Patent Publication Number: US-4255296-A

Title: Polybutadiene rubber composition

Description:
This invention relates to a composition comprising a polybutadiene rubber containing a polymer obtained by block or graft polymerization of cis-1,4-polybutadiene with syndiotactic(syn)-1,2-polybutadiene, alone or blended with the other diene rubber, said syn-1,2-polybutadiene being crystallized and in a short fiber-like shape, and a breaking resistance characteristic and reinforcing characteristic being considerably improved by specifying the diameter and length of said short fibers, which is particularly suitable for various parts of tyre or industrial materials such as belts, hoses, etc. 
     Furthermore, the present invention relates to a polybutadiene rubber composition comprising said polybutadiene rubber and said other diene rubber and further containing high quality carbon black, an abrasion resistance being improved thereby, so that such composition being suitable for rubber chafer (or rim cushion) and tread rubber. 
     Still further, the present invention relates to a polybutadiene rubber composition comprising said polybutadiene rubber and said other diene rubber and further containing low quality carbon black and an antioxidant of p-phenylenediamine, a flex resistance, crack growth resistance, weather resistance and workability being improved thereby, so that such composition is suitable as a rubber blend composition for side wall. 
     Since cis-1,4-polybutadiene has a good flex resistance, a good abrasion resistance, etc., it has been so far much used in various parts of tyre such as side wall, rubber chafer (or rim cushion) or tread, or the like, but it has a poor workability; for example, in appearance of roll baggy, in low strength after vulcanization or in high crack growth rate, etc. Owing to these disadvantages, it is used as blended with other diene rubber such as natural rubber, etc. but still a satisfactory result has not been obtained, and it is also difficult to use cis-1,4-polybutadiene in a high proportion. 
     On the other hand, a high speed performance is more and more required for tyres owing to the recent development of high speed way and the spread of radial tyres, and at the same time a rubber of high modulus of elasticity has been used around bead parts to prevent movements appearing at the bead parts. Thus, an increasingly large force is applied to a rubber chafer arranged between the im and bead, and the use of cis-1,4-polybutadiene so far used have been now limited because of said disadvantages. 
     Furthermore, in order to improve the high speed performance of tyre it is the current tendency to make the tread part and the bead part of tyre more elastic on one hand, and made the side wall part less elastic on the other hand, to weaken the impacts at the tread part. Thus, the flex resistance of the side wall part is more and more important, but the use of the conventional cis-1,4-plybutadiene has been now limited owing to the disadvantages as mentioned above. A high weather resistance such as ozone resistance, etc. is an important and necessary characteristic required for the side wall, but any satisfactory one has not been realized yet. 
     The present inventors have tried to make improvements by block or graft polymerization of cis-1,4-polybutadiene with syn-1,2-polybutadiene, but fluctuation in physical properties is large, though synthesized equally, and it has not been satisfactorily applied to light weight tyres or low fuel cost tyres which are recently in a larger demand. 
     As a result of further extensive studies to overcome said disadvantages, the present inventors have found that syn-1,2-polybutadiene to be block or graft polymerized with cis-1,4-polybutadiene is crystallized and is in a short fiber-like shape, and physical properties can be remarkably improved by specifying the diameter and length of the short fibers, with less fluctuation in physical properties. 
     The object of the present invention lies in a polybutadiene rubber composition comprising, as a main component, polybutadiene rubber containing polymers obtained by block or graft polymerization of cis-1,4-polybutadiene with syndiotactic (syn)-1,2-polybutadiene, alone or blended with other diene rubber in which a microstructures is of 78-93% by weight of cis-1,4-structures and 6-20% by weight of syn-1,2-structures, and at least 40% by weight of said syn-1,2-polybutadiene being crystallized and in a short fiberlike shape having an average diameter of 0.05-1μ and an average length of 0.8-10μ; 
     and further the object of the present invention lies in a polybutadiene rubber composition blended with the other diene rubber as mentioned above, especially suitable for a chafer of tyre, in which a carbon black as said filler having an iodine absorption number (IA) of 76 mg/g or more and a dibutyl phthalate oil absorption number (DBP) of 90 ml/100 g or more is further combined by 40 to 100 parts by weight relative to 100 parts by weight of blended polybutadiene rubber prepared by blending 40 to 95 parts by weight of said polybutadiene rubber and 5 to 60 parts by weight of said other diene rubber, so that a pico abrasion index defined by ASTM D2228 of the polybutadiene rubber composition is 210 or more; 
     and still further the object of the present invention lies in a polybutadiene rubber composition blended with the other diene rubber as mentioned above, especially suitalbe for a vulcanizable rubber composition for side wall, in which a carbon black as said filler having an IA of 30 to 90 mg/g and a DBP of 130 ml/100 g or less is further combined by 30 to 100 parts by weight relative to 100 parts by weight of blended polybutadiene rubber prepared by blending 40 to 95 parts by weight of said polybutadiene rubber and 5 to 60 parts by weight of said other diene rubber, and an antioxidant of p-phenylenediamine group is still further combined by 0.5 to 8 parts by weight relative to 100 parts of said blended polybutadine rubber; 
     and still further the object of the present invention lies in a polybutadiene rubber composition blended with the other diene rubber as mentioned above, especially suitable for tyre tread, in which a carbon black as said filler having an IA of 60 mg/g or more and a DBP of 70 ml/100 g or more is combined by 30 to 100 parts by weight relative to 100 parts by weight of blended polybutadiene rubber prepared by blending 40 to 95 parts by weight of said polybutadiene rubber and 5 to 60 parts by weight of said other diene rubber. 
     The polybutadiene rubber of the present invention can be produced basically according to the process disclosed in Japanese Patent Publication Nos. 17666/1974 and 17667/1974. However, it is difficult to obtain rubber having equal properties constantly, even if synthesized equally according to such a process. It has been made clear through the studies made by the present inventors that such difficulty is due to crystallization of syn-1,2-polybutadiene in a short fiber-like shape during the polymerization, and the shape of the short fibers of syn-1,2-polybutadiene gives a great influence upon the properties of the resulting rubber, and further a change in the shape of the short fibers remarkably depends upon stirring conditions (shearing speed distribution and shearing speed) during the polymerization. That is, in more concrete terms, a ratio of average length/average diameter of the short fibers is decreased when the shearing speed distribution is disturbed and stirring is carried out in a nearly turbulent state, whereas the ratio of average length/average diameter is increased when the stirring is carried out in a nearly laminar flow state. On the other hand, the diameter of short fibers is decreased when the stirring is conducted so as to increase the shearing speed, whereas the length of the short fibers is increased when the stirring is conducted so as to decrease the shearing speed. Thus, it is possible to freely and readily change the shape of syn-1,2-polybutadiene short fibers in a wide range on account of catching such phenomena. 
