Cross-linked polymer and production method for same

Abstract

This invention addresses the problem of providing a cross-linked polymer and a production method for the same, said cross-linked polymer having low residual monomer content and being excellent in reducing colorization properties after long-term storage or after long-term use, while maintaining excellent water absorbency and the like. Said problem is solved by means of a cross-linked polymer having a structural unit derived from acrylamide, wherein the included quantity of a transition metal excluding iron is 40 ppb or less, the yellowness index (YI) after coloring acceleration testing is 60 or less, and the pure water absorption capacity is 10 (g / g) or more.

Description

Crosslinked polymer and method for producing the same

[0001] The present invention relates to a crosslinked polymer and a method for producing the same.

[0002] Crosslinked polymers using acrylamide containing a trace amount of copper are known as raw materials for salt-resistant water-absorbent resins used in waterproofing materials and the like.

[0003] For example, Patent Document 1 discloses a method for producing an acrylamide polymer, in which an acrylamide monomer is aqueous solution polymerized under basic conditions to produce an acrylamide polymer, and the polymerization is carried out in the presence of copper ions in an amount of 1 ppb to 100 ppb relative to the acrylamide monomer.

[0004] Patent Document 2 discloses a method for producing a copolymer or the like by inverse emulsion polymerization of a mixture of acrylic acid and / or a salt thereof, and acrylamide as monomers in the presence of an optional crosslinking agent without adding a transition metal compound. The inverse emulsion polymerization is carried out in the presence of a predetermined initiator pair containing an oxidizing agent and a reducing agent.

[0005] Patent Document 3 discloses a method for producing a polymer, which includes at least one gel polymerization step (E) in which ethylenically unsaturated water-soluble monomers, a predetermined free radical source, and a radical polymerization inhibitor are contacted. Step (E) is carried out in such a manner that the concentration of the monomers in the reaction medium is high enough to cause gelation in the medium if the polymerization is carried out without the inhibitor. Patent Document 3 also discloses an embodiment in which the reaction medium in step (E) does not contain copper.

[0006] Japanese Patent Publication No. 62-39604 Publication of Japanese Special Publication No. 2007-502879 Publication of Japanese Special Publication No. 2013-538913

[0007] However, the above-mentioned conventional techniques have room for improvement in terms of quality, such as maintaining excellent water absorbency and reducing the amount of residual monomers, as well as colorability during long-term storage or use. In fact, no crosslinked polymer satisfying all of these requirements has been provided to date.

[0008] An object of one aspect of the present invention is to provide a crosslinked polymer that has a low monomer content after polymerization while retaining excellent water absorption properties and the like, and that is excellent in reducing discoloration after long-term storage or long-term use, and a method for producing the same.

[0009] That is, one embodiment of the present invention includes the following configuration.

[0010] A crosslinked polymer having structural units derived from acrylamide, wherein the content of transition metals excluding iron is 40 ppb or less, the yellowness index (YI) after a color acceleration test is 60 or less, and the pure water absorption capacity is 10 (g / g) or more, wherein the color acceleration test is carried out by exposing the crosslinked polymer to an atmosphere of a temperature of 80±1°C and a relative humidity of 80±1% for 11 days, the yellowness index (YI) is the yellowness index specified in ASTM D1925 in the Hunter Lab color system, and the pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) / (weight (g) of the crosslinked polymer before free swelling) (1).

[0011] Another embodiment of the present invention includes the following configuration.

[0012] A method for producing a crosslinked polymer, comprising: a polymerization step of polymerizing a monomer containing acrylamide, the content of transition metals excluding iron of which is 40 ppb or less, with a crosslinking agent to obtain a crosslinked polymer, wherein the crosslinked polymer has a pure water absorption capacity of 10 (g / g) or more. Here, the pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): Pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) / (weight (g) of the crosslinked polymer before free swelling) (1).

[0013] According to one aspect of the present invention, it is possible to provide a crosslinked polymer that has a low residual monomer content while maintaining excellent water absorption and the like, and that is excellent in reducing discoloration after long-term storage or long-term use, and a method for producing the same.

[0014] The present invention will now be described with reference to the best mode thereof. It should be understood that the terms used in this specification are used in the same manner as commonly used in the art unless otherwise specified. Therefore, unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the art to which this invention belongs. In the event of any conflict, the present specification (including definitions) shall prevail. The present invention is not limited to the following embodiments, and various modifications can be made within the scope of the claims.

[0015] [1. Definition of Terms] (1-1) "Crosslinked Polymer" In this specification, the term "crosslinked polymer" refers to a water-swellable, water-insoluble crosslinked polymer. In addition, in this specification, "water-swellable" means that the pure water absorption capacity defined in this specification is 5 (g / g) or more, and "water-insoluble" means that the soluble content defined in this specification is 50 mass% or less.

[0016] The crosslinked polymer can be appropriately designed depending on the application, and is not particularly limited, but is a hydrophilic crosslinked polymer having a structural unit derived from acrylamide. The hydrophilic crosslinked polymer is preferably a hydrophilic crosslinked polymer obtained by crosslinking acrylamide, or a hydrophilic crosslinked polymer obtained by crosslinking acrylamide and an unsaturated monomer having a carboxyl group. Furthermore, the total amount (100% by mass) is not limited to a polymer, and may be a surface-crosslinked polymer and / or a composition containing additives, etc., as long as the performance is maintained.

[0017] (1-2) Others In this specification, the range "X to Y" means "X or more and Y or less." In this specification, "weight" and "mass," "weight %" and "mass %, "parts by weight" and "parts by mass" are treated as synonyms. In this specification, unless otherwise specified, "ppm" means "ppm by mass" and "ppb" means "ppb by mass." In this specification, "acid (salt)" means "acid and / or its salt," and "(meth)acrylic" means "acrylic and / or methacrylic." In this specification, the unit of volume "liter" may be written as "l" or "L."

[0018] [2. Crosslinked Polymer] A crosslinked polymer according to one embodiment of the present invention (hereinafter, may be referred to as "the present crosslinked polymer") is a crosslinked polymer having structural units derived from acrylamide, wherein the content of transition metals excluding iron is 40 ppb or less, the yellowness index (YI) after a color acceleration test is 60 or less, and the pure water absorption capacity is 10 (g / g) or more; wherein the color acceleration test is carried out by exposing the crosslinked polymer to an atmosphere of a temperature of 80±1°C and a relative humidity of 80±1% for 11 days, the yellowness index (YI) is the yellowness index specified in ASTM D1925 in the Hunter Lab color system, and the pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): Pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) ÷ (weight (g) of the crosslinked polymer before free swelling) (1).

[0019] (2-1) Crosslinked polymer having structural units derived from acrylamide and content of transition metals other than iron therein The present inventors have found that by controlling the content of transition metals other than iron in the present crosslinked polymer to 40 ppb or less, it is possible to provide a crosslinked polymer that has excellent water absorption performance, is reduced in discoloration after long-term storage or use, and has a reduced content of residual monomers.

[0020] In the present specification, the term "structural unit derived from acrylamide" refers to a -CH ２CH (CONH ２ )- is a structural unit represented by

[0021] In addition, in this specification, the term "crosslinked polymer having structural units derived from acrylamide" refers to a polymer that has structural units derived from acrylamide and has a structure in which the structural units are crosslinked.

[0022] From the viewpoint of salt tolerance, the content of the acrylamide-derived structural units in the crosslinked polymer is preferably 50 mol% or more, more preferably 55 mol% or more, and even more preferably 60 mol% or more, based on all structural units (100 mol%) of the crosslinked polymer excluding the structures derived from the internal crosslinking agent. Also, from the viewpoint of pure water absorption, the content is preferably 95 mol% or less, more preferably 90 mol% or less, and even more preferably 75 mol% or less, based on all structural units.

[0023] The content of transition metals excluding iron in the crosslinked polymer is 40 ppb or less, preferably 30 ppb or less, more preferably 25 ppb or less, and even more preferably 20 ppb or less (the lower limit is 0 ppb). By setting the content of transition metals excluding iron contained in the crosslinked polymer within the above range, discoloration of the crosslinked polymer after long-term storage or long-term use is reduced, and whiteness can be maintained for a long period of time.

[0024] In this specification, the content of transition metals other than iron can be measured by ICP atomic emission spectrometry, for example, as described in the examples below.

[0025] The transition metal other than iron may be at least one element from each of Groups 3 to 11 of the periodic table. Examples include chromium, manganese, cobalt, nickel, and copper. In particular, in view of the magnitude of the influence that this metal has on the coloration and residual monomer content of the crosslinked polymer after long-term storage or long-term use, the transition metal other than iron is preferably copper.

