Process for producing an absorbent polymer by means of spread-drying

1. A process for preparation of an absorbent polymer, comprising the steps of:

i) polymerizing an aqueous monomer solution to obtain a polymer gel,

ii) comminuting the polymer gel to obtain a polymer gel granulate, and

iii) obtaining a mechanically loosened polymer gel granulate by loosening the polymer gel after the comminution in step ii), in the form of the polymer gel granulate obtained after the comminution; and

iv) drying the polymer gel granulate;

wherein the polymer gel granulate after step ii) has a spread including a first compressibility index κ 1 of the gel granulate that lies between about 10% and about 40%; and

wherein the spread behavior of the gel granulate after step ii) is characterized by at least one of the following features

(δ1) a second compressibility index κ 2 of the gel granulate lies between about 3·10 −5 Pa −1 and about 6·10 −5 Pa −1 ; or

(δ2) a first decompressibility index κ 1 ′ of the gel granulate lies between about 3% and about 15%; or

(δ3) a second compressibility index κ 2 ′ of the gel granulate lies between about 3·10 −5 Pa −1 and about 7·10 −5 Pa −1 .

2. The process according to claim 1 , wherein the spread behavior of the gel granulate or of the loosened gel granulate after step ii) is characterized by a cross section spread index Q of at least about 3 with a load with a total mass of about 1185 g.

3. The process according to claim 1 , wherein the spread behavior of the gel granulate or of the loosened gel granulate after step ii) is determined by a spread time constant τ of at least about 2 s with a load with a total mass of about 6185 g.

4. The process according to claim 1 , wherein the density of the undried gel granulated after step ii) is less than about 0.7 g/cm 3 .

5. The process according to claim 1 , wherein the gel granulate in step ii) is subjected to an at least three-step comminution with a cutting unit, a ripping unit and a “wolf” grinding unit.

6. The process according to claim 1 , wherein the polymer gel granulate comprises at least about 10 wt. %, based on the gel granulate, an absorbent polymer based upon:

(α1) from about 0.1 to about 99.999 wt. % of polymerized, ethylenically unsaturated, acidic groups-comprising monomers or salts thereof, or polymerized, ethylenically unsaturated monomers comprising a protonated or a quaternary nitrogen, or mixtures thereof,

(α2) from 0 to about 70 wt. % of polymerized, ethylenically unsaturated monomers which can be co-polymerized with (α1),

(α3) from about 0.001 to about 10 wt. % of one or more cross-linkers,

(α4) from 0 to about 30 wt. % of water-soluble polymers, as well as

(α5) from 0 to about 20 wt. % of one or more auxiliaries, respectively based on the dry absorbent polymer, wherein the sum of the component weights (α1) to (α5) amounts to 100 wt. %.

7. The process according to claim 1 , wherein the dried gel granulate has at least one of the following properties:

(φ1) the maximum absorption of a 0.9 wt. % aqueous NaCl solution lies within a range from about 10 to about 1000 g/g SAP granulate,

(φ2) the fraction that can be extracted with a 0.9 wt. % aqueous NaCl solution is less than about 30, based on the SAP granulate,

(φ3) the bulk density lies within the range from about 300 to about 1000 g/l,

(φ4) the pH value of about 1 g of the SAP granulate in about 1 liter of water is in the range from about 4 to about 10,

(φ5) the CRC value lies within the range from about 10 to about 100 g/g,

(φ6) the AAP value at a pressure of about 0.3 psi lies within the range from about 10 to about 60 g/g.