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Lightweight non-metallic composites exhibiting high thermal/electrical conductivity and high performance are favoured for a wide number of applications from high-volume commodity industrial applications to high-tech applications. Carbon nanofibres are one of several emerging nanomaterials available to boost the physical properties of composites and expand their applications and volume of use.
(Published on October 2008 – JEC Magazine #44)
PATRICK LAKE, PROJECT ENGINEER, AND
CARLA LEER, RESEARCH SCIENTIST,
APPLIED SCIENCES, INC.
The availability of nanocarbon materials, including single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs), carbon nanofibres (CNF) and nanographene platelets (NGP), offer the promise of capturing the ultra-high mechanical and transport properties of the graphite bond in a new generation of carbon fibre reinforced polymers which are of great technological importance. Carbon nanofibres (CNF) (see Figure 1) – a form of vapour-grown carbon fibres – are similar to carbon nanotubes in structure, with comparable mechanical and transport properties but a diameter of approximately 100 nm, and lengths of tens to hundreds of microns. High-purity CNF is available in commercial volumes at affordable prices from several suppliers worldwide.
The combination of these dimensions with low production costs and satisfactory availability allows CNF to bridge the gap between commercial continuous carbon fibre used widely for structural composites and single- or multi-wall nanotubes developed for sophisticated applications such as molecular electronics and sub-micron biomedical applications, both in terms of material processing and end applications. In the composite industry, the interest in carbon nanofibre stems from its potential to provide true multifunctional capabilities. CNF has been successfully used as an additive in polymer composites to modify several physical properties including thermal conductivity, coefficient of thermal expansion (CTE), absorption and scattering of electromagnetic radiation, electrical conductivity, flame retardancy, electron emission, and vibration damping. A recent review by Tibbetts et al. gives a useful overview of the achievements made thus far on CNF composite preparation and their properties .
Although much progress has been made, a bottleneck in achieving industrial applications for these composites is the homogeneous dispersion of the CNF into polymeric matrices. Ultimately, achieving superior transport properties in polymer composites comes down to a proper choice of fibre type, and geometry, and mastering the composite processing steps – more specifically the effective dispersion of the nanofillers in the continuous polymeric phase – or finding new product designs which overcome processing and dispersion difficulties.
One example is the new CNF preforms (Figure 2), which are easy to handle and overcome many of the difficulties attending to dispersion. Applied Sciences, Inc. has recently introduced a new fibre grade, PR-25-XT, that is easily dispersed in a wide range of solvents and polymeric matrices, has a higher surface area, shows unique transport properties due to the highly graphitic structure, and has numerous available sites for chemical functionalization (Figure 3).
CNF nanocomposites for aerospace applications
CNF nanocomposites are finding their way into numerous aerospace applications. CNF is used to impart electrical and thermal conductivity in adhesives for bonding. It is fairly well developed, and CNF-loaded thermal grease currently on the market provides unparalleled performance. Airframe manufacturers have identified prospects for several composite products enhanced by CNF, including structural reinforcement, structural health monitoring, thermal management, EMI shielding, lightning strike protection, vibration damping and de-icing [2, 3, 4]. A simple, low-cost alternative to metal films or foils on composites requiring EMI shielding is to incorporate carbon nanofibre into the composite structure to increase its conductivity and shielding effectiveness. Incorporating CNF eliminates the need for special handling during moulding and subsequent processing, thereby reducing complexity and cost. Furthermore, since CNF can be incorporated into the composite structure, concerns over surface scratches, delamination, or corrosion resistance of metal films are eliminated. Of course, CNF has the added benefit of improving mechanical properties and thermal conductivity in polymer composites as well, leading to potentially further weight reductions and enhanced thermal management. Adding CNF to polymers has been shown to provide lightning strike protection. When tested in lightning strike configuration, CNF-loaded composites have survived multiple lightning strikes without damage.
One estimate is that the use of such composites in large aircraft could ultimately make it possible to replace approximately 22.7 metric tons (MT) of aluminium with around 18 MT of composites, for a weight savings of about 4.7 MT per plane. Each kilogramme of weight saved provides significant fuel savings over the life of the airplane. The present worth of these savings is estimated at more than $200,000 per kilogramme saved.
CNF nanocomposites for automotive applications
Several of the current applications for CNF in the aircraft industry are increasingly being considered for the automotive industry. These include EMI shielding and thermal management for under-the-hood electronics, and use as an electrical conductivity additive for many polymer components. CNF nylon composites are already widely used for fuel tanks and lines in some vehicles; however, many additional opportunities are on the horizon. The automotive industry is particularly interested in weight reduction to improve fuel economy, and in lowering manufacturing costs for its vehicles. One area that has already shown increased demand is sheet moulding compounds (SMC) for body panels. SMC compounds are lightweight and produce “Class A” surface finishes that make them attractive for the various painting operations conducted to meet customer needs. The use of SMC reduces the weight of body panels by 20 to 40%, thus providing significant gains in vehicle mileage. The current technology is based on electrostatically spray-painting the body panel after it has been formed from SMC and sprayed with a conductive primer coat like a titanium-based paint. The longterm need in the automotive industry is to develop a conductive SMC avoiding the use of the expensive conductive titanium undercoat required to apply the electrostatic spray paint. Although prior attempts to use PAN- and pitch-based carbon fibres, conductive carbon blacks, and graphite flakes have been unsuccessful in producing a consistently reliable, electrostatically spray-paintable SMC with a Class A finish, the use of CNF for the electrical conductivity additive provides this essential property. Another rapidly growing automotive application which stands to benefit from CNF additives is long fibre thermoplastics (LFT). CNF can be readily incorporated into LFTs to impart thermal and electrical conductivity, while improving the mechanical properties of thermoplastic composites.
As composite technology develops to meet the opportunities for lightweight structural applications, CNF can be considered for extending the physical properties of conventional carbon and glass fibre composites for those applications. CNF can be used to achieve improved interlaminar mechanical properties, impart transport properties, tailor coefficients of thermal expansion, improve vibration damping, and enable structural health monitoring of composites, thus providing designers with a tool to achieve multi-functional benefits in new generations of lightweight structural composites .