Influence of Soap Concentration and Oil Viscosity on the Rheology

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Ind. Eng. Chem. Res. 2006, 45, 1902-1910

Influence of Soap Concentration and Oil Viscosity on the Rheology and Microstructure of Lubricating Greases M. A. Delgado, C. Valencia, M. C. Sa´ nchez, J. M. Franco,* and C. Gallegos Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Experimentales, UniVersidad de HuelVa, Campus UniVersitario del Carmen, 21071, HuelVa, Spain

The effects that both soap concentration and base oil viscosity exert on the rheology of lubricating greases and its relationship with grease microstructure are discussed in this work. With this aim, different lubricating grease formulations were manufactured by modifying the concentration of lithium 12-hydroxystearate and the viscosity of the base oil, according to an RSM statistical design. These lubricating greases were rheologically characterized through small-amplitude oscillatory shear (SAOS) and viscous flow measurements. In addition to these, scanning electronic microscopy (SEM) observations and mechanical stability tests were also carried out. It has been found that the structural skeleton (size and shape of the disperse phase particles) was highly influenced by the base oil viscosity. In this sense, the values of the viscoelastic functions in the linear viscoelastic region and the mechanical stability of the lubricating greases increase as the viscosity of the base oil decreases. An opposite tendency was observed during viscous flow tests at high shear rates, when the grease microstructure was mostly destroyed. On the other hand, the microstructural network of these greases becomes stronger as soap concentration increases. These results have been explained taking into account the balance between the solvency of the thickener in the base oil and the level of entanglements formed by soap fibers, which influence the lubricating grease network. 1. Introduction Although lubricating greases are widely used in many specific applications, their market only constitutes 3% of the total amount of lubricants processed in the world.1 This may be one of the reasons why, up to now, little attention has been paid to the rheology of lubricating greases.2-4 On the other hand, it is worth mentioning that the mechanical behavior of a grease in a lubricated surface is not yet fully understood. This is due to its complex rheological behavior in comparison with that of other liquid lubricants. Since lubrication is mainly a deformation and flow problem, the knowledge of the rheological properties of lubricating greases may contribute to elucidate one of the unresolved problems in tribology: the effect of viscoelasticity on lubrication characteristics,5 and, consequently, the way in which a lubricating grease behaves under operating conditions. Lubricating greases are generally highly structured suspensions, consisting of a thickener dispersed in mineral or synthetic oil. Fatty acid soaps of lithium, calcium, sodium, aluminum, and barium are most commonly used as thickeners. This component is added to increase the consistency of greases, preventing loss of lubricant under operating conditions and avoiding the penetration of contaminants, such as solid particles and water, without a significant reduction of the lubricating properties, which are mainly supplied by the oil. The thickener forms an entanglement network, which traps the oil and confers the appropriate rheological and tribological behavior to the grease.2 The performance of lubricating grease depends on the nature of its components and the microstructure achieved during its processing. Consequently, suitable structural and physical properties may be reached from a proper selection of the ingredients but, also, from a process optimization, as was previously reported.6 In this sense, it is relevant to understand * To whom correspondence should be addressed. Tel.: ++34 959 219995. Fax: ++34 959 219983. E-mail: [email protected].

how the development of the grease microstructure contributes to several functional and rheological properties of lubricating greases.7 Up to now, most of the studies carried out on lubricating grease rheology have been limited to greases in the middle range of the NLGI grades, which are typically used in roller bearings and are also susceptible to be pumped through centralized dispensing systems.8 In this paper, we discuss how the rheological behavior of a variety of greases, comprising a relatively wide range of NLGI grades, can be predicted in relation to its composition and microstructure. The effect of the type, size, and distribution of soap fibers has been analyzed. With these considerations, the main objective of this work was to study the rheological behavior and microstructure of lithium lubricating greases as a function of both soap concentration and viscosity of the base oil. 2. Experimental Section 2.1. Materials. 12-Hydroxystearic acid and anhydrous lithium hydroxide were used as saponifying agents. On the other hand, paraffinic lubricating oils, with different kinematic viscosities at 40 °C (ASTM D445 standard), were obtained by blending in different proportions base oils of minimum and maximum viscosities (i.e., 11 and 657 cSt, at 40 °C). All the components were kindly supplied by Verkol, S.A. (Spain). 2.2. Grease Manufacture. Greases were prepared in a stirred vessel using a controlled-rotational speed anchor impeller, coupled with an Ika-Visc torquemeter (Ika, Steufen, Germany), in batches of 4 kg. In the first step, the total amount of 12hydroxystearic acid necessary to reach the appropriate final soap content and, approximately, 54% (w/w) of the mineral oil was charged into the open vessel. The mixture formed was continuously stirred and, then, preheated up to the melting point of the fatty acid before the addition of the alkali solution. Once the mixture was preheated to 90 °C, the lithium hydroxide solution was added and the saponification reaction occurred until

