Antioxidant comprising a hydroxy aromatic compound containing sulfur and an organic phosphonate
This is a continuation-in-part of application Ser. No. 405,554, filed Oct. 11, 1973, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention generally relates to antioxidant compositions, and more particularly, to compositions especially suited for use with irradiated polyolefins and highly filled plastics, such as fire-retardant plastics. 
     Olefin polymers are particularly subject to severe deterioration from the oxidative action of air at elevated temperatures. They are also vulnerable to degradation by the action of ionizing radiation when large amounts of such radiation are deposited onto the polymer, e.g., amounts of 5×10 10  ergs per gram and higher. Oxidative deterioration at elevated temperatures, i.e., temperatures above 50° C., is manifested by surface crazing, cracking, and ultimately complete embrittlement. 
     Since large amounts of antioxidants, in the order of greater than 5%, result in increased cost and decrease in plastic strength, it is desirable to obtain an antioxidant system which will be effective at elevated temperatures over long periods of time in relatively low amounts. 
     A highly efficient antioxidant system is particularly desirable for irradiated plastics. Often, during irradiation, much of the antioxidant is consumed or rendered incapable of functioning as an antioxidant. It is therefore necessary that an antioxidant system be employed for plastics which are to be irradiated which will be effective and survive radiation when used in small quantities so as to prevent oxidation even after being subjected to irradiation. 
     Also, it is very desirable to have an antioxidant of high efficiency for highly filled plastics. High amounts of filler are often required in the formulation of fire-retardant plastics. It is necessary to incorporate a large amount of filler into the plastic to provide fire retardant properties. This generally results in a more permeable plastic composition which will more readily undergo oxidative degradation. Additionally, a large quantity of filler results in a decrease in plastic strength, so it is advantageous to have an efficient antioxidant which is effective in small amounts in the order of 5% or less of the plastic composition. 
     While the use of hydroxy aromatic compounds containing sulfur as an antioxidant is known, the use of phosphonates as an antioxidant has been known only when they contain a hydroxy aromatic compound. I have found that by combining a phosphonate compound with a hydroxy aromatic compound containing sulfur, a more efficient antioxidant system is obtained. 
     Thus, it is an object of this invention to provide an antioxidant composition suitable for use with various polymers sensitive to oxidation and which is more efficient at elevated temperatures than the antioxidants of the prior art. 
     It is a further object of this invention to provide an antioxidant composition which is especially suitable for use with irradiated polyolefins. 
     It is another object of this invention to provide a antioxidant composition particularly suited for use in fire-retardant plastic systems which have a large percentage of filler. 
     SUMMARY OF THE INVENTION 
     An antioxidant composition, especially suited for irradiated or fire-retardant plastics, comprising a hydroxy aromatic compound containing sulfur and an organic phosphonate compound. The hydroxy aromatic compound generally comprises phenol, cresol and naphthol groups with the sulfur preferably attached to at least one cyclic unsaturated compound. The cyclic unsaturated compound is a homoaromatic unsaturated ring compound wherein a ring carbon atom is attached to the sulfur. 
     Suitable phosphonate compounds have the formula ##STR1## wherein R, R&#39; and R&#34;, any two or all three of which may be the same or different, and which may contain further phosphonic moieties, each represents a substituted or unsubstituted alkyl, alkenyl, aryl, alkylaryl or heteroaromatic (e.g., a triazine) radical, or a polymeric radical, or R has the meaning above and R&#39; and/or R&#34; represent a hydrogen atom or are linked together to form a ring. 
     R&#39; and R&#34; are preferably non-aromatic, e.g., are alkyl and alkenyl groups, especially vinyl and allyl groups, because aromatic substituents have a greater tendency to be hydrolysed. 
     PREFERRED EMBODIMENT 
     The present invention provides an antioxidant composition comprising a mixture of (a) a non-metallic hydroxy aromatic compound containing at least one sulfur atom attached to an unsaturated carbocyclic group, and (b) an organic phosphonic acid or a diester or monoester thereof. The present invention also provides a polymer composition comprising one or more polymers which are susceptible to oxidation and an antioxidant composition as defined above. 
     The present invention is based on the surprising discovery that a blend of compounds (a) and (b) above exhibits better antioxidant properties than either compound used alone at the same total additive level. The antioxidants of the present invention have the further advantage of being very efficient at elevated temperatures. 
     Certain antioxidant compositions of this invention are particularly useful in plastics which are to be irradiated. The antioxidant composition of this invention has also been found well suited for use in highly filled fire-retardant plastic compositions. 
     Another surprising advantage exhibited by certain of the antioxidant compositions of the present invention is that they can be employed in small amounts, e.g., amounts of from 1 to 6%, in irradiated plastics compositions especially irradiated cross-linked olefin polymers, without substantially impairing either their antioxidant efficiency or the degree of the cross-linking of the material under irradiation. 
     Accordingly the present invention further provides a polymer composition comprising at least one irradiated cross-linked polymer and an antioxidant composition comprising (a) a hydroxy aromatic compound containing at least one sulfur atom and (b) an organic phosphonic acid or a diester or monoester thereof. 
     The hydroxy aromatic compound (a) is preferably non-metallic and the sulfur atom is preferably attached to an unsaturated carbocyclic, e.g., homoaromatic group, such as a phenol, cresol or naphthol group, but the irradiated cross-linked polymer compositions of the present invention may include antioxidant compositions in which compound (a) contains a sulphur atom attached to a carbon atom in a heteronuclear unsaturated ring, e.g., a triazine ring as in 6-(4-hydroxy-3,5-t-butylanilino)2,4-bis(n-octylthio)1,3,5-triazine. 
     Especially suitable hydroxy aromatic compounds have the formula R 1  --S--R 2  wherein R 1  and R 2 , which may be the same or different, each representing a substituted or unsubstituted phenol, cresol or naphthol groups. Amongst such compounds there may be mentioned, for example, 4,4&#39;-thiobis(6-t-butyl-m-cresol) or a mixture of oligomers of 4,4&#39;-thiobis(6-t-butyl-m-cresol) as described in British Pat. No. 1,241,582; 1,1&#39;-thiobis(2-naphthol); 2,2&#39;-thiobis(4-methyl-6-t-butyl-phenol); and the reaction product of phosphorus oxychloride and 4,4&#39;-thiobis(6-t-butyl-m-cresol) as described in U.S. Pat. No. 3,354,117. 
     Diesters of the hereinbefore mentioned organic phosphonic acid are especially preferred and amongst such compounds there may especially be mentioned, for example, diesters of 3-t-butyl-4-hydroxy-5-methyl benzyl phosphonic acid, amyl phosphonic acid, allyl phosphonic acid and α-ethyl-3,5-di-t-butyl-4-hydroxybenzyl phosphonic acid. These include, for example, di-n-octadecyl(3-t-butyl-4-hydroxy-5-methylbenzyl) phosphonate, diallyl allylphosphonate and diamylamylphosphonate. However free phosphonic acids may also be employed. 
     Other suitable phosphonates are 2,4,6-tris(dialkyl or dialkenyl phosphonato)1,3,5-triazines, e.g., 2,4,6-tris(diallyl phosphonato)-1,3,5-triazine, and the phosphonate of the formula ##STR2## 
     Amongst polymers into which the antioxidant compositions of the present invention may suitably be incorporated there may be mentioned thermoplastic olefin polymers such as polyethylene, polypropylene, ethylene/vinylacetate copolymers, ethylene/acrylic ester copolymers, ethylene/propylene copolymers and ethylene/propylene/diene terpolymers. Antioxidant compositions of the invention are also suitable for use with other polymers which are sensitive to oxidation including polyvinylchloride, polyvinylidenechloride, polyvinylidene fluoride, chlorinated polyethylene, acrylics, saturated and unsaturated olefin elastomers, polyethers and polyesters. 
     Other phosphorus containing antioxidants useful in polymer systems are found in my copending applications entitled &#34;Antioxidant Comprising a Hydroxy Aromatic Compound Containing Sulfur and an Organic Phosphite,&#34; Ser. No. 405,553, now U.S. Pat. No. 3,897,388 and &#34;An Irradiatable Polymer Composition with Improved Oxidation Resistance,&#34; Ser. No. 405,521, filed Oct. 11, 1973, now abandoned. 
     The present invention also provides shaped articles made from the polymer compositions, especially heat-recoverable articles made from the irradiated cross-linked polymers. 
     It will be appreciated by one skilled in the art that although the most convenient form of antioxidant composition according to the present invention is a mixture of compounds (a) and (b) as defined above a single compound containing both an organic phosphonate radical and a radical containing a hydroxy aromatic group and a sulfur atom could also be expected to be effective. 
    
