66
A iVAL Y TICBL E DI TI0,V
layer. It will be observed that the experimental and calculated values differ by as much as 54 per cent. Table I1 also includes hiding-thickness values as determined with the Pfund cryptometer. With the exception of the zinc oxide paste, which checks very closely, there exists about a 10 per cent difference between the value obtained from the experimental curve and that given by the crgptometer. This difference is in a direction which would indicate that in the case of the cryptometer the end point is realized a t a point where the wedge film is somewhat more than 98 per cent as bright as the infinitely thick layer. It is not impossible that, under the conditions existing in the cryptometer, the eye is capable of detecting smaller differences in brightness than the generally accepted 2 per cent limit. Brightness-film thickness curves were determined for basic carbonate of white lead and a small-particle-size zinc oxide-rubber pigment (Figure 4). These two pigments are of greatly different particle size and shape and therefore should differ considerably in light-reflecting and -scattering properties. The crossing of these two curves makes apparent the possibility of error that xould arise in assuming the empirical equation in this case. While this may be a rather severe test of the rigidity of the equation, it a t least indicates that the empirical formula has definite limitations. These curves mere not carried to complete hiding thickness because
VOl. 2, s o . 1
of the limitations imposed by the thickness of the spacers used in the wedge a t the time. Conclusions
By employing the channeled-spectrum phenomenon in making thickness measurements, the writers believe they have developed an extremely accurate method for the determination of brightness-film thickness curves. Although only applicable t o wet paints, the method should be of value in investigating the brightness and hiding power characteristics of pigments and in making comparative studies of paints. Although the theoretical and empirical equations discussed seem to express, in a general manner, the brightnessthickness relationship of paint films, the data presented indicate definite limitations. It would not seem safe to employ such equations without a more complete knowledge of these limitations. It is hardly to be expected that a theoretical or empirical formula, developed in the manner these equations have been, should express faithfully the complicated optical system presented by a suspension of pigment in oil. Literature Cited (1) Bruce, Bur. Standards, Tech. Papev 306. (2) Pfund, J . Optccal S O L A . m . , 20 (March, 1929).
Measurement of the Leveling Properties of Paints and Enamels' George S. Haslam and Lester D. Grady, Jr. THE NEW JERSEY
ZINC COMPANY, PALXERTOX, PA.
This paper shows that the leveling properties of a and simple method for the ONSIDERABLE diffipaint are not entirely expressed by consistency measdetermination of the leveling culty is experienced in properties for plant control. urements. The du Nouy tensiometer is suggested as g r a d i n g t h e leveling an instrument for measuring leveling properties. A Leveling is dependent on qualities of paints and enamcomparison of tensiometer results with results of five that property of a paint that els from a measurement of consistency measuring apparatus shows the value of permits flow after application their consistency characterthe tensiometer in measuring the leveling characterand results in the eradication istics. While it is true that istics of paints. of brush marks and other in general a paint of high Leveling is dependent on forces of flocculation and irregularities of the surface. yield value and low mobility forces of surface tension. The effect of these forces, It is a function of both the has poor leveling, and one flocculation of the pigment which are apparent only at low shearing stresses, of low yield value and and the surface forces of the h i g h m o b i l i t y h a s good appears to be measured by the du Nouy tensiometer. paint which govern flow after leveling, this is not always the cage. I n other words,' it is conceivable that an enamel the high shearing forces of -application have been reof heavy consistency may have better leveling properties moved. This force which produces leveling is not apparthan one of thinner consistency, so that consistency measure- ent during consistency measurements, and its effect is ments would lead to erroneous conclusions if used t o predict masked in these determinations. Since the force which causes leveling is essentially a surleveling qualities. The apparatus developed by Bonney (1) for the measure- face force, the du Xouy tensiometer was used in its measurement of this property gives a good indication of the leveling ment. The results give a numerical value to the leveling properties, provided the consistencies of the paint are the property which follows very closely the grading by a paintsame or nearly so. Large differences in the plastic proper- out test. ties, however, so seriously affect the results as t o render them Experimental Procedure invaluable. The technic developed for the operation of the du Xouy The work of Williamson ( 5 ) requires a very accurate determination of the plastic flow curve and should give a true tensiometer (4) was essentially the same as that used in the evaluation of the leveling properties. However, the present determination of surface tension. The circular rider was paper is an effort to give the average paint producer a rapid brought in contact with the paint surface and completely wet by the paint. The torsional force on the wire was inPresented before t h e Division of Paint 1 Received September 9, 1929. creased slowly a t a constant rate until the ring broke away a n d Varnish Chemistry at the 78th Meeting of t h e American Chemical from the surface, and the angle of rotation necessary to reSociety, Minneapolis, Mino., September 9 to 13, 1929.
