Zinc Soaps in Paints ZINC QLEATES WILLIAM HOWLETT GARDNER
A. E. JACOBSEN
Polytechnic Institute, Brooklyn, N. Y.
Titanium Pigment Corporation, South Amboy, N. J.
HYSICAL tests (6) have established that pronounced structural changes may occur in paint films during exterior exposure. Many of these changes can be ascribed only to chemical reactions between pigment and the vehicle, but none of the products have been definitely identified, even in the case of simple systems. Investigators have assumed (1,4,10) that when basic pigments were present, soaps were formed. No data have been previously obtained, however, to prove that this is true, although evidence of saponification (8) and the effect of the acidity of linseed oils upon paints containing different basic pigments (3) is such as t o suggest that these general assumptions are correct. This paper describes a study of certain zinc oxide paints and zinc oxide-oleic acid mixtures which demonstrated that the pigment can react with the vehicle under certain conditions. The products were isolated and identified in the case of the simpler systems consisting of zinc oxide and oleic acid. Although this study does not give a complete picture of what may take place in paints, it offers new views as to the types of products which may be formed during the aging of coatings containing basic pigments.
physical state. The films were then conditioned for 24 hours a t 75' F. (23.9' C.) and 50 per cent relative humidity just prior to testing. The test consisted of bending the film over mandrels varying in size from */16 to 3/8 inch in diameter in steps of 1/16 inch. The films exDosed for 1 dav could be readilv flexed over a 1/16-inchmandrel without rupture, whereas the films which had been removed after 3-week exposure could not be bent over the largest mandrel (3/8 inch) without breaking. The films exposed for 5 weeks readily broke between the fingers upon the slightest bending. These tests indicated that the films had become hard during this short exposure and that a t least some of the ingredients were transformed in such a way as to modify the internal structure of the film. MICROSCOPICAL STUDIES.Some idea of what may have occurred was obtained by observing slides of various dilute mixtures of zinc oxide and linseed oil with the aid of the microscope. A number of the zinc oxide particles dissolved in the oil and spherulite crystals were formed in their place. A dispersion of Red Seal zinc oxide in a linseed oil of acid number 12 showed spherulite crystal formation after standing for only 2 hours (Figure 1). Raw linseed oil of acid number 4 required a longer period before similar formations appeared. Zinc Oxide-Linseed Oil Paints Crystals of this type were also observed when oleic acid was used in place of linseed oil. The cross-hatched formations Paint films containing zinc oxide as the only pigment rapincreased with the amount of zinc oxide employed. This is idly become hard and brittle upon exterior exposure. This shown in Figure 2 for a zinc oxide-oleic acid mixture which marked change in film structure was observed by one of the exemplifies a graduation in crystal formation commensurate authors in a study of the flexibility of zinc oxide paint films. with gradation in density of the zinc oxide. The spherulite In this study (6) the paints were prepared by dispersing 24 crystals were too small in all instances to Dermit anv measureper cent of zinc oxide by volume in a linseed oil having an ments ofA optical "constants, acid number of 4. The paint but they show distinctive incontained the usual proport e r f e r e n c e p a t t e r n s when tion of a commercial leadviewed through crossed Nicols. manganese-cobalt n a p h t h e These interference figures were nate drier. Films which were Linseed oil paints containing only zinc oxide as the absent from fresh slides con0,003 inch thick were apwhite pigment became hard and brittle in a relataining only zinc oxide and plied t o tinned copper panels tively short time when exposed to the weather. linseed oil, or zinc oxide and by means of a pair of shims This would suggest that chemical reactions might oleic acid. Since only one and a straight edge used as have taken place within the film during exposure. type of pattern was observed, a draw knife, and when dry Microscopical examination of zinc oxide-linseed oil it would indicate that the difthey were exposed on a vermixtures revealed that the pigments used were ferent products had similar tical fence facing south a t definitely reactive, since some of the pigment parcryst alline lattices . Sayville, Long Island. Porticles dissolved in the oil and were replaced by . This microscopical study, tions were cut from the panels spherulite crystals. Similar spherulites were also however, gave no informaformed when zinc oxide was allowed to react with after 1-day, 3-week, and 5tion with respect to the idenweek exposure periods. The oleic acid. A study o f the reaction products of t i f i c a t i o n of t h e v a r i o u s films were loosened from the these two substances showed that two distinct p r o d u c t s . H e n c e , i t was metal by running mercury oleates can be formed-namely, a normal salt when followed by a more detailed around the edges of the stoichiometric quantities were used, and a product investigation of the simple panels and allowing it to of empirical formula Zn,C2,aH,380ts when zinc system zinc oxide-oleic acid, a m a l g a m a t e with the tin. oxide was used in excess. X-ray patterns of these with the hope of isolating at This permitted removal of two products differed appreciably. The hydrated least one of the products which the s l m s without subjecting soap at least is probably a coordinated compound. might be formed in a zinc them to any unusual stress paint. or otherwise affecting their
