Article pubs.acs.org/JPCC
Studies on the Aging Characteristics of Base Oil with Amine Based Antioxidant in Steel-on-Steel Lubricated Sliding Archana Singh,† Ravi T. Gandra,† Eric W. Schneider,‡ and Sanjay K. Biswas*,† †
Nanotribology Lab, Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India Chemical and Materials Systems Laboratory, General Motors Research and Development Center, 30500 Mound Road, Warren, Michigan 48090, United States
‡
S Supporting Information *
ABSTRACT: An industrial base oil, a blend of different paraffin fractions, is heated to 130 °C (1) in the ambient and (2) for use as a lubricant in a steel pin on a steel disk sliding experiment. The base oil was tested with and without test antioxidants: dimethyl disulfide (DMDS) and alkylated diphenylamine (ADPA). Primary and secondary oxidation products were monitored continuously by FTIR over a 100 h period. In addition, friction and wear of the steel pin were monitored over the same period and the chemical transformation of the pin surface was monitored by XPS. The objective of this work is to observe the catalytic action of the steel components on the oil aging process and the efficacy of the antioxidant to reduce oxidation of oil used in tribology as a lubricant. Possible mechanistic explanations of the aging process as well as its impact on friction and wear are discussed.
1. INTRODUCTION Oil used for lubrication in engineering machinery is generally a blend of paraffin, naphthene, and aromatic fractions. Depending on applications, different quantities of these fractions are blended to optimize viscosity, high temperature stability, and in general the life of the oil. A single fraction exposed to air at ambient and higher than ambient temperatures undergoes a series of chain reactions that gives peroxides as primary oxidation products, followed by their decomposition to alcohols and ketones. These products undergo further oxidation and condensation in time, leading to unacceptable acids, esters, and lactones.1,2 When time and temperature are the only two external drivers for change, the process is designated “autoxidation”. The effect of time and temperature on these fractions may be cooperative or antagonistic. Addition of aromatic rings to paraffin, for example, aid oxidation,1 while the presence of naphthalenes causes an inhibitory effect on oxidation. The property of a blend is however not summative, as the fractions may interact to yield properties much superior to those expected by a rule of mixtures.1 Base oil is often made “fully formulated” to add application-driven functionalities to the oil. Ofunne et al.3 in their high temperature study of crankcase oils and base oils found that paraffinic oils, on oxidation, generate more volatile acids, oil-soluble acids, and deposits than what are produced by blends (base oil). They further found that at temperatures above 260 °C the additives break down to accelerate deleterious catalytic polymerization reactions in the fully formulated oil. In most practical lubricating situations the presence of metal in oil is ubiquitous. The metal comes from the wear of © 2013 American Chemical Society
components or from breakdown of inorganic additives in operation.4−6 Oxidative or peroxidative attack occurs on the metal to form easily removable oxides.7,8 The rate of oxidation of oil when used as a lubricant may be significantly different from the rate of oxidation in autoxidation.9 The mechanism of oxidation is also found to be altered when tribology is imposed, necessitating a process designation of “tribooxidation”. The metal may exist in oil as oil-soluble organometallic salts or fresh solid particles. The catalytic action of a metal to promote oil oxidation is metal specific. Maduako et al.10 found zinc, nickel, and aluminum to be catalytic inhibitors which suppress polymerization and prolong oil life in autoxidation. Compounds of copper, lead, and iron11−14 on the other hand have large accelerating effects on the overall rate of oil oxidation. Colclough14 suggests that the effects of soluble copper on hydrocarbon oxidation depend very much on its concentration and on the presence or absence of soluble iron catalysts and sulfidic antioxidants. The antioxidant activity of copper happens because of a recycling process in which Cu+ and Cu2+ species are repeatedly involved in termination reactions with peroxy and alkyl radicals, and redox reactions with soluble iron, while the sulfur compounds present maintain a low hydroperoxide concentration. Antioxidants are used in the formulation of lubricants as they retard the oil aging process. Several types of molecules such as sulfur compounds,15 substituted phenols,16 zinc dialkyldithioReceived: October 4, 2012 Revised: January 3, 2013 Published: January 3, 2013 1735
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Scheme 1. Oxidation Inhibition Mechanism by Alkylated Diphenylamine
