Drying of Linseed Oil Paint Coordinated Metal Soaps as Oxygen Absorption and Drying Catalysts’ DOUGLAS G . NICHOLSON2 University of Illinois, Urbana, Ill.
tion studies on these paints nftcr aging for 95 days have shown that paints containing simple cobalt drier materials show increased induction period time intervals and a reduced rate of oxygen absorption, and that paints containing coordinated cobalt drier show practically no change in induction period and very little difference from the initial oxygen absorption rate exhibited by the respective paints. It appears that the observed shortening of induction periods and the production of oxygen absorption curves whose slope is practically independent of the pigment present may be attributed to a reduction of the tendency of the pigment particles to absorb coordinated drier metal atoms. No theory to account for the actiial drying mechanism of paints containing coordinated driers is postulated. Additional work is being done on this problem.
Cobaltous o-phenanthroline oleate has been used as a drying material in linseed oil paints pigmented with (a) zinc oxide, (b)zinc sulfide, and ( c ) anatase and rutile titanium dioxide. In all cases the induction period is materially shortened, as compared with a simple control, when the coordinated drier material is used. This effect is greatest in the case of paints pigmented with titanium dioxide. The slope of the “per cent gain in weight” curves is nearly constant and is independent of the pigment present when the Coordinated drier is used. This is not true when simple drier substances are present. Rutile titanium dioxide shows this shortened indurtion period when manganous, l e d , and cobaltic cobalt o-phenanthroline coordinated atoms are used as drier materials and compared with their respective simple controls. Data obtained on repeating the oxygen absorp-
ENERALLY the mechanics of the drying of a paint or varnish film is considered to involve a series of rather complex oxidation and polymerization reactions. Although several investigations have been carried out (3, 6, 6, 7,9,10, 11) in an attempt to learn more of the nature of these reactions, their actual operation remains somewhat of a mystery. Elm (4) showed changes in peroxide value, acid number, iodine value, etc., which took place as a film of trilinolenic glyceride dried. It is well known that the presence of certain metal soaps tends to accelerate the drying of oils containing unsaturated linkages. Cobalt soaps are generally considered as being surface drying agents, while manganese soaps apparently aid the drying processes beneath the film surface, and lead soaps are thought to facilitate drying throughout the entire film. Very little is known concerning the actual mechanics by which the presence of these soaps increases the rate of oxygen addition a t the double bonds in the oils. Jacohsen and Gardner (8) indicated that zinc soaps formed in zinc-oxidepigmented paint films probably contain coordinated linkages. Gebauer-Fuelnegg and Konopatsch (6) demonetrated that a complex compound must react with an oil in order to function as a drier. Anderson (1, 2) showed that platinous compounds are capable of coordinating with double bonds present in olefinic compounds and of thus forming addition com-
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Previous papers in this series have appeared in INDUSTRIAL AND E N ~ I CHEMISTRY as follows: 80, 114, 663 (1938); 51, 1300 (1939); Sa, 1259 (1940); 88, 1148 (1941). Present address, Chemical Warfare Servioe, U. 8. Army. I
NEERINQ
pounds of definite corgposition. These coordinated compounds are quite active and are capable of adding molecular hydrogen with the formation of a saturated hydrocarbon. Treating certain of these platinum-olefin compounds such as PtCla.C2H4 with other olefins such as styrene often results in the quantitative substitution of one olefin for the other in the complex. It is sometimes assumed that the drier atoms behave as oxygen curriers in paint films. Thus they fluctuate between their lower and higher valence states, acting as oxygen carriers, transporting oxygen to the double bonds in the oils present, and forming the peroxides which characterize the early stages of drying of a paint. This theory appears to be valid as long as one considers metal atoms which exhibit more than one valence. Zinc soaps are known to behave as “weak driers’’ in paint films. Zinc is generally Considered &g exhibiting but one valence. Zinc atoms form coordination compounds as do all other metals which behave as paint driers. The stability of these coordination compounds varies with the individual metal used. I n an effort to verify the idea that the drying mechanism may or may not involve coordination of the drier metal atom at the unsaturated bonds of the oil present in a paint film, it was decided to prepare driers having fully coordinated metal atoms and test their drying ability in paints as compared with other identical paints containing the simple metal compound. Cobalt compounds when fully coordinated and oxidized to the cobaltic state are extremely stable toward reduction.
