Oxidation of Petroleum Lubricants L. L. DAVIS, BERT H. LINCOLN, G. D. BYRKIT, AND W. A. JONES Continental Oil Company, Ponca City, Okla.
The oxidation of petroleum lubricants is an autocatalytic reaction, the initial phases of which have been investigated by an oxygen absorption test. Effects of crude source, refining, accelerators, and inhibitors may be thus studied. Two types of inhibitors are distinguished -true antioxidants and precipitants. The latter only are effective against metallic accelerators. Oxidation products are proximately separable by solvents and adsorption methods. Ultimate analyses of insolubles and oil-soluble resins, atepwise separated from used oil, show decreasing oxygen contents. Practical engine tests must finally determine the choice of inhibitors. Such tests emphasize the advantages of certain sulfurbearing derivatives over simple antioxidants.
0
NE of the most active problems being studied in petroleum and mechanical laboratories is the oxidation of lubricants in service and the effect of the oxidation produots on the equipment being lubricated. This problem is of immediate importance in all phases of lubrication but has reached serious proportions in the lubrication of internal combustion engines. This paper will present the methods of studying lubricant oxidation employed in our laboratories, and will discuss the relation between laboratory oxidation results and practioal engine tests. The study of oxidation of lubricants may be divided logically into three subjects: the oxidation of the oil and the factors which determine its rate, the quantity and kind of oxidation products produced, and the effect of the products of oxidation on the engine or equipment being lubricated.
Oxidation of Petroleum Lubricants I n general, the oxidation of the hydrocarbons found in lubricating fractions may be considered autocatalytic in character, where a chain mechanism is involved. Oxidation of oils by both laboratory methods and engines shows an initial or induction period of slow oxidation, a second or transition period in which the rate accelerates rapidly, and a third period in which oxidation continues a t a relatively high rate. Dornte (4) reported that, in the cases of oils showing autoxidation characteristics after oxidation starts, the square root of total oxygen absorption is a direct function of the time. Dornte (4) also showed that with some oils the oxidation
reaction is unaffected (either positively or negatively) by the oxidation products (6), and that with other oils the oxidation reaction is retarded by its oxidation products (6). I n practical tests with either laboratory oxidation machines or full-size engines, we have always found the autocatalytic type of reaction. This may be explained by the fact that the acidic oxidation products react chemically with the metals of the system t o form secondary products which accelerate oxidation at a much faster rate than the primary oxidation products of the oil. It appears, therefore, that the first factor in the study and control of oxidation is the initial or induction period. Moureu and.Dufraisse (16) discussed both the mechanics of oxidation and of antioxidation catalysts. Their method of test with a fixed volume of oxygen is rapid and easily satisfies the requirements for the study of the initiation of oxidation. Our modification of their method has proved of value in the study of the effect of source of crude, degree of refining, and use of accelerators and inhibitors on the oxidation of petroleum lubricants. The apparatus shown in Figure 1 consists of a Sligh flask (3) modified by the substitution of a 19/38 Btandard taper female joint at the to so that the flask is 20 om. high over-all. A 10gram sample o f the oil is delivered to the bottom of the flask by a calibrated Sligh pipet, care being taken t o avoid wetting the neck or sides. Air is displaced by a stream of oxygen, and an open-type mercury manometer of about 350-mm. pressure range is attached t h r o u g h a Celloseallubricated standard taper male joint. Springs are attached to avoid leakage, and the assembly is immersed to the middle of the neck of the flask in an oil bath held at 175 * 0.5" C. The pressure is read and recorded at 5 - m i n u t e i n t e r v a l s counted from the time of immersion. It rises t o a maximum of about 300 mm. because of the thermometric effect and after a time begins to drop as the oxygen is absorbed. The drop in pressure from the maximum is calculated for each time interval and recorded as a measure of the oxygen consumed.
FIQURE1. APPARATUSFOR STUDYOF OXIDATION
339
Figure 2 gives the pressure drop and therefore oxygen absorption for three SAE 30 crankcase oils and illustrates three typical forms of oxidation curves. Oil A is a heavily solvent-treated oil that has little, if any, natural a n t i o x i d a t i o n
INDUSTRIAL AND ENGINEERING CHEMISTRY
340
characteristics. The oxygen absorption accelerates rapidlyduring the first 40 minutes; then the rate becomes constant until a total pressure drop of 140 mm. is reached, as shown by the heavy straight line. After this period of constant rate of oxygen absorption, the rate decreases slowly because of the decreasing partial pressure of oxygen in the system.
I
/
reached by the time a 60-mm. pressure drop had occurred. Therefore the time in minutes required for a pressure drop of 60 mm. has been arbitrarily adopted as indicative of the oxidation induction period. Thus oils A, B, and C are reported to have induction periods of 69, 210, and 390 minutes, respectively. Effect of Metallic Accelerators. Oxidation of lubricants in actual service always occurs in the presence of metals and metallic compounds which are known to be accelerators of oxidation. Any laboratory method, therefore, must consider the effect of such accelerators. Figure 3 presents a group of oxygen absorption curves for oil 3 (Table I) , alone and with various amounts of iron naphthenate added as accelerator. The amount of iron naphthenate is reported as percentage of ferric oxide in the oil. Very small amounts of the iron salt greatly increase the rate of oxidation or shorten the induction period. Using the 60-mm. intercept as a measure of the induction period, the results show: Iron Naphthenate a s 70FeiOa None 0.0004 0,001
0.01 0.1
TIME - M/NUTES
FIGURE2.
OXYGET
AkBSORPTION BY
SAE 30 CRASKCASE OILS
THRED
Oil B is an exceptionally stable solvent-treated oil which gives a characteristic autoxidation type of curve. The induction period is very long, the oxygen absorption accelerating so slowly that 200 minutes are required to reach a constant rate. Oil C is oil A to which has been added an antioxidant. This oil shows a rather high initial oxygen absorption but a t a decreasing rate. After 300 minutes the rate becomes constant but is lower than in the case of oils A and B. These three curves show that with these oils the rate of oxygen absorption was constant by the time a 50-mm. drop had occurred and remained constant for some time thereafter. Examination of the data from several thousand determinations indicates that in all cases a constant rate was
T.4BLE
I.
