Water-absorbent resin composition, method for producing the same, and method for producing an absorber

Abstract

To provide a manufacturing method of a water absorbent resin composition which partially uses a polysaccharide as a sustainable raw material and is low cost, high in productivity, and excellent and simple in operativity, and a water absorbent resin composition excellent in adhesion moldability and less in coloration.SOLUTION: A manufacturing method of a water absorbent resin composition comprises a polymerization step in which a monomer containing acrylic acid (salt) as a main component is suspended in an oil phase containing a hydrocarbon dispersion medium and polymerized, and a separation step in which the polymer is separated from the hydrocarbon dispersion medium, and further comprises a step of adding starch particles.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a water-absorbing resin composition, a method for producing the same, and a method for producing an absorber, and more particularly to a water-absorbing resin composition containing a polysaccharide, a method for producing the same, and a method for producing an absorber.

Background Art

[0002] A water-absorbing resin is a water-swellable and water-insoluble cross-linked polymer, and is used in various water-absorbing articles such as disposable diapers, sanitary napkins, adult incontinence products (incontinence pads), pet sheets, soil water retainers for agricultural and horticultural use, and water stoppers for industrial use.

[0003] In recent years, from the viewpoints of reducing the burden on the global environment, resource conservation, carbon neutrality, SDGs (Sustainable Development Goals), etc., and further, it is preferable to have biodegradability. Generally, instead of depletable energy resources such as petroleum, there is an active movement to use so-called sustainable raw materials, which are renewable organic resources derived from animals and plants, which are components of living organisms. And also in the field of water-absorbing resins, research is being advanced to use at least a part of natural product-derived polysaccharides as sustainable raw materials.

[0004] For example, Non-Patent Document 1 describes that, for example, sodium starch graft polyacrylate obtained by graft-polymerizing sodium acrylate and starch is commercially available as a water-absorbing resin. Patent Document 1 describes a method for producing a water-absorbing resin composed of sodium starch graft polyacrylate. Non-Patent Document 2 describes that, as raw materials for commercially available acrylic acid-based water-absorbing resins, graft components such as starch and polyvinyl alcohol are optionally used, aqueous solution polymerization and inverse phase suspension polymerization are used as polymerization methods, and as required performances of the water-absorbing resin, the water absorption ratio, the water absorption ratio under pressure, the water absorption rate, the soluble content, etc. are within a predetermined range, and the ratio of fine powder having a particle diameter of 150 μm or less and 100 μm or less in the water-absorbing resin is small.

[0005] Patent Documents 2 to 3 describe that in the production of a water-absorbing resin, a hydrophilic polymer or the like can be added to the monomer, and examples of the hydrophilic polymer include starch, cellulose, derivatives thereof, water-soluble polyacrylic acid (salt), and the like.

[0006] Further, Patent Document 4 describes a method for producing a superabsorbent resin in which inverse suspension polymerization is carried out in the coexistence of 0.001 to 5% by mass (based on the monomer) of β-1,3-glucans.

[0007] Furthermore, Patent Document 5 describes a method for producing a hydrophilic polymer by polymerizing a hydrophilic monomer in a solution, in which a hydrophilic polymer such as starch is mixed and dispersed in a monomer solution and then the monomer is polymerized.

[0008] Also, Patent Document 6 describes a method for producing a water-absorbing resin composition in which a starch compound is added to an arbitrary production step of the water-absorbing resin.

[0009] Also, Patent Document 7 describes that since a graft polymer of starch or a graft polymer of cellulose has low heat resistance, a method for producing a water-absorbing resin using thin-film drying at 100 to 180 °C within 10 minutes after aqueous solution polymerization as a method for drying a hydrogel of these polymers is described.

[0010] Also, Patent Documents 8 to 11 describe a method for producing a water-absorbing resin in which a thickener such as polyacrylic acid (salt) and hydroxyethyl cellulose is added to a monomer for the purpose of controlling the particle diameter of the water-absorbing resin in inverse suspension polymerization.

[0011] Furthermore, Patent Document 12 describes a technique in which ethyl cellulose or the like is added as a dispersant to a hydrocarbon dispersion medium for the purpose of controlling the particle diameter of a water-absorbing resin in inverse suspension polymerization, and a method for producing a water-absorbing resin in which the dispersant is precipitated to aggregate the gel produced by polymerization.

[0012] When manufacturing an absorber used in a water-absorbing article such as a paper diaper, the water-absorbing resin is mixed with pulp and an adhesive as necessary and then molded. In recent years, in order to make the absorber thinner, the proportion of pulp used in the absorber has been decreasing, and as a result, adhesives are increasingly used for molding the absorber. Patent Document 13 describes a water-absorbing sheet composition as an absorber, and a hot-melt adhesive is disclosed as an adhesive here.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Patent Document 9

Patent Document 10

Patent Document 11

Patent Document 12

Patent Document 13

Non-Patent Documents

[0014]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0015] However, the prior art as described above is not sufficient in terms of manufacturing a water-absorbent resin composition containing polysaccharides in a simple method with low cost, high productivity, and good operability, and obtaining a water-absorbent resin composition with less coloring.

[0016] For example, the graft polymerization of sodium acrylate and starch requires an aqueous solution of starch, and the gelatinization (α - modification) process of starch for this purpose is complicated. Also, the aqueous solution of α - modified starch has a high viscosity and requires a relatively large amount of water for sufficient gelatinization. Therefore, due to the need for polymerization at a low concentration, it incurs a high drying cost and has low productivity.

[0017] Also, in the method for manufacturing a water-absorbent resin by polymerizing monomers in an aqueous solution, since it is necessary to remove a large amount of water to dry the water-containing gel particles obtained by pulverizing the polymerization gel, high temperature is required, and it is difficult to suppress the coloring derived from starch due to high temperature. Conversely, if the drying temperature is lowered to suppress coloring, the drying time becomes long and the productivity is low. Also, the thin-film drying described in Patent Document 7 had low productivity.

[0018] On the other hand, in inverse phase suspension polymerization, a method for manufacturing a water-absorbent resin in which polysaccharides such as β - 1,3 - glucans and starch compounds are added to a monomer solution is also known, but the addition amount of polysaccharides is small, and it is a method of dissolving polysaccharides in a monomer solution and performing polymerization.

[0019] Furthermore, the adhesives used in conventional absorbers generally have synthetic raw materials, which is one of the reasons why the proportion of sustainable raw materials used in the entire absorber does not increase sufficiently.

[0020] Therefore, there is a demand for a water-absorbing resin composition that uses at least a part of polysaccharides as a sustainable raw material, has little coloring, and has excellent adhesive moldability that can be easily molded into a desired absorber without using an adhesive. In addition, even when at least a part of polysaccharides is used as a sustainable raw material, there is a demand for a manufacturing method that can obtain a water-absorbing resin composition with little coloring and good adhesive moldability in a simple method with low cost, high productivity, and good operability. Furthermore, there is a demand for a manufacturing method of an absorber with a high proportion of sustainable raw materials used in the entire absorber and good adhesive moldability.

[0021] That is, one aspect of the present invention mainly aims to provide a manufacturing method of a water-absorbing resin composition that uses at least a part of polysaccharides as a sustainable raw material, is low-cost, has high productivity, and has good operability, a water-absorbing resin composition with little coloring and good adhesive moldability, and a manufacturing method of an absorber with a high proportion of sustainable raw materials used in the entire absorber and good adhesive moldability.

Means for Solving the Problems

[0022] In order to solve the above problems, the present invention is composed of the following configurations.

[0023] 〔1〕A method for producing a water-absorbing resin composition containing starch particles and polyacrylic acid (salt) - based water-absorbing resin particles, A polymerization step of suspending an aqueous solution of a monomer containing acrylic acid (salt) as a main component in an oil phase containing a hydrocarbon dispersion medium and performing polymerization, and A separation step of separating the polymer obtained in the polymerization step from the hydrocarbon dispersion medium, Furthermore, a method for producing a water-absorbing resin composition including a step of adding starch particles to the hydrocarbon dispersion medium at any stage from the polymerization step to the separation step.

[0024] The method for producing a water-absorbing resin composition according to [1], wherein the starch particles are not gelatinized.

