CA1289044C

CA1289044C - Pneumatic tire

Info

Publication number: CA1289044C
Application number: CA000521126A
Authority: CA
Inventors: Yasushi Kikuchi; Kazuyoshi Kayama; Yukiji Suzuki; Hiroyuki Watanabe; Shuichi Akita
Original assignee: Yokohama Rubber Co Ltd; Nippon Zeon Co Ltd
Current assignee: Zeon Corp; Yokohama Rubber Co Ltd
Priority date: 1985-10-23
Filing date: 1986-10-22
Publication date: 1991-09-17
Anticipated expiration: 2008-09-17
Also published as: KR870003885A; JPS6296546A; JPH0464624B2; DE3635587A1; DE3635587C2; KR960000901B1

Abstract

Abstract A pneumatic tire comprising a tread portion consisting of at least two layers including an outer surface side rubber layer and an inner side rubber layer, wherein said outer surface side rubber layer comprises natural rubber and/or polyisoprene rubber, a specified styrene-butadiene copolymer rubber, and a specified carbon black.

Description

~8~

PNEUM~TIC TIR~
Back~round The present invention relates to a pneumatic tire, which causes less heat build-up in the tire tread portion during running enables reduced fuel consumption, and which is remarkably improved in braking power not only on a wet road surface but also on a snow-covered or frozen road surface.
In order to meet the recent social needs for material resource saving and energy saving, a demand for reduced fuel consumption of automobiles is more and more increased. Thus, not only the development of car bodies themselves including engines which consume only a smaller amount of fuel has been made, but also studies of reduced lS fuel consumption tires which enables decreased energy loss have been increasingly made.
A rubber material having a smaller hysteresis loss has heretofore been sought as a raw material for a reduced fuel consumption tire. Among others, the tread 201 portion is said to be responsible ~or S0~ or more of the hysteresis loss~ Thus, natural rubber, polyisoprene rubber, polybutadiene rubber, styrene-butadiene copolymer rubber of a low glass transition temperature (Tg), and blends thereof, all of which have a small hysteresis loss, have been used as the rubber component of the . ' .

~2~

tread portion. In an aspect of the rubber composition~
there have heretofore been used rubber compositions containing a relatively small amount of carbon black having a relatively large particle size and as small an amount of a softener, such as an aromatic oil, as possible.
Recently, more and more kinds o functions on higher levels have been sought.for in a tire. For example, in the reduced fuel consumption tire developed as described above, a high braking performance capable of adapting the tire to a wet road surface and a snow-covered or frozen road surface from the viewpoint of safety without detriment to the reduced fuel consumption have been seriously demanded.
Where use is made of natural rubber, polyisoprene rubber, polybutadiene rubber, or low-Tg styrene-butadiene copolymer rubber, all having a small hysteresis loss as described above, however, a tire has ' a defect that the braking power particularly on a wet 20l road surface (wet skid resistance) is poor, leading to an extremely poor running stability. Where carbon black having a large particle size is used, deterioration of prop0rties such as braking power on a wet road surface and abrasion resistance cannot be avoided although the fuel consumption can be remarkably reduced. Further, ~289~

since the rubber composition used in the tread portion of a reduced fuel consumption tire contains a smaller amount of a softener blended therein, it is liable to harden at low temperatures, with the result that the braking power on a snow-covered or frozen road surface (ice skid resistance) is not on a fully satis~actory level.
On the other hand, high-vinyl polybutadiene and high-vinyl styrene-butadiene copolymer rubbers containing 50~ or more of 1,2-vinyl bonds have recently been proposed as a material satisfying both of the reduced fuel consumption and enhanced wet skid resistance.
However, since these rubbers have a high glass transition temperature ~Tg), they are poor in abrasion resistance and apt to harden at low temperatures, leading to notably poor braking power on a snow-covered or frozen road surface. Thus, they are also insufficient in satisfying all the above-mentioned properties.
As discussed above, there have been no i proposals for tires satisfying all the desired properties 20, such as reduced fuel consumption, and enhanced braking power on wet roads and on a snow-covered or frozen road surface. Particularly in winder when the road surface becomes slippery as a resu~t of snowfall or freezing, ordinary tires including reduced fuel consumption tires exhibit a very low braking power on the road surface as ~ . .

