Super absorbent polymer and method for producing same

Abstract

The super absorbent polymer comprises: a base polymer powder including a first crosslinked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups; and a surface crosslinked layer formed on the base polymer powder and including a second crosslinked polymer in which the first crosslinked polymer is further crosslinked via a surface crosslinking agent, wherein the super absorbent polymer has: a fixed height absorption (FHA) of 22.5 g/g to 29 g/g, a saline flow conductivity (SFC) of 35 (.Math.10.sup.−7 cm.sup.3.Math.s/g) or more, and T-20 of 180 seconds or less.

Claims

1. A super absorbent polymer comprising: a base polymer powder including a first crosslinked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups; and a surface crosslinked layer formed on the base polymer powder and including a second crosslinked polymer in which the first crosslinked polymer is further crosslinked via a surface crosslinking agent, wherein the super absorbent polymer has the following features: a fixed height absorption (FHA) (20 cm) for a physiological saline solution (0.9 wt % aqueous sodium chloride solution) of 22.5 g/g to 29 g/g, a saline flow conductivity (SFC) for a physiological saline solution (0.685 wt % aqueous sodium chloride solution) of 35 (.Math.10.sup.−7 cm.sup.3.Math.s/g) or more, and T-20 of 180 seconds or less which indicates the time required for absorbing 1 g of the super absorbent polymer to 20 g of aqueous solution of 0.9 wt % sodium chloride and 0.01 wt % alcohol ethoxylate having 12 to 14 carbon atoms under pressure of 0.3 psi, wherein the first crosslinked polymer includes a polymer in which the water-soluble ethylenically unsaturated monomer is crosslinked in the presence of an internal crosslinking agent, the internal crosslinking agent includes a polyolpoly(meth)acrylate-based first internal crosslinking agent, and an allyl(meth)acrylate-based second internal crosslinking agent, the polyolpoly(meth)acrylate-based first internal crosslinking agent is included in an amount of 0.4 to 1 part, and the allyl(meth)acrylate-based second internal crosslinking agent is included in an amount of 0.008 to 0.5 parts, by weight based on 100 parts by weight of a monomer composition including the internal crosslinking agent and the water-soluble ethylenically unsaturated monomer, and the allyl(meth)acrylate-based second internal crosslinking agent is allyl acrylate or allyl (meth)acrylate.

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 (0.9 wt % aqueous sodium chloride solution) for 30 minutes of 26 g/g to 34 g/g.

3. The super absorbent polymer according to claim 1, wherein the super absorbent polymer has a free swell rate (FSR) of 0.25 g/g/s or more when 1 g of the super absorbent polymer absorbs 20 g of a 0.9 wt % aqueous sodium chloride solution.

4. The super absorbent polymer according to claim 1, wherein the water-soluble ethylenically unsaturated monomer includes at least one selected from an anionic monomer, a nonionic hydrophilic monomer, or an unsaturated monomer; wherein the anionic monomer is an anionic monomer of acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloyl ethane sulfonic acid, 2-methacryloyl ethane sulfonic acid, 2-(meth)acryloyl propane sulfonic acid, or 2-(meth)acrylamide-2-methylpropane sulfonic acid, and a salt thereof; wherein the nonionic hydrophilic monomer is a nonionic hydrophilic monomer of (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxypolyethyleneglycol(meth)acrylate, or polyethyleneglycol(meth)acrylate; and wherein the unsaturated monomer is an unsaturated monomer containing an amino group of (N,N)-dimethylaminoethyl(meth)acrylate or (N,N)-dimethylaminopropyl(meth)acrylamide, and a quaternary compound thereof.

5. The super absorbent polymer according to claim 1, wherein the polyolpoly(meth)acrylate-based first internal crosslinking agent is selected from the group consisting of trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycoldi(meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentacrylate, glycerin tri(meth)acrylate, and pentaerythritol tetraacrylate.

6. The super absorbent polymer according to claim 1, wherein the super absorbent polymer has a particle diameter of 150 to 850 μm.

