Development of lignin-reinforced polyurethane adhesive for glued laminated timber

Abstract

Glued-laminated timber (Glulam), a structural engineered wood product, is mostly manufactured using structural adhesives including one-component polyurethane (PUR). Nevertheless, the concerns associated with PUR such as unsatisfactory gap-filling properties and lower resistance to delamination create opportunities for newer adhesives with enhanced bonding performance in outdoor environments. In this study, the use of fractionated lignin as a modifier in a polyurethane-based adhesive system was explored. Herein, the effect of lignin content (1, 2, and 3%) on the block shear strength (BSS), wood failure percentage (WFP), and delamination were evaluated. The results indicated that all the lignin-reinforced PUR specimens showed better adhesion performance compared to the controls (without lignin). Markedly, the lignin-PUR adhesive formulation containing 1% lignin addition exhibited superior adhesion properties than those with 2% and 3% lignin content. The statistical analyses also revealed that the lignin content influenced the BSS and WFP of the glulam specimens. Moreover, the lignin-modified PUR specimens showed increased delamination resistance and met the requirements for delamination stipulated in the ASTM D2559 standard, regardless of the lignin content. The lowest percent delamination (0.07%) was obtained from specimens bonded with 1% lignin. The presented data suggest that specimens bonded with 1% of lignin provided better bonding strength compared to other lignin-filled specimens. Thus, this study demonstrated the technical feasibility of fabricating glulam with enhanced adhesion performance using lignin-modified PUR adhesive.