The Scission of Polysulfide Crosslinks in Scrap Rubber Particles through Phase Transfer Catalysis

Abstract

Abstract Hydroxide ions transported by onium ions from water into swollen rubber particles rapidly break polysulfide crosslinks with little or no main chain scission. However, methylated monosulfide crosslinks do not cleave under these conditions. The reaction is not truly catalytic because the quaternary ammonium is decomposed. Model studies with N-methyl-N,N,N-tri-n-alkylammonium chlorides and di-2-cyclohexen-1-yl disulfide show that the decomposition reaction involves a highly selective methylation of the thiolate intermediate. However, catalyst decomposition can be inhibited with added alkylating agents. Alkylating agents also improve catalyst efficiency during devulcanization. The Hofmann reaction is yet another route for catalyst decomposition, but the rate is too slow to be competitive. Several onium salts have been examined in this process. In general, those having several large alkyl substituents are the better performers, consistent with the known partitioning/reaction rate behavior of such catalysts in simple, low viscosity solvents. However, with N-methyl-N,N,N-tri-n-alkylammonium chlorides, the rate decreases as the tri-n-alkyl substituents become very large, e.g., C18H37. This may originate from a slow, diffusion-controlled reaction that is peculiar to the network structure of the swollen vulcanizate.