PROCESS FOR THE PRODUCTION OF SUPERABSORBERS

Description

[0001] The present invention relates to a process for producing superabsorbent particles by polymerizing a monomer solution or suspension, comprising the steps of continuous pre-neutralization of a monomer under cooling and partial recycling of the pre-neutralized monomer to the pre-neutralization, continuous heating of the pre-neutralized monomer by means of a heat exchanger, and continuous post-neutralization of the heated monomer, wherein the monomer solution or suspension has a temperature of 70 to 99°C before entering the polymerization reactor and the temperature of the heating medium used in the heat exchanger is less than 160°C.

[0002] Superabsorbent polymers are used in the production of diapers, tampons, sanitary napkins, and other hygiene products, as well as water-retaining agents in horticulture. They are also known as water-absorbing polymers.

[0003] 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.

[0004] The properties of superabsorbents can be adjusted, for example, by the amount of crosslinker used. With increasing crosslinker quantity, the centrifugal retention capacity (CRC) decreases, and the absorption reaches a maximum at a pressure of 21.0 g / cm² (AUL < 0.3 psi).

[0005] To improve application properties, such as gel bed permeability (GBP) and absorption at a pressure of < 49.2 g / cm² (AUL 0.7 psi), superabsorbent particles are generally surface post-crosslinked. This increases the degree of crosslinking of the particle surface, thereby at least partially decoupling absorption at a pressure of < 49.2 g / cm² (AUL 0.7 psi) and centrifuge retention capacity (CRC). This surface post-crosslinking can be carried out in the aqueous gel phase. Preferably, however, dried, milled, and sieved polymer particles (base polymer) are coated on the surface with a surface post-crosslinker and thermally surface post-crosslinked. Suitable crosslinkers for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the polymer particles.

[0006] US 2011 / 0313113 and US 2012 / 0258851 disclose the polymerization of monomer solutions with dispersed gas bubbles. One method for generating the dispersed gas bubbles is to initiate the polymerization at very high temperatures.

[0007] The object of the present invention was to provide an improved process for the production of superabsorbents, in particular a reduction of the residence time of the monomer solution at very high temperatures before entering the polymerization reactor, while simultaneously ensuring a very high temperature of the monomer solution upon entry into the polymerization reactor.

[0008] The problem was solved by a process for producing superabsorbents through polymerization of a monomer solution or suspension containing a) at least one ethylene-unsaturated, acid-group-bearing monomer that is at least partially neutralized, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylene-unsaturated monomers copolymerizable with the monomers mentioned under a), and e) optionally one or more water-soluble polymers. comprising the steps i) continuous pre-neutralization of the monomer under cooling and partial recycling of the pre-neutralized monomer to the pre-neutralization, ii) continuous heating of the pre-neutralized monomer from step i) by means of a heat exchanger, iii) continuous post-neutralization of the heated monomer from step ii), iv) addition of the crosslinker to the monomer solution from step iii) and v) addition of the initiator to the monomer solution from step iv), wherein the monomer solution or suspension has a temperature of 70 to 99°C before entering the polymerization reactor and the temperature of the heating medium used in the heat exchanger in step ii) is less than 160°C.

[0009] In step i), the pre-neutralized monomer is cooled to a temperature preferably less than 45°C, particularly preferably less than 40°C, and most preferably less than 35°C. Lower temperatures reduce the polymerization tendency.

[0010] The pre-neutralized monomer is preferably recycled to 40 to 98%, particularly preferably to 60 to 95%, and most preferably to 80 to 90%, in step i). This recycling dilutes the fresh monomer with the pre-neutralized monomer solution and prevents overheating.

[0011] A suitable method for continuous pre-neutralization with recycling is described, for example, in WO 2007 / 028751 A2.

[0012] The temperature of the heating medium in step ii) is preferably less than 140°C, particularly preferably less than 120°C, and most preferably less than 110°C.

[0013] In the post-neutralization step iii), the degree of neutralization of the monomer a) is increased by preferably 5 to 45 mol-%, particularly preferably 10 to 35 mol-%, most preferably 15 to 25 mol-%.

