Method for producing polybasic acid salt-modified starch and polybasic acid salt-modified starch composition

Abstract

Provided are a method for easily obtaining a high-quality polybasic acid salt-modified starch when producing a polybasic acid salt-modified starch by reacting a starch with a polybasic acid and / or an acid anhydride thereof, and a high-quality polybasic acid salt-modified starch composition. A method for producing a polybasic acid salt-modified starch includes a step for obtaining a polybasic acid-modified starch using a raw material composition containing a starch and a polybasic acid and / or an acid anhydride thereof and a step for neutralizing the polybasic acid-modified starch, and is characterized in that the degree of crystallinity of the starch contained in the raw material composition is 20% or less.

Description

Method for producing polybasic acid salt-modified starch and polybasic acid salt-modified starch composition

[0001] The present invention relates to a method for producing a polybasic acid salt-modified starch and a polybasic acid salt-modified starch composition. More specifically, the present invention relates to a method for producing a polybasic acid salt-modified starch and a polybasic acid salt-modified starch composition that are used in various industrial products such as sanitary materials and detergents.

[0002] Conventional hydrophilic polymers derived from petrochemicals have been suitably used as raw materials for various industrial products for sanitary materials, detergents, etc. Among them, polyacrylic acid (salt)-based water-absorbent resins are widely used in sanitary materials such as disposable diapers and sanitary products, as well as in applications requiring water absorption and retention in the agricultural, horticultural, food, and industrial fields.

[0003] In recent years, in order to reduce the environmental load and realize a sustainable society, there has been a demand for hydrophilic polymers to be made from biomass-derived raw materials or to be biodegradable. Under these circumstances, methods for obtaining hydrophilic polymers by modifying starch, which is naturally present in large quantities, have been investigated (Patent Documents 1 to 3, Non-Patent Document 1).

[0004] Patent Document 1: Japanese Patent Publication No. 2007-222704 International Publication No. 2024 / 029246 Chinese Patent Application Publication No. 110981981

[0005] Carbohydrate Polymers Volume 64, Issue 2, 11 May 2006, Pages 345-349

[0006] In order to use starch as a raw material for various industrial products such as sanitary materials and detergents, it is necessary to efficiently improve the hydrophilicity of starch.

[0007] The present invention has been made in view of the above-mentioned problems, and aims to provide a method for easily obtaining high-quality (e.g., highly hydrophilic) polybasic acid salt-modified starch when producing the polybasic acid salt-modified starch by reacting starch with a polybasic acid and / or its acid anhydride.

[0008] Another object of the present invention is to provide a high-quality (e.g., highly hydrophilic) polybasic acid salt-modified starch (composition).

[0009] The present invention, which has solved the above problems, has the following configuration.

[0010] That is, one aspect of the present invention is: [1] a method for producing a polybasic acid salt-modified starch, comprising a step of obtaining a polybasic acid-modified starch using a raw material composition containing starch and a polybasic acid and / or an acid anhydride thereof, and a step of neutralizing the polybasic acid-modified starch, wherein the crystallinity of the starch contained in the raw material composition is 20% or less; [2] the production method according to the above [1], wherein in the step of obtaining the polybasic acid-modified starch, the raw material composition contains less than 5000 ppm by mass of at least one selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, and 4-dimethylaminopyridine, and the mass proportion of an ionic liquid having an imidazolium ion is less than 5000 ppm; [3] Preferably, the method for producing according to claim [1] or [2], wherein the raw material composition contains at least one functional group selected from the group consisting of a sulfoxide group, a pyridyl group, and an amide group, and the mass proportion of the organic solvent that is liquid in the step of obtaining the polybasic acid-modified starch is less than 5000 ppm; [4] Preferably, the method for producing according to any one of [1] to [3], wherein the raw material composition contains a nitrogen atom and / or a sulfur atom, and the mass proportion of the organic solvent and / or organic catalyst that is liquid in the step of obtaining the polybasic acid-modified starch is less than 5000 ppm; [5] Preferably, the method for producing according to any one of [1] to [4], wherein the total mass of the substances selected from the group consisting of water, organic solvents, and ionic liquids (excluding the organic solvent and / or organic catalyst that contains a nitrogen atom and / or a sulfur atom and is liquid in the step of obtaining the polybasic acid-modified starch) added in the step of obtaining the polybasic acid-modified starch is 10 mass% or less relative to the raw material composition; [6] Preferably, in the step of obtaining the polybasic acid-modified starch, the reaction temperature during the modification reaction between the starch and the polybasic acid salt and / or its acid anhydride is higher than 120°C in the production method according to any one of [1] to [5] above; [7] Preferably, in the production method according to any one of [1] to [6] above, the acid anhydride of the polybasic acid is a cyclic polybasic acid anhydride;[8] Preferably, the production method according to any one of [1] to [7] above, wherein the polybasic acid and / or its acid anhydride is a polybasic carboxylic acid and / or its acid anhydride; [9] Preferably, the production method according to any one of [1] to [8] above, wherein the polybasic acid and / or its acid anhydride is a dibasic acid and / or its acid anhydride;

[10] Preferably, the production method according to any one of [1] to [9] above, wherein the polybasic acid and / or its acid anhydride is succinic acid and / or its acid anhydride;

[11] Preferably, the production method according to any one of [1] to

[10] above, wherein the polybasic acid-modified starch is a polybasic acid monoesterified starch;

[12] Preferably, the production method according to any one of [1] to

[11] above, wherein a base catalyst is used in the step of obtaining the polybasic acid-modified starch, and the amount of the base catalyst used is less than 30% by mass relative to the mass of the polybasic acid and / or its acid anhydride;

[13] The method for producing a starch according to

[12] above is preferred, wherein the base catalyst is a solid alkali compound;

[14] The method for producing a starch according to

[13] above is preferred, wherein the solid alkali compound is at least one selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, and alkali metal hydrogencarbonates;

[15] The method for producing a starch according to any one of [1] to

[14] above is preferred, wherein the polybasic acid salt-modified starch has an average degree of substitution of polybasic acid (salt) per glucose residue of 0.5 or more;

[16] The method for producing a starch according to any one of [1] to

[15] above is preferred, wherein the polybasic acid salt-modified starch obtained in the step of neutralizing the polybasic acid-modified starch is further crosslinked;

[17] The method for producing a starch according to any one of [1] to

[16] above is preferred, wherein a kneader is used in the step of obtaining the polybasic acid-modified starch;

[18] The method for producing a starch according to

[17] above is preferred, wherein the kneader is a multi-screw kneader;

[0011]

[0023] Another aspect of the present invention is the polybasic acid salt-modified starch composition according to

[19] , wherein the average degree of substitution of the polybasic acid (salt) per glucose residue is 0.5 or more;

[20] The polybasic acid salt-modified starch composition according to

[19] , wherein the mass proportion of at least one selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, and 4-dimethylaminopyridine is less than 5000 ppm, and the mass proportion of the ionic liquid having an imidazolium ion is less than 5000 ppm;

[21] The polybasic acid salt-modified starch composition according to

[19] or

[20] , wherein the average degree of substitution per glucose residue is 1.0 or more;

[22] The polybasic acid salt-modified starch composition according to any one of

[19] to

[21] , wherein the polybasic acid is succinic anhydride;

[23] Preferably, the polybasic acid salt-modified starch composition according to any one of

[19] to

[22] , wherein the polybasic acid salt-modified starch is a cross-linked polybasic acid salt-modified starch;

[24] Preferably, the polybasic acid salt-modified starch composition according to any one of

[19] to

[23] , which is used as a raw material for hygiene materials, agricultural and horticultural supplies, cosmetics, or detergents;

[25] Preferably, the polybasic acid salt-modified starch composition according to any one of

[19] to

[23] , is used as a raw material for hygiene materials, agricultural and horticultural supplies, cosmetics, or detergents;

[26] Preferably, a method for producing hygiene materials, agricultural and horticultural supplies, cosmetics, or detergents, comprising using the polybasic acid salt-modified starch composition according to any one of

[19] to

[23] .

[0012] 1 is a diagram showing an example of a reaction formula between starch and a polybasic acid anhydride (succinic anhydride as an example) in the production method of the present invention. FIG. 2 is a diagram showing an example of each step in the production method of the present invention.

[0013] The present invention is described in detail below. A combination of two or more of the individual preferred embodiments of the present invention described below is also a preferred embodiment of the present invention. In this specification, the term "X to Y" indicating a range means "X or more and Y or less." Furthermore, in this specification, the term "polybasic acid (salt)" refers to a polybasic acid and / or a polybasic acid salt.

[0014] [Method for producing polybasic acid salt modified starch] Hereinafter, a method for producing polybasic acid salt modified starch will be described in detail.

[0015] The polybasic acid salt modified starch obtained by the production method of the present invention is composed of modified starch in which the starch molecules are densely modified with polybasic acid salts and which is partially or entirely soluble in water at the molecular level.

[0016] The present inventors have focused on the crystallinity of starch and found that highly hydrophilic polybasic acid salt-modified starch can be obtained by using starch with a crystallinity of 20% or less as a raw material.

[0017] Furthermore, when starch with a crystallinity of 20% or less was modified with a modifier (polybasic acid), it was unexpectedly found that highly efficient substitution with the modifier was possible even when a small amount of solvent was added or no solvent was added.

[0018] Starch generally exhibits high dispersibility and solubility in certain solvents such as water and dimethyl sulfoxide, and therefore modification with polybasic acids or their anhydrides in the solvent has been investigated. That is, a typical method for producing modified starch involves dispersing or dissolving starch in a solvent and then adding a modifying agent to cause the reaction to proceed.

[0019] Therefore, the addition of a large amount of solvent is required to modify starch uniformly and efficiently, and depending on the solvent, problems such as reduced productivity due to a decrease in reaction concentration and reduced reaction efficiency due to side reactions occurring between the modifying agent and the solvent have arisen. Furthermore, reactions using a solvent have issues such as the need to remove the solvent, the possibility of side reactions occurring between the solvent and the modifying agent, and extremely high production costs due to the dilute concentration during the reaction. Furthermore, reactions using a solvent can result in a decrease in quality due to residual solvent that cannot be completely removed. That is, a low content of residual solvent (e.g., organic solvent) is one element of high-quality polybasic acid salt-modified starch.

[0020] Commonly used raw starch extracted from natural sources tends to react inefficiently with modifiers when a small amount of solvent is added or when no solvent is added, resulting in a low average degree of substitution and lower hydrophilicity compared to conventional hydrophilic polymers derived from petrochemicals. Therefore, it is unsuitable for applications requiring high charge density, such as sanitary materials and detergents, and it has been found difficult to use it as a substitute for the above-mentioned hydrophilic polymers derived from petrochemicals.