DESCRIPTION OF OILS
Oil KO. Description 1 Mid-continent vacuum dist., solvent-refined. no inhibitor 1.157 “Methyl-S-linoleate” 2 Oil 1 0,5%’phosphite ester 3 Oil 1 4 Oil 1 0.1% polyhydroxy aromatic antioxidant o Commercial SAE 30 solvent-treated oil inhibited with proprietary mixt. of amine and phosphite ester 6 Acid-treated vacuum dist. from same source as 2 7 SAE 20 solvent-treated dist. from same crude as 1 8 Lightly solvent-trypted dist. from same source a s 1 containin Methyl-S-linoleate” 9 Commercial SA% 20 solvent-treated oil inhibited with DroDrietary mixt. of amine and phosphite esGr 10 Penna. S A E 20 mineral oil, no inhibitor 11 Solvent-treated SAE 20 mid-continent dist. 4 sulfurieed olefin 12 Solvent-treated SAE 20 mid-continent dist. 4phosphi.te ester
++ +
62 62 62
98 D8 98
62
98
67.6 62.6
101
78
54
102
61
92
55
112 100
55 54.3
102
54.3
102
Vol. 33, No. 3
Minutes for 60-hlm Drop in Pressure 159 126 83 32 18
These data are plotted on log-log paper (Figure 4) and include several points not given in Figure 3. The vertical line 0-0 represents the time for a 60-mm. drop for the oil without accelerator-that is, 159 minutes. As the amount of accelerator is increased, the induction time decreases, a straight-line relation being shown from the maximum induction at A to the minimum at B. With still larger amounts of accelerator the time is constant. Some oils do not show intercepts as well defined a t A and B as oil 3 but show curvature as indicated by the dotted line. I n such cases a straightline extrapolation is used to determine the intercepts. It appears, therefore, that an oil can tolerate a certain amount of iron naphthenate, as shown by intercept A , before its natural induction period is decreased. In the example given in Figure 4 this minimum amount of iron naphthenate which affects the induction period is 0.00023 per cent as ferric oxide. As the accelerator is increased, the induction period reaches a minimum a t intercept B , after which increased quantities of accelerator either make no further change or increase the induction period. In the above example the maximum amount of iron naphthenate which affects the induetion period is 0.04 per cent as ferric oxide. This method of analysis permits the comparison of the factors which affect oxidation, such as accelerators, inhibitors, and refining methods. The two left-hand graphs of Figure 5 show the relative effect of iron, lead, cadmium, copper, and silver naphthenates as accelerators; the two right-hand graphs show the effect of these same accelerators when “Methyl-S-linoleate” (13) is added as a sulfur-containing inhibitor. Owing to the fact that (a) iron is the metal common to alI practical lubricating systems, (b) through rusting it is readily available to form salts with organic acids, and (c) it permits the use of a wider range of concentrations in accelerator studies, it has been selected as the standard metallic accelerator. “Nuodex Certified Catalyst”, iron naphthenate containing 7.85 per cent iron, is used. Effect of Inhibitors. The use of addition agents in lubricants to inhibit oxidation or to decrease the effectof oxidation has become the rule rather than the exception. I n view of the complex action of these materials, we prefer calling them by the general term “inhibitors” rather than some term designating specific properties, such as “antioxidants” or “negative catalysts”.
March, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Using the 60-mm. pressure-drop induction period as a measure of the activity of an inhibitor, Figure 6 shows the relation between concentration of inhibitor and induction period. Oil 1 was blended with two types of inhibitors-a sulfurcontaining inhibitor and a proprietary phosphite ester. The practical value of an inhibitor is determined by its effectiveness in the presence of metallic accelerators. Figure 7 gives the iron tolerance curves for the two inhibitors shown in Figure 6. All three oils have the same base (Table I). Oil 1 contains no inhibitor, oil 2 contains “Methyl-S-linoleate”, and oil 3 contains a phosphite ester. The practical value of an inhibitor may be measured by the amount of accelerator required to bring the induction period of the inhibited oil back to the induction period of the original oil. Thus oil 1 with no inhibitor (Figure 7) has an induction period of 79 minutes and tolerates 0.0006 per cent ferric oxide without change. At this same induction time (79 minutes) oil 3, inhibited with phosphite ester, tolerates 0.0024 per cent ferric oxide, and oil 2, containing the “MethylSlinoleate”, tolerates 0.012 per cent. These inhibitors have at least two distinct actions on lubricating oil and probably another action on metal surfaces. First, they decrease oxidation rates or, as shown above, lengthen the time before oxidation starts. This action might be classed as true antioxidation and is probably the normal peroxide activity discussed by Moureu and Dufraisse (16) and others. The term “antioxidant” may be used to describe this action.
Many inhibitors show the first two actions in a varying degree, but relatively few form the protective coating against corrosion,
INDUCJION TIME
IRON NAPHTHENATE
a am
-
am4
f
I I
40
I
BO
I
,Yo
TIME - MINUTES
0
t
267
FIGURE 3. EFFECTOF IRONNAPHTHEKATE OK OXYGENABSORPTION OF OIL 3
The second action is the ability to counteract the effect of the metallic accelerators. I n all probability this is a true chemical effect in which the metal of the accelerator reacts with the inhibitor and is precipitated as an oil-insoluble inert compound-i. e., sulfide, phosphite, oxide, etc. This ability to remove or obviate the effect of the metallic accelerators is of primary importance in practical lubrication, since metal is always present and oil-soluble metal compounds will be formed through the chemical action of oxidation products on the metal. The term “precipitant” may be used to describe this action. A third probable action is the specific prevention of metallic corrosion by the formation of a protective film, such as a sulfide film. The question of corrosion prevention will be discussed later in this paper.
- MIN.
