UNIVERSITI PUTRA MALAYSIA

SHARIFAH MAZRAH BINTI SAYED MOHAMED ZAIN

FK 2009 112

ENHANCING TENSILE PROPERTIES OF CARBON FIBER-REINFORCED POLYPROPYLENE COMPOSITE USING CARBON NANOTUBE COATING

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ENHANCING TENSILE PROPERTIES OF CARBON FIBER-REINFORCED

POLYPROPYLENE COMPOSITE USING CARBON NANOTUBE COATING

By

SHARIFAH MAZRAH BINTI SAYED MOHAMED ZAIN

Thesis Submitted to the School of Graduate Studies, Universiti Putra Malaysia, in

Fulfilment of the Requirements for the Degree of Master of Science

November 2009

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DEDICATION

I dedicate this thesis to my beloved husband and twin daughters,

with love…

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Abstract of thesis presented to the Senate of Universiti Putra Malaysia in fulfilment

of the requirement for the degree of Master of Science

ENHANCING TENSILE PROPERTIES OF CARBON FIBER-REINFORCED

POLYPROPYLENE COMPOSITE USING CARBON NANOTUBE COATING

By

SHARIFAH MAZRAH BINTI SAYED MOHAMED ZAIN

November 2009

Chairman : Dr Suraya binti Abdul Rashid

Faculty : Engineering

Carbon fibers, when used without any surface treatments, will produce composites

with low interlaminar shear strength (ILSS) which attributed largely to weak bonding

between the fiber and the matrix. CNT-coating treatment was conducted to improve

carbon fiber-matrix interfacial bonding. This treatment was done by growing carbon

nanotubes (CNTs) directly on carbon fibers using chemical vapor deposition (CVD)

to create CNT-coated carbon fibers. The objectives was to study the microscopic

morphology of CNTs formation on the surface of carbon fibers at various treatment

conditions, to study the interfacial shear strength (IFSS) of CNT-coated carbon

fibers, tensile properties and thermal stability of CNT-coated carbon fiber composites

as well as comparison with untreated and commercial carbon fibers. The CNTs were

produced by a benzene-ferrocene gas reaction inside a high temperature tube furnace.

The reaction temperature, the carrier gas flow rate and weight of ferrocene were

varied at 700 oC, 800

oC and 900

oC; 100 ml/min and 300 ml/min; 0.3 g, 0.5 g and 1.0

g respectively and the reaction time for CNT growth was set at 30 minutes. The

microscopic morphology of CNTs formation on the surface of carbon fibers was

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observed using scanning electron microscopy (SEM) and transmission electron

microscopy (TEM) before it was fabricated into composites. The composites were

prepared by melt blending with polypropylene (PP) at different fiber content of 2, 4,

6, 8, 10 and 12 wt. %. It showed that CNTs were successfully grown on carbon fibers

at reaction temperature of 800oC and 900

oC. Interfacial shear strength of CNT-coated

fibers improved up to 499% compared to untreated fibers. Tensile properties

increased with the increase of fiber loading from 2 wt. % - 10 wt. % and decreased at

12 wt. % fiber content. With addition of 10 wt. % of CNT-coated CFPP composites,

the tensile strength and modulus increased up to 36% and 85%, respectively. CNT-

coated CFPP composites were more resistant to deformation, but had lower strength

when compared with commercial CFPP composites. The thermal stability of CNT-

coated CFPP composites showed an increment compared to the untreated CFPP

composite. As conclusion, CNT-coating treatment using parameters treated at

reaction temperature of 800oC; 300 ml/min hydrogen flow rate and 1.0 g of ferrocene

showed the most amounts of CNTs with fewer impurities which also exhibited the

best performance in IFSS, tensile properties and highest onset degradation

temperature, 325oC making it the best designation for this CNT-coating treatment

using current thermal CVD set-up.

