Superabsorbent polymer and preparation method thereof

Abstract

A superabsorbent polymer having optimized gel strength and exhibiting an improved absorption rate without increasing a specific surface area by chemical foaming or a physical method, and a preparation method thereof, are provided.

Claims

1. A superabsorbent polymer comprising: a base polymer powder comprising a first crosslinked polymer of a water-soluble ethylene-based unsaturated monomer having acidic groups which are at least partially neutralized; and a surface-crosslinked layer which is located on the base polymer powder, and comprises a second crosslinked polymer obtained by further crosslinking the first crosslinked polymer via alkylene carbonate, and a polycarboxylic acid-based copolymer, wherein the first crosslinked polymer has a neutralization degree of 70 mol % or less, the second crosslinked polymer comprises acidic groups which are neutralized with potassium salts and has a neutralization degree of more than 70 mol % and less than 100 mol %, and wherein the superabsorbent polymer has a gel strength of 8500 Pa to 10,500 Pa, as measured using a rheometer after being swollen with a 0.9 wt % physiological saline solution for 1 hour.

2. The superabsorbent polymer of claim 1, wherein the acidic groups of the first crosslinked polymer are neutralized with a basic material containing sodium, and the first crosslinked polymer comprises a sodium containing acidic group.

3. The superabsorbent polymer of claim 1, wherein the first crosslinked polymer has a neutralization degree of 50 mol % to 70 mol %.

4. The superabsorbent polymer of claim 1, wherein the alkylene carbonate is selected from the group consisting of ethylene carbonate, 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxepan-2-one.

5. The superabsorbent polymer of claim 1, wherein the polycarboxylic acid-based copolymer comprises a repeating unit represented by the following Formula 1a and a repeating unit represented by the following Formula 1b, and has a weight average molecular weight of 500 g/mol to 1,000,000 g/mol: ##STR00002## wherein, in Formulae 1a and 1b, R.sup.1, R.sup.2, and R.sup.3 are each independently hydrogen or an alkyl group having 1 to 6 carbon atoms, RO is an oxyalkylene group having 2 to 4 carbon atoms, M.sup.1 is hydrogen or a monovalent metal or non-metal ion, X is —COO—, an alkyloxy group having 1 to 5 carbon atoms, or an alkyldioxy group having 1 to 5 carbon atoms, m is an integer of 1 to 100, n is an integer of 1 to 1000, and p is an integer of 1 to 150 (when p is 2 or more, two or more repeating —RO— are the same as or different from each other).

6. The superabsorbent polymer of claim 1, wherein the surface-crosslinked layer has a thickness corresponding to 7 volume % to 20 volume % with respect to a total volume of the superabsorbent polymer.

7. The superabsorbent polymer of claim 1, wherein a bulk density is 0.4 g/ml to 0.8 g/ml, a content of fine particles of 100 mesh or less is 1% by weight or less, and an absorption rate is 20 s to 50 s.

8. The superabsorbent polymer of claim 1, wherein a content of particles having a particle size of 150 μm to 850 μm is 99% by weight or more, wherein the superabsorbent polymer has an absorbency under pressure (AUP) of 22 g/g to 25 g/g when measured under 0.7 psi using the 0.9 wt % physiological saline solution, and has a centrifugation retention capacity (CRC) of 29 g/g to 33 g/g when measured using the 0.9 wt % physiological saline solution.

9. A method of preparing the superabsorbent polymer of claim 1, comprising: performing crosslinking polymerization of the water-soluble ethylene-based unsaturated monomer having acidic groups which are at least partially neutralized, in the presence of an internal crosslinking agent, to form a water-containing gel polymer comprising the first crosslinked polymer having a neutralization degree of 70 mol % or less; drying, pulverizing, and size-sorting the water-containing gel polymer to form the base polymer powder; and reacting the base polymer powder with a surface crosslinking solution which is prepared by dissolving a surface crosslinking agent comprising potassium hydroxide, the alkylene carbonate, and the polycarboxylic acid-based copolymer in water to form the surface-crosslinked layer comprising the second crosslinked polymer obtained by further crosslinking the first crosslinked polymer on the surface of the base polymer powder via the alkylene carbonate, wherein the water is used in an amount of 2.5 parts by weight to 10 parts by weight, with respect to 100 parts by weight of the base polymer powder, and the potassium hydroxide, the alkylene carbonate, and the polycarboxylic acid-based copolymer are used in amounts satisfying the conditions of the following Equation 1:
0.05<a/((b×d)−c)≤1  [Equation 1] wherein, in Equation 1, a is a number of moles of the alkylene carbonate to be used, b is a number of moles of acidic group present in the first crosslinked polymer, c is a number of moles of the potassium hydroxide to be used, and d is a volume ratio of the surface-crosslinked layer with respect to a total volume of the superabsorbent polymer, provided that a and c satisfy the condition of 1<a/c<20.

10. The method of claim 9, wherein a/((b×d)−c) of Equation 1 is 0.1 or more and 0.5 or less, and a/c is 2 or more and 18 or less.

11. The method of claim 9, wherein the volume ratio of the surface-crosslinked layer is 0.07 to 0.2 with respect to the total volume of the superabsorbent polymer.

12. The method of claim 9, wherein the polycarboxylic acid-based copolymer is comprised in an amount of 0.01 parts by weight to 0.5 parts by weight with respect to 100 parts by weight of the base polymer powder.

