Super absorbent polymer and method for producing same

Abstract

A super absorbent polymer according to the present invention has an excellent discoloration resistance property even under high temperature/high humidity conditions, while maintaining excellent absorption performance, and is preferably used for hygienic materials such as diapers, and thus can exhibit excellent performance.

Claims

1. A super absorbent polymer comprising: a base polymer powder comprising a first cross-linked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups, wherein the first cross-linked polymer is prepared by crosslinking the water-soluble ethylenically unsaturated monomer in the presence of an internal crosslinking agent, a foaming agent and a surfactant, wherein the surfactant is sodium dodecylsulfate; and a surface cross-linked layer formed on the base polymer powder and comprising a second cross-linked polymer in which the first cross-linked polymer is further cross-linked via a surface crosslinking agent, wherein the super absorbent polymer has: a 15-min gel-AUL at 0.3 psi of 13 g/g or more, an absorbency under load (AUL) of 18 g/g or more, a gel bed permeability (GBP) of 30 darcy or more, and a vortex time of 45 seconds or less, wherein the vortex time means the amount of time required until the vortex disappears after adding 2 g of a super absorbent polymer to 50 mL of physiological saline solution and then stirring the mixture at 600 rpm.

2. The super absorbent polymer of claim 1, wherein the super absorbent polymer has a 15-min gel-AUL at 0.6 psi of 12 g/g or more.

3. The super absorbent polymer of claim 1, wherein the super absorbent polymer has a centrifuge retention capacity (CRC) of 29 g/g or more.

4. The super absorbent polymer of claim 1, wherein the super absorbent polymer has an average particle diameter of 300 μm to 600 μm.

5. The super absorbent polymer of claim 1, wherein in the super absorbent polymer, a super absorbent polymer having a particle diameter of 300 μm to 600 μm is contained in an amount of 45 to 85% by weight.

6. A method for producing a super absorbent polymer, comprising the steps of: crosslinking a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent, a foaming agent and a surfactant to form a hydrogel polymer containing a first cross-linked polymer, wherein the surfactant is sodium dodecylsulfate; coarsely pulverizing the hydrogel polymer to prepare a hydrogel polymer having an average particle diameter of 1.0 mm to 2.0 mm; drying and pulverizing the hydrogel polymer to form a base polymer powder; and heat-treating and surface-crosslinking the base polymer powder in the presence of a surface crosslinking agent to form a surface cross-linked layer on the base polymer powder, wherein the surface cross-linked layer comprises a second cross-linked polymer in which the first cross-linked polymer is further cross-linked via the surface crosslinking agent, wherein the superabsorbent polymer comprises: the base polymer powder comprising the first cross-linked polymer of the water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups; and the surface cross-linked layer formed on the base polymer powder, wherein the super absorbent polymer has: a 15-min gel-AUL at 0.3 psi of 13 g/g or more, a gel bed permeability (GBP) of 30 darcy or more, and a vortex time of 45 seconds or less, wherein the vortex time means the amount of time required until the vortex disappears after adding 2 g of a super absorbent polymer to 50 mL of physiological saline solution and then stirring the mixture at 600 rpm.

7. The method for producing a super absorbent polymer of claim 6, wherein the foaming agent is sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, or magnesium carbonate.

8. The method for producing a super absorbent polymer of claim 7, wherein the foaming agent is used in an amount of 1500 ppmw or less based on the weight of the water-soluble ethylenically unsaturated monomer.

9. The method for producing a super absorbent polymer of claim 6, wherein the surfactant is used in an amount of 300 ppmw or less based on the weight of the water-soluble ethylenically unsaturated monomer.

10. A super absorbent polymer comprising: a base polymer powder comprising a first cross-linked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups, wherein the first cross-linked polymer is prepared by crosslinking the water-soluble ethylenically unsaturated monomer in the presence of an internal crosslinking agent, a foaming agent and a surfactant, wherein the surfactant is sodium dodecylsulfate; and a surface cross-linked layer formed on the base polymer powder and comprising a second cross-linked polymer in which the first cross-linked polymer is further cross-linked via a surface crosslinking agent, wherein the super absorbent polymer has: a 15-min gel-AUL at 0.3 psi of 13 g/g or more, a gel bed permeability (GBP) of 30 darcy or more, and a vortex time of 45 seconds or less, wherein the vortex time means the amount of time required until the vortex disappears after adding 2 g of a super absorbent polymer to 50 mL of physiological saline solution and then stirring the mixture at 600 rpm.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) Hereinafter, preferred examples are provided for better understanding of the invention. However, these Examples are given for illustrative purposes only and are not intended to limit the scope of the present invention thereto.

