JP2015512990A - Method for hot surface post-crosslinking in a drum heat exchanger with reverse screw helix - Google Patents

Abstract

A method of post-crosslinking water absorbent polymer particles with a hot surface, wherein the polymer particles are coated with an aqueous solution, the coated polymer particles are deagglomerated, and the deagglomerated polymer particles are provided with a reverse screw helix. The invention relates to such a method in which a hot surface post-crosslinking is carried out using a drum-type heat exchanger.

Description

  The present invention is a method of post-crosslinking water-absorbing polymer particles, wherein the polymer particles are coated with an aqueous solution, the coated polymer particles are deagglomerated, and the deagglomerated polymer particles are converted into a reverse screw helix. To a hot surface post-crosslinking using a drum heat exchanger equipped with

  Water-absorbing polymers are used to produce diapers, tampons, menstrual bands and other sanitary products, but are also used as water retention agents in agricultural and horticulture. The water-absorbing polymer particles are also called superabsorbent.

  The production of water-absorbing polymer particles is described in the research paper “Modern Superabsorbent Polymer Technology”, F.A. L. Buchholz and A.M. T. T. et al. Graham, Wiley-VCH, 1998, pages 71-103.

The properties of the water-absorbing polymer can be adjusted, for example, by the amount of crosslinking agent used. As the amount of cross-linking agent increases, the amount of absorption under pressure of the Zentrifugen retention skapazitaet (CRC) and 21.0 g / cm ² (AUL 0.3 psi) takes a maximum value.

In order to further improve the use characteristics of the water-absorbing polymer particles, such as the permeability of the swollen gel layer in diapers (SFC) and the absorption under pressure of 49.2 g / cm ² (AUL 0.7 psi), The water-absorbing polymer particles are generally surface postcrosslinked. To that end, the degree of cross-linking of the particle surface is increased, whereby the absorption and centrifuge retention capacity (CRC) under pressure of 49.2 g / cm ² (AUL 0.7 psi) can be at least partially debindered. . This surface postcrosslinking can be carried out in the aqueous gel phase. However, among others, the dried, pulverized and sieved polymer particles (base polymer) are coated with a surface postcrosslinker and thermally postcrosslinked. Suitable crosslinking agents for this are compounds that can form covalent bonds with at least two carboxylate groups of the water-absorbing polymer particles.

  EP-A-1757645 A1 and EP-A-1757646 A1 disclose surface postcrosslinking of water-absorbing polymer particles in a rotating tube.

  German Offenlegungsschrift DE 102007024080 A1 teaches that the water-absorbing polymer particles are post-treated in a rotating vessel, for example with water.

  The object of the present invention was to provide an improved method for producing water-absorbing polymer particles, in particular improved surface postcrosslinking.

The subject is a method of post-crosslinking water-absorbing polymer particles with a hot surface, wherein the water-absorbing polymer particles a) at least one ethylenically unsaturated acid group which may be at least partially neutralized. A monomer having,
b) at least one cross-linking agent,
c) at least one initiator,
d) An aqueous monomer suspension or aqueous monomer suspension optionally containing one or more ethylenically unsaturated monomers and e) optionally one or more water-soluble polymers that can be copolymerized with the monomers described in a). In the process, wherein the polymer particles are coated with an aqueous solution, the coated polymer particles are deagglomerated, and the deagglomerated polymer particles are drum-shaped with a reverse screw helix. This has been solved by the method described above, where the hot surface is postcrosslinked using a heat exchanger.

  Preferably, the coating and the deagglomeration are carried out in a horizontal mixer, or the coating is carried out in a vertical mixer, and the deagglomeration is carried out in a horizontal mixer.

  In a preferred embodiment of the invention, the coated polymer particles are dried or heated during deagglomeration.

  The filling height of the drum heat exchanger is in particular 30 to 100%, particularly preferably 40 to 95%, particularly preferably 65 to 90%, relative to the height of the screw helix.

  The temperature of the water-absorbing polymer particles in the drum heat exchanger is in particular 120-220 ° C., particularly preferably 150-210 ° C., particularly preferably 170-200 ° C. and / or in the drum heat exchanger The residence time of the water-absorbing polymer particles at is in particular 10 to 120 minutes, particularly preferably 20 to 90 minutes, particularly preferably 30 to 60 minutes.

  The drum heat exchanger is usually electrically heated or heated using steam, in particular indirectly heated. Indirect heating means that the heating takes place through the rotating drum wall.

  A further subject of the present invention is an apparatus suitable for carrying out the method according to the invention. In particular, it includes a heatable horizontal mixer and a drum-type heat exchanger equipped with a reverse screw helix, a device for hot-surface postcrosslinking of water-absorbing polymer particles, and a vertical mixer, a heatable horizontal mixer And a hot surface post-crosslinking of water-absorbing polymer particles, including a drum heat exchanger with a reverse screw helix.

  The heatable horizontal mixer and drum heat exchanger, or the vertical mixer, heatable horizontal mixer and drum heat exchanger, among others, are connected directly in sequence.

  In a preferred embodiment of the present invention, a horizontal mixer that can be cooled is directly connected to the drum heat exchanger.

  Directly connected in series or directly connected means that the discharge from one device reaches the next device in the shortest possible path without intermediate storage.

  The present invention is based on the recognition that the result of hot surface postcrosslinking is very strongly influenced by residence time. In particular, in the case of water-absorbing polymer particles produced by the dropwise addition of a monomer solution, the permeability (SFC) of the swollen gel layer takes a noticeable maximum.

  Hot surface post-crosslinking in drum heat exchangers with reverse screw helices allows continuous hot surface post-crosslinking with a narrow residence time distribution and between individual helices under precise loading Remixing does not actually occur. The proportion of water-absorbing polymer particles having a too short residence time and a too long residence time and thus poor quality can thus be minimized.

  Next, the production of the water-absorbing polymer particles and the present invention will be further described.

  The water-absorbing polymer particles are produced by polymerizing a monomer solution or a monomer suspension, and are usually water-insoluble.