radicals to form the nitroxy free radical Ar2NO•, which is also a potent inhibitor.21 This is because it has the ability to terminate a large number of oxidation chains through catalytic action, which is represented by red arrows in Scheme 1. Jensen et al.24 proposed that during the inhibition of oxidation of secondary radicals aromatic secondary amines (Ar2NH), at elevated temperatures (above 120 °C), regenerate parent aromatic secondary amine via the corresponding nitroxy radical Ar2NO• (Scheme 1). The useful life of a lubricating oil is determined by its oxidative stability, which is enhanced by hydrogenation of the base oil and addition of various oxidation inhibitors. Various groups16,25−29 have examined the oxidation of fully formulated lubricants and base oils. The physical and chemical complexity of these systems, however, creates difficult-to-control reaction pathways, kinetics, and mechanisms. This complexity has driven researchers to perform much experimental work with model reactants such as paraffins.30−34 To develop our understanding of the effect of tribology on oil oxidation and the effect of oil oxidation on tribology, we have studied9 the performance of a simple model base fluid (hexadecane) when it is used to lubricate, with and without an antioxidant (dimethyl disulfide), a steel-on-steel sliding interaction. When temperatures are below 130 °C, we have noted that the tribological interaction has only a marginal effect of on the rate of oil oxidation. Given the expectation of a comparatively high antioxidant functionality promoted by the “radical scavenging” mechanism of alkylated diphenylamine, the present study explores the activities of this scavenger molecule in steel-on-steel tribology. In this paper, we record the changes in the structural characteristics by decomposition of a semisynthetic mineral oil, Yubase 4 (≥90% saturate), due to steel-on-steel sliding contact,
phosphates and zinc dialkyldithiocarbamates,17 and alkylated diphenylamines18−20 are known antioxidants. Antioxidants are usually classified in two families: peroxide decomposers and radical scavengers. 1.1. Peroxide Decomposers. An important class of these antioxidants consists of hydrocarbon molecules with sulfur linkages. Hydroperoxide decomposers reduce hydroperoxides to nonradical alcohols while they themselves become oxidized to higher oxidation levels and thus prevent the chain propagation reaction. Sulfur compounds, represented by alkyl sulfides, are converted into alkyl sulfoxides or alkyl sulfones when reacted with hydroperoxides. Sulfoxides may disproportionate or arrange thermooxidatively to form other sulfurcontaining products, such as sulfonic and sulfuric acids. These sulfur-containing products themselves are hydroperoxide decomposers.21 1.2. Radical Scavengers. Free radical scavengers are inhibitors that render free radicals innocuous, either by transferring a hydrogen atom to them or by an oxidation− reduction mechanism.22,23 Hydrogen transfer from the inhibitor to the free radical generates a new inhibitor-derived free radical. However, because of steric hindrance or resonance stabilization, the newly formed free radicals are incapable of propagating the oxidation process.21 Among all free radical scavenger compounds, alkylated diphenylamines (ADPA), because of their high stoichiometric efficiency, are some of the most employed molecules. ADPA react with peroxy radicals ROO•, generated from hydroperoxides in the first steps of oxidation, to form resonancestabilized Ar2N•. The Ar2N• free radicals are resonancestabilized and hence cannot start the new oxidation chains. However, they do react with hydroperoxides and peroxy free 1736
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which are distinct from those due to autoxidation. Given such changes in the oil, we explore the corresponding surface modifications of a steel pin rubbing a steel disk under lubricated conditions in a pin-on-disk machine. The results enable us to rationalize the transient and steady state phases of the wear of the pin.
Table 2. Tests Conducted for Tribooxidative Degradation of YB4
2. EXPERIMENTAL SECTION 2.1. Materials. Mineral oil Yubase 4 (YB4), supplied by General Motors, USA, was used as the base fluid in all experiments. Dimethyl disulfide (DMDS) (>99%), supplied by Merck, Germany, was used as an additive (antioxidant). Alkylated diphenylamine (ADPA), supplied by General Motors, USA, was used as another antioxidant. All chemicals were used without any further purification. 2.2. Tests. 2.2.1. Autoxidative Degradation of Oil. Oil (60 mL) was heated in a 100 mL three-neck round-bottom flask equipped with a condenser and a thermometer. To maintain constant air (oxygen) volume within the system, the condenser end was kept open to the ambient. A controller with a feedback loop was used to maintain the oil at a constant 130 °C. An electrically operated magnetic stirrer was used to stir the oil at all times during an experiment. Tests conducted for the autoxidative degradation of YB4 are given in Table 1. The maximum duration of a test was 120 h.
oil
additive
additive vol (% (v/v))
test code
1 2
YB4 YB4
− ADPA
− 0.1
YB4 (Aut) YB4 + ADPA-0.1 (Aut)
oil
additive
additive vol (% (v/v))
1 2 3
YB4 YB4 YB4
− DMDS ADPA
− 1 0.1 0.3 0.5
test code YB4 YB4 YB4 YB4 YB4
(Tr) + DMDS-1 (Tr) + ADPA-0.1 (Tr) + ADPA-0.3 (Tr) + ADPA-0.5 (Tr)
2.3. Oil Analysis. 2.3.1. FTIR Analysis of Oxidized Mineral Oils. To investigate the inception of oxidation in YB4 (with and without DMDS and ADPA) at 130 °C, FTIR spectra of oil samples were collected at different time intervals of the experiments and recorded. All spectra were recorded by a Perkin-Elmer GX spectrometer on a KBr cell. The instrument was equipped with a liquid nitrogen cooled deuterated triglycine sulfate (DTGS) detector. All IR spectra reported here are over 500 optimized scans at 4 cm−1 resolution by using a p-polarized beam. The sample and detector chambers were purged with nitrogen gas before starting the experiments and at regular intervals. The spectral analysis was carried out using spectrum software, version 3.02 (Perkin-Elmer, USA). 2.3.2. Gas Chromatography (GC)−Mass Spectrometry (MS) Analysis. GC−MS was used to characterize the mineral oil, YB4. A Trace GCMS (Thermo-Finnigan Corp.) was used for the analysis. The instrument was set up to perform GC− electron impact ionization (EI) MS. Positive ions were generated by electron impact ionization (EI) with 70 eV electron energy. A 30 m long, 0.25 mm inner diameter, and 0.25 μm thick wall coated VF-5MS stationary phase column provided the component separation. A 1 μL injection volume was used under the following conditions: The temperature program was initial temperature 40 °C, hold time 1 min, ramp 10 °C/min, final temperature 300 °C/min, and final hold time 20 min. Helium was used as a carrier gas. MS data were processed using Xcalibur Qualbrowser, version 1.2 (ThermoFinnigan Corp.), software. 