sisal fibers (10, 20, and 30 wt%) within the opti-mized UP-toughened epoxy blend. Scanning electronmicroscopy was used to analyze the morphologicalchanges of the fibers and the adhesion between thefibers and the UP-toughened epoxy system. The resultsshowed that the tensile and flexural strength of (alkali-silane)-treated fiber (30 wt%) -reinforced compositesincreased by 83% and 55%, respectively, as comparedwith that of UP-toughened epoxy blend. Moreover, ther-mogravimetric analysis revealed that the (alkali-silane)-treated fiber and its composite exhibited higher thermalstability than the untreated and alkali-treated fiber sys-tems. An increase in storage modulus and glass transi-tion temperature was observed for the UP-toughenedepoxy matrix on reinforcement with treated fibers. Thewater uptake behavior of both alkali and alkali-silane-treated fiber-reinforced composites is found to be lessas compared with the untreated fiber-reinforced com-posite. J. VINYL ADDIT. TECHNOL., 00:000–000, 2015. VC 2015Society of Plastics Engineers

INTRODUCTION

Natural fiber-reinforced thermoset composites have

generated considerable industrial interest over the past

decades. These materials have high specific strength-to-

weight ratio, low density, high strength and modulus-

enhanced energy recovery and biodegradability, which

allow them to compete with their synthetic counterparts

[1, 2]. Commercially available natural fibers, such as

hemp, jute, kenaf, sisal, flax, palm, silk, cotton, and

banana, have been widely used as reinforcements in poly-

meric materials. Sisal fibers are among the most widely

used natural fibers today because they are widely avail-

able, are relatively inexpensive, and possess excellent

mechanical and thermal properties [3–5]. Nevertheless,

the presence of cellulose, hemicelluloses, lignin, and

waxy substances allows absorbing moisture from the

environment, which leads to poor adhesion with the

hydrophobic polymer matrix [6, 7]. Various investigations

have been taken out on the modifications of natural fibers

to improve their compatibility with the polymeric material

[8]. Several authors [9–14] have reported different surface

treatments for fiber, which involves chemical techniques,

such as acetylation, alkali, benzoylation, grafting, silane

treatment, and alkali-silane treatments.

Epoxy resins are the most important thermosetting res-

ins, used widely in industry, which exhibits low shrinkage,

good mechanical and thermal properties, and excellent

adhesion to a variety of substrate materials [15–17]. The

major drawback of the cured epoxy systems is its brittle

nature, exhibiting poor resistance to impact and crack prop-

agation [18, 19]. To improve the working performance, the

epoxy resin has been blended with flexible polymers, such

as rubbers, thermoset, and thermoplastic polymers [20–23].

Unsaturated polyester (UP) resins can be applied to modify

the epoxy resins because of their low manufacturing cost

and ease of processing. The low-viscosity UP resin

improves the processibility of the epoxies, which amelio-

rates the impact and fracture properties with a decrease in

some of the thermal properties [24–27].

This work reports the effect of sisal fiber on the

mechanical, thermal, morphological, and water absorption

properties of the UP-toughened epoxy system. The

Correspondence to: Nagarjuna Reddy Paluvai; e-mail: nag1987@gmail.

com

Additional Supporting Information may be found in the online version

of this article.

DOI 10.1002/vnl.21491

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2015 Society of Plastics Engineers

JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2015

toughened epoxy blends have been prepared by using the

simple experimental method: mechanical mixing followed

by ultrasonication of UP resin to the epoxy monomer. In

order to enhance the properties of these materials, sisal

fibers were chemically modified with alkali and silane

solutions. The mechanical, thermal, and morphological

properties of fibers have been investigated by using a

Universal Testing Machine, thermogravimetric analysis

(TGA), and scanning electron microscopy (SEM)

analysis.

EXPERIMENTAL

Materials

Diglycidyl ether of bisphenol-A epoxy resin (viscosity:

11 6 2 Pa.s and epoxide equivalent: 189 6 5 g/eq), trie-

thylenetetramine, UP resin (viscosity: 0.45 6 0.05 Pa.s

and acid value: 25 6 3 mg KOH/g), and other reagents,

such as methyl ethyl ketone peroxide (initiator/catalyst)

and cobalt naphthenate (accelerator), were supplied from

Allied Agencies Ltd. (Hyderabad, India). 3-

Aminopropyltriethoxy silane (APTES) was procured from

Sigma-Aldrich (Bangalore, India). Sisal fibers used in this

work were obtained from tribal districts of Kheonjhar

(Odisha, India).

