JOHN D . F E R R Y 1 AND B O S T W IC K H . K E T C H U M W ood s H o le O ce a n o g r a p h ic I n s t it u t i o n , W ood s H ole, M ass.
A
N EFFECTIVE antifouling paint must release toxic continuously over a prolonged period. Selection of a toxic with a moderately low solubility, as described in the first two papers of this series (1, 2), can facilitate attainment of this result. How
ever, the paint must be formulated to provide a mechanism for the eventual dissolution of toxic particles which are originally buried deep below the surface.
One possible mechanism for the prolonged leaching of toxic from the interior of a paint is based on the use of a matrix which is sufficiently permeable so that water can enter the paint film and dissolve the toxic, and the toxic ions can subsequently diffuse to the surface and be released there. Experiments with certain matrices which possess substantial permeabilities to water vapor have been described by Young and Schneider (5). Successful paints can also be made with binders which have very low per
meability to water and ions and are insoluble and inerodible.
1 Present address, University of Wisconsin, Madison, Wis.
The mechanism of prolonged leaching in such paints, which re
quire a high loading of the toxic pigment to be effective, is dis
cussed in the present paper. Still another mechanism, which per
mits paints to be formulated with considerably lower toxic load
ings, will be described in a later paper of this series (S).
PAINTS W ITH H IG H T O X IC LOADING
The leaching behavior of paints with very high loadings of toxic can be explained on the basis of continuous contact of toxic par
ticles throughout the paint structure so that, as soon as one par
ticle is dissolved, another beneath it is exposed to solvent action.
These have been colloquially termed “ cannon ball” paints, since the particles are pictured as arranged roughly like the familiar structure of a pile of cannon balls, with the binder filling the in
terstices.
The high loading of toxic which is required to provide continu
ous contact demands a strong, tough binder to ensure that the
August, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 807 working under contract with the Office of Scientific Re
search and Development, and has made possible the prepa
ration of paints with very high loadings and excellent mechanical properties.
The properties of a seriesof
antifouiing paints with high ... ...
loadings of cuprous oxide are described hero. Evidence is
presented that such paints maintain their toxic leaching rates over long periods of time by virtue of continuous contact of the toxic particles.
M A TE R IA L S AND M ETH ODS
The paints used in these experiments were compounded with materials kindly supplied by the Bakelite Corporation— namely, cuprous oxide (electrolytic), polyvinyl chloride-acetate (Vinyl
ite VYH H), and rosin, the resins being dissolved in methyl ethyl ketone. In a series of nineteen different paints the proportion of Vinylite in the matrix varied from 25 to 100%, and the propor
tion of cuprous oxide varied from 60 to 90% of the total solids.
Panels of ground glass, 3 X 4 inches, were weighed, coated on both sides, allowed to dry, and reweighed. Their initial leaching rates in sea water were determined by the standardized tech
niques described in a previous paper (4). Later they were sub
mitted to various treatments, to be described, and their leaching rates were again measured. In certain cases an analysis of the toxic project from the solidified matrix, as shown schematically in Figure 1.
Maintenance o f the leaching rate by an antifouiing paint which contains a high loading o f toxic in an insoluble, impermeable matrix depends upon continuous contact of toxic particles throughout the paint film. W hen paints formulated with cuprous oxide and Vinylite are extracted either in citrated sea water in the laboratory or in the open sea, the toxic dissolves and leaves the matrix intact. The rate at which the leaching rate falls during extraction, as well as the total am ount o f toxic cxtractablc, is sensitive to the proportion o f toxic in the paint. This proportion m ust exceed 30% by volume to m ake particles below the surface accessible to solvent action. Antifouiing coatings of this type have the advantage o f being effective when ap
plied as a thin film, with good resistance to erosion.
Figure 1. Schem atic Cross Section o f the Surface o f a Fresh Coat o f Antifouiing Paint
Since paint vehicles wet the pigments suspended in them, it might be expected that each projecting toxic particle would be covered by a very thin layer of matrix. Actually most antifoui
ing paints containing cuprous oxide begin to release copper im
mediately (or within a few minutes) after immersion in sea water.
