Superabsorbent polymer and preparation method thereof

Abstract

The present invention relates to a superabsorbent polymer having an improved adsorption rate and a high bulk density, and a preparation method thereof.

Claims

1. A superabsorbent polymer comprising a base polymer powder containing a crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group in which at least a part thereof is neutralized, wherein a plurality of pores having a diameter of 1 μm or greater are formed in the base polymer powder, wherein the crosslinked polymer has a crosslinked structure and comprises a layered silicate-based particle dispersed in the crosslinked structure, and wherein the superabsorbent polymer has an bulk density of 0.55 g/ml or greater, and wherein the layered silicate-based particle has a columnar structure in which the maximum diameter of the orthogonal cross-section is 1 nm to 100 nm and the height is 0.01 nm to 20 nm.

2. The superabsorbent polymer of claim 1, wherein the plurality of pores having a diameter of 1 μm or greater formed in the base polymer powder comprise micropores having a diameter of 10 μm to 100 μm.

3. The superabsorbent polymer of claim 1, wherein a time required to remove a vortex generated by stirring 50 ml of a 0.9 wt % NaCl solution at a rate of 600 rpm at room temperature is 60 seconds or less.

4. The superabsorbent polymer of claim 1, wherein the layered silicate-based particle comprises a unit crystal which includes a metal oxide layer containing a metal oxide and a silica layer containing silica which is formed on at least one surface of the metal oxide layer.

5. The superabsorbent polymer of claim 1, wherein the layered silicate-based particle is contained in an amount of 0.01 parts by weight to 30 parts by weight based on 100 parts by weight of the base polymer powder.

6. The superabsorbent polymer of claim 1, wherein the water-soluble ethylenically unsaturated monomer comprises at least one selected from the group consisting of an anionic monomer of acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, and 2-(meth)acrylamide-2-methylpropane sulfonic acid, and a salt thereof; a nonionic hydrophilic monomer of (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol (meth)acrylate, or polyethylene glycol (meth)acrylate; and an unsaturated monomer containing an amino group of (N,N)-dimethylaminoethyl(meth)acrylate or (N,N)-dimethylaminopropyl(meth)acrylamide, and a quaternary compound thereof.

7. The superabsorbent polymer of claim 1, wherein the crosslinked polymer comprises a crosslinked structure in which polymer chains of the water-soluble ethylenically unsaturated monomer are crosslinked through a cross-linkable functional group of an internal crosslinking agent.

8. The superabsorbent polymer of claim 1, wherein the crosslinked polymer comprises a crosslinked polymer which is polymerized in the presence of an internal crosslinking agent containing a polyfunctional acrylate-based compound having the water-soluble ethylenically unsaturated monomer having a plurality of ethylene oxide groups.

9. The superabsorbent polymer of claim 7, wherein the internal crosslinking agent comprises at least one selected from the group consisting of polyethylene glycol diacrylate (PEGDA), glycerin diacrylate, glycerin triacrylate, unmodified or ethoxylated trimethylolpropane triacrylate (TMPTA), hexanediol diacrylate, and triethylene glycol diacrylate.

10. A method for preparing the superabsorbent polymer of claim 1, comprising the steps of: crosslinking a monomer composition which contains the layered silicate-based particle, a foaming agent, an internal crosslinking agent, and the water-soluble ethylenically unsaturated monomer having an acidic group in which at least a part thereof is neutralized, and which is stirred at a rate of 1000 rpm or greater, to prepare a hydrogel polymer; and drying, pulverizing, and classifying the hydrogel polymer to form a base polymer powder, wherein a concentration of the water-soluble ethylenically unsaturated monomer contained in the monomer composition is 40% by weight to 60% by weight.

