Influence of the Addition of Urea-Formaldehyde Adhesive to Liquefied Wood on Curing

Urea-formaldehyde adhesive is one of the most frequently used types of amino resins for wood bonding. However, due to its synthetic origin, more environmentally friendly adhesives are desired. Liquefied wood is one of the natural-based alternatives. In this research, wood was liquefied using a procedure in which low solvent content liquefied wood was obtained. For the purpose of this study, urea-formaldehyde adhesive was added to the liquefied wood in proportions of 0, 10, 20, 30 and 40 %. Differential scanning calorimetry and rheological oscillatory test techniques were used to analyze the curing process and the hardening behavior of different adhesive mixtures. Additionally, wood lamellas were bonded with the same adhesive mixtures, and the shear strength of the bonds was evaluated. It was found that the addition of urea-formaldehyde adhesive to liquefied wood led to the occurrence of multiple chemical reactions during the curing process, and that the higher amount of ureaformaldehyde adhesive lowered the temperature at which gelation of the adhesive mixture occurred. It was also found that the adhesive mixtures with lower portions of urea-formaldehyde adhesive did not contribute to higher bond shear strengths compared to specimens bonded with pure liquefied wood, and that none of the specimens met the standard requirements for non-structural applications under dry conditions.