I N D C S T R I A L S N D ESGINEERIIVG CHEAVISTRY
July 15, 1930
higher than those obtained with the falling-cylinder viscometer, while a t speeds of 1 to 10 r. p. m. the RfacMichael n values were about the same or somewhat lower than the n values obtained with the falling-cylinder viscometers. To see if the inconsistent results were due to the fact t h a t the rubber had been milled only slightly, rubber was milled 5, 15, and 30 minutes on a laboratory mill, and cements were made from each lot to a concentration of 21 per cent, using naphtha as solvent. The plasticity of the rubber was determined with the simplified Goodrich plastometer (3). The data obtained for these solutions are shown in Table IV. Table IV-Effect
CEMENT
TIME OF
MILLING
.Win. 1
2 3
5 15 30
of Milling Rubber o n Viscosity Constants of Its Solutions
PLASTICITY
OF
RUBBER
I
STEELCYLINDERS DIAM. = 0.086 CM.
n
6 . 8 1(1.58-1.68) 26.8 ! 1.14 53 3 j 1 07
GLASSCYLINDERS DIAM.= 1.106 CM.
k (10.10-13.98) 0.146 0 046
I
(1.61-1.73)
(260-310) 0.722 0.188
It is seen that the values of n and k for cement 1 are not constant for either cylinder. Cements 2 and 3 show much more consistent results, indicating that the more the rubber is masticated the more nearly Equation 7 is valid. The value of n for these cements could be checked within +0.03, and k could be checked to within about 5 per cent. The value of the exponent n cannot be determined with any degree of accuracy when the ratio of forces used is less than 2 , or when the time of fall for 15 cm. is less than 10 seconds, so the steel cylinders could not be used to test a 21 per cent solution of rubber masticated 90 minutes, the plasticity of
319
which was 79.8. However, this solution probably had a value of n very near 1, and i t was thin enough that it could be tested with sufficient accuracy in a viscometer of the Ostwald type. These viscometers have also been used to test asphalts both a t the temperature of boiling water and a t room temperature, and in all cases the fall of the viscometers could be represented by Equation 7 above. Since there is no trouble with evaporation in these materials, they can be tested quite satisfactorily even though the times of fall are nearly an hour for a distance of 5 cm. with the heavier weights. It was found that a t 25" C. the values of n for these asphalts were usually close to 1.0, while the values of k ranged from 300 to 900, giving viscosities from ::OO,OOO to 900,000 poises. These results indicate something of the possibilities of using this type of yiscometer in development as well as in control work. I n the latter type of work the instruments may, of course, be used for setting up empirical standards for a material even though the law of flow given by Equation 7 is not valid. Where the law does hold, two simple tests will determine the constants of the equation, allowing one to calculate the flowing properties of the material over a rather wide range of shearing stresses. Literature Cited Hatschek, "Viscosity of Liquids," p. 326, h-ew York, 1929. Herschel and Bulkley, Proc. A m . SOL. Testing Materials, 26, P t . 2, 621 (1926). Karrer, Davies, and Dieterich, I N D . ENG. CHEM., Anal. Ed., 2, 96 (1930). Kraemers, J . Rheology, 1, 76 (1929). Ostwald, Kolloid-Z., 47, 176 (1929). Smithsonian Tables, p . 156 (1921).
Modification of the Spinning-Film Hiding-Power Method' George S. Haslam THENEW
JERSEY ZINC COMPANY, P A L M E R T O N , PA
A technic has been developed for the measurement two can be determined for (e) of the hiding power of a dry paint film. The test is any given formula. the author d e s c r i b e s a essentially a refined paint-out test in which a uniform Accurate h i d i ng-p o w e r method for the measurefilm is applied to a contrasty panel by spinning on an methods are rapidly increasment of the hiding characoff-center spinning device. ing in i m p o r t a n c e . Paint t e r i s t i c s of pigment-in-oil Eye grading in north light is the final criterion of manufacturers are interested, mixtures, which permits the hiding and gives quite reproducible results. Dry hiding not only in improvements in evaluation of the contrastpower does not check wet measurements, as made on pigments that will permit obliterating and brightening the cryptometer, in every case and some of the excomplete hiding with one less power of the pigment alone. ceptions are discussed. coatof paint, b i t in small imThe present a&le describes provements that will permit the modifications that have been developed to measure the hiding power of paints re- greater hiding a t the same cost. Therefore, it is necessary that a test be developed which will evaluate these small differences. gardless of their pigment or vehicle composition. For many years the paint industry has been using the paintS o satisfactory single method has ever been offered for the determination of both wet and dry hiding power. The cryp- out test, in which a weighed amount of paint is applied to a tometer and other tests have been used to measure wet hiding panel on the surface of which have been painted black stripes power, and the method of Bruce, as well as the various paint- or dots. This test is extremely inaccurate and is subject to out tests, has been developed for evaluating the hiding char- misinterpretations. Unless the panel is prepared with exacteristics of the paint after drying. There is, however, no treme care, there is often a line or ridge where the black is single test that attempts to measure hiding power before and applied to the white, or, if the panel has been sanded, the after drying. The present test has been designed to obtain texture of the panel is different from place to place. -4great both of these values in order that the relationship between the deal of experience is necessary to apply paint by brushing so that a perfectly uniform film is applied, and deficiencies in 1 Received April 15, 1930 Presented before the Division of Paint brushing qualities, flow characteristics, and spreading rates and Varnish Chemistry a t the 79th Meeting of the American Chemical increase the difficulties. If the application of the paint is Society, 4 t l a n t a , Ga , hpril 7 to 11, 1930