[0025] The method for producing a water-absorbing resin composition according to [1] or [2], wherein the volume average particle diameter of the starch particles is 1 to 50 μm.

[0026] The method for producing a water-absorbing resin composition according to any one of [1] to [3], wherein the amount of the starch particles used is 3 to 100% by mass based on the amount of the monomer containing acrylic acid (salt) as a main component.

[0027] The method for producing a water-absorbing resin composition according to any one of [1] to [4], wherein the starch particles are added when the polymerization rate of the monomer containing acrylic acid (salt) as a main component is 10 mol% or more.

[0028] The method for producing a water-absorbing resin composition according to any one of [1] to [5], including a drying step of drying the polymer obtained in the polymerization step before, during, or after the separation step.

[0029] The method for producing a water-absorbing resin composition according to any one of [1] to [6], further including a surface crosslinking step after the polymerization step.

[0030] The method for producing a water-absorbing resin composition according to [7], wherein a polyglycidyl compound is added and crosslinked in the surface crosslinking step.

[0031] A water-absorbing resin composition containing 50 to 97% by mass of polyacrylic acid (salt)-based water-absorbing resin particles in a spherical shape or an aggregate thereof, and 3 to 50% by mass of starch particles.

[0032] The water-absorbing resin composition according to [9], wherein the starch particles are not gelatinized.

[0033] The water-absorbing resin composition according to [9] or

[10] , wherein the moisture absorption blocking rate when moisture-absorbed at 70 °C and 65% relative humidity for 60 minutes is 10% by mass or less.

[0034] 〔12〕A water-absorbing resin according to any one of 〔9〕 to 〔11〕, having a yellowness of 20 or less. Composition.

[0035] 〔13〕A method for producing a sheet-shaped absorber, comprising spraying a water-absorbing resin composition according to any one of 〔9〕 to 〔12〕 onto a sheet-shaped substrate, adding water, heating the water-absorbing resin composition, and gelatinizing starch.

[0036] 〔14〕The method for producing a sheet-shaped absorber according to 〔13〕, wherein the water is steam heated to 100 ° C. or higher, and the water-absorbing resin composition is heated by the steam.

[0037] 〔15〕The method for producing a sheet-shaped absorber according to 〔13〕 or 〔14〕, wherein the water-absorbing resin composition is heat-pressed and molded into a sheet shape while being heated.

Advantages of the Invention

[0038] According to one aspect of the present invention, there is provided a method for producing a water-absorbing resin composition which is low-cost, highly productive, and easy to operate, using polysaccharides as at least part of the sustainable raw materials, a water-absorbing resin composition having excellent adhesion moldability and little coloring, and a method for producing an absorber having a high ratio of sustainable raw materials used for the entire absorber and good adhesion moldability.

Brief Description of the Drawings

[0039]

Figure 1

Embodiments for Carrying Out the Invention

[0040] Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited thereto, and various modifications are possible within the described scope. Embodiments obtained by appropriately combining the technical means described in different embodiments are also included in the technical scope of the present invention. In this specification, unless otherwise specified, "A~B" representing a numerical range means "A or more and B or less", and "ppm" means "mass ppm". Further, "(meth)acrylic" means "acrylic and / or methacrylic". Furthermore, the mass of the water-absorbing resin composition and the like represents a value converted to a solid content unless otherwise specified.

[0041] 〔1〕Definition of Terms 〔1-1〕Water-absorbing Resin, Water-absorbing Resin Composition In this specification, the water-absorbing resin means a water-swellable crosslinked polymer, which is generally in the form of particles. Further, water-swellability means that the water absorption ratio (CRC) under no pressure defined in ERT 441.2-02 is 5 g / g or more.

[0042] Also, in this specification, the water-absorbing resin may refer to "a polymer crosslinked only inside, that is, a polymer having substantially the same crosslink density inside and on the surface" or "a polymer crosslinked inside and on the surface, that is, a polymer having a relatively high crosslink density on the surface compared to the crosslink density inside".

[0043] In this specification, the "polymer crosslinked only internally" and the "polymer crosslinked both internally and on the surface" are generally referred to as "superabsorbent resin" without distinction. However, when it is necessary to clearly distinguish between the presence or absence of surface crosslinking, since the "polymer crosslinked only internally" is before surface crosslinking, it is referred to as "superabsorbent resin before surface crosslinking" or "base polymer". Also, the "polymer crosslinked both internally and on the surface, that is, the polymer with a relatively high crosslinking density on the surface compared to the internal crosslinking density" is after surface crosslinking, so it is referred to as "superabsorbent resin after surface crosslinking" or "surface-crosslinked superabsorbent resin". Note that "before surface crosslinking" means "before adding the surface crosslinking agent" or "even after the surface crosslinking agent is added, before the surface crosslinking reaction by heat treatment starts".

[0044] In this specification, "superabsorbent resin" refers only to the resin component, and when it contains components other than the resin such as additives, it is referred to as "superabsorbent resin composition". Further, when the non-surface-crosslinked superabsorbent resin contains additives or the like, it is referred to as "superabsorbent resin composition before surface crosslinking" or "base polymer composition". Also, when the surface-crosslinked superabsorbent resin contains additives or the like, it is referred to as "superabsorbent resin composition after surface crosslinking" or "surface-crosslinked superabsorbent resin composition", respectively.

[0045] [1-2] Polyacrylic acid (salt) - based superabsorbent resin In this specification, a polyacrylic acid (salt)-based water-absorbing resin (hereinafter sometimes simply referred to as "water-absorbing resin") is a crosslinked polymer obtained by polymerizing a monomer containing acrylic acid (salt) as a main component. In other words, it is a crosslinked polymer having a structural unit derived from acrylic acid (salt) as a main component with respect to the entire structural unit constituting the polyacrylic acid (salt)-based water-absorbing resin, and is a crosslinked polymer having a graft component as an optional component. Note that the crosslinking may be carried out using a crosslinking agent or by self-crosslinking. Here, in this specification, "as a main component" preferably means 50 mol% or more, more preferably 70 mol% or more, still more preferably 90 mol% or more, and preferably 100 mol% or less, particularly preferably substantially 100 mol%.

[0046] Note that the structural unit constituting the polyacrylic acid (salt)-based water-absorbing resin includes a structural unit derived from an internal crosslinking agent, but in this specification, "monomer" means that it does not include an internal crosslinking agent.

[0047] 〔1-3〕 "EDANA" and "ERT" "EDANA" is an abbreviation of the European Disposables and Nonwovens Associations, and "ERT" is an abbreviation of the measurement method of water-absorbing resins of European standards (almost worldwide standards) (EDANA Recommended Test Methods). In the present invention, unless otherwise specified, the physical properties of the water-absorbing resin are measured in accordance with the ERT original (revised in 2002 / public known document).

[0048] Note that unless otherwise mentioned in this specification, the measurement methods of various physical properties of the water-absorbing resin composition follow the measurement methods in the following examples.

[0049] 〔2〕 Production method of water-absorbing resin composition In order to solve the above problems, the present inventors conducted intensive studies. As a result, an aqueous solution of a monomer containing an acrylic acid (salt) - based monomer as a main component is suspended in an oil phase containing a hydrocarbon dispersion medium and polymerized in a polymerization step, and a separation step of separating the polymer obtained in the polymerization step from the hydrocarbon dispersion medium is included. By adding starch particles to the hydrocarbon dispersion medium at any stage from the polymerization step to the separation step, it was found that the starch particles function as a dispersant. Furthermore, by leaving the starch particles in the water - absorbent resin composition, it was found that they also function as a moisture - absorption fluidity improver.

[0050] Also, according to the production method according to one embodiment of the present invention, there is no need to perform chemical modification or gelatinization (α - modification) of starch in advance, and since it is not necessary to set the monomer concentration to a low concentration as compared with the method of polymerizing the monomer in an aqueous solution, there is no drying cost and high productivity. Furthermore, since drying at a high temperature is not required, coloring derived from starch can be suppressed.

[0051] Also, according to the production method according to one embodiment of the present invention, since it is not necessary to dissolve starch in the monomer solution, the operability is good and simple. Moreover, even when the starch content is high, a water - absorbent resin composition containing starch particles and a polyacrylic acid (salt) - based water - absorbent resin can be preferably obtained.