7
FIGURE 4. I R O NN A P H T H E N A T E PLOTTED AGAINST INDCCTION TIME FOR OIL 3, ON A LOG-LOG SCALE
AS PERCENT OF FeQ
aom
341
Some inhibitors, however, show only the antioxidant characteristics. For example, certain polyhydroxy aromatic compounds and certain amines show excellent antioxidant qualities when tested with organic compounds in glass but show no precipitant properties. I n Figure 8, the curves for oils 1 and 2 are repeated for reference. Oil 4 is the same as oil 1, to which 0.1 per cent of a polyhydroxy aromatic compound was added; it gives substantially the same induction period as oil 2-176 minutes as compared with 178. When iron naphthenate is added, the induction period decreases immediately, reaching the original oil induction period of 79 minutes with 0.0006 per cent ferric oxide. This is the same amount of accelerator which the original oil 1 will tolerate before the induction period is reduced; therefore, although the hydroxy compound increased the induction period, it did not improve the tolerance to iron. Effect of Degree of Refining. This method of analysis was employed to study the effect of the degree of refining upon the oxidation stability of an oil. Figure 9 shows the effecton the induction period and iron tolerance obtained by solvent refining a mid-continent vacuum distillate having a Saybolt universal viscosity of 650 seconds a t 100’ F. The oil was refined to four different degrees, as measured by viscosity index, employing furfural as the solvent. As a basis of comparison, the oxidation curve of a pharmaceutical-grade white oil is shown (not from the same crude). The induction periods for the straight oils were:
Curve No.
Visoosity Index
Induction Period, Min.
Raw oil First extn. Second extn. 4. Third extn. 6. Fourth extn. 6. White oil
63 90 96 98 107 05
267 146 60 37
I. 2. 3.
70
18
INDUSTRIAL AND ENGINEERING CHEMISTRY
342
I)(
i
OIL NO.2 ACCELERATOR AS THE NAPHTHENATE -IRON 'tL L EA 0 XCADMIUM
-- --
-/RON
VLEAD )C.---CADMIUM
----
001 15
4
t
OIL NO 2 ACCELERATOR AS THE NAPM THENATE 0COPPER -SILVER
--V
__
9, + \
I
p -?-. ; -& bo.
P
The first light extraction apparently removes the more easily oxidized materials and leaves the natural inhibitors in the oil, thus giving the very high induction period of 267 minutes. Additional extractions reduce the induction periods step by step to a minimum of 37 minutes for the product of 107 viscosity index. The white oil may be considered as having been subjected t o the ultimate degree of refining and shows an induction period of only 18 minutes. I n general, the iron tolerance and original induction periods rate the oils in the same order. For any given amount of added iron naphthenate, the induction time for the lightest treated oil is greatest; and for the most heavily treated oil, the induction period is least. A similar effect is found in oils refined by sulfuric acid treatment. Figure 10 shows the oxidation curves for the same raw oil as was used in Figure 9, after a normal commercial treatment of 18 pounds and after a heavy treatment of 180 pounds of sulfuric acid per barrel. The light treatment shows an increase in stability over the raw oil, but the heavy treatment shows a very low induction period. The relative stability to oxidation of various lubricating oil cuts from the same crude is shown in Figure 11. Three distillates and a residual cut of the SAE 10, 20, 30, and 70 grades were produced from a mixed Oklahoma crude and refined to approximately 100 viscosity index by solvent treating. The distillates show induction periods of the same order but of increasing value for increasing viscosity. The residual bright stock of the SAE 70 grade shows much higher oxidation stability. Blends of the 30 and 70 grades to make the 50 and 60 grades show approximately the same induction period as the bright stock. I n closing the discussion on the oxidation of lubricants, it must be pointed out that the above method of study does not indicate actual value of a lubricant in service but merely provides a means of studying the factors which affect the in-
"'r !-
I
Vol. 33, No. 3
I -
-
001 1su 15
INDUCTION T/ME
- MINUTES
With Methyl-S-linoleate as inhibitor No inhibitor EFFECT OF FIVE NAPHTHENATES AS ACCELERATORS FIGURE5.
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1941
343
saponification number (D94-39T), respectively, although many laboratories use modifications of the standard methods to detect more accurately the end point in the dark-colored oxidized oils.
duction period. Although a long induction period is desirable, it must be remembered that in any lubricating system, particularly in an engine, the fresh oil is almost immediately contaminated with used oil, oil oxidation products, and fuel combustion products; therefore, oxidation of the oil takes place from the start. The actual value of an oil will be determined by its ability to counteract the contaminating materials by its action as a precipitant and the quantity and character of the oxidation products produced. A given oil may have a high induction period; but i t may slowly produce objectionable oxidation products such as corrosive acids, insoluble sludges, or lacquerlike resins. On the other hand, an oil may oxidize more rapidly but produce unobjectionable oxidation products.
.
OIL NO. 1 METHYL-5-LINOLEATI -PHOSPHITE ESTER X----
Products of Oxidation Mineral lubricating oils are composed of a great number of compounds; their exact chemical composition is unknown, but in all cases they include compounds of the aromatic, naphthenic, and paraffinic hydrocarbon series. Although considerable work has been reported on the chemistry of the oxidation of lubricants and of individual compounds which may be found in lubricants (.@A,81,89),the majority of investigators have used empirical or proximate methods of analysis to determine the quantity and character of oxidation products. These methods have usually included the chemical determination of acidic products, the physical separation of insolubles, and the change in oil characteristics such as viscosity and carbon residue. Acidic Products. The acidic products (i. e., free organic acids and esters) are usually determined by the A. S. T. M. methods for neutralization number (D188-27T) and
I
I
I
I
I
I l l
100 INDUCTION TIUE
1
1
- MINUTES
I
-
INHI8ITOR CONCENTRATION PERCENT
FIGURE 6. RELATION BETWEEN CONCENTRATION OF INHIBITOR A N D INDUCTION PERIOD
As yet there has been no clear definition of what the neutralization and saponification numbers signify. Burwell (9)showed that alkali-consuming products in oxidized hydrocarbon mixtures are more complex than simple acids and esters, and include lactones and polyhydroxy compounds. There is no relation between metallic corrosion and the “acids” as determined by the neutralization number. King (11) showed that acidic oxidation products may be of real benefit in improving the “oiliness” characteristics of the oil.