     The microstructure of the polybutadiene rubber according to the present invention can be determined from the following two values in the following manner; the polybutadiene rubber is dissolved in n-hexane, and the soluble matter and the insoluble matter are separated from each other by centrifuge. The microstructure of the soluble matter is determined by infrared absorption spectra after the n-hexane has been removed from the soluble matter and the residues are dissolved in carbon bisulfide, whereas the microstructure of insoluble matter is determined, after drying, by infrared absorption spectra according to a tablet method of potassium bromide. 
     In the polybutadiene rubber of the present invention the content of syn-1,2-structure is particularly important, and is 6-20% by weight. If it is less than 6% by weight, only physical properties almost equal to those of the conventional cis-1,4-polybutadiene rubber are obtained, whereas, if it is more than 20% by weight, the viscosity is so high that a considerably adverse effect is given to the workability. However, if cis-1,4-polybutadiene rubber is added thereto, even in such a case, to adjust the content of syn-1,2-structure to not more than 20% by weight on the whole, the polybutadiene rubber of such a content can be satisfactorily used. 
     In the polybutadiene rubber of the present invention, the average diameter and average length of syn-1,2-polybutadiene short fiber can be measured in a manner as described later, but the average diameter is 0.05-1μ and the average length is 0.8-10μ. If the average diameter is less than 0.05μ, the breaking resistance characteristic of polybutadiene rubber is lowered, whereas the average diameter above 1μ is not preferable, because the flex resistance of polybutadiene rubber is lowered. On the other hand, if the average length is less than 0.8μ, the crack growth resistance of plybutadiene rubber is lowered, whereas the average length above 10μ is not preferable, because the workability is considerably lowered. Furthermore, it is preferable that a ratio of average length/average diameter is at least 8, and when the ratio is above 8, the breaking resistance characteristic of polybutadiene rubber can be much more improved. 
     In the polybutadiene rubber of the present invention, it is necessary that at least 40% by weight of syn-1,2-polybutadiene is in short-fiber-like shape. If the proportion of short fiber is less than 40% by weight, the melting point of said shor fiber is lowered, while increasing a temperature dependency. As a result, the temperature dependency of polybutadiene rubber is also unpreferably increased. That is, the higher the tacticity of syn-1,2-polybutadiene, the more suitable the polybutadiene rubber composition of the present invention. 
     The plybutadiene composition of the present invention contains 40-95 parts by weight, preferably 50-95 parts by weight, of polybutadiene and 5-60 parts by weight, preferably 5-50 parts by weight, of other diene rubber, for example, such as natural rubber (NR), polyisoprene rubber (IR), styrenebutadiene copolymer rubber (SBR), cis-1,4-polybutadiene rubber, or mixtures thereof. If polybutadiene is less than 40 parts by weight, the flex resistance is poor, whereas if the diene rubber is less than 5 parts by weight, the workability is poor, and if it is more than 60 parts by weight, the amount of polybutadiene rubber is relatively reduced, and thus the flex resistance is unpreferably deteriorated. 
     In the polybutadiene composition of the present invention, a vulcanizing agent is sulfur, alkylphenol, disulfide, and 4,4&#39;-dithiodimorpholine, and preferably is sulfur. Its blending proportion is in a range as used in the ordinary rubber blending. 
     In the polybutadiene rubber composition of the present invention, the inorganic filler is anhydrous silicic acid, calcium carbonate, magnesium carbonate, talc, iron sulfide, iron oxide, bentonite, zinc oxide, diatomaceous earth, white clay, clay, alumina, titanium oxide, and carbon black. An appropriate blending proportion thereof is 20-120 parts by weight per 100 parts by weight of polybutadiene in view of reinforcement ability and workability. 
     In order to apply the polybutadiene rubber composition of the present invention to the rubber chafer, the carbon black as the filler must be a relatively high quality carbon black having an IA of 76 mg/g or more, preferably 86 mg/g or more, and a DBP of 90 ml/100 g or more, preferably 105 ml/100 g or more. The carbon black having the IA of less than 76 mg/g and the DBP of less than 90 ml/100 g is not preferable, because it fails to show a satisfactory reinforcement effect as the rubber chafer. 
     Pico abrasion index defined according to ASTM D2228 is restricted to 210 or more in such polybutadiene rubber composition for the rubber chafer, because, if it is less than 210, no remarkable effect is observed upon the improvement, as compared with the conventional rubber chafer, whereas, if it exceeds 210, the desired object as the rubber chafer can be satisfactorily attained. 
     In order to apply the polybutadiene rubber composition of the present invention to a rubber blending composition for side wall, the carbon black as the filler must be a low quality carbon black having the IA of 30-90 mg/g, preferably 45-75 mg/g, and the DBP of 130 ml/100 g or less. If the IA is less than 30 mg/g, the particle size is so large that the reinforcement effect is decreased, and the flex resistance is thus lowered. If it is more than 90 mg/g, the reinforcement effect can be expected, but the fatique orientation is liable to appear, and the flex resistance is also lowered. Thus, such is not preferable. Its blending proportion is 30-100 parts by weight per 100 parts by weight of the polybutadiene rubber containing the other diene rubber, and if it is less than 30 parts by weight, any satisfactory reinforcement effect cannot be obtained. If it exceeds 100 parts by weight, the workability and the flex resistance are lowered. 
     The antioxidant of p-phenylenediamine group to be blended in the polybutadiene rubber composition for the rubber blend composition for said side wall is N-phenyl-N&#39;-isopropyl-p-phenylenediamine, N,N&#39;-diphenyl-p-phenylenediamine, N-phenyl-N&#39;-(2,4-dimethyl) butyl-p-phenylenediamine, N-N&#39;-di-2-naphthyl-p-phenylenediamine, etc, or the mixtures thereof, and its blending proportion is 0.5-8 parts by weight per 100 parts by weight of the polybutadiene rubber containing the other diene rubber. If it is less than 0.5 parts by weight, the roll baggy cannot be improved and also the antioxidizing effect is not satisfactory. If it exceeds 8 parts by weight, the antioxidant is dissolved at the kneading in Banbury mixer and plays a role of lubricant. That is, since the rotor slides on the rubber, a satisfactory kneading cannot be obtained, and the effect upon the antioxidant is lowered to the contrary. 
     In order to apply the polybutadiene rubber composition of the present invention to tread rubber of tyre, it is necessary that the weight average molecular weight of cis-1,4-polybutadiene is 350,000 or more. If it is less than 350,000, a satisfactory abrasion resistance necessary for the tread cannot be obtained. Furthermore, as the carbon black, carbon black having the IA of 60 mg/g or more, and the DBP of 70 ml/100 g or more is used. If the IA is less than 60 mg/g, a satisfactory abrasion resistance cannot be obtained. If the DBP is less than 70 ml/100 g, a satisfactory reinforcement effect cannot be obtained. Thus, in any case less number is not satisfactory. Its appropriate blending proportion is 30-100 parts by weight per 100 parts by weight of the polybutadiene rubber in view of its blending effect, workability as well as fatigue resistance. 
     The polybutadiene rubber composition of the present invention is quite distinguished in its flex resistance, the crack growth resistance and the breaking resistance characteristics, and is suitable to the tread, the rubber chafer of tyre, and the rubber blend composition for the side and can be applied to almost all the other parts. For example, its application to coating rubber, bead filler, belt end coating rubber, buffer rubber between a reinforcement layers, or inner liner is expected. Thus, the present polybutadiene rubber not only can greatly improve the overall properties of tyre, but also provide a novel material indispensable for the development of tyres having various functions including the low fuel consumption tyre of light weight now in the increasing demand. 
     The present invention will be described in detail below, referring to the Examples and the comparative Examples. 
    
