A Practical Method for Determining Hiding Power of Paints A. E. JACOBSENAND C. E. REYNOLDS,Titanium Pigment Co., Inc., Brooklyn, N . Y.
A
METHOD for the determination of hiding power of paints which would be readily adaptable to practical use in the paint industry has been sought for some
years. In developing the method presented in this paper, consideration has been given to the requirements which should be met by a practical hiding-power test. It was assumed to be desirable that (1) a single coat of paint at an approximately normal spreading rate should be sufficient; (2) the hidingpower values should be obtainable from either a wet or dry film, and should give independent numerical values without reference to another paint or pigment; (3) the background should be visually obliterated; and (4) the effect of the ultimate brightness of the paint should be minimized. A method complying with these requirements would represent an advance over most visual obliteration methods thus far developed, which are subject to one or more of several limitations-for instance, the necessity of multiple coatings, indefinit,eness of end point, basing of results on observation of wet films, and particularly lack of consideration given to the effect of ultimate brightness of the paint. These limitations are inherent, not only in those methods which use a black and white contrast background, but also in those involving gray and white, if complete visual obliteration is used for the end point. Recently several methods have been developed which make use of the above-mentioned contrasts, but paint is applied t o only partial obliteration of the background. There are objections to the method developed by Bruce ( I ) , because of its inflexibility and because of the expensive equipment necessary. Sawyer (IO), on the other hand, has obtained hiding-power values on a more practical basis, by the partial obliteration of a gray and white background interpreted in terms of a comparison standard which has been evaluated for complete hiding over the same background. Although the partial obliteration method will give comparative values for very similar paints, correct evaluation by visual observation is difficult in the case of a gloss paint having a low pigment concentration in comparison with a flat paint of high pigment concentration. Sawyer (11) simplified his original method by making several paint-outs of increasing thickness and calculating the contrast ratio of each painted chart. Thus by obtaining several points he was able to draw a contrast-ratio filmthickness curve from which he obtained relative hidingpower values for paints or pigments for a chosen contrast
for an inexperienced operator to make severa paint-outs in order to obtain a check. In the observation of the panel some standard light source is advisable. It has been found that satisfactory results are obtained when the paint-out is held at arms’ length and in north light. The test can be made on a wet or dry basis, as the particular case warrants. In the case of flat paints or paints which contain a considerable percentage of volatile, it is advisable to make two or possibly three brush-outs in order to obtain a satisfactory end point, because of a change in hiding as the film dries. The simple weighing of the panel (or weighing of the brush and of the can of paint) before and after painting permits an easy calculation of the hiding power in desired terms. In the development of this hiding-power method, a study of obliteration curves of paints was made, like the curves obtained by Haslam (d), by applying a paint uniformly over a chart composed of black and several intermediate grays approaching a white. The paint was applied by brushing uniformly and the resulting brightness increase measured. The brightening power of the paint over each gray undersurface
ratio.
These methods are time-consuming and the paint industry i s interested in obtaining a more simple method. Therefore,
the development of a new and practical hiding-power method was undertaken. /NCREMENT UNITS
METHOD Paint is applied by brushing in a practical manner over a contrast background consisting of black (approximately 5 per cent brightness) and gray (25 per cent brightness). The .end point is reached when the obliteration of the contrast is just complete to the eye. Some care must be exercised in ,order not to go beyond the end point, and it may be advisable
FIGURE1.
- F/L#
TH/CHM€SS
VISUAL CONTRAST OBLITER.4TION
Black-25 per cent gray; black-60 per cent gray: and black-white
was studied. These data were compiled and a brightness obliteration curve for the paint was obtained. Although curves of this type are of little value for practical use, they furnish the means of studying the rates of change in brightness with change in film thickness. 393
ANALYTICAL EDITION
394
Let us consider a background composed of gray and black squares over which small amounts of white paint are applied successively and uniformly over both surfaces. Some point will be reached at which the darker surface ap roaches the original brightness of the lighter of the two undersurpaces, but at the same time an equivalent amount of paint is being a plied to the lighter contrast surface. The result is that the brigltness of the lighter surface also increases, but t o a relatively less degree than that of the darker surface. If the application of the paint is continued, at some point complete visual obliteration of the contrast will result.