I
l
500
l
I
I
I
l
l
,
,
I
100
-
INDUCTION TIME MINUTES
FIGURE 7 (Left).IRON TOLEIRANCII CURVESFOR Two INHIBITORS TOLERANCE CURVEOF POLYHYDROXY AROMATICANTIOXIDANT FIGURE 8 (Right). IRON
1
INDUSTRIAL AND ENGINEERING CHEMISTRY
344
.mol
’
’
I /NDUCTION
TIME
-
100
MINUTES
I
I
I
/
Vol. 33, No. 3
I
l
41
INDUCTION TIME -
l
I
I00 MINUTES
i
FIGURE 9 (Left). EFFECTOF DEGREEOF SOLVENT REFININGON OXIDATIOX STABILITY FIGURE10 (Right). EFFECTO F DEGREEO F SULFURIC ACID REFININGO S O X I D A T I O S STARIIJTY
Although the change in neutralization number of a n oil in use does not serve as a suitable basis for the prediction of the behavior of an oil in an engine, it is a valuable tool for the determination of the general trend of oxidation because it is easily and quickly determined and usually shows the same order of increase as do the other oxidation products. Insoluble Products. Since the oxidation products are less soluble than oil in certain organic solvents, they must be isolated and classified into the following arbitrary groups: CHLOROFORM INSOLUBLES. Chloroform is used to separate the insoluble contaminating material whose source is usually considered as extraneous to the lubricating oil system. These insolubles include the contaminating dust, metal particles, lead halides (from leaded gasoline), and carbon from the combustion zone. PETROLEUM NAPHTHA (ETHER)IXSOLUBLES. A. S. T. M. precipitation naphtha (DQ1-35) is usually used to determine the so-called sludge content of a used oil. Although this is a purely arbitrary classification of insolubles, its extensive use and correlation with engine conditions make it of real value. As yet no standard procedure has been adopted; and widely different results may be obtained, depending upon the fineness of the filter medium used. CHLOROFORM SOLUBLES.The material insoluble in naphtha but soluble in chloroform is sometimes reported as the chloroform solubles and represents the highly oxidized products which are either insoluble in oil or which, on slight additional oxidation, would have become insoluble. These are the materials which either precipitate in an engine to produce sludge or engine dirt or, through some peptizing action, remain in suspension in the oil and cause an excessive increase in viscosity. RESINS. Poell (IS), Noack ( 1 7 ) , Suida ($1, $2), Marcusson ( I 6 ) ? Schindler (19),and others have shown that, after the naphtha insolubles have been removed from an oxidized oil, there remains in solution a series of oxidized products which are plastic to solid in nature and which may be quantitatively removed by se+tive adsorption on decolorizing clay. The removal of these resins” from an oxidized oil leaves the remaining oil in practi-
cally its original condition, in so far as physical characteristics are concerned. In a highly oxidized oil the plastic t o solid resins may represent as much as 40 per cent by weight of the oil. As the used oil is returned to practically its original condition by the removal of the resins, it may be assumed that the sum of the chloroform solubles and the resins represents a quantitative measure of the total oxidation products. OIL INSOLUBLES. There have been many objections to applying the results from the above proximate analysis to practical problems because, it is contended, the material insoluble in the oil itself is the major cause of engine dirt. Thus Levin et al. (12) suggest the use of cotton as a filter medium to separate the true oil insolubles; however, since the oil may contain insolubles of every degree of fineness from coarse insoluble particles to colloid suspensions of such size that the particles will pass through cotton and an asbestos mat in a Gooch crucible, there seems at present to be no satisfactory definition for oil insolubles. INSOLUBLES IN OTHERSOLVENTS.It is obvious that the total oxidation products may be divided into any number of groups or degrees of insolubility, depending upon the solvents used. Thus Hall et al. (8) suggest the use of pentane instead of A. S. T. M. naphtha to determine the “undissolved sludge”. They also report that if liquid ethane or methane is used, more insoluble matter will be found than with propane; but the low critical temperature of the lighter solvents would greatly complicate their use. Pentane removes two to three times as many insolubles as does A. S. T. M. naphtha, but liquid propane still does not remove all of the remaining resins. We therefore prefer the use of naphtha to separate the least soluble fractions and the use of the adsorptive method for the separation of all of the remaining resins.
Effect on Oil Characteristics. The oxidation of lubricating oil effects great changes in the characteristics of the oil because of the presence of oxidation products. One of the important changes is in viscosity, which is an excellent measure of the extent of oxidation. All references to viscosity in this paper will be in Saybolt Universal seconds at 210” F. Another characteristic showing great change is the Conradson carbon residue. The removal of the total resins from a used
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1941
oil restores the color, viscosity, and carbon residue to practically the original values of the new oil.
TABLE 11. RATEOF OXIDATIONIN UNDERWOOD MACHINE Hours Oxidized Viscosity, sec. Carbon residue, Yo Neutralization No. Saponification No. CHCla insol. yo Naphtha insbl., % Resin, % Ultimate analysis, % Total inorganic Carbon Hydrogen Oxygen
New Oil 54.0 0.02 0.05 0.0 0.0
2 57.0 0.2
0.1
0.3 84.9 13.5 1.3
0.0 0.4
86.0 13.1 0.8
1.2 6.8 0.0 0.07 7.9
4 63.0 0.8 2.0 13.7 0.0 0.08 12.9
6 76.0 1.9 3.1 22.7 0.0 0.07 20.5
0.4 83.5 13.1 3.0
0.5 82.3 12.6 4.6
The relation between the quantity of oxygen absorbed and the production of oxidation products and change in oil characteristics is shown in Table I1 and Figure 12. Oil 7 , whose oxygen absorption characteristios are shown in Figure 11, curve 2, was oxidized in a laboratory modified Underwood apparatus (IS) a t 350" F. for 6 hours with 15 square inches of lead foil as a catalyst. Samples were analyzed by ultimate analysis for carbon, hydrogen, and oxygen, and by proximate analysis for the oxidation products. According to Figure 12, the oxygen absorption as determined by analysis of the oil shows a short induction period and then a fairly uniform oxidation rate over the test period. A study of the various oxidation products and of the changes in oil characteristics shows the same general type of increase. The oxidation was not carried to the point where insolubles
345
were produced, since the oil was of a heavily solvent-treated nonsludging type. However, the resins had increased to 20 per cent at the end of the test, with resultant increases in the viscosity and carbon residue of the oil. I n laboratory oxidation tests Haus (9) showed by ultimate analyses of the resin fractions that the oxygen content of the resin itself increases uniformly as the oxidation progresses. I n order to study the effects of oxidation during actual service, the same oil (No. 7 ) was used in a 1935 Pontiac 8 engine dynamometer set up under the following conditions : S eed, miles/hour gater-outlet temp., 0 F. Oil-gallery temp. F Oil pressure, lb./kq.,in. Duration of test, miles Make-up oil added EtaPb content of fuel, co./gal.
