Abstract:
The invention aims at providing a rubber composition for adhering to steel cords which is excellent in heat-resistant adhesion and moist heat-resistant adhesion as well as initial adhesion and has performance of a level similar to or higher than that of the related art in resistance to rubber fracture, wherein the following components (A) and (B) are blended to 100 weight parts of rubber components containing natural rubber and/or diene synthetic rubber:  
     Component (A): a mixture of a compound containing nickel and a compound containing molybdenum; or a compound containing nickel and molybdenum simultaneously in a metal-converted amount of 0.01 to 10 weight parts.  
     Component (B):  
     (b1) 0.2 to 20 weight parts of hexamethylenetetramine or a melamine derivative, and  
     (b2) 0.1 to 10 weight parts of at least one selected from the group consisting of phenol resins, resorcin, resorcin derivatives, and cresol resins.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a rubber composition for adhering to steel cords and particularly to a rubber composition for adhering to steel cords which is excellent in adhesion to steel cords.  
           [0003]    2. Description of the Related Art  
           [0004]    In order to increase the adhesive force of rubber to steel cords plated with brass, bronze, or zinc, a cobalt salt of an organic acid has exclusively been used so far. That is, there exists a technique where blending of the cobalt salt of an organic acid with a rubber composition promotes the formation of an adhesive interface layer onto steel cords, thus to increase the adhesive force.  
           [0005]    In recent years, problems have been generated such as deterioration in adhesive force caused by high exothermic heat accompanied by speeding up or power up of vehicles, deterioration in adhesive force caused by thermal history accompanied by prolonged lifetime of tires, or deterioration in adhesive force caused by moisture and heat over a period of time which elapses between manufacture and travel (a storing time from manufacture to use, containing a time of practical use), and thus adhesive properties of a higher level have been demanded.  
           [0006]    Although the cobalt is excellent in initial adhesion, heat deterioration (heat aging) causes serious deterioration in adhesive force as described above. The cobalt cannot be satisfying any more adhesion performance of a high level required in recent years. It is probable that the cobalt has a strong action to promote oxidation, and the action thereof to promote the formation of an adhesive layer causes increase in thickness of the adhesive layer in terminal travel to lead to fracture of the rubber layer.  
           [0007]    The cobalt is a rare metal, and countries producing the raw material thereof may be politically unstable. Accordingly, the cobalt is disadvantageous in high fluctuation in price and unstable supply of rubber products.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention has been carried out under the aforesaid circumstances. An aim of the invention is to provide a rubber composition for adhering to steel cords, composition which has excellent heat-resistant adhesion and moist heat-resistant adhesion as well as excellent initial adhesion and performance of a level similar to or higher than that in the related art as to resistance to rubber fracture.  
           [0009]    A rubber composition for adhering to steel cords as described in claim 1 is a rubber composition excellent in adhesion to steel cords. The rubber composition is prepared by adding the following components (A) and (B) to 100 weight parts of a rubber component containing natural rubber and/or a diene synthetic rubber.  
           [0010]    (A) A mixture of a compound containing nickel, and a compound containing molybdenum; or a compound containing simultaneously nickel, and molybdenum in a metal-converted amount of 0.01 to 10 weight parts.  
           [0011]    (B) (b1) 0.2 to 20 weight parts of hexamethylenetetramine or a melamine derivative, and (b2) 0.1 to 10 weight parts of at least one selected from the group consisting of phenol resins, resorcin, resorcin derivatives, and cresol resins.  
           [0012]    A composition as described in claim 2 is prepared by further adding a compound containing calcium (C) in the composition as described in claim 1.  
           [0013]    A composition as described in claim 3 is prepared by adding the aforesaid compound containing calcium (C) in an amount of 0.1 to 20% in weight ratio to the aforesaid component (A) in the composition as described in claim 2.  
           [0014]    A composition as described in claim 4 is prepared by further adding silica (D) in the composition as described in any one of claims 1 to 3.  
           [0015]    A composition as described in claim 5 is prepared by adding 1 to 100 weight parts of the aforesaid silica (D) to 100 weight parts of the rubber component in the composition as described in claim 4.  
