Super absorbent polymer and a preparation method thereof

A super absorbent polymer (SAP), comprising

• a base polymer powder comprising a crosslinked polymer of at least partially neutralized water-soluble ethylenic unsaturated monomer of acrylic acid;
• and
• a surface crosslinking layer formed on the base polymer powder,
• wherein the surface crosslinking layer is obtained by crosslinking the surface of the base polymer powder with a C2-C8 diol-based or glycol-based compound, and the surface crosslinking is performed by spraying a surface treatment aqueous solution comprising the C2-C8 diol-based or glycol-based compound on the base polymer powder with the spray angle of 10 to 30° and carrying out the surface crosslinking reaction at 120 to 250° C. for 10 to 120 minutes,
• wherein the SAP has the SPAN value of 1.2 or less, represented by the following Mathematical Formula 1, and
• satisfies the absorbing rate under load (ARUL) represented by the following Mathematical Formula 2:
• SPAN=[D(90%)−D(10%)]/D(50%)≦1.2 [Mathematical Formula 1]
• in Mathematical Formula 1,
• D(90%) is the diameter of the particle when the accumulated weight from the smallest particle becomes 90%, when the SAP particles are arranged in order of particle diameter measured according to EDANA method WSP 220.2,
• D(10%) is the diameter of the particle when the accumulated weight from the smallest particle becomes 10%, when the SAP particles are arranged in order of particle diameter measured according to EDANA method WSP 220.2, and
• D(50%) is the diameter of the particle when the accumulated weight from the smallest particle becomes 50%, when the SAP particles are arranged in order of particle diameter measured according to EDANA method WSP 220.2,
• 1.0>ARUL=AUP(10 min)/AUP(60 min)>0.70 [Mathematical Formula 2]
• in Mathematical Formula 2,
• AUP (10min) is the absorbency under pressure after 10 mins represented by the following Mathematical Formula 3, and AUP (60 min) is the absorbency under pressure after 60 mins represented by the following Mathematical Formula 3,
• AUP(g/g)=[Wb(g)−Wa(g)]/weight of absorbent polymer(g) [Mathematical Formula 3]
• in Mathematical Formula 3,
• Wa (g) is the sum of the weight of the absorbent polymer and the weight of the device capable of providing load to the absorbent polymer, and
• Wb (g) is the sum of the weight of the absorbent polymer in which moisture is absorbed under load (0.7 psi) for a given time of 10 mins or 60 mins and the weight of the device capable of providing load to the absorbent polymer.