Method for producing water-absorbent polymer particles with improved properties

0001The present invention relates to a process for producing water-absorbing polymer particles with an improved property profile, comprising thermal surface post-crosslinking in the presence of a salt of a polyvalent metal cation and a complexing anion and the subsequent post-treatment, wherein the post-treatment comprises coating with a salt of a polyvalent metal cation and a non-complexing anion and rewetting with renewed drying.

0002Water-absorbing polymer particles are used to manufacture diapers, tampons, sanitary napkins and other hygiene products, but also as water-retaining agents in agricultural horticulture. The water-absorbing polymer particles are often also referred to as "absorbent resin", "superabsorbents", "superabsorbent polymer", "absorbent polymer", "absorbent gelling material", "hydrophilic polymer", "hydrogels" or "superabsorbents".

0003The production of water-absorbing polymer particles is described in the monograph "<nplcit id="ncit0001" npl-type="b"><text>Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103</text></nplcit>, described.

0004The properties of the water-absorbing polymer particles can be adjusted, for example, by the amount of crosslinker used. As the amount of crosslinker increases, the centrifuge retention capacity (CRC) and the absorption under a pressure of 21.0 g/cm² (AUL0.3psi) passes through a maximum.

0005To improve the application properties such as liquid transfer coefficient (SFC), gel bed permeability (GBP) and absorption under a pressure of 49.2 g/cm² (AUL0.7psi), water-absorbing polymer particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, which increases the absorption under a pressure of 49.2 g/cm² (AUL0.7ps i) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface post-crosslinking can be carried out in an aqueous gel phase. Preferably, however, dried, ground and sieved polymer particles (base polymer) are coated on the surface with a surface post-crosslinker, thermally surface-post-crosslinked and dried. Suitable crosslinkers are compounds that can form covalent bonds with at least two carboxylate groups of the water-absorbing polymer particles.

0006<patcit id="pcit0001" dnum="WO2012045705A1"><text>WO 2012/045705 A1</text></patcit> discloses a process for producing thermally surface-postcrosslinked water-absorbing polymer particles, wherein the water-absorbing polymer particles are coated with at least one polyvalent metal salt before, during or after the thermal surface postcrosslinking and the polyvalent metal salt contains the anion of glycolic acid or the anion of a glycolic acid derivative. This document does not teach any further drying following the addition of a metal salt solution after thermal surface post-crosslinking or following the addition of water in the cooler.

0007<patcit id="pcit0002" dnum="WO2012107432A1"><text>WO 2012/107432 A1</text></patcit> discloses a process for producing water-absorbing polymer particles with a high swelling rate, comprising the steps of polymerization, drying, grinding, classification and thermal surface post-crosslinking, post-wetting and renewed drying.

0008<patcit id="pcit0003" dnum="WO2010149735A1"><text>WO 2010/149735 A1</text></patcit> teaches the addition of aluminum salt to superabsorbents, whereby the anion is selected from a list that contains both complexing and non-complexing anions. The addition can take place before, during or after surface post-crosslinking, but preferably it takes place before. D2 also teaches the possibility of re-moistening after surface post-crosslinking to adjust the desired water content. This document does not teach double addition of aluminum salt, once with complexing and once with non-complexing anion, and no drying after rewetting.

0009<patcit id="pcit0004" dnum="US2011059329A1"><text>US 2011/059329 A1</text></patcit> relates to a process for producing superabsorbents in which drops of a monomer solution are polymerized in a surrounding gas phase, whereby the superabsorbent particles achieve an average sphericity of 0.86 to 0.99, in contrast to superabsorbents which are ground and classified after solution polymerization and thereby achieve an average sphericity of the particles of 0.72 to 0.78. This document also teaches the addition of salts of multivalent cations during surface post-crosslinking, whereby the list of possible anions does not distinguish between complexing and non-complexing. It also teaches the addition of multivalent cations after surface post-crosslinking, whereby anions described there as "weakly complexing" are preferred. According to the teaching of D3, grinding and sieving after polymerization are avoided according to the invention and furthermore no drying takes place after the last addition of water.

0010<patcit id="pcit0005" dnum="WO2010057912A1"><text>WO 2010/057912 A1</text></patcit> also discloses a process for droplet polymerization. The resulting polymer particles are coated with a permeability enhancer and/or pretreated with steam. Polyvalent metal salts, or mixtures of such salts, can be used as permeability enhancers, with a list of anions not differentiated according to complexing or non-complexing being given. Pretreatment with steam is carried out, if necessary, before the addition of the permeability improver. This document is silent on grinding, sieving, surface post-crosslinking, twice adding a polyvalent metal salt, post-wetting or drying after post-wetting.

0011The object of the present invention was to provide a process for producing water-absorbing polymer particles with an improved property profile.

0012The object was achieved by a process for producing water-absorbing polymer particles by polymerization of a monomer solution or suspension containing<ol id="ol0001" compact="compact"><li>a) at least one ethylenically unsaturated monomer containing acid groups, which may be at least partially neutralised,</li><li>b) at least one crosslinker,</li><li>c) at least one initiator,</li><li>d) optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers mentioned under a) and</li><li>e) optionally one or more water-soluble polymers, wherein the polymer gel obtained is dried, ground and classified, the classified polymer particles are</li><li>f) at least one covalent surface post-crosslinker and</li><li>g) at least one salt of a polyvalent metal cation and a complexing acid anion</li></ol>thermally surface-crosslinked, characterized in that the surface-crosslinked polymer particles are subsequently post-treated and the post-treatment comprises the steps<ol id="ol0002" compact="compact"><li>i) coating with at least one salt of a polyvalent metal cation and a non-complexing acid anion,</li><li>ii) increasing the moisture content by 1 to 150 wt.% and</li><li>iii) Drying after increasing the moisture content</li></ol>includes.

0013Process steps i) and ii) can be carried out in any order. However, step i) is preferably carried out before step ii).

0014Suitable complexing acid anions for the at least one salt g) are carboxylic acid anions which, in addition to the carboxylic acid group, have at least one functional group suitable for complexing. Such functional groups have free electron pairs without themselves contributing to the charge balance of the polyvalent metal cation, such as, for example, hydroxy and amino groups. Preferred acid anions for the at least one salt g) are glycolate, glycinate, lactate, alanate, citrate, tartrate, tartronate and glycerate.

0015Suitable polyvalent metal cations for the salt g) are, for example, divalent cations such as Zn²⁺, Mg²⁺, Ca²⁺, Fe²⁺ and Sr²⁺, trivalent cations, such as Al³⁺, Fe³⁺, Cr³⁺, and Mn³⁺, tetravalent cations such as Ti⁴⁺ and Zr⁴⁺. Preferred polyvalent metal cations are Al³⁺, Ti⁴⁺ and Zr⁴⁺. The most preferred salt g) is aluminium lactate.

0016The amount of polyvalent metal cation used in the salt g) is preferably 0.001 to 1.5% by weight, particularly preferably 0.005 to 1% by weight, very particularly preferably 0.02 to 0.8% by weight, in each case based on the polymer particles.