    
     The following Examples illustrate the invention. 
     EXAMPLE I 
     Various mixtures of antioxidants were blended with a high density polyethylene (Marlex 6003, Philips Petroleum Company, density 0.096, M. I. 0.2) on a mill at 160° C. and hydraulically pressed into 6 in by 6 in by 0.020 in slabs at 170° C. and cooled, then irradiated to a dose of 20 Mrads. 
     To evaluate antioxidant efficiency, 1/8 in strips, 6 inches long were hung in a hot air circulating oven at 175° C. After varying periods of time, samples were withdrawn from the oven, allowed to cool for at least 4 hours to room temperature and then elongated in an Instron Tensile Tester. The initial jaw separation was 1 inch with an elongation rate of 2 in/minute. Jaw separation at rupture of the sample was used to indicate ultimate elongation. 
     The number of days of heat aging which resulted in rupture of the sample at or below 150% elongation on the Instron was selected as the time to failure. 
     Various amounts of 2,4,6 tris-(diallylphosphonato)-1,3,5 triazine were added to the Marlex 6003, together with anti-oxidants, the results of which are summarized in Table I. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Antioxidant Blend                                                         
          A   A:B                                                         
                 A:C                                                      
                    A:D                                                   
                       A:E                                                
                          A:F                                             
                             A:G A:H                                      
                                    A:I                                   
__________________________________________________________________________
Ratio in p.p.h. 0:3                                                       
          (3pph)                                                          
Days of Heat Aging                                                        
          &lt;˜6                                                       
               ˜8                                                   
                 13 12 &gt;13                                                
                           7 11   6 11                                    
to failure                                                                
pph 1:2                                                                   
Days of Heat Aging                                                        
               13                                                         
                 13 25 &gt;25                                                
                          13 18  13 11                                    
to failure                                                                
pph 2:1                                                                   
Days of Heat Aging                                                        
              &gt;13                                                         
                 13 21 &gt;25                                                
                          13 ˜15                                    
                                 18 11                                    
to failure                                                                
__________________________________________________________________________
 A is 2,4,6-tris-(diallyl phosphonato)-1,3,5-triazine                     
 B is 2,2&#39;-thiobis(4-methyl-6-t-butyl-phenol)                             
 C is tetrakis[3-(3,5-di-t-butyl-4-hydroxy phenyl) propionyloxymethyl]    
 methane                                                                  
 D is 4,4&#39;-thiobis(6-t-butyl-m-cresol)                                    
 E is oligomers of 4,4&#39;-thiobis(6-t-butyl-m-cresol) as described in       
 copending application Ser. No. 150,831, &#34;Compositions of Antioxidants of 
 Reduced Volatility&#34;.                                                     
 F is a high molecular weight sulfur containing hindered phenol cyanurate 
 (triazine derivative)                                                    
 G is 4,4&#39;-thiobis(6-t-butyl-o-cresol)                                    
 H is 1,1&#39;-thiobis(2-naphthol)                                            
 I is bis[(3,5-di-t-butyl-4-hydroxyphenyl) propionyl-2-oxyethyl] sulfide  
 