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FIGURE 2. PIGMENT HIDING-POWER VALUES FOR CHANGEIN PIGMENT CONCENTRATION The end point will vary with the contrast background used. Figure 1 shows a series of curves for three contrast backgroundsblack and 25 per cent gray, black and 60 per cent gray, and black and white (80 per cent). Assuming the contrast sensitivity of the eye as 1 per cent, the respective end points (at just visual obliteration) for the three backgrounds used are indicated on the graphs. (Kraemer and Schupp, 6,showed that in the case of a checkerboard design, the average person can just distinguish a contrast of slightly less than one per cent. Ex erimentally the authors have found this to be correct.) It wii be observed that the visual contrast obliteration takes place in the case of a black and 25 per cent gray background in the region where a comparatively small increment of paint (about 1 unit) is required t o obtain a 1 per cent increase of brightness; in the case of a black and 60 per cent gray background the visual obliteration is in the region where about 3 units of paint increment are required for 1per cent increase in brightness; in the case of a black and 80 per cent white background, almost 10 units of paint increment are re uired to obtain 1 per cent increase of brightness in the region the curve where visual contrast obliteration takes place.
01
In consideration of these factors a chart having a checkerboard area of one square foot was adopted upon which are printed alternate black (5 per cent) and gray (25 per cent) squares of approximately 2 X 2 inches, surrounded by a 0.375-inch border of the gray and an outer border of white. By means of the method developed it has been possible to determine hiding-power values a t visual obliteration for a number of paints and also to demonstrate in a practical manner some of the factors which affect hiding-power values.
Vol. 6, No. 6
FACTORSINFLUENCINQ HIDING-POWER VALUES Among the factors of importance is the role that the pigment volume concentration plays in a paint film. Pfund (7) states, “The dilution effect is observed when hiding-power measurements are carried out on mixtures whose pigmentvehicle ratio is varied through wide limits. It is found that unit mass of the pigment will hide increasingly greater areas as the pigment-vehicle ratio is decreased.’’ Sawyer (10, 11) also gives some data in support of this conception. In order to show the effect of pigment concentration on the hiding power of paints and a t the same time to compare hiding-power values of a series of paints, using the method described in this paper, a number of pigments were ground into pastes with linseed oil (containing drier) and mineral spirits (added to facilitate grinding of pastes of high pigment concentration; the exact amount remaining in the paste was determined by analysis). The following pigments were used: lithopone, titanium barium pigment, high strength lithopone, titanated lithopone, titanium calcium pigment, zinc sulfide, and titanium dioxide. In order to obtain paints of brushing consistency and of the desired pigment-vehicle ratios the pastes were diluted with oil or volatile in whatever proportion was necessary. Figure 2 shows the hiding-power curves obtained for dry lilms of the various pigments a t different pigment concentrations. I n all cases, the hiding power per unit of pigment increased as the pigment concentration decreased. In the consideration of a hiding-power method the factor of spreading rate plays a part; a method to be practical should employ an approximately normal spreading rate. This feature has been studied in connection with the method described in this paper and it has been found that the authors’ method permits hiding-power determinations at spreading rates very close to those of practical application. Table I gives spreading rates a t various pigment volume concentrations, based on observations of dry films. TABLEI. SPREADING RATES (Square feet per gallon)
TITANAT~D LITHOPONE VOLUME CONCEN- LITHO-
AND
TITANIUM HIGE- TITANIUM BARIUM STRENGTH CALCIUM TRALTION PONE PIGMENT LITHOPON~ PIQMENT 460 630 440 20 340 30 450 600 700 580 930 790 50 630 830 70 830 1070 1170 970 VOLUME ZINC TITANIUM DIOXIDB CONCENTRATION SULFIDE
As the pigment volume concentration increases the resulting spreading rate of paint increases. This is natural because an increase in pigment content of the paint decreases the relative amount necessary for hiding. The hidingpower spreading rate for the abnormal pigment volume concentration paints should not be confused with the practical pigment volume spreading rate paints. Paints between 20 and 30 per cent pigment volume concentration are practical gloss-type paints and paints of 50 to 70 per cent pigment volume concentration (in the dry film) are usually flat paints. There is generally a high percentage of volatile in flat paints; this is equivalent to a reduction in actual spreading rate (on pigment basis) , thus approaching a rate more consistent with painting practice, In the case of zinc sulfide and titanium dioxide paints, a practical pigment volume concentration of between 10 and 20 per cent is within the range of ordinary use.