2.4. Pin Surface Study. X-ray photoelectron spectroscopy (XPS; MULTILAB 2000, Thermofisher Scientific) was used to record the chemical state of the steel pin sample surface after tribological oxidation of mineral oil. The binding energy values corresponding to all the peaks in the XPS spectra are corrected in the study by a change corresponding to the C 1s peak at 285 eV. Quantitative analysis by XPS was performed using a sensitivity factor for each element. If the intensity of the ith element is Ii and the corresponding sensitivity factor is Si, then the atomic fraction/atomic concentration is
Table 1. Tests Conducted for Autoxidative Degradation of YB4 test
test
The heated oil was collected at regular intervals, centrifuged, and analyzed by Fourier transform infrared (FTIR) spectroscopy to determine the reaction products formed in the bulk lubricant after oil oxidation. 2.2.2. Tribological Activation of Oil Degradation. The tribological-oxidative oil degradation experiments of the mineral oil were performed on the same pin-on-disk machine (DUCOM, Bangalore, India) as used in hexadecane tribological-oxidative experiments.9 The pin and the disk both were made of heat treated EN31 steel. Similar to the hexadecane tribological-oxidative experiments, a 6 mm diameter pin was fixed to a holder at a 50 mm radius of the disk where a concentric groove of 34 mm width and 28 mm depth was machined a priori. The groove on the disk was flooded with 60 mL of the lubricating oil. The oil heating and temperature monitoring procedures were same as in the hexadecane tribological-oxidative experiments. The temperature at which the experiments were carried out was 130 °C. The tribological test conditions for all the experiments were normal load = 50 N (mean contact pressure = 1.76 MPa) and sliding velocity = 100 rpm (surface speed = 0.52 m/s). Similar to the autoxidation experiments, the lubricating oil samples were collected at regular time intervals, centrifuged, and characterized by FTIR spectroscopy. Tests conducted for the tribooxidative degradation of YB4 are given in Table 2. Each test was repeated three times. We present the average of three data sets. The scatter in the data is of the order of σ = ±2% of the average, where σ is the standard deviation.
Ni =
Ii Si
∑i
Ii Si
In the surface preparation, the worn surfaces were cleaned with hexane and dried before they were used for the XPS studies.
3. RESULTS 3.1. Characterization of Yubase 4. The mineral oil Yubase 4 (YB4) was characterized by FTIR spectroscopy and GC−MS analysis. The FTIR spectrum of YB4 is typical of paraffin (see the Supporting Information, Figure S1), which shows strong peaks at 2955, 2925, and 2854.5 cm−1, attributed 1737
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Figure 1. Oxidative product growth for autoxidative degradation of YB4 with and without ADPA (0.1% v/v) at 130 °C.
to the C−H stretching vibrations of −CH3 and −CH2 structures within the molecules. The spectrum shows two medium-intensity peaks at 1463 and 1377 cm−1 which correspond to bending vibrations of CH3 and CH2. A weak intensity peak is seen at 721 cm−1, because of the CH2 rocking vibration. The GC chromatogram of YB4 shows the presence of a mixture of heavy paraffins in the oil (see the Supporting Information, Figure S2). Application of gas chromatography coupled with mass spectrometry (GS−MS) allows clear characteristics of the paraffin separated from the oil by means of the corresponding m/z peaks and their abundances. On the basis of the mass spectrum analysis, it is found that YB4
contains a mixture of isoparaffins and n-paraffins of the C26− C31 carbon number range. Among all alkanes, heptacosane (C 27 H 56 ) and its isomers are the major constituents (approximately 50%) of YB4. 3.2. Autooxidative Degradation of Oil. At 130 °C FTIR peaks corresponding to OH (3600−2500 cm−1), CO (1900−1600 cm−1), and C−O (1500−900 cm−1) stretches appear, indicating the presence of secondary products: alcohols, ketones, aldehydes, carboxylic acids, and esters.21,35 Such peaks were not observed when the as-received oil was analyzed at room temperature. Figure 1 shows the evolution of peroxide and secondary products and demonstrates that the appearances of primary and secondary products are significantly delayed 1738
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when the additive ADPA is added to the base oil YB4. The firsttime appearance of the CO peak is used as a marker, in this work, for the inception of the oxidation process. Figure 2 shows that the inception time of the high molecular weight YB4 is significantly greater than that of the light oil hexadecane.
marginal improvement achieved in inception time of hexadecane by the addition of 1% v/v DMDS, addition of even a small amount of the additive ADPA (0.1% v/v) to YB4 gives a significant increase in the inception time. The latter increases steadily with an increase in the additive (ADPA) concentration (percent v/v) in the oil (Figure 3). A comparison (Figure 4) of the autoxidation and tribooxidation results
Figure 2. Inception time for carbonyl functions during autoxidative degradation of hexadecane and Yubase 4, with and without DMDS (1% v/v) and ADPA (0.1% v/v) antioxidants, at 130 °C.
Figure 4. Comparison of inception times for carbonyl functions in autoxidative and tribooxidative degradations of hexadecane and Yubase 4, with and without antioxidant, at 130 °C.