[0052] Therefore, even when at least a part of starch is used as a sustainable raw material, a water - absorbent resin composition with low cost, high productivity, good operability, and little coloring can be obtained by a simple method.

[0053] That is, the production method of a water - absorbent resin composition according to one embodiment of the present invention is a production method of a water - absorbent resin composition containing starch particles and polyacrylic acid (salt) - based water - absorbent resin particles, comprising a polymerization step of suspending an aqueous solution of a monomer containing acrylic acid (salt) as a main component in an oil phase containing a hydrocarbon dispersion medium and performing polymerization, and a separation step of separating the polymer obtained in the polymerization step from the hydrocarbon dispersion medium, The manufacturing method further includes a step of adding starch particles to a hydrocarbon dispersion medium at any stage from the polymerization step to the separation step.

[0054] 〔2-1〕Polymerization step (a) Monomer The monomer used in the present invention is a monomer containing acrylic acid (salt) as a main component. Examples of the salt include alkali metal salts, ammonium salts, and amine salts, with sodium salts, potassium salts, lithium salts, and ammonium salts being more preferable, and sodium salts being particularly preferable.

[0055] Also, acrylic acid (salt) is preferably neutralized in the range of 10 to 90 mol%, more preferably in the range of 40 to 80 mol%, and particularly preferably in the range of 60 to 75 mol%.

[0056] Therefore, acrylic acid (salt) is preferably neutralized with a neutralizing solution containing a hydroxide of an alkali metal such as sodium hydroxide, potassium hydroxide, or lithium hydroxide, a hydrogen carbonate such as sodium hydrogen carbonate or potassium hydrogen carbonate, or a monovalent basic compound such as ammonia, and particularly preferably neutralized with a neutralizing solution containing sodium hydroxide.

[0057] As monomers other than acrylic acid (salt) that may be contained in the monomer, among monomers having an unsaturated double bond (ethylenically unsaturated monomers), monomers containing an acid group are preferable. Specific examples of such monomers include (anhydrous) maleic acid, fumaric acid, crotonic acid, itaconic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, vinylsulfonic acid, styrenesulfonic acid, and their salts. These monomers may be used alone or in combination of two or more.

[0058] Examples of the salt include alkali metal salts, ammonium salts, and amine salts. Sodium salts, potassium salts, lithium salts, and ammonium salts are more preferable, and sodium salts are particularly preferable.

[0059] In addition, the monomer containing acrylic acid (salt) as a main component is preferably neutralized in the range of 10 to 90 mol%, more preferably neutralized in the range of 40 to 80 mol%, and particularly preferably neutralized in the range of 60 to 75 mol%.

[0060] Note that the monomer may be neutralized after polymerization. That is, the water-containing gel-like crosslinked polymer obtained in the polymerization step (hereinafter referred to as "water-containing gel") may be neutralized to obtain a neutralized product. However, considering the productivity of the water-absorbing resin, various physical properties, etc., it is more preferable to polymerize using a neutralized monomer to obtain a water-containing gel.

[0061] In addition, the monomer may optionally contain a hydrophilic or hydrophobic unsaturated monomer (hereinafter referred to as "other monomer") in addition to the monomers described above. Examples of the other monomer include 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 the like. The amount of the other monomer used may be such that it does not impair the physical properties of the resulting water-absorbing resin composition. Specifically, it is 50 mol% or less, more preferably 20 mol% or less, based on the monomer.

[0062] (b) Internal crosslinking agent In the present invention, it is preferable to use an internal crosslinking agent in order to obtain a polymer with a crosslinked interior. Examples of the internal crosslinking agent include compounds having at least two polymerizable double bonds in the molecule, such as N,N'-methylenebisacrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, (polyoxyethylene) trimethylolpropane tri(meth)acrylate, trimethylolpropane di(meth)acrylate, polyethylene glycol di(β-acryloyloxypropionate), trimethylolpropane tri(β-acryloyloxypropionate), poly(meth)allyloxyalkane, etc.; compounds that can react with carboxyl groups to form covalent bonds, such as polyglycidyl ethers (such as ethylene glycol diglycidyl ether), polyols (such as ethylene glycol, polyethylene glycol, glycerin, sorbitol, etc.). Compounds having at least two polymerizable double bonds in the molecule are more preferable. These internal crosslinking agents may be used alone or in combination of two or more.

[0063] Considering the physical properties of the resulting water-absorbent resin composition, the internal crosslinking agent is preferably used in the range of 5.0 mol% or less, more preferably in the range of 2.0 mol% or less, still more preferably in the range of 0.5 mol% or less, particularly preferably in the range of 0.1 mol% or less, and most preferably in the range of 0.001 to 0.1 mol% with respect to the total amount of the monomers.

[0064] (c) Hydrocarbon dispersion medium As the hydrocarbon dispersion medium, for example, aliphatic hydrocarbons such as n - hexane, n - heptane, 2 - methylhexane, 3 - methylhexane, 2,3 - dimethylpentane, 3 - ethylpentane, n - octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans - 1,2 - dimethylcyclopentane, cis - 1,3 - dimethylcyclopentane, trans - 1,3 - dimethylcyclopentane; aromatic hydrocarbons such as benzene, toluene, xylene, etc. can be used. These hydrocarbon dispersion media may be used alone or in combination of two or more. As the mixed hydrocarbon dispersion medium in which two or more hydrocarbon dispersion media are mixed, for example, Exxsol Heptane (manufactured by ExxonMobil, a mixed hydrocarbon dispersion medium containing n - heptane, 2 - methylhexane, 3 - methylhexane, and methylcyclohexane as main components) can be mentioned. Among these hydrocarbon dispersion media, n - hexane, n - heptane, cyclohexane, Exxsol Heptane, etc. are preferable in terms of easy industrial availability, stable quality, and low cost.

[0065] The usage amount of the hydrocarbon dispersion medium is not limited thereto, but from the viewpoint of removing the polymerization heat and facilitating the control of the reaction temperature of inverse phase suspension polymerization, it is preferably 50 to 600% by mass, more preferably 100 to 550% by mass, based on the total amount of the monomers.

[0066] (d) Monomer aqueous solution The monomer containing the acrylic acid (salt) as the main component and the internal cross - linker are preferably used in the form of an aqueous solution in order to be more efficiently dispersed in the hydrocarbon dispersion medium. In such an aqueous solution (hereinafter referred to as "monomer aqueous solution" in this specification), the concentration of the monomer is not limited thereto, but it may generally be 20% by mass or more and the saturation concentration or less, preferably 35% by mass or more and the saturation concentration or less, more preferably 35 to 70% by mass, and still more preferably 40 to 55% by mass, based on the total mass of the monomer aqueous solution (when the graft component is included, the mass excluding the graft component).

[0067] (e) Inverse phase suspension polymerization In the method for producing a water-absorbing resin composition according to an embodiment of the present invention, a monomer containing acrylic acid (salt) as a main component may be polymerized by an inverse phase suspension polymerization method in the presence of a polymerization initiator. At this time, if necessary, a polymer dispersant, a surfactant, or the like may be used.

[0068] The polymer dispersant is not particularly limited. Examples thereof include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, ethylene-maleic anhydride copolymer, ethylene-propylene-maleic anhydride copolymer, butadiene-maleic anhydride copolymer, oxidized polyethylene, ethylene-acrylic acid copolymer, ethyl cellulose, ethyl hydroxyethyl cellulose, and the like. These polymer protective colloids may be used alone or in combination of two or more.

[0069] Among them, from the viewpoint of the dispersion stability of the monomer containing acrylic acid (salt) as a main component, it is more preferable to use maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, etc. as the polymer protective colloid.

[0070] The amount of the polymer protective colloid used is not particularly limited. However, in order to maintain a good dispersion state of the monomer containing acrylic acid (salt) as a main component in the hydrocarbon dispersion medium and obtain a dispersion effect commensurate with the amount used, it is preferably 0.05 to 5% by mass, more preferably 0.1 to 3% by mass, based on the total amount of the monomer.

