Method for producing water-absorbent polymer particles with improved properties

Abstract

The present invention relates to a process for producing water-absorbing polymer particles having an improved profile of properties, comprising thermal surface postcrosslinking in the presence of a salt of a polyvalent metal cation and a complexing anion and subsequent aftertreatment, the aftertreatment comprising coating with a salt of a polyvalent metal cation and a non-complexing anion, and remoisturization with further drying.

Claims

1. A process for producing water-absorbing polymer particles comprising polymerizing a monomer solution or suspension comprising a) at least one ethylenically unsaturated monomer which bears an acid group and may be at least partly neutralized, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a) and e) optionally one or more water-soluble polymer, drying, grinding, and classifying a resulting polymer gel, and thermally surface postcrosslinking the classified polymer particles with f) at least one covalent surface postcrosslinker, and g) at least one salt of a polyvalent metal cation and a complexing acid anion selected from the group consisting of glycolate, glycinate, lactate, alanate, citrate, tartrate, tartronate, and glycerate, which comprises subsequently aftertreating the surface postcrosslinked polymer particles, the aftertreatment comprising i) coating with at least one salt of a polyvalent metal cation and a non-complexing acid anion selected from the group consisting of formate, acetate, propionate, methylsulfonate, sulfate, and chloride, ii) increasing the moisture content by 1 to 150% by weight, and iii) drying after the increase in the moisture content.

2. The process according to claim 1, wherein step ii) is conducted before step i).

3. The process according to claim 1, wherein the classified polymer particles are coated with 0.02 to 0.8% by weight of the polyvalent metal cation.

4. The process according to claim 1, wherein the surface postcrosslinked polymer particles are coated with 0.02 to 0.8% by weight of the polyvalent metal cation.

5. The process according to claim 1, wherein the polyvalent metal cation is selected from the group of Al.sup.3+, Ti.sup.4+, and Zr.sup.4+.

6. The process according to claim 1, wherein the surface postcrosslinked polymer particles, after increasing the moisture content, are dried at a temperature of less than 150° C.

7. The process according to claim 1, wherein the surface postcrosslinked polymer particles, after increasing the moisture content, are dried down to a moisture content of less than 10% by weight.

8. The process according to claim 1, wherein the water-absorbing polymer particles have a centrifuge retention capacity of at least 15 g/g.

Description

EXAMPLES

Production of the Base Polymer

Example 1

(1) A base polymer was prepared analogously to the continuous kneader process described in WO 01/38402 A1, in a List Contikneter reactor having a capacity of 6.3 m.sup.3 (LIST AG, Arisdorf, Switzerland). For this purpose, acrylic acid was neutralized continuously with sodium hydroxide solution and diluted with water, such 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 approx. 40.0% by weight. The crosslinker used was acrylated glyceryl triacrylate with triple ethoxylation overall (Gly-3 EO-TA), which had been prepared according to US 2005/0176910, in an amount of 0.348% by weight based on acrylic acid monomer. The crosslinker was added continuously to the monomer stream. For the calculation of the acrylic acid monomer content, the sodium acrylate present was considered theoretically as acrylic acid. The initiation was effected by likewise continuous addition of aqueous solutions of the initiators sodium persulfate (0.195% by weight based on acrylic acid monomer), hydrogen peroxide (0.002% by weight based on acrylic acid monomer) and ascorbic acid (0.0031% by weight based on acrylic acid monomer).

(2) The polymer gel obtained was dried on a belt drier, then the drier cake was crushed, ground by means of a roll mill and finally screened off to a particle size of 150 to 850 μm.

(3) The base polymer thus prepared had the following properties:

(4) CRC=36.0 g/g

(5) Extractables (16 h)=14.0% by weight

(6) Particle Size Distribution

(7) TABLE-US-00001 >850 μm <0.1% by weight 600-850 μm 3.61% by weight 300-600 μm 77.55% by weight 150-300 μm 18.8% by weight <150 μm <0.1% by weight

Example 2

(8) A further base polymer was prepared analogously to the continuous kneader process described in WO 01/38402 A1, in a List Contikneter reactor having a capacity of 6.3 m.sup.3 (LIST AG, Arisdorf, Switzerland). For this purpose, acrylic acid was neutralized continuously with sodium hydroxide solution and diluted with water, such 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 approx. 38.8% by weight. The crosslinker used was Gly-3EO-TA in an amount of 0.484% by weight based on acrylic acid monomer. The crosslinker was added continuously to the monomer stream. The initiation was effected by likewise continuous addition of aqueous solutions of the initiators sodium persulfate (0.14% by weight based on acrylic acid monomer), hydrogen peroxide (0.001% by weight based on acrylic acid monomer) and ascorbic acid (0.002% by weight based on acrylic acid monomer).

