Patent Publication Number: US-4550142-A

Title: Rubber composition

Description:
This application is related to application Ser. No. 630,399 filed July 13, 1984, now abandoned, which is a continuation-in-part of application Ser. No. 475,500 filed Mar. 15, 1983, now abandoned. 
     This invention relates to a rubber composition having an improved rebound. More specifically, this invention relates to a rubber composition comprising as a rubber component a highly unsaturated polymer rubber having a benzophenone introduced into its molecular chains. 
     In recent years, reduction of the rolling resistance of a tire and improvement of its braking property on a wet road surface, i.e. its wet skid resistance, have been strongly desired from the standpoint of the low fuel cost and safety of automobiles. Generally, these properties of a tire are considered correspondingly to the dynamic viscoelastic properties of a tread rubber material, and are known to be incompatible properties (see, for example, Transactions of I.R.I., Vol. 40, pages 239-256, 1964). 
     To reduce the rolling resistance of tires, the tread rubber material should have a high rebound. In view of the running condition of an automobile, the rebound should be evaluated at temperatures of from 50° C. to about 70° C. On the other hand, in order to improve the braking property of a tire on a wet road surface, which is an important property with regard to the safety of a car, the tire should have high wet skid resistance measured by a British portable skid tester. Accordingly, the tread rubber material is required to have a large energy loss as a frictional resistance which occurs when the tire is allowed to slide on a road surface while applying the brakes. 
     Heretofore, a blend of a styrene/butadiene copolymer rubber having a combined styrene content of 20 to 25% by weight and a high cis-1,4-polybutadiene rubber having a cis 1,4-linkage content of at least 80 mole% has been used to provide a compromise between these two incompatible properties. But this blend has not proved to be entirely satisfactory with regard to wet skid resistance. Attempts have been made to increase wet skid resistance by mixing this blend with a resin or rubber having a high glass transition temperature, but resulted in a reduced rebound. Hence, a further improvement has been desired. 
     It is an object of this invention to provide such an improvement. 
     The present inventors surprisingly found that a rubber composition containing at least 10% by weight, based on the entire rubber component, of a highly unsaturated polymer rubber having a benzophenone introduced into its molecular chains obtained by reacting an alkali metal-added highly unsaturated polymer rubber (the term &#34;alkali metal-added highly unsaturated polymer rubber&#34; denotes living diene-type polymer rubbers having an alkali metal bonded to the ends of the molecular chains, the highly unsaturated polymer rubbers having an alkali metal added randomly to the molecular chains) with a benzophenone has equal wet skid resistance to, and a much higher rebound than, a rubber composition containing as a rubber component a highly unsaturated polymer rubber having no benzophenone introduced thereinto. 
     The present invention provides a composition which, when used as a tire, reconciles rolling resistance and braking property on a wet road surface, i.e. wet skid resistance, which are important properties of tires in recent years, at high levels. If a benzophenone is introduced into a styrene/butadiene copolymer rubber having a combined styrene content of 20 to 25% by weight which has wet skid resistance at the required level, its rebound can be markedly improved without reducing its wet skid resistance. Thus, a composition having the above two properties satisfactorily at high levels can be obtained by using the aforesaid rubber alone. Furthermore, if a rubber obtained by introducing a benzophenone to polybutadiene rubber having a cis 1,4-linkage content of at least 80 mole% and a high rebound is combined with a styrene/butadiene copolymer rubber having a combined styrene content of at least 30% by weight which has a sufficient level of wet skid resistance but a low rebound, a composition which has the above two properties satisfactorily at high levels can be obtained. If a rubber obtained by introducing a benzophenone into a styrene/butadiene copolymer rubber having a combined styrene content of at least 30% by weight is blended with polybutadiene rubber having a cis 1,4-linkage content of at least 80 mole% by weight, a composition which has the aforesaid two properties satisfactorily at high levels can be obtained. Furthermore, if a rubber obtained by introducing a benzophenone into polybutadiene rubber having a cis 1,4-linkage content of at least 80 mole% is mixed with a rubber obtained by introducing a benzophenone into a styrene/butadiene copolymer rubber having a combined styrene content of at least 30% by weight, a composition which has the above two properties satisfactorily at high levels can be obtained. 
     The composition of this invention can also be used when a high rebound is required while a high wet skid resistance value is not particularly necessary. 
     The highly unsaturated polymer rubber into which a benzophenone is to be introduced in the present invention includes (co)polymer rubbers having diene units such as butadiene, isoprene and 1,3-pentadiene, and (co)polymer rubbers of cycloolefins such as cyclopentene and cyclooctene, which have a carbon-carbon double bond in the polymer chain, irrespective of the method of polymerization (solution polymerization, emulsion polymerization, etc.). Specific examples are polybutadiene rubber (having a high cis 1,4-linkage content, a low cis 1,4-linkage content, a low and a high 1,2-linkage content, etc.), polyisoprene rubber (having a high cis 1,4-linkage content, a low cis 1,4-linkage content, etc.), polychloroprene rubber, a styrene/butadiene copolymer rubber (having a combined styrene content of not more than 50% by weight, a low and a high 1,2-linkage content, etc.), a styrene/isoprene copolymer rubber (for example, having a combined styrene content of not more than 50% by weight), butadiene/isoprene copolymer rubber, acrylonitrile/butadiene copolymer rubber, polypentadiene rubber, butadiene/piperylene copolymer rubber, butadiene/propylene copolymer rubber, polypentenamer, polyoctenamer, and cyclopentene/dicyclopentadiene copolymer rubber. These examples are only illustrative and do not in any way limit the invention. 
