Polyester Varnish

varnish prepared from unsaturated oligoesters, mainly polymaleinates. Unlike most other varnishes, polyester varnishes contain reactive solvents, such as styrene, that copolymerize with the oligoester when the varnish is applied to a surface, forming an insoluble (irreversible) film 300–400 microns (μ) thick. Copolymerization is possible upon addition of an initiator-catalyst system (for example, cumene hydroperox-ide-cobalt naphthenate) to the polyester varnish, as well as upon exposure to ultraviolet rays in the presence of a photoinitiator such as benzoin or to a fast electron beam with an energy of 300–500 kilo electron volts. The coatings harden at room temperature in 3–24 hr in the first case, in 2–12 min in the second, and in no more than 2 sec in the third case.

The components of polyester varnishes usually include the following: (1) inhibitors, such as pyrocatechol and hydroqui-none, which prevent copolymerization during preparation; (2) thickeners, such as Aerosil silica or cellulose esters, which prevent the varnish layer from dripping down vertical surfaces; and (3) inert solvents, such as ketones and acetates, which are used to control the viscosity of the solution. Specific components of polyester varnishes containing styrene are additives such as paraffin, which form a surface film in the varnish layer; the film retards styrene volatilization and shields the coating from atmospheric oxygen, which inhibits copolymerization. Coatings with such additives require prolonged grinding and polishing. Additives are omitted when using solvents that polymerize upon exposure to oxygen, for example, triethylene glycol dimeth-acrylate.

Polyester varnishes are usually applied to the surfaces to be protected by pressure atomization or by means of varnish spreaders. Polyester varnish films can withstand temperature variations from –40° to 60°C but have low weather resistance. They take on a mirrorlike luster after polishing. Polyester varnishes, as well as pigmented materials based on them, such as polyester enamels and putties, are used mainly in the finishing of wood products (furniture, musical instruments, and television and radio cabinets).

Solutions of oligoester acrylates in inert organic solvents (for example, acetone), which form coatings 50–60 μ thick as a result of solvent volatilization and copolymerization of the oligoester acrylate molecules, are a special type of polyester varnish. Certain enamels based on these polyester varnishes are used to produce weatherproof coatings for metal articles.