Pneumatic radial tires
FIELD OF THE INVENTION 
     The present invention relates to pneumatic tires, and more particularly to pneumatic radial tires which can improve ride feeling by using a rubber composition which has a high flex resistance and produces an excellent damping effect in the sidewalls. 
     BACKGROUND OF THE INVENTION 
     Recently, resource and energy savings have been socially demanded. As a result, the development of so-called low fuel consumption tires has been eagerly carried out to reduce power loss. For this purpose, not only have tire structures been modified and the weight of tires been reduced, but also, almost all of the rubber compositions in all of the parts of the tire have been replaced with those yielding a lower energy loss. However, utilization of lower energy loss rubber compositions in all of the parts of the tire has resulted in very poor ride feeling of the tire. 
     Based upon consideration of the above-mentioned result and on the highly-advanced technology of lower energy consumption, it has been proposed to prepare a rubber composition having high energy loss in the sidewalls which will bring about an improvement in ride feeling. Such a composition having high energy loss can be prepared, for example, by (1) an increased addition of regular carbon black, (2) use of higher-quality carbon black or (3) an increased addition of softener. However, each of these approaches has unavoidable drawbacks. 
     An increased addition of regular carbon black means an increase in the modulus of elasticity of the rubber composition. Not only is it impossible by this means to improve the ride feeling but it also unfavorably lowers the flex resistance. 
     The use of higher-quality carbon black increases the storage modulus of elasticity E&#39; of the rubber composition and this is not desirable from the standpoint of cut growth which starts from a small incision on the sidewall. 
     An increased addition of softener greatly lowers the weather resistance of the composition. 
     SUMMARY OF THE INVENTION 
     A principal object of this invention is to provide a pneumatic radial tire which can improve the ride feeling by using an improved rubber composition possessing high energy loss in the sidewalls. 
     More particularly, this invention relates to a pneumatic radial tire having sidewalls, the vulcanizable rubber composition of which comprises, based on 100 parts by weight of rubber, 20 to 90 parts by weight of butadiene-piperylene copolymer and 80 to 10 parts by weight of at least one rubbery diene polymer selected from the group consisting of natural rubber, synthetic polyisoprene rubber, polybutadiene rubber, butadiene-styrene copolymer rubber, ethylene-propylene-diene ternary copolymer rubber and acrylonitrile-butadiene copolymer rubber. The vulcanizable rubber composition comprises 20 to 120 parts by weight of carbon black in the same manner as a usual rubber composition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows sidewall A of a tire and 
     FIG. 2 shows portion C in the sidewall which is particularly suitable for the arrangement of the rubber composition according to the present invention. Portion C extends, as shown in FIG. 2, from the position of maximum tire width W in sidewall A to bead B. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The butadiene-piperylene copolymers according to the present invention can be prepared by known methods (see U.S. Pat. No. 3,972,862, Canadian Pat. No. 1,042,141). 
     The average molecular weight of the copolymers varies between about 150,000 and 500,000. The amount of the butadiene-piperylene copolymer is limited to 20 to 90 parts by weight based on 100 parts by weight of rubber in the rubber composition according to the present invention because the sidewall in the tire of this invention cannot improve the damping effect when the amount is less than 20 parts by weight and, because sidewall cut resistance is lost when the amount is more than 90 parts by weight. The butadiene-piperylene copolymer is selected because polybutadiene rubber has excellent flex resistance but low tensile strength. 
     In order to improve the tensile strength, the microstructure of polybutadiene is changed from the cis to the trans form. Further, the possibility of crystallization of polybutadiene is prevented by disturbing the regularity of trans polybutadiene by copolymerization with 1,3-pentadiene so as to prevent any rapid change in the modulus of elasticity in the piperylene which arises as a result of melting the crystals when the tire is in operation. 
     1,3-pentadiene is particularly useful because it is a monomer which can be easily obtained commercially in the form of the C5 fraction from crude oil and because its polymer has such a structure as to comprise methyl groups which do not directly bond to the carbon atoms of the double bonds. Hence, 1,3-pentadiene never lowers the flex resistance of the composition. 
