Page 81 - Handbook of Adhesives and Sealants

Chapter
2

Theories of Adhesion

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 predic-
tions 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 uni-
versally applicable. By being familiar with these theories, one can de-
velop a knowledge base and an awareness of how adhesives and seal-
ants work in practical situations.

2.2 Forces Involved in Adhesion

The forces involved in holding adhesives and sealants to their sub-
strates or in holding adhesives and sealants together as a bulk ma-
terial arise from the same origins. These same forces are all around
us in nature. To understand what is happening in an adhesive or seal-
ant 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.

2.2.1 Adhesive and cohesive forces
Bond strength is not only the result of adhesion forces. Other forces
contribute to the strength of a joint. For example, molasses may have
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