Process for remoisturizing surface-postcrosslinked water-absorbing polymer particles

Abstract

A process for producing water-absorbing polymer particles, wherein surface postcrosslinked water-absorbing polymer particles are remoisturized and delivered pneumatically, and wherein the time between remoisturization and pneumatic delivery is less than one hour.

Claims

1. A process for producing water-absorbing polymer particles by polymerizing a monomer solution or suspension comprising at least one ethylenically unsaturated monomer which bears an acid group and may be at least partly neutralized, at least one crosslinker, at least one initiator, optionally one or more ethylenically unsaturated monomer copolymerizable with the ethylenically unsaturated monomer which bears an acid group, and optionally one or more water-soluble polymer, then drying, grinding, classifying, and surface postcrosslinking optionally additionally using polyvalent metal cations at 130 to 210 C. to provide dried surface postcrosslinked polymer particles, then remoisturizing the dried surface postcrosslinked polymer particles by adding an aqueous liquid to the particles, then pneumatically delivering the remoisturized surface postcrosslinked polymer particles, and optionally, classifying the remoisturized surface postcrosslinked polymer particles, wherein a time between the remoisturization and the pneumatic delivery is less than one hour.

2. The process according to claim 1, wherein the water-absorbing polymer particles are surface postcrosslinked by formation of covalent bonds.

3. The process according to claim 2, wherein polyvalent metal cations have been used additionally in the surface postcrosslinking.

4. The process according to claim 1, wherein the water-absorbing polymer particles supplied to the remoisturization have a temperature of 40 to 80 C.

5. The process according to claim 1, wherein remoisturization is effected using an aqueous solution or an aqueous dispersion comprising an inorganic particulate substance, an inorganic colloidally dissolved substance, an organic polymer, a cationic polymer, and/or a salt of a polyvalent metal cation.

6. The process according to claim 1, wherein the water-absorbing polymer particles supplied to the pneumatic delivery have a temperature of 40 to 80 C.

7. The process according to claim 1, wherein an initial velocity in the pneumatic delivery corresponds to a Froude number of from 10 to 18.

8. The process according to claim 1, wherein at least 95% by weight of the water-absorbing polymer particles have a particle size of at least 150 m.

9. The process according to claim 1, wherein at least 95% by weight of the water-absorbing polymer particles have a particle size of at most 600 m.

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

11. The process according to claim 1, wherein the time between the moisturization and the pneumatic delivery is less than 45 minutes.

12. The process according to claim 1, wherein the time between the moisturization and the pneumatic delivery is less than 30 minutes.

13. The process according to claim 1, wherein the time between the moisturization and the pneumatic delivery is less than 15 minutes.

Description

EXAMPLES

Example 1 (Production of the Water-Absorbing Polymer Particles)

(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 (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.

(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.

(5) 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): 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

(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. The residence time in the belt dryer was 37 minutes.

(9) The dried polymer gel was ground and screened off to a particle size fraction of 150 to 850 m. The resulting base polymer was surface postcrosslinked.

(10) 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 190 C. for 45 minutes.

(11) The following amounts were metered into the Schugi Flexomix: 7.5 t/h of base polymer 270.0 kg/h of surface postcrosslinker solution

(12) The surface postcrosslinker solution comprised 2.8% by weight of 2-hydroxyethyl-2 oxazolidone, 2.8% by weight of aluminum sulfate, 66.1% by weight of deionized water and 28.3% by weight of isopropanol.

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

(14) The resulting water-absorbing polymer particles had a centrifuge retention capacity (CRC) of 28.4 g/g.

Example 2

(15) A ProfiMixx 47 food processor (Robert Bosch GmbH; Gerlingen-Schillerhhe; Germany) was initially charged with 220 g of water-absorbing polymer particles from example 1, which were remoisturized with 5.5 g of water while stirring (level 4; approx. 500 rpm) and stirred for a further minute. The water was sprayed on by means of a peristaltic pump at a metering rate of 5 g/min. The tube had an internal diameter of 2.54 mm.