7. The super absorbent polymer according to claim 1, wherein the SFC is from 35 to 150.

8. The super absorbent polymer according to claim 1, wherein the T-20 is from 80 seconds to 180 seconds.

9. The superabsorbent polymer according to claim 3, wherein the FSR is from 0.25 g/g/s to 0.4 g/g/s.

10. A super absorbent polymer comprising: a base polymer powder including a first crosslinked polymer of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups; and a surface crosslinked layer formed on the base polymer powder and including a second crosslinked polymer in which the first crosslinked polymer is further crosslinked via a surface crosslinking agent, wherein the super absorbent polymer has the following features: a fixed height absorption (FHA) (20 cm) for a physiological saline solution (0.9 wt % aqueous sodium chloride solution) of 22.5 g/g to 29 g/g, a centrifuge retention capacity (CRC) for a physiological saline solution (0.9 wt % aqueous sodium chloride solution) for 30 minutes of 26 g/g to 34 g/g, a saline flow conductivity (SFC) for a physiological saline solution (0.685 wt % aqueous sodium chloride solution) of 35 (.Math.10.sup.−7 cm.sup.3.Math.s/g) or more, and a free swell rate (FSR) of 0.25 g/g/s to 0.40 g/g/s when 1 g of the super absorbent polymer absorbs 20 g of a 0.9 wt % aqueous sodium chloride solution, wherein the first crosslinked polymer includes a polymer in which the water-soluble ethylenically unsaturated monomer is crosslinked in the presence of an internal crosslinking agent, the internal crosslinking agent includes a polyolpoly(meth)acrylate-based first internal crosslinking agent, and an allyl(meth)acrylate-based second internal crosslinking agent, the polyolpoly(meth)acrylate-based first internal crosslinking agent is included in an amount of 0.4 to 1 parts, and the allyl(meth)acrylate-based second internal crosslinking agent is included in an amount of 0.008 to 0.5 parts, by weight based on 100 parts by weight of a monomer composition including the internal crosslinking agent and the water-soluble ethylenically unsaturated monomer, and the allyl(meth)acrylate-based second internal crosslinking agent is allyl acrylate or allyl (meth)acrylate.

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) As the manufacturing apparatus of a super absorbent polymer, a continuous manufacturing apparatus comprising a polymerization step, a hydrogel pulverizing step, a drying step, a pulverization step, a classification step, a surface cross-linking step, a cooling step, a classification step, and a transport step connecting respective steps can be used.

(3) (Step 1)

(4) 0.7 parts by weight (7000 ppm) of polyethylene glycol diacrylate (weight average molecular weight: ˜500 g/mol) as an internal crosslinking agent, 0.015 part (150 ppm) of allyl methacrylate and 0.01 part by weight of IRGACURE 819 as a photoinitiator were mixed to prepare a monomer solution. Subsequently, while continuously supplying 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. At this time, the temperature raised by the neutralizing heat was adjusted to 40° C. Further, 6 parts by weight of a 4 wt % aqueous solution of sodium persulfate was continuously subjected to line mixing, and then continuously supplied to a continuous polymerization reactor having a planar polymerization belt with a darn at each end. Thereafter, UV light was irradiated for 1 minute, and further thermal polymerization was carried out for 2 minutes to prepare a hydrogel. The water content of the hydrogel was confirmed to be 45% by weight.

(5) (Step 2)

(6) The hydrogel 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 holes having a diameter of 11 mm) together with a fine powder re-granulated body as shown in Table 1 below and pulverized under the respective conditions. Herein, the fine powder re-granulated body used the fine powder re-granulated body prepared in step 4 below, and the input ratio is shown in Table 1 as 20 weight % relative to the hydrogel.

(7) (Step 3)

(8) Then, the hydrogel pulverized in step 2 were dried in an oven capable of shifting airflow up and down. The hydrogel was 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.

