US 3340444 A
Description (OCR text may contain errors)
Sept. 5, 1967 w. A. SELKE 3,340,444
ELECTROLYTIC DEVICE COMPRISING F'IBROUS IONICALLY PERMEABLE SPACER Filed March 5, 1964 INVENTOR WILL/4M ,4. afis ATTORNEYS United States Patent 3,340,444 ELECTROLYTIC DEVICE COMPRISING FIBROUS IONICALLY PERMEABLE SPACER William A. Selke, Stockbridge, Mass., assignor to Kimberly-Clark Corporation, Neenah, Wis., a corporation of Delaware Filed Mar. 3, 1964, Ser. No. 349,130 4 Claims. (Cl. 317-230) ABSTRACT OF THE DISCLOSURE Porous paper spacer separating electrodes of an electrolytic device made of paper including phosphorylated cellulose fibers, spacer being impregnated with an electrolyte.
This invention relates generally to electrolytic devices, and has particular reference to devices of the kind in which a pair of electrodes are separated by a spacer carrying an electrolyte.
It is a general object of the invention to provide improved electrolyte-carrying spacing means for apparatus of the character referred to. Illustrative of such devices are electrolytic capacitors in which a pair of anodized aluminum or tantalum foils are separated by a web of material, e.g., paper, serving not only to keep the foils apart but also as a kind of wick for holding a liquid electrolyte. The electrolyte provides the circuit path between one foil and the dielectric oxide layer of the other.
, One of the disadvantages of such assemblies arises from the electrical resistance interposed by the electrolyte carrier. For example, when conventional papermaking fibers are immersed in electrolyte, the flow of electricity is predominantly through the free and interstitial liquid rather than through the fibers. If paper made of such fibers is used as an electrolyte carrier in an electrolytic capacitor, the resistance of the paper manifests itself in the form of dissipation or power factor or other performance deficiencies.
If this problem is sought to be overcome by using less fiber in the paper, then if the paper is made thin (as is preferable, to shorten the path of current flow) the mechanical strength of the paper sufiers, and there is an undesirable increase in the likelihood that spurs or irregularities on one of the foil surfaces might penetrate through the paper and abrade the other foil. On the other hand, if the low density paper is made thicker to obviate these difficulties the current path through the electrolyte, hence the resistance, is correspondingly and undesirably increased; and, further, the capacitor is more bulky.
It is an object of this invention to alleviate these prob lems by the provision of an electrolyte carrier which is itself ionically permeable. The invention includes within its scope a method of making such a carrier, and the provision of an improved electrolytic device employing such a carrier.
It is a more particular object to provide an electrolyte carrier of the character described, composed of paper. A coordinate object is to provide an improved paper suitable for use as an electrolyte carrier and spacer in an electrolytic device, e.g., a capacitor.
The attainment of this objective is predicated upon the employment of phosphorylated cellulose fibers in the manufacture of the paper. Such fibers have ion permeability, and they serve admirably as a constituent of a papermaking furnish. Preferably they constitute at least 50% of the papermaking mixture of fibers.
The drawing shows a typical electrolytic capacitor including paper spacers according to this invention.
3,340,444 Patented Sept. 5, 1967 The effect of phosphorylation upon cellulose in imparting conductance is shown by the following:
Example I Two pounds of Sunila unbleached kraft wood pulp were soaked for thirty minute in 20% sodium hydroxide. The fibers were then steeped in a mixture of 3,500 grams of urea and 1,200 cc. of phosphoric acid in 4,000 cc. of Water. After air-drying, the mass was heated in an oven for two hours at C. The ion exchange capacity was found to be 1.6 milli-equivalent per gram, determined by the method given by Selke and Bliss in Chem. Eng. Progr., 46 (1950), page 509.
From a chemical standpoint, the treatment described results in making the cellulose a substituted phosphate, capable of ionic reactions. The urea is an effective diamide, able to react with two of the valences of the phosphoric acid, leaving one to react with the cellulose.
