Super absorbent polymer and preparation method thereof

Abstract

The present disclosure relates to a super absorbent polymer and a preparation method of the same, which not only has excellent basic absorption performance, but also exhibits an improved absorption rate, and the like. The super absorbent polymer includes a base resin powder containing a first cross-linked polymer of a water-soluble ethylene-based unsaturated monomer having at least partially neutralized acidic groups; and a surface cross-linked layer containing a second cross-linked polymer in which the first cross-linked polymer is additionally cross-linked by a surface cross-linking agent on the base resin powder, wherein the super absorbent polymer contains less than 9.9% by number of super absorbent polymer particles having an aspect ratio, which is defined as shortest diameter/longest diameter of each super absorbent polymer particle, of less than 0.5, a vortex time is 5 to 55 seconds by a vortex method, and a surface tension is 50 to 80 mN/m.

Claims

1. A preparation method of a super absorbent polymer, comprising: preparing a monomer mixture comprising a water-soluble ethylene-based unsaturated monomer having at least partially neutralized acidic groups and an internal cross-linking agent; transferring the monomer mixture to a polymerization reactor along a transfer pipe having a diameter that varies from section to section; cross-linking and polymerizing the monomer mixture transferred to the polymerization reactor to form a hydrogel polymer containing a first cross-linked polymer; gel-pulverizing, drying, pulverizing and classifying the hydrogel polymer to form a base resin powder containing less than 9.9% by number of base resin powder having an aspect ratio, which is defined as shortest diameter/longest diameter of each base resin powder, of less than 0.5; and further cross-linking a surface of the base resin powder in the presence of a surface cross-linking agent to form a surface cross-linked layer; wherein during the transferring of the monomer mixture, the monomer mixture has a maximum transfer rate in a minimum diameter section of the transfer pipe and a minimum transfer rate in a maximum diameter section of the transfer pipe, and the maximum transfer rate is 2.5 times or more compared to the minimum transfer rate.

2. The preparation method of a super absorbent polymer of claim 1, wherein the monomer mixture further comprises a surfactant.

3. The preparation method of a super absorbent polymer of claim 1, wherein the monomer mixture is transferred in the minimum diameter section of the transfer pipe at a rate of 0.45 to 2.5 m/s, and is transferred in the maximum diameter section of the transfer pipe at a rate of 0.1 to 0.4 m/s.

4. The preparation method of a super absorbent polymer of claim 1, wherein the transfer pipe has a diameter of 0.002 to 0.01 m in the minimum diameter section, and has a diameter of 0.011 to 0.020 m in the maximum diameter section before the minimum diameter section.

5. The preparation method of a super absorbent polymer of claim 1, wherein the monomer mixture is transferred through the transfer pipe at a flow rate of 100 to 15000 kg/hr.

6. The preparation method of a super absorbent polymer of claim 1, wherein bubbles are generated in the monomer by the change in the transfer rate during the transfer of the monomer mixture, and a foaming polymerization proceeds by the generated bubbles during the cross-linking polymerization.

7. The preparation method of a super absorbent polymer of claim 1, wherein the surface cross-linking agent comprises at least one of polyalcohol-based compounds, polyepoxy-based compounds, polyamine compounds, haloepoxy compounds, condensates of haloepoxy compounds, oxazoline-based compounds, or alkylene carbonate-based compounds.

8. The preparation method of a super absorbent polymer of claim 1, wherein the surface cross-linking is carried out by increasing an initial temperature of 20° C. to 130° C. to a highest temperature of 140° C. to 200° C. over 10 minutes to 30 minutes, and maintaining at the highest temperature for 5 to 60 minutes for heat-treatment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an electron micrograph showing an example of a method for defining and measuring an aspect ratio of super absorbent polymer particles in a super absorbent polymer of one embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(2) Hereinafter, the present invention will be described in more detail with reference to examples. However, these examples are for illustrative purposes only, and the invention is not intended to be limited by these examples.

Example 1

(3) As a super absorbent polymer manufacturing apparatus, a continuous manufacturing apparatus including a polymerization process, a hydrogel pulverization process, a drying process, a pulverization process, a classification process, a surface cross-linking process, a cooling process, a classification process, and a transportation process connecting each process was used.

