REVIEW
A review on composite materials based on
recycled thermoplastics and glass fibres
L. Scelsi*
1
, A. Hodzic
1
, C. Soutis
1
, S. A. Hayes
2
, S. Rajendran
1
, M. A.
AlMa’adeed
3
and R. Kahraman
4
The purpose of this paper is to review recent work on composite materials based on recycled
thermoplastics and glass fibres (GFs). The high collection and separation cost of plastics waste,
and the legislative push to increase recycling rates, require the inclusion of increasing proportions
of low-quality plastic waste into recycled products. A robust method for upgrading mixed plastics
recyclates is the incorporation of fillers and reinforcements. In particular, addition of chopped GF
can lead to material systems with more favourable and consistent sets of mechanical properties.
Provided a good interfacial adhesion is achieved, the key structural properties of the composite
(stiffness and strength) are mainly dictated by the reinforcement. Therefore, a wide range of
polymers, including blends, are accessible for recycling into semistructural products. Glass fibres
are one of the most cost-effective ways of reinforcing recycled polymers, as testified by several
patents and commercial products which appeared in the last decade.
Keywords: Recycled polymer composites, Plastics recycling, Polymer blends, Glass fibre, Mixed plastic waste, Mechanical properties
Introduction
Plastics constitute a bulky and non-biodegradable
fraction of solid waste and are mainly produced using
non-renewable resources. Therefore, recycling of plastics
has become a worldwide environmental priority. An
estimated 3 million tonnes of plastics waste are
generated in UK alone each year, most of which are
found in the municipal solid waste (MSW) stream.
National targets in line with European Union Directive
94/62/EC require for UK recycling rates of 25?5% by
2010, in contrast with the 7% recycling rate achieved in
2002.
1
Traditionally, plastics recycling is concerned with
the production of second-grade pellets of a single-type
polymer. However, this approach can be applied only to
a limited number of items found in MSW, which must be
sufficiently well defined, of adequate size and present
low levels of contamination, i.e. plastic bottles and
containers,
2,3
or crates.
4
However, most plastic waste
(an estimated 56%) is used in packaging (three-quarters
of which is from households) and presents substantial
separation and cleaning challenges.
5
Moreover, as
already stated by several authors since the 1980s, the
key to successful mechanical recycling of plastics from
post-consumer waste is the development of suitable
markets for the recycled materials.
6
In western coun-
tries, the markets for most of the recycled products
produced today (i.e. low pressure pipes, traffic barriers,
outdoor furniture, dustbins) are becoming very compe-
titive
7
and new applications with higher ‘added-value’
must
be
found
for
recycled
thermoplastics.
A
long-standing challenge for polymer scientists and
engineers is the production of higher performance
recycled polymers using feedstocks that include increas-
ing proportions of low-quality post-consumer waste.
Literature reviews on recent advances in polymer
recycling have been written by La Mantia,
8
Brandrup,
5
and Vilaplana and Karlsson.
9
Several methods of
upgrading recycled plastics have been reported; mainly
involving blending recyclate with virgin polymers,
restabilisation using specialised additive formulations,
compatibilisation of mixed plastics, or addition of
elastomers and fillers. In particular, reinforcing recycled
polymers with glass fibres (GFs) or natural fibres can
lead to a composite material with a more favourable and
more consistent set of properties (as reported for
example by Xanthos et al.
10,11
). However, despite much
industrial work on the subject, as testified by patents and
publications in trade magazines, scientific literature on
recycled polymer composites is still fragmentary.
Low-cost composites with improved structural prop-
erties – in particular stiffness, heat deflection tempera-
ture and creep resistance – can be obtained from
recycled plastics by addition of rigid fillers or reinforce-
ments. Mineral or organic fillers can be successfully used
in recycled polymer systems. Apart from GFs, the most
widely reported inorganic fillers for recycled poly-
mers are CaCO
3
, talc or wollastonite.
12–19
Plant-based
organic fillers and reinforcements have also been
1
Department of Mechanical Engineering, The University of Sheffield,
Sheffield S1 3JD, UK
2
Department of Materials Science and Engineering, The University of
Sheffield, Sheffield S1 3JD, UK
3
Materials Technology Unit, Qatar University, P.O. Box 2713, Doha, Qatar
4
Department of Chemical Engineering, Qatar University, P.O. Box 2713,
Doha, Qatar
*
Corresponding author, email l.scelsi@sheffield.ac.uk
ß
Institute of Materials, Minerals and Mining 2011
Published by Maney on behalf of the Institute
Received 1 June 2010; accepted 5 June 2010
DOI 10.1179/174328911X12940139029121
Plastics, Rubber and Composites
2011
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1
increasingly used due to low cost and better environ-
mental performance. Owing to easier handling and
availability, wood flour
20,21
is much more widely used
than higher aspect ratio natural fibres.
22–27
Addition of fibres or rigid fillers can substantially alter
the properties of the recycled polymer. If the recycled
composite material is designed correctly, i.e. all the
phases present are well mixed and good interfacial
adhesion is achieved, key properties for semi-structural
applications (such as stiffness and strength) can be fairly
independent
from
the
recycled
polymer
matrix.
11
Compatibilisers and coupling agents might be added to
improve interfacial interactions; in that case, however,
the interplay among the different system components at
their respective interfaces must be studied to optimise
their respective effects.