0017Suitable non-complexing acid anions for the salt in step i) are organic acid anions which, in addition to the acid group, have no functional group suitable for complexing, or inorganic acid anions. Particularly preferred acid anions for the at least one salt in step i) are formate, acetate, propionate, methylsulfonate, sulfate and chloride.

0018Suitable polyvalent metal cations for the salt in step i) are, for example, divalent cations such as Zn²⁺, Mg²⁺, Ca²⁺, Fe²⁺ and Sr²⁺, trivalent cations, such as Al³⁺, Fe³⁺, Cr³⁺, and Mn3+, tetravalent cations such as Ti⁴⁺ and Zr⁴⁺. Preferred polyvalent metal cations are Al³⁺, Ti⁴⁺ and Zr⁴⁺. Particularly preferred salts in step i) are aluminium sulfate, sodium alum and potassium alum.

0019The amount of polyvalent metal cation used in the salt in step i) is preferably 0.001 to 1.5% by weight, particularly preferably 0.005 to 1% by weight, very particularly preferably 0.02 to 0.8% by weight, in each case based on the polymer particles.

0020The moisture content in step ii) is increased by preferably 2.5 to 100% by weight, particularly preferably 5 to 50% by weight, very particularly 10 to 25% by weight (post-moistening). The manner in which the moisture content is increased is not subject to any restriction. For example, the water-absorbing polymer particles can be brought into contact with water in liquid or gaseous form, for example by spraying or by aeration with moist gases (air, nitrogen, etc.). Alternatively, crushed ice or already moist water-absorbing polymer particles can be mixed in. Combinations of different addition forms are also possible, for example an aqueous solution of the salt of polyvalent metal cation and non-complexing acid anion and water vapor.

0021The product temperature during the increase in water content is, for example, 0 to 140°C, preferably 20 to 120°C, particularly preferably 50 to 100°C, most preferably 60 to 90°C.

0022The residence time between increasing the water content and subsequent drying is not critical and is, for example, less than 10 days, preferably less than 5 days, more preferably less than one day, particularly preferably less than 6 hours, most preferably less than 2 hours.

0023The water-absorbing polymer particles are then dried at temperatures of preferably less than 150°C, particularly preferably less than 130°C, very particularly preferably less than 110°, to a moisture content of preferably less than 10% by weight, particularly preferably less than 7% by weight, very particularly preferably less than 5% by weight.

0024The subsequent drying can be carried out statically or dynamically, ie the water-absorbing polymer particles are moved, for example stirred, or not. Dynamic drying is preferred. The pressure during drying is also not critical and corresponds, for example, to the ambient pressure or less (negative pressure). However, it is also possible to ventilate the water-absorbing polymer particles with a dry gas (air, nitrogen, etc.) for drying.

0025In a preferred embodiment of the present invention, the desired moisture content for the final product is adjusted during the drying according to the invention.

0026The present invention is based on the finding that the vortex and the permeability, ie the liquid conductivity (SFC) and the gel bed permeability (GBP) of water-absorbing polymer particles can be improved simultaneously by carrying out the thermal surface post-crosslinking in the presence of a salt of a polyvalent metal cation and a complexing acid anion and subsequently coating the surface-post-crosslinked polymer particles with a salt of a polyvalent metal cation and a non-complexing acid anion, swollen and dried again. The coating of the surface-crosslinked polymer particles with the salt of a polyvalent metal cation and a non-complexing acid anion can also be carried out after swelling and drying, ie the order of the two post-treatment steps is not important.

0027The production of the water-absorbing polymer particles is explained in more detail below: The water-absorbing polymer particles are produced by polymerization of a monomer solution or suspension and are usually water-insoluble.

0028The monomers a) are preferably water-soluble, ie the solubility in water at 23°C is typically at least 1 g/100 g water, preferably at least 5 g/100 g water, particularly preferably at least 25 g/100 g water, very particularly preferably at least 35 g/100 g water.

0029Suitable 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. Monomers a) that are produced from renewable raw materials are also very suitable.

0030Other suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

0031Impurities can have a significant impact on the polymerization. Therefore, the raw materials used should be as pure as possible. It is therefore often advantageous to specifically purify the monomers a). Suitable purification processes are described, for example, in the<patcit id="pcit0006" dnum="WO2002055469A1"><text>WO 2002/055469 A1</text></patcit>, the<patcit id="pcit0007" dnum="WO2003078378A1"><text>WO 2003/078378 A1</text></patcit> and the<patcit id="pcit0008" dnum="WO2004035514A1"><text>WO 2004/035514 A1</text></patcit> A suitable monomer a) is, for example, a<patcit id="pcit0009" dnum="WO2004035514A1"><text>WO 2004/035514 A1</text></patcit> purified acrylic acid with 99.8460 wt.% acrylic acid, 0.0950 wt.% acetic acid, 0.0332 wt.% water, 0.0203 wt.% propionic acid, 0.0001 wt.% furfurals, 0.0001 wt.% maleic anhydride, 0.0003 wt.% diacrylic acid and 0.0050 wt.% hydroquinone monomethyl ether.

0032The 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%.

0033The monomers a) usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers.

0034The monomer solution preferably contains up to 250 ppm by weight, preferably at most 130 ppm by weight, particularly preferably at most 70 ppm by weight, preferably at least 10 ppm by weight, particularly preferably at least 30 ppm by weight, in particular around 50 ppm by weight, of hydroquinone monoether, in each case based on the unneutralized monomer a). For example, an ethylenically unsaturated, acid group-bearing monomer with an appropriate content of hydroquinone monoether can be used to prepare the monomer solution.

0035Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and/or alpha-tocopherol (vitamin E).

0036Suitable crosslinkers b) are compounds with at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups that can be radically polymerized into the polymer chain and functional groups that can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts which can form coordinate bonds with at least two acid groups of the monomer a) are also suitable as crosslinkers b).

0037Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be radically polymerized into the polymer network. Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as in<patcit id="pcit0010" dnum="EP0530438A1"><text>EP 0 530 438 A1</text></patcit> described, di- and triacrylates, as in<patcit id="pcit0011" dnum="EP0547847A1"><text>EP 0 547 847 A1</text></patcit>, <patcit id="pcit0012" dnum="EP0559476A1"><text>EP 0 559 476 A1</text></patcit>, <patcit id="pcit0013" dnum="EP0632068A1"><text>EP 0 632 068 A1</text></patcit>, <patcit id="pcit0014" dnum="WO9321237A1"><text>WO 93/21237 A1</text></patcit>, <patcit id="pcit0015" dnum="WO2003104299A1"><text>WO 2003/104299 A1</text></patcit>, <patcit id="pcit0016" dnum="WO2003104300A1"><text>WO 2003/104300 A1</text></patcit>, <patcit id="pcit0017" dnum="WO2003104301A1"><text>WO 2003/104301 A1</text></patcit> and<patcit id="pcit0018" dnum="DE10331450A1"><text>DE 103 31 450 A1</text></patcit> Mixed acrylates containing ethylenically unsaturated groups in addition to acrylate groups, as described in<patcit id="pcit0019" dnum="DE10331456A1"><text>DE 103 31 456 A1</text></patcit> and<patcit id="pcit0020" dnum="DE10355401A1"><text>DE 103 55 401 A1</text></patcit> described, or crosslinker mixtures, such as in<patcit id="pcit0021" dnum="DE19543368A1"><text>DE 195 43 368 A1</text></patcit>, <patcit id="pcit0022" dnum="DE19646484A1"><text>DE 196 46 484 A1</text></patcit>, <patcit id="pcit0023" dnum="WO9015830A1"><text>WO 90/15830 A1</text></patcit> and<patcit id="pcit0024" dnum="WO2002032962A2"><text>WO 2002/032962 A2</text></patcit> described.

