Curing Agents (hardeners) for UF Adhesive

The curing agents, also called hardeners, accelerators, or catalysts, that are added to the resin by the user, have one thing in common. They are either acidic substances by themselves, or they are capable of liberating acid when mixed with the resin. The later class comprises ammonium salts of strong acids. Ammonium salts are more widely used than acids; they are cheap, convenient to handle, and give a high ratio of pot-life to setting time. As hardeners for use at both normal and elevated temperatures the ammonium salts of strong acids are in many respects ideal. They function as hardeners by reacting with the free formaldehyde in the resin, and/or the formaldehyde liberated under the conditions of curing, to give the corresponding acid, hexamethylene tetramine (hexamine) and water. The most commonly used ammonium salt is ammonium chloride, which liberates hydrochloric acid.

4 NH4CL + 6 CH2O ====> 4 HCl + (CH2)6N4 + 6 H2O

The immediate decrease in pH that takes place on addition of the salt is followed by progressive further decrease as formaldehyde is liberated from methylol groups. Incidentally the pH at the time of gelatin is not as low as that of the synergistic liquid that is expelled at a later stage of the hardening reaction.

The rate of liberation of formaldehyde and consequently the rate of fall in pH is sharply increased by an increase in temperature, which is one of the reaction why ammonium salt are excellent hot hardeners. Apart from the more common ammonium salts of strong inorganic acids, ammonium thiocyanate is a very efficient hardener, but is poisonous.

The effect of adding hardeners which are themselves acidic is to reduce the pH is one step to its final value. It therefore follows that with the progressive fall that occurs with an ammonium salt the rate of hardening is a more gradual process giving a higher ratio of pot-life time especially at normal temperature.