Water Immersion Testing of Metal Protective Paints ROLE OF OSMOSIS I S WATER A BSORP TIOiY AND BLISTERING W. W. KITTELBERGER AND A. C. ELJI The
.\-etL.
Jersey Zinc Company (ofP a . ) , Palmarton, Pa.
-4paint
system consisting of one coat of zinc chromate primer and one coat of low gloss, alkyd hull paint was applied to various substrate materials and immersed for varying periods of time in distilled water and in solutions of sodium chloride and sucrose with osmotic pressures of 6 to 30 atmospheres. The rate and degree of water absorption and blistering were determined by extensive measurements of weight and volume changes, and by visual examination. Regardless of t h e nature of the substrate and immersion bath, water absorption and blistering increased with increasing immersion time but decreased markedly with increasing osmotic pressure of t h e bath. Factual evidence developed here establishes
t h a t water is transferred through the face of the paint film under t h e influence of, and a t a rate determined b?, t h e difference in osmotic pressure of t h e bath solution arid of t h e solution formed within the paint sjstem br the water absorbed. If osmotic pressure of the external solution is high enough, blistering does not take place. The minimum osmotic pressure of the external solution required to preieiit blistering +aries with the nature of the substrate, and is suggested as a basis for measuring the adhesibn letel of the test coating for the substrate material. Some ebidence is pre-ented which indicates t h a t ~ pores hut water absorption does not take place b j w a of b y a process related to actual solution.
0
covering the ,mmc r:iiigc of wmot ic prrwures as the salt solutions in the first test series. This servcd to determirie whether there is any difference in principle in the beh:ivior of paint systems immcrsed in solut imia ivhirh h:ld the same oimot ic prwjures or escaping tendcncic.3 but contained 'different solutes. In thr third wrics the same p:iiiit system \vas applicd to stwl, duralumin, zinc, and p1 panels, and immerscd i n the same sugar ,solutiolis employed i l l the serond serics. This permitted a study of thc iiiAuencc ercrtc.d by the suhstr:ite and hclpcd t o strengtheii the general conccspt.c tlvrived flom t h r resiilts of the first t\vo t w t wries.
S E of the major types of coating failure when painted metal articles are immersed in fresh or sea water is blistcring and peeling. Some progress has been made in overcoming blistering by formulation; but it is doubtful that this prnhlc-m can be solved satisfactorily until more is known :ibout the fundsmental mechanism b y which blisters are formed when paint films are intermittently or continuously immersed in n-atcxr. In the course of work on the development of metal protective pigments and primers, extensive use was, made of water immersion teats, and consideration was given to the variables involved in this type of esposure. Some evidence n-as uncovered n-tiich indicated that the blistering of paint films immersed in fresh water is largely due to osmotic effects, \vliercas blistering in salt water is a combination of osmotic and elcctroendosmotic effects ( 5 ) ., This evidence, however, {vas insufficient to furnish a measure of the relative importance of osmotic and electroendosmotic influences and to formulate a valid theory of blistcr formation. Subsequent experiments have furnished the necessary data, and the results obtained in a study of osmotic influences are described and discussed in this paper. For the purpose of this report.osmosis is defined as the transfer of water through a paint film under thc influence of a solute concentration gradient; electroendosmosis applies to the movement of water through a film under the influence of an electrical potential gradient. This investigation dealt with the behavior of a paint system under purely osmotic influences. Electroendosmotic influences may be avoided by eliminating, as far as possihle, highly corrosive environments or corroding substrates \vhich could set up electrical potential gradients across the paint film and thus cause electroendosmosis. Both of these schemes were employed to make certain that the conclusions dran-n n-ere justified. Three series of tests m r e conductcd. In thc first aeries a paint system was applied to glass panels and immersed in sodium chloride solutions of varying solute content and, hence, of varying osmotic pressures. I n tKe second series the same paint system was applied to glass panels and immersed in sugar solutions
