Superabsorbent polymer and preparation method thereof

Abstract

Provided are a superabsorbent polymer exhibiting more improved absorption rate and liquid permeability as well as excellent basic absorption performance, and a preparation method thereof. The superabsorbent polymer includes a base polymer powder including a crosslinked polymer of water-soluble ethylene-based unsaturated monomers having acidic groups which are at least partially neutralized; and a surface crosslinked layer which is formed on the base polymer powder and in which the base polymer powder is additionally crosslinked via a surface crosslinking agent, wherein the superabsorbent polymer includes 10% by number or more of superabsorbent polymer particles each particle having an aspect ratio of less than 0.5, the aspect ratio defined as the shortest diameter/the longest diameter of the superabsorbent polymer particle, and has SFC in a predetermined range.

Claims

1. A superabsorbent polymer comprising: a base polymer powder including a first crosslinked polymer comprising a water-soluble ethylene-based unsaturated monomer having acidic groups which are at least partially neutralized; and a surface crosslinked layer which is formed on the base polymer powder and which includes a second crosslinked polymer comprising the first crosslinked polymer which is additionally crosslinked via a surface crosslinking agent, wherein the superabsorbent polymer includes 10% by number to 50% by number of superabsorbent polymer particles having an aspect ratio of less than 0.5, based on a total number of superabsorbent polymer particles, wherein the aspect ratio is defined as the shortest diameter/the longest diameter of the superabsorbent polymer particle, and the superabsorbent polymer has a saline flow conductivity (SFC) of 30.Math.10.sup.−7 cm.sup.3.Math.s/g or more, wherein the saline is a 0.685% by weight of an aqueous solution of sodium chloride.

2. The superabsorbent polymer of claim 1, wherein absorbency represented by the following Equation 1 is 46 g/g to 63 g/g:
Absorbency=CRC+AUP  [Equation 1] in Equation 1, CRC represents centrifuge retention capacity of the superabsorbent polymer for a physiological saline solution comprising 0.9 wt % aqueous solution of sodium chloride, for 30 minutes, and AUP represents absorbency under pressure of 0.7 psi of the superabsorbent polymer for the physiological saline solution for 1 hour.

3. The superabsorbent polymer of claim 2, wherein the CRC is 25 g/g to 35 g/g.

4. The superabsorbent polymer of claim 2, wherein the AUP is 21 g/g to 27 g/g.

5. The superabsorbent polymer of claim 1, wherein a 30-sec absorption rate for a physiological saline solution under a pressure of 0.3 psi is 1.5 mm/min or more.

6. The superabsorbent polymer of claim 1, wherein a surface tension is 60 mN/m to 75 mN/m.

7. The superabsorbent polymer of claim 1, wherein the surface crosslinking agent includes two or more kinds of alkylene carbonates having 2 to 5 carbon atoms, in which each of the two or more kinds of the alkylene carbonates have different carbon numbers.

8. The superabsorbent polymer of claim 1, wherein the water-soluble ethylene-based unsaturated monomer includes one or more of an anionic monomer selected from acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, or 2-(meth)acrylamide-2-methyl propane sulfonic acid, and salts thereof; a nonionic hydrophilic monomer selected from (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxy polyethylene glycol (meth)acrylate, or polyethylene glycol (meth)acrylate; or an amino group-containing unsaturated monomer selected from (N,N)-dimethylaminoethyl(meth)acrylate or (N,N)-dimethylaminopropyl(meth)acrylate, and a quaternary compound thereof.

9. The superabsorbent polymer of claim 1, wherein a 30-sec absorption rate for a physiological saline solution under a pressure of 0.3 psi is 1.7 mm/min to 3.0 mm/min.

10. A method of preparing a superabsorbent polymer of claim 1, comprising: carrying out a crosslinking polymerization of a water-soluble ethylene-based unsaturated monomer having acidic groups which are at least partially neutralized, in the presence of a foaming agent and an internal crosslinking agent to form a water-containing gel polymer including a first crosslinked polymer; gel-pulverizing, drying, pulverizing, and size-sorting the water-containing gel polymer to form a base polymer including 10% by number to 50% by number of base polymer powder having an aspect ratio of less than 0.5, the aspect ratio defined as the shortest diameter/the longest diameter of the base polymer powder; and carrying out a surface crosslinking of the base polymer by heat treatment in the presence of a surface crosslinking liquid containing a surface crosslinking agent and a liquid medium and having a surface tension of 30 mN/m to 50 mN/m at a temperature of 20° C. to 25° C.