           [0016]    The invention can provide a rubber composition for adhering to steel cords which has excellent heat-resistant adhesion and excellent moist heat-resistant adhesion as well as excellent initial adhesion and performance of a level similar to or higher than that in the related art as to resistance to rubber fracture.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0017]    Rubber Components  
           [0018]    Examples of rubber components used in the invention include natural rubber and synthetic rubber such as polyisoprene rubber (IR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), isoprene-isobutylene rubber (IIR), ethylene-propylene-diene rubber (EPDM), and modified rubber thereof. These can be used singly or as a mixture of two or more thereof.  
           [0019]    Component (A)  
           [0020]    The compound containing nickel, and the compound containing molybdenum can be either an inorganic compound or an organic compound, respectively.  
           [0021]    The inorganic compounds include nickel oxide, and molybdenum oxide; nickel sulfate, and molybdenum sulfate; nickel phosphate, and molybdenum phosphate; nickel phosphite, and molybdenum phosphite; nickel carbonate, and molybdenum carbonate; and nickel chloride, and molybdenum chloride.  
           [0022]    The organic compounds include nickel salts, and molybdenum salts of organic acids; and an organonickel sulfur compound, an organomolybdenum sulfur compound.  
           [0023]    Herein, preferred nickel salts of organic acids are nickel naphthenate, nickel octylate, nickel propionate, nickel abietate, and nickel acetate in view of excellent adhesion performance.  
           [0024]    Preferred molybdenum salts of organic acids are molybdenum naphthenate and/or molybdenum abietate in view of excellent adhesion performance.  
           [0025]    In component (A), a Ni/Mo ratio (weight converted to metals) is not particularly limited, but preferably is from 2/1 to 20/1. The ratio less than 2/1 may invite problems of being uneconomical (Mo is very costly as compared with Ni) and decreasing hardness (rigidity) of the resulting rubber. Exceeding 20/1 may invite a problem of being unable to acquire sufficient adhesion (Ni cannot compensate deterioration in adhesion).  
           [0026]    In these salts of organic acids, it also is possible to blend organic acids. For example, adhesion, particularly moist heat adhesion is preferably improved when component (A) is a mixture of nickel salts of naphthenic acid and octylic acid (naphthenic acid/octylic acid=1/1 to ¼ in weight percent) and a molybdenum salt of naphthenic acid.  
           [0027]    Naphthenic acid is preferred on the grounds that the high acid value of, for example, 250 mgKOH/g or more further improves the adhesion, particularly the heat-resistant adhesion.  
           [0028]    Component (B) A combination of the following components (b1) and (b2) composes component (B) of the invention.  
           [0029]    (b1) hexamethylenetetramine or melamine derivatives.  
           [0030]    (b2) at least one selected from the group consisting of phenol resins, resorcin, resorcin derivatives, and cresol resins.  
           [0031]    An example of the melamine derivatives is hexamethoxymethylmelamine.  
           [0032]    Examples of the resorcin derivatives include resorcin/alkylphenol/formalin copolymers and resorcin/formalin reaction products.  
           [0033]    Component (B) is used to improve markedly the adhesion, particularly the initial adhesion.  
           [0034]    Component (C)  
           [0035]    Use of compounds containing calcium (C) is preferred on the grounds of further improving the adhesion, particularly the heat-resistant adhesion.  
           [0036]    Examples of compounds containing calcium (C) include calcium borate and calcium neodecanoate, but are not limited to these salts.  
           [0037]    Compounds containing calcium (C) are preferably added to component (A) during the preparation thereof on the grounds that an effect of improving the hardness (rigidity) of the resulting rubber can be acquired (When component (C) is added to rubber singly apart from component (A), such effect can not be obtained).  
           [0038]    Component (D)  
           [0039]    Addition of silica (component (D)) can further improve the adhesion. Silica used include dry process silica (silicic acid anhydride) and wet process silica (hydrated silicic acid), but are not particularly limited.  
           [0040]    Carbon Black  
           [0041]    Carbon black usable in the invention is not particularly limited, and according to the classification by ASTMD1765, include SAF class having numbers of 100&#39;s, ISAF class having numbers of 200&#39;s, and HAF class having numbers of 300&#39;s which have a high reinforcing action, and concretely include N110, N121,N219,N220,N231,N330,and N339. These can be used singly or as a mixture of two or more thereof.  
