1 Introduction Today’s dependence on the internal combustion engines for daily transportation, generation of electricity and keeping the wheels of industrial commerce moving forward is an important factor in modern society. It is difficult to conceive civilized life today without the internal combustion engine and the everyday conveniences it provides. The demands placed on modern lubricants to provide quality performance under more stressful operating conditions over longer periods of time are increasing. The power output of modern, smaller, automotive engines has understandably increased engine operating stresses. These stresses - particularly when combined with longer engine oil drain intervals - produce faster and greater oxidation of this critically important lubricant. Modern steam and gas turbines used in electrical power generation expose the turbine to serious and costly lubricant malfunction. Higher temperatures are encountered in bearings, small reservoirs reduce residence times, and issues with varnish, deposit, and other failures have long been a major concern [1]. In the technical society that is closely in contact with reciprocating engines, it is widely understood that oxidation is perhaps the most challenging aspect of engine oils [2]. Understandably, inhibiting oxidation or at least controlling the rate of oxidation of engine oils has been a matter of serious effort for several decades following its application to automotive transportation. As another example of the negative effects of lubricant oxidation, the reliable operation of a power generating turbine and its associated equipment is dependent on the quality of the lubricant in use. To put this in perspective, according to a 2005 study by General Electric [3], malfunctioning turbines contribute on average to 20 percent of power plant all forced outages. Of this 20 percent, GE noted that 19 percent - that is, 95 % - of the turbine generator problems were associated with the lubricating oil system. In previous work [4, 5], the authors presented the interesting differences in the effects of three different oxida- Aus der Praxis für die Praxis 47 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 An in situ Sampling Study of the Chemistry of Oxidation of an Antioxidant-Treated Base Oil in an Isothermal Bench Test J.C. Evans, T.W. Selby, M. Manning, W. VanBergen* Engine oils, electric power industry’s turbine oils and many other lubricants serving civilization’s basic needs are vulnerable to oxidation. Thus, prevention of oxidation of these lubricants during use is of high importance and has challenged technical efforts for decades. Bench tests have been very useful and their development has been a serious technical commitment. Recently, a new approach in such testing has been developed using a specially designed isothermal reactor that permits in situ extraction of small samples for chemical and physical analysis during the oxidation test. This paper will present initial results of such studies using this isothermal instrument and in situ extractions. Keywords Base oil, oxidation, oxidation resistance, antioxidant, D2272, Isothermal reactor, Extraction, RPVOT Abstract * Jonathan C. Evans, Ph.D. Savant Group; Midland, Michigan, USA Theodore W. Selby, BS, MS Savant Group; Midland, Michigan, USA Marta Manning, Ph.D. Savant Laboratories; Midland, Michigan, USA William VanBergen, AS Chemistry Savant Laboratories; Midland, Michigan, USA Abbreviations: API: American Petroleum Institute ASTM: International American Society for Testing Materials EOR: End of Rapid Reaction FTIR: Fourier Transform Infrared; IR: infrared GE: General Electric kPa: kilo Pascals PTV: Percent Transmission Value psi: pounds per square inch RPVOT: Rotating Pressure Vessel Oxidation Test SAE: Society of Automotive Engineers T+S_2_2018_.qxp_T+S_2018 20.03.18 10: 36 Seite 47 an additional 5 mL of distilled water are put into the glass test beaker. The copper-wire catalytic coil used in the method is inserted into the test beaker as shown in Figure 2. The normal additional 5 mL of water required by the method is added to the metal cup in which the test beaker is placed for rotation during test. Rotation of the sample is begun at room temperature and the chamber is pressurized to 90 psi (620 kPa) with pure oxygen. At this point, heating is begun and the operating temperature is controlled to 150 ± 1 °C. At this temperature, the pressure will rapidly rise (in about 30 minutes) to