ANTIFOULING PAINTS Inherent Errors i n Determination of Leaching K a t e ALLEN L. ALEX4NDER, .J. B. BALLENTINE', iNL) 11. 0. YEEITEK' \aval Kesearch Laboratory, Wushinyton, 19.C . T h e mechanisms by which successful antifouling paints function hai e become more clear11 understood through t h e use of leaching rate determinations as a tool i n studying the effecti\e properties of toxic paints. I t appeared of interest, therefore, to ascertain more clearly t h e limitations associated with the standard leaching rate determination and to elaluate t h e effects of any likely variables on the validity of conclusions based on leaching rate values. Data based on a large number of determinations describe an aterape niean deliation t h a t may be expected in leaching rate balues cleterniined a t interials oler a 5-month period. i n ) deviation appearing between duplicate leaching rate determinations is smaller than other lari-
ables introduced in the preparation of duplicate batches of paint. Therefore, i t may be concluded t h a t the error introduced by leaching rate determination is likely to he less signifirant t h a n that resulting from t h e preparation of duplicate batches of paint. With this demonstration of t h e reproducibility of leaching rate measurements, values so obtained describing a critical property of toxic paints assume added significance. Because earlier research has established a minimum toxic solution rate for efficient copper-bearing paints, leaching rate values may now be used as a more reliable measure of this function in the formulation of successful antifouling paints.
A
poor results Fvhen cuprous oside containing excessive amounts of cupric oxide was included as toxic pigment. Marked color changes have been noted in the appearance of cuprous oxide diuing prolonged storage under improper conditions, the pigment discoloring to the black color characteristic of cupric oxide. To prevent this, normal manufacturing procedures require that the oxide particles be covered with some protective colloid which is compatible in the subsequent dispersion in paint vehicle. Because t h k is a rather common occurrence, the effect of increasing amounts of rupric oxide on leaching rates and performance wax studied.
S THE result of numerous researches reported iri recent
years, the mechanisnis by xhicli successful antifouling paints fmiction have become more clearly understood. T h e n.ork of Ketchum and associates ( d ) , establishing t h a t copper-bearing paints must yield copper ion to their surrounding niedia ut established minimum rates, has hecn usrd widely as a guide in t h e design of efficient toxic: paints. Leariling rates 1)rovide more quantitative informntion on the l~ctrformanc~~ of the paint in one respect thari acatual exposure to c,oiiditionb of high fouling intensity, because the loss of coppei' above t h e niinimuni rate required t o repress atttic~Iinir+iitand growth of fouling organisms represents a waste. Considerablu work ha3 been done t o establish the factors by means of which leaching rate m a y be regulated to provide adequate protection at greatest efficiency. The rate of matrix solubility has been described by Ketchum, Ferry, and Burns ( 5 ) ; methods and effects of controlling solubility have been dis( u s e d ( 3 ) . Film permeability ha.? been studied 11y Young and Schneidei, (6):nid Alexander and H ~ n m i c ~ l(i s2 )as a factor in the rate of r ~ l e a s cof toxic matcsrial t o wa watrr. Thc iriQuence of pigment part,irle size on leaching has I i c w i investigated ( 1 ), with the indication that only initial values are appreriably affected by marked variations in total surfare area of t h r pigment. In most of thew studies the effect of variations in thr paint system has been aswi.taiiiec1 b y drtriminatioli of leaching rate, as exposure esperimcwt Y iiidirate onl>-~ h c t l i sufficirnt c~ toxic is availahle t o suppress fouling. Because tlic value of I(xachinp rate measuremrnts lit13 I i w i i t~inphasizetl.it appcared of interest t o study inor(' closrl>- t t i c s ac.c~iir:ir~y of thci detni,mination itsrlf, to recognize fully an>-e i ~ o r siiihci,tmt in thc tr~c~hniqllc~. and to (~v;tluatotlic eflccti of such tji'i 01's on the vtililiitl- of c~oiicluiion~ 1) erty diwrilwtl t ) ~Icaching rate vziluc~i. 1). ( 2 ), pigmrnt particle sizt. \vas tlrterniiiirci prior to on into the pairit. a f t c a r . ir-1iic.h the, liroprties of tlir ~ C o r w k ~ t i wto stiidiv of Icarhi~igrate, it paint W C ~ I Ystudicd. appeai,id of iiit(wst to invcstigrttc thc effect of grinding time 011 particle size aiid the rcmlting 1c:nchin:: rat(.. . Evidence suhstantiates further t h e conclusions ( 2 ) that on11 initial leachirig rates are affectcd materially by estrr~iiielysmall particle size. requiring prolonged periods of grinding. Reports on t h e performailre of paints in service have indicated 1 9
I'resent addri.>\. -4iiiericnn Viscoie Corp.. M a r c u s H o o k , I'respnt utldrc;;-. 410 3 I m r i c k .il-e., i'an B r u n o , Calif.
