Drying of Linseed Oil Paint Effect of Atmospheric Impurities on Rate of Oxygen Absorption DOUGLAS G. NICHOLSON, University of Illinois, Urbana, Ill. REVIOUS papers in this series (3, 4,5, 6 ) have shown t h e effect of variation in drier concentration, in artificial light intensity striking the paint film, in pigment content as well as age of paint, and i n the concentration of several fatty acids upon the rate of oxygen absorption by linseed oil
P
paint films. Since it is generally considered that the drying of oil paint films is due t o a complex set of reactions involving oxidation and polymerization, a detailed investigation of each of the several factors affecting the rate of oxygen absorption of such films should include a study of the effect of gaseous
X V
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10090 OXYGEN 17.5% CO, 30% C02 80,590 Cor
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September, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Paint films pigmented with zinc oxide, zinc sulfide, or titanium dioxide ground in bodied linseed oil have been allowed to dry in artificial atmospheres of oxygen and either carbon dioxide, nitrogen, ozone, sulfur dioxide, or hydrogen sulfide. The rate of drying of these paints (as indicated by oxygen absorption) is not materially affected by rather wide variations in the carbon dioxide or nitrogen content of the atmosphere. The effect of traces of ozone is a definite shortening of induction periods, more rapid increase in weight, and definite wrinkling of all paints studied. Sulfur dioxide in the atmosphere reduces the drying time of zinc oxide paint films while it definitely lengthens that of paints pigmented with titanium dioxide or zinc sulfide. Hydrogen sulfide in the atmosphere reduces the rate of oxygen absorption of all paints. The latter effect is least pronounced in titanium dioxide paints and shows definite retardation of drying in films containing either zinc oxide or zinc sulfide.
1149
milliliter of this drier solution contained 0.0158 gram of cobalt, calculated as metal. The following concentrations of drier solution were used per 140 ml. of paint: zinc sulfide paint, 1 ml.; titanium dioxide paint, 2 ml.; zinc oxide paint, 3 ml. These drier concentrations were selected after a series of preliminary determinations had shown that fairly complete oxygen absorption data could be obtained in a 24-hour exposure period. A 60-watt Mazda lamp was illuminated continuously a t a distance 55 em. from the paint panel on the balance pan. This light corresponded to an intensity of 16 foot-candles a t the surface of the exposed paint anel. The gases were Elended in an auxiliary tank and allowed to enter the balance case a t a rate of 120-150 ml. per minute. The gas train was so arranged that samples could be obtained for analysis a t any time. In all determinations the balance case was initially filled with air. The artificial-atmosphere flow into the balance case was
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atmospheric impurities on the rate of oxygen absorption by these films. Since carbon dioxide and nitrogen are present in the atmosphere, any study involving atmospheric impurities should include these gases. Ozone has been mentioned as being a substance which accelerates the drying of paint and varnish (1, 2 ) . Much of the soft coal used as fuel contains sulfur which is converted t o sulfur dioxide when burned; therefore, small amounts of this gas may be considered an atmospheric impurity. M a n y communities located near certain industrial plants have gaseous hydrogen sulfide in the atmosphere. A detailed report concerning the effect of sulfur dioxide on plants and animals was recently published (8). Accordingly, a study was carried out i n which paint films were allowed t o d r y in artificial mixtures of oxygen and each of the five gases mentioned. The effect of various concentrations of each gaseous impurity was noted on linseed oil paint films containing titanium dioxide, zinc sulfide, or zinc oxide a s the pigment material.