3.3. Tribooxidative Degradation of Oil. Three sets of experiments were done under sliding conditions at 130 °C: (i) tribooxidative degradation of YB4 without additive (YB4), (ii) tribooxidative degradation of YB4 with DMDS, and (iii) tribooxidative degradation of YB4 with ADPA. As in the case of autoxidation (Figure 2), YB4 under sliding condition (Figure 3) shows an inception time which is greater than that of the tribooxidized hexadecane. Compared to the
indicates that tribology significantly reduces the CO peak inception time in hexadecane and YB4. Figure 4 also shows that the oxidation inception time in tribology is 3 times less than that corresponding to autoxidation. This reduction appears to be more or less insensitive to the use of an antioxidant. The tribooxidation results of hexadecane (with and without DMDS) and YB4 (with and without antioxidant) as shown in Figures 5 and 6 indicate that, in both cases, oil oxidizes to a peak concentration of alkyl hydroperoxide. The secondary products of carbonyl functions and alcohol are incepted exactly when the ROOH peak concentration is achieved. Initially, when the hydroperoxide starts to deplete rapidly in time (beyond the peak), there is a sharp rise in the generation of the secondary products. As the depletion continues steadily, the secondary product generation slows down considerably, but of course such products accumulate with time in oil. The trends in the rates of primary and secondary oxidative product generation in hexadecane and base oil YB4 are about the same, except that the rates of ketone and carboxylic acid generations are greater when the oil is base oil than when the oil is hexadecane. Dispersion of DMDS (1% v/v) in YB4 results in no change in the oxidation resistance of the oil (Figure 6). A very small addition of ADPA (0.1%) antioxidant in the oil, however, results in a dramatic improvement in oxidation resistance of YB4 (Figure 6). This addition increases the inception time of the CO peak from 6 to 18 h. After prolonged tribooxidation (48 h), however, the rates of primary and secondary oxidation products of the oil, oil with DMDS and oil with ADPA, are about the same.
Figure 3. Inception times for carbonyl functions in tribooxidative degradation of hexadecane and Yubase 4, with and without antioxidant, at 130 °C. 1739
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Figure 5. Log−log plots of oxidative product growth of Yubase 4 in comparison with those of hexadecane (with and without DMDS antioxidant) at 130 °C.
3.4. Tribology of Steel-on-Steel Lubricated Sliding. When hexadecane with and without DMDS additive is used to lubricate steel pin on steel disk sliding contacts at 130 °C, we have reported9 negative pin displacement at the very early stage of sliding. This was attributed to oxidation of steel. Figure 7 shows an absence of negative wear rate when YB4 (with DMDS and ADPA) is used as the lubricant. The wear characteristics of the pin lubricated by YB4 with DMDS and ADPA, with time, fall into two distinct regimes or phases. When DMDS is dispersed in the oil, phase I shows a positive wear (40 μm) with high wear rate up to 2.5 h. After 2.5 h (phase II), the wear rate
becomes nil up to 5 h. After 5 h, the wear profile shows a fall in wear of 20 μm by the end of the experiment. When ADPA is dispersed in the oil, phase I shows a positive wear (50 μm) with high wear rate up to 5 h. After 5 h (phase II), the wear rate becomes constant and results in a nearly flat curve. While the initial (positive) wear rate obtained using ADPA additive is higher than that obtained using DMDS additive, the wear rates obtained using both additives are practically nil after 25 h of sliding. Figure 8 shows that ADPA can reduce total wear as its concentration is increased up to 0.3% (v/v). Beyond 0.3% (v/ 1740
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Figure 6. Oxidative product growth plots for tribooxidative degradation of Yubase 4, with DMDS and ADPA antioxidants, at 130 °C.
v) concentration, change in total wear is negligible; we have observed the same wear when the concentration is 0.5% (v/v). The wear characteristics of the pin lubricated with YB4 along with ADPA (0.3% v/v and 0.5% v/v), with time, fall into three distinct regimes or phases. When ADPA (0.3% v/v) is dispersed in oil, phase I shows a positive wear (40 μm) with high wear rate up to 10 h. After 10 h (phase II), the wear rate remains constant and near zero up to 60 h. After 60 h (phase III) of sliding, the wear characteristic shows a sharp fall (30 μm) and results in 10 μm (average) wear in phase III. The wear characteristic of YB4 with 0.5% (v/v) ADPA concentration is similar to the wear characteristic of 0.3% (v/v) ADPA
concentration experiment. When 0.5% (v/v) ADPA is dispersed in oil, phase II ends at 170 h with 50 μm positive wear, followed by a sudden fall of 40 μm, and finally an average 10 μm wear in phase III. Figure 9 shows the change in the coefficient of friction by dispersing antioxidant in the lubricating oil YB4. Without additives the coefficient of friction after 40 h of running is about 0.075. This is brought down to 0.05 using ADPA (0.1% v/v) and to 0.02 using DMDS (1% v/v). The coefficient of friction decreases steadily with increasing ADPA concentration up to 0.3% (v/v), where the coefficient of friction recorded after 120 h of running is 0.015 (see the Supporting Information, Figure 1741
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Figure 9. Plot of coefficient of friction as a function of time for tribooxidation of YB4 (with and without DMDS and ADPA) at 130 °C.
Figure 7. Plot of wear (displacement) as a function of time for tribooxidation of YB4 (with and without DMDS and ADPA) and hexadecane with and without DMDS (in inset), at 130 °C.
°C after 48 h of sliding are shown in Figure 10b−d. The spectra are similar to those obtained from the without-antioxidant experiment, and fitted into four components at 285 ± 0.1, 286.6 ± 0.1, 288.2 ± 0.1, and 289.8 ± 0.1 eV. The signal at 285 eV of large intensity corresponds to C−C or C−H of an organic molecule.36−38 The peaks at 286.6, 288.2, and 289.8 are assigned to C−O or C−O/C−N when ADPA is present, C O, and OCO bonds, respectively.40−46 A four-peak deconvolution is shown in Figure 11 for the N 1s signal of steel pin surface lubricated by YB4 with 0.1% (v/v) and 0.5% (v/v) ADPA. The strongest signal at 399.9 ± 0.1 eV is related to the −C−NH−C− groups,47,48 and the peak at about 401.1 ± 0.1 eV is attributed to −N+− nitrogen.49 The peaks at about 402.2 ± 0.1 and 404.4 ± 0.1 eV are commonly attributed to oxidized nitrogen.50,51 The peak with the highest binding energy of 404.4 eV is assigned to NO surface contamination51 and is excluded here from further discussion. The weight concentrations (percent) of the components on the steel pin surface after prolonged tribooxidation were determined quantitatively from the XPS spectra and are summarized in Table 3.
