Method of preparing superabsorbent polymer

Abstract

A superabsorbent polymer according to the present invention has excellent initial absorption properties, and thus it may be used in sanitary materials such as diapers, etc., thereby exhibiting excellent performances.

Claims

1. A method of preparing a superabsorbent polymer, comprising: introducing a polymerization initiator and a foaming agent into a reaction mixture while controlling the temperature of the reaction mixture at about 40° C. to about 60° C., wherein the reaction mixture comprising an acrylic acid having at least partially neutralized acid groups, an internal crosslinking agent, and a surfactant; polymerizing the reaction mixture to form a hydrogel polymer containing a first cross-linked polymer, wherein a maximum foaming point is reached within 40 to 60 seconds from a polymerization initiation point of the acrylic acid; drying, pulverizing, and size-sorting the hydrogel polymer to form a base polymer powder; and surface-crosslinking the base polymer powder by heat treatment in the presence of a surface crosslinking solution to form superabsorbent polymer particles, wherein the superabsorbent polymer particles have 1 min-absorbency of 125 g/g to 165 g/g for distilled water, wherein the superabsorbent polymer particles have 1 min-absorbency of 30 g/g to 45 g/g for a 0.9 wt % sodium chloride aqueous solution, wherein the surfactant is sodium dodecyl sulfate and present in the reaction mixture in an amount of 0.01 to 0.1 parts by weight based on 100 parts of the acrylic acid, and wherein the foaming agent is sodium bicarbonate and present in an amount of 0.01 to 0.1 parts by weight based on 100 parts of the acrylic acid.

2. The method of claim 1, wherein the polymerization initiator is a thermal polymerization initiator, a photo-polymerization initiator, or an oxidation-reduction initiator.

3. The method of claim 1, wherein the base polymer powder is pulverized and size-sorted so that a content of the particles having a particle size of 150 μm to 850 μm is 90% or more.

4. The method of claim 1, wherein the surface crosslinking solution comprises one or more surface 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 glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, propanediol, dipropylene glycol, polypropylene glycol, glycerin, polyglycerin, butanediol, heptanediol, hexanediol, trimethylol propane, pentaerythritol, sorbitol, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, iron hydroxide, calcium chloride, magnesium chloride, aluminum chloride, and iron chloride.

5. The method of claim 1, wherein the superabsorbent polymer has an absorption rate of 45 seconds or less, as measured according to a vortex measurement method.

6. The method of claim 1, wherein the surface crosslinking solution includes silica and ethylene glycol diglycidyl.

7. The method of claim 6, wherein the internal surface crosslinking agent is polyethylene glycol diacrylate.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

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

Example 1

(2) In a 2 L-glass reactor surrounded by a jacket through which a heating medium pre-cooled at 25° C. was circulated, 500 g of acrylic acid, 0.05 g of IRGACURE 819, and 0.05 g of sodium dodecyl sulfate (SDS) were mixed with each other. 2.5 g of polyethylene glycol diacrylate (PEGDA, Mw=400) was injected thereto. 800 g of a 24% caustic soda solution was slowly added dropwise thereto, followed by mixing. A degree of neutralization of acrylic acid in sodium acrylate as a water-soluble ethylene-based unsaturated monomer thus obtained was 70 mole %.

(3) After confirming that a temperature of the mixture increased to 72° C. or higher by neutralization heat generated upon mixing the two solutions, the mixture was left until the reaction temperature reached 40° C. When the reaction temperature reached 40° C., 0.5 g of sodium bicarbonate (SBC) in a solid phase was mixed with monomers, and 54 g of a 2% sodium persulfate solution diluted with water was injected at the same time.

(4) The solution was poured in a tray (15 cm in width×15 cm) installed in a square polymerizer which had a UV irradiation device installed at the top thereof and had been preheated to 80° C., and polymerization was initiated by UV irradiation. 30 seconds later, gel was generated from the surface, and at a time point of 60 seconds after bubble formation was initiated, bubble formation reached a maximum point. After the bubble formation reached the maximum point, the reaction was allowed for additional 3 minutes, and a sheet-type polymer was cut in a size of about 5 cm×5 cm and then introduced into a meat chopper to pulverize the polymer. Finally, water-containing particles having a size of 1 mm to 10 mm were obtained.

(5) The water-containing particles were dried in a hot air oven. Specifically, the water-containing particles 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. After drying, the dried product had a water content of 2% or less.

(6) After drying, the product was pulverized using a food mixer and sorted by a standard test sieve, and particles having a size of 150 μm to 850 μm were selected to prepare a base polymer.

