SOME UK'USUAL PROPERTIES O F COLLOIDAL DISPERSIOSS' BY R. V. WILLIAMSOS
R e have been studying the viscous and plastic properties of dispersions for several years in order to increase our knowledge of their structure, During this time we have collected several dispersions that show rather striking characteristics when caused to flow under different conditions. The structural characteristics of these dispersions are only partially known, but t,heir flowing properties are of considerable interest to the manufacturer of colloidal dispersions, and a description of them may also interest those who are studying the problem of colloidal structure in a theoretical way. The Flowing Properties of Paints and Similar Suspensions. The paint manufacturer has known for a long time that the practical flowing and brushing behavior of paints cannot be estimated for their viscosities as ordinarily determined. This fact may be demonstrated with dispersions made by grinding 11 to 14 parts by volume of zinc oxide in I O O parts of alkali-refined raw linseed oil, blown linseed oil, and blown linseed oil thinned with turpentine until it has the same viscosity as the raw oil. The dispersions in the raw oil and in the thinned blown oil have the same apparent viscosity when stirred or when allowed to flow through an ordinary viscosity cup. Blown linseed oil is considerably more viscous than the raw oil; and the blown oil dispersion similarly exhibits a very much higher apparent viscosity than the other two. The flowing characteristics of these dispersions may be compared by painting glass plates with each of these dispersions, placing small cylinders (approximately one centimeter high and one-half centimeter inside diameter) open a t both ends on the freshly painted glass plates, filling each cylinder with the paint that was used to paint the glass plate upon which the cylinder rests, and then gently raising the cylinder and allowing the paint to flow out. The raw-oil dispersion flows out to a much smaller area than the other two although its apparent viscosity is comparatively low. The blown-oil and thinned blown-oil dispersions flow out to approximately the same area in spite of the difference in the viscosities of the two dispersions. The time required for the blown-oil dispersion to flow out is greater, however, than that required by the thinned blown-oil one. If the apparent viscosities of these dispersions are determined at different rates of flow through a capillary tube, the raw-oil dispersion shows a very high apparent viscosity at low rates of flow, but a t high rates of flow its apparent viscosity is about the same as that of the thinned blown-oil dispersion. I n other words, this dispersion shows marked plasticit'y. The other 1 Contribution No. 9 from the Experimental Station, E. I. du Pont de Semours and CO., Ino., Wilmington, belaware.
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two dispersions do not show much if any change in viscosity at different, rates of flow; i.e., they flow like ordinary viscous liquids. Photomicrographs of the specimens reveal a highly flocculated condition of the particles in the raw-oil dispersion, whereas the particles are highly dispersed in the other two dispersions. The areas covered by the dispersions as they flow out on the painted glass plates appear to be a function of the size of the flocculates. The degree of plasticity of the dispersions is also directly related to the sizes of the flocculates. One may say, therefore, that the flowing properties of a dispersion can be estimated from its plastic properties.? This is qualitatively true, but unfortunately scientists have not been able to find a satisfactory method for quantitatively expressing the plastic properties of disper,’sions. A mental picture of some of the factors that determine the areas of the flow-outs may be drawn as follows: A s the oil film advances the largest flocculates settle to the bottom , h t , and the friction on the glass plate prevents them from advancing farther. The smaller flocculates, however, are carried still farther. Finally the film advances until the friction of the smallest flocculates between the glass plate and the air-oil surface is great enough to overcome the “head” or stress due to the thickness of the oil film. If the forces of flocculation are sufficiently great to prevent the flocculates from being broken into smaller ones under this stress, the advance of the particles in the film is stopped. The control of the plastic characteristics of paints is important t,o the paint, manufacturer because to a large extent they determine the character of the surface of the paint film. In enamels, the plasticity must be very low so that brush marks and any uneven surface characteristics will disappear, and the paint will flow out to a mirror-like surface. I n flat wall-paints, printing inks, and paints for printed linoleum, a certain amount of plasticity is essential. Thixotropic Dispersions. The term thixotropy has been used by Freundlich3 and others to describe that property of dispersions that enables them to change repeatedly from a gel form to a liquid form under the stress of agitation and then to revert to the gel form again when allowed to remain at rest. The first thixotropic dispersion that I observed was prepared by Marion Yeasey a t the Cniversity of Wisconsin in 1 9 2 2 . This was an aluminum hydroxide dispersion. Freundlich and his eo-workers have described the conditions for preparing thixotropic dispersions of ferric oxide and aluminum hydroxide. I have observed several thixotropic dispersions of pigments. One of these was presented to me by C. K . Sloan of this laboratory. It is a dispersion of gas black in petroleum hydrocarbon which contains an organic dispersing agent. This dispersion remains very fluid for an hour or more after shaking, but it sets to a solid gel in a few hours. The time re-
* For further discussion of this point, see Killiamson: Ind. Eng. Chem., 21, 1108; Williamson, Patterson, and Hunt: I I I I (1929). Freundlich: Kolloid-Z., 46,289 (1928).
