Paints are coatings of fluid materials that are applied over surfaces. Paint is any liquid, liquefiable, or mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture to objects. Paint can be made or purchased in many colors—and in many different types, such as watercolor, artificial, etc. Paint is typically stored, sold, and applied as a liquid, but dries into a solid
Components
Binder, vehicle, or resins – The binder, commonly called the vehicle, is the film-forming component of paint. It is the only component that must be present. Components listed below are included optionally, depending on the desired properties of the cured film. The binder imparts adhesion and strongly influences properties such as gloss, durability, flexibility, and toughness.
Binders include synthetic or natural resins such as alkyds, acrylics, vinyl-acrylics, vinyl acetate/ethylene (VAE), polyurethanes, polyesters, melamine resins, epoxy, or oils. Binders can be categorized according to the mechanisms for drying or curing. Although drying may refer to evaporation of the solvent or thinner, it usually refers to oxidative cross-linking of the binders and is indistinguishable from curing. Some paints form by solvent evaporation only, but most rely on cross-linking processes.
Paints that dry by solvent evaporation and contain the solid binder dissolved in a solvent are known as lacquers. A solid film forms when the solvent evaporates, and because the film can re-dissolve in solvent, lacquers are unsuitable for applications where chemical resistance is important.
Paints that cure by polymerization are generally one or two package coatings that polymerize by way of a chemical reaction, and cure into a cross-linked film.
There are paints called plastisols/organosols, which are made by blending PVC granules with a plasticiser. These are staved and the mix coalesces. 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 re-soften or liquefy 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 electrostatic application of the dry powder.
Diluents or Solvent – The main purposes of the diluents is to dissolve the polymer and adjust the viscosity of the paint. It is volatile and does not become part of the paint film. It also controls flow and application properties, and in some cases can affect the stability of the paint while in liquid state. Its main function is as the carrier for the non volatile components. To spread heavier oils (for example, linseed) as in oil-based interior house paint, a thinner oil is required. These volatile substances impart their properties temporarily—once the solvent has evaporated, the remaining paint is fixed to the surface.
This component is optional: some paints have no diluent. Water is the main diluent for water-borne paints, even the co-solvent types.
Solvent-borne, also called oil-based, paints can have various combinations of organic solvents as the diluent, including aliphatic, 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.
Pigment and Filler – Pigments are granular solids incorporated in the paint to contribute color. Fillers are granular solids incorporate to impart toughness, texture, give the paint special properties, or to reduce the cost of the paint. Alternatively, some paints contain dyes instead of or in combination with pigments. Pigments can be classified as either natural or synthetic. Natural pigments include various clays, calcium carbonate, mica, silica, and talc. Synthetics would include engineered molecules, calcinated clays, blanc fixed, precipitated calcium carbonate, and synthetic pyrogenic 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 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.
Additives – Besides the three main categories of ingredients, paint can have a wide variety of miscellaneous additives, which are usually added in small amounts, yet provide a 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.
Additives normally do not significantly alter the percentages of individual components in a formulation.
Functions of paints
- Protects the surface from weathering effects of atmosphere and gases
- Prevents decay of timber and corrosion of metals
- Provides good appearance to the surface
- Provides smooth surface for easy cleaning
Characteristics of paints
- It should have good spreading power
- It should be economical
- It should have a reasonable drying period
- The paint must retain its colour for a fair amount of time
- It should form a hard and durable surface
- It should be resistant to wearing
- It should not affect the health of the workers
Varnishes
Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss sheens by the addition of “flatting” agents. Varnish has little or no color, is transparent, and has no added pigment, as opposed to paints or wood stains, which contain pigment and generally range from opaque to translucent. Varnishes are also applied over wood stains as a final step to achieve a film for gloss and protection. Some products are marketed as a combined stain and varnish.
After being applied, the film-forming substances in varnishes either harden directly, as soon as the solvent has fully evaporated, or harden after evaporation of the solvent through certain curing processes, primarily chemical reaction between oils and oxygen from the air (auto-oxidation) and chemical reactions between components of the varnish. Resin varnishes “dry” by evaporation of the solvent and harden almost immediately upon drying. Acrylic and waterborne varnishes “dry” upon evaporation of the water but experience an extended curing period. Oil, polyurethane, and epoxy varnishes remain liquid even after evaporation of the solvent but quickly begin to cure, undergoing successive stages from liquid or syrupy, to tacky or sticky, to dry gummy, to “dry to the touch”, to hard. Environmental factors such as heat and humidity play a very large role in the drying and curing times of varnishes. In classic varnish the cure rate depends on the type of oil used and, to some extent, on the ratio of oil to resin. The drying and curing time of all varnishes may be sped up by exposure to an energy source such as sunlight, ultraviolet light, or heat. Many varnishes rely on organic oils or resins for their binder in combination with organic solvents; these are highly flammable in their liquid state. In addition, all drying oils, certain alkyds, and many single-component polyurethanes produce heat during the curing process. Therefore, oil-soaked rags and paper can smolder and ignite hours after use if they are bunched or piled together, or, for example, placed in a container where the heat cannot dissipate.
Characteristics of varnishes
- It should render the surface glossy
- It should dry rapidly
- The colour of the varnish must not fade away easily
- It should be tough, hard and durable
- It should not shrink or show cracks after drying
Types of varnishes
- Oil varnishes
- Spirit varnishes
- Turpentine varnishes
- Water varnishes
Distempers
Distempers are paints that are prepared by mixing pigments with water and binders. The main objective of application of distempers is to provide the plastered surfaces a smooth surface. They are mostly cheaper than paints and varnishes.
Distempers consists of base, carrier, Colouring pigments and a size. Distempers are also available in powdered forms which need to be mixed with hot water before use. Examples of distempering: Whitewashing and colour washing.