Pa
931
PAINT PREPARATION
T h e paints were prepared in quart steel ball mills, in which the ball size varied between 0.375 and 0.5 inch. I n order t o avoid characterktics peculiar t o any single pigment type, five pigments of widely varying properties were selected (Table I).
TABLE S;n1,,plt:
A
n ( %
V 13
I.
PIGMESTS
Type C u p r u u s oxide Cuprous oxide Copper vrecipitare Cuprous oside Cuprous oside
Characterlatic or Origin Electrolytic Pyrochemical High inetallic copper Pyrocheinical PqTochoinical
Saniplrs A, I>, and 1; niet the Iequirenirmts of S a v y Department Spccification 52C4, Tvhrrras sample I3 was an impure material containing an excessive amount of iron and zinc oxide and sample C corisisted principally of copper metal, prrsumablv surrounded 1iy envelopes of cuprous oxide. I n addition, a considerable number of samples from stored cuprous oxide containing varying amounts of cupric oside were obtained from the Norfolk Saval Shipyard for electron nlicroscope studies and Ipac-hing rate measurpments. Grinds of each pignient were prrrJamI in each of formulas 1 and 2 (Table 11). K i t h t,hc single exception of pigment C, all formulas ground smoothly. However, with sample C all grinding ceased because of excessive hod>-ingaftrr24hours: additional thinner was required, rcsulting in lower solids. Duplicate batches were ground for 4 hours, 24 hours, and G days. Each batch was divided into equal parts, one of which was forwarded t o the Koods Hole Oceanographic Institution for leaching ]'ate nieasurement>s, and the
932
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE 11.
FORJfLTL.4S
Formula 1
Grama
Vehicle Aa Tricresyl phoaphate Chlorinated so1n.h Pigment (A-E, Table I I
.;-lo
60 40
-360
1000
Formula 2 W. W . rosin Hercolyn Pliolite S-1 Dicalite Coal tar naphtha Pigment (A-E, Table I I
211 106 14 71 176 422 1000 ‘ I Resin modified phenol-formaldehyde re.\in 1 . > O q I in coal tar naphtha. 6 125 centipoises chlorinated rubber 17.8 Coal tar naphtha 82.2 100.0
-
second was retaincd for particle size nicasurenicwt and csposuw studies. A careful chemical analysis (Specification 52C4) was made of each pigment, the results of which are shown in Table 111.
Total Copper,
TRPa,
%
70
%
86.61 80.08
99.98 92.42
0.49 1.55
91.71 197.54 87.34 98.00 87.97 100.64 Total reducing power.
81.35
Sample h B
C D E a
hIetallic Coppel,
1.25
1.15
Cu20
CuO,
%
b
98.81 88.93
In3c!lublc in 11x0~
sa
% i.71)
14.34 95.39 2’.’2’2 98.05 . . ,
Other 3Iaterial
0.27
.. .
...