, ~28~

mentioned above. Thus, the use of a snow tire is inevitable. This forces a user to spend a considerable time and labor in exchanging tires. In view of this, appearance of an all-season tire, which is essentially an ordinary summer season tire but which can satisfy the above-mentioned three desired properties, has been eagerly demanded.
Summary The present invention has been made with a view to meeting such a demand. Thus, an object of the present invention is to provide a pneumatic tire, which causes less heat build-up in the tire tread portion during running and enable reduced fuel consumption, and which is remarkably improved in braking power not only on a wet road surface but also on a snow-covered or frozen road surface. This tire is a pneumatic all-season tire capable of being used throughout the whole year irrespective of summer or winter.
; The inventors of the present invention have 20, made intensive investigations with a view to attaining the ob~ect and, as a result, have found that the use in the tread portion of a rubber composition containing a novel styrene-butadiene copolymer rubber can provide a pneumatic tire having a reduced fuel consumption and an excellent braking power on not only a wet road surface but also a snow-covered or frozen road surface. The present invention has been completed based on this finding.
Specifically, in accordance with the present invention, there i5 provided a pneumat.ic tire comprising a tread portion consisting of at least two layers including an outer surface side rubber layer and an inner side rubber layer, wherein the above-mentioned outer surface side rubber layer comprises (1) 100 parts by weight of a rubber component composed of 20 to 90 paxts by weigh-t of natural rubber and/or polyisoprene rubber, and 10 to 80 parts by weight of a styrene-butadiene copolymer rubber comprising 5 to 30 wt.% of bonded styrene units and 10 to 85 wt.% of 1,2-vinyl bonds in the butadlene unlts and having at a molecular terminal or in a chain thereof an atomic group introduced thereinto and represented by the following formula:

- CH - polymer chain I ~R
Rl - C - N \
¦ R3 OH (or SH) wherein Rl, R2 and R3 each stands ~or hydrogen or a substltuent; Rl may be bound with R2 or R3 to form a ring, and 30 to 80 parts by weight of carbon black serving as a ~2~

reinfcrcing agent and having a specific area (N2 SA) of 60 to 140 m /g and a dibutyl phthalate absorption (DBP
absorption) of 100 to 150 ml/100 g.
The foregoing object and other objects, and features of the present invention will become apparent from the following description.

~ ig. 1 is an illustrative meriodional semi-crosssectional view of one example of a tire according to the present invention.
The Preferred E~bodiment In Fig. l, the symbol T refers to a tread portion constituted of a cap layer 1 (outer sur~ace side rubber layer) and an underlayer 2 (inner~side rubber layer). Numeral 3 refers to a carcass layer stretched between a pair of left and right bead wires 4 and 4. In the tread portion T, a belt layer 5 is disposed in such a way that it surrounds the outer circumference of the carcass layer 3. Numerals 6 and 7 refer to a tread 20~ groove and a side portion, respectively.
(1) In the present invention, the outer surface side rubber layer 1 comprises 20 to 90 parts by weight of natural and/or polyisoprene rubber and lO to 80 parts by weight of specific styrene-butadiene copolymer rubber (total rubber component: 100 parts by weight). This is , because any formulation outside the range as specified above disadvantageously deteriorates any one of reduced fuel consumption,enhanced braking power on a wet road surface, and enhanced braking power on a snow-covered OI
frozen road surface. However, 30 parts by weight or less of other diene rubber such as polybutadiene, acrylonitrile-butadiene copolymer, or non-modified styrene-butadiene copolymer rubber may be ~urther incorporated.
In the styrene-butadiene copolymer rubber used herein, an atomic group represented by the following formula (I) is introduced into a molecular terminal or chain:
CH - polymer chain ,R2 R1 - C - N~ .--- (I) OH (or SH) wherein Rl, R2 and R3 each stands for hydrogen or a `
substituent; Rl may be bound with R2 or R3 to form a ring.
20, Introduction of the atomic group represented by the above-mentioned formula (I) is effected by reacting a compound (hereina~ter referred to as the 'tcompound A") represented by the Eollowing formula:

8~

~R2 Rl - C - N \

(whexein M stands for an O atom or an S atom Rl, R2 an~
R3 each stands for hydrogen or a substituent; Rl may be bound wi.th R2 or R3 to form a ring), with a styrene-butadiene copolymer.
Examples of the compound A include N, N-dimethylformamide and N,N-diethyl~ormamide; N,N-diethylacetamide; aminoacetamide, N,N-dimethyl-N',N'-dimethylaminoacetamide, and N-phenyldiacetamide;
N,N-dimethylacrylamide and N,N-dimethylmethacrylamide;
propionamide and N,N-dimethylpropionamide; 4-pyridylamide ;15 and N,N-dimethyl-4-pyridylamide; N,N-dimethylbenzamide, p-aminobenzamide, N',N'-(p-dimethylamino)benzamide, N,N-dimethyl-N'-(p-ethylamino)benzamide, and N-acetyl-N-2-naphthylbenzamide; nicotinamide and N,N-diethyl-nicotinamide; succinamide, maleamide, and N,N,N',N'- i -tetramethylmaleamide;succinimide, maleimide, N-methylmaleimide, N-methylphthalimide, 1,2-cyclohexane-dicarboximide, and N-methyl-1,2 cyclohexanedicarboximide;
oxamide, 2-furamide, N,N-N',N'-tetramethyloxamide, and N,N-dimethyl-2-~uramide; N,N-dimethyl-8-quinoline-carboxamide; N,N-dimethyl-p-aminobenzalacetamide and N,N-dimethyl-N',N'-(p'-dimethylamino)cinnamylidene-''; '`', , '`

`, ' ~L2~

acetamide; N,N-dimethyl-N',N'-(2-dime-thylamino)vinylamide and N'-(2-methylamino)vinylamide; urea, N,N'-dimethylurea, and N,N,N',N'-tetramethylurea; methyl carbamate and methyl N,N-diethylcarbamate; ~-caprolactam, N-methyl-s-caprolactam, N-acetyl--caprolactam, 2-pyrrolidone, N-methyl-2-pyrrolidone, N-acetyl-2-pyrrolidone, 2-piperidone, N-methyl-2-piperidone, 2-quinolone, N-methylquinolone, 2-indolinone, and N-methyl-2-indolinone; and isocyanuric acid and N,N',N"-trimethyl-10 isocyanuric acid; as well as their corresponding sulfur- -containing compounds. Among them, the most preferred are compounds having an alkyl group bonded to nitrogen.
Examples of the method of preparing a styrene-butadiene copolymer rubber having an atomic group introduced thereinto and represented by the above-mentioned formula (1) include (a) a process comprising polymerization of styrene and butadiene in the presence of an alkali metal based catalyst and/or an alkaline earth metal based catalyst, and addition of the compound 20l A to the polymerization reaction mixture solution; and (b) a process comprising addition-reaction of a styrene-butadiene copolymer with an alkali metal and/or an alkaline earth metal in a solution o~ the copolymer dissolved in an appropriate solvent, and addition of the compound A to the solution to further effect the reaction.

.~28~

Al~ali metal based catalysts that can be use~
in the polymerization and addition reactions as mentioned above include metals themselves such as lithium, rubidium, and cesium; and complexes thereof with a hydrocarbon compound or a polar compound (e.g., n-butyllithium, 2-naphthyllithium, potassium-tetrahydrofuran complex, and potassium-diethoxyethane complex). Examples of alkaline earth metal based catalysts include catalyst systems comprising as the main component a compound of barium, strontium, calcium, or the like, as disclosed in Japanese Patent Laid-Open Nos.115,590/1976, 9,090/1977, and 100,1~6/1982O These metal based catalysts are not particularly limited in so far as they can be used as a catalyst for ordinary solution polymerization.
After completion of the reaction, an unsaturated rubber polymer having compound A units introduced thereinto is recovered from the reaction mixture solution by an ordinary separation method such 20' as addition of a coa~ulating agent such as methanol, or stripping with steam. In the obtained unsaturated rubber polymer, the compound A is introduced in the form of an atomic ~roup represented by the following fo.rmula at a molecular terminal or in a chain of the pol~mer:

' .
.
,: . , . ~ . -:
. ~ .

