CA1119929A - Pneumatic radial tires for passenger cars - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A pneumatic radial tire for passenger cars having excellent wet skid resistance and cornering performance and a low rolling resistance is disclosed. This tire comprises a casing reinforced with a carcass of a radial construction and a belt superimposed about a crown portion of the carcass, and a tread of a cap-base rubber double laminate structure having a volume ratio of cap rubber to base rubber of 4:1 to 3:7. The cap rubber is a styrene-butadiene copolymer rubber having a composition and properties required for improving the wet skid resistance on wet road surface. The base rubber consists essentially of a polybutadiene rubber containing a polymer formed by block or graft polymerization of syn-1,2-polybutadiene with cis-1,4-polybutadiene having a weight average molecular weight of not less than 350,000, the polymer having a microstructure containing 6 to 20 wt%
of syn-1,2-structure, at least 40 wt% of the syn-1,2-poly-butadiene being crystallized and formed into short fibers having an average diameter of 0.05 to 1 µm and an average length of 0.8 to 10 µm, or is a blend consisting essentially of at least 10 wt% of the polybutadiene rubber and the remainder of the other diene rubber, and is compounded with 25 to 60 parts by weight of carbon black having an iodine absorption value of not less than 40 m?/g and a dibutyl phthalate absorption value of not less than 60 m?/100 g based on 1.00 parts by weight of the total rubber content of the base rubber.

Description

This invention relates to pneumatic raclial tires for passenger cars having excellent wet skid resistance and cornering performance and a low rolling resistance.
In general, the pneumatic radial tires for passenger cars are required to have excellent cornering perfomance, durability and economical value, and particularly wet skid resistance on road surface under wet conditions is required to be excellent in view o-f the sa-fety. Furthermore~ it is important that the durability during high-speed running is excellent with the advancement of highway networks.
Lately, the development of motor vehicles having less gasoline consumption, improvement of engines, weight-saving of motor vehicles and the like are attempted by the social demand for saving of resource and energy.
Particularly, the research and development of low rolling resistant tires are advanced in order to diminish power loss resulting from the tire.
Heretofore, it has been known that the reduction of the rolling resistance of the tire may contribute to save the gasoline consumption o-f the motor vehicle. Moreover, there are considered various factors exerting upon the rolling resistance of the tire, but it is particularly well-known from many articles that energy loss of rubber quality in a tread portion oE the t:ire greatly contributes to the rolling resistance oE the tire.
Now, it has been attempted to modify the quality o-f the tread rubber so as to make its energy loss as small as possible to thereby reduce the rolling resistance oE the tire. However, such a modification of tread rubber conversely tends to deteriorate the wet skid resistance. Therefore, it becomes a common sense that the improvemen~ of the rolling resistance conversely brings about the deterioration of the wet skid resistance in the pneumatic radial tire for passenger cars. In order to establish the contradictory performances of the rolling resistance and wet skid resistance together, there have hitherto been attempted various reforms and contrivances of tire construction, but unsatisfactory results are still obtained in view of practical uses, so that the establishment of the contradictory performances is said to be difficult till now.
The inventors have aimed at a tread of double laminate structure composed of a cap rubber and a base rubber (the part of the tread facing to ground is called as the cap rubber, and the remaining part thereof is called as the base rubber), which is usually practised in large-sized tires for truck and bus, in order to solve the above mentioned drawbacks of the prior art and establish the contradictory performances of rolling resistance and wet skid resistance in the pneumatic radial tire for passenger cars. That is, in the large-sized tires, there is a great problem that separation failure is liable to be caused due to heat build-up in the tread rubber during the running because the tread rubber gauge is -fairly thicker than that of the pneumatic radial tire for passenger cars. In order to solve this problem, a cap-base structure using a base rubber of a low energy loss composition is applied to the tread portion.
On the contrary, the cap-base structure as in the large-sized tire has not hitherto been much applied to the pneumatic radial tire for passenger cars from the following reasons;
that is, the separation failure due to heat build-up is little, and there are such problems in the production technique that the tread rubber gauge is fairly thin as compared with that of the large sized tire, the swelling difference between the cap rubber and the base rubber is large, and the like. And also, when the conventional rubber composition having a low energy loss is used as the base rubber, the modulus of elasticity lowers and as a result, there are caused problems relating to the cornering perform-ance, wear resistance, cut resistance and the like due to the reduction of rigidity in the tread portion, deterioration of high-speed durability and the reduction of rein-forcing effect.
As mentioned above, the inventors have aimed at the double laminate structure composed of cap rubber and base rubber in the tread portion of the large-sized tire and made various studies with respect to a means for establishing the contradictory performances o-f wet skid resistance and rolling resistance in the pneumatic radial tire for passenger cars and as a result, it has been confirmed that the cap-base structure is greatly effective even in the pneumatic radial tire for passenger cars. That is, the feature that the cap-base structure is e:Efective for establishment of the wet skid resistance and low rol.ling resistance is due to the :Eact that the wet skid resistance is largely dependent upon the friction coefficient of the cap rubber contactillg with ground surface, while the low rolling resistance is largely dependent upon the energy loss of the total rubber content including the cap r-ubber and base rubber, and hence the base rubber has an energy loss composition lower than that of the cap rubber.