[0021] On the other hand, in the production method of the present invention, by using starch with a low degree of crystallinity, the reaction between the starch and the modifier proceeds efficiently even under conditions where a small amount of solvent (other than the polybasic acid) is added or where no solvent (other than the polybasic acid) is added, resulting in a high degree of substitution and a polybasic acid salt-modified starch with a high charge density. This polybasic acid salt-modified starch is highly hydrophilic due to its high charge density, and can be synthesized at low cost because it can be produced under conditions where a small amount of solvent is added or no solvent is used. It can be used as an inexpensive, high-quality alternative to the above-mentioned petrochemical-derived hydrophilic polymers. Furthermore, the low residual solvent content results in a high-quality polybasic acid salt-modified starch.

[0022] The degree of modification of modified starch molecules can be evaluated by several techniques, such as measuring the average degree of substitution, which represents the amount of modifier bound per glucose residue of a starch molecule, and the proportion of modified starch molecules that have become molecular and dissolved in water. For such evaluations, pH titration, gel permeation chromatography (GPC), etc. are used. The methods for measuring and calculating the average degree of substitution are described in detail in the Examples.

[0023] <Step of Obtaining Polybasic Acid-Modified Starch> The production method of the present invention includes a step of reacting starch with a polybasic acid and / or its acid anhydride to obtain a polybasic acid-modified starch. In this specification, the reaction of starch with a polybasic acid and / or its acid anhydride is referred to as the "modification reaction," and the step of obtaining a polybasic acid-modified starch is referred to as the "modification reaction step."

[0024] (Starch) Examples of starch used in the present invention include starch obtained from living organisms, particularly plants, and modified starches thereof. The starch source is not particularly limited, but examples of plants include corn, potato, wheat, cassava, tapioca, waxy corn, rice, and sweet potato. The starch may be modified by esterification such as acetylation, etherification such as carboxyalkylation, phosphorylation, sulfation, phosphate cross-linking, enzyme treatment, etc., but preferably is a starch whose crystallinity has been reduced by physical modification without chemical modification, such as gelatinization or amorphization. A preferred form of starch used in the present invention is pregelatinized starch.

[0025] In addition to refined starch, unrefined raw materials containing starch may also be used directly. For example, all or part of starch-containing crops, such as grains such as rice and wheat, and rhizomes such as cassava and potato, may be used as raw materials in an unrefined state.

[0026] In the modification reaction, the starch is preferably amorphous with low crystallinity to enhance reaction efficiency. The starch used as a raw material before modification is in a solid form, such as granules, compressed flakes, or powder, with granules or powder being preferred. Furthermore, starch is hygroscopic, but the moisture contained in the starch is distinct from the water or organic solvent added separately during the reaction. The presence of water reduces the concentration during the reaction, thereby reducing reaction efficiency, and also promotes decomposition of the ester bonds and starch skeleton in the polybasic acid anhydride- or polybasic acid salt-modified starch as a modifying agent. Therefore, it is preferable that the starch be dried. The drying method is not particularly limited, but natural drying at room temperature or forced drying by heating is suitable. The drying temperature for heat drying is, for example, 80°C or higher and 200°C or lower, and the drying time is, for example, 1 minute to 12 hours.

[0027] The crystallinity of the starch in the modification reaction step is 20% or less. The crystallinity is preferably 15% or less, more preferably 12% or less, even more preferably 10% or less, and particularly preferably 8% or less. The lower limit of the crystallinity is not particularly limited, but is usually 0% or more, and may be 0.5% or more, 1% or more, 2% or more, or 3% or more. The crystallinity is preferably 0% or more to 20% or less, 0.5% or more to 15% or less, 1% or more to 12% or less, 2% or more to 10% or less, or 3% or more to 8% or less, in that order. By keeping the crystallinity of the starch within the above range, the effects of the present invention can be more effectively exhibited. The crystallinity of the starch is measured by the method described in the Examples.

[0028] Methods for producing starch with a low degree of crystallinity include adding water to starch, heating it to gelatinize it, and then drying it at high temperature to obtain an amorphous dried product, and pulverizing it with a heated shear pulverizer to reduce the degree of crystallinity (Japanese Patent Laid-Open Publication No. 2018-038368). Starch purified and collected from plants is generally called raw starch, but because raw starch generally has a high degree of crystallinity, it is desirable to carry out a step to reduce crystallinity. Commercially available starch having a degree of crystallinity of 20% or less may also be used.

[0029] (Polybasic Acid and / or Acid Anhydride Thereof) Examples of the polybasic acid and / or acid anhydride therefor used in the present invention include dibasic acids such as succinic acid, maleic acid, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, methyltetrahydrophthalic acid, methylhexahydrophthalic acid, endomethylenetetrahydrophthalic acid, methylendomethylenetetrahydrophthalic acid, and itaconic acid; polybasic acids such as trimellitic acid, citric acid, butanetetracarboxylic acid, and phosphoric acid; dibasic acid anhydrides such as succinic anhydride (also known as succinic anhydride), maleic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, endomethylenetetrahydrophthalic anhydride, methylendomethylenetetrahydrophthalic anhydride, and itaconic anhydride; and polybasic acid anhydrides such as trimellitic anhydride, citric anhydride, butanetetracarboxylic acid anhydride, and phosphoric anhydride. In this specification, acid anhydrides having a structure in which two or more molecules of a monobasic acid are condensed, such as acetic anhydride, are not considered to be polybasic acid anhydrides, since they are anhydrides of monobasic acids. In the production method of the present invention, the polybasic acid and / or its acid anhydride can also function as a solvent and can dissolve the polybasic acid-modified starch produced by the reaction.

[0030] The polybasic acid and / or its acid anhydride is preferably a polybasic acid anhydride. Furthermore, from the viewpoint of reactivity, the polybasic acid anhydride is preferably a cyclic polybasic acid anhydride. It is more preferable that the polybasic acid anhydride is in the form of an acid anhydride when added. Another preferred embodiment involves heating the polybasic acid in the system during modification to prepare a polybasic acid anhydride or a cyclic polybasic acid anhydride. In this case, the reaction efficiency can be improved by adding, in addition to the base catalyst, an inorganic catalyst that promotes the production of an acid anhydride, such as sodium hypophosphite. Furthermore, the polybasic acid and / or its acid anhydride is preferably a polybasic carboxylic acid and / or its acid anhydride, since this provides a high modification reaction efficiency. Furthermore, the polybasic acid and / or its acid anhydride is preferably a dibasic acid and / or its acid anhydride, more preferably succinic acid and / or its acid anhydride, and even more preferably succinic acid anhydride, since this provides a high modification reaction efficiency.

[0031] In the raw material composition, the mass proportion of the polybasic acid and / or its acid anhydride is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 100% by mass or more, relative to the mass of the starch, from the viewpoint of reactivity, etc. Furthermore, the upper limit of the mass proportion of the polybasic acid and / or its acid anhydride is not particularly limited, but is preferably 2000% by mass or less, more preferably 1000% by mass or less, even more preferably 550% by mass or less, and particularly preferably 400% by mass or less The mass proportion of the polybasic acid and / or its acid anhydride is preferably 50% by mass or more and 2000% by mass or less, more preferably 60% by mass or more and 1000% by mass or less, even more preferably 70% by mass or more and 550% by mass or less, and particularly preferably 100% by mass or more and 400% by mass or less, relative to the mass of the starch.

[0032] In the modification reaction, the molar ratio of the glucose units of starch to the polybasic acid and / or its acid anhydride can be set as appropriate. For example, the molar ratio of glucose units of starch to the polybasic acid and / or its acid anhydride is preferably 1:1 to 1:1000. A blending ratio within this range is preferred from the standpoint of yield and economic efficiency. The molar ratio is more preferably 1:1 to 1:500, even more preferably 1:1 to 1:300, and particularly preferably 1:1 to 1:100.

[0033] (Base Catalyst) In the step of obtaining the polybasic acid-modified starch, a catalyst (e.g., a base catalyst; an inorganic catalyst such as sodium hypophosphite that promotes the production of acid anhydrides) can be used, and the use of a base catalyst is particularly preferred. The base catalyst is not particularly limited as long as it has the effect of accelerating the modification reaction, and inorganic base catalysts can be used. Specific examples of inorganic base catalysts include hydroxides such as potassium hydroxide, sodium hydroxide, lithium hydroxide, calcium hydroxide, thallium hydroxide, tin hydroxide, lead hydroxide, and nickel hydroxide; carbonates such as potassium carbonate, sodium carbonate, rubidium carbonate, cesium carbonate, lead carbonate, zinc carbonate, and nickel carbonate; and bicarbonates such as potassium bicarbonate, sodium bicarbonate, rubidium bicarbonate, and cesium bicarbonate. These inorganic base catalysts can be used alone or in combination. Among these, carbonates and / or bicarbonates are preferred as the base catalyst, and carbonates are more preferred.

[0034] The base catalyst is preferably a solid alkali compound, since this can further promote the modification reaction. In one preferred embodiment of the present invention, the base catalyst is a solid alkali compound alone. The term "solid" refers to a state having a specific shape, such as granules, flakes, powder, or granules.

[0035] Examples of the solid alkali compound include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; alkali metal carbonates such as sodium carbonate and potassium carbonate; and alkali metal bicarbonates such as sodium bicarbonate and potassium bicarbonate. These solid alkali compounds can be used alone or in combination of two or more. Among them, the solid alkali compound is preferably at least one selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, and alkali metal bicarbonates, as it can further promote the modification reaction. Alkali metal carbonates and / or alkali metal bicarbonates are more preferred, alkali metal carbonates are even more preferred, sodium carbonate and / or potassium carbonate are even more preferred, and potassium carbonate is particularly preferred.

[0036] The amount of the base catalyst used is preferably 0.1 mol % or more and 200 mol % or less, more preferably 1 mol % or more and 100 mol % or less, even more preferably 2 mol % or more and 50 mol % or less, and particularly preferably 5 mol % or more and 25 mol % or less, in terms of base molar equivalent, relative to the polybasic acid and / or its acid anhydride as the modifying agent. When two or more types of the base catalyst are used in combination, the above substance amount ratio refers to the total substance amount ratio.

[0037] Here, the term "base molar equivalent" refers to the amount of the base catalyst used corrected by its valence as a base. For example, 1 mol% of sodium hydroxide, a monovalent base catalyst, is 1 mol%, and 1 mol% of sodium carbonate, a divalent base catalyst, is 2 mol%.

[0038] The amount of the base catalyst used is preferably less than 30% by mass, more preferably 0.1% by mass or more and less than 30% by mass, even more preferably 0.5% by mass or more and 25% by mass or less, and even more preferably 1% by mass or more and 20% by mass or less, based on the amount of the polybasic acid and / or its acid anhydride.

[0039] The amount of the base catalyst used is preferably equal to or greater than the lower limit, since it is possible to prevent a decrease in the reaction efficiency of the reaction composition, an increase in the reaction time, and a decrease in the average degree of substitution. On the other hand, the amount of the base catalyst used is preferably equal to or less than the upper limit, since it is possible to prevent an increase in the reaction time, a decrease in the degree of substitution, and overload or damage to the kneader, which are caused by the progression of unnecessary side reactions in the reaction composition and a decrease in the kneading uniformity due to an increase in the viscosity of the reaction composition.

