INDUSTRIAL AND ENGINEERING CHEMISTRY
806
Investigation so far points to the conclusion that p, not we,is a universal function of 7 and e. For instance, steam has a critical ratio of -1.3 as contrasted to the average value of 3.7, a difference of 16%. It conforms t o the p-chart with an average deviation of even less t.han 2YO. If we had correlated pro instead of p as a function of T and 8, the result would not have been so good as shown by Table I or the chart. An indirect proof o f the validity of the present modification is the tyork of Xcirton (2 1) on the fugacity chart. He showed that for twenty-four substances, deviating within 4% from the stanclard curvw, with some exceptions the i"ollo\ring relation holds:
f/P = S ( T , 8)
Vol. 38, No. 8
The use of the ideal reduced volume enables one to c o ~ ~ r c l a1lI ti r~. thermodynamic properties in terms of 'p and 0 or ip antl ?r \\.it11 the same success as in terms of IT and 8. It is hoped that, t l l c . present discussion may help in paving the way for more cxteiisivt. and confident use of the law of corresponding states in p r w t ical as well as in theoretical treatment of the thermodynamic, propi+ tim of real gases ACKNOWLEDGMENT
The author is grateful to Jamrs A. Ikattie antl I,zv& for advice and encouragement
IT'aritbii
I\
(101 '11)
Tht~i~efui~c~ t'imrii Kcwton's results we may conclude that t h t . pchart, p = ( ( 7 , e), or the modified law, 'p = F ( T , e), will be app1icd)le t o the tn-enty-four gases studied with about the sanit: degrw of accuracy. The question of constancy of the critical rat,io docls not enter the picture of the fugacity studies. This samc qiicstiori is not involved in the present generalization. Thc. 1)r(wnt inotlification of t,he lair of corresponding states, whic*ilslions that p and not p r c is a function of T and 8, may be consiilcrcd t o serve as a rat,ional basis for the p-rhart and othc,r relatoti correlations. Th?t this is a valid moclification is seeu from the rather wide divergencies of the known values of r0 fi,oni constancy. h number of other investigators used the law essentially in t>heniotlified form without explicitly pointing out that any modification was involved. Probably some of these investigators assumed that the validity of their correlation was limiteri by the lack of constancy of the critical ratio T ~ . With the removal of the constancy of the critical ratio as a necessary condition for the validity of the law of corresponding states, 0111' may feel that the validity of the p-chart and other related ror.rclations is not affected by the variation of critical ratio.
( 1 ) B e a t t i e , J . A , , a n d Bridgeinan, 0. C . , Pioc., A m . A c u d . I ' I * . \ r ~ . , 6 3 , 229 (1928). 1 2 ) B e a t t i e , J. .1.,a n d S t o c k m e y e r , W. H., P h y s . Soc. f < c ' / j l . / ' I O O , I ,\,. Physics, 7, 195 (1940). ( % Brown, G . G., Souders, 1I.,Jr., and S n i i t h , It. I,,, I ~ I I l .' ' r i m C H E Y . , 24, 515 (1932). i - i i ('ope, J. D., Lewis. TV. K . , and Weber, H. C., Iln'd.. 27. $v7 (1931). (5) D o d g e , B. F., Ibid.. 24, 1353 (1332). (6) K e e n a n , J. H., " T h e r m o d y n a m i c s " , p. 360, New York. JOIIII T i l e y 8: Sons, Inc., 1941. ( 7 ) K e y e s , F. G., J . Am. Chenz. Soc., 60,1761 (1935). ( 8 ) Lewis, W. K . , IXD.ESG. CHEN.,28, 257 (1936). (9) Lewis, W.K., a n d L u k e . C. D., I b i d . , 25, 725 (1938); Oil ./. 32, KO.40, 114 ( 1 9 3 4 ) . (IO) .\faron, S. H., a n d T u r n b u l l , D., IKD.l k ~ CHBX., . 33, 40s (1941); 34, 544 ( 1 9 4 2 ) ; J . -4m. C'hem. ,%IC., 64, 44, '210.5 * (1342). (11) S e w t o n , R. H., IND. E ~ GCHEM.. . 27, 302 ( 1 9 3 5 ) . (12) Onnes. H. K . , a n d Keesom, TI-. H., Encyc. del, l l a ~ l i .\\'I--., B a n d V , Teil I. 615 (1911). (13) d u , G. J., thesis. Mass.. Inst. Tech., June, 1937. (143 S u , G . J., a n d C h a n g , C . H., ISD. Esc. C H E h i . , 38, 800 ( I94kij (15) Su,G. J., a n d C h a n g , C. H., J . Am. C'hem. SOC., 68, 1080 ( 1 9 N (16) \Tatson. K . >I., and S m i t h , 11. L., iVatl. Petiolriim .\'cws. 28 S o . 97 (19361, (17) 'Teher, H . C., T h e r n i o d y ~ ~ a n i i cf usr Cheiiiical 1,:iigiIii Yew Yo1,k. .John \Viley & Sons, Ini~.,l9:JY. ~~~~
I
ACTION OF ANTIFOULING PAINTS Maintenance of the Leaching Rate of Antifouling Paints Formulated with Insoluble, Impermeable Matrices JOHN D. FERRY' AND BOSTWICK H. KETCHUR.1 Woods Hole Oceanographic Institution, V'obds Hole, Mass.
