Method for producing water-absorbing polymer particles with high swelling rate and high centrifuge retention capacity with simultaneously high permeability of the swollen gel bed

Abstract

A process for producing water-absorbing polymer particles having high free swell rate and high centrifuge retention capacity with simultaneously high permeability of the swollen gel bed by polymerization of an aqueous monomer solution in a polymerization reactor having at least two shafts (kneaders) which rotate in an axially parallel manner, subsequent extrusion at high temperatures and thermal surface postcrosslinking.

Claims

1. A process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising a) an 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 an ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a), and e) optionally one or more water-soluble polymer, in a polymerization reactor having at least two shafts rotating in an axially parallel manner, drying the resulting polymer gel, grinding the dried polymer gel, classifying and thermally surface postcrosslinking, which process comprises using at least 0.25% by weight of the crosslinker b), based on the unneutralized monomer a), extruding the polymer gel through a perforated plate prior to drying, the polymer gel during the extrusion having a temperature greater than 80° C. and less than 60 kWh/t of specific mechanical energy being introduced in the course of extrusion, wherein the extruder has a ratio of length to diameter of less than 5.

2. The process according to claim 1, wherein the polymer gel during the extrusion has a temperature of greater than 90° C.

3. The process according to claim 1, wherein an orifice ratio of the perforated plate of the extruder is in the range from 10 to 20%.

4. The process according to claim 1, wherein a pressure bearing on the perforated plate of the extruder is in the range from 15 to 35 bar.

5. The process according to claim 1, wherein at least 0.4% by weight of the crosslinker b), based on the unneutralized monomer a), is used.

6. The process according to claim 1, wherein extrusion is effected through holes having a diameter of 8 to 12 mm.

7. The process according to claim 1, wherein the extruder is trace-heated.

8. The process according to claim 1, wherein at least 50 mol % of monomer a) is partly neutralized acrylic acid.

9. The process according to claim 1, wherein monomer a) has been neutralized to an extent of 25 to 85 mol %.

10. A process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising a) an 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 an ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a), and e) optionally one or more water-soluble polymer, in a polymerization reactor having at least two shafts rotating in an axially parallel manner, drying the resulting polymer gel, grinding the dried polymer gel, classifying and thermally surface postcrosslinking, which process comprises using at least 0.25% by weight of the crosslinker b), based on the unneutralized monomer a), extruding the polymer gel through a perforated plate prior to drying, the polymer gel during the extrusion having a temperature greater than 80° C. and less than 60 kWh/t of specific mechanical energy being introduced in the course of extrusion, wherein an orifice ratio of the perforated plate of the extruder is in the range from 10 to 20%.

11. The process according to claim 10, wherein the polymer gel during the extrusion has a temperature of greater than 90° C.

12. The process according to claim 10, wherein a pressure bearing on the perforated plate of the extruder is in the range from 15 to 35 bar.

13. The process according to claim 10, wherein extrusion is effected through holes having a diameter of 8 to 12 mm.

14. The process according to claim 10, wherein the extruder is trace-heated.

15. The process according to claim 10, wherein at least 50 mol % of monomer a) is partly neutralized acrylic acid.

16. A process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising a) an 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 an ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a), and e) optionally one or more water-soluble polymer, in a polymerization reactor having at least two shafts rotating in an axially parallel manner, drying the resulting polymer gel, grinding the dried polymer gel, classifying and thermally surface postcrosslinking, which process comprises using at least 0.25% by weight of the crosslinker b), based on the unneutralized monomer a), extruding the polymer gel through a perforated plate prior to drying, the polymer gel during the extrusion having a temperature greater than 80° C. and less than 60 kWh/t of specific mechanical energy being introduced in the course of extrusion, wherein a pressure bearing on the perforated plate of the extruder is in the range from 15 to 35 bar.

17. The process according to claim 16, wherein the polymer gel during the extrusion has a temperature of greater than 90° C.

18. The process according to claim 16, wherein extrusion is effected through holes having a diameter of 8 to 12 mm.

19. The process according to claim 16, wherein the extruder is trace-heated.

20. A process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising a) an 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 an ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a), and e) optionally one or more water-soluble polymer, in a polymerization reactor having at least two shafts rotating in an axially parallel manner, drying the resulting polymer gel, grinding the dried polymer gel, classifying and thermally surface postcrosslinking, which process comprises using at least 0.25% by weight of the crosslinker b), based on the unneutralized monomer a), extruding the polymer gel through a perforated plate prior to drying, the polymer gel during the extrusion having a temperature greater than 80° C. and less than 60 kWh/t of specific mechanical energy being introduced in the course of extrusion, wherein extrusion is effected through holes having a diameter of 8 to 12 mm.

