Patent Publication Number: US-4550147-A

Title: Rubber composition with trithiol triazine and cobalt salt

Description:
This is a continuation of application Ser. No. 356,639, filed Mar. 10, 1982, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to rubber compositions and more particularly to a rubber composition having excellent moisture resisting adhesion and capable of preventing a decrease in the strength of adhesion with brass plate cords when exposed to high humidity. 
     2. Prior Art 
     Conventionally, as a means to improve the performance of an automobile tire, belt conveyor, etc., steel cords plated with brass have been widely used as a reinforcement. Also, composites for improving the adhesive property are compounded from one or more types of bonding agents selected according to requirements from the group consisting of resorcin, resorcin-formaldehyde resin, hexamethylenetetramine, melamine derivative, cobalt salt, etc., with various types of rubber composites have been known. These conventional rubber composites for steel cords, however, were disadvantages in that when exposed to high humidity, the rubber absorbs moisture, and particularly in an ambient temperature above 40° C. and a relative humidity at above 90 percent, the absorption of moisture by the rubber becomes substantially large. The absorption of moisture in the rubber causes an adverse effect on the interface of the adhesion between the steel cords and the rubber to allow the steel cord to separate from the rubber resulting in a shortened life for the product. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is the general object of the present invention to provide a rubber composition capable of preventing a decrease in the strength of adhesion between steel cords plated with brass and the rubber in an ambient environment of high humidity. 
     The above-mentioned object is achieved by utilizing a rubber composition obtained by compounding both cobalt salt and 1,3,5-triazine-2,4,6-trithiol whose bonding force with brass scarcely decreases even at high humidity and in other words which has moisture resistant adhesion to brass in a satisfactory amount. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The rubber composition of the present invention with excellent moisture resisting adhesion is characterized in that in various types of components compounded with 100 parts by weight of rubber are included 0.5 to 5 parts by weight cobalt salt (organic acid cobalt) and 0.1 to 5 parts by weight of 3,5-triazine-2, 4,6-trithiol represented by the following formula as the adhesive component. ##STR2## 
     Cobalt salts to be compounded, for example, is cobalt naphthenate, cobalt stearate, cobalt octylate and cobalt tallate. The compounding ratio of the cobalt salt is 0.5 to 5 parts by weight. When the content of cobalt salt is less than 0.5 parts by weight, an insufficient effect results; whereas when the content exceeds 5 parts by weight, this causes a weakening of the physical properties of the rubber. The compounding ratio of the 1,3,5-triazine-2,4,6-trithiol is 0.1 to 5 parts by weight. When the content of this is less than 0.1 parts by weight, an insufficient effect results; while when the content exceeds 5 parts by weight, not only are the physical properties of the rubber degraded but also another disdavantage results in that the costs are increased. 
     In addition; it was also discovered that the moisture resisting adhesive properties can be further improved by compounding not only cobalt salt and 1,3,5-triazine-2,4,6-trithiol together but also resorcinol or substituted phenol-formaldehyde resin and melamine derivative jointly for other components. 
     In other words, the present invention also provides a rubber composition having more desirable adhesive properties by employing with 100 parts by weight of rubber, 0.5 to 5 parts by weight of cobalt salt, 0.1 to 5 parts by weight of 1,3,5-triazine-2,4,6-trithiol, 0.5 to 5 parts by weight of resorcinol or substituted phenol-formaldehyde resin and 0.5 to 5 parts by weight of melamine derivative. 
     When the above mentioned resorcinol or substituted phenol-formaldehyde resin and melamine derivative are utilized in amounts less than the amounts given above, insufficient effect results; while when their amounts exceed those given above, the properties of the rubber is degraded. The substituted phenol-formaldehyde resins utilized are such as resorcin-formaldehyde condensation product, cresol-formaldehyde condensate, P-alkylphenol-formaldehyde condensate, a condensate of a mixture of resorcinol and alkylphenol and formaldehyde. 
     The melamine derivatives which can be utilized are represented by the formula shown below. ##STR3## 
     In the formula above, R 1 , R 2 , R 3 , R 4 , R 5  and R 6  are radicals selected optionally from the following groups: R 1 , R 2 , R 4  and R 5  from the group consisting of --CH 2  OH, --CH 2  OCH 3  and --CH 2  OC 2  H 5  ; R 3  selected from the group consisting of --OCH 3  and --OC 2  H 5  ; and R 6  from the group consisting of --CH 2  OCH 3  and --CH 2  OC 2  H 5  ; and n is within the range of 1 to 5. Particularly, the methylolmelamine derivatives wherein R 3  is --OCH 3  and the other radicals are --CH 2  OH and --CH 2  OCH 3  are preferable. Such melamine derivatives are obtained, for example, as starch syrup-like partially methoxylated methylol-melamine by methylating melamines with formaldehyde and further by methoxylating the objects with methyl alcohol. 
     In the rubber composition of the present invention the rubber component can be natural rubber and/or synthetic rubber. For the synthetic rubber, isoprene rubber is suitable. It goes without saying that other components such as vulcanizing agent, accelerator, antiozonant, carbon black, filler, etc. can be compounded together as ordinarily used according to necessity. 
     A further detailed description of the present invention will hereunder be given with reference to preferred examples. 
     In the examples preferred here, the results of tests conducted to evaluate the strength of adhesion with brass plated steel cords as well as the moisture resisting adhesion of the rubber composition of the present invention are shown. Shown in Table 1 are the various rubber compositions with their sample numbers and the test results are shown in Table 2. 
     
