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An Investigation on the Rheology, Morphology,
Thermal and Mechanical Properties of Recycled
Poly (ethylene terephthalate) Reinforced With
Modified Short Glass Fibers
I. Rezaeian, S.H. Jafari, P. Zahedi, S. Nouri
School of Chemical Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran
This work was done with the aim to solve an important
environmental issue regarding poly (ethylene tereph-
thalate), (PET) wastes. Samples of recycled PET (r-PET)
were reinforced with 10 to 30 wt% modified short glass
fibers (SGF) through a melt mixing process in an inter-
nal mixer and their performance were assessed and
compared with those of commercial glass reinforced
PET through investigation of their rheology, morphol-
ogy, thermal, and mechanical properties. It was found
that the mechanical properties of the glass reinforced
r-PET composites in most cases were comparable or
even higher than those of the commercial grades. The
impact strength of the 30 wt% SGF filled r-PET com-
posite was about 30% higher than the commercial
grades. This led to a conclusion that the PET wastes
can be successfully converted to easily moldable ther-
moplastic materials by incorporation of 30 wt% SGF
having a good balance of properties. Through investi-
gation of rheological and morphological properties the
optimum conditions for the best reinforcement per-
formance were determined. The r-PET with 30 wt%
glass fiber content showed the highest level of orienta-
tion and improved interaction with the r-PET matrix
while having an acceptable flow behavior and process-
ability. In spite of significant fiber breakage during the
melt mixing process, leading to about 20 times reduc-
tion in the fiber aspect ratio, the composites main-
tained their good mechanical properties and showed a
shear thinning behavior at high shear rates. The incor-
porated glass fibers acted as nucleating agents and
improved the crystallization rate of r-PET leading to an
overall increase in the crystallinity. POLYM. COMPOS.,
30:993–999, 2009.
ª
2008 Society of Plastics Engineers
INTRODUCTION
Poly (ethylene terephthalate), (PET), was produced for
the first time in 1946 and commercially used in textile
industry in 1953. PET has an excellent impermeability
against gases such as carbon dioxide and oxygen. Blow
molded PET containers and bottles were soon produced
and used for packaging of soft drinks [1]. Annually, about
six million tones of PET wastes are produced in the world
and most of it is not recycled. Two main obstacles in the
reuse of recycled PET (r-PET) are hygienic problems and
severe reduction in its mechanical properties because of
molecular weight decreases [2, 3]. There are different
methods of recycling PET and the choice of the most
suitable method depends on the final applications of r-
PET. Chemical recycling methods for PET are methanoly-
sis [4], glycolysis [5], and hydrolysis [6]. Mechanical
recycling method for PET is remelting [7] and mixing it
with other polymers or glass fibers in order to obtain the
desired properties. In the mechanical recycling method, r-
PET is grinded and washed with NaOH solution in order
to remove adhesives and labels and then granulated by
extrusion. r-PET granules are used for different applica-
tions such as partially oriented yarns, hollow fibers, and
filaments production [8]. Mechanically r-PET is cheaper
and has many secondary uses, especially when it is mixed
with glass fibers [9–11]. Finally, if there was no ecologi-
cal solution for PET wastes or technical and economical
justification for the reuse of r-PET, it can be burnt for
energy conservation.
Depaoli et al. [12] reported that in reprocessing of
PET, mechanical properties and the degree of crystalliza-
tion changes are considerable and there will be an
increase in the melt flow index and concentration of car-
boxyl groups which can lead to chemical and mechanical
degradation. Other studies show that the end carboxyl
groups after five times of injection molding increase about
three times and crystallization increases from 23% to
37% which affects the elastic modulus and impact proper-
ties [13]. Virgin PET has a ductile characteristic ([150%
elongation at break), but r-PET from bottles is brittle
(\20% elongation at break). These behaviors are due to
an increase in the degree of crystallinity, the presence of
impurities in r-PET and differences in thermal and me-
chanical properties of the virgin and r-PET [14]. One of
Correspondence to: I. Rezaeian; e-mail: rezaeian@ut.ac.ir
DOI 10.1002/pc.20647
Published online in Wiley InterScience (www.interscience.wiley.com).