Description

EXAMPLES

Example 1

(1) By continuously mixing water, 50% by weight sodium hydroxide solution and acrylic acid, a 42.7% by weight acrylic acid/sodium acrylate solution was prepared such that the degree of neutralization was 69.0 mol %. After the components had been mixed, the monomer solution was cooled continuously to a temperature of 30° C. by means of a heat exchanger and degassed with nitrogen. The polyethylenically unsaturated crosslinker used was 3-tuply ethoxylated glyceryl triacrylate (purity approx. 85% by weight). The amount used, based on the acrylic acid (boaa) used, was 0.35% by weight. To initiate the free-radical polymerization, the following components were used: 0.0008% by weight boaa of hydrogen peroxide, metered in as a 2.5% by weight aqueous solution, 0.13% by weight boaa of sodium peroxodisulfate, metered in as a 15% by weight aqueous solution, and 0.0023% by weight boaa of ascorbic acid, metered in as a 0.5% by weight aqueous solution. The throughput of the monomer solution was 800 kg/h.

(2) The individual components were metered continuously into a List ORP 250 Contikneter continuous kneader reactor (List AG, Arisdorf, Switzerland). In the first third of the reactor, 26.3 kg/h of removed undersize with a particle size of less than 150 μm were additionally added. The reaction solution had a feed temperature of 30° C. The residence time of the reaction mixture in the reactor was approx. 15 minutes.

(3) Some of the polymer gel thus obtained was extruded with an SLRE 75 R extruder (Sela Maschinen GmbH; Harbke; Germany). The temperature of the polymer gel in the course of extrusion was 95° C. The perforated plate had 12 holes having a hole diameter of 8 mm. The thickness of the perforated plate was 16 mm. The ratio of internal length to internal diameter of the extruder (L/D) was 4. The specific mechanical energy (SME) of the extrusion was 26 kWh/t. The extruded polymer gel was distributed on metal sheets and dried at 175° C. in an air circulation drying cabinet for 90 minutes. The loading of the metal sheets with polymer gel was 0.81 g/cm.sup.2.

(4) The dried polymer gel was ground by means of a one-stage roll mill (three milling runs, 1st milling run with gap width 1000 μm, 2nd milling run with gap width 600 μm and 3rd milling run with gap width 400 μm). The ground dried polymer gel was classified and a synthetic particle size distribution (PSD) with the following composition was mixed:

(5) 600 to 710 μm: 10.6% by weight

(6) 500 to 600 μm: 27.9% by weight

(7) 300 to 500 μm: 42.7% by weight

(8) 200 to 300 μm: 13.8% by weight

(9) 150 to 200 μm: 5.0% by weight

(10) The base polymer A thus obtained was analyzed. The results are entered in table 1.

Example 2

(11) 1.2 kg of base polymer A from example 1 were coated in a Pflugschar M5 plowshare mixer with heating jacket (Gebr. Lödige Maschinenbau GmbH; Paderborn, Germany) at 23° C. and a shaft speed of 200 revolutions per minute by means of a two-substance spray nozzle with 54.6 g of a mixture of 0.07% by weight of N-hydroxyethyl-2-oxazolidinone, 0.07% by weight of 1,3-propanediol, 0.7% by weight of propylene glycol, 2.27% by weight of a 22% by weight aqueous aluminum lactate solution, 0.448% by weight of a 0.9% by weight aqueous sorbitan monolaurate solution and 0.992% by weight of isopropanol, the percentages by weight each being based on base polymer A.

(12) After the spray application, the product temperature was increased to 185° C. and the reaction mixture was held at this temperature and a shaft speed of 50 revolutions per minute for 35 minutes. The resulting product was cooled to ambient temperature and classified again with a 710 μm sieve. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 2.

Example 3 (Comparative Example)

(13) The procedure was as in example 1, except that the resulting polymer gel was not extruded. The base polymer B thus obtained was analyzed. The results are entered in table 1.

Example 4 (Comparative Example)

(14) The base polymer B from example 3 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 2.

Example 5

(15) The procedure was as in example 1, except that the temperature of the polymer gel in the course of extrusion was 85°. The base polymer C thus obtained was analyzed. The results are entered in table 1.

Example 6

(16) The base polymer C from example 5 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 2.

Example 7 (Comparative Example)

(17) The procedure was as in example 1, except that the temperature of the polymer gel in the course of extrusion was 62°. The base polymer D thus obtained was analyzed. The results are entered in table 1.

Example 8 (Comparative Example)

(18) The base polymer D from example 7 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 2.

