Oxidation Characteristics of Some . Diester Fluids C. M. MURPHY AND HAROLD RAVNER Nasal Research Laboratory, Washington, D . C .
I
RECEpiT years the utility of diester-type lubricants has been demonstrated for such applications as turbo-jet engines, air-craft instruments, and other mechanisms where good Iowtemperature characteristics, nonvolatility, and high viscosity indexes are requisites. The development of these fluids as lubricants has already been reported ( 2 , S, 9), and it has been shown that a representative diester, bis(2-ethylhexyl) sebacate can be adequately inhibited against oxidation at temperatures up to 163" C. (16). Design trends in modern power plants and instrumentation are toward higher operating temperatures; therefore, the oxidation stability of fluids used for such applications is of interest. There has been some information published regarding the relative behavior of diesters in empirical oxidation tests ( 2 , 9),but no attempt has been made t o determine oxidation products or to study the kinetics of the reaction. The work here reported is part of a basic program for the study of the oxidation stability of diesters. Information is required on the nature and quantities of oxidation products, on the effect of molecular configuration on the reaction, and on reaction rates and activation energies. Eicosane, a typical paraffin of approximately the same molecular weight as the diesters, was included for purposes of comparison. pi
APPARATUS AND EXPERIMENTAL PROCEDURE
tesy of J. C. Geniesse of the Atlantic Refining Co. I n addition to low pressure distillation and countercurrent stripping, all samples were percolated just before use through a column containing'activated fuller's earth and alumina to remove traces of polar impurities whose presence might affect the subsequent oxidation. The physical constants denoting the puritv of the diesters are included in Table I . Oxidation determinations were normally made a t 125 O , 135 O , and 145" C. Under these conditions, reaction times varied from 1 or 2 days a t the lower temperature to 2 or 3 hours a t the upper level. At each temperature oxygen absorption runs were first made employing small samples. In most instances, these runs were repeated with larger samples from which portions could be withdrawn periodically for analyses of peroxide and total acid. The former was determined by the potassium iodide-sodium thiosulfate procedure and the latter by titration with alcoholic potassium hydroxide in a benzene-isopropanol medium ( 1). Water and carbon dioxide were determined gravimetrically in absorption tubes containing Drierite and Ascarite, respectively. The periodic determination of water and carbon dioxide required the oxygen flow t o be diverted t o an alternate train of absorbers so as not to interrupt the run. At the conclusion of each run, volatile acids and peroxides were determined in the condensate from the ice water trap.
All oxidations were carried out in a Dornte-type ( 6 ) cyclic apparatus previously reported (14). Oxygen gas in excess of that necessary t o saturate the sample (generally about 100 ml. per gram hour) was recycled through the system by means of a stainless steel diaphragm pump. Oxygen absorption was recorded on a time chart, and atmospheric pressure was maintained within the system by means of an automatic mercury leveling device. All gas volumes were converted t o 0' C. and 760 nim. pressure, Samples were oxidized in glass' cells fitted into a thermostated Dural block, the temperature of which was controlled by, and recorded on, a Brou-n Electronik potentiometer. The thermocouple was contained inside a well dipping into the oil sample, so t h a t close control ( ? ~ 0 . 2 5 'C.) was maintained throughout a run.