     Examples of benzophenones used in this invention are 4,4&#39;-bis(dimethylamino)-benzophenone, 4,4&#39;-bis(diethylamino)benzophenone, 4,4&#39;-bis(dibutylamino)benzophenone, 4,4&#39;-diaminobenzophenone and 4-dimethylaminobenzophenone. Benzophenones having at least one amino group, an alkylamino group or dialkylamino group on one or both benzene rings are especially preferred. Also useful are benzophenones which have at least one alkoxy group, halogen or hydrocarbon radical as a substituent, such as 4,4&#39;-diethoxybenzophenone, 3,4-dimethoxybenzophenone, 4,4&#39;-dimethylbenzophenone, 3,3&#39;-dicyclobenzophenone, 4-methyl-4&#39;-methoxybenzophenone, 2,2&#39;,3,3&#39;-tetramethylbenzophenone, and 2,2&#39;-dichlorobenzophenone. Benzophenone itself having no substituent can also be used. 
     Comprehensively, the aforesaid benzophenones are represented by the following general formula ##STR1## wherein R 1  and R 2  represent hydrogen, halogen, alkyl, alkenyl, alkoxy, amino, alkylamino or dialkylamino, and m and n represent integers whose total is from 1 to 10. 
     A highly unsaturated polymer rubber having a benzophenone introduced into its molecular chains can be prepared, for example, by a method which comprises adding the benzophenone into a solution of a living polymer rubber having an alkali metal bonded to the ends of its molecular chain obtained by polymerizing a diene-type monomer in the presence of an alkali metal-based catalyst, and reacting the living polymer rubber with the benzophenone; or by a method which comprises reacting a highly unsaturated polymer rubber in solution with, for example, an organic alkali metal compound to introduce the alkali metal, and thereafter reacting the polymer rubber with the benzophenone. The alkali metal-based catalyst may be those generally used in solution polymerization, for example lithium, sodium, rubidium, cesium, a complex of such a metal element with a hydrocarbon compound or a polar compound (such as n-butyllithium, 2-naphthyllithium, potassium-tetrahydrofuran complex, and potassium-diethoxyethane complex). 
     The amount of the benzophenone to be introduced into the highly unsaturated polymer rubber is, on an average, at least 0.1 mole, preferably at least 0.3 mole, more preferably at least 0.5 mole, especially preferably at least 0.7 mole, per mole of the rubber molecular chains. If it is less than 0.1 mole, no improvement in rebound can be obtained. The upper limit to the amount of the benzophenone introduced is 5 moles. If it is introduced in an amount of more than 5 moles, rubbery elasticity is lost. 
     The benzophenone introduced into the rubber molecular chains is bonded as an atomic grouping of the general formula ##STR2## (wherein R 1 , R 2 , m and n are as defined) to the carbon atom in the rubber molecular chains. 
     The site into which the benzophenone is introduced may be any of the sites of the rubber molecular chain, but preferably its ends. 
     The highly unsaturated polymer rubber having the benzophenone introduced into the polymer chains should account for at least 10% by weight, preferably at least 20% by weight, of the entire rubber component of the rubber composition. If its amount is less than 10% by weight, the effect of improving rebound is small, and the object of the present invention cannot be achieved. 
     The highly unsaturated polymer rubber having the benzophenone introduced thereinto can be used in combination with other rubbers. Examples of the other rubbers are styrene/butadiene copolymer rubber obtained by emulsion polymerization, polybutadiene rubber obtained by emulsion polymerization, polybutadiene rubber (having a high cis 1,4-linkage content, a low cis 1,4-linkage content, a low and a high 1,2-linkage content, etc.) obtained by solution polymerization catalyzed by an alkali metal-based catalyst, a Ziegler catalyst or an alfin catalyst), styrene/butadiene copolymer rubber (having a combined styrene content of not more than 50% by weight, a low and a high 1,2-linkage content, etc.), polyisoprene rubber (having a high cis-1,4-linkage content, a low cis 1,4-linkage content, etc.), polyalkenamers, and natural rubber. 
     The Mooney viscosity (ML 1+4 , 100° C.) of the highly unsaturated polymer rubber having the benzophenone introduced into the polymer chains is usually preferably 10 to 200, more preferably 20 to 150. If it is less than 10, mechanical properties such as tensile strength are inferior. If it exceeds 200, the miscibility of the highly unsaturated polymer rubber with another rubber is poor and processing of the rubber becomed difficult, with the result that the tensile strength and other properties of the resulting rubber composition are degraded. 
     The rubber component in the present invention may be used wholly or partly in the form of an oil-extended rubber. 
     The rubber composition of this invention is mixed, according to purposes and uses, with various compounding agents universally used in the rubber industry by using a mixer such as a roll mill or a Banbury mixer to form a rubber stock. The rubber stock is molded and vulcanized to produce the desired rubber product. Examples of the compounding agents include sulfur, stearic acid, zinc oxide, various vulcanization accelerators (for example, of the thiazole, thiuram and sulfenamide series), reinforcing agents such as various grades of carbon black (such as HAF and ISAF) and silica, fillers such as calcium carbonate and process oils. 
     Since the rubber composition of this invention provides a compromise between rebound and wet skid resistance at a high level, it is especially suitable as an automobile tire tread rubber material having improved safety and reduced fuel consumption. It can also be used as bicycle tires, shoe soles and floor materials. 
    