(19) TABLE-US-00001 TABLE 1 Influence of polymer gel temperature in the course of extrusion on the base polymer Polymer gel temp. CRC AUL0.3 psi FSR Ex. [° C.] [g/g] [g/g] [g/gs] 3*) — 37.7 11.0 0.31 1 95 34.4 16.4 0.38 5 85 34.2 17.0 0.37 7*) 62 37.2 10.3 0.25 *)comparative examples

(20) TABLE-US-00002 TABLE 2 Influence of polymer gel temperature in the course of extrusion on the end product Polymer gel temp. SFC CRC AUL0.7 psi FSR Ex. [° C.] [10−7 × cm.sup.3s/g] [g/g] [g/g] [g/gs] 4*) — 76 27.7 25.1 0.17 2 95 103 27.2 25.1 0.34 6 85 98 26.7 24.9 0.29 8*) 62 95 27.0 23.8 0.22 *)comparative examples

(21) Examples 1 to 8 show that the free swell rate (FSR) after surface postcrosslinking rises with the polymer gel temperature during extrusion.

Example 9 (Comparative Example)

(22) By continuously mixing water, 50% by weight sodium hydroxide solution and acrylic acid, a 42.7% by weight acrylic acid/sodium acrylate solution was prepared such that the degree of neutralization was 69.0 mol %. After the components had been mixed, the monomer solution was cooled continuously to a temperature of 30° C. by means of a heat exchanger and degassed with nitrogen. The polyethylenically unsaturated crosslinker used was 3-tuply ethoxylated glyceryl triacrylate (purity approx. 85% by weight). The amount used, based on the acrylic acid used, was 0.20% by weight. To initiate the free-radical polymerization, the following components were used: 0.002% by weight boaa of hydrogen peroxide, metered in as a 2.5% by weight aqueous solution, 0.1% by weight boaa of sodium peroxodisulfate, metered in as a 15% by weight aqueous solution, and 0.01% by weight boaa of ascorbic acid, metered in as a 0.5% by weight aqueous solution. The throughput of the monomer solution was 40 kg/h.

(23) The individual components were metered continuously into a List ORP 10 Contikneter continuous kneader reactor (List AG, Arisdorf, Switzerland).

(24) The reaction solution had a feed temperature of 30° C. The residence time of the reaction mixture in the reactor was approx. 15 minutes.

(25) Some of the polymer gel thus obtained was extruded with an SLRE 75 R extruder (Sela Maschinen GmbH; Harbke; Germany). The temperature of the polymer gel in the course of extrusion was 85° C. The perforated plate had 12 holes having a hole diameter of 8 mm. The thickness of the perforated plate was 16 mm. The ratio of internal length to internal diameter of the extruder (L/D) was 4. The specific mechanical energy (SME) of the extrusion was 26 kWh/t. The extruded polymer gel was distributed on metal sheets and a and dried at 175° C. in an air circulation drying cabinet for 90 minutes. The loading of the metal sheets with polymer gel was 0.81 g/cm.sup.2.

(26) The dried polymer gel was ground by means of a one-stage roll mill (three milling runs, 1st milling run with gap width 1000 μm, 2nd milling run with gap width 600 μm and 3rd milling run with gap width 400 μm). The ground dried polymer gel was classified and a synthetic particle size distribution (PSD) with the following composition was mixed:

(27) 600 to 710 μm: 10.6% by weight

(28) 500 to 600 μm: 27.9% by weight

(29) 300 to 500 μm: 42.7% by weight

(30) 200 to 300 μm: 13.8% by weight

(31) 150 to 200 μm: 5.0% by weight

(32) The base polymer E thus obtained was analyzed. The results are entered in table 3.

Example 10 (Comparative Example)

(33) The base polymer E from example 9 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 4.

Example 11

(34) The procedure was as in example 9, except that the amount of the crosslinker used, based on the acrylic acid used, was 0.28% by weight. The base polymer F thus obtained was analyzed. The results are entered in table 3.

Example 12

(35) The base polymer F from example 11 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 4.

Example 13

(36) The procedure was as in example 9, except that the amount of the crosslinker used, based on the acrylic acid used, was 0.35% by weight. The base polymer G thus obtained was analyzed. The results are entered in table 3.

Example 14

(37) The base polymer G from example 13 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 4.

Example 15

(38) The procedure was as in example 1, except that the amount of the crosslinker used, based on the acrylic acid used, was 0.43% by weight. The temperature of the polymer gel in the course of extrusion was 85°. The base polymer H thus obtained was analyzed. The results are entered in table 3.

Example 16

(39) The base polymer H from example 15 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 4.

