Water-absorbing resin

Abstract

This invention provides a water-absorbing resin with excellent water absorption and retention properties and high manufacturing efficiency. The invention relates to a water-absorbing resin containing a crosslinked polymer of a water-soluble polymer with acidic groups incorporated into starch partial decomposition products having a weight average molecular weight of less than 7.5 million.

Description

[Technical Field]

[0001] This invention relates to a water-absorbing resin. [Previous Technology]

[0002] Water-absorbing resins are widely used in various fields such as hygiene products, food, agriculture and forestry, and civil engineering. Among these water-absorbing resins, partially neutralized salts of polyacrylic acid or polymethacrylic acid are widely used. In addition, water-absorbing resins made from polysaccharides such as starch are also known.

[0003] Patent Document 1 discloses a method for manufacturing a water-absorbing resin by heating and drying carboxyl-alkylated starch to crosslink the starch molecules. It describes how the reduction in the molecular weight of the starch is better suppressed during the carboxylation reaction.

[0004] Patent document 2 discloses a method for manufacturing a water-absorbing resin by surface treatment of carboxyalkylated polysaccharide particles with non-crosslinking acids such as hydrochloric acid, followed by heating and drying or by using a crosslinking agent to crosslink the polysaccharides.

[0005] Patent Document 3 discloses a method for manufacturing a water-absorbing material by reacting starch with the anhydride of a polybasic acid in an extruder. It describes how, during the reaction with the anhydride of the polybasic acid, the decrease in the molecular weight of the starch is preferably suppressed. (Prior Art Documents, Patent Documents)

[0006] Patent Document 1: US Patent No. 5079354; Patent Document 2: Japanese Patent Publication No. 2010-504414; Patent Document 3: Japanese Patent Publication No. 2007-222704 [Summary of the Invention]

[0007] [The problem the invention aims to solve]

[0008] Conventional absorbent resins made from polysaccharides have insufficient absorbency. Furthermore, because they are made from high molecular weight polysaccharides, the material has high viscosity, causing problems with operability during manufacturing. The object of this invention is to provide an absorbent resin with excellent absorbency and water retention properties and high manufacturing efficiency. [Technical Means for Solving the Problem]

[0009] The inventors of this invention focused on the molecular weight of starch used as a raw material for a water-absorbing resin, and thus completed this invention. Specifically, this invention relates to a water-absorbing resin containing a crosslinked polymer of a water-soluble polymer incorporating acidic groups into partially decomposed starch products with a weight average molecular weight of 7.5 million or less.

[0010] The weight average molecular weight of the above-mentioned starch partial decomposition products is preferably above 50,000.

[0011] The dispersion (weight average molecular weight / number average molecular weight) of the above-mentioned starch partial decomposition products is preferably 5 or higher.

[0012] The weight-average molecular weight of the pullulan obtained by aqueous size exclusion chromatography analysis of the above-mentioned water-soluble polymer is preferably 500,000 to 50,000,000.

[0013] The above-mentioned acidic group is preferably a carboxyl alkyl group, a carboxyl alkenyl group, or a sulfonic acid alkyl group.

[0014] The above-mentioned water-absorbing resin preferably has the following characteristics: (a) the water absorption ratio of ion-exchanged water under no pressure is 100 to 400 g / g; (b) the water retention rate of ion-exchanged water is 80 to 300 g / g; (c) the water absorption ratio of physiological saline under no pressure is 20 to 70 g / g; and / or (d) the water retention rate of physiological saline is 7 to 60 g / g.

[0015] The ratio (A / B) of the water absorption ratio (A) of ion-exchanged water under no pressure to the absorption ratio (B) of physiological saline under no pressure is preferably 7 or less.

[0016] The water and / or hydrophilic solvent content of the above-mentioned water-absorbing resin is preferably 0.1% to 10%.

[0017] Preferably, the above-mentioned water-absorbing resin does not have an internal cross-linking structure formed by covalent bonds.

[0018] Furthermore, the present invention relates to a method for decomposing a water-absorbing resin, the method comprising the step of alkali treatment of the water-absorbing resin.

[0019] Furthermore, the present invention relates to an article containing the above-mentioned water-absorbing resin.

[0020] Furthermore, the present invention relates to a water-soluble polymer, which is formed by introducing acidic groups into a starch partial decomposition product with a weight average molecular weight of less than 7.5 million.

[0021] The dispersion (weight average molecular weight / number average molecular weight) of the starch partial decomposition products of the above-mentioned water-soluble polymer is preferably 5 or higher.

[0022] The weight-average molecular weight (Mw) of the water-soluble polymer obtained by aqueous size exclusion chromatography is preferably 500,000 to 50,000,000.

[0023] The above-mentioned acidic group is preferably a carboxyl alkyl group, a carboxyl alkenyl group, or a sulfonic acid alkyl group.

[0024] Furthermore, the present invention relates to a resin composition for manufacturing a water-absorbing resin, the resin composition containing the aforementioned water-soluble polymer. [Effects of the Invention]

[0025] The water-absorbing resin of the present invention has excellent water absorption and water retention properties. Furthermore, due to its low viscosity during manufacturing, it has excellent operability and improved production efficiency.

Implementation Method

[0027] <<Water Absorbent Resin>> The water absorbent resin of the present invention is characterized by containing a water-soluble polymer, wherein the water-soluble polymer introduces acidic groups into a starch partial decomposition product with a weight average molecular weight of less than 7.5 million.

[0028] <Partially Decomposed Starch Products> Partially decomposed starch products are mainly products resulting from the hydrolysis of some of the glycosidic bonds between α-glucose molecules that make up starch. However, there are no restrictions on the location or state of decomposition. The type of starch used as raw material is not particularly limited; examples include: waxy corn starch, tapioca starch, potato starch, corn starch (including waxy corn starch and high amylose), wheat starch, rice starch, sweet potato starch, etc.

[0029] The weight average molecular weight of the partially decomposed starch product is 7.5 million or less, preferably 5 million or less, more preferably 4.5 million or less, further preferably 4 million or less, and further preferably 3.5 million or less. If the weight average molecular weight exceeds 7.5 million, the viscosity increases, thus exhibiting a tendency for decreased operability in the reaction or purification steps when introducing acidic groups, and a tendency for decreased water absorption performance of the water-absorbing resin. There is no particular limitation on the lower limit of the weight average molecular weight of the partially decomposed starch product, but it is preferably 50,000 or more, more preferably 200,000 or more. If the weight average molecular weight is less than 50,000, there is a tendency for decreased water retention performance of the water-absorbing resin. Furthermore, there is no particular limitation on the method for determining the weight average molecular weight; for example, it can be determined based on a calibration curve of molecular weight versus dissolution time prepared using pullulan, whose molecular weight has been determined by aqueous size exclusion chromatography.

[0030] The number average molecular weight of partially decomposed starch products is not particularly limited, but considering viscosity, it is preferably below 1 million. Furthermore, the lower limit of the number average molecular weight of partially decomposed starch products is preferably above 10,000, and more preferably above 50,000. Moreover, the method for determining the number average molecular weight is not particularly limited; for example, it can be determined based on a calibration curve of molecular weight versus dissolution time prepared using pullulan with a known molecular weight in aqueous size exclusion chromatography.

[0031] The weight-average molecular weight or number-average molecular weight of the partially decomposed starch products can also be adjusted by mixing two or more partially decomposed starch products. In this case, the weight-average molecular weight or number-average molecular weight of the mixture is preferably within the above-mentioned range.

[0032] The dispersibility (weight average molecular weight / number average molecular weight) of partially decomposed starch products is not particularly limited, but is preferably 5 or higher, more preferably 7 or higher. The upper limit of the dispersibility (weight average molecular weight / number average molecular weight) of partially decomposed starch products is not particularly limited, but is generally 70 or lower. The viscosity of partially decomposed starch products is not particularly limited.

[0033] <Water-soluble polymer> The water-soluble polymer is formed by introducing acidic groups into the partial decomposition products of starch with a weight average molecular weight of less than 7.5 million. Regarding the acidic group, there are no particular limitations as long as it is a protic acid. Examples include: acidic groups with carboxyl groups such as carboxylalkyl and carboxylalkenyl; acidic groups with sulfonic acid groups such as sulfonylalkyl and sulfonylalkenyl; and acidic groups with phosphoric acid groups such as phosphoalkyl and phosphoalkenyl.

[0034] Carboxylalkyl refers to alkyl groups substituted with a carboxyl group. The number of carbon atoms in the alkyl group substituted with a carboxyl group is preferably 1 to 8, more preferably 1 to 5. The alkyl group can be either straight-chain or branched. Specific examples of alkyl groups include: methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, dibutyl, tributyl, n-pentyl, 1-methyl-n-butyl, 2-methyl-n-butyl, 3-methyl-n-butyl, 1,1-dimethyl-n-propyl, 1,2-dimethyl-n-propyl, 2,2-dimethyl-n-propyl, 1-ethyl-n-propyl, etc.

[0035] Specific examples of carboxyl alkyl groups include: carboxymethyl, carboxyethyl, carboxypropyl, carboxybutyl, carboxypentyl, etc.

[0036] Carboxylated alkenyl groups are alkenyl groups substituted with carboxyl groups. The alkenyl group substituted with a carboxyl group preferably has 2 to 8 carbon atoms, more preferably 2 to 4. The alkenyl group can be either linear or branched. Specific examples of alkenyl groups include: vinyl, 1-propenyl, 2-propenyl, 1-methyl-1-vinyl, 1-butenyl, 2-butenyl, 3-butenyl, 2-methyl-1-propenyl, 2-methyl-2-propenyl, 1-ethylvinyl, 1-methyl-1-propenyl, 1-methyl-2-propenyl, etc.

[0037] Specific examples of carboxyl alkenyl groups include: carboxyl vinyl, carboxyl propenyl, carboxyl butenyl, etc.

