LUO Yongyue et al., Thermal degradation of epoxidized natural rubber in presence of neodymium stearate 529
dation. T5 at which 5% decomposition occurs is an index of thermo-oxidative stability[10]. It is obvious that the ENR vulcanizates with 1 phr NdSt impart the highest T5 value during the thermo-oxidative degradation. The T5 value for the ENR25 vulcanizates with NdSt is higher than the samples without NdSt.
Table 4 gives the effect of NdSt concentration on ki- netics of thermo-oxidative degradation of ENR25. It can be found that from Table 4 that 1 phr NdSt gives the highest activation energy, which indicates that the ENR25 vulcanizate with 1 phr NdSt exhibits the better thermal oxidative aging resistance. This can be attributed to the Nd ion can capture free radicals result in the ter- mination of chain reaction. Another reason why the ENR25 vulcanizates with NdSt shows the better ther- mal-oxidative stability is that chelates formed in the vul- canization due to the coordination between oxygen atom and Nd ions[18]. These chelates can improve the stability of expoxide groups.
From the data of the thermal and thermal-oxidative degradation of ENR25 vulcanizates, the effect of NdSt on the thermal stability is more significant than the effect on thermal-oxidative stability.
2.3 FTIR-ATR analyses
The structure change of ENR25 vulcanizates before and after thermo-oxidative aging is analyzed by FTIR- ATR spectroscopy. Fig. 6(a) and (b) show the FTIR- ATR spectroscopy of ENR25 vulcanizates with 2 phr NdSt aged at 140 °C for different hours. It can be seen that the characteristic absorptions bands after thermo- oxidative aging have significantly changed. The absorp- tion bands at 3520 cm–1 (OH symmetric stretching vibra- tion), and 1737 cm–1 (C=O stretching vibration) become more intensive due to oxidation of ENR[19]. The absorp- tion band of CH2 symmetric stretching at 2848 cm–1 is unaffected in the process of thermal oxidative aging. So the ratio of the absorbance of C=O to that of CH2 can be used to evaluate the thermal-oxidative stability of ENR molecules[10]. The absorbance ratio (AC=O/ACH2) of the ENR25 with 2 phr NdSt and without NdSt aged at 140 °C for different time are presented in Fig. 7. It is clearly seen that the absorbance ratio (AC=O/ACH2) of ENR25 vulcani- zate with NdSt increases slower than that of pure vul- canizate. The incorporation of NdSt could enhance ther- mal-oxidative stability of the ENR25 vulcanizate. The
Table 4 Kinetic parameters of thermal degradation of
ENR25 with different loadings of NdSt
phr n A E/(kJ/mol) r
0 2 0.29×108 133.9 0.9995 0.5 2.1 1.18×108 142.1 0.9998 1 2.1 1.27×108 143.1 0.9997 1.5 1.9 0.28×108 134.5 0.9992 2 2 0.94×108 135.4 0.9997
Fig. 6 ATR-FTIR spectra of the ENR25 vulcanizates aged at 140 °C with 2 phr NdSt (a) and without NdSt (b)
Fig. 7 Absorbance ratio (AC=O/ACH2) of the ENR25 vulcanizates with 2 phr NdSt and without NdSt
result of FTIR-ATR is consistent with the analyses of TG analyses.
3 Conclusions
From this study, the following conclusion can be drawn:
(1) The thermal degradation of ENR25 in nitrogen was a one-step reaction regardless of NdSt content. The in- corporation of NdSt could significantly enhance the thermal stability of ENR25 and activation energy of