0038Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraalloxyethane, methylenebismethacrylamide, 15-fold ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine.

0039Very particularly preferred crosslinkers b) are the multiply ethoxylated and/or propoxylated glycerols esterified with acrylic acid or methacrylic acid to di- or triacrylates, as used, for example, in<patcit id="pcit0025" dnum="WO2003104301A1"><text>WO 2003/104301 A1</text></patcit> are described. Di- and/or triacrylates of 3- to 20-fold ethoxylated glycerol are particularly advantageous. Di- or triacrylates of 1- to 5-fold ethoxylated and/or propoxylated glycerol are very particularly preferred. The triacrylates of 3- to 5-fold ethoxylated and/or propoxylated glycerol are most preferred, in particular the triacrylate of 3-fold ethoxylated glycerol.

0040The amount of crosslinker b) is preferably 0.05 to 1.5 wt.%, particularly preferably 0.1 to 1 wt.%, very particularly preferably 0.3 to 0.6 wt.%, in each case based on monomer a).

0041All compounds that generate radicals under the polymerization conditions can be used as initiators c), 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. Preferably, mixtures of thermal initiators and redox initiators are used, such as sodium peroxodisulfate/hydrogen peroxide/ascorbic acid. However, disodium 2-hydroxy-2-sulfonatoacetate or a mixture of disodium 2-hydroxy-2-sulfinatoacetate, disodium 2-hydroxy-2-sulfonatoacetate and sodium bisulfite are preferably used as the reducing component. Such mixtures are known as Brüggolite^® FF6 and Brüggolite^® FF7 (Brüggemann Chemicals; Heilbronn; Germany).

0042Examples of ethylenically unsaturated monomers d) which are copolymerizable with the ethylenically unsaturated monomers a) containing acid groups are acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

0043As water-soluble polymers e) there can be used 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.

0044Usually an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75% by weight, particularly preferably from 45 to 70% by weight, very particularly preferably from 50 to 65% by weight. It is also possible to use monomer suspensions, ie monomer solutions with excess monomer a), for example sodium acrylate. As the water content increases, the energy required for subsequent drying increases and as the water content decreases, the polymerization heat can only be dissipated insufficiently.

0045The preferred polymerization inhibitors require dissolved oxygen for optimal effect. Therefore, the monomer solution can be freed of dissolved oxygen before polymerization by inerting, ie by flowing an inert gas, preferably nitrogen or carbon dioxide, through it. Preferably, the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

0046The monomer solution or suspension is polymerized. Suitable reactors are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel produced during the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating stirring shafts, as in<patcit id="pcit0026" dnum="WO2001038402A1"><text>WO 2001/038402 A1</text></patcit> The polymerization on the tape is described, for example, in<patcit id="pcit0027" dnum="DE3825366A1"><text>DE 38 25 366 A1</text></patcit> and<patcit id="pcit0028" dnum="US6241928B"><text>US 6,241,928</text></patcit> described. During polymerization in a belt reactor, a polymer gel is formed, which must be comminuted in a further process step, for example in an extruder or kneader.

0047To improve the drying properties, the comminuted polymer gel obtained by means of a kneader can be additionally extruded.

0048However, it is also possible to drop an aqueous monomer solution and polymerize the droplets produced in a heated carrier gas stream. The process steps of polymerization and drying can be combined, as in<patcit id="pcit0029" dnum="WO2008040715A2"><text>WO 2008/040715 A2</text></patcit> and<patcit id="pcit0030" dnum="WO2008052971A1"><text>WO 2008/052971 A1</text></patcit> described.

0049The acid groups of the resulting polymer gels are usually partially neutralized. Neutralization is preferably carried out at the monomer stage. This is usually done by mixing in 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%, it being possible to use the usual neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogen carbonates and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Sodium and potassium are particularly preferred as alkali metals, but sodium hydroxide, sodium carbonate or sodium hydrogen carbonate and mixtures thereof are very particularly preferred.

0050However, it is also possible to carry out the neutralization after polymerization at the stage of the polymer gel formed during 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 polymerization by adding part of the neutralizing agent to the monomer solution and setting the desired final degree of neutralization only after polymerization at the polymer gel stage. If the polymer gel is at least partially neutralized after polymerization, the polymer gel is preferably mechanically comminuted, for example by means of an extruder, whereby the neutralizing agent can be sprayed on, sprinkled over or poured on and then carefully mixed in. For this purpose, the resulting gel mass can be extruded several times for homogenization.

0051The polymer gel obtained is dried. There are no restrictions on the dryers. However, the polymer gel is preferably dried using a 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 determined according to test method No. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA. If the residual moisture is too high, the dried polymer gel has a glass transition temperature T_G and is difficult to process further. If the residual moisture is too low, the dried polymer gel is too brittle and undesirably large amounts of polymer particles with too small a particle size ("fines") are produced in the subsequent comminution steps. The solids content of the gel before drying is preferably between 25 and 90% by weight, particularly preferably between 35 and 70% by weight, very particularly preferably between 40 and 60% by weight. Alternatively, a fluidized bed dryer or a paddle dryer can also be used for drying.

0052The dried polymer gel is ground and classified, whereby single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills, can usually be used for grinding.

0053The average particle size of the polymer particles separated as a product fraction is preferably at least 200 µm, particularly preferably from 250 to 600 µm, most particularly from 300 to 500 µm. The average particle size of the product fraction can be determined using the test method No. WSP 220.2-05 "Particle Size Distribution" recommended by EDANA, whereby the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically. The mean particle size is the value of the mesh size that results for a cumulative 50 wt.%.

0054The proportion of particles with a particle size of at least 150 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, very particularly preferably at least 98 wt.%.

0055Polymer particles with too small a particle size reduce the liquid flow conductance (SFC). Therefore, the proportion of polymer particles that are too small ("fines") should be low.

0056Polymer particles that are too small are therefore usually separated and returned to the process. This is preferably done before, during or immediately after polymerization, ie before the polymer gel is dried. The polymer particles that are too small can be moistened with water and/or aqueous surfactant before or during the return.

0057It is also possible to separate out polymer particles that are too small in later process steps, for example after surface post-crosslinking or another coating step. In this case, the returned polymer particles that are too small are surface post-crosslinked or coated in another way, for example with pyrogenic silica.

0058If a kneading reactor is used for polymerization, the polymer particles that are too small are preferably added during the last third of the polymerization.

0059If the polymer particles that are too small are added very early, for example to the monomer solution, the centrifuge retention capacity (CRC) of the resulting water-absorbing polymer particles is reduced. However, this can be compensated for by adjusting the amount of crosslinker b) used, for example.

