New Ecological Lubricants on the Basis of Lyotropic Liquid Crystals

Article pubs.acs.org/IECR

New Ecological Lubricants on the Basis of Lyotropic Liquid Crystals Formed by Solutions of Maracuja Oil Ethoxylate Marian Włodzimierz Sułek† and Anna Bąk-Sowińska*,‡ †

Industrial Chemistry Research Institute, Rydygiera 8 Street, 01-793 Warsaw, Poland University of Technology and Humanities in Radom, Faculty of Materials Science, Technology and Design, Boleslawa Chrobrego 27 Street, Radom, Poland

‡

ABSTRACT: Aqueous 60% solutions of maracuja oil ethoxylated with 60 mol of ethylene oxide containing hexagonal mesophases were proposed as model biocompatible safe-to-use lubricants. Tests were carried out at steady loads using a four-ball tester (Tester T-02) (1−6 kN) and a ball-on-disk tribometer (Tester T-11) (10−50 N) in order to verify tribological properties of the solutions. At high loads (Tester T-02) a significant, almost 4.5-fold reduction in the coefficients of friction (μ) and about 2.5-fold decrease in wear scar diameter (d) were found relative to water as a base. Tester T-11 was used to study the suitability of the lubricant tested for lubrication of the following friction pairs: steel ball−steel disk, different types of ceramics, and polymers at lower loads. The ethoxylate solutions are excellent lubricants also for those pairs of materials. The interpretation of the results obtained was based on the proposed adsorptive-structural mechanism.

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“lubrication by water” was interpreted based on surface activity of the surfactants which, under static conditions, form micelles in the surface phase and/or lyotropic liquid crystals at higher concentrations.4,5,12−16 The fundamental research carried out on model lubricants (aqueous, micellar surfactant solutions) has formed a basis for designing formulations of hydraulic and cutting fluids.17−21 A number of aqueous solutions with high surfactant concentrations (50−70%) form liquid crystalline structures in the bulk phase.12 They are non-Newtonian liquids and they exhibit anisotropy of selected quantities, including the mechanical ones.5,12,22,23 The viscosity of the liquids decreases significantly with an increase in shear rate.24 The question is how the presence of LLC in the bulk phase will affect tribological properties of these aqueous solutions. Another reason why LLC should be studied as potential lubricants is a possibility to reduce corrosion resulting from the presence of water. Water is present in mesophases in various states: bound in liquid crystalline structures (bound water) and free (bulk, free water).25 The properties of the free water phase are similar to those of “pure water”; whereas bound water has a different activity, structure, mobility, and microviscosity.26−28 In our studies we are going to try to limit “free water” which may have a good effect on reducing corrosion of metals. Therefore, the solutions selected for testing have a high surfactant concentration (60%) in which the share of the liquid crystalline phase is the highest.

INTRODUCTION One of the most important trends in designing new lubricants is a search for the media which have a liquid crystalline structure or whose components have such a structure. The analysis of the properties of thermotropic liquid crystals (TLC) and lyotropic liquid crystals (LLC) indicates that they can have good lubricating characteristics.1−3 Results of the tests carried out on TLC are promising,2 although their applications may be limited due to high costs and application safety. The compounds forming this kind of mesophases may be toxic and difficult to utilization. LLC are a good alternative. The compounds forming this type of structure are produced, to a large extent, in high quantities, and their relatively low price does not constitute an impediment to their application. Lyotropic mesophases are built from amphiphilic compounds which are not harmful to the human body and the natural environment. Their safety has been confirmed repeatedly as demonstrated by their numerous applications as components of foodstuffs, pharmaceuticals, and cosmetics.3−5 Maracuja oil ethoxylated with 60 mol ethylene oxide is a mild, nonionic surface active agent. It can be used in compositions with anionic, cationic, and nonionic surfactants and is widely used in cosmetic and household chemistry preparations.32 Maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60) was selected as a representative amphiphilic compound forming liquid crystalline structures. Its ability to form mesophases has been confirmed and discussed in this paper by analyzing the results of polarized light microscopic examinations and their rheology. It is a safe raw material of plant origin which will ensure ecological safety and can be used in biotribological systems due to its biocompatibility. As it has been shown aqueous solutions of surfactants can act as lubricants characterized by low motion resistance and wear, as well as by antiseizure properties.1,9,10 This phenomenon of © 2013 American Chemical Society

