Superabsorbent Manufacturing Process

Abstract

The present invention relates to a process for producing a superabsorbent, in which a monomer solution M is cooled by a heat exchanger W, the neutralization is interrupted to wash the heat exchanger W, and the heat exchanger W is emptied, filled with water or an aqueous solution and blown dry.

Description

[Technical Field]

[0001] The present invention relates to a process for producing a superabsorbent, in which a monomer solution M is cooled by a heat exchanger W, neutralization is stopped for cleaning of the heat exchanger W, and the heat exchanger W is emptied, filled with water or an aqueous solution and cleaned by blowing. [Background technology]

[0002] Superabsorbents are used to make diapers, tampons, sanitary napkins and other hygiene articles, but are also used as moisture-retaining agents in commercial horticulture. Superabsorbents are also called water-absorbing polymers.

[0003] The production of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pp. 71-103.

[0004] Performance characteristics, such as gel bed permeability (GBP) and 2 It is common to surface postcrosslink superabsorbent particles to improve their absorbency under a pressure of 49.2 g / cm (AUL 0.7 psi). This increases the level of crosslinking on the particle surface, resulting in a 2 The absorbency at a pressure of 0.7 psi (AUL) and the centrifuge retention capacity (CRC) can be at least partially separated. This surface postcrosslinking can be carried out in an aqueous gel phase. However, it is preferred to surface-coat dried, ground, and sieved polymer particles (base polymer) with a surface postcrosslinker and then thermally surface-postcrosslink them. Suitable crosslinkers for this purpose are compounds capable of forming covalent bonds with at least two carboxylate groups of the polymer particles.

[0005] WO 2007 / 028751 A1 relates to a neutralization process. Summary of the Invention [Problem to be solved by the invention]

[0006] It is an object of the present invention to provide an improved process for producing superabsorbent particles, in particular for easier cleaning of the heat exchangers used. [Means for solving the problem]

[0007] This object is achieved by a process for producing a superabsorbent by at least partially neutralizing at least one ethylenically unsaturated monomer having an acid group with an aqueous base solution, cooling the resulting aqueous monomer solution M through a heat exchanger W, adding at least one crosslinking agent and at least one reaction initiator to the aqueous monomer solution M, then polymerizing the aqueous monomer solution M into a polymer gel, optionally extruding the polymer gel, drying the polymer gel, grinding, classifying and optionally thermally surface postcrosslinking the dried polymer gel, and stopping the neutralization to clean the heat exchanger W, which is then emptied, filled with water or an aqueous solution and cleaned by blowing.

[0008] The heat exchanger W is preferably filled with water or an aqueous solution at least twice, for example, twice, three times, or four times, and washed by blowing. The water or aqueous solution is preferably essentially free of polyvalent metal ions, in particular Ca. 2+ ions and / or Mg 2+ Essentially free of ions.

[0009] In a preferred embodiment, the heat exchanger W is further filled with an alkaline solution, heated, and emptied. The heat exchanger W is preferably filled with an alkaline solution at least twice, for example, two, three, or four times, heated, and emptied. The heat exchanger W may be previously emptied, filled with water or an aqueous solution, and cleaned by blowing. Prior to this, the heat exchanger W is preferably filled with water or an aqueous solution at least twice, for example, two, three, or four times, and cleaned by blowing.

[0010] The pH of the alkaline solution is preferably greater than 10, more preferably greater than 12, and most preferably greater than 14. An example of a suitable alkaline solution is sodium hydroxide solution. The sodium hydroxide content in the sodium hydroxide solution is preferably at least 10% by weight, more preferably at least 25% by weight, and most preferably at least 40% by weight.

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

[0012] [Figure 1] An example of a neutralization process is shown. Reference symbols are defined as follows: W Heat exchanger R Ring conduit P1 Pump in ring conduit P2 Pump to polymerization reactor B (Buffer) vessel Z1 Feed Z2 Feed Z3 Feed DETAILED DESCRIPTION OF THE INVENTION

[0013] The invention is based on the finding that the heat exchanger W can be cleaned in a simple manner and in many cases without stopping the polymerization. It is possible to dispense with complex mechanical cleaning.

