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Energy & Fuels 2004, 18, 952-959
Influence of Chemical Structures on Low-Temperature Rheology, Oxidative Stability, and Physical Properties of Group II and III Base Oils Brajendra K. Sharma,†,‡ Atanu Adhvaryu,†,‡ Sangrama K. Sahoo,§ Arthur J. Stipanovic,| and Sevim Z. Erhan*,‡ Department of Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USDA/NCAUR/ARS, Food and Industrial Oil Research, 1815 North University Street, Peoria, Illinois 61604, Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601, and Department of Chemistry, State University of New YorksCollege of Environmental Science and Forestry, Syracuse, New York 13210 Received September 19, 2003. Revised Manuscript Received March 18, 2004
The effect of base oil composition on lubricant properties has been studied to better understand low-temperature rheology, oxidative stability, and other physical properties. A simple approach has been adopted using low-temperature rheology to develop a multi-linear regression model to estimate Brookfield viscosity. Six base oils from API groups II and III were studied using different nuclear magnetic resonance spectroscopy methods. One method involved determination of average structural parameters from quantitative 13C NMR data, while the other method involved measurement of spin-lattice relaxation times at variable temperatures. The structural information obtained from these two methods was used to develop meaningful correlations for low-temperature, oxidative, and physical properties of these base oils. The study suggests that a deeper knowledge of chemical composition will help in selecting base oils to meet future product specifications.
1. Introduction Mineral oils used as lubricant base oil are essentially mixtures of n-paraffins along with iso-paraffins, cycloparaffins (also called naphthenes), and aromatics of various molecular weights. On the basis of recent refining trends, most of the finished oils in the market contain hydrocracked and iso-dewaxed base oils compared to traditional solvent-refined, solvent-dewaxed base oils. Even after catalytic hydrocracking and isomerization to reduce the concentration of n-paraffins from base oils that form wax crystallites at low temperatures, typical newer base oils still contain a finite concentration of n-paraffins. These base oils typically contain more than 90% paraffins (i.e., n-, iso- and cycloparaffins); therefore, their properties mostly depend on the nature and relative distribution of different paraffinic constituents. As a result, group II and III base oils (American Petroleum Institute group II designation indicates the oil has a viscosity index (VI) g 80 but e 120, while API group III designation requires a VI g 120. Both API group II and III have g90% saturates and e0.03% sulfur compounds) are now widely used in products requiring good low-temperature properties (LTPs), better oxidation stability, and improved fuel economy. * Corresponding author. Tel.: 309-681-6532. Fax: 309-681-6340. E-mail:
[email protected]. † Pennsylvania State University. ‡ USDA/NCAUR/ARS. § The University of Akron. | State University of New York.
Lubricant performance at low temperature depends on maintaining the appropriate lubricant rheology. As the temperature is lowered, the oil becomes supersaturated in n-paraffins at a specific temperature (Tc, onset temperature for microscopically visible wax crystallization); below that, paraffins in the oil nucleate into crystallites and then grow to form large wax crystals. At lower temperatures, wax crystal growth depends largely on the molecular weight distribution and concentration of paraffinic components and the composition of the oil.1,2 Crystal morphology and the steady-state flow properties of base oils are also affected by cooling rate.3 It has been described that the high activation energy region in the viscosity-temperature graph is related to gelation index and that corresponds to the crystallization temperature.1 The presence of wax crystals greatly affects the rheology of oil. In many applications, lubricants must meet lowtemperature performance standards for viscosity. Chemical additives such as pour point depressants (PPD) are required to achieve desired low-temperature performance specifications. Webber3,4 explained the effects of temperature history on low-temperature rheology. The effect of low-temperature lubricant rheology on engine (1) Webber, R. M.; George, H. F.; Covitch, M. J. Oil Flow Studies at Low Temperatures in Modern Engines, ASTM STP 1388; Shaub, H., Ed.; American Society for Testing and Materials: West Conshohocken, PA, 2000. (2) Webber, R. M. Ind. Eng. Chem. Res. 2001, 40, 195-203. (3) Webber, R. M. J. Rheol. 1999, 43, 911-931. (4) Webber, R. M. AIChE Topical Conference (International Conference on Petroleum Phase Behavior and Fouling) Preprints 1999, 138144.
