Basic Study of Extensional Flow Mixing for the Dispersion of Carbon Nanotubes in Polypropylene by Using Capillary Extrusion

Abstract

This study evaluated the mixing effect of simple uniaxial extensional flow for the dispersion of multiwalled carbon nanotubes (MWCNTs) into polypropylene (PP) as a nonpolar matrix. An only converging flow allowed for a high strain rate and was suitable for the compounding process. The extensional flow was characterized from the entrance pressure drop (ΔP0) at the converging section. Thus, in this study, capillary extrusion was employed to generate uniaxial extensional flow. Based on the hypothesis that the dispersion of nanofillers depends on the magnitude of flow-induced stress, ΔP0, which related to extensional stress, was measured directly during capillary extrusion by using an orifice die. The influences of the mass flow rate and the hole diameter in the orifice die, which affected ΔP0, on the extrusion of PP nanocomposites with an MWCNT loading of 1.0 wt.% were studied. The extruded samples were collected, and the dispersion state was evaluated based on the melt viscoelastic properties, volume resistivity, and morphological observations by optical microscopy (OM) and transmission electron microscopy (TEM). The agglomeration area of the MWCNTs decreased with higher ΔP0 (higher mass flow rate and smaller hole diameter), which increased the uniformity of the dispersion. Moreover, the influence of the length-to-diameter (L/D) ratio of the hole in the capillary die on the dispersion state of the MWCNTs was investigated. A higher L/D ratio of the capillary die did not improve the dispersion state, although shear and extensional stresses were provided.

A modified blister mixing element for generating extensional flow in a twin‐screw extruder: Process characterization and dispersion state of carbon nanotubes in cyclo‐olefin polymer

“…In a previous study, we showed that the simple uniaxial extensional flow generated by an orifice die in a capillary rheometer called “converging flow” could enhance the dispersion state of carbon nanotubes (CNTs) in a polypropylene matrix with a higher entrance pressure drop related to extensional stress. [ 4 ]…”

“…In a previous study, we showed that the simple uniaxial extensional flow generated by an orifice die in a capillary rheometer called "converging flow" could enhance the dispersion state of carbon nanotubes (CNTs) in a polypropylene matrix with a higher entrance pressure drop related to extensional stress. [4] In the TSE, kneading blocks (KB), which are a type of screw segment, produce an intensive shear flow field, and dispersive mixing and distributive mixing are controlled by an offset angle, a number of discs, and a specific disc width. [5] However, due to the limitation of the mixing capacity of KB for the dispersion of nanofillers, many researchers have attempted to increase the extensional flow components using convergence-divergence (C-D) geometry in the mixing segments and devices.…”