Molecular Distillation of Polymerized Drying Oils RICHARD S . MORSE' Distillation Products, Inc., Rochester, N. Y.
I
N RECENT years many advances have been made in high-vacuum distillation technique, and it is now com-
mon practice to perform commercial processes under pressures of a few millimeters of mercury. Although in this way fatty acids and other products of comparable vapor pressure may be treated, it has not proved feasible to distill the animal and vegetable glycerides. At much lower pressuresthat is, a t less than 0.01 mm.-the boiling point, or more properly the evaporation temperature, is again greatly reduced and the natural fats become more volatile. Even so, with conventional apparatus one encounters excessive decomposition, and the rate of distillation is too slow for practical purposes (6). With these low pressures it is thus necessary to employ many radical alterations in still design. One of the earliest molecular stills was described by Br@nsted and Hevesy (1) who employed it to separate the isotopes of mercury. One of the first successful applications of the principle to organic materials was made by C. R. Burch who forecast a revolution in the technology of distilling nonvolatile substances of high molecular weight. The principles of molecular distillation as they are thought of today were demonstrated independently by Burch, Washburn, and Waterman, and others. The essentials are: (a) The hot evaporating and cool condensing surfaces are situated opposite each other and are separated by unobstructed space. (b) The pressure of residual gas is reduced to such a low value that it has no effect upon evaporation from the distilling surface or on transport of the vapor to the condensing surface; this means that the distance between the hot and cold surfaces is maintained small compared with the mean free path of residual gas molecules. Many improvements have been made in unobstructed-path distillation apparatus, and the general technique of molecular distillation, especially of natural organic materials, has made notable advances (9). I n particular, various means have been devised for increasing the rate of distillation. This work has centered around the development of high-speed diffusion pumps and the design of distilling surfaces capable of exposing large quantities of distilland in the form of a thin film. Commercially, molecular distillation has come into use for the treatment of vitamin-bearing oils on a relatively large scale @)-that is, on the order of hundreds of gallons daily. The technique of commercial distillation has not, however, yet reached the point where it can be used competitively as an inexpensive means for refining animal and vegetable oils for edible and drying oil purposes. Nevertheless, the possibility of producing drying oils of improved characteristics from inexpensive sources with the aid of molecular distillation has continued to intrigue investigators. Some years ago Waterman and Oosterhof made a brief investigation of the method for the treatment of Polymerized linseed oil and demonstrated that a reduction of drying time can be achieved by the elimination of low-molecular-weight 1
Present address, National Research Corporation, Boston, Mass.
A series of fractions was distilled from a commercially processed and polymerized fish oil. By the removal of approximately 10 per cent of the low-molecular-weight constituents, a residue can be obtained which has greatly improved film-forming characteristics. Further improvements in drying time and film hardness may be obtained by rebodying polymerized oils from which lowmolecular-weight materials have been removed by distillation. The removal of distillate from polymerized oiticica, castor, and walnut oils gave residues which had drying times considerably less than those of the original polymerized oils.
constituents (8, IO). The processing of drying oils by highvacuum distillation has been the subject of numerous foreign and United States patents (6,8).The present work was undertaken to explore the possibilities of high-vacuum distillation in connection with the preparation of oils for paints and varnishes, in the belief that commercial molecular distillation will soon advance to a point where thia information may be applied. Before the full implications of high-vacuum distillation to the drying oil industry can be assessed, it will be necessary t o distill a large number of raw materials. The effect of bodying before and after distillation and the value of distillation as a step in longer processes of preparation are open to study. The purpose of the present work is to demonstrate that molecular distillation can improve a commercial-bodied fish oil selected more or less a t random from the market. It is not known exactly what methods were used to polymerize or otherwise prepare the oil before it was purchased for these experiments.
Distillation of a Bodied Fish Oil A series of fractions was distilled from a commercial heavybodied fish oil having the following characteristics: Acid value Refractive index a t 40° C. Viscositv a t 2 5 O C.. noises Iodine Value ' unsaponifiable matter aponifiostion number
r
3.78 1.4840 10.5 ii2.0 3.85
222.0
The bodied oil was distilled under an average pressure of one micron (10-8 -.) of mercury, and ten fractions representing 40 per cent of the original oil were removed. Samples of the residue were taken after the withdrawal of each distillate fraction. way it was possible to obtain a series of residues from which various percentages of the original oil 1039
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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
had been removed by distillation. Thus, except where otherwise indicated, the residues, and not the distillates, are the object of study, and the notation ' I % distillate removed" denotes the total quantity of material removed by distillation a t any point. After the removal of approximately 30 per cent distillate, some difficulties were encountered during distillation because of the polymerized material which formed on the distilling surface. Care should therefore be exercised in the interpretation of the data which represents the later stages of the distillation procedure.