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As the low energy loss composition, there is usually used a rubber composition consisting of at least one rubber selected from natural rubber, polyisoprene rubber, and cis-1,4-polybutadiene rubber, and carbon black of a lower filling amount as compared with the cap rubber compo-sition. Preferably, cis-1,4-polybutadiene rubber is used owing to excellent wear resistance, flexing resistance and the like, but there are problems in the working properties under unvulcanized state such as roll peeling property, swell value (shrinkage) and the like and in the cut resistance because the reinforcing effect after vulcanization is small.
Therefore, a rubber blend of cis-1,4-polybutadiene with other diene rubber is used, but it is still insufficient in practical uses. In any case, when the base rubber has the low energy loss composition as in the conventional tire, there are caused various problems as mentioned above, so that the cap-base structure can not e-ffectively be utilized : in the pneumatic radial tires for passenger cars.
Now, the inventors have made further studies in order to solve the above mentioned drawbacks due to the low energy loss composition of the base rubber and as a result, it was taken notice of the fact that syndiotactic (syn)-1,2-polybutadiene graft or block polymerized to cis-1,4-poly-butadiene is crystalli.zed and :Eormed into short :Eibers and found out that a polymer consisting essentially of a poly-butadiene rubber containing a graft or block polymer of syn-1,2-polybutadiene a:nd cis~ polybutadiene improves the reinforcing e-ffect, which has never been attained in the conventional cis-1,4-polybutadiene rubber, makes possible to have a resilience more higher than that o:f the conventional base rubber rein-folced with carbon black or resin, and is small in the swell value under unvulcanized state and also that when -thi.s polymer is applied to the base rubber o-f the tire having a cap-base rubber double laminate structure in the pneumatic radial tire for passenger cars, not only the contradictory performances of the wet ski.d resistance and low rolling resistance are established, but also the cornering performance, high-speed durability, cut resistance and the like are considerably improved as compared with the conven-tional pneumatic radial tires for passenger cars.
According to the invention, there is the provision of a pneumatic radial tire for passe:nger cars having excellent wet skid resistance and cornering performance and a low rol].ing resistance, comprising a casing reinforced with a carcass of a radial construction and a belt superimposed about an outer periphery o-f a crown portion of the carcass, and a tread of a cap-base rubber double l.aminate structure, characterized in that a volume ratio of cap rubber to base rubber is 4:1 to 3:7, said cap rubber is a styrene-butadiene copolymer rubber having a composition and properties required for improving the wet skid resistance on wet road surface, and said base rubber consists essentially o:E a polybutadiene rubber containing a polymer tormed by block or gra:Et poly-merization of syndiotac-t:ic (syn)-1,2-polybutadiene with cis-l,~-polybutad:iene havlng a weight average rrrolecular weight of not :Less than 350,000, the polybutcldiene rubber having a microstructure containing 6 to 20 wt% of syn-1,2-structure7 at least 40 w% of the syn-1,2-polybutadiene being crystal-lized and formed into short fibers having an average diameter ot 0.05 to 1 ~m and an average length of 0.8 to 10 ~m7 or is a blend consisting essentially of at ].east 10 wt% of said polybutadiene rubber and the remainder of the other diene rubber and is compounded with 25 to 60 parts by weight of carbon black having an iodine absorption value (IA) of not less than 40 mg/g and a dihutyl phthalate absorption value (DBP) of not less than 60 mQ/100 g based on 100 parts by weight of the total rubber content of said base rubber.
In a preferred embodiment of the invention, a rubberized layer of steel cords is used as the belt super-imposed about the outer periphery o-f the crown portion of the carcass .