(9) (Step 4)

(10) The polymer dried in step 3 was pulverized using a pulverizer and classified to obtain a base polymer having a size of 150 to 850 μm. On the other hand, through the above classification, the polymer particles having a particle size of less than 150 μm was granulated with water and used for the fine powder re-granulated body of step 2 described above.

(11) (Step 5)

(12) Then, 100 parts by weight of the base polymer prepared in step 4 was mixed with a crosslinking agent solution containing 4 parts by weight of water and 1 part by weight of ethylene carbonate and then subjected to a surface crosslinking reaction at 180° C. for 40 minutes. Then, the obtained product was cooled and classified to obtain a surface-crosslinked super absorbent polymer having a particle diameter of 150 to 850 μm.

Examples 2 to 8 and Comparative Examples 1 to 4

(13) A super absorbent polymer was prepared in the same manner as in Example 1, except that the content range of the internal crosslinking agent, the hole diameter of the perforated plate provided in the gel pulverizer, the water content of the hydrogel and the input ratio of the fine powder re-granulated body were changed as shown in Table 1 below.

(14) In the following Examples 1 to 8 and Comparative Examples 1 to 4, the gel strength of the hydrogel before and after the gel pulverization was measured by the method summarized in the following, and the measurement results are summarized and shown together in Table 1 below.

(15) *Measurement Method of the Gel Strength of Hydrogel

(16) A. Preparation of Samples to be Measured:

(17) First, a hydrogel sample to be measured (a hydrogel sheet before gel pulverization and a hydrogel after gel pulverization) was prepared to have a diameter of about 2.5 cm and a thickness of about 2 to 5 mm. The prepared sample was loaded on a microbalance, and an appropriate amount of distilled water was evenly sprayed using a sprayer (At this time, the initial water content of the hydrogel was measured in advance (water content measuring instrument condition: 180° C., 40 minutes), and the amount of water required to match the initial water content was calculated). In order to uniformly adjust the water content inside the sample, it was sealed in vinyl and stored at room temperature for 12 hours or more.

(18) B. Measurement Method:

(19) A hydrogel sample with controlled water content was loaded between two plates of the rheometer (ARES-G2), and the gap between the two plates was properly adjusted by pressing the plates with a force of 3 N so that the sample was contacted at the front face of the plate. Rest time was given for 5 minutes to stabilize the sample. At the time of measurement, first, a strain in the linear viscoelastic regime section where the storage modulus (G′) and the loss modulus (G″) were constant was found while increasing the strain at a frequency of 10 rad/s.

(20) After finding the strain value (usually 0.1%) in the linear regime section, the viscoelasticity (G′, G″) was measured for 60 seconds at a constant frequency (.Math.10 rad/s). After three or more measurements, the average value of G′ was calculated as the gel strength (G′).

(21) TABLE-US-00001 TABLE 1 Input Gel ratio of Gel Gel strength Hole fine strength of strength of ratio before Internal diameter Water powder hydrogel hydrogel and after crosslinking of content re- before gel after gel gel agent perforated of granulated pulver- pulver- pulver- (P/A*; plate hydrogel body ization ization ization ppm) (mm) (wt %) (wt %) (Pa) (Pa) (%) Example 1 7000/150 11 45 20 23300 18500 79.4 Example 2 7000/150 11 40 20 27700 20800 75.1 Example 3 7000/150 11 50 20 19200 15900 82.8 Example 4 8000/200 11 40 20 37400 26900 71.9 Example 5 5000/100 11 50 20 15600 10300 66.0 Example 6 7000/150 11 45 20 23100 17600 76.2 Example 7 7000/150 13 40 25 27800 21500 77.3 Example 8 7000/150 16 40 20 27600 23700 85.9 Comparative 5000/100 11 65 20  6500  6000 92.3 Example 1 Comparative 7000/150  7 45 20 23200  7900 34.1 Example 2 Comparative 7000/150 20 45 20 23300 22600 97.0 Example 3 Comparative 3000/50  11 45 20  8500  7000 82.4 Example 4 *Internal crosslinking agent P/A: polyethylene glycol diacrylate/allymethacrylate

(22) Referring to Table 1, it was confirmed that in Examples 1 to 8, the kind, content, water content and the like of the internal crosslinking agent are adjusted so that the hydrogel has a gel strength of 10,000 Pa or more, and the hole diameter and the water content of the perforated plate of the gel pulverizer are adjusted so that the gel strength after gel pulverization satisfies the range of 35 to 95% of the gel strength before gel pulverization.

