Process Optimization of Lignin-phenol-formaldehyde Resins for High-pressure Laminates: A Taguchi L9 Study on Temperature, Pressure, and Time

Lignin-phenol-formaldehyde (LPF) resins represent a promising route to reduce fossil phenol use in high-pressure laminate (HPL) production. However, lignin substitution affects cure kinetics, crosslink density, and hydrophilicity, requiring process optimization. This study evaluates the influence of temperature (120-160 °C), pressure (60-100.bar), and pressing time (20-60.min) on dimensional stability and surface quality of LPF-impregnated kraft paper sheets. A Taguchi L9 design was used to isolate main effects, and swelling was assessed after 2.h boiling-water immersion according to EN.438. Temperature was the dominant factor, followed by time and pressure. The best L9 condition (160 °C, 60.bar, 60.min) yielded 26.5% thickness swelling and 27.8% mass swelling. A model-predicted optimum (160.°C, 100.bar, 60.min) was validated experimentally, achieving 23.8% and 21.7% swelling. Although still above industrial HPL requirements (<10.%), the results establish a robust processing window and indicate that further improvements must come from resin formulation rather than press parameters alone. The findings are discussed in the context of lignin structure-reactivity relationships and previously published characterizations of industrial lignins.