Example 1

(2) (Step 1)

(3) A solution (solution A) in which 9 g of 0.5% IRGACURE 819 initiator (80 ppmw based on the monomer composition) diluted with acrylic acid and 40 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed was prepared. Then, a solution (solution B) in which 2.1 g of 5% allyl methacrylate diluted with acrylic acid was mixed was prepared.

(4) 490 g of acrylic acid, the solution A and the solution B were injected into a 2 L glass reactor surrounded by a jacket through which a heat medium pre-cooled at 25° C. was circulated. Then, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about 70 mol %. 5 g of 2% sodium dodecylsulfate solution (solution D-1) diluted with water was prepared as a surfactant. Further, 20 g of 4% sodium bicarbonate solution (solution D-2) diluted with water and 30 g of 4% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about 45° C., the solutions D-1, D-2 and E previously prepared were injected into the mixed solution and mixed.

(5) (Step 2)

(6) Then, the mixed solution prepared in step 1 was poured in a Vat-type tray (15 cm in width×15 cm in length) installed in a square polymerizer which had a light irradiation device installed at the top and whose inside was preheated to 80° C. Subsequently, the mixed solution was irradiated with light. It was confirmed that a gel was formed on the surface after about 20 seconds from light irradiation, and it was confirmed that polymerization reaction occurred simultaneously with foaming after about 30 seconds from light irradiation. Then, the polymerization reaction was performed for additional 2 minutes, and the polymerized sheet was taken out and cut into a size of 3 cm×3 cm. Then, the cut sheet was subjected to a chopping process using a meat chopper to prepare crumbs. The average particle diameter of the prepared crumbs was 1.5 mm.

(7) (Step 3)

(8) Then, the crumbs prepared in step 2 were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes, so that the dried product had a water content of about 2% or less. The dried crumbs were pulverized using a pulverizer and classified to obtain a base polymer having a particle diameter of 150 to 850 μm. The base polymer thus prepared had a centrifuge retention capacity of 35.4 g/g. The centrifuge retention capacity was measured according to Experimental Example described below.

(9) (Step 4)

(10) Thereafter, 100 g of the base polymer prepared in step 3 was mixed with a crosslinking agent solution obtained by mixing 4 g of water, 1 g of ethylene carbonate, and 0.1 g of Aerosil 200 (Evonik), and then surface crosslinking reaction was carried out at 190° C. for 30 minutes. The resultant product was pulverized and sieved to obtain a surface-crosslinked super absorbent polymer having a particle diameter of 150 to 850 μm. 0.1 g of Aerosil 200 was dry-added to the obtained super absorbent polymer and mixed in a dry state to prepare a super absorbent polymer.

Example 2

(11) (Step 1)

(12) A mixed solution was prepared in the same manner as in step 1 of Example 1, except that 1.3 g of the solution B (solution in which 2.1 g of 5% allyl methacrylate diluted with acrylic acid was mixed) was used and 30 g of the solution D-2 (4% sodium bicarbonate solution diluted with water) was used.

(13) (Step 2)

(14) Then, the mixed solution prepared in step 1 was poured in a Vat-type tray (15 cm in width×15 cm in length) installed in a square polymerizer which had a light irradiation device installed at the top and whose inside was preheated to 80° C. Subsequently, the mixed solution was irradiated with light. It was confirmed that a gel was formed on the surface after about 20 seconds from light irradiation, and it was confirmed that polymerization reaction occurred simultaneously with foaming after about 30 seconds from light irradiation. Then, the polymerization reaction was performed for additional 2 minutes, and the polymerized sheet was taken out and cut into a size of 3 cm×3 cm. Then, the cut sheet was subjected to a chopping process using a meat chopper to prepare crumbs. The average particle diameter of the prepared crumbs was 1.7 mm.