           [0042]    The Rest (Ratio of Blend and Others)  
           [0043]    The ratio of component (A) blended is from 0.01 to 10 weight parts (in conversion to metal) to 100 weight parts of the rubber component. The ratio of component (A) less than 0.01 weight part invites a problem of failing to obtain sufficient initial adhesion. Exceeding 10 weight parts results in promoting oxidative deterioration of rubber by metal and decreasing aging resistance of the rubber, and as a result offers a problem of decreasing adhesion after aging. The range is preferably from 0.01 to 5.0 weight parts, and more preferably from 0.01 to 2.0 weight parts (in conversion to metal).  
           [0044]    About component (B), the ratio of component (b1) blended is from 0.2 to 20 weight parts to 100 weight parts of the rubber component. The ratio of component (b2) blended is from 0.1 to 10 weight parts to 100 weight parts of the rubber component (The ratio varies depending upon purposes and kinds of resins used and cannot be unconditionally determined). When the ratio of component (b1) blended is less than 0.2 weight part or the ratio of component (b2) is less than 0.1 weight part to 100 weight parts of the rubber component, an effect of improving the adhesion cannot be sufficiently achieved. The ratio of component (b1) blended exceeding 20 weight parts or the ratio of component (b2) exceeding 10 weight parts makes the rubber resinous and fragile. Herein, a preferred range of component (b1) is from 0.5 to 10 weight parts, and that of component (b2) is from 0.5 to 5.0 weight parts.  
           [0045]    The ratio of a compound containing calcium (C) blended is from 0.1 to 20 percent in weight ratio to component (A). The ratio of component (c) blended is less than 0.1 weight percent fails to achieve sufficient improvement in heat-resistant adhesion whereas exceeding 20 weight percent makes it difficult to prepare component (A). Herein, a preferred range is from 1 to 10 weight percent.  
           [0046]    The ratio of sulfur blended is not particularly limited, and for example, from 1 to 10 weight percent. 
       
    
    
     EXAMPLES  
       [0047]    The invention is illustrated through examples, but is not to be construed as limited by the examples.  
       Examples and Comparative Examples  
       [0048]    Common components as described in the following “table 1” and respective components as described in the following “tables 2 to 5” were blended in ratios as described in the tables. Thereafter, these components were kneaded by use of a Banbury mixer according to a general procedure to prepare rubber compositions. About the resulting rubber compositions, an adhesion test (initial adhesion, heat-resistant adhesion, moist heat-resistant adhesion) and an aging resistance test were carried out and evaluated. The respective procedures of evaluation are as follows:  
         [0049]    (1) Adhesion Test  
         [0050]    Steel cords plated with brass were arranged at intervals of 12 lengths/25 mm and separated by seating rubber to be evaluated. Two sheets of such rubber were piled and vulcanized at 150° C. for 30 min to prepare a sample. Peel force of the sample of 25-mm width was measured by use of an autograph. A covering ratio by the steel cord rubber was evaluated by inspection with the naked eye.  
         [0051]    The initial adhesion, heat-resistant adhesion, and moist heat-resistant adhesion were evaluated (Heat-resistant adhesion: undergoing aging in a 160° C. oven to evaluate adhesion. Moist heat-resistant adhesion: A sample is placed in an autoclave containing water and subjected to aging in a 105° C. oven to evaluate adhesion). Results of measurements were estimated by indexes based on the results of Comparative Example 1 estimated as 100 and described in Tables 2 to 5.  
         [0052]    (2) Aging Resistance Test  
         [0053]    After aging in a Geer oven (after aging a sample in the 90° C. oven), a tensile test was carried out (according to JIS K6253). Retention degrees of breaking extension to the initial value were represented by indexes based on the result of Comparative Example 1 estimated as 100. Resulted are described in Tables 2 to 5.  