[0038] Sulfonic acid alkyl refers to alkyl groups substituted with sulfonic acid groups. Examples of alkyl groups substituted with sulfonic acid groups include those described in relation to carboxylic acid alkyl groups. Specific examples of sulfonic acid alkyl groups include: sulfomethyl, sulfoethyl, sulfopropyl, etc.

[0039] Sulfonyl alkenyl is an alkenyl group substituted with a sulfonyl group. As an alkenyl group substituted with a sulfonyl group, the alkenyl group described with respect to carboxyl alkenyl can be used. Specific examples of sulfonyl alkenyl groups include: sulfonyl vinyl, sulfonyl propenyl, etc.

[0040] Phosphoalkyl refers to an alkyl group substituted with a phosphate group. As an alkyl group substituted with a phosphate group, the alkyl group described with respect to carboxylalkyl can be used. Specific examples of phosphate alkyl groups include: methyl phosphate, ethyl phosphate, propyl phosphate, etc.

[0041] Phosphoalkenyl is an alkenyl group substituted with a phosphate group. As an alkenyl group substituted with a phosphate group, the alkenyl group described with respect to carboxyl alkenyl can be used. Specific examples of phosphate alkenyl groups include: phosphate vinyl, phosphate propenyl, etc.

[0042] Among the acidic groups, from the viewpoint of facilitating their introduction into starch partial decomposition products, it is preferable to have an acidic group having a carboxyl group or a sulfonic acid group, more preferably a carboxylalkyl group, a carboxylalkenyl group, or a sulfonic acid alkyl group, and even more preferably a carboxylalkyl group having 1 to 5 carbon atoms.

[0043] The molecular weight of the water-soluble polymer is not particularly limited. The weight-average molecular weight converted from pullulan obtained by aqueous size exclusion chromatography is preferably 500,000 to 50,000,000, more preferably 500,000 to 46,000,000, and even more preferably 700,000 to 20,000,000. If the above weight-average molecular weight is less than 500,000, the water retention capacity of the water-absorbing resin tends to decrease; if it exceeds 50,000,000, the water absorption capacity of the water-absorbing resin tends to decrease. Furthermore, the weight-average molecular weight converted from pullulan obtained by aqueous size exclusion chromatography can be determined based on a calibration curve of molecular weight versus dissolution time prepared using pullulan with known molecular weight in aqueous size exclusion chromatography.

[0044] The total acid value of the water-soluble polymer is preferably 50–350 mgKOH / g, more preferably 70–300 mgKOH / g. The total acid value refers to the acid value measured after neutralizing acidic groups to their unneutralized state, and represents the amount of all acidic groups introduced into the water-soluble polymer. If the total acid value is less than 50 mgKOH / g or exceeds 350 mgKOH / g, the absorbent resin will exhibit a tendency to show a decrease in the absorbency of aqueous solutions containing electrolytes, such as physiological saline.

[0045] When acidic groups are introduced into a water-soluble polymer by reacting a haloalkyl group having an acidic group with the hydroxyl group of starch or starch partial decomposition product, the amount of acidic group introduced can also be expressed by the degree of etherification. The degree of etherification of the water-soluble polymer is preferably 0.1 to 2.0, more preferably 0.2 to 1.5. The degree of etherification can be determined by ashing titration or the like. Furthermore, since the total acid value is detected by introducing acidic groups, if the starch or starch partial decomposition product used as the raw material does not contain acidic groups, it can be considered that the acidic groups detected by measuring the total acid value are equal to those introduced by the etherification reaction. Therefore, if the starch or starch partial decomposition product used as the raw material does not contain acidic groups, it can be easily calculated based on the above-mentioned total acid value. For example, when the acidic group is carboxymethyl and it is completely neutralized to a sodium salt, the degree of etherification can be calculated using the formula: (162 × total acid value) ÷ (56100 - 80 × total acid value). Furthermore, the unit of the total acid value in this case is mgKOH / g.

[0046] The free acid number of the water-soluble polymer is preferably 5 to 30 mgKOH / g, more preferably 7 to 25 mgKOH / g. The free acid number is the acid value measured against the unneutralized acid groups. If the free acid number is less than 5 mgKOH / g, the strength of the water-absorbing resin tends to become insufficient; if it exceeds 30 mgKOH / g, the water absorption performance tends to decrease.

[0047] The dispersibility (weight average molecular weight / number average molecular weight) of the water-soluble polymer is not particularly limited, but is preferably 5 to 110, more preferably 7 to 70. If it is less than 5 or exceeds 110, the water absorption performance of the water-absorbing resin tends to decrease. The number average molecular weight of the water-soluble polymer can be determined by aqueous size exclusion chromatography. The viscosity of the water-soluble polymer is not particularly limited.

[0048] <Crosslinking Structure of the Water-Absorbent Resin> The water-absorbing resin is a crosslinked product of the aforementioned water-soluble polymer. The crosslinking is formed by internal crosslinking and arbitrary surface crosslinking. The internal crosslinking structure is preferably formed by acidic groups present in the water-soluble polymer. Examples of internal crosslinking structures include: ionic bonds formed by acidic groups present in partially decomposed starch products, coordinate bonds formed by metal ions, and, when the acidic group is a carboxyl group, hydrogen bonds formed by the dimerization of carboxyl groups. As described below, from a decomposability perspective, the water-absorbing resin preferably does not have an internal crosslinking structure formed by covalent bonds. Examples of internal crosslinking using covalent bonds include: ester bonds, ether bonds, carbon-carbon single bonds (CC bonds), carbon-carbon double bonds (C=C bonds), etc., but are not limited to these. The structure of surface crosslinking and its formation method are described below.

[0049] <Any Component> The absorbent resin may also contain structural units other than the water-soluble polymers described above. Examples of structural units other than water-soluble polymers include polyacrylic acid partially neutralized crosslinked polymers, self-crosslinking polyacrylic acid partially neutralized polymers, starch-acrylic acid graft polymers, and other polyacrylic acid (salts). Examples of acrylic acid salts include sodium salts, potassium salts, and ammonium salts. Other structural units besides starch partial decomposition products include: methacrylic acid, maleic acid, vinyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamide-2-methylpropanesulfonic acid, 2-(meth)acrylamide ethanesulfonic acid, 2-(meth)acrylamide propanesulfonic acid, and other anionic unsaturated monomers and their salts; acrylamide, methacrylamide, N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, and (meth)acrylic acid. Nonionic unsaturated monomers containing hydrophilic groups, such as 2-hydroxypropyl ester, methoxy polyethylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acrylpiperidine, and N-acrylpyrrolidone; cationic unsaturated monomers such as N,N-dimethylaminoethyl methacrylate, N,N-diethylaminoethyl methacrylate, N,N-dimethylaminopropyl methacrylate, N,N-dimethylaminopropyl(meth)acrylamide, and their quaternary salts; linear cellulose; poly(γ-glutamic acid), etc.

[0050] When the water-absorbing resin contains structural units other than water-soluble polymers, its content is preferably 90% by weight or less, more preferably 50% by weight or less, and even more preferably 20% by weight or less, relative to the total amount of the water-soluble polymer used as the main component.

[0051] The water-absorbing resin may also contain water and / or a hydrophilic solvent. Examples of hydrophilic solvents include: methanol, ethanol, n-propanol, isopropanol, acetone, ethylene glycol, propylene glycol, diethylene glycol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol dimethyl ether, dimethyl sulfoxide, etc. The content of water and / or hydrophilic solvent in the water-absorbing resin is preferably 0.1% to 20%, more preferably 1% to 19%. If it is less than 0.1%, the water absorption rate tends to decrease; if it exceeds 20%, the water-absorbing resin particles tend to aggregate (become easily clumped) during water absorption. The content of water and / or hydrophilic solvent can be adjusted by heating or drying the water-absorbing resin.

[0052] <Physical Properties of the Water-Absorbent Resin> The water absorption ratio of the water-absorbing resin under no pressure can be determined by measuring the absorbency of physiological saline or ion-exchanged water when the water-absorbing resin is not under load, using the method described in the examples. In the solid state, the water absorption ratio of the water-absorbing resin of the present invention under no pressure for ion-exchanged water is preferably 100–400 g / g, more preferably 120–350 g / g. Furthermore, the water absorption ratio of the water-absorbing resin of the present invention under no pressure for physiological saline is preferably 20–70 g / g, more preferably 30–65 g / g.

[0053] The ratio (A / B) of the water absorption ratio (A) of the ion-exchanged water of the water-absorbing resin under no pressure to the absorption ratio (B) of physiological saline under no pressure is preferably 7 or less, and more preferably 5 or less.

[0054] Regarding the water retention rate of the absorbent resin, it can be determined by measuring the absorbency of physiological saline or ion-exchanged water when the absorbent resin is subjected to a load of 150 G using the method described in the examples. In the solid state, the water retention rate of the absorbent resin of the present invention for ion-exchanged water is preferably 80–300 g / g, more preferably 100–300 g / g. Furthermore, the water retention rate of the absorbent resin of the present invention for physiological saline is preferably 7–60 g / g, more preferably 10–60 g / g, and even more preferably 20–60 g / g.

[0055] <Method for Manufacturing Water-Absorbent Resin> The water-absorbent resin of the present invention is not particularly limited in its manufacturing method as long as it has the above-described structure. For example, it can be obtained by a step of partially decomposing starch, a step of introducing acidic groups, and a step of forming an internal cross-linked structure. Furthermore, the steps of partially decomposing starch and introducing acidic groups can be performed in different orders; either step can be performed first.

[0056] In the step of partially decomposing starch, in most cases, one of the α-1,4-glycosidic bonds of the α-glucose molecules that constitute starch is partially hydrolyzed. The method of partial decomposition is not particularly limited; examples include enzymatic treatment, acid treatment, and physical crushing of starch. Furthermore, these methods can be used in combination. The reaction apparatus can be a reaction vessel or an extruder, etc.