(16) 20 g of the remoisturized water-absorbing polymer particles in each case were subjected to mechanical stress after 0, 2, 4, 6, 8, 19, 30, 60 and 120 minutes respectively. To this end, the water-absorbing polymer particles were ground in a ball mill together with 30 ceramic bodies having round caps (approx. 7.4 g each) at 150 rpm for 5 minutes.

(17) Subsequently, the particle size distribution was measured. The percentages reported are percent by weight. The results are summarized in table 1:

(18) TABLE-US-00001 TABLE 1 Remoisturization with 2.5% by weight of water and stress period of 5 minutes SFC <150 150-180 180-300 300-600 600-850 >850 [10.sup.7 Minutes m m m m m m cm.sup.3s/g] 0 0.8% 0.9% 12.2% 51.5% 34.5% 0.2% 17 2 0.6% 1.0% 12.2% 49.0% 36.9% 0.2% 22 4 0.7% 0.8% 12.0% 50.9% 35.3% 0.2% 29 6 0.8% 0.8% 11.2% 48.9% 37.6% 0.8% 25 8 1.0% 1.0% 13.4% 49.0% 35.5% 0.2% 22 10 1.2% 1.1% 13.6% 49.6% 34.5% 0.2% 22 30 1.4% 1.2% 13.8% 48.9% 34.8% 0.2% 20 60 1.4% 1.2% 13.8% 48.8% 34.7% 0.1% 22 120 1.5% 1.4% 13.7% 48.5% 35.0% 0.1% 23

Example 3

(19) The procedure was as in example 2. Instead of grinding for 5 minutes with 30 ceramic bodies, 40 ceramic bodies were used for 15 minutes.

(20) Subsequently, the particle size distribution was measured. The percentages reported are percent by weight. The results are summarized in table 2:

(21) TABLE-US-00002 TABLE 2 Remoisturization with 2.5% by weight of water and stress period of 15 minutes <150 150-180 180-300 300-600 600-850 >850 Minutes m m m m m m 0 1.1% 0.1% 12.6% 64.8% 21.6% 0.0% 2 2.4% 1.5% 16.5% 58.2% 21.7% 0.0% 4 3.4% 1.9% 17.2% 56.9% 20.5% 0.1% 6 4.7% 2.3% 18.8% 56.3% 18.0% 0.0% 8 6.0% 2.6% 18.7% 54.9% 17.8% 0.0% 10 5.7% 2.7% 21.4% 56.2% 14.0% 0.0% 30 5.5% 2.7% 19.8% 55.5% 16.5% 0.0% 60 7.1% 2.7% 21.6% 55.1% 13.8% 0.0% 120 8.8% 3.2% 21.6% 55.6% 10.7% 0.0%

Example 4

(22) The procedure was as in example 3. Instead of remoisturizing with 5.5 g of water, 11.0 g of water were used.

(23) Subsequently, the particle size distribution was measured. The percentages reported are percent by weight. The results are summarized in table 3:

(24) TABLE-US-00003 TABLE 3 Remoisturization with 5% by weight of water and stress period of 15 minutes <150 150-180 180-300 300-600 600-850 >850 Minutes m m m m m m 0 0.6% 0.8% 12.1% 55.8% 30.7% 0.0% 2 0.3% 0.5% 8.6% 51.4% 39.0% 0.3% 4 0.5% 0.5% 9.0% 52.5% 37.4% 0.1% 6 1.0% 0.9% 13.7% 56.9% 27.7% 0.0% 8 0.9% 0.9% 11.8% 52.2% 34.3% 0.0% 10 0.7% 0.8% 11.6% 52.7% 34.1% 0.1% 30 0.8% 1.0% 13.7% 60.3% 24.4% 0.0% 60 1.2% 1.3% 15.0% 59.1% 23.3% 0.1% 120 1.6% 1.4% 15.7% 56.8% 24.7% 0.0%

(25) In each of tables 1 to 3, a significant rise in small polymer particles (<150 m) is discernible with increasing delay time between remoisturization and mechanical stress.