Abstract:
An electrolytic capacitor having aluminum anode and cathode members separated by a paper insulating spacer impregnated with a solution of mostly a glycol, water, unsaturated aliphatic dicarboxylic acid, aliphatic diene monocarboxylic acid, monohydric aliphatic alcohol, dodecanedioic acid, phosphoric acid, nitro aromatic compound having a benzene ring-substituted nitro moiety and a dialkylamine in an amount sufficient to provide a pH of 7.0-8.5.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/132.735 filed Oct. 6, 1993, now U.S. Pat. No. 5,519,567. 
    
    
     BACKGROUND OF THE INVENTION 
     It is desirable that the operating lives of electrolytic capacitors be as long as possible. The use of high voltage electrolytic capacitors particularly has increased considerably in recent years. These capacitors are used in large numbers in many devices such as power supply devices, inverters and charge-discharge devices. Increasingly, such devices require longer operating lives, low failure rates, lower ESR (equivalent series resistance) values, as well as high energy densities. 
     It is particularly important that during their operating lives the rate of failure of such capacitors be held to a minimum. The reason for this is that many times high voltage capacitors are used in a bank in which they are connected in parallel. Thus if one of the capacitors should short while the bank is at operating voltage the other capacitors discharge into it. As a result, the shorting capacitor is frequently totally destroyed. In such a case the device in which the capacitors are employed becomes inoperative. 
     The failure of such a device may have serious consequences, for example when the device is a power supply and is used in a computer. Here failure of the power supply may result in a loss of memory. 
     Besides the above-mentioned requirements it is an additional requirement that the electrolyte be free of toxic substances. 
     Capacitors provided with such an electrolyte are shown in Dapo, U.S. Pat. No. 5,111,365. The electrolyte shown in this patent contains N-methylformamide, 2-methoxyethanol, 2-ethoxyethanol, ethylene glycol or 1,2-propylene glycol, an aromatic dicarboxylic acid such as isophthalic acid or terephthalic acid and an aliphatic amine such as dimethylamine or monomethylamine. These capacitors have been found to be useful at relatively low voltage applications, for example about 55VDC. 
     Finkelstein, U.S. Pat. No. 4,373,176 shows electrolytic capacitors for use at operating voltages of 200VDC. In these capacitors the electrolyte employed contains a tertiary amine or a dipropylamine mono salt of dodecanedioic acid dissolved in a solvent consisting of ethylene glycol, N-methyl-2 pyrollidone and water. While these capacitors are free of toxic substances, their operating lives are too short or their ESR values are too high for many applications. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide capacitors free of toxic substances for use at high and low operating voltages that exhibit improved operating characteristics, particularly lower ESR values and longer operating lives. 
     It is particularly an object of this invention to provide electrolytic capacitors that are useful at operating voltages of 300VDC and higher. 
     According to one aspect of the invention a new and novel capacitor comprises anode and cathode members consisting of aluminum separated by a paper insulating spacer impregnated with an electrolyte consisting essentially of a solution containing, as the major ingredient, a glycol of 2-4 carbons, 1.00-12.00 wt. % of water, 0.10-5.00 wt. % of an unsaturated aliphatic dicarboxylic acid of 16-36 carbons, 0.00-4.00 wt. % of an unsaturated diene carboxylic acid of 5-9 carbons, 0.00-10.00 wt. % of a monohydric alcohol of 6-12 carbons, 0.00-10.00 wt. % of dodecanedioic acid, 0.00-0.50 wt. % of phosphoric acid, 0.0-5.0 wt % of a nitro aromatic compound having a benzene ring substituted nitro moiety and a dialkylamine wherein each alkyl is of 1-4 carbons in an amount sufficient to provide a pH 7.0-8.5. 
