Processing, Structure and Properties of Nanocarbon/Polypropylene Nanocomposites

Abstract

Multifunctional nanocarbon/polymer nanocomposites (NC) fibres have shown potential applications in electrostatic discharge, sensing and energy storage. Polymer NC fibres require being not only electrically conductive to provide functionalities but also mechanically strong to avoid damage during further processing and in service. This project focuses on nanocarbon/polypropylene (PP) NCs aiming to build up knowledge of their processing-structure-property relationships and develop PP NC fibres with electrical conductivity (EC) and mechanical properties sufficient for incorporation into conventional textile composites in combination with structural fibres, such as glass or carbon fibres. A cost-effective melt spinning technique and commercially available multi-walled carbon nanotubes (CNTs), graphene nanoplatelets (GNPs) and a fibre-grade PP were used in this study. Twin-screw extrusion compounding was used to produce PP-NCs. A multi-step masterbatch-dilution process using 3-step compounding with modified screws was developed to increase the dispersion of CNTs for melt spinning. The level of macro-dispersion of CNTs in melt compounded NCs was quantitatively characterised using optical microscopy (OM). A power law relationship between the level of macro-dispersion and the mechanical energy input has been found, which can be used for predicting the level of macro-dispersion of CNT NCs with higher loadings (5 wt.% and 7 wt.% CNT), which could not be characterised by OM analysis. The microstructure of moulded NCs has been studied using SEM and Raman spectroscopy. The EC of NCs was significantly affected by flow-induced orientation of CNTs and GNPs. Specifically, in the in-plane direction, compression moulded CNT NCs showed a concentration percolation threshold (Pc) of 0.81 wt.% due to a near-random structure while injection moulded CNT NCs showed a higher Pc of 1.44 wt.% due to a layered skin-core structure, resulting from fountain flow. Moulded hybrid CNT/GNP NCs showed higher EC than moulded CNT NCs at the same total loading, exhibiting a synergistic effect. For NC fibres, EC decreased upon increasing melt draw ratio and cold draw ratio due to increased orientation of CNTs and GNPs as evidenced by Raman spectroscopy and 2D-XRD. From an orientation percolation analysis, it has been found that CNTs are more efficient in forming a conductive network in fibres, whereas hybrid CNT/GNP system seems more efficient in withstanding the network break down under fibre drawing, ascribed to the planar face-to-face sliding of GNPs. NC fibres with a high molecular orientation exhibiting high Young’s moduli exceeding 20 GPa, high tensile strength values exceeding 1 GPa and EC exceeding 3×10-5 S/cm were developed successfully. However, the tensile strength of NC fibres was found to be limited by defects, both surface (surface roughness) and internal (the presence of large GNPs).

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Prasad Potluri (Supervisor) & Arthur Wilkinson (Supervisor)