Quantitative analysis of crosslinking junctions of vulcanized natural rubber through rubber-state NMR spectroscopy

Abstract

Rubber-state nuclear magnetic resonance (NMR) spectroscopy, a novel analytical method, was applied to quantitative analyses of crosslinking junctions and uncrosslinked pendant sulfidic groups for natural rubber (NR) vulcanizates. NR was vulcanized with two formulations: i.e., one including sulfur, ZnO, stearic acid and N-tert-butylbenzothiazole-2-sulfenamide (TBBS) as an accelerator and the other including sulfur, ZnO and stearic acid without the accelerator. The resulting NR vulcanizates were characterized by swelling methods and rubber-state NMR spectroscopy to determine crosslink density, number of sulfur atoms at crosslinking junctions and structure of crosslinking junctions, respectively. Small signals appearing in rubber-state 13C and 1H NMR spectra for the NR vulcanizates were assigned to crosslinking junctions, uncrosslinked pendant sulfidic groups and their neighboring groups by two-dimensional NMR spectroscopy. Based on the assignments, a proportion of the amount of CH linking to sulfur as the crosslinking junctions to total amount of CH linking to sulfur were estimated from intensities of the signals for the NR vulcanizates.