Surface morphology and electrical characteristics of direct fluorinated epoxy-resin/alumina composite

As basic insulating material, epoxy resins are widely used as the matrix of insulators in HVDC transmission system. In most cases, alumina of micro range is blended to improve mechanical properties as well as reduce expenses, etc. However, apart from benefits, the incorporation of micro-particles could alter surface electric behaviors due to the change in material structures and the surface trap distribution under operating condition. Therefore, the surface charge behavior under dc is changed as well, which can affect the surface flashover characteristic. Direct fluorination is a method to change surface properties by introducing stable C-F structure to the surface of the dielectric material. The newly formed fluorinated layer could bring changes to electrical characteristics, thus affecting surface electrical properties under dc stress. In this paper, samples are prepared using alumina casted epoxy resin composites. The two parts are blended using the same process used to produce cone-type insulators in gasinsulated substations (GIS) in industrial application. Experiment samples were treated with F2 /N2 mixture containing 12.5% F2 in volume at 50 °C under 0.1MPa. The scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR) were used to investigate surface physicochemical characteristics. Water and diiodomethane droplet contact angles were measured and surface energies were calculated. The distribution of the surface voltage potential, as well as the surface conductivity and DC flashover voltage were measured. Experiment results confirmed that the fluorination to epoxy resin and alumina composites can change the surface morphology by changing the exposed area of surface alumina particles. It also showed that fluorinated samples have higher DC flashover voltage in both air and SF6 due to the combined effect of the change in the surface morphology and the surface conductivity.