Abstract:
A rubber composition which comprises a vulcanizable rubber and 0.5 - 20 wt. parts of a metal salt of hydroxy benzoic acid per 100 wt. parts of the rubber. This composition exhibits excellent adhesive bonding to metallic material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rubber composition which is highly adhered to a metallic material by a vulcanization. Heretofore to promote adhesion between a rubber and a metallic material such as a steel cord, it has been known to plate the surface of the metallic material of steel or steel cord with a copper alloy such as brass and bronze. However, the need has increased for high speed, high load and high efficiency of motor car tires and conveyor belts, and driving belts, V-belts, hoses, etc., wherein metallic materials are used as tension members. Accordingly, a high adhesive bond between the rubber and the metallic material is needed. Recently, in order to improve the adhesive bond between a rubber and a metallic material, it has been proposed to add a mixture of a formaldehyde donor, a formaldehyde acceptor and a finely dispersed silica filler (it is called &#34;HRH&#34; system) to the rubber. However, rubber compositions including the HRH system, such as a mixture of fine silica, resorcinol and hexamethylenetetramine, have the disadvantages of generating a smog and bad smell to pollute environment and unsatisfactory workability when blended by a Bambury&#39;s mixer or a roller mixer because of the severe sublimation of resorcinol. The rubber compositions have also the disadvantage of unsatisfactory rolling workability because of low scorch safety and the disadvantage of deteriorated adhesive property caused by moisture absorption in the unvulcanized condition. 
     It has been proposed to add cobalt naphthenate in order to improve the adhesive property between a rubber and a metallic material. However, in said case, there is the disadvantage of deterioration of adhesive property in high temperature blending operation or high temperature vulcanization; (such as 150° - 190° C in the preparation) and also the disadvantage of deterioration of adhesive property and deterioration of other physical properties after heat aging; (such as about 100° C for several tens of hours of driving). 
     2. Summary of the Invention 
     It is an object of the present invention to improve the adhesive property between a rubber and a metal metallic material under vulcanization. It is another object of the invention to limit the deterioration of the adhesive bond caused by heat aging and to improve workability at the same time in the preparation of rubber products such as tires, belts and hoses. The objects of the invention have been attained by providing a rubber composition which comprises 0.5 - 20 wt. parts of a metal salt of hydroxybenzoic acid per 100 wt. parts of a rubber. (phr. : weights per hundred rubber). 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The inventors have found that the adhesive bond between a rubber and a metallic material has been remarkably improved by adding a metal salt of hydroxybenzoic acid to the rubber. The metal salts of hydroxybenzoic acid used in the invention include anhydrous and hydrated salts of hydroxybenzoic acid salts of Na, K, Mg, Ca, Al, Fe, Sn, Ni, Co, Zn, Cu and the like. The metal salt of hydroxybenzoic acid is added at a ratio of 0.5 - 20 wt. parts per 100 wt. parts of the rubber. (phr.). When it is less than 0.5 wt. phr., the desired effect of the invention is not attained. On the other hand, when it is higher than 20 phr., the disadvantageous phenomena of delay of vulcanization and deterioration of physical properties occur. The optimum amount of the metal salt of hydroxybenzoic acid is dependent upon the type of the rubber, the additives and the metallic material and is usually in a range of 0.5 - 7 phr. for improving the adhesive bonding to the metallic material. 
     The rubber compositions of the invention have excellent adhesive property to iron, copper, brass, zinc, bronze, aluminum, and the like. The typical rubbers used in the invention include diene-type rubbers such as natural rubber, polyisoprene rubber, polybutadiene rubber, styrenebutadiene copolymer, acrylonitrile-butadiene rubber, chloroprene rubber and mixtures thereof. 
     The vulcanizing agent used in the invention include the sulfur-vulcanization accelerator-type and the peroxide-type and the like. The vulcanization accelerator can be selected from a wide range of compounds such as sulfenamide-type compounds e.g. N-oxydiethylene benzothiazole sulfenamide; thiazole-type compounds e.g. 2-mercaptobenzothiazole; thiruam-type compounds e.g. tetramethyl thiuramdisulfide and the like. 
     It is possible to add the other additives such as zinc oxide, stearic acid, carbon black, fine silica (white carbon), depending upon the utility. The vulcanization temperature can be the conventional ones. The conditions of the present invention can be considered in accordance with the prior art such as: 
     Encyclopedia of Polymer Science and Technology Vol. 12 Pages 161-353 on rubbers; ibid Vol. 14 Pages 42-64 on tires and tire cord; ibid Vol. 8 Pages 184-185 on tire and cord dip; ibid Vol. 14 Pages 740-756 on Vulcanization; and 
     Rubber Chemistry and Technology 46 (4) Pages 981-998 on rubber-to-textile and rubber-to-steel cord adhesion. 
    
