Super absorbent polymer and method for preparing same

Abstract

The present invention relates to a super absorbent polymer. The super absorbent polymer contains polymer particles formed of large pores in a certain amount or more, and thus can exhibit large surface area and excellent initial absorption capacity. Therefore, when the super absorbent polymer is used, it can provide a sanitary material such as a diaper or a sanitary napkin which can quickly absorb body fluids and impart a dry and soft touch feeling.

Claims

1. A super absorbent polymer comprising: a base polymer powder including a cross-linked polymer obtained by crosslinking polymerization of a water-soluble ethylenically unsaturated monomer having an acidic group of which at least a part is neutralized in the presence of an encapsulated foaming agent; and a surface cross-linked layer that is further cross-linked from the cross-linked polymer and is formed on the base polymer powder, wherein M.sub.1 is a super absorbent polymer particle having three or more pores on its surface, which have a diameter of 20 to 200 μm, wherein M.sub.0 is a super absorbent polymer particle having a particle diameter of 300 to 425 μm, and wherein a ratio (M.sub.1/M.sub.0*100) is 10% or more, and wherein a distilled water absorption capacity of the super absorbent polymer is at least 150 g/g.

2. The super absorbent polymer according to claim 1, wherein the super absorbent polymer has a centrifuge retention capacity (CRC) for a physiological saline solution of 30 to 45 g/g.

3. The super absorbent polymer according to claim 1, wherein the ratio (M.sub.1/M.sub.0*100) is 20 to 70%, and wherein the distilled water absorption capacity is 150 to 500 g/g.

4. The super absorbent polymer according to claim 1, wherein the encapsulated foaming agent has a structure including a core containing hydrocarbon and a shell surrounding the core and formed of a thermoplastic resin, and has a maximum expansion size in air of 20 to 190 μm.

5. The super absorbent polymer according to claim 1, wherein the encapsulated foaming agent has an average diameter of 5 to 50 μm.

6. The super absorbent polymer according to claim 1, wherein the encapsulated foaming agent has an expansion ratio in air of 3 to 15 times.

7. The super absorbent polymer according to claim 4, wherein the hydrocarbon is at least one selected from the group consisting of n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, n-hexane, iso-hexane, cyclohexane, n-heptane, iso-heptane, cycloheptane, n-octane, iso-octane, and cyclooctane.

8. The super absorbent polymer according to claim 4, wherein the thermoplastic resin is a polymer formed from at least one monomer selected from the group consisting of (meth)acrylate, (meth)acrylonitrile, aromatic vinyl, vinyl acetate, vinyl halide and vinylidene halide.

9. A super absorbent polymer comprising: a base polymer powder including a cross-linked polymer obtained by crosslinking polymerization of a water-soluble ethylenically unsaturated monomer having an acidic group of which at least a part is neutralized in the presence of an encapsulated foaming agent; and a surface cross-linked layer that is further cross-linked from the cross-linked polymer and is formed on the base polymer powder, wherein a distilled water absorption capacity is at least 150 g/g.

10. A method for preparing the super absorbent polymer of claim 1 comprising: performing a crosslinking polymerization of the water-soluble ethylenically unsaturated monomer having an acidic group of which at least a part is neutralized in the presence of the encapsulated foaming agent to form a hydrogel polymer; drying, pulverizing and classifying the hydrogel polymer to form a base polymer powder; and further cross-linking a surface of the base polymer powder in the presence of a surface crosslinking agent to form a surface cross-linked layer, wherein the encapsulated foaming agent has a structure including a core containing hydrocarbon and a shell surrounding the core and formed of a thermoplastic resin, an average diameter before expansion is 5 to 50 μm, and a maximum expansion size in air is 20 to 190 μm.

11. The method for preparing a super absorbent polymer according to claim 10, wherein the encapsulated foaming agent has an expansion ratio in air of 3 to 15 times.

12. The method for preparing a super absorbent polymer according to claim 10, wherein the hydrocarbon is at least one selected from the group consisting of n-propane, n-butane, iso-butane, cyclobutane, n-pentane, iso-pentane, cyclopentane, n-hexane, iso-hexane, cyclohexane, n-heptane, iso-heptane, cycloheptane, n-octane, iso-octane, and cyclooctane.

13. The method for preparing a super absorbent polymer according to claim 10, wherein the thermoplastic resin is a polymer formed from at least one monomer selected from the group consisting of (meth)acrylate, (meth)acrylonitrile, aromatic vinyl, vinyl acetate, vinyl halide and vinylidene halide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an SEM image of a super absorbent polymer prepared according to Example 1.