  Monomers a) are particularly water-soluble, ie their solubility in water at 23 ° C. is typically at least 1 g per 100 g of water, in particular at least 5 g per 100 g of water, particularly preferably at least 25 g per 100 g of water, in particular Preferably at least 35 g per 100 g of water.

  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. Acrylic acid is particularly preferred.

  Further suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids such as styrene sulfonic acid and 2-acrylamido-2-methylpropane sulfonic acid (AMPS).

  However, some monomers a) may also be used, for example a mixture of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid.

  Contaminants can have a significant effect on polymerization. The raw materials used should therefore have the highest possible purity. Therefore, it is often preferred to specifically purify monomer a). Suitable purification methods are described, for example, in WO 2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. Suitable monomers a) are, for example, according to WO 2004/035514 A1, 99.8460% by weight acrylic acid, 0.0950% by weight acetic acid, 0.0332% by weight water, 0.0203% by weight propionic acid, 0% furfural. Purified acrylic acid having 0.0001% by weight, 0.0001% by weight maleic anhydride, 0.0003% by weight diacrylic acid and 0.0050% by weight hydroquinone monomethyl ether.

  The proportion of acrylic acid and / or its salt relative to the total amount of monomers a) is in particular at least 50 mol%, particularly preferably at least 90 mol%, particularly preferably at least 95 mol%.

  Monomers a) usually contain polymerization inhibitors, in particular hydroquinone half ethers, as storage stabilizers.

  The monomer solution is in each case up to 250 ppm by weight of hydroquinone half ether, preferably at most 130 ppm by weight, particularly preferably at most 70 ppm by weight, preferably at least 10 ppm, based on the unneutralized monomer a). It contains ppm by weight, particularly preferably at least 30 ppm by weight, in particular 50 ppm by weight. For example, for the preparation of the monomer solution, ethylenically unsaturated monomers with acid groups having a corresponding content of hydroquinone half ether can be used.

  Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and / or α-tocopherol (vitamin E).

  Suitable crosslinking agents b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which may be radically polymerised into the polymer chain and functional groups which can form covalent bonds with the acid groups of the monomers a). Furthermore, polyvalent metal salts capable of forming a coordinate bond with at least two acid groups of the monomer a) are also suitable as the crosslinking agent b).

  Crosslinker b) is a compound having at least two polymerizable groups which may be radically polymerized into the polymer network. Suitable crosslinking agents b) are, for example, as described in EP-A-0530438 A1, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, trimethylol. Allylamine, tetraallylammonium chloride, tetraallyloxyethane, European Patent Application Publication No. 0547847 A1, European Patent Application Publication No. 0559476 A1, European Patent Application Publication No. 0632068 A1, WO 93/21237 A1 , WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and German Patent Publication No. 10331450. Similar to the description of A1, diacrylates and triacrylates, in addition to the acrylate groups, as described in German Offenlegungsschrift 103 31 456 A1 and German Offenlegungsschrift 10355401 A1. Mixed acrylates containing ethylenically unsaturated groups or, for example, German Offenlegungsschrift 19543368 A1, German Offenlegungsschrift 19646484 A1, WO 90/15830 A1 and WO 2002 / 032962 A crosslinker mixture as described in A2.

  Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane, methylenebismethacrylamide, 15 times ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine. is there.

  Particularly preferred crosslinking agents b) are, for example, glycerol which has been esterified to diacrylate or triacrylate with acrylic acid or methacrylic acid and ethoxylated and / or propoxylated several times, as described for example in WO 2003/104301 A1. is there. Particular preference is given to diacrylates and / or triacrylates of glycerol which have been ethoxylated 3 to 10 times. Particular preference is given to glycerol diacrylates or triacrylates which are ethoxylated and / or propoxylated once to five times. In most cases, triacrylates of glycerol that have been ethoxylated and / or propoxylated 3 to 5 times, especially triacrylates of glycerol that have been ethoxylated 3 times, are preferred.

  The amount of cross-linking agent b) is in particular 0.05 to 1.5% by weight, particularly preferably 0.1 to 1% by weight, particularly preferably 0.2 to 0. 6% by mass.

  As initiator c) all compounds that form radicals under polymerization conditions, such as thermal initiators, redox initiators, photoinitiators can be used. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. In particular, a mixture of thermal initiator and redox initiator, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid is used. However, a mixture of sodium salt of 2-hydroxy-2-sulfinatoacetic acid, disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite is used as the reducing component, among others. This type of mixture is available as Brueggolite® FF6 and Brueggolite® FF7 (Brugegmann Chemicals, Heilbrunn, Germany). However, it is also possible to use the disodium salt of 2-hydroxy-2-sulfonatoacetic acid alone as a reducing component.

  Ethylenically unsaturated monomers unsaturated with acid groups a) copolymerizable with ethylenically unsaturated monomers d) are, for example, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate. Dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

  As water-soluble polymers e), polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified celluloses such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, in particular starch, starch derivatives and modified celluloses can be used.

  Usually, an aqueous monomer solution is used. The water content of the monomer solution is in particular 40 to 75% by weight, particularly preferably 45 to 70% by weight, particularly preferably 50 to 65% by weight. It is also possible to use monomer suspensions, ie monomer solutions with an excess of monomer a), for example sodium acrylate. As the water content increases, the energy costs for subsequent drying increase, and when the water content decreases, the heat of polymerization can still be derived only insufficiently.

  Preferred polymerization inhibitors are required for optimal action of dissolved oxygen. Thus, the dissolved oxygen can be removed from the monomer solution prior to polymerization by deactivation, i.e. through the flow of an inert gas, in particular nitrogen or carbon dioxide. In particular, the oxygen content of the monomer solution is reduced to less than 1 ppm by weight, particularly preferably less than 0.5 ppm by weight, particularly preferably less than 0.1 ppm by weight, before polymerization.