C
I S D C S T R I A L A N D ENGIiYEERI,VG CHEMISTRY
January 15, 1930
lease the ring was taken as a measure of the surface property of the paint which governs its leveling tendencies. In the investigation of this series of paints several consistency-measuring instruments were employed. A vacuum plastometer, which is a modification of the Bingham and Murray plastometer (S), was used. From measurement of the time of flow through a capillary tube and calculations of the shearing stresses a t regular intervals, the yield value and mobility were determined. As a relative measure of consistency on a given class of paints, it was found that the length of time required to pass through the tube under a constant vacuum was sufficient. This figure, designated as consistency index, is the time required for a paint to flow
centage readings of all three pigments increase a t different rates. With a low carbon-black content a high tensiometer reading is obtained, while a considerably larger volume content of lithopone is required to give the same tensiometer reading. It is well known that carbon black produces a "short" paint and that lithopone produces the least "shortness" of the three pigments. It is for the measurement of this property called "shortness" that the du Kouy apparatus is well suited. It is particularly applicable in the evaluation of lithographing inks and linoleum printing paints, where the ability of a paint to "level out" without "flowing out" is important. I n order to compare the results obtained on the du Nouy apparatus with consistency measurements, data were obtained on these general classes of commercial paints: A-Outside paints (1) Zinc paints (2) Lead paints B-Flat whites
AeVmr
Pr,?ctNr i
51
/b
voi W M T L_I
.2d
Figure 1
25 cm. through " a catdlarv of radius 0.054 cm. under E acuum of 6 inches (1.5 cm.) of mercury. It is a purely relative value and cannot be compared with results obtained under other conditions of vacuum or capillary dimensions. Results were also obtained on a standard Gardner mobilometer ( 2 ) . The intercept and slope of the curve, obtained by plotting weight against reciprocal of time, were taken as a measure of the yield value and mobility, respectively. Two other simple tests, the tube test and tail length test, were also included. The tube test consisted of immersing a tube of known inside diameter into a paint and a t the end of a specified time measuring the distance from point of immersion to the height of the paint inside the tube. After correcting for capillarity, the value thus obtained is a measure of the yield value of the paint. The tail length test measured the length of vertical flow of a specified number of drops of paint after a given time. This test is used in some paint laboratories and is considered a measure of the composite flow properties of paint. I
"
67
C-Enamels ( I ) Regular enamels ( 2 ) Four-hour enamels
Four samples were taken a t random in each class, practically all of them being in the range of painting consistency. A series of zinc oxide paints was also made up, the consistency being varied by varying the pigment-vehicle ratio, and included in this correlation. All the samples were run on the five instruments simultaneously, under the same conditions of temperature and humidity. All were wellaged samples, having been on the shelves for more than a year. The results are giyen in Table I, and for ease of visualization the values obtained by one method have been plotted against those obtained in another. I n Figure 2 - 4 tube test depression is plotted against the intercept on the pressure axis of the mobilometer curve, showing, in general, that an increase in the yield value as determined by hhe tube test is accompanied by an increase in the intercept. In Figure 2-B the tail length is plotted against the consistency index determined on the plastometer. Here again we find a fair agreement in trends. Figure 2-C, in which the mobilometer intercept is plotted against the vacuum plastometer value,
I (
Results and Discussion
Figure 2
Varying percentages of three pigments were ground in raw linseed oil and the resulting mixtures were measured on the du Noliy tensiometer. Figure 1 shows the tensiometer readings plotted against the percentage of pigment. Below a composition of 4 per cent pigment the tensiometer reading is essentially the same as i t would be for a surface tension determination on the oil, whereas above this per-
shows that the consistency index increases with the yield value. Similar curves would result if this value were plotted against the tube test results. I n Figure 2-D the slope of mobilometer curve is plotted against the plastometer results, showing that a mobility decrease accompanies an increase in length of time required to flow through the capillary.