Therefore, any such matrix skin must ordinarily bo too thin to interfere with dissolution of toxic and is, presumably, quickly re
moved after immersion. (In certain coatings applied as hot melts, there is some evidence of an initial surface coating which interferes with leaching and persists several days after immersion in sea water.) panel area) at the time the measurement is made. The mass of surface-exposed toxic may also be determined by complete ex
traction of the surface under certain conditions.
Ex t r a c t i o n. When n o mechanism is afforded to make the toxic particles beneath the matrix surface (open circles of Figure 1) accessible to solvent action, the total amount of extractable toxic is represented by that in the surface layer. Experiments were performed to show how the leaching rate changes when such a surface layer is gradually depleted by extraction in different ways.
Cuz 0 Extracted, g g ycm ?
Figure 2. Extraction o f Surface-Exposed Toxic from an Antifouiing Pain I Containing
40% Cuprous Oxide in a Hard Matrix
Panels were coated with a paint whose dry composition was 4 0 % cuprous oxide, 30% Vinylite, and 30% rosin. This mixture of resins is insoluble in sea water, and the volume fraction of toxic is so low (0.12) that isolation of the surface-exposed particles from one another and from underlying particles is assured. The leaching rates were measured, and the surface-exposed toxic was exhausted in two ways: (a) by leaching in six successive changes of sea water for 4-hour periods, the average leaching rate being calculated for each period; and (6) by extraction in sea water (containing 0.06% citric acid, adjusted with sodium hydroxide to pH 3.9) for various periods, followed always by a leaching rate measurement in plain sea water. The amount of cuprous oxide removed from the surface was calculated from integration of the leaching rate-time curve or from analysis of the citrated solutions.
The leaching rate is plotted against the total amount of cuprous oxide removed in Figure 2. It falls steadily as extraction pro
ceeds; the results in sea water and in acidified sea water are in reasonably good agreement.' By extrapolation, the leaching rate approaches zero when 57 micrograms of cuprous oxide per sq.
cm. of surface have been extracted. It may be concluded that, for this paint, the total mass of cuprous oxide particles originally exposed (black circles in Figure 1) was 57 micrograms per sq. cm.
fig. Cui 0 per cm.2 removed by extraction rate gives a rough estimate of the leaching life of a single layer of cuprous oxide particles2. In this case it was 57/22 or 2.6 days.
The surface was, in fact, virtually exhausted of toxic after 24 hours in sea water. It is clear that, if a paint is compounded of toxic particles of this type, many layers of particles will be neces
sary for the maintenance of leaching over a long period of time, and that dissolution of toxic must eventually take place from deep within the interior of the paint. The problem is, then, to make the particles buried deep in the paint accessible to solvent action. The following data show how the desired result may be obtained by increasing the toxic loading.
E X T R A C T IO N O F C U P R O U S O X I D E F R O M H E A V IL Y L O A D E D P A IN T S
Paints which contain higher loadings of toxic continue to leach after the surface layer of toxic has been dissolved. In order to carbonate; dissolution from the paint can proceed until substan
tial amounts of copper have accumulated in solution. After ex
traction, the panels were washed and their leaching rates were redetermined in ordinary sea water. Extractions in citrated sea water and leaching rate measurements in ordinary sea water were then repeated, alternately, several times.
The leaching rates are plotted in Figure 3 against the total amount of cuprous oxide removed as determined by analysis of the citrate solutions. This figure contrasts sharply with Figure 2, which showed that leaching ceases after extraction of 57
micro-* The mass of surface-exposed toxic and, therefore, the life of a single layer of particles might be expected to increase somewhat with particle diameter. Electrolytically prepared cuprous oxide such as that used in these experiments has an average particle radius of the order of 1 micron.
grams per sq. cm. when the volume fraction of toxic is 0.12.
When the volume fraction is 0.24, the leaching continues until 1000 micrograms per sq. cm. have been extracted; when the volume fraction is 0.45, it continues until more than 4000 micro
grams have been extracted. The leaching rate falls gradually, nevertheless, throughout the extraction.
Dissolution is evidently taking place lrom deep within the interior of the paint. Electrolytic cuprous oxide, such as that used in these paints, has an average particle radius of 1 to 2p.
