INDUSTRIAL AND ENGINEERING CHEMISTRY
1196
Vol. 21, No. 12
Relation of Oil Absorption to Consistency of Pigment-Oil Pastes’,’ R. V. Williamson EXPERIMENTAL S T A T I O N , E. I. DU PONT DE XEMOURS &
A study of the relation of oil absorption to apparent viscosity at different rates of shear has been made for five lithopone pigments and a single linseed-oil vehicle with an acid number of 10.4 and for a single zinc-oxide pigment and several linseed-oil vehicles which varied in acid number over a considerable range. The results show that the general rule that high oil absorption for a given pigment indicates high consistency for the ground pigment paste does not hold for the pigments studied. However, the consistencies of the zinc-oxide mixtures made by stirring the mixtures for five minutes with a spatula are shown to follow in a qualitative way the same order as the oilabsorption values.
COMPAKY,
WILMINGTON, DEL
The fact that no simple relationship exists between oil absorption and consistency is explained as follows: Two factors which act in opposition to each other-i. e., degree of wetting and soap formation-control the values obtained from oil-absorption measurements, whereas the consistency of the ground pastes is controlled primarily by soap formation alone. This factor acts continuously in the same direction as the concentration is increased. The degree of wetting of the pigment by the oil is not an important factor in the consistency of the ground pastes, because the energy supplied in the grinding process is sufficient to displace adsorbed films of moisture and air and cause the oil to wet the pigment.
...... HE relative consistencies of pigmenboil dispersions is usually considered to be in the same order as the oil absorptions of the pigments. For example, a high oil absorption is usually considered to indicate a high consistency for dispersions of the same concentration. Some laboratories have found so many exceptions to this general rule that they no longer consider that any general relationship exists between oil absorption and consistency. Baldwin (1) has concluded that no general relationship exists between oil absorption, yield value, and mobility. The present study was undertaken to determine whether or not these apparent inconsistencies could be explained on the basis of the variation in consistency of some pigment-oil dispersions with variation in rate of shear or flow.
T
Procedure
-
Prg/2; Shearing c ADDarent rate of shear 4d/rt where P = pressure in grams per square centimeter, 7 = radius of capillary in centimeters 1 = length in centimeters of advancing column of paint in capillary, d = distance in centimeters over which column advanced during the time, 1 , which was measured in seconds. Relative viscosity as used here = Load in grams used to cause cylinder t o revolve 1. time in seconds for 10 revolutions of cylinder
__
Materials
The pigments used were different types of lithopone and a single commercial grade of zinc oxide. The vehicles were mixtures of alkali-refined linseed oil and linseed-oil acids in different proportions. Data
The oil-absorption and consistency measurements were made in a room which was automatically maintained a t a temperature of 25” * 1’ C. and a humidity of 50 * 1 per cent. The data for the apparent viscosities were obtained with a Bingham-Murray plastometer. The relative viscosity data shown in Plate I11 and Table I were obtained with a Stormer viscometer. Apparent viscosity =
minutes. Two end points were observed. The first was the point a t which the pigment could be made to adhere into a solid mass. The mixture was still dry and crumbly a t this stage. The second end point was the point a t which the mixture changed from a crumbly mass into a paste that stuck to the spatula slightly or “tailed” when the ball of paste was shaken from the spatula. The results were recorded as the grams of oil to wet 100 grams of pigment.
The oil-absorption measurements were made by the spatula method. Approximately 75 per cent of the oil required to saturate the pigment was first added to the pigment. This was worked into the pigment on a smooth glass plate until an apparently uniform mixture was obtained. The mixture was still in a dry powder form a t this stage. Oil was then added in decreasing quantities with mixing after each addition of oil. The total time of mixing was approximately 20 1 Presented under the title “A Study of the Relation of Oil Absorption to Plasticity” before the Division of Paint and Varnish Chemistry a t the 78th Meeting of the American Chemical Society, Minneapolis, Minn., September 9 to 13, 1929. 2 Contribution No. 25 from Experimental Station, E. I. du Pont de Nemours & Company.
The lithopones used for the data in Plate I were furnished by the Grasselli Chemical Company, and the data from their control laboratory showed that sample I11 was an exception t o the general rule that low oil absorption is associated with low consistency. Their consistency measurements were made with a modified Stormer viscometer a t a single rate of shear. It was thought that an explanation of the abnormal behavior of sample I11 might be found if the apparent viscosities of these samples were determined a t different rates of shear. The oil-absorption values of these pigments were determined in this laboratory with an oil made from a mixture of alkalirefined linseed oil and linseed-oil acids. The mixture had an acid number of 10.4. Pastes were made by grinding 2 parts of pigment with 1 part by weight of oil on a buhrstone mill. The apparent viscosities of these pastes were determined over as wide a range of shearing rate as was possible with the Bingham-Murray plastometer. The apparent viscosities are plotted against apparent rate of shear in Plate I. The curves show the same relative order a t all rates of shear; therefore the apparent viscosities a t any particular rate of shear may be compared with the oilabsorption values. Such a comparison is made in Table I, Plate I. If one considers the first end point, samples I11 and V do not conform to the general rule that the order of consistency is the same as the order of oil absorption. If the second end point is taken as a measure of the oil absorption for these pigments, then samples IV and V come nearer
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1929
the right order, but sample I11 is still decidedly out of its proper order.