In the pneumatic radi.al tire for passenger cars according to the invention, the tread has a cap-base rubber double laminate structure usually used in the conventional large-sized tire in order to establish the contradictory performances of wet skid resistance and low rolling resist-ance. In this case, the volume ratio o-f cap rubber to base rubber is within a range of 4:1 to 3:7. When the volume ratio is outside the above range, even if both the cap and base rubbers satisfy the respective compositions as defined below, the establishment of the contradictory performances of wet skid resistance and low rolling resistance aiming at the invention can not be achieved.
~ccordl.n~ to the invention, the cap rnbber contri-butes to improve the wet skid resistance on wet road surface, so that it is necessary to take any improvements extending from a macro-cou:ntermeasure for widening a contact area of the tire with road surface or a ground contact area as far as possible to increase a dynamic friction coefficient between the tire and the road surface to a micro-countermeasure for adapting to fine unevenness of the road surface. In this connection, the cap rubber uses a styrene-butadiene copolymer rubber as a main ingredient. For exampleS there is used a rubber blend consisting o:E sytrene-butadiene copolymer rubber of at least 60 wt% based on the total rubber content and at least one of natural rubber and other diene rubbers such as polybutadiene rubber, polyisoprene rubber, ethylene-propylene-diene terpolymer rubber, halogenated butyl rubber ~ and the like.
; 10 With the cap rubber o-f the above compounding recipe, the wet skid resistance is improved, but the rolling resistance increases owing to the wide ground contact area.
As a result, the ~ase rubber is necessary to have a compound-ing recipe of low energy loss (i.e. large rebound resilience~
and improved durability and resistance to heat build-up in order to suppress the increase o-E the rolling resistance and ~ establish the contradictory performances of the wet skid ; resistance and low rolling resistance. According to the invention, therefore, the base rubber satisfying the above mentioned requirements consists essentially of a polybutadiene rubber containing a polymer formed by block or graft polymeri-zation of syn-1,2-polybutadiene with cis-1,4-polybutadiene having a weight average molecular weight of not less than 350~000, the polybutadiene rubber having a microstructure containing 6 to 20 wt% of syn l,2-structure, at least 40 wt%
oE the syn-L,2-polybutadiene being crystallized and formed into short fibers having an average diameter o:E 0.05 to 1 ~m and an average length of 0.8 to 10 ~m (this polybutadiene rubber being abbreviated as VC-BR hereinafter), or a blend consisting essentially of at least 10 wt% of VC-BR and the remainder of the other diene rubber and is compounded with 25 to 60 parts by weight of carbon black having an IA of not less than 40 mg/g and a DBP of not less than 60 mQ/100 g based on 100 parts by weight of the total rubber content.
According to the invention, it is important that the VC-BR constituting a main ingredient for the base rubber has a microstructure containing 6 to 20 wt% of syn-1,2-structure. When the syn-1,2-structure is less than 6 wt%, there is no great difference in properties between this base rubber and the usual cis-1,4-polybutadiene rubber containing no syn-1,2-structure, while when the syn-1,2-structure is more than 20 wt%, the viscosity becomes too higher and adversely exerts upon the working property.
The formation of syn-1,2-polybutadiene short fiber in the VC-BR improves the fatigue resistance of the base rubber according to the invention. Particularly, the improving effect is conspicuous when the short fibers have an average diameter of 0.05 to 1 ~m and an average length of 0.8 to 10 ~m. When the average diameter is less than 0.05 ~m~ the resistance to fracture is insufficient, while when the average diameter is more than 1 ~m, the flexing resistance lowers. Furthermore, when the average length is less than 0.8 ~m, the resistance to cracking is poor, while when the average diameter is more than 10 ~m, the working property is deteriorated.
Moreover, it is necessary that at least 40 wt% of the syn-1,2-polybutadienc are crystallized. When the crystallized amount is less than 40 wt%, the melting point of the short fiber becomes lower and hence the temperature dependence of the base rubber becomes large, so that it is not adaptable for the improvement of the durability.
Although the base rubber is originally located beneath the cap rubber without being exposed to the road surface, it becomes an exposed surface in the last running stage or due to eccentric wear and the like, so that the rubber is also required to have excellent wear resistance, cut resistance and the like. For this purpose, it is necessary that the cis~ -polybutadiene in the ~C-BR has a weight average molecular weight of not less than 350,000.
When the weight average molecular weight is less than 350,000, the wear resistance required as the tread rubber is not ensured.
When the VC-BR is blended with the other diene rubber, it is important that the blend contains at least 10 wt% of VC-BR. When the content of VC-BR is less than 10 wt%, the performance of syn-1,2-polybutadiene short fibers dispersed into the VC-BR is not developed, so that the content o VC-BR should be not less than 10 wt%. As the diene rubber, use may be made of, -for example, natural rubber, styrene-butadiene copolymer rubber, polybutadiene rubber, polyisoprene rubber and the like.
According to the invention, the VC-BR or the blend of VC-BR and the other diene rubber is compounded with carbon black having an IA o:E not less than 40 nng/g and a DBP
o:E not less than 60 m~/100 g. When the IA is :Less than ~0 mg/g, the specific surface area of carbon black is small and the wear resistance is insuffici.ent, while when the DBP
is less than 60 mQ/100 g, satisfactory reinforcing effect is not obtained. Moreover, carbon black is compounded in an amount of 25 to 60 parts by weight based on 100 parts by weight of the total rubber content including the VC-BR as a main ingredient in view of low energy loss (i.e., large rebound resilience), wear resistance and cut resistance, which are requirements as the base rubber. When the amount of carbon black is less than 25 parts by weight, the re-inforcing effect is insufficient and there is a practical problem in the durability, when the amount of carbon black is more than 60 parts by weight, the low energy loss composi-tion is not achieved satisfactori]y.
According to the invention, even when using the rubberized layer of steel cords as the belt, the tendency of degrading the gasoline consumption due to the increase of the weight as compared with the usual textile radial tire, which is a defect of the conventional steel belt tire, is considerably improved by the adoption of the cap-base structure, whereby the gasoline consumption o-f the vehicle can be more saved.
The invention will now be described in detail with respect to the following examples.
Examples 1-5, Comparative Examp]es 1-5 Penumatic steel belt radial tires for passenger cars having a tire size of 175 SR 1~ were manufactured by using a compounding recipe shown in the following Tab]e 1 as a tread rubber.
In order to judge the establishment Or the contra-dictory perEormances of wet skid resistance ancl low rolling resistance, a tire having a tread composed of styrene-butadiene copolymer rubber composition contributing to the improvement of wet skid resistance is used as a control, while Comparative Examples 1-5 show tires having a tread of cap-base structure wherein the cap rubber is the rubber composition of Control and the base rubber is a rubber composition used in large-sized tires~ and Examples 1-5 show tires having a tread of cap-base structure wherein the cap rubber is the same rubber composition of Control and the base rubber is a rubber composition using the VC-BR according to the invention. In this case, the volume ratio of cap rubber to base rubber is 13:7.
The thus obtained tires were inflated to an internal pressure of 1.7 kg/cm2, respectively. Then, the rolling resistance was measured by a drum tester and the wet skid resistance and cornering performance were measured according to practical model test. These measured results are also shown in Table 1.

