Degradation of Teflon/sup TM/ by electrical discharge in high-humidity air

Abstract

A long-term durability test of high-voltage AC electrodes for ozone generation in a novel type of advanced oxidation reactor has been conducted. In this reactor, ozone is produced by dielectric-barrier discharge through humid gas in a parallel-plate geometry, with water as the ground electrode. Teflon membranes were chosen as dielectric barriers for reasons of the highly aggressive nature of the plasma. The flat, metallic parts of the electrodes, parallel with the water surface, were embedded in a second dielectric, contiguous with the Teflon membranes. The test was carried out continuously for 20 weeks, with room-temperature air as the atmosphere, over stagnant deionized water, at 24 kV and 30 kV (rms), 60 Hz. A set of 32 small electrodes was used for each voltage. The lower of the two voltages was at the threshold of corona discharge (and ozone generation). Three different membrane thicknesses (0.01 in, 0.02 in, 0.03 in) were tested. The second dielectric was silicone transformer oil. Six 0.02-in and six 0.03-in membranes at 30 kV failed due to dielectric breakdown, as well as one 0.01-in membrane at 24 kV. A failure was always accompanied by a perforation of the membrane and a shutdown of the power supply. The frequency of the occurrence of failures increased with time. Scanning electron microscopy was done on some of the membrane perforations. In addition to the failures, a progressive deterioration of the membrane surfaces was observed. In a number of membranes this resulted in open cracks through which the silicone oil slowly leaked and polymerized on the outer membrane surfaces. Particularly susceptible were 0.01-in membranes, some of which had to be exchanged before the completion of the test. Interestingly, no actual dielectric breakdown occurred in these cases.