11. The method of claim 10, wherein the internal crosslinking agent comprises: a first internal crosslinking agent of poly(meth)acrylate of polyol; a second internal crosslinking agent of allyl(meth)acrylate; and a total content of the first and second internal crosslinking agents is 0.01 parts by weight to 2 parts by weight, based on 100 parts by weight of a monomer composition including the internal crosslinking agents and the water-soluble ethylene-based unsaturated monomer.

12. The method of claim 10, wherein the foaming agent includes one or more of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, or magnesium carbonate.

13. The method of claim 10, wherein the surface crosslinking agent included in the surface crosslinking liquid comprises two or more kinds of alkylene carbonates having 2 to 5 carbon atoms, in which each of the two or more kinds of the alkylene carbonates have different carbon numbers.

14. The method of claim 13, wherein the surface crosslinking liquid further includes a surfactant.

15. The method of claim 13, wherein the surface crosslinking liquid further includes a polycarboxylic acid-based copolymer having repeating units represented by the following Chemical Formula 1-a and Chemical Formula 1-b: ##STR00002## in Chemical Formulae 1-a and 1-b, 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, wherein when p is two or more, two or more of the repeating —RO— are the same as or different from each other.

16. The method of claim 13, wherein the liquid medium of the surface crosslinking liquid further includes an aliphatic alcohol having 6 or more carbon atoms.

17. The method of claim 10, wherein the surface-crosslinking of the base polymer is performed by heat treatment by raising an initial temperature of 20° C. to 130° C. to a maximum reaction temperature of 140° C. to 200° C. for 10 minutes to 30 minutes, and maintaining the maximum temperature for 5 minutes to 60 minutes.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an electron microscopic image showing definition of an aspect ratio of a superabsorbent polymer particle in a superabsorbent polymer of one embodiment and an exemplary method of measuring the same.

DETAILED DESCRIPTION OF THE EMBODIMENTS

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

Example 1

(3) As a manufacturing apparatus of a superabsorbent polymer, a continuous manufacturing apparatus consisting of a polymerization process, a water-containing gel pulverizing process, a drying process, a pulverizing process, a size-sorting process, a surface crosslinking process, a cooling process, a size-sorting process, and a transportation process connecting respective steps was used.

(4) (Step 1)

(5) 100 parts by weight of acrylic acid was mixed with 0.4 parts by weight of polyethylene glycol diacrylate (a weight average molecular weight of ˜500 g/mol) and allyl(meth)acrylate as internal crosslinking agents, 0.1 part by weight of sodium bicarbonate as a foaming agent, 0.01 parts by weight of sodium lauryl sulfate as a surfactant, and 0.01 part by weight of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator to prepare a monomer solution. Subsequently, while continuously feeding the monomer solution by a metering pump, 160 parts by weight of a 24 wt % aqueous solution of sodium hydroxide was continuously subjected to line mixing to prepare an aqueous monomer solution. Further, 6 parts by weight of a 4 wt % aqueous solution of sodium persulfate was continuously subjected to line mixing, and then continuously fed into a continuous polymerization reactor having a planar polymerization belt with a dam at each end. Thereafter, UV was irradiated to prepare a water-containing gel.

(6) (Step 2)

(7) The water-containing gel was cut to have an average size of about 300 mm or less, and then introduced into a pulverizer (equipped with a perforated plate including a plurality of pores having a diameter of 10 mm), followed by pulverization under respective conditions.

(8) (Step 3)

(9) Subsequently, the water-containing gel pulverized in the step 2 was dried in a drier capable of moving the air volume up and down. The water-containing gel was uniformly dried by allowing hot air of 180° C. to flow upward from downward for 15 minutes so that the water content of the dried powder was about 2% or less, and again allowing the hot air to flow downward from upward for 15 minutes.

(10) (Step 4)

(11) The polymer dried in step 3 was pulverized by a pulverizer and then size-sorted to obtain a base polymer having a size of 150 μm to 850 μm.

(12) (Step 5)

(13) Then, 1 g of ethylene carbonate and 1 g of propylene carbonate were mixed in 4 g of water to prepare a surface crosslinking liquid. The surface tension of the surface crosslinking liquid was measured as 45 mN/m.

(14) 6 g of the surface crosslinking liquid was sprayed onto 100 g of the base polymer powder prepared in step 4, and stirred at room temperature to allow uniform distribution of the surface crosslinking liquid on the base polymer powder. Subsequently, the base polymer powder mixed with the surface crosslinking liquid was introduced into a surface crosslinking reactor to perform surface crosslinking reaction.

