January 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
135
of Minnesota, and Carl Krieger of the Wisconsin Alumni Re-
TABLEVIII.
RESPONSETO
GROWTHFACTORSOTHERTHAN search Foundation for permission t o u8e unpublished data origi-
VITAMINBI24
Supplement
VitaminBI8 Weight at 8 y/Kg. die{ Weeks, Grama Trace 818 15-20 890 ao 9ao Trace 890
ao arsenic acid Basal and 0.45% aqimal protein factor, Brand A (antibiotic origin)b Basal and 1% qnjmsl protein factor, Brand A (antibiotic or1 in) b Basal and 0 , 2 6 6 animal protein factor (bac20 terial origin) Test erformed by Wisconsin Alumni Research Foundation. shire chi& per group. b Vitamin Bit content was not determined.
963
910 990
980 20 Leg-
ACKNOWLEDGMENT
The authors wish to acknowledge and thank Edward Stephenson of the University of Arkansas, George Briggs of the University
nating in their laboratories. LITERATURE CITED
(1) Briggs, G. M., Hill, E. G., and Giles, M . J., Poultry Sci., 29, No. 5. 723 - - (1950). (2) Halbrook, E. R., Cords, Fay, Winter, A. R., and Sutton, T. El., J . Nutritwn, 41,565 (1950). (3) Hall, H. H., Benjamin, J. C., Bricker, H. M., Gill, R. J., Hayne8, W. C.. and Tsuchiua. H . M.. Bact. Proc.. Abstracts 21 (1950). paper presented a t t h e 60th Meeting of the Society of American Bacteriologists. (4) Hendlin, D., and Ruger, M. L., Science, 111,542 (1950). (5) Lewis, J. C., Ijicki, K., Snell, N. S., and Garibaldi, J. A., U. S. Bur. Agr. Ind. Chem., A I C 254 (October 1949). (6) Stokstad, E. L. R., Page, A. C., Pierce, J., Franklin, A. L., Jukes, T. H.. Heinie. R. W.. Epstein. M.. and Welch, A. D., J . Lab. Clin. Med., 33,860 (1948).
-.
.----,
RECEIVED February 3, 1951. Presented before the Division of Agricultural CHPIMICAL and Food Chemistry at the 118th Meeting of the AMERICAN SOCIETY, Chicago, IU.
Stabilization of Dielectrics Operating under Direct Current Potential H. A. SAUER, D. A. MCLEAN, AND L. EGERTON Bell Telephone Laboratories, Inc., Murray Hill, N. J.
T
HE effectivenessof stabilizers in electrical capacitors impregnated with chlorinated aromatic compounds has been fully established. Of the stabilizers discovered, anthraquinone (9, fO) has been most widely used commercially, although its chloro derivatives and azoxybenzene have also been employed. Anthraquinone possesses the desirable characteristics of low volatility, low toxicity, high stabilizing effectiveness, and commercial availability in pure form at low cost. All quinones tested t o date have been found to have stabilizing action. Other compounds found to be effective are the nitroaromatics (6),maleic anhydride (6),sulfur (4,aromatic acid anhydrides, b e n d , and aromatic azo and azoxy compounds (I). In addition, it is claimed by Church and Garton (3)and by Church (2) that unsaturated aliphatic hydrocarbons which are readily reduced are stabilizers, octadecene in particular being mentioned. They indicate that the life of Chlorinated diphenyl capacitors has been extended more than ten times by the addilion of azobenzene or octadecene but under the conditions of the present work no stabilizing effect of octadecene could be noted, Much of the previous work has been rather exploratory in nature and in particular where test capacitors have been subjected t o accelerated aging tests, the number of samples has usually been small. This approach had no serious disadvantages in the early stages of the investigation since the effects observed were large and the question of precision therefore relatively unimportant. For example, ratios of the Iife of stabilized to unstabilized capacitors ranging from Bfold to about 1WfoId have been reported. This exploratary approach pennitted covering 8 wide range of compounds and selection of very effective stabilizers for commercid we at an early stage of the work. However, more thorough and precise experiments are necessary in order that both the fullest and most, effective use be ma&eof
