Process for producing superabsorbent particles

Abstract

A process for producing surface postcrosslinked superabsorbent particles, comprising polymerizing an aqueous monomer solution containing a small amount of reaction initiator to obtain a polymer gel, extruding the obtained polymer gel through a die plate, drying the extruded polymer gel on an air circulation belt dryer having one or more zones, grinding and classifying the obtained polymer particles, and then thermally surface postcrosslinking, wherein the temperature of the drying gas supplied in the process of drying in the front zone of the air circulation belt dryer is 120-160°C, and the speed of the supplied air is 1.2-3.0m / s.

Description

[Technical field]

[0001] The present invention relates to a process for producing surface postcrosslinked superabsorbent particles, which comprises polymerizing an aqueous monomer solution containing a small amount of reaction initiator to obtain a polymer gel, extruding the obtained polymer gel through a die plate, drying the extruded polymer gel on an air circulation belt dryer having one or more zones, grinding and classifying the obtained polymer particles, and then thermally surface postcrosslinking, in which the temperature of the drying gas supplied in the process of drying in the front zone of the air circulation belt dryer is 120-160°C, and the speed of the drying gas supplied is 1.2-3.0m / s. [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, pages 71-103.

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

[0005] EP 0 289 338 A2 describes a process for drying a polymer gel by means of a gas containing water vapour.

[0006] EP 1 002 806 A1 describes a process for drying a polymer gel in three defined drying compartments.

[0007] WO 2006 / 100300 A1 describes a process in which a polymer gel is dried on a belt dryer set up with a defined temperature profile.

[0008] EP 2557095 A1, WO 2014 / 118024 A1 and WO 2015 / 169912 A1 describe a process of gentle extrusion of polymer gels to improve saline flow conductivity (SFC) and free swell ratio (FSR).

[0009] WO 2018 / 114702 A1 and WO 2018 / 114703 A1 describe single screw extruders that are particularly suitable for the extrusion of polymer gels. Summary of the Invention [Problem to be solved by the invention]

[0010] The object of the present invention was to provide an improved process for producing surface postcrosslinked superabsorbent particles, in particular for producing surface postcrosslinked superabsorbent particles having a rapid liquid absorbency (T20) of 20 g / g or a rapid liquid volumetric absorbency (VAUL) under a pressure of 0.3 psi (2.07 kPa). In addition, the surface postcrosslinked superabsorbent particles must have a low residual monomer content. Meanwhile, the centrifuge retention capacity (CRC) and the centrifuge retention capacity (CRC) of 49.2 g / cm3 are not exceeded. 2 The total absorbency under pressure (AUHL) had to be at a maximum value. [Means for solving the problem]

[0011] The purpose of this is to a) at least one ethylenically unsaturated monomer having an acid group and which is at least partially neutralized; b) at least one cross-linking agent; c) at least one initiator; 1. A process for producing surface postcrosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension comprising: In a process for producing surface postcrosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension to obtain a polymer gel, extruding the obtained polymer gel through a die plate, drying, grinding and classifying the extruded polymer gel in an air circulation belt dryer having one or more zones, and then surface postcrosslinking the obtained polymer particles thermally, it is achieved by a process in which not more than 0.14% by weight of initiator c), based on monomer a), is used before neutralization, the temperature of the drying gas fed to the front zone of the air circulation belt dryer is 120-160° C. for at least 50% of the total residence time, the velocity of the drying gas fed to the front zone of the air circulation belt dryer is 1.2-3.0 m / s for at least 20% of the total residence time, and the front zone of the air circulation belt dryer is the zone in the air circulation belt dryer in which the moisture content of the polymer gel to be dried is at least more than 20% by weight at the beginning of the respective zone.

[0012] The present invention is based on the finding that the extrusion of the polymer gel before drying alone does not affect the liquid absorbency of 20 g / g (T20). Similarly, the extractables content seems to have a significant effect. The extractables content, in contrast, can be controlled by the amount of initiator during polymerization and the drying conditions. The important thing here is that the drying is carried out at a relatively low temperature and relatively quickly. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In a particular embodiment of the present invention, the velocity of the drying gas fed into the front region of the air circulation belt dryer is further 0.1-1.15 m / s for 10%-80% of the total residence time in the front region, with the lower velocity region upstream of the higher velocity region, which allows for more uniform and better drying of the polymer gel layer in the air circulation belt dryer.

[0014] The steam content of the drying gas fed to the front region of the air circulation belt dryer is in each case preferably at least 200 g, more preferably at least 250 g and most preferably at least 300 g per kg of dry drying gas.

[0015] The thermal surface postcrosslinking is preferably carried out at a maximum temperature of at least 180°C, more preferably at least 185°C, and most preferably at least 190°C.

[0016] In each case, preferably not more than 0.12% by weight, more preferably not more than 0.10% by weight, particularly preferably not more than 0.08% by weight, very particularly preferably not more than 0.06% by weight and most preferably not more than 0.04% by weight of initiator c), based on the monomer a) before neutralization, is used.

[0017] In the front region of the air circulating belt dryer, the temperature of the drying gas supplied is preferably 125-155°C, more preferably 130-150°C, and most preferably 135-145°C.

[0018] In the front region of the air circulating belt dryer, the velocity of the drying gas supplied is preferably 1.3-2.8 m / sec, more preferably 1.4-2.6 m / sec, most preferably 1.5-2.4 m / sec.

[0019] The front zone of the air circulating belt dryer is that zone of the air circulating belt dryer in which the moisture content of the polymer gel to be dried is preferably greater than 25% by weight, more preferably greater than 29% by weight, and most preferably greater than 32% by weight, at least at the start of the respective zone.

