Effects of Melamine and Ether Contents on the Curing and Performance Properties of Ureaformaldehyde (Uf) Resins as Binders for Particleboard

The objective of this study was to investigate the effects of melamine and ether contents on the curing and performance properties of UF resins as binders for wood composites. Various UF and UMF resins were synthesized with three different synthesis procedures. These resins were examined by 13C NMR, rheometer, and other methods and evaluated as particleboard binders. Three-layer particleboards were prepared with the resins catalyzed with various catalysts and levels, applied in face and core layers. The board test results were compared. Only about half of added melamine had reacted with formaldehyde. UMF resins were found to be catalyzed with stronger catalysts at suitable levels depending on melamine levels and on which layer of particleboard the UMF resins are to be applied. Even catalyzed with a stronger catalyst, the curing rates of UMF resins were still slower, and storage stabilities were shorter than UF resins, but the pot lives were longer, and internal bond strength and water resistance were higher. Moreover, resins synthesized with procedures 2 and 3 showed obviously longer storage times, longer pot lives, and longer gel times, and the particleboards bonded with these resins showed significant improvements in internal bond strength and water absorption values but the formaldehyde contents increased. The increased formaldehyde content test values indicated that linear methylene-ether groups in UF resins decompose in the hot-pressing of boards to emit formaldehyde, most of which is not captured back into the UF resin matrix. Uron-type methylene-ether groups decompose in the hot-pressing of boards to participate in the curing process and enhance the bonding of boards, but it could also emit extra formaldehyde which may not be effectively captured by UF resins but more effectively by UMF resins if the amount of melamine is high enough because of the increased reactive capacities of melamine. The results of this research offered a new hypothesis that the linear methyleneether bonds in UF resins might be a major contributor of the high free formaldehyde contents of particleboards. Decreasing the linear methylene-ether groups contents might effectively bring down the formaldehyde content of boards