All-season tire
The present invention relates to an all-season tire having high-wet grip properties, low rolling resistance, excellent low-temperature characteristics, and excellent on-ice and on-snow performance at a highly balanced level and with no sacrifice whatsoever of abrasion resisting characteristics. 
     Auto tires are individually required to have various different constructions and compositions having such characteristics as will meet various different weather conditions, such as, for example, intensely sunny conditions under which the road surface is dry, rainy conditions under which the road surface is wet, and snowy conditions under which the road surface is covered with snow. 
     Hitherto, it has been a general practice to use, according to different weather conditions and purposes, various auto tires which are entirely different in composition and construction, such as general-purpose tires, high wet-grip tires, and snow tires. 
     However, changing tires according to the change in weather and climate conditions involves considerable loss in respect of cost. It is necessary to have different sets of tires at hand, and tire changing requires much labor and cost. 
     For this reason, demand has been increasing for an all-season tire, that is, a tire available under any weather condition and which exhibits good performance under any service conditions regardless of weather and climate. 
     Generally, it is a predominant practice to use as tread rubber for winter-service tires a polymer blend of natural rubber (NR) and butadiene rubber (BR), with carbon black and oil loaded in relatively large proportions. Such tread rubber has good suitability for service at low temperatures because the glass transition temperature (Tg) thereof is low, but it has a disadvantage that its grip capability on a wet road, or coefficient of friction on such road (Wetμ), is low because of the presence of BR as a blend component. Another disadvantage is that a relatively large fill of carbon entails greater rolling resistance (RR) which brings energy loss. In other words, such tread rubber is disadvantageous for service under wet conditions and under general conditions, while it has good performance characteristics for winter service. 
     Tread rubber for summer-service tires is predominantly composed of a styrene-butadiene rubber produced by emulsion polymerization (E-SBR) as a main component. Tread rubber of such type exhibits fairly good characteristics under wet conditions, but it produces considerable energy loss due to RR and is inferior in low-temperature characteristics. 
     Attempts have been made to provide a tread rubber composition which is less liable to energy loss and which has excellent Wetμ. For example, there is known a blend of NR with high-vinyl styrene-butadiene rubber (SBR) in which the butadiene (BD) part has a large 1,2-vinyl structure (particularly, more than 50 mole% of 1,2-vinyl structure). Also known is a blend of NR with high-vinyl BR having a large 1,2-vinyl structure. Such blends have high Wetμ, but have a drawback in that their hardness at low temperatures is excessively high, that is, they are likely to become excessively stiffened so that their low-temperature characteristics are very unfavorable. 
    