[0014] A suitable method for continuous post-neutralization is described, for example, in WO 2007 / 028747 A1.

[0015] The present invention is based on the finding that the residence time of the monomer solution at very high temperatures before entering the polymerization reactor can be reduced by suitable reaction control and thus the polymerization tendency before entering the polymerization reactor.

[0016] In a particularly preferred embodiment of the present invention, a chelating agent is added to the monomer solution before step iii).

[0017] In a particularly preferred embodiment of the present invention, a chelating agent is added to the monomer solution before step ii).

[0018] The following explains the production of superabsorbents in more detail: Superabsorbents are produced by polymerization of a monomer solution or suspension and are usually insoluble in water.

[0019] The monomers a) are preferably water-soluble, i.e. the solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, particularly preferably at least 25 g / 100 g water, most preferably at least 35 g / 100 g water.

[0020] Suitable monomers a) are, for example, ethylene-unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is most preferred.

[0021] Other suitable monomers a) are, for example, ethylene unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

[0022] Impurities can have a significant impact on polymerization. Therefore, the raw materials used should be of the highest possible purity. It is therefore often advantageous to purify the monomers a) specifically. Suitable purification methods are described, for example, in WO 02 / 055469 A1, WO 03 / 078378 A1, and WO 2004 / 035514 A1. A suitable monomer a) is, for example, acrylic acid purified according to WO 2004 / 035514 A1 with 99.8460 wt% acrylic acid, 0.0950 wt% acetic acid, 0.0332 wt% water, 0.0203 wt% propionic acid, 0.0001 wt% furfurals, 0.0001 wt% maleic anhydride, 0.0003 wt% diacrylic acid, and 0.0050 wt% hydroquinone monomethyl ether.

[0023] The proportion of acrylic acid and / or its salts in the total amount of monomers a) is preferably at least 50 mol-%, particularly preferably at least 90 mol-%, most preferably at least 95 mol-%.

[0024] The monomers a) usually contain polymerization inhibitors, preferably hydroquinone semi-ethers, as a storage stabilizer.

[0025] The monomer solution preferably contains up to 250 ppm by weight, more preferably at most 130 ppm by weight, more preferably at most 70 ppm by weight, more preferably at least 10 ppm by weight, more preferably at least 30 ppm by weight, and particularly around 50 ppm by weight, hydroquinone semi-ether, in each case based on the unneutralized monomer a). For example, an ethylene-unsaturated, acid-group-bearing monomer with a corresponding hydroquinone semi-ether content can be used to prepare the monomer solution.

[0026] Preferred hydroquinone semi-ethers are hydroquinone monomethyl ether (MEHQ) and / or alpha-tocopherol (vitamin E).

[0027] Suitable crosslinking agents b) are compounds with at least two groups suitable for crosslinking. Such groups include, for example, ethylene-unsaturated groups that can be radically polymerized into the polymer chain, and functional groups that can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer a) are also suitable as crosslinking agents b).

[0028] Crosslinkers b) are preferably compounds with at least two polymerizable groups that can be radically polymerized into the polymer network. Suitable crosslinking agents b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- and triacrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, WO 03 / 104301 A1 and DE 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylene-unsaturated groups, as described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinking mixtures, as described for example in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0029] Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane, methylenebismethacrylamide, 15-fold ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine.

[0030] Particularly preferred crosslinking agents (b) are the multiply ethoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form di- or triacrylates, as described, for example, in WO 03 / 104301 A1. Di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol are particularly advantageous. Di- or triacrylates of 1- to 5-fold ethoxylated and / or propoxylated glycerol are especially preferred. Most preferred are the triacrylates of 3- to 5-fold ethoxylated and / or propoxylated glycerol, in particular the triacrylate of 3-fold ethoxylated glycerol.

[0031] The amount of crosslinker b) is preferably 0.05 to 1.5 wt.%, particularly preferably 0.1 to 0.8 wt.%, most preferably 0.15 to 0.5 wt.%, in each case calculated on the total amount of monomer a) used. With increasing crosslinker content, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g / cm² reaches a maximum.