10.1021/ie050826f CCC: $33.50 © 2006 American Chemical Society Published on Web 02/15/2006

Ind. Eng. Chem. Res., Vol. 45, No. 6, 2006 1903 Table 1. Correspondence between Experimental and Normalized Values of the Variables Studied (RSM Analysis) -1.414 8.00 11.0

normalized value (x and y) x (soap content (% (w/w)) y (oil viscosity (cSt))

-1 9.75 105.6

0 14.00 334.0

+1 18.25 562.4

+1.414 20.00 657.0

Table 2. Rheological and Tribological Parameters for the Lubricating Greases Studied normalized values of soap concentration and oil viscosity

η (Pa‚s) at 1.25 s-1

GN° (Pa)

k (Pa‚sn)

n

tan δ (at 1 rad/s)

ηrel

worked penetration (dmm)

unworked penetration (dmm)

penetration change (dmm)

0, 0 0, 0 +1.414, 0 -1.414, 0 0, +1.414 0, -1.414 +1, +1 +1, -1 -1, +1 -1, -1

361 403 637 178 392 388 541 521 210 178

18722.9 17420.5 69891.7 2847.2 12813.0 41765.2 77868.5 60237.3 6732.1 5465.9

554 554 951 251 535 571 749 676 275 258

0.13 0.12 0.09 0.14 0.16 0.06 0.13 0.07 0.16 0.09

0.109 0.109 0.107 0.123 0.151 0.070 0.177 0.115 0.186 0.128

0.98 0.97 0.99 0.93 0.96 0.99 0.99 0.99 0.97 0.96

386 375 303 444 429 325 347 341 480 437

373 371 305 434 371 301 335 324 454 461

-13 -4 2 -10 -58 -24 -12 -17 -26 24

neutralization was completed. Once the soap was formed, the temperature was increased up to 180 °C, for 60 min, and then the remaining lubricating oil was added during a controlled cooling ramp, in two batches, as described elsewhere.6 A final homogenization treatment at 8800 rpm, for 15 min, was applied in order to reduce crystal size. 2.3. Experimental Design for Grease Formulation. The experimental design of grease formulation for this study was carried out according to a central composite rotatable experimental design, with two factors at five levels, based on the response surface methodology (RSM), where the value of R was (1.414.9 Ten grease formulations were prepared, to fit a second-order response surface, resulting in greases with lithium soap concentrations of 8.00%, 9.75%, 14.00%, 18.25%, and 20.00% (w/w), prepared using paraffinic oils with kinematic viscosities of 11.0, 105.6, 334.0, 562.4, and 657.0 cSt at 40 °C. These soap concentration and oil viscosity values were selected in order to obtain significant differences in the typical composition range of commercial greases.10 As usual, the independent variables were normalized, and the corresponding values are shown in Table 1. 2.4. Characterization Tests. The rheological measurements were performed in both a controlled-stress Bohlin CS 50 (U.K.) and a controlled-strain Rheometric ARES (U.K.) rheometer, using cone-plate (40 mm, 4°) and plate-plate (25 mm diameter, 1 mm gap) geometries. Small-amplitude oscillatory shear (SAOS) measurements, inside the linear viscoelasticity regime, were carried out in a frequency range comprised between 0.01 and 100 rad/s at 25 °C. Stress sweep tests, at the frequency of 1 Hz, were previously performed on each sample to determine the linear viscoelasticity region. Viscous flow measurements were performed at 25 °C, in a shear rate range from 10-3 to 102 s-1. A serrated plate-plate geometry was used in order to eliminate the wall-slip effects usually observed in flow measurements with these materials.4 All the samples had the same recent past thermal history. At least two replicates were performed on fresh samples. Both unworked and worked penetration indexes were determined according to the ASTM D1403 standard, using the Seta universal penetrometer, model 17000-2 (Stanhope-Seta, U.K.), with a one-quarter cone geometry. The one-quarter-scale penetration values were converted into the equivalent full-scale cone penetration values according to ASTM D217. The samples were worked in a roll stability tester, model 19400-3 (StanhopeSeta, U.K.), according to the ASTM D1831 standard.