     Such a capacitor has been found to be particularly useful over the operating range of 300-600 VDC. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE in the drawing is a graph showing the relation of ESR to operating time for a capacitors of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It should be noted that Japanese Patent Application 63-218696 shows an electrolytic capacitor employing an electrolyte containing ethylene glycol or its ethers, water adipic acid or its salts and three or more acids or salts thereof of nitrobenzoic, phthalic, salicylic, benzoic, maleic, boric, phosphoric, oxalic, malic, citric, aspartic, glutamic acid or fatty dicarboxylic acids of 4-12 carbons. There is no teaching or suggestion in this patent of employing pelargonic acid or of employing N methylformamide. 
     There is also no teaching in this Japanese application that the use of the electrolyte shown therein significantly extends the operating lives of the capacitors. 
     Further, it should be noted that Example 4 of Dapo U.S. Pat. No. 5,111,365 shows an electrolyte for a capacitor containing 59.75 wt. % of ethylene glycol, 14.00 wt. % of N-methylformamide, 4.50 wt. % of water, 0.50 wt. % of pelargonic acid, 16.00 wt. % of isophthalic acid. 0.005 wt. % of phosphoric acid and 7.25 wt. % of dimethylamine. While capacitors containing this electrolyte are useful at operating voltages of up to 75VDC, they are not useful at voltages of 100VDC or higher as they short out when aging at this higher voltage. 
     Preferably a monobasic saturated aliphatic acid of 9-15 carbons is present in an amount not greater than 40 wt. % of all the acids present. Highly useful examples of such acids are pelargonic acid and lauric acid. 
     Although any monohydric aliphatic alcohol of 6-12 carbons such as n-octanol, 1-hexanol and n-heptanol may be used, it has been found that best results are achieved when this alcohol is n-octanol and it is present in an amount of at least 1.00 wt. %. 
     An example of a highly useful unsaturated aliphatic dicarboxylic acid of 16-36 carbons is dimer acid, particularly in amounts of about 0.40-0.80 wt. % and especially about 0.50 wt. %. 
     An example of an aliphatic diene monocarboxylic acid of 5-9 carbons that may be employed is sorbic acid (2,4-hexadieneoic acid). 
     To prevent gas formation at least 0.05% wt. % of a depolarizer, particularly in aromatic nitro compound having ring-substituted nitro moiety is present. Examples of such compounds that may be employed are o-nitroanisole, p-nitrobenzyl alcohol as well as other nitroaromatic alcohols, acids esters and amides thereof. 
     In place of all or part of the dimer acid phosphoric acid may be employed. 
     Examples of a glycol that may be employed are ethylene glycol, diethylene glycol and propylene glycol. Ethylene glycol is preferred especially when resistivity is a major concern. The other glycols, when used, are prefereably used as cosolvents with ethylene glycol. 
     Examples of the saturated aliphatic dicarboxylic acid that maybe employed include dodecanedioic acid and sebacic acid. 
     An electrolyte that is particularly useful for capacitors for operation in the 300-600VDC range contains about 87.00-88.00 wt. % of ethylene glycol, 1.00-2.00 wt. % of pelargonic acid, 0.75-2.00 wt. % of 1-octanol, 0.40-0.80 wt. % Of dimer acid, 0.00-0.10 wt. % of phosphoric acid, 0.10-0.50 wt. % of sorbic acid, 1.00-3.00 wt. % of dodecanedioic acid, 4.00-8.00 wt. % of water and 1.10-1.40 wt. % of dimethylamine. 
     For a more complete understanding of the invention the invention will now be described in greater detail with reference to the following examples, table and FIGURE of the drawing. 
     EXAMPLE I 
     The following electrolyte was prepared: 
     
