"Structure and long term properties of polyethylene monofilaments for artificial turf

applications"

"Structuur en lange termijn eigenschappen van polyetheen monofilamenten voor

kunstgrastoepassingen"

Blerina Kolgjini

Promotoren: Prof. dr. Paul Kiekens Prof. dr. ir. Gustaaf Schoukens,

Proefschrift ingediend tot het behalen van de graad van

Doctor in de Ingenieurswetenschappen: Textielkunde

Vakgroep Textielkunde

Voorzitter: Prof. dr. Paul Kiekens

Faculteit Ingenieurswetenschappen en Architectuur

Academiejaar 2011-2012

Legal Information

Supervisors

Prof. dr. Paul Kiekens

Department of Textiles

Faculty of Engineering

Ghent University (UGent)

Prof. dr. ir. Gustaaf Schoukens

Department of Textiles

Faculty of Engineering

Ghent University (UGent)

Members of the Examination Committee

Prof. Dr. Ir. Karen De Clerck UGent

Prof. dr. Paul Kiekens UGent

Prof. dr. ir. Gustaaf Schoukens UGent

Prof. dr. ir. Marc Verhaege UGent

Prof.dr. ir. Genti Guxho UPT

Prof. dr.ir. Ludwig Cardon Hogeschool Gent

Contact

Blerina Kolgjini

Department of Textiles

Faculty of Engineering and Architecture,

Technologiepark 907 - 9052 Zwijnaarde, Belgium

tel. +32 (0) 9 264 57 53 fax. +32 (0) 9 264 54 06

Funding BASILEUS project [2008-1799/001 – 001 MUN ECW]

TABLE OF CONTENTS

ENGLISH SUMMARY……………………………….......................................

NEDERLANDSE SAMENVATTING ………………………...........................

CHAPTER 1

Introduction…………………………………………….…………………….….

1.1. History of artificial turf…………………………………….………..........

1.2. Advantages and disadvantages of artificial turf…….………............

1.3. Application of artificial turf……………………………………………….

1.4. Goal and Outline.…………….…………………………………………….

CHAPTER 2

Artificial turf and components………………………………………………..

2.1. Introduction…………………………………………………………………

2.2. Pile layer, Polymers for monofilaments ………………………………

2.3. Manufacturing of turf fibres ………….………………………………….

2.4. Cross section of fibres/monofilaments………………………………..

2.5. Production line of artificial turf………………………………………….

2.6. Back-coating and finishing………………………………………………

2.7. Infill materials…………………………………………………….…………

2.8. Supporting layers, shock pad……………………………………….......

2.9. Supporting Layer – Subsoil……………………………………………...

CHAPTER 3

The test methods for artificial turf…………………………………………...

3.1. Test and recommendations for physical condition………………….

3.2. Player surface interaction………………..............................................

3.3. Ball surface interaction…………………………………………………..

CHAPTER 4

Test methods to measure the resilience of single monofilaments……

4.1. Introduction…………………………………………………………….......

4.2. Theoretical analysis for bending of monofilaments…………….…..

4.3. Test methods for resilience of pile layer measured in whole

system/artificial turf……………………………………………………….

4.4. Experimental set up for static bending test…………………………..

4.5. Mechanical properties……………………………………………..……..

4.6. Bending deformation of the monofilaments at higher

temperatures ………………………………………………………………

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21

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26

27

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37

42

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49

55

63

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4.7. Discussion…………………………………………………………………..

4.8. Conclusions ………………………………………………………………..

CHAPTER 5

Influence of stretching on bending behaviour…………………………….

5.1. Introduction……………………………………………………………..….

5.2. Experimental setup ……………………………………………………….

5.3. Characterization techniques…………………………………………….

5.4. Results and discussion ………………………………………………….

5.5. Conclusions…………………………………………………………………

CHAPTER 6

Effect of heat treatment on properties of monofilaments, bending

behaviour…………………………………………………………………………

6.1. Introduction…………………………………………………………………

6.2. Experimental set up……………………………………………………….

6.3. Results and discussion…………………………………………….…….

6.4. Conclusions…………………………………………………………………

CHAPTER 7

Three-phase characterization of uniaxially stretched linear low

density polyethylene…………………………………………………...………

7.1. Introduction…………………………………………………………..…….

7.2. Results and discussion…………………………………………………..

7.3. Third-Phase Characterization…………………………………………...

7.4. Mechanical properties related to the morphology ………………….

7.5. Resilience and deformation recovery of the different obtained

samples……………………………………………………..……………….

7.6. Conclusions……………………………………………………………...…

CHAPTER 8

Conclusions and recommendations for future research………………..

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i

ENGLISH SUMMARY

Introduction

Artificial turf as an alternative for natural grass has become more and more popular over the

last decades for sport applications. Several types of sports on different levels of game are

performed on these surfaces, such as hockey, tennis, rugby and football, both for training

purposes and for competitive games.

Synthetic surfaces were developed because of the need for surfaces which are independent

of climate, show less wear, allow a higher number of playing hours and have less operational

and maintenance costs.

The development of artificial surfaces started with the so-called first generation, consisting of

short-pile high density ‘full synthetic’ carpets without infill, and goes back to 1966. The

second generation of artificial turf was developed in the late 1980’s, with longer

monofilaments and sand infill materials and was followed later on by the third generation in

the 1990’s, consisting of carpets with longer fibres and with sand and rubber infill. This

shows the continuous attempts to improve the performance of these fields in terms of

interaction between players and surfaces and between ball and surfaces.

The developments include new fibre compounds, new infill materials and new techniques of

tufting and fixing the pile layer. But apart from advantages and the progress which was

made, concerns about different ball behaviour still exist.

However, the standards, in terms of quality, player safety and ball behaviour, do exist and

these fields have to be tested strictly and fulfil the parameters decided by the sport’s

governing bodies. New installed fields show very good results, comparable to the best

natural fields. Problems of quality still remain and they do show up after some months of

use. This is a very complex problem and involves the production companies of these fields.

The aim of this work is to conduct fundamental research into the behaviour of

monofilaments and their morphology structure developed at different stages of production

with the aim to improve the mechanical properties of the product under different conditions.

This includes firstly a theoretical analysis in understanding the possible factors influencing

the bending behaviour of monofilaments, understanding the possible deformation and

analysing the existing test methods to evaluate the long term properties of the pile layer at a

very early stage (before producing the final product, artificial turf). This leads to suggestions

for artificial turf manufacturers; how they can improve their products with the goal to offer

qualitative surfaces for the players, more specifically by improving for example the ball roll

behaviour as one of the very significant properties in fulfilling the quality demands required

by FIFA.

ii

In chapter 1, 2, 3 and 4, the components and test methods of investigating the bending

behaviour of monofilaments are explained. In chapter 5, 6 and 7, the influence of process

parameters on the bending behaviour of single monofilaments is examined. In chapter 7, the

influence of the third phase structure on the bending behaviour of single monofilaments is

examined.

Artificial turf and components

In chapter 2, the components of artificial turf and other sub-layers for football use are

explained starting from the top layer and finishing with the backing layer. The importance of

this chapter is in understanding the relation between the material parameters and the

properties of the end products in terms of the interaction between ball surface and player

surface, by emphasising the ball roll behaviour.

Test methods for artificial turf

Chapter 3 focuses on the standard test methods, used for different kinds of artificial fields.

This is important not only for general information of different test methods and their

improvements but also explains which components influence what and how. The

explanations for all the elements of artificial surfaces are as a parenthesis for chapter 4.

Test methods to measure the resilience of single monofilaments

In chapter 4, firstly a theoretical explanation of the main elements influencing the bending

behaviour of a single monofilament is given, this is followed by explaining the test methods

to measure the pile layer resilience and the resilience of single monofilaments. The detailed

explanation about the advantages and disadvantages of each test method shows the

importance of developing new practical test methods to perform measurements at elevated

temperatures. The new test method is evaluated by correlating results with the existing test

methods.

Influence of stretching on bending behaviour

Chapter 5 deals with the influence of processing production parameters on the mechanical

behaviour. The monofilaments produced in lab conditions and on the pilot line with different

stretching ratios are tested and their values are correlated with the cold draw ratio and the

morphology structure developed during production.

Influence of heat treatment on bending behaviour

In chapter 6, as a further step in the production, the influence of heat treatment under

different conditions on the behaviour of the monofilaments is investigated. The importance of

a stable product in combination with the improved bending behaviour is the main focus of

this chapter. Therefore, different temperatures and times of heat treatment are considered.

Third phase structure and bending behaviour

Chapter 7 presents a detailed analysis of the developed structures at different production

stages of the monofilaments. Differently from the classical ways of correlating the behaviour

iii

of polymers with crystalline and amorphous phases, this chapter focuses on the existence of

the intermediate or 3rd

phase. Combining measurements of Differential Scanning

Calorimetry, Raman and X-ray makes it possible to investigate the structure of the third

phase. The detailed information about the production parameters and the resulting structure

is an important step for the producers of the filaments and the carpet.

A new insight into the three phase morphology is obtained by combining the different

analytical tools and a new correlation between the different phases of the polyethylenes is

developed.

Conclusions and recommendations for future research

In chapter 8, the main conclusions of this work are summarised and recommendations for

future research are formulated.

iv

SAMENVATTING

Inleiding

Kunstgras als alternatief voor natuurlijk gras geniet een toenemende populariteit bij

sporttoepassingen. Kunstgras wordt gebruikt voor verschillende sportdisciplines op

verschillende niveaus (in een professionele context en op nationale en internationale

trainingen); voorbeelden zijn hockey, tennis, rugby en voetbal.

Synthetische velden werden ontwikkeld omwille van de behoefte aan velden die niet

onderhevig zijn aan het klimaat, minder slijtage vertonen, een groter aantal speeluren

toelaten en minder operationele en onderhoudskosten met zich meebrengen.

De ontwikkeling van kunstvelden begon in 1966 met de zogenaamde eerste generatie die

bestond uit kortpolige, zeer dichte ‘volledig synthetische’ tapijten zonder (in)vulmateriaal. De

tweede generatie kunstgras werd ontwikkeld op het einde van de jaren ’80 met langere

monofilamenten en zand als vulmateriaal. In de jaren ’90 volgde de derde generatie die

bestond uit tapijten met langere vezels en met zand en rubber als vulmateriaal. Dit illustreert

de voortdurende pogingen om de performantie van deze velden te verbeteren, met name

voor wat de interactie tussen de spelers en het veld en tussen de bal en het veld betreft.

De ontwikkelingen omvatten nieuwe vezelsamenstellingen, nieuwe vezelmaterialen en

nieuwe tuft- en fixeertechnieken. Afgezien van de voordelen en de vooruitgang die geboekt

werd, bestaat er nog bezorgdheid over het balgedrag.

Echter, kwaliteits - en veiligheidsvoorschriften evenals bepalingen omtrent balgedrag zijn

voorhanden waarbij de velden zorgvuldig getest worden en moeten voldoen aan de

parameters die opgelegd worden door overkoepelende sportorganisaties. Nieuwe velden

vertonen zeer goede resultaten die vergeleken kunnen worden met de beste natuurlijke

velden. Kwaliteitsproblemen komen nog voor na gebruik gedurende een aantal maanden. Dit

is een ingewikkeld aspect waarbij ook de producenten betrokken zijn.

Het doel van het werk bestaat erin om fundamenteel onderzoek te verrichten naar het

gedrag van monofilamenten en hun morfologiestructuur tijdens verschillende productiefases

met het oog op het verbeteren van de mechanische eigenschappen van het product onder

verschillende omstandigheden. Dit omvat in eerste instantie een theoretische analyse van

de mogelijke factoren die invloed kunnen uitoefenen op het buiggedrag van monofilamenten,

het verkrijgen van inzicht in het ontstaan van vervormingen en het analyseren van

bestaande test methoden om lange termijn eigenschappen van de poollaag in een zeer

vroeg stadium te beoordelen (vooraleer het eindproduct, het kunstgras, ontwikkeld wordt).

Dit leidt tot voorstellen voor kunstgrasproducenten hoe de producten verbeterd kunnen

worden om kwaliteitsvolle velden te verkrijgen door b.v. het balrolgedrag te verbeteren wat

een zeer belangrijk element vormt om te voldoen aan de kwaliteitseisen van FIFA.

v

In hoofdstukken 1, 2, 3 en 4 worden de componenten en test methoden uitgelegd om het

buiggedrag van monofilamenten te onderzoeken. Hoofdstukken 5, 6 en 7 bespreken de

invloed van de procesparameters op het buiggedrag van monofilamenten. Hoofdstuk 7

behandelt de invloed van de derde fase structuur op het buiggedrag van monofilamenten.

Kunstgras en componenten

In hoofdstuk 2 worden de componenten van kunstgras en andere onderlagen voor

voetbaltoepassingen uitgelegd beginnend bij de toplaag en eindigend bij de onderste laag.

Dit hoofdstuk onderzoekt de relatie tussen materiaalparameters en de eigenschappen van

eindproducten voor wat betreft de interactie tussen baloppervlak en speeloppervlak waarbij

de nadruk gelegd wordt op het balrolgedrag.

Test methoden voor kunstgras

Hoofdstuk 3 behandelt de standaard test methoden die gebruikt worden voor verschillende

soorten kunstgrasvelden. Dit is niet enkel van belang voor informatieve doeleinden

betreffende test methoden maar verklaart ook welke componenten invloed uitoefenen

waarop en hoe dat gebeurt. De uitleg draagt bij tot hoofdstuk 4.

Test methoden om de resiliëntie van monofilamenten te meten

Hoofdstuk 4 biedt een theoretische uitleg over de belangrijkste elementen die invloed

uitoefenen op het buiggedrag van een monofilament. Vervolgens komen de test methoden

aan bod om de resiliëntie van de poollaag en de monofilamenten te meten. De

gedetailleerde uitleg over de voor- en nadelen van elke test methode wijst op het belang om

nieuwe praktische test methoden te ontwikkelen om metingen bij hogere temperaturen uit te

voeren. De nieuwe test methode wordt beoordeeld door het correleren van de resultaten met

de bestaande test methoden.

Invloed van rek op het buiggedrag

Hoofdstuk 5 behandelt de invloed van productieparameters op het mechanische gedrag. De

monofilamenten die geproduceerd werden in het laboratorium en via de pilootlijn met

verschillende verstrekkingsgraden worden getest en hun waarden worden gecorreleerd met

de cold draw ratio en de morfologiestructuur ontwikkeld tijdens de productie.

Invloed van warmtebehandelingen op het buiggedrag

Hoofdstuk 6 onderzoekt de invloed van warmtebehandelingen onder verschillende

omstandigheden op het gedrag van de monofilamenten. Het belang van een stabiel product

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in combinatie met een verbeterd buiggedrag staat centraal in dit hoofdstuk. Daartoe worden

verschillende temperaturen en warmtebehandelingen in acht genomen.

Derde fase structuur en buiggedrag

Hoofdstuk 7 biedt een gedetailleerde analyse van de ontwikkelde structuren tijdens

verschillende productiefases van de monofilamenten. Los van de traditionele manieren om

het gedrag van polymeren te correleren met kristallijne en amorfe fases, behandelt dit

hoofdstuk de intermediaire of derde fase. Het combineren van metingen via Differential

Scanning Calorimetry, Raman en X-ray maakt het mogelijk om de structuur van de derde

fase te onderzoeken. De gedetailleerde informatie over de productieparameters en de

daaruit resulterende structuur is een belangrijke stap voor de producenten van filamenten en

tapijten.

Een nieuw inzicht in de derde fase morfologie wordt verkregen door het combineren van

verschillende analytische tools en een nieuwe correlatie tussen de verschillende fasen van

de polyethenen wordt ontwikkeld.

Conclusies en aanbevelingen voor verder onderzoek

Hoofdstuk 8 vat de voornaamste conclusies van het werk samen en levert aanbevelingen

voor verder onderzoek.

vii

LIST OF PUBLICATIONS

JOURNAL ARTICLES

“Influence of stretching on the resilience of LLDPE monofilaments for application in artificial

turf”, Journal Article (A1), Blerina Kolgjini, Gustaaf Schoukens and Paul Kiekens JOURNAL

OF APPLIED POLYMER SCIENCE (2012).

“Three-phase characterization of uniaxially stretched linear low density polyethylene”,

Journal Article (A2), Blerina Kolgjini, Gustaaf Schoukens and Paul Kiekens

INTERNATIONAL JOURNAL OF POLYMER SCIENCE (2011).

“Bending behaviour of LLDPE monofilaments in function of cold drawing and composition of

the LLDPE’s” Blerina Kolgjini, Ermira Shehi, Gustaaf Schoukens and Paul Kiekens, Article

under reviewer registered, No. 3639, FIBRES & TEXTILES IN EASTERN EUROPE.

CONFERENCE PRESENTATIONS

“The bending behaviour of linear low density polyethylene monofilaments” conference (C1),

Blerina Kolgjini, Gustaaf Schoukens and Paul Kiekens, 12th AUTEX Conference,

proceedings oral presentations (2012).

“Influence of third phase structure on resilience behaviour of monofilaments for artificial turf

application conference (C1), Blerina Kolgjini, Gustaaf Schoukens and Paul Kiekens

ICONTEX 2011: International Congress of Innovative Textiles: proceedings oral

presentations (2011).

“Resilience of monofilaments for artificial turf application at elevated temperatures”

conference (C3)Blerina Kolgjini, Paul Kiekens, Gustaaf Schoukens, Stijn Rambour and

Stefaan Janssens, 4th International Textile Conference Tirana, Albania: proceedings oral

presentation (2010).

“The effect of processing condition, annealing, on bending behaviour of monofilaments for

artificial turf application”, B. Kolgjini, S. Rambour, G. Schoukens, P. Kiekens , submitted to

The Grass yarn & Tufters Forum 2013, 18-20 February 2013 in Cologne, Germany.

viii

POSTER PRESENTATION

“Elevated temperature resilience of monofilaments for artificial turf application”, Blerina

Kolgjini, Paul Kiekens, Gustaaf Schoukens, Stijn Rambour and Stefaan Janssens, 12th FEA

PhD Symposium of Gent University, 2010

ix

1

1. Chapter 1

Introduction

1.1 History of artificial turf

The history of artificial turf started in the middle of the 60’s in the USA, when company

Astroturf installed, a first experimental synthetic field at Moses Brown School, Province, RI

[1]. In 1965, AstroTurf® installed 125 000 square feet (11 600 m2) of fabric (in total) to be

used for American football at the Houston Astrodome. These fields were based on nylon

fibres and were called the “first generation” of artificial turf (figure 1.1).

In Europe, Field Hockey was the driving force for the installation of no-filled surfaces. In

Switzerland, the first soccer/hockey pitch with artificial turf was installed in 1974 at

Heerenschürli. This type of surface was used for the first time for hockey in the Olympic

Games of 1976. Differently from the Astrodome, these carpets were made from

polypropylene fibre, by offering more comfortable surfaces. Polypropylene fibres are softer

than nylon fibres and also cheaper than nylon. Both of these surfaces now are called the

“first generation” of artificial turf.

At the beginning of the 1970’s, the “second generation” of artificial turf, with sand infill

material appeared. The surface had longer tufts spaced more widely apart and the presence

of sand infill was spread between the fibres to create sufficient firmness and stability for the

players. The second generation synthetic turf pitches provided a flatter playing surface than

natural grass gives better ball control and prevents balls from shooting off in unexpected

directions. This was a great improvement, especially for field hockey. Today, there are few

hockey clubs without an artificial field. This generation of synthetic fields was not very

successful for football fields, which were often installed with unsuitable sand and not properly

maintained.

The official soccer world did not accept these surfaces as an alternative to natural turf

because they did not meet the main sports-functional requirements for soccer. Thus, sand-

filled surface pitches were used mainly for winter training and school or leisure sports as a

substitute for non-existing, unavailable or unfeasible natural turf pitches.

The modern area of artificial turf called “third generation” goes back to 1997/98. The

development started when the Field Turf company patented a new technology with a rubber-

2

sand infill instead of pure sand. This surface has longer fibres which are spaced even further

apart in the backing. These fields are made of polyethylene which is even softer and

“friendlier” to the skin compared to nylon and polypropylene. The combination of fibre and

infill ensures a comfortable playing surface; sliding tackles are no longer a problem on these

fields as it was in the two other generations. As there is plenty of space between the turf

blades, studs sink well into the surface as is the cause with natural grass, put less stress on

the players joints and also allows the foot to get under the ball. These developments have

made third generation fields excellent for soccer.

Parallel to this, other artificial turf systems were also being developed which had no infill and

which reproduce the structural characteristics of natural turf more realistically [2]. A very

noticeable difference is the lack of granular displacement and airborne granules with the

unfilled systems when shoes or balls impact the surface.

The fourth generation of artificial turf, without infill material has been released recently by

DOMO® and has all the advantages of its predecessors and scores even higher in the wear

and resilience tests. It is an unfilled turf football pitch with a new profiled yarn-DOMO®

Champion Ascari. The absence of rubber infill makes installation, maintenance and recycling

easier. This turf gives the football player a peerlessly natural feeling and makes technically

high-quality football possible in all weather conditions [3].

FIFA the World football organization and UEFA organization, it European counterpart have

introduced standards for artificial turf, and aim to standardize the playing conditions, to

reduce the number of injuries and to increase the quality of the game.

3

Figure 1.1. Development of artificial turf in the years 1966 (1st generation), 1980 (2

nd

generation), 1996 (3rd

generation) [4].

In 2001, FIFA launched its FIFA Quality Concept for Artificial Turf [5].

In July 2004, the International Football Association Board (IFAB) introduced artificial

surfaces into the Laws of the Game by introducing two categories of performance.

Following the IFAB mandate to FIFA to create universal guidelines for football turf, the FIFA

Quality Concept has been further developed by introducing a FIFA recommended 2 Star

rating system. Based on the player’s feedback, medical research, test results and

information from the industry since the implementation in 2001, a more stringent standard

has been developed in addition to the existing level.

1.2 Advantages and disadvantages of artificial turf

Artificial turf was introduced as an alternative to completely replace natural turf in terms of

player comfort and safety. This is a bigger concern for regions where the natural turf has

difficulties to grow because of weather conditions or of closed areas. In 1965, the Astrodomo

Company developed these fields as a solution to obtain green and nice surfaces. The

stadium of baseball was completely covered, so natural turf did not grow since it needs

sunlight to thrive. A transparent roof was used but the gliding of the sun impeded the players

when catching high balls.

4

In Nordic countries or in winter time, many natural turf fields suffer from the cold, the snow or

the rain. As a result, those fields are muddy or even frozen and are easily damaged when

they are played on. Similarly, in countries with a warm climate, or in summer time, natural

turf fields suffer as well since grass cannot grow because of drought or disappears after a

few games. Moreover, for regions with freshwater shortage, the necessary watering

becomes less accepted and represents a substantial extra cost. One of the advantages of

artificial turf is its independency from the climate. The frequency of use is another

advantage, especially for professional clubs, by providing constant and uniform quality all

year round and across the world, which is not the case for natural turf. This gives another

advantage in densely populated areas where free space is expensive. They can use the

same surface of artificial turf for training and for official football games on national and

international levels.

Another advantage related to climate independence is that the wear of artificial turf is much

smaller than for natural grass.

The new generation of artificial surfaces is easier to maintain than natural grass fields [6].

The time for maintenance is much lower and the maintenance costs for a football turf field

can be significantly reduced.

Manufacturers have now developed football turf products that mirror the playing

characteristics of real grass and are resistant to difficult climates.

FIFA has recognized that all year round, improved and consistent playing conditions

worldwide bring enormous benefits to the global development of football.

Apart from all these benefits, pros and cons concerning the acceptance of artificial turf in

terms of injuries do exist. In some areas there has been a move back to natural grass in

some stadiums and a move towards newer artificial turf at others [7]. In some studies it was

concluded that artificial playing surfaces may be responsible for injuries. [8]. According to

Akkaya at al [9], football matches on synthetic fields can lead to serious orthopaedic injuries.

The American journal of sports medicine Incidence, Mechanisms, and Severity of Game-

Related College Football Injuries on Field Turf Versus Natural Grass: A 3-Year Prospective

Study (Meyers, 2010) concluded that Field Turf is in many cases safer than natural grass.

Meyers and Barnhill at al [10] proved the existence of similarities between Field Turf and

natural grass after a 5-year period of competitive play. Both surfaces were on the origin of

unique injuries. There were significant differences in injury time loss, injury mechanism,

anatomical location of the injury, and the type of tissue injured with different playing surfaces.

The hypothesis that high school athletes would not experience any difference in the

incidence, causes and severity of game-related injuries between Field Turf and natural

grass, was not supported. In another study by the Norwegian professional football

association [11], no significant differences were detected in injury rate or pattern between

5

3GAT and NG in Norwegian male professional football. The same conclusion has been

reached from other studies by showing no significant differences in the nature of overuse

injuries recorded on artificial turf and grass for either men or women [12-14].

In terms of surface performance (behaviour) some drawbacks remain. In 2008 ISA sport in

the Netherlands reported that the ball roll distance is higher than on natural turf and

increased in function of time or frequency of use of these fields. The same conclusion was

reported by Joosten [15] in 2003 and Baker[16] in 1990. Joosten reported that some players

perceive the ball roll speed and the ball bounces on artificial turf to be higher and different

compared to natural turf [15]. Other drawbacks of artificial turf are related to the wear of the

field. Severn et al. (2007) [17], Kieft (2009)[18] and Severn (2009) [19] have reported that the

properties of artificial turf change over time and stress the importance of regular

maintenance. Compared with the other generations, the third generation is more difficult to

be replaced and is not removable, so more attention should be paid on the installation. A

periodic disinfection of artificial turf is required as pathogens are not broken down by natural

processes in the same manner as with natural grass.

