Energy & Fuels 1999, 13, 493-498
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Application of Quantitative NMR Spectroscopy to Oxidation Kinetics of Base Oils Using a Pressurized Differential Scanning Calorimetry Technique A. Adhvaryu* and J. M. Perez Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
I. D. Singh Indian Institute of Petroleum, Dehradun, 248 005 India Received August 20, 1998. Revised Manuscript Received December 9, 1998
This study was primarily conducted to explore the effect of molecular composition of a base oil matrix on the oxidation kinetics and the variation of different thermodynamic parameters during oxidation. Pressurized differential scanning calorimetry was used because the method is fairly straightforward and highly repeatable. NMR-derived structural parameters and chromatographic and mass spectral data were able to explain the variation in the results obtained. Kinetic parameters of various systems were derived based on peak temperature (K) and program rate (β) relationship. This methodology allowed calculation of the activation energy, rate constant, and half-life period of the various test samples under study. Variation in the results of the onset temperature (OT) and start temperature (ST) for different base oils are explained in terms of their molecular composition obtained from quantitative FT NMR studies.
Introduction In the piston-ring zone the degradation of lubricating oil is mainly due to high-temperature oxidation and thermal decomposition, resulting in the formation of oxygenated compounds that polymerize on prolonged oxidation to form sludge particles. The quality of the base stocks plays a key role in the ultimate formulation of a premium-grade lubricant. Several factors responsible for the oxidation stability of lube base oil were studied separately1 and found to be dependent on their hydrocarbon-type composition, namely, saturates, diand tricyclic aromatics, total aromatics, sulfur and basic nitrogen content. Versatile thermoanalytical techniques such as differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and derivative thermogravimetry (DTG) have been used to study the oxidation stability of base oils2-4 and other petroleum products.5,6 Presently thermoanalytical techniques such as TGA and DTG are being extensively used to characterize narrow and wide distillation range base stocks in terms of onset and maximum oxidation temperature. Zhang et al.7 have reported the use of pressure differential scanning calorimetry (PDSC) for determining the piston deposit(1) Bhatnagar, A. K. in Proceedings of International Symposium on Production and Application of Lube Base Stocks; IIP, Dehradun; Singh, H., Rao, T. S. R., Eds.; Tata McGraw Hill Publication: New York, 1994. (2) Naga, A. E.; Salem, A. E. M. Lubr. Eng. 1985, 41, 470. (3) Naga, A. E.; Salem, A. E. M. Lubr. Eng. 1988, 44, 931. (4) Naga, A. E.; Salem, A. E. M. Lubr. Eng. 1986, 52, 210. (5) Bartlett, G. W.; Subero, D. J. Therm. Anal. 1987, 32, 1843. (6) Jain, M. C.; Kumar, D.; Rao, A. M.; Jain, S. K.; Srivastava, S. P. In IX National Symposium on Thermal Analysis, Goa, 1993.
forming tendency. TGA and DTG have also been found useful in determining the soot content in used lubricating oils.8 Considerable information on deposit and sludge formation has evolved over the years, as a variety of hyphenated analytical techniques have been employed to understand the mechanism of deposit formation. Seigel et al.9 studied valve deposits using a dualgas thermogravimetric analysis (TGA) and hyphenated infrared-gel permeation chromatography (GPC). Later, Ebert et al.10 used various microanalytical methods such as TGA and micro-FTIR to evaluate piston and cylinder head deposits. One major problem limiting an in-depth understanding of the role of oxidation in the base oil system is that most of the recognized oxidation tests are severely diffusion limited. DSC is a microsample technique that has wide application to study thermal and oxidative properties of materials including complex hydrocarbon mixtures.11,12 The static thin film of base oil in the DSC apparatus offers a constant surface area and a simple geometry which allows the separation of reaction kinetics and diffusional effects. Hence, this technique has also found application in basic studies of reaction kinetics. It has also been used regularly for quality control (7) Zhang, Y.; Perez, J. M.; Pei, P.; Hsu, S. M. Lubr. Eng. 1991, 48, 221. (8) Jain, M. C.; Kumar, D.; Basu, B.; Jain, S. K.; Srivastava, S. P. IOC R&D, Report 91181, 1991. (9) Seigl, W. O.; Zinbo, M. In The Chemistry of Engine Combustion Deposits; Ebert, L. B., Ed.; Plenum Press: New York, 1985. (10) Ebert, L. B. In Chemistry of IC Engine Deposits; Ebert, L. B., Ed.; Plenum Press: New York, 1985. (11) Wesoloski, M. Thermochim. Acta 1981, 46, 21. (12) Noel, F. Thermochim. Acta 1972, 4, 377.