    
     The invention will be further illustrated in detail by certain examples. 
     I. EXAMPLES 1-14; Standard Example 1; and Comparative Examples 1-2 
     The compositions stated in Table 1 and Table 3 were blended as follows. The test results of the rubber compositions of Examples 1 - 14, Standard Example 1; and Comparative Examples 1 - 2 are stated in Table 2 and Table 4. 
     Preparation and Vulcanization of Rubber Composition 
     The components of each master batch were blended by a Bambury&#39;s mixer and a vulcanizing agent was blended to the master batch on two roll mill to prepare each rubber composition. 
     Adhesive Strength Test 
     1. Adhesive Strength to Metal Material 
     Brass plated steel cords were arranged in parallel with each gap of 12.5 mm. Each rubber composition was coated from both sides of the steel cords to form each fabricated product in which each cord was embedded for 2 inches and the fabricated product was vulcanized under condition of 150° C-30 min. The pull-out test was carried out in accordance with ASTM D2229, under the following conditions. 
     1. to cool at a room temperature after vulcanization; 
     2. to cool at a room temperature after vulcanization and following heat aging at 100° C for 72 hours in a gear oven. 
     As is clear from the results of Table 2 and Table 4, the physical properties of the vulcanized rubber compositions of the invention are similar to those of the vulcanized rubber composition containing cobalt naphthenate however the adhesive strength to the brass plated steel cord and the retentive coefficient of adhesive strength after hot aging can be remarkably improved. 
     
                                           Table 1__________________________________________________________________________Unit: weight part (part by weight)                                       Stand.                                           Comp.           Exp.               Exp.                   Exp.                       Exp.                           Exp.                               Exp.                                   Exp.                                       Exp.                                           Exp.           1   2   3   4   5   6   7   1   1__________________________________________________________________________ (Master Batch)Natural rubber(RSS #1)           100 100 100 100 100 100 100 100 100Zinc oxide (#3) (JISK1410)           5   5   5   5   5   5   5   5   5Stearic acid    3   3   3   3   3   3   3   3   3Carbon black (HAF)           50  50  50  50  50  50  50  50  50Process oil*.sup.1           4   4   4   4   4   4   4   4   4Cobalt naphthenate           --  --  --  --  --  --  --  --  3Cobalt o-hydroxybenzoate           3   --  --  --  --  --  --  --  --Sodium p-hydroxybenzoate           --  3   --  --  --  --  --  --  --Magnesium p-hydroxybenzoate           --  --  3   --  --  --  --  --  --Zinc p-hydroxybenzoate           --  --  --  3   --  --  --  --  --Iron p-hydroxybenzoate           --  --  --  --  3   --  --  --  --Nickel p-hydroxybenozate           --  --  --  --  --  3   --  --  --Cobalt p-hydroxybenzoate           --  --  --  --  --  --  3   --  -- (Vulcanizing agent)Sulfur          4   4   4   4   4   4   4   4   4N-oxydiethylene-2-benzothia-zol sulfenamide 1   1   1   1   1   1   1   1   1__________________________________________________________________________ *.sup.1 Process oil: aromatic type process oil 
    
