I
FRANK R. M A Y 0 Stanford Research Institute, Menlo Park, Calif.
Some New ideas on Oxidation Development of improved surface coatings, of better aging tests for rubber and other polymers, of new tests for gasoline stability, and of long-lived antioxidants may come from a study of oxidation reactions
OXYGEN
reacts so rapidly with most free radicals (which have a free valence on carbon) that other reactions of these radicals are practically excluded when oxygen is present. Under such conditions, carbon-carbon bond formation by a free radical mechanism is largely prevented in vinyl polymerizations, in drying of oils, in cross linking of polymers, and in aging of rubber and gasoline. This article discusses reactions which can occur and how some of them should be studied further. Reaction of Oxygen with Free Radicals
Previous papers in this series (32-37, 39, 49) have considered the theoretical aspects of reactions of oxygen with unsaturated compounds. Some practical implications of these results will now be discussed. The very high rate of reaction of oxygen with free radicals will be considered first. In pure styrene a t 50’ C., each free radical in the polymerizing mixture adds about 1000 monomer units per second. O n a molar basis, oxygen is 1,000,000 to 20,000,000 times as reactive as styrene monomer toward these radicals, depending on the penultimate unit in the radical (33). In styrene in equilibrium with air, the molar concentration of oxygen is 0.0014M (34). about 1,’6000 that of pure styrene (8.43M). Therefore, at least 99.4% of the propagation reactions of polymer radicals ending in styrene units will involve oxygen rather than styrene monomer. Although styrene monomer is one of the most reactive of unsaturated compounds toward free radicals, even at concentrations as high as 8M it cannot compete with oxygen from air. However, when the concentration of free radicals approximates that of oxygen, the free radicals may react with each other before oxygen intervention. Such conditions apply where radicals do not react with scavengers, as in solvent cages
614
and in tracks produced by ionizing radiation. These relations are consistent with previous work on tetralin (3) and on unsaturated compounds ( 4 ) . Polymerization
Just as oxygen prevents normal free radical homopolymerization of styrene, all other free radical homopolymerizations also become copolymerizations in the presence of oxygen. Reports that normal polymers can be formed apparently result from failures to analyze the polyperoxides actually obtained ( 4 9 ) . Copolymerizations with oxygen may be either faster or slower than homopolymerizations in the absence of oxygen (37). These relations and also the retardations by oxygen of bulk and emulsion vinyl polymerizations (33, 37) have been discussed. Emulsion polymerizations are fast, because chain termination is restricted by the isolation of free radicals in emulsion particles. The “inhibition” by oxygen-by a factor of about 1000-of the emulsion polymerizations of styrene by butadiene is apparently due to destruction by oxygen of the advantage of emulsion polymerization; some oxygenated radicals are apparently more soluble and less isolated. The rate of copolymerization of styrene and oxygen in an emulsion formula corresponds fairly well to the bulk rate ( 3 3 ) . The initiation by oxygen of the high pressure polymerization of ethylene (78, 79) apparently depends on the formation of a little polyperoxide with early exhaustion of the oxygen. This peroxide initiates subsequent normal polymerization where there may be less need for high pressure. Oxygen must be used in proportions sufficient to form the initiator, but not in such proportions-depending on the pressure-as to give either a n uncontrollable reaction or a prolonged induction period. The latter corresponds to the very low rate of reaction of oxygen with vinyl acetate.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Drying Oils Only three mechanisms for air-drying of oils have recently received serious consideration : vinyl polymerization to give carbon-carbon bonds (70, 44) ; copolymerization of oxygen and oil with the formation of cross links involving oxygen (56); and a Diels.4lder reaction to produce carboncarbon cross links (2, 43). Condensation reactions requiring acid or base catalysis are assumed to be excluded in neutral oil films. Evidence that per0-0
I
!
