ROBERT M. EVANS1 AND EDWARD 6. BOBALEK Case Institute of Technology, Cleveland 6, Ohio
I n the S'iclrlund aqd Manowitz test, a piece of filter paper is coated on both sides, dried 48 hours, and inoculated with Aspergillus oryzae. I n the Kuodex laboratory test the malt agar plate is inoculated with Pullularia pullulans, because Go11 ( 7 ) , Reynolds ( l y ) , and Haensler ( 9 )found that it is the more important organism infecting exterior house paints.
ESPITE years of research, the vexing problem still remains of finding test methods that give a reasonable prediction of the relative mildew resistance of different paints in service. Biologically important variables are difficult to control, and also, all of the several convenient methods thus far proposed suffer from the limitation that no precisely quantitative rating scale has been devised to report the data. Although these limitations have long been recognized, no practical remedy has been suggested. Since, hon-ever, paint testing needs to be done, some decision must be made on procedures. Either one can reject as valueless all accelerated methods or one can use such of the several proposed methods as seem to be best, despite their imperfections. The first proposition, which has its proponents (2), mould rely solely on long-term Observations of paints under service conditions. This presents an impractical barrier to the progress of development work. If, on the other hand, one inclines to a less defeatist viev-point regarding accelerated tests, the questions still remain: What confidence can be placed in such tests? Which of those now practiced is best? These questions can be answered in only a limited way, since i t is necessary to confront the test data with the results they predict for long-term service exposure of coatings. To do this for all the available coating formulations is an obvious impossibility. Fortunately, however, long-term service tests are known for a number of very important formulations such as oil-base house paints. A comparison of the relative success of the more favored accelerated tests in confirming the service exposure tests would be useful in showing which accelerated tests have the fewest reversals in such correlations and which could, therefore, be used with greater confidence in testing paints of generally similar types; namely, paints containing drying oils as a large part of the vehicle. This work was undertaken to compare the performance of several accelerated laboratory methods in predicting mildew resistance of several paint formulations which already have been extensively service-tested by other investigators. I n the main, the data are analyzed to determine whether the paint formulation variables that influence mildew resistance of paints in service are also indicated positively by the accelerated tests. I n the course of the investigation, special advantages of the Hutchinson accelerated test became apparent, and some modifications are reported in order to improve this relatively little practiced test. Accelerated tests should be modified to include a schedule for weathering the paint film before measuring its mildev resistance.
Hutchinson ( 1 0 )of the Tropical Deterioration Administrations Committee of the National Research Council developed a test based on the hypothesis that the carbon requirements of the fungus should come only from the coating under test and demonstrated the advantages of using a nonnutrient agar as the culture medium in the t>est. He chose commercial glass string from which the starch sizing was removed by oxidation as a support for the coating film. The test organisms he recommends are Aspergallus niger, Aspergillus $avus, and Penicdlium luteum. An important feature of the Hutchinson inoculum is that it is very large; each organism is grown in a separate 16-ounce bottle. Large cultures are used in order to provide adequate nutrient for the spores both from the ungerminated spores, and the very neak carbohydrate solution they carry with them. The Hutchinson test also specified only 48 hours' drying time for the paint film. The procedures of the first three tests rrere conducted according to the specifications in the literature, except for the following variations: I n the Hutchinson string test the starch sizing fungi nutrient was removed from the string with hot nitric acid instead of potassium permanganate. I n the Vicklund-Rfanowitz test only Aspergallus oryzae n-as used, since it is preferred to the other organism A. niger. The samples were not ruled, and the stock culture was grown on a slant in a test tube rather than in a Petri dish. I n the Nuodex test, the only variation was that the dried paint samples rrere not leached. For the sake of uniformity, all observations of mildew growth were rated according to the scale proposed for the modified Hutchinson test. MODIFIED HUTCHINSON TEST IV