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quired for gelation can be shortened by reducing the amount of volatile vehicle until the dispersion will set immediately after shaking. This dispersion has been kept in the laboratory for a year, and the thixotropic property hati been demonstrated many times without any apparent change in this property of the dispersion. A dispersion with similar properties may be prepared by mixing zinc oxide (such as is used as a paint pigment) with gasoline in the proper proportions. If such a dispersion, contained in a small can, is shaken vigorously, it produces a sound resembling that produced by a very fluid liquid under similar circumstances. The dispersion does not flow out, however, if the shaking is stopped, the lid immediately removed, and the can turned bottom side up. I n this case, the effect is probably due to the highly flocculated condition that results from the poor wetting of the zinc oxide by the vehicle. This effect can be completely changed by adding a few drops of some good wetting agent, such as blown linseed oil. The dispersion then remains perfectly fluid after shaking, but the pigment settles rather rapidly to the bott’om of the container. Inverted Plasticity. The thixotropic dispersions described above are examples of highly plastic dispersions. Such dispersions are characterized by a very high apparent viscosity when stirred slowly and a lower viscosity when stirred rapidly. Several dispersions have been brought to my attention that show an inverted form of plasticity, compared to that described above. These dispersions flow readily when stirred slowly, but become quite stiff when stirred rapidly. I have used the term “inverted plasticity” in referring to this property. Victor Cofman called my attention to the fact that a dispersion of corn-starch and cold water in approximately equal proportions by weight shows this property to a marked degree. A striking demonstration can be made by inserting the fiuger into the dispersion, moving it around slowly to show the fluid character of the dispersion, and then jerking it out quickly, whereupon one receives the impression that the dispersion has solidified around the finger. If the concentration is right and the beaker is held firmly on the table, the whole mass of the dispersion will tend to be lifted as a solid. As soon as the violent agitation is stopped, the whole mass becomes fluid again. The peculiar properties of this dispersion can also be demonstrated by replacing the cylinder on a Stormer viscometer with a double-pronged stirrefi and determining the rates of stirring of the dispersion with different weights on the stirring device. For ordinary viscous liquids or dispersions the rate of stirring is proportional to the load on the stirring device. If the concentration of the starch dispersion is adjusted properly (approximately I :I by weight) and the same test is applied to it, the stirrer rotates a t a uniform rate with only the weight of the pan on the stirring device. If a thousand-gram weight is added to the pan, the stirrer continues to rotate a t the same rate.6 4 The Stormer viscometer was modified in this way by Booge and Steinbring in the duPont Laboratories. 6 The properties of this dispersion are described in greater detail in a paper with Heckert: Paint Oil Chem. Rev., 89, g (1930).
