OXIDATION OF LUBRICATING OILS Factors Controlling Oxidasion Stability' M. R. FENSICE, C. E. STEVENSOX, N. D. LAWSON, GLENN HERBOLSHEIMER, AND E. F. KOCH The Pennsylvania State College, State College, Penna.
A study of the oxygen absorption characteristics and the nature of the products obtained from oils oxidizing under controlled conditions has permitted the evaluation of a number of factors involved in lubricating oil deterioration. Investigation of selected fractions of a distillate stock has shown considerable variations in reactivity toward oxygen. The rate of oxygen absorption approximately doubles for each 10' C. temperature increase over the interval 140' to 180' C., provided it is not limited by physical factors. The presence or absence of water vapor in the oxidizing atmosphere has little effect on the oxidation of one type of oil. The oxidation of certain oils can be reduced greatly by adding natural or synthetic inhibitors. Clay treatment may either reduce or improve the stability of a n oil, depending upon its nature. Iron, copper, and lead are strong catalysts for oil oxidation when present either i n metallic form or as
naphthenate salts. Certain types of oils have a natural ability to counteract the catalytic effect of dissolved iron salts. The use of a phosphite additive controls or eliminates catalysis by certain iron, copper, or lead salts, but such additions are frequently accompanied by other changes in the oils so that the over-all effect is more complex than simply a reduced degree of oxidation. Addition agents are capable of making marked changes in the degree of oxidation occurring in oils. They may also influence the direction and character of other accompanying reactions, such a s polymerization, condensation, decarboxylation, dehydration, etc., with the result that many of the oxidation products may now be affected independently of the over-all degree of oxidation. I n general, addition agents must be studied thoroughly and i n detail in order to understand all the changes that are induced by their presence.
HE deterioration of the lubricant in an internal combustion engine appears to be the result of the simultaneous action of a number of factors, principal among which is the reaction with oxygen a t some temperature in the range of 100" to 200" C. This reaction is, or may be, affected as to both its rate and course by conditions imposed by the material and structure of the engine, and the manner in which it is operated. Some of the more obvious of these conditions are as follows: the presence of a variety of metallic catalysts, and of water vapor and other fuel combustion products; the partial pressure of oxygen in the crankcase atmosphere as a result of blow-by and ventilation; the effectiveness with which the bulk oil in the crankcase is agitated, and its exposure as a film on cylinder walls, etc., or as a spray; the effect of removing the volatile products, etc. I n addition, certain of the intermediate and final products of oxidation are more reactive than are the original hydrocarbon oil molecules and so may be affected to varying extents by pyrolysis and by reaction with the engine metals and fuel products. It is not surprising that no one of the variety of empirical oxidation tests which has been proposed is capable of evaluating the stability of oils for use in engines under all circumstances. Exactly which factors are significant and actually determine oil stability are not known. The relative importance of these factors undoubtedly varies greatly from one type of engine to another, with different engine operating conditions, and as the motor becomes older. The increasingly complex nature of modern lubricants confuses the problem of correlating laboratory tests and motor
results. I n addition to the variety of hydrocarbon types present in a n oil there are small but probably significant quantities of oxygen, sulfur, and nitrogen-containing compounds of natural origin. Specific demands made on the lubricant have resulted in the addition to i t of a variety of additives such as pour point depressants, viscosity index improvers, oiliness and film strength compounds, oxidation inhibitors, bearing corrosion inhibitors, materials classed as detergents and sludge dispersers, etc. The synthetic compounds used include condensation products of aromatic hydrocarbons or phenols with aliphatic compounds; highmolecular-weight polymers of unsaturated hydrocarbons and unsaturated esters; halogenated and sulfurized fatty oils and fatty acid esters; esters of phosphorous and phosphoric acids; various sulfur and phosphorus derivatives of aromatic compounds; organometallic derivatives of arsenic, antimony, tin, and lead; alkaline earth and similar metal salts of fatty acids and naphthenic acids; and an increasing variety of related compounds. The literature records numerous efforts to determine the effect of certain variables on the formation of oxidation products, among which may be mentioned the work of Haslam and Frolich ( 7 ) ,Meade (IO), and Weiss and Vellinger ( I S ) . Many such investigators were hampered by the limitations of their test methods in arriving a t an understanding of the degree of oxidation which actually took place. A preceding article in this series (6) described an apparatus and procedure which would permit the quantitative evaluation in an isolated system of some of the above-mentioned factors in oil deterioration. The work described here represents some of the results obtained by this means. Innumerable variations
T
1
For the first paper in this series, see literature citation 5 .
516
April, 1941
517
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
proach is the oxidation of components separated from one another as efficiently as possible by the application of modern exhaustive separation methods. The separation of a lubricant stock in this laboratory into 124 fractions by a combination of vacuum distillation and solvent extraction applied to a semirefined oil is to be described (4). The availability of these fractions for oxidation studies has made the second approach possible. While many phases of the work on the fractions are being investigated, the present report relates to the oxidation of a group of seven fractions or blends separated, as mentioned above, from a selected base oil. Figure 1 illustrates the approximate relation which these components bear to the base stock, while the properties of the selected fractions and of the over-all oil are given in Table I. The values for n and z in Table I refer to the carbon atom content and hydrogen deficiency in
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Component
TABLE I. SELECTED FRACTIONS OB A DISTILLATE LUBRICANT STOCK 05 B F c Q
Kinematic viscosity, centistokes At 100' F. At 210'. F. Saybolt viscosity, seconds At 100' F. 37 8' C ) At 210.' F. {98:9' C : ) Kinematic viscosity index Gravity, OA. P. 1. Viscosity-gravity constant Waterman anal sis Density, 20",&" C. Refractive index, n v Sp. refraction at 20' C. Aniline point, C. , aromatic rings naphtliene rings paraffin chains Rings per mol. Calcd. mol. wt. In formula CnHin + z n
f
X 0
37.56 5.76 176.4 44.3 103 30.3 0.520 0.5710 1.4841 0.3256 99.6 9 16 75 1.7 404 29.2 -4.6
130 5.70
101.8 9.05
601 54.8 -21 17.6 0.901
470 56.0 51 24.2 0.849
0.9451 1.5353 0.3296 47.3 40 7 53 2.8 341 25.4 -14.6
0.9052 1.5043 0.3273 78.2 25 15 60 3.1 440 32.4 -13.2
744 24.5 3458 116.7 7 17.9 0.877 0.9435 1.526 0.3254 62 37 20 43 4.9 495 36.9 -21.5
Vol. 33, No. 4
S
T
r
3.56
124.0 11.38
21.39 4.35
38.96 6.38
91.4 38.9 131 34.9 0.803
572 63.2 51 27.0 0.823
103 40.5 137 35.6 0.795
181 47.0 125 34.8 0.789
0,8468 1.4691 0.3290 103.2 2 18 80 1.2 343 24.6 -0.9
0.8888 1.4920 0.3264 97.3 15 20 65 3.1 517 37.6 -10.5
0.8432 1.4670 0.3291 108.8
0.8476 1.4693 0,3287 116.7
18 82 1.1 367 26.2 -0.3
15 85 1.2 443 31.6
18.53
0
0
-0.4
The original stock from which fractions were prepared by distillation and extraction.