(2) FIG. 2 is an SEM image of a super absorbent polymer prepared according to Example 2.

(3) FIG. 3 is an SEM image of a super absorbent polymer prepared according to Comparative Example 1.

(4) FIG. 4 is an SEM image of a super absorbent polymer prepared according to Comparative Example 2.

(5) FIG. 5 is an SEM image of a super absorbent polymer prepared according to Comparative Example 4.

(6) FIG. 6 is an SEM image of a super absorbent polymer prepared according to Comparative Example 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) Hereinafter, the actions and effects of the present invention will be described in more detail with reference to specific Examples of the present invention. However, these Examples are for illustrative purposes only, and the scope of the invention is not intended to be limited thereby.

Preparation Example 1: Preparation of Encapsulated Foaming Agent

(8) An encapsulated foaming agent was prepared having a core-shell structure, the core being iso-butane and the shell being formed of a copolymer of acrylate and acrylonitrile. The iso-butane was included in an amount of 25 wt % based on the total weight of the encapsulated foaming agent.

(9) The average diameter of the encapsulated foaming agent was 13 μm. The diameter of each encapsulated foaming agent was measured as the average Feret diameter through an optical microscope. The average diameter of the encapsulated foaming agent was determined and defined as the average diameter of the encapsulated foaming agent.

(10) 0.2 g of an encapsulated foaming agent was coated onto a glass Petri dish, and then left on a hot plate preheated to 150° C. for 10 minutes. The encapsulated foaming agent was gradually expanded by heat, and this was observed with an optical microscope, and the maximum expansion ratio and the maximum expansion size of the encapsulated foaming agent in air were measured.

(11) After heat was applied to the encapsulated foaming agent, the diameter of the top 10 wt % was measured in the order of many expanded particles and defined as the maximum expansion size. The ratio (D.sub.M/D.sub.0) of the average diameter (D.sub.M) of the top 10 wt % of many expanded particles after applying heat to the average diameter (D.sub.o) measured before applying heat to the encapsulated foaming agent was determined and defined as the maximum expansion ratio. The average diameter and average diameter all were measured in the same manner as in the diameter and average diameter of the encapsulated foaming agent.

(12) The maximum expansion ratio of the encapsulated foaming agent in air was about 9 times, and the maximum expansion size was about 80 to 150 μm.

Preparation Example 2: Preparation of Encapsulated Foaming Agent

(13) An encapsulated foaming agent was prepared having a core-shell structure, the core being iso-butane and the shell being formed of a copolymer of acrylate and acrylonitrile. The iso-butane was included in an amount of 25 wt % based on the total weight of the encapsulated foaming agent.

(14) The average diameter of the encapsulated foaming agent was 15 μm, the maximum expansion ratio in air was about 9 times, and the maximum expansion size was about 90 to 180 μm.

Preparation Example 3: Preparation of Encapsulated Foaming Agent

(15) An encapsulated foaming agent was prepared having a core-shell structure, the core being iso-pentane and the shell being formed of a copolymer of acrylate and acrylonitrile. The iso-pentane was included in an amount of 20 wt % on the total weight of the encapsulated foaming agent.

(16) The average diameter of the encapsulated foaming agent was 40 μm, the maximum expansion ratio in air was about 12 times, and the maximum expansion size was about 400 to 540 μm.

Preparation Example 4: Preparation of Encapsulated Foaming Agent

(17) An encapsulated foaming agent was prepared having a core-shell structure, the core being iso-octane and iso-pentane, and the shell being formed of a copolymer of acrylate and acrylonitrile. The iso-octane was included in an amount of 10 wt % and the iso-pentane in an amount of 5 wt %, based on the total weight of the encapsulated foaming agent.

(18) The average diameter of the encapsulated foaming agent was 30 μm, the maximum expansion ratio in air was about 8 times, and the maximum expansion size was about 200 to 300 μm.

Example 1: Preparation of Super Absorbent Polymer

(19) 100 g of acrylic acid was injected into a glass reactor, to which 123.5 g of a 32 wt % caustic soda solution was slowly added dropwise and mixed. During dropwise addition of the caustic soda solution, the temperature of the mixed solution was increased by the neutralizing heat and waited until the mixed solution was cooled. When the temperature of the mixed solution was cooled to 45° C., 0.2 g of sodium persulfate, 0.008 g of Irgacure TPO (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide), 0.3 g of the encapsulated foaming agent prepared in Preparation Example 1, 0.18 g of triethylene glycol diacrylate, 0.1 g of Cocamide monoethanolamine and 47 g of water were added to the mixed solution to prepare a monomer mixture.