Received: Revised: Accepted: Published: 16169

April 10, 2013 August 15, 2013 October 7, 2013 October 7, 2013 dx.doi.org/10.1021/ie401147u | Ind. Eng. Chem. Res. 2013, 52, 16169−16174

Industrial & Engineering Chemistry Research

Article

Figure 1. Kinds of lyotropic liquid crystals: (a) nematic phase, (b) lamellar phase, (c) hexagonal phase.

Figure 2. Dependence of dynamic viscosity coefficient (η) on concentration of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60) in water. Brookfield HADV-III Ultra, Helipath spindles, rotational speeds of the spindle: 5, 10, 50, 100 rpm, measurement temperature 25 °C.

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TEST RESULTS Aqueous solutions of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60) produced by Croda were used as model lubricants.25 Physicochemical Measurements. Liquid crystalline structures were identified in solutions using a polarizing microscope and their rheological properties were examined. Lyotropic liquid crystals (LLC) are built of micelles which, in turn, are built of molecules of amphiphilic compounds. As a result of long-range interactions, micelles become ordered and form various spatial and orientational structures. The most common mesophases are: nematic, lamellar, and hexagonal (Figure 1). Due to their optical anisotropy, LLC can be quite easily identified by means of polarized light microscopy. The regular mesophase is an exception as it does not exhibit anisotropy due to its kind of symmetry. Under polarized light, individual mesophases form characteristic textures. It results from an analysis of the images under polarized light for various concentrations of the maracuja oil ethoxylate (MEO60) that the most defined liquid crystalline structures are formed at about 60% surfactant concentration (Figure 2). The fanlike texture obtained (Figure 2a) indicates that hexagonal structures built of cylindrical micelles form in the solution.12 A schematic representation of the structures is shown in Figure 2b. Rheology of the Solutions. The presence of LLC, including hexagonal phases, can be confirmed also by their exceedingly high viscosity. Therefore, an analysis of changes in coefficients of viscosity as a function of surfactant concentration and various rotational speeds was carried out (Figure 2c). Figure 2c shows that the highest viscosity (ca. 1 000 000 mPa·s) was observed for 60% ethoxylate solutions and a well-

defined hexagonal phase can be expected at this concentration. The non-Newtonian character of the liquids examined can be confirmed by the observed considerable drop in viscosity with an increase in speed. It may be possible that Figure 2c does not convey a gradation of changes, because the Y-axis is shown in the exponential scale. The changes can be presented more vividly by comparing viscosities which are about 1 000 000 and 120 000 mPa·s at extreme rotational speed values (5 and 100 rpm), respectively. And so, a rotational speed increase from 5 to 100 rpm results in nearly a 8.5-fold decrease in viscosity. Antioxidative and Corrosion Properties. Due to predicted application oxidative and corrosion tests have been conducted. Thermooxidative ability was measured by differential scanning calorimetry with SETARM LAB System TG/ DSC device. The temperature of initiation of oxidation was 190 °C for ethoxylate and it was lower than for mineral oil (200 °C) and maracuja oil (230 °C). The temperature during usage of the lubricant will be much lower than 100 °C which is why temperature of oxidation will not be an obstacle for use. Corrosion tests were performed using a Ford method. Aqueous solution of maracuja oil ethoxylated with 60 mol of ethylene oxide was in a contact with gray iron for 2 h. Slight corrosion (1 point in a 7-point scale) has been observed. Due to elimination of the corrosion, it is required to add few hundredths of a percent of corrosion inhibitors. Tribological Properties. The tests were carried out at steady loads using two independent tribometers: a four-ball tester (Tester T-02)6−8 and a ball-on-disk tribometer (Tester T-11).9 Tester T-02. The measurements were made using 1/2 in. diameter bearing balls made of bearing steel 100Cr6 (surface roughness Ra = 0.32 μm, hardness 60−65 HRC). The tests 16170

