Impact of non-rubber components on the branching structure and the accelerated storage hardening in Hevea rubber

Abstract

Natural rubber (NR) contains cis-1,4-polyisoprene as well as a small quantity of non-rubber components, affecting the physical properties of NR, especially the storage hardening via gel formation. The free and/or linked non-rubber components were removed from NR by various treatments. The origin of gel formation in NR mainly resulted from proteins, which could be removed by enzymatic deproteinisation, surfactant washing or saponification treatment, and partly from phospholipids linked to NR molecule via H-bonding, which were decomposed by saponification or polar solvent treatment. The effect of the non-rubber components in NR on storage hardening was investigated by an accelerated storage hardening test (ASHT). The crude NR (CNR) containing proteins and lipids clearly showed the increase of gel content, suggesting the formation of crosslink structure during storage, resulting in the high plasticity number and Mooney viscosity after the ASHT. The gel content in protein-free NR did not change significantly only in washed NR (WSNR), but increased in the enzymatic deproteinised NR (DPNR). This resulted from the presence of residual proteins, such as oligopeptides. Interestingly, the gel content also reduced in the acetone extracted CNR (AE-CNR) and there was no significant effect on the storage hardening after ASHT, like WSNR. These were supposed to be due to the interaction or the participation of proteins and free fatty acids in the branching formation. Although the saponification could be decomposed branching at both terminal ends of the NR molecule, the protein and lipid residues occluded in the saponified NR (SPNR) affected the storage hardening. However, the flocculant remaining in the SPNR coagulated with acid and flocculant (AF-SPNR) would obstruct both gel formation and storage hardening.