    
     EXAMPLES 1-6 AND COMPARATIVE EXAMPLES 1-7 
     13 kinds of the polybutadiene rubber having different microstructures shown in Table 1--1 and Table 1-2 were used. 
     The average diameter and the average length of the short fibers were determined as follows: Various kinds of polybutadiene rubbers were extruded from a circular die having the diameter of 2 mm and the length of 10 mm to prepare the samples. The rubber component of the samples was dissolved in n-hexane by dipping for 48 hours, and the residues were freeze dried and dyed by osmium oxide (O s  O 4 ). Then, the resulting samples were enclosed by methyl methacrylate (MMA) and a very thin pieces were cut out from the samples by cutting them in the direction perpendicular to said extruding direction, and the diameters were measured by electron microscope. Likewise, very thin pieces were cut from the samples in parallel to said extruding direction, and the length were measured by electron microscope. 
     The average diameter and the average length were calculated according to the following formulae: ##EQU1## wherein r: average diameter of short fibers 
     ri: diameter of short fibers 
     l: average length of short fibers 
     li: length of short fibers 
     ni: number of short fibers having a diameter ri, or a length li. 
     Σni: 300 
     
                                           TABLE 1-1                               
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                       Sample                                             
Microstructure         1  2  3  4  5  6  7                                
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cis-1,4-structure                                                         
                 Weight %                                                 
                       94.0                                               
                          91.9                                            
                             85.0                                         
                                79.5                                      
                                   75.6                                   
                                      85.5                                
                                         81.3                             
trans-1,4-structure                                                       
                 Weight %                                                 
                       1.0                                                
                          1.4                                             
                             2.4                                          
                                2.8                                       
                                   3.2                                    
                                      2.2                                 
                                         2.5                              
syn-1,2-structure                                                         
                 Weight %                                                 
                       5.0                                                
                          6.7                                             
                             12.6                                         
                                17.7                                      
                                   21.2                                   
                                      12.3                                
                                         16.2                             
Average diameter of short fiber                                           
                 μ  0.3                                                
                          0.1                                             
                             0.25                                         
                                0.33                                      
                                   0.38                                   
                                      0.04                                
                                         0.08                             
Average Length of short fiber                                             
                 μ  3.2                                                
                          3.8                                             
                             3.1                                          
                                4.5                                       
                                   4.7                                    
                                      1.2                                 
                                         1.0                              
Average Length/average diameter                                           
                       10.7                                               
                          38.0                                            
                             12.4                                         
                                13.6                                      
                                   12.4                                   
                                      30.0                                
                                         12.5                             
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                                           TABLE 1-2                               
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                       Sample            Ubepol*                          
Microstructure         8  9  10 11 12 13 150                              
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cis-1,4-structure                                                         
                 Weight %                                                 
                       82.3                                               
                          86.3                                            
                             85.3                                         
                                84.4                                      
                                   83.1                                   
                                      87.1                                
                                         97                               
trans-1,4-structure                                                       
                 Weight %                                                 
                       2.6                                                
                          2.1                                             
                             2.0                                          
                                2.3                                       
                                   2.5                                    
                                      2.5                                 
                                         1.5                              
syn-1,2-structure                                                         
                 Weight %                                                 
                       15.1                                               
                          11.6                                            
                             12.7                                         
                                13.3                                      
                                   14.4                                   
                                      14.4                                
                                         (1.5)                            
Average diameter of short fiber                                           
                 μ  0.9                                                
                          1.2                                             
                             0.3                                          
                                0.42                                      
                                   0.55                                   
                                      0.1                                 
Average Length of short fiber                                             
                 μ  7.6                                                
                          9.8                                             
                             0.76                                         
                                11.5                                      
                                   4.2                                    
                                      6.7                                 
Average Length/average diameter                                           
                       8.4                                                
                          8.2                                             
                             2.5                                          
                                27.3                                      
                                   7.6                                    
                                      67.0                                
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 *cis-1,4-polybutadiene (Ube Industries Limited in Japan)                 
 
