SOLID PHASE STRUCTURE IN LUBRICATING GREASE BY E. W. J. MARDLES* AND I. E. PUDDING TON^ Received 5th April, 1951 In order that a system may have desirable properties as a lubricating grease, the ultimate solid phase particles should be highly anisometric. In addition, i t is important that the solid phase surface should promote aggregation of the individual particles to produce sufficient structure to give the system a yield value. This structure should be capable of reversible breakdown under con- ditions of shear and show little hysteresis in recovery. Conventional lubricating greases are dispersions of metallic soaps in lubricating oil. The most common forms are soft solids and contain about 10 yo of soap by volume. The usefulness of greases depends on a unique property ; at low stresses, the viscosity is very high-approaching infinity under the influence of gravity alone-but as the rate of shear is increased, the viscosity falls and finally reaches a value only slightly higher than that of the contained oil, so imposing low loads on the power supply, when the bearings operate at high speeds.It is desirable that the rheo- logical properties of grease remain relatively constant over the range of operating conditions encountered in use. The different behaviour of individual soap dispersions in this connection has led to a number of classes of lubricants for special purposes, with no one variety having sufficiently suitable properties to be classed as an all-purpose grease. Dispersions of materials, other than soaps that are now appearing for use as lubricants include carbon black, bentonite and silica.While practical considera,tions limit the choice of ingredients that may be used industrially, many combinations may be produced experimentally that have the de- sired properties. The similarity in refractive index of soap and oil makes optical micro- scopy of greases difficult. Dark field illumination and removal of the oil by selective solvents have been useful in some cases.l, Electron microscopy along with metal shadow casting 8, 4 have extended the range of examination considerably. These techniques show that the ultimate soap particles in soda, lime and lithium base greases are unbranched, flexible fibres with mean axial ratios in the vicinity of 20 to 30. The particles in aluminium soap greases do not appear to have been resolved.While the shape and size of the ultimate particles have a considerable influence on the flow properties of the dispersion, these are governed to a greater extent by the actual solid phase structure, and the ease with which this may be broken down and reformed under alternate conditions of strain and rest (thixotropy). This, in turn, is controlled by particle- particle interaction or flocculation in the particular liquid medium used. The most useful information on these systems is therefore, obtained from an examination of the flow properties. Rheological measurements on greases are complicated by the strong tendency of these systems to show plug flow. This is apparently due to * Royal Aircraft Establishment, Farnborough.t National Research Council, Ottawa, Canada. Farrington and Davis, Ind. Eng. Chem., 1936, 28, 414. Gallay, Puddington and Tapp, Can. J . Res. B., 1944, 22, 66. Birdsall and Farrington, J . Physic. Chem., 1948, 52, 1415. 4 Ellis, Can. J . Res. A , 1947. 25, 119. 4344 PHASE STRUCTURE IN LUBRICATING GREASE the combination of strong interparticle attraction and the relatively low viscosity of the liquid phase. In such cases the use of rotational visco- meters is limited to dispersions of low solid content, and most measurements have been made with capillaries having a large length to diameter ratio. A useful method when only a small amount of material is available involves the measurement of the variation of the distance between two parallel plates, with time, when a known volume of substrate is contained between them.This can be done with simple equipment and the viscosity calculated from the approximate equation where F is the force in dynes, d the distance between the plates, t the time elapsed, and G = 3v2q/8r, where ZI is the volume of liquid and the viscosity. From rate of shear against shearing stress curves, the yield value and mobility of the system can be found, The yield value may also be deter- mined separately by a rapid test involving the depth of penetration of a plate of known weight into the grease. This is actually a measurement of the structure of the grease and indicates its ability to " stay put " but tells little of the load that will be imposed on the bearings when it is used as a