Mechanism of HEAT BODYING LINSEED OIL H . E . Adams and P . 0 . Powers ARMSTRONG CORK COMPANY, LANCASTER, PA.
'
Changes in the properties of linseed oil during heat bodying have been studied in an attempt to correlate such changes with the chemical reactions which are believed to occur. In addition to properties usually given by other investigators, hydrogen absorption and heat of combustion at various stages of heat bodying have been determined. The data obtained are in substantial agreement with published results. Polymerization with the formation of a six-membered ring between two fatty acid groups is clearly
indicated. Three fatty acid groups may also combine, but no estimate of the extent of this reaction is possible at present. The formation of an intermediate compound is indicated by the results. This compound has a low iodine value but apparently is not a true polymer. It may be an intrapolymer formed by combination of two fatty acid groups in the same glyceride. Whatever the structure of the intermediate compound may be, it rearranges during later stages of heat bodying to form an interpolymer.
R
The trireactive glycerides in linseed oil may therefore be 28%, direactive &yo, monoreactive 24%, and nonreact,ive 4% if the arrangement is entirely random.
ECENTLY the chemistry of heat bodying of drying oils has been considerably clarified. It has been shown that, when drying oils or other esters of unsaturated acids are heated at ternperatures above 275' C., fatty acid groups combine with the formation of a six-membered ring. It has been suggested (8) that combination occurs by vinyl polymerization followed by ring closure, or that the reaction is a typical Diels reaction (7,11) where isolated double bonds rearrange to conjugated dienes before ring closure occurs. It has been shown that three and possibly more fatty acid groups may combine (9). The extent to which this reaction occurs in the heat bodying of linseed oil has not been established. This study was initiated to determine if the changes in the properties of linseed oil on heat bodying were consistent with a polymerization mechanism based on the formation of six-membered rings between fatty acid groups. Alkali-refined linseed oil was heat-bodied, and samples were taken periodically. Wijs iodine value, density, specific refractivity, viscosity, acetone insolubles, molecular weight, hydrogen absorption, and in some cases heat of combustion were determined on these samples.
MECHANISM OF POLYMERIZATION
An analysis has been made of the polymer structure to be expected if a triglyceride polymerizes by combination of fatty acid groups, with only two groups uniting. The assumption was made that any group was likely to react as any other; trireactive glycerides would thus be much more likely to polymerize than monoreactive glycerides. An equation waa derived for the distribution of dimers, trimers and higher polymers as the reaction proceeded:
where u = number of trireactive glycerides in polymer b = number of direactive glycerides in polymer c = number of monoreactive glycerides in polymer Ma& = fraction of reactive groups attached to an abc polymer M!,,, fraction of reactive groups attached to monomer trireactive glycerides at start. of reaction MEl,, = fraction of reactive groups attached to monomer direactive glycerides at start of reaction A l g o l = fraction of reactive groups attached to monomer monoreactive glycerides a t start of reaction p = extent of reaction
COMPOSITION OF LINSEED OIL