11. A method for preparing the superabsorbent polymer of claim 1, comprising the steps of: crosslinking a monomer composition which contains the layered silicate-based particle, a foaming agent, an internal crosslinking agent, and the water-soluble ethylenically unsaturated monomer having an acidic group in which at least a part thereof is neutralized, and which is stirred at a rate of 1000 rpm or greater to prepare a hydrogel polymer; and drying, pulverizing, and classifying the hydrogel polymer to form a base polymer powder, wherein the step of drying, pulverizing, and classifying the hydrogel polymer to form a base polymer powder comprises a course pulverization step of pulverizing the hydrogel polymer so that the particle diameter thereof is 2 mm to 10 mm, and wherein the course pulverizing step is carried out at a temperature of 50° C. or greater and a frequency of 15 Hz or greater.

12. A method for preparing the superabsorbent polymer of claim 1, comprising the steps of: crosslinking a monomer composition which contains the layered silicate-based particle, a foaming agent, an internal crosslinking agent, and the water-soluble ethylenically unsaturated monomer having an acidic group in which at least a part thereof is neutralized, and which is stirred at a speed of 1000 rpm or greater, to prepare a hydrogel polymer; and drying, pulverizing, and classifying the hydrogel polymer to form a base polymer powder, wherein the step of drying, pulverizing, and classifying the hydrogel polymer to form a base polymer powder comprises a course pulverization step of pulverizing the hydrogel polymer so that the diameter thereof is 2 mm to 10 mm, and wherein the method further comprises, before or after the course pulverization, a step of adding water in an amount of less than 20 parts by weight based on 100 parts by weight of the hydrogel polymer.

13. The method for preparing a superabsorbent polymer of claim 10, wherein the layered silicate-based particle is used in an amount of 1 to 50 parts by weight based on 100 parts by weight of the foaming agent.

14. The method for preparing a superabsorbent polymer of claim 10, wherein the foaming agent comprises at least one selected from the group consisting of azodicarbonamide, azodicarboxylamide, benzenesulfonyl hydrazide, dinitrosopentamethylenetetramine, toluenesulfonyl hydrazide, azobisisobutyronitrile, barium azodicarboxylate, and sodium bicarbonate.

15. The superabsorbent polymer of claim 8, wherein the internal crosslinking agent comprises at least one selected from the group consisting of polyethylene glycol diacrylate (PEGDA), glycerin diacrylate, glycerin triacrylate, unmodified or ethoxylated trimethylolpropane triacrylate (TMPTA), hexanediol diacrylate, and triethylene glycol diacrylate.

16. The method for preparing a superabsorbent polymer of claim 11, wherein the layered silicate-based particle is used in an amount of 1 to 50 parts by weight based on 100 parts by weight of the foaming agent.

17. The method for preparing a superabsorbent polymer of claim 12, wherein the layered silicate-based particle is used in an amount of 1 to 50 parts by weight based on 100 parts by weight of the foaming agent.

18. The method for preparing a superabsorbent polymer of claim 11, wherein the foaming agent comprises at least one selected from the group consisting of azodicarbonamide, azodicarboxylamide, benzenesulfonyl hydrazide, dinitrosopentamethylenetetramine, toluenesulfonyl hydrazide, azobisisobutyronitrile, barium azodicarboxylate, and sodium bicarbonate.

19. The method for preparing a superabsorbent polymer of claim 12, wherein the foaming agent comprises at least one selected from the group consisting of azodicarbonamide, azodicarboxylamide, benzenesulfonyl hydrazide, dinitrosopentamethylenetetramine, toluenesulfonyl hydrazide, azobisisobutyronitrile, barium azodicarboxylate, and sodium bicarbonate.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 schematically shows the structure of the unit crystals of the layered silicate-based particles used in examples.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(2) The present invention will now be described in more detail by way of examples. However, these examples are only for illustrative purposes, and the scope of the present invention is not limited by these examples.

Examples 1 and 2 and Comparative Examples 1 and 2: Preparation of Superabsorbent Polymer with Different Post-Addition Amounts of the Additives

Example 1

(3) 0.18 g of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide [IRGACURE 819] was added as a photopolymerization initiator to 226 g of acrylic acid and mixed for 5 minutes. Then, 4.1 g of polyethylene glycol diacrylate (Miramer M280) was added as a crosslinking agent thereto and mixed for 10 minutes to prepare a monomer solution.

