WATER-ABSORBENT RESIN COMPOSITION, METHOD FOR PRODUCING WATER-ABSORBENT RESIN COMPOSITION, AND METHOD FOR SLOWING WATER ABSORPTION RATE OF WATER-ABSORBENT RESIN PARTICLES

Abstract

Provided is a water-absorbent resin composition which slows the water absorption rate without increasing the particle diameter. A water-absorbent resin composition which contains water-absorbent resin particles having a median particle diameter of 200-600 μm and an acidic compound having a median particle diameter of 20-600 μm.

Claims

1. A water-absorbent resin composition comprising: water-absorbent resin particles with a median particle size of 200 to 600 μm; and an acidic compound with a median particle size of 20 to 600 μm.

2. The water-absorbent resin composition according to claim 1, wherein a difference (B−A) between an absorption rate A of the water-absorbent resin particles and an absorption rate B of the water-absorbent resin composition is 1 second or more.

3. The water-absorbent resin composition according to claim 1, wherein the absorption rate B of the water-absorbent resin composition is 4 to 130 seconds.

4. The water-absorbent resin composition according to claim 1, wherein a first acid dissociation constant of the acidic compound is 0.1 to 5.0.

5. An absorber comprising: water-absorbent resin particles with a median particle size of 200 to 600 μm; and an acidic compound with a median particle size of 20 to 600 μm.

6. An absorbent article comprising the absorber according to claim 5.

7. A method for producing the water-absorbent resin composition according to claim 1, the method comprising a step of mixing water-absorbent resin particles with a median particle size of 200 to 600 μm and an acidic compound with a median particle size of 20 to 600 μm.

8. The method for producing the water-absorbent resin composition according to claim 7, wherein a temperature in the mixing step is 0 to 90° C. and a relative humidity is 30 to 75%.

9. The method for producing the water-absorbent resin composition according to claim 7, wherein an amount of the acidic compound is 0.05 to 30 parts by mass relative to 100 parts by mass of the water-absorbent resin particles.

10. The method for producing the water-absorbent resin composition according to claim 7, wherein a ratio (T/S) of a median particle size T (μm) of the water-absorbent resin particles to a median particle size S (μm) of the acidic compound is 0.1 to 30.

11. A method for reducing an absorption rate of water-absorbent resin particles, the method comprising a step of mixing an acidic compound with a median particle size of 20 to 600 μm and water-absorbent resin particles with a median particle size of 200 to 600 μm.

Description

EXAMPLES

[0102] Hereinafter, the present invention will be described in detail with reference to the examples and comparative examples. However, the present invention is not limited to the examples.

[0103] The water-absorbent resin compositions obtained in the following examples and comparative examples were evaluated in various tests below. In addition, unless otherwise specified, the measurement was performed under an environment of a temperature of 25±2° C. and a humidity of 50±10%. Hereinafter, each evaluation test method will be described.

<Absorption Rate>

[0104] Absorption rates of each of the water-absorbent resin compositions and the water-absorbent resin particles were measured according to the following procedure based on a vortex method (JIS K7224-1996). To start with, 0.05 parts by mass of Blue No. 1 was mixed with 2000 parts by mass of ion-exchanged water to prepare colored ion-exchanged water, and a temperature was adjusted to 25±0.2° C. in a constant temperature water bath. The colored ion-exchanged water (50±0.01 g) was weighed in a beaker with a volume of 100 mL. Next, a stirrer (8 minφ×30 mm, no ring) was put in a beaker and stirred at a rotation speed of 600 rpm using a magnetic stirrer to generate a vortex. The time (seconds) from when 0.5±0.0002 g of the water-absorbent resin composition was put into the beaker until the stirrer was covered with the gelled ion-exchanged water was measured. The measurement was performed five times, and average values thereof were defined as the absorption rate B of the water-absorbent resin composition and the absorption rate A of the water-absorbent resin particles, respectively. A difference (B−A) between the absorption rate A of the water-absorbent resin particles and the absorption rate B of the water-absorbent resin composition is shown in Table 1.

