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Effect of Montmorillonite on Properties of Styrene–Butadiene–Styrene Copolymer Modified Bitumen
Jianying Yu, Lin Wang, Xuan Zeng, Shaopeng Wu, Bin LiSchool of Materials Science and Engineering, Wuhan University of Technology,Wuhan 430070, People’s Republic of China
Clay/styrene–butadiene–styrene (SBS) modified bitu-men composites were prepared by melt blending withdifferent contents of sodium montmorillonite (Na-MMT)and organophilic montmorillonite (OMMT). The struc-tures of clay/SBS modified bitumen composites werecharacterized by XRD. The XRD results showed thatNa-MMT/SBS modified bitumen composites may forman intercalated structure, whereas the OMMT/SBSmodified bitumen composites may form an exfoliatedstructure. Effects of MMT on physical properties,dynamic rheological behaviors, and aging properties ofSBS modified bitumen were investigated. The additionof Na-MMT and OMMT increases both the softeningpoint and viscosity of SBS modified bitumens and theclay/SBS modified bitumens exhibited higher complexmodulus, lower phase angle. The high-temperaturestorage stability can also be improved by clay with aproper amount added. Furthermore, clay/SBS modifiedbitumen composites showed better resistance to agingthan SBS modified bitumen, which was ascribed tobarrier of the intercalated or exfoliated structure to ox-ygen, reducing efficiently the oxidation of bitumen, andthe degradation of SBS. POLYM. ENG. SCI., 47:1289–1295,2007. ª 2007 Society of Plastics Engineers
INTRODUCTION
High-temperature rutting and low temperature cracking
of bitumen because of severe temperature susceptibility
limits its further application. Therefore, it is necessary to
modify bitumen [1]. The addition of polymers to bitumen
has been proved to be effective to improve the perform-
ance significantly. The pavement with polymer modified
bitumens (PMB) exhibits greater resistance to rutting and
thermal cracking, decreased fatigue damage, stripping,
and temperature susceptibility [2–4]. Among the polymer
modifiers of bitumen, styrene–butadiene–styrene (SBS)
block copolymer became the best modifiers of bitumen
because the physical and mechanical properties and rheo-
logical behavior of conventional bitumen can be improved
significantly with the addition of SBS. SBS exhibits a
two-phase morphology consisting of soft block and hard
block [5, 6]. The styrene, referred as the hard block, is
usually the dispersed phase, and provides the strength of
the material; while the butadiene, the soft block, contrib-
utes to the elasticity of SBS [7]. Unfortunately, SBS is
destined to separate from the bitumen when stored at high
temperature because of the poor compatibility between
SBS and bitumen [8]. Furthermore, SBS tends to degrade
by exposure to heat, oxygen, and UV light since SBS
contains an unsaturated bond, which may make a contri-
bution to the deterioration of bitumen pavement [9–11].
Layered silicate is a type of mineral, which consists of
layers of tetrahedral silicate sheets and octahedral hydr-
oxide sheets [12]. It mainly includes montmorillonite
(MMT), vermiculite (VMT), and kaolinite clay (KC).
Recently, layered silicates have been widely used for the
modification of polymers [13–15]. Polymer chains can be
intercalated into the interlayer of clay and make the clay
disperse into the polymer matrix at nanometer-scale,
which leads to significant improvements in thermal, me-
chanical, and barrier properties of polymers [16, 17].
SBS/KC composites have been successfully used to
improve the high-temperature storage stability of SBS
modified bitumen [7, 9]. The improvement in high-tem-
perature storage stability could be attributed to the KC in
the SBS/KC composites for it decreased the difference of
densities between SBS and bitumen. However, properties
except for high-temperature storage stability of SBS
modified bitumen cannot be improved obviously by KC,
which may be attributed that the layer of KC is not bene-
fit to be intercalated, not forming the nanocomposite.
Compared with KC, MMT has been proved to form nano-
composite easily.
In this article, clay/SBS modified bitumen composites
were prepared by the melt blending with Na-MMT and
OMMT, respectively. OMMT is the organic cationic sur-
factants modified MMT. In such case, the inorganic
actions in the galleries of the pristine Na-MMT are
exchanged for tallow composites/surfactants, which make
Na-MMT become compatible with the asphalt. The
effects of Na-MMT and OMMT on physical properties,
Correspondence to: Jianying Yu; e-mail: [email protected]
Contract grant sponsor: Ministry of Communications of the People’s
Republic of China and the Science and Technology Department of Hubei
Province, People’s Republic of China.
