Aus der Praxis für die Praxis 1 Introduction The rheological properties of lubricants in general have been extensively studied, since these properties control a lubricant’s ability to efficiently flow through mechanical systems and to form lubricating films [1-15]. However, most studies so far have focused mainly on crankcase lubricants and the related base stocks and base stock-polymer solutions [1-5, 7-9]. Low temperature rheology of crankcase oils is of particular interest since these lubricants need to be pumped throughout the engine when the engine is started. If the crankcase oil cannot be pumped at low temperatures when the engine is started, the lubricant will be unable to protect critical engine parts. This particular problem is also of interest to wind turbine operators. Wind turbines are often stopped under extreme conditions and then need to be restarted. At cold temperatures mechanical problems may occur when the wind turbine is restarted. In addition, if temperature is too low it may be difficult to restart the wind turbine due to the low temperature rheological properties of the wind turbine oils. Researchers studying crankcase oils have developed a series of standard tests to assess the low temperature rheology of these fluids as well as other lubricants [16- 27]. In fact, ASTM D2983 is used to assess the low temperature rheological properties of industrial fluids including wind turbine oils. However, lubricants can undergo complex structural changes at low temperatures. These phase transtions may involve formation of wax networks from constituents of the base oil or polymer networks from the polymers (viscosity modifiers) or esters added to oils. The formation of wax and polymer networks versus the natural increase in lubricant viscosity at low temperature can not be differentiatied by just measuring the low temperature viscosity of a lubricant [28-29]. Oscillatory rheometry can be used to determine whether oils form wax networks, polymer networks or have become extremely viscous at low temperatures [30-31]. This technique has been applied to the study of the low temperature rheology of wind turbine oils. These studies have made it easier to select the proper base oil and additive components for wind turbine oils. The result is optimized combinations of base oil and additive components with improved low temperature rheological properties versus current commercial wind turbine lubricants. 2 Standard Rheological Properties of Wind Turbine Oils Table 1 shows the 40 °C and 100 °C kinematic viscosities and -40 °C Brookfield viscosities of seven commercial wind turbine oils. Kinematic viscosities were measured using ASTM D445 [32], viscosity index was calculated using ASTM D2270 [33] and Brookfield viscosities were measured using ASTM D2983 [27]. The oils have similar kinematic viscosities at 40 °C. There is Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 61 * Kenneth J Garelick, B.S. Chemistry Jeffrey M Guevremont, Ph.D. Physical Chemistry William B. Anderson, Ph.D. Mechanical Engineering Mark T Devlin, Ph.D. Physical Chemistry Afton Chemical Corporation, Richmond, Virginia, USA Helen Ryan-Ph.D., Synthetic Inorganic Chemistry Afton Chemical Ltd, Bracknell, United Kingdom Warren Cates, M.S. Organic Chemistry Paul D. Savage, Ph. D Organometallic Chemistry Shell Global Solutions (US) Inc., Houston, TX Low Temperature Rheology of Wind Turbine Oils K. Garelick, J. Guevremont, W. Anderson, M. Devlin, H. Ryan, W. Cates, P. Savage* Wind turbines are often stopped under extreme conditions and then need to be restarted. If the temperature is too low it is difficult to re-start the wind turbine due to the low temperature rheological properties of the wind turbine oils. Typically, simple viscometric tests are used to measure the low temperature rheology of oils. However, lubricants can undergo complex structural changes at low temperatures that often make it difficult to determine the cause of differences in low temperature rheological results. Oscillatory rheometry has been used to examine the viscous and elastic behavior of wind turbine oils at low temperatures. These studies have made it easier to select the proper base oil and additive components for wind turbine oils. The optimized combinations of base oil and additive components have resulted in fluids that have improved low temperature rheological properties versus current commercial wind turbine lubricants. Keywords Rheology, Wind Turbine, Low Temperature, Base Oils, Esters, Viscosity Modifiers Abstract T+S_6_16 17.10.16 17: 01 Seite 61 Aus der Praxis für die Praxis a larger spread in the kinematic viscosities at 100 °C. However, the oils have significantly different viscosities at -40 °C. meter. There is excellent agreement between the results from these two techniques. With the exception of ASTM D5133 (Scanning Brookfield) all other commonly used ASTM methods must be run multiple times to produce plots of viscosity versus temperature. The oscillatory rheometer measures rheological properties at multiple temperatures and since the viscous and elastic properties of fluids are measured complex phase transitions in fluids at low temperature can be detected. Figures 2 and 3 are data from reference 30 showing how the rheometer can detect the formation of wax networks (Figure 2) and polymer networks (Figure 3) at low temperatures. Figure 2 shows a low temperature oscillatory rheometer trace for a Group I base oil that contains wax-like molecules [34]. The elastic modulus (open circles) and viscous modulus (closed circles) are plotted versus temperature. At approximately -15 °C both moduli increase rapidly indicating the formation of a wax network. Figure 3 shows a low temperature oscillatory rheometer