The functions of solvents are:

1.      Dissolve the binder.

2.      Reduce the viscosity.

3.      Adjust the solids content.

4.      Give good application properties.

5.      Evaporate completely after application.

They fall into three groups:

a)      True Solvents – Also known as active solvents, they completely dissolve the binder, different binders require different solvents, alkyds tolerate aliphatic and aromatic hydrocarbons, whereas acrylic requires aromatics and ketones and nitrocellulose and acrylic lacquers.

b)      Latent solvents – Have no real solvent power, but added to true solvents will then increase the total solvency power and can be seen by the effects of alcohols in nitrocellulose and acrylic lacquers.

c)      Diluents which have no solvency for a particular resin, but are often used to reduce solvent or product cost and may also moderate the aggressive qualities of stronger solvents. An excess of diluent such as hydrocarbon in a solvent mixture for nitrocellulose can cause incompatibility and the eventual precipitation of the resin.

What must be remembered is that what is termed a diluent for one resin may be a true or a latent solvent for another resin.

All solvents have evaporation rates ranging in speed from fast to medium to slow. There are solvents which evaporate so fast that they could not be considered useful for use in paint. Similarly others prove much too slow to be useful.

Solvents are also classified as low, medium or high boilers, this being the temperature at which they boil, which may give some indication of the evaporation rate.

The evaporation rate or volatility is critical when determining the drying speed for coatings, whether they be lacquer dry, oxidative or two components paints.

Flashpoint is a feature to be considered, this is the lowest temperature at which a liquid produces sufficient vapour to form an inflammable mixture with air and which can be ignited, under 23°C solvents are considered low flash and highly inflammable, thus needing special regulations for storage and use.

The term solvent is rather loosely used to describe any liquid used in the paint and tells us nothing about the liquid’s ability to dissolve the polymer in the paint.