13. The method of claim 9, wherein the alkylene carbonate is comprised in an amount of 0.2 parts by weight to 5 parts by weight with respect to 100 parts by weight of the base polymer powder.

14. The method of claim 9, wherein the potassium hydroxide is comprised in an amount of 0.01 parts by weight to 2 parts by weight with respect to 100 parts by weight of the base polymer powder.

15. The method of claim 9, wherein a foaming agent is further introduced during formation of the water-containing gel polymer.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) Hereinafter, preferred examples are provided for better understanding of the present invention. However, the following examples are for illustrative purposes only, and the present invention is not intended to be limited by these examples.

(2) <Preparation of Base Polymer Powder>

Preparation Example 1

(3) 400 g of acrylic acid was added to a 2 L glass beaker, to which 719.5 g of a 24.3% aqueous sodium hydroxide solution was slowly poured and mixed to prepare a first solution. At this time, neutralization heat was generated, and the mixed solution was stirred at room temperature and cooled to about 41° C.

(4) Subsequently, a second solution prepared by adding 0.200 g of polyethylene glycol diacrylate (PEGDA 600), 0.02 g of 1,6-hexanediol diacrylate, and 0.045 g of dioctyl sulfosuccinate sodium salt (AOT) to 50 g of acrylic acid; 35 g of a 0.31% aqueous ascorbic acid solution (a third solution); and a solution prepared by diluting 1 g of hydrogen peroxide and 0.69 g of potassium persulfate in 40 g of distilled water (a fourth solution) were sequentially added to the first solution, and mixed with each other by stirring.

(5) When the mixed solution stirred in the beaker was gelled and stirring was stopped, the gel was immediately poured in a vat-type tray (15 cm in width×15 cm in length). The poured gel was foamed at about 20 seconds, polymerized, and slowly shrunk. The sufficiently shrunk polymer was torn into 5 to 10 pieces and transferred into a kneader. The lid was closed and kneading was carried out for 5 minutes. In the kneading process, the lid was opened at the passage of 4 minutes from the beginning, 50 g of a 3.5% aqueous potassium persulfate solution was sprayed onto the polymer inside the kneader, and then the lid was closed.

(6) Thereafter, the polymer was passed through a hole having a diameter of 13 mm using a meat chopper to prepare crumbs.

(7) Subsequently, the crumbs were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes, so that the dried product had a water content of about 2% or less.

(8) Thereafter, the dried crumbs were pulverized using a pulverizer and size-sorted to obtain a base polymer powder having a particle diameter of 150 μm to 850 μm.

Preparation Example 2

(9) 400 g of acrylic acid was added to a 2 L glass beaker, to which 616.7 g of a 24.3% aqueous sodium hydroxide solution was slowly poured and mixed to prepare a first solution. At this time, neutralization heat was generated, and the mixed solution was stirred at room temperature and cooled to about 41° C.

(10) Subsequently, a solution prepared by adding 0.200 g of polyethylene glycol diacrylate (PEGDA 600), 0.02 g of 1,6-hexanediol diacrylate, and 0.045 g of dioctyl sulfosuccinate sodium salt (AOT) to 50 g of acrylic acid (a second solution); 35 g of a 0.31% aqueous ascorbic acid solution (a third solution); and a solution prepared by diluting 1 g of hydrogen peroxide and 0.69 g of potassium persulfate in 40 g of distilled water (a fourth solution) were sequentially added to the first solution, and mixed with each other by stirring.

(11) When the solution stirred in the beaker was gelled and stirring was stopped, the gel was immediately poured in a vat-type tray (15 cm in width×15 cm in length). The poured gel was foamed at about 20 seconds, polymerized, and slowly shrunk. The sufficiently shrunk polymer was torn into 5 to 10 pieces and transferred into a kneader. The lid was closed and kneading was carried out for 5 minutes. In the kneading process, the lid was opened at the passage of 4 minutes from the beginning, 50 g of a 3.5% aqueous potassium persulfate solution was sprayed onto the polymer inside the kneader, and then the lid was closed.

(12) Thereafter, the polymer was passed through a hole having a diameter of 13 mm using a meat chopper to prepare crumbs.

(13) Subsequently, the crumbs were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes, so that the dried product had a water content of about 2% or less.

(14) Thereafter, the dried crumbs were pulverized using a pulverizer and size-sorted to obtain a base polymer powder having a particle diameter of 150 μm to 850 μm.

Preparation Example 3

(15) 400 g of acrylic acid was added to a 2 L glass beaker, to which 781.2 g of a 24.3% aqueous sodium hydroxide solution was slowly poured and mixed to prepare a first solution. At this time, neutralization heat was generated, and the mixed solution was stirred at room temperature and cooled to about 41° C.

(16) Subsequently, a second solution prepared by adding 0.200 g of polyethylene glycol diacrylate (PEGDA 600), 0.02 g of 1,6-hexanediol diacrylate, and 0.045 g of dioctyl sulfosuccinate sodium salt (AOT) to 50 g of acrylic acid; 35 g of a 4% aqueous ascorbic acid solution (a third solution); and a solution prepared by diluting 1 g of hydrogen peroxide and 0.69 g of potassium persulfate in 40 g of distilled water (a fourth solution) were sequentially added to the first solution, and mixed with each other by stirring.

