II. CHEMISTRY OF UF RESINS: UF CONDENSATION
The reaction between urea and formaldehyde is complex. The combination of these two chemical compounds results in both linear and branched polymers, as well as tridimen- sional networks, in the cured resin. This is due to a functionality of 4 in urea (due to the presence of four replaceable hydrogen atoms) (in reality urea is only trifunctional as tetramethylolurea has never been isolated, except in the formation of substituted urons [2]) and a functionality of 2 in formaldehyde. The most important factors determining the properties of the reaction products are (1) the relative molar proportion of urea and formaldehyde, (2) the reaction temperature, and (3) the various pH values at which
condensation takes place. These factors influence the rate of increase of the molecular weight of the resin. Therefore the characteristics of the reaction products differ consider- ably when lower and higher condensation stages are compared, especially solubility, visc- osity, water retention, and rate of curing of the adhesive. These all depend to a large extent on molecular weights.
The reaction between urea and formaldehyde is divided into two stages. The alkaline condensation to form mono-, di-, and trimethylolureas. (Tetramethylolurea has never been isolated.) The second stage is the acid condensation of the methylolureas, first to soluble and then to insoluble cross-linked resins. On the alkaline side, the reaction of urea and formaldehyde at room temperature leads to the formation of methylolureas. When condensed, they form methylene–ether links between the urea molecules. The alkaline products from urea and formaldehyde, and from mono- and dimethylolureas, are as