Macromolecular piperazine/aluminum phosphate hybrid and its efficient intumescent flame retardant/thermal conductive polypropylene

Achieving a delicate balance between fire resistance and thermal conductivity in intumescent flame-retarded polypropylene (PP) poses a formidable challenge, primarily due to the susceptibility of intumescent barrier layers to functional fillers. To address this concern, a novel hybrid macromolecule (Al-PAP) was synthesized via polycondensation reaction, showcasing promising potential in constructing flame-retardant and thermally conductive PP. Remarkably, the inclusion of mere 14 wt% of the Al-PAP/melamine polyphosphate (Al-IFR) system raised the limiting oxygen index (LOI) of PP to 28.3 %, equivalent to that of PP containing 20 wt% of the conventional system, emphasizing the superior efficacy of the hybrid Al-PAP. Further, the PP with 20 wt% Al-IFR performed a 32.2 % of LOI, 850 °C of glow wire ignition temperature, >960 °C of glow wire flammable index and UL 94 V-0 ratings (3.2 mm & 1.6 mm), indicating the high flame retardancy. Subsequently, three thermal conductive fillers – aluminum oxide, boron nitride, and multi-walled carbon nanotubes – were evaluated based on a 20 wt% dosage of Al-IFR. Alumina emerged as the optimal candidate, enhancing flame retardancy and smoke suppression while imparting thermal conductivity to PP. Conversely, the other two fillers compromised fire-safety performance. Notably, the composite 5Al2O3/20Al-IFR/PP exhibited a 90.8 % lower peak heat release rate and an 88.2 % lower peak smoke production rate compared to PP. Additionally, its thermal diffusivity and thermal conductivity were 27.0 % and 48.9 % higher than those of PP, respectively. In short, this work combined molecular design and comprehensive screening to build the thermally conductive PP with high flame retardant efficiency, and relevant mechanisms were also investigated.