Лако-красочные материалы — производство

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 tridimen­sional 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 consider­ably when lower and higher condensation stages are compared, especially solubility, visc­osity, 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 follows (Formula 1):

The reaction also produces cyclic derivatives: uron, monomethyloluron, and dimethylo — luron.

On the acid side, the products precipitated from aqueous solutions of urea and formaldehyde, or from methylolureas, are low-molecular-weight methyleneureas [3]:

H2NCONH(CH2NHCONH)nH

These contain methylol end groups in some cases, through which it is possible to continue the reaction to harden the resin.

The monomethylolureas formed copolymerize by acid catalysis and produce poly­mers and then highly branched and cured networks (Formula 2):

The kinetics of the formation and condensation of mono — and dimethylolureas and of simple UF condensation products has been studied extensively. The formation of mono — methylolurea in weak acid or alkaline aqueous solutions is characterized by an initial fast phase followed by a slow bimolecular reaction [4,5]. The first reaction is reversible and is an equilibrium which proceeds to products due to the uptake of the products, the methy — lolureas, by the second reaction. The rate of reaction varies according to the pH with a minimum rate of reaction in the pH range 5 to 8 for a urea/formaldehyde molar ratio of 1:1 and a pH of 6.5 for a 1:2 molar ratio [6] (Fig. 1). The 1:2 urea/formaldehyde reaction has been proved to be three times slower than the 1:1 molar ratio reaction [7].

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 13kcal/mol when the reverse unimolecular reaction has an activation energy of 19kcal/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 x 10 4 (mol s) 1 for each site [8] and of the order of 3 x 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/[2(1 — p)] = exp[(—AGe)/(2RT)] (1)

where p is the degree of conversion at the equilibrium of the methylolation and demethy — lolation reactions, AGe is the standard Gibbs energy variation, T is the temperature in degrees kelvin, and R is a constant (1.987cal/gmol 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 = 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 formal­dehyde [11] and monomethylolurea [12], behaving like urea. The ability of methylenebi­surea 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.