Alkyl Phenols as Nondiscoloring Antioxidants for Svnthetic Rubber U
LELAND J. KITCHEN, HARRY E. ALBEKT, AND GEORGE E. P. SMITH, J R . The Firestone Tire &. Rubber Company, Akron, Ohio
Active nondiscoloring antioxidants for synthetic rubbers of the butadiene-styrene (GR-S) and butadiene-acrylonitrile (Buna N) types were the results of an intensive search for useful materials for this purpose at the close of World War 11. These new antioxidants are 2,4,6-suhstituted phenols, such as 2,4-dimethyl-6- tt-octylphenol, 2,4dimethy1-6-isohorny1pheno1, and 2,6- di- tert butyl -4methylphenol. The best materials were particularly effective in the N types of synthetic rubber, where they not only were nondiscoloring, but approached in activity the best aromatic secondary amine antioxidants in protecting both polymers and vulcankates against oxidative degradation. In GR-S, satisfactory polymer and vulcanizate protection was achieved, together with non-
discoloration and some degree of protection against weather checking, particularly the surface crazing that results from the natural weathering of a GR-S white stock. The study of phenolic antioxidants in synthetic rubher afforded an opportunity to investigate the relationship between chemical structure and antioxidant activity €or this type of compound. The results show that, in general, the 2,4,6-triallrylated phenols are somewhat more active antioxidants than lesser alkylated phenols and are more active than the corresponding 3,4,6-alkylated phenols. These results are discussed in terms of the relative ease of formation of ions and radicals, and of the mechanism by which these phenolic compounds may act as antioxidants in rubber.
G
agent, if of the polyphenol type, may or may not be practically nendiscoloring in the minute percentage used. However, if the nondiscoloring synthetic rubber is to be suitably stable for a long period of time, i t is necessary to add a special nondiscoloring antioxidant in larger proportion-Le., 1 to 2%. Phenols were among the f i s t materials found to have antioxidant properties. The protective action of various phenols for rubber was noted as early as 1870 (SO). Guaiacol and various polyhydric phenols were examined m antioxidants in 1921 ($7). With further investigation of the preservation of rubber with antioxidants, the aromatic amines became and have remained the most important rubber antioxidants. The search for nondiscoloring antioxidants for rubber led back t o the phenols, particularly monohydric phenols. 2-Naphthol w m found to have properties as a nondiscoloring antioxidant (19). With the development of the butadiene synthetic rubbers in Germany, many phenolic materials were examined as antioxidants. Among them were benzylphenols (SS), hydroxydiphenyl (16), phenols which have been substituted by condensation with styrene (17) and indene (22), and phenol sulfides (43) derived from phenol and cresols. More recently, as antioxidants for synthetic rubber there have been described the sulfides of alkyl phenols (3, 18), phenols and naphthols which have been substituted by condensation with aldehydes or ketones (%9), and substituted phenols obtained by condensing phenols with olefins and cyclo-olefins (2, 16). Since the present work was completed a patent (42) has been issued which describes the use of ditertiary alkyl phenols as stabilizers for synthetic rubber latex. The phenol mixtures recovered from cracked petroleum naphtha distillates have recently been reported as nondiscoloring antioxidants for synthetic rubber (12). Used in larger proportion, certain alkyl phenols were stated to be tackifying plasticizers in butadiene-acrylonitrile copolymers (13). I n the present investigation alkyl phenols were examined as nondiscoloring antioxidants for copolymers of butadiene with acrylonitrile and with styrene and for vulcanizates prepared from them. Activity was found to vary widely among alkyl
-
R-S and other butadiene copolymers contain high degrees of unsaturation and, in common with natural rubber hydrocarbon, are susceptible to oxidative deterioration. Consequently, mi antioxidant component is necessary in a butadiene-derived synthetic rubber for applications in which the rubber must withstand oxidative deterioration. The antioxidants of the secondary aromatic amine type, which are used generally in natural rubber compounds to improve their resistance t o deterioration in the presence of air, also inhibit the oxidative deterioration of the butadiene-derived synthetic rubbers. Phenyl-2-naphthylamine (phenyl-P-naphthylamine, PBNA), for example, is an eminently satisfactory antioxidant for butadiene-acrylonitrile and butadiene-styrene copolymers, and it or some similar amine antioxidant is commonly present in GR-S polymer. Rubbery materials containing phenyl-2-naphthylamine and other effective aromatic amine antioxidants have the disadvantage of undergoing varying degrees of discoloration on exposure to light or heat. Thus light-colored products which are nondiscoloring cannot be prepared when nitrogenous antioxidants of the discoloring type are present. Consequently, in the preparation of white or light-colored products from natural rubber, it was frequent prewar practice not to add antioxidant, but t o rely upon the naturally occurring antioxidants in the rubber to keep the products oxidation-resistant. The effectiveness of the natural antioxidants, which are nondiscoloring, is indicated by the stability of crude rubber contrasted with the high degree of instability, in the presence of air, of rubber hydrocarbon from which antioxidants have been removed by acetone extraction ($4). Because synthetic rubber contains no such natural antioxidant, it is necessary to add a stabilizer such as phenyl-2-naphthylamine; and if it is desired that the synthetic polymer and its compounds be nondiscoloring, a special nondiscoloring antioxidant must be added in place of phenyl-2-naphthylamine. Synthetic rubber as now prepared may have transient stability without antioxidant addition because of use, in the polymerization process, of a small amount of a “stopping agent” such as hydroquinone which itself may have antioxidant properties. The stopping
675
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676
TABLE I. ALKYLPHENOLS
A.
B.
AS
ANTIOXIDANTS FOR BUTAPRESE OVEN-AGING
Aging Conditions Material Polymer Time, T e m p , Tested a s Antioxidant Type days C. Miscellaneous None present (blank) NXM 4 100 Phenyl-2-naphthylamine (controls) N X N 4 100 NA 4 90 SXhl Heptylated diphenylamine 4 100 sA 4 90 h-A Tri henyl phosphite 4 90 KXM SulPUP 4 100 2,4,6-Trialkyl phenols 2,6-Di-tert-butyl-4-methylphenol UXM 4 100 NA 4 90 NA 10 95 NA 10 95 NA 4 90 NXXI 4 100 NA 4 90 NXM 4 100 SA 10 95 NXM 4 100 N. 4 4 90
C.
Pro erties of AgeBPolymersQuality Color Very poor Very good Very good Good Very good Very poor Fair
Dirty brown Dark brown Brown Brown Brown Dark brown Light brown
Very good Good Very good Good Very good Very good Very good Very good Very good Good Very good
Tan Cream Tan Light brown Yellowtan Light brown Cream Tan Dark tan T a n t o brown Nearlv white. cream tinge' Light cream T a n mottled with brown Cream mottled with brown
2j~ethyl-4,6-di-tert-butylphenol
NA NA
4
4
90 90
Good Fair
Camphene-0-cresol resin No. 2C
N '4
4
90
Fair
NXM NXM NA NA
4
2 4 6-Tri-tert-butylphenol
1Iiscellaneous alkyl phenols o-tert-Amylphenol p-Cresol p-tert-Butylphenol p-teit- Amylphenol p-tt-Octylphenol
N -4
100
90 96
Fair Fair t o poor Good to fair Fair
10
go 95
Fair Fair to poor
4
100
4 4
10 4
100
POLYiVlEHS DURING
Brown Brown Yellow Light brown, dark spots Dark brown Light brown, dark spots Brown Light brown, dark spots Brown Tan
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synthetic rubber, were carried out in two types of butadiene-acrylonitrile copolymer, Butaprene WA and Butaprene S X M (8). (Butaprene N is the trademark for synthetic rubbers of the nitrile type produced by the Firestone Tire & Rubber Company. Butaprene NA contains about 32% combined acrylonitrile. Butaprene SXM is somewhat higher in acrylonitrile, containing about 35%.) Because there is no single test suitable for evaluation of the resistance of a rubbery material to deterioration, a variety of tests was carried out on the polymers containing the materials being examined as stabilizers. They included oven-aging of polymer, used as a screening test. Those polymers which withstood the polymer-aging test well were compounded in carbon black stocks which were cured and subjected to oven and air-bomb aging. For discoloration properties, white stocks were prepared and exposed in sunlight, sun lamp, and Fadeometer tests.
