QUESTION: We have been advised to use an organosilane adhesion promoter in an acrylic adhesive to bond glass to metal. How do organosilanes act chemically in this application? How much water is necessary? What happens if we have an excess of water?

ANSWER: Organosilanes have been used for many years as so-called adhesion promoters, but they are normally used to enhance the durability of adhesive bonds, particularly to glass surfaces. In many cases, organosilanes do not necessarily improve the initial adhesion, but they prevent hydrolysis of the adhesive at the adhesive/glass interface. Organosilanes act as bridging molecules between the inorganic glass surface and the organic adhesive. Most organosilanes consist of a silicon atom with three alkoxy groups and a fourth group, such as vinyl, methacryloxy, or glycidoxy. This organic group is chosen for its compatibility with the adhesive and preferably its ability to copolymerize into the adhesive.

In contact with surface or atmospheric moisture, the alkoxy groups hydrolyze, and the silane forms Si-O-Si linkages with the glass surface. Note that this structure is essentially the same as the chemical structure of glass (e.g., silicon dioxide). Organosilanes are sometimes used as pre-treatments to glass surfaces, but are often just incorporated into the adhesive. You do not need to add extra water for the hydrolysis reaction to take place, but excess should not harm the organosilane. However, you have to be careful with excess water in, for example, a polyurethane system because of possible reaction with free isocyanate groups with the formation of carbon dioxide and subsequent foaming.