Method for preparing superabsorbent polymer and superabsorbent polymer prepared thereby

Abstract

The present invention relates to a method for preparing a superabsorbent polymer that progresses dry mixing of fine powders and a specific powder type of polymer binder when reassembling fine powders generated during the preparation process of a superabsorbent polymer, and thus obviates the necessity for a moisture drying process after reassembling fine powders, thereby reducing thermal losses, improving productivity, and obtaining a superabsorbent polymer having excellent basic absorption properties, and a superabsorbent polymer prepared by the method.

Claims

1. A method for preparing a superabsorbent polymer, comprising: conducting crosslinking polymerization of water soluble ethylenically unsaturated monomers comprising monomers having acid groups of which at least a part are neutralized, in the presence of an internal crosslinking agent, to form a hydrogel polymer comprising a first crosslinked polymer; gel grinding the hydrogel polymer to form a gel ground hydrogel polymer; drying the gel ground hydrogel polymer, and grinding and sieving the dried hydrogel polymer to form base polymer powders; and heat treating the base polymer powders to progress surface crosslinking, in the presence of a surface crosslinking agent to produce the superabsorbent polymer, wherein the method further comprises: recovering fine powders, after sieving the dried hydrogel polymer; reassembling the fine powders in the presence of a powder type of polymer binder to provide reassembled fine powders; and introducing the reassembled fine powders in the step of gel grinding the hydrogel polymer to mix the reassembled fine powders with the hydrogel polymer before drying, wherein the powder type of polymer binder consists of polyethylene oxide powders having a weight average molecular weight of 100,000 to 600,000 g/mol.

2. The method for preparing a superabsorbent polymer according to claim 1, wherein the fine powders have an average particle diameter less than 150 μm and wherein the reassembling the fine powders comprises dry mixing the fine powders and the powder type of polymer binder under a solvent-free condition, and then reassembling the fine powders through heat treatment.

3. The method for preparing a superabsorbent polymer according to claim 2, wherein the heat treatment is conducted at 105 to 180° C. for 10 to 20 minutes.

4. The method for preparing a superabsorbent polymer according to claim 1, wherein the powder type of polymer binder is used in an amount of 1 to 100 parts by weight, based on 100 parts by weight of the fine powders.

5. The method for preparing a superabsorbent polymer according to claim 1, wherein the reassembled fine powders are mixed in an amount of 10 to 30 parts by weight, based on 100 parts by weight of the hydrogel polymer before drying.

6. The method for preparing a superabsorbent polymer according to claim 1, wherein the gel grinding of the hydrogel polymer is conducted two or more times.

7. The method for preparing a superabsorbent polymer according to claim 1, wherein the water soluble ethylenically unsaturated monomers include one or more 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-methacryloylethane sulfonic acid, 2-(meth)acryloyl propane sulfonic acid, and 2-(meth)acrylamide-2-methylpropane sulfonic 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, and polyethylene glycol (meth)acrylate; or amino group-containing unsaturated monomers selected from (N,N)-dimethylaminoethyl (meth)acrylate, (N,N)-dimethylaminopropyl (meth)acrylamide, and quaternized products thereof.

8. The method for preparing a superabsorbent polymer according to claim 1, wherein the first crosslinked polymer includes a polymer formed by the crosslinking polymerization of the water soluble ethylenically unsaturated monomers, in the presence of a polyol poly(meth)acrylate-based first internal crosslinking agent selected from the group consisting of 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 pentaacrylate, glycerin tri(meth)acrylate, and pentaerythritol tetraacrylate; and an allyl(meth)acrylate-based second internal crosslinking agent.

9. The method for preparing a superabsorbent polymer according to claim 1, wherein the internal crosslinking agent includes a polyol poly(meth)acrylate-based first internal crosslinking agent and an allyl (meth)acrylate-based second internal crosslinking agent, and the first internal crosslinking agent is included in a content of 0.4 parts by weight to 1 part by weight, based on 100 parts by weight of a monomer composition comprising the internal crosslinking agent and water soluble ethylenically unsaturated monomers, and the second internal crosslinking agent is included in a content of 0.008 parts by weight to 0.5 parts by weight, based on 100 parts by weight of the monomer composition.