[0057] When starch is decomposed by enzyme treatment, there are no particular limitations on the enzyme used, as long as it can hydrolyze starch. However, from the viewpoint of efficiently reducing molecular weight, endo-type enzymes are preferred. Specific examples of enzymes include: α-amylase, cyclomaltodextrin glucanotransferase, 4-α-glucantransferase, 4,6-α-glucantransferase, amylo-transferase, neopullulanase, and amylopullulanase. These enzymes can also be used in combination. There are no particular limitations on the pH during enzyme treatment, but pH 5.0 to 7.0 is preferred. The pH can be adjusted by adding hydrochloric acid, acetic acid, sodium hydroxide, potassium hydroxide, etc. Preferably, the starch is heated and mixed at 70–110°C to gelatinize while simultaneously undergoing enzyme treatment. Enzyme treatment can be performed after starch gelatinization or simultaneously with gelatinization. As a method of performing enzyme treatment after starch gelatinization, an example is to first suspend the starch in water, heat it to gelatinize, and then add enzymes to initiate an enzymatic reaction. Alternatively, as a method of performing enzyme treatment simultaneously with starch gelatinization, an example is to suspend the starch in water, add enzymes, and heat the resulting mixture within a temperature range that will not completely deactivate the enzymes.

[0058] When partially decomposing starch by acid treatment, there are no particular limitations on the type of acid used, as long as it can hydrolyze the starch. Specific examples include hydrochloric acid, sulfuric acid, oxalic acid, acetic acid, formic acid, trifluoroacetic acid, etc. The preferred temperature for acid treatment is 150-160°C.

[0059] When starch is partially decomposed by physical crushing, specific methods may include: radiation irradiation, shearing, grinding, high pressure treatment, ultrasound, thermal decomposition, photodecomposition, and combinations thereof.

[0060] As a method for introducing an acidic group into a starch partial decomposition product, the starch partial decomposition product is reacted with a compound containing an acidic group or its precursor. By introducing the acidic group into the hydroxyl groups of the starch partial decomposition product, a water-soluble polymer can be obtained. The compound containing the acidic group is not particularly limited as long as it can be introduced with the aforementioned acidic group; examples include: haloalkyl compounds having an acidic group, haloalkenyl compounds having an acidic group, acid anhydrides, and salts thereof. Examples of halogens constituting haloalkyl compounds and haloalkenyl compounds include: chlorine and bromine.

[0061] Specific examples of compounds containing acidic groups include: monochloroacetic acid, monobromoacetic acid, 3-bromopropionic acid, 6-bromohexanoic acid, succinic anhydride, maleic anhydride, vinyl sulfonic acid, phosphoric acid, ethyl monochloroacetate, and their sodium and potassium salts. Specific examples of salts include: sodium monochloroacetate, potassium monochloroacetate, and sodium vinyl sulfonate.

[0062] Acrylonitrile can be cited as a precursor of a compound containing an acidic group. As a method of using acrylonitrile, for example, the following method can be cited: First, acrylonitrile is reacted with a partial decomposition product of starch under alkaline conditions, cyanoethyl is introduced, and the cyanoethyl is induced to form a amide group (Synthesis; 1989(12): 949-950), and the obtained amide is subjected to alkaline hydrolysis.

[0063] The reaction of partially decomposed starch products with monochloroacetic acid to generate water-soluble polymers is summarized in formula (I).

[0064] The reaction of partial decomposition products of starch with 3-bromopropionic acid to generate water-soluble polymers is summarized in formula (II).

[0065] The reaction of partial decomposition products of starch with 6-bromohexanoic acid to generate water-soluble polymers is summarized in formula (III).

[0066] The reaction of partially decomposed starch products with succinic anhydride to generate water-soluble polymers is summarized in formula (IV).

[0067] The reaction of partially decomposed starch with maleic anhydride to generate a water-soluble polymer is summarized in formula (V).

[0068] The reaction of partially decomposed starch with sodium vinyl sulfonate to generate a water-soluble polymer is summarized in formula (VI).

[0069] Formulas (I) to (VI) show water-soluble polymers formed by sodium salts in which all 6-position hydroxyl groups of glucose units are introduced with acidic groups, but hydroxyl groups that are not introduced with acidic groups may also remain. Furthermore, acidic groups that have not been neutralized by salt may also exist. The position of the acidic group is not limited as long as it is a hydroxyl group present in the partial decomposition of starch; it can be any of the hydroxyl groups at positions 1, 2, 3, 4, and 6.

[0070] The reaction conditions for the partial starch decomposition product and the acid-containing compound are not particularly limited, but it is preferred to carry out the reaction under alkaline conditions. When using a haloalkyl compound or a haloenyl compound as the acid-containing compound, it is preferable to use 1 to 1.5 equivalents of an alkaline agent relative to the haloalkyl compound or the haloenyl compound. Examples of alkaline agents include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, and sodium carbonate. The acidic group introduced into the partial starch decomposition product is preferably used to form a salt with sodium, potassium, lithium, ammonia, etc. derived from the alkaline agent. Therefore, the alkaline agent is preferably used in the amount required for the reaction of the haloalkyl compound or the haloenyl compound with the partial starch decomposition product and for neutralizing the acidic group of the haloalkyl compound or the haloenyl compound. For example, when using chloroacetic acid as the acid-containing compound, it is theoretically preferable to use 2 equivalents or more of an alkaline agent relative to chloroacetic acid. When using sodium chloroacetate, the acidic groups are pre-neutralized, so it is preferable to use an alkali of 1 equivalent or more relative to sodium chloroacetate.

[0071] The amount of the acid-containing compound used can be arbitrarily set according to the target total acid value (degree of etherification) of the water-soluble polymer. Generally, it is preferred to be 0.5 to 5 equivalents relative to 1 mole of the hydroxyl groups in the starch partial decomposition product, and more preferably 0.5 to 2.0 equivalents. When using a haloalkyl compound such as chloroacetic acid to prepare an aqueous solution for the reaction, the introduction reaction of the acid group competes with the hydrolysis reaction of the haloalkyl compound, so the haloalkyl compound must be used in excess of the theoretical value. Regarding the excess amount of haloalkyl compound in the aqueous solution reaction, it is preferred to set it to 5 equivalents or less relative to the theoretical value.

[0072] The reaction temperature of the starch partial decomposition product with the acidic compound is not particularly limited, but is preferably 0–120°C. The reaction time is not particularly limited, but is preferably 1–24 hours. The reaction can be carried out in water, but can also be carried out in a mixed solvent of water and methanol, ethanol, isopropanol, butanol, ethylene glycol, diethylene glycol, propylene glycol, ethylene glycol monoethyl ether, or other alcohols, or glycol ethers such as ethylene glycol dimethyl ether. Alternatively, the dried starch partial decomposition product powder can be dispersed in a hydrophilic solvent such as methanol, ethanol, isopropanol, butanol, or glycol ethers such as ethylene glycol dimethyl ether. When using a mixed solvent, the proportion of solvent other than water is preferably 50% by volume or less in the mixed solvent. The reaction apparatus can be a reaction vessel or an extruder, etc.

[0073] When using chloroacetic acid or other haloalkyl compounds or their salts as compounds containing acidic groups, there is no particular limitation on the reaction temperature with the partial decomposition products of starch, but it is preferred to be 0 to 100°C.

[0074] In particular, when using chloroacetic acid or its salt as the haloalkyl compound, it is preferable to carry out the reaction at 25–90°C to prevent hydrolysis caused by water in the reaction solution. The reaction time is preferably the time until the haloalkyl compound used as a raw material is consumed, and more preferably 1–12 hours for the stability of the haloalkyl compound and the efficiency of the process. The reaction can be carried out in water, but it can also be carried out in a mixed solvent of water and alcohols such as methanol, ethanol, isopropanol, butanol, and glycol ethers such as ethylene glycol dimethyl ether. Alternatively, the dried starch partial decomposition product powder can be dispersed in a hydrophilic solvent such as methanol, ethanol, isopropanol, butanol, and glycol ethers such as ethylene glycol dimethyl ether. When using a mixed solvent, the proportion of solvent other than water is preferably 50% by volume or less in the mixed solvent. The reaction apparatus can be a reaction vessel or an extruder, etc.

[0075] Furthermore, when using an anhydride as the compound containing an acidic group, the reaction can proceed simply by mixing the partially decomposed starch product with the anhydride and heating it. However, to promote the reaction, sodium carbonate, sodium hydroxide, tertiary amines such as triethylamine, imidazoles such as 2-methylimidazolium, quaternary ammonium salts such as tetrabutylammonium bromide, and phosphonium salts such as tetrabutylphosphonium bromide can also be used as catalysts. The amount of such catalysts added is preferably 0.1 equivalents or less relative to the compound containing the acidic group. These catalysts can be used alone or in combination of two or more.

[0076] The reaction time is preferably the time until the acid anhydride used as a raw material is consumed, more preferably 1 to 12 hours. The endpoint of the reaction can be determined by acid value measurement or IR measurement. The reaction can be carried out in water, but to prevent hydrolysis and alcoholysis of the acid anhydride, the reaction solvent is preferably an aprotic solvent such as dimethyl sulfoxide, dimethylformamide, dimethyl acetamide, or N-methylpyrrolidone. When using a mixed solvent, the proportion of solvent other than water is preferably 50% by volume or more in the mixed solvent. When the reaction is carried out in the absence of a solvent, the acid anhydride can act as a solvent, so the reaction temperature is preferably above the melting point of the acid anhydride. When a solvent is used in the reaction, the reaction temperature is preferably 50 to 100°C, and more preferably 70 to 90°C. The reaction apparatus can be a reaction vessel or an extruder, etc.

[0077] Regarding the salt of the acidic group that adds to the partially decomposed starch product, it is preferable to neutralize a portion of it. Through neutralization, a portion of the acidic group forming the salt is converted into a free acidic group. For example, when monochloroacetic acid is used as the aforementioned acidic compound and sodium hydroxide is used as the alkali, the sodium salt of the carboxyl group undergoes addition with the partially decomposed starch product. By adding an acid, a portion of the carboxyl group is converted into a free carboxylic acid.

