a synthetic adhesive produced from the initial materials for polyurethane synthesis. The primary constituents of polyurethan adhesives are aromatic or aliphatic isocyanates, containing not less than two NCO groups per molecule, and hydroxyl-bearing oligomers (for example, oligoesters synthesized from adipic acid and trimethylol propane).
Polyurethan adhesives may contain hardening initiators (small quantities of water, alcohol, or aqueous solutions of salts of organic acids or alkali metals) and fillers (titanium dioxide, zinc oxide, or portland cement). Solvents such as acetone are often added to polyurethan adhesives for better consistency and to control viscosity; synthetic resins, such as perchlorvinyl, are added to increase initial adhesiveness; and surface-active agents are added to improve wetting of the surfaces of the materials being bonded. The components of polyurethan adhesives are mixed at about 20°C immediately before application; the working life of the resultant pastelike compositions varies from 30 min to about 3 hr. The setting time for the adhesive layer, which is applied with a brush or spatula, is several hours to two days at room temperature or 2–6 hr at 60°–120°C. The adhesive compounds may be used at temperatures between –200° and 60°–120°C (up to 150°C or higher in the case of adhesives prepared from polynuclear aromatic isocyanates). They are resistant to oils, fuels, and molds and have good adhesion to metals, plastics, silica and organic glass, wood, and textiles. Thermoset adhesive compounds have exceptionally good mechanical properties, which may deteriorate slightly upon prolonged exposure to water.
Polyurethan adhesives are relatively expensive; certain source materials used in their synthesis (for example, toluene diisocyanates) are toxic. Polyurethan adhesives are used in machine building, aerospace technology, and construction.
REFERENCESEntsiklopediia polimerov, vol. 3. Moscow, 1977.
See also references under ADHESIVES.
A. B. DAVYDOV