Oxidation Characteristics of Lubricating Oils at High Temperatures H. DIAMOND, H. C. KENNEDY, AR'DR. G. LABSEN Shell Deeelopment Co., Emeryville, Calif.
E
XTENSIVE research has been conducted a t numerous laboratories on the deterioration of lubricating oils under condi-
tions t h a t simulate those in the crankcase of an engine or are but mildly accelerated (11). On the other hand, although some work has also been done on the composition of deposits in the top oylinder and the process of their formation ( 2 , 5 ) ,no really fundamental oxidation experiments have been made under these much more severe conditions until very recently ( 1 ) . A comprehensive summary of the general subject of lubricating oil evaluation (6) has pointed out that, rather than strive t o reach a compromise, future developments should aim a t tests specifically designed for measuring definite properties. Accordingly, a kinetic study of oil oxidation 'cyas made in the range above 220" C., the purpose being twofold-namely, to explain mare completely the behavioi of lubricants in service; and to extend existing knowledge of oxidation phenomena from the lowest temperatures a t which there is perceptible reaction up t o the point a t which ignition
is lowered, breaking off the a r m on the capsule and opening the upper vent hole, E , and lower drain hole, 8'; the oil then flows into the bottom of the glass cup, G, in a few seconds so that zero time is fixed precisely. The oxygen is circulated by a magnetic pump through an external absorption train to remove water, carbon dioxide, and any other volatile acidic oxidation products (as well as through a preheater). As reaction proceed?, pressure is maintained constant at one atmosphere by the manometer, buret, and automatic leveling device formerly adapted to the Dornte apparatus (IO),and the volume of oxygen absorbed is recorded continuously a- a function of time, EFFECT O F SAMPLE SIZE (OIL-FILM THICKNkSS)
In most stability tests involving circulation or agitation, amount of oxygen consumed or deterioration products formed ifi regarded as proportional t o the quantity of oil. It is thus tacitly mpurned, correctly or incorrectly, that all portions of the sample a i e rqually subjected t o various oxidizing influences-via., 0x1'genaiion 01 aeration, contact with catalysts, removal of volatilc QCCUIS oxidation products, etc. On the other hand, these conditions are EQUIPMENT AND EXPERI\lE%TAL PROCEDURE certainly not fulfilled in static tests such as the present one, and it i q therefoie importrtnt t o knoa how the results are affected h o The advantages of continuous oxygen-absorption meabui ev:iriation in sample size. ments have been amply discussed (3,$, 8, 1 0 ) Suffice it t o s a l 4ccordingly, experiments were performed a t three teinpt31:Lthat they provide the most pre tnd urii?quivocal criterion of tures upon an undoped, solvent-extracted, mid-continent, 120over-all deterioration. As to actu:il exprriniental arrangements, grade aircraft oil, employing both 3-ml. and 1.5-ml. sampleq one possibility is to adopt a niodificatioii of that designed bv (volumes measured a t rooni temperature) which, in the c u p , G, Dornte (3), and indeed this m i t i done by Iknison and I-larle ( 1 ) of Figure 1, of 48- t o 60-mm. inside diameter, give liquid clqiths Because of the danger in R oiking M ith laryc quantities of oil at of 1.8 and 0.9 mm , respectively, a t experimental tempeiature-. high temperatures in oxygen atmosphere, they employed small In cverv case, a Short induction period is terminated b r t i samplrs, and air as the ovidizing inediuni However, for the. brrab generallv leading t o a present investigation, another 4ightly concave-downward portype of apparatus vias devised tion of lhc, curve which is in turn in which the oil is oxidized in a fairly thin layer by pure follon ctl by a substantiallr constant rate until the final point ip oxygen; since a relatively large reached (taken a t approxiinntcdy surface of liquid is exposed to 60 in1 normal tcmperatuic and gas. agitation, which may be pressuie, total oxygen absorption). a variable factor, is not