This modification of the Hutchinson test used a desized Fiberglas tape instead of Fiberglas string. Although a dipped string method may be satisfactory for the very fluid insulating varnishes Hutchinson used, the dipped pigmented paints formed heavier films of uneven thickness which did not dry through homogeneously. Hutchinson objected to the use of Fiberglas tape or sheet, because on a planar surface i t seemed more difficult to distinguish between dead and living spores. This objection can be minimized by studying the surface mith either a stereomicroscope or with a monocular microscope a t 8OX using a strong light a t a grazing angle to the surface. Substrate. Fiberglas tape, Hess-Goldsmith Type B, 0.007 inch thick, 3/4 inch wide was de-sized a t 700" F. until all carbon was gone. Coating Substrate. A length of tape was cut. A volume of coating, measured out of a 2-ml. syringe in a quantity equivalent to 280 square feet coverage per gallon, was applied to the tape with a small beaver brush. After drying for 24 hours a second coat of the same paint was applied and leveled with the brush. Aging of Coating. All films Were dried in the laboratory for 2 weeks. Where so indicated, some laboratory aged films were
ACCELERATED T E S T METHODS
The methods compared are: I. Viclrlund and Manowitz test (81); 11. Nuodex laboratory test ( 1 4 ) ; 111. Hutchinson test ( 1 0 ) ; and IV, a modified Hutchinson test, which was developed in the course of this investigation. I n the more widely used test methods a piece of filter paper is painted, aged 24 to 72 hours a t various temperatures, placed on a n agar plate, and inoculated with some preferred organism. Two representative tests of this type are the Vicklund and Manowitz test (81) and the Nuodex laboratory test ( 1 4 ) : 1
Present address, Master Mechanics Go., Cleveland, Ohio.
122
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
January 1956
subjected to 21-day, Cleveland Club, degradation cycle ( 4 ) in a n Atlas Type X1A Weather-ometer. Organisms. A . niger, A . JEavus, Penicillium luteum, original cultures were obtained from Nuodex laboratories. Preparation of Inoculum. Pure culture techniques were employed. A 2-em. hole was cut into the top and liner of a 16-ounce, square, screw-top bottle. Fiberglas fabric was inserted between top and liner; 50 ml. of Bacto potato agar, prepared by label directions and adjusted to pH 5.6, was poured into bottle. After sterilization, the bottle was laid on its side, allowing the material in it to harden. About 5 ml. of sterile, distilled water mixed with a drop of Triton X-100 (Rohm 8: Haas Co., Philadelphia, Pa.) was added t o an agar slant of the desired organism, and some spores were dislodged into suspension with an inoculating loop. One ml. of the spore suspension was pipeted evenly over the agar surface in the 16-ounce bottle. Each organism was grown in a separate bottle under pure culture conditions. The inoculum was incubated for one meek a t 30" C., 85 to 95% relative humidity. It can then be stored for 2 months in a refrigerator. Inoculation and Incubation of Specimen. Culture medium formula was NH4N03
KHzPOi MgS04.7HzO KC1 Agar
Distilled
Hz0 pH adjusted to 5 . 0
grams
1.5 1.0
0.5 0.5
15-18 1000
Of this solution 20-ml. portions were poured into Petri dishes? sterilized, and allowed to harden. Preparation of Spore Suspension. Fifty 0.5-inch glass beads and 50 ml. of distilled water were put into a 125-ml. Erlenmeyer flask, along with 0.1% Triton X-100. After sterilization and cooling, solution and beads were poured into the 16-ounce bottle of inoculum and gently rocked back and forth to suspend spores. The suspension was then poured into a 250-ml., wide-mouthed Erlenmeyer flask. This procedure was repeated for each of the three cultures, A . niger, A . $avus, and P. luteum, with fresh beads and water, and all three spore suspensions were mixed in the same flask. Inoculation of Painted Tape. Samples were dipped into the mixed spore suspension for 1.5 minutes and then a 1-inch length was cut off, drained, and placed on the agar culture medium. The samples were incubated a t 30" C. for 4 weeks with weekly inspections. In the 30' C. oven aging no supplementary means were employed to maintain humidity. I n some experiments, the aging was done in a box a t a constant temperature of 38" C.,using a thermostatically controlled immersion heater in a water bath, and also including in the box two rag-packed beakers of water. This latter preferred procedure was varied also by dropping the temperature for 1-hour periods twice a day from 38" to 21" C., so as to produce occasional conditions of condensation. The variation of temperature and humidity conditions seemed to have no obvious influence on the rate or extent of mildewing. Apparently the organisms used are relatively insensitive to variations of incubation conditions within the limits reported. Rating Scale. The test results were evaluated according to the following scale:
