Super absorbent polymer and preparation method thereof

Abstract

The super absorbent polymer according to the present invention has reduced 3-hour saline solution re-wet while having a high absorption rate and absorption against pulp, and thus can be used for hygienic materials such as diapers, thereby exhibiting excellent performance.

Claims

1. A super absorbent polymer comprising: a base polymer powder containing a first crosslinked polymer of a water-soluble ethylenically unsaturated monomer having an acidic group in which at least a part thereof is neutralized; and a surface crosslinked layer formed on the base polymer powder and containing a second crosslinked polymer in which the first crosslinked polymer is additionally cross-linked via a surface crosslinking solution comprising a compound having two or more epoxy rings and a compound having two or more hydroxy groups in a weight ratio of 1:1 to 1:4, and water in an amount of 4 to 10 parts by weight based on 100 parts by weight of the base polymer powder, wherein the compound having two or more epoxy rings is ethylene glycol diglycidyl ether, and the compound having two or more hydroxy groups is propylene glycol, and wherein the super absorbent polymer has an absorption rate (vortex) of 50 seconds or less, an absorption against pulp of 18 g/g or more, a gel bed permeability (GBP) of 9 Darcy or more, and a 3-hour saline solution re-wet of 1.0 g or less.

2. The super absorbent polymer of claim 1, wherein it has an absorption rate (vortex) of 45 seconds or less.

3. The super absorbent polymer of claim 1, wherein it has an absorbency under pressure at 0.9 psi (0.9 AUP) of 9 g/g or more.

4. The super absorbent polymer of claim 1, wherein it has a centrifuge retention capacity (CRC) of 28 g/g or more.

5. A method for preparing the super absorbent polymer of claim 1, the method comprising the steps of: crosslinking the water-soluble ethylenically unsaturated monomer having an acidic group in which at least a part thereof is neutralized in the presence of an internal crosslinking agent to form a hydrogel polymer containing the first crosslinked polymer; drying, pulverizing and classifying the hydrogel polymer to form e base polymer power, and heat-treating and surface-crosslinking the base polymer powder in the presence of the surface crosslinking solution to form a super absorbent polymer particle.

6. The method for preparing a super absorbent polymer of claim 5, wherein the surface crosslinking solution comprises aluminum sulfate.

7. The method for preparing a super absorbent polymer of claim 5, wherein the surface crosslinking solution comprises an inorganic filler.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing an example of an apparatus for measuring a gel bed permeability (GBP) according to an embodiment of the present invention.

(2) FIGS. 2 and 3 are schematic views showing an example of a chamber and a mesh arrangement for measuring the gel bed permeability, respectively.

(3) FIG. 4 schematically shows a method of measuring an absorption against pulp according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) Hereinafter, preferred embodiments are presented to facilitate understanding of the invention. However, the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention thereto.

Example 1

(5) A solution in which 20.5 g (80 ppm) of 0.21% IRGACURE 819 initiator diluted with 500 g of acrylic acid was mixed, and a solution in which 21 g of 10% polyethylene glycol diacrylate (PEGDA, Mw=400) diluted with acrylic acid was mixed were injected. Then, 12 g of 2% SDS (sodium dodecyl sulfate) solution was mixed therein, to which 800 g of a 24% caustic soda solution was slowly added dropwise and mixed. The water-soluble ethylenically unsaturated monomer was thus obtained, and the degree of neutralization of acrylic acid in the sodium acrylate was 70 mol %.

(6) When mixing the two solutions, it was confirmed that the temperature of the mixture was increased by 72° C. or more by neutralizing heat. Then, when the temperature was cooled to 43° C., 13.4 g of a 4% sodium bicarbonate solution was mixed, and at the same time, 16 g of 4% sodium persulfate solution diluted with water was injected.

(7) Then, the above prepared solution was poured in a tray (15 cm in width×15 cm in length) installed in a square polymerizer which had a UV irradiation device installed at the top and whose inside was preheated to 80° C. Polymerization was initiated by irradiating an ultraviolet ray. The sheet-shaped polymer produced was cut into a size of about 5 cm×5 cm, and then put into a meat chopper to pulverize the polymer, thereby obtaining hydrogel particles having a size of 1 mm to 10 mm.

(8) The crumps were dried in an oven capable of shifting airflow upward and downward. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and again from the top to the bottom for 15 minutes, and thereby a water content of the dried product was set to 2% or less. After drying, the dried product was pulverized using a pulverizing device and classified into a size of 150 to 850 μm to obtain a base polymer.

