Assessment of Oxidation in Automotive Crankcase Lube Oil: Effects of

The changes in oxidation stability of two grades of crankcase lube oil, monograde (SAE 40) and multigrade (SAE 20W50), and their base oils were monito...
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Ind. Eng. Chem. Res. 2002, 41, 3473-3481

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GENERAL RESEARCH Assessment of Oxidation in Automotive Crankcase Lube Oil: Effects of Metal and Water Activity Felix Egharevba* and Albert U. C. Maduako Department of Chemistry, Ambrose Alli University, Ekpoma, Edo State, Nigeria

The changes in oxidation stability of two grades of crankcase lube oil, monograde (SAE 40) and multigrade (SAE 20W50), and their base oils were monitored by measuring their carbonyl peak index with respect to time in order to corroborate the results of sludge deposit and soluble acidity characteristics. The carbonyl peak index was calculated as the difference in the ratios of the absorbance of the carbonyl band at 1700 cm-1 and of the basic peak methyl stretching vibration at around 3000 cm-1 at oxidation time t and the start of the experiment. The results showed that the base oils were oxidized to a greater extent than the formulated oils. The monograde oil and its base oil had higher carbonyl peak indexes than their respective multigrade counterparts, showing that the monograde oils were less resistant to oxidation than the multigrade oils. However, results of sludge deposits and soluble acidity showed that the multigrade oils formed more oxidation products than the monograde oils, which also incidentally suggests that the multigrade oils deteriorate faster. The two conflicting observations therefore suggest that the types of oxidation products formed from the two oils were different even though initiated by similar mechanisms. The effects of metals (zinc, nickel, and aluminum), as well as the effect of water (on the activity of the metals), on the overall carbonyl peak index are reported. Introduction Lubricating oils in crankcase automotive engines are known to undergo oxidative degradation and wearmetal-catalyzed oxidation leading to products believed to be responsible for sludge deposits in used oils. A number of standard and proprietary laboratory bench test methods1-5 have been developed for monitoring the oxidation stability of the oils. Most of the methods use oil-soluble acidity and sludge deposit formation for assessing oil deterioration. Although there are no universally established standards for these parameters in all types of engines, evidence based on engine test data and product development experience6,7 showed that 3% sludge deposits was too high for most engines. Above this level, the engine was believed to run the risk of failure due to blockage of the oil filter and oil delivery tubes. Oil deterioration results in a loss of lubrication, with the signals shown by the appearance of sludge. Efforts aimed at reducing sludge formation may extend the life span of the lubricant and prevent failure in service. The latter, however, cannot be achieved without the understanding of the processes of sludge formation and the nature of the oxidized products.6 Although qualitative information is available, which reports the presence of acids, aldehydes, ketones, esters, and lactones in the oxidates of used crankcase lube oils, information on the extent of oxidation is scanty.8-10 From the literature it appears that infrared analysis and gas chromatography are the main chemical tools applied to highlight the major oxidation products. Infrared, in particular, gives an indication of the changes in the hydrocarbon matrix

of the oils at the molecular level. The areas of interest in the spectra are the hydroxyl absorption around 3400 cm-1 and the carbonyl absorption band at 1800 to 1700 cm-1. Others included the absorptions at 1710 and 1116 cm-1 due to viscosity improvers; 1180 and 1054 cm-1 due to sulfonate detergents (dispersant); a 1040 to 950 cm-1 band due to the antioxidants; and the olefinic absorption (around 1600 cm-1) arising from oxidative thermal cracking, which is also of interest. In view of the ease with which infrared can be used to monitor oil oxidation, several attempts have been made to obtain quantitative information from infrared spectra of used oils11 starting with the development of the double-beam instrument which takes the fresh oil as a baseline for comparison with the absorption obtained for the used oils. Such a procedure gives a direct indication of the extent of oxidation of the oil. Another method for determining the extent of oxidation of oil by measuring the ratio of the logarithm of the transmittances of the unoxidized and oxidized oil was developed by Barcello and Otero and used by Gunsel et al. and Spedding and Noel.12-14 Further developments by the use of a dispersive Fourier transform to enhance the magnification of small changes in spectral data were introduced by Coates and Setti.15,16 The results were remarked by the authors to be subject to the influence of sample volume, cell thickness variances, and instrument noise, which limits their precision and consequently their application. However, the older type instruments, which may now be resolved and assigned to definite functional groups, make it possible to easily sort out the carbonyl peaks commonly used to identify oil oxidation products. Using