A single layer of close-packed spheres, 2p in radius with the den
sity of cuprous oxide, would weigh 1460 micrograms per sq. cm.;
this figure represents a rough upper limit for the mass of surface- exposed toxic. A more reasonable figure might be obtained by multiplying by the volume fraction of toxic, which would reduce it to one half or one third this amount. By the time 4000 micro
grams per sq. cm. have been dissolved, therefore, the solvent has penetrated past several layers of particles. recovered copper expressed as cuprous oxide. These two quanti
ties are also plotted against each other in Figure 4.
It is evident that only the toxic is removed under these condi
tions, and that the matrix remains intact on the panel. As a matter of fact, after extraction the paints were visibly bleached, the original bright red having paled to pink or gray; it was
Cuprous Oxide Total Wt. Cu Recovered
in Soln. as
The extraction from the paint interior without loss of matrix, together with the sharp dependence upon the toxic volume frac
tion of the ability to release toxic beyond the surface layer, lead to the conclusion that, in the heavily loaded paints, the particles are in continuous contact. According to this interpretation, as soon as a particle becomes completely dissolved, another is un
covered beneath it. Even though the substance of the matrix be quite impermeable, extraction can proceed and toxic can even
tually be removed from deep in the interior of the paint. As the toxic is dissolved, a tenuous skeleton of the matrix remains on the surface, with voids where the toxic particles were previously lo
cated.
B E H A V IO R O F H E A V I L Y L O A D E D P A L N T S IN S E A Further information concerning the availability of toxic from the interior of heavily loaded paint films was obtained by studying their leaching in the sea. Panels coated with various paints, in which the proportion of cuprous oxide varied from 60 to 90% by weight, were immersed in the sea for 3 months, and were
tempo-August, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 809'
Fraction Leaching Rate, ns./sq. cm ./day, after;
CuiO“ 0 1 mo. 2 mo. 3 mo.
rarily removed at monthly intervals for leaching rate measure
ments in the laboratory. The panels were weighed and analyzed.
Table II gives the leaching rate measurements for five paints in which the matrix was pure Vinylite. The leaching rate falls off as soaking proceeds, and the smaller the volume fraction, the sharper this decrease. When the volume fraction is less than 0.3, the leaching rate tails to a negligible value within the first month of immersion, an indication that only the toxic particles originally exposed on the surface are available for dissolution. These re
sults thus parallel closely the data of Figure 3, obtained for ac
celerated extraction of paints in the laboratory.
The toxic dissolved could not be recovered for analysis in this experiment, but the total amount lost may be calculated from analysis of each panel at the end of the experiment. The loss of cuprous oxide, derived in this way, was then compared with the total loss of weight. Table III shows this comparison for three paints of the series of Table II and for several other paints of dif
ferent compositions. The values are closely similar in nearly in the sea. It is believed to be due to the partial reprecipitation of cupric salts on the paint surface. Basic cupric carbonate, formed from the oxidation of cuprous oxide in sea water, has a much lower solubility than the latter; cupric oxychloride, which is also formed, has a negligible solubility (2). The presence of deposits of these salts would therefore be expected to depress the leaching rate. It may also explain the tendency for the weight loss to be slightly less than the cuprous oxide loss (Table III).
To assess the effect of such deposits, leaching in the sea may be compared with leaching in citrated sea water in the laboratory, where the precipitation of cupric salts is prevented by the forma
tion of the highly soluble cupric citrate complex. This compari
son was made for a paint consisting of 90% cuprous oxide and 10% Vinylite. The leaching rates are plotted in Figure 6 against the total amounts of cuprous oxide removed. The latter quan
tities, for citrated sea water extraction, were obtained from analyses of the citrate solutions as before; for sea extraction, they were obtained by numerical integration of curves of leaching rate plotted against time, after temperature corrections (1) were made to convert the leaching rates as measured in the laboratory, to the prevailing temperatures of the sea. The validity of the
temperature corrections is indicated by the fact that the two curves for experiments in summer (average sea temperature over a 15-day period 23.5° C.) and winter (about 0 ° C. over a 90-day period) lie fairly close together.
Tfie curves for sea extraction coincide with that for citrate ex
traction up to removal of about 1000 micrograms per sq. cm. of cuprous oxide, but thereafter the leaching rate in the sea falls off more rapidly. After removal of 1500 micrograms cuprous oxide per sq. cm., thé sea-soaked panels developed a greenish color, whereas the citrate-extracted panels retained their red color;
even though the latter eventually became paler, they showed no evidence of deposit formation. It may be. concluded that the fall of leaching rate in the sea is due not only to the imperfections in continuous contact of particles, which are thought to be re
sponsible for the changes, during citrate extraction, but also partly to the formation of deposits which impede leaching.