<Median Particle Size of Water-Absorbent Resin Particles>

[0105] JIS standard sieves were combined in the following order from the top: a sieve with a mesh size of 850 μm, a sieve with a mesh size of 600 μm, a sieve with a mesh size of 500 μm, a sieve with a mesh size of 425 m, a sieve with a mesh size of 300 μm, a sieve with a mesh size of 250 μm, a sieve with a mesh size of 150 μm, and a receptacle. Water-absorbent resin particles (50 g) were placed on the uppermost sieve of combined sieves, and shaken for 10 minutes using a Ro-Tap type (Rotating and Tapping type) shaker to perform classification. After classification, a mass of the water-absorbent resin particles remaining on each sieve was calculated as a mass percentage with respect to a total amount, and a particle size distribution was determined. With respect to the particle size distribution, a relationship between a mesh size of the sieve and an integrated value of the mass percentage of water-absorbent resin particles remaining on the sieve was plotted on a logarithmic probability paper by integrating the particles on the sieve in a descending order of particle size. By connecting the plots on the probability paper with a straight line, a particle size corresponding to an integrated mass percentage of 50 mass % was defined as a median particle size of the water-absorbent resin particles.

<Median Particle Size of Acidic Compound>

[0106] An acidic compound (10 g) was sieved using a continuous fully automatic sonic vibration type sieving measuring instrument (Robot shifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.), a sieve with JIS standard mesh sizes of 850 μm, 500 μm, 425 μm, 300 μm, 212 μm, 106 μm, 75 μm, and 45 μm, and a pan under sieving conditions of a frequency of 80 Hz, a pulse interval of 1 second, and a classification time of 2 minutes. A mass of the acidic compound remaining on each sieve was calculated as a mass percentage with respect to a total amount. A mass percentage of the acidic compound remaining on each sieve was integrated in a descending order of particle size, and a relationship between a mesh size of the sieve and an integrated value of the mass percentage of the acidic compound remaining on the sieve was plotted on a logarithmic probability paper. By connecting plots on the probability paper with a straight line, a particle size corresponding to an integrated mass percentage of 50 mass % was determined, and this was taken as a median particle size of the acidic compound.

<Physiological Saline Absorption Amount>

[0107] In a plastic beaker of 500 mL, physiological saline of 500 g and a stirrer (8 mmφ×30 mm without ring) were put, and stirred at 600 rpm using a magnetic stirrer. Water-absorbent resin particles (2.0 g) were dispersed in the beaker, and sufficiently swollen by gentle stirring at 600 rpm for 1 hour. On the other hand, a mass Wa (g) of a standard sieve with a mesh size of 75 μm was measured, and an aqueous solution containing a swollen gel was filtered through a standard sieve with a mesh size of 75 μm. A standard sieve of 75 μm was allowed to stand for 30 minutes in a state where an angle formed with respect to the horizontal was inclined to be about 30 degrees, and excess physiological saline was removed from the water-absorbent resin particles. A sieve mass Wb (g) containing the swollen gel was measured, and a mass obtained by subtracting the mass Wa (g) of a 75 μm standard sieve from the mass Wb (g) was divided by the mass (2.0 g) of water-absorbent resin particles to calculate the absorption amount.

Absorption amount=(Wb−Wa)÷(mass of water-absorbent resin particles)

