Paint is a term used to describe a number of substances that consist of a pigment suspended in a liquid or paste vehicle such as oil or water. With a brush, a roller, or a spray gun, paint is applied in a thin coat to various surfaces such as wood, metal, or stone. Although it’s primary purpose is to protect the surface to which it is applied, paint also provides decoration.
Samples of the first known paintings, made between 20,000 and 25,000 years ago, survive in caves in
Paint was first used as a protective coating by the Egyptians and Hebrews, who applied pitches and balsams to the exposed wood of their ships. During the middle Ages, some inland wood also received protective coatings of paint, but due to the scarcity of paint, this practice was generally limited to store fronts and signs. Around the same time, artists began to boil resin with oil to obtain highly miscible (mixable) paints, and artists of the fifteenth century were the first to add drying oils to paint, thereby hastening evaporation. They also adopted a new solvent, linseed oil, which remained the most commonly used solvent until synthetics replaced it during the twentieth century.
The twentieth century has seen the most changes in paint composition and manufacture. Today, synthetic pigments and stabilizers are commonly used to mass produce uniform batches of paint. New synthetic vehicles developed from polymers such as polyurethane and styrene-butadene emerged during the 1940s. Alkyd resins were synthesized, and they have dominated production since. Before 1930, pigment was ground with stone mills, and these were later replaced by steel balls. Today, sand mills and high-speed dispersion mixers are used to grind easily dispersible pigments.
Perhaps the greatest paint-related advancement has been its proliferation. While some wooden houses, stores, bridges, and signs were painted as early as the eighteenth century, it wasn't until recently that mass production rendered a wide variety of paints universally indispensable. Today, paints are used for interior and exterior house painting, boats, automobiles, planes, appliances, furniture, and many other places where protection and appeal are desired.
Paint is composed of pigments, solvents, resins, and various additives. The pigments give the paint color; solvents make it easier to apply; resins help it dry; and additives serve as everything from fillers to ant fungicidal agents. Hundreds of different pigments, both natural and synthetic, exist. The basic white pigment is titanium dioxide, selected for its excellent concealing properties, and black pigment is commonly made from carbon black. Other pigments used to make paint include iron oxide and cadmium sulfide for reds, metallic salts for yellows and oranges, and iron blue and chrome yellows for blues and greens.
Solvents are various low viscosity, volatile liquids. They include petroleum mineral spirits and aromatic solvents such as benzol, alcohols, esters, ketones, and acetone. The natural resins most commonly used are lin-seed, coconut, and soybean oil, while alkyds, acrylics, epoxies, and polyurethanes number among the most popular synthetic resins. Additives serve many purposes. Some, like calcium carbonate and aluminum silicate, are simply fillers that give the paint body and substance without changing its properties. Other additives produce certain desired characteristics in paint, such as the thixotropic agents that give paint its smooth texture, driers, anti-settling agents, anti-skinning agents, defoamers, and a host of others that enable paint to cover well and last long.
Paint is generally custom-made to fit the needs of industrial customers. For example, one might be especially interested in a fast-drying paint, while another might desire a paint that supplies good coverage over a long lifetime. Paint intended for the consumer can also be custom-made. Paint manufacturers provide such a wide range of colors that it is impossible to keep large quantities of each on hand. To meet a request for "aquamarine," "canary yellow," or "maroon," the manufacturer will select a base that is appropriate for the deepness of color required. (Pastel paint bases will have high amounts of titanium dioxide, the white pigment, while darker tones will have less.) Then, according to a predetermined formula, the manufacturer can introduce various pigments from calibrated cylinders to obtain the proper color.
Making the paste
Dispersing the pigment
Thinning the paste
Canning the paint
Paint manufacturers utilize an extensive array of quality control measures. The ingredients and the manufacturing process undergo stringent tests, and the finished product is checked to insure that it is of high quality. A finished paint is inspected for its density, fineness of grind, dispersion, and viscosity. Paint is then applied to a surface and studied for bleed resistance, rate of drying, and texture.
