Water absorbent resin particles of crosslinked carboxyl-containing polymers with low monomer content

1. A process for the preparation of water-absorbent resin particles, the process comprising:

(I) polymerizing a polymerization mixture comprising:

(a) one or more ethylenically unsaturated carboxyl-containing monomers,

(b) one or more crosslinking agents,

(c) optionally one or more comonomers copolymerizable with the carboxyl-containing monomer, and

(d) a polymerization medium, to form a crosslinked hydrogel,

(II) contacting the crosslinked hydrogel with a peroxodisulfate salt,

(III) comminuting the hydrogel to particles prior to or after the peroxodisulfate salt is added in step (II), and

(IV) drying the hydrogel to form resin,

wherein step (II) is conducted in the substantial absence of a combination of surfactant and a material capable of adding across or reacting with a vinylic double bond so as to form reaction products which are incapable of vinylic polymerization via free radical initiation.

2. The process of claim 1 wherein the amount of peroxodisulfate salt is from 0.001 to 15 parts by weight per hundred parts by weight of the total amount of components (a), (b) and (c) of the polymerization mixture.

3. The process of claim 1 wherein the peroxodisulfate salt is in the form of an aqueous solution.

4. The process of claim 1 wherein at least part of the salt is employed as fines that were pretreated with an aqueous peroxodisulfate salt solution.

5. The process of claim 1 wherein the hydrogel is processed into particles having particle sizes of from 0.05 to 10 cm during the polymerization reaction in step (1).

6. The process of claim 1 further comprising heating the dried hydrogel from step (IV) to a temperature of from 170 to 250° C. for from 1 to 60 minutes.

7. The process of claim 6 wherein the heating is conducted in the presence of a chlorine-or bromine-containing oxidizing agent.

8. The process of claim 7 wherein the chlorine- or bromine-containing oxidizing agent is dissolved or dispersed in the polymerization mixture in an amount of from 10 to 2,000 ppm, based on the total weight of monomers, prior to initiation of the polymerization in step (1).

9. The process of claim 1 wherein the peroxodisulfate salt is contacted with the hydrogel in step (II) prior to comminuting the hydrogel in step (III).

10. The process of claim 9 wherein the contacting of the peroxodisulfate salt with the hydrogel in step (II) is conducted in the substantial absence of at least one of the following additives: reducing agents, water-insoluble fine inorganic particles, surfactants, organic solvents, and mineral oil.

11. The process of claim 1 wherein the peroxodisulfate salt is an alkali metal peroxodisulfate or ammonium peroxodisulfate.

12. The process of claim 7 wherein the chlorine- or bromine-containing oxidizing agent is selected from the group consisting of sodium chlorate, potassium chlorate, sodium bromate, potassium bromate, sodium chlorite and potassium chlorite or mixtures thereof.

13. The process of claim 1 wherein the peroxodisulfate salt is sodium persulfate.

14. A process for the preparation of water-absorbent resin particles, the process comprising:

(I) polymerizing a polymerization mixture comprising:

(a) acrylic acid,

(b) one or more crosslinking agents,

(c) optionally one or more comonomers copolymerizable with acrylic acid, and

(d) an aqueous polymerization medium, to form a crosslinked hydrogel,

(II) contacting the crosslinked hydrogel with a peroxodisulfate salt,

(III) comminuting the hydrogel to particles prior to adding the peroxodisulfate salt in step (II), and

(IV) drying the hydrogel to form resin,

wherein step (II) is conducted in the substantial absence of a combination of surfactant and a material capable of adding across or reacting with a vinylic double bond so as to form reaction products which are incapable of vinylic polymerization via free radical initiation.

15. The process of claim 14 wherein the peroxodisulfate salt is sodium persulfate.

16. The process of claim 14 wherein the amount of peroxodisulfate salt is from 0.001 to 15 parts by weight per hundred parts by weight of the total amount of components (a), (b) and (c) of the polymerization mixture.

17. A process for the preparation of water-absorbent resin particles, the process comprising:

(I) polymerizing a polymerization mixture comprising:

(a) acrylic acid,

(b) one or more crosslinking agents according to Formula I:

R 1 (—(R 2 O) n —C(O)R 3 ) k

wherein: R 1 is a straight- or branched-chain polyalkoxy radical with 1 to 10 carbon atoms, optionally substituted with one or more oxygen atoms in the backbone; R 2 is independently in each occurrence an alkylene group of 2 to 4 carbon atoms; R 3 is independently in each occurrence a straight- or branched-chain alkenyl moiety with 2 to 10 carbon atoms; n is a number from 1 to 20; and x is a number from 2 to 8

(c) optionally one or more comonomers copolymerizable with the carboxyl-containing monomer, and

(d) an aqueous polymerization medium, to form a crosslinked hydrogel,

(II) contacting the crosslinked hydrogel with 0.001 to 15 parts by weight per hundred parts by weight of the total amount of an alkali metal or ammonium peroxodisulfate salt,

(III) comminuting the hydrogel to particles prior to or after the peroxodisulfate salt is added in step (11), and

(IV) drying the hydrogel to form resin,

wherein step (II) is conducted in the substantial absence of a combination of surfactant and a material capable of adding across or reacting with a vinylic double bond so as to form reaction products which are incapable of vinylic polymerization via free radical initiation.