Table1
ApparentVi,. Oil Rbsorption . Rate q Shear-u, A~Cohercs%teTnlr 27.5
12.2
1n.h
41 .o
13.C
1q.b
. .
qs.0
13.1
14.6 .
117.0
11.2
1b.q .
117.0
14.5
1b.0
.
1197
of 10, and the oil-absorption values. The oil-absorption values in Plate I follow the general rule that high oil absorption corresponds to high consistency, but in Plate 11, with the exception of sample IV, this general rule is reversed. The order of consistencies shown in Plate I1 for the ground pastes was a surprise to the writer, because the consistencies of the pigment-oil mixtures before being passed through the buhrstone mill appeared to be in the same general order as the oil-absorption values. New pigment-oil mixtures were made as in preparation for grinding except that equal parts by weight of pigment and oil were used. These mixtures were stirred with a spatula for 5 minutes, and the relative viscosities were measured a t approximately the same rate of shear by means of a Stormer viscometer. The capillary viscometer could not be used for these measurements, because the pastes were too stiff, and the small lumps which were present would have clogged the capillary. Plate I11 shows the oil-absorption values, relative viscosities of the pigment-oil mixtures, and the apparent viscosities of the pastes after grinding, plotted against the acid number of the vehicle. Curve I shows that the oil absorption decreases with increase in acid number until a value of about 10 is reached and then increases as the acid number increases.
Plate I-Relation of Apparent Viscosity a t Different Rates of Shear to Oil Absorption for Different Lithopones
In order that the consistencies might be in the same order as the oil-absorption values, curves I1 and I11 would have to coincide approximately a t Rome particular rate of shear. Such a condition is illustrated by curves 111’and 111“for a rate of shear of 20. Curve I11 does not show any abnormal relationship to the other curves in Plate I; therefore, the abnormal relationship of the consistencies relative to that of the oil-absorption values cannot be explained on the basis of the variation in apparent viscosity with rate of shear. Plate I1 shows a set of curves similar to those in Plate I. The pigment used for these dispersions was a commercial grade of zinc oxide, and the vehicles were linseed oils with different acid numbers. As in Plate I, the curves may
10
I
0
10 8 4 30
9
50
60
70 BO
q0
IM 110 120 130
1
Plate 111-Relation of Viscosity of Ground and Mixed Zinc-Oxide Pastes t o Oil Absorption
Plate 11-Relation of Apparent Viscosity at Different Rates of Shear to Oil Absorption for Zinc Oxide Dispersed in Linseed 011 of Differe n t Acid Numbers
be compared a t any convenient rate of shear, because they show the same order of apparent viscosity a t all rates of shear. Table 11, Plate 11, shows the acid number of the vehicle, the apparent viscosities a t an apparent rate of shear
Curve I1 shows the oil-absorption values found bv using the “tailing value” as end point. This curve show; that-very little difference exists between the values for the two end points until the minimum point on the curve is reached, but with increasing acid number the difference in the values obtained by the two end points becomes increasingly great. It was not possible to obtain a soft paste with the vehicle having an acid number of 167, because the solid mass formed when the first end point was reached could not be mixed by means of a spatula with more of the high-acid oil. Curve I11 shows that the consistency curve for the pigment-oil mixtures prepared by stirring for a short time with a spatula has the same general shape as the oil-absorption curve. Curve IV shows that the order of consistencies changes after grinding to a continuous increase in consistency as the acid number of the vehicle is raised. These experiments show that no relation comparable to the general rule that high oil absorption corresponds t o high
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1198
consistency exists for dispersions of the zinc-oxide and linseedoil vehicles used in these experiments. Explanation of Results The oil-absorption values of the pigment for the different oils and the consistencies of the spatula-mixed pastes are dependent upon two opposing factors-degree of wetting and soap content. An increase in the degree of wetting reduces proportionally the oil-absorption values of the pigment for the different oils and the consistencies of the spatulamixed pastes. An increase in the amount of soap formed causes an increase in these properties. A neutral or very low acid oil does not readily wet the pigment because of a film of adsorbed moisture. A small increase in acid number of the oil causes a marked increase in the degree of wetting with the formation of only a small amount of soap. The result of these opposing factors for a slightly acid oil is a decrease in the oil absorption of the pigment and in the consistency of the spatula-mixed pastes. The increase in degree of wetting with increase in acid number approaches a maximum a t an acid number of 5 or 6. Further increase in acid number does not cause much increase in degree of wetting, but the amount of soap formed increases continuously as the acid number is increased. Therefore, the oil absorptions and consistencies of the spatula-mixed pastes which result from the use of linseed oil with an acid number slightly above 5i. e., from 5 to 15-are almost constant, because the two opposing factors of wetting and soap formation, for oils of these acid numbers, approximately balance each other. As the acid number is increased still further, the effect of soap formation relative to degree of wetting becomes increasingly greater. The oil-absorption values of the pigment and the consistencies of the spatula-mixed pastes increase, therefore, more and more rapidly as the acid number of the oil increases.