V
V
C2008 Society of Plastics Engineers
POLYMER COMPOSITES—-2009
the most important uses of r-PET is as reinforced compo-
sites. Usually, an unsaturated resin such as a polyester or
epoxy with chopped glass fibers are used for reinforced com-
posite production. Recently, there has been an increasing
tendency for the use of thermoplastic/glass fibers composites
[15]. The mechanical properties of thermoplastics reinforced
with glass fibers depend on interfacial adhesion between
polymeric matrix and the glass fibers, the length, and diame-
ter of the fibers and the volume ratio of the polymer to glass
fibers [16]. Also, the difference between hydrophilic and
hydrophobic properties of the polymer and the glass fibers
leads to incompatibility between the two phases, which can
be controlled by coupling agents and reactive additives [10].
Pegoretti et al. [17] reported the effect of hydrothermal age-
ing on thermomechanical properties of r-PET/SGF compo-
sites. Kim et al. [18] have studied the effect of morphology
on mechanical properties of poly (butylene terephthalate) re-
inforced with glass fibers in the injection molding process.
Velasco et al. [19, 20] investigated mechanical properties
and injection molding processing conditions for PET/SGF.
Since the mechanical properties in glass fibers rein-
forced composites are governed by morphological charac-
teristics of the composites, such as orientation of short
glass fibers (SGF) and extent of interaction between the
glass fibers and matrix and these in turn affect the rheo-
logical and thermal properties, there is a need to consider
all these important aspects when a r-PET is being rein-
forced with SGF. Therefore in this study, an attempt has
been made to investigate the mechanical, morphological,
thermal, and rheological properties of the r-PET/SGF
composites and to establish a correlation between these
properties. Moreover, the effects of modified SGF dimen-
sions on the mechanical failure and the optimum percent-
age of the glass fibers needed to achieve composites with
desired impact strength have been investigated.
Finally, a comparison was made between the mechani-
cal properties of 30 wt% glass fibers r-PET composites
with similar commercial grades.
EXPERIMENTAL
Materials
Granulated r-PET was supplied by Morvarid Sabz
Saveh Co. (M
n
¼15.35 kg/mol, M
w
¼31.37 kg/mol,
IV ¼0.61 dl/g, measured in 1,2 dichloro methane at 258C,
melting point ¼2508C and density ¼1.32 g/cm
3
). Impur-
ities in these r-PET granules were about 40 ppm PVC and
20 ppm polyolefins. Silane-modified SGF named 123 Cratec
supplied by Owens Corning LG Co. having an average
length of 4.5 mm, diameter of 11 lm, and L/Dequal to 409.
Antioxidant, Irganox B900, was obtained from Ciba Co.
Preparation of r-PET/SGF Composites
Granulated r-PET and modified SGF were dried in a
vacuum oven at 1208C and 908C for 24 h, respectively.
The mixing process of dried r-PET granules and SGF was
carried out in a Brabender internal mixer, equipped with a
Banbury model rotor, with a filling factor of 0.7 and a
volume of 300 cc. The mixing process of granulated r-
PET and modified SGF was carried out in this mixer for
12 min at 2708C. First, r-PET with 0.1 g Irganox was
melt mixed for about 2 min and then the modified SGF
was added to the r-PET melt. Samples containing 10, 20,
30 wt% modified SGF were molded by a hot press into
sheets and used for various tests according to ASTM
standards. Molding conditions for all the sheets were the
same i.e. at 2808C and 25 MPa.
Characterization of the r-PET/SGF Composites
Rheometric measurements were carried out by an
UDS200 model rotational rheometer (Paar Physica) with
two parallel plates, with a gap of 1 mm and diameter of
25 mm. The measurements were made in the linear visco-
elastic region (strain amplitude ¼1%), with a frequency
range of 0.1 to 500 rad/s at 2908C.
Scanning electron microscopy (SEM) tests were carried
out using a stereo scan scanning electron microscope
(S350 model, Cambridge Co.). After fracturing a portion
of the sheets in liquid nitrogen, they were etched by a
mixture of phenol/1,1,2,2-tetrachlorethane (6:4 wt) in
order to remove the PET phase, revealing the glass fibers.
The etched samples were then gold coated and the micro-
graphs were observed with different magnifications.
Glass transition temperature (T
g
), melting point (T
m
),
and crystallization (X) of the samples were measured by
a differential scanning calorimeter (DSC), DuPont Co.
