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Zhang, Yude, Liu, Qinfu, Xiang, Jingjing, & Frost, Ray(2014)Thermal stability and decomposition kinetics of styrene-butadiene rubbernanocomposites filled with different particle sized kaolinites.Applied Clay Science, 95, pp. 159-166.

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https://doi.org/10.1016/j.clay.2014.04.002

Thermal stability and decomposition kinetics of styrene-butadiene rubber 1

nanocomposites filled with different particle sized kaolinites 2

Yude Zhang 1∗, 3, Qinfu Liu 2, Jingjing Xiang 1, Ray L. Frost 3∗ 3

4

1 Cultivating Base for Key Laboratory of Environment-friendly Inorganic Materials in 5

University of Henan Province, School of Materials Science and Engineering, Henan 6

Polytechnic University, Jiaozuo 454000, P.R.China 7

2 School of Geoscience and Surveying Engineering, China University of Mining and 8

Technology, Beijing 100083, P.R.China 9

3 School of Chemistry, Physics and Mechanical Engineering, Science and Engineering 10

Faculty, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, 11

Australia. 12

ABSTRACT: A series of styrene-butadene rubber (SBR) nanocomposites filled with 13

different particle sized kaolinites are prepared via a latex blending method. The thermal 14

stabilities of these composites are characterized by a range of techniques including 15

thermogravimetry (TG), digital photos, scanning electron microscopy (SEM) and Raman 16

spectroscopy. These composites show some remarkable improvement in thermal stability 17

compared to that of the pure SBR. With the increase of kaolinite particle size, the residual 18

char content and the average activation energy of kaolinite SBR nanocomposites all decrease; 19

the pyrolysis residues become porous; the crystal carbon in the pyrolysis residues decrease 20

significantly from 58.23% to 44.41%. The above results prove that the increase of kaolinite 21

∗ Corresponding authors. E-mail addresses: zhangyudehn@hotmail.com (Y. Zhang), r.frost@qut.edu.au (R. L. Frost)

particle size is not beneficial in improving the thermal stability of kaolinite SBR 22

nanocomposites. 23

24

Keywords: Kaolinite; particle size; thermal stability; decomposition kinetic; rubber 25

nanocomposite. 26

27

28

1. Introduction 29

Polymers filled by layered clay mineral particles have gained a significant research 30

interest in the past two decades since the invention of nylon-clay based nanocomposites by 31

Toyota R&D group (Pavlidou and Papaspyrides, 2008). The clay minerals modified by 32

organic coupling agents can be endowed with good compatibility and dispersibility (Bergaya 33

et al., 2011). The incorporation of modified clay minerals can remarkably improve the 34

properties of clay polymer nanocomposites, such as mechanical, thermal and barrier 35

properties (Lagaly, 1999; Choudalakis and Gotsis, 2009; Coleman et al., 2011). The organo-36

clay minerals were used in various polymer systems (Bergaya et al., 2013). Rubber in 37

particular is an important class of polymer material due to their specific applications. Much 38

research about clay rubber nanocomposites has been carried out (Varghese and Karger-39

Kocsis, 2003; Shi et al., 2007; Liu et al., 2008; Gu et al., 2009; Yahaya et al., 2009; Vinay 40

Kumar and Prakash Chandra, 2010; Zhang et al., 2010; Galimberti, 2011). Some improved 41

results in mechanical and thermal properties of the rubber nanocomposites were observed. 42

Kaolinite has a wide variety of applications in industry, includingceramic materials, 43

paper fillers, coating pigments, an extender in water-based paints rubber fillers (Liu et al., 44

2008; Zhang et al., 2010; Bergaya et al., 2013) and other polymeric materials (Gardolinski et 45

al., 2000; Xia et al., 2010). Kaolinite is a 1:1 type layer structure with the basic unit 46

consisting of a tetrahedral sheet of SiO4 siloxane units and an octahedral sheet of AlO2 47

(OH)4. Kaolinite particles after modification can be evenly dispersed in polymer matrix. The 48

layered structure can act as the barrier cell in polymers and improve the thermal stability 49

(Mohan et al., 2011), gas barrier (Stephen et al., 2006), heat shielding and flame retardant 50

property of kaolinite polymer nanocomposites (Gilman, 1999; Gardolinski et al., 2000; Chen 51

et al., 2009). 52

In this paper a series of experiments are undertaken to investigate the relationships 53

between the thermal stability of the filled SBR nanocomposites and the kaolinite particle size. 54

