Melt spinning of poly(vinylidene fluoride) mono- and bicomponent fibres and yarns : the formation of piezoelectric beta-phase crystallinity

Sammanfattning: The present study investigates the possibilities of manufacturing piezoelectric polymer fibres or yarns for integration into textile materials. The polymers known to give the strongest piezoelectric effects are poly(vinylidene fluoride) (PVDF) and some of its copolymers. PVDF is used in commercially available piezoelectric products, mainly in the form of films. The processing of PVDF towards these applications includes three necessary steps: the formation of polar β-phase crystallites, the application of electrodes and poling under high voltage. As the melt spinning process largely influences the molecular structure of a fibre, the processing (spinning) parameters can be expected to influence the formation of β-phase crystallinity in PVDF. In this study, results from both small/lab scale and large/industrial scale melt spinning are presented. By combining a range of processing parameters, it is shown that the degree of crystallinity increases with increasing melt draw ratio and is not affected by solid state drawing. Solid state drawing is necessary to produce fibres with β-phase crystallinity, the content of which increases with increasing draw ratio. Further, for maximum β-phase content drawing should take place at a temperature between 70-90°C and the draw rate should be as high as possible.Next, melt spinning of novel bicomponent fibres with high β-phase PVDF in the sheath and a carbon black/polypropylene (CB/PP) compound in the core has been successfully realized. The yarn’s core is electrically conductive, this paving the way to using CB/PP as inner electrode in melt spinning of piezoelectric bicomponent fibres. The combined analyses by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) showed that DSC-measurements are not sufficient to provide information about the crystalline structure of PVDF.