Functionalized multi-walled carbon nanotube for improving the flame retardancy of ramie/poly(lactic acid) composite

Carbon nanotube (CNT) as a candidate of flame retardant additive has raised great interest among researchers. However, the insolubility and the aggregation of CNTs have severely limited their applications. This work aimed to functionalize CNT with a phosphorus-containing flame retardant to improve both flame retardancy and the dispersion. 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) had been successfully covalently grafted on the surfaces of multi-walled carbon nanotubes (MWCNTs) to obtain DOPO-linked MWCNTs (MWCNT–DOPOs) by a three-step process. MWCNT–DOPO was characterized by fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The thermal stability of MWCNT–DOPO was evaluated by thermogravimetric analysis (TGA). MWCNT–DOPOs in the ramie/poly(lactic acid) (PLA) composites were shown to be effective in improving flame retardancy according to UL94 test and limiting oxygen index (LOI) measurements. TGA results demonstrated that the char residues increased with the addition of MWCNT–DOPO. Moreover, the tensile strength of the ramie/PLA composite showed an improvement with the addition of MWCNT–DOPO.