(4) 1.6 g of Laponite RD was added as silicate-based particles to 155 g of ionized water and mixed for 30 minutes. Thereafter, 1.7 g of sodium persulfate was added as a thermal polymerization initiator thereto and dissolved in ionized water until completely dissolved. Then, 8.8 g of sodium bicarbonate was added as a foaming agent and mixed for 10 minutes to prepare a mixed aqueous solution.

(5) 192 g of ionized water was mixed with 661 g of 32% caustic soda (NaOH) to prepare a caustic soda solution.

(6) 483 g of acrylic acid was added to a 2 L-dual jacketed glass reactor, through which cooling water flowed at 20° C., and 54.7 g of the monomer solution was added thereto and mixed for 5 minutes. Then, the mixture was neutralized by introducing the caustic soda solution for 10 minutes. The temperature was raised to about 65° C. by neutralization heat, and the mixture was allowed to stand until it was cooled to 46° C. Then, 40.8 g of the mixed aqueous solution was added and mixed for 1 minute. Subsequently, the mixture was stirred by using a dynamic mechanical system (DMS) at a rate of 6000 rpm for a motor and 6900 rpm for a stirrer to prepare a monomer composition.

(7) The monomer composition was introduced into a feeder of a polymerization reactor consisting of a continuously moving conveyor belt, was irradiated with ultraviolet rays for 1 minute (irradiation amount: 2 mW/cm.sup.2) with a UV irradiator having intensity of 10 mW, was allowed to stand for 2 minutes, and was then cut to a size of 5 cm*5 cm, and then 100 g of ionized water (18.6 PHR) was added thereto and absorbed to obtain a hydrogel polymer.

(8) The hydrogel polymer was transferred to a cutting machine and then pulverized under conditions of 25° C. and 15.8 Hz. Subsequently, the pulverized hydrogel polymer was dried in a hot air dryer at 180° C. for 40 minutes, and the dried hydrogel polymer was pulverized with a hammer mill pulverizer. A polymer having a particle size (average particle size) of 150 μm to 850 μm was then classified using a sieve, and a polymer having a particle size (average particle size) of 300 μm to 600 μm was then classified to prepare a superabsorbent polymer.

Example 2

(9) A superabsorbent polymer was prepared in the same manner as in Example 1, except that no ionized water was added during the preparation of the hydrogel polymer.

Comparative Example 1

(10) A superabsorbent polymer was prepared in the same manner as in Example 1, except that 195 g (36.7 PHR) of ionized water was added during the preparation of the hydrogel polymer.

Comparative Example 2

(11) A superabsorbent polymer was prepared in the same manner as in Example 1, except that 145 g (27.0 PHR) of ionized water was added during the preparation of the hydrogel polymer.

Examples 3 to 5 and Comparative Example 3: Preparation of Superabsorbent Polymer with Different Monomer Concentrations

Example 3

(12) 0.45 g of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide [IRGACURE 819] as a photopolymerization initiator was added to 213.5 g of acrylic acid and mixed for 5 minutes to prepare a photoinitiator solution.

(13) 0.38 g of sucrose ester (Ryoto s-1670) was added to 150.3 g of acrylic acid, and then 10.8 g of polyethylene glycol diacrylate (Mw=508) [Miramer M280] was added as a crosslinking agent and mixed for 10 minutes to prepare a crosslinking agent solution.

(14) 1.9 g of Laponite RD was added as silicate-based particles to 154.7 g of ionized water and mixed for 30 minutes. Then, 3.5 g of sodium bicarbonate was added as a foaming agent and mixed for 10 minutes to prepare a mixed aqueous solution.

(15) 14.7 g of sodium persulfate was added as a thermal polymerization initiator to 132.6 g of ionized water and dissolved in ionized water until completely dissolved to prepare a thermal polymerization initiator solution.

