Polyacrylic acid (salt)-based water absorbent resin, and method for production thereof

Abstract

Provided is a method for producing a polyacrylic acid (salt)-based water absorbent resin, which is a convenient production method for a water absorbent resin for an absorbent suitable for practical use, the water absorbent resin having a reduced amount of residual monomers. Disclosed is a method for producing a polyacrylic acid (salt)-based water absorbent resin, the method comprising a polymerization step of polymerizing an aqueous monomer solution containing acrylic acid (salt) as a main component; a drying step of drying a water-containing gel-like crosslinked polymer obtained in the polymerization step; a surface crosslinking step of surface crosslinking the water absorbent resin under drying or the water absorbent resin which has been dried; and a packaging step of packaging the surface crosslinked water absorbent resin, wherein an iron content in the aqueous monomer solution in the polymerization step is 2 ppm (relative to the monomer(s)) or less, a moisture content of the water absorbent resin in the packaging step is 1% by weight or more, and the method further comprises, after the packaging step, a storage step of storing the packaged water absorbent resin for 3 days or longer.

Claims

1. A method of producing a polyacrylic acid (salt)-based water absorbent resin for use in an absorbent article, the method comprising: a polymerization step of polymerizing an aqueous monomer solution containing acrylic acid (salt) as a main component; a drying step of drying a water-containing gel-like crosslinked polymer obtained in the polymerization step; a surface crosslinking step of surface crosslinking the water absorbent resin under drying or the water absorbent resin which has been dried, in which a proportion of particles of the water absorbent resin that do not pass through a sieve having a mesh size of 850 m is 0% to 5% by weight, relative to the total amount of the water absorbent resin; a packaging step of packaging the surface crosslinked water absorbent resin; and a storage step of storing the packaged water absorbent resin in a powder state for 3 days or longer until the packaged water absorbent resin is shipped to a user, wherein an iron content in the aqueous monomer solution in the polymerization step is 2 ppm or less, a moisture content of the water absorbent resin in the packaging step is 1% by weight or more, a packaging container used in the packaging step is a plastic container or a container having a plastic inner bag, the packaging container capable of transporting the water absorbent resin in a powder state, instead of transporting an absorbent article, in a unit amount of 15 kg to 10 t, and an amount of residual monomers of the water absorbent resin that has been subjected to the storage step is reduced by 0.5% to 30%, relative to an amount of residual monomers in the packaging step.

2. The method according to claim 1, wherein the packaged water absorbent resin is stored for 3 days to 100 days.

3. The method according to claim 1, wherein a content of p-methoxyphenol in the aqueous monomer solution in the polymerization step is 5 ppm to 160 ppm by weight, and a content of p-methoxyphenol in the water absorbent resin in the packaging step is 5 ppm to 60 ppm by weight.

4. The method according to claim 1, wherein a total content of acetic acid and propionic acid in the aqueous monomer solution in the polymerization step is 1500 ppm by weight or less.

5. The method according to claim 1, wherein the moisture content of the water absorbent resin in the packaging step is 3% to 20% by weight.

6. The method according to claim 1, further comprising, during or before the packaging step, a step of adding at least one or more aggregation preventing agents selected from the group consisting of polyvalent metal salts, water-insoluble fine particles, and surfactants to the water absorbent resin.

7. The method according to claim 1, further comprising, during or before the packaging step, a step of adding at least one or more coloration preventing agents selected from the group consisting of -hydroxycarboxylic acid compounds, inorganic reducing agents, and chelating agents.

8. The method according to claim 1, wherein a travel distance for the water absorbent resin in the storage step is 10 km or less.

9. The method according to claim 1, wherein in the surface crosslinking step, surface crosslinking is carried out by using a crosslinking agent other than an epoxy-based crosslinking agent.

10. The method according to claim 1, wherein in the surface crosslinking step, surface crosslinking is carried out by using a dehydration reactive surface crosslinking agent.

11. The method according to claim 1, wherein the storage step is carried out in a storage place equipped with an apparatus for controlling at least one or more of air temperature and humidity.

12. The method according to claim 1, wherein a temperature and relative humidity in a place for storing the packaged water absorbent resin in the storage step is 0 C. to 35 C. and the 10% to 90%, respectively.

13. The method according to claim 1, wherein physical properties of the water absorbent resin are measured at least one or more times during the storage step.

14. The method according to claim 1, wherein the aqueous monomer solution in the polymerization step contains ammonium acrylate in an amount of equal to or more than 1% by mole and less than 90% by mole relative to the total amount of the monomer(s).

15. The method according to claim 1, wherein the water absorbent resin is stored in the storage step until an amount of residual monomers decreases by 10 ppm or more, and the amount of residual monomers in the water absorbent resin after the storage step is 500 ppm or less.

16. The method according to claim 1, further comprising a step of producing acrylic acid, wherein the acrylic acid production step is connected via pipelines with the process for producing the water absorbent resin in which the polymerization step through the packaging step are substantially connected; acrylic acid and/or vapor generated in the acrylic acid production step is supplied to the process for producing the water absorbent resin by using the pipelines; and the packaging step is contiguous with the storage step.

17. The method according to claim 16, wherein the acrylic acid production step is a distillation and/or crystallization step to produce acrylic acid, and the acrylic acid thus obtained is supplied to the polymerization step within 30 days after its production.

18. A method of producing a polyacrylic acid (salt)-based water absorbent resin for use in an absorbent article, the method comprising: a polymerization step of polymerizing an aqueous monomer solution containing acrylic acid (salt) as a main component; a drying step of drying a water-containing gel-like crosslinked polymer obtained in the polymerization step; a surface crosslinking step of surface crosslinking the water absorbent resin under drying or the water absorbent resin which has been dried, in which a proportion of particles of the water absorbent resin that do not pass through a sieve having a mesh size of 850 m is 0% to 5% by weight, relative to the total amount of the water absorbent resin; a packaging step of packaging the surface crosslinked water absorbent resin; and a storage step of storing the packaged water absorbent resin in a powder state at a temperature of 0 C. to 60 C. and at a relative humidity of 10% to 90% for 3 days or longer until the packaged water absorbent resin is shipped to a user, wherein an iron content in the aqueous monomer solution in the polymerization step is 2 ppm or less, a moisture content of the water absorbent resin in the packaging step is 1% by weight or more, a packaging container used in the packaging step is a plastic container or a container having a plastic inner bag, the packaging container capable of transporting the water absorbent resin in a powder state, instead of transporting an absorbent article, in a unit amount of 15 kg to 10 t, and an amount of residual monomers of the water absorbent resin that has been subjected to the storage step is reduced by 0.5% to 30%, relative to an amount of residual monomers in the packaging step.

19. The method of claim 18, wherein the packaged water absorbent resin is stored at a temperature of 20 C. to 30 C. and at a relative humidity of 30% to 70%.