[0026] As described above, in order to keep the content of transition metals excluding iron in the crosslinked polymer at 40 ppb or less, the content of transition metals excluding iron in acrylamide, which is the raw material monomer for the crosslinked polymer, is preferably 40 ppb or less. Furthermore, this content is more preferably 10 ppb or less, even more preferably 5 ppb or less, particularly preferably 3 ppb or less, and most preferably less than 1 ppb (the lower limit is 0 ppb). By keeping the content of transition metals excluding iron in acrylamide within the above range, the content (residual monomers) of acrylamide and other monomers (mainly acrylic acid or its salts) remaining in the crosslinked polymer can be reduced, and a crosslinked polymer with excellent safety can be obtained.

[0027] The transition metal other than iron contained in acrylamide is preferably copper, similar to the transition metal other than iron contained in the crosslinked polymer described above. Note that acrylamide used as a monomer is distributed as a solid (powder) or an aqueous solution of several tens of mass%, and the content of the transition metal other than iron in the acrylamide described above is preferably the above content relative to the solid content of acrylamide.

[0028] The crosslinked polymer having a structural unit derived from acrylamide may be a crosslinked (co)polymer obtained by copolymerizing acrylamide with a monomer other than acrylamide and crosslinking the copolymer. As the monomer, for example, an unsaturated monomer having a carboxy group is suitably selected. Among unsaturated monomers having a carboxy group, acrylic acid and / or an acrylic acid salt are preferred from the viewpoint of productivity. In other words, the present crosslinked polymer is preferably a crosslinked copolymer having a structural unit derived from acrylamide and acrylic acid and / or an acrylic acid salt. In this specification, "structural unit derived from acrylic acid" refers to a structural unit derived from -CH ２ CH(COOH)-, and the "structural unit derived from an acrylate salt" is -CH ２ CH(COOM)- (for example, M is a cation such as an alkali metal).

[0029] The content of structural units derived from acrylic acid and / or acrylate salts in the crosslinked polymer is preferably 5 mol% or more, more preferably 10 mol% or more, and even more preferably 20 mol% or more, based on all structural units (100 mol%) of the crosslinked polymer excluding the structures derived from the internal crosslinking agent, from the viewpoint of pure water absorption capacity. Also, from the viewpoint of salt tolerance, the content is preferably 50 mol% or less, more preferably 45 mol% or less, and even more preferably 40 mol% or less, based on all structural units.

[0030] The "acrylic acid" is not particularly limited, and known acrylic acids can be used. The known acrylic acid can be obtained, for example, by capturing gaseous acrylic acid obtained by catalytic vapor phase oxidation with a solvent such as water, and then purifying the resulting acrylic acid by distillation, crystallization, or the like. The acrylic acid may contain trace amounts of components such as a polymerization inhibitor and impurities.

[0031] The "acrylate" used is the above-mentioned acrylic acid neutralized with the following basic compound, but the acrylate may be a commercially available acrylate (e.g., sodium acrylate) or may be obtained by neutralizing acrylic acid in a crosslinked polymer production plant. Alternatively, the acrylate may be obtained by neutralizing part or all of the structural units derived from acrylic acid in the crosslinked polymer after the crosslinked polymer is obtained.

[0032] The monomer other than acrylamide is not limited to the above-mentioned unsaturated monomers, and other hydrophilic or hydrophobic unsaturated monomers can also be used. Examples of such unsaturated monomers include one or more monomers selected from the group consisting of methacrylic acid, maleic acid (anhydride), 2-(meth)acrylamide-2-methylpropanesulfonic acid, (meth)acryloxyalkanesulfonic acid, N-vinyl-2-pyrrolidone, N-vinylacetamide, methacrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, stearyl acrylate, and salts thereof.

[0033] For the monomers and crosslinking agents other than acrylamide, the content of transition metals excluding iron is preferably 40 ppb or less, more preferably 10 ppb or less, even more preferably 5 ppb or less, particularly preferably 3 ppb or less, and most preferably less than 1 ppb (the lower limit is 0 ppb).

[0034] The content of acrylamide in the monomers used to produce the crosslinked polymer is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 60% by mass or more, based on the total amount of monomers used in the polymerization. The content is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less, based on the total amount of monomers used in the polymerization. The monomers do not include a crosslinking agent.

[0035] The content of acrylic acid and / or acrylate salts in the monomers is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, based on the total amount of monomers used in the polymerization, and is preferably 60% by mass or less, more preferably 50% by mass or less, and even more preferably 40% by mass or less, based on the total amount of monomers used in the polymerization.

[0036] The content of the monomers other than acrylamide, acrylic acid, and acrylates in the monomers is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, based on the total amount of the monomers used in the polymerization, and is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less, based on the total amount of the monomers used in the polymerization.

[0037] However, the total mass % of acrylamide, acrylic acid and / or acrylic acid salts, and monomers other than acrylamide, acrylic acid and acrylic acid salts in the monomers used to produce the crosslinked polymer does not exceed 100 mass %.

[0038] The crosslinked polymer is preferably obtained by crosslinking an internal crosslinking agent with acrylamide and a monomer other than acrylamide, which is used as needed, as described later. That is, the crosslinked polymer preferably has a crosslinked structure derived from the internal crosslinking agent described later. As the internal crosslinking agent, the internal crosslinking agents exemplified in [4. Method for producing crosslinked polymer] can be suitably used.

[0039] (2-2) Yellowness index (YI) of the present crosslinked polymer after accelerated coloring test The present crosslinked polymer has a yellowness index (YI) of 60 or less after an accelerated coloring test. Because the present crosslinked polymer has this structure, it is excellent in reducing coloring during long-term storage or long-term use. This reduction in coloring is a necessary property for improving the appearance of a waterstop material such as a waterstop tape containing the present crosslinked polymer.

[0040] The crosslinked polymer is colorless or nearly transparent. The yellowness index (YI) after the accelerated coloring test is a parameter that represents the degree to which the hue of the crosslinked polymer changes toward yellow after the accelerated coloring test. The smaller the value of the yellowness index, the lower the degree of coloration and the closer it is to white. In this specification, the yellowness index (YI) is the yellowness index specified in ASTM D1925 in the Hunter Lab color system.

[0041] The coloring acceleration test is performed by exposing the crosslinked polymer to an atmosphere of 80±1°C and 80±1% relative humidity for 11 days. For example, the coloring acceleration test can be performed by the following method shown in the Examples below. A powder / paste sample holder with an inner diameter of 30 mm and a height of 12 mm is uniformly filled with 1.5 g of the crosslinked polymer, leaving no gaps at the bottom. The sample holder is then exposed to a thermo-hygrostat (Espec Corporation, product name: Small Environmental Tester, model SH-642) adjusted to an atmosphere of 80±1°C and 80±1% relative humidity for 11 days. The sample holder is then removed from the thermo-hygrostat, and the yellowness index (YI; yellowness index, ASTM D1925) is measured from the backside of the sample holder using a spectrophotometer SE7700 (Nippon Denshoku Industries Co., Ltd.) in the Hunter Lab color system. The measurement conditions are a 2-degree field of view and standard C illuminant.

[0042] The yellowness index (YI) after the accelerated coloring test is 60 or less, preferably 55 or less, and more preferably 50 or less (the lower limit is 0). When the yellowness index (YI) after the accelerated coloring test is 60 or less, the change in hue of the crosslinked polymer toward yellow over a long period of time is gradual, and there is an advantage that the colorability is excellently reduced.

[0043] Furthermore, the crosslinked polymer has a whiteness (WB) based on blue reflectance in the XYZ color system after the accelerated coloring test of preferably 25 or more, more preferably 27 or more, and even more preferably 30 or more (upper limit: 100). The larger the WB value, the lower the degree of coloring and the closer to white the polymer is.

[0044] When the WB is 25 or more, there are advantages in that the luminance of the crosslinked polymer is high over a long period of time and that coloration is reduced. The WB can also be measured using the spectrocolorimeter, as with the YI.

[0045] (2-3) Pure Water Absorption Capacity of the Crosslinked Polymer The crosslinked polymer has a pure water absorption capacity of 10 (g / g) or more. Because of this structure, the crosslinked polymer has sufficient water absorption capacity. Therefore, the crosslinked polymer can be suitably used as a constituent component of a water-stopping material.

[0046] The pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): Pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) / (weight (g) of the crosslinked polymer before free swelling) (1).

[0047] Specifically, the pure water absorption capacity is measured as follows. 0.050 g of the cross-linked polymer is placed in a non-woven bag (85 mm wide x 60 mm long), and the entire non-woven bag is submerged in deionized water (conductivity 1.0 mS / cm or less). The amount of deionized water is preferably in large excess relative to the cross-linked polymer. After the cross-linked polymer is immersed in the deionized water for 120 minutes (free swelling), the non-woven bag containing the water-absorbing gel is slowly removed from the liquid, drained for 20 seconds, and then weighed. Using a weight similarly measured using only the non-woven fabric without the cross-linked polymer as a blank, the pure water absorption capacity is calculated according to the following formula. The following formula (Formula 2) is the same as the above formula (Formula 1): Pure water absorption capacity (g / g) = (weight of the non-woven bag containing the water-absorbing gel (g) - weight of the cross-linked polymer 0.050 (g) - blank (g)) ÷ weight of the cross-linked polymer 0.050 (g) (2).

