Organic coatings are the most widely used coatings on metals and nonmetals. The coating provides a cover on the metal surface to safeguard against atmospheric corrosion. Appearance is also improved. These coatings have poor abrasion resistance and poor resistance to high temperatures. Some widely used organic coatings are discussed in the following.
Types of Organic Coatings
1. Oil paint:
Exterior surfaces are often finished with oil paints as it completely hides the surfaces to which it is applied. Paints require long time to dry. Oil paint contains base, vehicle, pigment, solvent or thinner and drier. This is used on both wood and metal surfaces.
These are produced by dissolving natural or synthetic resins in a drying oil. Natural resins (gums, amber, dammar) and synthetic resins (epoxy. alkyd, silicone, urea, melamine) are dissolved in drying oils such as castor oil, cotton seed oil, corn oil, soyabean oil, etc. Epoxy coatings are widely used for electrical insulation work. Alkyd type of varnishes are quick drying under heat. have good adherence to smooth surfaces and have very high exterior durability (against atmosphere).
These organic finishes are made by adding pigments to varnishes or a combination of a resin and a varnish. These may be air drying type or baking type. The enamel finishes are the most widely used coatings in all metal processing industries because of their good resistance to atmospheric corrosion, chemicals, ease in application, good dielectric and abrasion resistance, variety of colours, high heat resistant (alkyd and silicone types) and long lasting glossy finish.
These are noted primarily for their property of drying in a few minutes. Most lacquers are prepared from nitrocellulose as the main ingredient dissolved in some volatile organic solvent. Suitable pigments are added to have coloured lacquers. At least two coats of lacquers are required to give the protection that one coat of varnish or enamel affords. The main drawback of lacquers is that they do not adhere properly with metal surfaces. For this, metal surfaces are first coated with a non-lacquer primary coating followed by the application of lacquer. Poor exterior durability is another drawback of lacquers. But extremely fast-drying property of lacquers sometimes overweighs its drawback. Vinyl lacquers have properties that make them useful for lining food and beverage containers as this group is impermeable to water, chemical resistant and free from odour, taste and toxicity.
These are obtained by dissolving natural resin such as lac in alcohol and they dry quickly and often used as a sealing coat on wood since it gives a durable film. Lacquers and varnishes can be applied over it as shellac is soluble only in alcohol. The orange coloured shellac obtained by dissolving lac in alcohol can be converted into white shellac by bleaching.
6. Rubber-base coatings:
These are for metal surfaces and include (a) chlorinated rubber, (b) hypalon and (c) neoprene. Chlorinated rubber paint gives a protective coating with high resistance to water, alkalies, acids, mineral oils, grease, etc. It is used on such metal surfaces which are likely to be submerged in water for a long time. Hypalon is the trade name given to chlorosulphonated polyethylene. It gives high resistance to various oxidizing media and hence is used as oxidation resistant coating on metals. It gives good resistance to temperature between -62 and 155°C.
Neoprene finishes are obtained by dissolving neoprene in a solvent. After it is applied on the surface, the solvent evaporates leaving behind a film of neoprene on metal surface. It gives high resistance to salts, alkalies, oils and many acids.
These provide very stable coating (-150 to 70°C). Teflon is most widely used fluorocarbon as a coating. It is an expensive coating. Saw blades, valves and snow removing shovels often carry this coating.
8. Bituminous paints:
Coal-tar paint is an example of bituminous paint obtainees by dissolving coal-tar in a solvent. After its application, the solvent evaporate leaving behind a bituminous film on the metal surface. The coatings give very high resistance to water and thermal insulation.
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