Rubber composition for tire tread
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to rubber compositions for tire treads, which are suitable not only for use on ordinary paved roads, but also particularly suitable for use on icy and snow-covered roads. 
     2. Description of the Prior Art 
     Conventionally, for safe travel on icy and snow-covered roads, spike tires have been used in many cases. However, spike tires generate dust as well as noise pollution. In view of these environmental problems, the use of spike tires has become questionable. 
     Consequently, in recent years, a tire without spike pins, the so-called studless tire, was introduced. However, at its present state of development, this studless tire is inferior to the spike tire in three areas: grade climbing ability, braking performance, and standing start performance. Also, the studless tire usually uses a large amount of softening agent or plasticizer because of the necessity to lower the hardness of the tread portion. In most cases this softening agent or plasticizer migrates into other portions of the tire, such as the belt and side wall portion, which are located contiguous to the tread rubber, or it is lost from the tread rubber by vaporization, etc., thereby causing degradation of the original performance of the tread rubbers. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide rubber compositions for the tire tread which improve the grade climbing ability and braking performance, as well as the standing start performance of the tire tread when used on ice and snow. 
     Another object of the present invention is to provide rubber compositions for the tire tread which are capable of extending the life of the tire beyond tread rubber and reducing degradation of the tread rubber performance as much as possible those of the conventional studless tire. 
     In order to accomplish the objects mentioned above, the present invention employs a compositional structure that contains liquid polymers or a mixture of liquid polymers and other softening agents. Further, specific liquid polymers which have a flow point at or below 20° C. are used. Furthermore, the relationships between the contents of the constituents carbon black (x parts by weight), softening agent inclusive of liquid polymer (y parts by weight) and liquid polymer in the softening agent (z parts by weight) are (5/8)x-20≦y≦x and (y/5)+5≦z. 
     When the content y of the softening agent inclusive of the liquid polymer lies outside the range designated above relative to the carbon black content x, the performances which are required of the tire tread are unsatisfactory, and the processability suffers because of excessive stickiness. Carbon black content x is preferably in the range from 40 to 130 parts by weight. The liquid polymer is used alone or in combination with high aromatic, naphthene oil, plasticizer, etc. within the range designated above. When the liquid polymer content z lies outside the designated range, the hardness tends to be lower and even if the hardness can be maintained, an excessive increase in hardness after tire service cannot be prevented. 
     When the softening point of the liquid polymer exceeds 20° C., it cannot serve as a softening agent, and processability suffers. Consequently, it is preferable to select a liquid polymer with a flow point of 20° C. or less. 
     Desirable liquid polymers are liquid polybutadiene, liquid polybutene, liquid polyisoprene, liquid chloroprene, liquid polyisobutylene, liquid polystyrene butadiene, liquid polysopreneisobutylene, etc. 
     Therefore, through the use of a liquid polymer within a specified content range, the hardness can be lowered, and the area that contacts the road surface can be increased, thereby improving the grade climbing ability, braking and standing start performances. Furthermore, the liquid polymer possesses a relatively high molecular weight and a very low flow rate in comparison with oil, etc. so that migration into the other portions of the tire as well as vaporization, can be prevented or kept to a minimum. As a result, the life of a studless tire can be prolonged. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Table shows examples of the rubber compositions used in the present invention as well as examples for comparison. In addition to the constituents shown in the Table, 3 parts of zinc white, 2 parts of stearic acid, 1 part of Santoflex 13 (paraphenylenediamine system age resistor (antioxidant)), 1.5 parts of Nocceler MSA-G (sulfenamide system vulcanization accelerator), and 2 parts of sulfur are present in all the embodiments and examples for comparison. All of the constituents were mixed using a Banbury mixer to form a homogeneous mixture and the liquid polymer used was liquid polybutadiene. 
     Next, the rubber characteristics were determined, both for the compositions of the embodiments of this invention and of the examples for comparison. The results are also shown in the Table. 