[0048] The pure water absorption capacity is preferably 20 (g / g) or more, more preferably 30 (g / g) or more, further preferably 40 (g / g) or more, and particularly preferably 50 (g / g) or more. The upper limit of the pure water absorption capacity is not particularly limited, and is usually 1000 (g / g) or less, preferably 500 (g / g) or less, more preferably 250 (g / g) or less.

[0049] As described above, the present crosslinked polymer has a configuration in which the content of transition metals excluding iron is 40 ppb or less, the yellowness index (YI) after the accelerated coloring test is 60 or less, and the pure water absorption capacity is 10 (g / g) or more. The finding that the yellowness index can be reduced and the pure water absorption capacity can be increased by setting the content of transition metals excluding iron to 40 ppb or less was first discovered by the present invention. By having the above configuration, the present crosslinked polymer has excellent reduction in coloration after long-term storage or long-term use and sufficient water absorption. Therefore, the crosslinked polymer has excellent appearance even after long-term storage or long-term use and can exhibit sufficient water absorption properties when used as a waterstop material such as a waterstop tape.

[0050] The present inventors further found, while investigating the physical properties of the present crosslinked polymer, that the residual monomer content of the present crosslinked polymer is very low. Therefore, the physical properties of the present crosslinked polymer other than the colorability and water absorbency, such as the residual monomer content, will be explained below.

[0051] (2-4) Other physical properties of the present crosslinked polymer (2-4-1) Neutralization rate of acidic functional groups in the crosslinked polymer From the viewpoints of improving water absorption performance and reducing residual monomers, the neutralization rate of the acidic functional groups in the present crosslinked polymer is preferably 10 mol % or more relative to the acidic functional groups. In other words, the present crosslinked polymer is preferably such that 10 mol % or more of the acidic functional groups contained in the monomers are neutralized to form partially neutralized salts. Examples of the acidic functional groups include acidic functional groups such as carboxyl groups contained in the above-mentioned monomers other than acrylic acid and acrylamide.

[0052] The neutralization rate is more preferably 20 mol% or more, further preferably 30 mol% or more, and particularly preferably 40 mol% or more from the viewpoint of the pure water absorption capacity. The upper limit of the neutralization rate of the acidic functional group is not particularly limited.

[0053] The partially neutralized salt is not particularly limited, but from the viewpoint of water absorption performance, it is preferably a monovalent salt selected from alkali metal salts, ammonium salts, and amine salts. Furthermore, alkali metal salts are more preferred, and alkali metal salts selected from sodium salts, lithium salts, and potassium salts are even more preferred, with sodium salts being particularly preferred. Therefore, the basic substance used for the neutralization is not particularly limited, but preferred are monovalent basic substances such as alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, and carbonate (hydrogen) salts such as sodium carbonate (hydrogen) and potassium carbonate (hydrogen), with sodium hydroxide being particularly preferred.

[0054] The neutralization can be carried out before, during, or after polymerization, but it is more preferable to neutralize acrylic acid before polymerization. The neutralization can be carried out using a "basic compound." Specific examples include alkali metal carbonates, alkali metal bicarbonates, alkali metal hydroxides, ammonia, and organic amines. Among these, from the viewpoint of the physical properties of the resulting crosslinked polymer, a compound exhibiting strong basicity is preferably selected as the basic compound. That is, as the basic compound, more preferably, a hydroxide of an alkali metal such as sodium, potassium, or lithium is used, and even more preferably, sodium hydroxide is used. From the viewpoint of handleability, the basic compound is preferably in the form of an aqueous solution.

[0055] (2-4-2) Amount of Soluble Content of the Crosslinked Polymer The crosslinked polymer has a soluble content of 50% by mass or less. Because the crosslinked polymer has this structure, it has sufficient shape retention. Therefore, the crosslinked polymer can be suitably used as a constituent component of a water-stopping material. The soluble content can be measured by the method described in the examples below. The soluble content of the crosslinked polymer is preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 20% by mass or less. The lower limit of the soluble content may be 0.1% by mass or more.

[0056] (2-4-3) Acrylamide Content of the Crosslinked Polymer The acrylamide content of the crosslinked polymer is preferably 500 ppm or less. As described above, the present inventors have discovered that by setting the content of transition metals other than iron in the crosslinked polymer to 40 ppb or less, the yellowness index can be reduced and the pure water absorption capacity can be increased. At the same time, the present inventors have also discovered that the content of remaining monomers (residual monomers) in the crosslinked polymer is very low. Since there has been no previous knowledge regarding the correlation between the content of transition metals other than iron and the residual monomer content, the effect of the crosslinked polymer having a very low residual monomer content is an effect that even a person skilled in the art would not have predicted from the prior art. Furthermore, no crosslinked polymer has been known to date that can maintain a high pure water absorption capacity and reduce the yellowness index and the residual monomer content, as with the present crosslinked polymer.

[0057] From the viewpoint of safety, the acrylamide content of the present crosslinked polymer is preferably 500 ppm or less, more preferably 300 ppm or less, even more preferably 200 ppm or less, still more preferably 100 ppm or less, particularly preferably 50 ppm or less, and most preferably 30 ppm or less. The lower limit of the content is not particularly limited.

[0058] (2-4-4) Content of Acrylic Acid and / or Acrylate Salt in the Crosslinked Polymer When the monomer further contains acrylic acid and / or an acrylic acid salt in the crosslinked polymer, the content of the acrylic acid and / or the acrylic acid salt is preferably 500 ppm or less. The acrylic acid and / or the acrylic acid salt is the aforementioned residual monomer.

[0059] From the viewpoint of safety, when the content of acrylic acid and / or acrylic acid salts in the present crosslinked polymer is 500 ppm or less, the adverse effects of the acrylic acid and / or acrylic acid salts on the physical properties of the crosslinked polymer can be significantly reduced. From this viewpoint, the content of acrylic acid and / or acrylic acid salts in the present crosslinked polymer is more preferably 450 ppm or less, particularly preferably 400 ppm or less, and most preferably 350 ppm or less. The lower limit of the content is not particularly limited.

[0060] (2-4-5) Surface Tension of the Crosslinked Polymer The crosslinked polymer preferably has a surface tension value of 60 mN / m or more. By providing this configuration, the crosslinked polymer can further reduce the amount of water return when used as a waterproofing material or the like. From this perspective, the surface tension value is more preferably 65 mN / m or more, even more preferably 70 mN / m or more, and particularly preferably 72 mN / m or more. An upper limit of 75 mN / m is usually sufficient.

[0061] Surface tension is the work (free energy) required to increase the surface area of ​​a solid and / or liquid, expressed per unit area. The surface tension referred to in this application refers to the surface tension of an aqueous solution obtained when the crosslinked polymer is dispersed in a 0.90% by mass aqueous sodium chloride solution.

[0062] The method for measuring the surface tension of this crosslinked polymer is as follows. 50 ml of saline adjusted to 20°C is placed in a thoroughly washed 100 ml beaker. First, the surface tension of the saline is measured using a surface tensiometer (K11 automatic surface tensiometer manufactured by KRUSS). It is confirmed that the value is 71 to 75 (mN / m). Next, a thoroughly washed 25 mm-long fluororesin rotor and 0.5 g of crosslinked polymer are placed in the beaker containing the saline adjusted to 20°C after the surface tension measurement, and the mixture is stirred at 500 rpm for 4 minutes. After 4 minutes, stirring is stopped, and the hydrated crosslinked polymer is allowed to settle. The surface tension of the supernatant is then measured again using the same procedure. In one embodiment of the present invention, a plate method using a platinum plate is employed. The plate is thoroughly washed with deionized water and heated and cleaned with a gas burner before each measurement.

[0063] (2-4-6) Water absorption capacity of the present crosslinked polymer in a 3.5 mass% NaCl aqueous solution The present crosslinked polymer preferably has a water absorption capacity in a 3.5 mass% NaCl aqueous solution of 10 (g / g) or more, more preferably 15 (g / g) or more, and even more preferably 20 (g / g) or more. By having this configuration, the present crosslinked polymer has sufficient water absorption capacity.

[0064] The water absorption capacity of the 3.5 mass % NaCl aqueous solution can be measured by changing the amount of the crosslinked polymer used from 0.050 g to 0.200 g and by changing deionized water to the 3.5 mass % NaCl aqueous solution in the above-mentioned method for measuring the water absorption capacity of pure water.

