Development of recycled polymer composites
for structural applications

A.-M. Hugo

1

, L. Scelsi*

1

, A. Hodzic

1

, F. R. Jones

2

and R. Dwyer-Joyce

1

This paper is concerned with the formulation of composite materials for structural or semistructural
applications using thermoplastic polymer waste. The mechanical and thermal properties of a
proprietary blend of recycled polymers with a range of different fillers were investigated. The
effect varied with the aspect ratio of the filler and the mode of loading. Spherical calcium
carbonate gave a marginal improvement in modulus. Plate-like mica produced a significant
increase in modulus without reduction in strength. Glass fibres caused a significant increase in
modulus and strength while decreasing the linear coefficient of thermal expansion. Hybrid
systems containing glass fibre and a lower aspect ratio filler were also investigated to obtain a
material system which combines high properties and reasonably low cost. It was found that
addition of small quantities of mica to glass fibre reinforced blends exhibited a significant synergy
in tensile strength and modulus.

Keywords: Recycled, Thermoplastic, Mica, Glass fibre, Polymer blend, Composites

This paper is part of a special issue on Latest developments in research on composite materials

Introduction

In 2005, the annual consumption of plastic materials was
nearly 44 million tonnes for Western Europe alone, and it
has been rapidly growing.

1

Recycling of plastics has

therefore become a worldwide environmental priority.
The main barriers to plastics recycling are the high cost of
the recycled products relative to their performance and
the difficulty to identify satisfactory markets for these
materials.

2

In the UK, recycling targets for plastics of

26% for 2008 have been set by legislation.

3

Any post-use

plastic packaging contributes to this target, which has led
to a focus on the most cost effective and easily
recoverable streams, such as industrial and commercial,
rather than domestic, packaging.

Traditionally, recovered plastics are separated into

polymer types and used to produce ‘second grade’
pellets. However, the range of applications for these
pellets is limited due to their reduced properties and to
possible contamination, which prevents their use in food
contact applications. An alternative approach, explored
in this study, consists of upgrading plastic recyclates by
the addition of rigid fillers to improve their structural
properties and to make them suitable for long term
(semi)structural applications.

Polymers have many advantages compared to con-

ventional materials for structural applications, especially
if their stiffness and strength can be improved. Using
recycled materials reduces cost significantly; however,

polymer formulation and production can be challenging
to overcome the natural variability in feedstock.

2

Semicrystalline polyolefins have many desirable proper-
ties for structural applications: good toughness, high
fatigue resistance, chemical resistance, non-toxicity in
the environment, high electrical resistivity, low water
absorption, good corrosion resistance, UV stability,
lifetime of up to 50 years and recyclability. Conven-
tional building products have higher stiffness, better
creep resistance and lower coefficient of thermal ex-
pansion (CTE). This study investigates the use of fillers
to improve the properties of a proprietary blend of
plastics.

The polymer blend examined in this work consists

mainly of post-industrial plastic waste. Post-industrial
waste is the scrap from industrial processes, e.g. yarn
bobbins, jerry cans, end of runs, misprinted pots, etc. It
is usually clean and segregated by type, which presents
considerably less batch to batch property fluctuations
compared to domestic polymer waste. The blend is a
proprietary polymer formulation containing both semi-
crystalline and amorphous thermoplastics in order to
achieve a good balance between toughness and stiffness.
Although details on blend composition cannot be
disclosed, all the components were commodity polymers
typically very abundant in landfills, and the results
presented in this study are generally relevant for the
recycling

of

commingled

polymer

waste

streams.

Previous studies from Rutgers University

4

have shown

that it is possible to achieve polymer blend morphologies
with high mechanical properties without the addition of
compatibilisers through correct blend formulation and
processing.

The effect of filler on the mechanical properties will

depend upon its chemical composition, particle shape

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

*

Corresponding author, email [e-mail]

ß

Institute of Materials, Minerals and Mining 2011

Published by Maney on behalf of the Institute
Received 16 September 2010; accepted 19 September 2010
DOI 10.1179/1743289810Y.0000000008

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317

and size, size distribution, specific surface area, sur-
face chemistry, interparticle spacing and extent of
agglomeration.