dx.doi.org/10.1021/ie401147u | Ind. Eng. Chem. Res. 2013, 52, 16169−16174


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kN) and then, the system undergoes seizure. In the presence of a 60% ethoxylated oil solution, with an increase in load, the μ value drops to the order of several hundredths (0.08 for 5.5 kN) and is about 4.5-fold lower than the one for water at a considerably lower load (0.44 for 2.0 kN). Seizure occurs at 6.0 kN. This value is comparable to or higher than the ones for good quality lubricants.12 The dependence of wear scar diameter on load for water and a 60% aqueous solution of MEO60 has been shown in Figure 5. As expected, wear scar diameter (d) increases with an increase in load both for water (from 1.0 to 2.5 mm) and for the solution (from 0.5 to 1.3 mm) with the increase rate being considerably lower for ethoxylate solutions. It should be noted that the highest d value for the solutions at the load of 5.5 kN (1.3 mm) is lower than the one for water at 2.0 kN (1.8 mm). The analysis of Figures 4 and 5 indicates clearly that liquid crystalline structures show a remarkable capacity to reduce resistance to motion and wear. Ball-on-Disk Tribometer (Tester T-11). Tribological tests were also carried out on the aqueous 60% MEO60 solution in a steel ball−disk concentrated contact, with the disk being alternatively made of steel, polyamide 6, poly(methyl methacrylate), aluminum oxide, or zirconium oxide. The T-11 apparatus was used to assess the values of resistance to motion and wear of friction pairs under concentrated contact conditions of the ball-on-disk type.9 Balls 6.35 mm in diameter made of steel 100Cr6 were used as the upper element of the friction pair. The bottom element of the friction pair were disks 25.4 mm in diameter and 8 mm thick made of the following: • steel 100Cr6 (surface roughness Ra = 0.043 μm); • polyamide 6 (PA6) (surface roughness Ra = 0.0014 μm), producer Azoty Tarnów (Poland); • poly(methyl methacrylate) (PMMA) (surface roughness Ra = 0.031 μm), producer Profilex S.A.; • aluminum oxide (Al2O3) (surface roughness Ra = 0.148 μm); • zirconium oxide (ZrO2) (surface roughness Ra = 0.010 μm). An example of the dependence of the coefficient of friction on test duration for water and the solution tested for steel friction pairs has been shown in Figure 6. In the case of other disk materials the character of changes is analogous. The dependence μ (t) for a 60% aqueous solution of maracuja oil ethoxylate (MEO60) differs from the one for water. In the presence of the solution tested, the coefficient of friction

were carried out for 15 min at a steady load in the 1−6 kN range and the rotational speed of the spindle of 200 rpm. Figure 3 shows an example of the dependence of the coefficient of friction on time for water and a 60% aqueous solution of MEO60.

Figure 3. Example of the dependence of the coefficient of friction (μ) on time, for water and a 60% aqueous solution of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60): four-ball tester (T-02), rotational speed of the spindle 200 rpm, load 2.0 kN, test duration 900 s.

The effect of load on the coefficient of friction (μ) and wear scar diameters (d) for a 60% aqueous solution of maracuja oil ethoxylated with 60 mols of ethylene oxide (MEO60) has been examined and the results are given in Figures 4 and 5.

Figure 4. Dependence of the coefficient of friction (μ) on load for water and a 60% aqueous solution of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60): four-ball tester (T-02), rotational speed of the spindle 200 rpm, test duration 900 s.

Figure 5. Dependence of wear scar diameter of the balls (d) on load for water and a 60% aqueous solution of maracuja oil ethoxylated with 60 mols of ethylene oxide (MEO60): four-ball tester (T-02), rotational speed of the spindle 200 rpm, test duration 900 s.

Figure 6. Changes in the coefficient of friction (μ) as a function of time for water and a 60% aqueous solution of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60): steel ball−steel disk tribometer (T-11), load 50 N, linear velocity 0.1 m/s, friction path 90 m, test duration 900 s, temperature 25 °C.