(4) 0.4 parts by weight of polyethylene glycol diacrylate (weight average molecular weight: ˜500 g/mol) as an internal cross-linking agent, 0.01 parts by weight of sodium lauryl sulfate as a surfactant, and 0.01 parts by weight of phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator were mixed with 100 parts by weight of acrylic acid to prepare a monomer solution. Subsequently, while continuously supplying the monomer solution to a metering pump, 160 parts by weight of a 24 wt % aqueous solution of sodium hydroxide was continuously line-mixed to prepare a monomer aqueous solution. In addition, 6 parts by weight of a 4 wt % aqueous solution of sodium persulfate was continuously line-mixed to prepare a monomer mixture.

(5) The monomer mixture was first introduced through a single tube having a diameter (maximum diameter section) of 0.015 m at a flow rate of 240 kg/h. Secondarily, it was continuously transferred through a single tube (minimum diameter section) in which the diameter is changed to 0.008 m. In this transfer process, the transfer rate of each section was as summarized in Table 1 below.

(6) Through this transfer, the monomer aqueous solution was introduced into a polymerization reactor equipped with a movable conveyor belt, and UV polymerization was performed for 2 minutes by irradiating ultraviolet rays (irradiation amount: 2 mW/cm.sup.2) through a UV irradiation device to prepare a hydrogel polymer.

(7) After cutting the hydrogel to have an average size of about 300 mm or less, it was put into a pulverizer (equipped with a porous plate including a plurality of holes having a diameter of 10 mm) and pulverized.

(8) Subsequently, the pulverized hydrogel was dried in a dryer capable of changing wind direction up and down. The hydrogel was uniformly dried by flowing hot air at 180° C. from bottom to top for 15 minutes so that the moisture content of the dried powder was about 2% or less, and then flowing from top to bottom for 15 minutes.

(9) The dried polymer was pulverized using a pulverizer and then classified to obtain a base resin having a size of 150 to 850 μm.

(10) Thereafter, 6 g of a surface cross-linking aqueous solution containing 3 parts by weight of ethylene carbonate was sprayed onto 100 parts by weight of the prepared base resin powder, and stirred at room temperature to mix the surface cross-linking solution evenly on the base resin powder. Subsequently, the base resin powder mixed with the surface cross-linking solution was placed in a surface cross-linking reactor to perform a surface cross-linking reaction.

(11) The surface cross-linking reactor was operated such that the base resin powder was gradually heated from an initial temperature near 80° C. to reach a highest temperature of 190° C. after 30 minutes. After reaching the highest temperature, a further reaction was performed for 15 minutes, and then a sample of the super absorbent polymer finally produced was taken. After the surface cross-linking process, it was classified using an ASTM standard mesh to prepare a super absorbent polymer of Example 1 having a particle diameter of 150 μm to 850 μm.

(12) The base resin and super absorbent polymer obtained by the above method were analyzed by an electron microscope (see FIG. 1, etc.) to calculate the aspect ratio (a/b) of each base resin powder and super absorbent polymer particle. Thereafter, a ratio of particles (% by number) having an aspect ratio of less than 0.5 among the total base resin powder and super absorbent polymer particles was measured. As a result of the measurement, the ratio of particles having an aspect ratio of less than 0.5 among the total base resin powder and super absorbent polymer particles is shown in Table 1 below.

Example 2

(13) A super absorbent polymer of Example 2 was prepared in the same manner as in Example 1, except that the diameter of the single tube (transfer pipe) in the minimum diameter section was changed to 0.006 m during transfer of the monomer mixture to adjust the maximum transfer rate in the corresponding section as shown in Table 1 below.

Example 3

(14) A super absorbent polymer of Example 3 was prepared in the same manner as in Example 1, except that the flow rate of the monomer mixture was adjusted to 400 kg/h to adjust the transfer rate of the monomer mixture in each section as shown in Table 1 below.

Example 4

(15) A super absorbent polymer of Example 4 was prepared in the same manner as in Example 1, except that 0.005 parts by weight of the surfactant was included in the monomer mixture.

