Superabsorbent polymer and method for preparing the same

Abstract

The superabsorbent polymer has absorption under pressure/under no pressure, permeability, and absorption speed that are suitable for the application in thin hygienic goods, and simultaneously, inhibits the generation of dust in the preparation process of hygienic goods and does not exhibit blocking in the preparation process of the superabsorbent polymer.

Claims

1. A superabsorbent polymer comprising: a base polymer powder comprising a first crosslinked polymer of water soluble ethylenically unsaturated monomers having acid groups, wherein at least a part of the acid groups are neutralized; and a surface crosslink layer formed on the base polymer powder, comprising a second crosslinked polymer obtained by the additional crosslinking of the first crosslinked polymer by a surface crosslinking agent, wherein centrifuge retention capacity (CRC) is 26 g/g or more, absorption under pressure of 0.7 psi (0.7 AUP) is 18 g/g or more, permeability dependent absorption under pressure (PDAUP) is 15 g/g or more, absorption speed measured according to a vortex measuring method is 80 seconds or less, and anticaking efficiency is 30% or more, wherein the surface crosslink layer comprises hydrophilic inorganic particles, and hydrophobic inorganic particles are included on a surface of the surface crosslink layer.

2. The superabsorbent polymer according to claim 1, wherein a dust number of the superabsorbent polymer is 3 or less.

3. The superabsorbent polymer according to claim 1, wherein the centrifuge retention capacity (CRC) is 28 g/g or more.

4. The superabsorbent polymer according to claim 1, wherein the absorption under pressure of 0.7 psi (0.7 AUP) is 20 g/g or more.

5. The superabsorbent polymer according to claim 1, wherein the permeability dependent absorption under pressure (PDAUP) is 16 g/g or more.

6. The superabsorbent polymer according to claim 1, wherein the absorption speed measured according to a vortex measuring method is 75 seconds or less.

7. The superabsorbent polymer according to claim 1, wherein the anticaking efficiency is measured by the following Mathematical Formula 1:
Anticaking efficiency (%)=(W.sub.1)/(W.sub.1+W.sub.2)×100  [Mathematical Formula 1] wherein, in Mathematical Formula 1, W.sub.1 is a weight of the superabsorbent polymer dropped on the bottom, after 2 g of the superabsorbent polymer is uniformly sprayed onto a glass Petri dish with an inner diameter of 95 mm, and then moisturized in a constant temperature and humidity chamber of a temperature of 40° C. and relative humidity of 80% for 10 minutes, taken out, and turned over for 5 minutes, and W.sub.2 is a weight of the superabsorbent polymer remaining in the glass Petri dish.

8. The superabsorbent polymer according to claim 1, wherein the anticaking efficiency is 40% or more.

9. A method for preparing a superabsorbent polymer comprising: conducting crosslinking polymerization of water soluble ethylenically unsaturated monomers having acid groups, wherein at least a part of the acid groups are neutralized, in the presence of an internal crosslinking agent, to form a hydrogel polymer comprising a first crosslinked polymer (step 1); coarsely grinding the hydrogel polymer, and drying and grinding to form a base polymer powder (step 2); conducting surface crosslinking of the base polymer powder by heat treatment, in the presence of a surface crosslinking solution comprising hydrophilic inorganic particles, to form superabsorbent polymer particles (step 3); and coating hydrophobic inorganic particles on the superabsorbent polymer particles (step 4).

10. The method for preparing a superabsorbent polymer according to claim 9, wherein the hydrophilic inorganic particles are silica particles or metal oxide particles.

11. The method for preparing a superabsorbent polymer according to claim 9, wherein the step 3 comprises surface crosslinking the base polymer powder by raising a temperature to 180° C. for 10 to 50 minutes, and heat treating at a temperature above 180° C. for 10 to 50 minutes.

12. The method for preparing a superabsorbent polymer according to claim 9, wherein the hydrophobic inorganic particles are silica particles or metal oxide particles of which surfaces are treated with a compound having a hydrophobic group.

13. The method for preparing a superabsorbent polymer according to claim 9, wherein the hydrophilic inorganic particles are included in a content of 0.002 to 0.25 parts by weight, based on 100 parts by weight of the base polymer.