were subjected in separating the fractions. It was found, however, that by recombining all 124 fractions in the proportion in which they were obtained, a material was secured of approximately the same physical characteristics and oxygen susceptibility as oil 0. Fractions Q, T,and U in the first group illustrate a type of oxygen absorption curve described as inhibited or autocatalytic in nature. There is a so-called induction period during which the rate of oxidation is relatively low. At the end of this period there is a sharp increase in the rate of absorption which continues a t a relatively uniform rate until the oil is highly oxidized; then the oxidation rate declines. The maximum absorption rate is almost the same for all oils and appears to change very little with temperature. It probably represents a rate limited by physical factors such as diffusion and agitation, since a t this maximum only about 10 per cent of the oxygen supplied by the circulating device is being absorbed. The third phase relating to the decreasing oxygen absorption rate may be a result of a lower concentration of reactive molecules, or to oxidation inhibition by some product or products of the reaction. This type of o x i d a t i o n i s characterized by a " clean", highly =, acidproduct; that HOURS O f OX/DAT/ON is, little lacquer and oil-insoluble FIGURE 3. EFFECT OF TEMPERATCRE ON OXYGEN ABSORPTIOK material is produced, although acidity, the amount of oil-soluble oxygenated substances, and viscosity increase may be high. The four aromatic-containing fractions B, F, G, and X are characterized by oxygen absorption-time curves which are straight or almost straight lines. Base stock 0 belongs in this class also. A further characteristic is the production of significant quantities of oil-insoluble bodies and lacquer. At times this characteristic may not be too apparent, as in the case of 0 and F where the degree of oxidation is low, or of G where the degree of oxidation is high but the oxygenated material has apparently remained in soluble form, resulting in high viscosity increase
7€mr?ERATUP€
2.
FIGURE 4. EFFECT OF TEMPERATURE ON OXIDATIONCHARACTERISTICS 250 grams of oil 0 (Table I! oxidized for 20
hours
and acidity. The ratio of volatile acids produced to the neutralization number is also significantly lower for the aromaticcontaining fractions.
Effect of Temperature on Oxidation The results of experiments over a temperature range of 140" to 180" C. have indicated that for a wide variety of oils the reaction rate approximately doubles for a temperature increase of 10" C. in a fairly uniform manner. Although the absorption-time curves for many oils may appear to be linear, in no case has an absolute linear proportionality been found between millimoles of oxygen absorbed and time. The first portion of the curve may be convex either up or down, or it may even be slightly S-shaped. Consequently, curves obtained a t different temperatures can be compared as to rate only a t points of equal absorption, and then only
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1941
when the temperature change has not altered the course of the reaction. The effect of temperature on the oxidation of oil 0 may be seen in Figure 3. This behavior is typical of oils giving a n approximately linear absorption curve. For oils showing inhibited or autocatalytic oxidation, the length of the induction period is similarly affected; that is, the induction period is approximately halved for each 10" C. rise in temperature. Beyond the induction period, the subsequent behavior of autocatalytically oxidizing oils cannot be predicted by this means. One such oil was studied which showed a slightly lower oxygen absorption in 50 hours at, 150' C. than in 50 hours a t 140" C. There are apparent changes in the course of the reaction with increase in temperature as shown by the data on the distribution of the absorbed oxygen in Table I1 for the same group of runs on the oxidation of oil 0. Thus, with increasing temperature the proportion of the absorbed oxygen going to the formation of water has apparently decreased while that going to carbon dioxide, carbon monoxide, and volatile and fixed acids has increased. It is believed from other data that this is more a function of the amount of oxygen absorbed than it is of the temperature level a t which the oxidation is conducted, and that if this oil had been oxidized sufficiently long a t 150" C. so as to absorb the same amount of oxygen that it acquired in 20 hours a t 180" C., essentially the same distribution of the absorbed oxygen would be found a t the lower temperature. Not only is the rate of oxygen absorption approximately doubled for each 10" C. rise, but also the rate of production of lacquer and precipitables, volatile and fixed acids, etc., is similarly affected. This is indicated more clearly when the data for the amount of these products are plotted on a logarithmic scale against a linear temperature scale, as shown in Figure 4. The only data which deviate significantly from a simple log function are those for the production of precipitables. These deviations are better understood when it is considered that before oil insolubles appear, they must be present in sufficient quantity to exceed their solubility in the oxidized oil. Treatment with isopentane does not precipitate all the oxidation products, and a certain minimum occurs before oil-soluble isopentane-precipitable oxidation products appear. Effect of Water Vapor in Oxidizing A t m o s p h e r e It has been stated (IS) that for certain oils the rate and course of their oxidation are much affected by the humidity of the oxidizing atmosphere. I n particular, mention was made of decreased oil insolubles when oxidation occurred in the presence of water vapor. OF TEMPERATURE ON OILOXIDATION* TABLE 11. EFFECT
Temp. of oxidation, C. 150 Av. partial ressure Oz mm. Hg, 700 34 Millimoles 8 2 absorbed7250 g. oil Distribution of absorbed oxygen, 70 To Hr0 66.5 T o COz 3.8 T o GO 0 6 T o volatile acids 1.0 T o fixed acids 2.1 To isopentane-inso1.b 3.1 0.2 Neutralization No. of ,oxidized oil< 0.3 Milliequivalents volatile acid/250 g. oil Precipitable oxidation products A. T o t a l isopentane-insol., wt. % ' 0 09 B. Oil-sol., is0 entane-insol., wt. Yo 0.09 C. Oil-insol. wt. 0.00 D . M g . lacquer on 3 X 1 in. slide 0.9 Increase in vjscosity a t 100' F. (37.8' CJ, % Clarified oil 3.3 Isopentane-treated oil 3.3 0 250 grams oil 0 (Table I) oddiaed for 20 hour@. b Assuming is0 entane-insol. contains 15% oxygen. Milligrams per gram oil.
