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rubbing is necessary, pxekrably with the finger tips until a peculiar clinging feeling is produced: the same which is felt when the edge of a finger bowl is rubbed so as to give out a musical note. A surface cleaned in this way has some remarkable mechanical properties, the most striking being that two cleaned surfaces seize when pressed together with a relatively small force. This may be dernonstrated very simply by measuring the tangential force needed to produce slipping. That true seizing occurs is prowd by the ‘tearing of the surfaces which takes place in the act of slipping. The function of lubTkants is to keep the applied surfaces in the neutral condition by maintaining a “grease” film on each. Not all fluids, however, can act in this sense as lubricants for any particular surface such as that of glass. Water, ether, alcohol, benzene, and strong ammonia are apparently entirely incapable of maintaining a lubricating film on glass. Seizing occurs just as readily when they are present as it does with cleaned surfaces. Glycerol differs from the fluids mentioned above in the fact that though it wjll not maintain a lubricating film it does prevent seizing when present in excess. For instance, the maximal tangential force which a pair of cleaned surfaces would support without slipping was measured in grams, 5 5 , Flooding the surfaces with water, benzene, alcohol, etc., left this value unchanged. When a film of glycerol was deposited on the surfaces the force still stood a t 5 5 , but i t fell to 9 when the surfaces were fully flooded with glycerol. The expression “film” used above denotes a layer of fluid on the solid surface of the order IO-’ cm. in thickness. With a true lubricant ‘the facility for slipping is maximal when a layer of such excessive tenuity separates the solid faces and nothing is gained by increasing the thickness of the layer. Thus with castor oil the weight required just to start one face of glass slipping over another was I O g. when only the invisible film of fluid mentioned above was present, and it was still I O g. when the surfaces were flooded with oil. Some fluids indeed seem to lubricate better in thin than in thick layers; that is to say, to act in the contrary way to glycerol. Acids as a class behave in this way, the solid faces again being of glass. PULL IN GRAMS
..
FILM
FI,OODED
Acetic acid.. , , . . . . . . 40 Sulfuric acid ... . . 37
E.
If this result can be fully substantiated it will be an important and striking physical fact likely to throw much light upon the process of lubrication. One broad conclusion emerges from thes’e facts, namely, that lubrication depends wholly upon the chemical constitution of a fluid, and the fact that the true lubricant is able to render slipping easy when a film of only about one molecule deep is present on the solid faces, suggests that the true lubricant is always a fluid which is adsorbed by the solid face. If this be so, then the problem of lubrication is merely a special problem of colloid physics. Some solids, notably graphite, are lubricants themselves. But the chief function of graphite seems to be as an aid to lubrication by oil. The graphite is disintegrated in the bearing and besides filling up relatively large imperfections, its colloidal particles are adsorbed by the iron, forming in reality a graphite surface which has a lower surface tension against oil than iron has. Practically this means that the oil film adheres more strongly to the graphitized surface and therefore needs greater force or pressure to’break it down; i. e., the bearing will stand greater speed and pressure. Acheson’s “oildag” consists of a paste of colloidal graphite dispersed in oil. A small percentage is mixed with the ldbricating oil and feeds with it through the finest orifices. “Aquadag” is ’the corresponding water-soluble product, stabilized with tannin. CERAMIC PROCESSES ASSOCIATED PHENOMENA1
WITH COLLOID
By A. V. Bleininger PITTSBURGH, PSNNSYLVANIA
This paper is intended to present some of the aspects of colloid chemistry which have a bearing upon the technology of clays. Within the scope of this contribution it is not possible to consider 1 Published
by permission of the Director of the Bureau of Standards.
