Solvent-free non-isocyanate polyurethane thermoplastic films from lignin and castor oil: toward sustainable packaging applications.

Polyurethane (PU) films widely used in flexible packaging rely on petroleum-derived polyols and toxic isocyanates, raising significant environmental and safety concerns. Non-isocyanate polyurethanes (NIPUs), synthesized through the polyaddition of cyclic carbonates and amines, represent a promising alternative for developing safer and more sustainable polymer systems. In this study, fully bio-based thermoplastic NIPU films were prepared under solvent-free conditions using sebacic bis-cyclocarbonate (Seb CC), derived from castor oil, and lignin as a renewable aromatic component. Cyclocarbonated lignin (CC lignin) and native lignin were incorporated at varying Seb CC : lignin ratios (1 : 0.25-1 : 1) to evaluate the influence of lignin chemistry and loading on film structure and performance. The resulting films were characterized using FTIR, solid-state 13C CP/MAS NMR, thermal analysis (DSC and TGA), mechanical testing, permeability measurements (WVTR and OTR), scanning electron microscopy, and micro-computed tomography to investigate structure-property relationships. Lignin incorporation increased glass transition temperature, tensile modulus, and thermal stability, while significantly improving barrier performance. Native lignin formulations exhibited the lowest WVTR (0.33 g (100 in2·day)-1) and OTR (3.27-6.62 cc (100 in2·day)-1), approaching the performance of high-barrier biodegradable polymers. Micro-CT analysis revealed that barrier improvements were influenced not only by porosity but also by network rigidity and diffusion pathway tortuosity introduced by lignin's aromatic structure. These findings demonstrate that lignin-reinforced NIPU films provide a viable route toward fully bio-based, isocyanate-free materials with strong potential for sustainable packaging applications.