Synergistic antioxidant mixtures
TECHNICAL FIELD 
     This invention relates to antioxidant systems. More particularly, it relates to organic compositions stabilized against oxidative degradation by a two component system consisting of a novel compound and a phenolic antioxidant. 
     BACKGROUND ART 
     It is well-known that such organic materials as plastics, rubbers, lubricating oils, etc. are prone to oxidation and deterioration in the presence of oxygen. Oxidation of organic materials causes the loss of those intrinsic properties characteristic of the organic material. With a view to preventing deterioration a variety of antioxidants have been developed, however, these antioxidants fail to prevent completely the deterioration of the desired properties of the materials to which they are added. Thus, those skilled in the art are constantly searching for new and more effective antioxidant systems which are useful for the protection of polymers and other organic materials. 
     Antioxidant synergists have been known in the art for some time. For example U.S. Pat. No. 3,492,336 discloses a novel tetra-alkyl thioethyl thiodisuccinate compound for use with phenolic type antioxidants in the stabilization of polyolefins. 
     An antioxidant composition comprising a synergistic mixture of a phenol, an amine and a sulfone is described in U.S. Pat. No. 3,839,210. Specifically, U.S. Pat. No. 3,839,210 discloses an antioxidant composition comprised of a mixture of an oil soluble phenol, an oil soluble amine and a long chain alkylthioethyl sulfone compound in an oxidizable organic material, particularly petroleum oils. 
     In U.S. Pat. No. 3,398,116 a thiocarboxylic acid thioether ester is used in combination with a limited group of phenolic antioxidants to stabilize poly-alpha-olefins against oxidative degradation. It is taught in U.S. Pat. No. 3,398,116 that the antioxidant activity of phenolic antioxidants in poly-alpha-olefin systems rises to an unexpectedly high level when used in association with thiodicarboxylic esters. 
     A synergistic, terpene-based, antioxidant system is disclosed in U.S. Pat. No. 3,491,044 in which a rosin alcohol ester of thiodipropinonic acid is used in combination with a tris-isobornyl phenol antioxidant to stabilize natural and synthetic polymers, rubber, lubricants and oils, etc. 
     In addition, U.S. Pat. No. 3,758,549 discloses polyalkanol esters of alkylthio-alkanoic acids as synergists with phenolic antioxidants and U.S. Pat. No. 3,666,716 and U.S. Pat. No. 3,505,225 discloses derivatives of diphenylamine and the phenylnaphthylamines as synergists with dialkyl 3,3&#39;-thiodipropionates. 
     The art of using a combination of antioxidants has already been put into practice in industry and such antioxidant combinations are often highly effective. There has been prepared a variety of antioxidants which have been examined with a view towards obtaining more stable organic compositions. As a result it has been found that the combined use of the novel compounds of this invention and a phenolic antioxidant brings about an unexpectedly powerful antioxidative effect. None of the above cited patents or other literature in the art has disclosed or even suggests the compounds which are used in the practice of this invention. 
     DISCLOSURE OF THE INVENTION 
     The present invention relates to a stable organic composition which is prepared by mixing an organic material with a compound expressed by the general formula: ##STR2## wherein R is selected from the group comprised of (1) alkyl radicals having 6 to 20 carbon atoms; (2) phenyl radicals which may be substituted by 1 or 2 alkyl groups having 1 to 8 carbon atoms; (3) aralkyl radicals having 7 to 12 carbon atoms; and wherein R 1  is selected from the group comprised of (1) alkylene radicals having 2 to 6 carbon atoms; (2) cycloalkylene radicals having 5 to 12 carbon atoms; (3) alkylcycloalkylene radicals of the formula --R 3  --R 2  --(R 4 ) n , wherein R 2  is a cycloalkylene radical having 5 to 12 carbon atoms and R 3  and R 4  are alkylene radicals having 1 to 6 carbon atoms, and n equals 1 or 0; (4) or a group of the formula R 5  YR 6 , wherein R 5  and R.sub. 6 are alkylene radicals having 2 to 6 carbon atoms and Y is oxygen, sulfur or a 1,4-phenylene radical, 
     along with a phenolic antioxidant. 
     The present invention also relates to the compounds expressed by general formula I and the process for the synthesis of compounds expressed by general formula I. 