[0040] (Solvent) In one embodiment, in the modification reaction step, the raw material composition contains less than 5,000 ppm by mass of at least one organic solvent selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, and 4-dimethylaminopyridine, and less than 5,000 ppm by mass of an ionic liquid having an imidazolium ion. By substantially not using the solvent in the reaction step, the operation of removing the solvent after the reaction step can be omitted. Furthermore, this prevents the solvent, which has a strong toxicity and odor, from remaining in the resulting polybasic acid salt-modified starch, and allows for the production of a high-quality polybasic acid salt-modified starch that is sufficiently prevented from discoloration.

[0041] In this specification, when the raw material composition contains two or more predetermined organic solvents, the mass proportion of the organic solvents refers to the total mass proportion of the organic solvents. Furthermore, when the raw material composition contains two or more predetermined ionic liquids, the mass proportion of the ionic liquid refers to the total mass proportion of the ionic liquids. The raw material composition may not contain a predetermined organic solvent, or may not contain a predetermined ionic liquid. That is, the lower limit of the mass proportion of the predetermined organic solvent in the raw material composition is 0 ppm, and the lower limit of the mass proportion of the predetermined ionic liquid in the raw material composition is 0 ppm. For example, the raw material composition may not contain any of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, 4-dimethylaminopyridine, or an ionic liquid having an imidazolium ion.

[0042] The mass ratios mentioned above are measured by the method described in the examples or a method similar thereto, and are mass ratios relative to the raw material composition.

[0043] Examples of ionic liquids having imidazolium ions include 1-butyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, and 1-ethyl-3-methylimidazolium acetate.

[0044] In one preferred embodiment of the present invention, the raw material composition contains at least one functional group selected from the group consisting of a sulfoxide group, a pyridyl group, and an amide group, and the mass proportion of the organic solvent that is liquid (at 20°C and 1 atmospheric pressure) in the step of obtaining the polybasic acid-modified starch is less than 5,000 ppm.

[0045] In one preferred embodiment of the present invention, the raw material composition contains an organic solvent that contains sulfur atoms and / or nitrogen atoms and is liquid at 20° C. and 1 atmospheric pressure in an amount of less than 5,000 ppm by mass.

[0046] In another preferred embodiment of the present invention, the raw material composition contains less than 5,000 ppm by mass of an organic solvent and / or organic catalyst that contain sulfur atoms and / or nitrogen atoms and are liquid in the process for obtaining the polybasic acid-modified starch. Here, the organic solvent and organic catalyst contain sulfur atoms and / or nitrogen atoms and are liquid in the process for obtaining the polybasic acid-modified starch. Examples of such organic catalysts include pyridine and 4-dimethylaminopyridine.

[0047] In another preferred embodiment of the present invention, the raw material composition contains less than 5,000 ppm by mass of an organic solvent that contains sulfur atoms and / or nitrogen atoms and has a molecular weight of 130 or less. The lower limit of the molecular weight of the organic solvent is not particularly limited, but is usually 70 or more.

[0048] In another preferred embodiment of the present invention, the raw material composition contains less than 5000 ppm by mass of an organic solvent that contains sulfur atoms and / or nitrogen atoms and has a boiling point of 110° C. or higher at 1 atmosphere. There is no particular upper limit to the boiling point of the organic solvent at 1 atmosphere, but it is usually 300° C. or lower.

[0049] The mass proportion of the organic solvent (one or more of the following: at least one organic solvent selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, and 4-dimethylaminopyridine; an organic solvent containing at least one functional group selected from the group consisting of a sulfoxide group, a pyridyl group, and an amide group, and which is liquid in the process of obtaining a polybasic acid-modified starch; an organic solvent containing a sulfur atom and / or a nitrogen atom, and which is liquid at 20°C and 1 atmosphere; an organic solvent containing a sulfur atom and / or a nitrogen atom, and which is liquid in the process of obtaining a polybasic acid-modified starch; an organic solvent containing a sulfur atom and / or a nitrogen atom and having a molecular weight of 130 or less; or an organic solvent containing a sulfur atom and / or a nitrogen atom and having a boiling point of 110°C or higher at 1 atmosphere) in the raw material composition is preferably 4000 ppm or less, more preferably 3000 ppm or less, even more preferably 2000 ppm or less, and particularly preferably 1000 ppm or less. Furthermore, the mass proportion is most preferably equal to or less than the detection limit. For example, the raw material composition may be free of the organic solvent, and the mass proportion of the organic solvent may be 0 ppm.

[0050] In one preferred embodiment of the present invention, the raw material composition has a mass proportion of the ionic liquid containing at least one ion selected from the group consisting of ammonium, phosphonium, pyridinium, and imidazolium ions of less than 5000 ppm.

[0051] The mass proportion of the ionic liquid in the raw material composition is preferably 4000 ppm or less, more preferably 3000 ppm or less, even more preferably 2000 ppm or less, and particularly preferably 1000 ppm or less. Furthermore, the mass proportion is most preferably below the detection limit. For example, the raw material composition may be free of the ionic liquid, and the mass proportion of the ionic liquid may be 0 ppm.

[0052] The method for detecting the organic solvent and ionic liquid is not particularly limited, but common detection methods include nuclear magnetic resonance, gas chromatography, liquid chromatography, ultraviolet-visible spectroscopy, Raman spectroscopy, infrared spectroscopy, etc. Among these, detection methods using liquid chromatography are preferred from the standpoint of quantitativeness and simplicity of the measurement technique, and common devices can be used to perform measurements with a detection limit of less than 1000 ppm.

[0053] The modification reaction may be carried out using a solvent (for example, a solvent that does not contain nitrogen atoms and / or sulfur atoms) or under solvent-free conditions.

[0054] In the modification reaction step, the solvent may be present in an amount of 0 to 95% by mass in the raw material composition.

[0055] In the modification reaction, it is preferable to use substantially no solvent. That is, it is preferable to perform the modification reaction under solvent-free conditions. Note that the solvent in this case does not include the polybasic acid and / or its acid anhydride, which are the modifying agents. The absence of a solvent not only eliminates the need for solvent removal after the modification reaction, but also allows the modification reaction to be carried out at a high concentration, which has the advantage of enabling the modification reaction to be carried out efficiently. Adding water as a solvent can lead to problems such as hydrolysis of the ester bonds and starch backbone in the polybasic acid salt-modified starch, and, when an acid anhydride is used as the modifying agent, hydrolysis of the modifying agent. Furthermore, adding organic solvents or ionic liquids can cause problems such as the residue of toxic or odorous solvents in the resulting polybasic acid salt-modified starch, as well as problems such as increased heating time and coloration due to residual solvent. By substantially not using water, organic solvents, or ionic liquids, these problems can be fully prevented, resulting in a high-quality polybasic acid salt-modified starch. The term "solvent" refers to an optionally added solvent, and in the case of water, it refers to optionally added moisture, and does not include water resulting from elimination due to thermal condensation of starch and acid, or moisture present in undried raw materials. Specifically, "substantially no solvent" means that the total amount of solvent in the raw material composition is preferably 0.1% by mass or less, less than 5000 ppm, 4000 ppm or less, 3000 ppm or less, 2000 ppm or less, 1000 ppm or less, or below the detection limit. The solvent used herein is preferably liquid at 20°C and 1 atmosphere.

[0056] When the modification reaction is carried out using a solvent that does not contain nitrogen atoms and / or sulfur atoms, one or more solvents selected from the group consisting of water, organic solvents, and ionic liquids (but excluding solvents containing nitrogen atoms and / or sulfur atoms) can also be used. The solvent refers to any component in the raw material composition, excluding starch, polybasic acid, and base catalyst, that becomes liquid at the temperature and pressure during the reaction.

[0057] Even when a solvent is used in the modification reaction, it is preferable to use a small amount of solvent (solvent) from the viewpoint of the efficiency of the modification reaction. Therefore, the total amount of solvent, relative to the mass of the raw material composition used in the modification reaction, is preferably 10 mass% or less, more preferably 5 mass% or less, even more preferably 3 mass% or less, and further preferably 1 mass% or less, 0.5 mass% or less, 0.1 mass% or less, less than 5000 ppm, 4000 ppm or less, 3000 ppm or less, 2000 ppm or less, 1000 ppm or less, or below the detection limit. Furthermore, the total amount of one or more substances selected from the group consisting of water, organic solvents, and ionic liquids (excluding organic solvents or organic catalysts that contain nitrogen atoms and / or sulfur atoms and are liquid in the process of obtaining the polybasic acid-modified starch) is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, and even more preferably 1% by mass or less, 0.5% by mass or less, 0.1% by mass or less, less than 5000 ppm, 4000 ppm or less, 3000 ppm or less, 2000 ppm or less, 1000 ppm or less, or even below the detection limit. The raw material composition may not contain water, organic solvents, or ionic liquids as solvents. In the case of water, the amount of water used refers to optionally added water, and does not include water resulting from elimination by thermal condensation of starch and acid, or water present in the undried raw material.

[0058] Furthermore, in the modification reaction, since it is preferable to use a small amount of solvent (solvent) as described above, the solids concentration at 25°C of the raw material composition excluding the polybasic acid is preferably 70% by mass or more and 100% by mass or less, more preferably 90% by mass or more and 100% by mass or less, even more preferably 95% by mass or more and 100% by mass or less, and even more preferably 98% by mass or more and 100% by mass or less.

[0059] When the modification reaction is carried out at a reaction temperature exceeding the melting point of the polybasic acid and / or its acid anhydride, the polybasic acid and / or its acid anhydride can act as a solvent and dissolve the starch. Note that the mass proportion of the polybasic acid and / or its acid anhydride is not included in the mass proportion of the solvent.

[0060] (Modification Reaction Conditions) In the modification reaction, other components such as an antioxidant, a catalyst other than the base catalyst, etc. The mass proportion of the other components in the raw material composition used in the modification reaction is preferably 5 mass% or less, more preferably 1 mass% or less, and even more preferably 0.1 mass% or less.

[0061] The modification reaction is preferably carried out at a reaction temperature exceeding 120°C. This further improves the reactivity of starch. The modification reaction is also preferably carried out at a reaction temperature exceeding the melting point of the polybasic acid and / or its acid anhydride. The upper limit of the reaction temperature is not particularly limited, but is preferably 200°C or lower, more preferably 180°C or lower. The reaction temperature is preferably greater than 120°C and less than 200°C, more preferably greater than 120°C and less than 180°C.

[0062] The reaction time in the modification reaction is preferably 3 minutes or more, more preferably 5 minutes or more, and even more preferably 10 minutes or more. The reaction time may be 20 minutes or more, or even 30 minutes or more. The reaction time is preferably 12 hours or less, more preferably 6 hours or less, and even more preferably 3 hours or less. The reaction time is preferably 3 minutes or more and 12 hours or less, more preferably 5 minutes or more and 6 hours or less, and even more preferably 10 minutes or more and 3 hours or less.

[0063] (Kneader) In the modification reaction step, the raw material composition is preferably kneaded using a kneader such as a ball mill, a rod mill, a bead mill, a roller mill, a roll machine, a planetary mixer, a pan mixer, a kneader, a blender, etc. The kneader may be of a continuous type or a batch type.

