Method for producing super absorbent polymer

Abstract

A super absorbent polymer produced by the preparation method of the super absorbent polymer according to the present invention has excellent dryness while maintaining excellent absorption performance, and thus is preferably used for hygienic materials such as diapers and can exhibit excellent performance.

Claims

1. A method for producing a super absorbent polymer comprising the steps of: crosslinking acrylic acid monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent and a thermal polymerization initiator to form a hydrogel polymer; drying, pulverizing and classifying the hydrogel polymer to form a base polymer powder; and heat-treating and surface-crosslinking the base polymer powder in the presence of a surface crosslinking solution to form a super absorbent polymer, wherein the thermal polymerization initiator is used in an amount of 0.05 to 0.15 parts by weight based on 100 parts by weight of the acrylic acid, wherein the surface crosslinking solution comprises ethyleneglycol diglycidyl ether and propylene glycol in a weight ratio of 1:1.2 to 1:2.4, and the total amount of ethyleneglycol diglycidyl ether and propylene glycol ranges from 0.55 to less than 1 part by weight based on 100 parts by weight of the base polymer, wherein a dryness of the superabsorbent polymer ranges from 0.2 grams to 0.4 grams, the dryness being measured by swelling 2 grams of the superabsorbent polymer in 200 milliliters of water at a temperature of 22° C. to 24° C., removing the swollen superabsorbent polymer from the water and maintaining the swollen superabsorbent polymer at room temperature for 6 hours, pressuring a stack of five filter papers having a diameter of 5 centimeters at 0.2 psi for 1 minute against the swollen superabsorbent polymer, and determining the dryness from the weight of water absorbed by the filter paper, and wherein the superabsorbent polymer has a centrifugation retention capacity (CRC) of 30.7 g/g or more, and an absorbency under pressure (AUP) of 7.2 g/g to 10.7 g/g.

2. The method of claim 1, wherein the thermal polymerization initiator is sodium persulfate, potassium persulfate, ammonium persulfate, 2,2-azobis(2-amidinopropane)dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutylonitril, 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, or 4,4-azobis-(4-cyanovaleric acid).

3. The method of claim 1, wherein the surface crosslinking solution further comprises aluminum sulfate, or an inorganic filler.

4. The method of claim 1, wherein the super absorbent polymer has an absorption rate (vortex) of 35 seconds or less, as measured according to the measurement method of Vortex.

5. The method of claim 1, wherein a liquid permeability of the superabsorbent polymer ranges from 7 seconds to 11 seconds, the liquid permeability being measured by filling a chromatography tube having a glass filter and a valve at one end thereof with a 0.9% salt water, introducing a piston into the chromatography tube, opening the valve, and measuring a time (B) needed for the piston to reduce the level of the salt water from the 40 mL mark to the 20 mL mark of the chromatography tube, and then in a separate measurement, introducing 0.2±0.0005 grams of a sample of the superabsorbent polymer, the sample having been classified by a mesh ranging from #30 to #50, filling the chromatography tube having the classified sample, the glass filter and the valve at one end thereof with a 0.9% salt water, introducing the piston into the chromatography tube, opening the valve, and measuring a time (T1) needed for the piston to reduce the level of the salt water from the 40 mL mark to the 20 mL mark of the chromatography tube, and determining the liquid permeability as the difference of T1−B.

6. The method of claim 1, wherein the surface crosslinking solution further comprises aluminum sulfate and an inorganic filler.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) Hereinafter, preferred examples are provided for better understanding of the invention. However, these Examples are given for illustrative purposes only and are not intended to limit the scope of the present invention thereto.

