4NH4Cl þ 6HCHO ! 4HCl þ ðCH2Þ6N4 þ 6H2O Hexamethylene
tetramine
Ammonium chloride is a better hardener than hydrochloric acid, as the latter produces weaker joints. The effect of a fixed amount of ammonium chloride on the pH change and on the rate of resin curing as a function of time and temperature is shown in Fig. 2.
Often, particularly in cold-setting UF resins for joinery, hardeners consisting of mixtures of a salt such as ammonium chloride or ammonium sulfate with an acid such as phosphoric acid, citric acid, or others are used to regulate pot life and rate of curing. Both pot life and rate of curing of the resin can then be regulated (1) by varying the concentration of the hardener in the resin, (2) by changing the relative proportions of acid and salt, and (3) by changing the type of acid and/or salt composing the hardener. Acting on these three principles, setting times of between a few minutes and several hours can easily be obtained.