PREPAR~TION OF SAMPLES.Latex was withdrawn from a polymerization autoc l a v e d u r i n g butadiene-acrylonitrile p-tert-Decylphenol NA 10 95 emulsion copolymerization, when the ~-Cyc!ohexylphenol NA 4 90 Fair reaction had reached the desired converIIonoiaoborn lphenolf sA 4 Fair 90 sion but before stopping agent and inhil,l-Bis(p-hydiroxypheny1)-cycloFair hexane K -4 4 Cream bitor had been added. Material to be 90 2-tert-Butyl-4-methylphenol NXM 4 Good T a n , a few dark 90 tested as antioxidant and stabilizer was added as an aqueous emulsion. The SA Very good 2-tt-Octyl-4-methylphenol G K ? a n , a few 95 10 emulsions were prepared by dissolving spots Camphene-p-cresol resin0 NA Good 4 Tan 90 the test material in hot alcohol and 2-Methyl-4-tert-amylphenol SA 4 90 Good Brown pouring the alcohol solution, containing Camphene-0-cresol resin No. 1h N .?. 4 90 Good to fair T a n , a feabout 10% test material, into an equal spots SA 2 4-Di-tert-butylphenol 4 Good 90 Yellow volume of warm 10% sodium oleate N -4 2:4-Di-tert-amylphenol 4 Good 90 Cream solution in a Waring Blendor. The NA Camphene-phenol resini Fair 4 90 Tan 3,5-Dimethylphenol SA D a r k browii Fair 4 90 same amount of sodium oleate solution NA 2-Methyl-5-isopropylphenol 4 90 Light brown Good was used in every test, including blanks 3-Methyl-4-tert-butylphenol sA 4 90 Light brown Good to which no antioxidant was added. 3-Methyl-4,6-di-tert-butylphenol SXM 4 Good tcr €air 100 Brown SA 95 Good Light brown, Phenyl-2-naphthylamine was the control 10 dark spots antioxidant for every test or series of a Crude product of boiling point range 168' to 195' a t 5 mm. and ns* 1.5036, highly visoous liquid tests. All tests were made with an antiwhich slowly crystallized. tl-Octyl group is 1,1,3,3-tetramethylhutylresulting when alkylating agent oxidant concentration of 2% on the is diisobuLylene ( S I , SZ). weight of the dry polymer. b Crude resinous material of boiling point range 210' t o 245' at 3 mm., obtained by reaction of p cresol with oamphene-PhSOaH. It has been observed in this laboratory 0 Diisobutylene alkylate of 2,4-dimethylphenol ( 3 4 ) ; colorless liquid of b.p. 144-145.5' a t 10 m m . ; that the rate of deterioration and stiffenn 9 g 5 1.5106. ing of butadiene-acrylonitrile copolymer d Crude product, viscous oil of b.p. 142-147O a t 2.5 mm. and n2% 1.5366. is accelerated by the presence of iron in 6 Brown resin obtained from o-cresol a n d camphene-H2SOI; probably contains 2-methyl-4,6-diisoa manner similar to that already rebornylphenol. I From phenol a n d camphene-PhSOsH; highly viscous liquid of boiling point range 173O to 1 8 9 O a t ported for GR-S ( 1 ) ; and it was desired 10 mm. and ns4 1.5502 which slowly crystallized on standing a n d contained largely 2-isobornylphenol. to minimize the iron effect in deterioraFrom camphene a n d p-cresol. viscous red liquid of boiling point range 163O to 210' a t 3 mm.; tion tests. Hence, the latex samples probably contains 2-isobornyl-4-~ethylphenol. were coagulated with a solution of h Light yellow semiviscous oil of boiling point range 159O to 1 8 3 O at, 5 mm.; probably contains 2m~thyl-4-isobornylphenol. aluminum sulfate low in iron. I Solid resin from alkylation of phenol with camphene; probably contains some 2,4-diisobornylThe wet polymers were washed on a phenol. wash mill and dried in a circulating air oven a t 70" C. for 20 hours. Colors of the dry polymers ranged from white for those containing the best alkyl phenol antioxidants to light lavender-brown for the phenyl-2-naphphenols having different locations of substituents; and t,he effect thylamine controls. of structure upon antioxidant effectiveness was studied in order OVEK-AGING OF POLYMER. -4s a screening test for selection of to determine optimum configurations. stabilizers to be tested further, the polymer samples, about 0.2 Several alkyl phenols which were 2,4,6-trialkylphenols with inch thick, were oven-aged in a circulating air oven usually for 4 days at 90" C. for Butaprene NA and at 100' 6. !or Butaprene methyl in the para position seemed to have optimum structure; NXM. The samples were examined daily, by hanZj examination, these antioxidants were very effective replacements for phenyl-2for discoloration and evidences of deterioration, setup," hardnaphthylamine, particularly in butadiene-acrylonitrile copolyening, formation of soft spots, and surface lacquer formation. In mers, and were almost completely nondiscoloring. the case of fairly effective antioxidants, the only noticeable deterioration sign within a reasonable period of time was setup of the polymer, a partially cured condition caused by cross linking in the ANTIOXIDANTS FOR BUTAPRENE N polymer. The setup of the polymer invariably preceded more Testing Procedure. Antioxidant tests in butadiene-acryloprofound indications of deterioration such as weakening of the nitrile copolymer, the oil-resistant variety of butadiene-derived heat-cured specimen, hardening, and surface resinification. NA
SXM
Fair Fair
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INDUSTRIAL AND ENGINEERING CHEMISTRY
It was found inadvisable to carry out the oven-aging at temperatures much above 90” or 100’ C. because of the tendency a t higher temperatures for formation of a thin lacquerlike film which protected the interior of the sample against access of air and destroyed the validity of the test. On the other hand, the polymers were examined in cross section; a darker coloration on the surface than on the inside indicated incipient surface deterioration.
aging, were in excellent agreement with the hand-examination ratings of Table I, indicating that the latter values are sufficiently valid for the purpose of the test. INDUCTION PERIOD.The stabilities of autoxidizable materials which are protected by antioxidants frequently are expessed in terms of “induction periods.” The end of the induction period of a material is marked by an increase in rate of oxidation of the material. Although the induction phenomenon can be observed for natural rubber (34), it is less clearly defined for sgnthetic rubber, possibly because of the heterogeneous structure of synthetic rubber (80) and resultant variety of reactivities of various loci and because of the changes t h a t take place during aging in air which are inhibited b u t not entirely prevented by the antioxidant. However, some indication of induction periods in Butaprene NA polymer samples containing antioxidants was gained by noting the formation of spots, both on the surface of and within the sample. These appear during aging as dark-colored areas where extensive deterioration has taken place at a faster rate than in the main portion of the sample, comparable t o the increased rate of deterioration of natural rubber at the end of its induction period.
Table I lists a number of materials which were tested in Butaprene N polymers, along with aging conditions and the conditions and colors of the polymers after being heat-aged. The condition, or quality, was determined by hand examination; and the ratings given in Table I are based on comparisons with the phenyl-2-naphthylamine control made each day during the aging period. Controls were always rated “very good.”
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Very Good. Polymer in good condition after 4 days of ovenaging; somewhat setup, beginning second or third day of aging, but no other indications of deterioration. Good. Aged polymer in good condition, but inferior to control; setup beginning in first or second day of aging, and setup polymer possibly weakened somewhat by fourth day. Fair. Polymer sets up quickly and begins to harden or resinify before aging period is over. Poor. Polymer sets up even on drying, but does not resinify as quickly as unstabilized polymer. Very Poor. Polymer behaves as though no stabilizer were present, becoming set up during drying and quickly hardening and resinifying in the hot-air oven.