The thermal stability and thermal decomposition behaviour of the SBR nanocomposites filled 55

with different particle sized kaolinites are characterized by TG, DTG and thermal 56

decomposition kinetics parameters. The microstructure changes of the pyrolysis residues are 57

measured by digital photography, SEM micrographs and Raman spectroscopy which further 58

explains the influence of the kaolinite particle size on the thermal stability and thermal 59

decomposition behaviour of kaolinite SBR nanocomposites. 60

61

2. Experimental 62

2.1. Preparation of the kaolinite samples 63

The raw kaolinite originates from Zhangjiakou in China, and belonged to a kind of clay 64

with hydrothermal alteration origins. Four kaolinite samples are obtained by free settling 65

method. Firstly, the raw kaolinite is blended with water at a mass ratio of 1:4, and 0.5% 66

sodium hexametaphosphate is added as dispersant. The pH of the solution was adjusted to 67

10.0 using a sodium hydroxide solution. The solution was stirred for two hours. Then, the 68

upper solution is extracted to another container with a siphon method after settling for 10 69

min. The sand and mud at the bottom are separated from the kaolinite solution. The new 70

solution is resettled according to a specified set time. After settling for 480 min, the upper 71

solution of 2 cm height is extracted to a selecting container and labeled as Kaol-1; the 72

remaining solution is stirred for one hour and resettled for 184 min. The upper solution of 1 73

cm height is obtained through siphoning, and labeled as Kaol-2. The remaining solution is 74

stirred for one hour again and resettled for 114 min; the upper solution of 1 cm height is 75

extracted through siphoning, and labeled as Kaol-3. The residual solution is labelled as 76

sample Kaol-4. The particle size characteristics are shown in Table 1. 77

78

2.2. Preparation of the rubber nanocomposites 79

Firstly, 2.0% silane coupling agent (KH-Si69) is added into the kaolinite solution and 80

stirred for one hour using a mechanic mixer at around 80°C. The modified kaolinite samples 81

are blended with SBR1500 latex (22.4% solid content) for around 20 min under a low stirring 82

rate, and flocculated with the dilute hydrochloric acid solution (1.0%). The compounds are 83

dried in a vacuum drier at around 80°C. Then the dried compounds are processed into 84

kaolinite SBR nanocomposites using a two roll mill [26, 27]. Formulation of kaolinite SBR 85

nanocomposites (phr) are as follows: SBR, 100.00; zinc oxide, 3.00; stearic acid, 1.00; 86

accelerator NS, 1.00; sulfur, 1.75; kaolinite, 50; phr is the abbreviation of mass parts per 100 87

mass parts rubber. The preparation procedure of kaolinite rubber nanocomposite is briefly 88

described as follows: the dried compound contained kaolinite and raw rubber are plasticized 89

for 3-5 min in the XK-160 open mill around 40°C, the spacing between the rolls is about 90

7mm, and the roller rate is 6.98 m/min; then ZnO, stearic acid, accelerator NS and sulphur are 91

added into the plasticized compounds in turn, the roller spacing was adjusted to 3-4 mm and 92

mixed evenly for 12~15 min at 60°C. A small part of the obtained kaolinite rubber 93

compounds (12.0 g) are used to test optimum cure time in a ZWL-Ⅲ non-rotor vulkameter. 94

The rest of the kaolinite rubber compounds are put into a 150×150×2 mm mould and 95

vulcanized on a 25QLB vulcanizing machine at 153°C and 10.0 MPa pressure up to the 96

optimum cure time. The cured rubbers are cooled rapidly in air at the end of the curing cycle. 97

At last, four SBR nanocomposites filled by Kaol-1, Kaol-2, Kaol-3 and Kaol-4 are obtained, 98

and individually labelled as Kaol-1 SBR, Kaol-2 SBR, Kaol-3 SBR and Kaol-4 SBR. 99

2.3. Characterization 100

The particle size distribution of the four kaolinite samples is measured by using a 101