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Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai

memenuhi keperluan untuk Ijazah Master Sains

MENINGKATKAN SIFAT KETEGANGAN BAGI KOMPOSIT

POLIPROPILENA DIPERKUAT GENTIAN KARBON DENGAN

PENYALUTAN KARBON TIUB NANO

Oleh

SHARIFAH MAZRAH BINTI SAYED MOHAMED ZAIN

November 2009

Pengerusi : Dr Suraya binti Abdul Rashid

Fakulti : Kejuruteraan

Gentian karbon, apabila digunakan tanpa rawatan permukaan, ia akan menghasilkan

komposit yang mempunyai kekuatan ricih antara lamina (ILSS) yang rendah

seterusnya menyumbang kepada ikatan lemah antara gentian dan matriks resin.

Rawatan penyalutan karbon tiub nano keatas permukaan gentian karbon telah

dijalankan untuk meningkatkan ikatan antara muka gentian karbon dan matriks.

Rawatan ini dijalankan dengan menumbuhkan karbon tiub nano ke atas permukaan

gentian karbon menggunakan kaedah pemendapan wap kimia (CVD). Objektif kajian

ini adalah untuk mengkaji gambaran sifat karbon tiub nano yang terbentuk pada

keadaan rawatan berbeza, menganalisis kekuatan ricih antara muka (IFSS) gentian

karbon terawat, sifat ketegangan dan kestabilan terma komposit gentian karbon

terawat dan juga perbandingan dengan gentian karbon tidak dirawat dan gentian

karbon komersial. Karbon tiub nano yang terbentuk telah dihasilkan melalui tindak

balas wap antara benzena dan ferosena di dalam tiub relau bersuhu tinggi. Suhu

tindak balas, kadar alir gas pembawa (hidrogen) dan jumlah ferosena di jalankan

pada suhu 700 o

C, 800oC dan 900

oC; 100 ml/min dan 300 ml/min; 0.3 g, 0.5 g dan

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1.0 g, masing-masing dan masa tindak balas bagi pembentukan karbon tiub nano

ditetapkan pada 30 minit. Kajian morfologi mikroskopi karbon tiub nano yang

terbentuk dijalankan menggunakan mikroskop elektron imbasan (SEM) dan

mikroskop elektron transmisi (TEM) sebelum ia di jadikan komposit. Komposit

disediakan dengan campuran lebur polipropilena dan gentian karbon pada kandungan

berat peratus berbeza iaitu 2, 4, 6, 8, 10 and 12%. Keputusan menunjukkan karbon

tiub nano terbentuk dengan jayanya menyaluti permukaan gentian karbon pada suhu

rawatan 800oC and 900

oC. Kekuatan ricih antara muka gentian karbon terawat iaitu

disaluti karbon tiub nano telah meningkat sebanyak 499% berbanding gentian karbon

tidak dirawat. Sifat ketegangan meningkat dengan peningkatan peratus pengisian

gentian dari 2 - 10% dan menurun pada 12% kandungan gentian karbon. Dengan

penambahan sebanyak 10% gentian karbon terawat ke dalam polipropilena komposit,

kekuatan dan modulus ketegangan meningkat sebanyak 36% dan 85%, masing-

masing. Komposit gentian karbon terawat mempunyai rintangan lebih tinggi

terhadap canggaan, tetapi mempunyai kekuatan yang lebih rendah jika dibandingkan

dengan komposit gentian karbon komersial. Kestabilan terma bagi komposit gentian

karbon terawat menunjukkan peningkatan berbanding dengan komposit gentian

karbon tidak dirawat. Kesimpulannya, penyalutan karbon tiub nano pada parameter

800oC; kadar alir gas hidrogen 300 ml/min dan 1.0 g ferosena telah menunjukkan

pembentukan karbon tiub nano yang paling banyak dan kurang bendasing. Ia juga

menunjukkan prestasi terbaik bagi kekuatan ricih antara muka, sifat ketegangan dan

suhu penguraian tertinggi, 325oC menjadikan ia formula terbaik bagi rawatan ini

menggunakan set alat terma CVD yang sedia ada.

