JP2011231188A - Polyacrylic acid (salt) based water-absorbing resin and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water-absorbing resin in which too much purification process is not needed even if acrylic acid is biomass origin, which is a water-absorbing resin excessively consumed by a paper diaper or the like, and is water-absorbing resin which is gentle to earth environment and in which regenerating is possible, and in which whiteness is high, and coloring is little with the passage of time.SOLUTION: The polyacrylic acid (salt) based water-absorbing resin is a polyacrylic acid (salt) based water-absorbing resin which has an internal cross linkage structure obtained by carrying out polymerization of a water-soluble unsaturated monomer, and is characterized in that a carbon stable isotope ratio (δC) measured by an accelerator mass spectrometry is at least -20 per mill.

Description

  The present invention relates to a polyacrylic acid (salt) water-absorbing resin and a method for producing the same. More specifically, it is a water-absorbent resin made from monomers obtained from renewable animal and plant-derived organic resources, a polyacrylic acid (salt) -based water-absorbent resin having high whiteness and little coloration over time, and its It relates to a manufacturing method.

  In recent years, water-absorbing resins having a high water-absorbing property have been developed, and are widely used mainly in disposable applications as absorbent articles such as paper diapers and sanitary napkins, as well as water-holding agents for agriculture and horticulture, industrial water-stopping materials Has been.

  In recent years, due to problems in the global environment, instead of oil and other depletable energy resources (hereinafter referred to as “fossil resources”), the above-mentioned fossils are replaced by renewable organic and plant-derived organic resources that are constituents of living organisms. The movement to use resources excluding resources (hereinafter referred to as “biomass”) has become active. Also in the field of water-absorbing resins, research using monomers obtained from biomass as raw materials is in progress.

  Many rules (parameter measurement methods) have been proposed for water-absorbing resins from various viewpoints. For example, water absorption capacity under no pressure, water absorption capacity under pressure, water absorption speed, liquid permeability under no pressure, There are demands for improvements in physical properties such as liquid permeability, impact resistance, urine resistance, fluidity, gel strength, initial color tone (whiteness), coloration with time, and particle size. And many monomers and hydrophilic polymer compounds are proposed as the raw material. Of these, polyacrylic acid (salt) water-absorbing resins using acrylic acid and / or a salt thereof as a raw material are industrially most frequently used because of their high water absorption performance.

  However, although it is a water-absorbing resin developed for the purpose of improving the above physical properties, it is difficult to say that it still exhibits sufficient performance in the use of absorbent articles such as paper diapers. In particular, whiteness and coloration with time were significant when using a fossil resource-derived monomer.

  Then, paying attention to the raw material monomer and the method for treating the water-absorbent resin, proposals have been made for improving the whiteness of the water-absorbent resin and preventing coloring (Patent Documents 1 to 27, etc.). Specifically, a technique for controlling methoxyphenol in acrylic acid to 10 to 160 ppm (Patent Document 1), a technique for controlling hydroquinone in acrylic acid to 0.2 ppm or less (Patent Document 2), and a monomer for activated carbon (Patent Document 3), a technique using tocophenol as a polymerization inhibitor (Patent Document 4), a technique using an N-oxyl compound or a manganese compound as a polymerization inhibitor (Patent Document 5), methoxyphenol And the technique (patent documents 6, 7) etc. which use a specific polyvalent metal salt are known.

  In addition, as a technique for preventing coloring of the water-absorbent resin, a technique of adding a reducing agent such as hypophosphite (Patent Document 8), a technique of adding an antioxidant (Patent Documents 9 and 10), a metal chelate and, if necessary, Other techniques for adding a reducing agent and the like (Patent Documents 11 to 15), techniques for adding an organic carboxylic acid and other compounds as necessary (Patent Documents 16 to 19), and the like are known. Furthermore, a technique (Patent Documents 20 to 22) that focuses on a polymerization initiator in a polymerization process of a water-absorbent resin, or a technique that controls iron content in a reducing agent or the like by focusing on iron as a color-causing substance (Patent Documents) 23, 24), a technique using an ammonium acrylate salt as a monomer (Patent Document 25), and a technique (Patent Documents 26 and 27) for controlling the amount of oxygen in a drying process or a surface cross-linking process.

International Publication No. 2003/051940 Pamphlet US Pat. No. 6,444,744 International Publication No. 2004/052819 Pamphlet International Publication No. 2003/053482 Pamphlet International Publication No. 2008/096713 Pamphlet International Publication No. 2008/092843 Pamphlet International Publication No. 2008/092842 Pamphlet US Pat. No. 6,359,049 International Publication No. 2009/060062 Pamphlet International Publication No. 2009/011717 Pamphlet US Patent Application Publication No. 2005/085604 International Publication No. 2003/059961 Pamphlet European Patent No. 1645596 Japanese Registered Patent No. 3107873 International Publication No. 2009/005114 Pamphlet International Publication No. 2008/026752 Pamphlet JP 2000-327926 A Japanese Patent Laid-Open No. 2003-057422 JP 2005-186016 A JP-A-4-331205 US Patent Application Publication No. 2006/089611 US Pat. No. 7,528,291 International Publication No. 2007/072969 Pamphlet US Patent Application Publication No. 2006/074160 International Publication No. 2006/109882 Pamphlet US Patent Application Publication No. 2007/293632 International Publication No. 2006/008905 Pamphlet

  However, even when the anti-coloring technology disclosed in Patent Documents 1 to 27 is applied to biomass-derived monomers and water-absorbing resins, variations in the whiteness and coloration over time of the water-absorbing resins are still observed. It was difficult to say that the effect was sufficient.

  Furthermore, in the anti-coloring technology such as increasing the purity of the biomass-derived monomer, optimizing the polymerization conditions and drying conditions of the water-absorbent resin, or using a new anti-coloring agent (for example, a reducing agent), There is a possibility that problems such as an increase in production cost, a decrease in productivity, and a decrease in safety and water absorption characteristics due to the use of a coloring inhibitor may occur.

  The present invention has been made in view of the above problems, and its object is to provide a water-absorbing resin with high whiteness and little coloration over time without sacrificing productivity, manufacturing cost, safety, and the like. There is to do.

  A further object is a water-absorbent resin that does not require excessive purification steps even with biomass-derived acrylic acid, and is consumed in large quantities by paper diapers, etc., and is a renewable water-absorbent resin that is friendly to the global environment, An object of the present invention is to provide a water-absorbent resin having high whiteness and little coloration with time.

  As a result of diligent studies to solve such problems, when acrylic acid synthesized from biomass starting from C4 plants is used as a raw material for the water-absorbent resin, dihydroxyacetone by-produced during alcohol fermentation of C4 plants is the cause of coloring. It has been found that it is a substance, and by controlling the content of dihydroxyacetone in the acrylic acid to a specific amount, it has been found that it is possible to improve the whiteness of the resulting water-absorbent resin and to reduce coloring over time, and to completed.

That is, in order to solve the above problems, the present invention is a polyacrylic acid (salt) water-absorbing resin having an internal cross-linked structure obtained by polymerizing a water-soluble unsaturated monomer, which is measured by accelerator mass spectrometry. Provided is a polyacrylic acid (salt) -based water-absorbing resin characterized by having a stable carbon isotope ratio (δ ¹³ C) of −20 ‰ or more.

In order to solve the above problems, the present invention provides a polyacrylic acid (salt) -based water-absorbing resin comprising a step of polymerizing a water-soluble unsaturated monomer and a step of drying the resulting hydrogel crosslinked polymer. A polyacrylic acid (salt) system, characterized in that acrylic acid having a carbon stable isotope ratio (δ ¹³ C) measured by accelerator mass spectrometry of −20 ‰ or more is used as a monomer A method for producing a water absorbent resin is provided.

By using a biomass source as a raw material for the water-absorbing resin, the carbon stable isotope ratio (δ ¹³ C) of the obtained water-absorbing resin is -20 ‰ or more, a water-absorbing resin having high whiteness and little coloration with time is obtained. Obtainable.

  Hereinafter, the polyacrylic acid (salt) water-absorbing resin and the production method thereof according to the present invention will be described in detail. However, the scope of the present invention is not limited to these descriptions, and the present invention is not limited to the following examples. Modifications and implementations can be made as appropriate without departing from the spirit of the invention. Specifically, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the claims, and technical means disclosed in different embodiments are appropriately combined. Embodiments obtained in this manner are also included in the technical scope of the present invention.

[1] Definition of terms (1-1) "Water absorbent resin"
The “water-absorbing resin” in the present invention means a water-swellable water-insoluble polymer gelling agent. “Water swellability” means that the CRC (water absorption capacity under no pressure) specified by ERT441.2-02 is usually 5 [g / g] or more, and “water insolubility” It means that Ext (water-soluble content) specified by ERT470.2-02 is usually 0 to 50% by weight.

  The water-absorbent resin can be appropriately designed according to its use and is not particularly limited, but may be a hydrophilic cross-linked polymer obtained by cross-linking an unsaturated monomer having a carboxyl group. preferable. Further, the total amount (100% by weight) is not limited to the form of a polymer, and may contain additives and the like within a range that maintains the above performance.

(1-2) “Polyacrylic acid (salt) water-absorbing resin”
The “polyacrylic acid (salt) -based water-absorbing resin” in the present invention is a water-absorbing resin mainly composed of acrylic acid and / or a salt thereof (hereinafter referred to as “acrylic acid (salt)”) as a repeating unit. Means.

  Specifically, among the total monomers (excluding the crosslinking agent) used in the polymerization, a polymer containing usually 50 to 100 mol% of acrylic acid (salt) is mentioned, preferably 70 to 100 mol%, more preferably. Means a water-absorbing resin containing 90 to 100 mol%, particularly preferably substantially 100 mol%.