10.1021/ie000648k CCC: $22.00 © 2002 American Chemical Society Published on Web 06/12/2002

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Table 1. Properties of the Commercial Oilsa parameter SAE classification specific gravity at 15 °C kinematic viscosity at 40 °C (m2 s-1) kinematic viscosity at 100 °C (m2 s-1) viscosity index total base no. (mg of KOH/g) dynamic viscosity at 40 °C pour point (°C) zinc (wt %) calcium (wt %) magnesium (wt %) total acid no. (mg of KOH/g) paraffins content (wt %) naphthene content (wt %) aromatic content (wt %)

monograde oil OM1

multigrade oil OP1

SAE 40 0.89 154 14.80 97 5.30

SAE 20W50 0.89 131 17 130 6.30 4500 max

-12 max 0.11 0.03 0.07 0.39 38.96 27.65 37.39

0.13 0.04 0.09 0.44 62.77 23.56 9.66

monograde base oil OM2

multigrade base oil OP2

0.87 146.58 13.49 84

0.85 126.72 13.80 97

0.23

0.22

additives: sulfonate-type detergent/dispersants, zinc dithiophosphate antioxidants; in addition, multigrade has a poly(methyl acrylate) viscosity improver a

Source: ELF Oil Marketing Co. Ltd., Port Harcourt, Nigeria.

carbonyl peaks alone as bases for interpreting spectral results provides only qualitative information. Consequently, in this study, the carbonyl peak index (CPI) was introduced to provide quantitative results from the infrared spectra and by its design eliminates the drawbacks of the Barcello and Otero and the Coates and Setti approaches.12,16 Viscosity, which is used by lubricant manufacturers as a criterion for engine oil performance evaluation, is a physical method. The viscosity synonym in the chemical method is sludge deposit formation. Results of sludge deposit formation and changes in oil acidity17 are corroborated by CPI in this presentation. CPI eliminates the above-enumerated drawbacks by dividing the absorbance of the carbonyl band at 1700 cm-1 by the absorbance of the basic peak (CH3 stretching vibration) around 3000 cm-1. The advantages of using CPI are further that of simplicity, cheapness, and brevity and could be used to screen large samples before subjecting them to real engine and field performance tests. CPI would serve as a useful predictive tool in quality control and consistency checks as well as in comparative analysis of different lubricant products. Thus, CPI more accurately relates its values to the conditions generating the carbonyl compounds. The method is simple and less time-consuming. Experimental Section In Table 1 are shown the physical properties of the commercial oils studied. Oil Oxidation Stability Test. The oxidation stability test procedure was based on a modified version of the standard turbine oil oxidation test (IP280/80).1,3 A total of 250 cm3 (222 g) of the oil samples was treated in a three-necked oxidation flask, which was connected to a salt-ice-cooled flask containing 250 cm3 of a 0.01 M KOH solution The oil was heated to 180° C with an air flow of 1 L/min. Aliquot quantities of the oxidates were withdrawn at 5 h intervals for 65 h, quenched in ice, and analyzed for oil-soluble acidity and sludge deposits (hexane-insoluble in accordance with ASTM methods D664 and D893, respectively). Determination of Oil-Soluble Acidity. A total of 3.0 g of the oxidate (oxidized oil) in 125 cm3 of a 100: 99:1 toluene-2-propanol-water mixture was titrated potentiometrically with a 0.10 M 2-propanolic KOH using a Kent Electronic Instrument model EIL 7050

potentiometer with a combined glass-calomel electrode (Pye Unicam Ingold No. 44852). The 2-propanolic KOH was titrated in 0.10 cm3 portions into the magnetically stirred sample mixture, and the volume of KOH required to titrate the sample to 0 mV potential was taken as the end-point titer. A blank determination was made by titrating 125 cm3 of the toluene-2-propanol-water mixture with the KOH solution. Given that A is the volume of KOH required to titrate the sample to 0 mV potential, B, the volume corresponding to A for the blank titration, N, the molarity of the KOH solution, and W, the weight of oxidate sample, the soluble acidity was calculated as

SA (mg of KOH/g) )