         ______________________________________Chemical           Wt. %______________________________________ethylene glycol    87.65pelargonic acid    1.501-octanol          1.00dimer acid         0.5085% phosphoric acid              0.05hexadienoic acid   0.25dodecanedicarboxylic acid              1.85water              6.00dimethylamine      1.20______________________________________ 
    
     These ingredients were mixed together and then heated to 85° C. to dissolve all the solids. 
     The resultant electrolyte exhibited the following properties when measured at 30° C., resistivity=620 ohm.cm, pH=7.8. 
     This electrolyte was found to be particularly useful for employment in electrolytic capacitors designed for use in the 300-500 VDC range. 
     The electrolyte of Example I was used to impregnate paper separators in 400VDC computer style electrolyte aluminum capacitors with 630EFV anodes. 
     These capacitors were placed in an oven heated to 105° C. and the results after various periods of time are shown in the following table; 
     
                       TABLE______________________________________EVALUATION OF EXAMPLE I OF HIGH VOLTAGE FILLELECTROLYTE CAPACITOR TEST DATACAPACITOR VOLTS - 400                         ESR         IMPEDU/N   IL5 min. CAP(uF)  % INIT                         (ohms)                               % INIT                                     (m.ohm)______________________________________INITIAL VALUES (ALL LEAKAGE CURRENTS IN mA)132   1.473    2378     100.00                         .0210 100.00                                     .55838133   2.059    2376     100.00                         .0260 100.00                                     .55880134   1.350    2398     100.00                         .0290 100.00                                     .55384135   1.363    2463     100.00                         .0240 100.00                                     .53902136   1.396    2452     100.00                         .0250 100.00                                     .54147137   1.630    2462     100.00                         .0240 100.00                                     .53923138   1.488    2400     100.00                         .0270 100.00                                     .55328139   1.775    2398     100.00                         .0260 100.00                                     .55369AV    1.481    2416     100.00                         .0260 100.00                                     .54971500 HOUR VALUES132   1.936    2314     97.31 .0260 96.30 .57374133   1.731    2315     97.43 .0270 103.85                                     .57354134   1.726    2336     97.41 .0250 86.21 .56831135   1.855    2404     97.60 .0240 100.00                                     .55222136   1.637    2392     97.55 .0250 100.00                                     .55503137   1.633    2405     97.68 .0240 100.00                                     .55199138   1.795    2337     97.38 .0260 96.30 .56811139   1.148    2325     96.96 .0260 100.00                                     .57104AV    1.729    2354     97.42 .0254 97.83 .564251000 HOUR VALUES132   .298     2304     96.89 .0300 111.11                                     .57642133   .341     2305     97.01 .0300 115.38                                     .57617134   .199     2329     97.12 .0300 103.45                                     .57025135   .213     2395     97.24 .0300 125.00                                     .55458136   .200     2384     97.23 .0300 120.00                                     .55713137   .171     2397     97.36 .0300 125.00                                     .55412138   .203     2326     96.92 .0300 111.11                                     .57099139   .216     2324     96.91 .0300 115.38                                     .57148AV    .208     2346     97.08 .0300 115.80                                     .566392000 HOUR VALUES132   .148     2304     96.89 .0360 133.33                                     .57677133   .136     2304     96.97 .0360 138.46                                     .57677134   .158     2328     97.08 .0350 120.69                                     .57078135   .142     2393     97.16 .0320 133.33                                     .55516136   .134     2384     97.23 .0340 136.00                                     .55736137   .143     2398     97.40 .0320 133.33                                     .55400138   .145     2328     97.00 .0340 125.93                                     .57072139   .112     2326     97.00 .0350 134.62                                     .57127AV    .142     2346     97.09 .0343 131.96                                     .56660______________________________________ Notes: 1. U/N = unit number 2. IL5 = leakage current measured 5 minutes after rated voltage is reache 3. CAP = capacitance measured at room temperature, about 23° C., when measured 4. ESR = equivalent serves resistance measured at 120 Hz 5. 307S = identification of electrolyte 6. IMPED = impedance measured at 120 Hz 7. % INIT = % of initial value 
    
     An additional number of 400V rated aluminum electrolytic: capacitors having an average capacitance measured at room temperature of 260 uF were prepared employing the electrolyte of Example 1. These capacitors were also placed in an oven heated to 105° C. and the ESR values were measured after various periods of time. 
     The results are shown in the sole FIGURE of the drawing is a graph showing the relationship of the average ESR to time in the oven of these capacitors. 
     Additional electrolytes with the following composition were prepared: 
     EXAMPLE II 
     
         ______________________________________Chemical         Wt. %______________________________________Ethylene Glycol  89.15DI Water         3.45Pelargonic Acid  1.50n-Octanol        1.00Dimer Acid       0.50Dodecanedioic Acid            2.10o-Nitroanisole   1.00Dimethylamine    1.30______________________________________ 
    
     The initial properties of this composition are: Resistivity at 30° C. 750 Ω.cm pH=8.0. It can be used in capacitors rated up to 450 volts. 
     EXAMPLE III 
     
         ______________________________________Chemical         Wt. %______________________________________Ethylene Glycol  78.85Diethylene Glycol            12.00DI Water         3.48Lauric Acid      0.65n-Octanol        1.00Dimer Acid       0.50Dodecanedioic Acid            1.80o-Nitroanisole   1.00Dimethylamine    0.72______________________________________ 
    
     The initial properties of this composition are: resistivity at 30° C. 1120 Ω.cm, pH=7.1. 500 volt capacitors have been made with this composition, and tests are beginning. This composition was formulated for operation up to and including ratings of 600 volts.