Method for preparing super absorbent

Abstract

The invention relates to a method for producing a superabsorbent, in which a monomer solution M is cooled by means of a heat exchanger W, the neutralization process is interrupted in order to clean the heat exchanger W, and the heat exchanger W is emptied, filled with water or an aqueous solution and dried.

Description

[0001] The invention relates to a method for producing a superabsorbent, wherein a monomer solution M is cooled by means of a heat exchanger W, the neutralization process is interrupted to clean the heat exchanger W, and the heat exchanger W is drained, filled with water or an aqueous solution and blown dry.

[0002] Superabsorbents are used in the manufacture of diapers, tampons, sanitary napkins, and other hygiene products, but are also used as water-retaining agents in agriculture and horticulture. Superabsorbents are also known as water-absorbing polymers.

[0003] The preparation of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pp. 71-103.

[0004] To improve application properties such as Gel Bed Permeability (GBP) and 2 The absorption rate under pressure (AUL0.7psi) of the superabsorbent particles is usually post-crosslinked. This increases the degree of crosslinking on the particle surface, so that the absorption rate under pressure of 49.2g / cm 2 The absorbency under pressure (AUL 0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface postcrosslinking can be carried out in the aqueous gel phase. However, preferably, the surface postcrosslinker is applied to the surface of the dried, ground, and sieved polymer particles (base polymer) and then subjected to thermal postcrosslinking. Suitable crosslinkers for this purpose are compounds that can form covalent bonds with at least two carboxyl groups of the polymer particles.

[0005] WO 2007 / 028751 A1 relates to a neutralization method.

[0006] The object of the present invention is to provide an improved process for producing superabsorbent particles, in particular which makes it easier to clean the heat exchangers used.

[0007] The solution to the above-mentioned objects according to the invention is a process for producing a superabsorbent, wherein at least one ethylenically unsaturated monomer having acid groups is at least partially neutralized with an aqueous base, the resulting aqueous monomer solution M is cooled by means of a heat exchanger W, at least one crosslinker and at least one initiator are added to the aqueous monomer solution M, the aqueous monomer solution M is subsequently polymerized to form a polymer gel, the polymer gel is optionally extruded, the polymer gel is dried, and the dried polymer gel is comminuted, classified and optionally subjected to thermal surface postcrosslinking, characterized in that the neutralization process is interrupted for cleaning the heat exchanger W, the heat exchanger W is emptied, filled with water or an aqueous solution and blown dry.

[0008] Filling the heat exchanger W with water or an aqueous solution and blowing it dry is preferably performed at least twice, for example two, three or four times. Preferably, the water or aqueous solution is substantially free of polyvalent metal ions, in particular substantially free of Ca 2+ ions and / or Mg 2+ ion.

[0009] In a preferred embodiment, the heat exchanger W is additionally filled with an alkaline solution, heated, and drained. Filling the heat exchanger W with an alkaline solution, heating, and draining the heat exchanger W is preferably performed at least twice, for example, two, three, or four times. The heat exchanger W may have been previously drained, filled with water or an aqueous solution, and blown dry. Filling the heat exchanger W with water or an aqueous solution and blowing dry the heat exchanger W is preferably performed at least twice, for example, two, three, or four times.

[0010] The pH value of the alkaline solution is preferably greater than 10, particularly preferably greater than 12, and further particularly preferably greater than 14. A suitable alkaline solution is, for example, sodium hydroxide solution. The content of sodium hydroxide in the sodium hydroxide solution is preferably at least 10 wt %, particularly preferably at least 25 wt %, and further particularly preferably at least 40 wt %.

[0011] The alkaline solution is heated in the heat exchanger W to a temperature of preferably at least 30° C., particularly preferably at least 45° C., further particularly preferably at least 60° C. The alkaline solution is heated in the heat exchanger W for preferably at least 5 minutes, particularly preferably at least 15 minutes, further particularly preferably at least 25 minutes.

