Towards the production of conductive and piezoelectric bicomponent textile fibres

Licentiate thesis, 2013

In the present study electrical conductive and piezoelectric textile fibres have been manufactured and characterized. Potential applications of these fibres are for example textiles with heating and sensor properties. The flow behaviour during the melt spinning of the fibres was noted to greatly influence the final properties of polymer composite fibres. Several combinations of fillers were prepared using two different mixing methods. Graphite nanoplatelets (GNP) and carbon black (CB) were incorporated into a polypropylene matrix. Composites were manufactured, characterized and utilized in the form of conductive bicomponent fibres. The spinnability, the melt elasticity and the electrical conductivity were greatly influenced by the mixing route. The type of filler used also had a significant effect on the same properties. Synergistic effects of combining the different fillers were investigated. By replacing some of the CB with GNP, the melt elasticity was reduced without influencing the conductivity. However, orientation effects during melt spinning and cold drawing decreased the conductivity of the bicomponent fibres. The conductivity could partly be restored by a heat treatment. It is believed that the GNP particles are separated during the spinning leading to a reduced conductivity. Polarization of melt spun bicomponent fibres was conducted in order to produce poly(vinylidene fluoride) (PVDF) structures with piezoelectric properties. The sheath material was β-phase PVDF with a conductive composite of carbon black/high density polyethylene (HDPE) as the core material. Once the β-phase is poled it can convert mechanical deformation into electrical activity (piezoelectricity). Yarns were poled in a contact and a non-contact mode. The influence of the poling conditions on the piezoelectric properties was evaluated using axial tension in fibre direction. A high poling voltage at an elevated temperature was favourable for a high piezoelectric activity. The fibres were very sensitive (with regard to the activity) and even small mechanical deformations could be registered; for example a sensor prepared from woven textile could be employed in order to detect the heart beats of a human. The piezoelectric coefficient, g31, was shown to be 40% higher than for commercial film. A clear change of the electrical properties of the fibres were observed after poling as a result of the alignment of the electrical dipoles. In order to achieve a high piezoelectric activity of PVDF, the amount of β-phase should be as high as possible. This can be realized by adding different nanofillers. In the present study, a small amount of carbon nanotubes (CNT) was incorporated into PVDF as a β-phase enhancing agent. It was evident that this addition of CNTs could enhance the total amount of β-phase in the melt spun fibres.