     The compounds of this invention exhibit their novel properties when combined with the variety of stabilizers known as phenolics, many of which are commercially available and some of which are the subjects of patents. 
     DETAILED DESCRIPTION OF INVENTION 
     Typical of the phenolic antioxidants with stabilizing properties that are improved by the addition of compounds of the present invention are phenolic compounds having the geneal formulas: ##STR3## wherein R is a tert. alkyl radical having 4 to 8 carbon atoms, a cycloalkyl radical having 5 to 12 carbon atoms, or an aralkyl radical having 7 to 12 carbon atoms and wherein R 1  and R 2  are alkyl radicals having 1 to 12 carbon atoms, cycloalkyl radicals having 5 to 12 carbon atoms, or aralkyl radicals having from 7 to 12 carbon atoms; 
     or the formula: ##STR4## wherein R is an alkylidine radical having 1 to 4 carbon atoms, the group --O--, or the group --S--, and wherein R 1  and R 2  are alkyl radicals having 1 to 12 carbon atoms, cycloalkyl radicals having 5 to 12 carbon atoms, or aralkyl radicals having 7 to 12 carbon atoms. Preferably at least one of R 1  and R 2  is a tert.alkyl radical having 4 to 8 carbon atoms and is in a position ortho to the hydroxyl group, 
     or the formula: ##STR5## wherein ψ is a tert.butyl radical and wherein n is an integer from 1 to 4 and R is an alkyl radical having 8 to 20 carbon atoms, an alkylene radical having 2 to 6 carbon atoms, a thiodialkylene radical wherein each alkylene radical has 2 to 6 carbon atoms, a trivalent radical derived from a straight or branched chain hydrocarbon having 3 to 8 carbon atoms, or a tetravalent radical derived from a straight or branched chain hydrocarbon having 4 to 8 carbon atoms. 
     Typical phenolic antioxidants applicable in the present invention include: 
     2,6-di-tert.butyl-4-methylphenol 
     2,4,6-tri-tert.butylphenol 
     2,2&#39;-methylene-bis-(4-methyl-6-tert.butylphenol) 
     2,2&#39;-thio-bis-(4-methyl-6-tert.butylphenol) 
     4,4&#39;-thio-bis-(3-methyl-6-tert.butylphenol) 
     4,4&#39;-butylidene-bis-(6-tert.butyl-3-methylphenol) 
     Styrenated phenol 
     Butylated Octylated Phenol 
     Butylated α-methylstyrenated phenol 
     Styrenated butylated m, p-cresol 
     4,4&#39;-methylenebis (2,6-di-tert.butylphenol) 
     2,2&#39;-methylenebis[4-methyl-6-(1-methylcyclohexyl)phenol] 
     Butylated reaction product of p-cresol and dicyclopentadiene 
     Tetrakis[methylene 3-(3,5-di-tert.butyl-4-hydroxyphenyl)propionate]methane 
     1,3,5-trimethyl-2,4,6-tris(3,5-di-tert.butyl-4-hydroxy-benzyl)benzene 
     Thiodiethylene bis [3-(3,5-di-tert.butyl-4-hydroxyphenyl)propionate] 
     Octadecyl 3-(3,5-di-tert.butyl-4-hydroxyphenyl)propionate 
     The compounds of the present invention have as one of their characteristic properties the ability to vastly improve the effect of numerous compounds which are presently used as antioxidants for organic materials. Thus, while the compounds of the present invention may be considered as stabilizers in their own right, their properties are such that they would be more conventionally classified as &#34;synergists&#34;, in that, when combined with known phenolic stabilizers they exhibit the ability to increase stabilization to a degree far exceeding that which would be expected from the additive properties of the individual components. 
     The compounds represented by the general formula I may comprise from 10 to 90% of the antioxidant system, however, the maximum effectiveness of the antioxidant system is achieved when a compound represented by the formula I is combined with a phenolic antioxidant at ratios varying from 1:4 to 4:1. The optimum ratio of a given combination varies depending on the organic material to which it is added. 
