STUDY ON SEPARATION, RECYCLING, RE-MANUFACTURE AND REUSE OF CARBON FIBRE IN

CARBON FIBRE COMPOSITES

Charles Chinedu Isiadinso

October 31, 2015

Contents

1 INTRODUCTION 2

2 PROCESS 22.1 Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Recycling & Re-Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.4 Reuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 ANALYSIS 4

4 CONCLUSION 4

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1 INTRODUCTION

With demand growing by over 70% from 51,000 t in 2014, to just under 90,000 t by 2020 [1], theglobal carbon fibre market is predicted to be worth US$3.73 billion by 2020 [2]. While there is sufficienttheoretical capacity to accommodate this significant rise in demand (global carbon fibre productioncapacity was ≈ 104,000 t in 2014 [3]) the rise will bring with it environmental issues due to the energyused in carbon fibre production.

The rising global demand is due to increasing need for higher product strength to weight ratio (e.g.in wind turbine blades) and strict global green legislations especially in the automotive and aerospaceindustries. For example, EU legislation requires the fleet average achieved by all new cars by 2021 be 95g of CO2 per kilometre [4], compared to the 2014 average of 124.6 g/km [5]; thus there must be ≈ 24%reduction in the next 6 years. One way car manufactures are choosing to do this is by reductions invehicle weight via materials like aluminium and carbon fibre in place of steel wherever feasible [6] [7].

Carbon fibre composites offer significantly better specific strength (strength to weight ratio) over themajority of traditional materials such as steel, aluminium, titanium etc., for example, Carbon fibreT700S composite (with 250oF epoxy resin) manufactured by Toray has 2550 MPa tensile strength and1800kg/m3 density [8], while steel alloy AISI 5130 has 1275 MPa tensile strength and 7830 kg/m3

density [9]. However, the specific strength gains come at a price, the manufacturing process of carbonfibre requires a large amount of energy, 183-286 MJ/kg [10] embodied energy, compared to 30-60MJ/kg for steel [10], even polyester, which has a considerably high embodied energy, ≈ 70% higherthan steel [11], requires 30% − 57% less energy to manufacture than carbon fibre [10].

Carbon fibre composites have largely been used in long life cycle products like commercial jets,which traditionally have service lives as long as 30 years [12], wind turbine (≈ 25 years [13]), boats (4-8years [14]) etc., and thus usually offset their production footprint by the end of the lives. However,with the increasing use of carbon fibre composites in relatively short life cycle products like smartphoneand laptop cases, bicycle frames, sports equipment etc, and the scrapping of planes after just 8 yearsbecause of poor economics of running second-hand models [12], there is an increasing amount CO2 fromcarbon fibre manufacturing process, which are not offset; thus worsening the environmental implicationsof using carbon fibre composites.

This report looks into processes for separating and recycling carbon fibre in carbon fibre compositeslike carbon fibre reinforce polymers (CFRP), and methods of re-manufacturing and reusing the recoveredfibres in an effort to extend the life of carbon fibre used in short (and long) life cycle products.

2 PROCESS

2.1 Separation

The first step in recovering carbon fibres is separating them from the composite, for this, the processof Pyrolysis, which is considered to be the only carbon fibre recycling process capable of industrialapplication [15], is used. Carbon fibre composites are cut up into small manageable pieces and placed ongrids with predetermined sizes; smaller parts are placed directly on the grids without much preparation.The Pyrolysis unit is continuously supplied, via a conveyor belt, with carbon fibre composite materialsto minimise energy waste.

During Pyrolysis, thermal chemical division of organic compounds takes place at temperatures ex-ceeding 500oC. Long-chained molecules are broken into shorter-chained molecules using heat in anoxygen free environment. The high temperatures gasify other materials in the composite, like epoxyresin, leaving only clean carbon fibres, these gases are cleaned and fed into a burner to continue thePyrolysis process; thus, under stable conditions, the process does not require further external energy.Research into reducing the energy requirements of the Pyrolysis process are being carried out, a 2004feasibility study by Edward Lester et al [16] proved it was possible to extract carbon fibres from theircomposites using microwave heating. Microwave heating could reduce the amount of energy consumedin the Pyrolysis process by reducing heat-up times and thus reducing processing times.