(16) 117 g of ionized water was mixed with 624 g of 32% caustic soda (NaOH) to prepare a caustic soda solution.

(17) 428 g of acrylic acid was added to a 2 L-dual jacketed glass reactor, through which cooling water flowed at 20° C., and 17.5 g of the photoinitiator solution and 15.8 g of the crosslinking agent solution were added thereto and mixed for 5 minutes. The mixture was then neutralized by introducing the caustic soda solution for 10 minutes. The temperature was raised to about 65° C. by neutralization heat, and the mixture was allowed to stand until it was cooled to 46° C. Then, 9.2 g of the thermal initiator solution and 38.1 g of the foaming agent solution were added and mixed for 1 minute. The mixture was then stirred by using a dynamic mechanical system (DMS) at a rate of 6000 rpm for a pump motor and 6900 rpm for a stirrer to prepare a monomer composition. Herein, the concentration of the monomers included in the monomer composition was 45.9% by weight.

(18) The monomer composition was introduced into a feeder of a polymerization reactor consisting of a continuously moving conveyor belt, irradiated with ultraviolet rays for 1 minute (irradiation amount: 2 mW/cm.sup.2) with a UV irradiator having intensity of 10 mW, allowed to stand for 2 minutes, and then cut into a size of 5 cm*5 cm, and then 40 g of ionized water was added thereto and absorbed to obtain a hydrogel polymer.

(19) The hydrogel polymer was transferred to a cutting machine and then pulverized under conditions of 80° C. and 25 Hz. Then, the pulverized hydrogel polymer was dried in a hot air dryer at 180° C. for 40 minutes, and the dried hydrogel polymer was pulverized with a hammer mill pulverizer. A polymer having a particle size (average particle size) of 150 μm to 850 μm was then classified using a sieve, and a polymer having a particle size (average particle size) of 300 μm to 600 μm was classified again to prepare a superabsorbent polymer.

Example 4

(20) 0.43 g of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide [IRGACURE 8191] was added as a photopolymerization initiator to 204.8 g of acrylic acid and mixed for 5 minutes to prepare a photoinitiator solution.

(21) 0.36 g of sucrose ester (Ryoto s-1670) was added to 144.2 g of acrylic acid, and then 10.4 g of polyethylene glycol diacrylate (Mw=508) [Miramer M280] was added as a crosslinking agent and mixed for 10 minutes to prepare a crosslinking agent solution.

(22) 1.9 g of Laponite RD was added as silicate-based particles to 148.4 g of ionized water and mixed for 30 minutes. Then, 3.3 g of sodium bicarbonate was added as a foaming agent and mixed for 10 minutes to prepare a mixed aqueous solution.

(23) 14.1 g of sodium persulfate was added as a thermal polymerization initiator to 127.2 g of ionized water and completely dissolved in ionized water to prepare a thermal polymerization initiator solution.

(24) 163.4 g of ionized water was mixed to 598.5 g of 32% caustic soda (NaOH) to prepare a caustic soda solution.

(25) 410.7 g of acrylic acid was added to a 2 L-dual jacketed glass reactor, through which cooling water flowed at 20° C., and 16.8 g of the photoinitiator solution and 15.2 g of the crosslinking agent solution were added thereto and mixed for 5 minutes. Then, the mixture was neutralized by introducing the caustic soda solution for 10 minutes. The temperature was raised to about 65° C. by neutralization heat, and the mixture was allowed to stand until it was cooled to 46° C. Then, 8.8 g of the thermal initiator solution and 36.6 g of the foaming agent solution were added and mixed for 1 minute. The mixture was then stirred by using a dynamic mechanical system (DMS) at a rate of 6000 rpm for a pump motor and 6900 rpm for a stirrer to prepare a monomer composition. Herein, the concentration of the monomers included in the monomer composition was 44% by weight.

