Process for producing free-flowing superabsorbent particles

Abstract

The present invention relates to a process for coating surface-postcrosslinked superabsorbent particles, wherein the superabsorbent particles are coated at least with water and at least one polyethylene glycol having an average molar mass of 400 to 8000 g / mol.

Description

[0001] 241185

[0002] 1

[0003] Method for the production of flowable superabsorbent particles

[0004] The present invention relates to a method for coating surface-crosslinked superabsorbent particles, characterized in that the superabsorbent particles are coated at least with water and at least one polyethylene glycol with an average molar mass of 400 to 8000 g / mol.

[0005] 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.

[0006] 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.

[0007] To improve application properties, such as gel bed permeability (GBP) and absorption under a pressure of 49.2 g / cm² 2 (AUL 0.7 psi), superabsorbent particles generally undergo surface cross-linking. This increases the degree of cross-linking of the particle surface, thereby improving absorption under a pressure of 49.2 g / cm². 2 (AUL 0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface post-crosslinking can be carried out in the aqueous gel phase. Preferably, however, dried, milled, and sieved superabsorbent 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 superabsorbent particles.

[0008] WO 2007 / 121941 A1 discloses the coating of superabsorbent particles with high molecular weight polyethylene glycol.

[0009] The object of the present invention was to provide an improved method for coating surface-crosslinked superabsorbent particles, in particular to obtain particularly flowable superabsorbent particles.

[0010] The problem was solved by a method for coating surface-crosslinked superabsorbent particles, wherein the superabsorbent particles are coated with at least water and at least one polyethylene glycol with an average molar mass of 400 to 8000 g / mol.

[0011] Polyethylene glycol can also be referred to as PEG in the following text, where the subsequent number indicates the average molar mass. For example, polyethylene glycol with an average molar mass of 4000 g / mol is designated PEG4000. 241185

[0012] 2

[0013] The coating with polyethylene glycol according to the invention takes place after the surface crosslinking of the superabsorbent particles. Preferably, the coating with water is carried out before, after and / or together with the coating with PEG.

[0014] Preferably, the polyethylene glycol has an average molar mass of 750 to 7500 g / mol, more preferably of 1000 to 7000 g / mol, more preferably of 1500 to 6500 g / mol, and particularly preferably of 2000 to 6000 g / mol.

[0015] Preferably, the superabsorbent particles are coated with at least 0.5 wt.%, more preferably at least 1 wt.%, further preferably at least 2 wt.%, particularly preferably at least 3 wt.%, and especially preferably at least 5 wt.%, water, in each case based on the superabsorbent particles. This coating with water is also referred to as rewetting. The amount of water for coating can be added all at once or in stages (in partial amounts).

[0016] Preferably, the superabsorbent particles are coated with 200 to 2500 ppm, more preferably with 250 to 2000 ppm, further preferably with 300 to 1500 ppm, and particularly preferably with 500 to 1000 ppm, of polyethylene glycol, in each case based on the superabsorbent particles. These values ​​are ppm by weight.

[0017] Preferably, the superabsorbent particles have a temperature of at least 20°C, preferably at least 30°C, more preferably at least 50°C, and particularly preferably at least 70°C immediately before coating.

[0018] Preferably, the polyethylene glycol is used as a solution, more preferably as an aqueous solution. Preferably, the solution contains at least 10 wt% (10% solution), more preferably at least 20 wt% (20% solution), more preferably at least 30 wt% (30% solution), and particularly preferably at least 40 wt% (40% solution), based on the total amount of solution, polyethylene glycol. The amount of water in the PEG solution is included in the amount of water with which the superabsorbent particles are coated; i.e., if the PEG solution contains 0.1 wt% water, based on the amount of water used to coat the superabsorbent particles, then to achieve a coating with 1 wt% water, an additional 0.9 wt% water, based on the amount of water used to coat the superabsorbent particles, is used.

[0019] Preferably, the polyethylene glycol or the solution containing polyethylene glycol is sprayed onto the surface-crosslinked polymer particles for coating. Preferably, the polyethylene glycol or the solution containing polyethylene glycol is sprayed using at least one two-component nozzle or at least one hydraulic nozzle.

