From Lab to Pilot Scale: Melt Electrospun Nanofibers of Polypropylene with Conductive Additives
In this paper, the feasibility of fabricating polypropylene (PP) nanofibers was investigated using conductive additives such as sodium stearate (NaSt), sodium oleate (NaOl) and Irgastat during melt electro spinning with a single nozzle lab and a 600- nozzle pilot scale device. Varying PP grades of high melt flow indices (MFI=450-1200 g/10 min) were used with different amounts of additives. The effects of the additives on the fiber diameters, thermal properties, electrical conductivity and polymer degradation were investigated. On a lab scale, fiber diameters of less than 500 nm were achieved with the compound of PP HL712FB, 4 wt% NaSt and 2 wt% Irgastat. The lab scale device was extended by a heatable spinning chamber, which affects fiber diameter reduction. The fabrication of nanofibers was in principle attributed to the increase in electrical conductivity with the introduction of the additives. On a pilot scale, the smallest fiber diameter of 6.64 μm could be achieved with PP HL508FB and 2 wt% NaSt. The comparison between the production of the fibers with a single nozzle and the pilot scale plant has revealed that a transfer of results is not possible without further ado. Due to the higher dwell time in the nozzle, a strong thermal degradation of the polymer could be detected with the high temperature size exclusion chromatography, whereby NaOl had the strongest influence on the thermal degradation. The high melt flow PP HL712FB and its compounds could not be processed with the pilot scale device due to its low viscosity, resulting in an insufficient pressure built up within the spinneret. Another reason for the non-spinnability of the material is the higher thermal and mechanical stress caused by the preceding melts preparation in an extrusion step. Further adjustments to the pilot plant are necessary to ensure a constant temperature distribution in the nozzle plate to achieve uniform fiber cross sections. The implementation of an uneven collector has successfully led to an even deposition of the fibers to obtain an isotropic non-woven fabric.