(17) When the solution stirred in the beaker was gelled and stirring was stopped, the gel was immediately poured in a vat-type tray (15 cm in width×15 cm in length). The poured gel was foamed at about 20 seconds, polymerized, and slowly shrunk. The sufficiently shrunk polymer was torn into 5 to 10 pieces and transferred into a kneader. The lid was closed and kneading was carried out for 5 minutes. In the kneading process, the lid was opened at the passage of 4 minutes from the beginning, 50 g of a 3.5% aqueous potassium persulfate solution was sprayed onto the polymer inside the kneader, and then the lid was closed.

(18) Thereafter, the polymer was passed through a hole having a diameter of 13 mm using a meat chopper to prepare crumbs.

(19) Subsequently, the crumbs were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes, so that the dried product had a water content of about 2% or less.

(20) Thereafter, the dried crumbs were pulverized using a pulverizer and size-sorted to obtain a base polymer powder having a particle diameter of 150 μm to 850 μm.

Preparation Example 4

(21) 450 g of acrylic acid was added to a 2 L glass beaker, to which 799.5 g of a 24.3% aqueous sodium hydroxide solution was slowly poured and mixed to prepare a first solution. At this time, neutralization heat was generated, and the mixed solution was stirred at room temperature and cooled to about 41° C.

(22) Subsequently, a solution prepared by adding 0.225 g of polyethylene glycol diacrylate (PEGDA 600) and 0.045 g of dioctyl sulfosuccinate sodium salt (AOT) to 50 g of acrylic acid (a second solution); 26 g of a 4% aqueous sodium bicarbonate (NaHCO.sub.3) solution (a third solution); 35 g of a 0.31% aqueous ascorbic acid solution (a fourth solution); and a solution prepared by diluting 1 g of hydrogen peroxide and 0.69 g of potassium persulfate in 40 g of distilled water (a fifth solution) were sequentially added to the first solution.

(23) When the solution stirred in the beaker was gelled and stirring was stopped, the gel was immediately poured in a vat-type tray (15 cm in width×15 cm in length). The poured gel was foamed at about 20 seconds, polymerized, and slowly shrunk. The sufficiently shrunk polymer was torn into 5 to 10 pieces and transferred into a kneader. The lid was closed and kneading was carried out for 5 minutes. In the kneading process, the lid was opened at the passage of 4 minutes from the beginning, 50 g of a 3.5% aqueous potassium persulfate solution was sprayed onto the polymer inside the kneader, and then the lid was closed.

(24) Thereafter, the polymer was passed through a hole having a diameter of 13 mm using a meat chopper to prepare crumbs.

(25) Subsequently, the crumbs were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes, so that the dried product had a water content of about 2% or less.

(26) Thereafter, the dried crumbs were pulverized using a pulverizer and size-sorted to obtain a base polymer powder having a particle diameter of 150 μm to 850 μm.

Preparation Example 5

(27) 400 g of acrylic acid was added to a 2 L glass beaker, to which 616.7 g of a 24.3% aqueous sodium hydroxide solution was slowly poured and mixed to prepare a first solution. At this time, neutralization heat was generated, and the mixed solution was stirred at room temperature and cooled to about 41° C.

(28) Subsequently, a solution prepared by adding 0.200 g of polyethylene glycol diacrylate (PEGDA 600), 0.02 g of 1,6-hexanediol diacrylate, and 0.045 g of dioctyl sulfosuccinate sodium salt (AOT) to 50 g of acrylic acid (a second solution); 26 g of a 4% aqueous sodium bicarbonate (NaHCO.sub.3) solution (a third solution); 35 g of a 0.31% aqueous ascorbic acid solution (a fourth solution); and a solution prepared by diluting 1 g of hydrogen peroxide and 0.69 g of potassium persulfate in 40 g of distilled water (a fifth solution) were sequentially added to the first solution, and mixed with each other by stirring.

(29) When the solution stirred in the beaker was gelled and stirring was stopped, the gel was immediately poured in a vat-type tray (15 cm in width×15 cm in length). The poured gel was foamed at about 20 seconds, polymerized, and slowly shrunk. The sufficiently shrunk polymer was torn into 5 to 10 pieces and transferred into a kneader. The lid was closed and kneading was carried out for 5 minutes. In the kneading process, the lid was opened at the passage of 4 minutes from the beginning, 50 g of a 3.5% aqueous potassium persulfate solution was sprayed onto the polymer inside the kneader, and then the lid was closed.

(30) Thereafter, the polymer was passed through a hole having a diameter of 13 mm using a meat chopper to prepare crumbs.

(31) Subsequently, the crumbs were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 180° C. from the bottom to the top for 15 minutes and from the top to the bottom for 15 minutes, so that the dried product had a water content of about 2% or less.

(32) Thereafter, the dried crumbs were pulverized using a pulverizer and size-sorted to obtain a base polymer powder having a particle diameter of 150 μm to 850 μm.

(33) <Preparation of Polycarboxylic Acid-Based Copolymer>

Preparation Example 6

(34) A polycarboxylic acid-based copolymer was prepared according to a method of Preparation Example 1 disclosed in Korean Patent Publication No. 2015-0143167 (Korean Patent Application No. 2014-0072343).