The presence of rubber infill tends to retain heat from the sun and the surface of artificial turf

can be much hotter than natural grass with prolonged exposure to the sun. Water can be

used to cool down these surfaces but the used water can be on the origin of the leaching of

the heavy metals present in the rubber infill. The rubber infill is in most cases produced from

recycled car tires.

On the other hand, a study by Walker (2007) [20] has concluded that artificial turf in its full

lifetime cycle produces less greenhouse gas emissions than a natural grass surface when all

of the heavy environmental burdens in the form of irrigation, fertilizer, pesticide use and

mowing are taken into account.

In terms of financial and cost aspect, artificial turf is more profitable. Young (2009) [21]

mentions a cost of 520000 £. FIFA (2008) mentions figures between 430000 and 560000£.

This includes the ground works and the synthetic turf system, each representing about 40%

of the cost, as well as additional costs for floodlighting, fencing, maintenance equipment,

consultant fees and site investigations. Additional costs include maintenance and the

renewal of the artificial turf carpet when it is frequently used, typically after 7 to 10 years.

This is a large investment cost compared to a cost of about 100000 € for natural turf (Desso,

2010). On the other hand, the higher number of playing hours and the longer life span of

artificial turf makes it a profitable investment in several situations.

The newest generation of synthetic turf combines the playing characteristics and look and

feel of natural turf, with the advantages of synthetic turf. The use of synthetic turf in different

sports is now supported by UEFA, FIFA, the International Rugby Board (IRB), the

International Hockey Federation (FIH) and the International Tennis Federation (ITF).

6

1.3 Application of artificial turf

Artificial turf is used in many sports as listed below.

1.3.1 Artificial turf for football

Artificial turf has earned its place in the sports footballing world (figure 1.2). the European

football league has approved playing the UEFA Champions League, the UEFA Cup, the

qualification matches for the World Cup, and the European Championships on artificial turf.

This is due to several properties like more maintenance-friendly, saving of space and low

cost of maintenance. More details about artificial turf are presented in chapter 3.

Figure 1.2. An example of a football turf surface.

FIFA is also promoting artificial turf for football. At the beginning of 2001, FIFA introduced the

Quality Concept: an extensive test program to develop an international standard for artificial

turf, to ensure the safety of the players and to encourage developments in the artificial turf

industry.

1.3.2 Artificial grass for tennis (according to EN Standard 15330)

Artificial turf can also be used for tennis. In order to ensure the playing quality of the turf in

terms of player safety and ball behaviour, as well as the product quality are sufficient, the

sport’s governing bodies have established a set of standards. New installed fields need to

pass several tests, conducted by accredited test houses, in order to be approved for the use

for official games. The fibre length used in the pile layer should be between 10-20 mm, with

infill of sand and the fibres are to be produced as monofilaments.

7

The maintenance is an important aspect, especially related to the water permeability, which

should be made according to the specified requirements. Apart from EN standards, they

should be accredited by ITF.

1.3.3 Artificial turf for hockey

Two kinds of hockey surfaces exist: a non-filled surface (figure 1.3) and a sand-filled surface.

Non-filled surfaces are considered by the FIH as the highest category (class I). The surfaces

are used by spraying a lot of water on the surface (figure 1.4). The typical pile length is 10-

20mm. The infilled surfaces have a longer pile length (15-25 mm) and are infilled with sand,

they are considered as class II by the First class FIH.

Figure 1.3. Non in filled artificial surface for hockey sport use.

Second class hockey field.

Figure 1.4. Artificial surface with water for hockey sport use.

The synthetic surfaces designed to be used for hockey have to be in conformity with the

requirements concerning the interaction between the ball and surface to be played on. For

these fields, the FIH (Federation International de Hockey) has developed a similar set of test

methods and requirements (FIH, 2011).

8

1.3.4 Rugby puts pitches to the test

Synthetic turf surfaces designed to be used primarily for rugby (figure 1.5) must be in

conformity with the following requirements:

Figure 1.5. Rugby synthetic surface.

Fibre : monofilaments

Typical level and type of infill: Partly filled, rubber, sand

Infill height (%): 50-80

Typical pile height (mm): 55-70

Test pieces have be prepared in accordance with standards and the test results should be in

the value ranges required by these standards.

1.3.5 Surfaces designed for multi-sports use (EN Standard 15330)

For multi-sport surfaces (figure 1.6) the benefits are the limited use of space and the reduced

costs. For these surfaces also, standards exist but these surfaces are more used for training

or as sport surfaces at schools.

9

Figure 1.6. Figure of synthetic surface for multiple sports use.

1.3.6 Artificial turf for landscaping

Artificial grass is ideal for people with limited time but who want to enjoy green surroundings

(figure 1.7). Artificial grass has the appearance of a perfectly maintained lawn, in all

seasons. It solves the problem of shadowy corners, roof patios and areas around swimming

pools: all the places where natural grass grows poorly or not grows at all. With artificial

grass, the public space does not only obtain et a permanent appearance, the maintenance

costs are also significantly reduced. Artificial grass has a long life and is therefore a long-

term investment.

Figure 1.7. An example of landscaping artificial grass.(covering the terrace of a building in

the city).

1.4 Goal and Outline

The aim of this work is to conduct fundamental research into the behaviour of

monofilaments and the morphology structure developed at different stages of the production

line with the aim to improve the mechanical properties of the product under different

conditions.

10

This includes firstly, a theoretical analysis in understanding the possible factors influencing

the bending behaviour of monofilaments, understanding the possible deformation and

analysing the existing test methods to evaluate the long term properties of the pile layer at a

very early stage (before producing the final product, artificial turf). Secondly, analysing in

details the production parameters of monofilaments production and their influence on their

behaviour.

These are suggestions for artificial turf manufacturers; in order to improve their products with

the goal to offer qualitative surfaces for players, more specifically by improving the ball roll

behaviour as one of the main demands for these surfaces and as a quality criteria of the

FIFA standards.

In chapter 2 and 3, a review of the main components and test methods will be investigated.

In chapter 2 the components of artificial turf are analysed by understanding the influence of

each of them on the fundamental properties of artificial turf, emphasising the most widely

used polymer for the production of monofilaments the production line and the tufted carpet

methods.

In chapter 3 the focus will be on the test methods used to evaluate the entire system of

artificial turf based on the existing standards in function of the destination. The main focus

will be artificial turf for football use.

In chapter 4 the test methods to evaluate the resilience of the pile layer are described. The

current test methods are assessed and their weaknesses are identified. Based on this

analysis, another test method will be suggested by performing measurements at elevated

temperatures. From this method, in combination with the existing test method, (Favimat) will

be possible to obtain more realistic results about the monofilaments behaviour. Theoretical

analyses of single monofilaments on the bending behaviour will offer a clear view on the

factors influencing the deformation recovery.

In chapter 5 the bending behaviour of a single monofilament will be investigated in function

of the parameter production of the monofilaments stretching ratio. Measurements will be

performed on static bending and will be evaluated by using the Favimat apparatus. A

detailed morphology analyses will be performed by using a combination of Dynamic

Scanning Calorimetry, Raman scattering and X-ray measurements.

In chapter 6 the bending behaviour of monofilaments will be investigated at different stages

of the production line in function of the heat treatment. The focus will be on finding the most

proper combination between temperatures and production speed in order to obtain a stable

product and the best values for the bending behaviour. The detailed analysis of the

11

developed morphology structure will be correlated with the obtained properties, with the main

focus on resilience.

In chapter 7, the third developed structure at different stages of the production line and the

conditions of the product are correlated with the behaviour of the monofilaments.

In chapter 8, the main conclusions of the work are summarised and recommendations for

future research are formulated.

12

References:

[1] Information form AstroTurf, valid from:http://www.astroturfusa.com/ visited at

28.08.2012.

[2] Astroplay, Greenfields, XL-turf, Sportisca

[3] Information from: http://www.kestrelcontractors.co.uk/projects/whtc-4g-pitch.php.

[4] Deutscher Fussball-Bund (2006), DFB-Empfelungen für Kunststoffrasenplätze –

Fragen und Antworten,

http://www.dfb.de/uploads/media/DFB_Kunstrasenstudie_KF.pdf In 2001 FIFA

launched its FIFA Quality Concept for Artificial Turf.

http://www.fifa.com/mm/document/afdeveloping/pitchequip/fqc_football_turf_folder_3

42.pdf.

[5] A comparison of artificial turf, The Journal Of Trauma, Injury, Inflection and

Critical Care Volume 57, number 6 J. Trauma 2004; 57:1311-1314.

[6] Guskiewicz KM, Weaver ML, Padua DA, Garret WE. “Epidimiology of conclusion

in collegiate and high school football players”. Am J Sports Mad. 2000; 28:643-650.

[7] Lewis LM, Naunheim RS, Standeven J, “Quantitation of impact attenuation of

difference playground surfaces under various environmental conditions using a tri-

axial accelerator”. J.Trauma. 1999;5:935-935.

[8] Semih Akkaya, Mustafa Serinken, Nuray Akkaya, İbrahim Türkçüer, Emrah

Uyanık Joint Diseases & Related Surgery Vol. 22 • No 3 • 2011:155-159 Football

injuries on synthetic turf fields

[9] Meyers, Barnhill “Incidence, causes, and severity of high school football injuries

on Field Turf versus natural grass: a 5-year prospective study” (2004).

[10] Bjorneboe et al., 2010, “Risk of injury on third generation artificial turf in

Norwegian professional football” Br J Sports Med 2010;44:794-798 doi:

10.1136/bjsm. 2010.073783.

[11] Comparison of the incidence, nature and cause of injuries sustained on grass

and new generation artificial turf by male and female football players”. Part 2:

training injuries, Br J Sports Med 200741:i27-i32 doi:10.1136/ BJSM 2007.037275

[12] Jan Ekstrand, Martin Hägglund and C.W Fuller, “Comparison of injuries

sustained on artificial turf and grass by male and female elite football players, 2011”,

Scandinavian Journal of Medicine and Science in Sports, (21), 6, 824-832.

[13] J Ekstrand, T Timpka, M Hägglund “Risk of injury in elite football played on

artificial turf versus natural grass: a prospective two-cohort study” Br. J. Sports Med.

2006; 40:975-980 doi:10.1136/bjsm.2006.027623.

13

[14] Joosten T., “Players’ experiences of artificial turf, Stadia Turf Summit of the

International Association for Sports Surface Sciences, 2003”, Amsterdam, the

Netherlands, http://www.issssportsurfacescience.

[15] Baker S.W., “Temporal variation of selected mechanical properties of natural

turf football pitches”, Journal of the sports turf research institute, 1991, 67, 42-69.

[16] Severn K., Fleming P.R., Young C., James I.T. (2007),The play performance of

six water based field hockey pitches – spatial and temporal changes. In P. Fleming,

C. Young, S. Dixon, I. James, M. Carré & C. Walker (Eds.), Science, Technology

and research into sports surfaces (STARSS 2007). Loughborough University, UK.

[17] Kieft G., Quality monitoring of 50 artificial turf football fields, Sport SURF 7th

workshop, Loughborough, 2009, http://www.sportsurf.org/workshops/7/GJK.pdf

[18] Severn K., Physical properties vs. performance tests, Sport SURF 7th workshop,

Loughborough, 2009, http://www.sportsurf.org/workshops/7/KS.pdf.

[19] Kocher H., Neuere Erfahrungen mit Kunststoffrasen

org/downloads/documents/6EGR67DKG1_Joosten.pdf.

[20] C. Young, S. Dixon, I. James, M. Carré & C. Walker (Eds.), Grass is not always

greener: the application of life cycle assessment to natural and artificial turf sports

surfaces. In P. Fleming, Science Technology and research into sports surfaces

(STARSS 2007). Loughborough University, UK.

[21] Young C., Maintenance: cost benefits, Sport SURF 7th workshop, Loughborough,

2009, http://www.sportsurf.org/workshops/7/CY.pdf.

14

2. Chapter 2

Artificial turf and components

The development of third generations of artificial turf drastically improved the quality of

artificial playing surfaces, contributing to the exceptional performance of today’s third

generation artificial pitches. During these period various products of carpets were introduced;

woven, tufted, needled and knitted and a great variety of (sub) bases as well.

2.1 Introduction

Most of the third generation pitches consist of a carpet with tufted polyethylene (PE) yarns,

with infill of granulated styrene-butadiene rubber (SBR) in combination with silica sand, and

are installed on prepared surfaces [1-4].

Some of the bases incorporate textile products in the form of shock pads and geotextiles.

In figure 2.1 the main components of artificial turf are schematically presented.

Free pile layer, this is the pile layer above the infill material

Infill, a mixture of layers of sand and elastomeric material

Pile layer consisting of pile fibres and infill

Backing fabric

Backing coating of SBR latex or Polyurethane (PUR)

Sometimes the artificial turf can also consist of:

Elastic layer of synthetic foam or PUR bound rubber granules

Elastic supporting layer of PUR bound mixture of mineral and rubber granules

The sub base can consist of:

Asphalt supporting layer (bound supporting layer)

Mineral supporting layer (unbound supporting layer)

Sand layer with on top a layer of lava

The surfaces which are considered suitable for soccer can be divided in three basic types:

Non – filled carpet with elastic layer

Filled carpet without elastic layer (ES) or elastic supporting layer (ET)

Filled carpet with elastic layer (ES) or elastic supporting layer (ET) with low

thickness of pile layer

15

Figure 2.1. Schematic presentation of the composition of artificial turf used for football [5].

The performance of the entire system depends on the performance of each of these

components and the way of producing, installing and maintaining them. Therefore a brief

explanation is given for each element.

2.2 Pile layer, Polymers for monofilaments

The main polymers to produce monofilaments for the pile layer are polyethylene (PE),

Polypropylene (PP) or PP-copolymers and Polyamide (PA) [6]. These products are also

commercially available,

Historically, polyethylene was produced under a very high pressure and temperature. This

severe condition leads to many short-chains branching, leading to low density polyethylene.

On the other hand, the polyethylene synthesized by the low pressure method utilizing the

Ziegler catalyst (AlEt3 and TiCl4) is mostly linear and thus provides high density

polyethylene. The high density polyethylene has a higher degree of crystallinity and rigidity

than the low density polyethylene. The Ziegler catalyst was accidentally discovered when

Ziegler was examining the possibility of distilling metallo-organic molecules. The high density

polyethylene is used for water pipes, gas pipes, and packaging.

Polyethylene is the most common polymer used for monofilaments in artificial turf. Ethylene

is the monomer and is a rather stable molecule that polymerizes only upon contact with

catalysts. The conversion is highly exothermic, i.e. the process releases a lot of heat. The

16

most common catalysts consist of titanium (III) chloride, the so-called Ziegler-Natta catalysts.

Another common catalyst is the Phillips catalyst, obtained by depositing chromium (VI) oxide

on silica [7].

Polyethylene exists in many types, essentially having the same backbone of covalently

linked carbon atoms with pendant hydrogens; variations mainly arise from branches that

modify the nature of the material. The principal structure of polyethylene is illustrated in

figure 2.2.

Polyethylene (PE) is a rather soft and tough crystalline polymer, which is being

manufactured in three main types: Low density Polyethylene (LDPE) ( 0.92 g/cm3), High

Density of Polyethylene (HDPE) ( 0.95 g/cm3) and Linear Low Density Polyethylene

LLDPE ( 0.92-0.95 g/cm3).

The stiffness of the PE strongly increases with the increasing density [8]. All types gradually

lose their properties upon temperature increase, and melt at 105OC to 130

OC respectively.

Figure 2.2. Chemical structure of ethylene, the monomer of polyethylene (PE).

HDPE is defined by a density greater than or equal to 0.941 g/cm3. HDPE has a low degree

of branching and thus stronger intermolecular forces and tensile strengths. HDPE can be

produced by chromium silica catalysts, Ziegler-Natta catalysts or metallocene catalysts. The

lack of branching is ensured by an appropriate choice of the catalyst (for example, chromium

catalysts or Ziegler-Natta catalysts) and reaction conditions.

LLDPE is defined by a density range of 0.915–0.925 g/cm3. LLDPE is a substantially linear

polymer with significant numbers of short branches, commonly obtained by copolymerization

of ethylene with short-chain alpha-olefins (for example, 1-butene, 1- hexane and 1-octene).

LLDPE has a higher tensile strength than LDPE, it shows a higher impact and puncture

resistance than LDPE. Lower thickness (gauge) films can be blown, compared with LDPE,

with better environmental stress cracking resistance although it is not as easy to process.

LDPE is defined by a density range of 0.910–0.940 g/cm3. LDPE has a high degree of short

and long chain branching, which means that the chains do not pack into the crystal structure

so well. It has, therefore, less strong intermolecular forces as the instantaneous-dipole

17

induced- dipole the attraction is less. This results in a lower tensile strength and increased

ductility. LDPE is created by free radical polymerization. The high degree of branching with

long chains gives molten LDPE unique and desirable flow properties. LDPE is used for

plastic film applications and film wrap [8].

Polypropylene (PP), resembles PE, has an intermediate level of crystallinity between that of

Low Density polyethylene (LLDPE) and High Density Polyethylene (HDPE), but is somewhat

harder and stiffer than HDPE. Polypropylene is normally tough and flexible, especially when

copolymerized with ethylene. This allows polypropylene to be used as an engineering

plastic. It is crystalline as well, and has a melting point that ranges from 160 to 166 °C,

depending on the atactic material and crystallinity. Syndiotactic PP with a crystallinity of 30%

has a melting point of 130 °C. At lower temperature its impact strength is quite poor,

therefore PP is often modified with a certain amount of rubber (mostly built-in as a

copolymer). A special feature of PP is its ability to form integral hinges with a practically

unlimited resistance against repeated bending. Polypropylene has a good resistance to

fatigue.

Figure 2.3. Chemical structure of a polypropylene (PP) monomer.

Polyamide (PA) is generally known as “nylon”. This is a collection of polymers, which differ

in chain structure, and which are, according to the numbers of consecutive C-atoms in the

chain, designated as PA-6 and PA-6.6. Polyamides are crystalline polymers with relatively

high melting points (between 200 and 300OC). A polyamide is a polymer containing

monomers of amides (see figure 2.4) joined by peptide bonds. They possess good impact

strength, due to the fact that they absorb several percentages of water from the atmosphere.

Moreover, they have a good abrasion resistance which makes them suitable for technical

use. Quite often polyamides are reinforced with short glass fibres to improve their stiffness.

18

Figure 2.4. Chemical structure of a polyamide (PA) monomer.

2.3 Manufacturing of turf fibres

Pile layer is composed by the monofilaments or fibres which are made by extrusion of the

molten polymer, followed by drawing to orient the molecules [9 -10].

Monofilaments could be extruded (see figure 2.5) in the form of (mono-) tapes and then cut

in different thicknesses or in the form of fibrillated tapes, or produced as monofilaments with

different cross sections profiles as they are presented in figure 2.6 and figure 2.7.

Figure 2.5. The pilot line of production of monofilaments at the Textiles Department, Ghent

University.

The current production of fibrillated tapes or monofilaments for use in artificial turf is based

on post stretching in a solid condition at a controlled elevated temperature [11].

For the production of fibrillated fibres, thin films (split film) are extruded which contain parallel

splits or points of fracture by design (see figure 2.6 c). The final properties are created by

stretching and heating (for curled structures). The individual carpet tufts could be created by

six to eight fibres/ monofilaments composing the yarn.

19

Fibres are often selected based on their softness. But soft fibres often have a higher

coefficient of friction, which leads to a lower sliding distance and thus a higher temperature

at the surface [12].

Individual monofilaments are produced by extruding them as individual fibres (see figure 2.5

and 2.6). This gives the possibility to produce monofilaments with different thicknesses and

different cross sections by influencing the bending behaviour and the fibrillation. Their

thickness varies from 90 to 500 µm (0.5 mm). The individual production creates the

possibility to produce monofilaments from stiffer to softer material, dependent from the cross

section and it is possible to have a subsequent treatment to produce curled structures.

A combination of fibrillated fibres and monofilaments is possible in the production of pile

layers for sport surfaces.

a) monofilaments

b) curled monofilaments

c) fibrillated tapes

Figure 2.6. Three different types of monofilaments used as a pile layer for artificial turf.

For the fibre production, several additives are used for a wide variety of purposes, and may

be classified as fillers, anti-oxidants, stabilizers, plasticizers, fire retardants, pigments and

lubricants [13]. Additives are conventionally classified into two groups, processing additives

and functional additives.

20

Additives are introduced into the polymer during the processing. It is important to ensure that

the components are adequately mixed to create a homogeneous product. That is why they

are often added in the masterbatch.

Stabilizers are usually used in rather small quantities in order to prevent degradation of the

polymer when it is exposed to air, light and heat; the purpose is to maintain the properties of

the polymer rather than to modify them.

Lubricants may be used externally, to prevent adhesion of the polymer to the processing

equipment, or internally, either to aid flow during processing or to reduce friction between the

product and other materials. The stabilization of the pile fibres against UV light and heat is

crucial to the ageing behaviour. Light stabilizers ensure the durability of artificial turf over a

long service life [14-16]. Typical examples are hindered amino light stabilizers (HALS), due

to a low interaction of the product with other components in the compound or the production

process. [15]

The effect of the stabilization cannot be measured with simple and immediate tests and

visually it is not detectable as all artificial turf carpets are green when new. Therefore, a

diligent process of quality determination is vital.

2.4 Cross section of fibres/monofilaments

A number of synthetic products have been introduced to provide both indoor and outdoor

surfaces which simulate the appearance and mechanical properties of natural grass but they

should have a greater resistance to damage and require less maintenance.

The most common cross-section shape of monofilaments is a rectangular shape presented

in figure 2.7 a. The advantage of this shape is easy to produce and it is possible to create

fibrillated tape out of it. Parallel to this profile other profiles are introduced (see figure 2.7 b-

d). as well with the aim of having a better behaviour, especially the bending behaviour.

Figure 2.7 c-d shows two examples of another profile cross section invention. These profiles

do not show good properties and after 2000 cycles the fibrillation takes place exactly at the

points where the tensions in the filaments are high. This was shown by Abaqus simulations

[17]. In the figure the critical points are indicated by arrows.

a) rectangular profile

21

b) diamond profile

c) V-profile

d) V-profile

Figure 2.7. Examples of different profile cross section of monofilaments for synthetic grass

application.

2.5 Production line of artificial turf

Carpet Manufacturer

The techniques that are used for manufacturing textile sports surfaces are the same as for

the ordinary carpets.

The main techniques used to produce artificial turf are described below.

2.5.1 Knitting

The knitting technique was the first manufacturing method used for textile sport surfaces to

be used on a large scale [18]. The backing construction of a knitted carpet is rather open

which achieves the drainage requirements but an extra backing is required to achieve

dimensional stability. A backing fabric is knitted, usually from the polyester filament yarn,

with a multiplicity of needles mounted an a bar by forming a series of interlocking pillar

stitches. At the same time the pile yarn is laid in by passing round the needle and round a

22

looper to form a series of loops, which in case of textile sports surfaces are then cut to form

a grass-like pile.

Knitting is used for hockey surfaces (figure 2.8). A layer of polyurethane is added to provide

shock absorption and stability.

Figure 2.8. Example of a knitted carpet used for the hockey sport.

2.5.2 Tufting

Tufting is the most common way to make carpets, and is also the most common way to

manufacture textile sport surfaces (figure 2.9). The secondary backing process is needed to

bind the tufts and to provide dimensional stability. The backing fabric can be woven or

nonwoven textile made from polypropylene tape. A raw of needles carrying pile yarns

penetrates the backing fabric, and loops move in between the yarn and its needles. When

the needles withdraw, a raw of loops is left over the loopers. Then the process of cutting the

loops takes place thus forming the pile layer.

23

Figure 2.9. Presentation of the tufting process in industry.

Large sewing machine

Another possibility is needle-punched surfaces mainly used for outdoor multi-sport surfaces.

[19]. The artificial grass fibres are tufted into a polypropylene mat. The basic principle of

tufting has been derived from the sewing machine [20] as is shown in figure 2.10.

Figure 2.10. Large sewing machine for artificial turf production [20].

24

Across the whole width of the mat hundreds of needles are positioned, each of which pulls a

long artificial grass fibre through the mat. When the needle returns a loop is formed. The

loop is cut at the top so the fibres will stand straight up. Line markings can be tufted into the

mat in a colour of choice.

2.5.3 Weaving

The most common technique for weaving is the same technique as for Wilton carpets (see

figure 2.11). Woven sport surfaces have good dimensional stability, but a backcoating is

required for sport surfaces in order to ensure that the tufts are strongly bound. In wire Wilton

weaving, a backing fabric is woven, usually from polypropylene (PP) yarns, and the pile yarn

is introduced from the warp to form loops over the “wires” which are inserted weftwise. The

threaded needles are attached to a rod to the tufting machine. The needles are swiftly

pushed up and down through the primary backing. Then the carpet is mended and

inspected. Finally the carpet is gathered in rolls ready for the next step (to have latex applied

on the backing).

Figure 2.11. Schematic presentation of the Wilton loop formation.

The fibres are tufted (stitched) into the backing material in rows according to a patented wide

gauge spacing formula that enables studs to penetrate the infill material rather than the

surface fibre [21]. For fixing monofilaments on the backing, another layer is applied after

carpet production, known as coating.

Carpets used for football applications are made by dual face weaving machines of

Vandewiele. The introduced W-binding is so strong that no coating or latex is needed. A

research project on dual face weaving at the Department of Textiles showed that this type of

carpets is suitable for sport use and that it was not necessary to use latex or a coating layer.

25

Another method is face to face weaving (double plush Wilton) by weaving two backing

fabrics and looping the pile layer between the two. The sandwich is then cut to form two

carpets with levelled pile layer.

