1754
Ind. E n g . C h e m . Res. 1987, 26, 1754-1757
High-Temperature Antioxidant Capabilities of Base Oils and Base Oil-Additive Mixtures Milton D. Johnson,* Stefan Korcek, and Mikio Zinbo Ford Motor Co., Dearborn, Michigan 48121
Radical-trapping and peroxide-decomposing antioxidant capabilities of selected lubricant base oils and base oil-antioxidant additive combinations have been evaluated a t 160 “C by using a test procedure which simulates engine operating conditions. High-temperature antioxidant capabilities, HTACs, of base oils were found to vary in type as well as amount ranging from predominantly radical trapping t o substantial hydroperoxide decomposing capability and from no apparent HTAC to substantial HTAC. Some natural inhibitors were active initially and others only after their formation during the oxidation processes. Improvement of HTACs with addition of zinc dialkyldithiophosphates (ZDTPs) and 2,6-di-tert-butyl-4-methylphenol, separately or together, was base oil dependent. A secondary alkyl ZDTP improved HTAC in the presence of base oil more than a primary ZDTP. Assessment of HTAC provides a means of “fingerprinting” base oils based upon their natural antioxidant capabilities and their response t o supplementary antioxidants. The chemical composition of base oils plays an important role in determining the overall properties of automotive engine oils formulated from them. One important performance area affected by base oil composition is resistance to oxidative degradation. Depending on the relative proportions of various hydrocarbon structures present and the type and amount of heteroatoms, base oils have different propensities toward oxidation and, thus, differ in oxidizability. Some of the chemical species may act as natural antioxidants or may be converted to antioxidants during use. Both hydroperoxide-decomposing and radical-trapping antioxidant activity may be provided by these components. The nature and amount of these inhibitors depend upon the crude oil source and the method of refining. Oxidation resistance of engine oils depends, additionally, upon the presence of supplementary antioxidant additives. Zinc dialkyldithiophosphates, ZDTPs, provide both radical-trapping and peroxide-decomposing antioxidant activity, while amines and phenols provide additional radical-trapping capability. Various base oils respond differently to treatment with additives. A novel test procedure (Korcek et al., 1986) based upon a model for oil oxidation in internal combustion engines (Johnson et al., 1983) has been developed for determining high-temperature antioxidant capability, HTAC. The method, which simulates both the continuous interaction of combustion-derived free radicals with the oil in the piston cylinder areas of operating engines and engine operational temperatures, detects both radical-trapping and hydroperoxide-decomposing antioxidants. Thus, more information about total antioxidant capabilities is obtained than can be determined by using the procedure developed earlier in our laboratory (Mahoney et al., 1978) which measures only radical trapping antioxidant capacity at low temperature. The intent of this work was to utilize the method for characterization of the antioxidant capabilities, naturally present and formed during oxidation, of hydrocarbon base oils and to investigate combinations of base oils with representative antioxidant additives. The test procedure utilized for this study consists of investigating the effects of these components upon hexadecane oxidation while continuously adding preoxidized hexadecane containing a desired level of hydroperoxides. Hexadecane and hexadecyl hydroperoxides are representative of the types of materials present and of the hydroperoxides formed during oxidation of oils containing hydrocarbon groups. Results 0888-5885,I 87 ,I 2626-Im$Ol.50,I 0