(23) In contrast, it was confirmed that in Comparative Example 1, the water content of the hydrogel before gel pulverization is excessively high, so that the hydrogel before and after gel pulverization shows a low gel strength. Even in Comparative Example 4, the content of the internal crosslinking agent is excessively low, so that the hydrogel before gel pulverization shows a low gel strength. Further, in Comparative Examples 2 and 3, the hole diameter of the perforated plate of the gel pulverizer is not maintained at an appropriate level, and so the gel strength after the gel pulverization deviates from the appropriate range.

Experimental Example

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

(25) (1) Centrifuge Retention Capacity (CRC)

(26) 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. W.sub.0 (g, about 0.2 g) of the super absorbent polymer was 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. Using the respective weights thus obtained, CRC (g/g) was calculated according to the following Equation 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)}  [Equation 1]

(27) (2) Saline Flow Conductivity (SFC)

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

(29) (3) T-20

(30) 9 g of sodium chloride and 0.1 g of Lorodac (main component: linear alcohol ethoxylate having 12 to 14 carbon atoms, CAS #68439-50-9) were dissolved in 1 L of distilled water to make an aqueous solution, and the T-20 was calculated and measured with the time required for absorbing 1 g of the super absorbent polymer to 20 g of this aqueous solution under pressure of 0.3 psi. Specific measurement methods of T-20 were described in detail on pages 13 to 18 of European Patent Publication No. 2535027.

(31) (4) FSR (Free Swell Rate)

(32) The FSR of the base polymer powder or the super absorbent polymer was measured and calculated by using those classified into #30 to #50 (for example, those having a particle diameter of 300 to 600 μm) according to the method disclosed on pages 22 to 23 of European Patent Publication No. 2535027.

(33) (5) FHA

(34) The measurement was performed after absorbing the super absorbent polymer to a physiological saline solution (0.9 wt % aqueous sodium chloride solution) under pressure of 0.3 psi, which was measured and calculated as a fixed height absorption (FHA) (20 cm). The other specific measurement and calculation method was performed according to the method disclosed in Examples of U.S. Pat. No. 7,108,916.

(35) The physical property values of Examples 1 to 8 and Comparative Examples 1 to 4 measured by the above method are summarized in Table 2 below.

(36) TABLE-US-00002 TABLE 2 CRC FHA FSR SFC T-20 Unit g/g g/g g/g/s 10.sup.−7cm.sup.3 .Math. s/g s Example 1 28.5 24.5 0.32 55 140 Example 2 28.1 24.2 0.33 53 133 Example 3 29.0 24.6 0.30 56 145 Example 4 27.1 24.0 0.36 63 123 Example 5 28.8 23.7 0.30 47 147 Example 6 28.3 24.3 0.32 51 139 Example 7 28.6 24.5 0.30 55 146 Example 8 28.7 24.6 0.29 57 154 Comparative 29.0 22.0 0.19 18 233 Example 1 Comparative 25.6 21.1 0.33 23 134 Example 2 Comparative 27.1 23.3 0.18 57 241 Example 3 Comparative 29.1 22.8 0.20 18 213 Example 4

(37) Referring to Table 2, it was confirmed that in the case of Examples 1 to 8, the basic absorption performance defined by CRC is excellent, the suction force under pressure defined by FHA and the liquid permeability defined by the SFC are excellent, and the absorption rates defined by T-20 or FSR are also excellent.

(38) In contrast, it was confirmed that in the case of Comparative Examples 1 to 4, at least one of the liquid permeability, the absorption rate or the absorption under pressure is poor as compared with Examples.