     
         __________________________________________________________________________
Embodiment           Example for Comparison                               
1        2  3  4  5  1  2  3  4  5  6  7  8  9  10 11 12 13 14            
__________________________________________________________________________
Natural                                                                   
      70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70 70            
Rubber                                                                    
Cis 1.4 BR                                                                
      30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30  30           
Carbon                                                                    
      80 60 80 80 80 80 80 80 80 80 80 80 80 80 60 60 80 80  80           
(X) *1                                                                    
Aromatic                                                     60           
Oil                                                                       
Naphthene                                                                 
      20 15 20          45 65 25 10 25 60 65 60    10 10 60               
Oil                                                                       
L-BR A *2                                                                 
      30 15             15    5        30 20    10                        
L-BR B *3         30                               5                      
L-BR C *4   45 80                                                         
L-BR D *5            80    15    25 10       30       15                  
Total 50 30 65 80 30 80 60 80 30 35 35 90 85 90 10 15 25 60  60           
Amount of                                                                 
Softening                                                                 
Agent *6                                                                  
(5/8)X-20                                                                 
      30 17.5                                                             
            30 30 30 30 30 30 30 30 30 30 30 30 17.5                      
                                                   17.5                   
                                                      30 30  30           
(Y/5)+5                                                                   
      15 11 18 21 11 21 17 21 11 12 12 23 22 23 7  8  10                  
LBR Pour                                                                  
      -45                                                                 
         -45                                                              
            15 15 -55                                                     
                     25 -45                                               
                           25 -45                                         
                                 25 25 -45                                
                                          -45                             
                                             25 -45                       
                                                   -55                    
                                                      25                  
Point *7                                                                  
Hardness                                                                  
*8                                                                        
(Initial                                                                  
      51 52 52 50 56 52 47 40 62 66 64 32 35 39 62 60 67 48  52           
Period)                                                                   
(After                                                                    
      54 54 54 53 58 54 53 49 65 68 67 40 44 47 64 63 70 56  57           
Aging) *9                                                                 
Hardness                                                                  
Change *10                                                                
      3  2  2  3  2  2  6  9  3  2  3  8  9  8  2  3  3  8   5            
Pico-Abra-                                                                
      101                                                                 
         108                                                              
            104                                                           
               100                                                        
                  135                                                     
                     108                                                  
                        94 66 138                                         
                                 151                                      
                                    143                                   
                                       54 50 70 142                       
                                                   135                    
                                                      160                 
                                                         86  100          
sion Index                                                                
*11                                                                       
On-Ice                                                                    
Abrasion                                                                  
Index *12                                                                 
(Initial                                                                  
      112                                                                 
         110                                                              
            108                                                           
               109                                                        
                  105                                                     
                     95 115                                               
                           118                                            
                              88 80 84 129                                
                                          123                             
                                             116                          
                                                90 92 77 113 100          
Period)                                                                   
(After                                                                    
      107                                                                 
         107                                                              
            105                                                           
               104                                                        
                  102                                                     
                     92 100                                               
                           98 83 77 79 109                                
                                          100                             
                                             96 87 87 73 93  92           
Aging)                                                                    
Rolling                                                                   
      O  O  O  Δ                                                    
                  O  X  O  X  X  X  Δ                               
                                       X  X  X  Δ                   
                                                   Δ                
                                                      X  O   Δ      
Process-                                                                  
ability *13                                                               
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 Notes:                                                                   
 *1 Carbon black (ISAF)                                                   
 *2 Liquid polybutadiene flow point -45° C.                        
 *3 Liquid polybutadiene flow point -55° C.                        
 *4 Liquid polybutadiene flow point 15° C.                         
 *5 Liquid polybutadiene flow point 25° C.                         
 *6 Total amount of softening agent (y) = Amount of liquid polybutadiene  
 (x) + Amount of other softening agents                                   
 *7 The flow point was measured using the method in accordance with JIS   
 K2269.                                                                   
 *8 The hardness was measured in accordance with JIS K6301.               