2 33ke 8 . 2 6 ZnO 5.30Fe 1 . 6 3 Fe
~~~~~~
The method used for nieauririg particle size anti suliseyuen i conversion t o surface area has been descritied (I). The data describing the physical effect of various grinding h n e s xverc calculated to total surface area per 100 grams of pigment and are presented as averages of duplicate determinations. Thehe surface area dat,a involving the five pign1ent.s a t three grinding time. are shown in Table IV.
h
32.85 9.30 17.71 1) 11.63 E 17.90 a Square meters per
R
c
37.10 33.33 17.64 12.09 19.23 47.44 16.15 14.30 16 2G 11.21 18.75 18.18 16.94 24.92 16.07 100 grains iiignienrs
2H.03 18.68 13.Ii.i 17.28 2 1 70
21 -49 19.13
16.23 23.31 20.26
37,:: 4>,2$1 21.21 37.57 3i.45
Perhnps the most striking fact evident iiom TulJle I\- is t h a t sample A decreased in surface area approximately 50% after 4 hours’ grinding, \Thereas each of the other pigments remained about the same or showed slight increases. Furthermore, the rolc of vehicle in grinding is eniphasiaed by the fact that, 111 general, surface area increased more regularly and to a greater degree in formula 2 . The initial diminution in surface area of sample A may be explained by the fact t h a t finely divided cuproui oxide particles display a decided tendency to react with rosin in solution to form copper resinate, which in turn is readily soluble in the rosin solvents. Pigment A contains a high percentage of small particles initially, which react soon after contact n ith the rosin solution and disappear as soluble copper resinate. As grinding continues, the large particles are gradually reduced in size, with a corresponding and somewhat normal increase in total surface area. Further support of this view is given by the fact t h a t total surface area of the pigment in forniula 2 during 4 hours’
Vol. 43, No. 4
grinding did not diminish to so low a value as the same pigment in formula 1. I n formula 2 the ratio of cuprous oxide to rosin is 2 to 1 ( b y weight), whcreas in formula 1 this ratio is only slightly above 1. It is further significant that after 24 hours’ grinding formula 2, during which opportunity was afforded for the smallest particles to react with the rosin, particle size increased stcatlily with continued grinding. iilthough this continued to be the general trend in batches prepared by formula 1, pigments A antl D were no smaller after’6 days’ grinding than at the end of 24 hours. In general, it appears t h a t prolonged grinding beyond 24 hours in the matrix of formula 1 produces no appreciable increme in surface area except in the case of pigment B, which was shown to contain appreciable quantities (S.5Y0 by weight) of zinc oxide. This pigment probably accounts for the increase, as its size is easily diminished on grinding in a dispersing medi.ini antl it is often used as a standard for establishing fineness of grind. On the basis of these grinding data, several generalizations seem warranted. Extremely fine particles (0 to 2 microns) are undesirable in a toxic copper pigment, as they will react to form copper resinate which is less soluble in sea water than either copper metal or cuprous oxide. .4 grinding period of 24 hours is indicated t o ensure a fairly uniform dispersion of pigment; continued grinding does not result in uniform reduction in particlt, size of the currently used pignicnt and appears unwarranted on the basis of increased surface area. This substantiates further a view previously expressed ( 1 ) . The above grinding time is based on using quart jar mills: shorter periods are likely to be practicable with full scale mills. LE4CIIISG RATES
The detailed data of Table 1- are prewnted as illustrative of typiml leaching rate measurements; similar data covering all pigments in duplicat,e batches of forniulas 1 and 2 were used in deriving the percentage variations in the method diwussed below. Leaching rate has bccn defined t)y Ketchum (4)as the number of microgram of copper released daily per square ccntimcster of paint surfaw. d minimum of 10 niicrogranls ha.; becw - w o i l l i l i,eprcsent tlii: grvatrst margin of error attriburahle to this Iirac,ticaal application of the lrvu,hing rate tpchniqur:. Certain variations obviously are inlicrent in a sturiy of this nature, wrors arising from two sources-preparation of the paint, hatch. and the leaching rate determination itself. The ingretlient;; for duplicate small batches almost always arc taken from t h e same lot of ram materials, which is seldom the case in actual pro-
‘rSHI,E
Grindini: Tirne,
IIOWS 4
rr.