[0012] Figure 1 An example of a neutralization method is shown. The reference numerals have the following meanings:

[0013] W Heat Exchanger

[0014] R Ring pipe

[0015] P1 Pump in ring line

[0016] P2 Pump to polymerization reactor

[0017] B (buffer) container

[0018] Z1 delivery pipeline

[0019] Z2 delivery pipeline

[0020] Z3 delivery pipeline

[0021] The invention is based on the recognition that the heat exchanger W can be cleaned simply and generally without interrupting the polymerization process. Complex mechanical cleaning can be dispensed with.

[0022] The heat exchanger W used according to the present invention is an indirect heat exchanger, also known as a heat recovery device. Examples include plate heat exchangers, tube bundle heat exchangers, double-tube heat exchangers, and hybrids thereof. The heat exchanger W used is not limited. A preferred heat exchanger W according to the present invention is a plate heat exchanger. Plate heat exchangers consist of parallel plates, with the interstices alternately occupied by one medium and another. A spiral heat exchanger is a special form of plate heat exchanger in which spirally wound metal plates are used instead of flat plates.

[0023] The monomer solution M and the cooling medium can be conducted in countercurrent, cocurrent, crosscurrent, or cross-countercurrent. Countercurrent heat exchangers are preferred according to the present invention. In countercurrent heat exchangers, the substances are conducted so that they flow past each other in opposite directions. Ideally, the temperatures of the substance flows are exchanged, i.e., the originally cold medium reaches the temperature of the originally hot medium, and vice versa. However, in practice, a complete exchange of temperatures is not possible.

[0024] The preparation of superabsorbent is described in detail below.

[0025] Superabsorbents are prepared by polymerizing a monomer solution and are generally insoluble in water.

[0026] The ethylenically unsaturated monomer carrying an acid group is preferably water-soluble, i.e., its solubility in water at 23° C. is generally at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, particularly preferably at least 25 g / 100 g of water, and even more particularly preferably at least 35 g / 100 g of water.

[0027] Suitable monomers are, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is further particularly preferred.

[0028] Ethylenically unsaturated monomers carrying acid groups are usually partially neutralized. Neutralization is carried out at the monomer stage. This is usually achieved by mixing in a neutralizing agent in the form of an aqueous solution or preferably a solid. The degree of neutralization is preferably 40 to 85 mol %, particularly preferably 50 to 80 mol %, and even more preferably 60 to 75 mol %. Conventional neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates, and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Particularly preferred alkali metals are sodium and potassium, but sodium hydroxide, sodium carbonate, or sodium bicarbonate, and mixtures thereof, in particular sodium hydroxide, are particularly preferred.

[0029] The monomers generally contain a polymerization inhibitor, preferably a hydroquinone half ether which acts as a storage stabilizer.

[0030] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Examples of such groups include ethylenically unsaturated groups that can be free-radical polymerized into the polymer chain, and functional groups that can form covalent bonds with the acid groups of the monomers. In addition, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomers are also suitable as crosslinking agents.

[0031] Suitable crosslinkers are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 547 847 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 103 31450 A1, diacrylates and triacrylates as described in DE 103 31 456 A1 and DE 103 Mixed acrylates which contain ethylenically unsaturated groups in addition to acrylate groups, as described in 55401 A1, or crosslinker 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 the crosslinking agent is preferably 0.05 wt % to 1.5 wt %, particularly preferably 0.1 wt % to 1 wt %, and further particularly preferably 0.15 wt % to 0.6 wt %, based on the total amount of the monomers used. As the crosslinking agent content increases, the centrifuge retention capacity (CRC) decreases, and at 21.0 g / cm 2 The absorption rate under pressure (AUL0.3psi) passes through a maximum value.

[0033] All compounds that generate free radicals under polymerization conditions can be used as initiators, for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. Preference is given to using mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. Preferably, the disodium salt of 2-hydroxy-2-sulfinatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfinatoacetic acid and sodium bisulfite is used as the reducing component. Commercially available mixtures of this type are FF6 and FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0034] The water content of the monomer solution M is preferably 40 wt% to 75 wt%, particularly preferably 45 wt% to 70 wt%, and even more particularly preferably 50 wt% to 65 wt%. As the water content increases, the energy consumption in the subsequent drying process increases. When the water content decreases, the polymerization heat may not be fully dissipated.

