Patent Publication Number: US-4548980-A

Title: Tire tread rubber composition

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rubber composition having a specific furnace carbon black incorporated therein. More particularly, the invention relates to a tire tread rubber composition capable of providing a tire tread having small energy loss and a great coefficient (μw) of sliding friction on a wet road surface. 
     2. Description of the Prior Art 
     With the current worldwide concern for saving resources and energy, car manufacturers are making intensive efforts to manufacture automobiles which achieve better mileage economy. To meet this end, studies have been conducted by tire manufacturers to find effective ways to minimize energy loss through tires. 
     Two principal functions of tires are to support the weight of the automobile and to transmit the driving force of the wheels. As is well known, the energy loss resulting from the friction between a tire and the road surface or from the heat generated by the tire itself has an appreciable effect on the fuel consumption of the car. However, the energy loss which results from the tire itself, i.e., its rolling resistance, cannot be reduced without also decreasing the coefficient (μw) of its sliding friction against a wet road surface. In other words, a tire with a small rolling resistance has a great tendency to skid. 
     In order to attempt to resolve these contradictory factors, studies have been conducted which involve the incorporation of carbon black in large quantities into rubber to prepare a tire tread rubber composition which has a small rolling resistance while exhibiting a high coeffficient (μw) of sliding friction on a wet road surface. 
     It is generally known that the rolling resistance (RR) of a tire is a function of E&#34;/E&#39;=tan δ, wherein E&#34; is the loss elastic modulus and E&#39; is the storage elastic modulus. Tires having a low loss tangent (tan δ) also have a low rolling resistance (RR). The coefficient (μw) of sliding friction on a wet road surface is also closely related to the viscoelasticity property of rubber and, the higher the tan δ, the higher the μw. However, a rubber composition having a high tan δ has an undesirably high rolling resistance (RR). 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide a tire tread rubber composition of reduced energy loss properties. 
     Another object of the present invention is to provide an improved tire tread composition which, in addition to exhibiting a reduced energy loss, also exhibits a substantially large coefficient of sliding friction (μw) on a wet road surface. 
     Briefly, these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a rubber composition comprising from 20 to 150 parts by weight of carbon black incorporated in 100 parts by weight of rubber, said carbon black being a furnace carbon black having (1) a specific surface area (N 2  SA) within the range of 75 to 105 as measured by the nitrogen adsorption technique, (2) the difference between said N 2  SA value and the measured iodine adsorption (IA) being at least 15 (i.e., N 2  SA-IA≧15), and (3) the difference between said N 2  SA value and the measured specific surface area (CTAB) as determined by the cetyltrimethylammonium bromide adsorption technique being not greater than 5 (i.e., N 2  SA-CTAB≦5), said furnace carbon black further having a dibutyl phthalate adsorption value (24M4 DBP) of not greater than 110 (ml/100 g) and a tinting strength (Tint) of 90 to 110, said carbon black having a storage elastic modulus E&#39; of not greater than 12.5 MPa as specified by the ASTM-D 3191 method for carbon blacks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIGS. 1 and 2 are graphs which show the relationship between the rolling resistance (RR) of tires and the sliding friction coefficient (μw) of tires on a wet road surface using treads made from the rubber formulations identified in Tables 1 and 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A rubber composition using a furnace carbon black whose specific surface area abbreviated as (N 2  SA), as determined by the nitrogen adsorption technique is less than 75 has a low wear resistance and is not suitable for use as a tire tread. If the value of N 2  SA exceeds 105, the resulting rubber composition exhibits an increased tan δ value which leads to an excessive rolling resistance (RR). If the tinting strength (Tint) of the furnace carbon black is less than 90, the resulting rubber composition exhibits a poor wear resistance. If the Tint is greater than 110, the value of tan δ is undesirably increased. If the dibutyl phthalate adsorption (24M4 DBP) value exceeds 110 (ml/100 g), a rubber composition having an excessive storage elastic modulus E&#39; results. If Δ Tint is not more than -3, there is a tendency to decrease tan δ. 