20. The method according to claim 1, wherein the moisture content of the water absorbent resin in the packaging step is 10% to 20% by weight.

Description

EXAMPLES

(1) Hereinafter, the present invention will be more specifically described by way of Examples and Comparative Examples, but the present invention is not construed to be limited to these, and embodiments obtainable by appropriately combining the respective technical means disclosed in different Examples are also intended to be included in the scope of the present invention. Furthermore, for convenience, the unit liter may be indicated as L, and the unit % by weight as wt %.

(2) Meanwhile, unless particularly stated otherwise, an electrical equipment used in Examples and Comparative Examples used a power supply of 200 V or 100 V. Furthermore, unless particularly stated otherwise, various properties of the water absorbent resin of the present invention were measured under conditions of room temperature (20 C. to 25 C.) and humidity of 50 RH %. Also, the phrase measured quickly after sampling means that measurement operation was initiated within 1 to 3 hours, particularly within 1 hour, after sampling of a water absorbent resin.

(3) [Method for Measuring Physical Properties]

(4) (a) CRC (Absorption Capacity without Load)

(5) This was measured according to ERT441.2-02.

(6) 0.2 g (designated as weight WO [g]) of a water absorbent resin was weighed and uniformly placed in a bag (6060 mm) made of a non-woven fabric. The bag was heat sealed, and then the bag was immersed in 500 mL of a 0.9 wt % aqueous solution of sodium chloride regulated at 253 C. After 60 minutes passed, the bag was pulled up, and dehydration was carried out by using a centrifuge (centrifuge manufactured by Kokusan Co., Ltd., Model: H-122) under the conditions of 250G for 3 minutes. Thereafter, weight W1 [g] of the bag was measured.

(7) Furthermore, the same operation was carried out without placing the water absorbent resin, and weight W2 [g] of the bag at that time was measured. CRC (absorption capacity without load) was calculated from WO [g], W1 [g], and W2 [g] thus obtained according to the following formula.
CRC[g/g]={(W1W2)/W0}1[Mathematical formula 1]

(8) (b) AAP (Absorption Capacity Under Load)

(9) The AAP was measured according to the measurement method defined in ERT442.2-02, except that the load was changed to 4.83 kPa (0.7 psi).

(10) (c) Ext (Extractables)

(11) In accordance of ERT470.2-02, 1.000 g of a water absorbent resin was added to 200 ml of a 0.9 wt % aqueous solution of sodium chloride, and an amount of dissolved polymer (unit: % by weight) obtained after stirring for 16 hours was measured by pH titration.

(12) (d) RM (Residual Monomers)

(13) In accordance of ERT410.2-02, 1.0 of a water absorbent resin was added to 200 ml of a 0.9 wt % aqueous solution of sodium chloride, and an amount of dissolved monomers (unit: ppm) obtained after stirring for one hour at 500 rpm was measured by using HPLC (high performance liquid chromatography).

(14) (e) PSD (Particle Size Distribution) and D50 (Weight Average Particle Size)

(15) The PSD (particle size distribution) and D50 (weight average particle size) were measured by classifying a sample with standard sieves.

(16) That is, 10.0 g of a water absorbent resin was placed on JIS standard sieves (The IIDA TESTING SIEVE: inner diameter 80 mm) having mesh sizes of 2000 m, 1400 m, 1000 m, 850 m, 710 m, 600 m, 500 m, 425 m, 300 m, 212 m, 150 m, and 45 m under conditions of room temperature (20 C. to 25 C.) and humidity of 50 RH %, and the water absorbent resin was classified for 5 minutes in a RO-TAP sieve shaker (manufactured by Iida Seisakusho Japan Corp.; ES 65 type sieve shaker; SER. No. 0501), prior to measurement of PSD (particle size distribution).

(17) Furthermore, D50 (weight average particle size) means, as disclosed in U.S. Pat. No. 5,051,259, and the like, a particle size of a standard sieve corresponding to 50% by weight of all the particles. That is, a particle size distribution obtained by PSD (particle size distribution) measurement described above was used, and residual percentages (R) of various particle sizes were plotted on a logarithmic probability paper. From this graph, a particle size corresponding to R=50% by weight was read as D50 (weight average particle size).

(18) (f) SFC (Saline Flow Conductivity)

(19) The SFC was measured according to the method disclosed in U.S. Pat. No. 5,849,405.

(20) (g) Initial Color Tone and Color Tone Over Time

(21) In the present invention, color tone of a water absorbent resin was carried out with a Hunter's Lab color system. Furthermore, a spectrophotometric colorimeter, SZ-E80, manufactured by Nippon Denshoku Industries Co., Ltd. was used as a measuring apparatus (spectrophotometric colorimeter), and reflection measurement was selected as measurement conditions. Furthermore, a container for powder/paste sample (inner diameter: 30 mm, height: 12 mm), a standard round whiteboard for powder/paste No. 2, and a 30 translucent pipe were used.

(22) About 5 g of a water absorbent resin was packed in the container for the powder/paste sample, and in an atmosphere at room temperature (20 C. to 25 C.) and relative humidity of 50 RH %, L value, a value, and b value on a surface of the water absorbent resin were measured.

(23) In the present invention, color tone of a water absorbent resin immediately after production, or a water absorbent resin whose storage period in an atmosphere at air temperature of 30 C. or lower and relative humidity of 50 RH % or less was one year or less after production, was designated as initial color tone. L value measured at this time was designated as the lightness index before exposure.

(24) Furthermore, the following operation was carried out as a coloration acceleration test, and lightness index after exposure was measured.

(25) The coloration acceleration test was carried out by placing a container for powder/paste sample packed with about 5 g of a water absorbent resin, in a constant temperature constant humidity chamber (small-sized environmental testing machine manufactured by Espec Corp.; Model SH-641) that had been adjusted to an atmosphere at a temperature of 701 C. and relative humidity of 651 RH %, and exposing the water absorbent resin to this high temperature high humidity atmosphere for 7 days.

(26) Color tone for the water absorbent resin after exposure was designated as color tone over time, and L value measured at this time was designated as lightness index after exposure.

(27) Meanwhile, as L value was closer to 100, a degree of whiteness increased, and as a value and b value were closer to 0 (zero), a water absorbent resin was less colored and became whiter.

(28) (h) Moisture Content

(29) In an aluminum cup having a diameter of bottom of about 50 mm, 1.00 g of a water absorbent resin was weighed, and a total weight W8 [g] of a sample (the water absorbent resin and the aluminum cup) was measured.

(30) Subsequently, the sample was left to stand in an oven at atmosphere temperature of 180 C., and thus the water absorbent resin was dried. After 3 hours, the sample was taken out from the oven, and was cooled to room temperature in a desiccators. Thereafter, a total weight W9 [g] of the sample (the water absorbent resin and the aluminum cup) after drying was measured, a the moisture content (unit: [wt %]) was calculated according to the following formula.
Moisture content [wt %]=(W8W9)/(weight of water absorbent resin)100[Mathematical formula 2]

(31) Meanwhile, in the case of a water absorbent resin containing polyacrylic acid ammonium salt, ammonia is liberated at a high temperature. Therefore, the measurement is carried out except that vacuum drying was carried out with atmosphere temperature changed from 180 C. to 105 C. instead.

