IMPROVED PHENOTHIAZINE ANTIOXIDANTS FOR SYNTHETIC L U B R I C A N T S HAROLD 0 . STRANGE, JOSEPH J.
McGRATH,
AND JOHN P. PELLEGRINI, J R . New Products Division, Gulf Research Ci? Development Co., Pittsburgh, Pa.
Phenothiazine carboxylic acid esters are more effective antioxidants than phenothiazine on a molar basis No “phenothiazine sludge” occurs with these esters such as is observed with phenothiazine at temperatures above 300’ F. By means of a laboratory oxidation test, such inhibited oils (20-ml. samples) are found to be stable and sludge-free for up to 7 2 hours at air flow rates up to 8 liters per hour at 347’ F., and in the presence of metals at the standard air flow rate of 1 liter per hour. When tested at temperatures up to 425’ F., these inhibited oils showed a definite superiority to phenothiazine-inhibited blends by the complete absence of any oxidative sludge. These phenothiazine carboxylic acid esters probably yield soluble oxidation products but also may function as mild detergents or dispersants, the exact mechanism not being clear.
in diester oils.
HENOTHIAZINE (I) is a n effective antioxidant for diesterP t y p e lubricants a t temperatures u p to about 300’ F. At higher operating temperatures, a definite disadvantage in the use of phenothiazine and derivatives as antioxidants is their
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I tendency to form sludge, sometimes called “phenothiazine dirtiness” (3, 4, 5, 7, 7 7). Removal of this sludge by filtration would be required to prevent clogging of lubricating lines, filter screens, etc. These considerations preclude the use of phenothiazine derivatives as antioxidants for many applications, particularly in high temperature use. I n view of the excellent performance of phenothiazine a t temperatures below 300’ F., attempts have been made to synthesize derivatives of phenothiazine which would be effective at temperatures higher than 300’ F. and give no sludge (5, 7, 77). Some of these derivatives include substituents on the nitrogen (11), on one (or both) of the aromatic rings (111), or on both the nitrogen and the aromatic rings (IV). (The R
completely eliminated the sludging tendency or “phenothiazine dirtiness’’ of inhibited oils while maintaining a high degree of antioxidant activity. O n e report (5) states that “further study of phenothiazine derivatives in diesters may not be worthwhile when the purpose is to acquire a n antioxidant which does not give significant insoluble matter.” Other workers (7) found that with 3,7-dioctylphenothiazineformation of insolubles was eliminated but antioxidant activity was much less than the chemically equivalent amount of phenothiazines. They further claimed that greatly improved results could be obtained by using a second additive which acted synergistically. More recent work (2, 6, 73) indicates that the formation of insolubles remains an important problem in the use of diester oils in aircraft turbine engines. I n fact it appears to be the limiting factor for these oils (2). We believed further study of phenothiazine was warranted and prepared a number of derivatives, including esters of phenothiazine-1-carboxylic acid (V) and phenothiazine-2carboxylic acid (VI). We chose to prepare these esters as a means of obtaining phenothiazine derivatives with improved solubility. This paper describes the laboratory evaluation of
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these esters, which we found to be equivalent to phenothiazine as antioxidants but far superior with respect to eliminating the oxidative sludge, or phenothiazine dirtiness, formed a t elevated temperatures. Experimental
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IV groups include alkyl, aryl, or acyl.)
None of these variations
Apparatus. A micro-oxidation test (aluminum block test) similar to that employed by Cole (4)and others (7, 72) was employed as a screening test. Dry air was bubbled through 20 ml. of the sample blend contained in a borosilicate glass tube topped with a reflux condenser. T h e heat source was an electrically heated and thermostated aluminum block. Standard screening procedures were 48 hours, 347’ F., and 1 liter VOL. 6
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of dry air per hour bubbled through the samples. Time, temperature, and air flow were varied and some tests were run in the presence of metals. Compounds which passed this screening test were evaluated more thoroughly by the standard MIL-L-7808 oxidation-corrosion test (70). T h e oxidized oils were filtered when necessary, and viscosity and acid number data obtained; oxidation was indicated by increases in the viscosity a t 100' F. and in the acid number. Our experience indicates that this screening test yields data of more qualitative than quantitative significance. T h e sludge was rated by a visual observation of the filter residue. Materials. The phenothiazine carboxylic esters were synthesized in our laboratory. Phenothiazine-1-carboxylic acid was prepared by metallation of phenothiazine followed by carbonation ( 8 ); esters were prepared by simple esterification. Phenothiazine-2-carboxylic acid was prepared by treating 2chloroacetylphenothiazinewith pyridine followed by heating with alkali; the methyl ester was prepared by reaction with dimethyl sulfate in acetone (9). Higher esters were prepared by transesterification using sodium metal as the catalyst. T h e esters used as base stocks were Emolein 2957 (diisooctyl azelate) and a blend of Plexol 244 and Plexol 273, which are, respectively, diisooctyl and diisodecyl adipate. Results and Discussion
