INDUSTRIAL AND ENGINEERING CHEMISTRY
294
14) Cowan, J. C., IND.ENG.CHEM.,41, 294 (1949). 15) Cowan, J. C., perbonal communication to Don S. Bolley. 16) Craig, J.Am. Chem. Soc., 65, 1006 (1943). 17) Diels, O., andillder, K., Ann., 460, 98 (1928). 18) Dunlap, L. H., U. S. Patent 2,382,213 (Xug. 14, 1945). 19) du Pont de Nemours & Co., E. I., Brit,ishPatent 552,095 (Mar. 23, 1943).
(20) Zbid., 556,113 (Sept. 21, 1943). (21) Ellis, C., U. S. Patent 2,033,131 (Mar. 10, 1936). (22) Ibid., 2,033,132 (Mar. 10, 1936). (23) I b i d . , 2,146,671 (Feb. 7 , 1939). (24) Flint, R. B., and Rothrock, H. S.,Ibid., 2,276,176 (Mar. 10,
1942). (25) Forbes, W. C., and Neville, H. A , , ISD.ENG.CHEM.,32, 555-8 (1940). (26) Gardner, H. A , , U. 8. Patent 1,452,553 (1923). (27) Gerhart, H. L., Zhid., 2,361,018 (Oct. 24, 1914). (28) Ilemitt, D. H., and Armitage, F., J . Oil & Colour Chemists' Assoc., 29, 109 (1946). (29) Jordan, O., and Kollek, L., U. S. Patent 2,054,019 (Sept. 8, 1936) (30) Kass, J. P., presented before the Division of Paint, Varnish, and Plastics Chemistry at the Memphis Section Meeting of the AMERICAN CHEMICAL SOCIETY, Memphis, Tenn., 1942. I
Vol. 41, No. 2
(31) Lawler, W. D., Hable, G . J., and Steinle, J. V., U. S. Patent 2,353,910 (July 1944). (32) Lawson, W. E., and Sandborn, L. J., Ibid., 1,975,959 (Oct. 9, 1934). (33) Mighton, C. J.,Ibid., 2,346,858 (Agiil 18, 1944). (34) Ibid., 2,401,769 (June 11, 1946). (35) Milas, N. A., Ibid., 2,267,248 (Dee. 23, 1941). (36) Morrell, R. S., and Samuels, II., J . Chem. Soc., 1932, p. 2251. (37) Root, F. B., U.S. Patent 2,374,381 (April 24, 1945). (38) Rummelsburg, A. L., I b i d . , 2,370,689 (Mar. 6, 1945). (39) Scheiber, J., Ibid., 1,979,495 (Nov. 6, 1934). (40) Scheiber, J., British Patent 316,872 (Nov. 24, 1930). (41) Schwarcman, A., U. S. Patent 2,412,176 (Der. 3, 1946). (42) Zbid., 2,412,177 (Dee. 3, 1946). (43) Swain, R. A., Ibid., 2,304,288 (Dec. 8, 1942). (44) Teeter, H. M.,and Cowan, J. C., Oil h Soap, 22, 177-80 (1945). (45) Teeter, H. >I., Geerts, M. J., and Cowan, J. C., J . Am. Oil Chemists' Soc., 25, 158 (1948). (46) Wakeford, L. E., andHewitt, D. H., U. S. Patent 2,392,710 (Jan. 8, 1946). RFCFXYBD February 28, 1948
Isomeri
ctions ils J. C. Cowan
Northern Regional Research Laboratory, Peoria, I l l . T h e fundamental and practical aspects of the isonierization reactions of the unsaturated acids are discussed. Of particular interest to the drying oil chemist are a review of the methods of effecting conjugation and evaluating the conjugated oils, and discussions on the drying of oil films, the relation of isomerization to drying and copolymerization, and the factors responsible for after-tack. Particular attention is given to the problem of making a tung oil replacement, to the mechanisms of isomerization, to alkali isomerization, to nickel and iodide catalysts for isomerization, and to styrene copolymerization.
S
IKCE early in this century when the process for gasproofing tung oil was developed, the importance of the isomerization of conjugated and nonconjugated oils in protective coatings has steadily increased, Tung oil is easily isomerized and its isomerization has definite commercial importance. When tung oil is exposed to sunlight or treated with sulfur, selenium, or iodine a change from a liquid oil to a solid fat is effected. This chznge of the liquid oil to a solid is the result of the isomerization of t i e a-eleostearic acid to the @-eleostearicacid. ljeedless t o say, the producer who is now fortunate enough to have a supply of tung oil does not desire to have the physical state of his raw mat,erial changed since he would be unable to handle it in equipment normally available in his American plant (23). American tung oil when prepwed by extraction is readily isomerized and a heat treatment is necessary to stabilize it,. Expressed oil benefits from a similar treatment (66). When the supply of tung oi. became limited during the Japanese occupation of China, attempts to prepare replacements for t,ung oil were made by a large number of investigators. One of the direct methods of approach which might lead t o a tung oil replacement is the shifting of the unsaturated bonds in nonconjugated oils such as linseed or perilla oils to give conjugated unsatu-
ration. Although some efforts of coinmercial importance have been made to obtain extracted oils of higher polyunsaturated fatty acid content, more efforts have been toward isomerizing the oil t o produce conjugated systems since it was known that the conjugation was primarily reqponsible for the reactivity of tung oil. In addition to this interest in preparation of conjugated oils for industrial uce, studies on isomerization reactions havc resulted in a new method for analys's of oils and have extended the scope of research on the problem of the utilization of vegetable oils by the preparation of new derivatives. KOattempt has been made in this paper t o covcr all thc literature on isomerization and conjugation. The many known isomers of the fatty acids which have been reported in the literature are not included specifically. This paper deals primarily with the isomerization and conjugation as it is related to the drying oils and their reactions. Theoretical Aspects of Isomerization of Fat Acids
The term isomers is generally applied t o those compounds which have the same molecular formula but which differ in a t least one of their physical and chemical properties ($6). The term isomerization is applied t o reactions which effect changes among isomers. With the mono-unsaturated fatty acids, the isomeric forms are limited to the cis-trans isomers of the different positional isomers with the omega-unsaturated fatty acid existing in only one form. The different forms of mono-unsaturated fatty acids can be rcpresented as follows:
R
R
,/"=\ \
/"l
I
'
t.=C H
I1
/" 'El
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1949
Formula I would be considered the cis isomer; 11, the trans isomer. R equals H or alkyl radicals from methyl N t o pentadecyl and R1 equals any radical from carboxyl group to the hexadecanoate radical. Cycloparaffinic isomers also may exist. The isomers of octadecadienoic and octadecatrienoic acids differ considerably in their complexity and number as compared with the isomers of octadecenoic acid. For example, most positional isomers of linoleic acid may exist in four different cistrans forms as shown:
Iv
I11
RL
H
H
\
H H
\C=d
C=C
/
\
\$
V
H
\
/
c=c/c=c\ R'
VI
Cis-trans isomers of octadecenoic acid
I11 is a cis-trans; IV, a cis-cis; V, a trans-trans; VI, a transcis isomer. I n addition to these four forms of octadecadienoic acid where R is not hydrogen, there are two cis-trans forms when R = H, and three or more different types of positional isomers depending on the position of the double bonds with respect to each other. These may be classified as conjugated or a 1,3-butadiene (VII), e 1,4-pentadiene (VIII), and isolated diene types such as 1,5-hexadiene (IX):
R-CH=CH-CH=CH-CH2-CH*-R1
(VI1)
R-CH=CH-CH2-CH=CH-CHz-R1
(VIII)
R-CH=CH-CH~-CH~-CH=CH-R'
(IX)
Position isomers of octadecadienoic acid The allenic, acetylenic, and the cyclolefinic acids also, as well
as isomers in which the unsaturated bonds are conjugated with the carboxylic group, should be included. With the octadecatrienoic acids, there are eight possible isomers for most of the positional isomers, plus a number of different types of positional isomers. R-CH=CH-CH=CH-CH=CH-R'
(X)
R-CH=CH-CHz-CH=CH-CH=CH-R' R-CH=CH-CHe-CHz-CH=GH-CH=CH-R' R-CH=CH-CHz-CH=CH-CHz-CH=CH-R R-CH=CH-CH&H=CH-CH2-CHz-CH=CH-R'
(XI) ( XI1)
(XIII) (XIV)
Position isomers of octadecatrienoic acid These positional isomers may be classified as follows: A conjugated triene radical (X); a conjugated diene radical with a n allylic (XI) or with an isolated double bond (XII); a 1,4,7-octatriene radical (XIII); and a 1, 4-pentadiene radical with an isolated double bond (XIV). I n addition, all double bonds can be isolated one from another. The main difference between one isolated double bond and one in the 2,3 position to a second double bond is the ease with which the hydrogen may shift t o give conjugation. I n a 1,4-pentadiene radical (VIII) and (XIV), the hydrogen on the active methylene merely needs to undergo a 1,3 shift, whereas in a 1,bhexadiene (IX) and (XIV) or with isolated
295
double bonds a greater initial shift or a great number of 1,3shifts would be required. If a 1,3 shift occurred first to give a trans isomer, additional energy would be required to effect a second shift since the trans form would be more stable. Allenic, acetylenic, and cyclic isomers also can exist. Kumerous attempts have been made endeavoring to determine the mechanisms by which isomerizatio:i occurs in organic compounds. According to Marvel's chapter on cis-trans isomerization in Gilman's book ($ti), the following explanation is probably the best general mechanism for cis-trans isomerism: The double bond is polarized by the result of a collision with a n activating catalyst such as selenium. The polarized carbon atom and its adjoining carbon are now free to rotate and the forces are such as to make the deficient carbon atom essentially planar. When the system then moves to relieve these forces set up as a result of the collision, then both cis and trans isomers are formed. Ultraviolet light acts differently from most isomerizing agents. It usually converts a trans isomer to a cis isomer. Since ultraviolet light imparts energy to the mixture, the cis form is more easily achieved than by chemical methods. Shifting in polyunsaturated compounds to form isomeric compounds containing displaced unsaturation appears to depend on either the removal of an hydrogen atom or of a proton. Alkali is a strong catalyst for many rearrangements t o conjugated systems, and it appears to act as a proton acceptor. This leaves a molecule with a n excess of electrons and a 1,3 shift occurs (37) to a more stable form.