lubricant.Some light may be thrown on the process of flocculation by the follow- ing simple experiment. If equal amounts of dry talc are dispersed in two samples of toluene and about I yo of water, based on the talc, added to one, and an equal amount of oleic acid to the other, visible flocculation will be seen at once in the sample containing the water, while the particles in the second portion remain well dispersed. The aggregates will settle rapidly, leaving a clear supernatant liquid, to an equilibrium volume of about twice that occupied by the second sample. The unflocculated sample is comparatively slow in reaching its final condition. Shear, imposed by shaking or stirring reverses the process, causing temporary deflocculation. Aggregates form again in the sample containing the water, as soon as stirring is stopped.This same process occurs with many other two phase systems and it is of interest that the flocculation and sedimentation are much more rapid when the liquid medium is non- viscous. Removal of the supernatant liquid from the flocculated sample after sedimentation will give a solid with properties very similar to those of grease. Greases are thus composed of a minimal quantity of a solid material suspended in a liquid to give a scaffolding type of structure that enmeshes large quantities of the second phase, The effect of varying degrees of interparticle reaction on the viscosity of some suspensions is shown quantitatively in Tables I and I1 where the Einstein coefficients (theoretical 2-5 to 5 depending on particle shape) for various suspensions are compared.5 It is clear from these data that only with those liquids that are strongly absorbed by the solid is the predicted value of the Einstein coefficient approached.In the other cases the apparent overall particle geometry is radically changed. At higher concentrations this coefficient loses its usual significance, but the data in Table I1 show that the dispersion medium has a marked effect on the apparent particle shape. Experiments using a moving plate (edge-on) viscometer at unit rate of shear to determine the effect of the ultimate particle size were carried out by measuring the viscosity of 54 yo, by volume, suspensions of grain aluminium in kerosine. The suspension containing 300 mesh aluminium gave a viscosity of 150 poises compared to 10 poises for the 120 mesh particles.This indicates a higher degree of inter1 article reaction with increase in the specific surface. When suspensions axe sufficiently highly concentrated, interesting effects, such as apparent stress hardening, occur. These effects are due d4 = ct-lF-1, Mardles, Trans. Faraday SOC., 1940, 36, 1007, I 189.E. W. J. MARDLES AND I. E. PUDDINGTON 45 to interparticle friction and they can usually be eliminated or radically reduced by altering the liquid medium, so reducing the coefficient of friction between the particles, or by changing the shape of a portion of the suspended solid. Such behaviour is usually peculiar to substantially spherical particles and the addition of a small percentage of fibres has a large modifying effect.In more dilute dispersions, however, interparticle adhesion can take place by at least three mechanisms : (i) mechanical entanglement of long fibres ; (ii) adhesion on contact due to particle-liquid interfacial surface tension. This phenomenon is greatly enhanced by the presence of a second liquid phase that preferentially wets the solid. The sedimentation of talc in toluene, already described, is a typical example ; (iii) adhesion of the particles due to incipient surface swelling. TABLE I.-THE EINSTEIN COEFFICIENTS FOR SUSPENSIONS Liquid Medium n-Hexane . Oleic acid . Ethylene dibromide . Water . Lubricating oil . Boiled linseed oil . Silica (Irregular Grains) 4 9 5 6 I1 I2 Mica (Irregular Plates) 7 6 7 9 I0 - Aluminium (Flakes) Graphite (Plates) TABLE II.-THE RELATIVE VISCOSITY OF SUSPENSIONS OF 7.5 yo VOL.KAOLIN IN VARIOUS LIQUIDS Relative Einstein Viscosity Liquid Medium Light mineral oil . Benzyl alcohol . nz-Cresol . Diamyl phthalate . Aniline . Oleic acid rllrlo 4'8 1'9 1.7 1-6 1'5 2'2 51 16 9 8 7 I2 The first and third of these mechanisms would not be expected to be operative in the systems discussed so far, where the solids have been in- organic and the particles reasonably isometric. With the soap greases, however, they may assume considerable importance. Mechanism (iii) is a factor with all these greases at elevated temperatures and mechanical entanglement has a strong influence with long-fibred soda-soap greases. With these greases fibre length and toughness is increased by using