A Three-in-One Lignin-Derived Intumescent Flame Retardant towards Fire-Safe and Tough Polypropylene

Abstract

Polypropylene (PP) is highly flammable and suffers from severe melt dripping. The incorporation of traditional intumescent flame retardants often leads to poor compatibility and deterioration of the mechanical properties. Herein, we report a bio-derived “three-in-one” intumescent flame retardant (PLM), synthesized through a two-step reaction between lignin, melamine, and piperazine pyrophosphate, to integrate acid, carbon, and gas sources within a single molecular structure. When incorporated into PP at 30 wt%, PLM enabled the composite to achieve a UL-94 V-0 rating, increased the limiting oxygen index to 31.8%, and suppressed melt-dripping. Cone calorimetry revealed an 86% reduction in the peak heat-release rate and an 82% decrease in the total smoke production relative to neat PP. Microstructural analysis confirmed that PLM promoted the formation of a continuous and highly graphitized char layer while simultaneously strengthening the interfacial adhesion with the PP matrix. PP-PLM30 exhibited superior tensile strength and impact toughness compared to its physically blended counterpart, effectively mitigating the severe mechanical deterioration typically observed in high-loading flame-retardant systems. This study demonstrates a practical pathway for high-value lignin utilization and sustainable flame-retardant design for polyolefins.