Figure 8. Plot of wear (displacement) as a function of time for tribological oxidation of YB4 with varying concentration of ADPA at 130 °C.
4. DISCUSSION In attempting to prolong the life of lubricant oil in a machinery, one is confronted with three opposing trends which arise from the inevitabilities of the lubrication process. A simple single chain hydrocarbon is blended with many other hydrocarbon fractions to make the oil slow down oxidation. This is opposed by the presence of nascent iron debris (from component wear) in the oil. Their presence catalyzes oxidation. The oil oxidizes rapidly, and the life of the base oil is compromised. To neutralize this deleterious catalytic effect, antioxidants are added to the oil. These have the function to digest the radicals as they are generated and thereby restore the original life of the base oil. In the process the antioxidants are consumed, placing a limit on the prolongation of oil life. The action of the amine based scavenger molecules tends to suggest that the latter problem may be overcome and the antioxidant may continuously regenerate itself to promote a very long oil life. In this paper we have included all the three parameters as variablesoil composition and structure, tribology, and antioxidant inductionone at a time to note their impact on
S4). Increasing the antioxidant concentration to above 0.3% v/v brings no further benefit to the sliding interaction. 3.5. XPS Results: Slid Pin Surface. To observe change on the pin surface due to the oxidation of YB4, the slid pin surfaces were examined after 48 h (with and without DMDS), 80 h (with 0.1% ADPA), and 225 h (with 0.5% ADPA) of sliding. The high-resolution XPS spectra of pin samples for C 1s and N 1s core level were recorded and are shown in Figures 10 and 11, respectively. 3.5.1. YB4 without Antioxidant. The C 1s detailed spectrum is fitted into four components at 285 ± 0.1, 286.5 ± 0.1, 288.2 ± 0.1, and 289.2 ± 0.1 eV (Figure 10a). The signal at 285 eV of large intensity corresponds to C−H or C−C of an organic molecule.36−38 The peaks at 286.5, 288.2, and 289.2 eV are assigned to C−O, CO, and OCO bonds, respectively.36−41 3.5.2. YB4 with Antioxidant. The XPS C 1s core level spectra of YB4 with antioxidants (DMDS and ADPA) at 130 1742
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Figure 10. XPS C 1s detail spectra of steel pin lubricated with YB4 at 130 °C (a) without antioxidant, 48 h of sliding, (b) with DMDS, 48 h of sliding, (c) with ADPA (0.1% v/v), 80 h of sliding, and (d) with ADPA (0.5% v/v), 225 h of sliding.
Figure 11. XPS N 1s detail spectra of steel pin lubricated with YB4 at 130 °C (a) with ADPA (0.1% v/v), 80 h of sliding, and (b) with ADPA (0.5% v/v), 225 h of sliding.
base oil plays on autoxidation and tribooxidation, with and without antioxidant. 4.1. Autoxidation. The primary issue we discuss here pertains to the singular aging characteristics of the base oil. The
oil oxidation as well as on friction and wear of the substrate. We note the strong influence of base oil composition and tribology on oxidation and the importance of the antioxidant to generate protective tribo layers. We discuss below the possible roles the 1743
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higher viscosities within the same grade base oil tend to degrade at higher temperatures. 4.2. Tribooxidation. In our previous studies of aging of hexadecane9 it was noted that tribology strongly reduces the inception time for oxidation. A similar effect is observed for YB4 as tribology reduces the CO peak inception time by approximately 3 times (Figure 4). A possible reason for this difference between the process kinetics of autoxidation and the process kinetics of tribooxidation is the catalytic action of soluble iron which gives rise to unstable peroxy radicals.9 Decomposition of an alkyl hydroperoxide molecule occurs at temperatures close to ∼150 °C. Transition metal ions with two valence states such as Fe2+/3+, Pb2+/4+, and Cul+/2+ reduce the activation energy of this decomposition process. These ions must be present as metal soaps; otherwise they are not catalytically active.55 We have noted in Table 3 the formation of carboxylic acid as a product of steel-on-steel sliding interaction with YB4 as a lubricant. The precursor steps of soap formation, shown in reactions 2−5 with Fe as the metallic surface, arise from the attack of the metal surface by alkylperoxy radicals and alkylhydroperoxide.8,56
Table 3. Weight Concentrations (%) of the Components on the Steel Pin Surface in Different Experiments at 130 °C experiment
C−C/C−H
C−O
CO
OCO
YB4 (Tr) (48 h) YB4 + DMDS-1(Tr) (48 h) YB4 + ADPA-0.1(Tr) (80 h) YB4 + ADPA-0.5(Tr) (225 h)
74.39 53.87 57.54 68.14
15.49 25.15 22.58 18.1
5.91 14.67 13.84 9.13
4.19 6.28 6.02 4.62
base oil YB4 ages (or oxidizes) 3 times slower than what is done by a light oil hexadecane under conditions of autoxidation and tribooxidation. Aging studies of pure alkanes suggest that the rate of formation of hydroperoxides (primary oxidation product) increases with increasing concentration of unsubstituted methylene groups and with increasing number of carbons in the oil molecule.32,52 GC−MS analysis of YB4 (see the Supporting Information, Figure S2) shows that it contains a mixture of isoparaffins and n-paraffins of C26−C31 range. For such hydrocarbons one may therefore expect rapid inception of primary oxidation products and once incepted such products to be generated quickly. FTIR results (compare CO peak inception times for hexadecane and YB4 in Figures 3 and 4) presented here show that the base oil not only does not encourage oxidation; it actually slows it down when compared to what is done by a C16 hydrocarbon hexadecane. From the results shown in Figure 5 a clear difference can be noted between the time history of hydroperoxide generation in the hexadecane and that in the YB4 base oil. The following reaction presents the general hydroperoxide generation process where alkyl hydroperoxy radical abstracts hydrogen from the hydrocarbon present in the oil. ROO• + RH → ROOH + R•
Fe 2 + /3 +
2ROOH ⎯⎯⎯⎯⎯⎯⎯→ ROO• + RO• + H 2O
(2)
ROO• + Fe → RO• + FeO
(3)
ROOH + Fe → ROH + FeO
(4)
2ROOH + FeO → Fe(OCOR)2 + H 2O
(5)