5

Higher aspect ratio fillers give greater

reinforcement and produce higher stiffness, heat distor-
tion temperature and creep resistance.

The typical spherical fillers are calcium carbonate,

clay, glass beads, carbon black and alumina trihydrate.
Among these, calcium carbonate is the most widely used
filler as it is readily available and of low cost.

5

It reduces

warpage, increases modulus and, in virgin materials,
reduces the cost of the material. In such applications,
strength is normally reduced slightly. Impact toughness
is also reduced, with the exception of very fine additive
grades, which can act as impact modifiers.

5

Stearate

coatings are often used to improve surface bonding and
dispersion. The type of polymer is also important where
filler/matrix interfaces are considered. For example, the
coated filler increased the impact toughness in poly-
propylene (PP) homopolymer; however, it decreased the
toughness in high density polyethylene and PP.

6

Plate-like fillers are better reinforcements than sphe-

rical fillers. Examples are talc, mica and kaolin.

7

Modulus, shrinkage, warpage and heat distortion
temperature have been improved by the addition of all
these fillers to polymers. However, tensile strength,
impact strength and elongation at break tend to
decrease.

8

Mica has an aspect ratio only rivalled by fibrous

materials. For good bonding to non-polar plastics, it
needs to be silane treated or mixed with maleic
anhydride modified polymers. Most commercial appli-
cations do not justify the addition of expensive silane
treatment.

8

Mica has low CTE and good weathering

performance.

7

A synergy was found by some authors

9,10

when adding low quantities of mica to glass fibre
reinforced polyolefins to increase modulus, improve
dimensional stability and reduce cost. The increase in
properties was attributed to a positive effect of mica on
the fibre–matrix adhesion.

Fibre fillers have the highest aspect ratio and give

significant reinforcement. Examples are glass, carbon,
straw, flax, hemp and kenaf. The degree of reinforce-
ment is significantly affected by fibre modulus, aspect
ratio, length and orientation in the product. Glass fibre
is the most common reinforcement for polymers. As
reported by several industrial and academic studies, it
can be used to upgrade recycled thermoplastics into long
life products.

11

It improves strength, stiffness, fracture

toughness and heat resistance.

5,9

An increase in the heat

deformation temperature from 60 to 150

uC for a 40

wt-% loaded PP has been reported.

5

Titanate or silane

coatings and maleic anhydride or acrylic acid coupling
agents are required for optimum fibre–matrix bonding.
Fibre lengths .0?5 mm are required for optimum
strengthening, and the properties are dramatically
improved above 1 mm.

A study on 30 wt-% long glass fibre PP showed that

the addition of 20 wt-%CaCO

3

to the PP matrix gave an

increase of 10% in tensile modulus. Such an increase
exceeded the modulus enhancements predicted by the
rule of mixtures and was therefore attributed to
synergistic interactions between the glass fibres and
CaCO

3

. However, tensile strength and fracture tough-

ness decreased.

9

Short glass fibre and mica have been

shown to increase stiffness and reduce warpage.

10

In a

study of mica filled PP based glass mat thermoplastic,
the addition of up to 15 wt-% mica enhanced the fibre–
matrix adhesion while improving the tensile, flexural and
impact properties.

10

Fillers naturally have a lower CTE than polymers.

The CTE of filled compounds can depend upon particle
size, distribution and specific surface area.

12

Increasing

the interfacial area increases the constriction of the
matrix and decreases the CTE. However, poor adhesion
between filler and matrix can lead to an increase in the
thermal expansion coefficient.

13

For some systems (e.g.

silica filled epoxy composites), decreasing the filler
crystallinity decreases the CTE.

12

The majority of plastics are inherently flammable.

Reduced flammability is desirable for many building
applications and often mandatory for products designed
for indoor use. Conventional halogenated systems are
banned in many applications due to smoke toxicity and
environmental concerns. Zero halogen systems are
available, such as alumina trihydrate and magnesium
hydroxide. Loadings of 60 wt-% are required to produce
the same level of flame retardancy to the detriment of
strength and impact resistance. Intumescent phosphor-
ous based systems require ,30 wt-% loading. At this
level, the mechanical properties are still affected.

