Super absorbent polymer

Abstract

The present invention relates to a super absorbent polymer exhibiting more improved liquid permeability and/or absorption rate while maintaining excellent absorption performance. Such super absorbent polymer comprises a base polymer powder including a first cross-linked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups; and a surface cross-linked layer formed on the base polymer powder and including a second cross-linked polymer in which the first cross-linked polymer is further cross-linked through an alkylene carbonate having 2 to 5 carbon atoms, and satisfies predetermined physical properties.

Claims

1. A super absorbent polymer comprising: a base polymer powder including a first cross-linked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups; and a surface cross-linked layer formed on the base polymer powder and including a second cross-linked polymer in which the first cross-linked polymer is further cross-linked via an alkylene carbonate having 2 to 5 carbon atoms, wherein the super absorbent polymer has the following features: a centrifuge retention capacity (CRC) for a physiological saline solution (0.9 wt % sodium chloride aqueous solution) for 30 minutes is 25 to 35 g/g, an absorbency under pressure (AUP) under 0.7 psi for a physiological saline solution (0.9 wt % sodium chloride aqueous solution) for 1 hour is 23.5 to 30 g/g, a saline flow conductivity (SFC) for a physiological saline solution (0.685 wt % sodium chloride aqueous solution) (.sup..Math.10.sup.7 cm.sup.3.Math.s/g) is 60 to 130 (.sup..Math.10.sup.7 cm.sup.3.Math.s/g), a particle strength is 1.5 kgf to 2.5 kgf, and a horizontal gel strength (G), measured after absorbing and swelling a physiological saline solution for 1 hour, is 10,000 to 15,000 Pa, wherein the physiological saline solution is 0.9 wt % sodium chloride aqueous solution.

2. A super absorbent polymer comprising: a base polymer powder including a first cross-linked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups; and a surface cross-linked layer formed on the base polymer powder and including a second cross-linked polymer in which the first cross-linked polymer is further cross-linked through an alkylene carbonate having 2 to 5 carbon atoms, wherein the super absorbent polymer has the features: a centrifuge retention capacity (CRC) for a physiological saline solution (0.9 wt % sodium chloride aqueous solution) for 30 minutes is 25 to 35 g/g, an absorbency under pressure (AUP) under 0.7 psi for a physiological saline solution (0.9 wt % sodium chloride aqueous solution) for 1 hour is 23.5 to 30 g/g, a saline flow conductivity (SFC) for a physiological saline solution (0.685 wt % sodium chloride aqueous solution) (.sup..Math.10.sup.7 cm.sup.3.Math.s/g) is 60 to 130 (.sup..Math.10.sup.7 cm.sup.3.Math.s/g), T-20 indicating the time required for absorbing 1 g of the super absorbent polymer to 20 g of an aqueous solution of sodium chloride and alcohol ethoxylate having 12 to 14 carbon atoms, is 100 to 190 seconds, and a particle strength is 1.5 kgf or more, and a horizontal gel strength (G), measured after absorbing and swelling a physiological saline solution for 1 hour, is 10,000 to 15,000 Pa, wherein the physiological saline solution is 0.9 wt % sodium chloride aqueous solution.

3. The super absorbent polymer of claim 1 or 2, further comprising fumed silica particles and colloidal silica particles dispersed on the surface cross-linked layer.

4. The super absorbent polymer of claim 1 or 2, wherein the super absorbent polymer has a crosslinking ratio of 30% to 90%, wherein the crosslinking ratio is measured after swelling the super absorbent polymer in a physiological saline solution (0.9 wt % sodium chloride aqueous solution) to which 20 ppmw of Toluidine Blue 0 (TBO, CAS #92-31-9) has been added for 16 hours is 30% to 90%.

5. The super absorbent polymer of claim 1 or 2, wherein the first cross-linked polymer is formed by subjecting a monomer to a crosslinking polymerization in the presence of at least one internal crosslinking agent selected form the group consisting of bis(meth)acrylamide having 8 to 12 carbon atoms, poly(meth)acrylate of polyol having 2 to 10 carbon atoms and poly(meth)acrylate having 2 to 10 carbon atoms.

