Electrolyte containing a novel depolarizer and an electrolytic capacitor containing said electrolyte

Abstract: An improved electrolytic capacitor, having aluminium cathode and anode members, separated by an insulating spacer, impregnated with an electrolyte containing a depolariser is disclosed. The improvement consist in that said depolariser comprises an aliphatic conjungated diene monocarboxylic acid, its non-metallic salt, its amide, or its ester in a amount up to 20% by weight of the electrolyte. Said acid is preferably sorbic acid.

Description: The invention relates to a an electrolytic capacitor, having aluminum cathode and anode members separated by an insulating spacer impregnated with an electrolyte containing a depolarizer. The operating life of an electrolytic capacitor frequently is limited by the production of gas within the electrolyte employed in the capacitor. As a result of the formation of this gas, the equivalent series resistance (ESR) increases to an undesirable high value within a comparatively short time. Thus the life of the capacitor is shortened. It is known that production of the gas may result from a number of different mechanisms. One of these mechanisms is based on a thermal decomposition of the electrolyte. Another mechanism is due to the chemical reaction of the cathode, for example, aluminum with the electrolyte to produce hydrogen gas. It is also known that the electrochemical reduction of the electrolyte at the cathode produces an increase of the ESR in proportion to the amount of the gas produced at the cathode. Employing nitroaromatic compounds as cathode depolarizers is known e.g. from US-A-4 580 194 to prevent this gas production. It has been found however that nitroaromatic compounds limit the operating voltages of the aluminum electrolytic capacitors therefore, they are not useful as depolarizers, especially for high volt capacitors. It has also been found that nitroaromatic compounds are hazardous materials since they are toxic and may be absorbed through intact skin. For this additional reason, it is desirable that nitroaromatic compounds in electrolytes are replaced. U.S. Patent 2,089,683, shows an electrolytic capacitor, the electrolyte of which contains a straight chain aliphatic acid containing up to 4 carbon atoms, an example in this patent is shown to be propionic acid. There is no suggestion that these acids in any way act as depolarizers. U.S. Patent 3,539,881 shows an electrolytic capacitor in which the electrolyte employed contains monocarboxylic acid of less than 7 carbons, which acids are liquids. As examples of acids that are employed are shown propenoic, butenoic and pentenoic acids. However there is no suggestion in this patent that these acids act as depolarizers. U.S. Patent 3,611,056 shows an electrolyte for an electrolytic capacitor containing an aliphatic polyhydroxy carboxylic acid, for example citric acid. However, here too there is no suggestion that these acids function as depolarizers. European Patent Application 0,246,825 teaches electrolytic capacitors, the electrolytes of which contain an aliphatic carboxylic acid, said document discloses as examples mono unsaturated acids as crotonic acid, acrylic acid, octenoic acid and decenoic acid or dicarboxylic acids as 8,11-dimethyl-7,11-octadeca diene-1,18 dicarboxylic acid. Here, too, there is no suggestion that any of the acids disclosed therein are used as depolarizers. It is a principle object of this invention to provide an electrolyte particularly adapted for use as fill electrolytes in electrolytic capacitors having aluminum cathodes and anodes useful in a low to high voltage operating range which electrolyte contains an improved non-toxic depolarizer. According to the invention which is characterized by the features of claim 1, it has been found that the life of an electrolytic capacitor comprising aluminum cathode and anode numbers is significantly increased by employing as an electrolyte in such a capacitor an electrolyte containing an aliphatic conjugated diene monocarboxylic acid, its non-metallic salts, its amide or its ester, in an amount from 0.10 to 20% by weight of the electrolyte, said acid being preferably sorbic acid. While improved results have been found to occur with electrolytes containing aliphatic conjugated diene monocarboxylic acid and their non-metallic salts, amides or esters, it has been found that particularly improved results are achieved by use of electrolytes containing a sorbate compound particularly sorbic acid, an amine salt or an ammonium salt of sorbic acid. In this regard, it should be noted that the above-mentioned U.S. Patent 3,611,056 mentions the use of sorbic acid in an electrolyte for an electrolytic capacitor. However, this appears to be a typographical error since the "sorbic acid" is defined as a carboxylic acid which must have two vicinal hydroxy groups, column 3, lines 10-19. Since sorbic acid has no hydroxy groups it is apparent that the term "sorbic acid" employed in column 3, line 19 is a typographical error. Instead most likely ascorbic acid is meant since this acid has two vicinal hydroxy groups. The amount of the aliphatic conjungated dienic monocarboxylic acid or derivatives thereof that may be present in the electrolyte used in the capacitor according to the present invention ranges from 0.10-20 weight percent. When the sorbic acid is employed the amount of sorbic acid preferably may range from about 0.1 to about 15.0 weight percent. The solvents that may be employed in the electrolyte of the present invention include polar solvents, such as ethylene glycol, propylene glycol, 1-methyl-2-pyrrolidone, gamma-butyrolactone, dimethylformamide and dimethylacetamide and the mixtures of these materials. However, because of their relatively low cost and lack of toxicity, ethylene glycol and/or propylene glycol are preferred. In addition to the already mentioned aliphatic conjungated diene monocarboxylic acids compound, the electrolyte may contain additional carboxylic acids. Examples of these additional carbonxylic acids are acetic acid, propionic acid, azelaic acid, sebacic acid, adipic acid, itaconic acid, maleic acid, phthalic acid and benzoic acid in an amount of about 0.50-20.00 weight percent and in an amount such that the mol.% of the aliphatic conjungated diene monocarbonxylic acid compound of the combined amount of the aliphatic conjungated dienic monocarboxylic compound and the additional acid is 10-100 mol.%. In addition, the electrolytes used in the capacitor of the invention preferably contain up to 0.02 wt.% of phosphoric acid. To neutralize the phosphoric acid and other acids the electrolyte contains a sufficient amount of ammonia or an amine, for example dimethylamine, so that the pH of the electrolyte is between 7.0-10.0. To prevent foaming an anti-foaming agent such as di-2-ethylhexyl azelate might be employed. The azelate preferably is employed in an amount of about 0.01 to 0.25 wt. % depending on the amount of a dimer acid that is employed, such as dimerized oleric acid. Further the electrolytes generally contain about 5.0 to 8.00 wt.% of water. Particularly good results have been achieved with the use of an electrolyte comprising 85 to 86 wt.% of ethylene glycol, 5.00-7.00 wt.% of water, 0.60-0.80 wt.% of Dimer Acid, 0.04-0.06 wt.% of di-2-ethylhexyl azelate, 0.015-0.03 wt.% of phosphoric acid, 3-7 wt % Azelaic Acid, 1-2 wt.% of sorbic acid and 2.50-3.00 wt.% of dimethylamine. The invention will now be described in greater detail with reference to the following example and figures of the drawing; in which Fig. 1 shows the change in ESR (mn) as a function of time (hr) for a capacitor employing the electrolyte of the example 1 as compared to that employing a control electrolyte not containing a depolarizer when the capacitors are held at 105°C and 250V. Fig. 2 shows a similar comparison but with the capacitors held at 200V. Fig. 3 shows a similar comparison but with a different control electrode not containing a depolarizer and at a voltage of 350V. Fig. 4 shows similar a comparison but at a voltage of 200V and with a control electrolyte containing a nitroaromatic cathode depolarizer. Example An electrolyte was prepared by mixing the following ingredients: The ingredients were heated to 65°C-75°C while stirring to dissolve all solids and the pH was adjusted to 7.5 to 8.5 if necessary. The resistivity of the resultant electrolyte was found to be about 330 ohm.cm at 30°C. Capacitors were prepared by impregnating a paper spacer separating aluminum anode and cathode members with the electrolyte of the example and with previously employed (control) electrolytes. The lives of these capacitors were determined by measuring the increase of their ESR (equivalent series resistance) values as a function of time and the results are shown in the Figs. 1, 2, 3, and 4. The dashed line in each Figure corresponds with the ESR-graph of the capacitor according to the present invention. The solid line refers to control or prior art capacitors. The aluminum content of cathodes of the capacitors is 98 wt.%. As shown in the graphs, employment of the electrolyte provided a significant increase in the life span of the capacitors according to the present invention as compared to the lifespan of capacitors employing electrolytes not containing depolarizers; the lifespan of the invented capacitors is at least equal to the lifespan of capacitors provided with electrolytes containing the toxic nitroaromatic depolarizers of the prior art.