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_ Note) *l : styrene-butadiene copolymer rubber *2 : cis-1,4-polybutadiene rubber *3 : VC-BR is composed essentially of a polymer formed by graft polymerization of syn-1,2-polybutadiene with cis-1,4-polybutadiene, the polymer having a microstructure containing 12.6 wt% of syn-1,2-structure and the syn-1,2-polybutadiene being formed into shor~ fibers having an average diameter of 0.25 ~m and an average length of 3.1 ~m.
*4 : N-phenyl-N'-isopropyl-p-phenylenediamine *5 : N,N'-diphenylguanidine *6 : N-cyclohexyl-2-benzothiazolyl sulfenamide *7 : N,N-diisopropyl-2-benzothiazolyl sulfenamide *8 : The hardness of valcanized rubber with a thickness of 8 mm was measured by a spring-type JIS hardness meter.
*9 : The rebound resilience oE vulcanized rubber was measured at 30C by means of a Dunlop tripsometer defined in BS-903-223.
*10 : The test tire inflated to an internal pressure of ; 1.7 kg/cm2 was trained on a steel drum having a diameter oE 1707.6 mm and a width oE 350 mm~
which was rotated/by the driving oE a motor, at a speecl of 80 km/hr under a load o-f 385 kg for 30 minutes and thereafter the rotating speed of the drum was raised to 200 km/hr. l`hen, the driving of the motor was stopped to run the drum by inertia, during which the rolling resistance of the tire to the drum at a speed of 50 km/hr was measured on a basis of deceleration speed of the drum and time change. Next, a net rolling resistance of the tire was determined by subtract-ing the previously ca].culated drum resistance from the measured value~ The rolling resistance is evaluated by an index based on the tire of Control being 100. The higher -the index, the lower the rolling resistance.
*11 : A vehicle provided with the tes-t tire was run on a wet asphalt road at a speed o-f 40 km/hr, 70 km/hr or 100 km/hr and thereafter subjected to rapid braking at the above speed. Then, a running distance -for completely stopping the vehicle was measured at each speed of 40, 70 and 100 km/hr.
Thereafter, an average value was calculated from the measured values and evaluated by an index based on the tire of Control being 100.
The smaller the value, the poorer the wet skid resistance.
*12 : A vehicle provided with the test tire was run on an asphalt road provided with pyrons set at an interval of 25 m at a speed of 70-80 km/hr o-ver a given section. The cornering performance is estimated by the feeling and time required for passing through the above section ancl evaluated by an index based on the tlre of Control being 100.
The higher the index, the better the cornering performance.
*13 : A time till the occurrence of cracks was measured by using a de Mattia machine. The flexing resistance is evaluated by an index based Oll the - 1~ -tire of Control being 100. The higher the index, the better the flexing resistance.