(15) (Step 3)

(16) Then, the crumbs prepared in step 2 were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes, so that the dried product had a water content of about 2% or less. The dried crumbs were pulverized using a pulverizer and classified to obtain a base polymer having a particle diameter of 150 to 850 μm. The base polymer thus prepared had a centrifuge retention capacity of 36.1 g/g. The centrifuge retention capacity was measured according to Experimental Example described below.

(17) (Step 4)

(18) Thereafter, 100 g of the base polymer prepared in step 3 was mixed with a crosslinking agent solution obtained by mixing 4 g of water, 1 g of ethylene carbonate, 1 g of propylene carbonate and 0.1 g of Aerosil 200 (Evonik), and then surface crosslinking reaction was carried out at 190° C. for 30 minutes. The resultant product was pulverized and sieved to obtain a surface-crosslinked super absorbent polymer having a particle diameter of 150 to 850 μm. 0.1 g of alumina powder (Alu 130, Evonik) was dry-added to the obtained super absorbent polymer and mixed in a dry state to prepare a super absorbent polymer.

Example 3

(19) (Step 1)

(20) A mixed solution was prepared in the same manner as in step 1 of Example 1, except that 10 g of the solution D-1 (2% sodium dodecylsulfate solution diluted with water) was used.

(21) (Step 2)

(22) Then, the mixed solution prepared in step 1 was poured in a Vat-type tray (15 cm in width×15 cm in length) installed in a square polymerizer which had a light irradiation device installed at the top and whose inside was preheated to 80° C. Subsequently, the mixed solution was irradiated with light. It was confirmed that a gel was formed on the surface after about 20 seconds from light irradiation, and it was confirmed that polymerization reaction occurred simultaneously with foaming after about 30 seconds from light irradiation. Then, the polymerization reaction was performed for additional 2 minutes, and the polymerized sheet was taken out and cut into a size of 3 cm×3 cm. Then, the cut sheet was subjected to a chopping process using a meat chopper to prepare crumbs. The average particle diameter of the prepared crumbs was 1.2 mm.

(23) (Step 3)

(24) Then, the crumbs prepared in step 2 were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes, so that the dried product had a water content of about 2% or less. The dried crumbs were pulverized using a pulverizer and classified to obtain a base polymer having a particle diameter of 150 to 850 μm. The base polymer thus prepared had a centrifuge retention capacity of 34.2 g/g. The centrifuge retention capacity was measured according to Experimental Example described below.

(25) (Step 4)

(26) Thereafter, 100 g of the base polymer prepared in step 3 was mixed with a crosslinking agent solution obtained by mixing 4 g of water, 1 g of ethylene carbonate and 0.1 g of alumina powder (Alu 130, Evonik), and then surface crosslinking reaction was carried out at 190° C. for 30 minutes. The resultant product was pulverized and sieved to obtain a surface-crosslinked super absorbent polymer having a particle diameter of 150 to 850 μm. 0.1 g of alumina powder (Alu 130, Evonik) was dry-added to the obtained super absorbent polymer and mixed in a dry state to prepare a super absorbent polymer.

Comparative Example 1

(27) A solution (solution A) in which 9 g of 0.5% IRGACURE 819 initiator (80 ppmw based on the monomer composition) diluted with acrylic acid and 19 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed was prepared. Then, 13 g of a solution (solution B) of 5% trimethylolpropane triacrylate containing 9 mol % of ethylene oxide (Ethoxylated-TMPTA, TMP(EO)9TA, M-3190 manufactured by Miwon Specialty Chemical Co., Ltd.) diluted with acrylic acid was prepared.

(28) 470 g of acrylic acid, the solution A and the solution B were injected into a 2 L glass reactor surrounded by a jacket through which a heat medium pre-cooled at 25° C. was circulated. Then, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about 70 mol %. 30 g of 4% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about 45° C., the solution E previously prepared was injected into the mixed solution and mixed.