(35) In detail, 400 parts by weight of ion exchanged water was introduced into a 3 L 4-necked flask reactor equipped with a stirrer, a thermometer, a nitrogen inlet, and a circulating condenser, and the atmosphere inside the reactor was replaced by nitrogen under stirring, followed by heating to 75° C. under the nitrogen atmosphere.

(36) 2 parts by weight of ammonium persulfate was introduced into the reactor, and completely dissolved therein. Then, an aqueous monomer solution obtained by mixing 600 parts by weight of methoxypolyethylene glycol monomethacrylate (average addition mole number of ethylene oxide (EO): about 50 moles), 99.6 parts by weight of methacrylic acid, and 190 parts by weight of water, a mixed solution of 5 parts by weight of 3-mercaptopropionic acid and 60 parts by weight of water, and 150 parts by weight of a 3 wt % aqueous ammonium persulfate solution were continuously introduced for 4 hours at a constant speed. After completing the introduction, 5 parts by weight of a 3 wt % aqueous ammonium persulfate solution was further introduced at once.

(37) Thereafter, the internal temperature of the reactor was raised to 85° C., and maintained at 85° C. for 1 hour to complete the polymerization reaction.

(38) A weight average molecular weight of the polycarboxylic acid-based copolymer thus prepared was 40,000 g/mol, as measured by GPC (gel permeation chromatography).

(39) <Preparation of Superabsorbent Polymer>

Example 1

(40) The base polymer powder prepared in Preparation Example 1 was first introduced into a high speed mixer, and a surface crosslinking agent prepared by mixing 4 parts by weight of water, parts by weight of ethanol, 1 part by weight of ethylene carbonate (EC), 0.05 parts by weight of the polycarboxylic acid-based copolymer prepared in Preparation Example 6, 0.2 parts by weight of KOH, and 0.03 parts by weight of a discoloration inhibitor (Blancolen™ HP) with respect to 100 parts by weight of the base polymer powder was introduced into the high speed mixer, followed by stirring at 1000 rpm for 30 seconds. After stirring, a surface crosslinking reaction was performed while stirring the mixture in a planetary mixer at 190° C. for 60 minutes, thereby obtaining a superabsorbent polymer.

Examples 2 to 8 and Comparative Examples 1 to 11

(41) The superabsorbent polymers were obtained in the same manner as in Example 1, except that respective components as described in the following Table 1 were used.

(42) TABLE-US-00001 TABLE 1 Base polymer Composition of surface crosslinking Neutralization Use of solution (parts by weight, based on degree of first chemical 100 parts by weight of base polymer) crosslinked foaming Polycarboxylic polymer* agent** acid-based Type (mol %) (◯/X) Water EC copolymer KOH a/c a/((b × d) − c) Example 1 Preparation 70 X 4 1 0.05 0.2 3.186 0.337 Example 1 Example 2 Preparation 70 X 5 1.25 0.05 0.25 3.186 0.380 Example 1 Example 3 Preparation 60 X 5 1.25 0.05 0.25 3.186 0.270 Example 2 Example 4 Preparation 70 ◯ 4 1 0.05 0.2 3.186 0.337 Example 4 Example 5 Preparation 60 ◯ 5 1.25 0.05 0.25 3.186 0.270 Example 5 Example 6 Preparation 60 X 8 1.25 0.05 0.25 3.186 0.186 Example 2 Example 7 Preparation 70 X 4 1.5 0.05 0.1 9.557 0.480 Example 1 Example 8 Preparation 70 X 4 1.25 0.05 0.05 15.928 0.445 Example 1 Comparative Preparation 70 X 4 1 0.05 — — 0.304 Example 1 Example 1 Comparative Preparation 70 X 2 1 0.05 0.2 3.186 0.735 Example 2 Example 1 Comparative Preparation 76 X 4 1 0.05 0.2 3.186 0.440 Example 3 Example 3 Comparative Preparation 70 ◯ 4 1 0.05 — — 0.304 Example 4 Example 4 Comparative Preparation 70 X 4 3.5 0.05 0.2 11.150 1.178 Example 5 Example 1 Comparative Preparation 70 X 4 1 0.05 0.02 31.856 0.307 Example 6 Example 1 Comparative Preparation 70 X 4 1 — 0.2 3.185 0.735 Example 7 Example 1 Comparative Preparation 60 X 8 0.25 0.05 0.05 3.185 0.033 Example 8*** Example 2 Comparative Preparation 70 X 8 0.25 0.05 0.1 1.593 0.046 Example 9 Example 1 Comparative Preparation 70 X 4 0.3 0.05 0.2 0.956 0.138 Example 10 Example 1 Comparative Preparation 70 X 10.5 1 0.05 0.2 3.186 0.157 Example 11 Example 1 *In Table 1, the neutralization degree of the first crosslinked polymer was determined according to (4) a method of measuring the neutralization degree of the first crosslinked polymer described in Experimental Example 1 below. **In Table 1, the chemical foaming agent was sodium bicarbonate. ***In Table 1, with regard to Comparative Example 8, a base polymer having a low neutralization degree was used during surface crosslinking, and the input of water was increased, instead of reducing the input of KOH, to form a thick surface-crosslinked layer, such that the neutralization degree of the second crosslinked polymer in the prepared superabsorbent polymer was as low as less than 70 mol %.

