Drying of Linseed Oil Paint Effect of Artificial Visible Light‘ DOUGLAS 0 . NICHOLSON AND CHARLES E. HOLLEY, University of Illinois, Urbana, Ill.
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HE fact is generally accepted that light, particularly of the shorter wave lengths, has a definite effect on the drying of linseed and other oils (S, 4). The stability of dried fdms towards ultraviolet light has been the subject of several investigations (6, 8, 9). According to Stutz (10, I f ) , the exposure of wet oils to ultraviolet light produces an effect similar to that of heat or air blowing. Rogers and Taylor (7) made a study of the effect of visible light on the rate of oxidation of linseed oil. Very little quantitative information is available concerning the drying of pigmented oil films. Because it has been shown (6) t h a t films of bodied linseed oil pigmented with titanium dioxide dry much more slowly than the oil alone, a detailed study of the effect of artificial visible light upon similar pigmented linseed oil fdms was undertaken.
The effect of variations in artificial visible light intensity upon bodied linseed oil paint films pigmented with white lead, zinc oxide, and titanium dioxide has been studied in some detail. Generally speaking, an intense light tends to cause these films to gain in weight sooner than a less intense light. White lead shows the least effect, zinc oxide greater, and titanium dioxide the greatest differences in drying time as the light intensity is varied. It appears that the light is a catalyst favoring the removal of natural antioxidants present in the oil. tion and in a nonuniform distribution of the drier material. After such treatment the pigment concentration in the paint approached 25.5 per cent by weight and the drier content approached 0.0119 per cent cobalt (calculated as metal). This drier concentration was selected after a series of preliminary determinations had shown that the paints would dry at a rate which could be followed quite accurately with the experimental setup used.
Apparatus, Materials, and Procedure The rate of gain in weight of the paint films was followed accurately by means of a rather elaborate weighing device. The details of the method used were described previously (6):
The use of oleic acid as the solvent for the drier used was justified on the ground that this acid is a nonvolatile liquid of semidrying nature which will mix readily with the linseed oil used. The three pigments employed in this study represent materials showing rather wide differences in opacity as well as reactivity. Titanium dioxide is typical of a highly opaque nonreactive pigment; the white lead has low opacity and the zinc oxide is of the reactive type. The use of extended pigments was purposely avoided in this investigation.
The apparatus consisted essentially of a chain weight balance enclosed within a case so e uipped that changes in weight of a paint film could be followe% accurately from outside the case. A film of paint of uniform thickness was spread upon a glass plate of known weight, the painted plate was placed upon the balance pan, and the balance case was closed; it was not reopened until the particular experiment had been completed. Details concerning the preparation of the paint fdms were also described previously (6). Dry oxygen was constantly passed into the balance case at a rate of approximately 120 ml. per minute. An electric lamp socket was placed in a fixed position outside the balance case at an average distance of 55 cm. from the paint film. Lamps of loo-, 60-, and 25-watt intensities were inserted as desired. Foot-candle intensity readings, obtained by placing a Weston Junior foot-candle meter in the balance case, showed values of 14.5, 10.5, and 4 foot-candles, respectively, for the three lamps used. A hood of black oil cloth was used to cover the entire case so that the effect of excluding all light could be studied. The appropriate electric lamp was burned continuously throughout a particular observation. In those cases in which the drying was carried out in “complete darkness,” weighings were made with the aid of a 7.5-watt lamp for illumination purposes. This light was used for as short time intervals as possible. The tem erature within the balance case was maintained at 25 * 1” The paints used in this study consisted of titanium dioxide, French Process zinc oxide and white lead ground in body Q linseed oil. Each paint was pigmented with 3 pounds of pigment per gallon of paint. Two days prior to its use, 120 ml. of each of the paints were diluted with a mixture consisting of 19 ml. of body Q linseed oil and 1 ml. of oleic acid solution of cobalt naphthenate. This treatment was carried out so as to incorporate the drier in the paint completely. The direct addition of the drier solution to the zinc oxide paint resulted in some granula-
Discussion of Results The results are shown graphically in Figures 1 to 4. Figure 1 shows the data obtained by varying the light intensity which strikes films of unpigmented oil containing the s a m e d r i e r concentration. The DRYING TIME IN MINUTES FIGURE 1. EFFECT rate of gain in weight was quite OF LIQHT OF DIFFER- rapid although not appreciably affected by the intensity of the inENT INTENSITY UPON THE DRYING RATE OF cident light. CA-PIGMENTED OIL Figure 2 shows similar data for x = 100-watt lamp conthe pigmented oil films. A slight tinuously o = 60-watt lamp conloss in weight usually takes place tinuously A = 25-watt lamp oonimmediately after a particular study tinuously has begun. This fact was observed v = complete darkness
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1 A previous paper in thie aeriea was published in January, 1938, pages 114 to 116.