(9) 4 g of water, 4 g of methanol, 0.25 g of EX-810 (Ethylene Glycol Diglycidyl Ether), 0.3 g of propylene glycol (PG), 0.15 g of aluminum sulfate (Al—S), 0.1 g of fumed silica (AEROXIDE® Alu 130), and 0.05 g of a reducing agent (Na.sub.2S.sub.2O.sub.5) were mixed to prepare a crosslinking agent solution. The surface crosslinking agent solution was mixed with 100 g of the base polymer powder obtained above, and the mixture was placed in a convection oven at 140° C. and allowed to react for 35 minutes. The produced powder was classified with a standard mesh sieve according to ASTM standard to obtain a super absorbent polymer having a particle size of 150 to 850 μm.

Examples 2-5 and Comparative Examples 1-4

(10) The super absorbent polymer was prepared in the same manner as in Example 1, except that the composition of the surface crosslinking agent solution was changed as shown in Table 1 below.

Experimental Example

Evaluation of Physical Properties of Super Absorbent Polymer

(11) The physical properties of the super absorbent polymer prepared in the above Examples and Comparative Examples were evaluated by the following methods.

(12) (1) Centrifuge Retention Capacity (CRC)

(13) The centrifuge retention capacity(CRC) by water absorption capacity under a non-loading condition was measured for the super absorbent polymers of Examples and Comparative Examples in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 241.3.

(14) Specifically, W.sub.0(g) (about 0.2 g) of the super absorbent polymers of Examples and Comparative Examples were uniformly put in a nonwoven fabric-made bag, followed by sealing. Then, the bag was immersed in a physiological saline solution composed of 0.9 wt % aqueous sodium chloride solution 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 super absorbent polymer, and then the resultant weight W.sub.1(g) was measured.

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

(16) in Mathematical Formula 1,

(17) W.sub.0(g) is an initial weight(g) of the super absorbent polymer, W.sub.1(g) is the weight of the device not including the super absorbent polymer, measured after dehydrating the same by using a centrifuge at 250 G for 3 minutes, and W.sub.2(g) is the weight of the device including the super absorbent polymer, measured after immersing and absorbing the super absorbent polymer into a physiological saline solution at room temperature for 30 minutes and then dehydrating the same by using a centrifuge at 250 G for 3 minutes.

(18) (2) 3-Hour Saline Solution Re-Wet

(19) 2 g of the super absorbent polymer prepared in the above Examples and Comparative Examples was placed in a 500 mL beaker, and 60 mL of 0.9 wt % sodium chloride aqueous solution was added thereto at room temperature (about 22 to 24° C.) was added thereto and kept for 3 hours. Then, five sheets of filter papers with a diameter of 5 cm were stacked on the swollen super absorbent polymer, and a piston capable of uniformly applying a load of 0.2 psi was placed on the filter paper. After 1 minute, the weight of aqueous sodium chloride solution absorbed by the filter paper (3-hour saline solution re-wet) was measured.

(20) (3) Absorption Rate (Vortex)

(21) 50 mL of a 0.9 wt % sodium chloride aqueous solution was placed in a 100 mL beaker and 2 g of the super absorbent polymers prepared in the Examples and Comparative Examples were each added thereto while stirring at 600 rpm using a stirrer. Then, the amount of time until the vortex of the liquid caused by the stirring disappeared and a smooth surface was formed was measured, and the result was expressed by the vortex removal time (absorption rate; vortex).

(22) (4) Absorbency Under Pressure (AUP)

(23) The absorbency under Pressure (AUP) was measured for the super absorbent polymers of the Examples and Comparative Examples in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 242.3.

(24) Specifically, a 400 mesh stainless steel net was installed in the bottom of a plastic cylinder having an internal diameter of 60 mm. The absorbent polymers W.sub.0(g) (about 0.90 g) obtained in the Examples and Comparative Examples were uniformly scattered on the steel net under conditions of temperature of 23±2° C. and relative humidity of 45%, and a piston which can provide a load of 0.9 psi uniformly was put thereon. The external diameter of the piston was slightly smaller than 60 mm, there was no gap between the internal wall of the cylinder and the piston, and the jig-jog of the cylinder was not interrupted. In this case, the weight W.sub.3(g) of the device was measured.

(25) 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 0.90 wt % 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 120 mm was put thereon. The measuring device was put on the filter paper and the solution was absorbed under a load for about 1 hour. After 1 hour, the weight W.sub.4(g) was measured after lifting the measuring device up.

