On the Dielectric Losses of Polyethylene at Medium and High Frequencies
As mobile communication moves from 3G to 4G, the bandwidth used will be extended from 2 GHz to 5 GHz. The propagation loss is related to the 2.6:th power of frequency, thus a large increase in power consumption may be expected. The dielectric properties of the insulation in coaxial cables are of increasing importance for the propagation losses at high frequencies. “Electrically clean” polyethylene grades have very good dielectric properties up to 1 GHz after which the dielectric losses can increase. Foaming of polyethylene improves the dielectric properties even further. The aim of the present work is to gain an improved understanding of the dielectric losses in polyethylene foams.
The dielectric properties of solid high density polyethylene (HDPE) as well as foamed blends of low density polyethylene (LDPE) with HDPE were evaluated at medium and high frequencies using a broadband high-resolution dielectric spectrometer, a split post dielectric resonator and a cavity resonator equipped with a rectangular waveguide.
The foamed samples were prepared by extrusion through a slit die using a laboratory scale single screw extruder. It was found that foaming reduced both the dielectric constant and the dielectric loss factor at medium and high frequencies and increasing porosity led to a better performance. However, increasing the porosity further than 50% would require improved strain hardening properties of the melt. Useful die temperatures for foaming were between 195°C and 210°C. Incorporation of the chemical blowing agent azodicarbonamide increased significantly the dielectric loss factor of both the processed and unprocessed foaming blends. Deformation of foam cells occurred when the consolidating melt was subjected to drawing and compression. The cells became elongated and flattened which can reduce the dielectric losses.
In order to study the dielectric properties of solid HDPE, eight technical grades differing in molecular characteristics, catalyst types, co-monomer types and additive content were used. Samples were prepared by compression moulding the materials into plaques. The dielectric losses, as given by the dielectric loss tangent, decreased more or less linearly with an increasing degree of crystallinity for this series of polyethylenes. Lower dielectric losses were also associated with a higher melting point. An increased dielectric loss tangent could furthermore be related to a higher content of carbonyl and trans-vinylene groups. To some extent, this could be associated with a corresponding reduction in the crystallinity.
Key words: Dielectric properties, polyethylene, frequency, foaming, crystallinity, porosity.