a maximum of 190 psi or slightly more. This is shown in Figure 3. At some point of time after heat is applied to the pressure-chamber and its sample, the oxygen in the pressure chamber begins to attack the test sample and/ or any oxidation inhibitor it contains. For some test oils, the pressure remains fairly constant until the antioxidant is nearly exhausted as in Figure 3. However, as will be shown, for most modern lubricants the pressure begins to slowly drop shortly after the sample reaches maximum pressure. Aus der Praxis für die Praxis 48 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 tion inhibitors at different concentration on API Group I, Group II, Group III and Group IV base stocks. The effort was extended by examination of the role of water in the oxidation of API Group I-IV base oils [6]. The Quantum ® isothermal instrument shown in Figure 1, has been presented in previous work exploring differences in performance of three different oxidation inhibitors [4, 5]. This was done at different concentration on a range of American Petroleum Institute (API) Group I through Group IV base stocks. The study was interestingly extended by examining the inclusion of a small amount of water on the same base oils blends [6]. This paper presents a unique insight into the rate of degradation of an antioxidant during the oxidation process of a Group III base oil, by utilizing the in situ sampling capability of the Quantum ® isothermal instrument shown in Figure 1 with the small sample extraction device in place. 2 Experimental As the instrument of choice in earlier investigations [4, 5, 6], the Quantum ® isothermal instrument was selected for this study because of its unique in situ sampling feature, which, as noted, permits test sample extraction during the test. Ease of operation, good precision and versatility of the Quantum ® instrument has made it ideally suited for investigating oxidation reactions [7, 9]. As shown in Figure 2, the Quantum ® instrument consists of 1. a high-pressure reaction chamber, 2. heated to a selected controlled temperature, 3. a sample rotation system for sample exposure to oxidation, and 4. a sample-extraction attachment (shown in both Figures 1 and 2) that can be used for sampling during operation. (If desired, the Quantum ® instrument can be operated with the instrument positioned with a vertical pressure chamber and use of a magnetic stir bar for agitating the sample [8].) 2.1 Oxidation Reaction Conditions The experimental oxidation conditions chosen for this study were those of ASTM Test Method D2272, termed the Rotating Pressure Vessel Oxidation Test (RPVOT) [9]. This RPVOT method is primarily used for the evaluation of turbine oils for the power industry. In the present studies, the normal 50 ± 0.5 g of the test fluid containing the desired amount of antioxidant and Figure 1: The Quantum ® isothermal instrument shown with a syringe for sampling the test fluid Figure 2: Cutaway view of the Quantum ® isothermal instrument and sampling arrangement T+S_2_2018_.qxp_T+S_2018 20.03.18 10: 36 Seite 48 As would be expected, this change in the chamber pressure is closely related to both the oxidation susceptibility of the base oil and the effectiveness and manner of response of the antioxidant. To gain more information about the oxidation process, it was thought to be of interest to obtain the time at which the oxidation reaches its maximum rate of change. Reasonably, this was the critical point of the oxidation reaction at which maximum reaction of the test sample with the oxygen was occurring. Subsequent decrease in this oxidation rate could also be reasonably interpreted to indicate that both the oxygen and/ or the oxidation-prone components of the sample were decreasing in their effect on the chamber pressure. From this perspective, rate of oxidation would be expected to coincide with the derivative, ΔP/ ΔT, of the pressure chamber’s pressure exerted by the oxygen, water and sample. This derivative is also plotted in Figure 3. Thus, after a period of steady pressure of about 180- 190 °C (and related constancy of ΔP/ ΔT), the beginning of a sharp decrease in pressure - suggesting rapidly increasing oxidation - is accompanied by a sharp rise in the derivative. At the point of time at which the increasing rate of oxidation ends and begins to slow, the value of ΔP/ ΔT reaches its peak - a point of time which the authors termed the “End of Rapid Reaction” or EOR. As this oxidation ‘cleanup’ of remnant oxygen-prone test sample components proceeds, the pressure continues to decrease at a slower pace as evident in Figure 3 until it reaches a steady value of about 80 psi, of which 70 psi is attributable to water. 