Lightly colored and reactive maleic acid hydrotropic lignin: Characterization and polymeric applications

Acid hydrotropic fractionation using maleic acid, a US FDA approved indirect food additive, can rapidly (20 min) dissolve wood lignin at low temperatures (70-110 °C). This study characterized the physical and chemical properties, i.e., color, molecular weight, solubility, and chemical structure of maleic acid hydrotropic fractionation dissolved wood lignin (MAHL) obtained under various fractionation conditions. The study also demonstrated valorization of MAHL for two polymeric applications through direct mixing with (1) commercial sunscreen for its light color and UV protection capability, and (2) wood adhesives for its good reactivity. Sun protection factor (SPF) values of MAHL-containing sunscreens increased from 14 (control without lignin) upto 205 with just 5 wt% lignin supplementation. MAHL with low carbohydrates and good dispersion in sunscreen produced from moderate fractionation performed best in shielding UV light and yet maintaining light color to meet consumer preference. Lignin phenol formaldehyde (LPF) at 10% MAHL loading can meet the North American standards for interior plywood based on the ASTM D7998 test. MAHL lignin formaldehyde (LF) with phenol formaldehyde (LFPF) at 20% LF substitution resulted in a lignin-based wood adhesive with a wet bonding strength near 6 MPa pressed at 150 °C for 120 s, suitable for interior applications.