 *9 The hardness at 25°  C., after heat aging at 50° C., for
 two months.                                                              
 *10 The change of hardness shows the difference in hardness obtained by  
 subtracting the initial hardness from the hardness after aging.          
 *11 The picoabrasion index evaluation was carried out using a picoabrasio
 tester. The index is assigned a value of 100 for comparison example 14.  
 The abrasion resistance becomes higher as the numerical value of the inde
 increases.                                                               
 *12 The onice friction index evaluation was carried out using a portable 
 skid resistance tester, by covering the road surface with ice, and using 
 test pieces in the initial period and after aging. The index is assigned 
 value of 100 for the comparison example 14. A higher numerical value     
 indicates higher onice friction. The aging conditions employed were heate
 to 50° C. over a two month duration.                              
 *13 With respect to the roll processabilty, the rolling workabilty was   
 evaluated by assigning 3 grades for the respective rubber compositions   
 after mixing: O denotes &#34;good&#34;, Δ denotes &#34;acceptable (fairly      
 good)&#34;, and X denotes &#34;reject&#34;.                                          
 
    
     It is apparent from the Table, in comparison examples 13 and 14 which use conventional softening agents, such as aromatic oil and do not contain liquid polymers, that the change in hardness due to the aging is great, and also that the variation in the on-ice friction index is significant. Therefore, these compositions are not desirable. Comparison examples 1 through 14 show that even if a softening agent containing a liquid polymer is used, the liquid polymer must meet the compositional relationships represented by the specific inequalities mentioned previously, and must have a flow point of 20° C. or below. Otherwise, the balanced characteristics in terms of satisfactory hardness, pico-abrasion index, on-ice friction index, and rolling processability cannot be obtained. 
     For example, in comparison examples 1 and 5 which use softening agents containing the specified liquid polymers in amounts which satisfy the prescribed inequalities, the hardness and the change of hardness before and after aging are satisfactory, but the on-ice friction coefficient is rather lower than comparison examples 13 and 14, as it is the rolling processability, because liquid polymers having flow points of 20° C. or above are used. Also, in comparison example 2 in which the liquid polymer content in the softening agent is less than the specified amount, the change in hardness before and after aging is great, and a tendency for the hardness to increase after heat aging is observed. Consequently, the change of the on-ice friction index is great, and its decrease is significant. 
     On the other hand, in comparison example 3 the specified inequalities are satisfied only in terms of the relationship between the amount of carbon black and the total amount of the softening agent, the variation in hardness is very great, and also the pico-abrasion index is extremely low. Besides, reduction in the on-ice friction index is conspicuous, and the roll processability is inferior. 
     In comparison example 7, the liquid polymer has the required flow point, but is present in the softening agent in an amount close to the lower limit of the specified range and the total amount of softening agent is larger than the amount of carbon black. As a result, while the on-ice friction index initially is high, it decreases greatly during aging. In addition, the change in hardness is marked, the pico-abrasion index is low, and the roll processability is inferior. 
     Unlike those comparison examples mentioned above, embodiments 1 through 4 contain carbon black (x parts by weight), softening agent (y parts by weight) and liquid polymer in the softening agent (z parts by weight) according to the relationships of (5/8)x-20≦y≦x and (y/5)+5≦z relative to 100 parts by weight of diene system rubber, and use liquid polymers with flow points of 20° C. or below. For these compositions the hardness as well as its variations; the pico-abrasion index; the on-ice friction index as well as its variation; and the roll processabiliity are all satisfactory. This indicates that they are rubber compositions with specifically well-balanced characteristics. 
     As described above, this invention comprises a liquid polymer having a flow point below a specified temperature present in the softening agent above a specified amount and also limited in amount within a specified range relative to the amount of carbon black. As a result, all the performance parameters including grade climbing ability on ice and snow, braking performance and standing start performance are improved. Furthermore, the compositions specified in this invention extend the life of the rubber.