Rubber compositions of tire tread

A rubber composition of tire tread, which comprises an emulsion-polymerized styrene-butadiene rubber with a bonded styrene content of not less than 40% by weight which is contained in an amount of 10 to 100% by weight in a rubber component, and a sodium-containing basic inorganic compound and/or a basic sodium salt of organic acid which is blended in an amount of 0.5 to 5.0 parts by weight per 100 parts by weight of the rubber component. Safe driving at a high speed can be realized by using the rubber compositions.

FIELD OF THE INVENTION 
The present invention relates to a rubber compositions of tire tread, which 
can improve the steering stability of high performance tires for passenger 
cars, particularly sports cars, and can ensure safety during high speed 
driving. 
BACKGROUND ARTS 
Recently, in advances with high performance of tires for passenger cars, it 
has been demanded for tires to drive safely at a high speed. This calls 
for an intensive requirement for improved properties of tire treads. 
Particularly, the requirements for such tire treads are directing to 
gripping against road surface (grip property; being capable of responding 
at high speed driving), and high abrasion resistance. 
Until now, in order to satisfy both grip properties and abrasion 
resistance, reinforcing characteristics of carbon black have been 
improved, for example, by enlarging specific surface area of carbon black 
particles (using carbon black with a smaller diameter), and by making 
their flocculats (structures) larger. For ensuring the safety during high 
speed driving, a high grip property at high speed driving has been 
obtained by using a styrene-butadiene rubber (hereinafter referred to as 
"SBR") with a high bonded styrene content, as a rubber material for a 
rubber component used for tire tread part. 
When using a SBR with a high bonded styrene content and a highly 
reinforcable carbon black, a high grip property can be obtained. However, 
the use thereof increases an amount of heat generation of the tread part 
to soften the tread rubber due to the heat generated during driving, which 
makes the rigidity of the tread part lower and then, renders the steering 
stability unstable. 
An object of the present invention is to provide rubber compositions of 
tire tread which can ensure safety during high speed driving, by 
preventing the lowering of rigidity of the tread part due to the heat 
generation of the tread rubber, and the lowering of steering stability. 
As a result of the inventor's intensive research, it has been found that 
the specific sodium-containing inorganic compounds and/or sodium salts of 
organic acids have the ability to prevent the lowering of physical 
properties of the tread rubbers in the heat-generated state of tires, i.e. 
at a high temperature. 
SUMMARY OF THE INVENTION 
According to the present invention, there can be provided a rubber 
composition of tire tread, which comprises an emulsion-polymerized SBR 
with a bonded styrene content of not less than 40% (percent by weight; 
hereinafter the same, unless otherwise noted) which is contained in an 
amount of 10 to 100% in a rubber component, and a sodium-containing basic 
inorganic compound and/or a basic sodium salt of organic acid which is 
blended in an amount of 0.5 to 5.0 parts (parts by weight; hereinafter the 
same, unless otherwise noted) per 100 parts of the rubber component. 
DISCLOSURE OF THE INVENTION 
The rubber composition of the present invention may be prepared by 
blending, per 100 parts of the rubber component, 0.5 to 5.0 parts of the 
sodium-containing basic inorganic compound and/or the basic sodium salt of 
organic acid, carbon black, and other usual components which are generally 
blended with rubber compositions. 
In the rubber component, the emulsion-polymerized SBR with the bonded 
styrene content of not less than 40%, preferably 40 to 50% is contained in 
an amount of 10 to 100%, preferably 20 to 100%, most preferably 50 to 
100%. When the amount of the emulsion-polymerized SBR with the bonded 
styrene content of not less than 40% is smaller than 10% or the bonded 
styrene content is smaller than 40%, the grip property tends to be worse. 
