Method of Preparing Superabsorbent Polymer

Abstract

Provided is a method of preparing a superabsorbent polymer, which enables preparation of the superabsorbent polymer exhibiting an improved absorption rate while maintaining excellent absorption performances by minimizing a surface damage phenomenon of the superabsorbent polymer in a pneumatic conveying process during the preparation process of the superabsorbent polymer or in a pneumatic conveying process of the finally prepared superabsorbent polymer.

Claims

1. A method of preparing a superabsorbent polymer, the method comprising: 1) 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 a foaming agent, to form a water-containing gel polymer; 2) gel-pulverizing the water-containing gel polymer; 3) drying, pulverizing, and size-sorting the gel-pulverized water-containing gel polymer to form a base polymer powder; and 4) 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 perforated panel 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 55 to 75: $\begin{array}{cc}\mathrm{Chopping}\mathrm{index}=\frac{2\pi ⨯N}{60}⨯\frac{1}{A}⨯\frac{11}{R}& \left[\mathrm{Equation}1\right]\end{array}$ in Equation 1, wherein N represents a number of rotation (rpm) of a screw in the screw extruder, A represents porosity (πr.sup.2×n/πR.sup.2) of the perforated panel, wherein r represents a radius (mm) of the holes formed in the perforated panel, n represents a number of holes formed in the perforated panel, and R represents a radius (mm) of the perforated panel.

2. The method of claim 1, wherein the foaming agent includes one or more compounds selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, magnesium carbonate, azodicarbonamide, dinitroso pentamethylene tetramine, p,p′-oxybis(benzenesulfonyl hydrazide), p-toluenesulfonyl hydrazide, sucrose stearate, sucrose palmitate, and sucrose laurate.

3. The method of claim 1, wherein the chopping index is 60 to 70.

4. The method of claim 1, wherein a crush resistance index of the superabsorbent polymer is −0.3 to 0.

5. The method of claim 1, wherein CRC of the superabsorbent polymer is 27 g/g to 40 g/g.

6. The method of claim 1, wherein AUP of the superabsorbent polymer is 24 g/g to 30 g/g.

7. The method of claim 1, wherein a vortex of the superabsorbent polymer is 45 seconds or less.

8. The method of claim 1, wherein permeability of the superabsorbent polymer is 60 seconds or less.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0093] FIG. 1 shows a schematic illustration of an apparatus for measuring a crush resistance index in Examples of the present technology.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0094] Hereinafter, preferred exemplary embodiments are provided for better understanding of the present invention. However, the following exemplary embodiments are only for illustrating the present invention, and the present invention is not limited thereto.

[0095] Respective physical properties in the following Examples and Comparative Examples were measured by the following methods.

[0096] (1) Centrifugal Retention Capacity (CRC)

[0097] Measurement was performed in accordance with EDANA WSP 241.3. In detail, the superabsorbent polymer 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 (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 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.

CRC (g/g)={[W.sub.2 (g)−W.sub.1 (g)]/W.sub.0 (g)}−1  [Equation 2]

[0098] (2) Absorbency Under Pressure (AUP)

[0099] Absorbency under pressure (AUP) under 0.7 psi (or 0.18 psi) was measured for the superabsorbent polymers in accordance with EDANA WSP 242.3.

[0100] In detail, a 400 mesh stainless steel net was installed in the bottom of a plastic cylinder having an internal diameter of 25 mm. The superabsorbent polymer W.sub.0 (g) was uniformly scattered on the stainless steel net under conditions of room temperature and relative humidity of 50%. A piston which may uniformly provide a load of 0.7 psi (or 0.18 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 apparatus was measured.

[0101] 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% 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 90 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.4 (g) was measured after lifting the measurement apparatus up.

[0102] Absorbency under pressure (g/g) was calculated from the obtained weights according to the following Equation.

AUP (g/g)=[W.sub.4 (g)−W.sub.3 (g)]/W.sub.0 (g)  [Equation 3]

[0103] (3) Vortex (Absorption Rate)

[0104] The absorption rate was measured in seconds according to the method described in International Publication WO 1987-003208.