Morphological observations with a JEOL scanning electron microscope (SEM), model JSM-5410 (JEOL, Japan), were conducted at 15 kV. A small amount of each sample was immersed for 90 min in hexane to extract the oil. This operation was repeated until oil extraction was completed. Afterward, the sample was dried at room temperature. Finally, the samples were coated with gold. Micrographs at different magnifications (1000-10 000×) were taken on several samples. 2.5. Statistical Analysis. A statistical analysis was performed using the software Statistica (v. 6.0, StatSoft Inc., U.S.A.). The dependent variables considered in the RSM methodology were the logarithm of the plateau modulus (log GN°), the logarithm of the loss tangent (log(tan δ)), the fitting parameters of the power-law model (k and n), the relative deviation of the CoxMerz rule (ηrel), and the unworked and worked penetration values (results shown in Table 2). The parameters of the statistical equations shown in this paper, relating independent and dependent variables, were obtained by considering a statistical significance, i.e., not exceeding a significance level of 0.05 in the Student’s t test and having a 95% confidence interval excluding zero. 3. Results 3.1. SEM Observations. The lubricating greases studied in this work are fine dispersions of lithium soap in paraffinic oil. Thus, when a metal soap is dispersed in a mineral oil, the crystallized soap particles arrange themselves to form a characteristic microstructure depending on soap concentration, oil viscosity, and type of metal soap used, due to the balance of forces between the colloidal particles and the oil medium.11 These soap fibers are distributed in a random manner within a given volume, as can be seen in Figures 1 and 2 where selected micrographs of the systems studied, at magnifications of 5000× and 3500×, respectively, are shown. As can be observed, a wellformed entanglement network among structural units generally appears, although this microstructure is significantly affected by both thickener concentration and oil base viscosity. Important differences in the structural skeleton (size and shape of dispersed soap particles) are observed when greases with different soap concentration are compared (see Figure 1). Thus, large particles in the form of platelets are found for the lowest soap concentration, while entangled fibers clearly develop at concentrations higher than 14% w/w, the density of physical entanglements being larger as soap concentration increases, i.e., 20% w/w.

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Figure 1. SEM micrographs (5000×) of lithium lubricating greases with different soap concentrations: (a) 8% (w/w), (b) 14% (w/w), and (c) 20% (w/w) (kinematic oil viscosity, 334 cSt at 40 °C).

Figure 2. SEM micrographs (3500×) of lithium lubricating greases manufactured with oils having different kinematic viscosities: (a) 11 cSt, (b) 334 cSt, and (c) 657 cSt, at 40 °C (soap concentration, 14% w/w).

On the other hand, oil viscosity not only affects the shape and size of particles in the structural network (see, for instance, Figure 2, parts a and b) but also the size of hollow spaces among fibers where oil is trapped (see Figure 2, parts b and c).12 In this sense, longer fibers but also larger spaces were observed for the grease prepared with the oil having the highest viscosity (Figure 2c). Despite these longer fibers, this structure is spongier and the density of entanglements is lower than that obtained for greases prepared, for instance, with base oil of medium viscosity (Figure 2b). 3.2. Rheological Characterization. It is well-known that, when a shear stress or a shear rate is applied to a lubricating grease sample, a reorientation of fibers in the microstructural network takes place and, as a result, a shear-thinning flow behavior is observed.10 Figure 3a shows the viscous flow behavior of lithium greases, as a function of thickener concentration. The application on the lubricating greases studied of shear rates larger than those shown in Figure 3a produces the

fracture of the sample, as has been found with other greases.2,4 The power-law model describes fairly well the flow behavior observed within the shear rate range tested (r2 > 0.995):

