Resorcinol resin glues

Resorcinol resin glues set without the assistance of heat under mildly alkaline conditions. Therefore, no damage is caused to the wood by acid hardeners, and there are no restrictions on the moisture content of the wood up to 18 %. The resins can be cured with paraformaldehyde. Wood-to-wood glue joints with resorcinol resins are among the strongest and are weatherproof and resistant to solvents, oils, and fats. For reasons of economy, mixed condensates of resorcinol and phenol are used instead of pure resorcinol. 

In the manufacture of pure resorcinol resins, the reaction can be violently exothermic unless controlled by the addition of alcohols. Because the alcohols perform other useful functions in the glue mix, they are left in the liquid adhesive. PRF adhesives are generally prepared firstly by reaction of phenol with formaldehyde to form a PF resol polymer, that has been proved to be in the greatest percentage, and often completely, linear [95], In the reaction step that follows the resorcinol chemical is added in excess to the PF-resol to react it with the PF-resin -CH2OH groups to form PRF polymers in which the resorcinol groups can be resorcinol chemical or any type of resorcinol-formaldehyde polymer. 

Resorcinolic adhesives are either straight resorcinolic resins, or phenol-resorcinol copolymers. When military requirements dominated the field, during and after World War II, most of the beams were produced from oak, and the straight resorcinol resins were preferred. By the time of the Korean War, excellent phenol-resorcinol types had been developed. These glues were used in the production of the all-wood minesweepers used in naval operations. They demonstrated a high degree of durability in their many years of exposure to all the rigors of the Seven Seas. 

Over the years, various types of adhesives have been used in this application. Already in 1926, resins based on urea-formaldehyde were imported in the market by a company named LG. Farben (now BASF) and mark the beginning of structural adhesive technology in timber. Then, the resulting synthetic resin glues based on resorcinol and melamine have proven due to their excellent temperature and humidity resistance in work areas, where the bonded components are exposed to weathering or climatic conditions. The strength of the adhesive joint usually exceeds the inherent strength of the wood. 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

Results of a 2-year paint study indicate acetylated wood is a better painting surface (37) than untreated wood. UV radiation darkens unacetylated wood, but there is no change or a slight bleached effect with acetylated wood (37). In general, acetylation reduces the adhesive strength of wood (48). Adhesive strength is reduced with urea-formaldehyde resins (54, 55) and casein glues (55), but there is very little effect with resorcinol-formaldehyde resins (55). 

Work in Pretoria had shown that hardwoods like African kiaat and phenol-formaldehyde resin-impregnated European beech, glued with phenol-formaldehyde-resorcinol glues, could be expected to give reasonable service, and that weight and prices were reasonable. 

Figure 9 depicts the somewhat similar behavior for a room-temperaturesetting, urea-formaldehyde or phenol-resorcinol-formaldehyde resin-based glue. However, no initial heat-related drop in viscosity occurs during clamping, and the final polymerization hardening proceeds at a slower rate because it occurs at room temperature. 

With respect to application methods, blood glues can be spread on wood surfaces by most conventional means. These include roller, knife, and extrusion but do not include curtain coating or spray, for which the glues must be thinned below practical film retention levels. The major advantage of alkaline-dispersed blood glues over all other wood glues except resorcinol-based synthetic resin adhesives is their sensitivity to heat, resulting in extremely... 

Resins based on phenol and formaldehyde or a combination of phenol, resorcinol and formaldehyde are used in many glues and adhesives. The resins are used for decorative boards, laminated boards for floors, plywood, water-resistant boards, glass and mineral fibres for insulating, brake and clutch linings, abrasive cloth and paper, grinding wheels and moulds for casting metal and plastic. 

Phthalic anhydride, 1% (polyester resin component) Resorcinol, 2% petrolatum (adhesives and glues) Styrene, 1% (polyester resin cross-linker) Toluenediisocyanate, 2% petrolatum Turpentine, 1% olive oil (furniture polishes) Urea-formaldehyde resin, 10% petrolatum (fibre board glue)... 

A-ring of the tannin (194). Resin synthesis conditions varied in the approach to addition of formaldehyde. For example, the tannins could be crosslinked by reaction of a phenol-resorcinol-formaldehyde resin carrying little or no methylol functionality by addition of paraformaldehyde or by reaction of a phenol-formaldehyde prepolymer carrying a comparatively high methylol functionality. Adhesive formulations were similar to those described above. Typically, 100 parts of a 55% solids content water solution of wattle tannin were combined with 0.25 parts of defoamer, 7 parts of paraformaldehyde and 9 to 10 parts of coconut shell powder the pH was adjusted to 6.5 to 7.4 by addition of 40% NaOH. These adhesives also provided exterior quality glue lines. They are exceptional in their tolerance to high moisture content veneer and permit fast curing rates, subjects of particular interest in the plywood industry today. 

In the realm of boat building or repair such a glue would obviously be required, but there are few candidates that fit this specification. Undoubtedly one of the best and relatively easy to use is resorcinol/formaldehyde which is a two-part adhesive consisting of liquid resin and powder hardener. Mixed just before using, it is a dark red colour that shows up the glue line. 

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