US3091554A - Oxidation resistant cellulosic pellicle and process for the manufacture thereof - Google Patents

Description

United States Patent Office 3,091,554 Patented May 28, 1963 3,091,554 OXIDATION RESISTANT CELLULOSIC PELLICLE AND PROCESS FOR THE MANUFACTURE THEREOF Charles M. Rosser, Wallingford, and Richard A. Glinski, Clifton Heights, Pa., assignors to American Viscose Corporation, Philadelphia, Pa., a corporation of Delaware No Drawing. Filed July 1, 1960, Ser. No. 40,149

8 Claims.

This invention relates to improved oxidation resistant cellulosic sheets, methods for preparing said sheets, and storage batteries incorporating said sheets as separators.

Various processes and articles of manufacture wherein cellulosic sheets or films are used require that they have improved resistance to oxygen and oxides. A prime example of a use of a cellulosic sheet or film requiring improved oxidation resistance is in storage batteries particularly of the zinc-silver electrode type employing one or more layers of semi-permeable membranes such as regenerated cellulose as a separator between the electrodes and the alkaline electrolyte. The period of effective operation of this type of battery is limited by the deterioration of the separator caused by oxidation due in part to prolonged contact with the alkaline electrolyte and with electrolytically formed silver peroxide. Several methods have been employed to increase the life of the cellulosic membrane thereby permitting an increase in the number of charging cycles. These include increasing the number of layers of the film and/ or incorporating a protective coating on the surface of the film, and/or incorporating various antioxidant materials in the coat ing or in the semipermeable membrane.

Another use for cellulosic membranes having improved oxidation resistance is in dialyzing processes incorporating cellulosic semi-permeable membranes for the separation of various components in a solution. The need for improved oxidation resistant, semi-permeable membranes in dialyzing processes employing strong oxidizing material or material promoting oxidation is obvious.

It is an object of the present invention to provide an improved oxidation resistant, cellulosic pellicle.

It is another object of this invention to provide a process for the improvement of the oxidation resistance of cellulosic sheets and films.

It is still anotherobject of this invention to provide a method employing a series of steps each of which inv combination unexpectedly improves the oxidation resistance of cellulosic sheets and films.

It is a further object of this invention to provide storage batteries having a longer useful life.

It is a still further object of this invention to provide an improved storage battery employing a zinc-containing electrode, a silver-containing electrode, an alkaline electrolyte solution and a highly oxidation resistant cellulosic membrane separating the electrodes from the electrolyte solution.

In accordance with the present invention an improved oxidation resistant sheet comprises an alkali metal borohydride-treated, non-fibrous, semi-permeable cellulosic pellicle such as regenerated cellulose and hydroxyalkyl cellulose ethers preferably having alkyl groups with from 2 to 4 carbon atoms. The borohydride-treated cellulosic pellicle is further improved by the incorporation therein of a colloidal metal selected from the group consisting of copper, silver and gold, or by the incorporation of an amine-type antioxidant within the pellicle. Unexpected improvement in the oxidation resistance of the borohydride-treated cellulosic pellicle is experienced by impregnating it with both the colloidal metal and amine antioxidant.

The non-fibrous, semi-permeable cellulosic pellicles of this invention include homogeneous, continuous single sheets; composite sheets wherein the non-fibrous cellulosic pellicle is laminated to another or other sheet material; fibrous sheets, for example paper and pressed synthetic resin powders and fibers, impregnated and coated with regenerated cellulose or hydroxyalkyl cellulose to form a continuous, non fibrous surface; and non-fibrous, porous sheet material of synthetic resin material having cellulose impregnated pores, for example, porous polyvinyl chloride sheet material impregnated with regenerated cellulose as disclosed in U.S.P. 2,729,694.