Production Example 1

[0108] A round-bottomed cylindrical separable flask of 2 L having an inner diameter of 11 cm and equipped with a reflux condenser, a dropping funnel, a nitrogen gas introduction tube, and a stirring blade having two stages of 4-inclined paddle blades with a blade diameter of 5 cm was prepared as a stirrer. To this flask, n-heptane as a hydrocarbon dispersion medium of 293 g and maleic anhydride-modified ethylene-propylene copolymer of 0.736 g (Mitsui Chemicals, Inc., Hi-wax 1105 A) as a polymeric dispersant were added, and a temperature was raised to 80° C. with stirring to dissolve the dispersant, then a mixture was cooled to 50° C. On the other hand, an 80.5 mass % aqueous solution of acrylic acid of 92.0 g (1.03 mol) as the water-soluble ethylenically unsaturated monomer was placed in a beaker with an internal volume of 300 mL, and a 20.9 mass % aqueous solution of sodium hydroxide of 147.7 g was added dropwise while cooling with ice water from an outside to perform neutralization at 75 mol %. Then, hydroxyl ethyl cellulose of 0.092 g (Sumitomo Seika Chemicals Company, Limited, HECAW −15 F) as a thickener, the potassium persulfate of 0.0736 g (0.272 mmol) as a water-soluble radical polymerization agent, and ethylene glycol diglycidyl ether of 0.010 g (0.057 mmol) as an internal crosslinking agent were added thereto and dissolved, thereby preparing a first-stage aqueous solution. Then, the aqueous solution prepared above was added to a separable flask and stirred for 10 minutes, and then a surfactant solution obtained by heating and dissolving sucrose stearate ester of 0.736 g (Ryoto sugar ester S-370 manufactured by Mitsubishi Chemical Foods Corporation) with HLB3 as a surfactant in n-heptane of 6.62 g was further added. While a rotation speed of a stirrer was set to 550 rpm and stirring, an inside of the system was sufficiently replaced with nitrogen. Thereafter, the flask was immersed in a water bath at 70° C. and heated, and polymerization was performed for 60 minutes to obtain a first-stage polymerization slurry solution. On the other hand, an 80.5 mass % aqueous solution of acrylic acid of 128.8 g (1.43 mol) as the water-soluble ethylenically unsaturated monomer was placed in another beaker with an internal volume of 500 mL, a 27 mass % aqueous solution of sodium hydroxide of 159.0 g was added dropwise with external cooling to perform neutralization at 75 mol %, and then the potassium persulfate of 0.103 g (0.381 mmol) as a water-soluble radical polymerization initiator and ethylene glycol diglycidyl ether of 0.0116 g (0.067 mmol) as an internal crosslinking agent were added and dissolved, thereby preparing a second-stage aqueous liquid. An inside of a separable flask system was cooled to 25° C. while a rotation speed of the stirrer was set to 1000 rpm, and then a whole amount of a second-stage aqueous liquid was added to the first-stage polymerization slurry, and the inside of the system was replaced with nitrogen for 30 minutes. Then, the flask was immersed in a water bath at 70° C. again, the temperature was raised, and the polymerization reaction was performed for 60 minutes to obtain a hydrous gel polymer. A 45 mass % aqueous pentasodium diethylenetriamine pentaacetate solution (0.589 g) was added to a hydrous gel polymer after a second-stage polymerization under stirring. Thereafter, the flask was immersed in an oil bath set at 125° C., and water of 257.7 g was removed to the outside of the system while refluxing n-heptane by azeotropic distillation of n-heptane and water. Thereafter, a 2 mass % aqueous solution of ethylene glycol diglycidyl ether of 4.42 g (0.507 mmol) as a surface crosslinking agent was added to the flask, and the mixture was held at 83° C. for 2 hours. Thereafter, n-heptane was evaporated at 125° C. and dried to obtain polymer particles (dry product). The polymer particles were passed through a sieve with a mesh size of 850 μm to obtain 228.0 g of water-absorbent resin particles. A median particle size of the water-absorbent resin particles was 394 μm, and a physiological saline absorption amount was 61 g/g.

Example 1

[0109] L-tartaric acid (0.5 parts by mass) (manufactured by FUSO CHEMICAL INDUSTRY CO., LTD., product name: purified L-tartaric acid, a first acid dissociation constant pKa1=2.87, a second acid dissociation constant pKa2=3.97, a median particle size 280 μm) was added to 100 parts by mass of the water-absorbent resin particles obtained in Production Example 1, and was mixed for 30 minutes (conditions: a revolution speed 50 rpm, a rotation speed 50 rpm) under environment of a temperature of 25° C. and a relative humidity of 50% using a cross rotary mixer manufactured by Meiwa. Industries, Ltd., to obtain a water-absorbent resin composition.