14. The method for preparing a superabsorbent polymer according to claim 9, wherein the hydrophobic inorganic particles are included in a content of 0.001 to 0.15 parts by weight, based on 100 parts by weight of the base polymer.

15. The superabsorbent polymer according to claim 2, wherein the hydrophobic inorganic particles are surface-treated with hexamethyldisilazane, polydimethylsiloxane, or dimethyldichlorosilane.

16. The superabsorbent polymer according to claim 1, wherein water soluble ethylenically unsaturated monomers having acid groups is at least one of acrylic acid, methacrylic acid or a monovalent metal salt, a divalent metal salt, an ammonium salt, or an organic amine salt thereof.

17. The superabsorbent polymer according to claim 1, wherein a degree of neutralization is from 40 mol % to 95 mol %.

18. The superabsorbent polymer according to claim 1, wherein centrifuge retention capacity (CRC) is from 26 g/g to 45 g/g, absorption under pressure is from 0.7 psi (0.7 AUP) is 18 g/g to 28 g/g, permeability dependent absorption under pressure (PDAUP) is from 15 g/g to 24 g/g, absorption speed measured according to a vortex measuring method is from 20 seconds to 80 seconds, and anticaking efficiency is from 30% to 98%.

Description

EXAMPLE 1

(1) (Step 1)

(2) 100 parts by weight of acrylic acid was mixed with 0.2 parts by weight of polyethylene glycol diacrylate (weight average molecular weight: ˜500 g/mol) and 0.1 parts by weight of ethoxylated trimethylol propane triacrylate (weight average molecular weight: ˜700 g/mol) as internal crosslinking agents, and 0.01 parts by weight of IRGACURE 819 as a photoinitiator to prepare a monomer solution. Subsequently, the monomer solution was continuously fed with a metering pump, simultaneously with line mixing of 160 parts by weight of the aqueous solution of 24 wt % sodium hydroxide, thus preparing the aqueous solution of monomers. Here, a temperature increase by neutralization heat was controlled to 40° C. Further, after continuously line mixing 6 parts by weight of the aqueous solution of 4 wt % sodium persulfate, the solution was continuously fed to a continuous polymerization reactor having a planar polymerization belt at both ends. Thereafter, UV was irradiated for 1 minute, and then thermal polymerization was additionally conducted for 2 minutes to prepare a hydrogel.

(3) (Step 2)

(4) After cutting the hydrogel prepared in step 1 to the average size of about 300 mm or less, reassembled fine powders were additionally introduced into a grinder (equipped with a perforated panel including multiple holes having a diameter of 11 mm) and ground. Here, as the reassembled fine powders, the reassembled fine powders prepared in step 4 below was used, and the introduction rate was 18 wt % based on the hydrogel.

(5) (Step 3)

(6) The hydrogel ground in step 2 was dried in a dryer capable of transferring air volume up and down. Hot air at 180° C. was allowed to flow from the lower side to the upper side for 15 minutes, and to flow from the upper side to the lower side again for 15 minutes, so that the moisture content of the dried powders may become about 2%, thus uniformly drying the hydrogel.

(7) (Step 4)

(8) The polymer dried in step 3 was ground with a grinder and then sieved to obtain a base polymer with a size of 150 to 850 μm. Meanwhile, through the sieving, polymer particles having a particle diameter of less than 150 μm were assembled with water, and used as the reassembled fine powder of step 2.

(9) (Step 5)

(10) 100 parts by weight of the base polymer prepared in step 4 was mixed with a crosslinking solution including 3 parts by weight of water, 3 parts by weight of methanol, 0.5 parts by weight of 1,3-propanediol, and 0.1 parts by weight of Al.sub.2O.sub.3 particles (BET specific surface area 130 m.sup.2/g) as hydrophilic inorganic particles, and then the temperature was raised from room temperature to 180° C. for 25 minutes and maintained at 180° C. for 30 minutes, thus conducting surface crosslinking. The obtained product was cooled and sieved to obtain a surface-crosslinked superabsorbent polymer having a particle diameter of 150 to 850 μm.