0 = No growth 10 = Verv slight growth: either confined to a Dart of the edges or occakiond evydence of hyphae or living sporeclusters on scattered areas of the paint surface. Ordinarily microscopic examination is necessary to distinguish between 0 and 10. 20 = Sparse growth: May be confined as a heavy growth a t edges or very light patches may occur anywhere on sample. If the paint surface were ruled into about '/&xh squares, less than half the reference units should be infected. I n general, if to the naked eye, the evidence of contamination is definite, then the rating is 20 instead of 10. 30 = Profuse growth: A large part of the area is infected. With respect t o distribution, one half to three fourths of the reference '/&nch s uares may contain mildew growth; however, despite this wide Iistribution of the spore-bearing hyphae, a t
123
least half the total paint surface is not masked by the dark overgrowth. 40 = Severe growth: Distribution and density of spore bearing hyphae is so complete as to hide paint surface. Rating number 10 may be uncertain due to the difficulty in distinguishing between growing spores and the large number of dead spores resulting from the large inoculum. This uncertainty must be resolved by viewing the sample with grazing illumination using a 50 X microscope, or by using a stereoscopic microscope to make it easier to distinguish between dead and living spores. Where observation is sufficiently thorough, the remainder of the rating scale might be subdivided further, with emphasis on finer distinctions between the 0 t o 20 portion of the rating scale. AGING OF PAINT FILMS PRIOR TO MILDEW RESISTANCE T E S T S
Weathering of films in service can affect their mildew resistance. Certainly aged films suffer considerable chemical deterioration (3, 5, 6, 11, 16, 16, 28) which might alter its nutrient characteristics. In some cases fungicidal additives may be lost by leaching. Possibly, the aging effect is more complex, as was shown by the Leonard experiments that pentachlorophenol and phenylmercury salicylate promote oxygen absorption by the film (18). Whatever may be the cause, the net effect of weathering exposure should be included as a part of an accelerated test. Even in the valuation of fungicides, the relative permanence of such additives is as important as is their initial activity in fresh films. The Nicholl and Hoffman data (15)suggest that approximately 2 weeks are required before the properties of a paint film approximate maturity. Shortening this period by the use of heat can emphasize olefinic polymerization a t the expense of the normal autooxidative polymerization (16) and thus change the chemical and, possibly, the nutritive properties of the film. To minimize the possible variables of aging, all films were dried in a laboratory atmosphere for 2 weeks before fungicidal testing or before being subjected to artificial weathering. The accelerated weathering was done in a National X-1-A Weather-ometer using the Cleveland Paint and Varnish Production Club 21-day cycle ( 4 ) ,which has been specifically correlated with &month southern exposure a t Miami, Fla., for paints of the same general type as are used in this experiment. The samples were suspended in the Weather-ometer by nylon strings and after exposure, they were soaked in a dilute hydrochloric acid solution (pH 5.0) to eliminate any superficial hard water deposits which might contain iron, copper, or calcium. Such heavy metals, deposited from the water even in traces, might affect the growth of microorganisms to a greater extent than does the mildly acidic wash. PAINTED FORMULATIONS TESTED
Tables I and I1 list the identifying details of eight paint compositions (A to H ) for which thorough service test data are available in the literature. Also four coatings (J to L) are listed for which less complete service test observations are available but which are interesting because they illustrate the response of the accelerated tests to major changes in the type of the resinous binder in the paint. The cumulative experiences of independent investigators (8, 19) have established several principles of paint formulation with respect to the mildew resistance problem: 1. A paint containing zinc oxide is much more fungus resistant than one without. 2. In all oil paints the zinc oxide content is more important than the mercurial content. Possibly this applies also to other fungicides which have not been investigated as thoroughly as the mercurials. 3. 0.1% Phenylmercuric oleate is significantly effective in oil paints exposed for 1 year facing north; but on the panels a t 2 years south, this quantity is insufficient. Higher mercurial contents may give effective protection for 2 or more years even in southern exposure.
