SYNERGISTIC ANTIOXIDANTS FOR S Y N T H ET IC LU B R I CANTS T. G. D A V I S A N D J . W . THOMPSON Research Laboratories, Tennessee EaJtman Co., Division of Eastman Kodak Co., Kingsport, Tenn .
Laboratory oxidation tests disclosed the high activity of alkali metal salts of carboxylic acids and substituted phenols as synergists for arylamine antioxidants in ester-type synthetic lubricating oils for advanced jet engines. The results indicated that such oils would remain relatively stable and sludge-free a t considerably higher engine temperatures than oils containing previous antioxidant systems. Similar salts of various other metals were inactive. Extensive testing of a representative oil by the Air Force disclosed good correlation between bearing rig and laboratory oxidation tests a t 425” F.; however, the oil did not perform well in an actual jet engine test. An investigation of possible causes was inconclusive. One possible but unusual explanation was that the oil performed best when large volumes of air were passed through it. Such aeration occurred during the laboratory and rig tests, but not in the engine test. No mechanism for the interaction between the metal salt and the arylamine antioxidant is known as yet. However, several factors which may b e responsible are mentioned.
most jet aircraft turbine engines use a diestertype lubricant, such as bis(2-ethylhexy1)sebacate or 2,2,4trimethyl-1,3-pentanedioldinonanoate. Military specification MIL-L-7808D (77) outlines the properties required for the oils used in military turbojet engines. Turboprop engines use more viscous oils (78), which may be the same diesters plus suitable thickeners. Adequate oxidative stability is an extremely important requirement for jet lubricating oils. Oil oxidation is undesirable, since it causes viscosity increase, metal corrosion due to acid formation, and excessive oil consumption. Phenothiazine has been used widely as a n antioxidant in these oils, and has been considered reasonably satisfactory a t bulk oil temperatures up to about 300’ F. However, its utility a t higher temperatures has been questioned because of its sludging tendency and lower activity. The increase in jet aircraft speeds and engine temperatures has made oils superior to MIL-L-7808 oil essential ( 4 ) . During recent years, several military and civil target specifications for advanced oils have been issued, such as MIL-L-9236B (15), which called for less volatile, slightly thicker oils which would perform better a t a bulk oil temperature of 425’ F. A more recent specification, MIL-L-23699 (79)?describes an oil with slightly higher viscosity, better load-carrying ability, and better thermal stability than MIL-L-7808 type oils. After various fluid types, such as esters, silicones, phosphates, and mineral oil fractions, were investigated, it appeared that esters were the most suitable base stocks (20, 23). In view of the deficiencies of phenothiazine under the expected severe engine conditions, improved antioxidants were considered to be the key to hightemperature jet lubricant development; the need for improved antioxidants is illustrated by widespread research in this field (7, 9-72, 27, 22). Antioxidant properties stressed for use at 400’ to 500’ F. bulk oil temperatures include high potency and low sludging tendency. Extensive antioxidant studies for our MIL-L-9236B oil development program emphasized the difficulty of combining cleanliness and effectiveness in a high-temperature stabilizer. This led to the investigation of possible synergists for the clean, but insufficiently effective, arylamine antioxidants, such as T PRESENT
A
76
I B E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
N-phenyl-1 -naphthylamine, which had been tested. Synergism is defined as “the simultaneous action of separate agencies which, together, have greater total effect than the sum of their individual effects.” According to other investigators, the most probable reason for synergistic effect appears to be that one compound acts primarily as a peroxide decomposer and the other as an inhibitor of free radicals (73, 24). If the hydroperoxide formed by oxidation can be decomposed before it forms a free radical, the effectiveness of the free radical inhibitor might be increased. The peroxide decomposer itself may be subjected to attack by peroxy radicals and its own effectiveness would be improved by the free radical inhibitor (73). One compound could be beneficial to the other in autoxidation inhibition. A number of synergistic combinations of this type have been reported (6, 8, 72, 74, 24). In our work on synergistic mixtures, trace amounts of certain alkali metal salts appeared to be powerful synergists for arylamine antioxidants in ester lubricants a t temperatures of 347’ to 500’ F. This paper describes the laboratory evaluation of synergistic metal salt-arylamine antioxidant systems covering a variety of metal salts and antioxidants in a variety of ester-base oils. The utility of the more useful formulations in bearing rig and jet engine tests, as well as mechanisms by which the additive systems work, is also discussed. Experimental
Apparatus. An oxidation test proposed by the Air Force for determining the oxidative stability of jet engine lubricants at high temperatures was used to evaluate the metal saltarylamine antioxidant mixtures. The test is similar to one described in an Air Force technical report ( 7 ) . A 250-ml. sample of oil was heated at 425’ F. in the test cell shown in Figure 1 while air a t 96 liters per hour was blown through the oil. Freshly polished 1 X 1 x ‘ / 3 2 inch samples of steel, copper, aluminum, titanium, and silver, assembled as shown in Figure 2, were immersed in the oil. The tube was capped with a standard taper glass fitting which allowed volatile products to escape. The oil was heated in 20-hour cycles; at the end of each cycle, the tube was cooled, a 10-ml. sample was withdrawn, and the oil lost by evaporation was replaced.
