improved silica dispersibility with functionalized ssss
TRANSCRIPT
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IMPROVEDIMPROVEDIMPROVEDIMPROVED SILICASILICASILICASILICA DISPERSIBILITYDISPERSIBILITYDISPERSIBILITYDISPERSIBILITY WITHWITHWITHWITH
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IMPROVED SILICA DISPERSIBILITY WITH FUNCTIONALIZED S-SBR FOR
LOWER ROLLING RESISTANCE
Takeshi Yuasa, Takuo Sone
Elastomer Laboratory, Polymer Research Laboratories, JSR Corporation
Tetsuo Tominaga, Munetaka Iwano
Material Characterization & Analysis Laboratory, JSR Corporation
ABSTRACT
Dispersibility of silica filler affects hysteresis loss properties of rubber compounds.
Some modified styrene butadiene rubbers with functional groups at polymer chain ends have
been developed by solution polymerization system (S-SBRs) to achieve good interaction
with silica filler. They showed reduction in the hysteresis loss of silica reinforced
compounds. Especially, the modified S-SBR with both primary amino group and
alkoxysilyl group exhibited significantly lower hysteresis loss properties. In order to
observe the dispersibility of silica in vulcanizates with this modified S-SBR directly, the
measurements of transmission electron microscopy (TEM) and ultra small angle X-ray
scattering (USAXS) were carried out. As the results, the excellent dispersion state of silica
in the vulcanizates with this modified S-SBR was observed. This result coincided with
those obtained by dynamic viscoelstic measurement.
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INTRODUCTION
In recent years, reduction of rolling resistance has become one of important items
most often talked about in tire industry. Rolling resistance is closely related to hysteresis
loss of tire tread compounds. Therefore, vulcanizates having lower hysteresis loss are in
demand as materials for tire tread, in order to achieve lower rolling resistance.
An increase in interaction between polymer chain-end and filler is effective for the
reduction in hysteresis loss because of an improvement in dispersion of fillers in addition to
the decrease in free chain ends in polymer network. In order to increase the interaction with
filler such as carbon black and silica, various styrene butadiene rubbers polymerized in
solution (S-SBRs) with functional groups at chain-ends were developed. Functionalized
S-SBRs are believed to react with functional groups on the surface of filler. With this
hypothesis, from 1980s to 1990s, several S-SBRs modified with tin compounds [1, 2],
isocyanate compounds [3] or amino benzophenone compounds [4] for carbon black filler
were reported.
On the other hands, silica fillers have been widely used for tire tread compounds for
passenger cars, especially in Europe and Japan, because of their significant improvements in
the balance between rolling resistance and wet skid resistance. Intensive efforts to improve
the affinity of polymers with silica have been made by many researchers. S-SBRs with
functional groups at polymer chain-ends have been one of promising technologies to reduce
hysteresis loss of silica compounds. In 1990s, it was reported that S-SBRs functionalized
with alkoxysilyl groups at polymer chain-ends provided with low hysteresis of silica
compounds [5, 6]. Recently, Ozawa et al. reported that S-SBR with both amino and
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alkoxysilyl groups had shown improved hysteresis loss–abrasion balance [7]. We have
found that S-SBR with both primary amino and alkoxysilyl groups at the same chain end is
also effective for improvement in hysteresis loss [8].
As mentioned above, it has been shown that functionalized S-SBRs are an effective
tool for reduction in hysteresis loss derived from excellent dispersion of fillers such as carbon
black and silica in vulcanizates. Analysis of morphological structure of filler in
vulcanizates provides us with important information to understand relation between
hysteresis loss and filler dispersion. Ultra-small-angle-X-ray scattering (USAXS) is an
effective tool to study the hierarchical structure of fillers. Actually, structural analyses of
carbon black or silica in rubber composites with natural rubber or SBR were carried out using
USAXS measurement, and quantitative characterizations of aggregates and agglomerates of
fillers were reported [9]-[14].
In this paper, we report analysis of morphological structure of silica in vulcanizates
with functionalized S-SBRs by using transmission electron microscopy (TEM) and USAXS,
and discuss the correlation between the morphological structure and dynamic viscoelastic
properties.
EXPERIMENTAL
1. Preparation and characterization of Polymers
All styrene butadiene rubbers were polymerized by anionic polymerization
technique. Polymerization was carried out according to methods described in our previous
report [8]. The microstructure of S-SBR was analyzed by 1H-NMR (JEOL EX270) in
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CDCl3. TMS was used as a standard. The molecular weight of prepared polymer was
analyzed by gel permeation chromatography (GPC) based on calibration curve prepared with
polystyrene standards (Tosoh HLC8120, Columns: TSK gel GMHxl 30 cm x 2, 1 ml / min).
Measurement of GPC was carried out with RI detectors. The glass transition temperature
was measured according to ASTM standards D-3418 by differential scanning calorimeter
(DSC; TA Instruments DSC2910).
