Method for preparing super-absorbent polymer

Abstract

The present invention relates to a super-absorbent polymer having excellent properties, both centrifugal retention capacity (CRC) and absorption under pressure (AUP) having been improved by introducing a surface crosslinked layer crosslinked by surface-modified inorganic particles, and to a method for preparing the same. The super-absorbent polymer comprises: a base resin powder containing a crosslinked polymer of water-soluble ethylene-based unsaturated monomers having an at least partially neutralized acidic group; and a surface crosslinked layer formed on the base resin powder, wherein inorganic particles may be chemically bound to the crosslinked polymer contained in the surface crosslinked layer, via an oxygen-containing bond or a nitrogen-containing bond.

Claims

1. A method of preparing the superabsorbent polymer, the method comprising: polymerizing water-soluble ethylene-based unsaturated monomers in the presence of an internal crosslinking agent to form a hydrogel polymer, wherein the water-soluble ethylene-based unsaturated monomers have acidic groups which are at least partially neutralized; drying, pulverizing, and classifying the hydrogel polymer to form a base polymer powder; and crosslinking the surface of the base polymer powder in the presence of a surface crosslinking agent and surface-modified inorganic particles including surfaces modified with one or more functional groups selected from the group consisting of an epoxy group, a hydroxy group, an isocyanate group, and an amine group to form a surface crosslinking layer.

2. The method of claim 1, further comprising the step of modifying the surface of the inorganic particles with a modifier represented by the following Chemical Formula 1 to prepare surface-modified inorganic particles: ##STR00002## wherein R.sub.1 to R.sub.3 are each independently an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or halogen, and at least one of them is not an alkyl group, and R.sub.4 is a substituent having 2 to 10 carbon atoms, which has one or more functional groups selected from the group consisting of an epoxy group, a hydroxy group, an isocyanate group, and an amine group.

3. The method of claim 1, wherein the surface-modified inorganic particles are used in an amount of 0.01 to 10 parts by weight, based on 100 parts by weight of the base polymer powder.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1 to 3 illustrate an exemplary apparatus for measuring gel bed permeability and components equipped in the apparatus.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(2) 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 following Examples.

Preparation Example 1: Preparation of Surface-Modified Inorganic Particles (Introduction of Epoxy Group)

(3) 5% by weight of Aerosil 200 (Evonik) was dispersed in water to prepare 100 g of a solution. Subsequently, 1 mL of acetic acid was added to this solution to adjust pH to 3. Then, 2 g of (3-(glycidyloxy)propyl)trimethoxysilane was added thereto. To the solution thus obtained, 70 g of 1 mm bead (ZrO.sub.2) was added, and mixed for about 24 hours to modify the surface of the inorganic particles. A product thus obtained was washed with n-butyl acetate to obtain surface-modified inorganic particles.

Preparation Example 2: Preparation of Surface-Modified Inorganic Particles (Introduction of Epoxy Group)

(4) 200 g of IPA (iso-propyl alcohol) and 12 g of (3-(glycidyloxy)propyl)trimethoxysilane were added to 100 g of Ludox HSA (silica content: 30% by weight). To the solution thus obtained, 70 g of 1 mm bead (ZrO.sub.2) was added, and mixed for about 24 hours to modify the surface of the inorganic particles. A product thus obtained was washed with n-butyl acetate to obtain surface-modified inorganic particles.

(5) (Ludox HSA is colloidal silica having a particle size of 12 nm and a specific surface area of 215 m.sup.2/g)

Example 1: Preparation of Superabsorbent Polymer

(6) 100 g of acrylic acid, 38.9 g of caustic soda (NaOH), and 103.9 g of water were mixed. To this mixture, 0.01 g of a photo-polymerization initiator, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, 0.18 g of a thermal polymerization initiator, sodium persulfate, and 0.35 g of an internal crosslinking agent, polyethylene glycol diacrylate were added to prepare a monomer composition. Temperature of the monomer composition was maintained at 40° C. in a water bath.

(7) Meanwhile, for photo-polymerization and thermal polymerization of the monomer composition, used was an apparatus which was equipped with a biaxial rotary silicon belt and a mercury lamp placed on the belt and was able to provide hot air for an insulated space after UV irradiation.

