Rheol Acta (2004) 43: 482–490
DOI 10.1007/s00397-004-0385-4
Moisés Garcı́a-Morales
Pedro Partal
Francisco J. Navarro
Francisco Martı́nez-Boza
Malcolm R. Mackley
Crı́spulo Gallegos
Received: 21 January 2004
Accepted: 22 April 2004
Published online: 2 June 2004
Springer-Verlag 2004
This paper was presented at the first Annual European Rheology Conference
(AERC) held in Guimarães, Portugal,
September 11-13, 2003.
M. Garcı́a-Morales Æ P. Partal (&)
F. J. Navarro Æ F. Martı́nez-Boza
C. Gallegos
Departamento de Ingenierı́a Quı́mica,
Facultad de Ciencias Experimentales,
Universidad de Huelva,
Campus del Carmen,
21071 Huelva, Spain
E-mail: partal@uhu.es
M. R. Mackley
Department of Chemical Engineering,
University of Cambridge,
Cambridge, CB2 3RA, United Kingdom
ORIGINAL CONTRIBUTION
The rheology of recycled EVA/LDPE
modified bitumen
Abstract This paper describes linear
viscoelasticity, at low and intermediate temperatures, and the flow
behaviour, at high temperatures, of
polymer modified bitumen (PMB)
containing 5 and 9 wt% recycled
EVA/LDPE. The relationship between flow behaviour and microstructure of the modified bitumen
was also considered, by comparison
of experiments carried out in capillary and rotational rheometers and
photomicrographs taken using a
microscopy system whilst the sample
was being sheared. Blends of 60/70
penetration grade bitumen and
waste plastic (EVA/LDPE) were
processed in an open mixer using a
four blade propeller. Rheological
tests, differential scanning calorimetry (DSC) and microscopy showed
Introduction
Bitumen has different applications, and it is mostly used
in the road and paving industry. The performance of
such surfaces depends on the binding properties of
bitumen, as the bitumen constitutes the continuous
matrix and is the major component of the system that
deforms under load (Becker et al. 2003). Bitumen may
also have application in roofing membranes and in other
waterproofing materials (Garcı́a-Morales et al. 2004a).
It is generally assumed that bitumen, produced from
crude oil distillation processing, is a colloidal system in
which the asphaltenes are dispersed into an oily matrix
of the remaining components, the maltenes (Pérez-Lepe
et al. 2003), and this colloidal structure serves as a model
that the bitumen performance was
improved by adding the recycled
polymer. As a consequence, the use
of recycled EVA/LDPE in PMBs
can be considered a suitable and
interesting alternative from both an
environmental and economical point
of view. The experimental results
also show that pure bitumen has
shear-thinning characteristics. The
blending of polymer into the bitumen modifies the melt processing
characteristics of the blend, whilst
the viscoelastic properties of the
semi-solid composite are enhanced.
Keywords Modified bitumen Æ Recycled polymer Æ Flow
behaviour Æ Linear viscoelasticity Æ Capillary rheometry
to describe some of the observed thermo-mechanical
properties of bitumen. Other bitumen models have been
proposed, such as the SHRP model (Bonemazzi and
Giavarini 1999; Redelius 2000).
Extreme in-service temperatures as well as high traffic
loadings may cause significant problems in bituminous
pavements and roads, among which deformation at high
temperature (rutting), thermal cracking at low temperature, load-associated fatigue cracking and ageing are
the most common (Becker et al. 2003). For this reason, a
significant amount of work focused on the enhancement
of the performance of paving materials has been and is
being carried out.
Bitumen modification by polymer addition usually
improves the mechanical properties of the composite
483
and, therefore, the behaviour of road pavements: thermal susceptibility and rutting can be diminished, whilst
the resistance to low temperature cracking may increase,
since the binder undergoes a decrease in its effective glass
transition temperature (Garcı́a-Morales et al. 2004a).
Polyolefins such as polyethylene (HDPE, LLDPE or
LDPE) and copolymers, such as SBS and EVA have
been commonly employed (Airey 2003; Blanco et al.
1996; Yousefi 2003).
