Comparative Oxidation of Linseed
and Sardine Oils G . A. O'HARE AND W. J. WITHROWl Congoleum-Nairn, Inc., Kearny, New Jersey Linseed and sardine oils were oxidized by air blowing at 220" F. with improved blowing techniques which minimize polymerization. The conventional drying oil constants were used to follow the course of oxidation; dielectric constant and ultraviolet absorption measurements were also applied. Results indicate that these latter properties show characteristic and reproduciblq changes as oxidation progresses. The oxygen content of sardine oil was found to fluctuate as blowing proceeds. This is contrary to the linseed, which shows a steady rise in oxygen content with increasing blowing time. Sardine oil goes through highly reactive stages near the beginning of the blow; these are found at peaks in the fluctuating oxygen content curve of the oil. Comparative curves are given to illustrate the chemical and physical changes which take place during linseed and sardine oil oxidation.
D
RYING oil oxidation has been the subject of many investigations during the past years. Several approaches to this subject have been made, but as yet no well defined picture of the mechanism of the oxidation of a drying oil has been presented. One of the reasons for this is the complexity and nonuniformity of the starting raw oil. The distribution of the acid radicals in the triglyceride molecule varies' depending on the history of the oil, and this greatly influences the course of the oxygen addition to the molecule. Polymerization, which usually accompanies oxidation, also complicates the oxygen addition. Since oxidation promotes polymerization, the complication becomes greater as oxidation proceeds. The usual chemical and physical constants used to define a processed oil, such as iodine number, acid number, viscosity, etc., are affected by both polymerization and oxidation. For this reason, the use of these constants for following the course of oxidation is unsatisfactory. Hazelhurst (3) described in detail the use of the dielectric constant as a measurement of linseed oil oxidation. Since that time additional work was done in this laboratory using dielectric constant as a means of following drying oil processing, and some of this work is covered in the present paper. The physical and chemical changes which take place during the oxidation of linseed oil were studied extensively in this laboratory. As a result of that work more efficient methods of oil oxidation were found so that linseed oil can now be oxidized t o about 17-18% oxygen in a few hours with minimum polymerization. This is evidenced by the relatively low viscosity and refractive index obtained at high oxygen content. In the older methods of blowing it was difficult to put so much oxygen into the molecule before polymerization caused high viscosity and gelation. Patent applications are now pending on these improved oxidation methods. As soon as these are allowed, the details of the methods will be available in the patent literature. Some previous work was done on the oxidation of marine oils. Brockelsby and Denstedt ( I , @ and others studied rate of oxygen pickup, etc., during oxidation. Unfortunately, however, the high temperature (250" F.) and long blowing times (30 to 60 hours) in most of their work make it inevitable that a large amount of. polymerization complicates the oxidation reaction. This results in nonuniform oxidation products, and uniform reproducible results are exceedingly difficult to obtain. Consequently, it is useless to compare results of different investigations, 1
since the amount of polymerization always varies with the conditions involved. The improved oxidation technique of this laboratory was applied to sardine oil and the results were correlated with data on linseed oil obtained under the same conditions. I n each case it is felt that a minimum of polymerization was involved, and hence a much more accurate and representative picture of oil oxidative changes was obtained. The random triglyceride distribution of the higher polyethylenic acids found in sardine oil makes the problem of oxidation mechanism much more complex than that of linseed oil. The composition of marine oils has long been under study. During the past few years the methyl ester fractionation of their component acids has clarified the situation considerably. For the present work a west coast sardine oil was used. The approximate constants for this oil a t 77" F. follow: Refractive index Specific gravity Wijs iodine number Acid number Viscosity, centipoises
1.4806 0.9250 196 4.5 45
The oil was subjected to oxidation by air blowing a t 220' F No driers were used, and an air rate of 6 cubic feet per minute per 100 pounds of oil was maintained throughout the blow. Samples were withdrawn every 20 minutes during the oxidation. DETERMINATION OF PROPERTIES
VISCOSITY.A direct-reading Brookfield electroviscometer was used and values reported directly in centipoises a t 77" F. SPECIFIC GRAVITY.This property was measured at,77" F. with a set of precision hydrometers. ACID NUMBER. A conventional acid number determination was used with a 50-50 mixture of benzene-ethyl alcohol as the solvent. IODINE NUMBER.The Wijs method was used in this determination. MOLECULAR WEIGHT. The freezing point method with benzene as the solvent was used to determine the average molecular weight. OXYGEN CONTENT.Conventional combustion analysis for carbon and hydrogen was used to obtain oxygen content of the oils. DIELECTRICCONSTANT.The bridge method for dielectric constant measurement was used. A special test cell designed for liquid samples, a Schering type bridge, alternating current voltage source, and detector made up the apparatus. The values were measured at 200 kilocycles and 77" F. REFRACTIVE INDEX. The Abbe refractometer was used a t
77"F.