(1-3) “EDANA” and “ERT”
“EDANA” is an abbreviation for European Disposables and Nonwovens Associations, and “ERT” is an abbreviation for a method for measuring water-absorbent resin (EDANA Recommended Test Method), which is a European standard (almost world standard). is there. In the present invention, unless otherwise specified, the physical properties of the water-absorbent resin are measured in accordance with the ERT original (known document: revised in 2002).

(A) "CRC" (ERT441.2-02)
“CRC” is an abbreviation for Centrifugation Retention Capacity (centrifuge retention capacity) and means water absorption capacity without pressure (hereinafter also referred to as “water absorption capacity”). Specifically, it is the water absorption capacity (unit: [g / g]) after 30 minutes of free swelling with respect to a 0.9 wt% sodium chloride aqueous solution and further draining with a centrifuge for 0.2 g of water absorbent resin. .

(B) “AAP” (ERT442.2-02)
“AAP” is an abbreviation for Absorption Against Pressure, which means water absorption capacity under pressure. Specifically, the water absorption capacity (unit: [g / g]) after swelling under a load of 2.06 kPa (0.3 psi) for 1 hour with respect to a 0.9 wt% sodium chloride aqueous solution. Note that the load condition may be changed to 4.83 kPa (0.7 psi) for measurement.

(C) "Ext" (ERT470.2-02)
“Ext” is an abbreviation for Extractables and means a water-soluble component (water-soluble component amount). Specifically, it is a value (unit:% by weight) measured by pH titration after stirring 1 g of water absorbent resin at 500 rpm for 16 hours with respect to 200 g of 0.9 wt% sodium chloride aqueous solution. .

(D) "FSC" (ERT440.2-02)
“FSC” is an abbreviation for Free Well Capacity and means free swelling ratio. Specifically, the water absorption capacity (unit: [g / g]) measured without immersing 0.20 g of the water-absorbing resin in a 0.9 wt% sodium chloride aqueous solution for 30 minutes and then draining with a centrifuge. is there.

(E) “Residual Monomers” (ERT410.2-02)
“Residual Monomers” means the amount of monomer remaining in the water-absorbent resin. Specifically, 1.0 g of a water-absorbing resin was stirred for 1 hour at 500 rpm with respect to 200 cm ³ of a 0.9 wt% sodium chloride aqueous solution, and the amount of dissolved residual monomer was measured by HPLC (high performance liquid chromatography). Value (unit: ppm).

(F) "PSD" (ERT420.2-02)
“PSD” is an abbreviation for Particle Size Distribution, and means a particle size distribution measured by sieve classification. The weight average particle size (D50) and the particle size distribution range are the same as those described in “(1) Average Particle Diameter and Distillation of Particle Diameter” described in European Patent No. 0349240, page 7, lines 25-43. Measure with

(1-4) “Liquid permeability”
The flow of the liquid flowing between the particles of the swollen gel under load or no load is called “liquid permeability”. Typical measurement methods for this “liquid permeability” include SFC (Saline Flow Conductivity) and GBP (Gel Bed Permeability).

  “SFC (saline flow inductivity)” refers to the permeability of 0.69 wt% saline to a water absorbent resin at a load of 0.3 psi. It is measured according to the SFC test method described in US Pat. No. 5,669,894.

  “GBP” refers to the permeability of 0.69 wt% physiological saline to a water-absorbent resin under load or free expansion. It is measured according to the GBP test method described in International Publication No. 2005/016393 pamphlet.

(1-5) “Initial color tone and coloring over time”
The “initial color tone” in the present invention refers to the color tone of the water absorbent resin immediately after production or the water absorbent resin immediately after user shipment, and is usually managed by the color tone before factory shipment. Examples of the color tone measurement method include the methods described in International Publication No. 2009/005114 (Lab value, YI value, WB value, etc.).

  Further, “coloring with time” refers to a change in the color tone of the water-absorbent resin that occurs during long-term storage or distribution in an unused state. Since the water-absorbent resin is colored over time, the commercial value of the paper diaper can be reduced. Since coloration with time occurs in units of several months to several years, it is verified by an accelerated test (accelerated test under high temperature and high humidity) disclosed in International Publication No. 2009/005114.

(1-6) “Biomass”
“Biomass” in the present invention refers to an industrial resource derived from a living organism constituent material that is not a depleting resource, and refers to a renewable, organic organic resource excluding fossil resources.

  Biomass captures carbon dioxide in the atmosphere by photosynthesis during its growth process. Therefore, even if biomass is burned and carbon dioxide is discharged, the amount of carbon dioxide in the atmosphere does not increase as a whole. This property is called carbon neutral, which is a preferable property from the viewpoint of the global environment.

  Biomass may be derived from a single source or a mixture, for example, onion cobs and stems, grass and leaves. Biomass is not limited to biofuel crops, agricultural residues, municipal waste, industrial waste, papermaking Industrial sludge, pasture waste, timber and forest waste.

(1-7) “Carbon stable isotope ratio (δ ¹³ C)”
In the present invention, the “carbon stable isotope ratio (δ ¹³ C)” refers to three types of isotopes of carbon atoms existing in nature (abundance ratio ¹² C: ¹³ C: ¹⁴ C = 98.9: 1.11: 1.2 × 10 ⁻¹² units;%), the ratio of ¹³ C to ¹² C is good, and is a value defined by the following formula.

Here, PDB is an abbreviation for “Pee Dee Belemnite”, which means a fossil of an arrowhead composed of calcium carbonate, and is used as a standard of ¹³ C / ¹² C ratio. Further, the “carbon stable isotope ratio (δ ¹³ C)” is measured by an accelerator mass spectrometry (AMS method; Accelerator Mass Spectrometry).

(1-8) “C3 plant and C4 plant”
The “C3 plant and C4 plant” in the present invention refers to a classification based on the difference in plant photosynthesis mechanism, and a group of plants that perform C3 type photosynthesis is referred to as a C3 plant, and a group of plants that performs C4 type photosynthesis is referred to as a C4 plant.

  C3-type photosynthesis refers to synthesizing 3-phosphoglyceric acid having 3 carbon atoms from carbon dioxide taken into the plant body, and includes most land plants (about 90%).

  C4-type photosynthesis refers to the synthesis of oxalloacetic acid having 4 carbon atoms from carbon dioxide incorporated into the plant, and includes plants such as corn, sugar cane, and buckwheat.

  In addition to the C3 type photosynthesis and C4 type photosynthesis, there is a plant group that performs special photosynthesis called CAM type photosynthesis. This CAM type photosynthesis uses carbon dioxide that is taken into the plant body at night and stored in the form of malic acid, etc., and uses oxygen dioxide produced by decomposing it in the daytime. This applies to succulent plants such as cacti and pineapples. .

(1-9) Tracer / Traceability This refers to substances and properties of trace amounts added to follow up the state and range of propagation. In the present invention, a specific range of 13C, more preferably 14C, is used for the water-absorbent resin, but it has traceability within a range that can be distinguished from a commercially available or known water-absorbent resin by 13C (and 14C amount).

(1-10) Others In this specification, “X to Y” indicating a range means “X or more and Y or less”. Further, “t (ton)” which is a unit of weight means “Metric ton”, and unless otherwise noted, “ppm” means “ppm by weight” or “ppm by mass”. "Means.

[2] Production method of polyacrylic acid (salt) water-absorbing resin (2-1) Acrylic acid production process This step is an acrylic acid used as a raw material for polyacrylic acid (salt) -based water absorbent resin, in particular, acrylic from biomass. This is a step of obtaining an acid.

(Acrylic acid)
The acrylic acid used in the present invention is not particularly limited as long as the carbon stable isotope ratio (δ ¹³ C) of acrylic acid is −20 ‰ or more. For example, acrylic acid may be synthesized from propylene obtained by using ethanol, isopropanol, methanol or the like synthesized from a C4 plant such as corn or sugarcane as a starting material. Furthermore, natural oils and fats, preferably saponified products of vegetable oils and fats, particularly preferably a method of obtaining acrylic acid by dehydration / oxidation reaction of glycerin obtained from transesterification of natural fats and oils, β-alanine, lactic acid, glycerin, glucose, Examples thereof include a method of obtaining acrylic acid by dehydration reaction of 2-hydroxypropionic acid or 3-hydroxypropionic acid obtained by a fermentation method such as starch or cellulose. In addition to these, a method using acetylene obtained from biomass, a method by reaction of ethylene oxide obtained from biomass and carbon dioxide, a method by reaction of formalin obtained from biomass and acetic acid, and the like can be mentioned.

  Furthermore, as for the propylene currently used as a raw material for acrylic acid, a production method derived from biomass is exemplified instead of the cracking method of crude oil which is a fossil resource. Specifically, metathesis reaction of butene with ethylene, dehydration reaction of propanol, dehydration reduction reaction of glycerol, catalytic decomposition of butene, GTL synthesis reaction of biogas (synthesis gas), synthesis of methanol from biogas (synthesis gas) And MTO synthesis reaction via biomass, dehydrogenation reaction of biomass propane, and the like.

In the various production methods described above, ethylene, propanol, butene, glycerin, biogas, and the like are listed as starting materials. The production routes of these substances from biogas are as follows. That is, a method of obtaining ethylene and / or butene from biomass through ethanol, a method of obtaining butanol and / or butene from biomass through ethanol, a method of obtaining butene from biomass through butanol, and i-propanol from biomass through acetone. Examples thereof include a method, a method of obtaining n-propanol and / or iso-propanol from biomass, a method of obtaining BDF and glycerin from biomass, a method of obtaining synthesis gas (CO, H ₂ ) from biomass, and the like.

  About the manufacturing method of these acrylic acids derived from biomass, it is illustrated by international publication 2006/08024, 2007/119528, 2007/132926, US Patent application publication 2007/0129570, etc., for example. ing. International Publication No. 2006/08024 discloses the fact that propanal is by-produced when acrolein is obtained from glycerin, and the acrylic acid of the present invention can be easily obtained by oxidizing acrolein containing such propanal. .