2.2 Discussion and Study Information Information on oxidation susceptibility is critical in one of the fundamental provisions of modern civilization - electric power. In this is basic industry, reliable operation of the turbines generating electricity is critical. Unfortunately, as has been previously noted, a large percentage of unplanned turbine downtime is related to lubricant malfunction. Consequently, a comprehensive oil-condition monitoring program is imperative [10] wherever power turbines are used. When turbine oil is placed in service, oxidation resulting from the depletion of its antioxidant, results in the impairment of the turbine oil to provide adequate oil degradation protection. This degradation is primarily in deposit formation. As a consequence, in ASTM Test Method D4378 on In-Service Monitoring of Mineral Turbine Oils there is a recommendation for replacement of turbine oil when the antioxidant effectiveness - judged by use of a bench test such as the RPVOT - drops to 25 % of the original, new oil value [11]. This is, of course, a costly practice and most often when these oils are changed, the oil is otherwise serviceable in providing lubrication. Provision of better oxidation stability is the key to more reliable turbine lubrication [12]. 2.3 Determination and Comparison of Antioxidant Effectiveness Considering the desirability of determining antioxidant effectiveness in all of the varied applications in modern society - particularly for both the electric power industry and the very widely used reciprocating enginethe au thors elected to study the chemistry and physics of oxidation inhibitor deterioration. When equipped with a sample-extraction device, the Quantum ® instrument is well-suited for such study of antioxidant effectiveness and it seemed reasonable to use the ASTM D2272 oxidation test protocol in which the Quantum ® instrument had already proven its merits as a bench test. Accordingly, in this initial study a base oil representative of today’s Group III class of base oils was chosen to provide natural oxidation resistance to clearly demonstrate the response of the antioxidant. The Group III base stock oil was blended with 1.1 % of an aminic antioxidant. Care was taken that the antioxi- Aus der Praxis für die Praxis 49 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 Figure 3: Example of the response of a mineral oil to oxidation in the RPVOT test and also showing the derivative trace with its peak value signifying the end of rapid reaction (EOR) Figure 4: Response for chosen Group III base oil with 1.1 % aminic antioxidant to oxidation test conditions T+S_2_2018_.qxp_T+S_2018 20.03.18 10: 36 Seite 49 peaks at 1720 cm -1 is related to oxidation of the oil plus antioxidant sample and the other peak at 1520 cm -1 reflects the loss of the aminic antioxidant used. Since the increasing presence of the samples’ organic components being studied is shown by decreasing infrared light transmission through the sample, inverse peaks are generated by increasing concentrations. For the two peaks of interest the change in concentration of two components of interest are shown by arrows. However, it must be kept in mind that it is assumed that each peak has its own presumably linear, concentration/ percent-transmission relationship. 2.3.3 Analysis of Infrared Information As oxidation occurs in the lubricant, it depletes the antioxidants and produces oxygenated hydrocarbons such as carboxylic acids. In Figure 6, these acids can be observed in the aforementioned region of 1720 cm -1 Wave Number. Analysis of the percent transmission values (PTV) at 1720 cm -1 Wave Number of the Group III base oil with no added antioxidant can be compared to the Aus der Praxis für die Praxis 50 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 dant was well solvated in the base oil and that such solvency was carefully maintained over the length of the study. Figure 4 shows the oxidative response of this Group III base oil with 1.1 % of the aminic antioxidant. The oxidative resistance of the blend gave an EOR of 2480 minutes. It will be noted that, for this blend, the pressure began to decay immediately after reaching maximum and as commonly experienced, just prior to reaching the EOR, pressure