Facile preparation of graphene supported Co3O4 and NiO for reducing fire hazards of polyamide 6 composites

Graphene nanosheets coated with Co3O4 and NiO (denoted as GNS–Co3O4 and GNS–NiO) have been facilely synthesized and then melt-mixed with polyamide 6 (PA6) to investigate the fire retardant properties of the composites. The Co3O4 (20–40 nm) and NiO (8–15 nm) nanoparticles are homogeneously dispersed on the graphene surface, which has been demonstrated by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. Scanning electron microscopy clearly shows that graphene, GNS–Co3O4 and GNS–NiO are well dispersed in PA6 due to the electrostatic interaction. The visible increment in decomposition temperature and reduction in maximum decomposition rate are observed from thermogravimetric analysis for both PA6/GNS–Co3O4 and PA6/GNS–NiO composites. Testing in the cone calorimeter shows that incorporating GNS–Co3O4 and GNS–NiO confers PA6 with improved fire safety, as evidenced by the obvious delay in time to peak heat release and visible reduction in heat release rate, CO production rate and total smoke production. The result of the smoke density chamber indicates that GNS–Co3O4 and GNS–NiO perform better than bare graphene in inhibiting the smoke emission. The flammable gaseous products are largely reduced with the addition of GNS–Co3O4 and GNS–NiO as revealed from the thermogravimetric analysis/infrared spectrometry. The improvement in the fire safety is mainly due to the combination effect of physical barrier of graphene layers and catalytic carbonization of metal compounds.