trace for PAO containing a polymer that will aggregate at low temperature. PAO contains very few if any wax-like molecules and does not form a wax network at low temperature [30]. At approximately 0 °C, the elastic modulus 62 Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 Table 1: Kinematic and Brookfield Viscosities of Wind Turbine Oils Oil KV KV Viscosity B’field 40 °C 100 °C Index Viscosity mm 2 / s mm 2 / s mPa*s at -40 °C Oil A 337 37.7 161 447300 Oil B 323 34.4 151 589300 Oil C 329 42.2 184 249300 Oil D 335 38.0 163 512300 Oil E 337 39.5 169 548300 Oil F 321 35.0 154 731300 Oil G 327 39.6 173 292300 3 Oscillatory Rheometry 3.1 Low Temperature Scan An Anton-Paar model MCR302 Rheometer was used to simultaneously measure the viscous and elastic properties of oils as they were cooled at approximately 1 °C per minute. All measurements were made using parallelplate geometry. To eliminate moisture condensation on the sample and plates, the system was housed in a thermally controlled hood with a nitrogen purge. The rheometer was operated in the oscillatory mode with angular frequency of 2 radians per second and angular displacement of 1 milliradians. All data were collected in the linear viscoelastic region. From the viscous and elastic properties measured in the oscillatory rheometer, the complex viscosity of the wind turbine oils can be calculated. Figure 1 shows the correlation between -40 °C Brookfield viscosities and -40 °C complex viscosities measured using the oscillatory rheo- The oils have similar kinematic viscosities at 40 °C. There is a larger spread in the kinematic viscosities at 100 °C. However, the oils have significantly different viscosities at -40 °C. 3. Oscillatory Rheometry 3.1 Low Temperature Scan An Anton-Paar model MCR302 Rheometer was used to simultaneously measure the viscous and elastic properties of oils as they were cooled at approximately 1°C per minute. All measurements were made using parallel-plate geometry. To eliminate moisture condensation on the sample and plates, the system was housed in a thermally controlled hood with a nitrogen purge. The rheometer was operated in the oscillatory mode with angular frequency of 2 radians per second and angular displacement of 1 milli-radians. All data were collected in the linear viscoelastic region. From the viscous and elastic properties measured in the oscillatory rheometer, the complex viscosity of the wind turbine oils can be calculated. Figure 1 shows the correlation between -40 °C Brookfield viscosities and - 40 °C complex viscosities measured using the oscillatory rheometer. There is excellent agreement between the results from these two techniques. R² = 0.8869 0 200000 400000 600000 800000 0 200000 400000 600000 800000 Complex Viscosity (mPa*s) Brookfield Viscosity (mPa*s) With the exception of ASTM D5133 (Scanning Brookfield) all other commonly used ASTM methods must be run multiple times to produce plots of viscosity versus temperature. The oscillatory rheometer measures rheological properties at multiple temperatures and since the viscous and elastic properties of fluids are measured complex phase transitions in fluids at low temperature can be detected. Figures 2 and 3 are data from reference 30 showing how the rheometer can detect the formation of wax networks (Figure 2) and polymer networks (Figure 3) at low temperatures. Figure 2 shows a low temperature oscillatory rheometer trace for a Group I base oil that contains wax-like molecules [34]. The elastic modulus (open circles) and viscous modulus (closed circles) are plotted versus temperature. At approximately -15 °C both moduli increase rapidly indicating the formation of a wax network. Figure 3 shows a low temperature oscillatory rheometer trace for PAO containing a polymer that will aggregate at low temperature. PAO contains very few if any wax-like molecules and does not form a wax network at low temperature [30]. At approximately 0°C, the elastic modulus for the fluid increases rapidly indicating the formation of a polymer network. The fluids examined in Figures 1 and 2 would have very high Brookfield viscosities and by use of oscillatory rheometry the reason for the increase in viscosity is known. -3 0 -2 0 -1 0 0 1 0 1 0 -2 1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 E la stic M o d u lu s (G ' P a ) V isco u s M o d u lu s (G '' P a ) T em p erature, 0 C Figure 2: Oscillatory Rheometry Trace of Group I Base Oil [30] Figure 3: Oscillatory Rheometry Trace of PAO Containing a Polymer that Aggregates at Low Temperature [30] The detection of wax or polymer networks is easily observed by examining the ratio of the viscous and elastic moduli, referred to as tan d. Oils with high tan d values are just highly viscous at low temperature [30]. Fluids with low tan d values form wax or polymer networks. This distinction is critical if the low temperature rheological properties of oils need to be improved. In order to lower the -40 °C Brookfield viscosity of an oil with a high tan d value requires increasing the viscosity index of the oil so its viscosity does not increase rapidly as temperature changes. Polymers and esters that do not form networks at low temperature need to be identified, in order to improve the -40 °C Brookfield viscosities of oils with low tan d. Figure 1: Correlation between Brookfield and Complex Viscosities at -40 °C Figure 3: Oscillatory Rheometry Trace of PAO Containing a Polymer that Aggregates at Low Temperature [30] Figure 2: Oscillatory Rheometry Trace of Group I Base Oil [30] The oils have similar kinematic viscosities at 40 °C. There is a larger spread in the kinematic viscosities at 100 °C. However, the oils have significantly different viscosities at -40 °C. 3. Oscillatory