[0032] As initiators (c), any compound that generates radicals under the polymerization conditions can be used, for example, thermal initiators, redox initiators, and photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Preferably, mixtures of thermal and redox initiators are used, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. The disodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfonatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, and sodium bisulfite is preferably used as the reducing component. Such mixtures are available as Brüggolite® < FF6 and Brüggolite® < FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0033] Examples of ethylene unsaturated monomers that can be copolymerized with the ethylene-unsaturated, acid-group-bearing monomers a) and d) are acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, and diethylaminoethyl methacrylate.

[0034] Water-soluble polymers e) can include polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose.

[0035] An aqueous monomer solution is typically used. The water content of the monomer solution is preferably 40 to 75 wt.%, particularly preferably 45 to 70 wt.%, and most preferably 50 to 65 wt.%. It is also possible to use monomer suspensions, i.e., monomer solutions with a monomer (a) exceeding its solubility, for example, sodium acrylate. With increasing water content, the energy required for subsequent drying increases, and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.

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

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

[0038] To improve the drying properties, the crushed polymer gel obtained using a kneader can be additionally extruded.

[0039] The acid groups of the resulting polymer gels are usually partially neutralized. Neutralization is preferably carried out at the monomer level. This is typically done by mixing in the neutralizing agent as an aqueous solution or, more preferably, as a solid. The degree of neutralization is preferably 25 to 85 mol%, particularly preferably 30 to 80 mol%, and most preferably 40 to 75 mol%, whereby the usual neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal hydrogen carbonates, as well as mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Sodium and potassium are particularly preferred alkali metals, but sodium hydroxide, sodium carbonate, or sodium hydrogen carbonate, as well as mixtures thereof, are most preferred.Solid carbonates and bicarbonates can also be used in encapsulated form, preferably in the monomer solution directly before polymerization, during or after polymerization into the polymer gel, and before drying. Encapsulation is achieved by coating the surface with an insoluble or only slowly soluble material (for example, film-forming polymers, inert inorganic materials, or fusible organic materials), which delays the dissolution and reaction of the solid carbonate or bicarbonate so that carbon dioxide is only released during drying, and the resulting superabsorbent exhibits high internal porosity.

[0040] Optionally, a surfactant can be added to the monomer solution before or during polymerization, and the monomer solution can then be foamed with an inert gas or steam, or by vigorous stirring, either before or during polymerization. The surfactant can be anionic, cationic, zwitterionic, or non-ionic. A skin-friendly surfactant is preferred.

[0041] The polymer gel is then dried, usually in a circulating air belt dryer, until the residual moisture content is preferably 0.5 to 10 wt.%, particularly preferably 1 to 6 wt.%, and most preferably 1.5 to 4 wt.%, the residual moisture content being determined according to the EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". If the residual moisture content is too high, the dried polymer gel has a glass transition temperature (Tg) that is too low and is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle, and undesirably large quantities of polymer particles with an excessively small 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.%, particularly preferably between 35 and 70 wt.%, and most preferably between 40 and 60 wt.%.The dried polymer gel is then broken up and optionally coarsely crushed.

[0042] The dried polymer gel is then usually ground and classified, whereby single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills can usually be used for grinding.

[0043] The mean particle size of the polymer particles separated as the product fraction is preferably from 150 to 850 µm, particularly preferably from 250 to 600 µm, and most preferably from 300 to 500 µm. The mean particle size of the product fraction can be determined using the EDANA recommended test method No. WSP 220.2-05 "Particle Size Distribution", whereby the mass fractions of the sieve fractions are plotted cumulatively and the mean particle size is determined graphically. The mean particle size is the mesh size value obtained for a cumulative 50 wt.%.

[0044] The proportion of polymer particles with a particle size greater than 150 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, most preferably at least 98 wt.%.

[0045] Polymer particles with excessively small particle sizes reduce gel bed permeability (GBP). Therefore, the proportion of very small polymer particles ("fines") should be low.