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and reported previously (l), but it has not yet been adequately explained. Weight changes in the films were observed a t time intervals of such magnitude that smooth data curves could be obtained. The initial reading was taken as soon as possible after the paint film had been spread upon the glass panel used. The loss in weight generally took place very shortly after that. This loss was followed by either a gain in weight or a period during which no changes in weight were evident. This dormant period has been referred to as the
TIME OF MAXIMUM GAIN-MINUTES
FIGURE3. RELATION BETWEEN LIGHTINTENSITY AND TIME AT WHICH PAINTSHOWED MAXIMUM RATE GAINI N WEIGHT White lead French Process zinc oxide Titanium dioxide
induction period. Following the induction period, the gain per unit interval of FIGURE2. EFFECTOF ARTIFICIAL t i m e i n c r e a s e d VISIBLE LIGHTINTENSITY UPON THE steadily to a maxiRATEOF GAININ WEIGHTOF LINSEED mum, remained so for OIL PIGMENTED WITH VARIOUS MAa period of varying TERIALS length, and then x = 100-watt lamp A = 25-watt lamp o = 60-watt lamp . v. = complete darkness s t e a d i l y declined. A . White lead Care was taken to B . French Process zinc oxide C. Titanium dioxide obtain readings a t such intervals that the initial time of the maximum rate of gain could be accurately I observed. Figure 3 shows the relation between the light intensity and the initial time at which this maximum gain became evident. F i g u r e 2A shows that the i n d u c t i o n period for the white lead paint was very C I 3303 3EoO 3900 4200 4503 4800 short As a result, more difficulty was e x p e r i e n c e d i n observing time differences in the interval required for the paint to attain its maximum rate gain. This fact accounts for the rather odd shape of the curve in Figure 3A; Figures 3B and C give rather smooth curves for those pigments whose paints showed longer induction periods. According to Rogers and Taylor (Y), the change in rate of oxidation of linseed oil is not directly proportional to the light intensity. The results obtained in this study are in agreement with this fact. The explanation for the observed differences in lengths of the induction periods and in the rates a t which the different paints gained weight as the pigments were varied may possibly lie in the fact that natural antioxidants are present in the oil ( 2 ) . It is probable that light is a catalyst aiding in the rapid removal of these antioxidant materials. Since the unpigmented oil is virtually transparent, we should expect I
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FIGURE4.
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EFFECTOF EXPOSING SIMILARLY PIGMENTED FILMS FOR INTERMITTENT PERJODS
x = 100-watt lamp continuously = 100-watt lamp for first 100 min.
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that even a weak light would affect these antioxidants in a very short time. When such an oil is pigmented, a masking substance is introduced which would tend to reduce the catalytic effect of the light striking the film. Accordingly, as the pigment increases in opacity, we should expect a longer induction period; and as the intensity of the light is increased (holding the pigment constant), we should expect a shortening of this interval. The experimental data obtained with the pigments studied in this investigation tend to substantiate the explanation presented above. Since titanium dioxide paints showed the greatest "spreading" in the time a t which they gained weight as the light intensity was varied, an independent study was undertaken to show the effect of exposing similarly pigmented films for intermittent periods. Accordingly, a sample of titanium dioxide paint containing 0.0130 per cent cobalt by weight was exposed to the 100-watt lamp for the initial 100 minutes and then enclosed in darkness. A duplicate sample was retained in darkness for 540 minutes (9 hours) and then exposed to the 100-watt lamp for a 100-minute period before being again returned to the dark condition. Data obtained thus were compared with those obtained by exposing a control film to the 100-watt lamp continuously. These data appear in graphical form in Figure 4. From this representation it is evident that the exposure of these films to light a t different periods shows its greatest effect in reducing the length of the
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induction period, while not materially changing the rate of oxygen absorption by the film.
