Process for producing superabsorbent polymer particles

Abstract

The invention relates to a process for producing superabsorbent polymer particles, comprising surface postcrosslinking, classifying the surface postcrosslinked superabsorbent polymer particles, deagglomerating the separated oversize fraction using a roll crusher and recycling the disintegrated oversize fraction before or into the classification of the surface postcrosslinked superabsorbent polymer particles.

Claims

1. A process for producing superabsorbent polymer particles, comprising polymerization of 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 a resulting polymer gel, grinding, classifying, and surface postcrosslinking the resulting polymer particles, wherein the surface postcrosslinked superabsorbent polymer particles are classified into an undersize, midsize, and oversize fraction, the superabsorbent polymer particles of the oversize fraction are deagglomerated using a roll crusher, and the deagglomerated superabsorbent polymer particles are recycled before or into the classification of the surface postcrosslinked superabsorbent polymer particles, wherein the roll crusher is a double toothed roll crusher.

2. The process according to claim 1, wherein a surface of the roll crusher has a roughness R.sub.z of less than 25 μm.

3. The process according to claim 1, wherein the roll crusher has a gap width from 1 to 10 mm, wherein the gap width of the roll crusher is the smallest distance between the two surfaces that form the gap of the roll crusher.

4. The process according to claim 1, wherein a sieve for separating the oversize fraction and the midsize fraction has a mesh size from 600 to 1,200 μm.

5. The process according to claim 1, wherein a sieve for separating the undersize fraction and the midsize fraction has a mesh size from 100 to 300 μm.

6. The process according to claim 1, wherein a gap width of the roll crusher is from 50 to 800 μm larger than the mesh size of the sieve for separating the oversize fraction and the midsize fraction, wherein the gap width of the roll crusher is the smallest distance between the two surfaces that form the gap of the roll crusher.

7. The process according to claim 1, wherein the superabsorbent polymer particles of the undersize fraction are recycled into the polymerization.

8. The process according to claim 1, wherein the surface postcrosslinked superabsorbent polymer particles are classified using a tumbling screen machine.

9. The process according to claim 1, wherein the superabsorbent polymer particles have a centrifuge retention capacity of at least 15 g/g.

10. An apparatus for sizing of surface postcrosslinked superabsorbent polymer particles, comprising a tumbling sieve machine for classifying particles into an undersize, midsize, and oversize fraction, a roll crusher for deagglomerating the oversize fraction, and a line for recycling the deagglomerated superabsorbent polymer particles before or into the tumbling sieve machine.

11. The apparatus of claim 10, wherein the roll crusher is a double toothed roll crusher.

12. The apparatus of claim 10, wherein a surface of the roll crusher has a roughness R.sub.z of less than 25 μm.

13. The apparatus of claim 10, wherein the roll crusher has a gap width from 1 to 10 mm, wherein the gap width of the roll crusher is the smallest distance between the two surfaces that form the gap of the roll crusher.

14. The apparatus of claim 10, wherein a sieve for separating the oversize fraction and the midsize fraction has a mesh size from 600 to 1,200 μm.

15. The apparatus of claim 10, wherein a sieve for separating the undersize fraction and the midsize fraction has a mesh size from 100 to 300 μm.

16. The apparatus of claim 10, wherein a gap width of the roll crusher is from 50 to 800 μm larger than the mesh size of the sieve for separating the oversize fraction and the midsize fraction, wherein the gap width of the roll crusher is the smallest distance between the two surfaces that form the gap of the roll crusher.

17. A process for producing superabsorbent polymer particles, comprising polymerization of 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 a resulting polymer gel, grinding, classifying, and surface postcrosslinking the resulting polymer particles, wherein the surface postcrosslinked superabsorbent polymer particles are classified into an undersize, midsize, and oversize fraction, the superabsorbent polymer particles of the oversize fraction are deagglomerated using a roll crusher, and the deagglomerated superabsorbent polymer particles are recycled before or into the classification of the surface postcrosslinked superabsorbent polymer particles, wherein a surface of the roll crusher has a roughness R.sub.z of less than 25 μm.