2.6 Back-coating and finishing

Back-coating with some form of latex or resin is necessary to bind the tufts of grass-like

textile sport surfaces (figure 2.12). It is also necessary to improve the dimensional stability of

the backing material to prevent stretching and distortion during the installation and cracking

during use. Back-coating enhances the stability of all types of product considerably.

Back-coating is a common operation carried out as routine in the carpet industry and the

techniques and materials used for textile sports surfaces are often very similar to those used

for ordinary carpets. The backing is made of a combination of permeable woven and non-

woven polypropylene fabrics. Then the synthetic carpets go through a coating line. The

coating can be applied all over the back or for each row of stitching separately. The coating

secures fibres in their place and provides optimal drainage. The coating over all the back of

each row of stitching creates a chemical and mechanical bond, which seals the fibre rows

while leaving the rest of the backing totally permeable.

The fibres of artificial turf are traditionally fixed with a latex-based coating. Recently

polyurethane (PUR) has been used because of improved resistance to tearing and ageing.

The polyurethane (PUR) is distributed evenly among the fibres, which thus have better

adhesion to the backing. Compared to latex, polyurethane does not crumble or degrade.

Ageing tests have demonstrated that even under extreme weather conditions, polyurethane

will help the carpet to last longer. To anchor the fibres in the mat, a thick layer of liquid latex

is applied and cured in an oven at 120°C in order to allow the water to evaporate. The fibres

are secured and the artificial turf is then resistant to the heaviest sliding tackles. The artificial

turf is now ready for installation [22].

26

Figure 2.12. Presentation of the latex based coating layer on the back of artificial turf [19].

As previously stated, water permeability is created with all tufted artificial turf surfaces

through the perforation of the backing of holes which are drilled / burned in a grid pattern

with a typical spacing being 10 to 15 cm. Panel-like surfaces allow effective drainage through

the joint connection (which is – of course – effective if the supporting layer is sufficiently

permeable ).

2.7 Infill materials

After production and installation of all the elements, the infill materials should be added. The

infill is applied in several layers: normally it begins with a layer of sand to act as a ballast

layer to stabilize the surface by its weight, followed by a layer of elastomeric granules made

of:

Recycled Styrene Butadiene Rubber from car tires (SBR )

Styrene Ethylene Butadiene Styrene (SEBS )

Ethylene Propylene Diene Monomer (EPDM )

Thermoplastic Polyethylene (TPE) or Thermoplastic Polyethylene Vulcanized (TPV)

There are also surface systems in which a mixture of mineral and elastomeric granules is

applied between the 1st and the 2

nd layer of infill.

27

The granule size of the rubber may be between 0.1 and 2.5 mm, preferably between 0.5 and

2.5 mm (see figure 2.13). It is possible also to have the infill material in different colours, as

desired.

Figure 2.13. Infill material in different colour.

2.8 Supporting layers , shock pad

The presence of a shock pad is to reduce the amount of rubber infill and the pile length of

the carpet. For outdoor surfaces, the shock pad needs to be permeable to water to facilitate

drainage [23]. The resilience of the artificial turf system is controlled through a combination

of the type of material, the thickness of the layers and the length of the pile fibres, together

with the resilience of the elastic layer from Polyethylene (PE) or Elastic layers with stones on

it (ET).

Elastic layers (figure 2.14) are installed from prefabricated rolls or panels (PUR compound

foam, PE foam rolls or rubber compound mats) or they are paved on-site. Prefabricated

elastic layers are sensitive to dimensional changes. This means that movement of the elastic

layer elements (rolls or panels) and formation of wrinkles caused by exposure to heat or

sports stress must be avoided. Therefore, a gap is left between the prefabricated shock pad

rolls to let them change in dimensions.

Elastic layers are highly recommended since they provide a reasonable part of the available

resilience and do not require maintenance. Furthermore, elastic layers will outlast several

installations of turf.

Elastic layers cannot compensate for or replace insufficient supporting capability or

evenness of the subbase /subconstruction. Too much unevenness of the sub construction

28

leads to a non-uniform thickness of the elastic layer and thus to varying resilience of the

surfaces system.

Figure 2.14. Example of an polyethylene elastic layer and in-situ shock pad.

2.9 Supporting Layer – Subsoil

The supporting layer must provide the stability of the surface system (permanent evenness

even with acceptable loads) and water drainage. The design must follow the established

rules of sports surface construction [24]. Mineral supporting layers must be fully resistant to

frost (freeze-thaw heave). The top supporting layer should always be a bound supporting

layer (i.e. permeable asphalt).

Pure vertical drainage occurs during medium rainfall only. With heavy rain superficial

drainage to the sides of the pitch also takes place. It is necessary to provide drainage

trenches with enough drainage capacity.

Direct installation of artificial turf surfaces on top of unbound (mineral / gravel) supporting

layers is problematic. Unstable mineral materials are sensitive to deformation and shifting by

stepping on the turf surface and in the long run lead to unevenness which will be duplicated

on the turf surface. The stability is also important to keep the surface even during the

installation of the turf layer when vehicles transport the heavy turf rolls and the infill onto the

pitch.

29

References:

[1] Patents application WO9840559 A1, Synthetic Turf, (publication date:

17/09/1998).

[2] Patents application GB 2429171 A, “Artificial Turf” (publication date: 21/02/2007).

[3] Patents application US 6723412 B2, “Synthetic Turf” (publication date:

20/04/2004).

[4] Patents application US 6432505B1, Diamond cross section synthetic turf

filament. (publication date: 13/08/2002).

[5] Pictures reproduced from FIFA website

http://www.fifa.com/mm/document/footballdevelopment/footballturf/01/15/62/41/iatsm

anual2012edition.pdf

[6] Kenneth S. Whiteley,T,. Geoffrey Heggs, Hartmut Koch, Ralph L. Mawer,

Wolfgang Immel, Polyolefins in Ullmann's Encyclopedia of Industrial Chemistry

2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a21_487.

[7] Adrew Peacock, Handbook of polyethylene structures, properties and

applications, New York 2000.

[8] Patents application EP 1672020 A1, Polyethylene composition for artificial turf

(publication date: 21/06/2006).

[9] Patents application EP 1378592 A1, Artificial fibre as well as an artificial lawn for

sports fields provided with such fibre, (publication date: 07/01/2004).

[10] Patent application EP1734189 A1, Method for forming monofilaments, as well

as an artificial lawn built up there from, (publication date: 20/12/2006).

[11] Hans J. Kolitzus IST Switzerland Artificial Turf Surfaces for Soccer, What

Owners of Soccer Pitches should know about Artificial Turf 1 2 3, United States

Sports Surfacing Laboratory USSL www.isss.de/ist-ch.

[12] Rudy Verhelst, PhD, Study of Ball-Surface and Player-Surface Interaction on

Artificial Turf, Chapter 3, Frictional Characterisation of A Surface: Standing Start and

180° Turn.

[13] J. E. Mclntyre, editor, Synthetic fibres: nylon polyester, acrylic, polyolefin, 2005

publishing ltd Cambridge England ISBN 1 85573 588 1 Chapter 5, Polyolefine fibres

pg 235- 253.

[14] David I. Bower, An Introduction to Polymer Physics, Formerly at the University

of Leeds.

[15] Alban Glaser Simon Schambony, Light stabilization and more Additives plastic

Europe 9/2005, pp. 186-190.

[16] Schmidt H., Witkowska B., Kamińska I., Twarowska-Schmidt K., Wierus K.,

Puchowicz D, Comparison of the Rates of Polypropylene Fibre Degradation Caused

30

by Artificial Light and Sunlight, FIBRES & TEXTILES in Eastern Europe 2011, Vol.

19, No. 4 (87) pp. 53-58.

[17] A study performed at Department of Textiles, Ghent University, Belgium.

[18] G.H. Crawshaw, Textile sport surfaces and artificial grass, Elsevier Science

Publishers Ltd 1989.

[19] EN Standard, prEN15330-1 Surfaces for sports areas- Synthetic turf and

needle-punched surfaces primarily designed for outdoor use-Part 1: Specification for

synthetic turf, 04-2006.

[20] http://www.dessosports.com/en/all-about-artificial-turf/production-and-

installation-artificial-turf/

[21] Patent US 2010/0298073 A1, Artificial turf mat and method for manufacturing

thereof, (published Date: Nov.25,2010).

[22] http://www.mattex.com/grassBacking.html

[23] DIN 18035-6.

[24] DIN 18035-7.

31

3. Chapter 3

The test methods for artificial turf

In the previous chapter the composition of artificial turf is described. The performance of

entirely system depends on performance of each of these components mostly determined by

two parameters: playing characteristics and durability. These products should give the

appearance of grass and provide highest playing comfort and guarantee the highest quality

standard for football turf. To ensure that FIFA and UEFA have introduced the standards [1].

These standards are based on the best properties of natural grass used for football.

The test program for football turf assesses:

Physical conditions of the surfaced and identity of the surface

Sport technical performance: player surface interaction and ball surface interaction.

The series of tests includes laboratory tests and field tests. The field tests are conducted

after the installation of the surface/pitch. The player-surface interaction and the ball-surface

interaction tests are conducted in the laboratory and on the installed field.

3.1 Test and recommendations for physical conditions

3.1.1 Identification of the product

The identification of the product, based on EN and/or ISO test standards methods, include

the following criteria:

Mass per unit area and tufts per unit area.

Tuft withdrawal force- Measures how strongly the fibres are fixed into the backing of

the carpet.

Pile weight- Measured to ensure that not only the numbers of tufts are correct but

also that the correct dTex of yarn has been used.

Fiber identification- Can be identified by its melting point and so-called glass

transition temperature (type of polymer).

In-fill materials- Defines the various types of in-fill available for incorporation into the

gaps between the fibres of the synthetic turf (particle size/ particle shape/ bulk

density).

Optional- Compressive modulus and Optional shock pad under turf (a shock pad is

an impact-absorbing layer, which influences player comfort and ball response).

32

3.1.2 Sustainability

3.1.2.1 Abrasion Resistance (FIFA test method 9)

To determine the quality of the surface over years the Shock Absorption, Vertical

Deformation, Vertical Ball Rebound, and Rotational Resistance are measured before and

after wearing the specimen. The wearing equipment, Lisport, presented in figure 3.1 is made

by two studded rollers which travers over the test specimen to simulate the mechanical

abrasion of the surface that occurs on normal use during training or playing. The 5200 cycles

of wear is evaluated with a period of 5 years of use.

Figure 3.1. Lisport equipment, used for wearing artificially the turf specimen

3.1.2.2 Join Strength

This test is performed according to EN standard 12228:2002 and determines the strength of

the specimen where they are sewing (stitched seams) or adhered (bonded seams) with

adhesive. For this test is used Instron equipment presented in figure 3.2. The requirement

values are respectively 1000N/100mm and 25N/100mm.

33

a) b)

Figure 3.2. Instron equipment a) test of stitched seams b) test of bonded seams.

3.1.2.3 Climatic resistance

This test is performed to determine the resistance in colour, appearance and physical

properties on a specimen placed under the UV lamp, temperature and water. The standard

methods used are; for colour change of pile layer and infill, EN ISO 20 105 – A02, tensile

strength for pile layer EN 13864.

The evaluation of colour change is realized by using grey scale recommended to be less

than 3. The strength of pile yarn is measured after exposing under a UV radiation for certain

time and the percentage change to be no more than 50%.

3.2 Player surface interaction

3.2.1 Shock Absorption

Shock Absorption is a parameter which measures the percentage of the force absorbed by

field including here infill material, shock pad and the sub lave. In patent WO 2006/092337 is

specified the importance of the infill material and the influence on the shock absorption [2]. It

has been proved that the homogeneous distribution and a particle size of infill material

significantly affect on shock absorption, vertical deformation, vertical ball bounce and

traction.

This parameters leads to the safety of the players as it prevent injuries. The equipment used,

presented in Figure 3.3, consist of a weight connected with a sensor and the spring at the

bottom. The weight is allowed to fall on the ground or tested specimen. Force Reduction is

34

calculated by comparing the percentage reduction in this force relative to the reference force

on concrete. The recommended values are between 60 - 70%

Figure 3.3. Shock absorption equipment and other accessories to elaborate the test results.

3.2.2 Vertical Deformation

The stability of the field is another parameter effecting on the player comfort. A surface that

deforms excessively gives the impression of being unstable and it cause discomfort for

players. To determine this parameter the shock absorption is used but now when the weight

is dropped the force is recorded. Then the force reduction is calculated by evaluating it with

the recoded force on concrete. The recommended values are between 4 and 9 mm.

3.2.3 Slip Resistance

This test is performed to determine the ability of studs to slide through the surface without

causing the player to slip over. Slip resistance measures the deceleration experienced by the

players shoe as it makes contact with the surface. If the deceleration is too high, damages to

joints and ligaments may occur. The player must gain sufficient grip from the surface to

enable them to stop quickly. If there is insufficient grip the player will slip which could result

in a loss of balance with the danger of physical damage to muscle ligaments, soft tissue or

even bones. Conversely too much grip is also dangerous. When a player attempts to stop

forces are transmitted to joints and ligaments to decelerate the body’s forward momentum. If

the forces are transmitted too quickly then there is a danger that too high strain will be

imparted to the joints and ligaments resulting in damage.

The Slip Resistance Tester (figure 3.4) is possible to assess the surface in two ways. Firstly

how much horizontal movement the player will feel in the surface upon acceleration.

35

A studded test foot strikes the surface and comes to rest on a numbered scale. A low value

indicates a slippy surface and a high value a surface that doesn't allow sufficient movement

and is dangerous to the player. Values of 120-220 for horizontal movement have been found

to give sufficient grip. A second measurement is also taken with this apparatus. The

deceleration of the foot is measured as it stops. High decelerations will cause soft tissue

damage to the joints. Values between 3.0-6.0 g for this deceleration have been measured on

good natural turf. Lower values indicate a surface with low grip and higher values with high

grip.

Figure 3.4. The equipment of measuring the slip resistant of artificial turf.

3.2.4 Rotational Resistance

Another aspect of the interaction between the shoe sole and the surface is the ability to

change direction. Football is not a unidirectional sport but is one involving repeated changes

of direction. The player therefore needs to change direction on a regular basis as the game

moves around the field. The surface must allow sufficient grip to allow the player to

repeatedly change direction. Similarly, as for Slip Resistance, there is a need for an upper

and lower limit otherwise the aspect of safety playing will be affected. This property of the

surface is measured using a Traction Apparatus. The units of Torque are Newton meters are

abbreviated by N.M. Values for natural turf ore between 25–50 N.M., for good turf and

between 35– 45 N.M. for an “ideal” natural turf have been measured. The apparatus uses a

Torque Wrench and measures the amount of Torque necessary to start the motion of a

studded sole.

36

Figure 3.5. Equipment for measuring the rotation resistance of artificial turf.

3.2.5 Skin Abrasion/ Skin Friction

This test measures the abrasiveness and friction of artificial turf on the skin of the player

when sliding (figure 3.6). The development of this test came as a result of bad experience of

first and second generation synthetic turf. The developed device replicates the action of skin

rubbing on the artificial grass. A special silicone elastomer that simulates natural skin is

rubbed over the surface at speed. Afterwards, the damage to the silicone is assessed and

the friction between the silicone elastomer and the synthetic grass is recorded. If the surface

is abrasive, it damages the silicone elastomer, which can be assessed by measuring the

friction change in the silicone before and after the test procedure. A large change indicates

an abrasive surface. A surface that produces a high coefficient of friction is one that will lead

to friction burns. A certain amount of friction is, however, necessary to slow the player down

as he slides as well as to slow the ball down. Hence, a lower and an upper limit for the

coefficient of friction is needed

37

Figure 3.6. Equipment for measuring skin abrasion and frictions of artificial turf.

3.3 Ball surface interaction

There are three categories that define the performance of a ball on the surface. These are

Ball Bounce, Ball Roll and Angled Ball Behaviour.

3.3.1 Vertical ball rebound

The test consists on dropping the ball from two meters and measuring the height and the

rebound height of the ball will be measured. The performance of this test on natural turf give

values of between 50 and 100 cm, but an “ideal” natural turf will give values of between 60

and 85 cm. These are the recommended values for artificial turf as well. The importance of

this test is related with the quality of the game since it makes sure that the players can

control the ball. The equipment used is presented in figure 3.7. The height of the ball

rebound depends on the surface where it collides. If there is too much infill, it will bump the

ball higher values and the opposite happened when there is not enough infill material

Therefore it is necessary to measure the height to which a ball bounces when dropped onto

the surface from a certain specified height.

Another factor influencing on the height of the ball rebound is the also the use of the filed

which cause the compact of the infill material. The more compacted infill the higher ball

rebound will be.

38

Figure 3.7. Vertical ball rebound equipment.

3.3.2 Angled ball behaviour

The Angled Ball behaviour is a complex interaction between the ball and the surface

involving the friction between the ball and the impact of surface. The equipment used is

presented in figure 3.8. This test consist on hitting a ball at an angle with a certain speed, the

ball will bounce off the surface at a certain angle and speed. Then the measured values of

the speed after rebound and before rebound calculate the value of ball rebound of the tested

specimen. If the ball comes off the surface at a different trajectory and speed than

anticipated, it makes it difficult to control the ball. The recommended values are between 45

and 50%.

39

Figure 3.8. The equipment for measuring angel ball behaviour.

3.3.3 Ball Roll

This test determines the distance a ball can travels across the surface. The equipment is

presented in figure 3.9. A ball is rolled down a ramp and allowed to roll across the surface

until it comes to rest. The measured distance is an indication of the speed of the ball. This

has influence of the quality of the game. Their anticipation could be different it allows the

surface to be classified in terms of the speed of the surface or the deceleration of the ball

over the surface. The ball roll values for natural turf would vary between 4m and 10m,

natural turf in ideal conditions will give values from 4m to 8m. The lower the values of the ball

roll, the slower is the pitch. The behaviour of the ball roll is related to the friction between the

ball and the surface of the pile layer. On newly installed surfaces the ball roll distance is

between the limited values recommended by FIFA (between 4-10m). But this is not the case

for used surfaces. As the pile layer is flat it influences the ball to roll over the recommended

distance.

40

Figure 3.9. The equipment used to determine the ball roll distance.

After a certain time of use the pile layer will become flat by largely affecting the properties

related to the ball roll distance [3-4] the ability of the pile layer to recover, to return to first

position, is the key to have good properties of these fields. As it was mentioned in the

second chapter pile layer, composed by monofilaments, are tufted on the backing and later

secured their position by realizing coating with latex. Since the yarns are a fixed element in

the artificial turf construction, their properties should remain at a high level during use and for

long time in order to fulfil the FIFA and EUFA requirements.

41

References

[1] FIFA quality concept for football turf. Available on internet

http://www.fifa.com/mm/document/footballdevelopment/footballturf/68/52/24/fqctestm

ethodmanual%28may2009%29.pdf. 5 April 2012.

[2] Patent WO 2006/092337 “Artificial turf structure with granular infill”. Published 8

September 2006.

[3] H.J. Mink Kwaliteitszorg KNVB Kunstgras voetbalvelden ISA sport, MAART 2008.

[4] Joosten, T., 2003. Players experiences of artificial turf. ISSS Stadia turf summit,

Amsterdam. (ISSS publication), Available from:

http://www.isss.de/conferences/Amsterdam2003/Joosten.pdf visited at December

2010.

42

4. Chapter 4

Test methods to measure the resilience of

single monofilaments

In this chapter a new test method is described to measure the resilience of monofilaments

for artificial turf applications. Rather than measuring the resilience of the final product( the

resulting carpet) more practical tests can be used to measure the bending behaviour of the

monofilaments at a very early stage. Two methods are of interest hereby, namely the

dynamic and static bending test. The new test method was evaluated by using the results of

the Favimat R and12m-Lisport measurements.

4.1 Introduction

In chapter 3, the standard test methods to evaluate the synthetic surfaces for football sport

applications have been discussed. To guarantee the safety and quality performances of the

playing surfaces intended for sport applications, these surfaces have undergone enormous

improvements in the last years [1-2] which have been described in the previous chapters.

Despite these improvements, there is still quite some resistance among players and clubs

against artificial sport surfaces. These are partly based on experiences in the past, when

soccer was played on older types of artificial turf, not adapted to the specific demands of

soccer, but also partly due to shortcomings of the newest types of artificial turf, even

specifically designed for soccer applications.

One of the main complaints is that the ball roll behaviour is different from natural turf. In

general, players tend to perceive that the ball speed is higher and sometimes it is considered

too high. Joosten [3] found that 77% of the players experienced the ball speed and ball roll

capacity very high. In addition, the speed of the ball roll across the field has a great influence

on the execution speed and on the interaction with the surface (pile layer). On newly

installed artificial surfaces the behaviour of the ball is acceptable, within the limit of the FIFA

standards and even comparable to the one on natural turf. But after some times, the pile

layer will become flat, the ball will have less resistance to roll and therefore the speed and

the distance of rolling will be higher than expected [4]. Faster surfaces might be acceptable

for training purposes as some players prefer fields with a faster ball roll, especially, as it

facilitates a more technical way of playing. However, most players prefer a field with a

normal ball speed when it comes to official games.

43

As the pile layer is composed of monofilaments, fixed on the backing of the carpet as

explained in chapter 3, it is not possible to replace it, differently from infill materials.

Therefore, securing long term behaviour of monofilaments is the most important property for

the long term behaviour of the carpet itself.

Monofilaments are under several imposed deformations, as a result of ball-surface and

player-surface interaction. The most common deformations are the bending deformations,

which are considered in this research. The ability of the monofilaments to recover from

bending deformations, fully return to their first position, is called resilience. A product is

100% resilient if it can totally come back to its original position after deformation (action

during the game or training).

For artificial turf, an indication of the resilience can be obtained by measuring the ball roll

distance. As described in chapter 3, this test could be performed after installing the entire

system, which it will take time having e feedback. This makes the turf producers to wait.

Therefore, developing new test methods to measure the resilience/bending behaviour of

single monofilaments under different conditions is a real demand. A deeper insight of the

main parameters influencing the resilience or bending behaviour of monofilaments is

necessary.

Before explaining the resilience of monofilaments, it is of interest to explain the possible

deformation of monofilaments. As shown in figure 4.1, there are two possible ways of the pile

layer to become flat. For figure 4.1a, brushing up the filaments will help the pile layer to

recover to its original structure. So in this case the maintenance seems to be a good

solution. For figure 4.1b, brushing will not be efficient since an extra deformation is present

on the free pile layer, marked as a deformation between points 1 and 2. The second case

will be the focus of this research.

44

Figure 4.1. Possible deformations of the pile layer; a) deformation starting from the basis

which can be recovered by brushing up, b) deformation of the pile layer from the basis and

on the last contact point with the rubber infill, otherwise called the deformation of the free pile

layer.

4.2 Theoretical analysis for bending of monofilaments

Most of the formulas for strength of materials express the relations among the form and the

dimensions of a product, the loads applied thereto, and the resulting stress or deformation.

Any of such formulas are only valid within certain limitations and only applicable for certain

problems.

4.2.1 Beams; flexure of straight bars, assumptions for bending of

monofilaments

The formulas of straight beams [5] are based on the following assumptions:

1. The beam is of homogeneous material and the same modulus of elasticity is tension

and compression.

2. The beam is straight or nearly so; if it is slightly curved, the curvature is in the plane

of bending and the radius of the curvature is at least 10 times the depth;

3. The cross section is uniform

4. The beam has at least one longitudinal plane of symmetry

5. All loads and reactions are perpendicular to the axis of the beam and lie in the same

plane which is longitudinal plane of symmetry

6. The beam is long in proportion with its depth

45

7. The beam is not disproportionately wide

8. The maximum stress does not exceed the proportional limit.

During the static loading of an elastic system, the external work done by a constant force

acting thereon is equal to the internal work done by the stresses caused by that constant

force. This relationship is the basis of the method for finding the deflection of any given point

of an elastic system: a unit force is imagined to act at the point in a question and in the

direction of the deflection that is to be found. The stresses produced by such a unit force will

do a certain amount of internal work during the application of the actual loads. A unit force is

imagined to act to the specified point and the deflection is equal to the work used to deflect

the beam, since the applied force is constant.

4.2.2 Structural Beam Deflection

A beam of known cross section geometry will bend under the application of a specified load

and distribution (see figure 4.2).

Figure 4.2. The position of bending of a straight beam is schematically represented,

equivalent with a one side fixed monofilament on artificial turf.

Deflection at specified point:

Deflection at the un-supported end:

And the slope measured by θ:

46

Where:

E = Modulus of Elasticity (N/m

2)

I = Moment of Inertia (m4)

F = Load (N)

s = Stress at the cross-section being evaluated (N/m2)

y = Deflection (m)

l = Distance as indicated (m)

a = Distance as indicated (m)

The moment of inertia is function of the cross section and the distance from the neutral axis

to the edge of the beam geometry. The following formulas are specified.

Moment of Inertia for a:

Diamond shape:

Square:

Rectangular: or

Solid cycle:

Hollow cycle:

Sector of hollow

Or

47

Note that if t/R is small, α can exceed π to form an overlapped annulus.

Based on a study [6], carried out by the Department of Textiles at Ghent University, the

bending force shows an exponential correlation with the free pile length. The obtained

results, presented in figure 4.3, show that the measured bending force is decreased by

increasing the free length of the tested samples. The obtained results are in accordance with

the theoretical formulas mentioned above although the exponent of the power law correlation

obtained here (in absolute terms) is always smaller than expected, theoretically, F ~ 1 / L³.

This could be explained by the fact that the theory considers pure bending and small

deformations, assumptions which are not completely fulfilled in real conditions, as it is

proved [6].

Figure 4.3. The relationship between the measured bending force of monofilaments in

function of the free length of monofilaments. In the present graph, the results of two different

monofilaments produced from different polymeric materials are represented. For confidential

reasons the polymeric materials are not mentioned.