INDUSTRIAL AND ENGINEERING CHEMISTRY
124 Table I.
Identi6cation of Paints Used i n Accelerated Tests
Sample Reference Pigment volume, % of nonvolatile Pigment composition, weight % of Fez03 Diatomaceous silica Rutile TiOz Titanium calcium (rutile) Leaded zinc, 35% ZnO Basic carbonate white lead Talc ZnO Nonvolatile in vehicle, %
A
(21)
B ($f)
2 3 . 3 25.0 38.8 .. 23.0 . . 1i.i
., ..
3812
4i.6 28.5 11.8
75
70
..
Binder composition, % Refinrd linseed oil 64 Bodied oil and viscosity grade (Gardner-Holdt) Varnish 34h
C, D
(19) 28.5
.. ..
E, F
(f9) 28.5
..
21.7
22
.. 1 .
4i:o
..
78
42:5
72:6
73:4
,.
75
75
25(Y)
25(X)
.,
75
..
(25x1
..
J, Ka
..
L
33.0
..
..
0
.. .. 50 ..
..
..
..
.. .. .. .. ..
, .
..
..
..
..
55
45
G, H (18)
25 25 75
..
.. ..
..
75
..
Alkydb
26(22)
..
, I .
.
.. ..
R.1
..
0 ,.
..
..
, .
..
S'arnishc
.. ..
Fungicide, % ' of total vt. of paint as Phenylmercuric oleate 0 0 0 0.1 0 0 . 1 0 0 0 0 Phenylmercuric salicylate 0 0 0 0 0 0 00.33 0 0 5 Commercial product Sanitile of undisclosed formula which the manufacturer (Miaster Mechanics Co., Cleveland 13 Ohio) provided h i t h o u t fingicide (Sample J) and with fungicide (Sample K). This is typical of coatings used in interior painting of food processing plants where humidity, heat, and other conditions promote mildewing. b Reichhold Chemical 1331 Beckosol. 6 15 Gal. length, tung oil-p-phenylphenol formaldehyde resin, corresponds t o MIL-V-l73A, b u t v i t h o u t fungicide. d Conforms t o government specification TT-V-126, containing 40 gal. bodied linseed 4- 10 gal. tung/100 lb. of phenolic resin.
Vol. 48, No. 1 coincide exactly in rating number assigned t o an observation. I n Table 111, the numbers recorded represent a numerical average of three or more readings on separate reph a t e s , each being withdrawn from an incubation series and rat,ed before being identified. The degree of reproducibility of tests I to I11 is indicated in the references cited. For the modified Hutchinson test (IV), the reproducibility was estimated for sample pailits A to F in the unweathered state. I n each instance three incubation series mere prepared with 3, 2, and 2 replicates, respectively, in each serier. The standard deviation
x(z
-5)Z
112
u = ( n - 1 )
Table 11. Service Exposure Test Ratings Using Accelerated Test Scale
Paint Type
Av. rating
yr.
Panama ($1)
..
.. .1.