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the subject a t all completely and attention can be called only to certain of the more important topics. Clays are mixtures of finely divided hydrated aluminum silicates with granular matter such as quartz, feldspar, mica, etc. These represent all grades of subdivision, from the coarsest t o the finest. The property of plasticity, the ability of the substance to be molded when admixed with water and to retain the shape imparted to it, is to be attributed to the dispersed state of the aluminum silicates, principally A120a.zSi02.zH20,known under the general term of clay substance. The pure type mineral, known as kaolinite, sometimes found in crystalline form, rarely occurs in clays but has usually been reduced to particles apparently devoid of crystalline structure, of the magnitude of 5.u or smaller, admixed with colloidal material like ferric oxide and organic matter and frequently strained by absorbed salts. Clays are not necessarily plastic in the natural state but may have been indurated by pressure or heat, or set through other agencies so they cannot be made plastic without the aid of grinding. This is particularly the case with materials consisting of hydrated silicates other than A120&XO2.zH20, such as halloysite, pyrophylite, indianite, etc. The colloidal character of clays was recognized as early as 1874 by Schloesing’ and later more definitely by Rohland2 and C ~ s h m a n and , ~ it was agreed that a clay is very plastic, fat or sticky when the colloid matter is in excess or sandy; weak or non-plastic if the granular matter predominates. The colloidal characteristics of clays are substantiated most effectively by ultramicroscopic examination. Recent work of Jerome Alexander4 has shown that the Brownian movement is observed with practically every type of clay suspended in water, varying from the rapid motion of the finest particles to the more sluggish one of the larger particles or aggregates or when hampered by the presence of electrolytes.
47
.. ... .
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CLAY SUSPENSIONS
Clay suspensions in the deflocculated state when filtered through paper will show decided turbidity in the filtrate. Again, such a suspension, after the coarser portion has been allowed to settle out, will give rise to the familiar phenomenon of the Tyndall cone upon passing light through it. Clay suspensions are affected in a very pronounced manner by various reagents, both electrolytes and non-electrolytes. Probably the first striking phenomenon observed in this respect is that of absorption. Upon adding solutions of salts like “&I, BaC12, A12(SOd)s, and CuSOa, an appreciable amount of the basic ion is absorbed but practically none of the acid ion. Thus Sullivan6 found that I g. kaolin absorbed between 0 . 0 0 3 8 and 0.0169 g . of CuO from 50 cc. of a solution containing 2 g. of CuS04, but was itself slightly dissolved. Cushmane found the absorption of NH4 from NHdCl to be 0.077, of Ba from BaCl2 to be 0 . 3 7 3 , and of A1 from A12(504)3 to be 0.075 per cent after standing three days and using 0.1N solutions. Deeply colored solutions of metallic salts may thus be decolorized by passage through clay. Organic salt solutions like malachite green or methylene blue, consisting of large, complex molecules are absorbed to a very striking extent, and in fact Ashley’ employed the magnitude of absorption of the former as a measure of the colloid content or the plasticity of clay, using a solution containing 3 g. of the dye per liter. This type of absorption follows the general exponential equation with fair agreement. 1
Comfit. rend., 79 (1874), 376, 473.
* Z . anorg. Chem., 8 1 (1902),
158; 41 (1904), 325. a Trans. Am. Ceram. SOC.,6 , 6 5 ; Bureau of Chemistry, Bullet$ns 88 (1904) and 92 (1905). 4 Private communication. 6 U. S. Geological Survey, Bulletin 812. 6 Bureau of Chemistry, Bulletin 92. 7 U . S . Geological Survey, Bulletrn 888; Bureau of Standards, Techno logic Paper 98.