I
I
oxides o!’ the type -C-C-
(37) may be
intermediates in drying (44)is not convincing: the “moloxides” which have been reported are probably polymeric peroxides (27, 27, 28, 38). I t will now be considered how much oxygen is necessary to exclude vinyl polymerization and how readily oxygen penetrates into drying films. I n the polymerization of styrene in the presence of oxygen (33)?radicals ending in a styrene unit have an equal chance of reacting with styrene and with oxygen when the solution is in equilibrium with oxygen a t 1 mm. pressure, 0.6% of the pressure of oxygen in air. In methyl methacrylate saturated with air (35), five sixths of the methacrylate radicals react with oxygen, the rest with monomer. At about 30 mm. pressure of oxygen, one fifth of the osygen pressure in air, the rates of the t\vo reactions become equal. These are the only data known to the writer on how much oxygen is necessary to prevent polymerization. I n the conjugated, unsaturated systems responsible for the cross linking of drying oils, addition of radicals to double bonds rather than to oxygen may be favored by the hydroperoxide or ketone groups nearby, as in the case of methyl methacrylate. This tendency however should be more than offset by the high tendency of internal double bonds to copolymerize with oxygen rather than to homopoly-
merize (37) and by the low concentration of conjugated unsaturation in most drying oils. Therefore the chance of carbon-carbon bond formation in the airdrying of oils may be even poorer than with styrene, and 1% of the oxygen in air should largely exclude vinyl polymerization. Bragdon ( 7 7 ) presents analyses of sections of a film of dipentaerythrityl hexalinolenate after drying for 30 days. The excess oxygen content of the deepest 6 mils of a n 18-mil film was 58% of that in the 6 mils nearest the surface. Measurements of oxygen contents of entire films of various thicknesses u p to 18 mils when dried, after either 1 day or 30 days exposure, closely parallel the results on film sections. Thus, although the rate of oxygen absorption decreases abruptly after gelation ( 7 4 , there is no indication that the concentration of oxygen in drying films is sufficiently depleted to permit carbon-carbon bond formation. T h e cross linking of oils by the free radical copolymerization of oxygen and unsaturated compounds might involve either addition or transfer reactions (of hydrogen atoms) of unsaturated compounds with peroxide radicals : Addition : R-0:.
+ -C-CH=C-
I
by transfer (5),the effect of R in R-01. on the termination reaction also s e e m to be small. Thus the over-all rates for both addition and transfer in the table depend mostly on the unsaturated compound, little on the radical or termination. Over-All Rates of Oxidation of Unsaturated Compounds at 1M Concentrations in Inert Solvents in Presence of 0.0 1 M a,a'-Azobis(2-methylpropionitrile) Rates in Reactions by Reactions by Mole/L./Hr. Hydrogen Addition x 103 Transfer a-Methylstyrene Indene
15.8 15.3 9.7 8.2
Styrene P-Methylstyrene 1,l-Diphenylethylene
7.1 3.7 3.6 3.1 2.0 1.8
1.2 Acrylonitrile
1.0
Methyl methacrylate Butyl methacrvlate Vinylacetate Butyl acrylate
0.69
Ethyl linoleate 1,3,5-Trimethylcyclohexene 1-Methylcyclohexene Cyclohexene 4-Methyl-3heptene Methyl oleate Allylbenzene
0.79 0.62
0.57 0.19
o'20
'-Octene
H
'Transfer: R-o?,
-
+ -q-cH=L-
I
H -&-CH=(!-
There is one reservation in this conclusion. Russell (48)has shown that chain terminations among primary and secondary benzylperoxy radicals H
+ R-02-H
Rates for addition reactions in the table below are over-all rates of oxidation a t 503 C. at a nearly constant rate of initiation ( 3 7 ) . The products are polyperoxides, and sometimes also aldehydes or ketones. The rate for allylbenzene (37) is used as the basis for comparing all rates of reaction where hydrogen transfer predominates. The rates of oxidation by hydrogen transfer, relative to allylbenzene, at 55' C., without correction for the 5' difference in temperature (12). are based on oxidations \vith benzoyl peroxide. I n this column, the products are usually hydroperoxides. 1-Octene, in the transfer column, is unusual because it reacts by both mechanisms. At 75' C. (23): 67% of the peroxide radicals reacted by hydrogen transfer, but 31Yc reacted by addition. The table above is used to compare rates of reaction of various unsaturated compounds with peroxide radicals in general. The effects of R on propagation by RO,. are small because the substituents in R are separated from the free valence by a t least three atoms ( 3 7 ) . \)Vith unsaturated compounds which react either by addition (37) or
(Ph-b-O,.)
