Ind. Eng. Chem. Prod. Res. Dev. 1986, 2 5 , 621-627 Stone, J.; Burrus, C. A.; Wiesenfeld, J. M. Appl. fhys. Left. 1984, 4 5 , 212. Stone, J.; Chraplyvy, A. R.; Burrus, C. A. Opt. Lett. 1982, 7 , 297. Stone, J.; Wiesenfeld. J. M.; Marcuse, D.; Burrus, C. A.; Yang, S. Appl. fhys. Lett. 1985, 4 7 , 328. Stone, J.; Wlesenfeld, J. M.; Marcuse, D.; Burrus, C. A,; Yang, S., unpublished results. Uchida, N.; Uesugl, N.; Murakaml, Y.; Nakahara, M.; Tanifiji. T.; Inagaki, N. Ninth European Conference on Optical Communications, Geneva, Swttzer-
62 1
land, Oct 23-28, 1983; postdeadline paper. Urbach, F. fhys. Rev. 1959, 92, 1324. vanderSteen, G.H. A. M.; Papanlcholau, E. Wl//psRes. Rep. 1975, 30, 192. Wakafen, 0. E.; Samantha, S. R. J . Chem. Phys. 1978, 69, 493.
Received for review March 27, 1986 Accepted May 14,1986
Determination of the High-Temperature Antioxidant Capability of Lubricants and Lubricant Components Stefan Korcek;
Mllton D. Johnson, Ronald K. Jensen, and Mlklo Zlnbo
Research Staff, Ford Motor Company, Dearborn, Mlchigan 48 12 1
A laboratory procedure for assessment of the antioxidant capabilities of engine oils, base oils, and additives at elevated temperatures under conditions simulating those encountered in Internal combustion engines has been developed. The method includes gradual addition of hydroperoxides or hydroperoxide-producingspecies during the test in order to simulate their continuous formation in engine oils as a result of the interaction of combustionderived free radicals with the lubricant during engine operation. The procedure detects both radical-trapping and hydroperoxidedecomposing antioxidant species. Examples from evaluations of antioxidants (primary and secondary alkyl zinc dialkyl dithiophosphates and 2,6di-fe~-butyl-4-methylphenol),base oils, and new and used engine oils are presented.
Introduction Oxidation is one of the most important processes causing degradation of engine oils during service. Oil oxidation leads to formation of acidic products, insoluble materials, and sludge, depletion of additives, loss of dispersancy, increase of viscosity, etc. All of these undesirable changes are, however, also affected by other concurrent processes occurring in an operating engine such as thermal degradation, mechanochemical reactions, metal catalysis, and interactions with combustion products which result in nitration and hydrolysis. Contributions of such processes to degradation are the main reason that correlation of engine test results with results of laboratory oxidation tests is not always successful. Nevertheless, proprietary laboratory oxidation tests are often used by engine oil formulators in predicting directional trends and approximate engine test performance since improvement of engine oil resistance toward oxidation leads directionally to improved performance in engine testing. This work was not directed toward development of another of these tests; rather, the focus is understanding oxidation processes occurring in engines and using this knowledge in development of procedures that would evaluate various parameters contributing to oxidation stability of engine oils. Oxidation properties of an engine oil are determined by its composition. In this respect, contributing factors are compositions of base oils, additives, and additive diluent oils. Particularly important are the presence and antioxidant properties of synthetic antioxidant additives and of natural inhibitors in base and diluent oils. Consequently, antioxidant capability is one of the most important technological parameters determining the oxidation stability of engine oils. A procedure for determining the radical-trapping antioxidant capacity of new and used lubricants was published previously (Mahoney et al., 1978). That method 0196-432 lI86/1225-0621$01.50/0
provides useful information and has been applied to investigations of antioxidant consumption in engine oils during laboratory and service evaluations (Korcek et al., 1979,1981;Mahoney et al., 1980; Murray et al., 1982; Hsu et al., 1982). The method, however, measures only radical-trapping capacity at low temperature and, thus, does not reflect the contribution of peroxide-decomposing species or natural inhibitors that can be formed from oil components during oxidation; these types of species can play important roles at elevated temperatures. A new technique that is capable of detecting all types of antioxidants and which is conducted under oxidizing conditions simulating those in internal combustion engines has been developed and is described in this paper. A model for oxidation of oils in internal combustion engines on which this method is based will be discussed, principles of the experimental procedure will be presented, and application of the method to assessment of high-temperature antioxidant capabilities (HTAC) of antioxidants, base oils, and fully formulated engine oils will be described. Oxidation Model A model for oxidation of oils in internal combustion engines is shown in Figure 1 (Johnson et al., 1983). Engine oil oxidation is initiated by free radicals, which are derived either from combustion or from decompositionof primary oxidation produds such as hydroperoxides, ROOH. These free radicals react with the oil, RH, to abstract hydrogen and form alkyl radicals, R',which in the presence of oxygen form peroxy radicals, ROz'. In the absence of antioxidants, peroxy radicals further react with additional oil to form hydroperoxides (ROOH) and alkyl radicals (R*). This continues in a chain reaction process, which can result in the formation of a high concentration of hydroperoxides. The chain reaction process can be inhibited by adding radical-trapping antioxidants (AH) to the oil. In that case, 0 1986 American Chemical Society
622
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
----
COUBUSTIOY -T--
,
r
0 or A? 2
,
,
DECOMPOSING
PREOXIOIZED HD :.:...//.....