[0105] In detail, 2 g of the superabsorbent polymer was added to 50 mL of physiological saline at 23° C. to 24° C., and stirred with a magnetic bar (diameter of 8 mm and length of 31.8 mm) at 600 rpm, and a time taken for vortex to disappear was measured in seconds.

[0106] (4) Permeability

[0107] Permeability was measured according to the following Equation.

Perm=[20 mL/T1 (sec)]*60 sec  [Equation 4]

[0108] in Equation 4,

[0109] Perm represents permeability of the superabsorbent polymer,

[0110] T1 represents a time (sec) taken for 20 mL of a physiological saline solution (0.9% by weight of aqueous sodium chloride solution) to pass through the swollen superabsorbent polymer under a pressure of 0.3 psi, after placing 0.2 g of the superabsorbent polymer in a cylinder, and pouring the physiological saline solution whereby the superabsorbent polymer was completely submerged therein to allow the superabsorbent polymer to swell for 30 minutes.

[0111] In detail, a cylinder and a piston were prepared. With regard to the cylinder, an inner diameter thereof was 20 mm and a glass filter and a stopcock were provided at the bottom thereof. With regard to the piston, a screen having an outer diameter of slightly smaller than 20 mm to freely move up and down in the cylinder was disposed at the bottom thereof, a weight was disposed at the top thereof, and the screen and the weight were connected by a rod. The piston was equipped with a weight capable of applying a pressure of 0.3 psi due to the addition of the piston.

[0112] 0.2 g of the superabsorbent polymer was put while the stopcock of the cylinder was locked, and an excess of a physiological saline solution (0.9% by weight of aqueous sodium chloride solution) was poured so that the superabsorbent polymer was completely submerged. Then, the superabsorbent polymer was allowed to swell for 30 minutes. Thereafter, a piston was applied so that a load of 0.3 psi could be uniformly applied on the swollen superabsorbent polymer.

[0113] Subsequently, the stopcock of the cylinder was opened and a time taken for 20 mL of the physiological saline solution to pass through the swollen superabsorbent polymer was measured in seconds. At this time, the meniscus was marked when the cylinder was filled with 40 mL of the physiological saline solution, and the meniscus was marked when the cylinder was filled with 20 mL of the physiological saline solution, and thus a time taken to reach from the level corresponding to 40 mL to the level corresponding to 20 mL was measured to determine T1 of Equation 4.

[0114] (5) Crushing Method and Crush Resistance Index

[0115] 0.5 kg to 1 kg of the superabsorbent polymer with a diameter of 600 μm to 710 μm was transferred through a pneumatic conveying line (diameter: 1 inch, length: 30 m, 90 degree bending line 2ea) in a pneumatic conveying system (PCS) having a structure of FIG. 1 at a pneumatic velocity of 15 m/s to 20 m/s, and then re-classified using a classifier to recover superabsorbent polymers having a diameter of 600 μm to 710 μm.

[0116] Sphericity of the recovered superabsorbent polymers was measured using Malvern Morphology 4 equipment (Malvern Panalytical), and calculated according to the following Equation.

Crush resistance index=((Sphericity after crushing)−(Sphericity before crushing))×100  [Equation 5]

Example 1

[0117] 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 respective processes was used.

[0118] (Step 1)

