Lubricating Properties of breases from Petroleum Oils Ty
F. H. RHODES AICD HAROLD DWAINEALLEN, Cornel1 University, Ithaca, K.Y.
M
OST of the commonly T h e c o n s t r u c t i o n of t h e I n lubrication by soda-base greases the soap used l u b r i c a t i n g apparatus is shown in Figure 1. plays a n important part in the formation of the greases belong to one A s k e l e t o n p l a t f o r m 51 cm. lubricating Jilm. T h e glycerol present aids in or the other of two classes: the (20 inches) long and 7.6 cm. ( 3 stabilizing the structure of the grease, reduces lime-base or cup greases, which inches) wide, m a d e f r o m two consist essentially of petroleum long angle i r o n s 1.6 X 1.6 X the change in consistency on heating or working, oil stiffened with calcium soap, 0.16 cm. (j/8 X 5 / 8 X 1/16 inch) increases the lubricating power, eliminates the and the s o d a - b a s e o r f i b e r f a s t e n e d together with c r o s s increase in the coeflcient of static friction on greases in which the stiffening strips, is provided with a fixed heating, and reduces the susceptibility to moisture. agent is s o d i u m s o a p . Both b e a r i n g a t o n e end. A t the Greases containing a n amount of glycerol equal types of g r e a s e s are colloidal opposite end is a t t a c h e d t h e suspensions o r emulsions a n d mechanism for tilting and the to two or three times the quantity equivalent to owe their consistency to their scale for measuring the angle of the sodium soap present are much better lubrisuspensoid or emulsoid structure. tilt. cants than are those that contain merely the The flat s u r f a c e of the test Many greases contain a t least amount equioalent to the soap. With larger bearing is mounted a t a dissmall a m o u n t s of water, the quantities of glycerol, the lubricating value again presence of which may affect the tance of a b o u t 14 cm. f r o m physical properties of the mathe movable end of the frame. decreases. terial. Since greases are usually This flat surface is made up of a brass block in which are set made bv the direct saDonification of an animal or vegetable fat in the presence of a petro- three chromium-plated lugs. Into the block are drilled three leum oil, the glycerol formed as a by-product in this saponi- tapering holes 2.22 cm. (7/8 inch) deep, 2.38 cm. (15/16 inch) fication is also commonly present. The presence of this in diameter a t the upper end, and 2.06 cm. (13/16 inch) in component and its possible effect upon the structure and the diameter a t the lon-er end. These holes are located a t the behavior of the grease have often been overlooked. points of an equilateral triangle, the distance from center to Of particular importance is the subject of the effects of the center on the holes being about 4.3 cm. Into each hole is soap and the glycerol upon the coefficient of static friction inserted a tightly fitting brass lug projecting about 3 mm. between metallic surfaces lubricated by the grease. The con- above the upper surface of the block, held tightly in place by ditions under which greases are used are often such that it is a screw through the bottom of the block. The lugs are set difficult t o supply a large quantity of the lubricant continu- so that the flat surfaces of all three lugs are in the same horiously to the bearing, so that we must depend very largely zontal plane. While still in place in the block they are polupon the lubricating action of the film of material adsorbed ished successively on KO.0, No. 00, and No. 000 emery upon the bearing surface. It is well known that the fatty papers resting on a glass plate. The lugs are then removed oils and the free fatty acids are much more strongly adsorbed from the block and the upper surfaces are polished on a metal on metallic surfaces than are the paraffin hydrocarbons. lap covered with wool broadcloth charged with KO.600 There is some evidence to show that the soaps of the fatty Carborundum flour and kept wet with water, and are finally acids also are adsorbed to a considerable extent by metals, washed with water, dried, washed with petroleum ether, although perhaps not so strongly as are the free fatty acids dipped for a moment into a 25 per cent solution of sodium themselves. Since the lubricating greases always contain cyanide, washed, and dried. The polished brass lugs are soaps, it is to be expected that they will show higher specific then plated with chromium and polished with very fine emery lubricating powers than do the petroleum oils from which flour. The plated and polished lugs are reseated in the block, they are derived. The lubricating power may also be affected taking particular care to insure that the upper surfaces of all by the water or glycerol present. The extent and nature of three lugs are in the same horizontal plane. the effects of these various factors can be determined only The tops of the three lugs prepared in this way give, in effect, by experiment. a horizontal flat bearing surface that is very hard, fine grained, and smooth. Various other investigators have used bearings MEASUREMENT OF THE COEFFICIENT OF STATICFRICTION