From an environmental and economical point of
view, the addition of waste plastics to the road bitumen
is a good way of achieving waste disposal, also taking
into account that the cost is an important aspect when
selecting a polymer as a modifying agent (Garcı́a-Morales et al. 2004a, 2004b).
Knowledge of the rheological behaviour of bitumen
is necessary (Partal et al. 1999). However, the properties
of binders have usually been characterised using standard and/or modified standard test methods, which do
not necessarily accurately predict the full bitumen performance. The appearance of the Strategic Highway
Research Program (SHRP) protocol favoured the use of
the dynamic shear rheometry (DSR) to characterise the
mechanical behaviour of polymer modified bitumen over
a wide range of temperatures (Garcı́a-Morales et al.
2004b). Viscoelasticity measurements provide useful
information about the resistance of bitumen to traffic
loading (rutting and fatigue cracking) in the high inservice temperature region, as well as at low temperature
where thermal cracking is likely to happen. In addition,
viscous measurements, at higher temperatures, provide
information about the processing properties of the
bitumen where it is handled and mixed with mineral
aggregates, and finally applied on the road.
In this paper the use of a blend of recycled ethylenevinyl acetate copolymer and low-density polyethylene as
a modifying agent for the bituminous binder employed
in the construction of road pavements is examined.
Different rheological measurements were carried out on
neat and modified bitumen over both application and
processing temperature ranges. Linear viscoelasticity
tests were performed at low and intermediate temperatures (related to bitumen in-service properties) and viscous flow tests were carried out at high temperatures
(related to bitumen modification processing and road
paving: mixing characteristics, laydown, compaction).
Experimental
Bitumen of penetration grade 60/70, provided by Construcciones Morales S.A. (Spain), was used as a base
material for polymer modification. Waste plastic (EVA/
LDPE) from agriculture, provided by Egmasa (Spain),
was used as the modifying agent. Asphaltene content,
determined by the procedure outlined in ASTM D3279,
and penetration grade of the base bitumen, as well as
some physico-chemical characteristics of the polymer are
shown in Table 1.
Blends of bitumen and polymer, at 5 and 9 wt%
polymer concentrations, were prepared in an open
mixer, using an IKA RW-20 stirring device (Germany).
Samples were processed for 6 h, at 180 C, and at a
rotating speed of 1200 rpm. The modified bitumen was
compared with neat bitumen and bitumen processed
under the above described conditions, which will be
named as ‘processed bitumen’ hereafter. At a temperature of 180 C, both the bitumen and polymer are in the
melt phase and a dispersion of polymer drops is formed
in the bitumen matrix.
The rheological characterisation of neat and modified
bitumen was carried out using four different rheometers,
namely, a controlled-strain Rheometrics Scientific
ARES rheometer (USA), two controlled-stress Haake
RS150 and RS100 rheometers (Germany) and the
Cambridge MultiPass rheometer (MPR), a double piston capillary instrument developed by the Department
of Chemical Engineering at University of Cambridge
(Mackley et al. 1995).
Steady state flow curves at different temperatures and
temperature sweeps (1 CÆmin)1) at constant shear rate
(1 s)1) were carried out with the RS100 rheometer,
coupled to the heating system Haake TC501, and using a
plate-and-plate geometry (20 and 35 mm diameter, 1
and 2 mm gap). Frequency sweeps, at different temperatures, within the linear viscoelasticity region were
performed with the RS150 rheometer, using serrated
plate-and-plate geometries (10 and 20 mm diameter, 1
and 2 mm gap). Steady state flow curves at different
temperatures and temperature sweeps (1 CÆmin)1) in
oscillatory mode (1 rad s)1 and 1% strain) were
accomplished with the ARES rheometer, using a Couette geometry, having the inner and outer cylinders 32
and 34 mm diameter respectively, and a plate-and-plate
geometry (25 and 50 mm diameter, 1 and 2 mm gap).
The Cambridge MultiPass rheometer was used to perform steady shear experiments in ‘multipass’ mode, at
different temperatures, using a 2 mm diameter and
40 mm length capillary, and two packed beds with
spheres of different diameters (1 and 2 mm). At least two
runs of each test were performed on each instrument.
