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EPSRC: Risk assessment of novel CNT manufacturing processes

EPRSC_200IOM & Napier University undertook work to provide risk assessment for a EPSRC project led by Cambridge University which aimed to develop new production technologies for carbon nanotubes.

A summary is provided below:

Nanoscale particles can show properties which are different from the bulk, and are potentially advantageous for various applications. For example, nanoparticles of semiconductors have highly definable colours and hence are useful for security inks, while atoms of radionucleotides, used for tracer analysis in medicine can be held within bucky balls and kept separate from body chemistry. On the mechanical front, carbon nanotubes, which are not much larger in size than polymer molecules, show exceptionally axial strength and stiffness, the strength of an individual nanotube being at least ~10 times higher than that of any known fibre. The challenge is to make the nanotubes consistently and then to build them into fibres so that some of their brilliant mechanical properties can be translated into useful engineering materials.

A process to make carbon nanotube fibres in a single operation has recently been demonstrated by the Cambridge team. The potential of the process (announced in "Science") for making high performance fibres has led to considerable interest worldwide, both from the existing fibre industry, for whom it represents a disruptive technology, and from fibre users. However, the 'technology pull' is such that our insight into the process at a basic level needs to catch up. We need to be able to produce nanotubes of predetermined dimensions as the first stage towards a fibre product with highly consistent properties. The reason for the exceptional properties seen is not fully understood, nor is the relation between process parameters and the resultant structure. A deeper understanding is also necessary as a basis for scale-up strategies, which will be critical in estimating the likely industrial cost of the product, and thus the future risk.

The fibres made so far promise strengths and stiffnesses which will at least rival current carbon and aramid fibre products, while the energy absorption on fracture is several times that of these materials, commending the material for the burgeoning markets in body armour and vehicle 'hardening'. However, the intrinsic, one-step simplicity of the process indicates that the product should be very much cheaper than any equivalent currently available. Indeed, the process might be viewed as a highly refined version of that used to make carbon black, a commodity which sells for about 1/50th of the cost of carbon fibre. If this new cheaper fibre is successful in composites, it could bring down the cost of transport vehicles, enabling F1 structural technology to reach the family car.

The first stage of the project will be to build a fully instrumented production rig, to learn more about the nanotube growth and the origin of defects which are a source of inconsistency in measured properties. Key experiments will be undertaken to determine the best approach to scale-up, in particular a second reactor will be built to evaluate to miniaturise the process as a scale-up strategy. There is so much yet to be understood. Kilometre lengths of fibre will be produced so that the applications can be externally assessed.

Carbon nanoparticles provide opportunities for medicine: drug delivery and cancer treatment being two examples. However, the enthusiasm of pharmacologists and oncologists is balanced by cautionary notes from toxicologists. The properties which make nanoparticles unique lead to effects in vivo which may either be beneficial or detrimental. In the case of nanotubes, the latest toxicological studies indicate that they are no more toxic than particles from (say) a laser printer, however, it is recognised that the human body may have difficulty in eliminating nanotubes in the long term. From the business angle, any nanoparticle scare, whether well founded or not, may hold development back and at worst put investment at risk. As we take this work forward the materials researchers at Cambridge will work closely with toxicologists at Napier University and the IOM.

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