Process for producing a superabsorbent

Abstract

The present invention relates to a process for the continuous production of superabsorbents, in which an aqueous monomer solution is polymerized to form a polymer gel, and acrylic acid present in the off-gas from the polymerization and / or drying is washed out with the aqueous solution, the pH of which is between 9.0 and 12.5, and the acrylic acid-containing aqueous solution is used at least in part for the production of the monomer solution or is metered into the polymerization in parallel with the monomer solution.

Description

[Technical Field]

[0001] The present invention relates to a process for the continuous production of superabsorbents, in which an aqueous monomer solution is polymerized to form a polymer gel, and acrylic acid present in the off-gas from the polymerization and / or drying is washed out with the aqueous solution, the pH of which is between 9.0 and 12.5, and the acrylic acid-containing aqueous solution is used at least in part for the production of the monomer solution or is metered into the polymerization in parallel with the monomer solution. [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] EP 0922717A1 discloses a process for producing superabsorbents by static polymerization. This is carried out using an inert gas stream for cooling. The inert gas stream can be reused after condensation of water and acrylic acid. The dilute acrylic acid water obtained by condensation can be used to prepare a monomer solution.

[0005] WO 2010 / 040465 A1 discloses alkaline washing of off-gases obtained in the production of superabsorbents and recycling of the scrubber water to the process.

[0006] EP 1 178 059 A2, WO 2003 / 051415 A1, WO 2010 / 040466 A1 and WO 2011 / 120746 A1 likewise refer to the recycling of acrylic acid. Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention was to provide an improved process for producing superabsorbents, in particular an improved recycling of the acrylic acid off-gas discharged from the polymerization reactor. [Means for solving the problem]

[0008] This object was achieved by a process for the continuous production of superabsorbents by polymerizing an aqueous monomer solution containing partially neutralized acrylic acid to a polymer gel, optionally extruding the polymer gel, drying, grinding and classifying the polymer gel, and optionally thermally post-surface crosslinking the dried polymer gel, wherein the water content of the aqueous monomer solution is 40% to 75% by weight, the acrylic acid is neutralized to a range of 40 to 85 mol %, the acrylic acid present in the off-gas from the polymerization and / or drying is washed with an aqueous solution, the pH of which is 9.0 to 12.5, and the acrylic acid-containing aqueous solution is used at least in part for the production of the monomer solution or is metered into the polymerization in parallel with the monomer solution.

[0009] The pH of the aqueous solution is preferably 9.5 to 12.0, more preferably 10.0 to 11.5, and most preferably 10.5 to 11.0. The pH can be adjusted by mixing with 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. Particularly preferred alkali metals are sodium and potassium, while very particularly preferred are sodium hydroxide, sodium carbonate or sodium bicarbonate, and further mixtures thereof, especially sodium hydroxide. In the case of an acidic pH, correspondingly less neutralizing agent is used. The desired acidic pH is then necessarily determined by the acrylic acid washed.

[0010] The present invention is based on the discovery that the pH of the aqueous solution into which the acrylic acid is washed affects the stability and color of the resulting aqueous acrylic acid-containing solution.

[0011] The aqueous solution is preferably at a temperature of 40 to 80° C., more preferably 45 to 75° C., and most preferably 50 to 70° C. The amount of water that condenses can also be affected by temperature. DETAILED DESCRIPTION OF THE INVENTION

[0012] In a preferred embodiment of the invention, the acrylic acid is washed from the off-gas by a wash column. The wash column may have conventional internals. Random packing is preferred.

[0013] The gas velocity in the washing column is preferably 0.2 to 3.0 m / s, more preferably 0.5 to 2.5 m / s, and most preferably 1.0 to 2.0 m / s. In either case, the liquid hourly space velocity in the washing column is 1 / m² of the internal cross-sectional area of ​​the washing column. 2 Preferably 2 to 50 m 3 / h, preferably 5 to 40 m 3 / h, most preferably 10-30m 3 / h.

[0014] The aqueous solution in the washing column may be partially circulated, preferably 95% to 99.9%, more preferably 96% to 99.8%, most preferably 97% to 99.7%.

[0015] The acrylic acid-containing aqueous solution preferably contains less than 99% by weight water, more preferably less than 98% by weight, and most preferably less than 97% by weight water.

[0016] The content of acrylic acid and neutralized acrylic acid in the aqueous solution can be determined by online analysis, so the concentration in the monomer solution can be easily kept constant despite recycling.

[0017] For example, near-infrared spectroscopy is suitable for online analysis. The concentration of a component can be determined directly using a suitable calibration curve. The concentration of a component can also be determined indirectly via pH, density, and temperature. If the pH and temperature of the aqueous solution are kept sufficiently constant, the concentration of a component can also be determined solely by density.

