Solvents and diluents are used to lower the viscosity of epoxy resins systems to either permit easy compounding with other ingredients or to aid in application of the adhesive onto a substrate. Both solvents and diluents are low molecular weight liquid compounds that are chemically and physically compatible with epoxy resins and their curing agents. They differ primarily by their vapor pressure.
Solvents have a relatively high vapor pressure and will evaporate given a specific set of environmental conditions. Certain solvents will evaporate quickly at room temperature and atmospheric pressure, and others may require heating to elevated temperatures and pressures that are lower than atmospheric.
Diluents have much lower vapor pressures, and generally do not evaporate at ambient conditions. 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, to improve filler loading capacity, or to improve application properties.