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

ABSTRACT 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.