Silane Adhesion Promoters

Abstract

Silane adhesion promoters are organofunctional silicon compounds that promote adhesion of coatings to substrates, especially improving their resistance to debonding under humid conditions. It must be emphasized that these silanes are not related to polydimethylsiloxane fluids, which cause cratering and poor repaintability in coatings. Silane adhesion promoters may help overcome such problems of poor wetting and intercoat adhesion in coatings. Silane adhesion promoters are generally formulated into primers or added at relatively low levels to coatings, in contrast to silicone resins, which are typically used at about 30% level in silicone-alkyd or silicone-acrylic formulations. These silanes are ambifunctional compounds of the general structure (CH 3 O) 3 Si–R–X, where X is an organofunctional group chosen for compatibility with the organic coating and the alkoxysilane portion provides bonding to mineral substrates. 1 The alkoxysilane may be prehydrolyzed to silanols that are reactive with hydrate metal oxide surfaces and contribute siloxane cross-links. Methoxysilanes also react directly with metal hydroxides and then cross-link in the presence of atmospheric moisture. Silane adhesion promoters are often effective in improving the adhesion of coatings to plastic surfaces or to oily metal surfaces. As additives in paints, they may be useful in improving intercoat adhesion. A recent trend is to formulate adhesion promoters by mixing silane monomers, partially prehydrolyzing the silanes, or by mixing silanes with polymer precursors such as epoxies and melamines. Silane adhesion promoters are offered by silane manufacturers Dow Corning Corporation, Union Carbide Corporation, Petrarch Chemicals Division of Dynamit Nobel, and Peninsular Chemical Research, as well as by proprietary formulators such as Hughson Chemical Division of Lord Corporation. Adhesion promoters