16.3 Plastics Substrates in General

Plastics are usually more difficult to bond with adhesives than are metal substrates. Plastic surfaces can be unstable and thermodynamically incompatible with the adhesive. The actual bonding surface may be far different than the expected surface. The plastic part may have physical properties that can cause excessive stress in the joint. The operating environment can change the adhesive-plastic interface, the base plastic, the adhesive, or all three.

However, even with these potential difficulties, adhesive bonding can be an easy and reliable method of fastening one type of plastic to itself, to another plastic, or to a non-plastic substrate. Pocius, et. al., provides an excellent treatise on the use of adhesives in joining plastics.41 Many thermoplastic materials can also be solvent or thermally welded. In these cases, the resin from the plastic part being bonded acts as the adhesive.

The physical and chemical properties of both the solidified adhesive and the plastic substrate affect the quality of the bonded joint. Major elements of concern are the thermal expansion coefficient, modulus, and glass translation temperature of the substrate relative to the adhesive. Special consideration is also required of polymeric surfaces than can change during normal aging or on exposure to operating environments.

Significant differences in thermal expansion coefficient between the substrate and the adhesive can cause severe stress at the interface. This is common when plastics are bonded to metals because of the order of magnitude difference in thermal expansion coefficients. Residual stresses are compounded by thermal...