VOl. 21, No. 12
The consistencies of the ground pastes, on the other hand, are not appreciably affected by the degree of wetting of the pigment by the oils, because the energy supplied in the grinding process is sufficient to cause the oil to displace the film of adsorbed moisture and wet the pigment. The primary factor which influences the consistencies of the ground pastes is the amount of soap formed in the paste. Since an increase in acid number causes increase in soap concentration, and an increase in soap concentration causes an increase in consistency, the consistencies of the ground pastes must increase continuously as the acid number of the oil increases. Almost any relationship between oil absorption and consistency can be shown to exist by an inspection of Plate 111, if the conditions for obtaining the experimental data are properly chosen. For example, if one oil with an acid number of 5 and another with an acid number of 20 were chosen as the vehiclcs to be used with the zinc oxide employed in these experiments, an oil-absorption value of approximately 12 would be obtained in each case. The apparent viscosities of the ground pastes, however, would be, respectively, about 120 and 220. The difference in apparent viscosities is probably due to the unequal concentration of soap in the two pastes. Since different amounts of soap-forming constituents are known to exist in different types of lithopone, it is possible that the results obtained with the lithopone pigments are due to differences in the amounts of soapforming materials present in the samples. Lack of information a t present in regard to the chemical composition of these pigments prevents testing this hypothesis on these samples. The study will be continued on specially prepared lithopones. Literature Cited (1) Baldwin. IND. ENG.CHEM, 21, 1121 (1929).
Semi-Plant Scale Production of Gluconic Acid by Mold Fermentation' 0.E. May, H. T. Herrick, A. J. Moyer, and R. Hellbach COLORA N D FARM WASTEDIVISION, BUREAU OF CHEMISTRY A N D SOILS, WASHINGTON, D. C.
HE formation of glu-
Gluconic acid has been produced on a semi-plant quantities of citric and oxalic conic acid as a result scale by the action of Penicillium luteum purpuroacids were also formed. Bernof mold metabolism gefwm on solutions of commercial dextrose. Studies h a u e r , however, has develwas first reported by Molliard have been made of the effect of surface-volume ratio, oped a strain of Aspergillus in 1922 ( I O ) . In the course subsurface agitation, and PH. A high grade of aluminiger which, in the presence of an investigation on the pron u m has been established as a suitable material for ferof calcium carbonate and unmentation Pans. Using Seven aluminum Pans, 43 by 43 duction of citric and oxalic acder definite conditions of culids by a culture of Aspergilby 2 inches (108 by 108 by 5 cm.), about 36 kg. of gluconic ture, yields gluconic acid alZus niger on 5 per cent sucrose acid have been obtained on a n Il-day cycle, a yield cormost exclusively (3). solutions Molliard noted that responding to slightly more t h a n 57 per cent of theory. I n the course of a survey of the total acids recovered as the action of a large number of insoluble calcium salts from the fermented solutions did fungi on solutions of commercial dextrose it was found that an not account for the total titratable acidity. A third acid organism of the Penicillium luteum purpurogenum group was which he then isolated from the mother liquors was capable of oxidizing this sugar to gluconic acid in promising identified as d-gluconic acid from the properties of t h e yields without the formation of other acids (9). Further calcium and cinchonine salts, and the phenyl hydrazide, and study of some of the variables affecting the fermentation, by elementary analysis. Within the last five years Butke- such as temperature, concentration of dextrose, and composiwitsch ( d ) , Falck and Kapur ( 5 ) ,Bernhauer (Z), Wehmer ( I d ) , tion of the nutrient salts, resulted in increasing the yield to Takahashi and Asai (IS),and Amelung ( I ) have reported around 60 per cent of the theoretical and demonstrated that finding this acid among the products resulting from the mold even under widely varying conditions of culture no apprecifermentation of solutions of dextrose, sucrose, and maltose. able quantities of other acids were formed (6). In most cases the yields were comparatively low and varying I n 1919 Herzfeld and Lenart suggested that gluconic acid * Received October 25. 1929. might prove a valuable addition to the group of aliphatic
T