6. The super absorbent polymer of claim 1 or 2, wherein it has a particle size of 150 to 850 m.

7. The super absorbent polymer of claim 1 or 2, wherein the water-soluble ethylenically unsaturated monomer includes at least one selected from the group consisting of anionic monomers, non-ionic, hydrophilic group-containing monomers, and amino group-containing unsaturated monomers.

8. The super absorbent polymer of claim 7, wherein the anionic monomers are selected from the group consisting of acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, and salts thereof.

9. The super absorbent polymer of claim 7, wherein the non-ionic, hydrophilic group-containing monomers are selected from the group consisting of (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol(meth)acrylate, and polyethylene glycol (meth)acrylate.

10. The super absorbent polymer of claim 7, wherein the amino group-containing unsaturated monomers are selected from the group consisting of (N,N)-dimethylaminoethyl(meth)acrylate, (N,N)-dimethylaminopropyl(meth)acrylamide, and quaternary products thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a photograph showing examples of surface cross-linked particles in the measurement of the surface crosslinking ratio of the super absorbent polymer of Examples and Comparative Examples.

(2) FIG. 2 is a photograph showing an example of surface non-cross-linked particles in the measurement of surface cross-linking ratios of the super absorbent polymer of Examples and Comparative Examples.

(3) FIG. 3 is a photograph showing an example of a large particle in which several particles are aggregated without surface crosslinking in the measurement of the surface crosslinking ratio of the super absorbent polymer of Examples and Comparative Examples.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) Hereinafter, the preferred Examples are provided for better understanding of the invention. However, these Examples are given for illustrative purposes only and not intended to limit the scope of the present invention.

(5) In Examples and Comparative Examples, the contact angles of a surface crosslinking liquid and physical properties of the respective super absorbent polymers were measured and evaluated by the following methods.

(6) (1) Contact Angle of Surface Crosslinking Liquid

(7) After forming the surface crosslinking solution used in Examples and Comparative Examples, the contact angle was measured and evaluated by dropping on a PET substrate (Mitsuibishi Polyester Film, Grade name: O0300E).

(8) (2) Evaluation of Particle Size

(9) The particle sizes of the base polymer powders and the super absorbent polymers used in Examples and Comparative Examples were measured in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 220.3.

(10) (3) Centrifuge Retention Capacity(CRC)

(11) For the absorbent polymers prepared in Examples and Comparative Examples, the centrifuge retention capacity (CRC) by absorption magnification under a non-loading condition was measured in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 241.3.

(12) That is, after uniformly inserting W.sub.0(g) (about 0.2 g) of each polymer obtained in Examples and Comparative Examples in a nonwoven fabric-made bag and sealing the same, it was soaked in a physiological saline solution composed of 0.9 wt % sodium chloride aqueous solution at room temperature. After 30 minutes, water was removed from the bag by centrifugation at 250 G for 3 minutes, and the weight W.sub.2(g) of the bag was then measured. Further, the same procedure was carried out without using the polymer, and then the resultant weight W.sub.1(g) was measured.

(13) Using the respective weights thus obtained, the CRC(g/g) was determined according to the following Calculation Equation 1.

(14) $\begin{array}{cc}\mathrm{CRC}\left(g/g\right)=\left\{\left[W_2\left(g\right)-W_1\left(g\right)-W_0\left(g\right)\right]/W_0\left(g\right)\right\}& \left[\mathrm{Calculation}\mathrm{Equation}1\right]\end{array}$

(15) in Calculation Equation 1,

(16) W.sub.0(g) is an initial weight(g) of the super absorbent polymer,

(17) W.sub.1(g) is a weight of the device not including the super absorbent polymer, measured after soaking the same in a physiological saline solution for 30 minutes and dehydrating the same by using a centrifuge at 250 G for 3 minutes, and

(18) W.sub.2(g) is a weight of the device including the super absorbent polymer, measured after soaking the super absorbent polymer in a physiological saline solution at room temperature for 30 minutes, and then dehydrating the same by using a centrifuge at 250 G for 3 minutes.

(19) (4) Absorbency Under Pressure (AUP)

(20) For the absorbent polymers prepared in Examples and Comparative Examples, the absorbency under pressure was measured in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 242.3.

