DE3146334A1 - Electrochemical cell - Google Patents

Description

MEDTRONIC, ■ INC. 3055 Old Highway Eight, Minneapolis, Minn. 55440 / V.St.A.

Electrochemical cell

The invention relates to an electrochemical cell and, more particularly, to an improved cathode material for such cells.

One field of application of the present invention is the provision of electrical energy for inaccessible Devices such as implanted pacemakers. However, the invention is in a wide variety can be used by batteries and electrochemical cells. It is particularly suitable for primary elements that have a relatively high voltage and high energy density over a to provide long periods of time with low power consumption.

In certain cases, the cells of the invention can suitably be assembled and encapsulated in a dry atmosphere in dry rooms or dry areas with a relative humidity of less than about 2 %, using essentially anhydrous and / or dried components. All of the cells and cell tests explained here were produced or carried out in sealed enclosures which, although not necessarily completely hermetically sealed, provided essentially anhydrous components. In the case of series designs, the electrochemical cells according to the invention can preferably be in hermetically sealed enclosures, for example

welded stainless steel tanks, encapsulated with appropriate electrical feedthroughs are provided to allow electrical contact with the components of the cell in a known manner care for. Such cells are preferably assembled in a drying room.

The invention is based on the finding that certain organic oxygen-containing compounds (mentioned in detail below) as cathode materials for electrochemical Cells are particularly suitable when treated with iodine, bromine or certain interhalogens, for example IBr and the like (hereinafter referred to generally as halogen (e) for convenience) are mixed will. These compounds are beneficial because they form electrically conductive mixtures with halogens; H. Mixtures, in which the electrical conductivity is significantly greater than that of the halogen alone. These In the present case, mixtures are with regard to the composition of the components originally mixed with one another defined, since in some cases reaction products can be formed which, depending on the das Mixture-forming initial components can vary. In such mixtures and preferably below substantially In anhydrous conditions, the halogen can readily be used as the electrochemically active ingredient of the cathode material. The preferred halogen is iodine. The preferred oxygen-containing organic Compounds are polyethylene oxide (PEO), for example water-soluble resins with the trade name POLYOX (trademarks of Union Carbide Corporation), ethanol, polypropylene oxide, and vinyl acetate. Polymers and are monomeric (polymerizable and non-polymerizable) organic compounds and mixtures thereof used as the oxygen-containing organic component

• 3H6334

bar. The term "polymeric compound" is intended to include any organic compound that has two or more contain more monomer units.

The preferred anode of the electrochemical cells consists of the above-mentioned mixtures as the cathode material made of lithium. However, in principle any metal or alloy can be used that which forms an ion-conducting halide, e.g. silver, calcium, magnesium, sodium, lithium-magnesium alloys, Lithium-calcium alloys and the like. The monovalent metals are preferred.

The cells considered here are preferably like this built to form electrolytes in situ. For example, if the electrochemically active components are lithium and iodine, a solid lithium iodide electrolyte is formed between the anode and the cathode after the cell is assembled. Alternatively, the Electrolyte as a whole or partially prefabricated. For example, the following arrangement can be provided: Li / Lil (Al-O -) / PEO · nl-. One such arrangement is particularly suitable for modifying the self-discharge rate. Such alumina dispersions are known as such from CC. Liang, J. Electrochem. Soc. 120, page 128? (1973); CC. Liang and L.H. Barnette, J. Electrochem. Soc. 123, page 453 (1976) and US Pat. No. 3,713,897. Reference is made to these references for further details.

A further modification with regard to the functional relationship between the anode and cathode, which may be present provided the use of an anode which is (2-vinylpyridine) (P2VP) or another polymeric material coated with poly (U.S. Patent 3,957,533)

or the use of a self-supporting poly (2-vinylpyridine) body (U.S. Pat. No. 4,182,798). For further details, refer to the two cited patents referenced.

In the following, the invention is preferred by way of example Embodiments explained in more detail. In the accompanying drawings show:

Fig. 1 is a schematic representation of a

electrochemical test cell with improved cathode material accordingly the invention,

Fig. 2 is a schematic representation of a

hermetically sealed cell with a body made of poly (2-vinylpyridine) as

Figures 3 and 4 are voltage graphs

plotted against the output capacity (Q) for test cells according to the invention.