'"'' '' ~ v~

- CH - polymer chain ~R2 Rl - C - N ~
¦ R3 OH (or SH) wherein Rl, R2 and R3 each stands for hydrogen or a substituent; Rl may be bound with R2 or R3 to orm a ring.
Although the position of introduction of the compound A may be anywhere at the terminals or other positions of the molecular chain, the terminals of the molecular chain are preferred. The use of a polymer obtained in the reaction of a copolymer having a dienyl structure at the terminals of the molecular chain with the compound A contributes to further reduce fuel consumption.
An indispensable constituent of the present invention is incluslon of an atomic group represented by the above-mentioned formula (I) in the molecular chain of the styrene-butadiene copolymer rubber. A rubber 20l composition containing this styrene-butadiene copolymer shows a remarkably improved impact resilience as compared with a rubber compositi.on comprising an ordinary styrene butadiene copolymer rubber having no atomic group represented by the above-mentioned formula (I).
Therefore, a pneumatic tire using this rubber composition , - 12 ~

in the tread can significantly reduce the fuel consumption while maintaining other properties on high levels.
The styrene-butadiene rubber to be used in the present invention comprises 5 to 30 wt.~ preferably 10 to 30 wt.%, of bound styrene units and 10 to 85 wt.%, preferably 30 to 75 wt.%, of 1,2-vinyl bonds in the butadiene units.
When the amount of bonded styrene units is less than 5 wt.%, the wet skid resistance of the rubber composition is reduced, unfavorably weakening the braking power of the tire on a wet road surface~ On the other hand, when it exceeds 30 wt.%, the braking power on a snow-covered or frozen road surface as well as the lS abrasion resistance are disadvantageously deteriorated although the braking power on a wet road surface is increased.
When the amount of 1,2-vinyl bonds is less than 10%, not only is a difficulty encountered in manufac-20l turing, but also the effect of improving the brakingpower on a wet road sur~ace is small. On the other hand, when it exceeds 85%, not only is heat build-up increased, but also the braking power on a ~rozen road sur~ace and the abrasion resistance are disadvantageously decreased largely.

The styrene-butadiene copolymer rubber to be used in the present invention may contain a branched polymer having branches bonded with a tln-butadienyl bond in order to provide a good processability in tire manufacturing. However, when too many molecular terminals of the styrene-butadiene copolymer rubber are utilized for formation of tin-butadienyl bonds, the number of the effective molecular terminals to be utilized for introduction of an atomic group represented by the above-mentioned formula (I) is decreased. Thus, in order to secure the tire performance aimed at in the present invention while maintaining the good proces-sability in tire manufacturing, it is preferred that the ratio, A/B, of the branched polymer (A) having branches bonded with a tin-butadienyl bond to the polymer (B) having at least one atomic group introduced thereinto and represented by the above-mentioned formula (I) be within a range of 0.1 to ~Ø
(2) In the present invention, the outer surface 20l side rubber layer 1 contains as a reinforcing agent 30 to 80 wt.%, based on 100 parts by weight of the raw material rubber, of carbon black.
When the amount of carbon black is less than 30 parts by weight, neither sufficient braking power on a wet road surface nor sufficient abrasion resistance can be secured in the tire. On the other hand, when it exceeds 80 parts by weight, not only is the fuel consumption of the tire increased, but also a liability to tire slip on a frozen road surface ls unfavorably increased due to an increase in hardness of the tread portion at low temperatures.
The properties required o carbon black to be used herein include a specific surface area (N2 SA) of 60 to 140 m /g, preferably 75 to 120 m2/g, as measured by the nitrogen adsorption method, and a dibutyl phthalate absorption (DBP absorption) of 100 to 150 ml/
100 g, preferably ll0 to 140 ml/100 g.
When the N2 SA is less than 60 m2/g, the braking power o~ a wet road surface and abrasion resistance of the tire are notably deteriorated although the fuel consumption is reduced. On the other hand, when the N2 SA exceeds 140 m2/g, the fuel consumption of the tire is unfavorabl~ increased.
When the DBP absorption is less than 100 ml/
20l 100 g, not only is the abrasion resistance of the tire lowered, but also the controllability is disadvantageously deteriorated. When it e~ceeds 150 ml/100 g, the tread portion of the tire is disadvantageously liable to harden at low temperatures, leading to a decrease in braking pcwer on a snow-c~vered or froz~n road 2urfaFe-' .
', ~8~