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After the Pyrolysis process, further necessary separation processes, like using magnets to removemetal parts, are performed leaving only pure almost completely undamaged carbon fibres.

2.2 Recycling & Re-Manufacture

Because of the numerous preparation processes the carbon fibre went through in its past life, theresin free carbon fibres are sorted and either converted into pellets or powder (milled carbon fibres), orlarger coated plates (short carbon fibres) [17].

Recycled Carbon Fibre as Raw Material for Polymer Industry To produce CFRP fromreclaimed carbon fibres, treated carbon fibre powder is mixed into molten polymer in a twin screwextruder, extruded and pelleted in a Pelletiser. The resulting pellets are of high density due to thecarbon fibres and are thus suitable for very demanding applications.

Recycled Carbon Fibre as Woven Mat Apart from CFRP applications, it should be possible touse carbon fibres in a woven mat, similar to textile recycling. In textile recycling, after the garmentshave been chopped into manageable pieces, they are transported into a machine, which uses air to mixand separate the fibres [18]. From here the material is then moved into a chopping machine, whichcreates a fluffy material from the mixed garment [19]. Taking this process into carbon fibre recycling,after separating the fibres from resin, they should be moved into a mixer, which will attempt to pullapart the fibres, then into a crushing machine, which will reduce the fibres to a predetermined size. Thisshould then leave a fluffy material, similar to that in textile, which can then be rolled as flat as possibleand knitted together using virgin carbon fibres (or carbon fibre nano tubes which have significantlysmaller diameters) to form a quilted continuous mat, which can then be used like virgin carbon fibremats. There are methods similar to this, that aim to produce woven or non-woven recycled carbonfibre mats [20], however the use of long virgin fibres or carbon nanotubes especially to create quiltedrecycled carbon fibre mats has yet to be attempted.

2.3 Testing

To measure the strength of the recycle fibres, a hardness test should be conducted on a layer ofrecycled carbon fibre composite and compared with a layer of virgin fibres. Examination of recycledfibre behaviour in tension and compression could be useful; these could be performed using the same(or similar) method used by Fang Wang and Jiaxing Shao [21] to examine the material properties ofbamboo fibres in their 2014 paper, in which they glued both ends of a piece of bamboo fibre to a paperframe with a rectangular hole over the majority of the fibre. The paper frame was cut in half to allowthe fibre to stretch, and both halves were separately clamped and load was applied pulling the clampsapart until the fibre broke (see figure 1 in their report for a schematic of the assembly [21]). It is worthnoting there will be variation in the performances of different individual fibres due to a number ofdifferent factors e.g. non-uniformity of the recycling process [15]. In addition, material characterisationprocesses such as Transmission electron microscopy (TEM) or Scanning electron microscopy (SEM)could also be used to examine the surface of one layer of the stitched recycled carbon fibre mat andindividual fibres for defects and leftover resin.

2.4 Reuse

Injection Moulding & Additive Layer Manufacturing (ALM) Currently, recycled short carbonfibres are used in injection moulded CFRP parts, especially in the automotive industry. For this, pelletedCFRP is melted, injected into a mould of the desired part, cooled and post-processed.

With the rise of ALM technology, an alternative reuse of recycled carbon fibres could be in ALM parts.The new process would be similar to 3D printing, successive layers of the part would be built by eitherdepositing liquid CFRP onto a build surface layer by layer, taking care to ensure the layer hardens almostimmediately to avoid sinking, or by depositing chopped recycled carbon fibre strands into successivelayers of epoxy resin, which are cured immediately using UV light (similar to Stereolithography). Resinand carbon fibre will have to have separate build heads for this process.

