Durability of Soap-Treated Zinc Oxide Paints DOUGLAS G . NICHOLSON AND T. W. MASTIN
A durability study of zinc oxide paints
University of Illinois, Urbana, Ill.
containing various amounts of added zinc soaps applied to glass panels has shown: Small additions of zinc soap improve the durability of such films; larger additions of these soaps decrease the durability of similar films; acicular films are definitely more durable than either French or American process zinc oxide films; and distensibility data obtained from the films shortly after exposure show no relation to the ultimate durability of such films. Short-chain zinc salts (formate or propionate) do not behave similarly to the longer chain soaps when added to zinc oxide paints. Zinc oxide treated with carbon dioxide gas, prior to the preparation of paint, has produced a film which has indicated promise of greater durability than the untreated control after 13-month exposure at 45" south. Electron microscope photographs of the pigments used in this study indicate that increased durability is associated with larger particle size.
as typical of differences between all grades of these types of pigment. Merzbacher (11) reported that the drying of linseed oil films produces short-chain fatty acids (formic, acetic, propionic, etc.). Hydrolysis of the glycerides present in the drying oil would result in the formation of long-chain members of the series. Since zinc oxide, present in these films, would react with any acidic material present or generated in the drier films, definite amounts of several zinc soaps were added t o samples of the paint prior to its grinding. Table I below lists the composition of one set of paints. Since the type of zinc oxide was also considered as being a variable, it must be remembered that the total number of paints studied was three times that indicated in Table I. TABLE I. COMPOSITION OF PAINTS Soap Added t o Control None Zn stearate Zn oleate Zn oleate Zn oleate Zn linoleate Zn formate Zn formate Zn propionate Zn propionate
Amount Based on Pigment. %
...
0.1 0.5 1.0
2.0 5.0
0.25
i.0 0.33 1.00
The zinc formate and propionate were added to the pigment and mixed prior to grinding while the longer carbon chain soaps were added to the vehicle before the paints were ground. Application of Films Since the failure of zinc-oxide-pigmented films takes place by cracking and flaking, it was thought that 'soap formation could possibly account for the formation of a more brittle film which was incapable of expanding and contracting with the material on which the film was applied. In order to test this theory, it was necessary t o apply the films on surfaces from which sections of the paint could be removed for distensibility tests. The characteristic failure of zinc oxide films has been thought to be due to diffusion of moisture through the wood panels; a hydrostatic head of considerable magnitude is thus created which literally blows the film from the coated surface. Glass panels served to meet the requirement of a surface from which sections of the films could be removed and also to discount any possibility of failure due to moisture penetration. Individual differences, resulting from nonuniformities in wood panels, were eliminated by the use of glass panels. Double-strength window glass, 10 X 12 inches in area, was used for this exposure work. All glass was tested for uniformity in thickness prior to film application, The films were applied by placing the exposure glass on a sheet of plate glass and drawing a milled iron bar over the plate. The iron bar was machined so that it permitted O.dO7 inch clear-
Z
INC oxide has long been considered an important con-
stituent in exterior paint formulation. Exponents of the incorporation of this material in paint claim that it tends to produce a harder film and a t the same time acts to prevent mildew growth. The relative merits of any single pigment as compared with those of any other single pigment have been topics of several investigations (1-4, %io, 14-18). Zinc oxide paint film failure is characterized as the checking-cracking-flaking type. Since this material is considered basic in nature and thus capable of reacting with acidic materials present in the vehicle, one would expect the resulting zinc soaps to exhibit some effect on the durability of the resulting films. It is generally considered that blending of zinc oxide with other pigment materials tends to improve the durability characteristics of this material (6, 7 , 18, I S ) . In an effort t o learn the effect of soap formation on the durability of zinc oxide paint films, a rather detailed study was undertaken. I n this~woskone sample of each of the three types-French process, American process, and acicular zinc oxide materials-were ground in alkali-refined linseed oil a t 28 per cent pigment volume. Lead-manganese drier and mineral spirits thinner were added in definite quantities to all paints. The sample of American process zinc oxide used in this study is generally considered by pigment authorities as being more or less of the French variety. Thus differences noted between the French and American process materials used in this investigation should not be interpreted 996
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
August, 1942
ance from the surface of the glass panels. By spreading a film of the test paint on each panel, it was thus possible to obtain a uniform film approximating 0.004 inch in thickness. All films were aged in a constant-temperature humidity room for 7 days prior to being placed out of doors a t 45' south exposure. At the time of exposure, a6 well as a t periods of two weeks following exposure, sections of each film were stripped from the panels and tested for distensibility.