9
Statistical design of experiments
can be used to optimise the property set of recycled
thermoplastic composites containing both reinforcement
and compatibiliser,
28
or different types of reinforce-
ments.
29
For durable application it is also essential to
restabilise the recycled products.
30–32
In recent years
commercial formulations of heat and light stabilisers,
for instance Recyclostab from BASF (Ludwigshafen,
Germany), have been designed for commingled recycled
waste and allow in many cases to achieve a long term
stability equal or superior to that of virgin polymers.
4
Mechanical recycling of thermoplastics
polymers
Mechanical
recycling
consists
in recovering
waste
material
by
means
of
thermophysical
processes.
Plastics recycling operations generally consist of collec-
tion, separation and cleaning, followed by melt proces-
sing steps. Mechanical recycling of plastics presents
several technical, economical and marketing challenges.
In general, variability of product composition and
colour leads to difficulties in obtaining a product with
a consistent set of properties, and sourcing an adequate
supply of a reasonably clean feedstock is often the
crucial step in determining the economics of the
recycling process and the quality of the recycled
products.
5
Both processability and physical properties
of recycled products can be considerably deteriorated
with respect to virgin polymers. This is mainly due to the
following three phenomena:
33
(i) contamination: both by polymeric and non-
polymeric materials. Polymers can absorb low
molecular weight compounds, which dissolve
and migrate into the bulk of the material and
may cause discolouration, odour, toxicity, or
reduction in mechanical properties. Common
sources of contamination are labels, glue, print-
ing, product residue, etc.
(ii) Degradation: polymers are subject to negative
changes in their macroscopic properties due to
subtle modification of molecular structure that
can result from the following environmental
factors: UV light, thermal-oxidative processes
that can occur during moulding or even at room
temperature, attack by pollutant gasses, chemical
interaction with liquid contents, and others.
Degradative processes are often accelerated by
the presence of certain impurities
(iii) immiscibility: different polymer types are gen-
erally not mutually soluble. When two or more
polymer types are mixed, different materials tend
to form separate phases, and the resulting blend
typically has poor mechanical properties and
poor integrity.
Classification of relevant plastics waste streams
This review is concerned with mechanical recycling of
‘post-use’ thermoplastic waste, i.e. plastic waste origi-
nating from items which have been discarded after their
service life. Some of these plastics come from industrial
sources (such as industrial packaging); however, most of
them originate from collection of plastics fractions from
the MSW stream.
A large proportion of the plastics in the MSW arises
from domestic packaging in the form of films and
containers. Polyolefines (LDPE/LLDPE, HDPE and
PP) are the major components, followed by PS, PET,
PVC and other polymers. The proportions of each
polymer type are highly variable locally and nationally,
due to different consumption patterns, legislation and
the possible presence of separate collections schemes.
Depending on the nature of the waste and on the
framework designed for collecting and separating it,
different strategies can be adopted for the selection of
plastic fractions suitable for mechanical recycling.
Generally, the recovered material falls into one of the
following broad categories:
(i) ‘Single-grade polymer’. Such a waste stream
comes from a single original application and is
therefore constituted by one single polymer
grade or by very similar polymer grades. Being
from a single application source, it has a
relatively uniform and predictable level of
contamination and degradation and is generally
separated by colour. Some examples are HDPE
bottles,
3
bottle crates,
4
or LDPE/LLDPE films
from greenhouses
19
(ii) ‘Single polymer type’. A mixture of different
grades of the same polymer type, originating
from a range of applications. In practice there
are limitations to the degree of the separation of
plastics into types, thus each final product
stream would contain a preponderance of one
plastic type and some proportions of other
types.
6
Examples in this category can be LDPE
from packaging, or HDPE from different types
of containers. Excessive differences between the
constituent grades can lead to processability
problems and poor mechanical properties. In
the case of PP, which can be found in a variety
of both homopolymer and copolymer grades, a
more consistent melt flowrate can be obtained
by visbreaking the polymer chains by adding
small quantities of peroxides
28
(iii) ‘Polyolefinic fraction’. Using a simple sink/float
method, a light fraction, floating on water, and
a heavy fraction can be separated. The first
fraction is essentially made of LDPE, HDPE,
PP and high impact polystyrene (PS). The heavy
fraction is formed by polyvinylchloride (PVC),
crosslinked resins and high melting thermoplas-
tics. Although LDPE, HDPE, PP and PS are
incompatible polymers, it has been demon-
strated that reprocessing mixtures containing
Scelsi et al.
Composite materials based on recycled thermoplastics and GFs
2
Plastics, Rubber and Composites
2011
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more than 60% LDPE maintain good mechan-
ical properties, as the LDPE phase effectively
acts as a binder.
34
Generally, impact and
elongational properties of the polyolefinic frac-
tion are better than those of the mixed waste.
The improvement of tenacity is, in particular,
evident when considering impact resistance of
polyolefines as compared to mixed plastics.
Samples subjected to impact tests show an
increase in elongation at the elongation at break
from 7% to above 100%.
35
However, the elastic
modulus is reduced in this case. Gattiglia et al.
34
ground the heavy fraction to a particle size of
100 mm and used it as filler in a polyolefin
matrix to increase the elastic modulus without
substantially decreasing elongation at break.