In the presence of water, the coefficient of friction (μ) increases as a function of load from 0.38 (1.0 kN) to 0.44 (2.0 16171



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decreases within the first 100 s and then stabilizes at the level of 0.06. However, an increase from 0.18 to 0.38 can be observed for water. Tests were carried out at 5 loads (10, 20, 30, 40, 50 N) for steel−steel pairs. The results as a function of load are given in Figure 7.

Figure 9. Values of friction coefficient (μ) for various disk materials: steel, polyamide6 (PA6), poly(methyl methacrylate) (PMMA), aluminum oxide (Al2O3), and zirconium oxide (ZrO2) for water and a 60% solution of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60): ball-on-disk tribometer (T-11), load 50 N, linear velocity 0.1 m/s, friction path 90 m, test duration 900 s, temperature 25 °C. Figure 7. Dependence of the coefficient of friction (μ) on load for water and a 60% aqueous solution of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60). Steel ball−steel disk tribometer (T11), load 10−50 N, linear velocity 0.1 m/s, friction path 90 m, test duration 900 s, temperature 25 °C.

In the whole range of loads the coefficient of friction does not undergo changes in the presence of the solutions and ranges from 0.06 to 0.07. On the basis of that, it can be said that resistance to motion is at a very low level regardless of the load applied. The tribological tests were followed by measurement of wear scar diameters. Their dependence on the load applied is shown in Figure 8.

Figure 10. Values of wear scar diameter (d) for various disk materials: steel, polyamide (PA6), poly(methyl methacrylate) (PMMA), aluminum oxide (Al2O3), and zirconium oxide (ZrO2) for water and a 60% solution of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60): ball-on-disk tribometer (T-11), load 50 N, linear velocity 0.1 m/s, friction path 90 m, test duration 900 s, temperature 25 °C.

than for water in the case of all the materials examined (Figure 9). If the quotient of friction coefficients in water and solution (μH2O:μSOLUTION) is accepted as an assessment criterion for an increase in resistance to motion, then it will have the following values: 5.5 steel disk, 1.2 polyamide 6 disk, 6.7 poly(methyl methacrylate) disk, 2.3 aluminum oxide disk, and 3.4 zirconium oxide disk. A particularly large decrease in the μ value can be observed in the case of disks made of poly(methyl methacrylate). The lowest absolute values of μ were obtained for ethoxylate solutions in the case of disks made of PMMA (μ = 0.03) and steel (μ = 0.06). Wear scar values (d) are given in Figure 10. It can be seen (Figure 10) that the values of d for solutions are lower than those for water in the case of friction pairs with disks made of steel or zirconium oxide. A different trend can be observed in the case of the steel−aluminum oxide friction pair for which wear is lower for water. A similar dependence was observed for these pairs in the presence of other lubricants.12,28−30 Wear of a steel ball in contact with polymeric disks (PA6 and PMMA) was immeasurably low (Figure 10). Therefore, measurements of wear profiles were made for all the disks, including the polymeric ones (Figure 11). The results obtained (Figures 10 and 11) can be interpreted in terms of hardness of the materials in contact and friction pair geometry. In the case of a steel ball in contact with a disk made, alternatively, of steel, Al2O3, or ZrO2, no disk wear scar can be observed in the presence of water or the ethoxylate solution (Figure 11a−c). The steel ball is not worn (Figure 10).

Figure 8. Dependence of wear scar diameter (d) on load for water and a 60% aqueous solution of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60): steel ball−steel disk tribometer (T-11), load 10−50 N, linear velocity 0.1 m/s, friction path 90 m, test duration 900 s, temperature 25 °C.