*
May,
1920
T H E J O U R N A L O F I N D U S T R I A L A N D E N G I N E E R I N G CH E M I S T R Y
Salts as a class tend to coagulate clay while bases deflocculate it, as is common with all dispersed materials. This type of reaction is of considerable technical importance. Thus the action of NaOH, Na2C08 or NazSiOs in increasing the dispersion of the clay particles brings about a decided decrease in the viscosity of the system and, hence, according to Stokes' law makes possible a sharper and more prompt separation of the granular matter and the clay substance proper, a fact made use of in the washing and purification of kaolins, first proposed by Keppeler.' The clay thus treated and separated must later be coagulated with acid or salts to render possible its filter pressing. Another industrial application of the deflocculation of clays is connected with the so-called casting process in which the constituents of a porcelain or other ceramic body are stirred with water containing about 0 . 3 per cent of Na2C03 and NazSiOa in terms of the dry weight of the clay materials, to form a thick but still readily fluid suspension. Here the addition of the alkaline reagents is effective in reducing the water content required to keep the mass in the liquid condition. I n fact, the amount of water present is not greater than that found in the same mixture in the plastic state without the use of the alkali. This phenomenon may be readily demonstrated by adding to a plastic mass of kaolin a small amount of Na2C08 or NaOH. It will be noted that the plastic material is suddenly transformed into the fluid state. Upon the addition of acid or a salt like A12(S04)ait will revert to the plastic condition, thus illustrating the effect of deflocculation and coagulation. I n casting, the clay suspension is poured into a plaster mold which absorbs a sufficient amount of water to cause the mass to solidify into a layer conforming to the outlines of the mold. On the other hand, coagulating reagents, acids, and salts are used to increase the plasticity and strength of certain clays and to thicken suspensions of glazes and enamels. All the phenomena of deflocculation and coagulation occur in phases. Thus the addition of alkali to a clay suspension brings about first a decided viscosity minimum, followed by alternating maxima and minima until finally the phases merge in a definite direction. Similar conditions prevail also in a coagulation series with the phases in reverse sequence. Schulze's statement that the coagulating ions apparently increase in effectiveness proportionately with their valence is confirmed in the work with clays. Certain organic substances like the tannins react upon clay suspensions somewhat like the alkalies, though with less sharply defined phases. They may also assume the function of a protective colloid in that they render the clay less sensitive to the action of electrolytes. Kaolin suspensions are easily affected by the presence of impurities, and impure clays containing soluble sulfates deflocculate only with difficulty or not a t all. The fluidity of clay suspensions is probably the best means of following changes taking place in clay-water systems and for this reason technical viscosimeters have been used frequently for the estimation of the deflocculating or coagulating effect of reagents. While this practice is perhaps justified for suspensions containing a large excess of water it is not warranted for heavier ones, and the criticism of Bingham2 that such mixtures show evidence of viscous flow is justified, since the volume of the flow varies directly as, the pressure. It would seem then that viscosimeters of the Bingham type should be preferred for measurements of this kind even though the technical instruments suffice to mark the maxima and minima which it is necessary to establish. However, even such determinations of the viscosity or its reciprocal, the fluidity, of clay suspensions suffice to establish the relative plasticity of different clays inasmuch as equal concentrations will show fluidities inversely proportional to the general plastic nature of the materials. 1 Z. angew. Chem., 22 (1909), 5 2 6 ; Bureau of Standards, Technologic Paper 28; Bureau of Mines, Bulletin 128. Bureau of Standards, Scientific Paper "t.
43 7
One of the most interesting phenomena observed with clay suspensions is the electrical deposition of the particles by a direct current upon the positive electrode against the flow of water away from it. This is the basis of the invention of Schwerin for removing clay from positively charged particles (iron bearing minerals) and depositing the former upon a metallic belt or revolving drum. The addition of small amounts of sodium hydroxide increases apparently the charge of the clay particles and facilitates their deposition. As a matter of fact the separation is effected far more completely by previous washing treatment with the use of sodium hydroxide for bringing about maximum deflocculation. The principal application of the Schwerin process consists in the deposition of the material upon the moving positive electrode, thus replacing the filter press just as in the electro-osmose filtering apparatus devised by the same worker. It seems that the migration velocity is that of the heavier ions and that the voltage determines the degree to which water has been eliminated. These several phenomena of clays in the suspended state have been explained as being due primarily to the negative charge of the clay particles, so that the introduction of negative or OH-ions will bring about further dispersion or deflocculation and that of H-ions coagulation. It is apparent, however, that the case is not one of mere electrostatic repulsion or attraction. The presence of salt solutions not only brings about a change of the charges on the particles or neutralization but also a host of possible and complex interactions more or less complete as, for instance, in the addition of sodium carbonate to a clay already carrying absorbed calcium ions, and hydrolysis. Ashley compares these reactions with those of soap in that in either case we are dealing with a weak acid, only slightly soluble, the salts of which are all hydrolyzed and with the exception of those of the alkalies and ammonia but difficultly soluble. PLASTIC CLAY