[0014] The heat exchanger W used in accordance with the present invention is an indirect heat exchanger, also called a recuperator. It may be a plate heat exchanger, a shell-and-tube heat exchanger, a jacketed tube heat exchanger, or a mixture thereof. The type of heat exchanger W used is not subject to any restrictions. A preferred heat exchanger W according to the present invention is a plate heat exchanger. A plate heat exchanger is composed of parallel plates, with the gaps alternately occupied by one medium and another. A spiral heat exchanger is a special form of a plate heat exchanger, in which spirally wound panels are used instead of two-dimensional plates.

[0015] The monomer solution M and the cooling medium may be conducted in countercurrent, parallel, countercurrent, or cross-countercurrent flow. According to the present invention, a countercurrent heat exchanger is preferred. In a countercurrent heat exchanger, the materials are guided through the heat exchanger so that they flow in opposite directions. In an ideal case, the temperatures of the material flows are exchanged, meaning that the originally cold medium reaches the temperature of the originally hot medium, and vice versa. However, in practice, a perfect exchange of temperatures is not possible.

[0016] The manufacture of superabsorbents is described in detail below.

[0017] Superabsorbents are made by polymerizing a monomer solution and are typically water-insoluble.

[0018] The ethylenically unsaturated monomers having acid groups are preferably water-soluble, i.e., their solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, more preferably at least 25 g / 100 g water, and most preferably at least 35 g / 100 g water.

[0019] 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. Very particularly preferred is acrylic acid.

[0020] Ethylenically unsaturated monomers having acid groups are typically partially neutralized. Neutralization is carried out at the monomer stage. Neutralization is typically carried out by mixing a neutralizing agent as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably 40 to 85 mol %, more preferably 50 to 80 mol %, and most preferably 60 to 75 mol %. Conventional neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates, or mixtures thereof, can be used. Instead of alkali metal salts, ammonium salts can also be used. Particularly preferred alkali metals are sodium and potassium, but sodium hydroxide, sodium carbonate, or sodium bicarbonate, or mixtures thereof, especially sodium hydroxide, are very particularly preferred.

[0021] The monomer typically contains a polymerization inhibitor, preferably a hydroquinone monoether, as a storage stabilizer.

[0022] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Such groups include, for example, a free radical polymerizable ethylenically unsaturated group in the polymer chain and a functional group that can form a covalent bond with the acid group of the monomer. In addition, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.

[0023] Suitable crosslinking agents include, 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, 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, diacrylates and triacrylates, as described in DE 03 / 104301 A1, WO 03 / 104301 A1, and DE 10331450 A1; mixed acrylates which contain further ethylenically unsaturated groups as well as acrylate groups, as described in DE 10331456 A1, DE 10355401 A1, or crosslinker mixtures, as described, for example, in DE 19543368 A1, DE 19646484 A1, WO 90 / 15830 A1, and WO 02 / 032962 A2.

[0024] The amount of crosslinker, in each case calculated based on the total amount of monomers used, is preferably 0.05% to 1.5% by weight, more preferably 0.1% to 1% by weight, and most preferably 0.15% to 0.6% by weight. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases, reaching 21.0 g / cm. 2 The absorbency exceeds the maximum value under pressure (AUL 0.3 psi).

[0025] The initiator used can be any compound that generates free radicals under polymerization conditions, such as a thermal initiator, a redox initiator, or a photoinitiator. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Mixtures of thermal and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. The reducing component used is preferably the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, and sodium bisulfite. Such mixtures are available as Brueggolite® FF6 and Brueggolite® FF7 (Brueggemann Chemicals; Heilbronn; Germany).