    
     The following examples illustrate the present invention more specifically. 
     PRODUCTION EXAMPLE 
     Production of rubber having a benzophenone introduced thereinto: 
     (1) Reaction of styrene/butadiene copolymer rubber (S-SBR for short) obtained by using a lithium-based catalyst, with a benzophenone: 
     A 2-liter stainless steel polymerization reactor was washed and dried and purged with dry nitrogen. Then, it was charged with 112.5 g of 1,3-butadiene, 37.5 g of styrene, 820 g of benzene, 0.75 g of tetrahydrofuran and 2.0 millimoles of n-butyllithium (as an n-hexane solution). With stirring, the mixture was subjected to polymerization at 45° C. for 2 hours. Then, 3.0 millimoles of 4,4&#39;-bis(dimethylamino)benzophenone (to be abbreviated as MAB hereinafter) was added to the reaction mixture, and the mixture was stirred for 5 minutes to react the living polymer and MAB. The reaction mixture was then poured into a methanol solution containing 1 part by weight, of 2,6-di-t-butyl-p-cresol per 100 parts by weight of the (BHT) to coagulate the resulting polymer. The polymer was dried under reduced pressure at 60° C. for 24 hours, and its Mooney viscosity was measured [S-SBR (2*)]. The symbol * represents a rubber having a benzophenone introduced thereinto (this applies hereinafter). 
     For comparison, polymerization was carried out in accordance with the same polymerization recipe as above, and without reacting the polymer with MAB, the reaction mixture was poured into a methanol solution containing BHT to coagulate the resulting polymer. Thus, S-SBR having no MAB introduced thereinto was prepared [S-SBR (1)]. 
     (2) Reaction of polybutadiene rubber obtained by using a lithium-based catalyst (abbreviated as S-BR), with a benzophenone: 
     A 2-liter stainless steel polymerization reactor was washed, dried, and purged with dry nitrogen. It was then charged with 150 g of 1,3-butadiene, 820 g of benzene, 0.3 to 0.5 millimole of diethylene glycol dimethyl ether (diglyme) and 1.3 millimoles of n-butyllithium (as an n-hexane solution). With stirring, the mixture was subjected to polymerization at 40° C. for 1 hour. Then, 1.5 moles, per mole of the catalyst, of 4,4&#39;-bis(dimethylamino)benzophenone (to be abbreviated MAB) was added to the reaction mixture. The mixture was stirred for 5 minutes to react the living polymer with MAB. The reaction mixture was then poured into a 1.5% by weight methanol solution of 2,6-di-t-butyl-p-cresol (BHT) to coagulate the resulting polymer. The polymer was dried under reduced pressure at 60° C. for 24 hours, and its Mooney viscosity was measured ([S-BR(2*), S-BR(4*), S-BR(6*), and S-BR(8*)]. 
     For comparison, polymerization was carried out in accordance with the aforesaid polymerization recipe. Without reacting the polymer with MAB, the reaction mixture was poured into a methanol solution containing BHT to coagulate the polymer. It was dried to give S-BR having no MAB introduced thereinto [S-BR(1), S-BR(3), S-BR(5), and S-BR(7)]. 
     (3) Reaction of a lithiation product of styrene/butadiene copolymer rubber (E-SBR for short) obtained by emulsion polymerization or a lithiation product of cis-1,4-polybutadiene rubber (cis BR for short) obtained by using a Ziegler catalyst, with a benzophenone: 
     Styrene/butadiene copolymer rubber (combined styrene content 35% by weight) [E-SBR(1)] obtained by usual emission polymerization, styrene/butadiene copolymer rubber (combined styrene content 40% by weight) [E-SBR (2)] obtained by usual emulsion polymerization, or cis 1,4-polybutadiene rubber (cis 1,4-linkage content 98 mole%; Nipol 1220 made by Nippon Zeon Co., Ltd.) obtained by using a Ziegler catalyst was dissolved in toluene, and coagulated with methanol. This operation was repreated twice to remove impurities, and then the product was dried in the same way as in (1) above. 
     In 1300 g of dry benzene was dissolved 130 g of the purified E-SBR(2) or cis BR(1), and 4.6 millimoles of n-butyllithium and 4.6 millimoles of tetramethyl ethylenediamine were added. The mixture was reacted at 70° C. for 1 hour. Then, 6.9 millimoles of MAB was added and reacted for 5 minutes. The reaction product was coagulated and dried in the same way as in (1) above [E-SBR(3*), cis BR(2*)]. 
     Table 1 summarizes the properties of the polymer rubbers described in (1) to (3) above. The manner of bonding of the butadiene unit portion was measured by a usual infrared spectroscopic method. The amount of MAB introduced into the rubber molecular chains was measured by  13  C-NMR. 
     