     It is preferable that the butadiene-piperylene copolymer has a 1,3-pentadiene content between 15 to 50% by weight of the copolymer because it is impossible to sufficiently prevent the crystallization of trans-1,4-polybutadiene when the content of 1,3-pentadiene is less than 15% by weight. When crystallization is not prevented, the tire modulus of elasticity rapidly changes due to the melting of the crystal within the usual temperature range of tire operation (10° to 100° C.). This is not desirable because it becomes impossible to expect what the excellent flex resistance of polybutadiene is when the content of 1,3-pentadiene is more than 50% by weight. 
     The amount of carbon black to be used is preferably limited to 20 to 120 parts by weight because the modulus of elasticity and the tensile strength are not sufficient when the amount is less than 20 parts by weight and because the production operability for making the sidewalls is low and the flex resistance is greatly reduced when the amount exceeds 120 parts by weight. 
     The rebound resilience, measured at 30° C. with a Dunlop tripsometer, of the sidewall according to the present invention is preferably limited to not more than 50% because it is impossible to sufficiently improve the ride feeling when the resilience is higher than 50%. 
     The pneumatic radial tire of the present invention can improve the ride feeling when the vulcanizable rubber composition is arranged, as shown in FIG. 1, on sidewall A. The upper portion from the position of the maximum tire width W can greatly add to the rolling resistance of the tire, whereas the lower portion from the position of the maximum tire width W, i.e., the portion extending between the bead B and the position of maximum tire width W, can greatly add to the ride feeling performance. Thus, it is more desirable to provide a radial tire for low fuel consumption and excellent ride feeling by arranging the vulcanizable rubber composition according to the present invention all along part C, as shown in FIG. 2, extending from bead B to the position of maximum tire width W. 
     Since the vulcanizable rubber composition according to the present invention has (1) a markedly lower modulus of elasticity than that of any other similar composition of diene homopolymers and copolymers, (2) a flex resistance which is equal to or higher than that of cis-polybutadiene rubber even if the content of carbon black is increased, (3) a high tensile strength and (4) a high energy loss, it is highly suitable for obtaining a pneumatic radial tire with improved ride feeling performance. 
     The following non-limiting examples describe the invention in more detail. 
     EXAMPLES 1 TO 4 
     The butadiene-piperylene copolymer was prepared as follows: 
     In a 5 l flask equipped with a stirrer, a dropping funnel and a side cock (all of which were connected through ground connecting conical pipes), 1.3 l anhydrous toluene, 5 ml Al(C 2  H 5 ) 2  Cl, 1.365 g CCl 3  COOH, 300 g 1,3-butadiene and 50 ml piperylene were added to prepare a solution. The composition of piperylene used was as follows: 
     trans-1,3-Pentadiene--62.6% 
     cis-1,3-Pentadiene--27.5% 
     Cyclopentene--8.66% 
     Cyclopentadiene--0.68% 
     Other C 5  isomers--0.56% 
     This solution was cooled to 3° C. and another solution containing 60 mg vanadium triacetyl acetonate in 10 ml of toluene was added to the former solution. The polymerization was activated by the further addition of 70 ml of piperylene. After three hours, the polymerization was stopped, the reaction product was coagulated by alcohol and then dried through a hot roll. 
     The product was analyzed using an infrared absorption spectrometer. The analysis showed that the copolymerized pentadiene unit corresponded to 35% of the total polymer weight and its viscosity η was 233 dl/g in toluene at 30° C. 
     The butadiene-piperylene colpolymer thus obtained, hereinafter referred to as &#34;sample&#34;, was mixed with natural rubber, carbon black, etc., in the proportions as shown in Table 1 below. The characteristics of each of the compositions are also shown in Table 1 below. 
     Note, the values in Tables 1, 3 and 4 are in parts by weight unless otherwise indicated. 
