Super absorbent polymer and method for preparing the same

Abstract

The present invention relates to a super absorbent polymer having not only excellent absorption rate and absorbency under load but also excellent rewetting properties, and a method for preparing the same.

Claims

1. A super absorbent polymer comprising: a base polymer powder containing a first crosslinked polymer of a water-soluble ethylene-based unsaturated monomer having an acidic group in which at least a part thereof is neutralized; and a surface-crosslinked layer containing a second crosslinked polymer formed on the base polymer powder in which the first crosslinked polymer is additionally crosslinked via a surface crosslinking agent, wherein the superabsorbent polymer is prepared by: 1) performing thermal polymerization or photopolymerization of a monomer composition comprising the water-soluble ethylene-based unsaturated monomer having the acidic group in which at least the part thereof is neutralized, a carbonate forming agent, an anionic surfactant and a polymerization initiator to form a hydrogel polymer, wherein the anionic surfactant is dioctyl sodium sulfosuccinate, 2) drying the hydrogel polymer, 3) pulverizing the dried polymer, and 4) performing surface cros slinking reaction of the pulverized polymer; and wherein the super absorbent polymer has: an average particle diameter of 300 μm to 600 μm, CRC of 28 g/g to 35 g/g, AUL (0.9 psi) of 18 g/g to 25 g/g, 10-min WAUL of 15 g/g to 25 g/g, a gel bed permeability (GBP) for a physiological saline solution of 30 darcy or more, 5-min Gel-Vacuum AUL of 18 g/g to 25 g/g, and a vortex time of 5 seconds to 50 seconds, wherein the WAUL is measured according to the following Equation 1:
WAUL(g/g)=[W.sub.2(g)−W.sub.1(g)]/ W.sub.0(g)   [Equation 1] in Equation 1, 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, 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 below 15 cm from the super absorbent polymer under a load (0.7 psi) for 10 minutes, 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, and wherein the 5-min Gel-Vacuum AUL is measured according to the following Equation 5:
5-min Gel-Vacuum AUL(g/g)=[W.sub.8(g)−W.sub.7(g)]/ W.sub.0(g)   [Equation 5] in Equation 5, W.sub.0(g) is an initial weight(g) of the super absorbent polymer, W.sub.7(g) is the total sum of the weight of the super absorbent polymer and the weight of the device capable of providing a load to the super absorbent polymer, and W.sub.8(g) is the total sum of the weight of the super absorbent polymer and the 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 (0.3 psi) for 5 minutes and removing the remaining liquid with a vacuum device.

2. A super absorbent polymer comprising: a base polymer powder containing a first crosslinked polymer of a water-soluble ethylene-based unsaturated monomer having an acidic group in which at least a part thereof is neutralized; and a surface-crosslinked layer containing a second crosslinked polymer formed on the base polymer powder in which the first crosslinked polymer is additionally crosslinked via a surface crosslinking agent, wherein the first crosslinked polymer is a porous polymer containing a plurality of pores on a micron (μm) scale, wherein the superabsorbent polymer is prepared by: 1) performing thermal polymerization or photopolymerization of a monomer composition comprising the water-soluble ethylene-based unsaturated monomer having the acidic group in which at least the part thereof is neutralized, a carbonate forming agent, an anionic surfactant and a polymerization initiator to form a hydrogel polymer, wherein the anionic surfactant is dioctyl sodium sulfosuccinate, 2) drying the hydrogel polymer, 3) pulverizing the dried polymer, and 4) performing surface cros slinking reaction of the pulverized polymer; wherein the super absorbent polymer has an average particle size of 300 μm to 600 μm, CRC of 28 g/g to 35 g/g, AUL (0.9 psi) of 18 g/g to 25 g/g, 5-min Gel-Vacuum AUL of 18 g/g to 25 g/g, and a vortex time of 5 seconds to 50 seconds, wherein the WAUL is measured according to the following Equation 1:
WAUL(g/g)=[W.sub.2(g)−W.sub.1(g)]/ W.sub.0(g)   [Equation 1] in Equation 1, 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, 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 below 15 cm from the super absorbent polymer under a load (0.7 psi) for 10 minutes, 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, wherein the 5-min Gel-Vacuum AUL is measured according to the following Equation 5:
5-min Gel-Vacuum AUL(g/g)=[W.sub.8(g)−W.sub.7(g)]/ W.sub.0(g)   [Equation 5] in Equation 5, W.sub.0(g) is an initial weight(g) of the super absorbent polymer, W.sub.7(g) is the total sum of the weight of the super absorbent polymer and the weight of the device capable of providing a load to the super absorbent polymer, and W.sub.8(g) is the total sum of the weight of the super absorbent polymer and the 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 (0.3 psi) for 5 minutes and removing the remaining liquid with a vacuum device.

