Composition and Oxidation Stability of SAE 15W-40 Engine Oils

Tribology International 34 (2001) 127–134 www.elsevier.com/locate/triboint

Composition and oxidation stability of SAE 15W-40 engine oils J. Cerny *, Z. Strnad, G. Sebor Department of Petroleum Technology and Petrochemistry, Institute of Chemical Technology, Technicka 5, 16628 Prague, Czech Republic Received 28 February 2000; received in revised form 3 November 2000; accepted 4 December 2000

Abstract The modified IP 48 procedure was used for testing the oxidation stability of engine oils for ignition engines of the SAE 15W¨ MV, Petro-Canada, Shell, and 40 and minimum API SG/CD specifications. Eight engine oils, such as Agip, BP, Esso, Mogul, O Total, were collected and oxidised. Oils were also fractionated by using chromatographic methods, and group composition of base oils, amount of VI improvers and other additives were determined. Several differences in the oxidation stability were found between the collected oils. Viscosity characteristics, carbon residue, acid number, amount of pentane insolubles and mass losses during the test were chosen as parameters for evaluation of the oxidation stability of the oils. The most stable engine oil was blended from a hydrocracked base oil whereas most of the other oils were based on the solvent refined oils.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Engine oils; Chemical composition; Oxidation stability

1. Introduction In the literature, there are a great number of references about oxidation and thermooxidation stability of lubricating oils. Unfortunately, many of them are ambiguous or even contradictive [1–11]. Oxidation stability of lubricating oils is largely dependent on the oxidation test used [4]. There are several standardised laboratory tests, which are carried out at different temperatures, mostly in the temperature interval from 95°C to 200°C. Such an interval is very wide as a temperature of oxidation test, its duration and some other parameters are most likely the key points for assessment of oxidation stability of lubricating oils. Oxidation of oils undergoes three oxidation stages and the stage in which the oxidation test is stopped is decisive for evaluation of oil ageing [4]. The first oxidation stage is the inhibition period. In this stage, properties of the oil are relatively stable and the oxidation extent is very small. Duration of the inhibition period is predominantly affected by temperature and by the concentration of antioxidants. This stage ter-

* Corresponding author. Fax: +420-2-2431-0481. E-mail address: [email protected] (J. Cerny).

minates after depletion of synthetic antioxidants and oxidation of oil starts to be more pronounced. The second oxidation stage is the breakdown stage, in which oxidation is not influenced by synthetic antioxidants. The most pronounced effect on oxidation rate can be seen in a composition of the oil and in concentration of natural oxidation inhibitors [1,4,5,10]. Increased oxidation rate also deteriorates properties of the oil. The final oxidation stage is characterised by a slow oxidation rate due to a high oil viscosity. Oil contains a high concentration of oxidation products which can be partly polymerised and high oil viscosity then limits the access of air or oxygen into oil by a limited rate of diffusion [4,5]. An important factor in comparison of results from different oxidation tests is oxidation temperature and the stage in which the oxidation test is stopped [4]. That largely affects conclusions made from evaluation of tests. The amount of saturated hydrocarbons is a key parameter for stability of oils which are oxidised at low temperatures and/or for tests which are stopped during the inhibition stage, i.e. before antioxidants were depleted [4–7,12]. That is most likely due to the high dissociation energy of C–H bonds in saturated hydrocarbons. In high temperature tests which are stopped in the breakdown stage there is a very important parameter, the aromatics and sulphur contents in oils, as the sulphur

0301-679X/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 1 - 6 7 9 X ( 0 0 ) 0 0 1 5 0 - X