[0119] 100 parts by weight of acrylic acid was mixed with 0.6 parts by weight of polyethylene glycol diacrylate (weight average molecular weight: ˜500 g/mol) as an internal crosslinking agent and 0.01 part by weight of IRGACURE 819 as a photo-polymerization initiator to prepare a monomer solution. Subsequently, while continuously feeding the monomer solution by a metering pump, 140 parts by weight of a 31 wt % aqueous sodium hydroxide solution was continuously subjected to line mixing to prepare an aqueous monomer solution. At this time, after confirming that the temperature of the aqueous monomer solution raised to about 72° C. or higher by neutralization heat, the solution was left until it was cooled to 40° C. When the temperature was cooled to 40° C., solid-phase sodium bicarbonate as a foaming agent was added in an amount shown in Table 1 below, and simultaneously, 6 parts by weight of a 2 wt % aqueous sodium persulfate solution was added. The solution was poured into a tray in the form of Vat (width 15 cm×length 15 cm) installed in a square polymerization reactor on which a light irradiation device was mounted, and of which inside was preheated to 80° C., and light irradiation was conducted to photo-initiate. It was confirmed that gel was formed from the surface about 15 seconds after light irradiation, and that a polymerization reaction occurred simultaneously with foaming at about 30 seconds, and then, it was additionally reacted for 3 minutes to obtain a water-containing gel polymer in the form of a sheet.

[0120] (Step 2)

[0121] The water-containing gel polymer in the form of a sheet obtained in the step 1 was confirmed to have a temperature of 70° C. to 90° C. 200 ml of water at room temperature was sprayed onto the water-containing gel polymer. The water-containing gel polymer was cut into a size of 3 cm×3 cm, and then chopped under chopping index conditions as in Table 1 below while pushing the water-containing gel polymer into a perforated panel having a plurality of holes using a screw extruder equipped inside of a cylindrical pulverizing device.

[0122] (Step 3)

[0123] Then, the water-containing gel polymer chopped in the step 2 was dried in a dryer capable of transferring air volume up and down. Hot air at 180° C. was allowed to flow from the lower side to the upper side for 15 minutes, and to flow from the upper side to the lower side again for 15 minutes, thus uniformly drying the water-containing gel polymer so that the water content of the dried powder was about 2% or less.

[0124] (Step 4)

[0125] Subsequently, the polymer dried in the step 3 was pulverized with a pulverizer and then size-sorted to obtain a base polymer powder with a size of 150 μm to 850 μm.

[0126] (Step 5)

[0127] Then, 6 g of the aqueous surface crosslinking solution containing 3 parts by weight of ethylene carbonate was sprayed onto 100 parts by weight of the prepared base polymer powder, and stirred at room temperature to mix them so that the surface crosslinking solution was uniformly distributed on the base polymer powder. Subsequently, the base polymer powder mixed with the surface crosslinking solution was put in a surface crosslinking reactor to perform a surface crosslinking reaction.

[0128] In this surface crosslinking reactor, the base polymer powder was confirmed to be gradually heated from the initial temperature around 80° C., and was operated to reach a maximum reaction temperature of 190° C. after 30 minutes. After reaching the maximum reaction temperature, the reaction was further allowed for 15 minutes, and a sample of the superabsorbent polymer finally prepared was taken. After the surface crosslinking process, classifying was performed with a standard mesh sieve of ASTM standard to prepare a superabsorbent polymer having a particle size of 150 μm to 850 μm.

Examples 2 to 4 and Comparative Examples 1 to 5

[0129] Each superabsorbent polymer was prepared in the same manner as in Example 1, except that the amount of the foaming agent and chopping index were changed as in Table 1 below.

Experimental Example

[0130] Physical properties of the superabsorbent polymers prepared in Examples and Comparative Examples were measured and shown in Tables 1 and 2 below. In Tables 1 and 2, and N, A, and R represent variables of the above-described Equation 1, respectively.

