Preparation Method Of Super Absorbent Polymer

Abstract

The present invention relates to a method for preparing superabsorbent polymer. The method for preparing superabsorbent polymer according to the present invention enables providing superabsorbent polymer having excellent absorption ratio, absorption speed and permeability.

Claims

1. A method for preparing superabsorbent polymer comprising the steps of: conducting crosslinking polymerization of water soluble ethylenically unsaturated monomers having acid groups of which at least a part are neutralized, in the presence of an internal crosslinking agent and an expanding agent to form hydrogel polymer comprising a first crosslinked polymer, chopping the hydrogel polymer, drying the chopped hydrogel polymer, chopping the dried polymer, and surface-modifying the chopped polymer to obtain superabsorbent polymer particles, wherein the step of chopping hydrogel polymer is conducted under chopping index condition of 20 to 35 (/s) according to the following Calculation Formula 1, while pushing the hydrogel polymer to a perforated panel having a plurality of holes using a screw type extruder equipped inside of a cylindrical mill.
Chopping index=(TSC/A) [Mathematical Formula 1] in the Calculation Formula 1, is the angular velocity of a screw in the screw type extruder (2N/60 s), and N is the rotation number (rpm) of the screw. TSC is the solid content (%) of the hydrogel polymer introduced in the mill, and A is the aperture ratio (r.sup.2n/R.sup.2) of the perforated panel, and r is the radius (mm) of the holes formed on the perforated panel, n is the number of the holes formed on the perforated panel, and R is the radius (mm) of the perforated panel.

2. The method according to claim 1, wherein the expanding agent exists in the monomer composition at 1000 to 3000 ppm.

3. The method according to claim 1, wherein in the superabsorbent polymer particles, the rate of expanded particles in which concaves due to expansion occupy or more of the particle surface, is 10% or more of the total particle number.

4. The method according to claim 1, wherein in the superabsorbent polymer particles, the rate of the sum of expanded particles in which concaves due to expansion occupy or more of the particle surface, and porous particles comprising three or more pores or concaves having a depth of 10 m or more on the surface is 50% or more of the total particle number.

5. The method according to claim 1, wherein in the superabsorbent polymer particles, centrifuge retention capacity (CRC) to a saline solution (0.9 wt % sodium chloride aqueous solution) for 30 minutes is 26 g/g or more, absorbency under pressure (AUP) under 0.7 psi to a saline solution (0.9 wt % sodium chloride aqueous solution) for 1 hour is 21 g/g or more, saline flow conductivity (SFC) for a saline solution (0.685 wt % sodium chloride aqueous solution) is 35 (.Math.10.sup.7 cm.sup.3.Math.s/g) or more, and T-20 indicating a time taken for 1 g of superabsorbent polymer to absorb 20 g of an aqueous solution of 0.01 wt % C12-14 alcohol ethoxylate and 0.9 wt % sodium chloride is 190 seconds or less.

6. The method according to claim 1, wherein the internal crosslinking agent comprise one or more kinds compounds selected from the group consisting of N,N-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethyleneglycol di(meth)acrylate, polyethyleneglycol di(meth)acrylate, propyleneglycol di(meth)acrylate, polypropyleneglycol di(meth)acrylate, butanediol di(meth)acrylate, butyleneglycol di(meth)acrylate, diethyleneglycol di(meth)acrylate, hexanediol di(meth)acrylate, triethyleneglycol di(meth)acrylate, tripropyleneglycol di(meth)acrylate, tetraethyleneglycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, pentaerythritol tetraacrylate, triallylamine, allyl (meth)acrylate, ethyleneglycol diglycidyl ether, propylene glycol, glycerin, and ethylenecarbonate.

7. The method according to claim 1, wherein the expanding agent comprises one or more kinds of compounds selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium bicarbonate, magnesium bicarbonate, magnesium carbonate, azodicarbonamide (ADCA), dinitroso pentamethylene tetramine (DPT), p,p-oxybis(benzenesulfonyl hydrazide) (OBSH), p-toluenesulfonyl hydrazide (TSH), sucrose stearate, sucrose palmitate, and sucrose laurate.

