Method for producing superabsorbents

Abstract

The invention relates to a method for continuously producing superabsorbents, wherein an aqueous monomer solution is polymerized in order to form a polymer gel, the acrylic acid contained in the exhaust gas of the polymerization and / or drying process is scrubbed out using an aqueous solution, the pH value of the aqueous solution equals 9.0 to 12.5, and the aqueous solution loaded with acrylic acid is used at least partly to produce the monomer solution or is metered into the polymerization in parallel with the monomer solution.

Description

[0001] Process for producing superabsorbents

[0002] The present invention relates to a process for the continuous production of superabsorbents, wherein an aqueous monomer solution is polymerized to a polymer gel, the acrylic acid contained in the exhaust gas from the polymerization and / or drying is washed out by means of an aqueous solution, the pH of the aqueous solution is from 9.0 to 12.5 and the aqueous solution loaded with acrylic acid is used at least partly for the production of the monomer solution or is metered into the polymerization in parallel with the monomer solution.

[0003] Superabsorbents are used in the manufacture of diapers, tampons, sanitary pads, and other hygiene products, as well as as water-retaining agents in agricultural horticulture. Superabsorbents are also known as water-absorbing polymers.

[0004] 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.

[0005] EP 0 922 717 A1 discloses a process for producing superabsorbents by static polymerization. An inert gas stream is used for cooling. The inert gas stream can be recycled after condensation of water and acrylic acid. The dilute aqueous acrylic acid obtained by condensation can be used to prepare the monomer solution.

[0006] WO 2010 / 040465 A1 discloses the alkaline scrubbing of exhaust gases generated during the production of superabsorbents and the recycling of the scrubber water into the process.

[0007] EP 1 178 059 A2, WO 2003 / 051415 A1, WO 2010 / 040466 A1 and WO 2011 / 120746 A1 also mention the recycling of acrylic acid.

[0008] The object of the present invention was to provide an improved process for the production of superabsorbents, in particular an improved recycling of the acrylic acid discharged with the exhaust gas from the polymerization reactor.

[0009] The object was achieved by a process for the continuous production of superabsorbents, wherein an aqueous monomer solution containing a partially neutralized acrylic acid is polymerized to a polymer gel, the polymer gel is optionally extruded, the polymer gel is dried and the dried polymer gel is comminuted, classified and optionally thermally surface post-crosslinked, characterized in that the water content of the aqueous monomer solution is from 40 to 75 wt. %, the acrylic acid is neutralized to 40 to 85 mol. %, the acrylic acid contained in the exhaust gas from the polymerization and / or drying is washed out by means of an aqueous solution, the pH of the aqueous solution is from 9.0 to 12.5 and the aqueous solution loaded with acrylic acid is at least partly used to produce the monomer solution or is metered into the polymerization in parallel with the monomer solution.

[0010] The pH of the aqueous solution is preferably from 9.5 to 12.0, more preferably from 10.0 to 11.5, most preferably from 10.5 to 11.0. The pH can be adjusted by mixing in a neutralizing agent. Suitable neutralizing agents are, for example, alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and mixtures thereof. Sodium and potassium are particularly preferred as alkali metals, but very particular preference is given to sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof, in particular sodium hydroxide. For an acidic pH, correspondingly less neutralizing agent is used. The desired acidic pH is then established automatically by the washed-out acrylic acid.

[0011] The present invention is based on the finding that the pH value of the aqueous solution during the washing out of acrylic acid has an influence on the stability and color of the resulting aqueous solutions loaded with acrylic acid.

[0012] The aqueous solution preferably has a temperature of 40 to 80°C, more preferably 45 to 75°C, most preferably 50 to 70°C. The amount of condensing water can be influenced by the temperature.

[0013] In a preferred embodiment of the present invention, the acrylic acid is scrubbed from the offgas using a scrubbing column. The scrubbing column may have the usual internals. Packing is preferred.

[0014] The gas velocity in the wash column is preferably from 0.2 to 3.0 m / s, particularly preferably from 0.5 to 2.5 m / s, most preferably from 1.0 to 2.0 m / s. The liquid loading in the wash column is preferably from 2 to 50 m 3 / h, particularly preferably from 5 to 40 m 3 / h, especially preferably from 10 to 30 m 3 / h, each per m 2 internal cross-sectional area of ​​the wash column. The aqueous solution in the wash column can be partially recycled, preferably from 95 to 99.9%, particularly preferably from 96 to 99.8%, most preferably from 97 to 99.7%.

[0015] The aqueous solution loaded with acrylic acid should preferably contain only less than 99 wt.%, particularly preferably less than 98 wt.%, most preferably less than 97 wt.%, of water.