from evaluations of several base oils and combinations of two base oils with antioxidants, zinc dialkyldithiophosphates and a phenol, are presented. Experimental Section Procedure. The experimental procedures utilized for measuring high-temperature antioxidant capability, HTAC, have been described in detail elsewhere (Koreck et al., 1986). Briefly, the method involves determination of inhibition periods during oxidation of hexadecane in a model system while continuously adding hydroperoxides. The hydroperoxides, ROOH, added were prepared previously by oxidizing hexadecane in a stirred-flow reactor (Jensen et al., 1979) to a desired level of ROOH. The base oils or base oil-antioxidant mixtures were added to hexadecane in a glass batch reactor which was being purged with argon and was immersed in a constant-temperature bath. When the mixture reached the test temperature of 160 “C, oxygen and hydroperoxide flows were initiated. Two general procedures were followed. In mode I experiments, the initial volume in the reactor was 40 mL, and 2-mL samples were periodically withdrawn a t the same average rate as the preoxidized hexadecane was being added. The volume in the cell varied between 38 and 40 mL. This procedure provided samples for analyses but also resulted in removal of portions of the test materials and added hydroperoxides during the test. Inhibition periods could be determined from analytical data or from the point at which a temperature rise, due to exothermic uninhibited oxidation, occurred. Mode I1 used an initial mixture of hexadecane and sample totaling only 10 mL, no samples were removed, and all of the test materials remained in the cell. The inhibition times, in this case, were determined only from an increase of temperature. Analyses of samples obtained from mode I experiments can provide information regarding the type of antioxidant reactions which are occurring, hydroperoxide decomposing and radical trapping, while mode I1 provides a simpler means of obtaining relative HTAC. Hydroperoxide concentrations, [-OOH], were determined by using an iodometric procedure (Jensen et al., 1979),hexadecane oxidation products, [OP], by using gas chromatography (Zinbo et al., 1987), and low-temperature antioxidant capacity, n[AH], by using peroxy radical titration (Mahoney et al., 1978). Interpretation. Information regarding the type of antioxidant activity observed can be inferred from plots of ROOH and oxidation product concentrations vs. time. C 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1755 Table I. Antioxidant Capabilities of Base Oils at 160 OC inhibition time, s base oil S, wt 7'0 n[AH], mM from AT" from [0Plb 0.05 2.1 1300 2500 BO-1 0.15 0.9 600 1900 BO-2 0.46 1.8 1900 4000 BO-3 0.90 3.6 6400 7100 BO-4 0.80 1.5 6700 6600 BO-5 -300 BO-6 0.15 2000 BO-7 0.43 exptl conditions mode I mode I1 initial vol, mL HD 30 8 10 2 BO addition preox. HD, mL/min 0.6 0.3 [-OOH] in HD, mM 14 14
20
I 1 I
,
f
nDetermined from mode I1 experiments at AT = 2 "C (corrected for blank). bDetermined from mode I experiments at [OP] = 50 mM (corrected for blank).
Radical-trapping antioxidants give plots which nearly match the curves calculated based upon the rate of addition of preoxidized hexadecane. After the radical-trapping antioxidant is depleted, uninhibited oxidation will begin and the concentrations of ROOH and OP will increase rapidly. Hydroperoxide decomposers typically give curves showing ROOH concentrations below the calculated addition curve during the inhibition period. If oxidation is not inhibited at the same time, the concentration of OP may increase even though ROOHs are being decomposed. When efficient radical trapping and hydroperoxide decomposing are combined, the [-OOH] will be below the addition curve while [OP] will follow its addition curve since OP includes ROOH and their decomposition products. Materials. The antioxidants evaluated were a primary alkyl ZDTP, zinc di-n-octyldithiophosphate, [ (C8H170)2PS&Zn, n-CsZDTP; a secondary alkyl ZDTP, zinc diisopropyldithiophosphate, [ ((CH3)2CH0)2PS2]zZn, i-C,ZDTP; and a hindered phenol, 2,6-di-tert-butyl-4-methylphenol (MPH). n-C,ZDTP was synthesized in out laboratory by using a procedure previously described (Willermet et al., 1979), i-C3ZDTP was obtained from another laboratory, and MPH (Aldrich Chemical Co.) was further purified by recrystallization from methanol. Base oils evaluated were obtained from three base oil producers using various crude oil sources and refining processes. No attempt was made to analyze and chemically characterize these base oils in detail since correlations relating base oil composition to their oxidation properties are outside of the scope of this study. Base oils BO-1 and BO-3-BO-7 are either lOOSN or 150SN. Base oil BO-2 is a blend of 100SN and 300SN. Sulfur content and lowtemperature antioxidant capacities, n[AH], for these base oils are given in Table I.