(8) The monomer composition of which temperature was controlled in the water bath was fed to the belt of the apparatus. The monomer composition on the belt was irradiated with UV at an intensity of 10 mW for 60 seconds using the mercury lamp placed on the top of the belt. After UV irradiation, hot air was provided for the photo-polymerized polymer to maintain temperature at 90° C. for thermal polymerization. Thereafter, a hydrogel polymer discharged through a cutter was dried in a hot air dryer at 180° C. for 1 hour.

(9) The hydrogel polymer thus dried was pulverized using a pin mill pulverizer. Thereafter, the polymer was classified into a polymer having a particle size of less than about 150 μm and a polymer having a particle size of about 150 μm to about 850 μm using a sieve.

(10) Thereafter, a surface treatment solution including 1.0 part by weight of 1,3-propanediol, 1 part by weight of water, and 0.1 part by weight of the surface-modified inorganic particles prepared in Preparation Example 1, based on 100 parts by weight of the prepared base polymer powder, was sprayed and stirred at room temperature to uniformly distribute the surface crosslinking agent and the surface treatment solution in the base polymer powder.

(11) Thereafter, the mixture was fed to a surface crosslinking reactor set at about 180° C. to allow surface crosslinking reaction. In the surface crosslinking reactor, the temperature of the base polymer powder was gradually increased from the initial temperature near about 160° C., and reached a maximum temperature of about 180° C. after about 30 minutes. After reaching the maximum reaction temperature, additional reaction was allowed for about 20 minutes, thereby obtaining a final superabsorbent polymer sample. After the surface crosslinking process, a surface-crosslinked superabsorbent polymer having a particle size of about 150 μm to about 850 μm was obtained by using a sieve. The content of fine powder having a particle size of about 150 μm or less in the superabsorbent polymer was less than about 2% by weight.

Example 2: Preparation of Superabsorbent Polymer

(12) A superabsorbent polymer was prepared in the same manner as in Example 1, except that 0.3 parts by weight of the surface-modified inorganic particles was used, based on 100 parts by weight of the base polymer powder, in Example 1. The content of fine powder having a particle size of about 150 μm or less in the superabsorbent polymer was less than about 2% by weight.

Comparative Example 1: Preparation of Superabsorbent Polymer

(13) A superabsorbent polymer was prepared in the same manner as in Example 1, except that the surface-modified inorganic particles were not used in Example 1.

Comparative Example 2: Preparation of Superabsorbent Polymer

(14) 0.1 part by weight Aerosil 200 (Evonik) was added to and mixed with 100 parts by weight of the superabsorbent polymer prepared in Comparative Example 1.

Comparative Example 3: Preparation of Superabsorbent Polymer

(15) 0.3 part by weight Aerosil 200 (Evonik) was added to and mixed with 100 parts by weight of the superabsorbent polymer prepared in Comparative Example 1.

Experimental Example: Test of Physical Properties of Superabsorbent Polymer

(16) Portions of the superabsorbent polymers prepared in Example 1, Example 2, Comparative Example 2, and Comparative Example 3 were taken and classified for 10 minutes using a testing sieve having a mesh size of #100 to remove dust from the superabsorbent polymers (Dedusting process).

(17) Physical properties of the superabsorbent polymer before and after the dedusting process were measured by the following method and given in Table 1.

(18) (1) Centrifuge Retention Capacity (CRC)

(19) Centrifuge retention capacity (CRC) by absorbency under no load was measured for the superabsorbent polymers according to EDANA (European Disposables and Nonwovens Association) WSP 241.2.

(20) In detail, the polymer W0(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 0.9% by weight of sodium chloride aqueous solution. After 30 minutes, the bag was drained at 250 G for 3 minutes with a centrifuge, and the weight W2(g) of the bag was then measured. Further, the same procedure was carried out using no polymer, and the resultant weight W1(g) was measured.

(21) Thus, centrifuge retention capacity (CRC (g/g)) was calculated from these weights thus obtained according to the following Equation:
CRC(g/g)={[W2(g)−W1(g)]/W0(g)}−1  [Equation 1]

(22) wherein W0(g) is the initial weight (g) of the superabsorbent polymer,

(23) W1(g) is the weight of the empty bag which is measured after immersing the bag in the physiological saline solution at room temperature for 30 minutes and draining water off at 250 G for 3 minutes with a centrifuge, and

(24) W2(g) is the weight of the bag including the superabsorbent polymer, which is measured after immersing the bag including the superabsorbent polymer in the physiological saline solution at room temperature for 30 minutes and draining water off at 250 G for 3 minutes with a centrifuge.

