Paints & Coatings

An alkyd is a polyester resin modified by the addition of fatty acids and other components. Alkyds are derived from polyols and organic acids including dicarboxylic acids or carboxylic acid anhydride and triglyceride oils. The inclusion of a fatty acid confers a tendency to form flexible coatings. Alkyds are used in paints, varnishes and in moulds for casting. They are the dominant resin or binder in most commercial oil-based coatings. Approximately 200,000 tons of alkyd resins are produced each year. The original alkyds were compounds of glycerol and phthalic acid sold under the name Glyptal. These were sold as substitutes for the darker-colored copal resins, thus creating alkyd varnishes that were much paler in colour. From these, the alkyds that are known today were developed.

Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. Unsaturated polyesters are produced from multifunctional alcohols and unsaturated dibasic acids and are cross-linked thereafter; they are used as matrices in composite materials. Alkyd resins are made from polyfunctional alcohols and fatty acids and are used widely in the coating and composite industries as they can be cross-linked in the presence of oxygen. Polyesters are widely used as a finish on high-quality wood products such as guitars, pianos, and vehicle/yacht interiors. Thixotropic properties of spray-applicable polyesters make them ideal for use on open-grain timbers, as they can quickly fill wood grain, with a high-build film thickness per coat. It can be used for fashionable dresses, but it is most admired for its ability to resist wrinkling and shrinking while washing the product. Its toughness makes it a frequent choice for children’s wear. Polyester is often blended with other fibres like cotton to get the desirable properties of both materials. Cured polyesters can be sanded and polished to a high-gloss, durable finish.

Solution polymerization is a method of industrial polymerization. In this procedure, a monomer is dissolved in a non-reactive solvent that contains a catalyst or initiator. The reaction results in a polymer which is also soluble in the chosen solvent. Heat released by the reaction is absorbed by the solvent, reducing the reaction rate. Moreover, the viscosity of the reaction mixture is reduced, preventing autoacceleration at high monomer concentrations. A decrease in viscosity of the reaction mixture by dilution also aids heat transfer, one of the major issues connected with polymer production, since most polymerizations are exothermic reactions. Once the desired conversion is reached, excess solvent must be removed to obtain the pure polymer. Accordingly, solution polymerization is primarily used in applications where the presence of a solvent is desired anyway, as is the case for varnish and adhesives. Another application of polymer solutions includes the manufacture of fibers by wet or dry spinning or plastic films.

Many materials are produced via the conversion of synthetic resins to solids. Important examples are bisphenol A diglycidyl ether, which is a resin converted to epoxy glue upon the addition of a hardener. Silicones are often prepared from silicone resins via room temperature vulcanization. Alkyd resins are used in paints and varnishes and harden or cure by exposure to oxygen in the air. The word resin comes from French resine, from Latin resina “resin”, which either derives from or is a cognate of the Greek ῥητίνη rhētínē “resin of the pine”, of unknown earlier origin, though probably non-Indo-European. The word “resin” has been applied in the modern world to nearly any component of a liquid that will set into a hard lacquer or enamel-like finish. An example is nail polish. Certain “casting resins” and synthetic resins (such as epoxy resin) have also been given the name “resin”.

Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomers, and surfactants. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name “emulsion polymerization” is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are prevented from coagulating with each other because each particle is surrounded by the surfactant (‘soap’); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a ‘hairy layer’ around a particle that repels other particles, because pushing particles together would involve compressing these chains.