of steel, bismuth, or speculum metal. When comparatively The apparatus used in the experimental determination of soft metals are used, the investigators commonly note that the lubricating powers of the greases was developed in this some slight scratching of the surface takes place and usually laboratory by A. W. Lewis and was modeled after the in- remark that the scratching is so slight that they do not bestrument described by Wilson and Barnard ( 3 ) . It consists lieve it introduces any serious error in the measurement. It essentially of a light rider resting a t three points on a flat seems to the present authors that if any scratching whatsosurface lubricated by the grease under examination. The ever occurs there must be direct metal-to-metal contact, flat surface is so arranged that it can be tilted slowly and the and therefore the results obtained cannot measure the lubriangle of tilt can be measured a t any instant. The tangent cating power accurately. I n some preliminary experiments of the angle of tilt a t which the first slipping of the rider occurs carried out in this laboratory, attempts were made to use is taken as an index of the specific lubricating power of the lugs of steel, of nickel, and of brass. It is extremely difficult grease. With a grease of high lubricating power this initial to polish these metals highly without getting a t least a slight slip takes place, of course, a t a smaller angle than with a grease pitting, owing to variations in hardness among the different of low lubricating power. grains of which the steel is composed. The electrically de1275
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Immediately below these-i. e., toward the stationary end of the platform-is a movable contact plate, the exact position of which can be adjusted by means of a screw. One pole of a 4-volt electric circuit is connected through a relay with the frame; the other pole is connected with the contact plate. When sliding occurs, the circuit is closed and this actuates the relay 7and disconnects the motor. At the beginning DETAIL BLOCK e RIOER of the experiment the gap between the contact plate and the points of the rider is adjusted to about 0.4 mm. This adjustment must be made while the circuit is open; if made while the circuit is closed, the discharge that passes when the gap is first opened disrupts the film of lubricant and causes errors in L I . J H”’ 0the results later obtained in measuring the coeffii, cient of friction. The tilting of the frame is effected by a REIAV CIRCUIT 1 HEATING ClRWlT motor, connected (through a pair of geared W speed reducers, a clutch, and a pair of secondary reducers) with a steel shaft on which a cord is coiled. The cord runs over a pulley above the movable end of the platlorm and is then attached to the end of the platform. W h e n t h e m o t o r i s r u n n i n g , the cord is wrapped on the shaft and thus raises the end of the tilting frame. The rate of motion, measured a t the moving end of the frame, is about 0.6 cm. per minute. To the rising end of the platform is attached a scale to indicate the angle F I G U R E 1. APPARATUS FOR DETERMIX.4TIOX O F THE COEFFICIENTS OF S T A T I C FRICTION of tilt. The entire apparatus is mounted on t h i c k b l o c k s of rubber to minimize vibraA . Tilting frame K . Pivot f o r frame B. Brass block N . Contact points tion. C. Insulating box P. Upper pulley D. Scale for measuring angle of tilt &. Plated lug Preliminary experiments indicated that the reE . Thermometer R . Spindle sults obtained in the measurement of the coeffiF. Contact connection Y . .Aluminum tripod G . Heating coil Z. Contact bar cients of static friction are influenced t o some exI. Inlet for heated air tent by moisture and that even the exposure of the bearings to moist air during the progress of posited chromium is so fine grained that no such pitting took the determination may introduce errors. Inorder-to exclude place and therefore it was possible to obtain smooth and flat moisture and to make it possible to carry out the tests in a polished surfaces. In a few preliminary experiments samples dry atmosphere, a brass box with a glass cover was constructed of the same oils and greases were tested with these chromium- to enclose the bearing plate and the rider. This case is proplated lugs and with very carefully polished lugs of brass vided with an opening through which is introduced a current and of steel. Identical results were obtained, irrespective of air that has been passed through a long tube filled with of the nature of the metal a t the surface of the bearing. cotton, through three Friedrichs spiral wash bottles containing Most of the previous investigators in this field have used concentrated sulfuric acid, through a large U-tube containing a single plate of polished metal as the lower plate of the soda-lime, and finally through a U-tube filled with Dehydrite. bearing. It has been found that the polishing of a single The dry air passes through a long glass tube surrounded by large flat plate is difficult and tedious, and for this reason a heating coil of Chromel wire. The