[0020] The temperature of the polymer gel during the extrusion process is preferably 70 to 125°C, more preferably 80 to 115°C, and most preferably 90 to 105°C.

[0021] The water content of the polymer gel during the extrusion process is preferably 20% to 70% by weight, more preferably 30% to 65% by weight, and most preferably 40% to 60% by weight.

[0022] The hole openings in the die plate preferably have a diameter of 2 to 20 mm, more preferably 4 to 15 mm, and most preferably 6 to 10 mm.

[0023] The hole openings in the die plate preferably have a length of 15 to 45 mm, more preferably 20 to 40 mm, and most preferably 25 to 35 mm.

[0024] The manufacture of superabsorbents is described in detail below.

[0025] Superabsorbents are produced by polymerizing monomer solutions or suspensions and are typically water-insoluble.

[0026] The monomers a) 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.

[0027] Suitable monomers a) 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 particular preference is given to acrylic acid.

[0028] Monomer a) typically contains a polymerization inhibitor as a storage stabilizer, preferably hydroquinone monomethyl ether (MEHQ).

[0029] Suitable crosslinking agents b) are compounds that have at least two groups suitable for crosslinking.Such groups include, for example, free radically polymerizable ethylenically unsaturated groups in polymer chains and functional groups that can form covalent bonds with the acid groups of monomer a).In addition, polyvalent metal salts that can form coordinate bonds with at least two acid groups of monomer a) are also suitable as crosslinking agents b).

[0030] Suitable crosslinkers b) include, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described, for example, in EP 0 530 438 A1, EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, diacrylates and triacrylates, as described in DE 03 / 104301 A1, WO 03 / 104301 A1 and DE 10331450 A1, mixed acrylates which contain further ethylenically unsaturated groups as well as acrylate groups, as described in DE 10331456 A1, DE 10355401 A1 or crosslinker mixtures, as described for example in DE 19543368 A1, DE 19646484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0031] The amount of crosslinker b) is preferably 0.05% to 1.5% by weight, more preferably 0.1% to 1% by weight, and most preferably 0.3% to 0.6% by weight, in each case calculated based on the total amount of monomer a) used. As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases, reaching 21.0 g / cm 2 The absorbency exceeds the maximum value under a pressure of (AUL 0.3 psi).

[0032] The initiator c) used can be any compound that generates free radicals under polymerization conditions, for example thermal initiators, redox initiators or photoinitiators. Suitable thermal initiators are peroxomonosulfates and peroxodisulfates, as well as peroxomonophosphates and peroxodiphosphates. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. It is preferred to use a mixture of thermal initiators and redox initiators, for example sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. 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-sulfinatoacetic acid and sodium bisulfite. Such mixtures are available as Brueggolite® FF6 and Brueggolite® FF7 (Brueggemann Chemicals; Heilbronn; Germany).

[0033] The amount of reaction initiator c) is in each case based on the monomer a) before neutralization at most 0.14% by weight, preferably at most 0.12% by weight, more preferably at most 0.10% by weight, particularly preferably at most 0.08% by weight, very particularly preferably at most 0.06% by weight and most preferably at most 0.04% by weight.

[0034] Typically, an aqueous monomer solution is used. 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. It is also possible to use a monomer suspension, i.e. a monomer solution containing monomer a) in excess of its solubility, e.g. sodium acrylate. An increased water content leads to increased energy consumption in the subsequent drying, while a decreased water content may simply not remove the heat of polymerization sufficiently.

[0035] To function optimally, the preferred polymerization inhibitors require dissolved oxygen. Thus, the monomer solution may be inerted, i.e., flushed with an inert gas, preferably nitrogen or carbon dioxide, to remove dissolved oxygen prior to polymerization. The oxygen content of the monomer solution is preferably reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, and most preferably less than 0.1 ppm by weight prior to polymerization.

[0036] Suitable reactors for polymerization are, for example, kneading reactors or belt reactors. In kneaders, the polymer gel formed during polymerization of aqueous monomer solutions or suspensions is continuously comminuted, for example by counter-rotating stirrer shafts, as described in WO 2001 / 038402 A1. It is also possible to use kneaders with simultaneously rotating kneading shafts. Polymerization on belts is described, for example, in DE 3825366 A1 and US Pat. No. 6,241,928.

[0037] The resulting polymer gel is subsequently extruded through a die plate. The hole openings of the die plate are essentially not limited with respect to their shape and may be, for example, circular, elliptical, rectangular, triangular, hexagonal, star-shaped or irregular. The hole openings of the die plate are preferably circular. The diameter of the holes is preferably in the range of 2-20 mm, more preferably 4-15 mm, most preferably 6-10 mm. In the case of non-circular openings, the diameter of the holes is defined as the equivalent diameter on an area basis, i.e. as the diameter of a circle of the same cross-sectional area.

[0038] The length of the hole in the die plate is preferably in the range of 15-45 mm, more preferably 20-40 mm, and most preferably 25-35 mm. If the hole is a drilled hole in the die plate, the thickness of the die plate corresponds to the length of the hole. The opening may also be implemented in the form of a tubular insert in the die plate, which may protrude beyond the die plate. In this case, the length of the hole corresponds to the length of the insert.

[0039] The extruder typically consists of an elongated housing, an exit orifice in a die plate, and at least one screw shaft that rotates within the housing and conveys the polymer gel toward the exit orifice while generating back pressure. Generally, the polymer gel is extruded from within the extruder at high pressure through the die plate to the periphery. To prevent the polymer gel from being excessively cooled or heated during extrusion, the extruder is preferably trace heated, more preferably trace heated with steam, or trace cooled, as necessary. Extrusion can be carried out either continuously or batchwise.