(29) Then, the polymerization and chopping processes were carried out in the same manner as in Example 1. The average particle diameter of the prepared crumbs was 3.4 mm. Then, the drying and pulverizing processes were carried out in the same manner as in Example 1. The base polymer thus prepared had a centrifuge retention capacity of 36.4 g/g. The centrifuge retention capacity was measured according to Experimental Example described below. Then, the surface crosslinking and after-treatment were carried out in the same manner as in Example 1 to obtain a super absorbent polymer.

Comparative Example 2

(30) A solution (solution A) in which 11 g of 0.5% IRGACURE 819 initiator (80 ppmw based on the monomer composition) diluted with acrylic acid and 36 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed was prepared.

(31) 480 g of acrylic acid and the solution A were injected into a 2 L glass reactor surrounded by a jacket through which a heat medium pre-cooled at 25° C. was circulated. Then, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about 70 mol %. 30 g of 4% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about 45° C., the solution E previously prepared was injected into the mixed solution and mixed.

(32) Then, the polymerization and chopping processes were carried out in the same manner as in Example 1. The average particle diameter of the prepared crumbs was 3.8 mm. Then, the drying and pulverizing processes were carried out in the same manner as in Example 1. The base polymer thus prepared had a centrifuge retention capacity of 39.7 g/g. The centrifuge retention capacity was measured according to Experimental Example described below. Then, the surface crosslinking and after-treatment were carried out in the same manner as in Example 1 to obtain a super absorbent polymer.

Comparative Example 3

(33) A solution (solution A) in which 11 g of 0.5% IRGACURE 819 initiator (80 ppmw based on the monomer composition) diluted with acrylic acid and 42 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed was prepared.

(34) 470 g of acrylic acid and the solution A were injected into a 2 L glass reactor surrounded by a jacket through which a heat medium pre-cooled at 25° C. was circulated. Then, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about 70 mol %. 1.8 g of 1% Sugar ester (S-1670) (solution D-3) diluted with acrylic acid was prepared as a surfactant. Further, 15 g of 4% sodium bicarbonate solution (solution DA-4) diluted with water and 30 g of 4% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about 45° C., the solutions D-3, D-4 and E previously prepared were injected into the mixed solution and mixed.

(35) Then, the polymerization and chopping processes were carried out in the same manner as in Example 1. The average particle diameter of the prepared crumbs was 2.8 mm. Then, the drying and pulverizing processes were carried out in the same manner as in Example 1. The base polymer thus prepared had a centrifuge retention capacity of 38.4 g/g. The centrifuge retention capacity was measured according to Experimental Example described below. Then, the surface crosslinking and after-treatment were carried out in the same manner as in Example 1 to obtain a super absorbent polymer.

Comparative Example 4

(36) A solution (solution A) in which 11 g of 0.5% IRGACURE 819 initiator (80 ppmw based on the monomer composition) diluted with acrylic acid and 32 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed was prepared.

(37) 470 g of acrylic acid and the solution A were injected into a 2 L glass reactor surrounded by a jacket through which a heat medium pre-cooled at 25° C. was circulated. Then, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After confirming that the temperature of the mixture increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. The degree of neutralization of acrylic acid in the mixed solution thus obtained was about 70 mol %. 2.1 g of 1% Sugar ester (S-1670) (solution D-5) diluted with acrylic acid was prepared as a surfactant. Further, 30 g of 4% sodium persulfate solution (solution E) diluted with water were prepared. Then, when the mixed solution was cooled to about 45° C., the solutions D-3 and E previously prepared were injected into the mixed solution and mixed.

(38) Then, the polymerization and chopping processes were carried out in the same manner as in Example 1. The average particle diameter of the prepared crumbs was 4.4 mm. Then, the drying and pulverizing processes were carried out in the same manner as in Example 1. The base polymer thus prepared had a centrifuge retention capacity of 44.5 g/g. The centrifuge retention capacity was measured according to Experimental Example described below. Then, the surface crosslinking and after-treatment were carried out in the same manner as in Example 1 to obtain a super absorbent polymer.

Comparative Example 5

(39) For comparison, a product (product name: IM-930), produced and sold commercially by Sandia, was used as Comparative Example 5.

