F. W. J. van Hattum

Production and Assessment of Polycarbonate Composites Reinforced with Vapour-Grown Carbon Fibres

Abstract Vapour-grown carbon fibres were produced from methane in a flow reactor. The fibres were compounded with poly-carbonate in a co-rotating twin-screw extruder and the composites were subsequently injection moulded. The rheological characteristics of the polycarbonate composites reinforced with these vapour-grown carbon fibres were determined by using capillary and rotational rheometry. The tensile properties of the injection-moulded specimens were marginally better than those of the un-reinforced polycarbonate, but the impact resistance was severely diminished by the addition of the carbon fibres. This was attributed to the presence of polycyclic aromatic hydrocarbons on the surface of the fibres, that may impair the impact resistance of polycarbonate by chemical stress cracking.

The piezoresistive effect in polypropylene—carbon nanofibre composites obtained by shear extrusion

The piezoresistive effect on poly(propylene) (PP)–carbon nanofibre (CNF) composites fabricated by twin-screw extrusion and compression moulding has been investigated. The electrical and mechanical properties of PP/CNF composites have been obtained as a function of CNF concentration. Electrical conductivity exhibited low thresholds and values close to the required levels for EMI shielding applications at 2.4 vol%. Meanwhile the elastic modulus showed an enhancement with a maximum up to 130% for one of the composites at 0.9 vol% loading. Further, the piezoresistive response has been evaluated in four-point bending. Positive gauge factors between 2 and 2.5 have been obtained. The highest gauge factors are found within the percolation threshold. The characteristics of the materials and the production technique make them suitable for large scale applications.

Effect of filler dispersion on the electromechanical response of epoxy/vapor-grown carbon nanofiber composites

The piezoresistive response of epoxy/vapor-grown carbon nanofiber composites prepared by four different dispersion methods achieving different dispersion levels has been investigated. The composite response was measured as a function of carbon nanofiber loading for the different dispersion methods. Strain sensing by variation of the electrical resistance was tested through four-point bending experiments, and the dependence of the gauge factor as a function of the deformation and velocity of deformation was calculated as well as the stability of the electrical response. The composites demonstrated an appropriate response for being used as a piezoresistive sensor. Specific findings were that the intrinsic piezoresistive response was only effective around the percolation threshold and that good cluster dispersion was more appropriate for a good piezoresistive response than a uniform dispersion of individual nanofibers. The application limits of these materials for sensor applications are also addressed.

Polypropylene-Carbon Nanofiber Composites as Strain-Gauge Sensor

Polymeric materials have been replacing other materials in various applications, from structural to electronic components. In particular, since the discovery of conducting polymers and the beginning of the manufacture of conducting composites with carbon fillers, their use in electronics has been growing. A group of electronic components with large potential for industrial applications such as structural monitoring, biomedical, or robotics are sensors based on the piezoresistive effect, fabricated from conductive polymers and/or composites. The aim of this article is to characterize the piezoresistive effect of conductive polymer composites based on polypropylene filled with carbon nanofibers, and to demonstrate a way of fabricating strain gauges from these materials, using industrial techniques. With this purpose, some films are prepared by shear extrusion, which allows the composites to be produced industrially in a standard nonexpensive process. Then, the dependence of the electrical response both on the preparation conditions and on the mechanical solicitations is measured. The obtained gauge factor values, up to 2.5, and piezoresistive coefficients up to 0.0019 mm2/N, prove the viability of these materials for fabricating strain-gauges, where their main advantages are the lower price and the ability to deal with much higher deformations, when compared to metal or semiconductor strain-gauges.

Direct fibre simulation of carbon nanofibres suspensions in a Newtonian fluid under simple shear.

This work studies the effect of simple shear flows on the dispersion of carbon nanofibres (CNF) in a low viscosity Newtonian matrix. Analysis was performed by using a direct fibre simulation based on Particle Simulation Method (PSM) especially focusing on structure changes under shear flows. Suspensions of CNF/epoxy of different concentrations varying from 0.1 to 3.0 wt% were prepared by simple hand-mixing the as-received fibres with the epoxy resin. These suspensions were then subjected to different flow conditions (shear rates and strains) and their rheological behaviour was characterised. It is demonstrated that simulation can reproduce shear-thinning and structure developments induced by shear flow at different shear rates. Especially at low shear rates, flow induced helical bands were predicted by simulation, which was observed in experiments performed by other researchers and reported in the literature. From the results obtained it is concluded that the direct fibre simulator is a useful tool to design nano-scale fibre composites, allowing for optimisation of the critical parameters to determine the most adequate conditions leading to the required level of fibre dispersion.