     The antioxidant system according to the present invention can be added to said organic materials in various ways. For instance, it can be applied either after dilution with a solvent or directly as it is. Addition of the present antioxidant system to the organic material can be performed either by applying a mixture prepared in advance (said phenolic antioxidant with the compounds specified in the present invention) or by applying these ingredients individually. Mixing of the present antioxidant system with substances such as resins, plastics and rubbers, which are solid at room temperatures can be readily performed by means of conventional equipment, such as, mixers, kneaders and roll mills. 
     It has been found that addition of the antioxidant system of this invention to organic materials in the range from 0.1 to 5.0 parts per hundred of organic material by weight will effectively protect the organic material from deterioration. 
     As described above the antioxidant system according to the present invention comprises the novel compound expressed by the general formula I combined with a known phenolic antioxidant. The antioxidant system of the present invention demonstrates antioxidative activity superior to that of most conventional antioxidant systems prepared by combining two or more commercial antioxidants. 
     Best Mode For Carrying Out The Invention 
     The novel compounds of this invention having the general structural formula I are prepared by the addition of a compound of the formula R--S--H, (II), wherein R is defined in formula I to 2,3-epoxypropyl acrylate (or methacrylate). The resulting 2,3-epoxypropyl 3-(R-thio) propionate is then reacted with a compound of the formula HSR 1  SH, (III), wherein R 1  is as defined in formula I to yield the compounds of this invention. Both reactions are based catalyzed and may be carried out consecutively without isolation of the intermediate product. Suitable solvents for the reaction are alcohols, such as ethanol or 2-propanol, and both reactions may be carried out at a temperature between 10° C. or lower and the boiling point of the solvent. 
    
    
     The following examples are presented for illustration and not limitation. 
     EXAMPLE 1 
     A reaction vessel was charged with 20.2 grams (0.1 moles) of dodecanethiol and a solution of 12.8 grams (0.1 moles) of 2,3-epoxypropyl acrylate with 10 drops of Triton B® (Trademark for a 40% solution of benzyltrimethylammonium hydroxide in methanol) and 50 milliliters of 2-propanol was added over a period of 5 minutes during which the temperature of the reaction mixture rose to 43° C. The mixture was then stirred for 3 hours at ambient temperature. The reaction product was 2,3-epoxypropyl 3(dodecylthio)propionate. 
     Preparation of Compound of Structural Formula (I) 
     EXAMPLE 2 
     To the solution of 2,3-epoxypropyl 3(dodecylthio)propionate prepared as described in Example 1 above was added 0.5 grams (0.0075 moles) of potassium hydroxide. The potassium hydroxide was allowed to dissolve and 4.7 grams (0.05 moles) of 1,2-ethanedithiol was then added to the reaction mixture. A white solid precipitated from the mixture following the addition of the dithiol. The mixture was then stirred for 3 hours and the precipitated solid was filtered off. There was obtained 26.9 grams of 2,9-dihydroxy-4,7-dithiadecamethylene bis[3-(dodecylthio)propionate] with a melting point of 65°-68° C. 71% theoretical yield. 
     EXAMPLE 3 
     The procedure of Example 2 was repeated using 6.9 grams of 2-mercaptoethyl ether in place of the 1,2-ethanedithiol and there was obtained 26.3 grams (0.033 moles) of 2,12-dihydroxy-4,10-dithia-7-oxatridecamethylene bis[3-(dodecylthio)propionate] with a melting point of 62°-64° C. 
     EXAMPLE 4 
     The procedure of Example 2 was repeated using 5.4 grams (0.05 moles) of 1,2-propanedithiol in place of the 1,2-ethanedithiol. After stirring for a period of 3 hours, the reaction mixture was poured into 200 milliliters of water containing 5 milliliters of concentrated HCl. The organic oil was separated by extraction with hexane and hexane was removed from the extract by a rotary evaporator. The residue, a pale viscous liquid, weighed 37.5 grams. The NMR spectrum of the product indicated that it was 2,9-dihydroxy-4,7-dithia-5-methyldecamethylene bis[3-(dodecylthio)propionate]. 
     EXAMPLE 5 
     Substituting 8.8 grams (0.05 moles) of 1,5-cyclooctanedithiol for the 1,2-propanedithiol of Example 4 yielded 36.5 grams (0.0435 moles) of a pale viscous liquid shown by its NMR spectrum to be 3,3&#39;-(1,5-cyclooctanedithio)bis(2-hydroxypropyl)bis[3-(dodecylthio)propionate]. 