(iv) ‘Commingled polymer waste’. To avoid the
often costly separation step, commingled plastic
waste can be reprocessed directly. In this case, a
widely variable range of immiscible polymers
with different melt flow indexes and melting
temperatures can be formed into thick-sections
of so called plastic lumber. Such materials are
generally used as wood or concrete replacement
in low-performance applications, such as out-
door furniture, decking or traffic barriers. In
order to generate property sets adequate to
more demanding applications, the final blend
morphology must be controlled, through addi-
tion of compatibilisers and/or fillers and ade-
quate processing methods and parameters.
Life cycle analyses have shown that mechanical recycling
is the most favourable recovery route for easily
separable and relatively clean plastic waste-streams.
5
As shown by a recent study by Rajendran et al.,
36
for
mixed plastics the environmental gains of mechanical
recycling versus incineration are not evident, and
mechanical recycling makes both environmental and
economic sense only if the materials are not heavily
contaminated and if suitable markets for the recycled
products can be found. Reducing the separation and
cleaning steps to a minimum required level, and
upgrading through addition of fibres or fillers can be a
cost-effective and environmentally sustainable option
for recovery of plastic waste belonging, in particular, to
categories 3 and 4.
Upgrading recycled plastics by addition
of rigid reinforcements or fillers
Recycling waste thermoplastics into composites presents
the same type of challenges as recycling unfilled polymer
systems. However, due to the greater complexity of the
material system, which is composed by one or more
recycled polymers, the reinforcements/fillers and other
additives (compatibilisers, stabilisers, impact modifiers),
a quantitative understanding of these systems is still out
of reach. The morphology, and subsequent properties,
of the multiphase material need to be optimised
experimentally by tuning the composition and the
processing
parameters
on
an
individual
basis.
However, if a good inter-phase bonding is ensured, the
structural properties of the composite become less
dependent on the polymer matrix composition,
11
and
therefore recycled products with consistently higher
stiffness and strength can be obtained. Such products
can be used in semi-structural applications. The presence
of rigid fibres minimises creep and can greatly increase
heat deflection temperature (HDT).
37
Impact strength
and elongation at break follow less predictable trends,
which are briefly discussed.
Recycled polymer streams can often be considered
immiscible blends. Even single polymer type streams are
generally contaminated by other polymers (10% of
foreign materials is not unusual),
5
or are constituted
by a mix of grades which are not completely miscible.
This can result in a lack of adhesion at the phase
boundaries, which is one of the main reasons for the
brittleness of recycled plastics. The most striking effect
of recycling on short term properties is a drop in
elongation at break.
8
Compatibilisation is often the key
in improving both impact strength and toughness of
commingled plastics.
38
Alternatively, small particles of rubbery impact
modifiers (such as EPDM, SBS, SEBS and EVA) can
be added to recyclates to increase toughness, but
generally at the expense of stiffness.
39
Rigid fillers
greatly increase stiffness (which is generally the design
criteria for semi-structural applications), but tend to
further reduce elongation at break and, in some cases,
impact strength. Compatibilisers and coupling agents
can be added to improve adhesion respectively between
different polymer phases and between polymer and
reinforcement. In thermoplastics filled with wood flour
or natural fibres, effective compatibilisation is a key
factor in improving tensile and impact strength.
40
In general, for semistructural applications, which are
designed for stiffness with high safety factors, elongation
at break of a few percent is sufficient to guarantee a
satisfactory performance. Upgrading recycled mixed
plastic waste through addition of reinforcements is a
cost-effective way to produce materials that can be used
for a range of increasingly demanding applications.
Products already on the market range from fire-retarded
lightweight scaffolding boards,
41
to replacements for
automotive GMT composites,
42
to heavy-loaded bridges
with a guaranteed lifetime of 50 years.
43
The use of
reinforced recycled plastic in the construction of bridges,
in particular, shows the potential of these materials.
Researchers at Rutgers University, NJ, USA, showed
that the morphology of immiscible blends of recycled
polymers reinforced with a low percentage of GF can be
controlled during processing to obtain materials systems
with specific strength higher than steel.
43
Glass fibre reinforcement
Glass fibres are one of the most cost-effective reinforce-
ments for plastics. Chopped GFs can be compounded
with recycled thermoplastics to obtain recycled products
1
Schematics of physical compatibilisation (adapted from
Ref. 53)
Scelsi et al.
Composite materials based on recycled thermoplastics and GFs
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Table
1
Overv
iew
of
rese
arch
st
udies
on
GF-rein
forced
recy
cled
polym
ers
Aut
hors
Poly
mer
m
atrix
Reinf
orceme
nts/fillers/a
dditives
Comme
nts
Refere
nce
Xant
hos
et
al.
(1995
)
Mix
ed
plas
tics
w
aste
(M
PW)
(80%
PE)
20%
chop
ped
GF;
or
25%
m
ica,
or
40%
CaCO
3
,
o
r
4
0
%
talc
Unmodi
fied
and
m
aleated
recycle
d
blends
were
compo
unded
w
ith
vario
us
reinforc
ements
or
filler
s.
The
upgrad
ed
rec
yclate
s
had
propert
y
sets
compe
titive
with
virgin
HD
PE
m
oulding
compo
unds.
10
Xant
hos
et
al.
(1998
)
Ble
nds
of
vi
rgin
mixed
thermo
plati
cs
(TP),
with
high
prop
ortions
of
HD
PE
(.