.,..............
.......................................
CONTINUOUS ADDITION
t
Figure 1. Model for oil oxidation in internal combustion engines.
peroxy radicals react preferentially with the radical-trapping antioxidant to form nonradical products and keep the hydroperoxide concentration low. Formation of hydroperoxides is followed by radical formation from thermal and/or catalytic decomposition of hydroperoxides, which initiates and accelerates the oxidation. This initiation process can be prevented by the addition of peroxide-decomposing antioxidants, which convert hydroperoxides into nonradical products. Thus, to protect the oil against oxidative degradation in service, both types of antioxidants, radical trapping and peroxide decomposing, should be used in engine oil formulations. Hindered phenols and amines are typical representatives of the first type of antioxidants, while zinc dialkyl dithiophosphates (ZDTP) are believed to react by both mechanisms. In some engine oils ZDTPs are the only antioxidants used, while in other oils they are supplemented by other antioxidants, such as amines and hindered phenols. Natural inhibitors act mostly as peroxide-decomposing antioxidants. Their antioxidant properties, however, are very much dependent on the crude oil source and the method and degree of refining. From the above model for oxidation of oils in internal combustion engines, it can be seen that hydroperoxide products (ROOH) are continuously formed in engine oils during engine operation due to a continuous influx of free radicals from the combustion process even in the case when oxidation is inhibited by free-radical-trapping antioxidants. Since hydroperoxides, at elevated temperatures, initiate and accelerate further oxidation, their decomposition by peroxide-decomposingantioxidants is not only a very important inhibition process but also an important process of antioxidant consumption. Therefore, in order to simulate oxidative conditions encountered in operating engines, the antioxidant capabilities of engine oils, engine oil components, base oils, and additives should be evaluated under conditions of continuous influx of free radicals and continuous formation of peroxidic compounds at elevated temperatures. Experimental Section Materials. The antioxidants evaluated were a primary alkyl ZDTP, zinc di-n-octyl dithiophosphate [ ((CsH,70)2P02)zZn, n-CgDTP]; a secondary alkyl ZDTP, zinc diisopropyl dithiophosphate [ (((CH3)2CH0)zPS2)zZn, i-C3ZDTP]; and a hindered phenol, 2,6-di-tert-butyl-4methylphenol (MPH). n-CsZDTP was synthesized in our laboratory via the procedure previously described (Willermet et d.,19791, i-C3ZDTPwas obtained from another laboratory, and MPH (Aldrich Chemical Co.) was further purified by recrystallization from methanol. Hexadecane (Aldrich, 99+ %) was purified by a column chromatographic procedure, which was described in detail
REACTION MIXTURE
I
PERIODIC WITHDRAWAL
/
Figure 2. Schematic drawing of test cell.
