Abstract:
Phenolic age resistors such as 4-(3,5-ditertiary butyl-4-hydroxyphenyl)-2-butyl 2-methyl-4-thia-6-octanoyloxyhexanoate are used as antioxidants in diene rubbers, such as butadiene-styrene types and in polyolefins, such as polypropylene.

Description:
This is a division of application Ser. No. 739,687 filed Nov. 8, 1976 now U.S. Pat. No. 4,093,591. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the preparation and use of phenolic age resistors for polymers, both unsaturated and saturated. More particularly, it concerns compounds comprised of a disubstituted phenolic group attached at the para position to a carbon chain containing sulfur, a carbonyl group, and a carboxylic ester moiety. 
     The use of sulfur-containing hindered phenols is known, e.g., see Canadian Pat. No. 904,339; U.S. Pat. No. Re. 27,004 and U.S. Pat. No. 3,590,083 and British Pat. No. 1,396,469. 
     The art disclosing hindered phenols is extensive, including those containing carbonyl or ester groups, see e.g. Japanese Pat. No. 27,735/68, U.S. Pat. No. 3,753,943; U.S. Pat. No. 3,810,929, Japanese Pat. No. 0468/63; Rubber Chemistry and Technology, Vol. 46, No. 1, Pp. 96-105, March, 1973; U.S. Pat. No. 3,714,122 and Canadian Pat. No. 812,262. 
     SUMMARY OF THE INVENTION 
     Among the objects of this invention are: (1) to provide phenolic antioxidants for the protection of polymers, (2) to provide polymers stabilized against oxidative degradation, and (3) to provide a process for the preparation of said antioxidants. Other objects and advantages will hereinafter appear. 
     The new compounds have the following general formula: ##STR1## wherein R and R 1  are selected from the group consisting of tert. alkyl radicals having 4 to 8 carbons and cycloalkyl radicals having 5 to 12 carbons; R 2 , R 3 , R 4 , R 5 , R 6 , R 7  and R 9  are selected from the group consisting of H and alkyl radicals having 1 to 4 carbons; R 8  is selected from the group consisting of alkyl radicals having 1 to 20 carbons, alkenyl radicals having 2 to 20 carbons, cycloalkyl radicals having 5 to 12 carbons, phenyl, substituted phenyl and aralkyl radicals having 7 to 12 carbon atoms; x, y or z equals 0 to 12; x+y+z≦12; m equals 0 or 1; n equals 0 to 3 and X is 0 or NH. 
     The term &#34;substituted phenyl&#34; as used herein means a phenyl ring to which is attached one or more radicals illustrated by but not limited to halogens such as chlorine, alkyl radicals such as methyl, and hydroxyl. Preferably the substituted phenyl radical contains one or two substituents selected from the group consisting of methyl and other lower alkyls. 
     These compounds can be used in free form to stabilize polymers against oxidative degradation, and those in which R 8  is 1-alkenyl can be used as polymerizable antioxidants in diene polymers. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The compounds of this invention are prepared by first reacting a β-mercaptoethanol of structure ##STR2## with an unsaturated ester or amide of the structure ##STR3## The products of this reaction which are of the structure ##STR4## are then esterified by reaction with an acid chloride or anhydride derived from a carboxylic acid of the structure R 8  COOH. The first reaction is normally carried out in a low boiling alcohol, in the presence of a catalytic amount of a base, such as an alkali metal hydroxide, a tertiary aliphatic amine, or a quaternary ammonium hydroxide at a temperature between room temperature and the boiling point of the solvent. The esterification is accomplished by adding an acid chloride or anhydride to a solution comprising the products of the first reaction; an acid acceptor, such as pyridine or triethylamine and an aprotic solvent, such as ethyl ether, tetrahydrofuran, or carbon tetrachloride. Equimolar amounts of acid chloride (or anhydride) and the alcohol are preferred. The reaction, which is mildly exothermic, is normally carried out at ambient temperature, but may be carried out at any temperature between 0° C. and the boiling point of the solvent. 
     A typical preparation of one of this new class of compounds is shown in Example I which is intended to illustrate and not limit the methods by which the compounds of the present invention are made. Unless otherwise indicated, all parts are parts by weight. 
    
