Super absorbent polymer and method for preparing the same

Abstract

The present invention relates to a super absorbent polymer and a method for preparing the same. The present invention has features that it is possible to prepare a super absorbent resin that can have an improved centrifuge retention capacity (CRC) and a high absorbency under load (AUL) by controlling the shape and size of the chopper die holes during coarse pulverization of a hydrous gel phase polymer.

Claims

1. A method of preparing a superabsorbent polymer comprising: 1) carrying out thermal polymerization or photopolymerization of a monomer composition including a water-soluble ethylene-based unsaturated monomer and a polymerization initiator to form a hydrous gel phase polymer; 2) passing the hydrous gel phase polymer through a chopper die and pulverizing the polymer; 3) drying the pulverized hydrous gel phase polymer; 4) pulverizing the dried polymer; and 5) surface-crosslinking the pulverized polymer, wherein the chopper die is provided with a plurality of non-circular holes, the chopper die has an opening/ closing rate of 30 to 40%, and the chopper die satisfies the following Mathematical Formula 1:
c>2{square root over (0.4nA)}[Mathematical Formula 1] wherein A is the overall area of the upper surface of the chopper die with a unit mm.sup.2, n is the number of holes, and c is the total circumference of the plurality of holes, and wherein the value of c is 500 to 800 mm.

2. The method of claim 1, wherein the thickness of the chopper die is 5 to 15 mm.

3. The method of claim 1, wherein the n-holes have the identical shape with each other.

4. The method of claim 1, wherein n is an integer of 10 to 30.

5. The method of claim 1, wherein the pulverizing in step 2) is a step of pulverizing the hydrous gel phase polymer into a particle size of 2 to 10 mm.

6. The method of claim 1, wherein the drying in step 3) is carried out at a temperature of 120 to 250 C.

7. The method of claim 1, wherein the pulverizing in step 4) is carried out so that the particle size of the pulverized polymer becomes 150 to 850 m.

8. The method of claim 1, wherein the surface-crosslinking in Step 5) is carried out at a temperature of 100 to 250 C.

9. The method of claim 1, wherein the water-soluble ethylene-based unsaturated monomer is a compound represented by the following Chemical Formula 1:
R.sub.1COOM.sup.1[Chemical Formula 1] in Chemical Formula 1, R.sub.1 is a C.sub.2-C.sub.5 alkyl group including an unsaturated bond, and M.sup.1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

10. The method of claim 1, wherein the water-soluble ethylene-based unsaturated monomer includes one or more selected from the group consisting of: acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, 2-(meth)acrylamide-2-methyl propane sulfonic acid, (meth)acrylamide, N- substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxy polyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate,(N,N)-dimethylaminoethyl(meth)acrylate, and (N,N)-dimethylaminopropyl(meth)acrylamide.

11. The method of claim 1, wherein the surface crosslinking is carried out by reacting one or more crosslinking agents selected from the group consisting of ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, ethylene carbonate, ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, propanediol, dipropylene glycol, polypropylene glycol, glycerin, polyglycerin, butanediol, heptanediol, hexanediol, trimethylolpropane, pentaerythritol, sorbitol, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, iron hydroxide, calcium chloride, magnesium chloride, aluminum chloride, and iron chloride.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1 to 3 illustrate the chopper dies used in an example of the present invention, respectively.

(2) FIGS. 4 and 5 illustrate the chopper dies used in a comparative example of the present invention, respectively.

(3) FIG. 6 represents a SEM image of the surface of the absorbent polymer prepared an example and a comparative example of the present invention. FIG. 6(a) to FIG. 6(d) represent the absorbent polymers prepared in Comparative Example 1, Example 1, Example 2 and Example 3, respectively.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) Hereinafter, the preferred Examples are provided for better understanding. However, these Examples are given for illustrative purposes only and the invention are not intended to be limited by these Examples.

EXAMPLE 1

(5) 20 g of PEGDA (polyethyleneglycol diacrylate) and 14 g of ETTA (trimethylolpropane triacrylate) as the internal crosslinking agent were mixed with 490 g of acrylic acid, followed by mixing with 80 ppm of a photoinitiator (Irgacure 819). 670 g of 32% aqueous sodium hydroxide solution was added to the mixture to prepare a acrylic acid monomer composition with a neutralization degree of 70 mol %. The monomer composition was sufficiently stirred while maintaining at 421 C. Polymerization was initiated after about 30 seconds, and the resulting gel was divided by applying a shear force to the gel for 5 minutes to which 260 g of water was then added to prepare a gel. The gel thus prepared was coarsely pulverized for about 5 minutes, and the chopper dies (FIG. 1) used in the coarse pulverization were as follows. Diameter of a chopper die: 81 mm Thickness of a chopper die: 10 mm Center hole of a chopper die: 18.5 mm Diameter of the side groove of a chopper die: 2.5 mm Shape of holes: shamrock shape Number of holes: 10 Total circumference of holes: 550 mm

(6) (wherein the center hole of the chopper die is connected with spindles for rotating the chopper die, and the following Examples and Comparative Examples are likewise).