DOI 10.1002/pen.20802
Published online in Wiley InterScience (www.interscience.wiley.com).
VVC 2007 Society of Plastics Engineers
POLYMER ENGINEERING AND SCIENCE—-2007
dynamic rheological behaviors, and aging properties of
SBS modified bitumen were investigated.
EXPERIMENTAL
Materials
Bitumen, AH-90 paving bitumen was obtained from
SK Chemicals, Korea. The physical properties of the bitu-
men were listed in Table 1. SBS, Grade 791H, was pro-
duced by the Yueyang Petrochemical, China. It was a lin-
ear-like SBS containing 30% styrene, and the weight-av-
erage molecular weight was 120,000 g mol�1. The
sodium montmorillonite (Na-MMT), with interlayer cation
of Naþ and having a cation exchange capacity (CEC) of
90 meq/100 g, particle size of 200 mesh, and the aspect
ratio being 200–500, was supplied by Fenghong Clay
Chemical Factory, Zhejiang, China. The organophilic
montmorillonite (OMMT), which is MMT exchanged
with octadecylammonium ions and having a CEC of 130
meq/100 g and the same particle size with Na-MMT was
purchased from the same factory.
Preparation of MMT/SBS Modified Bitumen Composites
SBS modified bitumens were prepared using a high
shear mixer at 1708C and a shearing speed of 4000 rpm.
First, bitumen was heated to become a fluid in an iron
container, then upon reaching about 1708C, SBS was
added to the bitumen and sheared for 40 min to produce
SBS modified bitumen. After that, Na-MMT or OMMT
was added into SBS modified bitumen, and the mixtures
were blended at a fixed rotate speed about 60 min to pro-
duce MMT/SBS modified bitumen composites. When
compared with MMT/SBS modified bitumen, SBS modi-
fied bitumen in the absence of MMT was prepared under
the same conditions.
XRD Test
X-ray diffraction (XRD) graphs were obtained using a
Rigaku D/max 2400 diffractometer with Cu Ka radiation
(l ¼ 0.154 nm, 40 kV, 120 mA) at room temperature, the
diffract to grams were scanned from 1.58 to 158 in the 2yrange in 0.028 steps, scanning rate was 28/min.
Physical Properties Test
Classical Tests. The physical properties of bitumen,
including softening point, penetration (258C), and ductil-
ity (58C), were tested according to ASTM D36, ASTM
D5, and ASTM D113-86, respectively.
Brookfield viscometer (Model DV-IIþ, Brookfield En-
gineering, USA) was employed to measure the viscosity
of modified bitumens at 1358C according to ASTM
D4402.
High-Temperature Storage Stability Test. Static stor-
age tests were used to estimate high-temperature storage
stability of modified bitumens. The experimental system
consisted of a tube (32 mm in diameter and 160 mm in
height), vertically placed in an oven, at 1638C for 48 h
and, then it was taken out, followed by cooling down to
room temperature and being cut into three equal sections.
If the difference between the softening points of the top
and the bottom sections was less than 18C, the sample
was considered to have good high-temperature storage
stability. Otherwise, it was designated to be unstable.
Dynamic Rheological Characterization
Dynamic rheological measurements for all the samples
(with MMT and without MMT) were performed in plate–
plate mode (diameter 2.5 cm), in the Dynamic shear rhe-
ometer (Model AR2000, TA). Temperature sweeps (from
50 to 808C) with 18C increments were applied at a fixed
frequency of 10 rad/s and variable strain. The rheological
parameters were measured for calculating viscoelastic pa-
rameters such as complex modulus (G*), phase angle (d),and rut factor (G*/sind).
Aging Experiment
Thin film oven test (TFOT) and the rolling thin film
oven test (RTFOT) were used to simulate the change in
the properties of bitumen during the plant hot mixing and
the lay down process, and the pressure aging vessel
(PAV) that utilizes the residue from RTFOT was used to
simulate long-term aging after 5–10 years of service.