Clay in the plastic state likewise possesses some interesting properties most of which still require further elucidation. The measurement of plasticity itself has not been possible up to the present time except by empirical or indirect means, and it is only recently that progress has been made in this direction. The simplest mechanical conception of plasticity would be to consider it a case of viscosity according to the relation d = KW, where d = deformation, W = load, and K = constant. It seems, however, from the researches of Bingham that this is not the case but that we must differentiate between viscous and plastic flow in that the yield point of the former is zero and of the latter finite. I n a diagram correlating pressure with volume. of flow the curve of a viscous liquid will pass through the origin and in the case of a plastic substance we have an intercept on the pressure axis. In the case of clays studied by the writer' the pressure required to start initial flow through an orifice 0.25 in. in diameter varied from 40 to 75 lbs. per square inch according to the character of the clay. With decreasing water content the pressure required to produce flow increased very rapidly, and on the other hand with increasing amounts of water it dropped, so that the resulting curve was hyperbolic. Atterberg2 has made use of the conception of flow and lower plasticity limits. He determined the amount of water required to just cause flow of the clay and also the water content a t that stage where it can barely be rolled to form threads. The difference between these values he calls the plasticity number. Kinnison3 has found this factor to be quite characteristic, though he considers that it should be plotted against the water content of the plastic clay and the resulting diagram used for the classification of the different materials. The plasticity of clay is, without doubt, greatly affected by the Trans. A m . Ceuam. Soc., 16, 392. International Reports on Pedology, 1911. 8 Bureau of Standards, Technologic Paper 46. 1 2
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presence of organic matter, humus, tannins, etc., and it seems to be a fact that acidity is conducive to the greatest development of this property. The aging of clays does indeed improve their plastic working quality and it is believed that this is due in part to the formation of organic acids, caused by bacterial action or chemical processes. The addition of small amounts of reagents like A12C16 to kaolins brings about similar results provided the material is stored for some time, but no change is observed with the more impure materials. Tannic acid shows this action in a more marked degree. Alkalies, on the other hand, are quite active in inhibiting the plastic quality of clays t o a very pronounced extent. Electrolytes in general affect clays in that they increase or reduce the water required for the development of the plastic state and also the shrinkage upon drying. This effect depends obviously upon the kind and quantity of the salts already held by the materials and, as a rule, the purer clays are influenced to a greater extent than the more impure ones. Thus, upon adding up to 0.05 per cent of NaC1, CaC12, and A12C16 to Georgia kaolin the volume contraction due to drying was reduced by about I .o per cent. Here again the first small additions of the reagent show phases indicated by maxima and minima which gradually disappear as the concentration is increased. With most clays, the salts, chlorides, and sulfates increased the water content and drying contraction. DRYING SHRINKAGE
I n the drying process itself we have the contraction in volume of the plastic clay as a typical property of colloid materials which may show a magnitude of as high as IOO per cent, or as much as the true volume of the clay itself. The drying shrinkage is thus in a measure a criterion of the colloid nature of the clay, being the greater the more pronounced this development is. The presence of electrolytes affects not only the magnitude of the contraction in drying but the capillary flow of the water through the clay as well. Thus it is well known that the addition of sodium chloride in small quantity will accelerate the passage of the water to the surface and thus lessen the strain imposed upon the clay in drying. Upon drying clay and rewetting, it does not seem to recover its original plasticity a t once and this is the more pronounced the higher the temperature to which it is carried. The retempering of the dried clay is associated with evolution of heat. We are dealing therefore with a case analogous to that of a “set” gel, and a change which becomes irreversible a t a given temperature, usually a t about 500’ C., unless subjected to treatment with Steam a t high pressure. I t is interesting to note, likewise, that the drying shrinkage is the greater the more slowly evaporation of the water is allowed to take place, and vice versa.
