Stabilization of Capacitors Deterioration of capacitors occurs by different mechanisms in alternating current and direct current fields
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SIR: Paper capacitors are made, in general, by wrapping a n assembly of impregnated paper and metal foil tightly around a pair of terminals. The metal foil provides a high electrode area. After assembly, the paper is vacuum impregnated with a wax or liquid to improve the dielectric properties. Typical impregnants are mineral oil, chlorinated diphenyls, and chlorinated naphthalene. Under high operating temperature or voltages, these capacitors undergo a progressive degradation. This degradation can be retarded by adding a stabilizer to the impregnant. A few of these stabilizers-for example, sodium hydroxide, lead oxide, and Fuller’s earth-function by well defined mechanisms ( 2 ) . There are, however, a number of stabilizers whose action has not been adequately explained. These are of many types: quinones, sulfur, nitroaromatics, aromatic azoand azoxy-compounds, and others.
Discussion of Theories Some observers (5, 8) have presented evidence that stabilizers form protective films on the electrode metal, inhibiting the catalytic effect of the metal on decomposition of the impregnant. Others ( 2 ) contend that stabilization-with aluminum electrodes and chlorinated impregnants-consists of complex formation resulting in inactivation of the aluminum chloride formed by electrolysis of the decomposition products of the impregnant. T h e common feature is that these stabilizers are oxidizing agents. None of them are strong oxidizing agents, but they can be reduced quite readily chemically or electrically. Church and Garton (4) advanced a theory explaining the degradation and failure in terms of a cathodic reduction of the dielectric. For chlorinated impregnants, they postulate the formation of nascent hydrogen which then can remove the chlorine from the dielectric to form hydrochloric acid. They attribute stabilization, then, to the preferential combination of the hydrogen with the stabilizer. With this assump-
tion, they predict that unsaturated hydrocarbons are effective stabilizers. Indeed, they claim that octadecene does act as a stabilizer. Work in this laboratory (70) did not confirm this latter finding, so this theory must be either amended or discarded. This mechanism is essentially a free radical process, the hydrogen atoms being generated electrolytically. Church (3) also fails to take into account the relative ease of reduction of some of the chlorinated hydrocarbons. Chlorinated naphthalene, a t least in ethyl acetate-ethyl alcohol solution and a t a mercury electrode, is more easily reduced than hydrogen ion. If this holds true under the conditions imposed in capacitors, it follows that nascent hydrogen isn’t available. The reduction products would include hydrogen chloride unless the stabilizer was reduced instead. Simple unsaturated hydrocarbons, unlike the effective stabilizers listed, are not easily reduced electrically. Only when the double or triple bond is conjugated with another does the electrolytic reduction occur a t reasonably low potentials. Stating a hypothesis :
The degradation and failure of capacitors are the result of electrolytic processes and the stabilization of capacitors can likewise be an electrolytic process. The stabilizers of the type under consideration function not by acceptance of hydrogen but by being electrolytically reduced directly. The general description of this type of stabilizer is :
An effective stabilizer is a material which is more easily reduced electrolytically than the dielectric (or any other material in the system). The electrode is depolarized, thereby preventing degradation of the dielectric, so long as a material is present whose reduction potential and rate of reduction permit it to be reduced more readily than the dielectric. O n e chooses a stabilizer, of course, whose reduction products are not harmful.