(25) (2) Absorbency Under Pressure (AUP)

(26) Absorbency under pressure (AUP) was measured for the superabsorbent polymers according to EDANA (European Disposables and Nonwovens Association) WSP 242.2.

(27) First, a 400 mesh stainless steel net was installed in the bottom of the plastic cylinder having the internal diameter of 60 mm. The superabsorbent polymers W0(g) (0.90 g), of which absorbency under pressure was intended to be measured, was uniformly scattered on the steel net at room temperature and the humidity of 50%, and a piston which may provide a load of 4.83 kPa (0.7 psi) uniformly was put thereon, in which the 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. The weight W3(g) of the apparatus thus prepared was measured.

(28) 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 (0.9% by weight of sodium chloride aqueous solution) 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 on the glass filter.

(29) Subsequently, the prepared measuring apparatus was put on the filter paper, and the superabsorbent polymer in the apparatus was allowed to be swollen by the physiological saline solution under the load. After 1 hr, the weight W4(g) of the apparatus including the swollen superabsorbent polymer was measured.

(30) The weights thus obtained were used to calculate absorbency under pressure according to the following Equation 2:
AUP(g/g)=[W4(g)−W3(g)]/W0(g)  [Equation 2]

(31) wherein W0(g) is the initial weight (g) of the superabsorbent polymer,

(32) W3(g) is the total weight of the superabsorbent polymer and an apparatus capable of providing a load for the superabsorbent polymer, and

(33) W4(g) is the total weight of the superabsorbent polymer and the apparatus capable of providing a load for the superabsorbent polymer, which are measured after immersing the superabsorbent polymer in the physiological saline solution under a load of about 0.7 psi for 1 hour.

(34) (3) Gel Bed Permeability (GBP)

(35) The free swell gel bed permeability (GBP) of the superabsorbent polymer for the physiological saline solution was measured according to a method described in Patent Application No. 2014-7018005.

(36) In detail, to measure free swell GBP, an apparatus shown in FIGS. 1 to 3 was used. First, a plunger 536, with a weight 548 seated thereon, was placed in an empty sample container 530 and the height from the top of the weight 548 to the bottom of the sample container 530 was measured using a suitable gauge accurate to 0.01 mm. The force the thickness gauge applies during measurement was controlled to less than about 0.74 N.

(37) Meanwhile, a superabsorbent polymer to be tested for GBP was prepared from superabsorbent polymers which were prescreened through a US standard 30 mesh screen and retained on a US standard 50 mesh screen. As a result, a superabsorbent polymer having a particle size of about 300 μm to about 600 μm was prepared.

(38) Approximately 2.0 g of the superabsorbent polymer thus classified was placed in the sample container 530 and spread out evenly on the bottom of the sample container. Subsequently, the container without the plunger 536 and weight 548 therein was then submerged for about 60 minutes in the physiological saline solution which is 0.9% by weight of sodium chloride aqueous solution to swell the superabsorbent polymer under no restraining load. In this regard, the sample container 530 was set on a mesh located in the liquid reservoir so that the sample container 530 was raised slightly above the bottom of the liquid reservoir. The mesh did not inhibit the flow of the physiological saline solution into the sample container 530. Also, depth of the physiological saline solution during saturation was controlled so that the surface within the sample container was defined solely by the swollen superabsorbent polymer, rather than the physiological saline solution.

(39) At the end of this period, the plunger 536 and weight 548 assembly was placed on the swollen superabsorbent polymer 568 in the sample container 530 and then the sample container 530, plunger 536, weight 548, and swollen superabsorbent polymer 568 were removed from the solution. After removal and before being measured, the sample container 530, plunger 536, weight 548, and swollen superabsorbent polymer 568 were to remain at rest for about 30 seconds on a suitable flat, large grid non-deformable plate of uniform thickness. The height from the top of the weight 548 to the bottom of the sample container 530 was measured using the same thickness gauge used previously. The height measurement of the device where the plunger 536 and the weight 548 were placed in the empty sample container 530 was subtracted from the height measurement of the device including the swollen superabsorbent polymer 568 to obtain the thickness or height “H” of the swollen superabsorbent polymer.