R-CH=CH-CH=CH-VH-R'
+ NaOH + + H [NaOH 1 --+ + H + [NaOH] --+
R-CH=CH-CH=CH-CHz-R
+ NaOH
R-CH-CH-CHz-CH=CH-R' R-CH=CH-CH-CH=CH-R'
+
6 .
Mechanism of alkali isomerization This is one method of describing how this change occurs. The unsatisfied electrons are neutralized by the catalyst complex, and the proton is returned to the fatty acid molecule. By this mechanism, you would expect substantially complete conversion t o t h e conjugated isomer unless one of the unconjugated forms that might arise should be more stable than the conjugated forms or too stable to undergo isomerization with alkali. Actually, approximately 80 to 85% of a theoretical maximum has been achieved by careful alkali isomerism of linoleic acid (38). Recent work by Kass (30) and Riemenschreider and eo-workers (46) indicates that both 9,11- and 10, 12-linoleic acids are formed when linoleic acid is isomerized with alkali. Apparently only the bond which moves is isomerized in these reactions with alkali. Linolenic acid, an octadecatrienoic acid, presents a somewhat different situation since two active methylene groups are present. Actually linolenic acid gives 50% dienoic and 25% trienoic conjugation plus 25% cyclic isomers when isomerized with alkali (33). These products can be explained by the series of reactions shown below. The hydrogens may move in the directions indicated from either active methylene group t o give a 1,3 shift and a dienoic isomer which should be relatively stable. Formula XVI represents one of the two possible isomers. However, if the hydrogens move in the opposite directions to that indicated, a relatively unstable dienoic isomer (XVII) is obtained. It can undergo isomerization to form a conjugated trienoic acid (XVIII) or a cyclic isomer (XIX). A cyclic isomer also may result from the conjugated trienoic acid. R-CH=CH-
c c!Hr-CH=CH-CHa-CH=CH-R'
i t
(XV)
_I
R-CH=CH-CH=CH-CH~-CH~-CH=CH-R' (XVI or isomer)
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
296
+
1
R-CH~-CH~CH-ClI~C~I-C‘II=CII-C~I~-R
+
R-CHd-CH-CH=CH-C
,
(XT’III)
lI~-CH-CH=CH-II’ _I
(X I X )
Isomerization of an octadecatrienoic acid With certain other materials the mechanism appears to be different since different percentages of conjugation are achieved, and alkali-stable isomers lose conjugation on treatment with some of these catalysts. Examples are nickel-carbon and iodide catalysts. It appears likely that many more catalysts similar in type to these will be discovered since they probably isomerize by a free radical mechanism: R-CH=CH--CH2-CH=CH-Rf R-CH=CH-CH-CH=C€I-R’ R-CH=CH-CH=CH-:H-R’ R-CH=CH-CH=CH-CH2-R’
+ Ni -+ +X H + + U’IH --+
conjugated free radical (XX1a)whichcould enter into the film drying reaction. These chain reactions are probably of short duration since many possible chain termination and transfer reactions can occur but some should occur to make n = 2 to perhaps 5. This order of reaction is probably all that is needed since it has been shown that oils gel on heat polymerizat on when the order of polymerization is 2 to 3. This low order of polymerization is not serious when it is realized that Equation 4 represents a dimerization, and that this dimerization probably could occur between the free radical RO and XXIa as well as terminate a chain as shown for the hydroxyl group in Equation 6. Also, free radicals such as XXIa, could terminate the chain reaction by combining to form dimeric glycerides. RH
XX
+ 0 2 +ROOH XX --+ RO. (XXI) + *OH
XXII f RO.
Mechanism of nickel isomerization Tickel-carbon is a strong hydrogen acceptor and it produces approximately the per2entage of conjugation m hich would be expected from a free radical mechanism-for example, 66% from methyl linoleate (61). This samr approximate percentage is achieved R hen alkali-conjugated isomel s are the starting material and arc treated with a nickel-carbon catalyst. Table I shows the amounts of conjugation achieved with nickel-carbon catalyst and with alkali on dehydrated castor-fatty-acid radicals. This is believed to indicate that some equilibrium does exist in the reaction, but the exact nature of it is unknown. Quite possibly, the reaction is complex, having a number of different courses and with different end products resulting when a different catalysis is used.
xxm
Effects of Nickel-Carbon Catalyst on Dehydrated Castor Oil-Methyl Ester (51) Catalyst
None Nickel-carbon
Blkali
Alkali plus nickel-carbon
Conjugation, “/o 30.7
43.8 66.8 41.6
d similar free radical mechanism has been proposed t o explain the conjugation of methyl linoleate on autosidation. Approximately 66% of the initially oxidized molecules are conjugated (6). Relation of Isomerization to Drying Phenomena
Recent work on the oxidation of purified fatty acids has shonn that: oleic acid esters react with oxygen to give hydroperoxides ( 2 2 ) as shown by Equation 1; linoleic acid esters react with oxvgen to give a hydroperoxide in which conjugation is present ( 6 ) as shown by Equation 3; and linolenic acid esters react with oxygen to give conjugation when films are exposed to air (61). TTith these reactions as a background, it appears that a drying oil may undergo reactions similar to the following when a film drics: The hydroperoxide decomposes as shon n in Equation 2. Either of these free radicals react Ivith the conjugated peroxide generated from the linoleic acid ester to initiate a chain reaction which will continue until a chain transfer or termination reaction occurs, as shown by Equations 4 to 7. One chain termination reaction is shown in Equation 6. Chain transfer reactions are sholm in Equations 4, 5, and 7. Formation of hydrogen peroxide could be explained by the conibination of two hydroxyl radicals. Also a free radical such as the hydroxyl or the alcohol (XXI) might rcact with linoleic acid esters to give water or alcohol plus a nevi
(2)
+ +
-
0 2
( X X I I ) (3)
008
RO
+ xxII -+RO R-CIIOOH
XXIV
4- .OH --+- It0
XXIII
+ R-CH=CH-CH2-CH=CH-R’
--+
A I
It0 -
R-CHOOH CH--CII=CH-
Table I.
(1)
R-CH=CH-CH*-CI-I=CH -R‘ R-CH=CH-CH-CH-CH-R’
L
f Xi
Vol. 41, No. 2
H-
-13
R-CH=CII-C€I=(:H--(:rI.