However, during oil oxidation under lubrication conditions, the amount of oxygen in oil is probably less than that in the bulk oil as oxygen is consumed in metal surface oxidation. The oil oxidation process accelerates under lubrication condition because of the catalytic action of soluble iron. Comparing the results of autoxidation (Figure 1) and tribooxidation of YB4 (Figure 6), it is seen that iron also supports the formation of alcohols, ketones, and esters. The concentrations of alcohol and ketones are, however, much higher (even when these secondary products start to appear first in the oil) in the tribooxidatively degraded oil (Figure 6) than in that corresponding to autoxidative degradation (Figure 1, where iron is not present). Alcohols react with carboxylic acids (formed by the oxidation of ketones) and generate esters. In tribooxidation, the alcohol production becomes slow after 27 h of sliding and this is the time when the ester formation becomes rapid (Figure 6). This result indicates that alcohols are being consumed in ester formation. The observation is in good agreement with an earlier observation that esterification is favored by the presence of iron.57 Similar results are also reported by Gracia et al.58 The presence of iron (tribooxidation) also dictates the nature/type of lactone generated by oil oxidation and distinguishes it from the type of lactone generated by autoxidation. Figure S3 in the Supporting Information shows that the lactone peak (1785 cm−1) in autoxidatively degraded oil is shifted toward a lower frequency region (1772 cm−1) when the oil is degraded by tribooxidation.59 This FTIR results indicates that lactones generated in tribooxidatively degraded oil are unsaturated lactones in nature.59 Lactones generated in autoxidation are saturated. Figure 5 shows a higher concentration of aldehyde in hexadecane than in YB4 and a higher concentration of ketone in YB4 than in hexadecane. Chemistry of the alkoxy radicals generated in the propagation step of the free radical oil
(1)
The abstraction of a hydrogen atom by the peroxy radical to generate a hydroperoxide is the rate-determining step of oil oxidation.53 The rate of reaction 1 depends on the nature of the hydrocarbon (donor) as well as on the nature of the radical (acceptor). The peroxy radicals are relatively stable, and abstract preferentially only the most weakly bound hydrogen atom. Our previous results9 have shown that hexadecane when heated to 130 °C generates a significant quantity of branched hydrocarbon. This is not surprising when one considers the low boiling point of hexadecane. The branched hydrocarbons, in time, generate an excess of peroxy radicals which take part in rapid hydrogen extraction and hydroperoxide generation. It is also known that the reactivity of alkylperoxy radical strongly depends on its structure, being influenced by steric and polar effects. The reactivity in general increases as the electronwithdrawing capacity of the α-substituent increases. Because of the steric effect, primary and secondary peroxy radicals show a 3−5 times higher reactivity than what is shown by tertiary radicals.53 The fast rate of reaction that we have observed for hexadecane9 suggests generation of substantial quantities of primary and secondary peroxy radicals. We have no direct evidence for it, but the high boiling point of YB4 (in comparison with that of hexadecane) suggests that the peroxy radicals generated on heating of YB4 may mainly be tertiary radicals. This slows down the reaction and reduces the rate of hydroperoxide production. The multicomponent status of base oil1 and enhanced viscosity54 are two other reasons suggested to explain the high stability of base oil. Gamlin et al.54 suggests that base oils with 1744
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Figure 12. Schematic of possible interactions of ADPA with steel surface.
case of ADPA the induction time is only marginally affected by the quantity of the DMDS dispersed in oil. Apart from antioxidant function, an increase in ADPA concentration in YB4 also reduces the wear rate (Figure 8) and coefficient of friction (see the Supporting Information, Figure S4) of steel. ADPA has the ability to bond with iron in two ways: (i) with the lone pair of nitrogen and (ii) with delocalized π-electron aromatic systems of phenyl rings present in the molecule. Hence, it provides protection against wear and reduces the coefficient of friction. XPS results shown in Figure 11 indicate the presence of −C−N−H and −N+− nitrogen species on the pin surface. The nitrogen in −C−N−H form comes from the bonding of the delocalized π-electron aromatic systems of phenyl rings present in the ADPA molecule with iron, whereas −N+− nitrogen species comes from the bonding of lone pair of nitrogen with iron. The presence of a peak at 402.2 eV (N−O species, Figure 11) indicates that N-secalkoxydiphenylamine intermediate, generated during antioxidant action mechanism of ADPA, also bonds with iron. Moreover, in the present case, it is found that delocalized πelectron aromatic systems of phenyl rings present in the ADPA molecule dominate in bond formation with iron as we observe the largest peak area for −C−N−H species in Figure 11. Figure 12 shows a schematic of possible interaction of ADPA with the steel surface. It is possible that such chemical bonding of ADPA, N-sec-alkoxydiphenylamine, and that of carboxylic acid with iron, deposit organic materials on the surface which mask the iron and inhibit its catalytic action in oil in aid of oil oxidation. It is worthwhile to reiterate (Figure 4) at this stage that the antioxidant ADPA is as effective in retarding the base oil aging process in tribology experiments as it is in retarding the process in autoxidation. This, however, may be purely coincidental. We have no definitive evidence at this stage to suggest whichthe radical scavenging action, the masking action of ADPA to inhibit the catalytic action, or a combination of the twois the governing route for the antioxidant. XPS results with the presence of CO, C−O, and OC O components (Figure 10c,d) suggest the existence of oxidative products such as carboxylic acids and esters on the pin surface. All these triboactive oxidative products interact with the worn metal surface and form a protective layer on the surface which results in a low wear rate and a reduced coefficient of friction. While carboxylate soaps are known antifriction compounds,61 we are not aware of any published data on the lubricating properties of ADPA and N-sec-alkoxydiphenylamine intermediate generated by ADPA. The coefficient of friction data shown in Figure 9 suggest that ADPA provides an antifriction protective graft; the trend in friction with time suggests that coverage of such a graft increases with sliding time. This contention is supported by an observation (see the Supporting Information, Figure S4) that the coefficient of friction reduces with increasing concentration of ADPA in YB4.
