Low−temperature dielectric loss in polyethylene

Abstract

A calorimetric technique was used to accurately determine the dielectric loss tangent of both additive−free and commerical grades of polyethylene at liquid−helium temperature. We found that the loss tangent of polyethylene often exhibits a broadened dielectric relaxation peak. Although this peak is centered in the kilohertz region, its existence increases the loss tangent at 60 Hz. Thus, the possibility of the elimination or reduction of this peak is important in evaluating the suitability of polyethylene as an insulator for a superconducting ac power transmission line. We have measured the intrinsic dielectric loss tangent of polyethylne and have found it to be ∼5×10−6. In addition, by systematically studying the influence of chemical additives on the dielectric loss, we have found the conditions necessary to obtain polyethylene exhibiting this low loss using present industrial production methods.

Low temperature dielectric loss spectra of polyolefins

“…Over our seven samples c~18 m varied between 0.06 and 0.17rad/M with a mean value of 0.09rad/M (1 M = 1 g mole/litre). Thomas and King's [10] values were lower than any of ours and Yano's [15] was higher. Fig.…”

“…It suggests that the dielectric dispersion is attributable to individual BHT molecules relaxing independently. From the start the effect has been attributed to motion of the hydrogen atoms [10,15] and we have confirmed this by a deuteration experiment. 250 mg of BHT was dissolved in 15 ml of methanol CH 3 OD and refluxed for six hours then reprecipitated.…”

“…BHT is used to stabilise a wide variety of organic matter. According to two reports [10,15] its presence in polyethylene did not cause the low temperature relaxation, but according to another report [6,19] it did. We have now confirmed that it does [18], but also demonstrated that it can very easily be lost by evaporation and suggest that this sometimes happened inadvertently.…”

“…Amine derivatives are also sometimes used. The published literature up to early 1979 [6,10,[15][16][17] indicates that the low temperature relaxation is associated exclusively with phenol antioxidants, but otherwise is unclear and sometimes contradictory. Since that time it has become clear that the relaxation is always caused by antioxidants which are 2,6 di-tert-butyl phenol derivatives [18], and that previous reports to the contrary 1 were erroneous.…”

“…Typically as the samples are cooled towards liquid-helium temperature, 5 falls to an acceptable value then rises again. Thomas and King [10] gave the clue to the origin of the extra low-temperature loss when they reported that it took the form of a broadened Debye relaxation curve The approximate molar concentrations of our own samples are derived from information of our suppliers. Hitherto unpublished data are from three low-density polyethylene samples kindly supplied by Mr. J. Nury, ATO Chimie, France now been studied quite extensively.…”