Phytic Acid-Modified Lignin-Based Waterborne Polyurethane as a Bio-based Flame-Retardant for Polyvinyl Alcohol Films

Abstract

Herein, a lignin-based waterborne polyurethane modified with phytic acid (PA) was developed as a bio-based flame-retardant termed LPUPA to improve the flame resistance and mechanical performance of polyvinyl alcohol (PVA) films. Kraft lignin (KL) was initially functionalized with isophorone diisocyanate to form a lignin-based polyurethane (LWPU), followed by further phosphorylation using PA to obtain LPUPA. LPUPA exhibited excellent aqueous dispersibility and interfacial compatibility with PVA, enabling uniform film formation via a solvent casting method. Structural characterization via Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of urethane and phosphate groups. Compared with physically mixed KL, LPUPA showed improved dispersion, forming a more homogeneous film morphology and enhancing the tensile strength and toughness of PVA composites. Thermal analysis revealed that LPUPA lowered the onset decomposition temperature but significantly increased the maximum decomposition temperature and the amount of char residue under nitrogen and air. Combustion test results demonstrated that LPUPA effectively suppressed ignition and improved the limiting oxygen index of PVA films from 18.6% to 27.1%. The flame-retardant mechanism was attributed to condensed-phase char formation and gas-phase radical quenching as indicated by the Raman spectroscopy and XPS results of the char layer. The phosphorus-rich structure of LPUPA facilitated the formation of a stable, graphitized char layer through P–O–C and P–N–C linkages. These results demonstrated that LPUPA is a promising bio-based flame-retardant for sustainable polymer applications.