In the curing of the cold setting phenolic adhesives is that they are capable of giving good strength on curing at low temperatures, down to zero centigrade in fact. The setting of many other synthetic adhesives -even if they cure at all at low temperatures- is protracted and may result in a poor end product.
In the curing of the cold setting phenolic glues, the addition of an aqueous solution of an acid such as sulfuric, hydrochloric or acetic, causes the resin to be precipitated from solution. The acid commonly used as hardeners are therefore sulphonic acids derived from aromatic hydrocarbons, such as benzene or toluene sulphonic acid. These, or similar acids of certain aromatic compounds are compatible with the resin chiefly owing to the solubilising effect of the sulphonic acid group. Alternatively, however, a phenolic resin that has itself been unsulphonated resins, para-toluene sulphonic acid is most often used it is a strong acid, its normal solution having a pH of less than 0.5.
In a study of the effect of pH on cold setting resin prepared from formal 4-dehyde and monohydric phenols, it has been shown that a "barrier pH" exists, above which practically speaking, curing does not take place at room temperature. The barrier pH is roughly between 1 and 3 depending on the type of phenol; with ordinary phenol it is about pH 1.
While most acid set phenolic glues appear to give joints of good durability over long periods, both at high and low temperatures, there is evidence that a higher proportion fall below a specified minimum strength after a number of years than with resorcinol glues and that the acid hardeners used definitely attack the wood". A method of eliminating some of the danger of acid attracts on wood has been described. It evolves the addition of a "protective solution" to the acid catalyzed glue. This protective solution is designed to liberate an alkaline substance when the wood is subjected to an elevated temperature either during fabrication or in subsequent use. The composition of the solution is not stated.