Patent Publication Number: US-2018029421-A1

Title: Cord-rubber composite and pneumatic tire

Description:
TECHNICAL FIELD 
     The present invention relates to a cord-rubber composite in which a plurality of steel cords are coated with topping rubber, and a pneumatic tire using the cord-rubber composite. 
     BACKGROUND ART 
     A rubber product such as a pneumatic tire is often reinforced by a cord-rubber composite, for example, as disclosed in Japanese Patent Application Publication No. 2011-126338. For example, a cord-rubber composite comprises a cord array of parallel steel cords coated with unvulcanized topping rubber. Such cord-rubber composite is incorporated in a pneumatic tire as a ply of a tread reinforcing belt, a ply of a carcass and the like. The cord-rubber composite undergoes a vulcanization process, and the topping rubber is vulcanized and united with the steel cords. In order to improve durability of the rubber product, it is important to prevent separation between the steel cords and the topping rubber. 
     SUMMARY OF THE INVENTION 
     Problems to be Resolved by the Invention 
     However, steel cords have poor adhesion to rubber as compared with organic fiber cords. Thereby, conventionally, there has been a demand for improvement of the adhesion between steel cords and topping rubber. 
     The present invention was made in view of the above, and a primary object of the present invention is to provide a cord-rubber composite and a pneumatic tire using the same, in which the adhesion between the steel cords and the topping rubber is improved. 
     According to one aspect of the present invention, a cord-rubber composite comprises: 
     a cord array of parallel steel cords, and 
     an unvulcanized topping rubber coating the cord array and having a complex elastic modulus E of not less than 5 MPa after vulcanization, 
     the code array having a surface area parameter S (mm) of not less than 60 mm, wherein the surface area parameter S (mm) is defined by the following expression: 
     
       
      
       S=D×Pi×E,  
      
     
     wherein 
     E is a cord count of the steel cords per 5 cm width of the cord-rubber composite in a direction orthogonal to longitudinal directions of the steel cords, and 
     D is an average outer diameter (mm) of the steel cords per the 5 cm width. 
     In the cord-rubber composite according to the one aspect of the present invention, it is preferred that the steel cords include one ore more steel cords composed of steel filaments twisted together. 
     According to another aspect of the present invention, a pneumatic tire comprises a tire constructional member formed from the cord-rubber composite according to the one aspect of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a pneumatic tire as an embodiment of the present invention. 
         FIG. 2  is a perspective partial view of a cord-rubber composite as an embodiment of the present invention. 
         FIG. 3  is a cross-sectional partial view of the cord-rubber composite in  FIG. 2 . 
         FIG. 4  is a cross-sectional partial view of a cord-rubber composite as another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The cord-rubber composite according to the present invention is used in rubber products including pneumatic tires. 
     The pneumatic tire according to the present invention comprises a tire constructional member formed from the cord-rubber composite. The pneumatic tire can be embodied variously, for example, passenger car tires, motorcycle tires, heavy duty vehicle tires and the like. 
     Embodiments of the present invention will now be described in conjunction with accompanying drawings. 
       FIG. 1  shows a pneumatic tire  1  as an embodiment of the present invention. The pneumatic tire  1  in this embodiment is a radial tire for a passenger car. 
     The pneumatic tire  1  is composed of various tire constructional members  2  including a carcass  3 , a belt  4 , a tread rubber  5   a , sidewall rubbers  5   b , bead rubbers  5   c , bead cores  6 , and bead apex rubbers  7 . 
     The carcass  3  is composed of at least one carcass ply  3 A of carcass cords (in this embodiment, only one carcass ply). The carcass ply  3 A extends between the bead portions  1   c  through the tread portion  1   a  and the sidewall portions  1   b  and turned up around the bead core  6  in each bead portion from the axially inside to outside of the tire to form a pair of turned up portions  3   b  and a main portion  3   a  therebetween. 
     For example, the carcass cords in the carcass ply  3 A are arranged at an angle of from 80 to 90 degrees with respect to the tire equator C. 
     Between the main portion  3   a  and each of the turned up portions  3   b  of the carcass ply  3 A, the bead apex rubber  7  is disposed so as to extend radially outwardly from the bead core  6 . 
     The belt  4  comprises two cross plies  4 A and  4 B of parallel belt cords laid at, for example, an angle of 10 to 35 degrees with respect to the tire circumferential direction. 
     In the pneumatic tire  1  in this embodiment, each of the belt plies  4 A and  4 B is formed from the cord-rubber composite according to the present invention. 
       FIGS. 2 and 3  show a cord-rubber composite  11  as an embodiment of the present invention. 
     The cord-rubber composite  11  is made up of a cord array  12  and topping rubber  13 . The cord array  12  is composed of a plurality of steel cords  14  arranged parallel with each other. The lengths of the steel cords  14  are the same. The cord array  12  is coated with the topping rubber  13 .
 