esIt 1s notemorthv that the sinall sential. Furthermore, the oil is iamples shorn almost twicc the then distributed in a mannei rat? of oxygen absorption per unit somewhat approaching that in quantity of oil a3 do the large the upper region of an engine THERMOCOUPLE WELL Le., the absolute extmt of ahsorpAnother advantage is the geotion a t a given time is virtually metric simplicity of the reacting TO CIRCULATING AND ABSORPTION SYSTEM unaffected over the twofold varisystem, which may aid in t h r TO VACUUM OXYGEN INCLUDES PREHEATER ) SUPPLY, BURET AND ation in sample size Rerice it analmis of reaction mechanism. CONTROL MANOMETER Eollo~vs that oxidation rate is h closed glass system, shor7-n more strongly dependent on oil in Figure I, is used as reactor. surface exposed than on total Since reaction may be rapid quantity and that progress of reacfrom the very outset, degasbed tion should be expressed, say, in oil samples are first sealed in glass capsules, A , under carbon terms of volume of oxygen abdioxide or in vacuum. One of sorbedper 100 sq. cm. of oil surthese is placed in the large face rather than per 100 grame of vessel, B , which is then closed oil (as is cuptornary for Dornte exby the top, C, and heated t o temperature in a thermostated periments). Indeed, when this is Wood's metal bath, evacuated, done (Figure 2 and Table I), and filled with oxygen t o atvelocities fall almost within the Figure 1. Reaction System for High-Temperamospheric pressure. To start a limit of error a t 250" C., have a ture Oxygen-Absorption RIeasurements run, the magnetic hammer, D.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1952
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t
> Figure 2.
1835
>
TIME (MINUTES)
Effect of Oil-Film Thickness on Oxidation Rate of Aircraft Oil
TIME (MINUTES)
Figure 3.
Oxidation Kate o f Aircraft Oil a t Various Bath Temperatures In absenoo of added catalysts
In absence of added catalysts
EFFECT O F TEMPERATURE ON T H E UNCATALYZED REACTION OF OIL-FILMTHICKNESS ON OXIDATION R a w TABLE I. EFFECT OF AIRCRAFT OIL IN ABSENCEOF ADDEDCATALYSTS
Vol. of Oil Sample, Ml., Room Temp
Rstio of Reaction Velocities Stability, Velocity for Two Time", Constant, Film Xin. La Thicknesses 10.2 3 1 . 8 2 6 . 0 1 06 250 9.65 1.5 0.9 26 9 3 1.8 12.6 20.6 1.12 260 1.5 0.9 14.5 18.3 47.3 3 1.8 5.3 1.17 270 40.5 1.5 0.9 6.4 a Time required for absorption of 250 ml. (N.T.P.) of 0 2 per 100 sq. cm. of oil surface. b Slope of line oonnccting terminal point of curves in Figure 2 with end of induction period. Bath Temp., C.
Film Thickness, Ym.
ratio of 1.12 for the full-size t o half-size samples a t 260" C., and a ratio of 1.17 a t 270" C. Similar results were obtained in the presence of copper or crankcase catalyst.
The oxidation rate of 3-ml. samples (l.&mm. films) of aircraft oil was determined a t several temperatures from 220" t o 275' C.. with the results shown in Figure 3 and Table 11. At 280", spontaneous ignition occurred, not sufficiently severe t o damage the apparatus, but reaction was of course far too rapid t o measureLe., there is a critical explosion limit lying between 275" 2nd 280" C . An appreciable part of the oil volatilized and condensed on the side walls of the cup and top of the cell at 275" and 280' C., but not at any lower temperature. The general form of all the rate curves is the same up t o a total absorption of 280 to 300 ml. of oxygen per 100 sq. om. of oil surface when reaction is stopped. Short induction periods generally decreasing with rising temperature (1.5 to 3 minutes) are followed by almost linear curves which become steeper at higher tcmperatures. Conclusions are drawn only from the reaction velocity constants, k (taken as the slopes of lines connecting the terminal point with the end of the induction period), or from the 250-ml. reaction times (which are of course inversely proportional t o the slopes of lines connecting the 250-ml. points with the origin), not from the length of induction period or the slight curvature of the major portion, since these are not sufficiently reproducible t o warrant quantitative interprrtation.