  In addition, about the manufacturing method of the water absorbing resin using acrylic acid derived from biomass, international publication 2006/092711, 2006/0692272, 2006/136336, 2008/023039, Examples are 2008/023040 and 2007/109128. However, the above six patent documents do not disclose or suggest any method for producing the water absorbent resin of the present invention.

(Dihydroxyacetone)
In the present invention, it has been found that dihydroxyacetone has an adverse effect on the initial color tone and coloration over time of the water-absorbent resin.

  That is, the acrylic acid of the present invention preferably contains 0 to 500 ppm of dihydroxyacetone (as opposed to acrylic acid) as an impurity. The upper limit of the content of dihydroxyacetone is preferable in the order of 200 ppm, 100 ppm, 50 ppm, 20 ppm, 10 ppm, 5 ppm, 1 ppm, and 0.1 ppm. In addition, high purification has a problem of cost and yield of acrylic acid, and, as described in the examples below, since a predetermined amount may sometimes improve initial coloring (particularly WB), dihydroxy About 0.1 ppm of acetone may be present. When dihydroxyacetone exceeds 500 ppm in the monomer or acrylic acid used, a small amount of polymer is easily produced as a by-product in the monomer before polymerization, that is, the monomer before addition of the polymerization initiator, particularly the monomer during degassing, Not only is it unfavorable for industrial stable production, such as piping clogging for trace amounts of gel, but also the coloration of the resulting water-absorbent resin over time (coloration during storage and transportation from production to use) is reduced. It is not preferable.

  In the present invention, by reducing dihydroxyacetone, non-polymerizable acetic acid, which is an oxide thereof, can be reduced, so that the acid odor of the resulting water-absorbent resin can also be reduced.

  Conventionally, when the polymerization start temperature and concentration are increased for physical properties and productivity, the polymerization scale is increased, or the polymerization inhibitor is decreased to prevent coloring, partial monomer gelation occurs. It was happening. Therefore, in the present invention, it has been found that dihydroxyacetone decreases the stability of the monomer as the causal physical property. Dihydroxyacetone is preferably contained in an amount of about 0.1 to 500 ppm based on acrylic acid.

  That is, paying attention to the influence of dihydroxyacetone in acrylic acid, dihydroxyacetone reduces the stability of the monomer before polymerization, and is a cause of coloration with time of the water-absorbent resin, and is derived from C4 plants. Since acrylic acid contains a relatively large amount of dihydroxyacetone, it was found that the resulting water-absorbent resin was easily colored, and the present invention was completed. Moreover, since dihydroxyacetone is comparatively easy to be contained when dihydroxyacetone is obtained from a natural product, particularly C4 plant, dihydroxyacetone is relatively easily contained, the method of the present invention is suitably applied.

In addition, all the control methods (particularly paragraphs [0015] to [0039] and paragraphs [0042] to [0054] of Examples) disclosed in International Publication No. 2008/053646 are incorporated in the present application. Moreover, although the said patent documents 17 and 18 disclose the technique which superposes | polymerizes a water absorbing resin with acrylic acid derived from glycerol, similarly, a predetermined amount of phenol and hydroxyacetone are contained, and a water absorbing resin is manufactured in a specific process. The technology is not disclosed, and in combination with a specific polymerization inhibitor (preferably p-methoxyphenol 25 to 200 ppm) or Fe (preferably 0.005 to ₃ ppm / Fe ₂ O ₃ equivalent) in the control of phenol or hydroxyacetone Also not disclosed.

(Chelating agent)
The particulate water-absorbing agent of the present invention has, for example, further improved color stability (color stability when the particulate water-absorbing agent is stored for a long period of time under high temperature and high humidity conditions) and urine resistance (gel degradation prevention). For the purpose of improvement, a chelating agent (more preferably a water-soluble organic chelating agent) is preferably used.

  In view of the effect, the chelating agent is preferably a water-soluble organic chelating agent, more preferably a non-polymeric organic chelating agent having a nitrogen atom or a phosphorus atom, more preferably an amino polyvalent carboxylic acid. A system chelating agent or an amino polyvalent phosphoric acid chelating agent is used. Nonpolymeric organic compounds having a weight average molecular weight of 5,000 or less are preferred, and a molecular weight of 100 to 1,000 is more preferred, from the influence on polymerization and the properties obtained.

  Of these, compounds having a nitrogen atom or a phosphorus atom are preferred, more preferably two or more carboxyl groups in the molecule, more preferably 3 or more, preferably 3 to 100, more preferably 3 to 20, especially 3 to 10 An aminopolycarboxylic acid (salt) or an organic phosphoric acid (salt) compound having a phosphoric acid group is preferred. Organic polyvalent phosphoric acid compounds and amino polyvalent phosphoric acid compounds having an amino group are preferred.

  Examples of amino polycarboxylic acids (salts) having two or more carboxyl groups include iminodiacetic acid, hydroxyethyliminodiacetic acid, nitrilotriacetic acid, nitrilotripropionic acid, ethylenediaminetetraacetic acid, hydroxyethylenediaminetriacetic acid, hexamethylenediamine Examples include aminocarboxylic acid metal chelating agents such as tetraacetic acid, diethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic acid, and salts thereof.

  Examples of the organic polyvalent phosphoric acid compound or amino polyvalent organic phosphoric acid compound having three or more phosphoric acid groups in the molecule include ethylenediamine-N, N′-di (methylenephosphinic acid) and ethylenediaminetetra (methylenephosphinic acid). Polymethylenediaminetetra (methylenephosphonic acid), diethylenetriaminepenta (methylenephosphonic acid), 1-hydroxyethylidenediphosphonic acid, and salts thereof.

(Hydroxycarboxylic acid compound)
Furthermore, for the purpose of color stability effect, the lactic acid (salt) and citric acid (salt) exemplified in the international application 2007 / JP / 67348 (international application date August 30, 2007) which has not been published at the time of filing of the present application. Hydroxy carboxylic acid such as malic acid (salt), particularly non-polymeric hydroxy carboxylic acid (salt) may be used as the monomer or polymer thereof.

(Reducing inorganic salt)
Furthermore, for the color stability effect, reducing inorganic salts exemplified in US 2006/88115 may be used.

(Other impurities)
Among 6 types of impurities such as protoanemonin, allyl acrylate, allyl alcohol, aldehyde content (particularly furfural), maleic acid and benzoic acid in the acrylic acid, 1 or more, 2 or more, 3 or more, 4 or more, 5 or more 6 are each 0 to 20 ppm (mass basis, the same applies hereinafter). Preferably, each is 0 to 10 ppm, more preferably 0 to 5 ppm, still more preferably 0 to 3 ppm, particularly preferably 0 to 1 ppm, and most preferably ND (detection limit). Further, the total amount of protoanemonin, allyl acrylate, allyl alcohol, aldehyde, maleic acid and benzoic acid (with respect to acrylic acid mass) is preferably 100 ppm or less, more preferably 0 to 20 ppm, and even more preferably 0 to 10 ppm. preferable. As a suitable method for controlling these trace components and the amount of propionic acid, acrylic acid derived from the following non-fossil raw materials is used.

(2-2) Polymerization step In this step, an aqueous solution containing acrylic acid (salt) obtained in the above (2-1) acrylic acid production step as a main component is polymerized to form a hydrogel crosslinked polymer (hereinafter, This is a step of obtaining a “hydrogel”.

(Monomer) Excluding the cross-linking agent The water-absorbent resin obtained in the present invention uses an aqueous solution containing acrylic acid (salt) as a main component as a raw material (monomer), and is usually polymerized in an aqueous solution state. The The monomer concentration in the monomer aqueous solution is usually 10 to 90% by weight, preferably 20 to 80% by weight, more preferably 30 to 70% by weight, and further preferably 40 to 60% by weight.

  Moreover, it is preferable that the hydrogel obtained by superposition | polymerization of this aqueous solution is neutralized at least one part of the acid group of a polymer from a viewpoint of water absorption performance. The neutralization can be carried out before, during or after the polymerization of acrylic acid. For example, productivity of the water-absorbent resin, improvement of AAP (water absorption capacity under pressure) and SFC (saline flow conductivity) can be improved. From the viewpoint, neutralization is preferably performed before polymerization of acrylic acid. That is, it is preferable to use neutralized acrylic acid (that is, a partially neutralized salt of acrylic acid) as a monomer.

  The neutralization rate of the neutralization is not particularly limited, but is preferably 10 to 100 mol%, more preferably 30 to 95 mol%, still more preferably 50 to 90 mol%, and more preferably 60 to 80 mol% with respect to the acid group. Is particularly preferred. When the neutralization rate is less than 10 mol%, CRC (water absorption capacity under no pressure) may be particularly lowered, which is not preferable.

  In the present invention, when acrylic acid (salt) is used as a main component, a hydrophilic or hydrophobic unsaturated monomer other than acrylic acid (salt) (hereinafter also referred to as “other monomer”). Can also be used. Such other monomers are not particularly limited, but include methacrylic acid, (anhydrous) maleic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, (meth) acryloxyalkanesulfonic acid, N- Vinyl-2-pyrrolidone, N-vinylacetamide, (meth) acrylamide, 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. When using these other monomers, the amount used is not particularly limited as long as it does not impair the water-absorbing properties of the resulting water-absorbent resin, but is 50% by weight based on the weight of the total monomers. The following is preferable, and 20% by weight or less is more preferable.

(Neutralized salt)
The basic substance used for neutralization of acrylic acid as the monomer or polymer after polymerization (hydrogel) is not particularly limited, but alkali metals such as sodium hydroxide, potassium hydroxide and lithium hydroxide. Monovalent basic substances such as carbonates (hydrogen) salts such as sodium hydroxide and hydrogen carbonate, such as sodium carbonate and hydrogen carbonate, are preferred, and sodium hydroxide is particularly preferred. Moreover, it does not restrict | limit especially about the temperature at the time of neutralization (neutralization temperature), 10-100 degreeC is preferable and 30-90 degreeC is more preferable. In addition, about the neutralization process conditions other than the above, the conditions disclosed in International Publication No. 2006/522181 are preferably applied to the present invention.

