THE POTENTIAL OF WOLLASTONITE AS A NEW FILLER
FOR NATURAL RUBBER COMPOUNDS
HOANG THI BICH DIEP
UNIVERSITI SAINS MALAYSIA
2013
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THE POTENTIAL OF WOLLASTONITE AS A NEW FILLER
FOR NATURAL RUBBER COMPOUNDS
BY
HOANG THI BICH DIEP
Thesis submitted in fulfillment of the requirements for the degree of
Master of Science
October 2013
ii
ACKNOWLEDGEMENT
I would like to express my deepest and sincere gratitude to my main supervior and
also Dean of School of Materials and Mineral Resources Prof. Hanafi Ismail for his
supervision, advice, guidance and encouragement throughout my research project to
made this work succesful. My sincere appreciation to my co-supervisor Assoc. Prof.
Dr. Azura A. Rashid for her patience to proof read, comments and suggestions for
my thesis have been useful to my thesis. I warmly thank for their friendly helps.
My special thanks to AUN/Seed-net program for their generosity on giving me this
opportunity and financial support to pursue my master’s degree.
Many thanks to all the teachnician and staffs in School of Materials and Mineral
Resources Engineering especially Mr. Shahril, Mr. Joe, Mr. Mohammad Hasan, Mr.
Rashid, Madam Fong Lee Lee for their various contribution and assistance in
running the research.
I owe my loving thanks to my parents, husband, sisters, brothers and Assoc. Prof.
Nguyen Van Tu for their encourgement and understanding. I would like to convey
my thanks to all my friends for sharing their knowledge and creating a fun
environment during my study.
iii
TABLE OF CONTENTS
ACKNOWLEDGEMENT ....................................................................................... ii
TABLE OF CONTENTS ........................................................................................ iii
LIST OF TABLES ..................................................................................................vii
LIST OF FIGURES ................................................................................................. ix
LIST OF ABBREVIATION .................................................................................. xiv
LIST OF SYMBOLS .............................................................................................. xv
ABSTRAK ............................................................................................................ xvi
ABSTRACT ......................................................................................................... xvii
CHAPTER 1 - INTRODUCTION............................................................................. 1
1.1 Research Background ................................................................................. 1
1.2 Problem Statements .................................................................................... 2
1.3 Research Objectives .................................................................................... 4
CHAPTER 2 - LITERATURE REVIEW .................................................................. 5
2.1 Natural Rubber ........................................................................................... 5
2.1.1 General properties of Natural Rubber ................................................... 5
2.1.2 Standard Vietnamese Natural Rubber................................................... 6
2.2 Vulcanizing Ingredients .............................................................................. 7
2.2.1 Introduction ......................................................................................... 7
2.2.2 Sulphur ................................................................................................ 8
2.2.3 Activator.............................................................................................. 9
2.2.4 Accelerator .......................................................................................... 9
2.2.5 Antioxidant ........................................................................................ 10
2.3 Rubber Compounding ............................................................................... 11
2.3.1 Compounding process........................................................................ 13
2.3.2 Forming process ................................................................................ 13
iv
2.3.3 Vulcanization process ........................................................................ 14
2.4 Fillers ....................................................................................................... 15
2.4.1 Introduction ....................................................................................... 15
2.4.2 Wollastonite ...................................................................................... 16
2.4.3 Carbon black ..................................................................................... 18
2.4.4 Calcium carbonate ............................................................................. 20
2.5 Planetary Ball Mill .................................................................................... 22
2.6 Interface ................................................................................................... 23
2.6.1 Silane Coupling Agent ....................................................................... 24
2.7 Hybrid composites .................................................................................... 26
CHAPTER 3 - EXPERIMENTAL PROCEDURE .................................................. 28
3.1 Materials ................................................................................................... 28
3.1.1 Raw natural rubber ............................................................................ 28
3.1.2 Wollastonite ...................................................................................... 28
3.1.3 Commercial Fillers (Carbon black and Calcium carbonate) ................ 28
3.1.4 Sulphur .............................................................................................. 29
3.1.5 N-cyclohexyl-2-benzolthyazolsulfeamide (CBS) ............................... 29
3.1.6 Zinc oxide and Stearic acid ................................................................ 29
3.1.7 Toluene ............................................................................................. 29
3.1.8 N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD) .......................... 30
3.1.9 3-Aminopropyltriethoxysilane (APTES) ............................................ 30
3.2 Formulation .............................................................................................. 31
3.2.1 Wollastonite filled natural rubber compounds .................................... 31
3.2.2 Chemical treated wollastonite filled natural rubber compounds .......... 31
3.2.3 The partial replacement of wollastonite by carbon black or calcium
carbonate on the properties of NR compounds ................................................. 32
3.3 Equipments ............................................................................................... 33
3.3.1 Two roll mill...................................................................................... 33
3.3.2 Rheometer Monsanto ......................................................................... 34
3.3.3 Hot press ........................................................................................... 34