10. The method for preparing a superabsorbent polymer according to claim 1, wherein the surface crosslinking agent includes an alkylene carbonate-based compound or a polyhydric alcohol compound.

11. The method for preparing a superabsorbent polymer according to claim 1, wherein the powder type of polymer binder is used in an amount of 10 to 25 parts by weight, based on 100 parts by weight of the fine powders.

12. A superabsorbent polymer comprising: base polymer powders comprising a first crosslinked polymer of water soluble ethylenically unsaturated monomers having acid groups of which at least a part are neutralized; a surface crosslink layer formed on the base polymer powders, and comprising a second crosslinked polymer in which the first crosslinked polymer is additionally crosslinked by a surface crosslinking agent; reassembled fine powders; and a powder type of polymer binder consisting of polyethylene oxide powders having a weight average molecular weight of 100,000 to 600,000 g/mol; wherein a centrifuge retention capacity (CRC) of the superabsorbent polymer for a saline solution having 0.9 wt % sodium chloride in aqueous solution for 30 minutes is 30 g/g to 45 g/g.

13. The superabsorbent polymer according to claim 12, wherein the superabsorbent polymer has a particle diameter of 150 to 850 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows scanning electron microscope photographs of reassembled fine powders used in Examples 8 and 9 of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(2) Hereinafter, preferable examples are presented for better understanding of the present invention. However, these examples are presented only as the illustrations of the present invention, and the present invention is not limited thereby.

Examples 1 to 8

(3) As an apparatus for preparing a superabsorbent polymer, a continuous preparation apparatus performing a polymerization process, a hydrogel grinding process, a drying process, a grinding process, a sieving process, a surface crosslinking process, a cooling process, a sieving process, and a transportation process connecting each process, was used.

(4) (Step 1)

(5) Step of Forming Hydrogel Polymer

(6) 100 parts by weight of acrylic acid were mixed with 0.7 parts by weight (7000 ppm) of polyethylene glycol diacrylate (weight average molecular weight: ˜500 g/mol) and 0.015 parts by weight of allyl methacrylate as internal crosslinking agents, and with 0.01 parts by weight of IRGACURE 819 as a photoinitiator to prepare a monomer solution. Subsequently, while the monomer solution was continuously fed with a metering pump, 160 parts by weight of an aqueous solution of 24 wt % sodium hydroxide was line-mixed to prepare an aqueous solution of monomers. At this time, the temperature increase by the heat of neutralization was controlled to 40° C. After continuous line mixing of 6 parts by weight of an aqueous solution of 4 wt % sodium persulfate, the mixture was continuously fed to a continuous polymerization reactor having a planar polymerization belt equipped with embankments at both ends. Thereafter, UV was irradiated for 1 minute, and thermal polymerization was additionally progressed for 2 minutes to prepare a hydrogel polymer. The moisture content of such hydrogen polymer was confirmed to be 45 wt %.

(7) (Step 2)

(8) Step of Grinding Hydrogel Polymer

(9) The hydrogel polymer was primarily cut using a cutter such that the average size became about 300 mm or less, and then it was introduced into a chopper together with reassembled fine powders, and secondarily ground. Here, as the reassembled fine powders, the reassembled fine powders prepared in step 5 below were used, and the introduction rate was 20 parts by weight, based on 100 parts by weight of the hydrogel polymer.

(10) (Step 3)

(11) Step of Drying Gel Ground Hydrogel Polymer

(12) Subsequently, the hydrogel ground in step 2 was dried in a dryer capable of transferring wind up and down. Hot air was flowed from the lower side to the upper side for 15 minutes, and was flowed again from the upper side to the lower side for 15 minutes, so that the moisture content of dried powder became about 2% or less, thus uniformly drying the hydrogel.

(13) (Step 4)

(14) Step of Forming Base Polymer Powders

(15) The hydrogel polymer dried in step 3 was ground with a grinder and then sieved to obtain a base polymer with a size of 150 to 850 μm.

(16) Meanwhile, fine powders, i.e., polymer particles having a particle diameter of less than 150 μm, were recovered through the sieving, and then reassembled fine powders were prepared according to step 5 below, and used as the reassembled fine powders of step 2.