[0064] In particular, a kneader having multiple rotors can powerfully knead the entire raw material composition, and can easily improve production efficiency by making the process continuous or increasing the size. Therefore, it is preferable to use a multi-shaft kneader (kneader) with two or more shafts (multi-shaft). The number of shafts can be two, three, four, or eight. Examples of batch-type multi-shaft kneaders include a double-arm kneader, a pressure kneader, a Banbury mixer, and an internal mixer. Examples of continuous kneaders include a twin-screw kneader extruder, a twin-screw kneader, and a twin-screw extruder. The kneading tank in which the reaction composition is kneaded may be either an open type or a closed type. The rotors of the kneader may be tangential or intermeshing, but intermeshing types are preferred because they enable powerful kneading.

[0065] Specific examples of the kneading machine include a CKH-type continuous kneader (Honda Iron Works Co., Ltd.), a twin-screw extruder TEX (The Japan Steel Works, Ltd.), a twin-screw extruder TEXαIII (The Japan Steel Works, Ltd.), a continuous kneader (CONTINUOUS KNEADER, Dalton Co., Ltd.), a KRC hybrid reactor (KRC HYBRID REACTER, Kurimoto Iron Works Co., Ltd.), a KRC kneader (KURIMOTO-READCO CONTINUOUS KNEADER, Kurimoto Iron Works Co., Ltd.), a KEX extruder (KEX EXTRUDER, Kurimoto Iron Works Co., Ltd.), and a KEXD extruder (KEXD Examples of such extruders include a twin-screw extruder (KNEADER-RUDER, Kurimoto Iron Works Co., Ltd.), a twin-arm kneader-ruder (KNEADER-RUDER, Moriyama Corporation), a twin-screw kneader extruder TEX-SSG (Toshiba Machine Co., Ltd.), a twin-screw kneader extruder TEX-CS (Toshiba Machine Co., Ltd.), a twin-screw kneader extruder TEX-SX (Toshiba Machine Co., Ltd.), a twin-screw kneader extruder TEX-DS (Toshiba Machine Co., Ltd.), a twin-screw kneader extruder TEX-A (Toshiba Machine Co., Ltd.), a twin-screw kneader extruder TEX-B (Toshiba Machine Co., Ltd.), a twin-screw kneader extruder TEX-BS (Toshiba Machine Co., Ltd.), and a four-screw and eight-screw kneader extruder WDR series (Technovel Corporation). Therefore, in a preferred embodiment of the present invention, the apparatus used in the modification reaction step is a continuous double-screw kneader.

[0066] The raw material composition may be charged all at once into a reaction apparatus such as a reaction tank, or may be supplied to the reaction apparatus continuously or intermittently. The raw materials may be charged by sequentially charging each raw material, charging a plurality of raw materials at once, or charging premixed raw materials in a separate apparatus.

[0067] (Polybasic acid-modified starch) In the production method of the present invention, polybasic acid-modified starch is produced by reacting starch with a polybasic acid and / or its acid anhydride. The polybasic acid-modified starch is preferably polybasic acid monoesterified starch because of its high hydrophilicity. Note that "monoesterified" means that only one of the multiple acid groups of the polybasic acid is bonded to a hydroxy group of the starch, and the others are in the form of a free acid or an acid neutralized salt.

[0068] The polybasic acid-modified starch preferably has an average degree of substitution of polybasic acid per glucose residue constituting the starch of 0.5 or more. The average degree of substitution can be further increased by using a carbonate, bicarbonate, or the like as a base catalyst in the modification reaction. The average degree of substitution is usually 3 or less. The average degree of substitution is 0.3 or more, preferably 0.5 or more, more preferably 0.8 or more, even more preferably 1.0 or more, and particularly preferably 1.2 or more. By using a polybasic acid-modified starch having an average degree of substitution within the above range as a raw material, a polybasic acid-modified starch with appropriate hydrophilicity can be obtained.

[0069] In this specification, the average degree of substitution is measured by the method described in the Examples. Furthermore, the average degree of substitution of polybasic acid-modified starch can be converted into the mass percentage of the modifier relative to the starch, the mass percentage relative to the total mass, or the number of charges relative to the total mass, as necessary. Conversion can be performed using a general calculation method, and an example is described below. Furthermore, since the following calculation method can be used regardless of whether neutralization is present or not, conversion can also be performed using the same calculation for polybasic acid salt-modified starch. Incidentally, polybasic acids and / or their acid anhydrides are also simply referred to as modifiers. The mass percentage y of the modifier relative to the starch １ Conversion to (mass%) (Equation 1)

[0070]

[0071] Percentage by mass of modifier relative to total mass y ２ Conversion to (mass%) (Equation 2)

[0072]

[0073] The number of charges y derived from the acid group of the modifier relative to the total mass ３ Conversion to (mmol / g or meq) (Equation 3)

[0074]

[0075] In the above formulas (1) to (3), x represents the average degree of substitution, y represents the average degree of substitution, １ ~y ３ is the converted value, N is the degree of neutralization, M ｓ (g / mol) is the molecular weight of the glucose residue that constitutes the starch, M ｍ (g / mol) is the molecular weight of the modifier, M ｉ (g / mol) is the molar mass per charge of the counter cation resulting from the neutralization reaction of the acid group introduced into the starch by the modifying agent with a base catalyst, etc., and m is the valence of the modifying agent. The molecular weight of a hydrogen ion is set to 1 (g / mol).

[0076] Specifically, when the polybasic acid (salt) modified starch is a sodium partially neutralized salt of succinic acid modified starch, M ｓ is 162, m is 2, M ｍ is 100, M ｉ is 23. In the case of a partially neutralized magnesium salt, which is a divalent cation, M ｉ When multiple types of cations are used for neutralization, the average degree of substitution can be calculated by using the sum of the values ​​calculated from the degree of neutralization for each cation and the molar mass per charge, in the same way as in the calculation of the average degree of substitution by pH titration described below.

[0077] <Step of Purifying Polybasic Acid-Modified Starch> The method for producing polybasic acid salt-modified starch of the present invention preferably includes a step of purifying the polybasic acid-modified starch after the modification reaction step. The purification step is not particularly limited, but examples include raw material recovery, catalyst recovery, neutralization, filtration, decantation, extraction, washing, evaporation, distillation, and column chromatography. Each of the above operations can be performed alone or in combination of two or more. Among these, purification by filtration and washing is particularly preferred. In addition, organic solvents with low boiling points (e.g., 100°C or less) and easy removal (e.g., acetone) or water can be used as appropriate in the washing operation. In the purification step, the time and temperature can be appropriately set. The pressure is not particularly limited and may be normal pressure, increased pressure, or reduced pressure, but normal pressure or reduced pressure is preferred. Furthermore, to improve washing efficiency, the kneaded product may be pulverized before the washing operation.

[0078] <Step of Neutralizing Polybasic Acid-Modified Starch> The method for producing a polybasic acid-modified starch of the present invention includes a separate step of neutralizing the polybasic acid-modified starch obtained in the modification reaction step described above after the modification reaction step. This allows polybasic acid-modified starch to be obtained. By carrying out this neutralization step, some or all of the polybasic acids bonded to the starch become ionizable salts, which ionize in water and cause electrostatic repulsion between the modified starch molecules. This results in high hydrophilicity that allows the starch to be uniformly dispersed or dissolved in water. The polybasic acid-modified starch may be a fully neutralized salt or a partially neutralized salt. In one preferred embodiment of the present invention, the fully neutralized salt or partially neutralized salt is a neutralized product with a monovalent cation. Examples of monovalent cations include alkali metal salts such as sodium and potassium, and ammonium salts.

[0079] The step of neutralizing the polybasic acid-modified starch can be appropriately carried out using a metal compound, ammonia, an organic amine, or the like. Preferred examples of metal compounds include compounds containing alkali metals such as lithium, sodium, and potassium; and compounds containing Group 2 elements of the periodic table such as beryllium, calcium, barium, and magnesium. Specific examples include hydroxides, carbonates, bicarbonates, silicates, phosphates, and aluminates of these metals. In particular, if the solution becomes strongly alkaline during neutralization, decomposition of the ester bonds in the modified starch progresses. Therefore, it is preferred to select one or more carbonates, preferably sodium carbonate and / or potassium carbonate, as the base for neutralization. Furthermore, the base may be in the form of an aqueous solution for neutralization.

[0080] In the neutralization step, the time, temperature, and pressure can be set appropriately. The neutralization step can be carried out under solvent-free conditions or in water. In consideration of productivity, a high concentration of the polybasic acid-modified starch during neutralization is desirable, preferably 1% by mass or more, more preferably 5% by mass or more, and may be 10% by mass or more, 20% by mass or more, or 30% by mass or more. The degree of neutralization achieved in the neutralization step is preferably 50 mol% or more, more preferably 60 mol% or more, even more preferably 70 mol% or more, and even more preferably 80 mol% or more, since it is desirable that the liquid after neutralization be as close to neutral as possible. The upper limit of the degree of neutralization is preferably 100 mol% or less.

[0081] <Step of Crosslinking Polybasic Acid Salt-Modified Starch> The production method of the present invention preferably further includes a step of crosslinking the polybasic acid salt-modified starch. This crosslinking step can produce a crosslinked polybasic acid salt-modified starch. The crosslinked polybasic acid salt-modified starch is a water-swellable, water-absorbent polymer having the above-mentioned polybasic acid salt-modified starch in its main chain. One suitable application of the crosslinked polybasic acid salt-modified starch is a water-swellable, crosslinked polybasic acid salt-modified starch-derived water-absorbent resin. Here, "water-absorbent resin derived from crosslinked polybasic acid salt-modified starch" refers to a water-swellable crosslinked polymer obtained by crosslinking a hydrophilic polymer having a polybasic acid salt-modified starch as its main chain skeleton. Furthermore, "water-swellable" refers to an absorbency under no load (also referred to as centrifuge retention capacity (CRC)) of 5 g / g or more as defined in NWSP 241.0. R2(15).

[0082] The crosslinking reaction may be a reaction commonly used for crosslinking modified polysaccharides such as carboxymethylated cellulose. The crosslinking reaction may involve bonding functional groups of the polybasic acid-modified starch together, such as by ester condensation between hydroxy groups derived from cellulose and carboxylic acid (salt) groups derived from the polybasic acid and / or its anhydride added in the modification reaction, or by forming a bond with a crosslinking agent and reacting with it. Alternatively, the crosslinking reaction may involve bonding between crosslinkers bound to functional groups of the polybasic acid-modified starch. Alternatively, the crosslinking reaction may involve adjusting the degree of neutralization to form hydrogen bonds between acid groups and hydroxy groups, or a combination of multiple techniques. The term "acid (salt) group" refers to an acid group and / or an acid-base group. Examples of the acid group include a carboxylic acid group, a sulfonic acid group, a sulfate group, and a phosphate group. Examples of the acid-base group include metal salts of the acid group, such as alkali metal salts, alkaline earth metal salts, and transition metal salts, as well as organic amine salts and ammonium salts.

