Effect of Pre-coated Vs. Added Coupling Agents on the Vulcanization of Silica-filled Polyisoprene

Solid-state ¹³C NMR was utilized in analysis of the crosslink structure in silica-filled polyisoprene. Samples containing either a pre-coated coupling agent, a coupling agent added in the mixing process, or polyethylene glycol (PEG) were vulcanized and the type and density of crosslinking determined. The precoated coupling agent sample gave lower %swelling, lower extraction amounts, and a lower T ₂ relaxation than the mixed coupling agent, indicating increased crosslinking or increased filler –rubber interaction. The precoated 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 yielded a ¹³C spectrum similar to an unfilled sample indicating decreased silica adsorption of vulcanization agents. Neither coupling agent significantly decreased the total sulfurization nor the percent monosulfidic crosslinks from that of a silica-filled sample without coupling agent.