Layered charring agent with super strong carbonization for IFR system: Achieving highly efficient flame retardancy of PLA by ultra-low addition IFR

Traditional intumescent flame retardants (IFR) composed of ammonium phosphate (APP) and charring agent can endow polymers with excellent flame retardancy, but always at the cost of sacrificing the mechanical properties of polymers. In this work, we designed and synthesized a charring agent (named as PDM, 70% of residual mass at 700 °C) with super strong carbonization , and mixed it with APP to construct an IFR. This IFR endowed polylactic acid (PLA) with excellent flame retardancy under ultra-low loading conditions, while also enhancing and toughening PLA. Adding only 2 wt% of APP/PDM resulted in a UL-94 V-0 level and a 29.3% of limiting oxygen index (LOI), as well as a reduction of 40.4%, 39.6% and 34.1% in the peak of heat release (pHRR), total heat release (THR) and total smoke release (TSP) for PLA. In addition, the tensile and impact strengths were improved to 59.2 MPa and 13.1 kJ/m 2 from 58.8 MPa and 7.7 kJ/m 2 of pure PLA, respectively. The synergistic carbonization effect between PDM and APP was a key factor contributing to PLA's exceptional flame retardancy and smoke suppression performance.