are
much
faster
than
I
R among tertiary benzylperoxy radicals R' (Fh-(i--OI.).
On
the
other hand,
K this difference has not appeared with polyperoxide radicals (37) and may be unimportant among allylperoxy radicals. T h e table shows that addition of peroxide radicals to ethylene bonds is favored mostly by conjugated phenyl groups? and less by conjugated carbonyl and nitrile groups. Addition is also favored by 1,l-substitution ( 3 6 ) , but is hindered by 1,2-substitution-except in 5-membered rings. These conclusions are supported and extended by extensive data on copolymerization (30). T h e latter studies indicate also that conjugated phenyl and vinyl groups exert very similar effects in addition reactions -Le., that styrene and butadiene behave similarly, and that a peroxide radical is analogous to radicals in the polymerization of acrylonitrile or maleic anhydride. O n the other hand. hydrogen transfer is favored by accumulation of alkyl groups on both sides of the double bond. Most reactive in transfer
are single methylene groups between l,.l-double bonds? as in linoleic esters. Farmer proposed the generalization : conjugated systems add oxygen a t one end; others incorporate it in the allyl position (20). .4ddition processes by chain reactions are the most efficient cross-linking processes. Transfer processes by themselves cannot lead to cross linking, gelation, or drying. The only mechanism for cross linking by radicals, except by addition, is radical combination. The latter process is inefficient, because each cross link requires two radicals which must come from different molecules. In the presence of oxygen, they will be alkoxy or peroxide radicals, and they must react by coupling, not by disproportionation, to produce an ether or peroxide cross link. Thus decomposition of t\vo or more peroxide bonds is usually required to produce one cross link. The work of Kern, Heinz, and Hohr (25) on the reaction of oxygen, at one atmosphere, with methyl 9 , l l-octadecadienoate at 70' C. indicates what behavior may be expected of oils with conjugated unsaturation. Their product was a polyperoxide averaging about three ester units, 0.9 oxygen and one double bond per ester unit, and containing no hydroperoxide. The formation of this low polymer in a methyl ester should correspond to setting-gelationin a glycerol ester. Kern, Heinz, and Hohr found that both 1,2- and 1.4-addition occurred. By analogy with other 1,2-polyperoxides (37) thermal cleavage of a peroxide bond in a 1,2-polyperoxide should result in chain cleavage, ~
0.
I -CH-CH-CH=CH-
+
I
0
I
0 I
0
I1
-CH
HC--CH=C:H-
1'
0
0 and loss of one cross link. However, in bulk reactions, the latter loss might be compensated by addition of the two surviving alkoxy radicals to other units or by reaction with aldehydes (32). The 1,4-polyperoxides should be less susceptible to chain cleavage. Thermal decomposition of Kern's mixed polyperoxide a t 210' C. apparently produced alkoxy radicals and ether links. Peroxide links could not survive heating to 210' C., yet Kern observed no net change in the viscosity or gross composition of the oxidized ester. O n thc other hand, large losses of carbon and hydrogen from oil films during drying (75) point to cleavage of 1,2-poIyperoxides in films. VOL. 52, NO. 7
JULY 1960
615
Because many drying oils originally contain little or no conjugated unsaturation, their widespread usefulness needs to be accounted for. They are partially isomerized to conjugated oils during bleaching and bodying (43):and extensively isomerized during drying. Thus, the oxidation of unconjugated linoleic esters produces conjugated hydroperoxides (73,26,45,46.50) :
+ ROa. CH=CH-~H-CH=CH+
-CH=CH-CHz-CH=CH4-
ROzH
0
- CH=CH-CH=CH-~H-
2
I
( = RO~.)