I n our screening program we prepared and evaluated a number of phenothiazine derivatives as antioxidants for synthetic oils. T h e phenothiazine carboxylic esters of higher alcohols appeared to be unique in completely eliminating this sludge. Oils inhibited with these esters gave no evidence of sludge or phenothiazine dirtiness. Table I lists four of these esters, including the methyl ester, and their effect on the 100' F. viscosity and sludging of a diester oil. We found, as did Cole ( 5 ) ,that the methyl ester resulted in sludge; however, octyl and higher esters gave no sludge. For the prevention of sludge it appears that the alkyl ester should be greater than methyl. We believe the butyl ester to be a reasonable lower limit. O u r data show phenothiazine carboxylic esters to be more effective than phenothiazine on a molar basis. Thus, in Tables I to V, equivalent weights of phenothiazine and the esters denote a lower molar concentration of the phenothiazine esters. We next investigated the effect of several variables on the nonsludging tendency of these phenothiazine carboxylic esters. I n Table I1 are shown results of tests using a longer time period and an increased air flow. Even a t 8 liters per hour for 72 hours, no sludge was observed with the phenothiazine carboxylic esters. I n the presence of copper, magnesium, or steel, as well as a combination of all three metals for test times u p to 72 hours (Table 111), no sludge was obtained and the metal specimens were clear, bright, and unetched. A series of
Table I. Effect of Phenothiazine Carboxylic Esters on Oxidative Stability of a Diester Oil (347' F., 48-hour oxidation test, 1 liter of air per hour, 20-ml.
samples) 100' F. Viscosity Increase,
Antioxidant at 7 Wt. 7 0 None Phenothiazine Methyl phenothiazine-2-carboxylate Isooctyl phenothiazine-2-carboxylate Tridecyl phenothiazine-2-carboxylate Tridecyl phenothiazine-1-carboxylate
%
80 3.3 6.2 3.2 2.7 1, 7
Sludge None Heavy Medium None None None
Table II. Effect of a Phenothiazine Carboxylic Ester on Oxidative Stability of a Diester Oil at Different Air Flow Rates and Oxidation Times
(347 ' F. oxidation test, 20-ml. samples) Air Oxida100'F. Flow tion Viscosity Antioxidant Rate, Time, Increase, at 7 W t . % L./Hr. Hr. % Sludge None 8= 48 577 None Phenothiazine 2 48 1.8 Hpavv 8 48 5.7 Heav; 8 72 5.3 Heavy Tridecyl phenothiazine2 48 2.3 None 2-carboxylate 8 48 3.4 None 8 72 4.1 None a 8 liters per hour for a 20-ml. sample of oil is equal to 100 liters per hour for a 250-ml. sample.
tests a t temperatures up to 425' F. (Table IV) indicates further the excellent antioxidant activity and nonsludging tendency of phenothiazine carboxylic esters. Finally, in an MIL-L7808 oxidation-corrosion test (Table V), the tridecyl ester of phenothiazine-2-carboxylic acid and phenothiazine were compared. There was a notable absence of sediment in the oil inhibited with the phenothiazine ester and the metal specimens were clear and bright with only slight etching. (The small increase in acid number indicates that further work is necessary to meet this particular specification limit.) Studies (4, 5, 7, 77) of the mechanism by which phenothiazines exert their antioxidant activity suggest that phenothiazine sludge contains oxygenated polymeric substances of about five phenothiazine units. Presumably, the carboxyphenothiazine esters are oxidized to similar polymer substances which are soluble because of the pendant ester groups. I t is
Effect of Presence of Metals on Oxidation Stabilization of a Diester Oil by Phenothiazine Carboxylate Esters (347 ' F. oxidation test, 1 liter of air per hour, 20411. samples) 100" F . viscosity Test Antioxidant at 1 W t . 7 0 Meta Hours Increase, yo Sludge 72 159 Clear yellow oil
Table 111.
None
... ...
Cua
Isooctyl phenothiazine-2-carboxylate Phenothiazine Tridecyl phenothiazine-2-carboxylate
Cua Cua
...
cu
Mg Fe Cu-Fe-Mg * Copjer wire.
34
l&EC
PRODUCT RESEARCH
AND DEVELOPMENT
72 72 72 48 48 48 48 48 48
144 5.0 4.2 3.5 2.7 2.5)
Clear red oil None None Heavy None
None, metal panels bright and unetched 2.6
Table IV. Effect of Phenothiazine Carboxylic Esters on Oxidative Stability of Q Diester Oil at Different Temperatures (48-hour oxidation test, 1 liter of air per hour, 20-ml. samples) 10OOF. Test Viscosity Antioxidant Temp., Increase, wt. % OF. 70 Sludge
None 400 124 None Phenothiazine, 17G 400 7.7a Heavy Phenothiazine, 0 . 67G 400 3.6“ Heavy Isooctyl phenothiazine-2carboxylate, 17, 400 6.3 None Tridecyl phenothiazine2-carboxylate, 1yo 400 7.5 None None 425 120.5 None Phenothiazine, 17, 425 11.2 Very heavy Phenothiazine, 0.67, 42 5 29.1 Very heavy Tridecyl phenothiazine2-carboxylate, 17, 425 16.3 None a This “apparent” reversal of data in low viscosity range is within combined limit of error of both oxidation test method and microviscosity analysis.