3. The super absorbent polymer according to claim 1, wherein the super absorbent polymer has a rewetting amount of 0.05 g/g to 1 g/g, which is measured according to the following Equation 6:
Rewetting Amount (g/g)=[W.sub.10(g)−W.sub.9(g)]/W.sub.0(g)   [Equation 6] in Equation 6, W.sub.0(g) is an initial weight(g) of the super absorbent polymer, W.sub.9(g) is an initial weight(g) of the second filter paper, W.sub.10(g) is a weight(g) of the second filter paper that has absorbed a liquid leaking out from the super absorbent polymer swelled for 2 minutes under a load (0.7 psi), after the super absorbent polymers have absorbed 25 times their weight in a physiological saline solution for a sufficient time under no load condition.

4. A method of preparing a super absorbent polymer according to claim 1: 1) performing thermal polymerization or photopolymerization of a monomer composition comprising a water-soluble ethylene-based unsaturated monomer having an acidic group in which at least a part thereof is neutralized, a carbonate forming agent, an anionic surfactant and a polymerization initiator to form a hydrogel polymer, wherein the anionic surfactant is dioctyl sodium sulfosuccinate, 2) drying the hydrogel polymer, 3) pulverizing the dried polymer, and 4) performing surface cros slinking reaction of the pulverized polymer.

5. The method of preparing a super absorbent polymer according to claim 4, wherein the carbonate foaming agent is at least one selected from the group consisting of sodium carbonate, sodium hydrogen carbonate, magnesium carbonate, calcium carbonate, potassium carbonate, and potassium hydrogen carbonate.

6. The method of preparing a super absorbent polymer according to claim 4, wherein the carbonate foaming agent is used in an amount of 0.01 to 0.1% by weight relative to the water-soluble ethylene-based unsaturated monomer.

7. The method of preparing a super absorbent polymer according to claim 4, wherein the anionic surfactant is a carboxylic acid salt, a sulfonic acid salt, or a phosphate, containing 6 to 18 carbon atoms.

8. The method of preparing a super absorbent polymer according to claim 4, wherein the anionic surfactant is used in an amount of 0.05 to 3% by weight relative to the water-soluble ethylene-based unsaturated monomer.

9. The method of preparing a super absorbent polymer according to claim 4, wherein the surface crosslinking reaction is carried out by using at least one surface cros slinking agent selected from the group consisting of ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, ethylene carbonate, ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, propanediol, dipropylene glycol, polypropylene glycol, glycerin, polyglycerin, butanediol, heptanediol, hexanediol, trimethylolpropane, pentaerythritol, sorbitol, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, iron hydroxide, calcium chloride, magnesium chloride, aluminum chloride and iron chloride.

10. The method of preparing a super absorbent polymer according to claim 4, wherein the surface crosslinking reaction is carried out by further using silica.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1 to 3 are schematic views of an exemplary apparatus for measuring the gel bed permeability and parts provided in the apparatus.

(2) FIG. 4 is a schematic diagram of an exemplary apparatus for measuring WAUL.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(3) Hereinafter, preferred examples are presented to aid in understanding of the invention. However, the following examples are provided examples are provided for better understanding of the present invention, and the scope of the present invention is not limited thereto.