The invention is not directed to any particular cell or Battery construction, but rather the combination of the components that make up an electrochemical cell or battery directed, the cathode material being improved in the manner described herein. Consequently the invention is not limited to a specific construction or structural design of the electrochemical Cell restricted. In terms of physical arrangement, only anode and cathode arrangements need of the cell or battery with the cathode material explained here in the broadest sense in functional

relationship to each other. To be essentially anhydrous, the cells are assembled and encapsulated in a dry atmosphere, preferably in a dry room or area with a relative humidity of less than about 2 % , using essentially anhydrous and / or dried components.

Those illustrated and described below Exemplary embodiments are flat, cylindrical cells with flattened, disc-like components. It v-er. It is understood, however, that within the scope of the invention, the cell and its components have any desired shape and arrangement can have. In Figs. 1 and 2, the cathode is illustrated at 12, while the preferably off Lithium existing anode is denoted by 14. The anode and cathode are made with the help of inert current collectors 16 or 18 contacted, which can be made of metal, for example Hastelloy or stainless steel. In the illustrated cell, the lithium anode is ultrasonically peripheral around the current collector 18 formed around and welded to prevent contact between the collector and the cathode material 12. The electrochemically active components, i.e. H. the cathode 12 and the anode 14-, as well as the current collectors la and 18 are in a cup-shaped, chemically inert, non-conductive container 20 made of plastic such as a polyvinylidene fluoride with the trade name "Kynar" (trademark of Pennwalt Corporation) and a lid 22 of the same Material housed. These two components can be ultrasonically welded to one another or in some other way be tightly connected. Other insulating, inert materials, for example one from Allied Chemical Corporation under the trademark "Halar" on the

Market brought ethylene-chlorotrifluoroethylene, and a Copolymer of ethylene and tetrafluoroethylene with the name "Tefzel" (trademark of E.I. DuPont de Nemours) can be used for these components. Stainless steel leads 24 and 26 are in the Container members 20 and 22 molded thereinto; they make electrical contact with the current collectors 16 and 18 ago. To hermetically seal the cell, the entire unit housed according to FIG. 2 in a housing 30 made of stainless steel, with the electrical leads 24 and 26 are provided around glass / metal fusions 32 and 34 in a manner known per se are. The housing 30 can expediently, as illustrated, consist of two cup-shaped parts, which are welded together.

The cell of FIG. 2 has a body or film 36 made of poly (2-vinylpyridine) polymer based on the effective Surface of the anode 14 is seated, the effective surface being understood as the surface which is normally in contact with the cathode of the cell, at least initially.

Cells of the type described above require in their Initial assembly state no electrolyte. As a result, there is no electrolyte as such in the figures shown. However, after assembly, an electrolyte forms in situ. The electrolyte builds up due to the reaction between the anode metal and the iodine in the cathode between the cathode and the anode usually taking the form of a sheet. For example, a cell with a Lithium anode and iodine present in the cathode formed a lithium iodide electrolyte.

- ίο -

Iodine or another halogen or mixtures thereof and a selected oxygen-containing organic component or a mixture of such components are directly mixed with one another in various relative amounts to form the cathode material 12. In most cases, it is expedient to heat the mixture to relatively moderate temperature, for example about 125 ⁰ C. This is especially true when the oxygen-containing organic component is a polymeric compound. In contrast, a non-polymer such as tetrahydrofuran (THF) may not require heating when mixed with the halogen. POLYOX, a polyethylene oxide that is preferably used, is heated with iodine to about 125 ° C. for about an hour in order to achieve good results. .

Some caution is necessary when using different organic components are brought together with the halogen. For example, mixtures of starch and Iodine and some mixtures of polyethylene oxide and iodine violently at temperatures above about 125 ° C.

Iodine, a particulate material, can be found in either Form of coarse particles or of finely divided particles can be used, for example as powdered iodine. It is preferable to use finely divided Iodine worked.