    
     An object of the present invention is to provide an all-season tire having high wet-grip properties, low rolling resistance, excellent low-temperature characteristics, and excellent on-ice and on-snow performance, at a highly balanced level and with no sacrifice whatsoever of abrasion resisting characteristics. 
     The invention provides a tire having a tread, wherein a polymer component in the rubber composition constituting the tread comprises at least 30% by weight of a styrene-butadiene rubber produced by solution polymerization (S-SBR), the S-SBR comprising 10 to 20% by weight of styrene units and the balance of butadiene units; a content of butadiene units having 1,2-vinyl structure of 35 to 50 mole%; a ratios Mw/Mn as an indication of molecular weight distribution of 1.2 to 3.5; and a content of isolated styrene units of 40% by weight or more and a content of styrene units in the form of long chain blocks each consisting of 8 or more styrene units bound to each other, of 5% by weight or less, both contents being determined by decomposing the S-SBR with ozone and by applying a gel permeation chromatographic analysis to the ozone decomposition product, and wherein the pattern of the tread is such that the contact area ratio as defined by equation 1 is 60 to 67%, the lateral groove density as defined by equation 2 is within the range of 0.041 to 0.056 mm/mm 2 , and the lateral siping density as defined by equation 3 is within the range of 0.054 to 0.073 mm/mm 2  : ##EQU1## 
     In the equations, each of contact area, length of lateral groove or siping component and tread area is measured on the basis of a development elevation view of tread pattern. 
     According to the invention, it is possible to obtain an all-season tire of the invention by using a solution-polymerized styrene-butadiene rubber (S-SBR) having the above specified microstructure and by selecting a tread pattern having a proper contact area ratio, a proper lateral groove density, and a proper lateral siping density. 
     The S-SBR used for the purpose of the invention is a random copolymer and comprises 10 to 20% by weight of styrene units and has a content of butadiene units having 1,2-vinyl structure of 35 to 50 mole%. It may be used alone, or may be used in blend with NR. In the latter case, it is preferable to blend so that the blend consists of 30% by weight or more of S-SBR and 70% by weight or less of NR. Where the styrene content of S-SBR is within the above specified range, high tensile strength, low rolling resistance, and high abrasion resistance can be assured. Where the 1,2-vinyl structure is within the above specified range, high abrasion resistance, low rolling resistance, and favorable coefficient of sliding friction can be obtained in good balance. 
     Said S-SBR must be such that Mw/Mn as an indication of its molecular weight distribution, that is, the ratio between weight average molecular weight and number average molecular weight, is 1.2 to 3.5. If Mw/Mn is smaller than 1.2, no satisfactory processability is obtainable, and if it is larger than 3.5, low rolling-resistance is not obtainable to any satisfactory extent. Referring to the styrene unit distribution as analyzed by gel permeation chromatography (GPC), it is required that a content of isolated styrene units, namely 1-chain styrene units is 40% by weight or more of the entire bound styrene, and a content of styrene units in the form of long chain blocks each consisting of 8 or more styrene units bound to each other, of 5% by weight or less of the entire bound styrene. In any case, deviation from the above limits are undesirable because such advantageous characteristics of S-SBR according to the invention as low rolling resistance and good abrasion resistance cannot be obtained. 
     The S-SBR in the invention preferably has a molecular weight distribution of a two or more peak (multimodal) pattern as formed in the process of polymerization by using a coupling agent such as tin tetrachloride. Such S-SBR exhibits excellent low rolling resistance compared with those having a molecular weight distribution of one-peak (monomodal) pattern, hence preferable. 
     The advantageous features of the S-SBR according to the invention are not limited to well-balanced porperties such as RR, Wetμ, and abrasion resistance. The S-SBR exhibits excellent performance under low temperature conditions. It is less liable to stiffening at low temperatures. It shows a high coefficient of friction on ice (Iceμ). 
     In the present invention, attention is paid not only to rubber composition for the tread, but also to the tread pattern. It has been found that a tread pattern of an all-season tire should preferably be such that the contact area ratio as defined by the aforesaid equation 1 is 60 to 67%, the lateral groove density as defined by equation 2 is within the range 0.041 to 0.056 mm/mm 2 , and the lateral siping density as defined by equation 3 is within the range of 0.054 to 0.073 mm/mm 2 . 
     With the contact area ratio of 68 to 80% as in the case with ordinary tires for typical summer service, no sufficient grip on ice or snow surfaces in winter can be obtained. Whilst, if the tread has a contact area ratio of 50 to 59% as in the case of conventional snow tires, any sufficient traction cannot be obtained under ordinary summer conditions. For tread pattern groove, if the lateral groove density is greater than that specified herein, their edge effect will become greater, the result is increased edge effect, which means excellent on-snow performance, but unfavorable abrasion resistance. Conversely, if the density is below the lower limit specified, there results considerable decrease in edge effect, which means unsatisfactory on-snow performance. Same thing can be said with lateral siping density. A density greater than specified means inferior abrasion resistance. If it is lower than specified, unfavorable on-snow performance. 
     In this invention no particular limitation is put on the construction of tire tread. The tread portion may be composed of one layer (unit compound tread type), or of two layers, that is, a base and a cap placed thereon (cap/base type). In the case of cap/base construction, needless to say, the present invention is applied to the cap portion. 
     The invention will be described in more detail with reference to the Examples, but they are not to be construed as limiting the scope of this invention. 
     Table 1 shows a formulation used in the evaluation of S-SBR shown in Table 2. 
     It is noted that abrasion resistance, RR, Wetμ, and Iceμ as shown in Tables 2, 3 and 4 are given in terms of index numbers, with the following recipe taken as basis=100 (recipe wherein 100 parts by weight of SBR 1502 were used in place of S-SBR in Table 1). 
     