I
N A recent article
AN.4LY TICAL EDITIOAV
320
PAINT
VEHICLE
Per cent 29.7 31 3 34 36 38 38
5 6 2 7
PIGMEST Per cent 70 3 68 7 65 5 63 4 61 8 61 3
P%zKr
Table I
%
Per cent 0 10 20 30 40 50
carried through several coats so that complete hiding is accomplished, then a decision must be made between three and four coats, for instance, and the limit of accuracy is 25 per cent. If the paint is graded on a single-coat application, the results may he even more misleading. The hiding power, or contrast-obliterating power, of a paint is determined by the film thickness necessary to give 98 per cent of the ultimate brightness and is independent of the shape of the brightness-film thickness curve. Several characteristic curves are shown in Figure 1, in which film thickness or quantity of paint per unit area is plotted against the brightness over a black background. The brightness inI
I
UL?lMArE 3RIGHTNESS
V
HIDINGPOWER
ASBESTIKE
P e r cent 100 90 80 70
Vol. 2 , x o . 3
Sq, m . / l . 7 85 7 26 6 35 5 03 4 58 4 15
BKIGHTAESS
1
Wet
Sq.f t . / g a l . 320 296 259 205 187 169
Dry
Sq. m . / l , 7.19 6.44 5.47 4.24 3.75 2.58
Sq. f t . / g a l . 293 263 223 173 153 115
Per cent 92.8 92 4 92.2 92.0 91 3 90 5
finished surface that counts, the brightness should be expressed. An effort has been made in the development of this test to use equipment that is readily obtainable by any paint producer and to use a technic that is as rapid and as easy as the paint-out test. As a matter of fact, the present test is simply a more accurate paint-out test. Apparatus
Figure 2 shows the complete set-up, which includes a small motor on which has been mounted a panel holder for spinning uniform films, a chemical balance, a strong light for grading, a drying cabinet, and a set of standards. The spinning device. an ordinary motor stirrer in this case, and panel holder, with which the panel is spun off-center, has been found t o give very uniform results even with paints of quite high consistency. It is important that the paint lie free from foreign materials or unground lumps of pigment which would cauqe streaking in the spun film. This can be accomplished by straining the paint, prior to spinning, through a filter cloth or paint filter. The grading lamp is a 100-watt bulb that permits the eye to detect contrasts that are too slight to be detected in north-sky light. The drying cabinet is merely a dustproof steel cabinet equipped with shelves on n-hich the panels are placed and kept in a horizontal position during the drying period.
PAIN? FILM THICKNESS
creases rapidly at first and then becomes asymptotic to the line of ultimate brightness. At the point A both curves haye become 98 per cent of the ultimate brightness, a t which thickness the film would have complete hiding. It is obvious from a study of these curves that a coniparison of paints I and I1 made a t one-coat thickness would give a hiding-power evaluation that does not truly represent the relative merits of the two paints. I n this case the brightening power of paint I is greater than that of paint 11, but the contrast-obliterating power is the same. It is for this reason that the work of Bruce (1) is not sound. The measurement of the contrast ratio a t some point below complete hiding and the assumption of a general formula for the curve are not justified b y the experimental data a t hand. Were the determination of film thickness made a t the point of complete hiding (the point where the brightness of the film over the black half of the panel is 98 per cent of that over the white half), the method would then yield results of indisputable value. I n evaluating the hiding characteristics of a paint i t is well to remember that contrast obliteration is only one factor and that, unless the brightness is expressed, the hiding power figure is of little value. This is easily demonstrated by taking a white paint and adding to it enough black to make it gray. The hiding power or contrast-obliterating power is greatly increased, but the brightness of the resulting surface is decreased. Since, in most cases, it is the appearance of the
Figure 2--Apparatus
for Determining Hiding Power of a Dry Paint F i l m
The panel used in this hiding-power work is a 10.5-em. glass plate on which has been glued a piece of high-gloss photographic paper. I n preparing this paper a mask of black paper, in which have been cut a number of small geometric figures, is used. The photographic paper is exposed behind this mask and then del-eloped. The result is a TI hite hackground of approximately 80 per cent brightness, on which appear the black figures. This type of panel entirely eliminates the ridge effect where the black joins the white and gives a T ery sharp line as well. The standards for grading contrast were prepared in the following manner: Photographic paper was exposed behind
July 15, 1930
I,VDUSTRIAL A N D ENGINEERIATG CHEMISTRY
the mask described above for varying periods of time so that a series of ten panels, of varying degrees of contrast. mere prepared. This could be carefully controlled by cutting down the intensity of the light in the printer so that i t required approximately 40 seconds t o get complete exposure of the emulsion. With this illumination it was possible to get, with 1 or 2 seconds exposures, contrasts that were imperceptible when viewed in north light and just visible in the strong light.