Rheometry 3.1 Low Temperature Scan An Anton-Paar model MCR302 Rheometer was used to simultaneously measure the viscous and elastic properties of oils as they were cooled at approximately 1°C per minute. All measurements were made using parallel-plate geometry. To eliminate moisture condensation on the sample and plates, the system was housed in a thermally controlled hood with a nitrogen purge. The rheometer was operated in the oscillatory mode with angular frequency of 2 radians per second and angular displacement of 1 milli-radians. All data were collected in the linear viscoelastic region. From the viscous and elastic properties measured in the oscillatory rheometer, the complex viscosity of the wind turbine oils can be calculated. Figure 1 shows the correlation between -40 °C Brookfield viscosities and - 40 °C complex viscosities measured using the oscillatory rheometer. There is excellent agreement between the results from these two techniques. Figure 1: Correlation between Brookfield and Complex Viscosities at -40 °C With the exception of ASTM D5133 (Scanning Brookfield) all other commonly used ASTM methods must be run multiple times to produce plots of viscosity versus temperature. The oscillatory rheometer measures rheological properties at multiple temperatures and since the viscous and elastic properties of fluids are measured complex phase transitions in fluids at low temperature can be detected. Figures 2 and 3 are data from reference 30 showing how the rheometer can detect the formation of wax networks (Figure 2) and polymer networks (Figure 3) at low temperatures. Figure 2 shows a low temperature oscillatory rheometer trace for a Group I base oil that contains wax-like molecules [34]. The elastic modulus (open circles) and viscous modulus (closed circles) are plotted versus temperature. At approximately -15 °C both moduli increase rapidly indicating the formation of a wax network. Figure 3 shows a low temperature oscillatory rheometer trace for PAO containing a polymer that will aggregate at low temperature. PAO contains very few if any wax-like molecules and does not form a wax network at low temperature [30]. At approximately 0°C, the elastic modulus for the fluid increases rapidly indicating the formation of a polymer network. The fluids examined in Figures 1 and 2 would have very high Brookfield viscosities and by use of oscillatory rheometry the reason for the increase in viscosity is known. -3 0 -2 0 -1 0 0 1 0 1 0 -2 1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 E la stic M o d u lu s (G ' P a ) V isco u s M o d u lu s (G '' P a ) T em p erature, 0 C Figure 2: Oscillatory Rheometry Trace of Group I Base Oil [30] - 8 0 - 7 0 - 6 0 - 5 0 - 4 0 - 3 0 - 2 0 - 1 0 0 1 0 2 0 1 0 - 2 1 0 - 1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 E l a s t i c M o d u l u s ( G ' P a ) V i s c o u s M o d u l u s ( G '' P a ) T e m p e r a t u r e ( 0 C ) Figure 3: Oscillatory Rheometry Trace of PAO Containing a Polymer that Aggregates at Low Temperature [30] The detection of wax or polymer networks is easily observed by examining the ratio of the viscous and elastic moduli, referred to as tan d. Oils with high tan d values are just highly viscous at low temperature [30]. Fluids with low tan d values form wax or polymer networks. This distinction is critical if the low temperature rheological properties of oils need to be improved. In order to lower the -40 °C Brookfield viscosity of an oil with a high tan d value requires increasing the viscosity index of the oil so its viscosity does not increase rapidly as temperature changes. Polymers and esters that do not form networks at low temperature need to be identified, in order to improve the -40 °C Brookfield viscosities of oils with low tan d. The oils have similar kinematic viscosities at 40 °C. There is a larger spread in the kinematic viscosities at 100 °C. However, the oils have significantly different viscosities at -40 °C. 3. Oscillatory Rheometry 3.1 Low Temperature Scan An Anton-Paar model MCR302 Rheometer was used to simultaneously measure the viscous and elastic properties of oils as they were cooled at approximately 1°C per minute. All measurements were made using parallel-plate geometry. To eliminate moisture condensation on the sample and plates, the system was housed in a thermally controlled hood with a nitrogen purge. The rheometer was operated in the oscillatory mode with angular frequency of 2 radians per second and angular displacement of 1 milli-radians. All data were collected in the linear viscoelastic region. From the viscous and elastic properties measured in the oscillatory rheometer, the complex viscosity of the wind turbine oils can be calculated. Figure 1 shows the correlation between -40 °C Brookfield viscosities and - 40 °C complex viscosities measured using the oscillatory rheometer. There is excellent agreement between the results from these two techniques. Figure 1: Correlation between Brookfield and Complex Viscosities at -40 °C With the exception of ASTM D5133 (Scanning Brookfield) all other commonly used ASTM methods must be run multiple times to produce plots of viscosity versus temperature. The oscillatory rheometer measures rheological properties at multiple temperatures and since the viscous and elastic properties of fluids are measured complex phase transitions in fluids at low temperature can be detected. Figures 2 and 3 are data from reference 30 showing how the rheometer can detect the formation of wax networks (Figure 2) and polymer networks (Figure 3) at low temperatures. Figure 2 shows a low temperature oscillatory rheometer trace for a Group I base oil that contains wax-like molecules [34]. The elastic modulus (open circles) and viscous modulus (closed circles) are plotted versus temperature. At approximately -15 °C both moduli increase rapidly indicating the formation of a wax network. Figure 3 shows a low temperature oscillatory rheometer trace for PAO containing a polymer that will aggregate at low temperature. PAO contains very few if any wax-like molecules and does not form a wax network at low temperature [30]. At approximately 0°C, the elastic modulus for the fluid increases rapidly indicating the formation of a polymer network. The fluids examined in Figures 1 and 2 would have very high Brookfield viscosities and by use of oscillatory rheometry the reason for the increase in viscosity is known. -3 0 -2 0 -1 0 0 1 0 1 0 -2 1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 E la stic M o d u lu s (G ' P a ) V isco u s M o d u lu s (G '' P a ) T em p erature, 0 C Figure 2: Oscillatory Rheometry Trace of Group I Base Oil [30] Figure 3: Oscillatory Rheometry Trace of PAO Containing a Polymer that Aggregates at Low Temperature [30] The detection of wax or polymer networks is easily observed by examining the ratio of the viscous and elastic moduli, referred to as tan d. Oils with high tan d values are just highly viscous at low temperature [30]. Fluids with low tan d values form wax or polymer networks. This distinction is critical if the low temperature rheological properties of oils need to be improved. In order to lower the -40 °C Brookfield viscosity of an oil with a high tan d value requires increasing the viscosity index of the oil so its viscosity does not increase rapidly as temperature changes. Polymers and esters that do not form networks at low temperature need to be identified, in order to improve the -40 °C Brookfield viscosities of oils with low tan d. T+S_6_16 17.10.16 17: 01 Seite 62 Aus der Praxis für die Praxis for the fluid increases rapidly indicating the formation of a polymer network. The fluids examined in Figures 1 and 2 would have very high Brookfield viscosities and by use of oscillatory rheometry the reason for the increase in viscosity is known. The detection of wax or polymer networks is easily observed by examining the ratio of the viscous and elastic moduli, referred to as tan δ. Oils with high tan δ values are just highly viscous at low temperature [30]. Fluids with low tan δ values form wax or polymer networks. This distinction is critical if the low temperature rheological properties of oils need to be improved. In order to lower the -40 °C Brookfield viscosity of an oil with a high tan δ value requires increasing the viscosity index of the oil so its viscosity does not increase rapidly as temperature changes. Polymers and esters that do not form networks at low temperature need to be identified, in order to improve the -40 °C Brookfield viscosities of oils with low tan δ. For example, Figure 4 shows the low temperature oscillatory rheometry trace for an ester where the elastic and viscous moduli are converging indicating the formation of a polymer network. When this ester is blended into a fully-formulated oil the elastic modulus of the oil increases and the tan δ value decreases at low temperature. If the Brookfield viscosities for an oil containing this ester are too high then the oil would need to be formulated with an alternate ester to improve the low temperature rheological properties. Table 2 shows the viscous and elastic moduli for the seven wind turbine oils along with the tan δ values at -40 °C. Oils E and F have significantly lower tan δ values than any other oil indicating the formation of a network between constituents in the oil. The elastic moduli for these oils are also higher than the elastic moduli for the other oils. Oils A and D have slightly lower tan δ values indicating some network formation at low temperature but not as extensive as that for Oils E and F. Oils A, B, D, E and F have -40 °C Brookfield viscosities higher than 400,000 mPa*s. However, Oil B also has a low elastic modulus and high tan δ value. To improve the Brookfield viscosity of Oil B would require increasing the viscosity index of this fluid since there is no indication that the polymers in the oil form networks at low temperatures. To improve the Brookfield viscosity of Oils A, D, E and F requires changes to the esters or polymers in the fluids to ones that do not form networks at low temperature. 3.2 Effect of Temperature Cooling Rate It is known from research with crankcase oils and base oils that the rate of cooling of a sample influences the formation of the wax or polymer networks [16-23, 35]. An additional advantage of the oscillatory rheometer is that the viscous and elastic moduli and the complex viscosity of an oil can be measured at multiple temperature cooling rates. This is critical for wind turbine oils since cooling rates in the field may vary. Figure 5 shows the effect of cooling rate on the complex viscosity of Oils D and G. Cooling rate does not affect the complex viscosity of these wind turbine oils. The results for Oil H are also Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 63 For example, Figure 4 shows the low temperature oscillatory rheometry trace for an ester where the elastic and viscous moduli are converging indicating the formation of a polymer network. When this ester is blended into a fully-formulated oil the elastic modulus of the oil increases and the tan d value decreases at low temperature. If the Brookfield viscosities for an oil containing this ester are too high then the oil would need to be formulated with an alternate ester to improve the low temperature rheological properties. Figure 4: Oscillatory Rheometer Trace for Ester that Self-associates at Low Temperature Table 2 shows the viscous and elastic moduli for the seven wind