3.3.4 Shore type A ...................................................................................... 34
3.3.5 Wallace Die Cutter ............................................................................ 34
3.3.6 Instron Machine ................................................................................. 34
v
3.3.7 Thermal analyzer ............................................................................... 34
3.3.8 Scanning electron microscope (SEM) ................................................ 35
3.3.9 Physical ball mill ............................................................................... 35
3.3.10 XRF analysis ..................................................................................... 35
3.3.11 Particle size analysis .......................................................................... 35
3.4 Compounding Process .............................................................................. 36
3.5 Testing and characterizations .................................................................... 39
3.5.1 Curing Characteristics ........................................................................ 39
3.5.2 Tensile Test ....................................................................................... 40
3.5.3 Hardness Test .................................................................................... 41
3.5.4 Swelling Test ..................................................................................... 41
3.5.5 Thermogravimetric Analysis (TGA) .................................................. 42
3.5.6 Scanning Electron Microscopy (SEM) ............................................... 42
3.5.7 Flow Chart of Research Work ............................................................ 43
CHAPTER 4 - RESUTLS AND DISCUSSION ...................................................... 44
4.1 Characterization of wollastonite ................................................................ 44
4.1.1 The particle size distribution of wollastonite ...................................... 44
4.1.2 Scanning electron microscopy (SEM) of wollastonite ........................ 46
4.1.3 X-ray Fluorescence (XRF) ................................................................. 47
4.2 The effect of wollastonite loading on the properties of natural rubber (NR)
compounds .......................................................................................................... 48
4.2.1 Curing Characteristics ........................................................................ 48
4.2.2 Tensile Properties .............................................................................. 50
4.2.3 Hardness Properties ........................................................................... 52
4.2.4 Filler- Rubber Interaction................................................................... 53
4.2.5 Scanning Electron Microscopy (SEM) ............................................... 54
4.2.6 Thermogravimetric Analysis (TGA) .................................................. 56
4.3 The effect of untreated and ball mill physical treated wollastonite on the
properties of wollastonite filled natural rubber compounds .................................. 59
4.3.1 Curing Characteristics ........................................................................ 59
4.3.2 Tensile properties .............................................................................. 61
4.3.3 Hardness Properties ........................................................................... 65
4.3.4 Rubber-Filler Interaction.................................................................... 66
vi
4.3.5 Scanning Electron Microscopy (SEM) ............................................... 67
4.3.6 Thermogravimetric Analysis (TGA) .................................................. 69
4.4 The effect of silane coupling agent, 3-aminopropyltriethoxysilane (APTES)
on the properties of wollastonite filled natural rubber compounds........................ 72
4.4.1 Curing Charateristics ......................................................................... 72
4.4.2 Tensile Properties .............................................................................. 75
4.4.3 Hardness Properties ........................................................................... 79
4.4.4 Rubber-Filler Interaction.................................................................... 80
4.4.5 Scanning Electron Microscopy (SEM) ............................................... 81
4.4.6 Thermogravimetric Analysis (TGA) .................................................. 84
4.5 Effect of partial replacement of wollastonite by carbon black or calcium
carbonate on the properties of natural rubber compounds .................................... 87
4.5.1 Curing characteristics ........................................................................ 87
4.5.2 Tensile Properties .............................................................................. 90
4.5.3 Hardness Properties ........................................................................... 94
4.5.4 Rubber-Filler Interaction.................................................................... 95
4.5.5 Scanning Electron Microscopy (SEM) ............................................... 96
4.5.6 Thermogravimetric (TGA) Analysis ................................................ 101
CHAPTER 5 - CONCLUSIONS AND FUTURE WORKS ................................... 104
5.1 Conclusions ............................................................................................ 104
5.2 Future works ........................................................................................... 106
REFERENCES ..................................................................................................... 107
PUBLICATIONS ................................................................................................. 115
vii
LIST OF TABLES
Page
Table 2.1 SVR 3L properties and specifications
(http://www.phuanrubber.com/tcvn3769-2004_en.pdf)
7
Table 2.2 Accelerator Classes (Rodgers, 2004)
10
Table 2.3 The specific formulation (Ciesielski, 1999)
12
Table 2.4 Typical Chemical Composition of Commercial Wollastonite Products (Karian, 1999)
17
Table 2.5 Physical Properties of Wollastonite (Karian, 1999)
18
Table 2.6 Effect of Carbon Black on Rubber Properties (Rodgers, 2004)
20
Table 2.7 The properties of calcium carbonate (Wypych, 2000)
21
Table 3.1 Particle size, surface area and specific density of fillers
29
Table 3.2 List of the raw materials and their suppliers
30
Table 3.3 The formulation of wollastonite filled natural rubber compounds
31
Table 3.4 The formulation used to study the effect of silane coupling agent on the properties of NR compounds
32
Table 3.5 The formulation of partial replacement of wollastonite by carbon black or calcium carbonate of NR compounds
33
Table 3.6 Equipments used in the research
35
Table 3.7 Standard mixing for gum compounds (Muniandy, 2012)
37
Table 3.8 Standard mixing for filled compounds (Muniandy, 2012)
38
Table 4.1 Quantitative analysis of wollastonite using x-ray fluorescence
47
Table 4.2 Thermal stability parameters of wollastonite filled NR compounds
57
viii
Table 4.3 Thermal stability parameters of gum compound and wollastonite filled NR compounds with untreated and treated wollastonite
