PRI ageing and some related properties of constant viscosity natural rubbers
2002
Eng, A. H. | Chang, C. F. | Kawahara, S.
Constant viscosity natural rubber (NR) samples from different clones showed no correlation between copper content and Plasticity Retention Index (PRI) value. When the NR samples were stabilised with hydroxylamine, the degree of + storage hardening was low but the PRI remained high. The gel content of the NR samples decreased significantly after the PRI test. The extent of reduction in gel content was much greater than that of the reduction in molecular weight of the NR samples. This was attributed to the denaturation of the proteins, which led to the disintegration of crosslinks, or higher rate of oxidation due to the presence of a high level of metal ions in the gel. The extent of molecular weight reduction was slightly lower than that of plasticity value. The Wallace plasticity could be correlated with the geometrical mean of Mn and Mw, i.e. (MnMw)1/2. The correlations obtained from linear regression before and after PRI ageing were quite comparable probably because the NR samples were processed and aged under the same conditions. Activation energy of oxidation (AEO) determined by differential scanning calorimeter provided a more sensitive method of assessing the ageing resistance of NR samples than PRI. However, the correlation between PRI and AEO was insignificant.
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