The primary function of a diluent in an epoxy resin formulation is to reduce its viscosity to either make it easier to compound with fillers, improve filler loading capacity, or to improve application properties. Solvents, certain curing agents, and flexibilized epoxy resins can also lower the viscosity of epoxy adhesive formulations, but this is not their primary function. The effect of various diluents on the initial viscosity of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin.
Lower viscosity is important in applying the adhesive because it will determine what type of mixers and dispensers will be required and if the epoxy can be trowled, brushed, or sprayed. Lower viscosity is also important in achieving good adhesion in that it allows greater penetration of porous substrates and faster wetting of the micro-roughness on non-porous surfaces. Diluents also increase working life of the catalyzed epoxy system by (1) increasing the time that the mixture's viscosity is below a workable limit and (2) decrease the reactivity of the curing agent primarily because of dilution of the resin.