5.1 INTRODUCTION

The performance of a coating is what gives it its value. The term performance covers a great variety of effects but whatever it means, and this is the subject of the present chapter, the performance of a coating film is sensitive to its chemistry, its state after film formation and its end-use. Implicit in the performance property is an estimate of the service life of the coating.

Coatings fail because of irreversible changes which occur in the film as a result of exposure to a variety of possible stresses. The changes in a coating leading to failure are almost exclusively physical and may include phenomena such as gloss loss, colour change, dirt retention, chalking, cracking, delamination, blistering, fouling and corrosion. The stresses that give rise to the failure may be large and short lived ( e.g. impact) or small and long lived, often cyclic in nature. Moreover, the changes in the physical properties of the coating are often a consequence of underlying chemical processes occurring in the coating. In order to extend the service life of a coating, emphasis should be put on defining and understanding the mechanisms by which the controlling chemical changes occur. Without this knowledge of mechanism, it will not be possible to define or measure the controlling parameters. Unfortunately, while there are many techniques to study the 'damage' to coatings due to these chemical changes, at present there is no clear link between the chemical damage and the...