Influence of exfoliated graphite nanoplatelets on the flammability and thermal properties of polyethylene terephthalate/polypropylene nanocomposites

Abstract

Nanocomposites based polyethylene terephthalate (PET)/polypropylene (PP) (70/30 wt%) blends and exfoliated graphite nanoplatelets (GNP) as reinforcing fillers were developed using melt extrusion process. The filler concentration was varied between 0 -5.98 wt percent (%) (0–7 phr). The resulting nanocomposites were characterized in terms of flame retardancy, thermal conductivity, thermal behavior, morphology and structure. Cone calorimeter analysis, limiting oxygen index (LOI) and UL94 flame rating tests revealed that addition of GNPs to PET/PP improved the flame retardancy of PET/PP/GNP nanocomposites significantly. Cone calorimeter data show a significant reduction of peak heat release rate (PHRR), mass loss rate and delayed time to ignition (TTI) due to addition of GNPs to PET/PP blend. As much as 37% reduction in PHRR and 32% increase in TTI were observed for the maximum GNP loading. Enhancements of flammability properties were attributed to the development of compact, dense, uniform char layers on the surface of nanocomposites. The effective thermal conductivity was found to vary linearly with GNP loading which was attributed to the formation of effective interconnected heat conduction bridges formed by the GNPs. It was found that the effective thermal conductivity of the nanocomposites was increased by about 80%, i.e. from 1.2 W/m.K for the unreinforced PET/PP blend to 1.9 W/m K for the 5.98 wt% (7 phr) reinforced PET/PP/GNP nanocomposites. Differential scanning calorimetry results indicated that the addition of GNPs increased crystallization temperatures but decreased degree of crystallinity of PET/PP/GNP nanocomposites. However; the melting points remained essentially unaffected. Transmission electron microscopy and field emission scanning electron microscopy showed uniform dispersion of GNPs in the matrix with the formation of interconnected GNP sheets at 3 phr. Isolated instances of exfoliation of GNPs was also observed.