                       TABLE 1                                                     
______________________________________                                    
S-SBR               100    part by weight                                 
ISAF grade carbon black                                                   
                    50                                                    
Aromatic process oil                                                      
                    10                                                    
Stearic acid        2                                                     
Zinc oxide          5                                                     
Antioxidant         1.5                                                   
N--t-Butyl-2-benzothiazolyl-                                              
                    1                                                     
sulfenamide                                                               
Sulfur              1.6                                                   
______________________________________                                    
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
                 Example                                                  
                 1  2  3  4  5  6  7  8  9                                
__________________________________________________________________________
Amount of bound styrene (wt %)                                            
                 11.5                                                     
                    15 10 18 18 15 15 20 12                               
1,2-Vinyl structure in BD                                                 
                 45 43 37 44 45 45 50 37 48                               
(mole %)                                                                  
Cis-1,4 structure in BD                                                   
                 18 19 23 19 18 18 17 23 17                               
(mole %)                                                                  
Trans-1,4 structure in BD                                                 
                 37 38 40 37 37 37 33 40 35                               
(mole %)                                                                  
(--Mw/--Mn)      1.3                                                      
                    2.3                                                   
                       3.0                                                
                          1.5                                             
                             3.2                                          
                                1.8                                       
                                   2.0                                    
                                      2.8                                 
                                         3.4                              
Content of styrene unit                                                   
S1 (wt %)        42 51 43 48 42 52 45 42 48                               
S8 (wt %)        0.4                                                      
                    2.7                                                   
                       2.5                                                
                          1.8                                             
                             2.6                                          
                                4.0                                       
                                   4.3                                    
                                      2.8                                 
                                         4.7                              
Tg (°C.)  -59                                                      
                    -56                                                   
                       -69                                                
                          -52                                             
                             -51                                          
                                -54                                       
                                   -50                                    
                                      -57                                 
                                         -55                              
Roll processability (1)                                                   
                 G  E  E  G  E  E  E  E  G                                
Surface condition of                                                      
                 4  5  5  4  5  5  5  5  4                                
extruded tread (2)                                                        
Edge condition of                                                         
                 4  5  4  4  5  4  5  5  4                                
extruded tread (2)                                                        
Property of vulcanized rubber (3)                                         
Tensile strength (4)                                                      
                 220                                                      
                    230                                                   
                       209                                                
                          256                                             
                             224                                          
                                238                                       
                                   241                                    
                                      262                                 
                                         218                              
Abrasion resistance (5)                                                   
                 109                                                      
                    102                                                   
                       111                                                
                          100                                             
                             99 102                                       
                                   100                                    
                                      98 98                               
Tire property                                                             
RR (6)           78 83 75 86 86 84 84 86 80                               
Wet μ (7)     102                                                      
                    104                                                   
                       100                                                
                          112                                             
                             110                                          
                                105                                       
                                   106                                    
                                      108                                 
                                         105                              
__________________________________________________________________________
                 Comparison Example                                       
                 1  2  3  4  5  6  7  8  9  10                            
__________________________________________________________________________
Amount of bound styrene (wt %)                                            
                 18 18 14 17 12 19 23 8.5                                 
                                         12 16                            
1,2-Vinyl structure in BD                                                 
                 45 45 47 48 45 40 45 40 30 60                            
(mole %)                                                                  
Cis-1,4 structure in BD                                                   
                 12 18 17 17 17 18 18 20 24 13                            
(mole %)                                                                  
Trans-1,4 structure in BD                                                 
                 43 37 36 35 38 42 37 40 46 27                            
(mole %)                                                                  
(--Mw/--Mn)      1.17                                                     
                    4.1                                                   
                       1.4                                                
                          3.2                                             
                             2.8                                          
                                3.0                                       
                                   1.9                                    
                                      2.0                                 
                                         2.5                              
                                            3.1                           
Content of styrene unit                                                   
S1 (wt %)        41 43 35 28 41 42 42 43 45 41                            
S8 (wt %)        3.0                                                      
                    2.8                                                   
                       4.9                                                
                          4.5                                             
                             5.8                                          
                                7.5                                       
                                   4.3                                    
                                      3.0                                 
                                         2.8                              
                                            2.7                           
Tg (°C.)  -51                                                      
                    -51                                                   
                       -53                                                
                          -48                                             
                             -58                                          
                                -54                                       
                                   -43                                    
                                      -67                                 
                                         -74                              
                                            -38                           
Roll processability (1)                                                   
                 F  E  G  G  E  G  G  E  E  G                             
Surface condition of                                                      
                 3  5  4  4  5  4  4  5  5  5                             
extruded tread (2)                                                        
Edge condition of                                                         
                 2  5  4  4  5  4  4  5  5  4                             
extruded tread (2)                                                        
Property of vulcanized rubber (3)                                         
Tensile strength (4)                                                      
                 235                                                      
                    210                                                   
                       229                                                
                          240                                             
                             231                                          
                                238                                       
                                   252                                    
                                      212                                 
                                         198                              
                                            240                           
Abrasion resistance (5)                                                   
                 99 86 90 88 87 83 94 111                                 
                                         113                              
                                            93                            
Tire property                                                             
RR (6)           84 90 92 95 91 94 105                                    
                                      78 77 108                           
Wet μ (7)     111                                                      
                    109                                                   
                       106                                                
                          112                                             
                             103                                          
                                107                                       
                                   120                                    
                                      93 94 118                           
__________________________________________________________________________
 Notes to Table 2:                                                        
 (1) Roll wrapping properties, controllability, etc. were evaluated using 
 12 inch rolls. General evaluation is shown. &#34;E&#34; means excellent, &#34;G&#34; good
 and &#34;F&#34; fair.                                                            
 (2) Shown in 5point system (point 5 max).                                
 (3) Heated and press cured at 160° C. for 20 min.                 
 (4) Measured according to JIS K6310 (kg/cm.sup.2).                       
  (5) Evaluated according to ASTM D2228 and by using Pico abrasion tester.
 Values shown in index numbers, with SBR 1502, an ESBR, taken as 100. The 
 greater the value, the better.                                           
 (6) Tires of 185/70HR14 were made; recipes shown were used for treads.   
 Their performance characteristics were measured. Pattern used: contact   
 area ratio 64%, lateral groove density 0.049 mm/mm.sup.2, lateral siping 
 density 0.064 mm/mm.sup.2. Measu rements were made according to Twinroll 
 method, described in American Association of Automobile Engineers&#39;s      
 journal SAE No. 770875. Tires each fitted on a 5J×14 rim. Rolling  
 resistance (RR) measured under conditions of air pressure, 1.8 kg/cm.sup.
 , and load, 336 kg. SBR 1502=100. The smaller the value, the better.     
 (7) With same tires as those used in the rolling resistance tests, Wet   
 μ was measured according to the method specified under UTQGS, American
 tire quality grade standards. Tires, fitted on 5J×14 rim, were     
 mounted to a test trailer. Run on  wet asphalt road surface under        
 conditions of air pressure 1.8 kg/cm.sup.2 and load 336 kg. Wet μ was 
 measured when tire revolution was locked. Measurements shown in indices, 
 with SBR 1502=100. Larger values are more favorable. Content of styrene u
 its in polymer, and microstructure of butadiene were measured by Hampton 
 method using infrared spectrum. [Reference: Anal. Chem. 21, 923 (1949)] I
 the Table 2, S1 means a content of isolated styrene units and S8 means a 
 content of styrene units in the form of long chain blocks each consisting
 of 8 or more styrene units.                                              
 