50 175 253 35 4430
None 1
The engine run was continued without addition of makeup oil until the engine failed because of corroded wrist-pin bushings and insufficient lubrication, resulting from thelincreased viscosity of the oil.
50
t
0
0
4
2
6
HOURS
L
FIGURE 12. RELATION BETWEEN OXYGEN ABSORPTION,AND FORMATION OF OXIDATIONPRODUCTS AND CHANQE I N OIL CHARACTERISTICS
\
I
t
I
1
INDUCTION TIME
- MINUT€S
FIGURE11. RELATIVESTABILITY TO OXIDATIONOF VARIOUS LUBRICATING OIL CUTS
Proximate analyses were made on oil samples taken a t 1000-mile intervals and a t the end of the run. The results are plotted in Figure 13. The neutralization number showed acid formation a t an extremely rapid rate from the beginning of the test until a final value of 13 was obtained. The petroleum naphtha insolubles and resins indicated a short induction period and then a rapid increase a t a fairly uniform rate. Viscosity and carbon residue increased in a degree comparable with the insolubles and resin production. A sample of the final crankcase oil was analyzed in detail to show the oxygen distribution to the various oxidation products (Table 111). Line 1 shows the characteristics of the original new oil; line 4 gives the characteristics of the crankcase oil after the engine run. This used oil was subjected to proximate and ultimate analyses, and the results are presented in lines 5 to 10. The ultimate analyses were corrected for sulfur, chlorine, iron, lead, and silica to give the carbon,
INDUSTRIAL AND ENGINEERING CHEMISTRY
346
Vol. 33, No. 3
TABLE 111. ANALYSISOF USEDCRANKCASE OIL
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
16.
17.
18. 19. 20. 0.
Per Cent 100.0 0.4 99.6 100.0 0.2 99.8 2.3 97.5 41.7 55.8
New oil Resin in new oil Resin-free oil Used crankcase oil CHCls insolubles Insoluble-free oil Naphtha insoluble Insoluble-free oil Resins Resin-free oil Naphtha solubles 1st extn.: Resin Oil 2nd extn.: Reain Oil 3rd extn.: Resin Oil 4th extn.: Resin Oil Resin-free oil resin and insolubles Inorganic-free basis.
+
Viscosity, Seconds 54
...
3000
2000
3000
0.02
...
saponi- Ultimate Analysis", fication % No.
0
C 86.1
E 13.1
O 0.8
..
0 90
7s:9
ii:5
9:a
ii8
+:e i:o
13.0
83
ii:7
9:2
8.9
...
80
0.3
79:5 74.1 84.9
ii:s
0.0
.. ..7
7i:1
L:7 15.5 1.0
8.9
..
... ... 55
330
9.3
...
...
...
..
.. .. .. ..
22.0 5.9
27.5 5.3
21.0 4.4
16.5 3.6
56
20.0 0.2
8.5 1.5
11.8 51.5
... 52
3.4 0.0
6.3 0.2
.. .. ..
...
341
7.8
9.0
77
11.5 79.8
...
16.6 63.3
4000
5
OIL
7
IN
. . . . . .
0.05 13.0
... 181 ... 106
6.2 91.3
Neutrafization No. 0.05
6:Ol
Fractionation of Resin 330 6.0
97.5
MILES
OF OXIDATION O N FIGURE 13. EFFECTS ACTUALENGINE SERVICE
%
54 540
hydrogen, and oxygen percentage on an inorganic-free basis. To study the effect of the incremental removal of the resins,
0
Carbon Residue,
..
10.4 14.1
79.5 11.8
8.7
72.2 9.4 18.4 80.4 12.3 7.3 73.1 9.4 17.5 81.7 12.7 5.6 73.3 9.7 17.0 82.8 13.2 4.0 78 9 85.3
11.8 13.8
9.3 0.9
. . . . . .
the naphtha-soluble fraction (lines 8 and 11) was extracted with four increments of decolorizing clay ( 5 , 10, 20, and 30 grams per 10-gram sample). The results of this fractional extraction as suggested by Poell (18) and Haus (9) are given in lines 11 t o 19. The relations between oxygen content, quantity of resins, and oil characteristics are shown in Figure 14. All the oil characteristics show the same trend as the resin was removed. The viscosity of the used oil was 540 seconds. The removal of the insolubles decreased the viscosity to 330 seconds, and the removal of the resins reduced it to 52 seconds (2 seconds below the new oil). In similar manner the neutralization number, carbon residue, oxygen content, and saponification number approach the original ail characteristics upon removal of the resins. T h e new oil (Table 111, line 1) was contacted with 100 per cent by weight of clay; and an apparent resin content of 0.4 per O I , t I , I 0 IO 20 30 40 SO'" cent (line 2) was OX/DATfON PUOOUCTS REMOVED found. Theremoval PERCENT OF USED OIL of this "resin" made FIGURE 14. RELATIOX OF OXYpractically no GEN CONTENT, QUANTITY OF change in the oil RESIKS,AND OIL CHARACTERISTICS AFTER ENGINE SERVICE as characteristics
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1941
347
0
I
-
INDUCTION T/M& M/NUTES
I
I
l
1
t
1
I
IO0 INDUCTION TIME
-
t
I
400 MINUTES
FIQURE 15. IRON TOLERANCE OF FIVEOILS
shown in line 3. A second contact with the same quantity of clay showed only 0.1 per cent of an oily extract. The total resin and insolubles (lines 5, 7, and 9) were blended back in the resin-free oil (line 10). This blend was tested as reported in line 20. All the oil characteristics were similar to the naphtha soluble as given in line 8. It appears, therefore, that the resins went back into solution but the insolubles were not dispersed. We can conclude that the clay extraction removes the resins without appreciable change in their characteristics. The amount of resins as determined by adsorption seems to be a quantitative measure of the total oxidation products soluble in petroleum naphtha. Since the resins develop rapidly during the early period of use and as a rule are much greater in quantity than the naphtha insolubles, they appear to be a truer measure of the extent of oxidation than are the naphtha insolubles. The “free acids” and saponifiable material are also included in the resin fraction. As Haus (9) points out in detail, there is no sharp dividing line between the insolubles or asphalt and the resins. There seems to be a uniform series of products from the true oil to insoluble asphalts. When the total resins are separated in a single extraction, they vary from brittle solids to plastic tacky semisolids. In the resin fractionation reported above, the first fraction was a plastic solid, while the last fraction was a viscous tacky liquid.