70
Table 4.4 TGA data for NR gum and wollastonite filled NR compounds with and without APTES
85
Table 4.5 The thermal stability parameters of NR gum and commercial fillers of NR compounds
102
ix
LIST OF FIGURES
Page Figure 2.1 Chemical structure of natural rubber (Simpson, 2002)
5
Figure 2.2 Vulcanization the randomly oriented chains of natural rubber (Mark and Erman, 2005)
8
Figure 2.3 Sulphur vulcanization (Ciullo and Hewitt, 1999)
9
Figure 2.4 N-isopropyl-N’-phenyl-p-phenylenediamine (IPPD) (Roff et al., 1971)
11
Figure 2.5 Wollastonite structure (Ciullo and Robinson, 2002)
16
Figure 2.6 Structure of carbon black (Wang et al., 2003)
19
Figure 2.7 Speed ratios and grinding jar of Planetary Ball Mill
23
Figure 2.8
Schematic of silane mineral reaction to produce the modified wollastonite filler surface (Ma et al., 2011)
24
Figure 2.9 Interaction between filler and polymer matrix via silane coupling agent (Muniandy, 2012)
25
Figure 3.1 Cure curves for a maximum torque with reversion (Ciullo and Hewitt, 1999)
40
Figure 3.2 Dumbbell shaped test piece dimension
41
Figure 3.3 Flow chart of the research work
43
Figure 4.1 Graph of particle size distribution of wollastonite
44
Figure 4.2 Graph of particle size distribution of wollastonite after ball mill physical treated
45
Figure 4.3 SEM micrographs of wollastonite: (a) untreated (b) after treated at 400× magnification
46
Figure 4.4 Effect of wollastonite loading on cure and scorch times of wollastonite filled NR compounds
49
Figure 4.5 Effect of wollastonite loading on maximum torque of wollastonite filled NR compounds
49
x
Figure 4.6 Effect of wollastonite loading on tensile strength of wollastonite filled NR compounds
51
Figure 4.7 Effect of wollastonite loading on elongation at break of NR compounds
51
Figure 4.8 Effect of wollastonite loading on tensile modulus of NR compounds
52
Figure 4.9 Effect of wollastonite loading on hardness of NR compounds
53
Figure 4.10 Effect of wollastonite loading on filler-rubber interaction of NR compounds
54
Figure 4.11 SEM micrographs for tensile fracture surface of NR compounds: (a) Gum (b) 10 phr (c) 20 phr (d) 40 phr
55
Figure 4.12 TGA curve of wollastonite filled NR compounds
58
Figure 4.13 DTG curve of wollastonite filled NR compounds
58
Figure 4.14 Variation of cure time of natural rubber compounds with untreated and treated wollastonite loading
60
Figure 4.15 Variation of scorch time of natural rubber compounds with untreated and treated wollastonite loading
60
Figure 4.16 Variation of maximum torque of natural rubber compounds with untreated and treated wollastonite loading
61
Figure 4.17 Variation of treated and untreated wollastonite on tensile strength of wollastonite filled NR compounds
63
Figure 4.18 Variation of treated and untreated wollastonite on elongation at break of wollastonite filled NR compounds
63
Figure 4.19 Variation of treated and untreated wollastonite on tensile modulus M100 of wollastonite filled NR compounds
64
Figure 4.20 Variation of treated and untreated wollastonite on tensile modulus M300 of wollastonite filled NR compounds
64
Figure 4.21 Variation of treated and untreated wollastonite on on hardness of NR compounds
65
Figure 4.22 Variation of Qf /Qg value of NR compounds with treated and untreated wollastonite loading
66
xi
Figure 4.23 SEM micrographs of wollastonite filled NR compounds: (a) untreated 10 phr (b) treated 10 phr (c) untreated 20 phr (d) treated 20 phr (e) untreated 40 phr (f) treated 40 phr
68
Figure 4.24 TGA curves of treated and untreated wollastonite filled NR compounds
71
Figure 4.25 DTG curves of treated and untreated wollastonite filled
NR compounds
71
Figure 4.26 The effect of wollastonite loading and silane coupling agent on cure time of wollastonite filled NR compounds
73
Figure 4.27 The effect of wollastonite loading and silane coupling agent on scorch time of wollastonite filled NR compounds
73
Figure 4.28 The effect of wollastonite loading and silane coupling agent on maximum torque of wollastonite filled NR compounds
74
Figure 4.29 Effect of wollastonite loading and silane coupling agent on tensile strength of wollastonite filled NR compounds
77
Figure 4.30 The effect of wollastonite loading and silane coupling agent on elongation at break of wollastonite filled NR compounds
77
Figure 4.31 The effect of wollastonite loading and silane coupling agent on tensile modulus M100 of wollastonite filled NR compounds
78
Figure 4.32 The effect of wollastonie loading and silane coupling agent on tensile modulus M300 of wollastonite filled NR compounds
78
Figure 4.33 Effect of wollastonite loading and silane coupling agent on hardness of wollastonite filled NR compounds
79
Figure 4.34 Effect of wollastonite loading and silane coupling agent on rubber-filler interaction of wollastonite filled NR compounds
80
Figure 4.35 SEM images of wollastonite filled NR compounds: (a) without silane 10 phr (b) silane 10 phr (c) without silane 20 phr (d) silane 20 phr (e) without silane 40 phr (f) silane 40 phr
82
Figure 4.36 SEM micrograph shows better wollastonite-rubber interaction at 40 phr of NR compounds with silane coupling agent at magnification 400X
83
xii
Figure 4.37 TGA curve of wollastonite loading and APTES coupling agent of NR compounds
86
Figure 4.38 DTG curve of wollastonite loading and APTES coupling
agent of NR compounds
86
Figure 4.39 Effect of partial replacement of wollastonite by CB and CaCO3 on the cure time of NR compounds
88
Figure 4.40 Effect of partial replacement of wollastonite by CB and CaCO3 on the scorch time of NR compounds
89
Figure 4.41 Effect of partial replacement of wollastonite by CB and CaCO3 on the maximum torque of NR compounds
89
Figure 4.42 Effect of partial replacement of wollastonite by CB and CaCO3 on the tensile strength of NR compounds
91
Figure 4.43 Effect of partial replacement of wollastonite by CB and CaCO3 on the elongation at break of NR compounds
91
Figure 4.44 Effect of partial replacement of wollastonite by CB and CaCO3 on the tensile modulus M100 of NR compounds
93
Figure 4.45 Effect of partial replacement of wollastonite by CB and CaCO3 on the tensile modulus M300 of NR compounds
93
Figure 4.46 Effect of partial replacement of wollastonite by CB and CaCO3 on the hardness of NR compounds
94
Figure 4.47 Effect of partial replacement of wollastonite by CB and CaCO3 on the rubber-filler interaction of NR compounds
95
Figure 4.48 SEM micrographs of (a) wollastonite; (b) carbon black; (c) calcium carbonate at magnification 1.00 KX
96
Figure 4.49 SEM photographs of commercial filler/wollastonite: 10/30 phr (a) carbon black (b) calcium carbonate and (c) NR/wollastonite at 10 phr magnification 100X
98
Figure 4.50 SEM photographs of commercial filler/wollastonite: 20/20 phr (a) carbon black (b) calcium carbonate and NR/wollastonite at 20 phr at magnification 100X
99
Figure 4.51 SEM photographs of NR compounds at 40 phr of: (a) wollastonite; (b) carbon black (c) calcium carbonate at magnification 100X
100
Figure 4.52 TGA curve of CB/wollastonite and CaCO3/wollastonite filled NR compounds
103
xiii
Figure 4.53 DTG curve of CB/wollastonite and CaCO3/wollastonite filled NR compounds
103
xiv
LIST OF ABBREVIATION
NR Natural Rubber
SVR 3L Standard Vietnamese Natural Rubber
SEM Scanning Electron Microscopy