Example 1

(2) 100 g of acrylic acid, 0.35 g of polyethylene glycol diacrylate (PEGDA, Mw=523) as a crosslinking agent, 0.05 g of sodium persulfate (SPS) as a thermal initiator, 0.06 g of sodium bicarbonate (SBC) as a foaming agent, 0.02 g of sodium dodecylsulfate as a surfactant, 83.3 g of 50% caustic soda (NaOH) and 89.8 g of water were mixed to prepare a monomer aqueous solution composition. The monomer aqueous solution composition was subjected to a thermal polymerization reaction to obtain a polymerized sheet. The polymerized sheet was taken out and cut into a size of 3 cm×3 cm. Then, the cut sheet was subjected to a chopping process using a meat chopper to prepare crumbs. Then, the crumbs were dried in an oven capable of shifting airflow up and down. The crumbs were uniformly dried by flowing hot air at 185° 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 2% or less. After drying, the crumbs were pulverized using a pulverizer and then classified with an amplitude of 1.5 mm into three mesh sizes (combination of classified meshes: #30/#50/#100). The respective classified particles (10%/72%/18%) were collected, and a polymer having a particle size of about 150 μm to 850 μm was classified and obtained. The base polymer powder was obtained by the above method.

(3) Subsequently, to 100 parts by weight of the base polymer prepared, a surface crosslinking solution (4 parts by weight of water, 0.25 parts by weight of ethyleneglycol diglycidyl ether (EX-810), 0.3 parts by weight of propylene glycol (PG), 0.15 parts by weight of aluminum sulfate 18 hydrate, and 0.1 part by weight of silica (Aerosil A200)) was uniformly mixed, and then subjected to a surface crosslinking reaction at 140° C. for 30 minutes. After completion of the surface treatment, the resultant product was sieved to obtain a super absorbent polymer having a particle diameter of 150 to 850 μm.

Examples 2 to 9 and Comparative Examples 1 to 5

(4) A super absorbent polymer was obtained in the same manner as in Example 1, except that SPS, and the composition of the surface crosslinking solution were set as shown in Table 1 below.

Experimental Example: Evaluation of Physical Properties of Super Absorbent Polymer

(5) The physical properties of the super absorbent polymer prepared in Examples and Comparative Examples were evaluated by the following methods.

(6) (1) Extractable Contents (16 hr E/C)

(7) The extractable contents were measured for the base polymer in the preparation process of Examples and Comparative Examples by the same method as described in EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 270.2.

(8) Specifically, 1.0 g of the base polymer was put in 200 g of a 0.9 wt % NaCl solution, and then swollen for 16 hours while stirring at 500 rpm. The aqueous solution was filtered out through a filter paper. The filtered solution was primarily titrated to pH 10.0 with 0.1 N caustic soda solution, and then back-titrated to pH 2.7 with a 0.1 N hydrogen chloride solution. From the amount required during neutralization, the uncrosslinked polymer substance was calculated and measured as the extractable content.

(9) (2) Centrifuge Retention Capacity (CRC)

(10) The centrifuge retention capacity (CRC) by water absorption capacity under a non-loading condition was measured for the super absorbent polymers of Examples and Comparative Examples in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 241.3.

(11) Specifically, W.sub.0 (g, about 0.2 g) of the super absorbent polymers of Examples and Comparative Examples were uniformly put in a nonwoven fabric-made bag, followed by sealing. Then, the bag was immersed in a physiological saline solution composed of 0.9 wt % aqueous sodium chloride 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. Further, the same procedure was carried out without using the super absorbent polymer, and then the resultant weight W.sub.1 (g) was measured.

(12) Using the respective weights thus obtained, CRC (g/g) was calculated according to the following Mathematical Formula 1.
CRC (g/g)={[W.sub.2 (g)−W.sub.1 (g)−W.sub.0 (g)]/W.sub.0 (g)}  [Mathematical Formula 1]

(13) in Mathematical Formula 1,

(14) W.sub.0 (g) is an initial weight (g) of the super absorbent polymer, W.sub.1 (g) is the weight of the device not including the super absorbent polymer, measured after immersing and absorbing the same into a physiological saline solution for 30 minutes and then dehydrating the same by using a centrifuge at 250 G for 3 minutes, and W.sub.2 (g) is the weight of the device including the super absorbent polymer, measured after immersing and absorbing the super absorbent polymer into 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.