(11) (Step 6)

(12) Based on 100 parts by weight of the superabsorbent polymer particles prepared in step 5, as hydrophobic inorganic particles, 0.05 parts by weight of SiO.sub.2 particles (BET specific surface area 140 m.sup.2/g) of which surface was treated with hexamethyldisilazane was coated using a mixer, thus preparing a superabsorbent polymer.

EXAMPLES 2 TO 8

(13) Each superabsorbent polymer was prepared by the same method as Example 1, except that the hydrophilic inorganic particles, surface crosslinking reaction, and hydrophobic inorganic particles described in the following Table 1 were used.

(14) TABLE-US-00001 TABLE 1 Surface Inorganic particles mixed in a crosslinking time Inorganic particles mixed surface crosslinking solution Time for Time for during post treatment Kind of content temperature maintenance Kind of content inorganic Surface (parts by rise to above inorganic Surface (parts by particles treatment weight) 180(minutes) 180(minutes) particles treatment weight) Example 1 hyhdrophilic Non 0.1 25 30 hydrophobic HMDS.sup.1) 0.05 Al.sub.2O.sub.3 treated SiO.sub.2 Example 2 hyhdrophilic Non 0.1 15 35 hydrophobic HMDS 0.05 Al.sub.2O.sub.3 treated SiO.sub.2 Example 3 hyhdrophilic Non 0.1 35 25 hydrophobic HMDS 0.05 Al.sub.2O.sub.3 treated SiO.sub.2 Example 4 hyhdrophilic Non 0.05 25 30 hydrophobic HMDS 0.03 Al.sub.2O.sub.3 treated SiO.sub.2 Example 5 hyhdrophilic Non 0.15 25 30 hydrophobic HMDS 0.08 Al.sub.2O.sub.3 treated SiO.sub.2 Example 6 hyhdrophilic Non 0.1 25 30 hydrophobic DDS.sup.2) 0.05 Al.sub.2O.sub.3 treated SiO.sub.2 Example 7 hyhdrophilic Non 0.1 25 30 hydrophobic HMDS 0.05 SiO.sub.2 treated SiO.sub.2 Example 8 hyhdrophilic Non 0.1 25 30 hydrophobic DDS 0.05 SiO.sub.2 treated SiO.sub.2 .sup.1)HMDS: Hexamethyldisilazane .sup.2)DPS: Dimethyldichlorosilane

COMPARATIVE EXAMPLES 1 TO 8

(15) Each superabsorbent polymer was prepared by the same method as Example 1, except that the hydrophilic inorganic particles, surface crosslinking reaction, and hydrophobic inorganic particles described in the following Table 2 were used.

(16) TABLE-US-00002 TABLE 2 Surface Inorganic particles mixed in a crosslinking time Inorganic particles mixed surface crosslinking solution Time for Time for during post treatment Kind of content temperature maintenance Kind of content inorganic Surface (parts by rise to above inorganic Surface (parts by particles treatment weight) 180(minutes) 180(minutes) particles treatment weight) Comparative Hydrophilic Non 0.1 55 20 Hydrophobic HMDS 0.05 Example 1 Al.sub.2O.sub.3 treated SiO.sub.2 Comparative Hydrophilic Non 0.1 15 55 Hydrophobic HMDS 0.05 Example 2 Al.sub.2O.sub.3 treated SiO.sub.2 Comparative Hydrophilic Non 0.1 35 5 Hydrophobic HMDS 0.05 Example 3 Al.sub.2O.sub.3 treated SiO.sub.2 Comparative (not used) 25 30 Hydrophobic HMDS 0.08 Example 4 SiO.sub.2 Comparative Hydrophilic Non 0.15 25 30 (not used) Example 5 Al.sub.2O.sub.3 treated Comparative Hydrophilic Non 0.3 25 30 Hydrophobic HMDS 0.03 Example 6 Al.sub.2O.sub.3 treated SiO.sub.2 Comparative Hydrophilic Non 0.05 25 30 Hydrophobic HMDS 0.2 Example 7 Al.sub.2O.sub.3 treated SiO.sub.2 Comparative Hydrophilic Non 0.1 25 30 Hydrophilic Non 0.1 Example 8 Al.sub.2O.sub.3 treated Al.sub.2O.sub.3 treated Comparative Hydrophilic Non 0.1 25 30 Hydrophilic Non 0.1 Example 9 Al.sub.2O.sub.3 treated SiO.sub.2 treated Comparative Hydrophobic HMDS.sup.1) 0.1 25 30 Hydrophobic HMDS 0.05 Example 10 SiO.sub.2 SiO.sub.2 Comparative Hydrophobic DDS.sup.2) 0.1 25 30 Hydrophobic HMDS 0.05 Example 11 SiO.sub.2 SiO.sub.2 .sup.1)HMDS: Hexamethyldisilazane .sup.2)DDS: Dimethyldichlorosilane

