Dielectric Curable Adhesives

Epoxy adhesives should be 100 percent solids so as to eliminate the possibility of gassing and bubbles occurring in the bond line. However, dielectric heating is often used in the furniture industry to drive off water quickly from water-based adhesive emulsions. Water, being a polar material, heats rapidly in a microwave field, as every cook knows. 

Because of the highly polar nature of common epoxy resins and their curatives, most formulations can be cured via a microwave environment. A further advantage is that the uncured material absorbs microwave energy more strongly than that which is cured, so the cure occurs uniformly within the material. 

Microwave curing has been found to increase the glass transition temperature of epoxies and in some instances improve the mechanical properties. It is also possible to control the morphology of toughened epoxies more closely with microwaves.35 

Several studies have shown that a microwave cure cycle can be developed that provides equivalent performance properties to a thermal cure cycle. Table 14.9 shows the processing and performance characteristics of three commercial one-component epoxy adhesives cured via microwave and conventional thermal energy. Certain commercial epoxy adhesives could contain a large number of bubbles due to volatiles present during the cure cycle and the fast rate of cure. Therefore, specifically formulated adhesives for microwave curing may be necessary to optimize performance. 

There are several claims in the literature regarding the maximum speed of microwave curing of epoxy adhesives. Claims have been made of ten- to twenty fold cure time reduction when compared to conventional thermal heating. A more practical estimate, however, is that microwave joining using epoxy-based adhesive will significantly reduce the curing time 

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