  Suitable reactors are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel produced during the polymerization of the aqueous monomer solution or the aqueous monomer suspension is continuously finely pulverized by a plurality of opposed stirring shafts, as described in WO 2001/038402 A1. Polymerization on the belt is described, for example, in German Offenlegungsschrift 3825366 A1 and US Pat. No. 6,241,928. In the case of polymerization in a belt-type reactor, the polymer gel which has to be pulverized in a further process step is produced, for example, in an extruder or kneader.

  In order to improve the drying properties, the finely divided polymer gel obtained with a kneader can be further extruded.

  However, it is also possible to drop the aqueous monomer solution and polymerize the resulting droplets in a heated carrier gas stream. In this case, as described in WO 2008/040715 A2, WO 2008/052971 A1 and WO 2011/026876 A1, the process steps can include both polymerization and drying.

  The acid groups of the resulting polymer gel are usually partially neutralized. This neutralization is carried out in particular at the monomer stage. This is usually done by mixing the neutralizing agent as an aqueous solution, or advantageously by mixing the neutralizing agent as a solid. The degree of neutralization is in particular from 25 to 95 mol%, particularly preferably from 30 to 80 mol%, particularly preferably from 40 to 75 mol%, in which case conventional neutralizing agents, in particular alkali metal hydroxides, Alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates and mixtures thereof can be used. Instead of the alkali metal salt, an ammonium salt may be used. Sodium and potassium are particularly preferred as alkali metals, but sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof are particularly preferred.

  However, it is also possible to carry out the neutralization after the polymerization at the stage of the polymer gel produced during the polymerization. Furthermore, by adding a part of the neutralizing agent to the monomer solution and adjusting the desired degree of final neutralization only after polymerization at the polymer gel stage, the acid groups before polymerization can be reduced to 40 mol%, especially 10%. It is possible to neutralize ˜30 mol%, particularly preferably 15 to 25 mol%. If the polymer gel is at least partially neutralized after polymerization, the polymer gel is pulverized, especially mechanically, for example using an extruder, in which the neutralizing agent is sprayed, It can be sprayed or injected and then mixed thoroughly. To that end, the resulting gel material can still be extruded for homogenization several times.

Furthermore, the polymer gel is used, in particular using a belt reactor, until the moisture content is in particular from 0.5 to 15% by weight, particularly preferably from 1 to 10% by weight, particularly preferably from 2 to 8% by weight. This moisture content is determined by the test method no. EDANA recommended by EDANA. Measured according to WSP 230.2-05 “Mass Loss Upon Heating”. If moisture is too high, the dried polymer gel is difficult to post-process and has a too low glass transition temperature T _g. If the moisture is too low, the dried polymer gel is too brittle and undesirably results in large amounts of polymer particles having a too small particle size (“fine”) in the subsequent milling process. The solids content of the gel is, in particular, 25 to 90% by weight, particularly preferably 35 to 70% by weight, particularly preferably 40 to 60% by weight, before drying. However, alternatively, a fluid bed dryer or paddle dryer may be used prior to drying.

  Thereafter, the dried polymer gel is pulverized and classified, in which case a multi-stage roller mill, preferably a two-stage or three-stage roller mill, a pin mill, a hammer mill or a vibration mill, is used for pulverization. May be used.

  The average particle size of the polymer particles separated as product fraction is in particular at least 200 μm, particularly preferably 250 to 600 μm, particularly preferably 300 to 500 μm. The average particle size of the product fraction is determined by Test Method No. recommended by EDANA. It can be calculated using WSP 220.2-05 “Particle Size Distribution”, in which the mass fraction of the sieve fraction is cumulatively plotted and the average particle size is measured from the graph. In this context, the average particle size is an opening value, which results in a cumulative 50% by weight.

  The proportion of particles having a particle size of greater than 150 μm is in particular at least 90% by weight, particularly preferably at least 95% by weight, particularly preferably at least 98% by weight.

  Polymer particles having a particle size that is too small reduces the transmittance (SFC). Therefore, the proportion of polymer particles that are too small (“fine”) should be small.

  Thus, polymer particles that are too small are usually separated and returned to the process. This takes place inter alia before, during or immediately after the polymerization, i.e. before the polymer gel is dried. Too small polymer particles may be wetted with water and / or an aqueous surfactant before or during return.

  It is also possible to separate polymer particles that are too small in later process steps, for example after surface postcrosslinking or another coating step. In this case, the returned too small polymer particles are surface postcrosslinked or otherwise coated, for example with pyrogenic silicic acid.

  When using a kneading reactor for the polymerization, polymer particles that are too small are added especially during the last third of the polymerization.

  If polymer particles that are too small are added very early, for example already to the monomer solution, the centrifugal retention capacity (CRC) of the resulting water-absorbing polymer particles is thereby reduced. However, this can be offset by adapting the amount used to the cross-linking agent b).

  If polymer particles that are too small are added very later, for example, for the first time in an apparatus that is later connected to a polymerization reactor, such as in an extruder, the polymer particles that are too small will still be mixed into the resulting polymer gel. Can be difficult. However, poorly mixed, too small polymer particles are peeled off from the dried polymer gel during pulverization and are therefore separated again during classification and of too small polymer particles to be returned. The amount increases.

  The proportion of particles having a particle size of at most 850 μm is in particular at least 90% by weight, particularly preferably at least 95% by weight, particularly preferably at least 98% by weight.

  The proportion of particles having a particle size of at most 600 μm is in particular at least 90% by weight, particularly preferably at least 95% by weight, particularly preferably at least 98% by weight.

  Polymer particles having a particle size that is too large reduces the swelling rate. Therefore, the proportion of polymer particles that are too large should be low as well.

  Thus, polymer particles that are too large are usually returned to the pulverization of the separated and dried polymer gel.

  The polymer particles may be surface postcrosslinked to further improve the properties. Suitable compounds are, for example, polyvalent amines, polyvalent amines described in European Patent Application Publication No. 00830222 A2, European Patent Application Publication No. 0543303 A1 and European Patent Application Publication No. 0937336 A2. Divalent amidoamines, polyhydric epoxides, divalent, described in German Offenlegungsschrift 3314019 A1, German Offenlegungsschrift 3523617 A1, and European Offenlegungsschrift 0450922 A2. Alcohols or polyhydric alcohols or β-hydroxyalkylamides described in German Offenlegungsschrift 10 204938 A1 and US Pat. No. 6,239,230.

