Drying of Linseed Oil Paint J
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Effect of Acidity upon Rate of Oxygen Absorption' DOUGL4S G. NICHOLSOX, University of Illinois, Urbana, 111.
The effect of varying percentages of oleic, linoleic, linolenic, diheptylacetic, and capric acids upon the induction periods and rate of oxygen absorption of linseed oil paints pigmented with titanium dioxide, zinc sulfide, and zinc oxide, has been studied in some detail. Generally speaking, increased acidity shortens the induction period and increases the rate of oxygen absorption in the cases of the nonreactive pigments, with the reverse shown in the case of the reactive pigment studied. It appears that exposure to atmospheric oxygen and aging tend to remove natural antioxidants present in paint (drier absent), since the induction period of such exposed paints decreases while the rate of gain in weight due to oxygen absorption remains approximately constant.
GEKERALLY accepted fact is t h a t free fatty acids
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present in a linseed oil paint may react with basic pigment materials and result in the formation of metallic soaps. When formed, these soaps account for m a n y changes in physical and chemical properties of the paint, as well as of the d r y paint films. Droste ( 2 ) supported the above idea concerning longer chain fatty acids rvhereas v a n Hoek ( S ) maintained that only the short-chain fatty acids a r e formed in the drying processes. Previous studies in this series showed the effect of drier concentration ( 5 ) , intensity of artificial visible light (6), and pigmentation and age of the paint ( 4 ) upon t h e rate of oxygen absorption by paint films. Since very few data are available concerning the specific effect, if any, of varying concentrations of individual free f a t t y acids in a paint upon the rate of oxygen absorption b y t h e individual paint, i t was decided t h a t such a study would yield interesting information. Accordingly, definite volumes of paints pigmented with nonreactive materials, titanium dioxide and zinc sulfide, as well as with a reactive material, American process zinc oxide, mere diluted with varying amounts of several fatty acids. All factors, such as light intensity striking the paint film, drier concentration, pigmentation, thickness of film, and volume of paint under study, were held constant throughout t h a t portion of the study concerning a n y single pigment. I n this manner it was possible to assume t h a t a n y changes in rate of oxygen absorption could be attributed to the single varied factor-namely, the concentration of acid present. The apparatus used and method of film application were previously described ( 6 ) .
Preparation of Paint I n all cases 120 ml. of paint, consisting of the single pigment ground in body-& linseed oil, were diluted with a definite quantity of an oleic acid solution of cobalt naphthenate and various amounts of body-& linseed oil and of the particular acid under study in order that the volume resulting totaled 140 ml. Thus the drier concentration as well as concentration of pigment by volume was maintained constant throughout that portion of the study involving one pigment. All starting paints, before dilu1 F o r previous p a p e r s in t h i s series, see references 9, 4,and 6
tion with oil, drier solution, and acid, were pigmented at 3 pounds of pigment per gallon of paint. After films of these paints were prepared in the manner previously described (6), they were placed in the pan of a chainweight balance which was enclosed in an outer case. Changes in the weight of the films were followed a t suitable time intervals. -4Mazda light bulb was burned continuously an average distance of 55 cm. from the film. The average intensity of light striking the painted films was 1,0.5 foot-candles. Temperature was maintained constant at 25 * 0.5" C. Dry oxygen was continually passed into the balance case a t a rate varying between 120 and 150 ml. per minute. In all cases drier solution, oil, acid, and paint samples were mixed approximately 48 hours before each drying study was begun. The titanium dioxide and zinc oxide paints contained 0.0352 gram of cobalt (calculated as metal), and the zinc sulfide samples contained 0.0132 gram of this metal in each 140-ml. sample of paint prepared. These drier concentrations were selected after several preliminary determinations had indicated that the oxygen absorption of such paints could be satisfactorily followed in a 24-hour time interval. I n all cases samples containing concentrations of 5, 10, 15, and 18 ml. of the individual acids were used, while maintaining a total paint volume of 140 ml. These concentrations correspond to volume percentages of 3.57, 7.14, 