stabilizers and that completely reliable data for testing the various theories of stabilization be obtained. The work reported here, while still limited by the inherent wide dispersion of life test results, is an attempt t o determine certain relationships with more accuracy than most previous work. It can be divided into three phases: a study of the effect of concentration of anthraquinone on accelerated life test performance of chlorinated naphthalene-impregnated paper capacitor units; a comparison of the effectiveness of several stabilizers at 1% concentration; and investigation of the effect of stabilizers on the direct current life test performance of mineral oil-impregnated capacitor units. EXPERIMENTAL METHOD
The present work was facilitated by recent developments in testing technique which involve testing at higher temperature with a consequent higher factor of acceleration. The temperature of test is 130° C., this high temperature being justified because the deterioration process under direct current potential hss been shown t o be predominantly chemical and electrochemical; 1 therefore, the life is an exponential fanction of p (I, IO). If the exponent in the function is the same for various groups of samples representing different conditions of impregnant, stabilizer, and paper, then their relative performance should be the same a t high as at low temperature. That this is approximately true is indicated by recent experimental results (fS). Like results of m y highly accelerated test, these should be checked under conditione more nearly approximating operating conditions before designs are established. In this connection, caution should be exercised in extrapolating to conditions where the state of the dielectric changes, such as from the tiquid to the solid state, or in changing
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
136
the conditions such that the amount of stabilizer in solution is altered. I n this study the impregnant is in the liquid state and the stabilizer is completely in solution. The test method used was developed for rapid evaluation of the quality of capacitor paper and has been described elsewhere (14,16). The equipment used consists of an integrated test set comprising an o v e n , r w e r supply, circuit breakers, clocks, and the necessary safety evices.
0%
0.5*/0 AQ
AQ
0
100
5
"
0
AQ
100
I.O%
200
0
AQ
100
200
900
IO00
(01 2 0% AQ
2 100
25 ?
I.Oa/o
80 60 40 PO
(E) 4.0% AQ
I00 00
60 40
20
OO
100
200
300
500 600 TIME IN HOURS
400
700
000
Figure 1. Effect of Concentration of Anthraquinone (AQ) on Stabilizing Action in Chlorinated Naphthalene (Halowax 1001) 130° C., 450 volts direct current 2-layer, 0 . 4 - 4 1 kraft paper u n i t s Aluminum electrodes
Test units consisting of two layers of 0.4-mil paper interleaved with aluminum foil electrodes are used. The capacitance of each unit when dried and impregnated is approximately 1 microfarad and the test potential 450 volts direct current. The size of test unit and the potential gradient are chosen to avoid electrical overheatin The extreme severity of these test conditions can be realize%from the fact that the average life of unstabilized chlorinated naphthalene-impregnated test capacitors is from 3 to 6 houra. The process of preparing the test units and the purity of impregnant and stabilizer are carefully controlled. If the stabilizer as received appreciably affects the conductivity of impregnant, it is purified to remove impregnant-soluble ionic impurities. It is further essential in comparing the effectiveness of various stabilizers that the same lot of aper be used throu hout or that each result be compared with t t e performance of &e pa r under a standard set of conditions. I n the present work, effect of stabilizer type or concentration is studied by observing the performance of ten test units. More complete details of the processing and test are given in the articles previously referred to. The azoxybenzene-stabilized impre nant was the only one not prepared by the authors. Insteacf the material used was a commercial stabilized chlorinated naphthalene. This compound was used as received without purification.