(7) 1 kg of the coarsely pulverized gel was evenly spread on the dryer and dried. Drying was divided into a total of nine zones, among which the initial three zones were dried at a temperature of 185 C. and the remaining zones were dried at a temperature of 180 C., the respective zones being proceeded for about 4 to 5 minutes. The drying was carried out by setting the direction of the drying air four times from bottom to top and five times from top to bottom, and thus the drying was carried out for a total of 41 minutes. The dried polymer was pulverized by using a pulverizing device and size-classified through a standard mesh sieve according to ASTM standard to obtain an absorbent polymer powder having a particle size of 150 to 850 m.

(8) To 50 g of the resulting polymer powder, the surface crosslinking solution containing 0.4 g of ethylene carbonate as a surface crosslinking agent, 3.5 g of methanol; 3 g of water and 0.1 g of silica (Aerosil 380) was added and uniformly mixed, followed by drying in a hot air oven at 190 C. for 40 minutes. The dried powder was size-classified through a standard mesh sieve according to ASTM standard to obtain a super absorbent polymer having a particle size of 150 to 850 m.

EXAMPLE 2

(9) The super absorbent polymer was prepared in the same manner as in Example 1, with the exception that the following chopper dies (FIG. 2) were used. Diameter of a chopper die: 81 mm Thickness of a chopper die: 10 mm Center hole of a chopper die: 18.5 mm Diameter of the side groove of a chopper die: 2.5 mm Shape of holes: star shape Number of holes: 10 Total circumference of holes: 650 mm

EXAMPLE 3

(10) The super absorbent polymer was prepared in the same manner as in Example 1, with the exception that the following chopper dies (FIG. 3) were used. Diameter of a chopper die: 81 mm Thickness of a chopper die: 10 mm Center hole of a chopper die: 18.5 mm Diameter of the side groove of a chopper die: 2.5 mm Shape of holes: banana shape Number of holes: 10 Total circumference of holes: 640 mm

EXAMPLE 4

(11) The super absorbent polymer was prepared in the same manner as in Example 2, with the exception that the surface crosslinking solution containing 0.3 g of 1,3-propane diol as a surface crosslinking agent, 3 g of methanol, 3 g of water and 0.1 g of silica (Aerosil 200) was used.

EXAMPLE 5

(12) The super absorbent polymer was prepared in the same manner as in Example 3, with the exception that the surface crosslinking solution containing 0.3 g of 1,3-propane diol as a surface crosslinking agent, 3 g of methanol, 3 g of water and 0.1 g of silica (Aerosil 200) was used.

COMPARATIVE EXAMPLE 1

(13) The super absorbent polymer was prepared in the same manner as in Example 1 with the exception that the following chopper dies (FIG. 4) were used. Diameter of a chopper die: 81 mm Thickness of a chopper die: 10 mm Shape of holes: circular shape with a diameter of 14 mm Center hole of a chopper die: 18.5 mm Diameter of the side groove of a chopper die: 2.5 mm Number of holes: 10 Total circumference of holes: 440 mm

COMPARATIVE EXAMPLE 2

(14) The super absorbent polymer was prepared in the same manner as in Example 1, with the exception that the following chopper dies (FIG. 5) were used. Diameter of a chopper die: 81 mm Thickness of a chopper die: 10 mm Cater hole of a chopper die: 18.5 mm Diameter of the side groove of a chopper die: 2.5 mm Shape of holes: circular shape with a diameter of 8 mm Number of holes: 34 Total circumference of holes: 854 mm

EXPERIMENTAL EXAMPLE 1

(15) 1) Ratio of the Absorbency Under Load at 5 Minutes to the Absorbency Under Load at 60 Minutes (ARUL)

(16) ARUL (absorbing rate under load) was measured for the super absorbent polymers prepared in the Examples and Comparative Examples according to the following Equation:
ARUL=0.3AUL(5 min)/0.3AUL(60 min)[Equation 1]

(17) in Equation 1,

(18) 0.3AUL (5 min) and 0.3AUL (60 min) are the values of absorbency under load (AUL) at 5 minutes and 60 minutes shown in the following Equation 2, respectively,
0.3AUL(g/g)=[Wb(g)Wa(g)]/mass(g) of the absorbent polymer[Equation 2]

(19) in Equation 2,

(20) Wa(g) is the sum of the weight of the absorbent polymer and the weight of the device capable of providing a load for the absorbent polymer, and

(21) Wb(g) is the sum of the weight of the absorbent polymer in which moisture is absorbed after supplying water for the absorbent polymer under a load (0.3 psi) for 5 minutes or 1 hour, and the weight of the device capable of providing a load for the absorbent polymer.

(22) In this case, the absorbency under load was measured according to the EDANA WSP 242.2 method.

(23) A 400 mesh stainless steel net was installed in the bottom of a plastic cylinder having an internal diameter of 60 mm. 0.9 g of the absorbent polymer was uniformly scattered on the steel net at the room temperature and the humidity of 50%, and a piston which can provide a load of 0.3 psi uniformly was put thereon. The external diameter of the piston was slightly smaller than 60 mm, there was no gap between the internal wall of the cylinder and the piston, and the jig-jog of the cylinder was not interrupted. In this regard, the weight Wa(g) of the device was measured.

