Synergistic antioxidant combination of organic phosphite and amine
This is a division of application Ser. No. 391,235, filed Aug. 24, 1973, now U.S. Pat. No. 3,886,114. 
    
    
     BACKGROUND OF THE INVENTION 
     It is known to incorporate various compounds in natural and synthetic polymeric compounds, e.g., natural rubber, in order to prevent degradation resulting from atmospheric oxygen, ultraviolet light, and heat. In the past, these compounds have included phosphites, phenols, and amines (U.S. Pat. Nos. 3,080,338; 3,115,465; 3,243,394; 3,355,419; 3,369,059; 3,375,304; 3,377,315; 3,382,299; 3,406,143; 3,419,524; 3,425,987; 3,516,963; 3,535,277; 3,560,432; and 3,629,372; German Offenlegungsschrift No. 2,232,306; British Pat. Nos. 905,824 and 921,509). However, these compounds, under certain circumstances, have not provided the desired degree of protection of the polymer, especially against deterioration from exposure to atmospheric oxygen and heat. 
     SUMMARY OF THE INVENTION 
     It has now been discovered that elastomeric polymers, such as natural rubber, can be protected against oxidative deterioration by employing a synergistic combination of compounds. Thus, in accordance with one embodiment of the invention, natural rubber is compounded with a mixture of (1) an alkyl phenyl propylene glycol phosphite and (2a) a phenol or (2b) an amine or mixtures thereof. 
    
    
     DESCRIPTION OF THE INVENTION 
     The first component (1) in the synergistic combination is an alkyl phenyl propylene glycol phosphite. This compound is known in the aformentioned patents which are included herein by reference. Preferably, the phosphites have a molecular weight of between about 425 and 2100 and have a phosphorus content of between about 4.5 and 11.8%. Suitable compounds are decakis(nonylphenyl)heptakis (dipropylene glycol)octaphosphite (DHOP), tetrakis(nonylphenyl)polypropylene glycol 425 diphosphite (TPDP 425), polyphenyl dipropylene glycol phosphite (AHOP), tetraphenyl dipropylene glycol diphosphite (TP-DPG-DP) and mixtures thereof. 
     The synergistic combination also has either a phenol (2a), an amine (2b), or mixtures thereof. Both the phenolic and amine antioxidants are well-known in the art as evidenced by the aformentioned patents which are included herein by reference. The following phenolic antioxidants and other phenolic compounds, among others, are satisfactory: mixed polybutylated bisphenol A (4,4&#39;-isopropylidene diphenol) (Agerite Superlite), 2,6-di-t-butyl-4-methylphenol, styrenated phenol (Agerite Spar), 2,2&#39;-methylenebis(4-methyl-6-t-butylphenol) (American Cyanamid Co. Antioxidant 2246), hindered phenol (United States Rubber Co. Antioxidant 439), 4,4&#39;-dihydroxydiphenol, 4,4&#39;-thiobis(6-t-butyl-o-cresol) (Ethyl 736), p-butylphenol, p-isopropylphenol, p-(1,1,3,3-tetramethylbutyl)phenol, thymol, mixed m- and p-cresol, p-nonylphenol, other phenols, and cresols having alkyl substituents and mixtures thereof. Amine antioxidants may include, among others, the following: polymerized 1,2-dihydro-2,2,4-trimethylquinoline (Agerite Resin D), N,N -di(2-octyl)-p-phenylenediamine (Antozite 1), N,N&#39;-diphenyl-p-phenylenediamine, diphenylamine-acetone reaction product, phenyl-β-naphthylamine (Agerite Powder), other aromatic amines, diphenylamines, and mixtures thereof. Generally, the antioxidant combination has about 0.50 to 1.75, preferably about 0.75 to 1.75, parts phosphite to about 1.50 to 0.25, preferably about 1.25 to 0.25, parts phenol or amine antioxiant. 
     The antioxidant combination of the invention can be added to natural rubber or the like in an amount ranging from about 0.25 to 5 parts per hundred parts of rubber (phr). The synergistic combination can be employed with a rubber latex, a rubber solution, and a solid rubber to retard the oxidative deterioration of the rubbery polymer during polymerization. The components of the synergistic mixture may be combined before addition to the rubber or each component may be added separately to the rubber. The rubber may contain also conventional compounding ingredients, such as accelerators, stabilizers, antiozonants, carbon blacks, fillers, plasticizers, and mixtures thereof. 
     The new antioxidant combination, with or without other ingredients, may be included by any suitable method used in the rubber industry, such as mechanical mixing or other conventional compounding methods. The compounded rubber may be cured to a vulcanizate in accordance with a known procedure, such as press-curing at elevated temperatures. 
     Thus, in accordance with this invention, natural rubber is protected against oxidative deterioration effectively by using as antioxidant a combination of certain phosphites with certain phenols or certain amines. The synergistic results of the invention are evident when the polymer containing the new antioxidant combination is subjected to extended heat aging. The protection afforded by the antioxidant combination of the invention is excellent, whereas the same amount of the phosphite alone, the phenol alone, or the amine alone is not satisfactory under the same conditions. 
     Furthermore, synergism is produced by combining a phosphite containing propylene glycol residuals and a phenolic compound, such as p-nonylphenol or p-t-butylphenol, which heretofore usually exhibit low antioxidant characteristics. Besides being much less expensive, the new antioxidant combination provides a vulcanizate with improved mechanical and physical properties following heat aging as compared to using each component alone. 
     The following examples are submitted to illustrate, but not to limit, this invention. Unless otherwise indicated, all parts and percentages in the specification and claims are based upon weight. 
     EXAMPLE 1 
     Natural rubber was compounded with the following components and press-cured subsequently for 15 min. at 290°F. 
     