[0083] Specific reactants and reaction modes include the formation of esters of functional groups derived from polybasic acid salt-modified starch in an equivalent reaction using a condensing agent such as a carbodiimide, e.g., dicyclohexylcarbodiimide; the formation of esters by dehydration condensation of functional groups derived from polybasic acid salt-modified starch in a catalytic reaction using inorganic acids such as sulfuric acid and hydrochloric acid, organic acids such as citric acid, Lewis acids such as boron trifluoride, or metal salts such as titanium salts; and the formation of esters by dehydration condensation of functional groups derived from polybasic acid salt-modified starch in an equivalent reaction using an epoxy structure, oxetane structure, or oxazoline structure, e.g., (poly)ethylene glycol diglycidyl ether or glycerol diglycidyl ether. Ester formation by reaction of polybasic acid anhydrides such as carboxylic acid anhydrides such as succinic anhydride and maleic anhydride, or inorganic acid anhydrides such as polyphosphoric acid with the functional groups of polybasic acid salt-modified starch crosslinker; acetal formation of hydroxyl groups with compounds having multiple aldehyde groups such as glutaraldehyde and glyoxal; ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylenediaminetetraacetic acid ... Ester and / or amide formation between a carboxylic acid and a crosslinking agent having multiple hydroxyl or amine groups, such as polyethyleneimine, ethylene carbonate, propylene carbonate, or polyethyleneimine; ester formation between a hydroxyl group and a polyacid crosslinking agent capable of forming multiple ester bonds, such as an organic acid, such as succinic acid, maleic acid, trimellitic acid, or citric acid, or an inorganic acid, such as phosphoric acid; or ester formation between a hydroxyl group and a crosslinking agent having multiple bonds, such as acrylic acid, maleic acid, or vinylsilane, after binding the hydroxyl groups of polybasic acid salt-modified starch to the crosslinking agent. Examples of crosslinking methods include crosslinking by radical polymerization; crosslinking by reacting a silane coupling agent having a functional group reactive with a functional group derived from polybasic acid salt-modified starch, such as an epoxy group or an amino group, followed by condensation of the silane coupling agent; formation of hydrogen bonds between acid groups and hydroxy groups derived from crosslinked polybasic acid salt-modified starch by adjusting the degree of neutralization through the addition of an inorganic acid such as sulfuric acid or hydrochloric acid or an organic acid such as citric acid; and complex formation between polyvalent metal salts such as titanium salts, aluminum salts, and zirconium salts and acid groups derived from polybasic acid salt-modified starch.

[0084] Since the crosslinking step can proceed by the above-mentioned ester condensation even without a crosslinking agent, crosslinking may proceed in the drying step after the modification reaction step or the neutralization step. That is, the crosslinking step may be an independent step, or may be performed simultaneously with the modification reaction step or the drying step, or both. Furthermore, when the crosslinking step is an independent step, it may include a crosslinking agent addition step and a heat treatment step. The heating temperature in the heat treatment step is appropriately set depending on the type of crosslinking agent, and is, for example, 50°C or higher and 300°C or lower, preferably 100°C or higher and 200°C or lower. The heat treatment step may overlap with the drying step described below.

[0085] The amount of the crosslinking agent used is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, relative to the polybasic acid salt-modified starch, from the viewpoint of crosslinking reactivity. The amount used is preferably 10% by mass or less, more preferably 3% by mass or less, and even more preferably 1% by mass or less, from the viewpoint of the physical properties of the resulting crosslinked polybasic acid salt-modified starch. The amount of the crosslinking agent used is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.05% by mass or more and 3% by mass or less, and even more preferably 0.05% by mass or more and 1% by mass or less, relative to the polybasic acid salt-modified starch.

[0086] The amount of the crosslinking agent used is preferably 0.01 mol % or more, more preferably 0.05 mol % or more, and even more preferably 0.1 mol % or more, relative to the glucose residues of the polybasic acid salt-modified starch, and is preferably 10 mol % or less, more preferably 3 mol % or less, and even more preferably 1 mol % or less.

[0087] When two or more of the above crosslinking agents are used in combination, the amount used is the total amount used. In addition, in the crosslinking process, the time, temperature, and pressure can be appropriately set. When crosslinking by hydrogen bonding by adjusting the degree of neutralization, the degree of neutralization may be adjusted by adjusting the amount of base added at the stage of the neutralization process, or the degree of neutralization may be adjusted by adding an acid separately.

[0088] <Step of drying the cross-linked polybasic acid salt-modified starch> The production method of the present invention preferably further comprises a step of drying the cross-linked polybasic acid salt-modified starch. The drying temperature in the drying step is preferably 40°C or higher and 200°C or lower, more preferably 60°C or higher and 180°C or lower, even more preferably 80°C or higher and 160°C or lower, and particularly preferably 100°C or higher and 140°C or lower.

[0089] The drying time in the drying step is preferably 3 minutes to 24 hours, more preferably 5 minutes to 12 hours, even more preferably 7 minutes to 6 hours, particularly preferably 10 minutes to 3 hours, and most preferably 15 minutes to 90 minutes.

[0090] In the drying step, the pressure can be appropriately set, but is preferably normal pressure or reduced pressure. The drying method includes various methods such as heat drying, hot air drying, reduced pressure drying, infrared drying, microwave drying, drum dryer drying, band drying, and high-humidity drying using high-temperature steam.

[0091] <Step of Powdering Crosslinked Polybasic Acid Salt-Modified Starch> In the production method of the present invention, it is desirable to powder the dried product of the crosslinked polybasic acid salt-modified starch. The powdering method is not particularly limited, and may include forming a crosslinked polybasic acid salt-modified starch solution into particles before drying and then drying, pulverizing the dried product during drying, or pulverizing after drying. In other words, the powdering step may be an independent step, or may be performed simultaneously with a drying step, or may be a combination of multiple steps.

[0092] The powdered polybasic acid salt-modified starch may be used as is, or may be classified to a specific particle size. Classification to a specific particle size makes it possible to impart desirable physical properties depending on the application, such as adjusting the dissolution rate in the case of an uncrosslinked product, or improving the absorption rate and liquid permeability in the case of a crosslinked product when used as a water-absorbent resin. Non-crosslinked polybasic acid salt-modified starch may also be dried and powdered in the same manner as above.

[0093] <Step of surface-crosslinking powdery cross-linked polybasic acid salt-modified starch> The production method of the present invention preferably further comprises a step of surface-cross-linking the powdery cross-linked polybasic acid salt-modified starch. The surface-cross-linking step is a step in which a surface-cross-linking agent that reacts with the functional groups of the powdery cross-linked polybasic acid salt-modified starch (particularly the acid (salt) groups derived from the polybasic acid and / or its acid anhydride added in the modification reaction and the hydroxyl groups derived from the starch) is added to additionally cross-link the surfaces of the cross-linked powder particles, and is distinguished from a step in which the entire uncross-linked polybasic acid salt-modified starch is cross-linked.

[0094] The crosslinking agent and reaction method used for the surface crosslinking are not particularly limited, but the reactions described in the step of crosslinking the polybasic acid-modified starch can be suitably used. Furthermore, the reaction used for the surface crosslinking may be a single reaction, or a combination of multiple reactions. The crosslinking agent used for the surface crosslinking may be the same as the crosslinking agent used for the entire crosslinking of the polybasic acid-modified starch, or a different one may be used.

[0095] Furthermore, the addition of a surface cross-linking agent for forming surface cross-linking and the method for forming surface cross-linking are not particularly limited, and may be a method in which a polybasic acid salt-modified starch molded into a powder form before cross-linking is processed, a method in which surface cross-linking is carried out simultaneously with the entire cross-linking, a method in which surface cross-linking is carried out separately after the entire cross-linking, an independent step, a step carried out simultaneously with another step, or a method in which a plurality of steps are combined.

[0096] The above-mentioned surface crosslinking enables appropriate adjustment of physical properties depending on applications, and particularly in applications to water absorbent resins, it becomes possible to impart preferable physical properties such as strengthening of particle strength before and / or after swelling, and improvement or suppression of swelling ratio, absorption rate, liquid permeability, etc.

[0097] <Step of recovering and regenerating impurities removed by the purification step> In the production method of the present invention, the impurities removed by the purification step (for example, unreacted polybasic acid and / or its acid anhydride as raw materials) can be recovered as needed and recycled as reaction raw materials, etc. Furthermore, the solvent used in the purification can also be recovered as needed and recycled as a solvent. A distillation step can be suitably used for the recovery and recycling.

[0098] The conditions for the distillation step can be appropriately set. For example, the solvent can be recovered by distillation. Alternatively, for example, after removing a solvent having a boiling point lower than that of the polybasic acid and / or its acid anhydride by distillation, the polybasic acid and / or its acid anhydride can be recovered by vacuum distillation or the like.

[0099] The production method of the present invention may include other steps, etc., as long as it includes the above-mentioned modification reaction step. For example, the production method of the present invention may include a step of separating the base catalyst and the neutralizing agent.

[0100] [Polybasic Acid Salt-Modified Starch and Polybasic Acid Salt-Modified Starch Composition] In the present invention, the polybasic acid salt-modified starch composition may be substantially entirely (100% by mass) composed of the polybasic acid salt-modified starch, or may contain additives (added during the production process of the polybasic acid salt-modified starch) such as base catalysts and neutralizing agents, unreacted materials, etc. Here, "substantially" preferably means that substances other than the polybasic acid salt-modified starch are present in an amount of 0.5% by mass or less (lower limit: 0% by mass), 0.3% by mass or less, 0.1% by mass or less, or 0.01% by mass or less. Since it is difficult to purify the starch to 100% polybasic acid salt after production, the resulting polybasic acid salt-modified starch composition typically contains substances other than the polybasic acid salt-modified starch. Alternatively, the polybasic acid salt-modified starch composition may be a solvent dispersion containing the polybasic acid salt-modified starch and an aqueous solvent (e.g., water).

[0101] The polybasic acid salt-modified starch (composition) preferably has a soluble component ratio of 1% or more, more preferably 2% or more, even more preferably 5% or more, particularly preferably 10% or more, and most preferably 20% or more. The upper limit of the soluble component ratio is not particularly limited, but is preferably 100% or less, more preferably 90% or less, and even more preferably 80% or less. The polybasic acid salt-modified starch preferably has a soluble component ratio of 1% to 100%, more preferably 2% to 100%, even more preferably 5% to 100%, particularly preferably 10% to 100%, and most preferably 20% to 100%. A soluble component ratio above the lower limit is preferred because it results in high hydrophilicity. The soluble component ratio is determined by the method described in the Examples.

[0102] The preferred average degree of substitution of polybasic acid (salt) per glucose residue in the polybasic acid-modified starch (composition) is the same as the preferred average degree of substitution of polybasic acid per glucose residue in the polybasic acid-modified starch. The average degree of substitution of polybasic acid (salt) per glucose residue constituting the polybasic acid-modified starch is 0.3 or more, preferably 0.5 or more, more preferably 0.8 or more, even more preferably 1.0 or more, and particularly preferably 1.2 or more. The average degree of substitution is usually 3 or less. The average degree of substitution can be further increased by using a carbonate, bicarbonate, or the like as a base catalyst in the modification reaction. Polybasic acid-modified starch with an average degree of substitution within the above range is preferred because it has appropriate hydrophilicity.