η ) kγ˘ n-1

(1)

where “k” and “n” are the consistency and flow indexes, respectively. Figure 3a shows that viscosity clearly increases with soap concentration, yielding higher values of the consistency index, k. On the other hand, the flow index, n, decreases by increasing the thickener concentration, being quite close to zero for the highest concentration studied, which is representative of the typical yielding behavior shown by these materials.4 On the contrary, this flow behavior is almost unaffected by the base oil viscosity. As can be seen in Figure 3b, only slight differences in viscosity are found at low shear rates by modifying the viscosity of the base oil, the low shear rate

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Figure 3. Viscous flow curves, at 25 °C, for lubricating greases having different soap concentrations (a) and manufactured with oils having different kinematic viscosities (b).

viscosities of these greases being higher when they are manufactured with low-viscosity oil. This effect is also slightly detected at intermediate shear rates, as can be deduced from the values of the consistency index. Nevertheless, there is a tendency to obtain higher values of the highshear-rate-limiting viscosity when the base oil viscosity increases, which can be also deduced from the evolution of the flow index. More information about the microstructural network of these greases may be extracted from SAOS measurements. Figure 4 shows the mechanical spectra, in the linear viscoelasticity range, of lubricating greases as a function of soap concentration. As

can be observed, the linear viscoelasticity response is qualitatively similar for all the greases studied and also to that found with other commercial lubricating greases.13 The evolution of the storage and loss moduli with frequency for these lubricating greases (higher values of the storage modulus and a minimum in the loss modulus at intermediate frequencies) is characteristic of polymeric systems with physical entanglements and supports the idea that lubricating greases are highly structured systems, as has been otherwise detected by SEM techniques (see Figures 1 and 2). As can be observed in Figure 4a, the values of G′ and G′′ increase with thickener concentration, which is indicative of a stronger microstructural network. Nevertheless, the loss tangent, defined as the relationship between G′′ and G′, presents quite similar values in the whole frequency range studied (Figure 4b), showing similar relative elasticity in this range of soap concentration. Only greases having low thickener concentration, i.e., 8% (w/w), show higher values of the loss tangent in the high-frequency window. The values of the “plateau” modulus, GN°, a characteristic parameter for the intermediate entanglement region, are shown in Table 2. The plateau modulus, GN°, is defined as the extrapolation of the contribution of the level of entanglements to G′ at high frequencies.14 This evolution of the linear viscoelastic response of the lubricating greases with thickener concentration makes possible the application of an empirical superposition method, using the “plateau” modulus as a normalization factor, to obtain the master curves of both moduli as a function of soap concentration. This is illustrated in Figure 5, where the master curves for selected greases prepared with the same base oil are shown. Nevertheless, a shift factor has been used, due to the small deviation of the loss tangent values observed for the grease containing 8% (w/w) soap. Consequently, this shift factor depends on soap concentration, being quite similar for the most concentrated greases (i.e., 14% w/w and 20% w/w). On the other hand, the linear viscoelastic behavior of lubricating greases is highly affected by oil viscosity. Thus, the values of both moduli decrease as the viscosity of the base oil increases (see Figure 6a). In addition to this, the values of the loss tangent also depend on oil viscosity in the whole frequency range studied (Figure 6b), indicating that the relative elastic characteristics of a lubricating grease increase by decreasing oil viscosity.

Figure 4. Frequency dependence of the storage and loss moduli (a) and tan δ (b), in the linear viscoelasticity region, for lubricating greases containing different soap concentrations (filled symbols, G′; empty symbols, G′′).

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Figure 5. Frequency dependence of the normalized storage and loss moduli for lubricating greases containing different soap concentrations (filled symbols, G′/GN°; empty symbols, G′′/GN°). Inset: evolution of the horizontal shift factor with soap concentration.