TGA Thermogravimetric Analysis
DTG Derivative Thermal Gravimetry
CB Carbon Black
CaCO3 Calcium Carbonate
CBS N cyclohexyl-2-benzothiazole sulfonamide
IPPD N-isopropyl-N'-phenyl-p-phenylenediamine
APTES 3-Aminopropyltriethoxysilane
ZnO Zinc oxide
ASTM American Society for Testing and Materials
MDR Monsanto Moving Die Rheometer
C5H8 Cis-1,4 polyisoprene
Phr Part per hundred of rubber
M100 Stress at 100% elongation
M300 Stress at 300% elongation
xv
LIST OF SYMBOLS
t90 Cure time
ts2 Scorch time
MH Maximum torque
Eb Elongation at Break
Qf/Qg Swelling index
dNm Deci Newton Meter
T50% Degradation temperature at 50 wt%
µm Micrometer
min Minute
xvi
POTENSI WOLLASTONITE SEBAGAI PENGISI BARU UNTUK
SEBATIAN GETAH ASLI
ABSTRAK
Wollastonite atau kalsium silikat, (CaSiO3) berpotensi sebagai pengisi baru di
dalam sebatian getah asli (GA). Ciri-ciri, pematangan, sifat mekanik, kelakuan
pembengkakan, kestabilan haba dan morfologi sebatian getah asli terisi wollastonite
telah dikaji. Empat siri eksperimen telah dilakukan. Kesemua sebatian GA telah
disediakan menggunakan mesin penggiling bergulung dua dengan pelbagai
pembebanan pengisi daripada 0 hingga 40 bsg. Untuk siri pertama kesan pelbagai
pembebanan wollastonite ke atas sifat-sifat sebatian GA telah dikaji. Keputusan
menunjukkan masa pematangan (t90), masa skorj (ts2), kekuatan tensil dan
pemanjangan pada takat putus (Eb) berkurang, manakala tork maksimum (MH),
modulus tensil, kekerasan, kestabilan haba dan kelakuan pembengkakah meningkat
dengan penambahah pembebanan wollastonite. Untuk siri kedua, kesan rawatan
fisikal wollastonite menggunakah ‘ball mill’ ke atas sifat-sifat sebatian GA telah
dikaji. Wollastonite yang terawat telah meningkatkan sifat-sifat sebatian GA
berbanding wollastonite tidak dirawat. Untuk siri ketiga, kesan 1 bsg 3-
aminopropiltrietoksisilana (APTES) sebagai agen pengkupel telah dikaji. Kehadiran
APTES telah meningkatkan sifat-sifat sebatian GA. Ini disebabkah kehadirah
APTES telah meningkatkan interaksi getah-pengisi didalam sebatian GA. Untuk siri
terakhir, kesan penggantian separa wollastonite dengan hitam karbon atau kalsium
karbonat telah dikaji. Penggantian wollastonite dengan pengisi-pengisi komersial
telah meningkatkan dan memperbaiki sifat-sifat sebatian GA.
xvii
THE POTENTIAL OF WOLLASTONITE AS A NEW FILLER FOR
NATURAL RUBBER COMPOUNDS
ABSTRACT
Wollastonite or calcium silicate, (CaSiO3) has potential as a new type of filler
in natural rubber (NR) compounds. The curing characteristics, mechanical properties,
swelling behavior, thermal stability and morphology of wollastonite filled NR
compounds were investigated. Four series of experiments were performed. All NR
compounds were prepared using a two roll mill with different filler loading from 0 to
40 phr. In the first series, the effects of different wollastonite loading on the
properties of NR compounds were studied. Results indicated that the cure time (t90),
scorch time (ts2), tensile strength and elongation at break (Eb) decreased, whereas,
maximum torque (MH), tensile modulus, hardness, thermal stability and swelling
behavior increased with wollastonite loading. The second series, the effect of
physical ball mill treated wollastonite on the properties of NR compounds was
investigated. Treated wollastonite improved the properties of NR compounds as
compared to untreated wollastonite. The third series, the effect of 1 phr of 3-
aminopropyltriethoxysilane (APTES) as silane coupling agent was studied. The
presence of APTES showed an improvement in the properties of NR compounds.
This was due to the presence of APTES, which enhanced the rubber-filler interaction
of NR compounds. In the last series, the influence of partial replacement of
wollastonite by carbon black or calcium carbonate was investigated. The substitution
of wollastonite by commercial fillers have improved the properties of NR
compounds.
1
CHAPTER 1
INTRODUCTION
1.1 Research Background
Natural rubber (NR) is one of the main elastomers and widely used in
industrial and technology science, especially used for shock and vibration isolators
owing to its high elasticity. However, reinforcing filler is still needed to improve
properties of natural rubber compound products such as abrasion resistance, flex
resistance, hardness, tensile strength, tensile modulus, which gum compounds are
inadequate. NR is used in aero planes, buses and coaches, motor cars, bicycle, tyres,
footwear, belts, hose, seals and gaskets, all expect trains are highly dependent on
rubber (Simpson, 2002).
Various types of fillers are used in the rubber industry prominently to
reinforces, reduce material costs and improve processing (Rattanasom et al., 2007).
The common reinforcements obtained through usage of inorganic particle fillers
(mineral fillers) are effective method to enhance mechanical and physical properties
of NR compounds. The mineral fillers are wollastonite, mica, talc, clay, silica and
calcium carbonate can be used in amounts of 10% to 40% by weight (Hadal et al.,
2004; Ding et al., 2012).
Wollastonite is one of the most extensive mineral filler used in β-isotactic
polypropylene (iPP), which had begun to study by various scientists since 1980s. So
many investigations indicated that wollastonite is an effective and low cost filler in
2
thermoplastics, such as improved mechanical properties, as well as increased rate of
crystallinity and nucleation (Ding et al., 2012; Wang et al., 2001; Hawley and
Jaworski, 1998; Hanumantha Rao et al., 1998; Zhuang et al., 1997; Fu et al., 2002).
However, there are a little worked reported on the worth of wollastonite as a
filler in rubber compounds (Guan et al., 2011; Xu et al., 2004). According to Xu et
al. (2004), the resistance to wear, aging-resistant, fatigue-proof are enhanced after the
rubbers are filled with such surface modified needle-like wollastonite powders. The
chemical treated wollastonite use silanes or titanates to improve the interfacial
bonding energy between the wollastonite and rubber matrix (Hanumantha Rao et al.,
1998). The hydrophilic group of silane coupling agent chemically react with the
hydroxyl group on the surface of wollastonite and the hydrophobic group has a
certain degree of compatibility with the NR matrix.
The aims of this study is to investigate the potential of wollastonite on curing
characteristics, mechanical properties (tensile strength, tensile modulus, hardness),
swelling behaviour, morphology and thermal stability of wollastonite filled NR
compounds.
1.2 Problem Statements
Natural rubber (NR) is widespread used as an effectively in industrial
application. Fillers are incorporated into NR for reinforcement or cheaper rubber
products. In recently, carbon black and silica were used popular in natural rubber
compounds. Carbon black made from petroleum. It was added into NR compounds
to give excellent resistance to UV light and antiozonants. However, the trouble of
3
carbon black made black colour of rubber products, high cost and pollution. Silica is
mineral natural filler, which gives significant mechanical properties of NR
compounds and light colour of rubber products. But the disadvantages of silica are
difficult to disperse in NR compounds due to its hydrophilic surface. This filler have
to use silane coupling agent to improve the rubber-filler interaction, which expensive
cost one.