OXYGEN ABSORPTION AND REACTION RATES
All of the compounds examined exhibited typical autocatalytic oxygen absorption-time curves during the early stages of oxidation (Figure 1). In time, the concentration of degradation products became high enough t o retard the reaction. The autocatalytic stage has been studied in this work since it is the most susceptible to analytical treatment. The oxidation of an oil is a complex phenomenon, and following the course of any one oxidation product does not provide a complete insight into the operative mechanisms, although, as is The diesters were synthesized in this laboratory ( I S , 16). A shown later, the influence of peroxide level is profound. Over-all very pure sample of n-eicosane was obtained through the couroxygen consumption on the other hand is unambiguous, as Dornte states (61, provided the s a m ~ l e is saturated with gas a t 'all times. It has been demonT.4BLE I. P R O P E R T I E S OF COMPOUNDS INVESTIGATED strated t h a t the reaction velocity of some pefroleum oils Boilins P t . Refraotive Saponification -_____ Empirical Mol. Wt. M m . Index, Density a t s~mber is influenced by wall area (10). Formula (Theoretical) C. Ilg nso 20 O C . Calcd. FoundU This factor was not a variable Bis(3-methylbuty1)in the present work since the CsoH38O4 342.50 205-6 4 . 0 1 . 4 4 1 8 0.9252 327.6 328.2 sebacate Bis(2-ethylhexy1)wall areas of all sample cells CziH4oOa 356.53 165-8 1 . 5 1.4452 0.9290 314.7 313.0 glutarate Bis( 1,3-dirnethylbutyl)were of the same order of magC2zH4204 370.55 203-5 4 . 5 1 . 4 3 9 8 .... 302.8 305.7 sebacate nitude. Bis(2-ethylhexy1)-orjhthalate Czr H3804 390.54 183 0 . 3 1.4862 0.9849 287.3 287.1 The oxidation rate for all of Di(n-octy1)sebacate CnaHioO4 426,66 218 0 . 5 1.4492 0.9104 263.0 261.4 Bis(1-methylhepty1)-sebacateC~Haa0a 426 66 210 0 4 1.4456 0.9039 263.0 262.3 the diesters studied could be Bis(2-ethy1hexyl)sebacate CzaHboOl 426.66 224-6 2 . 0 1 4506 0.9132 263.0 260.2 characterized by the empirical 1,lO-Decanediol bis(2-ethylhexanortte) C26Hs 0 0 4 426.66 196 0 . 4 1.4469 0.9074 263.0 ,. .. b rate law Bis(3,5,5-trimethylhexyl)O
sebacate Eicosane
CzaHaaO? ClOH42
454.71 282.54
216e ,, ,
0.7
...
1.4504 1,4190d
0.9059
.,
,
.
246.8
...
245.7 ,...
Vn = k t
(1)
where V represents moles of oxygen gas absorbed per mole of diester, t is the time in hours,
1607
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1608
k is the rate constant relating V and t , and z has the value of 1/2. Dornte ( 6 ) found this square-root relationship t o hold for the oxidation of highly refined petroleum fractions.
Vol. 44, No, 1
frequency factors for the compounds investigated. The energies are all within the normal range of organic reactions. OXIDATION PRODUCTS
I
"I
I
TIME IN HOURS Figure 1.
Typical Oxygen Absorption Curves of Bis(l,3-dimethylbutyl)sebacate
I n the event of a n induction period, a second constant n-ould be included in the rate expression. I n the course of this investigation it n-as noted that with the removal of polar impurities from the compounds by percolation through activated fuller's earth and alumina, the induction period was substantially decreased and in most instances became negligible. Absence of an induction period was considered a criterion for the purity of the compounds. I n general, those trace impurities initially present did not materially affect the subsequent reaction rate. T h a t the rate constant, k , has energy significance is indicated by its general conformity with the Arrhenius equation -E -
k =AeRT
At the completion of each run, all of the compounds exhibited a strong odor characteristicaof oxidized oil. A definite yelloivish tint was usually evident, but in no instance was any lacquer or sludge formed. I n the case of bis(2-ethylhexyl)-o-phthalate, a crystalline material attributed to phthalic acid was deposited above the liquid level in the sample cell. There was some slight sublimation during all eicosane runs, but this was not sufficiently serious to disrupt the oxidation. The analyses for oxidized products did not permit an accounting of total oxygen consumption since alcohols, aldehydes, and ketones were not determined. In Table I11 are the results for those compounds on which periodic analyses were performed. The values reported are not concentration terms, but represent the percentages of absorbed oxygen recovered as peroxide, acid, carbon dioxide, and water, all except that for peroxide being cumulative. It was not practicable to withdraw samples for analysis a t equal oxygen absorptions; the data, however, were FO tabulated that values of oxygen absorption per mole of sample were reasonably comparable in magnitude, Oxygen recoveries were generally betaeen 80 and 90% for t h r diesters. I n the early stages of a run, the errors involved in the estimation of oxygen absorption and in the analyses for small quantities of oxygenated products were responsible for greater discrepancies. During the later stages, although these errors tended to decrease, oxygen recovery was generally less complete, probably as a result of the cumulative concentration of those oxidation products for Tvhich no analyses were performed. I n every series, the final analyses for peroxide and acid included the contents of the cold trap as n-ell as the bulk fluid. During the initial stages of these runs, peroxide accounted for a large percentage of absorbed oxygen. The concentration of peroxide increased to a maximum and then fell off sharply. However, the percentage of absorbed oxygen recovered as peroxide continually decreased, whereas that for acid increased, as a consequence of peroxide decomposition as well as of hydrolysis of the esters. The percentage of oxygen recovered as water remained essentially constant; that for carbon dioxide increased. In general, the amount of oxygen recovered as carbon dioxide was less than that for any other constituent. Reference to Table I11 shows that for comparable oxygen absorption per mole of sample, the proportions of oxygen recovered as acid, peroxide, water, and carbon dioxide were in reasonably good agreement for any compound a t all test temperatures. There was someahat less agreement in this respect between the various diesters, but not so much as to preclude the probability that their oxidation mechanisms n-ere essentially similar in the temperature range studied.