(I-B),
OH
160 698 68
170 685 150
TABLE 111. EFFECTOF WATERVAPORO N OXIDATIONCHARACTERISTICSa R u n number 237 238 249 Av. partial pressure Or mm. Hg 673 667 685 12 12 Av partial ressure H i 0 mm H g 0 Miilimoles absorbed/i50 g.' oil 131 139 150 Neutralization No. of oxidized oilb 0.8 0.7 0.7 Precipitable oxidation products: A. Total isopentane-insol., wt. 70 0.70 0.64 0.74 0.15 0.21 B. Oil-sol. isopentane-insol., wt. % 0.16 C Oil-insol (A-B) w t 70 0.54 0.49 0.53 D: Mg. lac4uer on 3' X i in. slide 3.5 3.3 3.2 Increase in viscosity at 100' F. (37.8' C.), % Clarified oil 10.1 9.5 12.8 Isopentane-treated oil 8.7 9.9 11.9 250 grams oil 0 were subjected t o oxidation for 20 hours a t 170' C . (338' F.) in the resence a n d absence of water vapor. b Milligrams $OH per gram oil.
8r
Q
Certain alterations in the apparatus and technique were required in order to oxidize with a humid atmosphere. For this purpose the circulating oxygen was bubbled through water before entering the oil. The bubbler was maintained a t about 14" C. which, if the oxygen became saturated, should have provided a water vapor partial pressure of 12 mm. in the circulating gas. The presence of this excess water over that produced by the oxidation made it impossible to determine the latter, and no water absorbent was used. Sodium hydroxide solution was utilized in place of ascarite to remove carbon dioxide from the circulating gas, but the amounts of carbon dioxide, carbon monoxide, and volatile acids produced were n o t determined. The material oxidized was oil 0 described in Table I. The effect of the presence of moisture @ on the oxidation and its productsis shown in Table 111. The ~y absorption of oxygen was slightly less than would be an8 ticipated from the decreased oxygen p a r t i a l pressure. The neutralization number a n d t h e amount of oil inHOURS AT /70'C (336-f) solubles and lacquer were substantiallv FIGURE 6 . EFFECT OF INHIBITORS the same, while viscosity increase was lower in proportion to oxygen absorption. I n general, i t appears that for this instance the presence of water 180 vapor had a negligible effect on both the rate 698 364 and course of the oxidation of this oil.
' 3 %
9 $
1
48.5 9.1 2.8 3.9 2.1 7.1
.O) 0.38 0.26 0.12 1.5 7.0 , 6 4
0.74 0.21 0.53 3.2 12.8 119
519
l;:; 2.22 0.68 1.54 9.4 32.5
28.2
Effect of Inhibitors I n the section on "Effect of the Nature of the Oil" it was noted that the fractions examined were more susceptible to oxidation than the original oil. This suggests that there must be present in the base stock materials which are capable of restricting the oxidation of other components. It has been found possible to separate fraction5 containing certain materials which have this inhibiting effect on the oxidation of susceptible stocks. The action of two such fractions or inhibitors will be described here.
INDUSTRIAL AND ENGINEERING CHEMISTRY
520
TABLE IV. Oil number Kinematic viscosity, centir,tokes A t 100 F. A t 210'. F. Saybolt viscosity, seconds A t 100' F. At 210' F. Kinematic viscosity index Gravity, 'A. P. I. Viscosity-gravity constant Waterman analysis Density, 2O0/4O C. Refractive index, n%O Sp. refraction a t 20' C. Aniline point, O C . aromatic rings naphthone rings paraffin chains Rings per mol. Calcd. mol. wt. I n formula CnHzri + z
PROPERTIES OF OILS AND OIL FRACTIONS STUDIED
1
2
3
5
8z
a b 0
5
8
7
6
32.94 5.50
26.40 4.81
26.30 4.80
35.59 5.93
27.72 4.99
312.5 17.1
287.5 16.35
248 49.9 104 29.7 0.818
154.0 44.2 117 33.6 0.801
125.9 42.0 123 34.1 0.802
124.6 42.0 123 34.0 0.803
179.4 45.6 108 31.2 0,809
130.8 42.6 123 34.3 0.801
1451 85.2 45 21.4 0.856
1328 83.1 46 21 . R 0.855
0.8740 1,4862
0.8534 1.4720
0.8509 1.4712 0.3286 107.5 2 17 81 1.2 379
0.8511 1.4717 0,3288 109.1 17 82 1.2 381
0.8660 1.4809 0,3286 103.6 7 15 78 1.6 410
0.8499 1.4704 0.3285 108.0 2 80 1 3 387
0.9216 1.5120 0.3251 74.0 29 20 51 4.1 494
0.9188 1.5138 0.3276 76.4 29 13 58 3.6 495
27.2 -2.3
27.3 -0.8
29.6 -3.9
27.7 -1.3
36.6 -17.0
36.6 -16.7
32.5 -5.4
28.8 -1.7
TABLE V. Inhibitor added yo b y weight Av. partial ressure 0 2 , mm. H g Millimoles absorbed/250 g. oil Distribution of absorbed oxygen, % T o HzO T o CO2 T o CO T o volatile acids T o fixed acids T o isopentane-inso1.b Neutralization No. of pxidised oilc Milliequivalents volatile aoid/250 g. oil Precipitable oxidation products A . Total isopentane-insol., wt. % B. Oil-sol. isopentane-insol., wt. % C . Oil-insol. ( A - B ) , wt. % D. Mg. lacquer on 3 X 1 in. slide Increase in viscosity a t 100' F. (37.8' C Clarified oil Isopentane-treated oil
4
53.36 7.24
8
1L
Vol. 33, No. 4
1
1s
EFFECTOF INHIBITORS" D
D
None 0 650 993
6 674 433
12 656 259
44.3 11.2 3.2 7.5 2.5 2.3 7.6 74.6
49.5 8.2 2.8 6.7 2.4 2.8 3.0 28.9
45.6 6.7 2.4 4.1 2.7 4.3 3.2 10.5
1.89 1.89 0.00 0.1 133 113
1.05 0.51 0.54 2.7 40.7 38.8
0.95 0.20
0.75 4.0
21.3 22.3
D
C 5
20 685
176
4Z.f 0.0
2.1 3.9 1.9 7.7 1.1 6.9 1.15 0.32 0.83 4.2
10 689 160
15 686 147
47.1 10.0 3.6 7.4 2.0 2.3 4.3 38.0
52.6 6.5 4.1 4.5 2.8 4.3 1.0 7.2
52.6 6.1 2.4 3.5 2.2 8.0 1.3 5.1
1.03 0.65 0.35 2.1
15.6 13.9
c
C
671 516
50.9 48.3
0.59 0.22 0.37 2.3 13.0 12.1
1.00 0.44 0.66 2.6 11.2 9.2
250 grams oil 2 (Table 11) plus inhibitor oxidized 20 hours a t 170" C. Assuming is0 entane-insol. opntains 15% oxygen. Milligrams $OH per gram oil.
