1. 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 2
2.1 Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Recycling & Re-Manufacture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.4 Reuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 ANALYSIS 4
4 CONCLUSION 4
1

2. 1 INTRODUCTION
With demand growing by over 70% from 51,000 t in 2014, to just under 90,000 t by 2020 [1], the
global carbon fibre market is predicted to be worth US$3.73 billion by 2020 [2]. While there is sufficient
theoretical capacity to accommodate this significant rise in demand (global carbon fibre production
capacity was ≈ 104,000 t in 2014 [3]) the rise will bring with it environmental issues due to the energy
used 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 aerospace
industries. For example, EU legislation requires the fleet average achieved by all new cars by 2021 be 95
g 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 in
vehicle 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 the
majority of traditional materials such as steel, aluminium, titanium etc., for example, Carbon fibre
T700S composite (with 250o
F epoxy resin) manufactured by Toray has 2550 MPa tensile strength and
1800kg/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 carbon
fibre requires a large amount of energy, 183-286 MJ/kg [10] embodied energy, compared to 30-60
MJ/kg for steel [10], even polyester, which has a considerably high embodied energy, ≈ 70% higher
than 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 (4-8
years [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 smartphone
and laptop cases, bicycle frames, sports equipment etc, and the scrapping of planes after just 8 years
because of poor economics of running second-hand models [12], there is an increasing amount CO2 from
carbon fibre manufacturing process, which are not offset; thus worsening the environmental implications
of using carbon fibre composites.
This report looks into processes for separating and recycling carbon fibre in carbon fibre composites
like carbon fibre reinforce polymers (CFRP), and methods of re-manufacturing and reusing the recovered
fibres 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 process
of Pyrolysis, which is considered to be the only carbon fibre recycling process capable of industrial
application [15], is used. Carbon fibre composites are cut up into small manageable pieces and placed on
grids 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 materials
to minimise energy waste.
During Pyrolysis, thermal chemical division of organic compounds takes place at temperatures ex-
ceeding 500o
C. Long-chained molecules are broken into shorter-chained molecules using heat in an
oxygen free environment. The high temperatures gasify other materials in the composite, like epoxy
resin, leaving only clean carbon fibres, these gases are cleaned and fed into a burner to continue the
Pyrolysis 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 2004
feasibility study by Edward Lester et al [16] proved it was possible to extract carbon fibres from their
composites using microwave heating. Microwave heating could reduce the amount of energy consumed
in the Pyrolysis process by reducing heat-up times and thus reducing processing times.
2

3. After the Pyrolysis process, further necessary separation processes, like using magnets to remove
metal 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, the
resin free carbon fibres are sorted and either converted into pellets or powder (milled carbon fibres), or
larger coated plates (short carbon fibres) [17].
Recycled Carbon Fibre as Raw Material for Polymer Industry To produce CFRP from
reclaimed carbon fibres, treated carbon fibre powder is mixed into molten polymer in a twin screw
extruder, extruded and pelleted in a Pelletiser. The resulting pellets are of high density due to the
carbon fibres and are thus suitable for very demanding applications.
Recycled Carbon Fibre as Woven Mat Apart from CFRP applications, it should be possible to
use carbon fibres in a woven mat, similar to textile recycling. In textile recycling, after the garments
have been chopped into manageable pieces, they are transported into a machine, which uses air to mix
and separate the fibres [18]. From here the material is then moved into a chopping machine, which
creates 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 pull
apart the fibres, then into a crushing machine, which will reduce the fibres to a predetermined size. This
should then leave a fluffy material, similar to that in textile, which can then be rolled as flat as possible
and knitted together using virgin carbon fibres (or carbon fibre nano tubes which have significantly
smaller diameters) to form a quilted continuous mat, which can then be used like virgin carbon fibre
mats. There are methods similar to this, that aim to produce woven or non-woven recycled carbon
fibre mats [20], however the use of long virgin fibres or carbon nanotubes especially to create quilted
recycled 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 of
recycled carbon fibre composite and compared with a layer of virgin fibres. Examination of recycled
fibre 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 of
bamboo fibres in their 2014 paper, in which they glued both ends of a piece of bamboo fibre to a paper
frame with a rectangular hole over the majority of the fibre. The paper frame was cut in half to allow
the fibre to stretch, and both halves were separately clamped and load was applied pulling the clamps
apart until the fibre broke (see figure 1 in their report for a schematic of the assembly [21]). It is worth
noting there will be variation in the performances of different individual fibres due to a number of
different factors e.g. non-uniformity of the recycling process [15]. In addition, material characterisation
processes 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 and
individual fibres for defects and leftover resin.
2.4 Reuse
Injection Moulding & Additive Layer Manufacturing (ALM) Currently, recycled short carbon
fibres are used in injection moulded CFRP parts, especially in the automotive industry. For this, pelleted
CFRP 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 either
depositing liquid CFRP onto a build surface layer by layer, taking care to ensure the layer hardens almost
immediately to avoid sinking, or by depositing chopped recycled carbon fibre strands into successive
layers of epoxy resin, which are cured immediately using UV light (similar to Stereolithography). Resin
and carbon fibre will have to have separate build heads for this process.
3

