Method of preparing superabsorbent polymer

Abstract

A method of preparing a superabsorbent polymer, which enables the preparation of the superabsorbent polymer exhibiting an improved absorption rate while maintaining excellent absorption performances is provided. The method of preparing the superabsorbent polymer includes carrying out a crosslinking polymerization of a water-soluble ethylene-based unsaturated monomer having acidic groups which are at least partially neutralized, in the presence of an internal crosslinking agent having a predetermined chemical structure to form a water-containing gel polymer, gel-pulverizing the water-containing gel polymer, drying, pulverizing, and size-sorting the gel-pulverized water-containing gel polymer to form a base polymer powder, and carrying out a surface crosslinking of the base polymer powder by a heat treatment in the presence of a surface crosslinking agent, wherein the gel-pulverizing is carried out by extruding the water-containing gel polymer through a porous plate having a plurality of holes using a screw extruder mounted inside a cylindrical pulverizer under a condition that a chopping index is 28 (/s) or more.

Claims

1. A method of preparing a superabsorbent polymer, the method comprising: carrying out a crosslinking polymerization of a water-soluble ethylene-based unsaturated monomer having acidic groups which are at least partially neutralized, in the presence of an internal crosslinking agent including a compound of Chemical Formula 1, to form a water-containing gel polymer; gel-pulverizing the water-containing gel polymer; drying, pulverizing, and size-sorting the gel-pulverized water-containing gel polymer to form a base polymer powder; and carrying out a surface crosslinking of the base polymer powder by heat treatment in the presence of a surface crosslinking agent, wherein the gel-pulverizing is carried out by extruding the water-containing gel polymer through a porous plate having a plurality of holes using a screw extruder mounted inside a cylindrical pulverizer under a condition that a chopping index according to the following Equation 1 is 28 (/s) or more: ##STR00003## wherein in Chemical Formula 1, R.sup.1 is a divalent organic group derived from alkane having 1 to 10 carbon atoms, and R.sup.2 is hydrogen or a methyl group,
Chopping index(C.I.)=ω×(TSC/A)×number of times of chopping(T)  [Equation 1] wherein in Equation 1, ω represents angular velocity (2π×N/60 s) of screw in the screw extruder, wherein in angular velocity (2π×N/60 s), N represents a number of rotation (rpm) of the screw, TSC represents a solid content (%) of the water-containing gel polymer fed into the pulverizer, and A represents porosity (πr.sup.2×n/πR.sup.2) of the porous plate, wherein in porosity (πr.sup.2×n/πR.sup.2), r represents a radius (mm) of the holes formed in the porous plate, n represents the number of holes formed in the porous plate, and R represents a radius (mm) of the porous plate.

2. The method of claim 1, wherein the water-soluble ethylene-based unsaturated monomer includes one or more selected from the group consisting of an anionic monomer including 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-methylpropane sulfonic acid, and a salt thereof; a nonionic hydrophilic monomer including (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxypolyethyleneglycol(meth)acrylate, or polyethyleneglycol(meth)acrylate; and an unsaturated monomer containing an amino group including (N,N)-dimethylaminoethyl(meth)acrylate or (N,N)-dimethylaminopropyl(meth)acrylamide, and a quaternary compound thereof.

3. The method of claim 1, wherein in Chemical Formula 1, R.sup.1 is methane-1,1-diyl, propane-1,3-diyl, propane-1,2-diyl, propane-1,1-diyl, n-butane-1,4-diyl, n-butane-1,3-diyl, n-butane-1,2-diyl, n-butane-1,1-diyl, 2-methylpropane-1,3-diyl, 2-methylpropane-1,2-diyl, 2-methylpropane-1,1-diyl, 2-methylbutane-1,4-diyl, 2-methylbutane-2,4-diyl, 2-methylbutane-3,4-diyl, 2-methylbutane-4,4-diyl, 2-methylbutane-1,3-diyl, 2-methylbutane-1,2-diyl, 2-methylbutane-1,1-diyl, 2-methylbutane-2,3-diyl, 3-methylbutane-1,2-diyl, or 3-methylbutane-1,3-diyl.

4. The method of claim 1, wherein the internal crosslinking agent includes 10% by weight to 100% by weight of the compound of Chemical Formula 1.

5. The method of claim 4, wherein the internal crosslinking agent further includes one or more additional internal crosslinking agents selected from the group consisting of N,N′-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentacrylate, glycerin tri(meth)acrylate, pentaerythritol tetraacrylate, triallylamine, allyl (meth)acrylate, ethylene glycol diglycidyl ether, propylene glycol, glycerin, and ethylene carbonate.