10.71, and 12.85 per cent, respectively. Since stearic acid is a solid a t ordinary temperatures, some difficulty would have been encountered in incorporating this material uniformly in the paints. Elevation of temperature for this solution process would possibly have resulted in a slight oxidation of the oil present. For this reason, stearic acid was not included. Accordingly, oleic, linoleic, 60 per cent pure linolenic (impurity largely linoleic acid), diheptylacetic (a 16carbon saturated acid), and capric acids were used. Thus, five acids in four different concentrations (five, including the control samples) in samples of each of the three pigmented paints constituted the samples used in this study. The drier (cobalt naphthenate) was added as an oleic acid solution. Since weight changes were taken as indicative of oxygen absorption, it was impossible to use a volatile solvent for the drier. Accordingly, 2 ml. of oleic acid drier solution of the proper concentration were added to all paints. Thus, while the control paints are taken as containing no free acid, they actually contained 2/140 or 1.43 per cent oleic acid by volume. The above mentioned individual acid volume percentages were in excess of this (drier solution) value. New control samples were prepared from the stock supply of the three paints at suitable time intervals. I n this way it was hoped that any changes occurring in the stock paint due to occasional exposure to atmospheric oxygen for short time intervals would be observed. 1239
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
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Observations Shortly after being placed on the balance pan, all paints exhibited the slight b u t definite loss in weight previously reported (6). This was followed b y the “induction periods”, during which no weight changes were evident. After these induction periods of varying length had elapsed, the films increased gradually in weight, the rate attaining a maximum value followed b y a decline. After 24 hours of such exposure the total increase in weight varied from 4 to 6 per cent.
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Curves were prepared in which the per cent gain in weight was plotted against the total drying time in minutes for each of the pigments, using each of the five acids in the concentrations mentioned. Figure 1 gives the curves obtained using oleic acid in the case of the three pigments studied. Lines were drawn tangent to each of the weight-time curves a t their points of inflection (maximum rate gain), and the slope of each was determined graphically. Figure 2 s h o w graphs resulting from plotting percentage acid by volume against the tangent of these angles for each of the five acids with each of the three pigments.
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0. ZNS
PAINT
0
0. ZNS PAINT-OLE\C ACID
V
a n
3 00
I
600
900
CONTROL P 3.57 m A C l D BY VOLUME+-
1200
I
L i a
DRYING TIME: -MINUTES
FIGURE 1. EFFECT OF INCREASE IN OLEICACID CONCENTRATION ON THE RATEOF OXYGENABSORPTION OF PAINTS PIGMENTED WITH TITANIUM DIOXIDE, ZINC SULFIDE, AND ZINC OXIDE
As was expected, the zinc oxide paints containing added acid showed definite bodying; those containing the higher percentages of the straight-chain acids exhibited a stiff consistency. I n the case of the diheptylacetic acid no apparent bodying was observed. Likewise, d a t a obtained from this acid when using zinc oxide paint samples showed practically no changes from those obtained with the control paints. Meanwhile when other acids were used under the same conditions, marked differences in the rate of oxygen absorption were observed.
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FIGURE 2. RELATIONOF SLOPEOF CURVES AT POINT OF MAXIMUMGAIN IN WEIGHTTO CONCENTRATION (BY VOLUME)OF ADDED ACID, FOR THREEPIGMENTS
SEPTEMBER, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
Discussion INDUCTION PERIOD.I n the cases of the nonreactive pigments a n increase in the amount of free acid present tended to shorten the induction period. This statement is general for all acids studied. The zinc oxide (reactive pigment) paints showed the reverse effect--namely, a n increase in acidity resulting in a lengthening of the induction period. This fact was observed for all acids except the diheptylacetic. It was probable t h a t , since these diheptylacetic-zinc oxide paints showed no bodying, the two 7-carbon chains protected the carboxyl group of the acetic acid from reaction with the zinc oxide present. Per cent gain in weight curves for the zinc oxide paint in the several concentrations of diheptylacetic acid show practically no difference, all being virtually superimposed. Figure 2C shows but little change in maximum slope for these paints.
