METHOD FOR PREPARING SUPERABSORBENT POLYMER

Abstract

The preparation method of superabsorbent polymer according to the present invention can increase suction power without degradation of other properties of superabsorbent polymer, and thus, the prepared superabsorbent polymer may be usefully used as material of hygienic goods such as a diaper.

Claims

1. A method for preparing superabsorbent polymer comprising the steps of 1) polymerizing and cross-linking a monomer composition comprising a water-soluble ethylene-based unsaturated monomer, a polymerization initiator, a first cross-linking agent and a blowing agent to form a hydrous gel phase polymer, 2) drying the hydrous gel phase polymer, 3) grinding the dried polymer, and 4) carrying out a surface cross-linking reaction by reacting the ground polymer at 180 to 250 C. for 50 minutes or more in the presence of a second cross-linking agent.

2. The method for preparing superabsorbent polymer according to claim 1, wherein the water-soluble ethylene-based unsaturated monomer is a compound represented by the following Chemical Formula 1,
R.sub.1COOM.sup.1[Chemical Formula 1] in the Chemical Formula 1, R.sub.1 is a C.sub.2-5 alkyl group comprising an unsaturated bond, M.sup.1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

3. The method for preparing superabsorbent polymer according to claim 1, wherein the first cross-linking agent is one or more selected from the group consisting of N,N-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethyleneglycol di(meth)acrylate, polyethyleneglycol (meth)acrylate, propyleneglycol di(meth)acrylate, polypropyleneglycol (meth)acrylate, butanediol di(meth)acrylate, butyleneglycol di(meth)acrylate, diethyleneglycol di(meth)acrylate, hexanediol di(meth)acrylate, triethyleneglycol hexanediol di(meth)acrylate, triethyleneglycol di(meth)acrylate, tripropyleneglycol di(meth)acrylate, tetraethyleneglycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, pentaerythritol pentaacrylate, triarylamine, ethyleneglycol diglycidyl ether, propylene glycol, glycerin, and ethylene carbonate.

4. The method for preparing superabsorbent polymer according to claim 1, wherein the blowing agent is one or more selected from the group consisting of sodium bicarbonate, 4,4-oxybis(benzenesulfonyl hydrazide), p-toluenesulfonyl hydrazine, sugar ester, and acetone.

5. The method for preparing superabsorbent polymer according to claim 1, further comprising the step of crushing the hydrous gel phase polymer to a particle diameter of 2 to 10 mm, before the step 2) of drying the hydrous gel phase polymer.

6. The method for preparing superabsorbent polymer according to claim 1, wherein the step 3) of grinding is carried out such that the particle diameter of the ground polymer becomes 150 to 850 um.

7. The method for preparing superabsorbent polymer according to claim 1, wherein the second cross-linking agent is one or more selected form 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, ferrous hydroxide, calcium chloride, magnesium chloride, aluminum chloride, and ferrous chloride.

8. The method for preparing superabsorbent polymer according to claim 1, wherein the surface cross-linking reaction is carried out at 180 to 200 C.

9. The method for preparing superabsorbent polymer according to claim 1, wherein the suction power of the superabsorbent polymer is 15.0 mL/g or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] FIG. 1 is a schematic diagram showing one example of an apparatus for measuring gel bed permeability (GBP) according to one embodiment of the present invention, and FIG. 2 and FIG. 3 are schematic diagrams respectively showing one example of a gel bed permeability measuring cylinder and mesh arrangement.

[0076] FIG. 4 shows an apparatus for measuring suction power.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0077] Hereinafter, preferable examples are presented for a better understanding of the invention. However, these examples are presented only to illustrate the present invention, and the scope of the invention is not limited thereto.

Comparative Example 1

[0078] A partially neutralized acrylic acid aqueous solution was mixed with 2300 ppm of polyethyleneglycol diacrylate (PEGDA) as an internal cross-linking agent, 1500 ppm of potassium persulfate (K.sub.2S.sub.2O.sub.8) as a thermal polymerization initiator, 100 ppm of hydrogen peroxide and 100 ppm of ascorbic acid, and thermal polymerization was progressed to obtain a polymerized sheet. The polymerized sheet was taken out and cut to a size of 3 cm3 cm, followed by chopping with a meat copper to prepare crumb. The crumb was dried in an oven capable of transferring air volume upward and downward. Hot air of 180 C. was flowed from the lower part to the upper part for 15 minutes, and from the upper part to the lower part for 15 minutes, thus uniformly drying, and drying was conducted such that the moisture content after drying became 2% or less. After drying, grinding was conducted with a grinder, followed by sieving to select those with particle diameters of 150 to 850 um, thus preparing a base resin. Thereafter, based on 100 g of the base resin, 4 g of water, 3.5 g of methanol, 0.15 g of ethyleneglycol diglycidyl ether, 0.10 g of Aerosil 200 were added and mixed, and then, reacted at a surface cross-linking temperature of 195 C. for 1 hour, and ground to obtain surface treated superabsorbent polymer with a particle diameter of 150 to 850 um using a sieve.