  Furthermore, DE 40 20 780 C1 describes cyclic carbonates as suitable surface postcrosslinking agents, and DE 19807502 A1 describes 2-oxazolidinone and its Derivatives, such as 2-hydroxyethyl-2-oxazolidinone, are described as suitable surface postcrosslinkers, and in German Patent 19807992 C1 bis-2-oxazolidinone and poly-2-oxazolidinone are suitable surfaces. Described as post-crosslinking agent, DE 198554573 A1 describes 2-oxotetrahydro-1,3-oxazine and its derivatives as suitable surface post-crosslinking agents. In Application Publication No. 19855454 A1, -Acyl-2-oxazolidinone is described as a suitable surface postcrosslinker, and German Offenlegungsschrift DE 10 204 937 A1 describes cyclic urea as a suitable surface postcrosslinker. JP-A-10334584 describes bicyclic amide acetals as suitable surface postcrosslinkers, and EP 1199327 A2 describes oxetanes and cyclic ureas which are suitable after surface. Written as a crosslinking agent, and WO 2003/031482 A1 describes morpholine-2,3-dione and its derivatives as suitable surface postcrosslinking agents.

  Preferred surface postcrosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, the reaction product of polyamide and epichlorohydrin, and a mixture of propylene glycol and 1,4-butanediol.

  Particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1,3-propanediol.

  In addition, surface postcrosslinking agents containing further polymerizable ethylenically unsaturated groups, as described in DE 37 13 601 A1, may be used. The amount of this surface postcrosslinking agent is in each case in particular from 0.001 to 2% by weight, particularly preferably from 0.02 to 1% by weight, particularly preferably from 0.05 to 0.2%, based on the polymer particles. % By mass.

  In a preferred embodiment of the present invention, in addition to the surface postcrosslinking agent, a polyvalent cation is applied to the particle surface before surface postcrosslinking, during surface postcrosslinking or after surface postcrosslinking.

  The polyvalent cations which can be used in the process according to the invention are, for example, divalent cations such as zinc, magnesium, calcium, iron and strontium cations, trivalent cations such as aluminum, iron, chromium, rare earth and manganese cations, tetra Valent cations such as cations of titanium and zirconium. As counter ions, hydroxides, chlorides, bromides, sulfates, hydrogen sulfates, carbonates, bicarbonates, nitrates, phosphates, hydrogen phosphates, dihydrogen phosphates and carboxylates such as acetic acid Salts, citrates and lactates are possible. Salts with various counter ions are also possible, for example basic aluminum salts, such as aluminum monoacetate or aluminum monobutyrate. Aluminum sulfate, aluminum monoacetate and aluminum lactate are preferred. Besides metal salts, polyamines may be used as polyvalent cations.

  The amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, in particular 0.005 to 1% by weight, particularly preferably 0.02 to 0.8% by weight, based on the polymer particles. It is.

  The surface postcrosslinking is usually carried out to the extent that the dried polymer particles are coated with an aqueous solution of the surface postcrosslinker by spraying a solution of the surface postcrosslinker onto the dried polymer particles. Subsequently, the polymer particles coated with the surface postcrosslinker are deagglomerated and hot surface postcrosslinked.

  Coating with a solution of the surface postcrosslinker is carried out, inter alia, in a mixing apparatus equipped with a movable mixing mold, in a screw mixer, a disk type mixer and a paddle type mixer. Suitable mixing devices are, for example, horizontal Pflugschar® mixers (Gebr. Loedige Machinenbau GmbH, Paderborn, Germany), Vrieco-Nata Continuous Mixer (Hosokawa MicroD, Hosokawa MicroD Mixer (Processall Incorporated, Cincinnati, USA) and Shugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands). However, it is also possible to spray the solution in a fluidized bed.

  Said deagglomeration is likewise carried out, inter alia, in a mixing device with a movable mixing mold, in a screw mixer, a disk mixer and a paddle mixer. Preferably, suitable mixing devices are, for example, horizontal Pflugschar® mixers (Gebr. Loedige Maskinenbau GmbH, Paderborn, Germany), Vrieco-Nata Continuous Mixer (Hosokawa MicroD And ProcessMixmill Mixer (Processall Incorporated, Cincinnati, USA).

  In the case of coating with an aqueous solution, the water-absorbing polymer particles tend to agglomerate (aggregate). In the vertical mixer, the water-absorbing polymer particles tend to agglomerate slightly during coating. This coating is therefore preferably carried out in a vertical mixer. Furthermore, the resulting agglomerates can be dissolved back with a moderate mechanical load. For this reason, horizontal mixers are more suitable for longer residence times. It is therefore possible to carry out the coating and deagglomeration in a horizontal mixer. The distinction between horizontal and vertical mixers is made by means of mixing shaft bearings, i.e. the horizontal mixer has a horizontally supported mixing shaft and the vertical mixer has a vertically supported mixing shaft. It has.

  The surface post-crosslinking agent is used as an aqueous solution. Depending on the content of the non-aqueous solvent or the total amount of solvent, the penetration depth of the surface postcrosslinker into the polymer particles can be adjusted.

  If only water is used as the solvent, a surfactant is preferably added. Thereby, the wetting behavior is improved and the tendency to agglomerate is reduced. However, inter alia, solvent mixtures are used, such as isopropanol / water, 1,3-propanediol / water and propylene glycol / water, the ratio of the mixed substances being in particular 20:80 to 40:60.

  Preferably, the water-absorbing polymer particles are still dried and / or heated before hot surface post-crosslinking. This is preferably done during deagglomeration, in particular in contact dryers, for example in paddle dryers and disk dryers. Suitable contact dryers include, for example, Hosokawa Bepex® horizontal paddle dryers (Hosokawa Micron GmbH, Leingarten, Germany), Hosokawa Bepex® disc dryers (Hosokawa MicronG, Inc.). , Germany), the Holo-Flite® dryer (Metso Minerals Industries Inc., Danville, USA) and the Nara paddle dryer (NARA Machinery Europe, Frechen, Germany).

