Adhesion in the bonding of wood to metal can be improved by adding a quantity of a polyvinyl acetal, in particular, 10-20% polyvinyl butyral; some care in the selection of solvent may be required. Alternatively, a solution of polyvinyl butyral can be applied to the metal and the resorcinol glue to the wood. The important subject of mixed thermosetting and thermoplastic adhesives is dealt with separately under the heading two-polymer adhesives.
Where the gluing of non-porous materials is concerned, it is natural to be careful about using adhesives containing much solvent especially if both adherents are non-porous. However resorcinol resins are used with great success in bonding certain plastic materials, especially melamine decorative laminates. Good results have also been obtained in bonding nylon with a resorcinol/phenol glue-results not unrelated perhaps to the solubility of nylon in phenol. As an example, the tensile shear strength obtained in bonding nylon 6 (polycaprolactam) to itself an to beech are of the order of 1450 psi and 750 psi respectively.
Resorcinol resins adhere well to concrete and brickwork, but special precautions need to be taken with concrete to achieve best results; this is because the 'laitance' (the loosely-bound surface commonly present on ordinary concrete) may be torn away through shrinkage stresses created during the setting of the adhesive. The precautions against this should include adding filler to the resin to reduce contractions, and abrading the surface of the concrete to expose the aggregate. A better way is to prevent formation of the laitance by vibrating the wet-mix in the course of laying the concrete. In other building applications, gluing to brickwork is generally satisfactory. And in gluing asbestor cement, resorcinol glues are highly successful. With asbestos cement as with concrete it is wise to allow for the reduction in shuffling time that alkalinity of cement may cause or, alternatively, neutralize the surface by applying a dilute solution of an acid.
Cylized rubber is readily glued with a resorcinol resin, especially to wood. In gluing rubber and other non-porous materials to wood, it is best to use an alcoholic, rather than an aqueous solution.
An important application of a resorcinol resin, although one that is perhaps marginal for consideration here, is in improving the adhesion of rubber to rayon and other synthetic fibres in the manufacture of automobile tyres, a mixed resorcinol resin and rubber latex adhesive being described wherein the non-acidity and water-solubility of the former permit compatibility with rubber latex.