321
a film on the panels, cutting the contrast considerably before spinning the final film. This is due to the skin-drying and wrinkling which occurs when too thick a coat is allowed to dry. These panels are then allowed to dry from 24 to 48 hours, depending on the type of paint, and are weighed as are the two panels containing the weighed amount of wet paint. From these weights the amount of wet paint which has been applied can be calculated. The panels are again compared with the standards under the light and the values recorded. This method of grading offers an opportunity of observing the decrease or increase in hiding power on drying if any occurs. Complete hiding has been established by grading the standards in bright north light and under the strong light; that contrast which can be observed to be rather strong under the lamp and is not visible in north light has been selected as having complete hiding. The numerical evaluation obtained by grading against the standards is plotted against the rate a t which the paint has been applied to the panel (square feet per gallon or square meters per liter) and a horizontal line drawn a t the numerical evaluation of complete hiding. If the film has been correctly spun one point will fall above the line, one below, and t h e third point either above or below but fairly close to the line. A curve is drawn through the three points and the amount of paint to give complete hiding is interpolated. The brightness of the films is then measured on the Pfund colorimeter or some other photometer and the hiding power expressed in square feet per gallon (square meters per liter) at a stated brightness.
(SOFT/ GAL.)
A series of exposures was prepared by varying the exposure from 1 to 10 seconds. This series was augmented by panels prepared during the work, which had contrasts that fell between the prepared panels, so t h a t there were about twentyfive panels in the final series. This type of panel has a very definite advantage in that i t conforms to the specifications set u p by Sub-committee VIII, Committee D-1 of the A. S. T. >I., in a tentative definition of hiding power. T h a t definition, Khich has been accepted for this article, is as follows: Hiding power of a paint is that quantity of paint that must be applied to a given area of impervious black and white background, the white to be 80 per cent * 2 per cent brightness and the black to be less than 8 per cent brightness, so as t o bring the brightness over the black background to within 98 per cent of that over the white background and shall be specified as being measured wet or dry. Experiment
.
Five panels are prepared by mounting the contrasty photographic print on glass and weighing. TWO grams of paint are weighed out on each of two of them, the weighing being done as rapidly as possible to minimize the loss of weight by evaporation of volatiles. After weighing, these two panels are placed in the drying cabinet. A film is then spun on one of the other three panels and graded at intervals during the spinning until the small geometric figures are just visible under the grading lamp. It is then compared with the standards and a numerical evaluation of the contrast given it. The same procedure is followed on the other two panels. An effort is made to have the films of such thickness that one is definitely visible in north light, a second is definitely invisible in north light, and the third about on the border line. In some cases, where a particularly thick film is required for complete hiding, i t has been found necessary first to spin
.B PERCENT INERT
It is admitted that the human eye as the final judge introduces a variable when more than one operator grades t h e panels, but those variations have proved to be very small when two operators who have had a little experience are allowed t o check against one another. Discussion of Data
Table I shows the data obtained on a series of paints prepared b y adding increasing amounts of inerts to a high hiding pigment and varying the pigment-vehicle ratio to maintain equal consistency. It will be noted that as the amount of inert is increased the total amount of pigment that could be incorporated is decreased and that the hiding power decreases for these two reasons. It is also interesting to note that the percentage decrease in hiding power or drying becomes greater as the percentage of inert is increased. This may be due to the presence of
ANALYTICAL EDITION
322
the inert, but i t is more likely to be due to the increase in the amount of volatile in the paint and the greater contraction of the film on drying. Whether or not this is a general relationship that exists for all pigments has not been established. Figure 3 shows the experimentally determined curve. The points below the line of complete hiding represent panels which when viewed with bright light showed slight contrast
5’01. 2, KO. 3
and when viewed in north light met the requirements of coinplete hiding. The panels shown above the line showed contrast in north light. Literature Cited Bur, Standards, Tech, Paper 305. (ZJ Haslam, IND. E X G . CHEM , Anal. ~ d .a ., 69 (1930).