                                           TABLE 1                                 
__________________________________________________________________________
             Amount of                                                    
       Amount of                                                          
             1,2-           Amount of MAB                                 
       combined                                                           
             linkage                                                      
                   Mooney   introduced (moles per                         
                                       Method of                          
       styrene                                                            
             unit  viscosity                                              
                            mole of the rubber                            
                                       introducing                        
       (wt. %)                                                            
             (mole %)                                                     
                   (ML.sub.1+4, 100° C.)                           
                            molecular chains)                             
                                       MAB                                
__________________________________________________________________________
S-SBR (1)                                                                 
       24.9  35.5  55       0                                             
S-SBR (2*)                                                                
       24.8  36.0  53       0.9        Method I                           
E-SBR (1)                                                                 
       35    --    80       0                                             
E-SBR (2)                                                                 
       40    --    70       0          Method II                          
E-SBR (3*)                                                                
       40    --    78       2.7                                           
Cis BR (1)                                                                
       --    --    42       0                                             
Cis BR (2*)                                                               
       --    --    45       2.7        Method II                          
S-BR (1)                                                                  
       --    68    71       0                                             
S-BR (2*)                                                                 
       --    68    69       0.9        Method I                           
S-BR (3)                                                                  
       --    70    40       0                                             
S-BR (4*)                                                                 
       --    70    58       0.9        Method II                          
S-BR (5)                                                                  
       --    63    69       0                                             
S-BR (6*)                                                                 
       --    63    70       0.9        Method I                           
S-BR (7)                                                                  
       --    75    67       0                                             
S-BR (8*)                                                                 
       --    75    68       0.9        Method I                           
__________________________________________________________________________
 Footnote to Table 1                                                      
 SSBR: Styrene/butadiene copolymer rubber obtained by polymerization in th
 presence of lithiumbased catalyst.                                       
 SBR: Polybutadiene rubber obtained by polymerization in the presence of a
 lithiumbased catalyst                                                    
 ESBR: Styrenebutadiene copolymer rubber obtained by emulsion             
 polymerization                                                           
 cis BR: Nipol 1220 (cis1,4 linkage content 98 mole %, made by Nippon Zeon
 Co., Ltd.)                                                               
 Method I: The living polymer rubber having Li bonded to the ends of the  
 molecular chain is reacted with MAB.                                     
 Method II: The polymer rubber is reacted with a lithium compound and then
 with MAB.                                                                
 
