702
INDUSTRlAL AND ENGINEERING CHEMISTRY
VOL. 29, NO. 6
show no free chloride generation when sealed in glass and heated with water for 6 hours or more at 150°C. No free chlorides are generated ?hen these compounds are refluxed with water, even when oxygen gas is passed through the liquid-water mixture. One of the most sensitive tests for the chemical instability of chlorinated organic compounds is the attack of the unstable chlorine, if present, on clean aluminum foil. The transformer liquids here described show no corrosive action on aluminum even though heated in contact with it in the presence of air and water at temperatures as high as 260" C. After having been heated a t 260" C, for 6 hours in contact with aluminum, the stability of the chlorinated compound suggested for transformer use is demonstrated not only by lack of metallic corrosion but by the substantially unchanged condition of the compound itself, the only change is a slight and almost imperceptible darkening in color. The high dielectric strength of these transformer compound mixtures is maintained even when directly exposed to atmospheric humidity changes over long periods. Figure 5 illustrates the stability in dielectric strength of a trichlorobenzenechlorinated diphenyl mixture exposed to atmospheric changes under conditions which were known to have caused the accumulation of water in the mixture itself. The water accumulated on the surface of the heavier chlorinated mixture without noticeable effects on the dielectric strength.
pounds of the type described has been successfully carried 0ut.l The practical elimination of the fire hazard heretofore associated with oil-filled electrical transformers has been recognized by the National Board of Fire Underwriters to the extent that the National Electric Code has been modified (12). The stringent restrictions and expensive vault requirements for the use of oil-filled transformers have been removed. This action appears fully justified in the light of the successful commercial use of these products during the past five years.
Industrial Application
RRCEIVED March 10, 1937. Presented before the Division of Industrial and Engineering Chemistry at the 93rd Meeting of the American Chemical Society, Chapel Hill, N. C., April 12 to 15, 1937.
The commercial introduction of the chlorinated cyclic, nonflammable, and nonexplosive dielectric hydrocarbon com-
Literature Cited (1) Bone and Coward, J. Chem. SOC.,93, 1197 (1924). (2) Cantelo, R. C., J. Phye. Chem., 28, 1036 (1924). (3) Clark, F. M., Trans. Electrochem. SOC.,65, 5 9 (1934). (4) Clark, F. M., U. 8.Patents 1,931,373 and 1,931,455 (1933). (5) Ibid., 1,944,730 (1934). (6) Ibid., 2,041,594 (1936). (7) Clark, F. M., and Kuta, W. M., Ibid., 2,012,301 (1935). (8) Ibid., 2,012,302 (1935). (9) Jenkins, R. L., and Sikarski, J. A., Ibid., 1,892,400 (1932). (10) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. I, p. 492, Vol 11, p. 173, Vol. V, p. 819, New York, Longmans, Green and Co., 1922-4. (11) Milbauer, J., Collection Czechoslov. Chem. Commun., 3 , 73 (1931). (12) Natl. Board Fire Underwriters, Natl. Electrio Code, pp. 282-8 (1935). (13) Norrish, R. J. W . , Proc. Roy. SOC.(London), 150, 36 (1935). (14) Rhodes, F. H., and Carey, J. T., IND. ENG. CHEM.,17, 909 (1926).
, Commercially
designated as Pyranol.
Lubricating Properties of Lime-Base Greases F. H. RHODES AND THOMAS ELLIOTT WANNAMAKER Cornell University, Ithaca, N. Y.
T
H E cup greases in common use consist essentially of petroleumlubricating oils thickened with calcium salts of the fatty acids. Water is also a normal component of commercial cup greases, the amount of water usually varying from 3 to 1 per cent. In most cases the lime-base greases are prepared commercially by direct saponification with calcium hydroxide of a fat in solution in a petroleum oil. Products thus prepared contain a t least a small amount of glycerol, which may have some effect upon the structure and properties of the grease. Since the conditions under which greases are used are frequently such as to make it difficult to maintain an excess of lubricant on the bearing, the characteristics of greases under conditions of film lubrication are of particular importance. It has been found (2) that in the case of the soda-base greases the lubricating characteristics of the base oil are greatly modified by the presence of soap, water, and glycerol. It is to be expected that, in the lime-base greases, corresponding effects although not necessarily similar ones, should be observed.