turbine oils along with the tan d values at - 40 °C. Oils E and F have significantly lower tan d values than any other oil indicating the formation of a network between constituents in the oil. The elastic moduli for these oils are also higher than the elastic moduli for the other oils. Oils A and D have slightly lower tan d values indicating some network formation at low temperature but not as extensive as that for Oils E and F. Table 2: Oscillatory Rheometry Results at -40 °C Oils A, B, D, E and F have -40 °C Brookfield viscosities higher than 400,000 mPa*s. However, Oil B also has a low elastic modulus and high tan d value. To improve the Brookfield viscosity of Oil B would require increasing the viscosity index of this fluid since there is no indication that the polymers in the oil form networks at low temperatures. To improve the Brookfield viscosity of Oils A, D, E and F requires changes to the esters or polymers in the fluids to ones that do not form networks at low temperature. 3.2 Effect of Temperature Cooling Rate It is known from research with crankcase oils and base oils that the rate of cooling of a sample influences the formation of the wax or polymer networks [16-23, 35]. An additional advantage of the oscillatory rheometer is that the viscous and elastic moduli and the complex viscosity of an oil can be measured at multiple temperature cooling rates. This is critical for wind turbine oils since cooling rates in the field may vary. Figure 5 shows the effect of cooling rate on the complex viscosity of Oils D and G. Cooling rate does not affect the complex viscosity of these wind turbine oils. The results for Oil H are also included in Figure 5. Oil H is an industrial oil formulated with mineral base stocks which contain wax-like materials that aggregate at low temperature. The results for Oil H are included to show that the rheometer can detect the effect of cooling rate on low temperature properties of oils. In the case of the wind turbine oils we examined cooling rate does not influence their low temperature rheological properties. 1 10 100 1000 10000 -40 -30 -20 -10 0 10 20 Complex Viscosity (Pa s) Temperature (°C) Oil H (1°C/ 1 min.) Oil H (1°C/ 10 min.) Oil D (1°C/ 1 min.) Oil D (1°C/ 10 min.) Oil G (1°C/ 1 min.) Oil G (1°C/ 10 min.) 3.3 Effect of Temperature Cycling In addition to changes in cooling rates in the field, wind turbine oils may be exposed to the cycling of temperature from high to low values. In crankcase oils once a fluid starts to show a tendency to form a complex structure at low temperature, that structure may remain even after the oil is exposed to a higher temperature. This “memory” effect is the reason that many low temperature ASTM methods for crankcase oil require a high temperature soak time to allow for any existing oil component associations to relax [16-23]. Figure 6 shows the complex viscosity for Oil H after the oil has been cycled from high to low to high temperature four times. The complex viscosity for Oil H Figure 5: Effect of Cooling Rate on Low Temperature Rheology For example, Figure 4 shows the low temperature oscillatory rheometry trace for an ester where the elastic and viscous moduli are converging indicating the formation of a polymer network. When this ester is blended into a fully-formulated oil the elastic modulus of the oil increases and the tan d value decreases at low temperature. If the Brookfield viscosities for an oil containing this ester are too high then the oil would need to be formulated with an alternate ester to improve the low temperature rheological properties. 0.01 0.1 1 10 100 1000 10000 -50 -40 -30 -20 -10 0 Modulus (Pa) Temperature (°C) Elastic Modulus Viscous Modulus Table 2 shows the viscous and elastic moduli for the seven wind turbine oils along with the tan d values at - 40 °C. Oils E and F have significantly lower tan d values than any other oil indicating the formation of a network between constituents in the oil. The elastic moduli for these oils are also higher than the elastic moduli for the other oils. Oils A and D have slightly lower tan d values indicating some network formation at low temperature but not as extensive as that for Oils E and F. Table 2: Oscillatory Rheometry Results at -40 °C Oils A, B, D, E and F have -40 °C Brookfield viscosities higher than 400,000 mPa*s. However, Oil B also has a low elastic modulus and high tan d value. To improve the Brookfield viscosity of Oil B would require increasing the viscosity index of this fluid since there is no indication that the polymers in the oil form networks at low temperatures. To improve the Brookfield viscosity of Oils A, D, E and F requires changes to the esters or polymers in the fluids to ones that do not form networks at low temperature. 3.2 Effect of Temperature Cooling Rate It is known from research with crankcase oils and base oils that the rate of cooling of a sample influences the formation of the wax or polymer networks [16-23, 35]. An additional advantage of the oscillatory rheometer is that the viscous and elastic moduli and the complex viscosity of an oil can be measured at multiple temperature cooling rates. This is critical for wind turbine oils since cooling rates in the field may vary. Figure 5 shows the effect of cooling rate on the complex viscosity of Oils D and G. Cooling rate does not affect the complex viscosity of these wind turbine oils. The results for Oil H are also included in Figure 5. Oil H is an industrial oil formulated with mineral base stocks which contain wax-like materials that aggregate at low temperature. The results for Oil H are included to show that the rheometer can detect the effect of cooling rate on low temperature properties of oils. In the case of the wind turbine