During recent years the curing temperature of the phenolic plywood flues has come down from the region of 130 - 140OC and 105 - 115oC, this is a decrease of considerable importance and has justified such glues being classified separately as medium temperature plywood glues.
The decrease in temperature has been made possible by a production technique that produces a resin of higher molecular weight than hitherto. In effect, the resin temperature advances the reaction and carried out in his reaction vessel a large part of the "curing," which would otherwise take place in the hot press. The molecular weights of "medium temperature" plywood adhesive are relatively high, and therefore the resins that can be isolated from them by neutralizing the solution are less easy to dissolve in common phenolic resin solvent such as ethyl alcohol and acetone, they may be considered to be approaching an incipient B stage. For solution to have good storage-life, a dilute (with respect to resin), but rather strongly alkaline solution is necessary; a resin content of 40-50% and a pH of 12-13 are therefore common. If the pH is of the order of 9.5-11 the glue will cured fairly but the storage-life, even at a resin concentration of only 40-50%, will be short. In raising the pH, by adding further strong alkali, an increase in storage life is obtained. Another effect of increase pH is reduced viscosity, which is probably the result only of improved solubility.
To rise the pH of a phenolic glue to 12-13, a relative large addition of strong alkali, usually sodium or potassium hydroxide, is required, much larger than that required to give the same pH in water alone. This doe to what may be described as the buffering effect that is characteristic of many resinous polymer. The addition of a small proportion of an alkylene polyamine such as ethylene diamine is claimed to improve the stability of aqueous solution of P.F. resins.