Acknowledgment The paints used in this investigation were prepared in the paint laboratory of Krebs Pigment and Color Corporation. The cobalt naphthenate drier was supplied by Nuodex Products, Inc.
Literature Cited (1) Elm, A. C., IND.ENQ.CHEM., 23,881 (1931). (2) Ibid., 26, 386 (1934). Fukushima, I., Horio, M., and Miki, T., J. SOC.Chem. Ind. Japan, 35,Suppl. Binding, 142 (1932). Gardner, H . A.. and Parks. H. C.. Paint Mfrs.' Assoc. U. S.. Circ. 172, 146 (1923). Nelson, H. A., Schmuta, F. C., and Gamble, D. L., Proc. Am. SOC.Testing Materials, 26,Pt. 2,663 (1926). Nicholson, D. G . , and Holley. C. E., IND.ENQ.CHEM.,30, 114 (1938). Rogers, W., and Taylor, H. S., J . Phys. Chem., 30, 1334 (1926). Schmuta, F. C., and Gamble, D. L., IND. ENQ.CHEM.,Anal. Ed., 1, 83 (1929). Schmuta, F. C., and Palmer, F. C.,IND.ENQ.CHEM.,22, 84 (1930). Stuta, G. F. A., Ibid., 18,1235 (1926). Ibid., 19,897(1927). R E C ~ I V EOctober D 10, 1937.
Vapor Pressures of Solvents' D. H. KILLEFFER 60 East 42nd Street, New York, N. Y.
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N THE preceding issue (April, 1938, pages 478 and 479) two convenient nomographic charts were given for the vapor pressures of common solvents boiling below 90" C. and between 90" and 150" C. The method there described has been used to construct the accompanying two nornographs (pages 566 and 567) covering two further groups of compounds boilingfrom 150" to 200" C. and above 200" C.' These bring the total number of compounds shown to 128. The graphs have been carefully checked with the best data available on the compounds included and are accurate within the limits of error of the original data. Readings taken from the nornographs (by running a straight line from scale to scale through the gage point for each compound) are accurate to approximately the degree of accuracy of reading the temperature scales. I n describing the previous graphs, the method of constructing the empirical temperature scales was given. The basic data for the temperature scale for the first graph were the vapor pressures of methyl isobutyrate. The second scale was constructed from data for isobutyl isobutyrate. Temperature scales for the third and fourth nomographs are composite. Bromobenzene was selected as the basic material, but data available on it did not cover the lower temperature range. Consequently, part of the scale from 30" to 250" C. was constructed From its vapor pressures, and the section from 0" to 30" was extrapolated by plotting available data on ethanol on this temperature scale from 30" to 80" C. and continuing the straight line thus obtained into the lower temperature range. 1 Those interested may obtain reproductions of these four nornographs on heavy Bristol board by sending 25 cents to Industrial and Engineering Chemistry, 706 Mills Building, Washington, D. C., to cover the cost of printing and postage. They will be mailed about May 1, 1938.
The agreement between the overlapping sections of the two curves was striking. The reason for selecting these compounds was the regularity of the curves of their vapor pressures and the fact that data on them were available from several sources for checking. I n selecting compounds for inclusion on the charts, every effort was made to include those most likely to be useful and also those on which reliable data are available. No apologies are offered for omissions, of which there are many, since the user of these charts can readily add any others he chooses, provided only that he has available two or three accurate points on their vapor pressure curves. The method of plotting points for additional compounds consists merely in determining the intersections of two or more lines connecting vapor pressures with their corresponding temperatures. If three or more lines drawn in this way fail to intersect in a point on the graph, there is some ground for questioning the data. I n this way i t has been possible to choose between conflicting data in the construction of these charts.
Acknowledgment Again, the author acknowledges his debt to D. S. Davis, whose method of constructing nomographic charts was modified for this use; to C. Hadlock of the Engineering Department of E. I. du Pont de Nemours & Company, Inc., whose compilation of vapor pressures supplied much of the data used; and to P. R. Rector of Carbide and Carbon Chemicals Corporation who supplied data and checked the completed graphs. RECEIVED February 12, 1938.
