US1892691A - Coating composition for dry cells - Google Patents

Description

Jam 1933- J. G. ZIIMMERMAN ,892,691

COATING COMPOSITION FOR DRY CELLS Filed April 19. 1929 INVENTOR Patented Jan. 3, 1933' UNITED STATES PATENT OFFICE JAMES GARFIELD ZIMMERMAN, OF MADISON, WISCONSIN, ASSIGNOR TO BURGESS BATTERY COMPANY, OF MADISON, WISCONSIN, A CORPORATION OF WISCONSIN COATING COMPOSITION FOR DRY CELLS Application filed April 19,

This invention relates to a coated, dry cell, depolarizing core and more particularly to a dip or solution'capable of depositing upon such a core a closely adherent protective and electrically conductive coating.

The ordinary bag type dry cell, as illustrated in the single figure of the accompanying drawing, comprises a zinc shell 1, and a core 2. The core consists of a depolarizing mixture of manganese dioxide or otherdepolarizer, powdered graphite, sal ammoniac, zinc chloride, and water. A carbon rod 3, having a brass cap 4, is embedded in the depolarizing mixture. The core 2, with its embedded carbon rod 3, may be termed a cathode core or simply a core. The core is set in a gelatinous electrolyte 5 which contacts with the zinc shell 1. The gelatinous electrolyte may be that described in United States Patent No. 1,292,764. Itusually consists of sal ammoniac, zinc chloride, a cereal such as starch, and water. A seal 6 of sealing wax or pitch, rests on washer 7 and closes the top of the cell. An expansion space 8 may be left between washer 7 and the top of the electrolyte 5.

In the production of core 2, a quantity of moist, loose depolarizing mixture iscompressed about the carbon rod in a mold. Usually there is no further cohesive force than that resulting from the moisture and compression. As a result, during factory manipulation, particles of the mixture frequently become dislodged at the surface of the core. These particles may bridge the intervening annular space occupied by electrolyte 5, make contact with zinc shell 1, and cause short circuit and local action within the cell. Prior to gelatinization of the electrolyte, the core 2 absorbs moisture therefrom and gradually becomes soggy. The sogginess of the core may increase to a point where, if gelatinization is greatly delayed, the core disintegrates and the cell is rendered useless.

To avoid such disintegration and short circuits, it is common practice to envelop the core 2 in a closely adherent bibulous wrapper, usually of cheesecloth. The cost of such a wrapper, together with the labor of applying it, represents a considerable item inthe 1929. Serial No. 356,319.

cost of the dry cell. The gauze wrapper is, in itself, expensive. It must be cut into rectangular sections of a proper size to encircle a core twice. It must be treated with an adhesive and must be wrapped carefully about the core to secure it in the correct position. Although gauze may be replaced by cheap tissue paper, the results are not entire- 1y satisfactory. Cells have been made without any wrapper but the omission of the wrapper requires careful technique and even then the percentage of rejects is high and the quality of the cells questionable.

It is an object of this invention to provide a retaining covering or dip for dry cell cores which is less expensive than a bibulous cloth wrapper and is more easily applied.

It is a further object of this invention to provide an improved covering for dry cell cores which imparts improved characteristics, such as less internal resistance and longer life to a dry cell.

Many attempts have been made to provide a dip or covering formed by dipping the depolarizing core into a liquid solution or suspension. Examples of such dips are described in United States Patents Nos. 1,316,597 and 1,370,052. Such dips comprise a non-conductive material, such as plaster of Paris, with or without an adhesive such as glue, starch and the like. To be of any service the coating of the core must be so thick as to result in excessive electrical resistance. If the coating is too thin it does not provide the desired protection for the core, especially after coming into contact with the liquid electrolyte prior to gelatinization.

My improved retaining covering or coating consists of a liquid medium or dip into which the core is dipped and which, upon removal of the core, adheres thereto to form a conductive layer 9 of uniform thickness thereon. The coating, after being allowed to dry, becomes sufliciently tough, cohesive and water-resistant to prevent sogginess and the dislodgment of particles of depolarizEng mixture from the body of the core. It, also, allows the necessary handling and stacking which the cores receive in the regular course of factory operations. It retains its conductivity so that the internal resistance of the cells is not increased.

My liquid coating medium comprises casein dissolved in an aqueous solution of a casein-dissolving reagent. Casein is readily soluble in dilute aqueous solutions of the alkali and alkaline earth hydroxides, and aqueous solutions of their soluble carbonates and bicarbonates. It is soluble in dilute solutions of mineral and organic acids. Any of the mentioned reagents, if kept weak enough not to impair the action of the cell, can be used for my purpose. Concentrated solutions are unsuitable since they hydrolize and precipitate the casein and introduce ex" cessive corrosion of the zinc electrode.