(15) In the surface crosslinking reactor, the base polymer powder was confirmed to be gradually heated at an initial temperature of about 80° C., and 30 minutes later, allowed to reach the maximum reaction temperature of 190° C. After reaching the maximum reaction temperature, the reaction was further allowed for 15 minutes. Then, a sample of the finally prepared superabsorbent polymer was taken. After the surface crosslinking process, the superabsorbent polymer was size-sorted using an ASTM standard sieve to prepare a superabsorbent polymer having a particle size of 150 μm to 850 μm of Example 1.

(16) The base polymer and the superabsorbent polymer obtained by the above method were analyzed by an electron microscopic image (see FIG. 1, etc.), and an aspect ratio (a/b) of each base polymer powder and superabsorbent polymer particle was calculated. A ratio (% by number) of particles having an aspect ratio of less than 0.5 in the entire base polymer powder and superabsorbent polymer particles was measured. As a result, it was confirmed that the ratio of the particles having an aspect ratio of less than 0.5 in the corresponding base polymer powder and superabsorbent polymer particle was about 10% by number.

Example 2

(17) A superabsorbent polymer of Example 2 was prepared in the same manner as in Example 1, except that 0.15 parts by weight of sodium bicarbonate was used as the foaming agent. The base polymer/superabsorbent polymer obtained by the above method were analyzed by an electron microscopic image, and a ratio (% by number) of particles having an aspect ratio of less than 0.5 in the entire base polymer powder and superabsorbent polymer particles was measured. As a result, it was confirmed that the ratio of the particles having an aspect ratio of less than 0.5 in the corresponding base polymer powder and superabsorbent polymer particle was about 33% by number.

Example 3

(18) A superabsorbent polymer of Example 3 was prepared in the same manner as in Example 1, except that 0.2 parts by weight of sodium bicarbonate was used as the foaming agent. The base polymer/superabsorbent polymer obtained by the above method were analyzed by an electron microscopic image, and a ratio (% by number) of particles having an aspect ratio of less than 0.5 in the entire base polymer powder and superabsorbent polymer particles was measured. As a result, it was confirmed that the ratio of the particles having an aspect ratio of less than 0.5 in the corresponding base polymer powder and superabsorbent polymer particle was about 45% by number.

(19) The subsequent surface crosslinking process was performed in the same manner as in Example 1 to prepare the superabsorbent polymer having a particle size of 150 μm to 850 μm of Example 3.

Example 4

(20) A superabsorbent polymer of Example 4 was prepared in the same manner as in Example 3, except that 0.02 g of polyoxyethylenesorbitan monopalmitate as a lubricant was added to the surface crosslinking liquid in step 5.

Example 5

(21) A superabsorbent polymer of Example 5 was prepared in the same manner as in Example 3, except that 0.3 g of aliphatic alcohol (monostearyl alcohol) as a lubricant was added to the surface crosslinking liquid in step 5.

Example 6

(22) A superabsorbent polymer of Example 6 was prepared in the same manner as in Example 1, except that 0.1 g of polycarboxylic acid-based copolymer as a lubricant which was prepared in the same manner as in Preparation Example 1 of U.S. Pat. No. 1,684,649 was added to the surface crosslinking liquid in step 5.

Example 7

(23) A superabsorbent polymer of Example 7 was prepared in the same manner as in Example 3, except that a surface crosslinking liquid prepared by mixing 1 g of trimethylene carbonate and 1 g of propylene carbonate in 4 g of water was used as the surface crosslinking liquid in step 5.

Comparative Example 1

(24) A base polymer of Comparative Example 1 was prepared in the same manner as in Example 1, except that sodium bicarbonate was not used as the foaming agent in step 1. The base polymer obtained by this method was analyzed by an electron microscopic image, and a ratio (% by number) of particles having an aspect ratio of less than 0.5 in the entire base polymer powder was measured. As a result, it was confirmed that the ratio of the particles having an aspect ratio of less than 0.5 in the corresponding base polymer powder was about 5% by number.

Comparative Example 2

(25) A superabsorbent polymer of Comparative Example 2 was prepared in the same manner as in Comparative Example 1, except that 100 parts by weight of the prepared base polymer powder and 5 g of a surface crosslinking liquid prepared by mixing 1 g of ethylene carbonate in 4 g of water were used. The surface tension of the surface crosslinking liquid was measured as 51 mN/m.