Results Base Oils. High-temperature antioxidant capabilities of selected base oils or their blends were evaluated by using both mode I and mode I1 procedures under the experimental conditions given in Table I. Results of mode I investigations are depicted in Figure 1 which in addition to [-OOH] and AT curves also includes [OP] curves. Total oxidation products, OP, include not only hydroperoxides but also other products formed from decomposition of hydroperoxides regardless of their chemical form. Thus, [OP] is a good indicator of total conversion and overall extent of oxidation.
TIME /
s
Figure 1. Antioxidant capabilities of base oils (mode I).
Review of results presented in Figure 1shows that antioxidant behaviors of base oils evaluated in this study differ significantly. In the presence of BO-1, in the initial stages, [-OOH] and [OP] more or less follow the addition curves. A t ca. 1200 s, [-OOH] and [OP] start to increase above the addition curve at similar rates which are, however, lower than an uninhibited rate (curve BLANK). The uninhibited rate is attained at ca. 2400 s. This is accompanied by a continuous and rapid increase in reaction temperature. These results suggest that BO-1 contains mainly radical-trapping antioxidant species and has very little peroxide decomposers. Radical-trapping species completely inhibit formation of additional oxidation products at the beginning and later only slow down the oxidation without decomposing hydroperoxides. When radical-trapping species are all consumed, uninhibited oxidation begins. In the presence of BO-2, in the early stages, [-OOH] seems to be unaffected; however, [OP] increases, more than that which would correspond to the addition curve, indicating formation and decomposition of ROOH. At about 400 s, both [-OOH] and [OP] start to increase very fast. A t about 1000 s, a significant1 drop in [-OOH] begins which is accompanied by some decrease in rate of formation of OP. Finally, at ca. 2800 s, a rapid increase of both [-OOH] and [OP] is resumed. These results suggest that BO-2 contains peroxide-decomposing species which initially react with added hydroperoxides to retard the oxidation. This first stage of peroxide decomposing is followed by a second stage attributable to secondary peroxide-decomposing species formed upon oxidation of the base oil. These secondary species slow down the oxidation by lowering hydroperoxide concentration and, thus, decreasing the rate of initiation due t o homolytic decompo-
1756 Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987
TIWE
1
5
Figure 2. Antioxidant capabilities of base oils (mode 11).
sition of ROOH until they are consumed. Despite the decrease in hydroperoxide concentration and decrease in rate of initiation, oxidation in the presence of BO-2 is inhibited for a much shorter period of time than would be suggested by [-OOH] curves. From [OP] and A T curves, it follows that the high-temperature antioxidant capabilities of BO-2 are less than those of BO-1. This finding suggests that altough [-OOH] curves provide valuable information about hydroperoxide decomposing reactions, their use in the determination of inhibition periods may be misleading and use of A T or [OP] curves is preferred. With BO-3-BO-5, [-OOH] remains below the addition curve a t all times during the inhibited oxidation, while [OP] is above it. The [-OOH] curves for all these oils indicate the presence of two-stage peroxide decomposition similar to BO-2 which is, however, much less pronounced. Thus, BO-3-BO-5 seem to act similarly as BO-2. However, the effectiveness of peroxide-decomposing species and/or their concentrations in these oils must be greater than in BO-2 since hydroperoxide levels during inhibited oxidation with these oils are lower than with BO-2. Consequently, the rates of initiation and rates of OP formation must be correspondingly lower. Based on the decrease of hydroperoxide level, antioxidant capabilities of BO-4 and BO-5 should be similarly high and greater than those of BO-3 and antioxidant capabilities of BO-3 greater than those of BO-2. This is reflected in inhibition times determined from [OP] curves (Table I). The order of antioxidant capabilities of base oils determined in the mode I experiments is similar to that obtained in mode I1 experiments (Figure 2) where inhibition periods were determined from A T (Table I) without any withdrawal of base oils from the reaction mixture during the experiment. Again, the best high-temperature antioxiddant capabilities were exhibited by BO-4 and BO-5, followed by BO-3, BO-1, and BO-2 in decreasing order. BO-6 and BO-7 were not evaluated in mode I experiments. In mode I1 experiments, BO-6 was found to accelerate oxidation, while BO-7 behaved similar to BO-2 except it produced a longer inhibition period than BO-2. This appears to be due to the presence of greater amounts of both original and secondary peroxide-decomposing species in BO-7 than in BO-2. Resulting antioxidant capabilities as expressed by inhibition time do not correlate with either low-temperature radical-trapping antioxidant capacity, n[AH], or %S in the base oil. This is not surprising. Low-temperature radical-trapping capacity does not reflect peroxide-decomposing effects or formation of secondary inhibiting species from base oil oxidation at elevated temperatures. The amount of sulfur, %S, cannot be a single factor in determining high-temperature antioxidant capabilities since the type of sulfur compounds and presence and/or formation
TIUE I s
Figure 3. Antioxidant capabilities of base oils and additives (mode 11).