The comparison between steady-state flow curves and complex viscosities obtained from SAOS measurements, the socalled deviation of the Cox-Merz rule, provides a quantification of the shear-induced structural breakdown. This comparison is shown in Figure 7, in the same range of shear rate and frequencies, as a function of soap concentration and oil viscosity, respectively. Taking into account that the power-law decrease is approximately the same in both types of measurements, the relative deviation of the Cox-Merz rule can be obtained, independently of the frequency or shear rate, as follows:

ηrel )

|

η* - η η* γ˘ )ω

(2)

The values shown in Table 2 indicate a significant structural breakdown in all cases. Nevertheless, it must be noticed that the lower deviations, i.e., lower shear-induced structural breakdown, were found for greases containing lower soap concentration and manufactured with base oil of higher viscosity. 3.3. Mechanical Stability. Lubricating greases should be physically and chemically stable under operation conditions, as for instance those found in roller bearings. In this work, the mechanical stability has been simulated by performing the

traditional penetration measurements before and after the standardized roll-stability test. In this sense, the mechanical properties of samples differing in both soap concentration and base oil viscosity have been studied, although it is well-known that these tests only represent an approximation to classify greases according to the ability to lubricate roller bearings under real conditions.12,15 Table 2 shows the penetration values obtained for unworked and worked lubricating greases. A lubricating grease is usually considered stable to the continuous shear of rolling elements when its variation of penetration before and after the roll-stability test is around zero. In this case, significant differences in mechanical stability have been found depending on composition and microstructure. As expected, an increase in soap concentration yields a decrease in the penetration values, due to the increase in consistency previously mentioned. On the contrary, the values of the penetration increment after working of greases containing different thickener concentration do not significantly depend on soap concentration, as can be observed in Table 2. On the other hand, the lower the base oil viscosity, the higher the grease consistency is. Finally, it is worth pointing out that a lubricating grease containing a high-viscosity oil shows a large negative increment of the penetration, similar to those found with unfinished greases and large crystal sizes.7 4. Discussion One of the most important tasks of the thickener is to confer solid or semisolid properties to a lubricating grease at low shear rates and a typical liquid behavior when lubrication is required. This is related to the yielding flow behavior of the grease, characterized by a decay of several decades in viscosity with a small increment of shear stress before a high-shear-rate-limiting viscosity is reached.4 This tendency to show a Newtonian behavior at high shear rates follows the theory of Van den Temple,16 which states that, for very large deformations, lubricating greases consist of noninteracting aggregates of soap particles dispersed into the lubricating oil. In this sense, the microstructure of the lubricating grease at high shear rate should be completely destroyed, and the presence of soap particles only produces a slight increment of the oil viscosity, depending on soap concentration. Consequently, the viscous flow behavior at high shear rates is controlled by the base oil. This would explain why, for the same soap concentration, the higher the

Figure 6. Frequency dependence of the storage and loss moduli (a) and tan δ (b), in the linear viscoelasticity region, for lubricating greases manufactured with oils having different kinematic viscosities (filled symbols, G′; empty symbols, G′′).

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Figure 7. Comparison between steady-state and complex viscosities for lubricating greases containing different soap concentrations (a) and manufactured with oils having different kinematic viscosities (b).

base oil viscosity is, the higher is the viscosity of greases at high shear rate. On the other hand, the consistency index, i.e., the viscosity value at 1 s-1, is mainly dependent on soap concentration, as can be deduced from the second-order response

surface and the statistical model obtained from multiple regressions (Figure 8a). On the contrary, the flow index statistically depends, almost linearly, on both variables (Figure 8b), which supports the importance of the base oil viscosity at high-shear conditions. More interesting, from a microstructural point of view, is the rheological response in the linear viscoelastic range, i.e., extremely low-shear conditions. Figure 9a shows the tendency of GN° to slightly decrease as oil viscosity increases and, mainly, to increase with soap concentration, as can be deduced from the parameters of the statistical model used. These results may be explained taking into account that the structural network of the lubricating greases becomes stronger as the soap concentration increases, due to the larger soap particle density and more entangled fibers at higher soap concentration (Figure 1), as previously reported for other greases.17 However, the microstructure of these greases presents higher shear dependence according to the fact that the relative deviation of the CoxMerz rule is mainly dependent on soap concentration, as can be observed in Figure 10. This result is in agreement with the fact that the higher soap contents are, the lower are the values of the flow index. On the contrary, the loss tangent is statistically dependent on oil viscosity and not on soap concentration (see Figure 9b), confirming that the high solvency (affinity of a liquid for a given solute) between the low-viscosity oil and the metal soap reinforces the network. In fact, tan δ could be selected as a key rheological parameter to evaluate the attraction forces between the lubricating oil and the thickener. As was pointed out before, the effect of the oil viscosity on the network seems to be more interesting than the effect of soap concentration, because this parameter not only affects the development and size of fibers but also the size of hollow spaces among fibers where oil is trapped, mainly affected by the solvency of the base oil.12 Thus, greases showing large fibers but a spongier microstructure, with relatively low level of entanglements, show less relative elastic

Figure 8. Response surface of the consistency (a) and flow (b) indexes of the lubricating greases studied, as a function of soap concentration and oil viscosity.