Mastersizer 2000 laser particle size analyser of Malvern company (wet, cycle injection mode, 102

and test time: 1-2 min). 103

Thermal stability of kaolinite and kaolinite SBR nanocomposites are characterized with 104

a Setaram Evolution 2400 analyser by heating from 21 to 600°C under a nitrogen atmosphere 105

at a heating rate of 3, 5, 10 and 20 K/min. In order to investigate the influence of kaolinite 106

particle size on the thermal stability of kaolinite SBR nanocomposites and the structure 107

properties of decomposition residues, kaolinite SBR nanocomposites are put in an alumina 108

crucible and heated for one hour under a nitrogen atmosphere in a tubular furnace (SK-109

G06123K) at the selected temperatures (550 and 600°C). The structural changes of the 110

pyrolysis residues are characterized using digital photograhpy and electron microscopic 111

examination. The digital photos are obtained using the FUJIFILM S2900 HD digital camera. 112

The pyrolysis residues of SBR composites are adhered to Cu stubs using a conductive 113

adhesive, and scanning electron microscopy (SEM) micrographs are obtained with a S4800 114

LV electron microscope under 30 kV accelerating voltage. 115

The Raman spectra are recorded at a resolution of 4 cm-1 using an inVia Laser confocal 116

Raman spectroscopy system, at conditions of 514.5 nm laser wavelength, 65 um slit-width, 117

10 s time constant and 3 scanning times. The laser power of the incident beam on the sample 118

is kept below 2 mW to prevent irreversible thermal damage to the sample. The samples are 119

scanned between 4000 cm-1 and 400 cm-1, with data acquisition time of 20 s. The spectral 120

regions of 1800-1000 cm-1 and 3400-2200 cm-1 are studied by curve fitting with PeakfitV4.0 121

software. Peak decomposition is accomplished using the Gaussian/Lorentzian functions. The 122

parameters are fitted to the original data using the standard error (SE < 10-2) and the 123

correlation coefficient (R2 > 0.999) as metrics for goodness of fit. The influence of the 124

baseline is reduced by performing baseline correction before curve fitting. The spectral 125

regions provide the most valuable data on the microstructure of carbons resulting from the 126

pyrolysis of kaolinite SBR nanocomposites. The band positions, intensities, widths and area 127

are determined. The defects of microcrystalline carbon for the samples are calculated from 128

the integrated intensities of the D and G bands on the Raman spectra, equal to the area ratio 129

of the D1 and G bands. The relative concentration of amorphous carbon is calculated from the 130

intensity of peaks at around 1530 cm-1 (D3) and 1200 cm-1 (D4) relative to that of D1 and G 131

bands (Reich and Thomsen, 2004; Potgieter-Vermaak et al., 2011). 132

2.4. Kinetics calculation 133

For kinetic analysis, it is assumed that a solid (polymer) will decompose into a new solid 134

and some gases. The thermogravimetric rate is given by Equation (1) according to the 135

Arrhenius equation and the non-isothermal kinetics theory (Flynn and Wall, 1966). 136

dCdT

= 𝐴𝛽

(1 − 𝐶)𝑛𝑒−𝐸/𝑅𝑇 (1) 137

Where C is defined as the degree of conversion, equals the mass of material loss divided by 138

the total mass loss as T or t; T, absolute temperature; β, constant heating rate; A, pre-139

exponential factor of the Arrhenius equation; E, activation energy; and R, the gas constant. 140

Equation (1) is the basic expressions for investigation of thermal decomposition kinetics. 141

According to the Flynn-Wall differential method (Flynn and Wall, 1966; Koga, 2013) and the 142

Ozawa integral method (Ozawa, 1965; Ozawa, 1992; Park et al., 2000), the thermal kinetics 143

parameters are obtained from the equation (2). 144

𝑙𝑔𝛽 = lg 𝐴𝐸𝑅− 𝑙𝑔𝐹(𝐶) − 2.315 − 0.4567 𝐸

𝑅𝑇 (2) 145

F(C) is a function of degree of conversion. Therefore, from mass loss vs. temperature 146

thermograms at several heating rates, one may determine the corresponding temperatures at a 147

constant mass loss. Then, from the slope of a plot of lg β vs. 1/T, the activation energy can be 148

calculated using equation (2). 149

2.5. Calculation of residual carbon 150

Based on the formulation of kaolinite SBR nanocomposites, the mass fraction of 151

kaolinite is about 31.90% in the SBR compound filled by 50 phr kaolinite, and the residual 152

kaolinite mass (Mrk) at 600°C is 31.90% (1-a), whereas a is the fraction of mass loss for 153

kaolinite at 600°C. In the pure SBR and kaolinite SBR nanocomposite, the mass fraction of 154

zinc oxide (MZnO) is about 2.81% and 1.91%, respectively. The residual mass of SBR 155

nanocomposite at 600°C is named as Mr. The residual char mass (Mc) for the kaolinite SBR 156

nanocomposites at 600°C is calculated by Mr subtracting Mrk and MZnO, namely Mc = Mr - 157