~ a h d Q d l o " ' " " LO" " " ' - '30
40
% DISTILLATE REMOVED
FIGURES 1 AND 2. DRYING TIME(MINUTES) AXD FILMHARDNESB OF RESIDUE
DRYINGTIME OF RESIDUES.T o samples of the residues were added 0.5 per cent lead, 0.05 cobalt, and 0.05 manganese as driers. I n the case of the residue from which as little as 0.05 per cent distillate had been removed, the cloudiness that was produced on addition of drier to the original oil was not observed. Films 0.001 inch (0.0254 mm.) thick were spread onto glass with a Parks filmograph, and the drying times determined by touching the film lightly with the finger. The film was considered to be dry when no visible mark could be transferred with the finger tip to a smooth glass surface. As Figure 1 shows, the drying time in minutes of the residue decreased rapidly with the removal of the first 10 per cent of distillate, and even the removal of 2 per cent of distillate was adequate to produce a drying time comparable with linseed oil to which identical quantities of drier had been added. Apparently the main bulk of the nondrying constituents present in the original polymerized oil-i. e., free fatty acids, sterols, and nonsaponifiable matter-was concentrated in the early distillates. It was apparently through the removal of such constituents (10 per cent) that the first improvement in film-forming properties was achieved. As the next portion (20 per cent) of the lower molecular weight materials was removed from the residue, no marked reduction in drying time was obtained. The removal of more than 36 per cent distillate reduced the drying time of the oil markedly. Althoughunpolymerized triglycerides were still present in the oil after removal of 10 per cent distillate, different concentrations of these substances did not significantly affect the time of drying. When more than 30 per cent distillate was removed from the original oil, however, further reduction in drying time was obtained; the residue after 40 per cent had been removed dried in a quarter of the time taken by the original oil. Some slight further polymerization of the distilland did, however, take place in the still during the distillation procedure, owing
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to the high temperatures involved, as mentioned above. Further work will be necessary before it can be determined to what extent the drying time was actually improved due only to the removal of more than 30 per cent of distillate. FILMHARDXESS OF RESIDUES. The residues withdrawn from the still a t progressive stages of distillation were spread on glass after the addition of drier, and hardness was determined with a Sward rocker (glass = 100). In general, the hardness characteristics of residues produced by removal of various percentages of distillate (Figure 2) corresponded with the drying times. Although the original oil, even after 7 2 hours, was unable to approach the hardness of linseed oil that had dried for 48 hours, removal of 2 per cent distillate produced a residue comparable to linseed oil in hardness. At 7 2 hours the hardness was still further improved, and none of the residues from which a t least 10 per cent distillate had been removed had the characteristic tackiness of commercial fish oils. FILM HARDNESS vs. DRYING TIMEOF RESIDUES.The data in Figure 3 indicate a change in film hardness with time for residues which had various amounts of distillate removed and illustrate the manner by which the film-forming properties of a polymerized fish oil may be improved through the removal of lower molecular weight fractions. The original oil dried in the manner typical of materials composed largely or entirely of fish oils. The hardness improved slowly during the first 25 or 30 hours and then reached a maximum, the final product being definitely soft as a result of the large percentage of nondrying components. ]Then these first fraction materials were removed by distillation, entirely different film-forming properties were achieved. From the data on molecular weights (Figure 12), free fatty acid values (Figure lo), and percentage of nonsaponifiable material (Figure ll), i t may be assumed that the resulting residues (oils from which 10-30 per cent distillate had been removed) were not only comparatively depleted of nondrying materials and antioxidants but consisted essentially of dimers of triglycerides.
