Correlation between space charge accumulation in polyethylene and its fluorinated surface layer characteristics

Abstract

Polyethylene (PE) samples were surface fluorinated by the F2/N2 mixture for 30, 60, 120 or 240 min to investigate the influence of the fluorinated layer characteristics on space charge accumulation. After polarization at 50 kV mm−1 dc electrical field and 40 °C for 240 min, the charge amounts of the samples fluorinated for the different times, normalized to the charge amount of the original sample, are 1.17, 0.51, 0.49 and 0.22, respectively, showing significant suppression of the charge accumulation by the longer treatments, especially for the 240 min treatment. Infrared analyses and SEM cross-section images of the fluorinated samples indicate the increases in degree of fluorination and thicknesses of the fluorinated layer with the treatment time, and the fluorinated layer thicknesses were determined to be 0.39, 0.45, 0.65 and 0.80 µm. Surface energy calculations show that the polar component increased from 2.9 to 14.7 mJ m−2 after the 30 min treatment, and subsequently decreased to 11.8, 11.5 and 9.5 mJ m−2 for the longer treatments of 60, 120 and 240 min. This suggests a similar change in surface layer permittivity with the treatment time. The fluorinations led to the shift of thermally stimulated discharge current peak from 156 °C of the original sample to 145, 142, 144 and 149 °C of the fluorinated samples, and thus reduced the trap depth of the surface layer. But, the longer treatments of 60, 120 and 240 min significantly improved the barrier properties of the surface layer to the diffusion of the chemical species from the semi-conductive electrode to the PE by the decrease in free volume of the surface layer. The chemical species diffused into the sample surface layer reduced the depth of surface traps. The decrease in free volume is more important in suppressing the charge accumulation than the increase in surface layer permittivity and the change in surface trap, because it would reduce the charge transport in the surface layer and the charge direct injection at both electrodes.

Fluorine gas treatment improves surface degradation inhibiting property of alumina-filled epoxy composite

“…[11][12][13] By controlling treatment factors during the direct fluorination process, a surface layer containing C-F bonds with preferable surface properties can be obtained. 14,15 In An's research, by controlling direct fluorination time and temperature, different surface conductivities are obtained in epoxy resins. [16][17][18] The dc surface flashover voltage of the epoxy resin is also found improved after direct fluorination.…”

Impact of interface between polyethylene and surface fluorinated polyethylene on space charge accumulation in the layered structures

“…It needs to be pointed out that there is a small oscillating signal just after the laser target electrode signal in measured current traces, which was found to be pertinent to the length of the wire connecting the HV electrode to the power supply isolating capacitor (in Figure 1). Although we have tried to shorten the wire length so that the oscillating signal is compressed and becomes closer to the target electrode signal, in comparison with our previous published results [19], it still has a small contribution to the signals just after the laser target electrode signal, as shown by the arrow in Figure 4b in the following text.…”

“…However, when oxygen as impurity was present in the fluorinating gas mixture at a low concentration, a thick fluorinated layer or a long treatment time was needed to reduce the space charge densities by the same amount [19]. We also found that fluorination time needed for effective suppression of the space charge could be significantly reduced by increasing fluorination temperature, for instance, from 240 minutes at 55 °C [19] to 120 minutes at 70 °C [20].…”

“…When a fluorinating gas mixture (F 2 /N 2 ) contained no detectable oxygen, a very thin fluorinated surface layer of PE or a surface fluorination for a short time could reduce space charge accumulation very effectively [17,18]. However, when oxygen as impurity was present in the fluorinating gas mixture at a low concentration, a thick fluorinated layer or a long treatment time was needed to reduce the space charge densities by the same amount [19]. We also found that fluorination time needed for effective suppression of the space charge could be significantly reduced by increasing fluorination temperature, for instance, from 240 minutes at 55 °C [19] to 120 minutes at 70 °C [20].…”

Enduring and Stable Surface Dielectric Barrier Discharge (SDBD) Plasma Using Fluorinated Multi-Layered Polyimide

“…The research works and applications are mainly concentrated on the improvements of the barrier properties, wettability, adhesiveness, chemical stability or biocompatibility of polymers [18,19] and dealing with the electrical properties of polymers. For example, An et al [20,21] indicated that surface fluorination of PE has a significant influence on the charge injection and accumulation. Du et al [22,23] found that surface fluorination is an effective method for modulating the electrical properties and restraining the surface charge and space charge accumulation of the PI film.…”