    
     As Table 2 shows, Comparison Example 1, where a polymer having a molecular weight distribution of 1.17 was used, had poor roll-workability, with unfavorable surface and edge conditions of extruded tread. In the case of Comparison Example 2 in which a polymer having a molecular weight distribution of 4.1 is used, abrasion resistance was unsatisfactory and rolling resistance was considerable. Table 2 proves that molecular weight distribution should be preferably 1.2 to 3.5. 
     It was found that there must be a content of bound styrene of 10 to 20% by weight, with a content of butadiene (BD) units having 1,2-vinyl structure of 35 to 50 mole%. If, as in Comparison Example 7, the bound styrene content accounts for as much as 23% by weight, or if, as in Comparison Example 10, the content of 1,2-vinyl structure amounts to as much as 60 mole%, the tire is subject to considerable rolling resistance, and accordingly to greater energy loss. If the bound styrene content is no more than 8.5% by weight as in Comparison Example 8, or if the content of 1,2-vinyl structure is no more than 30 mole% as in Comparison Example 9, the on-wet-surface skid resistance Wetμ is insufficient. 
     Referring next to chain length of styrene units, Comparison Examples 3 and 4 in which isolated (1-chain) styrene units constitute less than 40% by weight of the entire bound styrene content are cases in which the rolling resistance (energy loss) of the tire is considerable and the abrasion resistance is unfavorable. Same thing can be said with Comparison Examples 5 and 6 in which 8-or-more chain styrene units account for more than 5% by weight of the entire bound styrene. Therefore, it is clear that 1-chain styrene units should constitute 40% by weight or more of the entire bound styrene, and 8-or-more chain styrene units should constitute less than 5% by weight of the entire bound styrene. Further, as is apparent from Table 2, it is desirable to use a polymer having a glass transition temperature of -50° C. or below (Tg, measured by Model DSC-2, Perkin Elmer). A polymer having a Tg higher than -50° C. brings greater rolling resistance of the tire. 
     Table 3 shows characteristics of S-SBR/natural rubber blends, with comparison examples for reference. 
     In the recipes shown are used 100 parts by weight of a mixture of S-SBR and NR in place of 100 parts by weight of S-SBR in Table 1. 
     
                                           TABLE 3                                 
__________________________________________________________________________
                Example        Com. Example                               
                10 11 12 13 11 12 13 14 15 16 17                          
__________________________________________________________________________
S-SBR (wt parts)                                                          
                70 50 60 40 70 60 70 50 70 50 20                          
Amount of bound styrene (wt %)                                            
                15 15 18 19 13 18 25 7  18 16 19                          
1,2-Vinyl structure in BD                                                 
                47 47 37 35 28 57 46 38 48 38 35                          
(mole %)                                                                  
Cis-1,4 structure in BD                                                   
                17 18 23 24 25 13 18 22 17 23 21                          
(mole %)                                                                  
Trans-1,4 structure in BD                                                 
                36 35 40 41 47 30 36 40 35 39 44                          
(mole %)                                                                  
(--Mw/--Mn)     1.5                                                       
                   2.3                                                    
                      1.8                                                 
                         3.2                                              
                            1.6                                           
                               3.0                                        
                                  2.8                                     
                                     2.5                                  
                                        1.5                               
                                           1.8                            
                                              3.2                         
Content of styrene unit                                                   
S1 (wt %)       52 52 45 43 50 45 53 48 26 49 43                          
S8 (wt %)       1.8                                                       
                   1.8                                                    
                      2.5                                                 
                         3.4                                              
                            0.9                                           
                               0.8                                        
                                  2.0                                     
                                     3.1                                  
                                        4.0                               
                                           6.5                            
                                              3.4                         
Tg (°C.) -52                                                       
                   -52                                                    
                      -57                                                 
                         -57                                              
                            -73                                           
                               -36                                        
                                  -39                                     
                                     -71                                  
                                        -48                               
                                           -58                            
                                              -57                         
NR (wt parts)   30 50 40 60 30 40 30 50 30 50 80                          
Property of vulcanized rubber                                             
Tensile strength                                                          
                260                                                       
                   293                                                    
                      258                                                 
                         298                                              
                            220                                           
                               256                                        
                                  281                                     
                                     229                                  
                                        253                               
                                           231                            
                                              299                         
Abrasion resistance                                                       
                102                                                       
                   101                                                    
                      99 98 113                                           
                               97 95 117                                  
                                        85 82 90                          
Tire property                                                             
RR              83 86 84 86 80 104                                        
                                  102                                     
                                     78 100                               
                                           92 88                          
Wet μ        104                                                       
                   113                                                    
                      107                                                 
                         109                                              
                            92 117                                        
                                  115                                     
                                     93 110                               
                                           106                            
                                              98                          
__________________________________________________________________________
 