0060If the polymer particles that are too small are added very late, for example only in an apparatus downstream of the polymerization reactor, such as an extruder, the polymer particles that are too small can only be incorporated into the resulting polymer gel with difficulty. However, insufficiently incorporated polymer particles that are too small detach from the dried polymer gel during grinding and are therefore separated again during classification, increasing the amount of polymer particles that are too small to be returned.

0061The proportion of particles with a particle size of at most 850 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, very particularly preferably at least 98 wt.%.

0062The proportion of particles with a particle size of at most 600 µm is preferably at least 90 wt.%, particularly preferably at least 95 wt.%, very particularly preferably at least 98 wt.%.

0063Polymer particles that are too large reduce the swelling rate. Therefore, the proportion of polymer particles that are too large should also be low.

0064Polymer particles that are too large are therefore usually separated and returned to the grinding of the dried polymer gel.

0065The polymer particles are thermally surface-crosslinked to improve their properties. Suitable surface-crosslinkers f) are compounds that contain 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 in<patcit id="pcit0031" dnum="EP0083022A2"><text>EP 0 083 022 A2</text></patcit>, <patcit id="pcit0032" dnum="EP0543303A1"><text>EP 0 543 303 A1</text></patcit> and<patcit id="pcit0033" dnum="EP0937736A2"><text>EP 0 937 736 A2</text></patcit> described, di- or polyfunctional alcohols, as in<patcit id="pcit0034" dnum="DE3314019A1"><text>DE 33 14 019 A1</text></patcit>, <patcit id="pcit0035" dnum="DE3523617A1"><text>DE 35 23 617 A1</text></patcit> and<patcit id="pcit0036" dnum="EP0450922A2"><text>EP 0 450 922 A2</text></patcit> described, or ß-hydroxyalkylamides, as in<patcit id="pcit0037" dnum="DE10204938A1"><text>DE 102 04 938 A1</text></patcit> and<patcit id="pcit0038" dnum="US6239230B"><text>US 6,239,230</text></patcit> described.

0066Furthermore,<patcit id="pcit0039" dnum="DE4020780C1"><text>DE 40 20 780 C1</text></patcit> cyclic carbonates, in<patcit id="pcit0040" dnum="DE19807502A1"><text>DE 198 07 502 A1</text></patcit> 2-Oxazolidone and its derivatives, such as 2-hydroxyethyl-2-oxazolidone, in<patcit id="pcit0041" dnum="DE19807992C1"><text>DE 198 07 992 C1</text></patcit> Bis- and poly-2-oxazolidinones, in<patcit id="pcit0042" dnum="DE19854573A1"><text>DE 198 54 573 A1</text></patcit> 2-Oxotetrahydro-1,3-oxazine and its derivatives, in<patcit id="pcit0043" dnum="DE19854574A1"><text>DE 198 54 574 A1</text></patcit> N-acyl-2-oxazolidones, in<patcit id="pcit0044" dnum="DE10204937A1"><text>DE 102 04 937 A1</text></patcit> cyclic ureas, in<patcit id="pcit0045" dnum="DE10334584A1"><text>DE 103 34 584 A1</text></patcit> bicyclic amide acetals, in<patcit id="pcit0046" dnum="EP1199327A2"><text>EP 1 199 327 A2</text></patcit> Oxetanes and cyclic ureas and in<patcit id="pcit0047" dnum="WO2003031482A1"><text>WO 2003/031482 A1</text></patcit> Morpholine-2,3-dione and its derivatives are described as suitable surface post-crosslinkers f).

0067Preferred surface post-crosslinkers f) are ethylene carbonate, propylene carbonate, glycerol carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1,4-butanediol.

0068Particularly preferred surface postcrosslinkers f) are 2-hydroxyethyloxazolidin-2-one, oxazolidin-2-one and 1,3-propanediol.

0069Furthermore, surface post-crosslinkers f) can be used which contain additional polymerizable ethylenically unsaturated groups, as in<patcit id="pcit0048" dnum="DE3713601A1"><text>DE 37 13 601 A1</text></patcit> described

0070The amount of surface postcrosslinker f) is preferably 0.001 to 2 wt.%, particularly preferably 0.02 to 1 wt.%, very particularly preferably 0.05 to 0.2 wt.%, in each case based on the polymer particles.

0071The thermal surface post-crosslinking is carried out in the presence of at least one salt g) of a polyvalent metal cation and a complexing acid anion. The salt g) of a polyvalent metal cation and a complexing acid anion can be applied to the particle surface before or during the thermal surface post-crosslinking.

0072Surface post-crosslinking is usually carried out by spraying a solution of the surface post-crosslinker f) onto the dried polymer particles. Following spraying, the polymer particles coated with surface post-crosslinker f) are thermally dried, whereby the surface post-crosslinking reaction can take place both before and during drying.

0073The spraying of a solution of the surface post-crosslinker f) is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are very particularly preferred. The distinction between horizontal mixers and vertical mixers is made by the bearing of the mixing shaft, ie Horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft. Suitable mixers are, for example, horizontal ploughshare^® Mixer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix^® (Hosokawa Micron BV; Doetinchem; Netherlands). It is also possible to spray the surface post-crosslinker solution in a fluidized bed.

0074The surface post-crosslinkers f) are typically used as an aqueous solution. The penetration depth of the surface post-crosslinker f) into the polymer particles can be adjusted via the content of non-aqueous solvent or the total amount of solvent.

0075If only water is used as a solvent, it is advantageous to add a surfactant. This improves the wetting properties and reduces the tendency to clump. However, solvent mixtures are preferably used, for example isopropanol/water, 1,3-propanediol/water and propylene glycol/water, with the mixing ratio preferably being from 20:80 to 40:60.

0076The thermal surface post-crosslinking is preferably carried out in contact dryers, particularly preferably paddle dryers, most preferably disk dryers. Suitable dryers are, for example, Hosokawa Bepex^® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex^® Disc Dryer (Hosokawa Micron GmbH; Leingarten; Germany) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). In addition, fluidized bed dryers can also be used.

0077Thermal surface crosslinking can take place in the mixer itself, by heating the jacket or blowing in warm air. A downstream dryer, such as a tray dryer, a rotary kiln or a heatable screw, is also suitable. Mixing and drying in a fluidized bed dryer is particularly advantageous.

0078Preferred surface post-crosslinking temperatures are in the range 100 to 250°C, preferably 120 to 220°C, particularly preferably 130 to 210°C, very particularly preferably 150 to 200°C. The preferred residence time at this temperature in the reaction mixer or dryer is preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes, and usually at most 60 minutes.

0079After the thermal surface post-crosslinking, the post-treatment according to the invention is carried out.

0080The surface-crosslinked and post-treated polymer particles can then be classified again, with polymer particles that are too small and/or too large being separated and returned to the process.

0081The polymer particles can be additionally coated and re-moistened to further improve their properties.

0082The remoistening is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, very particularly preferably at 40 to 60°C. At temperatures that are too low, the water-absorbing polymer particles tend to clump together and at higher temperatures, water evaporates noticeably. The amount of water used for remoistening is preferably from 1 to 10% by weight, particularly preferably from 2 to 8% by weight, very particularly preferably from 3 to 5% by weight. Rewetting increases the mechanical stability of the polymer particles and reduces their tendency to become statically charged.