(Internal crosslinking agent)
In the present invention, it is particularly preferable to use a cross-linking agent (hereinafter also referred to as “internal cross-linking agent”) from the viewpoint of the water absorption performance of the resulting water-absorbent resin. The internal crosslinking agent that can be used is not particularly limited, and examples thereof include a polymerizable crosslinking agent with acrylic acid, a reactive crosslinking agent with a carboxyl group, and a crosslinking agent having both of them. Specifically, N, N′-methylenebisacrylamide, (poly) ethylene glycol di (meth) acrylate, (polyoxyethylene) trimethylolpropane tri (meth) acrylate, poly (meth) ary can be used as the polymerizable crosslinking agent. Examples thereof include compounds having at least two polymerizable double bonds in the molecule, such as roxyalkane. In addition, as reactive crosslinking agents, polyglycidyl ethers such as ethylene glycol diglycidyl ether; covalent crosslinking agents such as polyhydric alcohols such as propanediol, glycerin and sorbitol, and ionic bonds that are polyvalent metal compounds such as aluminum salts An example of the functional cross-linking agent. Among these, from the viewpoint of water absorption performance, a polymerizable crosslinking agent with acrylic acid is preferable, and acrylate-based, allyl-based, and acrylamide-based polymerizable crosslinking agents are particularly preferably used. These internal crosslinking agents may be used alone or in combination of two or more. The amount of the internal crosslinking agent used is preferably from 0.001 to 5 mol%, more preferably from 0.005 to 2 mol%, more preferably from 0.01 to 5 mol%, based on physical properties, with respect to the monomer excluding the crosslinking agent. 1 mol% is more preferable and 0.03-0.5 mol% is especially preferable.

  Therefore, it is impossible to obtain a water-absorbing resin having high physical properties with the uncrosslinked product exemplified in Patent Document 5 (internal crosslinking agent is not used). Therefore, in the present invention, a hydrogel crosslinked polymer is essential. .

(Other components in the monomer aqueous solution)
In order to improve various physical properties of the water-absorbent resin obtained in the present invention, the following substances can be added as optional components to the monomer aqueous solution. That is, water-soluble resin or water-absorbing resin such as starch, polyacrylic acid (salt), polyvinyl alcohol, and polyethyleneimine is, for example, 0 to 50% by weight, preferably 0 to 20% by weight, based on the monomer. Preferably 0 to 10% by weight, more preferably 0 to 3% by weight can be added. Furthermore, various foaming agents (carbonates, azo compounds, bubbles, etc.), surfactants, various chelating agents, additives such as hydroxycarboxylic acid and reducing inorganic salts are added to the monomer, for example, 0-5. % By weight, preferably 0 to 1% by weight can be added.

  Of these, chelating agents, hydroxycarboxylic acids, and reducing inorganic salts are preferably used for the purpose of suppressing coloration with time of the water-absorbing resin and improving urine resistance (preventing gel degradation). Particularly preferably used. The amount used in this case is preferably 10 to 5000 ppm, more preferably 10 to 1000 ppm, still more preferably 50 to 1000 ppm, and particularly preferably 100 to 1000 ppm with respect to the water absorbent resin. In addition, about the said chelating agent, hydroxycarboxylic acid, and a reducing inorganic salt, the compound disclosed by international publication 2009/005114, European Patent Nos. 2057228, and 1848758 is used.

(Polymerization initiator)
The polymerization initiator used in the present invention is appropriately selected depending on the polymerization form and is not particularly limited. For example, a thermal decomposition polymerization initiator, a photodecomposition polymerization initiator, a redox polymerization initiator, and the like can be given. Specifically, examples of the thermal decomposition 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; 2 , 2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, and the like. Examples of the photodegradable polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives, and azo compounds. Further, examples of the redox polymerization initiator include a system in which a reducing compound such as L-ascorbic acid or sodium hydrogen sulfite is combined with the above-described persulfate or peroxide. A combination of the thermal decomposition type polymerization initiator and the photodecomposition type polymerization initiator can also be mentioned as a preferred embodiment. 0.0001-1 mol% is preferable with respect to the said monomer, and, as for the usage-amount of these polymerization initiators, 0.001-0.5 mol% is more preferable. When the usage-amount of a polymerization initiator exceeds 1 mol%, since coloring of a water absorbing resin may be caused, it is unpreferable. Moreover, when the usage-amount of a polymerization initiator is less than 0.0001 mol%, since there exists a possibility of increasing a residual monomer, it is unpreferable.

  Instead of using the above polymerization initiator, polymerization may be carried out by irradiating active energy rays such as radiation, electron beam, ultraviolet rays, etc., and polymerization is carried out using these active energy rays and a polymerization initiator in combination. May be.

(Polymerization method)
In the present invention, when polymerizing the monomer aqueous solution, from the viewpoint of water absorption performance of the water-absorbing resin obtained and ease of polymerization control, aqueous solution polymerization or reverse phase suspension polymerization is usually employed. Preferably aqueous solution polymerization, more preferably continuous aqueous solution polymerization is employed. Especially, it is preferably applied to manufacture on a huge scale with a large production amount per line of water-absorbing resin. The production amount is preferably 0.5 [t / hr] or more, more preferably 1 [t / hr] or more, further preferably 5 [t / hr] or more, and particularly preferably 10 [t / hr]. That's it. Further, as preferred forms of the aqueous solution polymerization, continuous belt polymerization (disclosed in US Pat. Nos. 4,893,999, 6,241,928, US Patent Application Publication No. 2005/215734, etc.), continuous kneader polymerization, batch kneader polymerization (US Pat. No. 6987151, No. 6710141, etc.), among which continuous belt polymerization is particularly preferable.

  In the continuous aqueous solution polymerization, high temperature initiation polymerization with a polymerization initiation temperature of preferably 30 ° C. or higher, more preferably 35 ° C. or higher, further preferably 40 ° C. or higher, particularly preferably 50 ° C. or higher (the upper limit is the boiling point), or High monomer concentration polymerization in which the monomer concentration is preferably 35% by weight or more, more preferably 40% by weight or more, and further preferably 45% by weight or more (the upper limit is a saturated concentration) can be exemplified as a most preferable example. The polymerization initiation temperature is defined by the liquid temperature immediately before supplying the monomer aqueous solution to the polymerization machine. The conditions disclosed in US Pat. Nos. 6,906,159 and 7091253 are preferably applied to the present invention. can do.

  Furthermore, in the present invention, it is preferable to evaporate water during the polymerization from the viewpoint of improving the physical properties and drying efficiency of the resulting water-absorbent resin. That is, in the polymerization of the present invention, a water gel having a higher solid content may be obtained, and the solid content increase degree (“solid content of water gel after polymerization” − “monomer concentration before polymerization”) is 1 % By weight or more is preferable, 2 to 40% by weight is more preferable, and 3 to 30% by weight is further preferable. However, the solid content of the obtained hydrogel is preferably 80% by weight or less.

  These polymerizations can be carried out in an air atmosphere. However, from the viewpoint of preventing coloring, it is preferred to carry out under an inert gas atmosphere such as nitrogen or argon (for example, an oxygen concentration of 1% by volume or less). Moreover, it is preferable to polymerize after replacing the dissolved oxygen in the monomer or the solution containing the monomer with an inert gas (for example, dissolved oxygen concentration; less than 1 mg / L). Moreover, it can implement under any pressure of pressure reduction, a normal pressure, and pressurization.

(2-3) Gel Crushing Step This step is a step of crushing the hydrated gel obtained in the polymerization step to obtain a particulate hydrated gel (hereinafter referred to as “particulate hydrated gel”).

  The hydrogel obtained in the above polymerization step may be dried as it is, but in order to solve the problem, a crusher (kneader, meat chopper, cutter mill, etc.) is preferably used during or after the polymerization, if necessary. Use it to crush the gel to form particles. That is, a hydrated gel refinement (hereinafter also referred to as “gel crushing”) step may be further included between the polymerization step by continuous belt polymerization or continuous kneader polymerization and the drying step. In addition, when the gel is finely divided by dispersion in a solvent at the time of polymerization, such as reverse phase suspension polymerization, it is included in the fine particleization (fine particleization of the polymerization step) of the present invention. It is crushed using a crusher.

  From the viewpoint of physical properties, the temperature of the hydrogel at the time of gel crushing is preferably kept at 40 to 95 ° C. or more preferably 50 to 80 ° C. Moreover, the resin solid content of the particulate water-containing gel at the time of gel crushing or after crushing is not particularly limited, but is preferably 55 to 80% by weight from the viewpoint of physical properties. In the gel crushing step, water, polyhydric alcohol, a mixture of water and polyhydric alcohol, a polyvalent metal aqueous solution, or a vapor thereof is added as necessary for the purpose of improving crushing efficiency. May be. Further, when pulverizing a hydrogel having a high solid content concentration (for example, 55 to 80% by weight described above) to which the present invention can be preferably applied, even if ventilation air, preferably dry air is passed through the pulverizer. Good.

  The weight average particle diameter (D50) of the particulate hydrogel after gel crushing is preferably 0.2 to 4 mm, more preferably 0.3 to 3 mm, from the viewpoint of controlling the falling scattering rate low. 2 mm is more preferable. Since the weight average particle diameter (D50) of the particulate hydrous gel is within the above range, drying is performed efficiently, which is preferable. Moreover, 0 to 10 weight% of the whole particulate water-containing gel is preferable, and, as for the ratio of the particulate water-containing gel which has a particle size of 5 mm or more, 0 to 5 weight% is more preferable.

  In addition, the particle diameter of the said particulate water-containing gel is calculated | required by classifying with the sieve of a specific opening similarly to the particle diameter of the water absorbing resin after a grinding | pulverization process. Further, the weight average particle diameter (D50) can be determined in the same manner. However, when the classification operation of the particulate hydrous gel is difficult to measure due to aggregation or the like in the dry classification method, the wet classification method described in paragraph [0091] of JP-A-2000-63527 is used. To measure.