Further, it is preferred from the viewpoint ofreducing the fuel consumption that carbon black have a half-value width (~Dst), in the aggregate distri~ution, of 85 to 130 m~ as measured by the centrifugal sedimentation method.
Although the characteristic Eeature of the present invention consists in the use of a rubber composition comprising the above-mentioned rubber component and carbon black in the outer surface side rubber layer of a pneumatic tire having a tread portion consisting of at least two layers, this rubber composition may contain a vulcanizing agent, a vulcani-zation accelerator, a vulcanization aid, an antioxidant and/or a softener, alI of which are compounding ingredients commonly used in the rubber industry. The tire of the present invention can be applied to all tires including not only passenger car tires but also truck and bus tires without any particular limitation of the kind of tire.
20l Examples and Comparative Examples will now be described.
Examples and Comparative Examples:
The properties of a styrene-butadiene copolymer rubber (SB~) used herein are shown in Table I, while those of carbon black are shown in Table II.

:

`' ' .

Rubber compositions having respective formula-tions (in parts by weight) as listed in Table III were used in a cap layer of a tire having a two-layer tread structure as illustrated in Fig. 1 to produce 18 kinds of tires 165 SR 13. Each tire was evaluated in a wet road braking test, a frozen road braking test, and a `
rolling resistance test by using a stock car according to the following methods. The results are also shown in Table III.
(1) Wet road braking test:
Water was sprinkIed over the surface o~ an asphalt pavement. The brake distance was measured when the brake was applied at a speed of 40 km/hr. Evaluation was made in terms of an index relative to an index of 100 as defined for the tire of Comparative Example 1. The higher the index, the better the braking performance~
t2) Frozen road braking test:
The distance from brake application to complete stop of the car was measured when the brake was applied 20' in the course of running on a road surface perfectly frozen at an air temperature of -5 to -10 C at a speed of ~0 km/hr. Evaluation was made in terms of an index relative to an index of 100 as defined for the tire of Comparative Example 1. The higher the index, the better the braking performance.

;
'"

~2~

(3) Rolling resistance test:
Preliminary running was carried out on an indoor test drum of 707 mm in diameter with an internal tire pressure of 1.9 kg/cm2 under a load of 420 kg at a speed of 100 km/hr for 30 min. Thereafter, the rolling resistance was measured at a speed of 60 km/hr.
Evaluation was made in terms of an index xelative to an index of 100 as defined for the tire of Comparative Example 1. The lower the index, the lower the rolling resistance and hence the smaller the fuel consumption.

~ 2~

.
o C
a~
.c ~
a O ~ c rd ~ ~ u~ ~ O
O ~ H ~1 O O ~i 0 0 0 4~
O h O tlO r-l ?~
~rl ~ 6 ~i ~d O ~ O a o a ,, . o~ a~
6 a O O ~ O VO ^
4~ ~ o . ~. O
0 ~4 ~: ~ ~a o ' ~ ~ 6h ~ ~ ~o l l l E3 ~ a '6 H ~ ~ 6 a . ~ ~
a o c .,, ~ JJ~ ,, ..
~L~ ~ ~ ~rl ~ ~ ~2) ,_, 6 V O ~ 6 t~ ~ ~rl ~ ~ r1 o C a) ~ ~ C~
V ~d ~ ~ JJ JJ I
.c l ! -' Z Z Z
~ ~ o z ~ 4~ I z æz ~ ~
Z ZiZ Z 6 Z
Z; Z Z Z Z Z
. _ ~ .
V ~ ~ .
~ ? ~ ~ C ~ o ~ ~ o o o o o E C`~ C