[0046] Polymer particles that are too small are therefore usually separated and recycled into the process, preferably before, during, or immediately after polymerization, i.e., before the polymer gel is dried. The polymer particles that are too small can be moistened with water and / or aqueous surfactant before or during recycling.

[0047] It is also possible to separate excessively small polymer particles in later process steps, for example after surface recrosslinking or another coating step. In this case, the recycled excessively small polymer particles are surface recrosslinked or otherwise coated, for example with pyrogenic silica.

[0048] If a kneading reactor is used for polymerization, the undersized polymer particles are preferably added during the last third of the polymerization process. However, it is also possible to incorporate the undersized polymer particles into the polymer gel in a kneader or extruder downstream of the polymerization reactor.

[0049] If the polymer particles, which are too small, are added very early, for example already to the monomer solution, the centrifugal retention capacity (CRC) of the resulting polymer particles is reduced. However, this can be compensated for, for example, by adjusting the amount of crosslinker used (b).

[0050] The proportion of polymer particles with a particle size of at most 850 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, most preferably at least 98 wt.%.

[0051] The proportion of polymer particles with a particle size of at most 600 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, most preferably at least 98 wt.%.

[0052] Polymer particles that are too large reduce the swelling rate. Therefore, the proportion of excessively large polymer particles should also be low. Oversized polymer particles are thus typically separated and recycled back into the milling process.

[0053] 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 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US 6,239,230.

[0054] Furthermore, DE 40 20 780 C1 covers cyclic carbonates, DE 198 07 502 A1 covers 2-oxazolidinone and its derivatives, such as 2-hydroxyethyl-2-oxazolidinone, DE 198 07 992 C1 covers bis- and poly-2-oxazolidinones, DE 198 54 573 A1 covers 2-oxotetrahydro-1,3-oxazine and its derivatives, DE 198 54 574 A1 covers N-acyl-2-oxazolidinones, DE 102 04 937 A1 covers cyclic ureas, DE 103 34 584 A1 covers bicyclic amidoacetals, EP 1 199 327 A2 covers oxetanes and cyclic ureas, and WO 03 / 031482 A1 Morpholine-2,3-dione and its derivatives are described as suitable surface crosslinking agents.

[0055] Preferred surface crosslinking agents are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin, and mixtures of propylene glycol and 1,4-butanediol.

[0056] Particularly preferred surface crosslinking agents are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1,3-propanediol.

[0057] Furthermore, surface crosslinking agents containing additional polymerizable ethylene unsaturated groups, as described in DE 37 13 601 A1, can also be used.

[0058] The amount of surface crosslinking agent is preferably 0.001 to 3 wt.%, particularly preferably 0.02 to 1 wt.%, most preferably 0.05 to 0.2 wt.%, in each case based on the polymer particles.

[0059] Surface recrosslinking is typically carried out by spraying a solution of the surface recrosslinker onto the dried polymer particles. Following spraying, the polymer particles coated with the surface recrosslinker are surface recrosslinked and dried, whereby the surface recrosslinking reaction can take place both before and during drying.

[0060] Spraying a solution of the surface re-curing agent is preferably carried out in mixers with moving mixing tools, such as screw mixers, disc mixers, and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are especially preferred. The distinction between horizontal and vertical mixers is made by the orientation of the mixing shaft; that is, horizontal mixers have a horizontally mounted mixing shaft, and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, the Horizontal Ploughshare® mixer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), the Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands), the Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA), and the Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands). However, it is also possible to spray the surface re-curing agent solution in a fluidized bed.

[0061] Surface crosslinking agents are typically used as aqueous solutions. The penetration depth of the surface crosslinking agent into the polymer particles can be adjusted by varying the content of non-aqueous solvent or the total amount of solvent.

[0062] If water is used exclusively as the solvent, a surfactant is advantageously added. This improves the wetting properties and reduces the tendency to clump. Preferably, however, solvent mixtures are used, for example isopropanol / water, 1,3-propanediol / water, and propylene glycol / water, with the mixture mass ratio preferably being from 20:80 to 40:60.

