The pigment constitutes the solid portion of a wet paint. Pigments are insoluble in the vehicle and are generally heavier than the liquid vehicle portion. They may settle to the bottom of a container upon prolonged standing.
Natural earth pigments are generally much more stable to light than synthetic organic pigments.
The pigment portion of coatings contributes to the following desirable properties:
- Opacity (hiding)
- Corrosion inhibition
- Weather resistance
- Moisture resistance
- Level of gloss and hardness
- Film build and reinforcement
- The Main function of the pigment is to provide opacity (hiding) to obscure the substrate and protect the organic resin from degradation by the sun's ultraviolet light. Organic resins degrade to some extent in sunlight, some much more than others. Titanium dioxide is the pigment most frequently used to impart opacity to white paints and light tints, because it has high opacity. If a coat of paint does not completely obscure a surface, it is usually necessary to apply an additional coat.
- Another function of some pigments is corrosion control. Inhibitive pigments can be very effective in reducing the corrosion that would otherwise occur. Lead and chromate inhibitive pigments were commonly used in paints in the past but are now restricted because of adverse health effects. Examples of lead-chromate-containing pigments, and of those presently used, environmentally acceptable corrosion-inhibitive pigments are listed below:
- Common Inhibitive Pigments
- Relatively Hazardous Relatively Nonhazardous
- Red Lead Zinc Oxide
- White Lead Zinc Phosphate
- Zinc Chromate Zinc Molybdate
- Strontium Chromate Calcium Borosilicate
- Basic Lead-Silico-Chromate Calcium Phosphosilicate
- Zinc Phosphosilicate
- Barium Metaborate
- Pigments may improve adhesion and decrease moisture permeability. Leafing pigments such as aluminum tend to align themselves as parallel plates in the film to effectively increase film thickness by increasing the path that moisture must take to reach the substrate.
- Other things being equal, the greater the resin-to pigment ratio, the glossier will be the coating. The size of pigment particles (fineness of dispersion or grind) of the pigment in the vehicle also affects gloss. Other things being equal, the finer the dispersion, the glossier the cured film.
These cheaper pigments provide very little hiding. The pigment to resin ratio, generally expressed as pigment volume concentration (PVC), can vary widely. There can be no pigment,
or the pigment content can approach a value called the critical pigment volume concentration (CPVC). As this point is approached, there is insufficient binder to wet the individual pigment particles and bond them to the substrate. This may result in a poorly bonded or porous film or one with a mottled appearance.