Structure of Lubricating
Greases BRUCE B FARRINGTON A N D W N DAVIS Standard Oil Company of California, Richmond, Calif.
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HE r!.ajority of 1u I >r i c a t in g nease~coiisist of petroleuni luhrikating oil- absorbed in a soap matriy, tho h i ter being respnsible for the plae~icityand resistancr. to fh~wof this typo of lubricant Th,. chemistry of the soaps usually used in grmse manufacture-e.g., tile c;ilcimo, sodium, aluminuni, lead, and zinc soaps of aniroal and v e g e t a b l t , fats and fatty acid-is fairly ~ v l known, l but t h e d i s t r i b i i t i o i i of soap througho.it the various greases has apparently received scant attention. I J a11 ~ rittempt to obtain a knowledge of the fine structureof lubricating greases, the usiial micrescopic examination w i t h t r a n s mitted light was found to be unsatisfactory bei:ause of the t r a n s parent nature of the soap particles and the puzzling diffraction effects encountered. The uhe of dark-field t.echnie, however, made it possible bo ohtaiii g w d idea uf the fine structure of lubricating greases:
The nppmitus at first used for visual study was a standard microscope equipped with 8 cardioidaondenscr 3-mni. npochmm:rl, objeetivc lens xith iris and 10-mapnificat,ionocular. The
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and a i lamp for the Maadn projecbion lamp. The actual equipment used was a k i t e photomicroscopic apparai,u"(~nrlallographic type) nith 5 specially built condenser and mirror holder, hut the flexihilit,~and freedom from vibration of this apparatus is ~
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A s !m immersion oil, heavy liquid petrolatum was found satisfactory. Specimen smears were made by lacing a small piece of grease on B scrupulously clean glass slide 8 . 2 mm. thick), eovering with a clean cover glass, and pressing out flat. Immersion oil was placed between condenser and slide and between cover glass and object,ive lens. In order to study fiber detail, it was found necessary to thin out all hut the softest greasea by stirring into the grease placed on the &de B drop of liquid petrolatum; the hest results were obtained when microscopic pools of oil were dispersed throughout tho grease. Structural details are most easily observed along the borders of these oil pools because the latter a pear hlaek and afford the needed contrast with t.he lighted &em
Classification of Grease by Fiber Length Although there arc several methods of classifying lubricating grease---e.g., according to the alkali used for making the soap, 6he melting point, water resistance, method of maoufacture, ctc.-the texture of greases affordsa simple basis for arranging these products in a natural sequence, the roughest and most fibrous at one end and the smoothest and most salvelike at the other. Examination of lubricating greases by means of the dark-field microscope shows that this classification is by no means artificial, and that a continuous decrease in fiber length takes place RS the texture bccotnes smoother. LONG-FIIIEIR GIEASE. The coarsest structured greases are the soda-base "fiber greases." Figure 1 is a photomicrograph of a product of this type, a gear grease, enlarged 375 diameters. The long soap fibers are prominent and easily recognizable whereas t,hcrriint:ral oil appears black. From 0.01 to 0.03 mm. in diameter, the soap fibers extend for 1 mm. or more, and bend and double in graceful sinuous curves. In spite of their gauzy appearance ilie soap fibers arc tough and strong, and are made up of bundles of fibrils with their long axes practically parallel. A detailed view of these soap fibers is shown in Figure 2 ( X 1000). The fibrils making up the fibers measure about 0.5 micron (1 microii = 0.001 mm.) in diameter, altliough they may actually be slighdly less because of the diffraction effect. The length of the fibrils in the compound fiher has not yet been determined. Figure 1shows that, although fibers are comnion and characteristic featiires of fiher greases, they do not make up the entire body of such material. This is illustrated by the
The texture of lubricating greases is correlated with the length of soap fiber by means of photomicrographs taken with dark-field illumination. The soap fibers pictured vary in length from 1 mm. to a fraction of a micron with indications that some of the fibers are of ultramicroscopic dimensions. A system of classifying lubricating greases by means of soap fiber length is proposed, and an illustration of the applicability of the microscopic method to practical grease manufacture is given.
