EP2539381A1

EP2539381A1 - Method for producing water-absorbing polymer particles

Info

Publication number: EP2539381A1
Application number: EP11703228A
Authority: EP
Inventors: Rüdiger Funk; Thomas Pfeiffer; Matthias Weismantel; Stefan Blei
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2010-02-24
Filing date: 2011-02-15
Publication date: 2013-01-02
Also published as: WO2011104139A1; JP2013520539A; CN102762617A

Abstract

The invention relates to a method for producing water-absorbing polymer particles, in which an aqueous polymer gel is dried in a circulating air belt dryer on a continuous conveyor belt and the surface of the conveyor belt has a plurality of raised regions or recesses which are suitable for limiting the mobility of the aqueous polymer gel in the transversal direction.

Description

Process for the preparation of water-absorbing polymer particles

The present invention relates to a process for producing water-absorbing polymer particles, wherein an aqueous polymer gel is dried in a circulating air belt dryer on a circulating conveyor belt and the surface of the conveyor belt has a plurality of elevations or depressions, which are suitable to limit the mobility of the aqueous polymer Geis in the transverse direction.

Water-absorbing polymer particles are used in the manufacture of diapers, tampons, sanitary napkins and other sanitary articles, but also as water-retaining agents in agricultural horticulture. The water-absorbing polymer particles are also referred to as superabsorbers.

The preparation of water-absorbing polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103. The aqueous polymer gels obtained by polymerization are typically dried by means of a circulating air belt dryer As a result, as the drying progresses, the conveyor belt is no longer covered with polymer gel over the entire conveyor belt width, and at the edge areas, part of the drying air flows unused through the conveyor belt (by-pass ).

To solve this problem, the use of multi-stage circulating air belt dryer is proposed, wherein the drying material is redistributed on the next conveyor belt (see also in "Perry's Chemical Engineers' Handbook", 7th edition, McGraw-Hill, page 12-48).

The object of the present invention was to provide an improved process for drying aqueous polymer gels by means of a circulating air belt dryer. The object has been achieved by a process for producing water-absorbing polymer particles by polymerization of a monomer solution or suspension comprising a) at least one ethylenically unsaturated, acid group-carrying monomer which may be at least partially neutralized,

b) at least one crosslinker,

c) at least one initiator, d) optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers mentioned under a) and

e) optionally one or more water-soluble polymers, comprising drying the resulting aqueous polymer gel in a circulating air belt dryer by means of a circulating conveyor belt, grinding, classification, and optionally thermal surface post-crosslinking, characterized in that the surface of the conveyor belt has a plurality of elevations or depressions suitable The mobility of the aqueous polymer gels in the transverse direction must be limited.

The present invention is based on the finding that the shrinkage of the polymer gels during drying can be reduced without causing increased cracking. The reason for this is probably that the polymer gel to be dried has a temperature above the glass transition temperature TG during drying. Above the glass transition temperature TG, polymers are rubbery and tacky. Thus, the elevations and depressions to be used according to the invention are able to absorb the tensile forces that occur and the tackiness of the polymer gel particles prevents the formation of cracks.

Recirculating belt dryers suitable for the process according to the invention are described, for example, in the monograph "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 89 to 92. The shape of the elevations or depressions to be used according to the process of the invention is subject to no restriction.

Suitable elevations are mandrels with a height of preferably 0.01 to 5 cm, more preferably 0.05 to 2 cm, most preferably 0.1 to 1 cm. The mandrels may be nails or screws driven through the conveyor belt from the back or attached to the surface of the conveyor belt.

Suitable depressions are longitudinal and transverse grooves, for example, with a depth of 0.1 to 2 cm and a width of 0.1 to 2 cm.

The number and dimensioning of the elevations or depressions to be used in the method according to the invention can be determined by the skilled person by appropriate experiments.

Advantageously, circulating belt dryers with conveyor belts whose product-contacting surfaces are made of a stainless steel are used in the process according to the invention. Stainless steels usually have a chromium content of 10.5 to 13 wt .-% chromium. The high chromium content leads to a protective passivation of chromium dioxide on the steel surface. Other alloying components increase corrosion resistance and improve mechanical properties. Particularly suitable steels are austenitic steels with, for example, at least 0.08% by weight of carbon. In addition to iron, carbon, chromium, nickel and optionally molybdenum, the austenitic steels advantageously contain further alloy constituents, preferably niobium or titanium. The preferred stainless steels are steels with the material number 1.43xx or 1.45xx according to DIN EN 10020, where xx can be a natural number between 0 and 99. Particularly preferred materials are the steels with the material numbers 1 .4301, 1 .4541 and 1 .4571, in particular steel with the material number 1 .4301.

