Effect of gel on crystallization behavior of natural rubber after accelerated storage hardening test

Abstract

Crystallization of natural rubber (NR) and purified NR after storage hardening of the rubbers under dried condition with phosphorus pentoxide was investigated by dilatometry and polarized light microscopy. The purified NR was prepared by removing proteins and fatty acid ester groups from fresh NR (FNR) through deproteinization and transesterification, respectively. The rubbers were characterized in regard as gel content, ester content and crosslink density. Nucleation (I) and growth rates (V) of the gel fraction estimated from the number of spherulites and their sizes observed in the course of isothermal crystallization depended on supercooling (ΔT = Tm0 - T). Tm0 is equilibrium melting temperature and T is absolute temperature. Slope of linear line in a plot of logarithmic V versus 1/TAT for NR was independent of time for storage hardening, suggesting that the lateral surface free energy and diffusion were little function of branching and crosslinking. The rate of crystallization of FNR significantly decreased when the crosslink density increased. Since the overall crystallization is functions of chain mobility and surface free energy, the slower crystallization of well crosslinked FNR may be attributed to the less chain mobility and small value of ACT due to the formation of crosslinked and branched entities. © 2007 Wiley Periodicals, Inc.