Claims: An electrolytic capacitor, having aluminum cathode and anode members separated by an insulating spacer impregnated with an electrolyte containing a depolarizer, characterized in that said depolarizor is an aliphatic conjugated diene monocarboxylic acid, its non-metallic salt, its amide, or its ester in an amount from 0.10 to 20 % by weight the electrolyte. An electrolytic capacitor according to claim 1, wherein said acid is sorbic acid. An electrolytic capacitor according to claim 1 and 2, characterized in that said electrolyte comprises an additional carboxylic acid selected from the group consisting of acetic acid, propionic acid, azelaic acid, sebacic acid, adipic acid, itaconic acid, maleic acid, phthalic acid and benzoic acid, said carboxylic acid being present in an amount of about 0.50 to 20.0 wt. % whereby the mole % of the aliphatic conjungated diene monocarboxylic acid of the combined amount of the aliphatic conjungated diene monocarboxylic acid and the additional carboxylic acid amounts to 10 to 100 mole %, said electrolyte also comprising 2-20 wt % of water, up to 0.02 wt % of phosphoric acid and an amine or ammonia in an amount such that the pH of said electrolyte is between 7.0-10.0. An electrolytic capacitor according to claim 3, characterized in that the electrolyte comprising ethylene glycol or propylene glycol as a polar solvent. An electrolytic capacitor according to Claim 4 characterized in that the electrolyte further contains di-2-ethylhexyl azelate.

Inventors: DAPO ROLAND, DAPO, ROLAND
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1993
Main classification: H01G9
IPC codes: H01G9