Covalent grafting modification on the surface of nanotubes and its enhancement mechanism on anti-dripping-flame-retardant synergistic performance of polylactic acid composites

Polylactic acid (PLA), a biodegradable polymer derived from renewable resources, exhibits limited applicability in high-performance fields due to its inherently low melt strength and severe dripping behavior during combustion. To overcome these challenges, we developed a novel flame-retardant strategy by modifying carbon nanotubes (CNTs) with siloxane and epoxy polymer brushes via emulsion polymerization, yielding siloxane-functionalized CNTs (CNTs-Si) with a crosslinked siloxane shell. Under melt blending conditions, the cleavage of weak Si-O-Si bonds initiates a shear-induced dispersion mechanism-termed the “weak bond-triggered shear dispersion (WBTS)” effect-facilitating nanoscale distribution of CNTs throughout the PLA matrix. These well-dispersed CNTs-Si not only enhance crystallinity but also participate in in-situ ring-opening reactions with terminal PLA groups, forming a lightly crosslinked network that significantly reinforces melt strength and mechanical strength. When combined with intumescent flame retardants (IFR), the melt viscosity increased from 3.3 N·m to 6.2 N·m, and combustion-induced dripping is entirely suppressed. Moreover, structural characterization reveals that CNTs-Si catalyze the formation of oriented, graphitized char layers, working synergistically with P-based flame retardants to improve barrier performance. Consequently, the limiting oxygen index (LOI) increases to 30.7%, while peak and total heat release rates (HRR) are reduced by approximately 30% and 25%, respectively. This work presents an integrated strategy that combines enhanced dispersion, crystallization control, and flame-retardant efficiency, offering new insights into the design of multifunctional, high-performance PLA-based composites.