Otherwise, wollastonite’s source is the white mineral and very popular in the
worlds. It was used as filler for rubber, which are low cost, light colour rubber
products and high thermal stability of NR compounds.
By the way, one of the limitations of NR is its low temperature stability
(Menon et al., 1996). In semi-crystalline polymeric materials, wollastonite, talc,
calcium carbonate and clay are strongly influence the crystallization process, degree
of cystallinity and nucleation of crystallites. Reinforcement with each of the above
material increases crystallization temperature rate and lead to shorter processing time
in injection moulding (Hadal et al., 2004).
Although, a little study has been done on wollastonite filled rubber
compounds, there is no particular study focus on the filler loading and different
methods for fillers treatment. For this study, we used different methods such as ball
mill physical treated wollastonite, silane coupling agent (APTES) and hybrid
composites by commercial fillers, which could give significant mechanical properties
and thermal stability of NR compounds.
4
1.3 Research Objectives
This research was focused to study the potential of wollastonite as a new filler in NR
compounds. Hence, the research objective is divided in three phases:
i. To study the effect of wollastonite loading on the properties of natural rubber
compounds.
ii. To investigate the effect of ball mill physical and chemical (APTES)
treatment on the properties of wollastonite filled NR compounds.
iii. To examine the effect of partial replacement of wollastonite by commercial
fillers (carbon black, calcium carbonate) on the properties of NR compounds.
The characteristics of the wollastonite and NR compounds involved cure
characteristics, mechanical tests, swelling test, hardness test, thermal stability test,
and morphology. The cure characteristics parameter measured includes maximum
torque (MH), scorch time (ts2) and cure time (t90). The cure time is important to
determine the time for rubber vulcanizate. The mechanical tests include tensile
strength, elongation at break, tensile modulus at 100% and 300% elongations and
hardness. Swelling test was conducted by measuring the swelling percentage of the
NR compounds. Thermal test were focused on the thermogravimetric analysis (TGA)
and morphological. All of the tests are important to be conducted so that the effect of
the wollastonite filled natural rubber compounds can be understand.
5
CHAPTER 2
LITERATURE REVIEW
2.1 Natural Rubber
Natural rubber (NR) can be isolated from more than 200 different species of
plant; including surprising examples such as dandelions. However, only one tree
source Hevea Brasiliensis is one of the most commercial significant, which popular
used in industrial application (Simpson, 2002). The chemical name of NR is
polyisoprene (C5H8)n , which is polymerized from mono-isoprene (C5H8). NR is cis-
1,4-polyisoprene. In addition, the composition of NR also contains small amounts of
non-rubber substances, protein, sugar, fatty acids and resinous materials that function
as mild accelerator and activator for vulcanization. NR has excellent physical
properties, but NR also has disadvantage that is poor resistance to oxygen, heat and
ozone (Sirqueira and Soares, 2002). The chemical structure of natural rubber is
shown in Figure 2.1 (Simpson, 2002).
Figure 2.1: Chemical structure of natural rubber (Simpson, 2002)
2.1.1 General properties of Natural Rubber
The chemical structure of natural rubber (NR) contains double bond. It is
very important for chemically reacts of NR with other materials, especially which
determines the chemical reactivity and its ability react of NR with sulphur or other
ingredients in vulcanization rate (Treloar, 2005). NR has the ability to strain
crystallize when stretched which enables NR gum compounds (without filler) to have
6
good tensile strength. Moreover, NR has extremely high resilience, high tensile
strength, good tear resistance, and high fatigue failure in cured state.
Rubber is a material that when stretched return quickly to its approximate
original shape. This behaviour is called elasticity, and it can be explained by
reference to the rubber molecules. The important thing about these molecules, they
tend to be coiled up, not in any regular fashion, but unevenly twisted and twined,
very much like a plate of spaghetti or a tangled skein of wool will consist of just one
strand and in the tangled mass of many molecules (Huke, 1961).
2.1.2 Standard Vietnamese Natural Rubber
Vietnamese natural rubber is very popular in the world; in 2012 Vietnamese
natural rubber reached 955000 tons increase 17 percent as compared 2011.
Vietnamese natural rubber almost exports raw materials. Vietnam rubber group
reported that in 2010 to 2011 got 811600 tons of natural rubber from the crops.
Normally, Vietnamese natural rubber can export 816600, get $ 3.2 billion. This is the
second part only after rice, which can export among the top agricultural of Vietnam
agricultural. (http://www.globaltimes.cn/content/744010.shtml). Based on Table 2.1
showed the properties and specifications of SVR 3L
(http://www.phuanrubber.com/tcvn3769-2004_en.pdf).
7
Table 2.1: SVR 3L properties and specifications
(http://www.phuanrubber.com/tcvn3769-2004_en.pdf)
Parameter Value
Dirt (%wt, max) 0.03
Volatile matter (% wt, max) 0.8
Ash (%, max) 0.5
Nitrogen (% wt, max) 0.6
Initial plasticity (min) 35
Plasticity retention index (PRI) (min) 60
Lovibond colour index (max) 6
2.2 Vulcanizing Ingredients
2.2.1 Introduction
The curing or vulcanization is the process where the chains are chemically
linked together to form a network, is also known as crosslinking. However, the term
vulcanization covers not only the actual crosslinking process but also the method by
which the crosslinking is produced (Hofmann, 1967). This process increases
mechanical properties, abrasion resistance, decreases hysteresis and elongation.
8
Figure 2.2: Vulcanization of the randomly oriented chains of natural rubber (Mark
and Erman, 2005)
2.2.2 Sulphur
The most important vulcanizing agent for rubber is elementary sulphur,
which is marketed in a ground form as sulphur for vulcanization. The solubility of
sulphur varies from polymer to polymer. Sulphur dissolves relatively easily in
natural rubber and styrene-butadiene rubber at room temperature (Hofmann, 1967).
Sulphur with the raw gum rubber can react chemically together. It makes the long
chains like the bridge, which gives more dimensionally stable and less heat sensitive
product (Ciesielski, 1999). When rubber is vulcanized, sulfur molecules crosslink
the polymer strands. That cross-linking helps hold rubber products together and
allow them to resume their original shape after stretching (Freakley and Payne,
1978). Figure 2.3 shows the react chemically between polyisoprene and sulphur is
formed crosslinked polyisoprene.