    
     EXAMPLE I 
     A mixture of 12.5 grams of 3,5-ditert.butyl-4-hydroxyphenyl acrylate, 3.5 grams of 2-mercaptoethanol, 0.25 grams of potassium hydroxide, and 50 milliliters of ethanol was stirred at ambient temperature for 1-1/2 hours. The reaction mixture was poured into water, and the oil which precipitated was separated by extraction with benzene. The benzene was removed on a rotary evaporator, leaving 16.0 grams of a viscous oil which was identified through its NMR spectrum as 3,5-ditert.butyl-4-hydroxyphenyl 6-hydroxy-4-thiahexanoate. This product, along with 9.1 grams of triethylamine, was dissolved in 50 milliliters of tetrahydrofuran. To this solution was added, over a period of 5 minutes, 5.7 grams of methacryloyl chloride. During the addition, the temperature of the reaction mixture rose from 27° to 48° C. The reaction mixture was stirred for several hours and was then poured into water. The organic layer was separated by extraction with hexane. The hexane was removed on a rotary evaporator, leaving 17.2 grams of a viscous yellow oil, identified as 3,5-ditert. butyl-4-hydroxyphenyl 6-methacryloxy-4-thiahexanoate. 
     Typical examples of the radicals and substituents are: 
     (1) R and R 1  when tertiary alkyl radicals can be, for example, 1,1-dimethylpropyl; 1,1-dimethylbutyl; 1,1,2-trimethylpropyl; 1,1-dimethylpentyl; 1,1-dimethylhexyl; 1,1,3-trimethylpentyl; 1,1,3,3-tetramethylbutyl; 1-ethyl-1-methylbutyl; or tertiary butyl which is preferred; 
     (2) R,R 1  and R 8 , when cycloalkyl radicals can be, for example, cyclopentyl; cyclohexyl; 2-methylcyclohexyl; 4-methylcyclohexyl; 2,4-dimethylcyclohexyl; 4-tertiary butylcyclohexyl; or 2-methylcyclopentyl; 
     (3) R 2 , R 3 , R 4 , R 5 , R 6 , R 7  and R 9 , when alkyl radicals can be for example, methyl (which is preferred), ethyl, n-propyl, isopropyl, n-butyl or isobutyl; 
     (4) R 8 , when it is an alkyl radical can be, for example, those in number (3) above or pentyl, hexyl, octyl, 2-ethylhexyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, hexadecyl, heptadecyl, octadecyl, or eicosyl; 
     (5) R 8 , when it is a substituted phenyl can be, for example, p-tolyl; o-tolyl; p-ethylphenyl; p-isopropylphenyl; 2,4-xylyl; 3,5-xylyl; p-tert.butylphenyl; 2,4,6-trimethylphenyl; or 2,3,5-trimethylphenyl; 
     (6) R 8 , when it is an aralkyl radical can be, for example, benzyl; 2-phenylethyl; p-methylbenzyl; 3-phenylpropyl; 2-phenylpropyl; p-tert.butyl benzyl; or 2,4-dimethylbenzyl; and 
     (7) R 8 , when it is an alkenyl radical can be, for example, vinyl and isopropenyl (preferred for polymerizable antioxidants); allyl, 1-propenyl, 2-butenyl, 1-hexenyl, 2-methyl-2-propenyl, 1-octenyl, 1-undecenyl, 3-methyl-1-heptenyl, 2-dodecenyl, 8-tridecenyl, 8-heptadecenyl or 5-nonadecenyl. 
     Polymers susceptible to oxidative degradation in which these antioxidants are beneficial include vulcanized and unvulcanized, saturated and unsaturated, natural and synthetic, homopolymers and copolymers. The term &#34;copolymer&#34; as used herein, means a polymer which is comprised of two or more different monomers. Representative examples of each type are given for illustrative purposes; natural rubber in its various forms, e.g. smoked sheet and balata; synthetic polymers including homopolymers (e.g. polyisoprene and polybutadiene), copolymers, e.g. styrene-butadiene rubbers, acrylonitrile-butadiene rubbers, and copolymers of propylene, ethylene and dicyclopentadiene. Polyolefins such as polypropylene and polyethylene are also illustrative of polymers which benefit by the practice of the present invention as are copolymers of ethylene and propylene. 