[0026] The water content of the monomer solution M is preferably 40% to 75% by weight, more preferably 45% to 70% by weight, and most preferably 50% to 65% by weight. If the water content is high, the energy consumption in the subsequent drying step will increase, and if the water content is low, the heat of polymerization may simply not be sufficiently removed.

[0027] The temperature of the monomer solution M is preferably 10 to 90°C, particularly preferably 20 to 70°C, very particularly preferably 30 to 50°C.

[0028] To function optimally, preferred polymerization inhibitors require dissolved oxygen. Therefore, the monomer solution may be inerted, i.e., flushed with an inert gas, preferably nitrogen or carbon dioxide, to remove dissolved oxygen prior to polymerization. Preferably, the oxygen content of the monomer solution is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, and most preferably less than 0.1 ppm by weight prior to polymerization.

[0029] Suitable reactors for polymerization are, for example, kneader reactors or belt reactors. In kneaders, the polymer gel formed during the polymerization of aqueous monomer solutions or suspensions is continuously comminuted, for example, by counter-rotating stirrer shafts, as described in WO 2001 / 038402 A1. Belt polymerization is described, for example, in DE 3825366 A1 and U.S. Pat. No. 6,241,928. Polymerization in a belt reactor forms a polymer gel, which must be comminuted, for example, in an extruder or kneader.

[0030] To improve drying performance, the pulverized polymer gel obtained by the kneader may be further extruded.

[0031] The polymer gel is then dried, typically by air-circulating belt dryer, until the residual moisture content is preferably 0.5% to 10% by weight, more preferably 1% to 7% by weight, and most preferably 2% to 5% by weight, as measured by EDANA recommended test method No. WSP230.2-05 "Mass Loss on Heating." If the residual moisture content is too high, the glass transition temperature T of the dried polymer gel may be increased. g If the residual moisture content is too low, the dried polymer gel may become too brittle, and the subsequent grinding step will result in an unnecessarily large amount of excessively small polymer particles ("fines"). The solids content of the polymer gel before drying is preferably 25% to 90% by weight, more preferably 35% to 70% by weight, and most preferably 40% to 60% by weight. The dried polymer gel is then ground, and optionally coarsely pulverized.

[0032] The dried polymer gel is then typically ground and classified, and the equipment used for grinding may typically be a single or multi-roll mill, preferably a two or three roll mill, a pin mill, a hammer mill or a vibratory mill.

[0033] The average particle size of the polymer particles collected as the product fraction is preferably 150 to 850 μm, more preferably 250 to 600 μm, and most preferably 300 to 500 μm. The average particle size of the product fraction can be measured by EDANA recommended test method No. WSP220.2(05) "Particle Size Distribution," in which the mass ratios of the selected fractions are plotted in cumulative form, and the average particle size is determined graphically. The average particle size in this specification refers to the mesh size resulting from a cumulative weight of 50%.

[0034] To further improve properties, polymer particles can be thermally surface crosslinked.Suitable surface crosslinking agents are compounds that contain a group that can form covalent bonds with at least two carboxylate groups of polymer particles.Suitable compounds are, for example, multifunctional amines, multifunctional amidoamines, multifunctional epoxides as described in EP 0083022A2, EP 0543303A1 and EP 0937736A2, difunctional or multifunctional alcohols as described in DE 3314019A1, DE 3523617A1 and EP 0450922A2, or β-hydroxyalkylamides as described in DE 10204938A1 and U.S. Patent No. 6,239,230.

[0035] The amount of the surface postcrosslinker is preferably 0.001% to 2% by weight, more preferably 0.01% to 1% by weight, and most preferably 0.03% to 0.7% by weight, in each case based on the polymer particles.

[0036] In a preferred embodiment of the present invention, in addition to the surface postcrosslinker, a multivalent cation is applied to the particle surface.