The samples, sealed in an aluminum pan, heated at a rate
of 10 K/min from 300 to 600 K. The temperature was
calibrated by the melting point of ultra pure materials,
e.g., indium, corrections being made for thermal lag in
the specimens.
The tensile tests were measured on an Instron, model
6025. A cross head speed of 1 mm/min was used. Stand-
ard dumbbell-shaped specimens were cut directly from
the molded sheets. An average of at least five specimens
was used for each composition. The required energy for a
failure at high strain rate (3.5 m/s) was also investigated
using a Zwick impact tester at room temperature.
For length measurement of modified SGF, composite
samples were placed in an oven at 1200 K for 5 h. Under
these conditions, glass fibers ashes are left behind. These
glass fiber ashes were then viewed under a light micro-
scope and their average lengths were determined.
RESULTS AND DISCUSSION
Rheological Characterization
Torque Variations Studies in the Internal Mixer
(Brabender). Figure 1 shows the effect of addition of
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POLYMER COMPOSITES—-2009
DOI 10.1002/pc
10, 20, and 30 wt% modified SGF to r-PET on the tor-
que-time variations in the Brabender with a rotor speed of
80 rpm. The curves show two peaks, the first peak is due
to the addition of SGF to the molten r-PET and the sec-
ond peak is related to wetting, dispersion, and distribution
of the modified SGF in the polymeric matrix.
The SGF in the composite samples behave as filler and
results in a viscosity increase which is proportional to the
torque values. To investigate the effect of different rotor
speeds on the torque-time variations, three rotor speeds
40, 60, and 80 rpm were used. As expected, the higher
rotor speeds led to greater equilibrium torque values,
since the higher speed of rotation is accompanied by a
higher shear rate (see Fig. 2).
It is important to predict the parameters influencing the
processing conditions for the preparation of the r-PET/
SGF composites, especially by an extrusion process, e.g.,
feeding arrangement, time of materials addition in differ-
ent extruder zones with a special attention to the torque-
time graph for Brabender [21–24]. For example, the
amount of energy consumption which is of substantial im-
portance in any mixing device including a continuous
processing method such as extrusion can be calculated by
using the following equation, which was suggested by
Chen [25]:
w¼2pnZ
t2
t1
Mdt ð1Þ
Here, wis the mixer power, nis the rotor speed per mi-
nute, t
1
and t
2
are the primary and secondary times
respectively, and Mis the torque of the mixer.
Figure 3 shows the required energy calculated based
on the Chen method for the preparation of the r-PET/SGF
composites in the specific period of time necessary to
reach a homogenous state which is controlled by different
rotor speeds. The trend of increase in power consumption
is similar to the torque-time variations. It is observed that
by increasing the rotor speed and SGF content there is a
steady increase in the power consumption. At the highest
rotor speed (80 rpm), a four times increase in power con-
sumption is seen when the SGF content increase from 10
to 30 wt%. Similarly, almost a four times power is con-
sumed when at a fixed SGF content the rotor speed
increase from 40 to 80 rpm. This increase in power con-
sumption can be a limiting factor in processing of the
SGF reinforced r-PET.
Linear Viscoelastic Behavior Studies. The linear
viscoelastic responses as measured by the storage modu-
lus for the r-PET composites with different glass fiber
contents obtained using the lowest possible strain ampli-
tude (1%). Figure 4 demonstrates the pattern of changes
FIG. 1. Torque-time curves for various short glass fibers in a rotor
speed of 80 rpm for r-PET/SGF composites.
FIG. 2. Initial torque for 30 wt% modified SGF in a rotor speed of
80 rpm and the trend of the resulting change of the steady state torque
values.
FIG. 3. Power consumption as a result of glass fibers addition which is
parameterized by different rotor speeds.
FIG. 4. Patterns of changes of the storage modulus as a consequence
of changes of frequency.
DOI 10.1002/pc
POLYMER COMPOSITES—-2009
995
for the storage modulus as a result of the SGF addition.