                               TABLE 1                                       Natural Rubber   100   parts           Carbon Black (HAF)   60   parts           Zinc White   8   parts           Aging Preventive (6 C)   2   parts           Insoluble Sulfur   6   parts           Promoter (DZ)   1   part                      
 
         [0054]    [0054]                                                                           TABLE 2                                       Comparative Example                1   2   3   4   5                        Cobalt Stearate   ◯                       Ni/Mo Sample 1* 1         Ni/Mo Sample 2* 2         Ni/Mo Sample 3* 3         ◯       ◯   ◯       Ni/Mo Sample 4* 4         Ni/Mo Sample 5* 5         Ni/Mo Sample 6* 6         Ni/Mo Sample 7* 7         Ni/Mo Sample 8* 8         Amount of the aforesaid Metallic   0.2   0.2   0.005       15       Compound Blended to 100 Parts of       Rubber Component (Amount       Converted to Metal) (Part)       SUMIKANOL 620   2       2   2   2       (Sumitomo Chemical Co., Ltd.)* 9         SCILETTU 963L   4       4   4   4       (Mitsui Scitech)* 10         Initial Adhesion       Peel Force   100   82   76   73   101       Covering Ratio of Rubber   100   100   100   100   100       Heat-Resistant Adhesion       (160° C. × 96 h age)       Peel Force   100   75   88   110   82       Covering Ratio of Rubber   50   50   60   90   60       Moist Heat Adhesion       (105° 96 C. × h steam age)       Peel Force   100   83   105   106   88       Covering Ratio of Rubber   10   10   10   10   5       Aging Resistance (90° C. × 192 h   100   106   110   127   85       age)       Degree of Improvement                    
         [0055]    [0055]                                                                                   TABLE 3                                       Example                1   2   3   4   5   6                        Cobalt Stearate                               Ni/Mo Sample 1* 1     ◯       Ni/Mo Sample 2* 2         ◯       Ni/Mo Sample 3* 3             ◯       Ni/Mo Sample 4* 4                 ◯       Ni/Mo Sample 5* 5                     ◯       Ni/Mo Sample 6* 6                         ◯       Ni/Mo Sample 7* 7         Ni/Mo Sample 8* 8         Amount of the aforesaid   0.2   0.2   0.2   0.2   0.2   0.2       Metallic Compound Blended       to 100 Parts of Rubber       Component (Amount       Converted to Metal) (Part)       SUMIKANOL 620   2   2   2   2   2   2       (Sumitomo Chemical Co.,       Ltd.)* 9         SCILETTU 963L   4   4   4   4   4   4       (Mitsui Sciteck)* 10         Initial Adhesion       Peel Force   111   114   116   115   111   117       Covering Ratio of Rubber   100   100   100   100   100   100       Heat-Resistant Adhesion       (160° C. × 96 h age)       Peel Force   93   98   100   99   100   96       Covering Ratio of Rubber   80   80   90   80   70   90       Moist Heat-resistant Adhesion       (105° C. × 96 h steam age)       Peel Force   106   104   105   104   106   99       Covering Ratio of Rubber   20   15   10   10   10   20       Aging Resistance (90° C. ×   113   112   111   112   111   100       192 h age)       Degree of Improvement                    
         [0056]    [0056]                                                                                           TABLE 4                                       Example                7   8   9   10   11   12   13                        Cobalt Stearate                                   Ni/Mo Sample 1* 1         Ni/Mo Sample 2* 2         Ni/Mo Sample 3* 3             ◯   ◯   ◯   ◯   ◯       Ni/Mo Sample 4* 4         Ni/Mo Sample 5* 5         Ni/Mo Sample 6* 6         Ni/Mo Sample 7* 7     ◯       Ni/Mo Sample 8* 8         ◯       Amount of the afore-   0.2   0.2   0.2   0.2   0.2   0.2   0.2       said Metallic       Compound Blended       to 100 Parts of Rubber       Component (Amount       Converted to Metal)       (Part)       SUMIKANOL 620   2   2   8           2   2       (Sumitomo Chemical       Co., Ltd.)* 9         Resorcin/Formalin               2       Reaction Product       PENACOLITE RESIN       B19-S (Indospech Co.)       Resorcin (Sumitomo                   2       Chemical Co., Ltd.)       SCILETTU 963L   4   4   10   4   4       4       (Mitsui Scitech)* 12         Hexamethylene-                       4       tetramine       NOXELER H (Ouchi       Shinko Kagaku K. K.)       Silica (NIPSEAL AQ,                           10       Nippon Silica K. K.)       Initial Adhesion       Peel Force   105   114   116   114   118   114   105       Covering Ratio of   100   100   100   100   100   100   100       Rubber       Heat-Resistant       Adhesion (160° C. ×       96 h age)       Peel Force   99   104   114   102   98   98   97       Covering Ratio of   80   70   90   90   90   90   90       Rubber       Moist Heat-Resistant       Adhesion (105° C. ×       96 h steam age)       Peel Force   91   95   127   103   107   103   100       Covering Ratio of   30   10   30   10   10   15   40       Rubber       Aging Resistance   113   107   111   113   109   111   110       (90° C. × 192 h age)       Degree of       Improvement