(32) (i) Amount of p-Methoxyphenol Contained in Water Absorbent Resin

(33) An amount of p-methoxyphenol contained in a water absorbent resin of the present invention can be determined by analyzing a filtrate obtained by performing the same operation as in the measurement method for the (c) Ext (Extractables), except that the stirring time is changed from 16 hours to 1 hour. Specifically, when the filtrate obtained by the relevant operation is analyzed by high performance liquid chromatography, a content of p-methoxyphenol in a water absorbent resin can be determined. Meanwhile, the content of p-methoxyphenol is expressed in ppm (with respect to the water absorbent resin).

(34) (j) Amount of Reducing Agent (Sodium Hydrogen Sulfite) Contained in Water Absorbent Resin

(35) 50 g of pure water and 0.5 g of a water absorbent resin were placed in a 200-ml beaker, and the mixture was left to stand for one hour. Subsequently, 50 g of methanol was added thereto, and then 2.5 g of a solution prepared by dissolving 2 mmol of Malachite Green in an eluent that will be described below was added thereto. This solution was stirred for about 30 minutes and then filtered, and the filtrate was analyzed by high performance liquid chromatography, to determine an amount of a reducing agent contained in the water absorbent resin. Meanwhile, the eluent is prepared by mixing 400 ml of methanol, 6 ml of n-hexane, and 100 ml of 0.01 mol/l 2-N-morpholino-ethanesulfonic acid, sodium salt. Furthermore, a calibration curve was produced by analyzing samples prepared by spiking a reducing agent into a water absorbent resin that did not contain a reducing agent.

(36) (k) Iron Content in Aqueous Monomer Solution

(37) An aqueous monomer solution was diluted to about 100 times with ultrapure water, and an iron content was measured by an inductively coupled plasma (ICP) emission analysis method. An iron content was measured also for ultrapure water by an inductively coupled plasma (ICP) emission analysis method, and this value was used as a blank value.

(38) (l) Iron Content in Water Absorbent Resin

(39) 1.000 g of a water absorbent resin was weighed in a platinum crucible, and the water absorbent resin was incinerated by using an electric furnace (manufactured by Yamato Scientific Co., Ltd.; Muffle Furnace FO300). Subsequently, 5 ml of an aqueous nitric acid solution (manufactured by Wako Pure Chemical Industries, Ltd.; an aqueous solution prepared by mixing special grade nitric acid and ultrapure water at 1:1) was added to the platinum crucible taken out from the electric furnace, to dissolve the incineration product therein. Subsequently, 15 ml of ultrapure water was added thereto, to yield an aqueous solution of incineration product. Furthermore, the same operation as described above was carried out without introducing the water absorbent resin, to obtain a blank aqueous solution. The aqueous solutions obtained by the above operations were subjected to analysis by inductively coupled plasma (ICP) emission analysis method described in JIS K1200-6, to determine an iron content in the water absorbent resin.

(40) Hereinbelow, in Examples and Comparative Examples, acrylic acid having a moisture content of 900 ppm, and as an impurity, an acetic acid content of 770 ppm and a propionic acid content of 130 m was used, unless particularly stated otherwise. Furthermore, acrylic acid obtained in final purification step for acrylic acid was used within several hours (substantially no storage time).

Comparative Example 1

(41) 29.97 parts by weight of a 48.5 wt % aqueous solution of sodium hydroxide (containing 0.7 ppm of an iron component relative to sodium hydroxide), 35.87 parts by weight of acrylic acid (containing 70 ppm of p-methoxyphenol as a polymerization inhibitor), 0.78 part by weight of a 30 wt % aqueous solution of polyethylene glycol diacrylate (average molecular weight: 523) as an internal crosslinking agent, 0.88 part by weight of a 1 wt % aqueous solution of trisodium diethylenetriamine pentaacetate as a chelating agent, and 32.50 parts by weight of deionized water were supplied to a mixer, to prepare an aqueous monomer solution. At this time, the temperature of the aqueous monomer solution was 95 C. In the aqueous monomer solution, the amount of acetic acid was 630 ppm relative to the monomer(s), the amount of propionic acid was 110 ppm relative to the monomer(s), and the amount of iron component was 0.25 ppm relative to the monomer(s). The amount of p-methoxyphenol was 57 ppm relative to the monomer(s).

(42) Next, 1.99 parts by weight of a 3.0 wt % aqueous solution of sodium persulfate was added as a polymerization initiator to the resultant aqueous monomer solution, and polymerization was carried out, to obtain a sheet-like water-containing gel-like crosslinked polymer (hereinafter, also referred to as water-containing gel).

(43) The sheet-like water-containing gel thus obtained was continuously crushed by using a meat chopper (manufactured by Hiraga Kousakusho Co., Ltd.) having a screen with a diameter of 7.5 mm, to obtain a particulate water-containing gel. At this time, the moisture content of the particulate water-containing gel was 50% by weight.

(44) Subsequently, the particulate water-containing gel thus obtained was dried at 170 C. for 20 minutes by using a hot air circulation type dryer, to obtain a dried polymer. The dried polymer thus obtained was pulverized with a roll mill, and was further classified by using sieves having mesh sizes of 850 m and 150 m, to obtain a particulate water absorbent resin having a particle size of equal to or greater than 150 m and less than 850 m. The moisture content of the particulate water absorbent resin was 5.8% by weight, and the absorption capacity without load (CRC) was 31 [g/g].

(45) Subsequently, an aqueous solution of surface crosslinking agent containing 0.1 part by weight of ethylene glycol diglycidyl ether (product name: DENACOL 810) and 5 parts by weight of deionized water was added to 100 parts by weight of the resultant particulate water absorbent resin, and the mixture was mixed and heat treated for 30 minutes at 120 C. Subsequently, the mixture was cooled by standing for one hour, to obtain a water absorbent resin (a1). For the water absorbent resin (a1), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resin (a1) was 27 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 24 [g/g]. Furthermore, the iron content was 0.25 ppm, and the p-methoxyphenol content was 15 ppm.

Example 1

(46) The water absorbent resin (a1) obtained in Comparative Example 1 was packaged in an amount of 25 kg per paper bag, and the packages were stored for 38 days in a warehouse in the plant premise (the travel distance from the packaging place was 50 m). The container used for the packaging was a paper bag formed from an inner plastic bag and an outer paper bag, and the paper bag can be sealed by tying the tip of the inner bag. Furthermore, the environment inside the warehouse in the storage was controlled with an air conditioner to be at an air temperature of 20 C. to 30 C. and a relative humidity of 30% to 70%.