     
                                           TABLE 1                                 
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                 Comparative                                              
                        Example                                           
                             Example                                      
                                  Example                                 
                                       Comparative                        
                                              Example                     
                 Example 1                                                
                        1    2    3    Example 2                          
                                              4                           
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Natural rubber   85     80   50   10   5      35                          
Sample           15     20   50   90   95     50                          
SBR 1500.sup.1   --     --   --   --   --     15                          
HAF.sup.2        45     45   45   45   45     45                          
Aromatic oil     10     10   10   10   10     10                          
Antioxidant IPPD.sup.3                                                    
                 2      2    2    2    2      2                           
Stearic acid     2      2    2    2    2      2                           
ZnO              3      3    3    3    3      3                           
Accelerator MBTS.sup.4                                                    
                 --     --   0.2  0.2  0.2    0.2                         
Accelerator OBS.sup.5                                                     
                 0.7    0.7  0.6  0.7  0.7    0.7                         
Sulfur           1.5    1.5  1.5  1.5  1.5    1.5                         
100% modulus at 30° C. in kg/cm.sup.2                              
                 16     16   14   13   13     14                          
Rebound resilience at 30° C.                                       
                 50.3   48.6 47.3 41.2 40.6   45.8                        
Flex resistance  95     123  246  314  303    168                         
Ride feeling     3      4    5    5    5      5                           
Cut resistance   120    118  100  88   80     112                         
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 Note:                                                                    
 .sup. 1 SBR 1500 = Styrenebutadiene rubber 1500                          
 .sup. 2 HAF = Phil Black N 330                                           
 .sup. 3 Antioxidant IPPD: Ozonone 3C manufactured by Seiko Kagaku Corp., 
 corresponding to N--phenylN&#39;--isopropyl-p-phenylenediamine.              
 .sup. 4 Accelerator MBTS: Nocceler DM manufactured by Ouchi Shinko Kagaku
 Industries, corresponding to dibenzothiazyldisulfide.                    
 .sup. 5 Accelerator OBS: Nocceler MSA manufactured by Ouchi Shinko Kagaku
 corresponding to N--oxydiethylene2-benzothiazolesulfenamide.             
 
    
     Flex resistance was estimated from the following formula by determining the time elapsed until the initial crack appeared after flexing each sample without an initial incision, according to the flexing test, JIS K 6301: ##EQU1## 
     Ride feeling was estimated as follows: A vehicle with four test tires (size: 175 SR 14) was driven at the speed of 60 km/h over ten 10 mm diameter pipes arranged at regular intervals of 10 m. The noise and the vibrations produced by driving over the pipes were estimated and classified into five grades and averages. The larger the value the better the ride feeling. 
     Cut resistance was estimated as follows: An edge having an angle of 45° was shot at 8 cm thick vulcanized rubber composition blocks at a speed of 0.894 m/sec and an energy of 120 joules. The depth of the incision produced by the collision was then measured. An index of the cut resistance was calculated from the results according to the following formula: ##EQU2## 
     Index 100 corresponds to 38.5 mm which is equal to the depth of the incision in Example 2. The larger the index the better the cut resistance of the tire. The smallest index applicable to the tire sidewalls is 85. 
     The test data in Table 1 shows that the amount of the conjugated diolefin copolymer in the present invention is limited to 20 to 90 parts by weight. 
     EXAMPLES 5 TO 7 
     In a 5 l flask, equipped with a stirrer, a dropping funnel and a side cock (all of which were connected through ground connecting conical pipes), the following compounds were added as raw materials to prepare several solutions. 