Example 1

(4) A solution (Solution A) in which 11 g of 0.5% IRGACURE 819 initiator (110 ppm based on the monomer composition) diluted with acrylic acid and 28 g of 5% polyethylene glycol diacrylate (PEGDA, molecular weight 400) diluted with acrylic acid were mixed was prepared. Then, a solution (Solution B) of trimethylolpropane triacrylate containing 9 mol % of 5% ethylene oxide (Ethoxylated-TMPTA, TMP (EO) 9TA, M-3190 manufactured by Miwon Specialty Chemical Co., Ltd.) diluted with acrylic acid was prepared.

(5) 480 g of acrylic acid was injected into a 2 L glass reactor surrounded by a jacket through which a heat medium pre-cooled at 25° C. was circulated, and 37 g of the solution A and 14 g of the solution B were injected, respectively. Then, 700 g of 24% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After the temperature of the mixture increased to 72° C. or higher by neutralization heat upon dropwise addition of the solution C, 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 %.

(6) Meanwhile, 5% sodium bicarbonate solution (solution D) diluted with water and 28 g of 4% sodium persulfate solution diluted with in water were dissolved to prepare a solution (solution E-1). Further, as a surfactant, 6.7 g of 4% DOSS (Dioctyl sodium sulfosuccinate) solution (solution E-2) diluted with water was prepared.

(7) Then, when the temperature of the mixed solution was cooled to about 40° C., 14 g of the solution D previously prepared was injected into the mixed solution and mixed, and at the same time, the solution E-1 and the solution E-2 were injected.

(8) Then, the above prepared solution 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 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. Subsequently, the reaction was allowed 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 crumb through a chopping process using a meat chopper to prepare crumbs.

(9) The crumbs were then 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, and thereby the dried product had a water content of 2% or less.

(10) The dried product was 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 34.3 g/g and a water-soluble component content of 12.5% by weight. The centrifuge retention capacity was measured according to EDANA recommended test method No. WSP 241.2 and the water-soluble component content was measured according to EDANA WSP 270.2

(11) Thereafter, 100 g of the base polymer was mixed with a crosslinking agent solution obtained by mixing 3 g of water, 3 g of methanol, 0.4 g of ethylene carbonate, and 0.5 g of Aerosil 200 (EVONIK), and then surface crosslinking reaction was carried out at 190° C. for 30 minutes. The resultant was pulverized and sieved to obtain a surface-linked super absorbent polymer having a particle size of 150 to 850 μm. To 100 g of the surface-linked super absorbent polymer thus obtained was added 0.1 g of Aerosil 200 (EVONIK) and dry mixed to obtain a desired super absorbent polymer.

Example 2

(12) A super absorbent polymer was prepared in the same manner as in Example 1, except that the solution A was used in an amount of 34 g instead of 37 g, and 2.5 g of 5% allyl methacrylate solution diluted with acrylic acid was used instead of the solution B. The base polymer thus prepared had a centrifuge retention capacity of 34.6 g/g and a water-soluble component content of 12.1% by weight. The surface-crosslinked super absorbent polymer having a particle diameter of 150 to 850 μm was obtained in the same manner as in Example 1, by using the prepared base resin.

Example 3

(13) A surface-crosslinked super absorbent polymer having a particle diameter of 150 to 850 μm was obtained in the same manner as in Example 2, except that 6.7 g of 4% SDS (sodium dodecyl sulfate) solution diluted with water was used as the solution E-2 in Example 2.

Example 4

(14) A surface-crosslinked super absorbent polymer having a particle size of 150 to 850 μm was obtained in the same manner as in Example 3, except that 0.1 g of alumina (Alu 130, EVONIK) was added instead of 0.1 g of Aerosil 200 (EVONIK) in Example 3.

Comparative Example 1

(15) A solution (Solution A) in which 11 g of 0.5% IRGACURE 819 initiator (110 ppm based on the monomer composition) diluted with acrylic acid and 54 g of 5% polyethylene glycol diacrylate (PEGDA, molecular weight 400) diluted with acrylic acid were mixed was prepared.

(16) Then, 480 g of acrylic acid was injected into a 2 L glass reactor surrounded by a jacket through which a heat medium pre-cooled at 25° C. was circulated, and the solution A was injected. Then, 700 g of 24% caustic soda solution (solution C) was slowly added dropwise to the glass reactor and mixed. After the temperature of the mixture increased to 72° C. or higher by neutralization heat upon dropwise addition of the solution C, 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 %.