10.1021/ef0340615 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/04/2004
Physical Properties of Group II and III Base Oils
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Table 1. Physicochemical Data of API Group II and III Base Oils BO-II-1
BO-II-2
BO-II-3
density,a kg/L KV40,b cSt KV100,c cSt VId flash point,e °C
0.8551 20.23 4.08 99 218
0.8468 20.29 4.2 110 193
pour point,f °C Tcg BV@-40,h cP ZSVi, Pa‚s WAT,j °C
-18 -17.9 12256 16880 -17
Low-Temperature Properties -27 -24 -21.6 -25.1 20522 6916 30245 8348 -41 -22
OT,k °C insol. dep.l % volat. lossm %
186.5 5.3 94.66
189.1 5.8 93.2
Physical Properties 0.8369 20.9 4.35 118 229
Oxidation Properties 186.1 5.5 94.47
BO-III-1
BO-III-2
BO-III-3
0.823 17.2 4 133 190
0.8295 18.68 4.2 132 230
0.8328 22.08 4.6 125 233
-21 -22.3 146382 219150 -56
-18 -27.6 5417 7284 -26
-30 -25.8 7338 8965 -15
190.2 6.7 92.83
184.6 7.1 93.91
184.4 7.2 91.97
a Density at 15 °C, kg/L, D4052. b Kinematic viscosity at 40 °C, cSt, D445. c Kinematic viscosity at 100 °C, cSt, D445. d Viscosity index, D2270. e Flash point °C, COC, D92. f Pour point °C, D97. g Crystallization temperature. h Predicted Brookfield viscosity at -40 °C (cP). i Zero shear viscosity at -40 °C (Pa.s)-Thixotropic loop steady shear experiment. j Wax appearance temperature (Cryo-DSC). k Onset temperature, °C (PDSC). l Insoluble deposit % (PSMO). m Volatile loss % (PSMO).
performance is still not well understood. At low temperature when engines are started, oil supply can be impeded by high viscosity and in some cases crystallization of n-paraffins within the oil matrix results in fluid gelation. Low-temperature viscosity can be measured using cold-cranking simulator (ASTM D5293-02), mini-rotary viscometer (ASTM D6821-02 and D689603) and Brookfield viscometer (ASTM D2983-02b) measurements. The study of chemical variability of newer hydrocracked and isomerized base oils is required to understand the variation in LTPs and oxidation performance in these base oils. A diverse group of these base oils has been selected for characterization using nuclear magnetic resonance spectroscopy (NMR), routine physical properties using standard ASTM methods, and LTPs using rheometry. In addition, the oxidation performance of these base oils has been assessed using pressure differential scanning calorimetry (PDSC) and thin film micro oxidation (TFMO). An important LTP, Brookfield viscosity at -40 °C (BV@-40) has been correlated to data obtained from various rheometer experiments. The average structural parameters (ASP) from quantitative 13C NMR experiments and dynamic information from NMR spin-lattice relaxation measurements have been used to study the relationships of base oil composition with LTPs, oxidation, and other physical properties. 2. Experimental Section The mineral base oils studied are API group II and III base oils as shown in Table 1. Kinematic viscosity at 40 °C (KV40) and 100 °C (KV100) were determined according to ASTM D 445. VI was determined by ASTM D 2270. Pour point, flash point, and density at 15 °C were determined using the ASTM D 97, D 92, and D 4052 methods, respectively. BV@-40 for a different set of 9 base oils were determined in centipoise (cP) using a very low-shear Brookfield viscometer as per ASTM D2983 with 0.25 wt % of a PPD added to each base oil. In these nine base oils, BV@-40 values range from 3000 to 42000 cP. 2.1. Rheology. The base oil samples from different sources were analyzed after the addition of 0.25% (w/w) methacrylatetype PPD. Each sample was stirred thoroughly to ensure complete mixing. The samples were pretreated by heating to 80 °C for 2 h to minimize effects of unknown low-temperature
histories. In this study, we have utilized a dynamic oscillatory rheometer RDS II (Rheometric Scientific Inc., Piscataway, NJ) with parallel-plate (12.5 mm radius) geometry to study lubricant base oils under conditions of very low deformation and very low shear rate, to fully enable wax crystallites to develop without being disrupted by mechanical forces. 2.1.1. Dynamic Mechanical Analysis (DMA). Temperature sweep runs were performed at frequency 100 rad/s in DMA mode over a temperature range 15 to -45 °C, at a constant cooling rate of 2 °C/min. Dynamic mode was used to prevent the full development of macroscopic structures. In the dynamic mode, an oscillatory strain is applied to the sample, and resulting stress is measured. Using a strain sweep experiment, the linear viscoelastic range of strain was identified. It was found that critical strain level is 10%, up to which the rheological properties are independent of strain. The shear modulus was therefore measured for a strain of 2.0%. The socalled elastic or storage modulus, G′, is a measure of the solidlike character of a complex viscoelastic fluid, while loss modulus, G′′, reflects the liquid-like or viscous behavior of a fluid. Complex modulus, G*, can be obtained from G′ and G′′ using eq 1, and actually depicts the viscoelastic behavior.
