Chemical Modification of Expandable Graphite by Boric Acid and Its Flame Retarded Application in Polyethylene

The aim of this research is to get the graphite intercalation compound with high thermal stability and fire resistance. Firstly, the normal expandable graphite (EG) was prepared with potassium permanganate (KMnO4) as oxidant and sulfuric acid (H2SO4) as intercalator, respectively. Then, with (3-Aminopropyl)-trimethoxysilane (KH-550) as coupling agent, the boric acid (H3BO3) modified EG (EGB) was prepared by silane grafting method through H3BO3 bonded with EG. The analysis of X-ray powder diffractiometer and Fourier transform infrared spectroscopy testified the existence of EGB. EGB showed better thermal stability and flame retardancy for linear low density polyethylene (LLDPE) than the referenced EG. Addition of 13.0 wt% of EGB improved the limiting oxygen index (LOI) of 87.0LLDPE/13.0EGB to 24.6%, which was obviously higher than that of pure matrix of 17.5%. Whereas, the LOI of 87.0LLDPE/13EG was 22.7%. Furthermore, when EGB combined with ammonium polyphosphate (APP), the LOI and UL-94 level of 87.0LLDPE/8.7EGB/4.3APP reached 27.1% and V-0, respectively.