In terms of the paint's aesthetic components, color is checked by an experienced observer and by spectral analysis to see if it matches a standard desired color. Resistance of the color to fading caused by the elements is determined by exposing a portion of a painted surface to an arc light and comparing the amount of fading to a painted surface that was not so exposed. The paint's hiding power is measured by painting it over a black surface and a white surface. The ratio of coverage on the black surface to coverage on the white surface is then determined, with .98 being high-quality paint. Gloss is measured by determining the amount of reflected light given off a painted surface.
Tests to measure the paint's more functional qualities include one for mar resistance, which entails scratching or abrading a dried coat of paint. Adhesion is tested by making a crosshatch, calibrated to .07 inch (2 millimeters), on a dried paint surface. A piece of tape is applied to the crosshatch, then pulled off; good paint will remain on the surface. Scrubbability is tested by a machine that rubs a soapy brush over the paint's surface. A system also exists to rate settling. An excellent paint can sit for six months with no settling and rate a ten. Poor paint, however, will settle into an immiscible lump of pigment on the bottom of the can and rate a zero. Weathering is tested by exposing the paint to outdoor conditions. Artificial weathering exposes a painted surface to sun, water, extreme temperature, humidity, or sulfuric gases. Fire retardancy is checked by burning the paint and determining its weight loss. If the amount lost is more than 10 percent, the paint is not considered fire-resistant.
A recent regulation (California Rule 66) concerning the emission of volatile organic compounds (VOCs) affects the paint industry, especially manufacturers of industrial oil-based paints. It is estimated that all coatings, including stains and varnishes, are responsible for 1.8 percent of the 2.3 million metric tons of VOCs released per year. The new regulation permits each liter of paint to contain no more than 250 grams (8.75 ounces) of solvent. Paint manufacturers can replace the solvents with pigment, fillers, or other solids inherent to the basic paint formula. This method produces thicker paints that are harder to apply, and it is not yet known if such paints are long lasting. Other solutions include using paint powder coatings that use no solvents, applying paint in closed systems from which VOCs can be retrieved, using water as a solvent, or using acrylics that dry under ultraviolet light or heat. A consumer with some unused paint on hand can return it to the point of purchase for proper treatment.
A large paint manufacturer will have an in-house wastewater treatment facility that treats all liquids generated on-site, even storm water run-off. The facility is monitored 24 hours a day, and the Environmental Protection Agency (EPA) does a periodic records and systems check of all paint facilities. The liquid portion of the waste is treated on-site to the standards of the local publicly owned wastewater treatment facility; it can be used to make low-quality paint. Latex sludge can be retrieved and used as fillers in other industrial products. Waste solvents can be recovered and used as fuels for other industries. A clean paint container can be reused or sent to the local landfill.
A fluid, with viscosity, drying time, and flowing properties dictated by formulation, normally consisting of a vehicle or binder, a pigment, a solvent or thinner, and a drier, which may be applied in relatively thin layers and which changes to a solid in time. The change to a solid may or may not be reversible, and may occur by evaporation of the solvent, by chemical reaction, or by a combination of the two.
In modern technology, paint is classified in three major categories because of differing performance requirements: architectural paints, commercial finishes, and industrial coatings. A fourth category is artistic media.
Architectural paints are air-drying materials applied by brush or spray to architectural and structural surfaces and forms for decorative and protective purposes. Materials are classified by formulation type as solvent-thinned and water-thinned. The drying mechanism of solvent-thinned paints predominantly may be by solvent evaporation, oxidation, or a combination of the two. Solvent-thinned paints which dry essentially by solvent evaporation rely on a fairly hard resin as the vehicle. Resins include shellac, cellulose derivatives, acrylic resins, vinyl resins, and bitumens. In paints that dry by oxidation, the vehicle is usually an oil or an oil-based varnish. Water-thinned paints may be subdivided into those in which the vehicle is dissolved in water and those in which it is dispersed in emulsion form. Paints with water-soluble vehicles include the calcimines, in which the vehicle is glue, and casein paints, in which the vehicle is casein or soybean protein. Materials formed by emulsion polymerization are described as a latex, and products are called latex paints. See also Drier (paint); Drying oil; Polymer.