     Although the precise amount of antioxidant to be used depends on the polymer and conditions to which it will be exposed, generally the amount employed varies between 0.001 and 10 parts by weight per 100 parts by weight of polymer. A preferred range is about 0.05 to 1.75. The compounds are especially useful in polypropylene at a level of about 0.1 parts per hundred as they possess superior resistance to discoloration by heat and ultraviolet light. 
     As free form antioxidants, the compounds of this invention may be added to the polymer in any of the conventional ways, such as by addition to the latex or to a solution of the polymer or by mixing with the dry polymer in a Banbury mixer or on a rubber mill. 
     The compounds can also be made to be an integral part of the polymer itself by either grafting or polymerization techniques. Being part of the polymer itself prevents the antioxidant from being volatilized or solvent extracted from the polymer. 
     The grafting technique is accomplished by combining the antioxidant and the polymer together in the presence of a free radical initiator. The form in which the polymer is grafted (e.g., solution, latex or dry form such as on a mill or in a Banbury) can depend upon the free radical initiator being used. Free radical initiators include redox systems, azo compounds and peroxides. 
     The polymerization method involves the use of the antioxidant as a monomer in a free radical polymerization reaction. The formation of the resulting oxidation resistant polymer takes place during the normal solution or emulsion polymerization reaction in which the principal monomers combine in the presence of catalyst, solvent, modifier, and whatever other emulsifiers and reagents are required for the preparation of the particular polymer. Conventional monomer systems and polymerization recipes and techniques are disclosed or referenced in U.S. Pat. No. 3,714,122. The weight ratio of polymerizable antioxidant to the sum of the other monomers ranges from about 0.01 to 5 parts by weight of antioxidant to 100 parts by weight of other monomers. The preferred range is about 0.1 to 2 parts by weight per 100 parts of other monomer. Preferably at least 50 parts by weight of the monomer system consists of a conjugated diene monomer such as 1,3-butadiene or isoprene. 
     Representative examples of the age resistors of this invention are: 
     1. 3,5-ditert.butyl-4-hydroxyphenyl 4-thia-6-methacryloxyhexanoate; 
     2. 3,5-ditert.butyl-4-hydroxyphenyl 3-methyl-4-thia-6-methacryloxyhexanoate; 
     3. 3,5-ditert.butyl-4-hydroxyphenyl 2-methyl-4-thia-6-benzoyloxyhexanoate; 
     4. 2-(3,5-ditert.butyl-4-hydroxybenzyloxy)ethyl 4-thia-6-methacryloxyhexanoate; 
     5. 4-(3,5-ditert.butyl-4-hydroxyphenyl)-2-butyl 4-thia-6-methacryloxyhexanoate; 
     6. 4-(3,5-ditert.butyl-4-hydroxyphenyl)-2-butyl 2-methyl-4-thia-6-octanoyloxyhexanoate; 
     7. N-(3,5-ditert.butyl-4-hydroxyphenyl)-2-methyl-4-thia-6-methacryloxyhexanamide; 
     8. N-(3,5-ditert.butyl-4-hydroxybenzyl)-4-thia-6-methacryloxyhexanamide; 
     9. N-[1,1-dimethyl-2-(3,5-ditert.butyl-4-hydroxyphenyl)ethyl]-2-methyl-4-thia-6-methacryloxyhexanamide; 
     10. N-[1,1-dimethyl-2-(3,5-ditert.butyl-4-hydroxyphenyl)ethyl]-4-thia-6-isobutyroxyhexanamide; 
     11. 2,2-dimethyl-3-(3,5-ditert.butyl-4-hydroxyphenyl)propyl 2-methyl-4-thia-6-acetoxyhexanoate; 
     12. 3,5-di-t-butyl-4-hydroxyphenyl 4-thia-6-stearoyloxyhexanoate; 
     13. 3,5-di-t-butyl-4-hydroxyphenyl 4-thia-6-oleoyloxyhexanoate; 
     14. 3,5-bis(1-methylcyclohexyl)-4-hydroxyphenyl 2-methyl-4-thia-6-acetoxyhexanoate; 
     15. 2-ethyl-2-butyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propyl 4-thia-6-acryloxyhexanoate; 
     16. 4-(3,5-di-t-butyl-4-hydroxyphenyl)-2-butyl 4-thia-6-(phenylacetoxy) hexanoate; 
     17. 4-(3,5-di-t-butyl-4-hydroxyphenyl)-2 -butyl 4-thia-6-(4-methylbenzoyl) hexanoate. 
     For a better understanding of these complex structures, Table 1 is included, correlating the structures with the formulas of the example compounds: 
     