(20) The monomer mixture was fed at a feed rate of 500 to 2,000 mL/min onto a conveyor belt having a width of 10 cm and a length of 2 m and rotating at a speed of 50 cm/min. While the monomer mixture was fed, UV having an intensity of 10 mW/cm.sup.2 was irradiated, and polymerization reaction was performed for 60 seconds.

(21) The polymer obtained after the polymerization reaction was made into crumbs using a meat chopper. The crumbs were then uniformly dried in an oven capable of shifting airflow upward and downward, by flowing hot air at 185° C. from the bottom to the top for 20 minutes and again from the top to the bottom for 20 minutes. The dried product was pulverized using a pulverizer and then classified into a size of 150 to 850 μm to obtain a base polymer powder.

(22) Thereafter, to 100 g of the base polymer powder prepared above was added a solution obtained by mixing 5.0 g of ultrapure water, 6.0 g of methanol, 0.04 g of ethylene glycol diglycidyl ether and 0.02 g of silica (trade name: Aerosil 200). After mixing for 1 minute, the surface crosslinking reaction was carried out at 180° C. for 60 minutes.

(23) The resulting product was pulverized and classified to obtain a super absorbent polymer having a particle diameter of 150 to 850 μm.

Example 2: Preparation of Super Absorbent Polymer

(24) A super absorbent polymer was prepared in the same manner as in Example 1, except that the encapsulated foaming agent prepared in Preparation Example 2 was used instead of the encapsulated foaming agent prepared in Preparation Example 1.

Comparative Example 1: Preparation of Super Absorbent Polymer

(25) A super absorbent polymer was prepared in the same manner as in Preparation Example 1, except that the encapsulated foaming agent prepared in Preparation Example 1 was not added.

Comparative Example 2: Preparation of Super Absorbent Polymer

(26) A super absorbent polymer was prepared in the same manner as in Example 1, except that NaHCO.sub.3 was used instead of the encapsulated foaming agent prepared in Preparation Example 1.

Comparative Example 3: Preparation of Super Absorbent Polymer

(27) A super absorbent polymer was prepared in the same manner as in Example 1, except that a foaming agent in which NaHCO.sub.3 was encapsulated with polyethylene glycol was used instead of the encapsulated foaming agent prepared in Preparation Example 1.

Comparative Example 4: Preparation of Super Absorbent Polymer

(28) A super absorbent polymer was prepared in the same manner as in Example 1, except that the encapsulated foaming agent prepared in Preparation Example 3 was used instead of the encapsulated foaming agent prepared in Preparation Example 1.

Comparative Example 5: Preparation of Super Absorbent Polymer

(29) A super absorbent polymer was prepared in the same manner as in Example 1, except that the encapsulated foaming agent prepared in Preparation Example 4 was used instead of the encapsulated foaming agent prepared in Preparation Example 1.

Test Example: Evaluation of Super Absorbent Polymer

(30) The properties of the super absorbent polymers prepared in Examples and Comparative Examples were evaluated by the following methods, and the results are shown in Table 1 below.

(31) (1) Centrifuge Retention Capacity (CRC)

(32) The centrifuge retention capacity (CRC) for a physiological saline solution was measured in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. NWSP 241.0.R2.

(33) Specifically, W.sub.0 (g, about 0.2 g) of the polymer was uniformly put in a nonwoven fabric-made bag and sealed. Then, the bag was immersed in 0.9 wt % of sodium chloride aqueous solution (physiological saline 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. Meanwhile, the same procedure was carried out without using the polymer, and then the resultant weight W.sub.1(g) was measured.

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

(35) in Calculation Formula 1,

(36) W.sub.0(g) is an initial weight(g) of the polymer,

(37) W.sub.1(g) is a weight of a nonwoven fabric-made empty bag not containing the polymer, which is measured after immersing the empty bag not containing the polymer in a physiological saline solution at room temperature for 30 minutes and then dehydrating the same by using a centrifuge at 250 G for 3 minutes, and

(38) W.sub.2(g) is a weight of a nonwoven fabric-made bag including the polymer, which is measured after immersing and absorbing the empty bag containing the polymer in a physiological saline solution at room temperature for 30 minutes and then dehydrating the same by using a centrifuge at 250 G for 3 minutes.