(26) The monomer composition was introduced into a feeder of a polymerization reactor consisting of a continuously moving conveyor belt, irradiated with ultraviolet rays for 1 minute (irradiation amount: 2 mW/cm.sup.2) with a UV irradiator having intensity of 10 mW, allowed to stand for 2 minutes, and then cut into a size of 5 cm*5 cm, and then 40 g of ionized water (18.6 PHR) was added thereto and absorbed to obtain a hydrogel polymer.

(27) The hydrogel polymer was transferred to a cutting machine and then pulverized under conditions of 80° C. and 25 Hz. Subsequently, the pulverized hydrogel polymer was dried in a hot air dryer at 180° C. for 40 minutes, and the dried hydrogel polymer was pulverized with a hammer mill pulverizer. A polymer having a particle size (average particle size) of 150 μm to 850 μm was classified using a sieve, and a polymer having a particle size (average particle size) of 300 μm to 600 μm was classified again to prepare a superabsorbent polymer.

Example 5

(28) 0.41 g of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide [IRGACURE 819] was added as a photopolymerization initiator to 195.4 g of acrylic acid and mixed for 5 minutes to prepare a photoinitiator solution.

(29) 0.36 g of sucrose ester (Ryoto s-1670) was added to 137.6 g of acrylic acid, and then 9.9 g of polyethylene glycol diacrylate (Mw=508) [Miramer M280] was added as a crosslinking agent and mixed for 10 minutes to prepare a crosslinking agent solution.

(30) 2.2 g of Laponite RD was added as silicate-based particles to 175.4 g of ionized water and mixed for 30 minutes. Then, 3.9 g of sodium bicarbonate was added as a foaming agent and mixed for 10 minutes to prepare a mixed aqueous solution.

(31) 13.5 g of sodium persulfate was added as a thermal polymerization initiator to 121.4 g of ionized water and completely dissolved therein to prepare a thermal polymerization initiator solution.

(32) 212.9 g of ionized water was mixed with 571.2 g of 32% caustic soda (NaOH) to prepare a caustic soda solution.

(33) 392 g of acrylic acid was added to a 2 L dual jacketed glass reactor through which cooling water flowed at 20° C., and 16.1 g of the photoinitiator solution and 14.5 g of the crosslinking agent solution were added thereto and mixed for 5 minutes. Thereafter, the mixture was neutralized by introducing the caustic soda solution for 10 minutes. The temperature was raised to about 65° C. by neutralization heat, and the mixture was allowed to stand until it was cooled to 46° C. Then, 8.4 g of the thermal initiator solution and 34.9 g of the foaming agent solution were added and mixed for 1 minute. The mixture was then stirred by using a dynamic mechanical system (DMS) at a rate of 6000 rpm for a motor and 6900 rpm for a stirrer to prepare a monomer composition. Herein, the concentration of the monomers included in the monomer composition was 42% by weight.

(34) The monomer composition was introduced into a feeder of a polymerization reactor consisting of a continuously moving conveyor belt, irradiated with ultraviolet rays for 1 minute (irradiation amount: 2 mW/cm.sup.2) with a UV irradiator having an intensity of 10 mW, allowed to stand for 2 minutes, and then cut into a size of 5 cm*5 cm, and then 40 g of ionized water was added thereto and absorbed to obtain a hydrogel polymer.

(35) The hydrogel polymer was transferred to a cutting machine and then pulverized under conditions of 80° C. and 25 Hz. Subsequently, the pulverized hydrogel polymer was dried in a hot air dryer at 180° C. for 40 minutes, and the dried hydrogel polymer was pulverized with a hammer mill pulverizer. Then, a polymer having a particle size (average particle size) of 150 μm to 850 μm was classified using a sieve, and a polymer having a particle size (average particle size) of 300 μm to 600 μm was classified again to prepare a superabsorbent polymer.

Comparative Example 3

(36) A superabsorbent polymer was prepared in the same manner as in Example 3, except that the concentration of the monomers included in the monomer composition during the preparation of the hydrogel polymer was adjusted to 39% by weight.

