products of the reaction of urea with formaldehyde, which may be converted into cross-linked polymers in subsequent reactions.
The reaction of urea with formaldehyde takes place in a number of steps. Products that form at each step of the overall reaction may be capable of further conversions (methylol ureas) or incapable of entering further reactions (methylene ureas, which are reaction waste products), depending on the ratio of the reagents and the reaction conditions (the pH of the reaction mixture and the temperature). In practice, the synthesis of ureaformaldehyde resins is conducted in such a way as to completely eliminate the possibility of formation of methylene urea.
The molar ratio of urea to formaldehyde is usually 1 : 1.3 to 1 : 1.8. A temperature in the 30°-50°C range or the 70°-120°C range is selected for the preparation process, depending on the intended use of the resin (in the first case, as a binder for molding powder; in the second, as a binder for laminated plastics). The reaction begins in a neutral or weakly alkaline medium at pH 7–8 and ends in an acidic medium (pH 3–6.5). Buffer systems are introduced into the reaction vessel to stabilize the pH.
Urea-formaldehyde resins are produced as aqueous solutions. The resins are modified by partial esterification with various alcohols or by substitution of melamine or phenol for some of the urea to make them soluble in organic solvents, to improve their compatibility with other components of glues and enamels, and to increase the moisture resistance of articles made of the resins. Modified urea-formaldehyde resins are produced as alcohol solutions.
Urea-formaldehyde resins are used in the production of amino-plasts, glues (carbamide glue), and foams (mipora). Modified urea-formaldehyde resins are added to alkyd varnishes and enamels to give them greater hardness and luster and to improve their air-resistance.
REFERENCESTeknologiia plasticheskikh mass. Edited by V. V. Korshak. Moscow, 1972. Page 346.
Entsiklopediia polimerov, vol. 2. Moscow, 1974.
G. M. TSEITLIN