ZINC
STEARATE
ADDED
Results of Durability Tests Figure 1 shows the results of experiments carried out on Gardner-Parkes distensibility tester. From these data the following facts are noted: (a) In general, an increase in added zinc soap content produces a film having lower distensibility. (b) Acicular zinc oxide has the poorest distensibility. (c) All films decrease in distensibility on exposure and become
SOAP
/
AMERICAN
991
ZINC
OLEATE
ADDED
SOAP
PROCESS
:I\
"\
1
FRENCH
PROCESS
ACICULAR ACICULAR
.__.____..__ W
IMMED. 14 DAYS 2 8 DAYS
W
0..
%, P
I t
*lo
DISTENSIBILITY
ZINC
16
AT
20
24
BREAKING
LINOLEATE
ADDED
28
POINT
SOAP
ZINC
FORMATE
ZINC
PROPIONATE
I-
Y
ACICULAR
W
O/o
DISTENSIBILITY
AT
BREAKING
POINT
E
O/o
DISTENSIBILITY
AT
BREAKING
POINT
FIGTJREI 1. GRAPHICAL RECORD OF DISTENSIBILITY DATAON FILMSCONTAINING ZINCSOAPS
Films containing zinc stearate, zinc oleate, zinc linoleate, and respective controls were exposed a t 45" south on glass in February and March, 1939; zinc formate and propionate films were exposed on glass in late April, 1939. The entire series was applied by brush (three coats) to western red cedar panels and exposed on a south vertical fence in October, 1939.
very brittle after 4 weeks. (d) Zinc propionate and formate additions to zinc oxide paints show an inconsistent, indefinite distensibility record. Figure 2 gives typical fingerprint photographs of several of the test films on glass panels after 13-month exposure. The French process zinc oxide films showed the poorest
INDUSTRIAL AND ENGINEERING CHEMISTRY
998
Vol. 34, No. 8
French process control
French process 0.1 per cent zinc stearate
French process 0.5 pes cent zinc, stearate
American proceas rontrol
French piocess 1.0 per cent zinc stearate
French process 2.0 per cent zinc stearate
FIXGERPRINT PHOTEST FILMSO S GLASSP.ASELS AFTER ~ ~ - ; ~ I o N T H EXPOSURE FIUUHE2.
'I'OGRLPHS
French process 0.5 per cent zinc linoleate
French process 2.0 per cent zinc linol-ate
OF
August, 1942
INDUSTRIAL A
French process 0.1 per cent zinc oleate
French prooess 2.0 per cent zinc oleate
~ EDN G I N E E R I N G C H E M I S T R Y
American process 0.1 per cent zinc oleate
French proccss 1.0 per cent zinc oleate
French process 5.0 per cent zinc oleate
999
.
American process 5.0 per cent sin0 oleate
FIGURE 2 (Continued) general durability, while the American process films were slightly better. All films containing both of these types of pigment had completely failed after 18-month exposure. In general, the addition of a small amount (0.1-2.0 per cent) of soap to either type of zinc oxide resulted in a slight increase in durability when compared with the respective controls. Addition of larger amounts of soap caused increased and more rapid failure. No outstanding differences were noted in the durability effects of the various soaps on a single pigment. All acicular films applied .on glass were intact at.the time the other types of zinc oxide had failed completely. They remained intact over a total exposure period of 28 months. After 36-month exposure .the controls and films containing the lower concentrations of added soaps were still intact while those samples containing the higher concentrations of soaps were exhibiting severe failure. Figure 3 includes fingerprint photographs of some of these films after 36-month exposure. Films exposed south vertical on western red cedar panels have shown the same order of failure as the films exposed on glass. At 29-month exposure the majority of the French
process films had failed seriously, while the American process films were exhibiting slight to considerable checking and all acicular films were intact. Zinc oxide manufacturers have maintained for some time that the acicular variety of this material exhibited superior durability characteristics. This study has definitely supported their claim. Since the essential difference between these materials is particle size and shape, it follows that the cause of their differences in failure must be associated with some fundamental physical or chemical process encountered in exposure of their films. Figure 3 supports the claim of certain producers of zinc oxide that acicular films tend to crack in lines which parallel the stroke used in their application. Zinc oxide has a specific gravity of 5.42-5.78 while that of zinc carbonate is 4.4S4.45. Depew (6) indicated conclusively that zinc oxide is capable of reacting with moisture and atmospheric carbon dioxide when stored in a warehouse to form a basic carbonate. If such a reaction takes place in the atmosphere, it follows that similar reactions accompanied by the associated increase in volume should not be impossible in the semisolid gel structure of a paint film. The French
1000
INDUSTRIAL AND ENGINEERING CHEMISTRY
Control
1.0 per oent zinc oleate
0.1 per cent zinc stearate
2.0 per cent zinc oleate
Vol. 34, No. 8
2.0 per cent zinc stearate
5.0 per cent zinc oleate
FIGURE 3.