5

Experimental

Materials

The polymer used was a proprietary blend of common
amorphous and semicrystalline recycled polymers. The
plastics were shredded and granulated to 10 mm sized
flakes and then tumble mixed. The blend had a melt flow
index of 11?1 g/10 min at 230

uC. Four different addi-

tives were compounded with the blend.

(i) Omyalene 102M calcium carbonate from Omya

UK: an 86 wt-% stearic acid coated chalk
whiting in a polyolefin carrier. The particles
have an aspect ratio of 1 and an average particle
diameter of 2 mm. The specific surface area is
2?5 m

2

g

2

1

according to BET ISO 4652

(ii) Micro Mica W160 from Norwegian Talc AS

and distributed by Omya: a muscovite with
aspect ratio 20 : 1 and a median particle size of
13?5 mm (wet analysis Malvern Mastersizer X)
or 4?2 mm (X-ray analysis Sedigraph 5001). The
specific surface area is 6?8 m

2

g

2

1

according to

BET ISO 4652

(iii) 3299 EC13 chopped strand glass fibre from

PPG Industries: a silane treated fibre of 14 mm
diameter and 4?5 mm length. For additional
coupling, 2% Bondyram 1001 maleic anhydride
modified homo-polypropylene from Polyram
was added

(iv) 58578-M1-300 Superex POV0-HF flame retar-

dant masterbatch from Americhem, a proprie-
tary halogen free intumescent flame retardant in
low density polyethylene carrier.

Table 1 shows the additive combinations used with the
recycled blend in this study.

The flame retardant was added at a suitable level to

give UL 94 V0 rating.

14

Sample preparation

The materials were compounded using a Berstorff ZE25
co-rotating twin screw extruder with a temperature

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profile of 180–210

uC and a speed of 430 rev min

2

1

. The

compounded material was pelletised into 5 mm long
pellets. Standard test specimens were injection moulded
using a NegriBossi V55-200 with a 62 ton maximum
clamp force. The temperature profile was 200–230

uC.

The mould was not cooled.

Mechanical testing

Specimens were tested using a Hounsfield HK100-S.
Type 2 ISO 1367 dog bone specimens were tensile tested
at a speed of 5 mm min

2

1

. ASTM D 970 flexural

specimens (127612?763?2 mm) were tested to flexural
three-point bend test with a span of 51?2 mm at a speed
of 2 mm min

2

1

. The support noses had a diameter of

5?95 mm. The loading nose had a diameter of 6?3 mm.

Linear CTE

Two different methods were used to measure the linear
CTE. A Perkin Elmer Diamond thermomechanical
analyser was used over the range of 220–60

uC and at

a ramp rate of 2

uC min

2

1

. Measurement of the change

in length of the flexural test bars after conditioning at
2

18 and 55

uC was carried out using a standard

laboratory oven and freezer. Vernier callipers were used
to measure the change in dimensions.

Scanning electron microscopy

Flexural test bars were dipped in liquid nitrogen,
clamped in a vice and fractured by a hammer blow.
The samples were carbon coated during the sample
preparation procedure. The fracture surfaces were
examined using a Philips XL 40 in secondary electron
and backscattered electron modes.

Results

CaCO

3

and mica were added to the recycled polymer

blend to evaluate the potential property improvements
that can be achieved using low cost fillers. The proper-
ties resulting from the addition of a fire retardant are
also reported for comparison. Subsequently, the effect of
glass fibre reinforcement was evaluated to assess whether
the higher enhancement in properties justifies their
additional cost and processing complexity. A further
step was the investigation of hybrid systems containing
glass fibre and a lower aspect ratio filler in order to
obtain a wider range of property enhancement and
further improvement of certain properties through
synergistic effects.

Mechanical properties

The effect of each filler is dependent on the method of
loading. The particulate fillers increase tensile and
flexural

moduli

significantly

(Fig. 1).

The

higher

enhancement of tensile modulus for mica filled systems

is consistent with the literature. Mica has a far higher
aspect ratio than calcium carbonate, which increases the
contact area between the mica and the matrix and leads
to a more significant effect on properties. The increased
surface area enables improved stress transfer to the
filler.