Ordinarily, the test was continued for six cycles or 120 hours. Viscosity (2) in centistokes at 100' F. and acid number (3) (milligrams of K O H per gram of sample) were then determined for the 10-ml. samples. Oxidation was indicated by increases in viscosity and acid number. Sludging was determined by observation of deposits retained on a filter paper after filtering a 2-ml. oil sample and washing the residue with naphtha. For testing at 500' F., the same apparatus and procedure were used, except that stainless steel was substituted for the copper test sample. Materials. The arylamine antioxidants and metal salts used were obtained commercially or synthesized in our laboratories. The arylamines included various compounds, such as -V-phenylnaphthylarnines, diphenylamine derivatives, phenylenediamine derivatives, and phenothiazine. T h e alkali metal salts included potassium, sodium, lithium, and cesium salts of carboxylic acids and substituted phenols. A variety of phenolic compounds was used to prepare the phenoxide-type metal salts, such as potassium p-nonylphenoxide. The general structure is:
IO-MM. O.D. STD. WALL
\ IN' 1819 BALL J
O 2 IN.+ I N
T
4
4 7 4 IN.
U
i Figure 1 . Oxidation test cell and COP
64-MM. O.D.
TEST SPECIMENS
OM
I
in which M is an alkali metal and R is a n alkyl or aryl group or substituted derivative thereof. T h e low solubility of salts of aliphatic carboxylic acids and phenols in esters necessitated an extensive search for more soluble compounds. T h e most useful acid for providing soluble alkali metals salts was obtained by partial amidation of 1 mole of (ethylenedinitrilo) tetraacetic acid (EDTA) with 3 moles of a primary alkylamine, such as Primene 81R (Rohm and Haas Co.). This partial amide had one free carboxy group which was neutralized with potassium hydroxide to give a compound with the following structure : 0 0
II
II
CH2CNHR
RNHCCHp, NCHzCH N' RNHCCH,' I1
\CH,COK
II
0 where R is the alkyl group of Primene 81R. This salt-type synergist, hereafter referred to as K(EDTA-P81 R) salt, contained 4 weight % potassium. The esters used as base stocks were synthesized in our laboratories. They included bis(2-ethylhexyl) sebacate (a MIL-L7808 base stock) ; 2-butyl-2-ethyl-l,3-propanediol dinonanoate and 2-ethyl-2-hydroxymethyl-1,3-propanedioltriheptanoate, hereafter referred to as trimethylolpropane triheptanoate (candidate esters for MIL-L-9236B); and pentaerythritol nonanoate-2-ethylhexanoate (50/50) (a viscous, nonvolatile ester for 500' F. oxidation tests).
