Water absorbent material, method for production of surface cross-linked water absorbent resin, and method for evaluation of water absorbent material

1. A water absorbent material comprising a surface cross-linked water absorbent resin resulting from granular irregularly pulverized shaped particles of partial neutralization or whole neutralization of a water absorbent resin having acrylic acid or a metal salt thereof as a main component, which water absorbent material shows a metal atom concentration on the surface of said water absorbent material in the range of 0-10 atom % at 0 second value of polishing and in the range of 2 to below 35 atom % at 10 seconds of polishing as determined by subjecting said water absorbent material to Ar ion discharge polishing under a voltage of 500 V,

wherein the water absorbent resin having acrylic acid or a metal salt thereof as a main component has a ratio of partial neutralization in the range of 50-90 mol %, and wherein a fluorine atom ratio to the number of carbon atoms, F/C ratio, in the surface of said water absorbent material on which —COOH has been converted to —COOCH 2 —CF 3 , is not more than 0.03,

wherein the surface of the water absorbent resin is cross-linked in the presence of 0-0.3 mass part of water, based on 100 mass parts of the water absorbent resin,

wherein the proportion of the water absorbent material having a particle diameter of less than 106 μm is in the range of 0-10 mass %,

wherein the magnitude of the logarithmic standard deviation δζ which indicates the dispersibility of particle diameter is in the range of 0.25-0.45.

2. A water absorbent material according to claim 1 , which possesses a relation shown by the following formula between said metal atom concentration Y on the surface polished by the method of Ar ion discharge polishing under a voltage of 500 V and a polishing time X measured at 0 second, 10 seconds, 40 seconds, and 70 seconds respectively of said polishing:

Y=aX+b ,satisfying 0.30 ≦a≦ 0.60 and 0 <b< 20.

3. A water absorbent material according to claim 1 , wherein said value at 10 second value is not more than 30 atom %.

4. A water absorbent material according to claim 1 , wherein the relationship between the metal atom concentration Y and the polishing time X in seconds is as follows:

Y=aX+b ,satisfying 0.30 ≦a and 0 <b< 15.

5. A water absorbent material according to claim 1 , wherein the residual epoxy compound content of said water absorbent material is not more than 5 ppm.

6. A water absorbent material according to claim 1 , wherein the coefficient of absorption of physiological saline water is not less than 35 g/g and the coefficient of blood absorption is in the range of 70 to 100%.

7. A water absorbent material according to claim 1 wherein the speed to the physiological saline is not more than 30 seconds.

8. An absorbent article containing a water absorbent material set forth in claim 1 .

9. A water absorbent material according to claim 1 , wherein the surface of the water absorbent resin is cross-linked with a first cross-linking agent and a second cross-linking agent by contact with a surface cross-linking agent solution comprising the first cross-linking agent, the second cross-linking agent and 0-0.3 mass parts of water, based on 100 mass parts of the solid component of the resin,

wherein the first cross-linking agent is polypropylene glycol and the second cross-linking agent is ethylene glycol diglycidyl ether.

10. A water absorbent material according to claim 1 , wherein the water absorbent material consists of the surface cross-linked water absorbent resin.

11. The water absorbent material according to claim 1 , wherein the F/C ratio is 0 to 0.01.

12. The water absorbent material according to claim 1 , wherein the weight average particle diameter of the water absorbent resin is in the range of 200-300 μm.