In the unvulcanized state of the tire  1 , the topping rubber  13  is not yet vulcanized. In other words, the steel cords  14  are embedded in the unvulcanized topping rubber  13 .
 
The cord-rubber composite  11  is formed in the form of a sheet or strip. The thickness w 1  of the cord-rubber composite  11  is set to be 0.5 to 3.5 mm.
 
In this specification, the term “unvulcanized” includes all states before reaching complete vulcanization, therefore, the so-called semi-vulcanized state is included in “unvulcanized”.
 
     The cord-rubber composite  11  as each of the belt plies  4 A and  4 B constitutes a green tire (not shown) together with other raw tire constructional members  2 . 
     Through the process of vulcanizing the green tire, the topping rubber  13  of the cord-rubber composite  11  is vulcanized and united with steel cords  14 , therefore the cord-rubber composite  11  becomes the belt ply in the pneumatic tire  1 . 
     According to the present invention, a surface area parameter s of the cord-rubber composite  11  is set to be not less than 60 mm. 
     Here, the surface area parameter s is defined by the following expression: 
         S=D×Pi (=3.14 - - - )× E   (1)
 
     wherein 
     E is the cord count (count/5 cm) of the steel cords per 5 cm width of the cord-rubber composite in a direction orthogonal to the longitudinal directions of the steel cords, and 
     D is the outer diameter of the steel cords if the steel cords have the same outer diameter, or the average outer diameter of the steel cords if the steel cords have different outer diameters. 
     Thus, the term D×E in the expression (1) is equal to the total outer diameter of the steel cords existing in 5 cm width of the cord-rubber composite.
 
The surface area parameter S represents the total length of the outer peripheries of the steel cords existing in 5 cm width of the cord-rubber composite.
 
     Here, the outer diameter D of each steel cord is measured according to the “Testing methods for steel tire cords” specified in Japanese Industrial standard (JIS) G3510. 
     In the cord-rubber composite  11  according to the present invention, as the surface area parameter s is increased to at least 60 mm, the contact area between the steel cords  14  and the topping rubber  13  is increased, and the adhesion therebetween is improved. 
     In order to effectively improve the adhesion, the surface area parameter s is preferably set to be not less than 70 mm, more preferably not less than 80 mm. 
     However, if the surface area parameter s is too large, the weight of the cord-rubber composite  11  is increased, and the rolling resistance of the tire  1  is liable to deteriorate. Therefore, the surface area parameter s is preferably not greater than 400 mm, more preferably not greater than 300 mm. 
     The individual steel cord  14  can be a monofilament cord made up of a single steel filament  15  or a multifilament cord made up of a plurality of steel filaments  15  twisted together. 
     Although all the steel cords  14  of the cord array  12  can be monofilament cords, it is preferable that the steel cords  14  of the cord array  12  include at least one multifilament cord, more preferably include a plurality of multifilament cords, and still more preferably all the steel cords  14  are multifilament cords in order to improve the adhesion between the steel cords  14  and the topping rubber  13  by the increased surface area of the multifilament cord and the concavity and convexity of the surface of the multifilament cord into which the topping rubber  13  can penetrate. 
     In the case of the steel cord  14  being a multifilament cord, a single twist structure (1×N) is employed in this embodiment, wherein a number N of filaments  15  are twisted together once. In other words, a number N of filaments  15  are laid helically in one layer around the center. 
     By increasing the number N, it is possible to increase the adhesion between the steel cords  14  and the topping rubber  13 . However, if the number N exceeds 10, there is a possibility that the outer diameter D of the steel cord  14  becomes excessively large. Therefore, the number N of the filaments  15  is preferably set to be not more than 10.
 