Table V. Comparison of Phenothiazine and Tridecyl Phenothiazine-2-Carboxylate in MIL-L-7808D OxidationCorrosion Test Composition, W t . 70 Blend of Plexol 244 and 273 99 99 Phenothiazine 1. o ...
Tridecyl phenothiazine-;!carboxylate MIL-L-7808D oxidationcorrosion test Change in 100’ F. viscosity”, 7, Increase in acid number* FYeight change in metalsc, ~mg./sq.cm. Mg
... f6.8 0.6
1. o
+7.6 4.1
+0.20 +0.34 $0.24
-0.01 -0.01 cu -0.25 Fe f0.18 -0.01 +0.24 -0.06 A€? Visual observation Moderate varnish Slight varnish, and sediment no sediment a M I L - L - 7 8 0 8 D specification limit, -5 to 15%. * MTL-L-7808D spec$ication limit, 27,. M I L - L - 7 8 0 8 D specification limit, copper i 0.4, others 0.2%. A1
also possible that the carboxyphenothiazine esters act as detergents or dispersants. ‘The esters, per se or in some oxidized form, may solubilize the oxidative sludge as it is formed. This explanation is somewhart supported by the observation that the amount of sludge obt,ained using phenothiazine is reduced slightly when tridecyl phenothiazine-2-carboxylate is used as coadditive.
Regardless of the mechanism of action, we believe the nonsludging characteristics of phenothiazine carboxylic esters are of sufficient importance to renew interest in phenothiazine as high temperature antioxidants. Conclusions
Esters of phenothiazine carboxylic acid are effective antioxidants for diester oils a t temperatures ranging up to 425’ F. No phenothiazine sludge occurs, such as is observed with phenothiazine and derivatives when used in oils above 300’ F. T h e alkyl moiety of the esters might be butyl or higher for most effective results. T h e effectiveness of the phenothiazine carboxylic esters a t various temperatures, different air flow rates, and in the presence of metals was demonstrated in a diester oil. We are uncertain about the mechanism by which these esters prevent the formation of sludge. T h e oxidative “sludge” which forms may be soluble in the oil; or the phenothiazine esters (or an oxidized form) may exert a detergent or dispersant effect to solubilize the oxidative sludge. Whatever the mechanism, the nonsludging characteristics of oils containing phenothiazine carboxylic esters appear unique and should stimulate interest in the use of these antioxidants a t elevated temperatures. literature Cited
(1) Acton, E. M., Moran, K. E., Silverstein, R. M., J . Chem. Eng. Datu 6, 64 (1961). (2) Adamczak, R. L., Proceedings of USAF Aerospace Fluids and Lubricants Conference, San Antonio, Tex., April 16-19, 1963; ASTIA Document AD 604,278, pp. 12, 13, 16, 20, 21, 28 (1964). (3) Adams, H. FY., “Additive Studies in Research on High Temperature Gas Turbine Lubricants,” ASLE Meeting, Cincinnati, Ohio. Ami1 1960. ReDrint 60AM 3B-3. (4) Colk, j . FY., Jr., “High Temperature Antioxidants for Synthetic Fluids,” ASTIA Document AD 210,983 (1959). (5) Cole, J. W., Jr., “Studies of Antioxidants and Inhibitor Mechanisms at Elevated Temperatures,” Ibid., AD 260,345 (1961). ( 6 ) Dukek, FV. G.. J . Znst. Petrol. 50, 273 (1964). ( 7 ) Elliott, J. S., Edwards, E. D., Ibzd., 47, 39 (1961). ( 8 ) Gilman, H., Shirley, D. A,, Van Ess, P. R., J . A m . Chem. SOC. 66, 625 (1944). ( 9 ) Massie, S.P., Kadaba, P. K., Smith, C., J . Org. Chern. 24, 251 (1959). (10) Military Specification, Lubricating Oil, Aircraft Turbine Engine, Synthetic Base, MIL-L-7808D (USAF) (1959). (11) Murphy, C. M., Ravner, H., Smith, N. L., Ind. Eng. Chem. 42, 2479 (1950). (12) Murray, J. J., Scanlon, E. P., “Degradation of High Temperature Lubricants,” ASLE Meeting, Cincinnati, Ohio, April, 1960, Reprint 60AM 1A-2. (13) Yaffee, M. L., Aviation Week Space Technol. 55-63 (Jan. 11, 1965). RECEIVED for review August 25, 1966 ACCEPTED November 7 , 1966
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