Formation and Deterioration of Paint Films Changes in the Glyceryl Esters of Several Unsaturated Fatty Acids under Exposure to Ultraviolet Light J. L. OVERHOLT AND A. C. ELM The New Jersey Zinc Company (of Pa.), Palmerton, Penna.
bodying suffered by this ester during its preparation. It produced a wrinkled film which did not soften on prolonged exposure.
In previous papers of this series1 the authors reported the changes taking place in films of methanol and glycol esters of oleic, linoleic, linolenic, and eleostearic acids when exposed to ultraviolet light. Continuing this work the glyceryl esters of these acids were examined in the same manner. The course of the oxidation of the pure glyceryl esters is similar to that of the corresponding methyl and glycol esters. As anticipated from the higher degree of functionality of the glycerides, all but the oleate dried to tack-free films.
a
'.i
10
1
T
100 FIGURE 7.
1Ta-_l.
HE glyceryl esters of the four common fatty acids found in drying oils were prepared from the corresponding methyl esters by effecting an ester interchange between glyceryl triacetate and the methyl esters a t a relatively low temperature, distilling off the resulting methyl acetate, and washing out the mono- and diglycerides with alcohol. The purity of the resultant glycerides is indicated in Table I. The glyceryl eleostearate resembled the glycol eleostearate in physical appearance. It was a pasty solid of low melting point and exhibited a tendency to gel on aging, even though stored a t low temperatures. I t s oxygen content and saponification number were slightly higher and its diene value was definitely lower than theoretical. As the viscosity value of 2.2 poises indicates, this may have been caused by some heat 1
H O U R S EXPOSURE
W
60R
IODINE VALUES
0
1
H O U R S EXPOSURE
IO
I0,O
1000
IND. ENQ. CHEM..32, 378, 1348 (1940)
OF GLYCERYL ESTERS TABLE I. PROPERTIES
Oleate Linoleate Linolenate Eleoatearate
Acid No. 1.8 1.5 0.9 1.4 Density'J
Oleate Linoleate Linolenate Eleostearate
0.941
Saponification No. 196.2 193 194.6 210
Wija Iodine Value 85.2 165.1 251.4
...
Viscositya, Poisea 0.80 0.75 0.45 2.2
Maleic Anhydride Addition Value
..
.. 63:s Refractivc Indexa 1.4666 1 ,4758 1.4860
HOURS EXPOSURE
....
I
Curves 1, glycerol trioleate; 2, glycerol trilinoleate; 3, glyoerol hnolenate; 4, glycerol trieleostearate
a Determined a t 25' C.
658
INDUSTRIAL AND ENGINEERING CHEMISTRY
May, 1941
659
FIGURE 4. PEROXIDES
I_
HOURS EXPOSURE 10 100
I 0
IO00
FIGURE 5 . ALDEHYDES
.-2$
HOURS EXPOSURE IO 100
I
I
HOURS E X P O S U R E I O 100
1
I""
1000
FIGURE 10. V I S C O S I T Y m
I ooc HOURS E X P O S U R E 10 100
1
loo(
FIGURE I I . MOL. WEIGHT I T E R T - B U T Y L ALCOHOL
900
F I G U R E 6 . HYDROXYL OXYGEN
IO
1
100
1000
1503 p:
W
a
FIGURE
I
7. A C I D V A L U E S
0
100
*
0 I
1.46
50
>c
J
lop
J
S EXPOSURE IO0 loo(
IO
I
HOURS EXPOSURE 10 100
1,
FIGURE 8. ESTER VALUES
FIGURE 13. D E N S I T '
0.9
I
1
1
HOURS EXPOSURE 10 100
loo(
F I G U R E 14. SPECIFIC R E F R AC T I O N
t
0
I
HOURS IO
EXPOSURE 100
1000
Curves 1. Glycerol trioleate Curves 2. Glycerol trilinoleate
t
0.26 I
HOURS E X P O S U R E 10 100
Curves 3. Glycerol trilinolenate Curves 4. Glycerol trieleostearate
1001
660
’
INDUSTRIAL AND ENGINEERING CHEMISTRY
As anticipated, the increased complexity of the glyceryl esters is demonstrated by the fact that the linoleate, linolenate, and eleostearate formed dry, difficultly soluble films. This made the determination of a number of the physical properties by the chosen methods very difficult or impossible.