[0037] Polyvalent cations that can be used in the process of the present invention include, for example, divalent cations such as those of zinc, magnesium, calcium, and strontium; trivalent cations such as those of aluminum, iron, chromium, rare earths, and manganese; and tetravalent cations such as those of titanium and zirconium. Possible counterions are chloride, bromide, hydroxide, sulfate, hydrogensulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate, and carboxylates, such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0038] The amount of polyvalent cation used is, for example, 0.001% to 1.5% by weight, preferably 0.005% to 1% by weight, and more preferably 0.02% to 0.8% by weight, in each case based on the polymer.

[0039] Surface postcrosslinking is typically carried out by spraying a solution of the surface postcrosslinker onto dried polymer particles. After spray application, the polymer particles coated with the surface postcrosslinker are subjected to heat treatment.

[0040] The spray application of the solution of surface postcrosslinker is preferably carried out using a mixer with moving mixing implements, such as a screw mixer, a disk mixer, and a paddle mixer.Horizontal mixers, such as a paddle mixer, are particularly preferred, and vertical mixers are particularly preferred.Horizontal mixers and vertical mixers are distinguished by the installation 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 are, for example, the horizontal Pflugschar® plowshare mixer (Gebr. Loedige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixer (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands).However, it is also possible to spray the solution of surface postcrosslinker in a fluidized bed.

[0041] The surface post-crosslinking agent is typically used in the form of an aqueous solution. The penetration depth of the surface post-crosslinking agent into the polymer particles can be adjusted by the content of the non-aqueous solvent and the total amount of the solvent.

[0042] The heat treatment is preferably carried out in a contact dryer, more preferably a paddle dryer, and most preferably a disk dryer. Suitable dryers include, for example, the Hosokawa Bepex® horizontal paddle dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® disk dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Holo-Flite® dryer (Metso Minerals Industries Inc.; Danville; USA), and the Nara paddle dryer (NARA Machinery Europe; Frechen; Germany). Furthermore, a fluidized bed dryer can also be used.

[0043] Surface after crosslinking can be carried out in the mixer body by heating the jacket or blowing hot air.Also suitable is downstream dryer, such as tray dryer, rotary tube oven or heatable screw.It is particularly advantageous to mix in fluidized bed dryer and carry out thermal surface after crosslinking.

[0044] Preferred reaction temperatures are in the range of 100 to 250° C., preferably 110 to 220° C., more preferably 120 to 210° C., and most preferably 130 to 200° C. Preferred residence times at this temperature are preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and typically up to 60 minutes.

[0045] The surface postcrosslinked polymer particles can then be classified again to remove overly small and / or overly large polymer particles and recycle them into the process.

[0046] To further improve properties, the surface postcrosslinked polymer particles may be coated or rewetted.

[0047] Remoistening is preferably carried out at 30 to 80°C, more preferably 35 to 70°C, and most preferably 40 to 60°C. At excessively low temperatures, the polymer particles tend to form agglomerates, while at higher temperatures, the water evaporates prematurely to a noticeable extent. The amount of water used for remoistening is preferably 1 to 10% by weight, more preferably 2 to 8% by weight, and most preferably 3 to 5% by weight. Remoistening increases the mechanical stability of the polymer particles and reduces their tendency to become electrostatically charged. Remoistening is advantageously carried out in a cooler after thermal surface postcrosslinking.

[0048] Suitable coatings for improving swelling rate and gel bed permeability (GBP) include, for example, inorganic inert materials such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations. Suitable coatings for dust adsorption include, for example, polyols. Suitable coatings for combating the undesirable tendency of polymer particles to caking include, for example, fumed silica such as Aerosil® 200, precipitated silica such as Sipernat® D17, and surfactants such as Span® 20. [Example]

[0049] Example 1 (present invention) Monomer solution M was prepared by successively mixing deionized water, 50% by weight sodium hydroxide solution, and acrylic acid (see Figure 1) to a degree of neutralization corresponding to 72.0 mol %. The water content of monomer solution M was 57.0% by weight.