[0103] Here, the description of the polybasic acid (salt) is the same as that of the above-mentioned production method.

[0104] The preferred mass ratio of the solvent in the polybasic acid salt-modified starch (composition) of the present invention is the same as the preferred mass ratio of the solvent in the above-mentioned raw material composition.

[0105] The polybasic acid salt modified starch is preferably a solvent-free polybasic acid salt modified starch that does not use a solvent (solvent).

[0106] A preferred embodiment of the polybasic acid salt-modified starch composition comprises a polybasic acid salt-modified starch, and contains less than 5000 ppm by mass of at least one selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, and 4-dimethylaminopyridine, and less than 5000 ppm by mass of an ionic liquid having an imidazolium ion.

[0107] The polybasic acid salt-modified starch composition may not contain the above-mentioned dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, or 4-dimethylaminopyridine, and may not contain the above-mentioned ionic liquid. For example, the polybasic acid salt-modified starch composition may not contain any of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, 4-dimethylaminopyridine, or an ionic liquid having an imidazolium ion.

[0108] The polybasic acid salt-modified starch composition as described above has a sufficiently reduced amount of toxic and odorous solvents, is highly safe, and can be suitably used in sanitary materials and detergents that come into contact with the human body.

[0109] The polybasic acid salt-modified starch composition of the preferred embodiment contains polybasic acid salt-modified starch as a main component, and may further contain an organic solvent, an organic catalyst, and an ionic liquid, so long as the mass proportion of at least one selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, and 4-dimethylaminopyridine is less than 5000 ppm, and the mass proportion of the ionic liquid having an imidazolium ion is less than 5000 ppm. For example, the polybasic acid salt-modified starch composition may contain an organic solvent other than the six compounds described above, may contain ethyl acetate, or may contain an ionic liquid other than the ionic liquids described above.

[0110] In particular, the polybasic acid salt modified starch composition preferably has a total mass ratio of the organic solvent, organic catalyst, and ionic liquid of less than 5000 ppm.

[0111] For example, the polybasic acid salt-modified starch composition may contain no organic solvent, no organic catalyst, and no ionic liquid, and the total mass proportion of the organic catalyst, the organic solvent, and the ionic liquid may be 0 ppm.

[0112] The preferred average degree of substitution of polybasic acid salts per glucose residue in the polybasic acid salt-modified starch of the present invention is the same as the preferred average degree of substitution of polybasic acid salts per glucose residue in the above-mentioned polybasic acid-modified starch.

[0113] [Uses of Polybasic Acid Salt-Modified Starch (Composition)] The polybasic acid salt-modified starch or polybasic acid salt-modified starch (composition) having a specific average degree of substitution obtained by the production method of the present invention can be suitably used as a raw material for various industrial products, such as sanitary materials and detergents. For example, uncrosslinked polybasic acid salt-modified starch can be used as a thickener, binder, dispersant, stabilizer, metal sealant, film-forming agent, etc., and therefore can be suitably used in various applications. Specific application fields include papermaking, livestock feed, food, paint, building materials, adhesives, pressure-sensitive adhesives, electronic materials, pharmaceuticals, cosmetics, agriculture and horticulture, ceramics, civil engineering, and textiles. In addition, crosslinked polybasic acid salt-modified starch has properties such as water absorption, water retention, suction swelling, gelling properties, and thickening properties, making it suitable for use in the various applications mentioned above. Specific applications include sanitary products, disposable diapers, nursing pads, toilet absorbents, absorbents for pet sheets, soil water retention agents in agricultural and horticultural applications, seedling sheets, seed coating agents, artificial sphagnum moss, disintegration aids for pesticides, mushroom culture media, gelling agents for seeds, freshness preservation agents in food applications, drip absorbents, dehydrating agents, anti-condensation agents, ice packs in the distribution sector, anti-condensation agents in civil engineering and construction applications, water retention agents for spraying, concrete curing materials, lost circulation prevention agents, and silicides. Examples of uses include coating materials, cement admixtures, sludge solidification agents, excavation lubricants, drilling agents, antifriction agents, sandbag gelling agents, gel fragrances in cosmetics, sweat absorbents, thickeners, body warmers in the field of daily necessities, dressings in the medical field, absorbents for tampons, drug delivery agents, gelling agents for waste blood, poultices, water-stopping agents in electronic materials, gelling agents for battery electrolytes, thickeners in paints, water-swelling absorbents, emulsion stabilizers, etc. Among these, water-absorbent resins produced using polybasic acid salt-modified starch as a raw material are particularly suitable for use in hygiene materials such as disposable diapers, sanitary products, adult incontinence products (incontinence pads, etc.), and pet sheets, as well as in applications requiring water absorption and water retention in the fields of agriculture and horticulture (soil water retention agents, etc.), food (water-absorbent sheets, etc.), and industry (water-stopping agents, etc.).

[0114] In particular, in contrast to conventional polyacrylic acid-based products, which are highly residual in the environment and pose problems of fuel consumption and carbon dioxide emissions when incinerated, the polybasic acid salt-modified starch of the present invention has high biodegradability and therefore has the advantage of placing less strain on the environment throughout its entire life cycle, from raw materials to disposal.

[0115] FIG. 1 is a diagram showing an example of a reaction formula between starch and a polybasic acid anhydride according to the production method of the present invention. FIG. 1 illustrates a case where succinic anhydride is used as the polybasic acid anhydride. FIG. 2 is a diagram illustrating each step according to the production method of the present invention. FIG. 2 schematically shows a production method for a water-absorbent resin, which includes step 2 of reducing the crystallinity of starch 1, step 3 of modifying the starch with a polybasic acid and / or its acid anhydride (modifying agent), step 4 of purifying the starch to separate the modifying agent, and step 5 of neutralizing, crosslinking, and drying the starch to obtain starch-based water-absorbent resin (SAP) 6. Furthermore, by distillation step 7, the solvent used in the modification reaction step 3 and the purification step 4, as well as the modifying agent separated in the purification step 4, can be recovered and recycled, and reused in the modification reaction step 3 and the purification step 4.

[0116] Polybasic acid salt-modified starch (composition), which is a type of hydrophilic polymer and is obtained using the production method of the present invention or has a specific average degree of substitution, can be suitably used as a raw material for various industrial products such as sanitary materials and detergents. For example, water-absorbent resins obtained by crosslinking polybasic acid salt-modified starch can be suitably used in sanitary materials such as disposable diapers and sanitary products, as well as in applications requiring water absorption and retention in the agricultural, horticultural, food, and industrial fields. In such applications, the CRC in physiological saline is, for example, 5 g / g or more, and preferably 10 g / g or more.

[0117] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the examples below, and can be practiced with appropriate modifications within the scope of the above-mentioned and below-mentioned aims, and all such modifications are within the technical scope of the present invention. Furthermore, in the present invention, the measurement methods for each of the above physical properties are based on the measurement methods described in the examples unless otherwise specified. Each measurement was performed under conditions of room temperature (23±2°C) and humidity 35±5% RH.

[0118] <Methods for measuring physical properties> [Starch crystallinity] The crystallinity (unit: %) of the starch used as the raw material for the polybasic acid salt-modified starch of the present invention can be calculated by the Segal method from the diffraction intensity value obtained by X-ray diffraction, and is defined by the following formula.

[0119]

[0120] where x is the crystallinity of starch [%], I １７．１ is the diffraction intensity of the crystalline part (diffraction angle 2θ = 17.1°) in X-ray diffraction, I １５．８ indicates the diffraction intensity of the amorphous portion (diffraction angle 2θ = 15.8°). The diffraction angle is a value measured using an X-ray source of Cu / Kα (wavelength 0.154 nm). Therefore, if the X-ray source is changed, the angle can be calculated by appropriately correcting the angle according to the wavelength.

[0121] The X-ray diffraction was carried out using an apparatus having the following configuration.

[0122] Measurement device: Aeris manufactured by Malvern Panalytical X-ray source: Cu / Kα (0.154 nm) Tube voltage: 40 kV Tube current: 15 mA Measurement range: Diffraction angle 2θ = 5 to 50° X-ray scan speed: 1.67° / min

[0123] [Average Degree of Substitution] The average degree of substitution of the polybasic acid salt-modified starch of the present invention (hereinafter referred to as "sample") was measured by measuring the amount of acid in the sample by pH titration. The measurement solution was prepared as follows.

[0124] (Preparation of measurement solution) The entire amount of the sample to be measured was pulverized so as to pass through a JIS standard sieve with a mesh size of 300 μm. Then, 0.1 g of the pulverized sample was placed in a 100 mL glass beaker, diluted with 50 g of a 0.9 mass % sodium chloride aqueous solution, and a 30 mm long magnetic stirrer was placed in the beaker.

[0125] Next, to improve the uniformity of the measurement, a step of increasing the degree of neutralization of the sample and dispersing it in water was carried out. Specifically, an appropriate amount of 0.1 mol / L aqueous sodium hydroxide solution was added to the glass beaker, and the mixture was stirred at room temperature (20 to 25°C) at 350 rpm using a magnetic stirrer for 1 hour, and the pH of the solution was then measured using a handheld pH meter.

[0126] The amount of sodium hydroxide aqueous solution added was adjusted so that the pH of the solution after stirring was 5.5 to 7. If the pH of the solution after stirring was less than 5.5, additional sodium hydroxide aqueous solution was added and the solution was stirred again for 1 hour. This procedure was repeated until the pH of the solution after stirring was 5.5 to 7. On the other hand, if the pH of the solution exceeded 7, the measurement solution was prepared again.

[0127] The pH titration device used was an automatic titration device manufactured by HIRANUMA Co., Ltd. (formerly Hiranuma Sangyo Co., Ltd.) with the following configuration.

[0128] pH automatic titrator: Tight Station TS-980 Additional burette: B-900 Burette head: H-900 Automatic cycler: C-912 Control and analysis software: COM-2500 Acid titration solution: 0.1 mol / L hydrochloric acid-base titration solution manufactured by Kanto Chemical Co., Ltd. 0.1 mol / L aqueous sodium hydroxide solution manufactured by Kanto Chemical Co., Ltd.

