Composites: Part A 37 (2006) 1206–1215
www.elsevier.com/locate/compositesa
Recycling technologies for thermoset composite materials—current status
S.J. Pickering*
School of Mechanical, Materials and Manufacturing Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
Received 27 April 2005; accepted 13 May 2005
Abstract
The technologies for recycling thermoset composite materials are reviewed. Mechanical recycling techniques involve the use of grinding
techniques to comminute the scrap material and produce recyclate products in different size ranges suitable for reuse as fillers or partial
reinforcement in new composite material. Thermal recycling processes involve the use of heat to break the scrap composite down and a range
of processes are described in which there are various degrees of energy and material recovery. The prospects for commercially successful
composites recycling operations are considered and a new initiative within the European composites industry to stimulate recycling is
described.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer matrix composites; A. Thermosetting; E. Recycling
1. Introduction
Thermoset composite materials are used in a wide range of
applications in industries such as automotive and construction.
They come in a variety of forms. At the cheaper end, polyester
resins are combined with short glass fibres and low cost fillers
to produce moulding compounds for applications where high
mechanical properties are not required. For more demanding
uses, continuous carbon fibres and epoxy resins are used for
critical applications in the aerospace industry. In Europe,
approximately 1 million tonnes of composites are manufactured each year [1]. Although there are many successful uses
for thermoset composite materials, recycling at the end of the
life cycle is a more difficult issue. However, the perceived lack
of recyclability is now increasingly important and seen as a
key barrier to the development or even continued use of
composite materials in some markets [2].
1.1. Problems in recycling thermoset composites
The problems in recycling thermoset composites are as
follows.
Thermosetting polymers are cross linked and cannot
be remoulded, in contrast to thermoplastics which can
easily be remelted. Some thermosetting polymers can be
converted relatively easily back to their original
monomer, such as polyurethane. However, the more
common thermosetting resins, such as polyester and
epoxy are not practical to depolymerise to their original
constituents.
Composites are by their very nature mixtures of different
materials: polymer, fibrous reinforcement (glass or carbon
fibre) and in many cases fillers (these may be cheap mineral
powders to extend the resin or have some other function,
such as fire retardants). There are few standard formulations
and for most applications the type and proportion of resin,
reinforcement and filler are tailored to the particular end
use.
Composites are often manufactured in combination with
other materials. For example there may be foam cores to
reduce weight and cost or metal inserts to facilitate fastening
onto other components.
In addition to these specific problems, there are the other
problems associated with recycling any material from endof-life components, such as the need be able to deal with
contamination and the difficulty of collecting, identifying,
sorting and separating the scrap material.
1.2. The need to recycle
* Tel.: C44 115 951 3785; fax: C44 115 951 3800.
E-mail address: stephen.pickering@nottingham.ac.uk
1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compositesa.2005.05.030
Concern for the environment, both in terms of limiting
the use of finite resources and the need to manage waste
disposal, has led to increasing pressure to recycle materials
S.J. Pickering / Composites: Part A 37 (2006) 1206–1215
at the end of their useful life. Where it is economically cost
effective to recycle, materials recycling operations are
already well established and driven by economics, for
instance in the metals industries. Polymers are generally
more difficult to recycle and the economic incentives to
recycle are less favourable, particularly when waste disposal
in landfill is relatively cheap. Consequently, in order to
protect the environment, legislation has to be introduced
often combined with the use of economic instruments (such
as taxes) to encourage recycling to take place [1].
Waste management is now a high priority within the
European Union and there is a well known hierarchy of
routes for dealing with waste. In decreasing order of
desirability these are:
† Prevent waste through prevention at source during
manufacture
† Reuse a product
† Recycle material
† Incinerate waste
† With material and energy recovery
† With energy recovery
† Without energy recovery
† Landfill
For many years, and in the UK in particular,
composite waste has been disposed of in landfill. A
recent EU Directive on Landfill of Waste (Directive
99/31/EC) will result in a reduction in the amount of
organic material landfilled. As a consequence, it is
already illegal to landfill composites waste in many EU
countries.
Other directives deal with particular waste streams. The
End-of-life Vehicle Directive (Directive 2000/53/EC)
regulates the disposal of vehicles and the requirements are:
† From 2006, 85% of the weight of all end-of-life vehicle
must be re-used, recycled or subject to energy recovery
and only 15% may be disposed of in landfill.
† From 2015, 85% of the weight of all end-of-life vehicles
must be re-used or recycled, a further 10% may be
subject to energy recovery and a maximum of 5% of the
vehicle may be disposed of in landfill.
† From 2007, it is the responsibility of the vehicle
manufacturer to dispose of a vehicle.
