Method for producing super absorbent polymer and super absorbent polymer

Abstract

The present invention relates to a super absorbent polymer having more improved absorbency under pressure and liquid permeability, and a method for producing the same. The super absorbent polymer comprises a base polymer powder including a cross-linked polymer of a monomer containing a water-soluble ethylenically unsaturated compound or its salt; and a surface cross-linked layer that is formed on the base polymer powder and is further cross-linked from the cross-linked polymer, wherein a glass hollow particle having a micron-scale particle size is included on the surface cross-linked layer.

Claims

1. A method for producing a super absorbent polymer comprising the steps of: carrying out a crosslinking polymerization of a monomer composition containing a water-soluble ethylenically unsaturated compound or its salt in the presence of an internal crosslinking agent to form a cross-linked hydrogel polymer; drying, pulverizing, and classifying the cross-linked hydrogel polymer to form a base polymer powder; and surface-crosslinking the base polymer powder in the presence of one or more glass hollow particles having a micron-scale particle size and a surface crosslinking agent to form a surface cross-linked layer.

2. The method for producing a super absorbent polymer of claim 1, wherein the glass hollow particle is used in an amount of 0.01 to 20 parts by weight, based on 100 parts by weight of the monomer.

3. The method for producing a super absorbent polymer of claim 1, wherein the glass hollow particle has a particle size of 10 to 100 to and a density of 0.1 to 0.7 g/cc.

4. The method for producing a super absorbent polymer of claim 1, wherein the monomer includes at least one selected from the group consisting of anionic monomers of acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid or 2-(meth)acrylamido-2-methylpropanesulfonic acid, and their salts; non-ionic, hydrophilic group-containing monomers of (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol(meth)acrylate or polyethylene glycol (meth)acrylate; and amino group-containing unsaturated monomers of (N,N)-dimethylaminoethyl(meth)acrylate (N,N)-dimethylaminopropyl(meth)acrylamide, and their quaternary product.

5. The method for producing a super absorbent polymer of claim 1, wherein the internal crosslinking agent includes at least one selected from the group consisting of N,N-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentacrylate, glycerin tri(meth)acrylate, and pentaerythritol tetraacrylate.

6. The method for producing a super absorbent polymer of claim 1, wherein the surface crosslinking step is carried out in the presence of at least one surface crosslinking agent selected from the group consisting of a polyhydric alcohol compound, an epoxy compound, a polyamine compound, a haloepoxy compound, a condensation product of the haloepoxy compound, an oxazoline compound, a mono-, di-, or poly-oxazolidinone compound, a cyclic urea compound, a polyvalent metal salt, and an alkylene carbonate compound.

7. A super absorbent polymer comprising: a base polymer powder including a cross-linked polymer of a monomer containing a water-soluble ethylenically unsaturated compound or its salt; and a surface cross-linked layer that is formed on the base polymer powder and is further cross-linked from the cross-linked polymer, wherein one or more glass hollow particles having a micron-scale particle size is included on the surface cross-linked layer.

8. The super absorbent polymer of claim 7, wherein the glass hollow particle has a particle size of 10 to 100 m and a density of 0.1 to 0.7 g/cc.

9. The super absorbent polymer of claim 7, wherein the centrifuge retention capacity (CRC) for a physiological saline solution (0.9 wt % sodium chloride aqueous solution) for 30 minutes is 35 to 45 g/g and the absorbency under pressure (AUP) at 0.7 psi for a physiological saline solution (0.9 wt % sodium chloride aqueous solution) for 1 hour is 15 to 0.8 g/g.

10. The super absorbent polymer of claim 7, wherein the super absorbent polymer has an extractable content of 25% by weight or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an electron micrograph of the super absorbent polymer produced in Example 3, which shows that a glass hollow particle (GB) is contained on the surface cross-linked layer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(2) Hereinafter, preferred examples are provided for better understanding of the invention. However, these Examples are given for illustrative purposes only and are not intended to limit the scope of the present invention thereto.

