Silanization of technical lignins for amphiphobic coating and enhanced mechanical properties of thermoformed products

This study introduces a systematic approach to optimize methyltrimethoxysilane (MTMS) modification of alkali (Alk), kraft (Kl), and organosolv (Org) lignin at varying MTMS loadings of 0.5, 1, and 2.5 mol/g. The objective was to determine how lignin type, hydroxyl (-OH) composition, and MTMS loading influence lignin functionalization and, in turn, the barrier and mechanical performance of thermoformed lignin-coated sheets. By correlating lignin type, hydroxyl (-OH) composition, and MTMS loading, critical hydroxyl content ranges were identified (4 < OH < 5 mmol/g for Alk, Org, and Kl), establishing direct links between hydroxyl chemistry and water and oil repellency of the coated sheets. Optimal lignin functionalization was achieved at 0.5 mol/g MTMS for Alk lignin, 2.5 mol/g for Kl lignin, and 0.5-1 mol/g for Org lignin. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) and phosphorus-31 nuclear magnetic resonance (³¹P NMR) confirmed successful silanization and reduction of accessible hydroxyl groups. Thermogravimetric analysis (TGA) revealed enhanced thermal stability attributed to siloxane (Si-O-Si) network formation. These functionalized lignins were then applied as a coating on thermoformed fiber sheets, producing high water contact angle (97°-120°) and improved oil contact angle (58°-75°). The coated sheets also showed substantial improvements in strength and stiffness, with Young's modulus increase from 1,770 ± 2.3 MPa to 3,170 ± 1.6 MPa and tensile strength from 67 ± 3.7 MPa to 125 ± 1.2 MPa. These findings advance the chemistry of lignin functionalization for sustainable packaging applications.