Surface Modification of Sisal Fibers

Prior to surface modification, the fibers were washed

several times with ground water followed by a detergent-

diluted solution at 40–50oC to remove wax and other

impurities and subsequently air-dried. Fourier-transform

infrared (FTIR): O–H stretching (3,337 cm21), C5O

(1,732 cm21), C–O stretching (1,299 cm21), C–H asym-

metric and symmetric stretching (2,899 cm21), and CH2

(1,430 cm21) [28, 29].

Alkali Treatment of Sisal Fiber. The fibers, measuring

50 cm in length, were soaked in a solution containing 2

wt% alkali (NaOH) at room temperature for 4 h. Subse-

quently, the NaOH-treated fibers were washed three to

four times with distilled water containing a few drops (5–

10 wt%) of acetic acid to neutralize the excess sodium

hydroxide. The fibers were then dried in the oven at

1058C for 24 h [6]. FTIR: O–H stretching (3,300 cm21),

C–H asymmetric and symmetric stretching (2,884 cm21),

and CH2 (1,415 cm21) [28, 29].

Alkali-Silane Treatment of Sisal Fiber. The alkali-

treated fibers were immersed in the APTES solution (the

solution contains 6 wt% of silane mixed with ethanol/

water in a ratio of 8:2). The fibers were allowed to

endure for 1 h; subsequently, the ethanol-water was

drained out and the fibers were air dried for 30 min, fol-

lowed by drying in the oven [8, 11, 30, 31]. The interac-

tion of APTES with natural fibers mainly goes through

the following steps, as presented in Scheme 1. FTIR: O–

H stretching (3,295 cm21), C–H asymmetric and symmet-

ric stretching (2,844 cm21), CH2 (1,406 cm21), NH2

stretching (3,200 cm21), Si–O (820 cm21), and Si–O–Si

(1,239 cm21) [28, 29].

Fabrication of UP-Toughened Epoxy Systems and ItsSisal Fiber-Reinforced Composites

The composite specimens were made with three differ-

ent weight ratios of treated and untreated sisal fibers (10,

20, and 30 wt%). The mold was coated with a thin layer

of silicon spray, which acts as a good releasing agent.

The fibers were arranged in unidirectional orientation in a

mold measuring 25 cm 3 25 cm 3 0.3 cm in the form of

long fibers of an L/D ratio of 5:0.002.

The UP-toughened epoxy blends were fabricated by

using compression molding. A fixed amount of epoxy

resin (100 g) and varying amounts of UP resin (5, 10,

and 15 g) were mixed together in a 200-mL beaker at

room temperature with a mechanical shear stirrer for

about 2 h to give a homogeneous liquid. Then, the mix-

ture was sonicated for 30 min on pulse mode (15 s on/

15 s off). The external cooling system was employed by

submerging the beaker containing the mixture in an ice

bath to avoid the temperature rising during the sonication

process. Once the process was completed, a calculated

amount of hardener (20 wt%)/catalyst (2 wt%)/promoter

(2 wt%) was added to the above-sonicated solution [32,

33]. The solution was mixed thoroughly for 10 to 15 min

and then poured over the fibers, evenly passed and

pushed down with a roller to eliminate the air bubbles,

and silicon spray was used as a mold-releasing agent. The

fabrication process steps of the sisal fiber-reinforced UP-

toughened epoxy composites are depicted in Fig. 1 [34].

The mold was kept at room temperature for 24 h, the at

808C for 4 h, and postcuring was carried out at 1108C for

2 h and at 1408C for 15 min at 70.32 kg/cm2 pressure in

a compression machine. After curing, the samples were

removed from the mold and characterized for different

properties [35–37].

The same procedure was adopted to obtain UP-

toughened epoxy samples, except the addition of fibers

into UP/epoxy solution. FTIR of epoxy/UP: OH stretching

(3,541 cm21), methyl (–CH3) asymmetric stretch

(2,968 cm21), C5O stretching (1,724 cm21), asymmetric

and symmetric C–O–C stretching of aromatic ester (1,238

and 1,125 cm21), aromatic C5C (1,444 cm21), and C–

O–C oxirane group of epoxy peaks (908 and 823 cm21)

[28, 29].