[0040] Particularly suitable extruders are described, for example, in WO 2018 / 114702 A1 and WO 2018 / 114703 A1.

[0041] If the polymerization is carried out in a kneading reactor, the pressure drop across the die plate during extrusion is preferably 5-45 bar, more preferably 10-40 bar, most preferably 15-35 bar, and the aperture ratio of the die plate is preferably 5.0%-50%, more preferably 7.5%-30%, most preferably 10.0%-20%. The aperture ratio is defined as the ratio of the opening area of ​​the die plate (sum of the hole areas) to the maximum usable area of ​​the die plate.

[0042] If the polymerization is carried out by means of a belt reactor, the pressure drop across the die plate during extrusion is preferably 3-15 bar, more preferably 4-14 bar, most preferably 5-13 bar, and the aperture ratio of the die plate is preferably 35%-75%, more preferably 40%-70%, most preferably 45%-65%. The aperture ratio is defined as the ratio of the opening area of ​​the die plate (sum of the hole areas) to the maximum usable area of ​​the die plate.

[0043] The polymer gel is subjected to mechanical energy input during extrusion, in particular by the action of the rotating screw shaft. If the energy input is too high, damage to the internal structure of the polymer gel occurs.

[0044] The energy input can be influenced, for example, by the ratio of the internal length to the internal diameter (L / D) of the extruder, which is preferably 1 to 6.0, more preferably 2 to 5.5, and most preferably 3 to 5.0.

[0045] The specific mechanical energy (SME) introduced during the extrusion process is preferably 2.5-60 kWh / t, more preferably 5.0-50 kWh / t, and most preferably 10.0-40 kWh / t. The specific mechanical energy (SME) is the motor power (kW) of the extruder divided by the throughput of the polymer gel (t / h). This avoids damage to the polymer gel during the extrusion process.

[0046] During extrusion, the polymer gel preferably has a temperature in the range of 70 to 125°C, more preferably 80 to 115°C, and most preferably 90 to 105°C.

[0047] The water content of the polymer gel before passing through the die plate is preferably 20% by weight to 70% by weight, more preferably 30% by weight to 65% by weight, and most preferably 40% by weight to 60% by weight. Since extrusion may involve evaporation of water, the water content of the polymer gel generally decreases during extrusion. The ratio of the water content of the polymer gel after passing through the die plate to the water content of the polymer gel before passing through the die plate (FG post-extr / FG pre-extr ) is preferably at least 0.99, more preferably at least 0.95, and most preferably at least 0.91.

[0048] The acid groups of the polymer gel are typically partially neutralized. Neutralization is preferably carried out at the monomer stage. Neutralization is typically carried out by mixing a neutralizing agent as an aqueous solution or preferably as a solid. The degree of neutralization is preferably between 40 and 85 mol%, more preferably between 50 and 80 mol% and most preferably between 60 and 75 mol%, and customary 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. Instead of the alkali metal salts, it is also possible to use ammonium salts. Particularly preferred alkali metals are sodium and potassium, while sodium hydroxide, sodium carbonate or sodium hydrogen carbonate, as well as mixtures thereof, are very particularly preferred. Solid carbonates and hydrogen carbonates can also be introduced here in encapsulated form, preferably directly into the monomer solution before polymerization, into the polymer gel during or after polymerization and before drying them. Encapsulation is accomplished by coating the surface with an insoluble or slowly soluble material (e.g., a film-forming polymer, an inert inorganic material, or a meltable organic material), which retards dissolution and reaction of the solid carbonate or bicarbonate so that carbon dioxide is not released until during drying, and the resulting superabsorbent has high internal porosity.

[0049] The extruded polymer gel is then dried by an air circulating belt dryer with one or more zones until the moisture content is preferably 0.5-10 wt%, more preferably 1-7 wt%, and most preferably 2-5 wt%, as measured by EDANA recommended test method number WSP 230.2-05 "Mass loss on heating". The zones of the air circulating belt dryer are spatially separated areas where the drying conditions such as temperature, velocity and humidity of the drying gas can be individually adjusted. In the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, page 89, Figure 3.6, an air circulating belt dryer with five zones and one cooling zone is shown. If the humidity is too high, the glass transition temperature Tg of the dried polymer gel may become too low and it may simply be difficult to process further. If the humidity 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 water content of the polymer gel before drying is preferably 20% to 70% by weight, more preferably 30% to 65% by weight, and most preferably 40% to 60% by weight. The dried polymer gel is then crushed and optionally coarsely ground.

[0050] For a rapid liquid absorption (T20) of 20 g / g, the drying conditions in the front zone of the air circulation belt dryer are important, where the moisture content of the polymer gel to be dried is at least at the beginning of the respective zone above 20% by weight, preferably above 25% by weight, more preferably at least 29% by weight, most preferably at least 32% by weight.

[0051] In the front region of the air circulating belt dryer, the temperature of the drying gas supplied for at least 50% of the total residence time in the front region, preferably at least 60%, more preferably at least 70%, and most preferably at least 80%, is 120 to 160°C, preferably 125 to 155°C, more preferably 130 to 150°C, and most preferably 135 to 145°C.

[0052] In the front region of the air circulating belt dryer, the velocity of the drying gas supplied for at least 20% of the total residence time in the front region, preferably at least 30%, more preferably at least 40%, and most preferably at least 50%, is 1.2 to 3.0 m / sec, preferably 1.3 to 2.8 m / sec, more preferably 1.4 to 2.6 m / sec, and most preferably 1.5 to 2.4 m / sec.

