4.1 INTRODUCTION

Arguably the most important step in any coating process is film formation. This is the conversion of the coating from a liquid into an integral solid film after application so that the coated article can start to be usefully employed. An obvious example from most people's experience would be the painting of a domestic window frame. The first stage of film formation is completed when the window can be touched without the paint transferring to the decorator's finger. The coating is said to be 'touch dry' in this condition. Should the window be closed against its frame at this stage then it is likely that they would become stuck together under the pressure of closure.

The process of film formation must proceed further to avoid this problem and for the coating to behave more like a solid. When this condition is achieved, the coating is said to be 'hard dry' or block resistant under those particular conditions of time, temperature and pressure. For a window manufacturer wishing to stack frames in a hot warehouse, further advancement of film formation would be necessary before unwelcome adhesion or blocking could be avoided.

It is now not quite so obvious what we mean by film formation as a coating becomes 'solid'. It is clear, however, that the rate and extent of film formation are of the highest importance both in terms of the process - how quickly a coated article can be handled, overcoated, or stored,...