Paint
From Wikipedia, the free encyclopedia
- For other uses, see Paint (disambiguation).
Paint is the general term for a family of products used to protect and add color to an object or surface by covering it with a pigmented coating. As a verb, painting is the application of paint. Someone who paints artistically is usually called a painter, while someone who paints commercially is often referred to as a painter and decorator.
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 (lane markings), or as a preservative (to prevent corrosion or water damage). Paint is a semifinished product, as the final product is the painted article itself.
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[edit] Components
There are three primary components to a paint: binder, diluent, and additives. However, only the binder is absolutely required. The binder is the part which eventually solidifies to form the dried paint film. The diluent serves to adjust the viscosity of the paint. It is volatile and does not become part of the paint film. There are various additives, which components that are added to improve some property, such as color opacity and matness, pigment dispersion, or stability. Pigments or dyes are among the most common additives. They give a color to a paint. Pigments may also have the same functions as fillers: increase the thickness and hardness of the film and adjust the coloring power and opacity.
Typical binders include synthetic or natural resins such as acrylics, polyurethanes, polyesters, melamines, epoxy, or oils. Binders can be categorized into three sorts: those that dry, those that cure when they dry, and those that do not depend on drying for curing. Paints that dry contain a solid binder dissolved in a solvent; this forms a solid film when the solvent evaporates, and the film can dissolve in the solvent again. Latex paints, for example, cure irreversibly when they dry, since they undergo polymerization into irreversibly bound networked structures, so that the paint will not redissolve in the solvent. Recent environmental protection requirements discourage the use of evaporating solvents (VOCs), and alternative means of curing have been developed, particularly for industrial purposes. Epoxy coating, for example, is applied by mixing paint and hardener, which cure by forming a hard plastic structure. Such paints do not, strictly speaking, "dry" at all, but harden. In UV curing paints, the solvent is evaporated first, and hardening is then initiated by ultraviolet light.
Typical diluents include organic solvents such as petroleum distillate, alcohols, ketones, esters, glycol ethers, and the like. Water is a common diluent. Sometimes volatile low-molecular weight synthetic resins also serve as diluents.
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 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 emulsion of solid monomers. The term "latex" in the context of paint simply means a water emulsion; latex rubber (the sap of the rubber tree that has historically been called latex) is not an ingredient. When the water evaporates, the monomer undergoes emulsion polymerization 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.
Still other films are formed by cooling of the binder. For example, encaustic or wax paints are liquid when warm, and harden upon cooling.
[edit] Art
- Main article: Painting
Since the time of the Renaissance, siccative (drying) oil paints, primarily linseed oil, have been the most commonly used kind of paints in fine art applications; oil paint is still common today. However, in the 20th century, water-based paints, including watercolors and acrylic paints, became very popular with the development of acrylic and other latex paints. Milk paints (also called casein), where the medium is derived from the natural emulsion that is milk, were popular in the 19th century and are still available today. Egg tempera (where the medium is an emulsion of egg yolk mixed with oil) is still in use as well, as are encaustic wax-based paints. Gouache is a variety of watercolor paint which was also used in the Middle Ages and Renaissance for manuscript illumination. The pigment was often made from ground semiprecious stones such as lapis lazuli and the binder made from either gum arabic or egg white. Gouache is commercially available today.
Poster paint has been used primarily in the creation of student works, or by children.
[edit] Pigment
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- Main article: Pigment
Pigments, usually insoluble powders, are used both to provide color, and to make paint opaque, thus protecting the substrate from the harmful effects of ultraviolet light while also increasing a paint's hiding power.
Some pigments are toxic, such as those used in lead paint. Paint manufacturers replaced lead white with a less toxic substitute, which can even be used to color food titanium white (titanium dioxide) which was first used in paints in the 19th century. The titanium white used in most paints today is often coated with silicon or aluminum oxides for better durability.
Some newer paints—called prism paint—can produce effects where the color changes depending on the angle (orientation) at which it is viewed. Modern U.S. and Canadian banknotes, specifically the newer higher denomination notes, have this effect on them. This effect is produced by having pigment molecules that are long and thin and are meant to dry in a specific orientation, with different ends of the molecule being different colors...