As apparent from the results of Table 1, if it is intended to improve the rolling resistance o-f the tire of Control by adopting the cap-base structure to the tire tread and using the same rubber composition for base rubber as in the conventional large-sized tire, the rolling resistance is certainly improved, but the wet skid resistance somewhat reduces and also the cornering performance lowers. Therefore, it is obvious from Comparative Examples 1-5 that when the cap-base structure used in the large-sized tire is merely diverted to the pneumatic radial tire for passenger cars, the contradictory performances of the wet skid resistance and low rolling resistance can not be established.
On the other hand, it is apparent from Examples 1-5 that in the tires having a tread of cap-base structure wherein the VC-BR according to the invention is used in the base rubber, the wet skid resistance is maintained at a level equal to that of Control, the rolling resistance is considerably improved and hence the contradictory performances of the wet skid resistance and low rolling resista-nce is completely establishecl. Pa-rticularly, as shown in P~xamples 2-~, the hardness (modulus of elasticity) can be increased, while retaining the rebound resilience (cnergy loss) at substantially level as that of Control, by using the VC-BR
according to the invention together with the other diene rubber as the base rubber. As a result, the invention makes it possible to simultaneously improve three performances of the wet skid resistance, rolling resistance and cornering performance, which has hardly been achieved in the prior art.
In general, it is known that the cornering perform-ance is dependent upon the rigidity of the tread rubber.
Now, when the tread is made into a cap-base structure and the base rubber having a low energy loss (high rebound resilience) as compared with the cap rubber is used, the hardness (modulus of elasticity) conversely tends to reduce.
As apparent from Examples 1-5, however, when the base rubber is formed by using the VC-BR according to the invention, the hardness increases and the cornering performance is improved.
In the steel belt radial tire for passenger cars, it has been confirmed from Table 1 that the cornering perform-ance is improved as the hardness (modulus of elasticity) of the base rubber increases in the tread of cap-base structure.
Now, the wear resistance was tested by changing the kind and amount of carbon black to be compounded in the base rubber to obtain a result as shown in the following Table 2.
In this test, there was used a steel belt radial tire for passenger cars having a tire size of 175 SR 1~ and comprising a tread of cap-base structure (volume ratio of cap rubber to base rubber of 13:7) wherein the cap rubber is the same rubber composition as described in the tire of Control and the base rubber is made :Erom a compounding recipe shown in Table 2.