elsewhere (Jensen et al, 1979). Oxygen was Matheson UHP (min 99.99% purity), and argon was Matheson grade (min 99.9995% purity). Preoxidized n-hexadecane was prepared at 160 "C by using the stirred flow reactor technique (Jensen et al., 1979) with n-hexadecane flowing at 1 mL/min and O2 at 330 mL/min such that the residence time was ca. 400 s and the resulting hydroperoxide concentration was ca. 14 mM. Base oils evaluated were obtained from different crude oil sources. No attempt was made to analyze and chemically characterize these base oils in detail. BO-1 was a mixture of lOOSN and 150SN oils containing 0.15% sulfur, and BO-2 was a 150SN oil containing 0.80% sulfur. Engine oils used in this study were EO-1, commercial 1OW-30 SE/CC; EO-2, commercial 5W-20 SF/CC; EO-3, experimental 15W-40 formulated from BO-1 and DI package A; EO-4, commercial 1OW-40 SF/CC formulated from a base oil similar to BO-2 and DI additive package A; EO-5, experimental 15W-40 formulated as EO-3 plus supplementary antioxidant; and EO-6, experimental 1OW40 SE/CC. Method. High-temperature antioxidant capabilities of additives, base oils, and fully formulated engine oils were evaluated by using a model hydrocarbon oxidation system in which n-hexadecane (HD) was oxidized at 160 "C while preoxidized n-hexadecane containing hexadecyl hydroperoxides was continuously introduced. This simulates both engine operational temperatures and the continuous formation of hydroperoxides in an engine due to interactions of combustion-derived free radicals with the oil. Antioxidant capabilities of the above materials were assessed from the delay in the onset of rapid oxidation of n-hexadecane. That delay is attributable to antioxidant species present in, or formed from, the materials evaluated. The end of the delay was determined from a sudden and continuous increase in the rate of formation of hydroperoxides or total oxidation products or from an increase of reaction temperature. The experiments were run in a batch reactor (Figure 2) constructed from Pyrex glass with eccentric spheres and a gas inlet and a larger opening for addition of test materials and removal of samples. The test apparatus, including constant-temperature bath and gas flow system, was described in detail previously (Jensen et al., 1979). The argon or oxygen is directed into the inner sphere and
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1988
then through small holes into the test liquid such that thorough mixing occurs. Another inlet tube consisting of a glass tube with a capillary bore was used to provide a means for addition of preoxidized hexadecane from a reservoir using a Chromatronix Cheminert metering pump. Samples were removed by using disposable pipets, placed in vials, and quenched in an ice bath. The temperature of the reaction mixture was monitored by using a glasssheathed thermocouple. The test materials were added to n-hexadecane while the system was being flushed with argon and allowed to come to reaction temperature. Addition of oxygen and a continuous flow of peroxidized n-hexadecane was then begun, and the tests proceeded in one of two general modes. In mode I samples of reaction mixture for analyses were periodically withdrawn at the same average rate as preoxidized n-hexadecane was added. Typically, 40 mL of solution were present in the test cell initially. A 2-mL sample was withdrawn prior to the beginning of hydroperoxide addition and each time the reactor volume again reached 40 mL, so the volume in the cell cycled between 38 and 40 mL. This resulted in withdrawal of certain amounts of hydroperoxidesand also of evaluated materials from the system during the experiment. In mode I1 the initial volume in the reactor was typically 10 mL, and preoxidized n-hexadecane was added at a slower rate; no samples were withdrawn, so all evaluated materials and added hydroperoxides remained in the system and the beginning of uninhibited oxidation was determined only from temperature increase. Samples collected during mode I tests can be analyzed to provide detailed information regarding reactions that are occurring. Results reported for hydroperoxide concentrations, [-OOH], were obtained by using an iodometric titration procedure (Jensen et al., 1979). Concentrations of antioxidants and their reaction products were monitored by using HPLC which was conducted by using a 10-pm C18 radical pak cartridge (Waters Associates). The initial solvent was a 10/(10/10)/70 blend of CH2C12/(H20/ MeOH)/CH,CN flowing at 2 mL/min. Four minutes after injection, the composition was changed during a 4-min period to 30/0/70. A Model 750 solvent delivery system and a Model 752 ternary gradient programmer (both Micromeritics) were used. A Model 165 (Beckman) variable-wavelength ultraviolet detector was employed, and data handling was via a Perkin-Elmer Model 3600 data station. Peaks corresponding to ZDTP and ZDTP transformation products were monitored at 254 pm and MPH was monitored at 280 pm. Alternate Methods. The specific examples described in this paper were generated via the procedure and apparatus described above. Monitoring the effects of oils and oil components on the oxidation of hexadecane provides a means of assessing the observed effects in a well-characterized model system. Alternatively, the procedure and apparatus can be modified to simulate the continuous influx of radicals by various means, such as continuously adding low levels of suitable free-radical initiators, pure hydroperoxides, exhaust gases, or oxides of nitrogen, and to include other factors that may affect the oxidation such as metal catalysts. Also, it may be possible to evaluate oils and components in other suitable systems or even to evaluate them without dilution. These are areas for further investigation. Interpretation of Results Typical results from the measurement of high-temperature antioxidant capabilities using mode I experiments are presented schematically in Figure 3, where hydroper-
623
NO INHIBITION
[-OOH]
TIME Figure 3. Typical results from HTAC testing.