     
                                           Table 2__________________________________________________________________________                                       Stand.                                           Comp.           Exp.               Exp.                   Exp.                       Exp.                           Exp.                               Exp.                                   Exp.                                       Exp.                                           Exp.           1   2   3   4   5   6   7   1   1__________________________________________________________________________ (Physical properties)Tensile strength (Kg/cm.sup.2)           212 218 233 223 227 220 219 212 217Elongation (%)  390 410 460 415 415 390 415 380 410300% modulus(Kg/cm.sup.2)   156 161 152 158 160 180 158 157 153Hardness (Japanese Indust-rial Standard A)            71  71  72  71  71  74  72  70  70 (Adhesive strength)Brass plated steel cordbefore aging (Kg)           130 138 129 135 130 137 142  85 116after heat aging (Kg)            88  94  90  89  87  89 102  53  60Retentive coefficient (%)            68  68  70  66  67  65  72  62  52__________________________________________________________________________ 
    
     
                                           Table 3__________________________________________________________________________                                       Stand.                                           Comp.           Exp.               Exp.                   Exp.                       Exp.                           Exp.                               Exp.                                   Exp.                                       Exp.                                           Exp.           8   9   10  11  12  13  14  1   2__________________________________________________________________________ (Master Batch)Natural rubber (RSS #1)           100 100 100 100 100 100 100 100 100Zinc oxide (#3) (JISK1410)           5   5   5   5   5   5   5   5   5Stearic acid    3   3   3   3   3   3   3   3   3Carbon black (HAF)           50  50  50  50  50  50  50  50  50Process oil     4   4   4   4   4   4   4   4   4Cobalt naphthenate           --  --  --  --  --  --  --  --  6Cobalt o-hydroxybenzoate           6   --  --  --  --  --  --  --  --Sodium p-hydroxybenzoate           --  6   --  --  --  --  --  --  --Magnesium p-hydroxybenzoate           --  --  6   --  --  --  --  --  --Zinc p-hyroxybenzoate           --  --  --  6   --  --  --  --  --Iron p-hydroxybenzoate           --  --  --  --  6   --  --  --  --Nickel p-hydroxybenzoate           --  --  --  --  --  6   --  --  --Cobalt p-hydroxybenzoate           --  --  --  --  --  --  6   --  -- (Vulcanizing agent)Sulfur          4   4   4   4   4   4   4   4   4N-oxydiethylene-2-benzothiazolsulfenamide     1   1   1   1   1   1   1   1   1__________________________________________________________________________ 
    
     
                                           Table 4__________________________________________________________________________                                       Stand.                                           Comp.           Exp.               Exp.                   Exp.                       Exp.                           Exp.                               Exp.                                   Exp.                                       Exp.                                           Exp.           8   9   10  11  12  13  14  1   2__________________________________________________________________________ (Physical properties)Tensile strength (Kg/cm.sup.2)           209 200 212 210 215 205 210 212 200Elongation (%)  390 370 440 430 415 360 420 380 390300% modulus(Kg/cm.sup.2)   155 158 150 153 153 175 153 157 148Hardness (Japanese Indust-rial Standard A)            72  74  72  71  70  74  72  70  70 (Adhesive strength)Brass plated steel cordbefore aging (Kg)           127 144 133 130 152 128 160  85 126after heat aging (Kg)            82  97  92  86 102  85 112  53  64Retentive coefficient (%)            65  67  69  66  69  66  70  62  51__________________________________________________________________________ 
    
     II. EXAMPLES 15-23; Standard Example 2 and Comparative Examples 3-5 
     In accordance with the process of (I), the compositions stated in Table 5 were blended, vulcanized and the physical properties of the products were measured. The test results are shown in Table 6. As it is clear from the results of Table 6, the physical properties especially tensile strength under 300% elongation of the vulcanized rubber compositions of the invention were remarkably improved comparing with those of the vulcanized rubber composition containing cobalt naphthenate. The adhesive strengths to the zinc plated steel cord and the nonplated steel cord of the vulcanized rubber compositions of the invention were remarkably improved by the addition of the metal salts of hydroxybenzoic acid especially cobalt salt of p-hydroxybenzoic acid. The retentive coefficient of adhesive strength after heat aging were also remarkably improved. 
     
