US 2692210 A
Description (OCR text may contain errors)
Oct. 19, 1954 BURNHAM 2,692,210
PROCESS OF PURIFYING AND IMPREGNATING CELLULOSIC SPACERS FOR ELECTRICAL CONDENSERS Filed Dec. 10. 1949 JOHN BURNHAM,
BY awh- I gifi Patented Oct. 19, 1954 PROCESS OF PURIFYING AND IMPREGNAT- ING CELLULOSIC SPACERS FOR ELECTRI- CAL CONDENSERS John Burnham, Williamstown, Mass., assignor to Sprague Electric Company, North Adams,
Mesa, a corporation of Massachusetts Application December 10, 1949, Serial No. 132,345
(Gl. 1l7- l8) 6 Claims. 1
My present invention relates to the purification of cellulosic spacer materials. More particularly, it concerns a method of purifying regenerated cellulose films to make them specially adapted for use in the fabrication of electrolytic capacitors.
The use of a regenerated cellulose film as a spacer material for electrolytic condensers has been repeatedly suggested in the patent literabelow 25 parts per million by Weight, forcing an electrolytic condenser electrolyte through said roll in the same axial direction, and unwinding said regenerated cellulose film from said roll. According to another limited embodiment of the invention there is employed a process which comprises convolutely winding at least two regenerated cellulose films with at least two aluminum electrodes and at least two porous paper ture. Unfortunately, however, successful appli- 1O spacers into a cylindrical roll, whereby each re cation of the regenerated cellulose film to comgenerated cellulose film is separated from the mercial manufacturing operations, particularly other said films by a porous paper spacer and in the production of high capacity electrolytic an aluminum electrode, forcing boiling water condensers, has not been possible. This situathrough said roll in an axial direction until the tion results from the contamination of the regeneffiuent liquid has a chloride ion content suberated cellulose during its manufacture with stantially less than 25 parts per million and forcchloride ions, sulphate ions and other ionic irning an electrolytic solution in the same axial dipurities. Such impurities in the film may lead rection through said roll until said roll is satuto corrosion and failure of electrolytic capacitor rated with said electrolyte. structures in which the films are incorporated. According to my invention, I have devised a In the wet state following extrusion, regenerated novel and simple process for removing chloride, cellulose is extremely fragile and may be subsulphate and related ionic impurities from regenjected only with great difiiculty to thorough erated cellulose films without encountering the washing, in order to remove such ionic impurities. cost and equipment difficulties heretofore met in It has been suggested that dialysis may be emthe purification process. According to one employed to remove impurities of various kinds. bodiment of the invention boiling water or some While this process is to some extent successful, other solvent in which the ionic impurities are it requires extremely expensive and massive procsoluble is forced axially through a convolute roll essing equipment for efficiency in operation. consisting of the regenerated cellulose film and of It is an object of the present invention to a contacting porous spacing material, such as overcome the foregoing and related disadvantages 50 paper, cloth, glass, mat, etc. The axial washing of the regenerated cellulose films. A further ob process is conducted only for the length of time lect is to produce improved cellulosic spacer manecessary to reduce the chloride content of the terials for electrical devices. A still further obeffluent solvent to the maximum desired, e. g. ject is to provide a simple and inexpensive procqr about 25 parts per million in the case of regeness for producing regenerated cellulose film suberated cellulose films for use in electrolytic constantially free from ionic impurities. Additional densers as spacer materials. objects will become apparent from the following According to one of the limited embodiments description and claims. of the invention the washing process may he fol- These objects are attained in accordance with lowed by a treatment with a liquid which will be th present invention by employing a pur fication retained and/or absorbed in the regenerated cel- Process that comprises convolutely Winding a lulose film to act as a plasticizer and/or e1ectrosheet of regenerated cellulose spacing material 1m F l a l ti of ammgnium th a Contacting sheet of porou Spacing Illaieborate in glycol may be passed through the F m a f51indrica1r11 frcing a Solvent washed roll of regenerated cellulose film and ionic impurities through said roll in an axial di porous paper. Thereafter the T011 may be um rection, unwinding said You and drying the wound and optionally the regenerated cellulose Sulting p regenerated Cellulose fi In film may be dried prior to incorporation in caaccordance with one of the limited embodiments acit r assemblies, of t invention there is p y a p es According to one of the preferred embodiments which comprises convolutely winding a thin, reof my invention the wash roll consists of an generated cellulose film with a contacting porous electrolytic capacitor assembly in which the paper into a cylindrical roll, forcing boiling water spacing material separating the two aluminum through said roll in an axial direction until the electrodes comprises at least one layer of regenefiluent water possesses a chloride concentration erated cellulose and at least one contacting layer of a porous paper material, such as linen paper or kraft capacitor paper. If porous aluminum electrodes, such as sprayed aluminum on gauze electrodes, are used, it is sometimes possible to eliminate the porous paper spacer. By this process it is possible to wash the cellulose spacer to a safe chloride content and subsequently to impregnate the wound capacitor with the operating electrolyte. Normally the aluminum electrodes are formed with an oxide film prior to incorporation in the wash roll.
The regenerated cellulose film obtained in accordance with my invention is useful not only as a spacing material for electrolytic capacitors, but also as a dielectric spacer for electrostatic (non-electrolytic) condensers. In such instances, the regenerated cellulose has a much higher insulation resistance and results in capacitors with lower leakage currents than corresponding capacitors made with conventional, electrical grade, regenerated cellulose films.