    
     As described above, the tire tread of the invention exhibits excellent performance in all characteristics such as energy loss due to rolling resistance of the tire, friction force on wet road surface, abrasion resistance, and workability in production process. This is attributable to the fact that the S-SBR has a content of styrene units of 10 to 20% by weight and a content of butadiene units having 1,2-vinyl structure of 35 to 50 mole%, that Mw/Mn is 1.2 to 3.5, and that the S-SBR has a content of isolated styrene units of 40% by weight or more and a content of styrene units in the form of long chain blocks each consisting of 8 or more styrene units bound to each other, of 5% by weight or less, both contents being determined by decomposing the S-SBR with ozone and by applying a gel permeation chromatographic analysis to the ozone decomposition product. This rubber may be used alone, or may be used suitably in blend with natural rubber as shown in Table 3. It must be noted, however, as is apparent from a comparison of Example 13 and Comparison Example 17, 30% by weight or more of S-SBR is required where it is used in blend with NR. 
     The advantages of the S-SBR according to the invention are not limited to the above explained performance characteristics. The S-SBR has surprisingly good low-temperature characteristics. 
     Known blends of high-vinyl SBR or high-vinyl BR and NR intended to provide a tread composition liable to less energy loss and having excellent Wetμ, wherein the high-vinyl SBR has a larger content of butadiene units having 1,2-vinyl structure, that is, 50 mole% or more of such structure, or the high-vinyl BR has a larger 1,2-vinyl content, have a favorable balance of Wetμ and rolling resistance, but are liable to become stiffened at low temperatures; therefore, their low-temperature characteristics are rather poor. 
     Special winter-service compositions using a blend of NR, a polymer claimed to have good low-temperature characteristics, and a high-cis butadiene rubber (high-cis BR), with large amounts of carbon black and oils incorporated therein (generally, 70 parts by weight or more of carbon black and 30 parts by weight or more of oils), have good low-temperature characteristics, being less liable to become stiffened at low temperatures. However, their Wetμ is low because a high-cis BR is used, and further, they have considerable RR because their carbon content is large. They are advantageous as such for winter service, but their performance characteristics under wet conditions and general conditions are rather unfavorable. 
     In the field of all-season tire treads to which the present invention is directed, there have previously been used rubber blends containing emulsion-polymerized styrene-butadiene rubber (E-SBR) and BR. The tread-rubber composition of the all-season tire of the invention comprising rubber components including S-SBR or a mixture of S-SBR and NR have remarkable advantages over the conventional compositions in respect of various characteristics. 
     Table 4 below indicates that the tread rubber compositions of the all-season tire of the invention have an exceedingly favorable balance of performance characteristics under all conditions. The S-SBR used in Table 4 is shown in detail in Table 5. 
     