[0078] There is no particular limitation on the acid used for neutralization. When the acidic group is a carboxyl group, it is preferable to use an acid with a pKa of the same or lower as that of the carboxyl group, such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, acetic acid, formic acid, trichloroacetic acid, etc. When neutralizing sulfonic acid groups or phosphoric acid groups, it is preferable to use a strong acid, such as inorganic acids like hydrochloric acid or sulfuric acid, or a strong acidic ion exchange resin. Known apparatus such as a reaction vessel or extruder can be used for neutralization. After adding the acid, it is preferable to stir at 0 to 50°C for 0.2 to 1 hour to carry out the neutralization reaction. The neutralization reaction is preferably carried out under conditions of pH 6.8 to 7.2.

[0079] In the step of introducing the acidic group, or in the subsequent neutralization reaction, halogens derived from compounds containing acidic groups may sometimes form salts with metals or ammonia derived from alkali agents; therefore, desalination is preferable. As a desalination method, an example of washing can be performed by the following process: dissolving the water-soluble polymer in water to prepare an aqueous solution, adding this aqueous solution dropwise to a hydrophilic solvent such as methanol, ethanol, isopropanol, acetone, or acetonitrile, causing the water-soluble polymer to precipitate again, filtering and recovering it, then redispersing the recovered water-soluble polymer in aqueous methanol (with a water content of approximately 70-90%), stirring, and then filtering and recovering the water-soluble polymer particles. Alternatively, as a desalination method, an example can be performed by treating the aqueous solution of the hydrophilic polymer using a filter equipped with an ultrafiltration membrane. As the washing liquid during desalination, water, or a mixture of water and hydrophilic organic solvents such as methanol, ethanol, propanol, acetone, or acetonitrile, can be used. Desalination is preferably carried out until the salt concentration in the water-soluble polymer becomes less than 1%.

[0080] In the step of forming the internal cross-linked structure, the water-soluble polymers are cross-linked with each other. Regarding cross-linking, it can be formed without using a cross-linking agent by heating and drying, for example, a water-soluble polymer with a portion of its acidic groups being free acid groups, under aqueous conditions. The water-soluble polymer at the start of heating and drying can be an aqueous solution or a wet powder containing a water-containing solvent of 1% by weight or more. When drying a wet powder containing a water-containing solvent, the moisture content of the wet powder at the start of drying is preferably 1-85% by weight, more preferably 20-80% by weight, and even more preferably 55-75% by weight. Furthermore, the moisture content referred to here is the ratio of the combined amount of water and the hydrophilic solvent in the wet powder. The heating temperature is preferably 50-150°C, more preferably 60-130°C. The drying method is not particularly limited and can be performed using a rotary dryer, spray dryer, conical spiral mixer, etc.

[0081] When forming the crosslinked structure, solvents other than water may be used in addition to water. Examples of solvents other than water include: lower aliphatic alcohols such as methanol, ethanol, n-propanol, and isopropanol; ketones such as acetone; ethers such as dialkyl, tetrahydrofuran, and methoxy(poly)ethylene glycol; and amides such as ε-caprolactam and N,N-dimethylformamide. The proportion of solvents other than water in the total solvent is preferably adjusted according to the boiling point of the solvent. When the boiling point of the solvent is below 100°C, it is preferably 70% by volume or more, and when the boiling point is above 100°C, it is preferably 30% by volume or less.

[0082] The internal cross-linked structure can be formed without the use of a cross-linking agent as described above, but a cross-linking agent may also be used. Examples of cross-linking agents include: epoxy compounds, polyol compounds, polyamine compounds, polyisocyanate compounds, alkyl carbonate compounds, haloepoxy compounds, haloalcohol compounds, polyazoline compounds, carbodiimide compounds, silane coupling agents, and multi-metal compounds.

[0083] Examples of the above-mentioned epoxy compounds include: glycidyl succinate, sorbitol polyglycidyl ether, trimethylolpropane polyglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, dehydrated glycerol, etc.

[0084] Examples of the above-mentioned polyol compounds include: ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanediol, 1,2-cyclohexanediol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, ethylene-oxypropylene block copolymer, neopentyl terephthalate, sorbitol, etc.

[0085] Examples of the above-mentioned polyamine compounds include: ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, inorganic or organic salts of such polyamine compounds (such as azadinium salts), and polysaccharides with amino groups such as chitin.

[0086] Examples of the above-mentioned polyisocyanate compounds include 2,4-toluene diisocyanate and hexamethylene diisocyanate; examples of the above-mentioned polyazoline compounds include 1,2-epoxyethylbizoline.

[0087] Examples of the above-mentioned alkyl carbonate compounds include: 1,3-dioxolane-2-one, 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dialkyl-2-one, 4-methyl-1,3-dialkyl-2-one, 4,6-dimethyl-1,3-dialkyl-2-one, etc.

[0088] Examples of the above-mentioned halogenated epoxy compounds include epichlorohydrin, epibromohydrin, α-methylepicochlorohydrin or their polyamine adducts (e.g., KYMENE (registered trademark) manufactured by Hercules).

[0089] Furthermore, as other known crosslinking agents, aqueous carbodiimide compounds (e.g., Carbodilite manufactured by Nisshinbo Chemical Co., Ltd.), γ-glycidoxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane and other silane coupling agents, or multi-metal compounds such as hydroxides or chlorides of zinc, calcium, magnesium, aluminum, iron, zirconium, etc., may also be used.

[0090] In manufacturing the absorbent resin, in addition to internal crosslinking, surface crosslinking can also be performed. Surface crosslinking can improve the strength of the absorbent resin. The crosslinking agent used for surface crosslinking can be the same as that described above regarding internal crosslinking. Among these, epoxy compounds are preferred, and more preferably ethylene glycol diglycidyl ether, sorbitol polyglycidyl ether, or glycidyl succinate.

[0091] Surface crosslinking can be formed by spraying a surface crosslinking agent onto a water-absorbing resin, and then mixing it using a known method such as a cylindrical mixer, V-shaped mixer, belt mixer, spiral mixer, double-arm mixer, or pulverizing kneader to achieve crosslinking. A surfactant may be added as needed during spraying and mixing.

[0092] Furthermore, regarding the absorbent resin, in order to impart various functions, it may also contain disinfectants, deodorants, antibacterial agents, fragrances, various inorganic powders, foaming agents, pigments, dyes, hydrophilic short fibers, fertilizers, oxidants, reducing agents, water, and other additives such as salts. The amount of these additives can be appropriately selected by those skilled in the art.

[0093] <<Method for Decomposing Water-Absorbent Resin>> The method for decomposing water-absorbing resin according to the present invention is characterized by including a step of alkali treatment of the above-mentioned water-absorbing resin. In the alkali treatment step, the water-absorbing resin is placed under conditions preferably pH 9 or higher, more preferably pH 10 or higher. Through alkali treatment, the cross-linked structure or glycosidic bonds of the water-absorbing resin are broken and decomposed into water-soluble polymers, thereby reducing the environmental impact upon disposal.

[0094] There are no particular limitations on the alkaline agents used in the alkali treatment. Examples include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, sodium carbonate, and potassium carbonate. There are no particular limitations on the temperature during the alkali treatment, and it can be carried out at, for example, 5 to 50°C.

[0095] <<Articles Containing Absorbent Resin>> The articles of the present invention contain the aforementioned absorbent resin. Examples of such articles include: diapers, sanitary products, incontinence pads, portable toilets, waste disposal bags, animal excrement and urine treatment agents, medical treatment materials, wound dressings, and other sanitary materials; fertilizers and other agricultural materials; soil conditioners, sludge solidifying agents, waterproofing materials, and other civil engineering materials. Examples of sanitary products include laminates consisting of a bottom sheet, an absorbent core, and a top sheet stacked sequentially. The absorbent core contains the absorbent resin of the present invention, and may further contain absorbent paper or pulp if necessary.

[0096] <<Resin Composition for Water-Absorbent Resin>> The resin composition for water-absorbing resin of the present invention contains the above-mentioned water-soluble polymer. The content of the water-soluble polymer in the resin composition is preferably 5-80% by weight, more preferably 25-60% by weight.

[0097] Regarding the resin composition, in addition to water-soluble polymers, it may also contain any components such as solvents and crosslinking agents. As a solvent, water, methanol, ethanol, isopropanol, and other solvents mentioned above as used in forming the crosslinked structure can be used. As a crosslinking agent, those mentioned above as internal crosslinking agents or surface crosslinking agents can be used.

[0098] By heating and drying the resin composition of the present invention under aqueous conditions, an internal cross-linked structure of water-soluble polymers can be formed, thereby obtaining a water-absorbing resin. The heating and drying can be performed under the conditions described above for the step of forming the internal cross-linked structure of the water-soluble polymers. [Example]

[0099] Hereinafter, the present invention will be described by way of example, but the present invention is not limited to the following examples. Hereinafter, unless otherwise specified, "parts" or "%" means "parts by weight" or "% by weight".

[0100] (1)Materials used (1-1) Starch raw materials: corn starch, tapioca wax, corn potato

[0101] (1-2) Hydrolytic enzyme α-amylase (Spitase (registered trademark) HK / R manufactured by Nagase Kasei Corporation), 12,200 units / g; Branching enzyme (Branchzyme (registered trademark) manufactured by Novozyme), 25,000 units / g; Maltodextrose transglucosidase: Aerobic culture of Thermus thermophilus, centrifugation of the recovered bacterial cell fragments, and use of the supernatant as crude enzyme solution. Conventionally, the crude enzyme solution is supplied for column chromatography, and the sample purified to a single concentration by electrophoresis is used as purified enzyme solution.