dropped sharply and continued to equilibrium at about 90 psi. 2.3.1 in situ Sampling and Analysis As previously noted, the Group III Base Oil plus 1.1 % aminic antioxidant was chosen. The difference in the pressure trend immediately following maximum pressure shown by Figure 5 when compared to Figure 3 suggests that the manner of absorbing oxygen during oxidation in the oxygenated pressure chamber of the two different samples were fundamentally dissimilar. This added further interest in using the isothermal Quantum ® instrument and sample extraction and suggested interesting further studies of such differences. Five sample aliquots were extracted at various time intervals leading up to the EOR plus a final sixth sample extracted after the sample reached its lower pressure equilibrium after the EOR at 2569 minutes in the pressure chamber. Figure 5 depicts the time intervals of in situ extraction of each of the six samples. The samples were extracted at test intervals that were considered to give the pertinent information concerning the oxidation process taking place. This included the effort to obtain a sample just prior to the ‘hard break’ in decreasing oxygen pressure in the pressure chamber and also a sample after the reaction had ended and the pressure reached equilibrium. At this point the six extracted samples were prepared for infrared analyses. 2.3.2 Infrared Analysis One of the more informative analytical tools for following the chemical changes occurring during oxidation is the use of infrared spectroscopy by Fourier Transform (FTIR). This approach was applied to the six samples from the test oil whose extraction intervals during its oxidation pressure change were shown above in Figure 5. The Group III base oil and the six small 0.3 mL samples obtained through in situ extraction were analyzed by FTIR. From the total FTIR spectral information obtained, a Wave Number range of 1800 to 1450 cm -1 was selected. These data are shown in Figure 6. This Wave Number range was chosen to evaluate two important oxidation peaks reflecting oxidation effects on the Group III base oil containing the aminic antioxidant. One of the Figure 5: Plot of timing of in situ sample extractions for monitoring antioxidant resistance to oxidative test conditions Figure 6: Infrared spectrums of samples taken during oxidation test T+S_2_2018_.qxp_T+S_2018 20.03.18 10: 36 Seite 50 PTVs of the six extracted samples taken during the Quantum ® RPVOT analysis. Subtracting the PTV of each extracted sample from the PTV of the base oil is a reasonable measure of the degree of oxidation occurring. This information is shown in Figure 7. It is also reasonable to assume that the difference in PTV is a linear relationship. 2.3.3.1 Analysis of Base Oil plus Aminic Antioxidant Interestingly, the data of Figure 7 show that only after a period of about 2300 minutes does significant oxidation begin to show the presence of carboxylates. From Figure 5, this is the time at which the sample goes into rapid oxidation and produces an EOR time of 2480 minutes. This in turn leads to the question of why the onset of oxidation was retarded up to about 2200 minutes. Obviously from Figures 4 and 7, oxidation proceeds in a relatively slow and steady manner up to that time. Apparently, some comparatively slight oxygen depletion was occurring. It simply was not producing much, if any, hydrocarbon oxygenates according to the FTIR data until, over a comparatively short period of time, and an oxidation ‘storm’ began. The data collected led to a rationale concerning the behavior of the aminic antioxidant and a view of its mode of action in retarding of base oil oxidation during its period of effective behavior. 2.3.3.2 FTIR Analysis of Aminic Antioxidant Response The basic question posed by the response of the Group III base oil plus aminic antioxidant was in regard to the action of the antioxidant over the major time of exposure just prior to the relatively sudden onset of rapid oxidation of the oil. The infrared spectroscopic values shown in Figure 6 at 1520 cm -1 were chosen to evaluate the response of the aminic antioxidant. In doing this, it was considered reasonable to presume that the content of active aminic antioxidant was related to the PTV obtained for each particular sample of the six extractions. These values were compared to 1.1 % aminic antioxidant of the oil before the oxidation exposure. Figure 8 shows the interesting results of this analysis and it