In harnessing this phenomenon and procedure to the manufacture of paper to be used as an electrolyte carrier, any of various cellulosic papermaking fibers may be used, such as wood pulp, hemp, cotton, abaca, flax, or the like. By the chemical step of phosphorylation such cellulosic material can be provided with ion exchange groups, while still retaining the papermaking qualities of the original fiber. Papers can be made entirely of such modified fibers, but in the interest of flexibility in the papermaking process, and reduced cost, unmodified, and hence relatively nonconductive papermaking fibers, either cellulosic or synthetic, may be used up to about 50%. That is, in the paper sought to embody the qualities called for by this invention, the phosphorylated cellulose fibers are preferably present in an amount equal to at least 50%. Non-cellulosic fibers such as glass can be advantageously added to the .stock in small quantities to improve the strength of the wet web of paper, during manufacture. The furnish can be prepared in conventional papermaking equipment, with some reduction in the work done in the beater, since phosphorylated fibers require less beating than untreated fibers. The paper can be made in normal fashion on a Fourdrinier machine, and for capacitor purposes the paper may have a thickness from 0.75 to 3.0 mils, and a basis Weight from 6.5 to 20.0 pounds per ream (20 x 30 x 500).
Specific examples of the complete procedure are as fo1- lows:
Example 11 Two and one-half pounds of a kraft pulp made from Benares hemp (Crotalaria juncea) were soaked for one hour in a solution consisting of:
Urea grams 4,080 Phosphoric acid cc 1,632 Water cc 4,800
by repulping ordinary electrolytic condenser tissue. The stock composition was:
Percent Phosphorylated fiber 74.5 Repulped .003 Benares hemp paper 21.3 Glass microfiber, type 106 4.2
The stock was mixed in a Waring blender, then formed cell in which two electrodes were separated by two layers of the saturated paper.
Example III Phosphorylated fibers were prepared as described in Example II. They were mixed in equal proportions with untreated Benares hemp fibers, and formed into an aqueous slurry. They were beaten for ten minutes in a Valley Iron Works laboratory beater, and the furnish was the formed into paper sheets with a basis weight of 8.0 pounds per ream. After pressing, drying, and light calendering, the sheets had a thickness of 1.1 mils. The porosity was measured on a Greiner tester and found to be 80 cc. in 15 seconds. The resistance was tested as hereinbefore described and found to be only 40 ohms.
The invention thus provides a paper admirably suited to serve as an electrolyte carrierin an electrolytic device such as the one shown in the drawing. Anode and cathode foils and 11, respectively, are rolled up together with porous paper spacers 12. At least one of the foils is of a film forming metal and has an anodic dielectric film on its surface. The roll is enclosed within a metal container 13, having terminals 14 connected to the electrodes 10 and 11, and the container is filled with a film forming electrolyte. Since a substantial proportion of the paper fibers are of a chemically modified character which makes them ion permeable, i.e able to conduct an electric current when wet with electrolyte, a lower electrical resistance is obtained while at the same time preserving the benefit of having a wholly fibrous absorbent spacer interposed between the foils or other electrodes to keep them mechanically separated. Moreover, since the density of the paper need not be unduly diminished to reduce electrical resistance, the spacer may be made as thin as desired, without sacrifice of mechanical strength or danger of penetration by foil imperfections. It follows that electrolytic devices, e.g., capacitors, employing such spacers manifest lower dissipation and power factor losses and improved performance characteristics even at lower operating temperature at which the resistivity of the electrolyte itself may increase.
While the invention has been described in connection with electrolytic capacitors, because of its particular suitability for such devices, it is to be understood that the basic concept is of wider applicability and that fibers of the phosphorylated type, having ion exchange properties, can be used for other low-resistance separators such as those used in certain batteries, electrolytic cells and other electrochemical equipment.
It will also be understood, of course, that the examples described are merely illustrative of the basic procedures and advantages to which the invention relates, and that changes may be made in numerous respects without necessarily departing from the spirit and scope of the invention as expressed in the appended claims.
What is claimed is:
1. An electrolytic device comprising a pair of elec trodes, at least one being of a film forming metal and having an anodic dielectric film on its surface, a porous paper spacer separating said electrodes, said paper comprising a mixture of conductive phosphorylated cellulose fibers and non-phosphorylated non-conductive fibers, and said paper being impregnated with a film forming electrolyte.
2. A device as defined in claim 1 wherein the phosphorylated fibers are present in said paper spacer in an amount equal to at least 3. A device as defined in claim 1 wherein the nonphosphorylated paper fibers include cellulosic fibers.
4. A device as defined in claim 1 wherein the nonphosphorylated paper fibers include non-cellulosic fibers.
References Cited UNITED STATES PATENTS 7/1930 Edelman 317-230 7/1933 Ruben 317-230 OTHER REFERENCES JAMES D. KALLAM, Primary Examiner.