oxide concentration, [-OOH], and increase in reaction temperature, AT, are plotted as functions of reaction time. In the [-OOH] plots, an interrupted curve labeled ADDITION represents the hypothetical accumulation of hydroperoxides due to continuous addition of preoxidized n-hexadecane. Experimental data below this addition curve indicate that hydroperoxides added in the reaction solution are undergoing decomposition reactions. If [-OOH] increases above the addition curve, hydroperoxides must be formed in the reaction system due to oxidation reactions initiated by the formation of free radicals. In AT plots, any increase in temperature indicates exothermic reactions, and a decrease in temperature indicates endothermic reactions. If the decrease occurs during the experiment at a higher temperature than the initial one, such decrease could also indicate the end, or slowing, of exothermicreactions or a cooling of the mixture due to heat exchange with the constant-temperature bath. A rapid continuous increase in temperature, however, indicates the end of inhibition, and the onset of this increase indicates the beginning of chain reaction oxidation since oxidation reactions are exothermic. The uninhibited oxidation of n-hexadecane (NO INHIBITION) proceeds very rapidly due to homolytic decomposition of hydroperoxides (reaction l ) , which leads ROOH RO' + 'OH 02 (1) RO' + RH ROH R' ROz'
-
+
-
to formation of free radicals and initiation of oxidation. In the presence of a radical-trapping antioxidant (AH) such as 2,6-di-tert-butyl-4-methylphenol (MPH), oxidation is inhibited due to reactions of peroxy radicals (RO,') with antioxidant (reactions 2 and 3) until all antioxidant is AH + RO2' ROOH + A' (2)
A'
--
+ RO2'
AOOR
(3)
consumed and uninhibited oxidation begins. If reactions 1-3 are the only reactions involving hydroperoxides during the inhibited oxidation and AOOR is an inactive product, as it is at lower temperatures, then [-OOH] should follow the addition curve since each molecule of hydroperoxidedecomposed by reaction 1leads to formation of one molecule of hydroperoxide by reaction 2. The net result of these reactions is no change in hydroperoxide concentration over that resulting from hydroperoxide addition. In the actual case, however, a slight decrease of hydroperoxide concentration relative to that
824
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
Table I. Antioxidant Capabilities of MPH, n-CBZDTP,and i-C8ZDTPat 160 O C initial concn, mM run mode initial vol, mL MPH n-C8ZDTP i-C3ZDTP 1 I 40 1.11 2 1.04 3 1.09 4 1.13 1.05 5 1.10 1.05 6 4.20 4.26 1.06 8 1.07 4.38 I1 10 2.52 9 2.52 2.51 10 2.50 11 2.47 2.53 4.94 12
preox HD add. rate, mL/min [-OOH], mM 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.30 0.30 0.30 0.30
7
r, s
14.2 13.7 14.3 14.2 14.2 13.7 13.7 14.3 13.8 13.8 13.8 13.8
3500" llOOb llOOb
3200" 3500" 6000b 5800b 6100b 4500b 4200b 4700b 5700b
"From [-OOH], AT, and [MPH] plots. bFrom AT plots.
given by the addition curve (Figure 3) is observed in the presence of a radical-trapping antioxidant. This observation can be explained by second-order and/or induced decomposition of hydroperoxides or by preferential and fast decomposition of certain types of hydroperoxides present in added preoxidized n-hexadecane. In the presence of both peroxide-decomposing and radical-trapping antioxidants, such as combinations of ZDTP and MPH, hydroperoxide concentration must be lower than that corresponding to the addition curve when hydroperoxide decomposition processes prevail over processes of their formation. The overall effect of peroxidedecomposing antioxidants on the oxidation process and formation of total oxidation products, however, will depend on the type of hydroperoxide decompositionreactions and on the formation of free radicals in these reactions. Unlike the low-temperature antioxidant capacity procedure, which gives a quantitative measure of radicaltrapping antioxidant concentration by determining the number of peroxy radicals trapped, this procedure does not yield an exact quantity relatable to specific chemical reactions. This arises because the combined effects of peroxide decomposition and radical trapping are being assessed. The rate of initiation is not constant because of the addition of hydroperoxides and their decomposition during the test. Some of the hydroperoxide-decomposing reactions generate radicals while others do not. Also, not all of the hydroperoxides are decomposed. Relative HTAC at given experimental conditions can be expressed from inhibition time as, e.g., moles of hydroperoxide added/unit of sample, seconds, or seconds/unit of sample. Comparisons are valid only for values determined under identical conditions.