[0065] [3. Waterstop Material] A waterstop material according to one embodiment of the present invention contains the above-described present crosslinked polymer. The waterstop material may consist solely of the present crosslinked polymer, or may be a mixture of the present crosslinked polymer and other components in any ratio within a range that does not impair the performance of the waterstop material. Examples of the other components include one or more components selected from the group consisting of binders, inorganic or organic fine particles, fibrous substances, surfactants, and solvents.

[0066] As described above, the present crosslinked polymer has a low monomer content after polymerization and is excellent in reducing discoloration after long-term storage or use while retaining excellent water absorption, etc. Therefore, the water-stopping material can exhibit these properties, and therefore can be said to be a water-stopping material excellent in performance and appearance.

[0067] [4. Uses] The crosslinked polymer of the present invention has excellent water absorption properties and is therefore widely used in applications requiring water absorption, including as an absorbent. It is also used as an absorbent article containing the absorbent. The crosslinked polymer of the present invention is preferably used as an absorbent suitable for sanitary articles for absorbing body fluids such as urine and blood. It is also preferably used as an absorbent suitable for absorbent articles that are placed on an object (e.g., a floor, a bed sheet, etc.) and absorb human or animal body fluids such as urine and blood. Specific examples include disposable diapers, incontinence pads, sanitary napkins, pet sheets, waterproof nursing care sheets, and portable emergency toilets. Other examples of absorbent articles include cat litter, drip absorbents, freshness-preserving materials, and anti-condensation sheets.

[0068] Other uses include soil water retention agents, seedling sheets, seed coating materials, disposable hand warmers, cooling bandanas, ice packs, medical waste liquid solidification agents, soil solidification materials, water damage prevention waste liquid gelling agents, water-absorbing sandbags, compresses, thickeners for cosmetics, waterproofing materials for electrical and electronic materials and communication cables, gasket packing, sustained-release agents for fertilizers, various sustained-release agents (space disinfectants, air fresheners, etc.), wound protection dressings, anti-condensation building materials, oil moisture removers, food packaging materials, sealants, contact lenses, implant materials, medical materials, agricultural films, anti-fogging agents, tires, automotive-related materials, ventilation filters, thermal insulation materials, antibacterial and insect repellent materials, fire extinguishers, desiccants, humidity-regulating materials, food (juice) waste liquid solidification, thickeners, water treatment agents, cosmetics, heat storage materials, building materials, lubricants, cutting lubricants, and pore-forming materials. The crosslinked polymer of the present invention can also be used in applications such as paints; adhesives; antiblocking agents; light diffusing agents; matting agents; additives for decorative panels; additives for artificial marble; additives for toners; cement and concrete additives; additives for latent hydraulic materials such as geopolymers, fly ash, and blast furnace slag; additives for various hydraulic cements such as blast furnace slag cement, fly ash cement, LC3 cement (limestone-calcined clay cement), Portland limestone cement, and Portland cement; additives for various concretes such as 3D concrete printing, sprayed concrete, and precast concrete; pharmaceutical carriers, topical ointments, etc., by absorbing water and swelling and mixing with resins and / or substrates.

[0069] 4. Method for producing crosslinked polymer A method for producing a crosslinked polymer according to one embodiment of the present invention (hereinafter, may be referred to as "this production method") includes a polymerization step of polymerizing a monomer containing acrylamide, the content of transition metals excluding iron of which is 40 ppb or less, with a crosslinking agent to obtain a crosslinked polymer, wherein the crosslinked polymer has a pure water absorption capacity of 10 (g / g) or more. Here, the pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): Pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) / (weight (g) of the crosslinked polymer before free swelling) (1).

[0070] (Polymerization Form) The polymerization may be carried out in the absence of a solvent or in the presence of a solvent. The polymerization may be carried out by any of various conventionally known methods, such as bulk polymerization, solution polymerization, suspension polymerization, reversed-phase suspension polymerization, emulsion polymerization, reversed-phase emulsion polymerization, precipitation polymerization, cast polymerization, thin film polymerization, spray polymerization, etc. The polymerization method is preferably solution polymerization, more preferably aqueous solution polymerization.

[0071] That is, the polymerization step is preferably carried out as aqueous solution polymerization using an aqueous monomer solution. Furthermore, in one embodiment of the present invention, continuous aqueous solution polymerization may be carried out. Specific examples of continuous aqueous solution polymerization include continuous belt polymerization as described in U.S. Pat. No. 4,893,999 and continuous kneader polymerization as described in U.S. Pat. No. 6,987,151. By this continuous aqueous solution polymerization, a crosslinked polymer can be produced with high productivity.

[0072] In one embodiment of the present invention, examples of the aqueous solution polymerization include "high-temperature initiation polymerization," "high-concentration polymerization," and "foam polymerization." These polymerization methods can be carried out alone, or two or more of them can be used in combination. In this specification, "high-temperature initiation polymerization" refers to a polymerization method in which the temperature of the aqueous monomer solution at the start of polymerization is preferably 30°C or higher, more preferably 35°C or higher, even more preferably 40°C or higher, and particularly preferably 50°C or higher (the upper limit being the boiling point of the aqueous monomer solution). In this specification, "high-concentration polymerization" refers to a polymerization method in which the concentration of the aqueous monomer solution at the start of polymerization is preferably 30% by mass or higher, more preferably 35% by mass or higher, even more preferably 40% by mass or higher, and particularly preferably 45% by mass or higher (the upper limit being the saturated concentration). In this specification, "foam polymerization" refers to a polymerization method in which gas bubbles (particularly an inert gas, as described below) are dispersed in the aqueous monomer solution.

[0073] Although each of the above polymerization modes can be carried out in an air atmosphere, from the viewpoint of preventing discoloration of the resulting crosslinked polymer, it is preferably carried out in an inert gas atmosphere such as nitrogen or argon (for example, an oxygen concentration of 1% by volume or less). Dissolved oxygen in the aqueous monomer solution is also preferably sufficiently replaced with an inert gas (for example, the amount of dissolved oxygen is less than 1 mg / L).

[0074] Hereinafter, the method for producing the present crosslinked polymer will be described in detail using one example, but the method for producing the present crosslinked polymer is not limited to this example.

[0075] (i) Step of Preparing an Aqueous Monomer Solution This step is a step of preparing an aqueous monomer solution. The aqueous monomer solution can be prepared, for example, by dissolving the monomer and the internal cross-linking agent in water.

[0076] (Monomer) The present crosslinked polymer uses, as a monomer, a monomer containing acrylamide having a transition metal content excluding iron of 40 ppb or less. The monomer is as described above in "(2-1) Crosslinked polymer having a structural unit derived from acrylamide and its transition metal content excluding iron" in [2. Crosslinked polymer].

[0077] (ii) Polymerization Step As a method for polymerizing an acrylamide-containing monomer and a crosslinking agent, a method is preferably adopted in which the internal crosslinking agent is added to the aqueous monomer solution obtained in the above "(i) step of preparing an aqueous monomer solution" and a crosslinking reaction (crosslinking polymerization) is carried out simultaneously with the polymerization reaction. However, the present invention is not limited to this method, and a method may also be adopted in which the monomer is polymerized without adding an internal crosslinking agent, and then an internal crosslinking agent is added during or after the polymerization to cause crosslinking. As a result of the polymerization step, a hydrogel-like crosslinked copolymer (hereinafter sometimes referred to as "hydrogel") is obtained.

[0078] (Internal Crosslinking Agent) As the crosslinking agent (internal crosslinking agent) used in one embodiment of the present invention, for example, the internal crosslinking agents described in US Pat. No. 6,241,928 and the like can be used.

[0079] Specific examples include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, and glycidyl (meth)acrylate. One or more internal crosslinking agents are selected from these, taking into consideration reactivity and the like.

[0080] The amount of the internal crosslinking agent used is preferably set within the range of 0.001 mol% to 10 mol%, more preferably 0.01 mol% to 1 mol%, and even more preferably 0.10 mol% to 0.50 mol%, relative to the total monomer content. Setting the amount of the internal crosslinking agent used within this range is preferable because it increases the gel strength and reduces the water-soluble content, resulting in a crosslinked polymer that exhibits increased water absorption capacity and improved handleability after absorption. For example, "the amount of the internal crosslinking agent used is 0.01 mol% relative to the total monomer content" means that the value obtained by dividing the number of moles of the internal crosslinking agent by the number of moles of all monomers and multiplying this value by 100 is 0.01.

[0081] (Polymerization initiator) Examples of the polymerization initiator used in this step include a thermally decomposable polymerization initiator, a photodecomposable polymerization initiator, or a redox-based polymerization initiator used in combination with a reducing agent that promotes the decomposition of these polymerization initiators.

[0082] Examples of the photodegradable polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives, and azo compounds. Examples of the thermally degradable polymerization initiator include persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; peroxides such as hydrogen peroxide, t-butyl peroxide, and methyl ethyl ketone peroxide; and azo compounds such as 2,2'-azobis(2-amidinopropane) dihydrochloride and 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride. Examples of the redox polymerization initiator include a system in which the persulfates or peroxides are used in combination with a reducing compound such as L-ascorbic acid or sodium hydrogen sulfite. Furthermore, a combination of the photodegradable polymerization initiator and the thermally degradable polymerization initiator is also preferred.