E FJ G
Delaware test fence (19)
36 24 16 4
H
Fence in Long Island Author inspection of service in fungus infected. warm, humid bakery and brewery working areas No extensive service data reported-many paint formulators rate alkyd more mildew resistant t h a n linseed il, but less t h a n phenolic varnish Tests leading t o government spec. MIL-173.4
3
gl
Reference and Location
(18) J
K L
I
M
rating
Time, yr. 0.5 0 5
1 1
40 0 40 40 16 12
.. .. ..
.. .. ,. ..
24 14 10 0
3 3 0.5
..
..
,.
..
..
Titanium Pigment Corp. test
s
South Exposure
North Exposure
..
1
22 2
2
0 5
0 t o 10
4. The use of fungus resistant vehicles contributes a t least as much toward promoting fungus resistance in the complete paint as does the incorporation of zinc oxide. 5 . Different organic paint binders retain their relative rating with respect to fungus resistance throughout the exposure period-e.g., a phenolic varnish will remain a t all stages of exposure more resistant than a binder consisting only of drying oil. 6. The relative upgrading effect of supplementary toxic agents such as mercurials is greatest for paints which are already somewhat resistant because they contain inhibitive pigments or less nutrient binders. A useful accelerated test should indicate positively a t least these six variables which are important in service tests. All the paints listed in Table I were prepared in this laboratory and are not the identical samples for which service data were known. Any agreement betveen accelerated and service tests would indicate a typical response to details of formulation practice and hence should be of morevalue than if they represented correlations only for specific paint samples. DISCUSSIOE OF RESULTS
All observations of specific samples were recorded as either 0,
14 20, 30, or 40, and no intermediate numbers were assigned in any test, even though it may be quite feasible t o do so. Where finer rating scales are defined, different observers are unlikely to
for n = 7 , was invariably less than zk5 units for which is the average rating for each sample reported in Table 111. A similar error analysis was not made of the other methods, but' experience indicates that the precision is about the same in all the methods. In general, all the commonly used test's, when carefully executed, give results that are satisfactorily reproducible for all practical purposes sueh as rating relative order of mildew resistance of different paints. The length of incubation period before observation is important. If samples either contain no toxic agents or very effective quantities of toxic agents, the ratings after two and four weeks are substantially the same in all the tests. Where the toxic inhibition is slight, u for the 2-week series of observations may be as great as &15 but usually drops to 1.5 t o 0 for n = 7 after 4 m-eeks. Particularly where toxic inhibition is limited, as in Sample D (no zinc oxide, minimum quantity of mercurial), the growth rate is irregular. A profuse gron-th appears early on one replicate while the other remains clean. Eventually gron-th will start up and develop very fast on the next replicate. After 3 to 4 weeks, however, t,he appearance of all the replicates is about the same. The paints which are poorly inhibited with toxics show a gradual progression of growth, starting within 1 to 3 days and attaining within 2 weeks almost the same appearances as they will have after 4 weeks or more. Possibly something of value might result if careful observations were made of the change of appearance rating -4th time. Such observations, however, are tedious and disturb the samples, and determining the possible value of this scheme would represent a separate study not included in this program. AFa result of this work a minimum incubation period of 3 to 4 weeks is recommended. I n special instances, longer periods are desirable if samples are strongly inhibited, and lesser periods may do where there is no inhibition a t all. Four weeks seem t o be optimum for a variety of sample types. The possible prediction value of accelerated tests is shown also in Table 111. From the viewpoint of rating relative order of fungus resistance of different paints, with few exceptions the accelerated tests confirm service experience. For example, Test I reverses the order of E and F ; Test I1 reverses D and E, and E and F ; and Test IV seems incorrect on rating E and F in the weathered state. There seems t o be a tendency for Test I to underestimate the importance of differences in paint vehicle and of mercurial additives. By contrast, Test I1 seems possibly oversensitive to mercuriale in fresh paint films, All the tests showed that fungus resistance of some paints varied depending on whether the film was fresh or artificially weathered, but it was disturbing that all did not vary in the same direction.