                       Table 1______________________________________Correlation of Structure with Generic Formula Com-poundNo.   n     m     X    R.sup.5                       R.sup.6                            R.sup.8______________________________________1     0     0     O    H    H                             ##STR5##2     0     0     O    H    CH.sub.3                             ##STR6##3     0     0     O    CH.sub.3                       H                             ##STR7##4     1     1     O    H    H                             ##STR8##5     0     1     O    H    H                             ##STR9##6     0     1     O    CH.sub.3                       H    C.sub.7 H.sub.157     0     0     NH   CH.sub.3                       H                             ##STR10##8     0     1     NH   H    H                             ##STR11##9     0     1     NH   CH.sub.3                       H                             ##STR12##10    0     1     NH   H    H    C(CH.sub. 3).sub.211    0     1     O    CH.sub.3                       H    CH.sub.312    0     0     O    H    H    (CH.sub.2).sub.16CH.sub.313    0     0     O    H    H                             ##STR13##14    0     0     O    CH.sub.3                       H    CH.sub.315    0     1     O    H    H                             ##STR14##16    0     1     O    H    H                             ##STR15##17    0     1     O    H    H                             ##STR16##______________________________________ 
    
     The following examples illustrate the use of the compounds of the present invention as antioxidants and are not intended to be limitations thereof. 
     In a test of some of these compounds as polymerizable antioxidants, butadiene/acrylonitrile/antioxidant terpolymers were prepared using the following recipe: 
     
         ______________________________________Ingredient               Parts by Weight______________________________________Butadiene                67Acrylonitrile            33Water                    195Potassium soap of tallow fatty acids                    2.5Trisodium phosphate      0.2Ferrous sulfate          0.0144*Chelating agent         0.0568Sodium formaldehyde sulfoxylate                    0.0412Tert. dodecyl mercaptan  0.5**Cumene hydroperoxide (CHP)                    0.023Polymerizable Antioxidant                    1.5______________________________________ *90/10 mixture of tetrasodium salt of ethylenediaminetetraacetic acid and monosodium salt of N,Ndi(αN-di(α-hydroxyethyl)glycine. **70% cumene hydroperoxide in cumene. 
    
     The resulting polymers were coagulated, extracted 48 hours with methanol in a Soxhlet extractor, dried, and dissolved in benzene. Films were cast from the benzene solutions and oxygen absorption measurements were made on the films. The results of these measurements are listed in Table 2. 
     