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PANEL TESTS
The paint system used in this !vork consisted of uiie rout of zinc chromate primer (Kavy specific:ition 52-P-18) and one coat of low glow, gr:iy alkyd hull paint (mad< from 52-P-25 base). Although this pnrticulnr paint syFtcm is riot intended for use under w:iter, it n'as chosen because a considerable amount of d a t a is available from previoiis n-ater immersion tests with it, and because it is relatively easy io prepare and handle. The panels were approximately 3.3 X 2.6 inches in size; they varied in thickness depending upon the panel material. The glaas panels were cut from used spectrographic plates and were approximately 0.05 inch thick. After the photographic emulsion had been removed, the plates were cleaned in chromic acid solution, thoroughly rinsed first in t a p and then in distilled water, and finally dried by dipping in alcohol and ether. The metal panels were buffed t o a high degree of polish sevcrul days before being coated and stored in a desiccator over calcium chloride. On the day before the application of the test paint, thcsy were lightly buffed and then thoroughly cleaned in trichloroethylene vapor. 411 of the panels )\ere then n-eighed in both air and distilled water in order to permit their volumes to be calculated. The cdgcs and corners of a11 panels were \vel1 rounded. The test paints were applied by dip coating a t such a rate as t o yield primer and finish conts about 1.25 mils thick each. The
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the density and thermal expansion of the ceresin wax used Osmotic on the panel edges in order t o K a t e r Absorption, 70 Pressure of ,- 48-hr. immersion 192-hr. immersion 840-hr. immersion obtain correct values for paint Sugar S o h . , Atm. Fe A1 Zn Glass Fe A1 Zn Glass Fe A1 Zn Glass coating volumes. The glass 0 11.9 ... 1 1 0 11.2 16.0 l 5 , 3 1 5 . 7 l5,9 20.6 27.2 3 0 5 272 test panels were assumed t o 6 8.4 8.0 7.5 7.7 14.0 12.0 13.1 .... 1 6 . 6 13 7 17 5 14.8 12 5.4 5.3 5.0 5.0 10.0 8 5 8.6 9.2 13.8 11.9 12.7 .., . have the same density as t h a t 18 4 5 3 9 3 9 3 9 7 3 6 3 6 1 6 5 9 9 8 5 8 1 8 9 of a measured blank specimen 4 8 24 4 1 3 3 2 8 3 0 4 9 4 7 7 4 6 4 6 5 6 4 5 4 30 3 0 2 8 2 4 2 7 4 3 3 9 3 7 4 0 5 9 5 0 5 0 5 3 cut from the same lot of photographic plates. Since the thermal expansion of, this type of glass is relatively low, a pubTC K a t e r Absorption after Immersion for -Osmotic ' lished value for this property 4 8 hours 96 hours . I 9 2 hours 384 hours 840 hours Pressure oi 24 hours _____ 8oln.,Atm. NaCl Sugar KaC1 Sugar NaCl Sugar NaCl Sugar YaCl Sugar KaC1 bugar (3) was considered sufficiently 0 7.2 5.9 10.6 9.6 12.9 11.8 16.0 14.0 1 9 . 7 18 4 2 9 . 9 26 3 accurate for this work. PuhG 4.6 4.6 6.3 6.4 8.9 8.5 10.1 10.5 11 3 1 1 . 5 1 2 . 3 11 9 lished values were also accepted 12 3.4 3.5 4.5 4.6 5.9 5.7 7.1 6.9 8.:' 8 . 4 8.2 9 t 3 . 0 2 . 9 3 . 6 3 . 7 4 . 4 4 . 6 5 . 3 5 , 3 3 . 3 6 . 2 6 . 5 7 . 0 for t,he density and thermal ex2l 8 4 2.4 2.5 2.8 3.0 3 2 3.6 3.6 4.1 4.0 4.5 4.7 5 1 2.1 2.1 2.3 2.7 2.6 2.9 2.7 3.6 3.1 3 8 3.3 4.3 30 pansion of the Sichrome wire (2) used in the panel hooks and suspension rig employed in weighing. Immediately after a paint coats n'ere dried in a well ventilated glass-walled cabinet pnnel W:LS removed from the test solution and wipad with a soft maintained at about 32" C. Each test panel was weighed on sponge to remove adherent moisture, it was weighed repeatedly an analytical balance before and after application of each coat. over a measured period of time, usually 6 to 10 minutes. The reThe finished panels were also weighed submerged in the sultant weights were plotted as weight loss-time curves and estraproper salt or sugar solution, t o permit calculation of initial polated to zero time in order to obtain the superficially dry weights specimen volumes. T h e edges of the coated panels iw-e given of t h e specimens. This appenred t o be the most accuratc method additioiial protection by t w a dippings in molten ceresin wax. av+ilable for determining the specimen n-eight which changed A tiny hook of Nichrome wire was partially embedded in the 'ivas rapidly as a result of evaporation of water. Kext the psiicls were edging a t the center of one edge of each panel t o provide a means graded for blistering with the aid of A.S.T.M. Tentative Rli.qterof support for t h e weighing of specimens submerged in liquid. ing Standards ( I ) , but the assigned numerical values were revcarsed The panels were again weighed after the edges had been r a s e d so that 0 stood for no blistering and 10 for verv bad bli;tc,ring. T h e panels vere placed in a desiccatof.in which a relative humidand the small hooks attached. It seemed desirable to have t h e test solutions cover n rauge of ity of 50% was maintained with the aid of uliuric acid (specific osmotic pressures from t h a t of distilled water t o slightly above gravity, 1.327) arid a l l o w d I n come to q u i ibrium a t room temt h i t of sea water. 