January 1956
INDUSTRIAL AND ENGINEERING CHEMISTRY
125
by the Pullularia. Paint F, containing lead, zinc, and mercurial toxic was the only film that resisted mildewing. Paints C and D had a mildew rating of about 30, and B and E of about 10 after . . . . 10 0 2 weeks of incubation a t 33’ C. and 95% relative humidity. 40 33 10 While these exploratory tests suggest .. . . 40 .. .. that Pullulariacan be grown under labo30 13 30 20 ratory conditions, the details of its man.. ,. .. .. agement are insufficiently known t o allow . . .. .. .. .. its convenient use in a routine test pro. . .. cedure. Since other organisms, which 20 0 are more convenient t o use than Pullul.. .0. 10 10 0 aria, still give good correlation between accelerated and service tests-even with respect to the weathering effect-there seems to be no reason a t this time for specifying the use of Pullularia. However, should further experimentations show the desirability of doing so, its use is possible, a t least when testing is done on partially weathered paint films. These qualitative observations with Pullularia show that the growth of the most common mildew organism of outdoor exposure is dependent on weathering degradation t o an even greater degree than are the more convenient organisms used in the laboratory test. This further confirms the recommendation here proposed of conducting laboratory accelerated tests on films that are partly deteriorated by oxidation, ultraviolet, and water. The preferred method for accelerated aging may yet be in doubt, but its importance cannot be minimized as has been customary in the earlier history of mildew resistance testing,
Table 111. Comparison of Mildew Resistance Ratings of Coatings A to L in Accelerated and in Service Tests Rating order from service data (-4< B) (C < D < E < F) (G < H) (L < M) (J < K) Rating scale from service data Ratings from accelerated tests Test I-Vicklund and Manow1tz Fresh coating Aged coating Test 11-Nuodex Fresh coating Aged coating Test 111-Hutchinson Fresh coating Aged coating Test IV-Modified Hutchinson Fresh coating Aged coating
40
0
3G
24
16
4
24
14
40 33
10 10
40 40
40 40
13 7
15 17
20 7
27 7
37 30
20 20
37 35
10 37
17 20
13
. . 10 ..
40 40
23 13 10 10
0 0
20
7 13
40 40
23 40
13 0 17 23
20 27
40
40 40
0
. . .. .. ..
.. ..0
.
0
20
Ordinary experience in service indicates that paints suffer in mildew resistance as they are weathered. Tests I and 111indicate a contrary behavior in some instances, whereas tests I1 and IV better reflect experience in this regard. If it can be shown that weathered paints tend to be more susceptible to mildew, this fact would favor adoption of tests I1 and IV especially for evaluation of weathered films. Some further experiments were conducted t o clarify this issue. GROWING PULLULARIA ON THE FIBERGLAS TAPES
A different type Pullularia grows on weathered than on unweathered films. This organism exists in two variants-as yeastlike slime and as an articulated fungus with normal hyphae. I n the Nuodex test the slimelike variant grows on the malt aga and on the unweathered paint applied t o filter paper. However, on the weathered paint film the Pullularia grows with articulated, starlike hyphae, even when the paint is on filter paper. The type growing on artificially weathered films resembles more the growth habit observed on house sidings, While the modified Hutchinson test results were satisfactory with the recommended mold culture, it is generally agreed that the main infecting organism on exterior paint is Pullulariaand not the organisms used in the Hutchinson test. Therefore, if it is convenient, it would be