(A - B)N(56.1) W

Determination of Sludge Deposits. The sludge deposits were determined as a percentage of hexane insolubles. A total of 1.0 cm3 of the oxidates was weighed into clean, dried, 10 cm3 cone-shaped centrifuge tubes. A total of 7.0 cm3 of hexane was added to each tube and stirred with a microspatula until homogeneous. The mixtures were centrifuged at the maximum speed of an MSE minor 35 centrifuge for 20 min. The oil solution was decanted and the precipitate washed twice with hexane, dried, and weighed. The percentage of insolubles was calculated as

S)

B - A(100) W

using A for the weight of the centrifuge tube, B for the weight of dried insolubles and the centrifuge tube, and W for the weight of the oxidate. Metal-Organosalt Complex. The metal-organosalt complexes used for the catalytic experiments were prepared from their nitrates by dissolving 0.70 mol % zinc (1.99 g of Zn(NO3)2‚6H2O), 6.90 mol % nickel (20.35 g of Ni(NO3)2‚6H2O), and 13.30 mol % aluminum (50 g of Al(NO3)3‚9H2O) of the salts in a minimum volume of methanol to obtain orange, green, and yellow complexes, respectively. The metal content (wt %) of the complexes were determined to be 44.17 zinc (by EDTA titration), 25.25 nickel, and 13.80 aluminum by atomic absorption spectroscopy.

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Catalytic Studies. Without further purification, 100 mg/L of the zinc-m, nickel-m, and aluminum-m (in powdered forms) was added to 250 cm3 of the oil and subjected to the oxidation stability test. In the same vein, salt complexes (zinc-s, nickel-s, and aluminum-s) were added to the oil sample, and each mixture was heated until brown fumes of nitrous oxide appeared while the reaction was monitored for 1 h. The reaction mixture was cooled, and deposits on the sides of the condenser were washed into the flask with 50 cm3 of toluene. The precipitates obtained were filtered, washed with 25 cm3 of toluene, air-dried, and used for infrared analysis. The effect of water on the catalytic behavior of the metals was also investigated by treating the oil sample containing 100 mg/L of the powdered metal or the salt complex with 0.20% of water in the oxidation stability test. Determination of Kinetic Parameters. Based on the assumption that the oxidation process followed the simple mechanism

RH f P where RH ) hydrocarbon and P ) oxidation product n

rate ν ) k[RH] and

ln ν ) ln k + n ln [RH] where n is the order and k is the rate constant. On the basis of the above assumption, the kinetic data for the formation of acids and sludge deposits in the oil were calculated. Infrared Studies. The infrared spectra of the oxidate samples were obtained and recorded as neat films between sodium bromide plates using a Shimadzu model 40.8 I spectrophotometer. The percent transmittances at 1700 and 3000 cm-1 were recorded and the absorbances

A ) log

1 × 100 %T

where A is absorbance and T is transmittance. The CPI was calculated using the absorbance values as

CPI )

A1700t A1700t0 A3000 A3000

where A1700t is the absorbance at 1700 cm-1 at oxidation time t, A1700t0, the absorbance at 1700 cm-1 at the start of the experiment, and A3000, the absorbance of the methyl basic peak at 3000 cm-1. The samples of oil examined were monograde OM1 and multigrade OP1 and their base oils OM2 and OP2, respectively. Results and Discussion Results of Oil-Soluble Acidity and Sludge Deposit Formation. The results of the oil-soluble acidity obtained with time are shown in Figure 1. In the figure, the first 5 h of treatment showed erratic changes in the acidity of the formulated oils (OM1 and OP1), while the base oils (OM2 and OP2) did not show such changes. For the base oils, the acidity increased markedly from the 5th to the 20th hour. Similarly, the formulated oils showed a very rapid increase in acidity between the

Figure 1. Changes in the oil-soluble acidity with time (h).