of other non-sulfur-containing antioxidant species will also play their roles. Based on the results presented, it seems that this method has an advantage in assessment of high-temperature antioxidant capabilities from analyses for certain types of chemical species. We believe the method can be very useful in “fingerprinting” base oils and assessing base oil equivalency. At this point, we would like to stress, however, that higher values of antioxidant capabilities (longer inhibition times) do not necessarily indicate better overall base oil quality since there are other base oil properties which should also be considered in evaluation of overall oxidation behavior, such as oxidizability of base oil components, base oil-additive interactions, type of oxidation products formed, and their physicochemical properties (e.g., viscosity). Base Oils and Antioxidants. Interactions of base oils and antioxidants are known to produce effects upon inhibition times in oxidation tests (Hsu et al., 1986; Murray et al., 1984). Results from determinations of the HTAC of two ZDTPs and a phenol in the presence of either BO-2 or BO-5, shown in Figure 3 along with results for the antioxidants alone, illustrate some of these effects. These tests were done by using the mode I1 procedure starting with 10 mL of test solution and adding 14 mM ROOH at the rate of 0.3 mL/min. In these tests, only 1 mL of base oil was used; the remainder was hexadecane, so the inhibition times for base oil alone are shorter than those in Figure 2 because of the reduced concentration. Concentrations of antioxidants used and resulting inhibition times, taken as the time to a 2 OC temperature rise, are summarized in Table 11. The ratio of ZDTP to base oil corresponds to engine oil formulations containing ca. 0.1370 Zn. The radical-trapping antioxidant, phenol, level is high compared to most oils but gives radical-trapping activity, measured at 60 “C, comparable to the supple-
Ind. Eng. Chem. Res., Vol. 26, No. 9, 1987 1757 Table 11. High-Temperature Antioxidant Capabilities of Base Oils and Additives inhibition concn,mM time, s n-CRZDTP i-CSZDTP MPH T;(I AT? Hexadecane 3.42 1.81 1.81
3.44 1.69 1.69
3.40
0 4530 -360 3090 -170 3835
0 4530 -360 3090 -170 3835
2150 5800 2900 6050 4500 7760
0 3650 750 3900 2350 5610
-690 4160 620 4360 1700 6770
0 4850 1310 5050 2390 7460
BO-5 3.40 1.78 1.78
3.46 1.73 1.71
3.42 BO-2 3.42
1.83 1.71
3.42 1.71 1.71
a
3.38
Inhibition time, s, corrected for blank. *Change from base oil.