was found that the decrease in concentration of the molecular form of the antioxidant was slightly exponential and that the decrease in apparent concentration as calculated from its presumed PTV linearity was a correlation coefficient of R 2 = 0.996. The latter value is strong support for the assumption of a linear relationship between the concentration of active aminic antioxidant and the PTV of the FTIR. More important, however, is the information provided by the smooth, mildly exponential decrease in the concentration of the aminic antioxidant related to the essentially unoxidized base oil shown by Figure 7. It would seem as though the aminic antioxidant present in the oil performs as a sacrificial agent in preventing base stock oxidation until a certain lower concentration is generated at which time the antioxidant loses control. In this case, the particular form of aminic antioxidant used in this study loses Group III base oil oxidation control at a concentration of somewhat less than 0.5 %. Moreover, from the data of Figure 8 it seems evident that even though rapid oxidation of the base oil is beyond its control, a continued decrease of the aminic antioxidant is shown by the fact that the final in situ sample - extracted at 2569 minutes - also falls neatly as the final point on the curve of Figure 8. This seems to suggest a relationship of the level of active aminic antioxidant below which the base oil has the potential to pass into a readily oxidation-prone state. In this study that lowest level of effective aminic antioxidant seems below about Aus der Praxis für die Praxis 51 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 Figure 7: Oxidation intensity of the six oils extracted in situ from the Quantum ® RPVOT test on a Group III base containing 1.1 % aminic antioxidant Figure 8: Change in concentration of aminic antioxidant with exposure to oxidizing conditions T+S_2_2018_.qxp_T+S_2018 20.03.18 10: 36 Seite 51 3.1.2 Oxidation Response of the Aminic Antioxidant When the role of the aminic antioxidant was explored using their FTIR PTVs, it was interesting to note that the presence of the aminic antioxidant showed immediate and continuing decrease. At the same time, the sample was showing little or no oxidation. The two observations may be merged in that the aminic antioxidant’s chemical activity may be the factor in preventing oxidation of the base oil until the aminic antioxidant is reduced to a critical level as just discussed in the previous section. In summary, the chemical activity of the aminic antioxidant deflected oxidation attack on the mineral oil until the concentration of the aminic antioxidant fell to less than 0.5 % concentration. This initial study has thus suggested a mechanism of antioxidant effectiveness. 4 Conclusion This initial study using the Quantum ® Isothermal Reactor with its ability to take in situ sample extractions at will has shown the unique ability of the technique to provide samples capable of illuminating investigation of the chemistry of oxidation in mineral oils containing antioxidants. On the basis of this initial study, several paths of investigation are suggested and will be considered for future studies and potential publication. The FTIR analyses of the extracted samples of this particular Group III oil sample formulated with 1.1 % of an aminic antioxidant indicated that the observed slow decline in oxygen pressure - before the aminic antioxidant concentration fell below 0.5 % - may have been associated with the decline in concentration of the aminic antioxidant. It will be of interest to study this relationship in greater detail in studies following this initial work. The basic finding in this initial study conducted in the Quantum ® Isothermal Reactor was the value of being able to extract in situ small samples of an oxidation reaction as the reaction proceeded and from these samples to gain insight into the chemistry of the oxidation process. Future work will focus on evaluating additional antioxidants and blends of antioxidants with other base stocks to broaden the utility of this methodology. References [1] www.mobilindustrial.com, EN0756SH [2] Wurzbach, R.: Lubricant Oxidation Analysis and Control, Maintenance Reliability Group, Inc., 2010-02-14. Aus der Praxis für die Praxis 52 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 0.5 %. That is, the decrease in concentration of aminic antioxidant to a value of less than 0.5 % seems to allow rapid development of a rate or form of oxidation chemistry that is inhibited or sequestered by a higher aminic antioxidant concentration. These data from this initial study suggest several other interesting experiments that would explore the antioxidant/ base stock relationship. 