TABLE111. COMPARATIVE INDUCTION PERIODS FOR PHENOLIC ANTIOXIDANTS IN BUTAPRENE NA DURING OVEN-AGING,AS INDICATED BY SPOTFORNATION
BREAKDOWN TIMEON MILL. Because the quality ratings of Table I are based upon hand examinations, i t was desired to check some of the samples rated in this manner against some numerical value. Plasticity tests were found t o be unreliable for such partially cured polymers. A measure of the degree of aging was made by determining the time required t o reverse the cured condition by milling. Results are summarized in Table 11.
Time to Appearance of First
Antioxidant Present Phenyl-2-naphthylamine 2,6-Di-tert-butyl-4-methylphenol 2 6-Di-t t-octyl-4-methylphenol
2'4-Dimethyl-6-t t-octylphenol
Spots, Daw
..
..
Number of Spotsa after Oven-Aging a t 95O C. 4 8 10 None None None None None 16 14 45
None None None None 3
6 2~tt-Octyl-4-methylphenol 3 30 3-Methy1-4,6-di-tert-butylphenol 3 35 p-tert-Decylphenol 3 91 p-tt-Ootylphenol 2 135 96 p-tert-Butylphenol Per 180-gram sample of 0.2-inch thickness and 220 to 230 sq.
The cold (water-cooled) rolls (6 inches in diameter) were set a t a clearance of 0.005 inch. A 200-gram sheet of oven-aged polymer was folded twice and passed once between the rolls; it was then milled until the sheet on the roll contained no holes. The time in seconds, exclusive of the initial pass, was recorded. From the milling times there was devised the arbitrary quality scale of Table 11, with quality ratings ranging from A for control or control-equivalent, t o E for the blank. These ratings, which are a rough measure of resistance t o cross linking during the heat-
None None None hTone 22 3s Many Many om. area,
Several Butaprene N A samples, each with a n area of 225 sq. cm., were aged 10 days at 95 C. The experiment is summarized in Table 111. No spots formed on most of the polymers stabilized with those antioxidants which were rated very good in Tables I and 11; the other samples were in agreement TABLE11. BREAKDOWN TIMESO N MILLINQ OF OVEN-AQEDBUTAPRENE N with the ratings of the previous tests. POLYMERS GR-S polymers and vulcanizates conAging Milling Time, Rating taining certain antioxidants also have Conditions Seconds Comfrom given indications of induction periods, PBNA prtrative Polymer Polymer Time, Tzmz., Antioxidant Present Type days Test control Rating Aginga which are best indicated by oxygenNone (blank test) NXM 4 100 2105 30 E E absorption measurements (40). Sulfur NXM 4 100 75 23 D C V UL c A N I z ATE- AGING TESTS.The Heptylated diphenylamine NXM 4 100 30 23 B B 2,6-Di-tert-butyl-4-methylantioxidants listed in Table I which phenol NXM 4 100 20 23 A A 2,4-Dimethyl-6-tert-butylwere of sufficient interest for further 35 23 B A NXM 4 100 phenol examination were tested in carbon black 50 23 B B 2,4-Dimethyl-6-isobornylphenol NXM 4 100 2,4-Dimetbyl-6-tt-octylphenol NXM 4 100 14 12 A A stocks. p-Cresol 4 100 60 12 C
p-tt-Ootylphenot o-tert-Amylphenol 3-Methyl-4 6-di-tert-butylphenol p-tert-Butyiphenol p-tt-Octylphenol p-tert-Deaylphenol 2 6-Di-tert-but 1 4 methylphenol 3LMethyl-4 6-d%ert-but,ylphenol 2 6-Di-tt-oo6yl-4-methylphenol 2~tt-Octyl-4-methslphenol 2,4-Dimoth~ll-8-tt-octy3ghanol
NXM NXM NXM NA NA NA NA NA NA NA NA
4
4
4
1 0 lo 10 10 10 10 10 10
100 100 100 95 95 95 95 95 95 95 95
O
75
12
28 28 26 20 15 14 16 15
16
68 80
12 12 16
16
16 14 14 14 14
D C D B B B A B A A A
8-
-
C BC CC €3 A B A A
From drts of Table I on hand examination of aged pol mere; very good = A, good B, eta. Milling atoDDed et thin goint; aged polymer had hsrdIaned too much for gelling t o be reversible by milling. Q
Larger batches of the experimental p o l y m e r s w e r e prepared and compounded in stocks of the formula: polymer, 100.0 parts by weight; stearic acid, 3.5; zinc oxide, 5.0; E P C carbon black, 40.0; sulfur, 2.0; and N-cycle hexyl-2-benzothiazolesulfenamide, 1,3. The stocks were cured 60 and 80 minutes at 274“ F. Vulcanizate strips were oven-aged 4 days a t 100 c. and bomb-aged in air at 127” C. and 60 pounds per square inch for 10 hours for
INDUSTRIAL AND ENGINEERING CHEMISTRY
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1
I
-
-4-
n
a
i
n
z $33 0
I
I-
n-
22 a
-
I-
u)
1
UP AP UNAGED
UP AP OVEN-AGED
UP AP BOMB-AGED
UNAGED
Figure 1. Physical Properties of Butaprene NXM Vulcanizates Stabilized with 2,6-Ditert-butyl-4-me thylphenol
0
AT BREAK AT 300% ELONGATION
I
T
i
DIscoLoRanoN TESTSox WHITE VULCANIZATES. Polymers stabilized with the better of the alkyl phenol antioxidants examined in the experiment of Table I were remarkably resistant to disc\)loration. These nondiscoloring properties were found to cariy over to white vulcanized stocks. Representative alkyl phenolstabilized polymers were mixed in the formula: polymer, 100.0 parts by weight; cumar m.h. 2 1 / 2 resin, 7.5; sulfur, 2.25; magnesium oxide, 5.0; zinc oxide, 85.0; neutral clay, 20 0; titanium dioxide, 20.0; benzothiaxolyl disulfide, 1.1; and zinc diethyldithiocarbamate, 0.2. The stock was cured 40 minutes a t 280' F. Evposure tests of the cured stocks in the Fadeometer for 10 hours a t 126' F. or exposed to a G.E. sun lamp for 16 hours a t a distance of 7 inches turned the phenyl-2naphthylamine controls brown; but the stocks containing phenolic stabilizers discolored only slightly (Table IV). These nondiscoloring stocks, exposed a t 15% elongation to Florida sunlight for 2 months, February and March, were not discolored.
TABLEIT'. UNAGEO
OVEN-AGED
BOMB-AGED
Polymer and vulcanizate oven-aged 4 days a t 100° C Vulcanizate bomb-aged 15 hours a t 127' C in air a t 60 Ib per square inch U P Vulcanizate from unaged polymer .4P Vulcanizate from oven-aged polymer T Test stock C P B N A control stock
Polymer and vulcanizate oven-aged 4 days a t l0D0 @. Vulcanizate bomb-aged 15 hours a t 127' C. in air a t 60 Ib. per square inrh UP. Vulcanizate from unaged polymer A P . Vulcanisate from oven-aged polymer T. Test stock C. PBNA control stock
I
OVEN-AGED
Figure 2. Physical Properties of Butaprene NXM Vulcanizates Stabilized with 2,4-Dirnethyl-6-isoborn? lphenol
BOMB-AGED
Figure 3. Physical Properties of Bu taprene NXM Vulcanizates Stabilized with 2,P-Dimethyl-6- tt-octylphenol Polymer and vulcanizate oven-aged 4 days a t 100' C . Vulcanizate bomb-aged 15 hours a t 127' C. in air a t 60 lb. per square inch UP. Vulcanizate from unaged polymer A P . Vulcanisate from oven-aged polymer T. Test stock C. PBNA control stock
Butaprene NA stocks, and 15 hours for Butaprene NXM stocks. Similar stocks were prepared from the same polymers which had been subjected to the oven-aging described above; the vulcanisates from aged polymers were likewise oven- and bomb-aged. The physical properties of some of the more interesting antioxidants tested, particularly the more effective of the 2,4,6trialkylphenols, are presented graphically in Figures 1 t o 12. For graphing, the original data have been averaged with respect to cure; the stress at break for 60- and 80-minute cures and the values for stress at 300% elongation are averaged. Stress at 300% elongation is not indicated in the bar graphs for strips that broke at lower elongation.