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Thus by preparing such a cobaltic compound and using this material as a drier in a paint, one would expect practically no catalytic action by this material since the metal atom, being fully coordinated, would lie incapable of coordinating with the unsaturated linkages of the oil, and because coordinated cobaltic cobalt is very stable and resists reduction.
diamine compounds by addition of excess of the coordinating material to the metal oleate were unsuccessful. The ammoniated aqueous cobalt acetate was prepared bv addition of ammonium hydroxide to cobaltous acetate. After filtration this material was treated with a slight excess of potassium oleate. The meciaitate so formed was washed and . . dried. Attempts to dissolve this material in mineral or soybean oils were unsuccessful m the elevated temperature necessary to effect the solu7 I1 tion resulted in some 2 6 W decomposition of the product 5 with evolution of ammonia z gas. za T h e tri-o-phenanthroline u cobaltous acetate was prepared 6’ in aqueous solution by addition of the calculated amount of oH I G H TEMP t o (0 $&OLEATE LOW phenanthroline to an aqueous 0 solution of cobalt acetate. Ad300 600 900 I200 1500 la00 dition of aqueous potassium TIME OF D R Y I N G - M I N U T E S oleate to this coordinated cobalt solution resulted in the FIGURE 1. RATEOF OXYGENABSORPTIONBY formation of a tan-yellow oily PAINTSCONTAININGCOORDINATED COBALT OLEATE IN WHICHTHE DRIERWAS DISSOLVED precipitate. After being washed AT Low AND HIGHTEMPERATURES and dried, this material be-
Preparation of Coordinated Compounds Since oleic acid is rather readily available in a pure form, it was decided to prepare the c o o r di na t e d met a I oleates . Aqueous cobalt acetate was treated with potassium oleate until precipitation was complete. The resulting material was washed and dried in an oven a t 100’ C. I n an attempt to prepare the amine of this material, a sample was dissolved in mineral oil and treated with dry ammonia gas. A sticky insoluble m a t e r i a l was precipitated in the mineral oil. Attempts at preparing the coordinated pyridine or ethylene
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C O O R D I X A T E D AND U N C O O R D I N A T E D COBALT @ ~ E A T L O N RATBO F OXYGJEX A4B&ORPTIONO F W I T H \’ARIOlJS MATERIALS
FILMS PIGAIENrL’F:D
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1942
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DRYINQDISKSCOATED WITH RUTILETITANIUM DIOXIDEPAINT:(left) COBALT OLEATEAS DRIER, FIGURE 3. SANDERSON (right) COBALTOUS 0-PHENANTHROLINE OLEATE AS DRIER came darker in color. It dissolved in soybean oil without decomposition on heating to temperatures as high as 150" C. Analyses for metal content on all of these drying materials were carried out on a macro scale by ignition to the oxide. The corresponding manganese and lead o-phenanthroline oleates were prepared in the same manner as was the above described cobalt compound. The cobaltic o-phenanthroline acetate was prepared by oxidizing the cobaltous material by blowing a gentle stream of air through the complexed cobaltous aqueous solution for 12 hours. A slight excess of potassium oleate was added to this coordinated cobaltic material, The resulting oily precipitate was washed and dried.
titanium oxide, of zinc sulfide, and of zinc oxide ground in alkali-refined linseed oil so that there were 3 pounds of pigment in each gallon of paint. Thirty milliliters of each of these paints were thoroughly mixed with 3 ml. of each drier solution consisting of the drier dissolved in alkali-refined soybean oil. Table I presents a summary of the analytical data concerning the metal content of the driers as well as of the paints used. In order to effect a fairly rapid solution of some of these drier materials in the soybean oil, it was necessary to heat the oil to temperatures approximating 150' C. Some concern was felt that this high temperature might have caused changes in the constitution of the oil or a partial decomposition of the coordinated materials. A test was run in which a more dilute solution of coordinated cobaltous o-phenanthroline oleate was heated to 150' and also in which an equivalent solution was prepared by heating the oil below 45" C. Paints containing these two coordinated driers
Preparation of Paints Since the rate of oxygen absorption by the paint films was followed by a gravimetric method, it was impossible to use a volatile drier solution in this work. The paints consisted of
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TIME OF DRYING -MINUTES AND UNCOMPLEXED COBALT OLEATE ON UNPIGMENTED LINSEED OIL FIGURE 4. EFFECTOF COMPLEXED
Oxygen absorption data are also shown for the unpi mented oil oontaining no added drier as well as for a sample of the oil oontaining a n equivafent amount of the oomplexing agent only.