As previously mentioned, tribological tests were made for water and the 60% MEO60 solution at the highest load (50 N) using various materials. The friction pairs used consisted of a steel ball and disks made of steel, aluminum oxide (Al2O3), zirconium oxide (ZrO2), polyamide (PA6) or poly(methyl methacrylate) (PMMA). The test results obtained for water and MEO60 for various disk materials have been given in Figures 9 and 10. They do not form a basis for their interpretation in relation to the kind of material, as the materials applied differ in, for example, surface energy, roughness, elasticity, and brittleness, and the effect of these quantities on tribological properties is hard to predict. It can be noted, however, that the μ values are lower for the solutions 16172



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quantities, LLC to a small degree undergo deformation when subjected to load, whereas the presence of easy slip planes determines low resistance to motion. Solubilization of water in LLC also plays a substantial role. Test results1,10,12,28−30 indicate that easy slip planes can form in the water phase. Studies on solutions of amphiphilic compounds exhibiting liquid crystalline ordering will be continued. On the basis of results it can be stated that 60% aqueous solutions of maracuja oil ethoxylates are a good base for new, ecological, safe lubricants. Further research will be focused on selection of additional components (e.g., preservatives, corrosion and oxidation inhibitors, antifoaming agents) so that the lubricants obtained satisfy criteria for selected applications.

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SUMMARY AND CONCLUSIONS This paper presents possibilities to use 60% solutions of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60) as model lubricants. At that concentration, the presence of lyotropic liquid crystals (LLC) was confirmed experimentally (polarization microscopy, rheology). The results of tribological tests indicate that a considerable reduction in resistance to motion and wear relative to water as a base can be observed both at high loads of the order of kilonewtons (a fourball tester) and at lower loads of 10−50 N (a ball-on-disk tribometer). The coefficients of friction reach the values of the order of hundredths, which is characteristic of fluid friction.31 Besides, an extremely high load (up to 5.5 kNa four-ball tester) can be observed at which the mechanical system does not undergo seizure. This phenomenon is exceptional not only for water but also for commercial lubricants.11,12 To sum up, based on the analysis of the results obtained, it can be said that lyotropic liquid crystals may be used as lubricants characterized by low resistance to motion, low wear and high load-carrying capacity. Apart from having beneficial tribological properties they are also safe to use, because the basic components of LLC are oil ethoxylates which do not have a negative effect on the human body and the natural environment. Their safety has been verified repeatedly and this has become a basis for their application as ingredients in foodstuffs, pharmaceuticals, and cosmetics. They can thus be used as lubricants in machines and devices in the food, pharmaceutical, and cosmetics industries. Their application in biomaterials can also be expected.

Figure 11. Examples of disk wear profiles for the pairs: steel ball−disk made of (a) steel, (b) Al2O3, (c) ZrO2, (d) PA6, and (e) PMMA obtained after T-11 tests (load 50 N, linear velocity 0.1 m/s, friction path 90 m, test duration 900 s, temperature 25 °C) for water and a 60% solution of maracuja oil ethoxylated with 60 mol of ethylene oxide (MEO60): profilometer TOPO 01P.

However, wear of the disks made of polymers (PA6 and PMMA) can be observed (Figure 11d, e). The disks undergo higher wear in water. In the case of MEO60 solutions, wear is low and can be treated as a plastic deformation. The results of the measurements carried out on tester T-11 under moderate loads (10−50 N) indicate that resistance to motion and wear are considerably lower in the presence of the 60% solution of ethoxylated maracuja oil (MEO60) than in the presence of water. A beneficial effect of the ethoxylate as an additive modifying lubricating properties of water was found not only in the case of steel disks but also when the disks were made of ceramics (Al2O3, ZrO2) or polymers (PA6 and PMMA). Higher wear values for the steel−Al2O3 system were the only exception (Figure 10). Unexpectedly beneficial tribological properties of the 60% solution of (MEO60) can be interpreted on the basis of the adsorptive-structural mechanism. The ethoxylates (MEO60) are surface active compounds. They form liquid crystalline structures both in the surface phase and in the bulk phase at the concentrations applied (60% solutions).1,12 Mesophases fill microasperities which increases the real contact area and high load-carrying capacity. Due to anisotropy of mechanical

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financed by the Polish Ministry of Science and Higher Education within the framework of the Iuventus Plus Project in the years 2011−2013, no. IP2010 01870.

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REFERENCES

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New Ecological Lubricants on the Basis of Lyotropic Liquid Crystals

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