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ACKNOWLEDGEMENTS

With the completion of this thesis, I would like to express my deep and sincere

gratitude to my supervisor, Dr. Suraya bt Abdul Rashid, lecturer of Department of

Chemical and Environmental Engineering, Faculty of Engineering who introduced

me to the field of nano materials and her wide knowledge have been of great value

for me. Her understanding, encouraging, personal guidance and excellent advice

throughout this work have provided a good basis for the present thesis.

I also wish to express my warm and sincere thanks to my co-supervisors; Associate

Professor Dr. Robiah bt. Yunus (Department of Chemical and Environmental

Engineering, Faculty of Engineering) and Dr. Nor Azowa bt Ibrahim (Department of

Chemistry, Faculty of Science) for their constructive comments, valuable advice and

kind support that have been very helpful for this study.

I owe my most sincere gratitude to those who gave me the opportunity to work with

them in conducting the testing and analysis in the Advanced Materials Research

Centre (AMREC), SIRIM and Microscopy Unit, Institute of Bioscience (UPM).

During this work, I have collaborated with many colleagues for whom I have great

regard and I wish to extend my warmest thanks to all those who have helped me with

my work in the Department of Chemical Engineering (Faculty of Engineering) and

Department of Chemistry (Faculty of Science). I am grateful for all the valuable

advice and friendly help.

I owe my loving thanks to my husband Syed Mohamad Nazli bin Syed Ahmad, my

parents and families for their loving support. Without their encouragement and

understanding, it would have been impossible for me to finish this work.

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I certify that a Thesis Examination Committee has met on 20 November 2009 to

conduct the final examination of Sharifah Mazrah binti Sayed Mohamed Zain on her

thesis entitled “Enhancing Tensile Properties of Carbon Fiber-Reinforced

Polypropylene Composite using Carbon Nanotube Coating” in accordance with the

Universitites and University Colleges Act 1971 and the Constitution of the Universiti

Putra Malaysia [P.U.(A) 106] 15 March 1998. The Committee recommends that the

student be awarded the Master of Science.

Members of the Thesis Examination Committee were as follows:

Azni bin Idris, PhD

Professor

Faculty of Engineering

Universiti Putra Malaysia

(Chairman)

Luqman Chuah bin Abdullah, PhD

Associate Professor

Faculty of Engineering

Universiti Putra Malaysia

(Internal Examiner)

Mohd Amran bin Mohd Salleh, PhD

Faculty of Engineering

Universiti Putra Malaysia

(Internal Examiner)

Ishak bin Ahmad, PhD

Associate Professor

Faculty of Engineering

Universiti Putra Malaysia

(External Examiner)

BUJANG BIN KIM HUAT, PhD

Professor and Deputy Dean

School of Graduate Studies

Universiti Putra Malaysia

Date:

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This thesis submitted to the Senate of Universiti Putra Malaysia and has been

accepted as fulfilment of the requirement for the degree of Master of Science. The

members of the Supervisory Committee were as follows:

Suraya Abdul Rashid, PhD

Lecturer

Faculty of Engineering

Universiti Putra Malaysia

(Chairman)

Robiah Yunus, PhD

Associate Professor

Faculty of Engineering

Universiti Putra Malaysia

(Member)

Nor Azowa Ibrahim, PhD

Lecturer

Faculty of Science

Universiti Putra Malaysia

(Member)

HASANAH MOHD GHAZALI, PhD

Professor and Dean

School of Graduate Studies

Universiti Putra Malaysia

Date: 12 August 2010

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DECLARATION

I hereby declare that the thesis is based on my original work except for quotations

and citations which have been duly acknowledged. I also declare that it has not been

previously or concurrently submitted for any other degree at Universiti Putra

Malaysia or other institutions.