oils we examined cooling rate does not influence their low temperature rheological properties. Figure 5: Effect of Cooling Rate on Low Temperature Rheology 3.3 Effect of Temperature Cycling In addition to changes in cooling rates in the field, wind turbine oils may be exposed to the cycling of temperature from high to low values. In crankcase oils once a fluid starts to show a tendency to form a complex structure at low temperature, that structure may remain even after the oil is exposed to a higher temperature. This “memory” effect is the reason that many low temperature ASTM methods for crankcase oil require a high temperature soak time to allow for any existing oil component associations to relax [16-23]. Figure 6 shows the complex viscosity for Oil H after the oil has been cycled from high to low to high temperature four times. The complex viscosity for Oil H Figure 4: Oscillatory Rheometer Trace for Ester that Self-associates at Low Temperature Table 2: Oscillatory Rheometry Results at -40 °C Oil B’field Viscosity Elastic tan δ Viscous Modulus Modulus at -40 °C Pa Pa mPa*s Oil A 447300 964 4.3 222 Oil B 589300 1050 0.7 1500 Oil C 249300 382 0.7 531 Oil D 512300 683 2.5 279 Oil E 548300 1120 12.0 93 Oil F 731300 913 14.9 61 Oil G 292300 439 0.3 1416 T+S_6_16 17.10.16 17: 01 Seite 63 Aus der Praxis für die Praxis included in Figure 5. Oil H is an industrial oil formulated with mineral base stocks which contain wax-like materials that aggregate at low temperature. The results for Oil H are included to show that the rheometer can detect the effect of cooling rate on low temperature properties of oils. In the case of the wind turbine oils we examined cooling rate does not influence their low temperature rheological properties. 3.3 Effect of Temperature Cycling In addition to changes in cooling rates in the field, wind turbine oils may be exposed to the cycling of temperature from high to low values. In crankcase oils once a fluid starts to show a tendency to form a complex structure at low temperature, that structure may remain even after the oil is exposed to a higher temperature. This “memory” effect is the reason that many low temperature ASTM methods for crankcase oil require a high temperature soak time to allow for any existing oil component associations to relax [16-23]. Figure 6 shows the complex viscosity for Oil H after the oil has been cycled from high to low to high temperature four times. The complex viscosity for Oil H during all four cycles is identical so the low temperature rheology of Oil H is not affected by temperature cycling. Figure 7 shows the effect of temperature cycling (four passes from low to high to low temperature) on the complex viscosity for Oil C, which has a low Brookfield viscosity at -40 °C. The complex viscosities for Oil C during all four cycles are identical so the low temperature rheology of Oil C is not affected by temperature cycling. Therefore, unlike crank-case oils, the low temperature rheological properties of the wind turbine oils examined here are not affected by temperature cycling. 4 Conclusions Lubricants can undergo complex structural changes at low temperatures that often make it difficult to determine the cause of differences in low temperature rheological results. Standard low temperature rheological tests such as the Brookfield test can determine the low temperature properties of wind turbine oils but do not help identify the root cause of low temperature rheological issues. Oscillatory rheometry has been used to examine the viscous and elastic behavior of wind turbine oils at low temperatures. These studies have made it easier to select the proper base oil and additive components for wind turbine oils. References [1] J. Briant, J. Denis, G. Parc, “Rheological Properties of Lubricants”, Editions Technip, (1989). [2] R.M. Mortier, S.T. Orszulik, “Chemistry and Technology of Lubricants”, VCH Publishers, Inc., (1992). [3] D.N. Schulz, J.E. Glass, “Polymers of Rheology Modifiers”, ACS Symposium Series 462, (1991). [4] D.C. Venerus, E.E, Klaus, J.L. Duda, “Low Temperature Rheology of Hydrocarbon Lubricants”, SAE Paper 872048, (1987). [5] F.L. Lee, E.E. Klaus, J.L. Duda, “Measurement and Analysis of High-Shear Viscosities of Polymer Containing Lubricants”, SAE 881663, (1988). [6] S.P. Kemp, J.L. Linden, “Physical and Chemical Properties of a Typical Automatic Transmission Fluid”, SAE 902148, (1990). [7] R. Larsson, P.O. Larsson, E. Eriksson, M. Sjoberg, E. Hoglund, “Lubricant Properties for Input to Hydrodynamic and Elastohydrodynamic Lubrication Analyses”, Proc. Instn. Mech. Engrs Vol 214 Part J, (2000). [8] J.L. Duda, E.E. Klaus, S-C. Lin, “Capillary Viscometry Study of Non-Newtonian Fluids: Influence of Viscous Heating”, Ind. Eng. Chem. Res., Vol. 27 (2), (1988). [9] A.J. Moore, D. Cooper, T.M. Robinson, “Rheological Properties of Engine Crankcase and Gear Oil Components in Elastohydrodynamic Oil Films”, SAE 941977, (1994). [10] S. Bair, “The Variation of Viscosity with Temperature and Pressure for Various Real Lubricants”, Journal of Tribology Vol 123, (2001). 64 Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 during all four cycles is identical so the low temperature rheology of Oil H is not affected by temperature cycling. Figure 7 shows the effect of temperature cycling (four passes from low to high to low temperature) on the complex viscosity for Oil C, which has a low Brookfield viscosity at -40 °C. The complex viscosities for Oil C during all four cycles are identical so the low temperature rheology of Oil C is not affected by temperature cycling. Therefore, unlike crankcase oils, the low temperature rheological properties of the wind turbine oils examined here are not affected by temperature cycling. 