(17) (Step 5)

(18) Step of Providing Reassembled Fine Powders (Reassembly of Fine Powders with Moisture Content of 0%)

(19) Fine powders, i.e., polymer particles having a particle diameter of less than 150 μm, were recovered through the sieving, and then introduced into a mixer and dry-mixed under a solvent-free condition together with PEO powders (Mw=100,000 g/mol). The PEO powders were used while changing the content based on 100 parts by weight of the total fine powders. That is, the PEO powders were used in an amount of 1, 5, 10, 15, 25, 50, 75, and 100 parts by weight, respectively, which were divided as Examples 1 to 8. As the mixer, one equipped with a stirring means and a temperature control means was used.

(20) After the dry mixing was completed, heat treatment was progressed at 105 t for 10 minutes to prepare reassembled fine powders.

(21) (Step 6)

(22) Step of Forming Superabsorbent Polymer Particles

(23) Thereafter, 100 parts by weight of the base polymer prepared in step 4 were mixed with a crosslinking solution including 4 parts by weight of water and 1 part by weight of ethylene carbonate, and then subjected to a surface crosslinking reaction at 180° C. for 40 minutes. The obtained product then was cooled and sieved to obtain a superabsorbent polymer of which the surface was crosslinked, and having a particle diameter of 150 to 850 μm.

Example 9

(24) A superabsorbent polymer was prepared by the same method as Example 8, except that the heat treatment temperature was changed to 180° C., in the process of reassembling the fine powders of step 5.

Comparative Example 1

(25) A superabsorbent polymer was prepared by the same method as Example 8, except that after mixing the fine powders with water, reassembly was progressed, and a process of drying the moisture of the reassembled fine powders was progressed in a separate drier, in the process of reassembling fine powders of step 5.

(26) That is, based on 100 parts by weight of fine powders, 125 parts by weight of water were mixed with the fine powders in a mixer to progress reassembly of fine powders (using a kneader: 650 rpm, 1 minute). After mixing the fine powders with water, moisture drying was progressed (180° C., 1 hour).

Comparative Example 2

(27) A superabsorbent polymer was prepared by the same method as Example 8, except that based on 100 parts by weight of the fine powders, 10 g of the aqueous solution of polypropylene glycol (moisture content 10%) was used, in the process of reassembling the fine powders of step 5.

(28) However, due to the use of the polypropylene glycol aqueous solution, after reassembling the fine powders, a moisture drying process was progressed in a separate drier like Comparative Example 1 (moisture drying conditions: 180° C., 1 hour).

Comparative Example 3

(29) A superabsorbent polymer was prepared by the same method as Example 8, except that the heat treatment was progressed at 180° C. for 9 minutes, in the process of reassembling the fine powders of step 5.

(30) As the result of progressing heat treatment for less than 10 minutes, the reassembly efficiency of the fine powders was lowered to decrease productivity.

Experimental Example 1

(31) For the reassembled fine powders of Examples 8 and 9, average particle diameters were measured using a scanning electron microscope photograph, and are shown in FIG. 1.

(32) As shown in FIG. 1, it can be seen that the reassembled fine powders of Examples 8 and 9 have excellent binding and cohesion.

Experimental Example 2

(33) The reassembled fine powders of Examples 8 and 9 were ground and sieved by the method of the following Table 1, and then the reassembly efficiency and CRC were measured and the results are shown in Table 2.

(34) That is, the reassembled fine powders of the examples were ground using a hammer mill or a ball mill, and then, using a mesh sieve, they were divided into 3 or 7 stages, and the reassembly efficiencies and CRCs were measured (3 stages: sieved into stages of #30 top, #30˜50, #50 bottom according to the sieve particle size, 7 stages: sieved into #20 top, #20˜30, #30˜40, #40˜50, #50˜70, #70˜100, #100 bottom according to the sieve particle size).