'Since the osmotic pressure of a 3.57, sodium perature. .Liter a minimum of 10 days thcy were again weighed chloride solution, as calcdated from vapor pressure d a t a ( 4 ) , to determine final dry weiglit. It seemed desirable t o present some of the experimental data on a percentage basis; conseis 27.74 atmospheres, it was decided to carry out t h e immersion tests in solutions of approximately 0, 6, 12, 18, 24, and 30 atquently it Tvas necessary t o determine what portion of tllc total mospheres osmotic pressure. n-eight of the coating xac actually subjected t o the influmce of The experiments were so designed that one test panel would be the test solutions, because some of i t had been protected tiy the utilized for esGh set of measurements. In 'other words, six Tax edgings. Xfter soaking in water for a short time, t h paint specimens were immersed in each test solution to provide for the films could be cleanly removed from t h e actual test areas ivithin necessary measurements after six different periods of immersion. the boundaries of the \\-ax edges. Six of t h e specimen? were treated in this manner and, after beihg conditioned a t 50% Thus the experiments were begun with thirty-six test specimens, relative humidity, Jvere reweighed. This final dry weight subcontained in eighteen wide-mouth pint jars (three sets filled with six different solutions), each of which held two panel specitracted from t h e initial weight of coated and waxed specimens gave the initial weight of coatings on test areas. The area mens. T h e jars an$ their contents were held at constant temperature by immersion t o the lids in a water bath maintained a t of coating actually under exposure on each test panel was de25" C. The test specimens were examined 1, 2, 4, 8! 16, and 35 rived from the dimensions of the areas within the boundaries of days after the start of the immersion. I n order to allow for the waxed edges. Photographs of test panels were used for this accidents, extra specimens were'tested in some cases. purpose, and corrections were made for deviations from life size. During these immersion exposures, the test films absorbed STUDY OF VARIABLES water, increased in volume, blistered, and either gained or lost in dry weight. T h e magnitude of all these changes was determined T h e independent variables studied were nature of substrate, b y measuring weight and volume changes. All weighings were nature of solute, concentration of solute or osmotic pressure of bath, and length of time of immersion in test solution. The incarried out t o the nearest tenth of a milligram and converted to weight in vacuun1. Depending upon the nature of the substrate, vestigation was primarily a determination of t h e influence of the the panels weighed between about 15 and 30 grams. T'olume four independent variables upon the following five dependent variables: increase 'in weight or water absorption of coating, w m determined by weighing the panels in two media of known increase in volume, or swelling, loss of coating substance by but different densities. T h e media selected were air and the particular solution in which the panel was immersed. Since the leaching, density of material responsible for volume increase, and degree of blistering of paint film. T h e d a t a obtained are retemperatures of the solutions and inside the balance case could corded and discussed on the basis of this outline. Typical sets not be controlled within sufficiently narrow limits, the necessary temperature readings were made, ,and weight and volume of experimental results have been chosen t o illustrate the rclationships outlined. corrections calculated. T h e specific gravity of the solution used Table I illustrates the effect of variations in the nature of the as the denser medium in volume determinntions was checked substrate material upon degree and rate of water absorption by from time t o time with the aid of a clean gl t h e organic coating. A comparison of corresponding va1ur:s for weight and volume. Independent measurements were made of TABLE
I.