    
     EXAMPLE 1 
     Styrene/butadiene copolymer rubber obtained by polymerization with a lithium-based catalyst and reacted with MAB [S-SBR (2*)], the aforesaid styrene/butadiene copolymer not reacted with MAB [S-SBR (1)] or cis-1,4-polybutadiene rubber [cis BR(1)] obtained by polymerization with a Ziegler catalyst, and various compounding agents were kneaded in accordance with the compounding recipe shown in Table 2 in a Brabender-type mixer having a capacity of 250 ml to obtain rubber compositions. Sulfur and the vulcanization accelerator were added in amounts which would give an optimal vulcanized state when each of the rubber compositions was vulcanized. 
     Each of these rubber compositions was press-cured at 160° C. for 15 to 25 minutes to form test pieces. 
     
                       TABLE 2                                                     
______________________________________                                    
Compounding recipe                                                        
Ingredient           Parts by weight                                      
______________________________________                                    
Starting rubber (see Tables 3 to 7)                                       
                     100                                                  
HAF carbon black     50                                                   
Aromatic process oil 10                                                   
ZnO (No. 3)           3                                                   
Stearic acid          2                                                   
Sulfur                                                                    
N--oxydiethylene-2-benzothiazyl                                           
                     Varying amounts                                      
sulfenamide or N--cyclohexyl-2-                                           
                     (see Tables 3 to 7)                                  
benzothiazyl sulfenamide                                                  
______________________________________                                    
 
    
     The properties of vulcanizates of the resulting rubber compositions were measured. The results are shown in Table 3. 
     The rebound was measured at 53° C. by a Dunlop Tripso Meter. 
     The wet skid resistance was measured at 23° C. by a portable skid tester (made by Stanley Company, Britain) on a road surface of ASTM E-303-74 (a black safety walk, outdoor use type B by 3M Company). 
     
                                           TABLE 3                                 
__________________________________________________________________________
             Run No.                                                      
             Comparison  Invention                                        
Test item    1  2  3  4  5  6  7  8  9  10 11 12                          
__________________________________________________________________________
S-SBR (1)    100                                                          
                -- 70 50 -- 80 70 50 20 -- -- --                          
S-SBR (2*)   -- -- -- -- 100                                              
                            20 30 50 80 90 70 50                          
Cis BR (1)   -- 100                                                       
                   30 50 -- -- -- -- -- 10 30 50                          
Sulfur       1.8                                                          
                1.8                                                       
                   1.8                                                    
                      1.8                                                 
                         1.8                                              
                            1.8                                           
                               1.8                                        
                                  1.8                                     
                                     1.8                                  
                                        1.8                               
                                           1.8                            
                                              1.8                         
Vulcanization accelerator                                                 
             1.4                                                          
                1.4                                                       
                   1.4                                                    
                      1.4                                                 
                         1.4                                              
                            1.4                                           
                               1.4                                        
                                  1.4                                     
                                     1.4                                  
                                        1.4                               
                                           1.4                            
                                              1.4                         
Rebound (%)   59                                                          
                 61                                                       
                   59 60  65                                              
                            61 62 63 64 64 64 63                          
Wet skid resistance                                                       
              75                                                          
                 50                                                       
                   67 62  75                                              
                            75 75 75 75 72 67 62                          
Tensile strength (kg/cm.sup.2)                                            
             229                                                          
                185                                                       
                   213                                                    
                      196                                                 
                         228                                              
                            226                                           
                               230                                        
                                  230                                     
                                     225                                  
                                        220                               
                                           214                            
                                              202                         
Elongation (%)                                                            
             440                                                          
                430                                                       
                   440                                                    
                      440                                                 
                         420                                              
                            430                                           
                               440                                        
                                  430                                     
                                     430                                  
                                        430                               
                                           440                            
                                              430                         
300% Tensile stress                                                       
             130                                                          
                113                                                       
                   122                                                    
                      120                                                 
                         136                                              
                            133                                           
                               133                                        
                                  135                                     
                                     136                                  
                                        130                               
                                           124                            
                                              123                         
(kg/cm.sup.2)                                                             
__________________________________________________________________________
 Note:                                                                    
 The vulcanization accelerator is N--cyclohexyl2-benzothiazyl sulfenamide.
 
