Effects of highly branched polyurea on the water resistance and formaldehyde emission of urea-formaldehyde adhesive

JIANG Shuyang;HU Mengyang;ZHOU Xiaojian;DUAN Zhigang;DU Guanben;LI Taohong;Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products, Southwest Forestry University;College of Chemical Engineering, Southwest Forestry University;

highly branched polyurea;;urea formaldehyde resin;;wood adhesive;;water resistance;;free formaldehyde

Low molar ratio urea-formaldehyde resin(UF) was modified using highly branched polyurea(HBPU) with high molecular weight and urea terminals. The water resistance of the modified resin was improved and meanwhile the formaldehyde emission of plywood was decreased due to the reaction between HBPU and free formaldehyde, as well as the co-condensations of HBPU with UF components. As a result, the bonding performance and formaldehyde emission was well balanced. HBPU was synthesized through polycondensations between urea and tris(2-aminoethyl)amine(TAEA) without solvents and catalysts. The molecular weight distribution and structures of HBPU were characterized. Then the low molar ratio urea-formaldehyde resin was modified with HBPU aqueous solution by adding HBPU to UF system at the final alkaline stage, or by mixing HBPU with the UF resin after synthesis. The effects of the addition level of HBPU and adding procedure on bonding performance and free formaldehyde emission of the modified UF resin were investigated through the bonding strength tests and the determination of the formaldehyde emission of plywood. The gel permeation chromatography(GPC) and ~(13)C-nuclear magnetic resonance spectroscopy(~(13)C-NMR) results indicated that HBPU with high molecular weight, high branching degree, urea terminals and good water solubility was obtained by the synthesis method in this paper. As the molar ratio of U and TAEA increased, more urea-terminated polymers were formed. The electrospray ionization-mass spectrometry(ESI-MS) examination results showed that HBPU reacted with free formaldehyde and produced hydroxymethyl products. Co-condensation products, which were resulted from the reactions between hydroxymethylated HBPU and hydroxymethylurea, were also observed by the ESI-MS technique. The water resistance of the modified resins was improved by both of the two HBPU adding procedures, no matter what molar ratio of U and TAEA was used. Meanwhile, in contrast to the plywood prepared with the unmodified resin, the formaldehyde emission of the plywood prepared with the resin containing 5% HBPU was reduced by 41%. The main reason for the simultaneous improvement of bonding performance and reduction of formaldehyde emission is that HBPU not only increased the branching degree of the modified UF resins, but also captured the free formaldehyde. The key to balance bonding performance of the UF resin and formaldehyde emission from wood-based panel is to increase the branching degree of the resin and reduce the content of free formaldehyde. The introduction of polymers with high molecular weight, high branching degree and urea-similar reactivity is an effective way to improve adhesion performance and reduce formaldehyde emission.