cleaned, dried, and the scheme of using thrde separate lugs has been adopted. heated air is then introduced into the chamber containing With this arrangement it is much less trouble to prepare a the plate and rider. suitable bearing. Since the coefficient of static friction varies with the temThe sliding member of the bearing is composed of a light perature, provision is made for indicating and regulating the triangular piece of thin aluminum sheet with the central temperature of the bearing and the film of lubricant. Any portion cut away. I n each corner of this sheet is drilled a complete investigation of the lubricating properties of an small circular hole in which is set a polished-steel ball bearing. oil or grease involves the determination of the coefficient of The rider is of such dimensions that when in position one of static friction a t various known temperatures, and therefore the ball bearings rests on each of the polished lugs of the sup- it is necessary to provide a means of heating the testing surporting plate. The ball bearings are polished by hand with face. In order to permit the variation and control of the wool broadcloth carrying floated emery and then with broad- temperature, a heating element is mounted between the botcloth carrying levigated chromium sesquioxide. tom of the bearing block and the supporting platform. This The lugs are lubricated with the material to be tested, the heater consists of a set of small coils of Chromel wire set in rider is set carefully in place, and the frame is tilted until the slots cut in a sheet of Transite board and cemented in place first slipping of the rider occurs. In order to detect the first with Alundum cement. The wall of the chamber surroundslight motion of the rider, the apparatus is so constructed that ing the block is provided with an opening, directly opposite the slipping of the rider closes a relay circuit and shuts off the which a hole has been drilled into the block. A thermometer motor that actuates the tilting device. The rider itself is inserted into this hole registers the temperature of the block. provided with two contact points, as shown in Figure 1. The temperature is regulated by varying the current through
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INDUSTRIAL AND ENGINEERING CHEhIISTRY
the heating element. The temperature of the incoming air is kept the same as t h a t of the block by varying the amount of current through the heating coil in t'he inlet line through which the air is admitted. In the first few tests the bearings were not enclosed and no attempt was made t o control the humidity of the atmosphere surrounding the bearing surfaces. It was found that under these conditions the coefficient of static friction varied with the humidity. Experiments on a damp day gave lower apparent lubricating values for the oil than did experiments on a dry day. Maximum values were obtained when the air in the room was kept almost saturated with water vapor. I n a series of experiments in which the bearing surfaces were first dried in a desiccator, it mas found that the first results obtained with the freshly dried bearings gave a low value for the coefficient of static friction but that the apparent value increased as successive tests were made until finally a maximum was reached, The position of this maximum depended upon the humidity of the air. The variations and errors caused by the presence of moisture in the air and on the bearing surfaces were eliminated by drying the surfaces in an oven a t 120" C. and in a vacuum desiccator over phosphorus pentoxide; they were then enclosed in a box through which dried air was circulated as described above. At first the polished surfaces were cleaned with petroleum ether but this was found to be insufficient. It is necessary to supplement this n-ith boiling and rinsing in absolute alcohol to secure the meticulous cleaning required for accurate measurements. The final procedure adopted in making a determination of the coefficient of static friction of a n oil or grease is as follows: The lugs are plated and polished as described above and are set in the brass holding block. The block and the aluminum rider carrying the polished steel balls are immersed in petroleum ether and brought to a boil. This boiling v.-ith petroleum ether is repeated. Then the block and rider are removed, allowed to dry in the air, and boiled in absolute alcohol. After rinsing twice w i t h absolute alcohol they are dried for about one-half hour in an oven a t 1 2 0 " C . and transferred to a vacuum desiccator containing phosphorus pentoxide. The desiccator is evacuated and the block and rider are allowed to stand over t h e d e s i c c a n t for 12 h o u r s . The block is placed in the frame and the lubricant to be tested is applied to the polished s u r f a c e s of the lugs. TZ'77PL-RATURFLiquids are applied by FIGURE 2. VARIATION OF COEFFImeans of a m e d i c i n e CIENT OF STATIC FRICTION WITH dropper; greases are applied in the molten conTEMPERATURE dition. The rider is set 1. Oil alone in position, the contact 2 . Oil + 0.01 per cent sodium oleate 3. 4.