[0102] Furthermore, regarding the activity of maltodextrin, the following method was performed. Reaction solution 1, containing 10 w / v% maltotriose, 50 mM sodium acetate buffer (pH 6.0), and the enzyme, was incubated at 60°C for 20 minutes. Subsequently, the reaction was terminated by heating at 100°C for 10 minutes. The amount of glucose in the reaction solution was determined by the glucose oxidase method. Regarding the unit quantity of maltodextrin, maltodextrin activity that generates 1 μmol of glucose within 1 minute was defined as 1 unit.

[0103] (2)Preparation of partially decomposed starch products (Preparation Examples 1-17 and Preparation Example 54) Partially decomposed starch products were prepared by the following method. Furthermore, the weight-average molecular weight of the obtained partially decomposed starch products was determined based on a calibration curve of molecular weight versus dissolution time prepared by pullulan, whose molecular weight is known, in aqueous size exclusion chromatography.

[0104] Manufacturing Example 1: Partially Decomposed Starch from Corn Starch Corn starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare starch slurry. To this starch slurry, crude maltose transglucosidase was added at a concentration of 0.8 units per gram of starch solids, and branching enzyme was added at a concentration of 20 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, and then stirred while maintaining the temperature at 80°C for 6 hours to allow the reaction to proceed, thereby preparing liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 810,000.

[0105] Manufacturing Example 2: Partially Decomposed Starch from Corn Starch Corn starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare a starch slurry. To this starch slurry, crude maltose transglucosidase was added at a concentration of 1.6 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, then stirred while maintaining the temperature at 90°C for 3 hours, and subsequently maintained at 80°C for 17 hours to allow the reaction to proceed, thus preparing liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 663,000.

[0106] Manufacturing Example 3: Partially Decomposed Starch from Corn Starch Corn starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare a starch slurry. Maltodextrose transglucosidase purification enzyme solution was added to this starch slurry at a concentration of 1.6 units per gram of starch solids. After stirring at room temperature for 30 minutes, the mixture was kept at 80°C for 8 hours while stirring to allow the reaction to proceed, thus preparing liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 720,000.

[0107] Manufacturing Example 4: Starch Partially Decomposed Product from Corn Starch Corn starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare a starch slurry. Maltodextrose transglucosidase purification enzyme solution was added to this starch slurry at a concentration of 0.6 units per gram of starch solids, and α-amylase was added at a concentration of 0.03 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, then stirred while maintaining the temperature at 90°C for 4.5 hours, and then maintained at 100°C for 1.5 hours to allow the reaction to proceed, thereby preparing liquefied starch. The weight-average molecular weight of the obtained starch partially decomposed product was 1.46 million.

[0108] Manufacturing Example 5: Partially Decomposed Starch from Tapioca Starch Tapioca starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare a starch slurry. To this starch slurry, crude maltose transglucosidase was added at a concentration of 1.6 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, and then stirred while maintaining the reaction at 80°C for 20 hours to prepare liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 200,000.

[0109] Manufacturing Example 6: Partially Decomposed Starch from Tapioca Starch Tapioca starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare a starch slurry. To this starch slurry, crude maltose transglucosidase was added at a concentration of 0.4 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, and then stirred while maintaining the reaction at 80°C for 9 hours to prepare liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 1.18 million.

[0110] Manufacturing Example 7: Partially Decomposed Starch from Tapioca Starch Tapioca starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare a starch slurry. Maltodextrose transglucosidase purification enzyme solution was added to this starch slurry at a concentration of 0.4 units per gram of starch solids. After stirring at room temperature for 30 minutes, the mixture was kept at 80°C for 3 hours while stirring to allow the reaction to proceed, thus preparing liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 1.96 million.

[0111] Manufacturing Example 8: Partially Decomposed Starch from Waxy Corn Starch Waxy corn starch was suspended in tap water, and calcium chloride was added to adjust the pH to 6.0 to prepare a starch slurry with a concentration of approximately 15% by mass. α-Amylase was added to this starch slurry at a concentration of 0.78 units per gram of starch solids. After stirring for 30 minutes, the mixture was passed into a continuous liquefaction apparatus at a flow rate of 1 L / min. The starch slurry was heated in the continuous liquefaction apparatus at 100°C for 25 minutes, followed by heating at 140°C for 5 minutes to prepare liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 610,000.

[0112] Manufacturing Example 9: A partially hydrolyzed starch product derived from waxy corn starch was prepared using the same method as in Manufacturing Example 8, except that the amount of enzyme added was changed to 0.36 U. The weight-average molecular weight of the obtained partially hydrolyzed starch product was 1.33 million.

[0113] Manufacturing Example 10: Partially Decomposed Starch from Potato Starch A starch slurry was prepared by suspending potato starch at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0). Maltose transglucosidase purification enzyme solution was added to this starch slurry at a concentration of 0.8 units per gram of starch solids. After stirring at room temperature for 30 minutes, the mixture was continuously stirred and kept at 80°C for 4 hours to allow the reaction to proceed, thereby preparing liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 1.53 million.

[0114] Manufacturing Example 11: Starch Partially Decomposed from Corn Starch Corn starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare starch slurry. Maltodextrose transglucosidase purification enzyme solution was added to this starch slurry at a concentration of 0.6 units per gram of starch solids, and α-amylase was added at a concentration of 0.03 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, then stirred while maintaining the temperature at 90°C for 4.5 hours, and then maintained at 100°C for 1.5 hours to allow the reaction to proceed, thereby preparing liquefied starch. The weight-average molecular weight of the obtained starch partially decomposed product was 1.46 million.

[0115] Manufacturing Example 12: Partially Decomposed Starch from Corn Starch Corn starch was suspended at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0) to prepare a starch slurry. Maltodextrose transglucosidase purification enzyme solution was added to this starch slurry at a concentration of 0.6 units per gram of starch solids, and α-amylase was added at a concentration of 0.03 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, and then stirred while maintaining the reaction at 80°C for 6 hours to prepare liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 1.04 million.

[0116] Manufacturing Example 13: Partially Decomposed Starch from Tapioca Starch A starch slurry was prepared by suspending tapioca starch at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0). Maltose transglucosylase purification enzyme solution was added to this starch slurry at a concentration of 1.6 units per gram of starch solids. After stirring at room temperature for 30 minutes, the mixture was continuously stirred and kept at 80°C for 20 hours to allow the reaction to proceed, thereby preparing liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 190,000.

[0117] Manufacturing Example 14: Partially Decomposed Starch from Corn Starch A starch slurry was prepared by suspending corn starch at a concentration of 15% (w / w) in 20 mM acetate buffer (pH 6.0). Maltodextrose transglucosidase purification enzyme solution was added to this starch slurry at a concentration of 8.0 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, then stirred while maintaining the temperature at 90°C for 3.0 hours, and subsequently maintained at 80°C for 21 hours to allow the reaction to proceed, thereby preparing liquefied starch. The obtained partially decomposed starch had a weight-average molecular weight of 270,000.

[0118] (Manufacturing Example 15) A starch partially decomposed product derived from corn starch was prepared by suspending corn starch in tap water at a concentration of 30% (w / w), then adding 1 N sodium hydroxide to adjust the pH to 6.0, thus obtaining a starch slurry. A crude maltose transglucosidase enzyme solution was added to this starch slurry at a concentration of 0.2 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, and then reacted at 80°C for 6 hours while stirring, to prepare liquefied starch. The obtained starch partially decomposed product had a weight average molecular weight of 6.07 million and a dispersion of 35.4.

[0119] (Manufacturing Example 16) Corn starch was suspended in tap water at a concentration of 30% (w / w), and then 1 N sodium hydroxide was added to adjust the pH to 6.0 to obtain a starch slurry. Maltodextrin crude enzyme solution was added to the starch slurry at a concentration of 0.15 units per gram of starch solids, and α-amylase was added at a concentration of 0.04 units per gram of starch solids. The mixture was stirred at room temperature for 30 minutes, and then reacted at 80°C for 6 hours while stirring to prepare liquefied starch. The obtained partially decomposed starch had a weight average molecular weight of 1.34 million and a dispersion of 10.6. The aqueous solution of the obtained partially decomposed starch was vacuum dried at 80°C, and the dried product was pulverized to collect the dried powder that passed through a 1 mm mesh. The moisture content of the dried powder was 2.7%.

[0120] (Manufacturing Example 17) Corn starch was suspended in tap water at a concentration of 15% (w / w), and then 1 N sodium hydroxide was added to adjust the pH to 6.0 to obtain a starch slurry. α-Amylase was added to the starch slurry at a concentration of 0.5 units per gram of starch solids. After stirring at room temperature for 30 minutes, the mixture was reacted at 100°C for 20 minutes while stirring to prepare liquefied starch. The obtained partially decomposed starch had a weight average molecular weight of 1.37 million and a dispersion of 65.6.

[0121] (Manufacturing Example 54) Corn starch was suspended in tap water at a concentration of 30% (w / w), and then 1 N sodium hydroxide was added to adjust the pH to 6.0 to obtain a starch slurry. A crude maltose transglucosidase enzyme solution was added to the starch slurry at a concentration of 0.2 units per gram of starch solids. After stirring at room temperature for 30 minutes, the mixture was stirred while reacting at 80°C for 6 hours to prepare liquefied starch. The obtained partially decomposed starch had a weight average molecular weight of 1.91 million and a dispersion of 10.1.