                                           TABLE 4                                 
__________________________________________________________________________
Composition                                                               
           Example        Com. Example                                    
(parts by weight)                                                         
           14 15 16 17 18 18 19 20 21 22 23 24                            
__________________________________________________________________________
S-SBR-1    100                                                            
              50                                                          
S-SBR-2          70 50 50                                                 
S-SBR-3                         100                                       
S-SBR-4                            50                                     
SBR 1502                                 70 60                            
NR            50 30 50 50 70 60    50 50                                  
98% Cis BR                30 40          30 40                            
80% Vinyl BR                          50                                  
HAF grade Carbon                                                          
           50 60       50    90 50 60    70                               
black                                                                     
ISAF grade Carbon                                                         
                 50 60    80          50    60                            
black                                                                     
Aromatic oil                                                              
           10 20 10 20 15 30 50 10 20 10 30 10                            
Naphthenic oil            30                10                            
Zinc oxide 3  3  3  3  3  3  3  3  3  3  2  2                             
Stearic acid                                                              
           3  2  3  2  2  3  3  3  3  3  2  2                             
Wax        2  2  2  2  2  0  0  2  2  2  1  1                             
Antioxidant                                                               
           2  1.5                                                         
                 2  1.5                                                   
                       1.5                                                
                          3  3  2  1.5                                    
                                      1.5                                 
                                         1.5                              
                                            1.5                           
Vulcanization                                                             
           0.2                                                            
              0.2                                                         
                 0.2                                                      
                    0.2                                                   
                       0.2                                                
                          0  0  0.2                                       
                                   0.2                                    
                                      0.2                                 
                                         0.2                              
                                            0.2                           
accelerator D (1)                                                         
Vulcanization                                                             
           1.5                                                            
              1.5                                                         
                 1.5                                                      
                    1.5                                                   
                       1.5                                                
                          1.8                                             
                             1.8                                          
                                1.5                                       
                                   1.5                                    
                                      1.5                                 
                                         1.3                              
                                            1.3                           
accelerator MSA (2)                                                       
Sulfur     2  2  2  2  2  2  2  2  2  2  2.3                              
                                            2.3                           
Property of                                                               
vulcanized rubber                                                         
Abrasion   100                                                            
              101                                                         
                 103                                                      
                    105                                                   
                       98 62 85 93 92 83 90 101                           
resistance                                                                
Hardness (0° C.)                                                   
           61 59 59 56 58 47 54 65 65 63 68 64                            
(-20° C.)                                                          
           68 64 65 63 65 52 62 75 72 70 76 75                            
Tire property                                                             
RR         84 85 84 86 84 115                                             
                             120                                          
                                92 90 86 96 92                            
Wet μ   106                                                            
              109                                                         
                 108                                                      
                    112                                                   
                       115                                                
                          93 87 115                                       
                                   112                                    
                                      108                                 
                                         90 88                            
Ice μ   117                                                            
              123                                                         
                 120                                                      
                    121                                                   
                       118                                                
                          158                                             
                             143                                          
                                89 95 98 102                              
                                            104                           
Wet μ/RR                                                               
           1.28                                                           
              1.28                                                        
                 1.29                                                     
                    1.30                                                  
                       1.37                                               
                          0.81                                            
                             0.73                                         
                                1.25                                      
                                   1.24                                   
                                      1.26                                
                                         0.94                             
                                            0.96                          
__________________________________________________________________________
 (1) 1,3Diphenyl guanidine                                                
 (2) N--Oxydiethylene2-benzothiazylsulfenamide                            
 
    
     
                                           TABLE 5                                 
__________________________________________________________________________
                 S-SBR-1                                                  
                      S-SBR-2                                             
                           S-SBR-3                                        
                                S-SBR-4                                   
__________________________________________________________________________
Amount of bound styrene (wt %)                                            
                 15   18   22   15                                        
1,2-Vinyl structure in BD                                                 
                 45   40   48   60                                        
(mole %)                                                                  
Cis-1,4 structure in BD                                                   
                 18   21   17   13                                        
(mole %)                                                                  
Trans-1,4 structure in BD                                                 
                 37   39   35   27                                        
(mole %)                                                                  
(--Mw/--Mn)      1.5  2.5  1.8  2.5                                       
Content of styrene unit                                                   
S1 (wt %)        55   47   30   32                                        
S8 (wt%)         1.3  0.9  1.8  5.2                                       
Tg (°C.)  -54  -53  -40  -38                                       
__________________________________________________________________________
 Notes to Table 4                                                         
 Hardness . . . measured according to JIS K6301                           
 Ice μ . . . Onice friction coefficients at -10° C., shown in   
 indices. The recipe, in which 100 parts by weight of SBR 1502 were used i
 place of SSBR in Table 1 is taken as base=100. Larger values are more    
 favorable.                                                               
 
    
     As Table 4 shows, Comparison Examples 18 and 19 which represent special recipes for winter-service indicate less stiffening characteristics at low temperatures and relatively high Iceμ values, but are at a rather low level in RR and Wetμ. 
     Comparison Examples 20, 21 and 22, which are aimed at energy saving and high Wetμ, all show low RR and yet favorable Wetμ; but their hardness at low temperatures is rather high and their Iceμ is extremely low. As such, they are not suitable for low-temperature service. 
     Where S-SBR is used, but the content of 1,2-vinyl structure of the butadiene part exceeds 50 mole%, any satisfactory composition cannot be obtained which has a desired level of RR and Iceμ characteristics, even if the styrene content is within the range of 10 to 20% by weight (S-SBR-4). This is also true with S-SBR-3 in which the styrene content exceeds 20% by weight. Thus, it is essential that the styrene content must be within the range of 10 to 20% by weight and the 1,2-vinyl content of the butadiene part must be 35 to 50 mole%. 
     E-SBR and BR blends (Comparison Examples 23 and 24) previously known as typical all-season tire tread compositions have a balance of RR, Wetμ and Iceμ characteristics in low levels. In all respects, however, the tread composition of the all-season tire of the present invention have very remarkable advantages over the above conventional compositions. It is to be noted that plasticizers such as DOA (dioctyl adipate) may be suitably used in addition to such agents as softening agent, aromatic oil and naphthenic oil shown in the table. 
     In Tables 6 to 8 are shown the effects of tire tread pattern, that is, contact area ratio, lateral groove density and lateral siping density, which represent the results of research efforts directed by the present invention. The term &#34;contact area ratio&#34; referred to herein the ratio of area of the portion in contact with the road surface to the total projected area of the tread, as determined on the basis of a development eleva-tion view of tread pattern. That is, it refers to an entire tread portion excluding groove, siping, and like recesses. 
     
