Aus der Praxis für die Praxis 1 Brief Background The present civilization’s dependence on motorized transportation has been very evident for a number of years. It is difficult to conceive life today without this aspect. With the comparatively rapid development of this dependence over the last century has come a technical need. That is, the need to assure that the automotive engine - the heart of this mode of life - and its ‘life blood’ - the engine lubricant - is capable of meeting the highly variable uses of today’s societies throughout the world. Although such need has generated a rich technology associated with understanding lubrication, the rapidly changing complexities of civilization has generated even more need for knowledge. A major area of this need is to protect the engine lubricant from the ever more strenuous usage to which it is put and one of the more important aspects of this protection is in improving the oxidation resistance of the lubricant. 40 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 * Theodore W. Selby, BS, MS Savant Group; Midland, Michigan, USA Samina Azid, BS, PhD Savant Laboratories; Midland, Michigan, USA Jonathan C. Evans, BS, PhD Savant Group; Midland, Michigan, USA William VanBergen, AS Savant Laboratories; Midland, Michigan, USA Thomas Fischer, BS, PhD OelCheck, GmbH; Brannenburg, Germany Studies of Variation in the Oxidation Inhibition of Base Oils T. W. Selby, S. Azid, J. C. Evans, W. VanBergen, T. Fischer* Als Reaktion auf die weltweiten Bemühungen die Kraftstoffeffizienz und die Haltbarkeit zu verbessern, haben die Betriebsbeanspruchungen der Automobilmotoren in Folge von Konstruktionsänderungen an diesen Motorenzugenommen. Eine der Folgen ist, dass die Oxidationsbeständigkeit des Motoröls zunehmend an Bedeutung gewonnen hat. Mit diesem Anreiz wurde eine Studie auf einer grundlegenden Ebene mit drei ausgewählten Oxidationsinhibitoren initiiert, die jeweils in drei Konzentrationen einem Basisöl mit der natürlichen Oxidationsbeständigkeit der Gruppe II beigemischt wurden. Diese grundlegende Studie wurde in einem speziellen Hochdruck, iso-thermischen Reaktor, in dem in zukünftigen Studiendie Oxidationshemmung von Basisöl durchgeführt werden und Proben zur Analyse des Fortschritts der Reaktion des Testöls auf Oxidation genommen werden können. Das erste Papier zum Thema Mineralöl Oxidationsreaktion präsentiert die Grundlage in einer laufenden Studie über die Unterschiede in der Oxidationsreaktion mit Oxidationsinhibitoren, abhängig vom Basisöl und der Inhibitorkonzentration. Schlüsselwörter Basisöl , Oxidation, Oxidationsbeständigkeit , Antioxidationsmittel , D2272 In response to the world-wide efforts to improve fuel efficiency and durability, automotive engine operating stresses have increased with the changing design of these engines. One of the consequences is that engine oil oxidation resistance has become increasingly important. With this incentive, a study has been initiated at a basic level of the response of three selected oxidation inhibitors, each at three concentrations, in a base oil of Group II natural oxidation resistance. This basic study was conducted in a special high pressure, isothermal reactor within which, for contemplated future studies, the oxidation-inhibited base oil could be sampled at will for analysis of the progress of the response of the test oil to oxidation. This initial paper on the subject of mineral oil oxidation response presents groundwork in an ongoing study of the differences in oxidation response among oxidation inhibitors used depending on base oil and inhibitor concentration. Keywords Base oil, oxidation, oxidation resistance, antioxidant, D2272 Kurzfassung Abstract T+S_5_16 29.07.16 11: 28 Seite 40 Aus der Praxis für die Praxis 1.1 Purpose of Study The authors of this paper are well aware of the increasing level of concern regarding lubricant oxidation. At the same time, it is also apparent to many of how much more basic knowledge is needed to further improve its technical understanding. As a consequence, with the availability of a relatively new isothermal instrument, the authors have mounted a study to evaluate the essential performance and associated chemistry of oxidation of selected oxidation inhibitors. This paper is the first of several and will present a portion of the experimental approach to be applied at an introductory level. With this in mind, the authors thought it worthwhile to first mount a simple study of the basic responses of a selected base oil to two or three oxidation inhibitors in a simple isothermal environment to measure and compare their oxidation response. Studies that followed could be built on this preliminary investigation. 2 Instrument, Test Method and Fluids The choice of the appropriate instrument for the present and planned studies was important. Essentially, the instrument must have both the ability to conduct the experiments in a highly repeatable manner under different test conditions but should also be relatively simple to operate such that complex controls do not add a level of potential error from test to test. Lastly, if possible, the instrument should provide a way of both analyzing and altering the conduct of the experiment during tests. 