ACTION OF ANTIFOULING PAINTS Maintenance of Leaching Rate of Antifouling Paints Formulated with Soluble Matrices' BOSTWICB H. KETCHURI
i
h
JOHA ~ D. FERRl-2
Roods Hole Oceanogrnphir I n s t i t u t i o n . TToods Hole. I l o s *
-IRTHLR E. BIRXS, J H . Wiirr,
/\lorid \ - a i d Shipvtirrl. IIrirc 1,lirnrl. Calif
i n dntifouling paint containing rosin or other acidic resins in the matrix loses both toxic and niatria simultaneou-l? when immersed in the sea. The loss of matrix is 3how 11 to be the result of the dissolution of acidic resin in the slightly alkaline sea water. A paint of this sort maintains a uniform and adequate leaching rate because the dissol\ing matrix gradually exposes stores of toxic which were originally deep within the paint filni. The substitution of neutral resins which are insoluble in sea water. for rosin or other resins which are soliible, may decrease
or destroj the ability of the paint to maintain adequate leaching rates. .\ theoretical description of the simultaneous dissolution of toxic and matrix is presented; the effects of surface residues of insoluble matrix components on the dissolution of the paint arediscussed. The toxicit) of a paint which operates by this mechanism depends on the loading of the toxic ingredient and thesolution rate of the matrix. The opportunity of making independent adjustment of these two variables permits great flexihilit! in designing a paint with the desired leaching rate.
W E V I O C R paper described the mechanism which permits the continued solution of toxic from paints containing toxic particles in continuous contact ( 2 ) . I n order t o operate effectively through this mechanism, a paint must contain more than 30% cuprous oxide by volume. \Tally antifouling paints give satisfactory leaching rates fo; long periods with much smaller toxic loadings, and their operation must depend upon some other mechanism. Several possiblc hypot hews have been suggested to explain the adequate leaching rates of these paints. Permeability of the paint, film, erosion or exfoliation of the matrix, or dissolution of the matrix or one of its ingrcdicnt~might conceivably afford a mechanism for continu~ w releade b of toxic. With the exception of permeability, these invnl1.e the simultaneous loss of matrix and toxic from the paint tilni. Thiq papc'r n-ili describe the results obtained with various paints which lose rnatrix and toxic simultaneously when soaked in the +a, and prr5ent evidence that their maintenance of adequate ~t:acliing ratc.8 is associated 17-ith the rli=solution of thc matrix in
Kith that of the continuous contact paints described in the earlier paper ( 2 ) in which none of the insoluble matrix was lost during sea immersion; the weight losses from such paints were equal t,o the losses of cuprous oxide. That the loss of matrix from paints of the type referred to in Table I represents actual dissolution is suggested by the fact that inaiiy antifouling paints contain acidic resins, notably rosin. Such resins are soluble in mildly alkaline solutions, with formation of the corresponding salts, although they are insoluble in water or in acid solutions. I n sea water, a t p H 8.1, abietic acid and \VW rosin have heen found t,o have substantial solubilities of the order of several hundred micrograms per cc. (6). These solubilities have heen found to be proportional to the hydroxyl ion concentratioiis, as would be expected if a soluble salt is formed. Studies of neutral resins including ester gum and other essentially neutral roiin esters, copper resinate, cuniar resins, phenolic resins, and alkyd resins shon that the,v compoiinds have negligible solubilities at the pH of sea water. Studies of the rate of solution of matrix ingredients in sea Rater and in huffer solutions have afforded the folloiving additional inforiiintioii on matrix dissolution ( f i ) :
A
-tantially more weight than can be accounted for by the loss i f cuproii- oxide. The additional decrease in weight reflects , 1 1 1 , loss of the paint matrix. This behavior may be contrasted
It \vas alpo found that. the matrices of certain antifoujing paints npplird a.; clear films dissolve in sea water a t measurable rates. For example, the matrix of one paint of the cold plastic type dissolved in running sea water a t a rate of about. 100 micrograms per sq. cin. per day. Thus the hypothesis that the matrix, as well as the toxic. of an antifouling paint can undergo slow solution is supported by wliihility data.
' Previous papers i n this series appeared in June (page 612), .July (pagt6'39). and huguet (page 806). 7 P v w n t address, T'niversity of Wscnnsin. hladison, Wis.
93 1
INDUSTRIAL AND ENGINEERING CHEMISTRY.