[0016] The acrylic acid and neutralized acrylic acid content in the aqueous solution can be determined using online analysis. This makes it easy to keep the concentrations in the monomer solution constant despite recirculation.

[0017] NIR spectroscopy, for example, is suitable for online analysis. Using suitable calibration curves, the concentration of the components can be determined directly. The concentration of the components can also be determined indirectly via pH, density, and temperature. If the pH of the aqueous solution and the temperature are kept sufficiently constant, the concentration of the components can also be determined using density alone.

[0018] The acrylic acid-laden aqueous solution used to prepare the monomer solution can lead to significant changes in centrifuge retention capacity (CRC) and extractable content. Through online analysis and adjustment of the feed quantities of acrylic acid, water, and / or neutralizing agent, the concentrations in the monomer solution can easily be kept constant despite recirculation.

[0019] The production of superabsorbents is explained in more detail below:

[0020] The superabsorbents are produced by polymerizing a monomer solution and are usually water-insoluble.

[0021] The ethylenically unsaturated, acid-containing monomers 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, more preferably at least 25 g / 100 g water, most preferably at least 35 g / 100 g water. Suitable monomers include, 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 very particularly preferred.

[0022] The ethylenically unsaturated, acid-containing monomers are usually partially neutralized. Neutralization is carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably from 40 to 85 mol%, more preferably from 50 to 80 mol%, most preferably from 60 to 75 mol%, and the usual 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. Sodium and potassium are particularly preferred as alkali metals, but very particular preference is given to sodium hydroxide, sodium carbonate, or sodium bicarbonate, and mixtures thereof, in particular sodium hydroxide.

[0023] The monomers usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers.

[0024] Suitable crosslinkers are compounds with at least two groups suitable for crosslinking. Examples of such groups include ethylenically 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. Polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinkers.

[0025] 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 0 530 438 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 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated groups, as in DE 103 31 456 A1 and DE 103 55 401 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. The amount of crosslinker is preferably 0.05 to 1.5 wt. %, more preferably 0.1 to 1 wt. %, most preferably 0.15 to 0.6 wt. %, in each case calculated based on the total amount of monomer used.With increasing crosslinker content, the centrifuge retention capacity (CRC) and the absorption under a pressure of 21.0 g / cm decrease. 2 (AUL0.3psi) passes through a maximum.

[0026] Any compound that generates radicals under the polymerization conditions can be used as initiators, 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. Mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. 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 is preferably used as the reducing component. Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0027] The water content of the monomer solution is preferably from 40 to 75 wt.%, particularly preferably from 45 to 70 wt.%, and most preferably from 50 to 65 wt.%. As the water content increases, the energy required for subsequent drying increases, and as the water content decreases, the heat of polymerization can only be dissipated insufficiently.

[0028] The temperature of the monomer solution is preferably from 10 to 90°C, more preferably from 20 to 70°C, most preferably from 30 to 50°C.

[0029] The preferred polymerization inhibitors require dissolved oxygen for optimal effectiveness. Therefore, the monomer solution can be freed of dissolved oxygen before polymerization by inerting, i.e., by flowing an inert gas, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution before polymerization is preferably reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

[0030] Suitable reactors for polymerization include kneader reactors or belt reactors. In the kneader, 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. Belt polymerization is described, for example, in DE 38 25 366 A1 and US Pat. No. 6,241,928. Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example, in an extruder or kneader.

[0031] To improve the drying properties, the crushed polymer gel obtained by means of a kneader can be additionally extruded.

[0032] 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.%, most preferably 2 to 5 wt.%, wherein the residual moisture content is 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 too low. gand is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle, and undesirably large amounts of polymer particles with too small a particle size ("fines") are produced in the subsequent comminution steps. The solids content of the polymer gel before drying is preferably between 25 and 90 wt. %, more preferably between 35 and 70 wt. %, and most preferably between 40 and 60 wt. %. The dried polymer gel is then crushed and optionally coarsely crushed.

[0033] 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 be used for grinding.

[0034] The average particle size of the polymer particles separated as the product fraction is preferably from 150 to 850 pm, more preferably from 250 to 600 pm, and most preferably from 300 to 500 pm. The average particle size of the product fraction can be determined using the EDANA-recommended test method No. WSP 220.2 (05) "Particle Size Distribution," in which the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically. The average particle size is the mesh size value resulting for a cumulative 50 wt.%.

[0035] 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 14 019 A1, DE 3523617 A1, and EP 0 450 922 A2, or ß-hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230.