The invention also includes the method of improving the oxidation resistance of cellulosic sheets which comprises treating said sheets with an alkali metal borohydride solution and impregnating with either the colloidal metal or an amine antioxidant compound or both. Colloidal silver, gold or copper is advantageously applied to the cellulosic sheet by first treating the sheet with a reducible silver, gold or copper compound and then further treating the sheet with an alkali metal borohydn'de to reduce the metal compound. For example, a regenerated cellulose pellicle is first steeped in a dilute aqueous solution of silver nitrate and then immersed in a dilute aqueous solution of sodium borohydride to precipitate colloidal silver in situ Within the pellicle. Or, the cellulose pellicle is first steeped in a dilute solution of gold chloride or copper sulfate and then immersed in the borohydride solution to precipitate colloidal gold or copper within the pellicle. This method permits the reduction of the metal compound and the borohydride treatment of the cellulose pellicle in one step. Cellulosic sheets and pellicle may also be impregnated with colloidal silver, gold or copper in any known manner. A conventional mirroring process employing an ammoniacal solution of silver nitrate has been used for the deposition of colloidal silver. This solution is prepared by adding just enough additional ammonium hydroxide to nearly completely redissolve freshly precipitated silver hydroxide. The cellulose pellicle is steeped in this solution and then exposed to a reducing solution or atmosphere such as formaldehyde or its vapors. Colloidal gold is similarly deposited by the treatment of the pellicle withdilute gold salt or oxide solution which is reduced in situ by a reducing agentsuch as formaldehyde, hydrogen, carbon monoxide, hydrazine or hydroxylamine. The cellulosic sheet or pellicle is treated with the borohydride solution either before or after impregnating with colloidal metal.

The storage battery of this invention comprises electrodes, electrolyte and an electrolyte-permeable cellulosic membrane treated with an alkali metal borohydride, said membrane separating said electrodes and said electrolyte, and a battery casing therefor. The invention particularly concerns storage batteries having zinc-containing electrodes, silver-containing electrodes and alkaline electrolyte solutions. The electrolyte-permeable cellulosic membrane is treated in accordance with the method of this invention to produce superior oxidation resistant separators thereby greatly increasing the life of the battery.

The cellulosic sheets of the method of this invention include flexible cellulosic films and composite sheets having cellulosic surface components. Examples of cellulosic material include regenerated cellulose produced by denitration of cellulose nitrate, by the viscose process, by cuprammonium process, by deacetylation of cellulose acetate, etc, and films of hydroxyalkyl cellulose ethers and modifications thereof. It is preferred, particularly for use as battery separators, that the cellulosic sheets consist of semi-permeable, regenerated cellulose produced by denit-ration of cellulose nitrate since these sheets alone have demonstrated somewhat better resistance to alkaline oxidation.

The alkali metal borohydrides in accordance with this invention include sodium, potassium and lithium borohydrides which are applied to the cellulosic sheets in aqueous solutions at concentrations sufiicient to treat the cellulose at a weight ratio of about 1 part borohydride to from 2 to 50 parts of cellulose. Aqueous solutions containing from about .02 to about 1% alkali borohydride are usually sufficient for this purpose. Treatment time for the borohydride application to the cellulosic pellicle ranges from about 1 hour to 5 hours at room tempera ture.

It is realized that alkali metal borohydrides are known to have been used in the reduction of cellulose, particularly cellulose pulps. However, the improvement in oxidation resistance of cellulosic films afforded by the treatment thereof with the borohydrides as compared to cellulosic films employing other methods of reducing oxidation is entirely unexpected. In addition, the use of the borohydride treatment permits unexpected improvement in oxidation resistance when certain other known antioxidant treatments are used in combination therewith. It appears that the carbonyl groups in cellulosic sheets are reduced by the borohydride and that this reduction leads to stabilization of sensitive linkages in the cellulose molecule. There is no explanation, however, why this treatment in combination with certain other antioxidant treatments provides synergistic oxidation resistance.

The amine type antioxidant of this invention is preferably a C -C alkyl substituted diphenylamine but includes for example, naphthylamines, phenylene diamines, substituted derivatives of these amines and mixtures thereof. The amine antioxidant treatment of the cellulose pellicle generally involves the replacement of the water content of the cellulose with an organic solvent and then treatment of the film with the amine antioxidant in an organic solvent solution. Usually, the cellulosic pellicle is steeped in acetone which replaces the water in the cellulose. Then, the acetone impregnated pellicle is steeped in an organic solvent such as toluene which replaces the acetone. Thereafter the pellicle is treated with an amineantioxidant such as octylated diphenylamine in toluene at the desired concentration. Solutions containing from 0.5% and up to the limit of solubility of the antioxidant in the organic solvent are useful and a concentration of about 2% by weight is preferred.

In order to test the oxidation resistance of the treated cellulosic sheets of this invention under strong alkaline conditions, which would duplicate as far as possible the conditions met by the cellulosic membrane used as a zincsilver battery separator, a method for testing was used. This test consists of attaching a weight of 225 p.s.i. to a test film strip and suspending it in a flask containing a strong potassium hydroxide solution with or without the addition of silver peroxide so that one-half of the film remains above the liquid level while the remaining portion including the weight is submerged in the caustic solution at elevated temperatures. The silver peroxide may be added to the solution in order to further accelerate the test. The fiask containing the caustic solution is placed 4 in an oven at 135 F. and the length of time required for the film to break is recorded.