Attempts to measure absorption by 0.75 ml. of oil gave erratic results, absolute oxidation rates occasionally being as great as for 1.5 or 3 mi. The inconsistency probably lay in the failure of the smallest samples always t o spread to the same extent and in the fact t h a t they are unduly affected by the glass debris deposited on Reaction rate is multiplied &fold over the whole range from breaking off the arms of the capsule. -4 choice of 3-ml. samples 220" t o 275" C. The general trend in coefficients for successive as standard seems best justified on the ground that they are relatively inscnRitive to surh perturbations. TABLE IT. EmEcrr OR TEMPERATURE ON OXILLITI~X 1 1 4 ~ ~OF 3 AIRCRAFTOrr, IN Apparently, the prevailing rate of ABSENCE OF ADDET)C ITALYSTS diffusion limits access of gas t o the lower Aouarent portion of liquid. If oxygen were availhitivation Stability, Reaction Temperature Energyb, able throughout, then all the oil would Bath 250-M1. Velocity Coefficient, r E, Temp., !? lo3 Time, Constant, (Kg.-Cal./ Mole) B-Valuec be subject to oxidation and the data C. (T = I., City Chemical Corp., K e n . .) was a very weak pro-oxidaiit. 0
4
8
12
90 94 TIME, MINUTES
28
36
40
urenicnts inadc a t 260 C. with powiorcd c o p p t ~and crankcase catalyst are of sonic Figure 8. EtTect of Sheet Lead a n d Lead Bromide on Oxidation Kate of fundamental interest (Figure 9). Thus, conii i r c r a f t Oil at 230" C. parison of data at the two temperatures (Tablc V! vields values for temucmturc coefficient and apparent activatiou energy of the catalyzed ~ ( t ~ i c ivould have an influence, a run was made with 0.5% by weight of tion over this interval 11hich are appreciably lower than foi the powdered alumina (Merck's reagent) suspended in the oil, but uncatalyzed reaction (Table 11). Iikewise, from results a t each this was found inactive. Two conceivable explanations for the trmperature, it is calculated that the acceleration due t o (4,h discrepancy are: a more impervious coating is actually formed catalyst is definitely greater a t 260" than a t 260" C. Both thew not on the cup, but rather on the particles of powdered metal conclusion- are in accord n ith claqsical kinetic theory. during the procesa of manufacture; or the greater contents ot EFFECTOF SAMPLE SIZE Thr influence of variation i n oilcopper and iron (especially the former) shown by analysis to be film thickness on the rate of the catalyzed oxidation was cleterpresent in the sheet aluminum (Table IV) exert appreciable acinined a t 250" C., as shown in Table VI. Since the data obtained tion. The second alternative seems somewhat more plausible on 1.5-ml. samples in a sheet copper cup or with the addition of a although definite proof is lacking. \t tiny rate it is indicated sheet copper disk, powdered coppcr , or crankcase catalyst are ('lose that catalysis by aluminum alloys eriiploved in pistons, cylindrr to the corresponding values for 3 ml., the same relation holds as h(lads, etc., should not be disregarded in the case of the uncatalyxed oxidation. On the other hand, rither with a steel cup or witli a steel disk in a glass cup, the absolute oxygen-absorption rate is considerably greater for thc 01; TJIPLRITIES IN ALaivmuar TABLEIV. CONCENTRATION smaller-sized samples. Moreover, in sharp contrast, paralld Magexperimcnts performed with lead dkks showed appreciably slower Copper, Iron, nesium, Silicon, 'Type of Aluminum %w %w %w %N reaction for the 1.5-ml. samples, the acceleration here being very 0.23 0.62 0.04 0.05 Sheet metal (used in cups) amall. -4tentative explanation foi these phenomena is nffcicd Merck's C.P. powaer None 0.36 0.03 0.03 below. 16
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0
2
4
6
6 TIME
Figure 9.
10
I2
,
14
I
16
39
I
(MINUTES)
Effect of Powdered Catalysts on Oxidation Rate of Aircraft Oil at 260' C.
Figure 10.