[0053] The number of front zones is not limited in any way. If the air circulation belt dryer has, for example, only a single zone, and all zones meet the condition regarding moisture content, the front zone in the context of the present invention includes the entire air circulation belt dryer.

[0054] In a particular embodiment of the present invention, the drying gas supplied at a lower velocity is set in the first region of the air circulation belt dryer.Then, in the front region of the air circulation belt dryer, the velocity of the drying gas supplied in the range of 10% to 50%, preferably in the range of 15% to 70%, more preferably in the range of 20% to 60%, and most preferably in the range of 25% to 50% of the total residence time in the front region is further 0.1 to 1.15 m / s, preferably 0.3 to 1.10 m / s, more preferably 0.5 to 1.05 m / s, and most preferably 0.7 to 1.00 m / s.

[0055] If an air circulation belt dryer has a total of 10 zones, the residence time in each zone is 5 minutes, and the starting value of the moisture content of the polymer gel to be dried is met in only the first four zones, then the total residence time in the front zones is 20 minutes.

[0056] In a particular embodiment, if the velocity of the supplied dry gas is 1.00 m / sec in the first region and 2.0 m / sec in the three downstream regions, the residence time is calculated at a supplied dry gas velocity of 1.00 m / sec for 25% of the total residence time in the forward region, and at a supplied dry gas velocity of 2.0 m / sec for 75% of the total residence time in the forward region.

[0057] The air flowing over the polymer gel to be dried in the air circulation belt dryer can be from above or below. For uniform drying, it is suitable that the air flowing over the polymer gel to be dried is first from below and then from above during about 1 / 3 of the residence time in the air circulation belt dryer. Such a procedure and its advantages are described in WO 2006 / 100300 A1.

[0058] Suitable drying gases are, for example, air, nitrogen and mixtures of air and nitrogen. Alternatively, drying can be carried out using superheated steam as the drying gas, as described in Chapter 19 "Superheated Steam Drying" of "Handbook of Industrial Drying", 3rd edition, 2006, ISBN 9781420017618.

[0059] In the front region of the air circulation belt dryer, the water vapor content for at least 50%, preferably at least 60%, more preferably at least 70% and most preferably at least 80% of the total residence time in the front region is in each case preferably at least 200 g, more preferably at least 250 g and most preferably at least 300 g per kg of dried drying gas, which results in a good decomposition of the unconverted monomers a) during drying.

[0060] The water vapor content of the supplied drying gas can be achieved by actively supplying water by nozzles or sprayers, by supplying water vapor, or by wetting the drying material. It is also possible for the water vapor content to originate entirely or partly from the drying material itself, for example in terms of correspondingly controlling the fresh air supply and appropriately adjusting the ventilation of the drying material itself. For example, it is possible to use a lower drying temperature, a smaller fresh air supply, a slower flow through the drying material in the front region.

[0061] The dried polymer gel is then typically ground and classified, and the equipment used for grinding may typically be a single or multi-stage roll mill, preferably a two or three-stage roll mill, a pin mill, a hammer mill or a vibratory mill.

[0062] The average particle size of the polymer particles taken as the product fraction is preferably at least 200 μm, more preferably 250-600 μm, and very particularly 300-500 μm. The average particle size of the product fraction can be measured by EDANA recommended test method No. WSP 220.2-05 "Particle Size Distribution", in which the mass percentages of the selected fractions are plotted in cumulative form and the average particle size is determined graphically. The average particle size in this specification is the value of the mesh size that occurs at 50% cumulative weight.

[0063] The proportion of particles having a particle size above 150 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0064] Polymer particles that are too small will reduce the permeability (SFC), so the proportion of excessively small polymer particles ("fines") needs to be reduced.

[0065] Excessively small polymer particles are therefore typically removed and recycled into the process. This is preferably done before, during or immediately after polymerization, i.e., before drying of the polymer gel. Excessively small polymer particles can be wetted with water and / or aqueous surfactants before or during recycling.

[0066] It is also possible that the excessively small polymer particles are removed after a later process step, for example a surface postcrosslinking step or another coating step, in which case the recycled excessively small polymer particles are surface postcrosslinked or coated in another way, for example with fumed silica.

[0067] When a kneading reactor is used for the polymerization, the excessively small polymer particles are preferably added during the last third of the polymerization.

[0068] If too small polymer particles are added at a very early stage, for example, in the monomer solution, this will reduce the centrifuge retention capacity (CRC) of the water-absorbing polymer particles obtained.However, this reduction in centrifuge retention capacity (CRC) can be compensated for, for example, by adjusting the amount of crosslinker b) used.

[0069] If excessively small polymer particles are added at a very late stage, for example to a device connected downstream of the polymerization reactor, for example just before an extruder, the excessively small polymer particles may simply be difficult to incorporate into the resulting polymer gel. However, excessively small polymer particles that are not sufficiently incorporated will separate again from the dry polymer gel during grinding and will therefore be removed again in the process of classification, increasing the amount of excessively small polymer particles that are recycled.

[0070] The proportion of particles having a size of at most 850 μm is preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight.

[0071] The proportion of particles having a size of at most 600 μm is preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight.

[0072] Excessively large polymer particles reduce the free swell ratio, so the proportion of excessively large polymer particles should be low as well.

[0073] Therefore, excessively large polymer particles are typically removed and recycled to the grinding of the dried polymer gel.