Estimates of the fatty acid glycerides present in linseed oil have been revised in recent years. The values from the thiocyanogen value (18,19) and ultraviolet absorption of isomerized acids from linseed oil (17)agree fairly well, and show that the fatty acids from linseed oil of 185 iodine value consist of approximately 8% saturated acids, 25% oleic acid, 18% linoleic acid, and 49% linolenic acid. The high content of linolenic acid in linseed oil now seems to be well established. The distribution of the fatty acid groups between the glycerides has not been entirely established, but the best evidence indicates a random distribution (IS). Thus the content of linolenic triglyceride may be assumed to be O.4Qsor 11.8%. The total number of different glycerides is large (18), and the distribution has been simplified by classifying the acids as reactive (linolenic and linoleic) or unreactive (oleic and saturated) according to their tendency to polymerize on heating. Thus there are 67% reactive acid groups and 33y0 unreactive acid froups. The random distribution is given by the equation: aa f 3a*b
+ 3ab2 + b* = 1
where a (0.67) = Concentration of reactive groups b (0.33) = concentration of unreactive groups
The complete derivation is somewhat involved and will be reported later. I t is known that three fatty acid groups may combine with one another in the polymerization. An initial analysis of this process was made, and the equation has the same form, the functionality being increased as the relative amount of tri and higher linkages between fatty acid groups is increased. Equation 1 has been of great assistance in predicting the change in properties to be expected if polymerization follows the suggested pattern. If we consider a mixture of tri-, di-, mono-, and unreactive glycerides and consider the probable arrangements when two fatty groups combine, we have a picture of what may occur on heat bodying linseed oil. The unreactive glyceride acts only as a diluent and cannot take part in the polymerization. The monoreactive glycerides can act only as terminators to a polymer chain. 1124
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1944
Direactive glycerides form linear polymers, and trireactive glycerides when fully polymerized form cross-linked polymers which cause gel formation. However, if three fatty acid groups combine, the complexity is increased, monoreactive chains can enter into linear chaina, and, if three monoreactive groups combine, an unreactive polymer is formed. Direactive glycerides can form three-dimensional molecules while trireactive molecules will combine to give highly branched structures. Little is known about the amount of oomhination of three fatty acid groups in heat-bodied oils. It is apparent that a minor amount polymerizes in this manner but there is no estimate of what it actually is (6,6).
1125
of stopcocks so arranged that it was not necessary to break the vacuum entirely or interrupt the flow of nitrogen. Sample I was the only sample bodied to the gel stage. Wijs iodine numbers were determined according to the A.S. T.M. method (9). Refractive indices were determined a t 25" C. by an Abbe refractometer. Gardner-Holdt tubes were used t o measure viscosity. The densities were determined at 25" C. by an aluminum pycnometer of about 11 cc. capacity. Cryoscopic molecular weights were determined by measuring the depression of the freezing point of benzene. Hydrogen absorption was measured by a Burgess-Parr hydrogenator, using platinum oxide as a catalyst a t room temperature. To determine the percentage
TABLE I. SAMPLES I AND I1 BODIEDAT 305" C. UNDEB VACUUM Sample No.
Time, Hr.
Refractive Index
1-1 1-2 1-3 1-4
0.00 1.08 2.00 3.00 4.23
1.4796 1.4799 1.4838 1.4869 1.4893 1.4901 1.4912 1.4920 1 ,4920
1-6 1-6
1-7 1-8 1-9
6.00
6.00 6.92 7.00
Density
Visoosity.
0.9266 0.9266 0.9364 0.9491 0.9667 0.9682 0.9600
Iodine
No.
Sea.
Linseed Oil Sample I 184.6 726 179.6 761 162.6 844 136.2 1281 129.6 1443 126.6 1832 121.4 ...
1.0 1.2 2.6 10.7 36.6 70.6 229.6 3440.0
... ...
...
Gei
h1.01. Weight
.. *. ..
Sol. in Acetone,
%
100 100 100 66.3 39.7 32.2 23.9
... ...
Iodine No. of: Ha Heat of Acetone-sol. Acetone-insol. Absorption, Combustion, fraction fraction Lb. Cal./Gram
... ...
...
... ... ... ... ... ..... . ...
... ... ...
*.,
... ...
..*
7.8
...
6.3 3.9 2.6 2.0 1.6 1.2
...
9329 9281 9286 9336
...
9276
... ...
