Carbon nanotube reinforced polylactide/basalt fiber composites containing aluminium hypophosphite: thermal degradation, flame retardancy and mechanical properties

This work aims to develop modified carbon nanotube (m-CNT) reinforced polylactide/basalt fiber (PLA/BF) composites with enhanced thermal stability, flame retardancy, and mechanical properties using aluminium hypophosphite (AHP). Morphological observations revealed that both the m-CNT and AHP particles were homogeneously dispersed in the PLA/BF composites. The improvement of thermal stability and tensile strength/modulus strongly depended on the uniform dispersion of the mixed particles and the interactions between m-CNT and PLA. The presence of AHP and m-CNT significantly reduced the peak heat release rate of PLA/BF in microscale combustion calorimetery testing. The combustion behaviors were evaluated by limiting oxygen index (LOI), Underwriters Laboratories 94 (UL 94), and cone calorimetery. For PLA/BF containing 19 wt% AHP and 1 wt% m-CNT, it achieved a V-0 classification in UL 94 testing with a high LOI (31%). Additionally, the peak heat release rate and total heat release were respectively reduced by around 64% and 27% in comparison with PLA/BF. The residue analysis showed that many microspheres formed by the sufficient reaction between AHP and PLA under the physical barrier effect of m-CNT migrated to or accumulated on the surface of residues. The compact char layer composed of these microspheres could effectively reduce the heat conduction of basalt fibers and cut off the mass transfer path resulting in the weakening of the wick effect resulting in the significant improvement of the composites. Modified carbon nanotube reinforced polylactide/basalt fiber composites containing aluminium hypophosphite were prepared via melt blending method. The composites showed excellent thermal stability, flame retardancy, and mechanical properties.