10th and 20th hours. Within this period, the formulated oils attained a maximum acidity at the 15th hour, while the base oils showed different maxima at the 10th and 15th hours for the multigrade and monograde base oils, respectively. Irregular increases and decreases in the amounts of acids formed were observed for the oils after the 20th hour and up to the 35th hour for the formulated oils and the 45th for the base oils, after which very little changes in acidity were observed for all of the oils. A possible explanation for the erratic behavior of the formulated oils in the initial stage of the reaction (first 5 h) is that several reactions must have taken place such as thermal cracking of hydrocarbons in the presence of oxygen. A reaction such as this produces hydroperoxides and peroxides, which ultimately degrade into acidic products. Other likely reactions are inhibition of oxidation by the antioxidants and neutralization of initially produced acids by the basic additives in the formulated oils. The decomposition products of additives would also have contributed to changes in acidity. Hence, the first 5 h for the base oils and 10 h for the formulated oils are rather the induction periods necessary for the aging of the oils. In Figure 2 the results of the sludge deposit formation for the oils are plotted against time. It is seen for the formulated oils (OM1 and OP1) that little sludge was formed in the first 20 h of reaction. Sludge formation was enhanced after the 20th hour, and this was exponential for all of the oils, more especially the base oils, which on their part showed a rapid increase in sludge deposits from the 40th hour. Other observations on sludge formation are that the formulated oils maintained lower increases in sludge deposits than the base oils and the multigrade oil formed deposits more rapidly and maintained higher deposits than the monograde oils. The low levels of deposits found in all of the oils before the 20th hour is an indication that the formation of sludge in the oils is a secondary reaction in the process of oil degradation. Klaus et al.10 had confirmed that the formation of high molecular weight insoluble sludge is the final stage of oil deterioration. He also proposed that sludge is a secondary polymerization product resulting from aldol-type condensation reactions

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Figure 2. Plots of CPI and sludge deposit formation versus time (h).

of carbonyl compounds with mainly carboxylic acid functions. This assertion by Klaus confirmed that low levels of acidic materials are formed after the 20th hour of reaction, thus presenting the position that the acids may be participating in the sludge formation. Therefore, the rapid increase in sludge formation in all of the oils indicates increasing polymerization of the carbonyl compounds and therefore lowers the concentration of oil-soluble acidic products in the lubricants. However, the base oils showed a more rapid increase in sludge formation as from the 40th hour. The overwhelming higher sludge formation of the base oils relative to the formulated oils indicates that the presence of dispersants in the formulated oils would have hindered the settling of sludge, thus leading to lower levels of sludge deposits. When the formulated oils are compared, the sludge deposit of the multigrade oils was consistently higher than that of the monograde oils. This difference could be related to the compositional makeup of the oils. The monograde oil contained more substituted aromatic and naphthenic hydrocarbons whose activity in the system is hindered by steric factors. The multigrade oils contained, in addition to the straight chains paraffins base, a polyester viscosity improver [poly(methyl acrylate)], which may be interacting with the oil oxidation products in such a way as to promote the polymerization reactions. Results of Kinetic Studies. The results of the kinetic studies for oil acidity and sludge deposit formation are shown in Table 2. From the results, the base oils had a higher rate constant than the formulated oils (without metals), which showed that the former oxidized faster. Nevertheless, there is a similarity in the oxidation rates observed for the base oils. A similar phenomenon was also found in the formulated oils. These observations tend to suggest that the oxidation of these hydrocarbons has been initiated through a similar mechanism which corroborates the suggested thermal scission of the hydrocarbon C-C bonds. In that case, the lower rate constants obtained for the formulated oils relative to the base oil could only be explained by the presence of antioxidants in the formulated oils. It showed actually that the antioxidants were effective in inhibiting the