Vehicles manufactured now must meet the 2015
requirement as vehicles have a life expectancy of over
10 years. A directive on waste electrical and electronic
equipment has recently been issued (Directive
2002/96/EC) and it is likely that there will be regulations
concerning construction and demolition wastes soon.
As a consequence of increasing legislation there is a need
for recycling routes to be established and the potential
technologies are described next.
1207
2. Recycling technologies
A number of recycling technologies have been proposed
and developed for thermoset composite materials and these
are summarised in Fig. 1. There are fundamentally two
categories of process: those that involve mechanical
comminution techniques to reduce the size of the scrap to
produce recyclates; and those that use thermal processes to
break the scrap down into materials and energy. Each will
be considered.
2.1. Mechanical recycling
Mechanical recycling techniques have been investigated
for both glass fibre and carbon fibre reinforced composites,
but the most extensive research has been done on glass fibre.
The technique used is usually to initially size reduce the
scrap composite components in some primary crushing
process. This would typically involve the use of a slow
speed cutting or crushing mill to reduce the material to
pieces in the order of 50–100 mm in size. This facilitates the
removal of metal inserts and, if done in an initial stage
where the waste arises, the volume reduction assists
transport. The main size reduction stage would then be in
a hammer mill or other high speed mill where the material is
ground into a finer product ranging from typically 10 mm in
size down to particles less than 50 mm in size. Then a
classifying operation, typically comprising cyclones and
sieves, would be employed to grade the resulting recyclate
into fractions of different size [3,4,27,12].
In the mechanical recycling process, all of the
constituents of the original composite are reduced in size
and appear in the resulting recyclates which are mixtures of
polymer, fibre and filler. Typically the finer graded fractions
are powders and contain a higher proportion of filler and
polymer that the original composite. The coarser fractions
tend to be of a fibrous nature where the particles have a high
aspect ratio and have a higher fibre content. A number of
companies have been involved in developing the recycling
activity at an industrial scale, among them ERCOM in
Germany and Phoenix Fiberglass in Canada [3,12]. These
companies base their operations around the two most
common grades of thermoset glass fibre composite material:
Recycling Processes for
Thermoset Composites
Mechanical
Recycling
(comminution)
Powdered
fillers
Fibrous
products
(potential
reinforcement)
Thermal
Processes
Combustion
with energy
recovery
(and
material
utilisation)
Fluidised
bed process
Pyrolysis
Clean fibres
and fillers
with energy
recovery
Chemical
products,
fibres and
fillers
Fig. 1. Recycling processes for thermoset composite materials.
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S.J. Pickering / Composites: Part A 37 (2006) 1206–1215
bulk moulding compound (BMC) and sheet moulding
compound (SMC). These composites based generally on
polyester resins and comprise high proportions of filler,
usually calcium carbonate or the fire retardant alumina
tryhydrate. In the ERCOM process, a mobile shredder is
employed to undertake the initial size reduction. This is an
expensive piece of equipment and by making it mobile it
can be taken to various sites to perform an initial size
reduction to increase the bulk density of the material to
make transport more cost effective. The shredder reduces
the scrap into pieces of about 50 mm!50 mm in size with a
bulk density of about 330 kg/m3. At a central processing
site, a hammer mill is used to comminute the scrap material
further and it is graded using cyclones and sieves into a
number of powder and fibrous fractions as detailed in
Table 1. A hammer mill is an impact process and has the
advantage that, whilst there is abrasion on the hammers,
there are no blades that require regular sharpening. In the
Phoenix process [5], similar arrangement is used comprising
a two stage shredding and pulverising process followed by
grading of the product using screens and air classifiers.
Details of the recyclates produced by Phoenix are given in
Table 2.
A range of applications have been investigated for
recyclates. Those in the form of fine powders can be used as
a substitute for calcium carbonate filler in new SMC or
BMC. At loading levels of about 10% the reductions in
mechanical properties are tolerable. However, higher
proportions do give rise to processing problems, in that
the recyclate absorbs more resin and so increases the
viscosity of the moulding compound, and more significant
reductions in mechanical properties are also experienced [3,
6–8,27]. An advantage of the recyclate is that it has a lower
density than calcium carbonate, as it contains a significant
proportion of low density polymer, and so an SMC
component containing 10% recyclate as a filler substitute
can be 5% lighter than one using only calcium carbonate [3].