0083Suitable coatings for improving the swelling rate and liquid flow (SFC) include inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers and divalent or multivalent metal cations. Suitable coatings for dust binding include polyols. Suitable coatings against the undesirable tendency of the polymer particles to cake include pyrogenic silica, such as Aerosil^® 200, and surfactants such as Span^® 20.

0084A further object of the present invention are the water-absorbing polymer particles obtainable by the process according to the invention.

0085The present invention further provides water-absorbing polymer particles obtainable by polymerizing a monomer solution or suspension to form a polymer gel, drying, grinding and classifying the polymer gel to form polymer particles and thermally post-crosslinking the polymer particles on the surface, wherein the water-absorbing polymer particles have a moisture content of less than 10% by weight, a centrifuge retention capacity of at least 15 g/g, a fluid transfer of at least 80x10^-7cm³s/g, a gel bed permeability of at least 30 Darcies and a vortex of less than 70s.

0086The water-absorbing polymer particles according to the invention typically have a high liquid flow transmission (SFC) of at least 100 x 10^-7 cm³s/g, a high gel bed permeability (GBP) and a low vortex, for example a liquid conductance (SFC) of preferably at least 130 x 10^-7 cm³s/g, particularly preferably from 150 to 250 x 10^-7 cm³s/g, a gel bed permeability (GBP) of preferably at least 40 Darcies, more preferably at least 45 Darcies, most preferably from 50 to 100 Darcies, and a vortex of preferably less than 65 s, more preferably less than 62s, most preferably from 40 to 60s.

0087The water-absorbing polymer particles according to the invention have a centrifuge retention capacity (CRC) of typically at least 15 g/g, preferably at least 18 g/g, preferably at least 20 g/g, particularly preferably at least 22 g/g, very particularly preferably 23 to 40 g/g. The centrifuge retention capacity (CRC) is determined according to the test method recommended by EDANA No. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation".

0088The water-absorbing polymer particles according to the invention have an absorption under a pressure of 49.2 g/cm² of typically at least 15 g/g, preferably at least 18 g/g, preferably at least 20 g/g, particularly preferably at least 22 g/g, most preferably 23 to 40 g/g. The absorption under a pressure of 49.2 g/cm² is determined analogously to the test method No. WSP 242.2-05 "Absorption Under Pressure, Gravimetric Determination" recommended by EDANA, whereby instead of a pressure of 21.0 g/cm² a pressure of 49.2 g/cm² is set.

0089The water-absorbing polymer particles according to the invention have a moisture content of preferably less than 10% by weight, particularly preferably less than 8% by weight, very particularly preferably from 0.5 to 6% by weight, wherein the moisture content is determined according to the test method No. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA.

0090Another object of the present invention are hygiene articles containing water-absorbing polymer particles according to the invention.

0091The hygiene articles usually contain a water-impermeable back, a water-permeable top and, in between, an absorbent core made of the water-absorbing polymer particles according to the invention and fibers, preferably cellulose. The proportion of the water-absorbing polymer particles according to the invention in the absorbent core is preferably 20 to 100% by weight, more preferably 50 to 100% by weight.

0092The water-absorbing polymer particles are tested using the test methods described below.

0093The standard test methods designated "WSP" are described in: "<nplcit id="ncit0002" npl-type="b"><text>Standard Test Methods for the Nonwovens Industry", Edition 2005, jointly published by the "Worldwide Strategy Partners" EDANA (Avenue Eugene Plasky 157, 1030 Brussels, Belgium, www.edana.org) and INDA (1100 Crescent Green, Cary, NC 27518, USA, www.inda.org</text></nplcit>). This publication is available from both EDANA and INDA.

Methods:

0094Unless otherwise stated, measurements should be carried out at an ambient temperature of 23 ± 2 °C and a relative humidity of 50 ± 10 %. The water-absorbing polymer particles are thoroughly mixed before measurement.

moisture content

0095The moisture content of the water-absorbing polymer particles is determined according to the test method No. WSP 230.2-05 "Mass Loss Upon Heating" recommended by EDANA.

Centrifuge Retention Capacity

0096The centrifuge retention capacity (CRC) is determined according to EDANA recommended test method No. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation".

Absorption under a pressure of 0.0 g/cm² (Absorption under Load)

0097The absorption under a pressure of 0.0 g/cm² (AUL0.0psi) is determined analogously to the test method No. WSP 242.2-05 "Absorption Under Pressure, Gravimetric Determination" recommended by EDANA, whereby instead of a pressure of 21.0 g/cm² (AUL0.3psi) a pressure of 0.0 g/cm² (AUL0.0psi) is set.

Absorption under a pressure of 21.0 g/cm² (Absorption under Load)

0098The absorption under a pressure of 21.0 g/cm² (AUL0.3psi) is determined according to EDANA recommended test method No. WSP 242.2-05 "Absorption Under Pressure, Gravimetric Determination".

Absorption under a pressure of 49.2 g/cm² (Absorption under Load)

0099The absorption under a pressure of 49.2 g/cm² (AUL0.7psi) is determined analogously to the test method No. WSP 242.2-05 "Absorption Under Pressure, Gravimetric Determination" recommended by EDANA, whereby instead of a pressure of 21.0 g/cm² (AUL0.3psi) a pressure of 49.2 g/cm² (AUL0.7psi) is set.

Extractable

0100The content of extractable components of the water-absorbing polymer particles is determined according to the test method No. WSP 270.2-05 "Extractable" recommended by EDANA.

Free Swell Rate

0101To determine the swelling rate (FSR), 1.00 g (= W1) of the water-absorbing polymer particles are weighed into a 25 ml beaker and evenly distributed over the bottom. Then 20 ml of a 0.9 wt.% saline solution are dosed into a second beaker using a dispenser and the contents of this beaker are quickly added to the first and a stopwatch is started. As soon as the last drop of saline solution is absorbed, which is indicated by the disappearance of the reflection on the liquid surface, the stopwatch is stopped. The exact amount of liquid poured out of the second beaker and absorbed by the polymer in the first beaker is determined by weighing the second beaker back (=W2). The time required for absorption, measured by the stopwatch, is called t. The disappearance of the last drop of liquid on the surface is determined as time t.

0102The swelling rate (FSR) is calculated as follows:<maths id="math0001"><math display="block"><mi>FSR</mi><mspace width="1ex" /><mfenced open="[" close="]"><mi mathvariant="normal">G</mi><mo>/</mo><mi mathvariant="normal">G</mi><mspace width="1ex" /><mi mathvariant="normal">s</mi></mfenced><mo>=</mo><mi mathvariant="normal">W</mi><mn>2</mn><mo>/</mo><mfenced><mi mathvariant="normal">W</mi><mn>1</mn><mi>xt</mi></mfenced></math><img file="EP2870183B2_D0001.tif" /></maths>

0103However, if the moisture content of the water-absorbing polymer particles is more than 3 wt.%, the weight W1 must be corrected by this moisture content.

