2.1 Introduction

There is no unifying single theory of adhesion on which to accurately model all interactions that take place between the adhesive and the adherend. The existing theories of adhesion presented in this chapter provide methods by which one can rationalize practical observations. They are generally useful in understanding why adhesives stick and why, at times, they fail. Adhesion theories allow us to make predictions and even obtain a qualitative realization of joint strength.

There are several theories of adhesion that have endured the test of time. Each is applicable in certain circumstances, but none are universally applicable. By being familiar with these theories, one can develop a knowledge base and an awareness of how adhesives and sealants work in practical situations.

2.2 Forces Involved in Adhesion

The forces involved in holding adhesives and sealants to their substrates or in holding adhesives and sealants together as a bulk material arise from the same origins. These same forces are all around us in nature. To understand what is happening in an adhesive or sealant joint, we must first understand the forces that bind atoms and molecules together. Although there are many kinds of forces, it is mainly those of a physical and chemical nature that are important in understanding the development of adhesive and sealant joints.