[0074] To further improve the properties, the polymer particles are thermally surface crosslinked.Suitable surface crosslinkers are compounds that contain a group capable of forming a covalent bond with at least two carboxylate groups of the polymer particles.Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP-A-0083022A2, EP-A-0543303A1 and EP-A-0937736A2, difunctional or polyfunctional alcohols, as described in DE-A-3314019A1, DE-A-3523617A1 and EP-A-0450922A2, or β-hydroxyalkylamides, as described in DE-A-10204938A1 and US Pat. No. 6,239,230.Particularly suitable surface crosslinkers are ethylene carbonate and its derivatives, and 2-oxazolidones and its derivatives. Ethylene carbonate and N-(2-hydroxyethyl)-2-oxazolidinone are particularly preferred.

[0075] The amount of the surface postcrosslinking agent is preferably 0.001% by weight to 2% by weight, more preferably 0.02% by weight to 1% by weight, and most preferably 0.05% by weight to 0.2% by weight, in each case based on the polymer particles.

[0076] Similar to surface postcrosslinkers, it is possible to apply multivalent cations to the particle surface.

[0077] Polyvalent cations that can be used in the process of the invention are, for example, divalent cations such as zinc, magnesium, calcium and strontium cations, trivalent cations such as aluminum, iron, chromium, rare earth and manganese cations, tetravalent cations such as titanium and zirconium cations. Possible counterions are chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylates, for example acetate and lactate. Aluminum hydroxide, sulfate and lactate are preferred.

[0078] 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.

[0079] Surface postcrosslinking is typically carried out in such a way that the dried polymer particles are sprayed with a solution of the surface postcrosslinker. After spray application, the polymer particles coated with the surface postcrosslinker are subjected to a heat treatment.

[0080] The spray application of the solution of the surface postcrosslinker is preferably carried out in a mixer with moving mixing tools, 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 very particularly preferred. Horizontal mixers and vertical mixers are differentiated by the position of the mixing shaft, i.e. horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft. Suitable mixers are, for example, horizontal Pflugschar® ploughshare mixers (Gebr. Loedige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill mixers (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands). However, it is also possible to spray the solution of the surface postcrosslinker in a fluidized bed.

[0081] 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.

[0082] The heat treatment is preferably carried out in a contact dryer, more preferably a paddle dryer, most preferably a disk dryer. Suitable dryers are, for example, Hosokawa Bepex® horizontal paddle dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® disk dryer (Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® dryer (Metso Minerals Industries Inc.; Danville; USA) and Nara paddle dryer (NARA Machinery Europe; Frechen; Germany). In addition, fluidized bed dryers can also be used.

[0083] Surface postcrosslinking can be carried out in the mixer body by heating the jacket or blowing in hot air. Also suitable is a downstream dryer, such as a tray dryer, a rotary tube oven or a heatable screw. It is particularly advantageous to mix in a fluidized bed dryer and carry out thermal surface postcrosslinking.

[0084] The reaction temperature is preferably in the range of 180-250° C., more preferably 185-220° C., and most preferably 190-210° C. The preferred residence time at this temperature is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and typically up to 60 minutes.

[0085] Correspondingly, the higher the reaction temperature, the lower the total centrifuge retention capacity (CRC) and the lower the total centrifuge retention capacity (CRC) to 49.2 g / cm 2 Particularly high values ​​for absorbency under pressure (AUHL) are achieved.

[0086] When using a surface postcrosslinker having epoxy groups, such as ethylene glycol diglycidyl ether, the surface postcrosslinking temperature can be much lower.

[0087] 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.

[0088] To further improve properties, the surface postcrosslinked polymer particles may be coated or remoisturized.

[0089] The re-wetting is preferably carried out at 30-80°C, more preferably 35-70°C, most preferably 40-60°C. At too low temperatures the polymer particles tend to form agglomerates, while at higher temperatures the water evaporates already to a noticeable extent. The amount of water used for the re-wetting is preferably 1% to 10% by weight, more preferably 2% to 8% by weight, and most preferably 3% to 5% by weight. Re-wetting increases the mechanical stability of the polymer particles and reduces their tendency to become electrostatically charged. Re-wetting is advantageously carried out in a cooler after thermal surface post-crosslinking.

[0090] Suitable coatings for improving the swelling rate and gel bed permeability (GBP) are, 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 adsorption are, for example, polyols. Suitable coatings for combating the undesirable caking tendency of polymer particles are, for example, fumed silicas such as Aerosil® 200, precipitated silicas such as Sipernat® D17, and surfactants such as Span® 20.

[0091] method: The standard test methods described below and designated "WSP" are described in "Standard Test Methods for the Nonwovens Industry", 2005 edition, published jointly by the Worldwide Strategic Partners EDANA (Herrmann-Debrouxlaan 46, 1160 Oudergem, 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.

[0092] Unless otherwise stated, measurements should be carried out at an ambient temperature of 23±2° C. and a relative air humidity of 50±10%. The superabsorbent particles are thoroughly mixed before the measurements.

[0093] Residual Monomer Residual monomer content is determined by EDANA recommended test method number 210.2(05) "Residual Monomers".

[0094] moisture content Moisture content is measured by EDANA recommended test method number WSP 230.2(05) "Mass loss on heating". If the moisture content is greater than 5% by weight, the drying time at 105±2°C should be extended until constant weight is achieved.

[0095] The moisture content of the polymer gel to be dried is measured by drying 1.0-1.5 kg of polymer gel to constant weight at 105±2°C. Drying can be accelerated by moderately grinding the polymer gel.