Effects of Engines on Oxidation of Oils The final step in the study of the oxidation of lubricants is the effect of the oxidation products on the condition and operation of the equipment being lubricated. Laboratory test methods have not yet been developed to the point where dependable forecasts of automobile lubrication can be made; a
i t is therefore necessary to use actual engines for the final study. I n our laboratories eight engines consisting of two each of Pontiac 8, Pontiac 6, Chevrolet, and Dodge are used on dynamometer stands. At least one run and in some cases several runs on the whole battery are required to rate any given oil fully. To discuss the question of effects of oxidation products on engines, we will describe several specific cases rather than attempt generalizations or classifications of difficulties caused by oxidation products. Five SAE 30 oils, (1, 2, 5, 6, and 8, Table I) were selected; and the results obtained on the Chevrolet engines are discussed as typical examples of oil rating by engine test. Figure 15 gives the iron tolerances for the five oils. Oils 1, 2, and 6 have been discussed previously. Oil 5 was a commercial grade of solvent-treated oil in which a phosphite ester was used as an inhibitor. Oil 8 was from the same source as oil 1but had been lightly refined to a viscosity index of 92 and contained “Methyl-S-linoleate”. The oils were tested in 1939 Chevrolet engines for a period equal to 5000 miles under the following average conditions : Speed miles/hour Load ’horsepower Wrtte’r-outlet temp. 0 F. Oil-gallery temp O’F. EtrPb content oi’fuel, ml./gal.
GO
26.3
185 280 1.5
Figure 16 gives the proximate analysis of the crankcase oil sampled at 1000-mile intervals. Oils 1, 5, and 6 show a rapid increase in neutralization number, indicating that these oils start oxidizing rapidly at the beginning of the run. Oil 2 shows only a slight increase in neutralization number over the entire run, while oil 8
348
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
0
I IO00
I ZQDO
I 300U
_____----___-----
I 4CUO
Vol. 33, No. 3
. d U
DURATION Of TEST- MILES
0
ZOO0 3000 DURATION OF T E S T - M I L E S
IO00
4600
5U06
FIGURE 16. PROXIMATE ANALYSESOF OILS AFTER ENGINE TESTS
shows an induction period a t low rate; but after 2000 miles the neutralization number increases rapidly and remains constant at a value of 5 . The sludge or naphtha insolubles in oil 6 (the least refined of the five) shows a steady rise from the beginning. Oils 1, 5, and 8 show little insolubles until the last 1000-mile period, while oil 2 shows very little for the entire run. The resin formation is of particular interest. Oils 1 and 5 show a rapid increase in resin to a maximum at 2000-3000 miles, after which the quantity decreases. The decrease in resin content occurs approximately concurrently with the increase in naphtha insolubles. This indicates that after the resin reaches a maximum, it continues to oxidize to produce insolubles. The decrease in resins does not quantitatively equal the increase in naphtha insolubles. Thus in oil 1 a t the end of 2000 miles the resin content was 32 per cent, and a t the end of the run i t was only 12 per cent (a decrease of 20 per cent), while the insolubles showed an increase of only 5 per cent. This is due t o the fact that the oxidation does not stop a t the naphtha-insoluble phase but continues for the production of oil insolubles which precipitate from the oil and thus forms engine sludges or dirt. Such precipitated sludges are not found in the testing of the used oil. This drop in resin content after insolubles start forming is found frequently and seems to be the normal course of oxidation. Haus (9)believes that in many cases the precipitation is cyclic. Asphaltenes are produced from the resins until the limit of their solubility is reached, when they “rain” down and carry adsorbed resin with them, The resin content of the oil then increases to a new maximum, and the cycle repeats. I n less refined oils such as No. 6, the resin quickly reaches a
constant quantity and the insolubles are produced at a constant rate. The carbon residue and viscosity changes of the five oils folIow closely the relative production of resins and insolubles. Oils 1 and 5 show an excessive increase in viscosity. The latter became so viscous after 4000 miles that it was too heavy to measure in the Furol machine a t 210’ F.; and i t had the appearance of a thick, smooth, plastic asphalt. Table IV summarizes the rating for the five engine runs; 0 is assumed to be metal bright, and 100 to be so dirty that engine failure will result. Each engine point mentioned is individually rated, and the average is taken as the over-all ratTABLE IV. CHEVROLET F ~ G I N ETESTSO N SAE 30 OILS 1
Engine rating Valve gallery 50 Crank shaft 30 Engine interior 30 Oil pan 40 Oil screen 30 Piston skirt 10 Piston underhead 30 Cvlinder below rinx travel 10 Vilve stem 20 Connecting rod beaii n g s 20 Piston ring grooves 40 Connecting rods 10 Wrist pins 10 Average 25 Corrosion Wrist-pin bushings Severe Main bearings Severe Oil viscosity a t end of test, 8 8 0 . 672
Oil make-up added, quarts
4
2 20 10 20 20 0 5 10 5 10
10 10 5
Oil No. 5
60 10 10
so
10 0 10 0 20 10 30 10
5
0
10
18
None Xone
Bad Nonp Plastic solid 4
63
3
6 60 40 40 20 20 70 30 60 20 40 30 40
S 30 40 20 10 0 30
30 20
10 30 40
30 38
20 20 23
None None 117
h-one None 86
5
3
March, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
ing of the engine from a general cleanliness viewpoint. On this scale oil 2 rates best at 10, and oil 6 worst at 38. I n addition to the standard rating, a description of engine condition is required to evaluate the oils completely: Oil 1. This engine was dirtier in general appearance than the numerical ratin indicated, since the oil thickened to a greaselike consistency. &he engine parts were relatively free of lacquer, as is usually true when severe corrosion occurs. The wristpin bushings (bronze), connectin rod shims, and main bearings (Babbitt) were severely corrode$. Although this engine completed the 5000 miles, several engine tests on this oil have resulted in engine failure in 3000 to 4000 miles because of excessive bearing Corrosion. Oil 2. This engine was exce tionally clean, showing only slight lacquer on the pistons ancfengine interior. Deposits in the oil pan and valve gallery were slight and ap eared to be contaminating materials-i. e., lead compounds an%dust. Oil 5. This ensine aDDeared clean. showins no siens of lacquer or baked-on dkpositk: Corrosion’was eviaent on-the wristpin bushings. Although this oil had a high resistance to oxidation, in so far as oxygen absorption is concerned, its viscosity increased excessively, In other tests of this oil the viscosity increase was so great that two engines failed to complete the 5000 miles through lack of oil circulation; others showed loss of power because of viscous drag. Oil 6 . This engine was very dirty, showingconsiderable quantities of sludge in all parts. However, the engine completed the run of 5000 miles without loss of power or interference from the sludge. Oil 8. In general, the engine was fairly clean, and the drained oil was relatively low in viscosity and of good a pearance. The oil, however, had produced a granular type of fard sludge commonly called “coffee grounds”, which was carried freely by the oil. This is a characteristic of some oils when the refining has not been carried far enough to remove the slud e formers. This type of sludge is particularly bad because the oif appears t o be in good condition, but the small solid granules are carried into and plug the oil passages. In other tests of this oil three out of six engine runs failed between 3000 and 5000 miles because of burned-out bearings due to lack of oil.
