Cationic lignin derivatives with enhanced antibacterial and functional properties for sustainable PU-based packaging

Antibacterial materials play a critical role in mitigating infection risks, particularly in the context of escalating bacterial resistance and the growing need for sustainable alternatives. Lignin, a naturally occurring biopolymer with low toxicity, offers considerable promise in this area; however, its practical use is limited by its inherently low antibacterial performance. In this study, kraft lignin (KL) was functionalized through free radical polymerization using three cationic monomers: [2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (METAC), [2-(acryloyloxy)ethyl] trimethyl ammonium chloride (ATAC), and [3-(methacryloylamino)propyl] trimethyl ammonium chloride (MAPTAC). Among the resulting derivatives, KL-METAC demonstrated the highest charge density (3.76 mmol/g), along with superior solubility (96.4%), hydrophilicity, thermal stability, and UV-shielding capability. All cationic KL variants exhibited improved antibacterial performance against both Gram-positive and Gram-negative bacteria relative to unmodified KL, with KL-METAC and KL-MAPTAC showing the most pronounced activity, underscoring the influence of charge density on antibacterial efficacy. The potential applicability of KL-METAC—identified as the most promising derivative—was further validated through its incorporation into polyurethane (PU) films, which resulted in enhanced tensile strength (2.5–5.6 MPa), modulus (128–135%), water contact angle (52° to 62°), and UV resistance compared with pristine PU films. Remarkably, PU-KL-METAC films also exhibited elevated oxygen and water vapor permeability, indicating their suitability for breathable packaging applications, such as packaging for fresh produce. However, due to the presence of kraft lignin and its potential sulfur content, these materials are not intended for direct food contact. They are better suited for use in multilayer packaging systems, where a food-grade inner layer prevents direct contact. Further refinement of lignin derivatives is necessary to enable the development of lignin-based PU films for more demanding food packaging applications, such as dried products. • Lignin was polymerized to produce cationic lignin and characterized comprehensively. • Cationized lignin was blended with polyvinyl alcohol to fabricate a film. • The physicochemical properties of the films were analyzed for their use in food packaging. • The antibacterial, thermal, and mechanical properties of the films were improved. • The permeability and water absorption of the films should be further improved.