(17) Meanwhile, 28 g of 4% sodium persulfate solution diluted with water were dissolved to prepare a solution (Solution E-1). Then, when the temperature of the mixed solution was cooled to about 40° C., the solution E-1 previously prepared was injected into the mixed solution.

(18) The above prepared solution was then 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 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. Subsequently, the reaction was carried out 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.

(19) The crumbs were then 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, and thereby the dried product had a water content of 2% or less.

(20) 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 31.2 g/g and a water-soluble component content of 10.3% by weight. The centrifuge retention capacity was measured according to EDANA WSP 241.2 and the water-soluble component content was measured according to EDANA recommended test method No. WSP 270.2

(21) Thereafter, 100 g of the base polymer was mixed with a crosslinking agent solution obtained by mixing 3 g of water, 3 g of methanol, 0.4 g of ethylene carbonate, and 0.5 g of Aerosil 200 (EVONIK), and then surface crosslinking reaction was carried out at 190° C. for 30 minutes. The resultant was pulverized and sieved to obtain a surface-linked super absorbent polymer having a particle diameter of 150 to 850 μm. To 100 g of the surface-crosslinked super absorbent polymer thus prepared was added 0.1 g of Aerosil 200 (EVONIK) and dry mixed to obtain a desired super absorbent polymer.

Comparative Example 2

(22) A super absorbent polymer was obtained in the same manner as in Example 1, except that 5% sodium bicarbonate solution (solution D) diluted with water, and 4% DOSS (Dioctyl sodium sulfosuccinatesulfonate) solution (solution E-2) diluted with water as a surfactant were not used in Example 1.

Comparative Example 3

(23) A super absorbent polymer was obtained in the same manner as in Example 1, except that 6.7 g of 4% DOSS (dioctyl sodium sulfosuccinate) solution (solution E-2) diluted with water was first mixed with acrylic acid before the addition of caustic soda (solution C).

Experimental Example: Evaluation of Physical Properties of Super Absorbent Polymer

(24) The physical properties of the super absorbent polymers prepared in Examples and Comparative Examples were evaluated by the following methods.

(25) (1) WAUL (Wicking Absorbency Under Load)

(26) WAUL was measured using a measuring apparatus as shown in FIG. 4. Specifically, a tube with an inner diameter of 0.19 mm was horizontally connected, and a filter paper is placed on Kit, which had a hole in the bottom at a height of 15 cm and thus could suck water up from the bottom, and the AUL Kit (diameter of 1 inch) was placed on it. 0.16 g (W.sub.0) of a super absorbent polymer previously classified into a diameter of 300 to 600 μm is weighed. After evenly spreading on the bottom, a physiological saline solution was absorbed for 10 minutes under a condition that a load of 0.7 psi was applied. WAUL was calculated according to the following Equation 1:
WAUL(g/g)=[W.sub.2(g)−W.sub.1(g)]/W.sub.0(g)   [Equation 1]

(27) In Equation 1,

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

(29) 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,

(30) 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 below 15 cm from the super absorbent polymer under a load (0.7 psi) for 10 minutes.

(31) (2) Centrifuge Retention Capacity (CRC)

(32) The centrifuge retention capacity (CRC) was measured according to EDANA recommended test method No. WSP 241.2.

(33) Specifically, a super absorbent polymer having a particle diameter of 300 to 600 μm which was passed through a U.S. standard 30 mesh screen and retained on a U.S. standard 50 mesh screen was prepared from a super absorbent polymer for evaluating the centrifuge retention capacity. Then, the super absorbent polymer W.sub.0 (g, about 0.2 g) having a particle diameter of 300 to 600 μm was uniformly placed into a nonwoven fabric-made bag, followed by sealing. Then, the bag was immersed into 0.9% by weight of physiological saline solution at room temperature. After 30 minutes, the bag was dehydrated at 250 G for 3 minutes with a centrifuge, and the weight W.sub.4(g) of the bag was then measured. Meanwhile, after carrying out the same operation using an empty bag not containing a super absorbent polymer, the weight W.sub.3(g) was measured.

