16.4 Thermosetting Plastic Substrates

Thermosetting plastics cannot be dissolved in solution and do not have a melting temperature since these materials are crosslinked. Therefore, they cannot be heat or solvent welded. In some cases, solvent solutions or heat welding techniques can be used to join thermoplastics to thermoset materials. However, most thermosetting plastics are not particularly difficult to bond with adhesive systems. They are generally bonded with many different types of adhesives such as epoxies, thermosetting acrylics, and urethanes. Since thermosetting parts are often highly filled and rigid, a flexible adhesive is not so important as one that can resist the service environment and provide practical joining processes.

A list of typical tradenames and suppliers was given in Table 15.1 for the thermoset plastics discussed in this section. In general, unfilled thermosetting plastics tend to be harder, more brittle, and not as tough as thermoplastics. Thus, it is common practice to add filler to thermosetting resins. These fillers can affect the nature of the adhesive bond (either positively or negatively) and are a possible source of lot-to-lot and supplier-to-supplier variability.

Thermosetting plastics, being chemically crosslinked, shrink during cure. Sometimes the cure is not entirely complete when the part is bonded. In these cases, continued cure of the part during the bonding operation or even on aging will result in shrinkage and residual stresses in the joint. Depending on the nature of the cross-linking reaction, volatile byproducts could also generate due to post curing of the part and provide materials for...