(7) Thereafter, 100 g of the prepared base polymer was mixed with a mixture containing 3 g of water, 3 g of methanol, 0.2 g of ethylene glycol diglycidyl ether, and 0.05 g of aerosol 200 by using a high speed mixer, and reaction was allowed at 140° C. for 40 minutes. The mixture was cooled to room temperature, and then a superabsorbent polymer having a particle size of 150 μm to 850 μm was obtained by using a standard test sieve.

Examples 2 to 4

(8) A superabsorbent polymer was prepared in the same manner as in Example 1, except that in Example 1, the UV initiator (IRGACURE 819) and the surfactant stabilizer (SDS) were used in amounts as in the following Table 1.

Example 5

(9) In Example 1, when the temperature of the mixture reached 60° C., sodium bicarbonate and the sodium persulfate solution were injected, and polymerization was initiated in a polymerizer which had been preheated to 80° C. 25 seconds after light irradiation, gel was generated from the surface, and at a time point of 40 seconds after bubble formation was initiated, bubble formation reached a maximum point. Thereafter, in the same manner as in Example 1, a superabsorbent polymer was prepared.

Comparative Example 1

(10) In Example 1, when the temperature of the mixture reached 20° C., sodium bicarbonate and the sodium persulfate solution were injected, and polymerization was initiated in a polymerizer which had been preheated to 80° C. 35 seconds after light irradiation, gel was generated from the surface, and at a time point of 105 seconds after bubble formation was initiated, bubble formation reached a maximum point. Thereafter, in the same manner as in Example 1, a superabsorbent polymer was prepared.

Comparative Example 2

(11) In Example 1, 0.015 g of IRGACUR 819 was used, and when the temperature of the mixture reached 20° C., sodium bicarbonate and the sodium persulfate solution were injected, and polymerization was initiated in a polymerizer which had been preheated to 80° C. 65 seconds after light irradiation, gel was generated from the surface, and at a time point of 158 seconds after bubble formation was initiated, bubble formation reached a maximum point. Thereafter, in the same manner as in Example 1, a superabsorbent polymer was prepared.

Experimental Example: Evaluation of Physical Properties of Superabsorbent Polymers

(12) Physical properties of the superabsorbent polymers prepared in Examples and Comparative Examples were evaluated by the following methods.

(13) (1) Centrifuge Retention Capacity (CRC)

(14) Centrifuge retention capacity (CRC) by absorbency under no load was measured for the respective superabsorbent polymers of Examples and Comparative Examples in accordance with European Disposables and Nonwovens Association (EDANA) standard WSP 241.3.

(15) In detail, each polymer W.sub.0 (g, about 0.2 g) of Examples and Comparative Examples 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 sodium chloride aqueous solution of 0.9% by weight. 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 procedures were carried out without the superabsorbent polymer, and the resultant weight W.sub.1 (g) was measured.

(16) Thus, CRC (g/g) was calculated from the obtained weights according to the following Equation 1:
CRC(g/g)={[W.sub.2(g)−W.sub.1(g)−W.sub.0(g)]/W.sub.0(g)}  [Equation 1]

(17) in Equation 1,

(18) W.sub.0 (g) is an initial weight (g) of the superabsorbent polymer, W.sub.1 (g) is a weight (g) of an apparatus without the superabsorbent polymer, which is measured after immersing in a physiological saline solution for 30 minutes and draining water off at 250 G for 3 minutes using a centrifuge, and W.sub.2 (g) is the weight (g) of the apparatus including the superabsorbent polymer, which is measured after immersing the superabsorbent polymer in the physiological saline solution at room temperature for 30 minutes and draining water off at 250 G for 3 minutes using a centrifuge.

(19) CRC (BR CRC) was also measured for the respective base polymers prepared in Examples and Comparative Examples in the same manner as above.

(20) (2) Absorption Rate (Vortex)

(21) 50 mL of a 0.9 wt % NaCl solution and a magnetic bar with a size of 30 mm were placed in a 100 ml beaker. Under stirring at 600 rpm by a magnetic stirrer, 2.0 g of each of the superabsorbent polymers prepared in Examples and Comparative Examples was fed thereto when the NaCl solution reached 24° C. A time taken for a liquid vortex produced by stirring to disappear and for the liquid surface to be completely level was measured. This result was expressed as a vortex removal time (absorption rate; vortex).

(22) Further, from the respective base polymers prepared in the preparation processes of Examples and Comparative Examples, particles of 300 μm to 600 μm were taken, and absorption rates (BR Vortex) thereof were measured in the same manner as above.

(23) (3) 1 Min-Absorbency (0.9 wt % NaCl)

(24) 1 g of each of the superabsorbent polymers of Examples and Comparative Examples was placed in a 250 mL beaker, and immersed in 150 mL of 0.9 wt % sodium chloride aqueous solution at 24° C. for 1 minute. A 100-mesh standard sieve having a diameter of 90 mm was placed on the top of another 250 mL beaker, and all the immersed superabsorbent polymer was poured on the standard sieve, and then drained for 1 minute. The amount of the gel remaining on the standard sieve was measured, and regarded as 1 min-absorbency.