T h e conjugated hydroperoxides react readily (26) with more oxygen, by addition, to give polyperoxides without net loss of hydroperoxide (47, 56). Two other observations point to the need for producing a conjugated system before drying occurs. For set-to-touch, without metal drier, the glycerol ester of eleostearic acid-with three conjugated double bonds-required only 1.5 hours, while the ester of linolenic acid-with 9,12,15-unsaturation-required 42 hours (72). The failure of triolein to dry can be ascribed to its inability to produce conjugated unsaturation when it oxidizes. In summary, cross linking of airdried oils below 50' C. arises mostly from copolymerization of conjugated, unsaturated groups with oxygen. If there is little conjugation in the original oil, isomerization during oxidation is a necessary preliminary to drying. As the polyperoxides break down by the action of heat, light, or aging, they produce alkoxy radicals (20, 33, 34). Those which add to double bonds produce ether cross links and further oxidation of the film. Those which react by hydrogen transfer produce only further oxidation. Many of those alkoxy radicals which are fragments of 1,2-polyperoxides must cleave the carbon chain between the peroxide groups. In reactions of alkoxy radicals with double bonds, addition will be preferred with conjugated systems, hydrogen transfer with others. Thus films from drying oils should be held together at first by peroxide links, later by ether links, but not by carboncarbon bonds as long as oxygen at a few millimeters pressure is available. The above account suggests where further work in the field might lead to better understanding and better films. What are the effects of oxygen pressure on the rates and products of the drying process? At what film thickness does rate of diffusion of oxygen become important? What is the optimum amount of conjugation in an oil at the start of the drying process? Can conversion of
616
polyperoxides to polyethers be demonstrated during aging of a film? If an oxidized, but ungelled film is heated in the absence of oxygen, can carboncarbon bonds and better film properties be obtained? That drying of oils might occur through carbon-carbon bond formation and Diels-Alder reactions has been suggested (2, 43). Such reactions require isomerization of unconjugated oils, brought about by heat, oxidation, or catalysts (47). Diels-.4lder reactions of hydrocarbons such as isoprene are second order reactions and are slow even at 100' C. (54). Methyl eleostearate dimerizes only near 200' C. (47). These reactions would be far too slow in drying oils near room temperature, and they would nor be susceptible (53) to the knoun effects of catalysts, initiators, and oxygen. On the other hand, a Diels-Alder condensation may involve instead a 1,3-diene, formed by isomerization, and a double bond conjugated with a carbonyl group, -C=CH-CO--, re-
I
sulting from oxidation (43). few such Diels-Alder reactions involving maleic anhydride proceed spontaneously at room temperature ( d o ) , but other dienophiles react less easily. For reaction benreen methyl vinyl ketone and the eleostearic acids, heating at 150' C was continued for 16 hours ( 4 2 ) . Although conjugated octadecadienoic acids would react with maleic anhydride on refluxing in heptane for 4 hours. a temperature of 206' C. was used for reaction with acrylic acid (77). In experiments with octadecadienoic acid, acrolein and acrylic acid gave 80% or more of Dielsalder adducts at 100" to 200" C., but croton-aldehyde and crotonic acid reacted poorly even at 150' to 200' C. and no definite adducts could be isolated (57). The latter compounds, with 1,2-substituted double bonds, are closer to the structures expected in oxidized drying oils. These examples give no indication that Diels-Alder reactions can account for the drying of oils a t room temperature. An interesting test would be the reaction of an ester such as polyethylene fumarate with tung oil at room temperature in the presence of an inhibitor of vinyl polymerization. Ready gelation lvould indicate a Diels-Alder mechanism. Aging of Rubber The author's work on the oxidation of unsaturated compounds has applications to the mechanism of oxidation of unvulcanized natural rubber. First, the decomposition of simple alkoxy radicals, RR'R"-C-O. + RR'CO R".