9
Figure 2.3: Sulphur vulcanization (Ciullo and Hewitt, 1999)
2.2.3 Activator
Activator is used to enhance the accelerators efficiency. One of the
vulcanizing ingredients responsible for most of the conflicting theories existing is the
activator. This compound is experimentally proved to improve the processing and
final vulcanizate properties. In natural formulations, the activator most commonly
used is the zinc oxide. Not only use zinc oxide but also use stearic acid each other to
proceed more rapidly vulcanization as compared with vulcanization by sulphur alone
(Barlow, 1993).
2.2.4 Accelerator
The rate of vulcanization of a rubber compound is controllable by the choice
of accelerator. There are different types of accelerators used which vary to the
vulcanization systems, methods and temperature of curing. The accelerator is used to
reduce cure time by increasing cure rate of the rubber compounds (Simpson, 2002;
Barlow, 1993). Table 2.2 shows commonly accelerator use in rubbers (Rodgers,
2004).
10
Table 2.2: Accelerator Classes (Rodgers, 2004)
Class Response speed Acronyms
Aldehyde-amine Slow _
Guanidines Medium DPG, DOTG
Thiazoles Semi-fast MBT, MBTS
Sulfenamides Fast, delayed action CBS, TBBS, MBS, DCBS
Dithiophosphates Fast ZBPD
Thiurams Very fast TMTD, TMTM, TETD
Dithiocarbamates Very fast ZDMC, ZDBC
The accelerator used for:
Considerable scorch delay
Better cure rate
Cost effective
Improve appearance and avoid blooming on the products
Enhanced the resistance of degradation
Allow vulcanization process to occur at high or low temperature
2.2.5 Antioxidant
Antioxidants are used to protect rubbers from the effects of thermal
oxidation. Peroxide vulcanisates are usually protected with dihydroquinolines. Other
antioxidants react adversely with the peroxide inhibiting the crosslinking reaction
(Simpson, 2002). Different types of antioxidants possess different degree of chemical
reactivity with oxygen and ozone, different degrees of solubility, and different
diffusion rates in the rubber compounds. It determine how much protection a given
antioxidants will impart to the rubber vulcanizates and whether it will be long-term
11
or short-term proction p-phenylenediamine (PPD). It is used in rubber products such
as tires and mechanical goods, as an antiozonant, antioxidant, and antiflex agents
(Gent, 2001; Dick, 2003). Some common antioxidants or antiozonants derived from
PPD, N-isopropyl-N’-phenyl-p-phenylenediamine (IPPD) is one of the most popular
antioxidant being used in NR compounds (Malshe et al., 2006). Figure 2.4 shows the
chemical structure of IPPD.
HC
CH3
CH3
NH NH
Figure 2.4: N-isopropyl-N’-phenyl-p-phenylenediamine (IPPD) (Roff et al., 1971)
2.3 Rubber Compounding
The rubber compound is found by Goodyear and Hancock. Nowadays, the
natural rubber compounds are developed very large as materials for industrial
application (Ciesielski, 1999). Compounding is a process of combining raw rubber
and all compounding ingredients to ensure efficient manufacture of a promising
product with desired properties and performance. The compounding process consists
of the selection of the type and amount of various compounding ingredients used in a
mix, the manner of mixing and processing of the finished mixing (Dick and
Annicelli, 2009). The specific formulation is shown in Table 2.3.
12
Table 2.3: The specific formulation (Ciesielski, 1999)
Material Part per hundred of rubber
Raw gum elastomer 100
Sulfur from 0 to 4
Zinc oxide 5
Stearic acid 2
Accelerators from 0.5 to 3
Antioxidant from 1 to 3
Filler from 0 to 150
Plasticizer from 0 to 150
Each of ingredients has a specific function, either processing, vulcanization
or end use of the product. The various ingredients may be classified according to
their specific function in the following groups:
Fillers (carbon blacks and non-black fillers)
Plasticizers or softeners (extenders, processing aids, special plasticizers)
Age resistors or antidegradants (antioxidants, antiozonants, special age
resistors, protective waxes)
Vulcanizing ingredients (vulcanizing agents, accelerators, activators)
Special-purpose ingredients (coloring pigments, blowing agents, flame
retardants, odorants, antistatic agents, retarders, peptizers).
13
2.3.1 Compounding process
The polymers, fillers, processing aids, vulcanizing ingredients, and other
additives, which compounder has decided on, have to be mixed together. The two
basic machines for this process are the two-roll mill and the internal mixer. The
compounding of the ingredients into rubber often involves compromise. High
viscosity promotes high shear, which is required to break up filler aggregates; low
viscosity assists wetting of the particles, which is essential to achieve uniform
modulus and other properties. It has been said that the quality of the final product is
made in the mill room or compounding department because it is a uniform and high
level of dispersion and consistent rheological properties must be produced in the
batches of mixed compounds (Blow, 1971).
2.3.2 Forming process
After mixing, the green stock generally requires forming (shaping) into
blanks of suitable dimensions. At this stage the stock will retain the shape imposed
on it because it is predominantly plastic. Combined shaping and vulcanization
operation known as moulding produce the majority of products. Three distinct types
of moulding are used compression, transfer and injection (Nagdi, 1993).
For this research used compression moulding only. In the compression
moulding, a compression type mould, the rubber blank is placed directly into the
cavity mould where it is heated by conduction and caused to flow by application of
pressure. The mould is held between the heated platens of the hydraulic press. The
prepared quantity of moulding compound is placed in the mould and the moulding
placed in the press. The press closes with sufficient pressure to minimize flash at
14
mould parts line. The compound softens and flows to shape, chemical cure then
occurs as the internal mould temperature becomes enough high. The press is opened
and the mould removed. The vulcanization time is highly dependent on these three
factors i.e. temperature, size of product and their heat transfer (Blow and Hepburn,
1982).
2.3.3 Vulcanization process
The green stock (unvulcanize) need to be converted to an elastic material
after it has been formed to the desired shape. This can be achieved by the
vulcanization process, which usually takes place under pressure at elevated
temperature, using different techniques, such as: press vulcanization, open
vulcanization, continuous vulcanization and cool vulcanization. Chemically, the
process involves insertion of cross-links between polymer macromolecules through
the action of vulcanization ingredients. Without these chemical bonds, no improved
in the physical properties of the rubber mix can occur.
