COEXTRUSION OF POLYMERIC PIEZOELECTRIC FILAMENTS R.S.Martins and J.M.Nóbrega, IPC/I3N – Institute for Polymers and Composites, Univ. of Minho, Portugal R.Gonçalves, M.P.Silva, and S.Lanceros-Mendez, Centro/Dep. de Física, Univ. of Minho, Portugal J.G.Rocha, Dep.t of Industrial Electronics, Univ. of Minho, Portugal H.Carvalho, Centre for Textile Science and Technology, Univ. of Minho, Portugal Abstract The difficulties related to the development of industrial scalable production methodologies have limited the number of applications currently available in the field of interactive/electronic textiles, which are far below the anticipated a few years ago. In these areas the integration of piezoelectric materials, that possess sensing/actuating capabilities, such as poly(vinylidene fluoride), PVDF, and can be processed using conventional processing techniques, is very promising and has encouraged a large number of research works. However, until now, most of the developed production methodologies are difficult to adapt to the industrial scale. This work reports recent developments achieved in the framework of a research project, on the production of piezoelectric filament by coextrusion of PVDF and an electrically conductive thermoplastic. The developed production methodology is based on a conventional coextrusion line, for which a coextrusion die was designed to produce a multilayer filament, which comprises an inner layer of an electrically conductive Polypropylene grade and a middle layer of PVDF and is coated with an electrical conductive ink. The sensing capabilities of the produced filaments are also characterized. Introduction The exceptional pyro- and piezoelectric properties of Poly(vinylidene fluoride) (PVDF) motivated the interest of both industrial and academic communities, and its use in several applications that benefit from its sensor or actuator characteristics [1-3]. The PVDF properties depend both on its degree of crystallinity and orientation of its crystalline phase, which are strongly dependent on the processing conditions employed during production [1, 3-12]. PVDF is a polymorphic material that presents at least four crystalline phases, known as , ,  and . The - PVDF, which is non-polar, is obtained during cooling of the melt at high or moderate cooling speeds [1, 3]. From the point of view of its electrical activity, the -PVDF is the most effective, and can be obtained through the stretching of the -PVDF at 80ºC using a stretch ratio (R) between 3 and 5 [11, 12]. In order to optimize its electroactive properties, -PVDF is subjected to a poling process, by the application of a strong electrical field [1- 3], which further orients the crystallite dipolar moments. The obtained electrically active polymer, can generate an electrical potential when subjected to a mechanical excitation, or a mechanical action is produced when it is subjected to an electric field. These characteristics motivated the use of PVDF in sensors and actuators applications. A polymeric piezoelectric device comprises at least one piezoelectric layer and two electrically conductive layers, one at each side of the central layer, which are used as electrodes for the connection of electronic conditioning/drive equipment. In conventional processes piezoelectric PVDF based devices are produced in the form of films, starting from the extrusion of a PVDF layer that is subsequently stretched, poled and, finally, coated by metallization 13. Some research works available in the literature focus on the development of filament shaped PVDF sensors. Walter et al. [14,15] have studied extensively the phase transitions of extruded PVDF monofilaments, in which a composite part of simple PVDF filaments and epoxy resin was poled with a linear electric field, in a direction perpendicular to the fibers. The produced composite was shown to exhibit piezoelectric activity. In the comprehensive work done by Lund et al. and Ferreira et al. [16] for two-layered filaments, it was shown that the electroactive phase content is not affected by the conductive inner core. Additionally it was concluded that the β-phase content depends only on the processing temperature and stretch ratio, as happens for the single PVDF filaments. Sequential processing methods for the production of piezoelectric cables have been described by Mazurek et al [17,18]. They show that the cooling at -30ºC of PDVF, after stretching, increase the β-phase content, and the coating required for the placement of the outer electrode promotes a significant reduction.