Performance without compromise

at the nano-scale. Here, we look at the potential benefits and how research at the Department of Chemistry at Imperial College London is making such materials a reality.
Continuous fibre-reinforced polymer composites have revolutionised manufacturing due to their superior mechanical properties and light weight, combined with chemical and weather resistance. Although the stiff and strong continuous fibres impart impressive in-plane properties to the composite, the relatively weak, matrix-dominated compression and interlaminar properties remain major issues. Matrix toughening and Z-pinning (adding reinforcing fibres in the Z-axis) are promising approaches, but have various drawbacks. Interest is growing therefore, in carbon nanotube-based hierarchical composites, where nano-scale reinforcements are utilised alongside the traditional, micro-scale continuous fibres.

Carbon nanotubes (CNTs) are essentially one or more sheets of graphite rolled into cylinders. Individual, perfect CNTs possess exceptional stiffness, strength and strain at failure in the order of 1,000GPa, 60GPa and 5%, respectively, as well as the highest known thermal conductivity, versatile electronic conductivity and low density. Several techniques have been developed to spin them into macroscopic fibres, resulting in the strongest and toughest materials known, despite the as yet imperfect spinning processes currently available. These intrinsic properties have generated huge excitement in the use of CNTs in composite materials, either in the form of fibres, or as a nanoscale reinforcement for (usually polymer) matrices.

Building on a nano scale

The most common form of nanotube used in composites is multi-walled, grown by chemical vapour processes. Dimensions can vary significantly, but diameters of around 10-15nm and lengths in the order of tens of microns are common. These materials are relatively pure and cheap, but have a high defect concentration that reduces headline properties. Nevertheless, annual production capacities measure in thousands of tonnes, and they are available for immediate use.

Micro- and nano-fibre systems can be combined fundamentally through two different routes: adding CNTs into the polymer matrix or attaching them onto the primary fibres (see Fig 1). The intention is that the presence of CNTs may alleviate the drawbacks of conventional fibre composites associated with poor matrix properties. For example, they could offer both intralaminar and interlaminar reinforcement, improving delamination resistance and other through-thickness properties without compromising in-plane performance. In addition, they may reduce the tendency for fibre micro-buckling under compression and offer functional benefits such as electrical and thermal conductivity (lightning strike protection), solvent resistance, flame retardance and increased operating temperatures.

The performance of hierarchical composites containing CNTs, like that of pure nanocomposite systems, will be dictated by the intrinsic quality, dispersion, alignment and the interfacial chemistry of the CNTs. At present, the fabrication of optimised microstructures is largely limited by the processing methods available.

CNT filled resins can be produced by a variety of shear mixing methods. Traditional composite processing by resin transfer moulding (RTM) or resin film infusion (RFI) can produce hierarchical composites at low loadings of CNTs, offering a range of improved properties. For example, the addition of CNTs to the matrix has led to significant improvements in interlaminar shear strength (33%), as well as Mode I (100%) and Mode II (75%) fracture toughness. However, the high viscosity of CNT loaded resins and the tendency for self-filtration of the CNTs on the fibre tows has limited the CNT content typically to around 1-2wt%. If CNT loading can be increased while maintaining good dispersion and alignment, critical mechanical properties should increase.

Hairy tubes

The alternative approach to ensuring good CNT distribution and alignment is to have them protrude radially from the carbon fibre into the matrix, with the benefit that the reinforcement is focused on the matrix-fibre interface - a critical area when considering toughness - and in the likely optimum orientation. This also alleviates the problem of agglomeration commonly observed when CNTs are freely dispersed in resin. The feasibility of employing hairy fibres as a constituent in HCs has thus far been investigated by measuring the interfacial shear strength (IFSS), with promising results.

However, the direct grafting of CNTs onto the carbon fibres during carbon vapour deposition can significantly reduce the tensile strength of the fibres due to damage caused by the iron catalyst during growth, whilst other grafting approaches do not yield a radial orientation. Nevertheless, there are excellent prospects for developing CNT grafting methods that avoid such damage.

At Imperial College London, the problem of incorporating CNTs into traditional continuous fibre composites has been addressed at all stages from nanotube synthesis and modification to manufacture and testing of hierarchical composites. The Imperial team synthesises CNTs in a variety of controlled dimensions and a variety of surface functionalities. Shortening the tubes allows them to be more readily dispersed, while surface functionalisation allows their surface chemistry to be matched with a variety of polymer matrices.

Keeping it simple

Instead of carrying out chemical modifications of CNTs in the liquid phase, which creates challenges in purification and waste disposal, new patented techniques have been developed to use purely gas phase chemical cutting and modification methods that are compatible with the carbon vapour deposition method. The direct grafting of CNTs onto primary reinforcing fibres has been explored on both carbon and silica fibres.

The route is easily scalable with the nanotubes grown using vapour deposition and can be optimised by controlling catalyst concentration and growth parameters (furnace temperature, feed gas ratios, etc) so as to minimize damage. The IFSS of the grafted fibres improved by about 26% for IM7 and 150% for silica fibres compared to ‘as received’ fibres, while the loss in tensile strength was more moderate than in previous studies. These single fibre level results are encouraging, and support further development of the approach to a larger scale. Given that high performance IM7 carbon fibres are widely used in aerospace and F1 applications, these results suggest that existing systems could be enhanced using CNT grafting technology.

There has also been a thrust at Imperial College to produce hierarchical composites based on thermoplastic matrices for subsurface pipes used in oil and gas. Implementing a hierarchical reinforcement scheme in a chemically inert matrix would produce a material for use in applications where chemical resistance coupled with high strength are required. In addition to chemical resistance, PVDF is typically known to exhibit abrasion resistance, thermal stability, radiation resistance, excellent electrical insulation and gas barrier properties.

Tensile tests of the CNT reinforced matrix show that the addition of nanotubes increases the tensile modulus by about 10% and strength by 38%. Unidirectional carbon fibre reinforced PVDF composite and hierarchical composites were produced in prepreg form via a powder impregnation route. In compression tests of the hierarchical composites, the strength increased by as much as 28% while the modulus remained relatively unchanged.

The Imperial College team and others are continuing to develop promising hierarchical composite systems with current emphasis on scaling up production to produce larger specimens for comprehensive mechanical testing, relevant to the next stage of development.

www3.imperial.ac.uk/nanostructuresandcomposites