Types of Paint
There are many different types of paint, each with a different set of characteristics.
Primers. A primer, which is sometimes incorrectly termed an undercoat, is the first coat applied to the substrate, which is the surface which is to be painted. The chief function of the primer is to provide a good bond between the substrate and the finishing coats. Secondary functions of the primer may be to level the surface of the substrate, as in the "primer-surfacers" used on automobiles, or to seal the substrate, as in primers for resinous woods, such as pine, where exudation of natural oils from the wood could affect the finishing coat. Careful formulation to ensure the correct balance between wetting, penetration, and adhesion, and a choice of pigment to differentiate between the primer and finishing coats are important considerations.
Primers may be oil- or water-based. An oil-based primer is normally used only under oil-based top coats, but water-based primers may be used under either type of top coat.
Wood primers for exterior use are commonly oil-based paints with a blend of raw and modified oils with added resin for the solvent. Water-based formulations are also used for exterior as well as interior use. Unless the substrate is of an exceptionally resinous nature, the primer is applied directly to the clean, dry substrate and allowed to dry out for 48 hours (only 1 hour for a water-based paint).
The painting of outdoor surfaces should be done in warm, dry weather. When the temperature is below 7.5°C (45°F), or under high humidity conditions, drying is retarded and condensation on the drying paint is liable to occur. Temperatures above 35°C (9S°F) are also likely to have an adverse effect on drying. While water-based primers are less sensitive to moisture than oil-based materials, they should not be applied to water-soaked surfaces.
House Paints. House paints may be either oil-based or water-based. Regardless of the kind of paint used, it is important to follow the manufacturer's instructions, which will normally cover such points as the preparation of the surface to be painted, thinning of the paint, and conditions of application.
Although in broad terms it is true that the thicker the coat the greater its durability and the greater the protection it provides, there is an upper limit of coating thickness, which if exceeded will produce an inflexible coating that is liable to break away from the substrate. Interior coatings that serve a mainly decorative function need not be as thick as coatings that must stand a more rigorous outdoor environment; however, in both cases, several thin films are more effective than one thick one.
All paint manufacturers recommend a spreading rate, which is usually expressed as the area in square feet covered by a gallon of paint; this determines the film thickness of each coat. Recommended spreading rates take into account the nature of the substrate, the requirements of succeeding coats, and the composition and function of the paint. Adherence to the recommended spreading rate will guarantee optimum performance. Optimum dry film thicknesses for exterior house paints on wood are generally taken to be 0.004 to 0.005 inches. Dry film thicknesses of this order are normally obtainable from one primer coat spread at about 450 sq ft/gal (10.6 sq meters/liter), and two finish coats applied at about 650 sq ft/gal (15.4 sq meters/liter).
Masonry Paints. The use of concrete as a building material created a need for paints that are sufficiently free flowing to cover the relatively rough surface and yet not penetrate too deeply into the pores of the material. These paints must also be water and alkali resistant and adaptable to modern methods of application, such as spraying and rolling. Cement water-based paints consist of portland cement, lime, and often some siliceous aggregate (sand or gravel). About 1% calcium stearate is added to provide a water-repellant film, and titanium dioxide or other pigments are often added.
Water-based paints containing acrylic and polyvinyl acetate resins are probably the most widely used as masonry coatings. Their good resistance to alkalinity and water allow them to be used even on unaged concrete. Paints based on chlorinated rubber and on polyurethane and epoxy resins give tough, durable coatings; they are widely used, particularly on swimming pools. Similar resins are used in "seamless" floorings that often replace linoleum or rubber tiles.
Marine Paints. Marine paints for ships must be resistant to extreme environmental conditions, such as exposure to sunlight, temperature variations, and a salt-laden atmosphere. They must also protect steel- from saltwater corrosion and discourage "fouling"—the growth of marine plants and animals beneath the waterline.
Because the loss of speed caused by' fouling presents a serious economic problem, much work has been done on the development of compounds, which when mixed with paints to be applied below the waterline, will poison any plant or animal growth that attaches itself to the hull. These antifouling compounds, as well as being toxic to marine growth and soluble in sea water, must be compatible with the paint film and capable of being released slowly from the paint over a period of time. By the use of poisons of known toxicity and solubility and the control of paint film structure, compositions are made that will maintain an environment hostile to the growth of microorganisms for about a year.