Vortex

010450.0 ml ± 1.0 ml of a 0.9 wt.% aqueous sodium chloride solution are placed in a 100 ml beaker containing a magnetic stirrer bar measuring 30 mm x 6 mm. The sodium chloride solution is stirred at 600 rpm using a magnetic stirrer. 2.000 g ± 0.010 g of water-absorbing polymer particles are then added as quickly as possible and the time that elapses until the stirred cluster disappears due to the absorption of the sodium chloride solution by the water-absorbing polymer particles is measured. The entire contents of the beaker can still rotate as a uniform gel mass, but the surface of the gelled saline solution must no longer show any individual turbulence. The time required is reported as a vortex.

fluid conductivity (saline flow conductivity)

0105The fluid conductance (SFC) of a swollen gel layer under pressure of 0.3 psi (2070 Pa) is determined as in<patcit id="pcit0049" dnum="EP0640330A1"><text>EP 0 640 330 A1</text></patcit> described, as the gel layer permeability of a swollen gel layer of water-absorbing polymer particles, whereby the apparatus described in the aforementioned patent application on page 19 and in Figure 8 was modified in such a way that the glass frit (40) is no longer used, the stamp (39) is made of the same plastic material as the cylinder (37) and now contains 21 holes of the same size evenly distributed over the entire support surface. The procedure and evaluation of the measurement remains unchanged compared to<patcit id="pcit0050" dnum="EP0640330A1"><text>EP 0 640 330 A1</text></patcit>The flow is recorded automatically.

0106The fluid conductance (SFC) is calculated as follows:<maths id="math0002"><math display="block"><mi>SFC</mi><mspace width="1ex" /><mfenced open="[" close="]"><msup><mi>cm</mi><mn>3</mn></msup><mi mathvariant="normal">s</mi><mo>/</mo><mi mathvariant="normal">G</mi></mfenced><mo>=</mo><mfenced><mi>Fg</mi><mfenced><mi mathvariant="normal">t</mi><mo>=</mo><mn>0</mn></mfenced><mi>xL</mi><mn>0</mn></mfenced><mo>/</mo><mfenced><mi>dxAxWP</mi></mfenced><mo>,</mo></math><img file="EP2870183B2_D0002.tif" /></maths> where Fg(t=0) is the flow rate of NaCl solution in g/s, which is obtained from a linear regression analysis of the data Fg(t) of the flow determinations by extrapolation to t=0, L0 is the thickness of the gel layer in cm, d is the density of the NaCl solution in g/cm³, A is the area of ​​the gel layer in cm² and WP is the hydrostatic pressure above the gel layer in dyn/cm².

gel bed permeability

0107The gel bed permeability (GBP) of a swollen gel layer under a pressure load of 0.3 psi (2070 Pa) is determined as in<patcit id="pcit0051" dnum="US20050256757A"><text>US 2005/0256757</text></patcit> described (paragraphs [0061] and [0075]), as the gel bed permeability of a swollen gel layer of water-absorbing polymer particles.

examples

production of the base polymer

Example 1

0108A base polymer was prepared analogously to that described in<patcit id="pcit0052" dnum="WO0138402A1"><text>WO 01/38402 A1</text></patcit> described continuous kneading process in a reactor of the type List Contikneter with a volume of 6.3m³ (LIST AG, Arisdorf, CH). Acrylic acid was continuously neutralized with sodium hydroxide solution and diluted with water so that the degree of neutralization of the acrylic acid was 69 mol-% and the solids content (= sodium acrylate and acrylic acid) of this solution was approximately 40.0 wt.%. Glycerol triacrylate esterified with acrylic acid and 3-fold ethoxylated was used as a crosslinker (Gly-3 EO-TA), which was<patcit id="pcit0053" dnum="US20050176910A"><text>US 2005/0176910</text></patcit> was produced in an amount of 0.348 wt.% based on acrylic acid monomer. The crosslinker was continuously mixed into the monomer stream. To calculate the acrylic acid monomer content, the sodium acrylate contained was mathematically taken into account as acrylic acid. The initiation was also carried out by continuous addition of aqueous solutions of the initiators sodium persulfate (0.195 wt.% based on acrylic acid monomer), hydrogen peroxide (0.002 wt.% based on acrylic acid monomer) and ascorbic acid (0.0031 wt.% based on acrylic acid monomer).

0109The resulting polymer gel was dried on a belt dryer, then the dryer cake was broken, ground using a roller mill and finally sieved to a grain size of 150 to 850 µm.

0110The base polymer thus produced had the following properties:<ul id="ul0001" list-style="none" compact="compact"><li>CRC = 36.0 g/g</li><li>Extractable (16 h) = 14.0 wt.%</li></ul>

particle size distribution

0111<tables id="tabl0001" num="0001"><table frame="none"><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="23mm" /><colspec colnum="2" colname="col2" colwidth="26mm" /><tbody><row><entry>>850 µm</entry><entry>< 0.1 wt.%</entry></row><row><entry>600-850 µm</entry><entry>3.61 wt.%</entry></row><row><entry>300-600 µm</entry><entry>77.55 wt.%</entry></row><row><entry>150-300 µm</entry><entry>18.8 wt.%</entry></row><row><entry><150 µm</entry><entry>< 0.1 wt.%</entry></row></tbody></tgroup></table></tables>

Example 2

0112Another basic polymer analogous to that used in<patcit id="pcit0054" dnum="WO0138402A1"><text>WO 01/38402 A1</text></patcit> described continuous kneading process in a reactor of the type List Contikneter with a volume of 6.3m³ (LIST AG, Arisdorf, CH). For this purpose, acrylic acid was continuously neutralized with sodium hydroxide solution and diluted with water so that the degree of neutralization of the acrylic acid was 72 mol-% and the solids content (= sodium acrylate and acrylic acid) of this solution was approximately 38.8 wt.%. Gly-3EO-TA was used as a crosslinker in an amount of 0.484 wt.% based on acrylic acid monomer. The crosslinker was continuously mixed into the monomer stream. The initiation was also carried out by continuous addition of aqueous solutions of the initiators sodium persulfate (0.14 wt.% based on acrylic acid monomer), hydrogen peroxide (0.001 wt.% based on acrylic acid monomer) and ascorbic acid (0.002 wt.% based on acrylic acid monomer).

0113The resulting polymer gel was dried on a belt dryer, then the dryer cake was broken, ground on a roller mill and finally sieved to a grain size of 150 to 850 µm.

0114The base polymer thus produced had the following properties:<ul id="ul0002" list-style="none" compact="compact"><li>CRC = 33.6 g/g</li><li>Extractable (16 h) = 12.2 wt.%</li></ul>

particle size distribution

0115<tables id="tabl0002" num="0002"><table frame="none"><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="23mm" /><colspec colnum="2" colname="col2" colwidth="25mm" /><tbody valign="middle"><row><entry>>850 µm</entry><entry>0.02 wt.%</entry></row><row><entry>600-850 µm</entry><entry>26.1 wt.%</entry></row><row><entry>300-600 µm</entry><entry>48.3 wt.%</entry></row><row><entry>150-300 µm</entry><entry>24.9 wt.%</entry></row><row><entry><150 µm</entry><entry><0.1 wt.%</entry></row></tbody></tgroup></table></tables>

surface post-crosslinking of the base polymer

Example 3

0116In a Schugi^®-Flexomix type 100 D (Hosokawa-Micron BV, Doetichem, Netherlands) with gravimetric dosing and continuous mass flow-controlled liquid dosing via a liquid nozzle, base polymer from Example 1 was sprayed with a surface post-crosslinking solution. The surface postcrosslinker solution was a mixture of 0.07 wt% N-(2-hydroxyethyl)oxazolidinone, 0.07 wt% 1,3-propanediol, 0.50 wt% aluminum trilactate, 0.70 wt% propylene glycol, 1.00 wt% isopropanol and 2.22 wt% water, each based on the base polymer.