Experimental Example 1: Analysis of Superabsorbent Polymer

(43) The superabsorbent polymers prepared in the examples and comparative examples were analyzed by the following methods.

(44) (1) Thickness (μm) of Surface-Crosslinked Layer

(45) The thickness was measured according to a method disclosed in “Polymer 145(2018)174˜183”. The measured value was an average thickness.

(46) (2) Volume Ratios of Surface-Crosslinked Layer and Non-Surface-Crosslinked Layer

(47) A volume ratio (d) of the surface-crosslinked layer and a volume ratio of the non-surface-crosslinked layer were calculated, based on the average particle size (425 μm) of the superabsorbent polymer, using the above measured thickness of the surface-crosslinked layer, respectively.

(48) Meanwhile, the average particle size of the superabsorbent polymer was a weight average particle size which was measured according to the European Disposables and Nonwovens Association (EDANA) standard test method, EDANA WSP 220.3.

(49) (3) Neutralization Degree of Superabsorbent Polymer (RND)

(50) The neutralization degree of the superabsorbent polymer was determined by measuring an extractables content by back titration using a pH titrator according to the European Disposables and Nonwovens Association (EDANA) standard test method, WSP 270.3-10, and then calculating a final neutralization degree according to the following method.

(51) Carboxylate nCOOH (moles),
nCOOH=(VNaOH,s−VNaOH,b).Math.cNaOH

(52) wherein VNaOH,s represents a volume (ml) of NaOH which is required to titer a filtered sample solution to pH 10.0, VNaOH,b represents a volume (ml) of NaOH which is required to titer a blank solution containing no superabsorbent polymer to pH 10.0, and cNaOH represents a concentration (mol/L) of NaOH used in titration.
ntot=(VHCl,s−VHCl,b).Math.cHCl

(53) wherein VHCl,s represents a volume (ml) of HCl which is required to titer the filtered sample solution from pH 10.0 to pH 4.0, VHCl,b represents a volume (ml) of HCl which is required to titer the blank solution containing no superabsorbent polymer from pH 10.0 to pH 4.0, and cHCl represents a concentration (mol/L) of HCl used in titration.

(54) The neutralization degree was determined from nCOONa=ntot−nCOOH, and finally determined from the equation of the final neutralization degree (mol %)=nCOONa/ntot×100.

(55) (4) Neutralization Degree of First Crosslinked Polymer (BRND) (mol %)

(56) The neutralization degree of the first crosslinked polymer was determined from the amounts of acrylic acid and sodium hydroxide used in the preparation of the polymer.

(57) (5) Neutralization Degree of Second Crosslinked Polymer (SND) (mol %)

(58) The neutralization degree of the second crosslinked polymer was determined from the neutralization degree (RND) of the superabsorbent polymer, the neutralization degree (BRND) of the first crosslinked polymer, and the volume ratio of the surface-crosslinked layer, each previously measured, according to the following Equation 2.
Neutralization degree of second crosslinked polymer (SND)=(RND−BRND×Volume ratio of non-surface-crosslinked layer)/Volume ratio of surface-crosslinked layer  [Equation 2]

(59) (6) Content of Carboxylic Acid in Superabsorbent Polymer (RCA) (mol %)

(60) The content of carboxylic acid in the superabsorbent polymer was determined from the above measured neutralization degree (RND) of the superabsorbent polymer according to the following Equation 3.
Content of carboxylic acid in superabsorbent polymer (RCA)=100−RND  [Equation 3]

(61) (7) Content of Carboxylic Acid Present in Surface-Crosslinked Layer (SCA) (mol %)

(62) The content of carboxylic acid in the surface-crosslinked layer was determined from the content of carboxylic acid in the first crosslinked polymer (BRCA) (%), the content of carboxylic acid in the superabsorbent polymer (RCA), and the volume ratio of the surface-crosslinked layer according to the following Equation 4.
Content of carboxylic acid present in surface-crosslinked layer (SCA)=(RCA−(100−BRND)×Volume ratio of non-surface-crosslinked layer)/Volume ratio of surface-crosslinked layer  [Equation 4]

(63) In Equation 4, the content of carboxylic acid in the first crosslinked polymer (BRCA) (%) is a value of 100-neutralization degree (BRND) of the first crosslinked polymer, RCA, the volume ratio of the non-surface-crosslinked layer which is a volume ratio of a portion excluding the surface-crosslinked layer-formed portion from the superabsorbent polymer (i.e., corresponding to the volume ratio of the 1-surface-crosslinked layer), and the volume ratio of the surface-crosslinked layer are the values obtained according to the above-described methods.

(64) (8) Number of Moles of Carboxylic Acid Present in First Crosslinked Polymer (b)

(65) From the neutralization degree of the first crosslinked polymer, the weight of the acrylic acid in 100 g of the first crosslinked polymer was calculated to determine the number of moles of carboxylic acid.