     
                       TABLE 2(a)                                                  
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           A     B       C       D     E                                  
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Anhydrous toluene (l)                                                     
             1.3     1.3     1.3   1.3   1.3                              
Al(C.sub.2 H.sub.5).sub.2 Cl (ml)                                         
             5       5       5     5     5                                
CCl.sub.3 COOH (g)                                                        
             1.365   1.365   1.365 1.365 1.365                            
1,3-Butadiene (g)                                                         
             300     300     300   300   300                              
Piperylene (ml)                                                           
             50      50      50    80    100                              
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     Composition of piperylene used: 
     trans-1,3-Pentadiene--62.6% 
     cis-1,3-Pentadiene--27.5% 
     Cyclopentene--8.66% 
     Cyclopentadiene--0.68% 
     Other C 5  isomers--0.56% 
     Solutions A to E were cooled to 3° C. and another solution containing 60 mg vanadium triacetyl acetonate in 10 ml of toluene was added to the former solution. 0, 15, 70, 90 and 100 ml of piperylene was further added to these solutions, respectively. After three hours, each polymerization was stopped, the reaction product was coagulated by alcohol and then dried through a hot roll. 
     Each product was analyzed using an infrared absorption spectrometer. The analysis showed that the copolymerized pentadiene unit corresponded to the percentage of the total polymer weight and its viscosity value, as is shown in Table 2(b). 
     
                       TABLE 2(b)                                                  
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            A     B      C        D    E                                  
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Content in % by weight                                                    
              13      18     33     46   55                               
Viscosity [η] in                                                      
              1.83    2.01   2.33   1.68 2.51                             
toluene at 30° C.                                                  
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     The flex resistance values of the rubber compositions obtained by using these polymers are shown in Table 3. 
     
                                           TABLE 3                                 
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           Comparative                                                    
                  Example                                                 
                       Example                                            
                            Example                                       
                                 Comparative                              
                                        Comparative                       
           Example 3                                                      
                  5    6    7    Example 4                                
                                        Example 5                         
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Copolymer                                                                 
Sample A   50     --   --   --   --     --                                
Sample B   --     50   --   --   --     --                                
Sample C   --     --   50   --   --     --                                
Sample D   --     --   --   50   --     --                                
Sample E   --     --   --   --   50     --                                
Polybutadiene (BR 01)                                                     
           --     --   --   --   --     50                                
Natural rubber                                                            
           50     50   50   50   50     50                                
Carbon black HAF                                                          
           50     50   50   50   50     50                                
Aromatic oil                                                              
           10     10   10   10   10     10                                
Antioxidant IPPD                                                          
           2.0    2.0  2.0  2.0  2.0    2.0                               
Stearic acid                                                              
           2.0    2.0  2.0  2.0  2.0    2.0                               
ZnO        3.0    3.0  3.0  3.0  3.0    3.0                               
Accelerator MBTS                                                          
           0.2    0.2  0.2  0.2  0.2    0.2                               
Accelerator OBS                                                           
           0.6    0.6  0.6  0.6  0.6    0.6                               
Sulfur     1.5    1.5  1.5  1.5  1.5    1.5                               
Flex resistance                                                           
           86     151  243  186  93     100                               
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     The data in Table 3 suggests that a preferable relative amount of 1,3-pentadiene contained in the butadiene-piperylene copolymers is 15 to 50% by weight. 
     EXAMPLES 8 AND 9 
     The data in Table 4 below shows that with the same flex resistance of the rubber compositions of the present invention (Examples 8 and 9) there is a larger energy loss than when other conventional rubber compositions are employed for sidewalls. 
