Method of preparing superabsorbent polymer

A method of preparing a superabsorbent polymer, the method comprising:

• 1) Carrying out a crosslinking polymerization of a monomer composition comprising a water-soluble ethylene-based unsaturated monomer having acidic groups which are at least partially neutralized, in the presence of a foaming agent, to form a water-containing gel polymer;
• 2) Gel-pulverizing the water-containing gel polymer;
• 3) Drying, pulverizing, and size-sorting the gel-pulverized water-containing gel polymer to form a base polymer powder;
• and
• 4) Carrying out a surface crosslinking of the base polymer powder by heat treatment in the presence of a surface crosslinking agent,
• wherein the foaming agent is present in an amount of 500 to 1,500 ppm in the monomer composition, and
• the gel-pulverizing is carried out by extruding the water-containing gel polymer through a perforated panel having a plurality of holes using a screw extruder mounted inside a cylindrical pulverizer under a condition that a chopping index according to Equation 1 is 60 to 75:
• Chopping
• ⁢
• index
• =
• 2
• ⁢
• π
• ⨯
• N
• 60
• ⨯
• 1
• A
• ⨯
• 11
• R
• [
• Equation
• ⁢
• 1
• ]
• wherein in Equation 1,
• N represents a number of rotation (rpm) of a screw in the screw extruder,
• A represents porosity (πr2×n/πR2) of the perforated panel, wherein r represents a radius (mm) of the holes formed in the perforated panel, n represents a number of holes formed in the perforated panel, and
• R represents a radius (mm) of the perforated panel,
• wherein the superabsorbent polymer has a CRC of 27 g/g to 40 g/g as measured in accordance with EDANA WSP 241.3,
• a AUP of 24 g/g to 30 g/g as measured in accordance with EDANA WSP 242.3,
• a vortex of 40 seconds or less as measured by adding 2 g of the superabsorbent polymer to 50 mL of physiological saline at 23° C. to 24° C., and stirring with a magnetic bar (diameter of 8 mm and length of 31.8 mm) at 600 rpm, and a time taken for vortex to disappear being measured in seconds, and
• a permeability of 30 to 60 seconds measured according to Equation 4:
• Perm
• =
• [
• 20
• ⁢
• mL
• /
• T
• ⁢
• 1
• ⁢
• (
• sec
• )
• ]
• *
• 60
• ⁢
• sec
• [
• Equation
• ⁢
• 4
• ]
• wherein in Equation 4,
• Perm represents permeability of the superabsorbent polymer, T1 represents a time (sec) taken for 20 mL of a physiological saline solution (0.9% by weight of aqueous sodium chloride solution) to pass through the swollen superabsorbent polymer under a pressure of 0.3 psi, after placing 0.2 g of the superabsorbent polymer in a cylinder, and pouring the physiological saline solution whereby the superabsorbent polymer was completely submerged therein to allow the superabsorbent polymer to swell for 30 minutes.