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.
A novel bio-based carbon forming agent (Mg@PA-CS) containing P and N elements was were synthesized using the complexation characteristics of chitosan (CS) and phytate (PA). The flame retardant behavior of poly(lactic acid) (PLA)/Mg@PA-CS/APP composites (addition of 20 wt% of different ratios of Mg@PA-CS and APP to polylactic acid composites) were investigated by the limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimetry test (CCT), and thermogravimetric analysis (TGA). Due to the biphasic flame retardant and synergistic effect, since the 20 wt% flame retardant system (Mg@PA-CS:APP = 1:2), PLA composites passed the UL-94 test V-0 rating, reached 34% LOI value. The peak heat release rate (PHRR) and total heat release rate (THR) were reduced to 1/2 of the pure PLA, char residue could be as high as 11.49% at 800°C. Moreover, the flame-retardant mechanism of PLA composites during thermal decomposition was analyzed using a scanning electron microscope (SEM) and the coupling techniques of TGA linked with FT-IR (TG-FTIR).
A novel P-N containing hyperbranched poly(phosphorodiamidate) (HBPPDA) was synthesized by the A 2 +B 3 polymerization, which had high thermal stability, with greater than 40 wt% of the char residue left at 750 °C. The benzene and triazine rings in HBPPDA contributed to a good char-forming capacity. Furthermore, a large number of terminal primary amines could release incombustible gases to dilute the concentration of oxygen. Thus, HBPPDA could act as blowing agent and carbonization agent simultaneously. Only 6.25 wt% of HBPPDA could greatly enhance the flame-retarding efficiency of ammonium polyphosphate (APP), showing a remarkable synergy to improve flame retardancy and dripping resistance of polypropylene (PP). When 25 wt% of flame retardant (APP:HBPPDA = 3:1) were added into PP matrix, the composite achieved V0 rating for UL-94 test, and its limiting oxygen index (LOI) value was increased to 30.6%. The reduction of 76.2% and 41.5% were available for the peak heat release rate (PHRR) and the total heat release (THR). Meanwhile, the composite showed strong smoke suppression.
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