CHARACTERIZATION TECHNIQUES

Dynamic Mechanical Properties

Specimens of UP, epoxy, UP-toughened epoxy, and

untreated and treated reinforced UP-toughened epoxy

were measured at a frequency of 1 Hz and amplitude of

615 mm in the three-point bending mode using a

2 JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2015 DOI 10.1002/vnl

dynamic mechanical analysis (DMA) instrument (Q800;

TA Instruments, New Castle, DE) as per ASTM-D-5026

at a heating rate of 108C per min from 30 to 3008C. The

glass transition temperature (Tg) was calculated by the

maximum peak obtained from Tan delta versus the tem-

perature plot. The samples having dimensions of 63.5 mm

3 12.7 mm 3 3 mm were cut in a parallel direction with

respect to fibers.

SCHEME. 1. Possible reaction mechanism during the silane treatment of sisal fiber.

FIG. 1. Process flow chart for the fabrication of (sisal fiber)-reinforced UP-toughened epoxy composites.

DOI 10.1002/vnl JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2015 3

Interfacial Properties

Fourier-Transform Infrared Spectroscopy. FTIR

spectra of sisal fibers; uncured resin systems (UP, epoxy,

and epoxy/5 wt% UP) were recorded by an Agilent Cary

630 FTIR spectrometer (USA) with the attenuated total

reflectance technique.

Nuclear Magnetic Resonance Spectroscopy. The pro-

ton nuclear magnetic resonance (1H-NMR) spectra of the

epoxy, UP, and epoxy/UP samples were recorded on a

Bruker Avance III 500 MHz NMR spectrometer (MA).

Tetramethylsilane was used as internal standard, and

CDCl3 was used as solvent.

Scanning Electron Microscopy. The surface morphol-

ogy of the impact-tested samples and fibers was per-

formed by using a scanning electron microscope

(EVOMA 15; Carl Zeiss SMT, Germany). Prior to analy-

sis, all the fractured surfaces of samples were coated with

palladium using a sputtering system to eliminate electric

charging during SEM analysis.

Mechanical Properties

Tensile Testing. Tensile strength and modulus of

untreated and treated fibers were determined by using a

Universal Testing Machine (InstronVR 3382; Bucks, UK)

as per ASTM-D-3379 at 2 mm/min crosshead speed,

gauge length of 40 mm, and fiber length of 50 cm. The

blends and composites with dimensions of 200 mm 3

25 mm 3 3 mm were analyzed as per ASTM-D-5083

using the same Universal Testing Machine at a crosshead

speed of 5 mm/min and a gage length of 50 mm.

Flexural Testing. Specimens with dimensions of

127.5 mm 3 25 mm 3 3 mm were taken for flexural

tests under a three-point bending using the Universal

Testing Machine, in accordance with ASTM-D-790 at a

crosshead speed of 2 mm/min and a span length of

50 mm.

Impact Testing. The unnotched Izod impact strength of

samples was determined from specimens having dimen-

sions of 63.5 mm 3 12.7 mm 3 3 mm, using an impac-

tometer (Tinius Olsen, Inc, Horsham, PA) as per ASTM

D 256. The mechanical properties were conducted at

23 6 58C and 55% RH, using five specimens for each

test; the data provided were taken from an average of

five specimens.

Thermal Properties

Thermogravimetric Analysis. TGA thermograms of

samples were obtained by using a thermogravimetric ana-

lyzer (Q 50; TA Instruments) as per ASTM E 1868, and

samples of 10 mg or smaller were secured from 308C to

9008C at a heating rate of 58C/min in a nitrogen atmos-

phere, corresponding to initial and degradation tempera-

ture; % char was noted.

Water Absorption Test

The water absorption behavior of the samples is eval-

uated (according to Eq. 1) as per ASTM D 570. The

cured sisal fiber-reinforced UP-toughened epoxy compo-

sites with dimensions of 60 mm 3 60 mm 3 3 mm were

immersed in distilled water for 11 weeks. Specimens

were removed from the water and wiped with tissue paper

in order to remove the surface water on the sample and

weighed to an accuracy of 0.001 g taken for each interval

of 24 h

Wg %ð Þ5 Ww2Wd

Wd

� �3100 (1)

where Wg is the weight gain; Ww is the wet sample

weight; and Wd is the dry sample weight.

