Method for Producing Water-Absorbing Polymer Particles by Suspension Polymerization

Abstract

A process for producing water-absorbing polymer particles by suspension polymerization and thermal surface postcrosslinking, wherein the base polymer obtained by suspension polymerization has a centrifuge retention capacity of at least 37 g/g and the thermal surface postcrosslinking is conducted at 100 to 190° C.

1 . A process for producing water-absorbing polymer particles by polymerizing a monomer solution comprising
 a) at least one ethylenically unsaturated monomer which bears an acid group and optionally at least partly neutralized,   b) optionally one or more crosslinker,   c) at least one initiator,   d) optionally one or more ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a) and   e) optionally one or more water-soluble polymer,   the monomer solution being suspended in a hydrophobic organic solvent during the polymerization, and thermally surface postcrosslinking the resultant polymer particles using an organic surface postcrosslinker, wherein the amount of crosslinker b) is selected such that the polymer particles before the surface postcrosslinking have a centrifuge retention capacity of at least 37 g/g and the thermal surface postcrosslinking is conducted at 100 to 190° C.   
     
     
         2 . The process according to  claim 1 , wherein the amount of crosslinker b) is selected such that the polymer particles before the surface postcrosslinking have a centrifuge retention capacity of at least 40 g/g. 
     
     
         3 . The process according to  claim 1 , wherein the thermal surface postcrosslinking is conducted at 120 to 170° C. 
     
     
         4 . The process according to  claim 1 , wherein the organic surface postcrosslinker is selected from the group consisting of alkylene carbonates, 2-oxazolidinones, bis- and poly-2-oxazolidinones, 2-oxotetrahydro-1,3-oxazines, N-acyl-2-oxazolidinones, cyclic ureas, bicyclic amido acetals, oxetanes, and morpholine-2,3 -diones. 
     
     
         5 . The process according to  claim 1 , wherein from 1 to 5% by weight of organic surface postcrosslinker is used, based on the resultant polymer particles. 
     
     
         6 . The process according to  claims 1 , wherein a mean droplet diameter of the suspended monomer solution is from 200 to 500 μm. 
     
     
         7 . The process according to any of  claim 1 , wherein a dispersing aid is used in the polymerization. 
     
     
         8 . The process according to  claim 1 , wherein the resultant polymer particles are dewatered azeotropically after the polymerization. 
     
     
         9 . The process according to  claim 7 , wherein the resultant polymer particles are filtered and dried after the azeotropic dewatering. 
     
     
         10 . The process according to  claim 1 , wherein the thermal surface postcrosslinking is conducted in a mixer with moving mixing tools. 
     
     
         11 . Water-absorbing polymer particles obtained by a process of  claim 1 , having a centrifuge retention capacity of at least 37 g/g, an absorption under a pressure of 21.0 g/cm 2  of at least 30 g/g, an absorption under a pressure of 49.2 g/cm 2  of at least 14 g/g, a sum total of centrifuge retention capacity and absorption under a pressure of 21.0 g/cm 2  of at least 69 g/g, a sum total of centrifuge retention capacity and absorption under a pressure of 49.2 g/cm 2  of at least 54 g/g, and less than 20% by weight of extractables. 
     
     
         12 . Water-absorbing polymer particles according to  claim 11 , having a centrifuge retention capacity of at least 41 g/g. 
     
     
         13 . Water-absorbing polymer particles according to  claim 11 , having an absorption under a pressure of 21.0 g/cm 2  of at least 34 g/g. 
     
     
         14 . Water-absorbing polymer particles according to  claim 11 , having an absorption under a pressure of 49.2 g/cm 2  of at least 20 g/g. 
     
     
         15 . Water-absorbing polymer particles according to  claim 11 , having a sum total of centrifuge retention capacity and absorption under a pressure of 21.0 g/cm 2  of at least 74 g/g. 
     
     
         16 . Water-absorbing polymer particles according to  claim 11 , having a sum total of centrifuge retention capacity and absorption under a pressure of 49.2 g/cm 2  of at least 59 g/g. 
     
     
         17 . Water-absorbing polymer particles according to  claim 11 , having less than 14% by weight of extractables. 
     
     
         18 . Water-absorbing polymer particles according to  claim 11 , having a bulk density of at least 1.0 g/cm 3 . 
     
     
         19 . Water-absorbing polymer particles according to  claim 11 , wherein a proportion of particles having a particle size of 300 to 600 μm is at least 30% by weight. 
     
     
         20 . A hygiene article comprising
 (A) an upper liquid-impermeable layer,   (B) a lower liquid-permeable layer,   (C) a liquid-absorbing storage layer between layer (A) and layer (B), comprising from 0 to 30% by weight of a fibrous material and from 70 to 100% by weight of water-absorbing polymer particles,   (D) optionally an acquisition and distribution layer between layer (A) and layer (C), comprising from 80 to 100% by weight of a fibrous material and from 0 to 20% by weight of water-absorbing polymer particles,   (E) optionally a fabric layer directly above and/or beneath layer (C) and   (F) further optional components   wherein the water-absorbing polymer particles of (C) and (D) are according to  claim 11 .   
     
     
         21 . The hygiene article according to  claim 20 , wherein the water-absorbing polymer particles have a mean sphericity of at least 0.84.