Comparative Example 2

[0079] Superabsorbent polymer was prepared by the same method as Comparative Example 1, except that 4500 ppm of polyethyleneglycol diacrylate (PEGDA) was used as the internal cross-linking agent.

Example 1

[0080] Superabsorbent polymer was prepared by the same method as Comparative Example 1, except that 1000 ppm of SBC was used as a blowing agent.

Example 2

[0081] Superabsorbent polymer was prepared by the same method as Comparative Example 2, except that 1000 ppm of SBC was used as a blowing agent.

Comparative Example 3

[0082] Superabsorbent polymer was prepared by the same method as Comparative Example 1, except that the surface cross-linking temperature was 140 C.

Comparative Example 4

[0083] Superabsorbent polymer was prepared by the same method as Example 1, except that the surface cross-linking temperature was 140 C.

Comparative Example 5

[0084] Superabsorbent polymer was prepared by the same method as Comparative Example 2, except that the surface cross-linking temperature was 140 C.

Comparative Example 6

[0085] Superabsorbent polymer was prepared by the same method as Example 2, except that the surface cross-linking temperature was 140 C.

Experimental Example

[0086] For the superabsorbent polymer prepared in Examples and Comparative Examples, properties were evaluated as follows.

[0087] (1) Gel Bed Permeability (GBP)

[0088] For the superabsorbent polymer prepared in Examples and Comparative Examples, gel bed permeability (GBP) was measured. The measurement method of GBP is specified in U.S. Pat. No. 7,179,851.

[0089] First, the apparatus suitable for conducting a gel bed permeability test is shown in FIG. 1, and specifically shown in FIG. 2 and FIG. 3. The test apparatus (28) comprises a sample container (generally indicated as 30) and a piston (generally indicated as 35). The piston (35) comprises a cylindrical LEXANR 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 as 42 and 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.

[0090] 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).

[0091] 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 superabsorbent polymer sample (indicated as 68 in FIG. 2) is supported on the screen (66) within the cylinder (34) during testing.

[0092] 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 approximately 10 cm. Drainage holes (not shown) are formed in the sidewall of the cylinder (34) at a height of approximately 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 Ms 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.

[0093] 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.

[0094] 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.

[0095] 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 from the bottom of the weight (48) to the top of the cylinder (34) is measured using a caliper accurate to 0.01 mm. 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 (35) and weight (48) should be used for measurement when the sample (68) is later swollen following saturation.

[0096] The sample to be tested is prepared from superabsorbent 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 urn. The particles can be prescreened by hand or automatically. Approximately 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.

[0097] 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 thickness clipper or measuring instrument used previously (provided that the zero point is unchanged from the initial height measurement). The height measurement obtained from measuring the empty sample container (30), piston (35), and weight (48) is subtracted from the height measurement obtained after saturating the sample (68). The resulting value is the thickness, or height H of the swollen sample.

[0098] 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 the 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 twenty 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).

[0099] Permeability (darcy) is obtained by the following equation:

K=[QHMu]/[ARhoP][Equation 1]

[0100] where K is permeability (cm.sup.2), Q is flow rate (g/sec), H is height of sample (cm), Mu is liquid viscosity (poise) (approximately one centipoises for the test solution used with this Test), A is cross-sectional area for liquid flow (cm.sup.2), Rho is liquid density (g/cm.sup.3) (for the test solution used with this Test) and P is hydrostatic pressure (dynes/cm.sup.2) (normally approximately 3,923 dynes/cm.sup.2). The hydrostatic pressure is calculated from the following Equation 2.

P=Rhogh[Equation 2]

[0101] where Rho is liquid density (g/cm.sup.3), g is gravitational acceleration, nominally 981 cm/sec.sup.2, and h is fluid height, e.g., 7.8 cm for the Gel Bed Permeability Test described herein.

[0102] (2) Absorbency Under Load (AUL)

[0103] For the superabsorbent polymer prepared in Examples and Comparative Examples, Absorbency under Load (AUL) of 0.9 psi was measured as follows.