Experimental Example: Evaluation of Physical Properties of Super Absorbent Polymer

(40) The physical properties of the super absorbent polymer prepared in Examples and Comparative Examples were evaluated by the following methods, and the results are shown in Table 1 below.

(41) (1) Absorbency Under Load (AUL)

(42) The absorbency under load (AUL) at 0.9 psi for a physiological saline solution was measured for each super absorbent polymer prepared in Examples and Comparative Examples according to EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 242.2. Among the super absorbent polymers to be measured, super absorbent polymers which was passed through a US standard 30 mesh screen and retained on a US standard 50 mesh screen, were selectively classified to obtain the super absorbent polymers having a particle size of 300 μm to 600 μm, and the AUL thereof was measured.

(43) Specifically, a 400 mesh metal made of stainless steel was installed at the bottom of a plastic cylinder having an inner diameter of 25 mm. W.sub.0 (g, about 0.16 g) of a super absorbent polymer for measuring the absorbency under load were uniformly scattered on the screen at room temperature and relative humidity of 50%. Then, a piston capable of uniformly providing a load of 6.3 kPa (0.9 psi) was put thereon. At this time, the piston used was designed so that the outer diameter was slightly smaller than 25 mm and thus it could move freely up and down without any gap with the inner wall of the cylinder. Then, the weight W.sub.1(g) of the device thus prepared was measured.

(44) A glass filter having a diameter of 90 mm and a thickness of 5 mm was put inside a Petri dish having the diameter of 150 mm, and then 0.9 wt % of a physiological saline solution was poured in the Petri dish. At this time, the physiological saline solution was poured until the surface level became equal to the upper surface of the glass filter. Then, a sheet of filter paper having a diameter of 90 mm was put on the glass filter.

(45) Subsequently, the prepared device was placed on the filter paper so that the super absorbent polymer in the device was swelled by a physiological saline solution under load. After one hour, the weight W.sub.2(g) of the device containing the swollen super absorbent polymer was measured.

(46) Using the weight thus measured, the absorbency under load was calculated according to the following Mathematical Formula 1.
AUP(g/g)=[W.sub.2(g)−W.sub.1(g)]/W.sub.0(g)  [Mathematical Formula 1]

(47) in Mathematical Formula 1,

(48) W.sub.0(g) is an initial weight (g) of the super absorbent polymer, W.sub.1(g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, and W.sub.2(g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, after absorbing a physiological saline solution to the super absorbent polymer under a load (0.9 psi) for 1 hour.

(49) (2) Gel-AUL

(50) The Gel-AUL at 0.3 psi was measured using the same device as that used in the ‘(1) Absorbency under load (AUL)’. Among the super absorbent polymers to be measured, super absorbent polymers which was passed through a US standard 30 mesh screen and retained on a US standard 50 mesh screen, were selectively classified to obtain the super absorbent polymers having a particle size of 300 μm to 600 μm, and the Gel-AUL thereof was measured.

(51) Specifically, the resin W.sub.0 (g, 0.16 g) obtained in Examples and Comparative Examples was put into an AUL kit used in the ‘(1) Absorbency under load (AUL)’, and a piston capable of providing a load of 0.3 psi was put thereon. Then, the weight W.sub.3 (g) of the device thus prepared was measured. Subsequently, the piston was removed, and the super absorbent polymer was immersed in 1.5 g of physiological saline solution under no load to perform a primary swelling. Then, the physiological saline solution was absorbed under a load of 0.3 psi for 15 minutes and then subjected to a vacuum desorption under a vacuum pressure of 5 psi for 30 seconds to remove a physiological saline solution existing between gels. The weight W.sub.4 (g) of AUL kit including the physiological saline solution wholly containing gel therein was measured.

(52) Using the respective weights thus obtained, the 15-min Gel-AUL at 0.3 psi was calculated according to the following Mathematical Formula 2.
15-min gel-AUL at 0.3 psi(g/g)=[W.sub.4(g)−W.sub.3(g)]/W.sub.0(g)  [Mathematical Formula 2]

(53) in Mathematical Formula 2,

(54) W.sub.0(g) is an initial weight (0.16 g) of the super absorbent polymer,

(55) W.sub.3(g) is the total sum (g) of an initial weight of the super absorbent polymer and a weight of the device capable of providing a load of 0.3 psi to the super absorbent polymer, and

(56) W.sub.4(g) is the total sum (g) of a weight of the super absorbent polymer and a weight of the device capable of providing a load of 0.3 psi to the super absorbent polymer, after absorbing 1.5 g of a physiological saline solution to the super absorbent polymer under no load to perform a primary swelling, absorbing the physiological saline solution under a load of 0.3 psi for 15 minutes and then subjecting to vacuum desorption under a vacuum pressure of 5 psi for 30 seconds.