I
The ratio of water to calcium soap in a limebase grease has a very pronounced effect upon the consistency and upon the lubricating characteristics of the grease. The lubricating power may also be affected by the glycerol that is present.
I
Preparation of Greases The petroleum oil used in all of these greases was a distilled lubrication oil from paraffin-base (Pennsylvania) crude. It showed the following characteristics: density a t 20" C., 0.88; Saybolt viscosity a t 100" F., 445 seconds, and a t 210" F., 64 seconds. Preliminary experiments showed that pure dry calcium oleate, free from oleic acid, is practically insoluble in the petroleum oil. No true grease could be obtained by warming and stirring a mixture of the soap and oil to which a small amount of water had been added. On the other hand, when 7Qparts of the oil were warmed with 30 parts of a somewhat
JUNE, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
a. Effect of soap content and glycerol A . Oilalone 1. 10% soap 2,3. 15 and 20% soap 10. Containing glycerol
703
C.
Effect of free oleic acid A . Oil alone 7. No oleic acid 8. 2% oleic acid 9. 4 % oleic acid
FIGURE 1. VARIATIONOF FRICTION COEFFICIENT WITH TEMPERATURE
impure calcium oleate that had been allowed to stand for some time in the air and contained some free fatty acid, the soap passed completely into solution. The solution, when cool, was only slightly more viscous than the original oil. When water was added t o this solution, a stable but fluid emulsion resulted. Similar results were obtained when the neutral and undecomposed soap was warmed with petroleum oil to which a small amount of oleic acid had been added. Originally pure and neutral calcium oleate, when kept in the air for several weeks, acquired sufficient rancidity to become soluble to a considerable extent in the oil. The observation that calcium soaps can be dispersed by the addition of free fatty acid to form colloidal suspensions has been made by Boner ( 1 ) . The calcium oleate used in the ex,periments was prepared as follows : To a solution of pure neutral sodium oleate in hot water was added a slight excess of a hot dilute solution of calcium chloride. The precipitated calcium oleate was washed repeatedly by decantation with hot water and was then ground for 30 minutes in a mortar with frequently renewed portions of hot water. After filtration, the washed precipitate was allowed to stand for ashort time under renewed portions of absolute alcohol, filtered, washed with ether, dried quickly in the air, and stored in a desiccator in an atmosphere of dry nitrogen. This material remained neutral and was not soluble in petroleum oil, even after long standing. Mixtures containing 85 grams of petroleum oil, 15 grams of neutral dry calcium oleate, and known amounts of oleic acid were warmed to 110" C. for 30 minutes and then allowed to cool. In the mixtures containing more than 4.5 grams of free oleic acid, the soap dissolved completely to form solutions that, on cooling, were not much more viscous than the original oil. With only3 grams of the free acid, the soap dissolved only slowly; the cooled solution was very viscous. When only 1.5 grams of the free acid were present, the solution of the soap was incomplete and a part of the soap that dissolved on heating reprecipitated on cooling. In a similar series of experiments made with 75 parts of oil and 25 parts of calcium oleate, the addition of 10 grams of free oreic acid gave a fluid solution; when only 7.5 grams of acid were added, the solution remained clear but was distinctly more viscous than the original oil. When a mixture of 85 grams of oil, 15 parts of soap, and 5 parts of free acid was heated to 150" C. for 1 hour and then cooled to room temperature, a stiff translucent jelly resulted. In no case did any anhydrous mixture of petroleum oil, calcium oleate, and oleic acid give a product with the characteristics of a true grease. When water is stirred into a solution of calcium oleate in oil containing free fatty acid, a grease results. The consistency of the grease appears to depend much more on the relative concentration of the free fatty acid than on the amount of water. Mixtures containing large amounts of free acid give fluid emulsions, even with relatively small quantities of water; when the mixtures contain but little free acid, stiff