[0020] Two-fluid nozzles enable atomization into fine droplets or a spray mist. The atomization pattern is a circular or elliptical solid or hollow cone. Two-fluid nozzles can be externally mixed. 241185

[0021] 3. Two-component nozzles can be designed for external mixing or internal mixing. In external mixing two-component nozzles, the liquid and atomizing gas exit the nozzle head through separate openings. They are only mixed in the spray jet after exiting the nozzle. This allows for a wide range of independent control of droplet size distribution and flow rate. The spray cone of the nozzle can be adjusted via the air cap position. In internal mixing two-component nozzles, the liquid and atomizing gas are mixed within the nozzle, and the two-phase mixture exits the nozzle head through the same orifice (or through several parallel orifices). In internal mixing two-component nozzles, the flow rate and pressure are more closely coupled than in external mixing nozzles. Small changes in flow rate therefore lead to changes in the droplet size distribution. The desired flow rate is adjusted by selecting the appropriate cross-section of the nozzle orifice.

[0022] Suitable atomizing gases include compressed air or nitrogen at pressures of 0.5 bar and above. The droplet size can be individually adjusted via the nozzle geometry, nozzle type, the ratio of liquid mass flow to atomizing gas mass flow, as well as the gas and liquid pressure.

[0023] The present invention is based on the surprising finding that a coating with polyethylene glycol significantly improves the flow rate of surface-crosslinked superabsorbent particles.

[0024] The following section explains the production of superabsorbents in more detail:

[0025] The superabsorbent particles can be produced by polymerizing a monomer solution or suspension containing a) at least one ethylene-unsaturated, acid-group-bearing monomer, which may be at least partially neutralized, b) at least one crosslinker, and c) at least one initiator, wherein the aqueous monomer solution or suspension is polymerized to form a polymer gel, the resulting polymer gel is optionally comminuted, the polymer gel is subsequently dried, the dried polymer gel is optionally milled and classified, and the dried polymer gel is subsequently optionally surface-crosslinked thermally and cooled. The superabsorbents are typically insoluble in water.

[0026] 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, and most preferably at least 35 g / 100 g water. 241185

[0027] 4

[0028] 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 especially preferred.

[0029] 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-%.

[0030] The monomers a) typically contain polymerization inhibitors, preferably hydroquinone monomethyl ether (MEHQ), as a storage stabilizer.

[0031] 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 preferably around 50 ppm by weight, hydroquinone monomethyl ether (MEHQ), in each case based on the unneutralized monomer a). For example, an ethylene-unsaturated, acid-group-bearing monomer with a corresponding content of hydroquinone monomethyl ether (MEHQ) can be used to prepare the monomer solution.

[0032] Suitable crosslinkers 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 monomer a). Furthermore, polyvalent metal salts that can form coordinate bonds with at least two acid groups of monomer a) are also suitable as crosslinkers b). Other suitable crosslinkers b) are the “nano-clays” described in US 2017 / 0361305, the water glasses described in WO 00 / 31157 A1, and the aluminates described in WO 99 / 55767 A1.

[0033] 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 10331 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylene unsaturated groups, as described in DE 10331 456 A1 and DE 10355401 A1, or crosslinking mixtures, as described for example in DE 19543368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0034] Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane, methylenebismethacrylamide, 15-fold ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate, and triallylamine. 241185

[0035] 5

[0036] The amount of crosslinker b) is preferably 0.05 to 1.5 wt.%, particularly preferably 0.1 to 1 wt.%, and most preferably 0.15 to 0.6 wt.%, in each case calculated on the total amount of monomer a) used. With increasing crosslinker content, the centrifuge retention capacity (CRC) and the absorption at a pressure of 21.0 g / cm³ decrease. 2 goes through a maximum.

[0037] 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).However, pure 2-hydroxy-2-sulfonatoacetic acid or a salt thereof can also be used as a reducing component, especially when ascorbic acid is used in conjunction with it.

[0038] Chelating agents and 2-hydroxycarboxylic acids can be added to the monomer solution before or during polymerization, as described, for example, in WO 2017 / 170604 A1.