2. Compounding and Vulcanization
All compounds were prepared by 75 ml Brabender type laboratory mixer with the
recipe shown in Table 1. The compounds prepared were cured at 160 °C for a
predetermined period of time to obtain vulcanizates.
Table 1. Formulation for SBR/Silica compounds
PHRS-SBR 100Silica (Nipsil AQ, TOSOH SILICA) 50Silane (Si75, Degussa AG) 4Carbon Black; N339 (SEAST KH, TOKAI CARBON) 4Oil 10Stearic acid 2ZnO 3A.O.; N - (1,3-Dimethylbutyl)-N' -phenyl-p -phenylenediamine 1Acc; Diphenylguanidine 1.5Acc; N -Cyclohexyl-2-benzothiazylsulfenamide 1.8Sulfur 1.5
total 178.8
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3. Measurements
To evaluate the dispersibility of silica in the vulcanizates with the S-SBRs,
following measurements were carried out.
Dynamic Viscoelasticity
Dynamic viscoelasticity of the cured compounds were measured by a dynamic
mechanical analyzer (TA Instruments RSAIII) with cylindrical sample geometry and parallel
plate fixtures. The strain was varied from 0.1 to 10 % at 50 °C at a frequency of 100 rad/s.
Transmission electron microscopy
Sections of 100 nm thickness without staining were measured by a transmission
electron microscope (Hitachi H-7650) at 100 kV.
Ultra-small-angle-X-ray scattering (USAXS)
USAXS measurements were performed at the BL19B2 in the Spring-8. Our
USAXS configuration could cover the q range of 0.005 to 0.2 nm−1, with the X-ray energy
and camera length being set to 18 keV and 35 m respectively.
RESULTS and DISCUSSION
1. Structure of the functionalized S-SBRs
In order to investigate silica dispersibility in vulcanizates, three kinds of S-SBRs
were prepared by anionic polymerization technique. The structures of these S-SBRs are
shown in Figure 1. S-SBR [i] is a polymer without any modification. Other polymers
(S-SBR [ii] and S-SBR [iii]) have functional groups in polymer chain-ends which can
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interact with silica surface. Especially, S-SBR [iii] has two kinds of differential functional
groups in the same polymer chain-end. The analytical data of all polymers used in this study
is summarized in Table 2. We prepared a series of S-SBRs with different functional groups
having almost the same microstructure, molecular weight and glass transition temperature for
comparison.
Figure 1. Structures of functionalized S-SBRs
2. Hysteresis loss properties
Dynamic viscoelstic properties of vulcanizates with S-SBRs were measured by
strain sweep from 0.1 to 10 % at 50 °C. The results obtained are summarized in table 3 and
figure 2. Tanδ at 50 °C is considered to be a good indication of hysteresis loss. Tanδ at 50
°C decreased by introduction of functional groups at the polymer chain-ends in the order
Table 2. Polymers prepared for this study
S-SBR Functional group(s) Vinyl ST Mw Tg
(%) (%) (kg/mol) ( °C)
[i] none 62 21 199 -36[ii] Si-OR 60 20 204 -36[iii] Si-OR + NH2 61 20 194 -36
Si - OR
R'
Si - OR
R''- NH2
S-SBR [i] S-SBR [ii] S-SBR [iii]
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shown below.
Lower (Excellent); S-SBR [iii] >> S-SBR [ii] > S-SBR [i]
Primary amine & alkoxysilane alkoxysilane without modification
It is clear by comparing two modified S-SBRs that the S-SBR [iii] with both primary
amino group and alkoxysilyl group showed significantly larger reduction in the hysteresis
loss than S-SBR [ii] with only alkoxysilyl group. The effectiveness for the improvement in
hysteresis loss of each functional group agreed with our previous results [8].
It is generally understood that the dependence of G’ (∆G’) on the amplitude of the
applied strain, known as “Payne Effect’’, is related to dispersibility of fillers in compounds.
The value of ∆G’ is used as an index of filler dispersibility. Strain-G’ curves of silica
Table 3. Dynamic viscoelastic properties.
S-SBR [i] [ii] [iii]Functional group(s) non alkoxysilyl alkoxysilyl
+ aminoStrain Dependence 1% G' (MPa) 5.5 3.9 2.3
1% tanδ 0.129 0.095 0.078 at 50�C, 2% G' (MPa) 4.7 3.6 2.2 ω =100 rad/s 2% tanδ 0.155 0.113 0.082
3% G' (MPa) 4.2 3.3 2.23% tanδ 0.167 0.122 0.0855% G' (MPa) 3.6 3.0 2.25% tanδ 0.176 0.131 0.089Max tanδ 0.178 0.134 0.091min tanδ 0.082 0.071 0.071∆ tanδ 0.096 0.063 0.020∆ G' (MPa) 3.6 1.7 0.3
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compounds with S-SBRs are summarized in Figure 2. The vulcanizates of functionalized
S-SBRs showed lower ∆G’ than that of non-functionalized S-SBR. This result indicates the
improvement of silica dispersibility in compounds by the introduction of functional groups at
polymer chain-ends. This result and our previous study [8] strongly suggest that the
functionalized S-SBR can form chemical linkage with silica. As the result, share force
during compounding can be easily transmitted to filler aggregates through bound polymer.