Examples 6 and 7 and Comparative Example 4: Preparation of Superabsorbent Polymer with Different Pulverization Rates

Example 6

(37) 0.18 g of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide [IRGACURE 819] as a photopolymerization initiator, and 0.44 g of sucrose ester (Ryoto s-1670) were added to 220.2 g of acrylic acid and mixed for 5 minutes, and then 4.0 g of polyethylene glycol diacrylate [Miramer M280] was added as a crosslinking agent and mixed to prepare a monomer solution.

(38) 1.6 g of Laponite RD was added as silicate-based particles to 155 g of ionized water and mixed for 30 minutes. Then, 4.4 g of sodium persulfate was added thereto as a thermal polymerization initiator and completely dissolved, and then 8.8 g of sodium bicarbonate was added as a foaming agent and mixed for 10 minutes to prepare a mixed aqueous solution.

(39) 192 g of ionized water was mixed to 661 g of 32% caustic soda (NaOH) to prepare a caustic soda solution.

(40) 483 g of acrylic acid was added to a 2 L dual jacketed glass reactor, through which cooling water flowed at 20 CC, and 54.8 g of the monomer solution was added and mixed for 5 minutes. Thereafter, the mixture was neutralized by introducing the caustic soda solution for 10 minutes. The temperature was raised to about 65° C. by neutralization heat, and the mixture was allowed stand until it was cooled to 46° C. Then, 41.4 g of the mixed aqueous solution was added and mixed for 1 minute. Thereafter, the mixture was stirred by using a dynamic mechanical system (DMS) at a rate of 6000 rpm for a pump motor and 6900 rpm for a stirrer to prepare a monomer composition.

(41) The monomer composition was introduced into a feeder of a polymerization reactor consisting of a continuously moving conveyor belt, irradiated with ultraviolet rays for 1 minute (irradiation amount: 2 mW/cm.sup.2) with a UV irradiator having an intensity of 10 mW, allowed to stand for 2 minutes, and then cut into a size of 5 cm*5 cm, and then 100 g of ionized water was added thereto and absorbed to obtain a hydrogel polymer.

(42) The hydrogel polymer was transferred to a cutting machine and then pulverized under conditions of 90° C. and 25 Hz. Subsequently, the pulverized hydrogel polymer was dried in a hot air dryer at 180° C. for 40 minutes, and the dried hydrogel polymer was pulverized with a hammer mill pulverizer. Then, a polymer having a particle size (average particle size) of 150 μm to 850 μm was classified using a sieve, and a polymer having a particle size (average particle size) of 300 μm to 600 μm was classified again to prepare a superabsorbent polymer.

Example 7

(43) 0.18 g of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide [IRGACURE 819] was added as a photopolymerization initiator to 225.5 g of acrylic acid and mixed for 5 minutes, and then 4.1 g of polyethylene glycol diacrylate [Miramer M280] was added as a crosslinking agent and mixed for 10 minutes to prepare a monomer solution.

(44) 1.6 g of Laponite RD was added as silicate-based particles to 155 g of ionized water and mixed for 30 minutes. Then, 1.7 g of sodium persulfate was added thereto as a thermal polymerization initiator and completely dissolved, and then 8.8 g of sodium bicarbonate was added as a foaming agent and mixed for 10 minutes to prepare a mixed aqueous solution.

(45) 192 g of ionized water was mixed with 661 g of 32% caustic soda (NaOH) to prepare a caustic soda solution.

(46) 483 g of acrylic acid was added to a 2 L dual jacketed glass reactor, through which cooling water flowed at 20° C., and 54.7 g of the monomer solution was added and mixed for 5 minutes. Then, the mixture was neutralized by introducing the caustic soda solution for 10 minutes. The temperature was raised to about 65° C. by neutralization heat, and the mixture was allowed to stand it was cooled to 46° C. Then, 40.8 g of the mixed aqueous solution was added and mixed for 1 minute. Thereafter, the mixture was stirred by using a dynamic mechanical system (DMS) at a rate of 6000 rpm for a pump motor and 6900 rpm for a stirrer to prepare a monomer composition.