(47) For a water absorbent resin (A1) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resin (A1) was 27 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 24 [g/g]. Furthermore, the iron content was 0.25 ppm, and the p-methoxyphenol content was 15 ppm.

Comparative Example 2

(48) A water absorbent resin (a2) was obtained by the same operations as in Comparative Example 1, except that the drying temperature was changed from 170 C. to 160 C. For the water absorbent resin (a2), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Furthermore, the absorption capacity without load (CRC) of the water absorbent resin (a2) was 26 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 22 [g/g].

(49) Furthermore, the moisture content of the particulate water absorbent resin before surface crosslinking was 6.8% by weight, and the absorption capacity without load (CRC) was 28 [g/g].

Example 2

(50) The water absorbent resin (a2) obtained in Comparative Example 2 was packaged in the same manner as in Example 1 in an amount of 25 kg per paper bag, and the packages were stored for 38 days in a warehouse within the plant premise (the travel distance from the packaging place was 50 m). The environment inside the warehouse in the storage was also the same as in Example 1.

(51) For a water absorbent resin (A2) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resin (A2) was 26 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 22 [g/g].

Comparative Example 3

(52) 29.97 parts by weight of a 48.5 wt % aqueous solution of sodium hydroxide (containing 0.7 ppm of an iron component relative to sodium hydroxide), 35.87 parts by weight of acrylic acid (containing 70 ppm of p-methoxyphenol as a polymerization inhibitor), 0.78 part by weight of a 30 wt % aqueous solution of polyethylene glycol diacrylate (average molecular weight: 523) as an internal crosslinking agent, 0.88 part by weight of a 1 wt % aqueous solution of trisodium diethylenetriamine pentaacetate as a chelating agent, and 32.50 parts by weight of deionized water were supplied to a mixer, to prepare an aqueous monomer solution. At this time, the temperature of the aqueous monomer solution first increased up to 95 C. and then decreased to 85 C. Meanwhile, the amount of acetic acid in the aqueous monomer solution was 630 ppm relative to the monomer(s), the amount of propionic acid was 110 ppm relative to the monomer(s), and the amount of iron component was 0.25 ppm relative to the monomer(s).

(53) Subsequently, 1.99 parts by weight of a 3.0 wt % aqueous solution of sodium persulfate was added as a polymerization initiator to the resultant aqueous monomer solution, and polymerization was carried out, to obtain a sheet-like water-containing gel.

(54) The sheet-like water-containing gel thus obtained was continuously crushed by using a meat chopper (manufactured by Hiraga Kousakusho Co., Ltd.) having a screen with a diameter of 7.5 mm, and thus a particulate water-containing gel was obtained. At this time, the moisture content of the particulate water-containing gel was 50% by weight.

(55) Subsequently, the particulate water-containing gel thus obtained was dried at 180 C. for 40 minutes by using a hot air circulation type dryer, to obtain a dried polymer. The dried polymer thus obtained was pulverized with a roll mill, and was further classified by using sieves having mesh sizes of 850 m and 150 m, to obtain a particulate water absorbent resin having a particle size of equal to or greater than 150 m and less than 850 m. The moisture content of the resultant particulate water absorbent resin was 5.0% by weight, and the absorption capacity without load (CRC) was 34 [g/g].

(56) Subsequently, an aqueous solution of surface crosslinking agent containing 0.3 part by weight of 1,4-butanediol, 3 parts by weight of deionized water and 0.01 part by weight of polyoxyethylene sorbitan monostearate as a surfactant was added to 100 parts by weight of the resultant particulate water absorbent resin, and the mixture was mixed and heat treated for 30 minutes at 200 C. Subsequently, the mixture was cooled by standing for one hour, to obtain a water absorbent resin (a3). For the water absorbent resin (a3), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resin (a3) was 28 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 24 [g/g]. Furthermore, the iron content was 0.25 ppm, and the p-methoxyphenol content was 12 ppm.

Comparative Example 4

(57) The water absorbent resin (a3) obtained in Comparative Example 3 was packaged in an amount of 20 kg per container in a hard polyethylene container (holding 20 kg) having a capacity of 30 L, and the packages were stored for 90 days in a warehouse within the same plant premise as that used in Example 1. The container used for packaging was a hard polyethylene container and could be sealed with a lid. Furthermore, the environment inside the warehouse in the storage was controlled to be at an air temperature of 20 C. to 30 C. and a relative humidity of 30% to 60%.

(58) For a water absorbent resin (a4) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resin (a4) was 28 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 24 [g/g]. Furthermore, the iron content was 0.25 ppm.

Comparative Example 5

(59) While the water absorbent resin (a3) thus obtained in Comparative Example 3 was stirred, 0.1 part by weight of silicon dioxide (product name: AEROSIL 200) was added thereto as inorganic fine particles, and an aqueous liquid formed from 10 parts by weight of deionized water and 1 part by weight of propylene glycol as a mixing aid was added thereto to adjust a moisture content. The mixture was left to stand for one hour, to obtain a water absorbent resin (a5). For the water absorbent resin (a5), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resin (a5) was 26 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 23 [g/g]. Furthermore, the iron content was 0.23 ppm.

Comparative Example 6

(60) The water absorbent resin (a5) obtained in Comparative Example 5 was packaged in an amount of 20 kg per container in the hard polyethylene container used in Comparative Example 4, and the packages were stored for one day in a warehouse in the same plant premise as that used in Example 1. The container used for packaging was a hard polyethylene container and could be sealed with a lid. Furthermore, the environment inside the warehouse in the storage was controlled to be at an air temperature of 20 C. to 30 C. and a relative humidity of 30% to 60%.

(61) For a water absorbent resin (a6) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resin (a6) was 26 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 23 [g/g]. Furthermore, the iron content was 0.23 ppm.

Examples 3 to 6

(62) Water absorbent resins (A3 to A6) were obtained by the same operations as in Comparative Example 6, except that the period for storage in the warehouse was changed to 3 days (Example 3), 9 days (Example 4), 13 days (Example 5), and 85 days (Example 6)

(63) For water absorbent resins (A3 to A6) obtained after the passage of the storage periods, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resins (A3 to A6) was 26 [g/g] in all cases. The p-methoxyphenol content of the water absorbent resins (A3 to A6) was 10 ppm.

Comparative Example 7

(64) A water absorbent resin (a7) was obtained by the same operations as in Comparative Example 6, except that the period for storage in the warehouse was changed to 125 days.

(65) For the water absorbent resin (a7) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, it was observed that a portion of the water absorbent resin (a7) aggregated, and fluidity was decreased. Furthermore, the water absorbent resin turned yellow as compared with the water absorbent resins (A3 to A6).

Comparative Example 8

(66) A water absorbent resin (a8) was obtained by the same operations as in Comparative Example 5, except that 0.02 parts by weight of sodium hydrogen sulfite as a reducing agent was further added to the aqueous liquid formed from deionized water and propylene glycol. For the water absorbent resin (a8), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table.

