by Gautam Shah ➔
Coatings are thin surfacing. A coating comes into being mainly through a phase-change of a material. At pre-application-stage a coating material is in various phases such as solid, liquid or vapour, or a combination thereof like: suspension, solution, dispersion, emulsion, thermoplastic compound, thixotropic compound, etc. During application it changes to a Liquid phase, but once applied, it settles down to a heavier phase, usually (but not necessarily) a Solid phase.
Coatings are not exactly considered to be adhered-finishes, because the coating film is formed and (mainly) bonded through a lower to higher phase conversion process.
Coating is a thin Surface Finishing Technique. A coating system has Four main components.
1. Film forming substances
2. Solvents and Diluents
3. Pigments and Extenders
4. Other additives
■ Film forming substances are the prime materials of a coating system. Even if other components are not present, a coating (Clear coating) can still be created. Most of the film forming substances are in liquid form or convert to a liquid phase just prior to application. Film forming substances provide a film that binds all the ingredients, and also develop inter-facial adhesion between the coating and the substrate.
■ Solvents and diluents, impart solubility to specific film forming substances, allow dispersion and adjust the viscosity.
■ Pigments are chief colourants to coloured coatings. In some clear as well as coloured coatings, Dyes (dispersed or dissolved) are added to provide specific tinge (staining) to the coatings. Other colourants include opacifiers, whitening agents, fluorescent phosphorescence agents. These besides providing colour, impart transparency translucency, opacity, fluorescence, phosphorescent and luminance to the film. Extenders or fillers are low refractivity materials, usually white, but not necessarily. These are added to provide bulk to the coating, economise the use of pigments.
■ Additives help in manufacturing, storing, applying, drying and maintenance of a coating. These include surfactant, anti skin agents, wrinkle agents, dryers (metallic compounds that helps in polymerization of resins), thixotropic agents, foaming and de-foaming agents, catalysts, odour suppressants, mildewcide, insecticide, fungicide, plasticizers, anti setting agents, etc.
A clear coating (often called Varnish) has no pigments or extenders. For a specific tinge (transparent staining) dyes or very low proportion of pigments are used.
■ Surface treatments: Coatings are generally capable of forming a film on the relevant surface. But to receive a coating, the surface may require some modification or treatment. Surface treatments form a very important section of coating technology. Surface treatments are specific for the surface to be coated, such as cleaning, roughening, smoothening, etching, etc., or moisture proofing, rust inhibiting, barriers, static arresters, etc. Other surface treatments are designed to facilitate the application or deposition of the surface forming component and its setting or drying.
Coatings are deposited on objects by many different techniques and in variety of conditions. Common coatings are applied at normal environmental conditions. However, high end coatings require conditions that are usually unavailable at construction sites such as exact environment and controlled application, drying and handling techniques.