Vol. 44, No. 1
tion a progressive increase in life is observed. [The solubility of anthraquinone in chlorinated naphthalene at 130' C. (9) hm been found to be about 4.8aj0,] At 4% concentration, the value is 25-fold that of the group of capacitors containing only 0.5% anthraquinone. It is remarkable that the improvement over the unstabilized is by a factor of 270-fold. The test run on one member of this series (Figure 1, 1yoanthraquinone) is shown in duplicate to indicate the reproducibility of results. Figure 2 depicts graphically the concentration effect. The curves for both kraft and linen paper are slightly sigmoid in shape. During the life test of the group containing 4.0% anthraquinone (Figure 1E)the directxurrent conduction (leakage) current was frequently measured a t 100 volts. The total conduction current was divided by the number of capacitors on test giving the average current per capacitor. This value was plotted against time as curve A in Figure 3. The slight rise in conduction with time is followed by a more pronounced decrease in the latter stages of the test. This decrease occurs in spite of a progressive deterioration of the impregnant, which in itself might be expected to cause the conduction current to increase. Less complete information was obtained in the run with 2% anthraquinone content. Here, a marked increase in conduction current is observed in the early phases of the test. After the initial measurement no further measurements were made in the first 330 hours of test; this part of curve B in Figure 3, therefore, is shown as a dotted line. However, from this point on, a definite decrease in Conduction current is again evident. At the conclusion of the test (666 hours) the current per unit had decreased to its original value. I n the case of 4% concentration, the conduction current a t 1000 hours is well below the initial value. Generally, increase in conduction current does not precede failure in stabilized capacitots as is the case for unstabilized capacitors ( I S ) . Similar series of tests were performed employing linen paper instead of kraft. Charts of these data are not reproduced here but the life values are recorded in Table I. These results indicate that in the case of linen paper also a progressive increase in life is obtained as the anthraquinone concentration is increased. Comparing the stabilized with.the unstabilized samples, the factor of improvement is somewhat greater than that obtained in the case of kraft paper capacitors but the life attained is much less. The conduction current-time curve for the 4y0 anthraquinone concentration appears in Figure 3 as curve C . The conduction in stabilized linen paper capacitors is characterized by a high initial current and a continuous decrease with time. 1
I
I
I
1
I
I
I
I
I
I
1000
tg
a00
600
# 0
m 0 I-
DISCUSSION OF DATA
Charta ofathelife data for a series of concentrations of anthraquinone in chlorinated naphthalene (Halowax 1001) impregnated kraft paper capacitors appear in Figure 1. Tests have been conducted on samples containing 0.0, 0.5, 1.0, 2.0,and 4.oY0 anthraquinone. Contrary to the indications of previous work (fO), each increment of anthraquinone produces a pronounced extension in life under these test Conditions. A measure of the effectiveness of stabilizers may be obtained by Comparing the time required for 50% of the test capacitors to fail. These values are shown in Table I. As the concentration is increased t o satura-
w
z
F
200
0 0
1.0
2.0
AQ CONCENTRATION
Figure 2.
3.0
4.0
IN PERCENT
Relation between Concentration of Anthraquinone (AQ) and Life Time to 50% failure Data taken from Table I
SO
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TABLE I. EFFECT OF STAB~L~ZER CONCENTRATION CAPACITOR LIFEFOR VARIOUSSTABILIZE=AND IMPREGNANTS Impregnant Stabilizer Chlorinated naphthalene None Anthraquinone Anthraquinone Anthraquinone Ant6 raquinone Anthraquinone Ootadecene Tetraphenyltin Tetraphenyltin Dibutyltin dilaurate Dibutyltin dilaurate Benzil Azobenzbne 2,5-Di-tsr:-butylquione
Lifea Kraft Conaen- Tissue M115, tration, yo Hours
Ljfe" Linen Tiesue Hours'
Life" Kraft Tissue M134, Hours
41.7
2-Chloroanthraquinone 2-Chloroanthraquinone Azoxybenzene Cable Oil No. 1614
49 57 a7.2
None None Anthraquinone
Primol D Oil
a
87.1 1216
None Anthra uinone p-NitroIiphenyl Azobensene 2-Methylanthraquinone Sulfur Benzil Azoxybensene Time to 50% failure in a group of 10 samples.