DISCOLORATION OF WHITE BUTAPRENEKXM T'ULCANIZATESIN LIGHT-EXPOSURE TESTS Color after Exposure in Fadeometer 10 Hours at a t 1250 C.
Color after Exposure to G.E. Sun Lamp 16 Hours a t 7 Inches
Antioxidant Present 2,4-Dimethyl-6-tert-butylphenol, 2,4-dimethyl-6-tt-octylphenol, 2,4-dimethyl-6-isobornylphenol, or 2-tert-butvl-4-methvluhenol Off-white Off-white o-tert-Amylphcnol, p-tt-bcblphenol, 2 6-di-tert-buty1-4-methylphenolI 0; 2,6-di-tt-octyl-4-methylphenol Very light cream Off-white p-Cresol or 3-methyl-4,6-di-tertbutylphenol Light cream Off-white NoneQ Light cream Very light crranl Heptylated diphenylamine Light t a n Gray-brown Light t a n Phenyl-2-naphthylamine Brown a Polymer from which white stock was prepared was stabilized by initial incorporation of sulfur before drying fresh polymer.
Discussion of Results. Every alkyl phenol tested provided a t least some degree of antioxidant protection to butadiene-acrylonitrile type polymers and vulcanizates. However, wide differences in antioxidant activity were encountered according to the number and positions of the alkyl groups. Several groups of alkyl phenols gave only fair to mediocre protection against oven aging of polymer. Mono- and dialkyl
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C -
ii
K
c a i-
UP AP UNAGED
UP AP OVEN-AGED
UP AP BOMB-AGED
Figure 4. Physical Properties of Butaprene NXM Vulcanizates Stabilized with p - t t Octylphenol Polymer and vulcanizate oven-aged 4 days a t 100' C. Vulcanizate bomb-aged 15 hours at 127O C. in air at 60 l b per square inch UP. Vulcanizate from unaged polymer A P . Vulcaniaate from oven-aged polymer T. Test stock C. P B N A control stock
phenols containing large alkyl substituents such as tert-alkyl, cyclohexyl, and isobornyl in the para position tended to fall at the lower end of the scale of antioxidant activity. o-tert-Amylphenol also was relatively ineffective. Presence of a meta substituent did not seem to give any improvement in antioxidant activity in the few examples tested. Good antiqxidants were the 2,4-dialkyl phenols in which the para substituent was methyl and the ortho substituent tertbutyl, tt-octyl, and isobornyl. Still better were the various 2,4,6-trialkyl phenols, particularly those with methyl as the para substituent; they included 2 , 4 dimethylphenols which had tert-butyl, tt-octyl, and isobornyl in the 6 position and the 4methylphenols which were 2,6-disubstituted with tert-butyl, ttoctyl, and isobornyl (resinous product). The nondiscoloring 2,4,6-trialkyl phenol antioxidants not only were effective stabilizers for Butaprene N polymers, but also imparted to the vulcanizates (Figures 1 to 3, 6 t o 8, 10, and 11) age resistance approaching that obtained with the control antioxidant, phenyl-2-naphthylamine. In the latter respeat the trialkyl phenols were superior t o the structurally similar 2 , P dialkyl phenols, which were excellent for polymer aging but were surpassed by the 2,4,6trialkyl phenols in vulcanizate aging tests. Thus 2-tf-octyl-4-methylphenol was an excellent nondiscoloring antioxidant for polymers along with the structurally comparable 2,4dimethyl-6-tf-octylphenol and 2,6-di-ttoctyl-4-methylphenol, but the protection imparted to vulcanizates by the trialkylphenols (Figures 8 and 11) was better than that attained with the dialkyl phenol (Figure 12). Best of the nondiscoloring antioxidants tested was 2,6-di-tertbutyl-4-methylphenol, which has been found t o be satisfactory, from standpoints of both nondiscoloring nature and of antioxidant effectiveness, in the various Butaprene N polymers. It is effective both in polymers and cured stocks (Figures 1 and 6). Test and control stocks were practically equivalent in quality, indicating the trialkyl phenol to be a satisfactory phenyl-2naphthylamine replacement, so far as these test conditions are concerned, in cured stock as well as in uncured polymer. The 2,4dimethyl-6isobornyl- (Figures 2 and 7), and 2,4dimethyl-6-tf-octylphenols (Figures 3 and 8), were excellent nondiscoloring antioxidants for Butaprene N. 2,4DimethylGisobornylphenol was one of the best antioxidants found
UP AP UNAGEO
Ii UP
OVEN-AGED
AP
BOMB-AGED
Figure 5. Physical Properties of Butaprene NXM Vulcanizates Containing No Stabilizer (Blank Test) Polymer a n d vulcanizate oven-aged 4 days a t looo C. Vulcaniaate bomb-aged 15 hours a t 127O C. in air a t GO lb. per square inoh U P . Vulcaniaate from unaged polymer A P . Vulcanizate from oven-aged polymer T. Test stock C. P B N A control stock
AT BREAK
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-
300%ELONQATlON t
-T
a3-
d 0 z
323 0
I c v)-
Za '
-
tl
0UNAGED
OVEN-AGED
BOMB-AGED
Figure 6. Physical Properties of Butaprene NA Vulcanizates Stabilized with 2,6-Ditert-butyl-4-methylphenol Polymer oven-aged 4 days a t 90° C. Vulcaniaate ovenaged 4 days at looo C. Vulcanizate bomb-aged 10 hours a t 127' C.in air a t GO lb. per square inch U P . Vulcaniaate from unaged polymer A P . Vulcanizate from oven-aged polymer T. Test stock C. PBNA control stock
for Butaprene NA, whereas 2,6-di-tert-butyI-4-methylphenol was slightly better for Butaprene NXM and was more uniformly equivalent t o phenyl-2-naphthylamine. Generally speaking, the evaluations of carbon-black cured stocks (Figures 1 to 12) were in agreement with the evaluations (Tables I and 11) of the polymers. D a t a on p-it-octylphenol are given in Figures 4 and 9 as an example of an alkyl phenol of only moderate antioxidant activity. Figure 5 shows properties of cured stocks prepared from Butaprene NXM polymer which contained no added stabilizer whatsoever. The greatly lowered quality, compared with the
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BREAK
AT 3 0 0 %
ELONGATION
i UP AP UNAGED
UP AP OVEN-AGED
UP AP BOMB-AGED
Figure 7. Physical Properties of Butaprene NA Vulcanizates Stabilized with 2,4-Dimethyl-6-isobornylphenol Polymer oven-aged 4 days a t 90’ C. Vulcanizate ovenaged 4 days a t looo, C. Vulcanizate bomb-aged 10 hours a t 1 2 7 O C in air a t 60 Ib. per square inch U P . Vulcanizate from unaged polymer A P . Vulcsnizate from oven-aged polymer T. Test stock C P R N A control stock
control containing phenyI-2-naphthylamiiie, is indicated by stiffness-Le., moduli more than double the moduli of controlsand greatly lowered tensile strengths. As a group, the alkyl phenols are almost completely nondiscoloring in polymers and vulcanizates. Discoloration which took place during heat-aging of polymers (Table I) seemed due partly t o deteriorative changes in the polymers; thus polymers which contained the most effective antioxidants remained light in color during aging tests. The use of any of several of the antioxidants listed in Table I, B, enabled preparation of polymers which still were white after being oven-dried. White stocks containing the alkyl phenol stabilizers remained white or nearly white in light-exposure tests (Table IV). Among the test stocks containing phenols, those with p-cresol and 3methyl-4,6-di-te~t-butylphenol showed the greatest discoloration
tendency, though all stocks remained white when the exposure test was accompanied by weathering. In addition t o their nondiscoloring properties, the effective trialkyl phenol antioxidants have other advantageous characteristics. .41kyl phenols similar t o them have been described as being relatively nontoxic (37). The 2,4,6-trialkyl phenols of the types tested, being “hindered” phenols ( C I ) , are insoluble in aqueous alkali and therefore are not removed if the polymer is subjected t o a n alkali wash during plant processing. The alkali insolubility of the hindered phenols seems to be due to the shielding of the hydroxyl group by bulky ortho substituents, causing a decrease in the degree of phenolate ion formation in alkaline solutions (6). ANTIOXIDANTS FOR GR-S
