Method for preparing super absorbent polymer

Abstract

A method is provided for preparing a super absorbent polymer capable of exhibiting an excellent absorption rate by including a uniform porous structure through a simple and economical process. The method includes: performing a crosslinking polymerization of a water-soluble ethylenically unsaturated monomer to form a hydrogel polymer; drying and pulverizing the hydrogel polymer; classifying the pulverized polymer into polymer particles having a particle size of at least 10 to 150 μm, polymer particles having a particle size of 150 to 200 μm, and polymer particles having a particle size of 200 to 850 μm to form a base polymer powder having a particle size of 150 to 850 μm; and surface-crosslinking the base polymer powder, wherein in the crosslinking polymerization step, the foaming polymerization proceeds in the presence of polymer particles having an average particle size of 10 to 200 μm obtained in the classifying step and an anionic surfactant.

Claims

1. A method for preparing a super absorbent polymer comprising: performing a crosslinking polymerization of a water-soluble ethylenically unsaturated monomer having an acidic group of which at least a part is neutralized in the presence of an internal crosslinking agent to form a hydrogel polymer containing a first crosslinked polymer; drying and pulverizing the hydrogel polymer to form a pulverized polymer; classifying the pulverized polymer into polymer particles having a particle size of 10 to 150 μm, polymer particles having a particle size of 150 to 200 μm, and polymer particles having a particle size of 200 to 850 μm and combining the polymer particles having a particle size of 200 to 850 μm with at least a portion of the polymer particles having a particle size of 150 to 200 μm to form a base polymer powder having a particle size of 150 to 850 μm; and surface-crosslinking the base polymer powder, wherein the crosslinking polymerization is a foaming polymerization which proceeds in the presence of the polymer particles having a particle size of 10 to 150 μm obtained in the classifying step and an anionic surfactant without using a further foaming agent, wherein the polymer particles having a particle size of 10 to 150 μm is used in an amount of 0.1 to 3 parts by weight based on 100 parts by weight of the monomer, and wherein the super absorbent polymer has a bulk density of 0.55 to 0.65 g/ml, and an absorption rate measured according to Vortex measurement method of 30 to 53 seconds.

2. The method for preparing a super absorbent polymer according to claim 1, wherein the water-soluble ethylenically unsaturated monomer comprises: a methacrylic acid of which a part is neutralized, and an alkali metal salt thereof; and has a degree of neutralization of 55 to 95 mol %.

3. The method for preparing a super absorbent polymer according to claim 1, wherein the anionic surfactant includes one or more of sodium dodecyl sulfate, ammonium lauryl sulfate, sodium laureth sulfate, dioctyl sodium sulfosuccinate, perfluorooctane sulfonate, perfluorobutane sulfonate, alkyl-aryl ether phosphate, alkyl ether phosphate, sodium myreth sulfate or carboxylate salt.

4. The method for preparing a super absorbent polymer according to claim 1, wherein the anionic surfactant is used in an amount of 0.002 to 0.05 parts by weight, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer.

5. The method for preparing a super absorbent polymer according to claim 1, wherein during the crosslinking polymerization, a monomer aqueous solution containing the water-soluble ethylenically unsaturated monomer, the internal crosslinking agent, the polymer particles having a particle size of 10 to 150 μM and the anionic surfactant is used, and the monomer aqueous solution has a temperature of 30 to 60° C.

6. The method for preparing a super absorbent polymer according to claim 5, wherein the monomer aqueous solution further comprises an additive selected from the group consisting of a polyvalent metal salt, a photoinitiator, a thermal initiator, a polyalkylene glycol-based polymer, and any combination thereof.

7. The method for preparing a super absorbent polymer according to claim 6, wherein the additive is used in an amount of 2000 ppmw or less, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer.

8. The method for preparing a super absorbent polymer according to claim 1, wherein the super absorbent polymer has a centrifuge retention capacity (CRC) measured according to EDANA test method No. WSP 241.3 of 28 to 35 g/g, and an absorbency under load (AUL) at 0.9 psi measured according to EDANA test method No. WSP 242.3 of 16 to 23 g/g.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

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

Example 1

(2) (Step 1)

(3) 9 g (80 ppmw with respect to a monomer composition) of 0.5% IRGACURE 819 initiator diluted with acrylic acid and 40 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed to prepare a solution (solution A).

(4) Into a 2 L-volume glass reactor surrounded by a jacket in which a heating medium pre-cooled to 25° C. was circulated, 490 g of acrylic acid and the solution A were injected. To the glass reactor, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise and mixed. After confirming that the temperature of the mixed solution increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. A neutralization degree of acrylic acid in the mixed solution thus obtained was about 70 mol %. 5 g (1 wt % relative to acrylic acid) of the fine powder (polymer particles of the base polymer powder having a particle size of 10 to 150 μm) obtained in step 3 described later was added to the monomer aqueous solution. Further, 5 g (170 ppmw) of 2% sodium dodecylsulfate solution (solution D-1) diluted with water was prepared as a surfactant. Further, 30 g of 4% sodium persulfate solution (solution D-2) diluted with water was prepared. When the temperature of the mixed solution was cooled to about 45° C., the solutions D-1 and D-2 previously prepared were poured into the mixed solution and mixed.

