Thermal Polymerization of Softwood Kraft Lignin: Enhanced Adhesion of Lignin-Phenol-Formaldehyde Blends

Thermal polymerization improved the adhesion properties of softwood kraft lignin, making polymerized softwood kraft lignin a viable and sustainable partial substitute for phenol-formaldehyde resins used in engineered wood panels. This was confirmed by the increased shear strengths observed after pressing hardwood veneers with either alkaline aqueous lignin solutions or their blends with commercial phenol-formaldehyde resins. Plywood panels bonded with blends of polymerized kraft lignin and phenol-formaldehyde resins exhibited higher wood failure values compared with those bonded with phenol-formaldehyde resins alone. Extensive polymerization was achieved solely via nonoxidative thermal treatments. The impact of temperature and time on average molar mass, dispersity, and intrinsic viscosity was investigated by size-exclusion chromatography, revealing that the average molar mass of kraft lignin can be increased by more than 40 times its original value. Other critical parameters affecting the polymerization process such as pH and water content were identified, and their influence quantified. Hydroxyl content measured by ³¹P NMR before and after polymerization suggests that the formation of ethers between phenols and benzylic hydroxyl groups is mainly responsible for the observed increase in molar mass.