(66) b=Weight of acrylic acid in 100 g of first crosslinked polymer/molecular weight of acrylic acid

(67) TABLE-US-00002 TABLE 2 Neutrali- Neutrali- Neutrali- zation zation zation Content of Content of degree of degree of degree of carboxylic carboxylic Number of Volume Volume super first second acid in acid in moles of Thickness of ratio of ratio of absorbent crosslinked crosslinked super surface- carboxylic surface- non-surface- surface- polymer polymer polymer absorbent crosslinked acid in first crosslinked crosslinked crosslinked (RND) (BRND) (SND) polymer layer (SCA) crosslinked layer (μm) layer layer (d) (mol %) (mol %) (mol %) (RCA) (mol %) (mol %) polymer (b) Example 1 8 0.891 0.109 71.5 70 83.79 28.5 16.21 0.3430 Example 2 9 0.878 0.122 71.5 70 82.32 28.5 17.68 0.3430 Example 3 9 0.878 0.122 62.4 60 79.71 37.6 20.29 0.4692 Example 4 8 0.891 0.109 71.5 70 83.79 28.5 16.21 0.3430 Example 5 9 0.878 0.122 62.3 60 78.89 37.7 21.11 0.4692 Example 6 13 0.827 0.173 62.4 60 73.91 37.6 26.09 0.4692 Example 7 8 0.891 0.109 70.3 70 72.76 29.7 27.24 0.3430 Example 8 7 0.904 0.096 70.3 70 73.14 29.7 26.86 0.3430 Comparative 8 0.891 0.109 71.5 70 83.79 28.5 16.21 0.3430 Example 1 Comparative 4 0.945 0.055 71.4 70 95.26 28.6 4.74 0.3430 Example 2 Comparative 8 0.891 0.109 76.8 76 83.36 23.2 16.64 0.2703 Example 3 Comparative 8 0.891 0.109 71.4 70 82.87 28.6 17.13 0.3430 Example 4 Comparative 8 0.891 0.109 71.5 70 83.79 28.5 16.21 0.3430 Example 5 Comparative 8 0.891 0.109 70.3 70 72.76 29.7 27.24 0.3430 Example 6 Comparative 4 0.945 0.055 70.9 70 86.24 29.1 13.76 0.3430 Example 7 Comparative 14 0.815 0.185 60.4 60 62.16 39.6 37.84 0.4692 Example 8 Comparative 14 0.815 0.185 71.3 70 77.03 28.7 22.97 0.3430 Example 9 Comparative 6 0.918 0.082 71.2 70 84.57 28.8 15.43 0.3430 Example 10 Comparative 17 0.779 0.221 71.5 70 76.78 28.5 23.22 0.3430 Example 11

Experimental Example 2: Test of Physical Properties of Superabsorbent Polymer

(68) Physical properties of the superabsorbent polymers prepared in the examples and comparative examples were tested by the following methods, and the results are shown in Table 3.

(69) (1) Gel Strength

(70) A gel strength of the superabsorbent polymers prepared in the examples or comparative examples was measured by a method disclosed in Korean Patent No. 743274.

(71) In detail, the superabsorbent polymer sample (30˜50 Mesh) of the examples or comparative examples was passed through a sieve and 0.5 g thereof was weighed. The weighed sample was sufficiently swollen in 50 g of a physiological saline solution for 1 hour. Then, the solvent not absorbed therein was removed using an aspirator for 4 minutes. The solvent left on the surface of the same was evenly distributed and wiped once with a filter paper.

(72) 2.5 g of the swollen superabsorbent polymer was loaded between two parallel plates (a diameter of 25 mm, a lower plate thereof having a wall with a height of 2 mm for preventing the sample from leaking) of the rheometer, and the gap between the parallel plates was adjusted to 1 mm. At this time, the gap between the parallel plates was appropriately adjusted by pressing the parallel plates with a force of about 3 N so that the swollen superabsorbent polymer sample was contacted evenly at the face of the parallel plates. A linear viscoelastic regime section of strain where the storage modulus and the loss modulus were steady was found by using the rheometer while increasing the strain at an oscillation frequency of 10 rad/s. Generally, in the case of a swollen superabsorbent polymer sample, a strain of 0.1% is imparted in the linear viscoelastic regime section.

(73) The storage modulus and the loss modulus of the swollen superabsorbent polymer were measured by using the strain value of the linear viscoelastic regime section at a constant oscilation frequency of 10 rad/s for 60 seconds, respectively. The horizontal gel strength was obtained by averaging the obtained storage modulus values. For reference, the loss modulus was measured as a very small value as compared to the storage modulus.

(74) (2) Content of Fine Particles of 100 Mesh or Less After Ball-Mill Pulverization

(75) 100 g of the base polymer powder of the examples or Comparative examples was placed in a turbulizer mixer (a mixer customized by UTO engineering), which was then operated at 1000 rpm for 1 minute, and the superabsorbent polymer was completely recovered. The amount (wt %) of fine particles of 100 mesh or less (150 μm or less) in the recovered superabsorbent polymer was measured (based on the total weight of the superabsorbent polymer, wt %) using a sieving shaker (Retsch AS 200 model, amplitude: 1.5 mm/“g”, 10-min sieving).

(76) (3) Content of Fine Particles of 100 Mesh or Less After Surface Crosslinking

(77) The amount (wt %) of fine particles of 100 mesh or less (150 μm or less) in the superabsorbent polymer was measured (based on the total weight of the superabsorbent polymer, wt %) in the same manner as the method of measuring the content of fine particles of 100 mesh or less after ball-mill pulverization, except that the surface-crosslinked superabsorbent polymer of the examples or comparative examples was used.