greases can be obtained with as much a s 50 per cent water content. Although true greases could not be made by the addition of water to mixtures of neutral calcium oleate and petroleum oil, it was found that neutral greases with the typical texture and consistency of cup greases could be prepared by the following procedure : To 100 grams of a mixture of petroleum oil and neutral calcium oleate in the desired proportions, 2 grams of free oleic acid were added. The mixture was heated to 110-120° C . for an hour and allowed to cool t o 90" C. At this temperature a weighed quantity of a suspension of known concentration of pure calcium hydroxide in water was added, and the mixture was allowed to cool with constant stirring. The amount of calcium hydroxide added was just sufficient to react with the free oleic acid to form a neutral soap; the amount of water in the milk of lime was just enough (with that formed by the neutralization reaction) to give the desired amount Qf water in the hished grease. Samples of grease were prepared to contain, for each 80 grams of oil and 20 grams of soap, 0.1, 0.2, 0.5, 1.5, and 3.0grams of water, respectively. On standing, the mixtures that contained 0.1 and 0.2 gram of water formed gels that separated from part of the oil. With 0.5 gram of water a solid homogeneous gel was produced. When 1 gram of water was present, a grease was formed. On standing, this product appeared to dry out and shrinkage cracks appeared. The addition of 1.5 grams or larger amounts of water gave typical greases. When a large amount of water was used a few droplets of unemulsified water could be seen in the grease.
In the commercial preparation of cup grease, it is common practice to form the soap by the direct saponification of a fat in the oil. Probably the free fat, or the free fatty acid that may be present in small amounts near the end of the process, serves the same purpose as did the free fatty acid used herein the laboratory.
Lubricating Characteristics of Greases The coefficients of static friction for greases of various compositions were determined by the method and apparatus previously described by Rhodes and Allen (2) and Rhodes and Lewis (3). All results are expressed in terms of the tangent, p , of the angle a t which slip of the lubricated surfaces first occurred. The original petroleum oil used in the preparation of the greases showed a coefficient of 0.130 * 0.003 (11). (The indicated deviation is the arithmetical average deviation. The figure in parentheses indicates the number of individual measurements from which the average value was computed. The measurements were made a t various temperatures, from about 30" to 100" C.) The same oil, after being heated t o 110" C. for an hour and then cooled in a desiccator, gave a value of
*
INDUSTRIAL AND ENGINEERING CHEMISTRY
704
0.133 * 0.001 (11). Evidently the heating alone had no significant effect on the frictional characteristics. Oil that had been heated for an hour with an excess of calcium oleate, decanted from the undissolved s ~ o p ,and cooled, showed a coefficient of 0.148 f 0.0003 (14). Calcium oleate alone, even in the very low concentrations in which it is soluble in the oil, decreases the lubricating power; in this respect it differs from sodium oleate (2). When 95 grams of oil, 5 grams of calcium oleate, and 1.5 grams of oleic acid were heated for 30 minutes and cooled, the product gave a p value of 0.126 * 0.001 (12). A mixture of 80 grams of oil, 20 grams of soap, and 6 grams of oleic acid gave a p value of 0.126 * 0.001 (11). Petroleum oil containing 1.5 per cent oleic acid alone gave a coefficient of 0.076 * 0.004 (8). Apparently the calcium oleate that has been brought into solution or colloidal suspension in petroleum oil by the addition of oleic acid reacted with or, more likely, adsorbed the major portion of the free acid so that it was no longer available to be adsorbed on the bearing surfaces and to be effective in reducing the frictional coefficient. Greases containing 1.5 grams of water and the following weights of oil and soap were prepared and examined: Grease No, 1
Calcium Oleate, Grams 10
90 85 80
15
2 3
a
Petroleum Oil, Grams
20