Current production techniques would demand that the material must be adequately soluble in the dielectric. This restriction can be avoided by applying the stabilizer directly to the electrodes before assembly. Application of a comparatively insoluble stabilizer directly to the electrodes should also yield some information about the mechanism of the degradation. If the degradation occurs by electrolytic processes, the stabilizer should be effective, but if the process involves a free radical mechanism which can be initiated away from the electrode surface, the deterioration should be affected only slightly. A free radical process has been proposed by Basseches and McLean (7). They took the gassing of a mineral oil dielectric under alternating current stress as a measure of the degradation. Other investigators pointed out that deterioration occurs when alternating current is applied to the capacitor and that it becomes more rapid with increasing frequency. T h e materials that are effective stabilizers in direct current fields are effective in alternating current fields. T h e fact that degradation occurs on application of alternating current does not disprove the hypothesis offered above. As one applies a n alternating potential and increases its frequency, any electrolytic process will diminish but the possibility of introducing a new reaction exists. We might further postulate that under conditions that preclude electrolytic processes, a free radical process can occur. From this, it must be concluded that any materials which can react with free radicals to form more stable radicals will be at least reasonably effective stabilizers. As this class of materials includes all of the useful stabilizers and many more compounds as well, we can state that the known stabilizers for direct current uses will also be good stabilizers in alternating fields; but there will be other materials which are ineffective in direct current fields but effective in alternating current fields. The data of Sauer, McLean, and VOL. 53,
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Egerton (10) and those of Basseches and McLean ( 7 ) given in the table show a comparison of direct current with alternating current effects. Sauer, McLean, and Egerton fabricated capacitors-in groups of ten-with the dielectric and additive to be tested and subjected these to direct current stress of 450 volts a t 130° C. Basseches and McLean tested mineral oil with the desired additive by applying 10 kv. as 60 cycles. T h e actual stresses are 130 kv. per cm. in the latter work but more than 200 kv. per cm. in the direct current work. T h e stabilizing effect is shown by a n increase in life or a decrease in gassing. I t can seem immediately that the stabilizers previously mentioned are effective i n each set. Two sets of data for anthraquinone show that the uncertainty in the measurement is a t least loyo. Otherwise, the second column of results might lead us to believe that octadecene has some stabilizing effect in the direct current work. Reproducibility of results in the alternating current work appears to be no worse than 5y0. Also, octadecene has a marked effect in this work. Benzil is effective in both experiments but its reduction product, benzoin, is effective in the latter work but not in the former. Similarly, there is a marked difference i n the effects of dibutyl benzoquinone and its reduction product dibutyl hydroquinone. From this we can predict that the oxidized form will be a n effective stabilizer in direct current experiments while the hydroquinone form will not. From the experimental evidence we conclude : 0 T h e application of an alternating potential stress is not completely analogous to the use of direct current. T h e rapidly changing potential does not permit the normal electrode processes to occur. 0 Materials which react readily with free radicals will inhibit the degradation of dielectric in alternating current fields. These materials include some unsaturated aliphatic hydrocarbons, polynuclear aromatics, benzenoid hydrocarbons as well as the quinones, nitroaromatics: and others. Within this group there is a special class, easily reduced electrolytically, which will inhibit the degradation of dielectrics in a current field.
A simple test of the hypothesis stated would be a repetition of the experiments of Basseches and McLean using direct rather than alternating current. If the gassing data obtained with each of the several additives maintain the same general relation as in the work reported by Basseches and McLean, the free radical mechanism must be accepted. If, however, the gassing observed with the less easily reduced materials-e.g., benzoin and dibutyl-
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Effect of Additives on Performance of Capacitor Dielectric
Dielectric Chlorinated naphthalene (Halowax)
Mineral oil (Primol D)
Additive
Concentration
None Anthraquinone
0.0
Benzil Azobenzene Azoxybenzene Octadecene
1.0
None Anthraquinone p-Nitrodiphenyl Azobenzene Azoxybenzene Benzil Benzoin 2,5-Di-tert butylbenzoquinone 2,s-Di-tert-butylhydroquinone Diphenyl Octadecene
1.0 1.0 1.0 0.5 1.0
Direct Current" Alternating Currentb Life, kraft Life, kraft Pressure Per cent tissue tissue of presat 44 lM134, Mll5, hours, in sure of in hours mm. Hg control in hours 3.7 110 122
28.2 45.0 3.7
37.2 7.1 87.1 1216
0.0
0.5 0.5 0.09 0.5 0.08 1.0 1.0 0.10 0.10 0.10
6.4
55.9
100
2016 7.6
13.6
3.3
5.9
-500 -1000 481
0.52
0.9
7.0
12.5
0.92
1.6
0.10
15.0
26.9
0.07 0.12
31.0 40.2
55.5 72.0
Data of Sauer, RfcLean, and Egerton ( I O ) . Capacitors were tested at 130' C. with a 450-volt stress. "Life" is the time to 50% failure of a group of 10 capacitors. Data of Basseches and McLean (1). Oil was placed in a cell and the space above it evacuated. A 10,000volt 60-cycle ax. stress was applied to electrodes immersed in the oil. The pressure above the oil was measured at various times. The concentration of the additive in each case was 4.59 X 10-4 moles/100 grams of oil.