[0122] (3) Manufacturing of water-soluble polymers (Manufacturing Examples 18-34 and Manufacturing Example 55) (3-1) Manufacturing Example 18 125 g of a 15% by weight aqueous solution of the starch partial decomposition product manufactured in Manufacturing Example 1 (the hydroxyl group of the starch partial decomposition product is 0.35 mol) was added to a 500 ml separable flask equipped with a stirrer, thermometer, and condenser. Next, 42.7 g of a 48.8% sodium hydroxide aqueous solution (0.52 mol, which is 1.5 equivalents relative to the hydroxyl group of the starch partial decomposition product) was added, and the solution was stirred at a temperature below 60°C until completely homogeneous. After confirming that the solution was homogeneous, an aqueous solution obtained by dissolving 60.7 g of sodium monochloroacetate (0.52 mol, which is 1.5 equivalents relative to the hydroxyl group of the starch partial decomposition product) in 78.4 g of ion-exchanged water was added dropwise at a temperature of 50-60°C over 30 minutes. After adding the sodium monochloroacetate aqueous solution, the temperature was adjusted to 80-85°C, and the mixture was stirred for 1 hour. The endpoint of the reaction is achieved when the following condition is met: the reaction solution is sampled, and the chloride ion content in the reaction solution is determined by potentiometric titration using a 0.01 N silver nitrate solution, reaching at least 98% of the calculated chloride ion content of 6.0% when all sodium monochloroacetate has reacted. In this manufacturing example, the chloride content is 6.2%.

[0123] After the reaction is complete, the reaction solution is diluted with 28 g of deionized water. The diluted reaction solution is cooled to room temperature and added to 1 L of methanol over approximately 30 minutes to allow the water-soluble polymer to precipitate and then reprecipitate. After adding all the reaction solution, the mixture is stirred for 30 minutes, and the water-soluble polymer dispersed in methanol is separated into solid and liquid phases by vacuum filtration.

[0124] Subsequently, in order to remove sodium chloride from the water-soluble polymer, the recovered water-soluble polymer was redispersed in 0.7 L of aqueous methanol with a methanol / water ratio of 80 / 20 (volume ratio). After stirring and washing at room temperature for 30 minutes, solid-liquid separation was performed by vacuum filtration to recover the water-soluble polymer again. The chlorine content of the recovered water-soluble polymer was determined by potentiometric titration using 0.01 N silver nitrate solution. The washing process was repeated until the chlorine content was less than 1%. The total acid value of the obtained water-soluble polymer was 182 mgKOH / g, and the degree of etherification calculated from the total acid value was 0.71.

[0125] (3-2) Manufacturing Examples 19-32 Except for changing the raw materials and addition amounts as recorded in Table 1, the reaction was carried out in the same manner as in Manufacturing Example 18 to obtain a water-soluble polymer.

[0126] [Table 1] Water-soluble polymers Manufacturing Example 18 Manufacturing Example 19 Manufacturing Example 20 Manufacturing Example 21 Manufacturing Example 22 Manufacturing Example 23 Manufacturing Example 24 Manufacturing Example 25 Manufacturing Example 26 Manufacturing Example 27 Manufacturing Example 28 Manufacturing Example 29 Manufacturing Example 30 Manufacturing Example 31 Manufacturing Example 32 Starch partial hydrolysate 1 Manufacturing Example 1 Manufacturing Example 2 Manufacturing Example 3 Manufacturing Example 4 Manufacturing Example 5 Manufacturing Example 6 Manufacturing Example 7 Manufacturing Example 7 Manufacturing Example 8 Manufacturing Example 8 Manufacturing Example 9 Manufacturing Example 10 Manufacturing Example 11 Manufacturing Example 12 Manufacturing Example 15 Aqueous solution concentration (%) 15 15 15 15 15 15 15 15 25 25 25 15 15 15 30 Amount of aqueous solution (g) 125.0 115.0 125.0 125.0 100.0 96.4 125.0 114.8 82.5 95.2 93.7 115.0 112.5 77 66.3 Starch partial hydrolysate 2 - - - - - - - Manufacturing Example 13 - - - - Manufacturing Example 14 - - Aqueous solution concentration (%) - - - - - - - 15 - - - - 15 - - Amount of aqueous solution (g) - - - - - - - 12.8 - - - - 12.5 - - Alkali 48.8% NaOH aqueous solution (g) 42.7 27.5 42.7 42.7 23.9 32.9 42.7 43.6 5.5 47.4 53.4 27.5 29.9 - 23.4 48% KOH aqueous solution (g) - - - - - - - - - - - - - 75 - Aqueous solutions containing compounds with ionic functional groups Monochloroacetic acid (Na(g)) 60.7 44.7 60.7 60.7 38.9 46.8 60.7 61.9 8.9 77.0 75.8 44.7 48.5 - - Monochloroacetic acid (g) - - - - - - - - - - - - - 30.3 13.0 Water (g) used to dissolve compounds containing ionic functional groups. 78.4 63.9 78.4 78.4 55.6 59.7 78.4 79.9 10.4 150.0 150.0 63.9 69.4 40.4 3.3 Water used to dilute the reaction solution (g) 28 0 150 150 0 25 28 125 73 0 0 0 80 77 200 Reprecipitation was carried out using methanol (L). 1.0 0.8 1.5 1.5 0.7 0.8 1.0 1.4 0.6 1.0 1.0 0.8 1.2 1.0 1.0 Use aqueous methanol (L) for cleaning. 0.7 0.6 0.7 1.3 0.5 0.5 0.7 0.8 0.3 0.5 0.5 0.7 0.7 0.7 0.5

[0127] (Manufacturing Example 33) 20.0 g of starch partial decomposition powder (with 0.36 mol of hydroxyl groups in the starch partial decomposition product) prepared in Manufacturing Example 16 and 110 g of dimethyl sulfoxide (DMSO) were added to a 300 ml separable flask equipped with a stirrer, thermometer, and condenser, and dissolved. Next, 10.6 g of maleic anhydride (0.11 mol, equivalent to 0.30 mol of hydroxyl groups in the starch partial decomposition product) was added, and the mixture was stirred at 90–95 °C for 3 hours to allow the reaction to proceed. The reaction solution was sampled after stirring and neutralized and titrated with 0.1 N NaOH. The resulting acid value of the reaction solution was 70 mg KOH / g (the theoretical endpoint acid value was 43 mg KOH / g).

[0128] After the reaction is complete, the reaction solution is added to 2.5 L of acetone within about 30 minutes to allow the water-soluble polymer to precipitate and then reprecipitate. After adding all the reaction solution, the mixture is stirred for 30 minutes, and the water-soluble polymer dispersed in the acetone is separated into solid and liquid phases and recovered by vacuum filtration.

[0129] Subsequently, in order to remove unreacted maleic anhydride and maleic acid generated by hydrolysis from the water-soluble polymer, the recovered water-soluble polymer was redispersed in 500 mL of acetone, stirred and washed at room temperature for 30 minutes, and then subjected to solid-liquid separation by vacuum filtration to recover the water-soluble polymer again. The total acid value of the obtained water-soluble polymer was 90 mgKOH / g. The weight average molecular weight of the obtained water-soluble polymer was 3.1 × 10⁶, and the dispersibility was 39.6.

[0130] (Manufacturing Example 34) 27.2 g of starch partial decomposition product powder (0.47 mol of hydroxyl groups in the starch partial decomposition product) manufactured in Manufacturing Example 17 and 58.1 g of dimethyl sulfoxide (DMSO) were added to a 300 ml separable flask equipped with a stirrer, thermometer, and condenser, and dissolved. Next, 14.2 g of succinic anhydride (0.14 mol, 0.30 equivalents relative to the hydroxyl groups in the starch partial decomposition product) was added. The mixture was stirred at 70–75°C for 1 hour to allow the reaction to proceed. The reaction solution was sampled after stirring and neutralized and titrated with 0.1 N NaOH. The resulting acid value of the reaction solution was 85 mg KOH / g (the theoretical endpoint acid value is 80.1 mg KOH / g).

[0131] After the reaction was completed, 125 g of deionized water was added to the reaction solution for dilution. Then, 10.8 g (0.13 mol) of 48% NaOH aqueous solution was added to neutralize 93% of the theoretical amount of carboxylic acid introduced through the reaction with succinic anhydride, thus preparing the sodium salt. Subsequently, the obtained solution was added to 750 ml of methanol over approximately 30 minutes to allow the water-soluble polymer to precipitate and then reprecipitate. After adding all the reaction solution, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in methanol was separated from the solid and recovered by vacuum filtration.

[0132] Subsequently, in order to remove unreacted succinic anhydride and succinic acid generated by hydrolysis from the water-soluble polymer, the recovered water-soluble polymer was redispersed in 500 mL of methanol, stirred and washed at room temperature for 30 minutes, and then separated by vacuum filtration to recover the water-soluble polymer again. The total acid value and free acid value of the obtained water-soluble polymer were determined, and the results confirmed that it was partially neutralized with a total acid value of 138 mgKOH / g and a free acid value of 37 mgKOH / g. The weight-average molecular weight of the obtained water-soluble polymer was 3.7 × 10⁶, and the dispersibility was 13.8.

[0133] [Table 2] Water-soluble polymers Manufacturing Example 33 Manufacturing Example 34 Starch partial hydrolysate 1 Manufacturing Example 16 Manufacturing Example 17 Powder weight (g) 20 27.2 DMSO(g) 110 58.1 Acid anhydride (g) Succinic anhydride - 14.2 maleic anhydride 10.6 - Reaction temperature (°C) 90~95 70~75 Water used to dilute the reaction solution (g) - 125 48% NaOH aqueous solution (g) - 10.8 Solvent (L) for reprecipitation methanol - 0.75 acetone 2.5 - Cleaning solvent (L) methanol - 0.5 acetone 0.5 - Physical properties of water-soluble polymers Mw (×10) 6 ) 3.1 3.7 Mw / Mn 39.6 13.8 Total acid value (mgKOH / g) 90 138 Free acid value (mgKOH / g) - 37

[0134] (Manufacturing Example 55) 200 g of a 30% by weight aqueous solution of the starch partial decomposition product manufactured in Manufacturing Example 54 (the hydroxyl group of the starch partial decomposition product is 1.11 mol) was added to a 500 ml separable flask equipped with a stirrer, thermometer, and condenser. Next, 79.4 g of a 48.8% sodium hydroxide aqueous solution (0.96 mol, equivalent to 0.86 hydroxyl groups of the starch partial decomposition product) was added, and the solution was stirred at a temperature below 60°C until completely homogeneous. After confirming that the solution was homogeneous, 89.9 g of 6-bromohexanoic acid crystals (0.46 mol, equivalent to 0.4 hydroxyl groups of the starch partial decomposition product) were added little by little over 30 minutes at a temperature of 50–60°C. After adding 6-bromohexanoic acid, the temperature was adjusted to 45–50°C, and the mixture was stirred for 10 hours. The endpoint of the reaction is achieved when the following conditions are met: the reaction solution is sampled, and the bromide ion content in the reaction solution is determined by potentiometric titration using a 0.01 N silver nitrate solution, reaching at least 98% of the calculated bromide ion content of 9.8% when all 6-bromohexanoic acid has reacted. In this manufacturing example, the bromine content is 10.1%.