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Vol. 38, No. 9
in Figure 1 against the amount of toxic removed.
The leaching
ASD I,os5 rs WEIGHT ASD CCPROWS rate became zero after the dissolution of 240 micrograms of cuTABLEI. COPPERCOZTENT OXIDE O F COLD P L A S T I C P A I N T 5 4FTER S04KIK.G I N THE 8E.4 FOR prous oxide per sq. cm. This is the right. order of magnitude for 2 lIOUrH
3 -1
5 i 8 ‘3 4
h
Leaching Rate, @g./sq. cm./day, after Soaking in Sea Water for: ., 1 hr.a 1 hr.6 24 11r. 1; 14 1 2.2 4.7 14.5 4.9 8.9 2 , $5 8.1 12.5 4.g 5.6 12.7 I,, 3.4 ii:5 O.,o 2.0 1., 2.2 10.8 ?2.0 0.0 -e> . 7
CU20 Removed, ‘sq. cm. 41 64 74 83 116 153 172 172
wg.
Calculated on che basis of 1-hour leaohiup. Calculated on the hasis of 4-hour leaching. Running sea water; calculated on the basis ui &-hourleaching.
1)EF’ENDESCE OF AVAILABILII’Y O F TOXI(: O Y MAI’RIX
DISSOLLTION
E:vidrbiicc that the availability of toxic 111 paiiitr iv1iic.h lose btJth toxic anti matrix upon sea immersion is sssociar trcl \Tit11 iiiatrix dissolution is given by the following exptrimcnta. Panels coated with a cold plastic paiiit were‘ trcatcd l v i t l i stili. water acidificd with hydrochloric acid to pH -1. I‘nder these milditions the surface-exposed cuprous oxide is rapidly dissolvrd, whereas the solubility of the acidic matrix componc:rits is negligible. Accordingly, surface exhaustion of the toxic takes place, just as in the insoluble Vinylite paints described previously ( 2 ) . Leaching rates, measured by the standard procctlurc (3) in ordinary spa water after varying periods of acid extraction, itre plotted
the quantity of surface-exposed toxic on a freshly painted surface i2). Clearly, only the surface toxic has been dissolved, and the mechanism n-hich normally operates in the sea to maintain leaching from the interior of tmhepaint film has not functioned in the acidified solution. However, after the surface toxic has been exhausted, the ability o f this paint to leach a t an adequate rate can be rapidly restored by soaking in sea water. Panels which had been leached in the acid sea water (pH 1) were rinsed and returned to normal sea miter, and the leaching rates measured after various times. The results of this experiment are given in Table 11. The leaching rates of all of the panels increased during the first 4 hours of soaking in sea n.atrr, and all had reached values greater than 10 micrograms pear Z q . cm. per day by the ei:d of 24 hours in sea ivater.
I t is apparerit that the exhausted surface hat; been regenerated by sea water soaking, a new layer of cuprous oxide particles tias presumably been uncovered by removing the layer of matrix which remained after the acid treatment. The following experiments on regeneration in alkaline solution indicate that the dissolution of the matrix is responsible for the recovery of the leaching rat?. Panrls coated n.ith a cold plastic antifouling paint Lvere first extracted in sea water acidified with hydrochloric acid to pH 4 and then stirred a t 100 revolutions per minute in tap Rater adjusted to p H ’ l l . 5 with sodium hydroxide ( I ) . The acid treatment was carried out for 3 days in t,hrce successive changes of solvent. The leaching rates of the paint were measured after the acid extraction and after various times in the alkaline solution. The results (Table 111) show that the leaching rate increases rapidly after treatment with the strongly alkaline tap water. Comparison with the results in Table 11, which were obtained on the same formulation, shows that the action of the solution at pH 11.5 is iiiuch morc rapid than that of sea water a t pH 8.1. Panels coated with a number of effective antifouling painta ~ v e i ’ ecxstracted, as described, in sea water acidified t o p H 4 and tor 3 hours in tap water at pH 11.5. The results show that the kachiiig rate increases t o almost its initial value after 3 hours in the alkaline solution (Table IV). Thc increase in the leaching rate of an acid-exhausted paint resulting from treatment with alkali is attributed to the dissolution of N surface layer of matrix, which uncovers a new layer of cuprous oxide particles. The rapid regeneration of the leaching rat its steady-state leaching rate because of the soliibility of the matrix is given in Figure 3 to aid in visualizing the meailing of tlie areas a, and a,. If the assumption is made that toxic particales are disposed randomly, as Figure 3 shoii-s, ant1 the matrix surface is a plane intersecting them, then a,ld (neglecting exposed matrix in the pits) is calculated to be equal to o, the volrimo fraction of matrixa. Comhining with Equations 1 and 3,