The use of the coarser, fibrous recyclates in which the larger
pieces of recyclate contain significant amounts of intact
fibre, is generally reported to be more difficult and
reductions in strength and toughness are reported even
with modest additions of fibrous recyclate as a replacement
for filler. This is understood to be due to a lack of bonding
between the recyclate and the polymer and the larger
particles of recyclate acting as stress raisers in the composite
Table 1
Grades of SMC recyclate from ERCOM GmbH (Ref. [12])
Product
grade
RC1000
RC1100
RC3000
RC3101
Fibre length
(mm)
Glass content (%)
Bulk density (kg/m3)
!0.25
0.25–3
3–15
3–20
35
45
45
45
670
460
170
400
Table 2
Grades of SMC recyclate from phoenix fibreglass, Inc. (Ref. [3])
Recyclate grade
PHX-200 filler
fraction
MFX milled
fibres
CSX hybrid
fibres
Particle size
14 mm
z12 mm
Glass content
Filler and
organic content
13%
87%
0.8, 1.6 and 3.
1 mm
85%
15%
40%
60%
PHX-200 is a recyclate grade in a powder form suitable for use as a filler.
MFX is a recyclate grade in which there is a high glass fraction and the
fibres are grades in lengths from 0.8 to 3.1 mm. CSX is a recyclate grade
with much larger sized particles containing proportions of fibre, resin and
filler similar to the original material.
[6,8,27]. More recent work has shown that treating the
recyclate to increase the bonding can improve the
mechanical properties. In another study [9], the effect of
fibre length has been investigated and it has been found that,
in a short fibre moulding compound (BMC), fibrous
recyclate can be used successfully to partially replace
short glass fibres, provided the remainder of the virgin fibres
are replaced with longer fibres. Longer virgin fibres will in
any case give higher strengths and these can used to offset
the deleterious effects of the recyclate.
The use of thermoset recyclate has also been considered
in applications other than for recycling back into new
thermoset moulding compounds. Some investigations have
been done of the use of recyclate in thermoplastics [7] and
work at Brunel University [10,11,13,16] has included the
development of a novel twin-screw process for compounding thermoset recyclate and detailed research has been
undertaken into the properties of the materials produced. In
general, it has been found that recyclate has inferior
reinforcement properties to virgin reinforcement but that
with the use of grafting and coupling agents the
reinforcement properties of the recyclate can be increased
by between 45 and 65%—the treatment having the most
beneficial effect on improving tensile strength and toughness [11].
The value of recyclate can be enhanced if it can be used
in a way to exploit some of its unique properties. At
SICOMP in Sweden [17] a glass fibre based reinforcement
RECYCORE has been produced that consists of a core
containing a coarse recyclate. The recyclate core has a
particle size from 1 to 25 mm and can comprise up to 70%
of the weight of the reinforcement. The particular benefit is
that the recyclate gives the core a high permeability that
allows it to act as a flow layer along which resin can flow
during impregnation. At Bristol University [20] work has
been done to investigate the way in which recyclate can be
used to provide more damping in a composite so that it is
more effective in noise insulation.
A recent study by the Building Research Establishment
in the UK has investigated a number of applications for
thermoset recyclate in the construction industry such as in
the manufacture of plastic lumber from recycled
S.J. Pickering / Composites: Part A 37 (2006) 1206–1215
thermoplastics where recyclate can be used as an alternative
to wood fibre, or the manufacture of a recyclate reinforced
wood particle board [15,19]. Coarse recyclate has also been
used as a reinforcement in asphalt [14,18].
2.2. Thermal processing
2.2.1. Combustion with energy and material utilisation
Thermosetting polymers, like all organic materials, have
a calorific value and can be burned as a source of energy.
Measurements of calorific value have been reported for
polyester, vinylester, phenolic, urea formaldehyde and
epoxy resins [21]. Apart from urea formaldehyde, which has
a calorific value of 15,700 kJ/kg, the other resins considered
all have calorific values of approximately 30,000 kJ/kg. As
the most commonly used fibres and fillers are incombustible, the calorific value of a glass reinforced composite
generally depends only on the proportion of polymer as
illustrated in Fig. 2.
Some mineral fillers decompose and absorb energy
during combustion and fire retardants are used specifically
for that purpose. However, although the effect of fire
retardants is to reduce the initial ignition and flame spread,
the amount of energy absorbed is small compared with
calorific value of the resin. For example, the energy
absorbed by alumina trihydrate is 1000 kJ/kg, so, if there
is as much alumina trihydrate as there is polymer in a
composite, the calorific value will be decreased by 3.3%.
Similarly, calcium carbonate another widely used mineral
filler decomposes at temperatures between 700 and 900 8C
and absorbs 1800 kJ/kg. In a composite containing the same
weight of calcium carbonate as polymer, the calorific value
would thus be decreased by 6%. Combustion trials [21,22]
have shown that composites can successfully be burned for
energy recovery. Mixing scrap composites at 10% loading
with municipal solid waste has been shown to be a practical
way to dispose of scrap if landfill is prohibited [22].