+
is an inefficient process a t low to moderate temperatures. Polymers made from both styrene and methylstyrene at low
INDUSTRIAL AND ENGINEERING CHEMISTRY
oxygen pressures and at 50' C. contained ether groups, showing that some alkoxy radicals added to double bonds instead of decomposing (33, 34). Ethoxy radicals are said to react with each other a t low temperatures, but to decompose to methyl radicals and formaldehyde at high temperatures (55). Brook studied the reactions of tert-butoxy radicals in hydrocarbons a t 110' to 160' C. (76). In the solvent tetralin (probably comparable to isoprene units in reactivity), he found that only a quarter of the tertbutoxy radicals cleaved to acetone at 135' C. and even fewer at lower temperatures. However, recent work shows that 0-peroxyalkoxy radicals are much less stable (34). Such a radical from the thermal or photochemical cleavage of a-methylstyrene polyperoxide I hle
R-O-I-O-CH2
j
-I-C-O-/-O-CH, I Ph ! Me ; I .Me C-O-/-O-CH,-/-C-O' I Ph Ph
--
apparently decomposes explosively and completely into acetophenone, formaldehyde, and the terminal RO, radical. Xote that cleavage of the original terminal p-peroxyalkoxy radical produces another P-perosyalkox); radical, and so on. The reaction is essentially quantitative-xcept for the residual end groupbecause as much as 93y0 of the theoretical acetophenone (based on total oxygen in the copolymer) has been found after decomposition. The depolymerization reaction is considered to be explosive, because no intermediate radical -except the terminal radical-lived long enough to react with a-methylstyrene as solvent. The contrast in the stabilities of ordinary alkoxy and of pperoxyalkoxy radicals indicates that in the latter radical, the potential formation of formaldehyde facilitares the formation of the first acetophenone, and the potential formation of the second acetophenone facilitates the formation of the first formaldehyde, and perhaps the first acetophenone, and so on. As soon as the indicated polyperoxide-alkoxy radical is produced, the whole polymeric radical disintegrates. iinalogous radicals from styrene polyperoxide and methyl methacrylate peroxide behave similarly (33,35). These considerations, in conjunction with Bevilacqua's recent proposal for the mechanism of degradation of unvulcanized rubber ( 6 ) : suggest the scheme in the figure below. .Any radical-a peroxide radical is shownadds to Carbon No. 2. This is a rather unusual alternative to the common reaction by hydrogen transfer. Oxygen adds to the resulting radical on Carbon No. 3. Because of favorable steric relations, the resulting peroxide radical
OXIDATION REACTIONS
- CH, a
-
\Me
C = C H 7
6
Me
5
1
3
ve
/CHz--CH2
-CH,-c-c
9.
