Anhydrous formation of foamed silicone elastomers using the Piers–Rubinsztajn reaction

Abstract

Elastomeric silicone foams are generally produced by the generation of hydrogen through reaction of Si-H groups with active hydrogen compounds, including water and alcohols, in a process catalyzed by platinum or tin complexes. It can be very difficult to control the rate and magnitude of bubble formation, particularly because of adventitious water. Silicone foams in a variety of densities (0.08–0.46 g/cm3) were obtained using a newly developed Piers–Rubinsztajn reaction by combining α,ω-hydride-terminated poly(dimethylsiloxane) with an alkoxysilane crosslinker such as tetraethyl orthosilicate with catalysis by B(C6F5)3. A single reaction leads both to crosslinking and bubble evolution. The reaction is not significantly impacted by humidity: foams are generated by the release of alkane gases derived from the alkoxysilane crosslinker, typically methane or ethane, rather than hydrogen. It was found that crosslinker reactivity and concentration, and silicone molecular weight, can be used to effectively control bubble nucleation, coalescence, viscosity build and, therefore, final foam density and the formation of open or closed cell foams. Better quality foams normally resulted when hexane, which acts as a blowing agent, was added to the pre-foam mixture. In addition to these advantages, and excellent reproducibility, the Piers–Rubinsztajn reaction benefits from a very fast induction time.