    
     Various polybutadiene rubber compositions were obtained according to the following formula, using Samples 1-13 shown in Table 1-1 and Table 1-2, and Ubepol 150 (cis-1,4-polybutadiene rubber made by Ube Industries Limited in Japan) as the conventional product, individually. 
     The blending formula of various polybutadiene rubber compositions is given as follows: 
     
         ______________________________________                                    
Natural rubber    30 parts by weight                                      
Sample 1-13       Each 70 parts by weight                                 
(Ubepol 150)      (70 parts by weight)                                    
N-330-carbon black                                                        
                  50 parts by weight                                      
 IA: 82 mg/g                                                              
 DBP: 102 ml/100 g                                                        
Aromatic oil      10 parts by weight                                      
Stearic acid      2.5 parts by weight                                     
810 NA*.sup.1     2.0 parts by weight                                     
Wax               1.5 parts by weight                                     
Zinc oxide        4.0 Parts by weight                                     
DM*.sup.2         0.3 parts by weight                                     
Nob*.sup.3        0.4 parts by weight                                     
Sulfur            1.5 parts by weight                                     
______________________________________                                    
 *.sup.1 Nphenyl-Nisopropyl-p-phenylenediamine                            
 *.sup.2 di2-benzothiazyl sulfide                                         
 *.sup.3 Noxydiethyl-2-benzothiazolyl sulfeneamide                        
 
    
     Mooney viscosity (ML 1+4 ), Mooney scorch time (MST), polymer dispersion degree and roll workability, and hardness (Hd), breaking strength (Tb), breaking elongation (Eb), 300% modulus (300 Md), crack growth resistance, and flex resistance of various polybutadiene rubber compositions after vulcanization at 150° C. and 80 kg/cm 2  for 30 minutes. The results are shown in Table 2-1 and Table 2-2. 
     Test procedures: ML 1+4  and MST were determined according to the procedure described in JIS K6300. Polymer dispersion degree was determined by mixing each components and observing the resulting mixture by electron microscope (magnification: ×5,000, dyed by O s  O 4 ). Roll workability was evaluated by the appearance of roll baggy in 10-inch roll. Hd, Tb, Eb and 300 Md were measured according to JIS K6301. The crack growth resistance is evaluated by an index representing the time required for growth of the initial 2 mm cut to 12 mm in a de Mattia machine (300 cycles/min.), in presuming the time of the conventional case as 100. Thus, the higher value means a better crack growth resistance. Flex resistance is evaluated by an index representing the time till an appearance of a crack in a de Mattia machine, in presuming the time of the conventional case as 100. Thus, the higher value means a better flex resistance. 
     As is evident from Table 1-1, 1-2 and Table 2-1, 2-2, (1) the polybutadiene rubber composition of the present invention having 6-20% by weight of syn-1,2-structures had better results (Examples 1-3 and Comparative Examples 1-2), (2) the average diameter of short fibers in a range of 0.05-1μ, formed from syn-1,2-polybutadiene gave better results (Examples 4-5 and Comparative Examples 3-4), and (3) the average length of short fibers in a range of 0.8-10μ, and a ratio of average length/average diameter of 8 or more, formed from syn-1,2-polybutadiene gave better results (Examples 2, and 5-6, and Comparative Examples 5-7). 
     
                                           TABLE 2-1                               
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            Comparative              Comparative                          
                                            Comparative                   
            example 1                                                     
                   Example 1                                              
                         Example 2                                        
                               Example 3                                  
                                     example 2                            
                                            example 3                     
                                                   Example 4              
            (Sample 1)                                                    
                   (Sample 2)                                             
                         (Sample 3)                                       
                               (Sample 4)                                 
                                     (Sample 5)                           
                                            (Sample 6)                    
                                                   (Sample                
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                                                   7)                     
ML.sub.1+4  44     45    46    48    45     46     47                     
MST (min)   12     11    11    11    12     11     11                     
Dispersion of polymer                                                     
            good   good  good  good  No dis-                              
                                            good   good                   
                                     persion                              
Workability of roll                                                       
            slightly                                                      
                   good  good  good  good   good   good                   
            baggy                                                         
Hd (degree) 55     57    61    65    63     55     64                     
Tb (Kg/cm.sup.2)                                                          
            172    178   187   179   151    171    192                    
Eb (%)      568    551   560   552   376    570    565                    
300Md (Kg/cm.sup.2)                                                       
            72     79    81    89    85     67     87                     
Crack growth resistance                                                   
            100    236   472   560   82     115    498                    
(index)                                                                   
Flex resistance (index)                                                   
            120    145   710   860   55     105    745                    
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                                           TABLE 2-2                               
__________________________________________________________________________
                   Comparative                                            
                          Comparative                                     
                                  Comparative                             
                                         Comparative                      
            Example 5                                                     
                   example 4                                              
                          example 5                                       
                                  example 6                               
                                         example 7                        
                                                 Example                  
                                                        Conventional      
            (Sample 8)                                                    
                   (Sample 9)                                             
                          (Sample 10)                                     
                                  (Sample 11)                             
                                         (Sample 13)                      
                                                 (Sample                  
                                                        method            
__________________________________________________________________________
ML.sub.1+4  53     68     44      63     45      50     45                
MST (min)   11     11     12      11     12      11     11                
Dispersion of polymer                                                     
            good   good   good    good   good    good   good              
Workability of roll                                                       
            good   good   Slightly                                        
                                  Not easily                              
                                         good    good   good              
                          baggy   doctor-knifed                           
Hd (degree) 66     67     55      69     64      67     55                
Tb (Kg/cm.sup.2)                                                          
            169    154    149     170    170     184    171               
Eb (%)      547    405    438     318    540     561    565               
300Md (Kg/cm.sup.2)                                                       
            86     70     66      93     75      88     65                
Crack growth resistance                                                   
            481    262    95      230    180     580    100               
(index)                                                                   
Flex resistance (index)                                                   
            357    99     73      99     110     878    100               
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     EXAMPLES 7-9 AND COMPARATIVE EXAMPLE 8 
     The crack growth resistance, the flex resistance and the breaking resistance characteristics and abrasion resistance of various polybutadiene rubber compositions blended as in Table 4, using samples 14-17 and Ubepol 150 as the conventional product shown in Table 3 were investigated. 
     The weight average molecular weight of cis-1,4-polybutadiene (cis-1,4-structures) shown in Table 3 was determined by dissolving various kinds of polybutadiene rubber in n-hexane, separating soluble matter from insoluble matter by centrifuge, and measuring the soluble matter (cis-1,4-polybutadiene) according to a light scattering method. Breaking resistance characteristic is evaluated by an index representing a depth of scar on the rubber sample (15 cm×10 cm×5.5 cm) made by falling a weight of 700 kg having an edge (edge angle: 45°) from a definite level by gravity in a one-swing type testing machine, in presuming the depth of scar on the conventional case as 100. Thus, the higher value means a better breaking resistance characteristic. Abrasion resistance is evaluated by an index representing an evaluation by pico abrasion defined in ASTM D2228, in presuming the evaluation of the conventional case as 100. The larger value means a better abrasion resistance. 
     As is evident from Table 4, the polybutadiene rubber composition of the present invention having a weight average molecular weight of 350,000 or more also have a better alvasion resistance, and thus can be preferably applied to tread rubber of tyre. 
     