The rubber component may contain, other than the emulstion-polymerized SBR 
with the bonded styrene content of not less than 40%, a synthetic rubber, 
a natural rubber, or a mixture thereof. Examples of the synthetic rubber 
are, for instance, a SBR having silicon atom or tin- butadienyl bond and 
being prepared in a hydrocarbon solution in the presence of an organic 
lithium compound (so-called "solution-polymerized SBR"), an 
emulsion-polymerized SBR with the bonded styrene content of less than 40%, 
butadiene rubber (BR), isoprene rubber (IR), butyl rubber (IIR), 
halogenated butyl rubber, and the like. 
The bonded styrene content is measured according to the following method. 
About 5 ml of a sample which is ground as finely as possible is put into a 
beaker which is charged with about 100 ml of isopropyl alcohol to 
coagulate the finely ground sample. The coagulant is taken out and washed 
with isopropyl alcohol, and then the remaining alcohol is removed by 
clamping between filter paper sheets. The coagulant is put in 50 ml of a 
mixed solvent of toluene (90 parts by volume) and isopropyl alcohol (10 
parts by volume), and is heated on a hot plate of 80.degree. to 
100.degree. C. for 5 minutes. The obtained rubber solution is pored into 
isopropyl alcohol, and the precipitate is washed several times with 
isopropyl alcohol, then dried for one hour in a reduced pressure dryer of 
100.degree. C. under a vacuum of 24 to 25 mm Hg. The subsequent procedures 
are carried out according to JIS K 6383 (Testing Methods for Synthetic 
Rubber SBR). A bonded styrene content is calculated from a measured 
refractive index. 
Both the sodium-containing inorganic compound and the sodium salt of 
organic acid are basic compound which show a basicity pH of approximate 9 
to 14 when a 1N aqueous solution is measured. Examples of the 
sodium-containing inorganic compound are, for instance, sodium 
hydrogencarbonate, anhydrous sodium carbonate, sodium hydroxide, sodium 
polyphosphate, sodium metaphosphate and an admixture thereof. Compounds 
which contain sodium but are not basic such as sodium sulfate, sodium 
thiosulfate, sodium nitrate and sodium chloride are excluded. Sodium 
hydrogencarbonate is especially preferable in view of its less bad 
influences on human bodies, and since sodium hydrogencarbonate has a low 
basicity and is effective for preventing the lowering of physical 
properties of the tread rubbers in the heat-generated state of tires. 
Examples of the sodium salt of organic acid are, for instance, sodium 
acetate, sodium propinate, sodium butyrate, sodium benzoate, sodium 
oleate, sodium alginate, sodium carboxymethyl cellulose, and an admixture 
thereof. Sodium benzoate is especially preferable in view of its excellent 
reactivity with rubbers. The sodium-containing inorganic compound and the 
sodium salt of organic acid may be used in combination. 
The sodium-containing inorganic compound and/or the sodium salt of organic 
acid (hereinafter they may be referred to as "sodium-containing compound") 
is blended in an amount of 0.5 to 5.0 parts, preferably 0.5 to 4.0 parts, 
more preferably 1.0 to 4.0 parts per 100 parts of the rubber component. 
When the amount of the sodium-containing compound is less than 0.5 part, 
there is a tendency that the effect of preventing the lowering of physical 
properties at a high temperature cannot be obtained. When more than 5 
parts, the sodium-containing compound tends to be ununiformly dispersed 
and the resulting rubber composition tends to be inferior in abrasion 
resistance and crack resistance. 
As the carbon black, there may be used, for example, a furnace black which 
is usually utilized for reinforcing rubbers. 
In the rubber compositions of tire tread of the present invention, it is 
preferable that the carbon black is one which has an average particle size 
of not more than 25 nm (namely carbon blacks of ISAF or SAF, or a higher 
class), and that the carbon black is blended in an amount of not less than 
70 parts per 100 parts of the rubber component. In the present invention, 
the average particle size is number average particle size and measured by 
using a transmission electron microscope. For example, when a carbon black 
which has an average particle size of more than 25 nm (a carbon black 
so-called HAF or a lower class) is used alone, the required grip property 
cannot be obtained. Examples of the carbon black which has average 
particle size of not more than 25 nm are, for instance, ISAF (having an 
average particle size of 20 to 25 nm) or SAF (having an average perticle 
size of 11 to 19 nm), a fine carbon black having an average particle size 
of not more than 10 nm which belongs to a hard region of a smaller average 
particle size, and the like. The amount of the carbon black is more 
preferably 70 to 110 parts, most preferably 80 to 110 parts per 100 parts 
of the rubber component. When the amount of such carbon black is less than 
70 parts, there is a tendency that a high grip property cannot be 
obtained. 