8. The method according to claim 1, wherein the water soluble ethylenically unsaturated monomers comprise one or more kinds of compounds selected from the group consisting of anionic monomers and salts thereof selected from acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-(meth)acryloylethane sulfonic acid, 2-(meth)acryloyl propane sulfonic acid, or 2-(meth)acrylamido-2-methyl propane sulfonice acid; non-ionic hydrophilic group containing monomers selected from (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxy polyethylene glycol (meth)acrylate, or polyethylene glycol (meth)acrylate; and amino group containing unsaturated monomers selected from (N,N)-dimethylaminoethyl (meth)acrylate, (N,N)-dimethylaminopropyl (meth)acrylamide, and quarternarized products thereof

9. the method according to claim 1, wherein the superabsorbent polymer particle has a particle diameter of 150 to 850 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0120] FIG. 1 shows the SEM images of expanded particles in which concaves due to expansion occupy or more of the particle surface [FIG. 1(a)], porous particles that comprise three or more pores or concaves having a depth of 10 m or more on the surface [FIG. 1(b), and non-shear particles that have a smooth surface because deformation of particles does not occur during the chopping process [FIG. 1(c)] in the polymer particles prepared according to Example 1 of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0121] Hereinafter, the actions and the effects of the invention will be explained in detail through the specific examples. However, these examples are presented only as the illustrations of the invention, and the scope of the right of the invention is not limited thereby.

EXAMPLES AND COMPARATIVE EXAMPLES

[0122] As an apparatus for preparing superabsorbent polymer, an apparatus for continuous preparation consisting of a polymerization process, a hydrogel chopping process, a drying process, a chopping process, a sieving process, a surface crosslinking process, a cooling process, a sieving process, and transport processes connecting each process, was used.

[0123] (Step 1)

[0124] 100 parts by weight of acrylic acid were mixed with 0.4 parts by weight of polyethyleneglycol diacrylate (weight average molecular weight: 500 g/mol) and 0.1 parts by weight of hexandiol diacrylate as internal crosslinking agents, and 0.01 parts by weight of IRGACURE 819 as a photoinitiator to prepare a monomer solution. Subsequently, while continuously feeding the monomer solution with a metering pump, 160 parts by weight of 24 wt % sodium hydroxide aqueous solution were continuously line mixed to prepare a monomer aqueous solution. Here, it was confirmed that the temperature of the monomer aqueous solution raised to about 72 C. or more by neutralization heat, and then, waited until the temperature is cooled to 40 C.

[0125] When the temperature was cooled to 40 C., solid sodium bicarbonate, an expanding agent, was added to the monomer aqueous solution in the content described in the following Table 1, and simultaneously, 6 parts by weight of 2 wt % sodium persulfate aqueous solution was added.

[0126] The solution was poured into a tray in the form of Vat (tray, width 15 cmlength 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 photoinitiate. It was confirmed that gel was generated from the surface about 25 seconds after the photoinitiation, and that polymerization occurred simultaneously with expansion after about 50 seconds, and hydrogel polymer in the form of a sheet was obtained by additional reaction for 3 minutes.

[0127] (Step 2)

[0128] The hydrogel polymer in the form of a sheet obtained in step 1 was cut into a size of 3 cm3 cm, and then, chopped under chopping index condition as shown in the following Calculation Formula 1 and Table 1, while pushing the hydrogel polymer to a perforated panel having a plurality of holes using a screw type extruder equipped inside of a cylindrical mill.

Chopping index(C.I.)=(TSC/A) [Calculation Formula 1]

[0129] in the Calculation Formula 1,

[0130] is the angular velocity of a screw in the screw type extruder (2N/60 s), and N is the rotation number (rpm) of the screw.