Present address, College of the City of New York, New York, N. Y.
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102
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. DATAON OXIDIZED SARDINE OIL Time Sample Min.’ 0 0 1 30 2 53 3 75 4 95 5 115 6 135 7 156 8 175 9 195 10 215 11 235 12 255 13 275 14 295 I5 315 16 335 17 355
Viscosity, Centipoises 45 84 113 174 250 342 460 670 880 1170 1490 1950 2290 2790 3380 3800 4570 5800
Refractive Index 1.4805 1.4820 1.4824 1.4830 1.4835 1.4840 1.4842 1.4847 1.4850 1.4852 1.4857 1.4858 1.4859 1.4860 1.4860 1.4861 1.4861 1.4862
Dieleotrio Con- Iodine stant No. 3.23 195.0 3.57 173.5 3.72 167.6 3.91 158.5 4.08 1’52.0 4.17 142.5 4.25 136.5 4.33 132.0 4.41 122.5 4.45 123.0 4.48 120.5 4.50 119.5 4.52 117.5 4.64 116.5 4.57 115.5 4.57 114.5 4.57 113.6 4.57 112.0
Acid No. 4.5 5.5 5.9 6.5 7.2 7.6 8.0 8.2 8.3 8.6 8.7 8.8 8.9 9.0 9.0 9.1 9.2 9.4
Sp. Oxygen, Gr. % 0,925 14-87 0.940 16.38 0.946 16.80 0.952 16.27 0.960 17.71 0.966 18.33 0.970 17.35 0.976 17.93 0,980 18.46 0,983 19.81 0.985 18.25 0.988 19.75 0.991 19.15 0,996 18.77 1.001 19.38 1.005 19.67 1.007 19.39 1.010 19.89
Xol. Mt. 767 817 862 910 942 992 1010 1040 1060 1070 1090 1100 1110 1120 1130 1140 1160 1170
The increased reactivity of an oxidized oil may be explained by the reasoning that polymerization takes place a t the activated, oxidized position of the molecule. When sardine oil is blown, little or no induction period is experienced. A more rapid initial reaction, as evidenced by the heat given off, takes place than with linseed oil. The rate of oxygen addition again slows down, however, as polymerization starts .to become a factor. As noted in linseed oil, the addition of oxygen t o sardine oil does not follow the same general trend of increased oxygen content with increased blowing time.
1.4895
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1
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Table I gives complete data on oxidized sardine oil. The efficiency of the blowing procedure may be seen from the relatively low viscosity and refractive index values a t the relatively high oxygen contents. The viscosity, refractive index, specific gravity, acid number, iodine number, dielectric constant, and molecular weight show a definite trend as oxidation progresses (Table I). All these constants show an increase with increased blowing time, with the exception of the iodine number which, of course, decreases. These changes are similar to those which take place when linseed . oil is oxidized.
tI
I BLOWING TIME INCREASES
-
2 1.4835
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-
Y
I W
2000
0 y? vi
1.4015
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100 zoo BLOWING TIME IN MINUTES
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Figure 2. Effect of Blowing Time on Refractive Index, Specific Gravity, and Viscosity for Sardine Oil (closed lines) and Linseed Oil (broken lines)
The refractive index increase for blown sardine oil is analogous to that noted for linseed oil. A comparison of the curves is given in Figure 2. Little change is noted in refractive index near the end of the blowing. Figure 2 also illustrates the changes taking place in specific gravity and viscosity. Again the similaSity to linseed oil is evident. Figure 3 illustrates the acid number, iodine number, an’d molecular weight changes of linseed and sardine oils during oxidation. The curves for these oils have about the same slopes, and, in general, the changes taking place in the two oils are similar. Again the rate of change taking place near the end of the batch is much less than that a t the start. The use of the dielectric constant as a measure of linseed oil oxidation was mentioned previously in this paper. It was found that the dielectric constant measured a t 200 kilocycles and 77’ F. of linseed oil undergoes little change during heat polymer-
Figure 1. Course of Oxidation and Polymerization of Linseed Oil
The rate’of viscosity pickup increases near the end of the batch. This is a sign that heat polymerization becomes an important factor a t this stage of the blowing. In the production of a reactive oxidized oil for the varnish kettle it is essential to stop the blowing before polymerization builds up excessipe viscosity and cuts down the reactivity of the oil. The rapid viscosity increase near the end of the blowing cycle is also characteristic of linseed oil. Starting with a raw oil of 40 centipoises, a viscosity of 2500 centipoises is reached in apout 250 minutes. With linseed oil an induction period of about 15 to 45 minutes is usually experienced; this is followed by a rapid oxygen uptake during the first part of the cycle, and the rate of uptake decreases as blowing progresses. This is easily understood because oxidation promotes polymerization, which in turn retards further oxidation, An illustration of this is given in Figure 1.