desirable to use Pulluluriain the modified Hutchinson test. I n attempting to grow the Pullulariaon the modified Hutchinson substrate both fresh and artificially weathered paints on Fiberglas tape were inoculated. This attempt was quite unsuccessful because Pullulariawas blotted out by the seemingly more virile Aspergili and Penicillia,which entered as contaminants of the inoculuum. The reproductive capacity of the Pullulariaculture grew weaker with time so that eventually it was even difficult t o establish new cultures on malt agar slants. Since Pullulariais an ultraviolet resistant fungus, which will grow even on the south side of house siding in Florida (I?’), it seemed feasible t o use ultraviolet to kill the contaminants in the stock culture. Therefore, a 16-ounce bottle containing potato agar was inoculated with a culture from a stock bottle of Pullular i a which had been overrun with Aspergillus niger. The transplant was exposed t o that fraction of ultraviolet that would reach it when the radiation from a 15-watt, General Electric Co. germicidal lamp was reflected from a white painted walland through the glass side of a square, 16-ounce, green glass bottle. The radiation was sustained for 4 hours per day for the first 3 days of growth. This treatment caused the Pullularia t o grow vigorously enough to obscure obvious signs of other contaminating fungi. Using this purified Pullularia,samples of paint films, A to F, both weathered and unweathered, were inoculated. It was found that Pullulariacould be grown only on the weathered films, and even paint A, which is susceptible t o the other organisms in all stages of aging; was not attacied in its preweatheredcondition
I
ACKNOWLEDGMENT
The authors wish t o thank the Master Mechanics Co. and Herman W. Leggon for assistance with these experiments. REFERENCES
’
(1) Bauer, R., 2. Bacterial. Parisitenk. 98,No. 519 (April 12, 1928). (2) Buckman, J.,Paint Industry Magazine 69,No. 4,20(1954). (3) Chipault, J. R., and McMeans, E., Ofic. Dig. Federation Paint & Varnish Production Clubs 354,548 (July 1954). (4) Cleveland Paint and Varnish Production Club, Ibid., No. 276, 44 (January 1948); Part 11, No. 286, 813 (November 1948). (5) Dunbar, R. E., Ibid., No. 323,857(December 1951). (6) Farmer, E. H., J. Oil & Colour Chemists’ Assoc., No. 340, 343 (October 1948). and Coffey,G., Paint, Oil C h m . Rev. 111, No. 16, 14 (7) Goll, M., (Aug. 5,1948). (8) Goll, M.,and Hyde, R., Ibid., 115,No. 9,14 (April 24,1952). (9) Haensler, C. M.,thesis to Rutgers College, 1921. (10) Hutchinson, W. G.,Office of Scientific Research and Development, Rept. 5687, Oct. 31,1945. (11) IND.ENG.CHEM.46,No. 6,18 A (1954). (12) Leonard, J. M.,and Larson, H. L., Naval Research Lab. Rept. 4228, 1954. (13) Leonard, J. M., and Pitman, A. L., IND.ENG.CHEM.43, 2338 (1951). (14) Nuodex Products Co. Inc., Elizabeth, N. J., pamphlet, 1952. (15) Nicholls, R. V. V., and Hoffman, W. H., Ofic. Dig. Federation Paint & Varn$h Production Clubs 327, 245 (April 1952). (16) Payne, H. F., Organic Coatings Technology,” Wiley, New York, 1954. (17) Reynolds, E.S.,Mycologia 42,432-48 (1950). (18) Titanium Pigment Carp., visit t o test fences by author (1953). (19) Vannoy, W.G.,Oflc. Dig. Federation Paint & Varnish P r o d u e tion Clubs 277,163-75 (February 1948). (20) Verall, A. F., and Mook, P. V., IND.ENG. CHEM.42, 1350 (1950). (21) Vicklund, R. E.,and Manowitz, M., Engr. Research and Develop. Labs., Ft. Belvoir, Va., Rept. 1118, 1951. (22) Wheeler, D.H., Ofic. Dig. Federation Paint & Varnish Production Clubs 23,661 (November 1951). RECEIVEDfor review M a y 5, 1955.
ACCEPTED October 12, 1955.