[0039] 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 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.

[0040] The acid groups of the resulting polymer gels are usually partially neutralized. Neutralization is 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 40 to 85 mol%, particularly preferably 50 to 80 mol%, and most preferably 60 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, especially sodium hydroxide, are most preferred. The sodium hydroxide content in the sodium hydroxide solution is preferably 241185.

[0041] 6 at least 10 wt.%, particularly preferably at least 25 wt.%, most preferably at least 40 wt.%.

[0042] The temperature of the monomer solution is preferably from 10 to 90°C, particularly preferably from 20 to 70°C, and most preferably from 30 to 50°C.

[0043] 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.

[0044] The following explains solution polymerization:

[0045] Suitable reactors for solution 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 25366 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.

[0046] To improve the drying properties, the crushed polymer gel obtained using a kneader can be additionally extruded.

[0047] 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.%.

[0048] 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 7 wt.%, and most preferably 1.5 to 6 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 will have a glass transition temperature T that is too low. g It is difficult to process further. If the residual moisture content is too low, the dried polymer gel becomes too brittle, and subsequent comminution steps result in undesirably large quantities of superabsorbent particles with an excessively small particle size ("fines"). The dried polymer gel is then broken up and optionally coarsely ground. 241185

[0049] 7

[0050] 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.

[0051] The mean particle size of the superabsorbent particles separated as the product fraction is preferably 150 to 850 pim, particularly preferably 250 to 600 pim, and most preferably 300 to 500 pim. 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%.

[0052] The proportion of superabsorbent 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.%.

[0053] Superabsorbent particles with excessively small particle sizes reduce gel bed permeability (GBP). Therefore, the proportion of excessively small superabsorbent particles ("fines") should be low.

[0054] Therefore, excessively small superabsorbent particles are typically separated and recycled into the process, preferably before, during, or immediately after polymerization, i.e., before drying the polymer gel. The excessively small superabsorbent particles can be moistened with water and / or aqueous surfactant before or during recycling.

[0055] It is also possible to separate excessively small superabsorbent particles in later process steps, for example after surface recrosslinking or another coating step. In this case, the recycled excessively small superabsorbent particles are surface recrosslinked or otherwise coated, for example with pyrogenic silica.

[0056] If a kneading reactor is used for polymerization, the undersized superabsorbent particles are preferably added during the last third of the polymerization process. However, it is also possible to incorporate the undersized superabsorbent particles into the polymer gel in a kneader or extruder located downstream of the polymerization reactor.

[0057] If the excessively small superabsorbent particles are added very early, for example, already with the monomer solution, the centrifugal retention capacity (CRC) of the resulting superabsorbent particles is reduced. However, this can be compensated for, for example, by adjusting the amount of crosslinker used (b). 241185

[0058] 8

[0059] The proportion of superabsorbent 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.%.

[0060] The proportion of superabsorbent 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.%.

[0061] Superabsorbent particles with excessively large particle sizes reduce the swelling rate. Therefore, the proportion of excessively large superabsorbent particles should also be low. Excessively large superabsorbent particles are thus typically separated and recycled back into the milling process.

[0062] The following explains droplet polymerization:

[0063] In droplet polymerization, the monomer solution is metered into the reactor through at least one bore, forming droplets. Droplet polymerization is described, for example, in WO 2014 / 079694 A1 and WO 2015 / 110321 A1.

[0064] The bores can be located, for example, in a dripper plate. The number and size of the bores are selected according to the desired capacity and droplet size. The droplet diameter is typically 1.9 times the bore diameter. It is important that the liquid to be dripped does not pass through the bore too quickly or that the pressure drop across the bore is not too great. Otherwise, the liquid will not be dripped, but the liquid jet will be disrupted (sprayed) due to the high kinetic energy. The Reynolds number, based on the flow rate per bore and the bore diameter, is preferably less than 2,000, more preferably less than 1,600, particularly preferably less than 1,400, and most preferably less than 1,200.

[0065] The dropper plate preferably has at least 5, particularly preferably at least 25, most preferably at least 50, and preferably up to 750, particularly preferably up to 500, and most preferably up to 250, bores. The diameter of the bores is selected according to the desired droplet size.