As the other usual components, there may be used additives which are used 
in rubber kneading in usual production processes of tire treads, namely, 
vulcanizing agents, vulcanization accelerators, vulcanizing aids, rubber 
antioxidants, process oils and the like. Their blending amounts are not 
specifically restricted but within ranges which do not obstruct the 
purpose of the present invention. 
The rubber composition of tire tread of the present invention is prepared, 
usually, first by base-kneading the rubber component, the 
sodium-containing compound and the additives other than the vulcanizing 
agent and the vulcanization accelerator, with a banbury mixer, then 
admixing thereto the valucanizing agent and the valcanization accelerator 
with rolls or a banbury mixer. 
Thus prepared rubber composition of tire tread is extruded in a given form. 
After molding, a tread is produced by vulcanization. A tire having 
characteristic features regarding the rubber composition of the tire tread 
of the present invention can be obtained by a usual production process of 
tires.

The present invention is more specifically described and explained by means 
of the following Examples. The present invention, however, is not limited 
to the following Examples. 
EXAMPLE 1 
A rubber composition of tire tread was prepared by base-kneading a mixture 
of 137.5 parts (37.5 parts of an aromatic oil is extended) of an 
emulsion-polymerized SBR (available from Nippon Zeon Co., Ltd.) (referred 
to as SBR (A) in the following tables), 80.0 parts of a carbon black N110 
(having an average particle size of 18 nm), 0.5 part of sodium 
hydrogencarbonate, 2.0 parts of Sunnoc Wax, 2.0 parts of a rubber 
antioxidant 6c (N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine), 2.0 
parts of stearic acid and 3.0 parts of zinc oxide with a BR type banbury 
mixer (having a volume of 1.8 liters; available from Kobe Steel, Ltd.) for 
four minutes, and subsequently admixing 2.0 parts of sulfur and 1.0 part 
of a vulcanization accelerator CZ with a eight inches roll (available from 
Osaka Roll Kabushiki Kaisha). 
The resulting rubber composition of tire tread was shaped into a rubber 
sheet having a thickness of 2 to 3 mm with the above-mentioned roll. These 
rubber sheets were laminated to form a extrusion type tread. A steel 
radial tire (having a size of 225/50R16) was made by using the tread 
rubber of the extrusion type. 
The above-mentioned rubber composition of the tire tread and the tire were 
tested as reported in the following items. The results are shown in Table 
1. 
Viscoelasticity (E*) 
The rubber composition was vulcanized at a temperature of 170.degree. C. 
for 15 minutes, and shaped into a test specimen having a thickness of 2 mm 
and a width of 4 mm. 
Viscoelasticities E* at 50.degree. C. and at 90.degree. C. of the test 
specimen are measured respectively under conditions of an initial 
elongation of 10%, a dynamic strain of 0.5% and a vibration frequency of 
10 Hz with a viscoelastic spectrometer (available from Kabushili Kaisha 
Iwamoto Seisakusho). Also, a ratio of E* at 90.degree. C. to E* at 
50.degree. C. was calculated. 
Chemicals dispersibility 
The rubber composition was vulcanized at a temperature of 170.degree. C. 
for 15 minutes. A small chip of the resulting rubber was cut out and 
sliced into a thin film by a microtome (type 1400, available from LEITZ). 
Chemicals dispersibility was evaluated by observing the thin film with a 
microscope. 