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J. Cerny et al. / Tribology International 34 (2001) 127–134

aromatic compounds can act as natural inhibitors during the breakdown stage [1,4,10,13,14]. Oxidation stability is one of the serious properties of formulated oils influencing the oil quality within the oil drain interval. However, current engine tests included in specifications such as API SJ/SH and/or ILSAC GF-2 evaluate preferably piston deposits, engine sludge and varnish. These evaluation parameters are not directly associated with the oil oxidation and many other effects can play a role in formation of sludges and deposits. Moreover, there are some references that ageing during the engine tests, e.g. Sequence VIA, should be more extensive to be realistic [15]. For example, depletion of antioxidant capacity after the Sequence VIA test was found to be about 20%, whereas that after 6700 km mileage accumulation in vehicle testing was greater than 50% [15]. This work is aimed at testing the oxidation stability of fully formulated engine oils for spark ignition engines of the SAE 15W-40 specification. The high-temperature laboratory standard test IP 48 was modified for that purpose by prolongation of the oxidation time. Evaluation of the oxidation tests was made by using the standardised methods required by the IP 48 method, and several other parameters of the fresh and oxidised oils were also measured. Another goal of the work was to compare oxidation stability of several engine oils of the SAE 15W40 specification from important producers and to make some conclusion about oxidation stability relative to composition of base oils.

2.2. Fractionation of formulated oils Composition of oils was assessed by a combination of a size exclusion and adsorption chromatography. Preparative size exclusion chromatography was performed on a glass column 10 mm i.d. and 1000 mm long that was filled with a polystyrene–divinylbenzene gel BioBeads SX3. A mixture of toluene and ethylalcohol 9:1 v/v was used as a mobile phase. Two fractions were obtained after fractionation, i.e. fraction of polymer additives and the rest of oil. Separation of polar additives and polar oil compounds, and fractionation of the base oil was done by adsorption chromatography on a glass column 10 mm i.d. and 1000 mm long filled with silica gel and neutral alumina according to a modified procedure of Sawatzky [16]. Briefly, five fractions were obtained after fractionations, i.e. saturated hydrocarbons, three fractions of aromatics according to the ring number, and one fraction of polar compounds. 2.3. Modification of the IP 48 oxidation test The IP 48 oxidation test is usually used for testing base lubrication oils. To reach a sufficient oxidation extent of formulated oil the IP 48 method was modified by a prolongation of the oxidation time from two to four 6 h oxidation cycles (total oxidation time was 24 hours). Other conditions of the IP 48 were maintained, i.e. temperature of 200°C, air flow rate of 15 l h⫺1. A commercial apparatus from ANALIS was used for oxidation of all oils. A suitability of the modified IP 48 test for formulated oils is discussed below.

2. Experimental 2.4. Evaluation of the oxidation tests 2.1. Samples Eight samples of the SAE 15W-40 engine oils for spark ignition engines were collected. Their alphabetical list and corresponding specifications are presented in Table 1. All oils were obtained from official distributors in the Czech Republic and represented the highest quality SAE 15W-40 oils of each producer.

According to the IP 48 standard, oxidation tests should be evaluated by measuring kinematic viscosity at 40°C (ASTM D445) and Ramsbottom carbon residue of fresh and oxidised oils. As suitable apparatus for determination of the Ramsbottom carbon residue was not available, the Micro Carbon Residue Test (MCRT) according to ASTM D4530 was determined instead.

Table 1 SAE 15W-40 engine oils tested and their specification

Agip F1 Super BP Visco 2000 Esso Uniflo Mogul Felicia OMV Control Petro-Canada Gold HTX 600 Shell Helix Super Total Quartz 5000

API

ACEA

SJ/CD SJ/CF SJ/CF SH/CF SG/CF-4 SG/CF-4 SH/CD SH/CF

A3-96, A2-96, A2-96, A3-96,

CCMC B2-96 B2-96 B2-96 B3-96

A2-96, B2-96

G4, PD2 G4, PD2 G4, PD2 G4, D4, PD2


Additional analyses of fresh and oxidized oils were done by measurement of viscosity at 100°C and calculation of viscosity index (ASTM D2270), by acid number determination by potentiometric titration (ASTM D664), and by pentane insolubles (GOST 11063-64). FTIR spectra of oils were accumulated at 2 cm⫺1 resolution on the Nicolet 740 spectrometer. Oils were measured in a substance in a 0.1 mm KBr cell or in the form of film between two KBr discs in the 4000–400 cm⫺1 range. Sixty-four scans were acquired. The absorption bands of carbonyls at about 1700 cm⫺1 were evaluated to assess the oxidation extent.