Mrk - MZnO. The results of SBR nanocomposites filled by kaolinites with different particle 158

size were listed in Table 1. 159

160

3. Results and discussion 161

3.1. Thermogravimetric analysis 162

The thermal behavior of kaolinite has been studied in numerous papers and the results 163

of this work are similar to those reported in the literature see for example (Cheng et al., 2010; 164

Wang et al., 2011; Caglar et al., 2013). In the TG curve, the mass loss of four kaolinite 165

samples with different particle size are observed at temperature intervals of 26-150°C and 166

300-600°C, respectively (Fig. 1). The mass loss corresponds to the DTG peaks centred at 167

40°C and 500°C, respectively. The first mass loss was attributed to the removal of the surface 168

adsorbed water and the second mass loss is attributed to the dehydroxylation process with 169

formation of metakaolinite (Cheng et al., 2010). For the four kaolinites, the final mass loss is 170

about 15.36%, 14.83%, 14.57% and 13.64% respectively. With the increase of particle size, 171

the mass loss gradually decreases and the peak temperature increases. The changes result 172

from the hindering of the layered kaolinite particles to heat flow. 173

TG is a commonly employed approach to characterize the thermal stability and 174

thermal decomposition behavior (Durga and Narula, 2012). The data (Gilman, 1999; Ptáček 175

et al., 2010; Ptáček et al., 2011) including the temperature at which 5% mass loss occurs (T5, 176

a measure of onset temperature of decomposition), the temperature for 50% decomposition 177

(T50) as the mid-point of decomposition, the peak mass loss rate (Rp), the peak temperature 178

(Tp), the residual mass at 600°C (Mr) and the residual char mass (Mc) are considered as the 179

main indices in evaluating the thermal stability of clay polymer nanocomposites. 180

The shift of the TG and DTG curves of the pure SBR and kaolinite SBR nanocomposites 181

measured at heating rates from 3 to 20 K∙min-1 are shown in Fig.2. In Fig. 3 the 182

corresponding results of the decomposition temperatures (Ta) at different mass loss, Tp and 183

Rp at the heating rate of 3 K∙min-1 are reported. With the increase of the heating rate, the Ta 184

and Tp shift to higher location, and Rp increased gradually. Compared to the pure SBR, the 185

Rp of kaolinite SBR nanocomposites is significantly reduced due to the incorporation of the 186

kaolinite. The Tp decreases with the increase of kaolinite particle size. T5 of Kaol-1 SBR, 187

Kaol-2 SBR, Kaol-3 SBR, and Kaol-4 SBR, is 336.7, 338.6, 335.6, 330.9°C and increased by 188

42.1, 44.0, 41.0 and 36.3°C, respectively. T50 also increases by 24.8, 23.44, 21.7 and 18.96°C. 189

T5, T10 and T50 show the same change as for the Tp. With the increase of kaolinite particle 190

size, the thermal stability of the kaolinite filled SBR nanocomposite gradually reduces. 191

Based on the formulation of kaolinite SBR nanocomposites, for the SBR filled by 50 phr 192

kaolinite, the residual char mass (Mc) for the kaolinite SBR nanocomposites at 600°C can be 193

obtained by Mr subtracting Mrk and MZnO. The residual char results of SBR nanocomposites 194

filled with different particle sized kaolinites are listed in Table 2. Mc of Kaol-1 SBR is 7.71%, 195

which is the highest of the four kaolinite rubber samples. The char content in the residue 196

decreases with the increase of kaolinite particle size. For the Kaol-4 SBR nanocomposite 197

filled by the coarse kaolinite, the Mc is lower than that of pure SBR. The residual char layer 198

and kaolinite layer act as an insulation material to hinder the heat flow transport into the inner 199

of polymer matrix (Zhang et al., 2010) and is very beneficial to improve the thermal stability 200

