Thermal properties and crystallization behavior of highly deproteinized natural rubber

Natural rubber was deproteinized with a proteolytic enzyme and surfactant to reduce nitrogen content less than 0.02%. The deproteinized rubber was further purified by acetone-extraction followed by transesterification with sodium methoxide. The density of rubber decreased and the thermal expansion coefficient increased by deproteinization and transesterification. The glass transition temperature, T-g, was slightly reduced after deproteinization and transesterification. These were explained in relation to the decrease in gel content of the rubber. The equilibrium melting temperature, T-m(0), was practically unchanged by these treatments. The low-temperature crystallization between -30 and -10 degrees C was significantly influenced by free fatty acids present as a mixture and fatty acid groups linked to the rubber chain. The crystallization behavior was independent of T-g and T-m(0) values.