Method for preparing super absorbent polymer

1. A method for preparing a super absorbent polymer, comprising:

forming a first hydrogel polymer containing a crosslinked polymer of acrylic acid or its salt, and having a first centrifuge retention capacity (CRC; g/g) for a physiological saline solution measured in accordance with EDANA recommended test method No. WSP 241.2 and a first absorption rate (sec) measured according to the vortex test carried out under conditions of stirring 50 ml of 0.9 wt % NaCl solution at a rate of 600 rpm, wherein the first hydrogel polymer has a size of 0.01 to 3 mm;

forming a second hydrogel polymer containing a crosslinked polymer of acrylic acid or its salt, and having a second centrifuge retention capacity (CRC; g/g) in which a difference between the first centrifuge retention capacity and the second centrifuge retention capacity is −2 g/g to 2 g/g, and a second absorption rate (sec) in which a difference between the first absorption rate and the second absorption rate is 5 sec or more, wherein the second hydrogel polymer has a size of 0.5 to 5 mm;

forming a mixture of the first hydrogel polymer and the second hydrogel polymer;

drying the polymer mixture;

pulverizing the dried polymer mixture; and

surface-crosslinking the pulverized polymer mixture,

wherein the first hydrogel polymer and second hydrogel polymer have a bulk density of 0.5 to 0.7 g/m as measured according to EDANA (European Disposables and Nonwovens Association) WSP 250.3,

the super absorbent polymer has a centrifuge retention capacity (CRC) for a physiological saline solution of 36 to 50 g/g which is measured in accordance with EDANA recommended test method No. WSP 241.2, and

wherein the first hydrogel polymer and second hydrogel polymer have a water content of about 40 to 80% by weight.

2. The method of claim 1 , wherein

in the forming the first hydrogel polymer, the monomer aqueous solution further contains a foaming agent; or in the step of coarsely pulverizing the first and second crosslinked polymers, the first crosslinked polymer and the second crosslinked polymer are pulverized so as to have mutually different ranges of sizes.

3. The method of claim 1 , wherein

in the step of forming the first and the second crosslinked polymers, the respective monomer aqueous solutions contain mutually different amounts of polymerization initiators, foaming agents, or inorganic additives.

4. The method of claim 1 , wherein

in the step of forming the first and second crosslinked polymers, the respective monomer aqueous solutions are polymerized under mutually different temperatures, or polymerized under light irradiation with mutually different illuminance.

5. The method of claim 1 , wherein the first centrifuge retention capacity and the second centrifuge retention capacity are 25 to 50 g/g, respectively, the first absorption rate is 35 to 50 sec, and the second absorption rate is 45 to 80 sec.

6. The method of claim 1 , wherein the surface crosslinking step is carried out in the presence of a polyvalent epoxy-based compound having two or more epoxy groups per molecule, wherein the polyvalent epoxy-based compound is represented by the following Chemical Formula 1:

wherein, R 2 is C 1-3 alkylene.

7. The method of claim 1 , wherein the super absorbent polymer has an absorption rate of 35 to 55 sec which is measured according to the vortex test carried out under conditions of stirring 50 ml of 0.9 wt % NaCl solution at a rate of 600 rpm; an absorbency under pressure (AUP) for a physiological saline solution of 15 to 30 g/g which is measured in accordance with EDANA recommended test method No. WSP 241.3, and a bulk density of 0.5 to 0.7 g/ml as measured according to EDANA (European Disposables and Nonwovens Association) WSP 250.3.

8. The method of claim 1 , wherein the first and second hydrogel polymers are mixed after being formed in mutually different polymerization lines.

9. The method for preparing a super absorbent polymer super absorbent polymer of claim 1 , wherein

the forming the first hydrogel polymer includes: polymerizing a monomer aqueous solution containing acrylic acid or its salt in the presence of an internal crosslinking agent to form a first crosslinked polymer; and coarsely pulverizing the first crosslinked polymer; and

the forming the second hydrogel polymer includes: polymerizing a monomer aqueous solution containing acrylic acid or its salt in the presence of an internal crosslinking agent to form a second crosslinked polymer; and coarsely pulverizing the second crosslinked polymer.