10.1021/ef980176a CCC: $18.00 © 1999 American Chemical Society Published on Web 01/16/1999
494 Energy & Fuels, Vol. 13, No. 2, 1999 Table 1. Test Fluids Used in the Kinetic Study
Adhvaryu et al. Table 2. DSC Conditions Used for Base Oil Samples
fluid
description
conditions
values
BO-10 BO-10A BO-40 BO-Syn
base oil BO-10 + additive (ZDDP) base oil (with different molecular composition) synthetic blend (77 wt % saturates and 23 wt % aromatics)
sample size calorimetric sensitivity starting temp program rate temp sensitivity
0.5 mg (nominally) 5 mV/cm (0.5 mcal s-1/in.) 35 °C 5, 10, and 20 °C/min 0.2 mV/cm (10 °C/in.)
evaluations. Noel and Cranton13,14 were among the first researchers to recognize the versatility of DSC and PDSC for characterizing lubricating oils. They observed that the induction time increased with increasing inhibitor concentration, the effect of metal surfaces in lowering the threshold temperature, and variations of the threshold and induction temperature with different chemical additives. Later in a separate publication, Walker and Tsang15 reported the effect of sample size, oxygen pressure, and temperature on the induction time of base oils. It was also observed that the presence of olefinic hydrocarbons in the base matrix drastically reduced the induction time of oxidation. Thus, it is reasonable to assume that olefinic compounds when generated during oxidative decomposition of lube oil are reactive intermediates having a low but steady state concentration. This paper deals with the application of PDSC under programmed mode to study the thermo-oxidative behavior of neat, synthetic, and antioxidant additive-doped base oil. The kinetics of base oil oxidation has been studied using different heating program rates (β). The variation in the results is explained in terms of chromatographic, 13C NMR,16 and mass spectral data17 of base oils. The peak temperature and program rate (β) relationship was used to compute various kinetic parameters on the test samples. Structural information derived from 13C NMR studies was also used to explain variation in the results of onset temperature and start temperature of different base oils. Experimental Section All the experiments were carried out using a DuPont 9900 thermal analyzer. The DSC was first temperature calibrated using the melting point of indium (156.6 °C) at a 10 °C /min program rate. The indium sample was then programmed at other rates to be used; the difference between the melting point obtained and the theoretical value is the correction to be used. The test samples, Table 1, included neat base oils (BO-10, BO40), additive-doped base oil (BO-10A; 1.6 g of ZDDP in 100 g of total blend; 0.13% Zn in total oil formulation), and synthetic blend BO-Syn (mixture of 77 wt % saturates plus 23 wt % aromatics, obtained through repeated column chromatographic separation of BO-10). Approximately 0.5 mg of the oil sample was injected in a hermetically sealed aluminum sample pan, resulting in a film thickness of less than 1 mm in height. A pinhole was made in the lid of the sample cover for interaction of sample with reactant gas:oxygen. Table 2 presents the DSC conditions used for the present study. For kinetic measurements, the system was equilibrated to 35 °C and heated at three program rates (β) of 5, 15, and 20 °C/min. The oxygen (13) Noel, F. J. Inst. Pet. 1971, 57, 354. (14) Noel, F.; Cranton, G. E. In Analytical Calorimetry; Porter, Johnson, Eds.; Plenum: New York, 1974; Vol. 3, p 305. (15) Walker, J. A.; Tsang, W. SAE Tech. 1980, 801383. (16) Misbah, U. H.; Ahmad, B.; Mohammad, F. A. Fuel 1985, 64, 839. (17) ASTM D3239, Annual Book of ASTM Standards, Vol. 05.03, 1991.