Calorific Value of Thermoset Composites
35000
Calorific Value [kJ/kg]
30000
25000
20000
15000
10000
5000
0
0
20
40
60
80
Proportion of incombustible materials [%]
Fig. 2. Calorific value of thermoset composites.
100
1209
In order to recover some value from the incombustible
material, burning scrap composites in cement kilns is an
effective route as the glass reinforcement and mineral fillers
commonly used in composites contain minerals that can be
incorporated in cement. The effect of the these minerals on
the cement production process has been investigated [23]
and it was found that the only problem issue was the
presence of boron in the E glass fibre used for reinforcement
in the composite. Too much boron in cement can increase
the time for the cement to set, although the ultimate strength
is not affected. It was concluded from the study that as long
as the use of composites in cement manufacture did not
result in more than 0.2% boron oxide in the cement then
there would not be a significant effect on the performance of
the cement. Typically this would mean that no more than
about 10% of the fuel input to a cement kiln could be
substituted with polymer composite material.
Alternatively if scrap composites are co-combusted with
coal in a fluidised bed combustor [24] any calcium
carbonate filler in the composites will absorb oxides of
sulphur from the combustion of the coal and so reduce the
sulphur emissions. A trial in a commercially operating coalfired fluidised combustion boiler was undertaken over a
period of 4 days in which 730 kg of SMC and BMC were cocombusted with coal. It was found that the calcium
carbonate filler in the composite behaved in a similar way
to the addition of powdered limestone, which is used
commercially, in removing oxides of sulphur from the
combustion flue gases.
2.2.2. Fibre recovery using a fluidised bed thermal process
The fibre reinforcement has potentially the most
recoverable value in a composite. The theme of research
at the University of Nottingham over the past 10 years has
therefore been to develop a fluidised bed process to recover
high grade glass and carbon fibre reinforcement from scrap
glass and carbon fibre reinforced composites [25,26]. Scrap
composites are initially reduced in size to about 25 mm and
fed into a fluidised bed. This is a bed of silica sand with a
particle size of about 0.85 mm. The sand is fluidised
with a stream of hot air and typical fluidising velocities are
0.4–1.0 m/s at temperatures in the range of 450–550 8C. In
the fluidised bed, the polymer volatilises from the composite
and this releases the fibres and fillers to be carried out of the
bed as individual particles suspended in the gas steam. The
fibres and fillers are then separated from the gas stream,
which can then pass into a high temperature secondary
combustion chamber where the polymer is fully oxidised.
Energy may subsequently be recovered from these hot
combustion products. A diagram of the process is shown in
Fig. 3.
The process has been developed for the recovery of both
glass fibre and carbon fibre. The fibre product is in a fluffy
form comprising individual fibre filaments typically of
mean length (by weight) from 6 to over 10 mm. The fibres
are clean and show very little surface contamination. A glass
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S.J. Pickering / Composites: Part A 37 (2006) 1206–1215
a disperse form such as in bulk moulding compounds or
non-woven veil or tissue products [25].
2.2.3. Pyrolysis processes
Fig. 3. Fluidised bed recycling process.
reinforced polyester composite can be processed at 450 8C
at which temperature the polymer volatilises and releases
the fibres into the gas stream. Epoxy resins require higher
temperatures of up to 550 8C for rapid volatilisation of the
polymer.
Mechanical property measurements show that glass
fibres typically suffer a 50% reduction in tensile strength
but retain the same stiffness as the virgin fibre, when
processed at 450 8C. At higher temperatures there is
significantly greater reduction in mechanical strength,
resulting in a 90% reduction in strength at 650 8C. These
strength reductions match those reported for heat treated
glass fibre [43] and therefore may be interpreted as being
due to the effect of the high temperatures in the fluidised bed
process. It is worth noting that it may be supposed that heat
treatment in the fluidised bed will have removed any surface
treatment on the glass fibres (sizing) as well as the polymer
matrix. However, in experimental tests to investigate re-use
of the fibres in moulding compounds and veil, a silane size
was applied to some of the recycled fibres and it was found
not to give any improvement in mechanical properties [25].
Although it is likely that it is the lower tensile strength of the
recycled glass fibres that limits the properties in an
application rather that the fibre surface bonding.
Carbon fibres show a lower strength degradation of
typically 20% with retention of the original stiffness when
processed at 550 8C. Even though processed in air, the
carbon fibres do not show any measurable oxidation.
Analysis of the surface of the recycled carbon fibres also
shows that there is only a small reduction in surface oxygen
content, indicating that the fibres have good potential for
bonding to a polymer matrix if re-used in a composite.