[0063] Surface recrosslinking is preferably carried out in contact dryers, particularly paddle dryers, and most preferably disc dryers. Suitable dryers include, for example, the Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany), the Holo-Flite® dryers (Metso Minerals Industries Inc.; Danville; USA), and the Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used.

[0064] Surface post-crosslinking can occur within the mixer itself, by heating the jacket or blowing in warm air. A downstream dryer, such as a tray dryer, a rotary kiln, or a heated screw dryer, is equally suitable. Mixing and thermal surface post-crosslinking in a fluidized bed dryer is particularly advantageous.

[0065] Preferred reaction temperatures are in the range of 100 to 250°C, preferably 110 to 220°C, particularly preferably 120 to 210°C, and most preferably 130 to 200°C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, most preferably at least 30 minutes, and usually at most 60 minutes.

[0066] In a preferred embodiment of the present invention, the polymer particles are cooled after surface post-crosslinking. Cooling is preferably carried out in contact coolers, particularly preferably paddle coolers, and most preferably disc coolers. Suitable coolers include, for example, the Hosokawa Bepex® Horizontal Paddle Cooler (Hosokawa Micron GmbH; Leingarten; Germany), the Hosokawa Bepex® Disc Cooler (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® coolers (Metso Minerals Industries Inc.; Danville; USA), and the Nara Paddle Cooler (NARA Machinery Europe; Frechen; Germany). Fluidized bed coolers can also be used.

[0067] In the cooler, the polymer particles are cooled to preferably 40 to 90°C, particularly preferably 45 to 80°C, and most preferably 50 to 70°C.

[0068] Subsequently, the surface-crosslinked polymer particles can be reclassified, whereby polymer particles that are too small and / or too large are separated and recycled back into the process.

[0069] The surface-crosslinked polymer particles can be coated or re-moistened to further improve their properties.

[0070] Post-humidification is preferably carried out at 40 to 120°C, particularly preferably at 50 to 110°C, and most preferably at 60 to 100°C. At excessively low temperatures, the polymer particles tend to clump together, and at higher temperatures, a significant amount of water evaporates. The amount of water used for post-humidification is preferably 1 to 10 wt.%, particularly preferably 2 to 8 wt.%, and most preferably 3 to 5 wt.%. Post-humidification increases the mechanical stability of the polymer particles and reduces their tendency to accumulate static electricity. Advantageously, post-humidification is carried out in the cooler after thermal surface crosslinking. Suitable coatings for improving swelling rate and gel bed permeability (GBP) include, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations.Suitable coatings for dust binding include polyols. Suitable coatings to prevent the undesirable tendency of polymer particles to clump together include fumed silica, such as Aerosil® < 200, and surfactants, such as Span® < 20. Suitable coatings for dust binding, reducing clumping, and increasing mechanical stability include polymer dispersions, as described in EP 0 703 265 B1, and waxes, as described in US 5 840 321.

[0071] Subsequently, the coated and / or re-moistened polymer particles can be reclassified, whereby polymer particles that are too small and / or too large are separated and recycled back into the process. Examples

[0072] The examples were calculated using standard simulation programs. Example 1

[0073] In a continuous pre-neutralization process, 5715 kg / h (79.3 kmol / h) of acrylic acid, 3635 kg / h (45.4 kmol / h) of 50 wt% sodium hydroxide solution, and 5650 kg / h of water are mixed and cooled to 30°C using a shell-and-tube heat exchanger. 85% of the cooled mixture is recycled back into the pre-neutralization stage. The degree of neutralization is 57.3 mol%. The solids content is 44.8 wt%. The residence time of the monomer solution in the pre-neutralization stage is approximately one hour.

[0074] A shell-and-tube heat exchanger heats 15,000 kg / h of pre-neutralized monomer solution in counterflow. The heat exchanger has an internal surface area of ​​35 m². The temperature of the heating medium (inlet temperature) is 100°C (condensate). The flow rate of the heating medium is 12,100 kg / h. The temperature of the heating medium at the heat exchanger outlet is 60°C. The heating capacity of the heat exchanger is 562 kW. The temperature of the monomer solution rises to 75°C. The residence time of the monomer solution in the shell-and-tube heat exchanger is approximately 2.5 minutes.