flow rate was maintained at 100 mL/min at 100 psi pressure for each experiment. This is necessary to continuously remove the reaction product and ensure maximum contact between the sample and oxygen to get repeatable and reproducible results. The peak temperature (T, K) corresponding to each program rate was determined from the exotherm, and the inverse of T was plotted against log program rate (β). Using linear regression and subsequent computation of the data, various thermodynamic parameters were obtained for BO-10, BO-40, BO-Syn, and BO-10A. The NMR spectra of the base oil samples were recorded quantitatively in the Fourier transform (FT) mode on a JEOL FX 100 pulse spectrometer. All the spectra were obtained at an observing frequency of 99.5 MHz for 1H and 25.0 MHz for 13C nuclei. Deuterated chloroform (99.8% D) having 1% tetramethylsilane (TMS) was used as the solvent for making a 10-15% w/v solution for 1H and a 30-35% w/v solution for 13C NMR experiments, respectively. To get quantitative information, 13C NMR spectra were recorded by adding a relaxation agent [Cr(acac)3] in 0.1 M concentration, in the inverse-gated condition. A relaxation delay (D1) of 10 and 4 s was used for 1H and 13C NMR measurements, respectively. Typically, 16 scans for 1H and 3000 scans for 13C experiments were found to be optimum for enhancing the signal-to-noise ratio for obtaining quantitative spectra. Structural parameters on these oils were computed16,18 from the data obtained from 1H and 13C NMR measurements. Mass spectral data17 were obtained from a fast-atom bombardment mass spectrometer (FAB-MS) operating at 70 eV.
Results and Discussion Kinetic Studies. Hydrocarbon oxidation is the main chemical reaction resulting in the formation of high molecular weight oxidation products and ultimately an insoluble sludge. The PDSC thermal analysis technique is useful in obtaining kinetic parameters on the oxidative degradation of base oils. This method is often straightforward, and kinetic parameters can be derived based on the peak temperature and program rate (β) relationship. This method allows for calculation of the activation energy (Ea), Arrhenius frequency factor (Z), the rate constant (k), and half-life (t1/2) at various temperatures of interest. The suitability of this method is due to its linear program rates, high degree of base line stability, and most important of all, the inherent ability to directly measure the sample temperature by virtue of a thermocouple located in close proximity to the sample. DSC measures the differential heat (∆q) between the base oil sample and the reference, which is displayed on the Y-axis of the recorder as a direct function of sample temperature. To illustrate this technique, a set of four test fluids was selected for the present study. BO-10 and BO-40 are two base oils widely varying in their molecular composition. Their detailed analysis of hydrocarbon types, namely, saturates, aromatics, and polars as well as typical important physiochemical characteristics, are (18) Adhvaryu, A. Ph.D. Thesis, University of Roorkee, India, 1997.
Oxidation Kinetics of Base Oils
Energy & Fuels, Vol. 13, No. 2, 1999 495
Table 3. Chromatographic Analysis and Physiochemical Characteristics of Base Oils parameters
BO-10
BO-40
saturates (wt %)a aromatics (wt %)a polars (wt %)a aniline point, °Cb sulfur (wt %)b nitrogen (ppm)b mol weight (VPO)b
71.2 27.4 1.4 93.8 1.0 331 360
49.5 48.7 1.8 119.2 1.3 375 735
Z)
a Column Chromatographic data (ASTM D 2549). b Physiochemical data.
