PERMEABLE SUPERABSORBENT AND PROCESS FOR PRODUCTION THEREOF

Abstract

A highly permeable superabsorbent is prepared by a process comprising polymerizing an aqueous monomer solution comprising a) at least one ethylenically unsaturated monomer which bears acid groups and is optionally at least partly in salt form, b) at least one crosslinker, c) at least one initiator, d) optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers mentioned under a), and e) optionally one or more water-soluble polymers; drying the resulting polymer, optionally grinding the dried polymer and sieving the ground polymer, optionally surface postcrosslinking the dried and optionally ground and sieved polymer, adding x-ray-amorphous aluminum hydroxide powder after drying, grinding or sieving, and, if surface postcrosslinking is conducted, during or after this surface postcrosslinking, adding 0.1% to 10% by weight, based on the amount of polymer prior to the addition of the x-ray-amorphous aluminum hydroxide powder, of water after the addition of x-ray-amorphous aluminum hydroxide powder, wherein the water is added to the superabsorbent in at least three portions.

Claims

1. A process for producing a superabsorbent, comprising polymerizing an aqueous monomer solution comprising a) at least one ethylenically unsaturated monomer which bears an acid group and is optionally at least partly in salt form, 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 resulting polymer, optionally grinding the dried polymer and sieving the ground polymer, optionally surface postcrosslinking the dried and optionally ground and sieved polymer, adding x-ray-amorphous aluminum hydroxide powder after drying, grinding or sieving, and, if surface postcrosslinking is conducted, during or after this surface postcrosslinking, adding 0.1% to 10% by weight, based on the amount of polymer prior to the addition of the x-ray-amorphous aluminum hydroxide powder, of water after the addition of x-ray-amorphous aluminum hydroxide powder, wherein the water is added to the polymer in at least three portions.

2. The process according to claim 1, wherein the second and any consecutive portions of water are added after a time of at least 1 second following the addition of the previous portion.

3. The process according to claim 1, wherein 0.01% to 1% by weight, based on the amount of polymer prior to addition, of x-ray-amorphous aluminum hydroxide is added.

4. The process according to claim 3, wherein 0.1% to 0.5% by weight, based on the amount of polymer prior to addition, of x-ray-amorphous aluminum hydroxide is added.

5. The process according to claim 1, wherein the dried and optionally ground and sieved polymer is surface postcrosslinked with a postcrosslinker that forms covalent bonds with polar groups at the surface of the superabsorbent particles.

6. A superabsorbent obtained by the process of claim 1.

7. An article for absorption of fluids, comprising the superabsorbent obtained by the process of claim 1.

8. A process for producing articles for absorption of fluid, wherein the production of the articles involves addition of the superabsorbent obtained by the process of claim 1.

Description

EXAMPLES

[0174] The base polymer used in the examples 1-7 was prepared by polymerizing an aqueous monomer solution that comprised sodium acrylate and acrylic acid (corresponding to a neutralization level of the acrylic acid 71 mol %) in a concentration of 41% by weight (sodium acrylate plus acrylic acid based on the total amount), and also 0.75% by weight (based on unneutralized acrylic acid) of polyethylene glycol-4000 (polyethylene glycol having an average molar mass of 4000 g/mol) and 0.46% by weight (based on unneutralized acrylic acid) of triacrylate of triethoxylated glycerol. The initiator system used (based in each case on unneutralized acrylic acid) was 0.184% by weight of sodium persulfate, 0.0007% by weight of hydrogen peroxide and 0.0026% by weight of ascorbic acid. Polymerization was effected in a kneader. For better drying, the gel obtained was extruded and then dried and ground, and the sieve cut from 150 to 710 μm was obtained therefrom. The base polymer thus prepared had a CRC of 36.5 g/g and an AUL 0.3 psi of 14.6 g/g, and comprised 13.0% by weight of extractables. The particle size distribution obtained by means of sieve analysis was:

TABLE-US-00001 >850 μm <0.1% by weight 600-850 μm 10.6% by weight 300-600 μm 70.8% by weight 100-300 μm 18.0% by weight <100 μm <0.5% by weight

[0175] Base polymers of this kind are standard and also commercially available, for example from BASF SE, Ludwigshafen, Germany.

