Method for the production of absorbent polymer particles by polymerizing drops of a monomer solution

Abstract

A process for preparing water-absorbing polymer beads by polymerizing droplets comprising at least one monomer in a gas phase surrounding the droplets, the droplets being obtained by enveloping a first monomer solution with a second monomer solution and polymerizing the second monomer solution and polymerizing to give a more highly crosslinked polymer than the first monomer solution.

Claims

1. Water-absorbing polymer beads which have a centrifuge retention capacity of at least 30 g/g and a permeability of at least 3010.sup.7 cm.sup.3s/g, and less than 30% of a measured moduli of elasticity of an outer bead surface has a value of less than 60% of the mean modulus of elasticity, wherein the polymer beads have a crosslinking gradient and comprise at least one cavity in the bead interior.

2. The polymer beads according to claim 1, which have an absorbency under a load of 4.83 kPa (AUL0.7psi) of at least 20 g/g.

3. The polymer beads according to claim 1, wherein the outer bead surface of the polymer beads has a mean modulus of elasticity of at least 100 kPa.

4. The polymer beads according to claim 1, which comprise less than 10% by weight of extractables.

5. The polymer beads according to claim 1, wherein a ratio of maximum diameter of the cavity to a maximum diameter of the polymer bead is at least 0.1.

6. The polymer beads according to claim 1, wherein a quotient of mean modulus of elasticity of an outer bead surface and mean modulus of elasticity of an inner wall of the cavity is at least 2.5.

7. The polymer beads according to claim 1, which comprise at least partly neutralized polymerized acrylic acid to an extent of at least 50 mol %.

8. The polymer beads according to claim 1, which have a mean diameter of at least 200 m.

9. A hygiene article comprising the polymer beads according to claim 1.

Description

(1) FIG. 1 shows an example of a test setup on a microscope slide. In this FIGURE, the reference numerals have the following meanings: 1 to image evaluation 2 to pressure measurement 3 to pressure generation 4 solution 5 polymer bead

(2) When water-absorbing polymer beads which have the shape of hollow spheres are examined, it is also possible to measure the modulus of elasticity of the inner wall of the cavity. To this end, the swollen water-absorbing polymer beads are cut through by means of a scalpel.

(3) Centrifuge Retention Capacity (CRC)

(4) The centrifuge retention capacity of the water-absorbing polymer beads is determined by the EDANA (European Disposables and Nonwovens Association) recommended test method No. 441.2-02 Centrifuge retention capacity.

(5) Absorbency Under Load (AUL0.7 psi)

(6) The absorbency under load of the water-absorbing polymer beads is determined by the EDANA (European Disposables and Nonwovens Association) recommended test method No. 442.2-02 Absorption under pressure, using a weight of 49 g/cm.sup.2 (0.7 psi) instead of a weight of 21 g/cm.sup.2 (0.3 psi).

(7) Extractables

(8) The content of extractables of the water-absorbing polymer beads is determined by the EDANA (European Disposables and Nonwovens Association) recommended test method No 470.2-02 Extractable.

(9) The EDANA test methods are obtainable, for example, from the European Disposables and Nonwovens Association, Avenue Eugene Plasky 157, B-1030 Brussels, Belgium.

EXAMPLES

Example 1 (Comparative Example)

(10) 14.3 kg of sodium acrylate (37.5% by weight solution in water), 1.4 kg of acrylic acid and 350 g of water were mixed with 22 g of 15-tuply ethoxylated trimethylolpropane triacrylate. The solution was dropletized into a heated dropletizer tower filled with a nitrogen atmosphere (180 C., height 12 m, width 2 m, gas velocity 0.1 m/s in cocurrent). The metering rate was 16 kg/h. The dropletizer plate had 30200 m bores. Just upstream of the dropletizer, the initiator was metered into the monomer solution by means of a static mixer. The initiator used was a 3% by weight solution of 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride in water. The metering rate of the initiator solution was 1.1 kg/h.

(11) The water-absorbing polymer beads had the following properties:

(12) TABLE-US-00001 CRC 34.5 g/g AUL0.7psi 16.2 g/g Extractables 4.0% by weight SFC 0.9 10.sup.7 cm.sup.3s/g

(13) The mean modulus of elasticity of the outer bead surface was 60 kPa, the mean modulus of elasticity of the inner wall of the cavity was 30 kPa and the quotient of the mean moduli of elasticity was 2.

(14) The mean modulus of elasticity is the mean value from 20 individual measurements. A total of 4 individual measurements of the modulus of elasticity of the outer bead surface gave a value of less than 36 kPa.

(15) The mean bead diameter was 350 m.

Example 2 (Comparative Example)

(16) The water-absorbing polymer beads from example 1 were sprayed with a solution of 0.08% by weight of ethylene glycol diglycidyl ether, 1.75% by weight of water and 1.17% by weight of propylene glycol, based in each case on the water-absorbing polymer beads, and dried at 120 C. in a forced-air drying cabinet for 30 minutes.

(17) The water-absorbing polymer beads had the following properties:

(18) TABLE-US-00002 CRC 34.8 g/g AUL0.7psi 27.2 g/g Extractables 2.9% by weight SFC 16 10.sup.7 cm.sup.3s/g

(19) The mean modulus of elasticity of the outer bead surface was 150 kPa, the mean modulus of elasticity of the inner wall of the cavity was 40 kPa and the quotient of the mean moduli of elasticity was 3.75.

(20) The mean modulus of elasticity is the mean value from 20 individual measurements. A total of 10 individual measurements of the modulus of elasticity of the outer bead surface gave a value of less than 90 kPa.

(21) The mean bead diameter was 350 m.