    
     By comparing S-SBR (1) (Run No. 1) containing no benzophenone with S-SBR (2*) (Run No. 5) containing a benzophenone introduced therein, it is seen that the introduction of the benzophenone led to an improved rebound. Furthermore, by comparing a mixture of S-SBR(1) and S-SBR(2*) (Run No. 7) with a mixture of S-SBR(1) and cis BR(1) (Run No. 3) which is a conventional tire tread composition, it is seen that the former has markedly improved wet skid resistance and rebound over the latter. 
     EXAMPLE 2 
     Styrene/butadiene copolymer rubber obtained by polymerization with a lithium-based catalyst and having MAB introduced thereinto [B-SBR(2*)], the aforesaid styrene/butadiene copolymer rubber having no MAB introduced thereinto [S-SBR(1)], and polybutadiene rubber obtained by polymerization with a lithium-based catalyst and having MAB introduced thereinto [S-BR(2*), S-BR(4*), S-BR(8*)] or the aforesaid rubbers having no MAB introduced thereinto [S-BR(1), S-BR(3), S-BR(7)] and various compounding agents were kneaded in accordance with the compounding recipe shown in Table 2 in the same way as in Example 1 to form various rubber compositions. 
     Each of these rubber compositions was press-cured at 160° C. for 15 to 25 minutes to prepare test pieces which were tested in the same way as in Example 1. The results are summarized in Tables 4-1 and 4-2. 
     
                                           TABLE 4-1                               
__________________________________________________________________________
              Run No.                                                     
              Comparison  Invention                                       
                                   Comparison                             
                                            Invention                     
Test item     13 14 15 16 17 18 19 20 21 22 23 24 25                      
__________________________________________________________________________
S-SBR (1)     100                                                         
                 -- 90 50 -- 90 50 -- 90 50 -- 90 50                      
S-BR (1)      -- 100                                                      
                    10 50 -- -- -- -- -- -- -- -- --                      
S-BR (2*)     -- -- -- -- 100                                             
                             10 50 -- -- -- -- --                         
S-BR (3)      -- -- -- -- -- -- -- 100                                    
                                      10 50 -- -- --                      
S-BR (4*)     -- -- -- -- -- -- -- -- -- -- 100                           
                                               10 50                      
Sulfur (parts by weight)                                                  
              1.8                                                         
                  1 1.8                                                   
                       1.8                                                
                           1 1.8                                          
                                1.8                                       
                                    1  1 1.8                              
                                             1 1.8                        
                                                  1.8                     
Vulcanization accelerator                                                 
              1.4                                                         
                  2 1.4                                                   
                       1.4                                                
                           2 1.4                                          
                                1.4                                       
                                    2 1.4                                 
                                         1.4                              
                                             2 1.4                        
                                                  1.4                     
(parts by weight)                                                         
Rebound (%)    59                                                         
                  57                                                      
                    58 55  65                                             
                             60 63  55                                    
                                      58 56  60                           
                                               60 61                      
Wet skid resistance                                                       
               75                                                         
                  76                                                      
                    75 76  76                                             
                             75 76  78                                    
                                      75 76  78                           
                                               75 76                      
Tensile strength (kg/cm.sup.2)                                            
              229                                                         
                 185                                                      
                    212                                                   
                       196                                                
                          188                                             
                             215                                          
                                201                                       
                                   178                                    
                                      210                                 
                                         190                              
                                            175                           
                                               215                        
                                                  200                     
Elongation (%)                                                            
              440                                                         
                 400                                                      
                    430                                                   
                       420                                                
                          400                                             
                             430                                          
                                410                                       
                                   390                                    
                                      430                                 
                                         420                              
                                            385                           
                                               430                        
                                                  410                     
300% Tensile stress (kg/cm.sup.2)                                         
              130                                                         
                 126                                                      
                    130                                                   
                       128                                                
                          128                                             
                             130                                          
                                129                                       
                                   123                                    
                                      129                                 
                                         126                              
                                            130                           
                                               130                        
                                                  128                     
__________________________________________________________________________
 Note:                                                                    
 The vulcanization accelerator is N--cyclohexyl2-benzothiazyl sulfenamide 
 when its amount is 1.4 parts by weight, and N--oxydiethylene2-benzothiazy
 sulfenamide when its amount is 2 parts by weight.                        
 