(2-4) Drying step If the hydrated gel-like crosslinked polymer obtained in the polymerization step or the crushed gel obtained in the gel atomization step can be dried to the desired resin solid content, the method Although not particularly limited, for example, heat drying, hot air drying, vacuum drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, dehydration drying by azeotropy with a hydrophobic organic solvent, high-temperature steam was used Various drying methods such as high-humidity drying can be employed.

  Among these, hot air drying is preferable, hot air drying with a gas having a dew point temperature of 40 to 100 ° C. is more preferable, and hot air drying with a gas having a dew point temperature of 50 to 90 ° C. is more preferable. When hot air drying is used, the wind speed (velocity of the airflow passing perpendicularly to the drying object spreading horizontally) is preferably 0.01 to 10 [m / s], and preferably 0.1 to 5 [m / s. ] Is more preferable.

  In the present invention, the drying temperature to be applied is not particularly limited, but is preferably 50 to 300 ° C. (preferably performed under reduced pressure when it is 100 ° C. or lower), more preferably 100 to 250 ° C., and 150 to 200. More preferably. Moreover, as drying time, 10 to 120 minutes are preferable, 20 to 90 minutes are more preferable, and 30 to 60 minutes are more preferable. When the drying time is less than 10 minutes, the change occurring in the polymer chain inside the water-absorbent resin is small, and it is considered that a sufficient improvement effect cannot be obtained. On the other hand, when the drying time is 120 minutes or more, the water-absorbent resin is damaged, and the water-soluble content is increased, and the effects of improving various physical properties may not be observed.

  Furthermore, in order to achieve both physical properties and whiteness of the obtained particulate water-absorbing agent, the drying temperature is preferably within a range of 165 to 230 ° C., and the drying time is preferably within 50 minutes, particularly preferably 20 to 40 minutes. If the drying temperature or drying time is out of this range, the particulate water-absorbing agent may have a decrease in CRC (water absorption capacity under no pressure), an increase in water-soluble content, and a decrease in whiteness.

  Further, the resin solid content obtained from the loss on drying of the hydrogel crosslinked polymer or crushed gel (weight change when 1 g of the water-absorbent resin is heated at 180 ° C. for 3 hours) is preferably 80% by weight or more, 99 weight% is more preferable, 90 to 98 weight% is further more preferable, and 92 to 97 weight% is especially preferable. In the drying step, a dry polymer whose dry weight is adjusted within the above range is obtained.

  Furthermore, in order to reduce the residual monomer of the obtained particulate water-absorbing agent, prevent gel degradation (urine resistance), and prevent yellowing, the time from the completion of polymerization to the start of drying may be shortened. preferable. That is, regardless of the presence or absence of the above-described gel granulation step, the time from the completion of polymerization to the start of drying is preferably within 1 hour, more preferably within 0.5 hour, and within 0.1 hour More preferably. During this period, the temperature of the hydrogel crosslinked polymer or crushed gel is preferably controlled to 50 to 80 ° C, more preferably 60 to 70 ° C. By controlling within this temperature range, it is possible to achieve a reduction in residual monomer and low coloration.

(2-5) Crushing step and classification step This step is a step of crushing and classifying the dried product obtained in the drying step to obtain a water absorbent resin.

  In this step, the dried product obtained in the drying step can be used as a dry powder as it is, but it is preferable to control to a specific particle size in order to improve physical properties in the surface crosslinking step described later. The particle size control is not limited to the main pulverization step and the classification step, and can be appropriately performed in a polymerization step (particularly reverse phase suspension polymerization), a fine powder collection step, a granulation step, and the like. Hereinafter, the particle size is defined by a standard sieve (JIS Z8801-1 (2000)).

  The pulverizer that can be used in the pulverization step is not particularly limited, and a conventionally known pulverizer can be used. Specific examples include a roll mill, a hammer mill, a roll granulator, a jaw crusher, a gylet crusher, a cone crusher, a roll crusher, and a cutter mill. Among these, it is preferable to use a multistage roll mill or roll granulator from the viewpoint of particle size control.

  In the classification step, various classifiers such as sieve classification and airflow classification can be used.

  From the viewpoint of improving the physical properties of the water-absorbent resin obtained in this step, it is preferable to control the particle size to be as follows. That is, the weight average particle diameter (D50) of the water absorbent resin before surface crosslinking is preferably 200 to 600 μm, more preferably 200 to 550 μm, further preferably 250 to 500 μm, and particularly preferably 350 to 450 μm. Further, the ratio of fine particles passing through a sieve having a mesh size of 150 μm (JIS standard sieve) is preferably 0 to 5% by weight, more preferably 0 to 3% by weight, based on the whole water-absorbent resin, and 0 to 1 More preferred is weight percent. In addition, the ratio of huge particles that do not pass through a sieve having an opening of 850 μm (JIS standard sieve) is preferably 0 to 5% by weight, more preferably 0 to 3% by weight, based on the entire water-absorbent resin, and 0 to 1 More preferred is weight percent. Furthermore, the logarithmic standard deviation (σζ) of the particle size distribution of the water absorbent resin is preferably 0.20 to 0.40, more preferably 0.25 to 0.37, and further preferably 0.25 to 0.35. These particle sizes are disclosed in International Publication No. 2004/69915 and EDANA-ERT420.2. Measured by the method disclosed in -02 (Particle Size Distribution).

  Generally, when the particle size distribution is narrowed, that is, when the particle size is controlled so as to approach the upper and lower limits of the particle size, the coloring of the water-absorbent resin is conspicuous in color tone measurement. . Therefore, the particle size distribution of the water-absorbent resin obtained in the present invention is such that the proportion having a particle size of 150 to 850 μm is 95% by weight or more, preferably 98% by weight or more (upper limit is 100% by weight).

(2-6) Surface cross-linking step In this step, the surface vicinity of the water-absorbent resin obtained in the pulverization step and the classification step is cross-linked (surface cross-linking reaction) using a surface cross-linking agent in order to improve water absorption performance. It is a process. By this surface cross-linking treatment, a water-absorbing resin with little coloration and high whiteness can be obtained, and is particularly preferably applied to a water-absorbing resin with high-temperature surface cross-linking. Furthermore, when the water-absorbing resin obtained in the present invention is used as a raw material for sanitary goods (especially paper diapers), AAP (water absorption capacity under pressure) is preferably 20 [g / g] or more by the surface cross-linking treatment. You only have to increase it.

  Although it does not specifically limit as a surface crosslinking agent which can be used by this invention, Various organic or inorganic crosslinking agents can be mentioned. Among these, an organic surface crosslinking agent is preferable, and a combined use of an organic surface crosslinking agent and an ionic crosslinking agent is more preferable. Specifically, polyhydric alcohol compounds, epoxy compounds, polyvalent amine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, (mono, di, or poly) oxazolidinone compounds, alkylene carbonate compounds, particularly at high temperatures. A dehydrating ester-reactive crosslinking agent comprising a polyhydric alcohol compound, an alkylene carbonate compound, or an oxazolidinone compound that requires a reaction can be used. More specifically, compounds exemplified in US Pat. Nos. 6,228,930, 6071976, 6254990, and the like can be exemplified. For example, mono, di, tri, tetra or propylene glycol, 1,3-propanediol, glycerin, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, Polyhydric alcohol compounds such as sorbitol; Epoxy compounds such as ethylene glycol diglycidyl ether and glycidol; Alkylene carbonate compounds such as ethylene carbonate; Oxetane compounds; Cyclic urea compounds such as 2-imidazolidinone. The amount of the surface cross-linking agent used is suitably determined within the range of preferably 0.001 to 10 parts by weight, more preferably 0.01 to 5 parts by weight, with respect to 100 parts by weight of the water absorbent resin.

  Moreover, it is preferable to use water as a solvent when mixing the water absorbent resin and the surface cross-linking agent. The amount of water used is appropriately determined within a range of preferably 0.5 to 20 parts by weight, more preferably 0.5 to 10 parts by weight, with respect to 100 parts by weight of the water absorbent resin. Furthermore, in addition to the above water, a hydrophilic organic solvent may be used in combination, if necessary, and the amount used is preferably 0 to 10 parts by weight, more preferably 0 with respect to 100 parts by weight of the water absorbent resin. It is suitably determined within the range of ˜5 parts by weight. Furthermore, when mixing the surface cross-linking agent solution, water-insoluble fine particle powder or surfactant may be allowed to coexist to the extent that the effects of the present invention are not hindered. The type and amount of the fine particle powder and the surfactant are exemplified in U.S. Pat. No. 7,473,739, and the amount used is preferably 0 to 100 parts by weight of the water absorbent resin. It is appropriately determined within a range of 10 parts by weight, more preferably 0 to 5 parts by weight, and still more preferably 0 to 1 part by weight.

  In this step, the water-absorbent resin and the surface cross-linking agent are mixed, and then preferably heat-treated, and then cooled if necessary. 70-300 degreeC is preferable, the heating temperature at the time of the said heat processing has more preferable 120-250 degreeC, and 150-250 degreeC is further more preferable. When the said processing temperature is less than 70 degreeC, since heat processing time is extended and productivity falls, and a uniform surface crosslinked layer cannot be formed, it is unpreferable. Moreover, when the said processing temperature exceeds 300 degreeC, since a water absorbing resin deteriorates, it is unpreferable. The heating time during the heat treatment is preferably in the range of 1 minute to 2 hours. The said heat processing can be performed with a normal dryer or a heating furnace.

  European Patent Nos. 0349240, 0605150, 0450923, 0818733, 0450924, 0668080, JP-A-7-242709, 7-224304, US Patent Nos. 5,409,771, 5,597,873, 5,385,983, 5,610,220, 5,633,316, 5,674,633, 5,462,972, International Publication Nos. 99/42494, 99/43720, The surface cross-linking method disclosed in JP 99/42496 can be preferably applied to the present invention.

