Science Fair Project Encyclopedia
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. One who paints is called a painter.
Paint is very common and is applied to almost every kind of object. It is a method of producing art, an industrial coating, a driving aid (lane markings ), a preservative (rust-prone steel auto bodies), on interior walls, on exterior surfaces exposed to weather, and myriad other uses.
There are generally three parts to a paint: binder, diluent and additives. However, only one of these components, the binder, is absolutely required. The binder is that part of the vehicle 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. Anything else is an additive.
Typical binders include synthetic or natural resins such as acrylics, polyurethanes, polyesters, melamines, oils, or latex.
Typical diluents include organic solvents such as alcohols, ketones, esters, glycol ethers, and the like. Water is a common diluent. Sometimes volatile low-molecular weight synthetic resins also serve as diluents.
Typical additives include pigments, dyes, catalysts, thickeners, stabilizers, emulsifers, texturizers, adhesion promotors, 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. Other waterborne paints are emulsions of solid binders in water. When the diluent evaporates, the molecules of the binder coalesce to form a solid film. Still other films are fomed by cooling of the binder. For example, encaustic or wax paints are liquid when warm, and harden upon cooling.
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 latex and acrylic pigment suspensions. Milk paints (also called casein), where the medium is derived from milk, were popular in the 19th century and are still available today. Egg tempera (where the medium is egg yolk) 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.
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.
Lead paint: White pigment does not add color, but serves the very important function of increasing opacity and making the paint resistant to UV radiation. For centuries, the primary white pigment in paints was lead white (basic lead carbonate, 2PbCO3·Pb(OH)2), one of the oldest pigments known. The problem with lead white is that it is extremely toxic. See lead.
It wasn't until the middle of the 20th century that white lead was generally replaced by a less toxic substitute, titanium white, which was first used in paints in the 19th century. The titanium white used in most paints today is actually a mixture of titanium dioxide (pure titanium white) and zinc oxide (zinc white). See pigment.
Some newer paints can produce effects where the color changes depending on the angle (orientation) at which it is viewed. Modern U.S. and Canadian currency, 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.
Paint can be applied as a liquid, as a solid, or as a gaseous suspension. Techniques vary depending on the practical or artistic results desired.
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: DO NOT DO THIS with modern paints, they are highly toxic and this might cause death or permanent injury).
Paint may also be applied by flipping or spraying the paint, dripping , or by dipping an object in paint.
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.
Paint is often applied to walls with a roller. 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.
- 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.
- 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.
Ancient painted walls, to be seen at Dendara , 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.
- 20 recipes for homemade paint (text)
- Homemade Paint Recipes for Children
- For a glossary of terms used in the paint industry go to Oil & Colour Chemists' Association and click on Paintopedia.
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