4. Advanced Materials To build on work by Gui-Ming Song et al [22], further research into short
carbon fibre (especially reclaimed carbon fibre) reinforced ceramics for high stress, high temperature
applications 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 increases
in the firing temperature (the temperature of exhaust gasses leaving the engine’s fuel combustion
chamber) 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 873.15o
K
for commonly used Titanium) creates problems as the materials start to soften and eventually melt
while in operation. In an attempt to extend the upper temperature limit, there is a move towards Ultra
High Melting Temperature Ceramics (UHMTC) such as Hafnium diboride (HfB2), however these are
intrinsically brittle. One proposal to help reduce brittleness is short-carbon fibre reinforced UHMTC
composites. Research by Junjie Fei et al [23] proved it is possible to improve mechanical properties
of metal-based ceramic, titanium diboride–carbon (TiB2/C/Csf ), and thus it could be possible to
attempt using similar processes (hot pressing of wet ball-milled chopped recycled carbon fibres) to
produce carbon fibre reinforced UHMTCs.
3 ANALYSIS
Previous research into the effects of recycling methods on the mechanical properties of carbon fibres
show 10 − 20% reduction in tensile strength of recycled carbon fibres (RCF) when compared to virgin
carbon fibres (VCF) [15], however, UK based Coseley U.K. and German CFK Valley Recycling show
RCFs offer 20-40% cost savings over VCF, making them a considerable option. Also, with RCFs
produced by Pyrolysis reaching tensile moduli between 200 and 300 GPa and a minimum 3 GPa tensile
strength [24], RCFs are still significantly stronger than steel, aluminium and titanium alloys and with
the 20-40% reduction in cost, RCFs could cost between US$12 and US$16 per kg [25] making them
more 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 cost
of manufacturing virgin material. Research into energy consumption of carbon fibre recycling process
found that the recycling process required about 95% less energy (10MJ/kg for RCF [28] vs. 200MJ/kg
for VCF [29]) making the recycling process (via microwave heating) significantly cheaper (via cost
of electricity) and more eco-friendly (via CO2 emissions from energy production) than manufacturing
VCF.
One big problem with RCF is its random arrangement, one potential method of aligning the fibres
could be via centrifugal force. Photographic work by Fabian Oefner [30] show paint being scattered
outwards from a can by a fast rotating drill. From the images, the paint can be seen to form laminar
curved lines as they exit the can, and get turbulent the further from the can they got. Applying a
similar method, but in reverse, should see the fibres align, provided they are loose (i.e. a loosening
process is required), into straight curves as they get closer to the centre of the cause of the centrifugal
force (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 to
develop commercially deployable methods of extracting and recycling carbon fibre from carbon fibre
composites, and reusing them in other application. Luckily, for the past few years, there has been a
lot 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 of
carbon fibres as opposed to the current system of downcycling. With the increasing use of carbon fibre
in 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 fibre
composites before they are due to be scrapped in about 8 years [12].
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