Adhesion performance of melamine-urea-formaldehyde resins with various melamine contents for glued laminated timber by high frequency heating system

This work attempted to investigate adhesion performance of melamine-urea-formaldehyde (MUF) resin adhesives for bonding glued-laminated timber (Glulam). Two preparation methods were employed to formulate MUF resins with various melamine contents from 20 % to 50 %: one-step method of synthesizing MUF resins in a batch, and two-step method of mixing urea-formaldehyde (UF) resin with melamine-formaldehyde (MF) resin that had been synthesized separately. These MUF resin adhesives prepared by two methods were applied for bonding wood lamina of four softwood species to manufacture Glulam under high frequency (HF) heating system. A commercial phenolresorcinol-formaldehyde (PRF) resin was also used for a comparison. As the melamine content increased, the gelation time and peak temperature of MUF resins decreased. All Glulam samples except those of Korean red pine bonded with one-step method MUF resins with 50% melamine content satisfied the requirement of Korean standard in water soaking or boiling water delamination test as an exterior grade Glulam. The presence of resin canal in Korean red pine is believed to be responsible for its poor adhesion. These results showed that the one-step MUF resin with 50% melamine content provided acceptable water resistance with the Glulam.