A Unified Conductometric Method for the Determination of Ash in Refinery Sirups’ F. W. Zerban and Louis Sattler h’EW YORK SUGAR
TRADE LABORATORY, 80 SOUTII STREET, NEW Y O R Kh‘, , Y.
AW beet sugars produced in Europe show a fairly constant relationship between chemical ash and electrical conductivity (1). The ash can therefore be found easily b y multiplying the specific conductance by an average factor, termed the C-ratio. With raw cane sugars the case is not so simple, because the C-ratio varies from factory to factory and from country to country ( 2 ) . Unless the origin of a sample and the C-ratio for the particular factory or district are known, the ash content cannot be calculated from the specific conductance of the solution alone. It has been possible, however, to devise a conductometric method that applies to all raw cane sugars investigated so far ( 3 ) . This requires a second conductance determination in the presence of acid, b y which the conductance of the solution itself can be corrected. The corrected conductance equals 0.913 K 193.5 - 0.1 K1, where K is the specific conductance X lo6 of the solution itself, and K 1is that of the solution with added acid, under specified conditions. The corrected conductance, multiplied by the factor 0.001757, gives the percentage of ash for any raw cane sugar, irrespective of its origin. It has further been found that the same formula, referred to in this paper as the “simple conductometric formula,” also holds for sirups and molasses produced in the raw cane sugar factory, provided the experimental conditions are chosen so as to take into consideration the different range in the ash content of these products and the difference in the non-electrolyte concentration ( 4 ) . But the simple conductometric formula with the factor found for raw sugar products does not give correct results for refinery sirups. The factor b y which the corrected conductance must be multiplied is lower for final refinery sirups than for raw sugar molasses, still lower for unfiltered sirups, and lower again for filtered sirups. It also varies for different refineries. Thus the simple conductometric formula is found to be inapplicable unless the nature and origin of the refinery product are known. It is readily seen that a universal formula that can be used for any cane product irrespective of its nature or origin is desirable. The efforts in this direction, described in the present paper, have been partly successful. Since the factors to be used with refinery sirups decrease as the extent of the bone-black treatment increases, it was concluded that the bone-black filtration must in some way affect the relative proportions of the electrolytes. The known effect on the anions has already been corrected for
R
+
1 Received April 16, 1930. Presented before the Division of Sugar Chemistry at the 79th Meeting of the Smerican Chemical Society, Atlanta, Ga., April 7 to 11. 1930
by the simple conductometric formula. It would seem that in the bone-black treated products the cations also are involved. I n the study of raw cane sugars it had been observed that the conductometric titration curves obtained upon the addition of alkali ran very close together. But a solution of potassium chloride with enough sucrose added to simulate a raw sugar gave a much steeper displacement curve becacse the less mobile cations were absent. A few examples cited in the preceding paper ( 4 ) tended to show that raw sugar molasses would give a displacement curve with alkali similar to that of the raw sugars, but that the curves obtained with refinery sirups were steeper and approached that of the potassium chloride-sucrose solution. These findings pointed a way to a solution of the problem of refinery sirups. First, it was necessary to shorn- that treatment of raw sugar molasses with bone black actually has the effect observed with refinery sirups. Second, conductometric titrations with alkali had to be made on all the samples on hand in order to ascertain whether or not there exists a quantitative relationship which can be utilized to establish a more general conductometric formula. Experimental Procedure
The tests n i t h bone black were made as follows. ,4solution of 300 grams of raw sugar blackstrap in 2 liters of n a t e r was prepared and filtered through a mat of filter paper arid asbestos. The cheniical ash (sulfated ash minus 10 per cent) was determined in duplicate portions of the solution and calculated back to the original blackstrap concentration. Conductkity determinations were made on solutions containing 0.5 gram of the original molasses in 100 nil. l h conductance was measured on the solution itself, again after addition of 5 ml. of 0.25 X hydrochloric acid to 200 nil. of solution, and once more after addition of 5 ml. of 0.25 potassium hydroxide solution, instead of acid, to 200 rill. of a fresh portion. The rest of the original blackstrap solution was digested with 1000 grams of refinery bone black a t 80” C. for 15 minutes. The solution was then decanted from the b m e black, the latter sucked dry, and the liquid filtered. Ash and conductivity determinations were made on the filtrate as described above, using equivalent concentration3 of solids. The bone-black treatment was then repeated on the remaining solution and the same analyses made of the filtrate as before. These experiments were made with two blackstraps, one of Porto Rican and the other of Cuban origin. A\
,
