Kinetic analysis of organosilane hydrolysis and condensation

Failure at the bond interface could limit the lifetime and safe performance of adhesively bonded equipment. Consequently, there is great interest in increasing the durability of bonds in such stores. One method of achieving this is to pre-treat metal surfaces with a hydrolysed silane coupling agent. Hydrolysed silanes are commonly applied as acidified aqueous solutions. However, there are situations where the current chemistry of application cannot be used, because the application of acidified aqueous solutions is not permissible. Alternative methods to hydrolyse γγ-gylcidoxypropyltrimethoxysilane (GPS) in non-aqueous solutions have been investigated. A 1% solution of silane in 95% ethanol and 4% water hydrolysed at extremely low rates. Addition of glacial acetic acid did not accelerate the reaction significantly. A number of metal acetylacetonate and alkyl tin esters were found to catalyse the hydrolysis reactions. The two best catalysts, dibutyltin dilaurate (DBTDL) and dibutyltin diacetate catalysed the hydrolysis process at rates which were similar to that observed for the GPS hydrolysis in acidified water. Proton NMR spectroscopy identified silanol intermediates, confirming that the hydrolysis of GPS was a stepwise reaction. In the case of DBTDL, the hydrolysis was slow until the DBTDL had hydrolysed to the active catalyst species. Silicon-29 NMR confirmed that, post hydrolysis, the silanols slowly condensed to form oligomeric species.