Superabsorbent polymer compositions having color stability

1. A superabsorbent polymer composition comprising a superabsorbent polymer comprising:

a) from about 55% to about 99.9% by weight of the superabsorbent polymer of polymerizable unsaturated acid group containing monomer based on the superabsorbent polymer; and

b) from about 0.001% to about 5% by weight of internal crosslinking agent based on the polymerizable unsaturated acid group containing monomer;

c) from about 50 to 1000 ppm of a thermal initiator based on the polymerizable unsaturated acid group containing monomer wherein the thermal initiator is selected from sodium persulfate, potassium persulfate or ammonium persulfate; wherein the superabsorbent polymer has a degree of neutralization of from about 50 mol % to about 80 mol %; wherein elements a), b) and c) are polymerized into a crosslinked hydrogel, which is then prepared into superabsorbent polymer particles; the superabsorbent polymer composition further comprising the following surface additives to form superabsorbent polymer composition particles

i) from about 0.001% to about 5% by weight of surface crosslinking agent based on the superabsorbent polymer composition;

ii) from about 0.01% to about 5% by weight of an antioxidant based on the superabsorbent polymer composition is added to the polymerized product whether in crosslinked hydrogel or subsequent superabsorbent polymer particles or superabsorbent polymer composition particle form, wherein the antioxidant is selected from sulfinic acid, 2-hydroxy-2-sulfinatoacetic acid, 2-hydroxy-2-sulfonatoacetic acid, sulfamic acid, and sulfonic acid, and salts and derivatives of the foregoing and mixtures thereof, and

iii) from about 0.1% to about 1% by weight of the dry superabsorbent polymer composition of a chelating agent.

2. The superabsorbent polymer composition of claim 1 wherein the chelating agent is selected from an aminocarboxylic acid metal chelating agent iminodiacetic acid, hydroxyethyl iminodiacetate, nitrilotriacetic acid, nitrilotripropionic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, pentasodium diethylenetriaminepentaacetate, triethylenetetraminehexaacetic acid, trans-1,2-diaminocyclohexanetetraacetic acid, N,N-bis(2-hydroxyethyl)glycine, diaminopropanoltetraacetic acid, ethylenediaminedipropionic acid, hydroxyethylenediaminetriacetic acid, glycoletherdiaminetetraacetic acid, diaminopropanetetraacetic acid, N,N′-bis(2-hydroxybenzyl)ethylenediamine-N,N-diacetic acid, 1,6-hexamethylenediamine-N,N,N′,N′-tetraacetic acid and/or salts thereof; or a polyphosphoric acid metal chelating agent selected from pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, and salts thereof.

3. The superabsorbent polymer composition of claim 1 wherein the chelating agent comprises a polyphosphoric acid metal chelating agent.

4. The superabsorbent polymer composition of claim 2 wherein the polyphosphoric acid metal chelating agent is selected from pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, and salts thereof.

5. The superabsorbent polymer composition according to claim 1 wherein the antioxidant comprises sulfinic acid or derivative thereof.

6. The superabsorbent polymer composition according to claim 1 wherein the antioxidant comprises 2-hydroxy-2-sulfinatoacetic acid.

7. The superabsorbent polymer composition of claim 1 having a free swell gel bed permeability of at least about 6 Darcy as measured by the Free Swell Gel Bed Permeability Test.

8. The superabsorbent polymer composition according to claim 1 wherein the superabsorbent polymer composition, after storage for 42 days at 40° C. and 80% relative humidity, exhibits a Whiteness Index of at least about 7.5 as measured by the Whiteness Index Test.

9. A method to make a superabsorbent polymer composition comprising the steps of:

a) preparing a superabsorbent polymer by the process of polymerizing from about 55% to about 99.9% by weight of polymerizable unsaturated acid group containing monomer based on the superabsorbent polymer, and from about 0.001% to about 5% by weight of an internal crosslinking agent based on the polymerizable unsaturated acid group containing monomer, and from about 50 to 1000 ppm of a thermal initiator based on the polymerizable unsaturated acid group containing monomer wherein the thermal initiator is selected from sodium persulfate, potassium persulfate or ammonium persulfate, wherein the superabsorbent polymer has a degree of neutralization of greater than about 25%;

b) polymerizing the components of a) into a hydrogel; c) preparing superabsorbent polymer particles from the superabsorbent polymer; d) treating the superabsorbent polymer particles with surface additives comprising:

i) from about 0.01% to about 5% by weight of an antioxidant based on the superabsorbent polymer composition is added to the polymerized product whether in crosslinked hydrogel or subsequent superabsorbent polymer particles or superabsorbent polymer composition particle form, wherein the antioxidant is selected from a sulfinic acid, 2-hydroxy-2-sulfinatoacetic acid, 2-hydroxy-2-sulfonatoacetic acid, sulfamic acid, and sulfonic acid, and salts and derivatives of the foregoing; and

ii) from about 0.1% to about 1% by weight of the dry superabsorbent polymer composition of a chelating agent and wherein the superabsorbent polymer composition has a free swell gel bed permeability of at least about 6 Darcy as measured by the Free Swell Gel Bed Permeability Test.

10. The method of claim 9 wherein the chelating agent is selected from an aminocarboxylic acid metal chelating agent comprising iminodiacetic acid, hydroxyethyl iminodiacetate, nitrilotriacetic acid, nitrilotripropionic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, pentasodium diethylenetriaminepentaacetate, triethylenetetraminehexaacetic acid, trans-1,2-diaminocyclohexanetetraacetic acid, N,N-bis(2hydroxyethyl)glycine, diaminopropanoltetraacetic acid, ethylenediaminedipropionic acid, hydroxyethylenediaminetriacetic acid, glycoletherdiaminetetraacetic acid, diaminopropanetetraacetic acid, N,N′-bis(2-hydroxybenzyl)ethylenediamine-N,N-diacetic acid, 1,6-hexamethylenediamine-N,N,N′,N′-tetraacetic acid and/or salts thereof; or a polyphosphoric acid metal chelating agent selected from pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, and salts thereof.

11. The method of claim 9 wherein the chelating agent comprises a polyphosphoric acid metal chelating agent.

12. The method of claim 10 wherein the polyphosphoric acid metal chelating agent is selected from pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, and salts thereof.

13. The method of claim 9 wherein the antioxidant comprises 2-hydroxy-2-sulfinatoacetic acid.

14. The method of claim 9 having a free swell gel bed permeability of at least about 6 Darcy as measured by the Free Swell Gel Bed Permeability Test.

15. A sanitary article comprising the superabsorbent polymer composition of claim 1 .

16. A sanitary article comprising the superabsorbent polymer composition made by the method of claim 9 .

17. The superabsorbent polymer composition of claim 1 wherein the surface additives further includes iv) from 0.05 to about 2.0 wt % of a multivalent metal salt based on the dry superabsorbent polymer composition.

18. The superabsorbent polymer composition of claim 1 wherein the multivalent metal salt comprises aluminum sulfate.

19. The superabsorbent polymer composition of claim 1 wherein the surface cross linking agent comprises ethylene carbonate.