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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.

2

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

3

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

4

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

5

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

6

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]

7

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

8

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.

9

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]

10

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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