[0131] TSC is the solid content (%) of the hydrogel polymer introduced in the mill, and

[0132] A is the aperture ratio (r.sup.2n/R.sup.2) of the perforated panel, and r is the radius (mm) of the holes formed on the perforated panel, n is the number of the holes formed on the perforated panel, and R is the radius (mm) of the perforated panel.

TABLE-US-00001 TABLE 1 Expanding agent N r R C.I. (ppm) (rpm) TSC (mm) (mm) n (/s) Example 1 2000 180 0.44 4.5 42 24 30.1 Example 2 1600 180 0.44 4.5 42 24 30.1 Example 3 1400 180 0.44 5.0 42 24 24.4 Example 4 1200 180 0.44 5.0 42 24 24.4 Comparative 0 105 0.44 5.0 42 24 14.2 Example 1 Comparative 0 180 0.44 4.5 42 24 30.1 Example 2 Comparative 2000 105 0.44 4.5 42 24 17.6 Example 3

[0133] (Step 3)

[0134] Subsequently, the hydrogel polymer chopped in step 2 was dried in a drier capable of transferring air volume up and down. Hot air of 180 C. was allowed to flow from the lower side to the upper side for 15 minutes such that the moisture content of dried powder became about 2% or less, and then, it was allowed to flow again from the upper side to the lower side for 15 minutes, thus uniformly drying the hydrogel polymer.

[0135] (Step 4)

[0136] The polymer dried in step 3 was chopped with a mill, and then, sieved to obtain base polymer of 150 to 850 m.

[0137] (Step 5)

[0138] 100 parts by weight of the base polymer prepared in step 4 were mixed with 3 parts by weight of water, 3 parts by weight of methanol, and 0.5 parts by weight of ethylene carbonate, and then, surface crosslinking was conducted at 180 C. for 40 minutes. And, the obtained product was cooled, and then, sieved to obtain surface crosslinked superabsorbent polymer particles with a particle diameter of 150 to 850 m.

Experimental Example

[0139] The properties of each superabsorbent polymer prepared in Examples and Comparative Examples were measured and evaluated as follows.

[0140] (1) A Rate According to Particle Shape

[0141] The superabsorbent polymer particles were observed with scanning electron microscope (SEM), to calculate the rate of expanded particles in which concaves due to expansion occupy or more of the particle surface (classified as porous particles if the rate is less than ), porous particles that comprise three or more pores or concaves having a depth of 10 m or more on the surface, and non-shear particles that have a smooth surface because deformation of particles does not occur during the chopping process, in the total particles.

[0142] Among them, the SEM images of the expanded particles [FIG. 1(a)], the porous particles [FIG. 1(b) and the non-shear particles [FIG. 1(c)] in the superabsorbent polymer particles according to Example 1 were shown in FIG. 1.

[0143] (2) Centrifuge Retention Capacity (CRC)

[0144] A centrifuge retention capacity (CRC) by the absorption scale under no load was measured according to European Disposables and Nonwovens Association, (EDANA) Standard EDANA WSP 241.3

[0145] Specifically, W.sub.0 (g, about 0.2 g) of the superabsorbent polymer was uniformly put in an envelope made of non-woven fabric and sealed, and then, immersed in a 0.9 wt % sodium chloride saline solution at room temperature. After 30 minutes, the envelope was drained at 250 G for 3 minutes using a centrifuge, and then, the mass W.sub.2(g) of the envelope was measured. And, after the same operation without using superabsorbent polymer, the mass W.sub.1(g) at that time was measured. Using the obtained masses, CRC (g/g) was calculated according to the following Calculation Formula A, thus confirming centrifuge retention capacity.

CRC(g/g)={[W.sub.2(g)W.sub.1(g)W.sub.0(g)]/W.sub.0(g)}[Calculation Formula A]

[0146] (3) Absorbing Under Pressure (AUP)

[0147] Absorbing under pressure was measured according to European Disposables and Nonwovens Association Standard EDANA WSP 242.3.

