A lifeline for waste carbon fibres
Last month, Materials World carried a feature on the mechanisms being explored for recycling carbon fibre reinforced polymer composites. Rupal Mehta takes the story further with a report on a conference on carbon fibre recycling and reuse.
The announcement that Boeing is not necessarily ‘gearing towards [carbon fibre reinforced] composites’, might have surprised some delegates, but was characteristic of an insightful conference into carbon fibre recycling, held in Hamburg, Germany, from 3-4 November 2009.
Companies around the globe are being increasingly judged, and taxed, for the amount of waste they produce and the quantity that goes into landfill. Yet, while lightweight carbon fibre reinforced polymers (CRFP) boast other environmental credentials, reprocessing of waste CFRP produced during manufacturing or from end of life components is not well established. This poses complex questions for lifecycle assessors as heavier steel and aluminium do have strong recycling systems, and, furthermore, carbon fibre manufacture is said to require more energy per tonne than aluminium production from its ore.
The conference on Carbon Fibre Recycling and Reuse – Developing a Supply Chain for End of Life Carbon Fibre Reinforced Polymers was hailed as the first of its kind, perhaps indicating that enough progress is now being made to warrant the topic being the headline act.
Steve Line, CEO of Recycled Carbon Fibre Ltd, Dudley, UK, did not mince his words when he told delegates, ‘The research into how you recycle these materials is kids’ play compared to how you commercialise the process’.
The company claims to have established ‘the world’s first commercial scale continuous recycled carbon operation’ earlier this year, based on pyrolysis. Line revealed the problems, however, in ensuring market acceptance of the end product. ‘People won’t adopt the material unless they know what it is,’ he said. ‘It has to go through a technical review. [But] one of the challenges is how you get the feedstock – there needs to be education of suppliers. No-one supplies the fibre certificates, even for out-of-date prepregs, so you never know the actual fibre properties.
If you don’t know the source of material then you cannot guarantee what comes out at the end.’
Another issue is ‘The price of recycling’. Line asked delegates whether they would expect to pay a premium or a discount for recycled materials if scientific evidence proves that, though different materials, they perform as well as their virgin counterparts. When many of the attendees opted for the discount, he questioned the decision and admitted, ‘There is a paradigm – recycled equals inferior. The user [such as an automotive manufacturer] who is trying to get a credit for using recycled materials may be willing to pay a premium. But this is not working its way down to us as the compounder [who buys and processes the material] wants it cheap’.
Bill Carberry, Project Manager in Aircraft Recycling at Boeing, responded, ‘The value of recycled material comes back to the risk of the product. If you are using a recycled part in the car and it breaks down – it’s inconvenient.
A plane, on the other hand, falls out of the sky’.
Pete George, Carbon Fibre Recycling Principal Investigator, also at Boeing, added, ‘Carbon fibre will not be recycled back into aerospace structural components, [though] non-critical applications may be possible’.
The team from Boeing presented its work on creating proof-of-concept arm rests made from waste cured fuselage wings and uncured prepreg. The company, with its partners, has also manufactured the ‘first’ 100% recycled carbon fibre, low-cost, lay-up tool for composites made from scrap prepreg rolls.
‘The interiors [of aircrafts] are not recycled much currently,’ said Carberry, ‘If we could bring that up, that would be good.’ As Director of the Aircraft Recycling Association in the USA, he pointed out that 5,029 aeroplanes will leave service in 2028, and that the industry is working towards 90-95% of aircraft being recyclable by 2016.
Nonetheless, he questioned the assertion put forward in the pre-conference workshop that ‘aircraft manufacturing is gearing towards composites’ and therefore could be the key supplier of waste carbon fibre to recyclers, as well as providing an end market for recycled carbon fibre. He said, ‘That’s not true. The 787 is made from composites, but the next [Boeing aeroplane] will be made from the material that best helps that aircraft reach its mission. It might be aluminium or composites’.