b Y
B
hF O'
'
10
20
30
40
50
60
70
80
90
IW
DRWNG TINE-HOURS
us. DRYING TIME FIGURE 3. FILMHARDNESS
VISCOSITY OF RESIDUES.As might be expected, the Tiscosity values foi the residue varied inversely with the drying times (Figure 4). -4continual increase in viscosity nas noted until 10 per cent of distillate was removed. The viscosity remained essentially constant while the next 20 per cent R as removed and then rapidly increased. This later rapid increase in viscosity v a s probably the result both of further polymerization in the still and the removal of higher molecular weight material as distillate. REFRACTIVE INDEX OF RESIDUES.The curve in Figuie 5 , relating the refractive index to the amount of distillate removed from the residue, adhered closely t o the general form of those of drying time and viscosity. A gradual increase in refractiT-e index was found up t o the point of the 10 per cent
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1941
" 0 ' 2 4
FIGURES
e a i '
4 TO 13.
'
20
50 2 -
X DISTILLATE REMOVED PROPERTIES OF
30
20
0
40
7 . DISTILLATE REMOVED
DISTILLATE AND RESIDUE FR.4CTIONS REMOVED
distillatce. I n the region beyond 30 per cent distilland an increase mas again noted, indicating both concentration and formation of higher polymers. REFRACTIVE INDEXOF DISTILLATES. The first distillate fractions contained the bulk of the sterols (as well as free fatty acids), and the resulting refractive indices were correspondingly higher than the early fractions from which unsaponifiable material had been removed (Figure 6). Measurements made on the later fractions showed a gradual increase in the refractive index, probably due to the presence of more highly unsaturated components (Figure 8 ) . IODINE VALUEOF RESIDUES.Iodine values were determined for each of fhe residue samples. The standard Wijs
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AS A
FUXCTION OF P E R
CENT
DISTILLATE
method was modified by adding to the sample 10 cc. of a 2.5 per cent solution of mercuric acetate in glacial acetic acid directly after addition of the Wijs solution (4). The variation in iodine value for various residues did not appear particularly significant (Figure 7 ) . The maximum value occurred when 20 to 30 per cent of distillate had been removed. The lower degree of saturation found for the residue from which 40 per cent of the distillate had been removed was possibly caused by a loss of double bonds by further polymerization. IODINE VALUEOF DISTILLATE.Although the fatty acids present in the first fractions were largely of the saturated type, considerable unsaturation was noted in the nonsaponifi-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
able material; and as this was removed, the iodine value decreased (Figure 8). A gradual increase in iodine value was found in the distillate fractions appearing after the first 10 per cent had been removed. FREEFATTY ACIDIN DISTILLATE.Analyses indicated, as might be expected, that the main bulk of the free acids was concentrated in the first fractions of distillate (Figure 9). The material present in the first 10 per cent was solid a t room temperature, and the acid in the first fraction was nearly all palmitic. Most of the objectionable fishy odor was concentrated in the earlier distillates, particularly the first. FREEFATTY ACIDIN R E S I D ~ EThe . removal of the early fractions produced a marked improvement in the odor of the residues; it was perhaps best described as that of a spar varnish. A continued decrease in the acid value of the residue was also noted with the further removal of distillate. NONSAPONIFIABLE MATTERIN DISTILLATE.The original oil contained 3.85 per cent nonsaponifiable matter and the main portion was removed in the first distillate fractions (Figure 11). To some extent it was probably the presence of nonsaponifiable components as well as nondrying glycerides in the original oil which was so detrimental to successful film-forming properties of the polymerized product before distillation. MEANMOLECULAR WEIGHTOF RESIDUE. Mean molecular weights were determined by the rise in boiling point, according to a modification of the method of Menzies and Wright (7). Using carbon tetrachloride as a solvent, it was found that determinations of the molecular weights of both distillate and distilland could be made with ease. A sample of redistilled soybean oil was employed as a standard; a molecular weight of 880 was taken for this material since it had an iodine value equal to 130 and was assumed to contain chiefly triglycerides of Cla acids. The molecular weights so determined should be reasonably accurate or a t least useful for comparative purposes. Figure 12 shows that, after the removal of about 10 per cent distillate, there was little change in the mean molecular weights of the residues until a total of about 28 per cent had been distilled. Apparently the residues representing 70 t o 90 per cent of the original oil contain mainly dimeric triglycerides. After the removal of more than 30 per cent distillate, more highly polymerized components showed their presence in the residue. These were also formed to some slight extent by further polymerization during the distillation process. MEAN MOLECULAR WEIGHT OF DISTILLATE.Figure 13 indicates that the distillate fractions have WAVELENGTH- ~ J A mean molecular weights somewhat lower than the value expected for FIGURE 14. ULTRAVIOLET A~~~~~~~~ the unpolymerized triglycerides of SPECTRAOF RESIthe original oil. Because of the asDUE (0.00672 PER sociation of the fatty acids in the CEWT IN cyCLosolvent, the molecular weight deterHEXANE) minations as made on the fractions of high acid value are necessarily in error. The true molecular weights of the first fractions are therefore somewhat less than those shown in Figure 13. As distillation progressed, materials of higher mean molecular weight were removed, and in general, the distillate consisted of monomeric triglycerides. The last fractions had molecular weights which indicated the presence of some higher polymers. ULTRAVIOLET ABSORPTION SPECTRUM OF A RESIDUE. An ultraviolet adsorption curve was determined for a sample of the residue resulting after 20 per cent distillate had been re-
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moved. A maximum was found a t 232 mp indicating the presence of two conjugated double bonds in some of the fatty acids present in this residue sample ( 2 ) .