(57) In addition, the gel-AUL at 0.6 psi was measured in the same manner as described above, except that a load of 0.3 psi was changed to 0.6 psi.

(58) (3) Centrifuge Retention Capacity (CRC)

(59) The centrifuge retention capacity (CRC) by water absorption capacity under a non-loading condition was measured for the super absorbent polymers of Examples and Comparative Examples in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 241.3. Among the super absorbent polymers to be measured, super absorbent polymers which was passed through a US standard 30 mesh screen and retained on a US standard 50 mesh screen, were selectively classified to obtain the super absorbent polymers having a particle size of 300 μm to 600 μm, and the CRC thereof was measured.

(60) Specifically, W.sub.0 (g, about 0.2 g) of the super absorbent polymers of 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 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.6(g) of the bag was then measured. Further, the same procedure was carried out without using the super absorbent polymer, and then the resultant weight W.sub.5(g) was measured.

(61) Using the respective weights thus obtained, CRC (g/g) was calculated according to the following Mathematical Formula 3.
CRC(g/g)={[W.sub.6(g)−W.sub.5(g)−W.sub.0(g)]/W.sub.0(g)}  [Mathematical Formula 3]

(62) in Mathematical Formula 3,

(63) W.sub.0(g) is an initial weight (g) of the super absorbent polymer,

(64) W.sub.5(g) is the weight of the device not including the super absorbent polymer, measured after immersing and absorbing the device into a physiological saline solution for 30 minutes and then dehydrating the same by using a centrifuge at 250 G for 3 minutes, and

(65) W.sub.6(g) is the weight of the device including the super absorbent polymer, measured after immersing and absorbing the super absorbent polymer into a physiological saline solution at room temperature for 30 minutes and then dehydrating the same by using a centrifuge at 250 G for 3 minutes.

(66) (4) Gel Bed Permeability (GBP)

(67) The free swell Gel Bed Permeability (GBP) for a physiological saline solution was measured for each super absorbent polymer prepared in Examples and Comparative Examples according to the following method described in Korean Patent Application No. 10-2014-7018005.

(68) Specifically, the apparatus shown in FIGS. 1 to 3 was used to measure the free swell GBP. First, the plunger 536 installed with the weight 548 was placed in an empty sample container 530, and the height from the top of the weight 548 to the bottom of the sample container 530 was measured to an accuracy of 0.01 mm using an appropriate gauge. The force to which the thickness gauge applied during the measurement was adjusted to less than about 0.74 N.

(69) Meanwhile, among the super absorbent polymers for measuring GBP, super absorbent polymers which was passed through a US standard 30 mesh screen and retained on a US standard 50 mesh screen, were selectively classified to obtain the super absorbent polymers having a particle size of 300 μm to 600 μm.

(70) About 2.0 g of the super absorbent polymer classified in this way was placed in the sample container 530 and spread out evenly on the bottom of the sample container. Then, the container not containing the plunger 536 and the weight 548 therein, was submerged in 0.9 wt % physiological saline solution for about 60 minutes and allowed the super absorbent polymer to swell under no load condition. At this time, the sample container 530 was placed on the mesh located in a liquid reservoir so that the sample container 530 was raised slightly above the bottom of the liquid reservoir. As the mesh, those which did not affect the movement of the physiological saline solution into the sample container 530 were used. During saturation, the height of the physiological saline solution was allowed to be adjusted such that the surface within the sample container was defined by the swollen super absorbent polymer, rather than the physiological saline solution.