\vATER .~BSORPTION O F
C0.4TIXG
OX
DIFFERESTsL.BSTK.\TI:Y
FIGURE I -WATER ABSORPTION IN PER CENT VERSUS IMMERSION T I M E T H E NUMBERS ON THE CURVES INDI8ATE THE OSMOTIC PRESSURE OF T H E TEST SOLUTIONS I N PTMOSPHERES
1
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100
200
300
400
500
600
700
800
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I M M E R S I O N TIME-HOURS
various metal arid glass panels reveals that. t h e effect upoii hotti rate and di,grce of water absorption is relatively slight. It mutt be remembered t h a t water absorption i- a property of the coating which i q not wpected to be altered nintcrially by contact nitli the sub..trate. The cffcct of the nature of the substrate upon blistering, however, is of a different order and is discussed in tictail later in this paper. Table I1 compares the percentage x a t c r absorbed by the paint coating, after various immereion periods, from the salt and sugar solutions of corresponding osmotic pressures. The data shoiv t h a t t h e nature of the solutes used does not exert any significant influence and that the n-mot ic pressure of t h e solution is the controlling factor detclrmining both rate and amount of water taken into the film. Therefore, nicmtion of the nature of substrate and solute is eliminated from t h e discussion as far as possible, and observstihns arc interpicted in terms of length of immersion a n d osmotic pressure of the immersion solution. Regardless of the nature of the substrate or the immersion bath, water absorption increased with immersion time. These observations are illustrated by t h e curves in Figure 1. After a n initial high rate of water absorption lasting 100 t o 150 hours water as taken u p at, a steady rate. ..\ttention should be called t o the marked difference in behavior of t h e test film in distilled water and its behavior in all other solutions, including t h e onc having t h e lowest osmotic pressure. ;\lthough film.%immersed in various solutions approached saturation rather quickly, the film immersed in distilled water continued t o absorb water; the correTponding curve givcs no indication t h a t saturation might ever have been reached. When t h e snme d a t a were plotted against osmotic pressures of t h e test solutions, t h e curves of Figure 2 were obtained. Water absorption decreased sharply as osmotic pressure of t h e bath increased without, however, quite reaching zero. It is likely that the paint system used in this investigation would absorb a small quantity of water (about 1%) even from saturated solutions of salt or sugar. A s the typical results plotted in Figures 1 and 2 show, the force responsible for the absorption or imbibition of water by the paint coating is t h e difference in t h e osmotic pressures of the solution:: inside and outside of the film. Hoivever, although the experimental data are presented here in terms of osmotic pressures, t l w rc1:ttive vapor pressures or escaping tendencies of the solutions would have served as xell or better. In the case of n-ater transfer from the external to the internal solution, the osmotic pressure difference or gradient can be inc r e a s d either by increasing t h e internal or decreasing the external osmotic pressure. Therefore, as the water solubility of a paint costing increases (other propertios remaining the same) water should be absorbed in increasing amounts from a particular immerqion solution. 1,owry and Kohman (6) demonstrated that,
for rubber, water absorption increases with increasing content of water soluble materials. The paint coating with which our tests were carried o u t appeared t o behave in exactly t h e same manner as t h e rubber samples studied by Lowry and Kohrnan. Water absorption should cease only when the osmotic pressure gradient between the internal and external solutions becomes zero. In the ease in question, this equilibrium was most quickly reached with the most highly concentrated external solution. K i t h distilled water as the external liquid, hoivever, it was impossible to achieve equilibrium by diluting the internal solution with absorbed water; nevertheless, a . s t a t e of equilibrium could be reached even in this case if the mechaniqal pressure, exerted on the internal solution and resulting from t h e film's resistance t o swelling and deformation, becomes equal t o t h e osmotic pressure. ,This condition is a function of the mechanical and cohesive properties of the particular paint system under, test; although it does not materially alter the water absorption of most common paint systems, i t docs tend t o play a role in blistering as will tie shown later. NATURE O F THE FILM