The earliest of the antifouling paints contained copper or mercury compounds, and these are still used. Research with organometallic compounds has resulted in the manufacture of new substances, such as tributyl tin compounds, which are replacing the older compounds. Antifouling compounds, like other ship paints, are normally applied over anticorrosive primers containing lead or zinc pigment.
Paints for the superstructures, decks, and holds of ships require good anticorrosive properties and the ability to resist abrasion. Red lead in an oil-based paint, the traditional answer to corrosion, suffers the disadvantage of being extremely slow drying—it sometimes takes up to 6 months to dry completely to a hard film. Most marine paints are now based on synthetic resins with anticorrosive pigments. Paint systems with a minimum of four coats applied over a well-prepared surface are standard.
Fire-Retardant Paints. Fire-retardant paints are widely used in ships, offices, hotels, factories, and aircraft. They should not be confused with heat-resistant coatings, which are used on boilers, stoves, and hot pipes, and are designed to withstand continuous exposure to elevated temperatures without igniting or losing their protective or decorative qualities. The fire-retardant coating is designed to reduce the flammability of the surface to which it is applied. The most common paints of this type are nonflammable, intu-mescent coatings that swell or bubble on heating to form an insulating layer between the source of heat and the substrate. Even the best of such coatings will not protect a substrate for long against a full-scale fire. They will, however, provide a delay and slow the spread of the blaze. Among the most successful of the fire-retardant coatings are those containing brominated compounds; in addition to being nonflammable, these compounds release gases that exclude air from the surface and thereby extinguish flames.
Artists' Colors. Oil paints are usually made from pigments in a linseed oil base, although other vegetable oils are occasionally used. Some painters prefer cold-pressed oil, which is obtained without the use of heat. Also the larger particles obtained by hand-grinding pigments are preferred by some artists to the uniform, very small particles obtained by modern mechanical methods. Among the "pop" artists water-based house paints have achieved some popularity.
Artists' water colors consist of powdered pigments bound together with a vegetable gum, usually gum arabic or Senegal gum. The pigments used have changed little in the last 200 years, except for the replacement of toxic substances,, such as Paris green, or very expensive substances, such as lapis lazuli, with safer or cheaper counterparts.
Industrial Paints. Industrial paints are almost invariably based on synthetic resins, including higher polymers, which are organic compounds of a high molecular weight formed of repeating molecular structural units; these units usually form chains with a terminating group at each chain end. As many as 100,000 of these units may be combined in a single molecule. The chains may be linear, branched, or cross-linked; they impart varying degrees of flexibility, hardness, chemical resistance, and abrasion resistance to the films formed from them. Polymers that will give any or all of a set of desired characteristics may be synthesized for particular applications. Unlike paints already described, most industrial paints are dried, or cured, by the application of heat in an oven; this process is called baking, or staving.
"Two-pack" paints are based on epoxy or poly-urethane resins ; one pack contains the resin and the other a chemical curing agent. The two components are mixed before use and solidify within a few hours. Since these materials give hard, durable films, they are used as furniture finishes and for bowling alleys and shop floors.
Industrial paints are commonly applied by spray guns actuated by compressed air or by hydraulic pressure. The spray guns may be automatically controlled with the objects to be coated passing the spray guns on conveyors. A modification of the spray gun is the electrostatic spray, in which the article to be coated is grounded, and the atomized paint particles given a negative electrical charge as they leave the gun.
A recent development in the application of paint is the electrodeposition, or electrocoating, process used in the automobile industry. The article to be coated is immersed in a tank containing a water-based paint of low solids content. An electric current passed from cathodes in the tank to the object to be coated causes a uniform film of paint to be deposited evenly over the most complex shaped articles. Most automobile bodies in the United States and Europe are primed in this way.
A recent development in the curing of paints involves the use of a beam of accelerated electrons ; the electron beam cures the film in seconds, compared to 20 to 30 minutes baking time.