of kaolinite SBR nanocomposites (Ramesan, 2004; Chen et al., 2006). 201

The thermal analysis data indicate the thermal stabilities of the kaolinite SBR 202

nanocomposites are enhanced due to the incorporation of kaolinite compared to that of the 203

pure SBR, and the increase of kaolinite particles size damages the thermal stability of 204

kaolinite SBR nanocomposites. 205

206

3.2. Kinetics analysis of thermal decomposition 207

In this work, activation energy (E) is applied to investigate the thermal stability of SBR 208

nanocomposites filled with different particle sized kaolinites. The results of the 209

thermogravimetry at various heating rates are plotted against the temperature as illustrated in 210

Fig. 2. The logarithms of the heating rates (lgβ) are plotted against the reciprocal absolute 211

temperature (T-1) at different conversion degree (C), as shown in Fig. 4. According to the 212

slope of a plot of lg β vs. 1/T, the activation energy may be calculated by equation (2). The 213

activation energy at different conversion degree and the average activation energy of 214

kaolinite SBR nanocomposites are reported in Table 3. The values of average activation 215

energy of kaolinite SBR nanocomposites are enhanced significantly due to the incorporation 216

of modified kaolinite compared to the pure SBR. The kaolinite SBR nanocomposites are 217

more complex in the decomposition behaviour and need much more energy for degradation 218

than the pure SBR (Mohan et al., 2011). In addition, the average activation energy decreases 219

with the increase in the kaolinite particle size. The results also indicate that the increase of 220

kaolinite particle size is not beneficial to improve the thermal stability of the kaolinite SBR 221

nanocomposites. Kaol-1 SBR has the best thermal stability among all the samples. 222

223

3.3. Digital photos of residues 224

The digital photos of residues of the bulk pure SBR and SBR nanocomposites filled with 225

different particle sized kaolinites at 550°C are shown in Fig. 5. The pyrolysis residue of the 226

bulk pure SBR changes significantly in shape, its structure collapses completely, and a little 227

coherent and loose char is observed (Fig. 5a). For the Kaol-1 SBR filled with the finest 228

kaolinite, its pyrolysis residue keeps a good shape and doesn’t show any porosity (Fig. 5b). In 229

addition there is a more coherent and dense char layer. With the increase in kaolinite particle 230

size, the structure shape of the pyrolysis residue is gradually damaged; the gas pores and slits 231

increase and become bigger on the surface of the pyrolysis residues. A much lower coherent 232

and looser char is formed with the increase of kaolinite particle size. In addition, the 233

expansion rates of the char residues become much lower and the shrink is more serious with 234

the increase of kaolinite particle size. The kaol-1 SBR shows the highest expansion rate, and 235

has the best thermal stability. There is a good correlation with the changes of Ta, Tp, Rp and E 236

as measured by TG, DTG and kinetics analysis. 237

The thermal stability changes of four kaolinite SBR nanocomposites can be explained by 238

the structural differences of the pyrolysis products composed of the char layer and kaolinite 239

layer. When filled with the fine kaolinite particles, the relative amount of kaolinite layers 240

increases in the SBR nanocomposite (Zhang et al., 2014). The kaolinite layer is a good barrier 241

cell to heat flow. Therefore, the heat flow is more difficult to spread to the inner polymer at 242

the same pyrolysis temperature. In addition, the fine layered kaolinite particles with high 243

surface energy strongly interact with the rubber molecular chains. The interaction stabilizes 244

the organic molecules and prevents them from decomposing. The coherent and stable char 245

layer forms at higher temperature point. The kaolinite layer and char layer act as a hindrance 246

cell and improve the thermal stability of kaolinite SBR nanocomposites. These dense 247

hindrance layers can also restrict the decomposed small molecules to escape from the inner of 248

the polymer matrix, and the fewer gas pores and slits are formed in the residues of Kaol-1 249

SBR composite filled with the fine kaolinite. 250

251

3.4. SEM micrographs of residues 252

SEM micrographs of the pyrolysis residues of SBR nanocomposites filled by different 253

particle size kaolinite are shown in Fig. 6. The residues are mainly composed of the kaolinite 254

layers, char layers and some small particle materials. The small particles are ZnO. From 255