growth of the chain-propagating free radicals and consequently the decrease in the rate of the oxidation reactions. The acid production rate constants obtained for the oils in the presence of the metals are also shown in Table 2. All of the reactions were found to follow a firstorder reaction mechanism. In general, the acid production rate constants of the oils were higher in the presence of the metals. For zinc and nickel, the presence of the metals led to higher rate constants than the organometallic salts. Aluminum metal and its salt form had the same rate constants in the monograde oil. Nevertheless, the organometallic salt led to higher rate constants in the multigrade oil than the aluminum metal. The data also showed that the rate constants were higher in the monograde oil with the metals than in the multigrade oil, and the presence of nickel led to higher rate constants than zinc and aluminum. The higher rate constants obtained for the oils in the presence of the metals showed that these metals influence the formation of more acidic products and implied that the metals did catalyze the overall oxidation process. The greater activity of the metals relative to the salts suggested that the metals provided more active surfaces for catalysis than the salt forms. The explanation for the higher activity of the metals in the monograde oil suggests that the more aromatic and naphthenic hydrocarbon components of the monograde oil offered more coordination sites for the likely formation of more stable complexes with the metals relative to the multigrade oil. Similarly, the higher activity of nickel when compared to the other metals is explained by its ability to form a wide range of complexes,18 which would enhance its lability in the system. Table 2 also shows that the sludge deposits (in parentheses) followed a first-order reaction mechanism like the acid production, but the rate constants are higher than the acid production rate constants. It was seen that sludge deposits only began to appear in the oils from about the 20th hour (Figure 2), and this corresponded to the period of the most rapid increase in the acidity of the oils (Figure 1). This observation implies that sludge deposition was a secondary reaction that followed acid production consecutively. The higher sludge deposition rate constants suggest that acid production was the rate-limiting process, and this explains the relatively long induction periods taken for appreciable sludge deposit formation to begin in the oils. This induction period corresponds with the time required for the production of the threshold level of acidic products necessary for possible polymerization. Once this threshold was reached, the condensation reaction begins. The order of 1.00 (Table 2) obtained for the sludge deposit formation in the oils when compared to the slightly varying values obtained for acid production may suggest that there is only one type of functional group which participates in the condensation reactions leading to sludge deposit formation, and this may be the carboxylic acid function as reported by Spedding and Noel.14 In the presence of the metals, the reactions were all found to follow a first-order mechanism. In general, the presence of the metals (Figures 3 and 4) in the oils led to lower sludge deposition rate constants than were obtained in the oils without added metals. The lower sludge deposition rate constants of the oils in the presence of the metals showed that the reaction was

Ind. Eng. Chem. Res., Vol. 41, No. 14, 2002 3477 Table 2. Kinetic Data for Acidity and Sludge Deposit (in Parentheses) and for the Metal-Oil Combinationsa rate constant with metals oil

order

without metals

Zn-m

Zn-s

Ni-m

Ni-s

Al-m

Al-s

OM1 OP1 OM2 OP2

0.952 (1.00) 0.952 (1.00) 1.050 (1.00) 1.000 (1.00)

0.025 (0.050) 0.025 (0.052) 0.037 (0.071) 0.037 (0.072)

0.039 (0.055) 0.033 (0.039)

0.033 (0.042) 0.030 (0.047)

0.042 (0.045) 0.037 (0.045)

0.041 0.033

0.039 (0.043) 0.030 (0.049)

0.039 (0.047) 0.032 (0.045)

a

Zn-m: zinc metal. Zn-s: zinc salt. Ni-m: nickel metal. Ni-s: nickel salt. Al-m: aluminum metal. Al-s: aluminum salt.

Figure 3. Effect of zinc on the sludge deposition characteristics of the oils. Figure 5. Effect of water (w) on the activity of nickel metal.

Figure 4. Effect of water (w) on the activity of zinc metal.

inhibited. This phenomenon may be accounted for by either the initial inhibitory action of the metals in the formation of acidic oxidation products or a metaldirected obstruction of the polymerization reaction (8). The effect of water on the activity of zinc and nickel metals in the sludge formation is also shown (Figures 4 and 5). Result of Infrared Spectroscopic Analysis. The results of the infrared analysis of the oil oxidation products are shown in Figures 6 and 7. Figure 6 shows the spectra of oxidates for the first 10 h of the oxidation, while Figure 7 shows the molecular condition of the oils up to the 65th hour. Coates and Setti16 obtained similar results in the laboratory oxidation of a similar oil. Figure 6 shows that there was more intense absorption at 1700 cm-1 for the base oils than for the formulated oils. In all of the oils, prominent hydroxyl absorption featured around 3400 cm-1 and carbonyl absorption around 1700 cm-1, suggesting the presence of carboxylic acids, aldehydes, and ketones. In Figure 7, there was an increase in the intensity of the 1700 and 3400 cm-1 absorption for all of the oils. However, the increase in