     The value of Δ Tint is obtained by subtracting the calculated Tint from the measured Tint [i.e., Δ Tint=(measured Tint)-(calculated Tint)], and the calculated Tint is obtained from the following equation 1: 
     
         Calculated Tint=56+1.057×(CTAB)-0.002745×(CTAB).sup.2 -0.2596×(24M4 DBP)-0.201×(N.sub.2 SA-CTAB)    equation 1 
    
     see Rubber Chemistry and Technology, Vol. 48, page 538, 1975). 
     The furnace carbon black characterized above that is incorporated in the tire tread rubber composition of the present invention is used in an amount of 20 to 150 parts by weight, preferably 30 to 70 parts by weight, per 100 parts by weight of the rubber component. If the carbon black content is less than 20 parts by weight, the desired wear resistance is not obtained, and if the carbon black is used in an amount more than 150 parts by weight, an undesirably great energy loss occurs. 
     The furnace carbon black having the above properties can be prepared, for example, using a conventional oil-furnace type reaction furnace with suitably selecting thermal decomposition conditions of the raw material hydrocarbon, such as, a reaction temperature, an atmosphere in the reaction furnace, a residence time in the reaction furnace until cooling, and the like. In general, in order to increase the difference between the N 2  SA value and the IA value, the residence time should be shortened as compared with a conventional furnace carbon black. However, when the residence time is shortened, the difference between the N 2  SA value and the CTAB value is increased. Thus, the shortening of the residence time should be suitably selected so as to meet these two differences required in the present invention (i.e., N 2  SA-IA≧15 and N 2  SA-CTAB≦5). 
     When an oil is used in the preparation of the tire tread composition, the difference between the amount of the oil and that of the carbon black is preferably in the range of 30 to 40 parts [i.e., (carbon black amount)-(oil amount)=preferably 30 to 40 parts by weight]. If the difference is less than 30 parts by weight, the value of E&#39; increases thereby causing a drop in μw. If the difference is more than 40 parts by weight, the desired wear resistance of the tire tread is not obtained. 
     Needless to say, in addition to the carbon black characterized above, the rubber composition of the present invention may have incorporated therein any additive commonly used in rubber compounding such as a vulcanizing agent, a vulcanization accelerator, a vulcanization acceleration aid, an antioxidant, a softening agent or a filler. 
     As a result of incorporating the furnace carbon blacks described above in the present rubber composition, the same exhibits a small rolling resistance (RR), and tires which employ the composition at tire tread portion exhibit a high coefficient (μw) of sliding friction on a wet road surface. 
     The difference between the values of N 2  SA and IA should be at least 15 (N 2  SA-IA≧15), and the greater the difference, the greater the surface activity of the carbon black. The difference between the values N 2  SA and CTAB should not be more than 5 (N 2  SA-CTAB≦5), and the smaller the difference, the smaller the surface roughness of the carbon black. The great surface activity and the small surface roughness of the carbon black are both effective in changing the interaction between the carbon black and rubber in such a manner that the rubber blended with the carbon black has a reduced storage elastic modulus E&#39;. 
     The advantages of the present invention are described below in greater detail by reference to the results of tests conducted with actual formulations of rubber compositions. The formulations of the respective compositions and the characteristics of the carbon blacks used, as well as the properties of the vulcanized rubber compositions and their tire performance data are shown in Table 1. 