[0050] In the ring conduit R, the partially neutralized acrylic acid was circulated by means of pump P1 through heat exchanger W and vessel B. Water was metered in via feed Z1, sodium hydroxide solution was metered in via feed Z3, and acrylic acid was metered in via feed Z2. Pump P2 delivered the monomer solution M to the polymerization reactor.

[0051] The heat exchanger W used was 178m 2 The plate heat exchanger had an area of ​​10 ...

[0052] The crosslinking agent used was 3-ethoxylated glyceryl triacrylate (purity approximately 85% by weight). The amount used was 0.95 kg per 1 t of monomer solution M. In addition, 2.64 kg of polyethylene glycol (polyethylene glycol with an average molar mass of 4000 g / mol) and 9.69 kg of disodium 1-hydroxyethylidene-1,1'-diphosphonic acid (Cublen® K9012GR) were added to the monomer solution M in each case per 1 t of monomer solution M.

[0053] Free radical polymerization was initiated using 1.13 kg of a 0.25 wt % aqueous solution of hydrogen peroxide, 4.70 kg of a 15 wt % aqueous solution of sodium peroxodisulfate, and 1.06 g of a 1 wt % aqueous solution of ascorbic acid per 1 t of monomer solution M.

[0054] Monomer solution M was added to 6.3 ml 3 The monomer solution M was introduced into a List Contikneter continuous kneading reactor (LIST AG, Arisdorf, Switzerland) with a capacity of 1000 t / h. The throughput of the monomer solution M was approximately 22 t / h. The feed temperature of the reaction solution was 23.5°C.

[0055] Monomer solution M was inerted with nitrogen between the addition of the crosslinker and the addition of the hydrogen peroxide and sodium peroxodisulfate solutions. Ascorbic acid was metered directly into the reactor.

[0056] After about 50% of the residence time had elapsed, about 1000 kg / hour of polymer particles having a particle size of less than 150 μm, obtained by a manufacturing process by grinding and classification, were additionally metered into the reactor. The residence time of the reaction mixture in the reactor was about 15 minutes.

[0057] The resulting polymer gel was spread onto the conveyor belt of an air-circulating belt dryer by vibrating the conveyor belt. The air-circulating belt dryer had a length of 48 m. The conveyor belt of the air-circulating belt dryer had an effective width of 4.4 m. The aqueous polymer gel on the air-circulating belt dryer was subjected to a continuous flow of air / gas mixture and dried.

[0058] The dried polymer gel was crushed in a three-roll mill and sieved to a particle size of 150-700 μm. Polymer particles with a particle size of less than 150 μm were separated. Polymer particles with a particle size of more than 700 μm were recycled for crushing. Polymer particles with a particle size in the range of 150-700 μm were thermally surface post-crosslinked.

[0059] The polymer particles were coated with the surface postcrosslinker solution in Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried at 192.5°C for 45 min in a NARA paddle dryer (GMF Gouda, Waddinxveen, Netherlands).

[0060] The following amounts were weighed into a Schugi Flexomix®: 9.5t / hour of polymer particles 488.4 kg / hour of surface postcrosslinker solution

[0061] The surface postcrosslinker solution consisted of 1.36 wt. % 2-hydroxyethyl-2-oxazolidone, 1.36 wt. % propane-1,3-diol, 4.28 wt. % aluminum lactate, 54.04 wt. % water, 0.05 wt. % sorbitan monolaurate (Span® 20), and 38.91 wt. % isopropanol.

[0062] After drying, the surface postcrosslinked polymer particles were cooled to about 60° C. in a NARA paddle cooler (GMF Gouda, Waddinxveen, Netherlands), where the surface postcrosslinked polymer particles were coated with 285 kg / h of water, 1.67 kg / h of a 50 wt.% aqueous polyethylene glycol solution (polyethylene glycol with an average molar mass of 400 g / mol), 23.75 kg / h of a 1% aqueous sorbitan monolaurate solution, and 9.5 kg / h of silicon dioxide (Sipernat® 22S).