Short-term and long-term laboratory aging of SBS modi-
fied bitumen and MMT/SBS modified bitumen composites
were performed using RTFOT (ASTM D 2872) and PAV
(AASHTO PP1), respectively. The standard aging proce-
dures of 1638C and 75 min for the RTFOT and 1008C,2.1 MPa, and 20 h for the PAV were used.
RESULTS AND DISCUSSION
Structure of MMT/SBS Modified Bitumen Composites
Similar to polymer/layered silicate nanocomposites,
layered silicate modified bitumen has two structure types,
namely intercalated structure and exfoliated structure, as
TABLE 1. Physical properties of AH-90 bitumen matrix.
Physical properties AH-90 bitumen matrix
Penetration (258C, 0.1 mm) 92.6
Softening point (8C) 47.8
Ductility (158C, cm) >150
Ductility (58C, cm) 8.5
Viscosity (608C, Pa s) 187
1290 POLYMER ENGINEERING AND SCIENCE—-2007 DOI 10.1002/pen
shown in Fig. 1. The intercalated structures correspond to
well-ordered multilayered structures where the bitumen
chains are inserted into the gallery space between the sili-
cate layers. The exfoliated structures correspond to
delaminating structures where the individual silicate
layers are no longer close enough to interact with the gal-
lery cations [18].
The degree of exfoliation of silicate layers of Na-
MMT and OMMT in the bitumen was investigated by
using XRD techniques from the position, shape, and the
intensity of the basal reflections in the XRD patterns. Sev-
eral XRD curves for Na-MMT, OMMT, Na-MMT/SBS
modified bitumen composite, and OMMT/SBS modified
bitumen composite were shown in Fig. 2. As reported ear-
lier, when d001 peak shift to a lower angle, the interlayer
of the Na-MMT or OMMT will be widened [17]. The
interlayer spacing can be calculated according to the
Bragg equation (2d sin y ¼ l), which were given in Table
2. It can be found that the crystalline peak of the pristine
Na-MMT was at 2y ¼ 5.88 (d001 ¼ 1.50 nm) and it was
at 2y ¼ 3.02 (d001 ¼ 2.92 nm) in the Na-MMT/SBS
modified bitumen composite. Therefore, we can conclude
that the bitumen and SBS were intercalated into the
MMT gallery and Na-MMT/SBS modified bitumen com-
posite may form an intercalated structure. According to
Fig. 2, we could not observe any crystalline peak in XRD
for the OMMT/SBS modified bitumen composite, which
implied that the interlayer d-spacing of OMMT in the
OMMT/SBS modified bitumen composite was more than
4.4 nm. It may suggest that the layer of OMMT had al-
ready been peeled off and OMMT/SBS modified bitumen
composite may form an exfoliated structure.
The above-mentioned results can be explained by dif-
ferent microstructure between Na-MMT and OMMT. Na-
MMT layers were hydrophilic and the spaces between
them were small, which made the intercalation and peel-
ing of layers harder; while OMMT interlayer were already
enlarged by organic molecules, which made the micro-
structures of OMMT layers change and OMMT become
lipophilic [12]. This kind of structure of OMMT provided
the benefits for the insertion of bitumen molecules.
Effects of MMT on the Physical Properties of SBSModified Bitumen
Effects on Softening Point, Ductility, and Penetra-
tion. The effects of the Na-MMT and OMMT content
on the physical properties of SBS modified bitumens can
be seen in Table 3 as an increase in softening point and a
decrease in penetration with increasing MMT content.
The results showed that MMT can improve the high tem-
perature properties of SBS modified bitumen. When com-
pared with Na-MMT, OMMT showed better effect in
improving properties of SBS modified bitumen. This may
be the reason that OMMT/SBS modified bitumen compo-
sites formed an exfoliated structure, which made OMMT
FIG. 1. Schematic of structures of layered silicate/SBS modified bitumen.
FIG. 2. XRD patterns of MMT and MMT/SBS modified bitumen com-
posites.
TABLE 2. Interlayer spacing of MMT.
Samples 2y (8) d (nm)
Na-MMT 5.88 1.50
OMMT 2.42 3.64
Na-MMT in bitumen 3.02 2.92
OMMT in bitumen – >4.4
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2007 1291
disperse equably in bitumen. So, the properties of
OMMT/SBS modified bitumens composites were better
than Na-MMT/SBS modified bitumens composites.