This equation is, of course, only proximate in so far as the energy of activation and the so-called frequency factor, A , are presumed to be constant throughout the temperature range studied. The significance of this equation with respect to oxidation stability is discussed in a subsequent section. The activation energy, E, was calculated from the slope of the graph of log k us. the reciprocal of the absolute temperature. However. activation energies computed from these data are affected markedly dy relatively slight deviations in slope. It is estimated t.hat TABLE 11. ACTIVATION ENERGIES AND REACTION RATESOF DIESTERS the over-all experimental error is of the order of Activation Reaction Rate, kQ 1 2 . 0 kg.-cal. per mole. Energy, For the oxidation of eicosane, the value of 5 Kg.-Cal./Mole log A 125' C. 135O C. 145' C. 32.0 15.9 0.0207 0.0560 0.140 in Equation 1 was found to be 1/3. ExaminaBis(l-methylheptyl)sebacate Bis(3,5.5-trimethylhexyl)31.4 15.5 0.0197 tion of the data of Larsen et al. (11) indicates a sebacate 0.0512 0.133 similar cube-root relationship for the hydrocarl,lO-Deoanediol hexanoate) bis(2-ethy127.8 13.6 0.0247 0.0587 0.134 Di(n-octy1)sebacate 27.8 13.8 0.0327 0.0785 0.177 Dons decane and cetane. It is therefore evident Bis(3-methylbutyl)sebacate 25.8 12.6 0.0338 0.0755 0.162 0.0736 0.158 that the experimental rate constants for eicosane Bis(2-ethylhexy1)glutarate 26.7 12.5 0.0330 24.5 11.9 0.0254 0.0547 0.113 may not be compared directly with those for the ~ ~ I ~ : ~ : ~ ~ ~ ~ ~ 22.8 ~ ~ i 10.9 s ~ 0.0345 ~ ~ ~ 0.0668 ~ ~ l a0.136 t e ~i;~~;~imethylbutyl)sebacate 19.5 9.4 0 , 0 5 2 4 0.0968 0.172 diesters, but this factor would in no way affect the 26.2 12.9 0.03426 0.0793b 0.168b calculation of its activation energy. a Obtained as square-root function of oxygen absorbed. I n Table I1 are listed the calculated activation b Obtained as cube-root function of oxygen absorbed. energies, the rate constants, and the log of the
"Y
e m
:
09'9 1019 09""
"3':
9 t - m m m m w m ood.
m a wmm hmm wmh
. .. m i ; *
et-h
& ,
mm
:
.? 09" .3
.
y
o m . w
m hm
w
.. ,.
I N D U S T R I A L ANb
1610
gNBfN'Eel?l'N'c C H E M ' I S T ' R ' Y
I n the case of eicosane, the recovery of oxygen was substantially less than that for the diesters. This is probably a result of hydroperoxide decomposition to form ketones (8, 19), for which no accounting wae made.
//' I
I
I
A
I 2 MOLS PEROXIDE Oe/GRAM SAMPLE X IC'
0 Figure 2.
Relation of Oxygen Absorbed to Peroxide Concentration
Bia(Z-ethylhex*l)sebacate:
0 125" C.
0 135OC. 3 145OC.
Bis(3,5,5-trimethylhexy1)sebacate: X 125'C. 0 135OC. A 145OC.