These results are very important for the quality of the artificial turf field and demonstrate the

importance of the free length of the pile layer (monofilaments) in real conditions. This also

explains the importance of a very good maintenance of the artificial turf fields. Especially, the

results of the ball roll are directly correlated with the energy necessary to bend the

monofilaments. Beside these factors that influence the energy to bend the monofilaments

and the performance of the artificial turf fields, there are also other factors like the type of

y = 118,15x-2,138 R² = 0,9979

y = 77,273x-2,171 R² = 0,9922

0

1

2

3

4

0 2 4 6 8 10 12 14

F (

cN

)

Length (mm)

Product E Product H

48

polymeric material used for the production of the monofilaments production and the density

of the carpet (the number of monofilaments per meter square).

The polymeric materials, used for the production of monofilaments and having good

mechanical properties, should also be characterised by a low coefficient of friction. Some of

the polyolefin, such as High Density Polyethylene (HDPE) and Polypropylene (PP), are

characterized by a low coefficient of friction [7] which makes them the most suitable for

artificial turf monofilaments. The resulting coefficient of friction is an important parameter or

characteristic of the fibres for artificial grass applications.

The coefficient of friction is related to the nature of the polymer, polar or non-polar, and to

the mechanical properties such as the elastic modulus. The linear low density polyethylene

(LLDPE) is the most widely used polymer for the production of monofilaments for artificial

turf, as described in chapter 2, resulting from the best balance at this moment between the

mechanical properties, the coefficient of friction, the touch of the artificial turf field and the UV

and weather resistance.

The density of the carpet (artificial turf, stitches per square meter) directly influences the ball

roll distance. The higher the density, the more difficult to bend the pile layer and the higher

the energy needed for bending resulting in a lower ball roll distance. However, these

elements are considered to be constant in this research.

As can be understood from the above theoretical formulas, the deflection distance “y” seems

to be influenced by:

length (free length)

thickness

profile cross section (moment of inertia)

polymeric material used for monofilaments (

In function of polymeric materials there are also the parameters production of the

monofilaments.

49

4.3 Test methods for resilience of pile layer measured in whole system/artificial

turf

4.3.1 1m-Lisport

As it was described in chapter 3, the ball roll behaviour is tested in accordance to the FIFA

and UEFA standard ball roll test (figure 3.9). A ball is released from a 1 m ramp and the

rolling distance of the ball is measured. In order to get the FIFA one star certificate – which

corresponds to the level for community play, training and national level - this distance should

fulfil the FIFA requirements. In order to get the FIFA two star certificate, which corresponds

to an international level, the ball roll should be between 4 and 8 m [8]. This test can be

performed outdoor, in real conditions done on a newly installed field, as well as on a field

that has been played on for a certain period of time.

This test gives a clear indication of the degeneration of the ball roll quality, but it cannot be

used as an objective criterion to examine the effect of the ball roll degradation, as this test is

sensitive to external factors.

These external factors like wind speed and wind direction, slope of the field, wet or dry

conditions, etc. have an important impact on the ball roll distance. Moreover, the test result

depends a lot on the condition of the monofilaments: tests with the FIFA ball roll setup have

shown that the ball speed – and thus the ball roll – is affected to a great extent by brushing

the fibres. Beside this the feedback about the ball roll behaviour will take time, months or

even years, which is neither practical nor welcomed by the producers of artificial turf.

The indoor performance of this test has a lot of advantages especially regarding the quick

feedback compared with the test performed outdoor and avoiding also the external

influences. This has been possible due to artificially wearing the specimen by using the test

called Lisport [9] as described in chapter 3 (figure 3.1). This test simulates the wear and tear

of a field over time: 5200 cycles approximately correspond with 5000 playing hours. As a

result of this test, one gets a qualitative idea of the durability of the field, the (undesired)

fibrillation of the fibres and the amount of detached filaments. Therefore, it is used in the

FIFA regulations as a way to prepare the specimens for other tests: vertical ball rebound,

rotational resistance, shock absorption, energy restitution and vertical deformation are

determined before and after Lisport wearing. The limits for training pitches are raised to

20.200 cycles Lisport, because of their heavy usage.

This test is clearly very useful to simulate the degradation of the field after use for the

mentioned characteristics. But as the Lisport allows the analysis of artificial turf samples

restricted to 0.4 by 0.8 m, it is impossible to quantify the effect of the (visually observed)

50

degradation of the playing quality of the turf on the actual ball roll behaviour, as this can only

be tested with a turf sample of at least 10 m long. Due to the small sample size and the fact

that the sample has to be removed and disturbed before doing tests such as ball rebound or

shock absorption, some errors on these results may be expected. Also, it requires the

production of an artificial turf sample of 0.8 by 0.4 m, which is laborious and time-consuming

in the development process as yarn producing companies want immediate information on

the quality of their monofilaments/fibres.

4.3.2 12m-Lisport

In order to avoid the limitation of the dimensions of the samples for the ball roll test, which

should be at least 10m, 12m-Lisport equipment (figure 4.4) has been installed by the

Department of Textiles of Ghent University. The idea of artificially wearing with a 12m-Lisport

is the same as for the standard Lisport.

The 12m-Lisport is made in accordance with the European Standard [10] The rolls are

proportional to the small Lisport: 1m wide and weight of 100 kg each instead of 30 kg for 30

cm for the conventional Lisport. The stud configuration corresponds with the European

standard. The speed can be varied, but for these tests the speed is kept constant at 0.25

[m*s-1

]. One roll is rolling 40% faster than the other roll. There is no transversal movement

and the sample lies directly on concrete or by putting other supporting elements.

The advantage of a 12m-Lisport is that it gives the possibility to measure the ball roll in two

directions, but the production of an artificial turf specimen is still necessarily. Also it is not

clear if the degradation of artificial turf observed visually after the Lisport test, is due to the

fibre itself, the used infill or the combination between infill and fibre. Therefore, it is important

to test the filaments/ pile layer separately. This is of particular interest for artificial turf yarn

producing companies.

51

Figure 4.4. 12m-Lisport apparatus installed at the Department of Textiles of Ghent

University.

4.3.3 Test methods for bending behaviour applied only on single

monofilaments, Favimat R

As described in the first paragraph, during the interaction player surface and ball surface the

pile layer is subject to several deformations; pulling, rotation, torsion etc. In the presented

test methods, the bending deformation is considered as the most important deformation

since the tufted monofilaments are bended onto the infill material or in other words they do

not stay perpendicular with the horizontal plane (with the textile mat). The results are related

to the free pile length and with the position of bending as explained before.

The first test method developed to measure the resilience of monofilaments is the “Dynamic

Bending” test. This method creates the possibility to quantify the bending behaviour of the

artificial turf fibre over time. This method measures the resilience of a single monofilament or

several monofilaments at the same time without being influenced by other elements

presented in the real field such as the infill material. The apparatus used for this test is

Favimat R [11] which has been modified for this purpose, making it possible to flex a one-

side-clamped filament (see figure 4.5 a).

For this test one side of the monofilament(s) is clamped, while the other free side is

subjected to a perpendicular force, as described in figure 4.5.

52

a) b)

Figure 4.6. a) Schematic presentation of the Dynamic bending test Favimat R and b) test

setup.

The filament(s) has a free length of 17.5 mm (see figure 4.5.b) and is bended 2.87mm from

the clamped point. The filament(s) is bended 300 times at a speed of 100 mm/min. The limits

of the displacement are 2.0-8.0 mm. The total test duration for one repetition is 40 minutes.

Each test consists of four repetitions.

The force needed to cause bending is measured in both the advancing and receding part of

each cycle, thus obtaining hysteresis force-displacement loops (figure 4.6).

53

Figure 4.6. The typical hysteresis during the dynamic bending test of monofilaments

monitored during 300 cycles.

The resilience is expressed as the ratio between the maximal force of the last cycle of

bending (F300) and the maximum force (in the advancing part of the hysteresis) of the first

cycle of bending (F1), formula 1.

The number of flexing cycles is limited to 300 because of the memory of the machine.

However, 300 cycles seems very limited compared to the operational life of a pitch but the

sample is flexed during a very short time, 40 minutes. The chance that one filament is flexed

300 time in 40 minutes is very small. A filament(s) is 100% resilient if the force needed to

bend the filament(s) the 300th time is the same as the first time.

Figure 4.6 is an example of the measured force versus displacement. The results show that

from 15-20 cycles to 300 cycles, there is a slow decrease to a more or less constant value of

the maximum bending force.

The maximal force of each bending cycle is determined and is represented in a graph. The

loss in resilience is plotted as a function of the number of cycles (figure 4.7). Each test

results in 2 graphs; one is the graph with the absolute force (figure 4.7) and one with the

relative force (first force 100%, see figure 4.8).

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0 1 2 3 4 5 6 7 8 9

displacement (mm)

F (

N)

54

The most important graph is the relative graph, because this graph is related with the

increase in ball roll distance. The ball roll distance increase can indeed be predicted using

the resilience values.

Figure 4.7.The plot of the measured bending force in function of the number of cycles.

Figure 4.8. The relative bending force (in %) in function of the number of cycles.

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

0 50 100 150 200 250 300

F (

cN

)

Cycles

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300

F (

%)

Cycles

55

Although this test method is qualitative for monofilament, this test method is still limited as

the equipment allows performing the test at ambient temperature. Practically it is well known

that the temperature in the outdoor fields could go higher than the ambient temperature

(23±2 C), even up to 75°C. Therefore a new test method has to be developed in order to

quickly have results at other temperatures and to create the possibility to predict the

resilience of monofilaments at elevated temperatures.

The objective of this chapter is twofold, namely to develop a new testing method to assess

the bending behaviour of a single monofilament and to predict the behaviour of

monofilaments at elevated temperatures. The tested samples are randomly chosen from

industry. The results of the dynamic bending test and 12m-Lisport are more reliable to

predict the behaviour of the artificial turf field under real conditions.

4.4 Experimental set up for Static Bending test

In order to achieve a realistic bending of monofilaments, two U-shaped profiles, strictly with

the same height and very straight sides, are fixed together with two long screws which allows

fixing the monofilaments in between and securing in this way the perpendicular position of

the fixed monofilaments with the horizontal plane. For the free side of length 17.5 mm was

taken for the monofilaments, which corresponds with the length of the free part of the

monofilaments tested in the dynamic bending method (see figure 4.9). A flat plate is placed

on top of the monofilaments, initially located at the side of the fixed monofilaments and then

hooked up carefully until it becomes completely flat. Then a plate with a weight of 150 g,

which corresponds with the average weight each monofilament has in real fields, is placed

on top of the plate.

56

a) b)

Figure 4.9. a) Schematic presentation of the static bending test and b) the test set up for

static bending of monofilaments.

The remaining time of the fixed flat position of the monofilament is 40 minutes which

corresponds with the test set up for the dynamic bending and is almost the maximum time of

playing in the football game (half of the game in football is 45 minutes).

After removing the plate and the weight, monofilaments are freely relaxing. After a certain

time, pictures are taken for each of the samples. During the shooting of the picture, the

camera has to be in the same plane as the U-shaped profiles and in line with the fixed

monofilaments. This gives the possibility to avoid possible errors which might affect the

measurements of the position of the samples towards the horizontal line.

As the equipment is very simple and handy, it creates the possibility to perform the test at

elevated temperatures by placing the total equipment in a climate chamber.

4.4.1 Development of measurement techniques

For each test, the angle between the horizontal plane and the position of the monofilament is

measured. The recovery of the imposed deformation, defined as “Deformation recovery”, for

each specimen is calculated using the formula (2).

(2)

Where: the deformation recovery is expressed in percentage (%)

57

90° is the maximum angle, which corresponds with the perpendicular position of the

monofilaments at the beginning (t0),

Ø (tx) is the measured value of the angle at different relaxation times (tx),

tx is the relaxation time (5 min, 1 hour, 24 hours and 48 hours).

The angles of the yarn were measured for each monofilament and a mean value was

calculated.

Pictures are taken after 5 minutes, 1hour, 24 hours and 48 hours of relaxation. Then the

angle between the horizontal plate and the position of monofilaments is precisely measured,

as shown in figure 4.10.

Figure 4.10. An example of angle measurement for monofilaments during the static bending

test.

4.4.2 Results for new test method

The test is performed on three different samples, marked as B1, B2 and B3. These

monofilaments are produced from polyethylene, of the family of low density polyethylene

(LLDPE) and have different dTex, respectively 168.2; 92.5 and 126.4. The obtained results

will be discussed for each of them.

4.4.2.1 Deformation recovery at different relaxation times

From the calculated results, represented in figure 4.11, the deformation recovery was

increasing rapidly during the first hour of relaxation and then slowly increased for the rest of

α

58

the 24 hours of relaxation. From 24 hours until 48 hours of relaxation the deformation

recovery was practically the same and can be considered as a constant value (see figure

4.11). Therefore the relaxation time of 24 hours is the maximum relaxation time for the static

bending test [12].

Figure 4.11. Deformation recovery of monofilaments represented as a function of the

relaxation time. The three different samples are represented and the deformation recovery

seems to be constant after 24h of relaxation.

4.4.2.2 Correlation between static bending and dynamic bending

The deformation recovery and resilience for the different samples are represented in figure

4.12. Obviously, a very good correlation between these two is obtained (R2=0.98).

These samples have undergone all the possible steps of processing conditions and are

supposed to be ready to be used in the carpet production line.

0

20

40

60

80

100

1 10 100 1000 10000

Defo

rmati

on

Reco

very

(%

)

Relaxation time (log)

B-1 B-2 B-3

59

Figure 4.12. Correlation between deformation recovery and resilience for industrial products.

4.4.2.3 Correlations with the 12m-Lisport

It was possible to perform the 12m Lisport test on the carpet/ artificial turf based on samples

B1 and B2. The fibre height was 60 mm, 18 mm was filled with sand and 31 mm with SBR

granules. The resulting free height was 11 mm. After every 1000 cycles of 12m-Lisport

testing, the ball roll distance was measured. The ball roll distance of the artificial turf

structure with yarn B1 increases from 6 m to 7.8 m after 10000 cycles. The ball roll distance

of the artificial turf structure with yarn B2 increases from 6.5 m to 9.1 m after 7000 cycles and

further to 12.2 m after 10000 cycles.

A better image can be obtained by plotting the relative ball roll distance as function of the

number of applied cycles (figure 4.13). The relative ball roll distance is hereby defined as

measured ball roll distance divided by the initial ball roll distance.

B1

B2

B3

60

Figure 4.13. Relative ball roll distance as a function of the number of cycles.

In figure 4.14 the 1/relatively ball roll distance in function of the cycles for 12m-Lisport test is

presented. The obtained image is quite similar to the 1/bending force for Favimat R (figure

4.15). In both cases sample B1 shows a better behaviour compared with sample B2.

Figure 4.14. The 1/relative ball rolls distance in function of the number of cycles for 12m-

Lisport.

1

1,2

1,4

1,6

1,8

2

0 2000 4000 6000 8000 10000 12000

Rela

tiv

lery

Bo

ll R

oll d

ista

nce

Number of Cycles B1 B2

0

0,2

0,4

0,6

0,8

1

1,2

0 2000 4000 6000 8000 10000 12000

1/r

ela

tiv

e B

oll R

oll d

ista

nce

Number of Cycles B1 B2

61

Figure 4.15. The relative bending force in function of the number of cycles for the Favimat R

test.

To compare the results of the 12m-Lisport with the deformation recovery during the static

bending test, the inverse value of the relative ball roll distance has been calculated.

The measured value of the inverse value of the relative ball roll distance can be divided into

two parts: an asymptotic value reached after a sufficient number of cycles and a difference

between the inverse values of the measured values of the relative ball roll distance and the

asymptotic value. For the experiments with the B1 yarns, the asymptotic value equals 0.77

and the difference is 0.23. This means that 22.8% of the resilience is lost during the

experiment with the 12m-Lisport and explains the increase of the ball roll distance from 6m

to 7.78m.

For the B2 yarns, the asymptotic value is 0.66 and the difference is 0.34. This means that the

B2 yarns are losing 34% of their initial resilience in this experiment.

Afterwards, after 7000 cycles, the yarns started to fibrillate visually and the resilience is

further decreased due to this fibrillation. These asymptotic values of the relative ball roll can

be related to the reversible deformation after static bending. This relation is represented in

figure 4.16.

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0 50 100 150 200 250 300

1/F

(cN

-1)

Number of Cycles

B-1 B-2

62

Figure 4.16. Asymptotic value of the relative ball roll distance on the 12m-Lisport as function

of the reversible deformation after static bending at 23°C.

As a conclusion, the relative resilience behaviour of yarn B1 is better than yarn B2 supported

by the relative ball roll distance. This is confirmed by the change of 1/(relative ball roll

distance) as function of the applied cycles and related to the variation of the bending forces

and total bending energy as function of the applied cycles in the cantilever experiment.

Apart from showing a good correlation between these two test methods, the questions could

rise:

Why these samples have different values of resilience and different deformation recovery?

Why sample B1 have higher values of resilience and deformation recovery compared with

two other ones?

Referring to the theoretical explanation in paragraph 4.2 one of the reasons could be the fact

that samples have different thicknesses. But figure 4.17, shows that there is no correlation

between the bending force and the thickness of the product as expected. Another

explanation can be related with the structure of the product obtained for different parameters

during production of monofilaments [13]. Therefore, further explanations are needed and will

be discussed in details in the following chapters.

63

Figure 4.17. The relation between cross section and maximum bending force for the tested

samples.

4.5 Mechanical properties

4.5.1 Tensile test

Tensile testing is one of the most common forms of physical testing performed on

monofilaments. The data available from this test are very interesting with regard to the

applications of the monofilaments and for the fundamental knowledge of their internal

structure. The tensile test was performed in an Instron tensile apparatus with a load cell of

500 N, a length of the samples of 50 mm between the clamps and an elongation speed of

500 mm/min. The elastic region and the yield point for each sample after performing five

repetitions were analysed. The test was performed at ambient temperature and at higher

temperatures.

From these tensile tests, represented in figure 4.18, it is possible to calculate the following

properties:

Elastic Modulus characterising the first linear correlation between the stress and the

imposed strain;

Yield phenomena. A specimen is said to yield at the point beyond which the applied

strain is no longer fully recoverable.

0

0,5

1

1,5

2

2,5

0 0,05 0,1 0,15 0,2 0,25

F1 (

cN

)

Cross section (mm2)

B1 B2 B3

64

Figure 4.18. Stress-strain curves and the way of calculating the elastic modulus of the

tested samples.

4.5.2 Stress-Creep Relaxation

By using the Instron equipment another test method was performed, namely stress

relaxation in combination with creep relaxation (see figure 4.19). This method consists in

stretching the monofilaments up to a certain strain, 20 % in these experiments, a constant

strain for 10 seconds and relaxing freely the monofilaments to recover their reversible

deformation. This cycle was repeated 10 times and at different temperatures. The chosen

temperatures were 22⁰C, 40⁰C, 60⁰C, 80⁰C and 90⁰C. The maximum tenacity for the first

cycle, the second cycle and the last one was measured for each of the tested conditions.

By using the data acquired from this experiment, the tensile resilience was calculated by

dividing the maximum tenacity of the last cycle by the maximum tenacity of the first cycle

(formula 3).

Where: R tensile resilience, calculated in %

Maximum Tenacity 10 is the maximum tenacity at repetition 10

65

Maximum Tenacity 1 is the maximum tenacity at repetition 1.

Figure 4.19. Schematic presentation of the tensile strain at the beginning, at the end and

the maximum tenacity of the samples after different tensile deformation cycles.

4.5.3 Results for mechanical properties

4.5.3.1 Classical mechanical properties, elastic modulus

Referring to the obtained results, the mechanical properties of the tested monofilaments, B1

and B2, were quite different and related to differences in processing conditions of the

monofilaments. The internal stress as function of the stretching ratio was measured for the

two monofilaments. With the stretching ratio defined as λ, equal to the length of the tested

sample divided by the initial length, the measured internal stresses are reproduced in figure

4.20 as function of the first strain invariant, equal to formula 4:

This is the same as the one used in the theory of the rubber elasticity [14-16]

66

At 23°C, the modulus of elasticity was equal to 611 MPa for B1 and 1470 MPa for B2. The

maximum stress was 121 MPa for B1 and 269 MPa for B2 for the same elongation at break,

67%.

Figure 4.20. Relation between the measured internal stress (stretching force/section) as

function of the first invariant of deformation; full line-B2 and the broken line-B1.

The measured stress as function of the first strain invariant can be divided into two parts,

one part corresponding to the response of the crystalline structure composed of crystallites

adhered through inter-crystalline links and the second part corresponding to the elastic

behaviour of a crystalline enhanced amorphous matrix network [17]. The amorphous matrix

network can be characterised by the rubber deformation model as far as the elongation

temperature is above the glass transition temperature of the amorphous phase. The results

are represented in figures 4.21 and 4.22.

2-1/

67

Figure 4.21. Monofilament B1: full line-total stress, short broken line -stress corresponding to

the crystalline structure, long broken line-stress corresponding to the elastic or rubbery

behaviour of the tie-molecules

Figure 4.22. Monofilament B2: full line -total stress, short broken line - stress corresponding

to the crystalline structure, long broken line- stress corresponding to the elastic or rubbery

behaviour of the tie-molecules

The value of the yield stress is the same for the two monofilaments and equal to 90 MPa,

corresponding to the same degree of crystallinity. The elongation at yielding is equal to 35%

2-1/

2-1/

68

for B1 and 16% for B2. The results of the stretching experiments and the response of the

crystalline part indicate that the monofilaments B1 are characterized by a lower value of the

elasticity modulus than the monofilaments B2, but have a longer elongation before yielding.

The elastic response is characterized by an elasticity modulus of 20MPa for B1 and 75 MPa

for B2, both with the same elongation at break.

The measured values of the elasticity modulus in function of the temperature are

represented in figure 4.23. These values follow the Arrhenius law in function of the

temperature. The same relationship is obtained for the measured stress corresponding to a

stretching deformation of 20%, as shown in figure 4.24. in both cases, the highest values are

for the monofilaments B2 and the lowest for the monofilaments B1.

Figure 4.23. Elasticity modulus as function of the elongation temperature, □ -B2; ■ -B1.

69

Figure 4.24. Stress at 20% elongation as function of the elongation temperature, ■ -B2; □-B1.

4.5.3.2 Stress-creep recovery

By comparing the mechanical properties (table I) of the samples from stress- creep recovery

test, it is obvious that samples B2 and B3 show higher values of elastic modulus and tenacity

compared with sample B1. However they have almost the same degree of crystallinity,

around 43%. On the other hand, sample B1 shows higher values of resilience and

deformation recovery. The same relation is also observed for the resilience of elasticity and

tenacity of the samples on the stress-creep relaxation curves. The obtained results are

represented in figure 4.25. For comparison reasons, in figure 4.25 the resilience of each

sample after ten cycles. is shown. Sample B1 shows a better recovery as only 6 % of the

elasticity is lost after ten cycles, differently from samples B2 (13%) and sample B3 (28%).

70

Table I. Mechanical properties of samples for first and last cycles of stress- creep relaxation of samples at ambient temperature and at elevated

temperatures.

Sample Temperature (° C) Maximum tenacity Repetition

1 (cN/tex)

Maximum tenacity Repetition

10 (cN/tex) E-1 (cN/tex) E-10 (cN/tex)

B1

20

5.0 4.7 57.6 54.4

B2 12.5 11.0 130.9 109.6

B3 11.2 7.1 138.9 99.6

B1

40

3.2 3.0 32.2 28.9

B2 8.4 7.7 73.5 60.0

B1

60

2.4 2.3 19.2 17.9

B2 6.1 5.2 39.8 32.7

B1

80

1.9 1.8 12.0 11.1

B2 4.1 3.8 25.1 20.3

(Tenac10/Tenac1)

71

Figure 4.25. Comparison between resilience(F10/F1), resilience of elasticity (E10/E1) and

resilience of tenacity (Tenacity 10/Tenacity1) for B1, B2 and B3.

4.6 Bending deformation of the monofilaments at higher temperatures

By using the developed test method, as described above, it was possible to perform the

experiments at higher temperatures. The chosen temperatures are at ambient temperature,

at 40 and at 60°C. For each of the samples, three repetitions are performed and the

irreversible part of the deformation is plotted vs. relaxation time. The reproduced graphs are

presented respectively in figures 4.26- 4.28.

0

20

40

60

80

100

B1 B2 B3

(F10/f1) (E10/E1) (Tenc 10/Tenac1)F10/F1 E10/E1 Tenacity10/Tenacity1

72

Figure 4.26. Irreversible deformation at room temperature as function of the bending time; ■-

B2; □-B1

Figure 4.27. Irreversible deformation at 40°C as function of the bending time; ■-B2; □-B1

Irre

vers

ible

…

time (min)

73

Figure 4.28. Irreversible deformation at 60°C as function of the bending time; ■-B2; □-B1

The irreversible deformation varied in function of the logarithm of the time for the two

monofilaments and for all temperatures tested. The irreversible deformation followed the

Arrhenius law [18] in function of the temperature, as represented in figure 4.29. From figure

4.29, it appears that both selected samples show an increase of the irreversible deformation

by increasing the temperature. This is in the same line with the resilience of tenacity and

elastic modulus (see figure 4.30). Both samples show an increase of the irreversible

deformation for increased temperatures. As a result, the irreversible deformation for sample

B2 is higher than the irreversible deformation of sample B1 and, the ratio is around 2. Also, an

increase of the temperature from 23°C to 60°C results in an increase of the irreversible

deformation by a factor 2 (see figure 4.29).

74

Figure 4.29. The irreversible deformation after a bending time of 45 minutes as function of

the temperature; ■-B2; □-B1.

Figure 4.30. The irreversible deformation (%) as function of the temperature.

