METHOD FOR THE PRODUCTION OF SUPERABSORBENTS

Abstract

A process for producing superabsorbent particles by polymerizing a monomer solution or suspension, comprising drying of the resultant aqueous polymer gel in an air circulation belt drier, grinding, classifying, and optionally thermal surface postcrosslinking, wherein the aqueous polymer gel is introduced into the air circulation belt drier by means of an oscillating conveyor belt, the underside of the revolving conveyor belt is freed of adhering polymer gel by means of at least one stripper device, and the underside of the revolving conveyor belt is sprayed with water.

Claims

1. A process for producing superabsorbent particles comprising polymerizing a monomer solution or suspension comprising a) at least one ethylenically unsaturated monomer which bears an acid group and 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, drying a resultant aqueous polymer gel in an air circulation belt drier, grinding, classifying, and optionally thermal surface postcrosslinking, wherein the aqueous polymer gel is introduced into the air circulation belt drier by an oscillating conveyor belt, the underside of the revolving conveyor belt is freed of adhering polymer gel by at least one stripper device, and the underside of the revolving conveyor belt is sprayed with water by at least one spray nozzle.

2. The process according to claim 1, wherein the distance of the stripper device from a discharge end of the conveyor belt is less than 20% of the length of the conveyor belt, where the length of the conveyor belt is a distance of the pivot axis from the discharge end.

3. The process according to claim 1, wherein the stripper device mounted on the underside of the revolving conveyor belt is a scraper.

4. The process according to claim 3, wherein the scraper is inclined at 5° to 45° relative to the horizontal counter to the running direction of the conveyor belt.

5. The process according to claim 3, wherein a distance of the scraper from the underside of the revolving conveyor belt is from 0.1 to 5 mm.

6. The process according to claim 1, wherein a distance of the at least one spray nozzle from the discharge end of the conveyor belt is 1% to 50% of the length of the conveyor belt, where the length of the conveyor belt is the distance of the pivot axis from the discharge end.

7. The process according to claim 1, wherein a distance of the spray nozzles from the conveyor belt is from 5 to 50 cm.

8. The process according to claim 1, wherein from 2 to 20 kg/h of water is sprayed onto the underside of the revolving conveyor belt.

9. The process according to claim 1, wherein the water is sprayed onto the underside of the revolving conveyor belt by at least one two-phase nozzle.

10. The process according to claim 1, wherein the water is sprayed onto the underside of the revolving conveyor belt by at least one two-phase nozzle, and a weight ratio of atomizer gas to water is from 2 to 20.

11. The process according to claim 1, wherein the water is sprayed onto the underside of the revolving conveyor belt by at least two two-phase nozzles.

12. The process according to claim 1, wherein the at least one spray nozzle is beyond the stripper device in the running direction of the conveyor belt.

13. The process according to claim 1, wherein the conveyor belt has a length of 2 to 10 m, where the length of the conveyor belt is the distance of the pivot axis from the discharge end.

14. The process according to claim 1, wherein the surface of the conveyor belt at 23° C. has a contact angle with respect to water of at least 60°.

15. A hygiene article comprising superabsorbents produced by a process of claim 1.

Description

EXAMPLES

Example 1

[0096] 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 corresponded to 71.3 mol %. The solids content of the monomer solution was 38.8% by weight.

[0097] The polyethylenically unsaturated crosslinker used was polyethylene glycol-400 diacrylate (diacrylate 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.

[0098] To initiate the free-radical polymerization, pert of monomer solution, 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.

[0099] The throughput of the monomer solution was 20 t/h. The reaction solution had a feed temperature of 23.5° C.

[0100] The individual components were metered in the following amounts continuously into a List Contikneter continuous kneader reactor with a capacity of 6.3 m.sup.3 (LIST AG, Arisdorf, Switzerland):

TABLE-US-00001   20 t/h of monomer solution   40 kg/h of polyethylene glycol-400 diacrylate 82.6 kg/h of hydrogen peroxide solution/sodium peroxodisulfate solution   21 kg/h of ascorbic acid solution

[0101] Between the addition point for the crosslinker and the addition sites for the initiators, the monomer solution was inertized with nitrogen.

[0102] After about 50% of the dwell time there was an additional metered addition to the reactor of fines (1000 kg/h) which were obtained from the production process by grinding and sieving. The dwell time of the reaction mixture in the reactor was 15 minutes.

[0103] The aqueous polymer gel obtained was applied to the conveyor belt of an air circulation belt drier by means of an oscillating conveyor belt.

[0104] The air circulation belt drier had a length of 48 m. The conveyor belt of the air circulation belt drier had an effective width of 4.4 m.

[0105] The oscillating conveyor belt had a length of 5 m. The conveyor belt had a width of 0.8 m and an effective width of 0.5 m. The angle of repose of the aqueous polymer gel on the conveyor belt was about 15°. The cross section of the polymer gel bed on the conveyor belt was about 0.04 m.sup.2. The speed of the conveyor belt was 0.5 m/s.

[0106] Proceeding from one end position, the oscillating conveyor belt was accelerated through a first pivot angle β.sub.1 of 13° to an angular speed of 33°/s, decelerated through a second pivot angle β.sub.2 of 20° to an angular speed of 17°/s and decelerated through a third pivot angle β.sub.3 to the other end position. The total pivot angle was 50°. A double pass (from the first end position to the other end position and back) lasted about 7 s. The revolving conveyor belt had a surface of polytetrafluoroethylene (PTFE).

[0107] The temperature of the aqueous polymer gel on the oscillating conveyor belt was 90° C.

[0108] On the underside of the oscillating conveyor belt was a stripper device. The stripper device was a longitudinal scraper mounted transverse to the running direction of the revolving conveyor belt. The scraper was inclined at 20° against the running direction of the revolving conveyor belt. The distance of the stripper device from the discharge end was about 5 cm, meaning that the stripper device was in the region of the deflection roll. The distance of the stripper device from the revolving conveyor belt was 1 mm. The stripper device strips off aqueous polymer gel adhering to the outside of the revolving conveyor belt.

[0109] On the underside of the oscillating conveyor belt were additionally 3 two-phase nozzles. The two-phase nozzles were arranged transverse to the running direction of the conveyor belt. The distance between the two-phase nozzles was about 20 cm in each case. The distance of the two-phase nozzles from the discharge end was about 20 cm. The distance of the two-phase nozzles from the revolving conveyor belt was about 20 cm. A total of 10 kg/h of water and 100 kg/h of air were sprayed.

[0110] It was possible to efficiently clean the revolving conveyor belt by means of the stripper device. There was no significant caking at all on the conveyor belt over a period of 6 months.

[0111] On the air circulation belt drier, an air/gas mixture flowed continuously around the aqueous polymer gel and dried it. The dwell time in the air circulation belt drier was 37 minutes.

[0112] The dried polymer gel was ground and sieved to a particle size fraction of 150 to 850 μm.

[0113] The resulting water-absorbing polymer particles had a centrifuge retention capacity (CRC) of 34.9 g/g and an extractables content of 8.5% by weight.

Example 2 (Comparative Example)

[0114] The procedure was as in example 1, except that the spray nozzles on the oscillating conveyor belt were switched off.

[0115] It was possible to clean the revolving conveyor belt only with difficulty by means of the stripper device. There was significant caking on the conveyor belt, especially where polymer gel had already dried. After a few weeks of continuous production, the production had to be stopped and the conveyor belt cleaned, or the conveyor belt had to be exchanged owing to damage.