(34) Using the respective weights thus obtained, a centrifuge retention capacity was confirmed according to the following Equation 3:
CRC(g/g)={[W.sub.4(g)−W.sub.3(g)]/W.sub.0(g)}−1   [Equation 3]

(35) in Equation 3,

(36) W.sub.0(g) is an initial weight(g) of the super absorbent polymer having a particle diameter of 300 μm to 600 μm,

(37) W.sub.3(g) is a 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

(38) W.sub.4(g) is a weight of the device including a super absorbent polymer, measured after soaking and absorbing the super absorbent polymer having a particle diameter of 300 μm to 600 μm in 0.9% by weight of physiological saline solution at room temperature for 30 minutes, and then dehydrating the same by using a centrifuge at 250 G for 3 minutes.

(39) (3) Absorbency Under Load (AUL)

(40) The absorbency under load (AUL) for a physiological saline solution (0.9 psi) was measured according to EDANA recommended test method No. WSP 242.2.

(41) Specifically, a 400 mesh stainless steel net was attached to the bottom of a plastic cylinder having an inner diameter of 25 mm. W.sub.0 (g, 0.16 g) of a super absorbent polymer for measuring the absorbency under load were uniformly scattered on the screen under conditions of room temperature and relative humidity of 50%. Then, a piston which could provide a load of 6.3 kPa (0.9 psi) uniformly 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.5(g) of the device prepared in this way was measured. After putting a glass filter having a diameter of 90 mm and a thickness of 5 mm in a Petri dish having the diameter of 150 mm, 0.90% by weight of a physiological saline solution was poured in the 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. 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.6(g) of the device containing the swollen super absorbent polymer was measured.

(42) Using the weight thus measured, the absorbency under load was calculated according to the following Equation 4.
AUL(g/g)=[W.sub.6(g)−W.sub.5(g)]/W.sub.0(g)   [Equation 4]

(43) in Equation 4,

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

(45) W.sub.5(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

(46) W.sub.6(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.

(47) (4) Gel Bed Permeability (GBP)

(48) The gel bed permeability (GBP) for a physiological saline solution was measured according to the following method described in Korean Patent Application No. 10-2014-7018005.

(49) 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.

(50) Meanwhile, a super absorbent polymer having a particle diameter of about 300 to about 600 μm was obtained by selectively classifying a super absorbent polymer which was passed through a US standard 30 mesh screen and retained on a US standard 50 mesh screen.

(51) 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 could be adjusted such that the surface within the sample container was defined by the swollen super absorbent polymer, rather than the physiological saline solution.

(52) 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.

(53) 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 container 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 60 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).

(54) Using the data thus obtained, the GBP (cm.sup.2) was calculated according to the following Equation 3.
K=[Q×H×μ]/[A×ρ×P]  [Equation 3]

(55) in Equation 3,

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

(57) Q is a flow rate (g/sec)

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

(59) μ is a liquid viscosity (poise) (about 1 cP for the test solution used with this Test),

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

(61) 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

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

(63) 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)

(64) 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 cm.sup.2).

(65) (5) Vortex Time

(66) The vortex time of the super absorbent polymer was measured in the amount of time in seconds until the vortex disappeared after adding 2 g of a super absorbent polymer to 50 mL of physiological saline solution and then stirring the mixture at 600 rpm.

(67) (6) 5-Min Gel-Vacuum AUL

(68) The 400 mesh stainless steel screen was attached to the bottom of a plastic cylinder having an inner diameter of 25 mm. W.sub.0(g) of a super absorbent polymer for measuring the 5-min Gel-Vacuum AUL were uniformly scattered on the screen under conditions of room temperature and relative humidity of 50%. Then, a piston which could provide a load of 0.3 psi uniformly was put on the super absorbent polymer. 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.7(g) of the device prepared in this way was measured. After putting a glass filter having a diameter of 90 mm and a thickness of 5 mm in a Petri dish having the diameter of 150 mm, 0.9 wt % by weight of a physiological saline solution was poured in the dish. At this time, the physiological saline solution was poured until the surface level became equal to the upper surface of the glass filter. A sheet of filter paper having a diameter of 90 mm was put on the glass filter. 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 5 minutes, the remaining liquid was removed using a vacuum pump. At this time, the remaining liquid not absorbed between the swollen super absorbent polymer particles was removed. The weight W.sub.g(g) of the device containing the super absorbent polymer was measured. Using the weight thus measured, the 5-min Gel-Vacuum AUL was calculated according to the following Equation 5.
5-min Gel-Vacuum AUL (g/g)=[W.sub.8(g)−W.sub.7(g)]/W.sub.0(g)   [Equation 5]