SHARIFAH MAZRAH SAYED MOHAMED ZAIN

Date:

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TABLE OF CONTENTS

Page

DEDICATION ii

ABSTRACT iii

ABSTRAK v

ACKNOWLEDGEMENTS vii

APPROVAL viii

DECLARATION x

LIST OF TABLES xiv

LIST OF FIGURES xv

LIST OF ABBREVIATIONS xviii

CHAPTER

1 INTRODUCTION 1

1.1 Background Study 1

1.2 Problem Statement 3

1.3 Objectives of Study 5

1.4 Scope of Study 6

1.5 Thesis Structure 6

2 LITERATURE RIVIEW 8

2.1 Carbon Fibers Composites 8

2.2 Short-fiber Reinforced Composites 10

2.3 Composite Fabrication 12

2.3.1 Resin Matrix 13

2.3.2 Polypropylene

2.4 Carbon Fibers 17

2.5 Surface Properties of Carbon Fibers 18

2.6 Surface Treatments 19

2.7 Whiskerization 21

2.8 Carbon Nanotubes 23

2.8.1 Growth Mechanisme 26

2.8.2 Carbon Source 29

2.8.3 Catalyst 30

2.9 Carbon Nanotubes Growth by Chemical

Vapor Deposition (CVD) 30

2.10 Tensile Properties 38

2.11 Interfacial Shear Strength (IFSS) 39

2.12 Thermal Properties 42

3 METHODOLOGY 43

3.1 Introduction 43

3.2 CNT-coating Treatment 43

3.2.1 Carbon Fiber 44

3.2.2 CNT-coating Treatment using 44

the CVD rig

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3.2.3 Treatment Conditions 46

3.2.4 Average Weight 48

3.3 Characterization 49

3.3.1 Scanning Electron Microscopy 49

3.3.2 Transmission Electron Microscopy 50

3.4 Composite Processing 50

3.4.1 Polypropylene 51

3.4.2 Chopping Process 52

3.4.3 Melt Blending 52

3.4.4 Hot Compression Molding 53

3.5 Mechanical and Thermal Properties 54

3.5.1 Tensile Test 54

3.5.2 Interfacial Shear Strength (IFSS) Test 55

3.5.3 Thermogravimetric Analysis 57

4 RESULTS AND DISCUSSION 58

4.1 Introduction 58

4.2 Effect of Treatment Conditions on 58

Production of CNTs

4.3 Characterization of CNT-coated Carbon Fibers 61

4.3.1 Effect of Growth Temperature 69

4.3.2 Effect of Ferrocene 70

4.3.3 Effect of Hydrogen Flowrate 73

4.3.4 TEM of CNTs 74

4.4 Average Weight 75

4.5 Heating Effect 78

4.6 Interfacial Shear Strength (IFSS) Test 82

4.7 Tensile Test 85

4.7.1 Heated Carbon Fibers 85

4.7.2 Tensile Strength 87

4.7.3 Tensile Modulus 90

4.7.4 Percentage Improvement 93

4.7.5 Comparative with Commercial

Carbon Fiber 96

4.7.6 Fracture Surface of Composite 98

4.8 Thermogravimetric Analysis 99

4.8.1 Filler Loading Effect 100

4.8.2 TGA of Composite at 10 wt. % 101

Fiber Content

5 CONCLUSION AND RECOMMENDATIONS 103

5.1 Conclussions 103

5.2 Reccomendations 105

REFERENCES 108

APPENDICES 115

A1 Properties of Carbon Fiber 116

A2 Properties of Polypropylene 117

A3 Properties of Epolam 118

B1 Standard Test Method for Tensile Properties 120

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B2 Recommended Method for Tow Tensile Testing 134

C1 Thermal Gravimetric Analysis Datasheet 140

BIODATA OF STUDENT 150

LIST OF PUBLICATION 151

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LIST OF TABLES

Table Page

2.1. The estimated critical length for some short carbon fiber 12

reinforced thermoplastics (Carbon fiber diameter = 7.4µm.

tensile strength = 3.79GPa)

