Recycling Waste Lignin as Natural Adhesive to Prepare Sustainable Wooden Composite Materials

Wood is a green, renewable, and biodegradable polymer material, mainly used in fields such as artificial boards, papermaking, and biomass energy. However, its poor flammability and dimensional stability limit its application. Generally, surface treatment is required to achieve the substitution of wood materials for high-performance plastics and plywood. The aim of this study is to prepare bio-composite materials using lignin as a natural adhesive to improve the water resistance and heat resistance of the board surface. Both sides of poplar (Populus spp) boards were uniformly coated with sulfated lignin or dealkalized lignin, and the boards were hot-pressed for 1 h at 30 MPa and 180°C. The experimental results show that the hot-pressing treatment makes the interior of the board more compact; the mechanical strength, waterproof performance, and thermal conductivity are improved. The effect of the lignin-coated samples is more significant after hot-pressing. The type and proportion of lignin have a great influence on the mechanical properties of the material. Among them, 6% sulfate lignin and 6% dealkali lignin samples showed the best mechanical properties, with the maximum tensile strength of 408.06 and 549.86 MPa, and the maximum bending strength of 320.10 and 356.42 MPa, respectively. The sample of 10% dealkali lignin has good hydrophobicity, and the contact angle is 111°. It is of great significance to improve the preparation schemes of new materials such as green artificial boards and biodegradable plastics.