In these curves, storage modulus of composites at low fre-
quencies follows a nonterminal behavior in which a pla-
teau region is observed. Also, the storage modulus of the
samples increases as a consequence of increasing the
glass fiber contents. The storage modulus of the compo-
sites with higher glass fiber content shows less depend-
ency on the changes in frequency, especially at low fre-
quencies as compared to the low glass fiber content com-
posites as well as the neat r-PET. This is due to the fact
that at low frequencies, time is large enough for unravel-
ing of the entanglements so a large amount of relaxation
occurrence results in constant values of storage and loss
modulus. However, when the composite is deformed at
large frequencies, the entangled chains do not have time
to relax so modulus goes up. The increase in glass fiber
content increases the relaxation time and hence the less
dependency to frequency or a wider plateau region is
obtained for the composites with higher glass fiber
contents.
In Fig. 5, the changes of the complex viscosity of the
composite samples are presented. It is seen that the r-PET
shows a Newtonian like behavior. Such a behavior is
usual for virgin PET. This implies that the r-PET used in
this work, has maintained its structural identity and no
major structural changes due to degradation has happened
after the recycling. On the other hand, very significant
increase in viscosity is seen by incorporation of glass
fibers into the r-PET. The filled r-PET samples show
characteristic of a yield stress, which can be attributed to
the formation of network like structure of the glass fibers
in the r-PET matrix [26]. As the content of glass fiber
increases the composites show higher yield stress due to
the network formation. This network like structures get
disturbed and the composites show a shear thinning
behavior. The tendency of the composites for showing
yield stress and a shear thinning behavior become stron-
ger with increasing the glass fiber content.
FIG. 5. Comparison between complex viscosities.
FIG. 6. Response of the viscosity of the short glass fibers reinforced
r-PET.
FIG. 7. SEM micrograph of cryo-fractured surface of the r-PET/SGF composites prepared with (a) 10 wt% SGF; (b) 20 wt% SGF and (c) 30 wt%
SGF in rotor speed of 80 rpm.
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POLYMER COMPOSITES—-2009
DOI 10.1002/pc
To further determine the extent of yield stress for the
melt flow of the composite samples at shear stress above
its critical value, the flow curves at constant shear strain
were obtained for composites with different glass fiber
contents. Figure 6 presents the viscosity changes of the
samples due to controlled increases of the shear stress.
These results show that samples with a higher level of
glass fibers loading show greater values of the plateau
viscosity region at which the viscosity remains constant
and no flow occurs.
Morphology of the Composite Samples
Figure 7a–c shows the SEM micrographs of cryo-frac-
tured surface of the composite samples at low magnification,
which demonstrates the fracture behavior of the composites.
All the samples show indication of brittle type fracture hav-
ing a smooth matrix surface [27]. There are indications of
some slight fiber debonding and some fiber orientation. The
orientation is much more in the samples with higher glass
fiber contents. Moreover, some signs of fiber breakage are
also seen as there are fibers with different lengths. Further-
more, the SEM micrographs indicate that the fracture of the
glass fibers is directly proportional to the weight percentage
of the glass fibers in the samples. The extent of fiber break-
age determined by burning the composites in a heated oven
will be discussed subsequently. The reason for the observed
fiber orientation especially in the sample with the highest
glass content might be attributed to the sever fiber breakage
which facilitate their orientation in the flow direction. Fig-
ure 8 shows the SEM micrographs of r-PET/SGF with a
high magnification revealing the surface of the glass fiber
covered with the molten r-PET matrix. The diameter of the
covered fibers is almost double of the uncovered fibers. This
is an indication of a very good interaction between the glass
fibers and the r-PET matrix. This good interaction can lead
to a better load transfer from matrix to fiber resulting in
good mechanical properties.
Table 1 illustrates the length and aspect ratio of the
modified SGF in the composite samples made at 80 rpm,
2708C and mixed for 12 min in the Brabender internal
mixer. Practically, the aspect ratio of the fibers decreases
from 409 before mixing to about 20 after the mixing.
This large decrease in the aspect ratio due to the mixing
process may have a negative effect on the mechanical
properties, but it can have a positive impact on flow
behavior. The shorter the fiber, the higher is the possibil-
ity for their orientation in the matrix and also the lower is
their tendency to increase the melt viscosity. This can
improve the processability of the composites for higher
glass fiber content. Moreover, the tendency of the compo-
sites for showing a shear thinning behavior depends on
the reduction of the aspect ratio of the fibers. Therefore,
based on the rheological and morphological investigations
the optimum percentage of modified SGF in r-PET matrix
was taken 30 wt%, which results to a suitable aspect ratio
and also desirable mechanical properties for the compos-
ite samples as will be discussed subsequently.