mentary radical-trapping antioxidant present in some synthetic engine oils. Both ZDTPs in HD gave negative times to a temperature rise of 2 O C , when compared to HD alone, as did BO-2 when tested alone at this concentration. This occurred because, after short inhibition times, the subsequent temperature rises were faster than that obtained for the blank experiment. The greater rate indicates that oxidation is accelerated compared to noninhibited oxidation. This is consistent with previous observations of the behavior of ZDTPs when added to oxidizing hexadecane containing high levels of hydroperoxides (Johnson et al., 1983,1986) where inhibition was noted during the early stages but subsequent oxidation was accelerated due to radicals generated during induced decomposition of hydroperoxides. Also, the shorter inhibition time obtained for MPH with BO-2 present, 4160 s compared to 4530 s with HD only, indicates that additional initiation occurs with BO-2 present. The antioxidant responses as indicated by the increase of inhibition time relative to the base oil alone are generally greater in the case of BO-2 than for BO-5. Nevertheless, the inhibition times obtained with BO-5 plus antioxidant are greater than those of antioxidant plus BO-2 since BO-5 has greater HTAC by itself. Combinations of base oils and both n-C8ZDTP and iC3ZDTP give HTAC substantially improved compared to base oil alone. n-C8ZDTP provides greater improvement in BO-2 than in BO-5,1310 s vs. 750 s, while i-C3DTP gave similar results in both base oils, 2390 and 2350 s. The secondary alkyl ZDTP gives greater improvement than does the primary alkyl ZDTP.
Similarly, with base oil present, combinations of iC,ZDTP plus MPH gave substantially better HTAC than combinations of n-C8ZDTP and MPH which gave inhibition times similar to MPH alone. Combinations of both ZDTPs with MPH gave HTAC less than MPH alone when evaluated in the absence of base oil. Thus, a greater improvement of HTAC was obtained due to interactions between these base oils and i-C,ZDTP than occurred for the case of n-C,ZDTP. Conclusions Assessment of HTAC of base oils and base oil plus antioxidant combinations using a novel procedure which simulates the continuous formation of hydroperoxides due to interactions of combustion-derived free radicals with the oil in the piston cylinder areas of operating internal combustion engines has shown the following. HTACs of base oils vary considerably; one base oil gave no apparent inhibition and one showed mainly radicaltrapping antioxidant activity while others showed substantial hydroperoxide-decomposingability which may be accompanied by radical-trapping activity. Base oil natural inhibitors could be primary, present initially, and/or secondary, formed during oxidation of base oils. The relative response of base oils to antioxidant additives vary; a secondary alkyl ZDTP, i-C,ZDTP, improved the HTAC of evaluated base oil-antioxidant combinations to a greater extent than did a primary alkyl ZDTP, nC8ZDTP;in the presence of a radical-trapping antioxidant, MPH, i-C3ZDTP improved HTAC of these combinations while n-C,ZDTP did not. The HTAC method can be a useful tool in “fingerprinting” base oils and in assessing their equivalency. Registry No. n-C,ZDTP, 7059-16-7; i-C3ZDTP, 2929-95-5; MPH, 128-37-0.
Literature Cited Hsu, S. M.; Pei, P.; Ku, C. S.; Lin, R. S.; Hsu, S. T. Proceedings of the 5th International Colloquium on Additives for Lubricants and Operational Fluids, Technische Akademie Esslingen, West Germany, 1986, Vol. 1, p 3.14. Jensen, R. K.; Korcek, S.; Mahoney, L. R.; Zinbo, M. J . Am. Chem. Soc. 1979, 101, 7574. Johnson, M. D.; Korcek, S.; Zinbo, M. SAE Tech. Pap. Ser. 1983, No. 861 684.
Johnson, M. D.; Korcek, S.; Zinbo, M. ASLE Trans. 1986,29, 136. Korcek, S.; Johnson, M. D.; Jensen, R. K.; Zinbo, M. Ind. Eng. Chem. Prod. Res. Deu. 1986, 25, 621. Mahoney, L. R.; Korcek, S.; Hoffman, S.; Willermet, P. A. Ind. Eng. Chem. Prod. Res. Deu. 1978, 17, 250. Murray, D. W.; MacDonald, J. M.; White, A. M.; Wright, P. G. Proc. World. Pet. Congr. 1984, 11, 447. Willermet, P. A.; Mahoney, L. R.; Haas, C. M. ASLE Trans. 1979, 22, 301. Zinbo, M.; Jensen, R. K.; Johnson, M. D.; Korcek, S. Ind. Eng. Chem. Res. 1987,26, 902.
Receiued for review December 16, 1986 Accepted June 8, 1987