1
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8
12
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I
16 20 24 TIME (MINUTES]
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I
1
28
32
36
Effect of Copper Naphthenate on Oxidatiori Rate of Aircraft Oil a t 250" C. Low-concentration range
INDUSTRIAL AND ENGINEERING CHEMISTRY
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1839
rising copper content until, at 100 p.p.m., a maximum (5.81fold) acceleration is reached, Catalytic TemperaApparent and then falls off. Whereas AcidyCo:$e'nt, Activation all the rate curves are subStabiht Reaction Energy, r Ratio -i. 1 x 108 250-Mr' Velocity E stantially linear or slightly Temp., Time, Constant. k Cat (Kg.-Cal./ C. (T K.) Min. kS loglo k )-k( Mole)b B-Valueb concave downward (after very Copper powder (Baker L Adamson), l % w early stages of reaction) up 38.8 1.5888 3.80 250 1.9114 6.8 1.69 25.9 ,5660 to 500 p.p.m., a striking change 65.6 1.8169 3.41 260 1.8756 4.15 in form occurs a t higher conCrankcase catalyst, 0 . 5 % ~ centrationa. The curve for 250 1.9114 7.6 34.6 1.5391 3.39 I .77 28.0 6130 260 1.8756 4.45 61.1 1.7860 3.17 1000 p.p.m. has a smooth a Slope of line connecting terminal point of curves in Figures 6 and 9 with end of induction period. inflection (S-shape) which beb Not absolute values. comes much more abrupt at 2000 p.p.m.; here, what may actually be termed a second "break" occurs after 6 minutes, This is eliminated once more at 10,000 p.p.m. (1% by weight), a simple concavedownward curve being obtained. Average rate is fairly con-
ON OXIDATION RATE TABLEv. EFFECT O F TEMPERATURE CATALYSTS
OF
AIRCRAFT OIL
I N PRESENCE O F
-
TABLEVI. EFFECTO F OILFILM THICKNESS ON OXIDATION RATEOF AIRCRAFTOIL IN PRESENCE OF CATALYSTS AT 250" C.
Catalyst Xone Sheet copper cup Copper
TIME (MINUTES)
Figure 11. Effect of Copper Naphthenate on Oxidation Rate of Aircraft Oil at 250' C. High-concentrationrange
powder
&
Volume of Oil Sample, Ml., Room Temp. 3 1.5 3 1.5 3
i!%on), w Crankcase catalyst, 0 . 5 % ~ Noneb
k5
Copper disk
;.5
1.5 3
1.5 3 1.5 Steel disk 3 1.8 Lead disk 3 1.5 a Slope of line connecting tion period. b New batch of oil.
Sheet steel cup
Film Thickness, Mm. 1.8 0.9 1.5 0.8 1.8 0.9 1.8 0.9 1.8 0.9 1.8 0.9
Stability, 250-1M1. Time, Min. 26.0 26.9 8.8 8.2 6.8 7.7
Reaction Velocity Constant, k5
Ratio of Velocities for Two Film Thickneases
10.2 9.65 30.4 31.7 38.8 35.0
7.6 34.6 8.2 32.9 35 6 37.6 9.1 30.9 7.6 32.4 1.5 21.2 12.3 20.5 0.8 12.8 1.8 21.8 12.2 0.9 11.1 24.9 1.8 27.7 9.1 7.15 33.9 0.9 terminal point of rate curves with end
i:::
Dissolved Metal Catalysts. Supplementary t o the above work on oxidation in the presence of free metals, the effect of soluble soaps was investigated. Since the active material is already in solution at the start of a run, certain aspects of the mechanism of catalysis can be considered without the influence of factors that involve a solid-liquid interfacee.g., passivation and rate of solutionand conclusions derived t h a t may apply t o both types of catalyst. Two wellknown laboratory tests, Underwood and Davis, employ catalysts in soluble form. Furthermore, the action of dissolved metals was exhaustively studied under milder conditions several years ago, and it is interesting t o compare behavior at high and low temperatures. COPPER. Rate curves for the oxidation a t 250" C. of aircraft oil containing copper naphthenate are given in Figlyst -011 Batch It ures 10 and 11, and the general dependence of reaction velocity (as judged I 1 I I by average slope) on concentration, can 0.1 1 10 100 1000 be seen more clearly in the semilog CONCENTRATION OF METAL NAPHTHENATE, P.P.M. AS METAL plot, Figure 12. Even 0.1 p.p.m., as (LOGARITHMIC SCALE) copper, perceptibly increases the rate; Figure 12. Oxidation Rate of Aircraft Oil a t 250" C. the effect becomes more pronounced with A s function of dissolved metal content
0.96 1.11
1.05 1.03 0.95 0.60 0.49
1'27 of induc-
10.000
INDUSTRIAL AND ENGINEERING CHEMISTRY
1840
Vol. 44, No. 8