0

5

10

15

20

25

0 20 40 60 80 100

Irre

vers

ible

ten

sile r

esilie

nce f

or

10

cycle

s

Temperature (

C) B1 B2

75

The results at room temperature can be extrapolated to 40 or 60°C by taking the results of

the reversible deformations at these temperatures and using the correlation between the

reversible deformation and resilience or reversible deformation and the asymptotic value of

the relative ball roll measured by the 12m-Lisport test.

The asymptotic values of the ball roll for the monofilaments B1 at 40°C will be 8.45 m and 9.2

m at 60°C, starting from an initial value of 6 m. The asymptotic values of the ball roll for the

monofilaments B2 will be 10.3 m at 40°C and 13.4 m at 60°C [19], starting from an initial

value of 6m.These values are indicative of the quality of the artificial turf fields at higher

temperatures than room temperature and create an interesting tool to predict the playing

characteristics of the artificial turf fields at different temperatures.

4.7 Discussion

The bending of monofilaments seems to be very much influenced by the geometrical factors

like thickness, profile cross section and free pile length. Theoretically, the mechanical

properties of polymeric materials are influenced by the type of material they are composed

off but they are also influenced by the production parameters. Considering the theoretical

analysis, this seems to be very much influenced by the production parameters such as

drawing ratio, temperature, materials and the thickness of the product which are very

important for the morphological structures of the final product. Since the subject of this

chapter was to evaluate these test methods, a further explanation about the structure and

the relation with static bending and dynamic bending will be done in the following chapters.

The developments of test methods for measuring the resilience of single monofilaments

have several advantages such as a quick feedback for the producers and to study the

behaviour of the monofilaments at different production stages. However some hypotheses

are considered as discussed in section 4.2.

A static bending test creates the possibility to predict the behaviour of monofilaments at

elevated temperatures. In order to use the static bending test at elevated temperatures, it is

recommended that for each set of samples the correlation between two test methods at

ambient temperature should be determined for each set of samples. This unique correlation,

specifically for that set of samples, can then be used at higher temperatures.

The deformation recovery of the industrial samples (B1 and B2) shows a good correlation

with the 12m-Lisport and the static bending test can be considered as a useful test to predict

76

the behaviour of monofilaments at elevated temperatures, complemented by the correlation

between the static and dynamic bending behaviour.

4.8 Conclusions

In this chapter, the bending behaviour of monofilaments for sport surfaces has been studied.

A new test method has been developed to predict the resilience of single monofilaments and

this prediction could be extended at elevated temperatures. This new test method unlike

previous test methods, takes into account the real conditions in terms of temperatures of the

field. Combining both methods to measure the resilience of a single monofilament on

dynamic bending mode (Favimat) and static bending mode (new test method developed)

makes it possible to predict the effect of real imposed deformations on monofilaments

without the need to produce and to install the entire system of artificial turf.

77

References

[1] ASGi, association turf installers valid form webpage http://www.asgi.us/public-

library , visited 26/07/2012.

[2] Artificial Grass Market Study – 2007, valid from webpage

http://www.asgi.us/59.htm visited 26/07/2012 .

[3] Joosten T., Players’ experiences of artificial turf, Stadia Turf Summit of the

International Association for Sports Surface Sciences, 2003, Amsterdam,

Netherlands, http://www.issssportsurfacescience.

org/downloads/documents/6EGR67DKG1_Joosten.pdf.

[4] Isa sport “Kwaliteitszorg KNVB Kungras Voetbalvelden” Project number 28077000

Maart 2008.

[5] Warren C. Young, Roarks formulas for stress and strain, 6-th edition, ISBN 0-07-

072541-1, (1989).

[6] Confidential study from textiles department Gent University Belgium.

[7] G. Schoukens, “Developments in textile sport surfaces”, Advances in carpet

Manufacture, Woodhead Publishing Com., Cambridge, UK, 2009, Chapter 5, 102-

137.

[8] FIFA quality concept. Handbook of requirements for football turf. March 2006

edition [internet]. Available from:

http://www.fifa.com/documents/fifa/FQCturf/FQC_Requirements_manual_March

2006.

[9] Boisnard 1999 lisport Boisnard, D., The securitest: apparatus to measure the

interaction between foot and sport surface. 3d Technical Forum of the International

Association for Sports Surface Sciences, Mallorca, Spain, 1999

http://www.isss.de/conferences/Mallorca1999/forum3.html.

[10] ”Surfaces for outdoor sports areas ― Exposure of synthetic turf to simulated

wear” EN 15306 March 2007.

[11] Schoukens, G.; Rambour, S., International Conference on Latest Advances in

High -Tech Textiles and Textiles-based Materials. 23–25 September (2009), Belgium.

[12] B. Kolgjini, G. Schoukens, P. Kiekens, Journal of Applied Polymer Science, 124,

4081-4089(2012).

[13] Peacock A.J., Handbook of Polyethylene Structure Properties and Application.

New York 2000.

[14] P.D. Wu and E. van der Giessen,; Mechanical research communication, Vol.

19(5) 427-433, 1992.

78

[15] http://iopscience.iop.org/0031-9120/12/1/105.

[16] Marc R. Roussel, Department of Chemistry and Biochemistry, University of

Lethbridge, “Rubber elasticity” February 21, 2009.

[17] B. Kolgjini, G. Schoukens, P. Kiekens, S. Rambour, S. Janssens, 4th

International Textile Conference , Tirana Albania (2000).

[18] ScienTek Software1983-2006, www.StabilitySystem.com.

[19] B. Kolgjini, G. Schoukens, P. Kiekens; "Influence of the temperature on the

resilience and fibrillation resistance of monofilaments for artificial turf applications"

article submitted to Journal Polymer Engineering Science.

79

5. Chapter 5

Influence of stretching on bending behaviour

The influence of the stretching ratio of monofilaments on the bending behaviour is studied in

this chapter. The stretching ratio in solid state, modifies the structure of the product and the

corresponding performance of the monofilaments.

Rather than measuring the resilience of the final product, the developed test used to

characterize the bending behaviour of the monofilaments are used to characterise the

produced monofilaments in different stages of production line.

5.1 Introduction

In chapter 2, different types of polymers used for the monofilament production for artificial

turf were mentioned, including polyethylene (PE). The principal value of PE lies in its

desirable balance of physical properties in the solid state and in its chemical inertness.

These qualities in combination with its low costs and ready processability turn PE into the

material of choice for a wide variety of uses. Especially linear low density polyethylene

(LLDPE) became the raw material of choice for the grass in the third generation pitches [1].

LLDPE-based yarn offers reduced skin abrasion and superior player friendliness in sliding

and tackling compared to other yarn raw materials in combination with changes in the

construction of the pitch [1]. In the latest installation, longer piles with an elastic infill and a

shock absorbent system are used to provide improved player safety and comfort. [2]. But as

was mentioned in the previous chapters, the properties of the pile layer are changing in

function of the frequency of use of these fields. An indication for that is the speed of the ball

and the distance of the ball roll tested in accordance with the FIFA standards.

In chapter 4 were specified some of the parameters influencing on the bending deflection

and among them was also the type of polymeric material and the parameter production of

monofilaments.

The production of monofilaments, after extrusion, is based on post-stretching. A range of

properties can be obtained at the processing conditions.

Classical mechanical properties such as elastic modulus and stiffness of the product are well

described in literature. It has been recognized for quite some time that a very strong and stiff

product may be prepared from polymeric materials, in which molecules of infinite length are

arranged in an extended conformation parallel to a fibre axis [3]. With a higher draw ratio, the

elastic modulus and crystallinity increase, whereas the elongation at break is reduced

80

because of lamella rearrangement, and subsequently increase in crystallinity and amorphous

ordering [4-5].

Apart from the classical mechanical properties, not much is known about the resilience of

monofilaments. The objective of this chapter is to investigate the influence of the drawing

ratio (cold draw ratio) on the bending behaviour of monofilaments in order to assess the

optimal region of resilience and mechanical properties.

The tested monofilaments have almost identical thickness, the same cross section profile,

and are produced from the same polymeric material, in order to make possible to investigate

only the influence of the drawing ratio. Bending a filament will induce stress and strain, which

are highly influenced by the thickness of the filament. For example, doubling the thickness

will, theoretically, induce a bending stress which will be eight times higher (chapter 4).

For a structural investigation, different methods, such as DSC, X-Ray diffraction and Raman

spectroscopy, have been used in order to determine the different ordered phases, such as

crystallinity and interphase of semi-crystalline polymers. The morphology structure measured

with DSC, X-Ray and Raman spectroscopy is correlated with the obtained results.

5.2 Experimental setup

5.2.1 Material

The polymer material used in this study was obtained from Dow Chemical Company.

DOWLEX™ 2035G [6], linear low density polyethylene (LLDPE) with a density of 0.919

g/cm3 and a melt index of 6 g/10 min.

5.2.2 Production of the monofilaments

Extrusion of monofilaments

The filaments are extruded on a Haake Polydrive Extruder from Thermo Electronic

Corporation. It is a single screw extruder of 25 x D in length and a screw diameter of 19mm,

with a 3-zone heating system. The temperature in the first section (T1) was 140⁰C, in the

second section (T2) 180⁰C and in the third section (T3) 220⁰C. The temperature in the die

(T4) was 220⁰C. The die has 5 diamond-shaped openings with a cross section of 2.36mm2

each. After the melt stage, the filaments are pulled through a water bath for producing the

stretched monofilaments (see figure 1).

81

Figure 5.1. Schematic representation of the production of monofilaments in laboratory conditions (n, V2, V3, V4, V5 are speeds of the for rolls for

each step of production. f2 ,f3, f4, f5, are represent the cross section of monofilaments for each step. In this chapter the explanations are related only

for first stretching and second stretching (see table I). In chapter 6 are explained in detail the controlled shrinkage and annealed stages).

82

Table I. The production parameters of the monofilaments on MDR and CDR.

Sample

V1

(m/m

in)

V2

(m/mi

n)

Tex

(g/k

m)

f2

(mm2)

MDR

(Ø/f2)

V3

(m/min

)

Tex

(g/k

m)

f3

(mm2)

CDR

(f2/f3)

A3 2.35 0.80 3045 0.66 3.52 6.09 424 0.09 7.18

A4 2.57 0.80 2631 0.57 4.08 5.21 428 0.09 6.15

A5 2.79 0.80 2408 0.52 4.45 4.33 424 0.09 5.68

A6 3.01 0.80 2167 0.47 4.95 4.33 432 0.09 5.02

A7 3.23 0.80 1883 0.41 5.70 4.00 419 0.09 4.49

A8 3.45 0.80 1790 0.39 5.99 3.61 424 0.09 4.22

A9 3.67 0.80 1575 0.34 6.81 3.36 429 0.09 3.67

83

The complete line of production of monofilaments, schematically presented in figure 5.1, is

divided into four regions, but in this chapter it will be considered a melt stretching region (i),

from which the melt draw ratio (MDR) is calculated, followed by a solid state stretching

region (I), from which the cold draw ratio (CDR) is calculated. For the solid state stretching,

the filaments are conditioned in an air oven at 95°C. The MDR can be calculated by dividing

the cross section of the die opening (f1= 2.36 mm2) by the cross section (f2) of the filaments

after the melt stretching. The CDR is calculated by dividing the cross section before (f2) and

after (f3) solid state stretching (see table I). The final cross section (f3) is the same for all

samples.

Monofilaments with the same cross section but different draw ratios were produced by

changing the extrusion speed (n=V1) and the speed of the rolls (V2) and (V3), (see figure

5.1).

5.3 Characterization techniques

5.3.1 Dynamic and static bending test

The tests set up for bending characterization are the same as described in chapter 4. In both

cases experiments are performed at ambient temperature (23 ±2°C).

5.3.2 Tensile properties

The tensile tests were performe on an Instron 3369, with a load cell of 500N, samples were

clamped at L= 50mm and with a test speed v = 500mm/min. For each sample five

replications were performed. Tensile strength and elasticity modulus were calculated.

5.3.3 Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) was performed on a DSC Q 2000 (TA Instruments),

with a standard heating rate of 10⁰C/min in a nitrogen environment. Calibration is done with

indium and tin. An enthalpy of 290J/g for perfect crystalline polyethylene was used to

calculate the percentage of crystallinity.

84

5.3.4 Raman measurements

Raman measurements were performed on a FT-Perkin Elmer instrument. The

measurements range is from 3500 to 300 cm-1

. Three repetitions were done for each

sample, consisting of 32 scans, and a laser power of 800mW was used. The raw Raman

spectra were smoothed, and baseline corrected. The total integral intensity of the CH2

twisting region (1250-1350cm-1

) is independent of the degree of crystallinity and is used as

an internal standard [7]. The mass fraction of the crystalline phase (CR), the amorphous

phase (AR), and the interphase (TR) contained in the investigated samples was calculated

using the formulas (1) proposed by Strobel [7], after a Gaussian deconvolution.

CR= ; AR ; TR=1- (CR+ AR). (1)

I1417 and I1303 are the intensities at 1417cm-1

and 1303cm-1

. Itw is the integral intensity of the

whole CH2 twisting vibrations region (1250 – 1350cm-1

) and was used as an internal

standard.

5.3.5 WAXS measurements

The measurements were done on an ARL-XTRA, X-Ray diffractometer from Thermo Fisher

Scientific at the COMOC research group (Ghent University). The measurements are used to

characterize the crystalline microscopic structure of the polymer. The radiation source Cu K1

was operated at 45kW, 44mA. The scanning angle ranged from 5⁰ to 50⁰ (2θ), λ=1.54Ǻ, 0.02

step-size. The percentage of the amorphous, orthorhombic crystalline phase and the

monoclinic phase are calculated after deconvolution of the original spectra using the

Gaussian fit procedure.

5.4 Results and discussion

5.4.1 Correlation between the two bending methods

The resilience and the deformation recovery have been calculated for all the samples, using

two test methods: dynamic bending, which measures the maximum force for each cycle, and

static bending, which measures the deformation recovery. As can be concluded from figure

5.2, there is a good correlation (R2 =0.92) between the two methods for these samples.

85

Figure 5.2. Plot of deformation recovery from static bending versus resilience from dynamic

bending, for the samples with the same thickness, at ambient conditions.

5.4.2 Cold drawing effect on resilience and deformation recovery

The calculations of resilience obtained by dynamic bending and of deformation recovery by

static bending are summarized in table II. In figure 5.3, the resilience and the deformation

recovery are plotted as a function of the cold draw ratio(s).

The resilience and deformation recovery are decreasing slowly up to the CDR of 5.7, but an

increase above this value of CDR causes, for both methods, a significant decrease with

different gradients, the recovery deformation is more sharply decreased compared to the

resilience.

R² = 0,9151

0

20

40

60

80

100

0 20 40 60 80 100

Defo

rmati

on

reco

very

(%

)

Resilience (%)

86

Figure 5.3. Deformation recovery (static bending) and Resilience (Favimat R) vs. cold draw

ratio at ambient temperature.

This is due to a different way of acting or imposing deformation to the monofilaments when

comparing both methods. With dynamic bending, the samples/monofilaments are subjected

to a continuous flexural deformation for 40 minutes and for static bending, the deformation

imposed in the samples/monofilaments is a fixed one during 40 minutes.

When analysing the differences in CDR, the possible yielding of the monofilaments is more

important for monofilaments with a high CDR (above 5.7) than the variation of the resilience,

measuring the response time of the structure to the flexural deformations. This is an

indication of the increased orientation of the monofilaments and the transformation of the

crystalline structure into microfibrils.

As described in literature [8-9], the mechanical response to an applied deformation can be

correlated with the morphological and molecular characteristics of the polymers. The precise

morphology of an oriented sample is a function of many factors, such as deformation and the

process by which orientation was achieved. During the production of monofilaments, two

types of stretching will occur, melt stretching (initially) and solid state stretching. The melt

stretching can induce a certain degree of polymer chain orientation but an important

transformation of the crystalline structure is expected by solid state stretching of the

monofilaments. The orientation of the crystals phase is higher than the amorphous phase,

which can be explained by the easy relaxation of the amorphous chain [10].

0

10

20

30

40

50

60

70

80

3 4 5 6 7 8

Resilie

nce a

nd

Defo

rmati

on

re

co

very

(%

)

Cold draw ratio (f2/f3)

Resilience

Deformation Recovery

87

The stress - elongation curves for samples with different MDR’s is represented in figure 5.4.

The chosen samples are A3, A5 and A9, starting products before cold drawing. From this

figure, it is clear that the samples do not show a significant difference between them,

differently from the products after cold drawing as represented in figure 5.5. This figure

represents the true stress – true strain curves of samples for different CDR’s or different

degrees of solid state deformation. Solid-state deformation normally results in the destruction

of the crystallites of the original morphology, followed by a reordering to form new crystallites

[9].

Figure 5.4. Stress strain curves for samples with different melt draw ratio (MDR).

0

5

10

15

20

25

30

35

0 1000 2000 3000 4000 5000

Str

ess (

MP

a)

Strain (%)

A3 A5 A9

88

Figure 5.5. Stress strain curves for samples with different cold draw ratio. The arrow

indicates the increase of cold draw ratio.

In a general way, the samples show the same behaviour for both methods of bending.

However, the response for static bending is more influenced by the cold draw ratio than the

dynamic bending response. During static bending, the imposed deformation is constant for a

relatively long period of time (40 minutes) and the bended filaments have the possibility to

modify their structure and to create the same irreversible deformation in function of the time,

corresponding to the same kind of yielding. During dynamic bending, the deformations are

cyclic and the measurements of the force correspond more to the response time of stress on

the structure.

5.4.3 Correlation between bending tests and morphology structure

The values of crystalline and amorphous structures obtained by DSC, Raman and X-ray

measurements are summarized in table II and III. The crystalline fraction is increased by

increasing the cold draw ratio for all tested monofilaments, well known from literature [8-9],

but only in a limited way. Some variations are observed for the amounts of the interphase,

0

100

200

300

400

500

600

0 10 20 30 40 50 60 70 80 90 100 110 120

Tru

e S

tre

ss (

MP

a)

True Strain (%)

A9

A3

89

which is increased as the CDR is increased. The amorphous phase measured by Raman

spectra, (AR), and DSC (see table II and III) is decreased by increasing the CDR.

90

Table II. Bending behaviour. Thermal and tensile properties of samples at different CDR.

Sample Cold draw

ratio Resilience

(%)

Deformation

Recovery

(%)

DSC

Crystallinity (% )

DSC

Amorphous

(%)

E Modulus

(MPa)

Max Load

(N)

A3 7.2 24±5 33 51±1 49 270±3 100 ±9

A4 6.1 28±3 37 50±2 50 203±8 75±5

A5 5.7 34±3 59 47±0 53 165 ±9 57±4

A6 5.0 35±3 57 47±2 53 144±6 59±6

A7 4.5 35±3 69 47±1 53 125±4 55±5

A8 4.2 35±3 67 46±3 54 112±4 53±2

A9 3.7 38±0 72 46±0 54 103±3 45±3

91

Table III. Mass fraction of amorphous, crystalline, and interphase, determined with Raman and WAXS methods.

Sample

Raman X-Ray

CDR

(f2/f3) Amorphous

(%)

Crystalline

(%)

Transition

phase

(%)

Monoclinic

Crystalline

(%)

Orthorhombic

Crystalline

(%)

Amorphous

(%)

A3 7.2 36±4.0 46 ±3.1 18 16.0 ±0.4 47.9 ± 2.2 36.0

A4 6.2 43 ±3.3 42 ±4.5 15 11.4 ±0.1 51.6 ±1.9 36.0

A5 5.7 46 ±0.5 42 ±3.3 12 9.0 ±0.2 52.1± 3 38.9

A6 5.0 47 ±0.1 42 ±3.3 11 11.9 ±0.1 51.7 ± 1.7 36.3

A7 4.5 49 ±1.7 41± 3.4 10 10.8 ±0.1 51.6 ± 2.1 37.5

A8 4.2 50 ±4.9 41 ±6.6 9 12.4 ±0.1 51.1 ± 4 36.5

A9 3.6 51 ±1.8 41 ±6.0 8 11.3 ±0.1 51.9 ± 4 36.6

92

From the DSC curves (see figure 5.6), it is quite clear that the melting peak temperature is

around 125⁰C for all the samples, however, they show slightly different values of the melting

enthalpy as a result of the cold drawing. The arrow in the figure indicates the results of the

melting behaviour on an increasing cold draw ratio, going from 3.7 (A9) to 7.2 (A3).

Figure 5.6. DSC curves of all samples with different CDR. The arrow indicates the increase

of CDR.

By drawing above a CDR 5.7 (A5), an increase of the DSC crystallinity is observed and the

melting temperature of the extra crystalline fraction is between 70 and 90⁰C. This is the

result of the crystallization under stress or the orientation of the low melting part of LLDPE,

situated below the temperature of cold drawing of the monofilament (95⁰C). It seems that a

critical value of the cold draw ratio is necessary to induce this second crystallization and to

create a small concentration of very thin lamellae from the macromolecules containing a high

concentration of comonomers, typical of Ziegler Natta LLDPE’s.

The Raman spectra for all the samples, presented in figure 5.7, show a typical spectrum of

solid LLDPE in the 950-1500 cm-1

region [11-14], by superposition of three phases: an

93

orthorhombic crystalline phase, (CR), an amorphous phase (AR) and a disordered phase of

anisotropic nature (TR). An example of Gaussian deconvolution of Raman spectra is

presented in figure 5.8. In table IV, the assignments of the Raman bands are summarized

[14].

Comparison of the Raman spectra of these samples allowed the characterization of the

monofilaments as a function of CDR. The analysed data of the Raman measurements

confirmed that the crystalline phase is not very much influenced by the CDR, a part of

sample A3 (7.2). Besides that, as to the so-called amorphous (AR) and transition phases (TR),

by increasing the CDR, the amount of the so-called amorphous (AR) phase is increased from

25% for the non-oriented product up to 44% for the sample A5, cold draw ratio of 5.7, while

the amount of the transition phase (TR) is decreased. For the samples with a cold draw ratio

of 6.2 and 7.2, the so-called amorphous phase equals the amount of amorphous phase

measured by X-Ray spectroscopy. The two different zones of the Raman results as function

of the CDR are represented in figure 5.8. It follows from the discussion later on that these

three phases, calculated from Raman spectra according to the published equations [11-14],

do not correspond to the different phases according to the DCS and X-ray measurements.

However, a good correlation can be observed between the mechanical properties of the

monofilaments and the amount of the crystalline phase obtained from DSC-measurements

and the amorphous phase obtained from X-Ray spectroscopy.

Figure 5.7. Raman Spectra for samples A3, A5 and A9 after baseline correction and

normalized at 1240-1340cm-1

.

94

Figure 5.8. An example of Gaussian curve fitting of Raman spectrum for the phase

calculation (crystalline, amorphous and interphase as a difference between crystalline and

amorphous phase).

According to Lagaron [15], the 1130:1060cm-1

band intensity ratio from the Raman spectrum

reflects the degree of orientation. This measured ratio is increased with the CDR and is in

good correlation with the amounts of the transition phase. Also, the elastic modulus (see

table II and table III) is directly correlated to these ratios as presented in figure 5.9.

Gaussian deconvolution for Raman spectra

95

Table IV. Assignments of the Raman Bands of Polyethylene [14]

Bands (cm-1) Phase Mode

1060 C(A) Vs (C-C)

1080 A Vs (C-C)

1130 C(A) Vas (C-C)

1170 C(A) (CH2)

1296 C τ (CH2)

1310 A τ (CH2)

1370 C ω (CH2)

1418 C ω (CH2)

1440 A δ (CH2)

1460 A 2* (CH2)

C= Crystalline, A= Amorphous, V = stretching (s=symmetric & as=asymmetric), =rocking,

τ = twisting, ω =wagging, δ = bending.

A good correlation seems to exist between the elastic modulus and the amount of the

crystalline phase as measured by DSC-crystallinity and represented in figure 5.10. As a

result of this correlation, the elastic modulus is very low up to a degree of crystallinity of 42.5

wt% and is increasing linearly afterwards with the degree of crystallinity. The extra degree of

crystallinity above 42.5 wt% is linked by taut-tie molecules resulting in the increasing values

of the elastic modulus. The first 42.5 wt% of crystallinity is dispersed in the amorphous and

interphase of this LLDPE, without a link between these crystallites.

Another interesting result of the structure is obtained from the X-Ray spectroscopy, the

almost constant content of amorphous phase equal to 37 wt%. The difference between the

amount of amorphous phase obtained from DSC-measurements and the amount of

amorphous phase by X-Ray spectroscopy is the resulting amount of interphase inside the

LLDPE’s (see figure 5.11 and 5.12).

The starting samples, before cold drawing, have a degree of crystallinity of 44 wt%, 37 wt%

amorphous phase and 19 wt% interphase. With a cold draw ratio of 3.7, the structure is

composed of 46 wt% of crystallinity, 37 wt% of amorphous phase and 17 wt% of interphase.

For the highest cold draw ratio, equal to 7.2, the structure contains 51 wt% of crystalline

phase, 37 wt% amorphous phase and 12 wt% interphase. A part of the interphase is

transformed into a crystalline phase whereby the crystallites are connected by taut-tie

molecules. As a result of this transformation, the elastic modulus is increased but the

resilience and static bending recovery are decreased.

96

Figure 5.9. Elastic modulus versus molecular orientation calculated from Raman spectrum

measurements.

Figure 5.10. Elastic moduli as function of the degree of DSC-crystallinity of the samples after

cold drawing.

0

50

100

150

200

250

300

350

40 45 50 55 60

Ela

sti

c m

od

ulu

s (

MP

a)

DSC-crystallinity (%)

97

Figure 5.11. Amount of interphase (◊) and taut-tie molecule linked degree of DSC-

crystallinity (■).

Figure 5.12. Resilience and deformation recovery in function of the extra degree of

crystallinity, linked by taut-tie molecules; resilience ▲; deformation recovery □.