     
                       TABLE 1-1                                                   
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            Formulation No.                                               
Components and                                                            
            Example                                                       
Properties  1        2       3     4     5                                
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Components (in                                                            
parts by weight)                                                          
SBR1500     100      100     100   100   100                              
Zinc oxide  3        3       3     3     3                                
Sulfur      1.75     1.75    1.75  1.75  1.75                             
Stearic acid                                                              
            1        1       1     1     1                                
Carbon black                                                              
            50       50      50    50    50                               
TBBS        1        1       1     1     1                                
Properties of                                                             
Carbon Black Used                                                         
N.sub.2 SA  90       82      100   87    77                               
IA          70       65      83    63    55                               
CTAB        87       79      98    86    77                               
24M4 DBP    95       96      98    105   103                              
Tint        90       91      103   94    90                               
Δ Tint                                                              
            -12      -6      -4    -5    -4                               
Physical Proper-                                                          
ties                                                                      
tan δ at 60° C.                                              
            0.131    0.135   0.145 0.140 0.130                            
E&#39; (MPa) at 11.9     11.5    12.0  11.5  11.7                             
60° C.                                                             
Pico wear index                                                           
            94       93      112   95    90                               
Tire Performance                                                          
Index                                                                     
RR (index)  81       82      87    84    79                               
μw (index)                                                             
            114      117     114   119   116                              
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                       TABLE 1-2                                                   
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           Formulation No.                                                
Components and                                                            
           Comparative Example                                            
Properties 6       7        8      9     10                               
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Components (in                                                            
parts by weight)                                                          
SBR1500    100     100      100    100   100                              
Zinc oxide 3       3        3      3     3                                
Sulfur     1.75    1.75     1.75   1.75  1.75                             
Stearic acid                                                              
           1       1        1      1     1                                
Carbon black                                                              
           50      50       50     50    50                               
TBBS       1       1        1      1     1                                
Properties of                                                             
Carbon Black                                                              
Used                                                                      
N.sub.2 SA 70      110      90     90    100                              
IA         55      92       80     75    83                               
CTAB       69      106      88     82    99                               
24M4 DBP   95      100      100    105   115                              
Tint       90      95       90     95    100                              
Δ Tint                                                              
           -1      -15      -11    0     -4                               
Physical Proper-                                                          
ties                                                                      
tan δ at 60° C.                                              
           0.144   0.170    0.153  0.149 0.166                            
E&#39; (MPa) at                                                               
           12.9    11.3     13.8   13.7  12.6                             
60° C.                                                             
Pico wear index                                                           
           72      108      94     96    110                              
Tire Performance                                                          
Index                                                                     
RR (index) 88      107      89     89    105                              
μw (index)                                                             
           104     119      95     92    107                              
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                       TABLE 1-3                                                   
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              Formulation No.                                             
Components and                                                            
              Comparative Example                                         
Properties    11         12      13                                       
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Components (in                                                            
parts by weight)                                                          
SBR1500       100        100     100                                      
Zinc oxide    3          3       3                                        
Sulfur        1.75       1.75    1.75                                     
Stearic acid  1          1       1                                        
Carbon black  50         50      50                                       
TBBS          1          1       1                                        
Properties of                                                             
Carbon Black Used                                                         
N.sub.2 SA    90         82      93                                       
IA            73         65      91                                       
CTAB          88         80      91                                       
24M4 DBP      97         98      102                                      
Tint          86         116     109                                      
Δ Tint  -16        19      6                                        
Physical Proper-                                                          
ties                                                                      
tan δ at 60° C.                                              
              0.133      0.168   0.166                                    
E&#39; (MPa) at   11.6       13.0    13.4                                     
60° C.                                                             
Pico wear index                                                           
              73         115     100                                      
Tire Performance                                                          
Index                                                                     
RR (index)    80         113     100                                      
μw (index) 115        103     100                                      
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     Notes: 
     (1) The formulation components shown in the above table were in accordance with the &#34;Formulations for Evaluation of Carbon Black Using SBR&#34; specified in ASTM-D 3191 authorized by the American Society for Testing and Materials. SBR 1500: styrene-butadiene rubber, with bound styrene of 23.5%. 
     (2) The parameters tan δ and E&#39; were measured with a viscoelastic spectrometer (product of Iwamoto Seisakusho, K.K.). The measured values were a strain of 0.6%, 100 Hz and 60° C. for the values indicated in Table 1, and 1.0% strain, 100 Hz and 60° C. for those indicated in Table 2. 
