Effect of pre-coated vs. added coupling agents on the vulcanization of silica-filled polyisoprene

Solid-state C-13 NMR was utilized in analysis of the crosslink structure in silica-filled polyisoprene. Samples containing either a pre-coated couplin g agent, a coupling agent added in the mixing process, or polyethylene glyc ol (PEG) were vulcanized and the type and density of crosslinking determine d. The precoated coupling agent sample gave lower %swelling, lower extracti on amounts, and a lower T-2 relaxation than the mixed coupling agent, indic ating increased crosslinking or increased filler-rubber interaction. The pr e-coated also showed less cis-trans isomerization than the mixed, which may be due to its more uniform coating effect on the silica. The PEG sample yi elded a C-13 spectrum similar to an unfilled sample indicating decreased si lica adsorption of vulcanization agents. Neither coupling agent significant ly decreased the total sulfurization nor the percent monosulfidic crosslink s from that of a silica-filled sample without coupling agent.