In comparing the effectiveness of other stabilizers with that of anthraquinone, chlorinated naphthalene-impregnated kraft paper capacitors containing 1% additive were used except for the control group, in which the chlorinated naphthalene contained no additive. Figure 4 contains the charts of the life data from this study in the order of the stabilizimg effect of the additives. , The effect of tetraphenyltin and dibutyltin dilaurate is so small as to be of doubtful significance. Benzil and azobenzene were first described as stabilizers by Berberich and Friedman (I). 2,s di-tertbutylquinone approaches anthraquinone more closely than any of the others in this group in stabilizing properties. It, however, ia quite volatile at the temperature of impregnation (130' C.) and a perceptible amount was lost. Duplicate runs are presented for chlorinated naphthalene capacitors stabilized with tetraphenyltin, dibutyltin dilaurate, and anthraquinone. In addition, Figure 4 contains duplicate charts for capacitor units impregnated with No. 1514 CabIe Oil obtained in connection with the tests to be described. Table I contains a more complete list of materials tested for their stabilizing action. While 2methylanthraquinone and Zchloroanthraquinone approach anthraquinone in effectiveness, a t least under the conditions of these tests, octadecene does not stabilize chlorinakd naphthalene capacitors. Since azoxybenzene, another stabilizer discovered by Berberich and Friedman ( I ) , was run a t 0.5% concentration, the results are not directly comparable with those just discussed. The life -re given in Table I can be compared with the results for anthraquinone and anthraquinone derivatives at 0.5% concentration. As stated above, the 0.5% azoxybenzene compound tested is a commercial stabilized chlorinated naphthalene. Kraft paper test capacitors impregnated with mineral oils (Cable Oil No. 1514 and Primol D supplied by the Atlantic Refining Co. and Esso Standard Oil Co., respectively) were tested under the conditions described above. Each oil was tested unstabilized and stabilized with 0.5% anthraquinone. This concentration waa chosen because of the low solubility of anthraquinone (0.5 to 1% a t 130' C.) in the mineral oils. The life obtained in each case appears in Table I. The benefit realized by stabilization is an improvement in life by a factor of about 8 to 14, which' 'p approximately the same order of improvement obtained in chlo-
>500
2728 2266 48 1 > lo00
rinated naphthalene-impregnated kraft paper capacitors for the same concentration of stabilizer. Cable oil-impregnated kraft paper capacitors approximately saturated with anthraquinone (0.5%) have a life which closely corresponds to that obtained in chlorinated naphthalene-impregnated kraft paper capacitors containing 2% anthraquinone. 80 70 60 SO
40
30 20
2% ANTHRAQUINONE
t IO
I TIME IN HOURS
Figure 3.
Variation of Conduction Current w i t h
Time
Life t e s t reeulta shown in Table I
Since anthraquinone is sparingly soluble in mineral oils, it is to be expected that considerable advantage can be gained by the use of more soluble stabilizers. ZMethylanthraquinone seemed appropriate to try since it should retain practically all of the desirable properties of anthraquinone and has been found to poasesa about ten times the solubility in mineral oil. 'A test using 4% Z methylanthraquinone WBB run which resulted in a life of 2728 hours (Table 1)-an improvement factor of 31 as compared to 14 for 0.5% anthraquinone. Further experiments summarized in Table I indicate that azo and azoxy compounds, benzil, nitroaromatics, and sulfur also sre effective stabilizers in mineral 03 capacitors under these conditions of test. APPLZCATION OF RESULTS. The maximum practical advantage of these wsulta can be obtained by using stabilizers more soluble than anthraquinone. Figures 5 and 6 show the mlubility of
INDUSTRIAL AND ENGINEERING CHEMISTRY
138 1%
UNSTABILIZED 100
TETRA
1%
PHENYL T I N
Vol. 44, No. 1
DIBUTYL
T I N OILAURATE
Io/o BENZlL
1%
AZOBENZENE
80
60 40
20
'0
20
0 1%
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0
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20
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40
2 - 5 DlTERTlARY
20
0
40
60
80
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N a i s i 4 CABLE O I L
B U T Y L QUINONE
TIME IN HOURS
Figure 4. Comparison of Accelerated Life Tests on Test Capacitor Units Containing Chlorinated Naphthalene, Chlorinated Naphthalene Plus Additives, and Cable Oil 130' C., 450 volts direct current ?-layer, 0.4-mil kraft paper units Aluminum electrodes
several stabilizers in a mineral oil and in pentachlorodiphenyl (No. 1254 Aroclor). However, besides solubility, a stabilizer should possess stability, nontoxicity, low volatility, and should be available in pure form. It is unfortunate that anthraquinone, which has been used most commercially, while possessing all of the other desirable characteristics, is low in solubility. As an example of the need for a stabilizer having high solubility as well as the other desirable properties, it can be seen from Figure 5 that. the maximum amount of anthraquinone that can be put in a mineral oil capacitor is about 0.5%, which is the amount soluble a t 120' C. SMethylanthraquinone, on the other hand, is soluble to the extent of 51/270a t 120' C., but is not available in sufficiently pure form. It is quite likely that availability of pure 2-methylanthraquinone or other suitable stabilizers would increase the use of Aabilizers by the electrical industry.