P
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
October, 1941
IN ZINC OXIDE-LINSEED FIGURE 2. FIGURE 1. CRYSTAL FORMATION OIL MIXTURE
Zinc Oleates The usuztl procedure for preparing metallic soaps such as zinc oleate is by metathesis ( l a ) ,although it has been shown that metal soaps may be prepared by allowing metal oxides to react with fatty acids ( 7 ) . In this study both methods were used. DIRECTMETHOD. Red Seal zinc oxide and oleic acid were mixed in a small, tared, open glass bottle and allowed to stand after agitation until reaction caused the mixture to become semisolid. I n one series of experiments stoichiometric quantities were employed; in another, twice the amount of zinc oxide was used. These preparations were allowed to stand a t room temperature and were weighed frequently to determine any change in weight. At first there was a rapid loss, but a t the end of 72 hours no further significant change occurred. When stoichiometric amounts of the two reactants were used, the loss in weight was equivalent to the amount of water which would be formed in the preparation of a normal, monomeric zinc soap: ZnO
+ 2(CnH33COOH)+Zn(ClrH&0O)e + H20
The loss in weight was approximately half this amount with the preparations containing an excess of zinc oxide (Table I) A microscopical examination of the normal soap in polarized light showed that all of the zinc oxide had reacted but
TABLE I. WATEREVOLVEDIN REACTIONS BETWEEN ZINC OXIDEA N D OLEIC ACID
Red Seal Red Seal Kadox Kadox
Zn0, Grams 0.813 0.813 0.813 0.813
Red Seal Red Seal Kadox Kadox
1.627 1.627 1.627 1,627
Type of ZnO
Theory Based Loss in W t . upon Compn. Oleic Acid, by Reaction of Purified Grams Mixt., Gram Soaps, Gram 0.167 5.654 5.654 0.150 5.654 0.175 0.194 5.654 Average loss 0.172 0,180 5.664 0.078 0.079 5.654 0.070 5.654 0,080 5.654 Average loss 0.077 0.070
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CRYSTAL FORMATION IN ZINC OXIDE-OLEIC ACID MIXTURE
that there was an appreciable amount of unreacted zinc oxide left when an excess of this pigment was used. It was also observed that the products from both preparations consisted mainly of small crystals. These were somewhat acicular in form and of relatively low relief. The smallness of these soap crystals again made it impossible to determine optical properties except refractive index. The products of reaction were dissolved in a large quantity of diethyl ether, and the solutions were centrifuged twice to remove undissolved material. The resulting solutions were then allowed to evaporate spontaneously, whereupon the soaps crystallized. They were purified by recrystallization from this solvent in the usual manner. The normal soap had the following analysis and properties: Analysis calculated for CaeHssOaZn: C, 68.80; H, 10.59; Zn, 10.41. Found: C, 68.83, 68.51, 69.08; H, 10.70, 10.80, 10.24; Zn, 10.40, 10.32, 10.25. Melting point, 80" C . ; refractive index, 1.51.
In contrast, the crystalline product from the reaction mixture containing twice the amount of zinc oxide had the following characteristics: Analysis calculated for C&isa8Os&ly: c, 66.51; H, 10.43; Zn, 11.32. Found: C, 66.54, 66.23, 66.38, 66.91, 66.67; H, 10.54, 9.97, 10.07, 10.70, 10.37; Zn, 11.40, 11.42, 11.70, 11.56, 11.76. Melting point, 83" C.; refractive index, 1.51. DOUBLE-DECOMPOSITION METHOD. The procedure described by Whitmore and Lauro (12) was followed in the preparation of the metallic soaps by metathesis. I n these preparations aqueous solutions of zinc sulfate and of sodium oleate were used. As in the above experiments, stoichiometric quantities were employed for the preparation of both a normal soap and of a simple basic soap. The precipitated soaps were thoroughly washed with water, dried a t room tempcrature, and purified by crystallization from diethyl ether. They had the following properties: Analysis calculated for CssHea04Zn: Zn, 10.41. Found: Zn, 10.40. Melting point, 80" C . ; refractive index, 1.51. Analysis calculated for Cz8sHsssOssZny: C, 66.51 ; H, 10.43; Zn, 11.32. Found: C, 66.87, 66.46; H, 10.50, 10.26; Zn, 11.10, 11.15. Melting point, 83" C . ; refractive index, 1.51.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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When viewed through the microscope, both products appeared to be composed of spherical particles. No distinguishing difference was noted in these products from those obtained by the previous method. X-RAY ANALYSIS. X-ray powder diffraction patterns using Cu-K radiation were obtained for the two types of zinc soaps prepared by both methods. The samples were mounted on copper wire by means of a gum arabic binder. Table I1 summarizes the Debye and Schemer lines observed in the photographs obtained for each of the four different preparations. This table shows the two normal soaps to have similar, if not identical, molecular structures and to differ from the other two products, which were also similar to each other. It would appear, then, that the soaps were truly crystalline. The general weakness of the lines is undoubtedly due to the relatively large organic radical associated with the zinc atoms.
Vol. 33, No. 10
TABLE11. X-RAYSPECTRA OF ZIXC OLEATES Norilia1 SoapDouble-decompn. method
7 -
Direct method d
Intensity”
d
9.39 8.49 6.70 5.40 4.66
v. w.
