DRYING OILS AND RESINS Reactions Involving the Carbon-to-Carbon Unsaturation during the Thermal Treatment of Some Esters of Unsaturated C,, Fatty Acids’ THEODORE F. BRADLEY AND WILLIAM B. JOHNSTON American Cyanamid Company, Stamford, Conn.
When the methyl or ethyl esters of the unsaturated C1g fatty acids are heated to 300’ C., their unsaturation is reduced and
The authors appreciate that iodine values as well as molecular weight determinations are subject to doubt as a result of inherent limitations and easily effected analytical errors. The fact remains, however, that no alternative methods of analysis have appeared ,which can be generally applied as successfully as these methods. Conjugate unsaturation can best be determined by hydrogen values rather than by iodine values, yet evidence now appears that in the case of the natural drying oils and related esters conjugation is rapidly reduced and often totally destroyed during thermal treatment and that hydrogen is more difficult to add to the polymeric derivatives than are the halogen reagents. With the minimization of the negative errors introduced by conjugation (which can be avoided by the use of the ultraviolet spectra or in some cases of the maleic anhydride or “diene values” for detecting conjugation in advance) and by the use of the standard methods under which the positive errors introduced by substitution may be minimized, it is believed that iodine values remain as the most useful and acceptable analytical tool for the measurement of the carbon-to-carbon double bonds in the examination of drying oils and related esters, including their polymerized forms. This and two other papers (3, 4) provide experimental data which are believed to support these views.
numerous derivatives are formed. The major reaction products are polymers which are predominantly dimers. The physical and chemical constants permit of the recognition of three classes: The first originates from the octadecatrienates and possesses a bicyclic structure, the second is formed from the octadecadienates and has a monocyclic structure, and the third class is a heterogeneous group formed both from the oleic and the more unsaturated esters by partial thermal decomposition and recombination of certain of the derivatives. The nature of these products, the probable mechanisms of formation, and their influence on stand oils are discussed.
I
N T H E first application of general polymerization theory to the chemistry of drying oils ( I ) , it was shown that the gelation phenomena depended upon the active functionality of the molecules and that the entire double-bond systems of the drying oil acids behaved with respect to polymerization as though their functionality were but one. This strengthened the evidence favoring the bimolecular cyclization of fatty acid radicals as the active polymerization mechanism. (The recovery of dibasic acids from polymerized drying oils had previously been noted by some but disputed by others on the grounds that the molecular weight determinations were unreliable and were the consequence of colloidal association rather than of primary chemical reactions.) I n the second application of the general polymerization theory (8), the first approach was entirely neglected, and account was taken of the potential rather than of the active functionality of these systems. This required the measurement of the accumulated unreacted groups and of the molecular weights, allowance being made for the well-known effect of conjugation upon the iodine value in the case of tung oil. This treatment immediately showed that, while polymerization was the predominant reaction in each case, intramolecular reactions occurred also, especially in the case of linseed oil. This suggested that more attention should be paid to the nature of the intramolecular reactions and their effect upon stand oils. 1 Previoua
papers in this series appeared i n 1937, 1938, 1939, and May.
FIGURE I.
1940.
802
APPARATUS FOR HEATTREATING OF OILS
JUNE, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
In the present work a direct, experimental approach was made to ascertain whether the conclusions previously derived from the second application of polymerization theory are valid and to learn more about the type and degree of unsaturation which exists in these esters as well as the changes which result from thermal treatment under conditions which correspond to those used for the industrial production of stand oils. An adequate review of the prior investigations of others cannot be undertaken within the available space; therefore, the more pertinent recorded data are tabulated for conTenience. The data of Kino (7-14) and of Steger and van Loon (18-27) were found particularly helpful. Despite their extensive investigations and the related important work of others (6, 15, l o ) , present tabulations show that insufficient attention has generally been given to the determination of many of the physical and chemical constants of the substances reported. I n few, if any, cases are pure compounds represented since complex mixtures result from the thermal treatment. A considerable number of physical and chemical constants are then required to characterize these properly and to determine the results of such separations as have been attempted. The present work seeks to improve this situation and by collaboration with associates utilizes the ultraviolet absorption spectra as new and additional analytical means (4). Preparation of Unsaturated Esters The methyl esters of the total fatty acids from linseed, olive, soybean, and tung oils of standard commercial grades were prepared by metholysis. The fatty acids of dehydroxylated castor oil as representative of a product rich in the conjugated or 9,lloctadecadienic acid and the fatty acids of Armour and Company's "Neo-Fat 3R" as re resentative of a product rich in the nonconjugated or 9,12-octa$ecadienic (linoleic) acid were esterified directly with methanol. The first method of preparation is typified by the following example: 1500 grams of soybean oil, 1500 grams of methanol, and 2 grams of potassium hydroxide are refluxed on a steam bath during 2 hours (the mixture becoming homogeneous within half an hour). After cooling, 1500 grams of distilled water and 10 cc. of concentrated hydrochloric acid are added; the methyl esters are separated out, well washed with water, dehydrated over anhydrous calcium chloride, and distilled. Two batches (3000 grams of soybean oil) provided 2680 grams of methyl ester boiling from 155' to 165" C. at 1 mm. The amount of catalyst (potassium hydroxide) required for the ester interchange varied, with the different oils, from 0.0013 t o 0.004 per cent of the weight of oil. The second method of preparation may be illustrated by the following: 560 grams of dehydroxylated castor oil fatty acids, 1 liter of anhydrous methanol, and 25 grams of concentrated sulfuric acid are refluxed for 3.5 hours. When the reaction is nearly complete, the mixture forms two layers, the lower ester layer
803
being separated, washed with sodium bicarbonate solution and then with water, and finally dried over anhydrous calcium chloride. Distillation provided 382 grams of ester boiling at 160" to 162" C. at 1 mm. The analytical constants of the esters thus prepared are listed in Table I.