0
Results and Discussion
Laboratory Tests. The powerful synergism between a n alkali metal salt and a n arylamine in retarding ester oxidation is demonstrated by the results shown in Figures 3 and 4. These show the effect of oxidation tests a t 425' F. on the viscosity and acid number of 2-butyl-2-ethyl-l,3-propanediol dinonanoate with and without the stabilizers. T h e uninhibited ester oxidized rapidly in less than 20 hours. Neither N-phenyl-1-naphthylamine (PANA) a t a concentration of 2y0 nor K(EDTA-P81R) salt a t a concentration of 0.25y0 had much antioxidant value when tested separately; however, after 160 hours, the formulation containing a mixture of PANA a t a concentration of 2.0yo and K(EDTA-P81R) a t a concentration of 0.25Oj, showed no significant increase in viscosity or acid number Table I compares the effectiveness of various metal stearates at a concentration of 0.01% metal. In all cases, the oil used was 2-butyl-2-ethyl-1,3-propanedioldinonanoate stabilized
Figure 2. Test specimen and configuration
TEST SPECIMEN CONFIGURATION
with 1% PANA and 1% 5-ethyl-l0,lO-diphenylphenazasiline (5-10-10). The stearates of magnesium, calcium, barium, beryllium, aluminum, cadmium, zinc, vanadium, iron, and nickel were ineffective, since the oils containing them oxidized rapidly, as indicated by increases in the viscosity and acid number of the oil. I n contrast, the alkali metal stearates provided excellent stability. The alkali metal salts were unique in synergistic activity under these test conditions. Many salts of potassium, sodium, lithium, and cesium were found to be effective synergists. Table I1 shows the results of oxidation tests a t 425' F. on the above-mentioned ester when stabilized with various alkali metal carboxylates, phenoxides, and amines. In each instance, the addition of the salt greatly increased the oxidative stability of the oil. T h e useful range of alkali metal concentration appeared to be about 0.005 to 0.02 weight % for good performance a t 425' F. O n the basis of extensive oxidation and solubility studies, the K(EDTAP81R) salt was selected as the most useful synergist. The activity of arylamine antioxidants with the K(EDTAP81R) salt is shown in Table 111. T h e stabilizing value of each is improved by addition of the salt. The simple alkylated diphenylamines gave the cleanest systems. In order to determine whether the arylamine-alkali metal salt mixtures were useful in other ester types, oxidation tests a t 425' F. were conducted on esters of pentaerythritol, trimethylolpropane, and sebacic acid. The esters contained an antioxidant mixture of 1% PANA and 1% 5-10-10. The results in Table I V indicate that, in each case, the addition of 0,25yG K(EDTA-P81R) salt (O.OlOj, potassium) greatly increased the oxidative stability of the inhibited ester. T h e activity of the arylamine-metal salt mixtures a t higher temperatures was demonstrated by using the previously described Air Force oxidation test a t 500' F. Figure 5 indicates that the addition of the metal salt (0.1% potassium as VOL. 5
NO. 1
MARCH 1966
77
the salt of a naphthenic acid) to pentaerythritol nonanoate2-ethylhexanoate (75/25) containing 3% arylamine antioxidant (p,p '-dioctyldiphenylamine) gave a very dramatic increase in oxidative stability, considering t h i extremely high test temperature. A much higher potassium concentration was required at 500' F. than a t 425' F. Pronounced synergism was also observed during 347' F. oxidation tests of the type prescribed in MIL-L-7808.
Effect of Metal Salts on Oxidative Stability of Oilsa
Table I.
(425' F. oxidation test, 96 liters of air per hour) Metal Salt .Metal (concn., 0.07~o)
None Sodium Potassium Magnesium Calcium Barium Beryllium Aluminum Cadmium Zinc Vanadium Iron Nickel
Metal stearate concn., w t . yc
0,128 0.08 0.24 0.15 0.05 0.61 0.36 0.07 0.11 0.19 0 18 0 12
Oxidation Time, Hr.
