Preparation of Uniformly Vulcanized Stereoregular Natural Rubber through Critical Concentration Determination of Nitrogen-Containing Compounds

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.