The butadiene-styrene copolymers, exemplified by GR-SI are generally more resistant to stiffening during aging than the corresponding butadiene-acrylonitrile or Butaprene r\’ type polymers. This difference is in evidence in both polymer and vulcanizate aging. Antioxidants lyhich are active in Butaprene N are often active in GR-S, but the order of activity may vary considerably. The methods of evaluation are somewhat different in each case. Testing Procedure. I n many respects, the testing procedure used for evaluation of the antioxidants in GR-S resembled that used for Butaprene N. The GR-S polymer samples containing the desired antioxidants were prepared from uninhibited GR-S latex obtained from a plant polymerizer just before the addition of stopping agent. The antioxidant was added t o this latex, usually as a sodium oleate dispersion, and the coagulation was effected by the use of low-iron aluminum sulfate. The samples were dried at 75 C. in a forced circulation oven. Aging of the polymer samples was carried out in a forced circulation oven at a temperature of 110 or 120’ C. A blank (no antioxidant) and a phenyl-2-naphthylamine control were included in each set of samples. O
During oven-aging, a GR-S polymer passes through various stages of stiffening or cure and then starts to resinify. I n some cases there is an initial softening before this stiffening takes plaoe
0AT
0AT BREAK
Vol. 42, No. 4
BREAK
AT 300% ELONGATION
A T 300% ELONGATION
n C
UP AP UNAGED
UP AP OVEN-AGED
UP AP BOMB-AGED
UP AP UNAGED
UP AP OVEN-AGED
i
UP AP BOMB-AGED
Figure 8. Physical Properties of Butaprene NA Vulcanizates Stabilized with 2,4-Dimethyl-6- tt-octylphenol
Figure 9. Physical Properties of Butaprene NA Vulcanizates Stabilized with p - t t Qctylphenol
Polymer oven-aged 10 days a t 9 5 O C . Vulcaniaate ovenaged 4 days a t loo0 C. Vulcanizate bomb-aged 10 hours a t 127O C . in air a t 60 lb. per square inch UP. Vulcanieate f r o m unaged polymer A P . Vulcanizate from oven-aged polymer T. Test stock C. P B N A control stock
Polymer oven-aged 10 days a t 9 5 O C. Vulcanizates ovenaged 4 days a t 100° C . Vulcanizate bomb-aged 10 hours a t 1 2 7 O C. i n air a t BO lb. per square inch UP. Vulcanizate from unsged polymer A P . Vulcanizate from oven-aged polymer T. Test stock C. PBNA control stock
April 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
681
give a comparison between a given antioxidant and a control stock containing no antioxidant. A g i q Because natural weathering requires Conditions ~ i T:”~,, ~ ~ Properties , of Aged Polymers such a long period of time t o produce Material Tested as Antioxidanta days Qualityb Color significant results, it was of definite A. Miscellaneous Blank 2 110 Very poor Medium brown interest to compare surface checking Phenyl-2-naphthylamine 2 110 Very good Dark brown obtained in outdoor weathering with Heptylated diphenylamine 4 110 Good Dark brown that obtained on prolonged exposure in Triphenyl phosphite 2 110 Poor Yellow-t an Sulfur (1%) 2 110 Poor Brown the Weatherometer. For this compariB 2,4 6-Trialkyl phenols i,6-Di-tert-butyl-4-methylphenol 2 110 Good Light brown son, the outdoor weathering was ob4 110 Poor Light brown 2 6-Di-tt-octyl-4-methylphenol 2 110 Good Medium brown tained as described in the previous Light brown 2’4-Dimethyl-6-tertbutylphenol 2 110 Poor p a r a g r a p h a n d similar test strips, 2’4-Dimethyl-6-tt-octylphenol 2 110 Very good Yellow-tan 2’4-Dimethyl-6-isobornylphenol 2 110 Poor Medium brown stretched t o the same extent on a 2~4-Dimpthyl-6(o,~-dimethylbenayl)ph~nol 2 110 Good Medium brown 2 4 6-Tri-tert-butylphenol 2 110 Fair Light gray-brown wooden rack, were exposed for 96 hours 2~dethy1-4,B-di-tert-butylphenol 2 110 Fair Medium brown in the Weatherometer. e3. Miscellaneous alkyl phenols, substituted phenols p-tert-Amylphenol 2 110 Poor Medium brown Discussion of Results. The polymer p-ttOctylpheno1 2 110 Poor Medium brown aging results given in Table V show 3-Pentadecylphenol 2 110 Poor Yellow 1,l-Bis-(p-hydroxyphenyl)-cyclohexane 2 110 Fair to poor Medium Medium brown brown that many of the alkyl phenols gave 2-tert-Butyl-4-phenylphenol 2 110 Good 2,4-Di-tert-amyl henol 2 110 Good Yellow-tan good protection to the GR-S polymer. 2-ttOctyl-4-met~ylphenol 2 110 Good Medium brown Camphene-p-cresol resinC 2 110 Fair Medium brown Two of the 2,4,6-trialkyl phenols, 2,63-Methyl-4,6-di-tertbut~l~henol 2 110 Poor Medium brown di-tert-butyl-4-methylphenol a n d 2,4of 2% (based on dry polymer) used except where indicated. dimethyl-6tl-octylphenol, gave protec* Concentration Quality in com arison with PBNA control rated a s “very good.” Crude 2,6-diiso~orqyl-4-methylphenol, solid brown resin of boiling point range 210° to 245‘ a t 3 tion to the polymer during 2 days of mm., obtained by reacting p-cresol with camphene. aging a t 110’ C. which approached that given by phenyl-2-naphthylamine. Normal and aged tensile properties and when this occurs, the subsequent effect in a relatively thick of the white test stock vulcanizates showed only small differences between the antioxidants. Hence, although these data sample is often confined t o the surface. These stages of deteriorawere obtained for the compounds tested in Table V, only a tion can be detected readily by hand examination. The various antioxidants were rated according to the hand test results, relatively small amount of representative data is presented in Table VI. The tensile properties of the white test stock tend t o using phenyl-2-naphthylamine as the standard for comparison (Table V). be somewhat erratic. Both the Fadeometer and G.E. sun lamp exposure tests showed Because the alkyl phenols were principally of interest for nonthe alkyl phenols to be resistant t o discoloration (Table VII). discoloring purposes, vulcanizate evaluation was carried out in a They are vastly superior in this respect to phenyl-2-naphthylwhite stock, the formula of which is given in Table VI, Normal and aged tensile properties were obtained and representative data for this stock containing various antioxidants AND REPRESENTATIVE TENSILE TABLEVI. WHITE GR-S TESTSTOCKFORMULA appear in Table VI. DATAFOR THISSTOCK To determine the resistance of a White Stock Formula given antioxidant to discoloration in the Polymer 100.0 white test stock, both G.E. S-1 sun Cumar m.h. 21/r 10.0 Wax 2.0 lamp and Fadeometer exposure tests Magnesium oxide 8.0 0 Zinc oxide 100 were employed. Usually, small squares Ultramarine blue 0.1 or disks cut from 0.075 gage slabs were Titanium dioxide 30.0 Sulfur 4.0 exposed. Three cures of each stock Accelerator 11to2 0 were evaluated and the color after exElongation Antioxidant 200% Modulus Tensile posure, recorded in Table VII, corre30 5.0 70 30 50 70 30 50 70 sponds t o the average color for these min. min. min. min. min. mm. min. min. min. CUREDAT 290° F. NORMAL TENSILE PROPERTIES cures. I n some cmes, equal discolor& tion was obtained for the three cures. 2,6-Di-tert-butyl-4-methylphenol 225 300 400 1650 1350 1125 640 510 450 In other cases, the low Cure Was slightly Heptylated diphenylamine 225 275 375 1400 1175 975 580 470 430 more discolored and the high cure was AFTEROVEN-AQINQ 4 DAYSAT 212’ F. 2,6-Di-tert-butyl-4-methylslightly less discolored than the interphenol 600 700 800 1575 1425 1275 460 390 370 Heptylated diphenylamine 550 625 650 1625 1225 1025 455 380 360 mediate cure. F o r o u t d o o r weathering, tapered 300% Modulus Tensile Elongation
TABLE V. SUBSTITUTED PHENOLS AS ANTIOXIDANTS FOR GR-s POLYMERS DURING OVEN-AGING
# ‘
C
dumbbell strips 6 inches in length were out from 0.100 gage slabs given a n intermediate cure. These strips were tacked onto racks SO t h a t a 12.5% over-a11 was Obtained. They were examined €or discoloration and surface checking after various periods of weathering in Florida, California, or Akron. The results relating t o surface checking presented in Table VI11
30 60 9.0 30 60 90 30 min. min. min. min. min. mmn. min. CUREDAT 300° F. NORMAL TENSILBPROPERTIES
2,4-Dimethyl-6-tt-octylphenol 125 150 150 875 2,4-Dimethy1-6-tert-butylphenol 100 150 175 900 2,4-Di-tert-amylphenol 125 175 250 700 AFTEROVEN-AQINQ 4 DAYSAT 214-Dimethyl-6-tt-octylphenol 2,4-Dimethyl-6-tert-butylphenol 2,4-Di-tert-amylphenol
525
60 min.