2.2 A summary of CNTs production methods and the efficiency 25

2.3. List of notable accomplishment in CNT Synthesis using CVD 33

method

3.1. Carbon fibers properties 44

3.2. Preliminary testing conditions for CNT-coating treatment 47

on carbon fibers

3.3. Specimen designations for CNT-coating treatment and heating 48

conducted on carbon fibers

3.4. Typical polypropylene properties 51

3.5. Composition for composite compounding 53

4.1. Preliminary test of treatment conditions on carbon fiber to 59

identify the presence of CNTs

4.2. Specimen designations and treatment conditions for CNT-coating 61

treatment on carbon fibers

4.3. The average weight of CNT-coated carbon fibers 76

4.4. Percentage of improvement in IFSS for CNT-coated carbon fibers 84

As compared with untreated carbon fibers

4.5. The maximum percentage of improvement of CNT-coated 95

CFPP composites

4.6. Onset temperature of degradation (oC)

for pure PP and CNT-coated 100

CFPP composites type A3 at different carbon content

4.7. Onset temperature of degradation (oC)

for pure PP, untreated, 101

commercial and CNT-coated CFPP composites at

10 wt. % carbon content

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LIST OF FIGURES

Figure Page

2.1 The molecule structure of continuous polypropylene chain 16

2.2 Comparison of diameters of various carbon fibers, graphite 23

whiskers, vapor-grown carbon fibers, carbon nanotubes

and fullerenes

2.3 Eight allotropes of carbon: a) Diamond, b) Graphite, 24

c) Lonsdaleite, d) C60 (Buckminsterfullerene or buckyball),

e) C540, f) C70, g) Amorphous carbon, and h) single-walled

carbon nanotube or buckytube

2.4. Visualisation of a possible carbon nanotube growth mechanism 27

in substrate growth

2.5. Schematic representation of different types of growth morphologies 28

observed in carbon nanotubes (a) whisker-like, (b) branched,

(c) spiral, and (d) helical

2.6. Schematic of a thermal CVD apparatus 32

2.7. SEM micrographs of carbon fibers (a) before and (b) after nanotube 34

growth by CVD

2.8. SEM images :(a) sample grown on carbon fibers at 550oC on 35

(scale are 2 mm); (b) CNT grown on fibers at 750oC

(scale are 2 mm); and (c) carbon clusters grown at 850oC

on fibers (scale are 10 mm)

2.9a. CNT growth on carbon fibers before (left) and after immersion 36

in acetone for 5 min (right)

2.9b. CNT on carbon paper before (left) and after the ultrasonic bath 36

in acetone for 5 min (right)

2.9c. CNT on carbon fibers before (left) and after ultrasonic bath in 36

deionised water for 5 min (right)

2.10. Typical (a) single-fibre pull-out specimen and (b) force-displacement 41

pull-out curve

3.1. Diagram of CNT-coating treatment process conducted in a 46

CVD furnace

3.2. Restrained top loading method (RTC) 56

4.1. SEM micrograph of untreated carbon fibers 62

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4.2. SEM micrograph of CNT-coating treament on carbon fiber at 63