(69) in Equation 5,

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

(71) W.sub.7(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

(72) W.sub.8(g) is the total sum of the weight of the super absorbent polymer and the 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 (0.3 psi) for 5 minutes and removing the remaining liquid with a vacuum device.

(73) (7) Rewetting Properties

(74) A super absorbent polymer having a particle diameter of 300 to 600 pm which was passed through a U.S. standard 30 mesh screen and retained on a U.S. standard 50 mesh screen was prepared from a super absorbent polymer for evaluating the rewetting properties. Meanwhile, the 400 mesh stainless steel screen was attached to the bottom of a plastic cylinder having an inner diameter of 25 mm. Then, W.sub.0 (g, 0.16 g) of the previously prepared super absorbent polymer was uniformly scattered on the screen under conditions of room temperature and relative humidity of 50%, to thereby prepare a test assembly.

(75) Then, a first filter paper having a diameter of 25 mm was laid on the PE dish having a diameter of 80 mm, and the test assembly was placed thereon. Thereafter, 4 g of 0.9 wt % physiological saline solution was injected around the test assembly, so that the super absorbent polymer could absorb the physiological saline solution under no load condition. When the physiological saline solution was completely absorbed by the super absorbent polymer, it was left for 10 minutes so that the super absorbent polymer was swollen sufficiently.

(76) On the other hand, as Whatman Grade No. 4 filter paper, 10 sheets of filter papers having a diameter of 30 mm or more were overlapped to prepare a second filter paper. Then, the weight W.sub.5(g) of the second filter paper was measured. After lifting and removing the test assembly from the first filter paper, a piston capable of uniformly applying a load of 5.1 kPa (0.7 psi) onto the swollen super absorbent polymer was added. At this time, the piston 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 test assembly to which the piston was added was placed on the previously prepared second filter paper. After lifting and removing the test assembly to which the piston has been added after 2 minutes, the weight W.sub.6(g) of the second filter paper was again measured. Using each of the weights thus obtained, the rewetting amount (g/g) was calculated by the Equation 4.
Rewetting Amount (g/g)=[W.sub.6(g)−W.sub.5(g)]/W.sub.0(g)   [Equation 4]

(77) in Equation 4,

(78) W.sub.0(g) is an initial weight(g) of the super absorbent polymer, W.sub.5(g) is an initial weight(g) of the second filter paper, W.sub.6(g) is a weight(g) of the second filter paper that has absorbed a liquid leaking out from the super absorbent polymer swelled for 2 minutes under a load (0.7 psi), after the super absorbent polymers have absorbed 25 times their weight in a physiological saline solution for a sufficient time under no load condition.

(79) The results of the above measurement are shown in Table 1 below.

(80) TABLE-US-00001 TABLE 1 Average Vortex Rewetting pore size(4 CRC AUL WAUL Gel-AUL GBP time amount V/s)(μm) (g/g) (g/g) (g/g) (g/g) (darcy) (s) (g/g) Ex. 1 45 30.3 19.1 15.6 20.1 68 35 0.4 Ex. 2 48 30.8 19.4 16.1 20.2 53 29 0.6 Ex. 3 53 30.9 20.1 15.7 19.7 64 28 0.2 Ex. 4 44 30.4 21.1 16.3 19.6 72 34 0.6 Comparative 62 26.3 22.5 12.5 15.6 45 95 2.3 Ex. 1 Comparative 0.8 31.2 18.4 11.2 15.4 56 78 1.8 Ex. 2 Comparative 1.2 30.7 21.2 12.8 16.2 18 84 1.7 Ex. 3