Efficient flame retardancy of polypropylene by cobalt ions doped phytate melamine synergized with expandable graphite

Maintaining a delicate balance between flame retardancy and amount of expandable graphite (EG) in flame retardant polypropylene (PP) is still a formidable challenge, primarily due to the lower flame-retardant efficiency of EG. In order to address this concern, a novel cobalt-doped nanosheet flame retardant (PAMA-Co) was fabricated by a hydrothermal method utilizing phytic acid (PA) and melamine (MA), exhibiting a promising potential in constructing synergistic EG flame retardant PP. Significantly, the incorporation of only 7 wt% PAMA-Co/23 wt% EG system raises the limiting oxygen index (LOI) of PP to 27.3% and UL-94 V0 level, which indicates the excellent synergistic flame-retardant efficiency of PAMA-Co. Furthermore, both the peak heat release rate (PHRR) and the peak smoke production rate (PSPR) of PP composites with 7 wt% PAMA-Co/23 wt% EG are substantially reduced, exhibiting an 81.6% lower PHRR and 87.8% PSPR compared to pure PP. The excellent flame-retardant efficiency is attributed to the following mechanisms: Gas-phase dilution, catalytic cross-linking charring effect, and the suppression of the EG “popcorn effect” of PAMA-Co. In summary, this work not only advances an understanding of flame-retardant mechanisms but also offers a practical, green strategy for enhancing the safety and utility of PP in various applications.