Commercial finishes include air-drying or baking-cured materials applied by brush, spray, or magnetic agglomeration to kitchen and laundry appliances, automobiles, machinery, and furniture and used as highway marking materials. Industrial coatings are subdivided by their intended service: corrosion-resistant coatings, high-temperature coatings, and coatings for immersion service. See also Pigment; Surface coating.
A liquid solution of pigment in a suitable vehicle of oil, organic solvent, or water; liquid when applied but dries to form an adherent, protective, and decorative coating. Often categorized according to the solvent used for thinning, for example, water-thinned paint or solvent-thinned paint. Also see acrylic paint, cement-water paint, epoxy paint, latex paint, synthetic rubber-base paint, vinyl paint, water-based paint.
Columbia Encyclopedia: paint,
mixture of a pigment and a binding medium, usually thinned with a solvent to form a liquid vehicle. The term includes lacquer, portland cement paint, printing ink, calcimine, and whitewash. Paint is used to decorate or protect surfaces and is generally applied in thin coats which dry (by evaporation or by oxidation of the vehicle) to an adhesive film. Industrial finishes are usually applied by spraying or immersion and are often hardened by baking. Pigments, finely ground, impart color (including black and white) and affect the consistency, crack resistance, and flow characteristics of paint. They may be manipulated to produce glossy, satin, or flat finishes. Oil paints are pigments dispersed in a drying oil such as linseed oil, castor oil, or tung oil. These oils are diluted with a thinner, usually turpentine; metallic salts that catalyze oxidation of the oil may be added to increase the rate of drying. For water paints, pigment is dissolved in a mixture of water with a binder such as glue or casein, or emulsified in a latex polymer. Latex emulsion paint provides such excellent durability and color retention that it now dominates the paint market. Enamel paints contain varnish and usually dry to a hard, glossy finish. Industrial lacquers (widely used on automobiles and furniture) are valued for rapid drying to a hard finish. The vehicle is commonly pyroxylin in an organic solvent. Baked acrylic finishes have recently become popular for industrial products such as automobiles and appliances.
Paint is used to protect, decorate (such as adding color), or add functionality to an object or surface by covering it with a pigmented coating. An example of protection is to retard corrosion of metal. An example of decoration is to add festive trim to a room interior. An example of added functionality is to modify light reflection or heat radiation of a surface. Another example of functionality would be the use of color to identify hazards or function of equipment and pipelines.
As a verb, painting is the application of paint. Someone who paints artistically is usually called a painter or artist, while someone who paints commercially is often referred to as a painter and decorator, or house painter.
Paint can be applied to almost any kind of object. It is used, among many other uses, in the production of art, in industrial coating, as a driving aid (road surface marking), or as a barrier to prevent corrosion or water damage. Paint is a semifinished product, or intermediate good as the final product is the painted article itself.
Paint can also be mixed with glaze to create various textures and patterns. This process is referred to as faux finish and is quite popular with discerning homeowners, architects and interior designers.
Paint is also used for children's activities, such as finger painting.
There are three primary components to a paint:
Pigments impart such qualities as color and opacity (sometimes inappropriately called 'hiding'), and influence properties such as gloss, film flow, and protective abilities. Pigment can generally be categorized into two main types: Prime or hiding pigments and Inert or extender pigments.
The main modern white hiding pigment is Titanium dioxide. Zinc oxide is a weaker white pigment with some important usages. Color hiding pigments fall also into two main categories, those being Inorganic, mostly duller earth tone colors, and Organic, generally brighter but more expensive colors.
Inert pigments break down into natural or synthetic types. Natural pigments include various clays, calcium carbonate, mica, silicas, and talcs. Synthetics would include calcined clays, , precipitated calcium carbonate, and synthetic silicas.
