Method for the production of superabsorbers

Abstract

The present invention relates to a method for producing superabsorbers, wherein an ethylenically unsaturated monomer carrying acid groups is at least partially neutralized with sodium hydroxide solution, the sodium hydroxide solution is stored in a container immediately before the neutralization process, and the inner surface of the container is made of organic polymer.

Description

[0001] 240922

[0002] 1

[0003] Methods for the production of superabsorbents

[0004] The present invention relates to a process for the production of superabsorbents, wherein an ethylene unsaturated, acid-group-bearing monomer is at least partially neutralized with sodium hydroxide, the sodium hydroxide is stored in a container immediately before neutralization, and the inner surface of the container is made of organic polymer.

[0005] Superabsorbent polymers are used in the production of diapers, tampons, sanitary napkins, and other hygiene products, as well as water-retaining agents in horticulture. They are also known as water-absorbing polymers.

[0006] The production of superabsorbents is described in the monograph ''Modern Superabsorbent Polymer Technology«, FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103.

[0007] To improve application properties, such as gel bed permeability (GBP) and absorption under a pressure of 49.2 g / cm² 2 (AUL 0.7 psi), superabsorbent particles generally undergo surface cross-linking. This increases the degree of cross-linking of the particle surface, thereby improving absorption under a pressure of 49.2 g / cm². 2 (AUL 0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface post-crosslinking can be carried out in the aqueous gel phase. Preferably, however, dried, milled, and sieved polymer particles (base polymer) are coated on the surface with a surface post-crosslinker and thermally surface post-crosslinked. Suitable crosslinkers for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles.

[0008] WO 2007 / 028751 A1 concerns a neutralization process. WO 2010 / 052182 A1 describes coating with tannins.

[0009] The object of the present invention was to provide an improved process for the production of superabsorbent particles, in particular for the stable production of particularly white superabsorbent particles.

[0010] The problem was solved by a process for the production of superabsorbents by polymerization of an aqueous monomer solution or suspension containing a) at least one ethylene-unsaturated, acid-group-bearing monomer that is at least partially neutralized, b) at least one crosslinker, and c) at least one initiator, 240922

[0011] 2. The aqueous monomer solution is polymerized to form a polymer gel, the polymer gel is optionally extruded, the polymer gel is dried on a circulating air belt dryer, and the dried polymer gel is comminuted, classified, and optionally thermally surface crosslinked, characterized in that the ethylene-unsaturated, acid-group-bearing monomer a) is at least partially neutralized with sodium hydroxide solution, the sodium hydroxide solution is stored in a container immediately before neutralization, the container having a volume of 1 to 10,000 m³ 3 has a residence time of the sodium hydroxide solution in the container of 0.1 to 750 h, the inner surface of the container is made of organic polymer and the superabsorbent particles are not coated with a condensed and / or hydrolyzable tannin.

[0012] Immediately before neutralization means that the container with the sodium hydroxide solution is located in the immediate vicinity of the neutralization process, i.e., the spatial distance between the container and the neutralization process is typically less than 1,000 m, preferably less than 750 m, and more preferably less than 550 m.

[0013] The container has a volume of preferably 2 to 5,000 m³ 3 , especially preferred 5 to 2,000 m 3 , especially preferred 10 to 1,000 m 3 .

[0014] The residence time of the sodium hydroxide solution in the container is preferably 0.3 to 500 h, particularly preferably 0.4 to 250 h, and most preferably 0.5 to 150 h.

[0015] The throughput of sodium hydroxide solution in the container is preferably 1 to 30 t / h, particularly preferably 3 to 25 t / h, and most preferably 5 to 20 t / h, each based on 20 t / h of monomer solution.

[0016] The present invention is based on the finding that storing sodium hydroxide solution in excessively large containers and for excessively long residence times can lead to discolored superabsorbent particles. This can be prevented by using organic polymers as the container material.

[0017] The sodium hydroxide solution used has a content of preferably 10 to 60 wt.%, particularly preferably 15 to 55 wt.%, most preferably 20 to 50 wt.%.

[0018] The temperature of the sodium hydroxide solution in the container is preferably 25 to 75°C, particularly preferably 30 to 60°C, and most preferably 35 to 45°C.

[0019] The sodium hydroxide solution preferably contains less than 5 wt.%, particularly preferably less than 0.5 wt.%, most preferably less than 0.05 wt.% sodium chloride.