globular masses which appear in the lower left center of the picture. Long-fiber greases are most useful for the lubrication of gear sets because of the heavy film of lubricant built up by the matting action of the long soap fibers. The cushion of grease between the gear teeth helps considerably to take up the slack of worn gears, provides quiet operation, and prevents leakage from the gear housing. Figure 3 (X 1000) shows a detailed view of anotlier gear grease used in automotive transmissions and differentials. The slender type of soap fiber characteristic of this grease is shown clearly. In addition to the fibers, numerous small bright particles of a colloidal nature are scattered throughout. These are minute solid particles found only in dark colored, unfiltered oils. The adhesiveness of long-fiber greases is also made use of in universal joint lubrication in which case the fibers reduce the tendency of the grease to be thrown from the rotating parts, Figure 4 ( X 1000) shows a grease of this type. MEnrm-FmER GREASE. For certain purposes the clinging touglmess lent by long soap fibers is a disadvantage. Such is the case in the lubrication of ball and roller bearings, where the cohesiveness of a long-fiber grease causes it to be dragged from the rollers and wound in a ball. It is necesssary in this case to modify the fiber structure to an extent that will retain the resistance to the mechanical milling action of the rollers and the high melting point of fiber greases without the extreme stringineas characteristic of such products. Figure 5 ( X 1000) shows a product of this typo. The fibers here are not made up of bundles of fibrils as those of the long-fiber greases, but appear to be single, flat, twisted fibers measuring 60 to 80 microns in length (0.06 to 0.08 mm.), 1 micron in width, and 0.3 micron in thickness. The texture of medium-fiber greases resembles that of petrolatum, but a micrograph of snow-white petrolatum (Figure 6, X 1000) given for comparison shows that some of the fibers, in this case wax crystals, are considerably more rigid than the soap fibers existing in greqes. SHOET-FIBER GREASE.Short-fiber greases arc represented in Figure 7 ( X 1000). Here the fibers are sborter than any
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pictured so far; they range from 5 60 10 micronsinlength, and are about 0.4 micron in diameter. Included in the same class is the brick grease (Fignre 8, X 1000),aprodiictofthe rollgrease type manufactured a t high tomp e r a t u r e s . Roll greases are c o m m o n l y used in open-type 1,earings (paper mills) where consumption of the grease blocks must be kept to a low figure. Another i m p o r t a n t type of sbort-fiber grease is that used for the lubrication of locomot.ive driving journals and rod cups. Figure 9 ( X 1000) shows bhe structural details of a product of the latter type, t,he characteristic fibers of which are between l and 3 microns in length. MrcRoFInER GREASE. This group includes all greases with fiber lengths of 1 micron or less. S m o o t h - t e x t u r e cup greases (lime-soapbase products) are typical representatives of this class, as shown by Figure 10 ( X 1000). The soap fibers range from a few t,enths to 1 micron i n l e n g t h , a n d m e a s u r e 0.2 micron or less in d i a m e t e r . Measurements of fibers of this size are not reliable because of diffraction effects. By far the finest t e x t u r e d greases examined are the aluminum-soap-base products (Figure 11, X 1000). In outward a p p e a r a n c e s i m i l a r to cup greases, these p r o d u c t s show under the microscope a structure so fine as to be practically unr e s o l v e d by the microscopes available. The piet.nre shows a grain:, structure of some sort. which, from analogy with all other greases examined, is in all probability due to soap fibers of very small size; a guess a t their dimensions would be about 0.1 micron in length and 0.05 micron in diameter, allowing for diffraction effects. PROPOSED SYSTEM OF GREASE c L A S S 1 F I C A T 1 O N . Sufficient data have been presented to fibow the value of the dark-field microscopic method for classifying lubricating g r e a s e s b s
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means of fine structural detail and the length of soap fiber.
