A P/Al stoichiometry map for the flame-retardant synergy in polypropylene: Mechanism of phosphate evolution from metaphosphate to orthophosphate

Ammonium Polyphosphate (APP) based intumescent systems in polypropylene (PP) are widely used for halogen-free flame-retardant applications. They are often combined with mineral co-additives, such as aluminum trihydroxide (ATH), which are reported to be a beneficial-synergistic addition in some formulations and detrimental-antagonistic in others; a clear formulation guideline is still missing. In this work, PP composites containing APP and dipentaerythritol (DPER) along with ATH additions, across the 1-10% wt range, are prepared and analyzed. Flame-retardant performance is evaluated by LOI, and the condensed-phase mechanism is studied using FTIR-ATR, XRD, Raman spectroscopy, and SEM–EDS, including TGA derived char residues. The results show various compositions of the phosphate phase that form during decomposition, depending both on the initial composition and the thermal history of the sample. Specifically, solid residues are observed to shift from a metaphosphate-rich character towards a richer in aluminum ortho-phosphate one, as ATH loading and temperature increase. The results provide a nominal formulation based stoichiometric guide for understanding how APP/ATH ratios correlate with phosphate phase evolution and LOI response in the investigated PP/APP/DPER/ATH systems. Tracking these trends provides a practical formulation guide for the investigated system.