[0071] The surfactant is not particularly limited. For example, sucrose fatty acid esters, polyglycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkyl allyl formaldehyde condensed polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl glucamine, polyoxyethylene fatty acid amides, polyoxyethylene alkyl amines, phosphate esters of polyoxyethylene alkyl ethers, phosphate esters of polyoxyethylene alkyl allyl ethers, and other nonionic surfactants can be mentioned. These surfactants may be used alone or in combination of two or more kinds.

[0072] Among them, from the viewpoint of the dispersion stability of the monomer mainly containing acrylic acid (salt) in the hydrocarbon dispersion medium, it is more preferable to use sucrose fatty acid esters, polyglycerol fatty acid esters, etc. as the surfactant.

[0073] The amount of the above-mentioned surfactant used is not particularly limited. However, in order to maintain a good dispersion state of the monomer mainly containing acrylic acid (salt) in the hydrocarbon dispersion medium and obtain a dispersion effect commensurate with the amount used, it is preferably 0.05 to 5% by mass, more preferably 0.1 to 3% by mass, based on the total amount of the monomer.

[0074] In the manufacturing method according to an embodiment of the present invention, it is preferable to carry out a reaction by the reverse phase suspension polymerization method using a radical polymerization initiator. Such radical polymerization initiators include, but are not limited to, for example, persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; azo compounds such as 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-phenylamidinopropane)] dihydrochloride, 2,2'-azobis[2-(N-allylamidinopropane)] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid). These radical polymerization initiators may be used alone or in combination of two or more.

[0075] The amount of the radical polymerization initiator used is preferably 0.005 to 1 mol% based on the total amount of the monomers, although it is not limited thereto. If the amount used is 0.005 mol% or more, it is preferable because the polymerization reaction does not require a long time. When the amount used is 1 mol% or less, it is preferable because a rapid polymerization reaction is less likely to occur.

[0076] In addition, the radical polymerization initiator may be used in combination with a reducing agent such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid and used as a redox polymerization initiator.

[0077] In the production method according to an embodiment of the present invention, the reaction temperature of the inverse suspension polymerization varies depending on the radical polymerization initiator used, but is preferably 20 to 110 °C, more preferably 40 to 90 °C. If the reaction temperature is 20 °C or higher, the polymerization rate is high and the polymerization time does not become long, which is preferable. If the reaction temperature is 110 °C or lower, it becomes easier to remove the heat of polymerization, so the reaction can be carried out smoothly. The reaction time is usually preferably 0.1 to 4 hours.

[0078] In the production method according to an embodiment of the present invention, the amount of water in the monomer aqueous solution is preferably 10 to 200% by mass with respect to the hydrocarbon solvent. From the viewpoint of improving the dispersion state of the monomer containing acrylic acid (salt) as a main component, 10% by mass or more is preferable, and from the viewpoint of facilitating industrial production and being economically preferable, 200% by mass or less is preferable.

[0079] In the production method according to an embodiment of the present invention, the inverse suspension polymerization may be carried out in one stage, or may be carried out in two or more stages.

[0080] When carrying out inverse suspension polymerization in two or more stages, after carrying out the first-stage inverse suspension polymerization by the above-described method, a monomer containing acrylic acid (salt) as a main component is added to and mixed with the reaction mixture obtained in the first-stage polymerization reaction, and the second-stage and subsequent inverse suspension polymerizations may be carried out in the same manner as in the first stage.

[0081] In an embodiment of the present invention, the shape of the water-containing gel obtained in the polymerization step is not particularly limited, but is usually particulate. The particulate water-containing gel, in other words, the water-containing gel particles, may be primary particles, secondary particles in which the primary particles are aggregated, or a mixture of primary particles and secondary particles. More preferably, the water-containing gel particles are secondary particles in which the primary particles are aggregated. For example, before adding the starch particles described later, a flocculant such as silica may be added to flocculate the water-containing gel.

[0082] (f) Starch particles In one embodiment of the present invention, the starch particles are preferably gelatinizable (α - modified) starch. Examples of the starch particles that can be used in the present invention include corn starch, potato starch, wheat starch, tapioca starch, waxy corn starch, rice starch, sweet potato starch, and the like. Among them, potato starch and tapioca starch are easily gelatinized and thus excellent in adhesion moldability during absorber molding. On the other hand, corn starch is excellent as a dispersant and a hygroscopic fluidity improver.

[0083] Also, the starch may be chemically modified to the extent that it does not impair the content of the present invention. Examples of the modification method for obtaining modified starch include esterification such as acetylation treatment, etherification such as carboxyalkylation, phosphorylation, oxidation, sulfation, phosphoric acid cross - linking, adipic acid cross - linking, enzyme treatment, and combinations thereof. The type of substituent introduced by the modification may be one type or two or more types. Considering the physical properties of the resulting water - absorbent resin composition, the degree of substitution during modification is preferably 2 or less, more preferably 1 or less, still more preferably 0.5 or less, and particularly preferably 0.25 or less. When an acid group is introduced by chemical modification and the starch is hydrophilized, the water - absorption performance of the water - absorbent resin composition is improved. When an alkyl group is introduced and the starch is hydrophobized, the effect as a dispersant is improved.

[0084] Further, the starch may be crosslinked to such an extent that the content of the present invention is not impaired. The starch can be crosslinked by any known method. Specifically, it may be crosslinked using a crosslinking agent, or may be crosslinked using radiation (for example, radiation such as gamma rays, X-rays, electron beams, etc.) and / or heat. Examples of the crosslinking agent include N-methylol compounds having a cyclic moiety in the molecule (dimethylol ethylene urea, dimethylol dihydroxyethylene urea, etc.), polycarboxylic acids (tricarballylic acid citrate, butanetetracarboxylic acid, etc.), polyfunctional epoxy compounds (ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, glycerol diglycidyl ether, etc.), polyvalent metal ions (aluminum ions, chromium ions, etc.), polyfunctional amines (amino acids, polyamines, triamines, diamines, etc.), polyfunctional aldehydes (glutaraldehyde, glyoxal, etc.), and the like. These crosslinking agents may be used alone or in combination of two or more.

[0085] In the present invention, it is preferable that the starch particles added to the hydrocarbon dispersion medium are not gelatinized. By not being gelatinized, the starch particles exhibit the effect as a dispersant. Further, by allowing the starch particles added to the hydrocarbon dispersion medium to remain in the water-absorbing resin composition, the effect as a moisture absorption fluidity improver is exhibited.

[0086] From the viewpoint of exhibiting the effect as a dispersant or improving the moisture absorption fluidity, the volume average particle diameter of the starch particles is preferably in the range of 1 μm to 50 μm, more preferably in the range of 3 μm to 40 μm, and even more preferably in the range of 5 μm to 30 μm. The volume average particle diameter of the starch is measured by a laser diffraction particle size distribution measuring device with the polysaccharide dispersed in a solvent in which dissolution and swelling hardly occur.

[0087] The amount of starch particles added to the hydrocarbon dispersion medium is preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, particularly preferably 20% by mass or more, and most preferably 30% by mass or more, based on the monomer containing an acrylic acid (salt) monomer as a main component. The upper limit of the amount of starch particles used is preferably 100% by mass or less, and more preferably 50% by mass or less. When the amount of starch particles used is less than 3% by mass, the amount of starch used as a sustainable raw material decreases. When the amount of starch particles used exceeds 100% by mass, the water absorption ratio of the water-absorbing resin composition may decrease. For these reasons, when adding a polysaccharide to the aqueous monomer solution or the hydrocarbon dispersion medium even before the polymerization step, the total amount of the polysaccharide used before the polymerization step and the starch used during or after the polymerization step is preferably 100% by mass or less, and more preferably 50% by mass or less.

[0088] The timing of adding the starch particles is at any stage from the polymerization step to the separation step described later. Preferably, the starch particles are added when the polymerization rate of the acrylic acid (salt) monomer is 10 mol% or more, that is, added at the stage where the hydrogel, which is a polymer of the monomer, exists. The polymerization rate can be determined, for example, from the amount of the acrylic acid (salt) monomer in the reaction system.