A
-A EFFECTIVE: antifouling paint must release toxic contiiiuously over a prolonged period. Selection of a toxic wit11 ii moderately lorn solubility, as described in the first two papers of this series (f, g ) , can facilitate attainment of this result. Hox-
ever, the paint must be formulated t o provide a mechanism for the eventual dissolution of toxic particles which are originally buried deep below the surface. Orie possible mechanism for the prolonged leaching of toxic from the intcrior of a paint is based on the use of a matrix ivhich is sufhciently 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 permeability t o water and ions and are insoluble and inerodible. 1
Present address, University of Wisconsin, Madison, Wis.
The niechnnisni 0 1 pruluiiged 1t:acfiing in such prtirit., \rliic.ll rt'quire a high loading of the toxic pigment, to he effectirc:, is tii+ cussed in the present paper. Still another mechanism, ~hic.11 p('rmits paints to be formulated n-ith considerably lower tusii. i o : i t l b ings, will be described in a later paper of this series (.{). PAIBTS WITH HIGH TOXIC LOADING
The leaching behavior of paints with very high loadiiiga of t o 1 1 1 can be explained on the basis of continuous contact, of toxic p a r ticles throughout the paint structure so that, as soon as one p31ticle is dissolved, another beneath it is exposed to solvent ac.tiorr. These have been colloquially t,ermed "cannon ball" paints, sinc~, the particles are pictured as arranged roughly like tmhefamiliar structure of a pile of cannon balls, with the binder filling the interstices. The high loading of toxic which is required to provide coritinuous contact demands a strong, tough binder to ensure that ttria
808
INDUSTRIAL AND ENGINEERING CHEMISTRY
ceeds; the results iu sca water and in acidified sen witer 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 Figuic 1) was 57 micrograms per sq. cm.
=
i
I
I
1
0
y g . Cu,O
1000
per
cm.'
2000
3000
4000
r e m o v e d by extroction
Figure 3. Changes in Leaching Rates of Heavily Loaded Cuprous Oxide Paints during Extraction i n Citrated Sea Water Figures on curves refer to volume fractions of toxic in paints.
The ratio of mass of toxic initially exposed to initial leaching 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 &'as, 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 neceseary 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 t o solvent action. The following data show how the desired result may be obtained by increasing the toxic loading.
Vol. 38, No. 8
grams per sq. cm. when the volume fraction of toxic is 0.12. Klien the volume fraction is 0.24, the leaching continues until 1000 micrograms per sq. em. have been extracted; when the volume fraction is 0.45, it continues until more than 4000 micrograms have been extracted. The leaching rate falls gradually, nevertheless, throughout the extraction. Dissolution is evidently taking place from deep within the interior of the paint. Electrolytic cuprous oxide, such 35 that used in these paints, has an average particle radius of 1 to 2 p . A single layer of close-packed spheres, 2 p in radius with the density of cuprous oxide, would weigh 1460 micrograms per sq. em.; this figure represents a rough upper limit for the mass of surfaceexposed toxic. A more reasonable figure might be obtained by multiplying by the volume fraction of toxic, which would reduce it t o one half or one third this amount. By the time 4000 micrograms per sq. cm. have been dissolved, therefore, the solvent has penetrated past several layers of particles. The total amount of copper recovered in each set of citrated sea water extracts, calculated as cuprous oxide, was then compa'red with the loss in weight of the panel extracted. Table I s h o w this comparison for four paints of the series of Figure 3, and several other paints of different compositions. The loss of weight of the panel is practically identical in each case with the recovered copper expressed as cuprous oxide. These two quantities are also plotted against each other in Figure 4. I t is evident that only the toxic is removed under these conditions, 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 clearly apparent that only a skeleton of exhausted matrix remained.
TABLEI. EXTRACTIOS O F HEAVILYLOADED CUPROES OXIDE PAIXTS IS CITRATED SEAKATER Vinylite in Matrix,
Cuprous Oxide
70
% by a t .