(9) The polymer gel obtained was dried on a belt drier, then the drier cake was crushed, ground on a roll mill and finally screened off to a particle size of 150 to 850 μm.

(10) The base polymer thus prepared had the following properties:

(11) CRC=33.6 g/g

(12) Extractables (16 h)=12.2% by weight

(13) Particle Size Distribution

(14) TABLE-US-00002 >850 μm 0.02% by weight 600-850 μm 26.1% by weight 300-600 μm 48.3% by weight 150-300 μm 24.9% by weight <150 μm <0.1% by weight

Surface Postcrosslinking of the Base Polymer

Example 3

(15) In a Schugi® Flexomix 100 D (Hosokawa-Micron B.V., Doetinchem, the Netherlands) with gravimetric metering and continuous mass flow-controlled liquid metering via a liquid nozzle, base polymer from example 1 was sprayed with a surface postcrosslinking solution. The surface postcrosslinker solution was a mixture of 0.07% by weight of N-(2-hydroxyethyl)oxazolidinone, 0.07% by weight of 1,3-propanediol, 0.50% by weight of aluminum trilactate, 0.70% by weight of propylene glycol, 1.00% by weight of isopropanol and 2.22% by weight of water, based in each case on the base polymer.

(16) The moist base polymer was transferred directly from the Schugi® Flexomix, falling into a NARA Paddle-Dryer® NPD 1.6 W (GMF Gouda, Waddinxveen, the Netherlands). The throughput rate of base polymer was 60 kg/h (dry), and the product temperature of the steam-heated drier at the drier outlet was approx. 188° C. The drier was connected upstream of a cooler which rapidly cooled the product to approx. 50° C. The residence time in the drier was defined via the constant throughput rate of the base polymer and the weir height of 70%, and was approx. 60 minutes. The residence time necessary is determined by preliminary tests, which help to determine the constant metering rate which leads to the desired profile of properties. This is necessary in the continuous process since the bulk density changes constantly during the reaction drying. The properties of the resulting polymer are in table 1.

Example 4

(17) In a Schugi® Flexomix 100 D (Hosokawa-Micron B.V., Doetinchem, the Netherlands) with gravimetric metering and continuous mass flow-controlled liquid metering via a liquid nozzle, base polymer from example 2 was sprayed with a surface postcrosslinking solution. The surface postcrosslinker solution was a mixture of 0.11% by weight of Denacol® EX810 (ethylene glycol diglycidyl ether), 0.26% by weight of aluminum sulfate, 1.00% by weight of propylene glycol and 2.00% by weight of water, based in each case on the base polymer.

(18) The moist base polymer was transferred directly from the Schugi® Flexomix, falling into a NARA Paddle-Dryer® NPD 1.6 W (GMF Gouda, Waddinxveen, the Netherlands). The throughput rate of base polymer was 60 kg/h (dry), and the product temperature of the steam-heated drier at the drier outlet was approx. 180° C. The drier was connected upstream of a cooler which rapidly cooled the product to approx. 50° C. The residence time in the drier was defined via the constant throughput rate of the base polymer and the weir height of 70%, and was approx. 60 minutes. The residence time necessary is determined by preliminary tests, which help to determine the constant metering rate which leads to the desired profile of properties. This is necessary in the continuous process since the bulk density changes constantly during the reaction drying. The properties of the resulting polymer are in table 1.

(19) TABLE-US-00003 TABLE 1 Surface postcrosslinking of the base polymer CRC AUL0.7 psi AUL0.3 psi AUL0.0 psi SFC GBP Vortex FSR Ex. [g/g] [g/g] [g/g] [g/g] [10.sup.−7 cm.sup.3g/s] [darcies] [s] [g/gs] 3*) 24.2 22.0 26.0 35.4 119 17 86 0.17 4*) 29.7 21.8 28.3 42.8 45 22 95 0.20 *)comparative example

Aftertreatment after Surface Postcrosslinking

Example 5

(20) A Pflugschar® M5RMK shovel drier of capacity 5 l (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany) was initially charged with 1.2 kg of dry polymer from example 3. Subsequently, while stirring (60 rpm), within approx. 120 seconds, a nitrogen-driven two-phase nozzle was used to spray on a solution of 2% by weight of water and 0.50% by weight of aluminum sulfate, based in each case on the polymer used, with mixing for a total of 15 minutes. Finally, the product was screened through a 850 μm screen in order to remove lumps. The properties of the resulting polymer are in table 2.