RESULTS AND DISCUSSION

1H Nuclear Magnetic Resonance (NMR) Spectroscopy

1H-NMR spectra of epoxy, UP, and epoxy/UP systems

are shown in Fig. 2. UP spectra showed signal at 0.89–

1.5 ppm, 5.13–5.3 ppm (methylene and methyl groups of

FIG. 2. 1H-NMR spectra of UP, epoxy, and epoxy/UP systems.


propylene glycol), 3.65–4 ppm (methylene groups

attached to ether oxygen atom of diethylene glycol), 4–

4.6 ppm (methylene groups attached to ester oxygen atom

of diethylene glycol), 6.65–6.98 ppm (unsaturated vinyl

group of maleic anhydride), and 7.2–7.9 ppm (orthoph-

thelic acid). Similarly, epoxy/UP signals were found for

propylene glycol at 5.25 and 5.78 ppm and orthophthelic

acid at 7.3 and 7.4 ppm with the incorporation of UP in

the epoxy monomer. The other peaks of UP disappear

because a lower amount of UP (5 wt%) added within the

toughened epoxy system is much less. It confirmed that

the reaction between epoxy and UP was favorable [28,

29].

Scanning Electron Microscopy

Scanning Electron Microscopy Analysis of Fibers.

SEM micrographs of untreated sisal fiber (UTF), alkali

treated sisal fiber (ATF), and alkali-silane treated sisal

fiber (ASTF) are shown in Fig. 3a–c. It is noted that, in

the case of UTF, there are traces of impurities along the

longitudinal surface of the fiber. As can be seen from

Fig. 3b, c, the chemical treatment tends to remove wax

and other essences on fiber surface. The ATF were dis-

played as fibrillation and a coarser morphology, which

is clearly visible from the microporous structure on the

fiber. Unlike the UTF, the ATF shows a rough surface

because of the removal of hemicelluloses and pectin

groups that render uniform arrangement of the less dense

and rigid fibrils in the tensile direction. A similar phe-

nomenon was observed in the case of ASTF, which dem-

onstrated the presence of macrospores, fibrillation, as

well as rougher fiber surfaces. It is assumed that the

moisture in the fiber hydrolyzes the silanes to form sila-

nols, which finally forms covalent bonds or H-bonds

with the OH group of sisal fiber [6, 28, 29].

Scanning Electron Microscopy Analysis of the Blends.

The representative SEM micrographs of epoxy, UP, and

epoxy/UP systems are presented in Fig. 4a–c. The frac-

tured surface of the epoxy system is a smooth glassy and

homogenous microstructure with cracks in different

planes, indicating the brittle nature and poor impact

strength of the epoxy system (Fig. 4a). As can be seen

from Fig. 4b, the fractured micrograph of the UP resin is

less wide, and continuous rapid crack propagation has

taken place along the axis of crack growth. The fracture

surface is smooth with low long narrow hilltops. Con-

versely, the fractured surface of the UP system shows a

FIG. 3. SEM micrographs of (a) untreated, (b) 2% alkali treated, and (c) (alkali-silane)-treated fibers. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]

FIG. 4. SEM micrographs of (a) UP, (b) epoxy, and (c) epoxy/UP systems. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]


http://wileyonlinelibrary.com


homogenous structure. The miscible characteristics of the

epoxy/UP system are observed in Fig. 4c, suggesting

strong adhesion between the epoxy and UP and also

reducing the brittle nature of the matrix [26–29].

Scanning Electron Microscopy Analysis of Fiber-

Reinforced Epoxy/UP Composites. Figure 5a shows

the fractured surface of untreated sisal composite, indicat-

ing a clear pullout of fibers without any resin matrix

adhering to the fiber. This indicates poor adhesion

between the UTF and the matrix. The micrographs of

ATF-reinforced composites, shown in Fig. 5b, also show

pullout fibers along with the existence of cracks at the

broken surface. The interface is minimum and strong

adhesion exists between the fiber and matrix. Further-

more, the micrographs of (alkali-saline)-treated sisal com-

posites, depicted in Fig. 5c, show that there is a

separation of swelled fibrils of fracture lines and matrix

cracking. The fibers are pulled out together with the

matrix, which reveals improved adhesion at the interface

[9, 10, 37].