                                           Table 5__________________________________________________________________________                                      Stand.                                           Comp.                                                Comp.                                                     Comp.           Exp.              Exp.                 Exp.                    Exp.                       Exp.                          Exp.                             Exp.                                Exp.                                   Exp.                                      Exp. Exp. Exp. Exp.           15 16 17 18 19 20 21 22 23 1    3    4    5__________________________________________________________________________ (Master Batch)Natura rubber(RSS #1)           100              100                 100                    100                       100                          100                             100                                100                                   100                                      100  100  100  100Zinc oxide (#3) (JISK1410)           5  5  5  5  5  5  5  5  5  5    5    5    5Stearic acid    3  3  3  3  3  3  3  3  3  3    3    3    3Carbon black(HAF)           50 50 50 50 50 50 50 50 50 50   50   50   50Process oil     4  4  4  4  4  4  4  4  4  4    4    4    4Cobalt naphthenate           -- -- -- -- -- -- -- -- -- --   4    7    10Cobalt o-hydroxybenzoate           4  7  10 -- -- -- -- -- -- --   --   --   --Nickel p-hydroxybenzoate           -- -- -- 4  7  10 -- -- -- --   --   --   --Cobalt p-hydroxybenzoate           -- -- -- -- -- -- 4  7  10 --   --   --   -- (Vulcanizing agent)Sulfur          4  4  4  4  4  4  4  4  4  4    4    4    4N-oxydiethylene-2-benzothiazol sulfenamide           1  1  1  1  1  1  1  1  1  1    1    1    1__________________________________________________________________________ 
    
     
                                           Table 6__________________________________________________________________________                                      Stand.                                           Comp.                                                Comp.                                                     Comp.           Exp.              Exp.                 Exp.                    Exp.                       Exp.                          Exp.                             Exp.                                Exp.                                   Exp.                                      Exp. Exp. Exp. Exp.           15 16 17 18 19 20 21 22 23 1    3    4    5__________________________________________________________________________ (Physical properties)Tensile strength(Kg/cm.sup.2)           210              206                 189                    220                       211                          200                             227                                222                                   207                                      212  225  220  212Elongation (%)  400              390                 370                    400                       380                          360                             420                                420                                   400                                      380  430  450  470300% modulus(Kg/cm.sup.2)   156              154                 155                    177                       172                          157                             157                                143                                   140                                      157  152  141  125Hardness (JapaneseIndustrial Standard A)            71               72                  72                     70                        72                           71                              69                                 70                                    71                                       70   69   68   68 (Adhesive strength)Zinc plated steel cordbefore aging (Kg)           132               72                  65                     95                        84                           90                             154                                144                                   112                                       20  110   64   60after heat aging (Kg)            84               43                  33                     65                        58                           59                             108                                101                                    75                                       13   56   31   27Retentive coefficient (%)            64               60                  51                     68                        69                           66                              70                                 70                                    67                                       65   51   48   45Steel cord (non-plated)before aging (Kg)            36               38                  42                     44                        42                           54                              44                                 56                                    58                                       24   36   38   40after heat aging (Kg)            25               26                  27                     31                        29                           36                              33                                 40                                    41                                       15   19   19   20Retentive coefficient (%)            69               69                  65                     70                        70                           67                              75                                 72                                    70                                       63   53   50   61__________________________________________________________________________ 
    
     As stated above, the rubber compositions of the invention attain a remarkably high adhesive bond to the metallic material and prevent deterioration of adhesive bond caused by heat aging. The rubber compositions can be blended at high temperatures (hot blending) and vulcanized at high temperature (hot curing) to improve processability when compared with rubber compositions containing cobalt naphthenate. Accordingly, the rubber compositions of the invention can be utilized as an adhesive composition for rubber which is bonded to a metallic material.