Example 6

(21) A Pflugschar® M5RMK shovel drier of capacity 5 l (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany) was initially charged with 1.2 kg of dry polymer from example 4. Subsequently, while stirring (60 rpm), within approx. 120 seconds, a nitrogen-driven two-phase nozzle was used to spray on a solution of 2% by weight of water and 0.50% by weight of aluminum sulfate, based in each case on the polymer used, with mixing for a total of 15 minutes. Finally, the product was screened through a 850 μm screen in order to remove lumps. The properties of the resulting polymer are in table 2.

Example 7

(22) A Pflugschar® M5RMK shovel drier of capacity 5 l (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany) was initially charged with 1.2 kg of dry polymer from example 3. Subsequently, while stirring (60 rpm), within approx. 120 seconds, a nitrogen-driven two-phase nozzle was used to spray on a solution of 2% by weight of water and 0.50% by weight of aluminum trilactate, based in each case on the polymer used, with mixing for a total of 15 minutes. Finally, the product was screened through a 850 μm screen in order to remove lumps. The properties of the resulting polymer are in table 2.

Example 8

(23) 100 g in each case of the surface postcrosslinked polymer particles from example 3 were mixed at 90° C. and a relative air humidity of 75% in a climate-controlled cabinet for 90 minutes. The water absorption during the storage was approx. 6 to 8% by weight. Subsequently, the sample was introduced into a 500 ml plastic bottle and the mixture was homogenized by means of a Turbula mixer for 10 minutes. The sample was introduced into a round-bottom flask with baffles and dried at 80° C. under reduced pressure (27 to 30 mbar) in a rotary evaporator for 15 minutes. This was followed by screening off to a particle size of less than 850 μm. The dried polymer particles were analyzed. The results are summarized in table 2.

Example 9

(24) The 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

(25) The 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

(26) The 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

(27) A Pflugschar® M5RMK shovel drier of capacity 5 l (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany) was initially charged with 1.2 kg of dry polymer from example 8. Subsequently, while stirring (60 rpm), within approx. 120 seconds, a nitrogen-driven two-phase nozzle was used to spray on a solution of 2% by weight of water and 0.50% by weight of aluminum sulfate, based in each case on the polymer used, with mixing for a total of 15 minutes. Finally, the product was screened through a 850 μm screen in order to remove lumps. The properties of the resulting polymer are in table 2.

(28) TABLE-US-00004 TABLE 2 Aftertreatment after surface postcrosslinking CRC AUL0.7 psi AUL0.3 psi AUL0.0 psi SFC GBP Vortex FSR Moisture content Ex. [g/g] [g/g] [g/g] [g/g] [10.sup.−7 cm.sup.3g/s] [darcies] [s] [g/gs] [% by wt.] 5*) 24.1 21.2 26.0 39.7 145 54 84 0.18 3.0 6*) 28.4 20.4 26.8 45.1 54 88 80 0.22 4.0 7*) 25.7 22.5 26.5 37.0 166 19 106 0.16 3.1 8*) 25.3 22.9 26.6 36.1 130 16 90 0.19 5.2 9   24.9 21.5 26.4 39.6 187 53 58 0.22 1.6 10*)  28.3 19.6 26.0 43.0 46 97 57 0.28 7.3 11*)  25.3 21.0 25.6 35.9 182 21 74 0.19 6.3 12   25.3 21.6 26.1 39.6 186 81 60 0.19 6.2 *)comparative example

(29) TABLE-US-00005 TABLE 3 Overview of process conditions Examples Step A Step B Step C 1*) NO NO NO 2*) NO NO NO 3*) YES: Al lactate NO NO 4*) NO: Al sulfate NO NO 5*) YES: Al lactate YES: Al sulfate NO 6*) NO: Al sulfate YES: Al sulfate NO 7*) YES: Al lactate NO: Al lactate NO 8*) YES: Al lactate NO YES 9.sup.  YES: Al lactate YES: Al sulfate YES 10*)  NO: Al sulfate YES: Al sulfate YES 11*)  YES: Al lactate NO: Al lactate YES 12 .sup.  YES: Al lactate YES: Al sulfate YES Step A: surface postcrosslinking in the presence of a complexing acid anion Step B: aftertreatment with a non-complexing acid anion Step C: increasing the moisture content with subsequent drying Al sulfate: aluminum sulfate Al lactate: aluminum trilactate *)comparative example

(30) The results show that only on fulfillment of all process steps essential to the invention, i.e., surface postcrosslinking in the presence of a complexing acid anion, aftertreatment with a non-complexing acid anion and increasing the moisture content with subsequent drying, are water-absorbing polymer particles with high saline flow conductivity (SFC), high gel bed permeability (GBP) and low vortex obtained.