MPH
6
n-CgZDTP t MPH
I-C ZDTP t MPH 3
I-C.ZDTP
I
t MPH
A
'
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A
A
n
n
TITP
/
1
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Results and Discussion Antioxidants. High-temperature antioxidant capabilities of IVIF", representative primary and secondary zinc dialkyl dithiophosphates, n-C8ZDTP and i-C,ZDTP, respectively, and their combinations have been investigated by using both mode I and mode I1 experiments (Table I). Results obtained with each of the antioxidants at the same initial concentration of ca. 1.1 mM and for equimolar mixtures of the ZDTPs with MPH using mode I experiments (Figure 4) show that under these conditions, on an equimolar basis, antioxidant capabilities of ZDTPs are lower than those of MPH (see inhibition times, 7,in Table I). The [-OOH] plots for combinations with MPH show that i-C,ZDTP exhibits greater overall peroxide-decomposing capability than n-C,ZDTP. It is not only the original ZDTP that decomposes hydroperoxides since ZDTP, as such, is consumed very rapidly in the early stages of inhibition. For example, under
I
n-C ZDTP t MP IKQ\~C~ZDTP
8
t MPH
-A\ 1000
2000
3000
TIME /
4000
I1
5000
s
Figure 4. Antioxidant capabilities of MPH, ZDTP, and equimolar ZDTP + MPH combinations (mode I, runs 1-5) and changes in antioxidant concentrations.
our experimental conditions the ZDTPs investigated were completely converted to the corresponding dialkyldithiophosphoryl disulfides (DS) and presumably also to other species at ca. 750 s. These intermediates, and possibly also other products formed from them, have additional peroxide-decomposingcapabilities. This can be clearly seen in the case of n-C8ZDTP, which exhibits two distinguishable peroxide-decomposingstages. With i-C,ZDTP these
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 825 2.0
Table 11. Antioxidant Capabilities of Engine Oils at 160 "C: Experimental Conditions addition initial vol, preox mL HD, [-OOH]in finure mode HD EO mL/min HD, mM 0.60 13.7 2.4 37.6 6 I 14.0 2.0 0.30 7 I1 8.0 0.60 15.0 2.4 37.6 8 I
0
.. c a
I1
9
12
+ MPH
stages are not obvious. This is in agreement with previous observations (Johnson et al., 1983, 1986). Despite the substantially lower hydroperoxide concentration during inhibition in the presence of ZDTPs, the rate of initiation is not decreased as would be expected if reaction 1 were the only radical-producing reaction. The times at which MPH is completely consumed and inhibition periods determined from [*OH] and AT curves (Figure 4 and Table I) are for i-C,ZDTP MPH the same as for MPH alone, and for n-C8ZDTP MPH they are even shorter than those for MPH alone. This suggests that in the presence of both ZDTP and MPH free radicals are produced not only from reaction 1but also from ZDTP-induced decomposition of hydroperoxides. At equimolar concentrations, in the case of i-C3ZDTP MPH, the free-radical production from ZDTP-induced decomposition equally compensates the decrease in radical formation from reaction 1which occurs due to decrease in hydroperoxide concentration. In the case of n-C8ZDTP MPH the former radical production must be greater than the latter decrease in radical formation. From these data, it is not obvious whether ZDTPs and intermediates formed from them are also trapping free radicals. If so, then production of free radicals from ZDTP-induced decomposition is even greater than that described above. On the basis of previous studies (Johnson et al., 1986) it seems fair to assume that such radical trapping by ZDTP exists under the present reaction conditions. The results described above were obtained from mode I experiments in which substantial