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

[0036] To achieve optimal results, the preferred polymerization inhibitor requires dissolved oxygen. Therefore, prior to polymerization, the monomer solution can be purged of dissolved oxygen by inerting, i.e., by passing an inert gas, preferably nitrogen or carbon dioxide, through the monomer solution. Preferably, the oxygen content of the monomer solution is reduced to less than 1 wppm, particularly preferably to less than 0.5 wppm, and even more particularly to less than 0.1 wppm, prior to polymerization.

[0037] Suitable reactors for polymerization are, for example, kneading reactors or belt reactors. In kneaders, the polymer gel produced during the polymerization of an aqueous monomer solution or suspension is continuously comminuted, for example, by counter-rotating agitator shafts, as described in WO 2001 / 038402 A1. Polymerization in belt reactors is described, for example, in DE 38 25 366 A1 and US Pat. No. 6,241,928. Polymerization in belt reactors produces polymer gels that must be comminuted, for example, in an extruder or kneader.

[0038] To improve the drying properties, the comminuted polymer gel obtained by the kneader can additionally be extruded.

[0039] Subsequently, the polymer gel is usually dried using a belt dryer with circulating air to a moisture content of preferably 0.5 to 10 wt %, particularly preferably 1 to 7 wt %, and even more particularly preferably 2 to 5 wt %, wherein the residual moisture content is determined according to the test method No. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA. In the case of excessively high residual moisture, the dried polymer gel has an excessively low glass transition temperature T g, and further processing becomes difficult. If the residual moisture is too low, the dried polymer gel becomes too brittle, and an undesirably large amount of polymer particles with too small a particle size ("fines") is produced in the subsequent comminution step. The solids content of the polymer gel before drying is preferably 25 to 90 wt%, particularly preferably 35 to 70 wt%, and even more particularly preferably 40 to 60 wt%. The dried polymer gel is then broken up and optionally coarsely comminuted.

[0040] The dried polymer gel is then usually ground and classified, wherein grinding can usually be carried out using single-stage or multi-stage roller mills (preferably two-stage or three-stage roller mills), pin mills, hammer mills or vibration mills.

[0041] The average particle size of the polymer particles separated as the product fraction is preferably at least 150 μm to 850 μm, particularly preferably 250 μm to 600 μm, and even more particularly preferably 300 μm to 500 μm. The average particle size of the product fraction can be determined using the test method No. WSP 220.2 (05) "Partikel Size Distribution" recommended by EDANA, in which the sieved mass fractions are plotted cumulatively and the average particle size is determined graphically. The average particle size is the mesh width corresponding to 50% by weight of the cumulative mass.

[0042] To further improve the properties, the polymer particles can be subjected to thermal surface postcrosslinking. Suitable surface postcrosslinkers are compounds containing groups capable of forming covalent bonds with at least two carboxyl groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional aminoamines, polyfunctional epoxides as described in EP 0 083 022 A2, EP 0 543 303 A1, and EP 0 937 736 A2, difunctional or polyfunctional alcohols as described in DE 33 14 019 A1, DE 35 23 617 A1, and EP 0 450 922 A2, or β-hydroxyalkylamides as described in DE 102 04 938 A1 and US Pat. No. 6,239,230.

[0043] The amount of surface postcrosslinker, based on the polymer particles, is preferably 0.001 to 2% by weight, particularly preferably 0.01 to 1% by weight, further particularly preferably 0.03 to 0.7% by weight.

[0044] In a preferred embodiment of the present invention, in addition to the surface postcrosslinkers, polyvalent cations are additionally applied to the particle surface.

[0045] Polyvalent cations that can be used in the method of the present invention include, for example, divalent cations such as cations of zinc, magnesium, calcium, and strontium, trivalent cations such as cations of aluminum, iron, chromium, rare earth elements, and manganese, and tetravalent cations such as cations of titanium and zirconium. Possible counterions include chlorides, bromides, hydroxides, sulfates, hydrogensulfates, carbonates, hydrogencarbonates, nitrates, phosphates, hydrogenphosphates, dihydrogenphosphates, and carboxylates such as acetates and lactates. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferably used.