[0104] First, on the bottom of plastic cylinder of 25 mm inner diameter, stainless 400 mesh wire netting was installed. Superabsorbent polymer W.sub.0 (g, 0.16 g) was uniformly sprayed onto the wire netting under room temperature and 50% humidity, and a piston capable of uniformly giving 5.1 kPa (0.9 psi) load thereon has an outer diameter slightly less than 25 mm and does not have a gap with the inner wall of cylinder, and the upward and downward movement is not hindered. Wherein, the weight W.sub.3(g) of the apparatus was measured.

[0105] In the inner side of a petri dish of 150 mm diameter, a glass filter of 90 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 90 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.

[0106] Using each weight obtained above, AUL (g/g) was calculated according to the following Equation 3.

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

[0107] In the Equation 3,

[0108] W.sub.0(g) is the weight (g) of absorbent polymer,

[0109] W.sub.3(g) is the sum of the weight of absorbent polymer and the weight of the apparatus capable of giving a load to the polymer,

[0110] W.sub.4(g) is the sum of the weight of moisture-absorbed absorbent polymer and the weight of the apparatus capable of giving a load to the absorbent polymer, after supplying moisture to the absorbent polymer under a load (0.9 psi) for 1 hour.

[0111] (3) Suction Power (SP)

[0112] SP was measured with the measuring apparatus as shown in FIG. 4. Specifically, on the right side of the measuring apparatus, saline water (0.9% NaCl) was filled to the 0 mL gradation on a glass tube with 20 mm inner diameter. On the left side of the measuring apparatus, a 100 micrometer glass filter was installed on the bottom of a cylindrical funnel with 50 mm inner diameter, and 1.0 g of superabsorbent polymer was uniformly sprayed onto the glass filter under 23 C., 50% humidity conditions. Simultaneously with spraying superabsorbent polymer, the cock of a burette of the measuring apparatus was opened, and the amount of saline water (g) absorbed by 1 g of superabsorbent polymer for 5 minutes was measured.

[0113] (4) Centrifuge Retention Capacity (CRC)

[0114] According to EDANA method WSP 241.2, centrifuge retention capacity of the polymer of Examples and Comparative Examples were measured. That is, the polymer W(g) (about 0.2 g) respectively obtained through Examples and Comparative Examples was uniformly put in an envelope made of non-woven fabrics and sealed, which is then submerged into a saline solution (0.9 wt %) at room temperature. After about 30 minutes elapsed, the envelope was drained for 3 minutes under 250G condition using a centrifuge, and the weight of the envelope W.sub.2(g) was measured. Further, after the same operation without using polymer, the weight at that time W.sub.1(g) was measured. Using each obtained weight, CRC (g/g) was calculated according to the following Equation.

CRC(g/g)={(W.sub.2W.sub.1)/W}1[Equation 4]

[0115] (5) Vortex

[0116] 50.01.0 mL of a 0.9% NaCl solution was added to a 100 mL beaker. A cylindrical stirring bar (306 mm) was added, and the saline solution was stirred at 600 rpm on a stirring plate. 2.0000.010 g of superabsorbent polymer was added to the beaker as soon as possible, and at the beginning of the addition, a stopwatch was started. The stopwatch was stopped when the surface of the mixture became still state, which means that the surface does not have turbulent flow, at which time the mixture may still rotate but the whole surface of particles may rotate as one unit. The time indicated in the stopwatch was recorded as a vortex time.

[0117] The results are shown in the following Table 1.

TABLE-US-00001 TABLE 1 B/R.sup.1) P/D.sup.2) CRC Vortex CRC SP AUL GBP Vortex (g/g) (s) (g/g) (mL/g) (g/g) (darcy) (s) Comparative 40.3 87 36.4 15.2 18.0 27 95 Example 1 Example 1 38.4 70 31.5 17.4 19.6 36 84 Comparative 34.2 75 29.9 15.6 21.2 60 63 Example 2 Example 2 35.3 63 29.6 16.5 20.1 52 75 Comparative 40.3 87 36.7 15.0 16.9 15 91 Example 3 Comparative 38.4 70 32.1 17.0 18.0 19 81 Example 4 Comparative 34.2 75 29.9 15.5 20.1 43 82 Example 5 Comparative 35.3 63 29.7 16.3 19.4 39 71 Example 6 .sup.1)B/R: base resin before surface cross-linking .sup.2)P/D: final superabsorbent polymer prepared in Example or Comparative Example

[0118] As shown in the Table 1, it was confirmed that the superabsorbent polymer prepared according to the present invention has remarkably increased suction power (SP) compared to Comparative Examples, and there is no significant difference or rather there is an improvement in the properties other than suction power.

[0119] Therefore, the preparation method of superabsorbent polymer according to the present invention is characterized by increasing suction power without degradation of other properties.