Kaol-1 SBR to Kaol-4 SBR, the kaolinite particle size increases due to the agglomeration, 256

and this tendency is in good agreement with the results observed by the particle size analyser 257

(Table 1). In Kaol-1 SBR the kaolinite particles have a good dispersion, and the thickness of 258

the platelets is the smallest; there is little agglomerate phenomenon and the platelets are 259

exfoliated individually. The edge of kaolinite layers is comparatively vague due to the 260

coating of char layer compared to other samples. With the increase in particle size, the 261

agglomeration becomes more serious, and the edge of kaolinite layers is much clearer due to 262

the decrease of char layers. In addition, the pyrolysis residues became much looser, and the 263

pore size is much larger. The decrease of the compactness is the important reason results in 264

the decrease of the thermal stability of kaolinite SBR nanocomposites. 265

3.5. Raman spectroscopy of residues 266

The Raman spectra of the pyrolysis residues of kaolinite SBR nanocomposites at 550°C 267

and the fitted spectra in the ranges 1800-1000 cm-1 and 3400-2200 cm-1 are presented in Figs. 268

7-9. In Fig. 7 all the curves exhibit two narrow and overlapping peaks with intensity maxima 269

at ~1350 cm-1 and ~1590 cm-1 which correspond to the D and G bands of disordered and 270

ordered graphite (Sadezky et al., 2005). It is very similar to the Raman spectra of 271

carbonaceous materials. The G peak at ~1590 cm-1 displays the stretching vibrations of sp2 272

bonds in hexagonal graphite (Vidano et al., 1981; Reich and Thomsen, 2004). The D peak at 273

~1350 m-1 results from the broadening of G peak due to the introduction of the disordered 274

carbon. The high signal intensity between the two peak maxima is attributed to a band 275

between 1500 and 1550 cm-1, designated D3. The D3 band is associated with amorphous sp2-276

bonded forms of carbon (Cuesta et al., 1994). In addition, the broad D band conceals another 277

band of amorphous carbon at ~1200 cm-1, namely as D4. Then the region of 1800-1000 cm-1 278

is composed of four bands including D1, D3, D4 and G. 279

The Raman spectra of the pyrolysis residues of four kaolinite SBR nanocomposites are 280

analyzed by curve-fitting the four peaks (D1, D3, D4 and G) using Gaussian functions of the 281

PeakfitV4.0 software as shown in Fig. 8. Spectroscopic parameters such as peak positions, 282

integral area of intensity and full widths at half maximum (FWHM) obtained after curve-283

fitting are listed in Table 4. The FWHM of the G band ranges from 60 to 71 cm-1 which is far 284

larger than that recorded for highly oriented pyrolytic graphite of about 15-23 cm-1 (Cuesta et 285

al., 1994). This result suggests that the crystalline order degree is not high in the studied 286

samples. The defect degree of crystallite carbon (AD1/AG) in the pyrolysis residues calculated 287

from the integral intensity ratio of two bands at 1350 cm-1 (D1) and at 1590 cm-1 (G) ranges 288

from 0.94 to 2.09 (Table 4), and increases with the increase in kaolinite particle size. It 289

indicates that the decomposition behaviour is more complicated for the SBR nanaocomposite 290

filled with the fine kaolinite. The relative concentration of crystalline graphite carbon in the 291

residues is measured from peaks D1 and G intensity relative to that of the D and G peaks 292

(I(D1+G)/ITotal) (Ferrari and Robertson, 2000). The content of amorphous carbon (Cam) is the 293

peaks D3 and D4 intensity relative to that of the D and G peaks (I(D3+ D4)/ITotal). The 294

amorphous carbon (Cam) of four pyrolysis residues gradually increases with the increase of 295

kaolinite particle size, and the crystal carbon shows a reverse change tendency varying from 296

58.23% to 44.41%. It is suggested that the increase of kaolinite particle size is not beneficial 297

to form the crystalline carbon in the pyrolysis process, and brought on a negative influence in 298

improving the thermal stability of kaolinite SBR nanocomposites. 299

In the high wavenumber range from 2200 to 3400 cm-1, there are some bands attributed 300

to the second order scattering of the imperfect graphite and disordered carbon. The 3200 cm-301

