The action of tin compounds in condensation‐type RTV silicone rubbers

Abstract

Abstract The vulcanization of RTV (=room temperature vulcanizing) silicone rubbers under the action of organotin compounds is investigated. The mechanism for the cure is inferred from the chemical properties of a variety of relevant organotin compounds and is illustrated with the reactions that occur when dialkyltin dicarboxylates ( A ) are used as catalysts. The resulting organotin hydroxide ( B ) is shown to be the actual catalyst. With tetraethoxysilane ( 2 ) it forms organotin silanolates ( D ). Silanolysis of this compound results in the formation of a new siloxane linkage and recovery of the actual catalyst. Alkoxysilanes in which part of the alkoxy groups is replaced by siloxy groups, appear to be more reactive than the initial alkoxysilanes. At elevated temperatures side‐reactions like condensation of silanols with alcohols under the action of organotin compounds become increasingly prominent. Heating during cure effects, that part of the silanols becomes blocked by alkoxy groups. The mechanical properties of the resulting rubbers are usually far from optimum. During heating of cured rubber in a confined atmosphere, i.e., in the presence of the alcohol formed during cure, condensation of silanols with alcohol occurs too. Because these are equilibrium reactions, this process finally leads to complete degradation of the rubber.