         ______________________________________                                    
Components             Parts by Weight                                    
______________________________________                                    
Natural Rubber (Select A)                                                 
                       100                                                
Stearic Acid           2                                                  
ZnO                    5                                                  
TiO.sub.2              50                                                 
Sulfur                 2.75                                               
Benzothiazyl Disulfide (ALTAX)                                            
                       1.0                                                
Tetramethylthiuram Disulfide                                              
                       0.1                                                
(Methyl Tuads)                                                            
Antioxidant (s)        2                                                  
______________________________________                                    
 
    
     As shown in Table I, samples of vulcanized rubber contained as an antioxidant polybutylated bisphenol A alone, decakis(nonylphenyl)heptakis(dipropylene glycol) octaphosphite (DHOP) alone, and mixtures of both. There was also a control. The samples were aged in test tubes (Al blocks) for 4 days at 212°F. The percent tensile retention and percent elongation were determined according to ASTM D-412 Test Method. The results are listed in Table I. 
     This example demonstrates the synergistic effect obtained by using a mixture of the specified phosphite and the specific phenol as compared by using either alone. 
     
                                           Table I                                 
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Sample                                                                    
Antioxidant A  B   C   D   E   F   G   H   I                              
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Mixed polybutylated                                                       
bisphenol A (phr)                                                         
            -- 2.00                                                       
                   1.50                                                   
                       1.25                                               
                           1.00                                           
                               0.75                                       
                                   0.50                                   
                                       0.25                               
                                           --                             
Decakis(nonylphenyl)                                                      
heptakis(dipropylene                                                      
            -- --  0.50                                                   
                       0.75                                               
                           1.00                                           
                               1.25                                       
                                   1.50                                   
                                       1.75                               
                                           2.00                           
glycol) octaphosphite                                                     
(DHOP)                                                                    
Percent retained                                                          
tensile strength                                                          
aged for 4 days                                                           
             6 19  37  32  49  50  51  32  11                             
Percent retained                                                          
elongation at break                                                       
aged for 4 days                                                           
            32 44  60  58  66  71  75  66  46                             
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     EXAMPLE 2 
     The formulation of Example 1 was compounded as indicated in Table II with 1:1 mixtures of various phosphites with various phenols or with various amines. For comparative purposes, each formulation was compounded with only one component. There was also a control. The total amount of phosphite, phenol and amine in each sample was 2 phr. Each sample was press-cured to optimum cure at 290°F. The vulcanized rubber was aged in test tubes (Al blocks) for 2 days at 212°F. Tensile strength was determined according to ASTM D-412 Test Method. Table II specifies the results. 
     