Oil Oil
+ 0.05 per cent sodium oleate + 0.10 per cent sodium oleate
gap
an
he
Coefficient Of static friction determined.
The exact value for the coefficient of static friction of a particular sample of oil or grease varies somewhat with the length of time intervening between the application of the lubricant to the bearing and the actual measurement of the angle of slip. When the measurement is made immediately after the application of the lubricant, a rather high value is found; measurements made 5 or 10 minutes after the oil is put on the bearing give somewhat lower values. On further standing no further decrease takes place. The initial drop in the angle of slip appears t o be due to the fact that some time is required for the formation on the bearing surface of the firmly adsorbed and more or less regularly oriented film
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that is principally responsible for the lubrication. This is completely, or approximately completelg, formed in a short time so that longer standing has no further effect. That the values obtained with a specific lubricant at a fixed temperature do not change on long standing indicates that any error due to absorption of moisture has been eliminated. Variations of the amount of lubricant or the thickness of the layer of grease, within the limits of the ordinary experimental procedure, do not cause variations in the results. Tests made with this apparatus in its final form and with the procedure finally adopted gave consistent results. Two operators working independently of each other and using two different instruments of this type obtained, with t h e same oil, results that agreed closely. PREPARATION OF GREASES
All of the experimental work has been done with greases of the soda-base type. These are usually prepared by dissolving a fat in a petroleum oil and then cooking the mixture with a strong solution of sodium hydroxide until the fat is saponified. The cooking is usually continued for a sufficiently long time at a sufficiently high temperature to drive off all or almost a11 of the water present. Greases prepared in this way contain a mixture of the sodium soaps of various fatty acids in rather indefinite proportions. Various other substances, such as water, unsaponified fat, and glycerol, may also be present in undetermined amounts. Since such complex and indefinite mixtures do not lend themselves readily to the systematic investigation of the effects of individual factors, the present work was done with greases prepared from petroleum oil and pure sodium oleate. The oil used was a typical paraffin-base lubricating oil from Pennsylvania crude. It showed a density of 0.88 at 20" C. and possessed the following viscosity characteristics: TEUPERATFRE C. 30.55 37.9 54.45 98.7
' F. 87
100 130 210
S 4 Y R O L T VISCOSITY
Svconds 651 445 205.4 64
At first an attempt was made to prepare soda-base greases by dissolving pure, dry sodium oleate in the oil. The oil was heated to 100" C., and a known amount of pure dry sodium oleate was added. The mixture was heated a t looo C. with constant stirring for 45 minutes and was then chilled to room temperature in a n ice bath, without stirring. In no case was a true grease obtained. The chilled suspensions had about the same consistency as did the original oil. Those mixtures that contained a relatively large amount of soap showed a separation of solid immediately after cooling; all mixtures containing more than 0.1 per cent of sodium oleate contained some undissolved soap after standing for 12 hours. When dry sodium oleate is warmed in oil, only a small amount of soap passes into true solution or into permanent suspension and the solution or suspension thus obtained is not a true grease. It is evident that the soda-base greases are not simple solutions or suspensions of dry soap in oil. To obtain a true grease, some substance other than soap and mineral oil must be present. This additional necessary ingredient might conceivably be water, but in the preparation of many greases the cooking is effected a t so high a temperature that practically all of the water is expelled. It is also possible that t h e additional necessary substance is the glycerol formed as a by-product in the saponification of the fat. To test this hypothesis, a grease was prepared that contained 5 per cent of sodium oleate and an amount of glycerol equivalent to that which would have been formed if the sodium oleate had been prepared by the direct saponification of glycerol oleate.
