6.1 INTRODUCTION

Solvents and diluents are used to lower the viscosity of epoxy resins systems either to 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 even lower than atmospheric.

Diluents have much lower vapor pressures and generally do not evaporate at ambient conditions. However, they do have a finite vapor pressure, and given the right set of conditions (time, temperature, and pressure) they will vaporize. Two distinct classes of diluents are used with epoxy resins: nonreactive diluents and reactive diluents. Reactive diluents will enter into the crosslinking reaction with the primary resin, and nonreactive diluents will not. Nonreactive diluents primarily act as low-molecular-weight plasticizers for the epoxy composition.

Epoxy adhesive formulations demand a great variety of solvents and diluents with a wide range of evaporation rates, solvent strengths, and dispersion powers. These variations are required due to (1) the many types of epoxy resins, curing agents, and possible organic additives that can be used within a formulation and (2) the many different possible methods that can be used to apply the epoxy to...