  The water content of the water-absorbing polymer particles is increased by coating with the aqueous solution. This relatively high water content hinders hot surface postcrosslinking at higher temperatures. Therefore, it is preferable to dry the water-absorbing polymer particles before hot surface post-crosslinking and to heat to the reaction temperature.

  The water content of the coated and deagglomerated polymer particles is in particular less than 5 mass, particularly preferably less than 2 mass, particularly preferably less than 1 mass, before hot surface postcrosslinking.

  After deagglomeration, optionally dried and / or heated polymer particles are transferred into a drum heat exchanger equipped with a reverse screw helix for hot surface postcrosslinking.

  A drum type heat exchanger equipped with a reverse screw helix is a heatable and horizontal rotary drum. At this time, a reverse screw helix for forcibly conveying the product is incorporated in the inner wall of the drum. . The drum can be heated indirectly, for example via a drum wall. This heating is usually done electrically or using steam. Various wall temperatures can be adjusted within the drum via a plurality of independent heating zones along the drum longitudinal axis.

  Here, heating of the product directly in the interior space of the drum heat exchanger by installation of a burner or introduction of hot flue gas is usually not used.

  If a very uniform radial temperature distribution of the product is required in the drum, in addition to the reverse screw helix, radial mixing elements (eg in the form of entrained paddles or entrained webs) It can be installed on the peripheral surface of the drum inner wall. This mixing element facilitates radial mixing of the product in separate zones, formed on the basis of a reverse screw helix, and in particular a large drum diameter and a high filling height or a high screw helix. Used when working at height.

  FIG. 1 shows a drum heat exchanger having an indirect heating unit.

  FIG. 2 shows a cross section of the drum heat exchanger.

The symbols in these figures have the following meanings:
A Product inlet B Product outlet 1 Static jacket (isolated)
2 Rotating drum 3 Reverse screw helix 4 Heating zone 1 (electrical or steam)
5 Heating zone 2 (electrical or steam)
6 Heating zone 3 (electrical or steam)
a Product b Inner diameter of the drum c Height of the screw helix.

  The drum has a length of in particular 3 to 30 m, particularly preferably 5 to 25 m, particularly preferably 7 to 20 m. The inner diameter of the drum is in particular from 0.3 to 10 m, particularly preferably from 0.4 to 5 m, particularly preferably from 1 to 3 m.

  The screw helix has a height of 0.05 to 1 m, particularly preferably 0.1 to 0.8 m, particularly preferably 0.2 to 0.6 m. The lead of the screw helix is in particular from 0.05 to 0.5 m, particularly preferably from 0.1 m to 0.4 m, particularly preferably from 0.15 to 0.3 m. The height of the screw helix is the distance between the drum inner wall surface in the direction of the rotation axis and the highest point of the screw helix. The lead of the screw helix is a moving distance of the screw helix in the vertical direction when it is rotated once.

  The peripheral speed of the drum is in particular from 0.02 to 0.5 m / sec, particularly preferably from 0.03 to 0.3 m / sec, particularly preferably from 0.04 to 0.15 m / sec.

  The maximum filling height of the drum heat exchanger with the reverse screw helix is the filling height at which the product no longer reaches the next helix immediately beyond the height of the screw helix. is there. FIG. 2 shows the maximum filling height. This filling height corresponds to a filling height of 100%.

  The gradient of the longitudinal axis of the drum heat exchanger with reverse screw helix relative to the horizontal is in particular +10 to -10 °, particularly preferably +5 to -5 °, particularly preferably +1 to -1 °, In this case, a plus sign means a gradient rising in the transport direction, and a minus sign means a gradient descending in the transport direction.

  In a preferred embodiment of the invention, the water-absorbing polymer particles are cooled after hot surface postcrosslinking. This cooling is carried out inter alia in contact coolers, for example in paddle coolers and in disk coolers. Suitable coolers are, for example, the Hosokawa Bepex® horizontal paddle cooler (Hosokawa Micron GmbH, Leingarten, Germany), the Hosokawa Bepex® disc cooler (Hosokawa Micron GmbH, Germany) National), Holo-Flite® cooler (Metso Minerals Industries Inc., Danville, USA) and Nara paddle cooler (NARA Machinery Europe, Frechen, Germany).

  In the cooler, the water-absorbing polymer particles are cooled to 20 to 150 ° C., in particular 30 to 120 ° C., particularly preferably 40 to 100 ° C., particularly preferably 50 to 80 ° C.

  Subsequently, the surface post-crosslinked polymer particles may be reclassified, wherein too small and / or too large polymer particles are separated and returned to the process.

  The surface post-crosslinked polymer particles may be coated or post-wet for further improvement of the properties.

  This post-wetting is carried out in particular at 30 to 80 ° C., particularly preferably at 35 to 70 ° C., particularly preferably at 40 to 60 ° C. If the temperature is too low, the water-absorbing polymer particles tend to agglomerate, and if the temperature is higher, the water already evaporates significantly. The amount of water used for the post-wetting is in particular 1 to 10% by weight, particularly preferably 2 to 8% by weight, particularly preferably 3 to 5% by weight. The post-wetting increases the mechanical stability of the polymer particles and reduces the tendency of the electrostatic charging of the polymer particles. Preferably, the post-wetting is performed after hot surface post-crosslinking in a cooler.

  Suitable coatings for improving the swelling speed and the permeability (SFC) of the swollen gel layer are, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers and divalent metal cations or polyvalent metals. It is a cation. A suitable coating for dust binding is, for example, a polyol. Suitable coatings against the undesirable caking tendency of the polymer particles include, for example, zinc oxide, zinc carbonate, pyrogenic silicic acid, such as Aerosil® 200, and surfactants, such as Span®. 20.

  The water-absorbing polymer particles produced by the process according to the invention have a moisture content of 0-15% by weight, particularly preferably 0.2-10% by weight, particularly preferably 0.5-8% by weight. In this case, the moisture content is determined according to Test Method No. EDANA recommended by EDANA. Measured according to WSP 230.2-05 “Mass Loss Upon Heating”.