[0089] 〔2-2〕Separation step This step is a step of separating the polymer (hydrogel) obtained in the above-mentioned polymerization step from the hydrocarbon dispersion medium. Here, the separation method is not limited as long as the hydrogel can be separated from the hydrocarbon dispersion medium. For example, it can be carried out by one or more methods selected from the methods of filtration, removing the hydrocarbon dispersion medium by decantation, removing the hydrocarbon dispersion medium by distillation, and taking out the precipitated hydrogel from the system with an extruder. Among them, in order to reliably recover the starch particles, it is preferable to remove the hydrocarbon dispersion medium by distillation.

[0090] 〔2-3〕Drying step This step is to remove the moisture of the polymer (hydrogel) obtained in the above polymerization step before, during, or after the above separation step, and adjust the water content of the resulting water-absorbing resin composition. When distillation is performed in the above separation step, moisture also evaporates, so it also serves as a drying step.

[0091] In one embodiment of the present invention, the solid content ratio of the hydrogel is preferably 25 to 80% by mass, more preferably 40 to 70% by mass, and still more preferably 45 to 70% by mass. This results in high productivity without incurring drying costs. Furthermore, since drying at high temperatures is not required, coloring derived from starch can be suppressed.

[0092] Drying may be performed under normal pressure or reduced pressure, and in order to enhance the drying efficiency, it may also be performed under a gas flow such as nitrogen. When the drying step is performed under normal pressure, the drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, still more preferably 80 to 140°C, and particularly preferably 80 to 130°C. By controlling the temperature, coloring of the water-absorbing resin composition in the drying step can be prevented. In the case of reduced pressure, the drying temperature is preferably 60 to 100°C, and more preferably 70 to 90°C.

[0093] From the perspective of preventing coloring derived from starch, the drying time is usually 1 to 360 minutes, preferably 10 to 240 minutes, and more preferably 60 to 180 minutes.

[0094] More specifically, examples of the drying method include a method of performing dehydration by azeotropic distillation before the separation step; a method of performing drying under reduced pressure after the separation step, etc. Among them, due to the simplicity in the manufacturing process, a method of performing dehydration by azeotropic distillation before the separation step is preferred. In the above drying method, it is more preferable to dry the hydrogel with stirring. Specifically, azeotropic distillation with stirring and drying under reduced pressure with stirring are more preferable. By heating with stirring, drying can be performed efficiently.

[0095] 〔2-4〕Surface crosslinking step The manufacturing method according to an embodiment of the present invention may include a surface crosslinking step after the polymerization step. In the surface crosslinking step, a surface crosslinking agent is added to the water-containing gel particles obtained in the polymerization step, or the water-containing gel particles with an appropriately adjusted water content in the drying step or the water-absorbing resin powder after drying, and a surface crosslinking treatment is performed. Preferably, the water-containing gel particles with an appropriately adjusted water content in the drying step are surface crosslinked and further dried if necessary. The surface crosslinking step may be performed in a hydrocarbon dispersion medium before the separation step or after the separation step. Thereby, the surface crosslinking density is increased, and a water-absorbing resin composition having improved water absorption performance such as the water absorption ratio under load and gel strength can be obtained. Examples of such a surface crosslinking agent include compounds having two or more reactive functional groups. Examples thereof include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)ethylene glycol triglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; and carbonate compounds such as ethylene carbonate.Among these, polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)ethylene glycol triglycidyl ether, (poly)glycerol diglycidyl ether, (poly)glycerol triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether are particularly preferred. These surface crosslinking agents may be used alone or in combination of two or more.

[0096] The addition amount of the surface crosslinking agent is preferably in the range of 0.00001 mol to 0.03 mol, more preferably in the range of 0.00005 mol to 0.02 mol, and even more preferably in the range of 0.0001 mol to 0.01 mol, based on 1 mol of the total amount of the monomers mainly containing acrylic acid (salt) used in the polymerization step, so as not to reduce the water absorption magnification of the resulting water-absorbing resin composition and to enhance the crosslinking density near the surface and improve various properties.

[0097] The surface crosslinking agent may be added to the water-containing gel obtained in the polymerization step or the water-containing gel with appropriately adjusted water content in the drying step. However, it is preferably added in the presence of water in the range of 1 to 200% by mass, more preferably in the range of 5 to 100% by mass, and even more preferably in the range of 10 to 40% by mass, based on the solid content of the water-containing gel.

[0098] When using the surface crosslinking agent, water or a hydrophilic organic solvent may be used as a solvent as needed. Examples of the hydrophilic organic solvent include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; ketones such as acetone and methyl ethyl ketone; ethers such as diethyl ether, dioxane, and tetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxides such as dimethyl sulfoxide. These hydrophilic organic solvents may be used alone or in combination of two or more. Further, these hydrophilic organic solvents may be used as a mixed solvent with water.

[0099] The temperature in the surface crosslinking reaction is preferably 50 to 250°C, more preferably 60 to 180°C, still more preferably 60 to 140°C, and particularly preferably 70 to 120°C. Also, the reaction time of the surface crosslinking varies depending on the reaction temperature, the type and amount of the surface crosslinking agent, etc., and thus cannot be determined unconditionally. Usually, it is 1 to 300 minutes, preferably 5 to 200 minutes.

[0100] After the surface crosslinking step, the water-absorbing resin composition may be subjected to crushing of coarse aggregates or classification for removing coarse aggregates and fine powder. The mesh opening of the sieve used in the classification for removing coarse aggregates can be 1000 μm, 850 μm, or 710 μm. Here, the shape of the holes of the sieve is not limited to circular or square, and oval or rectangular shapes are also preferably used. In that case, the mesh opening refers to the minor axis or the length of the short side, and the aspect ratio meaning major axis / minor axis, or long side / short side is preferably 1 to 5.

[0101] 〔2 - 5〕Other steps The method for producing the water-absorbing resin composition according to one embodiment of the present invention may further include a step of adding additives such as a heat resistance stabilizer, an antioxidant, and an antibacterial agent as required.

[0102] The amount of the additive varies depending on the use of the water-absorbing resin composition, the type of the additive, etc. However, it is preferably 0.001 to 10% by mass, more preferably 0.01 to 5% by mass, and still more preferably 0.1 to 2% by mass with respect to the total amount of the monomers mainly containing acrylic acid (salt) constituting the water-absorbing resin composition.

[0103] 〔3〕Water-absorbing resin composition The water-absorbing resin composition according to one embodiment of the present invention is the water-absorbing resin composition obtained by the above manufacturing method. Further, the water-absorbing resin composition according to one embodiment of the present invention is a water-absorbing resin composition containing 50 to 97% by mass of polyacrylic acid (salt)-based water-absorbing resin particles in a spherical shape or an aggregate thereof, and 3 to 50% by mass of starch particles. The spherical shape or the aggregate thereof is generally obtained by inverse phase suspension polymerization. The starch particles contained in the water-absorbing resin composition are preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, particularly preferably 20% by mass or more, and most preferably 25% by mass or more. The upper limit of the content of the starch particles is preferably 50% by mass or less, more preferably 30% by mass or less. When the content of the starch particles is less than 3% by mass, the amount of starch used as a sustainable raw material decreases. When the content of the starch particles exceeds 50% by mass, the water absorption ratio of the water-absorbing resin composition may decrease. The water-absorbing resin composition of the present invention may further contain water, but the water content is preferably 20% by mass or less, more preferably 1 to 15% by mass, and still more preferably 3 to 13% by mass. Also, [2-5] it can also contain the components described in other steps.

[0104] The starch particles are distinguished from the starch integrated with the water-absorbing resin. When produced by the above-described manufacturing method, substantially the addition amount of the starch particles becomes the content of the starch particles with respect to the water-absorbing resin. However, when produced by other methods, the content of the starch particles can be determined by appropriately using classification, observation with an electron microscope or an optical microscope, elemental distribution analysis by an electron probe microanalyzer (EPMA), etc. The starch particles are preferably not gelatinized from the viewpoint of moisture absorption fluidity. As specific examples of the starch particles, those described in the [2-1] polymerization step can be used.

[0105] The mass average particle diameter of the water-absorbing resin composition according to one embodiment of the present invention can be selected in the range of 10 to 2000 μm according to the intended use. However, when it is intended for use in sanitary materials, the mass average particle diameter is 50 to 500 μm, more preferably 100 to 450 μm, and even more preferably 100 to 400 μm. Here, the mass average particle diameter is a value measured by the measurement method described in the examples below. Conventionally, a water-absorbing resin having a mass average particle diameter of about 300 to 600 μm has been preferred from the viewpoint of handling properties. However, the water-absorbing resin composition of the present invention is less likely to cake during moisture absorption and is further moldable. Therefore, the water-absorbing resin composition before molding can be handled with a smaller particle diameter than before.