[0176] The mixer used in the examples was a Pflugschar® 5R-MK plowshare mixer with capacity 5 L, model VT 5R-MK, with a heating jacket from Gebr. Lödige Maschinenbau GmbH; Elsener Strasse 7-9, 33102 Paderborn, Germany. To measure the temperature of the product in the mixer, a thermocouple was introduced into the opening provided for the purpose in the mixer to such an extent that its tip was at a distance from the heated inner wall of the mixer and was within the product, but could not be impacted by the mixing tools. For additional aluminum hydroxide in examples 1-6, an identical mixer but without heating jacket and thermocouple was used.

[0177] The x-ray-amorphous aluminum hydroxide used in the examples was aluminum hydroxide dried gel, catalog no. 511066100, batch number 3048632 from Dr. Paul Lohmann GmbH KG, Hauptstrasse 2, 31860 Emmerthal, Germany. By scanning electron microscope, the powder is found to be in the form of spherical particles having diameters in the region of 20-25 μm, but also some smaller spheres in the region of 5-10 μm. By x-ray diffractogram (measured with a D8 Advance Serie 2 diffractometer from Bruker Corporation, 40 Manning Road, Billerca, Mass 01821, U.S.A., with multiple sample changer, Cu anode, divergence slit 0.1° with ASS and Lynx-Eye, 3° aperture), no diffraction mines were measured, which indicates a size of the primary crystallites of distinctly smaller than 2 nm.

Example 1 (Comparative)

[0178] 1.2 kg of superabsorbent base polymer were initially charged in the mixer. At 23° C. and a shaft speed of 200 revolutions per minute, by means of a nitrogen-driven two-phase spray nozzle, a solution of 0.08% by weight of ethylene glycol diglycidyl ether, 2.5% by weight of propane-1,2-diol and 3% by weight of water, based in each case on the base polymer, was sprayed on. Subsequently, the shaft speed was reduced to 60 revolutions per minute, and the product temperature was increased to 130° C. and then maintained for 30 minutes.

[0179] Directly thereafter (the product temperature at that point was about 100° C.), the superabsorbent obtained was mixed in a further mixer at a shaft speed of 200 revolutions per minute with 0.1% by weight, based on the superabsorbent, of x-ray-amorphous aluminum hydroxide (mixing time about one minute). After the aluminum hydroxide had been mixed in, by means of a nitrogen-driven two-phase nozzle, 4.0% by weight, based on the superabsorbent, of water was also sprayed on (at a rate of 38 g/min) and mixed in. Thereafter, the sieve cut of 150-710 μm was obtained.

[0180] The superabsorbent thus obtained was analyzed; the measurements obtained are reported in table 1.

Example 2 (Comparative)

[0181] Example 1 was repeated, except that the total amount of water was added in two portions of 2.0% by weight each, the second after mixing in the first one (about fifteen seconds of mixing time). Samples were taken after each water addition step. The measurements obtained are reported in table 1.

Example 3

[0182] Example 1 was repeated, except that the total amount of water was added in four portions of 1.0% by weight each, each one after mixing in the previous one (about fifteen seconds of mixing time). The measurements obtained are reported in table 1.

Evaluation of Examples 1-3

[0183] Comparison between examples 1 to 3 shows that adding the same amount of water in at least three portions rather than at once improves GBP without significantly affecting CRC and AUL.

Example 4

[0184] Example 3 was repeated, except that the amount of aluminum hydroxide was increased to 0.3% by weight.

Example 5

[0185] Example 3 was repeated, except that the amount of aluminum hydroxide was increased to 0.5% by weight.

Evaluation of Examples 3-5

[0186] Comparison between examples 3 to 5 show the influence of aluminum hydroxide amount on GBP. For the superabsorbent and water addition method of this example, more than 0.3% by weight of aluminum hydroxide do not further increase GBP.

