Performance of Melamine-Urea-Formaldehyde Resin Adhesives at Various Melamine Contents for Bonding Glued Laminated Timber Under High Frequency Heating

Abstract

This work attempted to manufacture glued-laminated timber (Glulam) bonded with melamine-urea-form-aldehyde (MUF) resin adhesives at various melamine contents from 20% to 50% under high frequency (HF) heating for a very short time. Two preparation methods were employed to prepare MUF resin adhesives with different melamine contents: one-batch method of synthesizing MUF resins in a single batch, and two-batch method of mixing urea-formaldehyde (UF) resin with melamine-formaldehyde (MF) resin that had been synthe-sized separately. As the melamine content increased, the gelation time and peak temperature of MUF resins decreased. The adhesion performance of plywood showed that the one-batch MUF resin adhesive with 50% mel-amine content only satisfied the standard requirement of water resistance. Thus, the one-batch MUF resin adhe-sive with 50% melamine content was applied for bonding wood lamina from four softwood species such as Japanese larch, Korean red pine, Korean pine and Japanese cedar to manufacture Glulam under HF heating. All Glulam samples bonded with the one-batch MUF resin adhesives with 50% melamine content except those from Korean Red Pine satisfied the requirement in water soaking or boiling water delamination test as an exterior grade Glulam. The presence of rosin in Korean Red Pine was believed to be responsible for its poor adhesion. These results showed that the one-batch MUF resin adhesives with 50% melamine content provided acceptable water resistance with exterior grade Glulam manufactured under HF heating.