70%)
15%
chop
ped
GF;
coup
ling
ag
ent:
Krato
n
1
9
0
1
6
or
Polybo
nd
3009
A
set
of
mod
el
recycle
d
blends
with
varia
ble
composi
tion.
With
ad
dition
of
GF
and
adhe
sion
prom
oters,
the
blends
had
prop
erties
cons
istent
with
[2]
and
indepe
nden
t
of
blen
d
composi
tion.
11
Vezzo
li
et
al.
(1993
)
MPW
(39
%
PE,
19%
PVC,
15%
PP,
14%
PS,
13
%
PET)
30%
chop
ped
GF;
or
20%
ta
lc;
or
10%
SB
S
rubber
Addition
o
f
G
F
yielde
d
produc
ts
w
ith
very
high
stiffnes
s
(elas
tic
mod
ulus
5
2
. 8
GPa
with
30%
GF),
far
be
tter
than
th
at
of
the
origi
nal
blen
d
(0
. 95
GPa)
35
La
M
antia
(19
96)
Commi
ngled
PE,
PVC
and
PET
fro
m
rec
overed
plastic
contain
ers
for
liquids
10%
chop
ped
GF;
or
10%
CaCO
3
;
o
r
10%
sawd
ust
Stabilisers,
inert
fillers,
elastom
eric
modifiers
an
d
compa
tibilisers
can
imp
rove
proce
ssa
bility
and
propert
ies
of
rec
ycled
blen
ds.
39
Ma
&
L
a
Mantia
(19
94)
PET/HD
P
E
m
ixture
simula
ting
pa
ckagin
g
waste
10,
20
or
40
wt-%
chop
ped
GF
20%
GF
imp
roves
tens
ile
mod
ulus
(b
y
50%),
tensile
streng
th
(by
110%
),
and
impac
t
stre
ngth
(by
70%)
and
heat
deflect
ion
te
mper
ature
(HDT
).
Elonga
tion
at
break
is
uncha
nged
(1
. 5%
).
62
Vinci
and
La
Mantia
(19
96)
PP
extrud
ed
twi
ce
to
si
mulate
recyclin
g
Chopp
ed
GF;
wollas
tonite
and
calci
um
oxide
R-PP
fil
led
with
35
wt-%
GFs
show
high
te
nsile
modulu
s
and
HD
T,
bu
t
a
drop
in
elongat
ion
at
break.
Repla
cing
som
e
o
f
the
GF
with
one
of
th
e
o
ther
filler
s
slightly
decre
ases
mecha
nical
prop
erties
bu
t
imp
roves
elongat
ion
at
break
an
d
proce
ssabil
ity.
63
Tzan
kova
Dintch
eva
et
al.
(2001
)
Ligh
t
fra
ction
from
M
P
W
sep
arate
d
b
y
flota
tion
(75
–80%
PE,
20–2
5%
PP
and
0
. 3–0
. 5%
PET,
PVC,
PS)
20
w
t-%GF
,
woo
d
fibres
(W
F),
or
CaCO
3
20%
GF,
wood
fibre
s
(WF),
or
CaCO
3
all
great
ly
imp
rove
elastic
mod
ulus.
Tensile
and
impac
t
stre
ngth
are
almo
st
unaffec
ted
and
elon
gation
at
breaks
is
sl
ightly
de
creased
.
64
Hug
o
et
al.
(20
10)
Prop
rietary
bl
end
of
rec
ycled
thermo
plas
tic
po
lymers
GF,
CaCO
3
,
mica
,
fir
e
ret
ardan
ts
Up
to
30
wt-%
GF
grea
tly
improv
ed
tens
ile
an
d
flexura
l
moduli
and
streng
ths
and
redu
ced
therma
l
exp
ansion
by
60%.
Simultan
eous
addition
o
f
30%
GF
an
d
5
%
mica
appe
ared
to
have
a
syn
ergistic
effe
ct
in
enhan
cing
tensile
mod
ulus
(by
25
0%)
an
d
streng
th
(by
350%
).
70
Oh
and
Joo
(1998)
PP/EP
DM
blend
fro
m
car
bumpe
rs
fasc
ias
–
contam
inate
d
b
y
paint
Recy
cled
glas
s-mat
PP
fro
m
car
bum
pers
(co
ntain
ing
42
wt%
GF)
A
m
aterial
with
interes
ting
pr
opert
ies
was
ob
taine
d
by
adding
up
to
71%
GMT-
PP
(corresp
onding
to
30%
GF).
Tens
ile
mod
ulus
and
streng
th
increa
sed
cons
iderably
an
d
impact
streng
th
de
creased
.
45
Putra
et
al.
(20
09)
Po
st-consu
mer
waste
main
ly
com
posed
of
poly
olefines
7–26
vol.-
%
Gypsum
,
w
ollasto
nite,
talc,
or
GF
The
perfo
rmance
of
severa
l
inorg
anic
fil
lers/rei
nforcem
ents
was
compa
red.
Glass
fibre
s
g
a
v
e
the
be
st
imp
roveme
nts
of
stif
fness
and
stre
ngth,
follo
wed
by
ta
lc.
D
u
e
to
worse
disper
sion
and
ad
hesion
,
w
ollasto
nite
and
gyp
sum
lead
to
subst
antia
lly
sm
aller
improv
ement
s.