It is quite probable that the crosslink of the macromolecules together in such
a way that the whole mass becomes a single molecule. The cross-linking of rubber is
also referred to as curing, because it is a process whereby a raw material is converted
into a useful product (Nagdi, 1993). When rubber is vulcanized, sulfur molecules
crosslink the polymer strands. That cross-linking helps hold rubber products together
and allow them to resume their original shape after stretching (Freakley and Payne,
1978).
15
2.4 Fillers
2.4.1 Introduction
Filler incorporated into rubber compound for many purpose such as improve
processability, modify the physical properties and reduce cost of the rubber product
(Sae-oui et al., 2009). Some fillers are added in natural rubber with large amount of
fillers to improve mechanical properties of natural rubber compounds. Carbon black
and natural mineral such as silica, talc, and clay are common used filler in rubber
compounds. Some of these fillers have an influence on the rate of crosslink
generation due to their acidity or alkalinity. Silica and silicates are anomalous in their
effects on cure systems for one would expect them to behave as alkaline in nature
and thus accelerate sulphur vulcanization, in fact they contain substantial amounts of
combined oxygen, which represses or retards the reaction (Simpson, 2002).
The disadvantage of carbon black is black colour in the products. Hence, the
non black fillers are used for the purpose colour of the products. However, depending
on the purpose which we can decide the filler for the material ingredients of rubber
compounds. The non black fillers such as clays, calcium carbonate, talc, silica
usually use in natural rubber compounds, which improve the colour of rubber
product, flex resistance and abrasion resistance (Ciullo and Hewitt, 1999; Rodgers,
2004).
16
2.4.2 Wollastonite
Wollastonite is mineral filler. It has white colour, acicular crystal and
alkaline. The wollastonite composition is almost calcium oxide (CaO), silicon
dioxide (SiO2) and others a little metal oxide. The reaction to form wollastonite is:
CaCO3 + SiO2 → CaSiO3 + CO2↑ Wollastonite has the chemical composition CaSiO3. The molecular structure
of wollastonite is shown in Figure 2.5 (Ciullo and Robinson, 2002).
Figure 2.5: Wollastonite structure (Ciullo and Robinson, 2002)
Most of the world’s production comes from the United States, although
production comes from Finland, Mexico, and India is steadily increasing.
Wollastonite is the naturally occuring white mineral which is wholly acicular. The
length to diameter ratio typically varies from 3:1 to 20:1 but higher aspect ratios are
also available.
Furthermore, wollastonite has 9.8 pH value, low coefficient of thermal
expansion (6.5×10-6/ºC), and a low moisture content (less than 0.5%). Wollastonite is
becoming an increasingly important filler as an asbestos replacement but its most
17
important applications are due to its high brightness, low oil absorption, and
reinforcing effect. In latex coatings, its high pH helps in stabilizing pH of the latex
which improves the stability and shelf-life of the paint. In plastics applications,
wollastonite reinforces tensile strength, flexural strength, impact strength, scratch
resistance, improves thermal stability, increases welding strength, and decreases
warpage or shrinkage (Wypych, 2000; Robinson, 2003).
Wollastonite is very special in the reinforcing mineral area. This mineral is
the non-metallic, white mineral that is needle-like in particle shape. Wollastonite
reinforced in thermoset and thermoplastic resin-based compounds (Robinson et al.,
2002). Chemical treatment or silane coupling agent improve filler-rubber adhesion.
The stronger bonding between filler and natural rubber matrix improve mechanical
properties, weather resistance, and reduces shrinkage (Karian, 1999). The Table 2.4
and Table 2.5 showed chemical composition and physical properties of wollastonite.
Table 2.4: Typical Chemical Composition of Commercial Wollastonite Products
(Karian, 1999)
Component
(wt%)
Minerals
(New
York)
Minera
(Mexico)
Finland
India
China
Africa
Synthetic
grade
CaO
SiO2
Fe2O3
Al2O3
MnO
MgO
TiO2
K2O
Loss on ignition
47.5
51.0
0.40
0.20
0.10
0.10
0.02
0.05
0.68
46.1
51.4
0.25
0.66
0.05
0.50
0.02
0.32
0.65
44.5
51.8
0.22
0.44
0.01
0.56
0.05
0.01
2.20
47.0
49.5
0.43
0.60
0.29
0.20
0.01
0.11
1.79
45.3
50.6
0.34
0.68
0.06
0.70
0.02
0.12
1.60
44.6
50.5
0.42
0.80
0.50
0.09
0.13
2.32
45.7
52.6
0.26
0.47
0.04
0.55
0.13
0.38
0.36
18
Table 2.5: Physical Properties of Wollastonite (Karian, 1999)
Property Values
Morphology
Nominal aspect ratio
Particle size
Loose bulk density, kg/m3 (lb/ft3)
Tapped bulk density, kg/m3 (lb/ft3)
Specific gravity of solids
pH (10 wt% slurry)
Molecular formula
Color
Melting point (ºC)
Water solubility (g/100 cm3)
Acicular
20:1 to 3:1
Products vary from -10 mesh to -1250 mesh
Products vary from 220 to 1360 (14 to 85)
Products vary from 420 to 1440 (26 to 90)
2.87-3.09
8-10
CaSiO3 or alternatively CaO.SiO2
Brilliant white to cream
1540
0.0095
2.4.3 Carbon black
Carbon black is 90-99% elemental carbon with combined hydrogen and
oxygen. Typical functional groups which are located on the surface of blacks are
phenol, carboxy, lactol, quinine, ketone, pyrone and lactone. There also may be
traces of chemically combined sulphur present associated with the source of some
feedstocks. There is no evidence that this combined sulphur has any influence on
rubber crosslinking (Simpson, 2002). Carbon black plays quite important a role in
rubber compounding technology and in determining rheological and processing
behaviour. Carbon black has very good compatibility with rubbers (Yan et al., 2005).
Rattanasom et al., (2007) found that the mechanical properties and abrasion
resistance are increased, when CB was added in rubbers.