    
     
                       TABLE 4-2                                                   
______________________________________                                    
          Run No.                                                         
          Com-                                                            
          parison Invention                                               
Test item   26     27     28   29   30   31   32                          
______________________________________                                    
S-SBR (1)   --     50     --   50   --   --   --                          
S-SBR (2*)  --     --     --   --   100  50   50                          
S-BR (7)    100    50     --   --   --   50   --                          
S-BR (8*)   --     --     100  50   --   --   50                          
Sulfur       1     1.8     1   1.8  1.8  1.8  1.8                         
(parts by weight)                                                         
Vulcanization                                                             
             2     1.4     2   1.4  1.4  1.4  1.4                         
accelerator                                                               
Rebound (%)  55    55      65  63    65  61   65                          
Wet skid resistance                                                       
             78    77      78  77    75  77   77                          
Tensile strength                                                          
            180    199    190  205  228  197  210                         
(kg/cm.sup.2)                                                             
Elongation (%)                                                            
            390    420    400  420  420  410  430                         
300% Tensile stress                                                       
            125    130    129  132  136  134  131                         
(kg/cm.sup.2)                                                             
______________________________________                                    
 Note:                                                                    
 The vulcanization accelerators used were as indicated in the footnote to 
 Table 41.                                                                
 
    
     In the case of the combination of S-SBR having no MAB and S-BR having no MAB (comparison), the rebound is below additivity, but if at least one of these components has MAB introduced thereinto (invention), the rebound is above additivity, as shown in Tables 4-1 and 4-2. 
     EXAMPLE 3 
     Styrene/butadiene copolymer rubber (combined styrene content 35%) [E-SBR(1)] obtained by emulsion polymerization, E-SBR (3*) obtained by lithiating E-SBR(2) and then reacting the product with MAB, cis 1,4-polybutadiene rubber [cis BR(1)] obtained by using a Ziegler catalyst, or cis BR(2*) obtained by lithiating cis BR(1) and then reacting the product with MAB, and various compounding agents (sulfur was used in an amount of 1.8 parts by weight, and N-cyclohexyl-2-benzothiazyl sulfenamide as a vulcanization accelerator was used in an amount of 1.4 parts by weight) were kneaded by the same operation as in Example 1 in accordance with the compounding recipe shown in Table 2 to give various rubber compositions. Each of these rubber compositions was press-cured at 160° C. for 15 to 25 minutes to prepare test pieces, which were tested in the same way as in Example 1. The results are shown in Table 5. 
     
                                           TABLE 5                                 
__________________________________________________________________________
              Run No.                                                     
              Comparison                                                  
                       Invention                                          
                             Comparison                                   
                                    Invention                             
Test item     33 34 35 36 37 38 39  40 41 42 43                           
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E-SBR (1)     100                                                         
                 -- 50 -- 50 -- --  -- -- -- --                           
E-SBR (2)     -- -- -- -- -- 100                                          
                                50  50 -- -- --                           
E-SBR (3*)    -- -- -- -- -- -- --  -- 100                                
                                          50 50                           
Cis BR (1)    -- 100                                                      
                    50 -- -- -- 50  -- -- 50 --                           
Cis BR (2*)   -- -- -- 100                                                
                          50 -- --  50 -- -- 50                           
Rebound (%)    46                                                         
                  61                                                      
                    53  66                                                
                          60  42                                          
                                56  59  47                                
                                          60 63                           
Wet skid resistance                                                       
               81                                                         
                  50                                                      
                    73  50                                                
                          73  82                                          
                                73  73  82                                
                                          73 73                           
Tensile strength (kg/cm.sup.2)                                            
              280                                                         
                 185                                                      
                    226                                                   
                       178                                                
                          229                                             
                             275                                          
                                230 235                                   
                                       270                                
                                          230                             
                                             238                          
Elongation (%)                                                            
              490                                                         
                 430                                                      
                    460                                                   
                       420                                                
                          450                                             
                             500                                          
                                460 460                                   
                                       490                                
                                          450                             
                                             460                          
300% Tensile stress (kg/cm.sup.2)                                         
              148                                                         
                 113                                                      
                    129                                                   
                       119                                                
                          135                                             
                             150                                          
                                131 135                                   
                                       153                                
                                          136                             
                                             134                          
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     EXAMPLE 4 
     A 50:50 (by weight) mixture of natural rubber (abbreviated NR) and polybutadiene rubber obtained by polymerizing with a lithium-based catalyst [S-BR(1), S-BR(5) or S-BR(7)] or rubber obtained by introducing MAB into the aforesaid rubber [S-BR(2*), S-BR(6*) or S-BR (8*)], and various compounding agents were mixed by the same operation as in Example 1 in accordance with the compounding recipe shown in Table 2 to give various rubber compositions. Each of the rubber compositions was press-cured at 160° C. for 15 minutes to prepare test pieces, which were tested in the same way as in Example 1. The results are shown in Table 6. 
     