Example 6 (Comparative)

[0187] Example 3 was repeated, except that the amount of aluminum hydroxide was increased to 0.3% by weight.

Example 7 (Comparative)

[0188] Example 3 was repeated, except that the amount of aluminum hydroxide was increased to 0.3% by weight.

Evaluation of Examples 6-7

[0189] Examples 6 and 7, as compared to 4 and 5, respectively, show that adding the same amount of water in at least three portions rather than at once improves GBP without significantly affecting CRC and AUL also at higher aluminium hydroxide levels.

Example 8

[0190] 1.2 kg of a standard commercial surface-postcrosslinked superabsorbent polymer was charged in the mixer.

[0191] At 23° C. and a shaft speed of 200 revolutions per minute, 0.3% by weight, based on the su- perabsorbent, of x-ray-amorphous aluminum hydroxide were added to the superabsorbent and mixed for fifteen seconds. Then, by means of a nitrogen-driven two-phase nozzle, four portions of 1.0% by weight each, based on the super-absorbent, of water were also sprayed on (at a rate of 38 g/min) and mixed in. The mixing time following the addition of each portion of water but the last was fifteen seconds. Following the last portion, the superabsorbent was mixed for 300 seconds. Following each addition of water and mixing, samples were taken and the sieve cut of 150-710 μm thereof was obtained.

[0192] The samples thus obtained were analyzed; the measurements obtained are reported in table 2.

Example 9

[0193] Example 8 was repeated, except that the mixing time following the addition of each portion of water but the last was sixty seconds.

Example 10

[0194] Example 8 was repeated, except that the mixing time following the addition of each portion of water but the last was three hundred seconds.

Example 11

[0195] Example 8 was repeated, except that the mixing time following the addition of each portion of water but the last was six hundred seconds.

Evaluation of Examples 8-11

[0196] Examples 8-11 show that extending the time between adding the portions of water does not further improve or decrease GBP.

TABLE-US-00002 TABLE 1 Total Sample AUL0.9 Al(OH).sub.3 Water Water after water CRC psi GBP Example [% by wt.] [% by wt.] Portions portion no. [g/g] [g/g] [g/g] 1 (comparative) 0.1 4 1 1 28.9 21.1 65 2 0.1 4 2 1 28.5 21.6 45 2 28.3 20.7 65 3 0.1 4 4 1 28.7 22.1 42 2 28.7 21.9 57 3 28.2 21.2 67 4 28.3 20.6 74 4 0.3 4 4 1 29.0 22.2 60 2 28.4 21.1 82 3 28.5 20.5 91 4 28.2 20.2 100 5 0.5 4 4 1 28.7 21.3 69 2 28.2 20.4 82 3 28.0 19.8 97 4 28.1 19.3 98 6 (comparative) 0.3 4 1 1 27.9 20.1 84 7 (comparative) 0.5 4 1 1 28.0 19.6 94

TABLE-US-00003 TABLE 2 Water Mixing AUL0.9 Al(OH).sub.3 Portion time CRC psi GBP Example [% by wt.] [% by wt.] [s] [g/g] [g/g] [g/g] SAP used for — — — 30.0  2.6 11 Examples 8-  8 0.3 — 15 — — — 1 15 30.0 18.7 77 1 15 29.6 17.6 84 1 15 29.9 17.8 82 1 300 29.7 17.3 90  9 0.3 — 15 — — — 1 60 30.2 18.9 76 1 60 30.2 17.9 83 1 60 29.7 17.2 93 1 300 30.1 17.1 91 10 0.3 — 15 — — — 1 300 30.5 17.9 87 1 300 30.2 17.1 86 1 300 29.8 17.2 90 1 300 29.6 16.3 90 11 0.3 — 15 — — — 1 600 30.4 17.9 81 1 600 30.5 16.9 80 1 600 30.0 16.8 92 1 300 29.8 16.5 91