Thermal Properties
Table 2 shows DSC results for the composite samples
containing 10, 20, and 30 wt% modified SGF. These
results indicate that there is no significant change in T
g
and T
m
for these samples.
However, the crystallinity of the composite samples is
affected by the presence of the modified SGF in the poly-
mer matrix (see Table 2). The SGF can act as nucleating
agents, which enhances the crystallization rate leading to
a rapid formation of large number of crystallites at a
given period. This can increase the crystallization degree.
It is known that PET is a slow crystallizable polymer due
to its rigid backbone. This rigidity is due to the presence
of benzene ring and short aliphatic group in the backbone.
Incorporation of glass fibers can enhance its crystalliza-
tion rate by inducing nucleation.
FIG. 8. Adhesion and coverage of modified SGF between matrix and
fibers.
TABLE 1. Comparison of short glass fibers dimensions after mixing
with r-PET.
Aspect
ratio (L/D)
Length of modified
SGF after mixing (lm) Sample
19.72 216.96 r-PET70/SGF30
20.42 224.62 r-PET80/SGF20
22.49 247.42 r-PET90/SGF10
TABLE 2. DSC data for the r-PET/SGF composites.
Sample T
g
(8C) T
m
(8C) X
c
(%)
r-PET70/SGF30 75.3 251 23.2
r-PET80/SGF20 75.1 250.8 22.2
r-PET90/SGF10 74.6 250.8 18.6
r-PET 73.3 250.3 18.1
DOI 10.1002/pc
POLYMER COMPOSITES—-2009
997
The Mechanical Properties of the Composites
All the samples tested exhibit a semi ductile failure.
Table 3 and Fig. 9 show that samples containing 30 wt%
modified SGF having a critical toughness behavior gradu-
ally tend to be brittle. The elastic modulus of these sam-
ples shows a synergistic effect. The composite samples
containing 30 wt% SGF has the highest modulus value.
The yield stress variations for these samples were similar
to modulus variations. The stress values increases with
increasing SGF content in the samples up to 30 wt% glass
fibers while maintaining its ductile behavior. A tendency
for brittle behavior in these samples with more than
30 wt% glass fibers is expected. Figure 9 shows stress–
strain behavior for the composite samples. Maximum
elongation at break occurs for the samples containing
10 wt% SGF. The samples with 30 wt% modified SGF
have desirable mechanical and impact properties. In
Table 3, samples containing 30 wt% SGF from our
experiments give better mechanical properties compared
with virgin PET or r-PET with 30 wt% SGF commercial
grades. These results depend on the process conditions,
type of SGF and r-PET properties.
CONCLUSION
Studies on torque-time variations in the Brabender and
investigation of rheological properties of the SGF rein-
forced r-PET composites showed that the mixing torque,
shear stress, and shear viscosity and storage modulus of
the composites were increased with increasing amounts of
SGF. The SGF filled r-PET composites showed character-
istic of a yield stress which was attributed to the forma-
tion of network like structure becoming more prominent
at the higher SGF contents. The SEM studies and the
burning experiment leaving the fiber ashes revealed a
sever fiber breakage and about 20 times reduction in the
fiber aspect ratio as a result of the melt mixing process.
Moreover some signs of fiber orientation were observed
specially for the composites with the highest SGF content.
However, due to good interaction between the SGF and
the r-PET matrix revealed by the SEM studies and the
resulting fiber orientation the mechanical properties of the
r-PET composites in most cases were remained compara-
ble or even higher than those of the commercial grades
with the similar glass fiber contents. On the other hand,
due to their shear thinning behavior the r-PET composites
even with the highest SGF content showed very good
processability. This led to a conclusion that the PET
wastes can be successfully converted to easily moldable
thermoplastic materials with a good combination of prop-
erties by incorporation of 30 wt% SGF. And finally it was
shown that the SGF can act as nucleating agent which
could enhance the crystallization rate of r-PET leading to
an overall increase in the degree of crystallinity.
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TABLE 3. Mechanical properties of the r-PET/SGF composites and commercial grades.
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Elongation
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Tensile
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998
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