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with a decided increase in molecular volume. Upon further heating of the clay an exothermic reaction takes place a t about gooo C. which may be associated with an irreversible change of a colloidal nature, or the dissociation of the aluminum silicate, perhaps into A1203.Si02and silica. As the firing temperature is raised the external clay volume contracts, especially in the presence of fluxes, due to the effect of surface tension until the heat intensity is sufficient to bring about practically complete closing of the pore space. It is said then that the clay has become vitrified. With increased softening of the mass its lowered viscosity permits the surface-tension effect to become more and more prominent until finally the original shape has been lost and the surfaces curved analogous to the formation of a drop of water from a melting fragment of ice. I n the case of clay products the process, of course, is not allowed to proceed as far as this and the firing is stopped when the desired degree of vitrification has been reached, while with glass it is allowed to continue. At the same time the density of the vitrified or fused silicate decreases with increase in temperature, irrespective of the external contraction. The relation between temperature, time, and contraction in volume is therefore a very important one and expresses the result of the heat work done. It enables us to represent graphically the progress of condensation or vitrification and is equally important in the study of the firing processes of carbon, alumina, zirconia, thoria. etc. Although surface tension is probably the most important factor in bringing about the condensation of the substances with which we are here concerned, vapor pressure’ undoubtedly enters into the case in dealing with substances showing a vapor tension sufficiently high or in the admixture of inert materials wil h those of a more volatile character. Thus, the comparatively high vapor pressure of magnesia a t temperatures considerably below its fusion point is instrumental in bringing about its earlier condensation. Formation of sillimanite from kaolin and alumina may be brought about a t about 1500’ C. by the addition of a small quantity of boric acid, and the cementation of carbon from coal is likewise made possible by the presence of volatile constituents. I n all the silicate reactions at higher temperatures diffusion through the semi-rigid mass plays an important role. Finally, wherever the chemicai composition and the heat treatment permit it the end result is the partial elimination of the colloid phases and their replacement by crystalline identities. Thus, the clay substance decomposes into sillimanite and silica, glass yields calcium silicate, and cement its several calcium silicates and aluminates, but there is invariably present a residuum of an isotropic matrix.
FIRING O F CLAY
REPORTS
I n the firing of clay two points are of special interest during the early periods of the process, the expulsion of the hygroscopic and t h e chemically combined water. The removal of the former leaves the clay in a state exceedingly sensitive to the pre-ence of water vapor and gases, absorbing them with great avidity. This condition is accentuated after the evolution of most of the chemically combined water, when the material becomes an even better absorbent, taking up cagerly sulfur dioxide and trioxide. xn this state it possesses marked catalytic properties, as in the oxidation of sulfur dioxide to trioxide, the formation of ethylene, etc. I t is posgible that this characteristic of dehydrated clays may find uses in the technical reactions. hi^ material application of a number of shows considerable activity also when ground together with calcium oxide and moistened, producing a cement of good hydraulic setting qualities. The dehydration is necessarily endothermic and is associated
REPORT OF COMMITTEE APPOINTED TO DEFINE THE FIELD COVERED BY RESPECTIVE DIVISIONS AND SECTIONS OF THE AMERICAN CHEMICAL SOCIETY
The Committee Appointed to Define the Field Covered by Respective Divisions and Sections of the AMERICAN CHEMICAL SOCIETY has carried on correspondence with reference to the questions submitted, and now begs to report a? follows: I-It is believed that the designations which have been adoDted for the different divisions and -sections are themselves sufficient definitions of the field to be covered in each instance. 2-It is recommended that the established procedure of the soCrETY shall be to give authors their choice of the division or section in which they are to present their papers whenever it is possible to do SO The Secretary of the SOCIETY shall be empowered to assign the papers to other divisions or sections in case the paper is obviously out of place or when in his opinion it will be to the advantage of the division or to the pro-gram in general so to assign it. 3-When submitting the titles of papers authors must indi1 E. Podszus, Sprechsaal-Arch., No. 1 , 1912.