1 (D2΄) and 2420 cm-1 (D4΄) is assigned to an overtone of D2 (1620 cm-1) and D4 (1200 cm-1) 302

respectively, and the band (D1΄) at 2700 cm-1 has been explained as an overtone of D or D1, 303

whereas 2950 cm-1 (D3΄) is attributed to a combination of D and G. The peak positions, 304

integral area of intensity and FWHM obtained after curve-fitting are listed in Table 4. The 305

bands (D4΄, D3΄, D2΄) attributed to the amorphous carbon show an increase in intensity and a 306

decrease in wavenumber with the increase of kaolinite particle size. The band widths are very 307

different, whereas the band (D1΄) related to the defect crystalline carbon displays a decrease 308

with the increase of kaolinite particle size. The results in Table 5 are consistent with that in 309

Table 4, which also indicates that the crystalline carbon with high thermal stability decreases 310

with the increase of kaolinite particle size. Therefore, SBR nanocomposite filled with the fine 311

kaolinite particle has higher thermal stability in the pyrolysis process. 312

313

4. Conclusions 314

A series of kaolinite SBR nanocomposites are prepared by blending the latex styrene-315

butadiene rubber and four different particle sized kaolinites. Their thermal stabilities are 316

characterized by a range of techniques including TG, digital photos, SEM and Raman 317

spectroscopy. The thermal stability of kaolinite SBR nanocomposites are remarkably 318

improved compared to that of the pure SBR. With the increase of kaolinite particle, the 319

average activation energy of kaolinite SBR nanocomposite decreases gradually. The thermal 320

stability of kaolinite SBR nanocomposite is significantly reduced due to the decrease of the 321

char and crystalline carbon content in pyrolysis residues, the increase of agglomeration of 322

kaolinite particles and the decrease of the compactness of char layer. The increase of 323

kaolinite particle size is not beneficial to form the crystalline carbon in the pyrolysis process, 324

and brings on a negative influence in improving the thermal stability of kaolinite SBR 325

nanocomposites. 326

327

Acknowledgments 328

The authors gratefully acknowledge the financial support provided by the National 329

Natural Science Foundation Project of China (51034006 and 51104060), the Opening Project 330

of Henan Key Discipline Open Laboratory of Mining Engineering Materials (MEM11-2) and 331

the Ph.D. programs foundation of Henan Polytechnic University (648273) in China. 332

333

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LIST OF TABLES 436

Table 1 Particle size characteristic (D10、D50、D90) and specific surface area (SBET) 437

Table 2 Residual Mass of fillers and kaolinite SBR nanocomposites at different stages 438

Table 3 Activation energy of SBR nanocomposites filled with different particle sized 439

kaolinites 440

Table 4 Fitting parameters obtained from Raman spectra of the pyrolysis residues of 441

kaolinite SBR nanocomposites in region 1800-1000 cm-1 at 550°C 442

Table 5 Fitting parameters obtained from Raman spectra of the pyrolysis residues of 443

kaolinite SBR nanocomposites in region 3400-2200 cm-1 at 550°C 444

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LIST OF FIGURES 447

Fig. 1 TG and DTG curves of four different particle sized kaolinites at the heating rate of 448

3K/min 449

Fig. 2 TG and DTG curves of four different particle sized kaolinites at the different heating 450

rate 451

Fig. 3 Decomposition temperature at different mass loss, the peak temperature and the peak 452

mass loss rate at the heating rate of 3K/min 453

Fig. 4 Plots of lgβ versus T-1 for indicated conversion degree of the decomposition of SBR 454

Fig. 5 Digital photos of the residues of the bulk pure SBR and SBR nanocomposites filled 455

with different particle sized kaolinites at 550°C: a pure; b Kaol-1; c Kaol-2; d Kaol-3; e 456

Kaol-4. 457

Fig. 6 SEM photographs of the pyrolysis residues of SBR nanocomposites filled with 458

different particle sized kaolinites at 550°C: a Kaol-1; b Kaol-2; c Kaol-3; d Kaol-4. 459

Fig. 7 Raman spectra of the pyrolysis residues of kaolinite SBR nanocomposites at 550°C 460

Fig. 8 1800-1000 cm-1 fitting Raman spectra of the pyrolysis residues of kaolinite SBR 461

nanocomposites in region at 550°C 462

Fig. 9 3400-2200 cm-1 fitting Raman spectra of the pyrolysis residues of kaolinite SBR 463

nanocomposites in region at 550°C 464