Comparative Example 9

(67) The water absorbent resin (a8) obtained in Comparative Example 8 was packaged in an amount of 20 kg per container in the hard polyethylene container used in Comparative Example 4, and the packages were stored for one day in a warehouse in the same plant premise as that used in Example 1. The container used for packaging was a hard polyethylene container and could be sealed with a lid. Furthermore, the environment inside the warehouse in the storage was controlled to be at an air temperature of 20 C. to 30 C. and a relative humidity of 30% to 60%.

(68) For a water absorbent resin (a9) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the p-methoxyphenol content of the water absorbent resin (a9) was 7 ppm.

Examples 7 to 9

(69) Water absorbent resins (A7 to A9) were obtained by the same operations as in Comparative Example 8, except that the period for storage in the warehouse was changed to 3 days (Example 7), 6 days (Example 8), and 13 days (Example 9), and thus.

(70) For the water absorbent resins (A7 to A9) obtained after the passage of the storage periods, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table.

Comparative Example 10

(71) 46 parts by weight of 28 wt % aqueous ammonia (containing 0.1 ppm of an iron component), 49 parts by weight of a 48.5 wt % aqueous solution of sodium hydroxide (containing 0.7 ppm of an iron component relative to sodium hydroxide), 144 parts by weight of acrylic acid (containing 70 ppm of p-methoxyphenol as a polymerization inhibitor), 1.0 part by weight of a 10 wt % aqueous solution of polyethylene glycol diacrylate (average molecular weight: 523) as an internal crosslinking agent, 3.4 parts by weight of a 1 wt % aqueous solution of trisodium diethylenetriamine pentaacetate as a chelating agent, and 32 parts by weight of deionized water were supplied to a mixer, to prepare an aqueous monomer solution. At this time, the temperature of the aqueous monomer solution was 95 C. In the aqueous monomer solution, the amount of iron component was 0.15 ppm relative to the monomer(s), the amount of acetic acid was 650 ppm relative to the monomer(s), and the amount of propionic acid was 110 ppm relative to the monomer(s).

(72) Subsequently, 6 parts by weight of a 1.0 wt % aqueous solution of 2,2-azobis(2-amidinopropane)dihydrochloride as a polymerization initiator was added to the resultant aqueous monomer solution, and polymerization was carried out, to obtain a sheet-like water-containing gel.

(73) The sheet-like water-containing gel thus obtained was continuously crushed by using a meat chopper (manufactured by Hiraga Kousakusho Co., Ltd.) having a screen with a diameter of 7.5 mm, to obtain a particulate water-containing gel. At this time, the moisture content of the particulate water-containing gel was 32% by weight.

(74) Subsequently, the particulate water-containing gel thus obtained was dried at 170 C. for 20 minutes by using a hot air circulation type dryer, to obtain a dried polymer. The dried polymer thus obtained was pulverized with a roll mill, and was further classified by using sieves having mesh sizes of 850 m and 150 m, to obtain a particulate water absorbent resin having a particle size of equal to or greater than 150 m and less than 850 m. The moisture content of this particulate water absorbent resin was 6.2% by weight, and the absorption capacity without load (CRC) was 40 [g/g].

(75) Subsequently, an aqueous solution of surface crosslinking agent containing 0.1 part by weight of ethylene glycol diglycidyl ether (product name: DENACOL EX810) and 5 parts by weight of deionized water was added to 100 parts by weight of the resultant particulate water absorbent resin, and the mixture was mixed and heat treated for 30 minutes at 120 C. Subsequently, the mixture was cooled by standing for one hour, to obtain a water absorbent resin (a10). For the water absorbent resin (a10), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the iron content of the water absorbent resin (a10) was 0.15 ppm, and the p-methoxyphenol content was 15 ppm.

Example 10

(76) The water absorbent resin (a10) obtained in Comparative Example 10 was packaged in an amount of 20 kg per container in the hard polyethylene container used in Comparative Example 4, and the packages were stored for 35 days in a warehouse in the same plant premise as that used in Example 1. The container used for packaging was a hard polyethylene container and could be sealed with a lid. Furthermore, the environment inside the warehouse at the time of storage was controlled to be at an air temperature of 20 C. to 30 C. and a relative humidity of 30% to 60%.

(77) For a water absorbent resin (A10) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table.

Comparative Example 11

(78) 100 parts by weight of 28 wt % aqueous ammonia (containing 0.1 ppm of an iron component), 216 parts by weight of acrylic acid (containing 70 ppm of p-methoxyphenol as a polymerization inhibitor), 1.57 parts by weight of a 10 wt % aqueous solution of polyethylene glycol diacrylate (average molecular weight: 523) as an internal crosslinking agent, 4.93 parts by weight of a 1 wt % aqueous solution of trisodium diethylenetriamine pentaacetate as a chelating agent, and 79 parts by weight of deionized water were supplied to a mixer, to prepare an aqueous monomer solution. At this time, the temperature of the aqueous monomer solution was 95 C. In the aqueous monomer solution, the amount of iron component was 0.05 ppm relative to the monomer(s), the amount of acetic acid was 680 ppm relative to the monomer(s), and the amount of propionic acid was 120 ppm relative to the monomer(s).

(79) Subsequently, 9 parts by weight of a 1.0 wt % aqueous solution of 2,2-azobis(2-amidinopropane)dihydrochloride as a polymerization initiator was added to the resultant aqueous monomer solution, and polymerization was carried out, to obtain a sheet-like water-containing gel.

(80) The sheet-like water-containing gel thus obtained was continuously crushed by using a meat chopper (manufactured by Hiraga Kousakusho Co., Ltd.) having a screen with a diameter of 7.5 mm, to obtain a particulate water-containing gel. At this time, the moisture content of the particulate water-containing gel was 30% by weight.

(81) Subsequently, the particulate water-containing gel thus obtained was dried at 170 C. for 20 minutes by using a hot air circulation type dryer, to obtain a dried polymer. The dried polymer thus obtained was pulverized with a roll mill, and was further classified by using sieves having mesh sizes of 850 m and 150 m, to obtain a particulate water absorbent resin having a particle size of equal to or greater than 150 m and less than 850 m. The moisture content of the resultant particulate water absorbent resin was 6.0% by weight, and the absorption capacity without load (CRC) was 32 [g/g].

(82) Subsequently, an aqueous solution of surface crosslinking agent containing 0.1 part by weight of ethylene glycol diglycidyl ether (product name: DENACOL EX810) and 5 parts by weight of deionized water was added to 100 parts by weight of the resultant particulate water absorbent resin, and the mixture was mixed and heat treated for 30 minutes at 120 C. Subsequently, the mixture was cooled by standing for one hour, to obtain a water absorbent resin (a11). For the water absorbent resin (a11), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the iron content of the water absorbent resin (a11) was 0.05 ppm, and the p-methoxyphenol content was 17 ppm.