3 Discussion and Conclusions 3.1 Initial Study As is shown and illustrated, this initial study using the Quantum ® Isothermal Reactor instrument equipped with its in situ sample extractor produced some interesting potential insights regarding the oxidation chemistry of a mineral oil plus antioxidant. The oxidation protocol followed was that of the well-known ASTM Method D2272 Rotating Pressure Vessel Oxidation Test (RPVOT) in which the Quantum ® Isothermal Reactor has been tested and incorporated as Method B. In this initial study a Group IV base oil containing 1.1 % of a carefully solvated aminic antioxidant was selected for in situ extraction sampling during oxidation and subsequent FTIR analysis of these extracted samples. This approach has produced insights into interesting aspects of potential chemistry of this combination of mineral oil and antioxidant and encouraged further such tests to learn more about this and other such combinations of mineral oils and antioxidants. 3.1.1 Oxidation Response of the Mineral Oil This initial study showed that with 1.1 % concentration of the aminic antioxidant, the FTIR analyses of the Group III base oil showed very mild oxidation until somewhat above 2200 minutes. Slightly above 2400 minutes, a rapid rate of oxidation commenced and produced a climax of oxidation rate (the author’s so-called ‘End of Rapid Reaction’ (EOR)) peak at 2480 minutes. Analysis of the FTIR spectrograms obtained from the extractions of the mineral-oil/ aminic-antioxidant blend at a Wave Number of 1720 cm -1 showed that little oxidation had occurred until about 2100 minutes it began to be evident, although mild. The fact that virtually no significant oxidation occurred in the blend was interesting and suggested an interaction or molecular sequestering by the aminic antioxidant which was shown by the FTIR spectrograms at 1520 cm -1 Wave Number to be steadily diminishing in TPV during this period. Thus, it may be that aminic antioxidants may also work in a manner ancillary to the simple chemistry heretofore presented in literature. T+S_2_2018_.qxp_T+S_2018 20.03.18 10: 36 Seite 52 [3] Uddin, M.N.: Turbine Oil Cholesterol - A Silent Killer, LinkedIn, December 18, 2015. [4] Selby, T.W.; Azad, S.; J.C. Evans, J.C.; VanBergen, W.; Fischer, T.: Studies of Variation in the Oxidation Inhibition of Base Oils, OilDoc Conference & Exhibition, Bavaria, Germany, January 25-27, 2015. [5] Manning, M.; Selby, T.W.; Evans, J.C.; VanBergen, W.; Fischer, T.: Studies of Variation in the Oxidation Inhibition of Base Oils - Continuation of an Isothermal Study of Innate and Additive-inhibited Oxidation of Base Oils, STLE 70th Annual Meeting & Exhibition, Dallas, TX, USA, May 17-21, 2015. [6] Manning, M.; VanBergen, W.; Evans, J.C.; Selby, W. T.: Isothermal Study of the Influence of Water on Lubricant Oxidation, 19th International Colloquium Tribology Technische Akademie Esslingen, Ostfildern, Germany, January 12-14, 2016. [7] Selby, T.W.: Modern Instrumental Method of Accurately and Directly Measuring the Useful Life of Turbine Oils, OilDoc Conference and Exposition, Bavaria, Germany; February 2-3, 2011. [8] Selby, T. W.; Evans, J. C.; Azad, S.; VanBergen, W.: A Comparative Study of Grease Oxidation Using an Advanced Bench Test Technique, Proceedings of the 19t h International Colloquium Tribology - Lubricants, Materials and Lubrication, Technische Akademie Esslingen, Esslingen, Germany, January 22, 2014. [9] ASTM Method of Test D2272-14a: Standard Test Method for Oxidative Stability of Steam Turbine Oils by Rotating Pressure Vessel, Vol. 5, 2014. [10] Malcolm, M.: Turbine Oil Analysis Report Interpretation, Tribology & Lubrication Technology, Vol. 71, No 7, pp. 48-53, 2015. [11] ASTM Method of Test D4378-13: In-Service Monitoring of Mineral Turbine Oils for Steam, Gas and Combined Cycle Turbines, 2013. [12] Livingstone, G.; Ameye, J.: How to Double Turbine Oil Life, Lubes ’N’ Greases Europe-Middle East-Africa, 88, 2016, pp. 50-56. Aus der Praxis für die Praxis 53 Tribologie + Schmierungstechnik · 65. Jahrgang · 2/ 2018 expert verlag GmbH: Wankelstr. 13, 71272 Renningen Postfach 20 20, 71268 Renningen Tel. 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