Table 1 Physico-chemical characteristics of the bitumen and
recycled polymer
Bitumen
Asphaltene content (wt%)
Penetration grade (1/10 mm)
Recycled polymer
EVA/LDPE
vinyl acetate (wt%)
Black carbon (wt.%)
20.00
60–70
2/1
5
1
484
DSC measurements were carried out with a TA
Instruments Q100 (USA), using 10–20 mg of sample in
hermetic aluminium pans, and a heating rate of 10 C
min)1. The sample was purged with nitrogen at a flow
rate of 50 cm3min)1.
Optical microscopy was used to study the morphology of the modified bitumen. A Cambridge Shear
System 450, manufactured by Linkam Scientific Instruments (UK), coupled to a standard Olympus BH2
microscope was employed with that purpose, which
allowed observation of the sample under shear at different temperatures.
Results and discussion
Linear viscoelasticity and flow behaviour
Figure 1 shows oscillatory shear curves for neat and
modified bitumen at )10 and 50 C. As can be observed,
the elastic and viscous moduli increase as the temperature decreases, having an elastic modulus close to 108 Pa
at )10 C at high frequency. Polymer modification gives
rise to a significant increase in the viscoelastic functions,
mainly at 50 C, temperature at which the curves of the
linear viscoelasticity functions for modified bitumen
show important differences in relation to those for neat
and processed bitumen, particularly in the low frequency
region. The viscous modulus shows higher values than
the elastic modulus in the whole range of frequency, at
50 C, for 5 wt% modified bitumen, neat and processed
bitumen. The 9 wt% modified bitumen exhibits a quite
different behaviour. Thus, G¢ and G¢¢ values are very
similar within the frequency range studied and both
Fig. 1 Evolution of the linear
viscoelasticity functions with
frequency, at )10 and 50 C,
for neat, processed and polymer
modified bitumen (5 and
9 wt%)
curves present the same slope, 0.5±0.001, showing a
typical behaviour of a critical gel (De Rosa and Winter
1994).
At low temperature, the addition of polymer to the
bitumen does not produce a significant change in the
viscoelastic functions in relation to the neat or processed
bitumen, although the polymer modification may reduce
the bitumen stiffness at high enough frequencies. Thus,
modified bitumen shows a decrease in the slope of G¢
and, as a consequence, the glassy region of the
mechanical spectrum of these materials shifts to higher
frequencies or lower temperatures (Martı́nez-Boza et al.
2000). In addition, the difference between the neat and
processed bitumen curves increase at )10 C, which
proves that oxidation undergone by processing appears
more significantly at low temperatures. Oxidation is the
phenomenon which takes place during the stirring of
bitumen at the conditions described in the experimental
section, in which bitumen undergoes changes in its
chemical composition, in particular, a transformation of
aromatics into resins and resins into asphaltenes. It is
usually named as ‘primary ageing’, unlike the ‘secondary
ageing’ which takes place during the bitumen life-time
(Pérez-Lepe et al. 2003).
On the other hand, polymer modification makes the
viscoelastic functions increase at intermediate in-service
temperature, with the consequent benefit for the bitumen, improving ‘rutting’ resistance.
The flow behaviour of neat and modified bitumen at
higher processing temperatures is shown in Fig. 2 and 3,
for a wide range of shear rates, by combining rotational
and capillary rheometry (Barnes et al. 1989; Macosko
1994; Walters 1975). Flow curves of neat bitumen were
obtained, at different temperatures, from 10)2to 104 s)1,
485
Fig. 2 Flow curves of neat
bitumen, at different temperatures. Additional plots of MPR
bottom piston position and
pressure profiles
Fig. 3 Flow curves of 5 and
9 wt% polymer modified bitumen, at 100 and 180 C respectively. Additional plots of the
bottom piston position and
pressure profiles
486
Table 2 Cross model
parameters for neat and
modified bitumen at different
temperatures
Neat bitumen
g0[PaÆs]
k [s]
p
Modified bitumen
60 C
80 C
100 C
120 C
5 wt% 100 C
9 wt% 180 C
430.26
8.13·10)3
0.920
39.29
2.31·10)3
0.694
6.22
3.60·10)4
0.500
1.60
9.23·10-5
0.416
136.66
8.54·10)2
0.450
1.65
6.10·10)4
0.370
using controlled-strain and controlled-stress rheometers,
for low-medium shear rate values, and a capillary rheometer (MPR) in the high shear rate region (Fig. 2). The
bitumen shows a shear-thinning behaviour, with a region corresponding to a Newtonian viscosity at low
shear rate, g0, and a further drop in viscosity for higher
shear rates which became more apparent as the temperature was decreased. The flow behaviour shown by
the neat bitumen was represented fairly well by the Cross
model:
gg1
g0 g1
¼
1
1þðk_cÞp
ð1Þ
where g0 (Newtonian viscosity), k (characteristic time)
and p (related to the slope in the shear-thinning region)
increase as temperatures decreases. g¥ was considered to
be zero for the systems studied. Cross parameters for
neat bitumen are presented in Table 2.