Effects on Viscosity. Figure 3 showed that the viscosity
of MMT/SBS modified bitumen composites at 1358Ctends to increase with the increase in the content of
MMT. The increase in the viscosity may be due to the
formation of intercalated and exfoliated structure in
MMT/SBS modified bitumen composites, because the
movement of bitumen molecule chains was obstructed by
the layer of MMT at high temperature. In addition, the
viscosity of OMMT/SBS modified bitumen composites
was higher than Na-MMT/SBS modified bitumen compo-
sites at the same content of MMT. Such different behav-
iors can be explained by Na-MMT and OMMT respective
dispersing situations in bitumen.
Effects on High-Temperature Storage Properties. Be-
cause SBS is not totally compatible with bitumen, if a
mixture of SBS and bitumen is kept at high temperature,
the SBS will often separate out from the bitumen. When
the mixture is kept under quiescent conditions at high
temperatures, SBS will aggregate to form coarse particles,
which will result in the difference in properties between
top and bottom sections [7]. The high-temperature storage
stabilities of MMT/SBS modified bitumen composites
were shown in Fig. 4. The difference in softening point
between top and bottom was 2.28C for SBS modified bi-
tumen in the absence of MMT. When SBS modified bitu-
men including 1% OMMT or 2% Na-MMT, the differ-
ence in softening point between top and bottom was only
0.38C and 0.78C, respectively, which showed that the
storage stability of SBS modified bitumen was improved.
However, when the content of OMMT or Na-MMT
exceeded 1 or 2%, respectively, the difference in soften-
ing point increased with increasing MMT content. This
may be a consequence of the precipitation of excessive
MMT particles, which were not intercalated or exfoliated.
In addition, the storage stability of the OMMT/SBS modi-
fied bitumen composites was better than that of the Na-
MMT/SBS modified bitumen composites, which indicated
that OMMT had better compatibility and dispersing abil-
ity with SBS modified bitumen than Na-MMT.
Effects of MMT on Dynamic Rheological Propertiesof SBS Modified Bitumen
Dynamic shear tests are advantageous because the data
can be acquired within the linear range of the bitumen
blend in a loading mode that is similar to that of traffic
loading [19]. Figure 5 showed the curves of complex
modulus (G*) versus temperature for the MMT/SBS
modified bitumen composites. G* is defined as the ratio
of maximum shear stress to maximum strain and provides
a measure of the total resistance to deformation when
the bitumen is subjected to shear loading. According to
Fig. 5, increase in the G* value exhibited a more visco-
elastic behavior of MMT/SBS modified bitumen compo-
TABLE 3. Effects of MMT on the properties of SBS modified bitumen.
Kinds Content (%)
Softening
point (8C)Ductility
58C (cm)
Penetration
258C (0.1 mm)
Na-MMT 0 75.0 31.6 56.7
0.5 75.6 31.0 55.3
1 76.5 29.7 53.5
2 77.2 28.0 52.0
3 78.3 27.0 51.2
OMMT 0 75.0 31.6 56.7
0.5 76.1 30.8 52.5
1 77.6 29.3 51.2
2 78.4 28.5 50.2
3 80.1 27.7 49.6
FIG. 3. Viscosities of MMT/SBS modified bitumen composites versus
MMT contents at 1358C.FIG. 4. Effect of MMT content on storage stability of SBS modified
bitumen.
1292 POLYMER ENGINEERING AND SCIENCE—-2007 DOI 10.1002/pen
sites than that of SBS modified bitumen at high tempera-
ture. Moreover, it can be seen that with increasing MMT
contents, the G* value of the modified bitumens
increased. When compared with Na-MMT/SBS modified
bitumen composites, OMMT/SBS modified bitumen com-
posites exhibited higher complex modulus at the same
content of MMT, which may be caused by the exfoliation
of OMMT layers in SBS modified bitumen. These results
suggested that both Na-MMT and OMMT can improve
the viscoelastic behaviors of SBS modified bitumen.