Oil 2 of Table I V was the base stock used in preparing the inhibited blends. This was a light distillate also subjected to extensive extraction and clay treatment. The absorption curves in Figure 8 show that when oxidized a t 170" C., this oil had an exceedingly short induction period, of the order of a few minutes only. Figure 5 was constructed with an expanded ordinate scale below 200 millimoles absorbed. This was done in order that the lower portion of the curves would be better separated and defined. Low concentrations of the inhibitors (C and D) prolonged the induction period very little but produced some reduction in the total oxygen absorbed in 2 hours by shortening the period of rapid oxidation. As the amount of inhibitor was increased, the oxygen absorption of the blends was considerably reduced. At higher concentrations (10 per cent of C or 20 per cent of D) a limit was reached beyond which there was no further reduction in oxygen absorption rate. The absorption-time curves are then substantially linear and uniform. Other characteristics of the oxidation of the blends are affected by increased inhibitor concentration. Thus, the data in Table V for the same runs show that the amount of insoluble oxidation products is increased almost in proportion to the inhibitor content even though the total oxygen absorption may be reduced. Various other data, such as neutralization number, volatile acids, and viscosity increase, parallel the amount of oxygen absorbed. There are definite differences between the action of the two inhibitors. For the same percentage added, C is considerably more effective than D in reducing oxygen absorption and results in the production of less lacquer and oil-insoluble material.
Effect of Additives Additives for many purposes have been proposed for use in motor lubricants. In the present study interest is directed particularly toward additives which have as their purpose the restriction or inhibition of oxidation, although obviously those which purport to act as detergents, sludge dispersers, bearing corrosion inhibitors, etc., are also relevant t o oil deterioration. A list of sixty-nine antioxidant patents was given by Byers ( 2 ) . Since this publication the number of such patents has tripled or quadrupled. Most of them fall into one of the following classes: (a) hydroxy compounds, such as phenolic derivatives, naphthols, etc.; (6) nitrogen compounds, including naphthylamines, aniline derivatives, etc.; (c) sulfur compounds as typified by disulfides, thioethers, eto.; (d) compounds of the organometallic type; (e) compounds of halogens; and (f) compound involving the oxygen higher members and sulof fur groups in the periodic system, such as phosphorus, arsenic, antimony, selenium, and tellurium derivatives. From the list of commercially available additives three compounds ( M , N , P), -which consist
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9 400 2'3%M,
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N O 7. (336%;)
FIQURE 6. EFFECT O F ADDITIVES
April, 1941
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
perature. The wide disparity in the results obtained on oxidizing oils 7 and 8 indicates that the clay must have removed materials which were either very susceptible to oxidation or were potent promoters of the reaction. Large quantities (over 10 per cent) of oil-soluble isopentane-insoluble materials were formed in the untreated oil which then developed very high acidity and viscosity increase, whereas the degree of oxidation in the treated material was so low that few products of the oxidation were detectable. An opposite effect of clay is indicated by the results obtained from the treatment of oil 3 (Table IV). This fraction is a light distillate of high viscosity index and relatively low in aromatic ring content, and a t 150' C. showed an induction period of 6 hours before rapid oxygen absorption took place. Contacting of oil 3 with 3.3 weight per cent activated earth at 160' C. for 20 minutes produced oil 4 with almost identical physical properties. The induction period as shown in Figure 7 had been reduced to slightly over one hour. Since rapid oxidation started earlier with oil 4,it oxidized to a somewhat greater extent than oil 3. The result was a greater production of oil-insoluble isopentane-insoluble material, though the general oxidation characteristics were otherwise similar.
essentially of carbon, hydrogen, oxy$ gen, and sulfur, were 800 selected as typical inhibitors. Additive 8 N is believed to 600 be substantially dibenzyl disulfide. 400 The effect of these materials in reducing the oxygen ab200 sorption of highly $ susceptible oil 2 0 (Table IV) is shown in Figure 6 and ' j o ' ' 'do'' "Jo' ' ' 'A' 'd T a b l e V I . The HOURS AT /40%. similarity of their FIGURE 7. EFFECT OF CLAY action to that of the TREATMENT natural materials noted in the preceding section is evident. The induction period is prolonged indefinitely while significant quantities of oil-insoluble materials and lacquer are produced. Variations in the oxidation characteristics resulting from the use of the three additives are apparent. The blend containing P has absorbed the least oxygen, while N has produced the cleanest oxidation in terms of precipitables and lacquer. The use of either M or N resulted in substantially the same amount of oxidation, although M produced much more oil-insoluble materials and lacquer, and N had a considerably greater viscosity increase and greater amount of volatile acids. All three additives differed from the inhibitors discussed before in producing insignificant quantities of oil-soluble Additive oxygenated materials. Weight %
8 ~wo
$
t
e
!
