The curing agents, also called resin 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
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