9 196
Linseed Oil Sample I 1 11- 1 11- 2 11- 3
11- 4 11- 6 11- 6 11- 7 11- 8 11- 9 11-10
0.0 0.6
1.1 1.6 2.1 2.6 3.6 4.6 6.1 6.4
1,4797 1.4801 1.4812 1.4823 1.4835 1.4845 1.4863 1.4877 1.4883 1,4888
EXPERIMENTAL PROCEDURE
Three samples of alkali-refined linseed oil and one sample of linseed oil containing 1% litharge were bodied a t 305" C. The bodying was conducted in a %liter three-neck flask with groundglass joints. About 1500 cc. of oil were bodied a t a time. The oil w m well stirred and heated with a gas flame. Two of the h e e d oil eamples (I and 11) were bodied under a vacuum of about 5 mm. of mercury; the third linseed oil sample (111) and the sample containing litharge (IV) were bodied under an atmosphere of nitrogen. About 100-cc. samples were taken a t various times during heat bodying. These samples were withdrawn through a system
soluble in acetone, the Shuey method (W) was used. Five grams of oil were shaken with 45 cc. of acetone and, after standing 24 hours, the acetone layer was decanted off. Both fractions were freed of acetone by evaporation under vacuum. An estimate of the heat of reaction was obtained from the heats of combustion of the starting sample and the final gel. They were obtained by means of an Emerson bomb equipped with a Daniels adiabatic jacket. The constants of the ,original linseed oil follow:
m
TABLE 11. SAMPLES I11 Sample No,
111-1 111-2
111-3 111-4 111-6
I116
111-7 111-8 111-9
IV- 1 IV- 2 IV- 3 IV- 4 IV- 6
IV- 6 IV- 7 IV- 8 IV- 9
IV-10
IV BODIEDAT 305" C. NITROQEN
AND
Time, Refractive Hr. Index 0.6
1.0 1.67 2.47 3.0 4.0 6.0
6.0 7.0
0.0 0.68 1.18 1.68 2.08 2.68 3.08 8.68 4.08 4.68
Density
Viscosity, $eo.
Linseed Oil Sample 111 1.4799 0.9260 1.1 1.4810 0.9277 1.4 1.4821 2.0
..
i.iiso
1.4862 1.4874 1,4890 1.4900 Linseed Oil 1.4796 1.4820 1.4860 1.4868 1.4881 1.4896 1.4907 1.4915 1.4927 1.4938
Iodine No.
UNDER
Mpl. Weight
725 0.9265 1.4796 184.0 1.0 8.6 9329 77.65 11.09
The physical constants of the bodied oils are listed in Tables I and 11. PHYSICAL PROPERTIES
181.6 169.6 169.2
o.iia8 &.'I 14i.2 0.9468 11.1 129.9 123.7 0.9476 20.4 46.0 120.0 0.9520 70.1 117.8 0.8629 IV Containing 1% PbO im.7 0.9316 0.6 175.7 0.9422 0.6 168.3 0.9440 2.2 144.7 0.9610 4.3 137.6 10.0 0.9630 131.1 21.1 0.9606 127.3 46.0 0.9616 126.9 86.4 0.9676 197.2 127.6 0.9666 117.0 0.9699 1101.0
721 766 817
iio
1161 1261 1608 1661 784 826 904 1088 1109 1219 1360 1642 1889 2218
IODINEVALUE. Since the iodine value may be accurately determined and measures the unsaturation present in linseed oil, the values have been used to measure the extent of the combination of fatty acid groups, p. The Wijs iodine value istritiated by the presence of conjugated double bonds, but the amount of such is never structure, while present in heat-bodied linseed oil (6,27), large enough to influence the results greatly. Since the loss of one double bond is equivalent to a drop of 86.6 in iodine value, the actual drop divided by 86.6 indicates the extent of ring formation. HYDROGEN ABSORPTION.The hydrogen absorption of heatbodied linseed oil was surprisingly low, and the results indicate that a ring structure is formed on polymerization. Many unsaturated cyclic compounds (I, 9%)do not absorb hydrogen under the conditions employed, and it is apparent that the low results
1126
INDUSTRIAL AND ENGINEERING CHEMISTRY
F/ GUAE /. HYDAOGEN A BSOAPT/ON
