SML/ESMIS mixtures

Physicochemical and tribological test results for lubricants composed of water and SML/ESMIS mixtures are presented in sections 18.4 and 18.5 of this chapter, respectively. Particular attention was paid to the effect of the composition of the mixture on reducing resistance to motion and prevention of wear and seizure of the mating elements. An attempt was made to show that proper selection of the composition of the surfactants in a lubricating substance makes it possible to obtain lubricants capable of forming a stable adsorbed film during friction. 

PHYSICOCHEMICAL PROPERTIES OF AQUEOUS SOLUTIONS OF SML/ESMIS MIXTURES... 

Physicochemical studies were carried out for aqueous solutions of SML/ESMIS mixtures with the following ratios 1 9, 3 7, 5 5, 7 3, and for an aqueous solution of ESMIS (SML/ESMIS ratio = 0 10). The ratios stand for the weight ratio of the components. The selection of SML/ESMIS ratios was determined by the water solnbUi-ties of the mixtures obtained. The compositions with a large content of SML did not dissolve in water therefore, they were not examined. 

The first test was an assessment of surface activity of SML/ESMIS mixtures. Surface tension (o) measurements of their aqueous solutions at concentrations of 0.001%, 0.01%, 0.1%, 1%, 2%, and 4 wt% were made. A TDl Lauda tensiometer was used. The test was carried out at 20°C. The dependences of surface tension of aqueous SML/ESMIS solutions vs. concentration are presented in fig. 18.2. 

The results obtained confirm that the addition of SML/ESMIS mixtures to water causes a reduction in surface tension. For each of the mixtures tested, increasing the concentration in the range of 0.001% to 1% resnlted in a linear decrease in a. No significant changes in the value of a were observed at concentrations of 1%, 2%,... 

Determination of the effect of the SML/ESMIS ratio on the formation of microemulsions was the next test to confirm the ability of the mixtures to reduce interfacial tension. The following model systems were used in the study water-kerosene-SML/ ESMIS and water-vaseline oil-SML/ESMlS. Compositions containing a hydrophobic solvent (kerosene or vaseline oil) and an SML/ESMIS mixture were prepared at the weight ratios of 1 1 and 1 3 by adding portions of water to the resulting mixtures. Observations were carried out to find the water content at which microemulsions transformed into an emulsion. The test was carried out at 20°C. The results obtained can be seen in figs. 18.3 and 18.4. 

It was found that microemulsions formed much more readily in the case of a surfactant-mixture/hydrophobic-solvent composition with the 3 1 ratio. Besides, in both systems containing kerosene and in those containing vaseline oil, the largest share of water that did not lead to a transition of microemulsions into an emulsion occurred for an SML/ESMIS mixture with the 3 7 ratio. The fact that a maximum can be observed on the dependences obtained is also important. This shows that the most beneficial arrangement of molecules of both surfactants at the oil/water interface occurs only at a specific SML/ESMIS ratio. 

The changes in kinematic viscosity as a function of concentration indirectly prove the presence of micelles in solutions of SML/ESMIS mixtures. A noticeable increase in kinematic viscosity relative to water was not observed until the concentration reached 1%. In combination with the data from the surface-tension measurements, the CMC value can be expected around this concentration. It should also be added that there may be problems with an accurate determination of the CMC value when commercial products are used in the tests. This kind of material may contain unreacted reactants, e.g., fatty acids. As a result, changes in the quantities being measured are not very pronounced. In this case, it seems justified to quote a range or approximate concentration values at which CMC occurs. 

A relatively high effect of temperature was also discovered on the basis of measurements of kinematic viscosity of aqueous solutions of SML/ESMIS mixtures (table 18.1, fig. 18.7). Kinematic viscosity decreased by about 50% in all cases from 25°C to 60°C. 

Kinematic Viscosity for Aqueous Solutions of SML/ESMIS Mixtures at Various Temperatures... 

FIGURE 18.5 Dependence of kinematic viscosity on concentration of aqneons solutions of SML/ESMIS mixtures (25°C). The following SML/ESMIS ratios were investigated 0 10, 1 9, 3 7, 5 5 and 7 3. 

It has been assumed in this chapter that lubricant compositions will contain relatively low concentrations of additives, only up to a few percent. However, properties of aqueous solutions of SML/ESMIS mixtures at concentrations of about 40%-80% seem to be interesting. Liquid crystalline structures may form in such solutions. 

To confirm the presence of mesophases, dynamic viscosity measurements were made and microscopic photographs in polarized light were taken for aqueous solutions of SML/ESMIS mixtures at the concentrations of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90 wt%. 

FIGURE 18.8 Dependence of dynamic viscosity on concentration of aqneons solntions of SML/ESMIS mixtures with 1 9 and 3 7 ratios (Brookfield RV DVI+, temperatnre 25°C). 

FIGURE 18.9 Microscope photographs in polarized light of aqueous solutions of SML/ ESMIS mixtures with 1 9 (70%) and 3 7 (70% and 80%) ratios magnification 150x measurement temperature 25°C. 

The tests were conducted on aqueous solutions of SML/ESMIS mixtures at the concentrations of 0.01%, 0.1%, 1%, and 4 wt%. A T-11 tribometer (pin-on-disc) produced by ITeE Poland (Institute for Sustainable Technologies, National Research Institute, Radom, Poland) was used in the tests. The pins used were 3.0 mm in diameter and were made of LH15 steel or polyamide-6, while the discs were 25 mm in diameter and were made of polyamide-6. Before the tests, all components of the friction couples were thoroughly chemically cleaned. An ultrasonic cleaner was employed. The steel elements were cleaned in n-hexane, acetone, ethyl alcohol, and distilled water. The polyamide-6 elements were cleaned in ethyl alcohol and distilled water. After the cleaning process all friction-couple components were dried (50°C, 30 min). The time of the test run was 900 s, the friction-couple load was 10 N, and the sliding velocity was 0.1 m/s. The coefficient of friction was calculated from friction force measurements using... 