The oxygen-containing organic component can select from the following group:

• Ethers, such as polyvinyl methyl ether, tetrahydrofuran (THF), diethyl ether, butyl methyl ether, polyethylene oxide (PEO), polypropylene oxide (PPO) and polyvinyl butyral;

Alcohols such as polyvinyl alcohol, ethanol, phenol, methyl cellulose and starch;

Ketones such as acetone, polymethyl vinyl ketone and methyl ethyl ketone;

Esters, such as the following vinyl and polyvinyl esters: polyvinyl succinate, polyvinyl acetate, vinyl acetate, and Polyvinyl acetate;

Acids such as polyacrylic acid (PAA), acetic acid, and oxalic acid;

Salts such as sodium and ammonium salts of polyacrylic acid and sodium acetate;

Anhydrides, such as acetic anhydride and the following special compounds:

Propylene oxide

Poly (N-vinyl pyrrolidone)

Propylene carbonate

Triphenylphosphine oxide (TPP).

In addition, the above compounds can be used with each other or mixed with other materials, such as:

Polyethylene oxide + poly (2-vinylpyridine), for example in equal amounts (the number of oxygen atoms is equal to the number of nitrogen atoms), the mixture then being mixed with iodine in a ratio of 8 moles of iodine (I ₂ ) per mole of oxygen plus nitrogen will, and

- 12 polyethylene oxide + triphenylphosphine oxide, as above.

The presently provided oxygen-containing organic Compounds can generally be characterized as non-nitrogen-containing compounds in the sense that that if nitrogen is present, it is not present in the same way as the nitrogen, contained in the pyridine cathode material compounds such as poly (2-vinylpyridine), etc. In these last-mentioned compounds, the nitrogen is built into the compound in such a way that it carries electrons a halogen such as iodine. This is to be distinguished from the connection types provided here, for example the ammonium salts of acids of the type indicated above, where the nitrogen is not in the Is able to share electrons with a halogen such as iodine. That is, with the oxygen used according to the invention containing compounds it is assumed that the oxygen shares electrons with the halogen. An exception can be seen in this connection in the fact that poly- (N-vinylpyrroüdon) contains an active nitrogen in addition to an active oxygen.

Viscosity can be significant in certain applications in order to avoid any significant segregation of the various cathode components to prevent. This viscosity will vary depending on the particular cathode combination and the intended one Use of the same between that of a solid and a liquid. The viscosity can be influenced by either a thinning Solvent or a thickening agent (hereinafter referred to collectively as viscosity adjusting agent) the Cathode material is added depending on whether its viscosity is to be increased or decreased. Thinning solvents such as chloroform and orthodin

chlorobenzene, are examples of useful inert solvents. Alternatively, it is also possible to use a diluting solvent which is an oxygen-containing compound of the type according to the invention or a mixture of such oxygen-containing compounds. Examples of such oxygen-containing compounds which are suitable as diluting solvents include ethanol, acetone, tetrahydrofuran and diethyl ether. As inert thickeners, for example, smoked SiC> 2r finely divided AlI ₃ , finely divided Al ^ Og, sheet silicates such as montmorillonite, finely ground lithium iodide, hydrocarbon polymers, for example polystyrene, polybutene, poly (alpha-methylstyrene and styrene-butadiene rubbers). Instead, a thickener can be used which is an oxygen-containing compound of the type provided according to the invention or a mixture of such oxygen-containing compounds. Examples of compounds provided according to the invention which are suitable as thickeners include polyethylene oxide, polypropylene oxide and poly (N-vinylpyrrolidone).

Example I.

Cells of the type illustrated in Figure 1 were used for Manufactured for testing purposes. The cells had lithium anodes; as cathodes were mixtures of iodine and the various compounds specified in Table 1 are provided. The active area of the anode was 5.1 cm. The cathode materials were prepared in such a way that mixtures of iodine and each additive compound in a sealed Glass ampoule in a vibrating oven at 125 C for one hour were heated together for a long time, unless otherwise stated. The conductivities of the cathode materials are shown in Table 1. The mixtures were

prepared so that initially the ratio of iodine molecules (I ₉ ) to oxygen atoms of the additive was 8.0. All ratios are expressed in the same way unless otherwise specified. All cells were cathode-limited with a stoichiometric iodine capacity of 200 mAh. In all cells of this example, a 0.08 mm thick layer of poly (2-vinylpyridine) was placed between the anode and cathode in the manner illustrated in FIG. The cells were discharged at 37 C and a resistive load of 10 kO. All cells had open circuit voltages of around 2.8 V.

The mean capacity delivered (up to a limit of 100 mV, unless otherwise specified) is in Table 1 for each class of oxygenated additive compound listed based on the two best of three identical cells.