The data 50 Car presented indicate t h a t osmotic pressure causes n-atcr to cntcr the film; this would require t h a t the film act as a .wmipcrmeable membrane, as a source of soluble material t o form a solution Tvith a substantial osmotic pressure, and as a llolclcr for that ,colulion. Some circumstantial evidence rcgarding hvhnvior of thc film in certain of these respects mcrits discussiori
L
FIGURE 2 -WATER ABSORPTION IN PER CENT VERSUS OSMOTIC PRESSURE OF THE IMMERSION SOLUTION
I
-
T H E NUMBERS ON THE CURVES I W I C A T E IMMERSION TIME IN HOURS
I
6
12
18
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30
OSMOTIC PRESSURE- ATMOSPHERES
Calculations based on weight and volume increases of test films immersed in various salt solutions showed t h a t the material responsible for these increases had an average specific gravity of 0.9966 gram per cc. a t 25" C. .Utliough the individual density values ranged from 0.9600 to 1.0500 grams per cc., they showed no trend which appeared t o he related t o the concentration of sodium chloride in the test solutions. This would seem t o indicate t h a t little or no sodiiini chloride, but only pure water (specific gravity 0.9970 a t 25' C.), x a s absorbed by this coating during immersion in salt solution?. This conclusion has been supportcd by subsequcnt measurements of tlic clcctrical resistance of Fimilar paint systems exposed t o the same condition.-. K h c n thc paint films n'ere dried and reweighed after ininierpion tests had bcen completed, a major difference vas found hetween tliow wtiich 1i:scl 1,ecii immersed in salt solutions nnd those immersed in corrchpondiiig sugar solutions (Table 111); the former lost weight as the result of leaching, whcrcas the latter gained weight. T h e gain in n-eight slionn in the last column is not large, but t h e v:tl~ie rcprosents the difference betneen
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amount of material lost from the film by leaching and that of the material (other than water) absorbed from the sugar solution. I t seems reasonable t o assume t h a t the weight of material lost from the film by leaching was of the order of that lost i n the distilled water; consequently the total weight of the material absorbed from the sugar solution was about l.5yc. There is no doubt that the material responsible for this gain in weight \vas 21icrose.
-1.93 -1.69 -1.27 -1.17 - 1 04 -1.02
0 R 14
18 24
30
-1 -0
+o +o
02 13 40
31
+ O '78 T O 26
I t was observed t h a t a paint system immersed in salt solutions of different osmotic pressures absorbs water but no sodium chloritlc, x-hcreas a system immersed in sugar solutions of thc same o m o t i c pressures absorbs some sugar in addition to m t c r . This phenomenon cannot be explained by t h e assumption that absorption takes place by way .of pores in the-paint system; it is difficult t o visualize pores of such size or nature that the relatively largc, sugar molecules could enter whereas the sodium and chloride ions could not. T h e observations reported arc more easily explained wlien it is assumed that absorption is a process ak n t o solution. Thus inorganic sodium chloride is insoluble whereas organic sucrose is measurably soluble in the organic film binder. Up t o this point the discussion has dealt exclusively with the inhcrent properties of the paint system employed in these tests. T1wi-c ip no obvious reason why water alworption and loss of sub-tancc by leaching shoiilrl he measura!)ly affrctetl by the substrate t o which the paint system happen. t o be attarlicid. The results reported so far. have not changed t!iia pictiiri.. It was pointed out, however, that the nature of tlie . u h t r a t c do& affect one of the pht,nomen:i frequently encountc~rcdiv1ir.n n paintcd ti in water or aqueou- ~olu~ioii~-rianic~ly, t)listering.
[Iriiiotic
Pressure of 13:itch, . i t i i i .
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2-1 hr.
48 h r .
Grades for Rlisteriiig 96 hr. 19%Iir. 384 lir.
840 lir.
Steel Panels
0 Duralumin
ZlliC
N ~ 7.