amounts of antioxidants are withdrawn from the reaction mixture when samples are withdrawn for analyses (see dilution line in Figure 4, [MPH] part). This complicates interpretation of results and conceivablycan also affect thew. To verify effects of ZDTPs on inhibition in the presence of MPH, the mode I1 experiments were performed (Table I, runs 9-11; Figure 5,1:1 runs). In these experiments, inhibition periods from AT were determined without any sample withdrawal. Results obtained in this manner confirmed the overall findings from mode I experiments. The presence of ZDTPs in an equimolar amount is not adding to the high-temperature antioxidant capability of MPH under the conditions of gradual hydroperoxide addition. The same conclusions as those obtained with equimolar mixtures of ZDTPs and radical-trapping antioxidants can be made for antioxidant combinations where an excess of radical-trapping inhibitor is added. Antioxidant capabilities were not improved when the initial concentration of MPH was increased to ca. 4.3 mM while that of ZDTP
1'
4
1.61
EO-(
a
+ +
. I
4::hw /. , , ,
-1000
+
+
13.7
t
z l
\
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s Figure 5. Antioxidant capabilities of MPH and ZDTP combinations (mode 11, runs 9-12). TIME
0.10
1.0
29.0
,
2000
,
,
TIME I
~4000. ,
ANA
,
,
3000
,
,
. ,
,
,
5000
5
Figure 6. Antioxidant capabilities of engine oils (mode I; see Table
11).
remained at ca. 1.1mM (Table I, runs 6-8). An improvement in antioxidant capabilities could be obtained, however, when the concentration of ZDTP is greater than that of MPH. For example, when the concentration of i-C3ZDTP was greater by a factor of 2 than that of MPH, an increase in inhibition time from 4500 s (for MPH only) to 5700 s was observed (Table I, runs 9 and 12; Figure 5, i-C3ZDTP + MPH [2:1] vs. MPH). Presumably, maximum improvement could be achieved if the concentration of i-C3ZDTP would be such that all hydroperoxide added or formed would be decomposed by ZDTP during the entire inhibition period. This, however, needs to be further confirmed. Engine Oils. Determination of high-temperature antioxidant capabilities of engine oils should not be mistaken for an oxidation test, which predicts performance in engine tests such as the Sequence IIID test (ASTM, 1980) used in assessment of the resistance of engine oils to oxidative degradation as indicated by viscosity increase. Rather, high-temperature antioxidant capability should be viewed as an engine oil characteristic which can be used for mr iy practical purposes. Some examples of such practical e are described below. For illustration, typical [-OOH] and AT plots obtained with two types of engine oils using the mode I procedure (Table 11) are shown in Figure 6. Oil EO-1 is a typical representative of mineral oils containing only ZDTP type antioxidant, while EO-2 is a synthetic hydrocarbon engine oil containing both ZDTP and a supplementary radicaltrapping inhibitor. These results and results obtained with other oils (see below) suggest that determination of anti-
626
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 I '
A ~
3013
'
'
'
'
I
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580
.\.
MILES
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'
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i / A'
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,
.
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,
.
, 5000
TIME I s
Figure 9. Antioxidant capabilities of used engine oils (mode 11;see Table 11).
. . I'
8
1E-
8 i
l
'
1
1
-
8
i
+
.
*
+
-1
-r
6300 7
h"dE
i
i
Figure 8. Antioxidant capabilities of engine oils (mode I; see Table 11).
