X-Ray Diffraction Studies of Lubricants' Structures of Solid and of Unimolecular Films,
and Orientation Effects of Addition Agents GEORGE L. CLARK, ROBERT R. STERRETT? University of Illinois, Urbana, 111. AND BERT H. LINCOLN Continental Oil Company, Ponca City, Okla.
W
ELLS and Southcombe (8) discovered that the addition of a small amount of fatty acid reduced the static coefficient of friction and increased the oiliness of a mineral oil to a marked degree. Different types of experiments designed to investigate the causes producing such a beneficial effect indicated that this was the greatest in the range of boundary lubrication associated with very thin films, without doubt owing to the concentration of these polar molecules a t the oil-metal interface. Two explanations have been offered for the mechanism of the action of the polar molecules in t'his interfacial layer: 1. The polar ends of t,he fatty acid molecule tend t o interact with the secondary valence forces of the metal surfaces, thereby prevent,ing metal cohesion. 2. These molecules produce a layer on the surface which by its structure facilitates tangential slippage but is very resistant to rupture by a perpendicular force.
Bragg (9)and Trillat (7) gave some insight as to what may be the structure of the above-mentioned surface layer. From x-ray diffraction patterns of thin films of fatty acids it is deduced directly that the molecules are oriented with lengths perpendicular to the surface, carboxyls being adjacent in successive pairs of layers which serve as diffracting planes. Such a lubricating layer between rubbing surfaces behaves like a deck of playing cards. On the application of a tangential force the cards slip readily over one another, but a corresponding perpendicular stress produces little effect. This has been accepted as a suitable explanation for the action of fatty acids in lubricating oil blends. However, during recent years new types of addition agents have been developed which impart to the mineral oil some additional I Bssed on portions of a thesis submitted by R . R . Sterrett in partial fulfillment of the requirements for the Ph.D. degree, Cniversity of Illinois, February, 1936. 1 Continental Oil Research Fellow, Cniveraity of Illinois: present address, United States Rubber Company, Passaic, N. J.
beneficial properties. Dichloromethyl stearate is a good example, since much of the experimental work has been concerned with this compound. It possesses practical advantages of satisfactory nonvolatility, solubility, odor, lack of c o r r o s i v e n e s s , etc. If a S. A. E.30 mineral oil and a one per cent blend of methyl stearate and dichloromethyl stearate are compared for film strength by one of the familiar practical tests used by many oil companies (the so-called Timken test made under carefully controlled conditions), the follon-ing results are obtained: Pounds Load on the Arm 1% Dichloromethyl stearate blend blend S. -4.E. 30 mineral oil, base of blends
lYO Methyl stearate
28 14
8.5
Although these comparative values are not conclusive evidence of actual performance in service, a n increase in film strength due to' the chlorinated e s t e r i n 1 per A number of organic comcent blends is indicated. Other pounds, particularly chlochlorinated o r rinated derivatives, which ganic compounds possess some possibilities as also show charaddition agents to lubricatacteristic effects ing oils in enhancing film as measured by the Timken mastrengths and general perchine; some give formance, were subjected to higher and some x-ray diffraction analysis as lower values thin solid films on suitablethan 28 pounds metal surfaces. In many (6). The p u r p o s e cases t h e unsubstituted of the present recompound and chlorinated searches was to derivative orient e q u a l l y apply the x-ray well, but chlorinated esters diffraction of thelong-chain acid, shortm e t h o d to the investigation of c h a i n a l c o h o l t y p e are the structure of markedly superior to the lubricating films unchlorinated esters in preand the effects ferred molecular orientation of such addition and doubling of molecules agents as chloin each layer ; this accounts rinated e s t e r s which are able in part for increased film t o produce these strength of oil blends conimprovements in taining these addition lubrication, such agents. as reduction of The degree of molecular the coefficient of 1318
NOVEMBER. 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
friction and increases in film strength. With the studies of Bragg (2) and Trillat ( 7 ) as the only x-ray work relating to the theory of lubrication, it was necessary to develop new methods in order t o apply this technic to entirely new phases of the problem not included in their work. A study of orientation and structure of thin films of pure compounds was the first phase t o be investigated.
Structure of Solid Films of Pure Compounds Used as Addition Agents Bragg ( 2 )in his theory of lubrication reasoned that in layers of f a t t y acid on the metallic surface the double structure of the molecules was of great importance. This structure provided for the molecules to be held together in rather rigid layers by t h e carboxyl groups; the slippage would take place easily along those planes where the terminal methyl groups adjoined. The fundamental chemical and physical forces in lubrication and the practical selection of improved lubricants were considered in detail in papers presented before the American Petroleum Institute (S,5).
TABLE I.