(2-7) Fine powder recycling process This process is performed after separating the fine powder generated in the drying process and, if necessary, the pulverization process and the classification process (particularly the fine powder containing 70% by weight or more of the powder having a particle diameter of 150 μm or less). It refers to a process in which it is hydrated or recycled to a polymerization process or a drying process, and the methods described in US Patent Application Publication No. 2006/247351 and US Pat. No. 6,228,930 can be applied. By recycling the fine powder, the particle size of the water-absorbing resin can be controlled, and by adding the fine powder, a high solid content concentration can be easily achieved. Further, from the ventilation belt of the dryer, It is preferable because the water absorbent resin can be easily peeled off.

(2-8) Other steps In addition to the above steps, a polyvalent metal surface treatment step, an evaporation monomer recycling step, a granulation step, a fine powder removal step, and the like may be provided as necessary. Further, for the purpose of stabilizing the color tone over time and preventing gel degradation, the additives may be used as necessary in any or all of the above steps.

  The surface treatment step of the polyvalent metal salt is applied when liquid permeability under high pressure (SFC or GBP) is required. For example, the production methods described in US Pat. Nos. 6,605,673 and 6,620,899 are necessary. Applied.

[3] Polyacrylic acid (salt) water-absorbing resin The water-absorbing resin of the present invention is mainly composed of a polyacrylic acid (salt) water-absorbing resin and is intended for use in sanitary goods, particularly paper diapers. It can be obtained by the above-described polymerization method, surface crosslinking method, or the like. Further, the obtained water-absorbing resin preferably controls at least one or more of the physical properties listed in the following (2-1) to (2-7), and further includes two or more including AAP, In particular, it is preferable to control three or more physical properties. When the water absorbent resin does not satisfy the following physical properties, a high concentration diaper having a water absorbent resin concentration of 40% by weight or more may not exhibit sufficient performance.

(3-1) Initial color tone The water-absorbent resin obtained in the present invention is preferably a white powder because it is used as a raw material for sanitary goods such as paper diapers. Accordingly, in Hunter Lab color system measurement using a spectroscopic color difference meter, the L value (Lightness / lightness) is preferably 85 or more, more preferably 87 or more, and even more preferably 89 or more as the initial color tone. The a value is preferably -2 to 2, more preferably -1 to 1, more preferably -0.5 to 1, and particularly preferably 0 to 1. Further, the b value is preferably −5 to 10, more preferably −5 to 5, and further preferably −4 to 4. The upper limit of the L value is 100, but if it is 85 or more, there is no problem with color tone in sanitary goods. Further, the YI (Yellow Index) value is preferably 10 or less, preferably 8 or less, and more preferably 6 or less. Further, the WB (White Balance) value is preferably 70 or more, more preferably 75 or more, and further preferably 77 or more.

  The initial color tone refers to the color tone of the particulate water-absorbing agent after production, and generally refers to the color tone measured before shipment from the factory, but is stored under an atmosphere of 30 ° C. or less and a relative humidity of 50% RH. If it exists, the color tone measured within one year after production may be used.

(3-2) Temporal color tone As described above, the water-absorbent resin according to the present invention is used as a raw material for sanitary products such as paper diapers, and therefore has a clean white state even in a long-term storage state under high-temperature and high-humidity conditions. It is preferable to maintain. Therefore, in Hunter Lab color system measurement using a spectroscopic color difference meter, the L value (Lightness / lightness) is preferably at least 80, more preferably 81 or more, still more preferably 82 or more, and 83 or more as the color tone over time. Particularly preferred. Further, the a value is preferably −3 to 3, more preferably −2 to 2, and further preferably −1 to 1. Furthermore, 0-15 are preferable, as for b value, 0-12 are more preferable, and 0-10 are more preferable. In addition, although the upper limit of the said L value is 100, if it shows 80 or more, a real problem will not generate | occur | produce in the long-term preservation | save state on high temperature and humidity conditions.

  The color tone with time refers to a color tone measured after the water-absorbing resin is exposed for 7 days in an atmosphere at a temperature of 70 ± 1 ° C. and a relative humidity of 65 ± 1% RH.

(3-3) CRC (absorption capacity under no pressure)
The CRC (water absorption capacity under no pressure) of the water absorbent resin obtained in the present invention is preferably 10 [g / g] or more, more preferably 20 [g / g] or more, and further preferably 25 [g / g] or more. 30 [g / g] or more is particularly preferable. Although the upper limit of CRC is not particularly limited, it is preferably 50 [g / g] or less, more preferably 45 [g / g] or less, and still more preferably 40 [g / g] or less. When the CRC is less than 10 [g / g], the water-absorbing resin has a low water absorption amount and may not be suitable for use in absorbent articles such as paper diapers. In addition, when the CRC exceeds 50 [g / g], it is not preferable to use such a water-absorbent resin for the absorbent body because it may not be possible to obtain a sanitary product having an excellent liquid uptake rate. CRC can be appropriately controlled by the internal cross-linking agent and surface cross-linking agent described above.

(3-4) AAP (absorption capacity under pressure)
The AAP (water absorption capacity under pressure) of the water-absorbent resin obtained in the present invention is 1.9 kPa, more preferably under pressure of 4.8 kPa, as a means for achieving the above-mentioned drying in order to prevent leakage in paper diapers. 20 [g / g] or more is preferable, 22 [g / g] or more is more preferable, and 24 [g / g] or more is more preferable. The upper limit value of AAP is not particularly limited, but is preferably 40 [g / g] or less in view of balance with other physical properties. When the AAP is less than 20 [g / g], when such a water-absorbent resin is used for the absorbent body, the return of the liquid when pressure is applied to the absorbent body (usually, “Re-Wet”) This is not preferable because there is a possibility that a hygienic product with a small amount of the product may not be obtained. AAP can be appropriately controlled by the above-described surface cross-linking agent, particle size, and the like.

(3-5) SFC (saline flow conductivity)
The SFC (saline flow inductivity) of the water-absorbent resin obtained by the present invention is 1 [× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ] or more, preferably 10 [× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ] or more, more preferably 50 [× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ] or more. It is preferably 70 [× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ] or more, and most preferably 100 [× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ] or more. The upper limit of SFC is not particularly limited, but is preferably 3000 [× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ] or less, more preferably 2000 [× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ] or less. preferable. When the above SFC exceeds 3000 [× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ], it is preferable to use such a water absorbent resin for the water absorbent body, because there is a risk of liquid leakage in the water absorbent body. Absent. The SFC can be appropriately controlled by the above-described drying method or the like.

(3-6) Ext (water-soluble component)
The Ext (water-soluble content) of the water-absorbent resin obtained in the present invention is preferably 35% by weight or less, more preferably 25% by weight or less, further preferably 15% by weight or less, and particularly preferably 10% by weight or less. When the Ext exceeds 35% by weight, the gel strength of the obtained water-absorbent resin is weak and the liquid permeability may be inferior. Moreover, it is not preferable to use such a water-absorbing resin for the water-absorbing body because it may not be possible to obtain a water-absorbing resin with little liquid return (rewetting) when pressure is applied to the water-absorbing body. Note that Ext can be appropriately controlled by the internal cross-linking agent described above.

(3-7) Residual Monomers (residual monomers)
Residual Monomers (residual monomers) of the water-absorbent resin obtained in the present invention are preferably controlled to 0 to 400 ppm, more preferably 0 to 300 ppm, and still more preferably 0 to 200 ppm from the viewpoint of safety. Residual Monomers can be appropriately controlled by the polymerization method described above.

[4] Use of polyacrylic acid (salt) water-absorbent resin The use of the water-absorbent resin obtained by the production method according to the present invention is not particularly limited, and sanitary products such as paper diapers, sanitary napkins, incontinence pads, The water absorbent resin obtained in the present invention that can be used for absorbent articles such as agricultural and horticultural water retention agents, waste liquid solidifying agents, industrial water-stopping materials, etc. is an absorbent article that uses the water absorbent resin in a high concentration. Especially, excellent performance is exhibited. That is, the content (core concentration) of the water-absorbent resin in the absorbent body in the absorbent article is preferably 30 to 100% by weight, more preferably 40 to 100% by weight, further preferably 50 to 100% by weight, -100% by weight is even more preferred, 70-100% by weight is particularly preferred, and 75-95% by weight is most preferred. It is preferable to set the core concentration within the above range because the effects of the present invention can be further exhibited. In particular, when the water-absorbent resin obtained in the present invention is used for the upper part of the absorbent body within the above-mentioned core concentration range, it is highly permeable to urine and the like due to high liquid permeability (liquid permeability under pressure). Since the amount of absorption of the entire absorbent article such as a paper diaper is improved by efficient liquid distribution, it is preferable. Furthermore, it is preferable because it is possible to provide an absorbent article that maintains a hygienic white state.

In addition, the absorber preferably has a density of 0.06 to 0.50 [g / cm ³ ] and is compression-molded to a basis weight of 0.01 to 0.20 [g / cm ² ]. . Furthermore, it is possible to provide an absorbent article suitable for a thin paper diaper having a thickness of preferably 30 mm or less, more preferably 20 mm or less, and even more preferably 10 mm or less.

[5] Identification method / tracking method of water-absorbing resin In the present invention, the amount of ¹³ C in the water-absorbing resin may be quantified as a method for identifying a polyacrylic acid (salt) -based water-absorbing resin after production. For identification, the water-absorbing resin is taken out from the water-absorbing resin that is incorporated into a paper diaper or buried in the soil, and the amount of ¹³ C is quantified. In order to improve the identification or tracking system of the water-absorbent resin, the physical properties of the water-absorbent resin may be measured or the trace components may be quantified.