The revolving conveyor belt usually has a plurality of openings. In a preferred embodiment of the present invention, the circulating conveyor belt transversely to the transport direction a plurality of arranged in staggered rows slots having a length of preferably 5 to 50 mm, more preferably 10 to 40 mm, most preferably 15 to 30 mm, a Width of preferably 0.5 to 5 mm, more preferably 1 to 4 mm, most preferably 1, 5 to 3 mm, and a length to width ratio of preferably 2 to 20, more preferably 5 to 15, most preferably 8 to 12, has. The width of the circulating air belt dryer is preferably from 1 to 10 m, more preferably from 2 to 7.5 m, most preferably from 3 to 5 m.

The length of the circulating air belt dryer is preferably from 10 to 8 m, more preferably from 30 to 60 m, most preferably from 40 to 50 m.

The conveyor belt speed of the circulating air belt dryer is preferably from 0.005 to 0.05 m / s, particularly preferably from 0.01 to 0.35 m / s, very particularly preferably from 0.015 to 0.025 m / s. The residence time on the circulating air belt dryer is preferably from 10 to 120 minutes, particularly preferably from 20 to 90 minutes, very particularly preferably from 30 to 60 minutes.

The water content of the polymer gel bed in the feed zone is preferably from 25 to 90 wt .-%, particularly preferably from 35 to 70 wt .-%, most preferably from 40 to 60 wt .-%. The average particle size of the aqueous polymer gels is preferably from 0.1 to 10 mm, more preferably from 0.5 to 5 mm, most preferably from 1 to 2 mm.

The height of the polymer gel bed on the conveyor belt of the circulating air belt dryer in the feed zone is preferably from 2 to 20 cm, particularly preferably from 5 to 15 cm, very particularly preferably from 8 to 12 cm.

The gas inlet temperatures of the circulating air belt dryer are preferably from 150 to 200.degree. C., particularly preferably from 160 to 190.degree. C., very particularly preferably from 170 to 180.degree.

The gas stream used for drying may contain water vapor. However, the water vapor content should not exceed a value which corresponds to a dew point of preferably at most 50 ° C., particularly preferably at most 40 ° C., very particularly preferably at most 30 ° C.

The water content of the polymer gel after drying on the circulating air belt dryer is preferably from 0.5 to 15% by weight, more preferably from 1 to

10 wt .-%, most preferably from 2 to 8 wt .-%.

A particularly advantageous drying method is described in WO 2001/100300 A1.

The preparation of the water-absorbing polymer particles is explained in more detail below:

The water-absorbing polymer particles are prepared by polymerization of a monomer solution or suspension and are usually water-insoluble. The monomers a) are preferably water-soluble, i. the solubility in water at 23 ° C. is typically at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, more preferably at least 25 g / 100 g of water, most preferably at least 35 g / 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. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS). Impurities can have a significant influence on the polymerization. Therefore, the raw materials used should have the highest possible purity. It is therefore often advantageous to purify the monomers a) specifically. Suitable purification processes are described, for example, in WO 2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitable monomer a) is, for example, an acrylic acid purified according to WO 2004/035514 A1 with 99.8460% by weight of acrylic acid, 0.0950% by weight of acetic acid, 0.0332% by weight of water, 0.0203% by weight. % Propionic acid, 0.0001% by weight furfurale, 0.0001% by weight maleic anhydride,

0.0003% by weight of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl ether.

The proportion of acrylic acid and / or salts thereof in the total amount of monomers a) is preferably at least 50 mol%, particularly preferably at least 90 mol%, very particularly preferably at least 95 mol%. The monomers a) usually contain polymerization inhibitors, preferably hydroquinone half ethers, as a storage stabilizer.

The monomer solution preferably contains up to 250 ppm by weight, preferably at most 130 ppm by weight, more preferably at most 70 ppm by weight, preferably at least 10 ppm by weight, more preferably at least 30 ppm by weight, in particular

50 ppm by weight, hydroquinone half-ether, in each case based on the unneutralized monomer a). For example, an ethylenically unsaturated, acid group-carrying monomer having a corresponding content of hydroquinone half-ether can be used to prepare the monomer solution.