the absorption was less marked in the multigrade oil than in the monograde oil. The progressive increase in the intensity and broadness of both of the hydroxyl and carbonyl bands in all of the oils from the 10th hour to the 65th hour signified increased complexity of the oxidation products. Earlier reports10,12 have shown that the main reaction at the later stages of oil aging involves aldol-type condensation leading to the formation of polymeric materials and intramolecular esterifications leading to the formation of lactones. The products of the reaction comprising both oil-soluble and -insoluble materials reduce the infrared transmittance, thereby causing the broadening of the spectral bands. The bands occurring in the monograde oil appear to be more intense and suggest that the monograde and its base oil formed a more complex mixture of oxygenated compounds than the multigrade base oil. This observation would be explained by the difference in the hydrocarbon types that composed the oils. It appears that the straight-chain paraffins in the multigrade base oil formed simpler oxidation products than the more substituted aromatic and naphthenic hydrocarbons in the monograde oil. The interactions of the additives and the oxidation products in the multigrade oil would also result in complex products. The infrared spectra of the oils within the first 10 h in the presence of the various metals showed very little increase in the intensity of the carbonyl absorption. However, within the 10th to 65th hour period, the absorption in the carbonyl region became very rapid, though characterized by much less broad bands than those of the oils without added metals. There was no marked increase in the hydroxyl absorption, as was the case without added metals. It is also seen that absorptions at about 1000 cm-1 are virtually absent, indicating that the antioxidants had been converted to other products as in the oils without added metals. The low

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Figure 6. Infrared spectra of the oil oxidate samples for the first 10 h.

Figure 7. Infrared spectra of the oil oxidate samples from the 10th to 65th hour.

carbonyl absorption of the oils within the first 10 h in the presence of the metals would suggest a sort of inhibition of the oxidation process within this period. The relative lack of absorption at 1000 cm-1 in the 10th to 65th hour period showed that the antioxidants were actively involved in the oxidation inhibition process. Considering the reduction in soluble acidity, carbonyl and hydroxyl absorptions within the first 10 h, it seemed that the presence of the metals in the oils was acting synergistically with the antioxidants in the oxidation process, through scavenging of hydroperoxides in the formation of metal-hydroperoxide complexes. The rapid increase in the carbonyl intensity after the 10th hour suggests that the oxidation process had become catalytic, and this would be as a result of the complete decomposition of the antioxidants. In addition, the decomposition of the metal-hydroperoxide complexes formed at the inhibitory stage would have provided active free radicals, thereby contributing to catalysis of the oxidation process. The nature of the spectra of the oils at this later part of the oxidation showed that the oils experienced different stages of oxidation in the presence of different metals. The monograde oil with Zn-m and Al-s shows the spectra in very advanced stages of the degradation similar to those observed in the oils without added metals. The increased intensity of the hydroxyl absorption in the oils in the presence of Ni-m showed that the oils had begun to experience the advanced stages of degradation. For the rest of the oilmetal combinations, there were marked reductions in

the broadness of the carbonyl absorption bands and in the intensities of the hydroxyl absorptions. This observation signified that the oxidation products were derived from low levels of polymerization. The suggestion agrees with the low values of sludge deposits obtained from these systems (Figures 3-5). The infrared spectra of the oil oxidate samples from the oil-metal mixtures in the presence of water showed further reductions in the intensities and broadness of the carbonyl and hydroxyl absorptions relative to those of the oxidates of the oils alone (Figure 7) and the oilmetal mixtures without water. These results showed that the presence of water in the oil-metal mixtures led to a reduction of carbonyl oxidation products, which implies a deactivating effect on the catalytic behavior of the metals. Result of CPI Analysis. The plots of the CPI of the oils (OM1, OP1, OM2, and OP2) against time (h) are shown in Figure 2. The CPIs of all of the oils are shown to increase with time. The results obtained showed that the base oils (OM2 and OP2) were oxidized to a greater extent than the formulated oils. It is also seen that the monograde oil (OM1) and its base oil (OM2) had higher CPI values than the multigrade counterparts (OP1 and OP2), which suggests that the monograde oils formed more carbonyl oxidation products than the multigrade oils. However, earlier reports8,9 showed that the multigrade oils formed polymeric products and had more sludge formation than the monograde oils supported by this work (Figure 2). The latter suggests

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Figure 8. Effect of zinc on the CPI of the oils.