Figure 6 showed the result of phase angles (d) againsttemperature. The phase angle, defined as the phase differ-
ence between stress and strain in an oscillatory test, is a
measure of the viscoelastic balance of the material behav-
ior. Measurement of phase angle is generally considered to
be more sensitive to the chemical and physical structure
than complex modulus for the modification of bitumens
[5]. According to Fig. 6, we can see that obvious decrease
in phase angle with the addition of MMT at high tempera-
ture. The deduction in d value exhibits a more elastic
behavior of bitumen. The decreasing extent of phase angle
became greater when the content of MMT increased. Addi-
tionally, OMMT/SBS modified bitumen composites exhib-
ited lower phase angle than Na-MMT/SBS modified bitu-
men composites, which may be caused by their respective
dispersing structures in SBS modified bitumen.
In Strategic Highway Research Program (SHRP) speci-
fications, the rheological parameter, G*/sind, was selected
to express the contribution of the bitumen binder to
permanent deformation. This value reflects the total re-
sistance of a binder to deform under repeated loading
(G*) and the relative amount of energy dissipated into
nonrecoverable deformation (sind) during a loading cycle
[19]. The G*/sind value should be larger than 1 kPa at
10 rad/s (1.6 Hz) for the binder at a maximum pavement
design temperature. With a higher value of the parameter
rate, there is higher resistance to permanent deformation.
Figure 7 indicated that, when temperature ranges from 50
to 808C, there was a increase in rut factors of MMT/SBS
modified bitumen composites compared with SBS modi-
fied bitumen, which can be attributed to the increase in
G* and decrease in phase angle when MMT content
increases. Figure 7 also showed the isochronal plots of
G*/sind, revealing distinct differences because of the
ability of MMT modifiers to interact with bitumen. The
effect of Na-MMT and OMMT contents on the perform-
ance grade of the modified bitumen were listed in Table 4.
It can be seen that the performance grade of MMT/SBS
modified bitumen composites were improved with the
addition of MMT the MMT/SBS modified bitumen com-
posites had a higher performance grade than SBS modi-
fied bitumen. When G*/sind ¼ 1 kPa, the temperatures
of MMT/SBS modified bitumen composites were higher
than that of SBS modified bitumen composites, which
FIG. 5. Curves of G* versus temperature at 10 rad/s for MMT/SBS
modified bitumen composites.
FIG. 6. Curves of d versus temperature at 10 rad/s for MMT/SBS
modified bitumen composites.
FIG. 7. Curves of G*/sind versus temperature for MMT/SBS modified
bitumen composites.
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2007 1293
indicated that MMT is helpful for the improvement of rut-
ting resistance.
Effects of MMT on Aging Properties of SBSModified Bitumen
Aging is a very complex process in bitumen, and the
degree of complexity increases when PMB are involved.
Changes in the properties of aged PMB were dependent
on a combined effect of bitumen oxidation and polymer
degradation [7]. Figures 8–10 showed the changes in the
physical properties of SBS modified bitumen and SBS
modified bitumen with 3% MMT after aging.
Viscosity data often function as a window through
which other characteristics of bitumen may be observed
[20]. Accordingly, the change of viscosity before and af-
ter aging makes sense to the study on the aging process
of bitumen. The viscosity aging index (VAI) was used to
characterize the aging extent, and calculated by measuring
the viscosity of the samples before and after the test as
shown in Eq. 1 [21]. The higher the VAI value, the more
aged the sample is [10].
VAIð%Þ ¼ Aged viscosity value� Unaged viscosity value
Unaged viscosity value
� 100 ð1Þ
Figure 8 showed the VAI of modified bitumens after
RTFOT and PAV aging. An increase of the viscosities
was observed in all the samples after RTFOT and PAV;
however, the VAI of SBS modified bitumen decreased
obviously after the addition of MMT, which indicated that
MMT/SBS modified bitumen composites have better
aging resistance than SBS modified bitumen. It can be
concluded that resistance to aging of SBS modified bitu-
men can be improved by the addition of MMT, which
was ascribed to barrier of the intercalated or exfoliated
structure to oxygen, reducing efficiently the oxidation of
bitumen, and the degradation of SBS. When compared
with Na-MMT/SBS modified bitumen composite, OMMT/
SBS modified bitumen composite exhibited lower VAI
values. It indicated that exfoliated structure was more
effective than intercalated structure for enhancing the
aging resistance of SBS modified bitumen.