_~ ~ .a ~
O ~K ~e a ~qrC U7u~ ~ ~ co ~ ~ co ~a ?~ ~ l V V V ~ V V V
~; * * ~ o ~
cn a~ G 1 o ~ ~ cl ~ lo ~ d c~ 0 ~ ~d t~ ~; C~ F4 h ~ ~
_ .~

.

~2~

Notes):
*1 Nipol 1502 (Nippon Zeon Co., Ltd.) *2 Solprene 1204 (Asahi Chemical Industry Co., Ltd.) *3 Cariflex 1215 (Shell Chemical Co.) *4 measured by NMR using proton *5 The amount of a branched polymer was determined hy GPC (gel permeation chromatography). The amount of a polymer having an atomic group introduced thereinto and represented by the formula (I) was determined by 13 C-NMR (nuclear magnetic resonance method using Cl ).

, ` ' , ' .
, ~2~

Table II

. N S~ DBP absorption 5 QDst 6 Sample carbon black 22 (m /g)(mQ/100 g) (m~) _ ._ CB-l (N 339) 1 94 119 61 CB-2 (SAF) 145 116 82 CB-3 (FEF) 42 105 220 (trial product) _ Notes):
*l Seast KH (Tokai Carbon Co., Ltd.) *2 Diablack A (Mitsubishi Chemical Industries Ltd.) *3 HTC YloO (Chubu Carbon Co., Ltd.) *4 measured by the procedure as stipulated in ASTM

*5 measured by the procedure as stipulated in *6 The measurement of ~Dst was made using a disc centrifuge (manufactured by Joyce Loebl Company, 20~ United Kingdom). Carbon black was accurately weighed and admixed with an aqueous solution of ethanol in a volume of 20 times the weight of the carbon black and a surface active agent, followed by dispersion with supersonic waves.
Thus, a sample solution having a carbon black : ' . ' ', ' ' ' ' ~
.. . , , ~ , ... .

concentration of 5 mg/100 cc was prepared.
0.5 ml of the sample solution was poured into lO ml of a spin liquid consisting of distilled water. The centrifugal sedimentation was started all at once at a disc centrifuge revolution of 8,000 rpm. ~n aggregate distribution curve converted in Stokes' radius was prepared according to the light sedimentation method.
The aggregate distribution width at l/2 of the maximum frequency (maximum light absorption) in the histogram is defined as a half-value wid-th (~Dst).

.

~8~

_ n Ln Ln ~ n C~l Ln rl ~~ o ~ - - - ~ -~ _ o o Ln ~o n c~ n r~ I~ o r~ ~

.~ r~ Ln n n ~ ~ n r~ CO
s~, ~ ~
~ r~ n n ~ ~ ~ n r1~ 0 rl ~
_ _ , Ln ~ O ~ Ln C~l n r11~ 0 _ _ ~n ~ n n n ~ ,~ ~ n r~ I~ ~ ,n a) _ ~
a ~ O O Ln ~" Ln ~ Ln ,~ '` o o X _ _ O O n ~ Ln C~l Ln r~ r`

_ ~ Ln ~ n C~l Ln r1 1~ o C~l 00 H C~ Lon Ln Ln n n ~ '` o ~ol o ~ -- - - ~
, o~ LO~ Ln Ln ,~
-,a) ~ ~D L~O~ ~ n c~ Ln r~ n O C~l O
n n Ln ~ n C~l n r~ r~ 0 ~ -- - ~
`~ Ln ro~ n ~ ~n ~ Ln r-l n O Ln r~
J~ _ ..
~ ~ o~ - Lon ~ n C~ Ln r1 : ~ _ . . . _ . _ _ . __ . _ . . _ o ~ O Lon ~ n C~ Ln r~ 1~ O 00 ~
l _ . ~
~ r~ Ln _ r~ oo o Ln n ~ '` o o o - - - - - - - - - - - --- - - - -r~ r~ Ln ~ r~