[0046] The polyvalent cation is used in an amount of, for example, 0.001 to 1.5 wt %, preferably 0.005 to 1 wt %, particularly preferably 0.02 to 0.8 wt %, based on the polymer.

[0047] The surface postcrosslinking is usually carried out by spraying a solution of the surface postcrosslinker onto the dried polymer particles. After spraying, the polymer particles coated with the surface postcrosslinker are heat treated.

[0048] The solution of the surface postcrosslinker is preferably sprayed in a mixer with a moving stirring tool, such as a screw mixer, a pan mixer, or a paddle mixer. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are even more preferred. Horizontal mixers differ from vertical mixers in the way the stirring shaft is supported: horizontal mixers have a horizontally supported stirring shaft, while vertical mixers have a vertically supported stirring shaft. Suitable mixers are, for example, the Horizontal mixer. Mischer (Gebr. Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; The Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA), and Schugi (Hosokawa Micron BV; Doetinchem; The Netherlands). However, it is also possible to spray on the surface postcrosslinker solution in a fluidized bed.

[0049] The surface postcrosslinker is usually used as an aqueous solution. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by adjusting the content of the non-aqueous solvent or the total amount of solvent.

[0050] The heat treatment is preferably carried out in a contact dryer, particularly preferably in a paddle dryer, and further preferably in a tray dryer. Suitable dryers are, for example, Hosokawa Bepex Horizontal Paddle Dryer (Hosokawa Micron GmbH; Rheingarten; Germany), Hosokawa Disc Dryer (HosokawaMicron GmbH; Rheingarten; Germany), dryers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used.

[0051] Surface postcrosslinking can be carried out in the mixer itself by heating the outer casing or injecting hot air. Downstream dryers such as rack dryers, rotary kilns, or heated screws are also suitable. Mixing and thermal surface postcrosslinking are particularly advantageously carried out in a fluidized bed dryer.

[0052] Preferred reaction temperatures are in the range of 100° C. to 250° C., preferably 110° C. to 220° C., particularly preferably 120° C. to 210° C., further particularly preferably 130° C. to 200° C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, further particularly preferably at least 30 minutes, and generally at most 60 minutes.

[0053] The surface postcrosslinked polymer particles can subsequently be reclassified, with oversized and / or oversized polymer particles being separated off and fed back to the process.

[0054] To further improve the properties, the surface postcrosslinked polymer particles can be coated or remoistened.

[0055] Remoistening is preferably carried out at 30°C to 80°C, particularly preferably 35°C to 70°C, and even more preferably 40°C to 60°C. At temperatures too low, the polymer particles tend to agglomerate, while at higher temperatures, water evaporates significantly. The amount of water used for remoistening is preferably 1% to 10% by weight, particularly preferably 2% to 8% by weight, and even more particularly preferably 3% to 5% by weight. Remoistening enhances the mechanical stability of the polymer particles and reduces their tendency to electrostatic charge. Remoistening is preferably carried out in a cooler after hot surface postcrosslinking.

[0056] Suitable coatings for improving the swelling rate and the gel bed permeability (GBP) are, for example, inorganic inert substances (such as water-insoluble metal salts), organic polymers, cationic polymers and divalent or polyvalent metal cations. Suitable coatings for binding dust are, for example, polyols. Suitable coatings for counteracting the unfavorable agglomeration tendency of polymer particles are, for example, fumed silica (such as 200), precipitated silica (such as D17) and surfactants (e.g. 20). Example

[0057] Example 1 (according to the present invention )

[0058] A monomer solution M was prepared by continuously mixing deionized water, 50 wt% sodium hydroxide solution and acrylic acid (see Figure 1 ) so that the degree of neutralization is equal to 72.0 mol%. The water content of the monomer solution M is 57.0 wt%.

[0059] In loop line R, partially neutralized acrylic acid was circulated by pump P1 through heat exchanger W and container B. Water was added via delivery line Z1, sodium hydroxide solution via delivery line Z3, and acrylic acid via delivery line Z2. Pump P2 delivered monomer solution M to the polymerization reactor.