4
H\
\c
'0-0'
C = CH
/
2
-
CH
I 0,
-
CH,
-
1
- ai, -R
adds to Carbon No. 6, and oxygen immediately adds to Carbon S o . 7. The author's work suggests that this peroxide radical does not break down readily below 100' C.; it must first be converted to an alkoxy radical. The necessary alkoxy radical may be produced in a t least three ways: the peroxide radical may add to a double bond to produce a 0-peroxyalkyl radical; the latter radical may react with the &peroxide link to give an epoxide and the desired alkoxy radical ( 3 6 ) ; the peroxide radical may react with the rubber hydrocarbon by hydrogen transfer to produce a hydroperoxide group which subsequently produces the desired alkoxy radical by thermal or photochemical decomposition ; interaction of two peroxide radicals may produce two alkoxy radicals (8. 35). The last proposal is consistent with the recent hypothesis of Tobolsky and Mercurio that cleavage of rubber chains is associated with the chain termination step in oxidation (52). T h e alkoxy radical is now supposed to decompose quantitatively and explosively, like the polymeric radical from a-methylstyrene peroxide, into a cleaved rubber chain, one molecule of levulinaldehyde, and an alkoxy radical corresponding to the peroxide radical which originally attacked Carbon KO. 2. This type of attack requires practically the eventual formation of levulinaldehyde, the product emphasized by Bevilacqua. .4ttack by hydrogen transfer at Carbon S o . 1, as proposed by him, requires that the resulting allyl radical react at Carbon No. 3-instead of No. 1-and does not require that levulinaldehyde be produced readily or in one step, although the sequence would certainly lead to rupture of the rubber chain between Carbons 6 and 7. This scheme does not account for the formation of acetic acid, formic acid, or carbon dioxide. Perhaps competing variations of this mechanism a t Carbons l , 2, 7, and 8 will eventually account for the volatile products. Some difficulties may be expected when oxidations of model hydrocarbons a t 50' C.
are extrapolated to rubber at temperatures up to 140' C. This type of mechanism for the oxidative degradation of rubber a t moderate temperatures suggests why conventional testing procedures, employing elevated temperatures and sometimes elevated oxygen pressures, give a poor correlation with observations in service. T h e intermediates formed are not necessarily the same. T h e above considerations suggest that improved accelerated aging tests may be developed along the following lines. The exposure to oxygen should be at service temperatures and pressures. This absorption of oxygen may be accelerated by light, by photosensitizers, or by peroxides or azo compounds of stability appropriate for the testing temperatures. This treatment presumably produces the peroxides responsible for eventual service failure. To determine rapidly the effects on physical properties of the eventual disintegration of the peroxides, the test pieces may now be heated, in the absence of oxygen to avoid further aging, to whatever temperature is necessary to decompose the peroxides rapidly. Physical properties are then determined by a conventional test. Accelerated aging tests of other organic material subject to oxidation may be devised through similar considerations.
Irradiation of Polyethylene
The high reactivity of alkyl radicals with oxygen indicates that no carboncarbon cross links of long-lived radicals can be formed when polyethylene is irradiated in the presence of oxygen. This statement does not preclude carboncarbon bond formation by a nonradical mechanism or in localized areas where the concentrations of free radicals are higher than the concentration of oxygen. Lawton, Powell, and Balwit (29) have recently concluded that about five molecules of oxygen are consumed in converting one free radical to a carbonyl group. This conclusion is consistent with other findings by the author: peroxide radicals are fairly stable and do not cleave. They propagate oxidation chains or react with each other. Ordinary alkoxy radicals are not particularly efficient in cleavage. They probably arise from decomposition of peroxides, or after addition of peroxide radicals to double bonds.