REFLUX DEVICE
THERMOMETd
GAS
FIGURE 2. DIAGRAM
O F APPARATUS FOR OF OILS
HEATTREATVEKT
These oils were selected to provide a group of methyl esters whose known constituents, aside from minor proportions of saturated fatty acid esters, would represent materials in which each of the unsaturated C18fatty acid esters occurred in proportions such that the behavior of those predominating could be determined. Thus, from tung and linseed esters the influence of the conjugated us. unconjugated octadecatrienates could be ascertained; from dehydroxylated castor us. soybean and Neo-Fat 3R the influence of the conjugated us. unconjugated octadecadienates (linoleates) could be learned; finally in the case of the olive oil esters the absence of more than a small percentage of the polyene esters should allow the evaluation of oleic ester. For present purposes it was believed unnecessary to undertake the preparation of the esters of the individual pure acids, particularly in view of the work already accomplished by Kino (7-14), Steger and van Loon (18-27), and others. Analyses were conducted by conventional standard methods. All iodine values were conducted by the Kauffman method (1-hour), except in the case of tung and dehydroxylated castor oil esters where the Wijs (2-hour) method was substituted. TABLEI. ANALYTICAL COTSTANTS O F METHYL ESTERS Molecular weights were determined by the SaponifiRast camphor method. These analyses were Oil €3. P. cation Acid Iodine Rast Represented at 1 M m . ?&kg d:i,5 No. No. No. Mol. Wt. s u p p l e m e n t e d b y u l t r a v i o l e t absorption c. s p e c t r a w h i c h a r e p r e s e n t e d in another Olive 140-160 1.4493 0.8718 191 1.5 83.4 301 paper (4). Olein fraction 161-162 1.4508 0.8775 193.2 .. 85.6 from neat'sfoot
Soybean Neo-Fat 3 R Dehydroxylated castor Linseed Tung a
155-168 155-165 140-160 155-165 156-162 155-162 155-168 150-170 160-175 160-180
1.4549 1.4552 1.4552 1.4559 1.4655 1.4661 1.4608 1.4612 1.4928 1.4938
Uaed only for ultraviolet spectra.
0.8782
0.8803 0.8798 0.8780 0.8972 0.8903 0.8871 0.8857 0.8933 0.8953
211 206 190 194 191
...
193.5 191.0 192 194
...
1.9 0.9 2.0 1.0 1.0
129.2 134.5 136.5 140.0 131.0
...
... ...
1.:
176.2 183.1 164.0 156.1
299 287
...
2.,
1.3 3.8
303
...
...
...
...
Apparatus for Thermal Treatment of Esters The apparatus consisted of an electric oven of special design (Figures 1 and 2 ) in which was supported a three-necked round-bottom flask of suitable size serving as the reaction vessel and connected to a solid carbon dioxide trap for collecting any highly volatile material. The electric oven was made from a 15-gallon (56.7-liter) steel drum surrounded by a 1.5-inch (3.8-cm.) layer of asbestos and heated by six 150-watt strip heaters arranged vertically around
INDUSTRIAL AND ENGINEERING CHEMISTRY
804 TABLE
11.
POLYMERIZATION O F hfETHYL
ESTERS OF
FATTY ACIDS
V.4RIOUS
Q.aI L
B. P. a t 1 Mm.,
c.
ponification No.
Acid No.
Iodine No.
Rast .Mol. Wt.
Reacted
Hr.
Temp., O C.
Yield
83.4
301
..
...
...
525
5 5 24
325 325 300
20.0 79.5 9.3
300 443 432 427
24 24 24
300 300 300
78.8 1.9 4.8 3.9
n$?
d;:,$
140-160
1.4493
0.8718
191
...
ZOOa 100-150 300a
1.4686 0.9062 1.4478 0.8694 1.4742 0.9142
159
...
... ...
153
2.2
90.4
1 2 3 4
100-170 170-180 180-200 200-220
1.4476 1.4582 1.4612 1.4650
206 160 140 145
3.8 2.4 2.8 2.6
69.7 100.8 108.6 107.6
1
160-162 2005 130-160 160-162 200a 100-168 < 130 130-160 160-161 161-163 163-164 164-165 1657168 Residue
1.4655 1.4794 1.4555 1.4661 1,4783 1.4556 1,4427 1.4538 1,4575 1,4594 1,4612 1.4638 1.4665 1.4789
155-165
1.4552 1.4798 1.4532 1,4805 1,4478 1.4549 1,4801 1.4483 1.4483 1.4772 1.4453
Product
%
Olive Oil Acids Original ester Polymer Distillate Polymer Distillate Fraction Fraction Fraction Fraction Original Polymer Distillate Original Polymer Distillate Fraction Fraction Fraction Fraction Fraction Fraction Fraction Fraction
2 3 4 5 6 7 8
A. Original Polymer Distillate B. Polymer Distillate C. Original Polymer Distillate D. DistillateC Polymer Distillate
100-160
E. Original
155-165
Polymer Distillate Distillate Distillate Distillate F. Original Polymer Distillate 1 Distillate 2 Distillate 3 Distillate4 Original Polymer Distillate A. Original Polymer Distillate Fract. 1 Fract. 2 Fract. 3 Fract. 4 Fract. 5 Fract. 6
a
... 4
... 155-165 a
130-160 130;160
0
100-120 120-155 155-160 Residue 140;160 50-60 60-80 80-180 180-220
.a, . 150-160 155-168 (I
...