8.49 6.70 5.35 4.66 4.18 3.76 3.32
4.18
3.81 3.32 a
W.
v. w. v. w. m. m. v. w.
-High
d
Intensity“
d
9.29 8.03 7.36 6.60 4.50 4.20 3.28
v. w. v. w.
9.29
v. w.
7.42 6.60 4.54 4.13
W.
Intensitya W.
v. U’. v. U’.
m. m.
v. U’. W.
Zinc Content Soap--. Double-decompn. method -
Direct method
v. w.
.
U-
v. w. v. w. v. w.
8.18
Intensitya v. w . v. w. v. w. v. w .
W.
w. = weak, v. w. = very weak, in.
=
medium.
firmation of their possible polymeric structure was not obtained. No satisfactory explanation can be offered t o account for the same basic soap being obtained by both methods. Highly Discussion hydrated basic compounds of low molecular weight had been expected. It was most surprising, therefore, to find that The products prepared with zinc in excess of that required this was not the case, a t least when the soaps were prepared for the normal salt were not the simple basic soap expected by precipitation from aqueous solution. There is the posfrom the proportions of reactants used. They contained sibility that recrystallization from diethyl ether may have only an average of 11.4 per cent zinc instead of 17.8 per cent been responsible for the results obtained. This solvent, howas required for the basic compound, Zn(OH)z.Zn(ClsH3302)z. ever, could not have had any appreciable effect upon the The relatively sharp melting points of these soaps would insoaps resulting from the reaction of zinc oxide with oleic acid. dicate that they were not mixtures. Their zinc content The amounts of water evolved in these reactions (Table I) would preclude oxidation having taken place without some were within experimental limits that are required for an andistinct degradation. Any degradation of the oleate radical hydrous normal soap and for a hydrated soap of the above would have affected the ratio of carbon t o hydrogen, which composition. Pink’s failure (9) to obtain an anhydrous norwas the same in these hydrated products as that in the normal mal soap by metathesis might have been due to his use of a oleates. zinc chloride solution. It is well known that anions may have The simplest empirical formula which can repreaeiit the a marked effect upon the degree of hydration in the formation composition of these basic soaps is ZnsC2ssH63uOas.This of a complex salt of this type (11). Further studies of these would suggest that the products were a complex salt of reactions and those of other constituents of zinc paints are the general formula, Z~[Z~(CI~H~~OZ)~]~(H~O)~(OH)Z. If contemplated. written as a coordination compound containing sexacovalent While it may be expected that soaps of the type discussed zinc, such a complex may be represented graphically as in this paper are formed in paint films containing zinc oxide, follows: it is likely that other reactions also take place to produce somewhat different products. Each of these reactions would inHZO 01 01 01 01 01 01 01 01 OH2 fluence the properties of the coating and its behavior on aging.
Aclmowledgmen t
where 01 represents the oleate radical, and where the repeating unit, Zn(Ol)z, has the following structure: CHa(CH*)iCH=CH (CHz)sCH2
I
The authors are indebted to S. S. Cole for the x-ray data reported, and are grateful for the use of the laboratory facilities granted by J. L. Turner, of the Titanium Pigment Corporation. The analyses for carbon and hydrogen were carried out by E. B. Connor in the Micro-Chemical Laboratory, Polytechnic Institute of Brooklyn.
cI
Literature Cited Bunce, E. H., Symposium on Paint and P a i n t Product,s, p . 98, Am. Soo. Testing Materials, 1935. Droste, W. H., “Fachausschuss fur Anstrichtechnik”, Heft 2
(1929).
I
1
H,C(CHz)&H=CH(CHz)1CHq This formula is based upon the suggestions of Pfeiffer ( 8 ) that the two oxygen atoms of a carboxyl can be linked to different metal atoms, and that an ethylene group is capable of coordinating with a metal atoni such as zinc. The above compound mould have a molecular weight of 5196.35, but the soaps decomposed when dissolved in camphor so that con-
Droste, W. H., Verfkroniek, 9, 156 (1936). Hallett, R. L., and Rose, C. H., Symposium on P a i n t and Paint Products, p. 84, Am. SOC.Testing Materials, 1935. Jacobsen, A. E., Oficial Digest Paint & Varnish Production Clubs, 146, 215-28 (1935). Jacobsen, A. E., unpublished invcstigation. Licata, F. J., Drugs, Oils R. Paints, 50, No. 2, 66-8 (1929). Pfeiffer, P., “Organische 3lolekulerbindunge”, Berlin, Verlag von Ferdinand E n k e , 1927. Pink, R. C., J . Chem. Soc., 1939, 619. R y a n , L. R., Symposium on Paint and P a i n t Products, p. 99, Am. Soc. Testing Materials, 1935. Thomas, A. W.,“Colloid Chemistry”, pp. 141-9, Kew York, McGraw-Hill Book Co., 1934. Whitmore, W.F., and Lauro, M., IND. ENG.CHEM.,22, 646-9 (1930).