It can also be seen that Newtonian viscosity values
get closer as the temperature increases. Consequently,
the thermal susceptibility of the bitumen is reduced at
high temperature, due probably to a simpler structure,
as most of the resin stands dissolved into the maltenic
matrix. The bitumen behaves as a shear-thinning fluid at
high temperature in the high shear rate region, which
may be caused by its colloidal structure, with asphaltene
cores peptised by a shell of resin. According to the colloidal model, bitumen is a mixture of asphaltenes (black
particles) dispersed in an oily medium (maltenes) composed of saturates, aromatics and resins. Part of the
resins constitutes a shell around the asphaltene particles,
and a temperature-dependent equilibrium between the
resins around the asphaltenes and those dissolved in the
oily matrix is established (Lesueur et al. 1996).
The overall flow behaviour cannot be found by using
standard rotational rheometers with parallel plates alone
and capillary rheometry, which is able to reach sufficiently high shear rates, should be used for that purpose.
In the case of the MPR, two pistons operating in tandem
were used to drive the fluid sample through a small
capillary insert, and the pressure difference across the
capillary measured. Bottom piston positions and differential pressure profiles are included in Fig. 2, for the
neat bitumen at 100 and 120 C and the same conditions
of velocity (20 mm s)1), amplitude (10 mm) and idle
time (1 s) for the pistons, showing that pressure drop
across the capillary becomes different from zero just in
the period of time when the pistons move up or down.
Moreover, differential pressure is higher at low temperature as the bitumen has a higher viscosity.
High temperature flow curves for modified bitumen,
5 and 9 wt%, in a wide range of shear rates, are shown
in Fig. 3, for selected temperatures. Data sets for the 5
and 9 wt% modified bitumen were obtained by using a
controlled-strain, a controlled-stress and a capillary
rheometer. For the second bitumen, additional viscosity
tests were performed by changing the 2 mm diameter
capillary for two packed beds with steel spheres of two
different diameters (1 and 2 mm) in order to obtain
lower shear rates, close to those accomplished by rotational rheometers. The bitumen at high temperature was
made to pass through the packed bed, and the pressure
at its both sides was measured by pressure transducers
(Machac et al. 1998; Chhabra et al. 2001). As can be
observed in Fig. 3, the values obtained with the packed
bed are in good agreement with those obtained by using
a 2 mm capillary. The bottom piston position and
pressure differences profiles have been inserted in Fig. 3,
for an experience using 2 mm diameter spheres, with
9 wt% modified bitumen.
As for the neat bitumen, the flow curves present a
Newtonian region at low-medium shear rates, followed
by a drop in viscosity as the shear rate increases, and,
therefore, the flow behaviour can be represented by the
Cross model (Eq. 1). The values of the parameters g0, k
and p are shown in Table 2. The effect of polymer
addition at high temperature is clearly observed by
comparing neat bitumen and 5 wt% modified bitumen
at 100 C (Figs. 2 and 3). Thus, Table 2 shows that the
addition of 5 wt% EVA/LDPE increases g0 one order of
magnitude. Furthermore, higher values of k are obtained
and, as a consequence, the material becomes more
structured by the polymer addition. On the other hand,
it should be noticed that 9 wt% modified bitumen has a
value of the low-shear-rate viscosity, at 180 C, quite
similar to that shown by the neat bitumen at 120 C.
However, the modified bitumen is much more structured, as may be deduced from the values of the characteristic time (Table 2).