(17) Experimental Example: Evaluation of the Properties of Superabsorbent Polymer

(18) The properties of the superabsorbent polymers prepared in the examples and comparative examples were evaluated as follows.

(19) (1) Centrifuge Retention Capacity (CRC)

(20) For the superabsorbent polymers of the examples and comparative examples, centrifuge retention capacity (CRC) by absorption rate under no load was measured according to European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.3.

(21) Specifically, W.sub.0(g, 0.1 g) of the polymers of the examples and comparative examples were uniformly put in an envelope made of non-woven fabrics and sealed, and then soaked in a saline solution of a 0.9 wt % sodium chloride aqueous solution at room temperature. After 30 minutes, the envelope was drained using a centrifuge at 250 G for 3 minutes, and then the weight W.sub.2(g) was measured. After the same operation without using a superabsorbent polymer, the weight W.sub.1 (g) was measured.

(22) Using each mass obtained, CRC (g/g) was calculated according to the following mathematical formula.
CRC(g/g)={[W.sub.2(g)−W.sub.1(g)−W.sub.0(g)]/W.sub.0(g)}

(23) (2) Absorbing under Pressure (AUP)

(24) The absorption under pressure (AUP) of 0.7 psi of the superabsorbent polymers of the examples and comparative examples to a saline solution was measured according to the EDANA WSP 242.2 method.

(25) Specifically, on the bottom of a plastic cylinder having an inner diameter of 60 mm, a 400 mesh screen made of stainless was installed. Further, at room temperature and 50% humidity, W.sub.0 (g, 0.9 g) of the superabsorbent polymer of which absorption under pressure was to be measured was uniformly sprayed onto the screen. Subsequently, on the superabsorbent polymer, a piston capable of uniformly applying loads of 4.83 kPa (0.7 psi) was put. Here, a piston with an outer diameter of slightly smaller than 60 mm and no gap with the inner wall of the cylinder, and manufactured so as to freely move up and down, was used. The weight W.sub.1 (g) of the prepared apparatus was measured.

(26) Subsequently, inside a Petri dish having a diameter of 150 mm, a glass filer with a diameter of 90 mm and a thickness of 5 mm was put, and a 0.9 wt % saline solution was poured into the Petri dish. Here, the saline solution was poured until the surface of the saline solution became horizontal with the upper side of the glass filer. Further, on the glass filter, one piece of a filter paper with a diameter of 90 mm was put.

(27) Subsequently, the above-prepared apparatus was put on the filter paper, and the superabsorbent polymer in the apparatus was allowed to swell by the saline solution under load. After 1 hour, the weight W.sub.2 (g) of the apparatus containing the swollen superabsorbent polymer was measured. Using the measured weight, absorption under pressure was calculated according to the following mathematical formula.
AUP(g/g)=[W.sub.2(g)−W.sub.1(g)]/W.sub.0(g)

(28) (3) Permeability Dependent Absorption Under Pressure (PDAUP)

(29) The permeability dependent absorption under pressure of the superabsorbent polymers of the examples and comparative examples were measured according to EDANA WSP 243.1.