0.1 1 10 100 1000 10000 -50 -40 -30 -20 -10 0 10 20 Complex viscosity (Pa*s) Temperature (°C) Cycle 1 Cycle 2 Cycle 3 Cycle 4 Figure 7: Effect of Temperature Cycling on Complex Viscosity of Oil C 4. Conclusions Lubricants can undergo complex structural changes at low temperatures that often make it difficult to determine the cause of differences in low temperature rheological results. Standard low temperature rheological tests such as the Brookfield test can determine the low temperature properties of wind turbine oils but do not help identify the root cause of low temperature rheological issues. Oscillatory rheometry has been used to examine the viscous and elastic behavior of wind turbine oils at low temperatures. These studies have made it easier to select the proper base oil and additive components for wind turbine oils. References [1] J. Briant, J. Denis, G. Parc, “Rheological Properties of Lubricants”, Editions Technip, (1989). [2] R.M. Mortier, S.T. Orszulik, “Chemistry and Technology of Lubricants”, VCH Publishers, Inc., (1992). [3] D.N. Schulz, J.E. Glass, “Polymers of Rheology Modifiers”, ACS Symposium Series 462, (1991). [4] D.C. Venerus, E.E, Klaus, J.L. Duda, “Low Temperature Rheology of Hydrocarbon Lubricants”, SAE Paper 872048, (1987). [5] F.L. Lee, E.E. Klaus, J.L. Duda, “Measurement and Analysis of High-Shear Viscosities of Polymer Containing Lubricants”, SAE 881663, (1988). [6] S.P. Kemp, J.L. Linden, “Physical and Chemical Properties of a Typical Automatic Transmission Fluid”, SAE 902148, (1990). [7] R. Larsson, P.O. Larsson, E. Eriksson, M. Sjoberg, E. Hoglund, “Lubricant Properties for Input to Hydrodynamic and Elastohydrodynamic Lubrication Analyses”, Proc. Instn. Mech. Engrs Vol 214 Part J, (2000). [8] J.L. Duda, E.E. Klaus, S-C. Lin, “Capillary Viscometry Study of Non-Newtonian Fluids: Influence of Viscous Heating”, Ind. Eng. Chem. Res., Vol. 27 (2), (1988). [9] A.J. Moore, D. Cooper, T.M. Robinson, “Rheological Properties of Engine Crankcase and Gear Oil Components in Elastohydrodynamic Oil Films”, SAE 941977, (1994). [10] S. Bair, “The Variation of Viscosity with Temperature and Pressure for Various Real Lubricants”, Journal of Tribology Vol 123, (2001). [11] W.A Lloyd, J.M. Perez, D. Pruden, K.O. Henderson, D.L. Alexander, “Viscosity of Driveline Lubricants by a Special Mini-Rotary Viscometer Technique”, SAE 1999-01-3672, (1999). [12] J.W. Sprys, J. Doi, M. Furumoto, N. Hoshikawa, T. King, H. Hurashina, J. Linden, Y. Yasuhiro, F. Ueda, “Shear Stability of Automatic Transmission Fluids. Methods and Analysis. A Study by the International Lubricants Standardization and Approval Committee (ILSAC) ATF Subcommittee”, SAE 982673, (1998). [13] B. Kinker, “Automatic Transmission Fluid Shear Stability Testing and Viscosity Modifier Trends”, Proceedings International Symposium of Tribology of Vehicle Transmissions, (1998). [14] U.F. Schoedel, “Automatic Transmission Fluids (ATF)- The improvement of low temperature characteristics”, Lubrication Engineering 47(6), (1991). [15] J.W. Sprys, D.R. Vaught, E.L. Stephens, “Shear Viscosities of Automatic Transmission Fluids”, SAE 941885, (1994). [16] H. Shaub, M.F. Smith, Jr., C.K. Murphy, “Predicting Low Temperature Engine Oil Pumpability with the Mini-Rotary Viscometer”, SAE 790732, (1979). Figure 6: Effect of Temperature Cycling on Complex Viscosity of Oil H during all four cycles is identical so the low temperature rheology of Oil H is not affected by temperature cycling. Figure 7 shows the effect of temperature cycling (four passes from low to high to low temperature) on the complex viscosity for Oil C, which has a low Brookfield viscosity at -40 °C. The complex viscosities for Oil C during all four cycles are identical so the low temperature rheology of Oil C is not affected by temperature cycling. Therefore, unlike crankcase oils, the low temperature rheological properties of the wind turbine oils examined here are not affected by temperature cycling. Figure 6: Effect of Temperature Cycling on Complex Viscosity of Oil H 0.1 1 10 100 1000 -50 -40 -30 -20 -10 0 10 20 Complex viscosity (Pa*s) Temperature (°C) Cycle 1 Cycle 2 Cycle 3 Cycle 4 4. Conclusions Lubricants can undergo complex structural changes at low temperatures that often make it difficult to determine the cause of differences in low temperature rheological results. Standard low temperature rheological tests such as the Brookfield test can determine the low temperature properties of wind turbine oils but do not help identify the root cause of low temperature rheological issues. Oscillatory rheometry has been used to examine the viscous and elastic behavior of wind turbine oils at low temperatures. These studies have made it easier to select the proper base oil and additive components for wind turbine oils. References [1] J. Briant, J. Denis, G. Parc, “Rheological Properties of Lubricants”, Editions Technip, (1989). [2] R.M. Mortier, S.T. Orszulik, “Chemistry and Technology of Lubricants”, VCH Publishers, Inc., (1992). [3] D.N. Schulz, J.E. Glass, “Polymers of Rheology Modifiers”, ACS Symposium Series 462, (1991). [4] D.C. Venerus, E.E, Klaus, J.L. Duda, “Low Temperature Rheology of Hydrocarbon Lubricants”, SAE Paper 872048, (1987). [5] F.L. Lee, E.E. Klaus, J.L. Duda, “Measurement and Analysis of High-Shear Viscosities of Polymer Containing Lubricants”, SAE 881663, (1988). [6] S.P. Kemp, J.L. Linden, “Physical and Chemical Properties of a Typical Automatic Transmission Fluid”, SAE 902148, (1990). [7] R. Larsson, P.O. Larsson, E. Eriksson, M. Sjoberg, E. Hoglund, “Lubricant Properties for Input to Hydrodynamic and Elastohydrodynamic Lubrication Analyses”, Proc. Instn. Mech. Engrs Vol 214 Part J, (2000). [8] J.L. Duda, E.E. Klaus, S-C. Lin, “Capillary