                                           TABLE 3                                 
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                      Sample                  Ubepol                      
Microstructure        14    15    16    17    150                         
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cis-1,4-structure                                                         
                 weight %                                                 
                      85.2  86.7  88.2  79.8  97                          
trans-1,4-structure                                                       
                 weight %                                                 
                      2.2   2.1   2.0   3.2   1.5                         
syn-1,2-structure                                                         
                 weight %                                                 
                      12.6  11.2  9.8   17.0  (1.5)                       
Average diameter of   0.23  0.22  0.24  0.18                              
short fiber                                                               
Average length of     5.2   4.3   5.6   4.8                               
short fiber                                                               
Average length/Average diameter                                           
                      22.6  19.5  23.3  26.7                              
Weight average molecular weight                                           
of cis-1,4-polybutadiene                                                  
                      2.56 × 10.sup.5                               
                            3.57 × 10.sup.5                         
                                  5.72 × 10.sup.5                   
                                        7.24 × 10.sup.5             
                                              6.25 × 10.sup.5       
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                                           TABLE 4                                 
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            Comparative                Conventional                       
            example 8                                                     
                   Example 7                                              
                           Example 8                                      
                                 Example 9                                
                                       method                             
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Natural rubber                                                            
            50       50    50    50    50                                 
Styrene-butadiene                                                         
            20       20    20    20    20                                 
copolymer                                                                 
Sample 14   30                                                            
Sample 15            30                                                   
Sample 16                  30                                             
Sample 17                        30                                       
Ubepol 150                             30                                 
N-220 Carbon black*                                                       
            45                                                            
Aromatic oil                                                              
            5                                                             
Stearic acid                                                              
            2                                                             
810NA       1.5                                                           
Wax         1.0      &#34;     &#34;     &#34;     &#34;                                  
Zinc oxide  3.5                                                           
DM          0.4                                                           
Nobs        0.6                                                           
Sulfur      1.75                                                          
(Results)                                                                 
Crack growth resistance                                                   
(index)     548      562   561   555   100                                
Flex resistance (index)                                                   
            820      843   837   819   100                                
Breaking resistance                                                       
(index)     131      130   128   135   100                                
Abrasion resistance                                                       
(index)     85       105   128   130   100                                
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 *IA 121 mg/g, DBP 114 ml/100g                                            
 
    
     EXAMPLES 10-12 AND COMPARATIVE EXAMPLES 9-11 
     5 kinds of polybutadiene rubber having different microstructures shown in Table 5 were prepared, and Mooney viscosity (ML 1+4 ), Mooney scorch time (MST), Polymer dispersion degree, and hardnes (Hd), breaking strength (Tb), breaking elongation (Eb), 300% modulus (300 Md), pico abrasion index and crack growth resistance after vulcanization at 150° C. and 80 kg/cm 2  for 30 minutes were investigated for each polybutadiene rubber, as shown in Table 6. Tyres were made by using the rubber chafer of the resulting rubber compositions, and made to run on a drum, and then the degree of appearance of rim dislocation were investigated. The results are shown in Table 6. 
     
                       TABLE 5                                                     
______________________________________                                    
           Sam-    Sam-   Sam-  Sam- Sam-                                 
           ple     ple    ple   ple  ple                                  
Microstructure                                                            
           18      19     20    21   22    BR01*                          
______________________________________                                    
cis-1,4-structure                                                         
weight (%) 94.6    91.7   85.2  80.3 75.4  98.0                           
trans-1,4-structure                                                       
weight (%) 0.9     1.5    2.3   2.7  3.0   1.3                            
syn-1,2-structure                                                         
weight (%) 4.5     6.8    12.5  17.0 21.6  0.7                            
______________________________________                                    
 *cis-1,4-polybutadiene rubber (Japan Synthetic Rubber Co., Ltd.)         
 
    
     
                                           TABLE 6                                 
__________________________________________________________________________
           Comparative                Comparative                         
                                             Comparative                  
           example 9                                                      
                    Example 10                                            
                          Example 11                                      
                                Example 12                                
                                      example 10                          
                                             example 11                   
__________________________________________________________________________
NR         40       40    40    40    40     40                           
Sample 18  60                                                             
Sample 19           60                                                    
Sample 20                 60                                              
Sample 21                       60                                        
Sample 22                             60                                  
BR01                                         60                           
N-220 Carbon black*.sup.1                                                 
           70                                                             
Aromatic oil                                                              
           10                                                             
Stearic acid                                                              
           2                                                              
Tackyrol 130                                                              
           2        &#34;     &#34;     &#34;     &#34;      &#34;                            
Zinc oxide 5                                                              
RD*.sup.2  1                                                              
DM*.sup.3  1                                                              
Nobs*.sup.4                                                               
           1                                                              
Sulfur     3                                                              
(Results)                                                                 
ML.sub.1+4 70       73    78    82    80     70                           
MST (min)  12       11    11    12    11     11                           
Polymer                                                                   
dispersion degree                                                         
           good     good  good  good  No     good                         
                                      dispersion                          
Hd (degree)                                                               
           70       71    73    74    72     70                           
Tb (Kg/cm.sup.2)                                                          
           185      189   191   196   172    183                          
Eb (%)     204      206   201   198   182    206                          
300Md (Kg/cm.sup.2)                                                       
           150      151   153   155   152    150                          
Pico abrasion index                                                       
           200      214   224   232   218    180                          
Crack growth                                                              
resistance (index)                                                        
           99       170   425   490   210    100                          
Appearance degree of                                                      
           Slight   Not appearance           appear-                      
rim discrepancy                                                           
           appear-                           ance                         
           ance                                                           
__________________________________________________________________________
 *.sup.1 IA 121 mg/g, DBP 114 ml/100 g                                    
 *.sup.2 2,2,4trimethyl-1,2-dihydroquinoline polymer                      
 *.sup.3 di2-benzothiazyldisulfide                                        
 *.sup.4 Noxydiethylene-2-benzothiazolylsulfeneamide                      
 
