METHOD FOR PRODUCING SUPER ABSORBER PARTICLES

Abstract

A process for producing superabsorbent particles, comprising drying of a polymer gel, removal of incompletely dried polymer particles, comminution of the polymer particles removed, recycling of the comminuted polymer particles and storage of the recycled polymer particles.

Claims

1. A process for producing superabsorbent particles by polymerizing a monomer solution or suspension comprising a) at least one ethylenically unsaturated monomer which bears an acid group and optionally is 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, comprising i) drying a polymer gel to produce a dried polymer gel, ii) classifying the dried polymer gel from step i) to obtain at least one fraction of incompletely dried polymer particles and at least one fraction of dried polymer particles, and iii) comminuting the incompletely dried polymer particles from step ii), which comprises recycling the comminuted polymer particles from step iii) into the dried polymer gel before step ii) or into the dried polymer particles after step ii) and storing them together with the dried polymer gel before step ii) or the dried polymer particles after step ii) in a vessel for a period of at least 5 minutes.

2. The process according to claim 1, wherein the comminuted polymer particles from step iii) are recycled into the dried polymer gel before step ii) and stored together with the dried polymer gel before step ii) in a vessel for a period of at least 5 minutes.

3. The process according to claim 2, wherein the dried polymer particles from step ii) are ground by means of at least one multistage roll mill and classified by means of at least one tumbler screening machine.

4. The process according to claim 1, wherein the comminuted polymer particles from step iii) are recycled into the dried polymer particles after step ii) and stored together with the dried polymer particles after step ii) in a vessel for a period of at least 5 minutes.

5. The process according to claim 4, wherein the stored polymer particles are ground by means of at least one multistage roll mill and classified by means of at least one tumbler screening machine.

6. The process according to claim 3, wherein the ground and classified polymer particles are thermally surface postcrosslinked.

7. The process according to claim 1, wherein the storage period in the vessel is from 30 to 120 minutes.

8. The process according to claim 1, wherein the dried polymer particles obtained in step ii) have a moisture content of less than 5% by weight.

9. The process according to claim 1, wherein the incompletely dried polymer particles obtained in step ii) have a moisture content at least 10% by weight higher than the dried polymer particles obtained in step ii).

10. The process according to claim 1, wherein the incompletely dried polymer particles obtained in step ii) have a particle size of at least 8 mm.

11. The process according to claim 1, wherein the incompletely dried polymer particles are comminuted in step iii) to a particle size of less than 8 mm.

12. The process according to claim 1, to 11, wherein the incompletely dried polymer particles are comminuted in step iii) by means of a mill with sharp beaters or blades and a screen as grinding space boundary.

13. The process according to claim 1, wherein the polymer particles have a temperature in the range from 60 to 100 C. during the storage in the vessel.

14. The process according to claim 1, wherein the vessel used for storage is thermally insulated and/or trace heated.

15. The process according to claim 1, wherein ethylenically unsaturated monomer bearing an acid group is acrylic acid.

16. The process according to claim 5, wherein the ground and classified polymer particles are thermally surface postcrosslinked.

Description

EXAMPLES

Example 1

[0094] An acrylic acid solution that had been neutralized with sodium hydroxide solution to an extent of 75 mol % was polymerized together with polyethylene glycol diacrylate (diacrylate of a polyethylene glycol having an average molar mass of 400 g/mol) in a batchwise kneader. The resultant crumbly polymer gel was dried at 160 C. in an air circulation belt dryer. The dried polymer gel was then coarsely comminuted in the belt dryer by means of a spiked roller (polymer particles A).

[0095] By means of a screen having a mesh size of 10 mm, the incompletely dried polymer particles (polymer particles C) were removed. The proportion of the incompletely dried polymer particles removed that had a particle diameter of greater than 10 mm (polymer particles C) was 5% by weight. The polymer particles having a particle size of less than 10 mm were processed further as dried polymer particles (polymer particles B).

[0096] The moisture content of the dried polymer particles (polymer particles B) was 1.34% by weight (multiple determination).

[0097] The incompletely dried polymer particles having a particle diameter of greater than 10 mm that were removed (polymer particles C) were manually freed of the outer dry content by beating and comminuted with a cutting tool to a particle diameter of less than 10 mm.

[0098] The moisture content of the comminuted, incompletely dried polymer particles (polymer particles D) was 9.3% by weight (multiple determination).

[0099] 122.5 g of dried polymer particles (polymer particles B) and 14.5 g of comminuted, incompletely dried polymer particles (polymer particles D) were mixed and stored in a closed glass bottle at 80 C. for a period of 30 minutes.

[0100] The stored mixture was ground in three stages with a roll mill. The mixture was processible without difficulty.

[0101] The example shows that the combined storage of the dried polymer particles (polymer particles B) and of the comminuted, incompletely dried polymer particles (polymer particles D) made it possible to dispense with additional drying of the incompletely dried polymer particles.

Example 2 (Comparative Example)

[0102] The procedure was as in example 1. The dried polymer particles (polymer particles B) and the comminuted, incompletely dried polymer particles (polymer particles D) were mixed and ground immediately in three stages with a roll mill. The mixture comprised polymer particles having a relatively high moisture content that could not be ground and, when passed through the rolls, led to caking and to undesirable stress on the roller bearings.

Example 3

[0103] 245 g of polymer particles coarsely comminuted by means of a spiked roller (polymer particles A) and 29 g of comminuted, incompletely dried polymer particles (polymer particles D), each from example 1, were mixed in a closed glass bottle and stored at 80 C. for a period of 30 minutes.

[0104] By means of a screen having a mesh size of 10 mm, the incompletely dried polymer particles (polymer particles C) were separated from the mixture. The proportion of the incompletely dried polymer particles removed that had a particle diameter of greater than 10 mm (polymer particles C) was 4% by weight.

[0105] After removal of the incompletely dried polymer particles (polymer particles C), the stored mixture was immediately ground in three stages with a roll mill. The mixture was processible without difficulty.

[0106] The example shows that the combined storage of the coarsely comminuted polymer particles (polymer particles A) and of the comminuted, incompletely dried polymer particles (polymer particles D) makes it possible to dispense with additional drying of the incompletely dried polymer particles.

Example 4 (Comparative Example)

[0107] The procedure was as in example 3. The coarsely comminuted polymer particles (polymer particles A) and the comminuted, incompletely dried polymer particles (polymer particles D) were mixed. By means of a screen having a mesh size of 10 mm, the incompletely dried polymer particles (polymer particles C) were immediately separated from the mixture.

[0108] After removal of the incompletely dried polymer particles (polymer particles C), the mixture was immediately ground in three stages with a roll mill. The mixture comprised polymer particles having a relatively high moisture content that could not be ground and, when passed through the rolls, led to caking and to undesirable stress on the roller bearings.