12

In addition, mica has a higher tensile modulus

(over 100 GPa) compared to CaCO

3

(35 GPa).

Tensile strength decreases, and flexural strength

increases slightly (Fig. 2). The reduction of tensile
strength for the CaCO

3

filled systems indicated poor

interfacial adhesion for this system. The stearic acid
coating on CaCO

3

generally improves dispersion, but

has no or limited coupling effect.

5

For the mica filled

system, the tensile strength was practically unchanged,
as mica possesses better reinforcing ability than calcium
carbonate and typically does not depress strength
considerably.

5

In flexure, the stress is maximum at the

surfaces. The force is compressive on the loaded surface
with an equal and opposite tensile stress on the opposite
surface. The increase in flexural strength for both
particulate filled systems was in turn attributed to the
compressive component of the mechanical response. The
compressive strength of filled systems tends to increase
even for uncoupled systems. This is consistent with

1

Tensile and flexural modulus of systems developed in
this study: a tensile modulus and b flexural modulus
of particle reinforced recycled polymers

Table 1

Additive combinations compounded with proprietary recycled plastic blend used in this study (GF: glass fibre,
M: mica, FR: flame retardant)

Amount of additive in compound/wt-%

Additive

20%
CaCO

3

20%
Mica

Flame
retardant

15%
GF

15%GF
z5%C

15%GF
z5%M

30%
GF

30%GF
z5%M

Calcium carbonate

20

5

Mica

20

5

5

Glass fibre

15

15

15

30

30

Flame retardant masterbatch

40

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previous studies on filled thermoplastics, which reported
the compressive strength to be directly proportional to
Young’s modulus.

13

The addition of an intumescent flame retardant

increases tensile modulus, but is very detrimental to
strength and causes an unexpected decrease in flexural
modulus. The proprietary masterbatch (40 wt-%) was
required to give the required improvement on flamm-
ability properties. This high level would be expected to
have a significant effect on mechanical properties.
Intumescent flame retardant systems are not reported
to have a reinforcing effect, plus their hydrophilic nature
creates a poor interfacial bond with hydrophobic
polymers. Studies have reported an increase in modulus
and heat deflection temperature, but a decrease in
impact strength and other mechanicals.

5

Coupling

agents have been studied, showing improvements in
mechanical properties without a detrimental effect on
flammability.

15,16

Elongation in tension was reduced for all fillers, in

particular for glass fibres. However, elongation at break
for all systems was above 3?5%. Such values of
elongation at break are sufficient to guarantee a
satisfactory performance in semistructural applications,
which are generally designed for stiffness with high
safety factors.

17

Glass fibre significantly increased the strength and

modulus of the recycled polymer blend (Figs. 3 and 4).
Owing to the cost and processing limitations for the
recycled composite, the maximum amount of glass fibre
incorporated in the product was 30 wt-%. A secondary
filler was added to the glass fibre reinforced systems to
further

enhance

the

structural

properties

without

increasing the cost. Calcium carbonate had a similar
effect in the glass filled blend as with the pure polymer
blend. In both cases, the addition of calcium carbonate

caused a slight increase in the mechanical properties,
except for the tensile strength.

The addition of small proportions of mica to the glass

fibre reinforced blend resulted in an increase, rather than
a decrease (observed for CaCO

3

), in tensile strength.

Since 20 wt-% of mica alone did not alter the tensile

3

Tensile and flexural modulus of fibre reinforced materi-
als developed in this study: a tensile modulus; b flex-
ural modulus of recycled systems

2

Tensile and flexural strength of systems developed in
this study: a tensile strength; b flexural strength of
particle reinforced recycled polymers

4

Tensile and flexural strength of fibre reinforced sys-
tems developed in this study: a tensile strengths;
b

flexural strength modulus of recycled systems

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strength of the material (Fig. 2), it was concluded that a
synergistic interaction took place between mica and
glass fibre reinforcement. Such a positive effect was
observed both at 15 and 30 wt-% glass fibre loading and
was particularly marked during the tensile tests. The
addition of 5 wt-% mica into 30 wt-% glass fibre
reinforced blend resulted in an increase in tensile
strength of 20%. For the same material system, an even
more remarkable synergy was observed in terms of
tensile and flexural modulus, which increased by 35 and
7% respectively. An explanation for this effect has been
proposed in the section on ‘Discussion’.