Comparative Example 12

(83) The water absorbent resin (a11) obtained in Comparative Example 11 was packaged in an amount of 20 kg per container in the hard polyethylene container used in Comparative Example 4, and the packages were stored for one day in a warehouse in the same plant premise as that used in Example 1. The container used for packaging was a hard polyethylene container and could be sealed with a lid. Furthermore, the environment inside the warehouse in the storage was controlled to be at an air temperature of 20 C. to 30 C. and a relative humidity of 30% to 60%.

(84) For a water absorbent resin (a12) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table.

Examples 11 to 14

(85) Water absorbent resins (A11 to A14) were obtained by the same operations as in Comparative Example 12, except that the period for storage in the warehouse was changed to 3 days (Example 11), 7 days (Example 12), 14 days (Example 13), and 79 days (Example 14).

(86) For the water absorbent resins (A11 to A14) obtained after the passage of the storage periods, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table.

Comparative Example 13

(87) A water absorbent resin (a13) was obtained by the same operations as in Comparative Example 12, except that the period for storage in the warehouse was changed to 120 days.

(88) For the water absorbent resin (a13) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, it was observed that a portion of the water absorbent resin (a13) aggregated, and fluidity was decreased.

Example 15

(89) A water absorbent resin (A15) was obtained by the same operations as in Example 14, except that the period for storage in the warehouse was extended by one day to be 80 days. At this time, the temperature inside the warehouse during the extended storage period temporarily increased up to 40 C.

(90) For the water absorbent resin (A15) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, it was observed that a portion of the water absorbent resin (A15) aggregated, and fluidity was decreased.

Example 16

(91) A water absorbent resin (A16) was obtained by the same operations as in Example 15, except that 0.1 part by weight of silicon dioxide (product name: AEROSIL 200) as inorganic fine particles was added and mixed after the surface crosslinking step.

(92) For the water absorbent resin (A16) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, although the temperature inside the warehouse temporarily reached 40 C. during the storage period, aggregation of the water absorbent resin (A16) was not observed.

Comparative Example 14

(93) A water absorbent resin (a14) was obtained by the same operations as in Example 5, except that an aqueous solution of sodium hydroxide containing 8 ppm of an iron component was used in place of the aqueous solution of sodium hydroxide containing 0.7 ppm (relative to sodium hydroxide) of an iron component. Meanwhile, the amount of iron component in the aqueous monomer solution was 2.3 ppm relative to the monomer(s).

(94) For the water absorbent resin (a14) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, in the water absorbent resin (a14), yellowing was recognized by visual inspection as compared with the water absorbent resin (A5).

Example 17

(95) A water absorbent resin (A17) was obtained by the same operations as in Example 1, except that each of acetic acid and propionic acid were added to the raw material acrylic acid so as to give an amount of 1000 ppm. Meanwhile, the addition of the acetic acid and propionic acid was intended for assumption of acrylic acid obtained by a different purification method.

(96) For the water absorbent resin (A17) obtained after the passage of the storage period, the packaging container was opened at the time of sampling, and unpleasant odor (acid odor) was detected. Furthermore, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table.

Comparative Example 15

(97) A water absorbent resin (a15) was obtained by the same operations as in Comparative Example 1, except that acrylic acid having a p-methoxyphenol content of 200 ppm was used in place of acrylic acid having a p-methoxyphenol content of 70 ppm. Meanwhile, the amount of p-methoxyphenol in the aqueous monomer solution was 164 ppm relative to the monomer(s), and although the polymerization time was slightly delayed in the polymerization step, polymerization was carried out without any problem.

(98) For the water absorbent resin (a15), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the water absorbent resin (a15) was colored light brown, and the degree of whiteness was poor. Furthermore, the iron content of the water absorbent resin (a15) was 0.25 ppm, and the p-methoxyphenol content was 65 ppm.

Example 18

(99) The water absorbent resin (a15) obtained in Comparative Example 15 was packaged in an amount of 25 kg per paper bag in the same manner as in Example 1, and the packages were stored for 38 days in a warehouse within the plant premise (the travel distance from the packaging place was 50 m). Meanwhile, the environment inside the warehouse in the storage was also the same as that in Example 1.

(100) For a water absorbent resin (A18) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table.

Example 19

(101) The same operations as in Example 1 were carried out, except that a paper bag which did not have a vinyl inner bag was used as the container for packaging, and the packages were stored for 38 days in a warehouse within the plant premise (the travel distance from the packaging place was 50 m). Meanwhile, the environment inside the warehouse at the time of storage was the same as that in Example 1.

(102) For a water absorbent resin (A19) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, in the water absorbent resin (A19), a tendency of deterioration of fluidity was confirmed.

Example 20

(103) The same operations as in Example 1 were carried out, except that the storage place for the water absorbent resin was changed to a warehouse located at a travel distance of 100 km from the packaging place (transported by a truck), and the packages were stored for 38 days in that warehouse. Meanwhile, the environment inside the warehouse at the time of storage was the same as that in Example 1.

(104) For a water absorbent resin (A20) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, it was observed that in the water absorbent resin (A20), severe segregation occurred, and the water absorbent resin fine powder aggregated in the vicinity of the bottom of the packaging containers.

Comparative Example 16

(105) 27.24 parts by weight of a 48.5 wt % aqueous solution of sodium hydroxide (containing 0.7 ppm of an iron component relative to sodium hydroxide), 31.74 parts by weight of acrylic acid (containing 70 ppm of p-methoxyphenol as a polymerization inhibitor), 1.61 parts by weight of a 10 wt % aqueous solution of polyethylene glycol diacrylate (average molecular weight: 523) as an internal crosslinking agent, and 38.54 parts by weight of deionized water were supplied to a mixer, to prepare an aqueous monomer solution, and the resultant aqueous monomer solution was cooled. At this time, the temperature of the aqueous monomer solution was 30 C. Meanwhile, in the aqueous monomer solution, the amount of acetic acid was 630 ppm relative to the monomer(s), the amount of propionic acid was 110 ppm relative to the monomer(s), and the amount of iron component was 0.25 ppm relative to the monomer(s). The amount of p-methoxyphenol was found by calculation to be 57 ppm relative to the monomer(s).

(106) In a kneader equipped with two sigma-shaped blades, the aqueous monomer solution was introduced, and nitrogen gas was blown into the aqueous monomer solution, to reduce dissolved oxygen in the aqueous monomer solution, and at the same time, to replace the entirety of the interior of the kneader with nitrogen. Subsequently, while the blades of the kneader were rotated, cold water at 10 C. was circulated through the jacket, so as to adjust the temperature of the aqueous monomer solution to 20 C.

