Superabsorbent polymer and preparation method thereof

A superabsorbent polymer prepared by a method comprising:

• preparing a hydrogel polymer by polymerizing a water-soluble ethylenic unsaturated monomer, in the presence of a sole inner cross-linking agent and a polymerization initiators;
• drying the hydrogel polymer;
• pulverizing the dried polymer;
• and
• treating the surface of the pulverized polymer,
• wherein 0.001 to 0.01 parts by weight of the polymerization initiator and 0.2 to 2.0 parts by weight of the inner cross-linking agent are comprised, with respect to 100 parts by weight of the water-soluble ethylenic unsaturated monomer,
• wherein the step of treating the surface of the polymer includes the step of carrying out a surface cross-linking reaction of the pulverized polymer at 160 to 200° C. for 20 to 60 min by using a surface cross-linking solution including a surface cross-linking agent,
• wherein the water-soluble ethylenic unsaturated monomer includes one or more anionic monomers selected from the group consisting of maleic anhydride, fumaric acid, itaconic acid, 2-acryloyl ethane sulfonic acid, 2-methacryloyl ethane sulfonic acid, 2-(meth)acryloyl propane sulfonic acid, and 2-(meth)acrylamide-2-methyl propane sulfonic acid, and salts thereof;
• one or more nonionic hydrophilic monomers selected from the group consisting of (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxy polyethylene glycol(meth)acrylate, and polyethylene glycol (meth)acrylate;
• or one or more amino-containing unsaturated monomers selected from the group consisting of (N,N)-dimethylaminoethyl (meth)acrylate and (N,N)-dimethylaminopropyl (meth)acrylamide, and quaternary compounds thereof,
• wherein the inner cross-linking agent is one or more compounds selected from the group consisting of a C8-C12 bisacrylamide, bismethacrylamide, N,N′-methylene bisacrylamide, a poly(meth)acrylate of a C2-C10 polyol, a poly(meth)allyl ether of a C2-C10 polyol, N,N′-methylene bis(meth)acrylate, ethylene oxy(meth)acrylate, polyethylene oxy(meth)acrylate, propylene oxy(meth)acrylate, glycerin diacrylate, glycerin triacrylate, trimethylolpropane triacrylate, triallylamine, triallyl cyanurate, triallyl isocyanate, polyethylene glycol, diethylene glycol, and propylene glycol, and
• wherein the superabsorbent polymer has a degree of decrease in absorption capacity represented by the following Calculation Equation 1 is 0 to 1 (g/g):
• Degree of decrease in absorption capacity=absorption capacity during 10 min−absorption capacity during 3 h [Calculation Equation 1]
• (in said Calculation Equation 1,
• absorption capacity during 10 min is an absorption capacity value of the superabsorbent polymer measured according to the method of the following Calculation Equation 2, and
• absorption capacity during 3 h is an absorption capacity value of the superabsorbent polymer measured according to the method of the following Calculation Equation 3),
• Absorption capacity during 10 min (g/g)=[B(g)−(C(g))]/(A(g)) [Calculation Equation 2]
• (in Calculation Equation 2,
• A is the weight (g) of the superabsorbent polymer,
• B is the weight (g) of a nonwoven envelope (85×60 mm) including A (g) of the superabsorbent polymer therein, measured after soaking the same in artificial urine for 10 min, taking it out and draining the same for 5 s in the air, and removing moisture therefrom on 24 sheets of rectangular toilet paper for 10 s, and
• C is the weight (g) of a nonwoven envelope not including the superabsorbent polymer, measured after soaking the same in artificial urine for 10 min, taking it out and draining the same for 5 s in the air, and removing moisture therefrom on 24 sheets of rectangular toilet paper for 10 s), and
• Absorption capacity during 3 h (g/g)=[B(g)−(C(g))]/(A(g)) [Calculation Equation 3]
• (in Calculation Equation 3,
• A is the weight (g) of the superabsorbent polymer,
• B is the weight (g) of a nonwoven envelope (85×60 mm) including A (g) of the superabsorbent polymer therein, measured after soaking the same in artificial urine for 3 h, taking it out and draining the same for 5 s in the air, and removing moisture therefrom on 24 sheets of rectangular toilet paper for 10 s, and
• C is the weight (g) of the nonwoven envelope not including the superabsorbent polymer, measured after soaking the same in artificial urine for 3 h, taking it out and draining the same for 5 s in the air, and removing moisture therefrom on 24 sheets of rectangular toilet paper for 10 s).