THE POTENTIAL OF WOLLASTONITE AS A NEW FILLER FOR …

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


4.2.2 Tensile Properties .............................................................................. 50

4.2.3 Hardness Properties ........................................................................... 52

4.2.4 Filler- Rubber Interaction................................................................... 53



4.3 The effect of untreated and ball mill physical treated wollastonite on the

properties of wollastonite filled natural rubber compounds .................................. 59


4.3.2 Tensile properties .............................................................................. 61


4.3.4 Rubber-Filler Interaction.................................................................... 66

vi



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






4.5 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





4.5.6 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

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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)

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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

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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/).

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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.

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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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