SO
min
700
730
560
570
650 525 725 550 212O F.
740 680
580 550
540 460
250
300
300
600
525
650
480
400
420
200 250
250 400
300 500
700 550
600 576
550 675
510 470
430 360
390 340
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
682
TABLEVII.
Cream Dark tan Brown Cream
Miscellaneous alkyl phenols 3-Methyl-4,6-di-tert-butylphenol Light t a n 2,4-Di-tert-amylphenol Dark cream 2-tert-Butyl-4-phenylphenol N o change
4
Cream Light cream Light cream Very light cream Cream Cream Cream Cream Dark cream up AP UNAGED
UP AP OVEN-AGED
UP AP BOMB-AGED
Figure 11. Physical Properties of Butaprene NA Vulcanizates Stabilized with 2,6Di- tt-octylphenol-4-meth~lplaenol
AT BREAK AT 3 0 0 % ELONGATION
m a'
A T 3 0 0 % ELONOATION
4 I_-
Light cream Light brown Brown Light cream
B. 2.4.6-Trialkvlnhenols 2 ~ 6 - D i ~ ~ ~ ~ ~ ~ ~ ~ U t ~ - 4 - m eLight t h y l pcream henol 2,4-Dimethyl-6-teit-butylphenol Cream 2,4-Dimethyl-6-tt-octylphenol Cream 2,4-Dimethyl-6-isobornylphenol Cream 2-Methyl-4.6-di-tert-butylphenol Light tan 2,4,6-tri-tert-butylphenol Liiht tan
a
AT BREAK
G.E. 8-1 Sun Lamp, 16 Hours ai. 7 Inches
Fadeometer, 10 Hours a t 125O F.
Antioxidant A . Miscellaneous S o antioxidant Heptylated diphenylamne Phenyl-2-na hthylamne Triphenyl pgosphite
C
u
ARTIFICIAL EXPOSURE OF WHITE GR-S STOCK CONTAINING VARIOUS ANTIOXIDANTS
Val. 42, Ne. 41.
Polymer oven-aged 10 days a t 9 5 O C . Vulcaniza.te ovenaged 4 days a t 100' C. Vulcanizate bomb-aged 10 hours a t 127O C. in air a t 60 Ib. per square inch UP. Vulcaniaate from unaged polymer A P . Vulcanieate from oven-aged polymer T. Test stock C . PBNA rontrol stock
3
0
z a m 2
0 I
c2
The resistance of the white test stock to weather checking was noticeably improved by certain highly alkylated phenols. The best resulte mere obtained using 2,6-di-tert-butyl-4-methylphenol, 2,4-dimethyl-B-lert-butylphenol, and 2,Cdimethyl-tiIt-octylphenol. Some acbivity was shown by 2,4-dimethyI-S isobornylphenol. No improvement in checking resistance w m exhibited by 2,4di-tert-amylphenol or by 2-tert-buty1-4-phenylphenol. Triphenyl phosphite caused a slight increase in the amount of checking produced compared to a stw,k containing no antioxidant. I t was found that the Weatherometer does not give surface checking results which can he correlated with the checking ob-
m ma W $ 1
UP AP UNAGED
UP AP OVEN-AGED
UP AP BOMB-AGED
Figure 10. Physical Properties of Butaprene NA Vulcanizates Stabilized with 2,6Diisobornyl-4-methylphenol Polymer oven-aged 4 days a t 90' C . Vulcaniaate ovenaged 4 days a t 100' C. Vulcanieate bomb-aged 10 hours a t 127' C. in air a t 60 lb. per square inch C P . Vulcanieate from unaged polymer A P. Vulcanizate from oven-aged polymer T Test stock C. PBNA control stjock
amine and also much better than heptylated diphenylamine. Most of the highly alkylated p h e n o l s e v a l u a t e d caused no more discoloration in a white stock than was obtained in a similar stock that contained no antioxidant. Some of the small differences observed in the discoloration caused by various alkyl phenol antioxidants can be att,ributed to slight variations in the test procedure. There were some variations in the discoloration of the contro from time t o time The outdoor weathering tests summarized in Table VI11 check the artificial exposure tests in showing the 2,4,&trialky# phenols to be excellent from the standpoint of resistance to discoloration. As in the artificial exposure tests, these alkyl phenols were c o n s i d e r a b l y b e t t e r than phenyl-2naphthylamine and superior to heptylated diphenylamine.
TABLEVIII.
OUTDOORWEATHERING TESTSON i T 7 a 1 ~ OR-S ~ STQCKCONTAINING TTA4RIOUsh'TIOXIDANTS
Weathering
Antioxidant hfiscellaneous Kone h-one hTone Heptylated diphenylamine Phenyl-2-na hthylamine Triphenyl pfosphitn
Conditions _______ Months 2 4
4 4.5 2
4
4 4.5
Conditions of Strips after Weathering Cherkinga Color
Place
Florida Akron 4kronc Ca1if.d Florida Akron Akron Calif.
*
...~.....
Very light creitn:
,
White Light cream Tan
.
.. ., ... ....
...._n...
No improvement Large improvement Slightly increased checking
Dark gray Grayish cream
2,4,6-Trialkyl phenols 2,6-Di-tert-butyl-4-methyl2 Florida Moderate improveVery light cream phenol 4 Akron ment 2,6-Di-tert-butyl-4-methyl4.5 Calif. Moderate improveLight oreair, phenol ment 2,4-Dimethyl-6-tt-octyl4.5 Calif. Moderate improveLight cream phenol ment 2,4-Dimethyl-6-tert-butyl4.5 Calif. Moderate improveLight cream phenol ment 2,4-Dimethyl-6-isobornyl4,5 Calif. Blight improvement Light cream phenol C. Miscellaneous alkyl phenols 2,4-Di-tert-amylphenol 4 Akron No improvement Grayish cream 2-tert-Butyl-4-phenylphenol 4 Akron No improvement Light cream a Relative to control Containing no antioxidant. Exposed from February to April in Florida and then from M a y to September in Akron. C Exposed from M a y to September in Akron. d Exposed from January to M a y in California. 8.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1950
MECHANISM O F ANTIOXIDANT ACTION
tained m a result of outdoor weathering. For example, the
%his respect.