(25) (4) 1 Min-Absorbency (Distilled Water)

(26) 1.0 g (W.sub.3) of each of the superabsorbent polymers of Examples and Comparative Examples was placed into a nonwoven-fabric-made bag (15 cm×15 cm), which was immersed in 500 mL of distilled water at 24° C. for 1 minute. 1 minute later, the bag was taken from distilled water, and hung for 1 minute. Thereafter, the weight (W.sub.5) of the bag was measured. Further, the same procedures were performed without the superabsorbent polymers, and the weight (W.sub.4) was measured.

(27) 1 min-absorbency was calculated from the obtained weights according to the following Equation 2:
1 min-absorbency(distilled water)={[W.sub.5(g)−W.sub.4(g)−W.sub.3(g)]/W.sub.3(g)}  [Equation 2]

(28) in Equation 2,

(29) W.sub.3 (g) is an initial weight (g) of the superabsorbent polymer, W.sub.4 (g) is a weight (g) of an apparatus without the superabsorbent polymer, which is measured after immersing in distilled water for 1 minute, and W.sub.5 (g) is the weight (g) of the apparatus including the superabsorbent polymer, which is measured after immersing in distilled water for 1 minute.

(30) (5) Absorbency Under Pressure (AUP)

(31) Absorbency under pressure (AUP) was measured for the respective superabsorbent polymers of Examples and Comparative Examples in accordance with European Disposables and Nonwovens Association EDANA standard WSP 242.3.

(32) In detail, a 400 mesh stainless steel net was installed in the bottom of a plastic cylinder having an internal diameter of 60 mm. Each W.sub.0 (g, 0.90 g) of the superabsorbent polymers obtained in Examples and Comparative Examples was uniformly scattered on the stainless steel net under conditions of a temperature of 23±2° C. and relative humidity of 45%. A piston which may uniformly provide a load of 0.3 psi was put thereon, in which an external diameter of the piston was slightly smaller than 60 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.6 (g) of the apparatus was measured.

(33) 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 0.9% by weight of sodium chloride was poured until the surface level of the physiological saline solution became equal to the upper surface of the glass filter. A sheet of filter paper having a diameter of 120 mm was put on the glass filter. The measurement apparatus was mounted on the filter paper, thereby getting the liquid absorbed under the load for 1 hour. 1 hour later, the weight W.sub.7 (g) was measured after lifting the measurement apparatus up.

(34) AUP (g/g) was calculated from the obtained weights according to the following Equation 3:
AUP(g/g)=[W.sub.7(g)−W.sub.6(g)]/W.sub.0(g)  [Equation 3]

(35) in Equation 2,

(36) W.sub.0 (g) is an initial weight (g) of the superabsorbent polymer, W.sub.6 (g) is the sum of the weight of the superabsorbent polymer and the weight of the apparatus capable of providing a load for the superabsorbent polymer, and W.sub.7 (g) is the sum of the weight of the superabsorbent polymer after allowing the superabsorbent polymer to absorb the physiological saline solution under a load (0.3 psi) for 1 hour, and the weight of the apparatus capable of providing the load for the superabsorbent polymer.

(37) The measurement results are shown in the following Table 1.

(38) TABLE-US-00001 TABLE 1 Superabsorbent polymer Concentration 1 min- 1 min- Temperature of Maximum absorbency absorbency of UV foaming Base polymer (0.9% (distilled 0.3 mixture initiator SDS SBC point CRC Vortex CRC Vortex NaCl) water) AUP Unit ° C. ppmw ppmw ppmw sec g/g sec g/g sec g/g g/g g/g Ex. 1 40 100 100 1000 60 40 45 34 40 32 125 30 Ex. 2 40 100 500 1000 55 38 33 34 30 37 135 29 Ex. 3 40 100 1000 1000 54 36 30 33 25 40 140 28 Ex. 4 40 150 1000 1000 45 34 26 31 21 45 165 28 Ex. 5 60 100 100 1000 40 38 40 34 35 34 138 30 Comparative 20 100 100 1000 105 43 65 36 58 25 95 31 Ex. 1 Comparative 20 30 100 1000 158 42 70 36 62 23 92 31 Ex. 2

(39) As shown in Table 1, it was confirmed that the time taken to reach the maximum foaming point is associated with the absorption rate (vortex) and 1 min-absorbency of the superabsorbent polymer. Specifically, in the case of Examples of the present invention, the time taken to reach the maximum foaming point was short, as compared with that of Comparative Examples, and therefore, their absorption rate and 1 min-absorbency were remarkably improved, as compared with those of Comparative Examples.