(40) The GBP measurement was initiated by delivering a flow of the physiological saline solution (0.9% by weight of sodium chloride aqueous solution) into the sample container 530 with the swollen superabsorbent polymer 568, plunger 536, and weight 548 inside. The flow rate of the physiological saline solution into the sample container 530 was adjusted to cause saline solution to overflow the top of the cylinder 534, resulting in a consistent head pressure equal to the height of the sample container 530. The quantity of solution passing through the swollen superabsorbent polymer 568 versus time was measured gravimetrically using the scale 602 and beaker 603. Data points from the scale 602 were collected every second for at least sixty seconds once the overflow had begun. The flow rate, Q through the swollen superabsorbent polymer 568 was determined in units of g/sec by a linear least-square fit of fluid (g) passing through the swollen superabsorbent polymer 568 versus time (sec).

(41) Data thus obtained were used to calculate GBP (cm.sup.2) according to the following Equation 3:
K=[Q*H*μ]/[A*ρ*P]  [Equation 3]

(42) wherein K is a gel bed permeability (cm.sup.2),

(43) Q is a flow rate (g/sec),

(44) H is a height of the swollen superabsorbent polymer (cm),

(45) μ is a liquid viscosity (P) (approximately 1 cP for the physiological saline solution used in this test),

(46) A is a cross-sectional area for liquid flow (28.27 cm.sup.2 for the sample container used in this test),

(47) ρ is a liquid density (g/cm.sup.3) (approximately 1 g/cm.sup.3 for the physiological saline solution used in this test) and

(48) P is a hydrostatic pressure (dyne/cm.sup.2) (normally approximately 7,797 dyne/cm.sup.2).

(49) The hydrostatic pressure was calculated from P=ρ*g*h, wherein p is a liquid density (g/cm.sup.3), g is a gravitational acceleration (nominally 981 cm/sec.sup.2), and h is a fluid height (e.g., 7.95 cm for the GBP test described herein).

(50) At least two samples were tested, and a mean value of the results was used to determine the free swell GBP of the superabsorbent polymer. The unit was converted to darcy (1 darcy=0.98692×10.sup.−8 cm.sup.2), and given in Table 1.

(51) TABLE-US-00001 TABLE 1 Before dedusting process (After surface crosslinking) After dedusting process Kind of inorganic particles CRC AUP GBP CRC AUP GBP (Content) (g/g) (g/g) (darcy) (g/g) (g/g) (darcy) Example 1 Surface-modified inorganic 34.6 23.3 11 34.3 23.5  9 particles of Preparation Example 1 (0.1 part by weight, based on 100 parts by weight of base polymer powder) Example 2 Surface-modified inorganic 32.6 21.6 41 32.4 22.2 45 particles of Preparation Example 1 (0.3 parts by weight, based on 100 parts by weight of base polymer powder) Comparative — 34.5 22.7  3 — — — Example 1 Comparative Aerosil 200 35.1 21.0  8 34.1 21.6  4 Example 2 (0.1 part by weight, based on 100 parts by weight of superabsorbent polymer) Comparative Aerosil 200 34.6 16.7 48 33.4 17.6 31 Example 3 (0.3 parts by weight, based on 100 parts by weight of superabsorbent polymer)

(52) Referring to Table 1, the superabsorbent polymer according to an embodiment of the present invention was found to have excellent centrifuge retention capacity, absorbency under pressure, and permeability. In contrast, when any one physical property of centrifuge retention capacity, absorbency under pressure, and permeability of the superabsorbent polymers of Comparative Examples 1 to 3 was excellent, at least one of the other physical properties were poor.

(53) Further, the superabsorbent polymers of Examples 1 and 2 maintained excellent physical properties because the inorganic particles did not break away therefrom even after the dedusting process, whereas the physical properties of the superabsorbent polymers of Comparative Examples 2 and 3 were deteriorated.

(54) Accordingly, since the superabsorbent polymers according to an embodiment of the present invention have the surface crosslinking structure, their physical properties may be improved and deterioration in the physical properties may be minimized during the preparation and transportation of the superabsorbent polymers.

REFERENCE NUMERALS

(55) 500: GBP measuring apparatus 528: Test apparatus assembly 530: Sample container 534: Cylinder 534a: Region with outer diameter of 66 mm 536: Plunger 538: Shaft 540: O-ring 544, 554, 560: Hole 548: Annular weight 548a: Thru-bore 550: Plunger head 562: Shaft hole 564: 100 Mesh stainless steel cloth screen 566: 400 Mesh stainless steel cloth screen 568: Sample 600: Weir 601: Collection device 602: Scale 603: Beaker 604: Metering pump