Post–by Gautam Shah
Lacquer is a very generic term applied to a variety of coating systems. It indicates a surfacing system, where a film forming substance has been dissolved in or diluted with a solvent. On application the solvent evaporates leaving a thin film on the surface.
Lac or Shellac is a type of resin, produced by insects thriving on the cell sap of certain host plants. It is produced in Eastern parts of India and the SE Asia. The natural resin, as extracted contains internal and external impurities. These are removed mechanically, by melting and filtration processes. Two main forms are commercially available, Button and prills or flakes. Other custom products include coloured sticks for colouring and moulding craft items.
Wax is a natural constituent of Lac forming 2.5 to 5%. Lac Wax was often used, in Floor-polishes and cosmetics, later replaced by Carnauba wax. Lac Dyes were used for silk and wool, than Lac as a resin, in early 19th C before the availability of Synthetic Dyes.
Lac or shellac is bio-degradable, eco-friendly, tasteless and non-toxic material. So ideal for children toys, play-cards, book cover coatings, etc. It is highly inflammable, softens with and so can be moulded. Moulded lacquer is used for making bangles and jewellery embellishments. The moulding process involves heating, combining with additives, colours, kneading, hammering and shaping into a mass like dough.
The lacquer, on evaporation of its solvents changes the phase from Liquid to Solid, but does not convert itself into different substance. Lacquer films are soluble in the (original) solvent. Lacquers are also thermoplastic (will soften with an increase in heat) materials. The film forming substances of lacquers are linear polymers high in polymer weight with good solvent releasing property. Such substances are often very hard and brittle, so require addition of plasticizers to achieve better adhesion, flexibility and durability of the gloss.
Commercially the term Lacquer is applied to Solvent-based Coating systems (against water-based systems) that offer hard, glossy and durable film. In this sense, there are Two basic varieties:
1 Thermoplastics type: Nitro cellulose, Acrylics, Chlorinated Rubber, Vinyls, Epoxy.
2 Thermosetting type: Acrylics, Epoxy Resins, Polyurethanes, Vinyls, Alkyd, Melamine.
Cellulose is the most widely occurring, a natural polymer, available in almost all plants. Primary sources of industrial cellulose are cotton fibres and wood. Cellulose is a polyhydric-alcohol, and forms esters with organic acids. It also forms reaction products with inorganic acids. These cellulosic derivatives used in production of various types of lacquers are classified as:
1. esters of inorganic acids -nitro cellulose
2. esters of organic acids -cellulose acetate
3. others -methyl and ethyl cellulose
Lacquers can be classified into two basic groups:
NC Lacquers and Acrylic or Plastic Lacquers.
Nitro Cellulose Lacquers
NC Lacquers consist of Nitro cellulose as the main raw material to which a variety of plasticizer and resins are added, to provide wide range of film properties. PLASTICIZERS increase elasticity and extensibility of the film. Commonly used Plasticizers are, blown-castor oil, dibutyl phthalate, dioctyl phthalate, dibutyl sebacate, butyl stearate, chlorinated diphenyls. Additions of RESINS increase gloss and adhesion of the film. The resins are coconut and castor oil alkyds, maleic modified ester gum, cyclohexanone (ketonic) resins, acrylic resins, toluene sulfonamides formaldehyde resins.
NC Lacquers are produced by dissolving or diluting lacquer and suitable plasticizers in a mix of solvents. The SOLVENTS act in three ways: Active solvents are (ethyl acetate, butyl acetate), Latent solvents (ethyl alcohol, butyl alcohol, diacetone alcohol), and Diluents (toluene, xylene). A suitable mix of these basic three types of solvents is known as `Thinner‘. A very specific quality of Thinner is required for each application and weather condition.
NC Lacquers do not become yellow with ageing. It has no peeling problem. Lacquers provide very ‘water-white’ (no colouration or tinge) film. NC Lacquers are re-coatable and removable substances.
NC Lacquers are used on wood, metals, metals like gold, silver and copper, plastics, leather, paper, fabrics, as hair-fixers, and Nail-polishes. Lacquer is one of the best surface sealing material for wood products and so forms the primer coat for melamine, etc.
Solvent-based nitrocellulose lacquers (from resin of nitration of cotton (lint) and other cellulose based materials, were used commercially from the 19th C. These lacquers began to replace shellac lacquer and were used as protective or rust inhibitive clear coatings for brass musical instruments, gift items and furniture. These were also used in white-goods products (refrigerators, irons, cooking tops, etc. and in automobiles. These lasted for several decades till arrival of ‘Polymer based lacquers‘.
These materials are called Lacquers because the film is ‘water-white’. Acrylic lacquers of thermoplastic type are characterized by a transparent water white film, very good resistance to decolouration from temperature and UV light, good electrical properties, outdoor durability, excellent resistance to mineral oil-greases, resistance to chemical fumes, resistance to water based alkaline substances.
Acrylic lacquers convertible coating that is on drying covert to a different product, which is non-dissoluble and non-removable. These are used for: wood items, clear coating for brass, copper, and aluminium, as a top coat for final gloss and protection air-crafts, automobiles, toys, auto-parts, industrial components, consumer durables, TV cabinets. These are used as clear-invisible coating for exposed surfaces like bricks, stones and concrete. Acrylic lacquers with soluble dyes form transparent glass coatings. One of the major uses is for internal coatings of food and beverage cans.
Thermosetting Acrylics provide a hard and scratch proof coating, good colour retention, better water and detergent resistance, better gloss and chalking resistance.