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Very little difference appears in the rate of oxygen absorption of the unpigmented oil when complex or uncomplexed cobalt oleate of equal Grams Drier metal concentration is used (Figure 4). The slope of the level portion of the curves obtained of Paint o, 00008B8 when using complexed cobalt oleate is practically 0.0001005 o.0000868 independent of the pigment (if any) present. Some fundamental factor which is a charaotero.oooioo5 ~~:~~~~~~~ ~ $ istic ~ of $each ~of the~ pigments ~ ~studied has been 0.0000868 eliminated in the complexing of the drier atoms. Thus by this complesing process it amears that the oxygen absor*ption curves of ail paints trsted are- practically identical with those obtained from the unpigmented oil. A postulation to account for this effect could be attributed to the adsorption characteristics of each individual pigment. It is logical to expect that a metal atom which is surrounded by a large neutral group or groups would have less tendency to be adsorbed by pigment particles than would a similar uncoated metal atom. A previous study in this series (9)showed that an increase in the free fatty acid content in the vehicle of a paint tends to shorten the induction period of paints pigmented with titanium dioxide or with zinc sulfide. The reverse effect was exhibited by paints pigmented with zinc oxide. Since the zinc oside is a reactive pigment, it was assumed that large flocs of zinc soaps, formed in the paint on the addition of the fatty acids, enveloped the drier material producing the observed loss in drying rate. If it is assumed that the acid hydrogen atoms of these acids were adsorbed on the nonreactive pigment surface, it follows that the adsorption of the metal drier atom would be correspondingly reduced. Thus this adsorption postulation will explain facts previously noted as well as those reported here. Another possible explanation for the observations is to assume that the complexed drier materials are more soluble in the drying oils than are the corresponding uncomplexed controls. This idea WRS not supported in that the complexed cobaltous oleate in linseed oil exhibited a slight turbidity on standing. If such turbid paints were thoroughly stirred prior to the running of drying tests, no loss in drying time was observed.
TABLE I. METALCONTENTS OF DRIERS AND PAINTS Per Cent l\letal
Drier Co(o1este)g Co(0A)a (o1eate)z Mn(oleate)g Mn(0A)a (oleate)? Pb(0A)s Pb(oleateh (oleate)? Co(OA)8 (o1eate)s
Theory Actual 0.55 10.2 5.08 5.45 9.48 9.20 5.07 4.48 30.9 15.85 31.1 17.5 4.07 2 . 8 0
Grams of Drier Dis-
5 8 0 0 ~ 7 ~ ; ~ y - ~ p p e a r a n c eof Soybcan Oil
bean Oil 0.6000 1.2320
Soh. Clear Clear, turbid Turbid Turbid
01.294 .7200 1.7104
Clear
0,4682
0.8748
~
~
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;
slightly
showed almost identical oxygen absorption curves. These data appear in graphical form in Figure 1. Thus it was concluded that the heat necessary to dissolve the material did not materially decompose the coordinated metal atoms. The drier solution was added to and thoroughly mixed with the sample of individual paint approximately 24 hours before the drying of the film was studied.
Measurement of Oxygen Absorption by Paint Films The rate of oxygen absorption was noted by means of weight changes which took place in the individual films of paint, spread on at uniform thickness, on a glass plate placed on the pan of a chain-weight balance. The balance case was enclosed in an outer case in such a manner that weighings could be obtained without opening this case. Dry oxygen was admitted into the balance a t a rate approximating 120-150 ml. per minute. Complete details of this method have been published (11). A single glass plate of uniform thickness was used as the base plate for the paint films throughout this study. Temperature was m+intained at 25' * 0.5' C. A 50-watt Xlazda lamp was illuminated a t an average distance of 55 cm. from the paint film at all times. The concentration of the coordinated drier solutions in soybean oil were prepared so that 1 ml. of these solutions contained an amount of metal (based on actual analyses, Table I) equal to that in an equal volume of the soybean oil solution of the same simple metal.
Oxygen Absorption Figures 2, 3, and 4 are graphical records of the oxygen absorption data obtained in using complex and simple cobaltous oleate as the drier material. Rutile and anatase titanium dioxide materials (Figure 2) exhibit the same drying characteristics; the induction period of the simple material approximates 2100 minutes, and that of the paint containing the coordinated drier approximates 60 minutes. The oxygen absorption curves of the coordinated drier paint are very flat (constant slope) and about equal in rate over the initial 5 per cent increase in weight. The difference between the simple and complex drier atoms when pigmenting the paint with zinc sulfide and oxide, respectively, is definitely less than that exhibited by titanium-dioxide-pigmonted films at equal drier concentration. Practical drying tests (Figure 3) show clearly that the oxygen absorption data are definitely related to the actual drying times of the films.