0AT
tirely consistent. However, comparisons of the results in Tables I and I X indicate that in every case when methyl or hydrogen in the para position was replaced with tert-alkyl antioxidant activity was lowered.
BREAK
IAT 300%
683
ELONOATIOH
4, T
l
TABLE IX. ALKOXYAND HYDROXYPHENOLS AS ANTIOXIDANTS FOR BUTAPRENE NA POLYMER DURING & D A Y OVEN-AGINGAT 90" c. Antioxidant Phenyl-2-naphthylamine (oontrol) Catechol Hydroquinone 2-Methoxyphenol 2-Methoxy-4-methylphenol 2-Methoxy-4.6-tert-butylphenol 4-Methoxy-2,6-di-tert-butylphenol 4-Ethoxyphenol 4-Ethoxy-2,6-di-tert-butylphenol 4-Benzyloxyphenol
UP AP UNAGED
UP AP OVEN-AGED
Very good Very good Fair t o good Good Good Fair Very good Good Fair Very good
Brown Cream Tan Light brown
UP AP BOMB-AGED
Figure 12. Physical Properties of Butaprene NA Vulcanizates Stabilized with 2-tt-Octyl-4-methylphenol
25
Polymer oven-aged 10 days a t 95' C. Vulcanizate oven-aged 4 days a t 100' C. Vulcanizate bomb-aged 10 hours at 127' C. in air a t 60 lb. per square inch
UP. Vulcanizate from unaged polymer
AP. 2'. C.
Color of Aged Polymer
Rating
Vuloanizate from oven-aged polymer Test stock PBNA control stock
-
NO ANTIOXIDANT
-
4 METHYLPHENOL
2,6-M-TERT.-BUTYL-
20
-
0'
b N
a 1s-
!! Oxygen Absorption The oxygen absorption of QR-S polymers and vulcanizates has been studied lo? a t the Case Institute of Technology by J. R. Shelton r' 5 and Hugh Winn under the Firestone fellowship (39). The authors are indebted t o them for the oxygenabsorption results presented in Figures 13 and 14. Figure 13 shows that the raw GR-S polymer containing no antioxidant absorbs oxygen at a very rapid rate. The addition of triphenyl phosphite did not decrease this oxygen-absorption rate. The alkylphenol, 2,6-di-tert-butyl-4-methylphenol,definitely improved the resistance of the polymer to the absorption of oxygen; and phenyl-2-naphthylamine was still more effective in this respect. These results are in good agreement with ovenaging in that they indicate the same order of activity for the antioxidants tested. These polymers were coagulated with aluminum sulfate which completely inactivates triphenyl phosphite as a stabilizer. Triphenyl phosphite does produce a st& bilized salt-acid coagulated GR-S polymer. Oxygen-absorption studies likewise were carried out on white vulcanized stocks which corresponded to three of the polymers of Figure 13; the stocks had been mixed by the formula given in Table VI and cured for 40 minutes at 298" F. It is apparent from the results (Figure 14) that there was only a small difference from the standpoint of oxygen absorption between the two stocks that did not contain phenyl-2-naphthylamine. The stock containing phenyl-2-naphthylamine showed less absorption. These results are in agreement with the results of aging tests upon the same stocks (Table VIII). It is concluded that the white stock vulcanizates containing different antioxidants show a smaller change in aging properties than do the polymers.
PHENYL-/J-NAFHTHYLAMINE
I
I
Ly
IOO'C. 7 6 0 M M . CURED 40' AT 298OF. 0 TRIPHENYL PHOSPHITE
A 2,6-DI-TERT.-BUM-4-
6
1
I
20
40
I
I
60
80
I I00
P
I I20
I 140
I I 160 180
I I I I -I 2 W 220 240 280 280
HOURS
Figure 14.
Oxygen Absorption of White GR-S Stocks
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
684
Presence of ortho substitution did not seem to be necessary for good antioxidant activity. Although every effective alkyl phenol had at least one bulky ortho substituent, p-beneyloxyphenol, which has no ortho substitution, rated well in the preliminary test of Table IX. Among various optimum combinations of substituents of phenolic antioxidants for Butaprene Pi polymer (disregarding color) are the following, selected from the polymer-aging data of Tables I and IX. 2-Substituent nn
H
tert-Butyl tert-Alkvl Isobornkl
4-Substituent
__OCHz phenyl FT
OCHa
Methvl Met his1
__H
6-Substituent
tl
tert-Butyl H. methvl. or tert-alkyl H, rnethG1,’or isoborn Yl
Although the best alkyl phenol antioxidants studied in this investigation happen t o be “hindered” phenols-Le., they contain one or t n o bulky ortho substituents which tend to cover up the hydroxyl group and render it unreactive chemically-the above results with several alkoxy phenols indicate that i t is not necessary for the hydroxyl group of a monohydric phenol to be hindered for the phenol t o have optimum antioxidant properties. Thus, for monohydric phenols, in general, hindrance of the hydroxyl group is neither necessary for, nor prevents optimum antioxidant effectiveness. The superior antioxidant properties of the best trialkyl phenol antioxidants must be explained by some other method. Another suggestion has been advanced, that ortho-hydrogen plays an important part in some antioxidant processes (16). I n addition, it has been reported that the first steps in the oxidation of phenyl-%naphthylamine by potassium permanganate in acetone or by lead dioxide in benzene involve the removal of ortho-hydrogen atoms (38),a process which may or may not be an integral part of the antioxidation mechanism. However, ortho-hydrogen obviously is not required for the antioxidant processes involving 2,4,6-trialkylphenols, and, in fact, it may be concluded that an alkylated phenolic antioxidant containing a free ortho-hydrogen is actually less effective than a comparable one containing no unsubstituted ortho positions. Hence, the chemical activity of ortho-hydrogen atoms in aromatic antioxidants may have been misleading, and the corresponding oxidized forms-e.g., from phenyl-2-naphthylamine-probably are not involved in the antioxidation mechanism. Phenolic “stopping agents” or inhibitors for vinyl polymerizations are thought t o function as scavengers for the carbon radicals, R-,which are the chain carriers in these polymerizations.
H’ Phenolic antioxidants might act in a similar manner, because both R- and R 0 2 *radicals appear in the chain propagation steps during the autoxidation of hydrocarbons. However, the relationship of structure to activity as stopping agents among the alkylated phenols is the reverse of their relative activities as antioxidants. For example, the 2,4,6-trialkylation of phenol appears to enhance antioxidant activity in synthetic rubbers of the GR-S and Butaprene N types, but the 2,4,6-trialkyl phenols are almost completely ineffective as stopping agents for the GR-S butadiene-styrene emulsion polymerization system ($1). These results might point to the hypothesis that structural factors which aid ionization of the phenol molecule contribute t o the stopping activity, whereas alkylation, which inhibits ionization (6), contributes t o antioxidant activity. However, the ionization hypothesis falls down, as does any correlation with “hindrance” of
Vol. 42, No. 4
, the phenol, when groups other than alkyl are considered. 4 better suggestion may be that stopping activity involves the reaction of R. radicals with active positions on aromatic nuclei (35, $6) as well as with active hydrogen atoms; hence, covering active nuclear positions with inactive groups, such as alkyl groups, reduces the possible Kays by which the molecule of stopping agent can react with the chain carriers of the polymerization reaction. There appears to be no correlation whatsoever between stopping activity and the oxidation-reduction potentials of the stopping agents ($1). A number of attempts have been made to correlate antioxidant activity with oxidation-reduction potentials of hydroquinones and with critical oxidation potentials of phenols and aromatic amines (4,10, 11, E?). I n general, these correlations have been fairly good in the range of critical ovidation potentials, E , = 0.6 to 1.0 volt. Bolland and ten Have obtained an approximate linear relationship between the normal oxidation-reduction potential, E,, of phenols and hydroquinones and the logarithms of their relative efficiencies as oxidation chain terminators in the peroxide-catalyzed oxidation of ethyl linoleate. The lower the oxidation-reduction potential, the greater was the efficiency, down to the point (about 0.6 volt) where direct oxidation of the antioxidant by oxygen became appreciable. However, there have been a number of evceptions and deviations in the various series of compounds for which E , and E, have been measured; attempts to use this measurement as a means of predicting antioxidant activity have failed, for a number of compounds of different structure types have been found which have oxidationreduction potentials in the correct range without offering the predicted degree of antioxidant activity. It has been suggested, both for phenols (6)and for phenyl-2naphthvlamine ( 7 ) , that the antioxidant serves to break the oxidation chain by reacting with RO,. radicals.