T h e instantaneous reaction rate, k l , obtained from the slope of t h e oxygen absorption curve over small time intervals, was of t h e first order and dependent on peroxide concentration, giving rise t o the relationship '1
[Perox. ]
= const,ant
(3)
In Table IT- these values were computed for a few of the compounds studied and the constancy of
kl
seems reasonably
well established. Differentiating Equation 1 with respect to time, value of 1/2, gives
0;
Vol. 44, No. 7
oxygen per unit weight of oil. This method was not, considered satisfactory in t'he present instance, because if the stat,ed amount of oxygen were too large, the aut'ocatalytic stage of the reaction may have been passed and the apparent relat,ive order of stability of the compounds would be misleading. On the other hand, if . time necnessary the critical amount of oxygen were set too 1 0 ~the for the compounds to absorb this quantity of oxygen x-ould be too short to permit effective discrimination. The specific reaction rate IC could be utilized as a criterion of relative stability a t any one temperature, but, as will be shown. t,he order of relative reactivities may vary with temperature. Theoretically, the standard free energy of activation determines the specific reaction rate at, any temperature. I n general the higher the former, the slower is the reaction rate, although exceptions occur, particularly when long reaction chains are involved. To a first approximation, the experimental a d v a t i o n energy may be identified viith the standard free ene tion. It may thus be inferred that those fluids n-ith high activation energies would be the most reeist,ant to oxidation. In Table I1 it will be noted that, a t 12.5' C. the correlation between E and k n-as good but diminished as reaction temperatures were elevated. This lack of correlation is contrary t o what would be expected if the Arrhenius relation were generally valid for these reactions. It appears, however, that complicating factors are present which influence the reaction so that the expected correlation between E and k is not as complete as would be desired. The most notable lack of correlation in the present investigation concerned the only aroniatic diester, bis(2-ethylhexyl )-o-pht'halate, Several factors could account for the discrepancies noted. Apart from the experimental errors involved in the determination ,of k and the evaluation of E in the Arrhenius equation, the assumption was made that A was a constant. That A is not always constant has recently been pointed out b y Steacie and Szwarc (18). Variation in the value of A would tend to displace the graph of log k us. 1 / T so as to affect k , but would have no influence on the dope from which E is derived. This displacement may be sufficient to cause intersection of the graphs, vr-hich would account for inversions in the relative order of k ' s a t different temperature levels. The values of log A for the various esters are shown in
having t h e
'TABLEIv.
REACTIOS RATET O PEROXIDE CONCESTRATIOX
REL.4TIOX O F IXST.4XTAKEOGS
(4)
kl
Substituting
dY
for kl in Equatioa 3, and equating with
Equation 4 leads t o the expression V * I P= k , [Perox.]
(5)
This relationship, which is valid only during the autocatalytic stage, is independent of temperature and instantaneous reaction rate, and demonstrates that the amount of oxygen absorbed is a function of peroxide concentration. Equation 3 or its equivadV lent, -- = constant [Perox.], is applicable somewhat beyond the df point of nlaximum peroxide concentration. In Figure 2, the validity of Equation 5 is illustrated by the linearity of the plots of V1 '2 us. [Perox. 1 forbis(2-eth~~lhexyl)-o-phthalateand bis(3,5,5,trimethylhexy1)sebacate. Data for other esters also result in linear graphs. A similar dependence of reaction rate on peroxide concentration Bas noted by Denison ( 4 ) in the case of normally refined petroleum oils. OXIDkTION STABILITY 4ND RELATIOX TO STRUCTURE
There have been many attempts t o define satisfactorily the stability of an oil to oxidation. Some investigators have employed as a criterlon the time necessary t o absorb a given amount of
[Perox. 1" 145OC. 123OC.
123OC. 135OC Bis (2-ethylhexyl) - 0 phthalate 1.08 3.45 8.39 1.38 2.63 7.00 1.23 2,76 7.14 1.27 2.89 6.39 1.43 .. 6.02 1 35 .. 6.07
135'C.
145OC.
1.44 .. .. Av. 1.35 2.94 6.83 Bis(B,B,LtrimethyIhexy1)sebacate
Di(n-octy1)sebacate 1.68 3.96 7.90 1.51 3.88 8.27 1.77 3.69 7.24 1.99 4.54 7.04 .. 5.12 7 .R l 4.30 .. .. 4 30 .. 1.74 4.26 7.59 Bis(l,3-dimethylbuty1)sebacate
Av. 2 . 1 8
1 90
2.14 2.13 2.18 1.78 2.78 A v . 2 20
4.69 Eioosane 4.60 5.40 5,78 4 35
9.82
4.90
8 55
..
4.37
9.22 8.63 8.00 8.36
..
[Perox.] = moles peroxide oxygen per gram sample X 104.
