Modified phenolic adhesives hardening

Phenolics Phenol and resorcinol formaldehyde adhesives cure by condensation polymerisation with the elimination of water, and therefore require high curing pressures. They are normally available as two-component systems consisting of a paste resin and a liquid hardener. Traditional uses include wood bonding and plywood fabrication, but nowadays phenolics, especially modified phenolics, are also used in structural bonding of metals and plastics. 

In order to establish control values for the adhesives formulated using tannins, the initial work was done with phenol-resorcinol-formaldehyde (PRF) or resorcinol-formaldehyde (RF) resins on both surfaces, but modified for the honeymoon principle. The PRF resin chosen for this work was Borden s resin LT-75 with Borden s hardener FM-260. The RF resin used for a comparison was Chembond s RF-900. These resins have been used for wood gluing in the United States for more than two decades, especially for the manufacture of structural laminated timbers. 

Previous investigators have drawn attention to the beneficial effect of lime when added in small quantities to asphaltic bitumen. The lime helps retard oxidative hardening (13) and reduces the tendency towards water-stripping (4,11,12). Most asphalts are slightly acidic because of the presence of phenolic or carboxylic substituents and would therefore react with basic oxides to form insoluble salts. For example, Fromm (10) has described the use of iron salts of naphthenic acids as adhesion promoters to improve the water resistance of asphalt concretes. This promising approach is now undergoing commercial trials. The literature also describes methods of chemically modifying asphalt with maleic anhydride or acrylic acid (14), sulfur trioxide (15), sulfur dioxide (16), acetyl sulfate (17-21), and sulfuric acid (20). (For a recent review of the interfacial phenomena in asphaltic compositions see Ref. 4.)... 

Two types of m-aminophenol modified adhesives have been reported. The first is a multicomponent system that is not too easy to handle. " " Part I is a slow reacting phenol-resorcinol-formaldehyde resin (A) mixed with paraformaldehyde (B) as hardener and Part II is a phenol-m-aminoph-enol-formaldehyde resin (C) at a high pH mixed with a very slow hardener (D). Each part is applied to one surface and, after mating, the reactive components are mixed by diffusion and a cure results within minutes. 

The second is a two-component adhesive " component A is a m-aminophenol modified novolac at a pH of 5 (about 63% solids) and component is a formaldehyde donor used as hardener. The adhesive has gap-filling capability, impact resistance, and is thixotropic. A phenol-resorcinol laminating resin can be used as a primer, with good results. 

The compositions containing the bisphenol A-based epoxy resins and the phenolformaldehyde oligomers or dicyandiamide as the hardeners and the alcoholates of aluminum or sodium as the modifiers possessed excellent adhesion to the metals and, moreover, improved film-forming properties [266-269]. Sodium phenolate, aluminum trichloride, the hydroxides of the alkali and alkaline earth metals [270-274] and their halogenides [100,275,276] as well as the metal carboxilates [233,274,277-280] were used as the catalysts of the curing of epoxy oligomers with dicyandiamide, polyphenols, and polyalcohols. 

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