(78) (4) Powder Flowability

(79) The superabsorbent polymer prepared in the examples or comparative examples was mixed well to evenly mix the particles, and then 100.5 g of the sample was taken and poured into a 250 ml beaker. A density measuring cup was placed in the center under the funnel, then the hole of the funnel was closed, and the weighed sample was lightly poured in the funnel. A stopwatch was started at the moment the closed hole of the funnel was opened, and the time taken for the sample to completely reach the bottom of the funnel was measured. All procedures were performed in a constant temperature and humidity room (temperature of 23±2° C., relative humidity of 45±10%).

(80) (5) Bulk Density (B/D)

(81) About 100 g of the superabsorbent polymer prepared in the examples or comparative examples was placed in a funnel-type bulk density tester, and allowed to flow into a 100-ml container, and then the weight of the superabsorbent polymer received in the container was measured. The bulk density was calculated from (the weight of the superabsorbent polymer/the volume of the container, 100 ml) (unit: g/ml).

(82) (6) Absorption Rate (Vortex Time)

(83) The absorption rate of the superabsorbent polymers of the examples and comparative examples was measured in a second unit according to a method described in International Patent Publication No. 1987-003208.

(84) In detail, the absorption rate (vortex time) was determined by adding 2 g of the superabsorbent polymer to 50 mL of a physiological saline solution at 23° C. to 24° C., stirring the solution at 600 rpm with a magnetic bar (diameter of 8 mm, length of 31.8 mm), and measuring the time required for the vortex to disappear in the second unit.

(85) (7) Centrifuge Retention Capacity (CRC)

(86) Centrifuge retention capacity by absorbency under no load was measured for the respective polymers according to EDANA WSP 241.3.

(87) In detail, each of the superabsorbent polymers prepared in the examples and comparative examples was size-sorted through a sieve of #30-50. The initial weight W.sub.0 (g) (about 0.2 g) of the size-sorted superabsorbent polymer was uniformly put in a nonwoven fabric-made bag, followed by sealing. Then, the bag was immersed in a physiological saline solution (0.9 wt %) 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. By using the respective weights thus obtained, CRC (g/g) was calculated according to the following equation.
CRC (g/g)={[W.sub.2 (g)−W.sub.1 (g)]/W.sub.0 (g)}−1  [Equation 5]

(88) (8) Absorbency Under Pressure (AUP)

(89) The absorbency under pressure (AUP) of the polymers of the examples and comparative examples was measured according to the EDANA method WSP 242.3.

(90) In detail, a 400 mesh stainless steel net was installed in the bottom of a plastic cylinder having an internal diameter of 60 mm. The superabsorbent polymer (W (g) of about 0.90 g) was uniformly scattered on the steel net at room temperature and humidity of 50%, and a piston which may uniformly provide a load of 4.83 kPa (0.7 psi) was put thereon, in which an 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.a (g) of the apparatus was measured.

(91) 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 composed of 0.90 wt % sodium chloride was poured until the surface level of the physiological saline solution 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. The measurement apparatus was mounted on the filter paper, thereby having the liquid absorbed under the load for 1 hour. 1 hour later, the weight W.sub.b (g) was measured after lifting the measurement apparatus.

(92) Then, absorbency under pressure (g/g) was calculated from W.sub.a and W.sub.b according to the following equation.
AUP (g/g)={W.sub.b−W.sub.a}/W  [Equation 6]

(93) TABLE-US-00003 TABLE 3 Content of Content of fine particles fine particles of 100 mesh of 100 mesh or less after or less after Powder Gel ball-mill surface flow Bulk strength pulverizing crosslinking ability density Absorption CRC AUP (Pa) (% by weight) (% by weight) (g/s) (g/ml) rate (sec) (g/g) (g/g) Example 1 9550 5.7 0.6 10.3 0.61 45 30.5 24.8 Example 2 9530 5.6 0.7 10.3 0.61 37 30.2 24.7 Example 3 9630 5.6 0.5 10.2 0.61 50 30.2 24.8 Example 4 9060 5.9 1 10.2 0.53 31 30.5 24.0 Example 5 9120 6.0 0.9 10.1 0.53 36 30.1 24.1 Example 6 9680 5.4 0.4 10.4 0.61 49 30.0 24.9 Example 7 9210 5.4 0.8 10.3 0.61 51 30.2 24.8 Example 8 9160 5.5 0.8 10.2 0.6 51 30.2 24.7 Comparative 8210 7.8 0.7 10.2 0.61 61 30.5 24.7 Example 1 Comparative 8320 6.9 0.6 10.2 0.61 62 30.4 24.6 Example 2 Comparative 8250 7.4 1.1 10.3 0.61 55 30.5 24.9 Example 3 Comparative 8100 8.1 1.7 10.4 0.53 45 30.3 24.1 Example 4 Comparative 8110 4.7 0.4 10.5 0.61 60 27.1 21.2 Example 5 Comparative 8150 9.5 2.1 9.8 0.61 61 30.2 24.2 Example 6 Comparative 8340 6.4 0.5 10.4 0.61 48 30.2 22.2 Example 7 Comparative 7860 9.8 2.4 9.9 0.6 60 30.6 24 Example 8 Comparative 7880 9.9 2.3 9.9 0.61 57 30.7 24.1 Example 9 Comparative 7990 9.6 2.2 10.1 0.6 54 30.5 24.1 Example 10 Comparative 7830 6.5 0.3 9.6 0.61 58 31.2 22.6 Example 11

(94) As a result of the experiments, the superabsorbent polymers of Examples 1 to 8 showed bulk density and powder flowability equivalent to those of the comparative examples, while having high gel strength of 8500 Pa or more, specifically, 9000 Pa or more, and having a greatly reduced content of fine particles after surface crosslinking, and also showing more improved absorption rates while showing CRC and AUP equivalent to or higher than those of the comparative examples.