[0135] After the reaction was complete, the reaction solution was diluted with 250 g of deionized water. The diluted reaction solution was cooled to room temperature and added to 1.6 L of ethanol over approximately 30 minutes to allow the water-soluble polymer to precipitate and then reprecipitate. After adding all the reaction solution, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the ethanol was separated into solid and liquid phases by vacuum filtration.

[0136] Subsequently, in order to remove sodium bromide contained in the water-soluble polymer, the recovered water-soluble polymer was redispersed in 0.5 L of aqueous ethanol with a volume ratio of 90 / 10. After stirring and washing at room temperature for 30 minutes, solid-liquid separation was performed by vacuum filtration to recover the water-soluble polymer again. The bromine content of the recovered water-soluble polymer was determined by potentiometric titration using 0.01 N silver nitrate solution. The washing process was repeated until the bromide ion content was less than 1%. The total acid value of the obtained water-soluble polymer was 145 mgKOH / g, and the degree of etherification calculated from the total acid value was 0.64.

[0137] (4) Manufacturing of water-absorbing resin (Manufacturing Examples 35-51) 35 g of the water-soluble polymer (wet product containing water and methanol, with a moisture content of 65%) obtained by Manufacturing Example 18 was placed into a 300 ml beaker, and then 100 ml of water-containing methanol with a methanol / water ratio of 80 / 20 (volume ratio) was added to disperse the water-soluble polymer. While stirring with a magnetic stirrer, 3.7 ml of 1 N hydrochloric acid was slowly added using a pipette. After adding hydrochloric acid, the mixture was stirred for 15 minutes, and then filtered under reduced pressure to recover part of the water-soluble polymer that had been neutralized into free acid. The recovered water-soluble polymer was a wet crystal containing water and alcohol. The wet crystal was transferred to a petri dish and placed in a blower dryer set to 70°C for 12 hours to perform crosslinking treatment. During the drying process, methanol initially evaporates, and the water-soluble polymer temporarily dissolves in water, becoming maltose-like. Therefore, after drying, it becomes a sponge-like solid. The sponge-like solid is pulverized using a mortar and pestle and then sieved using 150 μm and 850 μm mesh sieves to recover particles with a diameter of 150–850 μm.

[0138] The water-absorbing resins of Manufacturing Examples 36 to 51 were obtained by performing the same operation as in Manufacturing Example 35, except that the raw materials and addition amounts were changed to those listed in Table 3.

[0139] [Table 3] Water-absorbing resin Manufacturing Example 35 Manufacturing Example 36 Manufacturing Example 37 Manufacturing Example 38 Manufacturing Example 39 Manufacturing Example 40 Manufacturing Example 41 Manufacturing Example 42 Manufacturing Example 43 Manufacturing Example 44 Manufacturing Example 45 Manufacturing Example 46 Manufacturing Example 47 Manufacturing Example 48 Manufacturing Example 49 Manufacturing Example 50 Manufacturing Example 51 Water-soluble polymers Manufacturing Example 18 Manufacturing Example 19 Manufacturing Example 20 Manufacturing Example 21 Manufacturing Example 21 Manufacturing Example 22 Manufacturing Example 23 Manufacturing Example 24 Manufacturing Example 24 Manufacturing Example 25 Manufacturing Example 26 Manufacturing Example 27 Manufacturing Example 28 Manufacturing Example 29 Manufacturing Example 30 Manufacturing Example 31 Manufacturing Example 32 Weight (g) 35 45 50 27 25 45 45 35 35 25 45 45 45 45 20 25 25 Moisture content (%) 65 69 69 64 64 66 66 65 65 67 68 63 73 60 69 66 66 Aqueous methanol (ml) 100 120 125 123 122 120 123 123 122 123 122 121 123 121 123 122 83 1N hydrochloric acid (ml) 3.7 4.9 2.3 1.7 3.1 4.8 2.3 2.4 3.5 1.8 3.0 4.3 2.5 3.6 1.6 2.8 1.0

[0140] (Manufacturing Example 52) 20 g of the water-soluble polymer (wet product containing aqueous acetone, moisture content 61%) obtained by Manufacturing Example 33 was added to a 300 ml beaker, and then 60 ml of aqueous acetone with an acetone / water ratio of 90 / 10 (volume ratio) was added to disperse the water-soluble polymer. While stirring with a magnetic stirrer, 19.3 ml of 1 N NaOH was slowly added using a pipette. After adding the NaOH aqueous solution, the mixture was stirred for 15 minutes, and then filtered under reduced pressure to recover a portion of the carboxylic acid introduced by maleic anhydride that was neutralized to sodium salt. The recovered water-soluble polymer was a wet crystal containing aqueous acetone. The wet crystal was transferred to a petri dish and placed in a blower dryer set to 70°C for 12 hours to perform crosslinking treatment. The solid obtained after treatment was crushed using a mortar and pestle, and then sieved using sieves with a mesh size of 150 μm and 850 μm to recover particles with a particle size of 150–850 μm.

[0141] (Manufacturing Example 53) 25 g of the water-soluble polymer (wet product containing water and methanol, with a moisture content of 63%) (wet crystals) obtained by Manufacturing Example 34 was transferred to a petri dish and placed in an air-cooled dryer set to 70°C for 12 hours to perform crosslinking treatment. After treatment, the obtained solid was pulverized using a mortar and pestle and sieved using sieves with mesh sizes of 150 μm and 850 μm to recover particles with a particle size of 150 to 850 μm.

[0142] (Manufacturing Example 56) 25 g of the water-soluble polymer (wet product containing aqueous ethanol, moisture content 61%) obtained by Manufacturing Example 55 was placed into a 300 ml beaker, and then 80 ml of aqueous ethanol with a volume ratio of 90 / 10 (ethanol / water) was added to disperse the water-soluble polymer. While stirring with a magnetic stirrer, 12.1 ml of 1 N hydrochloric acid was slowly added using a pipette. After adding hydrochloric acid, the mixture was stirred for 15 minutes, and then filtered under reduced pressure to recover part of the water-soluble polymer that had been neutralized into free acid. The recovered water-soluble polymer was a wet crystal containing aqueous ethanol. The wet crystal was transferred to a petri dish and placed in a blower dryer set to 150°C for 1 hour to perform crosslinking treatment. The solid obtained after treatment was pulverized using a mortar and pestle and sieved using 150 μm and 850 μm mesh sieves to recover particles with a particle size of 150–850 μm.

[0143] (5) Manufacturing of water-absorbing resin (Comparative manufacturing examples 1-2) Following the order disclosed in Example 2 of Patent Document 1 (US Patent No. 5079354), unde-molecularized corn starch was carboxymethylated and heated and dried under alkaline conditions to manufacture the water-absorbing resin of Comparative Manufacturing Example 1.

[0144] Following the order disclosed in Example 1 (paragraphs

[0146] to

[0147] ) of Patent Document 2 (Japanese Patent Publication No. 2010-504414), unreduced potato starch was crosslinked with epichlorohydrin and then carboxymethylated to produce the water-absorbing resin (obtained by hydrochloric acid treatment) of Comparative Manufacturing Example 2.

[0145] (6) Viscosity Evaluation Method for Partially Decomposed Starch: The viscosity of partially decomposed starch when mixed with NaOH aqueous solution is evaluated based on the following criteria. The mixing operation is performed as follows: In a 500 ml separable flask, using a device with a glass anchored stirring blade (blade diameter of 60 mm) mounted on a mechanical stirrer (Sany Motor BL600 manufactured by Shin-Tung Science & Technology Co., Ltd.), the addition ratios described in Manufacturing Examples 15-28 are used. The evaluation results are shown in Table 4. ○: The liquid viscosity is low, and it can be mixed uniformly. Δ: The liquid viscosity is high, or it becomes highly viscous during mixing, so it cannot be mixed uniformly. ×: The liquid viscosity is high, or it becomes highly viscous during mixing, so it cannot be mixed.

[0146] (7) Evaluation Method for Water-Soluble Polymers (7-1) Total Acid Value of Water-Soluble Polymers Here, the method for determining the total acid value of water-soluble polymers having carboxymethyl groups as acidic groups is explained. Accurately weigh approximately 0.3 g of the water-soluble polymer into a 100 ml beaker and dissolve it using 40 ml of deionized water. Place the aqueous solution into a potentiometric titration apparatus (AT-610 manufactured by Kyoto Electronics Industry Co., Ltd.) equipped with a glass electrode (C-171 manufactured by Kyoto Electronics Industry Co., Ltd.). When the sample is entirely sodium salt, the potential appears to be approximately 30 mV or less at this stage. Therefore, add 1 N hydrochloric acid until the potential reaches 320 mV or more, so that all carboxylic acid groups in the water-soluble polymer become free acid (becomes a state of excess hydrochloric acid). Once the potential reaches 320 mV or more, perform a neutralization titration using a 0.1 N NaOH aqueous solution. In this titration, two inversion points were detected: the first inversion point was detected near 220 mV, and the second inversion point was detected near 0 to -30 mV. The former is the neutralization point of excess hydrochloric acid in the sample, and the latter is the neutralization point of carboxylic acid in the water-soluble polymer. Therefore, the total acid value was calculated using the following formula 1: Total acid value (mgKOH / g) = [{(Vb-Va)×0.1×fa×56.11}÷Sa] / (1-wr) (Formula 1) Here, Va is the volume (ml) of 0.1 N NaOH consumed before reaching the first inversion point, Vb is the volume (ml) of 0.1 N NaOH consumed before reaching the second inversion point, fa is the valence of 0.1 N NaOH, and Sa is the sample volume. wr is the moisture content of the water-soluble polymer as determined by the following method.