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The sodium oleate was dissolved in a small amount of water, the glycerol was added, and the solution was evaporated on an oil bath until only a viscous mass remained. The petroleum oil, previously heated to 125' C., was then added in small uan tities and with constant stirring until about one-eighth the total oil had been introduced. During the addition of the oil, the temperature was raised gradually to 150' C. After holding the mixture at this tem erature for 45 minutes, the rest o f the oil, previously heated t o 120" C., was added slowly. Greases containing 25 per cent of soap and an amount of glycerol equivalent to the soap present were prepared in the same way, except that the heating at 150" C. was continued for 5 hours to insure dehydration. -4similar procedure was followed in prepari n g greases that contained two, three, and five times the amount of g l y c e r o l equivalent to the
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weight of the mixture remained clear when cold; with larger amounts of soap some undissolved material separated. For purposes of comparison, tests were also made on a sample of the oil alone, without soap, that had been heated to 100" C. for 45 minutes and then cooled. All tests were made in dry air. The results are shown graphically by the curves in Figure 2 . The presence of sodium oleate in solution in the oil markedly decreases the coefficient of static friction. The magnitude of the effect increases with the concentration of the soap in the solution up to the point a t which the oil is saturated with soap. SKth the materials used here, the limit of solubility was reached when the concentration of the dissolved soap was 0.1 per cent. The addition of soap in amounts larger than this produced no further effect upon the lubricnting power. The curves in Figure 2 all have about the same general soap. form. In each case the coefficient of friction remains about The grease containing 25 per constant or rises only slowly as the temperature is increased cent of sodium oleate with the until a more or less definite critical temperature is reached. FIGURE3. EFFECTOF corresponding amount of glyc- At this critical point the coefficient begins to increase rapidly. MOISTUREON COEFFI- erol had the characteristic tex- The exact position of the critical temperature and the sharpCIENTS OF STATICFRICture of the fiber greases. When ness of the break a t this point vary with the concentration of TION O F O I L AND O F worked between the fingers it soap in solution. Whenno SOLUTIONS OF SOAPIN OIL s h o w e d a long fibrous break. soap is present, the break 1. Heated oil i n updried air ./6 A grease prepared with 25 per occurs a t a rather low tem2. Saturated solutions of soap cent of soap and five times the perature (about 60" C.) and in oil, in undried air 3. Heated oil in dried air normal equivalent a m o u n t o f is not very sharp. I n the 4. Saturated solution of soap in dried air g l y c e r o l had a very smooth presence of 0.01 per cent of t e x t u r e a n d was softer than s o a p t h e critical temperaordinary fiber grease. It showed an exceedingly long break. ture is raised t o about SO" A grease containing 5 per cent of sodium oleate and the equiva- C. and becomes rather more lent quantity of glycerol 'was still fluid but had a higher ap- marked. Further increase parent viscosity than did the oil from which it was prepared. in concentration of s o a p With 5 per cent of soap and five times the equivalent amount again lowers the c r i t i c a l of glycerol the grease was pasty and had a characteristic p o i n t a n d diminishes the sharpness of the break. curdy texture. The formation of an adAttempts were made to prepare greases by a procedure similar to that described above, except that no glycerol was sorbed film of the type met added. It was found that in some instances fairly stiff with in lubrication may be FIGURE 4. EFFECT OF GLYCgreases could be obtained, but these decomposed on stirring considered as analogous, in EROL ON THE VARIATIONOF COEFFICIENTS OF STATIC and even on standing, with the separation of liquid oil. The some respects a t least, to the FRICTION OF GREASESWITH sample containing 5 per cent of soap was the more stable, crystallization of a s o l i d CHANGEIN TEMPERATURE trom a liquid. In both cases but even this separated on long standing. I n another ex1. Grease with five times normal periment, oil was heated to above 100" C. and to it was added, we have t o deal with the amount of glycerol, undried air equilibrium b e t ween t h e with constant stirring, a solution of sodium oleate in water. 