Figure 18.10 presents examples of dependences of friction coefficient changes as a function of time for water and 0.01% aqueous solutions of SML/ESMIS mixtures. The highest friction coefficient values were observed for the tests in which pure water was the lubricant. Over the test duration, the coefficient of friction increased from a value of 0.2 to about 0.3. Over the course of measurement, in the case of all the solutions containing the SML/ESMIS mixtures, only a slight reduction in resistance to motion occurred, and no changes in friction coefficient values were observed. However, the compositions analyzed differed in their ability to reduce p. A set of mean friction coefficient values from the whole 900-s test can be seen in fig. 18.11. 

Tests were then carried out for 0.1% solutions. The results are presented in fig. 18.12. The results obtained for 0 10 and 1 9 SML/ESMIS mixtures show similar values of the coefficient of friction ranging from 0.13 to 0.16. The highest resistance to motion was observed for the composition containing the highest share of SML (7 3) the mean p value equaled 0.24. 

A relatively high scatter in mean values of p was observed when 1% aqueous solutions of SML/ESMIS mixtures were examined (fig. 18.13). A friction coefficient value of 0.22 was obtained for the composition containing only ESMIS (0 10). Then the p value decreased to 0.2 for a 1 9 mixture. The lowest resistance to motion (p = 0.1) was observed for the 3 7 ratio. The solutions of the other two mixtures with 5 5 and 7 3 SML/ESMIS ratios exhibited higher resistances to motion the p values obtained equaled 0.16 and 0.22, respectively. 

Figure 18.14 shows the friction coefficient values obtained in tests carried out in the presence of 4% aqueous solutions of SML/ESMIS mixtures. No significant differences in friction coefficient values were observed for solutions of all the mixtures, the values ranged from 0.16 to 0.18. 

Figure 18.15 shows examples of dependences of p on time for water and 0.01% aqueous solutions of SML/ESMIS mixtures. In the case of water, during the conrse of the test the coefficient of friction increased slightly from the value of 0.12 to 0.16. The dependences obtained for solutions of all mixtures were similar the p values practically did not change dnring the course of the test. 

The relatively low p value (0.14) observed during tests in which pure water was the lubricant is worth noting. Addition of an SML/ESMIS mixture to water reduces the resistance to motion to some degree. Regardless of the SML/ESMIS ratio or mixture concentration, mean p values were close to the value obtained for water and ranged from 0.09 to 0.16. 

The tests were conducted for 1% aqueous solutions of SML/ESMIS mixtures. The T-02 tester was manufactured at the Institute for Sustainable Technologies-National Research Institute in Poland. The balls of /2-in. diameter were made of LH 15 bearing steel. Snrface ronghness was R, = 0.032 pm. The device was employed to measure motion resistance, wear, and antiseizure abilities in the presence of a lubricant. The methodology of the tests is described in the literature [130]. The values of the quantities measnred are arithmetic means of three independent measurements. The errors were evaluated with Student s 1-test at a confidence level of 0.90. 

The influence of aqueous solutions of SML/ESMIS mixtures on the coefficients of friction, wear, and temperature in the friction pair was evaluated under the following conditions rotational speed of the spindle, 200 20 rpm constant applied load, 3.0 kN (in the case of water, the tests were conducted at the load of 2.0 kN) test duration, 900 s. Data were automatically acquired every second. 

Changes in the coefficient of friction as a function of time observed for 1% aqueous solutions of SML/ESMIS mixtures are shown in fig. 18.20. Out of the lubricant compositions tested, the highest resistance to motion was observed for water. Its lubricating properties were so poor that, just a few seconds after starting the tester under the load of 3.0 kN, welding of the balls took place. Therefore, tests with pure water were carried out at a load of 2.0 kN. Under these conditions, the coefficient of friction was relatively high, fluctuating around 0.3. 

The addition of SML/ESMIS mixtures to water significantly improved its lubricity. At a load of 3.0 kN, seizure of balls was not observed. In the case of solutions of all the mixtures analyzed, a similar behavior of friction coefficient changes as a function of time was observed. In the initial phase of the test, the p values were the highest (about 0.2-0.3). Then, after about 100 s, the p value decreased to about 0.1. It is interesting that in the case of SML/ESMIS mixtures with the 3 7,5 5, and 7 3 ratios, the friction coefficient value was practically constant after the 100th second of the test. There were fluctuations in the p values for the other compositions (0 10 and 9 1). 

FIGURE 18.20 Friction coefficient changes vs. time for water and 1% aqueous solutions of SML/ESMIS mixtures at a load of 3000 N (four-ball tester). (Note Test for water at 2000 N.)... 

The character of changes in wear of the balls as a function of the SML/ESMIS ratio (fig. 18.23) is similar to the one observed for temperature (fig. 18.22). The lowest wear-scar value (0.95 nun) was obtained for the tests in the presence of a 1% aqueous solution of the SML/ESMIS mixture with the 5 5 ratio. Relatively low wear scar was also found for the 7 3 ratio. In the case of the other mixtures, wear scars of the balls ranged from 1.2 nun to 1.3 nun. In the tests for water (friction-pair load of 2.0 kN), the wear-scar diameter was 1.8 nun. 

Dependences of friction torque on time are presented in fig. 18.26, and the mean values of scuffing load, wear-scar diameters of the balls, and wear-scar profiles obtained after the tests in the presence of % aqueous solutions of SML/ESMIS mixtures are shown in figs. 18.27-18.29. 

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