Crystallization behavior and strength of natural rubber: Skim rubber, deproteinized natural rubber, and pale crepe

Crystallization behavior of natural rubber prepared by different procedures, such as skim rubber, deproteinized natural rubber (DPNR), and pale crepe, was investigated by dilatometry at -25 °C. DPNR was fractionated into four fractions by molecular weight. The high molecular weight fractions contained about 1.7 linked fatty acids per rubber molecule, while low molecular weight fraction showed an increase in quantity. The overall crystallization rate of the rubber decreased as the molecular weight decreased. Skim rubbers, purified by extraction with acetone, crystallized rapidly compared to acetone-extracted pale crepe, despite that the molecular weight of skim rubbers was about one-half of pale crepe. The quantity of linked fatty acid per rubber molecule of skim rubbers was less than 0.5, while that of pale crepe was 1.6. The difference in the rate of crystallization was presumed to be associated with the level of fatty acids linked to rubber molecule at the terminal and branch points present in pale crepe. The green strength of skim rubbers was significantly lower than those of untreated pale crepe and DPNR, but was comparable to transesterified DPNR, which contains no gel fraction and no linked fatty acids.