L'
"
"
Effect of Clay T r e a t m e n t The use of absorptive earths, such as fuller's earth and bauxite, is common and widespread in refining lubricating oils. An important function of this procedure is improvement in color to produce salable products. It is to be anticipated that the stability of the treated oil may be affected by removal both of susceptible materials such as cracked products, peroxides, oxygenated compounds, etc., and of other materials capable of influencing the course and extent of the oxidation reaction. Improvement of oxidation stability upon clay treatment is illustrated by the behavior of oils 7 and 8 in Figure 7 and Table VII. The properties of these fractions are given in Table IV. Oil 7 is a residual fraction of intermediate viscosity index and moderately high aromatic ring content, while oil 8 was produced by contacting oil 7 with 20 weight per cent activated earth (200-mesh Superfiltrol) for 10 minutes a t 175" C. Other than a slight reduction in density and viscosity, there was no msrked effect on the physical properties resulting from clay treatment. The curves for oils 7 and 8 in Figure 7 are not entirely comparable since the former was oxidized at 140' C. for 50 hours and the latter a t 150" C. for 20 hours. The 10' C. temperature increase is approximately equivalent to doubling the length of exposure; therefore the two curves were plotted assuming equal severity of conditions by compressing the time scale for the lower tem-
52 1
Catalysis by Metals Probably the most significant factor in oil deterioration in engines as opposed to laboratory oxidation tests is the presence in engines of a wide variety of metals in bulk form. These metals are subject to wear in varying degrees, with the
TABLE VI. EFFECT OF ADDITIVES~ None
To isopentane-inso1.b
M
N
P
0 650 993
3.0 666 236
0.2 700 268
0.2 699 163
44.3 11.2 3.2 7.5 2.5 2.3 7.6 74.6
61.2 9.2 1.9 2.5 1.7 6.2 1.6 5.7
54.0 7.9 2.4 4.5 1.9 1.1 1.6 12.0
58.9 7.0 2.5 4.5
i.89 1.89 0.00 0.1
D. Mg. lacquer on 3 X 1 &. slide Increase in viscosity at looo F. [37.S0 C.I* % Clarified oil 133 Isopentane-treated oil 113 a 250 grams oil 2 (Table 11) oxidized 20 hours a t 170' C. b Assuming is0 entane-insol. contains 15% oxygen. C Milligrams $OH per gram oil.
1.24 0.02 1.22 6.3 8.2 9.0
0.26 0.02 0.24 1.9 19.3
...
2.1
3.4 0.8 7.4 0.47 0.00 0.43 3.8 10.2 10.0
TABLE VII. EFFECT OF CLAYTREATMENT^ Oil number Clay treatment Temp. of oxidation O C. Duration of run hdurs Av partial reekre Oz mm Hg Miillmoles absorbed/250 'g. oil Distribution of absorbed oxygen, yo T o Hz0
8z
-
.-. .-. .. .
Isopentane-treated oil a Basis 250 grams oil. b Assuhing isopentane-insol. oontains 15% oxygen C Milligrams KOH per gram oil.
3
None 150 20 662 855
4
Treated 150 20 679 1052
7
8
140 50 622 729
150
None Treated 20 693 106
39.3 9.8 2.6 6.4 4.0 3.5 11.7 55 2.52 2.35 0.17 0.7
4.33 4.25 0.08 1.4
11.3 10.1 0.8 0.0
108 96
158 113
508 28
0.08 0.06 0.02 0.0 22.1 20.7
INDUSTRIAL AND ENGINEERING CHEMISTRY
522
s
600
b
% \
500
8400
%
$300
e 9 zoo 8
3
PO0 0 4 8 HOURS AT
0
/2 /6 N O ' C (338 "F)
20
FIGURE8. CATALYSIS BY METALS IX MASSAND IN SOLUTION Base Oils 1. Oil 0, Table I 2 Oil 5 Table I1 3: Oil 1: Table I1
Soluble Metals Added a8 Naphthenate Salts 36 p. p. m. Fe 20 p. p. m. Cu 500 p. p. m. Pb Massive Metals 7. 1 50 g. No. 20 iron wire 8. 5 17 g. No. 23 copper wire 9. 5 19 sq. cm. lead sheet
4. 0 6. 0 6. 0
+ + +
++ +
production of fine particles piesenting a large surface, and also to corrosion by the products of oxidation and combustion. Iron is present in greatest quantity, but copper, lead, zinc, tin, aluminum, cadmium, silver, nickel, chromium, and others may be present alone or as components of alloys. In addition, the fuel may introduce tetraethyllead or lead salts. Many oxidation tests have been devised involving metallic catalysts, and various investigators have attempted to evaluate the effect of various metals on oil deterioration (1, 3, 9). From the above list iron, copper, and lead have been selected for study as well-recognized catalysts whose occurrence in engines is almost invariable. It was recognized a t the outset that the action of metals in solution as salts could be quite different from that of the bulk or massive metals
VOl. 33, No. 4
which offer only a limited amount of surface. The effect of both types of catalysts are described in Figure 8 and Table VIII. Three different base oils were used in making these comparisons: oils 0, 1, and 5 . These oils are essentially similar as to souwe and refinement. Of the dissolved salts, copper appears to be the most effective and lead the least on a weight basis. The bulk metals are also potent catalysts and appear to be less readily inactivated by natural materials in the oil. This is due in all probability to the fact that a metal mass offers a continuously exposed surface, rhich in turn maintains the metal in solution. This would happen unless the surface was coated very effectively or otheriyise deactivated by some means so that the supply of soluble metal was eliminated. The copper and iron surfaces thus became less active as the oxidation progressed. The action of the lead surface vias quite different and can be correlated reasonably well with the known high oil solubility of lead salts of organic acids. The appearance of an induction period is provided since catalysis does not occur until the oxidation has progressed for some hours. A much larger quantity of lead must be dissolved to perform effective catalysis, and the lead probably goes into solution only after a sufficient amount of corrosive acids has been formed in the oxidizing oil. Once such catalysis sets in there is a cumulative or exponential effect until a rapid rate of oxygen absorption is reached which is then limited only by p h y si c a l factors. 3oo ('onsistent with this assumption, the \ copper and iron surfaces after the oxida2 tion was completed ZOO were covered with lacquer, and they lost negligibly in $'O0 weight. The lead surface mas bright and free from any deposit and lost 5.3 '0 L 8 /2 /6 20 grams per 250 grams HOURS AT /70 ' C ('S8 of oil. The effect of varyFIGURE9. CATALYTIC EFFECTOF ing t h e catalyst IRONADDED AS NAPHTHENATE
8
8
3
5 4
O F )
I
TABLE VIII. Oil No. Catalyst
0 None
., .