+I{‘
XXIa
(7)
Free radical reactions which inay occur in drying These reactions are proposed as one possible explanation for the drying of an oil film. I n connection with recent work a t the Northern Regional Itesearch Laboratory on after-tack in nickel-carbon conjugated oils, it has been suggested that oleic acid is responsible for the aftertack (62). Recently oleic acid esters have been isomerized wi:h nickel-carbon t o obtain what appears to be a mixture of trans forms of certain position isomers of oleic acid. A41thougholcic acid and elaidic acid and their esters oxidize a t approximately the same rat.e (I??), oleic a d d oxidizes much faster (Table 11) in the presence of ceriain catalysts such as hemin (56). Since elaidic acid oxidizes slo.ivly, it could be responsible for the development of tacky filnis by the liberation of oxidizcd fragments long after the oleic acid normally present in the film hrts oxidized and either participated in the drying of the film or the fragments escaped. Tests to support this hypothesis are being undertaken as well as tests on t’he effect of other conjugation catalysts on oleic acid. At least two catalysts, selenium and sulfur dioxide, are reported to be elaidizing agents as well as conjugating agents (4, S I , 67’). I t is quite possible that certain trans forms of the dienoic acids mag be much slower drying also and be partially responsible for after-tack. The occurrence of cis-trans9-12 linoleic acid in dehydrated castor oil might account for its slight after-tack (29). In addition, a dienoic acid containing iso-
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1949 Table 11. Time, Min. 30 120
Absorption of Oxygen by Oleic and Elaidic Acids
(35)
+
Hemin Elctidic Oleic 2a 3 61 2 5 250 240 14 13 423 300 19 18 475 Cubic mm. of oxygen a t 37' C. Oleic
Elaidic
+ Hemin 7
10
21 25
lated double bonds and existing in the trans form would be expected to behave like elaidic acid. Further support for the possible adverse effect of cis-trans isomerization is found in the oxidation rates of the conjugated trienoic acids (45). Table I11 shows how the oxygen absorption of three different conjugated octadecatrienoic acid varies with respect to differences in position and cis-trans isomerization.
Table 111. Oxidation of Octadecatrienoic Acids (45) Acid m-Eleostearic acid @-Eleostearic acid Peeudoeleostearic acid Methyl pseudoeleostearate Lineolenic acid E t h y l linolenate
Mole Ol/Mole Acid/100 Min. 2.68 1.02 0.63 0.41 0.52 0.24
a-Eleostearic acid, according to Morrell and Davis (@), is probably a trans-cis-cis and p-eleostearic acid is a cis-cis-trans. Morrell makes these selections on the basis of reactions with maleic anhydride t o give crystalline adducts. This selection assumes that cis forms do not react so readily with maleic anhydride to form the normal adduct. This assumption has been confirmed in other dienes such as piperylene or 1,3-pentadiene (16). Steric hindrance may be responsible for the failure of certain cis-forms of this conjugated diene to form maleic anhydride adducts and the steric effects in the fatty acid isomers certainly would be more powerful. Using the same reasoning as Morrell, Kass and Burr assigned a trans-trans-trans, or a trans-cis-trans configuration to pseudo-eleostearic acid since it gave a mixture of a- and p- type of maleic anhydride adducts (3%). Note that it is the slowest to react with oxygen of the three conjugated dienes. Since drying is established as an oxidative phenomenon, it appears sound to assume that although conjugated isomers do oxidize and dry faster than the unconjugated, trans isomers oxidize and dry much slower than the corresponding cis forms both in the conjugated and nonconjugated isomers. The information on conjugated isomers led Spi$zer, Ruthruff, and Walton (66)to suggest the explanation that certain anomalies in the drying of alkali-isomerized linseed oils might be due to position and cis-trans isomerization of conjugated acids. Since drying appears to be a polymerization phenomenon, further support for Spitzer's contention is found in the behavior of cis- and trans-piperylene toward oxygen and maleic anhydride (16). The cis form polymerizes and copolymerizes readily as compared with the trans form which polymerizes slowly or forms adducts. However, it appears more reasonable to assume that trans forms of octadecenoic acid, nonconjugated octadecadienoic acid, and possibly cyclic isomers of octadccatrienoic acid (98, 67, 69) are responsible for the unusual after-tack in certain isomerized oils. Methods of Conjugation
A number of methods of producing conjugation have been discovered that have been considered for or actually used on a commercial scale. These methods include the use of alkali, nickel-carbon, sulfur dioxide, iodide compounds, and oxygen. Certain other methods appear to have promise but some doubt remains concerning the extent of their effectiveness since reliable methods of measuring conjugation were not used. The best
297
method of determining the extent of conjugation in an organic compound is measurement of the ultraviolet light absorption b,v means of a spectrophotometer. This measurement can be accomplished either in a photoelectric (12) or photographic spectrophotometer. When the analysis for conjugation is conducted properly, an accurate measure of the conjugated diene, trienes, and the like can be made (98). The analysis is based on the absorptions of pure conjugated fatty acids isolated from dehydrated castor oil and alkali-isomerized linoleic and linolenic acids. Table IV lists some of the pure acids which have been measured and shows the extent which alkali effects conversion.
Table IV.
Peak Absorption of Conjugated Isomers (38)
Acid 9,11-Linoleic acid 10,12-Linoleio acid a-Eleostearic acid @-Eleostearicacid Alkali-isomerized linoleic acid Alkali-isomerized linolenic acid Values given for
Peak Absorption 2300-2400 Diene
1150 1150
..
870 j
600
Peak Absorptiona 2600-2800 Triene
.. 2ibo 1890 ii7
The extent of conjugation is found by comparing the absorptions a t the designated wave length with the absorptions of the pure conjugated fatty acids. Alkali-isomerization under standardized conditions has been proposed as a method for analyzing oils (40)but results are obtained with drying oils which indicate the presence of nonconjugating isomers ( d ) or inherent difficulties in the method. Maleic anhydride can be used also to determine extent of conjugation under certain specified conditions. Ellis and Jones (19) have shown that some conjugated dienes react with maleic anhydride at 130" C. to form a derivative. However, not all of the conjugated fatty acids react readily to give maleic anhydride adducts. Kass ($9)reports only one of three isolated 10,12-linoleic acids gives a n adduct with maleic anhydride which melts sharply, whereas the other two forms give liquid products or products of wide melting range. Consequently, a maleic anhydride number obtained in the manner suggested by Ellis does not measure the full amount of conjugation. Also, there is a further objection to tbe maleic anhydride number because of the reaction of the anhydride with alcoholic groups (5). Priest (60)reports that this effect of alcoholic groups is small. Another method of measuring the extent of conjugation is use of refractive index. This is the simplest method of any available, and it is based on the enhancement which conjugation gives to the molecular refractivity. However, this method is subject to great errors because other chemical changes also enhance molecular refractivity. When checked by the spectrophotometric method, and when no appreciable change in viscosity occurs, refractive index can be a suitable method of following a conjugation reaction (61). Still another method of detecting percentage of conjugation is the differential iodine number developed a t Woburn Degreasing Company (68). Again, it is a method which is inferior to the spectrophotometric method and may not give a really accurate picture of the amount of conjugation produced by a particular treatment. Alkali Isomerization of Drying Oils The most studied of all known methods of conjugation (excluding oxidation) is alkali-isomerization a t elevated temperatures. As early as 1840, Varrentrapp (66)treated oleic acid with fused potassium hydroxide and obtained palmitic and acetic acids and hydrogen. Consequently, it was believed for many years that oleic acid was a substituted acrylic acid. Apparently the fufied alkali isomerized the unsaturated bond until it became conjugated with the carboxyl group and then cleavage occurred.