a larger portion of oleate in the soap stock.The amount of grease that can be made from a given quantity of soap is considerably increased by this procedure, but so, too, is the difference in yield value when this is determined before and after the grease has been sheared. Indeed it is only with those soda greases where the fibres are kept purposely short that this difference is kept to a minimum, and this is accomplished at the expense of reduced consistency. If this procedure is not used, however, the useful properties of the grease soon deteriorate during service. Al- though individual greases vary a good deal, the data presented in Table I11 are illustrative. In contrast to soda soap greases, lime soap fibres are much tougher and smaller, though they also have a high axial ratio.High shear has46 None . Passed 3 times through a capillary at a mean shear rate of 2000 sec.-l Passed twice through a colloid mill at a mean shear rate 700.000 sec.-l . . PHASE STRUCTURE IN LUBRICATING GREASE 45,000 20,000 13,000 little effect on the individual particles and recovery in consistency after agitation is very rapid. These greases almost invariably contain small quantities of free water and it seems likely that the second mechanism of flocculation predominates in this case. TABLE III.-CHANGES IN YIELD VALUE OF 15 yo SODA BASE GREASE AFTER SHEARING Yield Value dynes/cmP Treatment I The importance of solid phase structure becomes very apparent with some greases containing no soap. Using silica aeragel,* for example, as little as 3 yo by volume dispersed in oil produces a grease of approximately the same consistency as 10 yo of soap and further additions of silica increase the yield value markedly with little effect on the mobility.Like the calcium soap greases, this dispersion shows almost no hysteresis after shearing and the worked and unworked penetrations are virtually identical. Dispersion of finely divided cellulose in oil behave similarly. If, however, the disperse phase is finely-divided carbon, a relatively large volume concentration is required and the solid phase structure appears to break down irreversibly with shear, at least recovery is slight and extremely slow. Increased concentrations of the carbon influence the yield value only slightly but decrease the mobility considerably and hence bearings lubricated with such dispersions are unduly loaded at high speeds.With both of these solid phases interparticle adhesion would not be influenced by mechanical entanglement or surface swelling. Illustrative data are pre- sented in Table IV. Values in Table IV were obtained from capillary flow measurements . TABLE IV.-YIELD VALUES OF SUSPENSIONS IN LUBRICATING OIL CONTAINING 10 yo SOLIDS CONCEN- TRATION BY VOLUME Solid Phase 1 Yield Value, dyneslcm2 Carbon black . Barium sulphate . Sodium stearate . Calcium soaps . Silica aeragel . 400 5,000 25,000 I 2 7,000 5,000 The variation of the yield value of a suspension with concentration of the solid phase is a most useful indication of the value of the solid in preparing greases with good rheological properties. Those solids that are efficient in contributing to the solid phase structure do not substantially decrease the mobility or increase the high shear viscosity. The reverse is true for those dispersions where a high solids concentration is required * From the method of preparation the ultimate particles in silica aeragel probably have a high length to breadth ratio.They are too small, however, t o be readily resolved microscopically. Kistler, J. Physic. Chem., 1932, 36, 52.E. W. J. MARDLES AND I. E. PUDDINGTON 47 to give a high yield value. A plot of yield value against volume concentra- tion of solids using petroleum jelly as a dispersion medium is shown in Fig. I. This gives a good indication of the variability of solids, when used as thickening agents. The aluminium grains used here would I A A 0 / I' 2 ' a / FIG. I.-The relation between log u (yield value g./sq. cm.) and volume addition of particles to B.P. petroleum jelly. I. Cotton fibres to z mm. length. 2, Hair fibres to z mm. length. 3. Glass fibres to 3 mm. length. 4. Glass fibres to z mm. length. 5. Graphite. 6. Aluminium grains (spheroidal). almost certainly have a surface coating of soap, making them hydrocarbon rather than metallic in their behaviour. The similarity of aluminium and graphite is obvious from the curves. The advantage of using solid par- ticles with a polar surface in the hydrocarbon medium when a high yield value is desired is also apparent. Royal Aircraft EstabEishment, Farnborough . National Research Council, Ottawa, Canada.