Classification process for superabsorbent polymer particles

Abstract

The invention relates to a classification process for superabsorbent polymer particles, comprising classifying the water-absorbent polymer particles in a screen machine under reduced pressure wherein the pressure above the uppermost sieve is from 0 to 4.0 mbar per sieve higher than below the undermost sieve.

Claims

1. A classification process for superabsorbent polymer particles, wherein the water-absorbent polymer particles are classified by using a tumbling screen machine having at least two sieves under reduced pressure and a pressure above an uppermost sieve is from 0.05 to 3.5 mbar per sieve higher than a pressure below an undermost sieve.

2. The process according to claim 1, wherein the pressure above the uppermost sieve is from 0.25 to 2.5 mbar per sieve higher than the pressure below the undermost sieve.

3. The process according to claim 1, wherein the pressure above the uppermost sieve is at least 2 mbar below ambient pressure.

4. The process according to claim 1, wherein a product fraction is removed by the at least two sieves of different mesh sizes.

5. The process according to claim 1, wherein an oversize is removed by the at least two sieves of different mesh sizes.

6. The process according to claim 1, wherein the superabsorbent polymer particles, during the classification, have a temperature of from 40 to 120 C.

7. The process according to claim 1, wherein the superabsorbent polymer particles are flowed over by a gas stream during the classification.

8. The process according to claim 7, wherein the water content of the gas stream is less than 5 g/kg.

9. The process according to claim 7, wherein the gas stream has a temperature of at least 40 C.

10. The process according to claim 7, wherein the gas stream is air.

11. The process according to claim 1, wherein the tumbling screen machine is partly or wholly thermally insulated.

12. The process according to claim 1, wherein a proportion of acrylic acid in a total amount of ethylenically unsaturated monomer in the superabsorbent polymer particles is at least 95 mol %.

13. The process according to claim 12, wherein a degree of neutralization of the ethylenically unsaturated monomer is from 65 to 80 mol %.

14. The process according to claim 12, wherein an amount of crosslinker based on the ethylenically unsaturated monomer is 0.2 to 0.6% by weight.

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

16. The process according to claim 1 wherein the at least two sieves of the tumbling screen medium are arranged over one another.

17. The process according to claim 1 wherein the at least two sieves of the tumbling screen medium are arranged in parallel.

18. The new process according to claim 1 wherein the tumbling screen machine has at least 3 sieves under reduced pressure.

19. The new process according to claim 1 wherein the tumbling screen machine has at least 4 sieves under reduced pressure.

20. The process according to claim 1 wherein the tumbling screen machine has as guide device deflecting the superabsorbent polymer particles to an exit orifice of each of the at least two sieves.

Description

EXAMPLES

Example 1

Comparative Example

(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 71.3 mol %. The solids content of the monomer solution was 38.8% by weight.

(2) The polyethylenically unsaturated crosslinker used was polyethylene glycol-400 diacrylate (di-acrylate proceeding from a polyethylene glycol with a mean molar mass of 400 g/mol). The amount used was 2 kg of crosslinker per t of monomer solution.

(3) To initiate the free-radical polymerization, 1.03 kg of a 0.25% by weight aqueous hydrogen peroxide solution, 3.10 kg of a 15% by weight aqueous sodium peroxodisulfate solution and 1.05 kg of a 1% by weight aqueous ascorbic acid solution were used per t of monomer solution.

(4) The throughput of the monomer solution was 20 t/h. The reaction solution had a temperature of 23.5 C. at the feed. The monomer solution was polymerized in a List Contikneter continuous kneader reactor with a capacity of 6.3 m.sup.3 (LIST AG, Arisdorf, Switzerland).

(5) Polyethylene glycol-400 diacrylate is continuously added to the monomer solution followed by addition of mixture of hydrogen peroxide solution and sodium peroxodisulfate solution. Ascorbic acid solution is separately continuously added to the kneader reactor.

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

(7) 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.

(8) 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.

(9) The dried polymer gel was ground and a 500 g-sample was screened by using a screen machine of type KS 1000 (Retsch GmbH, Haan, Germany) having four screen decks (1,000 m, 850 m, 300 m, and 90 m). The pressure above the uppermost sieve was below ambient pressure. The pressure above the uppermost sieve was 200 mbar (50 mbar per sieve) higher than the pressure below the undermost sieve. The pressure difference was kept constant during screening. The amount of polymer on the sieves after 1 minute sieving time at the end of the screening is recorded in table 1.

(10) The polymers on the 90 m sieve and on the 300 m sieve were combined. The 90 to 850 m sieve cut was analyzed analogous to the EDANA recommended test method No. NWSP 220.0.R2 (15) Determination of the Particle Size Distribution by Sieve Fractionation using six screen decks (850 m, 600 m, 300 m, 150 m, 106 m, and 45 m). The results are recorded in table 2.

Example 2

Comparative Example

(11) Example 1 was repeated, except that the pressure above the uppermost sieve was 20 mbar (5 mbar per sieve) higher than the pressure below the undermost sieve.

Example 3

(12) Example 1 was repeated, except that the pressure above the uppermost sieve was 8 mbar (2 mbar per sieve) higher than the pressure below the undermost sieve.

Example 4

(13) Example 1 was repeated, except that the pressure above the uppermost sieve was the same as the pressure below the undermost sieve.

Example 5

Comparative Example

(14) Example 1 was repeated, except that the pressure above the uppermost sieve was 2 mbar (0.5 mbar per sieve) lower than the pressure below the undermost sieve.

(15) TABLE-US-00001 TABLE 1 particle size fractions Example Pressure difference 1*) 2*) 3 4 5*) per sieve 50 mbar 5 mbar 2 mbar 0 mbar +0.5 mbar >1000 m 2.3 2.1 2.2 2.1 2.2 850-1000 m 3.6 3.6 3.6 3.6 3.5 300-850 m 93.5 69.2 69.2 69.2 69.1 90-300 m 0.5 25.0 20.1 20.1 22.7 <90 m 0.0 0.0 5.0 4.9 2.5 *)comparative

(16) TABLE-US-00002 TABLE 2 particle size distributions of 90 to 850 m sieve cut Example Pressure difference 1*) 2*) 3 4 5*) per sieve 50 mbar 5 mbar 2 mbar 0 mbar +0.5 mbar >850 m 0.2 0.2 0.2 0.1 0.2 600-850 m 25.0 23.4 25.3 25.0 24.9 300-600 m 52.1 50.2 53.2 52.4 50.2 150-300 m 11.0 12.2 12.4 12.3 12.5 106-150 m 5.4 7.0 5.6 5.8 6.2 45-106 m 4.7 5.7 3.2 4.2 4.9 <45 m 1.6 1.4 0.1 0.2 1.1 *)comparative