                       TABLE 1                                                     
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Rubber Compositions and Their Components                                  
                       Parts by Weight                                    
                        Comparison                                        
       Embodiment       Embodiment                                        
       Sample No.                                                         
Component                                                                 
         1      2      3    4    5    6    7    8                         
______________________________________                                    
Natural  100    100    100  70   70   100  70   70                        
Rubber                                                                    
Polyisoprene                                                              
         --     --     --   30   30   --   30   30                        
Rubber                                                                    
Carbon Black                                                              
         55     55     55   55   55   55   55   55                        
(HAF)                                                                     
Zinc Oxide                                                                
         7      7      7    7    7    7    7    7                         
Stearic Acid                                                              
         2      2      2    2    2    2    2    2                         
Antioxidant                                                               
         1             1    1    1    1    1    1                         
Silica   --     --     --   12   12   --   12   12                        
Resorcinol                                                                
         --     --     --   2.5  --   --   2.5  --                        
Cresol-form-                                                              
         --     --     --   --   2.5  --   --   2.5                       
aldehyde                                                                  
Resin                                                                     
Melamine --     --     --   2.5  2.5  --   2.5  2.5                       
Derivative                                                                
Cobalt   2.5    2.5    2.5  2.5  2.5  2.5  2.5  2.5                       
Naphthenate                                                               
1,3,5-Tria-                                                               
         0.5    1      3    1    1    --   --   --                        
zine-2,4,6-Tri-                                                           
thiol                                                                     
Sulfur   5      5      5    5    5    5    5    5                         
Vulcanization                                                             
         0.8    0.8    0.8  0.8  0.8  0.8  0.8  0.8                       
accelerator                                                               
______________________________________                                    
 Note:                                                                    
 As the melamine derivative shown above, the mixture wherein R.sub.1 is   
 --CH.sub.2 OH, R.sub.3 is --OCH.sub.3, R.sub.2, R.sub.4, R.sub.5 and     
 R.sub.6 are --CH.sub.2 OCH.sub.3 and n = 1 to 3 were utilized.           
 Antiozonant: Paraphenylenediamine series                                 
 Vulcanization accelerator: Sulfeneamide series                           
 
    
     Preparation of Samples and Peel Test 
     Samples were prepared by inserting the brass-plated steel cords laid out in parallel rows (20 cords/2.5 cm) between two sheets of unvulcanized rubber composition listed in Table 1 having a thickness of 1 mm to form one layer, accumulating on said layer another layer prepared in the same manner to form two layered composition and curring the two layered composition for 45 minutes at 140° C. in a metal mold. Then, a test was conducted on said samples by peeling off one layer from the other to measure the adhesive force in kg per 2.5 cm in width as well as rubber coverage to the steel cords. The rubber coverage referred herein is taken as follows. When all of the forty steel cords on both peeled off surfaces are covered 100 percent with rubber, this is rated as 100; while when twenty steel cords in total on both separated surfaces are completely exposed, this rating is zero. A measurement of the examples was carried out immediately after vulcanization and after aging for one week and two weeks by leaving the examples at 70° C. and at 93 percent relative humidity respectively. The peel test were conducted at a peel angle of 180° at a rate of 50 mm per minute. The steel cords used in the structure are 1×5×0.25 mm which are brass plated with a layer of brass with a thickness of 2,800 Å by diffusion plating. 
     
                       TABLE 2                                                     
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Test Results                                                              
                        Comparison                                        
       Embodiment       Embodiment                                        
Physical Sample No.                                                       
Property 1      2      3    4    5    6    7    8                         
______________________________________                                    
Immediately                                                               
after curing                                                              
Peeling  22.5   22.6   23.2 22.0 21.2 22.0 20.2 20.5                      
Force (kg)                                                                
Rubber   98     98     100  100  100  88   100  98                        
Coverage (%)                                                              
After one                                                                 
week of aging                                                             
Peeling  16.2   20.5   20.3 21.0 20.6 11.0 15.5 15.0                      
Force (kg)                                                                
Rubber   70     82     80   100  96   15   45   50                        
Coverage (%)                                                              
After two                                                                 
weeks of                                                                  
aging                                                                     
Peeling  13.2   15.9   16.3 19.2 20.0 7.8  12.5 13.1                      
Force                                                                     
Rubber   30     48     45   80   75   0    5    6                         
Coverage (%)                                                              
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     In Table 2 shown above, the samples 1 through 3 are examples of the present invention and it is apparent from them that compared with the sample No. 6 (comparison sample), the peel force and rubber coverage after aging with moisture and heat are improved. The samples Nos. 4 and 5 are further examples of the present invention and there is an evident improvement in the peel force as well as rubber coverage after aging with heat and moisture in comparison with the sample Nos. 7 and 8 (comparison samples) and also have an improvement in their performance even over the sample Nos. 1 through 3 (other examples of the present invention). 
     It should be clearly understood from the foregoing description that the rubber composite according to the present invention is provided with great effects which substantially prevents degradation in the adhesion of the rubber with brass in ambient conditions of high humidity.