It is well-known that the viscosity of a modified
bitumen should be controlled due to its strong effect on
bitumen workability and handling. In that sense, the
flow curves of neat, processed and modified bitumen, at
135 C, are compared in Fig. 4. As was previously
pointed out for oscillatory shear experiments, the differences between neat and processed bitumen are smaller
487
Fig. 4 Flow curves, at 135 C,
of neat, processed and polymer
modified bitumen (5 and
9 wt%)
as the temperature increases, showing that the oxidation
effects appear more clearly in the low and intermediate
temperature region. Figure 1 demonstrates that polymer
modification improves the mechanical properties of
bitumen at both low and medium in-service temperatures, with an increase in the resistance to permanent
deformation and thermal cracking. However, viscosity
should be kept low enough at higher handling temperatures of bitumen (Crockford et al. 1995) (laydown and
compaction temperatures) in order to allow an easy
application on the pavement. AASHTO MP1 (McGennis 1995) requires that the viscosity at 135 C should be
less that 3 Pa s. As can be seen in Fig. 4, a 5 wt%
modified bitumen stands below the above-mentioned
limit and, thus, it may be properly applied in pavement
building. Blends at 9 wt% exceed 3 Pa s and thus, the
pumping operations performed during the binder
application on the road might result quite difficult.
However, blends at 9 wt% might be successfully employed in the manufacture of roofing membranes, which
demand higher proportions of polymers (Fawcett and
McNally 2000). As a consequence, rheology and
microstructure of modified bitumen at 9 wt% are widely
studied in this paper.
Modified bitumen microstructure under shear
Figure 5 shows the results obtained during two
temperature sweep tests, performed on 9 wt% polymer
modified bitumen, carried out in oscillatory and
steady-shear mode at the conditions mentioned in the
experimental section. The frequency sweep test shows
the behaviour of the modified bitumen at very low
strain, that is, as the system stands nearly at rest and no
orientation effects take place on the dispersed polymer
(inset in Fig. 5). Temperature is the only parameter
causing a decrease in complex viscosity. In addition a
significant drop in viscosity occurs at about 105–110 C.
In order to explain this fact, standard calorimetry on
neat bitumen, 9 wt% modified bitumen and recycled
EVA were accomplished in a wide range of temperatures
(Fig. 6). Two melting peaks appeared during the
experiment carried out with the recycled polymer, a
larger one at 109 C and another one, much smaller, at
122 C. As shown in Table 1, the recycled polymer is a
blend of EVA copolymer with a low content in vinyl
acetate (peak at 109 C) and a minor proportion of
LDPE (peak at 122 C). As can be seen, a small peak at
about 109 C, corresponding to polymer melting, appears for the 9 wt% modified bitumen. It is worth
pointing out that other experiments carried out on a
similar blend of bitumen and polymer (Garcı́a-Morales
et al. 2004a, 2004b) not only gave rise to a larger peak,
but it was also shifted to a lower temperature, a fact
related to the polymer fraction swollen by bitumen light
components.
If a significant shear is applied to the modified bitumen, substantial structural changes are observed in a
shear rate sweep experiment (Fig. 5). Very long streaks
of polymer orientated in the flow direction, which do not
recover their original form after shear, appear at
the lowest temperatures (see Fig. 5), close to the melting point of the swollen polymer phase. At higher
488
Fig. 5 Evolution of viscosity
(1 s)1) and complex viscosity
(1 rad s)1) with temperature for
9 wt% polymer modified bitumen. Additional photomicrographs of the polymer modified
bitumen microstructure for different shear conditions
Fig. 6 DSC curves for recycled
EVA/LDPE, neat bitumen and
polymer modified bitumen
(9 wt%)
temperatures and moderate shear, the droplets of polymer, which remained in a spherical shape during the
oscillatory mode, tend to slightly deform in the flow
direction, adopting ellipsoid shapes which are transformed into spherical droplets after shear stops (inset in
Fig. 5). As a consequence of the structural changes in
polymer droplets, the resistance to the flow diminishes
and lower values in viscosity are obtained, at the same
temperature, in steady flow.
