Formaldehyde-Free Wood Adhesive Based on a Waterborne Maleic Anhydride Copolymer and Unmodified Kraft Lignin

Developing a formaldehyde-free adhesive with a wide range of raw material sources and excellent bonding performance in the wood-based panel industry remained a challenge for decades. In this work, styrene-maleamic acid copolymer (SMAA) and unmodified lignin are directly blended to prepare a formaldehyde-free wood adhesive. Effects of SMAA/lignin mass ratio and hot-pressing conditions on the bonding strength of plywood are systematically evaluated. The addition of lignin effectively enhanced the bonding strength. At an adhesive amount of 30 g/m2, the optimal mass fraction of lignin was 50 wt %. The adhesive met the plywood standard for grade II panel under processing conditions of 170 °C for 20 min, 180 °C for 10 min, or 200 °C for 5 min while satisfying grade I panel requirements at 180 °C for 30 min, 190 °C for 20 min, or 200 °C for 10 min. The filling of lignin at the interface and the formation of chemical bonds are proposed to be the reasons for achieving effective bonding. With advantages of easy preparation, no need for lignin modification, and high lignin content, this work provides an idea for the valorization of industrial lignin and the formaldehyde-free production of wood adhesive.