298
INDUSTRIAL A N D ENGINEERING CHEMISTRY
More recently, in 1914, Grun (66) heated polyunsaturated fatty acids with an excess of 40% sodium hydroxide in a n autoclave a t 270" to 280" C. for 3 hours. I n one experiment, the iodine value of linoleic acid was reducpd from 174 to 83.8. Although Grun believed that hydroxylation of the linoleic acid followed by formation of ether acid had occurred, it is apparent now that conjugation and polymerization had been effected. I n 1931, Morton, Heilbron, and Thompson (44)and others (24) reported on the unusually high absorptions obtained when certain vitamin A-containing oils weresaponifiedand the absorption of the total acid fraction obtained after ether cxtraction. Unfortunately, these absorptions were believed to result from acid decomposition products formed from materials associated with vitamin A. No study of the effect of the time or temperature on the saponification n a s made. Two yeais later, Dann and Moore ( I T ) reported their studies on the effect oE varying the duration of alcoholic potassium hydroxide saponification. They clearly demonstrated that the increased absorption resulting from the prolonged alkali treatment was achieved by some change in the polyunsaturated acids and that the most probable explanation was isomerization, Moore (41) later demolistrated that the linolenic acid n as convei ted to an isomeric material which had an absorption curve ~imilai.to cleostrw ic acid but not identical; Kass and Burr (99)demonstrated that this solid conjugated acid melting a t 77 C. was 10,12,14-octadecatrienoicacid. T h e n the work of ICass and Bmr was reported ( I O , YZ),it led to a number of American investigations of the reaction for possible commercial use. All of these investigations indicated that the reaction could be conducted in almost any medium M hidLpermitted strong alkali I O come in intimatr contact x\ith thc fatty acid radical a t elevated ternperatureq At lea3t one concern has used an alkali isomerization process to produce isomerized soybean and linseed oils commercially. It is difficult to make comparisons among different methods of alkali isomerization because of the lack of uniformity in the analysis of oils and in the ram materials treated. Bradley ( 8 , 9 )demonstrated that aqueous sodium or potassium hydroxide under pressure could be used. Hoaever, aqueous alkali isomerization is somewhat slower than isonierization when organic solvents are used; alcohols and glycols appear to be well suited as media for alkali isomerization. The ethers of the glycols appear t o be somewhat superior (IS)-in particular, glycol monomethyl ether appears to be superior to ethylene glycol. In addition, the conjugation can be effected with alkali in substantially anhgdroui conditions in short periods of time a t 285 to 300 C . (3'4). The behavior of the two most important acids in nonconjugated oils, linoleic acid and linolenic acid, toward alkali is considerably different. Whereas linseed oil contains substantially more linolenic acid than does soybean oil, the proportion of conjugated triene formed is not proportionate to this difference. Y o u will remember that under our discussion of linolenic acid that only 25% is converted to a conjugated triene, approximately 50% to a conjugated diene, and 25% is apparently converted to a cyclic isomer (52, 3 7 ) . With soybean oil a t least 80% of the linoleic is converted to conjugated forms and the resulting improvement is greater with soybean than with linseed oil. I n the all-important problem of finding a suitable replacement for tung oil, a reference to the composition of tung and isomerized soybean and linseed oils is enlightening. A reference to the composition of our available American-grown drying oils is particularly pertinent here since alkali does effect the highest amount of conjugation obtained in synthetic drying oils. If alkali isomerization to conjugation were complete, isomerized linseed oil would approach tung oil in the percentage of conjugated acids present but fall far short of the tung oil in the amount of conjugated triene present. Table T' shows that tung oil contains approximately 25 to 45oj, more triene than linseed oil. However, since only 25% of the nonconjugated triene is converted t o conjugated triene, alkali isomerized Iinsccd oil ron-
Vol. 41, No. 2
Table V. Composition of Soybean and Linseed Oils Before and After Isomerization Compared with Tung Oil (29) Acids Saturated acids Oleic acids Dienoic Nonconjugated Conjugated Trienoic Nonconjugated Conjugated diene Conjugated triene
Soybean -_____Before After
I5
I
Coinpoaition, yo Linseed Before After--
11
.
30 50
j. 2-5 .. ,,
22
10 40 e~
4 2
*.
Tung 2-7 1-20
16
j.
3 13
1-1 0
52
13 27
...
.. .
Y
13
... , . .
74-92
tains only approximately 13% of conjugated triene. Ho~vevcr, alkali-isomerized linseed oil does contain some 407, conjugated diene which results both from linoleic and linolenic acid. The main chemical disadvantages of the alkali prooess appear to bt: the formation of cyclic isomers from the linolenic acid and t l u resulting loss of unsaturation plus the loss of the glyceridc struoture to give fatty acids with the result,ing loss of functionality. Esterification to give glycerides or other polyalcohol esters IY*.sults in polymerization to give oils ol high viscosity suitable for varnishes and enamels. This polyniciriea,tionreduces vorsitf i l i i J' of the oils. Nickel Catalysts
Another method of achieving isorncrization which is comin~rcially feasible is the utilization of mctaliir catalysts. Probably the most v-idely used of these catalysts is uicliclwhich effectshydrugenatioii of vegetable oils rather than isomerization. However, to achieve fats of higher melting point without increased hydrogenation, the conditions of hydrogenation are frequently controlled to produce so-called iso-oleic acids. A review of the litera,ture on this subject is contained in one of the references (61). This ability of activated nickel to produce iso-oleic acid led J. P. Kass t o suggest that activated nickel with or wit,hout a suppork might produce conjugation in vegetable oils. Some of the important. data on catalyst studies which have been publislicd are (RO, 61, 62). Preliminary to studies of the nickel-carbon cat'algst, the research a t the Korthern Regional Research Laboratory included some studies of nickel-on-kieselguhr, activated carbon, and other active surface compounds. illthough some of these materials effect measurable conjugation, none is sufficiently activc t c s warrant possible commercial exploitation (Table VI).
Table VI.
Tsomerizatioris of Soybean Methyl Ester w i t h Active Surface Agents ( $ 1 ) Catalyst
None Acid washed kieaeIguhr Activated carbon, Nuchar XXX A1 silicate clay, ,Florex XXF Activated alumina, Nickel-on-kieselguhr
Diene Conitnourit, % iuaation, yo Poiyinei, Yc
..
23 23 10 23 30
1.0 11.3 12.8 3.9 4.7 17.3
5.6
14.4 18.5 30 3.4 19.5
Since activated carbon was one of the better isomerizing agent\, it was combined with nickel and the combination was found to possess enhanced catalytic power. This powrr was greatest at approximately 20% nickel or an atomic ratio of 20 carbon to 1 nickel. The choice of carbon black was most criticsl; cocatalytic activity was confined to certain carbons derived from waste products of the wood pulping industry. Although a large number oi other carbons were tested only one specific type was found suitable. The nickel-carbon catalyst was readily prepared by evaporating a solution of nickel formate mixed with the activated carbon, drying the r c d t i n g poivdcr, arid d(~orriliosingand activating
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
February 1949
the catalyst at 360' C. for 1 hour under an atmosphere of hydrogen. Lower or higher temperatures reduced activity of the catalyst. Normally the catalyst was used a t a concentration of 5 to lo%, this permitted use of 1 to 2% of nickel. The catalyst was active at temperatures as low as 150" C.; high percentages of isomerizations were obtained at 190" C. Since the temperatures of isomerization had considerable effect on the drying properties, temperatures above 200" C. were not investigated for drying oils (see Evaluation). Table VI1 summarizes the information on the effect of treating different vegetable oils with the nickel-carbon catalyst.
Table VII. Material
Nickel-Carbon Isomerization of Drying Oils (51 Time, Hours
Temperature,
4
c.
Conjugation, %
180
The time required for isomerization could be reduced readily with increasing amounts of catalyst, indicating possible continuous operation. By heating soybean oil with 25 grams of catalyst per 100 grams of oil, 33% conjugation was acheived in 20 to 30 minutes. From a theoretical standpoint the behavior of the nickel-carbon catalyst toward fatty acid derivatives of higher molecular weight is interesting. In addition t o the possibility of free radical mechanisms in this isomerization, apparently steric hindrance effects become important in the high molecular weight fatty acid molecules. For example, 46% conjugation is achieved with methyl esters of linseed fatty acids but only 0.9% is achieved with the polymerized glyceride. With increase in molecular weight of fatty acid derivative, conjugation was greatly reduced (Table VIII).
Table VIII.
Effect of Nickel-Carbon Catalyst on Linseed Fatty Acid Derivatives (51)
Derivative Methyl ester Glyceryl ester Pentaerythrityl ester Polymeric glyceride (X-viscosity)
Conjugation, % 46 33 10 0.9
€€e suggests that soybean oil might be treated as follows:
so2
Soybean oil
115" C. Stand oil
\ 290' C. liquid or
Sulfur Dioxide
Sulfur dioxide has been proposed by Waterman and his coworkers as a catalyst for the commercial isomerization of fats to produce: higher melting fats; and faster drying oils (69). Waterman describes this process as operating at three different temperatures: For cis-trans isomerization, 115" to 130" C. For conjugating, 160' to 220' C. For polymerization, 290 " to 300' C.
J
Fractionation
+ solid or margarine
I( 170' C. drying oil
fraction
Conjugated oil Isomerization with sulfur dioxide With linseed oil a t 160' to 220' C. a t 80 atmosphcres, the iodine value was reduced from 180 to 150 a n 4 the diene value increased t o 25.5. This oil polymerized very rapidly. Markley (26) reports that one major interest in this reaction in Holland is the production of margarine. No commercial exploitation of the reaction had been made up to the summer of 1945. However, the commercial scale polymerization of linseed oil with sulfur dioxide as a catalyst has been reported recently from Holland (10); the initial reaction a t 290" C. is a shift to conjugation. Use of Iodide Compounds A new process for the isomerization of polycne compounds to give conjugated oils was recently patented by Ralston and Turinsky of Armour and Company (63). This process involves the heating of fatty acids or thcir csters with iodide compounds in which the iodine is loosely combined. Representative catalysts are ammonium iodide, dodecylamine hydroiodide, triethanolamine hydroiodide, turpentine-iodine addition product, iodoform, phosphorus iodide, aluminum iodide, ctc. Although these compounds vary considerably in their isomerization activity, they all appear to be active to some extent. This recent development probably stems from earlier observations that iodine is a catalyst for cis-trans isomerizati'on of unsaturated fatty acids (49,68). From a commercial standpoint this process appears to be the simplest to operate and if conjugation can be effected without the production of undesirable side reactions, the process may become commercially important. Table IX indicates the success of the author's attempt t o repeat Ralston and Turinsky's work (47). The turpentine-iodine addition product appears t o offer an excellent oil for industrial use since it contains over 30% conjugation: the oil should be readily removable from the catalyst, and the oil has good color. The patents indicate that some aftertack was encountered which was caused apparently by failure to remove catalyst.