Amount of catalyst Av. partial ressure 0 2 . nim. Hg Millimoles absorbed/250 g. oil Distribution of absorbed oxygen, 7Q To Hz0 To COi To CO T o volatile acids To fixed acids T o isopentane-inso1.b Neutralization No. of oxidiped oilc Milliequivalents volatile acid/250 g. oil Precipitable oxidation products A . Total isopentane-insol., wt. % B . Oil-sol. isopentane-insol.. wt. yo C. Oil-inaol. ( A - B ) , wt. % D. Mg. lacquer on 3 X 1 in. slide Increase in viscosity at 100' F. (37.8' C Clarified oil Isopentane-treated oil
82
Q
b 0
d
+
%
0 Fe naph.
METALCATALYSIS~ 0 Cunaph.
0 Pb naph
36 p. p. m. 20 p p. m. 500 p. p. m. Fe cu Pb
1
None ,
..
1 Fe wire No. 20 50 grams
5
None
.. .
5
Cu wile No. 23 17 grams
5
Pb sheet 19 sq. om.
685 150
699 336
681 534
693 264
686 258
658 581
694 151
650 669
628 1162
53.2 5.6 2.2 3.5 2.0 5.8 0.7 5.3
53.5 10.9 3.1 4.3 2.2 8.3 2.4 14.4
50.0 11.4 2 .7 4.6 2.5 7.3 6.2 24.9
52.9 7.9 2.1 4.6 2.7 7.8 2.5 11.8
52.6 7.1 2.6 4.2 2.4 7.9 1.9 10.8
50.7 10.1 3.1 4.7 2.2 8.9 5.9 27.4
53.2 6.1 2.2 4.2 2.8 3.9 1.3 6.4
50.0 11.7 2.4 5.2 3.4 7.3 8.6 34.8
41.8 17.3 2.0 4.5 2.8 9.9 9.3 52.4
0.74 0.21 0.53 3.2 12.8 11.9
250 grams oil catalyst oxidized 20 hours at 170' C. Assuming isopentane-insol. cpntaina 15% oxygen. Millierams KOH Der eram oil. Lacqter peeled from aide (indeterminable).
2.39 0.48 1.91 3.5 24.0 21.1
3.34 1.10 2.24 d
34.1 28.0
1.77 0.46 1.31 1.1 22.2 19.4
1.74 0.51 1.23 4.9 19.9 17.5
4.38 1.55 2.83 42.5 31.0
0.50 0.09 0.41 2.2 12.3 13.5
4.15 1.07 3.08 41.6 29.3
9.83 8.42 1.41 5.9 215.5 95 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1941
c o n c e n t r a t i o n is shown by the data for iron naphthenate in Figure 9 and Table IX. The form of the oxygen absorption curves is of particular interest and can be interpreted reasonably by assuming that the iron salt is poisoned as a catalyst, or is removed from the field of action after a few hours of oxidation. That is, the curves FIGURE 10. CATALYSIS BY SOLUBLE for the catalyzed IRON OF BASEOIL 6 (TABLE 11) oxidations became substantially parallel to that for the base oil in the absence of catalyst after 3 to 8 hours of oxidation. The data in Table IX indicate that the catalyst has little effect in altering the direction in which the oxidation reaction proceeds but affects only the rate of the reaction. Thus, the distribution of the absorbed oxygen to water and carbon monoxide, volatile and fixed acids, and total insolubles, is substantially unchanged. The percentage which goes to form carbon dioxide is increased, but this generally occurs when the oxygen absorbed is in excess of 200 millimoles in the presence or absence of a catalyst, regardless of the nature of the oil. The catalyst also increases the production of oil-insoluble materials in a regular manner but does not affect the lacquer value. This would indicate that lacquer formation is not so much a function of the extent and rate of oxidation, but may be
'TABLE X.
523
CATALYSIS BY SOLUBLE IRON^ No Catalyst 684
as Naphthenate Av. partial ressure Os mm. H g 630 Millimoles absorbed/250 g. oil 318 1257 Distribution of absorbed oxygen, '36 27.3 34.8 T o Hs0 4.9 11.3 T o COz 0.8 1.9 T o CO 3.8 4.9 T o volatile acids T o fixed acids 5.8 6.3 T o isopentane-insol.6 0.3 4.4 Neutralization No. of oxidi!ed oil0 5.6 27.1 Milliequivalent6 voJatile acid/250 g. oil 12.1 61.1 Precipitable oxidation products 0.07 4.68 A . Total isopentane-insol., wt. 7% 0.07 4.61 B. Oil-sol. isopentane-insol., wt. % C . Oil-insol. ( A - B ) wt. % 0.00 0.07 D. M g . lacquer on s' X 1 in. slide 0.1 0.0 Increase in viscosity at, 100' F. (37.8' C.), % 43.9 204 Clarified oil 39.7 154 Isopentane-treated oil a 250 grams oil 6 (Table 11) catalyst oxidized 20 hours a t 130' C. b Assuming is0 entane-insol. cpntains 15y0 oxygen. c Milligrams 2 O H per gram 011.