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Figure 9. Response surface of the plateau modulus (a) and the loss tangent at 1 rad/s (b) of lubricating greases studied, as a function of soap concentration and oil viscosity.

Figure 10. Response surface of the relative deviation of the Cox-Merz rule, as a function of soap concentration and oil viscosity.

characteristics than greases with not largely developed fibers but a more compact structural skeleton. In this sense, a high oil viscosity acts as a mechanical barrier to the formation of physical entanglements and interactions among fibers occurring during crystal growth in the final steps of the manufacturing process, yielding larger spaces among fibers. Otherwise, this mechanical barrier dampens the shear-induced structural breakdown, as can be observed in Figure 10. Consequently, the effect of oil viscosity on the lubricating grease microstructure results from the balance between particle interactions, related to the development of physical entanglements, and the solvency between the thickener and the base oil, which is mainly affected

by the properties of the lubricating oil. As Couronne´ et al.12 has pointed out, a high solvency between the base oil and the metal soap yields a strongly interconnected microstructure and, therefore, a strong and more resistant network. On the contrary, lower solvency leads to a heterogeneous and weaker network. Thus, as can be seen in Figure 2, the paraffinic oil viscosity having the lowest viscosity favors a compact microstructure, which implies higher values of apparent viscosity at low shear rates and significant lower values of the loss tangent. In the same way, the microstructural network of the lithium grease with the highest oil viscosity is much weaker, showing large spaces among fibers and resulting in lower values of viscosity at low shear rates. On the other hand, the flow index, n, decreases with the base oil viscosity, which implies a higher shear dependence of the microstructure, probably due to an easier reorientation of the soap particles in a low-viscosity medium, which is corroborated with the Cox-Merz rule deviation found in these greases (Figure 10). The compact structural skeleton, favored by a low base oil viscosity, can hold relatively large amounts of lubricating oil because of the high solvency between the base oil and the metal soap. As a result, the microstructure easily tolerates the action of the rolling track in the roll-stability test, which implies stronger structural integrity, yielding lower values of penetration and relatively high mechanical stability (see Table 2). Moreover, the highly negative increment in penetration obtained with the grease containing a high-viscosity oil may be related to the presence of large fibers and low attraction forces between oil and thickener. In this case, the mechanical working induces a thickening of the grease, by reducing the crystal size and, possibly, favoring the agglomeration of the disperse phase, as has been previously observed in not well-milled greases.7 The penetration values for unworked greases are dependent on both soap concentration and base oil viscosity (Figure 11a), since the unperturbed network plays a key role in nonworked greases. On the contrary, the penetration values for worked greases only

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Figure 11. Response surface of the penetration for unworked (a) and worked (b) samples of the lubricating greases studied, as a function of soap concentration and oil viscosity.

depend on the soap content (Figure 11b), once the microstructure is almost destroyed. 5. Conclusions A statistical approach has been used to relate the oil viscosity and soap concentration with the rheological and mechanical properties of lithium greases, which was supported with a qualitative analysis of the microstructure. The lubricating grease microstructure is highly influenced by the base oil viscosity and the thickener concentration. Thus, large particles in form of platelets have been found at low soap concentration, while a high density of physical entanglements among fibers has been clearly observed at high concentrations. As a consequence, the apparent viscosity and the linear viscoelasticity functions increase with soap concentration. On the contrary, neither the relative elasticity nor the mechanical stability of the lubricating grease is affected by soap concentration. The oil viscosity not only affects the development and size of fibers but also the size of hollow spaces among fibers where oil is trapped. Large fibers, but a weaker structure and lower density of entanglements, are obtained for greases prepared with a high-viscosity base oil, yielding higher values of the loss tangent in SAOS measurements and poorer mechanical stability. However, the base oil viscosity does not exert a significant influence on the grease viscosity, although an opposite tendency has been observed at sufficiently high shear rates, which indicates that the viscous flow behavior is also influenced by the base oil viscosity when the grease microstructure has been sufficiently destroyed. In this sense, the oil viscosity mainly affects the rheological parameters related to the shear dependence of the microstructure or its relative viscous character, whereas the soap concentration specially influences the consistency of lubricating greases under moderate shear conditions. Despite the findings shown in this work, further investigations should be carried out in order to develop theory-based models relating the microstructure and grease properties.