reaction temperature of 800oC; 100 ml/min hydrogen flow rate

and (a) 1.0 g ferrocene (A1), (b) 0.5 g ferrocene (A2) and

(c) 0.3 g ferrocene

4.3. SEM micrograph of CNT-coating treament on carbon fiber at 64

reaction temperature of 800oC; 300 ml/min hydrogen flow rate

and (a) 1.0 g ferrocene (A3), (b) 0.5 g ferrocene (A4) and

(c) 0.3 g ferrocene

4.4. SEM micrograph of CNT-coating treament on carbon fiber at 65

reaction temperature of 900oC; 100 ml/min hydrogen flow rate

and (a) 1.0 g ferrocene (B1), (b) 0.5 g ferrocene (B2) and

(c) 0.3 g ferrocene

4.5. SEM micrograph of CNT-coating treament on carbon fiber at 66

reaction temperature of 900oC; 300 ml/min hydrogen flow rate

and (a) 1.0 g ferrocene (B3), (b) 0.5 g ferrocene (B4) and

(c) 0.3 g ferrocene

4.6. SEM micrograph of carbon fiber at reaction temperature of 700oC; 68

100 ml/min hydrogen flow rate (a) 1.0 g ferrocene;(b) 0.5 g ferrocene

4.7. TEM images with different magnification for a number of CNTs 75

grown on carbon fiber (a) CNTs with an open end (b) a bundle

of CNT (c) CNTs grown on carbon fiber with wavy morphology

(d) curved CNTs

4.8. Average weights of CNT-coated carbon fibers specimens 77

4.9. SEM micrograph of (a) untreated carbon fiber (b) carbon fiber 81

heated at 800oC and (c) carbon fiber heated at 900

oC

4.10. Interfacial Shear Strength (IFSS) of untreated, heated and 83

CNT-coated carbon fiber

4.11. Tensile strength of untreated and heated carbon fiber reinforced 86

polypropylene composites versus carbon content

4.12. Tensile modulus of untreated and heated carbon fiber reinforced 86

polypropylene composites versus carbon content

4.13. Tensile strength of untreated and CNT-coated carbon fiber reinforced 89

polypropylene at growth temperature of (a) 800oC, and (b) 900

oC

4.14. Tensile modulus of untreated and CNT-coated carbon fiber reinforced 92

polypropylene at growth temperature of (a) 800oC, and (b) 900

oC

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4.15. Relative improvements on the tensile properties of CNT-coated 94

CFPP composites (a) tensile strength (b) tensile modulus

4.16. Tensile properties of untreated CFPP, CNT-coated CFPP 97

type A3,and commercial CFPP composite (a) tensile strength

(b) tensile modulus

4.17. SEM micrograph of tensile fractures surface of the carbon fiber 98

composite with 10 wt.% fibers contents: (a) untreated carbon fiber,

(b) treated carbon fiber (type A3 at 10 wt.%)

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LIST OF ABBREVIATION

ABS Acrylonitrile butadiene styrene

ASTM American Society for Testing and Materials

Al2O3 Aluminium oxide

Ar Argon

CCD Charge coupled device

CF Carbon fiber

CNFs Carbon nanofibers

CNTs Carbon nanotubes

CFPP Carbon fiber reinforced polypropylene

CFRC Carbon-carbon fiber reinforced composites

CFRM Carbon fiber-metal reinforced composites

CFRP Carbon fiber-polymer matrix composites

CVD Chemical vapor deposition

Co Cobalt

C6H6 Benzene

E Modulus of elasticity

EDX Energy dispersive x-ray

Fe Iron

Fe(C5H5)2 Ferrocene

FRP Fiber reinforced polymer

GPa Gega Pascal

H2 Hydrogen

HRTEM High resolution transmission electron microscope

IFSS Interfacial shear stress

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ILSS Interlaminar shear strength

Lc Critical length

MFI Melt flow index

Mo Molybdenum

MW Molecular weight

MWNTs Multi-walled nanotubes

Ni Nickel

PAN Polyacronitrile

PC Polycarbonate

PEEK Polyetheretherketone

PP Polypropylene

PPS Polyphenylene sulfide

PS Polysulfone

RTC Restrained top loading

SEM Scanning electron microscope

SFRP Short-fiber reinforced polymer composites

SiC Silicon carbide

Si3N4 Silicon nitride

SWNTs Single-walled nanotubes

TiO2 Titanium dioxide

TEM Transmission electron microscope

TGA Thermo gravimetric analysis

VGCF Vapor grown carbon nanofibers

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CHAPTER 1

INTRODUCTION

1.1 Background Study

It had been established in recent years that polymer-based composites reinforced

with a small percentage of strong fillers could significantly improve the mechanical,

thermal and barrier properties of pure polymer matrix (Mahfuz et al., 2003). When

these fillers are rod-shaped, the surface area per particle will be higher than any other