Table IX.
Isomerizations with Iodine Compounds (47) Increase i n
This method, as well as others discussed later, does not produce Whereas alkali isomerizes 80 to 85% of linoleic acid, the nickel-carbon catalyst isomerizes only 65 to 70% of the methyl ester. This amount is further reduced when the acids are present in the glyoeride. Thc main advantage of this method, as well as of some of the others, is the retention of the glyceride structure without need for esterification.
Elaidinized oil
___)
Catalyst Used
8s much conjugation as the alkali-isomerization method.
299
CHIa
Turpentineiodine addition product
nD 30'
Reported 0.0057
0.0052 0.0086 PI1 0.0061 a Includes tetraene 0.10 t o
AlIs
Found 0.0039
0.0067 0.0080
0.0079 0.34%.
Conjuwtion, % Diene Triene Total 16.2 2.00 18.3a 31.0 28.0 26.9
0.12 4.99 4.63
Viscosity (Gardner)
. ..
E-F
31.12 Color5-6 33.33a B-C 31.7Qa C
Dehydrated Castor Oil
The industrial value of dehydrated castor oil in varnishes and other protective coatings is well established. Discussion here will be limited primarily to the isomeric fatty acids present in the oil and their relation to the problem of finding a tung oil replacement (5'9). Castor oil contains a high proportion of ricinoleic acid; it approaches the amount of eleosteaih acid present in tung oil. However, commercial methods of converting ricinoleic acid to an octadecadienoic acid have so far failed to convert a high percentage of this acid to the conjugated form; indeed, only 17 to 26% of the conjugated acid is formed (50). This percent-
INDUSTRIAL AND ENGINEERING CHEMISTRY
300
age of conjugated dienoic acid appears t,o be a mixture of cis-cisand cis-trans-9,ll-octadecadienoic acid (as). The nonconjugated isomers likewise are a mixture of the natural linoleic acid and a cis-trans isomer in which isomerization has occurred in 12, 13 posit,ion (29). As discussed under the relation of isomerization to the drying phenomenon, this isomer should oxidize more slowly than the natural linoleic acid and it may be responsible for the slight after-tack which occurs in filnis of dehydrated castor oil paints and varnishes. Comparison of the coniposition of dehydrated castor oil lvit,h that, of tung oil as shown in Table X clearly demonstrates that dehydrated castor oils is not a chemical replacement for tung oil. It is rather an oil which is intermediate in reactivity between tung oil and nonconjugated oils.
Table X.
Comparative Composition of Tung and Dehydrated Castor Oil (29)
F a t t y acid, % of Solid Oleic Conjugated diene Nonconjugated diene Conjugated triene
Dehydrated Castor, %
Tunr, %
3 14 27 43
2-7 1-20 1-10 74-94
Miscellaneous Methods of Conjugation
In addition t,o the methods already described, there are a number of others which produce conjugation, or which are reported to produce conjugation, but the amount of conjugat,ion has not, been checked by a reliable method. Of these methods, blowing wit,h air or oxygen is probably the oldest and is effective in promoting both conjugation and polymerization. Farmer (22) has shown that ethyl linolenate absorbs oxygen t o produce about 28.50/, diene conjugation when allowed to stand a t room temperat,ure, and recently Bolland and Koch (6) have shown that pure et,hyl linoleate a t 45 C. produces approximately 66 to 700/, conjugation in the first step of the autoxidation reaction. Continued exposure to oxygen promotes other reactions which reduce the apparent yield of conjugation but the actual yield approaches a substantial amount. Gunstone and Hilditch ( 2 7 ) have shown that blowing of methyl linolenate a t 80" C. (176" F.) produces approximately 18% conjugation in 4.5 hours. Bergstrom and Holman ( 3 ) recently have shown that in the presence of lipoxidase a t 0 " C., linoleic acid reacts with oxygen to form one pair of conjugated double bonds for every niolecule of osygen absorbed. O'Hare and Withrow (49) have shown that the blowing of linseed and sardine oils a t 220 O F. allows substantial oxygen absorption in the oils in a relat,ive short, time. Their reported absorption curves indicate that substantial anlourits of diene conjugation are formed. Xovak (48)has patented a process of conjugation which involves the blowing of linseed oil a t 65 O to 70' C. until the oil is bleached and then further blo\+inga t 40" to 45" C. Moist air is said to accelerate the change. Selenium dioxide oxidation of the linseed oil is report'ed to give an hydroxy-fatty-acid radical when 0.5 niole of the dioxide is reacted with 1mole of fatty acid radical (6.4). By dehydration of the hydroxylated oil, an oil is obt,ained which has a diene value of 24.8; t,his indicates substant,ial conjugation. Chemically this method appears to be sound, except that dehydration might not be responsible for the conjugation which occurs. Hydroxylation might take place a t the active methylene radical of the 1,4-pentadiene system. Dehydration to a conjugated system might be difficult to accomplish. However, rearrangement during hydroxylat,ion may occur and give conjugation (6). Boone ( 7 ) and Colbeth (15) have patented processes of oxidation and dehydroxylation which are reported to give conjugation. Certain bleaching eart,hs ( S Q , 65), magnesium silicates (6'51, selcniurn (nf), and tertiary amines (10) h a w bern reported t o produce conjugation in fatt,v acid radicals o f vegetable oils but
Vol. 41, No. 2
none of these methods appear t,o be promising for commercial operation. The pilot plant production of Corlinol, an isomerized linseed oil, is reported from England (62). On examination, it was found to cont,ain approximately 35% conjugation (47). Active halogen compounds such as tert-butyl hypochlorite have been shown recently to react, wit,h soybean oil to introduce active chlorine into the moleculeand t,oshift unsaturation to conjugation. Subsequent heating removed a port,ion of the halogen, and it increased the total conjugation to approximately 3oy0 (60). Evaluation of Conjugated Oils
The value of conjugated oils such as t,ung and oiticia oils has been demonstrated fully on a commercial scale. Their inain attributes are: a rapid polymerization rate; reactivity to1var.d cert,ain resins; rapid initial set in varnishes such a i ester gum and phenolic; rapid drying to free from tack in varnishes; excellent resistance to water and alkali; and ability to form wriiiltlt:d finishes. Although a number of diffcrent methods of isomcriztition have been discovered which convert nonconjugated oils t,o conjugated oils, no complete replacenlent for tung oil has been found. A( first thought, this may lead some to believe that thc. basic ideas-t,hat is, conversion of nonconjugated acids to conjugated acids-may not be a sound research approach. Early in the rwearch on isomerization a t t,his laboratory, an atternpi was made t o determine the feasibility of this basic idea. Kass ( 2 s ) prepared one of the high melting isomcrs of 10,12linoleic acid from alkali isomerization of 9,12-1inolcic acid and converted this isomer to the glyceride. Its films were compared with films of alkali-refined linseed oil by A. J. LeIYis ( 3 6 ) . Table XI summarizes t,he dat,a which 1,cwis collected on this comparison.
Table XI.