accelerating character of the decomposition reaction in the presence of aluminum electrodes, The aluminum chloride-catalyzed decomposition produces hydrochloric acid. The formation of aluminum chloride and hydrochloric acid are complementary reactions; formation of each additional amount of the one enhances the rate of formation of the ot,her. These reactions are accompanied by an increase in conduction current and by visible degradation of the paper. 4. These reactions proceed until a short circuit is established a t some weak point in the dielectric and the sample is said to have failed. Egerton and McLean (7) and McLean, Egerton, and Houtz (11) presented evidence that the quinones and other stabilizing compounds form continuous protective films on the electrode
THEORY OF STABILIZATION
Three explanations of the stabilizing action of quinones and other additives have been proposed. The findings of the present paper regarding effects of concentration and the stabilization of nonchlorinated compounds demand a re-examination of these theories. Because the theory of stabilization cannot be separated from the theory of deterioration of unstabilized impregnants, it is necessary to start the review with a consideration of the latter. Previous papers from this laboratory (7, 1.3) describe the deterioration occurring when direct voltages are applied to paper capacitors containing aluminum foil and chlorinated aromatic impregnants in the following steps: 1. Slight decomposition of the chlorinated impregnant by pyrolysis which is enhanced by catalytic action at the electrodes, particularly at faults in the protective oxide film or a t inclusions pf foreign metals. I n this pyrolysis traces of hydrochloric acid are formed. 2. Electrolysis of the hydrochloric acid thus produced by the direct current field yielding aluminum chloride a t the anode. 3. This aluminum chloride is the agent directly responsible for the principal degradation of the dielectric and for the self-
XI
60
Figure 5.
70
80
90
100 110 120 TEMPERATURE OC
I33
140
Is0
160
170
Solubility of Stabilizers in Primol D Mineral Oil
January 1952
INDUSTRIAL A N D ENGINEERING CHEMISTRY
139
prwent state of kuowledgz and may help to indicate the field6 where more data are needed. I n extending the theory of protective film formation to nonchlorinated impregnant paper-aluminum foil systems, it is necessary to Msume that electrode metals catalyze the pyrolysis of norrchlorinated as well as chlorinated dielectrics, and further to ~ m m that e the electrolysiis of conducting materials is inhibited by the presence of a high-resistance surface film on the electrode. Nonchlorinated imprcgnants are stabilieed by the same materiala and therefore, presurnably, through the same general mechanism as chlorinated impregnants, st le& te Borne degree. I n the opinion of the authors, thia is an argument against the complex formation thmry, tihich depends upon the notsble oomplexforming tendencies of aluminum chloride.
n~etal. They pstulste that thia f i l e r inhibit8 the catalytic effect of the metal on pyrolysis as ivell us the forniation of aluminum
Figure 7. Effeot of Moisture Content on Conduction Current Curvee of Unstabitized Chlorinated Naphthalene Test CapaCitOrS
chloride by electrolysis according to step 2. They state that a loOD c., 140 v o l t s d i m ~orraot possible secondary influence is inactivation of aluminurn chloride %layex, 0.4-rnil lion. p e p , unit. Aluminum dstmdar by formation of ccmplexcs bctwecn this c o m p u n d nnd the Moi~turem o t a g e s b a d en d q weight of psrtahiliaer. The mechsnism of deterioration and of stabilization were described in detail only for the aluminum electrode capaciThe theory of Church and Garton has aeveral attractivc BBtor. In the case of other electrode materials-.g., lead-tin ptcts. It explains scveiai observetions pr~viouslydescribed but foil----theelectrochcmical aspects (step 2) of the deterioration are not acwnntcd for in any of the earlier thwrii.8, notably that under net 80 pronounced, and the purely chemical aspects (Step 1) are F particular condit.ions of test, the paper next to the cathode much more important. is visibly decomposed before that next to the anode (IS) and that Ikrberich and Friedman (l), while accepting the above dethe stabilizers discovcrcd we all in a sense oxidizing agents (7). scription of the decomposition pmeem, contend on the basis of their experiments arid certain quantitative considerntiom that inaotivation of the alumin u m chloride catalyst by complex formation is the dominant stshilizing process. Chureh and Garton (3) and Church ( d ) contend that 80 essential reaction in the deterioration protea is the catliodie reduction of tha chlorinatcd impregnant to form hydrochloric acid. According to thein, "the &dizer combines prefererrtidy with the cathodically pmduced hydrogen, w i t h o u t formation of hermiul secondary products." They further state thst stabilization is produced by the presence of compounds such a8 the quinones and others whieli an. easily reduced, neither the compounds themselves nor their Eduction products hoing ionized. In all probability the behavior of dielcctriev in direct e u r ~ n fields t is s t i l l too poorly undemtood to permit B conclusive descrip. Figure 8. Effect of Moisture Content on Deterioration of Dielectric in tion of the mechanisms involved. However, Unstabilized Chlorinated Naphthalene Test Capacitors T %tiau rrom units r e f e d to:in 8 few comments may be justified by the
..