Example 5

(16) A super absorbent polymer of Example 5 was prepared in the same manner as in Example 1, except that the monomer mixture was first introduced through a single tube having a diameter (maximum diameter section) of 0.015 m at a flow rate of 240 kg/h, and continuously transferred through a single tube (minimum diameter section) in which the diameter is changed to 0.002 m. In this transfer process, the transfer rate in each section is summarized in Table 1 below.

Comparative Example 1

(17) A super absorbent polymer of Comparative Example 1 was prepared in the same manner as in Example 1, except that the diameter of the single tube (transfer pipe) in the minimum diameter section was changed to 0.012 m during transfer of the monomer mixture to adjust the maximum transfer rate in the corresponding section as shown in Table 1 below.

Comparative Example 2

(18) A super absorbent polymer of Comparative Example 2 was prepared in the same manner as in Example 1, except that the diameter of the single tube (transfer pipe) in the minimum diameter section was changed to 0.015 m during transfer of the monomer mixture to adjust the maximum transfer rate in the corresponding section as shown in Table 1 below.

Comparative Example 3

(19) A super absorbent polymer of Comparative Example 3 was prepared in the same manner as in Example 1, except that 0.02 parts by weight of the surfactant was included, and a 0.1 wt % sodium hydrogen carbonate blowing agent was further mixed in the monomer mixture.

Experimental Examples

(20) The properties of each super absorbent polymer prepared in Examples and Comparative Examples, and various properties in the manufacturing process were measured and evaluated in the following manner.

(21) (1) Transfer Rate of Monomer Aqueous Solution (m/s)

(22) The transfer rate of the monomer aqueous solution was calculated from the following equation by obtaining a cross-sectional area from a diameter of the transfer pipe in the transfer section and measuring a flow rate of the monomer mixture in the same section:
Transfer rate (m/s)=flow rate (m.sup.3/hr)/cross-sectional area (m.sup.2)
(2) Measurement of Aspect Ratio and Particle Distribution of Base Resin Powder and Super Absorbent Polymer Particles

(23) As shown in FIG. 1, the shortest diameter (a) and the longest diameter (b) of each powder/particle were calculated by an electron microscope, and the aspect ratio of each powder/particle was measured therefrom. Thereafter, the ratio (% by number) of the number of powder/particles having an aspect ratio of less than 0.5 among the total powder/particles obtained in each Example/Comparative Example was calculated.

(24) (3) Centrifuge Retention Capacity (CRC)

(25) The centrifuge retention capacity (CRC) by absorption ratio under a non-loading condition was measured according to the EDANA (European Disposables and Nonwovens Association) WSP 241.3 method. After inserting W.sub.0 (g, about 0.2 g) of the super absorbent polymer uniformly in a nonwoven fabric envelope and sealing the same, it was soaked in saline (0.9 wt % aqueous solution of sodium chloride) at room temperature. After 30 min, the envelope was centrifuged at 250 G for 3 minutes to drain, and the weight W.sub.2 (g) of the envelope was measured. Further, after carrying out the same operation without using the super absorbent polymer, the weight W.sub.1 (g) of the envelope was measured. Then, CRC (g/g) was calculated by using the obtained weight values according to the following Formula 1.
CRC(g/g)={[W.sub.2(g)−W.sub.1(g)−W.sub.0(g)]/W.sub.0(g)}  [Formula 1]
(4) Absorbency Under Pressure (AUP)

(26) The absorbency under pressure (AUP) of each super absorbent polymer prepared in Examples and Comparative Examples was measured according to the EDANA (European Disposables and Nonwovens Association) WSP 242.3 method.

(27) First, a 400 mesh stainless steel screen was installed in a cylindrical bottom of a plastic having an inner diameter of 60 mm. W.sub.0 (g, 0.90 g) of the polymer prepared in each of Examples 1 to 6 and Comparative Examples 1 to 4 was uniformly scattered on the screen at a temperature of 23±2° C. and a relative humidity of 45%. Thereafter, a piston which can uniformly provide a load of 4.83 kPa (0.7 psi) was placed on the polymer. Herein, the outer diameter of the piston was slightly smaller than 60 mm, there was no gap with the inner wall of the cylinder, and jig-jog of the cylinder was not interrupted. At this time, the weight W.sub.3 (g) of the device was measured.