Hiding pigments, in making paint opaque, also protect the substrate from the harmful effects of ultraviolet light.
Some pigments are toxic, such as the lead pigments that are used in lead paint. Paint manufacturers began replacing white lead pigments with the less toxic substitute, which can even be used to color food, titanium white (titanium dioxide), even before lead was functionally banned in paint for residential use in 1978 by the U.S. Consumer Product Safety Commission.
Titanium dioxide was first used in paints in the 19th century. The titanium dioxide used in most paints today is often coated with silicon or aluminum oxides for various reasons such as better exterior durability, or better hiding performance (opacity) via better efficiency promoted by more optimal spacing within the paint film. Opacity is also improved by optimal sizing of the titanium dioxide particles.
The binder, or resin, is the actual film forming component of paint. It imparts adhesion, binds the pigments together, and strongly influences such properties as gloss potential, exterior durability, flexibility, and toughness.
Note that drying and curing are two different processes. Drying generally refers to evaporation of vehicle, whereas curing refers to polymerization of the binder. Depending on chemistry and composition, any particular paint may undergo either, or both processes. Thus, there are paints that dry only, those that dry then cure, and those that do not depend on drying for curing.
Paints that dry by simple solvent evaporation contain a solid binder dissolved in a solvent; this forms a solid film when the solvent evaporates, and the film can re-dissolve in the solvent again. Classic nitrocellulose lacquers fall into this category, as do non-grain raising stains composed of dyes dissolved in solvent.
Paints that cure by oxidative crosslinking are generally single package coatings that when applied, the exposure to oxygen in the air starts a process that crosslinks and polymerizes the binder component. Classic alkyd enamels would fall into this category.
Paints that cure by catalyzed polymerization are generally two package coatings that polymerize by way of a chemical reaction initiated by mixing resin and hardener, and which cure by forming a hard plastic structure. Depending on composition they may need to dry first, by evaporation of solvent. Classic two package epoxies or polyurethanes would fall into this category.
Latex paints cure by a process called coalescence where first the water, and then the trace, or coalescing, solvent, evaporate and draw together and soften the latex binder particles together and fuse them together into irreversibly bound networked structures, so that the paint will not redissolve in the solvent/water that originally carried it.
Recent environmental requirements restrict the use of Volatile Organic Compounds (VOCs), and alternative means of curing have been developed, particularly for industrial purposes. In , the solvent is evaporated first, and hardening is then initiated by ultraviolet light. In powder coatings there is little or no solvent, and flow and cure are produced by heating of the substrate after application of the dry powder.
The main purpose of the vehicle is to adjust the viscosity of the paint. It is volatile and does not become part of the paint film. It can also control flow and application properties. Its main function is as the carrier for the non volatile components.
Water is the main vehicle for water based paints.
Solvent based, sometimes called oil based, paints can have various combinations of solvents as the vehicle, including aliphatics, aromatics, alcohols, and ketones. These include organic solvents such as petroleum distillate, alcohols, ketones, esters, glycol ethers, and the like. Sometimes volatile low-molecular weight synthetic resins also serve as diluents.
Besides the three main categories of ingredients, paint can have a wide variety of miscellaneous additives, usually added in very small amounts. Some examples include additives to improve wet edge, improve pigment stability, impart antifreeze properties, control foaming, control skinning, etc. Other additives might be thickeners, coalescent solvents, or biocides to fight bacterial growth.
Fillers serve to thicken the film, support its structure and simply increase the volume of the paint. Not all paints include fillers. Pigments that also function as fillers are called simply "pigments"; "fillers" are generally color-neutral and opaque. It is necessary to adjust the resulting off-white color with pigments to give the desired color. Common fillers are cheap and inert, such as talc, lime, baryte, clay, etc. Depending on the paint, most of the paint film may consist of pigment/filler and binder, the rest being other additives.
Besides pigments and dyes, other types of additives include catalysts, thickeners, stabilizers, emulsifiers, texturizers, adhesion promoters, flatteners (de-glossing agents), and the like.