(107) Subsequently, 0.734 part by weight of a 3 wt % aqueous solution of sodium persulfate as a polymerization initiator, and 0.132 part by weight of 1 wt % L-ascorbic acid were added to the aqueous monomer solution, and polymerization was initiated. The system was further stirred for 30 minutes, and was aged, to obtain as a polymerization product a water-containing gel-like polymer having a weight average particle size (D50) of about 2.0 mm.

(108) Subsequently, the water-containing gel-like polymer thus obtained was dried at 170 C. for 20 minutes by using a hot air circulation type dryer, to obtain a dried polymer. The dried polymer thus obtained was pulverized with a roll mill, and was further classified by using sieves having mesh sizes of 850 m and 180 m, to obtain a particulate water absorbent resin having a particle size of equal to or greater than 180 m and less than 850 m. The moisture content of the resultant particulate water absorbent resin was 5.1% by weight, and the absorption capacity without load (CRC) was 33 [g/g].

(109) Subsequently, an aqueous solution of surface crosslinking agent containing 0.1 part by weight of ethylene glycol diglycidyl ether (product name: DENACOL 810) and 5 parts by weight of deionized water was added to 100 parts by weight of the resultant particulate water absorbent resin, and the mixture was mixed and heat treated for 30 minutes at 120 C. Subsequently, the mixture was cooled by standing for one hour, to obtain a water absorbent resin (a16). For the water absorbent resin (a16), the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. Meanwhile, the absorption capacity without load (CRC) of the water absorbent resin (a16) was 28 [g/g], and the absorption capacity under load (AAP 0.7 psi) thereof was 23 [g/g]. Furthermore, the iron content was 0.25 ppm, and the p-methoxyphenol content was 13 ppm.

Example 21

(110) The water absorbent resin (a16) obtained in Comparative Example 16 was packaged in an amount of 20 kg per container in a hard polyethylene container in the same manner as in Example 6, and the packages were stored for 85 days in a warehouse within the plant premise (the travel distance from the packaging place was 50 m). Furthermore, the environment inside the warehouse in the storage was also the same as that in Example 6.

(111) For a water absorbent resin (A21) obtained after the passage of the storage period, the amount of residual monomers and the like were measured quickly after sampling. The results are presented in the following table. It was confirmed that the resultant water absorbent resin (A21) turned yellow during the storage.

Examples 22 to 24

(112) Water absorbent resins (A22) to (A24) were obtained by the same operations as in Example 1, except that acrylic acid which was obtained by storing acrylic acid obtained in the final purification step for acrylic acid for 30 days (Example 22), 10 days (Example 23), and one day (Example 24) was used as a raw material acid for the water absorbent resin.

(113) The residual monomers in the water absorbent resins (A22) to (A24) thus obtained were measured, to be confirmed that the amount of residual monomers increased by about 20 ppm per day during the storage of acrylic acid.

Examples 25 to 27

(114) Water absorbent resins (A25) to (A27) were obtained by the same operations as in Examples 22 to 24, except that instead of acrylic acid (100 wt %), a 80 wt % aqueous solution of acrylic acid was used. Meanwhile, the acrylic acid was stored in a state of a 80 wt % aqueous solution of acrylic acid.

(115) The residual monomers in the water absorbent resins (A25) to (A27) thus obtained were measured, to be confirmed that the amount of residual monomers increased by about 50 ppm per day during the storage of the aqueous solution of acrylic acid.

Example 28

(116) A water absorbent resin (A28) was obtained by the same operations as in Example 5, except that an aqueous liquid formed from 18 parts by weight of deionized water and 1 part by weight of propylene glycol as a mixing aid was used in place of the aqueous liquid formed from 10 parts by weight of deionized water and 1 part by weight of propylene glycol as a mixing aid. The absorption capacity without load (CRC) of the water absorbent resin (A28) thus obtained was 24 [g/g], and the absorption capacity under load (AAP 0.7 psi) was 21 [g/g]. Furthermore, the amount of residual monomers was 268 ppm, and the moisture content was 21% by weight.

(117) TABLE-US-00001 TABLE 1 After standing for 1 hour After passage of storage period Change in residual monomers Residual monomers {circle around (1)} Residual monomers {circle around (2)} Corrected Corrected Corrected value value Storage value Change in relative Water Moisture relative step Moisture relative moisture Measured to solid absor- content Measured to solid Storage content Measured to solid content value content bent (1) value content period (2) value content (2) (1) {circle around (2)} {circle around (1)} (1 {circle around (2)}/{circle around (1)}) 100 resin [wt %] [ppm] [ppm] [day(s)] [wt %] [ppm] [ppm] [wt %] [ppm] [%] [%] Comp. Ex. 1 a1 6.1 704 750 0 Example 1 A1 38 7.7 680 737 1.6 24.0 3.4 1.7 Comp. Ex. 2 a2 7.3 541 584 0 Example 2 A2 38 8.6 529 579 1.3 12.0 2.2 0.8 Comp. Ex. 3 a3 0.8 341 344 0 Comp. Ex. 4 a4 90 1.3 339 343 0.5 2.0 0.6 0.1 Comp. Ex. 5 a5 12.8 307 352 0 352 Comp. Ex. 6 a6 1 12.8 305 350 0 2.0 0.7 0.7 Example 3 A3 3 12.8 303 347 0 4.0 1.3 1.3 Example 4 A4 9 12.9 302 347 0.1 5.0 1.6 1.5 Example 5 A5 13 13.2 300 346 0.4 7.0 2.3 1.8 Example 6 A6 85 14.1 298 347 1.3 9.0 2.9 1.5 Comp. Ex. 7 a7 125 15.0 294 346 2.2 13.0 4.2 1.8 Comp. Ex. 8 a8 13.2 201 232 0 232 Comp. Ex. 9 a9 1 13.2 194 224 0 7.0 3.5 3.5 Example 7 A7 3 13.3 185 213 0.1 16.0 8.0 7.9 Example 8 A8 6 13.3 183 211 0.1 18.0 9.0 8.9 Example 9 A9 13 13.4 183 211 0.2 18.0 9.0 8.7

(118) TABLE-US-00002 TABLE 2 After standing for 1 hour After passage of storage period Change in residual monomers Residual monomers {circle around (1)} Residual monomers {circle around (2)} Corrected Corrected Corrected value value Storage value Change in relative Water Moisture relative step Moisture relative moisture Measured to solid absor- content Measured to solid Storage content Measured to solid content value content bent (1) value content period (2) value content (2) (1) {circle around (2)} {circle around (1)} (1 {circle around (2)}/{circle around (1)}) 100 resin [wt %] [ppm] [ppm] [day(s)] [wt %] [ppm] [ppm] [wt %] [ppm] [%] [%] Comp. Ex. a10 4.3 106 111 0 10 Example 10 A10 35 5.5 89 94 1.2 17.0 16.0 15.0 Comp. Ex. a11 5.1 165 174 0 165 11 Comp. Ex. a12 1 161 4.0 2.4 12 Example 11 A11 3 152 13.0 7.9 Example 12 A12 7 5.5 151 160 0.4 14.0 8.5 8.1 Example 13 A13 14 6.2 151 161 1.1 14.0 8.5 7.4 Example 14 A14 79 6.5 149 159 1.4 16.0 9.7 8.3 Comp. Ex. a13 120 6.7 149 160 1.6 16.0 9.7 8.1 13

