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Gemma Harkrider
Paint is a liquid pigment that, after applied to a solid material and allowed to dry, adds a film-like layer, in most cases to create an image, known as a painting. Paint can be made in many colors and types. Most paints are either oil-based or water-based, and each has distinct characteristics.
For one, it is illegal in most municipalities to discard oil-based paint down household drains or sewers.[citation needed] Clean-up solvents are also different for water-based paint than oil-based paint.[3] Water-based paints and oil-based paints will cure differently based on the outside ambient temperature of the object being painted (such as a house). Usually, the object being painted must be over 10 C (50 F), although some manufacturers of external paints/primers claim they can be applied when temperatures are as low as 2 C (35 F).[4]
In the 13th century, oil was used to detail tempera paintings. In the 14th century, Cennino Cennini described a painting technique utilizing tempera painting covered by light layers of oil. The slow-drying properties of organic oils were commonly known to early European painters. However, the difficulty in acquiring and working the materials meant that they were rarely used (and indeed, the slow drying was seen as a disadvantage[12]). The paint was made with the yolk of eggs, and therefore, the substance would harden and adhere to the surface it was applied to. The pigment was made from plants, sand, and different soils. Most paints use either oil or water as a base (the diluent, solvent, or vehicle for the pigment).
In 1718, Marshall Smith invented a "Machine or Engine for the Grinding of Colours" in England. It is not known precisely how it operated, but it was a device that dramatically increased the efficiency of pigment grinding. Soon, a company called Emerton and Manby was advertising exceptionally low-priced paints that had been ground with labor-saving technology:
In the 21st century, "paints" that used structural color were created. Aluminum flakes dotted with smaller aluminum nanoparticles could be tuned to produce arbitrary colors by adjusting the nanoparticle sizes rather than picking/mixing minerals to do so. These paints weighed a tiny fraction of the weight of conventional paints, a particular advantage in air and road vehicles. They reflect heat from sunlight and do not break down outdoors. Preliminary experiments suggest it can reduce temperatures by 20 to 30 degrees Fahrenheit vs conventional paint. Its constituents are also less toxic.[15]
Thermoplastic mechanisms: Some films are formed by simply cooling the binder. For example, encaustic or wax paints are liquid when warm, and harden upon cooling. In many cases, they re-soften or liquify if reheated.
The paint type known as Emulsion in the UK and Latex in the United States is a water-borne dispersion of sub-micrometer polymer particles. These terms in their respective countries cover all paints that use synthetic polymers such as acrylic, vinyl acrylic (PVA), styrene acrylic, etc. as binders.[21] The term "latex" in the context of paint in the United States simply means an aqueous dispersion; latex rubber from the rubber tree is not an ingredient. These dispersions are prepared by emulsion polymerization. Such 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 binder particles and fuse them together into irreversibly bound networked structures, so that the paint cannot redissolve in the solvent/water that originally carried it. The residual surfactants in paint, as well as hydrolytic effects with some polymers cause the paint to remain susceptible to softening and, over time, degradation by water. The general term of latex paint is usually used in the United States, while the term emulsion paint is used for the same products in the UK, and the term latex paint is not used at all.
Recent environmental requirements restrict the use of volatile organic compounds (VOCs), and alternative means of curing have been developed, generally for industrial purposes. UV curing paints, for example, enable formulation with very low amounts of solvent, or even none at all. This can be achieved because of the monomers and oligomers used in the coating have relatively very low molecular weight, and are therefore low enough in viscosity to enable good fluid flow without the need for additional thinner. If solvent is present in significant amounts, generally it is mostly evaporated first and then crosslinking is initiated by ultraviolet light. Similarly, powder coatings contain little or no solvent. Flow and cure are produced by the heating of the substrate after electrostatic application of the dry powder.[citation needed]
Combination mechanisms: So-called "catalyzed" lacquers" or "crosslinking latex" coatings are designed to form films by a combination of methods: classic drying plus a curing reaction that benefits from the catalyst. There are paints called plastisols/organosols, which are made by blending PVC granules with a plasticiser. These are stoved and the mix coalesces.
Solvent-borne, also called oil-based, paints can have various combinations of organic solvents as the diluent, including aliphatics, aromatics, alcohols, ketones and white spirit. Specific examples are organic solvents such as petroleum distillate, esters, glycol ethers, and the like. Sometimes volatile low-molecular weight synthetic resins also serve as diluents.
Pigments are granular solids incorporated in the paint to contribute color. Dyes are colorants that dissolve in the paint. Fillers are granular solids incorporated to impart toughness, texture, give the paint special properties,[24] or to reduce the cost of the paint. During production, the size of such particles can be measured with a Hegman gauge. Rather than using only solid particles, some paints contain dyes instead of or in combination with pigments.
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 cheap and inert materials, such as diatomaceous earth, talc, lime, barytes, clay, etc. Floor paints that must resist abrasion may contain fine quartz sand as a filler. Not all paints include fillers. On the other hand, some paints contain large proportions of pigment/filler and binder.
Some pigments are toxic, such as the lead pigments that are used in lead paint. Paint manufacturers began replacing white lead pigments with titanium white (titanium dioxide), before lead was banned in paint for residential use in 1978 by the US Consumer Product Safety Commission. The titanium dioxide used in most paints today is often coated with silica/alumina/zirconium for various reasons, such as better exterior durability, or better hiding performance (opacity) promoted by more optimal spacing within the paint film.[25]
Micaceous iron oxide (MIO) is another alternative to lead for protection of steel, giving more protection against water and light damage than most paints. When MIO pigments are ground into fine particles, most cleave into shiny layers, which reflect light, thus minimising UV degradation and protecting the resin binder. Most pigments used in paint tend to be spherical, but lamellar pigments, such as glass flake and MIO have overlapping plates, which impede the path of water molecules.[26] For optimum performance MIO should have a high content of thin flake-like particles resembling mica. ISO 10601 sets two levels of MIO content.[27] MIO is often derived from a form of hematite.
Various technologies exist for making paints that change color. Thermochromic ink and coatings contain materials that change conformation when heat is applied or removed, and so they change color. Liquid crystals have been used in such paints, such as in the thermometer strips and tapes used in aquaria and novelty/promotional thermal cups and straws.
Color-changing paints can also be made by adding halochromic compounds or other organic pigments. One patent[29] cites use of these indicators for wall coating applications for light-colored paints. When the paint is wet it is pink in color but upon drying it regains its original white color. As cited in patent, this property of the paint enabled two or more coats to be applied on a wall properly and evenly. The previous coats having dried would be white whereas the new wet coat would be distinctly pink. Ashland Inc. introduced foundry refractory coatings with similar principle in 2005[30][31] for use in foundries.
Electrochromic paints change color in response to an applied electric current. Car manufacturer Nissan has been reportedly working on an electrochromic paint, 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.[32] 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. This new technology has been used to achieve glare protection at the touch of a button in passenger airplane windows.
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, new water-based paints such acrylic paints, entered the market 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 common in the 19th century and are still used. Used by the earliest western artists, Egg tempera (where the medium is an emulsion of raw egg yolk mixed with oil) remains in use as well, as are encaustic wax-based paints. Gouache is an opaque variant of watercolor, which is based around varying levels of translucency; both paints use gum arabic as the carrier and water as a thinner. Gouache is also known as 'designer color' or 'body color'.
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