The rapid initial addition reaction of urea and formaldehyde is followed by a slower condensation, which results in the formation of polymers [7]. The rate of condensation of urea with monomethylolurea to form methylenebisurea (or UF ‘‘dimers’’) is also pH dependent. It decreases exponentially from a pH of 2 to 3 to neutral pH value. No condensation occurs at alkaline pH values.
The initial addition of formaldehyde to urea is reversible and is subject to general acid and base catalysis. Different energies of activation are reported for the forward methylolation and backward demethylolation reaction. The forward bimolecular reaction is reported to have an activation energy of 13 kcal/mol when the reverse unimolecular reaction has an activation energy of 19 kcal/mol [5]. Other sources report values of 17.5 and 17.1 kcal/mol for the same reactions, respectively [8]. If one considers that the reaction of monomethylloation of urea at pH 7 is of the order of 1 10 4 (mol s) 1 for each site [8] and of the order of 3 10 4 (mol s) 1 at rather alkaline pH it is possible to deduce what occurs at alkaline pH when urea reacts with formaldehyde to form methylolated ureas. The inverse reaction of decomposition of the methylolurea will limit somewhat, however, the proportion of methylolated urea prepared, the reaction running to completion only as methylolated ureas react to form dimers and higher oligomers when the pH is lowered in the condensation phase. If the condensation phase is not effected a calculation of the degree of advancement of the reaction of methylolation of urea under alkaline conditions can be carried out by the use of the formula [9]
p=½2ð1 pÞ ¼ exp½ð Gy Þ=ð2RT Þ ð1Þ
where p is the degree of conversion at the equilibrium of the methylolation and demethy- lolation reactions, Gy is the standard Gibbs energy variation, T is the temperature in degrees kelvin, and R is a constant (1.987 cal/g mol K). When introducing the reported activation energies of the urea forward methylolation reaction (17.5 kcal/mol) [8] and of the methylol urea demethylolation reaction (17.1 kcal/mol) [8] one obtains a degree of advancement p ¼ 0.60, hence at equilibrium under the conditions used 60% of the urea is present as methylolureas [9]. This compares well with a degree of conversion of 65%, at the equilibrium, of the more reactive melamine extrapolated by reported kinetic values [10] to the same conditions used herewith. The advancement of the reaction may eventually proceed to even higher degrees of conversion, even in alkaline environments, only as a consequence of the subsequent formation of methylene ether-linked oligomers.
The rates of introduction into the urea molecule of one, two, and three methylol groups have been estimated to have the ratio 9:3:1. The formation of N,N0 -dimethylolurea from monomethylolurea is three times that of monomethylolurea from urea.
Methylenebisurea and higher oligomers undergo further condensation with formal- dehyde [11] and monomethylolurea [12], behaving like urea. The ability of methylenebi- surea to hydrolyze to urea and methylolurea in weak acid solutions (pH 3 to 5) indicates the reversibility of the amidomethylene link and its lability in weak acid moisture. It explains the slow release of formaldehyde over a long time in particleboard and other wood products manufactured with UF resins.