    
     As can be seen from Table I, the phosphonate alone (antioxidant A) or the hydroxy aromatic compound containing sulfur alone generally failed in less than 13 days at the 3 pph level. However, a 3 pph level of a blend of phosphonates and hydroxy aromatic compound containing sulfur gave substantially better results in the order of 25 days, indicating a synergestic result from the blending of the two ingredients. Also, where a non-sulfur containing phenol (antioxidant C) and a phenol with the sulfur atom not attached to an aromatic group (antioxidant I) were evaluated, little or no increased life was obtained. Finally, where the sulfur atom is attached to an unsaturated heterocyclic compound such as a triazine ring (F), only a slight increase in life was obtained. Thus, it appears that the preferred system is a phosphonate with a hydroxy aromatic compound containing sulfur which has at least one of the sulfur atoms attached to an aromatic ring. 
     EXAMPLE II 
     Stauffer E719 from the Stauffer Chemical Company having the formula ##STR3## was blended with antioxidant E of Table I and Marlex 6003. Failure was measured as generally described in Example I but the temperature of heat aging was 200° C. and failure was defined as an elongation at or below 50%. 
     4 pph of antioxidant E alone and a 1:3 ratio of E to E719 gave approximately 100 hours before failure. E719 alone resulted in failure in less than 24 hours. However, a 3:1 ratio of E to E719 gave 148 hours while 2:2 ratio resulted in 138 hours until failure. 
     EXAMPLE III 
     A fire retardant plastic of the following formula was used to evaluate antioxidant efficiency. 
     