                       Table 2______________________________________Oxygen Absorption of NBR (CHP Initiator)Containing Bound AntioxidantCompound     Hours to Absorb 1% O.sub.2 @ 100° C.______________________________________1            652            974            805            387            2658            1519            149______________________________________ 
    
     Two butadiene/acrylonitrile rubbers were prepared from monomer systems containing one of the compounds of the present invention. Azoisobutyronitrile was used as the initiator. The polymers were extracted and oxygen absorption measurements made. The results are listed below: 
     
                       Table 3______________________________________Compound     Hours to Absorb 1% O.sub.2 @ 100° C.______________________________________5            2389            265______________________________________ 
    
     Table 4 contains additional data on samples of emulsion polybutadiene prepared from a monomer system containing 1.5 parts by weight of a polymerizable antioxidant per 100 parts by weight of butadiene. 
     
                       Table 4______________________________________Compound     Hours to Absorb 1% O.sub.2 @ 100° C.______________________________________1            2835            2577            3138            2099            246______________________________________ 
    
     Compounds 1 through 11 were each used in free form to stabilize an SBR polymer (1006) by addition to a benzene solution of SBR-1006 at a concentration of 1 part per 100 parts rubber. Oxygen absorption tests were made on the films by evaporation of the benzene. The testing procedure is of the type described in further detail in Industrial and Engineering Chemistry, Vol. 43, page 456 (1951) and Industrial and Engineering Chemistry, Vol. 45, p. 392 (1953). 
     Oven aging tests at 140° C. were also carried out on samples of polypropylene at 0.1 parts by weight free form antioxidant per 100 parts by weight polypropylene. The tests of the compounds used as free form antioxidants are presented in Table 5. 
     
                       Table 5______________________________________         Hours to Absorb                        Days to Failure         1% Oxygen      at 140° C. inCompound      SBR-1006       Polypropylene______________________________________1             715            --2             773            --3             467            154             638            --5             676            106             589            217             485             88             850            --9             960            --10            538            1311            633            --Commercial PhenolicAntioxidant   --             39______________________________________ 
    
     The antioxidants of this invention are equal or superior to commercially available products in discoloration characteristics. Several commercial controls and several of the compounds of this invention were mixed at the 0.1 pph level into polypropylene. Original color and color of the plastic after five days oven exposure at 140° C. was measured to test thermal discoloration resistance. The results are shown in Table 6. 
     
                                           Table 6__________________________________________________________________________   Original Color               Aged 5 days @ 140° C.Compound   Rd a  b  CI*               Rd  a  b    CI*__________________________________________________________________________No Anti-oxidant 51.6      -1.3         +5.8            7.3               Failed                   -- --   --3       53.3      -1.7         +5.6            7.3               51.1                   -1.6                      +5.3 7.05       55.3      -1.2         +6.0            7.7               53.5                   -1.3                      +5.7 7.66       53.0      -1.4         +6.2            7.0               53.2                   -1.5                      +6.6 6.67       53.0      -1.5         +7.5            5.9               48.1                   -1.5                      +10.8                           3.910      53.2      -2.8         +4.5            4.5               49.7                   -3.5                      +12.8                           3.8CommercialPhenolicAntioxidantControl 1   56.0      -1.5         +6.1            7.4               53.4                   -1.6                      +6.4 6.7CommercialPolycyclicAntioxidantControl 2   54.4      -1.7         +6.0            7.1               49.4                   -1.6                      +6.5 6.1CommercialControl 3   52.8      -1.7         +5.8            7.0               49.9                   -1.7                      +7.4 5.5Wingstay L   52.7      -1.7         +7.5            5.7               43.7                   -4.4                      +30.6                           1.2__________________________________________________________________________ *CI is Color Index 
    
     Three of the experimental antioxidants (3, 5 and 6) were as good or better than the commercial controls, and all of them were superior to Wingstay L. 
     Ultraviolet light discoloration and stain tendencies were measured in an SBR based test formulation at the 1.5 pphr level. Measurements were taken of the original color of the rubber compounds and color after 120 hours Fade-O-Meter exposure both with and without a coating of white nitrocellulose lacquer on the samples. This lacquer manifests staining tendencies. The results of these discoloration tests are shown in Table 7. 
     