(3) In the Preparation Examples, Examples and Comparative Examples described below, the physical properties of the super absorbent polymers were measured and evaluated by the following methods.

(4) (1) Evaluation of Particle Sizes

(5) The particle sizes of the base polymer powders and the super absorbent polymers used in Examples and Comparative Examples were measured according to EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 220.3.

(6) (2) Centrifuge Retention Capacity (CRC)

(7) 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.

(8) That is, W.sub.0 (g, about 0.2 g) of the 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 polymer, and then the resultant weight W.sub.1 (g) was measured.

(9) Using the respective weights thus obtained. CRC (g/g) was calculated according to the following Calculation Equation 1, thereby confirming the centrifuge retention capacity.
CRC (g/g)={[W.sub.2 (g)W.sub.1 (g)W.sub.0 (g)]/W.sub.0 (g)}[Calculation Equation 1]

(10) in Calculation Equation 1,

(11) W.sub.0 (g) is an initial weight (g) of the super absorbent polymer.

(12) W.sub.1 (g) is a weight of the device, which is measured after immersing and absorbing the same into a physiological saline solution for 30 minutes and then performing dehydration by using a centrifuge without the super absorbent polymer at 250 G for 3 minutes, and

(13) W.sub.2 (g) is a weight of the device including the super absorbent polymer, which is measured after immersing and absorbing the super absorbent polymer in a physiological saline solution at room temperature for 30 minutes and then performing dehydration by using a centrifuge at 250 G for 3 minutes.

(14) (3) Absorbency Under Pressure (AUP)

(15) The absorbency under pressure (AUP) was measured for each super absorbent polymer of Examples and Comparative Examples according to EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 242.3.

(16) First, a 400 mesh metal made of stainless steel was installed at the bottom of a plastic cylinder having an inner diameter of 60 mm. W.sub.0 (g, about 0.90 g) of the polymer obtained in Examples 1 to 6 and Comparative Examples 1 to 3 were uniformly scattered on the screen under conditions of a temperature of 232 C., and a relative humidity of 45%. Then, a piston capable of uniformly providing a load of 4.83 kPa (0.7 psi) was put thereon, in which 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. At this time, the weight W.sub.3 (g) of the device was measured.

(17) A glass filter having a diameter of 125 mm and a thickness of 5 mm was placed in a Petri dish having the diameter of 150 mm, and then a physiological saline solution composed of 0.90% by weight of 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 for 1 hour under the load. After 1 hour, the weight W.sub.4 (g) was measured after lifting up the measuring device.

(18) The AUP (g/g) was calculated from the weights thus obtained, according to the following Calculation Equation 2, thereby confirming the absorbency under pressure:
AUP (g/g)=[W.sub.4 (g)W.sub.3 (g)]/W.sub.0 (g)[Calculation Equation 2]

(19) in Calculation Equation 2,

(20) W.sub.0 (g) is an initial weight (g) of the super absorbent polymer,

(21) 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

(22) 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 (0.7 psi) for 1 hour.

(23) (4) Content of Extractables

(24) Among the super absorbent polymers produced in Examples and Comparative Examples, 1.0 g of sample having a particle size of 150 to 850 m was put in a 250 mL Erlenmeyer flask, and 200 mL of a 0.9 wt % sodium chloride aqueous solution was added to a physiological saline solution and subjected to free swelling for 1 hour while stirring at 250 rpm. The aqueous solution was then filtered with a filter paper. The filtered solution was subjected to first titration up to pH 10 with 0.1 N caustic soda solution and then subjected to back titration to pH 2.7 with 0.1N hydrogen chloride solution. The content (wt %) of extractables in the super absorbent polymer was measured from the obtained proper amount according to EDANA recommended test method No. WSP 270.3.

(25) (5) Gel Bed Permeability (GBP)

(26) The free swell Gel Bed Permeability was measured according to the method disclosed in U.S. Pat. No. 7,179,851.

Example 1: Preparation of Super Absorbent Polymer

(27) 3M glass hollow particles iM30K (particle size: 15.3 m (D50), density: 0.6 g/cc) manufactured by 3M Company were used.

