On the production of polyethylene dielectrics and conductive polymer composite fibres

Doktorsavhandling, 2010

Foamed cable insulations and conductive polymer composite fibres are two applications in polymer processing where the electrical properties of a material may be altered significantly during processing. Their common denominator is that the outcome depends to a large degree on the flow behaviour of the polymer melt, in particular the melt extensional properties. In communication cables, the speed of propagation can be increased and the power lost in the transmission line decreased by foaming the cable insulation. Measurements on foamed polyethylene at 34 kHz and 11.2 GHz, using a dielectric spectrometer and a cavity resonance technique respectively, showed that the dielectric constant and the dielectric loss factor decreased with increasing porosity. However, the incorporation of a blowing agent significantly increased the dielectric loss factor. Comparing the loss factors measured at 1.8 GHz using a split-post dielectric resonator of 8 commercial high density polyethylene grades with the presence of specific molecular groups showed correlations between low dielectric losses and low amounts of carbonyl and trans-vinylene groups. Increasing the conductivity of polymer composite textile fibres may extend their use in various applications like heating, sensing and shielding. Several combinations of conductive fillers and polymers were prepared by melt mixing in a mixing chamber. Multiwalled carbon nanotubes (MWNT) and carbon black (CB) were used as conductive fillers. Conductivity was measured and the melt spinnability was evaluated with a capillary viscometer. Spinnability was closely related to melt elasticity and yield stress phenomena which in turn could be related to the formation of a filler network. The type of MWNT and method of their dispersion had a large influence on melt elasticity and conductivity. Elongation during melt spinning and drawing significantly decreased fiber conductivity. This effect was more pronounced for MWNT than for CB. However, conductivity lost due to elongation could be restored by a heat treatment in case of CB, but not for MWNT. An important finding was that bi-component melt spinning technology (core/sheath structure) facilitated the use of higher filler concentrations producing fibres with higher conductivity. A conductive polymer blend, showing double percolation, prepared with and without a compatibiliser was also explored. The compatibiliser improved the spinnability of the blend significantly and also gave a slight improvement in the conductivity.