2.1 Isothermal Reactor To keep the experimental conditions as simple and straightforward as possible, the authors selected the isothermal instrument known as the Quantum which was originally developed to include such studies [1, 2]. Because of its precision and freedom from need for a liquid bath, the instrument has also been included in ASTM Test Method D2272 [3] for turbine oil oxidation standards and has been applied in other such applications. The instrument is shown in Graphic 1 and its cut-away is sketched in Graphic 2 to show the manner in which experiments may be conducted using its isothermal, nonbath design. Whatever operating temperature is selected by the operator is precisely provided for the length of the test. In the experiments to be reported in the present work, the operating conditions were those of the ASTM Test Method D2272. This is a test condition familiar to many to measure the oxidation resistance of turbine engine lubricants. If and when desired, the Quantum instrument permits small samples of the test oil to be taken without significant interference with the progress of the test. In this initial study, however, only the pressure response under consistent temperature control was sought to establish the foundation of the study. Both the pressure and temperature of the pressure chamber are electronically measured and conveyed to a computer program for later analysis. 2.2 Data Collection and Analysis As in ASTM D2272, oxidation of the test oil ultimately leads to the exhaustion of the oxidation inhibitor additive. This produces a pressure trace reflecting the response of the test fluid to such oxidation. The pressure and temperature data collected with a computer program also permitted simultaneously computing another piece of information regarding the time at which the oxidation inhibitor was essentially completely ex- Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 41 Graphic 1: Quantum isothermal reactor Graphic 2: Internal lay-out of Quantum iso-thermal test instrument. T+S_5_16 29.07.16 11: 28 Seite 41 Aus der Praxis für die Praxis hausted. As will be later shown in Graphic 3, this exhaustion point becomes the peak of the recorded pressure-time derivative and was termed the end of reaction or EOR. 2.3 Test Fluids To simplify this first portion of the projected study, one base oil was evaluated. This base oil was representative of today’s commonly used Group II class of more highly refined base oils. This oil was individually blended with three antioxidants, each at three concentrations - 0.20 %, 0.65 % and 1.10 %. Considerable care was taken to assure that the antioxidants were well solvated in the base oil and carefully maintained for experiment. 3 Experimental Technique 3.1 Set-up Expediently, the isothermal test technique of the Quantum was set up in the manner of ASTM D2272. Specifically, 50 ±0.5 grams of the test fluid plus the indicated percent content of the antioxidant are weighed into the glass sample cup and the copper-wire catalytic coil of the method properly cleaned and inserted into the test oil as shown in Graphic 2. As in ASTM D2272, in each test 5 mL of water are put in the sample cup and 5 mL of water are added to the pressure chamber outside of the glass cup. The glass cup is then placed in the magnetic cup holder shown in Graphic 2 and the assemblage put into the pressure chamber and the pressure chamber sealed appropriately. At this point, the pressure chamber is flushed with 99.5 % pure oxygen at 90 PSI (pounds per square inch) three times before final filling to 90 ±0.2 PSI according to ASTM D2272 [3]. Test temperature is set at 150 ±0.1 °C and recorded continuously together with the pressure in the chamber during the test. 3.2 Operation After pressure and temperature connections are made, the test is begun. Since a test can take from less than 100 minutes to several thousand minutes, reasonably close attention is paid to the pressure recording to be sure to obtain the full data from a test without letting the test run unnecessarily long after completion of oxidation response. However, for full information and understanding of the influence of oxidation on the pressure in the chamber, the pressure recording is continued for about an hour after it drops to equilibrium at the lowest recorded value. An example of the record from a test run is shown in Graphic 3. In addition to the traces for temperature and pressure is a trace showing the derivative, ΔPressure/ - ΔTime, which relates to the end of antioxidant function, termed ‘EOR’. 3.2.1 Oxidation and Generation of the EOR The end of typical pressure trace in Graphic 3 shows a point of time at which the test fluid rapidly loses its resistance to oxidation. Consequently, the oxygen pressure begins to fall more and more rapidly as the fluid is oxidized until much of the mass of test fluid has reacted. At this stage, the reaction reaches its peak and begins to wane and this point has been termed the end of reaction, EOR. It is also at this point that the derivative of pressure reaches its highest value as evident in Graphic 3. 