13
Geo
rge
and
Dillman
(2000
)
Mix
ed-co
lour
post-con
sumer
H
DPE
flak
e
fro
m
rec
ycled
de
tergent
bottles
GF
fro
m
recove
red
fibr
eglass
compo
sites;
and
woo
d
flour
20–2
5%
GF:
increa
se
in
stiffnes
s
cons
istent
w
ith
th
e
perfo
rmance
of
virgi
n
GF;
ho
wever,
the
recove
red
fibre
s
caus
ed
a
sharp
redu
ction
in
impact
streng
th.
Repla
cing
a
proport
ion
of
GF
with
wood
flour
decre
ased
the
modulu
s,
but
restore
d
impact
streng
th.
29
Scelsi et al.
Composite materials based on recycled thermoplastics and GFs
4
Plastics, Rubber and Composites
2011
VOL
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Au
thors
Polym
er
matrix
Re
inforcements/
fillers/additives
Com
ments
Re
ferenc
e
Ad
ewol
e
&
Wo
lkowic
z
(20
03)
Blend
of
post-c
onsu
mer
HDPE
(from
contain
ers)
and
PP
(mixtu
re
of
homo-p
olymer
and
copo
lymer
industr
ial
off-gra
de
materia
ls)
20
%
choppe
d
GF;
couplin
g
agent:
M
ontell
HG20
1;
compa
tibilis
er:
Kra
ton
Both
un-reinf
orced
and
20%
GF
rei
nforced
blends
with
diff
erent
co
mpositio
ns
of
cons
tituent
polym
ers
and
com
patibliser
w
ere
prep
ared.
The
prop
erty
set
w
a
s
tailo
red
to
specif
ic
applica
tion
requ
irement
s.
20
%
G
F
allows
pr
oductio
n
o
f
a
significa
ntly
stif
fer,
but
more
bri
ttle
range
of
m
aterials
.
28
H
agbe
rg
and
D
ickerso
n
(1997)
Carpet
Waste
(Nylon
,
C
aCO
3
,
PP,
SBR)
Compat
ibilised
w
ith
10%
M
aleic
Anhydri
de
graf
ted
PP
[PolyBo
nd
31
50]
15
,
3
0
%
choppe
d
G
F
The
ad
dition
of
30
%
G
F
impr
oved
the
tensile
streng
th
(by
18
0%),
tens
ile
mod
ulus
(b
y
190%)
and
Izod
impac
t
stre
ngth
(by
130%)
.
The
prop
erties
of
the
glass
fil
led
car
pet
waste
were
compe
titive
with
sev
eral
comme
rcial
resin
s.
65
Pe
goret
ti
and
Pe
nati
(20
04)
PET
from
bevera
ge
bo
ttles
recover
ed
fro
m
MSW
15
,
3
0
%
Chopp
ed
GF
Rem
arkable
increa
se
in
modulu
s,
tensile
strength
,
imp
act
stre
ngth
an
d
HDT
in
the
reinforc
ed
prod
uct.
El
ongatio
n
a
t
brea
k
remains
low.
Hygro
therma
l
a
g
ing
can
be
an
issue
for
du
rable
ap
plica
tions.
66
C
antwel
l
(19
99)
Bottle-grade
rec
overed
PET
10
,
2
0
,
30,
40,
50%
load
ings
of
eith
er
woven
glass
fa
bric
or
chop
ped
stra
nd
m
a
t
(CSM)
Hig
h-qua
lity
lamina
tes
were
manufa
cture
d
via
compre
ssion
mou
lding.
Th
e
mec
hanical
propert
ies
were
si
milar
to
lamina
tes
ba
sed
on
virgin
PET.
Goo
d
fracture
resist
ance,
compe
titive
with
toug
h
syst
ems
,
such
as
carbo
n-reinfo
rced
PEE
K.
67
Re
zaeian
et
al.
(20
08)
Bottle-grade
rec
overed
PET
10
–30%
choppe
d
G
F
The
proc
essability
and
orien
tation
of
GF
in
r-PET
at
diff
erent
load
ings
was
studied.
68
Liu
et
al.
(20
02)
Recycle
d
ABS
and
po
lyamide
(PA)
from
ca
r
compo
nents
Th
e
P
A
compo
nent
w
a
s
reinf
orced
w
ith
shor
t
G
F
The
imp
act
stre
ngth
for
blen
ds
of
recycle
d
ABS
and
GF-
reinforce
d
P
A
was
lower
than
that
expect
ed.
This
was
attribu
ted
to
po
orly
bo
nded
GFs
in
the
blend
mat
rix.
69
Tab
le
1
continue
d.
Scelsi et al.
Composite materials based on recycled thermoplastics and GFs
Plastics, Rubber and Composites
2011
VOL
40
NO
1
5
with improved property sets. They are easily obtainable
from a range of manufactures and their production
process is quite energy efficient, so that their use into
recycled products does not significantly affect the
environmental performance. According to calculations
based on data from the life cycle analysis database
IDEMAT using the software SimaPro, the cumulative
energy demand of GF is only 8?7 (MJ kg
2
1
)
Eq
,
compared with 65?8 (MJ kg
2
1
)
Eq
for PP.
44
Equally,
the price of commercial chopped strand GF is in the
range of $1?7–3/kg, making them a cost-effective option
for upgrading plastics recyclates. An interesting alter-
native is to use post-consumer GF-filled polymers (such
as glass-mat PP
45
) as a source of GF. Recent studies
have shown that thermoplastic composites of improved
performance can also be obtained by using GF
recovered from thermoset composites after granula-
tion.