2. Same as 1, dried air 3. Grease with normal amount of After the addition of the soap solution the heating and stir- irregularly oriented m 01 e ring were continued until the water was expelled. The mix- cules of the liquid phase and 4. ture remained quite fluid until the concentration of water the regularly arranged mole5. dropped to about one per cent. At this point the mass thick- cules in a solid or adsorbed 6. Same as 4,dried air ened rapidly and assumed the consistency of cup grease. On phase. In both cases a rise in temperature tends to shift continued stirring and heating the mass again became fluid the eqklibrium in the direction that will make the liquid phase and the soap separated as a solid precipitate. These results indicate that in normal fiber greases the glyc- the more stable one. As long as the interatomic orintermolecuerol present plays an important part in determining the con- lar forces that hold the component units together in the space sistency and the stability of the material. In some cases a lattice are stronger than those that tend to disintegrate the small amount of water may be present and may be partly crystal or the adsorbed layer, the material remains in the responsible for the consistency. Greases containing only solid form; as the temperature is increased, the disruptive water and free from glycerol are not very stable but tend to forces become stronger and n point is reached a t which liquefaction occurs. I n the case of a true crystal the internal forces thin out or to separate on working or on standing. that hold together the component units of the space lattice are uniform throughout, and therefore the temperature a t which EFFECT OF SODIUM OLEATEo s THE COEFFICIENT OF STATIC the solid phase ceases to be stable is sharply defined and the FRICTIOX OF PETROLEUM OIL material shows a definite melting point. With the oriented Solutions of soap in the mineral oil were prepared by adding adsorbed films which are encountered in lubrication, the inknown amounts of pure sodium oleate to known amounts of ternal forces are not symmetrical. The molecules in the inner the oil, heating to 100" C. for 45 minutes with constant stir- layer next t o the metal are firmly bound, but as we pass toring, and finally chilling in an ice bath. Those mixtures in ward the liquid side the intermolecular forces become weaker which the amount of soap did not exceed 0.1 per cent of the and weaker and the orientation in the successive layers be-
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come more and more irregular. At no one interface is there an abrupt change from the regular orientation of the film to the disorganized structure of the liquid. Consequently there is no sharp point a t which the film suddenly disappears as the temperature is increased. The film simply becomes thinner and thinner as the more weakly oriented layers remote from the solid surface are liquefied. The critical temperature a t which the adsorbed film begins to show marked reduction in thickness and the coefficient of static friction begins t o rise sharply is not, therefore, as well defined as is the melting point of a solid. The exact position of the break in the curve will depend upon several factors-the nature of the adsorbed
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may be assumed that a t relatively low temperatures the moisture, either by being itself adsorbed on the bearing or in some other manner, interferes with the development of a normal lubricating film. At a critical temperature of about 80" C. this interference disappears and, through a short range of temperature, a film of very high lubricating power is formed. This may be a film containing both moisture and oil, possibly with the components in alternate layers. At about 100" C. this film is destroyed and the coefficient begins to rise toward that for dry oil. A portion of the oil saturated with sodium oleate a t room temperature was tested in the usual way while undried air was passed through the case surrounding the bearings. The results, as shown in Figure 3, were similar to those obtained with the oil alone, although in the presence of the soap the coefficients of friction were lower, both in dry air and in moist air, than those obtained with the pure oil.