shape of fillers. Carbon fibers have extremely high mechanical strength and stiffness

in axial direction and because of this, they are considered the most interesting fillers

for advanced applications composites (Gordeyev et al., 2001). Composite materials

have two major advantages among many others; improved strength and stiffness,

especially when compared with other materials on a unit of weight basis. For

example, composite materials can be made to have the same strength and stiffness as

high-strength steel and yet 70% lighter. Other advanced composite materials such as

rural material, yet weigh only 60% as much. Composite materials can be tailored to

efficiently meet design requirements of strength, stiffness and other parameters, all in

various directions (Jones, 1999).

Good composite performance often depends on the degree of adhesion between the

fiber and the resin binder. The physical and chemical properties of carbon fibers play

a major role in determining the degree of adhesion between the fiber and the resin

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matrix. Adhesion is usually controlled by chemical bonding due to functional groups

and by mechanical interlocking due to surface morphology. This leads researchers to

develop a number of surface treatments that could improve the fiber matrix polymer

interfacial bonding. In order to improve the bonding properties of carbon fibers,

various surface treatment approaches can be applied.

Carbon fiber surfaces are chemically inactive and must be treated to form surface

functional groups that promote good chemical bonding with resin matrix. Surface

treaments for carbon fibers are of two types, oxidative and non-oxidative. Oxidative

surface treatments produce acidic functional groups on the carbon fiber surface. Non-

oxidative surface treament was developed by coating the carbon fiber surface with an

organic polymer that has funtional groups capable of reacting with the resin matrix

(Mallick, 1993).

One of the non-oxidative surface treatment methods is whiskerization. It was an

affective way to increase the shear strength of carbon-epoxy composite materials by

400% (Kowbel et al., 1997). Whiskerization technique on carbon fiber appears to

have positive effect in improving the interfacial adhesion between carbon fibers and

the matrix (Kowbel et al., 1997; Rebouillat, 1984; Katsuki et al., 1987 and Ismail,

1993). Whiskerization involves growing carbon nanotubes (CNTs) on the surface of

carbon fibers. By growing the nanotubes on the surface of carbon fibers, the total

surface area would increase and enhance the mechanical interlocking between fibers

and resin matrixes. Over a decade, carbon nanotubes have attracted great interest in

the scientific field because of their unique structure, remarkable mechanical, thermal

and electrical properties and potential applications. Based on their exceptional tensile

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strength and modulus, CNTs are deemed to be able to improve the tensile properties

of the composite material and to increase the fracture energy once a crack is initiated.

A lot of studies and applications had recommended that nanotubes to be grown

directly on substrate (carbon fiber) to minimize contact resistance between the

nanotubes and the substrate, especially in large area at low cost, which however is

still a challenge (Li et al., 2000). There are many techniques in producing CNTs and

chemical vapor deposition (CVD) had been found to be efficient and selective for

either single-walled or multi-walled carbon nanotubes and had demonstrated several

advantages in growing the carbon nanotubes (Zhu et al., 2003). It had been

established in recent years that polymer-based composites reinforced with the

superior properties of carbon nanotubes (CNTs) could enhance and significantly

improve the performance of the polymer matrix and numerous composites.

1.2 Problem Statement

It is well known that fiber-matrix adhesion strength plays an important role on the

mechanical properties of fiber-reinforced polymer composites (Mallick, 1993). When

load is applied to composites, it will be distributed and transferred through fiber-

matrix interfaces. A strong bonding promotes a better involvement of more fibers

which increases the strength of composites accordingly. However, carbon fibers

usually experience a poor bonding behavior to polymer matrix due to their nature of

smoothness and chemical inertness. This had been attributed largely to poor adhesion

or weak bonding between carbon fiber surface and matrix molecules. Carbon fibers,

when used without any surface treatment, produce composites with low interlaminar

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shear strength (ILSS) (Zhang et al., 2004). Kowbel and co-worker reported an

improvement of 200% - 300% in interlaminar shear strength (ILSS) of CNT-coated

carbon fiber-reinforced epoxy composites. CNTs which increase the interfacial area

of carbon fibers (Thostensen et al., 2001) provide a larger number of contact points

for fiber-matrix bonding which attributed to the improvements in ILSS (Mallick,

1993).