Comparative Evaluation of Film from Conjugated Glyceride (35)
Characteristics Viscositv Color " Conjugation Drying tests Set-to-touch After 9 days Cold-water immersion (24 hours) Hot-water immersion (60 min,) .ilkah immersion tests 5p% S a O H
Linseed oil
Tri-lO,12-linoloin J J100% (some loht i n
9.5 hours hloderat,e tack Very white Dull 18 hours after removal White and soft White after 24 hours Completely dissolved in 1.5 hours
3.75 hours Tack-free Slightly cloudy Recovered in 2 minutes
A 6 None
preparation)
Very slightly cloudy Immediate recovery I n t a c t after 1.5 hours
The film of 10,12-trilinoleate dried set-to-touch rapidly and was tack-free a t 9 days. It had excellent water and alkali rcsistance, and excellent durability in accelerated weathering tests. Although this sample was too small t o prepare a varnish, this evidence favored the belief that conjugation itself was beneficial and that objectionable features of conjugation processes might be due to side reactions. The rapid rates of polymerization which have been reported for the different, conjugated oils are t o be expected from their composition. For example, an alkali-isomerized linseed oil having LI viscosity of 135 poises (Gardncr Z-6) is reported t o have a gelation time a t 600' C. of 12 minutes, whereas that for tung oil is 6 minutes, and that for linseed oil of 2-6viscosity is 113 minutes. Table XI1 shows the relative bodying rates of nickel-carbon isomerized oils as compared with dehydrated castor oil and alkalirefined linseed and soybean oils (20). Kickel-carbon isomerized linseed oil is approximately five times as fast as the parent oil (alkali-refined linseed oil). Closcly reb t e d i,o oil bodying or polymerization is the kettle time rcquired
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1949
3Q1
Relative Bodying Rates (20)
Table XII. Oil Isomerized linseed Dehydrated castor Isomerized soybean Alkali-refined linseed Alkali-refined soybean
Relative Times Required t o Body Oil 2-15 poises a t 2-46.4 poises 550' F. 560' F. at 590° F. 0.21 0.35 0.93 1.00 2.23
0.37 0.35 0.72 1.00
*.
0.22 0.27 1.01 1.00 2.43
to prepare a varnish. In general, the polymerization speed of the isomerized oil effects a substantial reduction in the required cooking time. This is shown in Table XI11 which gives cooking times of three different varnishes with different oils: dehydrated castor; nickel-carbon isomerized linseed; nickel-carbon isomerized soybean; and alkali-refined linseed. These data may indicate more reactivity toward resins on the part of isomerized oils. However, that question cannot be fully answered since the polymerization rates of the oils themselves is much faster. The Chicago Club of the Federation of Paint, Varnish, and Lacquer Clubs reports data showing that a commercially isomerized linseed oil was a rapid bodying varnish oil (1.4). Their study included ten modified linseed oils.
Table XIII. Oil Dehydrated castor ' Isomerized linseed Isomerized soybean Alkali-refined linseed
Figure 1. Plates from Sanderson Drying Meter Test with Isomerized Linseed Oil Paints
Kettle Times for Varnishes (20) Phenyl phenolic 98 83 160 145
Time, Minutes Maodified phenolic
Rosin
The third attribute of tung oil finishes-that is, the rapid initial set of varnishes-is found to be improved in isomerized oils as compared with parent drying oils. Oil films and paints from isomerized soybean and linseed dry set-to-touch much more rapidly than paints and varnishes from the parent oils. Figure 1 shows plates from Sanderson drying meter tests which contained nickel-carbon isomerized and unisomerized linseed oil paints, Plates 1, 2, 4, and 5 are isomerized linseed oil paints, and plate 3 the parent oil. The isomerized oils used were prepared a t 160 ', 170 ', 180 ', and 190' C., respectively. The minimum in drying time was with the oil isomerized at 170'. The parent oil paint film required 6 to 7 hours (1 revolution of the disk equals approximately 9 hours) whereas the isomerized-oil paint film required 2.5 to 3.5 hours to dry set-to-touch. This rapid initial set-to-touch drying of the isomerized and conjugated oils also is found in the varnishes prepared from them. Table XIV shows that isomerized linseed is a faster drying oil than the original linseed in three different varnishes but that neither is equal to dehydrated castor oil ($0).
Table XIV.
Drying Rates of Varnishes from Isomerized Oils (20)
Varnish %Phenyl phenoljc I odified phenollc Ester g u m
Set-to-Touch Time, Hours Dehydrated Isomerized castor oil linseed oil Linsoed oil 3.5 5.3 5.8 3.0 3.3 5.1 3.7 2.5 5.0
1 160' C.; 2 = 170' C.; 3 = control (alkali-refined linseed oil); 4 = 180' C . ; 5 = 190' C.; complete circle repreeente 8 hours 49.2 minutes
set hard as the varnishes from many other oils, including the parent oil. Indeed, the varnish films from isomerized linseed are generally softer than those from varnishes of the parent oil. In the study by the Chicago Club (1.4) the commercially isomerized linseed oil usually gave the softest varnish film of the ten oils tested. With a hydrocarbon resin, however, t gave the hardest film. I n studies on nickel-carbon isomerized linseed oAl(oil 2) the varnishes from the parent unisomerized oils (oil 4) were generally harder. In Table XV the hardness of films from comparable varnishes of dehydrated castor oil (oil 1) and isomerized soybean (oil 3) are also given.
Table XV.
Sward Hardness5 of Varnish Films (20)
Oil 16 Oil 2 Oil 3 Oil 4 Varnish At 24 A t 336 At 24 A t 336 A t 24 At 336 .4t 2 4 A t 336 Series hr. hr. hr. hr. hr. hr. hr. hr. A 19 42 14 39 4 22 18 44 8 19 B 10 20 2 8 10 25, C 22 37 16 33 7 19 18 34 D 15 37 16 35 10 26 22 34 E 4 10 10 16 8 12 8 12 a Plate glass hardness = 100. b Oil 1 = dehydrated castor oil; 2 = isomerized linseed; 3 = irornwized soybean; and 4 = alkali-refined linseed.
The varnishes from alkali-refined linseed oil are usually 4 to 6 Sward units harder than the isomerized linseed oil; the dehydrated castor oil varnishes are harder than isomerized soybean and linseed but softer or equal to linseed varnishes. This failure to dry hard is one of the major drawbacks to isomerized oils and it has been called after-tack. Table XVI. Comparative Hot-Water and Alkali Resistance of Isomerized Oils (20)
Just as there is a carry-over of the rapid bodying characteristic of the isomerized oil to the varnish cooking, varnishes from isomerized oil usually set-to-touch faster than the varnishes from the parent oil. As with the cooking times and polymerization times, the beneficial effect is reduced. However, what is probably more important, the varnishes from isomerized oils do not
Oil Isomerized linseed Linseed Isomerized soybean Soybean
Hot Water Cloudiness Recovery, Minutes Slight 15 White Over 7 days Clear Cloudy 20'ipitted)
0.1 N Wrinkling Severe Severe Moderate Moderate
NaOH Removal, 50 in 192 hr. 100 in 6 hr. 75 in 192 hr. 100 in 72 hr.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
302
Figure 2.
B
4
Plates from Sanderson Drying Meter Test with Isomerized Soybean Oil Paints
= control (alkali-refined s o y b e a n oil); 2 = 160' C.; 3 = 170" C.; 180° C.: 5 = 190' C.; 6 = control (nonbreak soybean oil); complete circle represents 8 hours 49.2 m i n u t e s
=
The water and alkali resistance of films of isomerized oils arc decidedly superior to films of the parent oils. Table XVI gives comparisons between linseed and soybean oils. However, this iniproved resistance is not carried over to the varnishes a3 much as might be hoped. T h e Chicago Club rcports that the commercially isomerized linseed oil was probably t'hird in cold-water and fourth in hot-r-iat,er resistance of ten oils tested. Studies on varnish films containing nickel-isomerized linseed and alkali-refined linseed oils indicate that sonie superior alkali resistance i s found in t,he varnish fiIm of the isomerized oil but, in general, t,he varnish films of the parent linseed oil and of isomerized oil are approxiinately equal in their behavior. However, the films of the varnishes from dehydrated castor oil were consistently better in hot- and cold-water resistance and in alkali resistance when compared with the nickel-carbon isomerized soybean and linseed oils as well as with the original linseed. I t appears that dehydrated castor oil is superior to both nickclcarbon isomerized linseed and soybean oils as a varnish vehicle and that it, is comparable to alkali-isomerized linseed oil if ease of manufacture and versatility of the oil are considered. Dehgdratcd castor oi! can be prepared with fairly low viscosities such as Qardner J or K, and it is not necessary to esterify the fat acid to obtain the glycerides or other esters. Of all the oils containing isomeric linoleic acids, dehydrated castor oil has proved thus far t o be the most useful. In 1944 over 70,000,000 pounds of this oil QWT used in the drying oil indust,ry (f),
Pionbreak soSbean oil with baqic carbonate white lead
Raw Iinsred oil with ha& oarbona t e white lead
Lime-treated conjugated soybean oil with basic carbonate white lead
Conjugated soybean oil with basic carbonate white lead and 5.5% calcium oxide
Figure 3.