Comparative Example 3

(26) A superabsorbent polymer of Comparative Example 3 was prepared in the same manner as in Example 1, except that 100 parts by weight of the prepared base polymer powder and 5 g of a surface crosslinking liquid prepared by mixing 1 g of ethylene carbonate in 4 g of water were used.

Comparative Example 4

(27) A superabsorbent polymer of Comparative Example 4 was prepared in the same manner as in Example 3, except that 100 parts by weight of the prepared base polymer powder and 5 g of a surface crosslinking liquid prepared by mixing 1 g of ethylene carbonate in 4 g of water were used.

Comparative Example 5

(28) Preparation and drying of the water-containing gel polymer were performed according to a method described in Preparation Example of Korean Patent Publication No. 2015-0132035. Thereafter, a base polymer was prepared and subjected to surface crosslinking according to a method described in Example 1 of Korean Patent Publication No. 2015-0132035, thereby preparing a superabsorbent polymer of Comparative Example 5.

Experimental Example

(29) Physical properties of the respective superabsorbent polymers prepared in Examples and Comparative Examples, and all physical properties during the preparation processes were measured and evaluated by the following methods.

(30) (1) Measurement of Aspect Ratio and Particle Distribution of Base Polymer Powder and Superabsorbent Polymer Particle

(31) The shortest diameter (a) and longest diameter (b) of each powder/particle were calculated by electron microscopy as in FIG. 1, and an aspect ratio of each powder/particle was calculated therefrom. A ratio (% by number) of powder/particles having an aspect ratio of less than 0.5 in the entire powder/particles obtained in each of Examples/Comparative Examples was calculated.

(32) (2) Centrifuge Retention Capacity (CRC)

(33) The centrifuge retention capacity (CRC) by water absorption capacity under no load was measured in accordance with EDANA (European Disposables and Nonwovens Association) standard test method WSP 241.3. After uniformly introducing W.sub.0(g) (about 0.2 g) of the superabsorbent polymer in a nonwoven fabric-made bag and sealing the same, it was immersed in a physiological saline solution composed of 0.9 wt % aqueous solution of sodium chloride at room temperature. After 30 minutes, the bag was dehydrated by using a centrifuge at 250 G for 3 minutes, and then the weight W.sub.2(g) of the bag was measured. Further, after carrying out the same operation without using the superabsorbent polymer, the weight W.sub.1(g) of the bag was measured. CRC (g/g) was calculated by using the obtained weight values according to the following Calculation Formula 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 Formula 1]

(34) (3) Absorbency Under Pressure (AUP)

(35) The absorbency under pressure (AUP) of the superabsorbent polymers of Examples and Comparative Examples was measured in accordance with EDANA (European Disposables and Nonwovens Association) standard test method WSP 242.3.

(36) First, a 400 mesh stainless steel net was installed in the cylindrical bottom of a plastic having an internal diameter of 60 mm. W.sub.0(g, 0.90 g) of each of the superabsorbent polymers of Examples 1 to 6 and Comparative Examples 1 to 4 was uniformly scattered on the steel net under conditions of temperature of 23±2° C. and relative humidity of 45%, and a piston which can uniformly provide a load of 4.83 kPa (0.7 psi) was put thereon. The external diameter of the piston was slightly smaller than 60 mm, there was no gap between the cylindrical internal wall and the piston, and the jig-jog of the cylinder was not interrupted. At this time, the weight W.sub.3(g) of the apparatus was measured.

(37) After putting a glass filter having a diameter of 125 mm and a thickness of 5 mm in a Petri dish having a diameter of 150 mm, a physiological saline solution composed of 0.90 wt % sodium chloride was poured in the dish until the surface level became equal to the upper surface of the glass filter. The measuring apparatus was put on the glass filter and the solution was absorbed under a load for about 1 hour. After 1 hour, the weight W.sub.4(g) was measured after lifting up the measuring apparatus.

(38) Using the respective weights thus obtained, AUP(g/g) was calculated according to the following Calculation Formula 2, thereby confirming the absorbency under pressure.
AUP(g/g)=[W.sub.4(g)−W.sub.3(g)]/W.sub.0(g)  [Calculation Formula 2]

(39) in Calculation Formula 2, W.sub.0(g) is an initial weight (g) of the superabsorbent polymer,

(40) W.sub.3(g) is the total sum of a weight of the superabsorbent polymer and a weight of the apparatus capable of providing a load for the superabsorbent polymer, and

(41) W.sub.4(g) is the total sum of a weight of the superabsorbent polymer and a weight of the apparatus capable of providing a load to the superabsorbent polymer, after immersing the superabsorbent polymer in a physiological saline solution under a load (0.7 psi) for 1 hour.