LEACIIIXG RATESFOR
Sample NO.
1.i 1B
2A 24
2B 1.i 1B 2A 2R
Furface .irean
18.52 16 7 5
3i.19 37.12
PIG\fEST
A,
FOR\IULI 1
Leaching Rates, Months Ini2 tial weeks 1 2 3 4 5 6 . 6 22.1 2 1 . 3 9 . 1 13.7 8 . 9 4 7 . 4 2 2 . 7 22.1 1 0 . 0 1 ~ 5 . 6 9 . 0 4 0 . 0 2 3 . 6 22.5 10.0 15.3 9 . b 3 8 . 0 3 2 . 2 20.1) 9 . 7 14 7 9 . 5 51.8 19.4 19.7 9 . 1 1 0 . 3 6.3 45.a 2 1 . 8 18.1 7 . 3 10.0 5 . 2 44.317.816.2 6.1 8.76.3 51.2 l L 6 14.3 0.1 9.1 5.8
Square meters per 100 qrama dry pigiiient.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1951
A
0
O I
SAMPLf
20 30
50 60
10
90 100 I10 120 130 140 150
80
HOURS
Figure 1 .
may be subjected t o uneven rates of erosion or abrasion which could affect markedly the rate of solution of soluble ingredients. After errors likely to arise from the limiting conditions of the experiment have been recognized fully, i t is necessary t o consider them carefully, and if possible to determine the extent to which they may be expected to influence the end results of leaching rate determinations. As shown in Table V, four values for leaching rate data were obtained, c,omprising check leaching rate determinations for each formulation. Obviously, by the theoretical elimination of all errors these four values would be in close agrecinent; hovever, in view of the factors already pointed out, such agreement \vas not to be expect,ed and the actual differences hi.tween these determinat,ions serve as a inemure of the over-all accuracy of the leaching rate determination, along with any diqcrepancy arising in the preparation of duplicate batches of identical formulations. After four values had been established for each pigment in each grinding period, the average value was (:a]culated and each difference from the mean w m tabulat,ed along with the percentage deviation. This was done for all of the data, only part of u-hich is shown in Table V, and the averages are recorded in Table VI for formulas 1 and 2. Thus, the variat,ions of Table VI represent the mean deviation for all grinding periods a t the soaking times indicated. The number of samples studied is sufficient to permit the assumption that, the values listed represeri t a mean deviation most lilcely t>obe encountered in practice, anrl therefore the accuracy of any leaching rate determination may be assumed to fall within the limits indicated for the matrices studied. It is evident from these data t h a t marked differences exist i l l the accuracy of leaching rate determinations on paints containing identical pigments, but dispersed in different media. Leaching rate values for formula 1 vary between much closer limits than (lo those for formula 2, indicating that formula 1 provides for a moi'e uniform dispersion and distribution of the cuprous oxide pigment. Froin examination of these formulas a possible explanation can be advanced. A relatively low pigment-volume ratio characterizes formula 1 and the matrix is composed principally of a solution of rosin and phenolic resin! each of which is well known for its wetting and dispersing properties. The ingredients of forniula 2
A
4C
GRINDING
1,rarhing Hate 1's. Grinding Time Formula 1
0
IO
1 2C
! 3C
l 40
! 50
1 60
1 70
/
80 90
l
1 I 0 3 110
1
933
i
1 / ~ IZC 13C I40 I50
H O U R S GRINDING
I'igiire 2.