(21) First, a 400 mesh stainless steel net was installed in the cylindrical bottom of a plastic having an internal diameter of 60 mm. W.sub.0(g, 0.90 g) of the absorbent polymers prepared in Examples 1-6 and Comparative Examples 1-3 were uniformly scattered on the steel net under conditions of temperature of 232 C. and relative humidity of 45%, and a piston which can provide a load of 4.83 kPa (0.7 psi) uniformly was put thereon. The external diameter of the piston was slightly smaller than 60 mm, there was no gap between the cylindrical internal wall and the piston, and the jig-jog of the cylinder was not interrupted. At this time, the weight W.sub.3(g) of the device was measured.

(22) After putting a glass filter having a diameter of 125 mm and a thickness of 5 mm in a Petri dish having a diameter of 150 mm, a physiological saline solution composed of 0.90 wt % of sodium chloride was poured in the dish until the surface level became equal to the upper surface of the glass filter. A sheet of filter paper having a diameter of 120 mm was put thereon. The measuring device was put on the filter paper and the solution was absorbed under a load for 1 hour. After 1 hour, the weight W.sub.4(g) was measured after lifting the measuring device up.

(23) Using the respective weights thus obtained, AUP(g/g) was calculated according to the following Calculation Equation 2, thereby confirming the absorbency under pressure.

(24) $\begin{array}{cc}\mathrm{AUP}\left(g/g\right)=\left[W_4\left(g\right)-W_3\left(g\right)\right]/W_0\left(g\right)& \left[\mathrm{Calculation}\mathrm{Equation}2\right]\end{array}$

(25) in Calculation Equation 2,

(26) W.sub.0(g) is an initial weight(g) of the super absorbent polymer,

(27) W.sub.3(g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, and

(28) W.sub.4(g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, after absorbing a physiological saline solution to the super absorbent polymer under a load (0.7 psi) for 1 hour.

(29) (5) Particle Strength

(30) Using the Texture Analyzer (model name: XT2 Plus, TA), the particle strength of the super absorbent polymers of Examples and Comparative Examples was measured as follows.

(31) First, the super absorbent polymer samples (30 to 50 mesh) of Examples and Comparative Examples were sieved off to collect samples having a particle size of 600 to 850 m. A single particle of the super absorbent polymer was extracted from the collected samples. After a plate module was placed on the above measuring device, the single particle was placed on the plate module. A cylinder with a diameter of 8 mm was lowered parallel to the plate module just above the single particle. While lowering the cylinder at a speed of 0.01 mm/s, it was pressurized at a constant speed. The force with which a single particle can withstand with time was measured, and the maximum force applied to the particle until crushing was measured. The maximum force during crushing thus obtained was determined several times through experiments, and the average force thereof was calculated and defined as the particle strength.

(32) (6) Gel Strength (G)

(33) For the absorbent polymers/base polymer powders prepared in Examples and Comparative Examples, the horizontal gel strength was measured.

(34) First, the absorbent polymer samples (30-50 mesh) prepared in Examples and Comparative Examples were sieved off and 0.5 g of the samples were weighed. The weighed samples were sufficiently swelled in 50 g of a physiological saline solution for 1 hour. After that, the solvent not absorbed therein was removed by using an aspirator for 4 minutes, and the solvent left on the surface of the same was evenly distributed and wiped once with a filter paper.

(35) 2.5 g of the swelled super absorbent polymer was loaded between two parallel plates (parallel plates with a 25 mm diameter, a lower plate thereof having a wall with a 2 mm height for preventing the sample from leaking) of the rheometer, and the gap (1 mm) between the parallel plates was adjusted. At this time, the gap between the parallel plates was properly adjusted by pressing the plates with a force of about 3 N so that the swelled sample was contacted evenly at the face of the plates.

(36) A linear viscoelastic regime section of strain where the storage modulus and the loss modulus were steady was found by using the rheometer while increasing the shear strain at a 10 rad/s oscillation frequency. Generally, in the case of a swelled super absorbent polymer, a strain of 0.1% is imparted in the liner viscoelastic regime section.

(37) The storage modulus and the loss modulus of the swelled super absorbent polymer was measured by using the strain value of the linear viscoelastic regime section at an oscillation frequency of 10 rad/s for 60 seconds. The horizontal gel strength was obtained by taking an average of the obtained storage modulus. For reference, the loss modulus was measured as a very small value as compared to the storage modulus.