L1 = w ~ R ? u ~ / u ~
Vol. 38, No. 9
The intrinsic rate of solution of the toxic does not enter into this expression. Qualitatively this resul t would be expected when the matrix dissolves much more slowly than the toxic, as i t usually does in practice, and t,herefore solely determines the over-all wlution rate. I n the hypothetical case of a n insoluble pigment, the scheme on which Equation 4 is derived provides for the undermining of the pigment particles, which then drop off as the matrix recedes. I n this case Equation 4 xould still be fulfilled, hut the pigment would not pass into solution and L, woultl I)(> :t measure of the rate of loss of the insoluble pigment. By nieans of Equation 4 the intrinsic rate of solution of the matrix of an antifouling paint may be calculated from the toxic Icacliirig rate data. This has been done for the cold plastic paints containing various amounts of cuprous oxide (as in Figure 2) usiiig the average of the copper leaching rates measured between tlie hccond a.nd fifth riiontlis of exposure. The rcsiilts of these i.;zIrulatjor~.: a w g i w n i i i Tahlc YITT,
'TIABLE
\-III.
STE.ADY-ST.&TE LEA'CHING PL.ASTIC P.4INTS
Average Leaching Rate (2-5 M a ) , rrg./sq. cm./das 2.2 4.3 6.9 9.2 11.3 12.7
P.11 ti t
.
s0 4
0.04 0.08
6 7 B B
0.14 0.20 0 24 0.30
______
r
~~~
IC,
0.74
0.72 0.70 0.68 0 66 0 64
~~______
~
The matrix area ie, therefore, az
-
A
-
4
o,A
rraNA = VIA
-
WA
______
The calculated values for the intrinsic leaching rate of the matrix are about 40 micrograms per sq. cm. per day, which is certainly the right order of magnitude, a value of about 100 having heen obtained for the rate of dissolution of the pure matrix in running sea water. However, the fact t,hat R, is not constant for the entire series indicates that the simplified t,heory presented liere is not strictly applicahle. Equation 4 gives an indication of the matrix dissolution rate required t o maintain an adequate toxic leaching rate in a paint of given composition. For example, the intrinsic matrix solution rates required to maintain a copper leaching rate of 10 micrograms per sq. cm. per day are given in Table IX for various loadings of cuprous oxide in a matrix whose density is 1.25. The required value decreases rapidly with increase in loading and beromps very small as the proport,ion of cuprous oxide approaches that required for continuous contact ( 2 ) . Table IX also showe that the w i g h t of paint per unit area and the thickness required for a life of one year decrease rapidly lvith increasing loading wit,h cuprous oxide. SURFACE RESIDUES OF INSOLUBLE MATRIX COMPONENTS
When some satisfact,ory cuprous oxide paints are immersed in the sea, the soluble materials may be dissolved from the surface to leavi, a rcidual skeleton of insoluble ingredients. Insolu-
Q -
*
3
pg./sq.
cm./dar 46 44 39 36 39 31
0
(4)
T h e exposed matrix area is taken as t h e area, A , of t h e plane surface in Figure 3, minus t h e circles c u t out by intersections with toxic partirles. T h e aum of t h e latter areas, assuming random distribution of particles ( S per unit volume), is:
--
COLD
Rt (Calcd.)
~________
which is L: theoretical expression for the sttwly-state leacliirig rate controlled en:irely by matrix ciiswlutioii.
~ N A ~ ~ ~- (r *2) d zi r
RATESOF
01 = Sum
of
0 2 = Exposed
Exposed
Toxic Areas
Matrix Area
-
0
Figure 3. Schematic Diagram of Paint Which Operates through Mechanism of Solution nf Matrix
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
September, 1946
c; Figure 4.
935
D
( : r o w Sections of Cold Plastic Antifouling Paint Films Applied to Steel PanelThe layers are identified as: (a) embedding material, (b) antifouling paint. (c-) anticorrosive pairit. (d) strel. A, B, C, D aredesrribed in the text.