EFFECTOF GLYCEROL AND WATERVAPORON COEFFICIENTS GREASES A synthetic grease was prepared that contained 5 per cent of sodium oleate and an amount of glycerol equal to that which would have been formed if the sodium oleate had been produced by the direct saponification of olein in the oil. The exact procedure followed in preparing this grease is described above. The product was similar in consistency to a typical fiber grease. No separation of solid soap took FIGURE 5. PLASTICITY GRAPHS FOR GREASES place, even when the material was allowed to stand for a COXTAINJNGDIFFERENTCONCENTRATIONS OF long time a t room temperature. The lubricating power of GLYCEROL this material was determined both in dried and in undried CURVE CURYE CURVE GLYCEROL AT 60.5' C. AT 25.8' C. AT O o C. air a t several different temperatures. The results are shown None 1 la lb in Figure 4. At 20" C. Normal amount 2 2a 2b 5 times normal amount 3 3a 3b the grease showed coefficients of friction considermaterial, the firmness with which the material is adsorbed ably less than those obo n the solid surface of the bearing, the thickness of the ad- tained with the oil alone sorbed film, etc. The introduction of even a very small under similar conditions. amount of soap (0.01 per cent) iiito.the oil raises the critical As the temperature was intemperature, since the soap film is more firmly adsorbed than creased the coefficient deis the film from the original oil. With increasing amounts creased rapidly. At about of soap the critical temperature falls again, since the adsorbed 90" C. there was a very films are thicker and the outer layers are less subject to the sharp drop, followed by a orienting action of the metal surface and are therefore more more gradual d e c r e a s e susceptible to the disintegrating action of the higher tempera- a b o v e 1 0 0 " C . T h e tures. When only a very small amount of soap is present greases containing two and and, therefore, the film is very thin, even a slight disintegra- three times the amount of tion of the film causes a rapid increase in the coefficient of glycerol equivalent to the static friction. Consequently the break in the curve is a soap used gave practically sharp one. With large amounts of soap the films are thicker identical results. These and the disintegration of the outer layers has relatively less were appreciably b e t t e r lubricants than the mixeffect, so that the breaks in the curve are more gradual. ture containing merely the FIGURE 6. EFFECT OF WORKEFFECT OF MOISTUREUPON THE COEFFICIENT OF STATIC normal quantity of glycIKG UPON PLASTIC CHARICerol. The drop in fricFRICTION OF OILS AND GREASES TERIBTTCS OF GRE.4SES tional coefficientsa t about CCRVE To determine the extent to which the presence of traces of 8 0 " C. w a s v e r y p r o Before After moisture may affect the coefficient of static friction of a n o u n c e d , and a t higher GLYCEROL stirring stirring lubricant, a sample of the lubricating oil that had been heated temperatures the l u b r i None la lb Normal amount 2s 2b t o 100" C. for 45 minutes was tested while ordinary undried cating effect was so great 5 times normal amount 3a 3b air was passed through the apparatus. The results are shown that its measurement was in Figure 3. The data obtained in the presence of undried difficult. Even with the air were much less consistent among themselves than those most careful leveling of the bed plate, difficulty was experiobtained with dry air, but the agreement among the large enced in placing the rider so that it would not slide before number of individual tests was sufficiently close to indicate tilting was started. definitely the form of the graph relating temperature and Unexpected results were obtained with the grease containcoefficient of friction. ing five times the normal amount of glycerol. At low temTo explain the unexpected effects of traces of moisture upon peratures this product had about the same lubricating power the coefficient of static friction, the writers have only a tenta- as the oil alone and was a much poorer lubricant than the tive hypothesis unsupported by independent evidence. It greases containing less glycerol. As the temperature was OF
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INDUSTRIAL AND ENGINEERING CHE~lIITlRY
increased from 20" to 70" C., the coefficient of friction remained constant. At 70" C. the frictional coefficient began to rise rapidly.
PLASTICITY OF GREASES Although the lubricating greases are usually regarded as plastics, the relationships between unit stress intensity and rate of flow of a grease are not exactly those specified by the laws of ideal plastic flow. There is no very definite yield point or minimum stress intensity below which no deformation or flow of the grease occurs, and the mobility or rate of change of deformation with change in stress intensity is not constant but increases with the rate of shear ( I ) . As the stress on a grease is gradually increased from zero, the rate of shear increases continuously (at first slowly and then more rapidly) until finally the ratio of the rate of deformation to the stress intensity becomes approximately constant. The plasticities of several greases were measured, using the plastometer described by Rhodes and Wells ( 2 ) . The efflux tube of this plastometer was 5.77 cm. long and had a radius of 0.1928 cm. The pressure required to force the grease through this tube was provided by applying nitrogen, under pressure, to the surface of the charge in the plastometer. The instrument was so constructed that the grease within the supply chamber could be stirred if desired. Plasticity measurements mere made on three samples containing 25 per cent of sodium oleate and, respectively, no glycerol, an amount of glycerol equivalent to the soap present, and five
Vol. 23, Yo. 11
times this amount of glycerol. The stirring mechanism was detached, so that the data indicate the characteristics of the material in the unmorked condition. The results are shown in Figure 5 . At 0" C. the presence of the normal amount of giycerol stiffens the mass; with large concentrations of glycerol the grease becomes even thinner than when none is present. At 25.8" C. somewhat similar effects are observed but the differences between the samples are smaller. At 60.5" C. the sample containing no glycerol is much more fluid than either of the others. With increasing concentration of glycerol there is a decrease in the change in consistency with temperature. Determinations were also made (at 25.8" C.) in which the grease was stirred. The agitator in the plastometer was rotated a t 50 revolutions per minute. The results are shown in Figure 6. The sample containing no glycerol broke down quickly and ran through the efflux tube under its own hydrostatic head. The grease containing the normal amount of glycerol showed much less tendency to disintegrate; the one containing five times the normal amount showed practically no change in consistency. The glycerol serves to minimize the change in consistency of the grease with working.