[0106] The shape of the water-absorbing resin composition according to one embodiment of the present invention is spherical or an aggregate thereof. Here, the spherical shape includes not only a perfect sphere but also a substantially spherical shape with an aspect ratio of 1.0 to 1.2.

[0107] Further, the water-absorbing resin composition according to one embodiment of the present invention has a non-pressurized absorption ratio of 20 g / g or more, more preferably 25 g / g or more, and even more preferably 30 g / g or more. The upper limit value of the non-pressurized absorption ratio is not particularly limited, but is 70 g / g or less, more preferably 60 g / g or less, and even more preferably 50 g / g or less.

[0108] Further, the water-absorbing resin composition according to one embodiment of the present invention has a pressurized water absorption ratio (AAP0.7) under a pressure of 0.7 kPa of 10 g / g or more, more preferably 12 g / g or more, and even more preferably 15 g / g or more. The upper limit value of the pressurized water absorption ratio (AAP0.7) under a pressure of 0.7 kPa is not particularly limited, but is 40 g / g or less, more preferably 35 g / g or less, and even more preferably 30 g / g or less.

[0109] Further, the water-absorbing resin composition according to one embodiment of the present invention preferably has a moisture absorption blocking rate of 10% by mass or less, more preferably 7% by mass or less, and even more preferably 5% by mass or less.

[0110] In addition, the water-absorbing resin composition according to an embodiment of the present invention has a yellowness index (YI) of 20 or less, more preferably 15 or less, still more preferably 10 or less, and particularly preferably 6 or less. The lower limit value of the yellowness index is not particularly limited, and the closer it is to 0, the more preferable it is, but it is 1 or more, more preferably 2 or more. The smaller the yellowness index, the less colored it is and the closer it is to substantially white.

[0111] 〔4〕Method for manufacturing a sheet-shaped absorber The water-absorbing resin composition described in 〔2〕 and 〔3〕 can be used as an adhesive by gelatinizing starch and molded into a sheet-shaped absorber. That is, it is a method for manufacturing a sheet-shaped absorber in which the water-absorbing resin composition is sprayed in a sheet shape, water is added, and the water-absorbing resin composition is heated to gelatinize the starch.

[0112] The heating temperature (temperature of the heat medium) is preferably 100°C or higher, more preferably 110°C or higher. From the viewpoint of preventing coloring, the upper limit is preferably 200°C or lower, more preferably 150°C or lower.

[0113] The heating time is preferably 10 seconds or longer. From the viewpoint of preventing coloring, the upper limit is preferably within 1 hour, more preferably within 10 minutes.

[0114] As the heating method, a method of pressing and crimping a heated solid, and / or a method of spraying steam heated to 100°C or higher can be adopted. In continuous production, the "heated solid" may be a heating roll. By pressing a solid against the water-absorbing resin composition, the water-absorbing resin composition can be efficiently heated and the absorber can be molded. In addition, by spraying steam heated to 100°C or higher, water addition and heating can be efficiently performed.

[0115] The water may be added either as a liquid or as steam, but preferably liquid water and steam are added.

[0116] The absorber of the present invention may contain fibers such as pulp. However, since the present invention can be applied to thin absorbers, the fiber content in the absorber is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less.

[0117] The absorber of the present invention may further sandwich or surround a molded product of a water-absorbing resin composition with a non-woven fabric. The non-woven fabric may be used after molding the water-absorbing resin composition, or a non-woven fabric that can withstand the above heating may be used before molding the water-absorbing resin composition.

[0118] 〔5〕Effect According to one aspect of the present invention, there is provided a method for producing a water-absorbing resin composition that is low-cost, highly productive, and easy to operate, using at least a part of a polysaccharide as a sustainable raw material, and a water-absorbing resin composition that is excellent in adhesive moldability and has little coloring. Further, there is an effect that a method for producing an absorber having a high use ratio of a sustainable raw material with respect to the entire absorber and good adhesive moldability can be provided.

[0119] Further, according to the method for producing the water-absorbing resin composition described above, in the reverse phase suspension polymerization, it is possible to reduce troubles in the polymerization step, separation step, and drying step, which are likely to cause troubles such as agglomeration of the hydrogel. Furthermore, the water-absorbing resin composition of the present invention also has an effect of excellent moisture absorption fluidity, which has been considered to be contrary to adhesive moldability.

[0120] Conventionally, a water-absorbing resin composition produced using reverse phase suspension polymerization has had problems such as being difficult to fix and mold due to a smooth surface shape and factors such as the remaining surfactant used. However, when the absorbent resin composition of the present invention is used, it can be easily molded into an absorber by gelatinizing starch. Furthermore, the absorber of the present invention also has an effect that, compared with an absorber using a generally hydrophobic synthetic adhesive, the adhesive does not inhibit liquid absorption and has good liquid wettability.

Example

[0121] The present invention will be described more specifically according to the following examples and comparative examples. However, the present invention should not be construed as being limited thereto, and examples obtained by appropriately combining the technical means described in each example are also included in the scope of the present invention. Each experiment was conducted under the conditions of room temperature of 20 to 25 °C and humidity of 30 to 60% RH. The electrical equipment used for measurement used electricity of 100 V and 60 Hz.

[0122] <Evaluation method> 〔CRC (Absorption capacity under non-pressurized conditions)〕 CRC (Absorption capacity under non-pressurized conditions) was measured in accordance with the EDANA method (ERT441.2-02). Specifically, 0.2 g of the water-absorbing resin composition was placed in a bag made of non-woven fabric, immersed in a large excess of 0.9 mass% sodium chloride aqueous solution for 30 minutes to allow the water-absorbing resin composition to swell freely, and then dehydrated for 3 minutes using a centrifuge (centrifugal force: 250 G), and CRC (Absorption capacity under non-pressurized conditions) (unit: g / g) was measured.

[0123] 〔AAP (Absorption capacity under pressurized conditions)〕 AAP (Absorption capacity under pressurized conditions) was measured in accordance with the EDANA method (ERT442.2-02) except that the load was changed from 21 g / cm 2 to 49 g / cm 2 Specifically, using a large excess of 0.9 mass% sodium chloride aqueous solution, 0.9 g of the water-absorbing resin composition was swollen under a pressure of 4.8 kPa (49 g / cm2, 0.7 psi) for 1 hour, and then AAP (Absorption capacity under pressurized conditions) (unit: g / g) was measured. The result measured under the pressurized condition of 0.7 psi is denoted as AAP0.7.

[0124] 〔Water content and solid content ratio〕 The water content was measured in accordance with the EDANA method (ERT430.2-02). In the measurement, the mass of the sample was changed to 1 g, the drying temperature was changed to 180 °C, and the drying time was changed to 3 hours. Specifically, after putting 1 g of the sample into an aluminum cup with a bottom diameter of 50 mm, the total mass W1 (g) of the sample (hydrous gel and aluminum cup) was accurately weighed. Next, the sample was left still in an oven set at an ambient temperature of 180 °C. After 3 hours had passed, the sample was taken out of the oven and the total mass W2 (g) was accurately weighed. When the mass of the sample used in this measurement was M (1 g), the water content (100-α) (mass%) of the sample was determined according to the following formula (1). Here, α is the solid content ratio (mass%) of the sample.

[0125] (100-α) (mass%) = {(W1 - W2) / M} × 100 Formula (1) 〔Mass average particle diameter of the water-absorbing resin composition〕 An 80-mm-diameter JIS standard sieve was combined from the top in the order of 850 μm, 600 μm, 425 μm, 300 μm, 212 μm, 150 μm, 106 μm, 75 μm, 45 μm, and a receiving tray. 10.0 g of the water-absorbing resin composition was put into the top sieve of the combination and classified by shaking for 5 minutes using a rotary tap shaker (IIDA SIEVE SHAKER, TYPE: ES-65 type). The mass of the particles remaining on each sieve was calculated as the mass percentage with respect to the total amount, and the relationship between the sieve opening and the integrated value of the mass percentage of the particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle diameter corresponding to an integrated mass percentage of 50 mass% was read as the mass average particle diameter (D50).