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

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

Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be incorporated in the polymer network in free-radically polymerized form. Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- and triacrylates, as in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 A1 and DE 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated groups, such as in

DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinker mixtures, as described, for example, in DE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/032962 A2.

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

Very particularly preferred crosslinkers b) are the polyethoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form di- or triacrylates, as described, for example, in WO 2003/104301 A1. Particularly advantageous are di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol. Very particular preference is given to diacrylates or triacrylates of 1 to 5 times ethoxylated and / or propoxylated glycerol. Most preferred are the triacrylates of 3 to 5 times ethoxylated and / or propoxylated glycerol, in particular the triacrylate of 3-times ethoxylated glycerol.

The amount of crosslinker b) is preferably from 0.05 to 1, 5 wt .-%, particularly preferably 0.1 to 1 wt .-%, most preferably 0.3 to 0.6 wt .-%, each based on Monomer a). As the crosslinker content increases, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g / cm ² (AUL 0.3 psi) passes through a maximum.

As initiators c) it is possible to use all compounds which generate free radicals under the polymerization conditions, for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. Preference is given to using mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. However, the reducing component used is preferably a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn; DE).

For example, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, ethylenically unsaturated monomers d) which can be copolymerized with the ethylenically unsaturated acid group-carrying monomers a) are lat, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

As water-soluble polymers e) it is possible to use polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose.

Usually, an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75 wt .-%, particularly preferably from 45 to 70 wt .-%, most preferably from 50 to 65 wt .-%. It is also possible monomer suspensions, i. Monomer solutions with excess monomer a), for example sodium acrylate, use. With increasing water content, the energy expenditure increases during the subsequent drying and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.

The preferred polymerization inhibitors require dissolved oxygen for optimum performance. Therefore, the monomer solution may be polymerized prior to polymerization by inerting, i. Flow through with an inert gas, preferably nitrogen or carbon dioxide, are freed of dissolved oxygen. Preferably, the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, most preferably less than 0.1 ppm by weight. Suitable reactors are, for example, kneading reactors or belt reactors. In the kneader, the aqueous polymer gel formed in the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating stirring shafts, as described in WO 2001/038402 A1. The polymerization on the belt is described, for example, in DE 38 25 366 A1 and US Pat. No. 6,241,928. During the polymerization in a belt reactor, an aqueous polymer gel is formed, which must be comminuted in a further process step, for example in an extruder or kneader.

To improve the drying properties, the comminuted aqueous polymer gel obtained by means of a kneader can additionally be extruded.

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

However, it is also possible to carry out the neutralization after the polymerization at the stage of the aqueous polymer gel formed during the polymerization. Furthermore, it is possible to neutralize up to 40 mol%, preferably 10 to 30 mol%, particularly preferably 15 to 25 mol%, of the acid groups before the polymerization by adding a part of the neutralizing agent already to the monomer solution and the desired final degree of neutralization only after the polymerization is adjusted at the stage of the aqueous polymer Geis. If the aqueous polymer gel is at least partially neutralized after the polymerization, the aqueous polymer gel is preferably comminuted mechanically, for example by means of an extruder, wherein the neutralizing agent can be sprayed, sprinkled or poured on and then thoroughly mixed in. For this purpose, the gel mass obtained can be extruded several times for homogenization.

The aqueous polymer gel is then dried with a circulating air belt dryer until the residual moisture content is preferably 0.5 to 15% by weight, particularly preferably 1 to 10% by weight, very particularly preferably 2 to 8% by weight, the residual moisture content being the EDANA recommended test method no. WSP 230.2-05 "Moisture Content". If the residual moisture content is too high, the dried polymer gel has too low a glass transition temperature T _g and is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle and in the subsequent comminution steps undesirably large quantities of polymer particles having too small a particle size ("fines") are produced. , particularly preferably from 35 to 70 wt .-%, most preferably from 40 to 60 wt .-%.

The dried polymer gel is then ground and classified, wherein for grinding usually one- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibratory mills, can be used.

The average particle size of the polymer fraction separated as a product fraction is preferably at least 200 μm, more preferably from 250 to 600 μm, very particularly from 300 to 500 μm. The mean particle size of the product fraction can be determined by means of the EDANA recommended test method No. WSP 220.2-05 "Particles Size distribution ", whereby the mass fractions of the sieve fractions are cumulatively applied and the average particle size is determined graphically, the average particle size being the value of the mesh size which results for accumulated 50% by weight.