[0066] The diameter of the bores is preferably 50 to 500 mm, particularly preferably 70 to 300 mm, and most preferably 100 to 200 mm. The spacing between the bores is preferably 50 to 500 times, particularly preferably 70 to 300 times, and most preferably 80 to 200 times the bore diameter. Too small a spacing leads to the formation of agglomerates, while too large a spacing reduces the yield. 241185

[0067] 9

[0068] The temperature of the monomer solution when passing through the bores is preferably from 5 to 80°C, particularly preferably from 10 to 70°C, and most preferably from 30 to 60°C.

[0069] The reactor is percolated by a carrier gas. The carrier gas can be guided through the reactor in cocurrent or countercurrent flow to the freely falling droplets of the monomer solution, preferably in cocurrent flow, i.e., from bottom to top. Preferably, after each pass, at least part of the carrier gas, preferably at least 50%, and particularly preferably at least 75%, is recirculated into the reactor. Typically, a portion of the carrier gas is removed after each pass, preferably up to 10%, particularly preferably up to 3%, and most preferably up to 1%.

[0070] The oxygen content of the carrier gas is preferably from 0.5 to 15 vol.%, particularly preferably from 1 to 10 vol.%, and most preferably from 2 to 7 vol.%.

[0071] The carrier gas preferably contains nitrogen in addition to oxygen. The nitrogen content of the carrier gas is preferably at least 80% by volume, particularly preferably at least 90% by volume, and most preferably at least 95% by volume. Gas mixtures can also be used. The carrier gas can also be loaded with water vapor and / or acrylic acid vapors.

[0072] The gas velocity is preferably set so that the flow in the reactor is directed, for example, there are no convection vortices opposing the general flow direction, and is usually 0.1 to 2.5 m / s, preferably 0.3 to 1.5 m / s, preferably 0.5 to 1.2 m / s, particularly preferably 0.6 to 1.0 m / s, and most preferably 0.7 to 0.9 m / s.

[0073] The carrier gas flowing through the reactor is expediently preheated to the reaction temperature before entering the reactor.

[0074] Advantageously, the gas inlet temperature is controlled such that the gas outlet temperature, i.e., the temperature at which the carrier gas leaves the reactor, is typically between 90 and 150°C, preferably between 100 and 140°C, more preferably between 105 and 135°C, particularly preferably between 110 and 130°C, and most preferably between 115 and 125°C. The reaction can be carried out under positive or negative pressure; a negative pressure of up to 100 mbar relative to ambient pressure is preferred.

[0075] The reaction gas, i.e., the gas leaving the reactor, can be cooled, for example, in a heat exchanger. During this process, water and unreacted monomer (a) condense. The reaction gas can then be at least partially reheated and returned to the reactor as a recirculated gas. A portion of the reaction gas can be removed and replaced with fresh carrier gas, whereby the water and unreacted monomers (a) contained in the reaction gas can be separated and recycled. 241185

[0076] 10

[0077] A heat transfer system is particularly preferred, i.e., part of the waste heat from cooling the exhaust gas is used to heat the circulating gas.

[0078] The reactors can be heated externally. The heating system is set so that the wall temperature is at least 5°C above the reactor's internal temperature, reliably preventing condensation on the reactor walls.

[0079] The reverse suspension polymerization is described below:

[0080] In reverse suspension polymerization, the monomer solution is suspended in a hydrophobic solvent during polymerization. Reverse suspension polymerization is described, for example, in WO 2008 / 068208 A1 and WO 2015 / 062883 A2.

[0081] All solvents known to those skilled in the art for use in suspension polymerization can be employed as hydrophobic solvents. Aliphatic hydrocarbons, such as n-hexane, n-heptane, n-octane, n-nonane, n-decane, cyclohexane, or mixtures thereof, are preferred. Hydrophobic solvents exhibit a solubility in water of less than 5 g / 100 g at 23°C, preferably less than 1 g / 100 g, and particularly preferably less than 0.5 g / 100 g.