Mechanical Properties

Mechanical Properties of Sisal Fibers. The diversity

of mechanical properties of UTF, ATF, and ASTF is

shown in Table 1. It is evident that the alkali treatment

improves the mechanical properties of the fiber nearly

21% in the tensile strength and 23% in Young’s modulus,

obtained at 2 wt% of NaOH concentration as compared

with UTF. This behavior is probably due to the alkali

treatment removing the impurities on the fiber surface,

which leads to fibrillation of sisal fiber, thereby produc-

ing a rough surface topology of improved tensile proper-

ties as compared with UTF. However, beyond 2 wt% of

NaOH concentration, the fiber strength was reduced

because of delignification. Higher alkali concentration (15

wt% of NaOH) results in the fiber losing the cementing

material (i.e., lignin), thus decreasing the tensile strength

and tensile modulus to 23% and 3% as compared with 2

wt% ATF. On the contrary, silane treatment on the ATF

found substantial improvement in the tensile properties.

Tensile strength of ATF increased from 503 MPa to 531

MPa and a tensile modulus from 17.3 GPa to 19.6 GPa

with the silane treatment. This phenomenon is attributed

to the establishment of H-bonds as well as covalent bond-

ing between APTES and sisal fiber, which makes the

fiber surface more hydrophobic and rougher than the

alkali treatment.

Mechanical Properties of Blends. The mechanical

properties of epoxy, UP, and epoxy/UP are presented in

Table 2. It is evident that incorporating UP resin into the

epoxy acts as a toughening agent and significantly

increases the mechanical properties as compared with that

of the unmodified resin; the detailed toughened mechanism

is shown in Scheme 2. The toughened epoxy at 5 wt% UP

resin displayed the optimum increase in tensile strength

and modulus of 16% and 34%, respectively, as compared

with the unmodified resin. A similar increase in the flex-

ural strength and modulus of 19% and 25% was observed.

This behavior is due to the formation of hydrogen bonding

between the epoxy and UP. Impact strength of epoxy/5%

UP is 37% higher than that of the unmodified resin (Table

2). It is concluded that the UP resin helps to absorb the

impact energy and prevents crack propagation during the

fracture. Furthermore, addition UP resin (beyond 5 wt%)

in the epoxy reduces the mechanical properties that con-

firm the plasticization effect in the UP-epoxy system [38,

39]. The toughened epoxy at 5 wt% of UP has been

TABLE 1. Mechanical properties of untreated and treated sisal fibers.

Type of

treatment for

sisal fiber

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Elongation

at break

(%)

Untreated 393 6 26 14.1 6 0.11 1.5

1% alkali treated 435 6 32 16.8 6 0.05 1.76

2% alkali treated 503 6 15 17.3 6 0.04 2.86

5% alkali treated 427 6 25 16.5 6 0.05 2.21

10% alkali treated 386 6 21 15.3 6 0.07 2.3

15% alkali treated 320 6 11 13.7 6 0.07 1.35

Alkali-silane treated 531 6 30 19.6 6 0.14 3.45

FIG. 5. SEM micrographs of (a) untreated, (b) alkali treated, and (c) (alkali-silane)-treated (sisal fiber)-reinforced UP-toughened epoxy composites at

30 wt% fiber content. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]



optimized based on the mechanical properties and was con-

sidered a base matrix for further studies.

Mechanical Properties of Sisal Fiber-Reinforced

Epoxy/UP Composites. The mechanical properties of

sisal fiber-reinforced epoxy/UP composites are depicted

in Table 3. It is evident that the mechanical properties of

epoxy/UP increases with the incorporation of both treated

and untreated sisal fibers from 10 to 30 wt%. The compo-

sites prepared at 30 wt% untreated and treated sisal fibers

exhibited optimum mechanical performance; beyond that,

there was deterioration in the properties (Table S1). This

behavior reveals the agglomeration in the composite

because of inefficient interface between fiber and matrix,

which leads to the development of microcracks, thus

resulting in weak mechanical strength.

Comparing the mechanical performance of ATF com-

posites with the UTF samples, there is an enhancement in

tensile strength and tensile modulus and a decrease in

impact strength to 29%, 9%, and 14%. A similar incre-

ment in flexural strength (9%) and flexural modulus

(14%) is observed. It reveals that the alkali treatment

improves the fibrillation in fibers, which thereby increases

the surface contact with the matrix, thus contributing to

enhanced stress transfer at the interface. Similarly, it is

assumed that alkali treatment improves the surface adhe-

sive characteristics of the fiber by removing the natural

and artificial impurities.

