Rubber compositions containing polymeric phenol derivatives of 1,3,4-thiadiazole
BACKGROUND OF THE INVENTION 
     This invention relates to antioxidants for rubber compositions and their use in the stabilization of said compositions. More particularly, the invention relates to rubber compositions that possess a high degree of resistance to the detrimental effects of oxidative aging over prolonged periods of time. 
     Essentially all types of rubber are known to be susceptible to deterioration resulting from prolonged exposure to atmospheric oxygen. The major cause of deterioration is the attack of oxygen on the olefinic unsaturation bonds contained in rubber. To prevent deterioration, various antioxidants are added depending on the severity of processing and service of the finished article. Thus, there is a need for novel antioxidants which produce commercially acceptable articles. Among the known antioxidants, phenolic type antioxidants are widely used to protect rubber against oxidative effects. However, many of the phenolic antioxidants may be volatilized at high temperature applications. 
     It has been now discovered that certain phenol derivatives of 1,3,4-thiadiazole prevent the detrimental effect of oxygen on rubber compositions over long periods of time at elevated temperatures. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a rubber composition having incorporated therein a 1,3,4-thiadiazole compound selected from the group consisting of 
     (a) compounds of the structural formula ##STR1## wherein R 2  represents t-butyl group, R 4  represents C 1-4  -alkyl groups, and R 1 , R 3  and R 5  represent hydrogen and hydroxy groups provided that R 1  and R 5  are hydrogens when R 3  is hydroxy and R 3  is hydrogen when R 1  and R 5  are hydroxy groups and (b) reaction products of 4,4&#39;-(1-methylethylidene)bisphenol and 3-hydroxymethyl-5-hydroxymethylthio-1,3,4-thiadiazolidine-2-thione in the molar ratio of 1:1 to 2:1. 
     Another aspect of the invention is a method of protecting rubber compositions against oxidative deterioration by incorporating therein the above described 1,3,4-thiadiazole compounds. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The antioxidants of the invention may be prepared by reacting a phenol with 3-hydroxymethyl-5-hydroxymethylthio-1,3,4-thiadiazolidine-2-thione according to the following reaction scheme. ##STR2## Alternately, the compounds may be prepared by reacting the phenol with 2,5-mercapto-1,3,4-thiadizole and formaldehyde in the presence of an acid catalyst. In both methods, the catalyst may be selected from strong acids such as p-toluenesulfonic acid, sulfuric acid and hydrochloric acid. 
     The antioxidants of the invention may be incorporated into rubber in the amount of about 0.2 to 5 parts per hundred parts rubber (PHR). The effective amount will depend to some extend on the type and grade of rubber used and on the severity of the deteriorating conditions to which the finished article will be exposed. 
     Rubber that can be protected with the antioxidants includes natural rubber such as balata and gutta percha rubber in various forms. 
     The rubber may be of the black variety containing carbon black fillers and of light variety containing inorganic and mineral fillers such as, among others, titanium dioxide, calcium carbonate, silicate minerals, particularly kaolin clay, whiting, talc and wollastonite. Other compounding ingredients may be added as necessary. The following compounding ingredients, among others, are applicable: accelerators, plasticizers, peptizers, lubricants and processing aids. 
     The data hereinbelow are intended to illustrate, but not to limit the scope of the invention. Unless otherwise stated, all parts and percentages in the specification and claims are by weight. 
    
    
     EXAMPLE I 
     The reactor was charged with 42.0 g (0.20 moles) 3-hydroxymethyl-5-hydroxymethylthio-1,3,4-thiadiazolidine-2-thione, 45.6 g (0.20 moles) p,p&#39;-isopropylidenediphenol, 150 ml toluene, and 0.50 g p-toluenesulfonic acid catalyst. The reaction mixture was refluxed for 11/2 hours. Water, 7.2 g (0.40 moles) was removed by azeotropic distillation. After cooling to 90° C., toluene was stripped and the molten reaction product was removed. Upon cooling it became a brittle yellow solid. The yield of the copolymer was 76.5 g. 
     EXAMPLE II 
     The reactor was charged with 65.6 g (0.40 moles) 2-t-butyl-4-methylphenol, 30.0 g (0.20 moles) 3,5-dimercapto-1,3,4-thiadiazole, 32.4 g (0.40 moles) 37% aqueous solution of formaldehyde, 200 ml toluene, and 0.50 g p-toluenesulfonic acid catalyst. About 28.0 g (1.555 moles) water was collected by azeotropic distillation. The product was treated with 15 ml of 20% aqueous solution of sodium carbonate to remove unreacted catalyst and toluene was stripped. The product, 3-(2-hydroxy-3-t-butyl-4-methylbenzylthio)-1,3,4-thiadiazolidine-2-thione, was a yellowish brown solid weighing 97.5 g. 
     EXAMPLE III 
     Test specimens were prepared by compounding two batches of natural rubber with antioxidants of the invention and other conventional compounding ingredients. For each batch a sample containing no antioxidant was used as control. The samples were press cured for 15 minutes at 153° C. After 4 day aging in test tubes at 100° C., the elongation and tensile strength were determined according to ASTM D-412 method. The physical data compiled in Tables I and II showed that the specimens containing the compounds of the invention retained the desired properties under oxidative conditions. 
     The above embodiments and illustrations have shown various aspects of the present invention. Other variations will be evident to those skilled in the art and such modifications are intended to be within the scope of the invention as defined in the appended claims. 
     