[0082] The hydrophobic solvent boils in the range of preferably 50 to 150°C, particularly preferably 60 to 120°C, and most preferably 70 to 90°C.

[0083] The ratio between hydrophobic solvent and monomer solution is 0.5 to 3, preferably 0.7 to 2.5 and most preferably 0.8 to 2.2.

[0084] The mean diameter of the monomer solution droplets in the suspension, if no agglomeration is carried out, is preferably at least 100 pim, particularly preferably from 100 to 1000 pim, particularly preferably from 150 to 850 pim, most preferably from 300 to 600 pim, wherein the droplet diameter can be determined by light scattering and represents the volume-averaged mean diameter.

[0085] The diameter of the monomer solution droplets can be adjusted via the applied stirring energy and by using suitable dispersing aids.

[0086] Dispersing agents are preferably added to disperse the aqueous monomer solution in the hydrophobic solvent or to disperse the resulting superabsorbent particles. These agents can be anionic, cationic, nonionic, or amphoteric surfactants, or natural, semi-synthetic, or synthetic polymers.

[0087] Anionic surfactants include sodium polyoxyethylene endodecyl ether sulfate and sodium dodecyl ether sulfate. A cationic surfactant is, for example, trimethylstearylammonium chloride. An amphoteric surfactant is, for example, carboxymethyldimethylcetylammonium. Nonionic surfactants include, for example, sucrose fatty acid esters, such as sucrose monostearate and sucrose dilaurate; sorbitan esters, such as sorbitan monostearate; and polyoxyalkylene compounds based on sorbitan esters, such as polyoxyethylene sorbitan monostearate.

[0088] The dispersing agent is usually dissolved or dispersed in the hydrophobic solvent. The dispersing agent is used in amounts between 0.01 and 10 wt.%, preferably between 0.2 and 5 wt.%, and particularly preferably between 0.5 and 2 wt.%, based on the monomer solution. The diameter of the monomer solution droplets can be adjusted by the type and amount of the dispersing agent.

[0089] For polymerization, it is advantageous to connect several stirred reactors in series. The subsequent reaction in further stirred reactors can increase the monomer conversion and reduce backmixing. It is also advantageous if the first stirred reactor is not too large. As the size of the stirred reactor increases, the size distribution of the dispersed monomer solution droplets inevitably broadens. A smaller first reactor therefore enables the production of superabsorbent particles with a particularly narrow particle size distribution.

[0090] The reaction is preferably carried out under reduced pressure, for example at a pressure of 800 mbar. The boiling point of the reaction mixture can be adjusted to the desired reaction temperature by changing the pressure.

[0091] If polymerization is carried out under sufficient reflux, inerting can be omitted. In this process, the dissolved oxygen is removed from the polymerization reactor along with the evaporating solvent.

[0092] The superabsorbent particles can be azeotropically dehydrated in the polymer dispersion, separated from the polymer dispersion, and the separated superabsorbent particles can be dried to remove the adhering residual hydrophobic solvent.

[0093] The following describes surface recrosslinking:

[0094] The superabsorbent 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 superabsorbent 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, β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US 6,239,230, or oxazolines, as described in EP 0 999 938 A2.

[0095] Furthermore, cyclic carbonates such as ethylene carbonate, propylene carbonate, and glycerol carbonate are covered in DE 40 20 780 C1, EP 4 289 888 A1, KR10-2021 -0038252 A and EP 3 424 988 A1; 2-oxazolidinone and its derivatives, such as 2-hydroxyethyl-2-oxazolidinone, are covered in DE 198 07 502 A1; bis- and poly-2-oxazolidinones are covered in DE 198 07 992 C1; 2-oxotetrahydro-1,3-oxazine and its derivatives are covered in DE 198 54 573 A1; N-acyl-2-oxazolidinones are covered in DE 198 54 574 A1; and DE 102 04 937 A1 Cyclic ureas, in EP 2 204 388 A1 oxetanes, in DE 103 34 584 A1 bicyclic amidoacetals, in EP 1 199 327 A2 oxetanes and cyclic ureas and in WO 03 / 031482 A1 morpholine-2,3-dione and its derivatives are described as suitable surface crosslinking agents.

