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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: [email protected] (Y. Zhang), [email protected] (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
445
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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