(39) (2) Ratio of Pore Particles

(40) Among the super absorbent polymers, a sample having a particle size of 300 to 425 μm, passing through a US standard 40 mesh screen and not passing through a US standard 50 mesh screen, was prepared.

(41) The diameter of the pores formed on the surface of the sample was measured through a scanning electron microscope (SEM). The diameter of the pores was measured as the average Feret diameter.

(42) Among the sample, the polymers having three or more pores with a diameter of 20 to 200 μm on the surface were selected, and the ratio (M.sub.1/M.sub.0*100) of the polymer (M.sub.1) having three or more pores with a diameter of 20 to 200 μm on the surface to the super absorbent polymer (M.sub.0) having a particle diameter of 300 to 425 μm was calculated.

(43) SEM images of the super absorbent polymers prepared in Examples 1 and 2 are shown in FIGS. 1 and 2, respectively, and SEM images of the super absorbent polymers prepared in Comparative Examples 1, 2, 4, and 5 are shown in FIGS. 3 to 6, respectively. Referring to FIGS. 1 to 4, it is confirmed that a large number of particles having large pores exist in the super absorbent polymer, whereas particles having large pores do not exist in the super absorbent polymers prepared in Comparative Examples 1 and 2.

(44) In addition, looking at the SEM images of the super absorbent polymers prepared in Comparative Examples 4 and 5 using an encapsulated foaming agent that expands to a large size when expanded in air, it is confirmed that the size of the formed pores is larger than the size of the super absorbent polymer particles, and thus pores are not properly formed on the surface.

(45) (3) Distilled Water Absorption Capacity

(46) In order to measure the absorption capacity of the super absorbent polymer in distilled water, W.sub.0 (g, about 1.0 g) of the polymer was uniformly put in a nonwoven fabric-made bag (width: 16.5 cm, length: 27.5 cm) and sealed. Then, the bag was put into a 2 L plastic beaker, and 1 L of distilled water at 24° C. was added thereto. After swelling the polymer for 1 minute, the bag was lifted from the distilled water, the distilled water kept in the bag for 1 minute was allowed to drop by gravity for 1 minute, and then the weight W.sub.4(g) of the bag was measured. Meanwhile, the same procedure was carried out without using an empty bag not containing the polymer, and then the resultant weight W.sub.3(g) was measured.

(47) Using the respective weights thus obtained, the distilled water absorption capacity was calculated according to the following Calculation Formula 2:
Distilled water absorption capacity (g/g)=[W.sub.4(g)−W.sub.3(g)]/W.sub.0(g)  [Calculation Formula 2]

(48) in Calculation Formula 2,

(49) W.sub.0(g) is an initial weight(g) of the polymer,

(50) W.sub.3(g) is a weight of a nonwoven fabric-made empty bag not containing the polymer, which is measured after immersing the empty bag not containing the polymer in distilled water at 24° C. for 1 minute, and then lifting the bag for 1 minute and naturally dehydrating the same, and

(51) W.sub.4(g) is a weight of a nonwoven fabric-made bag containing the polymer, which is measured after immersing and absorbing the empty bag containing the polymer in distilled water at 24° C. for 1 minute, and then lifting the bag for 1 minute and naturally dehydrating the same.

(52) TABLE-US-00001 TABLE 1 Pore distilled water CRC particle absorption capacity [g/g] [%] [g/g] Example 1 37.9 32 231 Example 2 37.5 53 235 Comparative 37.3 0 74 Example 1 Comparative 38.7 0 107 Example 2 Comparative 39.4 0 109 Example 3 Comparative 38.2 0 111 Example 4 Comparative 37.1 7 137 Example 5

(53) Referring to Table 1, it is confirmed that in Examples 1 and 2, as a super absorbent polymer is prepared using a foaming agent capable of expanding to a proper size, a large amount of pore particles is formed, and very excellent distilled water absorption capacity is exhibited.

(54) It is also confirmed that in the case of not using an encapsulated foaming agent, or using a conventional carbonate foaming agent, or using an encapsulated foaming agent that expands too large, as in Comparative Examples 1, 2, 4 and 5, polymer particles in large pores are formed are not produced in a mass, and thus good distilled water absorption capacity is not exhibited.

(55) It is confirmed that in the case of the foaming agent in which carbonate is encapsulated with polyethylene glycol as in Comparative Example 3, polyethylene glycol stabilizes the carbonate and only delay the foaming time and does not expand to a desired size, whereby polymer particles in which pores of a desired size are formed are not produced in a mass.