Phenolic Resin Adhesive

any of the adhesives based on novolac or resol phenolic resins. Phenolic resin adhesives are manufactured as liquid compositions and films. The liquid adhesives, which are the most common, are resin solutions (usually in alcohol) sometimes containing a hardener (for novolac resins), for example, paraformaldehyde and hexamethylenetetramine, or a hardening catalyst (for resol resins), for example, organic sulfonic acids; the solution may also contain a filler (wood flour or mineral powders). Liquid phenolic resin adhesives may be thermoplastic or thermosetting. With the former, joints are cured at ordinary temperatures; with the latter, curing is carried out at 115°–150°C. Adhesives in film form are produced by impregnating sulfate paper with thermosetting adhesive and then drying the paper at 80°–100°C. Because of their limited storage life, film adhesives, like thermoplastic adhesives, are prepared at the site of use.

Phenolic resin adhesives are used most commonly for bonding wood (usually thermoplastic types) and making plywood (thermosetting types). In both cases, the adhesive is applied to the surface with a brush and allowed to stand for 5–15 min to permit evaporation of the solvent; the joint is then assembled and cured under pressure. When adhesives in film form are used, the films are put in place and the surfaces are immediately joined.

Phenolic resin adhesives have high water resistance. The strength of the thermosetting types is 2–4 meganewtons/m² (20–40 kilograms-force/cm²), while that of the thermoplastic types, for example, adhesives based on phenol-formaldehyde resin and organic sulfonic acids or on phenol-formaldehyde resins using resorcinol and paraformaldehyde, is at least 13 meganewtons/m² (130 kilograms-force/cm²). The cost of phenolic resin adhesives is relatively low. One drawback, however, is brittleness. To eliminate this disadvantage, the adhesives are modified with polyvinyl acetals (butvar-phenolic adhesives), rubbers, and other polymers. Work with the adhesives requires adequate ventilation and protective clothing.