(5) (Step 2)

(6) Then, the mixed solution prepared in step 1 was poured in a Vat-type tray (15 cm in width×15 cm in length) installed in a square polymerizer which had a light irradiation device installed at the top and was preheated to 80° C., and the mixed solution was then subjected to light irradiation. It was confirmed that at about 20 seconds after light irradiation, gel was generated from the surface, and at about 30 seconds after light irradiation, polymerization reaction occurred concurrently with foaming. Then, the polymerization reaction was performed for additional 2 minutes, and the polymerized sheet was taken and cut in a size of 3 cm×3 cm, and then subjected to a chopping process using a meat chopper to prepare the cut sheet as crumbs. The average particle size of the prepared crump was 1.5 mm.

(7) (Step 3)

(8) Subsequently, the crumbs prepared in step 2 were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes such that the dried crumbs had a water content of about 2% or less. The dried crumbs were pulverized using a pulverizer and classified by size to obtain a base polymer having a size of 150 to 850 μm. The remaining polymer particles of the base polymer powder having a particle size of 10 to 150 μm were recycled to step 1 described above and used.

(9) (Step 4)

(10) Thereafter, 100 g of the base polymer prepared in step 3 was mixed with a crosslinking agent solution which was prepared by mixing 4 g of water, 1 g of ethylene carbonate, and 0.1 g of Aerosil 200 (EVONIK), and then surface crosslinking reaction was performed at 190° C. for 30 minutes. The resulting product was pulverized and then passed through a sieve to obtain a surface-crosslinked super absorbent polymer having a particle size of 150 to 850 μm. 0.1 g of Aerosil 200 was added by a dry process to the obtained super absorbent polymer and mixed in a dry state to prepare a super absorbent polymer.

Example 2

(11) (Step 1)

(12) 9 g (80 ppmw with respect to a monomer composition) of 0.5% IRGACURE 819 initiator diluted with acrylic acid and 40 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed to prepare a solution (solution A).

(13) Into a 2 L-volume glass reactor surrounded by a jacket in which a heating medium pre-cooled to 25° C. was circulated, 490 g of acrylic acid and the solution A were injected. To the glass reactor, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise and mixed. After confirming that the temperature of the mixed solution increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. A neutralization degree of acrylic acid in the mixed solution thus obtained was about 70 mol %. 15 g (3 wt % relative to acrylic acid) of the fine powder (polymer particles of the base polymer powder having a particle size of 10 to 150 μm) obtained in step 3 described later was added to the monomer aqueous solution. Further, 5 g (170 ppmw) of 2% sodium dodecylsulfate solution (solution D-1) diluted with water was prepared as a surfactant. Further, 30 g of 4% sodium persulfate solution (solution D-2) diluted with water was prepared. When the temperature of the mixed solution was cooled to about 45° C., the solutions D-1 and D-2 previously prepared were poured into the mixed solution and mixed.

(14) Subsequently, steps 2 to 4 were carried out in the same manner as in Example 1 to prepare a super absorbent polymer.

Example 3

(15) (Step 1)

(16) 9 g (80 ppmw with respect to a monomer composition) of 0.5% IRGACURE 819 initiator diluted with acrylic acid and 40 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed to prepare a solution (solution A).

(17) Into a 2 L-volume glass reactor surrounded by a jacket in which a heating medium pre-cooled to 25° C. was circulated, 490 g of acrylic acid and the solution A were injected. To the glass reactor, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise and mixed. After confirming that the temperature of the mixed solution increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. A neutralization degree of acrylic acid in the mixed solution thus obtained was about 70 mol %. 25 g (5 wt % relative to acrylic acid) of the fine powder (polymer particles of the base polymer powder having a particle size of 10 to 150 μm) obtained in step 3 described later was added to the monomer aqueous solution. Further, 5 g (170 ppmw) of 2% sodium dodecylsulfate solution (solution D-1) diluted with water was prepared as a surfactant. Further, 30 g of 4% sodium persulfate solution (solution D-2) diluted with water was prepared. When the temperature of the mixed solution was cooled to about 45° C., the solutions D-1 and D-2 previously prepared were poured into the mixed solution and mixed.

(18) Subsequently, steps 2 to 4 were carried out in the same manner as in Example 1 to prepare a super absorbent polymer.

Comparative Example 1

(19) (Step 1)

(20) 9 g (about 80 ppmw with respect to a monomer composition) of 0.5% IRGACURE 819 initiator diluted with acrylic acid and 40 g of 5% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid were mixed to prepare a solution (solution A).