140
INDUSTRIAL AND ENGINEERING CHEMISTRY
This theory can, of course, be extended to cover nonchlorinated dielectrics by assuming the existence of degrading electrochemical reduction reactions when direct current potentials are applied to cellulose, mineral oils, and other nonchlorinated organic compounds. The theory of Church and Garton is fortunately amenable to quantitative test, since reduction products should be detectable in quantities related to the charge transported by the leakage current. Furthermore, it should be possible to isolate and identify the reduction products of the stabilizers. In this connection, Church and Garton (3) and Church ( 2 ) claim t o have identified oxanthrol in aged anthraquinone-stabilized capacitors. At the present time, there is no wholly acceptable theory of stabilization. There are a large number of experimental observations which any such theory must account for. Some of the more important ones are discussed below. The above data indicate that the life of chlorinated naphthalene-impregnated test capacitors under conditions of test used in this work is not a linear function of the stabilizer concentration. However, more data are needed on the relation between life and stabilizer concentration. The relation is difficult to determine precisely owing t o the large dispersion in life test results. Investigation in connection with the ower factor of capacitors containing lead-tin alloy electrodes a n a the stabilization of such capacitors with sulfur (11) demonstrate the importance of barrier action due to films formed on the electrode surface. It is conceivable that this is a secondary result not closely related to stabilizer action. The observation that under certain conditions deterioration begins a t the cathode ( l a ) appears to be adequately explained by electrochemical reduction. However, when lead-tin alloy electrodes are used, in sharp contrast to the aluminum case, there was an absence of noticeable deterioration of the paper in test samples of comparable life. This suggests the entrance of factors such as s cific electrode effects as was discussed in previous papers from tf$ laboratory, in which special significance was attached to the action of aluminum chloride in the presence of aluminum electrodes, Furthermore, it is now clear that certain test conditions result in no visible early preferential decomposition at the cathode. As a matter of fact, in many units which were examined in the present work any deterioration observed was more marked a t the anode than a t the cathode. Electrolytic reduction theory indicates, according to Church and Garton that readily hydrogenated aliphatic hydrocarbons should be edective stabilizers. Work done in this laboratory with octadecene under the conditions of test described herein and life tests performed a t lower temperatures with a number of other unsaturated aliphatics have failed to support this view. It is possible that under other test conditions or in other dielectric systems, stabilization. by unsaturated aliphatics will be observed. Experiments carried out several years ago show that at least under particular conditions moisture, always considered detrimental in dielectrics, is actually a stabilizer. The results of these experiments have not been published previously, but have a bearing on the present discussion. These experiments were stimulated by the observation that within certain limits, using aluminum electrodes and unstabilized chlorinated impregnants, the more carefully and thoroughly the units were dried, the more rapid their deterioration on direct current potential. Important details and findings of these experiments are described below. Units consisting of linen capacitor paper aluminum electrodes, and chlorinated naphthalene were prepared with various amounts of moisture uniformly distributed through them using techniques previously described (IS). They were placed on test a t 120 volts direct current while held at 100" C. Figure 7 is a plot of the conduction current against time. The initial conductivity correlates in the expected way with