hydroquinone-is significantly increased, the electrolytic mechanism is confirmed. These hypotheses and conclusions are a by-product of the development of a n analytical procedure for anthraquinone ( 6 ) . From these conclusions, other useful chemical stabilizers should also be easily reduced electrolytically and hence determinable i.n the same or a similar polarographic: system. This further conclusion has been tested. Some other quinones, azo- and azoxybenzene, and p-nitrodiphenyl yield welldefined polarographic waves and their diffusion currents are proportional to concentrations. Estimated half-wave potentials us. mercury pool electrode in methanolchloroform-hydrochloric acid solution are 1,2-naphthoquinone, - 0.05; 1,4-naphtho2-tert-hutylanthraquiquinone, -0.00; none, -0.05; anthraquinone, -0.05; p nitrodiphenvl, -0.35; and azobenzene, -0.0 volt. Azoxybenzene appears to yield a double wave with the first obscured by a maximum. The diffusion plateau is reached by -0.5 volt. Benzil and sulfur may also be determined polarographically at rather low potentials. Pasternak (9)found a halfwave potential of -0.31 volt os. N.C.E. for benzil at pH 1.3. Hall (7) reports that sulfur, at apparent pH 6 has a halfwave potential of -0.50 us. aqueous S.C.E. Both reductions involve two electrons and the materials will reduce at potentials about 30 mv. more positive for each unit decrease of pH. These two materials, in common with the other chemical stabilizers mentioned, can be reduced at a low potential and xvill, therefore, be reduced preferentially over even the chlorinated dielectrics.
INDUSTRIAL AND ENGINEERING CHEMISTRY
T h e fact that each of the chemical stabilizers is electrolytically reducible at a low potential is a prerequisite to its utility as a stabilizer. T h e varying degrees of effectiveness of the several useful materials may be related to differences in reaction rates or diffusion coefficients but the major criterion is the reduction potential. This datum is not readily obtainable under the operating conditions but the polarographic half-wave potentials can be used as a reasonable substitute. literature Cited (1) Basseches, H., hlclean, D. A , IXD. END.CHEM.47, 1782-94 (1955). (2) Berberich, L. J.: Friedman, R., Zbid., 40. 117 11948)
(3)-khurc$, H.' F., J. Znst. Blec. Engrs. (London) 98, 113 (1951). (4) Church, €3. F.. Garton, C. G., :Vatwe 162, 301 (1948). (5) Egertonj L.>' McLean, D. A,, IND. ENG.CHEM.38, 512 (1946). (6) Garn, P. D., Bott: ?A. C., A n d . Chem. 33, 84-5 (1961). (7) Hall, M. E., Zbid., 22, 1137 (1950). (8) McLean, D. A , , Egerton, L., Kohman, G. T., Brotherton, M., IND.ENG.CHEM. 34, 101 (1942). (9) Pasternak, R., Helu. Chim. Acta 31, 753 (1948). (10) Sauer, H. A., McLean, D. -4., Egerton, L., IND.EXG.CHEM.44, 135 (1952).
PAULD. GARN Bell Telephone Laboratories, Inc. Murray Hill, N. J.