  The water-absorbing polymer particles produced by the process according to the invention have a proportion of particles having a particle size of 300 to 600 μm, in particular at least 30% by weight, particularly preferably at least 50% by weight, particularly preferably at least 70% by weight. Have.

  The water-absorbing polymer particles produced by the process according to the invention are typically at least 15 g / g, in particular at least 20 g / g, preferably at least 22 g / g, particularly preferably at least 24 g / g, very particularly preferably at least It has a centrifuge retention capacity (CRC) of 26 g / g. The centrifugal retention capacity (CRC) of the water-absorbing polymer particles is usually less than 60 g / g. The centrifuge retention volume (CRC) is a test method no. Recommended by EDANA. Measured in accordance with WSP 241.2-05 “Fluid Retention Capacity in Salin Solution, After Centrifugation”.

The water-absorbing polymer particles produced by the process according to the invention are typically at least 15 g / g, in particular at least 20 g / g, preferably at least 22 g / g, particularly preferably at least 24 g / g, very particularly preferably at least It has an absorption of 26 g / g under a pressure of 49.2 g / cm ² . The absorbed amount of the water-absorbing polymer particles under a pressure of 49.2 g / cm ² is usually less than 35 g / g. The amount of absorption under a pressure of 49.2 g / cm ² is the test method No. recommended by EDANA. Measured according to WSP 242.2-05 "Absorption Under Pressure Gravimetrie Determination ", instead of a pressure of 21.0 g / cm ² in time, is adjusted to a pressure of 49.2 g / cm ^2.

Method:
Standard test methods, referred to below as “WSP”, are “Worldwide Strategies Partners” EDANA (Avenue Eugene Plusy, 157, 1030 Brussel, Belgium, www.edana.it100) and INDA1it100. 115, Cary, North Carolina 27518, USA, www.inda.org), “Standard Test Methods for the Nonvendors Industry”, 2005 edition. This publication is available from both EDANA and INDA.

  Unless otherwise stated, the measurements should be performed at an ambient temperature of 23 ± 2 ° C. and a relative air humidity of 50 ± 10%. The water-absorbing polymer particles are thoroughly mixed before the measurement.

Residual monomer The residual monomer content of the water-absorbing polymer particles is determined by test method No. EDANA recommended by EDANA. Measured according to WSP 210.2-05 “Residual Monomers”.

Centrifugal Retention Capacity
This centrifuge retention volume (CRC) is determined by Test Method No. recommended by EDANA. Measured in accordance with WSP 241.2-05 “Fluid Retention Capacity in Saline, After Centrifugation”.

Absorption under pressure of 49.2 g / cm ² (Absorption under Load) The absorption under pressure of 49.2 g / cm ² (AUL 0.7 psi) was determined by test method No. EDANA recommended by EDANA. WSP 242.2-05 "Absorption Under Pressure, Gravimetrie Determination" is measured in accordance with, instead of a pressure of 21.0g / cm ² (AUL0.3psi) during its, 49.2 g / cm ² of (AUL 0.7 psi) Adjusted to pressure.

Extractability The content of the extractable component of the water-absorbing polymer particles is determined by the test method No. recommended by EDANA. Measured according to WSP 270.2-05 “Extractable”.

Permeability (Saline Flow Conductivity)
The permeability (SFC) of the swollen gel layer under a pressure load of 0.3 psi (2070 Pa) is the same as that described in EP 0640330 A1, and is swelled with water-absorbing polymer particles. In this case, the apparatus described in the European Patent Application on page 19 and FIG. 8 is not used with a glass frit (40) anymore, and the plunger (39 ) Is made of the same plastic material as the cylinder (37) and is now modified to include 21 equally sized holes evenly distributed across all mounting surfaces. The means and evaluation of this measurement remain unchanged compared to EP-A-0640330 A1. The flow rate is automatically detected.

The transmittance (SFC) is calculated as follows:
SFC [cm ³ s / g] = (Fg (t = 0) × L0) / (d × A × WP)
In the above equation, Fg (t = 0) is the flow rate of the NaCl solution at g / s obtained by extrapolation to t = 0 for linear regression analysis of flow rate measurement data Fg (t), and L0 is , Cm is the thickness of the gel layer in cm, d is the density of the NaCl solution in g / cm ³ , A is the area of the gel layer in cm ² , and WP is dyn / cm ² The hydrostatic pressure on the gel layer.

Schematic which shows the drum type heat exchanger provided with the indirect heating part. 1 is a schematic view showing a section of the drum heat exchanger.

Example Example 1
The base polymer was produced in a co-current spray dryer with an integrated fluidized bed (27) and an outer fluidized bed (29) according to FIG. 1 of WO 2011/026876 A1. The cylindrical part of the spray dryer (5) had a height of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB) had a diameter of 3.0 m and a weir height of 0.4 m. The external fluidized bed (EFB) had a length of 3.0 m, a width of 0.65 m and a weir height of 0.5 m.

  Dry gas was fed to the end of the spray dryer via the gas distributor (3). A part of this dry gas was supplied through the fabric filter (9) and the washing tower (12) (circulation gas). Nitrogen having an oxygen content of 1-5% by volume was used as the drying gas. Prior to initiation of the polymerization, the apparatus was flushed with nitrogen until an oxygen content of less than 5% by volume. The amount of gas in the cylindrical part of the spray dryer (5) was 16.170 kg / h. The pressure inside the spray dryer was 4 mbar under ambient pressure.

  The starting temperature of the spray dryer was measured at three positions at the lower end of the cylindrical part, as described in FIG. 3 of WO 2011/026876 A1. Three individual measurements (47) were used to calculate the average starting temperature. The circulating gas was heated and metering of the monomer solution was started. From this point on, the average starting temperature was adjusted to 116 ° C. by adjusting the gas inlet temperature with the heat exchanger (20).