[0020] The organic polymer is not subject to any restrictions. Suitable organic polymers include polyolefins, polyesters, polyamides, polytetrafluoroethylene, polyvinyl chloride, epoxy resins, and silicone resins. The container can be made from 240922.

[0021] The container consists of three organic polymers, for example reinforced by glass fibers. Preferably, the container is made of steel and lined with the organic polymer.

[0022] The following section explains the production of superabsorbents in more detail:

[0023] Superabsorbents are produced by polymerization of a monomer solution and are usually insoluble in water.

[0024] The ethylene unsaturated, acid-group-bearing monomers a) are preferably water-soluble, i.e., the solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, particularly preferably at least 25 g / 100 g water, and most preferably at least 35 g / 100 g water.

[0025] Suitable monomers include, for example, ethylene-unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Acrylic acid and methacrylic acid are particularly preferred monomers. Acrylic acid is especially preferred.

[0026] The ethylene-unsaturated, acid-group-bearing monomers a) are usually partially neutralized. Neutralization is carried out at the monomer stage. This is typically done by mixing in the neutralizing agent as an aqueous solution or, more preferably, as a solid. The degree of neutralization is preferably 40 to 85 mol%, particularly preferably 50 to 80 mol%, and most preferably 60 to 75 mol%, using sodium hydroxide solution.

[0027] The monomers typically contain polymerization inhibitors, preferably hydroquinone semi-ethers, as a storage stabilizer.

[0028] Suitable crosslinking agents (b) are compounds with at least two groups suitable for crosslinking. Such groups include, for example, ethylene-unsaturated groups that can be radically polymerized into the polymer chain, and functional groups that can form covalent bonds with the acid groups of the monomer. Furthermore, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.

[0029] Suitable crosslinking agents b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- and triacrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, WO 03 / 104301 A1 and DE 10331 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylene unsaturated groups, as described in DE 103 31 240922

[0030] 4

[0031] 456 A1 and DE 103 55 401 A1, or crosslinking mixtures, as described for example in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0032] The amount of crosslinker (b) is preferably 0.05 to 1.5 wt.%, particularly preferably 0.1 to 1 wt.%, and most preferably 0.15 to 0.6 wt.%, in each case calculated on the total amount of monomer used. With increasing crosslinker content, the centrifuge retention capacity (CRC) and the absorption at a pressure of 21.0 g / cm³ decrease. 2 (AUL0.3psi) passes through a maximum.

[0033] As initiators (c), any compound that generates radicals under the polymerization conditions can be used, for example, thermal initiators, redox initiators, and photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Preferably, mixtures of thermal and redox initiators are used, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. The disodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfonatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, and sodium bisulfite is preferably used as the reducing component. Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0034] The water content of the monomer solution M is preferably 40 to 75 wt.%, particularly preferably 45 to 70 wt.%, and most preferably 50 to 65 wt.%. With increasing water content, the energy required for subsequent drying increases, and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.

[0035] The temperature of the monomer solution M is preferably from 10 to 90°C, particularly preferably from 20 to 70°C, and most preferably from 30 to 50°C.

[0036] The preferred polymerization inhibitors require dissolved oxygen for optimal effectiveness. Therefore, the monomer solution can be purified of dissolved oxygen prior to polymerization by inerting, i.e., by passing an inert gas, preferably nitrogen or carbon dioxide, through it. Preferably, the oxygen content of the monomer solution is reduced to less than 1 ppm by weight prior to polymerization, particularly preferably to less than 0.5 ppm by weight, and most preferably to less than 0.1 ppm by weight.

[0037] Suitable reactors for polymerization include kneading reactors and belt reactors. In a kneading reactor, the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating agitator shafts, as described in WO 2001 / 038402 A1. Polymerization on a belt is described, for example, in DE 38 25 366 A1 and US 6,241,928. Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example, in an extruder or kneader. 240922

[0038] 5

[0039] To improve the drying properties, the crushed polymer gel obtained using a kneader can be additionally extruded.

[0040] The polymer gel is then typically dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10 wt.%, particularly preferably 1 to 7 wt.%, and most preferably 2 to 5 wt.%, the residual moisture content being determined according to the EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". If the residual moisture content is too high, the dried polymer gel will have a glass transition temperature T that is too low. gThe dried polymer gel is brittle and difficult to process further. If the residual moisture content is too low, the dried polymer gel becomes too brittle, and subsequent comminution steps result in undesirably large quantities of polymer particles with an excessively small particle size ("fines"). The solids content of the polymer gel before drying is preferably between 25 and 90 wt.%, particularly preferably between 35 and 70 wt.%, and most preferably between 40 and 60 wt.%. The dried polymer gel is then broken up and optionally coarsely comminuted.

