Industrial application of hardwood kraft lignin as phenol substitute in high-pressure decorative laminate resin manufacturing

Abstract

Replacing petroleum-based phenol (P) with renewable polyphenols such as lignin in High Pressure Decorative Laminates (HPDL) offers promising environmental and economic advantages.HPDL are widely used in construction and furniture and are composed of kraft paper layers impregnated with a resol-type phenolformaldehyde (PF) resin, topped with a melamine-formaldehyde decorative surface. Their manufacture involves resin synthesis, paper impregnation, drying, and hot pressing.Lignin, an abundant polyphenolic by-product from the pulp and biofuel industries, remains largely underutilised, with less than 2% of kraft lignin (KL) directed towards value-added products. In particular, hardwood KL— prevalent in South America from species such asEucalyptusspp.—exhibits low reactivity and solubility, limiting its direct application in thermosetting resins. However, chemical modification—specifically hydroxy-methylation—can enhance its reactivity and compatibility with PF resin systems [1].This study investigates the incorporation of hardwood KL into lignin-phenol-formaldehyde (LPF) resols for use in HPDL production. Seven formulations, with P substitution levels ranging from 0 to 80 wt.%, were synthesised and characterised in terms of pH, solids content, flow time, gel time, free formaldehyde content, and molecular weight distribution by size exclusion chromatography (SEC). Kraft papers were impregnated with LPF resins, dried to obtain prepregs, and hot-pressed at 150 °C to produce laboratory-scale HPDL and HPL (without decorative surfaces). Performance was evaluated using dynamic mechanical thermal analysis (DMTA), boiling water resistance, and statistical methods.HPDL containing up to 60 wt.% KL exhibited satisfactory boiling water resistance. However, formulations above 60 wt.% KL with number-average molecular weights over 750 g/mol showed blistering and delamination after the boiling water immersion test, likely due to reduced flow during curing. Interestingly, viscoelastic properties improved with increasing KL content up to 50 wt.%. The storage modulus (E′) at 150 °C rose from 2.72 GPa (0 wt.% KL) to 14.15