GB686595A - Improvements in or relating to heat-curing adherent resins and resin films - Google Patents

Abstract

A stable liquid film-forming composition is made by dissolving a heat-advancing phenolformaldehyde resin in a volatile solvent and adding additional formaldehyde to form an unstable, highly heat-reactive mixture (1), separately dissolving in a volatile solvent a polyvinyl acetal containing free hydroxyl groups and having a preponderance of acetal groups of at least four carbon atoms (2), and combining (1) and (2) to form a solution of 25 to 50 parts of the polyvinyl acetal, the additional aldehyde in (1) being stoichiometrically equivalent to 50 to 175 per cent of the free hydroxyl groups in (2). A suitable phenol-formaldehyde resin is made by heating equimolar amounts of phenol and formalin with barium hydroxide as catalyst at 130 DEG to 140 DEG F. until the free formaldehyde is reduced to 5 to 6 per cent. Solid carbon dioxide is added to inactivate the catalyst and the resin dehydrated by heating at 200 DEG to 215 DEG F. After cooling aqueous sodium hydroxide is added and the reaction checked by addition of ethylene glycol monoethyl ether when the viscosity had risen to 400-600 centipoises at 80 DEG C. Suitable solvents are a mixture of isopropyl alcohol, diacetone alcohol and water or of diacetone alcohol and ethylene glycol monoethyl ether. A suitable polyvinyl acetal is polyvinyl butyral containing 7 per cent free hydroxyl but no residual acetate groups. The compositions may be used for bonding metal and wood and asbestos facings to metal clutch plates, as adhesives for paper or cloth abrasive belts, and as electrically insulating tape for cables and coils. They may be formed into flexible, elastic films which on heating to 200 DEG F. become non-blocking and may be solvent-activated with acetone and may be cured at 125 DEG to 180 DEG C. to form hard tough cherry-red products having a power factor at 60 cycles per sec. not greater than 0.007.