(119) TABLE-US-00003 TABLE 3 After standing for 1 hour After passage of storage period Change in residual monomers Residual monomers {circle around (1)} Residual monomers {circle around (2)} Corrected Corrected Corrected value value Storage value Change in relative Water Moisture relative step Moisture relative moisture Measured to solid absor- content Measured to solid Storage content Measured to solid content value content bent (1) value content period (2) value content (2) (1) {circle around (2)} {circle around (1)} (1 {circle around (2)}/{circle around (1)}) 100 resin [wt %] [ppm] [ppm] [day(s)] [wt %] [ppm] [ppm] [wt %] [ppm] [%] [%] Example 15 A15 5.1 165 174 80 6.5 137 147 1.4 28.0 17.0 15.7 Example 16 A16 5.1 165 174 80 6.4 139 149 1.3 26.0 15.8 14.6 Comp. Ex. a14 13.5 431 498 13 15.1 411 484 1.6 20.0 4.6 2.8 14 Example 17 A17 6.4 623 666 38 7.7 601 651 1.3 22.0 3.5 2.2 Comp. Ex. a15 6.3 857 915 0 15 Example 18 A18 38 8.0 827 899 1.7 30.0 3.5 1.7 Example 19 A19 6.1 704 750 38 8.3 654 713 2.2 50.0 7.1 4.9 Example 20 A20 6.1 704 750 38 7.8 666 722 1.7 38.0 5.4 3.7 Comp. Ex. a16 6.8 599 643 0 16 Example 21 A21 85 8.2 580 632 1.4 19.0 3.2 1.7

CONCLUSIONS

(120) In Example 1 and Example 2, the moisture content increased by 1.6% by weight (Example 1) and 1.3% by weight (Example 2), respectively, while the residual monomers had further decreased, even after the influence of increase in the moisture content was excluded by calculating the amount of residual monomers in terms of the solid content.

(121) In Example 1 and Example 2, the moisture content increased by 1.6% by weight (Example 1) and 1.3% by weight (Example 2), respectively, and the amount of residual monomers decreased by 24 ppm (Example 1) and 12 ppm (Example 2), respectively. In regard to the residual monomers, even the corrected values based on the solid content in which the influence of the increase in the moisture content was excluded, also decreased. That is, it was confirmed that the residual monomers decreased by the storage for 38 days.

(122) In Examples 3 to 6 and Comparative Examples 5 to 7, the water absorbent resin having a moisture content of 12.8% by weight was stored for 0 days to 125 days. In the storage for 0 days to less than 3 days, the residual monomers tended to decrease, but the values were not stabilized, while after 3 days, the amount of residual monomers were stabilized in a reduced state. On the other hand, in the storage for 125 days in Comparative Example 7, the increment in the moisture content was 2% by weight or more, aggregation occurred in a portion of the water absorbent resin, and coloration was also observed.

(123) In Examples 7 to 9 and Comparative Examples 8 and 9, sodium hydrogen sulfite was added to the water absorbent resin having a moisture content of 13.2% by weight, and the water absorbent resin was stored for 0 to 13 days. In the storage for 0 days to less than 3 days, the residual monomers tended to decrease, but the values were not stabilized, while after 3 days, the amounts of residual monomers were stabilized in a reduced state. Furthermore, in Comparative Examples 5 and 6 and Examples 3 to 5 in which no sodium hydrogen sulfite was added, the absolute amount of residual monomers and the decrement thereof increased.

(124) In Examples 10 to 14 and Comparative Examples 10 to 13, by using ammonium acrylate in an amount of 40% by mole or 55% by mole relative to the total amount of monomer(s), in the storage for 0 days to less than 3 days, the residual monomers tend to decrease, but the value was not stabilized, while after 3 days, the amount of residual monomers was stabilized in a reduced state. Furthermore, in Comparative Examples 5 to 6 and Examples 3 to 5 wherein the same operations except for the polymerization step were carried out, the absolute amount of residual monomers and the decrement thereof increased.

(125) From the results of Example 15 and Example 16, it was confirmed that aggregation can be prevented by the addition of inorganic fine particles.

(126) In Comparative Example 14, a large amount of iron component induced deterioration of color tone of the water absorbent resin during the storage.

(127) In Example 17, a large total content of acetic acid and propionic acid as of 2000 ppm (relative to the monomer(s)) induced saturation of foul odor in the container during the storage, and when the containers were opened, unpleasant odors were detected.

(128) From the results of Comparative Example 15 and Example 18, it was confirmed that a large amount of p-methoxyphenol induced deterioration of color tone of the water absorbent resin.

(129) In Example 19, it is speculated that since a paper bag which lacked a vinyl inner bag was used, the water absorbent resin absorbed moisture in the atmosphere during the long-term storage, and to induce decrease in fluidity of the water absorbent resin.

(130) In Example 20, it is speculated that as the water absorbent resin was transported over a long distance by a truck, the water absorbent resin inside the paper bag was segregated. Furthermore, the segregated fine powder tended to produce aggregates easily during the long term storage.

(131) Comparative Example 16 and Example 21 are an example in which -hydroxycarboxylic acid, a reducing agent, or a chelating agent was not used as a coloration preventing agent. Physical properties of the water absorbent resins obtained in the Example and Comparative Example were almost equivalent to those of Example 6, but it was confirmed that color tone was deteriorated during the long-term storage.

(132) It was noted from Examples 22 to 24 and Examples 25 to 27 that further reduction of residual monomers can be attained by shortening a storage period of acrylic acid or an aqueous solution of acrylic acid. That is, it is preferable to connect (connected via a pipeline, particularly connected over the distance described above) the production process for acrylic acid (particularly, a final purification step involving distillation or crystallization) and the production process for the water absorbent resin. Furthermore, it is noted that it is preferable to connect the production process for acrylic acid as 100% acrylic acid but not in an aqueous solution state with the production process for the water absorbent resin.

(133) Since in Example 28, a moisture content was high, CRC and AAP decreased, suggesting a risk of aggregation.

(134) As mentioned above, it was confirmed that residual monomers could be decreased during the storage step according to the present invention. Furthermore, in regard to deterioration of fluidity or color tone that may occur in the storage step, it is preferable to employ predetermined storage conditions, and to use an aggregation preventing agent or a coloration preventing agent.

INDUSTRIAL APPLICABILITY

(135) The water absorbent resin obtained by the production method of the present invention can be advantageously applied to a hygienic material such as a paper diaper, a sanitary napkin, and an incontinence pad.