The striking decrease in complex viscosity observed
above 90 C can be related to the melting process of the
swollen polymer phase. In addition, during steady flow
at temperatures close to the polymer melting point,
shear makes the polymer drops extend into very long
streaks. Thus, the viscosity decreases due to polymer
elongation. At higher temperatures and moderate strain
rate, the polymer droplets lengthen into ellipsoid particles and recover their spherical shape after shearing
(Luciani et al. 1997).
The effect of shear on polymer droplets has been
confirmed by other experiments that combine shear flow
and sample relaxation. Thus, Fig. 7 illustrates the results
489
Fig. 7 Evolution of viscosity
with pistons idle time, corresponding to MPR experiments,
for neat bitumen (160 C)
and 9 wt% polymer modified
bitumen, at 140 and 160 C
respectively. Additional photomicrographs of the droplet
relaxation process
of an experiment using the MPR rheometer and a
packed bed with spheres, aiming to show the way that
polymer droplet relaxation may affect the viscosity of
the modified bitumen. Pressure drop of bitumen across
the packed bed was measured for a group of experiments
in which the idle time of pistons between their up and
down movements is increased. Although polymer
droplets deform passing through the packed bed, the
relaxation process may make them recover their original
spherical shape if a long enough idle time is considered.
As has already been mentioned, spherical droplets give
rise to higher viscosity than elongated drops which easily
orientate towards the flow direction. Hence, for short
idle times, which do not allow polymer drops to take
their initial shape back, the viscosity of the modified
bitumen is lower. Once the polymer particle has thoroughly returned to its original state, the viscosity value
keeps constant although experiments at higher idle times
are considered. Plots of viscosity vs idle time, as a
function of temperature, are shown in Fig. 7 for experiments carried out, with 1 mm diameter spheres and the
same piston velocity (60 mm s)1), on 9 wt% modified
bitumen and neat bitumen. It can be seen that an increase in viscosity occurs for modified bitumen as longer
idle times are considered. For the case of neat bitumen
the viscosity value remains constant for experiments at a
fixed piston velocity.
Some photomicrographs showing the relaxation
process of a polymer droplet in the modified bitumen
have been included in Fig. 7. Previously, shear was applied on the modified bitumen in order to produce
droplet deformation. Then shear was stopped and the
evolution of the shape for a selected droplet (marked by
a short-dashed line) was followed along time. It is worth
mentioning that the relaxation time is not the same for
all the droplets, but the velocity at which the phenomenon takes place depends on the ratio r/r0, where r is the
interfacial tension and r0 the radius of the droplet
(Luciani et al. 1997; Wolf et al. 2000). Hence, for a
considered temperature, shorter relaxation times are
obtained for the smallest polymer particles.
Conclusions
Polymer modification using recycled EVA/LDPE
improves the mechanical properties of bitumen in the
temperature region where the material is used as a surface coating. In relation to processing, the 5 wt%
modified bitumen has viscosity values, at 135 C, that
allow easy application on the road. On the contrary, the
viscosity of the 9 wt% modified bitumen is too large for
processing at that temperature. Neat bitumen has normally been considered to be a Newtonian fluid at high
temperature. However, a drop in viscosity takes place
when experiments are performed at very high shear rate,
which were obtained by using capillary rheometry. For a
modified bitumen, a significant decrease in the complex
viscosity with increasing temperature was observed in
oscillatory shear and this was related to the melting
process of the swollen polymer phase. Such a decrease in
viscosity is less evident when steady shear is applied on
the sample, because the modified bitumen microstructure is strongly affected by shear.
490
Surprisingly, we have found that the bitumen-polymer system is very well suited for optical observation
during shear. Providing the optical depth of the bitumen
is sufficiently small, good contrast between the ‘‘dark’’
bitumen and the ‘‘transparent’’ polymer can be
achieved. This in turn enabled detailed studies to be
made on polymer droplet extension and relaxation in an
optical shear cell.
The addition of recycled EVA/LDPE to bitumen
enhances the materials mechanical properties, as well as
providing a useful environmental alternative for the
disposal of waste plastics.
Acknowledgements This work is part of a research project sponsored by a MCYT-FEDER programme, (Research Project
MAT2001-0066-C02-02). The authors gratefully acknowledge its
financial support.
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