                       TABLE 6                                                     
______________________________________                                    
            Com. Ex.                                                      
                    Ex.        Com. Ex.                                   
            25   26     19      20   27   28                              
______________________________________                                    
Contact area ratio                                                        
              50     55     62    65   72   80                            
Tire characteristics                                                      
RR            100    114    119   123  141  153                           
Wet μ      100    104    115   120  122  118                           
Wet μ/RR   1.00   0.91   0.97  0.98 0.87 0.77                          
Ice μ      100    105    112   114  114  117                           
On-snow performance                                                       
              5.0    5.0    5.0   4.5  3.5  3.0                           
______________________________________                                    
 Note:                                                                    
 (1) Compositions of tire tread rubber in Tables 6 to 8 are shown below.  
 
    
     
         S-SBR-1 (see Table 5)                                                     
                    50    (parts by weight)                               
NR                  50                                                    
Carbon black (ISAF grade)                                                 
                    55                                                    
Aromatic oil        15                                                    
Zinc oxide          3                                                     
Stearic acid        3                                                     
Wax                 2                                                     
Antioxidant         2                                                     
Vulcanization accelerator                                                 
                    1.5                                                   
CZ (1)                                                                    
Sulfur              2                                                     
 The above (1) is N--cyclohexyl2-benzothiazyl-sulfenamide.                
 (2) Tire size is 185/70HR14. Lateral groove density of pattern is 0.049  
 mm/mm.sup.2, and lateral siping density is 0.064 mm/mm.sup.2.            
 (3) In Table 6, RR, values for Wet μ and Ice μ are given in terms o
 indices as calculated on the basis of Comparison Example 25. The smaller 
 the value of RR, the better, while the greater the value of Wet μ or  
 Ice μ, the better.                                                    
 (4) Onsnow performance is indicated by 5point method (point 5 max) on the
 basis of onsnow steering stability measurements.                         
 
    
     According to our researches, contact area ratio is generally 68 to 80% in the case of ordinary summer-service tires, and is 50 to 59% in the case of snow tires. 
     As Table 6 shows, the rolling resistance (as an indication of energy loss) of a tire becomes unfavorable as the contact area ratio becomes greater. This tendency is remarkable particularly in the case of Comparison Examples 27 and 28 (contact area ratios 72% and 80% respectively). 
     On-wet-surface friction coefficient Wetμ tends to improve as the contact area ratio increases, but tends to rather grow worse when the contact area ratio exceeds 67%. 
     On-ice friction coefficient Iceμ tends to show little or no improvement when the contact area ratio of about 60% is reached. Where contact area ratio is large (as in Comparison Examples 27, 28), no favorable on-snow performance is achievable, but as can be seen from Examples 19 and 20, satisfactory on-snow performance can be attained, if contact area ratio is 67% or below. 
     From the standpoint of balancing of above four characteristic elements, an all-season tire tread should preferably have a contact area ratio of 60 to 67%. 
     Table 7 shows the effects of lateral groove density. If lateral groove density is smaller than 0.041 mm/mm 2   as in Comparison Example 29, edge effect is small, and accordingly, on-snow performance of the tire is rather poor. If lateral groove density is larger than 0.056 mm/mm 2  as in Comparison Example 30, the tire can exhibit satisfactory on-snow performance, but its abrasion resistance is unfavourably low. As can be seen from Examples 21, 22 and 23, therefore, it is preferable that the lateral groove density of an all-season tire tread should be within the range of 0.041 to 0.056 mm/mm 2 . 
     Table 8 shows the influence of lateral siping density. The effect of lateral siping density shows same tendency as seen with the effect of lateral groove density. If lateral siping density is smaller than 0.054 mm/mm 2  as in Comparison Example 31, edge effect is naturally small, and accordingly the on-snow performance of the tire is unfavorable. If lateral siping density is larger than 0.073 mm/mm 2  as in Comparison Example 32, the on-snow performance of the tire is excellent, but its abrasion resistance is extremely low. In order to obtain well-balanced tire characteristics, therefore, as can be clearly seen from Examples 24, 25 and 26, the lateral siping density should preferably be within the range of 0.054 to 0.073 mm/mm 2 . 
     Table 9 shows examples of cases where the invention may be advantageously applied to the cap portion of a cap/base structure. Examples 27 and 28 in which the rubber composition of the invention is applied to the cap portion, are advantageously well balanced in respect of RR, Wetμ, Iceμ, on-snow performance and abrasion resistance, as compared with Comparison Example 33 in which a rubber composition other than the one of the invention is applied to the cap portion only, or Comparison Example 34 in which such rubber composition other than the invention is applied to both cap and base portions (unit compound tread type). 
     