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RESULTS
5000
0.900
1.4875 x 0 w
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ULTRAVIOLET ABSORPTION.Optical density measurements were made in ethyl ether solution with the Beckman spectrophotometer. Duplicate runs were made and were in very good agreement.
Vol. 39, No. 1
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200
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550
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360
I
100 2 00 BLOWING TIME IN MINUTES
300
Figure 3. Effect of Blowing Time on Molecular Weight, Iodine Number, and Acid Number for Sardine Oil (closed lines) and Linseed Oil (broken lines)
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1947
ization. A sample of linseed oil which has been heat-bodied to about 4000 centipoises has about the same dielectric constant as the unbodied oil, which is 3.25. As linseed oil is oxidized, the dielectric constant increases regularly with oxygen content to a value of about 5.00 a t an oxygen value of about 17.2% (Figure 4). Since the dielectric constant docs not change with straight heat polymerization as do the other constants, this characteristic offers an accurate and reproducible method of differentiating between oxidation and polymerization during linseed oil processing. It has actually been found to be a satisfactory continuous control measurement for the commercial production of oxidized linseed oils. 5.5
r
I
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Figure 4.
100 200 BLOWING TIME IN MINUTES
MO
Effect of Blowing Time
on Dielectric Constant at 200
Kilo-
103
wave length for a series of blown linseed oils. As the oxygen content of the oil increased, the extinction coefficient also increased at any given wave length. Ultraviolet absorption curves for the oxidized sardine oil show no such regular increase as blowing progresses (Figure 7). This is again a definite indication that the mechanism of sardine oil oxidation is different from that of linseed oxidation. DISCUSSIOIV OF RESULTS
I n attempting to explain the behavior of sardine oil on oxidation, the two things which come t o mind are high saturates content and higher polyethylenic acids present in the oil. It is possible that these successive stages of oxygen increases and decreases represent a course of reaction by which the higher unsaturated acids (four or five double bonds) are selectively oxidized a t one or more of their double bonds, with the formation of an unstable oxygen linkage a t this point. This would result in loss of unsaturation and increase in oxygen content. The oxygenated bond could then break down with polymerization or condensation with other functional molecular groups with the elimination of oxygencontaining volatile side products. This could then account for a decrease in oxygen content of the oil if the loss were greater than the oxygen being absorbed from the incoming air a t that time. As a result of this type of reaction, higher double bond acids could successively be changed to lower double bond acids. Thus the sardine oil oxidation could be a stepwise reaction, starting with acids of five double bonds and going successively through acids of four and two double bonds.
cycles and 77' F. for Sardine Oil (closed line) and Linseed Oil (broken line)
With this background of dielectric constant measurement on linseed oil, the method was applied to the oxidation of sardine oil. Again a reproducible increase of dielectric constant from 3.23 of the raw oil t o about 4.60 of the oxidized oil was found. This indicates that dielectric constant could be used as a measurement of the changes taking place during sardine oil oxidation. However, this was not a direct measure'of the oxygen content of the oil as it was in linseed. Cltimate analyses on these samples of oxidized sardine oil brought out many interesting and unusual points. Figure 5 shows the oxygen content of linseed and sardine oil as blowing progresses. The oxygen content of the sardine oil did not follow the relatively uniform increase of the linseed. A change from about 11% to about 17.2% is noted for linseed oil. The sardine oil plot shom successive increases and decreases in oxygen content as blowing progresses. This suggests a stepwise oxygen uptake in which some oxygen is added to the molecule and then lost as further blowing is carried out. Close examination of the curve indicates three different instances of oxygen peaks which are followed by appreciable oxygen lyses. Other instances are noted, but they are small enough to be within the limit of error of the combustion analyses. Since these results were somewhat different from those expected, several other batches of sardine oil were blown and oxygen pickup was determined. In all cases curves of the same shape were obtained with the oxygen peaks falling a t about the same points on the plot. The oxygen content found for blown sardine oil-the only one of the conventional oil constants that did not follow the usual smooth linseed oil curves-made it apparent that some changes were taking place which were not being measured by these conventional constants. In an attempt to find some other measurement which would indicate the changes taking place as the oxygen content fluctuates, the ultraviolet absorption of the samples was determined. It had been found previously that an ultraviolet spectral absorption change accompanied the blowing of linseed oil. In Figure 6 the extinction coefficient is plotted against
17
z 16 0 x
c
y
16
Y E -
0
14
13
i
I2
I ' II
0
100
200 BLOWING TIME IN MINUTES
