Resorcinol Phenol Formaldehyde Adhesive

Resorcinol alone was used for some years as the phenolic substance. Later it was realized that the important properties of a resorcinol formaldehyde adhesive could be retained, and the cost reduced, by replacing part of the resorcinol with ordinary phenol. Cresol is less suitable but limited amounts of vegetable tannins may be used. All the earlier remarks relating to "pure" resorcinol, apply broadly to resorcinol/phenol adhesives, at least in the amounts in which phenol is normally used to replace resorcinol. The usual replacement is up to one half the molecular proportion of total phenolic constituents, but as much as 80% has been suggested. Without sacrificing too many desirable properties it is certainly possible to replace considerably more than one half, but the process of room temperature hardening after gelatin becomes progressively slower as the proportion of phenol is increased. Another disadvantage of using too much phenol is that the characteristic phenolic smell (completely absent with resorcinol) may be pronounced, even in the fully hardened glued joint.

There are two principal methods of preparing resorcinol/phenol resins; formaldehyde may be reacted with a mixture of resorcinol and phenol, or the resorcinol and phenol may be reacted with formaldehyde in separate stage. Both methods can lead to similar products; properties of the product depend much more on the molecular ratio and pH of the reaction than on anything else.