Process to make water-absorbing polymer structure having superabsorbent polymer index

A process for the preparation of a particulate water-absorbing polymer structure, comprising the process steps of:

• i) providing an untreated water-absorbing polymer structure wherein the structure is a particle wherein the untreated water-absorbing polymer structure has a degree of neutralization of at most 70 mol % and the particulate water-absorbing polymer structure has an average particle size in the range of 150 to 600 μm;
• and
• ii) bringing the untreated particulate water-absorbing polymer structure into contact with from 2.5 wt % to 7.5 wt % based on the untreated particulate water-absorbing polymer structure of a an acidic component selected from anisic acid, benzoic acid, formic acid, valeric acid, citric acid, glyoxylic acid, glycollic acid, glycerol phosphoric acid, glutaric acid, chloroacetic acid, chloropropionic acid, cinnamic acid, succinic acid, acetic acid, tartaric acid, lactic acid, pyruvic acid, fumaric acid, propionic acid, 3-hydroxypropionic acid, malonic acid, butyric acid, isobutyric acid, imidinoacetic acid, malic acid, isothionic acid, methylmaleic acid, adipic acid, itaconic acid, crotonic acid, oxalic acid, salicylic acid, gluconic acid, gallic acid, sorbic acid or p-oxybenzoic acid, wherein the untreated particulate water-absorbing polymer structure is brought into contact with the acidic component at a temperature of from 50° C. to 100° C.;
• and wherein the treated particulate water-absorbing polymer structure comprises an inner region and an outer region surrounding the inner region, wherein the inner region has a degree of neutralization of at least 60% or more and the outer region has a degree of neutralization of at least 10% less than the degree of neutralization of the inner core, and the particulate water-absorbing polymer structure has superabsorbent polymer (SAP) index of at least 140×10−7 cm3s/g, wherein the SAP index is defined as
• SAP index=(RET×SFC)/pH
• wherein
• RET=the retention determined in accordance with ERT 441.2-02,
• SFC=the permeability determined in accordance with the test method described herein and
• pH=the pH determined in accordance with ERT 400.2-02.

A particulate water-absorbing polymer structure comprising an inner region and an outer region surrounding the inner region, wherein the outer region of the particulate water-absorbing polymer structure has been brought into contact with from 2.5 wt % to 7.5 wt % based on the particulate water-absorbing polymer structure of a an acidic component and with an inorganic;

• wherein the inner region has a degree of neutralization of 60 mol % or more and the outer region has a degree of neutralization of at least 10 mol % less than the degree of neutralization of the inner core;
• and wherein the water-absorbing polymer structure has the following properties:
• (β1) a retention, determined in accordance with ERT 441.2-02, of at least 27 g/g;
• (β2) an absorption, determined in accordance with ERT 442.2-02, under a pressure of 50 g/cm2 of at most 20 g/g;
• (β3) pH, determined in accordance with ERT 400.2-02, of less than 6.5 and
• (β4) an ammonia-binding capacity, determined in accordance with the test method described herein, of at least 98 mg/g.