(38) (7) Saline Flow Conductivity (SFC)

(39) The saline flow conductivity was measured in accordance with the method disclosed in Columns 54 to 59 of U.S. Pat. No. 5,562,646.

(40) (8) Free Swell Rate (FSR)

(41) The FSR of the base polymer powder or the super absorbent polymer was measured and calculated according to the method disclosed on pages 22 to 23 of European Patent Publication No. 2535027 using those classified into #30 to #50 (for example, those having a particle size of 300 to 600 m).

(42) (9) T-20

(43) 9 g of sodium chloride and 1 g of Lorodac (main component: alcohol ethoxylate having 12 to 14 linear carbon atoms, CAS #68439-50-9) were dissolved in 1 L of distilled water, and the T-20 was calculated and measured with the time required for absorbing 1 g of the super absorbent polymer to 20 g of this aqueous solution. Specific measurement method of T-20 is described in detail on pages 13 to 18 of European Patent Publication No. 2535027.

(44) (10) Surface Crosslinking Ratio

(45) 0.5 g of a super absorbent polymer (powder having a particle size of 300 to 600 m was classified and taken) was swollen in a physiological saline solution (0.9 wt % sodium chloride aqueous solution) to which 20 ppmw of Toluidine Blue O (TBO, CAS #92-31-9) had been added for 16 hours. After swelling, the solution and the super absorbent polymer powder were poured into a Petri dish, and then the super absorbent polymer powder was observed with a Stereotype Microscope, and the surface crosslinking ratio of the super absorbent polymer was evaluated as follows.

(46) First, the photographs (image resolution: 1024768, transmission mode, stereotype) of the whole particles of the super absorbent polymer after swelling at 10 magnification are photographed, and the size and length of the photographs were adjusted so that the total number of particles in the photographs was 200 to 300. Next, the paper size was set to A4 size with Microsoft's Power Point 2010 version, the photographed picture was pasted, and the picture size was adjusted to 19 cm25.33 cm. Then, the contrast of the photograph was adjusted to about 40% in black and white, and the brightness of the photograph was adjusted at an appropriate brightness, for example, at a reflectance of about 18%.

(47) As a result, for example, photographs as shown in FIGS. 1 to 3 were obtained. In the photographs of the images containing such whole particles, particles having stripes with a width of 2 mm or more and a length of 5 mm or more formed on the particles were observed. For reference, the stripes having a width of 2 mm or more and a length of 5 mm or more are observed while the SHELL of the super absorbent polymer of the surface cross-linked layer was broken. The degree of surface crosslinking could be confirmed and evaluated by the traces that the SHELL was broken by swelling.

(48) As shown in FIG. 1, particles having stripes with a width of 2 mm or more and a length of 5 mm or more formed on the particles were evaluated as surface cross-linked super absorbent particles, and as shown in FIGS. 2 and 3, particles without stripe formation can be evaluated as surface non-cross-linked particles. For reference, FIG. 2 is a photograph showing an example of surface non-cross-linked super absorbent polymer particles in which stripe patterns were not formed on the single particle. FIG. 3 is a photograph showing an example of a large particle in which several particles were aggregated. The stripe patterns were not observed also on such a large particle, and it was evaluated that it was not surface-cross-linked.

(49) Under this criterion, the ratio of numbers of the surface cross-linked particles among the particles of the entire super absorbent polymer produced under the same conditions was calculated by the surface cross-linkage ratio. The surface cross-linking ratio was calculated according to the following formula: Surface crosslinking ratio (%)=[Number of particles evaluated as surface cross-linked particles/Number of total particles in photograph]100.

Example 1

(50) 100 g of acrylic acid, 0.5 g of polyethylene glycol diacrylate (Mw=523) as a crosslinking agent, 83.3 g of 50% caustic soda (NaOH), and 89.8 g of water were mixed to prepare a monomer aqueous solution composition having a monomer concentration of 45% by weight.