that the multigrade oils deteriorated more rapidly. This counter information indicates that the types of carbonyls formed from the two types of oils were different. We have noted above17 that the monograde oils generally contained more substituted aromatic and naphthenic hydrocarbons, which are stabilized, while the multigrade contained more straight-chains paraffins, which favor polymerization.19 The oxidation products of the two oils on this basis are different because it appears that the straight-chain paraffins in the multigrade oils formed simpler oxidation products than the more substituted aromatic and naphthenic hydrocarbons in the monograde oils. It seemed from the results that, although the monograde oil formed more carbonyl products, the multigrade oil formed more stereochemically favored carbonyls, which acted as active monomers and therefore polymerized. It might also be that the involvement of the carbonyl functions in the polymer linkages reduced the amount of free carbonyls in the multigrade oil, thereby resulting in the apparent lower carbonyl absorption intensities. The effects of metals and their organosalt complexes on the CPIs of the oils are shown in Figures 8-10 as plots of CPI versus time. In general, the presence of metals in the oils was found to lead to lower CPIs at the initial stages of the reactions. However, in most cases the CPIs increased rapidly with time and eventually overshot the CPIs of the oils without added metals. The CPIs of the monograde oil with Zn-s (Figure 8), the multigrade oil with Ni-s (Figure 9), and the multigrade oil with Al-m (Figure 10) remained lower than the CPIs of the oils without added metals. For zinc and nickel, the CPIs of the oils with metal were generally higher than those of the oils with the organometallic salt, while for aluminum, the reverse was the case. Further, the CPIs of the oils were generally higher with the metals in the monograde oil than for metals in the multigrade oils. It should be noted that the oil-soluble organometallic compounds were used to simulate oxidative wear products. The lower CPIs observed for the oils with added metals relative to those without metals at the initial stages of reaction suggest that lower amounts of carbonyl oxidation products were formed, which implies that the oxidation was inhibited. This would suggest

Figure 9. Effect of nickel on the CPI of the oils.

Figure 10. Effect of aluminum on the CPI of the oils.

that the metals acted as radical scavengers at this period of the reaction. Similarly, the higher values of the CPI observed for the oils with added metals at the later part of the reactions showed that higher amounts of carbonyl oxidation products were formed, implying invariably that the oxidation had become catalytic. The result agrees with the proposed mechanism,19 which showed the decomposition of metal-hydroperoxide complexes formed during radical-scavenging processes would have led to enhanced production of reactive free radicals which acted as chain propagators, thereby catalyzing the oxidation process. Moreover, the higher CPI values observed for the oils with zinc and nickel metals relative to their organometallic salts confirm that these metals provided more reactive surfaces in their heterogeneous form, while aluminum appeared to be more reactive in

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Figure 11. Effect of water (w) on the activity of zinc metal.

Figure 13. Relationship of CPI with sludge deposit formation.

Figure 12. Effect of water (w) on the activity of nickel metal.

the homogeneous (organosalt) form. In addition, the higher activity of the metals in the monograde oil than the multigrade oil showed that the mode and extent of oxidation were functions of the type of hydrocarbon in the oils. This appeared to confirm the suggestion that the more substituted aromatic components of the monograde oil provided more stable complexes8 with the metals relative to the multigrade oil. In the presence of water, however, the CPIs of the oil-metal mixtures were altered as shown in Figures 11 and 12 for zinc and nickel. The results showed that the presence of water in the zinc-oil mixture led to lower CPI values and, for the nickel-oil mixtures, it led to lower CPI values in the monograde oil. Thus, the presence of water generally led to a reduction in the oxidation of the hydrocarbons, which confirmed that water had a deactivating effect on the catalytic activity of the metals during oil oxidation. This action of water is not understood yet because water is believed to catalyze the oxidation of hydrocarbons without added metals.6 It is, however, suggested that water molecules could have competed with the oil for the metal surface or could have participated in the process of radical scavenging. It is suggested further that water participation occurs in the metal-oil interphase. The amount of water that can compete for the metal surface depends on how much water could be complexed in the metal-water-oil system, and this also depends on how much of the water is emulsified in the oil phase. The true nature of this suggested complex is not yet understood.

Figure 14. Relationship of CPI with oil-soluble acidity.

Moreover, in Figure 2, the plot of CPI versus time was related to sludge deposits with time. It is seen that the layout of the profiles of both plots appears to measure the same property in the oils as one increases in quantity in consonance with the other. In Figure 13, the CPI is related to sludge deposit formation. The plots showed clearly the response of CPI with sludge formation, and thus CPI may prove to be a useful measure of oil degradation in crankcase lubricating oils. However, in Figure 14, CPI is observed to decrease with oil-soluble acidity. This will be the case because oil acidity decreases with time after the threshold limit is reached, which also agrees with the plots of sludge deposit with oil-soluble acidity (Figure 15), while sludge deposits decrease sharply with oil-soluble acidity. The models that fit the relation of CPI with sludge deposit and oilsoluble acidity could be worked out with the appropriate regression equation. Conclusion In conclusion, the oxidation stability of formulated oils was found to be a function of the hydrocarbon type in the oils. The monograde types formed more oxidation

