Origin of characteristic properties of natural rubber - Synergistic effect of fatty acids on crystallization of cis-1,4-polyisoprene .2. Mixed and esterified fatty acids in natural rubber

Crystallization behavior of deproteinized natural rubber at -25 degrees C was investigated by dilatometry in connection with the effect of long-chain fatty acid groups, esterified to the rubber chain, as well as free fatty acids and their esters. The overall crystallization rate decreased after removal of the acetone-extractable free daily acids and their esters, while it increased significantly when the fatty acid groups esterified to natural rubber molecule were removed by transesterification with sodium methoxide. Both the acetone-extracted and transesterified rubbers showed a significant increase in the overall crystallization rate after the addition of 1 wt% stearic acid. The crystallization of acetone-esterified rubber was accelerated by the addition of 1 wt% methyl linoleate, a plasticizer of natural rubber, whereas it was suppressed in the case of transesterified rubber in which the esterified fatty acid groups were removed completely. The fatty acid groups esterified to natural rubber molecule gave rise to the accelerated crystallization of the rubber in conjunction with free fatty acids and their esters.