TABLE 1

Iodine cathodes with oxygen-containing components: conductivity and operating behavior the cell

Great Oxygenated
link Cathodes
conductive
speed
(Ohm cm) Medium
At first
idle
tension
(V) Medium
submitted
capacity
(mAh) Medium
Cathode
utilizing
(%) I. ether Polyethylene oxide (PEO) l, 79xlO ~ ⁴ 2.80 172 86 Oi Polypropylene oxide (PPO) 1.98x10 " ⁴ 2.80 176 88 ¹ 0.7 / 0.3 PEO / PPO copolymer ■ 2.80 178 89 Tetrahydrofuran (THF) 1.27 x 10 " ⁴ 2.87 130 65 Polyvinyl butyral 4,13xlO ~ ⁴ 2.80 148 74 Alcohols Polyvinyl alcohol (PVA) l, 826 x 10 ^-3 2.94 158 79 Methyl cellulose 2.77 153 77 phenol 5.0 χ 10 ~ * 2.80 146 73 strength 2.83 180 90 Ethanol 5,496xlO ~ ⁵ 2.80 180 90

Ketones

Methyl ethyl ketone

1.5 χ 10

-4

2.80

80

cn co co

TABLE 1 (continued)

Great Oxygenated
link Cathodes
conductive
speed,
(Ohm cm ") Medium
At first
idle
tension
(V) Medium
submitted
capacity
(mAh) Medium
Cathode
utilizing I.
, ι t ( Ester Polyvinyl succinate ___ 2.78 1 62 81 Polyvinyl acetate 5.38 x 10 " ⁶ 2.78 164 82 I '. Vinyl acetate 4, 17x10 " ⁶ 2.78 182 91 1 f
«* (Ι i Acids Polyacrylic acid (PAA) 2.71 169 80 . 't acetic acid 3.7 χ 10 " ⁶ 2.73 166 83 Oxalic acid 2.69 168 80 t * I Salts Sodium salt of PAA 2.76 155 78 Ammonium salt of PAA 2.78 176 88 Approx
MIl Sodium acetate 2.1 χ ΙΟ " ⁴ .2.95 159 80 Anhydrides Acetic anhydride 6.5 χ 10 " ⁵ 2.79 178 89 Further Poly (N-vinyl pyrrolidone) 2.7 χ 10 " ⁵ 2.79 186 93 Fabrics Triphenylphosphine oxide (TPP) 2.7 χ ίο " ⁵ 2.78 130 65

Example II

Mixtures of iodine and polyethylene oxide (PEO) were prepared within a starting composition range of 0.50 to 40.0 molecules of iodine (I ₂ ) per oxygen atom of the PEO. The polyethylene oxide contemplated in these blends was POLYOX (from Union Carbide Corporation). The water content of the PEO _7, determined according to the Karl Fischer analysis, was 0.21 %.

Table 2 shows the variation in electrical conductivity and the density as a function of the composition, measured on pellets pressed from mixtures which were prepared in sealed glass ampoules and placed in a vibrating oven under the specified Conditions were heated. For each composition shown in Table 2, three identical, cathode-limited Cells built with a stoichiometric iodine capacity of 200 mAh. An additional group of cells was produced in an otherwise identical manner for each composition without a P2VP layer between anode and cathode. The cells were discharged at 37 ° C. with ohmic loads of 10 kA. The given capacities above the limit value of 100 mV (unless otherwise stated) are listed in Table 3. the Mean discharge curves are illustrated in FIGS. 3 and 4 for cells with and without P2VP layers, respectively.

Table 2

Manufacture and properties of cathode material (! «/ PEO)

Together
settlement
(I _2/0) Manufacturing
Temp. ( ⁰ C) Time (h) 1.0
1.0 conductivity
(Ohm " ¹ cm" ¹ ) ΙΟ " ⁴
ΙΟ " ⁴ density
(g / cm) 0.5
1.0 100
100 1.0 2.0 χ
4.8 χ ΙΟ " ⁴ 2.62
3.54 2.0 100 1.0 3.9 χ ΙΟ " ⁵ 4.35 8.0 125 1.0 7.3 χ 10 " ^ά 4.77 20.0 125 1.0 4.5 χ 10 " ⁷ 4.80 40.0 125 2.0 χ 4.88