40 120 120 40 40 40 40 40 40 40 20 20
40
+
Ester l,Oyc h'-phenyl- I-naphthylamine diphenylphenazasiline. a
Acid N o . , Viscositv Increask,
YO
299 30 17 231 254 299 334 540 353 126 >lo00 693 21 2
+
1.07,
Me. KOfi/G. Sample 7.50 0.87 0.54 7.16 8.32 8.92 10.06 6.84 8.73 7.29 14.53 10 73 6 22 5-ethyl- 10,lO -
Activity of Various Metal Salts with Amine Antioxidantsa (425' F. oxidation test, 96 liters of air per hour) Oxidation Viscosity Alkali Metal Salt in Ester Concn., Time, Increase, ( M e t a l Concn., 0 . 0 7 ~ o ) Wt. % Hr. 70 20 190 None 120 11 0 046 Potassium salt of 1-naphthol Potassium salt of l-phknylazo-2120 21 naphthol 0.073 Potassium salt of 2,2-dimethyl120 18 undecanoic acid 0.065 Potassium salt of a naphthenic acid 0.088 120 22 120 22 Sodium propionate 0.04 120 38 Sodium nonanoate 0.08 140 22 Potassium (EDTA-P81R ) 0.25 120 17 Potassium (p-nonylphenoxide) 0.065 120 34 Sodium (p-nonylphenoxide) 0.11 Potassium (EDTA-2-ethylhexyl100 24 amine) 0.14
Table II.
Bearing Rig and Engine Tests, I n view of the encouraging oxidation test results, a MIL-L-9236B candidate was sent to the Air Force (Wright Air Development Division) for the bearing rig test which is used to qualify oils for the turbine engine test. T h e oil was 2-butyl-2-ethyl-l,3-propanediol dinonanoate stabilized with 1 weight % Goodrite Stalite (an octylated arylalkylated diphenylamine, B. F. Goodrich Co.) plus 0.50 weight % K(EDTA-P81R) salt (0.02 weight yo potassium in the oil). This formulation, which was chosen after a series of laboratory tests indicated these to be the most effective antioxidant concentrations in terms of stability and cleanliness, met the physical property requirements of MIL-L-9236B with minor exceptions. Bearing rig conditions were : 100 hours' duration, 425' F. bulk oil temperature, 525" F. bearing temperature, 8180-r.p.m. shaft speed, 500-pound bearing load, 600-cc. per minute oil flow, 1.47-cu. feet per minute air flow through bulk oil. After several tests, the oil in this rig was given a n excellent rating in terms of very low increases in viscosity (from 14.8 to 18.1 cs. a t 100' F.) and acidity (from 0.03 to 0.26 acid number). I t was also given a very low deposit rating. T h e oil was then subjected to the 100-hour turbine engine test specified in MIL-L-9236B. However, the engine test was terminated after 20 hours because of 100% increase in viscosity. I n addition, the oil had a high acid number, and the engine was rather dirty. Subsequent laboratory oxidation tests and other tests on the oil sent to the Air Force indicated that no change had taken place in the oil due to storage which could have caused this poor performance. Figure 6 shows the performance of the oil in 425' F. laboratory oxidation, bearing rig, and engine tests, where viscosity increase was used as a measure of oxidation. There was good correlation among resltlts obtained from laboratory oxidation tests, bearing rig tests, and engine tests on non-salt-containing formulations previously evaluated by the Air Force; therefore, there is apparently some inherent oxidative characteristic of the arylamine-alkali metal salt system that is not in other antioxidant systems, and this characteristic is accented by some difference between the bearing rig test and the engine test. O n e explanation may be that the oils containing potassium salts performed well only in tests in which larue amounts of air
~
+
a Ester 1.07,JV-phenyl- I-naphthylamine f 7.07, 5-ethyl- 10,lO diphenylphenarasiline.
%?
>
Table 111.
Activity of Various Amine Antioxidants with Metal Salt"
(425' F. oxidation test, 96 liters of air per hour) Salt Antioxidant in Ester Base Concn., at 2 W t . yG W t . 7, .V,,V '-di-2-naphthyl-p-phenylenediamine 0 0 25 V-phenyl-1-naphthylamine 0 0 25
Octylated arylalkylated diphenylamine
0
Phenothiazine*
0
0.50 0.25
a
78
K( E D TA-P8 7 R ) salt.
A t 1 ut.
Oxidatton Vzscosity Time, Increase, Hr. %
40 40 140
167 40 256 16
20 140 40 40
47 1 22 81 25
80
YOconcentration.