0

2

4

6

8

10

12

14

16

18

20

0 10 20 30 40 50 60

Am

ou

nt

of

ph

ase (

%)

λ2 -1/λ

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20

Resilie

nce (

%)

or

defo

rmati

on

reco

very

(%

)

Extra degree of crystallinity (%), linked by taut-tie molecules

98

From these results, a LLDPE with a structure containing 37 wt% amorphous phase, 1.5 wt%

interphase and 61.5 wt% crystalline phase, composed of 42.5 wt% dispersed and 19.5 wt%

linked crystalline phase, is characterized by a zero value of resilience and deformation

recovery. This means that the transformation of the measured interphase into a linked

crystalline structure increases the classical mechanical properties, such as the elastic

modulus, but decreases the resilience and the deformation recovery. The limit for

deformation recovery is obtained for LLDPE containing 37% of amorphous phase (X-Ray)

and 63% ordered phase (44 wt% crystalline and 19 wt% transition phase) as measured by

DSC and X-Ray spectroscopy with a corresponding value of resilience around 40%. As the

resilience seems to be linearly influenced by the amount of linked crystalline structure, there

is a more complex behaviour of the deformation recovery as function of this structure. There

seem to be two zones for the deformation recovery as function of the interphase or the

corresponding degree of extra linked crystalline phase.

Figure 5.13 represents the X-ray diffraction patterns of monofilaments obtained with different

CDR. All patterns show typical orthorhombic crystal phase diffraction peaks at about 21.6⁰,

23.8⁰, and 36.3⁰ corresponding to (110), (200), and (020) crystal planes respectively [16].

The characteristic diffraction peaks of the monoclinic crystal phase are perceptible at 19.3⁰

and 23.2⁰, and 25.1⁰ (see figure 5.14), which corresponds to planes (001), (200) and (-201).

The calculations of the percentage of the monoclinic and orthorhombic crystalline phases are

summarized in table III.

99

Figure 5.13. WAXS patterns of pellet and monofilaments with cold draw ratios 3.7 (A9), 5.7

(A5) and 7.2 (A3).

Figure 5.14. The typical picks for X-ray scattering of LLDPE, after Gaussian deconvolution.

Gaussian deconvolution for X-ray measurements

100

It is clear that the increase in CDR causes an increase in the percentage of orthorhombic

crystalline phase until a CDR of 5.7. The amount of amorphous phase obtained by X-ray

measurements is nearly constant (36%) for all the samples, however, the amount of the

different crystalline structures are changing as a function of the cold draw ratio. Further

details of the complex structure of these different phases and differences in structures

analysed by Raman spectroscopy and X-ray measurements will be explained in chapter 7.

It could be concluded that the definitions of amorphous phase and interphase from the

Raman spectra do not correspond to the same phases according to the DSC and X-ray

measurements.

Regarding the results concerning the percentage of crystalline (see table II and table III),

different values are observed. This difference is due to the fact that different techniques have

been used and that these techniques detect different types of structures.

5.5 Conclusions

The purpose of this chapter was to investigate the influence of the cold draw ratio on the

bending properties. To this end, two methods were used, which show a good mutual

correlation: deformation recovery determined by static bending and resilience determined by

dynamic bending.

Monofilaments produced from the same polymer and with the same cross section are

characterized by a totally different bending behaviour in function of the cold draw ratio. By

increasing the cold draw ratio from 3.7 to 7.2, the tensile properties, such as the elastic

modulus is increased from 103 to 270 MPa, due to an increased amount of linked crystalline

structures and a decrease of the interphase, measured by DSC and X-Ray spectroscopy

with a nearly constant amount of the dispersed crystalline phase. This is a remarkable result

for the correlation between the mechanical properties of the polyethylenes and the content of

interphase and taut-tie linked crystallites. These results indicate that the mechanical

properties of the polyethylene are not only influenced by the total degree of crystallinity but

also by the connections between the crystallites, measured as taut-tie linked crystalline

structures and by the amount of transformation of the interphase into an ordered crystalline

phase.

The resilience and deformation recovery are a decrease function of the cold draw ratio of the

monofilaments. The obtained results indicate that these decreases of resilience and

101

deformation recovery are dominated by the amount of linked crystalline phase. Also, the

influence of the crystalline phase can be split up into two parts and the transformation of the

interphase into an ordered crystalline structure is very important for the different mechanical

properties. These are the results from the complex structure of the LLDPE monofilaments

obtained by the combination of melt and solid state stretching. Regarding the results

concerning the amounts of the different structures, crystalline, amorphous or interphase,

different values were observed from Raman spectroscopy and X-Ray measurements. This

difference is due to the fact that these techniques detect different types of structures and a

combination of these two techniques is necessary in order to obtain a complete analysis of

the complex structure of the semi-crystalline polymers.

102

References:

[1] Schoukens, G. Chapter 5; ‘‘Development in textile sports surfaces’’, in Goswami,

K. K., 2009, ‘‘Advanced in Carpet Manufacture”Cambridge Woodhead Publishing in

Textiles, ltd.

[2] Peter Sandkuehler, Enrique Torres, Thomas Allgeuer; Procedia Engineering 2

(2010) 3367-3372.

[3] W. H Carothers and J. W. Hill , J Am. Cham Soc ., 54 (1932).

[4] Failla, M. D.; Carrella, J. M.; De Micheli, R. J Polym Sci Polym Phys 1988, 26,

2433.

[5] Clements, J.; Jakeways, R.; Ward, I. M. Polymer 1978, 19, 639. 11.

[6] DOWLEX TM 2035 G (Cast Film), Polyethylene Resin. Tarragona Technical

Center, Tarragona Spain.

[7] Strobel, R. G.; Hagerdon, W. J Polym Sci Polym Phys Ed 1978, 16, 1181.

[8] Rabiej, S.; Binias, W.; Binias, D. Fibres Text East Eur 2008, 16, 5762.

[9] Peacock, A. J. Handbook of Polyethylene Structure Properties and Application;

New York, 2000, Chapters 6 and 8.

[10] Zuo, F.; Burger, C.; Chen, X.; Mao, Y.; Hsiao, B. S. Macromolecules 2010, 43,

1992.

[11] Sato, H.; Shimoyama, M.; Kamiya, T.; Amari, T.; S ˇ asˇic, S.; Ninomiya, T.;

Siesler, H. W.; Ozaki, Y. J Appl Polym Sci 2002, 86, 443.

[12] Lagaron, J. M. J Mater Sci 2002, 37, 4101.

[13] Paradkar, R. P.; Salhalkar, S. S.; He, X.; Ellison, M. S. J Appl Polym Sci 2003,

88, 545.

[14] Maxfield, J.; Stein, R. S.; Chen, M. C. J Polym Sci Polym Phys Ed 1978, 16, 37.

[15] Lagaron, J. M.; Dixon, N. M.; Reed, W.; Pastor, J. M.; Kip, B. J. Polymer 1999,

40, 2569.

[16] Russell, K. E.; Hunter, B. K.; Heyding, R. D. Polymer 1997, 38, 1409.

103

6. Chapter 6

Effect of heat treatment on properties of

monofilaments, bending behaviour

In this chapter the bending behaviour of monofilaments are investigated in function of the

temperature treatment for each step of production line. A stable monofilament product is very

important to guarantee long term behaviour of the final product. Therefore two different

temperatures of heat treatment are considered.

6.1 Introduction

In chapter 5, the influence of the orientation of the monofilaments on their properties was

discussed. Drawing polymeric monofilaments results in a high strength, but when heated

they begin to contract. These thermal properties may limit the maximum usage temperature

but also higher temperatures reduce the desirable tensile properties.

The limitation of use depends on the polymer. In various tables of materials the maximum

temperature of use in the long term and in the short term exposure at elevated temperatures

[1]

In case of thermoplastics the main limitation is that they become brittle at low temperatures

(Tg) which makes them lose their impact strength. The other properties are not affected but

in most cases the elastic modulus and the tensile strength improve upon decrease of the

temperature [2].

For semi-crystalline polymers the brittleness is found below their transition temperature. For

example, polypropylene (PP) becomes brittle at about -10⁰C, polyethylene (PE) retains its

ductile nature down to very low temperatures. Polyamide (PA) has a brittleness temperature

just some degrees below the room temperature. [3].

The pile layer of artificial turf as a polymeric product is likely to show poor retention, poor

creasy resistance and poor shape retention. The reason is that they are under relatively high

temperatures during the coating process and in real life, after installing, the monofilaments

are constantly under different temperatures and actions at the same time, which makes it

even easier to destroy the product. (These elements are described in chapter 2 and chapter

3).

104

Therefore, in order to prevent the instability in the dimensional, physical and mechanical

properties, monofilaments go through heat setting or annealing. In addition, heat setting or

annealing is a process known to eliminate the internal tensions generated during fabrication,

as stretching. The process itself will induce ductility, soften the material, relieve internal

stress, refine the structure by making it more homogeneous and improve cold working

properties. The heat fixes the material in a relaxed state and thus avoids subsequent

shrinkage as the internal stresses are relieved.

In case of semi-crystalline polymers, the morphological modification of crystal segments

occurs via lamellae thickening during heating. Two mechanisms were reported to be

responsible for the process of the thickening of lamellae which are localized solid-state

diffusion of the polymer chain segments and partial melting/ recrystallization [4-5]

The structural change depends on the temperature, on the time and also on the dimension of

the product. Annealing/heat setting may lead to changes in the crystalline and non-crystalline

phase. Various parameters were reported to control the morphology of annealed polymers

including annealing time, molecular weight, molecular weight distribution, annealing

temperature, heating rate, cooling rate, etc. [6-8]

If the rate of heating is too high, the shrinkage increases because disintegration of the

unstable regions is more rapid than consolidation of the structure by post crystallization. At

higher temperatures the equilibrium is attained more quickly.

Practically, monofilaments of LLDPE pass minimum three times an oven after extrusion.

Knowing the relation between thermal history processing condition and microstructural

changes of these monofilaments, it is very important to control their long term behaviour.

The mechanical properties are well described in several books [9-10] and papers but not

much is known about the influence of heat setting/ annealing on the bending behaviour.

The objective of this chapter is to investigate the influence of heat setting on the bending

behaviour of monofilaments in different stages of the production. The transformed

morphology supports the correlations between structural parameters and the properties of

the product.

It is well known that the structural changes of the product occur at the crystallization

temperature and above the melting temperature. Therefore, the investigations are performed

at different temperatures, 100 C and 120 C. The heat treatment is performed with controlled

shrinkage of monofilaments and with fixed ends of monofilaments. Annealing is performed

with fixed ends for ten seconds for both temperatures.

105

The questions are:

1. What is the influence of heat setting / annealing on the bending behaviour of

monofilaments?

2. Which is the advised temperature for having good properties of classical mechanical

properties, i.e. bending behaviour combined with shrinkage dimensional stability?

3. What is the influence of time heat setting/ annealing in combination with the

temperature heat setting/ annealing?

4. Out of all of the possible combinations which one is the most convenient and why?

6.2 Experimental set up

For a realistic approximation of the monofilaments, the production of monofilaments is

performed on a pilot monofilament line from Oerlikon Barmag type 3E/24D installed at the

Department of Textiles, Ghent University. A selection was made of for four products with

different cold draw ratios based on the previous samples produced by small extruder

(chapter 5). The selected monofilaments are those with cold draw ratio 7.2; 6.2; 5.5 and 3.3,

with dTex per monofilaments (dpf) approximately of 1980 [g/km]. These samples showed a

significant difference on their behaviour.

The extruder has a single screw diameter of 30mm and a length of 24D. The temperature in

the die was 220°C. The die has 24 diamond-shaped openings with a cross section of 70mm2

each. Monofilaments after the melt stage (section i in figure 5.1) were pulled through a water

bath and then passed through the oven, with air circulations at a temperature of 100°C

(section I in figure 5.1), this way finishing the first stage of the production line, cold drawing

(CDR) of monofilaments.

After the first stage of the production of monofilaments (section I in figure 5.1), the controlled

shrinkage is performed at temperatures of 100 C and 120 C (section II in figure5.1). For

heat setting performed at 100 C, the controlled shrinkage is 10% and for heat setting at

120 C, the shrinkage is 15 %. Shrinkage is controlled by changing the speeds of the rolls

before and after the oven. Heat setting at 120 C is also performed with fixed ends. After

shrinkage, samples are annealed at controlled temperatures, i.e. at 100 and 120 C for ten

seconds (section III in figure5.1).

106

The analysis of the product per each stage, are performed on macro and micro scale. For

macro scale analyses the dynamic bending, static bending and tensile tests are performed

for all the samples with the same procedure as described in the previous chapter.

The morphological analyses are performed by analysing the measurements of DSC, Raman

and X-ray. The tests set up for each test are the same as in the chapter 5. The shrinkage is

performed at 75 C per 15 minutes based on the internal standard modified for polyethylene

material [11].

6.3 Results and discussion

Tests are performed on samples at different stages of the production line.

cold drawing samples (refers to stage I indicated in figure 5.1).

heat treatment with fixed ends at 120 C, samples (stage II indicated in figure 5.1, but

the speed of the rolls are the same as sample is fixed).

controlled shrinkage at different temperatures at 100 and 120 C samples (stage II

indicated in figure 5.1).

annealing with fixed ends at 100 and 120 C for ten seconds samples (stage III

indicated in figure 5.1).

6.3.1 Mechanical properties, E-Modulus and Maximum Load

The mechanical properties of the produced monofilaments are the first important

characteristics determining the structural changes of monofilaments induced during their

processing and characterizing the performance of the final product. The measurements of

the elastic modulus deal with the initial slope between stress and strain, where stress is

proportional with the strain. In table I, the measurements of the mechanical properties at

different stages of the production line and treated at different conditions; temperatures,

controlled shrinkage and fixed ends are summarised.

Figure 6.1a to 6.1d represents the relation of the elastic modulus of different samples at

different stages in the production with the drawing ratio presented by using the formula ( ²-

1/ ). It is noteworthy that the values of the elastic modulus are increasing by increasing the

107

draw ratio of the product. This relation remains the same for all the samples although they

have undergone heat treatment at different temperatures under different conditions.

When the original samples and those heat treated with fixed ends at 120 C are compared

the values of the elastic modulus are found to have no significant difference. This can be

explained by the concept of “tout tie molecules” (TTM). The blocks connected by the tout tie

molecules (TTM) and hence the microfibrils are prevented from moving in such a direction

that the end to end distance of the TTM would be reduced [10].

Table I. Mechanical properties of samples at different stages of production and treated at

different temperatures.

CDR Original product Heat treatment 120° fixed ends

E-Modulus (MPa) Max Load (N) E-Modulus (MPa) Max Load (N)

7,2 216±3 34±1 219±3 359±1

6,2 177±4 197±5 171±3 256±4

5,5 159±4 197±5 122±7 118±6

3,3 63±4 85±3 63±2 90±9

CDR

10% shrinkage at 100°C 15 % shrinkage at 120°C

E-Modulus (MPa) Max Load (N) E-Modulus (MPa) Max Load (N)

7,2 104±3 327±9 122±17 318±28

6,2 87±5 237±3 90±7 225±9

5,5 75±4 203±10 71±6 177±6

3,3 44±1 96±4 45±3 88±8

CDR

Annealed at 100 °C Annealed at 120 °C

E-Modulus (MPa) Max Load (N) E-Modulus (MPa) Max Load (N)

7,2 146±12 241± 139±7 231±19

6,2 138±12 214± 120±5 170±6

5,5 112±7 183± 91±4 155±14

3,3 60±5 86± 53±2 79±4

In case of controlled shrinkage, 10% and 15%, the values of elastic modulus are decreased

compared with the starting products (figure 6.1b).

The decrease of the elastic modulus after shrinkage is something expected, heat setting with

free ends relaxes the TTM and decreases the end-to-end distance. Peterlin [10] described

108

that the elastic values drop quite substantially for heat treated samples compared with the

values of samples before the heat treatment.

Annealing at different temperatures with fixed ends and for 10 seconds does not make any

differentiation (figure 6.1c) and the values are increased, approaching the values of the

original samples (figure 6.1d). This might be as a result of increase of enhanced crystallinity

and increased crystalline size of the annealed samples. In addition the thickness of a crystal

increases only on annealing temperature, [12-13] near (but below) the melting temperature,

whereas at lower annealing temperatures no changes are detected [10]. The crystalline

bridges obtained after annealing with fixed ends are almost formed by fully extended

interfibrillar TTM increasing the elastic modulus.

The effective cold draw ratios were used in the different graphs, namely for the samples with

10% shrinkage is the effective cold draw ratio 90% of the starting cold draw ratio and for

samples with 15% shrinkage is the effective cold ratio 85% of the starting cold draw ratio.

Figure 6.1a. Elastic modulus of monofilaments after cold drawing (stage I in figure5.1) and

after cold drawing and heat treatment with fixed ends (stage II in figure 5.1) in function of

drawing ratio represented by ( ²-1/ ).

0

50

100

150

200

250

300

0 10 20 30 40 50 60

Elas

tic

mo

du

lus

(MP

a)

λ2 - 1/λ

Original product

Fixed heat tretment 120

109

Figure 6.1b Elastic modulus of the samples after cold drawing (stage I in figure 5.1) and

shrinkage (stage II in figure 5.1).

0

50

100

150

200

250

300

0 10 20 30 40 50 60

Elas

tic

mo

du

lus

(MP

a)

λ2 -1/λ

10% shrinkage

15% shrinkage

110

Figure 6.1c Elastic modulus of the samples after cold drawing (stage I in figure 5.1),

shrinkage and annealing with fixed ends (stage III in figure 5.2).

0

50

100

150

200

250

300

0 10 20 30 40 50 60

Elas

tic

mo

du

lus

(MP

a)

λ2 -1/λ

10%Shr+Anneald

15%Shr+Anneald

111

Figure 6.1d Comparison of the elastic moduli of the different produced samples, upper line

after cold drawing (stage I), middle line after cold drawing, shrinkage and annealing (stage

III), lowest line after cold drawing and shrinkage (stage II).

The temperature in the core of the product depends on the temperature and the time in the

oven, the speed of producing and the type of the polymer. By knowing the temperature in the

oven, the speed and the heat capacity of the product, it is possible to calculate the

temperature in the core of the product, using the formula for heat transfer with convection:

To is the starting temperature of the monofilaments;

Tf the temperature of the monofilaments after heat transfer;

Toven the temperature of the oven.

0

50

100

150

200

250

300

0 10 20 30 40 50 60

Elas

tic

mo

du

lus

(MP

a)

λ2 -1/λ

112

So, samples treated in the oven at 100 C will attain a final temperature of 78 C. This

temperature is sufficient to influence on the molecules or lamellae of the product which melt

at this temperature. A heat treatment at 120°C increases the temperature up to 92°C and will

have more possibilities to rearrange the structure of the product.

There is a difference between heat treatment with free ends (controlled shrinkage) and

annealed (with fixed ends). Whereas in the former case, the loss of the high elastic modulus

is permanent, the loss is smaller and only short lived in the latter case. The elastic modulus

recovers to a value higher than that obtained from previous heat treatments. Simultaneously,

the density increases as a consequence of the slow partial recrystallization at room

temperature which brings the elastic modulus almost back to its values before annealing.

This is reported to reduce the flexibility of microfibrill composing the product, monofilament

[10].

6.3.2 Resilience of monofilaments at different stages of production line of

monofilaments

The results obtained for resilience of the monofilaments at the dynamic bending mode are

summarized in table II. In figure 6.2 the relation of resilience at different stages of the

production with drawing ratio are shown

113

Table II. Resilience of all tested monofilaments at different stage of production line.

CDR

Original product Heat treatment 120°C fixed ends

F₁ (cN) R (%) F₁ (cN) R (%)

7,2 2,9±1 20,0±1.5 2,2±0.4 20,2±1.8

6,2 2,1±0.4 21,2±1.7 2,2±0.8 27,8±4.1

5,5 1,7±1.3 24,6±3.9 0,8±0.4 39,2±2.6

3,3 0,9±0.4 39,4±4.7 0,8±0.3 43,6±3.6

CDR

10% shrinkage at 100°C 15 % shrinkage at 120°C

F₁ (cN) R (%) F₁ (cN) R (%)

7,2 2,2±0.7 21,0±0.7 1,8±0.2 33,7±1.4

6,2 1,8±0.6 24,7±0.9 1,6±0.6 38,0±3.0

5,5 1,6±0.3 25,2±0.9 1,5±0.9 35,5±3.9

3,3 0,9±0.9 36,5±1.8 0,9±0.5 45,5±3.5

CDR

Annealed at 100 °C Annealed at 120 °C

F₁ (cN) R (%) F₁ (cN) R (%)

7,2 2,3±0.6 37,0±0.5 2,2±0.6 37,3±0.4

6,2 1,8±1.0 39,3±0.2 1,7±0.5 37,0±0.5

5,5 1,8±0.4 41,8±0.4 1,4±0.3 38,5±0.2

3,3 1,1±0.3 44,6±1.0 1,2±0.3 41,0±0.1

When the results of the resilience at different stages of the production line are compared, it is

obvious that the behaviour of the monofilaments after heat treatment is significantly

improved compared with the original samples. This can be explained based on the

morphological change which might take place.

There are several factors affecting morphological changes, such as the temperature and

time of annealing (length of annealing time). At elevated temperatures the orientation of the

crystallographic axes becomes random by influencing the behaviour of the product [12, 14-

16]. Annealing at 110 C or below causes a morphological transformation accompanied by

tilting the molecular chain around the b-axis [13]. Annealing above 110 C, single crystals

assume a spherulitic structure with random orientation of the crystallographic axes and the

114

increase of thickness of lamellae which was explained by melting of the thinner lamellae

followed by recrystallization. According to [17-19] the branches and connection between

lamellae and molecules will be replaced with new connections.

By analysing step by step the obtained values of the resilience, the results are as follows:

First of all the relation between the resilience and drawing ratio is confirmed as described in

chapter 5. By increasing the drawing ratio the resilience is decreased significantly.

The measured values of the bending force for the first cycle on the dynamic bending test (F1)

do not show a remarkable difference for the different stages of production (see figure 6.2).

Besides this, the correlation with drawing ratio remains as for the elastic modulus, if the

effective cold draw ratio is used. The decease of the elastic modulus after shrinkage is

compensated by the increase in thickness and the final results of F1 are completely defined

by the effective cold draw ratio.

Figure 6.2 Variation of F1 in function of drawing ratio for different stages of the production

line and at different temperatures.

0

0,5

1

1,5

2

2,5

3

3,5

0 10 20 30 40 50 60

Forc

e F

1 (c

N)

λ2 -1/λ

115

The measured values of the bending force F300 after 300 cycles are reproduced in figure 6.3.

From this figure can be concluded that the obtained result of F300 can split up into two lines:

In the first line, original samples and 10% shrinkage at 100°C show the same relation with

effective cold draw ratio. In the second line, samples with 15% shrinkage at 120°C and

samples annealed, respectively, at 100 and 120°C show the same correlation with the

effective cold draw ratio, by showing better behaviour compare with the first line This means

that the final temperature of the monofilaments, for the samples in the first line (78°C) is not

sufficient to modify the bending behaviour of the monofilaments on dynamic bending mode.

But samples in the second line reach the temperature of the core of 92°C or higher up to

120°C and as a result bending behaviour is improved. For samples with 10% shrinkage at

100 C it is necessarily to pass the annealing step as well in order to rise the temperature

(more than 78 C), a sufficient temperature for rearrangement of the structure of the product.

Figure 6.3 The measured forces F300 after 300 cycles on the dynamic bending test ( F300

for original samples, F300 for 10% shrinkage, F300 for 15% shrinkage, F300 for

annealed samples at 100 C, F300 for annealed samples at 120 C). The full line represents

the results of F1.

0

0,5

1

1,5

2

2,5

3

3,5

0 10 20 30 40 50 60

Forc

e (

cN)

λ2 -1/λ

116

The results of the dynamic bending can be split up into two parts, one part related to the

irreversible deformation and another part by the change of the elastic modulus as function of

the bending cycles. During bending, the maximum force was obtained after a displacement

of 4 mm of the clamp of the Favimat R and an irreversible deformation of the monofilaments

was observed of 2 mm after 300 bending cycles (figure 4.6). If the elastic modulus is

constant during bending experiment, the force F300 will be 50% of the starting value of F1.

The differences between these expected results of F300, with a constant value of the elastic

modulus, and the measured values are due to a modification of the elastic modulus.

The calculated ratio of the elastic modulus E300/E1 is represented in figure 6.4 as function of

the effective cold draw ratio for the two series of results.

Figure 6.4 Calculated ratio of the elastic moduli E300/E1 as function of the effective cold draw

ratio ( cold drawing and cold drawing followed by 10% shrinkage at 100°C, ▲ other heat

treated samples).

The relative decrease of the elastic modulus under dynamic bending is less for the samples

which has reached a temperature of at least 92°C. The relative decrease of the elastic

modulus seems to be related to the ln (λ) as represented in figure 6.5.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 10 20 30 40 50 60

E30

0/E 1

λ2 -1/λ

117

Figure 6.5 Relative elastic moduli as function of ln(CDR); cold drawing and cold drawing

followed by 10% shrinkage at 100°C, ▲ other heat treated samples.

These differences in behaviour are probably related to the more stable connections between

the microfibrlils for the monofilaments treated at temperatures above 92°C. This explains

also the improvement of mechanical properties.

Comparing results for different annealing temperatures it is obvious that there is not a

significant change. In figure 6.6 are represents the values of resilience of samples with

different cold draw ratio at different stage of production.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

1 10

E30

0/E

1

Effective cold draw ratio (ln CDR)

118

Figure 6.6 Comparison of resilience of the samples with different CDR at different stages of

production.