[0096] Preferred surface crosslinking agents are ethylene carbonate, ethylene glycol diglycidyl leather, reaction products of polyamides with epichlorohydrin, and mixtures of propylene glycol and 1,4-butanediol.

[0097] Particularly preferred surface crosslinking agents are ethylene carbonate, 2-hydroxyethyl 1-2-oxazolidinone, 2-O-xazolidinone and 1,3-propanediol.

[0098] Furthermore, surface crosslinking agents containing additional polymerizable ethylene unsaturated groups, as described in DE 37 13 601 A1, can also be used.

[0099] The amount of surface crosslinking agent is preferably 0.001 to 2 wt.%, particularly preferably 0.02 to 1 wt.%, most preferably 0.05 to 0.7 wt.%, in each case based on the superabsorbent particles.

[0100] In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface crosslinking agents.

[0101] The polyvalent cations that can be used in the process according to the invention are, for example, divalent cations such as zinc, magnesium, calcium, and strontium; trivalent cations such as aluminum, iron, chromium, rare earth elements, and manganese; and tetravalent cations such as 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.

[0102] 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. 241185

[0103] 13

[0104] Surface recrosslinking is typically carried out by spraying a solution of the surface recrosslinker onto the dried superabsorbent particles. Following spraying, the superabsorbent particles coated with the surface recrosslinker are thermally treated, whereby the surface recrosslinking reaction can take place both before and during drying.

[0105] 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.

[0106] Surface crosslinking agents are typically used as aqueous solutions. The penetration depth of the surface crosslinking agent into the superabsorbent particles can be adjusted by varying the content of non-aqueous solvent or the total amount of solvent.

[0107] If water is used exclusively as a solvent, a surfactant is preferably 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, propylene glycol / water, 2-methyl-1,3-propanediol / water, ethylene glycol / water, diethylene glycol / water, triethylene glycol / water, tetraethyl glycol / water, or polyethylene glycol / water, wherein the mixture mass ratio is preferably from 20:80 to 40:60.

[0108] 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.

[0109] Surface post-crosslinking can take place within the mixer itself, by heating the jacket or blowing in warm air. A downstream dryer, such as a tray dryer (241185), is also suitable.

[0110] 14

[0111] Rotary kiln or a heated screw conveyor. Mixing and thermal surface crosslinking in a fluidized bed dryer are particularly advantageous.

[0112] 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 15 minutes, most preferably at least 20 minutes, and usually at most 60 minutes.

[0113] In a preferred embodiment of the present invention, the superabsorbent 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.

[0114] In the cooler, the superabsorbent particles are cooled to preferably 40 to 90°C, particularly preferably 45 to 80°C, and most preferably 50 to 70°C.

[0115] The superabsorbent particles can be coated or re-moistened to further improve their properties:

[0116] Post-humidification is preferably carried out at 30 to 120°C, particularly preferably at 35 to 110°C, most preferably at 40 to 100°C, and especially preferably at 50 to 90°C. At excessively low temperatures, the superabsorbent 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 superabsorbent particles and reduces their tendency to accumulate static electricity.

[0117] 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, for example, polyols. Suitable coatings to prevent the undesirable tendency of superabsorbent particles to clump together include, for example, fumed silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20. 241185

[0118] 15

[0119] Another object of the present invention is superabsorbent particles coated with at least water and at least one polyethylene glycol with an average molar mass of 400 to 8000 g / mol.

[0120] Preferably, the polyethylene glycol has an average molar mass of 750 to 7500 g / mol, more preferably of 1000 to 7000 g / mol, more preferably of 1500 to 6500 g / mol, and particularly preferably of 2000 to 6000 g / mol.

[0121] Preferably, the superabsorbent particles are coated with at least 0.5 wt.%, preferably with at least 1 wt.%, further preferably with at least 2 wt.%, particularly preferably with at least 3 wt.%, based on the superabsorbent particles, water.

[0122] Preferably, the superabsorbent particles are coated with 200 to 2500 ppm, more preferably with 250 to 2000 ppm, further preferably with 300 to 1500 ppm, and particularly preferably with 500 to 1000 ppm, of polyethylene glycol, in each case based on the superabsorbent particles. These values ​​are ppm by weight.

