PHENOL-UREA-FORMALDEHYDE (PUF) CO-CONDENSED WOOD ADHESIVES

The reaction of urea with methylolphenol under acidic conditions was i nvestigated. The alternating copolymer of urea and phenol could be syn thesized by the reaction of urea and 2,4,6-trimethylolphenol. The reac tions of urea with polymethylolphenol mixtures also were investigated by changing the reaction conditions, such as the molar ratio and acidi ty. The co-condensates were analysed by carbon-13 nuclear magnetic res onance spectroscopy to determine the ratios of co-condensation. Anothe r synthetic method to obtain co-condensed resins was developed by reac ting phenol with UF concentrate, which is a type of urea-formaldehyde resin prepared with a high molar ratio of formaldehyde to urea (F/U) s uch as greater than 2.5. Further, it was found that resol-type co-cond ensed resins can be synthesized by alkaline treatment of the co-conden sed resins that were once prepared under acidic conditions. The curing process and thermal properties of resol-type co-condensed resins were investigated by torsional braid analysis. The co-condensed resins dis played almost the same curing behavior and heat resistance as a commer cial resol. Their adhesive performance against plywood was evaluated b y conducting a cyclic 4 h boil test and a 72 h boil test. The plywood test results passed the requirements for JAS special-grade structural plywood. (C) 1998 Elsevier Science Ltd. All rights reserved.