  The product was collected in the internal fluidized bed (27) until the level of the weir. A dry gas having a temperature of 132 ° C. was supplied to the internal fluidized bed (27) via the pipe (25). The amount of gas in the internal fluidized bed (27) was 10,000 kg / h.

  The exhaust gas from the spray dryer was supplied to the washing tower (12) through the fabric filter (9). The liquid level in the wash tower (12) was kept constant by pumping excess liquid. The liquid in the washing tower (12) is cooled using the heat exchanger (13) and conveyed counter-currently by the nozzle (11), so that the temperature in the washing tower (12) is adjusted to 45 ° C. did. In order to wash and remove acrylic acid from the exhaust gas, the liquid in the washing tower (12) was adjusted to be alkaline by adding caustic soda solution.

  The washing tower exhaust gas was distributed over lines (1) and (25). The temperature was adjusted using heat exchangers (20) and (22). The heated drying gas was supplied to the spray dryer via a gas distributor (3). This gas distributor consisted of a series of plates and had a gas loss of 5-10 mbar, depending on the amount of gas.

  The product was transferred from the inner fluidized bed (27) into the outer fluidized bed (29) using a rotary feeder (28). A drying gas having a temperature of 60 ° C. was supplied to the external fluidized bed (29) via the pipe line (40). The drying gas was air. The amount of gas in the external fluidized bed (29) was 2500 kg / h.

  The product was conveyed from the external fluidized bed (29) onto the sieve (33) using a rotary feeder (28). Using a sieve (33), particles (aggregates) having a particle size of more than 3000 μm were separated.

  In order to prepare the monomer solution, first, glycerin triacrylate (crosslinking agent) that was ethoxylated three times was added to acrylic acid, followed by addition of 37.3% by mass of an aqueous sodium acrylate solution. The temperature of the monomer solution was maintained at 10 ° C. by pumping through a heat exchanger. In the pump circuit, there was a filter having an opening of 150 μm behind the pump. The initiator was added to the monomer solution in front of static mixing devices (41) and (42) via conduits (43) and (44). Sodium peroxodisulfate is fed via conduit (43) at a temperature of 20 ° C. and Brueggolit® FF7 (Brugegmann Chemicals, Heilbrunn, Germany) is fed via conduit (44) at a temperature of 5 ° C. Supplied. All initiators were pumped into the circuit and metered into all drip units via a control valve. A filter having an opening of 100 μm was present behind the static mixing device (42). In order to supply the monomer solution to the end of the spray dryer, three dropping units were used as described in FIG. 4 of WO 2011/026876 A1.

  The dropping unit was composed of an external tube (51) and a dropping cassette (53) as described in FIG. 5 of WO 2011/026876 A1. The dropping cassette (53) was connected to the internal tube (52). The inner tube (52) had a PTFE seal (54) at the end and could be withdrawn for maintenance purposes during operation.

  FIG. 6 of WO 2011/026876 A1 describes the internal structure of the dropping cassette. The temperature of the dropping cassette (61) was adjusted to 25 ° C. using the cooling water in the passage (59). This dripping cassette had 256 holes. These holes had a diameter of 2.5 mm at the inlet and a diameter of 170 μm at the outlet. The holes were arranged in 6 rows, where the distance between the holes in one row was 12.38 mm and the spacing between the rows was 14 mm. The dropping cassette (61) had a flow passage (60) that did not include a dead space in order to evenly distribute the premixed monomer solution and initiator solution on the two dropping plates (57). . The holes were distributed on two dropping plates (57) each having 128 holes, that is, the two dropping plates (57) each had three rows of holes. The two drop plates (57) had an angled arrangement with an angle of 3 °. Each drip plate (57) was made of special steel (material No. 1.4571) and had a length of 530 mm, a width of 76 mm and a thickness of 15 mm.

  The amount supplied to the spray dryer was 10.25% by mass of acrylic acid, 32.75% by mass of sodium acrylate, 0.035% by mass of glycerol triacrylate ethoxylated twice (about 85% by mass), Brueggolit (Registered trademark) FF7 0.00285% by mass (Brüggemann Chemicals, Heilbrunn, Germany), sodium peroxodisulfate 0.266% by mass and water. Brueggolit® FF7 was used as a 5 wt% aqueous solution and sodium peroxodisulfate was used as a 15 wt% aqueous solution. The degree of neutralization was 71%. The supply rate per hole was 1.6 kg / h.

The basic polymer obtained has a bulk density of 74.4 g / 100 ml, an average particle size of 392 μm, a width of particle size distribution of 0.48, an average sphericity of 0.91 and a centrifuge retention capacity of 21.4 g / g. (CRC), 17.9 g / g of 49.2 g / cm ³ (AUL 0.7 psi) pressure absorption and 2.75 wt% residual monomer content.

Example 2
1300 g of the base polymer from Example 1 was added at about 23 ° C. in a Pfluschar® mixing device (Gebr. Loedige Machinery GmbH, Paderborn, Germany) with a heating jacket of type Loe 5 for hot surface postcrosslinking. And coated with the following solutions using a twin screw spray nozzle at a shaft speed of 250 rpm (each time for the base polymer):
0.10% by mass of 2-hydroxyethyl-2-oxazolidinone,
0.10% by mass of 1,3-propanediol,
1.00% by weight of 1,2-propanediol,
1.00% by mass of water,
Aluminum tributyrate aqueous solution (22% by mass) 3.00% by mass.

  After spraying, the product temperature was increased to 185 ° C. and the reaction mixture was maintained for a total of 150 minutes at the temperature and a shaft speed of 60 rpm per minute. Samples were removed after various times. All samples were sieved with a particle size of 150-850 μm before analysis.

Example 3
1500 g of the base polymer from Example 1 was added at about 23 ° C. in a Pflugschar® mixing device (Gebr. Loedige Machinery GmbH, Paderborn, Germany) with a heating jacket of type Loe5 for hot surface postcrosslinking. And coated with the following solutions using a twin screw spray nozzle at a shaft speed of 250 rpm (each time for the base polymer):
1.00% by mass of 1,3-propanediol,
1.00% by mass of water,
Aluminum tributyrate aqueous solution (22% by mass) 3.00% by mass.

