Ask Dr. Dave
Question: I need to bond glass to steel but I’m worried about thermal stresses on the bond lines and long-term durability in an outdoor environment. What type of adhesives would you recommend?
Answer: This type of application can often be tricky because you have to deal with large differences in thermal expansion coefficients between the glass, the steel and the adhesive. An ideal adhesive would have its own thermal expansion coefficient midway between that of the glass and the steel, but this is difficult to achieve. Epoxies are the traditional adhesives used for this application, but they should be used with care. I have seen serious cracking of the glass in certain applications, particularly with epoxy adhesives cured at high temperatures. 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. The extreme flexibility of silicones minimizes bond line stresses, and they are usually the best bet if you are looking at low-temperature operating environments.
Tough thermosetting polyurethanes have also been used successfully in the bonding of automotive windshields. 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 and acrylic adhesives are very resistant to possible UV degradation, and will withstand high-humidity conditions well, especially if you add some silane adhesion promoters to the adhesives to prevent hydrolysis at the adhesive-glass interface. Epoxies withstand humidity similarly well but are somewhat sensitive to UV radiation; this is why coatings chemists don’t recommend them for use in outdoor coating applications.