4 Experimental Results The Group II base oil alone and as modified by the three antioxidants, each at three concentrations, were tested in the Quantum instrument using the test protocol of ASTM Method D2272 previously described. 4.1 Base Oil Graphic 4 shows the response of the base oil used in these first tests (a common Group II base oil). Without an antioxidant, oxidation was rapid and the base oil had an EOR of only 30 minutes. From experience in application of the Quantum isothermal instrument to base oils with and without antioxidant protection, this response was typical. 42 Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 Graphic 3: Example of a test fluid analysis in the Quantum instrument. T+S_5_16 29.07.16 11: 28 Seite 42 Aus der Praxis für die Praxis 4.2 Response of Base Oil to Antioxidant A As previously noted, three increasing levels of Antioxidant A were made using the Group II Base Oil. These concentrations of 0.20 %, 0.65% and 1.10 % antioxidant were used in all subsequent blends of each antioxidant. Graphic 5 shows the progressive response of the Group II base oil to increasing content of Antioxidant A. Values of EOR for these tests are given in the Legend for the graphic. Although each increase in concentration improves the base oil’s oxidation resistance and related EOR somewhat, the overall comparative level of resistance is not impressive. 4.3 Response of Base Oil to Antioxidant B Antioxidant B was next used to add oxidation resistance to the Group II base oil and results are shown in Graphic 6. In comparison to the previous results with Antioxidant A, Antioxidant B is much more effective, at least at the levels of 0.65 % and 1.10 %. 4.3.1 Pressure Constancy during Test An interesting observation was that both antioxidants seem to be effective at holding oxidation to a minimum during the period of test. This was evidenced by the comparatively small change in oxygen pressure until just before the onset of high rate of oxidation with the exhaustion of the antioxidant’s protection. 4.4 Response of Base Oil to Antioxidant C In view of the widely different responses of Antioxidants A and B in the Group II base oil, it was of evident interest to evaluate another identified for this study as Antioxidant C. It is apparent from Graphic 7 that Antioxidant C is similar in effectiveness to Antioxidant B at the highest concentration of 1.10 %. However, at the lower concentrations of 0.20 % and 0.65 %, Antioxidant C is shown to be markedly more effective than Antioxidant B. Tribologie + Schmierungstechnik 63. Jahrgang 5/ 2016 43 Graphic 4: Response of chosen Group II base oil to oxidation test conditions Graphic 7: Effect of Antioxidant C on Base Oil Graphic 5: Effect of Antioxidant A on Base Oil Graphic 6: Effect of Antioxidant B on Base Oil T+S_5_16 29.07.16 11: 28 Seite 43 Aus der Praxis für die Praxis This is shown more clearly in Graphic 8, a comparison of EOR versus concentration for the three antioxidants tested. 5 Discussion and Conclusions This initial study of the effects of three antioxidants on a single Group II base oil was made to improve the extraction of oxidation information on lubricants. The technical procedure used at this stage was that applied to oxidation tests of turbine lubricants, namely ASTM Method D2272. At the heart of this opening study was the application of the isothermal Quantum instrument for the tests generating and analyzing relevant oxidation response of nine simple formulations of the base oil and three antioxidants. This first study was surprisingly informative concerning the similarities and differences in just these three additives made for a common purpose. One of the benefits of the precision with which these tests were run (the authors replicated a number of the results and power curves) was that calculation of the defined End of Reaction (EOR) pressure/ time derivative value gave a clear comparison of the true antioxidation contribution of the additive, particularly when combined with concentration effects. The information and techniques presented in this study suggest that a broad study of antioxidants and their chemistries and reactions under various conditions might be helpful in sharpening both their present applications and, particularly, clarifying the direction that should be taken in forthcoming chemistry. References [1] T.W. Selby, S.W. Froelicher, J. Secrist: Studies of the Oxidation Dynamics of Turbine Oils - A New Form of the Rotating Pressure Vessel Oxidation Test, ASTM International Symposium on Oxidation and Testing of Turbine Oils, ASTM STP1489, Published: 2008, Pages: 9 [2] T.W. Selby: Modern Instrumental Method of Accurately and Directly Measuring the Useful Life of Turbine Oils, OilDoc Conference of 2011 [3] ASTM Method of Test D2272, Oxidative Stability of Steam Turbine Oils by Rotating Pressure Vessel, Published: 2011 44 Tribologie + Schmierungstechnik 63. 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