Table 4. NMR-Derived Structural Parameters and Mass Spectral Analysis of the Base Oils parameters/structures
BO-10
BO-40
%Cnp %Cipa %Cna %Carb %Car,alkb %Cnb ACLb condensed 3 ring cycloparaffins (wt %)c naphthenobenzenes (wt %)c dinaphthenobenzenes (wt %)c acenaphthenes/dibenzofurans (wt %)c fluorenes (wt %)c phenanthrenes (wt %)c naphthenontherenes (wt %)c
45.9 29.7 19.1 22.7 4.0 19.8 7.9
43.9 21.1 30.7 12.1 5.0 31.5 11.7
15.2 4.1 3.6 2.5 1.5 0.3 0.7
32.6 5.4 4.1 3.9 3.2 1.4 1.3
a
a NMR data on base oils. b NMR data on aromatic concentrate of base oils. c Mass spectroscopic data ACL ) average chain length.
presented in Table 3. The 1H and 13C NMR derived parameters of these base oils and their aromatic concentrates are presented in Table 4 along with their mass spectroscopic data. For comparison, BO-Syn, a synthetic blend of 77% saturates and 23% aromatics in weight proportion, did not contain any polar compounds (sulfur- and nitrogencontaining heterocyclics). These were removed by column chromatography (ASTM D 2549). BO-10A is an antioxidant (ZDDP) additive-doped base oil, BO-10. A series of three program rates (5, 10, and 20 °C/min) were used, and the corresponding peak temperatures were recorded for each sample. Figure 1a shows the plot of log program rate (β) versus the inverse peak temperature of the base oil sample BO-10. Linearity of this plot indicates that the reaction is of first order. The slope of the line δ log β/δ(1/T) is generated by linear regression using an IBM PC. This model assumes a constant thin film thickness, and Arrhenius-type equations can be applied to such systems. Thermal evaporative loss is reduced significantly in the PDSC module because the average oxygen concentration on the film increases as the film thickness decreases. The plot in Figure 1a shows a high value of the determination coefficient (R2) for the system. Similar results were obtained for each of the samples studied. Using the slope of the line thus generated, the individual activation energy (Ea) for each system is calculated from the following equation
δ log β Ea ) 2.19R δ(1/T)
ments. From the value of Ea obtained in eq 1, computation of the preexponential factor (Z) is possible using eq 2 Calculation of Z from the above equation is
(1)
where R is the gas constant (1.987 cal/mol K) and β is the program rate (°C/min) used in the different experi-
βEaeE/RT RT2
(2)
primarily based on the fact that the reaction is first order in such systems. The subsequent calculations for the rate constant (k) and half-life (t1/2) can be obtained using the standard Arrhenius and half-life equations. Analysis of Kinetic and Thermodynamic Results in Terms of Composition and NMR-Derived Parameters. A critical analysis of the data in Table 5 at a particular program rate (β ) 10 °C/min) show that BO-10 and BO-40 have a significant difference in their kinetic and thermodynamic parameters. BO-10 has a high activation energy (Ea ) 83.9 kcal/mol) compared to BO-40 (Ea ) 13.0 kcal/mol), calculated from the slope of δ log β/δ(1/T) (Figure 1b). Column chromatographic data (Table 3) show that BO-10 has a high saturate (71.2%) and low aromatic (27.4%) content as compared to the low saturate (49.5%) and high aromatic content (48.7%) in BO-40. The high thermo-oxidative stability of saturates is well documented in the literature.19 A separate study using HPLC for ring-type distribution of BO-10 and BO-40 has shown that BO-10 contains predominantly mono- and diaromatics, while BO-40 contains mainly tri- and polyaromatic structures.18 The molecular weight determination using vapor pressure osmometry (VPO) shows BO-40 has an average molecular weight twice that of BO-10 and, therefore, contains larger ring systems in their matrix. As a result of this, the complexity of the ring structures increases in BO40, making these compounds more susceptible to thermal and oxidative