The ASTF-reinforced composites displayed an opti-

mum mechanical performance as compared with all other

systems. The increase in tensile strength (34%), tensile

modulus (17%), flexural strength (14%), and flexural

modulus (17%) of ASTF-reinforced UP-toughened epoxy

composites was observed as compared with the UT com-

posites and 6% diminution in impact strength at 30 wt%

fiber loading. This behavior is due to the silane molecule

reacting with suitable bonding groups of UP-toughened

epoxy system. Thus, chemical reactions occur between

sisal fiber, matrix, and silane-coupling agent. Interfacial

properties are improved by the resultant chemical bond-

ing. The coupling mechanism of APTES-grafted fiber

with UP-toughened epoxy composites has been proposed

by several authors [30, 40], as summarized in Scheme 3.

The average impact strength of the natural composites

is mentioned in Table 3 as a function of fiber content.

Impact strength results indicate that the fiber surface

treatments do affect the impact energy as expected; the

firm interfaces lead to reduced impact properties. It is

evident that the impact energies of the UT-reinforced UP-

toughened epoxy composites are considerably higher than

that of ATF- and ASTF-reinforced composites. This is

due to the fact that chemical treatments improve the

roughness and adhesive characteristics of sisal fiber sur-

face by partial removal of wax, hemicelluloses, and lig-

nin. Examination of the fracture surfaces indicates that

fiber and matrix fractures and fiber pullout are the pri-

mary failure mechanisms. UT-reinforced composites have

more pullout fibers compared with ATF- and ASTF-

reinforced composites. This enables the untreated compo-

sites to absorb more impact energy during fracture.

Thermogravimetric Analysis

Thermogravimetric Analysis of Untreated and Treated

Fibers. The thermal stability of sisal fibers as a function

of alkali and alkali-silane treatments was examined by

TABLE 3. Mechanical properties of fiber-reinforced composites.

(Sisal fiber)-reinforced

epoxy/UP composites

Fiber

loading

(%)

Tensile

strength

(MPa)

Tensile

modulus

(GPa)

Flexural

strength

(MPa)

Flexural

modulus

(GPa)

Impact

strength

(J/m)

Untreated 0 65 6 2 2.64 6 0.02 119 6 3 3.56 6 0.02 207 6 4

10 69 6 2 2.99 6 0.04 128 6 4 5.12 6 0.03 246 6 3

20 75 6 2 3.4 6 0.03 141 6 2 6.33 6 0.03 298 6 2

30 89 6 2 3.83 6 0.03 158 6 3 7.46 6 0.03 326 6 3

Alkali treated 10 73 6 1 3.16 6 0.03 133 6 4 6.06 6 0.03 239 6 2

20 88 6 2 3.8 6 0.04 152 6 5 7.14 6 0.03 247 6 1

30 115 6 2 4.2 6 0.06 172 6 3 8.5 6 0.02 280 6 2

Alkali-silane treated 10 76 6 1 3.2 6 0.03 138 6 2 6.2 6 0.03 243 6 3

20 93 6 2 3.91 6 0.03 159 6 3 7.33 6 0.03 266 6 2

30 119 6 2 4.48 6 0.04 180 6 3 8.74 6 0.03 304 6 2

TABLE 2. Mechanical properties of UP, epoxy, and epoxy/UP systems.

Epoxy/UP

composition

Tensile strength

(MPa)

Tensile modulus

(GPa)

Flexural strength

(MPa)

Flexural modulus

(GPa)

Impact strength

(J/m)

100/0 56 6 2 1.97 6 0.01 100 6 4 2.83 6 0.02 151 6 5

0/100 32 6 4 0.72 6 0.02 45 6 2 1.31 6 0.02 101 6 3

95/5 65 6 2 2.64 6 0.02 119 6 3 3.56 6 0.02 207 6 4

90/10 57 6 4 1.9 6 0.02 97 6 4 2.94 6 0.05 214 6 5

85/15 44 6 3 1.46 6 0.03 53 6 1 1.56 6 0.02 180 6 4


using TGA/differential thermogravimetric (DTG) curves.

The weight loss as a function of temperature for untreated

and treated sisal fibers exhibits a three-stage decomposi-

tion, as shown in Fig. 6a, b. The ATF and ASTF exhib-

ited a higher thermal decomposition temperature as

compared with UTF. The weight loss of the samples from

room temperature to 150oC is related to loss of moisture

and other volatiles. The alkali-silane treatment results in

the partial extraction of hemicelluloses, which are highly

hydrophilic and considered the greatest factor responsible

for water absorption in lignocellulosic fibers.