ROY
+ ArOH +ROzH + ArO.
The kinetic data for both phenols and hydroquinones in the peroxidic oxidation of ethyl linoleate (6)are conclusive. Alkyl groups in the hydroquinones tend to lower the oxidation-reduction potentials of the quinone-hydroquinone systems, leading t o more effective antioxidants in the ethyl linoleate system ( 4 ) The evidence from the authors’ antioxidant studies is consistmt with this mechanism. I n the synthetic rubber systems the introduction of an alkoxy or alkyl group into the phenolic nucleus, particularly in the 2 and 4 positions, increases the antioxidant activity and decreases the oxidation-reduction potential of the phenol, in fact bringing the oxidation-reduction potential down into the proper range for optimum antioxidant activity ( I O ) . Expressed in terms of structure, alkyl groups in the 2, 4,and 6 positions of a phenol probably allow the formation, with lower activation energy, of resonating semiquinone type radicals of greater stability ( 5 ) . Typical contributing structures may he represented as follou~s:
The greater effect of the methyl group relative t o any larger alkyl group in the 4 position may be ascribed to hyperconjugation (9). I n the aromatic p-phenylenediamine series, it has been shown (6, pp. 331-2) that o-alkyl groups sterically prevent the resonance stabilization, and hence, the formation of semiquinone-type nitrogen radicals, an effect that does not occur in the formation of the corresponding oxygen semiquinones. This difference in the
April 1950
INDUSTRIAL A N D ENGINEERING CHEMISTRY
stereochemistry of the formation of semiquinone-type nitrogen and oxygen radicals probably explains why para substitution by alkyl, alkoxy, and alkylamino groups is desirable for enhanced antioxidant activity i n both aromatic secondary amines and phenols, whereas ortho substitution is desirable in the phenol series, b u t is highly deleterious t o antioxidant activity in the aromatic secondary amine antioxidants. The formation of semiquinone radicals from hydroquinones has been shown experimentally b y means of potentiometric curves (16,28) and polarographic d a t a (88, 44). It is postulated t h a t this type of data, obtained on substituted phenols and phenolic compounds, may correlate with antioxidant activities. The formation of resonance-stabilized aryl oxygen or nitrogen (or sulfur, selenium, etc.) radicals may be considered t o be another condition, together with a n oxidation-reduction potential in the proper range, for the formation of an antioxidant of optimum activity in a given oxidizable polymer or material. The fate of the radicals formed from antioxidant molecules is uncertain. The semiquinones derived from hydroquinones may react further in an antioxidant manner t o form quinones, which are colored and probably are lost to further reaction. The semiquinone type radicaIs from alkoxy and alkyI phenols cannot react further i n this manner, and thus are advantageous from the standpoint of discoloration. These radicals generally do not have the power t o pull many hydrogen atoms from polymer chains or they automatically would become oxidation catalysts; however, they certainly may react with R. radicals, becoming tied up in the polymer structure. Semiquinone type radicals may exhibit antioxidant activity i n their own right. If the semiquinone radicals could acquire hydrogen atoms from some readily available source other than from polymer chains, then the antioxidant molecule would be reformed and ready t o work again; however, BolIand and ten Have ( 4 ) found no evidence of any such regenerative process in the ethyl linoleate-peroxidehydroquinone system. SUMMARY
*
Alkyl phenols were found to be antioxidants for butadienederived copolymers and vulcanizates. I n addition, the alkyl phenol antioxidants contributed no discoloration t o polymers or stocks containing them; in this respect they were unsurpassed by any other class of compounds that have been evaluated in this laboratory. A large variety of alkyl phenols were tested in order to determine optimum structural requirements for satisfactory nondiscoloring antioxidants. In effectiveness the materials ranged from only fair for p-tert-alkyl phenols up to very good for 2,4-dialkyl phenols and excellent for 2,4,6-trialkyl phenols having methyl as the para substituent. The better antioxidants for the Butaprene N type synthetic rubbers, such a~ 2,4dimethyl-6-isobornylphenol, 2,6-di-tertbutyl-4-methylpheno1, and 2,6-di-tt-octyl-4-methylphenol, were not only nondiscoloring, but approached phenyl-%naphthylamine in effectiveness in protecting polymers and vulcanizates against oxidative deterioration. , In raw GR-S, the degree of effectiveness of the alkyl phenols in protecting against polymer deterioration varied considerably. Some of the better compounds, such as 2,6-di-tert-butyl-4methylphenol and 2,4dimethyl-6-tt-octylphenol, gave polymer protection approaching that produced by phenyl-2-naphthylamine and, in the subsequent GR-S vulcanizates, such 2,4,6trialkyl phenols were nondiscoloring and afforded some protection against weather-checking. ACKNOWLEDGMENT
The authors wish to express their appreciation t o the Firestone Tire and Rubber Company for permission to publish this work. They also wish to thank J. R. Shelton and Hugh Winn for supplying the data on oxygen absorption, which were obtained under the
685
Fireatone fellowship a t the Case Institute of Technology. They wish t o acknowledge the valuable assistance of L. 0. Bentz of this laboratory in carrying out some of the laboratory work, and t o thank L. B. Wakefield and E. F. KIuchesky for helpful discussions of their work on stopping agents. LITERATURE CITED (1) Albert, H. E., Smith, G. E. P., Jr., and Gottschalk, G. W., IND. ENG.CHEM.,40, 482 (1948). (2) Anon., “Summary Report on Production and Performance of
German Synthetic Tires and Other Transportation Items,” Rubber Bureau, WPB, and ORR, Washington, D. C., Rept. CC-31, 1945; Rubber Age (N. Y . ) ,57, 568 (1945). (3) Beaver, D. J . , U. S. Patent 2,364,338 (Dec. 5, 1944). (4) Bolland, J. L., and ten Have, P., Trans. Faraday SOC.,43, 201
(1947) ; Faradau Soc. Discussion, €jo. 2, 252 (1947). (5) Branch, G. E. K., and Calvin, M., Theory of Organic Chem-
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(8) Crossley, R. H., and Cashion, C. G., Rubber Age ( N . Y.), 58, 197 (1945). (9) Deasy, C . L., Chem.Rev., 36, 145 (1945). (10) Doede, C. M., in “Proceedings of the Rubber Technology Conference,” ed. by Dawson and Scott, pp. 730-5, Cambridge, England, W. Heffer & Sons, 1938. (11) Elley, H. W., Trans. Electrochem. SOC., 69, 195 (1936). (12) Frolich, P. K., U. 5 . Patent 2,379,482 (July 3, 1945). (13) Fryling, C. F., Ibid., 2,360,864 (Oct. 24,1944). (14) Greenbank, G. R., and Wright, P. A,, “Correlation between
Structure and Electronic Properties of Organic Compounds and Their Antioxygenic Activity,” presented a t 110th Meeting of AMERICAN CHEMICAL SOCIETY, Chicago, 1946. (15) I. G. Farbenindustrie, A.-G., Belg. Patent 447,225 (1942). (16) I. G. Farbenindustrie, A.-G., Brit. Patent 350,563 (March 14, 1930). (17) Ibid., 463,194 (March 23, 1937).
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