Methods of Studying Weight Changes The rate of absorption of oxygen by the paint films was followed by noting changes in weight of the film (applied on a glass plate) placed on the pan of a chain weight balance. The entire balance was enclosed in an outer case. Extensions wer'e fitted to the beam and pan releases as well as chain mechanism of the balance so that wei hings could be obtained from outside the outer case. Details o f t h e balance setup and of the method of applying the paint films were previously described ( 5 ) . The actual increasg in weigh6 noted were the net result of a composite of increases in weight due to oxygen absorbed, less the loss in weight due to volatile oxidation products. The three paints were pigmented with 3 pounds of pigment per gallon. This pigmentation is e uivalent t o 9.27 per cent titanium dioxide, 8.57 per cent zinc sulzde, and 6.37 per cent zinc oxide by volume. The vehicle was body-Q linseed oil. No thinner or volatile solvent was used. Cobalt naphthenate drier was added to each paint sample as an oleic acid solution. Each
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FIGURE 2. DATAON DRYINGPAINTFILMSIN OXYGENWAS OPERATED OZONEATMOSPHEREWHEN THE OZONIZER FOR VARYING PERIODS
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INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE3.
Vol. 33, No. 9
WRINKLING OF TITANIUM DIOXIDEPAINTFILMEXPOSED TO OZONE-OXYGEN ATMOSPHEREFOR 3 HOURS
T h e film was photographed 24 hours later: a film of the same paint dried in oxygen atmosphere showed no wrinkling after a similar exposure period.
started as soon as possible after the initial (aero time) weighing was obtained. Thus an error of constant magnitude was encountered in all cases. Since the analyses of the gases entering the balance case were determined, it follows that these values were not the same as the actual composition of the atmosphere in the balance case. Values given in this report concerning atmosheric composition were obtained from analyses of incoming lended mixtures and should not be interpreted as being the actual atmospheric composition within the balance case. Since it was not possible to blend definite quantities of ozone with oxygen, dry oxygen was passed through a laboratory ozonizer for varying periods of time. Thus the ozone concentration in
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Convex watch glass
the oxygen entering the balance case was practically constant during the time the ozonizer was operating but dropped t o zero immediately after the ozonizer was turned off. Some difficulty was encountered in analyzing for the ozone content in these oxygen-ozone mixtures due to the rapid failure of all rubber tubing in the analysis system after contact with ozone.
Results with Different Atmospheres CARBONDIOXIDEAXD NITROGEN.Figure 1 shows the small variations in the rates of increase in oxygen absorption
Concave watah glass
OF OZONE ON WRINKLING OF FILMS OF VARYIXGTHICKNESS FIGURE 4. EFFECT
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INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1941
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as the oxygen concentration of the gas entering the balance case is varied throughout rather wide limits. A logical assumption is that the rate-controlling reaction in the drying process is not dependent upon the concentration of oxygen in the atmosphere surrounding the paint film. Figure 1 also indicates that the pigment material present in the film does not exert any great effect when the oxygen diluent is carbon dioxide or nitrogen. OZONE. Figure 2 is a quantitative representation of the effect of minute traces of ozone in the oxygen atmosphere on the rate of gain of these paint films. Analysis for ozone was carried out by allowing a known volume of the ozone-oxygen mixture to stand in contact with potassium iodide solution.