[0126] 〔Color evaluation of the water-absorbing resin composition (yellowness index: YI)〕 The coloration evaluation of the water-absorbing resin composition was carried out using a spectroscopic color difference meter SZ-Σ80 COLOR MEASURING SYSTEM manufactured by Nippon Denshoku Industries Co., Ltd. Under the set conditions (reflection measurement / attached powder / paste sample stage (inner diameter 30 mm, height 12 mm / standard powder / paste standard white plate No. 2 / 30Φ projection light pipe)), 5 g of the water-absorbing resin composition was filled into the attached sample stage (filling about 60% of the attached sample stage), and the yellowness index (YI) of the surface of the water-absorbing resin composition was measured with the above spectroscopic color difference meter.

[0127] 〔Moisture absorption blocking rate〕 For the evaluation of the moisture absorption blocking rate of the water-absorbing resin composition, 2 g of the sample was evenly spread on the bottom of an aluminum cup with a bottom diameter of 50 mm and a height of 22 mm, and immediately placed in a thermo-hygrostat (manufactured by ESPEC, model: SH-641) previously adjusted to 70°C and 65% relative humidity, and left for 60 minutes. Then, the moisture-absorbed water-absorbing resin composition was transferred to a JIS standard sieve with a diameter of 80 mm and a mesh opening of 2000 μm. At this time, if the moisture-absorbed water-absorbing resin composition adheres strongly to the aluminum cup and cannot be transferred to the sieve, the water-absorbing resin composition in the state of moisture absorption blocking is peeled off and transferred to the sieve while taking care not to break it as much as possible. Immediately, this was sieved for 8 seconds with a vibration classifier (IIDA SIEVE SHAKER, TYPE: ES-65 type), and the mass W3 (g) of the water-absorbing resin composition remaining on the sieve and the mass W4 (g) of the water-absorbing resin composition passing through the sieve were measured. The moisture absorption blocking rate (mass%) was calculated by the following calculation formula. The lower the moisture absorption blocking rate, the better the moisture absorption fluidity, and the handling properties of the powder, etc. are improved.

[0128] Moisture absorption blocking rate (mass%) = {mass W3 (g) / (mass W3 (g) + mass W4 (g))} × 100 〔Example 1〕 A 2 L round-bottom cylindrical separable flask equipped with a stirrer, two-stage paddle blades, a reflux condenser, a dropping funnel, and a nitrogen gas inlet pipe was prepared. Also, a cooling water circulation device (manufactured by Tokyo Rika Kikai Co., Ltd.: EYELA COOL ACE CA-1112) was used in the cooling part to circulate a coolant at 5°C.

[0129] 340 g of n-heptane was placed in this flask, and 0.45 g of sucrose stearate (manufactured by Mitsubishi Chemical Foods Co., Ltd., Ryoto Sugar Ester S-370) with an HLB of 3 was added as a surfactant. While stirring the mixture of n-heptane and the surfactant in the flask, the temperature was raised to 70 °C to dissolve the surfactant, nitrogen gas was blown in to expel dissolved oxygen, and then it was cooled to 55 °C.

[0130] On the other hand, 74.06 g (1.03 mol) of acrylic acid was taken, and while cooling from the outside, 163.5 g of a 19% by mass aqueous sodium hydroxide solution was added dropwise to carry out 75 mol% neutralization. Then, 2.2 g (0.406 mmol) of 5% by mass potassium persulfate as a radical polymerization initiator and 0.184 g (0.053 mmol) of 5% by mass ethylene glycol diglycidyl ether as an internal cross-linking agent were added to and mixed with this 75 mol% neutralized product to prepare a monomer aqueous solution.

[0131] After blowing nitrogen gas into the monomer aqueous solution to sufficiently replace the inside of the system with nitrogen, the entire amount of the monomer aqueous solution was added to the separable flask and dispersed by stirring at 500 rpm. Thereafter, when the bath temperature was raised to 70 ° C, polymerization started. Thereafter, the bath temperature was maintained at 70 ° C for 1 hour. The polymerization rate of the monomer was 99 mol% or more. 30 g (33% by mass based on the monomer solid content) of ungelatinized corn starch (manufactured by Sanwa Starch Co., Ltd., Sanwa Corn Starch Y, volume average particle diameter 15 μm, yellowness YI = 7.2) was added, a dehydrating tube was attached to the separable flask, the stirring rotation speed was set to 1000 rpm, the bath temperature was raised to 95 ° C, and 120 g of water was extracted (the water content was 25% by mass in calculation). No generation of aggregates during dehydration and no fluctuation of the stirring power were observed. Further, n-heptane was distilled off to obtain a water-absorbing resin composition comprising a water-absorbing resin, ungelatinized starch particles, and a surfactant. This was dried under reduced pressure at 100 ° C and passed through a JIS standard sieve with an opening of 850 μm to obtain a water-absorbing resin composition (1) containing 25% by mass of ungelatinized starch particles. The mass average particle diameter of the water-absorbing resin composition (1) was 70 μm, the water content was 4% by mass, the CRC was 34.2 g / g, the yellowness YI was 8.5, and the moisture blocking rate was 0% by mass. When observed with an electron microscope, spherical or football-shaped particles were seen. The spherical particles were regarded as the water-absorbing resin, and the football-shaped particles were regarded as starch. No particles in which the starch was fused to the water-absorbing resin were seen.

[0132] Example 2 In Example 1, after raising the bath temperature to 95 ° C and extracting 120 g of water, 4.0 g of a 2% by mass ethylene glycol diglycidyl ether aqueous solution was injected with a syringe and heated at 80 ° C for 1 hour. Otherwise, the same operation as in Example 1 was carried out to obtain a water-absorbing resin composition (2) containing 25% by mass of ungelatinized starch particles. The mass average particle diameter of the water-absorbing resin composition (2) was 80 μm, the water content was 5% by mass, the CRC was 27.3 g / g, the AAP(0.7) was 20.9 g / g, the yellowness YI was 8.7, and the moisture blocking rate was 0% by mass.

[0133] Example 3 In Example 2, the procedure was the same as in Example 2 except that the amount of starch used was changed to 10 g (11% by mass based on the monomer), and a water-absorbing resin composition (3) containing 10% by mass of ungelatinized starch particles was obtained. The mass average particle diameter of the water-absorbing resin composition (3) was 90 μm, the water content was 4% by mass, the CRC was 34.1 g / g, the AAP(0.7) was 22.5 g / g, the yellowness YI was 9.0, and the moisture blocking rate was 3% by mass.

[0134] 〔Example 4〕 In Example 2, the procedure was the same as in Example 2 except that the amount of starch used was changed to 3 g (3% by mass based on the monomer), and a water-absorbing resin composition (4) containing 10% by mass of ungelatinized starch particles was obtained. The mass average particle diameter of the water-absorbing resin composition (4) was 90 μm, the water content was 4% by mass, the CRC was 35.6 g / g, the AAP(0.7) was 25.5 g / g, the yellowness YI was 9.3, and the moisture blocking rate was 4% by mass.

[0135] 〔Comparative Example 1〕 In Example 2, the procedure was the same as in Example 2 except that corn starch was not added, and a comparative water-absorbing resin composition (1) was obtained. The mass average particle diameter of the comparative water-absorbing resin composition (1) was 90 μm, the water content was 4% by mass, the CRC was 36.1 g / g, the AAP(0.7) was 26.5 g / g, the yellowness YI was 9.3, and the moisture blocking rate was 15% by mass.