Table 3

Operating behavior of the cell with varying cathode composition (1 ,, / PEO) *

Cathodes
together
settlement
(I ₂ ZO) Submitted
capacity
(mAh) 58 (12) theoretically Exploitation
{% of Ge
velvet iodine) Electrodes
volume ***
(cm ³ ) Specific
capacity
(mAh / cm) with P2VP location 97 (17) 0.5 128 (7) 29 (6) 0.625 .93 (19) 1.0 171 (1) 49 (8) 0.450 216 (38) 2.0 182 (8) 64 (4) 0.373 343 (19) 8.0 177 (1) 86 (1) 0.339 505 (3) 20.0 91 (4) 0.335 544 (24) 40.0 88 (1) 0.331 534 (2) 0.327 610

Table 3 (continued)

Cathodes
together
settlement
(I _2/0) Submitted
capacity
(mAh) 52 (12) theoretically Exploitation
(% of Ge
velvet iodine) Electrode
.volume ***
(cm ³ ) Specific
capacity
(mAh / cm) without P2VP location 72 (7) 0.5 141 (6) 26 (6) 0.586 89 (20) 1.0 155 (2) 36 (4) 0.411 175 (17) 2.0 ** 143 (12) 71 (3) 0.334 422 (17) 8.0 ** 117 (17) 78 (1) 0.300 516 (7) 20.0 ** 72 (6) 0.296 483 (41) 40.0 ** 59 (9) 0.292 400 (58) 0.289 692

* Mean deviations are given in brackets.

** Limit of 100 mV not reached. .

*** Includes volume of P2VP location, if any.

Example III

Cells of the type described in the above examples were produced with I ₂ : O ratios of 8.0 and / or 20.0 at preparation temperatures below 125 ° C. Oxygen-containing compounds with different molecular weights were provided in conjunction with iodine in order to obtain the cathode materials. The heating time was one hour, except for the one sample that was prepared by mixing the components only at room temperature. The variations investigated and the utilizations achieved for 3-cell groups are shown in Table 4. All of these cells were provided with a 0.08 mm thick sheet of poly (2-vinylpyridine) that was placed between the anode and cathode. The cells were discharged at 37 ° C under resistive loads of 10 kilograms.

No significant variations in cathode utilization as a function of molecular weight (MW) were observed. The cells were found to be satisfactory have been discharged even if the cathode is at its Preparation was not heated at all. Consequently, the heating, although it is in the preparation of the cathode may be desirable, not a compulsory feature.

Table 4

Preparation temperature / operating behavior of the cell with polyethylene oxide of varying molecular weight

Cathode
preparation
temperature
( ⁶ C) Cathodes
together
settlement
(I _2/0) PEO type Cathodes
from use
(*) 25th 20th POLYOX 87 100 8th POLYOX 91 100 8th MW 5,000,000 * 90 100 20th MW 600,000 * 91 100 . 8th MW 20,000 * 90

* Obtained from Polysciences, Warrington, PA.

Example IV

Six different cathode materials were produced from mixtures of PEO with a molecular weight of 20,000 and 5,000,000 in different proportions as an oxygen-containing component. The proportions of PEO with the two molecular weights were varied in steps of 20% between 0 and 100%. The initial ratio of the I _? -Molecules to the O atoms was set to 20 in each case. The PEO compounds were purchased from Polysciences, Warrington, PA. based. Each mixture was

3H6334

de mixed and then heated to 125 ° C for one hour as explained above. Cells were divided into groups of 3 constructed, with a 0.08 mm thick sheet of poly (2-vinylpyridine) between the anode and the cathode was ordered. All cells were discharged at 37 ° C under loads of 10 kü. The middle submitted Capacities for these cells are listed in Table 5 .

Table 5

Performance of the cell with mixed polyethylene oxides of two molecular weights

Composition of the PEO
Component (% MW 20,000,
Remainder MW 5,000,000-PEO) Example V Cathode utilization
(*) 0 86 20th 90 40 86 60 88 80 90 100 90

There were mixtures of iodine and oxygen-containing organic Prepared compounds, which in turn have been mixed with a nitrogen-containing compound. The parts were heated in a sealed glass ampoule in a vibrating oven at 125 ° C for one hour long in the case of 2-ethylpyridine and aniline and 24 hours, respectively long in the case of poly (4-vinylpyridine). These materials were tested as cathodes in cells which, like those in Example I, have a stoichiometric iodine

had a capacity of 200 mAh. For each cathode were
three identical cells with an initial composition of 8 iodine molecules per sum of oxygen plus nitrogen atoms according to Table 6 were tested. The cells were discharged as described in Example I. The capacities delivered above 100 mV are listed in Table 6 .

