Selective oxidative devulcanization of hydrosilylation-cured silicone elastomers: enhanced circularity

Oxidative bromination of hydrosilylation cured silicone elastomers occurs much more rapidly than acid-catalyzed depolymerization. The β-effect facilitates SiC cleavage permitting decrosslinking to oils that can be reprocessed into RTV elastomers. In commerce, silicone elastomers are typically cured by platinum-catalyzed hydrosilylation or radical induced crosslinking (high temperature vulcanization (HTV)), both of which lead to crosslinks comprised of 2 or 3 carbon fragments. Alternatively, room temperature vulcanization (RTV) utilizes catalyzed nucleophilic substitution to give elastomers with Si–O–Si crosslinks. At end of life, the best current recycling protocols use aggressive acid/base conditions to regenerate cyclic monomers and mixtures of crosslinkers/fillers, etc . Over longer time periods all three types of elastomers undergo acid-catalyzed depolymerization initiated by HBr derived from bromine. However, we report that oxidative de-crosslinking with bromine of the Pt-cured elastomers occurs in minutes to hours to regenerate long chain silicone polymers, terminated with SiOH groups. It is therefore possible to devulcanize Pt-cured elastomers in the presence of RTV elastomers and reuse the high molar mass materials directly in new RTV elastomers, avoiding complete depolymerization, and thereby increasing the circularity of silicones.