46,47
Several patents, commercial products and publica-
tions in trade magazines on GF reinforced recycled
products
have
appeared
in
recent
years.
41,42,48–60
However, apart from a series of contributions from
Xanthos and co-workers,
10,11
who focused on reinfor-
cing commingled waste with GF and other fillers and to
improve phase adhesion by compatibilisation, published
academic work on recycled GF reinforced thermoplas-
tics is quite fragmentary.
The main interest in using GFs in polymer products is
their better and more consistent performance compared
to other fillers and reinforcements. Generally, GF
loadings of 10–40% result in substantial increase in
elastic modulus, accompanied by an increase in tensile
strength. This is due to the high aspect ratio of the fibres
(whose diameter is in the range of 10–20 mm and final
modal length in the range of 0?5 mm), which translates
in an excellent reinforcing ability, and to relatively
mature technologies available to couple commercial
silane-treated GFs and polymer matrices. For virgin
thermoplastics, GFs considerably reduce elongation at
break and can either increase or decrease impact
strength. However, the overall trend is different in the
case of recycled thermoplastics, which often are mixtures
of poorly-bonded polymers, and where elongation at
break is almost unaffected by addition of GF,
61
while
impact strength generally increases. The increase in
material integrity and performance when adding GF to
commingled plastics has been interpreted by some
authors in terms of physical compatibilisation, i.e.
binding of dissimilar resin domains through the fibres.
This concept, illustrated in Fig. 1 was used to explain
the good performance of recycled thermoplastics com-
posites fabricated through the RadLite technique (GE
Plastics).
53
Some of the key academic and industrial studies are
listed in Tables 1–3. Xanthos et al.
10,11
focused on
reinforcing commingled waste with GF and other fillers
and to improve phase adhesion by compatibilisation,
and showed that products with improved property sets
and acceptable processability could be obtained. These
authors reported improved and more consistent proper-
ties with the addition of GF. For non-compatibilised
polymer matrix, stiffness and tensile strength imp-
roved considerably, with almost no effect on notched
impact strength. Compatibilisation by addition of
maleated HDPE improved notched impact strength. A
Table
2
Som
e
rece
nt
US
paten
ts
on
GF
-reinforc
ed
rec
ycled
polym
ers
Authors
In
vention
Comme
nts
Refere
nce
Phillips
et
al.
(2002)
Stru
ctural
recycle
d
plas
tic
lumbe
r,
fro
m
wast
e
th
ermopla
stics
(TP)
and
w
aste
GF
reinforc
ed
therm
osets
(TS)
The
TS
(gen
erally
from
recover
ed
fibre
glas
s)
is
granu
lated
and
th
en
extrud
ed
w
ith
th
e
T
P
.
Coupli
ng
ag
ents
and
foamin
g
agents
can
be
adde
d.
48
Khrisna
wswam
y
et
al.
(2004)
Th
ermopl
astic
compo
site
lumbe
r
ha
ving
reinforci
ng
lami
nate
of
un
idirecti
onal
fibre
s
A
T
P
lumbe
r
with
at
leas
t
o
n
e
rei
nforcing
laye
r
o
f
unidirec
tiona
l
lami
nate.
Both
lumbe
r
mat
rix
an
d
laminat
e
matrix
prefe
rably
made
of
r-PE
and
r-P
P.
The
laminat
e
layers
are
heat
bond
ed
to
the
TP
cor
e.
49
Muzzy
(20
04)
Fibr
e-reinfor
ced
recycle
d
thermo
plastic
co
mposi
te
and
metho
d
A
c
o
mposi
te
includi
ng
a
comm
ingled
TP
matrix
(mainly
fro
m
carpe
t
waste
)
and
a
varie
ty
of
high
modulu
s
fibre
s
(prefera
bly
GF
GMT
or
long
fibre
s),
combin
ed
under
low
stress
co
nditions
to
preserve
fibre
length.
50
Nosker
an
d
Renfree
(1998)
C
ompos
ite
building
m
aterials
fro
m
rec
yclable
waste
A
c
o
mposi
te
buildin
g
materia
l
suit
able
fo
r
railroa
d
ties
,
made
of
co
mming
led
plas
tic
wast
e
(cu
rbside
tailings
–
mos
tly
H
DPE)
and
suit
ably
coated
fibres.
Both
compo
nents
are
granu
lated
and
subse
quen
tly
extrus
ion
co
mpounde
d.
51
Scelsi et al.
Composite materials based on recycled thermoplastics and GFs
6
Plastics, Rubber and Composites
2011
VOL
40
NO
1
Table
3
Exam
ples
of
com
mercia
l
p
roducts
ma
de
of
GF-re
inforc
ed
rec
ycled
polym
ers
Compa
ny
Poly
mer
matr
ix
Re
inforcement
C
omposi
te
prod
uct
Re
ferenc
e
Axion
Inte
rnationa
l
Holdin
gs
Inc.
