Structural characterization of sulfur vulcanized deproteinized natural rubber by solid-state¹³C NMR spectroscopy

The structural changes in unfilled and filled vulcanized deproteinized natural rubber (DPNR), which is a highly purified natural rubber (NR), were investigated by solid-state ¹³C NMR spectroscopy. The chemical shifts of new signal were assigned by calculation and compared with those observed by Koenig et al. for conventional vulcanized NR. The crosslink structure of sulfur vulcanized DPNR were similar to that of sulfur-vulcanized NR for CV system assigned by Koenig et al. The increase in the relative intensities of signals between 45 and 58 ppm, which was assigned to monosulfidic and polysulfidic bonds, was concerned with the increase in crosslink density. In this case, the crosslink density was increased after aging, which might be due to incomplete curing. Furthermore, the solid-state ¹³C NMR spectrum of the aged sulfur vulcanizate DPNR showed new signals at around 73 ppm, which is assignable to carbon attached to hydroxyl group because of thermal oxidation. It can provide more complete understanding of the final vulcanizate crosslink structure of DPNR vulcanizates.