FINGERPRINT PHOTOGRAPHS OF ACICULARFILMS ON GLASS PANELS AFTER 36-
MOKTHEXPOSURE
2.0 per cent zinc linoleate
5.0 per cent zinc linoleate
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1942
Carbonated zinc oxide
Zinc oxide control FIGURE
PHOTOGRAPHS 4. FINGERPRINT
OF
FILMSON
WESTERN
process material, exhibiting maximum surface, should be most reactive and the acicular, exhibiting least surface, should be least reactive toward this change. Figure 4 shows fingerprint photographs of two films painted on western red cedar, exposed at 45" south, containing zinc oxide paint ground in alkali-refined linseed oil pigmented a t 28 per cent pigment volume. The upper picture shows the condition of a film prepared from untreated control zinc oxide after 13-month exposure; the lower presents the condition a t 13 months of a similar film prepared from the aame pigment after carbonation by exposure to carbon dioxide
French process zinc oxide
1001
RED CEDAR
EXPOSED AT 45'
SOUTH FOR
13 MONTHS
gas for approximately 200 hours. This carbonation was carried out by bubbling a slow stream of carbon dioxide gas through water and then passing this moist gas into a roundbottom flask containing the dry pigment. A stirring mechanism making about 50 revolutions per minute kept the dry zinc oxide in motion. The analysis of this carbonated material indicated the presence of approximately 40 per cent zinc carbonate. I n order to show the relation of particle size to durability in the pigments included in this study, electron microscope photographs of samples of the three types of zinc oxide were
American prgcess zinc oxide
Uncarbonated zinc oxide (used a8 control in carbonated zino oxide portion)
PANELS
Acicular zinc oxide
Carbonated zino oxide prepared for this study
MICROSCOPE PHOTOGRAPHS OF VARIOUS ZINC OXIDES FIGURE 5. ELECTRON
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
1002
obtained. In addition the control as well as carbonated zinc oxide used was photographed by the electron microscope. These photographs appear in Figure 5. From this information it is readily seen that the Carbonated material as well as the acicular crystals of zinc ovide have much less surface than the uncarbonated control or the zinc oxide materials of smaller particle size.
Aclcnowledgment The paints used in this study were prepared in the Allied Research Laboratory of The Sherwin-Williams Company. The Graflex fingerprint camera and the constant temperature-humidity room, used to age the panels prior to exterior exposure, are the property of the United States Regional Soybean Laboratory. The electron microscope photographs included in this paper were obtained with the assistance of Martha Barnes, of the University of Illinois.
Literature Cited (1) Anderson, A. W., P a i n t , Oil Chem. Rev., 97, No. 8 , 42-3 (1935). (2) Anderson, G., Paint Varnish Prodiiclion Mgr., 15,2 2 - 4 (1936).
