Detection of isomerization in commercial natural rubber

Abstract

Detection of isomerization in commercial natural rubber (NR) was performed through Nuclear Magnetic Resonance (NMR) spectroscopy in conjunction with structural analysis of pure NR. Three similar grades of commercial NR, which are technically specified rubbers from Vietnam (SVR10), India (SIR10) and Indonesia (ISNR10) and the pure NR, which is prepared by precipitation of rubber latex tapping from Hevea brasisliensis, were used in this work. All the commercial NRs were purified by acetone extraction and the pure NR were prepared by deproteinization and acetone extraction. The ¹³C-NMR analysis showed that the main characteristic ¹³C-signals of cis-1,4-isoprene unit in the polymer Chain are unchanged, however, the ¹³C-signal at 40 ppm, which is distinguished to the signal at 39.8 ppm, is detected for all commercial rubbers. This signal is assigned to the methyl group of trans-1,4-isoprene units as observed in Carifrex-309 as a model. The appearance of this signal implied the occurrence of isomerization during Processing of the commercial NR. The degradation is found to weaken the network structure and result in poor mechanical properties of commercial NR.