Within the binder are dispersed one or more pigments. Primary pigments offer opacity, therefore covering power, colour, and in some cases some other functionality, like anticorrosive protection, antifouling protection etc. There are also the so-called secondary pigments, also called with other names (fillers, extenders). These fillers help the primary pigments develop their requested properties, while keeping the cost at reasonable levels.
Usually pigments are inorganic materials in powder form. Some pigments, as e.g. the most red or blue pigments, are organic. The most usual pigments are:
- Whites: Titanium dioxide, zinc oxide, lithopone
- Blacks: Carbon black, iron oxide
- Blues: Phthalocyanine, milori (Prussian blue)
- Yellows: Iron oxide, lead chromate, organic yellows
- Reds: Iron oxide, organic lakes, molybdenum salts, cadmium salts
- Greens: Chrome oxide, phthalocyanine, combination of yellow and blue pigments
- Metallic: Aluminium, Zinc, Bronze powders and pastes
- Anticorrosive: Lead oxide, zinc chromate, zinc phosphate
- Fillers: Barytes (barium sulphate), calcium carbonate, talc, mica, silica salts, kaolin
Solvents have only a temporary use, as long as the paint is in a liquid form. However, they also offer some more use to the paints, such as the even flow, etc.
The most usual solvents in paint manufacture are:
- Hydrocarbons: White spirit, xylene, toluene, naphtha
- Alcohols: Butanol, isopropanol
- Esters: Ethyl acetate, butyl acetate, etc.
- Ketones: MIBK, ΜΕΚ
- Glycol ethers and others.
The general trend today is towards using the least possible content of volatile organic compounds (V.O.C.) so that the environment is not needlessly burdened. This effort is achieved with different manners.
One way is to use resins soluble in water. This does not necessarily mean that these paints do not harm the environment. In order to make a resin soluble in water, different VOCs are used, such as amines and other additives, which may also harm the environment. A life-cycle assessment (LCA) must be run from cradle to grave, not only for the paints, but for the painted objects overall. Water-dilutable paints in general have not reached an acceptable level of properties, except for the architectural emulsion paints and some electrophoresis paints, mostly used in the automotive industry.
Another class of paints developed in order to lower the VOCs are the high solids paints. The resins for these paints are specially produced to have a lower viscosity while keeping the same properties, resulting in the need of lower amounts of solvents for the production and the application of the paints.
Of course powder coatings, mentioned earlier, need no solvents at all.
Besides the above mentioned raw materials, paints contain also some special ingredients, additives, in smaller percentage. This, however, does not mean that their role is minor for the performance of the paint.
Additives include the driers (catalysts for the oxidation), anti-skin agents, anti-settling agents, surface additives (usually surfactants), wetting agents for the effective mixing of inorganic pigments with the organic binders, anti-foaming agents etc.
There are two general forms of paint application:
- Buildings and constructions which are painted once and may later need some simple maintenance, and
- OEM painting where products are painted, usually in a production line.
Paints can be applied in many ways. The most popular paint application methods are: by brush, spray, airless spray, roller, dipping.
Brush application is the most widely known method. It is mainly used for small areas, mostly for the undercoats, because the brush helps the paint fill the pores of the substrate.
Spray guns are used for larger areas and for fast drying paints, where the paint cannot be well laid on the surface with a brush. Air is blown through a nozzle, and the paint is sucked from the container by the vacuum produced by the Venturi effect.
For application with an airless spray gun, the paint is contained in a pressure vessel. Pressure is applied over the surface of the paint, thus pushing the paint through a jet over the substrate. This method is widely used in industrial maintenance painting, marine paints, high-viscosity paints such as High Build intermediate coatings etc.
Rollers are used to coat large surfaces without edges and corners.
Dipping is used for fast coating, relatively small objects which need to be painted completely. This method is mainly used in production lines.
Another sort of spray gun is the electrostatic paint gun. The paint is made conductive by using special additives. It is then electrically charged before leaving the gun. The substrate is charged with the opposite charge, thus selectively attracting the paint droplets. This method is used in production lines, for painting oddly-shaped objects.
Electrophoresis is used with water based special paints, in a large bath, mostly in industrial production lines such as in the automotive industry.
When studying the coating of a construction or an object, coatings are usually taken as coating systems.
A coating system usually consists of:
- Undercoats (intermediate coats)
- Finish coats
Primers come into direct contact with the substrate and offer adhesion. If the substrate is steel, primers should also provide corrosion protection.
Undercoats offer the suitable to the coating system and also provide the necessary adhesion between the primer and the finish.
Finish coats give to the system the required colour, the final texture and the resistance to the environment where the system will be exposed, whether it is water, chemicals, the weather, sun radiation, abrasion, impact etc.
For a coating system to perform well, a good adhesion to the substrate is of utmost importance. The primer is the most important factor for the good adhesion.
The dry film thickness of each coat is specified by the paint producer. For conventional alkyd, epoxy, polyurethane or chlorinated rubber paints it is usually between 30-40 microns, while for conventional vinyl or acrylic paints even less, usually 20-30 microns.
In order to reduce the cost of application, some paints have been developed, which can be applied in higher thicknesses. These paints are called High Build Coatings. They are usually applied at a dry film thickness of 80-100 microns.
Steel structures need to have a total dry film thickness above 100-150 microns in order to be fully protected for corrosion. In some extreme cases of harsh environments, a higher film thickness may be required, sometimes reaching 300-600 microns.