     (3) The pico wear index was determined in accordance with ASTM-D-2228, and is expressed in terms of relative values to Formulation No. 13 as standard, with the reciprocal of the wear of Formulation No. 13 being taken as 100. The smaller the index, the greater the pico wear. 
     (4) For the measurement of rolling resistance RR (energy loss) and coefficient (μw) of sliding friction on a wet road surface, tire samples (185/70 HR 14) were prepared using the tread formulations shown in Table 1. The respective values of RR and μw were expressed as indices, with the values for Formulation No. 13 taken as 100. The smaller the value of RR, the better. On the other hand, the greater the value of μw, the better. 
     (5) TBBS: N-t-butyl-2-benzothiazole sulfenamide. 
     (6) The specific surface area of carbon black as measured by the nitrogen adsorption technique (N 2  SA), iodine adsorption (IA), and the specific surface area as measured by the cetyltrimethylammonium bromide adsorption technique (CTAB) are indices for the surface area of carbon black particles in terms of the nitrogen, iodine or cetyltrimethylammonium bromide adsorbate on the particles. The value of the specific surface area (N 2  SA) according to the nitrogen adsorption technique is indicated in m 2  /g and is determined by the method specified in ASTM D-3037-73; the value of iodine adsorption abbreviated as (IA) is indicated in mg/g and is determined by the method specified in ASTM-D 1765-739; and the value of the specific surface area as determined by the cetyltrimethylammonium bromide adsorption technique (CTAB) is indicated in m 2  /g and is determined by the method specified in ASTM-D 3765. The dibutyl phthalate adsorption (24M4 DBP) and tinting strength (Tint) values were measured in accordance with ASTM-D 3493-79 and ASTM-D 3265-75, respectively. 
     The results of the evaluation of various carbon black samples are shown in Table 1. 
     As is clear from Table 1, Formulation Nos. 1 to 5 exhibited superior characteristics. A carbon black with an N 2  SA value exceeding 105 provided a rubber composition having an increased tan δ value and hence an undesirably great RR value (Formulation No. 7). A carbon black having a 24M4 DBP value of more than 110 also provided a rubber composition having an undesirably great RR value (Formulation No. 10). Formulation Nos. 12 and 13 having a large Δ Tint, especially a Δ Tint value greater than -3, again produced rubber compositions having substantially large RR values. Formulation No. 12, having a Tint value exceeding 110, is particularly undesirable because of the excessively large RR value. Formulation No. 6, having an N 2  SA value of less than 75, produced a rubber composition having a relatively low RR value, but this rubber had an impracticably low level of wear resistance. Formulation No. 11 having a Tint value of less than 90 produced a rubber composition of low wear resistance. 
     Formulation Nos. 8 and 13, which exhibit a difference between N 2  SA and IA values of less than 15 (N 2  SA-IA≧15), and Formulation No. 9, which exhibits a difference between N 2  SA and CTAB values of greater than 5 (N 2  SA-CTAB≦5), all showed increased values of E&#39;, and their μw values were correspondingly low. 
     Only Formulation Nos. 1 to 5 were satisfactory both in the wear resistance and the balance between rolling resistance (RR) and the coefficient (μw) of sliding friction on a wet road surface. These formulations had E&#39; values of not more than 12.5 MPa, beyond which the value of μw would decrease thereby resulting in insufficient gripping or braking action on a wet road surface. 
     As is apparent from the data shown above, in order to achieve the desired advantages of the tire tread rubber composition according to the present invention, the tire composition must contain a furnace carbon black having the following characteristics: (1) and N 2  SA value in the range of 75 to 105 (m 2  /g) provided that the difference between N 2  SA and IA values is at least 15 (N 2  SA-IA≧15) and the difference between N 2  SA and CTAB is not greater than 5 (N 2  SA-CTAB≦5); (2) a 24M4 DBP value of not greater than 110 (ml/100 g); and (3) a Tint value in the range of 90 to 110, with Δ Tint being not greater than -3. Particularly good results are obtained by a rubber composition having a storage elastic modulus (E&#39;) of not greater than 12.5 MPa with the formulation specified in ASTM-D 3191 for the evaluation of carbon black. Such a rubber composition has a small energy loss, provides a substantial grip on wet road surfaces and exhibits a high wear resistance. 