After application, the paint solidifies and becomes tack-free. Depending on the type of binder, this hardening may be a result of curing (polymerization), evaporation, or even phase change brought about by cooling. In oil-based paint, curing takes the form of oxidation, for example oxidation of linseed oil to form linoxin to create a varnish. Other common cured films are prepared from crosslinkers, such as polyurethane or melamine resins, reacted with acrylic polyester or polyurethane resins, often in the presence of a catalyst which serves to make the curing reaction proceed more quickly or under milder conditions. These cured-film paints can be either solvent-borne or waterborne.
Latex paint is a water-based dispersion of sub-micron polymer particles. The term "latex" in the context of paint simply means an aqueous dispersion; latex rubber (the sap of the rubber tree that has historically been called latex) is not an ingredient. These dispersions are prepared by emulsion polymerization. When the water evaporates, the polymer particles coalesce to form a solid film. The polymer itself resists water (and typically some other solvents). Residual surfactants in the paint as well as hydrolytic effects with some polymers cause the paint to remain susceptible to softening and, over time, degradation by water.
Paint can be applied as a solid, a gaseous suspension (aerosol) or a liquid. Techniques vary depending on the practical or artistic results desired.
As a solid (usually used in industrial and automotive applications), the paint is applied as a very fine powder, then baked at high temperature. This melts the powder and causes it to adhere (stick) to the surface. The reasons for doing this involve the chemistries of the paint, the surface itself, and perhaps even the chemistry of the substrate (the overall object being painted). This is commonly referred to as "powder coating" an object.
As a gas or as a gaseous suspension, the paint is suspended in solid or liquid form in a gas that is sprayed on an object. The paint sticks to the object. This is commonly referred to as "spray painting" an object. The reasons for doing this include:
In the liquid application, paint can be applied by direct application using brushes, paint rollers, blades, other instruments, or body parts. Examples of body parts include fingerpainting, where the paint is applied by hand, whole-body painting (popular in the 1960s avant-garde movement), and cave painting, in which a pigment (usually finely-ground charcoal) is held in the mouth and spat at a wall (Note: some paints are toxic and might cause death or permanent injury).
Rollers generally have a handle that allows for different lengths of poles which can be attached to allow for painting at different heights. Generally, roller application takes two coats for even color. A roller with a thicker nap is used to apply paint on uneven surfaces. Edges are often finished with an angled brush.
After liquid paint is applied, there is an interval during which it can be blended with additional painted regions (at the "wet edge") called "open time." The open time of an oil or alkyd-based emulsion paint can be extended by adding white spirit, similar glycols such as Dowanol™ (propylene glycol ether) or commercial open time prolongers. This can also facilitate the mixing of different wet paint layers for aesthetic effect. Latex and acrylic emulsions require the use of drying retardants suitable for water-based coatings.
Paint may also be applied by flipping the paint,, or by dipping an object in paint.
Interior/exterior house paint tends to separate when stored, the heavier components settling to the bottom. It should be mixed before use, with a flat wooden stick or a paint mixing accessory; pouring it back and forth between two containers is also an effective manual mixing method. Paint stores have machines for mixing the paint by shaking it vigorously in the can for a few minutes.
Water-based paints tend to be the safest, and easiest to clean up after using -- the brushes and rollers can be cleaned with soap and water.
It is difficult to reseal the paint container and store the paint well for a long period of time. Store upside down, for a good seal, in a cool dry place. Protect from freezing.
Proper disposal of paint is a challenge. Avoid acquiring excess paint. Look for suitable recycled paint before buying more. Try to find recycled uses for your left over paint. Paints of similar chemistry can be mixed to make a larger amount of a uniform color. Old paint may be usable for a primer coat or an intermediate coat.
If you must dispose of paint, small quantities of water based paint can be carefully dried by leaving the lid off until it solidifies, and then disposing with normal trash. But oil based paint should be treated as hazardous waste, and disposed of according to local regulations.