(40) TABLE-US-00003 TABLE 3 Influence of crosslinker on the base polymer Crosslinker CRC AUL0.3 psi FSR Ex. [% by wt.] [g/g] [g/g] [g/gs]  9*) 0.20 37.3 8.8 0.31 11 0.28 34.3 11.8 0.35 13 0.35 33.5 15.1 0.35 15 0.43 32.5 22.9 0.40 *)comparative examples

(41) TABLE-US-00004 TABLE 4 Influence of crosslinker on the end product Crosslinker SFC CRC AUL0.7 psi FSR Ex. [% by wt.] [10.sup.−7 × cm.sup.3s/g] [g/g] [g/g] [g/gs] 10*) 0.20 106 25.8 23.2 0.24 12 0.28 121 26.4 25.0 0.26 14 0.35 135 26.3 23.9 0.29 16 0.43 122 26.4 24.6 0.32 *)comparative examples

(42) Examples 9 to 16 show that the free swell rate (FSR) after surface postcrosslinking rises with the amount of crosslinker used in the extruded base polymers.

Example 17 (Comparative Example)

(43) The procedure was as in example 9, except that the resulting polymer gel was not extruded. The base polymer I thus obtained was analyzed. The results are entered in table 5.

Example 18 (Comparative Example)

(44) The base polymer I from example 17 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 6.

Example 19 (Comparative Example)

(45) The procedure was as in example 11, except that the resulting polymer gel was not extruded. The base polymer J thus obtained was analyzed. The results are entered in table 5.

Example 20 (Comparative Example)

(46) The base polymer J from example 19 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 6.

Example 21 (Comparative Example)

(47) The procedure was as in example 13, except that the resulting polymer gel was not extruded. The base polymer K thus obtained was analyzed. The results are entered in table 5.

Example 22 (Comparative Example)

(48) The base polymer K from example 21 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 6.

Example 23 (Comparative Example)

(49) The procedure was as in example 15, except that the resulting polymer gel was not extruded. The base polymer L thus obtained was analyzed. The results are entered in table 5.

Example 24 (Comparative Example)

(50) The base polymer L from example 23 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 6.

(51) TABLE-US-00005 TABLE 5 Influence of crosslinker on the base polymer Crosslinker CRC AUL0.3 psi FSR Ex. [% by wt.] [g/g] [g/g] [g/gs] 17*) 0.20 38.0 8.9 0.33 19*) 0.28 34.9 11.7 0.32 21*) 0.35 34.2 13.8 0.31 23*) 0.43 34.2 17.3 0.31 *)comparative examples

(52) TABLE-US-00006 TABLE 6 Influence of crosslinker on the end product Crosslinker SFC CRC AUL0.7 psi FSR Ex. [% by wt.] [10.sup.−7 × cm.sup.3s/g] [g/g] [g/g] [g/gs] 18*) 0.20 152 26.4 23.6 0.18 20*) 0.28 160 26.1 23.9 0.18 22*) 0.35 138 25.9 23.7 0.20 24*) 0.43 107 26.4 23.7 0.18 *)comparative examples

(53) Examples 17 to 24 show that the amount of crosslinker used in the base polymers, in the absence of extrusion, does not have any significant influence on the free swell rate (FSR) after surface postcrosslinking.

Example 25 (Comparative Example)

(54) The procedure was as in example 15, except that some of the polymer gel thus obtained was extruded with an OEE 8 extruder (AMANDUS KAHL GmbH & Co. KG; Hamburg; Germany). The temperature of the polymer gel in the course of extrusion was 85°. The perforated plate had 8 holes having a hole diameter of 8 mm. The thickness of the perforated plate was 15 mm. The ratio of internal length to internal diameter of the extruder (L/D) was 6.3. The specific mechanical energy (SME) of the extrusion was 89 kWh/t.

(55) The base polymer M thus obtained was analyzed. The results are entered in table 7.

Example 26 (Comparative Example)

(56) The base polymer M from example 25 was thermally surface postcrosslinked as in example 2. The fraction with a particle size of less than 710 μm was analyzed. The results are entered in table 8.

(57) TABLE-US-00007 TABLE 7 Influence of SME on the base polymer SME CRC AUL0.3 psi FSR Ex. [kWh/t] [g/g] [g/g] [g/gs] 15 26 32.5 22.9 0.40 25*) 89 30.4 18.9 0.33 *)comparative example

(58) TABLE-US-00008 TABLE 8 Influence of SME on the end product SME SFC CRC AUL0.7 psi FSR Ex. [kWh/t] [10.sup.−7 × cm.sup.3s/g] [g/g] [g/g] [g/gs] 16 26 122 26.4 24.6 0.32 26*) 89 133 25.9 24.7 0.26 *)comparative example

(59) Examples 16 and 26 show that extrusion with excessively high specific mechanical energy (SME) lowers the free swell rate (FSR) after surface postcrosslinking.