Actual car test 
The steel radial tires made in the same manner as disclosed above were 
attached to a domestic 3000 cc passenger car. A steering stability and 
response property of the actual car and a grip limit height of the actual 
car were tested on the Okayama testing course of Sumitomo Rubber 
Industries, Ltd. with two passengers. The results of sensory evaluations 
are represented as an index to the blank value (100) of Comparative 
example 1 in which no sodium-containing compound is blended. The grip 
limit height of the actual car was evaluated by running the car on an 
asphalt road circularly at a radius of 40 m and measuring a speed at which 
the circle having a radius of 40 m cannot be traced by the car even if the 
driver tries to maintain the circle by turning the steering wheel. 
EXAMPLES 2 to 3 AND COMATIVE EXAMPLES 1 to 3 
Rubber compositions of tire tread were prepared in the same manner as in 
Example 1 by using the same blend as that of Example 1 except that sodium 
hydrogencarbonate in a powder or a granular state was used in the 
respective amounts described in Table 1. Steel radial tires were made in 
the same manner as in Example 1 by using these rubber compositions of tire 
tread. 
The same tests as in Example 1 were carried out with respect to the 
resulting rubber compositions of tire tread and tires. The results are 
shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Comparative 
Comparative Comparative 
Example 1 
Example 2 
Example 1 
Example 2 
Example 3 
Example 3 
__________________________________________________________________________ 
SBR (A) .sup.*1 
137.5 137.5 137.5 
437.5 
137.5 
137.5 
Carbon black N110 
80.0 80.0 80.0 80.0 80.0 80.0 
Sodium hydrogencarbonate 
-- 0.3 0.5 2.0 5.0 10.0 
E* @50.degree. C. 
210 212 221 214 216 221 
E* @90.degree. C. 
90 97 111 113 116 117 
E* @90.degree. C./E* @50.degree. C. 
0.43 0.46 0.50 0.53 0.54 0.53 
Chemicals dispersibility 
-- good good good Slightly 
bad 
bad 
Steering stability and 
100 100 105 108 110 109 
response property of 
actual car 
Grip limit height 
100 100 100 101 100 100 
of actual car 
__________________________________________________________________________ 
EXAMPLES 4 to 5 AND COMATIVE EXAMPLE 4 
Rubber compositions of tire tread were prepared in the same manner as in 
Example 1 by using the same blend as that of Example 2 except that the 
rubber component was changed as shown in Table 2. In Table 2, SBR (A) is 
an emulsion-polymerized SBR with a bonded styrene content of 45% 
(available from Nippon Zeon Co., Ltd.), SBR (B) is an emulsion-polymerized 
SBR with a bonded styrene content of 40% (available from Nippon Zeon Co., 
Ltd.), SBR (C) is an emulsion-polymerized SBR with a bonded styrene 
content of 35% (available from Nippon Zeon Co., Ltd.), SBR (D) is a 
solution-polymerized SBR with a bonded styrene content of 29% and a vinyl 
content of 39% by mole (available from Nippon Zeon Co., Ltd.). Steel 
radial tires were made in the same manner as in Example 1 by using these 
rubber compositions of tire tread. 
The same tests as in Example 1 were carried out with respect to the 
resulting rubber compositions of tire tread and tires. The results are 
shown in Table 2. 
TABLE 2 
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Comparative 
Example 4 
Example 4 Example 5 
______________________________________ 
SBR (A) .sup.*1 27.5 
SBR (B) .sup.*2 
137.5 
SBR (C) .sup.*3 137.5 
SBR (D) .sup.*4 110.0 
Carbon black N110 
80.0 80.0 80.0 
Sodium hydrogencarbonate 
2.0 2.0 2.0 
E* @50.degree. C. 
225 230 226 
E* @90.degree. C. 
116 122 124 
E* @90.degree. C./E* @50.degree. C. 