(51) Subsequently, 810 g of the monomer aqueous solution was first mixed with 30.54 g of a 0.18% ascorbic acid solution and 33 g of a 1% sodium persulfate solution, and the mixture was fed through a feed section of a continuous polymerization reactor with a kneader, together with 30.45 g of a 0.15% hydrogen peroxide solution, so as to perform polymerization. At this time, temperature of the polymerization reactor was maintained at 80 C., and the maximum polymerization temperature was 110 C. and the polymerization time was 1 min and 15 s. Thereafter, kneading was continuously performed, and polymerization and kneading were performed for 20 min. Thereafter, the size of the polymer produced was distributed to less than 0.2 cm. At this time, the water content of the hydrogel polymer finally formed was 51% by weight.

(52) Subsequently, the gel pulverization was performed by using Meat Chopper (SL Company, SM3-2) pulverizing device, and using a gel-pulverizing device in which an inverter for axis control (for example, manufacturer: LS Industrial Systems, model name: iG5A) was installed, an S-13 mm perforated plate (for example, an aperture ratio of 25%) having a diameter of 10 mm was installed. More specifically, the gel pulverization was performed by rotating the shaft at a speed of 60 Hz while passing the hydrogel polymer through a pulverizing device.

(53) Subsequently, the resulting polymer was dried in a hot-air dryer at a temperature of 175 C. for 30 minutes, and the dried hydrogel polymer was pulverized by the following method.

(54) First, as the pulverizing device, a food mixer type pulverizing device (manufacturer: HANIL, model name: HMF-30005) was used, 500 g of the dried polymer powder was placed in the pulverizing device, and pulverized for 15 seconds with strength weak, and additionally 500 g was further used and pulverized by the same method.

(55) 1 kg of the polymer powder thus pulverized was put in a classifier (Restsch, AS200) and classified with an amplitude of 1.5 mm into five mesh sizes (combination of classified meshes: #25/#30/#50/#100/PAN), and respective classified particles were collected. Then, the large particles collected on the #25 sieve were additionally pulverized for 15 seconds, and such additional pulverization was two times.

(56) Through the above-described method, a polymer having a particle size of about 150 m to 850 m was classified and obtained. After the base polymer powder was obtained by the above method, the FSR thereof was measured. As a result, it was confirmed to be 0.24 g/g.Math.s.

(57) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 0.04 part by weight of fumed silica particles (Aerosil 200), 0.01 part by weight of colloidal silica particles (ST-O), and 0.05 part by weight of the polycarboxylic acid-based copolymer disclosed in Preparation Example 1 of Korean Patent Laid-open Publication No. 2015-0143167 (Korean Patent Application No. 2014-0072343) were mixed to form a surface treatment solution. Such a surface treatment solution was sprayed onto the base polymer powder, stirred at room temperature, and mixed so that the surface treatment liquid was evenly distributed on the base polymer powder.

(58) Thereafter, the base polymer powder was placed in a surface crosslinking reactor and the surface cross-linking reaction was performed.

(59) In the surface crosslinking reactor, it was confirmed that the base polymer powder was gradually heated at an initial temperature near 20 C. After 20 minutes elapsed, operation was performed so as to reach the maximum reaction temperature of 185 C. After reaching the maximum reaction temperature, additional reaction was carried out for 40 minutes, and a sample of the finally produced super absorbent polymer was taken. After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Example 2

(60) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(61) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 0.04 part by weight of fumed silica particles (Aerosil 200), 0.02 part by weight of colloidal silica particles (ST-O), and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Then, surface crosslinking was carried out in the same manner as in Example 1.

(62) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Example 3

(63) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(64) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 0.04 part by weight of fumed silica particles (Aerosil 200), 0.03 part by weight of colloidal silica particles (ST-O), and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Then, surface crosslinking was carried out in the same manner as in Example 1.

(65) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Example 4

(66) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(67) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 0.04 part by weight of fumed silica particles (Aerosil 200), 0.04 part by weight of colloidal silica particles (ST-O), and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Then, surface crosslinking was carried out in the same manner as in Example 1.

(68) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Example 5

(69) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(70) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 0.02 part by weight of fumed silica particles (Aerosil 200), 0.06 part by weight of colloidal silica particles (ST-O), and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Subsequently, surface crosslinking was carried out in the same manner as in Example 1.