(26) Using the respective weights thus obtained, the AUP (g/g) was calculated according to the following Mathematical Formula 2.
AUP(g/g)=[W.sub.4(g)−W.sub.3(g)]/W.sub.0(g)  [Mathematical Formula 2]

(27) in Mathematical Formula 2,

(28) W.sub.0(g) is an initial weight(g) of the super absorbent polymer, W.sub.3(g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, and W.sub.4(g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, after absorbing a physiological saline solution to the super absorbent polymer under a load (0.9 psi) for 1 hour.

(29) (5) Gel Bed Permeability (GBP)

(30) The gel bed permeability (GBP) was measured for the super absorbent polymer prepared in the Examples and the Comparative Examples. The measurement method of GBP is disclosed in in U.S. Pat. No. 7,179,851.

(31) First, the apparatus suitable for carrying out the gel bed permeability test is shown in FIG. 1. The test apparatus 28 comprises a sample container, generally indicated at 30, and a piston, generally indicated at 35. The piston 35 comprises a cylindrical LEXAN shaft 38 having a concentric cylindrical hole 40 bored down the longitudinal axis of the shaft. Both ends of the shaft 38 are machined to provide upper and lower ends respectively designated 42, 46. A weight, indicated as 48, rests on one end 42 and has a cylindrical hole 48a bored through at least a portion of its center.

(32) A circular piston head 50 is positioned on the other end 46 and is provided with a concentric inner ring of seven holes 60, each having a diameter of about 0.95 cm, and a concentric outer ring of fourteen holes 54, also each having a diameter of about 0.95 cm. The holes 54, 60 are bored from the top to the bottom of the piston head 50. The piston head 50 also has a cylindrical hole 62 bored in the center thereof to receive end 46 of the shaft 38. The bottom of the piston head 50 may also be covered with a biaxially stretched 400 mesh stainless steel screen 64.

(33) The sample container 30 comprises a cylinder 34 and a 400 mesh stainless steel cloth screen 66 that is biaxially stretched to tautness and attached to the lower end of the cylinder. A gel particle sample, indicated as 68 in FIG. 3, is supported on the screen 66 within the cylinder 34 during testing.

(34) The cylinder 34 may be bored from a transparent LEXAN rod or equivalent material, or it may be cut from a LEXAN tubing or equivalent material, and has an inner diameter of about 6 cm (e.g., a cross-sectional area of about 28.27 cm.sup.2), a wall thickness of about 0.5 cm and a height of about 10 cm. Drainage holes (not shown) are formed in the sidewall of the cylinder 34 at a height of about 7.8 cm above the screen 66 to allow liquid to drain from the cylinder to thereby maintain a fluid level in the sample container at approximately 7.8 cm above the screen 66. The piston head 50 is machined from a LEXAN rod or equivalent material and has a height of approximately 16 mm and a diameter sized such that it fits within the cylinder 34 with minimum wall clearance but still slides freely. The shaft 38 is machined from a LEXAN rod or equivalent material and has an outer diameter of about 2.22 cm and an inner diameter of about 0.64 cm.

(35) The shaft upper end 42 is approximately 2.54 cm long and approximately 1.58 cm in diameter, forming an annular shoulder 47 to support the weight 48. The annular weight 48 has an inner diameter of about 1.59 cm so that it slips onto the upper end 42 of the shaft 38 and rests on the annular shoulder 47 formed thereon. The annular weight 48 can be made from stainless steel or from other suitable materials resistant to corrosion in the presence of the test solution, which is 0.9 wt % sodium chloride solution in distilled water. The combined weight of the piston 35 and annular weight 48 equals approximately 596 g, which corresponds to a pressure applied to the sample 68 of about 0.3 psi, or about 20.7 g/cm.sup.2, over a sample area of about 28.27 cm.sup.2.

(36) When the test solution flows through the test apparatus during testing as described below, the sample container 30 generally rests on a 16 mesh rigid stainless steel support screen (not shown). Alternatively, the sample container 30 may rest on a support ring (not shown) diametrically sized substantially the same as the cylinder 34 so that the support ring does not restrict flow from the bottom of the container.

(37) To conduct the gel bed permeability test under “free swell” conditions, the piston 35, with the weight 48 seated thereon, is placed in an empty sample container 30 and the height is measured using a suitable gauge accurate to 0.01 mm with the platen removed. It is important to measure the height of each sample container 30 empty and to keep track of which piston 35 and weight 48 is used when using multiple test apparatus. The same piston 36 and weight 48 should be used for measurement when the sample 68 is later swollen following saturation.