                                           Table 7__________________________________________________________________________   Original Color               Fade-O-Meter Exposed                           Fade-O-Meter Exposed   Tinting     120 hrs. discoloration                           120 hrs. - stainCompound   Rd a  b  CI*               Rd a  b  CI*                           Rd  a  b   CI*__________________________________________________________________________No AO   87.8      -0.8         +5.1            14.9               88.1                  -0.8                     +4.7                        16.0                           91.3                               -0.3                                  +3.6                                      23.43       88.0      -0.9         +5.3            14.2               87.6                  -1.3                     +6.6                        11.1                           91.1                               -0.3                                  +4.2                                      20.25       88.2      -0.9         +5.3            14.2               87.9                  -0.8                     +5.6                        13.7                           91.3                               -0.2                                  +3.6                                      24.06       89.2      -0.9         +5.6            13.7               89.7                  -0.8                     +5.3                        14.5                           91.5                               -0.3                                  +3.9                                      21.87       89.3      -1.2         +7.2            10.6               88.0                  -0.5                     +6.3                        12.4                           91.1                               -0.2                                  +3.8                                      22.810      89.0      -0.8         +5.0            15.3               88.7                  -0.9                     +5.3                        14.3                           91.3                               -0.2                                  +4.0                                      21.7Wingstay L   87.6      -0.9         +6.1            12.5               87.3                  -1.3                     +7.2                        10.3                           91.2                               -0.3                                  +4.0                                      21.2CommercialAntioxidantControl 87.2      -1.2         +6.5            11.3               82.6                  -0.4                     +9.4                        8.4                           **85.4                               -0.3                                  13.4                                       6.2__________________________________________________________________________ *CI is Color Index **Visual stain at 48 hours 
    
     All of the experimental materials exceeded Wingstay L and the other control in discoloration resistance, and only compound 7 and the commercial control appeared to tint the original color slightly. 
     In Tables 6 and 7 color measurements are based on the Hunter Coordinate System in which an Rd of 100 is perfect reflectance and an Rd of 0 is a black body (perfect absorbance). Positive a represents red; negative a green; positive b yellow; and negative b blue. The greater the absolute value of a or b, the greater is the intensity of color. 
     Color Index is Rd/(|a|+|b|). The greater the color index, the closer color is to white. 
     The preferred reaction temperature for the reaction between the unsaturated ester or amide and the β-mercaptoethanol depends on the particular ester or amide used. Generally, the reaction proceeds well at the reflux temperature of the solvent (78° C. in the case of ethanol). The amount of base used is generally less than five moles per 100 moles of reactants, preferably less than one mole per 100 moles of reactants. The two reactants are preferably present in stoichiometric amounts, i.e. equimolar, although other ratios can be utilized. 
     Examples of unsaturated esters and amides which can be used in the first reaction are: 
     3,5-ditert.butyl-4-hydroxyphenyl acrylate 
     3,5-ditert.butyl-4-hydroxyphenyl 2-butenoate 
     3,5-ditert.butyl-4-hydroxyphenyl methacrylate 
     3,5-ditert.butyl-4-hydroxyphenyl methacrylamide 
     3,5-ditert.butyl-4-hydroxyphenyl acrylamide 
     Examples of the solvents which may be used are: ethanol, methanol and 1-propanol. 
     Examples of the bases which may be used are: potassium hydroxide, sodium hydroxide, triethylamine, and trimethyl benzyl ammonium hydroxide. 
     The acid acceptor in the esterification reaction is used in amounts equimolar with the acid chloride, although some excess can be used. Equimolar amounts of acid chloride (or anhydride) and the alcohol are preferred, although different ratios may be used and a 10 percent excess of acid chloride over the equimolar amount is sometimes desirable. 
     Examples of acid chlorides and anhydrides which may be used are acetic anhydride; acetyl chloride, methacryloyl chloride; isobutyryl chloride; acryloyl chloride; methylbenzoyl chloride and benzoic anhydride. 
     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 therein without departing from the spirit or scope of the invention.