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
701
0
I:,,,, +
0.25% K(EDTA-PE1R)SALT (0.01% K)
ESTER
30v , ,
10
+
ESTER
2.0% N-PHENYL-1-NAPHTHYLAMINE
, , + p'"",
K ( Y P E 1 R I S A L T (0.01% K )
40 EO 120 HOURS TESTED AT 425°F.
0
Figure
3.
160
Effect of potassium salt on viscosity
. 2 20 ESTER
+
ESTER
= o
2.0% N-PHENYL-1-NAPHTHYLAMINE
+ +
0
Figure 4. number
2.0% N-PHENYL-1-NAPHTHYLAMINE 0.25% K(EDTA-PE1R)SALT (0.01% K)
EO 120 40 HOURS TESTED AT 425°F.
160
Effect of potassium salt on acid
Figure 3. ttrecr OT merai salts and antioxidants at high temperature Ester Ester f 0.970 potassium salt of naphthenic acid (0.10% Kl Ester 3.070 Vanlube 81 Ester 0.9y0 potassium salt of naphthenic acid (0.10% K) 3.0% Vanlube 81
__/---~
60 50
----
40
2o
4o M) 120 140 HOURS TESTED AT 500°FF.
+
+
+
Table IV.
Effect of Metal Salt-Antioxidant Mixtures on Esters (425' F. oxidation test, 96 liters of air per hour) Salt Oxidation Viscositv Concn.,j Time, Increase, Stabilized Estera W t . 70 Hr. 70 Pentaerythritol tetrahexanoate 0 40 198 25 0.25 120 Trimethylolpropane triheptanoate 0 20 76 0.25 60 7 Bis( 2-ethylhexyl) sebacate 0 60 215 0.25 120 30 a ly0 ,N-phenyl- 1-naphthylamine lyG 5-ethyl- 10,lO-diphenylphenazasiltne. h K ( E D TA-P87R) salt.
+
,
-LABORATORY
40
-_--BEARING -.-
RIG
ENGINE
Figure 6. Laboratory, bearing rig, and engine tests on lubricant containing metal salt
::pz; __-----
0
0
40
80
120
160
HOURS TESTED
were blown through the bulk oil during the test. Our laboratory oxidation test involves blowing air through the oil, and the bearing rig test involves blowing air through the bulk oil; in the engine test, the bulk oil is not aerated. It is not known why air is beneficial; the general opinion has been that the higher the contact rate between the air and the oil, the faster the oil degrades. The high rate of air flow may sweep out volatile degradation products (although the use of a condenser and low air flow did not indicate this, Table V) or catalyze a change in the alkali metal salt or the alkylated diphenylamine antioxidant. A laboratory test was carried out in which the flow rate of air was reduced from 96 to 5 liters per hour, and both the acid number of the oil and the sludge increased somewhat, but not so much as in the engine test. Several variations of the oxidation test were used to determine why the lubricant performed poorly in the engine test. The data in Table V show that a very low air flow (2 liters per hour) or low air flow in the presence of large metal specimens with the exception of brass, galvanized iron (zinc-coated), and lead did not cause degradation. I t is not known how much, if any, brass or zinc comes in contact with the oil in the test engine. If a high air flow is used, however, the lubricant is considerably more stable, even in the presence of brass. The engine used in the test had a lead coating on the bearings; the presence of a large lead specimen in the vapors above the oil in the laboratory test caused rapid oil degradation, whereas lead below the oil surface did not. O n the theory that acidic vapors corroded the lead to form salts which catalyzed oxidation, lead stearate was added, but it had no effect on degradation. A recent paper by Chakravarty (8) indicates that peroxides rather than acids are responsible for lead corrosion in hydrocarbon oils. Such a mechanism may be applicable in these synthetic systems. A composition of pentaerythritol nonanoate-2-ethylhexanoate (75125) stabilized with 3.0% p,p'-dioctyldiphenylamine and 0.90% potasrium naphthenate was tested as a possible candidate for the very severe jet lubricant specification M I L L-27502 (76). This oil performed well in 500' F. high-airflow oxidation tests in our laboratories and a t Southwest Research Institute. However, after the sample was tested in a nonaerated bearing rig? its viscosity was high, its acid number wa5 increased, and it was extremely dirty. The mechanism by which the metal salt-arylamine antioxidant mixtures inhibit the formation of sludge and acidic
Table V.