[0018] The acrylic acid-containing aqueous solutions used in the preparation of the monomer solution can result in distinct variations in centrifuge retention capacity (CRC) and extractables content. By online analysis and adjustment of the amounts of acrylic acid, water and / or neutralizing agent raw materials, it is easily possible to keep the concentration of the monomer solution constant despite recycling.

[0019] The manufacture of superabsorbents is described in detail below.

[0020] Superabsorbents are made by polymerizing a monomer solution and are typically water-insoluble.

[0021] 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.

[0022] 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.

[0023] 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 with a neutralizing agent, either 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 %. Customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates, and mixtures thereof. Instead of alkali metal salts, ammonium salts can also be used. Particularly preferred alkali metals are sodium and potassium, while very particular preference is given to sodium hydroxide, sodium carbonate or sodium bicarbonate, and mixtures thereof, especially sodium hydroxide.

[0024] The monomer typically contains a polymerization inhibitor, preferably a hydroquinone monoether, as a storage stabilizer.

[0025] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Such groups include, for example, an ethylenically unsaturated group that can undergo free radical polymerization to become a 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.

[0026] 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, and the like. diacrylates and triacrylates, as described in DE 03 / 104301 A1, WO 03 / 104301 A1 and DE 10331450 A1, mixed acrylates which contain not only acrylate groups but also ethylenically unsaturated groups, as described in DE 10331456 A1 and 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.

[0027] The amount of crosslinker 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, calculated in each case based on the total amount of monomer used. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases, reaching 21.0 g / cm. 2 The absorbency under pressure (AUL 0.3 psi) exceeds the maximum value.

[0028] The initiator used may 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).

[0029] The water content of the monomer solution 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 process will increase, and if the water content is low, the heat of polymerization may simply not be sufficiently removed.

[0030] The temperature of the monomer solution is preferably 10 to 90°C, particularly preferably 20 to 70°C, and very particularly preferably 30 to 50°C.

[0031] To function optimally, preferred polymerization inhibitors require dissolved oxygen. Therefore, the monomer solution can be inerted, i.e., flushed with an inert gas, preferably nitrogen or carbon dioxide, to remove dissolved oxygen before 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, before polymerization.

[0032] 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.

[0033] To improve the drying performance, the pulverized polymer gel obtained by the kneader may be further extruded.

[0034] The polymer gel is then typically dried using an air-circulating belt dryer until the residual moisture content is preferably 0.5-10 wt. %, more preferably 1-7 wt. %, and most preferably 2-5 wt. The residual moisture content is determined 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 exceeded. g If the residual moisture content is too low, the dried polymer gel will be 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 crushed and, optionally, coarsely ground.

[0035] The dried polymer gel is then typically ground and classified; 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.

[0036] The average particle size of the polymer particles removed as the product fraction is preferably 150 to 850 μm, more preferably 250 to 600 μm, and most particularly 300 to 500 μm. The average particle size of the product fraction may be determined by EDANA recommended test method No. WSP220.2(05) "Particle Size Distribution," in which the mass fractions 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 value resulting from a cumulative weight of 50%.

[0037] To further improve the 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.

[0038] The amount of the surface postcrosslinking agent 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, based on the polymer particles in each case.

[0039] In a preferred embodiment of the present invention, in addition to the surface postcrosslinker, a multivalent cation is applied to the particle surface.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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 position of the mixing shaft, that is, horizontal mixers have horizontally mounted mixing shafts, and vertical mixers have vertically mounted mixing shafts.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.

[0044] 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.

[0045] 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, fluidized bed dryers may also be used.

[0046] 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.

[0047] 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.

[0048] The surface postcrosslinked polymer particles can then be classified again to remove overly small and / or overly large polymer particles and reuse them in the process.

[0049] To further improve properties, the surface postcrosslinked polymer particles may be coated or rewetted.

[0050] 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.

[0051] 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]

[0052] A monomer solution is prepared by successively mixing deionized water, 50% by weight sodium hydroxide solution and acrylic acid to a degree of neutralization corresponding to 72.0 mol %. The water content of the monomer solution is 57.5% by weight.

[0053] The crosslinking agent used was 3-tuply ethoxylated glyceryl triacrylate (purity: approximately 85% by weight), and the amount used was 1.2 kg per ton of monomer solution.

[0054] Free radical polymerization is initiated using 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 g of a 1 wt. % aqueous ascorbic acid solution per ton of monomer solution.