6.3.3 Deformation recovery of monofilaments at different production stage

The obtained results for the deformation recovery on static bending are summarised in table

III. As expected the deformation recovery shows the same relation, as previously stated with

cold draw ratio. By increasing cold draw ratio, the deformation recovery is decreased.

The deformation recovery of samples after a heat treatment at 100 and at 120 C with

controlled shrinkage shows almost the same values as the values of the original product. But

for samples with heat treatment at 120 C with fixed ends, the deformation recovery is

significantly improved and values are almost the same as for annealed samples.

The annealing temperature seems not to have influence on the behaviour of monofilaments

on static bending mode as the obtained results show no difference for both temperatures.

0

5

10

15

20

25

30

35

40

45

50

7,2 6,2 5,5 3,3

Resilience vs. CDR at different stages of production line

Original product Heat treatment 120°C fixed ends

10% shrinkage at 100°C 15 % shrinkage at 120°C

Annealed at 100 °C Annealed at 120 °C

119

Table III. Deformation recovery of samples at different production stags of and at different

temperatures of heat treatment.

CDR Original product Fixed ends 120 C

Deformation recovery (%) Deformation recovery (%)

7,2 51±6 79±9

6,2 63±6 84±3

5,5 66±6 82±5

3,2 74±5 85±5

CDR

Controlled shrinkage at:

100 C 120 C

Deformation recovery (%) Deformation recovery (%)

7,2 57±3 55±5

6,2 64±3 68±7

5,5 75±6 75±6

3,2 76±6 77±6

CDR

Annealed at :

100 C 120 C

Deformation recovery (%) Deformation recovery (%)

7,2 78±3 76±2

6,2 76±3 76±3

5,5 81±2 76±2

3,2 81±3 81±3

In figure 6.7 and 6.8 the values of the deformation recovery are plotted vs. the drawing ratio

at different stages of production line. The relation of deformation recovery with drawing ratio

is very similar with the relation of resilience with drawing ratio (Figure 6.6) although the

values for the deformation recovery are much higher than for the resilience. This

demonstrates the correlation between these two methods, however, referring to figure 6.9

the correlation is not always strong (R2=0.98÷0.92), as it was presented in chapter 4 and

chapter 5.

120

Figure 6.7 Deformation recovery of monofilaments at the static bending mode of

monofilaments at different production stages in function of the drawing ratio presented by

( ²-1/ ).

0

20

40

60

80

100

0 10 20 30 40 50 60

Defo

rmati

on

Reco

very

(%

)

²-1/

Originalproduct

Fixed endsat 120°C

10%shrinkage at100°C

15%shrinkage at120°C

Annealed at100°C

Annealed at120°C

121

Figure 6.8. The influence of heat treatment on the deformation recovery for samples with

different CDR at different stages of the production line.

Figure 6.9. Resilience vs. deformation recovery of the monofilaments at different stages of

the production line.

0

10

20

30

40

50

60

70

80

90

7,2 6,2 5,5 3,3

Deformation recovery vs. CDR at different stage of production line

Original product Fixed ends 120°C

Controlled shrinkage at 100°C Controlled shrinkage at 120°C

Annealed at 100°C Annealed at 120°C

0

20

40

60

80

100

0 20 40 60 80 100

Defo

rmati

on

Reco

very

(%

)

Resilience (%)

Original product

Fixed ends at120°C

10% shrinkageat 100°C

15% shrinkageat 120°C

Annealed at100°C

Annealed at120°C

122

Another important element showing by these results is the fact that the effect of the drawing

ratio, after heat treatment with fixed ends and after being annealed, is completely

diminished. The samples show almost the same values of deformation recovery differently

from resilience. This behaviour is more visible for samples with fixed ends heat treatment at

120 C and annealed at both temperatures (see figure 6.8).

6.3.4 Effect of a heat treatment on the dimensional stability of monofilaments,

shrinkage.

As it was mentioned in the introduction, another important aspect of the samples is the

dimensional stability. This is important not only for aesthetic reasons but also because the

loss in the length of pile the layer influences the properties of the ball roll. This aspect is

explained more in detail in chapter 4. In table IV the measured values of shrinkage are

summarized for all the samples.

Table IV. Shrinkage of the monofilaments at different temperatures and measured on all the

samples at different stages and with different CDR.

CDR Original product Fixed ends at 120°C

505 g 5 g 505 g 5 g

7.2 9.5±0.3 8.9±0.2 9.1±0.5 8.2±0.9

6.2 8.3±0.4 6.9±0.3 5.8±0.3 5.3±0.6

5.5 9.6±0.3 8.3±0.6 5.2±2.4 3.7±1.5

3.3 7.6±2.2 5.2±1.1 8.4±0.9 6.3±0.2

CDR 10% Shrinkage at 100°C 15 % Shrinkage at 120°C

505 g 5 g 505 g 5 g

7.2 7.2±0.3 6.0±0.7 2.3±0.3 1.6±0.7

6.2 2.9±0.3 0.8±0.4 1.1±0.9 0.8±0.6

5.5 3.3±0.5 1.6±0.4 1.4±0.4 1.4±0.1

3.3 3.0±0.6 1.0±2.2 2.0±0.8 1.4±0.1

CDR Annealed at 100 °C Annealed at 120 °C

505 g 5 g 505 g 5 g

7.2 6.2±0.1 5.4±0.0 1.6±0.5 1.1±0.9

6.2 3.4±0.1 1.7±0.5 2.0±0.3 0.7±0.6

5.5 4.2±0.2 2.0±0.2 1.6±0.2 0.6±0.3

3.3 2.5±0.0 1.0±0.0 1.0±0.9 0.0±0.2

123

Original samples and samples with heat treatment with fixed ends do not show a significant

difference; in both cases the values of the shrinkage are very high. Stability in the product

seems to be obtained for samples after 10% and 15% shrinkage treated respectively at 100

and 120 C. After performing annealing the stability of the product are improved compare

with the previous step. The annealed at 120°C show better values than other temperature.

Figure 6.10. The relation between the shrinkage and CDR of samples at different stages of

the production line.

Considering the shrinkage of the product, it is obvious that samples are not stable after

controlled shrinkage and after heat treatment with fixed ends, but only after passing these

steps and performing the annealing at elevated temperatures.

0

1

2

3

4

5

6

7

8

9

10

7,2 6,2 5,5 3,3

Shrinkage of the product in function of CDR and stages of production line

Original product Heat treatment 120°C fixed ends

10% Shrinkage at 100°C 15 % Shrinkage at 120°C

Annealed at 100 °C Annealed at 120 °C

124

6.3.5 Morphology structure

Investigation of the morphology of polymeric fibres is of a fundamental and practical

significance providing an excellent example of a unique microscopic structure that produces

outstanding macroscopic properties.

The polymers used in fibres are linear, so the molecules are a few nanometers wide and

several hundred nanometers long. For non-oriented materials, the molecules are coiled and

folded into loose isotropic spheres. When a fibre is oriented, by drawing for example, the

molecular chains become aligned parallel to the fibre axis (uniaxial fibre orientation), and

stiffness and strength improve. In most fibres, the molecules are still coiled and folded,

although they are oriented. Only in ultrahigh modulus fibres or in fibres formed from liquid

crystalline precursors the molecules are highly elongated and extended parallel to the fibre

axis.

The morphology of PE has been studied by various techniques. X-ray scattering, thermal

analysis Differential Scanning Calorimetry (DSC) and Raman spectroscopy are among the

many techniques that complement microscopy investigations. However, even for the

crystallinity the results are not consistent. In this chapter three different techniques are

considered: thermal analysis presented by DSC, and Spectroscopy presented by X-ray and

Raman.

A basic element of semicrystalline fibres is the microfibril. They may be bundled into fibrils,

about several hundred nanometers thick, and are known to exist in most fibres and are also

known to be present in drawn single crystals, such as single crystal polyethylene (PE) mats,

melt extrudates, and solid state extrudates. X-ray diffraction suggested an arrangement of

fine structures about 50nm long and 5nm wide in semicrystalline fibres [5, 6]. Peterlin

observed the formation of fibrils and microfibrils by the deformation [7,8]. The fibrillation

during deformation is explained by a mechanically weak boundary between the fibrils [9-10].

6.3.5.1 DSC measurements at different of production stages

The DSC crystallinity values of samples are summarized in table V. As it was expected, the

percentage crystalline phase is increased by increasing CDR, however not enormously. But

after performing heat treatment with fixed ends and also in case of controlled shrinkage and

annealed at 100 and 120°C the percentage of crystalline phase is almost constant. The

possible changes in the morphology of the monofilaments which could arise from heat

treatment or annealing were qualitatively investigating the DSC curves. A close inspection of

125

the DSC curves obtained for different annealing conditions, represented in Figure 6.10,

shows that a weak and blurred peak appearing in the vicinity of 40 °C for not annealed

samples is changing for the annealed ones. This peak, characteristic of the LLDPE product,

presumably reflects the melting of very small crystalline entities. Moreover, the shape and

intensity of the melting peak centred in the vicinity of 120 °C do not show a noticeable

dependence on the annealing temperature.

Another difference in the DSC curves between original, heat treated and annealed samples

is the change in the shape of the endothermal part. As a result of heat treatment, samples

show broader melting features compared to the original samples. In addition, multiple

melting peaks are characteristic for LLDPE materials, because of the presence of a broad

distribution of crystal sizes and due to a highly heterogeneous structure [20]. Because of

heat treatment, a disorientation of molecules is observed for both cases (see figure 6.10),

although not very visible. More visible is the difference in the endotherms between isotropic

samples (not stretched) and heat treatment at 75 C (see figure 6.10 a). These results are in

the same line as reported by Lagaron et al [21]. Apart from these differences, there is no

significant influence on the degree of crystallinity (see table V).

In table V, the degree of crystallinity of the samples shows no difference compare with the

original product, but mechanical properties of annealed samples are improved compared to

heat treatment samples under controlled shrinkage. This is in the same line with the

publications [9, 17]. It has been noted that annealing bulk polymer is a convenient method

for raising its density and tensile modulus. Annealing is known to enhance the degree of

crystallinity and molecular orientation of semicrystalline thermoplastic polymers which in turn

directly affect the physical and mechanical properties.

As it was reported et al. [22] the increase in crystal perfection could reduce the number of tie

by molecules which act as stress-transfer unit between the crystalline zones. Thus, a

crystallinity threshold may exist and when it is overcome a decrease in toughness appears

due to the lack of tie molecules.

Taking into consideration the relation CDR and DSC crystallinity, for annealed samples, it is

obvious that only the orientation of the product contributes to the mechanical properties, as

the correlation between CDR and the Elastic Modulus (see figure 6.1 a-d) still remains but

the values of DSC crystallinity (see table V) are almost constant for all the samples.

126

Table V. The amount of crystalline and non-crystalline parts calculated from DSC

measurements for samples at different stages of the production line.

CDR Original product Fixed ends 120°C

Crystallinity (%) Non Crystalline (%) Crystallinity (%) Non Crystalline (%)

7,2 51 49 48 52

6,2 47 53 49 51

5,5 48 52 47 53

3,2 43 57 44 56

CDR

Controlled shrinkage at:

100°C 120°C

Crystallinity (%) Non Crystalline (%) Crystallinity (%) Non Crystalline (%)

7,2 49 51 49 51

6,2 44 56 48 52

5,5 48 52 44 56

3,2 42 58 42 58

CDR

Annealed at:

100°C 120°C

Crystallinity (%) Non Crystalline (%) Crystallinity (%) Non Crystalline (%)

7,2 51 49 51 49

6,2 48 52 49 51

5,5 49 51 49 51

3,2 46 55 49 51

The crystal morphology changes occur during annealing at temperatures slightly above the

original crystallization temperature of the crystals and far below their melting temperature.

Annealing at 120 C, close to the melting temperature, could cause a recrystallization of the

product which might result in decreases of the lamella thickness and degree of crystallinity.

Compan et al [5] suggested that the morphological changes occurring in the films during

annealing seem to favour the formation of molecular packing defects, and the changes that

annealing may produce in the free volume, are not important enough to affect the

morphological change in a decisive way. Annealing processes may favour the formation of

micro cavities (molecular packing defects) in the crystalline-amorphous interface that can

accommodate individual site molecules without disturbing the normal dissolution process in

the rubbery region of the polymer matrix. According to this interpretation, annealing may

cause crystallite thickening that hinders but also may provide packing defects in the

127

crystalline amorphous interface [5]. In our case the packing defect might be most probable

as the melting temperature is not changing.

LLDPE DOWLEX 2035G

LLDPE DOWLEX 2606G

Figure 6.10. Influence of annealing in the DSC curves, more emphasises on the region at

40 C a) heat treatment at 75 C and b) heat treatment at 120 C.

128

6.3.5.2 X-ray measurements

In table VI the calculated values of orthorhombic, amorphous and monoclinic structure are

summarized. The orthorhombic structure is increased by performing a heat treatment with

controlled shrinkage at 100 C, which is not the case for a heat treatment at 120 C. The

increase of amount of this structure is at the expense of monoclinic structure as the

amorphous phase is not changing.

For a heat treatment at 120 C, the orthorhombic structure is not changing but the monoclinic

structure is increased and the amorphous phase is decreased.

From x-ray (WAS) measurements, a change in intensity is reported, as a result of annealing,

for orthorhombic crystalline structures at the orthorhombic peaks (21.5 and 23.5 of 2θ.) as

presented in figure 6.11. These seem to be in line with other publications, where this change

is reported [23-24]. It is also a slight shift towards the lower values of 2θ as a result of

annealing. A small peak can be seen at an angle around 20 in the stretched sample,

assigned as monoclinic crystalline structure coming from a phase transformation from

orthorhombic crystals, reported also by Lagaron et al [21].

The monoclinic peak is not detected on the x-ray scattering for the other samples either

because it is not present or because it is masked by the amorphous halo, which is most

probable as by using Gaussian curve fitting was possible to calculate this structure. The

results are summarised in table VI.

129

Figure 6.11. Comparison of the x-ray intensity of the original sample and the annealed one

for the same CDR.

0

5000

10000

15000

20000

25000

18 19 20 21 22 23 24 25 26

Inte

nsit

y

2 θ

Original

Heat tretment

Annealed

Monoclinic

Orthohombic

Orthohombic

130

Table VI. X-Ray (WAXS) measurements performed for all the samples

Original samples (with CDR)

CDR Orthohombic crystalline (%) Amorphous (%) Monoclinic crystalline(%)

7.2 38 43 19

6.2 44 40 16

5.5 48 37 15

3.3 44 37 20

10% Shrinkage at 100°C

7.2 53 41 6

6.2 54 40 6

5.5 52 40 8

3.3 49 39 12

15% Shrinkage at 120°C

7.2 40 33 27

6.2 44 34 23

5.5 50 34 16

3.3 37 31 32

Heat treatment at 120°C with fixed ends

7.2 33.4 42.9 23.7

3.3 43.0 33.5 23.5

Annealed at 120°C/10"

7.2 53 37 13

5.5 46 39 15

3.3 42 34 24

Annealed at 100°C /10"

7.2 51 36 12

6.2 48 38 14

5.5 50 36 14

3.3 38 37 26

131

6.3.5.3 Raman measurements

In table VII the Raman measurements are summarised. It is shown that the degree of

crystallinity of the samples is lower than for the other two methods X-ray and DSC. It has

been noted that the observation provided is in disagreement with the standard argument [25]

that the Raman crystallinity and the DSC crystallinity yield similar results as they both

measure the core crystallinity. The difference between crystallinity measured by these two

methods has also been reported by Lagaron et al [21], and can be explained by the fact that

Raman crystallinity excludes a fraction of crystallinity, which most likely has an undeveloped

morphology.

In general, the amount of interphase is reduced for samples after passing a heat treatment

with controlled shrinkage and after being annealed.

The amount of crystalline structure is increased for samples with heat treatment with

controlled shrinkage at 120 C, and annealed in both temperatures compared to the original

product, differently from heat treatment with controlled shrinkage at 100 C, where no

significant change is reported. Annealing at 120 C allows materials to have a more

homogeneous crystalline morphology made by thick lamellae with effective lateral order of

chains, which apparently seems not to be the case for annealing at 100 C.

The amount of amorphous phase could be considered as not being influenced by CDR and

the different steps of heat treatment in the production line.

Theoretically, the intensity at the orthorhombic structure (I1417) is increased by a heat

treatment (see figure 6.12).

Considering the formula for molecular orientation I1130/ I1060, samples at all the steps of the

product line do show a correlation with CDR. By increasing the orientation the molecular

orientation, is increased and this is improved for heat treatment as presented in figure 6.13.

Only the sample with CDR 7.2 stays at the same level of orientation as with the original

product.

132

Table VII. Raman measurements for all the samples at different stage.

Original product

CDR Crystalline (%) Amorphous (%) Interface (%)

7.2 35 21 44

6.2 35 25 39

5.5 39 25 36

3.3 31 25 44

15 % Shrinkage at 120 ⁰C

7.2 39 27 39

6.2 40 28 32

5.5 34 27 38

3.3 35 26 39

10% Shrinkage at 100 ⁰C

7.2 33 30 37

6.2 39 30 31

5.5 32 29 39

3.3 30 29 41

Heat treatment with fixed ends at 120 ⁰C

7.2 30 27 42

6.2 39 28 34

5.5 31 27 42

3.3 39 25 35

Heat treatment at 120⁰C/10''

7.2 41 27 33

5.5 44 26 30

3.3 34 28 38

Heat treatment at 100⁰C/10''

7.2 39 29 32

6.2 38 26 36

5.5 41 27 32

3.3 36 26 38

133

Figure 6.12. Comparison of the Raman intensity of the original sample and the annealed

one with the same CDR.

Figure 6.13. Raman molecular orientation of samples at different stages of the production

line, presented by I1130/I1060.

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

3 4 5 6 7 8

Ram

an

mo

lecu

lar

ori

en

tati

on

(1130/1

060)

Cold draw ratio

Original 10% shrinkage Annealed

134

6.4 Conclusions

By employing tensile testing it was found that heat treatment or annealing of polymeric

materials causes changes in the properties of the materials which can be explained by

changes in the morphological structures. However, these changes are not detected neither

by DSC, Raman and either by x-ray as the calculated values of presented phases seem not

to change enormously. Increasing annealing temperatures does not influence the melt

temperatures.

Annealing carried out under proper conditions enhances the molecular orientation of

semicrystalline polymers, which in turn directly affect the physical properties and mechanical

properties of the product.

The bending behaviour in the static bending and in the dynamic bending mode seems to be

improved.

Heat treatment and annealing was found to have a remarkable influence on the dimensional

stability of the final product and on their properties.

135

References:

[1] A.K. Van der Vegt, From Polymers to plastics, Chapter 8,. Valid from

http://www.vsd.nl.hf.m008htm.

[2] Man-Made fibres, Science and technology, Volume 3, Edited by H.F.Mark,

S.M.Atlas, E. CerniaLondon, 1968.

[3] A.K. Van der Vegt, From Polymers to plastics, Chapter 3, valid from

http://www.vsd.nl.hf.m008htm.

[4] J. Kong, X. Fan, Y. Xie, and W. Qiao, “Study on the molecular chain heterogeneity

of linear low‐density polyethylene by cross‐fractionation of temperature rising elution

fractionation and successive self‐nucleation/annealing thermal fractionation,” Journal

of Applied Polymer Science, vol. 94, no. 4, pp. 1710–1718, Sep. 2004.

[5] V. Compañ, A., Andrio, M. L., López, C. Alvarez, and E. Riande, “Effect of time of

annealing on gas permeation through coextruded linear low-density polyethylene

(LLDPE) films”, Macromolecules, vol. 30, no. 11, pp. 3317–3322, 1997.

[6] M. El Kindi and H. P. Schreiber, “Morphological responses in thermally

conditioned linear low density polyethylene”, Polymer Engineering & Science, vol. 32,

no. 12, pp. 804–809, 1992.

[7] D. R. Rueda, J. Martinez-Salazar, and F. J. Balt-Calleja, “Annealing effects in

lamellar linear polyethylene as revealed by microhardness,” Journal of Materials

Science, vol. 20, no. 3, pp. 834–838, Mar. 1985.

[8] Z.-G. Wanga, B.S. Hsiaoa, B.X. Fua, L. Liua, F. Yeha, B.B. Sauerb, H. Changc,

J.M. Schultzd, “Correct determination of crystal lamellar thickness in semicrystalline

poly (ethylene terephthalate) by small-angle X-ray scattering” Polymer 41 (2000)

1791–1797.

[9] A. J Peacock, Hanbook of Polyethylene Structure Properties and Applications;

New York, 2000.

[10] Anagnostis E. Zachariades and Roger S. Porter, editors, The strength and

stiffness of polymers., Chapter 3, Anton Peterlin, “Mechanical and transport

properties of drawing semicrystalline polymers”., isbn 0-8247-1846-1 New York

10016.

[11] Vargroep Textielkunde hot air retraction test voor polyester PM/214 B.

[12] D.A. Blackadder, P.A. Lewe, ”Properties of polymer crystal aggregates. 2)

Annealing of polyethylene crystal aggregates” Polymer, Volume 11, Issue 3 March

1970, page 147-167.

[13] M. Tian and J. Loos, “Investigations of morphological changes during annealing

of polyethylene single crystals,” Journal of Polymer Science Part B: Polymer Physics,

vol. 39, no. 7, pp. 763–770, 2001.

136

[14] H.E. Bair, R. Salovey, T.W. Huseby, “Melting and annealing of polyethylene

single crystals, Polymer Volume 8, 1967, page 9-20.

[15] D.A. Blackadder,P.A. Lewell, Properties of polymer crystal aggregates. (1)

Comparison of polyethylene crystal aggregates with bulk crystallized polyethylene;

Polymer, Volume 11, Issue 3, March 1970, Pages 125–146.

[16] D.A. Blackadder, P.A. Lewell, “Properties of polymer crystal aggregates: Part 3.

Comparison of the annealing behaviour of bulk-crystallized polyethylene with that of

aggregates of polyethylene crystals; Polymer, Volume 11, Issue 12, December 1970,

Pages 659–665.

[17] H. E. Bair, R. Salovey, and T. W. Huseby, “Melting and annealing of polyethylene

single crystals”, Polymer, vol. 8, no. 0, pp. 9–20, 1967.

[18] Melting and annealing of single crystal aggregates of trans-polydodecenamer E.

Martuscelli and V. Vittoria Laboratorio di Ricerche su Tecnologia dei Pofimeri e

Reologia, CNR,Via Toiano 2, Arco Fefice, Napoli, Italy.

[19] Mingwen Tian, Joachim Loos Eindhoven Investigations of Morphological

Changes during Annealing of Polyethylene Single Crystals Polymer Laboratories,

Dutch Polymer Institute, Technical University of Eindhoven, P.O. Box 513, Eindhoven

5600 MB, The Netherlands

[20] M. Zhang, D.T. Lynch, S.E. Wanke “Effect of molecular structure distribution on

melting and crystallization behavior of 1-butene/ethylene copolymers” Polymer 42

(2001) 3067±3075

[21] J.M. Lagaron, S.Lopez-Quintana, J.C. Rodriguez-Cabello, J.C.Merino, J.M.

Pastor.“ Comparative study of the crystalline morphology present in isotropic and

uniaxially stretched conventional and metallocene polyethylenes” Polymer 41 (2000)

2999-3010

[22] D. Ferrer- Balas, M.Li.Mospoch, A.B.Martinez, O.O. Santana “Influence of

annealing on the microstructural, tensile and fracture properties of polyethylene

films”, Polymer 42 (2001) 1697-1705.

[23] E. W. Fischer and G. F. Schmidt, “Long Periods in Drawn Polyethylene,”

Angewandte Chemie International Edition in English, vol. 1, no. 9, pp. 488–499,

1962.

[24] J. B. Jackson, P. J. Flory, and R. Chiang, “Thermodynamic stability of solution-

crystallized polyethylene,” Trans. Faraday Soc., vol. 59, no. 0, pp. 1906–1917, Jan.

1963.

[25] L. Mandelkern, “Physical properties of polymers” ACS Professional reference

book 1993.

137

7. Chapter 7

Three-phase characterization of uniaxially

stretched linear low density polyethylene

In this chapter, a detailed morphological study of the polyethylene monofilaments at the

different stages of the production line is performed by Raman spectroscopy, Differential

Scanning Calorimetry (DSC) and X-ray measurements. The structure of the three-phase

morphology of the linear low density polyethylene monofilaments was investigated by

combining these measurements.

7.1 Introduction

Polyethylene is one of the most extensively studied polymers and the understanding of its

structure-properties relationship has been one of the main topics of fundamental research

over the past few decades.

Polyethylene in the solid state, as part of the polyolefin family, is a semi-crystalline polymer,

which consists of three-phase morphology: a crystalline phase surrounded by a non-

crystalline phase comprising a partially ordered layer (third phase) adjacent to the crystallites

and a disordered phase (amorphous phase) in the intervening spaces [1-3]. The third phase

or transition phase is an intermediate component in addition to the crystalline and

amorphous phases. The character of the third phase, also referred to as the interface, the

intermediate phase or rigid amorphous phase, has been the subject of discussions in several

papers.

Some authors [4-5] assume the intermediate phase has mechanical properties slightly stiffer

than a purely amorphous phase. The larger the length-to-thickness ratio of the lamellae, the

stronger the reinforcing effects of the lamellae were on the amorphous matrix [5].

During uniaxial deformation, polymeric materials are often subjected to large plastic

deformations, giving rise to a preferential orientation of the macromolecules and the

morphology, which may result in a high anisotropy of the structure, thus resulting in improved

mechanical properties. Furthermore, especially in thin films, a preferred orientation of the

crystalline component produced by trans crystallization during cooling and/or by spin casting

138

may give rise to a strong anisotropy and can therefore have a profound influence on the

mechanical properties of these films [6-8]. Normally, uniaxially oriented filaments [4,9] show

good mechanical properties in the direction of orientation and their relaxation behaviour is

explained by the presence of an amorphous phase. By accepting the presence of a third

phase described as a rigid amorphous phase, it was possible to compute the effective elastic

properties of polymeric materials.