(71) At the end of this period, the assembly of the plunger 536 and weight 548 was placed on the swollen super absorbent polymer 568 in the sample container 530 and then the sample container 530, plunger 536, weight 548 and swollen super absorbent polymer 568 were removed from the solution. Thereafter, before GBP measurement, the sample container 530, plunger 536, weight 548 and swollen super absorbent polymer 568 were placed on a flat, large grid non-deformable plate of uniform thickness for about 30 seconds. The height from the top of the weight 548 to the bottom of the sample container 530 was measured again by using the same thickness gauge as previously used. Then, the height measurement value of the device in which the plunger 536 equipped with the weight 548 was placed in the empty sample container 530 was subtracted from the height measurement value of the device including the swollen super absorbent polymer 568, thereby obtaining the thickness or height “H” of the swollen super absorbent polymer.

(72) For the GBP measurement, 0.9 wt % physiological saline solution was flowed into the sample container 530 containing the swollen super absorbent polymer 568, the plunger 536 and the weight 548. The flow rate of a physiological saline solution into the sample container 530 was adjusted to cause the physiological saline solution to overflow the top of the cylinder 534, thereby resulting in a consistent head pressure equal to the height of the sample container 530. Then, the quantity of solution passing through the swollen super absorbent polymer 568 versus time was measured gravimetrically using the scale 602 and beaker 603. Data points from the scale 602 were collected every second for at least sixty seconds once the overflow has started. The flow rate (Q) passing through the swollen super absorbent polymer 568 was determined in units of grams/second (g/s) by a linear least-square fit of fluid passing through the sample 568 (in grams) versus time (in seconds).

(73) Using the data thus obtained, the GBP (cm.sup.2) was calculated according to the following Mathematical Formula 4.
K=[Q×H×μ]/[A×ρ×P]  [Mathematical Formula 4]

(74) in Mathematical Formula 4

(75) K is a gel bed permeability (cm.sup.2),

(76) Q is a flow rate (g/sec),

(77) H is a height of swollen super absorbent polymer (cm),

(78) μ is a liquid viscosity (poise) (about 1 cP for the physiological saline solution used with this Test),

(79) A is a cross-sectional area for liquid flow (28.27 cm.sup.2 for the sample container used with this Test),

(80) ρ is a liquid density (g/cm.sup.3) (about 1 g/cm.sup.3, for the physiological saline solution used with this Test), and

(81) P is a hydrostatic pressure (dynes/cm.sup.2) (normally about 7,797 dyne/cm.sup.2).

(82) The hydrostatic pressure was calculated from P=ρ×g×h, where ρ is a liquid density (g/cm.sup.3), g is a gravitational acceleration (nominally 981 cm/sec.sup.2), and h is a fluid height (for example, 7.95 cm for the GBP Test described herein).

(83) At least two samples were tested and the results were averaged to determine the free swell GBP of the super absorbent polymer, and the unit was converted to darcy (1 darcy=0.98692×10.sup.−8 UW) and shown in Table 1 below.

(84) (5) Absorption Rate (Vortex Time)

(85) The absorption rate of the super absorbent polymers prepared in Examples and Comparative Examples was measured in second unit according to the method described in International Publication WO1987/003208.

(86) Specifically, the absorption rate (vortex time) was calculated by a process in which 2 g of the super absorbent polymer was added to 50 mL of physiological saline solution at 23° C. to 24° C., and stirred at 600 rpm by a magnetic stirring bar (diameter 8 mm, length 31.8 mm), and the time required for the vortex to disappear was determined in second unit.

(87) The above measurement results are shown in Table 1 below.

(88) TABLE-US-00001 TABLE 1 Gel-AUL (g/g) CRC AUL GBP Vortex 0.3 psi 0.6 psi (g/g) (g/g) (darcy) (sec) Ex. 1 15.2 12.3 31 18.5 50 43 Ex. 2 18.5 14.1 30.5 19.3 62 41 Ex. 3 19.1 15.4 30 19.7 48 36 Comparative Ex. 1 13.8 11.3 31.5 19 55 85 Comparative Ex. 2 12.3 9.3 34 15 25 85 Comparative Ex. 3 14.8 6.9 34 11 25 35 Comparative Ex. 4 12.6 8.8 37 12 15 90 Comparative Ex. 5 14.6 10.8 30 16 30 30