     EXAMPLE 6 
     2,3-epoxypropyl 3-(phenylthio)propionate was prepared by adding 11.0 grams (0.1 moles) of benzenethiol to a solution of 12.8 grams of 2,3-epoxypropyl acrylate with 10 drops of Triton B® in 50 milliliters of 2-propanol. The addition was carried out over a period of 5 minutes during which the temperature of the reaction mixture rose to 54° C. The mixture was then stirred for 3.5 hours at ambient temperature. To this reaction mixture was added 0.5 grams of potassium hydroxide. After the potassium hydroxide had dissolved there was added 5.4 grams of propanedithiol and the reaction mixture was stirred for 3.5 hours. The mixture was then poured into a solution of 5 milliliters of concentrated hydrochloric acid in 100 milliliters of water. The organic oil was separated by extraction with toluene and toluene was then removed from the extract by rotary evaporator. There was obtained 26.6 grams (0.091 moles) of a viscous liquid shown by its NMR spectrum to be 2,9 -dihydroxy-4,7-dithia-5-methyldecamethylene bis[3-(phenylthio)propionate]. 
     EXAMPLE 7 
     Substitution of 12.4 grams (0.1 moles) of benzyl mercaptan and 4.7 grams (0.05 moles) of 1,2-ethanedithiol for the benzenethiol and 1,2-propanedithiol in the procedure described in Example 6 yields 31 grams of a viscous oil identified by its NMR spectrum to be 2,9-dihydroxy-4,7-dithiadecamethylene bis[3-(benzylthio)propionate]. 
     Testing of Compounds of Structural Formula (I) 
     The antioxidative activity of the compounds and mixtures of this invention was determined by means of the oxygen absorption test. The testing procedure is of the type described in detail in Industrial and Engineering Chemistry, Vol. 43, Page 456, (1951), and Industrial and Engineering Chemistry, Volume 45, Page 392 (1953). A mixture of 0.5 parts of each compound from Examples 2 to 7 and 0.5 parts of butylated octylated phenol was added to 100 parts of SBR 1006 and oxygen absorption measurements were made on the samples at 100° C. In addition, each compound from Examples 2 through 7 of the present invention and butylated octylated phenol was added individually to SBR 1006 at a concentration of 0.5 parts per hundred and these samples were tested in the same way. The data obtained from these tests were used to calculate the synergistic effect (SE). This quantity is defined by G. Scott on page 205 of Atmospheric Oxidation and Antioxidants, Elsevier Publishing Company, (1965) as ##EQU1## where M equals the activity of the mixture; &#34;a&#34; equals the activity of component &#34;a&#34;; and &#34;b&#34; equals the activity of component &#34;b&#34;. 
     The time, in hours, required for each sample to absorb 1% oxygen by weight and the calculated SE values are listed in the Table I below. 
     
                       TABLE I                                                     
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Synergistic Activity of the Compounds                                     
of this Inventon                                                          
Synergist Product Hours to Absorb 1% Oxygen                               
of Example Additive   Additive + B.O.P.*                                  
                                     SE                                   
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2          62         720            168                                  
3          60         667            150                                  
4          53         862            232                                  
5          61         750            180                                  
6          45         616            144                                  
7          31         470             97                                  
B.O.P.*    207        --             --                                   
DLTDP**    48         498             90                                  
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 *B.O.P. = Butylated Octylated Phenol                                     
 **DLTDP = Dilauryl 3,3thiodipropionate  the most widely used commercial  
 synergist                                                                
 
    
     Had no antioxidant been present, the SBR 1006 would have absorbed 1% oxygen in 5 to 10 hours. 
     It is evident that the compounds of this invention are much more effective than the well-known synergist DLTDP, which is now the synergist of choice in the industry. 
     In addition, the compound of Example 4 was combined with six commercially available antioxidants (0.5 parts of each by weight) and the mixtures were added to SBR-1006 at a concentration of 1 phr. The samples were subjected to the oxygen absorption test cited above with the following results: 
     
                       TABLE II                                                    
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Antioxidant                                                               
         A.O. Alone                                                       
                   A.O. plus Ex. 4                                        
                                 Synergistic Effect                       
______________________________________                                    
1        134       654          301                                       
2        148       488          176                                       
3        340       1038         181                                       
4        249       722          160                                       
5        595       855           37                                       
6        372       653           63                                       
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     The identity of the antioxidants used in Table II are set out below. 
