Improving Polyurethane Adhesives by Partially Replacing Hydroxypropyl-Maleate Modified Soda Lignin with Polyethylene Glycol: A Sustainable Approach to Biomass-Based Adhesive Technology

Polyurethane adhesives provide strong adhesion, durability, and versatility across various substrates but rely on non-renewable petrochemical polyols. To improve sustainability, researchers seek to discover renewable biopolymer alternatives rich in hydroxyl groups, making it a promising bio-based polyol for polyurethane formulations. In this study, Soda Lignin (SL), a sulfur-free, renewable, and highly functional biopolymer, was modified with maleic anhydride esterification and subsequent propylene carbonate oxyalkylation to produce hydroxypropyl maleate-modified soda lignin (HOMSL), enhancing its hydroxyl content and reactivity toward isocyanates. To ensure effective crosslinking and minimize porosity, the NCO/OH molar ratio was precisely calculated in the adhesive formulations. Then, Enhanced polyurethane (E-PU) adhesives were synthesized by partially replacing PEG-200 with different weight ratios of HOMSL (20–40 wt%). Structural analysis by FTIR and GPC confirmed successful modification, with HOMSL exhibiting increased molecular weight (Mw = 3708 Da) and polydispersity (PDI = 3.4). Thermogravimetric analysis showed that E-PU adhesives with 30–40 wt% HOMSL exhibited a two-stage degradation process with improved thermal stability due to the aromatic lignin backbone. Mechanical testing revealed a peak tensile strength increase from 2.4 MPa (E-PU1) to 4.8 MPa (E-PU4), while elongation decreased with increased HOMSL, indicating higher crosslink density. The lap shear strength on aluminum substrates improved significantly, from 0.006 MPa (Al/E-PU1) to 0.0496 MPa (Al/E-PU2) when 20 wt% HOMSL was used. However, higher HOMSL content led to interfacial detachment due to limited isocyanate availability. These results demonstrate that HOMSL can effectively enhance the thermal, mechanical, and adhesive performance of PU systems, offering a scalable and eco-friendly strategy for biomass-based adhesive production.