(95) Specifically, Examples 1 and 4 including potassium hydroxide in the surface crosslinking agent showed powder characteristics and absorption performances similar to those of Comparative Examples 1 and 4 prepared in the same manner except for potassium hydroxide, while showing great improvement in terms of the absorption rate, indicating improvement of the absorption rate of the superabsorbent polymer by use of potassium hydroxide in the surface crosslinking agent.

(96) In particular, the superabsorbent polymer of Comparative Example 4, which was prepared by using only the known chemical foaming agent without using potassium hydroxide in the surface crosslinking agent, showed an absorption rate equivalent to that of Example 1. However, after pulverizing and surface crosslinking, generation of fine particles was greatly increased, and thus the superabsorbent polymer was further deteriorated in terms of process stability. These results indicate that the absorption rate may be sufficiently improved by only using the potassium hydroxide in the surface crosslinking agent without using the chemical foaming agent, and at the same time, the problem of generating a large amount of fine particles due to the use of the chemical foaming agent may be solved.

(97) Meanwhile, Comparative Example 2 having the same composition of the surface crosslinking agent while not satisfying the condition of the water content in the surface crosslinking solution had an excessively thin surface-crosslinked layer, and as result, its absorption rate was greatly reduced, as compared with Example 1. These results indicate that optimization of the treatment conditions of the surface crosslinking solution is required during formation of the surface-crosslinked layer in order to improve the absorption rate of the superabsorbent polymer.

(98) Meanwhile, when Examples 2 and 3 are compared with each other, it can be seen that as the neutralization degree of the base polymer was increased, better effects in terms of absorption rate were obtained, but the content of fine particles was increased. Comparative Example 3, in which the neutralization degree of the base polymer was 76%, showed powder characteristics and absorption performance almost equivalent to those of Example 1 which was prepared under the same conditions except for the neutralization degree. However, the content of fine particles was greatly increased, and the effects of improving the absorption rate were greatly reduced.

(99) These results indicate that the neutralization degree of the base polymer influences the absorption rate and the content of fine particles of the superabsorbent polymer, and in order to achieve a balance between the increase of the absorption rate and the decrease of the content of fine particles which have a trade-off relationship therebetween, it is necessary to optimize the neutralization degree in the base polymer.

(100) Comparative Example 5 (a/((b×d)−c)=1.178) satisfying the composition conditions of ethylene carbonate, the polycarboxylic acid-based copolymer, and KOH while not satisfying the content conditions (0.05<a/((b×d)−c)=1) defined by Equation 1 during preparation of the surface crosslinking agent showed a great reduction in the content of fine particles of 100 mesh or less after surface crosslinking, but showed increased powder flowability and decreased gel strength, as compared with the examples, and as a result, the absorption rate of the superabsorbent polymer was greatly reduced, and CRC and AUP were also deteriorated, as compared with the examples. Comparative Example 9 (a/((b×d)−c)=0.045) showed low gel strength of 8500 Pa or less, a high content of fine particles of 100 mesh or less after pulverizing and surface crosslinking, and a great increase in the absorption rate.

(101) Comparative Example 6 (a/c=30.6) not satisfying the content conditions (1<a/c<20) of ethylene carbonate and KOH during preparation of the surface crosslinking agent showed a reduction in powder flowability and gel strength, a great increase in the content of fine particles of 100 mesh or less after pulverizing and surface crosslinking, and a great reduction in the absorption rate of the superabsorbent polymer, as compared with the examples.

(102) Further, Comparative Example 7 including no polycarboxylic acid-based copolymer during preparation of the surface crosslinking agent was similar to the examples in terms of the content of fine particles of 100 mesh or less after surface crosslinking, powder flowability, bulk density, and absorption rate of the superabsorbent polymer, but its absorbency under pressure was reduced.

(103) Further, Comparative Example 8, in which the second crosslinked polymer in the prepared superabsorbent polymer had a neutralization degree of less than 70 mol % by using the base polymer having a low neutralization degree during formation of the surface-crosslinked layer and by performing surface crosslinking under conditions of forming a thick surface-crosslinked layer by increasing the use of water while reducing the input of KOH, showed reduced gel strength, a great increase in the content of fine particles of 100 mesh or less after pulverizing and surface crosslinking, slightly reduced powder flowability, and a reduced absorption rate.

(104) Comparative Example 10, in which the molar ratio (a/c) of the ethylene carbonate (a) and potassium hydroxide (c) was less than 1 during formation of the surface-crosslinked layer, showed reduced gel strength, a great increase in the content of fine particles of 100 mesh or less after pulverizing and surface crosslinking, and a reduced absorption rate.

(105) Further, Comparative Example 11, in which excess water was used during preparation of the surface crosslinking agent, showed reduced gel strength and a reduced absorption rate.