  The physical properties to be analyzed include water absorption capacity, water absorption capacity under pressure, particle size distribution, soluble components, particle size, liquid permeability, etc., and the minor components to be analyzed include residual monomers and residual saturated organic acids (particularly propionic acid). The remaining cross-linking agent may be identified and / or traced by comparing with a past standard sample.

[Example]
EXAMPLES Hereinafter, although this invention is demonstrated according to an Example, this invention is not limited to these. For convenience, “liter” may be described as “L” and “wt%” may be described as “wt%”. The physical properties of the water-absorbent resin of the present invention were measured under conditions of room temperature (20 to 25 ° C.) and humidity of 50 RH% unless otherwise specified.

[Measurement Example 1] CRC (absorption capacity under no pressure)
After weighing 0.2 g of water absorbent resin (weight W0 [g]), uniformly putting it in a non-woven bag (60 × 60 mm) and heat-sealing, 0.9 wt% chloride adjusted to 25 ± 3 ° C. It was immersed in 500 mL of aqueous sodium solution. After the elapse of 60 minutes, the bag was pulled up and drained using a centrifuge (centrifuge manufactured by Kokusan Co., Ltd., type: H-122) at 250 G for 3 minutes. Thereafter, the weight W1 [g] of the bag was measured.

  The same operation was performed without adding the water-absorbing resin, and the weight W2 [g] of the bag at that time was measured. CRC (water absorption capacity under no pressure) was calculated from the obtained W0 [g], W1 [g], and W2 [g] according to the following formula.

[Measurement Example 2] AAP (Water absorption capacity under pressure)
Using the measuring apparatus shown in FIG. 1, the AAP (water absorption magnification under pressure) of the water absorbent resin of the present invention was measured. In addition, although AAP was measured according to the measuring method prescribed | regulated to ERT442.2-02, only the load was changed to 4.83 kPa. Hereinafter, the measurement method will be described with reference to FIG.

  A plastic support cylinder (inner diameter: 60 mm) to which a 400-mesh metal mesh 101 (mesh size: 38 μm) was fused was prepared. Under an atmosphere of 20 to 25 ° C. (room temperature) and a relative humidity of 50 RH%, 0.900 g of the water absorbent resin 102 was uniformly placed on the wire mesh 101. Next, the piston 103 and the load (weight) 104 were placed in this order on the water absorbent resin 102, and the weight Wa [g] of the measuring device set was measured. The piston 103 and the load (weight) 104 had no gap with the support cylinder 100 and had an outer diameter slightly smaller than 60 mm so that the vertical movement was not hindered. Furthermore, a load could be uniformly applied to the water absorbent resin.

  Next, a glass filter 106 (diameter 90 mm) (manufactured by Mutual Riken Glass Co., Ltd .; pore diameter 100 to 120 μm) was placed in a Petri dish 105 (diameter 150 mm), and the temperature was adjusted to 20 to 25 ° C. 0.9 wt% The aqueous sodium chloride solution was poured until it was flush with the upper surface of the glass filter 106. Next, one sheet of filter paper 107 (diameter 90 mm) (manufactured by ADVANTEC Toyo Co., Ltd., product name: JIS P 3801, No. 2, thickness 0.26 mm, reserved particle diameter 5 μm) was placed on the glass filter 106, and the entire filter paper 107 was It was allowed to get wet and excess 0.9 wt% sodium chloride aqueous solution 108 was removed. Thereafter, the set of measuring devices was placed on the filter paper 107 and the 0.9 wt% sodium chloride aqueous solution 108 was absorbed by the water absorbent resin 102.

  After 1 hour, the measuring device set was lifted and its weight Wb [g] was measured. AAP (water absorption capacity under pressure) was calculated from the obtained Wa [g] and Wb [g] according to the following formula.

[Measurement Example 3] SFC (saline flow conductivity)
The SFC (physiological saline flow inductivity) of the water-absorbent resin of the present invention was measured using the measuring apparatus shown in FIG. SFC (physiological saline flow inductivity) is a value indicating the liquid permeability of the swollen water-absorbent resin, and the larger the value, the higher the liquid permeability. In addition, the measurement of SFC was performed according to the method disclosed in US Pat. No. 5,849,405.

  First, the SFC measuring apparatus shown in FIG. 2 will be described.

  The SFC measuring device used in the present invention is a 0.69 wt% sodium chloride aqueous solution (hereinafter referred to as “physiological saline”) storage tank 31 and its associated equipment, and physiological saline is passed through the swelling gel 44 under load. The liquid passing device 40 is configured to include a measuring unit that measures the weight of the physiological saline that has passed through the swelling gel.

  The storage tank 31 is provided with a glass tube 32 having an open end in order to transfer the physiological saline 33 to the liquid device 40 at a constant hydrostatic pressure. The glass tube 32 is inserted into the physiological saline 33 through a rubber plug (lid) so that the level of the physiological saline in the cylinder 41 is 5 cm above the bottom of the swollen gel 44.

  Further, the storage tank 31 is provided with an L-shaped tube 34 with a cock for transferring the physiological saline 33 to the liquid passing device 40. The L-shaped tube 34 with a cock is disposed below the surface of the physiological saline 33 in the storage tank 31, and the flow rate of the physiological saline can be controlled by the cock 35.

  The liquid passing device 40 includes a cylinder 41 (inner diameter 6 cm) and a piston 46, a stainless steel wire mesh 42 (mesh opening 38 μm) at the bottom of the cylinder 41, and a glass filter 45 at the bottom of the piston 46. Are attached so that the water-absorbent resin or swollen gel 44 cannot move. Further, a sufficient hole 47 is formed in the lower part of the piston 46 so that physiological saline can pass therethrough. Further, the liquid passing device 40 is placed on a table composed of a stainless steel wire mesh 43.

  The measurement unit includes a container 48 that collects the physiological saline that has passed, and an upper pan balance 49 that measures the weight of the collected physiological saline.

  Next, an SFC measurement method will be described.

  A liquid passing device 40 was prepared in which 0.900 g of a water-absorbing resin was uniformly placed in a cylinder 41 and a piston 46 on which a weight was placed so as to have a load of 2.07 kPa was installed. After the water absorbent resin in the liquid flow device 40 was swollen with artificial urine for 60 minutes, the gel layer height of the swollen gel 44 was measured. In addition, the said swelling method was performed based on the AAP (water absorption magnification under pressure) measuring method. The artificial urine contains 0.25 g of calcium chloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, and phosphorus. A solution prepared by adding 0.15 g of diammonium hydrogen hydride to pure water and making the total weight 1000.00 g was used.

  The liquid passing device 40 after swelling with the artificial urine is installed as shown in FIG. 2, and the physiological saline 33 is loaded under a load of 2.07 kPa, and the L-shaped tube 34 with a cock from the storage tank 31 at a constant hydrostatic pressure. Then, the liquid was passed through the swelling gel 44. Starting from the time when the cock 35 was opened, the weight of physiological saline passing through the gel layer at intervals of 20 seconds was recorded for 10 minutes. The value obtained by dividing the increased liquid volume [g] every 20 seconds by the elapsed time [s] (20 seconds) is the flow rate Fs (T) of physiological saline passing through the swollen gel 44 (mainly between the particles) [g / s], and the flow rate Fs (T) was plotted against the elapsed time.

  The time point at which a stable flow rate was obtained was read from the plot, and that point was defined as Ts. Using only the flow rate data obtained from the Ts until the end of the measurement (10th minute), Fs (T = 0), that is, the initial flow rate through the gel layer is set to the result of the least squares method as T = 0. After calculation by extrapolation, SFC (saline flow conductivity) was calculated according to the following formula 3.

here,
Fs (t = 0): flow rate [g / s]
L0: Gel layer height [cm]
ρ: Density of aqueous sodium chloride solution (1.003 [g / cm ³ ])
A: Area above the gel layer in the cylinder 41 (28.27 [cm ² ])
ΔP: hydrostatic pressure applied to the gel layer (4920 [dyn / cm ² ])
It is.

[Measurement Example 4] Ext (water-soluble component)
A water-absorbing resin (1.00 g) and a 0.90 wt% sodium chloride aqueous solution (184.3 g) were put in a plastic container with a cap having a capacity of 250 mL and stirred for 16 hours to extract a water-soluble component in the water-absorbing resin. This extract was filtered using one filter paper (ADVANTEC Toyo Corporation, product name: JIS P 3801, No. 2, thickness 0.26 mm, retention particle diameter 5 μm), and 50.0 g of the obtained filtrate was used for measurement. A liquid was used.

  The measurement solution was titrated with a 0.1N NaOH aqueous solution until pH 10 and then titrated with a 0.1N HCl aqueous solution until pH 2.7, and titrated ([NaOH] mL, [HCl] mL). ) Moreover, the same operation was performed only with respect to 0.90 wt% sodium chloride aqueous solution, and empty-quantity ([bNaOH] mL, [bHCl] mL) was calculated | required.

  Ext (water-soluble content) was calculated from the titration amount obtained by the above operation and the average molecular weight of the monomer according to the following formula 5. In addition, when the average molecular weight of the monomer was unknown, the average molecular weight of the monomer was calculated using the neutralization rate calculated according to the following formula 6.

[Measurement Example 5] Residual Monomer (residual monomer)
The residual monomer contained in the water-absorbent resin of the present invention is obtained by subjecting a filtrate obtained by performing the same operation except that the stirring time in the Ext (water-soluble) measurement method is changed from 16 hours to 2 hours. It is calculated | required by analyzing by liquid chromatography. The residual monomer is expressed in ppm (vs. water-absorbing resin).

[Measurement Standard 6] Water content 1.00 g of water absorbent resin was weighed into an aluminum cup having a bottom diameter of about 50 mm, and the total weight W8 [g] of the sample (water absorbent resin and aluminum cup) was measured.

  Next, the sample was allowed to stand in an oven having an atmospheric temperature of 180 ° C., and the water absorbent resin was dried. After 3 hours, the sample was taken out of the oven and cooled to room temperature in a desiccator. Thereafter, the total weight W9 [g] of the dried sample (water absorbent resin and aluminum cup) was measured, and the water content (unit: [wt%]) was calculated according to the following formula.