November 15,1934
I N D UST R I AL A N D E N G I N EE R I N G CH E M I ST R Y
In case of a complete obliteration method as described in this paper, a decrease in the ultimate brightness of a paint results in higher hiding-power value figures, as observed a t visually complete contrast obliteration. This effect of decreased brightness has been previously demonstrated by Pfund (S), Gardner (g), and Rhodes and Fonda (9). Morrison (6) found that there was an increase in the hiding power of a paint when raw linseed oil was substituted for refined linseed oil. This was ascribed by him to the presence of coloring matter in the raw oil; obviousIy it was due to the decreased brightness of the paint. T h e interdependhiding value is illustrated in Figure 3. A practical linseed oil paint of 90 per cent ultimate brightness was reduced in brightness successively to 84 and 78 per cent by the addition of a coloring
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EVALUATION OF PIGMENTS FROM HIDING-POWER VALUES For the purpose of comparison, hiding values of paints of 28 per cent pigment volume concentration were selected for composite pigments because this concentration is considered standard in pigmentation of most linseed oil paints. On the other hand, zinc sulfide and titanium dioxide are used a t low pigment concentration only, and were therefore compared on the basis of 15 per cent pigment volume concentration. Hiding-power values determined by the authors are as follows (Figure 2):
TABLE11. HIDINGPOWER VALUES 28 PER CENTPIQMENT VOLUMECONCENTRATION
HIDINQ POWER Sq. ft.llb.
Lithopone Titanium barium pigment High-strength lithopone Titanated lithopone Titanium calcium pigment 15 PER CENTPIQMENT VOLUMECONCEITRATION Zi,nc sulfide Titanium dioxide
43
58 67 67 74 10s
180
Hiding-power values a t pigment concentrations other than those given above may be readily obtained by drawing a vertical line through hiding-power curves a t any desired pigment concentration. (It is interesting to note that the hidingpower relations of various pigments shown above are consistent with those obtained by Hallett, 3.)
ACKNOWLEDGMENT Acknowledgment is due to Jos. L. Turner, director of research, and D. W. Robertson, sales engineer, both of the Titanium Pigment Co., Inc., for their valuable help and suggestions in the preparation of this paper.
LITERATURE CITED (1) Bruce, H. D., Bur. Standards, Tech. Paper 306 (1926).
Gardner, Sward, and Levy, Am. Paint Varnish Mfrs. Assoc., Sci. Sect., Circ. 362 (1930). (3) Hallett, R. L., Proc. Am. SOC.Testing Materials, 30, Part I1
(2)
(1930).
G.S., IND.ENG.CHEM.,Anal. Ed., 2,69 (1930). Kraemer and Schupp, Jr., “Determination of Hiding Power of White Paints,” presented before the 85th Meeting of the American Chemical Society, Washington, D. C,, March 26 t o 31, 1933. (6) Morrison, R.A.,OiKcial Digest Fed. Paint & Varnish Prod., 112,
(4) Haslam,
(6)
t h o u g h t h i s influence can be clearly traced even a t comparatively low pibment concent r a t i o n s , it manifests itself in a most pronounced manner
8
‘
po
f
$ a
745 (1932).
(7) Pfund, A. H., Am. Soc. Testing Materials, Tech. Papers 30,878 +-
(1930). (8) Pfund, A. H., J. Franklin Inst., 196,69 (1923). (9) Rhodes and Fonda, IND.ENQ.CHEM., 18, 130 (1926). (10) Sawyer, R. H., Ibid., Anal. Ed., 6 , 113 (1934). (11) Sawyer, R.H., “A Method for Measuring the Hiding Power of Dry Paint Films,” presented before the 86th Meeting of the American Chemical Society, Chicago, Ill., September 10 t o 15. 1933. RECEIVED April 28, 1934. Presented before the Division of Paint and Varnish Chemiatry a t the 87th Meeting of the American Chemical Society, 8t. Peteraburg, Fla., March 26 to 30,1934.
STARCH FROM WASTESWEETPOTATOES MAYFINDUSE IN PAPER INDUSTRY. According to the results of experimental tests made by the Bureau of Standards sweet potato culls offer some possibility as a source of starch for sizing paper and for paper adhesive. The culls constitute a huge waste in the southern states, and the Bureau of Standards and the Bureau of Chemistry and Soils are jointly endeavoring t o find ways of profitably using them. Corn starch and cassava starch are used extensively in book pa ers to improve their printing quality. The Bureau of Standar& made and tested wood-fiber book papers sized with these starches and with sweet potato starch and the latter compared very favorably with the others.
