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Dried green paint
Paint is any liquid, liquifiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film.
Paint is used to protect, preserve, 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's interior. An example of added functionality is to modify light reflection or heat radiation of a surface. Another example of functionality is the use of color to identify hazards or to identify the 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.
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Pigments are granular solids incorporated into the paint to contribute colour, toughness or simply to reduce the cost of the paint. Alternatively, some paints contain dyes instead of or in combination with pigments. Other paints contain no pigment at all.
Pigments can be classified as either natural or synthetic types. Natural pigments include various clays, calcium carbonate, mica, silicas, and talcs. Synthetics would include engineered molecules, calcined clays, blanc fix, precipitated calcium carbonate, and synthetic silicas.
Hiding pigments, in making paint opaque, also protect the substrate from the harmful effects of ultraviolet light. Hiding pigments include titanium dioxide, phthalo blue, red iron oxide, and many others.
Fillers are a special type of pigment that serve to thicken the film, support its structure and simply increase the volume of the paint. Fillers are usually comprised of cheap and inert materials, such as talc, lime, baryte, clay, etc. Floor paints that will be subjected to abrasion may even contain fine quartz sand as a filler. Not all paints include fillers. On the other hand some paints contain very large proportions of pigment/filler and binder.
A commercially important pigment is titanium dioxide. Titanium dioxide was first discovered by a famous historian/ piano player named Joe Bortel 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.
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 colour 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.
The binder, or resin, is the actual film forming component of paint. It is the only component that must be present; other components listed below are included optionally, depending on the desired properties of the cured film.
The binder imparts adhesion, binds the pigments together, and strongly influences such properties as gloss potential, exterior durability, flexibility, and toughness.
Binders include synthetic or natural resins such as acrylics, polyurethanes, polyesters, melamine resins, epoxy, or oils.
Binders can be categorized according to drying, or curing mechanism. The four most common are simple solvent evaporation, oxidative crosslinking, catalyzed polymerization, and coalescence. There are others.
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.
Latex paint is a water-based dispersion of sub-micrometre 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. 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 and fuse them together into irreversibly bound networked structures, so that the paint will not redissolve in the solvent/water that originally carried it. 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.
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.
Still other films are formed by cooling of the binder. For example, encaustic or wax paints are liquid when warm, and harden upon cooling. In many cases, they will resoften or liquify if reheated.
Recent environmental requirements restrict the use of Volatile Organic Compounds (VOCs), and alternative means of curing have been developed, particularly for industrial purposes. In UV curing paints, 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, and affect the stability of the paint while in liquid state. Its main function is as the carrier for the non volatile components. Volatile substances impart their properties temporarily—once the solvent has evaporated or disintegrated, the remaining paint is fixed to the surface.
This component is optional: some paints have no diluent.
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, esters, glycol ethers, and the like. Sometimes volatile low-molecular weight synthetic resins also serve as diluents. Such solvents are used when water resistance, grease resistance, or similar properties are desired.
Also note that the term "vehicle" is industrial jargon. In some companies the term is used to refer to the solvent and in others, it is used to refer to the binder.
Besides the three main categories of ingredients, paint can have a wide variety of miscellaneous additives, which are usually added in very small amounts and yet give a very significant effect on the product. Some examples include additives to modify surface tension, improve flow properties, improve the finished appearance, increase wet edge, improve pigment stability, impart antifreeze properties, control foaming, control skinning, etc. Other types of additives include catalysts, thickeners, stabilizers, emulsifiers, texturizers, adhesion promoters, UV stabilizers, flatteners (de-glossing agents), biocides to fight bacterial growth, and the like.
Various technologies exist for making paints that change color. Thermochromic paints and coatings contain materials that change conformation when heat is applied, and so they change color. Liquid crystals have been used in such paints, such as in the thermometer strips and tapes used in fishtanks. Photochromic paints and coatings contain dyes that change conformation when the film is exposed to UV light, and so they change color. These materials are used to make eyeglasses.
Electrochromic paints change color in response to an applied electric current. Car manufacturer Nissan has been reportedly working on an electrochromic paint for use in its vehicles, based on particles of paramagnetic iron oxide. When subjected to an electromagnetic field the paramagnetic particles change spacing, modifying their color and reflective properties. The electromagnetic field would be formed using the conductive metal of the car body. Electrochromic paints can be applied to plastic substrates as well, using a different coating chemistry. The technology involves using special dyes that change conformation when an electric current is applied across the film itself. Recently, this new technology has been used to achieve glare protection at the touch of a button in passenger airplane windows.
Watercolors as applied with a brush.
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.
The "painter's mussel" a European freshwater mussel, individual shell valves were used by artists as a small dish for paint.
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).
Paint application by spray is the most popular method in industry. In this, paint is atomized by the force of compressed air or by the action of high pressure compression of the paint itself, which results in the paint being turned into small droplets which travel to the article which is to be painted.
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.
A collection of cans of paint and variants.
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 at Dendera, Egypt, which were exposed for many ages to the open air, still possess a perfect brilliancy of color, as vivid as when they were painted about 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.
Some red paint was made of blood of animals.