(15) In addition, with respect to the respective base polymers produced in the preparation process of Examples and Comparative Examples, CRC (BR CRC) was measured in the same manner as described above.

(16) (3) Dryness

(17) 2 g of the super absorbent polymer was put in a 500 mL beaker and 200 mL of distilled water at 22 to 24° C. was added thereto. After the super absorbent polymer was swollen, it was taken out and left at room temperature for 6 hours. Subsequently, after stacking five filter papers with a diameter of 5 cm, the filter paper was placed on the super absorbent polymer, and pressurized at 0.2 psi for 1 minute. The weight (dryness, g) of the distilled water absorbed by the filter pater was measured.

(18) (4) Absorption Rate (Vortex)

(19) 50 mL of a 0.9 wt % NaCl solution was put in a 100 mL beaker, and then 2 g of each super absorbent polymer prepared in Examples and Comparative Examples was added thereto while stirring at 600 rpm using a stirrer. Then, the vortex time was calculated by measuring the amount of time until a vortex of the liquid caused by the stirring disappeared and a smooth surface was formed, and the result was shown as the vortex removal time.

(20) 50 mL of a 0.9 wt % NaCl solution was put in a 100 mL beaker, to which a magnetic bar with a size of 30 mm (polygon type with a length of 30 mm and a thickness of 8 mm) was added. While stirring at 600 rpm using a magnetic stirrer, 2.0 g of the super absorbent polymers prepared in Examples and Comparative Examples were respectively added. Then, the vortex time was calculated by measuring the amount of time until a vortex of the liquid caused by the stirring disappeared and a smooth surface was formed, and the result was shown as the vortex removal time (absorption rate; vortex).

(21) (5) Absorbency Under Pressure (AUP)

(22) The absorbency under pressure (AUP) of the super absorbent polymers of Examples and Comparative Examples was measured in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. WSP 242.3.

(23) Specifically, a 400 mesh stainless screen was installed at the bottom of a plastic cylinder having an inner diameter of 60 mm. W.sub.0 (g, about 0.90 g) of the super absorbent polymers obtained in Examples and Comparative Examples were uniformly scattered on the stainless screen under a condition of a temperature of 23±2° C. of and a relative humidity of 45%. Then, a piston capable of providing a load of 0.9 psi uniformly was designed so that the outer diameter was slightly smaller than 60 mm and thus it could move freely up and down without any gap with the inner wall of the cylinder. At this time, the weight W.sub.3 (g) of the device was measured.

(24) 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 a physiological saline solution composed of 0.90 wt % sodium hydroxide aqueous solution was poured until the surface level became equal to the upper surface of the glass filter. Then, a sheet of filter paper having a diameter of 120 mm was placed on the glass filter. The measuring device was placed on the filter paper, so that the liquid was absorbed under load for one hour. After one hour, the measuring device was lifted and the weight W.sub.4 (g) was measured.

(25) Using the respective weights thus obtained, AUP (g/g) was calculated according to the following [Mathematical Formula 2.
AUP (g/g)=[W.sub.4 (g)−W.sub.3 (g)]/W.sub.0 (g)  [Mathematical Formula 2]

(26) in Mathematical Formula 2,

(27) W.sub.0 (g) is an initial weight (g) of the super absorbent polymer, W.sub.3 (g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, and W.sub.4 (g) is the total sum of a weight of the super absorbent polymer and a weight of the device capable of providing a load to the super absorbent polymer, after absorbing a physiological saline solution to the super absorbent polymer under a load (0.9 psi) for 1 hour.