[0123] Preferably, the superabsorbent particles have a flow rate ratio after the coating according to the invention to the flow rate before or without coating, flow rate measured according to EDANA-NWSP 251.0.R2 (19), of at least 1.03, preferably at least 1.04, more preferably at least 1.05, and particularly preferably at least 1.06. Preferably, the above-mentioned ratio is from 1.03 to 1.5, more preferably from 1.04 to 1.4, more preferably from 1.05 to 1.3, and particularly preferably from 1.06 to 1.25.

[0124] Preferably the superabsorbent particles have a flow rate measured according to EDANA-NWSP 251.0.R2 (19) of at least 8.0 g / s, preferably at least 10.0 g / s, more preferably at least 12.0 g / s, and particularly preferably at least 13.0 g / s.

[0125] Another object of the present invention is hygiene articles containing superabsorbent particles according to the invention.

[0126] Methods:

[0127] The standard test methods described below, designated “NWSP”, are described in: “Nonwovens Standard Procedures”, 2019 edition, jointly published by EDANA (Avenue Eugene Plasky, 157, 1030 Brussels, Belgium, www.edana.org) and INDA (1100 Crescent Green, Suite 115, Cary, North Carolina 27518, USA, www.inda.org). This publication is available from both EDANA and INDA. 241185

[0128] 16

[0129] Unless otherwise specified, measurements should be carried out at an ambient temperature of 23 ± 2 °C and a relative humidity of 50 ± 10%. The superabsorbent particles should be thoroughly mixed before measurement.

[0130] Examples

[0131] Surface-crosslinked superabsorbent particles

[0132] A Lödige plowshare mixer (type MR5) with heating jacket, oil circuit and nitrogen device was preheated with an oil circuit temperature of 105°C (nitrogen purge approx. 80L / h).

[0133] After approximately 60 minutes of preheating, 1000 g of commercially available surface-crosslinked superabsorbent particles (SAVIVA® B 400; BASF SE) were added to the preheated Lödige mixer and a rotation speed of 60 rpm was set.

[0134] After the product temperature (of the superabsorbent particles) reached 80°C, the oil bath with oil circulation was switched off (starting point for a total residence time of 13.5 minutes). Two minutes after the product temperature of 80°C was reached, a solution of polyethylene glycol (as a 50% aqueous solution) and deionized water, or deionized water only (the respective amounts of PEG solution, PEG, and water are given in Table 1), was sprayed with nitrogen at 60 rpm using a two-fluid nozzle (nitrogen pressure 200 L / h / preset 1.5 bar, pump setting 700 (type Ismatec BVP, hose type Tygon R3607 as an intermediate piece) = 55 g / min). Seven minutes after reaching the product temperature of 80°C, another 10 g of deionized water was sprayed, and 12 minutes after reaching the product temperature of 80°C, a further 10 g of deionized water was sprayed.

[0135] After a total residence time of 13.5 minutes following the reaching of a product temperature of 80°C, the product was discharged into a plastic tray and, after approximately 5 minutes, sieved using a sieve machine (Retsch, model AS 200, with additional Retsch sieves, d = 200 mm, h = 50 mm) for 5 minutes at an amplitude of 1.44 mm to a particle size of < 850 pim. The sample was then filled into a PE bottle. The flow rate of the sample was determined according to the EDANA method NWSP 251.0.R2 (19). The results are also summarized in Table 1. 241185

[0136] 17

[0137] Table 1:

[0138] *) Comparative example

[0139] **) not a free-flowing product

[0140] The examples show that coating surface-crosslinked superabsorbent particles with polyethylene glycol significantly improves the flow rate (see comparative example 1), with the molar mass and the amount of PEG used playing a role (see comparative examples 6, 12, 15). Base polymer particles (not surface-crosslinked)

[0141] A Lödige plowshare mixer (type MR5) with heating jacket, oil circuit, and nitrogen system was preheated to an oil circuit temperature of 105°C (nitrogen purge approx. 80 L / h). After approximately 60 minutes of preheating, 1000 g of base polymer particles of commercially available surface-crosslinked superabsorbent particles (SAVIVA® B 400 base polymer before surface crosslinking; BASF SE) were added to the preheated Lödige mixer and a rotational speed of 60 rpm was set.