[0036] The amount of surface postcrosslinker is preferably 0.001 to 2 wt.%, particularly preferably 0.01 to 1 wt.%, very particularly preferably 0.03 to 0.7 wt.%, in each case based on the polymer particles.

[0037] In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers.

[0038] The polyvalent cations usable in the process according to the invention include, for example, divalent cations, such as the cations of zinc, magnesium, calcium, and strontium; trivalent cations, such as the cations of aluminum, iron, chromium, rare earths, and manganese; and tetravalent cations, such as the cations of 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.

[0039] 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.

[0040] Surface post-crosslinking is typically performed by spraying a solution of the surface post-crosslinker onto the dried polymer particles. Following spraying, the polymer particles coated with the surface post-crosslinker are thermally treated.

[0041] The spraying of a solution of the surface post-crosslinker 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 most preferred. The distinction between horizontal mixers and vertical mixers is made by the bearing of the mixing shaft, i.e. horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, Horizontale Pflugschar® mixers (Gebr. Lödige 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 surface post-crosslinker solution in a fluidized bed.

[0042] Surface postcrosslinkers are typically used as an aqueous solution. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by varying the non-aqueous solvent content or the total solvent quantity.

[0043] The thermal treatment is preferably carried out in contact dryers, particularly preferably paddle dryers, and most preferably disc dryers. Suitable dryers include, for example, Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA), and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used.

[0044] Surface post-crosslinking can occur in 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 heatable screw, is also suitable. Mixing and thermal surface post-crosslinking are particularly advantageous in a fluidized-bed dryer.

[0045] Preferred reaction temperatures are in the range from 100 to 250°C, preferably from 110 to 220°C, particularly preferably from 120 to 210°C, very particularly preferably from 130 to 200°C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes, and usually at most 60 minutes.

[0046] The surface-crosslinked polymer particles can then be classified again, with polymer particles that are too small and / or too large being separated and returned to the process.

[0047] The surface-crosslinked polymer particles can be coated or re-moistened to further improve their properties.

[0048] Re-moistening 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 temperatures that are too low, the polymer particles tend to clump together, and at higher temperatures, water evaporates noticeably. The amount of water used for re-moistening is preferably from 1 to 10 wt.%, particularly preferably from 2 to 8 wt.%, and most preferably from 3 to 5 wt.%. Re-moistening increases the mechanical stability of the polymer particles and reduces their tendency to static charge. Re-moistening is advantageously carried out in the cooler after thermal surface post-crosslinking.

[0049] Suitable coatings for improving the swelling rate and gel bed permeability (GBP) include inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or multivalent metal cations. Suitable coatings for dust control include polyols. Suitable coatings for counteracting the undesirable caking tendency of polymer particles include fumed silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20.

[0050] Example

[0051] By continuously mixing deionized water, 50 wt.% sodium hydroxide solution, and acrylic acid, a monomer solution is prepared so that the degree of neutralization corresponds to 72.0 mol%. The water content of the monomer solution is 57.5 wt.%.

[0052] Triple-ethoxylated glycerol triacrylate (approx. 85 wt.%) was used as the crosslinker. The amount used was 1.2 kg per t of monomer solution.

[0053] To initiate the radical polymerization, 1.39 kg of a 0.25 wt.% aqueous hydrogen peroxide solution, 3.58 kg of a 15 wt.% aqueous sodium peroxodisulfate solution and 1.28 kg of a 1 wt.% aqueous ascorbic acid solution are used per t of monomer solution.

[0054] The monomer solution is fed into a List Contikneter reactor with a volume of 6.3m 3 (LIST AG, Arisdorf, Switzerland). The throughput of the monomer solution is approximately 20 t / h. The reaction solution has a temperature of 23.5°C at the inlet.

[0055] Between the addition point for the crosslinker and the addition points for the hydrogen peroxide and sodium peroxodisulfate solutions, the monomer solution is mixed with 4 m 3 / h of nitrogen is used to render the reactor inert. The monomer solution is metered into the reactor without separating the nitrogen. The ascorbic acid solution is metered directly into the reactor in parallel with the monomer solution. After approximately 50% of the residence time, an additional 1,000 kg / h of polymer particles with a particle size of less than 150 pm, which arise from comminution and classification during the production process, are metered into the reactor. The residence time of the reaction mixture in the reactor is approximately 15 minutes.

[0056] The resulting polymer gel is fed onto the conveyor belt of a circulating air belt dryer via an oscillating conveyor belt. The circulating air belt dryer is 48 m long, and the conveyor belt of the circulating air belt dryer has an effective width of 4.4 m. On the circulating air belt dryer, the aqueous polymer gel is continuously circulated with an air / gas mixture (approx. 175°C) and dried. The residence time in the circulating air belt dryer is 37 minutes.