A particular advantage of the fluidised bed process is that
it is very tolerant of mixed and contaminated materials [25].
Mixtures of composites of any polymer type can be
processed and the process is tolerant of painted surfaces
or the presence of foam cores in composites of sandwich
construction. Metal inserts moulded into a composite do not
have to be removed before being fed into the fluidised bed as
any metals are retained in the bed and could be separated by
regrading the sand. The most promising applications for
the recovered fibres are those that require short fibres in
2.2.3.1. Glass fibre composites. In a pyrolysis process, a
combustible material is heated in the absence of oxygen. In
these conditions, it breaks down into lower molecular
weight organic substances (liquids and gases) and there is
also a solid carbon char product. Pyrolysis offers a method
of recovering material from the polymer in a scrap
composite that has the potential to be used as a feedstock
for further chemical processing. A diagram of a typical
pyrolysis process is shown in Fig. 4, in which the gases
evolved are used as fuel to provide heat for the process.
Pyrolysis processing of scrap composites has been
investigated by a number of workers. In the USA, the
SMC Automotive Alliance undertook research on pyrolysis
in the early 1990s [27]. The processing temperature was
700–1000 8C in a batch processing plant designed for tyre
pyrolysis. The process produced a fuel gas used to provide
heating for the pyrolisation chamber, some liquid oil
products and a solid residue, which comprised the inorganic
fibres and fillers and a char residue. The solid residue was
ground to a powder and investigations of its use as a filler in
new SMC were carried out. It was found that up to 30% of
the ground solid residue could be incorporated into an SMC
without adversely affecting the processing or the mechanical properties of the moulded parts. A detailed chemical
analysis of the pyrolysis oil was not carried out but it was
reported to have a composition similar to heavy crude oil.
Recognising that there is potential value in the solid
products if the glass fibres could be recovered in good
condition, a low temperature pyrolysis process was also
investigated [28]. This involved the use of temperatures of
about 400 8C and the presence of steam was found to
increase the rate of polymer degradation and enable
Fig. 4. Pyrolysis process.
S.J. Pickering / Composites: Part A 37 (2006) 1206–1215
the fibres to be separated more easily from the solid
pyrolysis products. The resulting solid products were then
cleaned by acid digestion to remove the calcium carbonate
filler, for recovery as calcium chloride flake, the fibres were
then separated and cleaned. The mechanical properties of
the fibres were measured in terms of specific fracture energy
retention. A pyrolysis temperature of 400 8C was found to
be necessary to breakdown the polymer, but the fibres only
retained about 50% of their specific fracture energy.
More recently, pyrolysis of composites has been
investigated at the University of Leeds [31] and at the
School of Engineering in Bilbao, Spain [29]. The work
reported in Spain concerned the pyrolysis of a polyester
SMC at temperatures from 300 to 700 8C. The composition
of the SMC is given in Table 3 and is a typical polyester/
glass fibre formulation. Pyrolysis below 400 8C was
unsatisfactory as the polymer did not completely decompose. However, the pyrolysis products formed in the
temperature range of 400–700 8C were remarkably consistent, as shown in Table 4. The pyrolysis gases were found to
contain mainly CO2 and CO with a hydrocarbon gas content
of less than 10% and have a low calorific value of about
14 MJ/Nm3. The liquid products were found to contain a
complex mixture of organic compounds containing typically 66% aromatic compounds and about 25% oxygenated
compounds such as ketones, carboxylic acids, alkylbenzenes and aryl naphthalenes with calorific values of about
37 MJ/kg and are similar to fuel oil.
The work undertaken at Leeds University involved
pyrolysis of a range of composite materials at temperatures
ranging from 350 to 800 8C. The range of composites
investigated used polyester, vinylester, epoxy and phenolic
thermosetting resins as well as polypropylene and polyethylene terephthalate thermoplastics in various combinations with glass and carbon fibre reinforcement and
calcium carbonate filler. The pyrolysis investigations
involved heating the composites in a fixed bed reactor in
the absence of air. A comparison of the materials and the
pyrolysis products when processed at a temperature of 500
or 550 8C is shown in Table 5. It was found that polyester
resins have decomposed fully at a temperature of 450 8C,
whereas the other resins generally need a higher temperature
of 500–550 8C. The composites generally yielded between 1
and 10% gaseous products. Carbon dioxide was the main
gas produced but significant proportions of the combustible
gases carbon monoxide, hydrogen and other hydrocarbons
were produced. Gross calorific values of the gaseous
Table 3
Composition of SMC used in pyrolysis trial (Ref. [29])
Component
Proportion (by weight) (%)
Ortho-phthalic polyester resin and
other additives
Calcium carbonate filler
Glass fibre
28.3
46.7
25
1211
Table 4
Pyrolysis products from SMC (Ref. [29]), expressed as % (weight) of SMC
Temperature
400 8C
500 8C
600 8C
700 8C
Solid yield
(%)
Liquid yield
(%)
Gas yield
(%)
75.2
74.9
73.9
72.6
14.5
14.2
14.9
13.7
10.5
11.0
11.5
12.8
products were generally less than 18 MJ/kg, except for gas
from the epoxy resin composite which was rich in methane
and from the polypropylene thermoplastic composite which
was rich in the monomer propene. The gaseous products
from these materials had high calorific values between 42
and 44 MJ/kg.
