Biomass-Based Flame Retardants, Nitrogen/Phosphorus Hybrid Modified Lignin in Thermoplastic Polyurethane (TPU) Applications

A phosphorus–nitrogen hybrid flame retardant (L–CS–P 2 O 5 ) was developed using a two-stage synthesis method, based on lignin, chitosan, and phosphorus pentoxide as the main components. It was then incorporated into thermoplastic polyurethane (TPU) to enhance its flame-retardant properties. The performance under different additions was investigated. Data indicated that adding 15 phr of L–CS–P 2 O 5 to TPU composite materials was the optimal ratio for achieving flame retardancy. At this ratio, the TPU/15 phr L–CS–P 2 O 5 mixture exhibited excellent flame retardancy, with a limiting oxygen index of 24.6% and V-2 rating in the UL-94 test. Compared with pure TPU, the addition of 15 phr of L–CS–P 2 O 5 significantly reduced the peak heat release rate (PHRR) and peak smoke production rate (PSPR) by 63.26% and 64.62%, respectively. The impressive flame-retardant performance stemmed from a powerful synergy between condensed and gas-phase mechanisms, combining processes like dehydration, char formation, dilution of flammable gases, and free radical quenching effects. In addition, the incorporation of L–CS–P 2 O 5 induced a plasticizing effect in TPU, resulting in a 45.32% enhancement in elongation at break. This significant mechanical improvement substantially broadened the potential application scope of TPU composites in more fields. Graphical Nitrogen/Phosphorus Hybrid Modified Lignin: Enhanced Flame retardancy of Thermoplastic Polyurethane.