7.00
July 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table 11, and the inconstancy of A is apparent. The effect of long reaction chains on the validity of the equation is well known. Since the reactions studied here are autocatalytic, it is probable t h a t the reaction chains involved are not without some bearing on the discrepancies. The inversion of the expected correlation between E and /c is evidently not limited to the oxidation of diesters. A similar phenomenon can be noted in the data of Denison (6) and of Dornte (6, 7 ) on the oxidation of petroleum oils. It has been shown that there is a direct relation between the ease of oxidation of normal hydrocarbons and their chain length ( l a ) . The presence of side chains in most of the compounds included in the present investigation precluded the testing of this hypothesis for the diesters. However, some conclusions could be reached concerning the effect of the substituent groups and the ester linkages on oxidation stability. Except in the case of 1,lO-decanediol bis( 2-ethylhexanoate), the bifunctional portion of the ester molecules was an acid. In three compounds, the monofunctional portion was derived from 2-ethylhexanol, with the bifunctional portion varying in structure from the aromatic o-phthalic acid to the aliphatic glutaric and sebacic acids. Their net activation energies (Table 11) ranged from 22.5 kg.-cal. for the sebacate to 25.7 kg.-cal. for the glutarate, the difference being somewhat larger than the experimental error. Despite the similar temperature coefficients of k , the magnitude of the individual velocities varied considerably, that of the phthalate being lowest, while those of the aliphatic diesters were quite comparable. Since the oxidation mechanisms of these compounds have been shown to be similar, the aromatic structure of the phthalate must serve t o slow down the oxidation reaction. The three isomeric octyl sebacates differed greatly in activation energies and reaction rates; this must necessarily be a result of structural differences. Di(n-octy1)sebacate had a n activation energy (27.8 kg.-cal.) which was intermediate between that of the bis(1-methylheptyl) ester (32.0 kg.-cal.) and that of the bis(2ethylhexyl) ester (22.5 kg.-cal.). At 125 O C. the reaction rates of these compounds corresponded with what would be expected from the Arrhenius equation-e.g., t h a t of bis( 1-methylhepty1)sebacate was the lowest whereas t h a t of the bis(Pethylhexy1) ester was the highest. At more elevated temperatures, the reaction rate of the latter compound increased less rapidly than did those of the other esters. The relative order of reactivity of these compounds is somewhat surprising in view of the generally accepted hypothesis concerning the vulnerability of a tertiary C-H bond in the liquid phase as compared with a secondary C-H bond (19, 90). It seems reasonable t o assume, therefore, that a t lower temperatures the carbonyl oxygen provides a shielding action to protect the adjacent tertiary C-H bond; the protection decreasing with distance and having little if any effect beyond two carbon atoms. At more elevated temperatures, increased thermal agitation and the increasing role of side reactions may obscure the protective effect. Some evidence for the shielding action may be deduced from differences in the ease of esterification of normal alcohols as compared to secondary alcohols containing branched chains adjacent t o the functional group. Comparison of the bis( 1-methylhepty1)- and the bis( 1,3-dimethy1butyl)sebacates indicates that the latter was the more reactive a t all temperatures and is in fact the least stable of all the esters studied. The presence of two tertiary C-H bonds is undoubtedly responsible for this increased reactivity. If the C-H structure in the 1 position were completely protected by the carbonyl oxygen, his( 1,3-dimethylbutyl)sebacateshould be the equivalent in stability to the analogous bis(3-methylbutyl) ester. Since the latter is the more stable, it must be concluded t h a t the carbonyl oxygen is not sufficiently large t o afford complete shielding to the adjacent tertiary C-H bond. Bis(3,5,5-trimethylhexyl)sebacate would be expected t o be relatively unstable, but the compound was on all accounts quite the
1611