(21) Into a 2 L-volume glass reactor surrounded by a jacket in which a heating medium pre-cooled to 25° C. was circulated, 490 g of acrylic acid and the solution A were injected. To the glass reactor, 850 g of 24% caustic soda solution (solution C) was slowly added dropwise and mixed. After confirming that the temperature of the mixed solution increased to about 72° C. or higher by neutralization heat, the mixed solution was left until it was cooled. A neutralization degree of acrylic acid in the mixed solution thus obtained was about 70 mol %. 5 g (170 ppmw) of 2% sodium dodecylsulfate solution (solution D-1) diluted with water was prepared as a surfactant. Further, 30 g of 4% sodium persulfate solution (solution D-2) diluted with water was prepared. When the temperature of the mixed solution was cooled to about 45° C., the solutions D-1 and D-2 previously prepared were poured into the mixed solution and mixed. Then, an SBC solution was prepared at a concentration of 4 wt %, and about 6.25 g (500 ppm with respect to the monomer composition) was injected into the mixed solution and mixed.

(22) Subsequently, steps 2 to 4 were carried out in the same manner as in Example 1 to prepare a super absorbent polymer.

Comparative Example 2

(23) A super absorbent polymer was prepared in the same manner as in Example 1 except that sodium dodecylsulfate solution (solution D-1) was not used in step 1.

Comparative Example 3

(24) A super absorbent polymer was prepared in the same manner as in Example 2 except that sodium dodecylsulfate solution (solution D-1) was not used in step 1.

Experimental Example: Evaluation of Physical Properties of Super Absorbent Polymer

(25) The physical properties of the super absorbent polymers prepared in Examples and Comparative Examples were measured and evaluated by the following methods, and the results are shown in Table 1 below.

(26) (1) Bulk Density

(27) About 100 g of the super absorbent polymer was added to a funnel-shaped bulk density tester and flowed down in a 100 ml container, and then the weight of the super absorbent polymer contained in the container was measured. The bulk density was calculated as (weight of super absorbent polymer)/(volume of container, 100 ml). (Unit: g/ml).

(28) (2) Absorption Rate (Vortex Time)

(29) The absorption rates of the super absorbent polymers of Examples and Comparative Examples were measured in seconds in accordance with a method described in International Patent Publication WO 1987/003208.

(30) In detail, the absorption rate (or vortex time) was calculated by measuring in seconds the time required for the vortex to disappear, after adding 2 g of the super absorbent polymer to 50 mL of a physiological saline solution at 23° C. to 24° C. and then stirring it a magnetic bar (diameter 8 mm, length 31.8 mm) at 600 rpm.

(31) (3) Centrifuge Retention Capacity (CRC)

(32) The centrifuge retention capacity (CRC) by water absorption capacity under a non-loading condition was measured for each polymer according to EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 241.3.

(33) In detail, in the polymers respectively obtained through Examples and Comparative Examples, a polymer classified with a 30-50 mesh sieve was obtained. W.sub.0 (g) (about 0.2 g) of these polymers were uniformly put in a nonwoven fabric-made bag, followed by sealing. Then, the bag was immersed in a physiological saline solution composed of an aqueous sodium chloride solution (0.9 wt %) 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.

(34) Using the respective weights thus obtained, CRC (g/g) was calculated according to the following Equation.
CRC (g/g)={[W.sub.2 (g)−W.sub.1 (g)]/W.sub.0 (g)}−1  [Equation 1]

(35) (4) Absorbency Under Load (AUL)

(36) Absorbency under load (AUL) at 0.9 psi was measured for each polymer according to EDANA test method WSP 242.3.

(37) First, at the time of measuring the absorbency under load, a polymer classified at the time of CRC measurement was used.

(38) In detail, a 400 mesh stainless steel screen net was installed in the bottom of a plastic cylinder having an internal diameter of 25 mm. W.sub.0 (g) (0.16 g) of the super absorbent polymer was uniformly scattered on the steel net at room temperature and humidity of 50%, and a piston which can uniformly provide a load of 0.9 psi was put thereon, in which an external diameter of the piston was slightly smaller than 25 mm, there was no gab between the internal wall of the cylinder 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.

(39) 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 for 1 hour under the load. After 1 hour, the weight W.sub.4 (g) was measured after lifting up the measuring device.

(40) The weights thus obtained were used to calculate absorbency under load (g/g) according to the following Equation 2:
AUL (g/g)=[W.sub.4 (g)−W.sub.3 (g)]/W.sub.0 (g)  [Equation 2]

(41) The results measured as above are shown in Table 1 below.

(42) TABLE-US-00001 TABLE 1 Bulk density Vortex (#30-50) CRC (#30-50) 0.9 AUL (g/ml) (sec) (g/g) (g/g) Example 1 0.62 41 30.4 18.3 Example 2 0.60 37 30.1 18.5 Example 3 0.59 55 30.5 18.7 Comparative 0.61 47 30.3 18.7 Example 1 Comparative 0.63 52 30.5 18.6 Example 2 Comparative 0.62 51 30.3 18.6 Example 3

(43) Referring to Table 1 above, it was confirmed that the super absorbent polymers of Examples exhibited excellent absorption rate as compared with Comparative Examples.