TABLE-US-00001 TABLE 1 (Unit) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Preparation Foaming agent (ppm) 1500 1000 500 1000 conditions Chopping index 63 63 63 72 N rpm 208 208 208 208 A — 0.38 0.38 0.38 0.37 R mm 10 10 10 9 Physical properties CRC (g/gSAP) 33.2 33.8 34.2 32.5 of base resin Vortex (sec) 46 50 53 44 Crush resistance Sphericity — 0.70 0.70 0.70 0.70 (#600-710 um) (before crushing) Sphericity (after — 0.69 0.7 0.69 0.68 crushing) Crush resistance −1.0 0.0 −1.0 −2.0 index Physical properties CRC (g/gSAP) 28.5 28.9 29.1 28.3 of superabsorbent AUP (g/gSAP) 24.9 25.1 25.1 24.7 polymer before CRC + AUP (1) 53.4 54 54.2 53 crushing Vortex (sec) 32 37 40 37 Permeability (sec) 35 36 44 60 Physical properties CRC (g/gSAP) 28.2 29 28.9 28.2 of superabsorbent AUP (g/gSAP) 25 24.9 25.2 24.7 polymer after CRC + AUP (2) 53.2 53.9 54.1 52.9 crushing Vortex (sec) 33 37 40 37 Permeability (sec) 37 39 45 62 Deterioration of Δ(1) − (2) −0.2 −0.1 −0.1 −0.1 physical properties

TABLE-US-00002 TABLE 2 Comparative Comparative Comparative Comparative Comparative (Unit) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Preparation Foaming agent (ppm) 1000 500 0 1000 1000 conditions Chopping index 45 45 63 53 83 N rpm 208 208 208 208 208 A — 0.38 0.38 0.38 0.38 0.36 R mm 14 14 10 12 8 Physical properties CRC (g/gSAP) 35.5 36 37.5 35 31 of base resin Vortex (sec) 61 66 70 56 40 Crush resistance Sphericity — 0.70 0.71 0.72 0.70 0.70 (#600-710 um) (before crushing) Sphericity (after — 0.65 0.7 0.71 0.69 0.68 crushing) Crush resistance −5.0 −1.0 −1.0 −1.0 −2.0 index Physical properties CRC (g/gSAP) 29.5 29.9 30 29.4 27 of superabsorbent AUP (g/gSAP) 25.1 25.3 24.8 25 23.1 polymer before CRC + AUP (1) 54.6 55.2 54.8 54.4 50.1 crushing Vortex (sec) 41 52 63 42 35 Permeability (sec) 60 85 95 60 23 Physical properties CRC (g/gSAP) 29.3 29.6 29.9 29.2 27.1 of superabsorbent AUP (g/gSAP) 24.9 25.2 24.8 25.1 23.1 polymer after CRC + AUP (2) 54.2 54.8 54.7 54.3 50.2 crushing Vortex (sec) 42 51 64 44 34 Permeability (sec) 64 86 96 62 25 Deterioration of Δ(1) − (2) −0.4 −0.4 −0.1 −0.1 0.1 physical properties

[0131] As shown in Table 1, it was confirmed that when the superabsorbent polymers were prepared by the preparation method according to the present technology using the foaming agent and the chopping index in the range according to the present technology, the crush resistance index was high and CRC, AUP, vortex, and permeability were also excellent. It was also confirmed that due to the high crush resistance index, most of the superabsorbent polymers maintained the equivalent physical properties even after crushing.

[0132] However, as shown in Table 2, it was confirmed that Comparative Examples 1 and 2 showed low crush resistance index due to the chopping index lower than the range according to the present technology, and therefore, after crushing, the physical properties of the superabsorbent polymers were mostly deteriorated. Further, Comparative Example 3 showed deterioration in the absorption rate and permeability, although the crush resistance index was equivalent to that of Examples of the present technology. This is because gel pulverization was not uniformly performed to affect the physical properties of the superabsorbent polymer.

[0133] Further, Comparative Example 4 showed deterioration in the absorption rate and permeability, because no foaming agent was used and thus sufficient pores were not formed inside the superabsorbent polymer. Comparative Examples 5 and 6 showed deterioration in the absorption rate and permeability (Comparative Example 5), or deterioration in the absorption ability (CRC and AUP), although the crush resistance index was equivalent to that of Examples of the present technology, like in Comparative Example 3.

[0134] Accordingly, it was confirmed that when the preparation method according to the present technology is performed using the foaming agent and the chopping index in the range of the present technology, the crush resistance index is high and various physical properties of the superabsorbent polymer are improved at the same time.