6. The method of claim 1, wherein the internal crosslinking agent is used in an amount of 0.01 part by weight to 5 parts by weight with respect to 100 parts by weight of the water-soluble ethylene-based unsaturated monomer.

7. The method of claim 1, wherein the base polymer powder has a particle size of 150 μm to 850 μm after the size-sorting.

8. The method of claim 1, wherein the surface crosslinking agent includes one or more polyols selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; one or more alkylene carbonate-based compounds selected from the group consisting of ethylene carbonate and propylene carbonate; epoxy compounds including ethylene glycol diglycidyl ether; oxazoline compounds including oxazolidinone; polyamine compounds; mono-, di- or polyoxazolidinone compounds; or cyclic urea compounds.

9. The method of claim 8, wherein the surface crosslinking agent further includes one or more inorganic materials selected from the group consisting of silica, clay, alumina, silica-alumina composite, titania, zinc oxide and aluminum sulfate.

10. The method of preparing the superabsorbent polymer of claim 1, wherein the surface crosslinking agent is used in an amount of 0.01 part by weight to 3 parts by weight with respect to 100 parts by weight of the base polymer powder.

11. The method of claim 1, wherein the heat treatment of the surface crosslinking is carried out at a temperature of 80° C. to 200° C. for 20 minutes to 2 hours.

12. The method of claim 1, wherein the superabsorbent polymer has a centrifuge retention capacity (CRC) of 35 g/g to 50 g/g for a physiological saline solution having a content of 0.9% by weight of sodium chloride aqueous solution for 30 minutes.

13. The method of claim 5, wherein the superabsorbent polymer has an absorption rate of 35 sec to 70 sec, as measured by a vortex method.

Description

EXAMPLES

(1) Hereinafter, preferred examples are provided for better understanding of the present invention. However, the following Examples are only for illustrating the present invention, and the present invention is not limited thereto.

Example 1

(2) As a device for preparing a superabsorbent polymer, a continuous preparation system consisting of a polymerization process, a water-containing gel pulverizing process, a drying process, a pulverizing process, a size-sorting process, a surface crosslinking process, a cooling process, a size-sorting process, and a transport process connecting each process was used.

(3) To 100 parts by weight of acrylic acid, 123.5 g of 32 wt % caustic soda (NaOH), 0.2 g of sodium persulfate as a thermal polymerization initiator, 0.008 g of diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide as a photo-polymerization initiator, 0.6 g of 3-methylbutane-1,3-diyl diacrylate (a compound of Chemical Formula 1), 0.18 g of laponite, and 55.0 g of water were added and mixed with each other, thereby preparing a monomer composition having a total solid content of 43.8% by weight.

(4) The monomer composition was fed to a rotary belt having a width of 10 cm and a length of 2 m and rotating at a speed of 50 cm/min at a feed rate of 500 mL/min to 2,000 mL/min. While feeding the monomer composition, ultraviolet light was irradiated at an intensity of 10 mW/cm.sup.2 to allow a photo-polymerization reaction for 60 seconds. After cutting the sheet-shaped water-containing gel polymer obtained by the polymerization reaction to a size of 3 cm×3 cm, pulverizing/chopping were carried out by extruding the water-containing gel polymer through a porous plate having a plurality of holes using a screw extruder (meat chopper) mounted inside a cylindrical pulverizer under the conditions of a chopping index shown in the following Equation 1 and Table 1:
Chopping index(C.I.)=ω×(TSC/A)×number of times of chopping(T)  [Equation 1]

(5) in Equation 1,

(6) ω represents angular velocity (2π×N/60 s) of screw in the screw extruder, wherein N represents the number of rotation (rpm) of the screw, TSC represents a solid content (%) of the water-containing gel polymer fed into the pulverizer, and A represents porosity (πr.sup.2×n/πR.sup.2) of the porous plate, wherein r represents a radius (mm) of the holes formed in the porous plate, n represents the number of holes formed in the porous plate, and R represents a radius (mm) of the porous plate.

(7) Thereafter, the gel-pulverized water-containing gel polymer was dried using an air-flow oven at 185° C. for 40 minutes, and pulverized and size-sorted into a particle size of 150 μm to 850 μm.

(8) To 100 g of the base polymer powder thus formed, a mixed solution of a surface crosslinking agent of 3.2 g of ultra-pure water, 4.0 g of methanol, 0.088 g of ethylene carbonate, and 0.01 g of silica (DM30S) was added, and mixed for 1 minute. This mixture was heat-treated at 185° C. for 90 minutes to allow surface crosslinking, followed by size-sorting. Thus, superabsorbent polymer particles having a particle size of 150 μm to 850 μm were prepared.

