Urea-formaldehyde, a thermoset, is obtained by the condensation of urea [(NH2 )2 CO] or derivatives such as hydroxymethylurea and formaldehyde (HCHO) in the presence of a proper catalyst either basic or acid. The general equation of the first reaction is given below:
NH2 -CO-NH2 + HCHO → NH2 -CO-NH-CH2 OH + OHCH2 -NH-CO-NH-CH2 OH
A wide variety of urea-formaldehyde resins can be obtained by careful selection of the pH, reaction temperature, reactant ratio, amino monomer, and degree of polymerization. If the reaction is carried far enough, an infusible polymer network is produced. The condensation proceeds in several consecutive stages: First it yields a liquid and transparent resin easily soluble in organic solvents. Secondly, the condensation continues and the resin becomes easy to mold. Thirdly, upon heating, the resin hardens yielding a hard solid, non fusible and insoluble in organic solvents. Industrially, the condensation is maintained in the second state in order to be able to mold the resin easily. The major fillers consist of pure cellulose, caseine or cotton flocks, in order to not alter the whiteness. Once cured, urea-formaldehyde exhibits good mechanical and dielectric properties and good chemical resistance.
