Room-Temperature-Cured Polydimethylsiloxane Elastomers from Aqueous Dispersion

The observations are consistent with the formation of grafts of hydrophilic silicates to the silanol chain ends at the surface of the PDMS emulsion particles prior to removal of the water. The silicates may be introduced into the system directly or through the alkaline colloidal silica, which contains appreciable amounts of dissolved silicates. Monomeric, or possibly dimeric, silicate grafted to more than one chain end could migrate to the interior of the PDMS emulsion particle to form further crosslinks. Grafts to higher molecular weight silicates and silicate grafts bearing anionic charge would be solvated with water and tend to remain at the oil-water interface. This could provide steric as well as ionic stabilization to the PDMS particles, and the system would no longer respond to electrolytes as a simple, ionically stabilized, hydrophobic colloid but more as a hydrophilic colloid resembling silica. The role of the dioctyltin dilaurate is to interact in the water phase with the water-soluble silicates to render them, in effect, hydrophobic and to transport them to the oil-water interface. There the dialkyltin species can catalyze condensation between silanol on the silicate with silanol on the PDMS chain ends. Alkyltin acylates are known to hydrolyze in aqueous alkaline media, and the product can condense with silanol on the silicate to form silastannoxane intermediates. These are relatively stable to hydrolysis, provided bulky groups are present on tin as in dioctyltin dilaurate.