sorption. Suitable correctioii was therefore applied to the data plotted in Figure 11. Further investigation revealed that a superatmospheric pressure of carbon dioxide had been built up in t h r capsule during the initial pi'riod 100 100 of warming up and filling the apparatus with osygen, probably by partial decarboxylation of the naphthciiate radical a t so high a temperature; on breaking the capsule, the carbon dioxide 10 10 was releaved and actuat>ed the measuring device in reverse before its removal by t,he absorption train. In agreement with sucli an explanation, special oil samples, when heated :it 1L I I I I I1 250" C. in vacuo, evolved 0.1 1 10 100 1000 10,030 carbon dioxide and developed CONCENTRATION OF COPPER NAPHTHENATE, P.P.M. AS Cu a reddish turbidity resem(LOGARITHMIC SCALE) bling a fine precipita,te of Figure 13. Stability of Aircraft Oil a t 250" C. and S..4.E. 30 Mid-continent copper or cuprous osidtt. Oil a t 150" C. What bearing these ntwi,v:LA s function of di+sol\ed copper Content tions have on the mechitriism of catalysis is not clcai,. Soau that has liberated mcttrl in insoluble form callnot s t ~ i c t l y6e termed a homogeneous ntant above 3000 p.p.m., but, the ourve forms 101. 3000 and catalyst; however, if a solid phase is exceedingly fine and well 6000 p.p.m. are intermediate to 2000 and 10,000 p.p.m., so Lhat dispersed, it, may conceivably approach a true or colloidal solute t,he transition is gi.adu:tl :tiid continuous though nonctheless in its action. Xlost prohal)ly, only a minor portion of the cardrastic:. boxylate had been decomposed before oxidation was started, so that interpretation of thc, ~ e r u l t sneed not be seriously altered. Stabilities, as measured by time loqriired for the absorption of 250 ml. of oxygen, are plotted on a log-log scale, Figure 13, to IRON. Corresponding data were also obtained on iron iiaphfacilitate comparison with earlier \vork on a mid-continent motor t~henate,as shown in Figure 14; the eoncentratioii-depc.ndenc:ct oil in the Dornte apparat,us a t 150" (1. (7'). Of course there are curve is included in Figure 12. II'hereas 0.1 p.p.m. (as iron) marked differences in ttquipnicnt and tt:c:hnique between the in-o produces a negligible effect (less than the experimental crmr). types of experiment othor than tcnipc,rature, and the t,wo oils, catalytic action rises steadily v i t h increasing concentration ovc'r though of similar origin, differ in viscosity grade (S.A.E. 30 I:S. 60). the entire range studied uiit'il, :tt 10,000 p.p.m.-ix., 1% 115' It is therefore all the more noteworthy that these two curves weight-a 12.CJ-fold accelei.ation is reached, in contrast to c:oppor possess the same general c h a r n c t e r i s t i c s - - i i ~ e l ~a, slow decrease which exhibits a maximum a t 100 p.p.m. Although copptti~is in stability with rising copper content t o a fairly flat minimum, more active than iron bclow 300 p.p.m., the reverse is tmt. i,o a followed by a sharper increase. Tho major distinctions lie in thc. very marked degree a t highcr concentrations. Furtlic~riiiorc~, absolute position of the minirnum ctt 100 p.p.m. a t 250' as against unlike the case of copper, :?I1 thr, rat'e curves for iron xi'c n1riiosl 300 p.p.m. a t 150" C. and, as might have been expected, a much linear. lesser over-all variation in rate----i v:i,talyt,ic cfficienc*y--at. the higher temperature. Significantly, the accclerstloll ploduced by 100 p.p.m. of soluble copper is greater than for the solid metal, sheet or powder (according to the regular procedure), and even as little a8 5 or 10 p.p.m. gives a comparable effect. Since all concentrations of dissolved copper that may reasonably be attained were found t o accelerate oxidation at least to some extent, there is no confirmation for the statement by Matthijsen (9) t h a t copper is an antioyidant above 230" c. looO
I I looO
h n unexpected phenomenon occurred with the two highest concentrations of copper naphthenate. At the beginning of an experiment, there was a sudden pressure increase in the system equivalent to t h e evolution of 7.5 ml. (absolute volume, not corrected or calculated in any manner) of gas at 10,000 p.p.m. and 2.5 ml. a t 6000 p.p.m.; then, following an induction period of about 0.5 minute, oxidation proceeded normally as evidenced by oxygen ab-
0
Figure 14.
