Any surface coating pigment may require some or all of the following properties:

1) To provide colour.

2) To obliterate previous colour.

3) To improve the strength of the paint film.

4) To improve the adhesion of the paint film.

5) To improve the durability and weathering properties.

6) To improve protection against corrosion.

7) To reduce gloss.

8) To modify flow and application properties.

To choose a pigment to carry out the selected functions, we must know about the following properties of the pigment:

i)                    Tinting strength of a coloured pigment is related to that of some standard pigment of similar hue. Relatively transparent pigments can have high tinting strengths, since it is usually a measure of how much pigment is required to move white to a known shade.

ii)                   Light fastness is a measure of how long a pigment may last. Many pigments fade or darken or change shades badly in the light. This is because the ultra-violet rays in sunlight are sufficiently energetic to break certain chemical bonds and thus change molecules. A change in the chemical structures means a change in the ability to absorb light in the visible region of the spectrum, with consequent loss of colour or variation in hue. If the pigment can absorb ultra-violet rays without breakdown, it will protect the binder with the energy dissipated harmlessly as heat.

iii)                  Bleeding characteristics is the effect when white paint is applied over a red and the white paint turns pink. What happens is that the solvents in the white paint dissolve some of the red pigment in the background coat and carry it into the white overlayer. Organic reds are particularly prone to this fault and theoretically it can occur with any colour.

iv)                 Hiding power - ideally one coat of paint should obliterate any colour, but frequently more are needed and the trend by car manufacturers to cleaner brighter colours makes this more difficult.

The total film thickness of paint applied should be within specification limits, insufficient film thickness leading to poor protection and appearance. Excessive film thicknesses leading to poor mechanical properties, slower drying, more sink age.

The pigment must prevent light from passing through the film to the previous coloured layer and back to the eye of the observer. The pigments do this by absorbing and scattering light.

The hiding power of a paint is often expressed as the number of square metres covered by one litre of paint to produce complete hiding. Hiding power depends largely upon the wavelengths and the total amount of light that the pigment will absorb, on its refractive index and also on its particle size and shape.

i)                    Refractive index is a measure of how lightrays are bent e.g. holding a straight stick in a pond, the stick appears bent because air and water have different refractive index, the light travelling faster in air than in water.

Lightrays may be bent by pigment particles and returned to the eye (explained more fully under heading of colour). The lightrays suffer refraction, diffraction and reflection by transparent particles that have a refractive index differing from that of the film in which they lie.

White pigments are transparent in large lumps, but white in powder form, because they have high refractive indices (2.0 - 2.7), which is greater than that of the film formers (1.4 - 1.6). So a pigment which is largely transparent, can give excellent covering power by use of differences in refractive index.

Extender pigments are also transparent in bulk and white in powder form, but they do not colour paints because their refractive index scarecely differs from those of the film formers.

TiO_2, Titanium Dioxide pigments in particular have excellent hiding power because of their refractive index being higher than the film former.

This is seen when clearcoat is flatted in water, clearcoat appears totally transparent, even when coated with water it is totally transparent. As the clearcoat is then flatted, it appears white in the flatting water because of different refractive properties.

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i)                     Particle size - there is an ideal particle diameter for maximum scattering of light at interfaces and this is approximately equal to the wave-length of the light in the particle. As a rough guide, the optimum diameter is approximately half the wavelength of the light in air i.e. about 0.2 - 0.4um (micrometres). Below this size, the particle loses scattering power, above it the number of interfaces in a given weight of pigment decreases; the hiding power of a transparent pigment is reduced.

Pigments have particle diameters varying from 0.01pm (Carbon Black) to approximately 50µm (some extenders). No sample contains pigment particles all of an identical size, rather there is a mixture of sizes with an average diameter.

Connected with particle size are surface area and oil absorption. Oil absorption is a method of adding to a 100g sample of pigment sufficient oil to turn the mixture into a paste, the amount of oil giving an indication of the surface area of pigment particles.

It is interesting to note that one gram of Titanium Dioxide (particle diameter 0.2 - 0.3µm) has a surface area of 12m² , one gram of silica (particle diamter 0.015 - 0.2µm) has a surface area of 190m² , about the area of a singles court at tennis.

The most important part of the pigment particle is its surface. At the surface are those-chemical groups that will make contact with the chemical groups of the resins. If these groups on the pigment surface attract each other strongly, the pigment particles will tend to cluster, resisting wetting and dispersion and setting up a loosely bound structure of particles in the paint, called flocculation, which will affect its application properties.

In general, the larger the surface area, the more active the pigment will be.

ii)                   Particles shapes for pigments can be spherical, cubic, nodular (rounded irregular shape), acircular (needle or rod like) or laminar (plate like). Since particle shape affects pigment packing, it therefore affects hiding power.

Rod shaped particles can reinforce paint films or poke through the surface reducing gloss. Such rough surfaces may help the next coat to stick more easily, so this type of pigment is more useful in undercoats. Plate shaped particles tend to overlap one another, like tiles on a roof, making it more difficult for water to penetrate the film.

iii)                 Specific gravity (Sg) - this is the weight of a substance in grams divided by its nett volume in millilitres. Titanium Dioxide has a Sg of 4.1 white lead 6.6.  An expensive pigment (per Kg) may prove economical if its specific gravity is low - a little goes a long way.

The Sg of the pigment will affect the weight of the final product, high density pigments producing heavier paints. Extender pigments are not only cheap, but have low specific gravities, that is why they are used to increase pigment volume.

i)                    Chemical reactivity - some pigments (e.g. Zinc Oxide) may react with resin reactive groups (e.g. acid groups) causing the resin to crosslink causing excessive viscosity increase on storage.

On the other hand, some pigments are included particularly because they are reactive, excellent examples are the anti-corrosive pigment chromates of zinc, lead and strontium. When the paint film is permeated by water, these pigments slowly release chromate ions because of their low, but measurable solubilities.

Another anti-corrosive pigment, red lead reacts with oil or alkyd based paints, due to the formation of lead soaps of azelic acid. Both lead and chromate pigments are relatively toxic and the search is on for less toxic alternatives.

ii)                   Thermal stability of the pigment should be known so as to prevent pigment decomposition or to alter its nature, either from the processing methods or from the inservice exposures, which the paint is likely to experience.