Question: Can you comment on the stresses created by structural adhesive bonding?
Answer: The primary forces are shrinkage of the adhesive on curing and large differences in thermal expansion coefficients between the substrates and the adhesive. High stresses can cause delamination of bonds or damage to the substrates, such as glass cracking.
Shrinkage during curing is a phenomenon that is not always recognized but can be significant. It is caused by the density of polymers being higher than that of the corresponding monomers. For example, acrylic monomers can shrink as much as 10-12% and epoxies 1-5% during curing. The use of non-reactive plasticizers or fillers can lower this shrinkage significantly.
Regarding thermal expansion coefficients, large differences are found between metals and ceramics and organic adhesives (which cure to form plastics). It is not uncommon for plastic material to have up to 10 times the thermal expansion coefficient of a metal or ceramic. Stresses become particularly important when bonding dissimilar materials such as metals to plastics or metals to glass. In bonding steel to glass, for example, 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.
Epoxy adhesives have traditionally been used for this application but should be used with care. I have seen serious cracking of the glass in certain applications, particularly with epoxy adhesives cured at high temperatures. One way to mitigate this effect is to lower the modulus of the adhesive (i.e., make it more flexible and tougher). Some degree of flexibility in the adhesive will help to dissipate stresses in the bond line. Intrinsically flexible adhesives like rubber-toughed acrylics or polyurethanes are very good candidates. ASI
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