It noted that, up to 245oC, untreated and treated sisal

fibers exhibit thermal stability, and the mass loss of the

samples was small. Beyond 300oC, the samples show a

drastic loss in mass that is due to decomposition of cellu-

lose and hemicelluloses. Above 370oC, degradation

occurs as a result of delignification of fibers. Further-

more, it is found that the initial, 50%, and the final

decomposition temperature and weight of the residue

increased with silane treatment (Table S2). Maximum

weight loss of UTF, ATF, and ASTF is observed at

515oC (weight loss is 98.82%), 552oC (weight loss is

98.93%), and 752oC (weight loss is 86%), respectively. It

reveals that the ASTF improves the thermal resistance as

compared with both ATF and UTF, wherein 86% weight

loss was observed at 600oC. This shows that alkali-silane

treatment was efficacious in improving the thermal stabil-

ity of the sisal fibers. Furthermore, it confirms that the

treatment reduces the cementing material and removes the

moisture on natural fibers, which makes the fiber more

thermally stable than that of UT and ATF.

TGA of UP, Epoxy, and Epoxy/UP Systems. From

Fig. 7a, b, it can be concluded that there was no signifi-

cant weight loss seen in the UP-toughened epoxy matrix

due to the thermal stability decreasing as effective cross-

linking density decreases. The initial, 50%, and final

weight decomposition temperatures of epoxy resin

SCHEME. 3. Possible reaction between fiber and epoxy/UP system.

SCHEME. 2. Possible toughened mechanism between UP and epoxy resins.


decrease with the addition of UP resin (Table S3). Maxi-

mum weight loss for UP, epoxy, and UP-toughened epoxy

was observed at 394.6oC (weight loss is 97%), 480.4oC

(weight loss is 98.93%), and 475oC (weight loss is 86%),

respectively. Hence, it is evident that the incorporation of

UP within the epoxy does not cause an appropriate varia-

tion in the thermal stability of the system.

TGA of Untreated and Treated Fiber-Reinforced

Epoxy/UP Composites. TGA thermograms of sisal

fiber-reinforced composites are shown in Fig. 8a, b. The

initial weight loss up to 240oC indicates the removal of

moisture and other volatiles from the fiber in the compo-

sites. The weight loss from 240 to 380oC is primarily due

to degradation of the composites. The incorporation of

both treated and untreated sisal fibers increases thermal

stability in the UP-toughened epoxy system. Tmax

increased from 475 to 525oC (Table S4) with the incorpo-

ration of UTF within the matrix that additionally

increased to 570.8oC with the incorporation of ATF to

the matrix, thus indicating that the effective adhesion

between the fiber and matrix is good as compared with

the UTF. The ASTF-reinforced composites show opti-

mum thermal stability as compared with the UTF and

ASTF-reinforced composites. The initial, 50%, and the

final decomposition temperature increased in the case of

ASTF-reinforced composites, which further confirm

strong interfacial balance because of the establishment of

hydrogen and covalent bonds. Furthermore, the percent-

age of char in the case of alkali-silane-treated composite

was tuned to 13.56% and observed the improvements in

the flame retardance of the system. Thermogravimetric

studies revealed that the ASTF-reinforced composites

show more thermal resistance in comparison with ATF-

and UTF-reinforced composites, where 86% weight loss

was observed at 700oC.

Dynamic Mechanical Analysis

Storage Modulus. The variation in storage modulus

(E’) of UP-toughened epoxy and its treated and untreated

composites as a function of temperature is presented in

Fig. 9a. It is evident that E’ decreases with an increase in

temperature for all systems, because of the increased

chain mobility, which lowers the crosslink density of the

polymer [26, 28]. The UP-toughened epoxy exhibited an

optimum magnitude of E’ of 3.88 GPa at 30oC that

FIG. 7. (a) TGA, and (b) DTG thermograms of UP, epoxy, and epoxy/

UP systems.

FIG. 6. (a) TGA, and (b) DTG thermogram curves of treated and

untreated fibers.


drastically decreased beyond 50oC, thus representing the

Tg region of the matrix. Furthermore, the E’ substantially

increases with the incorporation of treated and untreated

sisal fibers in a matrix over the entire experimentally

investigated range of temperature. This behavior reveals

the hindrance in the chain mobility of the matrix polymer

caused by the presence of the fibers that increases the E’

value, which further improved wettability of the fibers

within the matrix. It contributed to efficient stress transfer

from the fibers to the matrix at the interface. The DMA

results can also be elaborated on from the mechanical

findings depicted in the earlier section. The magnitude of

E’ in the samples varies in the following order:

E0Epoxy=UP < E0UTF composite < E0ATF composite

< E0ASTF composite

Tan delta (Tan d). The variation in tan delta of UP-

toughened epoxy and the treated and untreated composites

are displayed in Fig. 9b. It is observed that UP-toughened

epoxy displays Tg around at 101oC, which is increased to

124oC on reinforcement with untreated sisal fibers. Fur-

thermore, the Tg additionally increases in the case of ATF-

and ASTF-reinforced composites to 130oC and 138oC,

respectively. This effect can be related to an enhancement

of the matrix/fiber interaction caused by the fibers’ surface

treatments. In addition, the intensity of tan delta in the

polymer matrix reduced drastically in the composites. This

behavior is possibly due to segmental immobilization of

the matrix chains in the presence of sisal fiber. These facts

are well in agreement with increased E0 magnitude in the

fiber-reinforced composites.

Water Absorption Test

The water absorption behavior was determined in

terms of weight increase for composite specimen

immersed in water at 23oC as per ASTM D 570 [41]. The

percentage weight gain of all samples with respect to the

square root of time is shown in Fig. 10. The order of

water uptake is Untreated composite>Alkali-treated

composite>Alkali-silane-treated composite>Epoxy 5

Epoxy/UP>UP. The water resistance increases with the

addition of treatments to the fibers because of their

hydrophilic nature and to decrease the amount of either

impurities or hemicelluloses. The ASTF-reinforced UP-

toughened epoxy composites gave better water resistance

FIG. 9. (a) Temperature-dependence storage modulus of the compo-

sites; and (b) temperature-dependence tan delta of the composites.

FIG. 8. (a) TGA, and (b) DTG thermograms of (sisal fiber)-reinforced

composites having 30% fiber content.


as compared with the UTF- and ATF-reinforced compo-

sites. This behavior is due to the hydrophobic nature of

Si–O–Si linkage and to the complete removal of hemicel-

luloses that, leading to a better adhesion between the fiber

and matrix, reduces interfacial voids.

CONCLUSIONS

The present study confirmed that the UP resin (5 wt%)

has been used to modify the brittle epoxy matrix without

diminution of mechanical properties.

Alkali and alkali-silane treatments improve the interfa-

cial bonding between the sisal fibers and UP-toughened

epoxy. Reasonable enhancements of mechanical proper-

ties were observed with alkali-silane-treated sisal fibers as

compared with 2% alkali-treated sisal fibers. This behav-

ior is due to the grafting of the silane-coupling agent onto

the alkali-treated fiber surface, which changes the mor-

phology of the fibers by removing hemicelluloses and

moisture, forming silanol, and facilitates the formation of

H-bond or covalent linkage with an OH group of fiber at

the interface. It was further confirmed from TGA studies

that alkali and alkali-silane-treated sisal fibers are ther-

mally more stable than untreated fibers. The SEM of the

treated fibers revealed that the fiber became porous and

fibrillated by alkali and alkali-silane treatment.

It is observed that the mechanical properties of treated

fiber-reinforced UP-toughened epoxy composites

increased as compared with the untreated fiber-reinforced

UP-toughened epoxy composites with respect to 10, 20,

and 30 wt% fiber loading. Alkali-silane-treated fiber-

reinforced composites have proved that there is an

enhancement in mechanical properties as compared with

the alkali-treated fiber-reinforced composites. This is due

to the alkali-silane-treated fibers having more surface area

compared with untreated and alkali-treated fibers, which

increase the adhesion between the fibers and hydrophobic

matrices and increase the mechanical properties. How-

ever, there is a slight decrease in the impact strength of

the composites as compared with the untreated samples,

which are primarily due to strong interfaces that lead to

reduced impact properties. It was further confirmed by

TGA studies that alkali-silane-treated sisal fiber-rein-

forced composites are thermally more stable than

untreated and alkali-treated fiber-reinforced composites.

The composites possess good dynamic mechanical behav-

ior at dry conditions as is suggested by the DMA analy-

sis. The alkali-silane-treated fiber-reinforced UP-

toughened epoxy composites show better water-resistant

performance because of the hydrophobic nature of Si–O–

Si linkage.

ACKNOWLEDGMENTS

The authors acknowledge the financial support under

Center of Excellence for Green Transportation Network

(CoE-GREET) sponsored by the Department of Chemicals

and Petrochemicals, Ministry of Chemicals and Fertilizers,

Govt. of India.

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