The liquid and solid condensable products from the
pyrolysis contain a mixture of different classes of organic
materials as shown in Table 5 and so the oil has potential for
use as a fuel (typical calorific value was in the region of
30 MJ/kg) and also chemical feedstock. Of particular note
were the products from the polyester composite [30] as
shown in Table 6. The condensable liquid contained 26%
styrene and the solid condensable product was 96% phthalic
anhydride. Both of these materials are potentially valuable
feedstock for the manufacture of polyester resins.
The residual solid products from the pyrolysis were the
glass and carbon reinforcement fibres and along with any
mineral fillers and char from the decomposition of the
polymer. The polyester composite produced a solid residue
reported to contain 16% char. In a second stage, oxidation
process at 450 8C this was removed to yield clean glass
fibres. These have suffered a 50% degradation in mechanical strength, similar to findings of other workers [25] and
were incorporated successfully as 25% substitution for
virgin short glass fibre in a polyester dough moulding
compound with little reduction in mechanical properties of
the composite.
2.2.3.2. Carbon fibre composites. Pyrolysis has also been
investigated for the recycling of carbon fibre composites.
Some laboratory investigations were undertaken in Japan
[36], where samples of carbon fibre composite based on
epoxy and phenolic resin were investigated. The work
focused on the properties of the carbon fibre following
heating in air and pyrolysis. There is some uncertainty in the
experimental method as the pyrolysis was unusually
described as taking place in a stream of air. Heating was
done for extended periods of several hours at temperatures
of 400, 500 and 600 8C and tensile strength measurements
were made and compared to virgin fibre. The modulus of the
fibre was not measured. The results indicated that for a
carbon fibre composite heated under pyrolysis conditions at
500 8C, there was little degradation of the tensile strength of
the carbon fibre. However, when carbon fibre on its own was
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S.J. Pickering / Composites: Part A 37 (2006) 1206–1215
Table 5
Pyrolysis products from various composites (Ref [31]), expressed as % weight of composite
Composite
Temperature (8C)
Solid yield (%)
Oil/wax yield
(%)
Polyester resin with calcium carbonate, alumina
trihydrate fillers and glass fibre
Phenolic resin (24%) with calcium carbonate filler
and glass fibre
Epoxy resin with carbon fibre
Polyester resin with (70–80%) with glass fibre
Polypropylene (60%) with glass fibre
Poly ethylene terephthalate (PET) (60%) with glass
fibre
Vinyl ester resin (30%) with glass fibre
500
45.8
45.7
8.5
500
90.2
8.8
1.0
500
550
550
550
67.4
30.0
44.8
74.4
31.3
59.4
46.8
13.0
1.2
10.6
8.4
12.6
550
83.4
15.0
1.6
heated in air the tensile strength decreased by about 25%
and this was understood to be due to more severe oxidation
as the fibres did not have a protective layer of resin. At
600 8C there was much more severe oxidation of the carbon
fibre and under pyrolysis conditions the tensile strength of
the fibre reduced by over 30%.
A catalytic pyrolysis process has been developed by
Adherent Technologies, Inc. in the USA [32,34,35] for
carbon fibre composites based on epoxy resins. The
pyrolisation takes place at a low temperature (typically
around 200 8C) in the presence of a proprietary catalyst and
the polymer is completely degraded into low molecular
weight hydrocarbons in liquid or gaseous form and the
remaining carbon fibres are substantially free from resin.