Y
Similar Chevrolet engine tests on three SAE 20 automotive oils are shown in Figure 17. Oil 9 is a well-known commercial solvent-treated oil inhibited with a phosphite ester. Oil 10 is a Pennsylvania grade mineral oil (containing no recognizable inhibitor), and oil 11 is a solvent-treated midcontinent distillate (oil 7) inhibited with “Thialkene”, a sulfurized olefin derived from petroleum wax by halogenation, dehalogenation, and sulfurization (14). I n view of the previous discussion, detailed comments on the oil changes given in Figure 17 are not required; however, it will be of interest to compare the results from oil 11 containing the sulfur inhibitor with the same oil without the inhibitor as reported in Figure 13. The engines using oils 9, 10, and 11 were relatively clean. Oil 9 showed an over-all rating of 14 and was an exceptionally clean engine. The wrist-pin bushings were moderately corroded, and the front and rear main bearings were slightly corroded. The oil thickened considerably, and there was a coating of greaselike material in the valve gallery which was easily cleaned with gasoline. Oil 10 gave an engine with an over-all rating of 27 and was considerably dirtier than oil 9. The wrist-pin bushings showed corrosion, and the oil-ring drain holes were heavily carboned. Oil 11 gave an engine with an over-all rating of 14 (the same as oil 9) but no signs of corrosion. There was very little lacquer and carbon in this engine. Deposits seemed to be largely lead gray in color. This engine would have rated better except for a black discoloration of the connectingrod bearings which, however, were free and showed no signs of distress. It may be concluded that unless a n oil is completely inhibited with an addition agent having a strong precipitating action on metallic accelerators (as oils 2 and l l ) , the oil is
349
certain to oxidize rapidly in service even if i t has a high original resistance to oxidation as measured by the induction period. The objectionable oxidation products may be: 1. Insoluble in oil (as in No. 6), and may precipitate in the engine and thus roduce an excessively dirty engine. 2. Insoluble, gut in such small quantity that the precipitated material gathers in articles or “coffee rounds”. These may the oil passages of an cause engine failure tecause of pluggin otherwise clean engine, with the oil itsefSf in relatively good condition. This is ty ical of oil 8. 3. Insoluble, \ut under conditions where the insolubles do not precipitate but, owing to a peptizing action, are carried in suspension in the oil. This is typical of oil 10. In such cases the oil may become so viscous that power loss or engine failure may occur. 4. Soluble (such as resins , produced in oils 1, 5, and 9. The oil may become so viscous t a t en ine failure will occur. It is impossible to isolate the relative e8ect of items 3 and 4 in viscosity rise; but in most cases of excessive viscosity the resin content will be very high. 5. Corrosives, such as shown by oils 1, 5, 9, and 10.
09
h
Effects of Oil Oxidation Products on Engines I n addition to the troubles arising from engine deposits, oxidation products may also cause serious difficulties through the corrosion of engine parts, particularly bearings. The factors causing corrosion are not yet well established, but it is known that engine corrosion results from products both of combustion and of the oxidation of the lubricant. Boerlage et al. ( I ) and Duff (7) reported that cylinder corrosion results from products of combustion, particularly during the starting-up period and under low temperature operation. Many laboratory tests such as Underwood’s (93) have shown
-------OIL
NO I O NO.11
---O/L
* _ . e -
--------------------
/--
--------___
/--
1000
ZOO0
DURATION OF
3000
4000
SmO
rmr- MILES
FIGURE 17. PROXIMATE ANALYBES OF THREE SAE 20 OILSAFTER ENGINETESTS
350
INDUSTRIAL AND ENGINEERING CHEMISTRY
conclusively that some lubricants oxidize to produce acids which are highly corrosive to alloy bearings, such as cadmium-silver and copper-lead. Although it is difficult to determine the relative importance of combustion products and lubricant oxidation products in actual engine tests, it is possible to study the relative corrosiveness of different oils by rigidly keeping the fuel, rate of fuel consumption, and engine conditions constant. I n the work discussed above, all engines were equipped with tin-Babbitt bearings which are not particularly sensitive to corrosion; however, as mentioned above, corrosion of wrist-pin bushings and main bearings occurred with certain oils. As a general rule the used crankcase oil has been found to contain relatively large amounts of iron when corrosion is observed, indicating that not only the bearings but also the cylinder walls, piston rings, etc., have been corroded. Keller (10) and others have suggested the measurement of cylinder corrosion or “wear” by the iron content of the used oil. The results of the iron determinations on the used oils from the engine runs discussed above are given in Table V. The iron content is given for the oil, both as drained from the engine and after filtration to remove all oil insolubles. It is assumed that the oil-soluble iron is available as an oxidation accelerator, and that the insoluble iron in suspension in the oil is the result either of abrasion or of precipitation from soluble compounds.