Similar to the VAI, the change of softening point is an
indication of the sensitivity to aging of bitumen. It can be
calculated as Eq. 2. A smaller change of softening point
reflects lower influence of aging.
Change of softening point ðoCÞ ¼ aged softening point
� unaged softening point ð2Þ
FIG. 8. VAI of modified bitumens after RTFOT and PAV aging.
FIG. 9. Changes of softening point of modified bitumens after RTFOT
and PAV aging.
FIG. 10. Retained ductility of modified bitumens after RTFOT and
PAV aging.
TABLE 4. Effect of MMT contents on the performance grade of modified
bitumen.
Sample
Content
(%)
Temperature
when G*/sin d ¼1 kPa (8C)
SBS modified bitumen 0 74.6
Na-MMT/SBS modified bitumen 1 76.1
Na-MMT/SBS modified bitumen 3 78.7
OMMT/SBS modified bitumen 1 76.7
OMMT/SBS modified bitumen 3 80.2
1294 POLYMER ENGINEERING AND SCIENCE—-2007 DOI 10.1002/pen
Figure 9 revealed the changes of softening point of the
modified bitumens after RTFOT and PAV. At the end of
the RTFOT, the softening point of the samples increased.
However, the softening point of all the samples decreased
after PAV. Two possible reasons for the changes are (1)
the molecular weight of the base bitumen is increased due
to oxidation, which increased the softening point; (2)
polymer molecules are degraded in size, and as a result,
the bitumen-polymer interactions may be reduced dramat-
ically, which induce the reduction of the softening point
[22]. The increase of softening point after RTFOT means
that effect of the oxidation of bitumen exceeded influence
of the degradation of SBS, while the result was contrary
after PAV.
We can also see from Fig. 9, the change values of soft-
ening point increased in the order: OMMT/SBS modified
bitumen composites, Na-MMT/SBS modified bitumen
composites, and SBS modified bitumen in the absence of
MMT. These results indicated that aging resistance of
SBS modified bitumen had been improved by the addition
of MMT.
The retained ductility can also be used to evaluate the
resistant ability to aging. It can be calculated as Eq. 3. Ahigher value of retained ductility shows better resistance
to aging.
Retained ductility ð%Þ ¼ aged ductility
unaged ductility� 100: (3)
The retained ductility of modified bitumens after aging
can be seen in Fig. 10. The difference of retained ductility
of all the samples was rather small after RTFOT aging.
However, MMT/SBS modified bitumens, especially
OMMT/SBS modified bitumen, showed larger retained
ductility compared to SBS modified bitumen after PAV
aging, which indicated that the resistance to long-term
aging of modified bitumens was improved with the addi-
tion of MMT. OMMT/SBS modified bitumen exhibited
the highest retained ductility, owing to the formation of
exfoliated structure.
CONCLUSIONS
Clay/SBS modified bitumen composites were prepared
by melt blending with different amounts of Na-MMT and
OMMT. The XRD results showed that the Na-MMT/SBS
modified bitumen composite may form an intercalated
structure, whereas the OMMT/SBS modified bitumen
composite may form an exfoliated structure.
The addition of MMT to SBS modified bitumen
increased both the softening point and viscosity. However,
a decrease was shown on the values of ductility due to
the addition of MMT. The high-temperature storage sta-
bility can be improved by MMT with a proper amount
added. Both Na-MMT and OMMT can improve the
dynamic rheological properties of SBS modified bitumen.
MMT/SBS modified bitumen composites exhibited higher
complex modulus, lower phase angle, and higher rutting
resistance. MMT/SBS modified bitumen composites
showed better resistance to aging than SBS modified bitu-
men, which was ascribed to barrier of the intercalated or
exfoliated structure to oxygen, reducing efficiently the ox-
idation of bitumen and the degradation of SBS. When
compared with Na-MMT, OMMT had greater effects in
improving properties of SBS modified bitumen, which
indicated that exfoliated structure was more effective than
intercalated structure in enhancing properties of SBS
modified bitumen.
ACKNOWLEDGMENTS
We are grateful to Dr. Shanjun Gao and Dr. Lili Wu
for helping with this study.
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