G* ) [(G′)2 + (G′′)2]1/2
(1)
2.1.2. DMA Single Point. In this measurement, when the temperature of -40 °C is reached using the same cooling rate, single-point measurements of G′, G′′, and G* were made in the DMA mode at a frequency of 100 rad/s and 2% strain. 2.1.3. Steady Shear-Thixotropic Loop. In steady shear mode a continuous rotation is used to apply a shear rate, and the resultant stress is measured as a function of shear rate. Base oil samples were typically cooled from 15 to -40 °C at a constant cooling rate of 2 °C/min. Once the target temperature was reached, the wax crystal dispersions were subjected to a shear rate ramp over the range from 0 to 500 s-1 and then back down to 0.0125 s-1 in 600 s, forming a loop. The zero shear viscosity was obtained from the value of viscosity (η) obtained approaching zero shear rate (0.0125 s-1). 2.2. NMR. All spectra were recorded on a Bruker DRX 300 NMR spectrometer operating at 300.13 MHz for 1H and 75.47 MHz for 13C using a C/H dual 5 mm probe. 50% (w/w) solutions were prepared using the 99.8% deuterated CDCl3 (from Sigma Aldrich). Cr(acac)3 (0.01 M) was used as relaxation agent to reduce the spin-lattice relaxation time. In quantitative 13C NMR experiments an inverse gated decoupling pulse sequence was used to suppress the unwanted nuclear Overhauser effect. In this case, a 13C π/2 pulse width of 5.5 µs, a sweep width of
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18.8 kHz, and relaxation delay of 5 s were employed. A total of 4000 scans were co-added for each sample. The aromatic region was not considered for analysis as group II and III base oils are rich in saturates (>99%). The gated spin-echo (GASPE) 13C NMR method was used for spectral editing. The experimental details are described elsewhere.5 After normalization, three spin-echo experiments generated the basic free induction decays to be combined and yielded three CHn subspectra using the Bouquet methodology.6 Spin-lattice relaxation time (T1) measurements were done using the inversion recovery method on neat base oils without any solvent at 27 °C. The interpulse delay used was 60 s. An 8-value variable delay list from t ) 0.01 s to t ) 20 s was used with 32 scans for each relaxation delay. The 2D data set was acquired using 8K data points. The peak maxima for desired NMR peaks were selected to calculate their peak intensities. The peak intensities obtained for different peaks were plotted against variable delay values and fitted to a simple exponential to give T1 values directly.7 T1 measurements on a representative base oil were performed at different temperatures: -33, -13, 7, and 27 °C. 2.3. DSC. All the PDSC experiments were done using TA instruments DSC 2910 thermal analyzer (TA Instruments, New Castle, USA) as described elsewhere.8,9 Sample weights of 1.5 mg were used in hermetically sealed-type aluminum pans with pinhole lids. The system was equilibrated at 35 °C and heated at a rate of 10 °C/min under a constant pressure of 425 psi of oxygen gas (dry, 99% pure) in the PDSC cell. The resulting thermogram was used to obtain an onset temperature (OT) representing the temperature when rapid increase in oxidation rate is observed. For cryogenic DSC experiments, the temperature was first programmed from 30 to -150 °C at a rate of 10 °C/min and then the heating curve for the sample was obtained with a temperature program from -150 °C to 50 °C at a rate of 10 °C/min. 2.4. TFMO. The test oil (25 mL) spread as a thin film on an activated high carbon steel catalyst surface was oxidized inside a bottomless glass reactor on a hot plate at constant temperature (175 °C) with a steady flow (20 cm3/min) of dry air passing over the sample. A constant flow of air ensured removal of volatile oxidation products as well as a steady concentration of air blanket on the oil film.9 After a specific time interval, the catalyst with the oxidized oil sample was removed from the oxidation chamber and cooled rapidly under a steady flow of dry N2 and weighed for sample loss due to thermal evaporation (volatile loss). The catalyst with the oxidized oil sample was soaked (30 min) with tetrahydrofuran (THF) to dissolve the soluble portion of oxidized oil, placed in desiccator for complete removal of any trace solvent, and then weighed to determine insoluble deposit.
