Interaction modulation of diphosphate piperazine and melamine phosphate for enhanced water resistance and flame retardancy in polypropylene

This study reports our recent effort to address the key challenges of hydrophilicity and inefficiency for conventional intumescent flame retardants (IFRs) through the regulation of interaction modes. Three interaction modes are designed through physical blending (DPAPI/MP, astatic binding), core–shell structure (DPAPI@MP, weak chemical interaction), and covalent bonding (DPAPI-MP, strong chemical interaction). The effect of different interaction modes between piperazine diphosphate (DPAPI) and melamine phosphate (MP) on flame retardancy is analyzed. The introduction of only 20 wt% DPAPI-MP into the PP matrix significantly enhances its flame retardant properties, raising the limiting oxygen index (LOI) increase from 17.2 % to 27.8 % and achieving a UL-94 V-0 rating. The peak heat release rate (PHRR) and total smoke release (TSR) decreased by 79 % and 54 %, respectively. The residual char increases to 29.4 % from 0. The flame retardant mechanism involves a topological rearrangement of the triazine ring, which releases non-combustible gases, while acidic compounds facilitate the development of a three-dimensional P-O-C cross-linked structure. For water-resistant performance the PP/DPAPI-MP/RDP cretained the UL-94 V-0 rating even after 168 h of hydrothermal aging at 70 °C. The precipitation rate is reduced to 1.18 wt%, and the tensile strength decreases by only 1.5 %. The PP/DPAPI-MP/RDP exhibit an excellent balance between flame retardancy and water resistance. This study proposes a novel design concept for regulating the interaction modes of flame retardants, offering new insights into the development of water resistant and flame retardant PP.