13.1 INTRODUCTION

Epoxy adhesives are most commonly used as liquids or pastes. However, certain types of epoxies can be employed in the form of a solid. The components in these adhesives are mixed and processed to a stage where the resulting adhesive product is in a solid but still fusible (uncrosslinked) state. When the applied solid adhesive is heated, it melts, flows, and wets the substrate. Additional heating time then causes the adhesive to cure completely into a strong, thermosetting structure.

Solid epoxy adhesives can be formulated in various ways. The more common methods are described below.

1. Latent curing agents such as dicyandiamide are dissolved into solvent solutions of solid epoxy resins. This is then followed by evaporation of the solvent.

2. Soluble curing agents are added into liquid epoxy resins and cured until a B-stage condition is reached. The B stage is a solid, thermoplastic stage. When given additional heat, the B-stage epoxy will flow and continue to cure to a crosslinked condition or C stage.

3. Reactive powdered resins and curing agents can be combined by dry blending. These powder blends can then be preformed into various shapes by dry compression pressing.

The main mechanism in all these methods is the physical separation and restriction of molecular mobility of the epoxy resin and the curing agent that are imposed by the solid state of the product. These adhesive systems generally provide a shelf life of up to 6 months at room temperature depending on the reactivity of the curing agent...