Gum in Gasoline When gasoline is stored for considerable periods with limited access of air, small proportions of products are formed which produce troublesome deposits in
the intake systems of gasoline engines. Thus far, little success has been attained in correlating the origin of the gasoline and its stability in storage. Such a correlation may be devised by considering how free radicals. may be generated, and then what they can do. This discussion is based on the premise that gum is formed essentially by steps already known in autoxidation, vinyl polymerization and coupling reactions of free radicals. Radicals may be generated by thermal initiation without oxygen, by thermal initiation with oxygen, and by decomposition of peroxidic oxidation products. A few unsaturated compounds such as styrene (24; 37): indene; and mixtures with maleic anhydride, produce free radicals spontaneously below 100' C. At 50' C. thermal initiation of styrene saturated with oxygen a t 1 atmosphere is about 38 times as fast as in the absence of oxygen (39). In spontaneous initiation with oxygen, indene seems to be more active than styrene (37), while a-methylstyrene seems less active. Rates of initiation with saturated hydrocarbons and other unsaturated compounds are largely unknown, but they cover a wide range, All peroxides decompose on heating, usually into radicals, but the rates and yields of radicals vary. At 50" C. the peroxide formed by the autoxidation of styrene has no significant catalytic effect, but the peroxides formed from indene and a-methylstyrene have important effects (37). At higher temperatures. styrene polyperoxide may have some low catalytic activity. After the radicals are generated by one of the above routes, they can react in three ways by, oxidation, polymerization, and coupling of two radicals, or (with little net effect) chain transfer. In the absence of oxidation and polymerization, for practical purposes no chains are possible and the radicalsor their descendants resulting from hydrogen transfer-couple or disproportionate. This coupling reaction increases the size of the initial molecules and might lead to gum, but it requires a t least one pair of radicals to be generated for each coupling reaction. These considerations have suggested a new test for the stability of a gasoline in storage. Controlled production of radicals, by use of a thermally unstable azo compound or peroxide, or a photosensitizer, should greatly accelerate the gum forming process and reduce, or eliminate, differences among gasolines in thermal initiation and autocatalysis. Preliminary results by G. C. Bassler and J. R. Smith in this laboratory suggest that such study of gum-forming potentials in gasolines, independent of initiation phenomena, may be a useful measure of stability in storage. VOL. 52, NO. 7
0
JULY 1960
617
Antioxidants and Inhibitors
each molecule of retarder stopped many chains. Retarders of indefinite life lvould of course be useful also in low-temperature oxidations. Progress along these lines requires further measurements of the number of chains stopped by representative retarder molecules under various conditions.
Alkylphenols and aromatic amines are commonly used as antioxidants or oxidation inhibitors for free radical processes. T h e work of Bickel and Kooyman (7) and of others suggests that peroxide radicals react with trialkylphenols in the following manner
.&.
H
X
H
X
H
X
H
X
+ m,. H
X
-
Thus, one molecule of phenol destroys two oxidation chains. hctive hydrogen is not necessary for antioxidant action, because a molecule like h’,2V’-tetramethyl-fi-phenylenediamine reacts with two peroxide radicals, first bv apparently reversible complex formation with one radical, then irreversibly with a second (22). \Vith a few exceptions. phenols and amines stop only two oxidation chains per inhibitor’ molecule (9). If antioxidants suitable for use in fluids are a n d polymers a t 200’ to 400’ to be found. a new approach is neces.. sary. As the service temperature increases, the rate of chain initiation increases rapidly and the kinetic chain length becomes shorter, T h e first factor increases the rate of consumution
c.
of antioxidant a n d the second increases the proportion of antioxidant required to reduce the rate of oxidation-by a given fraction. ”hat is needed is a retarder which has a n indefinite life. It would function by complexing with the peroxide radical, retaining it as a deactivated radical until the complex collided with a second peroxide radical. The two peroxide radicals would then destroy each other in a normal fashion. and the retarder would be regenerated without change. There are two kinds of evidence that such action is possible. In the polymerization of styrene in bromobenzene as solvent a t 155’ C., the bromobenzene functions as a chain transfer agent without being incorporated in the polymer ( 3 7 ) . Apparently the polymer radical complexes with bromobenzene. but the latter is regenerated in the transfer step Recently, certain polynuclear hydrocarbons, notably dibenzo-(a, h)-anthracene, benzo(o)pyrene, and pervlene, have been found to retard strongly the gelation of a silicon oil in air a t 270’ C. ( 7 ) . Such compounds are known to react with free radicals, and their service life in these exPeriments-48 hours a t 270’ c. lvith 0.2 weight % of retarder-suggests that
-
618
RO
H X
Acknowledgment
Ronald Swidler, G. C . Bassler, and J. R. Smith, all of Stanford Research Institute, E. M. Bevilacqua of the LT.S. Rubber Research Center, and P. 0. Pojvers of the Pennsylvania Industrial Chemical Corp. have provided useful discussions and references. literature Cited
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INDUSTRIAL AND ENGINEERINO CHEMISTRY
