Carbon microsphere-modified MXene: A high-efficiency hybrid flame retardant for simultaneously enhancing fire resistance, smoke suppression and thermal conductivity in poly (L-lactic acid)

Two-dimensional layered MXene nanosheets represent a promising environmentally friendly and efficient flame retardant. However, MXene tends to aggregate readily in polymer matrices, which compromises the mechanical and flame-retardant properties of the resulting composites. Carbon microspheres (CMSs), with their diverse surface-active functional groups, provide an effective solution to address this challenge. Furthermore, the carbon-rich CMSs enhance the flame retardancy of composites through their catalytic carbonization effect. In this work, we achieve for the first time the uniform loading of CMSs onto MXene nanosheets via electrostatic self-assembly. This innovative strategy effectively resolves the dispersion challenges of MXene in polymer matrices and enables its incorporation into poly (l-lactic acid) (PLA) systems. Remarkably, with only 5 wt% MXene/CMSs loading, the PLA/MXene/CMSs composite exhibited improved mechanical properties, and the thermal conductivity of the PLA composite was increased to 0.31 W∙m⁻¹∙K⁻¹, which was 47.6% higher than that of Neat PLA. In addition, the limiting oxygen index (LOI) increased to 26.6%, achieving a V-0 rating in the UL-94 vertical burning test. Compared to neat PLA, the composite demonstrated reductions of 28.1% and 15% in the peak heat release rate (pHRR) and total heat release (THR), respectively. Notably, the total smoke release (TSR) decreased significantly by 70.4% relative to Neat PLA. Additionally, a more stable and compact char layer formed during combustion. This enhancement primarily originates from the markedly improved dispersion of flame retardants in the PLA matrix, the catalytic carbonization effect mediated by transition metal Ti in MXene, and the efficient carbonization capability of CMSs. This work provides an advanced and eco-friendly solution to address the trade-off between MXene aggregation in polymers, high flame-retardant efficiency, smoke suppression, thermal conductivity and mechanical performance.