The proportion of particles having a particle size of at least 150 μm is preferably at least 90% by weight, more preferably at least 95% by weight, very particularly preferably at least 98% by weight. Polymer particles with too small particle size lower the permeability (SFC). Therefore, the proportion of too small polymer particles ("fines") should be low.

Too small polymer particles are therefore usually separated and recycled to the process. This preferably occurs before, during or immediately after the polymerization, i. before drying the aqueous polymer gel. The polymer particles that are too small can be moistened with water and / or aqueous surfactant before or during the recycling.

It is also possible to separate small polymer particles in later process steps, for example after surface postcrosslinking or another coating step. In this case, the recycled too small polymer particles are surface postcrosslinked or otherwise coated, for example with fumed silica. If a kneading reactor is used for the polymerization, the too small polymer particles are preferably added during the last third of the polymerization.

If the polymer particles which are too small are added very early, for example already to the monomer solution, this lowers the centrifuge retention capacity (CRC) of the water-absorbing polymer particles obtained. However, this can be compensated for example by adjusting the amount of crosslinker b).

If the polymer particles which are too small are added very late, for example only in an apparatus connected downstream of the polymerization reactor, for example an extruder, then the polymer particles which are too small can only be incorporated into the resulting aqueous polymer gel with difficulty. Insufficiently incorporated too small polymer particles, however, dissolve again during the grinding of the dried polymer gel, are therefore separated again during classification and increase the amount of recycled too small polymer particles. The proportion of particles having a particle size of at most 850 μm, is preferably at least 90% by weight, particularly preferably at least 95% by weight, very particularly preferably at least 98% by weight. The proportion of particles having a particle 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.

Polymer particles with too large particle size reduce the swelling rate. Therefore, the proportion of polymer particles too large should also be low.

Too large polymer particles are therefore usually separated and recycled to the grinding of the dried Polymergeis. The polymer particles can be postcrosslinked to further improve the properties. Suitable surface postcrosslinkers are compounds containing groups that can form covalent bonds 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 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230. Furthermore, in DE 40 20 780 C1 cyclic carbonates, in DE 198 07 502 A1

2-Oxazolidinone and its derivatives, such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 992 C1 bis- and poly-2-oxazolidinone, in DE 198 54 573 A1 2-Oxotetrahydro-1, 3-oxazine and its Derivatives, in DE 198 54 574 A1 N-acyl-2-Oxazolidinone, in

DE 102 04 937 A1 describes cyclic ureas, in DE 103 34 584 A1 bicyclic amidoacetals, in EP 1 199 327 A2 oxetanes and cyclic ureas and in WO 2003/031482 A1 morpholine-2,3-dione and its derivatives as suitable surface postcrosslinkers ,

Preferred surface postcrosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1,4-butanediol.

Very particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1,3-propanediol. Furthermore, it is also possible to use surface postcrosslinkers which contain additional polymerisable ethylenically unsaturated groups, as described in DE 37 13 601 A1. The amount of surface postcrosslinker is preferably from 0.001 to 2% by weight, particularly preferably from 0.02 to 1% by weight, completely more preferably 0.05 to

0.2 wt .-%, each based on the polymer particles.

In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers before, during or after the surface postcrosslinking.

The polyvalent cations which can be used in the process according to the invention are, for example, divalent cations, such as the cations of zinc, magnesium, calcium, iron and strontium, trivalent cations, such as the cations of aluminum, iron,

Chromium, rare earth and manganese, tetravalent cations, such as the cations of titanium and zirconium. As a counterion, chloride, bromide, sulfate, hydrogen sulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylate, such as acetate, citrate and lactate, are possible. Aluminum sulfate and aluminum acetate are preferred. In addition to metal salts, polyamines can also be used as polyvalent cations.

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

The surface postcrosslinking is usually carried out so that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. Subsequent to spraying, the polymer particles coated with surface postcrosslinker are thermally dried, whereby the surface postcrosslinking reaction can take place both during and after drying.