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FIGURE 6. PHOTOGRAPH OF SANDERSON DISKSAFTER DRYING
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The resulting iodine liberated was titrated with standard thiosulfate solution. The average ozone content in this artificial atmosphere was 0.091 per cent by volume. The effect of the ozone addition was noticed immediately in the weight changes of all films. Bfter the paint films were exposed to the ozone-oxygen atmosphere for 90 to 120 minutes, wrinkling of the films appeared, first at the edges of the plate and then progressing toward the inner areas. Such a wrinkled film is shown in Figure 3. It was thought that this initial wrinkling at the edges of the film was due to uneven coating of the paint film a t the edges of the panel. Accordingly two watch glasses, one concave side u p and the other convex side up, were coated with 0.5-ml. of the experimental zinc sulfide paint. The convex watch glass presented a paint film which was thick at the outer edges, while the concave glass gave a film which was much thinner at the outer edges. Figure 4 is a photograph of these films after exposure t o the ozone-oxygen atmosphere for 150 minutes. Both show definite wrinkling, from the outside edges toward the center. Apparently the factors associated with wrinkling in such films are the result of film-solid interface forces. I n the case of the ozone-oxygen atmosphere the type of pigment present was not of material significance in affecting the velocity of the increase in weight. SULFURDIOXIDE.Figure 5 is a plot of the data obtained when using the sulfur dioxide-oxygen atmosphere. Apparently the presence of sulfur dioxide in the atmosphere tends to retard the rate of drying of paints pigmented with zinc sulfide or titanium dioxide; at the same time it increases the rate of drying of paint pigmented with zinc oxide. One possible explanation of these data appears to be t h a t sulfur dioxide reacts with traces of moisture formed during the drying of the film to produce sulfurous acid. This acid reacts with the cobalt naphthenate drier t o form cobalt sulfite and naphthenic acid. This theory was supported by the following experiment: Dry sulfur dioxide was passed through a n oleic acid solution of cobalt naphthenate with no observable effect. When a trace of moisture was introduced into the reaction tube in continued contact with sulfur dioxide, a crop of crystals formed rapidly. This crystalline material was filtered, mashed, and found t o be cobalt sulfite. In the case of the zinc oxide paint, a portion of the observed increase in weight could be attributed to reaction of the zinc oxide pigment with sulfur dioxide to form zinc sulfite. Since sulfur dioxide coming in contact with the film was thus converted to zinc sulfite, it follows t h a t the cobalt drier material present was not appreciably affected by this gas and was available as a soluble catalyst to aid in the drying of the film; the fact t h a t zinc oxide films showed definite increase in rate of gain in weight as the concentration of sulfur dioxide was increased seemed t o support this theory. However, analyses of exposed zinc oxide and titanium dioxide films did not show the expected high zinc sulfite content. One of the titanium dioxide pigmented films had a higher sulfite content than did e zinc oxide films tested. e I summarizes a portion of the analyses carried out exposed films. It appears t h a t the sulfur dioxide content of the film increased in proportion to the concentration of this gas in the atmosphere. I n other words, the nature of the pigment did not materially affect the amount of sulfur dioxide absorbed by the paint film. The sulfur dioxide content in the films was determined by acidifying a portion of the material with phosphoric acid, warming, and passing the evolved vapors into bromine water. The resulting sulfate was determined gravimetrically as barium sulfate. All zinc-oxide-pigmented films showed a tan-white color (darkening) when allowed to dry i n a sulfur dioxide-oxygen atmosphere. This tint became evident after 7 to 10 hours of exposure in the artificial atmosphere.
Vol. 33, No. 9
TABLEI. SULFUR DIOXIDECONTENT OF PAINTFILMSDRIED IN OXYGEN-SULFUR DIOXIDEATMOSPHERES Pigment in0 TiOz TiOn TiOz
’% SO2 in AtrnosDhere 19 12
% ’ SOz in Film, after
2.6 0.00
0.241 0.0017
24-Hr. Drvinp: . . 0.466 0.505
Figure 6 shows the result of Sanderson drier tests run on three films exposed t o a 10 per cent sulfur dioxide atmosphere for a n hour, after which they were placed in a constant temperature-humidity room in the Sanderson apparatus. Three similar films were applied and retained in the chemical laboratory (contact with air) for a n hour prior to exposure in the Sanderson apparatus. One revolution of the sand track on these films is equivalent to approximately 9 hours of drying time. Since these films were exposed 1 hour prior to exposure in the Sanderson apparatus, the total drying time was actually 1 hour more than the figures in Table 11. Table I1 presents the approximate time of drying of the films shown in Figure 6. Attention is called to the fact t h a t the actual drying tests (Sanderson) check the oxygen absorption curves shown in Figure 5-namely, t h a t zinc oxide films do d r y more rapidly and t h a t zinc sulfide and titanium dioxide films dry much more slowly in a sulfur dioxide-oxygen atmosphere than they do in oxygen alone.