3. Results and Discussion Detailed physicochemical data for six base oils are presented in Table 1. Base oils BO-II-1 and BO-II-2 belong to group II; BO-II-3, high VI group II base oil; while BO-III-1, BO-III-2 ,and BO-III-3 are group III base oils. Thus, the performance properties of these oils depend mainly on the variation in their saturate constituents rather than aromatics. (5) Sahoo, S. K.; Pandey, D. C.; Singh, I. D. International Symposium on Fuels and Lubricants, Symposium papers, 2nd, Srivastava, S. P., Ed.; Tata McGraw-Hill: New Delhi, India, 2000; pp 273-278. (6) Bouquet, M.; Bailleul, A. Fuel 1986, 65, 1240-1246. (7) Sharma, B. K.; Stipanovic, A. J. Ind. Eng. Chem. Res. 2002, 41, 4889. (8) Adhvaryu, A.; Erhan, S. Z.; Liu, Z. S.; Perez, J. M. Thermochim. Acta 2000, 364, 87-97. (9) Adhvaryu, A.; Erhan, S. Z. Biobased Industrial Fluids and Lubricants; Erhan, S. Z., Perez, J. M., Eds.; American Oil Chemical Society: Champaign, IL, 2002; pp 1-19.
Sharma et al.
Figure 1. Temperature-dependence of storage (G′) and loss (G′′) modulus for BO-II-2 and BO-II-3 base oils.
3.1. Rheology. Figure 1 displays the plot of storage and loss moduli versus temperature for BO-II-2 and BOII-3 base oils. The samples exposed to this strain history (strain ) 2%) during cooling showed loss-dominant behavior, G′ < G′′, over the entire range of temperature sweep experiments. The absence of elastic-solid response (G′) at higher temperatures (> -20 °C) implies that it requires structuring that span length scales equivalent to that of the rheological geometry. This happens only when enough crystalline wax has been precipitated to form a contiguous network structure on the rheological length scale, a sort of rheological percolation threshold.2 Webber2 found that a critical weight fraction of wax crystals for a given base oil is required to observe the elastic solid response. This critical weight fraction depends on the nucleation and growth processes and, therefore, n-paraffin composition and cooling rate as well as the morphology of the crystallites. As the temperature is lowered in temperature sweep experiments, two base oils BO-II-2 and BO-II-3 behave in a similar fashion (same G′ and G′′ values) down to -20 °C; but below -20 °C, there is rapid increase in G′ and G′′ of BO-II-2, while in BO-II-3 there is relatively slow increase in G′ and G′′ (Figure 1). Similar trends were obtained for base oils BO-II-1, BO-III-2, and BOIII-3, as shown in Figure A in Supporting Information. This difference in BO-II-2 and BO-II-3 samples can be ascribed to the volume fraction of crystallized wax, crystallite shape morphology, the number density, and size distribution of crystallites.1 The transition temperature at which G′ and G′′ began to increase rapidly with decreasing temperature is the crystallization temperature (Tc). Tc is the temperature at which crystals become microscopically visible within the sample; it also corresponds to the temperature at which the activation energy increases significantly.1 A lower Tc (approximately -25.1 °C) as in BO-II-3 increases the supersaturation for a given concentration and molecular weight distribution of n-paraffinic components and thus increases the nucleation rate compared to BO-II-2 (approximately -21.6 °C).2 At temperatures lower than this Tc but greater than the glass transition temperature (approximately -80 °C for mineral base oils) of the oil, the oil approaches a thermodynamically steady state.4 Temperature decrease produces an increase of both the viscous (G′′) and elastic (G′) component, but
Physical Properties of Group II and III Base Oils
Energy & Fuels, Vol. 18, No. 4, 2004 955
