Up to 50% Phenol Reduction in High-Pressure Laminate Production: Hardwood Kraft Lignin Valorization

Abstract

"Replacing petroleum-based phenol with renewable polyphenols in High-Pressure Laminates (HPL) offers economic and environmental benefits. High-Pressure Decorative Laminates (HPDL) used in construction and furniture are made from Kraft paper with phenol-formaldehyde (PF) resin and surface decorative paper with melamine-formaldehyde (MF) resin. Lignin, a by-product of paper and biofuel production, has potential as a sustainable alternative due to its polyphenolic structure. Softwood lignin is preferred for phenolic resins, but hardwood lignin is more common in South America. To enhance KL’s reactivity, hydroxy-methylation is used in resols [1]. This work investigates the impact of increasing phenol substitution with hardwood KL in ligninphenol-formaldehyde (LPF) resins on the physical and viscoelastic properties of HPL. Seven LPF resols with varying phenol substitutions (0-80 wt.%) were synthesized using eucalyptus KL. The PF resin (0 wt.%) was synthesized in one step, while LPF resols were prepared in two stages, including hydroxy-methylation, and subsequent phenol addition. Only PF resin exhibited phase separation, and its aqueous phase was discarded before adjusting the flow index of all resins to 13.5 s with alcohol. The resins were characterized in terms of pH, solid content, flow time, gel time, free formaldehyde content, and molecular weights using GPC. The resols were employed for the impregnation and drying of Kraft papers. The HPL were thermocompressed at 150 °C, meanwhile a white decorative paper with MF resin was added as surface layer for HPDL. HPL performance was evaluated using DMTA and boiling water resistance test, with statistical analysis, while HPDL performance was assessed with boiling water test. As phenol is replaced with hydroxy-methylated KL, the gel time initially increased but then decreased at higher concentrations, indicating changes in the reaction rate and cross-linking. The resistance to boiling water immersion decreased as the KL levels increased. HPDL containing up to 60 wt.% KL met the AS/NZS 2924.1:1998 standard specifications. However, HPDL with higher KL content and a number average molecular weight exceeding 750 g/mol, exhibited blistering and delamination. The viscoelastic performance of HPL improved with KL levels with elastic modulus at 150 °C of 2.72 GPa and 14.15 GPa for 0 wt.% and 50 wt.% KL, respectively. Modifications to the formulations or curing conditions are required to replace phenol with KL at levels above 50 wt.%."