50
70 75
80 85
90
Volume fraetiona 0.33 0.38 0.45 0.54 0.65
Total Wt. Loss of Paint, rg./sq. om. 3500 4600
5300 6700 7900
Cu Recovered in S o h . as CutO, p g . / s q . cm. 3100 4200 4600 6400
8000
EXTRACTION OF CUPROUS OXIDE FROM HEAVILY LOADED PAINTS
Paints which contain higher loadings of toxic continue t o leach after the surface.layer of toxic has been dissolved. I n order to study the behavior of such paints, a series of mixtures was prepared in which the dry volume fraction of cuprous oxide ranged from 0.24 to 0.45 (60 to 80% by weight). The matrix was composed of 3 parts Vinylite and 1 part rosin. The initial leaching rates of these paints were measured, and they were then extracted in sea water containing O.lyosodium citrate. The citrate forms a highly soluble complex with thc copper, which is therefore not reprecipitated as basic cupric carbonate; dissolution from the paint can proceed until substantial amounts of copper have accumulated in solution. After extraction, the panels Jvere 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 cit'rate solut'ions. 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. Electrolyticaliy prepared cuprous oxide such as that used in these experiments has an average particle radius of the order of 1 micron
1
Calculated assuming the density of the matrix to be 1.2;.
The extraction from the paint interior without loss of matrix, together with the sharp dependence upon the toxic volume fraction 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 so011 as a particle becomes completely dissolved, another is uncovered beneath it. Even though the substance of the matrix be quite impermeable, extraction can proceed and toxic can eventually 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 Tvere previously located. BEHAVIOR OF HEAVILY LOADED PAINTS IN SE-I
Further information concerning the availability of toxic iron1 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, Xvere immersed in the sea for 3 months, and were tempo-
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
August, 1946
TABLE 11. LEACHISGRATES OF C TPROUS OXIDE-VISYLITE PAIlVTS .4FTER IMMERSION I N SEA \yATER FOR ~-.&RIOL-S PERIODS
wt. % CunO
n-+,.5;
Vinylite
Volume ti^^ CuzOa
Leaching Rate, 0
cm./day, after: 2mo. 3mo.
pg./’sq.
1 mo.
Calculated assuming the density of the matrix to be 1.25.
TABLE111.
Ek? 70 70 70 80 80 80 90 90
LOSSES O F TOTAL \ITEIGHT ASD O F CUPROUS OXIDE DI-RING 3 MONTHS IT SEAKATER
Nt. % Vinylite 15
22.5 30 10 15 20
5 10
Wt. % Rosin 15
7.5 0 10 5 0 6 0
Volume Fraction Cu204 0.33 0.33 0.33 0.45 0.45 0.45
d S q . Total wt. CulO
0.65 0.65
Calculated assuming the density of the matrix to be 1.25.
rarily removed a t monthly intervals for leaching rate measurements in the laboratory. The panels were weighed and analyzed. Table I1 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 falls 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 results thus parallel closely the data of Figure 3, obtained for accelerated 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 a t the end of the experiment. The loss of cuprous oxide, derived in this way, was then compared with the total loss of weight. Table I11 shows this comparison for three paints of the series of Table I1 and for several other paints of different compositions. The values are closely similar in nearly every case, an indication that ,in the sea, just as in citrated sea water in the laboratory (Table I), only the cuprous oxide disaolves while the matrix remains intact. The data are plotted in Figure 5. These paints, originally bright red, develop a greenish color in the sea. It is believed to be due to the partial reprecipitation there is n-astagc at first, tiecaiisc the initial leaching rate is many times greater than the minimum critical value. Howevcr, the deposition of cupric- s:ilts 011 the paint, surface depresses the leaching rate and tliiis :irtr to wrne extcnt n.: a n automatic riuh i o thi- twistagc. IJTEHATURE CITED (1) l~’e11’5’.J . D . .
:tiid
( : : w i t t . L). f1.. ISD. I~:No. CHLV.. 38, 612
(1946).
( 2 ) F e r r y , J. D., and Riley. G . d.,Ihid., 38,699 (1946). (3) K e t c h u r n , B. H.. Ferry, J. D., a n d B u r n s , 8 . E., J r . . !/id..to be niil-rli.ihed. -~-~~ ~(4) Ketchurn, B. H., F e r r y , J. D., Redfield, A. C., a n d B u r n s . 4 , E.,
Jr.. I b i d . , 37, 456 (1945). ( 5 ) Yoiing. G . H., and Schneider, I\-.Ti.. Itiid., 35, 4%; ( l $ W i i .
CONTRIBUTION No. 346 of the Woods Hole OceanograpIiic Iri>titution. This work was done under a contract between the Institution and the Bureau of Ships, Navy Department, which has approvcd its publication. The interest of Captain H. A. Ingram. U.S.N., in initiatiug the study is gratefully B C knowledged. The opinions presented here are those of the authorn and do not necessarily reflect the official opinion of the Navy Department nr the naval service a t large.