[0055] The monomer solution was added to 6.3 ml 3 The monomer solution was fed into a List Contikneter continuous kneading reactor (LIST AG, Arisdorf, Switzerland) with a capacity of 1000 t / h. The throughput of the monomer solution was approximately 20 t / h. The feed temperature of the reaction solution was 23.5°C.

[0056] Between the addition point of the crosslinker and the addition points of the hydrogen peroxide and sodium peroxodisulfate solutions, 4 ml of the monomer solution was added. 3 The reactor is inerted with nitrogen at 2000 kJ / h. The monomer solution is metered into the reactor without nitrogen separation. The ascorbic acid solution is metered directly into the reactor in parallel with the monomer solution.

[0057] After about 50% of the residence time, about 1000 kg / h of polymer particles having a particle size of less than 150 μm, obtained in a manufacturing process by grinding and classification, are additionally metered into the reactor. The residence time of the reaction mixture in the reactor is about 15 minutes.

[0058] The resulting polymer gel is added to the conveyor belt of an air circulation belt dryer by an oscillating conveyor belt. The total length of the air circulation belt dryer is 48 m. The conveyor belt of the air circulation belt dryer has an effective width of 4.4 m. In this air circulation belt dryer, an air / gas mixture (approximately 175°C) is continuously passed around the aqueous polymer gel to dry it. The residence time in the air circulation belt dryer is 37 minutes.

[0059] The dried polymer gel is crushed in a two-stage roll mill and sieved to a particle size of 150 to 850 μm. Polymer particles with a particle size of less than 150 μm are separated. Polymer particles with a particle size of more than 850 μm are recycled for crushing. Polymer particles with a particle size in the range of 150 to 850 μm are thermally surface post-crosslinked.

[0060] The polymer particles are coated with the surface postcrosslinker solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried at 185° C. for 45 minutes in a NARA paddle dryer (GMF Gouda, Waddinxveen, Netherlands).

[0061] The following amounts are weighed out and metered into a Schugi Flexomix®: 7.5t / h polymer particles 348.75 kg / h of surface post-crosslinking agent solution

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

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

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

[0065] Too low a pH value of the aqueous solution results in poor separation of acrylic acid from the off-gas stream, whereas too high a pH value of the aqueous solution results in poorly reproducible discoloration and polymer gel in the recycled aqueous solution.

Claims

1. 1. A process for continuously producing a superabsorbent by polymerizing an aqueous monomer solution containing partially neutralized acrylic acid to a polymer gel, optionally extruding the polymer gel, drying, grinding, classifying the polymer gel, and optionally thermally surface postcrosslinking the dried polymer gel, wherein the water content of the aqueous monomer solution is 40% to 75% by weight, the acrylic acid is neutralized to a range of 40 to 85 mol %, acrylic acid present in the off-gas from the polymerization and / or drying is washed with an aqueous solution, the pH of which is 9.0 to 12.5, and the acrylic acid-containing aqueous solution is at least partially used for the production of the monomer solution or is metered into the polymerization in parallel with the monomer solution.

2. 2. The process of claim 1, wherein the pH of the aqueous solution is from 9.5 to 12.

0.

3. 2. The process of claim 1, wherein the pH of the aqueous solution is from 10.0 to 11.

5.

4. 2. The process of claim 1, wherein the pH of the aqueous solution is from 10.5 to 11.

0.

5. The process of any one of claims 1 to 4, wherein the pH of the aqueous solution is adjusted with sodium hydroxide solution.

6. The process of any one of claims 1 to 5, wherein the aqueous solution is at a temperature of from 50 to 70°C.

7. 7. The process of any one of claims 1 to 6, wherein the acrylic acid is washed from the off-gas by a wash column.

8. 8. The process of claim 7, wherein the gas velocity in the scrubbing column is 1.0 to 2.0 m / s.

9. 9. The process of claim 7 or 8, wherein random packing is used in the wash column.

10. The liquid hourly space velocity in the washing column is 1 / m² of the internal cross-sectional area of ​​the washing column. ２ 10 to 30 m per ３ / h.

11. The process of any one of claims 7 to 10, wherein the aqueous solution is partially circulated through the wash column.

12. 12. The process of claim 11, wherein 97% to 99.7% of the aqueous solution is partially circulated in the wash column.

13. 13. The process of claim 11 or 12, wherein the aqueous solution exiting the wash column comprises less than 97% by weight water.

14. 14. The process of any one of claims 1 to 13, wherein the content of acrylic acid and neutralized acrylic acid in the monomer solution is determined by online analysis, and the amount of acrylic acid, water and / or neutralizing agent used in the monomer solution is adjusted accordingly.

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