Repolymerization of Distilled Residue The above data show that the removal of 10 per cent distillate from a polymerized fish oil produces a drying oil having film-forming properties of improved characteristics. With the thought in mind that all of the nondrying constituents, as well as antioxidants, might be concentrated in the first 10 per cent distillate, samples of the fourth and seventh fraction residues, representing 10 and 30 per cent distillate, respectively, were further bodied. This process was carried out under vacuum (0.075 mm.) at an average temperature of 260' C.
x
A i
LL
I
I
,I
1
2
4
-
5
6
7
8
TIME OF REBODYING DRYINGTIVE AND FILnr HARDNESS OF RESIDUE
FIGUREB15
PLOTTED
3
TIME OF REBOWING HRS. (250'- 270.CJ AND AGAISST
16.
DRYINGTIMESOF BODIEDRESIDUES. For a given degree of bodying, a comparable reduction in drying time was obtained for residues resulting from the removal of either 10 or 30 per cent of distillate (Figure 15). By such treatment the previous drying time of 80 minutes (Figure 1) was further reduced to 20 minutes. FILMHARDNESS OF REBODIED RESIDUES. Hardness tests were made on the rebodied residue resulting from the removal of 10 per cent of distillate. Considerable improvement in? hardness was achieved by further polymerization of the material left after the removal of some of the nondrying components (Figure 16). Apparently the maximum effect of rebodying was obtained after 3 hours.
Applications to Other Types of Oils The principle of eliminating nondrying components in order to improve film-forming properties should be applicable to other common types of polymerized vegetable or marine drying oils. A preliminary investigation was made to demonstrate the effect of removing low-molecular-weight material§ from some of the more common bodied oils. The effect of such a process upon the drying time and refractive index of several products is shown in the following table: Original Bodied Oil
7 Dist
Refractive Drying Time, Roemoved Index, 40' C. RIin. 1.5058 30 7 25 1.5070
.. .. 25 .. 40
1.4818
00
1.4819
15 150
1.4800 1.4814
50
Molecular distillation technique appears to be a new means available for the analysis of the various components present i n
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1941
drying oils. It is hoped that this useful technology may now not only be applied to further our knowledge of structure of oils in general, but will eventually be employed on a commercial basis for the production of new and improved materials. The data presented here are but a brief introduction to possible applications of such a high-vacuum process.
Acknowledgment The author wishes to express appreciation for the assistanoe given by members of the staff of Distillatian Products, Inc., for carrying out some of the chemical analyses and also to Joseph J. Mattiello of the Hilo Varnish Company for many helpful suggestions.
Literature Cited (1) BrZnsted and Hevesy, Phil. Mag., 43, 31 (1922). (2) Farmer and Van den Heuvel, J . SOC.Chem. Ind., 57, 24 (1938); Moore, Biochem. J . , 31, 138 (1937). (3) Hickman, IND. ENQ.CHDM.,29, 1107 (1937). (4) Hoffman and Green, Oil & Soap, 16, 236 (1939).