T H E O R Y O F CONTINUOUS CONTACT
In the case of simple extraction (as in citrated sea water), with
out the formation of deposits on the surface of the paint film, a rough theoretical description may be made of the process of leaching by continuous contact of toxic particles. In a close- packed arrangement of spheres, each particle touches several others, and there is a complete chain of contacts extending through the structure. In hexagonal close packing, the volume fraction occupied is 0.74. For a cuprous oxide-Vinylite system, a volume fraction of 0.74 corresponds to 93% of cuprous oxide by weight, and this should be the maximum amount which could be compounded if the particles were spherical. In practice the volume fraction of cuprous oxide in the systems studied is always less than 0.74. The particles are undoubtedly not in perfect continuous contact; on the average, the probability that a given particle touches at least one other is less than unity.
C o p p e r R e c o v e re d in S o lu t io n E x p r e s s e d o s jug. C u g 0 per c m ?
Figure 4. Loss o f W eigh t during Ex
traction in Citrated Sea W ater vs.
Copper Recovered in Solution
To examine further the concept of contact probability, the toxic particles in a paint film may be thought of as arbitrarily classified into “ layers” parallel to the surface, the thickness of a layer being equal to the average particle diameter. A layer contains, on the average, N particles per sq. cm. The probability that a particle in one layer touches (i.e., approaches within some small critical distance) at least one particle in another layer is denoted by p.
Then the number of particles in the first subsurface layer which make contact with the surface layer is pN , and the number of particles in the nth layer which are connected by chains of con
tacts with the surface is N pn.
When extraction has proceeded to the nth layer, only those par
ticles which were originally in contact chains leading to the
sur-Loss of C u ^ O, by Anal ys i s , fjgVcm?
Figure 5. Loss o f W eight during Im m ersion in the Sea vs. Loss o f Cuprous Oxide as Determined by
Analysis
face will be exposed to solvent action; the others at that level will bo isolated by being enveloped in matrix. Hence the ex
posed particles at this point number N pn. The leaching rate is proportional to the total area of exposed particles and, hence, on the average to their number:
Ln/U = p n where La = initial leaching rate
L , " leaching rate after extraction has reached nth layer The smaller p is, the more rapidly the leaching rate will fall els
extraction proceeds deeper into the paint. Since p should de
crease with decreasing volume fraction of toxic, the results of Figure 3 are qualitatively explained by this theory, although the data do not warrant a quantitative calculation.
The weight of extractable toxic in n subsurface "layers” of paint, plus the surface layer, is roughly given by:
dPv i(l + p + p2 + • • • + p") where d — average particle diameter
p “ density of toxic
Ci ■= volume fraction of toxic in paint
The series converges for infinite n, to give dpvi /( l — p). It follows that only a limited amount of toxic can be dissolved from a paint with the postulated structure, no matter how long the extraction is continued. The data of Figure 3 suggest that this is indeed the case. The maximum extractable toxic may bo esti
mated by extrapolating these data to zero leaching rate; it ranges from 1500 micrograms per sq. cm. at Vi — 0.24 to 5000 at »i — 0.45. The corresponding values of p, the contact prob
ability, can be calculated from the preceding equation, taking d «*
2 microns and p => 6 grams per co. The results are p = 0.81 at a volume fraction of 0.24 and p =» 0.89 at a volume fraction of 0.45. This indicates how contact probabilities of only a little less than unity may account for limited toxic extractability and hence limited paint life, and how very small differences in this probability may result in large differences in paint behavior.
An estimate may also be made of the depth to which extraction proceeds before the leaching rate falls below the critical anti- fouling value of 10 micrograms per sq. cm. per day. In Figure 3 this point is reached for a toxic volume fraction of 0.45 after 4700
An estimate may also be made of the depth to which extraction proceeds before the leaching rate falls below the critical anti- fouling value of 10 micrograms per sq. cm. per day. In Figure 3 this point is reached for a toxic volume fraction of 0.45 after 4700