[0041] The dried polymer gel is then usually ground and classified, whereby single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills can usually be used for grinding.

[0042] The mean particle size of the polymer particles separated as the product fraction is preferably from 150 to 850 pim, particularly preferably from 250 to 600 pim, and most preferably from 300 to 500 pim. The mean particle size of the product fraction can be determined using the EDANA recommended test method No. WSP 220.2 (05) "Particle Size Distribution", wherein the mass fractions of the sieve fractions are plotted cumulatively and the mean particle size is determined graphically. The mean particle size is the mesh size value obtained for a cumulative 50 wt%.

[0043] The polymer particles can be thermally surface-crosslinked to further improve their properties. Suitable surface crosslinkers are compounds containing groups that can form covalent bonds with at least two carboxylate groups of the polymer particles. Suitable compounds include, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04938 A1 and US 6,239,230.

[0044] The amount of surface crosslinking agent is preferably 0.001 to 2 wt.%, particularly preferably 0.01 to 1 wt.%, and most preferably 0.03 to 0.7 wt.%, in each case based on the polymer particles. 240922

[0045] 6

[0046] In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface crosslinking agents.

[0047] The polyvalent cations that can be used in the process according to the invention are, for example, divalent cations such as zinc, magnesium, calcium, and strontium; trivalent cations such as aluminum, iron, chromium, rare earth elements, and manganese; and tetravalent cations such as titanium and zirconium. Possible counterions include chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate, and carboxylates such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0048] The amount of polyvalent cation used is, for example, 0.001 to 1.5 wt.%, preferably 0.005 to 1 wt.%, particularly preferably 0.02 to 0.8 wt.%, in each case based on the polymer.

[0049] Surface recrosslinking is typically carried out by spraying a solution of the surface recrosslinker onto the dried polymer particles. Following spraying, the polymer particles coated with surface recrosslinker are thermally treated.

[0050] Spraying a solution of the surface re-curing agent is preferably carried out in mixers with moving mixing tools, such as screw mixers, disc mixers, and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are especially preferred. The distinction between horizontal and vertical mixers is made by the orientation of the mixing shaft; that is, horizontal mixers have a horizontally mounted mixing shaft, and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, the Horizontal Ploughshare® Mixer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), the Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands), the Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA), and the Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands). However, it is also possible to spray the surface re-curing agent solution in a fluidized bed.

[0051] Surface crosslinking agents are typically used as aqueous solutions. The penetration depth of the surface crosslinking agent into the polymer particles can be adjusted by varying the content of non-aqueous solvent or the total amount of solvent.

[0052] The thermal treatment is preferably carried out in contact dryers, particularly preferably paddle dryers, and most preferably disc dryers. Suitable dryers include, for example, the Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA), and the Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used. 240922

[0053] 7

[0054] Surface post-crosslinking can occur within the mixer itself, by heating the jacket or blowing in warm air. A downstream dryer, such as a tray dryer, a rotary kiln, or a heated screw dryer, is equally suitable. Mixing and thermal surface post-crosslinking in a fluidized bed dryer is particularly advantageous.

[0055] Preferred reaction temperatures are in the range of 100 to 250°C, preferably 110 to 220°C, particularly preferably 120 to 210°C, and most preferably 130 to 200°C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, most preferably at least 30 minutes, and usually at most 60 minutes.

[0056] Subsequently, the surface-crosslinked polymer particles can be reclassified, whereby polymer particles that are too small and / or too large are separated and recycled back into the process.

[0057] The surface-crosslinked polymer particles can be coated or re-moistened to further improve their properties.

[0058] Post-humidification is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, and most preferably at 40 to 60°C. At excessively low temperatures, the polymer particles tend to clump together, and at higher temperatures, a significant amount of water evaporates. The amount of water used for post-humidification is preferably 1 to 10 wt.%, particularly preferably 2 to 8 wt.%, and most preferably 3 to 5 wt.%. Post-humidification increases the mechanical stability of the polymer particles and reduces their tendency to accumulate static electricity. Advantageously, post-humidification is carried out in the cooler after thermal surface crosslinking.

[0059] Suitable coatings for improving swelling rate and gel bed permeability (GBP) include, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. Suitable coatings for dust binding include, for example, polyols. Suitable coatings to prevent the undesirable tendency of polymer particles to clump together include, for example, fumed silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20.