     
                                           TABLE 4                                 
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            Example                                                       
                 Example                                                  
                      Comparative                                         
                             Comparative                                  
                                    Comparative                           
                                           Comparative                    
                                                  Comparative             
            8    9    Example 6                                           
                             Example 7                                    
                                    Example 8                             
                                           Example 9                      
                                                  Example                 
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                                                  10                      
Natural rubber                                                            
            50   50   50     50     50     50     50                      
Polybutadiene (BR 01)                                                     
            --   --   50     50     50     50     50                      
Sample C    50   50   --     --     --     --     --                      
Carbon black FEF                                                          
            50   55   40     45     50     55     45                      
Aromatic oil                                                              
            8    8    8      8      8      8      15                      
Antioxidant IPPD                                                          
            2.0  2.0  2.0    2.0    2.0    2.0    2.0                     
Stearic acid                                                              
            2.0  2.0  2.0    2.0    2.0    2.0    2.0                     
ZnO         3.0  3.0  3.0    3.0    3.0    3.0    3.0                     
Accelerator MBTS                                                          
            0.2  0.2  0.2    0.2    0.2    0.2    0.2                     
Accelerator OBS                                                           
            0.6  0.6  0.6    0.6    0.6    0.6    0.6                     
Sulfur      1.5  1.5  1.5    1.5    1.5    1.5    1.5                     
100% modulus at 30° C.                                             
            14   14.5 14     17     20     23.5   14.5                    
in kg/cm.sup.2                                                            
Flex resistance                                                           
            253  221  185    136    108    71     119                     
Weather resistance                                                        
            B-1  B-1  C-2    B-2    B-1    B-1    C-4                     
(Ozone cracking test)                                                     
Rebound resilience                                                        
            47.6 45.2 56.5   53.2   50.6   48.2   49.8                    
at 30° C.                                                          
Ride feeling                                                              
            5    5    3      2      1      2      4                       
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     Weather resistance was measured with a weathering test apparatus, Model OMS-R-2, manufactured by Suga Shikenki K.K., according to JIS K 6301. 
     Measuring conditions: 
     Duration--48 hr 
     Elongation--30% strain 
     Temperature--40° C. 
     Ozone concentration--50 pphm 
     The state of the weather resistance was recorded on the basis of the characteristics shown in Table 5. 
     TABLE 5 
     Number of cracks 
     A: Small in number 
     B: Large in number 
     C: Innumerable in number 
     Size and Depth of cracks 
     1: Invisible to the naked eye but visible when using a 10X magnifying glass 
     2: Visible to the naked eye 
     3: Cracks are deep and relatively large (less than 1 mm) 
     4: Cracks are deep and large (from 1 mm to less than 3 mm) 
     5: Cracks are 3 mm or more 
     EXAMPLES 10 AND 11 
     In these examples, the rubber composition according to the present invention was applied to tire sidewalls to observe its effects. 
     In Example 10, the rubber composition according to Example 7 was applied to tire sidewall A as shown in FIG. 1 to prepare a 175 SR 14 tire. 
     In Example 11, the rubber composition according to Example 7 was applied to portion C between the position of maximum tire width and bead B, and according to Comparative Example 6 was applied to portion D above portion C to prepare a 175 SR 14 tire. 
     The results obtained are shown in Table 6 below. 
     
                       TABLE 6                                                     
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         Example 10                                                       
                   Example 11                                             
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Portion C  Composition of                                                 
                       Composition of                                     
           Example 7   Example 7                                          
Portion D  Composition of                                                 
                       Composition of                                     
           Example 7   Comparative Example 6                              
Rolling    100         104.5                                              
resistance (control)                                                      
Ride feeling                                                              
           5           5                                                  
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     Rolling resistance was measured under the following conditions: The test tire, subjected to an internal pressure of 1.7 kg/cm 2 , was trained on a steel drum with a diameter of 1,707.6 mm and a width of 350 mm that was rotated by driving a motor at a speed of 80 km/hr under a JIS 100% load (385 kg) for 30 minutes. Thereafter, the rotating speed of the drum was raised to 100 km/hr. Then, the driving of the motor was stopped and the drum was allowed to run 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 the deceleration speed of the drum and the time change. Next, the net rolling resistance of the tire was determined by subtracting the previously calculated drum resistance from the measured value. Moreover, the rolling resistance of the test tire was evaluated by the following formula: ##EQU3## wherein the rolling resistance of Example 10 was utilized as a control. 
     It can be seen from the results in Table 1, Table 4 and Table 6 that the pneumatic radial tire of the present invention yields an excellent ride feeling without lowering the flex resistance, cut resistance or weather resistance. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.