He highlighted figures released by Zoltek, specialists in commercialising carbon fibre, headquartered in Bridgeton, USA. The firm’s market analysis up to 2013 shows that aerospace use of carbon fibre is decreasing over time as a percentage of total demand, with sectors such as construction and wind energy showing the largest growth.
Those industries were not represented at the event, indicating that further collaboration is needed to develop a commercially viable supply chain for end-of-life CFRP. Thomas Hunter of Firebird Advanced Materials Inc, in Raleigh, USA, proposed this when he gave a recycler’s perspective on reprocessing prepreg production scrap.
Firebrand has developed a pilot-scale continuous microwave CFRP recycling process. Hunter echoed Line’s comments about the need for high quality feedstock and long-term supply contracts. He said, ‘The waste forms vary from tacky material to hardened warehouse-aged baled scrap. It’s a challenge for the recycler’. There could also be potential contamination of the fibre waste, for example, the prepreg scrap could contain tape from the lay-up or polymer release films. These need separating.
He highlighted the need for better cooperation between waste generators and recyclers to reduce the capital burden of sorting and shipping the waste. Scrap sorting, size reduction and densification at the source before transporting for reprocessing could achieve this.
However, Hunter insisted, that firms ‘should not densify their waste without some thought of how to separate the material afterwards’, and refuted suggestions from a delegate to co-locate small-scale recyclers at the site of waste generation to cut costs further. He emphasised the need for larger economies of scale for recyclers to make a return, but added, ‘I am an advocate for regional facilities when there is enough material supplied and the industry grows [rather than exporting waste overseas]’.
With a large capital investment needed to get this industry moving, the science has got to back it up. Research is ongoing to probe the potential for producing high quality recycled carbon fibre material for end use.
A team at The University of Nottingham, UK, is also exploring an alternative chemical method using supercritical fluid. Talking to Materials World after the event, Dr Steve Pickering at Nottingham, explained, ‘We have developed the fluidised bed process, but there is some degradation in the strength of the fibres and also it oxidises the polymer. We wanted to achieve some useful material from the polymer especially given the End of Live Vehicle Directive’. The EU Directive aims to reduce the amount of waste generated when cars are scrapped.
Using propanol at 50bar and 300ºC, the researchers have removed the epoxy, producing clean fibres that, they say, retain up to 97% of the tensile strength of the virgin material, with no change in the modulus. ‘As there is not a mechanical process taking place, there is less degradation in the fibre strength and fibre damage,’ added Pickering.
As the recycled fibres are short and discontinuous, ensuring limited damage and producing continuous fibres that are highly aligned is said to be the solution to achieving high fibre volume fraction (about 40% and above) for reinforcing the matrix and re-creating composites with the best structural properties. The team at Nottingham, among others, is investigating techniques, such as wet hydrodynamic processes, for fibre alignment, as using high moulding pressures to achieve high fibre volume fraction causes fibre breakage.
Researchers at Imperial College London, UK, meanwhile, have been investigating the failure modes of recycled CFRPs. Soraia Pimenta and Silvestre Pinho aim to create computational programs to model the materials and predict the response of structural components. This will hopefully give engineers the tools to design recycled structures, promoting a new market for them, while helping recyclers optimise their processes accordingly.
One of the novel findings at Imperial, which drew great interest at the conference, was that fibre bundles – held together by residual matrix or char – enhance the fracture toughness of the composite. ‘This was surprising,’ said Pimenta, as usually recyclers try to eliminate these bundles. She explained, ‘The bundle is an obstacle to the crack, while single fibres are easily pulled out. If we have the the optimum number and geometry of bundles in the recyclate, we can optimise its mechanical properties’. Pickering suggests the finding may be due to the ‘high volume fraction in the bundles’.
Ultimately, though, the conference was about environmental benefit, Pickering cautions that this golden ticket should not be forgotten by those researching in the area. He said, ‘Lifecycle analysis is important. We are recycling to reduce the environmental impact, so the recycling operations must have low environmental impact’. On that note, the work continues.
Further information: CFR Recycling