The regenerated cellulose employed in accordance with the invention may be made in the usual manner without special processing equipment or control. Other polar resinous films are contemplated for use in accordance with the present teachings, provided that appropriate wash solvents, wash temperatures and rates are employed.
When the basic washing process of the invention is to be practiced, it is generally desirable to wind up the contacting sheet of porous spacing material as part of the finishing operation in the manufacture of the regenerated cellulose film. The wide rolls e. g. using 56" paper sheets may be subjected to the treating process without further handling. If a cloth or fabric spacer is used, it may, of course, be reused until failure occurs, for example, tearing, fraying, etc.
In addition to water, mixtures of water and polyhydroxy compounds, such as glycol and glycerine, may be employed as solvents. It is apparent that numerous other solvents may be employed dependent upon the ionic impurities to be removed, type of film to be processed, etc. The rate of solvent flow across the regenerated cellulose surface, the diffusion rate of ions from from the inner parts of the film, the temperature of the solvent and film are determining factors in the processing control. While I am not fully aware of the exact phenomenon involved, I believe that the primary action is a diffusion controlled process and, therefore, depends upon the diffusion rate for chloride or other ions which may be present within the regenerated cellulose film.
Reference may be made to the appended drawing which illustrates a simplified cross section of a typical washing apparatus suitable for carrying out the process of my invention. Convolutely wound roll 9 consists of a porous spacer sheet IE) and a regenerated cellulose film ll. This roll is fitted into the container !2. Cap i3 is attached to a wash water supply. The wash water [4 flows axially through the section 9 in the direction of the arrows, forming an efliuent i5 which is exhausted. A conductivity cell 16 is connected to bridge 11, as a process control for the amount of ionic impurities in the emuent i5. It is generallypreferable'to force wash water [4 through section 9 by means of simple hydrostatic pressure. However, it is also possible to pull the wash water through the section by means of a vacuum pump.
In most instances involving-large rolls of an- 4 preciable width it is desirable periodically to reverse the axial direction of the wash solution. In this way the diffusion is equalized at each end of the section.
As a specific example of the practice of my invention, 0.0005" cellophane of 12 width was convolutely wound with 0.003" porous linen paper on a 1" core to a total diameter of 5". The core was stoppered and the wound roll placed in a column. Boiling tap water was passed through the wound roll in an axial direction at a rate of 2 liters per minute for 30 minutes. Following this, a liter of boiling distilled water was passed through the section in the same axial direction. Following the distilled water rinse, 500 cc. of a solution of 25% ethylene glycol in water was passed through the roll. The regenerated cellulose film was unwound and dried at 70 C. Prior to the washing operation the regenerated cellulose had a chloride ion concentration of 250 parts per million. After the washing process the chloride ion content was only 15 parts per million. The glycol was added as a plasticizer and electrolyte solvent for the film.
It should be noted that the final treatment with an aqueous glycol solution described in the foregoing specific example is ordinarily omitted, when electrostatic spacing material is being produced.
For some purposes the contacting sheet of porous spacing material may be composed of metallic cloth or screen. The material employed for this purpose is not critical, provided that it is porous in nature and chemically inert, i. e., does not impart ionic impurities to the regenerated cellulose film.
My new purification process requires a minimum quantity of solvent such as distilled water. It provides the user of the highly purified, regenerated cellulose film with a simple and inexpensive means for producing such film from the commercially available regenerated cellulose films containing an excessive amount of ionic impurities. If the manufacturer of the regenerated cellulose film should try to remove such ionic impurities, this would require several additional washings, a much greater amount of floor space and a great quantity of distilled water. This would greatly increase the cost of manufacture, and the relatively small demand for the highly purified, regenerated cellulose film would hardly justify the heavy investment for the additional equipment that would be required to include such purification in the regular production of the film.
As many apparently widely diiferent embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.
1. A purification process which comprises convolutely winding a sheet of regenerated cellulose having soluble impurities with a contacting sheet of porous spacing material into a cylindrical roll, and forcing a solvent for the impurities through said roll in an axial direction to cause the impurities to diffuse out of the regenerated cellulose sheet and into the solvent.
2. A purification process which comprises convolutely winding a thin, incompletely washed" regenerated cellulose filrn with a contacting porous paper into-a'- cylindrical roll, forcing boil- 5 ing water through said roll in an axial direction until the effluent water possesses a chloride concentration below 25 parts per million by weight, forcing an electrolytic condenser elec trolyte through said roll in the same axial direction to saturate the resulting film with said electrolyte, and unwinding said regenerated cellulose film from said roll.
3. A purification process which comprises convolutely winding at least two incompletely washed regenerated cellulose films with at least two electrode foils and at least two porous paper spacers into a cylindrical roll having each regenerated cellulose film separated from the other of said films by a porous paper spacer and an electrode foil, forcing boiling water through through said roll in an axial direction until the efliuent liquid has a chloride ion content substantially less than 25 parts per million and forcing an electrolytic capacitor electrolyte in the same axial direction through said roll until said roll is saturated with said electrolyte.
4. A process which comprises convolutely winding a sheet of regenerated cellulose having soluble impurities with a contacting sheet of porous spacing material into a cylindrical roll and forcing an aqueous aliphatic polyhydroxy solvent for the impurities through said roll in an axial direction to cause the impurities to diffuse out or" the regenerated cellulose sheet and into the solvent.
5. The process of claim 4 wherein the polyhydroxy compound is glycol.
6. The process of claim 4 wherein the polyhydroxy compound is glycerine.
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