UNIVERSITI PUTRA MALAYSIApsasir.upm.edu.my/51540/1/FK 2009 112RR.pdf · Abstract of thesis...
Transcript of UNIVERSITI PUTRA MALAYSIApsasir.upm.edu.my/51540/1/FK 2009 112RR.pdf · Abstract of thesis...
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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