reverse. This stability is confirmed by unpublished work on t h e excellent oxidation stability of carboxylic acid esters of 3,5,5trimethylhexanol-1. Phosphorus esters of this alcohol were also found to be more oxidation stable than the corresponding 2ethylhexanol-1 esters (17y. The stability of this structure may be explained by consideration of the fact that quaternary carbon atoms cannot form hydroperoxides and are therefore unreactive. The tertiary C-H bond, which should be reactive, is probably hindered from ready hydroperoxide formation by the shielding effect of the neighboring methyl substituents. Bearing in mind the dependence of reaction rate upon peroxide concentration, a s was indicated in a previous section, the relatively low reactivity of such compounds with oxygen would follow. 1,lO-Decanediol bis( 2-ethylhexanoate) is commonly regarded as the inverse ester of bis( 2-ethylhexyl)sebacate, but the analogy is not strictly correct, as may be seen from the two structural formulas:
0 (
c-c-c-c-c-c-o-c-~’-c-c-c)2 I
C
I
C Bis(2-ethylhexy1)sebacate 0
I’ (c-c-c~c-c-c-o-c-c-c-c-c)* cIt I
C 1,lO-Decanediol bis(2-ethylhexanoate) The sebacate ester was found to be less stable than the corresponding glycol ester. This is in accordance with previous reasoning concerning the protective or screening action afforded by carbonyl oxygen. I n the 1,lO-decanediol ester, this effect should be even more pronounced than in the dibasic acid ester, owing to the closer proximity of the carbonyl oxygen to the tertiary C-H bond. Although the reaction rates of eicosane, a normal paraffin, could not be compared directly with those of the diesters, its activation energy of 26.2 kg.-cal. was in the middle range of those of the diesters. Presumably, its relative reactivity would also be of an intermediate order. The activation energies of the diesters investigated ranged from 20 t o 30 kg.-cal. per mole and are of the same magnitude as those of highly refined white petroleum oils ( 6 ) ,which are essentially napthene hydrocarbons. The improved oxidation stability of less highly refined petroleum oils was shown by Denison ( 4 ) to result from the presence of naturally occurring inhibitors, primarily sulfur compounds. SUMMARY AND CONCLUSIONS
The oxidation of diesters has been shown to be autocatalytic. At all temperatures the square root of the oxygen absorbed was a direct function of peroxide concentration which increased t o a maximum and then dropped sharply. At equal oxidation stages, the nature and amounts of oxidized products were comparable. I n the diesters, absorbed oxygen was accounted for t o a large extent as peroxides, acids, carbon dioxide, and water. For the hydrocarbon eicosane, oxygen recovery was less complete, probably owing to the formation of compounds containing carbonyl oxygen for which no analyses were performed. Reaction rates of the diesters were a square-root function of the oxygen absorbed. Activation energies were computed from the Arrhenius equation, which though not strictly applicable t o a complicated reaction, could be utilized to evaluate the stability of the compounds. The energies ranged from 20 to 30 kg.-cal. per mole and were in the same range of values as the paraffins and highly refined petroleum oils.
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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
Vol. 44, No. 7
Variations in the stabiiitieq of the diesters could be related to molecular configurations. Tertiary C-H bonds contributed t o reactivity, but their proximity to such structures as carbonyl oxygen or a quaternary carbon made for more stable compounds. This stability was probably due to a screening effect.
Dornte, R. W., Ibid., 28, 26 (1936). Dornte, R. IT.. and Ferguson, C. V., Ibid., 28, 863 (1936). George, P., Rideal, E. K., and Robertson, A, Nature, 149,
ACKNOWLEDGMENT
Larsen, R. G., Thorpe, R. E., and Armfield, F. h., IND.ENG.
The authors wish to express their appreciation to their colleagues, J. G. O’Rear, for his synthesis of a number of the diester fluids and George Cohen, for his assistance in some of the experimental work.
Lewis, J. S., J . Chem. Soc., 1927, 1555. Miller, R. IT., Craig, P. N., and Wolfe, J. K., NRL Rept P-
LITERATURE CITED
O’Rear, J. G., X R L R e p t . 3891 (1951). Shell Development Co. Tech. Bept. No. 5 ; 13239 (Oct. 22, 1950). Steacie, E. IT7. R., and Szwarc, M., J . Chem. Phvs., 19, 1309
(1) Am. SOC.Testing Materials, “Standards on Petroleum Products and Lubricants,” Designation D 974-48T, Philadelphia, 1950.
(2) Atkins, D. C., Baker, H. R., Murphy, C. &I., and Zisman, W. A, IND. ENG.CHEM., 3 9 , 4 9 1 (1947). (3) Bried, E. M., Kidder, H. F., Murphy, C. M., and Zisman, W. A., Ibid., 3 9 , 4 8 4 (1947). (4) Denison, G. H., Jr., Ibid., 36,477 (1944). (5) Denison, G. H., Jr., and Harle, 0. L., Ibid., 41, 934 (1949).