(US
A)
PP/HD
PE
blend
of
rec
ycled
car
bumpe
rs
and
milk
jugs
C
hopp
ed
GFs
Rai
lroad
ties
mad
e
o
f
glas
s-filled
PP/H
DPE
blend
52
I-B
eam
of
PP/HDPE
used
for
tank
bridge
s
GE
Pl
astics
(USA
);
RadL
ite
Tech
nology
Commin
gled
plas
tics-po
wdered
and
for
med
into
rolls
afte
r
mixing
with
GF
in
aq
ueous
dispe
rsion
C
hopp
ed
GFs
U
nfoame
d
:
SMC
compo
sites,
with
similar
prop
erties
to
virgin
ones
;
53
Fo
amed:
pa
rticle
board,
sound
ba
rriers
or
othe
r
cons
truct
ion
mat
erials
Saly
p
N
V
(Belg
ium)
PP
mixed
grades
fro
m
auto-
shredd
er
residu
e
(ASR
)
1/
2
inch
GFs
C
a
r
bumpe
rs
or
other
automo
tive
parts
54
Uniq
Extr
usions
(UK)
Blend
of
th
ermopla
stics
fro
m
post-in
du
strial
and
post-c
onsu
mer
stream
s
C
hopp
ed
GFs
Sc
affoldi
ng
bo
ards.
Holl
ow
core
and
there
fore
light
weigh
t,
no-sl
ip.
Can
be
fire-re
tarde
d
41
Greyst
one
Logist
ics
(USA
)
Blend
of
PE
and
PP
fro
m
p
o
st-indu
strial
and
po
st-consu
mer
stream
s
Fibre
glas
s
rods
inserte
d
af
ter
mou
lding
Rack
able
pallets.
Pe
rform
ance
an
d
cost
co
mpetitive
with
wood
palle
ts
55
Envi
roKare
Tech
(Can
ada)
Recy
cled
PP
20
%
long
GF
Pa
llets
with
cost
co
mpetitive
to
woo
den
palle
ts
an
d
superio
r
stre
ngth.
The
produc
t
w
as
mad
e
usin
g
the
innovat
ive
proce
ss
of
thermo
plas
tic
flow-m
oulding
56
Geo
rgia
Comp
osites
Inc.
(US
A)
Mixed
carpe
t
waste
40
%
choppe
d
GF,
or
glass
m
a
t
sup
plied
as
a
rol
l
Mat
erial
of
sup
erior
prop
erties
than
comme
rcial
GMT
compo
sites
for
automo
tive
ap
plica
tion
42
,
5
0
,
57
PP
from
carpe
t
wast
e
Lapinu
s
Fibres
BV
(The
Net
herlan
ds)
HDPE/
PP;
or
PA
6;
with
compa
tibliser
2
0
%
Rockfil
mineral
fibres
PA
6
z
20%
fibre
s
incre
ased
modulu
s
(by
25%),
imp
act
strength
(by
60%)
an
d
HDT
(from
75
to
15
6
. 5
uC
).
Fibre/co
mpa
tibiliser
m
ix
marke
ted
un
der
th
e
n
a
m
e
R
o
cknet
58
Poly
merix
(US
A)
Post-c
onsum
er
commo
dity
or
enginee
ring
plas
tic
scra
p
C
hopp
ed
GF
Plas
tic
lumber
with
GF
reinforc
ement
an
d
a
foame
d
core,
whic
h
prov
ides
deflect
ion
load
prop
erties
si
milar
to
the
southe
rn
yellow
pine
59
Comp
osite
Techn
olog
ies
Corp.
(USA
)
Blend
of
rec
ycled
po
st-consu
mer
and
post-in
du
strial
PP
and
PEs
15
wt-%
GF
rec
overed
from
regro
und
fibre
glas
s
Back
board
syst
ems
w
ith
a
high
impact
stre
ngth
an
d
a
high
flexura
l
m
odulus
60
Scelsi et al.
Composite materials based on recycled thermoplastics and GFs
Plastics, Rubber and Composites
2011
VOL
40
NO
1
7
further series of publications by La Mantia and co-
workers
34,39,62–64
evaluated the performance of a range
of additives and fillers in upgrading recycled plastics.
Glass fibres appeared to consistently outperform other
fillers and to cause simultaneous improvements in elastic
modulus, tensile or flexural strength, notched impact
strength and heat deflection temperature. These authors
also compared the performance of different compound-
ing methods and reported substantially higher properties
for composites processed in a twin-screw extruder due to
better distribution of the dispersed phase.
64
Other
solitary studies
13,28,29,45,65–70
appeared in the literature
in more recent years, all reporting a substantial increase
of properties with the addition of up to 40 wt-%
chopped GF. The mechanical properties of most of
these systems are summarised in Fig. 2 to illustrate
common qualitative trends.
Elastic modulus and tensile strength increase with the
addition of GF in the recycled polymers as shown
respectively in Fig. 2a and b.
Despite the low elongation at break, in many
instances impact strength appears to increase as shown
in Fig. 2c. Among the studies reported in Table 1, a
substantial decrease in impact strength was observed by
George and Dillman
29
(who directly used recovered GF
from thermoset composites after granulation) and Liu
et al.,
69
and was attributed in both cases to poor
bonding between the fibres and the polymer matrix. Oh
and Joo
45
also reported a decrease in impact strength
by adding glass-mat reinforced PP to an EPDM-
toughened PP.
In almost all cases, elongation at break of GF filled
products drops to values of 2–4% as shown in Fig. 2d.
This value is generally much lower than typical values of
elongation at break for virgin polyolefines or for single
type unfilled recycled polymers; however, it is not very
different from the elongation at break of uncompatibi-
lised commingled plastic waste.