0.52 0.53 0.55 
Chemicals dispersibility 
good good good 
Steering stability and re- 
109 108 108 
sponse property of 
actual car 
Grip limit height 
98 92 99 
of actual car 
______________________________________ 
.sup.*1 Emulsion-polymerized SBR with a bonded styrene content of 45% 
.sup.*2 Emulsionpolymerized SBR with a bonded styrene content of 40% 
.sup.*3 Emulsionpolymerized SBR with a bonded styrene content of 35% 
.sup.*4 Solutionpolymerized SBR with a bonded styrene content of 27% and 
vinyl content of 39% by mole 
All for SBR containing 37.5 parts of extending oils per 100 parts of the 
polymer 
EXAMPLES 6 to 7 AND COMATIVE EXAMPLES 5 to 9 
Rubber compositions of tire tread were prepared in the same manner as in 
Example 1 by using the same blend as that of Example 2 except that sodium 
hydrogencarbonate was changed to the inorganic compounds shown in Table 3. 
Steel radial tires were made in the same manner as in Example 1 by using 
these rubber compositions of tire tread. 
The same tests as in Example 1 were carried out with respect to the 
resulting rubber compositions of tire tread and tires. The results are 
shown in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Comparative 
Comparative 
Comparative 
Comparative 
Comparative 
Example 6 
Example 7 
Example 5 
Example 6 
Example 7 
Example 8 
Example 9 
__________________________________________________________________________ 
SBR (A) 137.5 
137.5 
137.5 137.5 137.5 137.5 137.5 
Carbon black N110 
80.0 80.0 80.0 80.0 80.0 80.0 80.0 
Anhydrous sodium 
2.0 
carbonate 
Sodium hydroxide 
2.0 
Magnesium carbonate 2.0 
Calcium carbonate 2.0 
Sodium sulfate 2.0 
Sodium thiosulfate 2.0 
Sodium nitrate 2.0 
E* @50.degree. C. 
221 183 188 185 163 159 169 
E* @90.degree. C. 
120 107 92 89 81 78 80 
E* @90.degree. C./E* @50.degree. C. 
0.54 0.58 0.49 0.48 0.50 0.49 0.47 
Chemicals good good good good bad bad bad 
dispersibility 
Steering stability and 
109 112 102 98 96 -- -- 
response property of 
actual car 
Grip limit height of 
101 99 100 99 98 -- -- 
actual car 
__________________________________________________________________________ 
EXAMPLES 8 to 12 
Rubber compositions of tire tread were prepared in the same manner as in 
Example 1 by using the same blend as that of Example 2 except that the 
rubber component, the carbon black and the aromatic oil were blended as 
shown in Table 4. The carbon black N110 has an average particle size of 18 
nm, the carbon black N220 has an average particle size of 22 nm, and the 
carbon black N330 has an average particle size of 28 nm. Steel radial 
tires were made in the same manner as in Example 1 by using these rubber 
compositions of tire tread. 
The same tests as in Example 1 were carried out with respect to the 
resulting rubber compositions of tire tread and tires. The results are 
shown in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Example 8 
Example 9 
Example 10 
Example 11 
Example 12 
__________________________________________________________________________ 
SBR (A) 137.5 
110.0 
110.0 110.0 110.0 
Natural rubber 20.0 
BR 20.0 20.0 20.0 
Carbon black N110 .sup.*1 
80.0 80.0 
Carbon black N220 .sup.*2 
80.0 110.0 70.0 
Carbon black N330 .sup.*3 
Aromatic oil 7.5 7.5 37.5 
Sodium 2.0 2.0 2.0 2.0 2.0 
hydrogencarbonate 
E* @50.degree. C. 
231 230 233 243 215 
E* @90.degree. C. 
121 123 126 123 116 
E* @90.degree. C./E* @50.degree. C. 
0.52 0.53 0.54 0.51 0.54 
Chemicals dispersibility 
good good good good good 
Steering stability and 
110 108 109 110 105 
response property of 
actual car 
Grip limit height of 
97 98 98 108 97 
actual car 
__________________________________________________________________________ 
.sup.*1 average particle size: 18 nm 
.sup.*2 average particle size: 22 nm 
.sup.*3 average particle size: 28 nm 
EXAMPLES 13 to 23 
Rubber compositions of tire tread were prepared in the same manner as in 
Example 1 by using the same blend as that of Example 2 except that sodium 
hydrogencarbonate was changed to the sodium-containing compounds as shown 
in Table 5. Steel radial tires were made in the same manner as in Example 
1 by using these rubber compositions of tire tread. 