601 (1942).
Glavis, F. J., and Stringer, H. R., “Symposium on Synthetic Lubricants,” A S T M Special Pub. 77, 16 (1947). Kreulen, D. J. W., and Kreulan Van Selms, F. G., J . I n s t . Petroleum, 34, 930 (1948).
CHEM.,34,183 (1942). 2573 (1945).
Murphy, C. M., and Ravner, H., Ibid., C-3380 (1948). Murphy, C. M., Ravner. H., and Smith, N. L., IND.E m . CHEM.,42,2479 (1950).
(1951).
Walsh, A. D., T r a m . Faraday SOC., 42,269 (1946). Ibid,, 43,297 (1947). RECEIVED for review December 8, 1951.
~ ~ C C E P T E February D 26, 1962. The opinions or assertations contained in this paper are t h e authors’ and are not to be construed as official or reflecting t h e views of the Navy Department.
Autoignition Properties of Certain Diesel Fuels J. ENOCH JOHNSON, JOHN W. CRELLIN, AND HOMER W. CARHART Naval Research Laboratory, Washington 25, D . C .
T THE present time the evaluation of Diesel fuels in terms of ignition quality is accomplished by means of the C F R engine method. This procedure requires relatively large quantities of fuel, which presents a severe handicap in any research on fuels, additives, pure hydrocarbons, and other compounds when only small amounts of material are available. Furthermore, the engine provides data under limited conditions, which are often difficult t o interpret, although in recent years new techniques, such as t h a t used by Levedahl and Howard (Q),have widened its scope. Various methods have been used t o simulate the engine and yet eliminate Pome of the difficulties associated with it by the design of special devices such as the rapid compression machines (8, 1 4 ) and the constant-volume bombs (4,11) which restrict themselves t o the development of a single combustion cycle or a fraction thereof. Studies of ignition and slow oxidation in combustion tubes under both static and flow conditions have provided data which have been valuable in the interpretations of ignition phenomena (1,16). To achieve a better understanding of the relationship between fuel composition and ignition properties and to study other factors influencing ignition, it was desired t o have a laboratory method capable of utilizing small quantities of material. A considerable amount of information has been published by many investigators on the temperatures of self-ignition of combustibles as measured in apparatus of various designs based on the Moore oildrop method. Much of this data has been obtained using air a t atmospheric pressure, although some of the work has been done with atmospheres other than air. Although ignition of fuel in Diesel engines occurs a t relatively high pressures, it was felt t h a t investigations a t atmospheric pressure, having the advantage of simplicity of equipment and operational procedure, would be of considerable value in studying ignition phenomena
APPARATUS AXD PROCEDURES
The Jcntzsch ignition apparatus ( 5 )is a convenient and versatile instrument for studying ignition properties of materials a t atmospheric pressure, as has been demonstrated at various laboratories ( 3 , 6 , 17, 18). Certain modifications of t,he apparatus Tvere made from time t,o time in order to make the instrument more suitable for specific investigations. Essentially, the apparatus consists of an electrically heated ignition chamber coupled with an accurate oxygen-mctcring system. In the original Jentzsch method, the flow rate of oxygen supplied to the ignition chamber is controlled by means of a fine adjustment needle valve and is measured in terms of bubbles per minute by passing t’he oxygen through a water bubbler. As the bubble rate is decreased, however, more and more air diffuses into the chamber, which lowers the effective oxygen concentration. Therefore, this method was modified by metering pure oxygen and nitrogen into the ignition chamber by means of independent bubblers. The bubble rates were so adjusted that their sum was 300 per minute or about 25 cc. per minute. The oxygen concentration of the dried gas stream \vas measured continuously by means of a Pauling oxygen meter (1.2). The stainless stecl ignition chamber of the Jentzsch apparatus contains four cylindrical cells open a t the top, three of which are interconnected and receive oxygen from a central inlet near the bottom. The fourth cell is used for the measurement of the block temperature and contains a thermocouple made of No. 24 B &- S gage, Fiberglas-insulated, duplex iron-constantan wire, shielded by a two-hole ceramic tube over the portions of the wire inside the ignition chamber. At the bottom of the ceramic tube, the wires arc spread and spot-welded t o a stainless steel crucible of the type used for ignition tests. During the ignition tests, one of the shallow ignition crucibles is placed in the bottom of each cell as a fuel receptacle. K i t h the
