Boron-Based Polyphosphazene-Functionalized Mxene Nanosheets for Polypropylene Composites with Improved Mechanical Properties and Flame Retardancy Applications

Developing high-performance resins with exceptional thermal oxidation stability, flame retardancy, smoke release suppression, and mechanical properties is an important industrial challenge. However, current flame-retardant design strategies often compromise other composite material properties. Especially when using polyolefin, unsaturated polyester, and other noncharred materials, it is usually necessary to add large amounts of flame-retardant fillers. In this study, a nanosynergist (Ti3C2Tx@PPD) for functionalizing Ti3C2Tx nanosheets with boron-based polyphosphazene was designed and adopted for a piperazine pyrophosphate/polypropylene (PAPP/PP) system as an application example. By controlling the chemical environment of cyclotriphosphazene, the condensed phase characteristics of polyphosphazene were preserved, but also an atypical vapor phase flame-retardant mechanism was activated. The combination of P/N/B elements and Ti3C2Tx exhibited excellent catalytic char-forming performance compared to others in the literature. Only 2% of incorporated Ti3C2Tx@PPD reduced the total heat released from the composite by 66.3%, the total smoke released by 71.8%, and the fire growth index by 49.4%. The incorporation of Ti3C2Tx@PPD inhibited deterioration of the mechanical properties of the composite. In addition, the pyrolysis path of Ti3C2Tx was revealed under a special environment. This study lays the foundation for the functional design of Ti3C2Tx nanomaterials that can be used in various applications that require high-performance resins.