Synergistic effect between a novel silane-containing hyperbranched polyphosphamide and ammonium polyphosphate on the flame retardancy and smoke suppression of polypropylene composites

A novel hyperbranched polyphosphamide with abundant terminal groups of silane (HBPPA-Si) was synthesized from phosphorus oxychloride (POCl 3 ), 4, 4′-diaminodiphenylmethane (DDM) and (3-aminopropyl) triethoxysilane (APTES), which had high thermal stability with a residual weight of 44.0 wt% at 800°C in nitrogen atmosphere. By incorporating 25 wt% of APP and HBPPA-Si with an optimum mass ratio of 1:1 into polypropylene (PP) composites, the PP/APP/HBPPA-Si composite reached V-0 rating at the UL-94 test and also obtained a maximum LOI value of 27.5%. Meanwhile, their peak heat release rates (PHRR), total heat release (THR), and total smoke production (TSP) reduced dramatically by 72.5%, 35.8%, and 62.7% respectively, when compared with virgin PP. The improved flame retardancy and smoke suppression of PP/APP/HBPPA-Si composite were ascribed to their excellent synergistic effect working efficiently in both condensed-phase and gas-phase actions. It is noticeable that the high-quality and high-quantity char residue possessing thermal-stable P- and Si-containing cross-linked structures can shield the underlying polymeric materials from further hydrolysis and simultaneously insulate the volatile fuel and heat transfer. Moreover, the weak polar silane shell of HBPPA-Si improved the interfacial compatibility of APP within the PP matrix, leading to enhanced mechanical properties with increasing HBPPA-Si concentration.