Uniformly vulcanized stereoregular natural rubber was prepared by accelerated sulfur vulcanization of deproteinized natural rubber (DPNR) with an optimal amount of octadecylamine (ODA) to open DPNR research field as a green, sustainable chemistry. The DPNR prepared from natural rubber latex of the Para rubber tree ( Hevea brasiliensis ), was vulcanized using standard recipe containing stearic acid, ZnO, vulcanization accelerator, and sulfur, together with various amounts of ODA (0.2–6.0 phr). The cure characteristics measured with a rheometer depended on the amount of ODA. The resulting vulcanized DPNR was characterized by swelling measurements and rubber-state NMR spectroscopy. The crosslink density estimated from the swelling ratio was highest at 2.6 phr ODA, corresponding to a concentration of approximately 0.10 mol/kg-rubber, although the intensity ratios of signals assigned to C-S linkages as crosslinking junctions were the lowest. The tensile strength of the vulcanized DPNR was also markedly improved at 2.6 phr ODA. From the results, 0.10 mol/kg-rubber (2.6 phr) was a critical ODA concentration for preparing the uniformly vulcanized stereoregular natural rubber with the highest crosslink density and the smallest amount of C-S linkage. The resulting uniformly vulcanized stereoregular natural rubber exhibited superior mechanical properties with effective strain-induced crystallization. • Uniformly vulcanized stereoregular natural rubber was prepared from DPNR by incorporating octadecylamine into a conventional sulfur vulcanization formula. • A critical concentration of octadecylamine of 0.10 mol/kg-rubber (2.6 phr) was found. • Rubber-state NMR spectroscopy was performed to characterize structure of crosslinking junctions. • Superior mechanical properties were obtained through enhanced strain-induced crystallization in uniformly vulcanized stereoregular natural rubber. • Direct correlations were found between mechanical properties and structure of crosslinking junctions.
Nguyen et al. (Fri,) studied this question.