To increase cost-effectiveness of the recycled materi-
als, hybrid composites can be formulated by simulta-
neous addition of GF and a lower aspect ratio filler. By
replacing a proportion of GF with a low cost filler, a
new balance of properties is obtained. Generally elastic
modulus,
tensile
strength
and
HDT
are
slightly
decreased, whereas elongation at break increases. Vinci
and La Mantia
63
also reported substantially better
processability for hybrid materials based on recycled
PP. Impact strength can also increase, as in the case of
George and Dillman,
29
who partially replaced recovered
GFs with wood flour in a recycled HDPE. Sometimes,
synergistic interaction can be observed between the GF
and the secondary filler. Recent work from Hugo et al.
70
on a commingled recycled blend showed that composites
containing 5% wt mica in addition to 15–30% GF had
remarkably better tensile properties than those contain-
ing only GF (Fig. 3).
Another
important
property
for
semi-structural
application is the coefficient of thermal expansion. As
2
Influence of GF on mechanical properties of recycled polymers: a elastic modulus for GF-filled recyclates. In some
cases the tensile modulus was measured, in others the flexural modulus. b tensile strength; c impact strength. Due to
different measurement methods for impact strength between authors, impact strength relative to unfilled samples has
been reported here. d elongation at break
Scelsi et al.
Composite materials based on recycled thermoplastics and GFs
8
Plastics, Rubber and Composites
2011
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40
NO
1
shown in Fig. 3, addition of 30 wt-% GF can reduce the
thermal expansion coefficient to values approaching
those of steel and concrete.
70
A number of recent patents on GF reinforced recycled
thermoplastics, manufactured using a variety of proces-
sing methods, and commercial products which can
compete with virgin plastics or with wood in load-
bearing applications, testify the effectiveness of GF
reinforcement in upgrading commingled thermoplastic
waste. Tables 2 and 3 present some examples of the key
inventions and durable products made of recycled GF
reinforced plastics. A variety of GF reinforced plastic
lumbers are available in the US alone, most of which
produced without addition of compatibilisers or cou-
pling agents, as the marginal improvement in structural
properties for these systems often does not justify the
increase in cost. However, coupling agents and compa-
tibilisers, such as maleated polyolefines, are now
becoming very competitive
71
and are likely to become
more widely used in the near future, as increasingly
demanding applications are being targeted.
Discussion
Recent studies on recycled thermoplastics filled with
GFs have been summarised. In most of these studies,
GFs were one of several low-cost rigid filler options used
to upgrade thermoplastic waste. Glass fibres outper-
formed all other fillers and reinforcements, which
included wood fibre, wood flour, CaCO
3
, talc and
wollastonite, by significantly increasing stiffness and
tensile strength. When reinforcing recycled thermoplas-
tics with virgin GF, an increase in impact strength was
also generally observed, while elongation at break was
only marginally reduced. In most of the industrial
products these improvements were obtained by using
commercial silane-treated GF without additional com-
pounds that improve the matrix-fibre coupling, such as
maleated polyolefines. Recovered GF from thermoplas-
tic or granulated thermoset composites can also be used
to reinforce waste plastics, however in this case impact
strength is more likely to decrease.
Property enhancement by addition of GF to recycled
polymers varied widely from one system to another.
However, the final properties of GF-filled recycled
polymer composites have been shown by some authors
to be more dependent on the processing method and
parameters, rather than on the composition of the
matrix polymer blend.
11,64
Therefore, addition of GF
appears to be an effective way to obtain products with a
more consistent set of properties from recovered
polymer feedstocks of variable composition. The cost
of virgin chopped GF is in the order of $2/kg, making
them cost-effective in upgrading commingled polymer
waste, considering that products competitive with virgin
polymers can generally be produced by direct incorporation
of 20% GF, and that the costs of separating the mixture
into single-polymer streams are avoided. According to an
estimate from Brandrup
5
the sorting cost for post-
consumer plastics into single type polymers in Germany
in the 1990s amounted to over 30% of the total cost of
recycling and was equivalent to the cost of the subsequent
reprocessing operations.
A disadvantage of filled systems, and in particular of
GF filled polymers is the increase in melt viscosity. Such
a decrease in processability, which results in higher
energy consumption and reduced throughput, is toler-
able if the reprocessed parts are extruded or injected into
relatively thick walled sections. Polymer processing
simulations can help in identifying suitable products
that can be obtained from recycled GF-reinforced
plastics.
11
Moreover, multiple reprocessing of GF-filled
products can be problematic, due to fibre breakage,
which occurs at each processing step, and polymer
matrix degradation. Most studies on glass-fibre rein-
forced thermoplastics have been performed on engineer-
ing polymers such as nylon
72
or polycarbonate
73
and
have shown that degradation upon reprocessing is
aggravated due to the higher viscosity of filled systems.
For this reason, durable applications must be targeted to
extend the useful life of the material before incineration
or disposal.
Acknowledgement
The authors would like to acknowledge Qatar Science
and Technology Park for funding the collaborative
research between the University of Sheffield and Qatar
University.
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3
Tensile modulus (TM), tensile strength (TS) and coeffi-
cient of thermal expansion (CTE) for a recycled blend
reinforced with different proportions of glass fibres
(GFs), CaCO
3
(C) and mica (M) (adapted from Ref. 70)
Scelsi et al.
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