Effect of Test Variation on Oil Containing Metal Salt" (425' F. oxidation test, 5 liters of air per hour) Oxidatzon Viscosity T i m e , Increase, Test Variation Hr. 7c None 120 13 Air, 2 liters/hr. 10 120 11 Condenser on tube 120 30 0 . 1yo Copper stearate added 120 Brass strip 13l/2 X 11/2 X inch in place of standard metals 100 165 Brass strip 131/z X 1'/2 X l / 3 2 inch in place 50 of standard metals; air, 96 liters/hr. 120 Lead strip 13'/2 X l l / p X l / 1 6 inch in place of standard metals 40 > 1000 Lead strip 3l/2 X 11/2 X l / 1 8 inch suspended above oil in place of standard metals below oil surface 40 > 1000 inch in place of Lead strip 3l/2 X 11/2 X standard metals 120 21 0.593 lead stearate added 120 15 Galvanized iron strip 13ljz X 11/2 X l / a 2 inch in place of standard metals 120 113 a Ester l.OcG Goodrite Stalite 0.5Yc K ( E D T A - P 8 7 R ) salt.
+
+
oxidation products in lubricating oils is not known. the following interactions may be occurring :
However,
The alkali metal salts may have a capacity to direct the oxidation of the base oil to form oxidation products other than acids or sludges. The alkali metal salts and amine antioxidants in the presence of heat and air (oxygen) may form complexes whicb are more potent antioxidants than the original material. The salts may form free radicals or catalyze the antioxidants to form free radicals which in some manner interfere with the sequence of chain reactions in the autoxidation of the oils. Conclusions
Certain alkali metal salts are effective synergists for arylamine antioxidants a t 347' to 500' F., especially under laboratory and bearing rig conditions where air-to-oil contact rate is high. Their effectiveness was demonstrated in various ester-type base oils covering this temperature range. As the air-flow rate in oxidation tests is increased, these antioxidant systems become more effective; and as oxidation progresses a t 500' F., the inhibitory effect of these systems increases. Liquid esters containing these antioxidant systems appear to be promising as high-temperature lubricants in applications where the bulk oil can be aerated. Acknowledgment
The authors thank the Aeronautical Systems Division, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio, for permission to cite bearing rig and engine test results. VOL. 5
NO. 1
MARCH 1966
79
References
(1) Adams, H. E., “Materials for High Temperature Jet Engine Lubricants,” Wright Air Development Center, M’ADC Tech. Rept. 59-244 (1959). (2) American Society for Testing Materials, Standard Method D 445-61. (3) Ibzd., D 974-58T. (4) Berkey, K. L., “Developments in Aircraft Turbine Lubricants,” SAE National Aeronautics Meeting, New York, N. Y . , 1958. (5) Black, H . C., Johnson, J . H., U. S. Patent 2,680,122 (1954); C. A . 48, 9729 (1954). (6) Brunier, R., “Development of High Temperature OxidationCorrosion Inhibitors to Improve Stability of High Temperature Hydraulic Fluids and Lubricants,” .4rmour Research Foundation, Illinois Inst. of Technology, Chicago, Ill., WADC Tech. Rept. 58-171,Part I1 (1959). (7) Cantrell, T. C., Smith, H. G., U. S. Patent 2,707,172 (1955); C. A . 49, 9921 (1955). (8) Chakravarty, N. K., J . Inst. Petrol. 51, 98 (1965). (9) Cohen. G., Murphy, C. M., O’Rear, J. G., Ravner, H., Zisman, M:. A., Ind. Eng. Chem. 45, 1766 (1953). (10) Cole, J. W., Jr., “High Temperature Antioxidants for Synthetic Fluids,” ASTIA Document A D 210-983, ASTIA Document Service Center, Dayton, Ohio, PB 142507, 1959. (11) Dukek, LV. G., J . Inst. Petrol. 50, 273 (1964). (12) Elliott, J. C., Edwards, E. D., Ibid., 47, 39 (1961). (13) Ingold, K. U., Ibid., 47, 375 (1961). (14) Knapp, G. G., Orloff, H. D., Ind. Eng. Chem. 53, 63 (1961). (1 5) Military Specification, “Lubricating Oil, Aircraft Turbine Engine, 400’ F.,” MIL-L-9236B (USAF), 1960.
