7.1 INTRODUCTION

We have seen that the presence of organic solvents in coatings is something of a liability, as they impart flammability, toxicity and cost yet are (almost) entirely lost from the final coating. It is little wonder that a generation of coatings chemists has busied itself trying to reduce their level or remove them altogether.

There are broadly two strategies for solvent removal, as distinct from replacement with a more benign liquid such as water ( cf. Chapter 8). Either the binder can be made more fluid, so that the application viscosity can be reached with less solvent or none at all, or it can be made more solid, so that it can be applied as a powder. In both cases, severe demands are placed on its design. Binders for high solids liquids have to be highly fluid, but still cure to an intractable solid, whilttst those for powder coatings must be solid enough to retain their particulate identity at high ambient temperatures, yet be fluid enough to coalesce into a smooth film on stoving.

It is also possible to identify an intermediate position, where solid particles are dispersed in a liquid that may or may not be organic. The viscosity of the composition is in that way decoupled from the viscosity of the binder, as explained in Chapter 8, and high molecular weight materials can be used. However, the geometry of particle packing ensures that solids of above about 70% (including any...