Table 6

Operating behavior of cells with mixed oxygen / nitrogen components

With P2VP film

Without P2VP film

Organic components Cathodes
together
settlement
(I _2/0 / N) Medium
submitted
capacity
(mAh) Medium
Cathode
utilizing
(%) Medium
submitted
capacity
(mAh) Medium
Cathode
utilizing
(%) N>
CO POLYOX, 2-ethyl-
pyridine 8 / 0.5 / 0.5 143 71.5 147 73.5 POLYOX, poly
vinyl pyridine) 8 / 0.5 / 0.5 167 83.5 173 86.5 PEO (Molecular Ge
weight 20,000) *,
Poly (4-vinyl pyridine) 8 / 0.5 / 0.5 139 29.5 165 82.5 POLYOX, aniline 8 / 0.5 / 0.5 170 85

Obtained from Polysciences, Warrington, PA.

CD CjJ GO

L θ e r s e i t e

Claims (18)

Expectations 1 / Electrochemical cell with an anode arrangement and a functionally related cathode arrangement, characterized in that the cathode arrangement (12) at least partially consists of a mixture a halogen component and an oxygen-containing organic component, which consists of the Ethers, alcohols, ketones, vinyl and polyvinyl esters, salts, anhydrides, propylene oxide, poly (N-vinylpyrrolidone), Propylene carbonate, triphenylphosphine oxide and mixtures of the same existing group. 2. Cell according to claim 1, characterized in that the halogen component is at least partially composed of iodine consists. 3. Cell according to claim 1 or Z, characterized in that the oxygen-containing component consists at least partially of polyethylene oxide. 4. Cell according to one of the preceding claims, characterized in that the anode arrangement (14) Having lithium. 5. Cell according to one of claims 1 to 3, characterized in that that the anode assembly (14) comprises a monovalent metal. 6. Cell according to claim 1, characterized in that the oxygen-containing organic component of the cathode TELEPHONE: 089/6012039 · CABLE ·. ELECTRICPATENT MUNICH - 2 de (12) is essentially polymeric. 7. Cell according to claim 1, characterized in that the oxygen-containing organic component of the cathode (12) is essentially monomeric. 8. Cell according to claim 1, characterized in that the oxygen-containing organic component of the cathode (12) has a polymer / monomer mixture. . 9. Cell according to one of the preceding claims, characterized in that the cathode mixture is more Halogen component as an oxygen-containing component contains. 10. Cathode material for electrochemical cells, characterized by one consisting essentially of a halogen component and polyethylene oxide Combination. 11. Cathode material according to claim 10, characterized in that that iodine is at least partially provided as a halogen component, 12. Cathode material according to claim 10 or 11, characterized characterized in that the amount of the halogen component is greater than the amount of the polyethylene oxide. 13. Cell according to one of claims 1 to 9, characterized in that that for establishing the functional relationship one of the anode arrangement (14) is assigned Polymer component is provided. 14. Cell according to claim 13, characterized in that the polymer component assigned to the anode is poly (2- vinylpyridine). 15. Cell according to claim 14, characterized in that the poly (2-yynylpyridine) in the form of an anode coating is present. 16. Cell according to claim 14, characterized in that the poly (2-vinylpyridine) in the form of a self-supporting Body is present. 17. Cell according to one of claims 1 to 9 and 13 to 16, characterized by a means for adjusting the viscosity. 18. Cell according to one of claims 1 to 9 and 13 to 17, characterized in that the oxygen-containing organic compound polyvinyl methyl ether, tetrahydrofuran, diethyl ether, butyl methyl ether, polyethylene oxide, Polypropylene oxide, polyvinyl butyral, polyvinyl alcohol, ethanol, phenol, methyl cellulose, Starch, acetone, polymethyl vinyl ketone, methyl ethyl ketone, polyvinyl succinate, polyvinyl acetate, vinyl acetate, Polyacrylic acid, acetic acid, oxalic acid, sodium and ammonium salts of polyacrylic acid, sodium acetate, Acetic anhydride or mixtures of these substances are provided.