[0148] Specifically, on the bottom of plastic cylinder of 60 mm inner diameter, stainless 400 mesh wire netting was installed. Superabsorbent polymer W.sub.0 (g, 0.90 g) was uniformly sprayed onto the wire netting under temperature of 232 C. and 45% humidity, and a piston capable of uniformly giving 4.83 kPa (0.7 psi) load thereon has an outer diameter slightly less than 60 mm and does not have a gap with the inner wall of cylinder, and the upward and downward movement is not hindered.

[0149] At this time, the weight W.sub.3(g) of the apparatus was measured. In the inner side of a petri dish of 150 mm diameter, a glass filter of 125 mm diameter and 5 mm thickness was placed, and a saline solution consisting of 0.90 wt % sodium chloride was placed to the same level as the upper side of the glass filter. One filter paper of 120 mm diameter was loaded thereon. The measuring apparatus was loaded on the filter paper, and the liquid was absorbed under load for 1 hour. After 1 hour, the measuring apparatus was lifted, and the weight W.sub.4(g) was measured. Using each weight obtained above, AUL (g/g) was calculated according to the following Calculation Formula B, thus confirming absorbency under pressure.

AUP(g/g)=[W.sub.4(g)W.sub.3(g)]/W.sub.0(g) [Calculation Formula B]

[0150] (4) Saline Flow Conductivity (SFC)

[0151] A saline flow conductivity was measured and calculated according to the method disclosed in column 54 to column 59 of U.S. Pat. No. 5,562,646.

[0152] (5) T-20

[0153] An aqueous solution was prepared by dissolving 9 g of sodium chloride and 0.1 g of Lorodac (main ingredient: linear C12-14 alcohol ethoxylate, CAS #68439-50-9) in 1 L of distilled water, and T-20 was calculated as a time (seconds) taken for 1 g of superabsorbent polymer to absorb 20 g of the aqueous solution. The specific measurement method of T-20 is describe in detail in pages 13 to 18 of European Patent Publication No. 2535027.

TABLE-US-00002 TABLE 2 Expanded Porous Non-shear particles(%) particles(%) particles(%) Example 1 11 53 36 Example 2 12 55 33 Example 3 14 42 44 Example 4 10 46 44 Comparative 2 20 78 Example 1 Comparative 1 34 65 Example 2 Comparative 15 22 63 Example 3

TABLE-US-00003 TABLE 3 CRC (g/g) AUP (g/g) SFC (.Math.10.sup.7cm.sup.3 .Math. s/g) T-20 (sec) Example 1 28.1 25.6 60 116 Example 2 28.5 26.1 55 132 Example 3 28.1 23.4 37 170 Example 4 27.4 24.3 41 165 Comparative 29.5 23.5 30 240 Example 1 Comparative 27.0 23.8 30 180 Example 2 Comparative 25.8 24.3 70 114 Example 3

[0154] Referring to Tables 1 to 3, since the superabsorbent polymers according to Examples 1 to 3 were obtained through the polymerization process introducing expansion and the hydrogel chopping process under appropriate chopping index, the rate of the expanded particles was 10% or more of the total particle number, and the rate of the sum of the expanded particles and porous particles was 50% or more of the total particle number.

[0155] To the contrary, in the superabsorbent polymer of Comparative Examples 1 and 2, the rate of non-shear particles was as high as 65% or more of the total particle number, and the rates of the expanded particles and porous particles were low. In the superabsorbent polymer of Comparative Example 3, the rate of the expanded particles was as high as 15% of the total particle number, but the rate of the porous particles was as low as 22%.

[0156] And, compared to the superabsorbent polymers according to Comparative Examples 1 to 3, the superabsorbent polymers according to Examples 1 to 3 had equivalent absorbing under pressure (AUP), but exhibited excellent centrifuge retention capacity (CRC), saline flow conductivity (SFC) and T-20 property.