Examples 2 and 3

(9) Superabsorbent polymer particles of Examples 2 and 3 were prepared in the same manner as in Example 1, except for varying the chopping index and conditions to achieve the same during gel pulverization as in the following Table 1.

Comparative Example 1

(10) Superabsorbent polymer particles of Comparative Example 1 were prepared in the same manner as in Example 1, except that 3-methylbutane-1,3-diyl diacrylate (the compound of Chemical Formula 1) was not used, and 0.24 g of polyethylene glycol diacrylate (PEGDA) was used.

Comparative Example 2

(11) Superabsorbent polymer particles of Comparative Example 2 were prepared in the same manner as in Comparative Example 1, except that 0.45 g of polyethylene glycol diacrylate (PEGDA) was used.

Comparative Examples 3 and 4

(12) Superabsorbent polymer particles of Comparative Examples 3 and 4 were prepared in the same manner as in Example 1, except for varying the chopping index and conditions to achieve the same during gel pulverization as in the following Table 1.

(13) TABLE-US-00001 TABLE 1 Number of times of Chopping chopping T N TSC R R index (times) (rpm) (%) (mm) (mm) n (/s) Example 1 1 180 45 5 42 19 31.5 Example 2 1 150 45 4 42 24 32.5 Example 3 2 120 45 8 42 10 31.2 Comparative Example 1 1 180 45 5 42 19 31.5 Comparative Example 2 1 180 45 5 42 19 31.5 Comparative Example 3 1 120 45 8 42 10 15.6 Comparative Example 4 1 180 45 9 42 7 26.4

Experimental Example

(14) Each of the superabsorbent polymers prepared in Examples and Comparative Examples was measured and evaluated for physical properties by the following method.

(15) (1) Centrifuge Retention Capacity (CRC)

(16) The centrifuge retention capacity (CRC) by absorbency under no load was measured in accordance with European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.3.

(17) In detail, the superabsorbent polymer (or base polymer powder; the same applies hereinafter) W.sub.0 (g, about 2.0 g) was uniformly placed into a nonwoven-fabric-made bag, followed by sealing. Then, the bag was immersed at room temperature in a physiological saline solution which is a 0.9% by weight sodium chloride aqueous solution. After 30 minutes, the bag was drained 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 the superabsorbent polymer, and the resultant weight W.sub.1 (g) was measured. From these weights thus obtained, CRC (g/g) was calculated according to the following Equation A to confirm the water retention capacity:
CRC(g/g)={[W.sub.2(g)−W.sub.1(g)−W.sub.0(g)]/W.sub.0(g)}  [Equation A]

(18) (2) Absorption Rate by Vortex Method

(19) The absorption rate of each superabsorbent polymer (or base polymer powder; the same applies hereinafter) of Examples and Comparative Examples was measured in seconds in accordance with a method described in International Patent Application No. 1987-003208.

(20) In detail, the absorption rate (or vortex time) was calculated by measuring a time in seconds, which was required until a vortex disappears, after adding 2 g of the superabsorbent polymer to 50 mL of a physiological saline solution at 23° C. to 24° C. and then agitating it using a magnetic bar (8 mm in diameter and 31.8 mm in length) at 600 rpm.

(21) The physical properties of Examples/Comparative Examples measured by the above-described method are summarized in the following Table 2.

(22) TABLE-US-00002 TABLE 2 Example Example Example Comparative Comparative Comparative Comparative 1 2 3 Example 1 Example 2 Example 3 Example 4 CRC (superabsorbent 42.2 38.4 39.3 Gel pulverization 31.2 43.2 38.2 polymer; g/g) not available Absorption rate 68 45 57 46 107 89 (superabsorbent polymer; sec)

(23) Referring to Table 1, Examples 1 to 4 were confirmed to exhibit excellent CRC and absorption rate, since the thermally degradable compound of Chemical Formula 1 was used as the internal crosslinking agent and gel pulverization was carried out under the predetermined chopping index condition. In contrast, in Comparative Example 1, in which a common internal crosslinking agent was used in a reduced amount, instead of the compound of Chemical Formula 1, in order to increase CRC, gel pulverization itself was not available due to the increased instrumental load. Further, it was confirmed that Comparative Example 2 showed poor absorption performances, as compared with Examples, and Comparative Examples 3 and 4 which were out of the appropriate chopping index range showed poor absorption rates.