degradation. The nomenclature of various NMR-derived structural parameters used in this study is presented in Table 6. The carbon-type measurement from 13C NMR experiments shows that the percent normal paraffin (%Cnp ) 45.9) and isoparaffin (%Cip ) 29.7) in BO-10 is much higher than the corresponding value in BO-40 (Table 4). It is observed that n- and isoparaffinic hydrocarbons degrade very little during the inhibited stage and then oxidize at similar and slow rates during the breakdown stage. The general features of this profile have been observed in a wide range of laboratory and engine tests in which oxidation is important. Thus, increasing the saturate content of a base stock will increase the duration of the oxidation-inhibited stage. Moreover, BO-40 also has a higher nitrogen content (375 ppm) (Table 3). Basic nitrogen compounds, which are present in the base oil as nitrogen heterocyclics, are known to accelerate oil oxidation and reduce the oxidation life of formulated products.20 Earlier studies using NMR-derived parameters19 have shown that most of these nitrogen-containing heterocyclics are mainly attached to polyaromaticand naphthenoaromatic-type structures present in the base oil matrix. Table 4 also presents the 13C NMR data on the aromatic concentrate of BO-40. The results show that (19) Murray, D. W.; Clarke, C. T.; MacAlpine, G. A.; Wright, P. G. SAE Spec. Publ. 1982, 15, 821236. (20) Joshida, T.; Igarashi, J.; Watanabe, H.; Stipanovic, A. J.; Thiel, C. Y.; Firmstone, G. P. SAE Tech. 1998, 981405.
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Figure 1. Plot of log β verses peak temperature for (a) base oil BO-10 and (b) base oil BO-40. Table 5. Kinetic and Thermodynamic Parameters of Base Oils Obtained from PDSC Studies at Program Rate (β) ) 10 °C/min sample ID
peak height temp (K)
1/T (×103)
Ea (Kcal/mol)
E/RT
ln Z
k (min-1)
t1/2
BO-10 BO-40 BO-Syn BO-10A
598.20 608.20 599.30 586.50
1.67 1.64 1.67 1.70
83.90 13.00 23.20 51.50
70.60 10.80 19.40 44.07
70.76 9.07 18.32 43.78
11.7 × 10-1 0.18 2.13 × 10-6 0.75
0.59 min-1 3.91 min-1 5425.8 h-1 0.88 min-1
Table 6. Nomenclature of NMR-Derived Structural Parameters parameters
nomenclature
%Car %Car,alk %Cnp %Cip %Cn ACL
percent aromatic carbons percent alkyl substituted (not methyl) aromatic carbons percent normal paraffinic carbons percent isoparaffinic carbons percent naphthenic carbons average chain length of the aliphatic hydrocarbons present in the system (both in saturates and attached to aromatic and naphthenic rings)
BO-40 has a higher percentage of naphthenic carbons (%Cn) attached to aromatics than BO-10. Further, the average chain length (ACL) and Car,alk (area integral in the spectral range 137-160 ppm)21 indicate high aromatic substitution by long-chain alkyl groups in BO40. The presence of such long alkyl substitution on polyaromatic and naphthenoaromatic structures in BO40 makes it more susceptible to thermo-oxidative degradation. The above conclusion is further corroborated from the mass spectroscopic data on the two base oils BO-10 and BO-40 (Table 4). Base oil BO-40 has a higher weight percent of condensed three-ring cycloparaffins, naphthenobenzenes, fluorenes, phenanthrenes, and naphthenontherenes than base oil BO-10. The presence of such complex polyaromatic structures and associated large alkyl substitution on them (observed from NMR data) makes BO-40 thermally more unstable than BO10. Thus, it can be concluded from the foregoing discussion that the difference in the basic hydrocarbon composition of BO-10 and BO-40 accounts for the variation in their activation energy during oxidation (Table 5). BO-Syn (with no polars) has a very low rate of reaction (2.13 × 10-6 min-1) and an unusually long halflife period (Table 5). Polar compounds present in the (21) Stephane, G.; Patrice, R.; Jean, J. D.; Jean, C. E.; Patrice, V. Fuel 1981, 60, 226.
