Preparation of expandable graphite using a hydrothermal method and flame-retardant properties of its halogen-free flame-retardant HDPE composites

This article reports a new way to prepare expandable graphite using a hydrothermal method. Natural flake graphite was added to a concentrated sulfuric acid and nitric acid mixture in a flask with mechanical stirring at room temperature. This was then placed in a Teflon-lined autoclave. The autoclave was sealed and quickly heated to the desired temperature, where it was maintained for 1 h before it was cooled to room temperature. The results showed that the expanded volume of EG obtained using the hydrothermal method was higher than those of EGs obtained through conventional liquid phase synthesis and ultrasound irradiation for the same acid concentrations in the mixture. X-ray diffraction patterns were used to analyze the structure and confirm that expandable graphite had indeed been prepared. A scanning electron microscope was utilized to observe the morphologies of the expandable graphite and expanded graphite. A cone calorimeter was used to investigate the flame-retardant properties of the halogen-free composites. The data show that the peak HRR values of the composites containing 30 wt% EG are dramatically lower than those of the pure HDPE resin and composites containing 30 wt% NG. The residual mass increases to 8.0 wt%, 30.1% and 37.6%, respectively, for pure HDPE, 30 wt% NG composites and 30 wt% EG composites. These results show that the HDPE/EG composite possesses excellent flame-retardant and thermal properties.