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(5) Imperial Chemical Industries, Ltd., Brit. Patent 422,941 (Jan. 22, 1935); Imperial Chem. Ind., Ltd., and Hill and Walker, Ibid., 428,864 (May 16, 1935); Fawcett and Walker, Ibid., 442,000 (Jan. 31, 1936), U. 8. Patent 2,128,354 (Aug. 30, 1938); Hickman (to Eastman Kodak Co.), U. 8. Patent 2,126,466 (Aug. 9, 1938). (6) Krafft et al., Ber., 28, 2583 (1895), 29, 1316, 2240 (1896), 32, 1623 (1899), 36, 4339 (1903); Fischer and Harries, Ibid., 35, 2158 (1902); Erdman, Ibid., 36, 3456 (1903); von Rechenberg, J. prakt. Chem., 80, 547 (1909); Houben, Ber., 52, 1460 (1919); Walden, 2. anorg. Chem., 112, 225 (1920); Waterman and Rijks, 2. deut. Ole- u. Fett-Id., 46, 177 (1926). (7) Mennies et al., J . A m . Chem. Soc., 43, 2309, 2314 (1921); Coulson, Analyst, 57, 757 (1932). (8) Oosterhof, van Vlodrop, and Waterman, U. S. Patent 2,065,728 (Dec. 29, 1936). (9) Waterman et al., Chem. Weekblad, 24, 268 (1927), 26, 469 (1929); Burch, Proc. Roy. SOC.(London), A123, 271 (1929); Washburn, Bur. Standards J . Research, 2, 467 (1929); Hickman, J . Optical Soc. A m . , 18, 69 (1929); Hickman and Sanford, J. Phys. Chem., 34, 637 (1930). (10) Waterman and Oosterhof, Rec. trav. chim., 52, 895 (1933). PRFSENTED before the Division of Paint and Varnish Chemistry at the 99th Meeting of the American Chemical Society, Cincinnati, Ohio.
Reaction Mechanism of the Acid Hydrolysis of Fatty Oils J
J
J
TZENG-JIUEQ SUEN AND TSUN-PU CHIEN The Tung Li Oil Works, Chungking, China
NE of the rapid methods for obtaining free fatty acids
0
from a fatty oil is to heat the oil first with a few per cent of concentrated sulfuric acid for a short time and then steam it to bring about hydrolysis (1, 9). Recently large amounts of free fatty acids were desired in this plant, and this acid hydrolysis process was tried. The results on the rate of hydrolysis indicated that a study of the reaction kinetics would prove of value. I n studying the rate of hydrolysis of an ester which is partially miscible with water in aqueous solution, Lowenhers (6, 8) showed that the reaction is taking place practically in the water phase. If an acid is used as the catalyst, a constant amount of ester is changed per unit of time; i. e., the reaction is apparently of zero order. In his study of the kinetics of soapmaking, however, Smith (6) showed that the saponification proceeds in the soap phase. Reaction a t the interface is of no significant magnitude except a t the beginning of the saponification in which no soap is present initially. Lascaray’s work on the hydrolysis of tallow with alkaline catalysts (9) agrees with Smith’s results, He concluded that the bulk of the reaction proceeds in the oil phase, with the help of the dissolved water, The activity of the alkaline catalysts was explained by their power to increase the solubility of water in the oil phase.
Experimental The apparatus is shown in Figure 1. A 500-cc. three-necked flask was fitted with a mercury-sealed electrical stirrer, E, directly driven by stirring motor M , with a reflux condenser, C , a thermometer, TI, and a sam ling tube, Y. The various oils and fats studied were obtained 8om the local market. The oil t o be hydrolyzed was first charged into the flask and heated with
stirring t o 115’ C.; 3 or 4 per cent by weight of strong sulfuric acid was added with further heating at 115’ C. for 10 minutes. It was then allowed t o cool while the apparatus was being transferred to a thermostat, B, the temperature of which was meas-
~~
~
Fatty oils were hydrolyzed by the sulfuric acid method, and the reaction course was followed by determining the acid number of the samples at various time intervals. By plotting the logarithm of the difference of saponification number and acid number, of the oils hydrolyzed, against time, straight lines were obtained. Since this is practically the same as plotting the logarithm of the concentration of unhydrolyzed oil in the oil phase against time, the reaction appears to be first order. Hydrolysis with different oil-water ratios yielded the same results. By assuming that the reaction takes place in the oil phase, all the data can be satisfactorily interpreted. Some of Lewlrowitsch’s data on hydrolysis of oils with hydrochloric acid as the catalyst can be interpreted in the same way.