the moisture content-the higher the moisture content the higher the conductivity. However, the conductivities of the samples of higher moisture content are relativel stable or decrease with time, whereas the conductivit of the dEy (0.06% water) samples increases until it passes that ofythe wet units. Not only this, but the dry samples failed by short circuit in an average of 250 hours, while all of the samples containing moisture were removed from test without failure after 497 hours. Photographs of the unwound units taken after test and with the aluminum foil electrodes still in place are shown in Figure 8. The dry samples a t the end of 250 hours show the decomposition pattern typical of dry samples containing unstabilized chlorin-
Vol. 44, No. 1
ated compounds. This coneivts of spots ranging from pin-point size up to as much as l,', inch in diameter, usually concentrated along the center line of the sheet. These spots are brown, brittle, have a caramel odor, and fluoresce under ultraviolet light. The samples which contained moisture also showed decomposition, but the higher the content the less severe and localized the decomposition. The above results can be explained by protective film formation since water reacts rapidly with aluminum to form aluminum hydroxide. Also the results may be explained by inactivation of aluminum chloride, since this compound is most active in the anhydrous state. To avoid any misunderstanding, the authors wish to state that experiments on moisture were limited to a specific set of conditions. They would not expect to find this effect generally, since in their opinion it is dependent on reactions of aluminum or aluminum chloride with water. Furthermore, they would not expect to find this result unless, as in this case, the voltage is kept so low that temperature rise in the unit by joule heating is small. For these reasons, the results are primarily of theoretical interest and are not to be interpreted to mean that moisture in insulation is desirable. Some investigators (8) have attempted to accelerate capacitor life tests by deliberately introducing water. I n view of the above results, the present authors believe this to be an unsafe procedure, since the presence of water alters the usual decomposition processes. Additional fundamental studies would be helpful in resolving the conflict between the various theories of stabilization. Most helpful in this connection, in the authors' opinion, would be precise knowledge of the nature of the charge carriers in insulating materials, of the mechanism of conduction in heterogeneoue dielectrics such as impregnated paper, and of the nature of the electrode reactions. LITERATURE CITED
(1) Berberich, L. J., and Friedman, Raymond, IND.ENG.CEEM.,
40, 117 (1948). (2) Church, H.F.,J . Inst. EZec. Engrs. (London),98, 113 (1951). (3) Church, H.F., and Garton, C. G., Nature, 162,301 (1948). (4) Egerton, L. (to Bell Telephone Labs., Inc.), U. S. Patent 2,287,421 (June 23, 1943). (5) Ibid., 2,391,685(Dec. 25, 1945). (6)Ibid., 2,391,689(Dec. 25, 1945). (7) Egerton, L., and McLean, D. A., IND. ENQ.CHEM.,38, 512 (1 946). (8) ICristensen, H.K., Trans. Danish Acad. Tech. Sci., No.6 (1949). (9) McLean, D. A. (to Bell Telephone Labs., Inc.), U. S. Patent 2,339,091(Jan. 11, 1944). (10) McLean, D. A., and Egerton, L., IND.ENO. CHEM.,37, 73 (1945). (11) McLean, D. A., Egerton, L., and Houtz, C. C., IND.ENQ. CHEM.,38, 1110 (1946). (12) McLean, D. A., Egerton, L., Kohman, G . T., and Brotherton, M., Ibid., 34, 101 (1942). (13) McLean, D. A., and Kohman, G. T., J. Franklin Inst., 229, 223 (1940). (14) Sauer, H.A., Bell Labs. Recmd, 25, 17 (1947). (15) Sauer, H . A., and McLean, D. A., Proc. Inst. Radio E n g ~ s . ; Waves and EZectrons, Sect. I , 37, 927 (1949). RECEIVED March 28,1951.
Correction In the 1951 Materials of Construction Data section on carbon and graphite [IND.ENQ.CHEM.,43, 2285 (1951)], the first two columns a t the bottom of the page were incorrectly headed. The first column should be headed Electrical Resistivity, Ohm-Cm. (1) and the second column Specific Heat, B.t.u./Lb./' F. (1).
