Solid and Semisolid DGEBA Resins

The higher-MW semisolid (EEW of 225 to 280) and solid epoxy resins (EEW 450) may be blended into lower-MW resins to improve flexibility and decrease reactivity. They also improve adhesion due to the higher concentration of hydroxyl groups along the molecular chain. Ten percent of a higher-MW epoxy resin blended into a conventional liquid epoxy resin (EEW of 190) can significantly improve flexibihty, but a reactive diluent frequently needs to be added to the formulation to counteract the increased viscosity caused by the addition. 

The highest-MW DGEBA epoxy resins are termed phenoxy resins. They are highly linear molecules that are used primarily as thermoplastic coating resins. However, they can be blended with lower-MW epoxy resins for the improvement of specific properties such as flexibility, impact and fatigue resistance, and thermal cycling. Phenoxy resins are sometimes used alone as a thermoplastic hot melt adhesive generally in film form. 

However, one of the greatest advantages of epoxy resin chemistry is that epoxy resins can provide low-viscosity formulations without solvents. As a result, these 100 percent solids systems elude environmental regulations and production precautions that are required for a flammable material. 

Solvents are used, however, for special applications. For example, solvents may be added to reduce viscosity and assist penetration on porous substrates. On certain polymeric substrates, solvents may be added to improve adhesion by assisting the diffusion of the adhesive molecules into the substrate. On nonporous substrates, volatile solvents must be evaporated before cure because the solvent could interfere with the degree of crosslinking, and under certain curing conditions, gaseous bubbles could form in the bond line and degrade joint strength. 

Solid epoxy resins are usually formulated as solvent solutions and blends with lower-MW resins for the production of liquid adhesive systems. However, solid epoxy resins are also often employed in the manufacture of adhesive systems having solid form. There are several forms of solid epoxy adhesives that find application. The most common are supported or unsupported film, powder, and solder stick. Formulations for these adhesives are detailed in Chap. 13. 

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