3. Results and discussion 3.1. Composition of SAE 15W-40 oils As mentioned in the Experimental section, composition of the oils was analysed by size exclusion and adsorption chromatography, both in a preparative mode. Size exclusion chromatography led to a determination of the amount of polymer additives in oils, and adsorption chromatography gave insight into a composition of base oil used for production of the oils. Results are summarised in Table 2. As can be seen from Table 2, the amount of polymer additives was very similar in almost all oils analysed and lay between 7 wt% and 8 wt%. Some exceptions were found for the oils C and H, in which the amount of polymer additives was above or below this range, respectively. As to the absolute values of the polymer additives amount, it can be somewhat overestimated due to the mechanism of the size exclusion chromatography. Some of the most polar low molecular weight additives elute from Bio-Beads gels very quickly and can be eluted together with the polymeric material. It was especially the case of some metal carboxylates, most likely overbased alkylsalicylates, which were detected by FTIR spectroscopy in almost all polymer fractions. Detection of

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alkylsalicylates was supported by absorption bands near 1560 cm⫺1 (carboxylate ions) and 1390 cm⫺1 (carbonates). Very intensive and well resolved absorption bands of alkylsalicylates were found in the FTIR spectrum of the polymer fraction of the oil C and that might cause an overestimation of the amount of VI improvers in that oil (see Table 2). Very similar was the case of alkylphenolates which were also detected in the FTIR spectra of the polymer fraction. (C–O groups around 1230 cm⫺1). Total amount of alkylsalicylates and alkylphenolates separated in the polymer fraction can be assessed as about 2 wt% to 3 wt%, showing thus that the real concentration of viscosity modifiers in the oils was about 5 wt% to 6 wt%. Qualitative evaluation of the FTIR spectra of the polymer additives gave evidence that VI improvers in almost all oils were either olefin copolymers or hydrogenated styrene–diene copolymers. Distinction between these two types of VI improvers is very difficult by FTIR. Polymethacrylate polymers were only detected in one of the oils, namely the oil G. It was the only oil with significant absorption bands near 1730 cm⫺1 and 1150 cm⫺1, which are typical for methacrylates. Further fractionation of the oils free of polymer additives gave results also shown in Table 2. Group composition of the hydrocarbon matrix of oils allowed a comparison of base oils used for production of formulated oils. In the fractions of polar compound, there were concentrated some polar compounds of the base oils and almost all low molecular weight additives. A purity of the polyaromatic fractions was checked by FTIR spectroscopy and no remarkable bands of additives were observed in the FTIR spectra. It is thus believed that almost all additives left in oil after the size exclusion chromatography were concentrated in the fractions of polar compounds. As to the composition of base oils, there were two types of base oils from which the engine oils were produced. For five of the engine oils, the composition of their base oils was very similar to typical solvent refined

Table 2 Chromatographic fractionation of the SAE 15W-40 oils Polymer additives in oil (wt%)

Oil Oil Oil Oil Oil Oil Oil Oil

A B C D E F G H

8.0 8.2 10.4 8.1 7.2 7.4 8.0 6.4

Composition of base oil+low MW additives (wt%)