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Advanced Materials To build on work by Gui-Ming Song et al [22], further research into shortcarbon fibre (especially reclaimed carbon fibre) reinforced ceramics for high stress, high temperatureapplications like automotive brake disks, land based gas turbine engine stator vanes and rotor blades,aerospace engines etc. is recommended. In the case of gas turbine engines, relatively small increasesin the firing temperature (the temperature of exhaust gasses leaving the engine’s fuel combustionchamber) leads to significant increases in power output, thus making the engine more efficient. However,increasing the firing temperature past the material operation temperature range (sub zero to 873.15oKfor commonly used Titanium) creates problems as the materials start to soften and eventually meltwhile in operation. In an attempt to extend the upper temperature limit, there is a move towards UltraHigh Melting Temperature Ceramics (UHMTC) such as Hafnium diboride (HfB2), however these areintrinsically brittle. One proposal to help reduce brittleness is short-carbon fibre reinforced UHMTCcomposites. Research by Junjie Fei et al [23] proved it is possible to improve mechanical propertiesof metal-based ceramic, titanium diboride–carbon (TiB2/C/Csf ), and thus it could be possible toattempt using similar processes (hot pressing of wet ball-milled chopped recycled carbon fibres) toproduce carbon fibre reinforced UHMTCs.

3 ANALYSIS

Previous research into the effects of recycling methods on the mechanical properties of carbon fibresshow 10 − 20% reduction in tensile strength of recycled carbon fibres (RCF) when compared to virgincarbon fibres (VCF) [15], however, UK based Coseley U.K. and German CFK Valley Recycling showRCFs offer 20-40% cost savings over VCF, making them a considerable option. Also, with RCFsproduced by Pyrolysis reaching tensile moduli between 200 and 300 GPa and a minimum 3 GPa tensilestrength [24], RCFs are still significantly stronger than steel, aluminium and titanium alloys and withthe 20-40% reduction in cost, RCFs could cost between US$12 and US$16 per kg [25] making themmore competitive (pricewise) to steel, US$0.77/kg [25], titanium, US$4.93/kg [26], and aluminium,US$1.5/kg [27] when the strength and weight benefits are considered.

When recycling any material, it is worth checking that the cost of recycling is not higher than the costof manufacturing virgin material. Research into energy consumption of carbon fibre recycling processfound that the recycling process required about 95% less energy (10MJ/kg for RCF [28] vs. 200MJ/kgfor VCF [29]) making the recycling process (via microwave heating) significantly cheaper (via costof electricity) and more eco-friendly (via CO2 emissions from energy production) than manufacturingVCF.

One big problem with RCF is its random arrangement, one potential method of aligning the fibrescould be via centrifugal force. Photographic work by Fabian Oefner [30] show paint being scatteredoutwards from a can by a fast rotating drill. From the images, the paint can be seen to form laminarcurved lines as they exit the can, and get turbulent the further from the can they got. Applying asimilar method, but in reverse, should see the fibres align, provided they are loose (i.e. a looseningprocess is required), into straight curves as they get closer to the centre of the cause of the centrifugalforce (e.g. a spinning barrel); from with the straightened fibres can be extracted.

4 CONCLUSION

In conclusion, growing demand and supply of carbon fibre means there is an ever increasing need todevelop commercially deployable methods of extracting and recycling carbon fibre from carbon fibrecomposites, and reusing them in other application. Luckily, for the past few years, there has been alot of interest in this field, and a number of processes have already been commercially implemented,however, these still need to be improved, and new processes are needed to achieve a true recycling ofcarbon fibres as opposed to the current system of downcycling. With the increasing use of carbon fibrein products like Boeing’s 787, of which there currently exist 329 (and over 760 yet to be completed) [31]each with over 23 t of carbon fibre [32], now is the time to perfect methods of recycling carbon fibrecomposites before they are due to be scrapped in about 8 years [12].

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