  After spraying, the product temperature was increased to 180 ° C. and the reaction mixture was maintained for a total of 120 minutes at the temperature and a shaft speed of 60 rpm per minute. Samples were removed after various times. All samples were sieved with a particle size of 150-850 μm before analysis.

  Examples 2 and 3 show a significant effect of residence time on the results of surface postcrosslinking. The transmittance (SFC) takes a noticeable maximum value.

Example 4
In a further test, the residence time distribution was measured in the equipment previously used for hot surface postcrosslinking. Paddle mixers of the type Nara Paddle Dryer 1.6W (GMF Gouda, Waddinxveen, The Netherlands) and Hysorb® M7055 (BASF SE, Ludwigshafen, Germany) were used. The number of revolutions was 37 rpm, and the weir height was 87% (in the case of 100%, the weir is the same height as the top of the paddle). The throughput of Hysorb (registered trademark) M7055 was 60 kg / h. At the entrance of the paddle dryer, for the test, a specially colored particle of the product Hysorb® M7055 (BASF SE, Ludwigshafen, Germany) is added and the time of the colored particle The distribution was analyzed at the outlet of the paddle dryer. Cumulative frequency distributions T ₁₀ , T ₅₀ and T ₉₀ were calculated from the frequency distribution by integration (T ₁₀ corresponds to 10% of the cumulative frequency distribution, etc.).

The following width of the frequency distribution was calculated:
T ₉₀ / T ₅₀ = 1.31
T ₁₀ / T ₅₀ = 0.78
The average residence time (T ₅₀ ) was 42 minutes.

Example 5
The method was carried out as in Example 4. The throughput was increased from 60 kg / h to 80 kg / h.

The following width of the frequency distribution was calculated:
T ₉₀ / T ₅₀ = 1.31
T ₁₀ / T ₅₀ = 0.79
The average residence time (T ₅₀ ) was 34 minutes.

Example 6
The method was carried out as in Example 4. The degree of filling was reduced from 87% to 62% by reducing the weir height.

The following width of the frequency distribution was calculated:
T ₉₀ / T ₅₀ = 1.33
T ₁₀ / T ₅₀ = 0.77
The average residence time (T ₅₀ ) was 31 minutes.

Example 7
The method was carried out as in Example 4. Instead of a paddle dryer, a drum heat exchanger with a reverse screw helix was used. The drum heat exchanger had an inner diameter of 700 mm, a length of 6000 mm, and 60 screw threads. These spiral threads had a height of 100 mm and a lead of 100 mm. The number of revolutions was 0.9 rpm, the throughput was 108 kg / h, and the filling height was 100% of the height of the screw helix.

The following width of the frequency distribution was calculated:
T ₉₀ / T ₅₀ = 1.07
T ₁₀ / T ₅₀ = 0.96
The average residence time (T ₅₀ ) was 52 minutes.

Example 8
The method was carried out as in Example 7. The rotation speed was increased from 0.9 rpm to 2.5 rpm. The throughput was increased from 108 kg / h to 224 kg / h. The filling height was 72% of the screw helix height.

The following width of the frequency distribution was calculated:
T ₉₀ / T ₅₀ = 1.01
T ₁₀ / T ₅₀ = 0.99
The average residence time (T ₅₀ ) was 24 minutes.

  Examples 4-8 demonstrate that the residence time distribution of a drum heat exchanger with a reverse screw helix is clearly narrower.

Claims (15)

A method of post-crosslinking a water-absorbing polymer particle, wherein the water-absorbing polymer particle is a) at least one ethylenically unsaturated acid group-containing monomer, which may be at least partially neutralized.
b) at least one cross-linking agent,
c) at least one initiator,
d) An aqueous monomer suspension or aqueous monomer suspension optionally containing one or more ethylenically unsaturated monomers and e) optionally one or more water-soluble polymers that can be copolymerized with the monomers described in a). Manufactured by polymerizing a liquid, wherein
The polymer particles are coated with an aqueous solution, the coated polymer particles are deagglomerated, and the deagglomerated polymer particles are hot surface postcrosslinked using a drum heat exchanger with a reverse screw helix. Characterized by the above.   The method according to claim 1, wherein the coating and the deagglomeration are carried out in a horizontal mixer.   The process according to claim 1, characterized in that the coating is carried out in a vertical mixer and the deagglomeration is carried out in a horizontal mixer.   4. A method according to any one of claims 1 to 3, characterized in that the coated polymer particles are dried during deagglomeration.   5. A method according to claim 1, wherein the coated polymer particles are heated during deagglomeration.   The method according to claim 1, wherein a filling height of the drum heat exchanger is 65 to 90%.   The temperature of the water-absorbing polymer particles in the drum-type heat exchanger is 170 to 200 ° C. and / or the residence time of the water-absorbing polymer particles in the drum-type heat exchanger is 30 to 60 minutes. 7. A method according to any one of claims 1 to 6, characterized in that it is.   The method according to any one of claims 1 to 7, characterized in that the drum heat exchanger is heated electrically or using steam.   9. A process according to any one of claims 1 to 8, characterized in that the monomer a) is at least 95 mol% partially neutralized acrylic acid.   10. A method according to any one of claims 1 to 9, characterized in that the water-absorbing polymer particles have a centrifuge retention capacity of at least 26 g / g.   An apparatus for post-crosslinking hot absorbent polymer particles comprising a drum-type heat exchanger with a heatable horizontal mixer and a reverse screw helix.   12. The apparatus according to claim 11, wherein the horizontal mixer and the drum heat exchanger are directly connected in sequence.   An apparatus for post-crosslinking hot absorbent polymer particles comprising a vertical mixer, a heatable horizontal mixer, and a drum heat exchanger with a reverse screw helix.   The apparatus according to claim 13, wherein the drum type heat exchanger including the vertical mixer, the horizontal mixer, and the reverse screw helix is directly connected.   15. A device according to any one of claims 11 to 14, wherein a coolable horizontal mixer is directly connected to the drum heat exchanger.

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