[0096] In the case of relatively small quantities, for example drying in a belt dryer simulator, it is also possible to carry out intermediate weighing operations with the total amount of polymer gel. In this case, the amount of polymer gel may also be less than 1.0 kg. In the last step, drying is carried out at 105±2° C. until constant weight is reached. The moisture content of the polymer gel in the intermediate weighing operations is then ascertained by calculation.

[0097] Centrifuge Retention Capacity Centrifuge Retention Capacity (CRC) is measured by EDANA recommended test method number WSP 241.2(05) "Liquid Retention Capacity in Saline After Centrifugation."

[0098] 21.0g / cm 2 Absorbency under pressure (absorbency under load) 21.0g / cm 2 Absorbency under Pressure (AUL 0.3 psi) is measured by EDANA recommended test method number WSP242.205 "Absorbency under Pressure, Gravimetric."

[0099] 49.2g / cm 2 Absorbency under high pressure (absorbency under high load) 49.2g / cm 2 The absorbency under pressure (AUHL) is 21.0g / cm 2 (0.3psi) pressure instead of 49.2g / cm 2 The absorbency test is measured by a method similar to EDANA recommended test method number WSP 242.2(05) "Absorbency Under Pressure, Gravimetric" except that a pressure of 0.7 psi (1.0 psi) is set.

[0100] extract The extractables content of water-absorbing polymeric particles is determined by EDANA recommended test method number WSP 270.2(05) "Extractables".

[0101] Liquid absorbency of 20g / g (T20) The liquid absorbency at 20 g / g (T20) is measured by the "K(t) Test Method (Dynamic Effective Permeability and Uptake Rate Measurement Test Method)" described in EP 2535027 A1, pages 13-18.

[0102] Volumetric Absorption of Liquids at 0.3 psi (2.07 kPa) Pressure (VAUL) For volumetric absorbency (VAUL) of liquid under a pressure of 0.3 psi (2.07 kPa), the τ value is measured by the "Volumetric Absorbency Under Load (VAUL)" test method described in EP 2922882 B1, page 22. In this specification, the τ value is referred to as the "characteristic swelling time."

[0103] Saline flow conductivity Saline Flow Conductivity (SFC) is measured by the Urinary Permeability Measurement (UPM) Test Method, a test method described on pages 19-22 of EP 2535698 A1. EXAMPLES

[0104] Examples 1 to 13 polymerization: An acrylic acid / sodium acrylate solution is prepared by successively mixing deionized water, a 50% by weight sodium hydroxide solution and acrylic acid, so as to obtain a degree of neutralization corresponding to 71.0 mol %. The solids content of the monomer solution is 41.0% by weight.

[0105] The crosslinker b) used is 3-ethoxylated glyceryl triacrylate (purity about 85% by weight). The amount used is 0.45% by weight, based on the acrylic acid used. Furthermore, the monomer solution is composed of 0.75% by weight of polyethylene glycol-4000 (polyethylene glycol with an average molar mass of 4000 g / mol), based on the acrylic acid used.

[0106] Free radical polymerization is initiated using 0.0005% to 0.0020% by weight of hydrogen peroxide, 0.06% to 0.15% by weight of sodium peroxodisulfate, and 0.0076% by weight of ascorbic acid, in each case based on the acrylic acid used. The exact conditions for the individual examples can be found in Table 1.

[0107] The monomer solution was added to 6.3 ml 3 The monomer solution was then introduced into a List Contikneter continuous kneading reactor (LIST AG, Arisdorf, Switzerland) with a capacity of 1000 t / h. The throughput of the monomer solution was approximately 20 t / h.

[0108] The monomer solution was inerted with nitrogen between the addition of the crosslinker and the addition of the hydrogen peroxide and sodium peroxodisulfate solutions. Ascorbic acid was metered directly into the reactor.

[0109] After about 50% of the residence time, about 1200 kg / h of polymer particles obtained in a manufacturing process by grinding and classification, having a particle size of less than 150 μm, were additionally metered into the reactor. The residence time of the reaction mixture in the reactor is about 15 minutes.

[0110] Extrusion: The resulting polymer gel is metered into a 650 EX extruder (ECT-KEMA GmbH, Girbigsdorf, Germany).

[0111] The temperature of the polymer gel during the extrusion process is about 115-130 °C. The die plate has 2764 holes, and the diameter of the holes is 8 mm. The thickness of the die plate is 33 mm. The opening ratio of the die plate is 42%. The ratio of the internal length to the internal diameter (L / D) of the extruder is 4. The pressure drop across the die plate is about 27-28 bar.

[0112] Drying: The polymer gel samples are taken while still hot and loosely layered in a stationary belt dryer simulator.

[0113] The belt dryer simulator is a cylindrical stainless steel pot with a sieve plate. Here, the flow of drying air through the polymer gel is either from below or from above. It is possible to control the direction, temperature, humidity (steam injection) and amount (=speed) of the drying air. The simulator is programmable and can continuously generate any different drying profile, even as the drying operation progresses over time. The belt dryer simulator is capable of simulating drying in an air circulation belt dryer with one or more zones.

[0114] When introducing the polymer gel, it must be ensured that the dry air can flow through the porous polymer gel bed. The height of the polymer gel bed is 9 cm. For this, 1285 g of extruded polymer gel is required.

[0115] The polymer gel is dried for 25 minutes. The air flow through the polymer gel is first from below for 1 / 3 of the time, then from above. The temperature of the drying air is 135-170°C. The velocity of the drying air is 1.0-2.0 m / sec. The drying air contains 100-700 g of water vapor per kg of drying air. The exact conditions for the individual examples can be found in Table 1.