Saturates

Monoaromatics

72.5 71.4 69.6 67.3 83.7 84.7 93.6 73.1

17.7 15.2 16.9 18.3 10.9 7.2 2.9 16.4

Diaromatics 2.7 2.7 3.3 4.2 1.7 1.4 0.5 3.3

Polyaromatics 1.6 2.9 3.2 3.4 1.6 0.6 0.3 2.3

Polar compounds 5.5 7.8 6.9 6.8 2.1 6.1 2.8 4.8

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oils with a content of saturated hydrocarbons about 70 wt%. The other three engine oils (Oils E, F, and G) had much lower content of aromatic hydrocarbons and this gives evidence that a hydrocracked oil was used for their production. Content of total aromatics of the Oil E was 14.2 wt%. This value is too high for most hydrocracked oils, and the amount of polyaromatics is also increased. It can therefore be deduced that the base oil for a production of the Oil E was mixed from hydrocracked and solvent refined oils. Another possibility is the use of moderately hydrocracked oil only. Composition of the base oil of the Oil F is typical for hydrocracked oils, but not the VHVI grade. Content of total aromatics did not exceed 10 wt% and the amount of polyaromatics was also very low. Finally, the base oil of the Oil G was clearly hydrocracked oil of the VHVI grade. Content of total aromatics was about 3.7 wt% and most of that amount can originate from the solvent oil of the additive package. The use of VHVI oil for production of the Oil G was also reflected in a high viscosity index of the oil (see below). Special attention should be paid to the amount of polar compounds (Table 2). From our experience, the amount of polar compounds in most solvent refined oils is in the range from 3.5 wt% to 5.0 wt%. It can be assumed from the amount of polar compounds in Oils A, B, C, D, and H that the concentration of pure oil additives, other than VI improvers alkyl salicylate and alkylphenolates, in the oils was in the range from 2 wt% to 3 wt%. This amount of additives corresponds well to the concentration of polar compounds in Oils E and G made from hydrocracked base oils. The content of natural polar compounds in hydrocracked base oils is indeed very low or even negligible and the polar compounds of the Oils E, F, and G were mostly the oil additives. Together with metal carboxylates and alkylphenolates detected in the polymer fractions, the total amount of additives in the oils was thus at the level of 4 wt% to 6 wt% in addition to about 5 wt% to 6 wt% of viscosity modifiers. The amount of polar compounds in the Oil F was found to be relatively high thus showing an increased concentration of additives.

imeter. High mass losses of volatile oil components can lead to a digestion of the oil tested and to a viscosity increase which was not caused by oxidation. Despite the limitations described above, an effort to modify the IP 48 test for oxidation of formulated oils was made with the Oil E and results are shown in Fig. 1. All conditions of the IP 48 test were maintained except the oxidation time which was variable from 12 to 30 hours in 6 h oxidation cycles. Similar tests were done with nitrogen instead air to check the effect of oil digestion due to mass losses. Although the mass losses of the oil at the end of the test exceeded even 20 wt% in both runs, it is clear from Fig. 1 that the viscosity change due to oxidation was significantly greater than that in the blind experiment with nitrogen blowing. It was believed that the difference in the viscosity values gives a satisfactory space for an evaluation of oxidation severity. The Oil E oxidised for 30 hours was also analysed for the amount of pentane insolubles, and their concentration was found to be negligible. That led us to a conclusion that the modified IP 48 oxidation test can be used for an assessment and comparison of oxidation stability of the SAE 15W-40 engine oils. Four 6 h oxidation cycles were chosen as a suitable oxidation time. 3.3. Oxidation of engine oils of the SAE 15W-40 specification Oxidation tests were performed according to the modified IP 48 procedure described above. To meet specifications of the IP 48 test, oxidation stability of oils was evaluated by determining a kinematic viscosity at 40°C and the amount of carbon residue (MCRT). In addition, kinematic viscosity at 100°C, mass losses during the test, amount of pentane insolubles, and acid number were used as complementary evaluation parameters. FTIR spectra of fresh as well as oxidised oils were also taken. Viscosity characteristics of fresh oils were very simi-

3.2. Modification of the IP 48 oxidation test High temperature oxidation test IP 48 is a suitable test for engine oils as the oil in cylinders and on piston rings is exposed to temperatures above 200°C. However, the test is not recommended for oils formulated with ash forming additives. The second limitation of the test is that the mass losses of the oil during test should not exceed 10 wt%. Both limitations are probably associated with the problems with a viscosity measurement. Decomposition of some ash forming additives can lead to a formation of solid and/or insoluble products that can disturb the oil flow through the capillary of the viscos-

Fig. 1. Viscosity changes of the Oil E during air oxidation and nitrogen blowing.