                       TABLE 6                                                     
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       Run No.                                                            
       Comparison    Invention                                            
Test item                                                                 
         44     45     46   47   48   49   50   51                        
______________________________________                                    
NR       100    50     50   50   --   50   50   50                        
S-BR (1) --     50     --   --   --   --   --   --                        
S-BR (2*)                                                                 
         --     --     --   --   --   50   --   --                        
S-BR (5) --     --     50   --   --   --   --   --                        
S-BR (6*)                                                                 
         --     --     --   --   100  --   50   --                        
S-BR (7) --     --     --   50   --   --   --   --                        
S-BR (8*)                                                                 
         --     --     --   --   --   --   --   50                        
Sulfur   2.0    1.5    1.5  1.5  1.5  1.5  1.5  1.5                       
Vulcanization                                                             
         0.8    1.4    1.4  1.4  1.4  1.4  1.4  1.4                       
accelerator                                                               
Rebound (%)                                                               
          63    62     62   60    65  66   66   65                        
Wet skid  68    73     71   74    74  73   71   74                        
resistance                                                                
Tensile  301    211    212  213  185  194  205  193                       
strength                                                                  
(kg/cm.sup.2)                                                             
Elongation                                                                
         530    450    440  440  400  400  410  390                       
(%)                                                                       
300% Tensile                                                              
         164    122    122  125  126  133  131  132                       
stress (kg/cm.sup.2)                                                      
______________________________________                                    
 Note:                                                                    
 The vulcanization accelerator is N--oxydiethylene2-benzothiazyl          
 sulfenamide.                                                             
 
    
     EXAMPLE 5 
     Styrene/butadiene copolymer rubber having 4,4&#39;-diaminobenzophenone introduced thereinto [S-SBR (2&#39;*) having the same properties as S-SBR(2*)] was prepared by repeating the procedure of Section (1) of Production Example except that the aforesaid benzophenone was used instead of MAB. Furthermore, polybutadiene rubber having 4,4&#39;-diaminobenzophenone introduced thereinto [S-BR(2&#39;*) having the same properties as S-BR(2*)] was prepared by repeating Section (2) of Production Example except that the aforesaid benzophenone was used instead of MAB. 
     Each of these rubbers and various compounding agents were mixed in the same way as in Example 1 in accordance with the compounding recipe shown in Table 2 to give various rubber compositions. Each of these rubber compositions was press-cured at 160° C. for 15 to 25 minutes to prepare test pieces, which were tested in the same way as in Example 1. The results are shown in Table 7. 
     
                       TABLE 7                                                     
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          Run No.                                                         
          Comparison Invention                                            
Test item   52     53     54   55   56   57   58                          
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S-SBR (1)   100    --     50   --   --   50   80                          
S-SBR (2&#39;*) --     --     --   100  --   --   20                          
S-BR (1)    --     100    50   --   --   --   --                          
S-BR (2&#39;*)  --     --     --   --   100  50   --                          
Sulfur      1.8     1     1.8  1.8   1   1.8  1.8                         
Vulcanization                                                             
            1.4     2     1.4  1.4   2   1.4  1.4                         
accelerator                                                               
Rebound (%)  59     57    55    64   64  61   61                          
Wet skid resistance                                                       
             75     76    76    75   76  76   75                          
Tensile strength                                                          
            229    185    196  225  188  199  228                         
(kg/cm.sup.2)                                                             
Elongation (%)                                                            
            440    400    420  430  400  410  430                         
300% Tensile stress                                                       
            130    126    128  136  130  130  131                         
(kg/cm.sup.2)                                                             
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 Note:                                                                    
 The vulcanization accelerators used were as indicated in the footnote to 
 Table 41.