Dirt Collection of Paints from Conjugated and Other Oils
Vol. 41, No. 2
Observations Concerning After-Tack During the course of the investigations on nickel-carbon isonieriaed oils, it was noted that the temperature of isomcrizatioii had an appreciable effect on the quant,ity of after-tack, Talilt, VI shows t'hat the isomerization, as measured by total corijugrtion, increased with temperature, However, the tackiness of tlic derived oil and paint films (61, 62) go through a minimum, ant1 the initial drying rate through a maximum, at a tcmpcrat,ure of npproxinitely 170" C. Figure 2 shows plates from a Sanderson drying metcr tost. Plates 2, 3, 4,and 5 show paint films from isomerized soybean oils (temperatures 160°,170", 180", and 190" C.) while plates 1. and ii show alkali-refined and nonbrealc soybean oil, respcct,ivolv. Comparison of the plates shows that number 3 dried set-to-touch in 4 to 6 hours whereas the unisomerized oils dried set-to-touch iri 8 to 11 hours. These plates show that temperature affects initial drying rates arid subsequent vork s h o w d lhat films from oils isomerized a t a higher temperature had an objectionable residual tack. Although the films from oils isomerizcd a t 170' C. had the least amount, of residual or after-tack, the paint films were excellent dirt collrctors. Tests on the Sanderson meter indicated that zinc oxide reduced this after-tack but that it was still objectionahle. Sincc the zinc oxide-pigmented films of the conjugat,ed oils ~ e r superior c to the other conjugated soybean oil films, ol,hcr alkaline earth oxides were tested. It was found that, calcium oxide substantially reduced this objeetionablc aftm-i acli so that the dirt collection on t,hose films cont,aining calcium oxitlcr was no inore than on a comparable linseed oil paint film ( 3 7 ) . Figure 3 shows paint panels that were cxpo'sed ai. Peoria, Ill., and in which t,he only oil vehicle was nickel-carbon isomeriztid soybean oil. The lime-t,reated soybean oil paint film is almost, black, whereas t'he one containing 5 ~ 5 %calcium oxide i s whit>c. Soybean and linseed paint films were included for comparison. All of the allralinc-eart,h oxides tested substantially reduced this objectionable after-tack but no other matciials appear to act. in a similar manner. Additional work indicates tho calcium oxide hydra1 e s t n calcium hydroxide and then the charaet,cristic x-ray diffraction liiics of calcium hydroxide gradually disappear from the film. I t would appear that the calcium hydroxide, as well as t h o other alkaline earth oxides, form salts with the organic aciJs present. I n the presence of aluminum oxide, lhe ame film? absorb water and become exceedingly tacky ( 4 7 ) . In the presence of most pigments, except alkaline earth oxides, the nickel-carbon isomerized soybean paint films become exceedingly tacky and acidic. It, now appears that the formation of elaidinized derivatives of oleic acid may be responsible for this after-t'a4r, since these forms are known t o oxidize slowly in the presence of o x i d d o n catalysts and t o be present in the nickel-carbon isomerized soybean oil. Styrene Copolymerization One possible outlet for the catalytically conjugated oils is the production of varnish vehicles by copolymerization with styrene. X recent publication by Hewitt (28) reviews this subject wit,h respect to tung, dehydrated castor, and linseed oils. From a chemical standpoint, conjugated fatty acid radicals and nonconjugated fatty acid radicals differ in their react,ivity toward styrene. When an oil such as soybean or linseed is copolymerized with styrene, the nonconjugated fatty acid radicals act as cffect.ivc chain transfer or chain terminat,ion reagents. This reaction can be represented schematically. XXV may be considered as an activated molecule, such as a free radical from a peroxide, which initiates a chain reaction with styrene to give a polymeric free radical XXVI. XXVI might react with a nonconjugated dienoic fatty acid to destroy the free radical and give the polymer XXVII. A new free radical XXVIII would appear which would be reactive toward reactioninitiating radicals. Achally, little polymer is formed vvhcn large amounts of nonconjugatcd fatty acids arc prcscnt,; this indicafos
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
February 1949
R-
CH-CH2
Gelled products are not obtained with oils such as linseed, but incompatible polymers are obtained readily. These linseed oil bh l x copolymers do not give clear films. The explanation appears t o XXVI be that copolymers of varying molecular weight are formed in the R.CH2.R’ RCH-CHz -H R-CH-R’ reaction. Unpolymerized linseed oil gives practically no viscosXXVI ity increase whereas polymerized linseed gives polymers which --+ [Ah ]x separate into two parts during the course of the reaction. XXVIII XXVII I n Table XVII, the influence of conjugation with respect t o Copolymerization with nonconjugated fat acids combining power with styrene is indicated by amounts of styrene combined with different viscosities of dehydrated castor oil. that reactions of the nonconjugated fatty acid with XXVmay occur It is reported that a copolymer of styrene, with 100-poise deto destroy any molecules which might initiate the polymerization. hydrated castor oil containing %yo combined styrene, gave a clear film drying within 0.5 hour; R’-CH=CH-CH=CH-R” Ph-CHXCH2 ---+ this hardened by further oxidation and yielded a film equal in water resistance t o a tung oil phenolformaldehyde resin spar varnish. In connection with the reaction between dehydrated castor oil and styrene, high molecul’ar weight acidic materials of approximately 4000 and 1000 molecular weight have becn isolated by the saponification of the copolymer. The 4000 molecular weight Lhh ]x fraction was tough and hard, whereas the 1000 molecular weight XXIX fraction was soft and balsamlilte. Copolymers of styrene with conjugated isomers R.
xxv
Ph-CHZCH2
303
_____f
[
-*
+
Summary
The conjugated fatty acid radicals may act as chain propagation agents and promote reaction between the fatty acid radical and styrene in a manner somewhat similar ’to butadiene. I n this sequence of reactions, the polymeric free radical XXVI reacts with the conjugated fatty acid to give another polymeric free radical without destroying the activated center in the molecule. Consequently, polymerization may continue, and gelation of the reaction mixture containing glyceride ester comes rapidly after a certain degree of copolymerization is obtained. If a trifunctional molecule such as a glyceride is visualized as reacting with styrene, then the rapid gelation after some degree of reaction is achieved is understandable. One scheme of depicting oil-styrene crosslinlcing is as follows: Copolymerization with conjugated oils
-R-I
I-R-
-R-1
1-R-
crosslinked polymer Fronia practicalstandpoint, anoil which contains some amounts of both types of fatty acid radicals is preferred. Dehydrated castor oil and catalytically conjugated linseed or soybean oil are suitable for the polymerization. However, films from styrene copolymers of nickel-carbon conjugated soybean and linseed oils might give films which exhibit residual or after-tack. With some blown oils such as blown linseed oil, compatible copolymers with styrene can be obtained. Oils blown at 100’ to 120’ C. give better results than oils blown a t lower temperatures. Recent investigations by Gunstone and Hilditch ( S 7 ) and others (6, 48) indicate that these blown oils contain considerable amounts of conjugation.
Table XVII.
Copolymerization of Dehydrated Castor Oil with Styrene (28) Viscosity, Poises 20 36 60
100
Combined
Styrene, %
80
70 62 35
There now are available a number of methods which will convert nonconjugated vegetable oils to conjugated oils. The alkali method produces the highest conjugation, whereas the dehydration of castor oil appears to be the most useful method which has been discovered to date. Recently discovered methods have one advantage over alkali in that the glyceride molecule is not disrupted. Their utility will depend on the methods found t o control production of undesirable isomers such as the elaidic and position isomers of octadecenoic acid, or on the utilization of materials which remove or appreciably reduce theafter-tack in the derived protective coatings. Copolymerization of conjugated oils with styrene may not remove the objectionable after-tack, but this approach should lead to new and interesting materials. Literature Cited AgIicultural Statistics, U. S. Dept. d g r . (1945). Bailey and Fisher, Oil & Soap, 23, 14 (1946). Bergstrom and Holman, Nature, 161, 55 (1948). Betram, U. S. Patent 2,165,530 (1939). Bickford, Dollear, and Markley, OiE & Soap, 15, 256 (1938). Bolland and Koch, J . Chem. SOC.(London),1945, p. 445. Boone, U. S. Patent 2,308,152 (1943). Bradley, U. S. Patent 2,350,583 (1944). Bradley and Richardson, IND. ENO.CHEnr., 34, 237 (1942). Burr, U.S. Patent 2,242,230 (1941). Cannegieter, Paint, Oil Chem. Rev., 110, No. 4, 17, (1947). Cary and Beckman, J . Optical Soc. Am., 31, 682 (1941). Cawley, U. S. Patent 2,343,644 (1944). Chicago Club Report, Federation of Paint and Varnish Clubs, Convention a t Home Daily (Oct. 28, 1943). Colbeth, U. 6. Patent 2,388,122 (1945). Craig, J . Am. Chem. Soc., 65, 1006 (1943). Dann and Moore, Biochem. J . , 27, 1166 (1933). Ellis, Ibid., 26, 791 (1932). Ellis and Jones, Analyst, 61, 812 (1936). Falkenburg, Schwab, Teeter, and Cowan, IND.ENO. CHEM., 38, 1002 (1946).