Example 2

[0110] A water-absorbent resin composition was obtained in the same manner as in Example 1 except that L-tartaric acid was changed to 1.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles in Example 1.

Example 3

[0111] A water-absorbent resin composition was obtained in the same manner as in Example 1 except that L-tartaric acid was changed to 2.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles in Example 1.

Example 4

[0112] A water-absorbent resin composition was obtained in the same manner as in Example 1 except that L-tartaric acid was changed to 3.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles in Example 1.

Example 5

[0113] A water-absorbent resin composition was obtained in the same manner as in Example 1 except that L-tartaric acid was changed to 5.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles in Example 1.

Example 6

[0114] A water-absorbent resin composition was obtained in the same manner as in Example 1 except that L-tartaric acid was changed to 10.0 parts by mass relative to 100 parts by mass of the water-absorbent resin particles in Example 1.

Example 7

[0115] Citric acid (1.0 part by mass) (manufactured by FUSO CHEMICAL INDUSTRY CO LTD., product name: Fuso citrate (anhydrous), a first acid dissociation constant pKa1=2.90, a second acid dissociation constant pKa2=4.35, a third acid dissociation constant pKa3=5.69, a median particle size 236 μm) was added to 100 parts by mass of the water-absorbent resin particles obtained in Production Example 1, and the mixture was mixed under environment of a temperature of 25° C. and a relative humidity of 50% for 30 minutes (conditions: a revolution speed 50 rpm, a rotation speed 50 rpm) using a cross rotary mixer manufactured by Meiwa Industries, Ltd., to obtain a water-absorbent resin composition.

Example 8

[0116] DL-malic acid (1.0 part by mass) (manufactured by FUSO CHEMICAL INDUSTRY CO., LTD., product name: Fuso malate, a first acid dissociation constant pKa1=3.23, a second acid dissociation constant pKa2=4.77, a median particle size 156 μm) was added to 100 parts by mass of the water-absorbent resin particles obtained in Production Example 1, and mixed for 30 minutes (conditions: a revolution speed 50 rpm, a rotation speed 50 rpm) under environment of a temperature of 25° C. and a relative humidity of 50% using a cross rotary mixer manufactured by Meiwa Industries, Ltd., to obtain a water-absorbent resin composition.

Example 9

[0117] Fumaric acid (1.0 part by mass) (manufactured by FUSO CHEMICAL INDUSTRY CO., LTD., product name: fumaric acid, a first acid dissociation constant pKa1=3.07, a second acid dissociation constant pKa2=4.58, a median particle size 161 μm) was added to 100 parts by mass of the water-absorbent resin particles obtained in Production Example 1, and mixed for 30 minutes (conditions: a revolution speed 50 rpm, a rotation speed 50 rpm) under environment of a temperature of 25° C. and a relative humidity of 50% using a cross rotary mixer manufactured by Meiwa Industries, Ltd., to obtain a water-absorbent resin composition.

Comparative Example 1

[0118] The water-absorbent resin particles obtained in Production Example 1 were used as they were as water-absorbent resin particles of Comparative Example 1.

TABLE-US-00001 TABLE 1 Water-absorbent resin composition Water-absorbent resin Difference Acidic compound particles between Addition Median Median absorption amount particle particle Absorption Absorption rates (parts by size size rate A rate B (B-A) Type mass) (μm) (μm) (seconds) (seconds) (seconds) Example 1 L-tartaric acid 0.5 280 394 29 31 2 Example 2 L-tartaric acid 1 280 394 29 33 4 Example 3 L-tartaric acid 2 280 394 29 36 7 Example 4 L-tartaric acid 3 280 394 29 35 6 Example 5 L-tartaric acid 5 280 394 29 44 15 Example 6 L-tartaric acid 10 280 394 29 56 27 Example 7 Citric acid 1 236 394 29 33 4 Example 8 DL-malic acid 1 156 394 29 33 4 Example 9 Fumaric acid 1 161 394 29 30 1 Comparative — — 394 29 — — Example 1