[0116] Grinding and sieving: The dried polymer gel was coarsely crushed, pulverized by a three-stage roll mill, and sieved to a particle size of 150 to 700 μm. The sieving is carried out so that at least 95% by weight of the polymer particles have a particle size of 150 to 700 μm.

[0117] [Table 1]

[0118] [Table 2]

[0119] Surface post-crosslinking: 1.2 kg of the classified polymer particles are coated with a mixture of 1.41% by weight of isopropanol, 3.13% by weight of water, 0.07% by weight of N-hydroxyethyl-2-oxazolidinone, 0.07% by weight of propane-1,3-diol and 0.5% by weight of aluminum lactate (solution A) or a mixture of 2.54% by weight of water and 2.00% by weight of ethylene carbonate (solution B) by two-phase spray nozzle in a VT 5R-MK ploughshare mixer (Loedige Maschinenbau GmbH; Paderborn, Germany) equipped with a heating jacket at 23 ° C. and a shaft speed of 200 revolutions per minute, in each case based on the polymer particles used. The exact conditions of the individual examples can be found in Table 3.

[0120] After spray application, the product temperature is increased to 175°C to 185°C and the reaction mixture is held at this temperature for 45 minutes at a shaft speed of 50 revolutions per minute. The product obtained is cooled to ambient temperature and again sieved through a 700 μm sieve. The fraction with a particle size less than 700 μm is analyzed. The results are given in Table 3.

[0121] [Table 3]

[0122] A comparison of Example 1 and Example 8 shows that reducing the amount of initiator used in the polymerization results in improved T20.

[0123] Comparing Example 4 with Example 1 shows that extrusion prior to drying results in improved T20.

[0124] Comparing Example 5 with Example 1 shows that reducing the temperature of the drying air results in improved T20.

[0125] Comparing Example 6 with Example 5 shows that if the drying air velocity is too slow, drying is retarded.

[0126] Comparing Example 7 with Example 5 shows that increasing the dry air velocity results in improved T20.

[0127] A comparison of Example 1 and Example 2 shows that increasing the water vapor content of the drying air results in improved residual monomer content.

[0128] Comparing Example 3 with Example 1 shows that increasing the thermal surface postcrosslinking temperature resulted in improved aggregate CRC and AUHL.

[0129] Inventive Examples 8-13 have a better T20 of at least 12 seconds than Examples 1-6 (Comparative Examples).

[0130] Examples 14 to 18 polymerization: An acrylic acid / sodium acrylate solution is prepared by successively mixing deionized water, a 50% by weight sodium hydroxide solution and acrylic acid, so as to obtain a degree of neutralization corresponding to 71.0 mol %. The solids content of the monomer solution is 41.0% by weight.

[0131] The crosslinker b) used is 3-ethoxylated glyceryl triacrylate (purity about 85% by weight). The amount used is 0.45% by weight, based on the acrylic acid used. Furthermore, the monomer solution is composed of 0.75% by weight of polyethylene glycol-4000 (polyethylene glycol with an average molar mass of 4000 g / mol), based on the acrylic acid used.

[0132] Free radical polymerization is initiated using 0.0005% by weight hydrogen peroxide, 0.06% by weight sodium peroxodisulfate and 0.0076% by weight ascorbic acid, in each case based on the acrylic acid used. The exact conditions for the individual examples can be found in Table 1.

[0133] The monomer solution was added to 6.3 ml 3The monomer solution was then introduced into a List Contikneter continuous kneading reactor (LIST AG, Arisdorf, Switzerland) with a capacity of approx. 20 t / h. The throughput of the monomer solution was approx. 20 t / h.

[0134] The monomer solution was inerted with nitrogen between the addition of the crosslinker and the addition of the hydrogen peroxide and sodium peroxodisulfate solutions. Ascorbic acid was metered directly into the reactor.

[0135] After about 50% of the residence time, about 1200 kg / h of polymer particles obtained in a manufacturing process by grinding and classification, having a particle size of less than 150 μm, were additionally metered into the reactor. The residence time of the reaction mixture in the reactor is about 15 minutes.

[0136] Extrusion: The resulting polymer gel is metered into a 650 EX extruder (ECT-KEMA GmbH, Girbigsdorf, Germany).

[0137] The temperature of the polymer gel during the extrusion process is about 115-130 °C. The die plate has 2764 holes, and the diameter of the holes is 8 mm. The thickness of the die plate is 33 mm. The opening ratio of the die plate is 42%. The ratio of the internal length to the internal diameter (L / D) of the extruder is 4. The pressure drop across the die plate is about 27-28 bar.

[0138] Drying: The polymer gel samples are taken while still hot and loosely layered in a stationary belt dryer simulator.

[0139] The belt dryer simulator is a cylindrical stainless steel pot equipped with a sieve plate. Here, the flow of drying air through the polymer gel is either from above or below. It is possible to control the direction, temperature, humidity (steam injection) and amount (=speed) of the drying air. The simulator is programmable and can continuously generate any different drying profile, even as the drying operation progresses over time. The belt dryer simulator is capable of simulating drying in an air circulation belt dryer with one or more zones.

[0140] When introducing the polymer gel, it must be ensured that the dry air can flow through the porous polymer gel bed. The height of the polymer gel bed is 9 cm. For this, 1285 g of extruded polymer gel is required.

[0141] The polymer gel is dried for 25 minutes. Air flow through the polymer gel is first from below and then from above in the first three zones. Residence time in each zone is 2.5 minutes. The last zone is the cooling zone. The temperature of the drying air is 140-198°C. The velocity of the drying air is 1.0-2.0 m / sec. The drying air contains 75-350 g of water vapor per kg of drying air. The exact conditions for the individual examples can be found in Table 4.