Scrap composites in the form of scrap material from aircraft
and prepreg, using both unidirectional and woven carbon
fibre fabric were evaluated. After initial shredding, the
material was fed into the continuous pyrolysis reactor and
were processed in 5 min. Prepreg material with a backing
paper was also treated and the backing paper was also found
to be completely degraded in the process. The liquid
hydrocarbon products recovered had the composition shown
in Tables 7 and 8 shows the composition of the gaseous
products although the proportion of liquid and gaseous
products was not reported. The chemical species recovered
from the epoxy resin were as expected from the constituents
used in the resin manufacture. The recycled carbon fibres
were characterized in terms of tensile strength and surface
chemistry. It was found that the strength degradation of the
recycled fibres varied between 1 and 17%. The surface of
the recovered carbon fibre was analysed and variable results
were found. In one case [34], the surface oxygen content of
the recovered fibres was 83% higher than the virgin and in
another [35] it was found that there was a reduction in the
surface oxygen content of about 18%. In both cases, the
oxygen was present in similar bonds to the virgin carbon
fibre and therefore the recycled carbon fibres would also be
suitable for bonding to a polymer matrix in a composite. In a
more recent publication, it is understood that the catalytic
process is being further developed in collaboration with
Boeing [37].
Gas yield (%)
A gasification process has also been developed for
carbon fibre composites [33]. In this process, the scrap is
heated in a controlled flow of oxygen at temperatures of
600 8C. The polymer is converted to shorter chain
hydrocarbons and gases (H2 and CO) and the carbon fibres
can be recovered for reuse. In the process, some char residue
from the polymer remains on the fibres, but this is generally
less than 10%. The recycled fibres have been used as a
substitute for glass fibre in a bulk moulding compound and
have been shown to give an improved tensile strength (of
over 25%) relative to glass fibre.
A pyrolysis/gasification process is currently being
developed in Denmark for the recycling of composite
wind turbine blades [40]. In this process, the wind turbine
blade is cut up into pieces of about 1 m in size and placed in
a large batch reactor where they are heated in the absence of
air. Air is introduced towards the end heating cycle to
oxidise any char so that clean glass fibre and fillers are
recovered. The gaseous and liquid products of the pyrolysis
are used as a source of energy and the recovered solid
products (glass fibre and filler) have potential for use as
reinforcement in a similar way to the fibrous fractions
produced from the mechanical recycling processes.
Table 6
Products from pyrolysis of polyester composite (Ref. [30])
Composite composition (by weight)
Polyester resin and additives 63%, glass fibre 30%, calcium carbonate filler
7%
Pyrolysis products (450 8C) (by weight)
Solid
39.3%
Oil/solid organic
39.6/15.4%
Gas
5.8%
Composition of oil and solid organic products (by weight)
Species
Oil
Solid organic
Ethylbenzene/xylene
3.6%
0.3%
Styrene
26.2%
1.0%
a-Methylstyrene
5.5%
0.3%
Phthalic anhydride
2.7%
96.2%
Dimethyl phthalate
2.5%
0.3%
1,3-Diphenylpropane
5.1%
1.1%
Other
44.6%
1.8%
S.J. Pickering / Composites: Part A 37 (2006) 1206–1215
Table 7
Liquid products from pyrolysis of epoxy carbon fibre composite (Ref [35])
Product
Proportion (by mole) (%)
Acetic acid
Pyridine-SO3 complex
Phenol
Aniline
p-Toluidine
Isoquinoline
6.1
6.1
5.1
73.9
4.4
4.3
3. Discussion
Mechanical recycling processes are suitable for scrap
composite material which is relatively clean and uncontaminated and from known origin. The technologies
developed produce powder and fibrous recyclates, which
have potential for reuse. However, the powder recyclates
have limited potential for reuse back into the thermoset
compounds from which they originated. Although they are
of lower density, there are other drawbacks to their use in
terms of ease of processing and lower mechanical properties
in the products in which they are used. The fibrous
recyclates have some potential as reinforcement materials,
but they are not as good as the virgin reinforcement, and
there are problems associated with the bonding of the
recyclate with polymers and the tendency for the larger
pieces of recyclate to be stress raisers and act as failure
initiation sites.
The thermal recycling processes have the advantage of
being able to tolerate more contaminated scrap materials.
The fluidised bed process produces a very clean fibre
product, but it is not in the same form as an existing virgin
fibre products. Development work is therefore needed to
identify the ways in which the material can be reprocessed
into cost effective new products. The same is also true of the
fibre products developed from the pyrolysis processes.
These may have varying degrees of char on the recycled
fibres, which may limit the reuse options or require further
processing to remove it. The pyrolysis processes are
generally more complex in principle than the fluidised bed
process but do produce potentially useful organic products
from the polymer. These would need further processing to
Table 8
Gaseous products from pyrolysis of epoxy carbon fibre composite (Ref.
[35])
Product
Proportion (by mole) (%)
1-Propene
Water
Sulphur dioxide
Hydrogen cyanide
1-Butene
1,3-Butadiene
Bromomethane
Acetone
Acetonitrile
1,3-Cyclopentadiene
14.7
41.9
10.8
5.5
2.3
5.0
1.4
13.4
2.5
2.6
1213
separate them from the mixture of products produced and it
seems likely that this would only be cost effective on a large
scale.
