Method for the production of superabsorbers

Abstract

A process for producing superabsorbents, comprising polymerization of a monomer solution and thermal surface postcrosslinking, wherein the monomer solution comprises at least 0.75% by weight of a hydroxyphosphonic acid or salts thereof, calculated on the basis of the total amount of monomer used, and at least 0.09% by weight of aluminum cations, calculated on the basis of the total amount of polymer particles used, is added to the polymer particles before, during or after the thermal surface postcrosslinking.

Claims

1. A process for producing superabsorbent particles by polymerizing a monomer solution or suspension comprising a) at least one ethylenically unsaturated monomer which bears an acid group and may have been 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, comprising i) polymerizing the monomer solution to give a polymer gel, ii) optionally comminuting the resulting polymer gel, iii) drying the polymer gel, iv) grinding and classifying the dried polymer gel to give polymer particles, v) thermally surface postcrosslinking the classified polymer particles, vi) optionally reclassifying the surface postcrosslinked polymer particles, vii) optionally recycling polymer particles removed in step iv) upstream of step iii) and viii) optionally recycling polymer particles removed in step vi) upstream of step iii), which comprises adding to the monomer solution prior to step i) at least 0.75% by weight of a hydroxyphosphonic acid or salts thereof, calculated on the basis of the total amount of monomer a) used, and adding to the polymer particles between step iv) and step vi) at least 0.09% by weight of aluminum cations, calculated on the basis of the total amount of polymer particles used.

2. The process according to claim 1, wherein the polymer particles removed in step vi) and recycled upstream of step iii) have a particle size of less than 150 μm.

3. The process according to claim 1, wherein 1-hydroxyethylidene-1,1′-diphosphonic acid is used as the hydroxyphosphonic acid.

4. The process according to claim 1, wherein the aluminum cations are added prior to step v).

5. The process according to claim 1, wherein the aluminum cation is used in the form of aluminum sulfate.

6. The process according to claim 1, wherein at least 0.8% by weight of a hydroxyphosphonic acid or salts thereof, calculated on the basis of the total amount of monomer a) used, is added to the monomer solution prior to step i).

7. The process according to claim 1, wherein at least 0.85% by weight of a hydroxyphosphonic acid or salts thereof, calculated on the basis of the total amount of monomer a) used, is added to the monomer solution prior to step i).

8. The process according to claim 1, wherein at least 0.1% by weight of aluminum cations, calculated on the basis of the total amount of polymer particles used, is added to the polymer particles after step iv).

9. The process according to claim 1, wherein at least 0.11% by weight of aluminum cations, calculated on the basis of the total amount of polymer particles used, is added to the polymer particles after step iv).

Description

EXAMPLES

Example 1

(1) A monomer solution which is composed of 406.8 g of acrylic acid, 4271.4 g of aqueous sodium acrylate solution (37.3% strength by weight), 130.4 g of water and 5.86 g of 3-tuply ethoxylated glycerol triacrylate (purity about 85% by weight) and has been freed of atmospheric oxygen with nitrogen gas for 30 minutes was polymerized in an LUK 8.0 K2 polymerization reactor having two axially parallel shafts (Coperion Werner & Pfleiderer GmbH & Co. KG; Stuttgart, Germany). Additionally added to the monomer solution were 77.29 g of an aqueous solution of the disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid (20% strength by weight). The polymerization was initiated by adding 15.84 g of aqueous sodium peroxodisulfate solution (15% strength by weight), 1.41 g of aqueous hydrogen peroxide (1% strength by weight) and 91.12 g of aqueous ascorbic acid solution (0.5% strength by weight). The polymerization had ended after about 30 minutes. The resulting polymer gel was ground three times with the aid of a commercial meat grinder with a 6 mm die plate, and dried in a laboratory drying cabinet at 175° C. for 90 minutes. The dried polymer was ground and sieved to a particle size of 150 to 710 μm. The base polymer thus produced had a centrifuge retention capacity (CRC) of 38 g/g.

(2) For surface postcrosslinking, 1000 g of base polymer were coated in an M5R plowshare mixer (Gebrüder Lödige Maschinenbau GmbH; Paderborn; Germany) at 23° C. and a shaft speed of 200 revolutions per minute by means of two two-phase spray nozzles with the following solutions:

(3) Solution I: 0.50 g of N-(2-hydroxyethyl)-2-oxazolidinone 0.50 g of propane-1,3-diol 33.10 g of aqueous aluminum sulfate solution (26.8% strength by weight)

(4) Solution II: 10.0 g of isopropanol

(5) After the spray application of the two solutions, the temperature was increased to 195° C. and the reaction mixture was held at this temperature and a shaft speed of 60 revolutions per minute for 75 minutes. Subsequently, the reaction mixture was resieved at 23° C. to a particle size of 150 to 710 μm.

(6) The superabsorbent particles obtained were analyzed. To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 1.

Example 2 (Comparative Example)

(7) The procedure was as in example 1. Rather than 77.29 g of an aqueous solution of the disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid (20% strength by weight), only 36.61 g of an aqueous solution of the disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid (20% strength by weight) were added to the monomer solution.

(8) The superabsorbent particles obtained were analyzed. To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 1.

Example 3 (Comparative Example)

(9) The procedure was as in example 1. Surface postcrosslinking was accomplished using, rather than 42.90 g of aqueous aluminum sulfate solution (26.8% strength by weight), only 11.10 g of aqueous aluminum sulfate solution (26.8% strength by weight).

(10) The superabsorbent particles obtained were analyzed. To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 1.

Example 4 (Comparative Example)

(11) The procedure was as in example 1. The monomer solution did not comprise any disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid. Instead, 15.46 g of the disodium salt of 2-hydroxy-2-sulfonatoacetic acid were added to the monomer solution.