(16) Military Specification, “Lubricating Oil, Aircraft Turbine Engine, 500’ F.,” MIL-L-27502 (USAF), 1961. (17) Military Specification, “Lubricating Oil, Aircraft Turbine Engine, Synthktic Base,” MIL-L-7808D (USAF), 1959. (18) Military Specification, “Lubricating Oil, Aircraft Turbine Engines, Syn. Type (OX-38),” Ministry of Supply, Air Division, Directorate of Engine Research and Development, Material Specification No. D. Eng. R. D. 2487, 1957. (19) Military Specification, “Lubricating Oil, Aircraft Turboprop and Turboshaft Engines, Synthetic Base,” MIL-L-23699 (Wep), 1963. (20) Murphy, C. M., O’Rear, J. G., Ravner, H., Sniegoski, P. J., Timmons, J., J . Chem. Eng. Data 4, 344 (1959). (21) Murphy, C. M., Ravner, H., Smith, N. L., 2nd. Eng. Chem. 42, 2479 (1950). (22) Silverstein, R. M., “Synthesis and Evaluation of High Temperature Antioxidants for Synthetic Hydraulic Fluids and Lubricants,” WADC Tech. Rept. 58-335,Wright Ai: Development Center, Dayton, Ohio, 1959. (23) Sokas, M. M., “Review of the Air Force Materials Research and Development Program,” WADC Tech. Rept. 53-273, Supplement 2, Wright Air Development Center, Dayton, Ohio, 1956. (24) Stuart, W. T., Stuart, F. A., Advan. Petrol. Chem. Rejning 7, 10 (1963). RECEIVED for review June 10, 1965 ACCEPTED November 22, 1965 Division of Petroleum Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965.
ANTIOXIDANTS, FREE RADICAL CHAIN TERMINATORS HANS LOW Shell Oil Research Laboratory, Wood Riucr, Ill.
A chemical system was devised to evaluate antioxidants in terms of their ability to terminate a free radical chain reaction as distinct from other functions such as the decomposition of peroxides. The reaction used in these studies was that between a thiol and an olefin, its progress being followed by gas-liquid chromatographic analysis. The addition of compounds known to be antioxidants or expected to perform as such changed the rate of the free radical addition reaction of thiol and olefin. The ratio of the uninhibited to the inhibited rate provides a measure of the effectiveness of antioxidants to scavenge free radicals and interrupt oxidative free radical chain reactions. The effect of several oxidation inhibitors on this reaction and their effectiveness in conventional air-oxidation tests are discussed.
oxidation of organic compounds and the inhibition of (74,28). T h e prevention of oxidation is, of course, of primary industrial concern and antioxidants are used in a great variety of commercial products. Such materials have been classified roughly according to their presumed chemical function as primary and auxiliary inhibitors (72> 75, 20). T h e former, also known as chain breakers or chain terminators, include mostly phenols and aromatic amines. The latter group is made u p of several types of compounds which share the functional characteristic HE
Tsuch processes have been described extensively
80
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
of decomposing peroxides or hydroperoxides in such a manner as to minimize the propagation of the oxidative chain reaction promoted by these peroxides. Among compounds known to decompose peroxides are amines, sulfides, disulfides, bases, acids, metal ions, and esters of phosphorus acids. The chemical reactions involved in the inhibited oxidation processes are complex. Antioxidants can conceivably engage in more than one type of reaction leading to inhibition of oxidation. Thus, it is of considerable theoretical as well as practical interest to clarify the mechanism by which a chemical