18. A process for producing superabsorbent polymer particles, comprising polymerization of 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 a resulting polymer gel, grinding, classifying, and surface postcrosslinking the resulting polymer particles, wherein the surface postcrosslinked superabsorbent polymer particles are classified into an undersize, midsize, and oversize fraction, the superabsorbent polymer particles of the oversize fraction are deagglomerated using a roll crusher, and the deagglomerated superabsorbent polymer particles are recycled before or into the classification of the surface postcrosslinked superabsorbent polymer particles, wherein a sieve for separating the oversize fraction and the midsize fraction has a mesh size from 600 to 1,200 μm.

19. A process for producing superabsorbent polymer particles, comprising polymerization of 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 a resulting polymer gel, grinding, classifying, and surface postcrosslinking the resulting polymer particles, wherein the surface postcrosslinked superabsorbent polymer particles are classified into an undersize, midsize, and oversize fraction, the superabsorbent polymer particles of the oversize fraction are deagglomerated using a roll crusher, and the deagglomerated superabsorbent polymer particles are recycled before or into the classification of the surface postcrosslinked superabsorbent polymer particles, wherein a sieve for separating the undersize fraction and the midsize fraction has a mesh size from 100 to 300 μm.

20. A process for producing superabsorbent polymer particles, comprising polymerization of 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 a resulting polymer gel, grinding, classifying, and surface postcrosslinking the resulting polymer particles, wherein the surface postcrosslinked superabsorbent polymer particles are classified into an undersize, midsize, and oversize fraction, the superabsorbent polymer particles of the oversize fraction are deagglomerated using a roll crusher, and the deagglomerated superabsorbent polymer particles are recycled before or into the classification of the surface postcrosslinked superabsorbent polymer particles, wherein a gap width of the roll crusher is from 50 to 800 μm larger than the mesh size of the sieve for separating the oversize fraction and the midsize fraction, wherein the gap width of the roll crusher is the smallest distance between the two surfaces that form the gap of the roll crusher.

21. A process for producing superabsorbent polymer particles, comprising polymerization of 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 a resulting polymer gel, grinding, classifying, and surface postcrosslinking the resulting polymer particles, wherein the surface postcrosslinked superabsorbent polymer particles are classified into an undersize, midsize, and oversize fraction, the superabsorbent polymer particles of the oversize fraction are deagglomerated using a roll crusher, and the deagglomerated superabsorbent polymer particles are recycled before or into the classification of the surface postcrosslinked superabsorbent polymer particles, wherein the surface postcrosslinked superabsorbent polymer particles are classified using a tumbling screen machine.

Description

EXAMPLES

Example 1

(1) By continuously mixing deionized water, 50% by weight sodium hydroxide solution and acrylic acid, an acrylic acid/sodium acrylate solution was prepared, such that the degree of neutralization corresponds to 71 mol %. The solids content of the monomer solution was 41.5% by weight.

(2) 3-tuply ethoxylated glycerol triacrylate was used as crosslinker. The amount of crosslinker was 1.06 kg per t of monomer solution.

(3) The free-radical polymerization was initiated by adding 1.3 kg of a 0.25% by weight aqueous hydrogen peroxide solution, 1.54 kg of a 27% by weight aqueous sodium peroxodisulfate solution, and 1.4 kg of a 1% by weight aqueous ascorbic acid solution, each based per t of monomer solution.

(4) The throughput of the monomer solution was 18 t/h. The monomer solution had a temperature of 30° C. at the feed.

(5) The individual components were metered in the following amounts continuously into a continuous kneader reactor with a capacity of 6.3 m.sup.3 (LIST AG, Arisdorf, Switzerland):

(6) TABLE-US-00001 18.0 t/h of monomer solution 19.08 kg/h of 3-tuply ethoxylated glycerol triacrylate 51.12 kg/h of hydrogen peroxide solution/sodium peroxodisulfate solution 25.2 kg/h of ascorbic acid solution

(7) Between the addition point for the crosslinker and the addition sites for the initiators, the monomer solution was inertized with nitrogen.

(8) After approx. 50% of the residence time, a metered addition of fines (1000 kg/h), which were obtained from the production process by grinding and screening, to the reactor additionally took place. The residence time of the reaction mixture in the reactor was 15 minutes.