[0129] (Calculation of average degree of substitution) The test solution was titrated using the above-mentioned automatic titrator. First, the test solution was titrated with aqueous sodium hydroxide until the pH reached 10.5, and then with hydrochloric acid until the pH reached 2.8, and the amount of titration solution required for each was recorded. In addition, a similar blank measurement was performed without adding the sample, and the amount of acid or base required for each neutralization was calculated from the difference in the amount of titration solution added in the sample measurement and the blank measurement. The amount of acid in the modified starch comes from the semi-esterified and bonded modifier and the modifier in the free acid form remaining in the sample. When the modifier is an acid anhydride, the following formula holds:

[0130]

[0131] where x is the average degree of substitution, W is the mass (g) of the sample used in the pH titration, s is the proportion (mass%) of the remaining solvent in the sample calculated from the HPLC measurement results, a is the proportion (mass%) of the remaining modifier in the sample, and M ｓ is the molar mass per unit of glucose constituting starch (g / mol), M １ is the molar mass of the acid anhydride of the modifying agent (g / mol), M ２ is the molar mass of the remaining free acid form of the modifier (g / mol), m ｉ is the molar mass per charge (g / mol) of the cationic species i derived from the base catalyst added during the modification reaction or the base used for neutralization, n is the acid valence of the modifying agent, N ｉ is the degree of neutralization derived from the cationic species i of the sample, Σ is the summation symbol indicating that the calculations for each cationic species involved in neutralization are summed up, A is the amount of hydrochloric acid (mL) required for the acid titration of the sample, A ｂ is the amount of hydrochloric acid (mL) required for the blank acid titration. The molar mass of hydrogen ions was set to 1 g / mol.

[0132] When the modifying agent is succinic anhydride, M １ is 100 g / mol, M ２ is 118 g / mol and n is 2. In the case of starch, the repeating unit is a glucose unit, so M ｓ In the case of polybasic acid modified starch using only potassium carbonate as the base catalyst, the cationic species involved in neutralization is only potassium, and mＫ (m ｉ In this case, i is potassium (K), which is the molar mass of potassium, 39 g / mol. Ｋ (N ｉ In this case, i (potassium (K)) can be determined from the results of pH titration.

[0133] If sodium carbonate is then used for post-purification neutralization to obtain polybasic acid salt-modified starch, the cationic species involved in the neutralization will be potassium and sodium. Ｎａ (m ｉ In this case, i is sodium (Na), which is the molar mass of sodium, 23 g / mol. The degree of neutralization by potassium NK is calculated by the value calculated by the measurement before neutralization, and the degree of neutralization by sodium N Ｎａ (N ｉ where i is sodium (Na) is the degree of neutralization of the entire sample calculated by pH titration, N to N Ｋ It can be found by subtracting

[0134] The degree of neutralization N of the entire sample is calculated by adding the total amount B (mL) of sodium hydroxide solution required to adjust the pH of the sample solution, the total amount B (mL) of sodium hydroxide solution required for base titration by the titrator, and the total amount B (mL) of aqueous solution required for titration of the blank measurement. ｂ (mL) using the following formula:

[0135]

[0136] The above formula was transformed to calculate the average degree of substitution x using the following formula:

[0137]

[0138] When a polybasic acid having a free acid group is used as the modifying agent instead of an acid anhydride, M １ Subtract the molecular weight of water, 18, from M ２ This can be calculated by adding 18 to the above.

[0139] [Residual Solvent Proportion, Residual Modifier Proportion] In measuring the average degree of substitution, the "residual solvent proportion" and "residual modifier proportion" in the sample required are quantified as the mass proportion of the residual solvent or residual modifier relative to the total sample mass, calculated from the area value of the peak detected in a differential refractive index (hereinafter referred to as RI) chromatogram of high performance liquid chromatography (hereinafter referred to as HPLC).

[0140] When the amount of sample injected in HPLC is the same, the peak area value detected by the RI detector is proportional to the mass concentration of the sample in the measurement solution, and therefore the amount of remaining solvent or remaining modifier in the sample can be calculated from the relationship between the mass concentration of the remaining solvent or remaining modifier determined by pre-calibration and the peak area value. Furthermore, when a polybasic acid anhydride is used as the modifier and HPLC measurement is performed with the eluent described below, the modifier is detected as a hydrolyzed free acid.

[0141] (Sample Preparation) A measurement solution was prepared according to the following procedure.

[0142] The entire sample to be measured was pulverized so that it would pass through a JIS standard sieve with a mesh size of 300 μm. Then, 0.1 g of the pulverized sample was weighed into a 100 mL glass beaker equipped with a 30 mm long magnetic stirrer, and 10 g of pure water was added. Stirring was initiated at 350 rpm using a magnetic stirrer at room temperature (20 to 25°C). After 30 minutes of stirring, 40 g of a 0.25% by weight aqueous phosphoric acid solution was added to the beaker, and stirring was continued for 30 minutes to obtain a 0.2% by weight sample dispersion.

[0143] Thereafter, the sample dispersion was passed through a filter (manufactured by Membrane Solutions: PTFE syringe filter, hydrophilic 25A, pore size 0.22 μm) to obtain a measurement solution.

[0144] (HPLC Measurement) Measurement was carried out using an Alliance HPLC System manufactured by Waters. The device was equipped with an ion exclusion chromatography column and an RI detector. The measurement device and measurement conditions were as follows.

[0145] Pump / autosampler: Alliance HPLC System manufactured by Waters Guard column: Shim-pack SCR(H) guard column (manufactured by Shimadzu GLC) Column: Shim-pack SCR-101H (manufactured by Shimadzu GLC) RI detector: 2414 differential refractometer (manufactured by Waters) Eluent: 0.4% by mass phosphoric acid aqueous solution Flow rate: 1 mL / min Injection amount: 50 μL Measurement temperature: 35°C.

[0146] During HPLC measurement, a sufficient amount of eluent was passed through the instrument, and the detector baseline was stabilized. Data collection from the RI detector and analysis of the RI chromatogram were performed using Empower3 software manufactured by Waters. In addition, the peak area values ​​of the impurities, remaining solvent or remaining modifier, were obtained from the RI chromatogram and used to calculate the remaining ratio.

[0147] (Calculation of Residual Solvent Ratio and Residual Modifier Ratio) The solvent or modifier alone was measured in the same manner as in the sample measurement, and the relationship between the RI peak area value and the concentration of the solvent or modifier was calculated. Specifically, the solvent or modifier was diluted 10,000 times with a 0.4 mass% aqueous phosphoric acid solution and then filtered to obtain a measurement solution with a concentration of 0.01 mass%, which was then measured by HPLC. The elution time and RI peak area value of the solvent or modifier were obtained from the RI chromatogram. The obtained RI peak area value was divided by the mass concentration of the measurement solution to calculate the coefficient C (1 / mass%) of the solvent or modifier concentration in the measurement solution and the RI peak area value. Subsequently, the concentration of the residual solvent or residual modifier in the measurement solution was calculated using the coefficient C from the RI peak area value of the measurement solution, and the product by the dilution factor of the sample was obtained to calculate the proportion of the residual solvent or residual modifier in the sample.

[0148]

[0149] Here, x is the mass fraction (mass%) of the remaining solvent or modifier in the sample, A is the RI peak area value detected from the measurement solution of the sample, and c is the mass concentration (mass%) of the sample in the measurement solution.

[0150] [Ratio of soluble components] The ratio of soluble components in the polybasic acid salt modified starch (sample) of the present invention was calculated from the value detected in the RI chromatogram of gel permeation chromatography (hereinafter referred to as GPC).

[0151] (Sample Preparation) A solution was prepared by diluting the sample to be measured so that the concentration of the sample to be measured was 0.1% by mass based on the solid content, and the diluted solution was filtered through a filter (manufactured by Membrane Solutions: PTFE syringe filter, hydrophilic 25A, pore size 0.22 μm) to obtain a measurement solution. GPC measurement was performed on this measurement solution under the following conditions.

[0152] (GPC Measurement) Measurement was carried out using a Viscotek TDA305 manufactured by Malvern Panalytical. The instrument was equipped with a gel permeation chromatography column, an RI detector, a light scattering detector, and a capillary viscometer. The measurement instrument and measurement conditions were as follows:

[0153] Pump / autosampler: GPCmax manufactured by Viscotek Guard column: OHpak SB-G (manufactured by Showa Denko) Column: Two OHpak SB-806MHQ (manufactured by Showa Denko) connected in series Detector: Viscotek TDA305 manufactured by Malvern Panalytical (temperature inside the system was maintained at 30°C) Eluent: aqueous solution of 60 mM sodium dihydrogen phosphate dihydrate, 20 mM disodium hydrogen phosphate dodecahydrate, and 400 ppm sodium azide (pH 6.35 to 6.38) Flow rate: 0.5 mL / min Injection volume: 100 μL Measurement temperature: 30°C.

[0154] The water used in this GPC measurement was ultrapure water purified using a Millipore Simplicity UV (Merck). Furthermore, during the GPC measurement, a sufficient amount of eluent was allowed to flow through the instrument, ensuring a stable baseline for the detector. In particular, the measurement was performed under conditions where there were no noise peaks in the light scattering detector.

[0155] The instrument was calibrated using polyoxyethylene glycol (mass average molecular weight (Mw) 22,396, molecular weight distribution (Mw / Mn = 1.0), refractive index increment with concentration (dn / dc) = 0.132, solvent refractive index 1.33) as a standard sample. The differential refractive index of the soluble components of the polybasic acid salt-modified starch to be analyzed was measured using the values ​​for dextran ((dn / dc) = 0.147, solvent refractive index 1.33), which is also a polysaccharide and is commonly used in GPC configurations. Data collection and analysis of RI, light scattering intensity, and viscosity (DP) were performed using Malvern Panalytical's OmniSEC 5.3 software. The molecular weight of the soluble components versus elution time was calculated from the RI, light scattering intensity (7° angle), LALS, and viscometer data, and the RI area value of peaks present at molecular weights of 1,000 or more was obtained from the RI chromatogram.

[0156] The soluble component ratio in GPC of the measurement object (polybasic acid salt-modified starch) was calculated using the following formula. Carboxymethylcellulose sodium salt (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), which has the same main skeleton as starch, a glucose polymer, cellulose, and is a completely water-soluble polymer, was used as the standard substance. The soluble component ratio was calculated based on the area value of the RI peak obtained by GPC measurement under the same conditions. To correct the soluble component ratio, the remaining solvent ratio measured by HPLC was excluded from the calculation.

[0157]

[0158] Here, x is the proportion of dissolved components (% by mass), Ap is the RI peak area value of the measurement sample having a molecular weight of 1,000 or more, Ac is the RI peak area value detected in the measurement of carboxymethylcellulose sodium salt as a standard substance, s is the proportion of remaining solvent (% by mass) calculated from the HPLC measurement results, and a is the proportion of remaining modifier (% by mass) calculated from the HPLC measurement results.

[0159] [CRC (absorbency without load)] The CRC (absorbency without load) of the crosslinked polybasic acid salt-modified starch of the present invention in physiological saline (hereinafter abbreviated as "saline") was determined in accordance with NWSP 241.0.R2(15). Specifically, 0.2 g of a sample was placed in a nonwoven bag and immersed in a large excess of saline (0.9% by mass sodium chloride aqueous solution) for 30 minutes to allow the sample to freely swell. The sample was then dehydrated using a centrifuge (centrifugal force: 250 G), and the CRC (absorbency without load) (unit: g / g) was measured. Pure water CRC was also measured using pure water instead of saline. The pure water CRC was measured using pure water instead of saline, and the sample amount was changed from 0.2 g to 0.1 g, but the measurement was otherwise performed in the same manner as for saline CRC.