(12) During the polymerization, the polymer gel formed wound around the shafts of the polymerization reactor. The experiment had to be stopped.

Example 5 (Comparative Example)

(13) The procedure was as in example 1. The monomer solution did not comprise any disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid.

(14) The superabsorbent particles obtained were analyzed. To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 1.

Example 6

(15) The procedure was as in example 1. No aluminum sulfate was used for surface postcrosslinking. Instead, 80.8 g of aqueous aluminum trilactate solution (18.9% strength by weight) were used.

(16) The superabsorbent particles obtained were analyzed. To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 1.

(17) TABLE-US-00001 TABLE 1 CRC AUL0.7 psi GBP Ex. Monomer solution SXL [g/g] [g/g] [darcies] L a b 1 9500 ppm Cublen 1500 ppm Al.sup.3+ 29.8 21.4 109 74 5.3 12 (as sulfate) 2*) 4500 ppm Cublen 1500 ppm Al.sup.3+ 30.4 21.5 101 63 7.3 15 (as sulfate) 3*) 9500 ppm Cublen  500 ppm Al.sup.3+ 31.1 23.3 26 85 0.9 8.6 (as sulfate) 4*) 9500 ppm Blancolen **) — — — — — — 5*) No Cublen 1500 ppm Al.sup.3+ 29.9 20.6 118 50 9.1 16.7 (as sulfate) 6*) 9500 ppm Cublen 1500 ppm Al.sup.3+ 29.3 25.3 17 79 3.7 11 (as lactate) SXL: surface postcrosslinking Cublen: disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid Blancolen: disodium salt of 2-hydroxy-2-sulfonatoacetic acid *)comparative experiment **) not possible, polymerization stopped

(18) The results of examples 1 to 3 show that superabsorbents having high color stability and high gel bed permeability (GBP) are obtained only with a large amount of hydroxyphosphonic acid (Cublen) and a large amount of aluminum cations.

(19) Example 4 shows that 2-hydroxysulfonic acids (Blancolen), in the amounts in the monomer solution that are necessary for color stabilization, interfere in the polymerization.

(20) Examples 1 and 6 show the advantages of sulfate over lactate as counterion.

Example 7

(21) By mixing water, aqueous sodium hydroxide solution and acrylic acid, a 43.0% strength by weight acrylic acid/sodium acrylate solution was prepared. The degree of neutralization of the monomer solution thus prepared was 72.0 mol %.

(22) The monomer solution was degassed with nitrogen at about 23° C. The polyethylenically unsaturated crosslinker used was 3-tuply ethoxylated glyceryl triacrylate (purity about 85% by weight). The use amount of 3-tuply ethoxylated glycerol triacrylate, calculated on the basis of the amount of acrylic acid used, was 0.16% by weight.

(23) Additionally added to the monomer solution was a 20% by weight aqueous solution of the disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid. The use amount of disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid, calculated on the basis of the amount of acrylic acid used, was 0.30% by weight.

(24) To initiate the free-radical polymerization, the following components were used: hydrogen peroxide (0.002% by weight of a 1.0% by weight aqueous solution), sodium peroxodisulfate (0.15% by weight of a 15% by weight aqueous solution), and ascorbic acid (0.01% by weight of a 0.5% by weight aqueous solution). The percentages by weight were calculated based on the total amount of acrylic acid used.

(25) The individual components were metered continuously into a List ORP 10 kneader reactor (List AG, Arisdorf, Switzerland). The throughput of the monomer solution was 40 kg/h.

(26) The reaction solution had a feed temperature of about 23° C. The dwell time of the reaction mixture in the reactor was about 15 minutes. After polymerization and gel comminution, 1 kg of aqueous polymer gel in each case was applied to drying sheets with a base of 300 μm sieve mesh and dried in an air circulation drying cabinet at 175° C. for 60 minutes. The dried polymer gel was ground and classified by means of a roll mill.

(27) To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 2.

Example 8

(28) The procedure was as in example 7. In the first third of the reactor, 0.86 kg/h of superabsorbent with a particle size of less than 150 μm was additionally added. The added superabsorbent particles had been thermally surface postcrosslinked and did not comprise any disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid.

(29) To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 2.

Example 9

(30) The procedure was as in example 7. In the first third of the reactor, 1.72 kg/h of superabsorbent with a particle size of less than 150 μm were additionally added. The added superabsorbent particles had been thermally surface postcrosslinked and did not comprise any disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid.

(31) To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 2.

Example 10

(32) The procedure was as in example 7. In the first third of the reactor, 2.58 kg/h of superabsorbent with a particle size of less than 150 μm were additionally added. The added superabsorbent particles had been thermally surface postcrosslinked and did not comprise any disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid.

(33) To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 2.

Example 11

(34) The procedure was as in example 7. In the first third of the reactor, 3.44 kg/h of superabsorbent with a particle size of less than 150 μm were additionally added. The added superabsorbent particles had been thermally surface postcrosslinked and did not comprise any disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid.

(35) To determine color stability, the superabsorbent particles were stored at 70° C. and a relative humidity of 80% for 14 days. The results are compiled in table 2.

(36) TABLE-US-00002 TABLE 2 Superabsorbent added Ex. (calculated based on polymer produced) Discolorations 7 none + 8  5% by weight − 9 10% by weight − 10 15% by weight −− 11 20% by weight −−− No visible point discolorations + Few visible point discolorations − Distinct, visible point discolorations −− Very distinct, visible point discolorations −−−

(37) The results of examples 7 to 11 show that the thermally surface postcrosslinked superabsorbent particles no longer absorb any disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid and do not become discolored during storage. The point discolorations increase with the amount of added superabsorbent.