All paints and products made for the surface coating industry broadly consist of three parts
- Pigments and extenders
- Vehicle or medium
- Metal Protective Coatings
plus other miscellaneous raw materials included for special purpose such as driers, anti-settling agents, thickeners and fungicides.
1. Pigments and Extenders
- Pigments are white or coloured powders, consisting of inorganic and organic chemical compounds, obtained either naturally (from the earth) or synthetically (from chemical factories), and they perform a variety of functions depending for what purpose they are used. All pigments provide opacity and colour and by careful blending, an extremely wide range of colour can be obtained. Certain neutral pigments impart durability against weather and other chemically active pigment form water resistant, and hence corrosion resistant when incorporated in the correct medium or binder. Commonly used pigments are titanium dioxide (white) oxides or iron (red, yellow and black) lead and zinc chromes (yellow, orange and red) carbon black and a variety of organic chemicals which provide blues, greens, reds and yellows.
- Extenders are generally inert inorganic minerals in fine powder form such as clays, chalk and other natural deposits which, when incorporated in paint become transparent due to their low refractive power. Hence, the name ‘extender’ or ‘filler’ as they are sometimes known, as their main purpose is to extend the contents of the paint to produce ‘body’, adjust the gloss of the finish (the more extender the lower the gloss) and, generally, to provide a useful cheap ingredient invaluable to the paint chemist. Commonly used mineral extenders are China Clay (Aluminium Silicate), Calcium Carbonate and Barytes (Barium Sulphate).
2. Vehicle or Medium
- Commonly known as the binder this part of paint is a liquid constituent, serving to bind together the pigments and extenders, which after application dries and solidifies by one or all of three methods – evaporation, oxidation and polymerisation (chemical reaction).
- In oleo-resinous paints such as enamels, primers, etc. the binder can consist of a wide variety of raw materials such as Linseed Oil, Varnishes made from vegetable oils with either natural resins (resin and copals from trees and fossils) or, now more commonly, synthetic resins (phenolic resins, ester gums and alkyds – all of which are complex chemical derivatives).
- In water based products such as emulsion paints, the binder can consist of an emulsion of oil in water (glue solution) or a dispersion in water of a synthetic resin such as styrene butadiene, acrylic or vinyl resins. There are all polymer resin and products of complex organic chemical reactions.
3. Metal Protective Coatings
- The name is self explanatory and the purpose of the solvent present in a paint formulation is to dissolve and thin the binder, and therefore the paint, to render the consistency suitable for application by whatever means is chosen – either brush, roller, spray or even dipping.
- In oil or varnish paints white spirit is by far the most common solvent but in more specialized products many other types of thinners and diluents are used including alcohols, ketones, acetates and a wide range of aromatic compounds. Another extremely well known paint thinner is, of course, water which is used in the manufacture of emulsion paints.
- Evaporation of the solvent from the applied paint film is the first stage in the drying process when the surface becomes sticky, rather than wet, and the process then proceeds chemically until the coat of paint is hard.