You don’t indicate which substrate is on the outside, and this does make a difference in the possible choice of adhesive. Epoxy adhesives were traditionally used for this application, but they should be used with care as they may be too rigid. In addition, if the glass is exposed to daylight, epoxies are somewhat sensitive to UV radiation and may discolor or degrade.

This type of application can often be tricky, particularly because you have to deal with the large differences in thermal expansion coefficients between the glass, the steel, and the adhesive. The ideal adhesive would have its own thermal expansion coefficient midway between that of the glass and the steel, but this is difficult to achieve and cracking of the glass may occur in certain applications. Some degree of flexibility in the adhesive will help to dissipate stresses in the bond line.

If there are no high tensile or shear stresses on the bond, you should seriously consider a silicone adhesive for this type of application. The extreme flexibility of silicones minimizes bond line stresses, and they are usually the best bet if you are looking at extremely low-temperature operating environments.

Tough, thermosetting polyurethanes have also been used very successfully, notably in the bonding of automotive windshields. If the glass is on the outside, modern UV-curing acrylic adhesives have been widely used for this type of application and can be formulated with the right degree of flexibility.

Regarding bond durability outdoors, silicones, polyurethanes, or acrylic adhesives will be very resistant to possible UV degradation and will withstand high humidity conditions well, particularly if you add some silane adhesion promoters to the adhesives to prevent hydrolysis at the adhesive-glass interface. Epoxies will withstand humidity well, but the exposure to UV radiation might be an issue. This is why coatings chemists don’t recommend using epoxies as a topcoat in outdoor coating applications. 

Any views or opinions expressed in this column are those of the author and do not represent those of ASI, its staff, Editorial Advisory Board or BNP Media.