(30) Specifically, on the bottom of a plastic cylinder having an inner diameter of 60 mm, a 400 mesh wire netting made of stainless was installed. At room temperature and 50% humidity, W.sub.0 (5.0 g) of the superabsorbent polymer was uniformly sprayed onto the wire netting, and a piston capable of uniformly applying loads of 4.83 kPa (0.7 psi) was put thereon. The piston has an outer diameter of slightly smaller than 60 mm and no gap with the inner wall of the cylinder, and can freely move up and down. The weight W.sub.1 (g) of the apparatus was measured. Inside a Petri dish having a diameter of 150 mm, a glass filer with a diameter of 90 mm and a thickness of 5 mm was put, and a 0.9 wt % saline solution was poured into the Petri dish until the surface of the saline solution became the same level as the upper side of the glass filer. On the glass filter, one piece of a filter paper with a diameter of 90 mm was put. The above measuring apparatus was put on the filter paper, and the superabsorbent polymer was allowed to absorb the solution under load for 1 hour. After 1 hour, the measuring apparatus was lifted, and the weight W.sub.2 (g) was measured. Permeability dependent absorption under pressure was calculated according to the following Formula 3.
PDAUP(g/g)={(W.sub.2(g)−W.sub.1(g)}/W.sub.0(g)

(31) (4) Absorption Speed (Vortex Time)

(32) The absorption speed of the superabsorbent polymers of the examples and comparative examples were measured in the unit of seconds, according to the method described in International Patent Publication No. 1987-003208.

(33) Specifically, for the absorption speed (or vortex time), the superabsorbent polymer (2 g) was put in 50 mL of a saline solution at 23° C. to 24° C., the solution was stirred with a magnetic bar (diameter 8 mm, length 30 mm) at 600 rpm, and a time taken until the vortex disappeared was measured in the unit of seconds.

(34) (5) Anticaking Efficiency

(35) The anticaking efficiency of the superabsorbent polymers of the examples and comparative examples were measured.

(36) Specifically, in a glass Petri-dish with a diameter of 95 mm, the superabsorbent polymer (2 g) was uniformly distributed. The petro dish was put in a constant temperature and humidity chamber which is maintained at a temperature of 40° C. and a relative humidity of 80%, left for 10 minutes, and the Petri dish was turned over. After 5 minutes, the weight (W.sub.1) of the polymer dropped on the bottom and the weight (W.sub.2) of the superabsorbent polymer remaining in the Petri dish were measured, and the anticaking efficiency was calculated according to the following mathematical formula.
Anticaking efficiency (%)=(W.sub.1)/(W.sub.1+W.sub.2)×100

(37) (6) Dust Number

(38) Using a Dustview II device of Palas, Germany, dust numbers of the superabsorbent polymers (30 g) of the examples and comparative examples were measured.

(39) The measured results are shown in the following Table 3.

(40) TABLE-US-00003 TABLE 3 Anticaking CRC AUP PDAUP Vortex Dust Efficiency (g/g) (g/g) (g/g) (sec) Number (%) Example 1 30.3 23.2 18.1 62 0.6 83 Example 2 26.9 22.8 18.0 64 0.5 88 Example 3 30.1 18.5 15.3 63 0.7 80 Example 4 29.8 23.3 18.4 70 0.4 39 Example 5 30.2 21.6 16.9 60 1.8 93 Example 6 30.2 23.3 18.1 63 0.5 89 Example 7 30.1 22.5 17.5 62 0.4 84 Example 8 30.2 22.9 17.8 63 0.4 82 Comparative 25.8 17.5 9.0 84 2.3 25 Example 1 Comparative 23.5 15.5 10.3 98 2.8 29 Example 2 Comparative 33.3 13.3 5.4 81 2.1 18 Example 3 Comparative 28.4 22.9 14.3 95 2.5 28 Example 4 Comparative 27.9 21.5 14.9 83 2.4 2 Example 5 Comparative 28.5 13.3 8.8 75 4.8 22 Example 6 Comparative 26.3 14.9 9.3 86 10.5 83 Example 7 Comparative 27.7 16.4 13.5 77 2.3 13 Example 8 Comparative 27.8 16.2 13.1 75 2.2 15 Example 9 Comparative 25.8 17.3 14.5 81 3.3 65 Example 10 Comparative 25.5 17.8 14.7 82 3.6 63 Example 11