(47) The monomer composition was introduced into a feeder of a polymerization reactor consisting of a continuously moving conveyor belt, irradiated with ultraviolet rays for 1 minute (irradiation amount: 2 mW/cm.sup.2) with a UV irradiator having an intensity of 10 mW, allowed to stand for 2 minutes, and then cut into a size of 5 cm*5 cm, and then 100 g of ionized water was added thereto and absorbed to obtain a hydrogel polymer.

(48) The hydrogel polymer was transferred to a cutting machine and then pulverized under conditions of 90° C. and 15.8 Hz. Subsequently, the pulverized hydrogel polymer was dried in a hot air dryer at 180° C. for 40 minutes, and the dried hydrogel polymer was pulverized with a hammer mill pulverizer. Then, a polymer having a particle size (average particle size) of 150 μm to 850 μm was classified using a sieve, and a polymer having a particle size (average particle size) of 300 μm to 600 μm was classified again to prepare a superabsorbent polymer.

Comparative Example 4

(49) A superabsorbent polymer was prepared in the same manner as in Example 6, except that the hydrogel polymer was pulverized under conditions of 90° C. and 10 Hz after transferring it to a cutting machine.

Experimental Examples: Measurement of Physical Properties of Superabsorbent Polymers Obtained in Examples and Comparative Examples

(50) The physical properties of the superabsorbent polymers prepared in the examples and comparative examples above were measured by the following methods, and the results thereof are shown in Tables 1 to 3.

Experimental Examples 1. Centrifuge Retention Capacity (CRC) for Physiological Saline Solution

(51) The centrifuge retention capacity (CRC) by water absorption capacity under a non-loading condition was measured for the superabsorbent polymers of the examples and comparative examples in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 241.2.

(52) That is, W.sub.0(g) (about 0.2 g) of the super absorbent polymers of the Examples and Comparative Examples were uniformly put in a nonwoven fabric-made bag, followed by sealing. Then, the bag was immersed in a physiological saline solution composed of a 0.9 wt % aqueous sodium chloride solution at room temperature. After 30 minutes, water was removed from the bag by centrifugation at 250 G for 3 minutes, and the weight W.sub.2(g) of the bag was then measured. Further, the same procedure was carried out without using the polymer, and then the resultant weight W.sub.1(g) was measured.

(53) Using the respective weights thus obtained, CRC (g/g) was calculated according to the following Mathematical Formula 1, and thus, the centrifuge retention capacity was confirmed.
CRC(g/g)={[W.sub.2(g)−W.sub.1(g)]/W.sub.0(g)}−1  [Calculation Formula 1]
In the above Calculation Formula 1,

(54) W.sub.0(g) is an initial weight (g) of the superabsorbent polymer,

(55) W.sub.1(g) is the weight of the device not including the super absorbent polymer, measured after dehydrating the same by using a centrifuge at 250 G for 3 minutes, and

(56) W.sub.2(g) is the weight of the device including the super absorbent polymer, measured after immersing and absorbing the super absorbent polymer into a 0.9 wt % physiological saline at room temperature for 30 minutes and then dehydrating the same by using a centrifuge at 250 G for 3 minutes.

Experimental Example 2. Extractable Content (EC)

(57) The amounts of the extractable content for the superabsorbent polymers of the Examples and Comparative Examples were measured in accordance to EDANA method No. WSP 270.3.

Experimental Example 3. Absorption Rate (Vortex-Test)

(58) 50 ml of a 0.9 wt % NaCl solution was added to a 100 ml beaker, and 2.00 g of each of the superabsorbent polymers according to the examples and comparative examples was added while stirring at 600 rpm using a stirrer. Then, the time until the vortex of the liquid caused by the stirring disappeared and a smooth surface was formed was measured.