oxidant capabilities can be used in "fingerprinting" engine oils and assessing consistency in engine oil formulation. Additives, Base Oils, and Engine Oils. BO-1 and a base oil claimed to be similar to BO-2 were used in blending two engine oils, EO-3 and EO-4, containing the same amounts of an identical dispersant-inhibitor additive package. AT curves for these oils (Figure 7) show that the antioxidant capability of EO-3 formulated from BO-1, is substantially greater than that of BO-1, while the antioxidant capability of EO-4 is much lower than that of BO-2. This is the only time we have observed in our work that the resulting engine oil exhibits much lower antioxidant capability than the base oil from which it is blended. Thus, this result casts some doubt on the similarity of the base oil used in formulation of EO-4. Although it is possible that "dilution" by VI improvers and diluent oil could decrease antioxidant capability of the original base oil, it is not likely that such decrease would have such a magnitude. Nevertheless, the data show that base oil composition plays a very important role in determining antioxidant capabilities of the final product. This can be further confirmed by the results of the Sequence IIID tests in which EO-4 passed the requirement limiting the viscosity increase while EO-3 failed it. The use of additional supplementary radical-trapping inhibitor, however, gave an oil, EO-5, that passed Sequence IIID testing and also exhibited increased antioxidant capability (Figure 8). Despite the fact that these particular formulations showed similar trends in our antioxidant capability test as in the Sequence IIID test, we would like to point out that such correlation should not be generalized. Base oils of different origin and additives of different composition
may behave differently, and engine oils exhibiting similar high-temperature antioxidant capabilities may not necessarily perform similarly in the Sequence IIID test. Used Engine Oils. In previous investigations (Korcek et al., 1979,1981; Mahoney et al., 1980) the peroxy radical titration method (Mahoney et al., 1978) was used to follow the decay of low-temperature antioxidant capacity in used engine oils. That method yielded information on residual content of radical-trapping inhibitors. The present method provides similar information which, however, includes both radical-trapping and peroxide-decomposing high-temperature antioxidant capabilities. High-temperature antioxidant capabilities of engine oils in laboratory, engine, and service testing decrease with time or mileage accumulation. For illustration, results of measurements of high-temperature antioxidant capabilities of used engine oils obtained from vehicle testing under unique high-severity conditions are shown in Figure 9. Inhibition times (fiist increase of temperature) for oil EO-6 after 580 mi dropped from the original value of 3300 to 1950 s and at 3013 mi further decreased to 1000 s. Thus, this method can be utilized to monitor changes of antioxidant capabilities during testing or service. Summary High-temperature antioxidant capability is one of the most important parameters characterizing oxidation properties of engine oils. In this work, a laboratory method for assessment of high-temperature antioxidant capabilities has been developed and used in evaluation of antioxidants, base oils, and engine oils. The evaluation was performed by using a model hydrocarbon oxidation system in which n-hexadecane was oxidized at 160 "C while preoxidized n-hexadecane containing hydroperoxides was continuously introduced. This approximates temperatures experienced by the oil in the piston-cylinder area of an operating internal combustion engine and simulates continuous formation of hydroperoxides due to interactions of combustion-derived free radicals with oil components. The technique provides information on both radical-trapping and peroxide-decomposing antioxidants present or formed in the above materials. The mode I procedure provides samples for analysis that can yield detailed information about the chemistry of the system being studied, while mode I1 is a simpler procedure which allows qualitative comparison of various components or formulations. Investigations with a radical-trapping antioxidant (2,6di-tert-butyl-4-methylphenol(MPH)) and representative primary and secondary zinc dialkyl dithiophosphates (ZDTPs) (zinc di-n-octyl dithiophosphate (n-C8ZDTP)and zinc diisopropyl dithiophosphate (i-C,ZDTP), respectively) lead to the following conclusions:
627
Ind. Eng. Chem. Prod. Res. Dev. lB86, 25, 627-630
On an equimolar basis, ZDTPs exhibit lower antioxidant capabilities than MPH. Antioxidant capabilities of equimolar mixtures of ZDTPs and MPH are determined by capabilities of MPH and are not improved by the presence of ZDTPs despite a significant decrease of hydroperoxide concentration due to ZDTP-induced decomposition. This behavior of ZDTPs could be explained by free radical formation from ZDTP-induced decomposition of hydroperoxides and by depletion of ZDTP before MPH capabilities are exhausted. Antioxidant capabilities of MPH can be improved by addition of i-C,ZDTP in an amount greater than the amount of MPH. It was not determined what this limiting amount must be; however, when the amount of i-C3ZDTP was greater by a factor of 2 (on a molar basis), significant improvement of antioxidant capabilities was observed. Similarly, the method can be used to characterize the antioxidant capabilities of base oils, to assess consistency of engine oil formulation, and to determine oxidation properties of engine oils. The effects of base oil-additive package interactions and formulation changes on hightemperature antioxidant capability can be investigated.
