Lignin as a multifunctional chain extender for high-performance waterborne polyurethane: A comparative investigation on strategic utilization beyond filler applications

As the most abundant natural aromatic polymer, lignin demonstrates unparalleled potential in advancing sustainable waterborne polyurethane (WPU) systems. This work redefined the role of enzymatic lignin (EL) by strategically integrating it as a chain extender (C-LWPU) and a reinforcing filler (F-LWPU), transcending its conventional use as a filler. When employed as a chain extender, EL enabled a remarkable incorporation of 20 wt%, yielding LWPU films with a tensile strength of 31.73 MPa, a 772 % enhancement over pristine WPU, along with exceptional bioactivity (free radical scavenging ratios of 99.4 % for ABTS + and 91.3 % for DPPH), superior UV shielding, and thermal stability. In contrast, the F-LWPU films achieved a marginally higher tensile strength (34.37 MPa) but required only 1 wt% EL loading, while with reduced bioactivity (free radical scavenging ratios of 92.1 % for ABTS + and 20.9 % for DPPH). Crucially, the C-LWPU system outperformed in balancing high EL utilization, cost-effectiveness, and multifunctionality, underscoring EL's capacity to act as a multifunctional structural unit rather than a mere filler. By resolving the longstanding trade-off between high biomass content and performance, this work establishes EL as a pivotal enabler of next-generation WPU materials, offering a scalable pathway toward fully bio-based polymers with tailored properties for industrial applications.