TABLE V. Oil No.
1
2 0
7
11 12 9
10
Corroeion Wrist-pin bushings Bearings Severe Heavy None None None None Heavy Severe None None Trace Heavy Moderate Light None Light
ENGINE CORROSION c/o Be in Used Oil Unfiltered Filtered 0.034 0.016 0,002 0,002
0.008 0.210 0.018 0.072
0.003 0.176 0,002 0.061
0.032 0.019
0.030 0,004
Viscosit Enqine of U s e 2 Rating Oil, Sea. 25
10
675 63
14
540 56
20 20 27
108 260 124
38 22
117
Oils 2, 6, and 11, which showed no signs of corrosion in the engine, have very low total and soluble iron content. The other oils, all of which showed engine corrosion, have relatively high iron contents. It is particularly interesting to note that those oils having high iron content show excessive oxidation as measured by the final viscosity. The effect of added inhibitors is well shown in oils 7, 11, and 12. Oil 7, a solvent-treated SAE 20, showed severe corrosion, high iron content in the used oil, and excessive viscosity increase. Oil 11, the same oil inhibited with a sulfurized olefin, showed no corrosion, low iron, and almost no viscosity rise. Oil 12, the same oil inhibited with a phosphite ester, showed improvement over oil 7 but was not so completely inhibited as oil 11. The effectiveness of the sulfur-bearing inhibitors appears to be due to two causes-the precipitation of metallic accelerators as discussed above, and the direct protection of the metal surfaces from corrosion. Chemical examination of the bearing surfaces after use of the sulfur inhibitors always shows the release of hydrogen sulfide in the presence of an acid, which indicates the presence of the sulfide. Simard et al. (% in I studying ), extreme pressure lubricants, showed by electronic diffraction the presence of a metallic sulfide on metal surfaces after using a sulfur-bearing lubricant. This was particularly true of the copper-alloy bearing materials. It is probable that the presence of this same film gives the improved load-carrying or am-strength characteristics to lubricants containing sulfur additives.
Vol. 33, No. 3
Conclusions Many of the hydrocarbons found in lubricating fractions oxidize with ease, and their behavior in use is improved by the presence of oxidation inhibitors, either naturally occurring or added. These inhibitors may have three distinct actions: a true antioxidant characteristic which increases the induction period, a “precipitating” action which removes or counteracts the effect of metallic accelerators, and the formation of protective films on metal surfaces which retard corrosion and therefore the formation of oil-soluble metallic accelerators. Sulfur-bearing esters of the “Methyl-S-linoleate” type and sulfurized olefins derived from petroleum wax have all three characteristics to a marked degree. So-called antioxidants, such as those of the amine and polyhydroxy aromatic types, may have a remarkable effect by increasing the induction period but have no effectiveness against metallic accelerators. The final proof of ability of a lubricant to withstand oxidation must be in practical lubrication tests such as actual engine runs. The physical and chemical changes in the oil during use are an accurate measure of the rate of oxidation, but the final determination is the effect the oxidation products have on the engine.
Acknowledgment The authors wish to acknowledge the work done by various members of the Continental Oil Company research staff in preparing the data used in this article. All engine test work was carried out under the direction of B. E. Sibley and was supervised by Robert D. Best.
Literature Cited (1) Boerlage, G. D., and Gravesteyne, B. J. J., Brit. Motorship, 13, 150 (Aug., 1932). (2) Burwell, A. W., IND.ENG.CHBM.,26, 204 (1934). (2A) Chernozhukov, N. I., and Krein, S. E., Neftgunoe Khoz., 1932, 242-50, 280-90; 1933, 35-9, 102-5, 1815, 2775; 1934, 3001; 1935, 3467; 1938, 59-66. (3) Clayden, A. L., Proc. Am. SOC. Testing Materials, 30, Part I, 460 (1927). (4) Dornte, R. W., IND. ENG.CHEM.,28, 26 (1936). (5) Dornte, R. W., and Ferguson, C. V., Ibid.,28, 863 (1936). (6) Dornte, R. W., Ferguson, C. V., and Haskins, C. P., Ibid.,28, 1342 (1936). (7) Duff, W. N., Engineer, June 23 and 30, 1933. (8) Hall, F. W., Levin, Harry, and MoMillan, W. A., IND.ENQ. CHEM.,Anal. Ed., 11, 183 (1939). (9) Haus, Erich, Oel Kohle Erdoel Teer, 14, 299, 321 (1938). (10) Keller, G. H., Automotive I d . , 72, 484 (1935). (11) King, R. O., J. Inst. Petroleum Tech., 20, 97 (1934); Oil @a8 J., 32, No. 37, 13 (1934). (12) Levin, Harry, and Towne, C. C., IND. ENQ.CHQM.,Anal. Ed., 11, 181 (1939). (13) Lincoln, B. H., and Steiner, W. L., U. S. Patents 2,113,810-11 (April 12, 19381, 2,186,646 (Jan. 9, 1940). (14) Lincoln, B. H., Steiner, W. L., and Byrkit, G. D., Ibid., 2,218,132 (Oct. 15, 1940). (15) Marcusson, J., Burchartz, H., and Wilke, P., “Die naturlichen und kunstlichen Asphalte”, 2nd ed. rev., Leipzig, Wilhelm Engelmann, 1931. (16) Moureu, Charles, and Dufraisse, Charles, Chemistrll & Induatrg, 47, 819, 848 (1928). (17) Noack, K., Oel Kohle Erdoel Teer, 13, 959 (1937). (18) Poell, H., Erdoel u. Tern, 7 , 360, 366 (1931). (19) Schindler, Hans, and Bondi, A,, Petroleum Z., 34, No. 10, Motorenbetrieb u . Maschinen-Schmierung, 11, No. 3, 2-6 (1938). (20) Simard, G. L., Russell, H. W., and Nelson, H. R., A. C. S., Petroleum Div., Reprints of Papers, Boston Meeting, p. 119 (1939). (21) Suida, Hermann, OeZ Kohl6 Erdoel TeeT, 13, 201, 225 (1937). (22) Suida, Hermann, and Poell, H., Petroleum Z.,33, No. 10, 1 (1937). (23) Underwood, A. F., S. A. E. JWT?IU~, 43, 3851’ (1938).