Test films of regenerated cellulose prepared by the denitration of cellulose nitrate and having a thickness of about 2 mils were prepared as follows.

Test film A was first washed to remove glycerine and then steeped in acetone. The acetone impregnated film was placed in toluene for 3-0 minutes and then placed in a toluene solution of octyl substituted diphenylamine at a concentration of about 2% by weight for about minutes. The film was then dried.

Test film B was first washed to remove glycerine and then steeped in an ammoniacal solution of silver nitrate (4 gm./250 ml.). This film was then exposed to formaldehyde vapors for 10 minutes to precipitate colloidal silver therein. The colloidal silver impregnated film was then dried.

Test film C was first washed to remove glycerine and then steeped in an aqueous solution of sodium borohydride (2 gm./250 ml.) for 3 hours at room temperature. This solution gave aratio of 1 part borohydride to 2 parts cellulose. The film was removed from the borohydride solution and water washed. The washed film was rewashed with a 1% acetic acid solution, Washed again with water and dried.

Test film D was prepared as was test film C except that a less concentrated sodium borohydride solution was used. The solution concentration was 0.4 gm./250 ml. of water which gave a ratio of 1 part borohydride to 10 parts cellulose.

Test film E was prepared as were test films C and D except that a less concentrated sodium borohydride solution was used. The solution concentration was 0.08 gm./ 250 ml. of water which gave a ratio of 1 part borohydride to 50 parts of cellulose.

Test film F was first treated as test film D and then further treated as test film A.

Test film G was first Washed to remove glycerine and then steeped in a plain aqueous solution of silver nitrate at a concentration of 4 gm./250 ml. of water. When the film was completely impregnated with silver nitrate solution it was placed in an aqueous solution of sodium borohydride having a concentration of 0.4 gm/250 m1. of water for 2 hours at room temperature. Thereafter, the film was Water washed, rewashed in 1% acetic acid solution, washed again with water and dried.

Test film H was prepared as was film G and then further treated as was test film A.

' The control test film was untreated having the glycer- 1ne removed by water washing.

The test films were subjected to the previously described oxidation resistance test wherein a 45% solution of KOH was used. The results of this test are set forth in the following table.

Table 1 Test Film Hours in 45% KOH at F.

Control (Amin A B O. D. E. (NaBHi) B F. (NaBH4+ Amin 2 G. (Colloidal Silver NaBH H. (oilloidal Silver NaBHi Amine) 288, 363, 503

1 Borohydride treatment with a ratio of 1 part borohydride to 2 parts cellulose. ceaBlorohydrlde treatment with a ratio of 1 part borohydride to 10 parts u ose.

Table 11 Test Film Hours in 45% KOH and AgZOZ at 135 F.

is, 25, 22, 23, 25, 22, 2s 20 28 If 1 69, 70 (NaBHi Amine) 1 45 (Colloidal Silver N 3BH4) 90 (Colloidal Silver NaBHi Amine) l (Colloidal Silver Amine) 1 Borohydride treatment with a ratio of 1 part borohydride to 10 parts cellulose.

To demonstrate this invention employing colloidal gold and copper as antioxidant agents the following data are given.

Test film J was first washed to remove glycerine and then steeped in a dilute aqueous solution of gold chloride (4 gm./250 ml.) for 2 hours. This film was then exposed to formaldehyde vapors for about 10 minutes to precipitate the gold particles therein. Thereafter, the gold impregnated sheet is further impregnated with octyl substituted diphenylamine in the manner set forth for test film A.

Test film K was prepared by first washing the regenerated cellulose film to remove glycerine plasticizer. It 0 was then steeped in aqueous gold chloride solution (4 gm./250 ml.) for 2 hours. The saturated film was then steeped in an aqueous solution of sodium borohydride (0.4 gm./ 250 ml.) for 3 hours at room temperature. The film was then water washed, washed with 1% acetic acid, rewashed with water and dried.

Test film L was prepared in the same manner as test film K and then impregnated with the amine antioxidant in the manner set forth for test film A.

Test film M was prepared in the same manner as test film L except that the gold chloride solution was replaced with an aqueous copper sulfate solution (4 gm./ 250 ml.).

The above test films were subjected to the test procedures described for Tables I :and II and the results are set forth in the following table.