[0057] The dried polymer gel is ground using a two-stage roller mill and sieved to a particle size of 150 to 850 pm. Polymer particles with a particle size of less than 150 pm are separated. Polymer particles with a particle size greater than 850 pm are returned to the grinding process. Polymer particles with a particle size in the range of 150 to 850 pm are thermally surface-crosslinked.

[0058] The polymer particles are coated with a surface post-crosslinker solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried in a NARA Paddle Dryer (GMF Gouda, Waddinxveen, Netherlands) for 45 minutes at 185°C.

[0059] The following quantities are dosed into the Schugi Flexomix®:

[0060] 7.5 t / h polymer particles

[0061] 348.75 kg / h surface post-crosslinker solution

[0062] The surface postcrosslinker solution contains 2.2 wt% 2-hydroxyethyl-2-oxazolidone, 2.2 wt% 1,3-propanediol, 29.0 wt% 1,2-propanediol, 3.2 wt% aluminum sulfate, 56.9 wt% water and 6.5 wt% isopropanol.

[0063] After drying, the surface-crosslinked polymer particles are cooled to approximately 60°C in a NARA paddle cooler (GMF Gouda, Waddinxveen, Netherlands).

[0064] The offgas is fed to a scrubbing column. The offgas consists essentially of the drying offgas and the polymerization offgas. The scrubbing column has a diameter of 5.9 m and a height of 19 m. The scrubbing column contains packing. At the top of the scrubbing column, approximately 530 m 3 / h aqueous solution is added. The aqueous solution consists of the bottom liquid of the wash column, approx. 4.7 m 3 / h water and approximately 50 kg / h 48 wt.% sodium hydroxide solution. The pH of the aqueous solution is 10.5 to 11.0. The temperature of the aqueous solution is 60°C. The remaining aqueous solution is discharged and used to prepare the monomer solution. The content of acrylic acid and neutralized acrylic acid in the aqueous solution is determined using NIR. The amounts of water, 50 wt.% sodium hydroxide solution, and acrylic acid in the neutralization are adjusted accordingly.

[0065] Excessively low pH values ​​of the aqueous solution lead to insufficient separation of acrylic acid from the exhaust stream. Excessively high pH values ​​of the aqueous solution lead to poorly reproducible discoloration and polymer gel in the recirculated aqueous solution.

Claims

Patent claims 1. A process for the continuous production of superabsorbents, wherein an aqueous monomer solution containing a partially neutralized acrylic acid is 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 thermally surface-postcrosslinked, characterized in that the water content of the aqueous monomer solution is from 40 to 75% by weight, the acrylic acid is neutralized to 40 to 85 mol%, the acrylic acid contained in the exhaust gas from the polymerization and / or drying is washed out by means of an aqueous solution, the pH of the aqueous solution is from 9.0 to 12.5, and the aqueous solution loaded with acrylic acid is used at least partly to produce the monomer solution or is metered into the polymerization in parallel with the monomer solution.

2. Process according to claim 1, characterized in that the pH of the aqueous solution is from 9.5 to 12.

0.

3. Process according to claim 1, characterized in that the pH of the aqueous solution is from 10.0 to 11.

5.

4. Process according to claim 1, characterized in that the pH of the aqueous solution is from 10.5 to 11.

0.

5. Process according to one of claims 1 to 4, characterized in that the pH of the aqueous solution is adjusted with sodium hydroxide solution.

6. Process according to one of claims 1 to 5, characterized in that the aqueous solution has a temperature of 50 to 70°C.

7. Process according to one of claims 1 to 6, characterized in that the acrylic acid is washed out of the exhaust gas by means of a washing column.

8. Process according to claim 7, characterized in that the gas velocity in the washing column is from 1.0 to 2.0 m / s.

9. Process according to claim 7 or 8, characterized in that packings are used in the washing column.

10. Process according to one of claims 7 to 9, characterized in that the liquid load in the washing column is from 10 to 30 m 3 / h per m 2 internal cross-sectional area of ​​the wash column.

11. Process according to one of claims 7 to 10, characterized in that the aqueous solution in the washing column is partially circulated.

12. Process according to claim 11, characterized in that from 97 to 99.7% of the aqueous solution in the washing column is partially circulated.

13. A process according to claim 11 or 12, characterized in that the aqueous solution discharged from the wash column contains less than 97% by weight of water.

14. A process according to any one of claims 1 to 13, characterized in that the content of Acrylic acid and neutralized acrylic acid in the aqueous solution is determined by online analysis and the amounts of acrylic acid, water and / or neutralizing agent used in the monomer solution are adjusted accordingly.