4
20 24 28 32 36 40 TIME, MINUTES Effect of Iron Naphthenate on Oxidation Rate of Aircraft Oil a t 8
19.
16
250" C.
August 1952 1000
INDUSTRIAL AND ENGINEERING CHEMISTRY
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1000 2
S c
II
0 m
N o Catalvst
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S A E 30 M i d - C o n l i n e n !
No Catnlyst
-
Aircraft Oil
011
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1841 have been compared as function of lead content (Figure 17); here, too, the similarity in general trend a t the two temperatures is striking in view of the differences in base stock and experimental technique. OXIDIZED OIL AND GAS AN A LYSE S
After oil oxidation, the gas remaining in the system was generally 97 to 98% oxygen by volume, quite free of carbon diSoluble Iron - SAE 30 bltd-contxnent 011 oxide and other acid gases; the carbon monoxide conI I I I I I 1 0.1 1 10 100 1000 10,000 tent lay between 1 and CONCENTRATION OF IRON NAPHTHENATE (P.P.M. AS Fe) 275, on the whole being (LOGARITHMIC SCALE) greater a t the highest temFietire 15. Stability of Aircraft Oil a t 250" C. and S.A. E. 30 Mid-continent 'Oil a t 150" C. peratures, but the trend A s function of dissolved iron content was not altogether regular. The oxidized oil was considerably darker in color and more On the whole, the two dissolved metals appear to possess the viscous than the original. Saponification number seemed to fit11 same general characteristics (of course, not absolute activity) at with rising temperature of reaction, from 17 mg. of potassium low concentrations but differ markedly a t high concentrations, hydroxide per gram a t 220' to 12 a t 275" C . ; acid numbers Figure 12. The high rate a t 10,000 p.p.m. of iron may be close fluctuated about a mean value of 2.0, perhaps being slightly to the upper limit that can be measured by the apparatus, so that higher a t the lowcst temperatures. A simple explanation would an undetectable maximum could conceivably exist a t some higher be that carboxylic acids, esters, lactones, etc. , are decarboxylated concentration. As with soluble copper, carbon dioxide was or otheiwise decomposed a t the highest temperatures studied. evolved by decomposition of the iron naphthenate but in much As expected, the saponification number of the oxidized oil wah smaller quantity (1.5 ml. absolute volume a t 10,000 p.p.m. iron). nearly twice as great for the 1.5-ml. samples as for the 3-ml. at In analogous fashion t o the comparison made for copper, highthe termination of either catalyzed or uncatalyzed reactions, temperature stability-i.e., 250-mI. t i m e i s plotted on a log-log since they were here carried twice as far per unit quantity (not scale, Figure 15, against iron content together with earlier data for surface) of oil. a mid-continent motor oil in the Dornte apparatus a t 150" C. (7). In no case could it be definitely established that the composition Thc trend in stability is very similar a t both temperatures from of the gas or oil after an experiment carried to the usual extent of 0 to 100 p.p.m. but not a t higher concentrations. However, the oxidation was significantly altered by the presence of a solid minimum at 150" C . is quite flat and results were unfortunately catalyst except, of course, for the metal that was dissolved in thc not obtained above 1000 p.p.m. so that the lack of parallelism oil by corrosion. The copper contents of 3-ml. or 1.5-ml. oil here cannot be firmly established. LBAD. Similarly, data for lead naphthenate are given in Figures 12 and 16. Whereas 100 and 300 p.p.m. (as lead) have no effect, catalytic action is appreciable a t 600 p.p.m. and rises rapidly with increasing concentration so as t o give a maximum a t about 2000 p.p.m. (acceleration ratio = 14.4), then falling off rapidly up t o 10,000 p.p.m.