Paint & Coatings

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, decorate, 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 would be the use of color to identify hazards or functions of equipment and pipelines.

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.

Contents 1 Components 2 Color-changing Paint 3 Application 4 Product Variants 5 External Links

Components

Pigment

Pigments are granular solids incorporated into the paint to contribute color or 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 increase the volume of the paint. Fillers are usually comprised of cheap and inert materials, such as talc, lime, baryte, and clay. 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 and was 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).

Some pigments, such as the lead pigments that are used in lead paint, are toxic. Paint manufacturers began replacing white lead pigments with the less toxic substitute, titanium white (titanium dioxide)—which can even be used to color food—even before the US Consumer Product Safety Commission  functionally banned lead in paint for residential use in 1978.

Binder

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, and oils.

Binders can be categorized according to drying, or curing, mechanism. The four most common are simple solvent evaporation, oxidative cross-linking, 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; the binder forms a solid film when the solvent evaporates, and the film can redissolve 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 submicron 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, in which 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 cross-linking are generally single-package coatings. When these paints are applied, the exposure to oxygen in the air starts a process that cross-links 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. These paints 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.

Volatile Vehicle or Solvent

The main purpose of the vehicle, or solvent, is to adjust the viscosity of the paint. the vehicle 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 it is in the liquid state. The main function of the vehicle is as the carrier for the non volatile components. Volatile substances impart their properties temporarilyâ?”once the solvent has evaporated or disintegrated, the remaining paint is fixed to the surface. This component is optional; some paints have no solvent.

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 solvents. Such solvents are used when water resistance, grease resistance, or similar properties are desired.

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.

Additives

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 have 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.

Color-changing Paint

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. These paints sometimes contain liquid crystals. Thermochromic paints can be used for such applications as  thermometer strips and tapes for 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 reportedly been 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.

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 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 to the surface. The reasons for using the solid form have to do with the chemistries of the paint, the surface itself, and perhaps even the chemistry of the substrate (the overall object being painted). This process is commonly referred to as "powder coating."

As a gas or as a gaseous suspension, the paint is suspended in solid or liquid form in a gas that is sprayed onto an object. This process is commonly referred to as "spray painting." Spray painting might be used for any of the following reasons:

• 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.
• Special stylisitc results can be achieved this way.
• A chemical (typically a solvent) can be sprayed along with the paint to dissolve together both the delivered paint and the chemicals on the surface of the object being painted.

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 method, 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 that 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"). This period is 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. Extending the open time 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.

Interior/exterior house paint tends to separate when stored; the heavier components will settle 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 also have the easiet clean-up—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 leftover 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.

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.
• Varnish and shellac provide a protective coating without changing the color. They are paints without pigment.
• 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.
• 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 paints contain either glass powder or tiny metal flake fragments instead of the color pigments found in standard oil-based paints. Enamel paint is also mixed with varnish to increase shine as well as to assist in the hardening process.
• A glaze is an additive used with paint to slow drying time and increase translucency, as in faux painting and art painting.
• A roof coating is a fluid-applied membrane that has elastic properties that allow it to stretch and return to its original shape without damage. It provides UV protection to polyurethane foam and is widely used as part of a roof restoration system.
• Inks are similar to paints, except they are typically made using finely ground pigments or dyes, 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 visibility conditions.
• Anti-graffiti paints are used to defeat the marking of surfaces by graffiti artists. There are two categories—sacrificial and nonbonding. Sacrificial coatings are clear coatings that allow the removal of graffiti, usually by pressure washing the surface with high-pressure water, thereby removing the graffiti and the coating (hence, the coating itself is "sacrificed"). They must be reapplied afterward for continued protection. Sacrificial coatings are  most commonly used on natural-looking masonry surfaces, such as statuary and marble walls, and on rougher surfaces that are difficult to clean. Nonbonding coatings are clear, high-performance coatings, usually catalyzed polyurethanes, that allow the graffiti very little to bond to. After the graffiti 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 nondrying 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.
• No-VOC paints, which are solvent-free paints that do not contain volatile organic compounds, have been available since the late 1980s. Low-VOC paints, which typically contain anywhere from 0.3% to 5.0% VOCs as coalescent, or coalescing solvent, have been available since the 1960s. 0-VOC paints were introduced to the market place around 2006.