Synergistic barrier effect of aluminum phosphate on flame retardant polypropylene based on ammonium polyphosphate/dipentaerythritol system

The synergistic barrier effect of aluminum phosphate (AlPO₄) on the char-formation behavior of flame-retarded polypropylene (FR-PP) based on the ammonium polyphosphate (APP)/dipentaerythritol (DPER) system was investigated. The flame-retardant property and combustion behavior of FR-PP was studied by means of the limiting‑oxygen index, UL 94 vertical burning and cone-calorimeter tests. Results indicate that the partial replacement of APP with AlPO₄ in the FR-PP composites showed significant synergistic effects during combustion. The interactions between AlPO₄ and APP were examined by means of thermogravimetry and thermogravimetry-Fourier transform infrared (FTIR) analyses. FTIR spectra and X-ray diffraction spectra were obtained to detect residues of APP/AlPO₄ mixtures at different temperatures. Scanning-electron microscopy and energy dispersive spectrometry was used to analyze the condensed-phase residues from cone-calorimeter tests. Scanning-electron microscopy showed that the crystallized aluminum metaphosphate that was derived from the crystal transition of AlPO₄ sealed the char residue surface and contributed to the formation of a more uniform and compact char. Thus, surface char with excellent barrier properties will protect the underlying polymer matrix from further degradation and combustion.