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 degree s C was investigated by dilatometry in connection with the effect of l ong-chain fatty acid groups, esterified to the rubber chain, as well a s free fatty acids and their esters. The overall crystallization rate decreased after removal of the acetone-extractable free daily acids an d their esters, while it increased significantly when the fatty acid g roups esterified to natural rubber molecule were removed by transester ification with sodium methoxide. Both the acetone-extracted and transe sterified rubbers showed a significant increase in the overall crystal lization rate after the addition of 1 wt% stearic acid. The crystalliz ation of acetone-esterified rubber was accelerated by the addition of 1 wt% methyl linoleate, a plasticizer of natural rubber, whereas it wa s suppressed in the case of transesterified rubber in which the esteri fied fatty acid groups were removed completely. The fatty acid groups esterified to natural rubber molecule gave rise to the accelerated cry stallization of the rubber in conjunction with free fatty acids and th eir esters.