Urea-Formaldehyde Resins
These thermosetting resins find applications in coatings, adhesives, laminating, and molding compositions. The materials are formed in water at a pH above 7 at the start of the reaction, because the methylol derivatives that form condense rapidly at acidic conditions. The initial step, where urea undergoes a nucleophilic addition of formaldehyde, can be shown as follows:
In the past, it was believed by some that further condensations that take place at pH below 7 include formations of cyclic intermediates. This, however, was never demonstrated [151]. NMR spectra of urea-formaldehyde resins show [152] that condensations under acidic conditions proceed via formations of methylene linkages:
Under alkaline conditions, on the other hand, dimethylene ether groups form instead [152]:
In addition, the more highly condensed water-soluble resins contain hemiformal groups. Further reactions may not result in formation of polymeric materials [153]. This is especially true when the ratios of formaldehyde to urea are low. Some are of the opinion that linear oligomeric condensates form instead. These urea-formaldehyde condensates separate as colloidal dispersions that are stabilized by association with excess formaldehyde [153]. The cross-linking reaction consists of agglomeration of colloidal particles with an accompanying release of formaldehyde. This opinion is supported by several observations: (1) when one plots the logarithm of solution viscosity against time during the polymerization, the plot exhibits a sharp break. Also, the plot differs from similar ones for phenol-formaldehyde condensation reactions that show continuous increases in viscosity. (2) Scanning electron micrographs of the fully cured resins show surface characteristics that resemble more the surfaces of coagulated and coalesced colloidal particles than those of high molecular weight polymers. (3) X-ray diffraction patterns and laser Raman spectra of the cross-linked resins show that there are crystalline areas in the material and absence of water. Similar patterns are obtained from hydrogen-bonded proteins with close chain packing. On the other hand, FT-IR studies [154] show that methylene and ether cross-links are present in the cured resin. There are also indications of the presence of cyclic ether units. The above information also suggests that the final structure of the urea formaldehyde resin may be a function of the feed ratio and the pH at which it was formed. Urea-formaldehyde resins for surface coatings are commonly modified for solubility in organic solvents by reacting them with alcohols to form ether groups. Usually, n-butyl alcohol is used. The reaction is carried out under basic conditions, before acidification:
After etherification, the reaction mixture is acidified and the resin is further reacted to acquire the desired degree of condensation. A typical butylated urea-formaldehyde resin contains 0.5–1.0 moles of butyl ether groups per mole of urea.