[0083] The amount of the polymerization initiator used is preferably 0.0001 mol% to 1 mol%, more preferably 0.0005 mol% to 0.5 mol%, based on the total monomers. A use amount of the polymerization initiator of 1 mol% or less is preferable because it can suppress deterioration in the color tone of the water-absorbent resin. Furthermore, a use amount of the polymerization initiator of 0.0001 mol% or more is preferable because it can suppress an increase in residual monomer. Note that the low residual monomer content of the present crosslinked polymer cannot be achieved simply by using the polymerization initiator in an amount of 0.0001 mol% or more.

[0084] The pH of the aqueous solution (aqueous monomer solution) is preferably 6 to 12, more preferably 7 to 11, even more preferably 7.5 to 10.5, and particularly preferably 8 to 10.

[0085] The polymerization step is preferably carried out while crushing the crosslinked polymer and / or its precursor obtained during the polymerization. For example, when kneader polymerization such as the continuous kneader polymerization is carried out, the polymerization step and the crushing of the crosslinked polymer and / or its precursor (gel crushing step) are carried out simultaneously.

[0086] The gel crushing step is a step of crushing (gel crushing) the crosslinked polymer obtained in the polymerization step to obtain a particulate hydrogel (hereinafter referred to as "particulate hydrogel"). The "gel crushing" refers to crushing the crosslinked polymer into particles using a gel crusher such as a kneader, meat chopper, or cutter mill. Regarding the embodiments and conditions of gel crushing, for example, the contents described in International Publication No. 2011 / 126079 and the like are applicable to one embodiment of the present invention.

[0087] However, the present production method is not limited thereto, and may include a step of crushing the crosslinked polymer after the polymerization step. For example, the polymerization step may be carried out in an aqueous solution, and the hydrogel-like crosslinked polymer obtained after the polymerization is completed may be crushed. For example, when the continuous belt polymerization is used, the crosslinked polymer is crushed after the polymerization step.

[0088] When a particulate hydrogel is obtained in the polymerization process, such as in reversed-phase suspension polymerization or droplet polymerization, it is preferable not to carry out the gel crushing step.

[0089] An example of the continuous kneader polymerization is a method in which the aqueous monomer solution is fed into a kneader with a lid, a polymerization initiator is added, and the polymerization is initiated by heating to room temperature or 30° C. to 70° C., and the hydrogel polymer produced as the polymerization proceeds is polymerized while being broken down into small pieces by the shear force caused by the rotation of the kneader blades. The polymerization vessel used in the kneader polymerization is not particularly limited, but is preferably one that has a lid on the top so as to maintain an atmosphere inert to the polymerization reaction during polymerization, and that can be purged with an inert gas.

[0090] When a double-arm kneader is used, it is preferable to use two rotating agitators that rotate in opposite directions at equal or unequal speeds. The rotating agitator shafts can be any of Sigma, S, Banbury, and fishtail types.

[0091] The crosslinked polymer obtained in the polymerization step is subjected to the next step, the drying step, either in the form of particles as they are or in the form of particles obtained by gel crushing. The mass average particle diameter (D50) of the particulate hydrogel is appropriately set within a range of preferably 0.1 mm to 50 mm, more preferably 0.2 mm to 10 mm, and even more preferably 0.5 mm to 5 mm, from the viewpoint of drying efficiency.

[0092] The present production method may include any steps in addition to the (i) step of preparing an aqueous monomer solution and the (ii) step of polymerization, such as a drying step, a pulverizing step, a classification step, a granulation step, a post-crosslinking step, etc., in addition to the gel-crushing step.

[0093] (iii) Drying Step The present production method preferably includes a drying step.

[0094] The drying step is a step of drying the particulate hydrogel obtained in the polymerization step or the particulate hydrogel obtained in the gel crushing step to a resin solid content within a desired range to obtain a dried polymer. The desired resin solid content range is preferably 80% by mass or more, more preferably 85% to 99% by mass, even more preferably 90% to 98% by mass, and particularly preferably 92% to 97% by mass. Here, the "resin solid content" is a value calculated from the loss on drying (the change in mass when 1 g of sample is dried at 105 ° C. for 3 hours) and is calculated using the following formula (Formula 3): Resin solid content (mass%) = (mass of sample (1 g) - mass of sample after drying (g)) / (mass of sample (1 g)) × 100 (3).

[0095] The drying method in one embodiment of the present invention is not particularly limited, and examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, high-humidity drying using high-temperature water vapor, etc. Among these, from the viewpoint of drying efficiency, hot air drying is preferably selected, and more preferably band drying in which hot air drying is performed on a ventilated belt is selected.

[0096] The drying temperature in this step is not particularly limited, but is appropriately set within a range of preferably 120°C to 200°C, more preferably 140°C to 180°C, from the viewpoint of drying efficiency and the color tone of the crosslinked polymer. The drying time is appropriately set within a range of preferably 10 minutes to 300 minutes, more preferably 20 minutes to 240 minutes, and even more preferably 30 minutes to 200 minutes. The drying temperature is usually determined by the temperature of the heat medium (for example, in the case of hot air drying, it is determined by the temperature of the hot air), but in the case of drying that cannot be determined by the temperature of the heat medium, such as microwave drying, it is determined by the temperature of the particulate hydrogel. The drying temperature may be a constant temperature or may be changed as appropriate during drying.

[0097] Drying conditions other than the drying temperature and drying time may be appropriately set depending on the moisture content of the particulate hydrogel, the amount of the particulate hydrogel to be supplied to the drying step, the target resin solid content, etc. When band drying is selected, the conditions described in, for example, WO 2006 / 100300 and the like are applied to one embodiment of the present invention.

[0098] (iv) Pulverization step, classification step The present production method may include a pulverization step and a classification step. In this step, the dried polymer obtained in the drying step is pulverized (pulverization step) and adjusted to a particle size within a desired range (classification step). Note that this pulverization step differs from the gel-disintegration step in that the dried polymer to be pulverized has been subjected to a drying step.

[0099] When the present production method includes a drying step, the pulverization step may be carried out before, during, or after drying, but is preferably carried out after drying. More preferably, the pulverization step is carried out after the gel-crushing step and the drying step.

[0100] The equipment (pulverizer) used in the pulverization step is not particularly limited, and examples thereof include high-speed rotary pulverizers such as roll mills, hammer mills, screw mills, and pin mills, as well as vibration mills, knuckle-type pulverizers, and cylindrical mixers. Among these, a roll mill is preferably selected because of the ease of controlling the particle size distribution.

[0101] The method for adjusting the particle size in the classification step is not particularly limited, but preferred examples include sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)) and air classification. Among these, sieve classification is preferably selected from the viewpoint of classification efficiency.

[0102] [5. Summary] One embodiment of the present invention may include the following inventions [1] to

[16] .

[0103] [1] A crosslinked polymer having structural units derived from acrylamide, wherein the content of transition metals excluding iron is 40 ppb or less, the yellowness index (YI) after a color acceleration test is 60 or less, and the pure water absorption capacity is 10 (g / g) or more; wherein the color acceleration test is carried out by exposing the crosslinked polymer to an atmosphere of a temperature of 80±1°C and a relative humidity of 80±1% for 11 days, the yellowness index (YI) is the yellowness index specified in ASTM D1925 in the Hunter Lab color system, and the pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) / (weight (g) of the crosslinked polymer before free swelling) (1).

[0104] [2] The crosslinked polymer according to [1], further comprising a structural unit derived from acrylic acid and / or an acrylate salt.

[0105] [3] The crosslinked polymer according to [2], wherein the neutralization rate of the acidic functional groups of the crosslinked polymer is 10 mol % or more.

[0106] [4] The crosslinked polymer according to any one of [1] to [3], wherein the acrylamide content is 500 ppm or less.

[0107] [5] The crosslinked polymer according to any one of [2] to [4], wherein the content of the acrylic acid and / or acrylic acid salt is 500 ppm or less.

[0108] [6] The crosslinked polymer according to any one of [1] to [5], wherein the transition metal other than iron is copper.

[0109] [7] The crosslinked polymer according to any one of [1] to [6], having a surface tension of 60 mN / m or more.

[0110] [8] The crosslinked polymer according to any one of [1] to [7], wherein the pure water absorption capacity is 10 to 250 (g / g).

[0111] [9] A water-stopping material comprising the crosslinked polymer according to any one of [1] to [8].

[0112]

[10] A method for producing a crosslinked polymer, comprising: a polymerization step of polymerizing a monomer containing acrylamide, the content of transition metals excluding iron of which is 40 ppb or less, with a crosslinking agent to obtain a crosslinked polymer, wherein the crosslinked polymer has a pure water absorption capacity of 10 (g / g) or more. Here, the pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): Pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) / (weight (g) of the crosslinked polymer before free swelling) (1).