(35) Hammer mill (grinding conditions: 650 rpm, grinding until a torque value of 4 is reached)

(36) Ball mill (grinding conditions: 300 rpm, 20 minutes, using 10 ceramic balls)

(37) In case the reassembled fine powders are sieved into 3 stages or 7 stages, the reassembly efficiency can be measured according to the following calculation formulae.
3 stage classification: reassembly efficiency=(the amount of #30 top+the amount of #30˜50)/total amount of fine powders*100  [Calculation Formula 2]
7 stage classification: reassembly efficiency=(the amount of #20˜30+the amount of #30˜40+the amount of #40˜50+the amount of #50˜70+the amount of #70˜100)/total amount of fine powders*100  [Calculation Formula 3]

(38) TABLE-US-00001 TABLE 1 Content of binder (parts Rates according to particle size (wt %) by #20 #100 Example weight) top #20~30 #30~40 #40~50 #50~70 #70~100 bottom 1 1 0.56 0.41 0.71 1.62 2.28 9.02 85.40 2 5 0.80 0.35 0.80 1.50 2.89 12.36 81.31 3 10 1.46 1.36 3.98 9.77 12.4 15.16 55.82 4 15 5 25 0.66 0.61 2.29 7.88 13.68 19.63 55.24 6 50 7 75 8 100

(39) TABLE-US-00002 TABLE 2 Content of binder (parts Reassembly Reassembly Reassembly by efficiency 1 efficiency 2 efficiency 3 CRC Example weight) (%) (%) (%) [g/g] 1 1 14.0 3.3 31.6 2 5 17.9 6.3 25.2 3 10 42.7 29.0 13.2 22.0 4 15 20.7 21.2 5 25 44.1 25.1 40.7 21.6 6 50 55.6 15.9 7 75 62.4 12.6 8 100 74.1 9.7 Reassembly efficiency 1: using ball mill grinding and 7 stage classification method Reassembly efficiency 2: using ball mill grinding and 3 stage classification method Reassembly efficiency 3: using hammer mill grinding and 3 stage classification method

Experimental Example 3

(40) For the reassembled fine powders of Comparative Examples 1 and 2, the reassembly efficiencies, CRCs, and water absorption speeds were measured by common methods. The results are shown in the following Table 3.

(41) TABLE-US-00003 TABLE 3 Content of PPG aqueous solution binder Moisture (parts Reassembly Water Comparative content by efficiency CRC absorption Example (%) weight) (%) [g/g] speed 1 56 68 35.1 1 2 10 10 No reassembly effect

(42) As shown in the Table 3, although Comparative Example 1 fulfilled the reassembly efficiency and CRC of certain levels, a separate drying process should be progressed due to a high moisture content. Further, Comparative Example 2 required a drying process despite a low moisture content, and particularly, it could not exhibit the reassembly effect, and thus was inefficient.

(43) Thus, it can be seen that in the case of Comparative Examples 1 and 2, a process is lengthened due to the separate drying process, and energy loss is generated.

Experimental Example 4

(44) The CRC of each superabsorbent polymer prepared in Example 1 and Comparative Example 2 was measured and evaluated as follows.

(45) Centrifuge Retention Capacity (CRC)

(46) Centrifuge retention capacity (CRC) according to absorption rate under no load was measured according to EDANA (European Disposables and Nonwovens Association) standard EDANA WSP 241.3. W.sub.0 (g, about 0.2 g) of the superabsorbent polymer were uniformly put in an envelope made of a non-woven fabric, and the envelope was sealed. The envelope was then soaked in a 0.9 wt % sodium chloride aqueous solution (saline solution) at room temperature. After 30 minutes, the envelope was drained at 250 G for 3 minutes using a centrifuge, and the mass W.sub.2 (g) of the envelope was measured. After the same operation without using superabsorbent polymer, the mass W.sub.1 (g) at that time was measured. Using the obtained weights, CRC (g/g) of superabsorbent polymer was calculated according to the following Calculation Formula 4, 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 4]

(47) The property values of Example 1 and Comparative Example 2 measured by the above method are summarized in the following Table 4.

(48) TABLE-US-00004 TABLE 4 CRC Unit g/g Example 31.6 1 Comparative 23.0 Example 2

(49) Referring to Table 4, it can be seen that Example 1 of the present invention exhibits properties that are equivalent to or much better than those of Comparative Example 2, and particularly, exhibits much better basic absorption performance defined as CRC.