0117The wet base polymer was taken directly from the Schugi^®-Flexomix falling into a NARA paddle dryer^® Type NPD 1.6 W (GMF Gouda, Waddinxveen, Netherlands). The throughput rate of base polymer was 60 kg/h (dry) and the product temperature of the steam-heated dryer at the dryer outlet was approx. 188°C. A cooler was connected downstream of the dryer, which quickly cooled the product to approx. 50°C. The residence time in the dryer was determined by the constant throughput rate of the base polymer and the weir height of 70% and was approx. 60 minutes. The necessary residence time is determined by preliminary tests with the help of which the constant dosing rate is determined that leads to the desired property profile. This is necessary in the continuous process because the bulk density changes continuously during the reaction drying. The properties of the resulting polymer are contained in Table 1.

Example 4

0118In a Schugi^®-Flexomix type 100 D (Hosokawa-Micron BV, Doetichem, Netherlands) with gravimetric dosing and continuous mass flow-controlled liquid dosing via a liquid nozzle, base polymer from example 2 was sprayed with a surface post-crosslinking solution. The surface post-crosslinking solution was a mixture of 0.11 wt.% Denacol^® EX810 (ethylene glycol diglycidyl ether), 0.26 wt.% aluminum sulfate, 1.00 wt.% propylene glycol and 2.00 wt.% water, each based on the base polymer.

0119The wet base polymer was taken directly from the Schugi^®-Flexomix falling into a NARA paddle dryer^® Type NPD 1.6 W (GMF Gouda, Waddinxveen, Netherlands). The throughput rate of base polymer was 60 kg/h (dry) and the product temperature of the steam-heated dryer at the dryer outlet was approx. 180°C. A cooler was connected downstream of the dryer, which quickly cooled the product to approx. 50°C. The residence time in the dryer was determined by the constant throughput rate of the base polymer and the weir height of 70% and was approx. 60 minutes. The necessary residence time is determined by preliminary tests with the help of which the constant dosing rate is determined that leads to the desired property profile. This is necessary in the continuous process because the bulk density changes continuously during the reaction drying. The properties of the resulting polymer are contained in Table 1.<tables id="tabl0003" num="0003"><table frame="all"><title>Tab. 1: Surface post-crosslinking of the base polymer</title><tgroup cols="9"><colspec colnum="1" colname="col1" colwidth="11mm" /><colspec colnum="2" colname="col2" colwidth="14mm" align="center" /><colspec colnum="3" colname="col3" colwidth="21mm" align="center" /><colspec colnum="4" colname="col4" colwidth="21mm" align="center" /><colspec colnum="5" colname="col5" colwidth="21mm" align="center" /><colspec colnum="6" colname="col6" colwidth="21mm" align="center" /><colspec colnum="7" colname="col7" colwidth="19mm" align="center" /><colspec colnum="8" colname="col8" colwidth="16mm" align="center" /><colspec colnum="9" colname="col9" colwidth="15mm" align="center" /><thead valign="middle"><row><entry align="center">example</entry><entry>CRC [g/g]</entry><entry>AUL0.7psi [g/g]</entry><entry>AUL0.3psi [g/g]</entry><entry>AUL0.0psi [g/g]</entry><entry>SFC [10-⁷cm³g/s]</entry><entry>GBP [darcies]</entry><entry>Vortex [s]</entry><entry>FSR [g/gs]</entry></row></thead><tbody valign="middle"><row><entry align="center">3*)</entry><entry>24,2</entry><entry>22,0</entry><entry>26,0</entry><entry>35,4</entry><entry>119</entry><entry>17</entry><entry>86</entry><entry>0,17</entry></row><row><entry align="center">4*)</entry><entry>29,7</entry><entry>21,8</entry><entry>28,3</entry><entry>42,8</entry><entry>45</entry><entry>22</entry><entry>95</entry><entry>0,20</entry></row><row><entry namest="col1" nameend="col9" align="justify">*) Comparison example</entry></row></tbody></tgroup></table></tables>

post-treatment after surface cross-linking

Example 5

0120In a plowshare^®1.2 kg of dry polymer from Example 3 were placed in a paddle dryer type M5RMK with a volume of 5 l (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany). A solution of 2 wt.% water and 0.50 wt.% aluminum sulfate, each based on the polymer used, was then sprayed on with stirring (60 rpm) within approx. 120 seconds using a nitrogen-operated two-fluid nozzle and mixed for a total of 15 minutes. Finally, the mixture was sieved through an 850 µm sieve to remove lumps. The properties of the resulting polymer are shown in Table 2.

Example 6

0121In a plowshare^®1.2 kg of dry polymer from Example 4 were placed in a paddle dryer type M5RMK with a volume of 5 l (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany). A solution of 2 wt.% water and 0.50 wt.% aluminum sulfate, each based on the polymer used, was then sprayed on with stirring (60 rpm) within approx. 120 seconds using a nitrogen-operated two-fluid nozzle and mixed for a total of 15 minutes. Finally, the mixture was sieved through an 850 µm sieve to remove lumps. The properties of the resulting polymer are shown in Table 2.

Example 7

0122In a plowshare^®1.2 kg of dry polymer from Example 3 was placed in a paddle dryer type M5RMK with a volume of 5 l (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany). A solution of 2 wt.% water and 0.50 wt.% aluminum trilactate, each based on the polymer used, was then sprayed on with stirring (60 rpm) within approx. 120 seconds using a nitrogen-operated two-fluid nozzle and mixed for a total of 15 minutes. Finally, the mixture was sieved through an 850 µm sieve to remove lumps. The properties of the resulting polymer are shown in Table 2.

Example 8

0123100 g of the surface-crosslinked polymer particles from Example 3 were stored in a climate-controlled cabinet for 90 minutes at 90°C and a relative humidity of 75%. The water absorption during storage was approximately 6 to 8% by weight. The sample was then filled into a 500 ml plastic bottle and homogenized for 10 minutes using a Turbula mixer. The sample was filled into a round-bottomed flask with baffles and dried in a rotary evaporator for 15 minutes at 80°C under vacuum (27 to 30 mbar). It was then sieved to a particle size of less than 850 µm. The dried polymer particles were analyzed. The results are summarized in Table 2.

Example 9

0124The procedure was as in Example 8. Instead of polymer from Example 3, polymer from Example 5 was used. The results are summarized in Table 2.

Example 10

0125The procedure was as in Example 8. Instead of polymer from Example 3, polymer from Example 6 was used. The results are summarized in Table 2.

Example 11

0126The procedure was as in Example 8. Instead of polymer from Example 3, polymer from Example 7 was used. The results are summarized in Table 2.