(28) (6) Liquid Permeability

(29) In a state where a piston was introduced in a chromatography tube (F20 mm), the liquid surface was displayed as a 20 ml mark line and a 40 ml mark line. Then, water was inversely introduced in a chromatography tube so that bubbles are not generated between a glass filter and a cork at the bottom of the chromatography tube, filling the tube for approximately 10 ml, and the chromatography tube was washed 2 to 3 times with salt water and filled with 0.9% salt water up to 40 ml or greater. A piston was introduced in the chromatography tube, a valve at the bottom was opened, and then the time (B) taken for the liquid surface decreasing from a 40 ml mark line to a 20 ml mark line was recorded.

(30) 10 mL of salt water was left in the chromatography tube, to which 0.2±0.0005 g of classified (30 # to 50 #) sample was added. Salt water was added thereto so that the salt water volume became 50 ml, and then the result was left for 30 minutes. After that, a piston with a weight (0.3 psi=106.26 g) was introduced in the chromatography tube, and the result was left for 1 minute. After opening a valve at the bottom of the chromatography tube, the time (T1) taken for the liquid surface decreasing from a 40 ml mark line to a 20 ml mark line was recorded. Thereby, the liquid permeability (time of T1−B) was measured.

(31) The results of the above measurement are shown in Table 1 below.

(32) TABLE-US-00001 TABLE 1 Composition of Physical surface properties of Physical properties of super crosslinking agent base polymer absorbent polymer EX- BR 16 hr Liquid SPS Water 810 PG CRC EC CRC Dryness Vortex AUL permeability Unit ppmw phr phr phr g/g wt % g/g g sec g/g sec Ex. 1 500 4 0.25 0.3 34.0 8.1 31.1 0.4 31 8.4 7 Ex. 2 500 4 0.25 0.6 34.0 8.1 30.7 0.3 32 9.1 8 Ex. 3 1000 4 0.25 0.3 35.1 10.7 32.3 0.4 33 7.2 8 Ex. 4 1000 4 0.25 0.6 35.1 10.7 32.0 0.3 30 8.1 8 Ex. 5 1500 4 0.25 0.3 34.9 11.6 31.8 0.4 31 9.7 11 Ex. 6 1500 4 0.25 0.6 34.9 11.6 31.5 0.2 30 10.7 10 Ex. 7 1500 4 0.25 1.0 34.9 11.6 30.2 0.5 33 13.1 16 Ex. 8 2000 4 0.25 0.3 34.8 12.5 31.9 0.7 30 9.1 14 Ex. 9 2000 4 0.25 0.6 34.8 12.5 31.4 0.6 32 9.6 16 Comparative 1500 4 0 0.25 34.9 11.6 35.5 3.0 31 6.5 0.7 Ex. 1 Comparative 1500 4 0.25 2.0 34.9 11.6 29.0 2.7 31 14.9 23 Ex. 2 Comparative 2500 4 0.25 0.6 34.3 15.5 33.2 2.5 35 7.1 4 Ex. 3 Comparative 3000 4 0.25 0.6 34.5 19.2 34.2 2.6 37 8.6 8 Ex. 4 Comparative 1500 4 0.25 0 34.9 11.6 31.4 0.6 30 10.0 9 Ex. 5

(33) From the results of Table 1, it was confirmed that the super absorbent polymers of Examples according to the present invention had excellent dryness while maintaining other physical properties equal to or higher than those of the super absorbent polymers of Comparative Examples.

(34) On the other hand, in the case of Comparative Examples, it was confirmed that various physical properties were deteriorated according to the use amount of the thermal initiator or the composition of the surface crosslinking agent. Specifically, in the case of Comparative Examples 1 and 2, since the use amount of propylene glycol as a surface crosslinking agent was small or large, the dryness was decreased. In the case of Comparative Examples 3 and 4, the use amount of the thermal initiator (SPS) was large and thus the dryness was decreased. In addition, in the case of Comparative Example 5, since propylene glycol as a surface crosslinking agent was not used, the dryness or liquid permeability was relatively lowered as compared with those of Examples.