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From the results of Table 2, it can be seen that the practical use is sufficiently accomplished when using carbon black having an IA o~ not less than 40 mg/g and a DBP
of not less ~han 60 mQ/100 g. As apparent Erom Comparative Example 6 using carbon black N660 having an IA of less than 40 mg/g and a DBP of less than 60 mQ/100 g~ there is practically a problem in the wear resistance level.
In the above mentioned tread of cap-base structure, it is considered that the base rubber is not necessary to have a wear resistance because it is not originally exposed at the tread surface. However, the base rubber is actually exposed from the tread surface in the middle and last stages of the tread wear or due to eccentric wear and the like.
Now, the feature that the exposed base rubber is rapidly worn becomes unacceptable as a practical tire. Particularly, the tread gauge of the pneumatic radial tire for passenger cars is thinner than that of the large-sized tire, so that it is required to give a sufficient wear resistance to the base rubber. In order to satisfy this requirement, carbon black having an IA of not less than 40 mg/g and a DBP of not less than 60 mQ/100 g is compounded into the base rubber using the VC-BR according to the invention as apparent from the results of Table 2.
As a~parent from the comparison of Comparative P.xamples 7, 8 and 9 with Examples 6, 10 ancL 11 in Table 2, when the amount oE carbon black is less than 25 parts by weight, the rolling resistance is improved (which is coinci-dent with the increase oE the rebound resilience), but the wear resistance is considerably poor and there is caused a problem in practical uses, while when the amount of carbon ~ 2 ~

black is more than 60 parts by weight, the wear resistance is improved, but the rebound resilience conversely lowers and the improvement of the rolling resistance is not achieved.
Therefore, it can be seen from the results of Table 2 that the amount of carbon black used is within a range of 25 to 60 parts by weight per 100 parts by weight of the total rubber content of the base rubber in order to achieve a practical balance between the rolling resistance and the wear resistance.
Examples 12-16, Com_arative Examples 10-12 Pneumatic steel belt radial tires for passenger cars having a tire size of 175 SR 1~ were manufactured by changing the volume ratio of cap rubber to base rubber as shown in the following Table 3 in the tread of cap-base structure wherein the cap rubber is the same rubber composi-tion as described in the tire of Control and the base rubber is the same rubber composition as described in Example 1.
The influence of the volume ratio of cap rubber to base rubber on the running performances of the thus obtained tire was observed to obtain a result as shown in Table 3.