[0142] After the product temperature (of the base polymer particles) reached 80°C, the oil bath with oil circulation was switched off (starting point for a total residence time of 13.5 minutes). Two minutes after the product temperature of 80°C was reached, a solution of polyethylene glycol (as a 50% aqueous solution) and deionized water, or deionized water only (the respective amounts of PEG solution, PEG, and water are given in Table 2), was sprayed with nitrogen at 60 rpm using a two-fluid nozzle (nitrogen pressure 200 L / h / preset 1.5 bar, pump setting 700 (type Ismatec BVP, hose type Tygon R3607 as an intermediate piece) = 55 g / min). Seven minutes after reaching the product temperature of 80°C, another 10 g of deionized water was sprayed, and 12 minutes after reaching the product temperature of 80°C, another 10 g of deionized water was sprayed.

[0143] After a total residence time of 13.5 minutes following the reaching of a product temperature of 80°C, the product was discharged into a plastic tray and, after approximately 5 minutes, sieved using a sieve machine (Retsch, model AS 200, with additional Retsch sieves, d = 200 mm, h = 50 mm) for 5 minutes at an amplitude of 1.44 mm to a particle size of < 850 pim. The sample was then filled into a PE bottle. The flow rate of the sample was determined according to the EDANA method NWSP 251.0.R2 (19). The results are also summarized in Table 2.

[0144] Table 2:

[0145] Comparative example

[0146] The comparative examples 16 and 17 show that the coating of base polymer particles with polyethylene glycol has no noticeable effect on the flow rate.

Claims

241185 19 Patent claims 1. Method for coating surface-crosslinked superabsorbent particles, characterized in that the superabsorbent particles are coated with at least water and at least one polyethylene glycol with an average molar mass of 400 to 8000 g / mol.

2. The method according to claim 1, characterized in that the polyethylene glycol has an average molar mass of 2000 to 6000 g / mol.

3. Method according to claim 1 or 2, characterized in that the superabsorbent particles are coated with at least 3 wt.% water.

4. Method according to one of claims 1 to 3, characterized in that the superabsorbent particles are coated with 200 to 2500 ppm, based on the superabsorbent particles, of polyethylene glycol.

5. Method according to one of claims 1 to 3, characterized in that the superabsorbent particles are coated with 500 to 1000 ppm, based on the superabsorbent particles, of polyethylene glycol.

6. Method according to one of claims 1 to 5, characterized in that the superabsorbent particles have a temperature of at least 30°C immediately before coating.

7. Method according to one of claims 1 to 5, characterized in that the superabsorbent particles have a temperature of at least 70°C immediately before coating.

8. Method according to any one of claims 1 to 7, characterized in that the polyethylene glycol is used as a solution.

9. The method according to claim 8, characterized in that an aqueous solution is used.

10. Superabsorbent particles obtainable according to a method of claims 1 to 9, wherein the superabsorbent particles are coated at least with water and at least one polyethylene glycol having an average molar mass of 400 to 8000 g / mol.

11. Superabsorbent particles according to claim 10, characterized in that the polyethylene glycol has an average molar mass of 2000 to 6000 g / mol. 241185 20 12. Superabsorbent particles according to claim 10 or 11, characterized in that the superabsorbent particles are coated with 200 to 2500 ppm, based on the superabsorbent particles, of polyethylene glycol.

13. Superabsorbent particles according to any one of claims 10 to 12, wherein the superabsorbent particles have a ratio of flow rate after coating to flow rate before coating, each measured according to EDANA-NWSP 251.0.R2 (19), of at least 1.

03.

14. Superabsorbent particles according to any one of claims 10 to 13, wherein the superabsorbent particles have a flow rate measured according to EDANA-NWSP 251.0.R2 (19) of at least 8.0 g / s.

15. Hygiene articles containing superabsorbent particles according to one of claims 10 to 14.

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