| The prepress properties of urea-formaldehyde (UF) resin was related to the plywood performance and the production efficiency. In order to investigate the prepress properties of UF resin with low molar ratio, the pH value, thermal curing behaviors, resin structure, crystallinity, viscosity, and pot life had been studied to reveal the effects of assembly time, melamine modifier, molar ratio, and curing agent on the prepress properties of UF resin in this paper. The main conclusions were shown as follows:(1) Increasing the placing time could improve the prepress properties of UF resin. With the increase of placing time, the curing reaction of UF resin began to make molecular weight increase as the reduction of pH value, which improved the viscosity and stickiness, leading to better prepress properties; with the increase of placing time, the initial and peak temperature decreased, which reduced the curing temperature and improved precure degree, leading to better prepress properties;(2) Adding melamine or incresing molar ratio, the hydroxyl methyl gradually increased and the methylene reduced, which contributed to a strong cross-linking linkages in the resin structure and more hydrogen bonds with wood surface, leading to better prepress properties.(3) After adding ON-50 system (the ratio of ammonium chloride and citric acid was 1 tol) in UF resin, the prepress bonding performance of resulting plywood bonded by UF resin reached to the best. The curing rate increased resulting in a short pot life, and the initial and peak temperature reduced leading to a heavy viscosity. As a result, the prepress properties of UF resin improved.(4) The prepress propertie improvement of UF resin was related to the resin viscosity, the amount of hydroxyl methyl and methylene in resin structure, and the curing reaction rate. Increasing viscosity, changing the amount of hydroxyl methyl and methylenem or raising curing reaction rate could improve the prepress properties of UF resin. |
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