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Note) *l : Durability according to a drum test of FMVSS No. 109 The tire was trained on a drum at a speed of 81 km/hr for 120 minutes and thereafter the running speed was raised at a constant rate (8 km/hr) every 30 minutes. In this way, the limiting speed till the fracture of the tire was measured to evaluate the level of high-speed durability~
*2 : The tire o:E Comparative Example 10 and test tire were mounted on left and right front and rear wheels of the passenger car, respectively.
This car was run while interchanging the left and right tires at the front and rear wheels every the running distance of 5000 km. Every the given running distance~ the depth of the remaining groove was measured to evaluate the wear resistance.

In Table 3, each performance of the tire of Comparative Example 10 is indicated as an index of 100, and the performances of the other tires are calculated therefrom.
The higher the value, the more the improvement of the performance.
From the results of Table 3, it can be seen that the volume ratio of cap rubber to base rubber is within a range oE 4:1 to 3:7 in view of practical tire per-formances.
Particularly, the volume ratlo of cap rubber to base rubber is preferable within a range of 7:3 to 2:3 considering the balance of the tire performances.
In the volume ratio of cap rubber to base rubber o-f 85/15 shown in Comparative Example 11, the contribution of the base rubber to the rolling resistance is small, while ~ 3~ ~

when the volume ratio of cap rubber to base rubber is 25/75 as shown in Comparative Example 12, a part of the base rubber is exposed from the tread surface in the running distance of 20,000-30,000 km and completely exposed in the running distance o-f 30,000-40,000 km, so that the wear resistance is considerably poor and there is caused a problem in the practical use.
Moreover, it is apparent from Table 3 that the high-speed durability is improved by the low energy loss composition of the base rubber ~heat build-up is small) in the tread of cap-base structure.

- 2~ -

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined follows:-

1. A pneumatic radial tire for passenger cars having excellent wet skid resistance and cornering performance and a low rolling resistance, comprising a casing reinforced with a carcass of a radial construction and a belt super-imposed about an outer periphery of a crown portion of said carcass and a tread of a cap-base rubber double laminate structure, characterized in that a volume ratio of cap rubber to base rubber is 4:1 to 3:7, said cap rubber is a styrene-butadiene copolymer rubber having a composition and properties required for improving the wet skid resistance on wet road surface, and said base rubber consists essentially of a polybutadiene rubber containing a polymer formed by block or graft polymerization of syndiotactic (syn)-1,2-polybutadiene with cis-1,4-polybutadiene having a weight average molecular weight of not less than 350,000, the polybutadiene rubber having a microstructure containing 6 to 20 wt% of syn-1,2-structure, at least 40 wt% of the syn-1,2-polybutadiene being crystallized and formed into short fibers having an average diameter of 0.05 to l µm and an average length of 0.8 to 10 µm, or is a blend consisting essentially of at least 10 wt% of said polybutadiene rubber and the remainder of the other diene rubber and is compounded with 25 to 60 parts by weight of carbon black having an iodine absorption value of not less than 40 mg/g and a dibutyl phthalate absorption value of not less than 60 m?/100 g based on 100 parts by weight of the total rubber content of said base rubber.

2. A pneumatic radial tire for passenger cars as claimed in claim 1, wherein said belt is composed of a rubberized layer of steel cords.

3. A pneumatic radial tire for passenger cars as claimed in claim 1, wherein said volume ratio of cap rubber to base rubber is 7:3 to 2:3.