Example 3

(22) 14.3 kg of sodium acrylate (37.5% by weight solution in water), 1.4 kg of acrylic acid and 350 g of water were mixed with 22 g of 15-tuply ethoxylated trimethylolpropane triacrylate (first monomer solution). 14.3 kg of sodium acrylate (37.5% by weight solution in water), 1.4 kg of acrylic acid and 350 g of water were mixed with 44 g of 15-tuply ethoxylated trimethylolpropane triacrylate (second monomer solution). The solutions were dropletized into a heated dropletizer tower filled with nitrogen atmosphere (180 C., height 12 m, width 2 m, gas velocity 0.1 m/s in cocurrent). The metering rate of the first monomer solution was 16 kg/h. The metering rate of the second monomer solution was 1.6 kg/h. The dropletizer plate had 30200 m bores, and each bore was surrounded by an annular gap. Just upstream of the dropletizer, the initiator was metered into the monomer solutions by means of static mixers. The initiator used was a 3% by weight solution of 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride in water. The metering rate of the initiator solution into the first monomer solution was 1.1 kg/h. The metering rate of the initiator solution into the second monomer solution was 0.1 kg/h.

(23) The water-absorbing polymer beads had the following properties:

(24) TABLE-US-00003 CRC 34.5 g/g AUL0.7psi 22.1 g/g Extractables 3.5% by weight SFC 30 10.sup.7 cm.sup.3s/g

(25) The mean modulus of elasticity of the outer bead surface was 90 kPa, the mean modulus of elasticity of the inner wall of the cavity was 40 kPa and the quotient of the mean moduli of elasticity was 2.25.

(26) The mean modulus of elasticity is the mean value from 20 individual measurements. A total of 3 individual measurements of the modulus of elasticity of the outer bead surface gave a value of less than 54 kPa.

(27) The mean bead diameter was 360 m.

Example 4

(28) The procedure of example 3 was repeated. The second monomer solution was prepared by using 88 g of 15-tuply ethoxylated trimethylolpropane triacrylate.

(29) The water-absorbing polymer beads had the following properties:

(30) TABLE-US-00004 CRC 34.9 g/g AUL0.7psi 27.9 g/g Extractables 3.5% by weight SFC 50 10.sup.7 cm.sup.3s/g

(31) The mean modulus of elasticity of the outer bead surface was 140 kPa, the mean modulus of elasticity of the inner wall of the cavity was 50 kPa and the quotient of the mean moduli of elasticity was 2.8.

(32) The mean modulus of elasticity is the mean value from 20 individual measurements. A total of 4 individual measurements of the modulus of elasticity of the outer bead surface gave a value of less than 84 kPa.

(33) The mean bead diameter was 370 m.

Example 5

(34) The procedure of example 3 was repeated. The second monomer solution was prepared by using 176 g of 15-tuply ethoxylated trimethylolpropane triacrylate.

(35) The water-absorbing polymer beads had the following properties:

(36) TABLE-US-00005 CRC 34.0 g/g AUL0.7psi 29.3 g/g Extractables 3.0% by weight SFC 90 10.sup.7 cm.sup.3s/g

(37) The mean modulus of elasticity of the outer bead surface was 180 kPa, the mean modulus of elasticity of the inner wall of the cavity was 60 kPa and the quotient of the mean moduli of elasticity was 3.

(38) The mean modulus of elasticity is the mean value from 20 individual measurements. A total of 5 individual measurements of the modulus of elasticity of the outer bead surface gave a value of less than 108 kPa.

(39) The mean bead diameter was 380 m.

Example 6 (Comparative Example)

(40) The procedure of example 6 of WO 2006/077054 A1 was repeated.

(41) The water-absorbing polymer beads had the following properties:

(42) TABLE-US-00006 CRC 21.2 g/g AUL0.3psi 17.6 g/g Extractables 17.5% by weight SFC 12 10.sup.7 cm.sup.3s/g

(43) The mean modulus of elasticity of the outer bead surface was 80 kPa, the mean modulus of elasticity of the inner wall of the cavity was 40 kPa and the quotient of the mean moduli of elasticity was 2.

(44) The mean modulus of elasticity is the mean value from 20 individual measurements. A total of 4 individual measurements of the modulus of elasticity of the outer bead surface gave a value of less than 48 kPa.

(45) The mean bead diameter was 210 m.

Example 7 (Comparative Example)

(46) 14.275 kg of sodium acrylate (37.5% by weight solution in water) and 1.367 kg of acrylic acid were mixed with 0.358 kg of water, 22 g of 15-tuply ethoxylated trimethylolpropane triacrylate and 80 g of EDTA (10% by weight solution of the sodium salt of ethylenediaminetetraacetic acid in water). After addition of 33 g of 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride (3% by weight solution in water) and 110 g of hydrogen peroxide (3% by weight solution in water), the solution was dropletized into a heated dropletization tower filled with nitrogen atmosphere (180 C., height 12 m, width 2 m, gas velocity 0.1 m/s in cocurrent). The metering rate was 16 kg/h. The dropletizer plate had 37170 m bores. The diameter of the dropletizer plate was 65 mm. Just upstream of the dropletizer, the initiator was mixed with the monomer solution by means of a static mixer.

(47) The water-absorbing polymer beads had the following properties:

(48) TABLE-US-00007 CRC 33.0 g/g AUL0.7psi 25.0 g/g Extractables 7.0% by weight SFC 10 10.sup.7 cm.sup.3s/g

(49) The mean modulus of elasticity of the outer bead surface was 90 kPa, the mean modulus of elasticity of the inner wall of the cavity was 40 kPa and the quotient of the mean moduli of elasticity was 2.25.

(50) The mean modulus of elasticity is the mean value from 20 individual measurements. A total of 3 individual measurements of the modulus of elasticity of the outer bead surface gave a value of less than 54 kPa.

(51) The mean bead diameter was 360 m.