[0136] 〔Comparative Example 2〕 An aqueous polymerized absorbent resin composition was prepared. Specifically, 37.1 g (0.51 mol) of acrylic acid, 1.02 g (0.0195 mmol) of a 1% by mass aqueous solution of polyethylene glycol diacrylate (average molecular weight 523), and 0.23 g of 1% by mass pentasodium diethylenetriaminepentaacetate were mixed in a 250 ml polypropylene resin container to prepare an aqueous acrylic acid solution. While stirring this aqueous acrylic acid solution with a magnetic stirrer, 15.0 g of corn starch (manufactured by Sanwa Starch Industry Co., Ltd., Sanwa Corn Starch Y, volume average particle diameter 15 μm) as a polysaccharide was added and dispersed to prepare a monomer dispersion. Then, 29.7 g (0.36 mol) of a 48.5% by mass aqueous sodium hydroxide solution and 25.0 g of water were mixed and heated to 42°C, and the resulting aqueous sodium hydroxide solution was mixed with the above monomer dispersion. As a result, the temperature of the mixed solution immediately rose to 103°C due to the heat of neutralization. Due to this temperature rise, the dispersed corn starch dissolved, but the viscosity of the aqueous monomer solution increased rapidly, and stirring with a magnetic stirrer became impossible.

[0137] When the temperature of this aqueous monomer solution dropped to 95°C, 2.6 g of a 2% by mass aqueous sodium persulfate solution was added as a polymerization initiator. Polymerization started 3 seconds after the addition of the aqueous sodium persulfate solution. After 5 minutes, the hydrated polymer was taken out, but since the polymerization initiator could not be uniformly mixed, a hydrated gel-like polymer with a mottled appearance of polymerized portions and remaining monomer portions was obtained. This hydrated gel-like polymer was cut into pieces approximately 1 to 5 mm in size with scissors, placed on a wire mesh, and dried with hot air at 150°C for 30 minutes. The dried product was pulverized with a roll mill and classified using a JIS standard sieve with an opening of 850 μm and a JIS standard sieve with an opening of 150 μm to obtain a comparative water-absorbent resin composition (2). The mass average particle diameter of the comparative water-absorbent resin composition (2) was 472 μm, the CRC was 27.1 g / g, the yellowness YI was 35.12, and the moisture blocking rate was 100% by mass.

[0138] [Example 5] In Example 2, 0.3 g of fumed silica (Aerosil (trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) was added before adding the starch particles, and the water-absorbing resin aggregated. Subsequently, by operating in the same manner as in Example 1, a water-absorbing resin composition (5) was obtained. The mass average particle diameter of the water-absorbing resin composition (5) was 465 μm, the yellowness index YI was 8.6, and the moisture blocking rate was 0% by mass.

[0139] [Example 6] [Molding of Absorbent] A cylinder (height 20 mm, inner diameter 30 mm) was placed in the center of an aluminum foil (10 cm × 10 cm), and 1 g of the water-absorbing resin composition (2) obtained in Example 2 was uniformly sprayed. 0.42 g of deionized water was sprayed from 20 cm above by atomization, and a 200-mesh nylon mesh (10 cm × 10 cm) was covered. Then, a household steam iron (mass 900 g) in the steam high (hemp) mode was immediately placed for 1 minute to spray steam. The moisture-absorbed product of the obtained water-absorbing resin composition (2) had solidified in a sheet shape. This was designated as a sheet-shaped absorbent (1).

[0140] [Comparative Example 3] In Example 6, the operation was the same as in Example 6 except that the comparative water-absorbing resin composition (1) obtained in Comparative Example 1 was used instead of the water-absorbing resin composition (2) obtained in Example 2. The moisture-absorbed product of the obtained comparative water-absorbing resin composition (1) had solidified in a sheet shape. This was designated as a comparative sheet-shaped absorbent (1).

[0141] [Evaluation of Adhesive Moldability] It is evaluated in the same manner as the moisture absorption blocking rate. Specifically, the formed sheet-like absorbent is quickly (0 minutes) transferred to a JIS standard sieve with a diameter of 80 mm and an opening of 2000 μm. At this time, if the sheet-like absorbent is firmly attached to the aluminum foil and cannot be transferred to the sieve, the sheet-like absorbent in the state of having undergone moisture absorption and blocking is peeled off with care so as not to break it as much as possible and transferred to the sieve. Immediately, it is sieved for 8 seconds by a vibration classifier (IIDA SIEVE SHAKER, TYPE: ES-65 type), and the mass W5 (g) of the sheet-like absorbent remaining on the sieve and the mass W6 (g) of the water-absorbing resin composition that passed through the sieve are measured. The degree of molding was calculated according to the following formula. After the classification, the sheet-like absorbent was left standing on the sieve, and the classification was also carried out 10 minutes and 30 minutes after molding. That is, in the measurement after 30 minutes, the same sample was classified 3 times.

[0142] Degree of molding (mass %) = {mass W5 (g) / (mass W5 (g) + mass W6 (g))} × 100 〔Evaluation of liquid wettability〕 A 0.9 mass% aqueous sodium chloride solution colored with Food Blue No. 1 was dropped onto the sheet-like absorbent in one drop (0.05 g) using a pipette, and the liquid wettability was visually confirmed.

[0143]

Table 1

[0144] [Summary] In Examples 1 to 5, the amount of sustainable raw materials used increased compared to Comparative Example 1, the moisture absorption blocking rate decreased, and the coloring decreased. It is presumed that the coloring decreased because it was covered with white starch particles. In Examples 1 to 5, the coloring during production decreased compared to Comparative Example 2 (aqueous solution polymerization). Example 6 was able to achieve permanent molding and also improved the liquid wettability compared to Comparative Example 3.

Industrial applicability

[0145] The water-absorbing resin composition according to one embodiment of the present invention is used in disposable diapers, sanitary napkins, for adults Disposable incontinence products (incontinence pads), sanitary materials (sanitary products) such as pet sheets, and soil for agriculture and horticulture It can be suitably used in various water-absorbing articles such as soil water retention agents and industrial water stop agents.

Claims

1. A method for producing a water-absorbent resin composition containing starch particles and polyacrylic acid (salt)-based water-absorbent resin particles, comprising the steps of: A polymerization step in which an aqueous solution of a monomer containing acrylic acid (salt) as a main component is suspended in an oil phase containing a hydrocarbon dispersion medium to carry out polymerization; A separation step of separating the polymer obtained in the polymerization step from the hydrocarbon dispersion medium, The method for producing a water absorbent resin composition further comprises a step of adding starch particles to a hydrocarbon dispersion medium at a time point when a polymerization rate of a monomer containing acrylic acid (salt) as a main component is 10 mol % or more in any stage from the polymerization step to the separation step.

2. The method for producing a water-absorbing resin composition according to claim 1, wherein the starch particles are not gelatinized.

3. The method for producing a water-absorbing resin composition according to claim 1 or 2, wherein the starch particles have a volume average particle size of 1 to 50 μm.

4. The method for producing a water absorbent resin composition according to any one of claims 1 to 3, wherein the amount of the starch particles used is 3 to 100 mass% relative to the amount of the monomer containing acrylic acid (salt) as a main component.

5. The method for producing a water-absorbing resin composition according to any one of claims 1 to 4, further comprising a drying step of drying the polymer obtained in the polymerization step before, during, or after the separation step.

6. The method for producing the water-absorbing resin composition according to any one of claims 1 to 5, further comprising a surface cross-linking step after the polymerization step.

7. The method for producing a water-absorbing resin composition according to claim 6, wherein in the surface cross-linking step, a polyglycidyl compound is added to perform cross-linking.

8. The water-absorbent resin composition contains 50 to 97 mass % of polyacrylic acid (salt)-based water-absorbent resin particles which are spherical or aggregates thereof, and 3 to 50 mass % of starch particles, and has a moisture absorption blocking rate of 10 mass % or less when allowed to absorb moisture for 60 minutes at 70°C and a relative humidity of 65%.

9. The water-absorbing resin composition according to claim 8, wherein the starch particles are ungelatinized.

10. The water-absorbing resin composition according to claim 8 or 9, which has a yellowness index of 20 or less.

11. A method for producing a sheet-like absorbent body, comprising: spraying the water-absorbent resin composition according to any one of claims 8 to 10 into a sheet; adding water to heat the water-absorbent resin composition; and gelatinizing starch.

12. The method for manufacturing a sheet-like absorbent body according to claim 11, wherein the water is steam heated to 100° C. or higher, and the water-absorbing resin composition is heated by the steam.

13. The method for manufacturing a sheet-like absorbent body according to claim 11 or claim 12, wherein the water-absorbing resin composition is pressure-bonded while being heated to form a sheet shape.

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