Tfihlt IV shows the results obtaincd i n one of the experiment? on bliytcring of the paint s 'm immersed in solutions of different osmotic pressures. .Blistering was rated by visual examination antl hy comparison with the tentative A.S.T.lI. blisterinK standard- (with tlie difference t h a t 0 designated no blistering and 10 very bad blistering). The rolatively rough surface of thP low gloss finish coat made it difficult for incipient blistering t o be recognized. The result was that minute surface irregularities were likely t o he grntletl a.5 blisters when actually they were not. Thus blistc~ringrating9 of 0.1 or 0.2 assigned t o some of the specimen. arc. of no practical Aignificance. T a h k I V sliows that the tltygec of bli?tcring suffered by tho t c . d coating on any one of the four diffcrent substrates dccreascxl a, the osmotic pressure of thc bath solution increased. antl increased a. tlie ininicrsion time increased. It also show? thnt the nature of the suhstratc. is re.-ponsible for marked diffrr(mrcs in 3usccptil)ility of coating. t o blistering. The paint film ov ktwl panels did n,)t blister signifirantly in any of the solutions antl c'xhi1)itc.d significant blistering in the distilled water bath only after 16 days of inimersio,n; the same paint system applied t o the other substratc materials, however, blistered q u i t e sevrrc'ly even in the 6- antl 12-atmosphere sotutions. The relative rcsistancc to blistering exhibited by this p i i n t system on four diffcrent substrate surfaces folloivs practical vxperiencc and may be explained on the basis of diffcrrnccs i n ailhesion of the paint coating t o the substrates te5te.d.' I t should therefore be possiblr t o base upon this phenomenon a mcthotl for determining relative levels of adhesion of a given coating t o various substrate materials. The determination of 'relative adhesion levels of different paint systems t o the same substrate mnttrrial, howwer, is more compliratcd. A N A L Y S I S OF BLISTER FORMATION
The forces respbnsible for blistering must be large enough to overcome the film's rcsistance t o swelling and deformation, as well as its adhesion t o the substrate before blistering can result. .In analysiq of these forces is simplified if we assume the condition i n which the first trace of liquid water has penetrated through t hc film t o the substrate. W e aasume further t h a t the paint film acts as a semipermeable membrane and t h a t the estornal liquid is a salt solution. Cnder these initial conditions, if the water in the incipient blister between the paint film and the substrate were pure, the transfer tendency due t o osmotic preisure would be outward from the, incipient blister through the film. It appears necessary, therefore, t o premise the presence of a solute in thc water of the blister derived eit,her from the paint film or from the substrate. It is necessary to premise further that the amount and character of this solute are sufficient to raise the osmotic p r e s u r e of the solution forming t h s blister t o a value greater than t h a t of the external solution. When such a condition exisrs, osmotic pressures tend t o result in the transference of water from the outside of the paint film, through the paint film, and into the blister, and thus cause it to grow; therefor?, t h e maximum osmotic pressure causing growth of the blistrr \vi11 be the amount by which the osmotic pressure of the solution i n tlie blister exceeds t h a t of the external solution. If P B is the osmotic pressure of the solution in the blister, P E the osniotic pressure of the external solution, and P.u the total mechanic:il force rc&ting t)listwing,
Pg - P E Glass
voi. 38,
= P.11,
t,r P E = P B - P.u
thc o3motic pressure of the external solution necessary to just inliilit blistering, may therefore be considered the difference betivbc'n the osmotic pressure Jvithin the blister, which tends to cause grorvth, and the mechanical forces of adhesion and film strength, Ivhirh resist growth. In t h e particular ease described, resista n w t o .s\vcslling or dcformation \vas the samcin all specimcnr he-