[0075] The heated monomer solution is post-neutralized with 1123 kg / h (14.0 kmol / h) of 50 wt% sodium hydroxide solution. The sodium hydroxide solution temperature is 40°C. Due to the heat of neutralization, the temperature rises to 95°C. The degree of neutralization is 75.0 mol%. The solids content is 43.6 wt%. The residence time of the monomer solution in the post-neutralization solution is approximately 12 seconds.

[0076] Subsequently, crosslinker and initiator are added successively to the post-neutralized monomer solution, the monomer solution is transferred to your polymerization reactor and polymerized. Example 2 (comparative example)

[0077] In a continuous pre-neutralization process, 5715 kg / h (79.3 kmol / h) of acrylic acid, 3635 kg / h (45.4 kmol / h) of 50 wt% sodium hydroxide solution, and 5650 kg / h of water are mixed and cooled to 30°C using a shell-and-tube heat exchanger. 85% of the cooled mixture is recycled back into the pre-neutralization stage. The degree of neutralization is 57.3 mol%. The solids content is 44.8 wt%. The residence time of the monomer solution in the pre-neutralization stage is approximately one hour.

[0078] 15,000 kg / h of the pre-neutralized monomer solution are post-neutralized with 1,123 kg / h (14.0 kmol / h) of 50 wt% sodium hydroxide solution. The temperature of the sodium hydroxide solution is 40°C. Due to the heat of neutralization, the temperature rises to 53.4°C. The degree of neutralization is 75.0 mol%. The solids content is 43.6 wt%. The residence time of the monomer solution in the post-neutralization bath is approximately 12 seconds.

[0079] A shell-and-tube heat exchanger heats the post-neutralized monomer solution. The heat exchanger has an internal surface area of ​​35 m². The temperature of the heating medium (inlet temperature) is 100°C (condensate). The flow rate of the heating medium is 12,100 kg / h. The temperature of the heating medium at the heat exchanger outlet is 73°C. The heating capacity of the heat exchanger is 378 kW. The temperature of the monomer solution rises to 83.7°C. The residence time of the monomer solution in the shell-and-tube heat exchanger is approximately 2.5 minutes.

[0080] Subsequently, crosslinker and initiator are added successively to the heated monomer solution, the monomer solution is transferred to your polymerization reactor and polymerized. Example 3 (comparative example)

[0081] In a continuous pre-neutralization process, 5715 kg / h (79.3 kmol / h) of acrylic acid, 3635 kg / h (45.4 kmol / h) of 50 wt% sodium hydroxide solution, and 5650 kg / h of water are mixed and cooled to 30°C using a shell-and-tube heat exchanger. 85% of the cooled mixture is recycled back into the pre-neutralization stage. The degree of neutralization is 57.3 mol%. The solids content is 44.8 wt%. The residence time of the monomer solution in the pre-neutralization stage is approximately one hour.

[0082] 15,000 kg / h of the pre-neutralized monomer solution are post-neutralized with 1,123 kg / h (14.0 kmol / h) of 50 wt% sodium hydroxide solution. The temperature of the sodium hydroxide solution is 40°C. Due to the heat of neutralization, the temperature rises to 53.4°C. The degree of neutralization is 75.0 mol%. The solids content is 43.6 wt%. The residence time of the monomer solution in the post-neutralization bath is approximately 12 seconds.

[0083] A shell-and-tube heat exchanger heats the post-neutralized monomer solution. The heat exchanger has an internal surface area of ​​35 m². The temperature of the heating medium (inlet temperature) is 111°C (1.5 bar heating steam). The flow rate of the heating medium is 897 kg / h. The temperature of the heating medium at the heat exchanger outlet is 60°C. The heating capacity of the heat exchanger is 520 kW. The temperature of the monomer solution rises to 95°C. The residence time of the monomer solution in the shell-and-tube heat exchanger is approximately 2.5 minutes.