Since carbon fibers produce composites with poor interfacial shear strength when

used without any surface treatments, it has led investigators to develop a number of

surface treatments (Rebouillat, 1984). The importance of surface treatment is due to

the surfaces of carbon fibers that are chemically inactive. Surface functional groups

would formed on carbon fibers surface by treatment that would enhanced good

bonding with the resin matrix, hence improving the mechanical properties of

composites produced. With better mechanical properties of composites, it attracts

many manufacturers to develop various types of usage in industrial, recreational and

engineering field.

This research intends to study the surface treatment on carbon fibers using one of the

surface treatments. The as-received PAN carbon fibers were treated by using the

concept of whiskerization. This treatment was done by growing carbon nanotubes

directly on carbon fibers using chemical vapor deposition (CVD) to create CNT-

coated carbon fibers. Each individual carbon fiber, which is several microns in

diameter, is surrounded by carbon nanotubes. The morphology of CNTs grown on

carbon fiber substrate was observed at various treatment conditions and the

interfacial shear strength of these treated fiber were measured. Although there is

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much research done on the production of CNTs on carbon fibers, none had studied

the tensile behavior of CNT-coated carbon fiber-reinforced polypropylene

composites using short fibers. The effect of CNT-coating treatment in the

enhancement of tensile properties of the treated carbon composite was studied.

1.3 Objectives of Study

The objectives and scope of study had been clearly defined to achieve the goal of this

research and are listed as follow:

1. To study the microscopic morphology of CNTs formation on the surface of

carbon fibers by CNT-coating treatment at various treatment conditions.

2. To determine the effect of CNT-coating treatment on carbon fibers by

studying the interfacial shear strength (IFSS) of CNT-coated carbon fibers,

tensile properties and thermal stability of CNT-coated carbon fiber

composites.

3. To investigate the comparison of tensile properties and thermal stability

between CNT-coated carbon fiber composites with the commercial carbon

fiber composites.

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1.4 Scope of Study

The scope of study of this research are as follow:

1. CNT-coating treatment was conducted using CVD method by varying the

reaction temperature (700 o

C, 800oC and 900

oC), hydrogen flow rate (100

ml/min and 300 ml/min) and weight of ferrocene (0.3g, 0.5g and 1.0g). The

microscopic morphology of CNTs formation on the surface of carbon fibers

was observed using scanning electron microscopy (SEM) and transmission

electron microscopy (TEM).

2. Untreated, CNT-coated and commercial carbon fibers were fabricated into

composites by reinforcing them with polypropylene at different fiber

content. The carbon fiber content was varied between 2 wt. % and 12 wt. %.

The improvement in tensile properties at different treatment conditions and

fiber load were determined.

3. The fiber matrix adhesion was assessed by the interfacial shear strength

(IFSS) test and the thermal stability of untreated and CNT-coated carbon

fiber composites was determined by its onset degradation temperature.

1.5 Thesis Structure

The thesis structure is organized into five chapters. Chapter Two consists of literature

review, which encompasses background information and review of previous work

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done which is related to carbon fibers, carbon nanotubes growth by CVD and using

carbon fiber as fillers in a polymer based matrix. It also includes fundamental theory

regarding mechanical properties of carbon fiber reinforced polymer matrix

composites. In Chapter Three, the type of materials incorporated, instruments and

apparatus used, formulas as well as the procedure for sample preparation is

presented. Detailed results of CNT-coating treatment on carbon fibers, morphology

study, and experimental data obtained are presented in Chapter Four with discussions

on data analysis and interpretation. Finally, Chapter Five concludes the overall work

by reporting the important findings from the research done and recommendations for

future work.

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