The Chemistry of Paint

Paint has a wide range of applications and the composition is essential to it having the desired properties. The colour comes from pigments, which often are made from transition metals.

Composition

There are 3 main components making up paint, these are:

Pigment – giving the paint its colour
Binding Medium – a polymer to hold pigment in place
Solvent – disolves the binding medium and reduces viscosity to make it easier to apply

Paint is a colloid, this means it contains small particles dispersed within another substance, but are not dissolved. In this case, the pigment is dispersed in the solvent, water or an organic solvent. They will not separate easily because the pigment particles are so small.

Water and oil based paints

Water-based paints are also known as emulsions, they use water as the solvent. They usually dry much quicker than oil-based paints as they only require the water to evaporate for this process to be complete.

Oil paints use an organic substance such as naptha. They dry when the solvent evaporates but then the oil oxidises to leave a protective layer

Colours

The pigment is the chemical added to give the paint its colour. A wide range of chemicals can be used as pigments in paint but often transition metal salts are responsible for providing different colours, for example:

Titanium dioxide – white
Iron oxide – red or yellow
Chromium oxide – green

These colours are made in transition metals due to the metal ions absorbing certain wavelengths of light. Thetransition metals have partially filled d-orbitals and this is key in producing the bright coloured salts. When they bond with other atoms, the electrons in the d-subshells are repelled and some of these electrons raise in energy.

Not all d-subshell electrons will rise to a higher energy level, when white light is shone through some of this energy is then used to promote the other electrons into the higher energy state. The energy required to do this corresponds to particular wavelengths of light, which are then absorbed. This is what causes different colours to be produced.

Some transition metals will produce a range of different colours, such as iron. This is because they can be in different oxidation states (meaning they lose more or less electrons). This will alter the way the d-orbital splits and therefore the energy of light absorbed.