-i.e., 1% by weight. The maximum is both higher and sharper than for copper and would possibly be even somewhat more pronounced were it not for limitations of t,he app& Numbers on curves denole ratus. Concentration of Pb ~n ppm In one sense, lead is less powerful than either copper or iron in that a higher concentration must be reached before appreciable effect is obtained. On the other hand, lead is more powerful than either copper or iron over the whole concentration range investigated in that the observed peak value is greater. The rate curves are either concave downward or 0 4 8 I Q 16 20 94 28 32 36 40 linear and show more variation in form TIME (MINUTES) than does iron but not so much as copFigure 16. Effect of Lead Naphthenate on Oxidation Rate of Aircraft Oil at 250' C. per. Again, stabilities a t 250" and 150" C. 110 -
1842
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 8
mused a minor undulation in t>he curve, the initial high rate was substantially maintained for more than half the total duration--i.e., through interval V I I d o r e it began to fall off appreciably. The slopes of thc curve were d c u l a t e d over l h e first and last, intcrvals ai: 35.6 and 6.7 (inl. per 100 sq. cm. per min10 A i r c r a f t Oil ute), respectively, the a rate for the entire tiin M i d -Cant inen t Oil 22.8. A t the end of t,he experiiiitint, the gas remaining in t,he I I 1 tem was found to havc :LC0.1 1 10 100 1000 10,000 cumulated as much as 13% of CONCENTRATION OF LEAD NAPHTHENATE (P.P.M. AS Pb) carbon monoxide. If appropri(LOGARITHMIC SCALE) ate correction is applied to t,hc Figure 17. Stability of Aircraft Oil a t 250" C. and S.A.E. 30 \lid-Continent Oil a t 150" C. ~crininal oxygen-absorl)( ion A s function of dissolved lead content value as measured, t,liis is increased by 7.5% an(1 t h r samples oxidized in contact n ith a copper cup or disk (and then cuivc would be alteied a5 d i o ~ nby the dashed portion. T o filtered in benzene solution) were grouped about a mean value of hc exact, a further small corrcctiori should be made for the rccluction in final reaction rate because of dilution-i.e., only 85% approximately 200 p.p.m.; with a steel cup or disk, the iron conoxygen is present a t tlic end of the run-but the dependence on tent was about 30 p.p.m. for 3-ml. and 90 p.p.m. for 1.5-1111. samples; on a lead disk, 3-ml. samples gained 5000 p.p.m. of disoxygen pressure has no1 Iriwn detc,iniiried. Alternatively, provision could have btmi mtde for carbon monoxide removal. solved metal and the 1.5-ml. as much as 19,000 p.p.m. However, it is not felt t h l anv convlusions t o be derived M ould In general, analyses of the oxidized oils were not very precise lie scriously affected. or reproducible, especially in regard to dissolved metal content The oil was oxidiztd t o J black opaque mass which, on cooling, However, no conclusions have been drawn t h a t would requirr 1)eu:tnie quite hard and re~eiiibledasphalt; a saponification numknowledge of the results to a high degree of accuracy. ber of 90 and acid neutrolization number of 13 were reach~d. BEHAVIOR OF OIL UPON PROLONGED OXIDATION Obviouqly, the formation of such inaterial in the power 8ectioii of
