Electrospun nanofibre veils and carbon fibre-based veils for enhanced electrical conductivity of carbon fibre reinforced polymer composites

Abstract

Carbon fibre reinforced polymer (CFRP) composites are comprised of high performance reinforcement carbon fibres and a polymer resin matrix. CFRP composites have been used by the commercial aerospace industry to manufacture structural composites due to their high mechanical strength, impact resistance, and thermo-mechanical properties. Carbon fibres are electrically conductive, however, epoxy-carbon fibre laminates have limited electrical conductivity in the through-thickness direction due to the insulating nature of the epoxy matrix. Enhanced through-thickness electrical conductivity is now desired for modern composites applications requiring lightning strike protection, electrostatic dissipation, and electromagnetic shielding.

Various technologies and materials such as resin modification, fibre surface modification, and interleaving have been investigated with the target to increase CFRP performance and through-thickness electrical conductivity. Interleaving, where a thin interleaving material is inserted between the dry carbon fabric plies or prepreg layers, is a common method for improving the interlaminar fracture toughness (IFT) and electrical conductivity of CFRP. Interleaves are also easy to add to the production process. Interleaving reinforcement is expected to improve the mechanical performance of composites under structural loads.

This thesis addresses the effect of conductive veil interlayers on enhancing the through-thickness electrical conductivity of CFRP composites. The research work carried out in this thesis was focused on investigating the use of electrospun nanofibres and microfibrous carbon fibre-based veils interleaved in carbon fibre reinforced polymer composites. These materials were used to assess the potential of the veils to improve the through-thickness electrical properties without compromising the mechanical properties of the composites. Electrospun nanofibre veils are well suited to resin infusion due to their high porosity, which makes it easier for resin to flow and wet well the nanofibres, providing favourable interfacial properties and mechanical robustness to the end-product.

The research work started with the design and development of a low cost nozzle-free electrospinning setup that can efficiently and evenly deposit the electrospun nanofibres at a higher production rate over a large surface of carbon fibre fabric. The equipment uses a rotating mandrel partially immersed within a polymer solution to produce fibres in an upward motion by inducing the formation of multiple Taylor cones and subsequently multi-jetting out of an electrified open surface. This lab-scale, high-throughput device has provided an alternative, economical route for nozzle-free electrospinning research, in contrast to the high costs associated with industrially available upscaling equipment. Among the device’s technical specifications, a key feature is a cryo-collector mandrel capable of collecting nanofibres at sub-zero temperatures, which can induce ultra-porous nanostructures with wider pores. A multi-channel gas chamber allows the conditioning of the atmosphere, temperature, and airflow, while the chamber’s design averts user exposure to the high-voltage components.

The processability of conductive polymers is an important research area in the field of polymer science. There have been several techniques reported in the literature on this topic. However, the scalability of many of those techniques and their manufacturability on an industrial scale are not always practically viable. The electrospinning of conductive polymers, in particular, has not been extensively studied, despite the potential commercial applications of electrospun nanofibers in areas such as sensors, anti-static coatings, membranes, biocompatibility, and energy storage. Polyaniline (PANI) has significant advantages among conducting polymers, it based on a low-cost aniline monomer. This polymer is easy to synthesise, has tuneable electrical conductivity, and high environmental stability.

PANI has a low molecular weight, low solubility, an infusible nature, and a rigid backbone structure, which makes it necessary to be blended with other electrospinnable polymers to produce nanofibres. The conductive forms of PANI, polyaniline emeraldine base (PANI-EB), and polyaniline emeraldine salt (PANI-ES) were synthesised and characterised. Polyvinylpyrrolidone (PVP) has good solubility, mechanical strength, film forming, electrospinability, high molecular compatibility with other polymers, and the ability to form thermally stable composites. The PVP chemical interaction with resin could provide good compatibility and enhance the mechanical properties of the CFRP composites.

In order to achieve the conductive electrospun nanofibres, polyaniline (PANI) in its emerald base (PANI-EB) and salt (PANI-ES) forms was combined with PVP to produce nanofibres in a one-step electrospinning process using a nozzle-free electrospinning setup to achieve the conductive electrospun nanofibres. The surface morphologies and chemical structures of the PANI-EB/PVP and PANI-ES/PVP nanofibres were characterised using different techniques. The incorporation of different contents of PANI-EB and PANI-ES in the PVP solution and their impact on the structure and properties of the nanofibres were investigated. Through electrical characterisation, It was found that a high amount of PVP compared to PANI (18:1 W/W) was required in order to electrospin the PANI/PVP solutions into nanofibres. This could be the reason that PANI/PVP electrospun nanofibre veils and their interleaved CFRP composites were not conductive; the presence of an insulating carrier polymer restricted the nanofibre veils' ability to reach the required electrical conductivity percolation threshold and their possible use in aerospace applications requiring higher electrical conductivity. A higher amount of PANI in the PANI/PVP solution was found to be unsuitable for electrospinning due to PANI’s higher viscosity and electrical conductivity. Therefore, an alternative route was explored based on prefabricated veils.

As the electrospun nanofibre veils were not conductive, pre-fabricated microfibrous carbon fibre-based veils were used as interleaving in CFRP composites. These veils were made from loosely connected carbon fibre and nickel-coated carbon fibre with a binder, and their random orientation forms a porous network. Carbon fibre (CF) or nickel-coated carbon fibre (NiCF) veils were used as interlayers between standard carbon fibre reinforcement fabrics. The through-thickness electrical conductivity of the interleaved composites with CF or NiCF veils improved over 50 fold, from 0.18 to 9.47 and 9.16 S/cm, respectively, compared to the control composites. The introduction of conducting veils facilitated the creation of electrical pathway between the carbon fabric plies by reducing the non-conducting resin rich zone in the interlaminar region. However, the interleaved specimens exhibited a ca. 20-24% reduction in their interlaminar shear strength (ILSS) and flexural strength.

In conclusion, the development of a nozzle-free electrospinning setup has proven its ability to overcome limitations and drawbacks associated with single and multi-nozzle spinneret configurations, such as low yield, limited production capacity, non-uniform electric field distribution, and clogging. This lab-scale high-throughput device can provide an alternative, economical route for needleless electrospinning research, in contrast to the high costs associated with industrially available upscaling equipment. This can help electrospun materials advance research in tissue engineering, wound healing, energy storage, energy harvesting, and other applications in a wide range of industrial sectors. This research revealed the difficulties associated with the production of conducting electrospun veils based on PANI, but other types of veils such as the prefabricated microfibrous carbon fibre-based veils show considerable promise. This research shows that conducting veil-interleaved CFRP composites can meet the functional integration requirement of the aerospace sector for electrical properties and can find applications in lightning strike protection, electromagnetic shielding, electrostatic dissipation, and structural health monitoring. The interdisciplinary work presented in this thesis opens up new pathways for exploration in regards to the fields of materials science, composites, and nanotechnology.