                       TABLE 7                                                     
______________________________________                                    
            Com. Ex.                                                      
                   Example      Com. Ex.                                  
            29     21     22     23   30                                  
______________________________________                                    
Lateral groove                                                            
              0.035    0.043  0.048                                       
                                   0.053                                  
                                        0.062                             
density (mm/mm.sup.2)                                                     
Tire characteristics                                                      
RR            100      101    100  99   101                               
Wet μ      100      101    101  100  100                               
Ice μ      100      100    99   102  101                               
On-snow performance                                                       
              3.5      4.5    5.0  5.0  5.0                               
Abrasion resistance                                                       
              100      98     97   95   82                                
______________________________________                                    
 Notes to Table 7:                                                        
 (1) Tire size is 185/70HR14. Contact area ratio of pattern is 64%, and   
 lateral siping density is 0.064 mm/mm.sup.2.                             
 (2) In Table 7, RR, values for Wet μ and Ice μ are given in terms o
 indices as calculated on the basis of Comparison Example 29. The smaller 
 the value of RR, the better, while the greater the value of Wet μ or  
 Ice μ, the better.                                                    
 (3) Abrasion resistance is evaluated by abrasion amounts after the tire  
 ran 10,000 km and is given in terms of indices as calculated on the basis
 of Comparison Example 29. Larger values are more favorable.              
 
    
     
                       TABLE 8                                                     
______________________________________                                    
            Com. Ex.                                                      
                   Example      Com. Ex.                                  
            31     24     25     26   32                                  
______________________________________                                    
Lateral siping                                                            
              0.049    0.056  0.064                                       
                                   0.071                                  
                                        0.078                             
density (mm/mm.sup.2)                                                     
Tire characteristics                                                      
RR            100      100    101  101  101                               
Wet μ      100      100    99   101  100                               
Ice μ      100      101    100  101  99                                
On-snow performance                                                       
              3.5      5.0    5.0  5.0  5.0                               
Abrasion resistance                                                       
              100      99     98   96   84                                
______________________________________                                    
 Notes to Table 8:                                                        
 (1) Tire size is 185/70HR14. Contact area ratio of pattern is 64%, and   
 lateral groove density is 0.049 mm/mm.sup.2.                             
 (2) In Table 8, RR, values for Wet μ, Ice μ and abrasion resistance
 are given in terms of indices as calculated on the basis of Comparison   
 Example 31.                                                              
 
    
     
                                           TABLE 9                                 
__________________________________________________________________________
           Example       Com. Ex.                                         
           27     28     33     34                                        
__________________________________________________________________________
Cap portion                                                               
           Rubber com-                                                    
                  Rubber com-                                             
                         Rubber com-                                      
                                Rubber com-                               
           position A                                                     
                  position B                                              
                         position C                                       
                                position D                                
Base portion                                                              
           Rubber com-                                                    
                  Rubber com-                                             
                         Rubber com-                                      
                                Rubber com-                               
           position E                                                     
                  position E                                              
                         position E                                       
                                position D                                
Tire characteristics                                                      
RR          81     82     85    100                                       
Wet μ   116    120    118    100                                       
Ice μ   114    116     97    100                                       
On-snow performance                                                       
           5.0    5.0    4.0    4.0                                       
Abrasion resistance                                                       
           109    112     93    100                                       
__________________________________________________________________________
 Notes to Table 9:                                                        
 (1) Tire size is 185/70HR14. Contact area ratio of pattern is 64%, latera
 groove density is 0.049 mm/mm.sup.2 and lateral siping density is 0.064  
 mm/mm.sup.2.                                                             
 (2) In Table 9, RR, values for Wet μ, Ice μ and abrasion resistance
 are given in terms indices as calculated on the basis of Comparison      
 Example 34.                                                              
 (3) Rubber compositions used in cap and base portions are as follows:    
 
    
     
         Rubber composition A:                                                     
                 Example 14 in Table 4                                    
Rubber composition B:                                                     
                 Example 16 in Table 4                                    
Rubber composition C:                                                     
                 Com. Ex. 22 in Table 4                                   
Rubber composition D:                                                     
                 Com. Ex. 23 in Table 4                                   
Rubber composition E:                                                     
NR 80 (parts by weight),                                                  
                   BR    20                                               
Aromatic oil                                                              
              5,   Zinc oxide                                             
                         3                                                
Stearic acid                                                              
            2.5,   Antioxidant                                            
                         1.0                                              
GPF carbon   35,   Sulfur                                                 
                         2                                                
Vulcanization accelerator D                                               
                   0.2                                                    
Vulcanization accelerator MSA                                             
                   1.5