Dielectric Loss Spectra of Polyethylenes

Abstract

The polymer tape to be used in the construction of an ac superconducting power transmission cable should have a dielectric loss factor no greater than about 10⁻⁵. The polymers with the most desirable mechanical properties, such as polyesters, polyimides, and polycarbonates, all have losses several times too large even at low electric stresses, so any possibility of reducing these losses would be worth investigating. Also under investigation are ways of increasing the mechanical strength of low-loss polymers, such as polyethylene. Pure unoxidised polyethylenes, either without electrodes or pressed between metallic plates, usually have dielectric loss factors below 10⁻⁵ from 10 Hz to 10 GHz and below 20 K.^1,2 It also appears that they may be modified by copolymerizing the ethylene with carbon monoxide or by chlorinating them to form a rubber without substantial loss factor enhancement,^3,4 even though these procedures result in the formation of carbonyl and chloride substituent groups, which greatly increase the room temperature losses.⁵ It is likely that the introduction of dipolar cross-linking groups would also be permissible, and certainly also tert-butyl or other hydrocarbon side branches⁶ and all textural modifications. In fact, substantially increased low temperature loss factors appear to result only from certain very specific causes, but because these are the oxidation, accidental or deliberate, of the polyethylene^7,8 and the addition of certain antioxidants,⁹ they require investigation. The present paper reports low voltage dielectric studies of (a) deliberately oxidised polyethylenes, (b) polyethylenes prepared with various antioxidants, and (c) various “lossy” polymers and glasses.