Synthesis, Structure, And Properties Of Modified Urea-Formaldehyde Resins Based On Multiple Copolycondensation

The formaldehyde emission and poor water resistance of urea-formaldehyde (UF) resin are closely related to its molecular structure. Based on copolycondensation theory, polyfunctionality compounds (melamine, aldehydes, larch tannin, and hyperbranched polymers) were used to modify UF resin by regulating molar ratio and synthetic process with the purpose of optimizing the molecular structure of resin. The chemical structures of the modified UF resins were measured by Fourier transform infrared spectroscopy, matrix-assisted laser desorption/ionization-time of flight mass spectrometry, and 13C nuclear magnetic resonance, then the reaction mechanism of copolycondensation was discussed. The curing behaviors of these resins were characterized by differential scanning calorimetry and thermomechanical analysis. The bonding properties of these co-condensed resins were evaluated by the performance of the wood-based panels. The main conclusions are summarized as follows:(1) The molar ratio of formaldehyde to urea (F/U) had an obvious effect on the structure and performance of UF resin. As it decreasing from 1.2 to 0.9, the contents of hydroxymethyl and free formaldehyde decreased, as well as the degree of polycondensation, the curing activation energy, the thermomechanical properties, and the dispersion component in the surface free energy of the cured resin. Besides, the bond strength of the wood-based panels decreased linearly, and the formaldehyde emission declined exponentially with the decrease of F/U molar ratio.(2) The performance of the wood-based panels could be effectively improved by the addition of melamine into UF resins as wood adhesives. The melamine addition stage had an obvious influence on the structure and properties of the MUF resins. Adding melamine in the early stage yielded high cross-linking degree and better plywood performance; the polycondensation of melamine derivatives, which was easy to occur, had an inhibitory effect on the polycondensation of urea derivatives. When melamine was incorporated in the late stage, the triazine ring couldn’t be effectively embedded in the skeleton of the MUF resin; the curing activation energy of MUF resin increased; the curing degree decreased as well as the performance of plywood. The optimal pH values in basic stages and F/(U+M) molar ratios in acid stage were 7.0-8.0 and 1.6-1.8, respectively.(3) Glutaraldehyde and furfural could be successfully introduced into the resin system by co-condensing with urea and formaldehyde. Glutaraldehyde could improve the flexibility of the resins by its long carbon chain. When added in the early stage, glutaraldehyde played a positive role in reducing the formaldehyde emission of the plywood; when added in the late stage, it could enhance the bonding properties of the plywood, while reducing the curing time and the curing activation energy of the resin significantly. The incorporation of furan ring of furfural led to low molecular weight, low cross-linking density, and long curing time. However, the free formaldehyde content of resin reduced significantly. Under the optimized parameters, the bond strength of the plywood can reach the type Ⅱ grade of plywood requirement and the formaldehyde emission of it can meet the Eo standard.(4) Larch tannin, which is mainly composed of prodelphinidin units, was introduced into UF resin by the methylene linkage on nucleophilic A ring. The incorporation of polyphenol structure promoted the rigidity and flexibility of the glued system, increased the molecular weight of the resin, and upgraded the properties of the plywood as well. Under the optimized parameters, the bond strength of the plywood can reach the type II grade of plywood requirement and the formaldehyde emission of it can meet the Ei standard.(5) Poly(amide-amine) hyperbranched polymers could highly improve the structure of the resins due to their abundant amino groups and highly branched structures. HB(MA-EDA)1 and HB(MA-EDA)3 as additives could upgrade the water resistance of the MUF resins. The modified UF resin, with employing very finite amount of HB(MA-EDA)1 as a basic catalyst, yielded considerable improvement of the performance of the produced particleboards. The hyperbranched structures of HB(MA-EDA)1, which introduced a large amount of high reactivity amino groups, could be used as resin frame structures to improve the cross-linking degree and stability of the resin and decrease the curing activation energy of the resin as well.