A,VALYTICAL EDITION
68
These data show that the results on any one of the instruments is comparable with the results on any other and that the flow characteristics are relatively the same regardless of the type of instrument used. Although the results of the painters' grading show quite good agreement in any given class, they are not concordant when they grade an enamel against an outside paint of the same flow characteristics. There is evidently a factor, or factors, that has not been effective in the determination of the flow constants. If we now compare the observations using the du Kouy tensiometer and those obtained with the flow constants, a n interesting correlation is apparent. The paints fall in three general classes-namely, the outside paints with high tensiometer readings, the enamels with low readings, and the flat whites intermediate between the two. Figure 3 shows the du Kouy readings plotted against the flow properties. It is t o be noted that as the flow constants indicate an increase in the consistency there is an increase in the tensiometer reading. However, the principal value of these curves is the differentiation between paints of the same flow constants. It was possible to select two paints having the same flow properties but widely different surface properties. and likewise two paints with the same surface properties but unlike flow characteristics. It was interesting to put a drop of one of these paints on the other and observe the action. I n the case of the two with unlike tensiometer readings and the same flow characteristics there was little, if any, admixing; but in the case of the two with like tensiometer readings the admixing was a t once apparent. Table I
SAMPLE
PLAS.
DIT
421 108 366 119
66.0 55.5 65.0 56.0
1 4 2 3
650 280 29 239
60.0 53.0 51.5 52.0
1 2 4 3
597 53 37 8
48.0 47.5 41.0 39.5
1 2 3 4
MOBILTOMEGER NOUY PAINTOMETER MOBIL- CONSIST- TENSI- ERS' ENCY OMETER JUDGTAIL TUBE INTER- OMETER INDEX READING MENT b LENGTH TEST CEPT SLOPEa Sec. Degrees Cm. Cm. Grams LEAD PAlNTS
A2 A3 A4
A1
2.69 5.00 3.83 4.40
2 35 1.33 1.90 1.00
70 51 67 44
B1 B2 B3 B4
3.91 4.40 4.80 4.39
1.25 0.80 1.00 0.75
85 83 42 82
C1 C2 C3 C4
5.84 14.21 16.01 26.28
0.50 0.25 0.00 0.00
0.066 0.141 0.074 0.137
FLAT PAINTS
0.070 0.082 0.088 0.100
(-HOUR ENAMELS
62 26 18
0.006 0.038 0.037 0.153
0
ZINC PAINTS
D1 D2 D3 D4
5.48 3.43 3.40 3.83
1.80 2.15 2.70 2.15
El E2 E3 E4
2.77 14.18 3.66 7.85
2.60 0.00 1.55 0.00
29 36 44 36
0.032 0.043 0.034 0.051 0.006 0.008 0.005 0.005
T. H.c 174 H. 565
T.
T . H .
56.0 46.0 52.5 47.5
2 4
1 3
PAINTS
T. H. 0.025 5.40 T. H. 0.054 2.95 T. H. 278 50 0.078 4.27 1.40 20 8.53 24 0.129 1.00 9 0.149 0.20 a 17.61 a Mobilometer slope = lOO/T(P intercept). b Heaviest test No. 1. c T. H. = too heavy.
F2 F3 F4 F5 F6
-
yield value and lowest mobility. Practical paint-out tests, however, showed sample 3 to have the best leveling qualities and the other four samples to be about the same. Tensiometer results fell directly in line with the practical paintout tests, sample 3 having lowest tensiometer reading and the other four samples being equal.
/
I
-' ~ o s L o w L - ~N T~ i
T. H.
114.0 65,O 55.0 51.0
1 2 3 4 5
These curves show, in general, that the du Noiiy tensiometer measures a factor that is of considerable importance in grading paints from one class to another and in defining the consistency characteristics of paints in which the nature of the flow after application is of prime importance. I n order further to substantiate the claim that the du pu'oiiy tensiometer measures the leveling qualities of paints, the data in Table I1 are presented. Five enamel paints were submitted for the grading of leveling properties. Consistency measurements showed sample 3 to have the greatest
D
C S
EDT
TLBE
L
