Green synthesis of ammonium polyphosphate-grafted lignin/piperazine pyrophosphate for enhancing the flame retardancy and UV weather resistance of HDPE composites

This study reports the green synthesis of ammonium polyphosphate-grafted lignin (APP@Lig) without organic solvents. The synthesis employed ammonium dihydrogen phosphate, urea, and phosphoric acid through nucleophilic substitution and polycondensation. Piperazine pyrophosphate (PAPP) and APP@Lig were mixed at a 3:1 mass ratio and added to a high-density polyethylene (HDPE)/polyolefin elastomer (POE) composite system (PEPO) as flame retardants. With 30 wt% total flame retardant content, the composite achieved a UL-94 V-0 rating and a limiting oxygen index (LOI) of 28.9 %. Cone calorimetry confirmed that the addition of APP@Lig reduced heat release significantly. Compared with pure PEPO, the composite showed 36.3 % lower total heat release (THR) and 82.9 % lower peak heat release rate (pHRR). The peak CO₂ release rate also decreased by 0.4 g·s⁻¹ . In addition, the fire performance index (FPI) increased by 76.5 % compared with the system containing only 30 % PAPP, indicating improved fire safety. The composite showed excellent UV resistance (UPF 44.31) due to lignin’s aromatic structures and phenolic hydroxyl groups. Overall, this work introduces a practical strategy for combining inorganic and bio-based flame retardants to enhance both flame retardancy and weather resistance in HDPE-based composites.