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cause the same coating was employed; the opposing forces were. on the one hand, attraction between the paint system and the substrate and, on t h e other hand, difference between the osmotic pressures of the internal solution and t,he external solution (immersion bath). This osmotic pressure gradient was greatest when the barh contained distilled water, and blistering resulted in all cases. It was smallest when the bath had a n osmotic pressure of 30 stmosphercs, and no significant blistering was observed in ,any caw. The osmotic pressure of tlie bath solutions just able to prevent blistering of the paint system used in t h e w tests on the four different substrate materials, therefore, may serve as a relative measure of the attractive forces between the paint system and the substrates employed. (This assumes t h a t t h e substrate does not influence the internal osmotic pressure of the blisten, P B . ) MECHANISM O F PROCESS
Tile preceding analysis of forces involved in blistering assumed that some water had reached the coating-substrate interface; no mention was made, however, of t h e mechanism of this process. T h e observations made in the’eourse of this investigation furnish a n adequate explanation of t h e forces causing transfer of water through t,he face of t h e film, hut fail t o explain why, under certain conditions, water passes entirely through t h e film and btxcomes concentrated in blisters a t the interface. Since a definite force exists, which is represented in niogt cases by a n wcaping tendency gradient drawing water into the coating, a still greater force ivould seem t o be required t o draw it o u t again into blisters at the interface wit+ the substrate, especially since the forces of adhesion must also be overcome in the process. Horvever, it is possible t h a t the blisters do not form exactly a t the interface, but t h a t a thin, perhaps monomolecular layer of the coating remain. attached t o the substrate. I n this case absorbed \vater would not have t o leave the coating t o cause blistering. It is likely t h a t the concentration of dissolved film substance in the absorbed n.ater xould increase &s water m o w s from the facv of
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the film toivard the layer next t o the subdtmte. This concectration gradient should keep the water moving through tlie film t o tlie intvrface, or t o the last layer next t o t h e interface. There it should form blisters as ,soon as tho pressure becomes high enough to overcome the cohesivc strrngth a t the interface between monomolecular layer ant1 film. This assumption, however, makes it mow difficult t o unilcrstand \Thy there are .such marked differences in blistering propertics of a given coating tcm on different substrate materials, unlcw it is a s s u n i d that the character of this monornolccular layer is grr,atly motlificd by the nature of tlic .substratum. ACKNOW’LEDG11EST
This work was conducted at the request of and tinder conrrart with thc Bureau of Ships, S a v y Depart,ment. Tlie assistance of the head and various membcrh of ti;e Rosearcli Section, Bureau of Ships, and permission by the S a v y Department t o publish the results of this investigation, are gratefully acknowlvdged. We also thank W.hI. Peirce and D . 1,. Gamble, as well as other members of the Technical Department of The S e i v Jerst’y Zinc Company, for their constant interest and advice, and C. L. Blose, L. H. Farbcr, and H. E. Green for carrying out most of the experimental work. LITERATURE CITED ( 1 ) A m . Soc. for Testing Slaterial.;, S t a n d a r d s , S u e c . D714-43T, pp. 1559-62 (1944). (2) Handbook of Cheiiiistry and P h y s i c s . p..1207, C l c v e l a n d . Chem.
liuhber Pub. Co., 1942. (3) I f l i d . , p. 1655. (4) I n t e r n a t i o n a l Critical Tahles. Yol. 111, p , % S i (1928). (5) Kittelberger, If-. W., ISD. Exti. CHEM., 34, 943-8 (1942). (6) L o n r y , H . H . , a n d I,ii:lt. i ’ a r n i - h , and I’lustics Chemistry a t the 109th \leetirig of t h e . i \ i E H i C . * s CIIF.\IIC.AL SOCIETY A ,t l n n t l c C i t y , S . J.
ACTION OF ANTIFOULING PAINTS Solubilities of Antifouling Toxics in Sea Water JOHX D. FERRY’ AND GORDOS A. RILEY r o o d s Hole Oceanographic I n s t i t u t i o n , IPbods H o l e , Mass. T h e solubilities i n sea water of iarious copper arid mercur> compounds which have been proposed as antifouling toxics coier a \cry wide range, from l o p 3 to oier lo5micrograms of metal per cc. Cuprous oxide, the toxic which has been nio4t widely used, has a solubility of 5 . & micrograms of copper per CC., high enough to be effectiie but not jo high that its leaching rate from antifouling paints i h too difficult to control.
T h e solubility and rate of solutiori of c u p r o u , ~oxitlr in >(saivater wcre reported in thc first paper of this series (.5). Altliough no other toxic substance has bcen .