[0113]

[11] The method for producing a crosslinked polymer according to

[10] , wherein the transition metal other than iron is copper.

[0114]

[12] The method for producing a crosslinked polymer according to

[10] or

[11] , wherein the polymerization step is carried out in an aqueous solution.

[0115]

[13] The method for producing a crosslinked polymer according to any one of

[10] to

[12] , wherein the polymerization step is carried out while crushing the crosslinked polymer and / or its precursor obtained during the polymerization.

[0116]

[14] The method for producing a crosslinked polymer according to any one of

[10] to

[13] , further comprising, after the polymerization step, a step of crushing the crosslinked polymer and a step of drying the crushed crosslinked polymer.

[0117]

[15] The method for producing a crosslinked polymer according to any one of

[10] to

[14] , wherein the monomer further contains acrylic acid and / or an acrylate salt.

[0118]

[16] The method for producing a crosslinked polymer according to any one of

[12] to

[15] , wherein the pH of the aqueous solution is 6 to 12.

[0119] Hereinafter, one embodiment of the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0120] [Measurement Methods] The measurement and evaluation methods for various items carried out in the examples and comparative examples will be explained below.

[0121] (Pure Water Absorption Capacity of Crosslinked Polymer) 0.050 g of the crosslinked polymer was accurately weighed and placed in a nonwoven bag (85 mm wide x 60 mm long). The entire nonwoven bag was submerged in a large excess (250 mL or more) of deionized water (electrical conductivity 1.0 mS / cm or less). After immersion (free swelling) for 120 minutes, the nonwoven bag containing the water-absorbent gel was slowly removed from the liquid, drained for 20 seconds, and then weighed. Using a weight similarly measured using only the nonwoven fabric without the crosslinked polymer as a blank, the pure water absorption capacity was calculated according to the following formula (Formula 2): Pure Water Absorption Capacity (g / g) = (Weight of Nonwoven Bag Containing Water-Absorbent Gel (g) - Weight of Crosslinked Polymer 0.050 (g) - Blank (g)) ÷ Weight of Crosslinked Polymer 0.050 (g) (2).

[0122] (3.5% NaCl Water Absorption Capacity of Crosslinked Polymer) 0.200 g of the crosslinked polymer was accurately weighed and placed in a nonwoven bag (85 mm wide x 60 mm long). The entire nonwoven bag was submerged in a large excess (250 mL or more) of 3.5% by mass NaCl aqueous solution. After immersion (free swelling) for 120 minutes, the nonwoven bag containing the water-absorbent gel was slowly removed from the solution, drained for 20 seconds, and then weighed. The weight of the nonwoven fabric without the crosslinked polymer was used as a blank, and the pure water absorption capacity was calculated according to the following formula (Formula 4): 3.5% NaCl Water Absorption Capacity (g / g) = (weight of nonwoven bag containing water-absorbent gel (g) - weight of crosslinked polymer 0.200 (g) - blank (g)) ÷ weight of crosslinked polymer 0.200 (g) (4).

[0123] (Soluble content of crosslinked polymer) 0.5 g of accurately weighed crosslinked polymer, 1000 g of deionized water, and a stirrer (50 mm) were placed in a 1150 mL cylindrical plastic container (φ60 mm), and stirred at 500 rpm for 16 hours to extract the soluble content of the crosslinked polymer. The obtained extract was filtered using filter paper (product name: JIS P 3801, No. 2, thickness 0.26 mm, retention particle size 5 μm / manufactured by ADVANTEC Toyo Co., Ltd.). A portion of the obtained filtrate (W Ｌ (g) was dried under reduced pressure and the weight of the remaining solids (WＳ The soluble content (mass%) was calculated according to the following formula (5): soluble content (%) = (weight of remaining solids W Ｓ (g) ÷ Filtrate weight before vacuum drying W Ｌ (g)) × (Extract weight 1000.5 (g) ÷ Crosslinked polymer weight 0.5 (g)) × 100 (5).

[0124] (Residual Monomer Content of Crosslinked Polymer) 1 g of crosslinked polymer, 200 g of 0.9% by mass sodium chloride aqueous solution, and a stirrer (35 mm) were placed in a 250 mL cylindrical plastic container (φ60 mm) and stirred at 500 rpm for 16 hours to extract the residual monomer in the crosslinked polymer. The resulting extract was filtered using a sheet of filter paper (product name: JIS P 3801, No. 2, thickness 0.26 mm, retention particle size 5 μm / manufactured by ADVANTEC Toyo Co., Ltd.). The resulting filtrate was passed through a filter (GL Sciences, GL Chromatodisc, aqueous 25A, pore size 0.45 μm) and subjected to high-performance liquid chromatography (HPLC). The residual monomer content (unit: ppm) was calculated from the obtained peak area and the prepared calibration curve.

[0125] A calibration curve used to measure the residual monomer content of a crosslinked polymer was prepared as follows. Specifically, first, a solution was prepared by dissolving each residual monomer (acrylamide, acrylic acid) in a 0.9% by mass aqueous sodium chloride solution at concentrations of 0.025 ppm (5 ppm relative to the crosslinked polymer), 0.25 ppm (50 ppm relative to the crosslinked polymer), 1.0 ppm (200 ppm relative to the crosslinked polymer), 2.5 ppm (500 ppm relative to the crosslinked polymer), and 5 ppm (1000 ppm relative to the crosslinked polymer). The solution was then subjected to HPLC in the same manner as above, and a calibration curve was prepared from the results. The residual monomer content was calculated based on the calibration curve. Residual monomer content values ​​of 5 ppm or less in terms of concentration in the crosslinked polymer were treated as Trace (below the detection limit).

[0126] The HPLC measurement conditions for each residual monomer are as follows.

[0127] Measurement condition 1: acrylamide <Eluent>: 0.25 vol% aqueous phosphoric acid solution <Column>: Shodex RSpak DE-413 (manufactured by Showa Denko K.K.) <Column temperature>: 50°C <Flow rate>: 1 ml / min <Injection amount>: 0.2 ml <Detector>: UV, wavelength 210 nm.

[0128] Measurement condition 2: acrylic acid <Eluent>: 20 vol% aqueous phosphoric acid solution <Column>: Shodex RSpak DM-614 (manufactured by Showa Denko K.K.) <Column temperature>: 23±2°C <Flow rate>: 0.7 ml / min <Injection amount>: 50 μl <Detector>: UV, wavelength 210 nm.

[0129] (Coloration over time after accelerated coloration test of crosslinked polymer) Coloration evaluation of the crosslinked polymer was carried out using a spectrophotometer SE7700 manufactured by Nippon Denshoku Industries Co., Ltd. Measurement was carried out under standard C light source conditions with a 2-degree field of view, with reflection measurement selected. An attached powder / paste sample stage with an inner diameter of 30 mm and a height of 12 mm was used, and a standard round white plate for powders and pastes No. 2 was used as a standard, and a 30Φ projection pipe was used. Approximately 1.5 g of the crosslinked polymer was uniformly filled into the sample stage so that there were no gaps at the bottom.

[0130] A coloration acceleration test was performed by exposing the sample stage filled with the crosslinked polymer for 11 days in a thermo-hygrostat (manufactured by Espec Corporation, product name: small environmental tester, model SH-642) adjusted to an atmosphere of 80±1°C and 80±1% relative humidity. After the test, the YI value (yellowness index, ASTM D1925) and WB (whiteness index based on blue reflectance) in the Hunter Lab color system were measured from the backside of the sample stage using the spectrocolorimeter. These measured values ​​were defined as "coloration over time (80°C, 80% RH, 11 days)." A lower YI value indicates less coloration and a color that is essentially closer to white, while a higher WB value indicates less coloration and a color that is essentially closer to white.

[0131] (Concentration of copper in monomer) The concentration of copper in the monomer was measured by ICP optical emission spectroscopy in accordance with JIS K0116:2014 General rules for optical emission spectroscopy. Specifically, the copper content in the monomer was quantified using an iCAP6500 Duo (manufactured by Thermo Fisher Scientific) and yttrium as an internal standard. A calibration curve was prepared by diluting an ICP general-purpose mixed standard solution (SPEX XSTC-622B) with ultrapure water. After preparing the calibration curve, the spectrum of copper in the ultrapure water used for dilution was measured at n = 10, the standard deviation was calculated, and the lower limit of quantitation was set to 10 times the standard deviation, and the lower limit of detection was set to 3 times the standard deviation. As a result of the measurement, the lower limit of detection of copper was 1 ppb or less.