Viscometry Study of Non-Newtonian Fluids: Influence of Viscous Heating”, Ind. Eng. Chem. Res., Vol. 27 (2), (1988). [9] A.J. Moore, D. Cooper, T.M. Robinson, “Rheological Properties of Engine Crankcase and Gear Oil Components in Elastohydrodynamic Oil Films”, SAE 941977, (1994). [10] S. Bair, “The Variation of Viscosity with Temperature and Pressure for Various Real Lubricants”, Journal of Tribology Vol 123, (2001). [11] W.A Lloyd, J.M. Perez, D. Pruden, K.O. Henderson, D.L. Alexander, “Viscosity of Driveline Lubricants by a Special Mini-Rotary Viscometer Technique”, SAE 1999-01-3672, (1999). [12] J.W. Sprys, J. Doi, M. Furumoto, N. Hoshikawa, T. King, H. Hurashina, J. Linden, Y. Yasuhiro, F. Ueda, “Shear Stability of Automatic Transmission Fluids. Methods and Analysis. A Study by the International Lubricants Standardization and Approval Committee (ILSAC) ATF Subcommittee”, SAE 982673, (1998). [13] B. Kinker, “Automatic Transmission Fluid Shear Stability Testing and Viscosity Modifier Trends”, Proceedings International Symposium of Tribology of Vehicle Transmissions, (1998). [14] U.F. Schoedel, “Automatic Transmission Fluids (ATF)- The improvement of low temperature characteristics”, Lubrication Engineering 47(6), (1991). [15] J.W. Sprys, D.R. Vaught, E.L. Stephens, “Shear Viscosities of Automatic Transmission Fluids”, SAE 941885, (1994). [16] H. Shaub, M.F. Smith, Jr., C.K. Murphy, “Predicting Low Temperature Engine Oil Pumpability with the Mini-Rotary Viscometer”, SAE 790732, (1979). Figure 7: Effect of Temperature Cycling on Complex Viscosity of Oil C T+S_6_16 17.10.16 17: 01 Seite 64 Aus der Praxis für die Praxis [11] W.A Lloyd, J.M. Perez, D. Pruden, K.O. Henderson, D.L. Alexander, “Viscosity of Driveline Lubricants by a Special Mini-Rotary Viscometer Technique”, SAE 1999-01-3672, (1999). [12] J.W. Sprys, J. Doi, M. Furumoto, N. Hoshikawa, T. King, H. Hurashina, J. Linden, Y. Yasuhiro, F. Ueda, “Shear Stability of Automatic Transmission Fluids. Methods and Analysis. A Study by the International Lubricants Standardization and Approval Committee (ILSAC) ATF Subcommittee”, SAE 982673, (1998). [13] B. Kinker, “Automatic Transmission Fluid Shear Stability Testing and Viscosity Modifier Trends”, Proceedings International Symposium of Tribology of Vehicle Transmissions, (1998). [14] U.F. Schoedel, “Automatic Transmission Fluids (ATF)- The improvement of low temperature characteristics”, Lubrication Engineering 47(6), (1991). [15] J.W. Sprys, D.R. Vaught, E.L. Stephens, “Shear Viscosities of Automatic Transmission Fluids”, SAE 941885, (1994). [16] H. Shaub, M.F. Smith, Jr., C.K. Murphy, “Predicting Low Temperature Engine Oil Pumpability with the Mini-Rotary Viscometer”, SAE 790732, (1979). [17] M.F. Smith, Jr., “Better Prediction of Engine Pumpability Through a More Effective MRV Cooling Cycle”, SAE 831714, (1983). [18] G.M. Schmidt, M.T. Olsen, M.I. Michael, “Low Temperature Fluidity of Lubricating Oils Under Slow Cool Conditions”, SAE 831718, (1983). [19] K.O. Henderson, E.E. Manning, C.J. May, R.B. Rhodes, “New Mini-Rotary Viscometer Temperature Profiles That Predict Engine Oil Pumpability”, SAE 850443, (1985). [20] R.L. Stambaugh, J.H. O’Mara,“Low Temperature Flow Properties of Engine Oils”, SAE 821247, (1982). [21] T. Selby, “Further Consideration of Low-Temperature, Low Shear Rheology Related to Engine Pumpability - Information from the Scanning Brookfield Technique”, SAE 852115, (1985). [22] T. Selby, “The Use of the Scanning Brookfield Technique to Study the Critical Degree of Gelation of Lubricants at Low Temperatures”, SAE 910746, (1991). [23] T. Selby, “Problems in Bench Test Prediction of Engine Oil Performance at Low Temperature”, SAE 922287, (1992). [24] ASTM D5293, “Standard Test Method for Apparent Viscosity of Engine Oils and Base Stocks Between -10 °C and -35 °C Using Cold - Cranking Simulator”. [25] ASTM D4684, “Standard Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low Temperature”. [26] ASTM D5133, “Standard Test Method for Low Temperature, Low Shear Rate, Viscosity/ Temperature Dependence of Lubricating Oils Using a Temperature-Scanning Technique”. [27] ASTM D2983, “Standard test Method for Low-Temperature Viscosity of Lubricants Measured by Brookfield Viscometer” [28] H.H. Winter, F. Chambon, “Analysis of Linear Viscoelasticity of a Crosslinking Polymer at the Gel Point”, Journal of Rheology, 30(2), 367-382, (1986). [29] C. Schwittay, M. Mours, H.H. Winter, “Rheological Expression of Physical Gelation in Polymers”, Faraday Discussions, 101, 93-104, (1995). [30] M.A. Batko, D.W. Florkowski, M.T. Devlin, S. Li, D.W. Eggerding, W.Y. Lam, T.F. McDonnell and T-C. Jao, “Low Temperature Rheological Properties of Aged Crankcase Oils”, SAE 2000-01-2943, (2000). [31] S. Li., M.T. Devlin, G.P. Liesen, C.T. West and T-C. Jao, “Low Temperature Rheology of Engine Lubricants: Investigation of High Used Oil Pumping Viscosity”, SAE 2000-01-2944, (2000). [32] ASTM D445, “Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids”. [33] ASTM D2270, “Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100 °C”. [34] B.K. Sharma, A.J. Stipanovic, “Predicting Low Temperature Lubricant Rheology Using Nuclear Magnetic Resonance Spectroscopy and Mass Spectrometry”, Tribology Letters, Vol. 16, Nos.1-2, (2004) [35] R.M. Weber, “Low Temperature Rheology of Lubricating Mineral Oils: Effects of Cooling Rate and Wax Crystallization on Flow Properties of Base Oils”, J. Rheol., 43(4), (1999). Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 65 Nutzen Sie auch unseren Internet-Novitäten-Service: www.expertverlag.de mit unserem kompletten Verlagsprogramm, über 800 lieferbare Titel aus Wirtschaft und Technik T+S_6_16 17.10.16 17: 01 Seite 65