Sle salt3 oi copper tmxme iui~orpvratedin this layer. After the developnient of this iurf:rce d i p s i t , the paint hits a light grcen or gray color which can he scraped off to expost. tlic original red color of the paint beneath. Recent experiments hy Saroyan at the 3Iare Iiland S a v a l Shipyard iiidicatc that, on hot and cold plastic paints which contain rosin :inti cuprous oxide, the green surface layer is largely coniposed of copper resinate. The character of the surface deposita has been studied by microscopic examination of wction$ prcpared hy Lucas of the Bell Telephone Lahoratories ( 5 ) . Photographs of sections of paint films, including the steel plate, anticorrosive paint, and antifouling paint, are reproduced in Figure 4. Photograph A shows a n untreated, cold pIa3tic type paint. The antifouling paint is densely and evenly pigmented. B and C she\>- sections of different preparations of the same paint after 1 and 6 months, respectively, of soaking in the i w . The toxic pigment has been
cxtractcd froill tile surI:iw layer W I I I ~ I I , OII v i s u d cwmiiiiation nppwrs green. &4fter6-rnontli smking tlicz 1,x;tr;irtion of cuprous uxitie has ertc,nth:tl nt):iri>-thwugli thc entire depth of the, film of antifouling paitit. D $ h o w tlic appearance of a fouling paint n-Iiirh ii;i(I heen ao:tkeil for 3 days in a solution contairiing 0.025 .I! > o d i x i i glycinate and 0.48 111 sodium chloride IpI1 10.5). =\i tle*erihcil t w l i r r in t h e article, this solution disr ; o l ~ e sboth c~uprou-:osiilc. and soluhle matrix about, a hundred times as f a h t iis they arc' dissolved in sea water. The cuprous oxide has been extracted from tlic surface of the paint by this solution and leave? a lighter colored layer which is pale yellow rather than the green observed on paints exposed in the sea. The surface layer of the glycine-extracted paints is apparently composed of the insoluble portions of the matrix. I n the sea the interstices of this structure become filled with precipitated deposits of ineoluhli. copper salts.
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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Vol. 38, No. 9
In every case the value of AZ'/wl is greater than that of A M
TABLF,IX. INTRINSIC MATRIXDISSOLUTION RATESREQUIRED wz,which shows that toxic has been lost in excess of its proporTU MAINTAINA COPPERLEACHINQ RATE OF 10 MICROGWS tion in the paint,. In fact, in several cases there has actually PER SQ.CM. PER DAY, CALCULATED FROM EQUATION 4 Weight F/o of Cuprous Oxide 10 20 30 40 50 60 70
Matrix S o h . Rate Required pg./sq. cm./ day
Weight per Unit Area Required for Life of 1 Year, r g . / s q . cm.
Thickness Required for Life of 1 Year, milm 10 5 3 2 i I 1
TABLEX. ACCUMULATION OF SURFACE RESIDUESB Y PAINTS OF I~EUTRAL RESINS CONTAINING VARIOUSAMOUNTS
-
0.36; vn = 0.64; 28-day period between fourth and eighth week of immersion) Surface Acruniu% Neutral *.M lation, Total Resin Neutral Wt. Lost, CB./ Resin' WI .wt Bq. cm. Added p d s q . cm. b t e r gum 25 1030 1250 910 220 670 -109b 500 30 170 1970 1000 620 Copper resinate 14 1350 3190 1670 970 18 2220 Hercolyn 13 616 1140 320 525 670 470 18 935 1410 -175b 715 24 228 945 435 -49b 310 29 126 335 820 840 36 516 6 Including neutral material present in the roain. b Apparent gain in matrix.