(28) Subsequently, a glass filter having a diameter of 125 mm and a thickness of 5 mm was placed in a petri dish having a diameter of 150 mm, and saline (0.9 wt % sodium chloride) was poured in the dish. At this time, the saline was poured until the surface level of the saline became equal to the upper surface of the glass filter. After the measuring device was mounted on the glass filter, the liquid was absorbed for 1 hour under a load. After 1 hour, the measuring device was lifted, and the weight W.sub.4 (g) was measured.

(29) Then, AUP (g/g) was calculated by using the obtained weight values according to the following Formula 2.
AUP(g/g)=[W.sub.4(g)−W.sub.3(g)]/W.sub.0(g)  [Formula 2]

(30) In Formula 2,

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

(32) W.sub.3 (g) is a sum of a weight of the super absorbent polymer and a weight of the device providing a load to the polymer, and

(33) W.sub.4 (g) is a sum of a weight of the super absorbent polymer and a weight of the device providing a load to the polymer, after making the super absorbent polymer absorb the saline for one hour under a load (0.7 psi).

(34) (5) Absorption Rate by Vortex Method (Vortex Time)

(35) The absorption rate (vortex time) of each super absorbent polymer of Examples and Comparative Examples was measured in seconds according to the method disclosed in International Patent Publication No. 1987-003208.

(36) Specifically, the absorption rate (or vortex time) was calculated by adding 2 g of the super absorbent polymer to 50 mL of saline at 23° C. to 24° C., stirring a magnetic bar (8 mm in diameter and 31.8 mm in length) at 600 rpm, and measuring the time until vortex disappeared in seconds.

(37) (6) Surface Tension of Super Absorbent Polymer

(38) All procedures were conducted in a constant temperature and humidity room (temperature 23±0.5° C., relative humidity 45±0.5%). 150 g of saline composed of 0.9 wt % sodium chloride was placed in a 250 mL beaker and stirred with a magnetic bar. 1.0 g of the super absorbent polymer was added to the stirring solution, stirred for 3 minutes, and then allowed to stand for 15 minutes or more so that the swollen super absorbent polymer settled on the bottom.

(39) Thereafter, the supernatant (the solution immediately below the surface) was extracted with a pipette, and transferred to another clean cup to measure the surface tension using a surface tension meter (Kruss K11/K100).

(40) The physical properties of Examples 1 to 5 and Comparative Examples 1 to 3 measured by the above method are summarized in Table 1 below.

(41) TABLE-US-00001 TABLE 1 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 1 Ex. 2 Ex. 3 Flow rate (kg/hr) 240 240 400 240 240 240 240 240 pipe 1 Dia.(m) 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 (Dia. in maximum diameter section) Transfer rate in maximum 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32 diameter section (m/s) Pipe 2 Dia.(m) 0.008 0.006 0.008 0.008 0.002 0.012 0.015 0.015 (Dia. in minimum diameter section) Transfer rate in minimum 1.1 1.8 2.1 1.1 1.3 0.65 0.32 0.32 diameter section (m/s) Particle distribution 8.8 9.5 9.2 5.5 5.8 7.2 10.5 45.0 (aspect ratio less than 0.5; % by number) CRC (g/g) 30.5 28.7 30.1 31.0 29.0 29.2 29.5 27.7 AUP (g/g) 26.0 25.5 24.0 25.2 24.2 24.5 24.7 24.5 Absorbency (g/g) 56.5 54.2 54.1 56.2 53.2 53.7 54.2 53.5 Surface tension (mN/m) 70.2 69.8 67.2 70.8 70.0 70.1 69.7 63.3 Vortex (sec) 48 39 35 55 32 65 63 49

(42) Referring to Table 1, the super absorbent polymers of Examples 1 to 5 in which the transfer rate was controlled during the transfer of the monomer aqueous solution exhibited water retention capacity, absorbency under pressure, and surface tension equal to or higher than those of Comparative Examples, along with an improved absorption rate.

(43) It was confirmed in Comparative Example 3 that the absorption rate was improved to some extent, but the absorbency was reduced compared to Examples due to the use of a blowing agent and a surfactant. In addition, as the particles having a small aspect ratio were contained in a large amount in Comparative Example 3, breakage of particles or deterioration in physical properties is expected to be very significant in the process of transferring the super absorbent polymer or applying the polymer to products.