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FIGURE 7. RESULTS OF EXPOSING PAINTFILMS.TO HYDROGEN SULFIDE-OXYGEN ATMOSPHERES
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1941
HYDROGEN SULFIDE.Figure 7 shows that the presence of hydrogen sulfide in the atmosphere definitely reduces the rate of oxygen absorption by paint films. This effect increases in intensity as the concentration of the gas increases for paint pigmented with titanium dioxide. With ainc oxide or zinc sulfide as pigments, the effect is definite when using extremely low concentrations of hydrogen sulfide. Increasing the concentration of this impurity in the atmosphere does not materially reduce the rate of oxygen absorption.
SULFIDEANALYSESBY TABLE 111. HYDROGEN PIERCE AND HAENISCH (7) Pigment in0 ZnO ZnO TiOz Ti08 Ti02
Yo HISin Atmomhere 11:5 2.0 00.0 16.0 6.5 00.0
1153 THE
METHODOF 91 HIS
i,“ Film 0.570 0.563 0.188 0.610 0.599 0.122
Acknowledgment ‘TABLE 11. DRYINGTIMEFOR PAINTFILMSIN DRYER Pigment
Ti02 ZnS ZnO
Air Exposure
THE
SANDERSON
Drying Time after 1 Hr. in 10% Sot Followed by Air Exposure
HT.
Min.
HT.
Min.
5 5 16
20 20 50
77 57 14
0 30 50
The samples of paint used in this study were prepared in the paint laboratory of Krebs Pigment and Color Corporation. The Sanderson apparatus belonging to the United States Regional Soybean Laboratory, Urbana, Ill., was used in the drying tests.
Literature Cited It is possible that the hydrogen sulfide in the atmosphere reacts with the cobalt drier present to precipitate cobalt sulfide. For some unexplained reason the removal of the catalyst is apparently more complete with zinc pigments than in the case of titanium dioxide. No positive proof is available t o support this idea. Analyses of several of the films exposed to the hydrogen sulfide-oxygen atmospheres of Table 11, show the presence of a n appreciable hydrogen sulfide content (Table 111).
Gardner, H. A., “Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors”, 7thzed.,p. 442, Washington, Inst. of Paint and Varnish Research, 1937. Hopkins, B. S., “General Chemistry for Colleges”, p. 73, Boston, D. C. Heath and Co., 1937. Nicholson, D. G., IND. ENG.CHEM..31, 1300 (1939). Ibid., 32, 1259 (1940). Xicholson, D. G., and Holley, C. E., Ibid., 30, 114 (1938). Ibid., 30,563 (1938). Pierce, W. C., and Haenisch, E. L. “Quantitative Analysis”, p. 177, New York, John Wiley & Sons, 1937. Setterstrom, Carl, IND. ENQ.CHEM.,32,473 (1940).
THE ALCHEMIST . . . . . After Teniers AGAIN
we are indebted t o the late Sir William J. Pope for an addition to the Berolzheimer series of Alchemical and Historical Reproductions (No. 129). The original on canvas is 23 by 191/2 inches and is not signed. This is undoubtedly a copy of “Le Grimoire de Hypocrate”, No. 47 in the series, although i t is slightly larger than the latter, which is 23 by 15 inches. It may well be that David Teniers, the Younger, started this painting, rejected it for one reason or another, and then turned it over for completion to one of the considerable number of students who worked under him. Those parts of the composition which appear in both paintings are identical except for the facial expression of the Alchemist. It is obvious to any one making a comparison that the boy and a number of objects have been added in the center foreground, at the right, and elsewhere. As a result the scene appears “cluttery”. The drawing tacked on the back wall is also new and is undoubtedly a sketch subsequently used in one of Teniers‘ alchemical series. D. D. BEROLZHEIMER 50 East 4lst Street New York, N. Y.
The lists of reproductions and directions for obtaining copiea appear a8 follows:
1 to 96, January, 1939 issue, page 124. 97 to 120, January, 1941, page 114. An addtional reproduction appears each month.