In the example shown in  FIGS. 2 and 3 , the number N is two.
 
     As another twist structure of the steel cord  14 , it is possible to employ a layered twists structure (N+M). 
       FIG. 4  shows a cord-rubber composite  11  as another embodiment of the present invention, wherein a 3+8 layered twists structure is employed in each steel cord  14 .
 
In the layered twists structure (N+M), the cord is composed of a core of N filaments which are usually twisted together but may be not twisted, and
 
a sheath of M filaments which are laid helically in one layer around the core.
 
     Compared with the steel cord  14  of the single twist structure, the steel cord  14  of the layered twist structure can form a lot of recesses and gaps through which the topping rubber  13  can enter between the filaments  15 . Thereby, the layered twist structure can further improve the adhesion between the steel cords  14  and the topping rubber  13  as compared with the single twist structure. 
     The number N of the filaments  15  of the core  17  and the number M of the filaments  15  of the sheath layer  18  can be arbitrarily set. However, it is preferable that the numbers N and M are set to be 2 to 10 in the same way as the number N of the filaments of the single twist structure. 
     In order to improve the adhesion between the steel cord  14  and the topping rubber  13 , the surface  15   o  of each steel filament  15  can be coated with a plating layer (not shown). The plating layer is formed by, for example, brass plating, bronze plating, or the like. 
     The topping rubber  13  is a rubber compound  13 G having a complex elastic modulus E of not less than 5 MPa after vulcanization. 
     The complex elastic modulus E* is measured by using a viscoelastic spectrometer in accordance with Japanese Industrial standard JIS-K6394 under the following conditions: 
     temperature: 30 degrees C. 
     frequency: 10 Hz 
     initial strain: 10% 
     amplitude: plus/minus 2% 
     strain mode: tensile 
     such topping rubber  13  can decrease difference in rigidity between the topping rubber  13  and the steel cords  14  after vulcanization. Thereby, the cord-rubber composite  11  can reduce stress concentration that is likely to occur at the interface  16  between the steel cords  14  and the topping rubber  13 , therefore, it is possible to improve the adhesion between the steel cords  14  and the topping rubber  13 . 
     In order to effectively improve the adhesion between the steel cords  14  and the topping rubber  13 , the complex elastic modulus E* of the topping rubber  13  after vulcanization is preferably not less than 5.5 MPa, more preferably not less than 6 MPa. However, if the complex elastic modulus E* is too high, the viscosity of the topping rubber  13  during vulcanization becomes excessively high, and there is a possibility that the productivity is decreased. Therefore, the complex elastic modulus E* is preferably set to be not greater than 25 MPa, more preferably not greater than 20 MPa. 
     As described above, in the cord-rubber composite  11 , the surface area parameter s is set be not less than 60 mm to increase the contact area between the steel cords  14  and the topping rubber  13 , and the complex elastic modulus E* is set to be not less than 5 MPa to decrease the stress concentration which is likely to occur in the interface  16  between the steel cords  14  and the topping rubber  13 . As a result, the cord-rubber composite  11  can be greatly improved in the adhesion between the steel cords  14  and the topping rubber  13 . 
     In the pneumatic tire  1  in which the cord-rubber composite  11  is incorporated as the belt plies  4 A and  4 B, it is possible to prevent separation between the steel cords  14  and the topping rubber  13  over a long period of time. Therefore, a separation failure in the tread portion  1   a  can be effectively prevented to improve the durability of the tire. 
     The pneumatic tire  1  according to the present invention can include the cord-rubber composite  11  as the carcass ply  3 A or other reinforcing plies (not shown) alternatively or in addition to the belt plies  4 A and  4 B. Such tire  1  can be improved in the durability. 
     Incidentally, the cord-rubber composite  11  can be used for rubber products other than pneumatic tires. 
     While detailed description has been made of the preferred embodiments of the present invention, the present invention can be embodied in various forms without being limited to the illustrated specific embodiments. 
     WORKING EXAMPLES 
     Cord-rubber composites (working examples Ex. 1 and Ex. 2, comparative examples Ref. 1 to Ref. 4) having structures as shown in  FIG. 2  were made according to the specification listed in Table 1. 
     using each of the cord-rubber composites as the belt plies of the tire structure shown in  FIG. 1 , a green tire was formed and vulcanized. Thereby, pneumatic tires (size 185/70R14 for passenger car) having the respective cord-rubber composites vulcanized and the tire internal structure shown in  FIG. 1  were experimentally manufactured.
 