~ d U u ~r ~1 ~ ICI ~ û ~IJ CJ 1 U~ S ~ S S 5,,,~
(I) ~ O ~rl X IJ C) h ,C~ S N 4 r1 L.) ~ ~ a~ ,~ ~ a~ L J ~ ~ s~ S~ s~ o ~
r1 ~ O C~ ,q V ~ O 0 r~ 2 Ll Ll-l r~ 51 ~J
o ~s V~rl L~
P~ ~ N U~
_ _ _ _ _ .

~2~3~0~

Note):
*1 TTR-20 (TTR: Thailand Tested Rubber) *2 N-isopropyl-N'-phenyl-p-phenylenediamine *3 N-cyclohexy1-2-benzothiazylsulfenamide It will be apparent from Table III that the tires of Examples 1 to 5 had a rolling resistance lower and hence a fuel consumption far lower than those of the tire of Comparative Example 1, while the braking power on a wet road surface and a snow-covered or frozen road surface was improved. On the other hand, in the tires of Comparative Examples 1 to 5 using a single polymer in the tread portion thereof, all of the braking power on a wet road surface, the braking power on a snow-covered or frozen road surface, and the rolling resistance could not be satisfied. Even in the systems of a blend with natural rubber, the use of a styrene-butadiene copolymer rubber having no atomic groups introduced into the molecular chain thereof and represented by the afore-, mentioned formula (I) (Comparative Examples 6, 7, and 9) or a styrene-butadiene copo}ymer rubber having more than 30 wt.~ o the bonded styrene units (Comparative Example 8) provided an insuficient improvement. The use of SAF
carbon having an N2 SA of more than 140 m2/g (Comparative Example 10) provided a poor rolling resistance, while the ~2~

- 2~ -use o~ FEF carbon having an N2 SA of less than 60 m /g (Comparative Example 11) provided no sufficient braking power on a wet road surface. In contrast, the use of N339 carbon or carbon black in Examples 6 and 7 provided a tire remarkably improved in all properties simultaneously.
As described above, according to the present invention, since a rubber composition comprising a novel styrene-butadiene copolymer rubber is used in the tread portion, there can be obtained an all-season pneumatic tire having a remarkably lowered fuel consumption and simultaneously satisfying the braking power on a wet road surface and that on a snow-covered or frozen road surface.

', ' , '

Claims

1. A pneumatic tire comprising a tread portion consisting of at least two layers including an outer surface side rubber layer and an inner side rubber layer, wherein said outer surface side rubber layer comprises (1) 100 parts by weight of a rubber component composed of 20 to 90 parts by weight of natural rubber and/or polyisoprene rubber, and 10 to 80 parts by weight of a styrene-butadiene copolymer rubber comprising 5 to 30 wt.% of bonded styrene units and 10 to 85 wt.% of 1,2-vinyl bonds in the butadiene units and having at a molecular terminal or in a chain thereof an atomic group introduced thereinto and represented by the following formula:

wherein R1, R2 and R3 each stands for hydrogen or a substituent; R1 may be bound with R2 or R3 to form a ring, and 30 to 80 parts by weight of carbon black serving as a reinforcing agent and having a specific area (N2 SA) of 60 to 140 m2/g and a dibutyl phthalate absorption (DBP
absorption) of 100 to 150 ml/100 g.

2. A pneumatic tire as claimed in claim 1, wherein said outer surface side rubber layer further comprises 30 parts by weight or less of polybutadiene rubber, acrylonitrile-butadiene copolymer rubber, or non modified styrene-butadiene copolymer rubber.

3. A pneumatic tire as claimed in claim 1, wherein said styrene-butadiene copolymer rubber contains a branched rubber having branches bonded with a tin-butadienyl bond.

4. A pneumatic tire as claimed in claim 1, wherein said carbon black has a half-value width (.DELTA.Dst), in the aggregate distribution, of 85 to 130 mµ as measured by the centrifugal sedimentation method.