Curing of urea-formaldehyde resins with a molar ratio of formaldehyde to urea less than one

Abstract

In the course of studies, it was found that the curing of urea-formaldehyde resins (UFR), with the formation of a spatially crosslinked structure, at a molar ratio of formaldehyde to urea less than one, occurs due to the formation of an inert compound – methylenediurea (MDU). For the experiment, the laboratory UFR (LUFR) was synthesized according to the following scheme. In the first stage, the molar ratio of urea : formaldehyde (U : F) = 1 : 2. The pH value is 7.5...8. At the second stage, the pH is 4,2...4,5. The third stage is the introduction of additional urea to the molar ratio U : F = 1 : 0.9, the pH is 7...8. The synthesis of MDU was carried out at a molar ratio of U: F = 1: 0.5. Urea was introduced into the formalin solution (pH 3), with constant stirring, and the latter was completely dissolved at 20 °C. After that, the reaction temperature was raised to 50 °C, and kept until a dense precipitate was formed. The resulting precipitate was washed on a paper filter with distilled water until a neutral reaction. The sample was dried at 105 °C to a constant mass. In a slightly alkaline environment, at the third stage of synthesis, when urea is introduced to the molar ratio U : F = 1 : 0.9, monomethylolurea is formed, which under acidic curing conditions will react primarily with urea, which is in a significant excess. This leads to the formation of MDU. Moreover, the latter does not enter into either the addition reaction with formaldehyde or the condensation reaction with the hydroxymethyl groups of UFR. As a result, MDU does not participate in the formation of the polymer structure, being an inert filler, and its components by weight, urea and formaldehyde, can be excluded from the final molar ratio. Thus, at the initial molar ratio of the initial components U : F = 1 : 0.9, during a series of chemical transformations, the final molar ratio increases to U : F = 1 : 1.012,which allows the formation of a spatially crosslinked polymer.