3.1 INTRODUCTION

Physical chemistry is an important factor, which leads to the exceptionally good performance properties of epoxy adhesives. Physical chemistry deals with the physiochemical and surface chemistry aspects of the system. Here system is defined as the adhesive, its constituents, the substrates, and the interface region of the joint. The physical chemistry can determine the success or failure of an epoxy adhesive application.

These properties have a profound effect on the processing properties of the uncured adhesive and on the end properties of the fully cured product. The properties determined by physical chemistry affect both the cohesive strength of the adhesive film as well as the degree of adhesion to the substrate. They also affect the permanence and durability of the adhesive bond once it is placed into service.

To understand the overall effect of physical chemistry, one must consider the adhesive system in three stages:

1. Uncured

2. As it cures

3. When finally cured

In all these stages the adhesive properties will be determined primarily by the molecular structures of the epoxy resin and curing agent or catalyst employed. Additives and modifiers, of course, will also affect these properties, and they are discussed in later chapters.

All three stages markedly influence overall joint performance. The degree of interfacial adhesion is greatly determined by the first two stages. The final two stages determine the degree of cohesive strength within the cured adhesive film. From a practical standpoint, the last stage is where the ultimate performance of the adhesive or sealant...