[0160] Example 1 Commercially available pregelatinized starch (Corn Alpha Y, manufactured by Sanwa Starch Industry Co., Ltd.) was dried in an oven at 150°C for 1 hour to obtain an absolute dry state (bone-dry state). During the drying process, the sample was appropriately weighed to observe the change in mass and confirm that a constant weight had been reached. After drying, the crystallinity of the pregelatinized starch was measured and found to be 6%.

[0161] 100 parts by mass (9 g) of the bone-dry pregelatinized starch, 300 parts by mass of succinic anhydride, and 15 parts by mass of potassium carbonate were charged into a batch twin-screw mixer and mixed for 15 minutes. The temperature inside the mixer was heated to 125°C using an electric heating jacket. After mixing, the contents of the mixer were in the form of a homogeneous thermoplastic resin.

[0162] Subsequently, the contents of the kneading tank after the above kneading were removed from the kneader and pulverized using a Force Mill FM-1 manufactured by Osaka Chemical Co., Ltd. Next, the pulverized material was added to a five-fold amount of acetone and stirred at room temperature for 5 minutes or more, followed by vacuum filtration and acetone washing to remove unreacted modifier. Thereafter, the mixture was vacuum dried at 45°C until it became completely dry, yielding succinic acid-modified starch.

[0163] One part by mass (0.4 g) of the succinic acid-modified starch obtained by the above procedure was diluted with 19 parts by mass of pure water, and then a 10% by mass aqueous solution of sodium carbonate was added dropwise with stirring to neutralize the mixture, yielding an aqueous dispersion of succinate-modified starch (polybasic acid-modified starch composition). Note that the amount of base required to achieve a degree of neutralization of 90 mol% was determined in advance by pH titration.

[0164] After the neutralization, the resulting aqueous dispersion of succinate-modified starch was subjected to pH titration, HPLC measurement, and GPC measurement, and the average degree of substitution and the proportion of soluble components were calculated. The sample mass was determined by the solids concentration determined by drying the aqueous dispersion to constant weight at 120°C. The measurement results are shown in Table 1. Furthermore, the polybasic acid salt-modified starch composition contained dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, 4-dimethylaminopyridine, and an ionic liquid containing an imidazolium ion below the detection limit.

[0165] Comparative Example 1 A succinate-modified starch was obtained in the same manner as in Example 1, except that commercially available raw starch (Corn Starch Y, manufactured by Sanwa Starch Industry Co., Ltd.) was used as the raw starch. The crystallinity of the bone-dry raw starch was 32%. The measurement results of the average degree of substitution and the proportion of soluble components of the obtained succinate-modified starch are shown in Table 1.

[0166]

[0167] In Example 1 and Comparative Example 1 in Table 1, the reaction was carried out under conditions in which no solvent was added during the reaction. When corn alpha starch with a low degree of crystallinity was used, the esterification reaction between succinic anhydride and starch proceeded well, resulting in a high average degree of substitution of 1.58 and a high proportion of soluble components.

[0168] On the other hand, when cornstarch with a high degree of crystallinity was used, the reaction between succinic anhydride and starch was not good, resulting in a significantly lower average degree of substitution of 0.12, and most of the polymer was insoluble in water.

[0169] Example 2 100 parts by mass (4.3 g) of the same bone-dry pregelatinized starch as in Example 1, 185 parts by mass of succinic anhydride, 10 parts by mass of potassium carbonate, and 2,600 parts by mass of dimethyl sulfoxide (DMSO) were placed in a separable flask and placed in a batch twin-screw kneader, followed by stirring at 80°C for 3 hours.

[0170] After the reaction was completed, acetone in an amount 10 times the total volume of the reaction composition was added, and the mixture was stirred at room temperature (20°C to 25°C) to precipitate succinic acid-modified starch. Unreacted succinic anhydride was removed by vacuum filtration and washing with acetone.

[0171] Thereafter, the mixture was vacuum dried at 55° C. under 10 hPa to obtain unneutralized succinic acid-modified starch.

[0172] One part by mass of this succinic acid-modified starch was added to 1,000 parts by mass of the same sodium phosphate buffer solution used in the GPC measurement, and after thorough stirring to achieve a uniform solution, a 10% by mass aqueous solution of sodium carbonate was added dropwise with stirring to neutralize (neutralization rate: 90 mol%), yielding a 0.1% by mass solution of neutralized succinic acid-modified starch (polybasic acid salt-modified starch composition). After neutralization, the resulting succinic acid-modified starch dispersion was subjected to pH titration, HPLC measurement, and GPC measurement, and the average degree of substitution and the proportion of soluble components were calculated.

[0173] The measurement results are shown in Table 2.

[0174]

[0175] By carrying out the modification without a solvent as in Example 1, no solvent remains after acetone washing and vacuum drying as in Example 2, resulting in a polybasic acid salt-modified starch with fewer problems such as odor.

[0176] Example 3 A polybasic acid salt-modified starch composition was obtained in the same manner as in Example 1, except that 100 parts by mass (24 g) of pregelatinized starch, 100 parts by mass of succinic anhydride, 5 parts by mass of potassium carbonate, and 20 parts by mass of water were charged into a batch twin-screw kneader.

[0177] After kneading, the contents of the kneading tank were found to be a uniform resinous substance having thermoplastic properties. The measurement results are shown in Table 3.

[0178]

[0179] Compared to Example 3, by carrying out modification with a modifying agent without a solvent as in Example 1, a polybasic acid salt-modified starch with a higher average degree of substitution and a higher proportion of soluble components was obtained.

[0180] Example 4 To 100 parts by mass (4.0 g, solids concentration 10% by mass) of the aqueous dispersion of succinate-modified starch obtained in Example 1, 6 parts by mass of a 0.1% by mass aqueous solution of ethylene glycol diglycidyl ether (Denacol EX-810, manufactured by Nagase ChemteX Corporation) as a crosslinking agent was added, followed by stirring until homogeneous. The mixture was then heated and dried in a 120°C fan oven for 45 minutes to obtain crosslinked succinate-modified starch. The CRC of the obtained crosslinked succinate-modified starch was measured after pulverization, and found to be 11.2 g / g in saline and 31.3 g / g in pure water.

[0181] [Comparative Example 2] An attempt was made to prepare a cross-linked succinate-modified starch from the succinate-modified starch obtained in Comparative Example 1 using the same method as in Example 4. However, the succinate-modified starch obtained in Comparative Example 1 had a low soluble component ratio and was not soluble in water, so cross-linked succinate-modified starch could not be synthesized.

[0182] This application is based on Japanese Patent Application No. 2024-154683, filed on September 9, 2024, the disclosure of which is hereby incorporated by reference in its entirety.

Claims

1. A method for producing a polybasic acid-modified starch, comprising the steps of obtaining a polybasic acid-modified starch using a raw material composition containing starch and a polybasic acid and / or its acid anhydride, and neutralizing the polybasic acid-modified starch, wherein the crystallinity of the starch contained in the raw material composition is 20% or less.

2. The manufacturing method according to claim 1, wherein in the step of obtaining the polybasic acid-modified starch, the raw material composition contains less than 5000 ppm by mass of at least one selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, and 4-dimethylaminopyridine, and less than 5000 ppm by mass of an ionic liquid having an imidazolium ion.

3. The manufacturing method according to claim 1 or 2, wherein the raw material composition contains at least one functional group selected from the group consisting of a sulfoxide group, a pyridyl group, and an amide group, and the mass proportion of the liquid organic solvent in the process for obtaining the polybasic acid-modified starch is less than 5000 ppm.

4. The manufacturing method according to claim 1 or 2, wherein the raw material composition contains nitrogen atoms and / or sulfur atoms, and the mass proportion of the organic solvent and / or organic catalyst that is liquid in the process of obtaining the polybasic acid-modified starch is less than 5000 ppm.

5. The manufacturing method according to claim 1, wherein in the step of obtaining the polybasic acid-modified starch, the total mass of the substances selected from the group consisting of water, organic solvents and ionic liquids (excluding organic solvents or organic catalysts that contain nitrogen atoms and / or sulfur atoms and are liquid in the step of obtaining the polybasic acid-modified starch) added is 10 mass% or less of the raw material composition.

6. A manufacturing method according to claim 1 or 2, wherein in the step of obtaining the polybasic acid-modified starch, the reaction temperature during the modification reaction between the starch and the polybasic acid salt and / or its acid anhydride is higher than 120°C.

7. The method according to claim 1 or 2, wherein the acid anhydride of the polybasic acid is a cyclic polybasic acid anhydride.

8. The method according to claim 1 or 2, wherein the polybasic acid and / or its acid anhydride is a polybasic carboxylic acid and / or its acid anhydride.

9. The method according to claim 1 or 2, wherein the polybasic acid and / or its acid anhydride is a dibasic acid and / or its acid anhydride.

10. The method according to claim 1 or 2, wherein the polybasic acid and / or its acid anhydride is succinic acid and / or its acid anhydride.

11. The method according to claim 1 or 2, wherein the polybasic acid-modified starch is a polybasic acid monoesterified starch.

12. A manufacturing method according to claim 1 or 2, wherein a base catalyst is used in the step of obtaining the polybasic acid-modified starch, and the amount of the base catalyst used is less than 30 mass% relative to the mass of the polybasic acid and / or its acid anhydride.

13. The method of claim 12, wherein the base catalyst is a solid alkaline compound.

14. The method according to claim 13, wherein the solid alkaline compound is at least one selected from the group consisting of alkali metal hydroxides, alkali metal carbonates, and alkali metal hydrogen carbonates.

15. The method of claim 1 or 2, wherein the average degree of substitution of polybasic acid (salt) per glucose residue of the polybasic acid salt-modified starch is 0.5 or more.

16. The method of claim 1 or 2, further comprising a step of cross-linking the polybasic acid salt-modified starch obtained in the step of neutralizing the polybasic acid-modified starch.

17. The manufacturing method according to claim 1 or 2, wherein a kneader is used in the step of obtaining the polybasic acid-modified starch.

18. The method of claim 17, wherein the kneader is a multi-shaft kneader.

19. A polybasic acid salt-modified starch composition comprising a polybasic acid salt-modified starch having an average degree of substitution of polybasic acid (salt) per glucose residue of 0.5 or more.

20. The polybasic acid salt-modified starch composition according to claim 19, wherein the mass proportion of at least one selected from the group consisting of dimethyl sulfoxide, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, pyridine, and 4-dimethylaminopyridine is less than 5000 ppm, and the mass proportion of the ionic liquid having an imidazolium ion is less than 5000 ppm.

21. The polybasic acid salt-modified starch composition of claim 19, wherein the average degree of substitution is 1.0 or more per glucose residue.

22. The polybasic acid salt modified starch composition of claim 19, wherein the polybasic acid is succinic anhydride.

23. The polybasic acid salt-modified starch composition of claim 19, wherein the polybasic acid salt-modified starch is a cross-linked polybasic acid salt-modified starch.

24. The polybasic acid salt-modified starch composition according to any one of claims 19 to 23, which is used as a raw material for hygiene materials, agricultural and horticultural supplies, cosmetics, or detergents.

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