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FIGURE 5. RATE OF OXYGENABSORPTION OF RUTILETITANIUM DIOXIDI: FILMSCONTAINING COMPLEXED A N D UNCOMPLEXED MANGANOUSOLEATh AND LEADOLEATE AS DRYINQ CATALYSTS
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induction periods were due to increased solubility of the drier materials. Figure 5 also shows data obtained when rutile titanium dioxide pigmented films are allowed to dry with complesed and uncomplexed lead oleate as the drying material While the difference between the two drier materials is not so great as in the case of the cobalt or manganese compounds, i t is sig7 nificant to note that the complexed material 6 favors oxygen absorption. I n order to show the loss of drying time (if any) of these paints on can storage, the paints containing complexed and uncomplexed cobaltous drier materials were permitted to age 95 days after the initial drying test. Figure 6 gives data obtained in these tests superimposed on the initial oxygen absorption wa curves (Figure 2). Data obtained in the 300 600 800 1200 I500 1800 2100 2400 2 7 0 0 ) ) 3€ I repetition of the aged zinc oxide and zinc sulfide paints as well as of the anatase titanium dioxide TIME OF DRYING-MINUTES m i n t were similar in nature. Attention is FIGURE6. OXYGEN ABSORPTION DATAOBTAINED IN DRYING COMPLEXED called to the loss in oxygen absorption rate in AND UNCOMPLEXED COBALTOUS OLEATE-RUTILE TITANIUM DIOXIDE PAINTS the aged sample when uncomplexed cobaltous AFTER %-DAY AGING drier 7s used,- whereas very Gttle decrease in rate of oxygen absorption is exhibited by films agcd an equal time period containing complexed drier metal. Effect of Different Driers An aqueous sample of cobaltous o-phenanthroline acetate Since the differences between the induction periods in was oxidized to the cobaltic state by bubbling oxygen through paints containing complexed and uncomplexed cobaltous the solution for 12 hours. Addition of potassium oleate to oleate were greatest for the titanium dioxide paints, tests usthis material resulted in the formation of an oil-like precipitate ing lead or manganese materials as driers were carried out which was different in properties and appearance from the on paints pigmented with this material alone. corresponding cobaltous material. Analyses of this material Figure 5 shows graphical data obtained in drying rutile showed a metal content considerably less than that expected titanium dioxide paint containing manganese compounds &S from the postulated formula of the material. Since coordidriers. The complexed material again shows a much shorter nated cobaltic compounds are known for their stability (reinduction period than does the uncomplexed control. Since sistance to reduction) and the cobalt atoms present were fully the complexed manganese drier solution in soybean oil excoordinated, one would expect this material to have very hibited a tendency to settle, tests were run in which some of little effect as a paint drier. Figure 7 shows the oxygen the clear supernatant solution was tested for drying ability, absorption data obtained when using this material as the drier. Here again one notes the linear nature of the curve and the relatively short induction period.
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Acknowledgment
7
The paints used in this study were prepared in the paint laboratories of K,rebs Pigments Division of E. I. du Pont de Nemours & Company, Inc. The Sanderson drier tests mentioned in this report were run on the Sanderson drier belonging to the United States Regional Soybean Products Laboratory, Urbana, Ill.
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Literature Cited 0
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FIGURE7. OXYGEN ABSORPTIONDATAOBTAINED WITH COBALTIC 0-PHENANTHROLINE AS DRIER IN RUTILE TITANIUM DIOXIDE PAINT
in which the solution was stirred thoroughly before a sample was obtained, and in which the lower turbid settled portion of the solution was used as the drier. Figure 5 indicates that as the amount of manganese material in the drier solution (even if insoluble) increased, the induction period of the resulting paint decreased. These data tend to discredit any possibility that the increased drying rate and shorter
(1) Anderson, J. S.,J . Chem. Soc., 1934,971. (2) Ibid., 1936, 1041. (3) Carrick, L.L.,Mitchell, F. S., and Hemp, N. A., Am. Paint J , 26, No.4,50 (1941). (4) Elm, A. C.,IND.ENG.CHEM., 23,881 (1931). (5) Gardner, W.H., and Waddell, R. B., Ibid., 33,629 (1941). (6) Gehauer-Fuelnegg, E.,and Konopatsoh, Gottfried, Ibid., 23. 163 (1931). (7) Ingle, H., J. SOC.Chem. I d . , 37,319 (1917). (8) Jacobsen, A. E., and Gardner, W. H., IND. ENQ.CHHIM., 33,1254 (1941). (9) Nicholson, D.G., Ibid., 31, 1300 (1939). (10)Ihzd., 32, 1259 (1940). (11) Nicholson, D.G., and Holley, C. E., Ibid., 30, 116 (1938). P B S E ~ N T Eunder D the title ”Complexed Metal Soaps as Oxygen Absorption and Drying Catalysts” before the Division of Paint, Varnish, and Plastics Chemistry at the 103i-d Meeting of the AMSRICAN CHEMICAL SOOIETY, Memphis, Tenn.