The spraying of a solution of the surface postcrosslinker is preferably carried out in mixers with agitated mixing tools, such as screw mixers, disc mixers and paddle mixers. Particularly preferred are horizontal mixers, such as paddle mixers, very particularly preferred are vertical mixers. The distinction in horizontal mixer and vertical mixer via the storage of the mixing shaft, ie horizontal mixer have a horizontally mounted mixing shaft and vertical mixer have a vertically mounted mixing shaft. Suitable mixers are, for example, Horizontal Pflugschar® mixers (Gebr. Lödige Maschinenbau GmbH, Paderborn, DE), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV, Doetinchem, NL), Processall Mixmill Mixer (Processall Incorporated, Cincinnati, US) and Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, NL). However, it is also possible to spray the surface postcrosslinker solution in a fluidized bed.

The surface postcrosslinkers are typically used as an aqueous solution. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by the content of nonaqueous solvent or total solvent amount.

If only water is used as the solvent, it is advantageous to add a surfactant. This improves the wetting behavior and reduces the tendency to clump. However, preference is given to using solvent mixtures, for example isopropanol / water, 1,3-propanediol / water and propylene glycol / water, the mixing mass ratio preferably being from 20:80 to 40:60.

The temperature of the water-absorbing polymer particles in the dryer is preferably from 100 to 250 ° C, more preferably from 130 to 220 ° C, most preferably from 150 to 200 ° C. The residence time in the dryer is preferably from 10 to 120 minutes, more preferably from 10 to 90 minutes, most preferably from 30 to 60 minutes. The degree of filling of the dryer is preferably from 30 to 80%, more preferably from 40 to 75%, most preferably from 50 to 70%. The degree of filling of the dryer can be adjusted by the height of the drain weir. Subsequently, the surface-postcrosslinked polymer particles can be classified again, wherein too small and / or too large polymer particles are separated and recycled to the process.

The surface-postcrosslinked polymer particles can be coated or post-moistened for further improvement of the properties.

The post-wetting is preferably carried out at 30 to 80 ° C, more preferably at 35 to 70 ° C, most preferably at 40 to 60 ° C. If the temperatures are too low, the water-absorbing polymer particles tend to clump together and at higher temperatures water is already noticeably evaporating. The amount of water used for the rewetting is preferably from 1 to 10 wt .-%, particularly preferably from 2 to 8 wt .-%, most preferably from 3 to 5 wt .-%. By rewetting the mechanical stability of the polymer particles is increased and their tendency to static charge reduced.

Suitable coatings for improving the swelling rate and the permeability (SFC) are, for example, inorganic inert substances, such as water insoluble metal salts, organic polymers, cationic polymers and di- or polyvalent metal cations. Suitable coatings for dust binding are, for example, polyols. Suitable coatings against the undesirable tendency for the polymer particles to cake are, for example, fumed silica, 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 preferably 0 to 15 wt .-%, particularly preferably 0.2 to 10 wt .-%, most preferably 0.5 to 8 wt .-%, wherein the Moisture content according to the EDANA recommended test method No. WSP 230.2-05 "Moisture Content".

The water-absorbing polymer particles prepared according to the method of the invention have a centrifuge retention capacity (CRC) of typically at least 15 g / g, preferably at least 20 g / g, preferably at least 22 g / g, more preferably at least 24 g / g, most preferably at least 26 g / g, up. The centrifuge retention capacity (CRC) of the water-absorbing polymer particles is usually less than 60 g / g. Centrifuge retention capacity (CRC) will be determined according to the EDANA recommended test method

No. WSP 241.2-05 "Centrifuge Retention Capacity".

Examples

Example 1 (Preparation of the aqueous polymer gel)

By continuously mixing deionized water, 50% by weight sodium hydroxide solution and acrylic acid, an acrylic acid / sodium acrylate solution was prepared so that the degree of neutralization was 75 mol%. The solids content of the monomer solution was 40.9% by weight.

Polyethylene glycol 400 diacrylate (diacrylate starting from a polyethylene glycol having an average molecular weight of 400 g / mol) was used as the polyethylenically unsaturated crosslinker. The amount used was 2 kg of crosslinker per t of monomer solution.

To initiate the free-radical polymerization, 1.03 kg of a 0.25% strength by weight aqueous hydrogen peroxide solution, 3.10 kg of a

15 wt .-% aqueous sodium peroxodisulfate solution and 1, 05 kg of a 1 wt .-% aqueous ascorbic acid used.

The throughput of the monomer solution was 20 t / h. The reaction solution had a temperature of 23.5 ° C. at the inlet. The individual components were metered in the following amounts continuously into a reactor of the type List Contikneter with a volume of 6.3 m ³ (LIST AG, Arisdorf, CH):

20 t / h monomer solution

40 kg / h of polyethylene glycol 400 diacrylate

82.6 kg / h hydrogen peroxide solution / sodium peroxodisulfate solution

21 kg / h ascorbic acid solution

Between the addition point for the crosslinker and the addition points for the initiators, the monomer solution was rendered inert with nitrogen.