     
         ______________________________________                                    
Antioxidant No.      Chemical Name                                        
______________________________________                                    
1                    Styrenated phenol                                    
2                    Butylated α-methylstyrenated phenol            
3                    Butylated reaction product of                        
                     p-cresol and dicyclopentadiene                       
4                    Styrenated butylated m, p-cresol                     
5                    2,2&#39;-methylenebis(4-methyl-6-tert.-                  
                     butyphenol)                                          
6                    4,4&#39;-Thiobis(3-methyl-6-tert.buty-                   
                     phenol)                                              
______________________________________                                    
 
    
     It is evident from Table II that the compound from Example 4 exhibits synergistic activity with a number of commercially available antioxidants. 
     Additional synergistic activity of the compounds of this invention was determined by means of the circulating hot air oven test. This testing procedure is one commonly employed in the industry and was conducted using the products from Examples 2, 4 and 5. 
     The products from Examples 2, 4 and 5 were evaluated alone and in combination with the commercially available phenolic antioxidant Tetrakis [methylene 3-(3,5-di.tert.butyl-4-hydroxyphenyl)propionate] methane (hereinafter known as TMPM) in polypropylene. 
     The testing procedure consists of dissolving the compound or compounds in a suitable solvent such as toluene. The solution was deposited on a commercially available unstabilized polypropylene known as Profox® 6501 at a concentration of 0.10 pph by weight if evaluated alone and in a concentration of 0.05/0.05 pph blend by weight when evaluated with TMPM. The materials were blended with a Henschel blender until all the solvent had evaporated. The stabilized resin was then injection molded to form a dumbell shaped test specimen using appropriate injection molding conditions. 
     The resulting test specimens were then oven aged in triplicate at 140° C. in a circulating hot air oven and observed daily. The first signs of degradation that were noted is crazing. The failure point was embrittlement to flexing, that is, bending by hand to less than 90°. Failure times are the average for the three samples. 
     Table III is test data for the products of Examples 2, 4, and 5 alone at a concentration of 0.1 pph by weight of polypropylene, and in combination with TMPM at a concentration of 0.05 pph for the example and 0.05 pph for the TMPM in the polypropylene. In addition, distearyl thiodipropionate was tested, evaluated alone and in combination with TMPM at the concentrations as stated above. Distearyl thiodipropionate is a commercially available synergist marketed under the name Plastanox STDP®. 
     
                       TABLE III                                                   
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SYNERGISTIC ACTIVITY IN POLYPROPYLENE                                     
         Days to Crazing                                                  
                      Days to Failure                                     
Synergist Product   Additive         Additive                             
of Example Additive + TMPM    Additive                                    
                                     + TMPM                               
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2          10       71        11     72                                   
4           3       73         4     75                                   
5           3       80         3     81                                   
TMPM*      21       --        23     --                                   
0.005 pph                                                                 
TMPM*      86       --        88     --                                   
0.1 pph                                                                   
STDP**      2       60         2     60                                   
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 *Tetrakis [methylene 3(3,5-ditert.butyl 4hydroxy-phenyl)propionate]      
 methane                                                                  
 **Distearyl thiodipropionate                                             
 
    
     From Table III it is evident that the products of Examples 2, 4 and 5 of this invention exhibit synergistic activity greater than a known and commercially accepted synergist. 
     Industrial Applicability 
     From the testing data obtained it is evident that the compounds disclosed herein significantly enhance the stability of SBR 1006 and polypropylene when combined with a known phenolic antioxidant, (i.e. butylated octylated phenol). The industrial applications are readily apparent in light of the high synergistic activity of these novel compounds when used in conjunction with known phenolic antioxidants. Use of the compounds of this invention would significantly reduce the amount of costly phenolic antioxidant that is needed to provide the desired stability of the organic material. In addition, the use of these novel antioxidant systems would lessen the undesirable effects of color formation and discoloration. 
     While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made herein without departing from the spirit or scope of this invention.