The cold setting acid hardening phenolic glues have a smaller but more varied use than the hot setting alkaline ones, being confined almost entirely to the assembly gluing of wood. In the preparation of the resins, phenol itself is normally used, generally with a molecular ratio between 1.5 and 2.2 of formaldehyde to one of phenol. After heating under alkaline conditions, the resin is neutralized or made weakly acid, to obtain a homogenous solution in a non-alkaline medium it is necessary to remove most of the water and re-dissolved the resin in an organic solvent such as ethanol, iso-propanol or acetone.
Where it is possible to produce phenolic glues that are quite strongly alkaline yet stable-that is in terms of both pot-life and shelf life- the addition of a strong acid results in a short pot-life, which of course is the reason why the one is hot setting and the other cold setting. The later is therefore supplied as two part adhesive, a resin and a hardener.
The addition of the acid hardener required to effect setting at room temperature generally causes an exothermic reaction; the lower the pH and the higher the resin concentration, the higher the rise in temperature. The increase in temperature may be enough to shorten the pot-life considerably; on the other hand, the quantity of heat generated in a glue line of normal thickness is unfortunately not enough to accelerate setting of the adhesive in the joint. The exothermic reaction makes it necessary to observe accuracy in weighing or measuring-two much hardener and pot life is short, too little and the rate of hardening is disproportionately increased.
One advantage of cold setting phenolic adhesive 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 adhesive-even if they cure at all at low temperatures is protracted and may result in a poor end-product.