Superabsorbent polymer and a preparation method thereof
The present invention relates to a superabsorbent polymer that has more improved gel strength and is improved in both water retention capacity (centrifuge retention capacity, CRC) and absorption ability under pressure (AUP) because of an optimized cross-linked structure of a base resin powder inside a cross-linked surface layer, and a preparation method thereof. The superabsorbent polymer includes a base resin powder including a 1st cross-linked polymer of a water-soluble ethylenic unsaturated monomer having acid groups of which at least a part is neutralized, and a cross-linked surface layer that includes a 2nd cross-linked polymer further cross-linked from the 1st cross-linked polymer and is formed on the base resin powder, wherein an inorganic particle is chemically bonded to the 1st cross-linked polymer by the medium of a cross-linking bond, an oxygen-containing bond (—O—), or a nitrogen-containing bond (—NR—, where R is hydrogen or a C1-C3 alkyl or an amide bond).
1. A superabsorbent polymer, including: a base resin powder including a 1st cross-linked polymer of a water-soluble ethylenic unsaturated monomer having acid groups of which at least a part is neutralized; and a cross-linked surface layer that includes a 2nd cross-linked polymer further cross-linked from the 1st cross-linked polymer and formed on the base resin powder, wherein an inorganic particle is chemically bonded to the 1st cross-linked polymer by the medium of a cross-linking bond, an oxygen-containing bond (—O—), or a nitrogen-containing bond (—NR—, where R is hydrogen or a C 1 -C 3 alkyl or amide bond).
2. The superabsorbent polymer according to claim 1 , wherein the water-soluble ethylenic unsaturated monomer includes one or more monomers selected from the group consisting of: an anionic monomer of acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloyl ethane sulfonic acid, 2-methacryloyl ethane sulfonic acid, 2-(meth)acryloyl propane sulfonic acid, or 2-(meth)acrylamide-2-methyl propane sulfonic acid, and a salt thereof; a nonionic hydrophilic monomer of (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxy polyethyleneglycol(meth)acrylate, or polyethyleneglycol (meth)acrylate; and an amino-containing unsaturated monomer of (N,N)-dimethylaminoethyl (meth)acrylate, or (N,N)-dimethylaminopropyl (meth)acrylamide, and a quaternary compound thereof.
3. The superabsorbent polymer according to claim 1 , wherein the 1st cross-linked polymer includes a cross-linked polymer prepared by polymerizing the water-soluble ethylenic unsaturated monomer in the presence of an inner cross-linking agent including a multi-functional acrylate-based compound having a plurality of ethylene oxide groups.
4. The superabsorbent polymer according to claim 1 , wherein the 1st cross-linked polymer includes a cross-linked structure in which the polymer chains polymerized from the water-soluble ethylenic unsaturated monomer are cross-linked with the cross-linkable functional group of the inner cross-linking agent by the medium of a cross-linking bond, an oxygen-containing bond, or a nitrogen-containing bond of the inorganic particle.
5. The superabsorbent polymer according to claim 3 , wherein the inner cross-linking agent includes one or more compounds selected from the group consisting of polyethyleneglycol diacrylate (PEGDA), glycerin diacrylate, glycerin triacrylate, unmodified or ethoxylated trimethylol triacrylate (TMPTA), hexanediol diacrylate, and triethyleneglycol diacrylate.
6. The superabsorbent polymer according to claim 1 , wherein the inorganic particle is a silica nanoparticle or an alumina nanoparticle.
7. The superabsorbent polymer according to claim 1 , wherein the inorganic particle is surface-modified with a cross-linkable or hydrophilic functional group including at least one functional group selected from the group consisting of a (meth)acrylate-based functional group, an allyl group, a vinyl group, an epoxy group, a hydroxy group, an isocyanate group, and an amine group.
8. The superabsorbent polymer according to claim 1 , wherein the 2nd cross-linked polymer includes a polymer that is formed by further cross-linking the 1st cross-linked polymer with a surface cross-linking agent.
9. The superabsorbent polymer according to claim 8 , wherein the surface cross-linking agent includes one or more compounds selected from the group consisting of ethyleneglycol, 1,4-butanediol, 1,6-hexanediol, polypropyleneglycol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropyleneglycol, glycerol, ethylene carbonate, and propylene carbonate.
10. The superabsorbent polymer according to claim 1 , wherein the centrifuge retention capacity (CRC) to a saline solution is 20 to 40 g/g, the absorption ability under pressure (0.7 psi) (AUP) to a saline solution is 15 to 35 g/g, and the gel strength is 4000 to 20000 Pa.
11. The superabsorbent polymer according to claim 1 , wherein 0.01 to 30 parts by weight of the inorganic particle is bonded to 100 parts by weight of the base resin powder.
12. The superabsorbent polymer according to claim 1 , having a particle diameter of 150 to 850 μm.
13. A method of preparing the superabsorbent polymer of claim 1 , including the steps of: preparing a hydrogel polymer by carrying out cross-linking polymerization of a water-soluble ethylenic unsaturated monomer having acid groups of which at least a part is neutralized, in the presence of an inner cross-linking agent and an inorganic particle of which the surface is modified with a cross-linkable or hydrophilic functional group including at least one functional group selected from the group consisting of a (meth)acrylate-based functional group, an allyl group, a vinyl group, an epoxy group, a hydroxy group, an isocyanate group, and an amine group; preparing a base resin powder by drying, pulverizing, and classifying the hydrogel polymer; and forming a cross-linked surface layer by further cross-linking the surface of the base resin powder in the presence of a surface cross-linking agent.
14. The method of preparing the superabsorbent polymer of claim 13 , wherein 2 to 200 functional groups per inorganic particle are introduced to the surface-modified inorganic particle.
15. The method of preparing the superabsorbent polymer of claim 13 , wherein the inorganic particle has a specific surface area of 5 to 600 m2/g and a diameter of 5 to 500 nm.
16. The method of preparing the superabsorbent polymer of claim 13 , wherein the surface-modified inorganic particle includes the inorganic particle of which the surface is bonded to the compound of Chemical Formula 1:
wherein, in Chemical Formula 1, R 1 to R 3 are independently a C 1 to C 10 alkyl group, a C 1 to C 10 alkoxy group, or a halogen, and at least one of them is not an alkyl group, R 4 is a C 2 to C 20 aliphatic functional group including at least one functional group selected from the group consisting of a (meth)acrylate-based functional group, an allyl group, a vinyl group, an epoxy group, a hydroxy group, an isocyanate group, and an amine group at the end, or a C 2 to C 20 hetero-aliphatic functional group of which at least one carbon is substituted with oxygen or nitrogen, including at least one functional group selected from the group consisting of a (meth)acrylate-based functional group, an allyl group, a vinyl group, an epoxy group, a hydroxy group, an isocyanate group, and an amine group at the end.