(38) The sample to be tested is prepared from super absorbent material particles which are prescreened through a U.S. standard 30 mesh screen and retained on a U.S. standard 50 mesh screen. As a result, the test sample comprises particles sized in the range of about 300 to about 600 μm. The particles can be prescreened by hand or automatically. About 2.0 g of the sample is placed in the sample container 30, and the container, without the piston 35 and weight 48 therein, is then submerged in the test solution for a time period of about 60 minutes to saturate the sample and allow the sample to swell free of any restraining load.

(39) At the end of this period, the piston 35 and weight 48 assembly is placed on the saturated sample 68 in the sample container 30 and then the sample container 30, piston 35, weight 48, and sample 68 are removed from the solution. The thickness of the saturated sample 68 is determined by again measuring the height from the bottom of the weight 48 to the top of the cylinder 34, using the same clipper or gauge used previously, provided that the zero point is unchanged from the initial height measurement. The height measurement value obtained from measuring the empty sample container 30, piston 35, and weight 48 is subtracted from the height measurement value obtained after saturating the sample 48. The resulting value is the thickness, or height “H” of the swollen sample.

(40) The permeability measurement is initiated by delivering a flow of the test solution into the sample container 30 with the saturated sample 68, piston 35, and weight 48 inside. The flow rate of test solution into the container is adjusted to maintain a fluid height of about 7.8 cm above the bottom of the sample container. The quantity of solution passing through the sample 68 versus time is measured gravimetrically. Data points are collected every second for at least 20 seconds once the fluid level has been stabilized to and maintained at about 7.8 cm in height. The flow rate Q through the swollen sample 68 is determined in units of grams/second (g/s) by a linear least-square fit of fluid passing through the sample 68 (in grams) versus time (in seconds).

(41) Permeability in Darcy is obtained by the following Mathematical Formula 3.
K=[Q×H×Mu]/[A×Rho×P]  [Mathematical Formula 3]

(42) in Mathematical Formula 3, K is a permeability (cm.sup.2), Q is a flow rate (g/sec), H is a height of sample (cm), Mu is a liquid viscosity (poise) (approximately one centipoises for the test solution used with this Test), A is a cross-sectional area for liquid flow (cm.sup.2), Rho is a liquid density (g/cm.sup.3) (for the test solution used with this Test) and P is a hydrostatic pressure (dynes/cm.sup.2) (normally approximately 3,923 dynes/cm.sup.2). The hydrostatic pressure is calculated by the following Mathematical Formula 4.
P=Rho×g×h  [Mathematical Formula 4]

(43) in Mathematical Formula 4, Rho 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., 7.8 cm for the Gel Bed Permeability Test described herein.

(44) (6) AAP (Absorption Against Pulp, Ability to Absorb Moisture from Pulp)

(45) The ability to absorb moisture from the pulp (AAP) was measured by the method as shown in FIG. 4. Specifically, 6 sheets of tissues with a size of 107 mm×107 mm tissue (Kimberly Kimtec Science Wipers, small size) were placed in plastic disposable dish with a size of 140 mm×140 mm. 1 g of the super absorbent polymer prepared in the Examples and Comparative Examples was uniformly sprayed as a whole on the tissue, and then six sheets of tissues were placed thereon. 30 mL of 0.9% sodium chloride aqueous solution was poured onto the tissue and swollen for 5 minutes, and then the weight (AAP) of the swollen gel between the tissues was measured.

(46) The above measurement results are shown in Table 1 below.

(47) TABLE-US-00001 TABLE 1 Physical property of super absorbent polymer Composition of surface Saline crosslinking solution solution EX- re- 0.9 810 PG Al—S Silica CRC wet Vortex AUL GBP AAP Unit g g g g g/g g sec g/g darcy g/g Ex. 1 0.25 0.3 0.15 0.1 34.0 0.3 32 10 11 22 Ex. 2 0.25 0.3 0.20 0.02 35.2 0.5 40 10 9 19 Ex. 3 0.25 0.6 0.15 0.1 33.8 0.3 31 11 10 21 Ex. 4 0.25 0.6 0.20 0.02 34.7 0.6 42 10 9 18 Ex. 5 0.25 1.0 0.15 0.1 32.0 0.7 30 13 16 18 Comparative 0 0 0.15 0.1 38.5 3.0 31 9 1 — Ex. 1 Comparative 0 0.25 0.15 0.1 38.5 3.1 30 7 1 17 Ex. 2 Comparative 0.1 0 0.20 0.03 32.2 1.1 36 11 31 18 Ex. 3 Comparative 0.1 0 0.45 0.03 34.3 1.1 33 10 23 17 Ex. 4 Comparative 0.25 2.0 0.15 0.1 30.0 2.7 33 15 23 16 Ex. 5