oxidation mechanism governs the type of oxidative product formation. Secondary alkoxy radicals (in abundance in hexadecane) prefer to form aldehydes, whereas tertiary alkoxy radicals (in abundance in YB4) prefer to form ketones. We have already noted here the possibility that tribooxidation of YB4 generates more tertiary radicals than that in the case of hexadecane. 4.3. Antioxidant. DMDS is an antiwear compound and a friction modifier. The sulfur−carbon bond of DMDS cleaves under extreme pressure (EP) conditions to give an inorganic sulfur-containing layer.60 This inorganic sulfur-containing layer protects the surface from wear (Figure 7) and reduces the friction (0.02 after 40 h sliding) (Figure 9). Disulfides have also been reported as an effective antioxidant. In the present case, however, DMDS (Figure 6) fails as an effective antioxidant. The boiling point of DMDS is 110 °C, which is lower than the present experimental temperature (130 °C). We believe that DMDS evaporates at the experimental temperature (130 °C), resulting in oxidation inception at 6 h, the same as in the case of YB4 alone (Figure 6). ADPA acts as a free radical scavenger. Among all free radical scavenger compounds, the molecule most employed in lubrication is alkylated diphenylamine, because of its high stoichiometric efficiency. The reaction mechanism of ADPA is given in Scheme 1. Jensen et al.24 proposed that during the inhibition of oxidation of secondary radicals by aromatic secondary amines (Ar2NH), at elevated temperatures (above 120 °C), the parent aromatic secondary amine regenerates via the corresponding nitroxyl radical Ar2NO• (Scheme 1). Thus, at high temperatures, one molecule of ADPA can catalytically scavenge a large number of radicals before the nitroxyl radical is destroyed. It has been reported that this regeneration process can provide ADPA with a stoichiometric efficiency of more than 12 radicals per molecule.24 Therefore, a small amount of ADPA (0.1%) is very effective in delaying the oxidation process (Figure 6). An increment in ADPA concentration improves the situation as YB4 starts to oxidize at 60 and 175 h with 0.3% (v/ v) ADPA and 0.5% (v/v) ADPA, respectively (Figure 3). The radical scavenger ADPA competes successfully with the ratedetermining step of the propagation reaction and forms resonance-stabilized diphenylamino radicals from alkylperoxy radicals. Because of resonance stabilization, diphenylamino radicals cannot start new oxidation chains. However, they do react with hydroperoxides and peroxy free radicals to form the nitroxyl free radical, which is also a potent inhibitor. ADPA therefore delays hydroperoxide generation and the oxidation process. Once oxidation commences, the oxidation characteristics (for all secondary products) with time do not change due to the addition of ADPA (from 0 to 0.5% v/v). This implies that ADPA is totally consumed in the induction period. The regenerative character of ADPA is highlighted when its performance is compared with that of DMDS. Unlike in the 1745
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oil. We acknowledge the help given by Mrs. Namita and Mr. H. S. Shamasunder of the IISc in carrying out this work.
The protective layer also inhibits the attack of the substrate iron by other chemically corrosive products of oil degradation. ADPA is a well-known anticorrosion surfactant.62 Addition of ADPA to oil thus reduces the wear of iron (Figure 8). Iron soaps accelerate oxidation and polycondensation/polymerization reactions with increasing concentration.63,64 We note the presence of both C−O and CO and that of iron soaps (OCO) on the pin surface (Figure 10d) after 225 h of sliding. It is possible that this soap induces polymer generation at these times, providing the pin surface with an antiwear film. Such films which come into being only at such times could give rise to sudden drop in wear as is seen in Figure 8.