T H E rate of gain in weight of the glyceryl esters (Figure 1) is almost identical with that of the methyl and glycol esters. This is evidence that the initial step in oxidation is independent of the alcohol combined with the fatty acids. However, on prolonged exposure the glycerides lost less weight than did the glycol esters. The rate of gain in weight falls in the order oleate, linoleate, eleostearate, linolenate ; the last shows the greatest gain in weight and the greatest resistance to decomposition. The iodine values (Figure 2) indicate that the glyceryl oleate oxidized a t about the same rate as did the methyl and glycol esters. The curves for the more unsaturated glycerides, however, show that the glycerides oxidize in four rather than three stages-namely, the induction period, a period of rapid oxidation ending in drying, a third stage of slow continued oxygen absorption, and a fourth period in which little change occurs. Figure 3 shows the maleic anhydride values. I n the preceding two papers it had been suggested that peroxide interfered with the maleic anhydride or diene values. This view has found additional support in the results of this investigation. On oxidation glyceryl oleate, linoleate, and linolenate show a maximum diene value a t the exposure interval of the maximum peroxide values. It is noteworthy that the maximum peroxide concentration of the glyceryl linolenate and the maximum hydroxyl, acid, and ester values and total oxygen content (Figures 4, 6, 7, 8, and 9) of the glyceryl linoleate, linolenate, and eleostearate which formed solid films were lower than the corresponding values of the methyl and glycol esters. I n spite of a lower degree of unsaturation, or perhaps because of it, the oxidized oleate showed a slightly higher total oxygen content than did the other esters. The dry point, which is clearly indicated by the viscosity curves (Figure lo), preceded by a short time the point of maximum peroxide and aldehyde concentration (Figures 4 and 5 ) . The oleate, linoleate, and linolenate became very
Vol. 33, No. 5
viscous after 500, 14 and 5 hours of exposure, respectively, so that it was not possible to make further viscosity measurements. The glyceryl oleate did not dry but remained a viscous liquid throughout the whole exposure period of 1000 hours. Much like the viscosity, the refractive index and density values (Figures 12 and 13) increased sharply at the end of the induction period. The specific refraction (Figure 14), which is derived from the refractive index and density, indicated a much faster reaction for the glycerides than was observed with the methyl or glycol esters. The specific refraction of the glyceryl linoleate between 10- and 24-hour exposure is represented by a dotted line because of the uncertainty of the values obtained during this period. There is little doubt, however, that the specific refraction decrease* sharply in the neighborhood of the dry point. The molecular weight of the glyceryl oleate in tert-butyl. alcohol decreased sharply after about 110 hours of exposure. indicating appreciable decomposition. The glyceryl linoleate and linolenate decreased in molecular weight to about half the original value during the first few hours of exposure, but a t a point which coincided roughly with a sharp rise in viscosity the films quickly became insoluble. Although actual molecular weight measurements could not be obtained after that, it is assumed that a rapid increase in molecular weight preceded solidification of the film. Some support for this assumption is furnished by the glyceryl linoleate values. S o molecular weight measurements could be obtained on the oleostearate. There is considerable evidence that the peroxides or peroxide decomposition products are the important links in the formation of films. If a dry film forms, the ester is moderately resistant to further oxidation, but if a dry film does not form, it oxidizes further and decomposes to acid and hydroxyl compounds. Acknowledgment The authors wish to thank J. G. Smull and E. P. Clocker of Lehigh University who prepared the esters used in this investigation and A. J. Farber who carried out the analytical work. PrmammEn before the Diviaion of Paint and Varnish Chemistry a t the 100th Meeting of the American Chemical Society, Detroit, Mich
FURFURALASASOLVENT S h SELECTIVE solvent, furfural has become important industriallv in two rather widely divergent fields. These are the treatment of lubricating oils(1) and rosin refining ( 3 ) . I n the first example, furfural is employed to remove undesirable aromatic and sludge-forming constituents; in the latter it acts to remove the dark colored components normally present in crude rosin. In spite of the fact that furfural has gained such large industrial uses as a result of its solvent action, a survey of the literature indicated that data as to its solvent properties are rather incomplete. Since the very term “selective solvent” implies that such properties are limited, the purpose of this paper is to point out some of those limitations.
A
Preparation of Pure Furfural
K i t h the idea in view that such data as are presented here might be of value in industrial processes, it seemed advisable
FLOYD TRIMBLE,
Quaker Oats Company, Chicago, 111.
to use furfural of a purity that might readily be obtained by commercially feasible means rather than to strive for the utmost in purity and therefore obtain data which might not be of practical value. Of the various commercially feasible procedures to follow in purifying furfural, careful vacuum fractionation of the ConlInercial grade appeared best. The apparatus used for this purification was so designed and operated that the significant impurities known to be present in the starting furfural would be effectively eliminated. The acidity of the furfural was reduced by this distillation from 0.050 to 0.002 Der cent. calculated as acetic acid. Such slight acidity as the iatter vaiue is probably beneficial in the staglization of furfural, as the work of Groth ( 2 ) indicates. Furfural obtained by alkali treatment or from the bisulfite addition product seemed to be less stable, as evidenced by a