[0084] Subsequently, crosslinker and initiator are added successively to the heated monomer solution, the monomer solution is transferred to your polymerization reactor and polymerized. Example 4

[0085] In a continuous pre-neutralization process, 5715 kg / h (79.3 kmol / h) of acrylic acid, 3635 kg / h (45.4 kmol / h) of 50 wt% sodium hydroxide solution, and 5650 kg / h of water are mixed and cooled to 30°C using a shell-and-tube heat exchanger. 85% of the cooled mixture is recycled back into the pre-neutralization stage. The degree of neutralization is 57.3 mol%. The solids content is 44.8 wt%. The residence time of the monomer solution in the pre-neutralization stage is approximately one hour.

[0086] A shell-and-tube heat exchanger heats 15,000 kg / h of pre-neutralized monomer solution in counterflow. The heat exchanger has an internal surface area of ​​9.4 m². The temperature of the heating medium (supply temperature) is 150°C (4.8 bar heating steam). The flow rate of the heating medium is 960 kg / h. The temperature of the heating medium at the heat exchanger outlet is 60°C. The heating capacity of the heat exchanger is 562 kW. The temperature of the monomer solution rises to 75°C. The residence time of the monomer solution in the shell-and-tube heat exchanger is approximately 40 seconds.

[0087] The heated monomer solution is post-neutralized with 1123 kg / h (14.0 kmol / h) of 50 wt% sodium hydroxide solution. The sodium hydroxide solution temperature is 40°C. Due to the heat of neutralization, the temperature rises to 95°C. The degree of neutralization is 75.0 mol%. The solids content is 43.6 wt%. The residence time of the monomer solution in the post-neutralization solution is approximately 12 seconds.

[0088] Subsequently, crosslinker and initiator are added successively to the post-neutralized monomer solution, the monomer solution is transferred to your polymerization reactor and polymerized. Example 5 (comparative example)

[0089] In a continuous pre-neutralization process, 5715 kg / h (79.3 kmol / h) of acrylic acid, 3635 kg / h (45.4 kmol / h) of 50 wt% sodium hydroxide solution, and 5650 kg / h of water are mixed and cooled to 30°C using a shell-and-tube heat exchanger. 85% of the cooled mixture is recycled back into the pre-neutralization stage. The degree of neutralization is 57.3 mol%. The solids content is 44.8 wt%. The residence time of the monomer solution in the pre-neutralization stage is approximately one hour.

[0090] 15,000 kg / h of the pre-neutralized monomer solution are re-neutralized with 1,123 kg / h (14.0 kmol / h) of 50 wt% sodium hydroxide solution. The temperature of the sodium hydroxide solution is 40°C. Due to the heat of neutralization, the temperature rises to 53.4°C. The degree of neutralization is 75.0 mol%. The solids content is 43.6 wt%. The residence time of the monomer solution in the nebulizer is approximately 12 seconds.

[0091] A shell-and-tube heat exchanger heats the post-neutralized monomer solution. The heat exchanger has an internal surface area of ​​9.4 m². The temperature of the heating medium (inlet temperature) is 150°C (4.8 bar heating steam). The flow rate of the heating medium is 960 kg / h. The temperature of the heating medium at the heat exchanger outlet is 60°C. The heating capacity of the heat exchanger is 446 kW. The temperature of the monomer solution rises to 89°C. The residence time of the monomer solution in the shell-and-tube heat exchanger is approximately 40 seconds.

[0092] Subsequently, crosslinker and initiator are added successively to the heated monomer solution, the monomer solution is transferred to your polymerization reactor and polymerized. Example 6 (comparative example)

[0093] In a continuous pre-neutralization process, 5715 kg / h (79.3 kmol / h) of acrylic acid, 3635 kg / h (45.4 kmol / h) of 50 wt% sodium hydroxide solution, and 5650 kg / h of water are mixed and cooled to 30°C using a shell-and-tube heat exchanger. 85% of the cooled mixture is recycled back into the pre-neutralization stage. The degree of neutralization is 57.3 mol%. The solids content is 44.8 wt%. The residence time of the monomer solution in the pre-neutralization stage is approximately one hour.