                                           TABLE I                                 
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                     Vulcanizates, parts by weight                        
Ingredients          1    2    3    4    5                                
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Smoked sheet rubber  100.0                                                
                          100.0                                           
                               100.0                                      
                                    100.0                                 
                                         100.0                            
K-STAY G.sup.(1)     5.0  5.0  5.0  5.0  5.0                              
Stearic acid         2.0  2.0  2.0  2.0  2.0                              
Zinc oxide           5.0  5.0  5.0  5.0  5.0                              
Carbon black         50.0 50.0 50.0 50.0 50.0                             
Sulfur               2.5  2.5  2.5  2.5  2.5                              
N--Oxydiethylene-2-benzothiazole-                                         
                     0.5  0.5  0.5  0.5  0.5                              
sulfenamide                                                               
3-(2-Hydroxy-3-t-butyl-5-methyl-                                          
                     --   2.0  --   --   --                               
benzyl)-5-(2-hydroxy-3-t-butyl-5-                                         
methylbenzylthio)-1,3,4-thiadiazolidine-                                  
2-thione                                                                  
Copolymer of p,p&#39;-isopropylidenediphenol                                  
                     --   --   2.0  --   --                               
and 3-hydroxymethyl-5-hydroxymethylthio                                   
1,3,4-thiadiazolidine-2-thione (1:1)                                      
Copolymer of p,p&#39;-isopropylidenediphenol                                  
                     --   --   --   2.0  --                               
and 3-hydroxymethyl-5-hydroxymethylthio-                                  
1,3,4-thiadiazolidine-2-thione (2:1)                                      
3-(2,6-dihydroxy-3,5-di-t-butylbenzyl)-                                   
                     --   --   --   --   2.0                              
5-(2,6-dihydroxy-3,5-di-t-butylbenzyl-                                    
thio)-1,3,4-thiadiazolidine-2-thione                                      
Physical porperties after cure                                            
5 Tensile, psi       3420 3080 2210 2240 3280                             
Elongation           600  590  530  520  560                              
Hardness             57   58   56   58   57                               
Physical properties after aging                                           
Tensile, percent retained                                                 
                     25   47   46   58   48                               
Elongation, percent retained                                              
                     45   51   57   60   64                               
Hardness, points changed                                                  
                     0    +7   +3   +2   +3                               
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 .sup.(1) Rubber processing aid distributed by R. T. Vanderbilt Company,  
 Inc.                                                                     
 
    
     
                       TABLE II                                                    
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                   Vulcanizates,                                          
                   parts by weight                                        
Ingredients          6         7                                          
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Smoked sheet rubber  100.0     100.0                                      
K-STAY G             5.0       5.0                                        
Stearic acid         2.0       2.0                                        
Zinc oxide           5.0       5.0                                        
Carbon black         50.0      50.0                                       
Sulfur               2.5       2.5                                        
N--Oxydiethylene-2-  0.5       0.5                                        
benzothiazolesulfenamide                                                  
3-(3,5-Di-t-butyl-4-hydroxybenzyl)-5-                                     
                     --        2.0                                        
(3,5-di-t-butyl-4-hydroxybenzylthio)-                                     
1,3,4-thiadiazole-2-thione                                                
Physical properties after cure                                            
Tensile, psi         3470      3550                                       
Elongation           580       570                                        
Hardness             58        60                                         
Physical properties after aging                                           
Tensile, percent retained                                                 
                     14        42                                         
Elongation, percent retained                                              
                     26        56                                         
Hardness, points changed                                                  
                     -4        0                                          
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