[Measurement Example 7] Weight average particle diameter (D50), weight percentage less than 150 μm, logarithmic standard deviation of particle size distribution (σζ)
10.00 g of the water-absorbent resin was classified using a JIS standard sieve having the following openings, the weight of each sieve was measured, and the weight percentage with a particle diameter of less than 150 μm was calculated. Further, the residual percentage R of each particle size was plotted on several probability papers, and the particle diameter corresponding to R = 50 wt% was read from this graph as the weight average particle diameter (D50). Note that. The weight average particle diameter (D50) refers to the particle diameter of a standard sieve corresponding to 50% by weight of the whole particle as disclosed in US Pat. No. 5,051,259. Further, the logarithmic standard deviation (σζ) of the particle size distribution was calculated according to the following formula 8. The logarithmic standard deviation (σζ) of the particle size distribution means that the smaller the value, the narrower the particle size distribution.

  Here, X1 is a particle diameter corresponding to R = 84.1% by weight, and X2 is a particle diameter corresponding to R = 15.9% by weight.

  The JIS standard sieves (The IIDA TESTING SIVE: inner diameter 80 mm) were prepared with openings of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, 106 μm, and 45 μm. Moreover, it classified for 5 minutes with the low tap type sieve shaker (Iida Seisakusho make; ES-65 type sieve shaker).

[Measurement Example 8] Color tone evaluation of water-absorbing resin The color tone evaluation of the water-absorbing resin was carried out in the Hunter Lab color system. Note that LabScan (registered trademark) XE manufactured by HunterLab was used as a measurement apparatus (spectral color difference meter), and reflection measurement was selected as the measurement condition. In addition, powder / paste sample containers (inner diameter 30 mm, height 12 mm), standard round white plate No. 1 for powder / paste. 2 and 30Φ floodlight pipes were used.

  The powder / paste sample container is filled with about 5 g of a water-absorbent resin, and the L-value of the surface of the water-absorbent resin is measured with the spectral color difference meter at room temperature (20 to 25 ° C.) and a relative humidity of 50 RH%. Lightness: brightness index), a value, and b value were measured.

  In the present invention, the color tone of the water-absorbent resin immediately after production or the water-absorbent resin in which the storage period in an atmosphere at a temperature of 30 ° C. or less and a relative humidity of 50 RH% or less is within one year after production is referred to as “initial color tone”. The L value measured at this time is referred to as “lightness index before exposure”.

  In addition, as a “coloring promotion test”, the following operation was performed, and “lightness index after exposure” was measured.

  The above color acceleration test was carried out in a constant temperature and humidity chamber (Espec Co., Ltd., small environmental tester; model SH-641) adjusted to an atmosphere of 70 ± 1 ° C. and relative humidity of 65 ± 1 RH%. A powder / paste sample container filled with was placed and exposed for 7 days.

  The color tone of the water-absorbent resin after exposure is referred to as “temporal color tone”, and the L value measured at this time is referred to as “lightness index after exposure”.

  Note that the whiteness increases as the L value approaches 100, and the closer the a and b values are to 0 (zero), the lower the color and the substantially white.

[Measurement Example 9] δ ¹³ C (carbon stable isotope ratio)
Place 500 mg of copper oxide and 10 mg of water-absorbing resin in a quartz glass tube with an outer diameter of 9 mm, an inner diameter of 6 mm, and a length of 350 mm with one side closed, and then 5 silver wires (diameter 0.1 mm, length 5 mm) I put it in.

  Next, the quartz cotton that has been heat-treated at 1000 ° C. is loosely rolled and placed in the middle of the quartz glass tube so that the water-absorbent resin is not released outside the tube when the inside of the glass tube is evacuated. I made it. The vacuum operation was performed using an oil rotary pump and a liquid nitrogen cold trap. The glass tube containing the water-absorbing resin and the auxiliary combustor was evacuated to high vacuum and sealed to a length of 300 mm using a gas burner.

  Thereafter, the quartz glass tube was heated at 900 ° C. for 3 hours to burn the sample and generate carbon dioxide and the like.

  Combustion gas containing carbon dioxide generated by the above operation was introduced into a vacuum apparatus, and purified and separated using liquid nitrogen to recover pure carbon dioxide.

  This carbon dioxide is recovered as graphite on the iron powder acting as a catalyst by the hydrogen reduction method. Furthermore, the recovered graphite and iron powder were mixed and pressed into an aluminum target holder to obtain an analysis sample of a tandetron accelerator mass spectrometer.

  Thereafter, the carbon isotope ratio of the sample was measured using a tandetron accelerator mass spectrometer.

[Example 1]
Acrylic acid was obtained starting from sugarcane, a C4 plant.

  A NaOH aqueous solution was added to the acrylic acid obtained above at a neutralization temperature of 60 ° C. to obtain a sodium acrylate aqueous solution having a neutralization rate of 75 mol% and a concentration of 35 wt%. At this time, the content of 3-hydroxypropionic acid was 2100 ppm (relative to a sodium acrylate aqueous solution).

  Monomer (1) was obtained by dissolving 0.05 mol% (as compared with sodium acrylate) of polyethylene glycol diacrylate as an internal crosslinking agent in the sodium acrylate aqueous solution. 350 g of the monomer (1) was put into a cylindrical container having a volume of 1 L within 3 minutes, and nitrogen bubbling was performed at 2 [L / min] for 20 minutes to deaerate. Next, sodium persulfate 0.12 [g / mol] (with respect to monomer) and L-ascorbic acid 0.005 [g / mol] (with respect to monomer) in the form of an aqueous solution were mixed with the above-mentioned single amount. Polymerization was started by adding to the body (1). Stirring was stopped after the initiation of polymerization, and a standing aqueous solution polymerization was performed. When 14 minutes had elapsed after the start of polymerization, a peak polymerization temperature of 110 ° C. was exhibited. Thereafter, after 30 minutes, the polymer was taken out from the cylindrical container to obtain a hydrogel crosslinked polymer (1).

  The obtained hydrogel crosslinked polymer (1) was subdivided by a meat chopper (pore diameter: 8 mm) under an atmosphere temperature of 45 ° C. and then introduced into the dryer within 3 minutes. The drying was performed by blowing hot air having a wind speed of 1.8 [m / sec] and a temperature of 170 ° C. for 20 minutes. The dried polymer (solid content concentration: about 95% by weight) obtained by the drying operation was pulverized with a roll mill and classified into 850 to 150 μm with a JIS standard sieve to obtain a particulate water-absorbing resin (1). .

  0.05 weight of ethylene glycol diglycidyl ether (trade name Denacol (registered trademark) EX-810; manufactured by Nagase Kasei Co., Ltd.) as a surface cross-linking agent with respect to 100 parts by weight of the obtained particulate water-absorbing resin (1) After spraying a mixture consisting of 1 part by weight / 1 part by weight of isopropyl alcohol / 3 parts by weight of water, heat treatment was performed in an oil bath at 195 ° C. for 60 minutes to obtain a surface-crosslinked water-absorbent resin (1).

  The L value of the Hunter Lab color system of the obtained water absorbent resin (1) was 89. Further, the L value after the coloring promotion test was 80, and coloring with time was small.

  In addition, the stable carbon isotope ratio obtained in Measurement Example 9 was −13 ‰. The dihydroxyacetone content was 250 ppm.

[Comparative Example 1]
The same operation as in Example 1 was performed except that acrylic acid was obtained using cassava, which is a C3 plant, as a starting material.

  The L value of the Hunter Lab color system of the obtained comparative water absorbent resin (1) was 76. Further, the L value after the coloring acceleration test was reduced to 62.

  In addition, the stable carbon isotope ratio obtained in Measurement Example 9 was −25 ‰. The dihydroxyacetone content was 1460 ppm.

  By using the biomass-derived material, carbon dioxide in the atmosphere does not increase even if it is burned, and a water-absorbing resin that is friendly to the global environment can be obtained. Furthermore, by using a biomass source derived from a C4 plant, it is possible to provide a water-absorbing resin having high whiteness and little coloration with time.

Claims (9)

A polyacrylic acid (salt) water-absorbing resin having an internal cross-linked structure obtained by polymerizing a water-soluble unsaturated monomer,
A polyacrylic acid (salt) -based water-absorbent resin, wherein a carbon stable isotope ratio (δ ¹³ C) measured by accelerator mass spectrometry is −20 ‰ or more.   The water absorbent resin according to claim 1, wherein the dihydroxyacetone content is 500 ppm or less.   The water-absorbent resin according to claim 1 or 2, wherein the saturated aliphatic carboxylic acid content is 2000 ppm or less.   The water-absorbent resin according to any one of claims 1 to 3, wherein the saturated aliphatic carboxylic acid is at least one selected from acetic acid, propionic acid, and butyric acid.   The water-absorbent resin according to any one of claims 1 to 4, wherein the water-soluble unsaturated monomer is mainly composed of acrylic acid obtained from a biomass source derived from a C4 plant. The water-absorbent resin according to any one of claims 1 to 5, wherein the acrylic acid has a carbon stable isotope ratio (δ ¹³ C) of -20 ‰ or more.   The water-absorbent resin according to any one of claims 1 to 6, wherein a content of dihydroxyacetone in the acrylic acid is 500 ppm or less. A method for producing a polyacrylic acid (salt) water-absorbing resin, comprising a step of polymerizing a water-soluble unsaturated monomer, and a step of drying the resulting hydrogel crosslinked polymer,
Production of polyacrylic acid (salt) -based water-absorbing resin, characterized in that acrylic acid having a carbon stable isotope ratio (δ ¹³ C) measured by accelerator mass spectrometry of −20 ‰ or more is used as a monomer Method.   The production method according to claim 8, wherein the water-soluble unsaturated monomer is mainly composed of acrylic acid obtained from a biomass source derived from a C4 plant.