Leaching Rate r.+. Grinding Time Formula 1
tiuc,tion, anti this voultl a("nnit for some variation in the final properties ol t h e paint. In the curreiit study, great care was exerc.isctl i n the preparation of each hatch, and all duplicates were grouri(l ~iinultanro~isly on itirntical Inills at the sanir tcmperat,ure and for r \ a c t l > - ~ q i i a lperiods of time. Therefor?, it niiist be a s s u m d that the results shown are as reproduciblrl as reasoriably ma!. I)(, expected from practical considerations. Similar considerations hold, perhaps to a lessrr degrec~, in evaluating these paints by a careful study of their learhing rate properties. A lack of quantitative methods for the preparation of panels appears to open a possibility of considerable error. For example, in coating tu.0 small panels \vith films of a plastir paint, the same degree of ac20 ci1rac.y may hardly be esprrted as a-hen duplicat.r samplcxs arcs neighed from an analytical balance in a quantitative detrrniinxtion; yet in each case the sample is equally iniportant, as it affects the accuracy of the final results. ildditional variations must ai,ise from slight differences or uiievrnness in dr~-ing,although uniform at12 mowpheric conditions are maintained throughout 0 IOtho period of panel preparation. Further differences may be expected from incomplete e dispersion orwetting of the pigment and tosicmaterial. Finally, t h r sample panels are allowed t o soak in the owan for estrntlrd period,%,1\-11c~rvthry
Initial 2 week.:
i . I .-I
2.08
h .39
24 .8!)
2 3
n
4
IJ 43
6 18 5 7u
2 77 1 71
SL' !m 2 1 2fi 10. o ;, 17.61 14 i l
1.41
8 2Y
3 81
20.48
1
.iv.
1 81 1.39 I 11
10 2 8 73
65
75
4.73 01 2 23
I.'
i l ,
0 0
0
0
0
0
0
I
I
I
I
,
I
934
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 4.
0% Cupric Oxide
coutaiu ii highrr percvntagc, of cuprous oxide as weli a- sarnr. cyclized rubber, which possrss somewhat poorer wetting properties. Thus, it would seem that a matrix providing for more uniform and even dispersion and wetting would yield mor? reatli1~reproducible leaching rate values. Table VI1 lists the average differences between duplicate lc%c,hirig rate determinations made from the same hatch of paint. In this case, the percentage error may be attributed solely t o ttc leaching rate technique, which includes the error introduced i n panel preparation. These data were derived by averaging all thc ,differences b e h e e n duplicate values determined for each paint. :md were tabulated according to periods a t which the det,ermina'rions were made and classified for t,ht t x o matrices. Re,sults of this study show good correlation with those of Table VI, except that, as might be expected, t,he over-all error is smaller. Again t,he variance is much smaller for the paints prepared in the matrix of formula 1 than for those of formula 2, lending further evidenw t,o the hypothesis that irregularities in leaching rate cleterminwtioiiv may be attributed to the inadequate dispersion or wetting of the pigment by the matris. Thus, the assumption appears justified that the major factor limiting the accurarv of the l e a ~ h iiig rate tcchniqur i s t h r preparation of t r s t panc1.q :inti their S U ~ -
Figure 6 .
Vol. 43, No. 4
970 Cupric Oxide
Soaking
Period, Months
Initial
2 rveeks 1 >
..-
Formula I-.
.i\.. diff.
2.77
1.76 1.57
0.55
.\ Y .
0.77 0.54 1.33
- ~ A v . 70 diff.
8.08 7.84 8.79
__
- E'orinula 2
.Zv.&E. 2.85 2.00
1.92
0.88 1,54
5.67 6.82 6.94
0.98
7.38
1.70
IY.% diff. 9.06 !.42 I
.79
i 89
9.60 13.46 9.20
On c o i n p r i n g Table VI, which shows the over-all variation. Ivith Table VIT, indicating the variation due to leaching rate rleterminatioii alone, it is evident that the latter wror is low and idls safely n-ithin the limits normally accepted in studies of this nature. So c-alculations on leaching rate (lata were made for .