Table 2. Average Structural Parameters Derived from Inverse Gated 13C NMR Spectrum and GASPE-Edited Spectrum samples BO-II-1 BO-II-2 BO-II-3 BO-III-1 BO-III-2 BO-III-3 Cara Csatb Cpc CNd Cnpe Cipf CN,CH3g CN,othersh CHi CH2j CH3k t-CH3l b-CH3m ACLn
0.2 99.8 69.0 30.8 25.1 43.9 4.9 25.9 19 64 17 7.6 9.4 18.86
0.4 99.6 62.1 37.5 19.8 42.2 4.7 32.9 20 63 17 6.9 10.1 20.09
0.6 99.4 78.3 21.1 30.8 47.5 3.5 17.7 15 67 18 8.8 9.2 18.55
0.4 99.6 86.8 12.9 39.4 47.4 2.1 10.8 12 70 18 9.6 8.4 19.02
0.1 99.9 82.5 16.5 35.0 47.5 3.2 13.4 14 68 18 8.9 9.1 18.22
0.4 99.6 84.6 15.0 34.3 50.3 2.5 12.6 13 69 18 9.3 8.7 18.63
a Aromatic carbons. b Saturate carbons. c Paraffinic carbons. Naphthentic carbons. e n-Paraffinic carbons. f Iso-paraffinic carbons. g Methyl-substituted naphthenic carbons. h Other (except methyl) naphthenic carbons. i CH carbons from GASPE. j CH2 carbons from GASPE. k Methyl carbons from GASPE. l Terminal methyl carbons. m Branched methyl carbons. n Average chain length. d
the variation of viscous component is higher (Figure 1). As a consequence, at lower temperatures the sample will retain a higher viscous component than elastic component. For example, at lower temperatures BO-II-2 retains a higher viscous component compared to BOII-3. The high viscosity of BO-II-2 at lower temperature (-40 °C) (compared to BO-II-3) is due to its more sollike and less structured nature at lower temperatures. The compositional difference between these oils, including molecular weight distribution of n-paraffins, affect the development of supersaturation, wax crystal nucleation, and crystal growth processes in oils, so each oil has undoubtedly unique rheology caused by the nparaffins composing the wax. One would expect that BO-II-3 having high n-paraffinic carbons (Cnp, Table 2) should have high Tc compared to BO-II-2. However, in an earlier study,1 the differences in Tc of oils were explained on the basis of their n-paraffin molecular weight distribution and not as a function of the amount of n-paraffins. An index of the n-paraffin molecular weight distribution in base oil can be obtained by the average chain length (NMR parameter). A low Tc value in BO-II-3 thus can be explained by a low ACL (18.55) compared to BO-II-2 with high ACL (20.09) resulting in high Tc. This trend follows with other base oils, as shown in Table 1. The nucleation transition and Tc are nominally visible at concentrations of PPD used in these experiments. The presence of PPD surely helps to reduce total increase in viscosity in the nucleation region at low temperatures. The values of G′, G′′, and G* at -40 °C obtained in the DMA temperature sweep and single-point experiments for a set of 9 base oils were used in developing a correlation for estimating BV@-40. The repeatability of a DMA single-point measurement is better than the DMA temperature sweep experiment. The coefficient of variation of G′, G′′, and G* in DMA single-point measurements is 0.98) using NMR-derived ASP as well as T1 times. However, oxidation stability tests such as PDSC and TFMO could be predicted only to an accuracy level of 88 and 82%, respectively. The physical properties such as density, viscosity, VI, and flash point can be predicted well from both NMR approaches, i.e., ASP (R2 > 0.96) and T1 times (R2 ) 0.79-0.96). It is possible that performance properties not evaluated in this study can also be predicted from parameters obtained using NMR methods. Acknowledgment. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. Supporting Information Available: Figure A: Temperature-dependence of loss (G′′) modulus for different base oils; Figure B: Viscosity change with shear rate in steady shear experiment at -40 °C; and Figure C: GASPE subspectra to estimate individual CHn (n ) 0-3) groups. This material is available free of charge via the Internet at http://pubs.acs.org. EF0340615