base oils are mainly oxygen-, nitrogen-, and sulfurcontaining heterocyclics. Though some sulfur compounds are believed to have antioxidant character, polar compounds in general (mainly basic nitrogenous compounds) increase the rate of oxidation. In a separate study on jet fuels, it is reported that the removal of sulfur, nitrogen, and unsaturates through extensive hydrotreating increased the oxidative stability of these fuels.22 The presence of such compounds will make the system more susceptible to oxidation. These facts support the low oxidation rate constant for BO-Syn. The different kinetic parameters computed on antioxidant additive-doped BO-10 is shown in Table 5 as BO-10A. It is noticed that the rate of the oxidation reaction has significantly lowered to k ) 0.75 min-1, compared to k ) 1.17 min-1 observed in BO-10. This indicates that the rate of oxidative degradation in BO-10 is reduced due to the presence of antioxidant additive (ZDDP). The ZDDP additive is known to function both as a radical scavenger and a peroxide decomposer during base oil oxidation.23-26 The removal of ROO• species (22) Cummings, A. L.; Pei, P.; Hsu, S. M. ASTM Spec. Publ. 1984, 809. (23) Howard, J. A.; Ohkatsu, Y.; Chenier, J. H. B.; Ingold, K. U. Can. J. Chem. 1973, 51, 1543. (24) Howard, J. A. In Frontiers of Free Radical Chemistry; Academic Press: New York, 1980; p 237.
Oxidation Kinetics of Base Oils Table 7.
Energy & Fuels, Vol. 13, No. 2, 1999 497
13C
NMR-Derived Average Structural Parameters of Base Oilsa
parameters BO-10 BO-20 BO-30 BO-40 BO-50 BO-60 BO-70 % Car % Car,alk % Csat % Cnp % Cn % Cip
5.3 1.6 94.7 45.9 19.1 29.7
5.6 2.2 94.4 42.2 2.4 29.7
4.0 1.5 96.0 45.3 22.9 27.9
4.3 1.8 95.7 43.9 30.7 21.1
3.3 0.7 96.7 44.1 23.0 29.6
4.6 1.5 95.4 43.5 26.9 25.0
2.5 0.6 97.4 40.1 29.3 28.0
a Error/precision of 13C NMR-derived structural parameters are within the precision limit of IP 392 method.
Figure 3. Variation of PDSC onset temperature with %Csat.
Figure 2. Variation of PDSC onset temperature with %Car.