Example 12

0127In a plowshare^®1.2 kg of dry polymer from Example 8 were placed in a paddle dryer type M5RMK with a volume of 5 l (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany). A solution of 2 wt.% water and 0.50 wt.% aluminum sulfate, each based on the polymer used, was then sprayed on with stirring (60 rpm) within approx. 120 seconds using a nitrogen-operated two-fluid nozzle and mixed for a total of 15 minutes. Finally, the mixture was sieved through an 850 µm sieve to remove lumps. The properties of the resulting polymer are shown in Table 2.<tables id="tabl0004" num="0004"><table frame="all"><title>Tab.2: Post-treatment after surface post-crosslinking</title><tgroup cols="10"><colspec colnum="1" colname="col1" colwidth="10mm" /><colspec colnum="2" colname="col2" colwidth="11mm" align="center" /><colspec colnum="3" colname="col3" colwidth="18mm" align="center" /><colspec colnum="4" colname="col4" colwidth="18mm" align="center" /><colspec colnum="5" colname="col5" colwidth="18mm" align="center" /><colspec colnum="6" colname="col6" colwidth="21mm" align="center" /><colspec colnum="7" colname="col7" colwidth="16mm" align="center" /><colspec colnum="8" colname="col8" colwidth="13mm" align="center" /><colspec colnum="9" colname="col9" colwidth="11mm" align="center" /><colspec colnum="10" colname="col10" colwidth="24mm" align="center" /><thead valign="middle"><row><entry align="center">example</entry><entry>CRC [g/g]</entry><entry>AUL0.7psi [g/g]</entry><entry>AUL0.3psi [g/g]</entry><entry>AUL0.0psi [g/g]</entry><entry>SFC [10^-7cm³g/s]</entry><entry>GBP [darcies]</entry><entry>Vortex [s]</entry><entry>FSR [g/gs]</entry><entry>moisture content [wt.%]</entry></row></thead><tbody valign="middle"><row><entry align="center">5*)</entry><entry>24,1</entry><entry>21,2</entry><entry>26,0</entry><entry>39,7</entry><entry>145</entry><entry>54</entry><entry>84</entry><entry>0,18</entry><entry>3,0</entry></row><row><entry align="center">6*)</entry><entry>28,4</entry><entry>20,4</entry><entry>26,8</entry><entry>45,1</entry><entry>54</entry><entry>88</entry><entry>80</entry><entry>0,22</entry><entry>4,0</entry></row><row><entry align="center">7*)</entry><entry>25,7</entry><entry>22,5</entry><entry>26,5</entry><entry>37,0</entry><entry>166</entry><entry>19</entry><entry>106</entry><entry>0,16</entry><entry>3,1</entry></row><row><entry align="center">8*)</entry><entry>25,3</entry><entry>22,9</entry><entry>26,6</entry><entry>36,1</entry><entry>130</entry><entry>16</entry><entry>90</entry><entry>0,19</entry><entry>5,2</entry></row><row><entry align="center">9</entry><entry>24,9</entry><entry>21,5</entry><entry>26,4</entry><entry>39,6</entry><entry>187</entry><entry>53</entry><entry>58</entry><entry>0,22</entry><entry>1,6</entry></row><row><entry align="center">10*)</entry><entry>28,3</entry><entry>19,6</entry><entry>26,0</entry><entry>43,0</entry><entry>46</entry><entry>97</entry><entry>57</entry><entry>0,28</entry><entry>7,3</entry></row><row><entry align="center">11*)</entry><entry>25,3</entry><entry>21,0</entry><entry>25,6</entry><entry>35,9</entry><entry>182</entry><entry>21</entry><entry>74</entry><entry>0,19</entry><entry>6,3</entry></row><row><entry align="center">12</entry><entry>25,3</entry><entry>21,6</entry><entry>26,1</entry><entry>39,6</entry><entry>186</entry><entry>81</entry><entry>60</entry><entry>0,19</entry><entry>6,2</entry></row><row><entry namest="col1" nameend="col10" align="justify">5 *) Comparison example</entry></row></tbody></tgroup></table></tables><tables id="tabl0005" num="0005"><table frame="all"><title>Tab. 3: Overview of the process conditions</title><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="28mm" /><colspec colnum="2" colname="col2" colwidth="37mm" align="center" /><colspec colnum="3" colname="col3" colwidth="37mm" align="center" /><colspec colnum="4" colname="col4" colwidth="27mm" align="center" /><thead valign="middle"><row><entry align="center">examples</entry><entry>Step A</entry><entry>Step B</entry><entry>Step C</entry></row></thead><tbody valign="middle"><row><entry align="center">1*)</entry><entry>NO</entry><entry>NO</entry><entry>NO</entry></row><row><entry align="center">2*)</entry><entry>NO</entry><entry>NO</entry><entry>NO</entry></row><row><entry align="center">3*)</entry><entry>JA: Al-Lactate</entry><entry>NO</entry><entry>NO</entry></row><row><entry align="center">4*)</entry><entry>NO: Al-sulfate</entry><entry>NO</entry><entry>NO</entry></row><row><entry align="center">5*)</entry><entry>JA: Al-Lactate</entry><entry>JA: Al-Sulfate</entry><entry>NO</entry></row><row><entry align="center">6*)</entry><entry>NO: Al-sulfate</entry><entry>JA: Al-Sulfate</entry><entry>NO</entry></row><row><entry align="center">7*)</entry><entry>JA: Al-Lactate</entry><entry>NO: Al-Lactate</entry><entry>NO</entry></row><row><entry align="center">8*)</entry><entry>JA: Al-Lactate</entry><entry>NO</entry><entry>YES</entry></row><row><entry align="center">9</entry><entry>JA: Al-Lactate</entry><entry>JA: Al-Sulfate</entry><entry>YES</entry></row><row><entry align="center">10*)</entry><entry>NO: Al-sulfate</entry><entry>JA: Al-Sulfate</entry><entry>YES</entry></row><row><entry align="center">11*)</entry><entry>JA: Al-Lactate</entry><entry>NO: Al-Lactate</entry><entry>YES</entry></row><row><entry align="center">12</entry><entry>JA: Al-Lactate</entry><entry>JA: Al-Sulfate</entry><entry>YES</entry></row><row rowsep="0"><entry namest="col1" nameend="col4" align="justify">Step A: Surface post-crosslinking in the presence of a complexing acid anion</entry></row><row rowsep="0"><entry namest="col1" nameend="col4" align="justify">Step B: Post-treatment with a non-complexing acid anion</entry></row><row rowsep="0"><entry namest="col1" nameend="col4" align="justify">Step C: Increasing the moisture content followed by drying</entry></row><row rowsep="0"><entry namest="col1" nameend="col4" align="justify">Al-sulfate: aluminum sulfate</entry></row><row rowsep="0"><entry namest="col1" nameend="col4" align="justify">Al-Lactate: Aluminum Trilactate</entry></row><row><entry namest="col1" nameend="col4" align="justify">*) Comparison example</entry></row></tbody></tgroup></table></tables>

0128The results show that only when all process steps essential to the invention are fulfilled, ie, surface post-crosslinking in the presence of a complexing acid anion, post-treatment with a non-complexing acid anion and increasing the moisture content with subsequent drying, water-absorbing polymer particles with high liquid transfer (SFC), high gel bed permeability (GBP) and low vortex are obtained.