1319
MOLECCLAR ARRANGEMENTS dh'D SPACINGS IN OF ADDITIOXAGENTS
Orders Present Stearic acid Odd Dichlorostearic acid Odd .Methyl stearate .411 Methyl palmitate A11 Dichloroethyl stearate Mixed Dichloromethyl stearate Mixed Dichlorocetyl acetate -411 Dichloroactadecyl stearate Odd Trichlorophenol Odd 'Trichloronaphthalene Monochlorodiphenylene oxide
Not oriented
...
Spacing,
Molecular lrrangement
A.
39.5 39.2 26.5 22 2 51.5 49.0 27.6 42.9 7.95
... IO.5 5
FILMS
Double Double Single Single Double Double Single Single or double Plane of ring parallel to surface
..
'
.
.
.
.
.
I
.
Plane of ring parallel to surface
s h o w that both oriented equally well and indicated the same double-molecule structure. A considerable difference, however, appeared in the case of the esters. The chlorinated ethyl stearate exhibited a double-molecule structure, and the molecules must then have been held a t the polar ends by strong forces since tlie electrons seemed to be concentrated in a n exceedingly well-defined plane. The evidence for this latter statement TWLS found in EXPERIMENT .IL To examine the molecular structure of laver5 of some of these chlorinated addition compounds, the solids TT ere the normal decrease oi intensity in successive orders and, melted onto cylindrical brass rods, and the x-ray beam TI as pacsed therefore, also in the great number of orders present. A tangential to the surface. BYthe use of such a curved specimen similar structure was indicated in patterns given by chlorinthe necessity of rotation or o&illation of a plate covered with the ated methyl stearate taken in the lowtemperature camera. film was avoided. The effect on the planar spacing of a change in the diameter of the cylinder was investigated nith a pure Therefore, a double-niolecule structure may be expected compound ivhich gave twentv orders of reflection from the long when the ester is of the long-chain acid, short-chain alcohol spacing. The size of the cylinder between 1 and 0 2 cm. (0.39 type. With all the unsubstituted esters studied, as well as and 0.079 inch) diameter had no effect. The diqtance from >amwith those reported in the literature, the layers were formed of ple to photographic plate was made nearly 5 cm. (1.97 inches) and calculated exactly from a diffraction line due to the pure single molecules. A similar arrangement was evidenced in metal. The error in this quantity was about 1 0 . 1 mm. (0 0039 the layers of the chlorinated esters of the short-acid, longinch). The accuracy of pattern measurement nas such that it alcohol or the long-acid, long-alcohol types. We may then enabled the calcuconclude that differences in film strength among the 1 per lation of the int e r p l a n a r diecent chlorinated aliphatic ester-oil blends can be accounted tance to w i t h i n for in part by the fact that certain of these compounds show 1 0 . 2 6. orientation depends upon the double-molecule structure. the metal used in the bearOf the three chlorinated ring compounds, two showed RESULTS.4ND ing surface, as shown by definite molecular orientation n-ith the plane of the ring paralC O N C L U S I O N S . lel to the surface. 9 n y other arrangement would not be in comparisons of films on Table I conaccord with the known size of the rings. Evidently these copper, iron, zinc, cadmium, tains t h e l o n g highly polar molecules adhere to the surface in a layer conand brass. s p a c i n g corresisting of flat molecules that slide readily over one another. sponding to moFilms built up from uniTrichlorophenol, which forms a layer resembling a perfect lecular l e n g t h molecular layers by a technic crystal, has by far the highest value for film strength of these and layer structhree materials. Trichloronaphthalene does not orient and originally proposed by Blodture for certain has a l o a value in this regard. Monochlorodiphenylene gett and Langmuir to a long-chain acids oxide is intermediate in both respects. The following are known thickness have been and e s t e r s and Timken values for blends containing one per cent by weight of three compounds studied in terms of fundathe compound in a S. A. E. 30 mineral oil: of the aromatic mental behavior in diffractPounds Load type. For the Addition Comuound on the Arm ing x-rays, and correlation substituted acids Trichlorophenol 49.0 of structure and orientation 34.8 Monochlorodiphenylene oxide and esters, chlo27 0 Trichloronaphthalene with l u b r i c a t i n g mecharination was Evidently, then, orientation is important but is not the c a r r i e d out on nism. only factor in determining film strength. This is indicated the correspondThe remarkable ability of by the fact that some of the chlorinated and unsubstituted ing unsubstipolar addition compounds, compounds are nearly equivalent in the matter of preferred tuted compound; such as methyl and ethyl niolecular orientation but vary widely in ability to increase the chlorines film strength in oil blends. esters of chlorinated longp r o b a b l y took u p positions on chain acids when present Orientation of Films on Different Metals the CY and @ carin very small amounts, to bons of the acid. The measurement of the length of the molecules and their improve the orientation of arrangement-i. e., double- or single-molecular layers-as reA comparison hydrocarbon molecules in corded previously is not dependent on the type of metal used. of t h e s u b s t i films is demonstrated from However, two differences in the patterns are obtained from t u t e d a n d unfilms of the same compound formed in identical manner on diffraction patterns. substituted acids
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