Method for producing water-absorbing polymer particles by polymerizing droplets of a monomer solution

Abstract

A process for producing water-absorbing polymer particles by polymerizing droplets of a monomer solution, where the monomer solution comprises at least two initiators, one of the initiators is a peroxide with the exception of persulfate, and the molar ratio of peroxide to further initiator is at least 1:8, the water-absorbing polymer particles themselves, and hygiene articles comprising these water-absorbing polymer particles.

Claims

1. A process for producing water-absorbing polymer particles comprising providing droplets of a monomer solution in a carrier gas, the monomer solution comprising a) at least one water-soluble ethylenically unsaturated monomer, b) a crosslinker, c) at least one peroxide initiator with the exception of persulfate, wherein the at least one peroxide initiator consists of hydrogen peroxide and an organic peroxide, d) at least one further initiator wherein the further initiator d) is selected from the group consisting of an azo compound and/or a photoinitiator and/or a redox initiator, and e) water, wherein the molar ratio of initiator c) to further initiator d) is at least 16:1, and polymerizing the droplets of the monomer solution in the carrier gas surrounding the droplets, wherein the monomer solution is metered into the carrier gas at a speed of less than 0.2 m/s.

2. The process according to claim 1, wherein the monomer a) has at least one acid group.

3. The process according to claim 2, wherein the acid groups of the monomer a) are neutralized at least partly.

4. The process according to claim 1, wherein the monomer solution comprises less than 0.5% by weight of the crosslinker b) based on the monomer a).

5. The process according to claim 1, wherein the organic peroxide of initiator c) is a hydroperoxide.

6. The process according to claim 1, wherein the further initiator d) is an azo compound and/or a photoinitiator.

7. The process according to claim 1, wherein the droplets have a mean diameter of at least 200 μm.

8. The process according to claim 1, wherein at least 90%, by weight, of the droplets have a diameter of at least 100 μm.

9. The process according to claim 1 wherein the molar ratio of initiator c) to further initiator d) is 16:1 to 32:1.

10. The process according to claim 1 wherein the organic peroxide of the initiator c) is selected from the group consisting of acetylacetone peroxide, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butyl perpivalate, tert-butyl perneohexanoate, tert-butyl perisobutyrate, tert-butyl per-2-ethylhexanoate, tert-butyl perisononanoate, tert-butyl permaleate, tert-butyl perbenzoate, di(2-ethylhexyl) peroxydicarbonate, dicyclohexyl peroxydicarbonate, di(4-tert-butylcycloahexy) peroxydicarbnate, dimyristyl peroxydicarbonate, diacetyl peroxydicarbonate, allyl perester, cuyml peroxyneodecanoate, tert-butyl per-3,5,5-trimethylhexanoate, acetylcyclohexylsulfonyl peroxide, dilauryl peroxide, dibenzoyl peroxide, and tert-amyl perneodecanoate.

11. The process according to claim 1 wherein the initiator c) consists of hydrogen peroxide and tert-butyl hydroperoxide.

Description

EXAMPLES

Example 1 (Comparative)

(1) 14.275 kg of sodium acrylate (37.5% by weight solution in water) and 1.367 kg of acrylic acid are mixed with 0.358 kg of water, 22 g of 15-tuply ethoxylated trimethylolpropane triacrylate and 80 g of EDTA (10% by weight 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), the solution was dropletized into a preheated dropletizing tower filled with a nitrogen atmosphere (180° C., height 12 m, width 2 m, gas velocity 0.1 m/s in cocurrent). The metering velocity was 16 kg/h. The dropletizer plate had 37 drillholes of 170 μm. The diameter of the dropletizer plate was 65 mm. The initiator was mixed with the monomer solution by means of a static mixer just upstream of the dropletizer.

(2) The water-absorbing polymer particles had the following properties:

(3) TABLE-US-00001 Residual monomers 0.19% by weight CRC: 40 g/g AUL0.7 psi 16 g/g Extractables   12% by weight

Example 2

(4) The procedure of Example 1 was repeated. In addition, 55 g of hydrogen peroxide (3% by weight solution in water) were metered in.

(5) The water-absorbing polymer particles had the following properties:

(6) TABLE-US-00002 Residual monomers 0.16% by weight CRC: 36 g/g AUL0.7 psi 20 g/g Extractables   9% by weight

Example 3

(7) The procedure of Example 1 was repeated. In addition, 110 g of hydrogen peroxide (3% by weight solution in water) were metered in.

(8) The water-absorbing polymer particles had the following properties:

(9) TABLE-US-00003 Residual monomers 0.12% by weight CRC: 33 g/g AUL0.7 psi 25 g/g Extractables   7% by weight