[0132] (Copper Concentration in Crosslinked Polymer) 0.2 g of crosslinked polymer and decomposition reagent (5 ml of 60% nitric acid (ultrapure reagent) 1.38% + 0.2 ml of hydrochloric acid (ultrapure reagent)) were added to a quartz insert (Milestone General) to obtain a mixture. The quartz insert was placed in a TFM decomposition vessel (Milestone General), and an anti-seize reagent (1 ml of hydrogen peroxide + 4.2 ml of ultrapure water) was added to the TFM decomposition vessel. Next, the TFM decomposition vessel containing the mixture was treated using a microwave pretreatment device (ETHOS UP, Milestone General) under the decomposition program conditions shown in Table 1 to obtain a decomposition solution.

[0133] In the table, T1 is the temperature inside the TFM decomposition vessel measured by an internal temperature sensor, and the temperatures in the table are the temperatures reached after the completion of the step. After the completion of step 4, the TFM decomposition vessel was cooled with a fan for 30 minutes. After the T1 temperature fell below 40°C, the TFM decomposition vessel was opened, and two bottles of the decomposition solution were placed in a plastic container (ICP measurement vessel). The TFM decomposition vessel was washed with ultrapure water, and then the solution was adjusted to a total of 20 g with ultrapure water to obtain Preparation Solution B. Note that the time in Table 1, for example, "0:02:00," indicates 2 minutes.

[0134] The copper concentration in the crosslinked polymer was measured by ICP-MS analysis in accordance with the standard addition method described in JIS K0116: 2014. Specifically, a general-purpose mixed standard solution for ICP (SPEX XSTC-622B) was added to Preparation Solution B to give different concentrations, and the resulting solution was diluted with ultrapure water (resistivity: 18 MΩ cm or higher) to give a plurality of Preparation Solutions C. The copper concentration in the crosslinked polymer was measured using an Agilent 7700S (Agilent Technologies).

[0135] [Embodiment 1] An embodiment of the present invention will be described below.

[0136] 241.8 parts by mass of a 40% by mass aqueous solution of acrylamide (copper content in the aqueous solution <1 ppb, copper content in acrylamide (solid content) <2.5 ppb), 148.2 parts by mass of a 37% by mass aqueous solution of sodium acrylate, and 27.8 parts by mass of deionized water were mixed. 29.7 parts by mass of a 1.5% by mass aqueous solution of methylenebisacrylamide, an internal crosslinking agent, was mixed with the resulting mixture to prepare a reaction solution. The pH of the reaction solution was 9.6.

[0137] The resulting reaction solution was degassed by bubbling with nitrogen gas (10 L / min) for 30 minutes. Next, the reaction solution was fed into a jacketed stainless steel double-arm kneader having two sigma-type blades, and the system was purged with nitrogen gas while maintaining the temperature at 20°C (nitrogen gas replacement was continued until the end of polymerization). Next, while stirring the reaction solution, 2.7 parts by mass of a 25% by mass aqueous solution of sodium persulfate and 3.4 parts by mass of a 2% by mass aqueous solution of L-ascorbic acid were added, and after about 1 minute, the temperature rose and polymerization began. Sodium persulfate and L-ascorbic acid are polymerization initiators. The maximum temperature during polymerization was 95°C. After reaching the peak polymerization temperature, stirring was continued for 30 minutes at a jacket temperature of 60°C to obtain a hydrogel polymer (1).

[0138] The hydrogel polymer (1) was fragmented into approximately 3 mm pieces. This fragmented hydrogel polymer (1) was spread on a 50-mesh wire netting (opening 300 μm) and dried at 160°C for 130 minutes to obtain a dried polymer (1) with a water content of 4% by mass. The water content was calculated from the loss on drying of the obtained dried polymer (1) (the change in mass when 1 g of the sample was dried at 105°C for 3 hours) as in the evaluation of the resin solids content described above, as follows: water content (% by mass) = 100% - resin solids (% by mass).

[0139] The dried polymer (1) was pulverized using a hammer-type pulverizer and classified using a metal sieve (sieve size: 20 mesh) to obtain a 20 mesh-pass classified product. Subsequently, the 20 mesh-pass classified product was subjected to a fine pulverization treatment using a mill-type pulverizer to obtain a crosslinked polymer (1) as a 60 mesh (opening size: 250 μm) pass 100 mesh (opening size: 150 μm) on-classified product. The soluble content of the crosslinked polymer (1) was 11% by mass. The evaluation results of the crosslinked polymer are shown in Table 2.

[0140] In this example, the same reaction solution as in Example 1 was used, and the conditions from preparation of the reaction solution to the initiation of polymerization were also the same as in Example 1. The maximum temperature during polymerization was 96°C. After reaching the peak polymerization temperature, stirring was continued for 20 minutes at a jacket temperature of 60°C. After stirring was temporarily stopped, 32.6 parts by mass of a 5% by mass aqueous solution of sodium hydrogen sulfite was added, and stirring was continued for an additional 10 minutes at a jacket temperature of 60°C, yielding a hydrogel polymer (2).

[0141] The hydrogel polymer (2) was fragmented into pieces of about 3 mm. This fragmented hydrogel polymer (2) was dried under the same conditions as in Example 1 to obtain a dried polymer (2) with a water content of 4 mass%.

[0142] The dried polymer (2) was pulverized and classified under the same conditions as in Example 1 to obtain a 20 mesh-pass classified product. Subsequently, the 20 mesh-pass classified product was subjected to a fine pulverization treatment using a mill-type pulverizer to prepare a crosslinked polymer (2) as a 60 mesh (opening 250 μm) pass 100 mesh (opening 150 μm) on-classified product. The soluble content of the crosslinked polymer (2) was 13% by mass.

[0143] The crosslinked polymer (2) was evaluated in the same manner as in Example 1. The evaluation results of the crosslinked polymer (2) are shown in Table 2.

[0144] Example 3 In this example, 284.0 parts by mass of a 40% by mass aqueous solution of acrylamide (copper content in the aqueous solution <1 ppb, copper content in acrylamide (solid content) <2.5 ppb), 102.6 parts by mass of a 37% by mass aqueous solution of sodium acrylate, and 30.1 parts by mass of deionized water were mixed. 30.6 parts by mass of a 1.5% by mass aqueous solution of methylenebisacrylamide, an internal crosslinking agent, was mixed with the resulting mixture to prepare a reaction solution. The pH of the reaction solution was 9.3.

[0145] The polymerization was carried out under the same conditions as in Example 1 until the initiation of polymerization. The maximum temperature during polymerization was 98°C. After the polymerization peak temperature was reached, stirring was continued for 20 minutes at a jacket temperature of 60°C, and stirring was temporarily stopped. Next, 32.6 parts by mass of a 5% by mass aqueous solution of sodium hydrogen sulfite was added, and stirring was continued for an additional 10 minutes at a jacket temperature of 60°C, yielding a hydrogel polymer (3).

[0146] The hydrogel polymer (3) was fragmented into 3 mm pieces. This fragmented hydrogel polymer (3) was dried under the same conditions as in Example 1 to obtain a dried polymer (3) with a water content of 4 mass%.

[0147] The dried polymer (3) was pulverized and classified under the same conditions as in Example 1 to obtain a 20 mesh-pass classified product. Subsequently, the 20 mesh-pass classified product was subjected to a fine pulverization treatment using a mill-type pulverizer to prepare a crosslinked polymer (3) as a 60 mesh (opening 250 μm) pass 100 mesh (opening 150 μm) on-classified product. The soluble content of the crosslinked polymer (3) was 12 mass%.

[0148] The crosslinked polymer (3) was evaluated in the same manner as in Example 1. The evaluation results of the crosslinked polymer (3) are shown in Table 2.

[0149] Example 4 In this example, 330.0 parts by mass of a 40% by mass aqueous solution of acrylamide (copper content in the aqueous solution: <1 ppb, copper content in acrylamide (solid content): <2.5 ppb), 52.9 parts by mass of a 37% by mass aqueous solution of sodium acrylate, and 32.7 parts by mass of deionized water were mixed. 31.6 parts by mass of a 1.5% by mass aqueous solution of methylenebisacrylamide, an internal crosslinking agent, was mixed with the resulting mixture to prepare a reaction solution. The pH of the reaction solution was 8.7.

[0150] The polymerization was carried out under the same conditions as in Example 1 until the initiation of polymerization. The maximum temperature during polymerization was 99°C. After the polymerization peak temperature was reached, stirring was continued for 20 minutes at a jacket temperature of 60°C, and stirring was temporarily stopped. Next, 32.6 parts by mass of a 5% by mass aqueous solution of sodium hydrogen sulfite was added, and stirring was continued for an additional 10 minutes at a jacket temperature of 60°C, yielding a hydrogel polymer (4).

[0151] The hydrogel polymer (4) was fragmented into 3 mm pieces. This fragmented hydrogel polymer (4) was dried under the same conditions as in Example 1 to obtain a dried polymer (4) having a water content of 4% by mass.