Linear CTE

Thermomechanical analysis measurements produced
complex results due to the number of transitions for
the separate polymers. Measurement of the test speci-
mens produced reasonably consistent results (Fig. 5).
The standard deviation was appreciable due to the small
changes in size. Particulate fillers appeared to increase
the linear CTE, while it was significantly reduced in the
presence of fibre reinforcement. The increase in CTE for
the particulate systems can be attributed to the poor
adhesion between these fillers and the polymer matrix.

13

The reduction in CTE for the glass fibre reinforced
systems is consistent with a strong coupling between
glass fibres and the polymer matrix, which was achieved
by the addition of silane coating and maleic anhydride
grafted PP.

Scanning electron microscopy

Scanning electron microscopy showed a well dispersed
blend of different polymers. The orientation of the fibres
in the direction of process flow (perpendicular to the
fracture surface) can be observed at the fractured surface
(Fig. 6). The calcium carbonate addition showed good
distribution with a little agglomeration (Fig. 7). The
maximum agglomerate size observed was below 10 mm
(Fig. 7b).

Discussion

Stearate coated calcium carbonate behaved as predicted
from the earlier reported literature.

5

The modulus was

increased by 24%, and the strength was decreased
slightly. In flexural mode, the modulus was increased
by 40%, and the strength was also increased slightly. The
higher improvement in flexural properties was due to the
combination of tensile and compressive modes.

The uncoated mica resulted in an increased reinforce-

ment effect as expected with the increase in aspect ratio.

The tensile modulus was increased by 86%, and the
strength decreased marginally. This improvement is in
line with other studies that reported 50–100% higher
properties compared to talc or calcium carbonate, with
little or no reduction in impact strength.

5

In flexural

mode, the modulus increased by 114% and strength
increased slightly.

The silane treated glass fibre with maleic anhydride

polypropylene compatibiliser significantly improved the
strength and modulus of the blend, as predicted. The
15 wt-% glass fibre increased the tensile strength by 70%
and the elastic modulus by 63%. The flexural strength
was again increased by 60% and the flexural modulus by
210%. The 20 wt-% mica increased the tensile modulus
to the same degree as 15 wt-% glass fibre, however
without the increase in strength. Mica could be used as
an alternative to glass fibre for certain applications. The
30 wt-% glass fibre increased the tensile strength by 85%
and the modulus by 240%. The flexural strength was
increased by 115% in this case, and the flexural modulus
by 445%, as expected in a well oriented and consolidated
glass fibre composite.

The addition of 5 wt-% calcium carbonate to 15 wt-%

glass fibre in the recycled polymer blend increased the
tensile and flexural moduli by further 20%, while the
strength was decreased in tension. The 15 wt-% glass
fibre reinforced PP (GRPP) samples reinforced with
5 wt-% mica presented similar property enhancements,
with the difference that mica improved the tensile
strength. However, a much more pronounced synergistic
effect was observed for the addition of mica to 30 wt-%
filled glass fibre blend. For this system, mica improved
the tensile modulus by as much as 35% and flexural

5

Linear CTE for systems developed in this study

6

Scanning electron microscopy images showing fracture
surface morphology of 30 wt-% glass fibre and 5 wt-%
mica filled compound: a secondary electron mode and b
backscattered electron mode showing filler distribution

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modulus by 7%, with a marginal increase in strength. A
similarly remarkable effect has been observed for mica
filled glass fibre reinforced PP by Zhao and co-workers

10

The authors found that the addition of moderate loadings
of mica to glass fibre mat reinforced thermoplastic PP led
to a substantial increase in tensile and flexural modulus (in
the order of 100%), combined with a moderate improve-
ment of strength. This synergy was explained by the
increase in radial compressive residual stresses on the glass
fibres (and therefore an enhanced fibre–matrix adhesion)
brought about by the addition of mica. During solidifica-
tion, the difference in CTE between polymer matrix and
glass fibres generated a compressive radial stress at the
interface, which is proportional to the difference in
thermal expansion coefficient and to the elastic modulus
of the matrix.