     The carbon black used in the present invention and having the characteristics shown above is applicable not only to styrene-butadiene rubber (SBR), it is also applicable with equal results to natural rubber or any kind of synthetic rubber or blends thereof such as natural rubber (NR), or a blend of styrene-butadiene rubber (SBR) and natural rubber (NR), or a blend of SBR, NR and butadiene rubber (BR). See Table 2 below. 
     
                       TABLE 2-1                                                   
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            Formulation No.                                               
Components and                                                            
              Example     Comparative Example                             
Properties    14      15      16     17                                   
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Components (in parts by                                                   
weight)                                                                   
Natural rubber (NR)                                                       
              100     100     100    100                                  
Styrene-butadiene rubber                                                  
              --      --      --     --                                   
(SBR)                                                                     
Butadiene rubber (BR)                                                     
              --      --      --     --                                   
Carbon black  50      50      50     50                                   
Aromatic oil  15      15      15     15                                   
Zinc oxide    3       3       3      3                                    
Stearic acid  2       2       2      2                                    
Curing accelerator                                                        
              1.5     1.5     1.5    1.5                                  
Sulfur        2       2       2      2                                    
Properties of Carbon                                                      
Black Used                                                                
N.sub.2 SA    80      92      90     93                                   
IA            60      70      72     91                                   
CTAB          80      89      88     91                                   
(N.sub.2 SA)-(IA)                                                         
              20      22      18     2                                    
(N.sub.2 SA)-(CTAB)                                                       
              0       3       2      2                                    
24M4 DBP      102     95      98     102                                  
Tint          92      98      80     109                                  
Δ Tint  -5      -5      -22    6                                    
Physical Properties                                                       
tan δ at 60°  C.                                             
              0.128   0.131   0.139  0.151                                
E&#39; (MPa) at 60° C.                                                 
              5.81    5.77    6.23   6.81                                 
Pico wear index                                                           
              102     103     75     100                                  
Tire Performance Index                                                    
RR (index)    84      85      87     100                                  
μw (index) 115     117     108    100                                  
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                       TABLE 2-2                                                   
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              Formulation No.                                             
Components and  Example  Comparative Example                              
Properties      18       19      20    21                                 
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Components (in parts by                                                   
weight)                                                                   
Natural rubber (NR)                                                       
                50       50      50    50                                 
Styrene-butadiene rubber                                                  
                50       50      50    50                                 
(SBR)                                                                     
Butadiene rubber (BR)                                                     
                --       --      --    --                                 
Carbon black    60       60      60    60                                 
Aromatic oil    20       20      20    20                                 
Zinc oxide      3        3       3     3                                  
Stearic acid    2        2       2     2                                  
Curing accelerator                                                        
                2        2       2     2                                  
Sulfur          2.5      2.5     2.5   2.5                                
Properties of Carbon Black                                                
 Used                                                                     
N.sub.2 SA      82       112     90    93                                 
IA              65       90      85    91                                 
CTAB            79       108     84    91                                 
(N.sub.2 SA)-(IA)                                                         
                17       22      5     2                                  
(N.sub.2 SA)-(CTAB)                                                       
                3        4       6     2                                  
24M4 DBP        96       105     95    102                                
Tint            91       106     102   109                                
Δ Tint    -6       -4      2     6                                  
Physical Properties                                                       
tan δ at 60° C.                                              
                0.130    0.192   0.150 0.161                              