[0094] 15,000 kg / h of the pre-neutralized monomer solution are re-neutralized with 1,123 kg / h (14.0 kmol / h) of 50 wt% sodium hydroxide solution. The temperature of the sodium hydroxide solution is 40°C. Due to the heat of neutralization, the temperature rises to 53.4°C. The degree of neutralization is 75.0 mol%. The solids content is 43.6 wt%. The residence time of the monomer solution in the nebulizer is approximately 12 seconds.

[0095] A shell-and-tube heat exchanger heats the post-neutralized monomer solution. The heat exchanger has an internal surface area of ​​9.4 m². The temperature of the heating medium (supply temperature) is 165°C (7 bar heating steam). The flow rate of the heating medium is 920 kg / h. The temperature of the heating medium at the heat exchanger outlet is 60°C. The heating capacity of the heat exchanger is 520 kW. The temperature of the monomer solution rises to 95°C. The residence time of the monomer solution in the shell-and-tube heat exchanger is approximately 40 seconds.

[0096] Subsequently, crosslinker and initiator are added successively to the heated monomer solution, the monomer solution is transferred to your polymerization reactor and polymerized.

[0097] The examples show that the inventive method enables lower flow temperatures in the heat exchanger. However, higher flow temperatures increase the fouling of the heat exchanger by polymer deposits. Furthermore, this reduces the residence time at higher temperatures.

Claims

1. A process for producing superabsorbent particles by polymerizing a monomer solution or suspension comprising a) at least one ethylenically unsaturated monomer which bears acid groups and is at least partly neutralized, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers mentioned under a) and e) optionally one or more water-soluble polymers, comprising the steps of i) continuously preneutralizing the monomer while cooling and partly recycling the preneutralized monomer into the preneutralization, ii) continuously heating the preneutralized monomer from step i) by means of a heat exchanger, iii) continuously postneutralizing the heated monomer from step ii), iv) adding the crosslinker to the monomer solution from step iii) and v) adding the initiator to the monomer solution from step iv), wherein the monomer solution or suspension is at a temperature of 70 to 99°C prior to entry into the polymerization reactor and the temperature of the heating medium used in the heat exchanger in step ii) is less than 160°C.

2. The process according to claim 1, wherein the preneutralized monomer is cooled in step i) to a temperature of less than 45°C.

3. The process according to claim 1 or 2, wherein the preneutralized monomer is cooled in step i) to a temperature of less than 40°C.

4. The process according to any of claims 1 to 3, wherein the preneutralized monomer is cooled in step i) to a temperature of less than 35°C.

5. The process according to any of claims 1 to 4, wherein between 40% and 98% of the preneutralized monomer is recycled into step i).

6. The process according to any of claims 1 to 5, wherein between 60% and 95% of the preneutralized monomer is recycled into step i).

7. The process according to any of claims 1 to 6, wherein between 80% and 90% of the preneutralized monomer is recycled into step i).

8. The process according to any of claims 1 to 7, wherein the temperature of the heating medium used in the heat exchanger in step ii) is less than 140°C.

9. The process according to any of claims 1 to 8, wherein the temperature of the heating medium used in the heat exchanger in step ii) is less than 120°C.

10. The process according to any of claims 1 to 9, wherein the temperature of the heating medium used in the heat exchanger in step ii) is less than 110°C.

11. The process according to any of claims 1 to 10, wherein the neutralization level is increased in step iii) by 5 to 45 mol%.

12. The process according to any of claims 1 to 11, wherein the neutralization level is increased in step iii) by 10 to 35 mol%.

13. The process according to any of claims 1 to 12, wherein the neutralization level is increased in step iii) by 15 to 25 mol%.

14. The process according to any of claims 1 to 13, wherein monomer a) is acrylic acid.

15. The process according to any of claims 1 to 14, wherein the superabsorbent particles are surface postcrosslinked.