(71) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Example 6

(72) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(73) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 3.5 parts by weight of methanol, 0.02 part by weight of fumed silica particles (Aerosil 200), 0.06 part by weight of colloidal silica particles (ST-O), and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Subsequently, surface crosslinking was carried out in the same manner as in Example 1.

(74) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Example 7

(75) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(76) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 3.5 parts by weight of methanol, 0.04 part by weight of fumed silica particles (DM30S), 0.06 part by weight of colloidal silica particles (ST-O), and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Then, surface crosslinking was carried out in the same manner as in Example 1.

(77) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Example 8

(78) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(79) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 3.5 parts by weight of methanol, 0.04 part by weight of fumed silica particles (Aerosil 200), 0.06 part by weight of colloidal silica particles (ST-AK), and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Subsequently, surface crosslinking was carried out in the same manner as in Example 1.

(80) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Example 9

(81) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(82) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, 3.5 parts by weight of methanol, 0.04 part by weight of fumed silica particles (DM30S), 0.06 part by weight of colloidal silica particles (ST-AK), and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Then, surface crosslinking was carried out in the same manner as in Example 1.

(83) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Comparative Example 1

(84) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(85) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of ethylene carbonate, 3.5 parts by weight of water, and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Then, surface crosslinking was carried out in the same manner as in Example 1.

(86) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Comparative Example 2

(87) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(88) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of 1,3-propanediol, 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 and 3.5 parts by weight of water were mixed to form a surface treatment solution. Then, surface crosslinking was carried out in the same manner as in Example 1. After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

Comparative Example 3

(89) A base polymer powder was prepared in the same manner as in Example 1. The FSR thereof was measured, and as a result, it was confirmed to be 0.24 g/g.Math.s.

(90) Thereafter, based on 100 parts by weight of the base resin powder, 0.4 part by weight of 1,3-propanediol, 3.5 parts by weight of water, 0.5 part by weight of aluminum sulfate and 0.05 part by weight of the same polycarboxylic acid-based copolymer as disclosed in Example 1 were mixed to form a surface treatment solution. Then, surface crosslinking was carried out in the same manner as in Example 1.

(91) After the surface crosslinking step, a surface cross-linked super absorbent polymer having a particle size of about 150 to 850 m was obtained by using a sieve. The content of the fine powder having a particle size of about 150 m or less in the product of the super absorbent polymer was less than 1.0 wt %.

(92) For the super absorbent polymers of Examples 1 to 9 and Comparative Examples 1 to 3, the physical property measurement and evaluation of CRC, AUP, SFC, gel strength(G), T-20 and surface crosslinking ratio were carried out, and the measured physical property values are shown in Table 1 below.

(93) TABLE-US-00001 TABLE 1 Surfaces SFC Particel Gel crosslinking CRC AUP (.Math.10.sup.7 T- strength strength ratio(%) (g/g) (g/g) cm.sup.3 .Math. s/g) 20(s) (kgf) (Pa) Example 1 35.7 27.2 24.1 69 173 1.61 11405 Example 2 40.8 27.4 23.5 84 152 1.73 11047 Example 3 43.1 27.3 23.8 94 138 1.82 11153 Example 4 45.6 27.6 24.4 116 145 2.01 12553 Example 5 46.2 27.1 24.1 112 152 1.91 12027 Example 6 42.1 27.3 23.7 90 140 1.58 11305 Example 7 47.3 27.4 23.8 98 152 1.52 10069 Example 8 42.1 27.9 23.6 87 156 1.55 11203 Example 9 46.2 27.3 23.5 91 145 1.53 11032 Comparative 26.0 26.2 21.6 47 160 1.23 9006 Example 1 Comparative 22.1 26.4 21.1 43 156 1.17 9001 Example 2 Comparative 26.3 25.8 21.8 48 133 1.10 9332 Example 3

(94) Referring to Table 1, it was confirmed that Examples exhibited not only high surface crosslinking ratio, excellent absorption performance (particularly, absorbency under pressure) and liquid permeability, but also excellent particle strength and gel strength and the like as compared with Comparative Examples. Particularly, it was confirmed that Examples exhibited excellent physical properties by using various colloidal silica particles and fumed silica particles during surface crosslinking.