[0142] Grinding and sieving: The dried polymer gel was coarsely crushed, pulverized by a three-stage roll mill, and sieved to a particle size of 150 to 700 μm. The sieving is carried out so that at least 95% by weight of the polymer particles have a particle size of 150 to 700 μm.

[0143] [Table 4]

[0144] [Table 5]

[0145] In Examples 14 to 17, a loose polymer gel layer was obtained. A loose layer may cause the drying air to bypass the polymer gel particles in an irregular flow path. In Example 18, the relatively slow drying air velocity immediately after the start of drying, combined with the relatively low temperature of the drying gas, resulted in a dense polymer gel layer.

[0146] [Table 6]

[0147] Surface post-crosslinking 1.2 kg of the classified polymer particles are coated with a mixture of 1.41 wt.-% isopropanol, 3.13 wt.-% water, 0.07 wt.-% N-hydroxyethyl-2-oxazolidinone, 0.07 wt.-% propane-1,3-diol and 0.5 wt.-% aluminum lactate (solution A) or with a mixture of 3.0 wt.-% water, 1.5 wt.-% propane-1,2-diol and 0.04 wt.-% ethylene glycol diglycidyl ether (solution B) by a two-phase spray nozzle in a VT 5R-MK ploughshare mixer (Loedige Maschinenbau GmbH; Paderborn, Germany) equipped with a heating jacket at 23 °C and a shaft speed of 200 revolutions per minute, in each case based on the polymer particles used. In solution A, classified polymer particles with a particle size of 100 to 600 μm were used. In solution B, classified polymer particles with a particle size of 300 to 600 μm were used. The exact conditions for the individual examples can be found in Table 6.

[0148] After spray application, the product temperature is increased to 185°C (solution A) or 160°C (solution B) and the reaction mixture is kept at this temperature for 45 minutes (solution A) or 30 minutes (solution B) at a shaft speed of 50 revolutions per minute. The product obtained is cooled to ambient temperature and again classified through a 700 μm sieve. The fraction with a particle size less than 700 μm is analyzed. The results are given in Table 6.

[0149]

Table 7

Claims

1. a) at least one ethylenically unsaturated monomer having an acid group and which is at least partially neutralized; b) at least one cross-linking agent; and c) at least one initiator; 1. A process for producing surface postcrosslinked superabsorbent particles by polymerizing an aqueous monomer solution or suspension comprising:

1. A process for producing surface-postcrosslinked superabsorbent particles by polymerizing the aqueous monomer solution or suspension to obtain a polymer gel, extruding the obtained polymer gel through a die plate, drying, crushing and classifying the extruded polymer gel in an air-circulating belt dryer having one or more zones, and then thermally surface-postcrosslinking the obtained polymer particles, wherein before the neutralization, not more than 0.14% by weight of initiator c), based on the monomer a), is used, the temperature of the drying gas fed to the front zone of the air-circulating belt dryer is 120-160°C for at least 50% of the total residence time, and the velocity of the drying gas fed to the front zone of the air-circulating belt dryer is 1.2-3.0 m / s for at least 20% of the total residence time, and the front zone of the air-circulating belt dryer is a zone in the air-circulating belt dryer in which the moisture content of the polymer gel to be dried is greater than 20% by weight, at least at the start of each zone.

2. 10. The process of claim 1, wherein the velocity of the drying gas supplied to the front region of the air circulation belt dryer is further between 0.1 and 1.15 m / sec for 10% to 80% of the total residence time in the front region, the lower velocity region being upstream of the higher velocity region.

3. 10. The process of claim 1, wherein the velocity of the drying gas supplied to the front region of the air circulation belt dryer is further between 0.7 and 1.00 m / sec for 25% to 50% of the total residence time in the front region, the lower velocity region being upstream of the higher velocity region.

4. The process according to any one of claims 1 to 3, characterized in that the drying gas fed to the front region of the air circulation belt dryer has a steam content of at least 200 g / kg of dried drying gas.

5. 4. The process according to any one of claims 1 to 3, wherein the thermal surface postcrosslinking is carried out at a maximum temperature of at least 180°C.

6. 4. The process according to claim 1, wherein not more than 0.04% by weight of initiator c), based on monomer a) before neutralization, is used.

7. 4. The process of any one of claims 1 to 3, wherein the temperature of the drying gas supplied to the front region of the air circulation belt dryer is 135 to 145°C for at least 80% of the total residence time in the front region.

8. 4. The process of claim 1, wherein the velocity of the drying gas supplied to the front region of the air circulation belt dryer is from 1.5 to 2.4 m / s for at least 50% of the total residence time in the front region.

9. 4. The process according to any one of claims 1 to 3, wherein the front zone of the air circulation belt dryer is a zone of the air circulation belt dryer in which the moisture content of the polymer gel to be dried is greater than 32% by weight at least at the start of the respective zone.

10. The process according to any one of claims 1 to 3, wherein the temperature of the polymer gel during extrusion is between 70 and 125°C.

11. The process according to any one of claims 1 to 3, wherein the temperature of the polymer gel during extrusion is between 90 and 105°C.

12. The process according to any one of claims 1 to 3, wherein the water content of the polymer gel during extrusion is between 20% and 70% by weight.

13. The process according to any one of claims 1 to 3, wherein the water content of the polymer gel during extrusion is between 40% and 60% by weight.

14. The process according to any one of claims 1 to 3, wherein the hole openings in the die plate have a diameter of 2 to 20 mm.

15. The process according to any one of claims 1 to 3, wherein the hole openings in the die plate have a length of 15 to 45 mm.

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