8,
+
10. Table X indicates that the uncatalyaed oxidation of this oil was not productive of either oil i n s 0 1u b 1e s 0 r lacquer, nor did the catalyst lead to significant changes in oil insolubles, in spite of the high degree ofoxidation in the latter case.
*zoo
$ L
O
3!$ vr 2~y /00 V,
93 5o
30
3
/O
20
0
4 8 / 2 / 6 HOURS AT /SO 2. (320 'E)
FIGURE11. Oil 0 (Table I)
NAPHTRENATE~ TABL IX. ~ EFFECTOF IRON Iron naphthenate, wt. yo Iron, p. p. m. Av. partial ressure 0 2 , mm Hg, Millimoles absorbed/25O'g. oil Distribution of absorbed oxygen, % T o HzO T o COz T o CO T o volatile acids T o fixed total Fcids isopentane-inso1.b
8,
0,000 0
0,005
685 150 53,2
694 201 55,7
4
0.010 9 694 231 54,5
18 P. P. M . F e
0.020 18 691 253 56,0
0.040 36 699 336 53,5
0 2 0
SUPPRESSION OF IRON CATALYSIS
+ thenate 36 p. p. m. iron as naph-
Possible Mechanism for Catalyst Suppression
The oxygen absorption-time curves obtained when oil insolubles are formed during the oxida5.6 8.7 9.6 9.7 10.9 tion of oils in the presence of small quantities of 2.2 3.0 3.0 3.0 3.1 3.5 3.5 3.5 3.9 4.3 iron salts usually show a short period of rapid :p +:f $ : : : absorption followed by a moderately sharp break Neutralization No. of oxidized oilc 1.0 1.0 1.2 1.2 2.4 in the curve, with the absorption rate decreasing 5.6 7.1 8.0 9.8 14.4 Milliequivalent8 volatile acid/250 g. oil and then proceeding a t about the same rate as Precipitable oxidation products B A. oil-sol. Total isopentane-insol. isopentane-insoi. wt. wt. % 8:;: E:: that of the uncatalyzed oxidation of the same C: Oil-insol. ( A - B ) , wt. %' 0.53 0.90 1.17 1.32 1.91 oil. This suggests the inactivation of the cataD . Mg. lacquer on 3 X 1 in. slide 3.2 3.4 3.9 3.7 3.5 lyst by a mechanism which involves the formaIncrease in viscosity a t 100' F.(37.8' C.), % ' Clarified oil if:! i::: i:: tion of a complex between the iron and some Isopentane-treated oil oxidation product which is insoluble in the hot a 260 grams,base oil 0 (Table I) oxidized 20 hours a t 170' C . b Assuming isopentane-insol. contains 15% oxygen. oil. To test this assumption, a catalyzed oxidaMilligrams KOH per gram oil. tion was performed in which the oil sampling tube was equipped with a fine (Ace Glass Company porosity D) sintered glass filter sealed to its lower end. This was done so that samples could be withdependent upon the time a surface is exposed to the oxidizing oil and upon the temperature. drawn periodically that were free of materials insoluble in the The effect of an iron salt on the oxidation of a n oil oil .at the temperature of oxidation. These samples were which shows an induction-period type of oxidation is analyzed for iron content by a photometric method (11). shown in Figure 10. This oil was a well-refined distilThe results so obtained for iron content and oxygen ablate of high viscosity index and low aromatic ring content sorption are typified in Figure 11. The break in the absorption curve after about 3-hour oxidation corresponds to a (oil 6, Table IV). The oxidation was conducted a t the low temperature level of 130" C . in order to secure a long considerable decrease in the concentration of dissolved iron as shown by the bar graph, which refers to the auxiliary induction period in the uncatalyzed oxidation. With 18 p. p. m. of iron present as naphthenate, the induction scale a t the right of Figure 11. It is thus apparent that iron period was eliminated completely as shown in Figure salts must remain in solution to be catalytically active, and
!:A
::::
0
A:$
;:::
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
524
t h e suggested mechanism of deactivation is apparently confirmed.
molecules of acids to form anhydrides and between molecules of alcohols to form ethers, and (c) loss of water from an alcohol in forming an olefinic linkage. The higher water production is also accompanied by a lower viscosity increase. This may be seen clearly in Table XI, where with similar amounts of oxygen absorbed the viscosity increase in the presence of phosphite was only half as great, and the proportion of the absorbed oxygen recovered as water was 15 per cent higher, than in its absence. The lower viscosity increase is an anticipated result of either reaction a or c, since acids and alcohols are far more viscous than the corresponding esters and unsaturated hydrocarbons (6). The data presented here show the complex nature of the many factors affecting lubricating oil deterioration. The marked changes in the stability of oils produced by small changes in basic composition, by catalysts present in little more than traces, and by the introduction of various addition agents all enhance the difficulties in interpreting the results of relatively simple oxidation tests. I n addition, one or more of these factors may influence and change the direction or character of the main oxidation reaction. Further, these changes may extend to the supplementary reactions of condensation, dehydration, polymerization, etc. I n the main, these factors are essentially well-known chemical and physical phenomena, such as the effect of temperature on reaction rate, the action of inhibitors, and of catalysts. Difficulties in understanding the nature of the changes taking place are due rather to the number of simultaneous reactions, and to the variation in the importance of these reactions as a given oxidation progresses. Other factors in lubricating oil oxidation remain to be studied. Those already described need extension and correlation with service conditions. The composition of lubricating oil seems a logical basis on which to coordinate all such knowledge. When this is done, it is reasonable to suppose that the gap between laboratory and service tests will be narrowed. Such oxidation studies should be extended to include higher molecular weight pure hydrocarbons and their mixtures, for in this way the nature of the higher molecular weight fractions of petroleum can be further clarified and understood.