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Table 3 Viscosity characteristics of engine oils of the SAE 15W-40 specification Viscosity at 40°C Oil Oil Oil Oil Oil Oil Oil Oil

A B C D E F G H

107.88 106.22 107.93 110.25 99.19 110.95 106.52 105.73

Viscosity at 100°C VI 13.91 14.03 14.36 14.97 13.94 15.09 15.70 14.30

129 133 136 141 143 142 157 138

Fig. 4.

Fig. 2.

Increase in oil viscosity at 40°C after oxidation.

lar in almost all cases and are presented in Table 3. Viscosity at 40°C ranged mostly from 105 mm2 s⫺1 to 111 mm2 s⫺1, the only exception was the Oil E whose viscosity was below 100 mm2 s⫺1. Viscosity at 100°C was about 14 mm2 s⫺1 except the Oils D, F, and G whose viscosities were slightly higher with a consequence of a correspondingly increased viscosity index. Whereas there were only minor differences between the fresh oils, after the oxidation test the viscosity characteristics differed significantly. The viscosity changes after oxidation are shown in Figs. 2–4, in which

Fig. 3.

Increase in oil viscosity at 100°C after oxidation.

Change in viscosity index of oils after oxidation.

the oils are ordered according to increased changes. Remarkable differences were found for the viscosity changes at 40°C. A percentage increase of viscosity ranged from 7% to 90% (Fig. 3), the lowest increase exhibited the Oils F and C. Viscosity increase at 100°C was somewhat lower than that at 40°C (Fig. 4). After oxidation of the Oil F even a decrease in the viscosity was found. A very small viscosity increase was again found for the Oil C. Evaluation of oxidation stability of oils by viscosity measurement is affected by a thermal stability of viscosity modifiers. Their degradation during thermal and oxidative ageing (C–C bond breaking and lowering the molecular weight) can lead to a viscosity decrease while low or moderate extent of oil oxidation proceeds. That effect is more pronounced at higher temperatures of viscosity measurements. During oil use in engines, viscosity increase due to oxidation is additionally compensated for by a mechanical shear-down of polymers and fuel dilution. In fact, viscosity of used oils is frequently lower than that of fresh oils. Figure 4 shows the changes of viscosity index after oxidation. The viscosity index of each oxidised oil was lower than that of the fresh oil. However, the percentage decrease in VI oscillated mostly in a relatively narrow range between 15% and 20%. It is therefore believed that a change of VI during a high temperature oxidation is caused by a thermal stability of viscosity modifiers rather than by oxidative ageing of oil. The amount of carbon residue after oxidation of oils reflects thermal rather than oxidation changes in oils and it is most likely associated with the amount of aromatics and other carbon forming compounds such as some oxidation products. From that viewpoint it can be understood that the lowest increase in carbon residue was found after oxidation of the Oils F and G which contained the lowest content of aromatics, Fig. 5. Carbon residue of individual fresh oils was determined between 1.1 wt% and 1.6 wt%.

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Fig. 5. oils.


Increase in the amount of carbon residue after oxidation of

Weight losses of oils after the oxidation test IP 48 are shown in Fig. 6 and relatively large differences between the oils were found. Weight losses for the individual oils lay in the range between 10 wt% and 20 wt%, the lowest were again found for the Oils F and C. That gives to the oils a good potential for low consumption during their use in engines. Besides the volatility of the base oil, mass losses of oils during the oxidation test are most likely also influenced by an oxidative deterioration of oils, which can lead to a formation of volatile low molecular oxidation products. It was additionally proved during oxidation tests of oils of another viscosity specification. Oxidation stable oils showed low mass losses during test, and there was no correlation with volatile losses according to Noack. Pentane insolubles are most likely formed by a condensation of polar oxidation products. Their amount in oxidised oils is shown in Fig. 7. For half of the oil the amount of pentane insolubles was almost negligible. However, the second half of the oxidised oils contained a significant amount of pentane insolubles that in the case of Oils D and A reaches the values of 4 wt% and

Fig. 6.