Examples of casein-dissolving reagents include mineral acids, such as hydrochloric and sulphuric acids, organic acids, including those that are relatively strong, such as formic and acetic acids, and those that are comparatively weak, such as citric, lactic and malic acids. My preferred casein-dissolving reagent is citric acid which dissolves casein qu te readily and does not exert a deleterious action upon the dry cell. The precise action of citric aci d upon casein is not known. It is believed, however, that a compound of the two is formed which compound is soluble in water but is believed to be colloidal.

I prefer to use only as much citric acid as is necessary to effect substantially complete solution of the casein. I may vary the proportion of casein in the solution from three to eight percent, but more than this maximum proportion results in a solution which is too thick for satisfactory dipping at ordinary temperatures. I prefer to use a live to six percent solution of casein. It is desirable to limit the proportion of citric acid in order to prevent unnecessary corrosion of the zinc electrode and I find that citric acid in amounts substantially equal to the amounts by weight of casei is satisfactory. Thus, I may vary the proportion of citric acid from three to eight percent, while my preferred proportion is from five to six percent, based upon the crystalline form of the acid.

In making my improved coating solution or dip I proceed to mix the desired proportions of powdered casein and water, preferably by starting with the casein, adding cold water, and slowly agitating the mixture to maintain a smooth consistency. T he mixture may be heated and the desired quantity of citric acid added while heating. The temperature should not be allowed to exceed C., or objectionable chemical changes will take place in the casein. After the casein has become substantially completely dissolved, which should not require more than thirty minutes, the solution is ready for the dipping operation.

Casein solutions are viscous, and they be come more viscous as the proportion of casein increases while they become more limpid as the temperature increases. I have found that a five to six percent solution will deposit a very satisfactory coating at ordinary room temperature. The core should be immersed for a period of about live seconds in order that a uniform coating be secured. A momentary dip results in the lower portion being in the solution a period of time comparatively longer than the upper portion with the result that a thicker coating forms upon the lower portion. This effect is increased by the natural draining of the solution toward the bottom before the coating sets. A drip bead usually forms upon the bottom of the core and this may be removed by contact with any surface, preferably one which is slightly roughened. Momentary wiping contact with a brush removes the bead very efi'ectively. The dipped core is then dried in the ordinary atmosphere. The usual expedients may be used to accelerate drying. The temperature may not be materially increased during the early stages of drying while the coating is still wet since the coating will become thinner and will run. The temperature may not exceed 70 C. at any time or permanent injury to the casein will result. Moisture migrates from the coating into the core to contribute to the drying action. After the coating is dried the core may be dipped into a fused wax dip to insulate the bottom of the core and form a spacing collar 10 thereon. The core may be dipped into the insulating wax before the coating is applied. This wax collar 10 prevents contact between the zinc can and the core. Instead of forming such an insulating bottom upon the core, an insulating washer may be used in the bottom of the can.

The salts in the depolarizing core, namely sal ammoniac and zinc chloride, precipitate the casein and assist in forming a hard, leathery, porous wrap. The precipitated casein retards the movement of moisture from the coating to the core and prevents disintegration of the mix before the coating has set. The pores are very small which renders it impossible for loose framents of the depolarizing mix to go through the coating and contact with the zinc can to cause short circuits, which is possible with a gauze wrapper. The coating, while possessing ample strength to withstand the necessary factory manipulation, is still thinner than the bibulous gauze wrapper and provides increased space for electrolyte. The citric acid in the coating provides electrical conductivity and decreases the internal resistance of the cell. My coating slowly disintegrates in water but disintegration is delayed for a number of minutes. In contact with a fresh liquid electrolyte such as is referred to heretofore, distintegration takes place more slowly and, since the electrolyte sets into a firm gelatinous condition within a few minutes, the coating preserves the core in compact form until after the electrolyte has set. After this the electrolyte supports the core and prevents deformation'thereof.

It has also been proposed to provide a coat ing composition for dry cell cores comprising a liquid suspension of starch in a water solution of gelatinized starch, a specific composition being a suspension of 30% to 55% of starch in solution of 0.5% to 1.5% of gelatinized starch. I have discovered that my coating composition may be very advantageously modified by admixing therewith varying proportions of the starch coating composition. I may add from 5% to 15% ammonium chloride to the composition to decrease the viscosity of the solution. The resulting composition is given a very smooth consistency and, upon dipping, it distributes itself quite uniformly over the surface of the core.

While I have described my improved dip in its relation to a cylindrical dry cell core, it may be applied to the cores of other forms of dry cells, as for instance, those used in the plate type of battery. The method of application also may be varied. It may be applied to the different types of cathodes by painting, spraying, or by any other suitable method.

I claim:

1. A composition for coating dry cell cores comprising a liquid suspension of starch in a Water solution of gelatinized starch, casein and a casein-dissolving reagent.

2. A composition for coating dry cell cores comprising a liquid suspension of starch in a Water solution of gelatinized starch, casein and a weak organic acid.

3. A. composition for coating dry cell cores comprising a liquid suspension of starch in a water solution comprising gelatinized starch, ammonium chloride, casein and citric acid.

In testimony whereof I aiiix my signature.

JAMES GARFIELD ZIMMERMAN.

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