LITERATURE CITED (1) A n e s o n , ISD. E ~ CHEV., G 24, 71 (1932) (2) Rhodes a n d Wells, Ibzd., 21, 1273 (1929). (3) Wilson a n d B a r n a r d , Ihid., 14, 683 (1922) RECEIVED M a y 15, 1933
Reaction between Urea and Gypsum COLINW. WHITTAKER, FRARK 0. LUNDSTROM, AND STERLIXG B. HENDRICKS Bureau of Chemistry and Soils, Washington, D. C.
M
Crea reacts with gypsum i n the presence of moisture to set free the water of hydration the
opportunity for reaction between the urea and various s u b s t a n c e s present, e v e n in gypsum and form the complex CaSO4.4CO(NHd2. the absence of a visible liquid The Physical, optical, and chemical properties Of phase, This paper presents the this compound are described. It has beenfound results of a s t u d y of the reto be less hygroscopic than urea and therefore a c t i o n between urea and one would not in itself impair the mechanical conof the c o m p o n e n t s of superp h o s p h a t e , calcium s u l f a t e phosphate the with the bedifion of a fertilizer mixture in which if was dihydrate or gypsum, and the comes intimately mixed formed. If the reaction Were to 90 to completion properties of the c o m p o u n d other solids present. A kn0w.lin a n acerage commercial fertilizer containing formed in that reaction. edge of the reactions that may Urea and superphosphate the increase in free The u r e a a n d calcium s d occur b e t w e e n u r e a and the fate dihydrate used were C. P. various constituents of supermoisture would be only 0.2 to 0.4 per cent. p h o s p h a t e is fundamental to grade. lt7hen heated to conitant weight a t 350" to 400" C., a proper understanding of this problem. The use of urea in place of other forms of nitrogen the calcium sulfate lost weight equivalent to a water content in mixed fertilizers raises new problems among which is the of 20.8 per cent. The theoretical water of crystallization is effect of urea on the mechanical condition or drillability of the 20.93 per cent. Saturated salt solutioiis in desiccators furnished the various relative humidities used in the hygromixture containing it. Urea is known to form complexes with many salts, especially scopicity work and in determining the loss of moisture from with those having water of crystallization, although the fact urea-gypsum mixtures. The desiccators were kept in a that a given salt has no hydrates does not preclude the for- constant-temperature room a t 30" C. mation of a urea complex with that salt. A few typical REACTION BETWEE?; UREA.LVD C a L c I u I l r SULFATE IX THE examples of urea complexes with inorganic salts are as follows PRESENCE OF A LIQUID PHASE (8): Ca(N03)2,4CO(r\"2)2, KH4C1CO(NH2)2, CaI2.6COEqual amounts of calcium sulfate dihydrate and urea were (MI&,C a I ~ C 0 ( N H 2 ) , . 2 H ~HgCl2.CO(NH2)2, O, AgKO&O( S H & The spraying of superphosphate with a solution of added to an aqueous solution of the latter saturated a t 30" urea in ammonia (6) deposits the urea in a finely divided form C., and the whole was maintained a t that temperature in a on the granules of the superphosphate, thus providing thermostat with occasional shaking. Microscopic examina-
ODERN processes of
ureamanufactureprod u c e a s o l u t i o n of urea in a m m o n i a that can be economically used to ammoniate s u p e r p h o s p h a t e . The ammonia reacts with the acid c o n s t i t u e n t s of the s u p e r -