This mechanism leads to the better dispersion of filler. The improvement of dispersibility of
silica in compounds provides with the reduction in hysteresis loss. The order of Payne
Effect of S-SBRs is summarized as follows.
Lower (Excellent); S-SBR [iii] >> S-SBR [ii] > S-SBR [i]
Primary amine & alkoxysilane alkoxysilane without modification
3. Direct observation of silica dispersibility in vulcanizates
3-1. Transmission Electron Microscope (TEM) observation
10-1 100 101106
107
Strain / %
G' /
Pa
non modified S-SBR [i] S-SBR with alkoxysily l group [ii] S-SBR with amino and alkoxysily l groups [iii]
Figure 2. Strain-G’ dependence of vulcanizates.
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The measurement of TEM was carried out in order to observe silica dispersibility in
vulcanizates directly. The images obtained are shown in Figure 3. The relatively large
silica agglomerates were observed in S-SBR [i] vulcanizate and these agglomerates were
dispersed inhomogeneously. On the other hand, S-SBR [iii] with both primary amino and
alkoxysilyl groups clearly showed better dispersibility of silica in comparison with S-SBR [i]
and [ii]. These observations agree with the results obtained from dynamic viscoelasticity.
3-2. Ultra-small-angle-X-ray scattering (USAXS) measurement
USAXS measurements of vulcanizates with S-SBRs prepared were carried out.
Figure 4 shows the USAXS profiles of silica aggregates in vulcanizates. S-SBR [i] showed
power-low scattering profile and no shoulder in the measured q range. This result indicates
that silica aggregates with various sizes were dispersed in S-SBR [i] vulcanizate. On the
other hand, the scattering profiles of S-SBR [ii] and [iii] with functional groups were
different from that of S-SBR [i]. Specifically, S-SBR [iii] with two kinds of functional
groups showed characteristic scattering profile with a shoulder in the q range of 0.05 to 0.1.
Figure 3. TEM images of cured S-SBR [i] ~ [iii] compounds.
S-SBR [iii] S-SBR [ii] S-SBR [i]
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This characteristic scattering profile indicates the existence of uniform silica aggregates (or
agglomerates) on size scale of < 100nm. These results obtained by USAXS experiments
suggest good correlation between silica dispersibility and hysteresis loss. The vulcanizate
with better silica dispersion showed higher degree of improvement in hysteresis loss
properties.
In order to estimate the size of the silica aggregates, we analyzed the scattering
curve of S-SBR [iii] based on unified Guinier/power-low approach, applied by Koga et al. for
the analysis of carbon black hierarchical structure [14]. They approximated the scattering
profiles as a unified function (eq 1) expressed by sum of the two components, that is, the
mass-fractal power-low scattering and the form factor F’ (q).
)(')3/exp()( 22 qBFqqRAqI mDg +−≅ − (1)
10-2 10-1
q / nm-1
Inte
nsity
/ a.
u.
non modified S-SBR [i] S-SBR with alkoxysily l group [ii] S-SBR with amino and alkoxysily l groups [iii]
Figure 4. USAXS profiles of silica particles in S-SBR [i] ~ [iii].
11
in which A and B are numerical constants, Dm is mass-fractal dimension and Rg is the radius
of gyration of silica aggregate. As the result of the best-fit between eq 1 and the data (shown
by the symbols in figure 5), Rg and Dm were estimated to be 83 nm and 2.3 respectively.
CONCLUSIONS
The functionalized S-SBR (S-SBR [iii]) with both primary amino and alkoxysilyl
groups at the same chain-end showed significant improvement in hysteresis loss and much
lower degree of strain dependence of G’ (Payne Effect). These behaviors indicate excellent
dispersion of silica in vulcanizates. The USAXS measurement of S-SBR [iii] showed a
characteristic scattering profiles in the q range of 0.05 to 0.1 and this result indicated that the
aggregates of silica on size scale of < 100nm were uniformly dispersed in compounds. The
size of silica aggregates calculated by using Koga adopted model was 83 nm of Rg. The
S-SBR vulcanizates with well dispersed aggregates of silica provided with excellent
10-2 10-1
q / nm-1
Inte
nsity
/ a.
u.
S-SBR [iii] unified function
Figure 5. Best-fitting result of unified function.
12
hysteresis loss. These results validated the results obtained by dynamic viscoelstic
measurements. Therefore, it can be said that the functionalized polymer is a very effective
tool for the improvement in hysteresis loss. Furthermore, the study on dispersibility of silica
in compounds of functionalized S-SBRs will lead us to further development of next
generation S-SBR.
ACKNOWLEDGEMENT
The synchrotron radiation experiments were performed at the BL19B2 in the
SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI)
(Proposal No. 2007A1907, 2007B1942).
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