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The same property has been found in a number of dispersions of ordinary paint pigments in organic vehicles by Heckert and Sloan at the du Pont Experimental Station. Since the effect appears only within certain concent’ration limits, it is probably related to the increase in volume that occurs when granular masses are deformed, owing to differences in the relative pore-space for different arrangements of the grains or particles. This phenomenon was named “dilatancy” and explained by Reynolds.6 Inverted plasticity is not, however, determined simply by the relation of the volumes of dispersed solid and vehicle, for it occurs only in those dispersions in which the vehicle has a high dispersing action on the particles or, in other words, wets the particles well. Sodium Silicate Dispersions. The viscous, plastic, and elastic properties of sodium silicate dispersions vary greatly with the relative proportions of sodium, silica, and water. A dispersion that exhibits properties similar to the inverted plasticity described above can be prepared by evaporating the water from an ordinary water-glass solution until the proper concentration of sodium silicate is reached. The proper concentration can be readily determined by removing a piece of the dispersion from the mass by means of a spatula, drawing it out very slowly with t,he hands, and laying it on the table to show that it has no tendency to return to its original shape, and then cutting a fresh piece from the original dispersion, rolling it into a ball, and throwing it to the floor with considerable force. If it has a suitable concentration, it bounces from six to ten feet in the air. If an attempt is made to stretch the ball rapidly, it breaks with the conchoidal fracture characteristic of a non-crystalline solid. Different types of flow are shown by other sodium silicate dispersions. For example, one may prepare a dispersion (with a higher ratio of sodium to silica and a lower proportion of water) which appears to flow under low stresses in much the same manner as the one described above, but which behaves quite differently under high sudden stresses. The similarity as well as the difference in properties of the two dispersions can be shown by wrapping a sample of each dispersion in tin-foil and applying the following tests. If the samples are squeezed gently between the thumb and finger, both of them feel soft and flow slowly and in much the same manner. But if the samples are thrown to the floor, the difference in properties is revealed. The first sample bounces like a rubber ball, and the second sounds like a brick instead of the soft sodium silicate dispersion. If the sample that bounces is subjected to a sudden pull, it breaks with a conchoidal fracture, whereas the one that strikes like a brick stretches like a good grade of pulling taffy. A slight modification of the above demonstration serves very well to illustrate how much alike different dispersions may be under one set of conditions and how different under another. For example, the two sodium silicate disPhil. Mag., 20, 496 ( r 8 8 j ) ; Proc. Roy. Inst., (1886); “Scientific Papers,” 2 , 203, 2 1 7 (1901).
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persions described above may be placed in rubber bags made from toy balloons and a third specimen added containing a paste of ordinary molding clay and water. If the concentrations of these dispersions are properly adjusted, they all feel soft and deform to about the same degree when squeezed gently. But when they are thrown to the floor, the first bounces like a rubber ball, the second does not bounce but, sounds like a rock hitting the floor, and the third squashes like a bag of wet meal. The first sodium silicate dispersion exhibits the property of inverted plasticity described above in connection with starch and water suspensions. Attention was there called to the similarity of the conditions of concentration necessary for producing inverted plasticity and “dilatancy”. The same similarity exists in the case of the sodium silicate dispersions, for the property is noticeable only within a limited range of concentration. Attention was called, however, in the case of the starch and water suspensions to the difficulty of accounting for the propert,y of inverted plasticity on the basis of the volume relation of starch and water alone, because the property is specific for given liquids. The difficulty is even more pronounced in the case of the sodium silicate dispersion, for the dispersion has no obvious suspended particles. Also, the second sodium silicate dispersion is several times as concentrated as the first, yet it does not exhibit inverted plasticity. I am indebted to James G. Tail of the Philadelphia Quartz Company for the sodium silicate demonstrations. Liquejaction of i’iscose Gels by loweiing the Temperature. Most materials become more fluid as the temperature is raised and become solid or freeze when the temperature is lowered sufficiently. Tiscose is rather unusual in this respect inasmuch as freshly prepared viscose sets to a gel within a few minutes when heated to the temperature of boiling water and liquefies again when placed in an ice bath. Liquefaction of the gel does not take place, however, unless it is placed in the ice bath immediately after gelation occurs. The phenomenon is probably due to changes in hydration of the components of the dispersion brought about by lowering the temperature. If the chemical changes that produce gelation proceed too far, the changes in hydration produced by lowering the temperature are not sufficient to redisperse the gel again. I t is essential that only a small amount’ of viscose is used for demonstrating this property so that the time required to change the temperature of the whole mass is short. JVe ordinarily use about five cubic centimeters contained in a test tube. The test tube is placed in a glass vessel that contains boiling water. The viscose is stirred with a glass rod, and the heating is continued until the material shows a definite crack that does not flow together when the stirring rod is drawn through the material while the end of the rod is in contact with the wall of the test tube. This is considered t o be the point of gelation. If the gel is then taken out and immediately placed in an ice bath, it returns to a fluid condition.
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summary
A number of experiments are described to show the marked changes in the plastic and elastic properties of dispersions which may be produced simply by changes in temperature or in the magnitude and the rate of application of stresses. The experiments may be useful to the manufacturer of colloidal materials because they illustrate some of the unexpected results that may occur from simple changes in the physical environment of the materials. They should also be useful to those interested in the fundamental study of structure in colloidal dispersions because they furnish a number of facts which must be explained by any comprehensive theory of colloidal structure. E. I . du Pont d e Nemours arid Company, Wdmangton, Delaxare.