         ______________________________________                                    
Low density polyethylene  40%                                             
Ethylene-ethyl acrylate copolymer                                         
                          7%                                              
Ethylene-propylene-diene terpolymer                                       
                          8%                                              
Flame retardant (brominated aromatic)                                     
                          21%                                             
Antimony oxide            10%                                             
Thermal Stabilizers       6%                                              
Trimethylolpropane trimethylacrylate                                      
                          3%                                              
Antioxidant Combination   5%                                              
______________________________________                                    
 
    
     An organic phosphonate of the formula di-n-octadecyl-3-t-butyl-4-hydroxy-5-methylbenzyl-phosphonate was blended with other antioxidants, and tested for resistance to oxidation, the results of which are summarized on Table II. 
     As can be seen from Table II, a synergistic result is obtained with the fire retardant plastic when an organic phosphonate (J) is blended with a sulfur containing substituted phenol. When the phosphonate was blended with a sulfur containing compound not containing a phenolic substituent (K), no synergism was found. Also, when the phosphonate was blended with a hydroxy aromatic compound containing a sulfur group not attatched to a ringed compound (I), poorer results were obtained. Also, when a nickel salt of a phosphonate (M) was blended with a hydroxy aromatic compound containing sulfur (E) synergism was much less pronounced than where a metallic salt was not part of the combination, i.e. J:E. 
     
                                           TABLE II                                
__________________________________________________________________________
Antioxidant Blend                                                         
            J:E  J:L    J:F J:I  J:K  E:M   E:N                           
__________________________________________________________________________
Ratio in p.p.h.                                                           
          6:0                                                             
Days at 175° C. for                                                
            less than                                                     
                 less than                                                
failure     14 days                                                       
                 14 days              11-14 days                          
                                            11-13 days                    
Ratio     5:1                                                             
Days at 175° C. for                                                
failure     24 days                                                       
Ratio     4:2                                                             
Days at 175° C. for                                                
                 greater than                                             
                            less than                                     
                                 less than                                
failure     27 days                                                       
                 30 days                                                  
                        26 days                                           
                            14 days                                       
                                 12 days                                  
                                      12 days                             
Ratio     3:3                                                             
Days at 175° C. for                                                
failure     30 days                                                       
                 23 days              13 days                             
                                            19 days                       
Ratio     2:4                                                             
Days at 175° C. for                                                
failure     17 days                                                       
                 30 days                                                  
                        16 days                                           
Ratio     1:5                                                             
Days at 175° C. for                                                
failure          16 days                                                  
Ratio     0:6                                                             
Days at 175° C. for                                                
                 less than            less than                           
                                            less than                     
failure     12 days                                                       
                 16 days                                                  
                        15 days       7 days                              
                                            7 days                        
__________________________________________________________________________
 *A to I are the materials given in Table I.                              
 K is dilauryl dithiopropionate                                           
 J is di-n-octadecyl-3-t-butyl-4-hydroxy-5-methylbenzyl phosphonate       
 L is the reaction product of 3 moles of 4,4&#39;-thiobis(6-A-butyl-(m-cresol)
 with 1.05 moles of phosphorus oxychloride as described in U.S. Pat. No.  
 3,354,117.                                                               
 M-Nickel bis [o-ethyl (3,5-di-t-butyl-4-hydroxybenzyl]phosphonate        
 N-Free acid of bis [o-ethyl (3,5-di-t-butyl-4-hydroxybenzyl)]phosphonate 
 
    
     
                                           TABLE III                               
__________________________________________________________________________
Parts of                                                                  
J       B    C   D   E   H      F      G   I                              
__________________________________________________________________________
  0                                                                       
Days at 175° C.                                                    
        less than    about                                                
                         about  about  about                              
for failure                                                               
        6 days                                                            
             7 days                                                       
                 7 days                                                   
                     20 days                                              
                         9 days 6 days 6 days                             
                                           6 days                         
  1                                                                       
Days at 175° C.                                                    
                     about                                                
                         about                                            
for failure                                                               
        7 days                                                            
             7 days                                                       
                 12 days                                                  
                     24 days                                              
                         9 days 11 days                                   
                                       13 days                            
                                           8 days                         
  2                                                                       
Days at 175° C.                                                    
                     about                                                
                         greater than                                     
                                greater than                              
for failure                                                               
        13 days                                                           
             11 days                                                      
                 19 days                                                  
                     30 days                                              
                         19 days                                          
                                16 days                                   
                                       18 days                            
                                           14 days                        
__________________________________________________________________________
 
    
     EXAMPLE IV 
     Approximately 12 mil thick strips of Marlex 6003 containing 3 parts total antioxidants was irradiated to 15 Mrads. Various blends of di-n-octadecyl-3-t-butyl-4-hydroxy-5-methylbenzyl phosphonate were evaluated as summarized in Table III. Failure was defined as in Example II. A sample containing 3 parts of J only failed after 13 days. 
     EXAMPLE V 
     The experiments of Example IV were generally repeated using diallylallyphosphonate in place of di-n-octadecyl-3-t-butyl-4-hydroxy-5-methylbenzyl phosphonate with similar results. 
     EXAMPLE VI 
     The experiments of Example IV were generally repeated using diamylamylphosphonate in place of di-n-octadecyl-3-t-butyl-4-hydroxy-5-methylbenzyl phosphonate with similar results.