A complete series of grease micrographs has been given to illustrate the wide range of fiber length encountered (1 mm. down to less than a few tenths of a micron). A system of grease classification is proposed, depending on the length of soap fiber: Classification Long fiber Medium fiber Short fiber Microfiber
Fiber Length Microns 100 or more 10 to 100 1 to 10 1 or less
Texture Fibrous ropy Clingin& slightly rough Slightly rough, short Smooth, unctuous
Effect of Heat Processing on Grease Structure As an example of the uses to which the dark-field microscopic method can be put, Figures 12 and 13 ( X 1000) show what
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happens to certain kinds of fiber greases on continued heating and stirring. Figure 12 shows a short-fiber grease heated for 1 hour, and Figure 13 the same grease after 4 hours of heating and stirring. The fibers in the latter case are only about one-fifth of the length of the original. Under other conditions of manufacture, the fibers can be built u p to some extent.
Acknowledgment Grateful acknowledgment is made to Paul M. Viemont for assistance in the preparation of the photomicrograph*. RECEIVED October 18, 1935. Presented before the Division of Petroleum Chemistry a t the 90th Meeting of the American Chemical Society, San Francisco, Calif., August 19 to 23, 1935.
Durability of Paint on Wood Effect of Extractive Substances in Certain W o o d s F. L. BROWNE Forest Products Laboratory, Madison, Wis.
OR many years differences in the durability of house paints on various woods were commonly attributed chiefly to unfavorable effects of extractive substances, often indiscriminately called “resins,” present in some woods. Many manufacturers of paint still write directions for application in which that idea is clearly reflected. Careful studies a t the Forest Products Laboratory of the painting characteristics of woods (2), however, proved that physical structure is the dominant property of wood affecting the durability of house paint and that the importance of extractive substances has been greatly exaggerated ( 1 ) . Certain minor trends attributable to a n effect of extractive substances were observed, but they were not always unfavorable. I n redwood and southern cypress, extractives seemed to prolong the life of paint coatings; in the white pines and yellow pines, extractives seemed to shorten the life of coatings of a mixed-pigment paint containing zinc oxide but to exert little if any effect on the life of pure white lead paint. The extractive substances in cypress, redwood, and the pines differ in nature far too seriously to be lumped together under any one designation such as “resin$.” Only those extractives of wood that are reasonably closely related to the
When the extractive substances of redwood, southern cypress, and ponderosa pine, respectively, are transferred to the surfaces of boards of eastern hemlock, a wood lacking in characteristic extractive substances of its own, the hemlock acquires some of the painting characteristics of the wood from which the extract is taken. The experiments substantially confirm an earlier deduction that extractive substances in certain woods affect the durability of paint coatings, sometimes favorably, sometimes unfavorably, although the effect of such extractive substances is much less important in pqint life than the. physical structure of the wood.
resin of southern yellow pine ought to be called resins. Such extractives consist of a solid rosin and a volatile oil, usually turpentine. The principal extractive substances characteristic of the white pines and the yellow pines are of this type. The resin of ponderosa pine, which was used in the experiments described in this report, contains a rosin that is chiefly abietic acid (7) and a turpentine. Softwoods other than the pines contain resins but in smaller amounts and usually in localized deposits rather than diffused generally throughout the wood. The characteristic extractive of southern cypress is oily in nature. It contains cyprall, which is probably an aldehyde, and the sesquiterpene cypressene ( 5 ) . Chemically and physically the oily substance in cypress differs markedly from the resins of the pines. Cypress may properly be called an oily wood, but it should not be classed with the pines as a resinous wood. The characteristic extractives of redwood are soluble in water and consist of a complex mixture of Ieuco bases of certain dyes (dihydropyrene derivatives) (IO),tannins, and
