Ammonium polyphosphate@melamine phytate-layered double hydroxides: a loaded core–shell flame retardant for flame retardancy and smoke suppression in polypropylene

Conventional intumescent flame retardants (IFRs) have demonstrated remarkable efficacy in augmenting flame retardancy but often increase smoke release. To improve the smoke suppression effect of IFRs, this study employed a one-pot method to prepare a loaded core–shell flame retardant material, namely APP@PMA–LDH. This was achieved by supramolecular assembly of phytic acid (PA) and melamine (MA) to form melamine phytate (PMA), which was coated onto ammonium polyphosphate (APP) and loaded with layered double hydroxide (LDH). APP@PMA–LDH was then compounded with a carbonized foam agent (CFA) and polypropylene (PP) to produce PP composite materials. The experimental results showed that the sample containing APP@PMA-LD H1.0 (with 1 wt% LDH) exhibited the highest effects, with a substantial reduction of 68.4% in the peak heat release rate and a 59.5% decrease in total smoke production relative to PP. Furthermore, the sample attained a limiting oxygen index of 30.3% and achieved the UL-94 V-0 rating. The analysis revealed that APP@PMA–LDH promotes the development of a dense and stable graphitic char layer on the surface of PP, reducing internal porosity and effectively hindering the emission of flammable volatile substances, thus increasing the flame retardancy and smoke suppression of PP.