from the system arrest the chain propagation step (necessary for further degradation of hydrocarbons) and subsequently reduce the rate of reaction. This is also evident from the half-life calculated for this system (t1/2 ) 0.88 min-1), which increased from 0.59 min-1 observed in the case of BO-10. However, failure to follow the theoretical relation between Ea and k could possibly be due to large changes in the entropy of complex molecular structures of base oils during oxidation. The speed of a particular reaction step is determined by its activation energy requirement and is the result of competition among the several individual kinetic rate constants observed in the oxidation reaction. This observation also clarifies that it is not possible to predict a specific reaction sequence for a particular chemical structure in this mixture at any one time. The final product distribution of the starting mixture is governed by the thermodynamics of the system and the probability distribution. This was verified by studying a series of base oils with various chemical compositions (Table 7). Analysis of Results from Programmed Temperature PDSC Studies. The PDSC oxidation onset temperature relates to the resistance of base oil to autooxidation. A higher OT means greater oxidation resistance for that matrix. It is observed that as the amount of aromatic carbon (Car) increases, the onset temperature decreases gradually (Figure 2). An opposing trend was observed with Csat (Figure 3). Separate benchtop oxidation studies18 on lube base oils revealed that aromatic hydrocarbons, particularly naphthenoaromatics and polynuclear aromatics, have quite low oxidation
stability. Any unscavenged radical generated during oxidation will tend to preferentially attack aromatics rather than saturated hydrocarbons because of resonance stabilization of the resultant aromatic radical. The greater slope of the variation of onset temperature with Car,alk (Figure 4) compared to that of Car indicates that the presence and abundance of alkyl substitution on aromatic rings significantly affects oxidation. It is clear, therefore, that increasing the saturate carbon content in the base oils should result in an extension of the oxidation inhibition stage. This explains the opposing trend followed by the onset temperature with Car and Csat. The base oil carbon type also has a significant influence on the oxidation start temperature obtained from PDSC exotherm. Normal paraffins are highly stable toward oxidation; ST increases linearly with its carbon content in the base oil (Figure 5). The low reactivity of paraffin hydrocarbon is due to the higher activation energy required for radical formation through C-C chain scission. Hoo et al.27 have also reported that n-paraffins undergo comparatively few reactions and only under very vigorous conditions. Thus, only an extremely reactive particle, a free radical, can attack a n-paraffin molecule. The temperature at which such a molecule would oxidize will be high. It was also shown
(25) Bridgewater, A. J.; Dever, J. R.; Sexton, M. D. J. Chem. Soc., Perkin II 1980, 1006. (26) Kennerly, G. W.; Patterson, W. L. Ind. Eng. Chem. 1956, 48, 1917.
(27) Hoo, G. H.; Lewis, E. In Proceedings of International Symposium on Production and Application of Lube Base Stocks; IIP, Dehradun; Singh, H., Rao, T. S. R., Eds.; Tata McGraw-Hill Publication: New York, 1994.
Figure 4. Variation of PDSC onset temperature with %Car,alk.
498 Energy & Fuels, Vol. 13, No. 2, 1999
Adhvaryu et al.
Figure 5. Variation of PDSC start temperature with %Cnp.
Figure 7. Variation of PDSC start temperature with %Car,alk.
Figure 6. Variation of PDSC start temperature with %Csat.
Figure 8. Variation of PDSC start temperature with %Car.
through kinetic studies that an increase in the saturate content increases the activation energy (Ea), resulting in the increase in ST for oxidation (Figure 6). Further, alkyl substitution on the benzene ring (having long chain lengths) makes aromatic structures highly susceptible to oxidative degradation, resulting in a lowering of ST (Figure 7). Similarly, an increase in the aromatic carbon of base oil (as polyaromatic structures) results in lowering the oxidative stability of the system. This is reflected in the lowering of PDSC ST (Figure 8). It may thus be concluded from the foregoing discussion that lesser the stability of a hydrocarbon or carbon type, the lower the ST of oxidation and vice versa.
saturate content (mainly n-paraffin-type hydrocarbons) increases the activation energy of oxidation, while polar compounds affect the rate of reaction. NMR-derived data on carbon-type distribution can explain the variation in thermodynamic parameters as well as the onset and start temperatures of different base oils. Normal paraffin structures are usually stable toward oxidation, and the start temperature increases linearly with its carbon content in the base oil. Finally, the presence and abundance of aromatic rings having long alkyl substitution affect base oil oxidation by decreasing the onset as well as the start temperature.
Conclusions Chromatographic and spectroscopic techniques are reliable tools to explain the variation in the oxidation kinetics of different base oils. The oxidation usually follows first-order kinetics, and an Arrhenius equation can be applied to such systems to calculate various thermodynamic parameters. The study shows that
Acknowledgment. The authors thank Mr. D.C. Pandey for his help in running PDSC. A.A. thanks C.S.I.R., New Delhi, for a research fellowship during the course of this work. EF980176A