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(1) Zuidema, H. H. Oxidation of lubricating oils. Chem. Rev. 1946, 38, 197−226. (2) Russell, G. A. Fundamental processes of autoxidation. J. Chem. Educ. 1959, 36, 111−118. (3) Ofunne, G. C.; Maduako, A. U.; Ojinnaka, C. M. Studies on the effects of temperature on the chemical characteristics of automotive crankcase oils and their base oils. Tribol. Int. 1991, 24, 173−178. (4) Challen, J. M.; Oxley, P. L. B. An explanation of the different regimes of friction and wear using asperity deformation models. Wear 1979, 53, 229−243. (5) Black, J. A.; Kopalinsky, E. M.; Oxley, P. L. B. An investigation of the different regimes of wedge slides over a soft surface: the influence of wedge angle, lubrication and prior plastic working of the surface. Wear 1988, 123, 97−114. (6) Sychra, V.; Lang, I.; Sebor, G. Analysis of petroleum and petroleum products by atomic absorption spectroscopy and related techniques. Prog. Anal. At. Spectrosc. 1981, 4, 341−426. (7) Appeldoorn, J. K.; Goldman, I. B.; Tao, F. F. Corrosive wear by atmospheric oxygen and moisture. ASLE Trans. 1969, 12, 140−150. (8) Newley, R. A.; Spikes, H. A; Macpherson, P. B. Oxidative wear in lubricated contact. J. Lubr. Technol. 1980, 102, 539−544. (9) Singh, A.; Gandra, R. T.; Schneider, E. W.; Biswas, S. K. Lubricant degradation and related wear of a steel pin in lubricated sliding against a steel disc. ACS Appl. Mater. Interfaces 2011, 3, 2512− 2521. (10) Maduako, A. U. C.; Ofunne, G. C.; Ojinnaka, C. M. The role of metals in the oxidative degradation of automotive crankcase oils. Tribol. Int. 1996, 29, 153−160. (11) Davis, L. L.; Lincoln, B. H.; Byrkit, G. D.; Jones, W. A. Oxidation of petroleum lubricants. Ind. Eng. Chem. 1941, 33, 339−350. (12) Fenske, M. R; Stevenson, C. E.; Lawson, N. .D.; Herbolsheimer, G.; Koch, E. F. Oxidation of lubricating oilsfactors controlling oxidation stability. Ind. Eng. Chem. 1941, 33, 516−524. (13) Larsen, R. G.; Armfield, F. A. Catalysis in the oxidation of lubricating oil. Ind. Eng. Chem. 1943, 35, 581−588. (14) Colclough, T. Role of additives and transition metals in lubricating oil oxidation. Ind. Eng. Chem. Res. 1987, 26, 1888−1895. (15) Denison, G. H., Jr.; Condit, P. C. Oxidation of lubricating oils mechanism of sulfur inhibition. Ind. Eng. Chem. 1945, 37, 1102−1108. (16) Kajiyama, T.; Ohkatsu, Y. Effect of meta-substituents of phenolic antioxidantsproposal of secondary substituent effect. Polym. Degrad. Stab. 2002, 75, 535−542. (17) Du, D. C.; Kim, S. S.; Chun, J. S.; Suh, C. M.; Kwon, W. S. Antioxidation synergism between ZnDTC and ZnDDP in mineral oil. Tribol. Lett. 2002, 13, 21−27. (18) Gatto, V. J.; Moehle, W. E.; Cobb, T. W.; Schneller, E. R. The relationship between oxidation stability and antioxidant depletion in turbine oils formulated with Groups II, III and IV base stocks. J. Synth. Lubr. 2007, 24, 111−124. (19) Wiklund, P. The response to antioxidants in base oils of different degrees of refining. Lubr. Sci. 2007, 19, 169−182. (20) Gatto, V. J.; Elnagar, H. Y.; Moehle, W. E.; Schneller, E. R. Redesigning alkylated diphenylamine antioxidants for modern lubricants. Lubr. Sci. 2007, 19, 25−40. (21) Cochrac, G. J.; Rizvi, S. Q. A. In Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing; Totten, G. E., Westbrook, S. R., Shah, R. J., Eds.; Manual Series MNL37WCD; ASTM International: West Conshohocken, PA, 2003; p 787. (22) Hsu, S. M.; Ku, C. S.; Pei, P. T. Oxidative degradation mechanisms of lubricant. In Aspects of Lubricant Oxidation; Stadmiller, W. H., Smith, A. N., Eds.; ASTM STP 916; ASTM International: West Conshohocken, PA, 1986; p 27. (23) Rizvi, S. Q. A. Lubricant additives and their functions. In Friction, Lubrication, and Wear Technology; Blau, P. J., Vol. Chairman;
5. CONCLUSIONS 1. Aging of a base oil consisting of many paraffinic fractions is significantly slower than the aging of a pure n-alkane. One of the reasons for this effect, we suggest, may be the fact that the former generates with time and heating more tertiary radicals than what is done by the pure alkane which on heating predominantly generates primary and secondary radicals. 2. Steel-on-steel tribology accelerates the aging of heated base oil over that observed when the oil is heated in a flask open to the ambient. This acceleration of aging has been attributed to by earlier workers and the present authors, when the lubricant was hexadecane, to the catalytic action of iron. 3. Dimethyl disulfide because of its low boiling point (110 °C) is an ineffective agent to prevent aging of base oil heated to 130 °C. 4. Alkylated diphenylamine is an effective antioxidant in tribology conducted using hot oil. The present work, however, is not conclusive on whether it is radical scavenging or masking of catalysis by iron which is the more important driver of antioxidant action. 5. Secondary oxidation products in the presence of ADPA generate a protective tribofilm, which can over a time frame of 120 h reduce the coefficient of friction to 0.015 and the wear rate of the pin to a value close to zero.
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1, FTIR spectra of mineral oil Yubase 4; Figure S2, GC chromatogram of mineral oil Yubase 4; Figure S3, FTIR of YB4 (without antioxidant) degraded by autoxidation and tribooxidation process; Figure S4, plot of coefficient of friction as a function of time for tribooxidation of YB4 with varying concentration of ADPA at 130 °C. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Tel.: +918022932589. Fax: +918023600648. E-mail: skbis@ mecheng.iisc.ernet.in. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to General Motors (R&D) Warren, MI, for providing the financial support that has made this work possible. We gratefully acknowledge Prof. Colin D. Bain and Dr. Jackie Mosely of the University of Durham, U.K., for our discussion related to this work and GC−MS analysis of the base 1746
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