300
Figure 5. Oxygen Content of Sardine Oil (closed line) and Linseed Oil (broken line)
Holman and Burr (4) reported that unsaturated carbo2yl compounds show intense absorption near 2300 and 2700 A. This suggests the possibility of the formation of unsaturated carbonyl compounds as oxygen adds to the higher polyethylenic acids of sardine oil, with resultant absorption increase. These unsaturatedcarbonylstructures could then break down withlossof oxygen and accompanying'loss of absorption. Samples 3 and 2 of Figure 7 seem t o substantiate this theory. Sample 3 shows $wer oxygen content and absorption than does sample 2 a t 2300 A. The fact that a t 2700 A. sample 3 has a higher absorption than sample 2
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
104
suggests the possible formation of conjugated trienes as a result of the carbonyl breakdown. From this point on the oxidation could proceed through the formation of acids of three double bonds until finally only acids aontaining one and two double bonds exist. Since the possibility for triene conjugation would be gone after the stage of two double bonds was reached, diene conjugation could become an effective factor and account for absorption increases a t 2300 A. The loss of absorption shown by sample 6 could also be explained by a loss in unsaturated carbonyl structure which had been building up
Vol. 39, No. 1
that two samples reacted more vigorously than the others. It was also found that these two samples showed a much greater viscosity and refractive index change than did the other samples. One was a t an oxygen peak on the curve, whereas the second was the next sample in line from this peak. It is interesting to note that both samples were taken near the beginning of the blowing cycle. As an example of the viscosity changes involved, the highly reactive sample went from a viscosity of 340 to 2200 centipoises on heating to 550OF. in 40 minutes. This represents a change about 6.5 times the original viscosity. A later sample
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2600
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Figure 6. Absorption Spectra for Oxidized Linseed
Oil
Samples 1 through 5 represent increasing blowing time; sample 1, raw oil
through samples 4 and 5. A steady decrease of absorption which cqrresponds to the slow rate of change of many of the other constants on these samples was noted beyond sample 7. This may be caused by the destruction of the highly absorptive conjugated diene or carbonyl structures by polymerization, which becomes a critical factor a t this late stage of oxidation. Since linseed oil has no polyethylenic acids above linolenic, there is no possibility that the described reactions of the four and five double bond acids take place. A relatively stable oxygen addition product is formed, and no significant loss of oxygen takes place during the blowing; a t least the oxygen loss would be lefs than the amount being absorbed from the oxidizing air. Net oxygen content would then increase as blowing progresses (Figure 5). The authors believe that it is this type of stable OXYgen addition that is being measured by the dielectric constant in both oils. This accounts for the fact that the dielectric constant of the sardine oil kcreases even though the net oxygen content decreases because of other types of decomposition and polymerization reactions involving polyethylenic acids. Sinck sardine oil during oxidation seems to show stages which are relatively unstable and gives off oxygen-containing products on decomposition, it i s probable that highly reactive molecular structures exist a t these stages. Therefore, a highly reactive oil should exist a t this sta,ge of processing, or ht the oxygen peaks on the curve. As a check, samples from the present oxidation series were heat-bodied under the same conditions of time and temperature. Viscosity and refractive index measurements were made on these samples before and after heat bodying. It was noted 1
0 2400
Figure 7.
2500
2600
2700
2800
2900
i.
Absorption Spectra of Oxidized Sardine
Oil
Sample. 1 through 7 represent increasing blowing time; sample 1, raw oil
taken near the end of the batch went from 1950to 5300 centipoises under the same conditions, a change of only 2.7 times the original viscosity. The latter figure (2.7) is a slightly higher value than is obtained with oxidized linseed oil under identical bodying conditions. Since spectrophotometric background on oxidized oils of this type is meager and the data presented are somewhat limited, it should berecognized that theexplanationsoffered must of necessity be highly speculative. The writers feel sure, however, that many valuable contributions to the theory of oil oxidation mechanism will result from further spectrophotometric work. They also feel certain that the high saturated acid content of sardine oil plays a significant role in oxidation. Further work on segregated sardine oils should clarify this. ACKNOWLEDGMENT
The authors wish to express their appreciation for the technical assistance of Adam Kopacki in carrying out this investigation. LITERAT,URE CITED
(1) Brockelsby, H. N.,and Denstedt, 0. F., Contrib. Can. B i d . Fisheries, 8,321 (1934). 12) . , Denstedt. 0. F.. and Brockelsby, H. N., J. BioZ. Board Can.,1, 487 (1936). ' (3) Ha~elhurst, E.H., Paint Manuf., 13,275 (1943). (4) Holmm, R.T., and Burr, G . 0.. Arch. BbAent., 7,47 (1945). PRESENTED before the Division of Paint, Varnish, and Plastica Chemistry at the 109th Meeting of the A M ~ R I C A CHEMICAL N SOCIETY, Atlantic City, N . J .
