a varnish prepared from source materials used in the synthesis of polyurethane.
Polyurethan varnishes are divided into double- and single-packed types. Double-packed varnishes are usually composed of two solutions, one with a hydroxyl-containing oligomer (an oligoether or oligoester or an epoxy or alkyd resin), and the other with a low-molecular-weight product of the reaction of an excess of a diisocyanate and a dihydric or trihydric alcohol (for example, toluene-2,4-diisocyanate and diethylene glycol). The solution is mixed immediately before use to avoid gelation of the varnish, whose components react quickly at room temperature.
Single-packed polyurethan varnishes contain a solution of an oligoester and a “blocked” or “hidden” isocyanate (for example, a product of reaction of the isocyanate with phenol), which reacts with the oligoester only at high temperatures; the blocking agent undergoes volatilization during the process. Solvents and thinners for polyurethan varnishes include ketones, esters (primarily acetates), and aromatic hydrocarbons that contain no traces of water or alcohol; tertiary amines and organic acid salts catalyze the hardening process of the coatings.
Polyurethan varnishes are applied to the surface to be protected by spraying (including spraying in an electric field) or electrodeposition. Film coatings of single-packed systems are dried at 150°-350°C, whereas those of double-packed systems are dried at ordinary temperatures (sometimes in a humid atmosphere) or at 80°-120°C. The coatings are insoluble (irreversible) and are distinguished by good adhesion to metal, wood, plastics, leather, textiles, concrete, and plaster and by high electrical insulating properties and abrasion resistance. They are stable in fresh and salt water, inorganic acid vapors, and hydrocarbon solvents, and they retain their luster for up to several years; pigmented materials, such as polyurethan enamels, also retain their color tone. Polyurethan coatings are used to protect chemical and radio engineering equipment, ship and airplane parts, concrete structural components, floors, furniture, and sporting goods.
REFERENCESEntsiklopediia polimerov, vol. 3. Moscow, 1977.
See also references under VARNISHES.
M. M. GOL’DBERG