[0063] The delivery rate of pump P1 in normal operation is 300 t / h. After a while, the delivery rate of pump P1 in neutralization dropped to below 200 t / h due to fouling in the plate heat exchanger W.

[0064] While the polymerization was continuing, the feed and discharge of the monomer solution in heat exchanger W were closed off. Heat exchanger W was evacuated through a separate conduit having an internal diameter of about 7.5 cm. At the same time, heat exchanger W was vented through a further conduit having an internal diameter of about 2.5 cm.

[0065] Thereafter, the heat exchanger W was filled with demineralized water within 2 minutes. After about another 2 minutes, the demineralized water was blown out using compressed air. The process of filling with demineralized water and blowing it out was repeated 2 to 4 times.

[0066] Thereafter, the supply and discharge of the monomer solution to the heat exchanger W were opened again, taking a total of about 40 minutes.

[0067] If the above washings did not bring about any obvious improvement, the polymerization was stopped and the neutralizer was emptied, after which the feed and discharge of the monomer solution in the heat exchanger W were closed.

[0068] The heat exchanger W was then filled with demineralized water within 2 minutes. After a further 2 minutes, the demineralized water was blown out using compressed air. The process of filling with demineralized water and blowing it out was repeated three times.

[0069] Heat exchanger W was then filled with 50 wt % sodium hydroxide solution within about 3 minutes, heated to about 70° C. for about 30 minutes, and emptied. The process of filling with sodium hydroxide solution, heating, and emptying was repeated three times in the same manner.

[0070] The heat exchanger W was then filled with demineralized water within 2 minutes. After a further 2 minutes, the demineralized water was blown out using compressed air. The process of filling with demineralized water and blowing it out was repeated three times.

[0071] Neutralization and polymerization were then resumed, taking a total of about 6 hours.

[0072] Example 2 (not according to the invention) The procedure was the same as in Example 1. The heat exchanger W was disassembled and mechanically cleaned in a complex manner.

[0073] Neutralization and polymerization were then resumed, taking a total of about 12 hours.

Claims

1. 1. A method for producing a superabsorbent, comprising at least partially neutralizing at least one ethylenically unsaturated monomer having an acid group with an aqueous base solution, cooling the resulting aqueous monomer solution M by means of a heat exchanger W, adding at least one crosslinking agent and at least one initiator to the aqueous monomer solution M, then polymerizing the aqueous monomer solution M into a polymer gel, optionally extruding the polymer gel, drying the polymer gel, grinding, classifying and optionally thermally post-surface crosslinking the dried polymer gel, wherein the neutralization is stopped for cleaning of the heat exchanger W, which is then emptied, filled with water or an aqueous solution and cleaned by blowing.

2. 2. The method of claim 1, wherein the heat exchanger W is filled with water or an aqueous solution at least twice and cleaned by blowing.

3. 3. The method according to claim 1 or 2, wherein the heat exchanger W is further filled with an alkaline solution, heated and emptied.

4. 4. The method of claim 3, wherein the heat exchanger W is filled, heated, and emptied with alkaline solution at least twice.

5. 5. The method according to claim 3 or 4, wherein the alkaline solution used is a sodium hydroxide solution.

6. 6. The method of claim 5, wherein the sodium hydroxide solution has a content of at least 40% by weight.

7. The method according to any one of claims 3 to 6, wherein 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, wherein 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, wherein a plate heat exchanger is used as the heat exchanger W.

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

11. The method according to any one of claims 1 to 10, wherein the ethylenically unsaturated monomer having an acid group used is acrylic acid.

12. The process according to any one of claims 1 to 11, wherein the base used is an alkali metal hydroxide, alkali metal oxide, alkali metal bicarbonate, and / or alkali metal carbonate.

13. The process according to any one of claims 1 to 12, wherein the base used is sodium hydroxide.

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

15. 15. The method according to any one of claims 1 to 14, wherein the water content of the monomer solution M is from 50% to 65% by weight.

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