[0060] As heat exchanger W, a 178m 2 Plate heat exchanger.

[0061] Triethoxylated glycerol triacrylate (about 85 wt%) was used as a crosslinking agent. The amount used was 0.95 kg per ton of monomer solution M. 2.64 kg of polyethylene glycol (having an average molecular weight of 4,000 g / mol) and 9.69 kg of 1-hydroxyethylidene-1,1'-diphosphonic acid disodium salt ( K9012GR), each calculated as M per ton of monomer solution.

[0062] To initiate free radical polymerization, 1.13 kg of a 0.25 wt% aqueous hydrogen peroxide solution, 4.70 kg of a 15 wt% aqueous sodium peroxodisulfate solution and 1.06 kg of a 1 wt% aqueous ascorbic acid solution were added per ton of monomer solution M.

[0063] The monomer solution M is added to a List Controller with a 6.3m 3 The reactor was a 1.5-ton reactor (LISTAG, Aristof, Switzerland). The throughput of the monomer solution M was about 22 t / h. The temperature of the reaction solution at the inlet was 23.5°C.

[0064] Between the addition point of the crosslinking agent and the addition point of the hydrogen peroxide and sodium peroxodisulfate solution, the monomer solution M was inerted with nitrogen. Ascorbic acid was added directly to the reactor.

[0065] After about 50% of the residence time, about 1,000 kg / h of polymer particles with a particle size of less than 150 μm, resulting from comminution and classification during the production process, were additionally fed into the reactor. The residence time of the reaction mixture in the reactor was about 15 minutes.

[0066] The resulting polymer gel was conveyed via a vibrating conveyor belt to the conveyor belt of a circulating air belt dryer. The circulating air belt dryer was 48 m long and had an effective width of 4.4 m. The aqueous polymer gel was continuously circulated and dried in the circulating air belt dryer using an air / gas mixture.

[0067] The dried polymer gel was comminuted using a three-stage roller mill and sieved to a particle size of 150 to 700 μm. Polymer particles with a particle size of less than 150 μm were separated. Polymer particles with a particle size of greater than 700 μm were returned for comminution. The polymer particles with a particle size range of 150 to 700 μm were subjected to thermal surface postcrosslinking.

[0068] In Schugi The polymer particles were coated with a surface postcrosslinker solution in a NARA Micron (Hosokawa Micron BV, Doetinchem, The Netherlands) and subsequently dried in a NARA Paddle Dryer (GMF Gouda, Wadinksveen, The Netherlands) at 192.5° C. for 45 min.

[0069] Add the following amount of materials to Schugi

[0070] 9.5t / h polymer particles

[0071] 488.4 kg / h surface post-crosslinker solution

[0072] The surface post-crosslinker solution contained 1.36 wt% of 2-hydroxyethyl-2-oxazolidinone, 1.36 wt% of 1,3-propylene glycol, 4.28 wt% of aluminum lactate, 54.04 wt% of water, 0.05 wt% of sorbitan monolaurate, and 38.91 wt% of isopropyl alcohol.

[0073] After drying, the surface postcrosslinked polymer particles were cooled to about 60° C. in a NARA Paddle-Cooler (GMF Gouda, Wadinksveen, The Netherlands). 285 kg / h of water, 1.67 kg / h of a 50 wt % aqueous polyethylene glycol solution (polyethylene glycol with an average molecular weight of 400 g / mol), 23.75 kg / h of a 1 wt % aqueous sorbitan monolaurate solution and 9.5 kg / h of silica were used. Surface postcrosslinked polymer particles were coated.

[0074] The delivery rate of pump P1 is 300 t / h during normal operation. After a period of time, the delivery rate of pump P1 drops below 200 t / h due to contamination in plate heat exchanger W during the neutralization process.

[0075] During the polymerization, the inlet and outlet of the monomer solution were closed at the heat exchanger W. The heat exchanger W was emptied via a separate line with an internal diameter of approximately 7.5 cm. The heat exchanger W was vented via another line with an internal diameter of approximately 2.5 cm.