The results are shown by Figure la. At low temperatures the greases have lower lubricating values than does the oil alone; a t higher temperatures the greases are the better lubricants. The data appear to indicate that an increase in the content of soap increases the lubricating power; the results of further experiments show that the decrease in lubricating power is not due directly to the increased concentration of sosp but is to be attributed to the decrease in the ratio of water to soap. Greases containing soap and oil in constant ratio and water in varying amounts, as follows, were prepared and tested: Crease No. Oil, grams Soap, grams Water, grams
4 85 1.5 1.5
6 85 1.5
3
6 85
15 5
The results are shown in Figure lb. At relatively low temperatures the lubricating power increased with increase in the water content; a t higher temperatures the frictional coefficients for all three greases approached the same low value. At some rather definite and high temperature, with each grease there appeared to be a change resulting in an abrupt decrease in the coefficient of static friction. This critical temperature decreased with increase in the water content of the soap, It appears that with the lime-base greases the water is in some way involved in the formation of the adsorbed lubricating film and is important in determining the characteristics of the film. The polished lugs of the apparatus used in the determination of the coefficients of friction remained bright a t all temperatures below the critical values a t which the abrupt drop in the static coefficient occurred; a t temperatures above this critical value a thin dull "greasy" film appeared on the metal.
VOL. 29, NO. 6
To determine the effect of free fatty acid, the following greases were tested (Figure IC) : Grease No. Soap, grams Oil grams Oleh acid, grams
7 15 85 0
8 15 85 2
9 15 86 4
The addition of oleic acid improved the lubricating power a t low temperatures but decreased it a t higher temperatures. In the presence of the free acid there was no sharply pronounced decrease in the frictional coefficient as the temperature was raised. The samples containing oleic acid showed no deposition of a film on the surface of the metal at higher temperatures. In the ordinary cup greases made by the direct saponification of fat in oil by means of lime, the glycerol formed by the reaction remains in the finished product and may conceivably affect its lubricating characteristics. To a grease prepared in the usual way and containing 85 grams of oil, 15 grams of soap, and 1.5 grams of water, 5.5 grams of glycerol were added. The frictional characteristics of this material are shown by curve 10, Figure l a . The presence of glycerol lowered the coefficient of friction a t all temperatures below about 90" C. The grease containing glycerol did not show the increase in lubricating power between 30" and 60" C. that is exhibited by greases with a high ratio of water to soap. I n the presence of glycerol the deposition of a greasy film a t high temperatures was observed.
Conclusions The phenomena involved in the formation and the behavior of the lime-base greases are so complicated that any attempt to explain the characteristics of such products on the basis of the simple assumption that they are suspension of micellar aggregates of calcium oleate in oil is inadequate. The water present in the grease plays an important part in determining its lubricating characteristics; for maximum lubricating power the proper ratio of water to soap is essential. Probably the water is actually built into the laminated adsorbed film that forms on the surface of the metals and that is responsible for the lubricating power of the grease. The glycerol formed during the process of manufacture may have a distinct effect on the effectiveness of the grease. The presence of free oleic acid in the finished product appears to be of little real benefit. In this work the effects of the concentration of soap, water, and free oleic acid on the static coefficient of friction of greases prepared from a rather viscous paraffin-base oil and calcium oleate have been of primary interest. Commercial greases are usually made from mixed soaps and are often prepared from oils differing somewhat in viscosity and character from that used here. These variations in the nature of the soap and the oil may be expected to have definite effects on the lubricating characteristics.
Literature Cited (1) Boner, IXD.ENQ.CHIOM., 27, 665 (1935). (2) Rhodes and Allen, Ibid., 25, 1275 (1933). (3) Rhodes and Lewis, Ibid., 26, 1011 (1934). RECEIVED
July 10, 1936.