The proper name for the effects of "prism paint," a made up name, is "interference."
[edit] Application
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 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).
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. The reasons for doing this include:
- the application mechanism is air and thus no solid object ever touches the object being painted;
- the distribution of the paint is very uniform so there are no sharp lines
- it is possible to deliver very small amounts of paint or to paint very slowly;
- a chemical (typically a solvent) can sprayed along with the paint to dissolve together both the delivered paint and the chemicals on the surface of the object being painted;
- some chemical reactions in paint involve the orientation of the paint molecules.
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, dripping, 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.
- http://muextension.missouri.edu/xplor/wasteman/wm6001.htm "Safe Use, Storage and Disposal of Paint"
- http://www.epa.state.oh.us/pic/facts/hhwpaint.html "Storage and Disposal of Paint Facts"
[edit] Product variants
- Primer is a preparatory coating put on materials before painting. Priming ensures better adhesion of paint to the surface, increases paint durability, and provides additional protection for the material being painted.
- Wood stain is a type of paint that is very "thin," that is, low in viscosity, and formulated so that the pigment penetrates the surface rather than remaining in a film on top of the surface. Stain is predominantly pigment or dye and solvent with little binder, designed primarily to add color without providing a surface coating.
- Varnish and shellac provide a protective coating without changing the color. They are paints without pigment.
- Lacquer is usually a fast-drying solvent-based paint or varnish that produces an especially hard, durable finish.
- An enamel paint is a paint that dries to an especially hard, usually glossy, finish. Enamel can be made by adding varnish to oil-based paint.
- Fingerpaint
- Inks are similar to paints, except they are typically made using dyes exclusively (no pigments), and are designed so as not to leave a thick film of binder.
- Titanium dioxide is extensively used for both house paint and artist's paint, because it is permanent and has good covering power. Titanium oxide pigment accounts for the largest use of the element. Titanium paint is an excellent reflector of infrared, and is extensively used in solar observatories where heat causes poor seeing conditions.
- Anti-Graphiti paints are used to defeat the marking of surfaces by graphiti artists. There are two categories, sacrificial and non-bonding. Sacrificial coatings are clear coatings that allow the removal of graphiti, usually by pressure washing the surface with high-pressure water, removing the graphiti, and the coating (hence, sacrificed.) They must be re-applied afterward for continued protection. This is most commonly used on natual-looking masonry surfaces, such as statuary and marble walls, and on rougher surfaces that are difficult to clean. Non-bonding coatings are clear, high-performance coatings, usually catalyzed polyurethanes, that allow the graphiti very little to bond to. After the graphiti is discovered, it can be removed with the use of a solvent wash, without damaging the underlying substrate or protective coating. These work best when used on smoother surfaces, and especially over other painted surfaces, including murals.
- Anti-climb paint is a non-drying paint that appears normal while still being extremely slippery. It is usually used on drainpipes and ledges to deter burglars and vandals from climbing them, and is found in many public places. When a person attempts to climb objects coated with the paint, it rubs off onto the climber, as well as making it hard for them to climb.
[edit] History
Cave paintings drawn with red and yellow ochre, hematite, manganese oxide and charcoal may have been made by early homo sapiens as long as 40,000 years ago.
Ancient painted walls, to be seen at Dendera, Egypt, although exposed for many ages to the open air, still possess a perfect brilliancy of color, as vivid as when painted, perhaps 2000 years ago. The Egyptians mixed their colors with some gummy substance, and applied them detached from each other without any blending or mixture. They appeared to have used six colors: white, black, blue, red, yellow, and green. They first covered the field entirely with white, upon which they traced the design in black, leaving out the lights of the ground color. They used minium for red, and generally of a dark tinge.
Pliny mentions some painted ceilings in his day in the town of Ardea, which had been executed at a date prior to the foundation of Rome. He expresses great surprise and admiration at their freshness, after the lapse of so many centuries.
Paint was made with the yolk of eggs and therefore, the substance would harden and stick onto the surface applied.
[edit] See also
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