Farmer, Koch, and Sutton, J. Chem. SOC.(London), 1943, p. 541. Farmer and Sutton, Ibid., p. 119. Gardner and Sward, “Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors,” 10th ed., p. 429, Henry A. Gardner Laboratory Inc., 1946. Gillam, Heilbron, Hilditch, and Morton, Biochem. J., 25, 30 (1931).
Gilman, “Organic Chemistry,” 2nd ed., Vol. I, pp. 218, 455-6, New York, John Wiley & Sons, 1943. Goss and Markley, U. S. Dept. Commerce, OTS Rept. PB 1254, pp. 55-59 (1945).
Gunstone and Hilditch, J. Chem. SOC., 1945, p. 836. Hewitt and Armitage, J. Oil Colour & Chemists’ Assoc., 29, 109 (1946).
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(29) Kass, presented before the Division of Paint, Varnish, and Plastics Chemistry at the Memphis Section Meeting of the AMERICAN CHEMICAL SOCIETY, Memphis, Tenn., 1942. (30) Kass, presented as a part of the Symposium on Drying Oils before the Division of Paint, Varnish, and Plastics Chemistry at the h4innesot.a Section Meeting of the AMERICAN CHEMICAL SOCIETY, Minneapolis, Minn., 1947. (31) Kass and Burr, J . Am. Chem. Soc., 61, 1062 (1939). I b i d . , p. 3292. Kass and Skell, presented before the Division of Organic Chemistry at the Detroit Section Meeting of the ANERICAN CHEYICAL SOCIETY, Detroit, Mich., 1943. (34) Kirschenbauer, U. 5 . Patent 2,389,260 (1945). (35) Kuhn and Meyer, 2 . Physiol. Chem., 185, 204 (1929). (36) Lewis, unpublished work, Northern Regional Research Labora8 .
in,.., rYI J
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(37) Lewis, Moser, and Cowan, unpublished xork, Nortlie1,ri Re-
gional Research Laboratory. (38) Mattiello, “Protective and Decorative Coatings,” 1701. IT’, Chap. 12, New York, John Wiley & Sons, 1944. (39) Mitchell and Kraybill. J . Am. Chem. Soe., 64, 988 (1942). (40) Mitchell, Kraybill, and Zcheile, IND. EXG.CHEX.,ANL. F h . , 15, l(1943). (41) Moore, Biochem. J . , 31, (1937). (42) Morrell and Davis, J . Chem. Soc. (London),1936, p. 1481. (43) Morrell and Davis, Trans. Faradau Soc., 32, 209 (1936). (44) Morton. Heilbron. and Thompson, Biochem. J., 25, 20 (1931) (45) Myers, Kass, and Burr, Oil &-Soap, 18, 107 (1941). (46) Nicols, Herb, and Riemenschreider, unpublished woik, Easter 11
Regional Laboratory. (47) Korthern Regional Research Laboratory, unpublished WOI k. (48) Xovak, U.S. Patent 2,178,604 (1939). (49) O’Hare and Withrow, IKD.ENG.CHEM, 39, 101 (1947). (60) Priest and Von Mikusch, Ibid.. 32, 1314 (1940).
Vol. 41, No. 2
(51) Radlove, Teeter, Boild, Cowan and Kass, I b i d . , 38, 997 (1946). (52) Radlove, Teeter, and Cowan, Oficial Digest, Federation Paint & V a r n i s h Production Clubs, 265, 74 (1947). (53) Kalston and Turinsks-, U. S. Patents 2,411,111-3 (1946). (64) Rose, Freeman, and hIoKinney, IND.EKG.CHEDI.,34, At2 (1942). (55) Schicht and Grun, German Patent 287,660 (1914). (56) Spitzer, Ruthruff, and Walton, Am. Paint J . , 26, No. 12, 68 (1941). (57) Steger and Van Loon, Fettchem. Umschau, 43, 17 (1936). (58) Strain, J . Am. Chem. Soc., 63, 3448 (1941). (59) Sunderland, J . Oil & Coloztr Chemists’ Assoc., 28, 137 (f945). (60) Cowan, Teeter, Bachman. and Bell, IXD. ENG. CHeivr., in press. (61) Teeter, Radlove, and Cowan. unpublifihed work, Northern Regional Research Laboratory. (62) Touchin, Paint Manvf. 16 (No. 7 ) , 237 (1946). (63) Turk, and Boone, U. 8 . Patent 2,405,380 (1946). (64) Turk, Damson, and Soloway, Am. Paint J . , 28, No. 9 16 (1943). (65) Turk and Feldman, Paint, Oil, Chem. Rev. 106, No. 13, 10 (1943). (66) Varrentrapp, Ann., 35, 196 (1840). (67) Vlodrop, van, Chem. Weekblad, 38, I50 (1941). (68) Von Mikusch and Frazier, IND.ENG.CHEIM., ilN.\I.. ED., 15, 109 (1943). (69) Waterman and van Vlodrop, Alien Propert>- Custodian, 359,978 (Filed 1940) ; Waterman and co-workers, Verfkronielz, 13, 130-6, 180-2 (1940). RECEIVEDFebruary 2 6 , 1948. Keceiit Fork on isomers of conjugated octadecadienoic acid indicates that revised constants are neressary i n spectrophotometric determinations of polyunsaturated acid8 ( P a p e r 19. Am. Oil Chemists’SUc., New York, Kov. 15-17, 1948; also Papers 16. 18, 3 3 , 5 5 , and 56 on isomerization of drying oils).
P. 0. Powers Battelle Menzorial Institute, Columbus, Ohio
T h e steps encountered in the oxidation of drying oils by air are peroxide formation, decomposition of peroxides, polymerization, and degradative oxidation. A n induction period occurs prior to uptake of oxygen and this has been shown to be due to the presence of inhibitors. Recent work has shodn the presence of hydroperoxides, particularly when oxidation is conducted under mild conditions, but there is also considerable evidence for the presence of cyclic peroxides. Peroxides apparently decompose by dehldration and by reduction, I t seems possible that peroxide decomposition and polymerization are intimately associated. Polymerization inevitably occurs subsequent to oxidation but it has not been established clearly whether the polymer chains are formed through either ether or peroxide linkages or through carbon-to-carbon bonds. Considerable evidence for the latter structure has been accumulated.
U R I S G recent yeais a careful study has been made of the oxidation of animal and vegetable oils; still, the statement of Xilas ( 2 4 ) made in 1932, ‘‘ , the mechanism of the oxidation of unsaturated oils is still relatively obscure owing to the complexity of the oxidation products and the uncertainty of their structure,” remains an accurate estimate of our present knowlpdgr. Progress has been apparent in the intervening 15 years and much valuable work has been done by university, government, and industiial laboratories both here and abroad. In bpite of all this artivin , it must be admitted that an entirely batisfactory
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explanation of all the phenomena observed in the oxitfatiun of drying oils has not been achieved. This failure often has been attributed to the complexit,y of the drying oils themselves, arid it is only in recent years that an accurat,e analysis of the composition of linseed oil was possible. However, our knowledge of the composition of t,he common drying oils is now on a quantitat,ive basis, and the accuracy of the met,hods of analysis has been greatly improved. A great deal of progress has been made in the separation of the pure fatty acids. Methyl esters of these acids serve as useful methods of interpreting t,hebehavior of the drying oils on oxidation. The study of the behavior of mixtures of esters does not always para,llel the behsvior of the pure ester since the oxidation of oleate esters is greatly increased ( 1 8 ) in the presence of a small amount of a linoleic acid ester. Rate of oxidation is increased, however, with the unsaturation (33),and when mixtures of trienoic and dienoic esters ( 1 5 ) are present, the trienoic esters are oxidized first. Also, when dienoic and oleic esters are present,, the dienoic esters are oxidized first. I n one respect the methyl esters are not completc models for the drying oils. They do not dry and thus remain susceptible t o oxidation to a greater degree than do the glycerides. Thus, greater amounts of oxygen may be absorbed than will be absorbed by the corresponding glyceride (38). Such infornation may bc applicable t o the study of deterioration but is a further stage of oxidation than usually is encountered in the drying process. The complexity of the products of oxidation of the drying oils is well known and is the principal reason for t,he difficulties c:n-