(42) (4) Saline Flow Conductivity (SFC)

(43) The saline flow conductivity (SFC) was measured and calculated according to the methods disclosed in columns 54 to 59 of U.S. Pat. No. 5,562,646.

(44) (5) 30-Sec Absorption Rate

(45) 30-sec absorption rate and porosity were measured by swelling about 0.16 g of the superabsorbent polymer in a physiological saline solution fed through a mesh in the bottom of a cylindrical cylinder under a pressure of 0.3 psi. A change of a height of an upper plate of a rheometer according to volume expansion of the superabsorbent polymer was measured in real time, and from a value obtained by dividing the height of the upper plate at 30 sec by the absorption time (30 sec), the 30-sec absorption rate was measured and calculated. Further, porosity was calculated by the following method: when swelling of the superabsorbent polymer was completed, the total volume inside the cylinder (final absorption height*the bottom area of the cylindrical cylinder) was calculated, and from this value, the amount of the physiological saline solution absorbed by the superabsorbent polymer which was measured by a water content meter was subtracted.

(46) (6) Surface Tension of Surface Crosslinking Liquid and Superabsorbent Polymer

(47) All procedures were carried out in a constant temperature and humidity room (temperature of 23±0.5° C., relative humidity of 45±0.5%).

(48) First, the surface crosslinking liquid was pipetted and transferred to another clean cup, and then the surface tension of the surface crosslinking liquid was measured by using a tensiometer (surface tensionmeter Kruss K11/K100).

(49) Next, the surface tension of the superabsorbent polymer was measured as follows. 150 g of physiological saline composed of 0.9 wt % sodium chloride was put in a 250 mL beaker, and directly stirred with a magnetic bar. 1.0 g of the superabsorbent polymer was added to the solution under stirring, and stirred for 3 minutes. Stirring was stopped and the swollen superabsorbent polymer was allowed to settle to the bottom for 15 minutes or longer.

(50) Then, the supernatant (the solution just below the surface) was pipetted and transferred to another clean cup and measured using a tensiometer (surface tensionmeter Kruss K11/K100).

(51) The values of physical properties of Examples 1 to 7 and Comparative Examples 1 to 5 which were measured by the above methods are summarized and shown in Table 1 below.

(52) TABLE-US-00001 TABLE 1 Surface tension Particle (surface distribution 30-sec Surface tension crosslinking (aspect ratio of absorption (superabsorbent liquid) less than 0.5) CRC AUP Absorbency SFC rate polymer) Unit mN/m % by number g/g g/g g/g .Math.10.sup.−7 cm.sup.3 .Math. s/g mm/min mN/m Example 1 45 10 28.3 25.0 53.3 53 1.9 70 Example 2 45 33 28.0 24.6 52.6 50 2.2 68 Example 3 45 45 26.4 24.8 51.2 49 2.4 66 Example 4 33 45 27.9 25.4 53.3 42 2.4 60 Example 5 33 45 27.0 25.3 52.3 50 2.4 60 Example 6 42 10 27.6 24.5 52.1 50 1.9 69 Example 7 45 45 27.7 24.1 51.8 43 1.9 66 Comparative 45 5 27.7 25.7 53.4 46 1.4 70 Example 1 Comparative 51 5 28.7 24.7 53.4 44 1.4 71 Example 2 Comparative 51 10 27.8 23.2 51 20 1.7 69 Example 3 Comparative 51 45 27.7 23.0 50.7 25 2.4 68 Example 4 Comparative 45 2 33.1 24.2 57.3 5 0 70 Example 5

(53) Referring to Table 1, Examples 1 to 7 were confirmed to satisfy predetermined particle distributions and to exhibit excellent liquid permeability defined as 35 (.Math.10.sup.−7 cm.sup.3.Math.s/g) or more. It was also confirmed that Examples 1 to 7 showed excellent basic absorption performances defined by absorbency, etc., and also showed optimized particle distributions while having excellent liquid permeability, thereby showing excellent absorption rate defined by 30-sec absorption rate.

(54) In contrast, one or more of liquid permeability and absorption rate were poor in Comparative Examples 1 to 5, as compared with Examples.