lwi
-
The presence of surface residues and deposits give rise to t.hanges in the paint which are not adequately described by the simplified theory presented in the preceding section. The magiiitude of the deviations from the simplified theory can be determined by total analysis of the paint film after various times of soaking. The total loss of weight and of cuprous oxide from the paint can be measured; the loss of matrix is taken as the difference between these two values. If Equation 1 is fulfilled a t all times, even though the leaching rates of both toxic and matrix may change with time, over any given time interval the following relation should hold:
ATlwl
E
M/WZ
(5)
where A T and AM are the quantities of toxic and matrix dissolved, respectively. If, however, a residue of insoluble matrix components or a deposit derived from the toxic is left on the paint surface as the soluble components are dissolved, AT/wl will always be greater than AM/@,and their difference is a measure of the magnitude of the deposit and/or residue. The total amount of accumulated material is given by
been an apparent gain in weight of the matrix over the interval specified. Table X also gives the total weight of material, S , which accumulated on the surface during the period of immersion, calculated by Equation 7. I t is of considerable magnitudr and, for some of the paints, even exceeds the total loss in weight during the interval of iniinersion. If w ; is taken as 0.11 instead of 0, the values of S are increased by a factor of 1.44. Many other measurements of this sort have shown that, in the simultaneous dissolution of toxic and matrix, the amount of dissolved matrix as calculated by difference is almost always less than that specified by the simple relation of Equation 1. Paints for which this is true have accumulated surface residues which may include insoluhle matrix ingredients as well as precipitated salts of copper. DISCUSSION
The toxic leaching rate of a paint which function8 by virtue of a soluble matrix reaches a fairly constant value after a short period of soaking. The absolute value depends on both the rate of solution of the matrix and the proportion of toxic compounded in the paint. This permits the formulation of paints which have uniform copper leaching rates in excess of 10 micrograms per sq. cm. per day, the value necessary for the prevention of fouling, without an excessive waste of toxic. Furthermore, the leaching rate may be maintained a t constant level as long as any paint remains on the surface. The life of paints formulated with soluble matrices can, therefore, be extended by increasing the thickness of the paint film. In contrast to these observations, the leaching rate of a paint of the continuous contact type (9)decreases gradually with immersion and falls below the minimum value as soon as the critical depth is exceeded. Young, Schneider, and Seagren (7) presented theoretical curves of toxic losses from antifouling paints: their curve depicting a paint with a permeable matrix shows a steady decrease similar to the observed behavior of continuoue contact paints. The opportunity of making independent adjustments of two variables-the degree of toxic loading and the matrix dissolution rate-permits great flexibility in designing a paint with the desired leaching rate. I t follows that a variety of satisfactory paints can be formulated with the same basic ingredients. The major disadvantage of soluble matrix paints is that matrices of suitable solubility frequently have rather poor film characteristics. Careful formulation is required to obtain good physical durability and prevent premature failure by cracking, alligatoring, blistering, or erosion. I n the design of paints with soluble matrices, the proportion of toxic should be the minimum which, in a matrix with a given dissolution rate, will provide a n adequate steady-state leaching rate. Any excess above this minimum will be wasted since the leaching rate will be unnecessarily high. I n order to achieve maximum life, the thickness of the paint film should be the maximum consistent with economy and mechanical strength and integrity. LITERATURE CITED
(1) Burns, -4.E., Jr.. Interim Report 4 (unpublished), Navy Y a r d .
Mare Island, 1943. (2) . . Ferrv. J. D.. and Ketchum, B. H.. IND.ESG. CHEM.,38, 80+ (1946).
'
where w ; = weight fraction of copper (expressed as cuprous oxide) in accumulated materials. If the accumulation were pure copper abietate, w ; would be 0.11, which is B probable upper limit. If the accumulation is largely matrix residue, w ; E 0 and the equation reduces to
S z - WAup T - A M 10,
Results for a series of paints made up with cuprous oxide, rosin, and various amounts of different neutral resins (Hercolyn, ester gum, and copper resinate) are given in Table X. Here values of A T / W and A M / w 2 have been calculated for the period from the fourth t o the eighth week of immersion in the sea.
(3) Ketchum, B. H . , Ferry, J. D., Redfield, -4.C., and Burns, A. E., Jr., Ibid., 37, 456-60 (1945). (4) Ketchum, B. H., Todd, D., Reynolds, B., and Mott. B., 7tb Rept. from Woods Hole Oceanographic Inst. to Bur. of Ships
Paper 7 (unpublished), 1944. Report iron1 Bell Telephone Lab. t o Bur. of Ship,. (unpublished), 194.5. (6) Todd, D., and Ketchurn, B. H., 8th Rept. from Woods H o l e Oceanographic Inst. t o Bur. of Ships (unpublished), Chap. 7 , Part I1 (1945). (7) Young, G. H., Schneidei, W. K., and Seagren, G. W.. IND.ENCI. (6) Lucaa, F. F.
CHEM.,36, 1130 (1944).
C o s T n x n c T i u s No, 347 of the Woods Hole Oceanographic Institution.
Trrt experiments at X o o d s Hole were conducted under contract with the BureaL of Ships, S a v y Department, which has given permission for their publicetion. The opinions presented here are those of the authors and do not nececaarily reflect the official opinion of the Navy Department or the naval aervicc at large.