[0060] Methods:

[0061] Unless otherwise specified, measurements should be carried out at an ambient temperature of 23 ± 2°C and a relative humidity of 50 ± 10%. The superabsorbent particles should be thoroughly mixed before measurement. 240922

[0062] 8

[0063] Colour value (CI E colour numbers [L, a, bl])

[0064] The color value is measured using a colorimeter model "LabScan XE Spectrometer" (HunterLab; Reston; USA) according to the CIELAB method (Hunterlab, Volume 8, 1996, Issue 7, pages 1 to 4). Colors are described by the coordinates L, a, and b of a three-dimensional system. L characterizes the brightness, where L = 0 is black and L = 100 is white. The values ​​for a and b describe the position of the color on the red / green and yellow / blue color axes, respectively, where positive a values ​​represent red colors, negative a values ​​represent green colors, positive b values ​​represent yellow colors, and negative b values ​​represent blue colors.

[0065] The Hunter 60 value (HC60) is a measure of the whiteness of surfaces and is defined as L-3b, meaning the lower the value, the darker and yellower the color.

[0066] The test was performed using a tissue culture dish (diameter of 35 mm and height of 10 mm) and a port plate opening of 0.5 inches.

[0067] The color value is measured in accordance with the tristimulus method according to DIN 5033-6.

[0068] Examples

[0069] Example 1

[0070] 150 g of a 25 wt% sodium hydroxide solution were stored together with a material sample for 5 weeks at 50°C under argon. The material sample was made of polypropylene and had dimensions of 50 mm x 20 mm x 1 mm.

[0071] The sodium hydroxide solution stored in this manner was used to produce superabsorbents. For this purpose, 88.00 g of 25 wt% sodium hydroxide solution and 24.73 g of ice (made from ultrapure water) were placed in a 250 ml polypropylene beaker and neutralized with 39.63 g of acrylic acid (stabilized with 0.02 wt% hydroquinone monomethyl ether) while cooling, ensuring that the temperature did not exceed 30°C. Subsequently, 0.223 g of triple ethoxylated glyceryl triacrylate (Laromer® PO 9044V, BASF SE, Ludwigshafen, Germany) was added to the previously prepared, neutralized solution along with a further 16.19 g of acrylic acid, acting as a crosslinking agent. The resulting monomer solution was then inerted for 30 minutes using a glass frit with a nitrogen flow rate of 150 l / h. The degree of neutralization was 71%, and the solids content was 40 wt%.

[0072] To initiate radical polymerization, 0.502 g of a 10 wt% aqueous sodium peroxodisulfate solution, 0.112 g of a 1 wt% hydrogen peroxide solution, and 0.279 g of a 1 wt% ascorbic acid solution were added successively. 240922

[0073] 9

[0074] The resulting polymer gel was ground up (using a meat grinder with a 6mm perforated disc; KitchenAid Professional Europe Inc., Belgium, type 5KSM7990X). The ground polymer gel was then spread evenly on a drying tray (approx. 60g of gel per tray with a surface area of ​​1.350 cm²). 2The polymer was distributed and dried for 60 minutes at 170°C in a circulating air drying oven. It was then milled using a roller mill (Bauermeister Zerkleinerungstechnik GmbH, Norderstedt, Germany, type LRC 125 / 70). This was carried out in a three-stage milling process with gap widths of 1,000 µm, 600 µm, and 400 µm. The resulting polymer was then sieved to a particle size of 150 to 850 µm using a vibrating screen (type Retsch AS 200, screen diameter 200 mm, 1.45 mm amplitude for 10 min).

[0075] The color values ​​L, a, b and HC60 were determined for these polymer particles. The results are summarized in Table 1.

[0076] Example 2

[0077] 150 g of a 25 wt% sodium hydroxide solution were stored together with a material sample for 5 weeks at 50°C under argon. The material sample was made of polytetrafluoroethylene and had dimensions of 50 mm x 20 mm x 1 mm. The stored sodium hydroxide solution was used for the production of superabsorbents.

[0078] The superabsorbents were produced analogously to Example 1.

[0079] Example 3 (not according to the invention)

[0080] 150 g of a 25 wt% sodium hydroxide solution were stored together with a material sample for 5 weeks at 50°C under argon. The material sample was made of martensitic steel with the material number 1.4021 according to DIN EN 10088-3 (12.0 to 14.0 wt% chromium and 0.16 to 0.25 wt% carbon) and had dimensions of 50 mm x 20 mm x 1 mm. The stored sodium hydroxide solution was used for the production of superabsorbents.