Common specifications of the steel cords are as follows:
 
     structure: 1×2 single twist 
     outer diameter of filaments: 0.295 mm 
     outer diameter of cord: 0.59 mm 
     For each of the cord-rubber composites incorporated in the pneumatic tires, the adhesion between the steel cords and the topping rubber was evaluated. Test methods are as follows. 
     &lt;Initial State Adhesion&gt; 
     From each of the vulcanized tires, the belt plies were taken out. Then, according to a method of test for measuring the force required to separate, by stripping, two plies bonded with rubber specified in Japanese Industrial standard JIS K6256-1, the belt plies were separated with a stripping speed of 50 mm/min.
 
From the separated belt plies, the coverage (%) of the surfaces of the steel cords with the topping rubber was determined. The results are shown in Table 1, wherein the larger the numerical value, the better the adhesion between the steel cords and the topping rubber.
 
     &lt;Aged State Adhesion&gt; 
     By placing the vulcanized pneumatic tires in an environment of 70 degrees Celsius and of 95% relative humidity for 10 days, the tires were aged, and the belt plies were taken out from each of the aged tires. Then, as explained above, the coverage (%) was determined.
 
The results are shown in Table 1, wherein the larger the numerical value, the better the adhesion.
 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Ref. 1 
                 Ref. 2 
                 Ref. 3 
                 Ref. 4 
                 Ex. 1 
                 Ex. 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 steel cord count E (/5 cm) 
                 32 
                 36 
                 46 
                 32 
                 36 
                 46 
               
               
                 surface area parameter S (mm) 
                 59 
                 67 
                 85 
                 59 
                 67 
                 85 
               
               
                 complex elastic modulus E* (MPa) 
                 4.5 
                 4.5 
                 4.5 
                 8.5 
                 8.5 
                 8.5 
               
               
                 adhesion (coverage %) 
               
               
                 initial state 
                 50 
                 60 
                 70 
                 50 
                 90 
                 100 
               
               
                 aged state 
                 20 
                 40 
                 50 
                 30 
                 80 
                 90 
               
               
                   
               
            
           
         
       
     
     From the test results, it was confirmed that the cord-rubber composites Ex. 1 and Ex. 2 had better adhesion between the steel cords and the topping rubber in both the initial state and the aged state as compared with the cord-rubber composites Refs. 1 to 4. Thus, in the pneumatic tires including the cord-rubber composites Exs. 1 and 2 as the belt plies, it is possible to prevent the separation between the steel cords and the topping rubber over a long period of time, therefore, it was confirmed that the durability can be improved. 
     REFERENCE SIGNS LIST 
     
         
           11  cord-rubber composite 
           12  cord array 
           13  topping rubber 
           14  steel cord