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Figure 15. Changes in sludge deposit formation with oil-soluble acidity.

products than the multigrade oils, as indicated by the CPI, but the multigrade oils formed more oxidation products, as indicated in the sludge deposit profile. The metals (used to simulate wear particles in automotive crankcase lube oils) could at the initial stage (first 10 h) of the reaction act as radical scavengers but would catalyze the oxidation for the rest of the period. Water (used to simulate water contamination and the high relative humidity of the tropical environment) was found to have a deactivating effect on the catalytic activity of the metals. Acknowledgment The authors are grateful to Professor F. E. Okieimen of the Department of Chemistry, University of Benin, Benin City, Nigeria, for reading through the manuscript. Literature Cited (1) Hsu, S. M. Review of laboratory bench tests in assessing the performance of automotive crankcase oils. Lubr. Eng. 1981, 37 (12), 722-730.

(2) Cecil, R. High-temperature thickening of motor oils. J. Inst. Pet. 1973, 59 (569), 201-210. (3) Petroleum products and lubricants. Turbine Oil Oxidation Text (ASTM D-943) in the Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1982; Parts 23-25. (4) Meyer, R. G. Lubricating testing apparatus. U.S. Patent 3,143,877, 1964. (5) Cvitkovo, E.; Klaus, E. E.; Lockwood, F. A. Thin film test for measurement of the oxidation and evaporation of ester type lubricants. ASLE Trans. 1979, 22, 395. (6) Elisha, A., Jr. Lubricant deterioration in service. In CRC Handbook of Lubrication; Booser, E. R., Ed.; CRC: Boca Raton, FL, 1983; Vol. 1, pp 517-532. (7) Assef, P. A. Used engine oil analysissReview; SAE Paper No. 70642; SAE: Warrendale, PA, 1977. (8) Ofunne, G. C.; Maduako, A. U.; Ojinaka, C. M. Studies in the Ageing characteristics of automotive crankcase oils. Tribol. Int. 1989, 22 (6), 401-404. (9) Ofunne, G. C.; Maduako, A. U.; Ojinaka, C. M. High temperature oxidation stability automotive crankcase oils and their base oils. Tribol. Int. 1990, 233. (10) Klaus, E. E.; Cho, L.; Dang, H. Adaptation of Penn state microoxidation Test for the evaluation of automotive lubricants; SAE Paper No. 801362; SAE SP90-473; SAE: Warrendale, PA, 1980; Vol. 980, p 83. (11) Frassa, K. A.; Sarkis, A. B. Diesel engine condition through oil analysis; SAE Paper No. 680759; SAE: Warrendale, PA, 1968. (12) Barcello, J. R.; Otero, C. A spectrophotometric method for studying the oxidation of lubricating oils. J. Inst. Pet. 1964, 50 (481), 15. (13) Gunsel, S.; Klaus, E. E.; Duda, J. L. High-temperature deposition characteristics of mineral oil and synthetic lubricant base stocks. Lubr. Eng. 1988, 44 (8), 703-708. (14) Spedding, H.; Noel, S. F. W. Development of techniques for the analysis of piston lacquers by Infrared spectroscopy. Tribology 1972, 31-33. (15) Coates, J. P.; Setti, L. C. Condition monitoring of crankcase oils using computer aided Infrared spectroscopy; SAE Paper No. 841372; SP589; SAE: Warrendale, PA, 1983; p 37. (16) Coates, J. P.; Setti, L. C. Infrared spectroscopic methods for the study of lubricant oxidation products. ASLE Trans. 1985, 29 (3), 394. (17) Egharevba, F.; Maduako, A. U. C. Correlation model between sludge deposits formulation and oil acidity in lubricating oils oxidation. Jamaican J. Sci. Technol. 1998, 9, 37-44. (18) Cotton, F. A.; Wilkinson, G. A. Advanced inorganic chemistry; Wiley: New Delhi, India, 1972; p 878. (19) Shieldon, R. A.; Kochi, J. K. Metal catalyzed oxidation of organic compounds in the liquid phasesA mechanistic approach. Catalysis 1979, 25, 272.

Resubmitted for review October 23, 2001 Revised manuscript received April 12, 2002 Accepted April 16, 2002 IE000648K