[0076] Subsequently, the heat exchanger W was filled with demineralized water over a period of 2 minutes. After approximately another 2 minutes, the demineralized water was blown out with compressed air. The process of filling with demineralized water and blowing dry was repeated two to four times.

[0077] Subsequently, the inlet and outlet for the monomer solution were reopened at the heat exchanger W. The time required was approximately 40 minutes in total.

[0078] In the case that the above cleaning did not significantly improve the situation, the polymerization was interrupted and the neutralization was drained. Subsequently, the inlet and outlet of the monomer solution of the heat exchanger W were closed.

[0079] Subsequently, the heat exchanger W was filled with demineralized water over a period of 2 minutes. After approximately another 2 minutes, the demineralized water was blown out using compressed air. The process of filling with demineralized water and then emptying was repeated three times.

[0080] Afterwards, the heat exchanger W was filled with 50 wt% sodium hydroxide solution over about 3 minutes, then heated to about 70° C., maintained for about 30 minutes, and emptied. The process of filling with sodium hydroxide solution, heating, and evacuating was repeated three times.

[0081] Subsequently, the heat exchanger W was filled with demineralized water over a period of 2 minutes. After approximately another 2 minutes, the demineralized water was blown out using compressed air. The process of filling with demineralized water and then emptying was repeated three times.

[0082] Neutralization and polymerization were then restarted, taking a total of about 6 hours.

[0083] Example 2 (not according to the present invention )

[0084] The operation was carried out as in Example 1. The heat exchanger W was disassembled and subjected to a laborious and time-consuming mechanical cleaning.

[0085] Neutralization and polymerization were then restarted, taking a total of about 12 hours.

Claims

1. A method for producing a superabsorbent, wherein at least one ethylenically unsaturated monomer having acid groups is at least partially neutralized with an aqueous base, the aqueous monomer solution M obtained is cooled by means of a heat exchanger W, at least one crosslinker and at least one initiator are added to the aqueous monomer solution M, the aqueous monomer solution M is subsequently polymerized to form a polymer gel, the polymer gel is optionally extruded, the polymer gel is dried, and the dried polymer gel is comminuted, classified and optionally subjected to thermal surface postcrosslinking, characterized in that To clean the heat exchanger W, the neutralization process is interrupted and the heat exchanger W is drained, filled with water or an aqueous solution and blown dry.

2. The method according to claim 1, characterized in that The heat exchanger W is filled with water or an aqueous solution and blown dry at least twice.

3. The method according to claim 1 or 2, characterized in that The heat exchanger W is additionally filled with alkaline solution, heated and emptied.

4. The method according to claim 3, characterized in that Filling the heat exchanger W with the alkaline solution, heating and emptying are performed at least twice.

5. The method according to claim 3 or 4, characterized in that Sodium hydroxide solution was used as the alkaline solution.

6. The method according to claim 5, characterized in that The sodium hydroxide solution has a content of at least 40 wt%.

7. The method according to any one of claims 3 to 6, characterized in that The alkaline solution is heated in the heat exchanger W to at least 60°C.

8. The method according to any one of claims 3 to 7, characterized in that The alkaline solution is heated in the heat exchanger W for at least 25 minutes.

9. The method according to any one of claims 1 to 8, characterized in that As the heat exchanger W, a plate heat exchanger is used.

10. The method according to any one of claims 1 to 9, characterized in that As the ethylenically unsaturated monomer having an acid group, an ethylenically unsaturated carboxylic acid is used.

11. The method according to any one of claims 1 to 10, characterized in that Acrylic acid was used as the ethylenically unsaturated monomer having an acid group.

12. The method according to any one of claims 1 to 11, characterized in that Alkali metal hydroxides, alkali metal oxides, alkali metal hydrogencarbonates and / or alkali metal carbonates are used as bases.

13. The method according to any one of claims 1 to 12, characterized in that Sodium hydroxide was used as the base.

14. The method according to any one of claims 1 to 13, characterized in that The ethylenically unsaturated monomer having an acid group is neutralized to 60 mol % to 75 mol %.

15. The method according to any one of claims 1 to 14, characterized in that The water content of the monomer solution M is 50 wt % to 65 wt %.

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