The nonlinear stress-strain behaviour of polyethylene material is governed by the relative

proportion of the crystalline and non-crystalline phases, consisting of the amorphous and

oriented amorphous phase, their orientation and their connectivity with respect to one

another. Based on a nonlinear viscoelastic model [10], the amorphous phase is in a liquid-

crystalline state. Young’s modulus and the strength of semi-crystalline polymers are primarily

affected by quasi-amorphous inter-lamellar regions [11], where several types of molecules,

such as loops, tails and bridges joining up with lamellae can be distinguished. The

intermediate or third phase, which may be summarized as being similar to linking lamellae

and amorphous phase [12], forms a surface layer around each lamella acting as a wrapping

membrane.

Raman spectroscopy was introduced to characterize the three-phase morphological

structure of semi-crystalline polyethylene [9]. The Raman spectra can be used to assess the

level of the three morphological components, by giving cumulative information with respect

to all phases [9, 13-17]. Such investigations demonstrated that chains involved in the third

phase or in the anisotropic disordered phase were stretched, but lacked lateral order. The

same conclusion was also observed from other results [18], showing that the non-crystalline

inter-lamellar phase is anisotropic and exhibits properties that are intermediate between that

of a crystalline solid and of an amorphous melt.

In the classical analysis of the Raman spectra, these spectra are resolved into three

components corresponding to the orthorhombic crystalline phase, a liquid-like amorphous

phase and a third partially ordered phase. The total, integral intensity of the CH2-twisting

vibrations range (1250 – 1350 cm-1

) is independent of the degree of crystallinity and used as

a standard with which the intensities of the other bands can be compared. In the classical

approach [15], the mass fractions of the crystalline (CR), amorphous (AR) and transition

phase (TR) are calculated using the integral intensities of the bands located at 1416 cm-1

and

1303 cm-1

.

X-ray measurements provide a model of the three-phase morphology [19]. Wide angle

diffraction (WAXS) scans the sample and the scattering intensity is plotted as function of the

angular position 2θ. The results of the X-ray measurements suggested that the amorphous

139

halo of a polyethylene in a solid state is the sum of scattering from a completely amorphous,

liquid-like phase and from the intermediate, better-ordered regions that originate during

crystallization.

The structure of the interphase, described as anisotropic [18] and having properties

intermediate between that of semi- crystalline solid and amorphous melt, is still not clear

however.

The objective of this chapter is to obtain a better insight into the structure of the third phase

by combining DSC, Raman and X-ray measurements. The structure of the intermediate or

rigid amorphous phase is changed by uniaxially stretching the polyethylene samples. For this

reason, several monofilaments of linear low density polyethylene with different draw ratios

and treated at 100 and 120 C were investigated and the results analyzed.

7.2 Results and discussion

7.2.1 DSC Measurements.

From DSC measurements as described in chapter 5 and 6, it was observed that the range of

the melting temperatures and the final melting temperature was nearly constant,

independent of the draw ratio, as represented in figure 7.1. The DSC melting endotherms

are characterized by a broad melting range of temperatures between 30°C and 144°C. Such

a broad melting range of temperatures is characteristic of LLDPE materials and is the result

of the broad distribution of crystal sizes. This is further attributed to a highly heterogeneous

structure that results from non-random incorporation of the comonomer during the

polymerization with a Ziegler-Natta catalyst. The total melting range, from 30°C up to 144°C,

was used to calculate the melting enthalpy and the corresponding calculated degree of

crystallinity. The percentage of crystallinity is only increasing with a small fraction by

increasing the draw ratio and not changing enormously after heat treatment as shown from

the result in table III, chapter 5 and table V chapter 6. From the DSC curves (see figure 7.1),

it is quite clear that the highest melting peak temperature is around 123°C for all the

samples; however, they show slightly different values for the melting enthalpy as a result of

the cold drawing.

140

Figure 7.1. DSC curves for sample A3 A5 and A9 at melting stage.

7.2.2 Raman Spectra.

Figure 7.2 shows the recorded Raman spectra in the region of 950-1500 cm-1

, corresponding

to the spectra commonly observed for semicrystalline polyethylenes. As represented in

figure 7.2, the measured Raman spectra were decomposed into individual bands using

Gauss functions in the region between 1250 and 1500 cm-1

. A very good approximation of

the measured spectra by the Gauss curve fitting, using the indicated individual bands, is

obtained as indicated in figure 7.2.

141

Figure 7.2. Gaussian deconvolution of Raman spectra.

The total integral intensity ITW of the CH2-twisting region (1250-1350 cm-1

) is independent

from the degree of crystallinity and is used as an internal standard [15]. The spectrum in this

twisting region can be deconvoluted into a narrow band cantered at 1295 cm-1

and a broader

component having its maximum intensity at 1303 cm-1

. In the classical approach [15], the

mass fraction of the crystalline phase (CR) is calculated using the integral intensity of the

band located at 1416 cm-1

and the mass fraction of the amorphous phase (AR) is calculated

using the integral intensity of the band located at 1303 cm-1

. In another approach, the

integral intensity of the band located at 1295 cm-1

is used to calculate the all-trans molecules

[14]. The structures calculated from these Raman approaches are not necessarily identical

with structures formed during crystallization and drawing of LLDPEs, as characterized by the

other analysing methods. Based on these published results [14-20] with the cited

approaches, we define the amount of the gauche-conformations as the ratio between the

total integral intensity at 1303 cm-1

(I1303), relative to ITW, and the corresponding amount of

the trans-conformations as the ratio between the total integral intensity at 1295 cm-1

(I1295)

relative to ITW,, according to the following equations (1 and 2):

%Trans = (I1295/ITW) * 100 (1)

% Gauche = (I1303/ITW) * 100 (2)

The results calculated for the different LLDPE monofilaments are summarized in Table 1.

0

0,2

0,4

0,6

0,8

1

1250 1300 1350 1400 1450 1500

Ram

an

In

ten

sit

y

Raman shift (cm-1)

Gaussian deconvolution

Originalcurve

1295Trans

1303Amorphous1417Orthohombic1435

1457

SUM

142

7.2.3 X-Ray Spectra.

The recorded x-ray spectra were used to calculate the amount of the amorphous phase in

the different LLDPE monofilaments after the Gauss decomposition of diffraction. The

characteristic diffractions also treated in previous chapters are presented in figure 7.3 and

the peak band of the amorphous phase around 21.5°, which is typical for polyethylene

polymers, was used to calculate the amount of the amorphous phase. The results of the

amount of the amorphous phase are summarized in table I.

Figure 7.3. Measured X-ray spectra of LLDPE monofilament and Gaussian deconvolution.

The result of the decomposition for sample A3 is shown in figure 7.3. Both the crystalline

peaks and the amorphous halos were represented by Gaussian profiles. These results

showed that the monoclinic or triclinic phase was present in the cold-drawn material.

However, the amount of monoclinic material, determined by X-ray measurements, is rather

small. But the presence of this monoclinic material is an indication of the presence of a

partially ordered component of the polymer structure, with a lower degree of order than the

true crystalline structure.

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

16 17 18 19 20 21 22 23 24 25 26

X-r

ay I

nte

nsit

y

2θ ( )

Gaussian deconvolution

Original curve

16.9Monoclinic

21.4Orthohombic

21.5Amorphous

23.6Orthohombic

Sum

Difference

143

The results obtained from DSC, Raman and X-ray measurements for samples with CDR are

summarized in Table I, together with the amount and structure of the third phase calculated

as the difference between the non-crystalline structures from DSC and the amorphous

fraction from X-ray measurements.

Table I Structure analysis of the three phases of the LLDPE cold draw ratio monofilaments.

CDR

DSC Raman

CRY (T) (%) Amorphous +3rd

phase (%) Trans structure(%) Gauche structure (%)

7.2 51±1 49 64 36

6.1 50±2 50 59 41

5.7 47±0 53 54 46

5.0 47±2 53 53 47

4.5 47±1 53 51 49

4.2 46±3 54 50 50

3.7 46±0 54 49 51

CDR X-ray Amorphous (%) Structure of 3

rd phase

3rd Phase (%) Trans (%) Gauche (%)

7.2 36.0±2 13.0 0.0 13.0

6.1 36.0±2 9.0 5.0 14.0

5.7 38.9±2 7.0 7.1 14.1

5.0 36.3±2 6.0 10.7 16.7

4.5 37.1±2 4.4 11.5 15.9

4.2 36.5±2 4.0 13.5 17.5

3.7 36.6±2 3.0 14.4 17.4

7.3 Third-Phase Characterization.

The Raman measurements together with the X-ray and DSC measurements will be used to

obtain a better and further insight in the amount and structure of the non-crystalline fraction

of the monofilaments. The crystalline phase is obtained from the DSC measurements and

the difference is the non-crystalline phase, containing the amorphous and intermediate or

third phase. The content of gauche and trans molecules is obtained from the Raman

measurements, by following the formulas 1and 2, explained in 7.2.2 paragraph.

The crystalline phase, calculated from the DSC measurements, contains only trans-

conformations. The amorphous phase, containing only gauche molecules and described as

a mobile gauche-containing amorphous component [21], was calculated from the X-ray

measurements. The difference between the non-crystalline fraction calculated from DSC and

the amorphous phase calculated by X-ray yields the resulting percentage of the intermediate

phase present in the different samples.

144

The percentage of the third phase is slowly decreasing by increasing the cold draw ratios.

The differences between the gauche content from Raman and the fraction of the amorphous

phase by X-ray were calculated and these values correspond to the amount of gauche

molecules present in the third phase. The content of trans molecules increases with the draw

ratio; and the maximum draw ratio is obtained if the third phase contains only trans

molecules. At this limit, all the polymer chains in the third phase are completely stretched in

the draw ratio.

A better insight into the structure of the third phase is obtained by combining the DSC,

Raman and X-ray measurements and by splitting the Raman CH2-twisting vibration region

into trans and gauche molecules. As a result, the splitting of the CH2-twisting region in the

Raman spectra corresponds to the splitting of the conformers into trans and gauche

molecules and not into amorphous and ordered structures. This situation represents a major

difference with the published interpretations of the Raman spectra.

The same analysis was also performed for the samples produced with the pilot line. The

calculated values of the third phase obtained by a difference of % of the amorphous phase

with DSC and % of the amorphous phase with X-ray are correlated with the drawing ratio

respectively for samples after cold draw ratio, heat settings with fixed ends at 120°C, with

controlled shrinkage and annealed for ten seconds at 100°C and 120°C, are presented

respectively for each temperature in the figures 7.4 and 7.5.

145

Figure 7.4. The relation of 3rd

phase structure with drawing ratio for CDR samples

represented for the original samples and heat treated at 100 C ( original samples, 10%

shrinkage, annealed at 100 C).

0

5

10

15

20

25

0 10 20 30 40 50 60

3rd

ph

ase (

%)

²-1/

146

Figure 7.5. The relation between 3rd

phase structure and drawing ratio of the original

samples and the samples treated at 120 C under different conditions ( original samples,

annealed at 120 C, 15 % shrinkage, heat treatment with fixed ends at 120 C).

The samples obtained after cold drawing and those after heat treatment at 100 C are

characterized by a decreased amount of third phase by increasing the cold draw ratio and

only small differences are measured between the original samples and the heat treated

samples at 100°C. As the amount of amorphous phase is constant, a certain amount of the

third phase is transformed into a crystalline phase. When the samples are treated at 120 C,

a different behaviour is observed. Samples treated at 120°C with fixed ends are comparable

with the original samples. Samples with controlled shrinkage (15%) at 120 C show an

increase of the amount of the third phase and the annealed samples at 120 C, after 15%

shrinkage at 120°C, show a decrease of the third phase and the values are even lower than

for the samples annealed at 100 C. These results indicate the complex behaviour of partial

melting and recrystallization of the oriented monofilaments, which will be explained in more

detail by discussing the results of the mechanical properties of the different samples.

0

5

10

15

20

25

30

0 10 20 30 40 50 60

3rd

ph

ase (

%)

²-1/

147

7.4 Mechanical properties related to the morphology

Upon stretching, the amount of the trans segments in the third phase increases, with a

nearly similar amount of the crystalline and amorphous phase. This is related to the

extension of the tie-molecules in the third phase and has a direct influence on the

mechanical properties of the oriented monofilaments. Figure 7.6 shows the influence of the

amount of the trans and gauche segments on the elasticity modulus of the oriented

monofilaments for cold draw samples.

Figure 7.6. Elasticity modulus as function of the amount of trans segments in the LLDPE

monofilaments.

The elasticity modulus is a linear function of the amount of trans segments in the

intermediate phase, starting from a value of 45 MPa for the intermediate phase containing

100% of gauche molecules and increasing to 280 MPa for 100% trans segments. The third

phase is the interconnecting phase between the crystallites; and the mechanical property of

this third phase is directly related to the overall mechanical properties of the monofilaments.

The elasticity modulus of the third phase can then be calculated by using this new approach

of the structure of the polyethylenes, namely that the third phase is making the link between

the crystalline structures and can be directly related to the final mechanical properties of the

polyethylenes. The influence of the cold draw ratio on the elasticity modulus of the third

phase is demonstrated in table II and figure 7.7.

0

50

100

150

200

250

300

0 5 10 15

Ela

sti

c M

od

ulu

s (

MP

a)

Amount of Trans molecules in the 3rd phase (%)

148

The monofilaments with a cold draw ratio of 7.2, containing 13% of the third phase and an

elasticity modulus of 270 MPa, can be characterized with an elasticity modulus of the third

phase of 2080 MPa

Table II. Elasticity modulus of the 3rd

phase as function of the cold draw ratio

CDR E Modulus (MPa) of the

monofilaments Amount of the 3

rd

phase (%) E Modulus (MPa) of the 3

rd

phase

3.7 103 17.4 591

4.2 112 17.5 640

5.0 144 16.7 860

5.7 165 14.1 1180

6.1 203 14.0 1450

7.2 270 13.0 2080

Figure 7.7. Calculated elasticity modulus of the 3rd

phase as function of the deformation

related to the cold draw ratio

The relationship between the elasticity modulus of the monofilaments produced with the pilot

line and the measured amount of the third phase can also be used to calculate the elasticity

modulus of the third phase for these obtained monofilaments. These calculated values of the

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60Ela

sti

cty

mo

du

lus o

f th

e 3

rd p

hase (

MP

a)

λ2 -1/λ

149

elasticity modulus of the third phase are reproduced on figure 7.8, together with the values

already calculated for the other monofilaments (see figure 7.7).

Figure 7.8. Calculated values of the elasticity modulus of the 3rd

phase as function of the

deformation related to the cold draw ratio (∆ LLDPE DOWLEX 2035G; ■ pilot line, LLDPE

DOWLEX 2606G).

The same correlation between the elasticity modulus of the third phase is obtained for the

two LLDPEs as function of the imposed deformation related to the cold draw ratio and these

results are the confirmation of the proposed structural model of the polyethylenes, consisting

of a three-phase structure and the links between the crystallites are related to the third

phase.

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60

Ela

sti

cit

y m

od

ulu

s o

f th

e 3

rd p

hase

(MP

a)

λ2 -1/λ

150

Figure 7.9. Calculated elasticity modulus of the 3rd

phase as function of the imposed

deformation related to the cold draw ratio (■ original samples after cold drawing, ∆ after 10%

shrinkage at 100°C, ▲ after 10% shrinkage at 100°C and annealing for 10 seconds at

100°C)

The calculated elasticity modulus of the third phase is decreased after the 10% shrinkage at

100°C and more decreased than the amount of shrinkage imposed on the monofilaments as

represented in figure 7.9. This can be the result of the partial melting of the crystalline

structure, resulting in an increase of the distance between the lamellae and some

transformation of the trans segments into gauche segments. After annealing at 100°C for 10

seconds, the final temperature of the monofilaments is 100°C and the elasticity modulus of

the third phase is increased and is approaching the values of the original samples. This

means that a part of the third phase, located at the interface with the lamellae, is

recrystallized, and the distance between the lamellae has decreased and a certain amount of

gauche segments is transformed into trans segments.

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60

Ela

sti

cit

y m

od

ulu

s o

f 3rd

ph

ase (

MP

a)

λ2 -1/λ

151

Figure 7.10. Calculated elasticity modulus of the 3rd

phase as function of the imposed

deformation related to the cold draw ratio (■ original samples after cold drawing, ∆ after 15%

shrinkage at 120°C, ▲ after 15% shrinkage at 120°C and annealing for 10 seconds at

120°C).

The importance of the processing parameters is even more expressed at the treatment

temperature of 120°C, as represented in figure 7.10. An important decrease of the elasticity

modulus of the third phase is calculated for the 15% shrinkage at 120°C, due to a more

important partial melting of the lamellae at the interface between the crystalline and the third

phase. Annealing at 120°C during 10 seconds increases the elasticity modulus of the third

phase to its original values, resulting from the recrystallization at the interphase between the

two phases.

In conclusion, the mechanical behaviour of the stretched LLDPE monofilaments is rather

complex but an analysis of this behaviour is possible by incorporating the presence of the

third phase in the total structure. The third phase creates the links between the crystalline

lamellae and is responsible for the final mechanical properties of the monofilaments.

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60

Ela

sti

cit

y m

od

ulu

s o

f 3rd

ph

ase (

MP

a)

λ2 -1/λ

152

7.5 Resilience and deformation recovery of the different obtained samples

The possible correlations between the resilience or the deformation recovery and the content

of the third phase are represented in the figures 7.11 and 7.12, respectively for the heat

treatments at 100 or 120 C.

For the resilience of the original monofilaments and the one with 10% shrinkage at 100°C or

15% shrinkage at 120°C, the values are decreasing from 65% at a content of the third phase

of 37% to a value of zero for a zero value of the third phase. The % of deformation recovery

is increasing from 42% at a 0% third phase to 100% at a 37% content of the third phase.

For the samples treated at 100°C or 120°C for 10 seconds, the percentage of resilience is

going from 0 at 0% third phase to 100% at 37% third phase. The deformation recovery is

going from 64% at 0% third phase to 100% at 37% third phase.

Figure 7.11. Correlation of 3rd

phase with resilience or deformation recovery for samples

heat treated at 100 °C ( resilience for original samples, resilience for 10% shrinkage,

resilience after annealing samples, deformation recovery for original samples,

deformation recovery for 10% shrinkage and deformation recovery after annealing).

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40% R

esilie

nce o

r %

Defo

rmati

on

reco

vey

% 3rd phase

153

Figure 7.12. Correlation of 3rd

phase with resilience or deformation recovery for samples

heat treated at 120 °C ( resilience for original samples, resilience for 15% shrinkage,

resilience after annealing samples, deformation recovery for original samples,

deformation recovery for 15% shrinkage and deformation recovery after annealing

samples).

The content of the third phase is very important for the resilience, the deformation recovery

and of course for the long term behaviour of the monofilaments. The thermal treatment

during 10 seconds is very important for better long term properties and for the internal

characteristics of the third phase.

7.6 Conclusions

LLDPE monofilaments with different cold-drawn ratios were produced and the three-phase

morphology was characterized. The three-phase morphology contains a crystalline phase,

an amorphous phase and a third, or intermediate, phase. The combination of Raman

spectroscopy, DSC and X-ray measurements creates the possibility to characterize the

amounts and composition of the three phases. The amount of the amorphous phase is

nearly constant, independent of the cold-drawn ratio. The amount of the intermediate phase

is function of the cold-drawn ratio. The content of gauche and trans segments is strongly

influenced by the cold draw ratio and a linear variation of the content of trans segments is

obtained with the cold-drawn ratio. The mechanical property of the oriented monofilaments is

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40

% R

esi

lien

ce o

r %

De

form

atio

n r

eco

very

% 3rd phase

154

directly correlated with the properties of the intermediate phase and confirms that the

intermediate phase is the linking phase between the crystallites. These results suggest that

the elasticity modulus is determined by trans-segment in the intermediate phase and that the

intermediate phase is related to the tie molecules. It was found that the two peaks in the

Raman spectra, respectively at 1303 and 1295 cm-1

, can be correlated with the amount of

gauche and trans molecules in the polyethylene monofilaments. A constructive and new

insight into the three-phase morphology was obtained by combining the DSC and X-ray

measurements with the amounts of trans and gauche molecules calculated from the Raman

spectra analysis.

155

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157

Chapter 8

Conclusions and recommendations for future

research

Artificial turf, used for different sport applications, is considered as more than only a good

alternative for natural grass. These surfaces can be used due to their advantages such as

climate independence, less maintenance, more economical and more homogeneous playing

characteristics. There are many applications for different sports including football, rugby,

tennis, golf, etc. Most of the research concerning artificial turf is focused on the behaviour of

the pile layer, composed by monofilaments, and on football applications. This PhD work

focuses on the development of a new test method and on the study of the behaviour and

structure of the monofilaments at different stages of their production.

The drawbacks of the current test methods, to measure the bending behaviour and

fibrillation of the monofilaments, make it necessary to develop a new test method. These

drawbacks are the following:

The Lisport test can only be performed on the final product, on the artificial turf. This makes it

difficult to characterize the external factors influencing the resilience.

The Favimat R test makes it possible to test a single monofilament and give a quick

feedback for the product. This is very useful information for the producers, but the

measurements and results can only be obtained at ambient temperatures. The developed

test method was firstly evaluated at ambient temperature by combining the obtained results

of the Lisport tests and the resilience at ambient temperatures before extending this new

method to higher temperatures.

Theoretical analyses of geometrical factors, including thickness, profile cross sections and

free length of monofilaments in the artificial turf fields clarify their influence on their bending

behaviour, although several hypotheses are considered. For example, increasing the

thickness of the product by a factor of two induces an increase in the bending force by a

factor between six to eight. During this research, one profile cross section is considered with

constant thickness in order to realize a scientific study of the relationship between the

structure and the processing of the monofilaments.

Another factor influencing the bending behaviour of the monofilaments is the used polymer.

There are several polymeric materials which can be used for the pile layer of the artificial

158

turf, such as polypropylene (PP) and polyamides (PA), polyethylenes (PE), polyesters.

Linear low density polyethylene (LLDPE) is the most widely used polymer for the production

of monofilaments at this moment due to their possible optimal properties of elastic modulus,

resilience, softness, weather resistance and melt processing. The macromolecular properties

of LLDPE can be modified between large limits due to their different possible polymerization

reactions, for example the used catalyst can be a Ziegler-Natta based on a metallocene

catalyst. This makes the linear low density polyethylene the best choice of material for

research.

The processing parameters used during the different steps of the production of the

monofilaments are also very important for their final results. These processing parameters

are studied in detail, especially the influence of the drawing ratio in the solid phase and the

heat treatment of the products after cold drawing. The post stretching of the monofilaments

after extrusion is the most important process parameter for the final structure of the obtained

products and on their behaviour.

A combination of different test methods, such as Differential Scanning Calorimetry, Raman

and X-ray spectroscopy, creates a better insight into the structure of the monofilaments

during the different steps of production. It was possible to calculate the third phase or

intermediate phase located between the crystalline phase and amorphous phase by the

combination of these analytical test methods and to make a link between the characteristics

of this third phase and the mechanical behaviour of the final monofilaments.

The relationship between the bending behaviour and elastic modulus and the process

parameters of the monofilaments is well explained by the existence of the third phase in

addition to the crystalline and amorphous phase. The mechanical properties of this phase is

increased by increasing the cold draw ratio and this third phase is the link between the

crystalline lamellae and the amorphous phase. The amount and mechanical properties of

this phase shows a good correlation with the elastic modulus of the monofilaments and their

resilience.

The heat treatment of the product shows a significant influence on the behaviour of

monofilaments but the annealing time seems to be important as well, by allowing the core of

the product to reach the necessary temperature and time to rearrange the structure. This

rearrangement of the crystalline and third phase structure creates a better link between

these two phases and better final mechanical properties.

Based on the obtained results, it can be concluded that the fibrillation and resilience of the

product shows a better result for the non-stretched samples than the cold drawn ones. But

the elastic properties of the non-stretched samples are not sufficient to allow them to pass all

159

the necessary production steps of the carpet/ artificial turf. For this reason, cold drawing of

the monofilaments is always necessary.

As a conclusion, multilayered monofilaments will be an optimal solution and will be the

monofilaments of the future. For the multilayered monofilaments, one polymer is located in

the core and a different polymer forms the outer layer of the multi-layered structure.

The following conclusions can be drawn: the chosen polymer for the core layer should have

a melting temperature higher than the polymer used for the outer layer and the two different

polymers have to be compatible. The temperature used during the post stretching, the cold

drawing, must be situated between the melting temperatures of the polymers combined. The

result will be that the core is cold stretched and will have relatively high mechanical

properties and the outer layer is only stretched in the molten phase inducing less or in the

limit no orientation. The high elastic modulus of the monofilaments will be obtained by the

core material and the resilience, fibrillation resistance and softness will be obtained with the

outer layer. In addition, the monofilaments produced on this way can be connected with the

coating layer by welding if the same polymer is used for the outer and the coating layer.

A good combination of polymers for the multi-layered monofilaments could be polypropylene

(PP) and metallocene-catalyzed linear low density polyethylene (m-LLDPE). These two

polymers are compatible and the window of stretching temperatures is situated between 130

and 155°C, quite large and industrially usable.

This research and the obtained results, apart from giving information about the structure

development of the product in the different production stages, are also very useful for the

producers of the monofilaments.

However, further research and studies have to be done for defining the optimal combinations

between different polymeric materials to be used in multi-layered monofilaments for artificial

turf applications. A possible combination and choice of two different polymers is already

recommended and can be the fundamental choice for the next generation of artificial turf

fields.