The adsorption of alkoxysilanes on oxidised aluminium substrates

As part of an international project on organosilane adhesion promoters, the interaction of two commercial alkoxysilanes (gamma-glycidoxy propyl trimethoxy silane (GPS) and gamma-amino propyl triethoxy silane (APS)) with oxidised aluminium surfaces has been investigated. In the case of GPS, studies have been undertaken to establish the effect of the water content (0-10 v/v%) in the methanolic solution on the adsorption process. XPS has been employed to construct adsorption isotherms and it is shown that the uptake of this molecule increases with the water content. Results have been interpreted in terms of lack of water (less than the stoichiometric amount required for hydrolysis of the silane molecule) or slow kinetics of the hydrolysis reaction. It is shown that the mono-hydrolysed molecule yields a diagnostic radical cation at m/z = 222 in the ToF-SIMS spectra of these specimens. The variation in the intensity of this ion provides a useful information about the progress of the hydrolysis reaction. APS has been used for preliminary studies into the effect of rinsing on the nature of the silane layer deposited. The results show very different adsorption behaviour depending on the rinsing protocol employed (that is water, methanol or no rinsing), and in the thickness of the film which ranges from 2 to >5 nm, and chemistry of the deposited layer. Rinsing in water of the APS film leads to the formation of a polysiloxane-like layer, as a result of the rapid hydrolysis, and subsequent polymerisation, of APS molecule. Evidence is given that, contrary to other observations, this polymer does not originate from any source of contamination. (C) British Crown Copyright 1998/DERA Published with permission of the Controller of Her Britannic Majesty's Stationary Office.