Also, the degree of oxidative degradation of used oils during service or laboratory testing can be monitored. Registry No. MPH, 128-37-0;n-C8ZDTP, 7059-16-7;iCSZDTP, 2929-95-5. Literature Cited ASTM Spectral Technical Publication, 315H, Part 2, "Multlcyllnder Test Sequences for Evaluating Automotive Englne Oils"; American Society for Testing and Materials: Philadelphia, PA, 1980. Hsu, S. M.; Ku, C. S.;Becker, D. A. SA€ Tech. Pap. Ser. 1082, No. 821240. Jensen, R. K.; Korcek, S.;Mahoney, L. R.; Zinbo, M. J . Am. Chem. SOC. 1870, 101 7574-7584. Johnson, M. D.; Korcek, S.; Zinbo, M. SA€ Tech. Pap. Ser. 1083, No. 83 1 684. Johnson, M. D.; Korcek. S.; Zinbo, M. ASLE Trans. 1986, 2 9 , 136-140. Korcek, S.; Mahoney, L. R.; Johnson, M. D.; Hoffman, S. SA€ Trans. 1070, 8 7 , 3568-3596. Korcek, S.;Mahoney. L. R.; Johnson, M. D.; Siegl, W. 0. SA€ Tech. Pap. Ser. 1081, No. 810014. Mahoney, L. R.; Korcek, S.;Hoffman, S.; Willermet, P. A. Ind. €ng. Chem. Prod. Res. Dev. 1878, 17, 250. Mahoney. L. R.; Otto, K.; Korcek, S.;Johnson, M. D. Ind. Eng. Chem. Prod. Res. Dev. 1080, 19, 11-15. Murray, D. W.; Clarke, C. T.; MacAlpine, G. A.; Wright, P. G. SA€ Tech. Pap. Ser. 1082 No. 821236. Wlllermet, P. A. ASLE Trans. 1070, 22, 301-306. I
Received for review March 28, 1986 Accepted July 31, 1986
Corrosivity of Diethanolamine Solutions and Their Degradation Products Amitabha Chakma and Axel Meisen" Department of Chemical €ngineering, The University of Britlsh Columbia, Vancouver, British Columbia V6T 1 W5, Canada
Weight loss and potentiodynamic tests were performed to determine the corrosivity of AISI-ASE 1020 carbon steel in aqueous solutions containing diethanolamine (DEA) and/or its principal degradation products. The solutions were either free from or saturated with CO,. The amine solutions were found to be corrosive in the presence of CO,, and corrosion rates at 100 O C and atmospheric pressure r a n w from approximately 1.6 to 2.1 mm/year for DEA concentrations of 30-60 wt %, respectively. 3-(Hydroxyethyl)-2-oxazolidone, which is a major degradation compound of DEA, was found to be most corrosive. The corrosivity of DEA solutions in the presence of C02 is explained qualitatlvely in terms of the reduction in solution pH and the enlargement of the corrosion region in the Pourbaix diagram due to metal complexing.
Introduction Corrosion is a major problem for alkanolamine-based gas-sweetening planta since it may Tesult in metal failure (especially in heat-exchanger tubes and absorber/regenerator trays), equipment fouling, and foaming. Diethanolamine (DEA) plants generally experience less corrosion than mqmoethanolamine plants (Kohl and Riesenfeld, 1979), but corrosion is also a concern for DEA planta (Polderman and Steele, 1956; Moore, 1960; Fitzerald and Richardson, 1966; Smith and Younger, 1972; Hall and Barron, 1981). It is now well established (McMin and Farmer, 1969; Maddox, 1977) that the corrosion rate increases with temperature and "acid gas loading", i.e., the concentration of C02 and H2S in solution. Industrial solutions usually contain, apart from DEA, water, and absorbed acid gases, significant quantities of amine degradation products. The latter are formed by irreversible reactions which DEA undergoes mainly with C02. The 0196-4321/06/ 1225-0627$01.50/0
corrosivity of degradation products is a matter of considerable controversy and practical importance. In 1956 Polderman and Steele reported simple experiments which indicated that the products corrode steel. Moore (1960) subsequently published some industrial data and showed that the corrosion rate increases with the concentration of degradation products. Values as high as 1 mm/year (40 mpy) were found for carbon steel. The corrosivity of degradation products has also been reported by others (Smith and Younger, 1972; Nonhebel, 1972). However, Blanc et al. (1982) recently published data in support of their claim that DEA degradation products are not corrosive under conditions typically encountered in gas-treating plants. Their main argument was that the pH of a DEA solution of 30%-all percentages used in this paper are weight percent-(or 3 mol/L) lies between 11.5 and 10 for temperatures ranging from 20 to 100 "C; the precise pH value depends on the concentration of the 0 1986 American Chemical Society