18

The main effect of adding mica to the

polymer matrix is an increase in elastic modulus, whereas
the variation in CTE is only moderate. This can lead to a
stronger interfacial interaction between the glass fibres
and the polymer matrix, leading to a significant increase in
stiffness. The present work seems to indicate that to obtain
this effect, mica does not need to be surface treated. The
untreated mica used in this study tended to increase the
CTE of the polymer blend, which could lead to a further
increase in compressive stress at the interface and a more
significant improvement in the elastic modulus. It is
expected that the fibre–matrix adhesion strength would be
significantly decreased in the presence of higher loadings
of mica because of the contact of the glass fibres and the
mica flakes at the interface. However, it is still unclear why
this synergy was observed only for 30% and not for 15%
glass fibre loading and why similar trends have not been
previously reported by other authors on a variety of highly

reinforced glass fibre thermoplastic systems with the
incorporation of particulate fillers.

The proprietary intumescent flame retardant reduced

all mechanical properties by 10–30%, except the tensile
modulus, which was increased by 20%. The particles had
little reinforcing effect in this case. Use of flame
retardants is undesirable due to the reduction in
mechanical properties and higher cost. Reinforcement
additives and impact modifiers could be added to
counterbalance the property reduction.

5

The glass fibre gave a significant reduction in CTE.

The 15 wt-% glass fibre gave 30610

2

6

uC

2

1

, which is

close to that of wood across the grain.

19

The 30 wt-%

glass fibre gave 18610

2

6

uC

2

1

values in line with steel,

concrete and wood along the grain.

19–21

The fibres were

oriented parallel to the direction of the flow, and the
large interfacial area constricted the expansion of the
matrix. The increase in CTE for calcium carbonate is
unexpected compared to another study, which gave a
12% reduction in a 40 wt-% filled PP blend,

12

and was

attributed to the weak interfacial adhesion.

Conclusions

The effect of the addition of various commercially
available fillers to a blend of recycled polymers was
investigated to evaluate the simultaneous enhancement
of key structural properties (stiffness, strength and CTE)
of the blend.

Varying results were obtained by the incorporation of

a single filler (CaCO

3

, mica or glass fibre), depending on

the aspect ratio of the filler and the method of loading.
Hybrid systems containing glass fibre and a lower aspect
ratio filler achieved a wider range of property enhance-
ment and further improvement of certain properties
through synergistic effects.

Spherical calcium carbonate gave a modest increase in

tensile and flexural modulus to the detriment of tensile
strength, while plate-like mica increased the moduli
significantly with minor improvements in strength. Glass
fibre reinforcement contributed a significant increase in
strength and moduli, particularly in flexural mode. The
20 wt-% mica increased the modulus to the same degree
as the 15 wt-% glass fibre without causing an increase in
strength. Mica could be used as a replacement for glass
fibre in certain applications. The addition of mica to
glass fibre resulted in a further improvement of
mechanical properties, particularly in tensile mode.

The substantial increase in modulus with the addition

of 5% mica and 30% glass fibre to the recycled polymer
blend was explained in terms of improved interfacial
adhesion between glass fibres and polymer matrix due to
the effect of mica on the polymer properties. Further
investigations will be needed to comprehensively under-
stand such a remarkable effect. The addition of
particulate fillers without chemical coupling to the
polymer blend seemed to increase the CTE, but material
systems containing

adequately

coupled

glass

fibre

reinforcement showed a reduction in the CTE to the
same level as concrete, steel and wood.

Acknowledgements

The authors wish to acknowledge Z. B. Marzuki for his
assistance in the mechanical testing and C. Magnus for
his assistance in dynamic mechanical analysis testing. In

7

Images (SEM) showing fracture surface morphology of
20 wt-% calcium carbonate filled compound: a second-
ary electron mode; b back scattered electron mode
showing calcium carbonate distribution

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addition, the authors wish to acknowledge DTI-KTP for
funding of the project.

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