In addition, after about 50% of the residence time, a metered addition of fine grain (1000 kg / h) from the production process by grinding and sieving into the reactor took place. The residence time of the reaction mixture in the reactor was 15 minutes.

The obtained aqueous polymer gel was used for the following experiments.

Example 2 (comparative example)

1 .201 g aqueous polymer gel from Example 1 were introduced into a drying pan and dried for 370 minutes at 175 ° C in a convection oven. The drying pan consisted of a 5 cm high stainless steel frame with an internal dimension of 23.7 x 23.7 cm and a replaceable PTFE base plate. The PTFE base plate was square and was inserted precisely into the stainless steel frame.

The stainless steel frame was previously sprayed with PTFE spray to avoid possible adhesion. The aqueous polymer gel in the drying pan was weighted with a suitable stainless steel plate before drying to prevent the edges of the polymer gel from bending up during drying. The stainless steel plate had parallel rows of slots with an opening of 2 x 20 mm.

The polymer gel shrank three-dimensionally during drying. For the evaluation of the shrinkage only the two-dimensional or area shrinkage was determined. For this purpose, the dried polymer gel was placed as a complete block on a black base, photographed from a distance of 56 cm from the top and recorded the percentage shrinkage via a pixel evaluation.

The shrinkage was 18%. Example 3

The procedure was as in Example 2. 1 .245 g of polymer gel from Example 1 were used and the smooth PTFE bottom plate was replaced by a PTFE bottom plate with 17 rectangular longitudinal grooves. The longitudinal grooves were 5.0 mm deep and 8.0 mm wide.

The shrinkage was 10%. Example 4

The procedure was as in Example 2. There were used 1.246 g of polymer gel from Example 1 and the smooth PTFE bottom plate was replaced by a PTFE bottom plate with 17 rectangular longitudinal and transverse grooves. The longitudinal and transverse channels were each 5.0 mm deep and 8.0 mm wide.

The shrinkage was 6%.

Claims

claims

Process for the preparation of water-absorbing polymer particles by polymerization of an aqueous monomer solution or suspension comprising a) at least one ethylenically unsaturated, acid group-carrying monomer which may be at least partially neutralized,

b) at least one crosslinker,

c) at least one initiator,

d) optionally one or more with the monomers mentioned under a) copolymerizable ethylenically unsaturated monomers and

e) optionally one or more water-soluble polymers, comprising drying the resulting aqueous polymer gel in a circulating air belt dryer by means of a circulating conveyor belt, grinding, classification, and optionally thermal surface post-crosslinking, characterized in that the surface of the conveyor belt has a plurality of elevations or depressions suitable the mobility of the aqueous polymer geis in the transverse direction must be limited.

A method according to claim 1, characterized in that the surface of the conveyor belt has a plurality of thorns with a height of 0.01 to 5 cm,

A method according to claim 1 or 2, characterized in that the circulating conveyor belt is austenitic steel.

A method according to claim 1 or 3, characterized in that the circulating conveyor belt has a plurality of openings.

Method according to one of claims 1 to 4, characterized in that the circulating conveyor belt transversely to the transport direction, a plurality of slots arranged in staggered rows with a length of 5 to 50 mm, a width of 0.5 to 5 mm and a ratio of length to width of 2 to 20 has.

6. The method according to any one of claims 1 to 5, characterized in that the circulating conveyor belt has a width of at least 1 m. 7. The method according to any one of claims 1 to 6, characterized in that the conveyor belt speed of 0.005 to 0.05 m / s.

8. The method according to any one of claims 1 to 7, characterized in that the water content of the polymer geis prior to drying in the circulating air belt dryer is from 30 to 70 wt .-%.

Method according to one of claims 1 to 8, characterized in that the water content of the polymer geis after drying in the circulating air belt dryer is from 0.5 to 15 wt .-%.

Method according to one of claims 1 to 9, characterized in that the height of the polymer gel filling on the circulating conveyor belt is from 2 to 20 cm.

1 1. Process according to any one of Claims 1 to 10, characterized in that the water-absorbing polymer particles have a centrifuge retention capacity of at least 15 g / g.