Conductive long fibre reinforced thermoplastics by using carbon nanofibres

Abstract In the present work conductive semistructural composites were produced by injection moulding, combining the mechanical properties of long (glass) fibre thermoplastics (LFTs) with the conductive properties of carbon nanofibres (CNFs). The moulded LFT/CNF parts exhibit percolation behaviour between 2 and 3 wt-%CNF loading, characterised by a sharp drop in electrical resistivity from 1012 to 102 Ω cm, accompanied by a step increase in the shear viscosity at low shear rates. At the same time, the main LFT properties are maintained: the modulus increased slightly, strength and viscosity, and thus processability, remained unaffected, while the impact resistance decreased. In addition, a significant improvement in surface quality was observed, largely eliminating the glass fibre printthrough, typically found in LFTs. The results thus show the successful use of small amounts of CNFs in making LFTs conductive, which becomes suitable for, e.g. electrostatic painting at limited additional cost.

Cost reduction in manufacturing of aerospace composites

Abstract Advanced fibre reinforced composites are already extensively used in modern aircraft, due to advantages offered for weight reduction, durability, mechanical performance, etc. However, composite structures in aerospace are usually not associated with low costs. Traditional design methods are very time consuming, materials and production processes very expensive as well as labour intensive. To make full use of the potential of composites, a complete redesign of aircraft structures is necessary. Owing to qualification and certification procedures the introduction of low cost materials or low cost production processes is hindered. The current paper describes the results of a collaborative research project aimed at achieving significant cost reductions in the manufacturing of advanced composites for aerospace applications. A highly integrated redesign of a generic wingbox section was made and a low cost, room temperature, open mould (non‐autoclave) resin infusion production process was developed using a room temperature curing resin system for the production of the demonstrator.

Thermal diffusivity measurements of vapour grown carbon fibre composites, using the optical beam deflection technique

Abstract Carbon fibres are used widely as reinforcements for polymeric matrices in high-tech applications by virtue of their good mechanical, electrical and thermal properties. Vapour grown carbon fibres (VGCF) are a new type of fibres whose technological applications, namely reinforcements for thermoplastic matrix composites, are yet to be developed. Due to their potentially low cost and high production rates, VGCFs show promising new possibilities in this area. Recently, in industry worldwide, the potential of VGCFs in thermal and electrical applications such as solid-state batteries and electrostatic painting of high volume automotive plastic parts is increasingly being recognised. As a result, easy and non-destructive measurement of thermal and electrical properties of VGCF-based composites has become an important issue for use in industrial applications. In this study the optical beam deflection technique was used for the evaluation of the thermal diffusivity of vapour grown carbon fibres (VGCF) – polymer composites. It is shown that, for low diffusivities, simple approaches such as the “phase method” or the “zero crossing method” are not valid and a multiparameter fitting to the amplitude values has to be used instead. The thermal diffusivity of the composites, measured along the direction of the fibre preferential orientation, increases with fibre content. This increase is not very marked, possibly due to poor adhesion between fibre and matrix.

Tailored shear extrusion of carbon nanofibre/ polyamide composites and its effect on electrical percolation threshold

Abstract Carbon nanofibre (CNF) thermoplastic composites can offer permanent conductivity at electrostatic discharge levels with very low filler levels, without affecting processing and other beneficial properties of the host polymer. This capacity makes these materials of particular interest for, for example, electrostatically painted automotive parts. However, ultimate properties depend largely on the processing history of the material. The present paper describes a methodology to optimise the production of nylon 6 composites reinforced with CNFs by balancing fibre dispersion and fibre degradation during melt compounding. The optimisation approach used combines a modelling routine of the compounding process with a multiobjective evolutionary algorithm able to deal with multiple objectives simultaneously. The electrical properties of the composites, compounded and subsequently injection moulded at several fibre loadings, revealed direct relationships with the modelling optimisation parameters. In this way, the potential of tailoring operating conditions and/or screw design during compounding to obtain desired final product properties was demonstrated.

Continuous Production of Polycarbonate-Carbon Nanotube Composites

One of the main obstacles in current research in polymer carbon nanotube composites is the high price of carbon nanotubes and thus their availability in large quantities. This critically limits the range of techniques that can be used to prepare the composites. As such, the development of methods for their continuous production in small quantities can bring great benefit to the research in this area. The present text describes an investigation on the continuous, laboratory-scale, production of polymer/carbon nanotube composites by extrusion. The objective of the study was not only to make new materials, but also, through appropriate design, to enable the scaling-up of the production technique. Multi-wall carbon nanotubes (MWNTs) were made by chemical vapour deposition of acetylene on alumina supported transition metal catalysts. Composites were then produced in a purpose built micro-extruder and characterized at the nano- and macroscopic scales. It was found that important processing parameters of the current micro-extruder were comparable to those of industrial extruders. Thus, it can be anticipated that the quality of composites produced with it will be comparable to that of composites of the same system produced in large-scale equipments.