(9) The resulting polymer gel was placed onto a belt dryer. On the belt dryer, an air/gas mixture flowed continuously around the polymer gel and dried it.

(10) The dried polymer gel was ground and screened off to a particle size fraction of 150 to 850 μm.

(11) The resulting base polymer exhibited a centrifuge retention capacity (CRC) of 38.7 g/g, an absorbency under high load (AUL0.7 psi) of 7.3 g/g and a PSD of

(12) TABLE-US-00002 >850 μm 2.5 wt. % 300-850 μm 82.6 wt. % 150-300 μm 14.1 wt. % <150 μm 0.8 wt. %.

(13) The resulting base polymer was surface postcrosslinked:

(14) In a Schugi Flexomix® (Hosokawa Micron B.V., Doetinchem, the Netherlands), the base polymer was coated with a surface postcrosslinker solution and then dried in a NARA® paddle dryer (GMF Gouda, Waddinxveen, the Netherlands) at 185° C. for 45 minutes. The paddle dryer was heated with steam having a pressure of 24 bar (220° C.).

(15) The following amounts were metered into the Schugi Flexomix®:

(16) TABLE-US-00003 7.5 t/h of base polymer 282.75 kg/h of surface postcrosslinker solution

(17) The surface postcrosslinker solution comprised of 1.59% by weight of 2-hydroxyethyl-2 oxazolidone, 1.59% by weight of propandiole-1.3, 13.25% by weight of propandiole-1.2, 0.08% by weight of Sorbitanmonolaurat (Span® 20), 11.13% by weight of aluminum trilactate, 51.95% by weight of deionized water, and 20.41% by weight of isopropanol.

(18) After being dried, the surface postcrosslinked base polymer was cooled to approx. 60° C. in a NARA® paddle cooler (GMF Gouda, Waddinxveen, the Netherlands).

(19) After the cooling, the superabsorbent polymer particles were again screened off to from 150 to 850 μm (SXL polymer).

(20) The SXL polymer was analysed and exhibited a centrifuge retention capacity (CRC) of 30.1 g/g, an absorbency under high load (AUHL) of 23.4 g/g, and a saline flow conductivity (SFC) of 52×10.sup.−7 cm.sup.3 s/g.

(21) The oversize fraction (SXL overs) was deagglomerated using a roll crusher having a corrugated surface and a gap width of 0.85 mm (deagglomerated SXL polymer). The SXL polymer and the deagglomerated SXL polymer were combined (SXL product).

(22) The deagglomerated SXL polymer was screened off to a particle size fraction of 150 to 850 μm and analysed. The results are shown in table 1.

Example 2

(23) Example 1 was repeated. The SXL overs were deagglomerated using a roll crusher having a corrugated surface and a gap width of 1.00 mm. The deagglomerated SXL polymer was screened off to a particle size fraction of 150 to 850 μm and analysed. The results are shown in table 1.

Example 3

(24) Example 1 was repeated. The SXL overs were deagglomerated using a roll crusher having a corrugated surface and a gap width of 1.20 mm. The deagglomerated SXL polymer was screened off to a particle size fraction of 150 to 850 μm and analysed. The results are shown in table 1.

Example 4

(25) (Comparative Example)

(26) Example 1 was repeated. The SXL overs were deagglomerated using a rotor mill having a rotor-ring sieve system having round-shaped holes of 10 mm as grinding space bound. The deagglomerated SXL polymer was screened off to a particle size fraction of 150 to 850 μm and analysed. The results are shown in table 1.

Example 5

(27) (Comparative Example)

(28) Example 1 was repeated. The SXL overs were deagglomerated using a rotor mill having a rotor-ring sieve system having round-shaped holes of 6 mm as grinding space bound. The deagglomerated SXL polymer was screened off to a particle size fraction of 150 to 850 μm and analysed. The results are shown in table 1.

(29) TABLE-US-00004 TABLE 1 Properties of the deagglomerated SXL overs CRC AUL0.7 psi SFC Example [g/g] [g/g] [10.sup.−7 cm.sup.3s/g] 1 28.8 22.8 45 2 28.7 22.0 50 3 29.1 22.6 53   4*) 29.9 20.3 22   5*) 30.9 19.5 10 *)comparative example