4. Environmental acceptability
Although the hierarchy of waste management routes
appears to give preference to those recycling routes that
maximise material recovery, the mechanical recycling of
valuable fibres and resins as fillers does not necessarily give
the best environmental return. Pyrolysis process has the
potential to produce chemical feedstocks from the polymer
but these may be difficult to refine from the mixture of
products produced. If a pyrolysis process only produces
chemicals suitable for use as fuels then an energy recovery
process with high quality fibre recovery may be more
acceptable. Only environmental audits of the recycling
routes can identify which are the most acceptable and
although some work has been reported [39] much more
analysis of the recycling processes needs to be done.
5. Prospects for commercial operation
Although there has been much research into different
methods for recycling thermoset composites a truly
successful commercial operation has yet to be achieved
anywhere in the world. A number of studies have
investigated the likely cost effectiveness of composites
recycling. An early study by the SMCAA [4] concluded that
the recycling of 20,000 tonnes of scrap SMC per year (a
mixture of both cured and uncured waste) could be cost
effective given that suitable markets for the recyclates
produced could be found. The ERCOM project [3] has
found that the filler substitute recylates are more expensive
by weight than traditional fillers (calcium carbonate) but
that they are cheaper per unit volume on account of the
lower density. However, Phoenix Fiberglass in Canada
ceased operating in 1996 and the ERCOM company and
other similar operations have not been able to find sufficient
markets for recyclate to operate at commercially viable
levels of production [18]. A cost analysis was undertaken of
the fluidised bed recycling process and this concluded that,
when processing glass reinforced composites, an annual
throughput of at least 10,000 tonnes per annum would be
needed for the operation to become cost effective [25].
At a recent seminar on composite recycling held at
SICOMP in Sweden in 2003 a workshop was held to
identify the barriers to composites recycling and a
questionnaire was circulated to obtain the opinion of the
delegates [42]. The clear message was that cost and lack of
markets were the main barriers to the implementation of
composites recycling operations but that new legislation
was the main driver towards recycling. The key issue is that
the costs of recycling operations, whether they be
1214
S.J. Pickering / Composites: Part A 37 (2006) 1206–1215
mechanical recycling or other forms of thermal recycling
mean that the recyclates currently produced are too
expensive to give a clear market advantage over alternative
existing materials [19,42]. Furthermore, commercially
viable operations require large throughputs and markets
need to be developed to consume these quantities of
recyclate. Either the recyclates must find higher value end
markets and this may mean developing new higher grade
recyclates or the cost of the recyclate must reduce to allow
the recyclates to penetrate further into existing markets. A
recent study investigating recycling routes for scrap
thermoset composites from the construction industry [15]
has reported that significant incomes could be earned if
recyclates were used as filler materials replacing woodchip
or recycled plastics. However, it was also noted [19] that the
difficulty of using the recyclates in processes already
optimised for existing products was a barrier. The future
therefore lies in developing markets into which the recycled
products can be sold at profitable prices and some ongoing
studies are focussing on this aspect [38]. Glass fibre
composites comprise the bulk of the thermoset composites
currently manufactured. The production volumes of carbon
fibre composites are several orders of magnitude lower but
carbon fibre is a much more valuable material, typically at
least ten times the cost of glass fibre. The prospects are
therefore potentially more attractive for carbon fibre
recycling.
Recent changes to waste management legislation and
likely future directions mean that recycling routes are
urgently needed to be in place for thermoset composite
materials if they are to continue to have a place in the
market. Recognising that composite recycling activities
need stimulation and financial assistance if they are to
succeed, the European Composites Industry Association
(formerly the GPRMC) is proposing a European Composites Recycling Concept (ECRC) [41]. Under this scheme a
‘Green label’ will be given to composites from manufacturers who adopt this scheme and this will guarantee that the
components will be recycled appropriately in accordance
with the legislative requirements at the time. Composites
manufacturing companies, which join the scheme will pay
money into a fund that will manage composites recycling on
a pan-European scale and finance the recycling activities.
The scheme is currently in its early stages and will initially
focus on composites in the automotive industry.
6. Conclusions
A considerable amount of research has been done to
investigate potential recycling techniques for thermoset
composite materials and some of the key work in the area of
mechanical and thermal recycling processes has been
described in this paper. Despite this there is no commercially viable composites recycling activity anywhere in the
world, largely as a result of markets not being available at
the right price for the recycled materials that are produced.
European legislation now requires that recycling routes are
available for composites, as other waste management
methods will not be allowed. The European composites
industry is responding with a European Composites
Recycling Concept to manage waste from composites and
stimulate recycling activities.
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