E&#39; (MPa) at 60° C.                                                 
                7.22     7.63    9.15  8.13                               
Pico wear index 97       110     88    100                                
Tire Performance Index                                                    
RR (index)      82       111     89    100                                
μw (index)   111      107     90    100                                
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                       TABLE 2-3                                                   
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            Formulation No.                                               
Components and                                                            
              Example     Comparative Example                             
Properties    22      23      24      25                                  
______________________________________                                    
Components (in parts by                                                   
weight)                                                                   
Natural rubber (NR)                                                       
              40      40      40      40                                  
Styrene-butadiene rubber                                                  
              30      30      30      30                                  
(SBR)                                                                     
Butadiene rubber (BR)                                                     
              30      30      30      30                                  
Carbon black  55      55      55      55                                  
Aromatic oil  20      20      20      20                                  
Zinc oxide    3       3       3       3                                   
Stearic acid  2       2       2       2                                   
Curing accelerator                                                        
              1.5     1.5     1.5     1.5                                 
Sulfur        2       2       2       2                                   
Properties of Carbon                                                      
Black Used                                                                
N.sub.2 SA    100     82      100     93                                  
IA            82      64      82      91                                  
CTAB          98      80      99      91                                  
(N.sub.2 SA)-(IA)                                                         
              18      18      18      2                                   
(N.sub.2 SA)-(CTAB)                                                       
              2       2       1       2                                   
24M4 DBP      98      100     120     102                                 
Tint          100     92      97      109                                 
Δ Tint  -7      -5      -5      6                                   
Physical Properties                                                       
tan δ at 60° C.                                              
              0.144   0.120   0.188   0.159                               
E&#39; (MPa) at 60° C.                                                 
              6.31    6.72    6.77    7.20                                
Pico wear index                                                           
              113     98      110     100                                 
Tire performance Index                                                    
RR (index)    88      80      115     100                                 
μw (index) 116     109     107     100                                 
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     Notes 
     Samples marked &#34;Example&#34; in Table 2 were prepared according to the present invention using carbon blacks having the characteristics defined above. Samples marked &#34;Comparative Example&#34; refer to comparative samples which used carbon blacks outside the scope of the present invention. 
     The pico wear, RR and μw index values are each expressed in terms of relative values to Formulation No. 17 as standard in Table 2-1, Formulation No. 21 as standard in Table 2-2 and Formulation No. 25 as standard in Table 2-3, said standard values being taken as 100. 
     The values of the tire rolling resistance (RR) and the coefficient (μw) of sliding friction on a wet road surface shown in the test results of Tables 1 and 2 are correlated in the graphs of FIGS. 1 and 2. Each graph plots μw and RR values on the x-axis and y-axis, respectively. The open dots (o) in FIG. 1 refer to the formulations that satisfy the requirements recited in the claims of the present invention, whereas the solid trianges () and diamonds () refer to the formulations outside the scope of the present invention, and those indicated by the solid diamonds () had a low wear resistance. 
     The values moving further in the direction indicated by the closed fist represent generally preferred results except for those samples having insufficient wear resistance. Formulation No. 11 (FIG. 1) and Formulation No. 16 (FIG. 2) provided rubber compositions having relatively good balance between the values of RR and μw but poor wear resistance moved further in the direction indicated by the closed fist. 
     In FIG. 2, the open dots (o), diamond () and triangles () refer to the formulations that satisfy the ranges defined in the claims. The formulations identified by the solid and open dots (  and o) contained natural rubber, while the formulations identified by the solid and open diamonds (  and  ) contained an NR/SBR blend. The formulations identified by the solid and open triangles (  and  ) contained a blend of three elastomers, i.e., NR, SBR and BR. The solid marks all refer to formulations outside the scope of the present invention. The solid star represents Formulation Nos. 17, 21 and 25. These formulations were used as reference indices for comparing the characteristics of the other formulations. 
     As shown in FIGS. 1 and 2, the tire tread rubber composition of the present invention is advantageous in that it has a small rolling resistance (reduced energy loss) and exhibits a substantial coefficient of sliding friction on wet road surfaces. 
     Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.