[0152] The dried polymer (4) was pulverized and classified under the same conditions as in Example 1 to obtain a 20 mesh-pass classified product. Subsequently, the 20 mesh-pass classified product was subjected to a fine pulverization treatment using a mill-type pulverizer to prepare a crosslinked polymer (4) as a 60 mesh (opening 250 μm) pass 100 mesh (opening 150 μm) on-classified product. The soluble content of the crosslinked polymer (4) was 14 mass%.

[0153] The crosslinked polymer (4) was evaluated in the same manner as in Example 1. The evaluation results of the crosslinked polymer (4) are shown in Table 2.

[0154] Example 5 In this example, 235.9 parts by mass of a 40% by mass aqueous solution of acrylamide (copper content in the aqueous solution <1 ppb, copper content in acrylamide (solid content) <2.5 ppb), 144.7 parts by mass of a 37% by mass aqueous solution of sodium acrylate, and 0.3 parts by mass of deionized water were mixed. 66.9 parts by mass of a 1.5% by mass aqueous solution of methylenebisacrylamide, an internal crosslinking agent, was mixed with the resulting mixture to prepare a reaction solution. The pH of the reaction solution was 9.5.

[0155] The polymerization was carried out under the same conditions as in Example 1 until the initiation of polymerization. The maximum temperature during polymerization was 92°C. After the polymerization peak temperature was reached, stirring was continued for 20 minutes at a jacket temperature of 60°C, and stirring was temporarily stopped. Next, 32.6 parts by mass of a 5% by mass aqueous solution of sodium hydrogen sulfite was added, and stirring was continued for an additional 10 minutes at a jacket temperature of 60°C, yielding a hydrogel polymer (5).

[0156] The hydrogel polymer (5) was fragmented into 2 mm pieces. This fragmented hydrogel polymer (5) was dried under the same conditions as in Example 1 to obtain a dried polymer (5) with a water content of 4 mass%.

[0157] The dried polymer (5) was pulverized and classified under the same conditions as in Example 1 to obtain a 20 mesh-pass classified product. Subsequently, the 20 mesh-pass classified product was subjected to a fine pulverization treatment using a mill-type pulverizer to prepare a crosslinked polymer (5) as a 60 mesh (opening 250 μm) pass 100 mesh (opening 150 μm) on-classified product. The soluble content of the crosslinked polymer (5) was 8% by mass.

[0158] The crosslinked polymer (5) was evaluated in the same manner as in Example 1. The evaluation results of the crosslinked polymer (5) are shown in Table 2.

[0159] Comparative Example 1 A hydrogel polymer (C1) was obtained by the same procedure as in Example 1, except that a 40% by mass aqueous solution of acrylamide (the copper content in the acrylamide (solid content) was 215 ppb) was used. The hydrogel polymer (C1) was fragmented into particles of about 3 mm. This fragmented hydrogel polymer (C1) was dried under the same conditions as in Example 1 to obtain a dried polymer (C1) with a water content of 4% by mass.

[0160] The dried polymer (C1) was pulverized and classified under the same conditions as in Example 1 to obtain a 20 mesh-pass classified product. Subsequently, the 20 mesh-pass classified product was subjected to a fine pulverization treatment using a mill-type pulverizer to obtain a crosslinked polymer (C1) as a 60 mesh (opening 250 μm) pass 100 mesh (opening 150 μm) on classified product. The soluble content of the crosslinked polymer (C1) was 10 mass%. The crosslinked polymer (C1) was evaluated in the same manner as in Example 1. The evaluation results of the crosslinked polymer (C1) are shown in Table 2.

[0161] Comparative Example 2 A hydrogel polymer (C2) was obtained by the same procedure as in Comparative Example 1, except that a 40% by mass aqueous solution of acrylamide (the copper content in the acrylamide (solid content) was 103 ppb) was used. The hydrogel polymer (C2) was fragmented into 3 mm particles. This fragmented hydrogel polymer (C2) was dried under the same conditions as in Example 1, to obtain a dried polymer (C2) with a moisture content of 4% by mass.

[0162] The dried polymer (C2) was pulverized and classified under the same conditions as in Example 1 to obtain a 20 mesh-pass classified product. Subsequently, the 20 mesh-pass classified product was subjected to a fine pulverization treatment using a mill-type pulverizer to prepare a crosslinked polymer (C2) as a 60 mesh (opening 250 μm) pass 100 mesh (opening 150 μm) on classified product. The soluble content of the crosslinked polymer (C2) was 12 mass%. The crosslinked polymer (C2) was evaluated in the same manner as in Example 1. The evaluation results of the crosslinked polymer (C2) are shown in Table 2.

[0163] [Discussion] (1) From the results of Examples 1 to 5, it can be seen that the crosslinked polymer according to one embodiment of the present invention, which has a transition metal content excluding iron of 40 ppb or less, has a low residual monomer content and exhibits the effect of excellent reduction in discoloration after long-term storage or long-term use. Furthermore, from the results of the pure water absorption capacity and the 3.5% NaCl water absorption capacity, it can be seen that the crosslinked polymer exhibits the above-mentioned effect without adversely affecting basic water absorption properties such as absorption capacity.

[0164] (2) It is found that the crosslinked polymers obtained in Comparative Examples 1 and 2 have a higher residual monomer content than the crosslinked polymers of Examples and are crosslinked polymers inferior in colorability after long-term storage or long-term use. In other words, it is found that even if a crosslinked polymer is obtained by polymerizing a monomer containing acrylamide with a crosslinking agent, a crosslinked polymer having a transition metal content other than iron of more than 40 ppb cannot solve the problem of the present invention.

[0165] According to one embodiment of the crosslinked polymer and the method for producing the same, the monomer content after polymerization is low, and even when stored or used for a long period of time, coloration can be reduced and the content of residual monomers can be reduced. Therefore, the crosslinked polymer can be suitably used in applications such as water-stopping materials that require high quality and reduced coloration.

Claims

1. A crosslinked polymer having structural units derived from acrylamide, The content of transition metals other than iron is 40 ppb or less, The yellowness index (YI) after a coloring acceleration test is 60 or less, A crosslinked polymer having a pure water absorption capacity of 10 (g / g) or more: The coloring acceleration test is carried out by exposing the crosslinked polymer to an atmosphere having a temperature of 80±1° C. and a relative humidity of 80±1% for 11 days. The yellowness index (YI) is the yellowness index defined in ASTM D1925 in the Hunter Lab color system, The pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): Pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) ÷ (weight (g) of the crosslinked polymer before free swelling) (1).

2. The crosslinked polymer according to claim 1 , further comprising structural units derived from acrylic acid and / or an acrylate salt.

3. The crosslinked polymer according to claim 2, wherein the neutralization rate of the acidic functional groups contained in the crosslinked polymer is 10 mol % or more.

4. 2. The crosslinked polymer according to claim 1, wherein the acrylamide content is 500 ppm or less.

5. 3. The crosslinked polymer according to claim 2, wherein the content of acrylic acid and / or acrylate salts is 500 ppm or less.

6. 2. The crosslinked polymer of claim 1, wherein said transition metal other than iron is copper.

7. 2. The crosslinked polymer according to claim 1, having a surface tension value of 60 mN / m or more.

8. The crosslinked polymer according to claim 1, wherein the pure water absorption capacity is 10 to 250 (g / g).

9. A water-stopping material comprising the crosslinked polymer according to any one of claims 1 to 8.

10. The method includes a polymerization step of polymerizing a monomer containing acrylamide having a transition metal content excluding iron of 40 ppb or less with a crosslinking agent to obtain a crosslinked polymer, The crosslinked polymer has a pure water absorption capacity of 10 (g / g) or more. Here, the pure water absorption capacity is the water absorption capacity when 0.05 g of the crosslinked polymer is allowed to freely swell in deionized water for 120 minutes, and is a value calculated by the following formula (Formula 1): Pure water absorption capacity (g / g) = (weight (g) of the crosslinked polymer after free swelling - weight (g) of the crosslinked polymer before free swelling) ÷ (weight (g) of the crosslinked polymer before free swelling) (1).

11. The method for producing a crosslinked polymer according to claim 10, wherein the transition metal other than iron is copper.

12. The method for producing a crosslinked polymer according to claim 10, wherein the polymerization step is carried out in an aqueous solution.

13. The method for producing a crosslinked polymer according to claim 10, wherein the polymerization step is carried out while crushing the crosslinked polymer and / or its precursor obtained during the polymerization.

14. The method for producing a crosslinked polymer according to claim 10, further comprising, after the polymerization step, a step of crushing the crosslinked polymer, and a step of drying the crushed crosslinked polymer.

15. The method for producing a crosslinked polymer according to claim 10, wherein the monomer further contains acrylic acid and / or an acrylate salt.

16. The method for producing a crosslinked polymer according to any one of claims 12 to 15, wherein the aqueous solution has a pH of 6 to 12.

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