The hot setting phenolic glues have their important place in making plywood. They are usually made with a molar ratio of about 1.5 - 2.25 formaldehyde to 1 of phenol. If prepared under weakly alkaline conditions, as formation of the resin proceeds, separation into two layers takes place an aqueous layer above. Under more strongly alkaline conditions, above pH of about 10 but depending on molecular ratio and degree of condensation, a homogeneous solution is formed, this makes the resin appear water soluble, whereas it is in fact soluble in the aqueous alkali. By comparison with pure phenol resins, cresol resins require higher concentrations of alkali to ensure solubility. Nowadays, phenolic resin plywood glues are applied mainly as aqueous alkaline solution, and to a small extent as film glues.
Of the phenolic substances that may be used to replace ordinary phenol in adhesives, mixed-isomer cresol, as distinct from meta-cresol-and mixed isomer xylenols are most used. In subsequent polycondensation of the resultant methylol phenols (the hardening reaction), further shown that the rates of polycondensation although lower are of the same relative order.
The curing of the solution form of phenolic wood glues at a high temperature say 130oC, is attended by certain disadvantages; one is the evaporation of moisture from the wood, another is the absorption of the resin by the wood, resulting in excessive penetration. The latter is influenced by the porosity of the wood and by its moisture content. So far as the glue itself is concerned there are three factors that contribute to penetration; low initial viscosity on heating, a rather slow rate of gelation (compared with urea glue), and the fact that the resin is of low average molecular weight compared with many other polymers. The likehood of penetration is decreased as the curing temperature is lowered. It is not easy to overcome all these defects in one resin, and at the same time ensure it having satisfactory storage life.