Acknowledgment This work is part of two research projects (PPQ2001-2822 and CTQ2004-02706) sponsored by MEC-FEDER programs. The authors gratefully acknowledge their financial support. Literature Cited (1) A° stro¨n, H. Grease in Elastohydrodynamic Lubrication. Ph.D. Thesis, Luleå University of Technology, Sweden, 1993. (2) Mas, R.; Magnin, A. Rheology of colloidal suspensions: Case of lubricating greases. J. Rheol. 1994, 38, 889-908. (3) Gow, G. Lubricating grease. In Chemistry and Technology of Lubricants, 2nd ed.; Mortier, R. M., Orszulik, S. T., Eds.; Blackie Academic & Professional: London, 1997; pp 306-319. (4) Balan, C.; Franco, J. M. Influence of the geometry on the transient and steady flow of lubricating greases. Tribol. Trans. 2001, 44, 53-58. (5) Williamson, B. P.; Walters, K.; Bates, T. W.; Coy, R. C.; Milton, A. L Viscoelastic properties of multigrade oils and their effect on journalbearing characteristics. J. Non-Newtonian Fluid Mech. 1997, 73, 115126. (6) Franco, J. M.; Delgado, M. A.; Valencia, C.; Sa´nchez, M. C.; Gallegos, C. Mixing rheometry for studying the manufacture of lubricating greases. Chem. Eng. Sci. 2005, 60, 2409-2418. (7) Delgado, M. A.; Franco, J. M.; Valencia, C.; Sa´nchez, M. C.; Gallegos, C. Relationship among microstructure, rheology and processing of a lithium lubricating grease. Chem. Eng. Res. Des. 2005, 83, 10851092. (8) Hutton, J. F. The rheology of petroleum-based lubricating oils and greases: a review. In The Rheology of Lubricants; Davenport, T. C., Ed.; Applied Science Publishers Ltd.: London, 1973; p 16. (9) Montgomery, D. C. Design and Analysis of Experiments, 4th ed.; John Wiley & Sons: New York, 1996. (10) Vinogradov, G. V.; Sinitsyn, V. V. Electron microscopy of lubricating greases. J. Inst. Pet. 1961, 47, 357-363. (11) NLGI. Lubricating Greases Guide; National Lubricating Grease Institute: Kansas City, MO, 1994. (12) Couronne´, I.; Vergne, P.; Mazuyer, D.; Truong-Dinh, N.; Girodin, D. Effects of grease composition and structure on film thickness in rolling contact. Tribol. Trans. 2003, 46, 31-36. (13) Madiedo, J. M.; Franco, J. M.; Valencia, C.; Gallegos, C. J. Modeling of the nonlinear rheological behavior of lubricating grease at low shear rates. J. Tribol. 2000, 122, 590-596.

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(14) Baumgaertel, M.; De Rosa, M. E.; Machado, J.; Masse, M.; Winter, H. H. The relaxation time spectrum of nearly monodisperse polybutadiene melts. Rheol. Acta 1992, 31, 75-82. (15) Lundberg, J.; Ho¨glund, E. A new method for determining the mechanical stability of lubricating greases. Tribol. Int. 2000, 33, 217223. (16) Papenhuijzen, J. M. P. The role of particle interactions in the rheology of dispersed systems. Rheol. Acta 1972, 11, 73-88.

(17) Hamnelid, L.; Axel Christiernsson, A. B. Consistency consist in sy or “The cone penetration’s conclusive condemnation”. NLGI Spokesman 1998, 62, 17-30.

ReceiVed for reView July 13, 2005 ReVised manuscript receiVed December 7, 2005 Accepted January 24, 2006 IE050826F