Vol. 34, No. 8
(3) Brandt, R.W., Metal Cleanzng Finishing, 3,499 (1931). (4) Bunce, E. H., Am. Paint V a r n i s h Mfrs.Assoc. Circ. 319, 541 (1927). (5) Depew, H. A., P a i n t , Oil Chem. Rev., 102,No. 17, 9 (1940). (6) Eide, A. C., and Depew. H . A., Am. P a n t J.,20, 7 (1936). (7) Foulon, A., Allgem. Oel- u . Fett-Ztd., 34, 131 (1937). (8) Hoek, C. P. van, Farben.-Ztg., 33, 2789 (1928). (9) Kamp, Paul, Farbe u. Lack. 1930.291. (10) Kekwick, L. O., and Pass, A, Paint Varnish Production Mgr., 13, 296 (1938). (11) Merabacher, S., Chem. Umschau Fetts, Oele, Wachse Harze, 35, 17.1. flR38) (12) Nelson, H. A,, Am. Paint J . , 15, No. 28, 20 (1931). (13) Newman. R. M., Paint, Oil Chem. Reo., 97,No. 21, 56 (1935). (14) Pupil, F., Recherches inventions, 17, 226 (1936). (15) Rigolot, H., Bull. soc. encour. i n d . n d . , 9, 990 (1907). (16) Rosser, E. O., Kunststoffe, 16, 170 (1926). (17) Tauber, Ernst, Farben-Ztg., 17, 1888 (1912). (18) Trott, L. H., New Jersey Zinc Co., Research Bull., 1928. \ _ _ _ _
PRBSENTED before the Division of Paint, Varnish, and Plastics Chemistry a t CHEMICAL SOCIETY, Memphis, Tenn. the 103rd Meeting of the AMERICAX A portion of this work constituted part of the thesis submitted by T. W. Mastin for the degree of master of science in chemistry in the Graduate School of the University of Illinois.
The System Nitric Acid-Sulfuric Acid-Water J
ENTHALPY-TEMPERATURE NOMOGRAPH J. LLOYD MCCURDY AND CLYDE MCKINLEY
I
N THE preparation of mixed acids for nitration processes, as well as in the nitration reactions themselves, the temperature and, as a result, the heat balance of the system is of prime importance. Recently a correlation of all available heat content, specific heat, and heat of mixing data for the ternary system nitric acid-water-sulfuric acid was presented as a summarized enthalpy and specific heat plot, with examples for its use in heat balance calculations1. Owing to the importance of such reactions it has seemed advisable to present this data in a nomograph t o facilitate its use. A solution is presented in Figure 1 to calculate the change in enthalpy of mixed-acid ternary mixtures with increase or decrease in temperature. Figure 2 shows the relative enthalpy of the ternary system referred to each pure component at 32" F. That is, each component in the pure state has zero enthalpy a t 32" F. By the use of Figure 2 in combination with Figure 1 it is possible t o determine the enthalpy of any acid mixture a t any temperature relative to the above base and the consequent change in enthalpy with change in temperature. With these charts it is relatively simple to make heat balance calculations for any system containing these components. The nomograph (Figure 1) solves graphically the relation:
H where H C,
At = cfkqge in 1
=
CpAt
= change in enthalpy of 1 lb. of mixture = s ecific heat of 1 lb. of mixture
temperature of 1 lb. of mixture
McKinley, C., and Brown, G. G., Chem. &. M e t . Eng. 49, 142 (1942)
(1)
University of iMichigan, Ann Arbor, Mich. Since the specific heat is a function of acid composition,
C, does not appear on the nomograph; instead, acid compositions are plotted in such a way that Equation 1 is solved, To find the relative enthalpy of one pound of mixed acid containing 60 per cent sulfuric acid, 20 per cent nitric acid, and 20 per cent water a t 100" F., for example, it is necessary to read the relative enthalpy of this mixture a t 32" F. from Figure 2 and then add the change in enthalpy for the temperature change, 32' to 100" F., as given by Figure 1:
% "Os
(anhydrous or water-free basis)
yototal acid
=
20
+ 60 = 80%
=
20/80 X 100 = 25%
From Figure 2 for 25 per cent nitric acid (anhydrous basis) and 80 per cent total acid, the relative enthalpy a t 32" F. is read as -108 B. t. u. per pound of solution. On Figure 1 the acid composition is located a t 80 per cent total acid and 25 per cent nitric acid in the anhydrous acid is extended horizontally t o reference line 0, and thus locates a point representative of the specificheat of the mixture. Extending a line through this point and a point on temperature scale N corresponding to 68" F. (100" - 32"), we read the change in enthalpy on scale M as 32 B. t. u. per pound of solution. The relative enthalpy of the mixture, therefore, a t 100" F. is -108 +32 B. t. u. or -76 B. t. u. per pound of solution. The smallest scale possible should be used in reading the temperature difference since this leads to the greatest accuracy in the erithalpy readings. Following is an example of the use of the charts in the calculation of heat balances commonly encountered in the preparation of mixed acids for nitration: A mixed acid containing