    
     As is evident from Table 6, the content of syn-1,2-structure is desirably in a range of 6-20% by weight, and the polybutadiene rubber composition can satisfactorily attain the object of rubber chafer when the pico abrasion index of the rubber composition is 210 or more. 
     Test procedures: ML 1+4  and MST were determined according to the procedure described in JIS K6300. The polymer dispersion degree was determined by mixing the components and observing the resulting mixture by electron microscope (magnification: ×5,000, dyed by O s  O 4 ). Hd, Tb, Eb and 300 Md were measured according to JIS K6301. The crack growth resistance is evaluated by an index representing the time required for the growth of the initial 2 mm cut to 12 mm in a de Mattia machine (300 cycles/min.), in presuming the time in Comparative Example 11 as 100. Thus, the higher value means a better crack growth resistance. 
     The degree of appearance of rim dislocation was determined by mounting a rubber chafer made from a rubber composition of Table 6 at a tyre size 165 SR13, making it run on a steel drum in an equivalent distance of 10,000 km at a tyre inner pressure of 2.3 kg/cm 2 , a 150% over-load over the designed rated load and a speed of 100 kg/hr, and then dissecting the tyre to inspect a rim dislocation on the rubber chafer surface. 
     EXAMPLES 13-15 AND COMPARATIVE EXAMPLES 12 AND 13 
     The workability (roll baggy), the pico abrasion index, and the crack growth resistance of the rubber compositions shown in Table 7 were evaluated in the same manner as in Example 10, while changing the blend proportion of the diene rubber. Results are shown also in Table 7. The crack growth resistance is evaluated by an index based on presuming that of Comparative Example 11 as 100. 
     
                                           TABLE 7                                 
__________________________________________________________________________
          Comparative                Comparative                          
          example 12                                                      
                   Example 13                                             
                         Example 14                                       
                               Example 15                                 
                                     example 13                           
__________________________________________________________________________
NR        70       55    35    5     0                                    
Sample 20 30       45    65    95    100                                  
N-220 Carbon black                                                        
          70                                                              
Aromatic oil                                                              
          6                                                               
Stearic acid                                                              
          2                                                               
Tackyrol 130                                                              
          2                                                               
Zinc oxide                                                                
          5        &#34;     &#34;     &#34;     &#34;                                    
RD        1                                                               
DM        1                                                               
Nobs      1                                                               
Sulfur    3                                                               
Workability                                                               
(roll baggy)                                                              
          good     good  good  fairly                                     
                                     not                                  
                               good  good                                 
Pico abrasion                                                             
          203      211   230   252   256                                  
index                                                                     
Crack growth                                                              
resistance                                                                
          490      460   360   108   89                                   
__________________________________________________________________________
 
    
     As is evident from Table 7, a good result can be obtained in the application to the rubber chafer by 40-95 parts by weight of the polybutadiene rubber of the present invention in 100 parts by weight of the rubber. 
     EXAMPLES 16-18 AND COMPARATIVE EXAMPLES 14-16 
     The rubber compositions shown in Table 9 using various kinds of carbon black shown in Table 8 were evaluated in the same manner as in Example 10, and results are given in Table 9. The crack growth resistance is evaluated by an index based on presuming the value of Comparative Example 11 as 100. 
     
                       TABLE 8                                                     
______________________________________                                    
N-351      N-339   N-332    N-327 N-234 N-219                             
______________________________________                                    
IA      67      90      84    86    118   118                             
mg/g                                                                      
DBP    120     120     102    60    125    78                             
ml/100 g                                                                  
______________________________________                                    
 
    
     
                                           TABLE 9                                 
__________________________________________________________________________
          Comparative          Comparative  Comparative                   
          example 14                                                      
                   Example 16                                             
                         Example 17                                       
                               example 15                                 
                                      Example 18                          
                                            example 16                    
__________________________________________________________________________
NR        40                                                              
                   &#34;     &#34;     &#34;      &#34;     &#34;                             
Sample 20 60                                                              
N-351 Carbon black                                                        
          70                                                              
N-339 Carbon black  70                                                    
N-332 Carbon black        70                                              
N-327 Carbon black              70                                        
N-234 Carbon black                     70                                 
N-219 Carbon black                           70                           
Aromatic oil                                                              
          6                                                               
Stearic acid                                                              
          2                                                               
Tackyrol 130                                                              
          2                                                               
Zinc oxide                                                                
          5                                                               
RD        1        &#34;     &#34;     &#34;      &#34;     &#34;                             
DM        1                                                               
Nobs      1                                                               
Sulfur    3                                                               
Pico abrasion index                                                       
          190      220   216   210    228   246                           
Crack growth                                                              
resistance (index)                                                        
          210      350   190    96    450   84                            
__________________________________________________________________________
 
    
     As is evident from the results of Table 9, a good result as the rubber chafer can be obtained in the present invention by using carbon black having the IA of 76 mg/g or more, and the DBP of 90 ml/100 g or more. 
     EXAMPLES 19-21 AND COMPARATIVE EXAMPLES 17-19 
     5 kinds of polybutadiene having different microstructures shown in Table 10 were prepared, and Mooney viscosity (ML 1+4 ), Mooney scorch time (MST), and the polymer dispersion degree, and after vulcanization at 150° C. and 80 kg/cm 2  for 30 minutes, hardness (Hd), breaking strength (Tb), breaking elongation (Eb), 300% modulus (300 Md), crack growth resistance, and flex resistance of each rubber after blended as shown in Table 11 were measured. Results are given in Table 11. 
     