Table III The results set forth in the above tables demonstrate conclusively the excellent oxidation resistance of the films and sheets of this invention and the unexpected improvement provided by the disclosed process. Known treat ment of films to reduce oxidation are comparatively in effective when used alone or even when combined as with test film I. However, the oxidation resistance of films treated with the borohyd-ride is greatly improved while the combined treatments including the borohydride produce a synergistic improvement well over the expected result.

Colloidal silver impregnation with reduction of the silver salt by formaldehyde or the like appears to be far inferior in producing oxidation resistance than when the borohydride is used to simultaneously reduce the silver salt and treat the cellulose film or pellicle.

Impregnation with both gold particles and diphenylamine produces unexpected improvement in oxidation resistance either when the gold salt is reduced with ordinary reducing agents such as formaldehyde or with sodium borohydride. However, the great increase when using the borohydride treatment is totally unexpected even when compared to the excellent results obtained using formaldehyde as the reducing agent for the gold salt.

Further improvement in the method of this invention by treatment of cellulosic pellicles with aqueous alkali metal borohydrid-e solutions in the presence of a water soluble salt of a metal of the group including lithium, magnesium, calcium, strontium and barium is expected in accordance with U.S.P. 2,898,333. Thus, in the presence of these salts smaller amounts or less concentrated aqueous solutions can be used to treat the cellulosic pellicle to obtain the desired result.

Various changes and modifications may be made in practicing the invention without departing from the spirit and scope thereof and, therefore, the invention is not to be limited except as defined in the appended claims.

We claim:

1. A method of improving the oxidation resistance of a non-fibrous cellulosic pellicle which comprises impregnating said pellicle with a reducible compound of a metal selected from the group consisting of gold, silver and copper, reducing said compound of a metal to precipitate a colloidal metal in situ within said pellicle, and treating said pellicle with an alkali metal borohydride solution at a weight ratio of about 1 part borohydride to from 2 to 50 parts cellulose.

2. A method of improving the oxidation resistance of a non-fibrous oellulosic pellicle which comprises impregnating said pellicle with a borohydride reducible compound of a metal selected from the group consisting of gold, silver and copper, and reducing said compound of a metal to precipitate a colloidal metal in situ within said pellicle by treating said pellicle with an alkali metal borohydride at a weight ratio of about 1 part borohydride to from 2 to 50 parts cellulose.

3. The method of claim 2 wherein the metal compound is silver nitrate.

4. The method of claim 2 wherein the metal compound is gold chloride.

5. The method of claim 2 wherein the cellulosic pellicle is regenerated cellulose film prepared by denitration of cellulose nitrate.

6. The method of claim 2 wherein the cellulosic pellicle is also treated with an amine antioxidant compound.

7. The method of claim 6 wherein the film is treated with an alkyl substituted diphenylamine dissolved in an organic solvent.

8. An oxidation resistant sheet comprising an alkali metal borohydride-treated, non-fibrous, semi-permeable cellulosic pellicle selected from the group consisting of regenerated cellulose and hydroxyalkyl cellulose ethers, said pellicle impregnated with (l) a colloidal metal selected from the group consisting of gold and silver, and (2) an alkyl substituted diphenylamine.

References Cited in the file of this patent UNITED STATES PATENTS 2,475,538 Baird July 5, 1949 2,511,472 Kmecik June 13, 1950 2,520,963 Reeves Sept. 5, 1950 2,602,757 Kantrowitz et a1. July 8, 1952 2,696,515 Koren et a1. Dec. 7, 1954 2,729,540 Fisher Jan. 3, 1956 2,898,333 Jullander Aug. 4, 1959 3,013,099 Mendelsohn Dec. 12, 1961

Claims (1)

1. A METHOD OF IMPROVING THE OXIDATION RESISTANCE OF A NON-FIBROUS CELLULOSIC PELLICLE WHICH COMPRISES IMPREGNATING SAID PELLICLE WITH A REDUCIBLE COMPOUND OF A METAL SELECTED FROM THE GROUP CONSISTING OF GOLD, SILVER AND COPPER, REDUCING SAID COMPOUND OF A METAL TO PRECIPITATE A COLLOIDAL METAL IN SITU WITHIN SAID PELLICLE, AND TREATING SAID PELLICLE WITH AN ALKALI METAL BOROHYDRIDE SOLUTION AT A WEIGHT RATIO OF ABOUT 1 PART BOROHYDRIDE TO FROM 2 TO 50 PARTS CELLULOSE.