Mass losses of oils after oxidation.

Fig. 7.

The amount of pentane insolubles in oxidised oils.

6 wt%, respectively. These oils also showed a high increase in the amount of carbon residue. That can have a detrimental effect on the cleanliness of some of the engine parts. However, it should be noted that the increased amount of pentane insolubles in Oils D and A did not cause any difficulties with a reproducibility of viscosity measurements and that can mean that the pentane insolubles were soluble in oil. The next two analyses of oxidised oils were directed to compare the amount of oxygen incorporated into the oils. Acid number of oxidised oils is graphically presented in Fig. 8. Whereas acid numbers of fresh oils were almost equal for all the oils, i.e. between 2.5 mg KOH g⫺1 and 3.0 mg KOH g⫺1, differences between acid numbers of oxidised oils were more significant ranging from about 7 mg KOH g⫺1 to 14 mg KOH g⫺1. Similar to the amount of pentane insolubles, the highest acid numbers were also determined for the Oils D and A. There is probably a direct relation between these parameters as more acidic oxidation products of oils can easily condensate to form pentane insoluble compounds. Almost the same tendency as for acid numbers was obtained for an increase in carbonyl bonds concentration

Fig. 8.

Acid number of oxidised oils.


Fig. 9. Increase in integral FTIR absorbance of carbonyl groups after oxidation of oils.

in oxidised oils. Results are shown in Fig. 9. Determination of the carbonyl intensity increase by FTIR spectroscopy could reliably be done as the intensity of carbonyl bands in fresh oils was low and did not exceed 10 abs. units cm⫺1. Intensities and spectral shape of carbonyl bands in fresh and oxidised oil are compared in Fig. 10. As well as in the previous measurements, the most oxidised oils were found to be the Oils A and D. It is interesting to note that the Oils C and F, which after the oxidation test kept excellent viscosity characteristics and other properties, showed only a moderate oxidation stability expressed by an acid number and increase in intensity of CO band absorption.

4. Conclusions A modified IP 48 standard test was used for testing the oxidation stability of eight engine oils of the SAE 15W-40 and minimum API SG/CD specifications. The conditions of oxidation were chosen on purpose as more

Fig. 10. FTIR spectra of fresh and oxidised oils in the region of carbonyl absorption.

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severe than would be expected in common use of oil in an engine. Therefore, the differences in oxidation stability between the oils tested give no evidence for the quality of the oils within the oil drain interval. Rather, the results from the oxidation tests showed a potential of the oils for prolongation of the oil drain interval, which is currently recommended by car producers to be between 10 and 15 thousand km. Most of the eight oils tested were produced from solvent refined base oils. Only two of the oils were made from hydrocracked base oil, and base oil of another oil was most likely made from a mixture of hydrocracked and solvent refined oils. None of the oils had superior properties in all characteristics tested. However, it can be concluded that the Oils F and C were the oils with the highest oxidation stability among the SAE 15W-40 engine oils tested and, therefore, with a good potential for a prolongation of oil drain interval. The oil F was produced from a hydrocracked base oil.

Acknowledgements The authors thank the refinery Koramo a.s. in Kolin for financial support of the work. Many thanks also to Dr I. Va´clavickova and Dr N. Vinklarkova for their help and interest. Part of the work was supported by grant no. 104/00/0576 obtained from Grant Agency of the Czech Republic.

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Composition and Oxidation Stability of SAE 15W-40 Engine Oils

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