(28) 537.45 g of acrylic acid, 0.32 g of Laponite RD, 0.86 g of polyethylene glycol diacrylate (Mw=598) as a crosslinking agent, 653.17 g of 30% caustic soda (NaOH), 0.04 g of diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide as UV initiator, 1.07 g of sodium persulfate, and 206.41 g of water were mixed to prepare a monomer composition having a monomer concentration of 36.7 wt % (degree of neutralization of acrylic acid: 70 mol %).

(29) Subsequently, the monomer composition was stirred for 25 seconds under the condition of 300 rpm and subjected to foaming.

(30) Then, the monomer composition was fed to the continuously moving conveyor belt reactor through the feeder, and then irradiated with UV rays (irradiation dose: 2 mW/cm.sup.2) for 2 minutes through a UV radiation device to conduct UV polymerization for 1 minute and thermal polymerization for 2 minutes, thereby producing a hydrogel polymer.

(31) The hydrogel polymer was transferred to a cutting machine and then cut to a size of 0.2 cm. At this time, the water content of the cut hydrogel polymer was 50 wt %.

(32) Subsequently, the hydrogel polymer was dried in a hot air dryer at 185 C. for 40 minutes, and the dried hydrogel polymer was pulverized by using a pin mill. Then, it was classified into the polymer having the particle size (average particle size) of less than 150 m and the polymer having the particle size of 150 m to 850 m by using a sieve.

(33) After a base polymer powder was obtained through the classification, 0.154 g of 1,3-propanediol as a surface crosslinking agent, and 0.4 g of the glass hollow particle (0.04 parts by weight based on 100 parts by weight of the monomer) were added to 2.8 g of water and 3.5 g of methanol and mixed to prepare a surface crosslinking solution. Thereafter, the surface crosslinking solution was sprayed onto the base polymer powder and mixed with stirring at room temperature so that the surface crosslinking solution was uniformly distributed on the base polymer powder. Then, the base polymer powder mixed with the surface crosslinking solution was put into the surface crosslinking reactor and subjected to the surface crosslinking reaction.

(34) In this surface crosslinking reactor, the surface crosslinking was carried out on the base polymer powder at 185 C. for 40 minutes to produce a super absorbent polymer of Example 1. After the surface crosslinking step, the obtained super absorbent polymer was classified using a standard mesh based on ASTM standard to produce a super absorbent resin of Example 1 having a particle size of 150 m to 850 m.

Example 2: Production of Super Absorbent Polymer

(35) The super absorbent resin of Example 2 having a particle size of 150 m to 850 m was produced in the same manner as in Example 1 except that the surface crosslinking time was changed to 50 minutes.

Example 3: Production of Super Absorbent Polymer

(36) The super absorbent resin of Example 3 having a particle size of 150 m to 850 m was produced in the same manner as in Example 1 except that the surface crosslinking time was changed to 60 minutes.

Comparative Example 1: Production of Super Absorbent Polymer

(37) The super absorbent resin of Comparative Example having a particle size of 150 m to 850 m was produced in the same manner as in Example 1 except that the glass hollow particle was not used during formation of a surface crosslinking solution.

(38) The measurement results of the physical properties of Examples and Comparative Example are summarized and shown in Table 1 below.

(39) TABLE-US-00001 TABLE 1 Compar- Exam- Exam- Exam- ative ple 1 ple 2 ple 3 Example 1 Glass hollow particle 0.04 0.04 0.04 0 (part by weight) CRC 49.9 49.9 49.9 49.9 (base polymer powder; g/g) GBP (darcy) 2.7 2.8 3.5 1.8 CRC(g/g) 40.5 39.7 38.3 40.5 AUP(g/g) 18.9 21.2 24.1 17.2

(40) Referring to Table 1, it was confirmed that the super absorbent polymers of Examples exhibit a centrifuge retention capacity and liquid permeability equal to or higher than those of Comparative Example, and exhibit improved physical properties such as absorbency under pressure.