Strongly adsorbed rubber at the surfaces of the carbon black reduces the
rubber’s mobility and becomes bound rubber (Simpson, 2002). The incorporation of
19
carbon black with high surface area in an elastomer results in a high level of
reinforcement and higher tensile strength, tear strength, and abrasion resistance, but
also results in a compound with high hysteresis, high cost, and one which is more
difficult to mix and process (Evans, 1981). The improvement in properties such as
tensile strength, modulus, tear strength, and abrasion resistance when carbon black
was added to elastomers has motivated much research into the mechanisms of such
reinforcement. There are likely both chemical and physical interactions between
carbon back and the rubber matrix, resulting in property improvement, but the
understanding of the nature of carbon black reinforcement is still growing (Barlow,
1993). The structure of carbon black is shown in Figure 2.6 (Wang et al., 2003).
Figure 2.6: Structure of carbon black (Wang et al., 2003)
20
Table 2.6: Effect of Carbon Black on Rubber Properties (Rodgers, 2004)
Rubber Property
Effect of increase in carbon black properties Surface Structure Loading
Uncured properties Mixing temperature Die swell
Increases Decreases
Increases Decreases
Increases Decreases
Mooney viscosity Increases Increases Increases Dispersion Decreases Increases Decreases Loading capacity Decreases Decreases _ Cured properties 300% Modulus Insignificant Increases Increases Tensile strength Increases Insignificant Increasesa Elongation Insignificant Decreases Decreases Hardness Tear resistance Hysteresis Abrasion resistance Low strain dynamic modulus High strain dynamic modulus
Increases Increases Increases Increases Increases Insignificant
Increases Decreases Insignificant Insignificant Insignificant Increases
Increases Increasesa
Increases Increasesa
Increases Increases
a Increases to an optimum, then decreases.
2.4.4 Calcium carbonate
Calcium carbonate (CaCO3) is one of the common mineral filler used
in polymer composites that can improve thermal resistance properties, could control
viscosity, reduce shrinkage, have light colour and easy to be compounded (Etelaaho
et al., 2011). Calcium carbonate is non-reinforcing filler, which increase impact
properties of polymer composites, inexpensive and used to provide colour and
minimize the price of the products (Deodhar et al., 2011).
There are two main types of calcium carbonate available that is ground
natural limestone and precipitated calcium carbonate. The ground lime is prepared by
grinding mineral limestone, whilst the precipitated calcium carbonate is obtained by
chemical precipitation from salt solution (Mittal et al., 2011). The ground limestone
21
has less than 100 meshes and used as an inert diluents and cheapening filler for
rubber compounds (Simpson, 2002).
There are so many application of calcium carbonate, the major polymer
applications such as PVC, PE, PS, silicone, acrylic, rubber, polyester and many more
(Wypych, 2000). Calcium carbonate can show high brightness and high gloss in the
products. It also imparts some disadvantages to the products include lowered tensile
strength and elongation, and low resistance to organic acids (Maier and Calafut,
1998). The common properties of calcium carbonate are listed in Table 2.7 (Wypych,
2000).
Table 2.7: The properties of calcium carbonate (Wypych, 2000)
Property Value
Density (g/cm3)
Decomposition temp (°C)
Hardness (Mohs scale)
Melting point (°C)
Moisture content (%)
Chemical resistance
pH of water suspension
Brightness
Particle shape
Oil absorption (g/100g)
Specific surface area (m2/g)
2.7-2.9
1150
3-4
1339
0.01-0.5
Reacts with acids
9-9.5
82-94
Irregular
13-21
5-24
22
2.5 Planetary Ball Mill
The purpose of the ball mill physical treated is reduction particle size using
Planetary Ball Mill (model Retsch PM 100). The particle size is one of a factor to
enhance the mechanical properties such as tensile strength, tensile modulus of the
polymer composites (Kauly et al., 1996). Consequently, Planetary Ball Mill is finer
one of the methods to reduce particle size. This is one of the most popular mills used
for synthesizing almost all materials. In this type of mill, the milling has considerably
high energy.
The working principle of the Retsch PM 100 is based on the relative
rotational movement between the grinding jar and the sun wheel diameter and speed
of rotation this speed ratio is decisive for the energy input and therefore for the
results of the size reduction process. There are two speed ratio settings in this
machine. For example, a ratio of 1:-1 means that each time that the sun wheel rotates
the grinding jar also rotates exactly once in the opposite direction. With a speed ratio
of 1:-2 the grinding jar rotates twice for each sun wheel rotation. In order to follow
the rotational movement of the grinding jar you have to imagine that you are
standing at the center of the sun wheel. During the sun wheel rotation you will see
the red reference point exactly twice, i.e. the grinding jar has rotated twice (see
Figure 2.7) (http://www.retsch.com/products/milling/planetary-ball-mills/pm-100/).
23
Figure 2.7: Speed ratios and grinding jar of Planetary Ball Mill
2.6 Interface
The function of silane coupling agent are chemically link between organic
and inorganic materials. The study of mechanism of the silane coupling agents was
begun in 1960s. The main applications of silanes were the bonding between glass
fibres and thermoplastic resins, which result improve the reinforcement (Rodgers,
2004). The mineral fillers such as clay, silica, and silicates, which have surface silica
groups can hydrolyzed to silanols (-SiOH). These silanol groups behave as acids (-
SiO-H+) are active. If the fillers have higher surface area, they have more silanol and
give more active, which show increases mechanical properties and improve abrasion
resistance. The methoxy or ethoxy groups react during mixing with silanol groups on
the surface of silica (SiO2) particles to give better bonding between filler and NR
matrix (Ciullo and Hewitt, 1999). The chemically reaction of wollastonite and silane
coupling agent facilitates improvement the properties of NR compounds is shown in
Figure 2.8 (Ma et al., 2011). Meng and Dou, (2008) found that the pimelic acid (PA)
treated wollastonite filled polypropylene composites improve the interfacial
interaction of the filler into matrix.
24
Figure 2.8: Schematic of silane mineral reaction to produce the modified wollastonite
filler surface (Ma et al., 2011)
2.6.1 Silane Coupling Agent
Silane coupling agents improve properties of compounds containing silica
and silicate fillers by forming chemical bond across the filler and the rubber
interface. They can also give improved properties with other materials, such as
carbon blacks. They can be used with clays, which do not form strong chemical
bonds with the polymer, thus bringing them into the general category of useful
processing additives. The improved compound properties result from a better
compatibility and linkage of the rubber and filler. The coupling agent can be added
direct to the mixer with the filler (Ren et al., 2006; Poh and Ng, 1998; Simpson,
2002).
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