[0081] The superabsorbents were produced analogously to Example 1.

[0082] Example 4 (not according to the invention)

[0083] 150 g of a 25 wt% sodium hydroxide solution were stored together with a material sample for 5 weeks at 50°C under argon. The material sample was made of ferritic stainless steel with the material number 1.4104 according to DIN EN 10088-3 (15.5 to 17.5 wt% chromium, 0.2 to 0.6 wt% molybdenum, 0.15 to 0.35 wt% sulfur and 0.10 to 0.17 wt% carbon) and had dimensions of 50 mm x 20 mm x 1 mm. The stored sodium hydroxide solution was used for the production of superabsorbents.

[0084] The superabsorbents were produced analogously to Example 1. 240922

[0085] 10

[0086] Example 5 (not according to the invention)

[0087] 150 g of a 25 wt% sodium hydroxide solution were stored together with a material sample for 5 weeks at 50°C under argon. The material sample was made of ferritic steel with the material number 1.0425 according to DIN EN 10088-3 (0.8 to 1.4 wt% manganese and at least 0.2 wt% aluminum) and had dimensions of 50 mm x 20 mm x 1 mm. The stored sodium hydroxide solution was used for the production of superabsorbents.

[0088] The superabsorbents were produced analogously to Example 1.

[0089] Table 1: Measurement of color values *) not according to the invention

Claims

240922 11 Patent claims 1. A process for producing superabsorbent particles by polymerizing an aqueous monomer solution or suspension, comprising a) at least one ethylene-unsaturated, acid-group-bearing monomer that is at least partially neutralized, b) at least one crosslinker, and c) at least one initiator, in which the aqueous monomer solution is polymerized to a polymer gel, the polymer gel is optionally extruded, the polymer gel is dried on a circulating air belt dryer, and the dried polymer gel is comminuted, classified, and optionally thermally surface-crosslinked, characterized in that the ethylene-unsaturated, acid-group-bearing monomer a) is at least partially neutralized with sodium hydroxide solution, the sodium hydroxide solution is stored in a container immediately before neutralization, the container having a volume of 1 to 10,000 m³ 3has a mean residence time of the sodium hydroxide solution in the container of 0.1 to 750 h, the inner surface of the container is made of organic polymer and the superabsorbent particles are not coated with a condensed and / or hydrolyzable tannin.

2. Method according to claim 1, characterized in that the container has a volume of 10 to 1,000 m³ 3 has.

3. Method according to claim 1 or 2, characterized in that the average residence time of the sodium hydroxide solution in the container is from 0.5 to 150 h.

4. Method according to one of claims 1 to 3, characterized in that the sodium hydroxide solution has a content of 10 to 60 wt.%.

5. Method according to one of claims 1 to 3, characterized in that the sodium hydroxide solution has a content of 20 to 50 wt.%.

6. Method according to one of claims 1 to 5, characterized in that the sodium hydroxide solution has a temperature of 25 to 75°C.

7. Method according to one of claims 1 to 5, characterized in that the sodium hydroxide solution has a temperature of 35 to 45°C.

8. Method according to any one of claims 1 to 7, characterized in that the sodium hydroxide solution contains at most 5 wt.% sodium chloride. 240922 12 9. Method according to any one of claims 1 to 7, characterized in that the sodium hydroxide solution contains at most 0.05 wt.% sodium chloride.

10. Method according to any one of claims 1 to 9, characterized in that the organic polymer is a polyolefin, a polyester, a polyamide, polytetrafluoroethylene, polyvinyl chloride, an epoxy resin or a silicone resin.

11. Method according to any one of claims 1 to 10, characterized in that the container is made of steel and the inner surface of the container is coated with the organic polymer.

12. Method according to one of claims 1 to 11, characterized in that an ethylene-unsaturated carboxylic acid is used as the ethylene-unsaturated, acid-group-bearing monomer.

13. Method according to one of claims 1 to 11, characterized in that acrylic acid is used as an ethylene-unsaturated, acid-group-bearing monomer.

14. Method according to any one of claims 1 to 13, characterized in that the ethylene unsaturated, acid-group-bearing monomer is neutralized to 40 to 85 mol%.

15. Method according to any one of claims 1 to 13, characterized in that the ethylene unsaturated, acid-group-bearing monomer is neutralized to 60 to 75 mol%.

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