Experimental Example 4. Bulk Density (B/D)

(59) 100 g of the superabsorbent polymers of the examples and comparative examples were flowed through a standard fluidity measuring device orifice and received in a container with a volume of 100 ml, the superabsorbent polymer was cut so as to be horizontal, and the volume of the superabsorbent polymer was adjusted to 100 ml. Then, the weight of only the superabsorbent polymer excluding the container was measured. The weight of only the superabsorbent polymer was then divided by 100 ml, which is the volume of the superabsorbent polymer, to obtain the bulk density corresponding to the weight of the superabsorbent polymer per unit volume.

Experimental Example 5. Moisture Content (%)

(60) With respect to the hydrogel polymers obtained in the examples and comparative examples, the moisture content was obtained as a value calculated by measuring the weight loss according to evaporation of water in the polymer during the drying process through infrared heating. In this case, the drying conditions were set so that the temperature was increased from room temperature to 180° C. and then the temperature was maintained at 180° C., and the total drying time was 20 minutes, including 5 minutes for the temperature rising step.

(61) TABLE-US-00001 TABLE 1 Experimental example results of superabsorbent polymers prepared in Examples 1 and 2 and Comparative Examples 1 and 2 Post- Centrifuge addition Retention Moisture amount Capacity Extractable Absorption B/D content Category (PHR) (g/g) content (%) rate (s) (g/ml) (%) Example 1 18.6 52.4 25.1 48 0.56 50.8 Example 2 0 56.2 28.1 42 0.55 47.1 Comparative 36.7 47.8 20.1 44 0.51 52.9 Example 1 Comparative 27.0 52.2 24.7 46 0.53 51.4 Example 2

Example 2

(62) As shown in Table 1, the superabsorbent polymers obtained in Examples 1 and 2 had a bulk density of 0.56 g/ml and 0.55 g/ml, respectively, confirming that the bulk density was increased compared to the superabsorbent polymers of Comparative Examples 1 and 2, which had a bulk density of 0.51 g/ml and 0.53 g/ml, respectively.

(63) TABLE-US-00002 TABLE 2 Experimental example results of superabsorbent polymers prepared in Examples 3 to 5 and Comparative Example 3 Concentration Centrifuge of monomer Retention Moisture composition Capacity Extractable Absorption B/D content Category (wt %) (g/g) content (%) rate (s) (g/ml) (%) Example 3 45.9 42.2 18.1 47 0.59 47.4 Example 4 44.0 45.2 18.6 54 0.58 50.4 Example 5 42 49.5 22.8 52 0.56 52.5 Comparative 39 52.0 24.3 50 0.54 54.1 Example 3

(64) As shown in Table 2, the superabsorbent polymers obtained in Examples 3 to 5 had a bulk density of 0.59 g/ml, 0.58 g/ml, and 0.56 g/ml, respectively, confirming that the bulk density was increased compared to the superabsorbent polymer of Comparative Example 3 which had a bulk density of 0.54 g/ml.

(65) Meanwhile, when the concentration of the monomer composition was excessively increased to more than 45.9% by weight, precipitation occurred in the reactants during the neutralization process, and the polymerization of the superabsorbent polymers could not be carried out.

(66) TABLE-US-00003 TABLE 3 Experimental example results of superabsorbent polymers prepared in Examples 6 and 7 and Comparative Example 4 Centrifuge Retention Moisture Pulverization Capacity Extractable Absorption B/D content Category rate (Hz) (g/g) content (%) rate (s) (g/ml) (%) Example 6 25 50.4 31.2 54 0.60 45.0 Example 7 15.8 49.7 22.9 45 0.56 45.5 Comparative 10 47.1 24.6 42 0.54 45.2 Example 4

(67) As shown in Table 3, the superabsorbent polymers obtained in Examples 6 and 7 had a high bulk density of 0.60 g/ml and 0.56 g/ml, respectively, confirming that the bulk density was increased compared to the superabsorbent polymer of Comparative Example 4, which had a bulk density of 0.54 g/ml.