Incorporating Carbon Nanotubes into Polypropylene Fibers

Abstract

Carbon nanotubes (CNT) are an exciting new carbon based material discovered in 1991 by Iijima. The size, crystalline structure and conductivity make them an exciting choice for use in a composite fiber. The purpose of this study is to explore the possibilities of melt spinning carbon nanotubes compounded in polypropylene (PP) using conventional spinning equipment.

Carbon Nanotubes, pre-compounded in 30 melt flow rate polypropylene, were purchased from Hyperion Catalysis International at 15% concentration. Let downs from this concentration were spun into fibers using a single screw extruder. However, the resultant fibers exhibited a rough texture and distinct lumps of aggregated carbon nanotubes due to inadequate mixing and dispersion of the concentrated CNT/PP and virgin PP. To address this issue a twin screw extruder was used to compound the polymer into several lower concentrations and a second attempt at spinning yielded greater success. Fibers containing up to 3% CNT were spun as well as some bicomponent fibers. The fibers spun were slightly smoother than those of the initial trial although lumps along the fiber surface are still evident, especially at higher loadings.

Imaging the fibers under optical and scanning electron microscope reveals the extent of the nanotube agglomerate formation and the severe deformation of the fibers. The aggregates of carbon nanotubes appear in all composite fibers and cause the tensile properties to suffer by acting as stress concentration sites, leading to fiber failure. Conductivity is not achieved even with the highest loading of 3% carbon nanotubes. A uniform distribution of the nanotubes in the polypropylene is believed to be critical to spinning uniform fibers with good mechanical characteristics and to reaching percolation at low loading. CNT aggregation remains a challenge to the processing of these composite fibers.