The same tests as in Example 1 were carried out with respect to the 
resulting rubber compositions of tire tread and tires. The results are 
shown in Table 5. 
TABLE 5 
__________________________________________________________________________ 
Example 13 
Example 14 
Example 15 
Example 16 
Example 17 
Example 18 
__________________________________________________________________________ 
SBR (A) 137.5 137.5 137.5 137.5 137.5 137.5 
Carbon black N110 
80.0 80.0 80.0 80.0 80.0 80.0 
Sodium polyphosphate 
2.0 
Sodium metaphosphate 
2.0 
Sodium acetate 2.0 
Sodium butyrate 2.0 
Sodium propionate 
Sodium benzoate 2.0 
Sodium oleate 
Sodium alginate 
Sodium carboxymethyl 
cellulose 
E* @50.degree. C. 
217 228 225 226 224 221 
E* @90.degree. C. 
109 120 126 130 131 121 
E* @90.degree. C./E* @50.degree. C. 
0.50 0.53 0.56 0.58 0.58 0.55 
Chemicals dispersibility 
permissible 
permissible 
good good good good 
Steering stability and 
105 109 110 110 111 108 
response property of 
actual car 
Grip limit height 
101 103 104 104 105 101 
of actual car 
__________________________________________________________________________ 
Example 19 
Example 20 
Example 21 
Example 22 
Example 23 
__________________________________________________________________________ 
SBR (A) 137.5 137.5 137.5 137.5 137.5 
Carbon black N110 
80.0 80.0 80.0 80.0 80.0 
Sodium polyphosphate 
Sodium metaphosphate 1.0 
Sodium acetate 1.0 1.0 
Sodium butyrate 
Sodium propionate 
Sodium benzoate 
Sodium oleate 
2.0 1.0 
Sodium alginate 2.0 
Sodium carboxymethyl 2.0 
cellulose 
E* @50.degree. C. 
215 220 220 225 219 
E* @90.degree. C. 
114 117 120 121 118 
E* @90.degree. C./E* @50.degree. C. 
0.53 0.53 0.55 0.54 0.54 
Chemicals dispersibility 
good good good permissible 
permissible 
Steering stability and 
108 108 110 108 109 
response property of 
actual car 
Grip limit height 
100 101 102 102 101 
of actual car 
__________________________________________________________________________ 
As is clear from Table 1, when sodium hydrogencarbonate is not contained in 
the rubber composition, or blending amount is less than 0.5 part, 
viscoelasticity at 9.degree. C. is low, and when the blending amount is 
more than 5 parts, chemicals dispersibility is bad. As is clear from Table 
2, when emulsion-polymerized SBR with a bonded styrene content of not less 
than 40% is not contained in the rubber composition, grip limit height of 
the actual car is worse. As is clear from Table 3, when the inorganic 
compound does not contain sodium, or the inorganic compound is not an 
appropriate compound even if it contains sodium, viscoelasticity at 
90.degree. C. is low. As is clear from Table 4, when not less than 70 
parts of a carbon black having an average particle size of not more than 
25 nm is contained in the rubber composition, grip limit height of the 
actual car is not worse and steering stability and response property are 
excellent. As is clear from Table 5, when a basic sodium salt of organic 
acid is used, viscoelasticities at 50.degree. C. and 90.degree. C. are 
especially high and chemicals dispersibility is also excellent. 
When the rubber composition of tire tread of the present invention is used, 
safe driving at a high speed can be realized, since a lowering of the 
rigidity of the tread part due to heat generation of the tread is small, 
steering stability and response property of the actual car are excellent, 
and grip limit height of the actual car is high.