Effect of Phosp h i t e on S o l u ble Metal Catalysis An additive inXOURS AT /70C ' (338 ?) tended t o reduce FIGURE 12. EFFECT OF A PHOSPHITE lubricating Oil deON METAL CATALYSTS terioration should counteract or 1. Oil 0 (alone) 2. 011 0 + 17 tributyl phosphite the cats3. Oil 0 + 1% tributyl phosphite + 18 m. F e l y t i c e f f e c t of 4. O i ? ' 8 ' + 1% tributyl phosphite + 20 metals. This may p. m. C u be accomplished 5. OiPO f 1% tributyl phosphite + 500 p. p. m. P b in a manner similar t o that naturally occurring in some oils, such as by the iron deactivation mechanism just described. That is, by some means the catalyst is rendered inactive and insoluble, or i t is somehow poisoned. Esters of acids derived from phosphorus are alleged to overcome the action of metals in causing oil deterioration ( 3 ) . Figure 12 and Table XI show that the use of tributyl phosphite rendered the metal naphthenate catalysts (iron, copper, and lead) practically inert in promoting the oxidation of oil 0, whereas Figure 8 shows that these metal salts are quite active in increasing the oxidation of the oil. Only in the case of copper naphthenate was a slight initial catalytic activity evident in the presence of phosphite. The concentration of phosphite was perhaps relatively high (one per cent), and this amount had some effect in reducing oxidation of the stock in the absence of catalysts. The use of this much phosphite in all cases resulted in the production of a black precipitate and lacquer deposit, and of a relatively large amount of lacquer because of adhesion of a fine black powder to the lacquered slide surface. Another effect of the phosphite addition is an increase of 15 to 25 per cent in the proportion of the absorbed oxygen which is recovered as water (Table XI). This is not illogical in view of the known powerful dehydrating action of phosphoric acid. The increased water formation may be the result of one or more of the following reactions: (a) loss of water in the reaction between acid and alcohol groups in the formation of esters and lactones, ( b ) loss of water between
TABLE XI.
8,
To Hz0 T o COa T o CO
T o volatile acids T o fixed acids T o isopentane-inso1.b Neutralization No. of ,oxidized oilc Milliequivalents volatile acid/250 g. oil Precipitable oxidation products A . Total iaopentane-insol., wt. 96 B . Oil-sol. isopentane-insol., wt. 70 C. Oil-insol. ( A - B ) , wt. % D. Mg. lacquer on 3 X 1 in. slide Increase in viscosity a t 100" F. (37.8' C J , % Clarified oil Isopentane-treated oil b c
Acknow-ledgment The cooperation and financial assistance of the Pennsylvania Grade Crude Oil Association is gratefully acknowledged. Literature Cited
(1) Anonymous, Oil Gas J . , 38, No. 30, 54 (1939). (2) Byers, J. H., Natl. Petroleum News, 28, No. 51, 78-84 (1936). (3) Downing, F. B., Holbrook, G. E., and Fuller, J. H., Oil Gas J . , 38, No. 5 , 70 (1939). EXG.C H E M . , (4) Fenske. M R., and Hersh, R. E., IND. t o be published. AL CATALYSIS5 (xi MET EFFECT OF A PHOSPHITE (5) Fenske, M. R., Stevenson, C. E., Rusk, R. A.,
Catalyst p p. m. as naphthenate Tributyl'pgosphite, wt. 3 '% Av partial ressure Os,mm. H g Miillmoles absorbed/250 g. oil Distribution of absorbed oxygen, %
a
+
-
0.00 685 150
1.0 704 99
...
18 F e 1.0 700 99
20 C u 1.0 699 137
500 P b 1.0 700
53.2 5.6 2.2 3.5 2.0
5.8 0.7 5.3
74.1 7.8 2.7 -. . 1.6 3.9 9.8 1.1 1.6
76.1 8.6 2.7 1.4 4.1 11.2 1.2 1.4
68.5 7.3 3.2 1.0 4.4 10.4 1.5 1.4
81.4 7.2 1.9 1.5 4.6 9.0 1.7 1.2
0.74 0.21 0.53 3.2
0.83 0.09 0.74 23.0
0.95 0.11 0.84 6.9
1.21 0.15 1.06 21.9
0.86 0.11 0.55 8.1
5.3 5.1
5.6
6.8 6.1
5.3 5.7
...
12.8 11.9
5.5
250 grams oil 0 (Tab1,e I) addition agents oxidized 20 hours a t 170" C. Assuming is0 entane-insol. contains 15% oxygen. bcilligrams per gram oil.
OH
Vol. 33, No. 4
81
Lawson, N. D., Cannon, M. R., and Koch, E. F., Ibid., Anal. Ed., 13, 51 (1941). (6) Goheen, G . E., IND. ENQ.CHEhl., 32, 503 (1940). 17) . , Haslam. R. T.. and Frolich. P. K.. Ibid.., 19.. 292 (1927).
(8) Keith, J. R.,and Roess, L. C., Ibid.. 29, 460 (1937). (9) Matthijesen, H. L., 72-89 (1940).
J. Inst. Petroleum Tech., 26,
(10) Meade, B. et d.,IND. ENG.CHEhl., 1 9 , 1 2 4 0 (1927). (11) Rescorla, A. R., Fry, E. M., and Carnahan, F. L., Ibid., Anal. Ed., 8 , 242 (1936). (12) Vlugter, J. C., Waterman, H. I., and Van Westen, H. A., J . Inst. Petroleum Tech., 21, 661 (1935). (13) Weiss, H., and Vellinger, E., Proc. W o r l d Petrol e u m Congr., L o n d o n , 1988,2, 423. PRESENTED before the Division of Petroleum Chemistry a t t h e 100th Meeting of the American Chemical Society, Detroit, Mich.