                                           TABLE II                                
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                                                Percent Retention         
                                                of Tensile Strength       
                                                After Aging for           
Sample                                                                    
         Phosphite         Phenol or Amine      2 Days                    
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A   None                None                    26.6                      
B-1 Decakis(nonylphenyl)heptakis(dipro-                                   
    pylene glycol) octaphosphite (DHOP)                                   
                        None                    31.0                      
B-2 None                2,6-Di-t-butyl-4-methylphenol                     
                                                40.4                      
B-3 DHOP                2,6-Di-t-butyl-4-methylphenol                     
                                                73.8                      
C-1 DHOP                None                    31.0                      
C-2 None                Styrenated phenol (Agerite Spar)                  
                                                57.5                      
C-3 DHOP                Styrenated phenol (Agerite Spar)                  
                                                70.5                      
D-1 DHOP                None                    31.0                      
D-2 None                2,2&#39;-Methylenebis(4-methyl-6-t-butylphenol)       
                                                42.5                      
D-3 DHOP                2,2&#39;-Methylenebis(4-methyl-6-t-butylphenol)       
                                                72.5                      
E-1 DHOP                None                    31.0                      
E-2 None                Hindered phenol (Antioxidant 439)                 
                                                45.0                      
E-3 DHOP                Hindered phenol (Antioxidant 439)                 
                                                61.1                      
F-1 DHOP                None                    31.0                      
F-2 None                4,4&#39;-Dihydroxydiphenol  27.6                      
F-3 DHOP                4,4&#39;-Dihydroxydiphenol  59.9                      
G-1 DHOP                None                    31.0                      
G-2 None                4,4&#39;-Thiobis(6-t-butyl-o-cresol)                  
                                                47.8                      
G-3 DHOP                4,4&#39;-Thiobis(6-t-butyl-o-cresol)                  
                                                74.9                      
H-1 DHOP                None                    31.0                      
H-2 None                Polymerized 1,2-dihydro-2,2,4-trimethyl-          
                        quinoline               59.0                      
H-3 DHOP                Polymerized 1,2-dihydro-2,2,4-trimethyl-          
                        quinoline               78.1                      
J-1 DHOP                None                    31.0                      
J-2 None                N,N&#39;-di(2-octyl)-p-phenylenediamine               
                                                59.2                      
J-3 DHOP                N,N&#39;-di(2-octyl)-p-phenylenediamine               
                                                80.4                      
K-1 DHOP                None                    31.0                      
K-2 None                N,N&#39;-Diphenyl-p-phenylenediamine                  
                                                49.7D)                    
K-3 DHOP                N,N&#39;-Diphenyl-p-phenylenediamine                  
                                                83.7D)                    
L-1 DHOP                None                    31.0                      
L-2 None                Phenyl-β-naphthylamine                       
                                                51.4                      
L-3 DHOP                Phenyl-β-naphthylamine                       
                                                83.5                      
M-1 DHOP                None                    41.1                      
M-2 None                Diphenylamine-acetone reaction product            
                                                72.0                      
M-3 DHOP                Diphenylamine-acetone reaction product            
                                                89.3                      
N-1 Polyphenyl dipropylene glyol                                          
    phosphite (AHOP)    None                    51.9                      
N-2 None                Polybutylated bisphenol A                         
                                                31.8                      
N-3 AHOP                Polybutylated bisphenol A                         
                                                68.1                      
O-1 Tetraphenyl dipropylene glycol                                        
    diphosphite (TP-DPG-DP)                                               
                        None                    39.4                      
O-2 None                Polybutylated bisphenol A                         
                                                31.8                      
O-3 TP-DPG-DP           Polybutylated bisphenol A                         
                                                59.9                      
P-1 Tetrakis(nonylphenyl) polypropylene-                                  
    glycol 425 diphosphite (TPDP 425)                                     
                        None                    52.1                      
P-2 None                Polybutylated bisphenol A                         
                                                31.8                      
P-3 TPDP 425            Polybutylated bisphenol A                         
                                                78.0                      
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     It is manifest from this example that an equal amount of combined antioxidants produced rubber compounds with considerably better physical properties following aging than those containing only the phosphite, the phenol or the amine. 
     EXAMPLE 3 
     By following the procedure of Example 1, vulcanizates were prepared with 1:1 mixtures of DHOP and phenols which normally possess low antioxidant properties. Each component also was used alone and a control was prepared. Each sample has a total of 2 phr of phosphite and phenol. The percent retention was determined as described in Example 2. The resulting data are found in Table III. 
     
                                           Table III                               
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                                           Percent Retention              
                                           of Tensile Strength            
                                           After Aging for                
Sample                                                                    
         Phosphite         Phenol or Amine 2 Days                         
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A   None                None               16.0                           
B-1 Decakis(nonylphenyl)heptakis(dipro-                                   
    pylene glycol) octaphosphite (DHOP)                                   
                        None               42.6                           
B-2 None                p-t-Butylphenol    50.0                           
B-3 DHOP                p-t-Butylphenol    66.1                           
C-1 DHOP                None               42.6                           
C-2 None                p-Isopropylphenol  41.6                           
C-3 DHOP                p-Isopropylphenol  51.1                           
D-1 DHOP                None               42.6                           
D-2 None                p-(1,1,3,3-Tetramethylbutyl)phenol                
                                           47.7                           
D-3 DHOP                p-(1,1,3,3-Tetramethylbutyl)phenol                
                                           51.5                           
E-1 DHOP                None               42.6                           
E-2 None                Thymol             31.8                           
E-3 DHOP                Thymol             50.8                           
F-1 DHOP                None               42.6                           
F-2 None                Mixed m- and p-cresol                             
                                           36.3                           
F-3 DHOP                Mixed m- and p-cresol                             
                                           57.0                           
G-1 DHOP                None               42.6                           
G-2 None                p-Nonylphenol      46.4                           
G-3 DHOP                p-Nonylphenol      60.6                           
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     Synergism is shown in the antioxidant combinations tested in this example. 
     Having set forth the general nature and specific embodiments of the present invention, the true scope is now particularly pointed out in the appended claims.