TOTAL DRYING TIME
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ual periods of 4 to 5 minutes and tightly closed after each sample had been withdrawn. Figure 3, A and B, shows data obtained from the aging of the titanium dioxide and zinc oxide stock paints. The zinc sulfide study showed essentially the same type of data. dttention is called to the fact t h a t the slopes of the initial and aged paint curves are essentially the same a t the point of maximum rate gain. I n other words, the rate of gain is equal in the aged samples to t h a t initially obtained. The induction period is shortening in all cases shown. Comparison of curves I C and 3B shows that the zinc oxide causes progressive lengthening of the induction period and loss in slope as the acidity is increased (IC), whereas a shortening in induction period and very little change in slope occurs with air exposure (no drier present) and aging only. From these data we can conclude that exposure to atmospheric oxygen
MINUTES
DRYING
TIME
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O F AGINGPAINTS AFTER S E V E R A L S.4MPLES H A D BEENREMOVED FROhI STOCK SAMPLES FIGURE 3. EFFECT These curves represent data obtained by repeating new control samples on the aged stock paint.
SLOPEAND RATE OF OXYGEKABSORPTION.Generally (Figures 1,A and B , as well as 2, A and B ) ,a n increase in acidity resulted in a n increase in the rate of oxygen absorption in the cases of the nonreactive pigments, whereas the reverse was true when zinc oxide paints were used (Figures 1C and 2C). Krumbhaar (I) showed t h a t lead soaps are crystalline in nature whereas the corresponding zinc soaps are large flocs. Accordingly, i t seems probable t h a t the formation of such voluminous flocs of zinc soaps in the paint would envelope a portion of the cobalt drier material present and result in a partial loss in effectiveness of this catalyst. The data observed in this study are essentially the same as those shown when the amount of drier is reduced in a particular paint ( 5 ) . The fact t h a t the zinc oxide paints containing diheptylacetic acid did not exhibit bodying and did not show this progressive increase or decrease in slope or change in induction period, tends to support the idea t h a t zinc soaps of this acid were not formed. Additional work is planned on this problem. CHANGESIN DRYISG RATEOF COXTROI,PAINTS.Each sample used in this study was prepared by withdrawing definite quantities of the starting paints from one-gallon stock samples. These portions were then treated with drier, acid, and oil in the manner described. During the course of the study it was decided to run duplicate control samples some days after the stock gallon samples had been initially opened. Thus, changes in the rate of oxygen absorption of the resulting “aged” sample paints could be attributed to changes which had taken place in the stock samples caused by exposure to atmospheric oxygen for different time intervals. These gallon samples were open to the atmosphere for individ-
does not materially increase the long-chain free acids present, although a shorter induction period (probably due to a partial removal of natural antioxidants in the oil) does result. S o absolute evidence is a t hand to support this idea, although these data appear t o point definitely to the existence of these conditions. It is also evident from these data that the variations in induction periods in different samples of the same type of oil encountered by different investigators may be due to differences in time of atmospheric exposure of the samples. I n other words, the length of the induction period is an inverse function of the past history of the oil-namely, the longer a sample is exposed to atmosphere (absence of drier), the shorter the induction period.
Aclcnowledgment The paints and linseed oil used in this study were prepared and supplied by the Paint Laboratory of the Research Dirision of Krebs Pigmeiit and Color Corporation. The cobalt naphthenate drier used was supplied by Yuodex Products, Inc. The linolenic and capric acids used in this study were furnished by Armour and Company.
Literature Cited Blom, A. V., and Krumbhaar, W., Paint Varnish Production 19.5fi l-R_3 _ R L_ 19, _.,__ 56 ((1939). Droste, N. H., H.,’barhen-Ztg., Farhen-Ztg., 41, 771 (1936). Hoek, C.P.van, I b i d . , 41, 330 (1936). Nicholson, D.G.,IKD.ENO.CHEM., 31,1300 (1939). Nicholson, D.G.,and Holley. C.E., I h i d . , 30, 114 (1938). Ihid., 30,563 (1938). ~
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