(24) After putting a glass filter having a diameter of 90 mm and a thickness of 5 mm in a Petri dish having a diameter of 150 mm, a physiological saline solution composed of 0.9 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 90 mm was put thereon. The measuring device was put on the filter paper and the solution was absorbed under a load. After 5 minutes and 1 hour, the weight Wb(g) was measured after lifting the measuring device up. 0.3AUL (5 min) and 0.3AUL (60 min) were calculated from the above measured results, respectively.

(25) The measured results are shown in Table 1 below.

(26) 2) Absorbency Under Load (0.9AUL (60 Min))

(27) The absorbency under load (0.9AUL (60 min)) was measured for the absorbent polymers prepared in the Examples and Comparative Examples according to the following Equation 3:
0.9AUL(g/g)=[Wb(g)Wa(g)]/weight(g) of the absorbent polymer[Equation 3]

(28) in Equation 3,

(29) Wa(g) is the sum of the weight of the absorbent polymer and the weight of the device capable of providing a load for the absorbent polymer, and

(30) Wb(g) is the sum of the weight of the absorbent polymer in which moisture is absorbed after supplying water for the absorbent polymer under a load (0.9 psi) for 1 hour, and the weight of the device capable of providing a load for the absorbent polymer.

(31) In this case, the absorbency under load was measured according to the EDANA WSP 242.2 method, and this was measured in the same manner in the previous 0.3AUL (60 min), with the exception that a piston capable of uniformly further providing a load of 0.9 psi.

(32) 3)CRC(Centrifuge Retention Capacity)

(33) The CRC of the absorbent polymers prepared in Examples and Comparative Examples was evaluated by the following method.

(34) In accordance with EDANA WSP 241.2 (European Disposables and Nonwovens Association, EDANA), the retention capacity by absorbency under no load was measured for the absorbent polymers prepared in Examples and Comparative Examples.

(35) Specifically, the polymer W.sub.0 (g, about 0.2 g) prepared in Examples and Comparative Examples was uniformly put in a nonwoven fabric-made bag, followed by sealing. Then, the bag was immersed in 0.9 wt % of a physiological saline solution at room temperature. After 30 minutes, moisture was removed from the bag at 250 G for 3 minutes with a centrifuge, 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. Thus, CRC (g/g) was calculated from the respective weights thus obtained, according to the following Equation.
CRC(g/g)={[W.sub.2(g)W.sub.1(g)]/W.sub.0(g)}1[Equation 4]

(36) in Equation 4,

(37) W.sub.0(g) is a weight (g) of the absorbent polymer,

(38) W.sub.1(g) is a weight (g) of the device which is measured after draining water off at 250 G for 3 minutes with a centrifuge, without using the absorbent polymer, and

(39) W.sub.2(g) is a weight (g) of the device including the absorbent polymer, which is measured after immersing the absorbent polymer in 0.9% by weight of the physiological saline solution at room temperature for 30 minutes and then draining water off at 250 G for 3 minutes with a centrifuge.

(40) The results thus obtained are shown in Table 1 below. For comparison, the loads (ampere) of the pulverizing device when pulverizing with chopper dies in Examples and Comparative Examples are also shown together.

(41) TABLE-US-00001 TABLE 1 CRC (g/g) ARUL (%) 0.9AUL (60 min) Before After Before After (g/g) surface surface surface surface After surface Ampere (A) crosslinking crosslinking crosslinking crosslinking crosslinking min max Average Ex. 1 38.9 30.7 35.5 61.3 18.0 Ex. 2 39.3 31.1 40.3 71.4 19.1 1.6 2.6 2.1 Ex. 3 41.4 31.6 42.2 69.8 18.5 1.7 2.5 2.1 Ex. 4 39.4 31.0 40.5 70.5 19.0 1.6 2.6 2.1 Ex. 5 40.2 31.2 39.9 70.3 18.7 1.5 2.6 2.0 Com. Ex. 1 39.6 31.4 39.5 53.9 17.5 Com. Ex. 2 39.5 31.8 39.7 70.0 17.7 1.8 3.0 2.4

(42) As shown in Table 1, it could be confirmed that the absorbency under load (0.9AUL) of Examples was excellent as compared to Comparative Examples. In addition, ARUL of Examples was significantly excellent as compared to Comparative Examples, and Comparative Example 2 exhibited the same level of ARUL as Examples, but since the load applied to the chopper die was high, it was difficult to apply to the actual process.

EXPERIMENTAL EXAMPLE 2

(43) The morphology of the super absorbent polymer prepared in Examples and Comparative Examples was confirmed by SEM image, and the results are shown in FIG. 6.

(44) As shown in FIG. 6, it could be confirmed that the surface of the super absorbent polymer of Examples became more rough form than that of Comparative Examples, thereby increasing the centrifuge retention capacity (CRC) and the absorbency under load (AUL) of the super absorbent polymer.