                       TABLE 10                                                    
______________________________________                                    
           Sam-    Sam-   Sam-  Sam- Sam-                                 
           ple     ple    ple   ple  ple                                  
Microstructure                                                            
           23      24     25    26   27    BR01*                          
______________________________________                                    
cis-1,4-structure                                                         
weight (%) 94.6    91.7   85.2  80.3 75.4  98.0                           
trans-1,4-structure                                                       
weight (%) 0.9     1.5    2.3   2.7  3.0   1.3                            
syn-1,2-structure                                                         
weight (%) 4.5     6.8    12.5  17.0 21.6  0.7                            
______________________________________                                    
 *cis-1,4-polybutadiene rubber (Japan Synthetic Rubber Co., Ltd.)         
 
    
     
                                           TABLE 11                                
__________________________________________________________________________
           Comparative                Comparative                         
                                             Comparative                  
           example 17                                                     
                    Example 19                                            
                          Example 20                                      
                                Example 21                                
                                      example 18                          
                                             example 19                   
__________________________________________________________________________
NR         30       30    30    30    30     30                           
Sample 23  70                                                             
Sample 24           70                                                    
Sample 25                 70                                              
Sample 26                       70                                        
Sample 27                             70                                  
BR01                                         70                           
N-550 Carbon black*.sup.1                                                 
           60                                                             
Aromatic oil                                                              
           20                                                             
Stearic acid                                                              
           2                                                              
810NA*.sup.2                                                              
           2                                                              
Wax        2        &#34;     &#34;     &#34;     &#34;      &#34;                            
Zinc oxide 4                                                              
DM*.sup.3  0.3                                                            
Nobs*.sup.4                                                               
           0.4                                                            
Sulfur     1.5      1.4   1.4   1.3   1.3    1.5                          
(Results)                                                                 
ML.sub.1+4 44       45    46    48    45     43                           
MST (min)  12       11    12    11    11     11                           
Polymer dispersion                    No                                  
degree     good     good  good  good  dispersion                          
                                             good                         
Hd (degree)                                                               
           55       55    55    55    55     55                           
Tb (Kg/cm.sup.2)                                                          
           170      175   180   184   142    168                          
Eb (%)     568      558   555   563   475    574                          
300Md (Kg/cm.sup.2)                                                       
           65       65    66    65    63     65                           
Crack growth                                                              
resistance (index)                                                        
           100      300   455   540   240    100                          
Flex resistance                                                           
(index)    120      315   610   825   94     100                          
__________________________________________________________________________
 *.sup.1 IA 43mg/g, DBP 121mg/100g                                        
 *.sup.2 Nphenyl-Nisopropyl-p-phenylenediamine                            
 *.sup.3 di2-benzothiazyldisulfide                                        
 *.sup.4 Noxydiethylene-2-benzothiazolylsulfeneamide                      
 
    
     As is evident from Table 11, a good result can be obtained in a range of 6-20% by weight of syn-1,2-structures. 
     Test procedures: ML 1-4  and MST were determined according to the procedure described in JIS K6300. The polymer dispersion degree was determined by mixing the components and observing the resulting mixture by electron microscope (magnification:×5,000, dyed by O s  O 4 ). Hd, Tb, Eb and 300 Md were measured according to JIS K6301. The crack growth resistance is evaluated by an index representing the time required for growth of the initial 2 mm cut to 12 mm in a de Mattia machine (300 cycles/min.), in presuming the time of Comparative Example 19 as 100. Thus, the higher value means a better crack growth resistance. Flex resistance is evaluated by an index representing the time till an appearance of a crack in a de Mattia machine, in presuming the time of Comparative Example 19 as 100. Thus, the higher value means a better flex resistance. 
     EXAMPLES 22-24 AND COMPARATIVE EXAMPLE 20 AND 21 
     The workability (roll baggy), the flex resistance and the crack growth resistance of rubber compositions shown in Table 12 were evaluated in the same manner as in Example 19 by changing a blend ratio of the diene rubber, and results are shown in Table 12. The flex resistance and the crack growth resistance are indicated based on presuming the values of Comparative Example 19 as 100. 
     
                                           TABLE 12                                
__________________________________________________________________________
          Comparative                Comparative                          
          example 20                                                      
                   Example 22                                             
                         Example 23                                       
                               Example 24                                 
                                     example 21                           
__________________________________________________________________________
NR        70        55    35    5     0                                   
Sample 25 30        45    65   95    100                                  
N-550 Carbon black                                                        
          60                                                              
Aromatic oil                                                              
          20                                                              
Stearic acid                                                              
          2                                                               
810NA     2                                                               
Wax       2        &#34;     &#34;     &#34;     &#34;                                    
Zinc oxide                                                                
          4                                                               
DM        0.3                                                             
Nobs      0.4                                                             
Sulfur    1.4                                                             
(Results)                                                                 
Workability                                                               
          good     good  good  fairly                                     
                                     not                                  
(roll baggy)                   good  good                                 
Crack growth                                                              
resistance (index)                                                        
          490      490   450   86     42                                  
Flex resistance                                                           
(index)   72       250   600   690   690                                  
__________________________________________________________________________
 
    
     As is evident from Table 12, a good results can be obtained by 40-95 parts by weight of polybutadiene rubber of the present invention in 100 parts by weight of rubber. 
     EXAMPLES 25-27 AND COMPARATIVE EXAMPLES 22-24 
     Blends of the polybutadiene rubber of the present invention with various kinds of diene rubber were evaluated in the same manner as in Example 19, and the crack growth resistance and the flex resistance of Examples 25 in Table 13 are indicated based on presuming those of Comparative Example 22 as 100, those of Example 26 on those of Comparative Example 23 as 100, and those of Example 27 on those of Comparative Example 24 as 100. 
     
                                           TABLE 13                                
__________________________________________________________________________
                Comparative    Comparative  Comparative                   
          Example 25                                                      
                example 22                                                
                       Example 26                                         
                               example 23                                 
                                      Example 27                          
                                            example 24                    
__________________________________________________________________________
NR        20    20                    20    20                            
SBR       30    30                                                        
IR                     30      30     10    10                            
Sample 25 50           70             70                                  
BR01            50             70           70                            
N-550 Carbon black                                                        
          55    55     60                                                 
Stearic acid                                                              
          2     2      2                                                  
Aromatic oil                                                              
          20    20     20                                                 
810NA     2     2      2                                                  
Wax       2     2      2       &#34;      &#34;     &#34;                             
Zinc oxide                                                                
          3     3      4                                                  
DM        0.5   0.5    0.3                                                
Nobs                   0.4                                                
DPG       0.3   0.3                                                       
Sulfur    1.4   1.5    1.4     1.5    1.4   1.5                           
(Results)                                                                 
Crack growth                                                              
resistance (index)                                                        
          490   100    430     100    470   100                           
Flex resistance                                                           
(index)   725   100    570     100    600   100                           
__________________________________________________________________________
 
    
     As is evident from Table 13, the polybutadiene rubber of the present invention is better, even if blended with various kinds of the diene rubber, as compared with the conventional cis-1,4-polybutadiene rubber. 
     EXAMPLES 28-30 AND COMPARATIVE EXAMPLES 25-27 
     The crack growth resistance and the flex resistance of rubber compositions shown in Table 15 were evaluated in the same manner as in Example 19 by using various kinds of the carbon black shown in Table 14 by way of indicating based on presuming those of Comparative Example 19 as 100, and results are given in Table 15. 
     
                       TABLE 14                                                    
______________________________________                                    
       Carbon black                                                       
Properties                                                                
         N-762   N-650   N-568 N-363 N-326 N-285                          
______________________________________                                    
IA       26       36      45   66    82    102                            
mg/g                                                                      
DBP      62      125     132   68    71    126                            
ml/100 g                                                                  
______________________________________                                    
 
    
     
                                           TABLE 15                                
__________________________________________________________________________
         Comparative    Comparative        Comparative                    
         example 25                                                       
                  Example 28                                              
                        example 26                                        
                               Example 29                                 
                                     Example 30                           
                                           example 27                     
__________________________________________________________________________
NR       30                                                               
                  &#34;     &#34;      &#34;     &#34;     &#34;                              
Sample 25                                                                 
         70                                                               
N-762    60                                                               
N-650              60                                                     
N-568                   60                                                
N-363                           60                                        
N-326                                 60                                  
N-285                                      60                             
Aromatic oil                                                              
         20                                                               
Stearic acid                                                              
         2                                                                
810NA    2                                                                
Wax      2                                                                
Zinc oxide                                                                
         3        &#34;     &#34;      &#34;     &#34;     &#34;                              
DM       0.3                                                              
Nobs     0.4                                                              
Sulfur   1.4                                                              
(Results)                                                                 
Crack growth                                                              
resistance (index)                                                        
         340      200   85     470   290   97                             
Flex resistance                                                           
(index)  82       180   320    650   420   150                            
__________________________________________________________________________
 
    
     As is evident from Table 15, a satisfactory result can be obtained by using carbon black having the IA of 30-90 mg/g and the DBP of 130 ml/100 g or less. 
     EXAMPLES 31-33 AND COMPARATIVE EXAMPLES 28-34 
     The workability (roll baggy), aging by heat and aging by ozone were investigated by blending various kinds of the antioxidant shown in Table 16, and the results are given in Table 16. 
     Aging by heat were represented by percent retention of Tb and Eb after aging at 100° C. for 72 hours, compared with those before aging, according to JIS K6301. Aging by ozone was evaluated by way of the time till breaking of a sample according to ASTM D-3395-75. 
     As is evident from Table 16, not only the workability but also the aging by heat and ozone can be considerably improved by a synergistic effect by blending the polybutadiene rubber of the present invention with the antioxidant of p-phenylenediamine group. 
     
                                           TABLE 16                                
__________________________________________________________________________
                           Comp.                                          
                               Comp.                                      
                                   Comp.                                  
                                       Comp.                              
                                           Comp. Comp.                    
                                                     Comp.                
             Ex. 31                                                       
                   Ex. 32                                                 
                       Ex. 33                                             
                           ex. 28                                         
                               ex. 29                                     
                                   ex. 30                                 
                                       ex. 31                             
                                           ex. 32                         
                                                 ex. 33                   
                                                     ex.                  
__________________________________________________________________________
                                                     34                   
NR           70                            70                             
Sample 25    30                                                           
BR01                                       30                             
N-550 Carbon black                                                        
             60                            60                             
Aromatic oil 20    &#34;   &#34;   &#34;   &#34;   &#34;   &#34;   20    &#34;   &#34;                    
Stearic acid 2                             2                              
Wax          2                             2                              
810 NA       2                             2                              
Santflex 13*.sup.1 2                             2                        
DP*.sup.2              2                                                  
D*.sup.3                   2                         2                    
RD*.sup.4                      2                                          
No crack SP*.sup.5                 2                                      
MB*.sup.6                              2                                  
Zinc oxide   3                                                            
DM           0.3                                                          
Nobs         0.4   &#34;   &#34;   &#34;   &#34;   &#34;   &#34;   &#34;     &#34;   &#34;                    
Sulfur       1.4                                                          
(Result)                                                                  
Workability (Roll baggy)                                                  
             good  good                                                   
                       good                                               
                           not not not not not   not not                  
                           good                                           
                               good                                       
                                   good                                   
                                       good                               
                                           good  good                     
                                                     good                 
Tb Retention %                                                            
             90    91  87  71  73  67  62  69    71  71                   
Eb Retention %                                                            
             82    84  78  70  72  58  51  71    71  69                   
Dynamic ozone test (hrs)                                                  
             13    15  13  10  7   6   5   9     10  9                    
__________________________________________________________________________
 *.sup.1 Nphenyl-N(2,4-dimethyl)butyl-p-phenylenediamine                  
 *.sup.2 N,Ndiphenyl-p-phenylenediamine                                   
 *.sup.3 Phenyl2-naphthylamine                                            
 *.sup.4 2,2,4trimethyl-1,2-dihydroquinoline                              
 *.sup.5 [Mono(or di, or                                                  
 *.sup.6 2mercapto benzoimidazole