[0147] (7-2)The degree of etherification of the water-soluble polymer is calculated using the total acid value mentioned above, using the following formula 2. Degree of etherification = (162×TAV) ÷ (56100-80×TAV) (Formula 2) Here, TAV is the total acid value of the water-soluble polymer (unit is mgKOH / g).

[0148] (7-3) Moisture Content of Water-Soluble Polymer Moisture content refers to the percentage (%) of weight reduction relative to the initial weight of the sample when dried at a drying temperature of 130°C, obtained using a halogen moisture meter. In this embodiment, 0.5–1.0 g of water-soluble polymer was placed in a halogen moisture meter HC103 manufactured by Mettler-Toledo Corporation, and measured at a drying temperature of 130°C, a shutdown reference of 1 mg / 50 seconds, and %MC mode (displaying MC value = (initial sample weight – dried weight) ÷ initial sample weight × 100). The displayed MC value was set as the moisture content.

[0149] (7-4) Free Acid Value of Water-Soluble Polymer The free acid value is the acid value defined when crosslinking of a water-soluble polymer is performed by acid treatment. In the examples, the crosslinked polymer is manufactured after acid treatment, making it difficult to directly quantify by titration. Therefore, the value calculated by the following calculation is defined as the free acid value. In the acid treatment, an acid with a lower pKa than the carboxylic acid of the water-soluble polymer is used. Therefore, the amount of acid added during acid treatment is substantially equal to the free acid value. Therefore, the free acid value is calculated using the following formula 3. Free acid value (mgKOH / g) = (Vc × N × fb × 56.11) ÷ Sb (Formula 3) Here, Vc is the volume (ml) of the acid solution used in the acid treatment, N is the equivalent concentration of the acid solution, fb is the valence of the acid solution, and Sb is the weight (purity) of the water-soluble polymer added in the acid treatment.

[0150] (8) Evaluation method of water-absorbing resin (8-1) Water absorption ratio under no pressure (physiological saline) 1.0 g of the test sample was placed into a tea bag (20 cm long and 10 cm wide) made of nylon mesh with a mesh size of 63 μm (JIS Z8801-1:2006), and immersed in 1,000 ml of physiological saline (salt concentration of 0.9% by weight) for 3 hours without stirring. Then, it was hung for 10 minutes to drain the water. The weight (h1) including the tea bag was measured, and the water retention was calculated according to the following formula. In addition, the physiological saline used and the temperature of the test environment were 25℃±2℃. FSC (g / g)=(h1)-(h2) In addition, (h2) is the weight of the tea bag measured by the same operation as above in the case where there is no test sample. Here, FSC is short for Free Swell Capacity, which refers to the water absorption ratio without pressure.

[0151] (8-2) Water absorption ratio under no pressure (ion-exchanged water) Except that 0.2 g of the test sample was placed into a tea bag and ion-exchanged water was used instead of physiological saline, the weight (h1') of the tea bag after soaking was measured in the same manner as for the water absorption ratio under no pressure (physiological saline), and the water retention was calculated according to the following formula. Furthermore, (h2') is the weight of the tea bag measured by the same procedure as above in the case where no test sample is available. FSC (g / g) = {(h1') - (h2')} / 0.2

[0152] (8-3) Water Retention Capacity (physiological saline) After determining the water absorption ratio under no pressure as described above, the tea bag and the tea bag were placed in a centrifuge and centrifuged at 150 G for 90 seconds to remove the remaining liquid components. The weight (h3) including the tea bag was measured, and the water retention capacity was calculated according to the following formula: CRC (g / g) = (h3) - (h4) Furthermore, (h4) is the weight of the tea bag measured by the same operation as above in the case where no sample was measured. Here, CRC is an abbreviation for Centrifuge Retention Capacity, which represents the centrifugation retention capacity, referring to the water retention rate.

[0153] (8-4)Water Retention Rate (Ion-Exchanged Water) After determining the water absorption ratio under no pressure as described above, the tea bag and the tea bag were placed in a centrifuge and centrifuged at 150 G for 90 seconds to remove the remaining liquid components. The weight (h3') including the tea bag was measured, and the water retention was calculated according to the following formula: CRC (g / g)={(h3')-(h4')} / 0.2 Furthermore, (h4') is the weight of the tea bag measured by the same operation as above in the case where no sample was measured.

[0154] (8-5) Alkali decomposition: Approximately 5 g of the absorbent gel, after undergoing a water retention test using physiological saline, was placed into a 200 ml beaker, and a few drops of 48.8% NaOH aqueous solution were added on top. Then, the mixture was stirred using a stainless steel pharmaceutical spoon to spread it thinly at the bottom of the beaker, and the dissolution state was visually confirmed. When the gel disintegrates, the insoluble matter disappears, and a clear solution is formed, it is evaluated as "dissolved." When the gel state is maintained or some insoluble matter remains, it is evaluated as "insoluble."

[0155] (9) Evaluation of water-absorbing resins (Examples 1-20, Comparative Examples 1-2) The water absorption properties and alkali decomposition properties of each water-absorbing resin obtained by manufacturing examples 35-53, manufacturing example 56, and comparative manufacturing examples 1-2 were measured. The results are shown in Table 4.

[0156] [Table 4] Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Example 11 Example 12 Example 13 Example 14 Example 15 Example 16 Example 17 Example 18 Example 19 Example 20 Comparative Example 1 Comparative Example 2 Water-soluble polymers Manufacturing Example 18 Manufacturing Example 19 Manufacturing Example 20 Manufacturing Example 21 Manufacturing Example 21 Manufacturing Example 22 Manufacturing Example 23 Manufacturing Example 24 Manufacturing Example 24 Manufacturing Example 25 Manufacturing Example 26 Manufacturing Example 27 Manufacturing Example 28 Manufacturing Example 29 Manufacturing Example 30 Manufacturing Example 31 Manufacturing Example 32 Manufacturing Example 33 Manufacturing Example 34 Manufacturing Example 55 Comparative Manufacturing Example 1 Comparative Manufacturing Example 2 Water-absorbing resin Manufacturing Example 35 Manufacturing Example 36 Manufacturing Example 37 Manufacturing Example 38 Manufacturing Example 39 Manufacturing Example 40 Manufacturing Example 41 Manufacturing Example 42 Manufacturing Example 43 Manufacturing Example 44 Manufacturing Example 45 Manufacturing Example 46 Manufacturing Example 47 Manufacturing Example 48 Manufacturing Example 49 Manufacturing Example 50 Manufacturing Example 51 Manufacturing Example 52 Manufacturing Example 53 Manufacturing Example 56 Types of starch corn starch corn starch corn starch corn starch corn starch Cassava Cassava Cassava Cassava Cassava Waxy corn Waxy corn Waxy corn potato corn starch corn starch corn starch corn starch corn starch corn starch corn starch potato Cross-linking (acid or base) HCl HCl HCl HCl HCl HCl HCl HCl HCl HCl HCl HCl HCl HCl HCl HCl HCl NaOH NaOH HCl none (epicochlorohydrin) Starch partial hydrolysate 1 Manufacturing Example 1 Manufacturing Example 2 Manufacturing Example 3 Manufacturing Example 4 Manufacturing Example 4 Manufacturing Example 5 Manufacturing Example 6 Manufacturing Example 7 Manufacturing Example 7 Manufacturing Example 7 Manufacturing Example 8 Manufacturing Example 8 Manufacturing Example 9 Manufacturing Example 10 Manufacturing Example 11 Manufacturing Example 12 Manufacturing Example 15 Manufacturing Example 16 Manufacturing Example 17 Manufacturing Example 54 - - Usage (by weight) 100 100 100 100 100 100 100 100 100 90 100 100 100 100 90 100 100 100 100 100 - - Mw (×10) 6 ) 0.81 0.66 0.72 1.46 1.46 0.20 1.18 1.96 1.96 1.96 0.61 0.61 1.33 1.53 1.46 1.04 6.07 1.34 1.37 1.91 - - Mw / Mn 6.4 7.4 8.7 9.8 9.8 2.9 22.6 8.0 8.0 8.0 35.2 35.2 19.1 9.9 9.8 10.6 35.4 10.6 65.6 10.1 - - Starch partial hydrolysate 2 - - - - - - - - - Manufacturing Example 13 - - - - Manufacturing Example 14 - - - - - - - Usage (by weight) 0 0 0 0 0 0 0 0 0 10 0 0 0 0 10 0 0 0 0 0 - - Mw (×10)6 ) - - - - - - - - - 0.19 - - - - 0.27 - - - - - - - Mw / Mn - - - - - - - - - 2.94 - - - - 4.40 - - - - - - - Counteracting ions Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Sodium Potassium Sodium Sodium Sodium Sodium Sodium Sodium Physical properties of water-soluble polymers Mw (×10) 6 ) 3.4 2.0 2.3 4.0 4.0 0.8 5.3 11.5 11.5 9.4 1.6 2.3 4.5 9.3 3.4 1.9 45.6 3.1 3.7 4.8 - - Mw / Mn 9.3 8.7 8.5 7.2 7.2 10.1 16.8 19.3 19.3 25.3 7.4 22.3 14.6 20.9 16.1 11.2 104.9 39.6 13.8 14.0 - - Total acid value (mgKOH / g) 182 170 180 161 161 142 165 170 170 172 71 192 191 162 134 138 154 90 138 145 - - Degree of etherification 0.71 0.65 0.70 0.60 0.60 0.51 0.62 0.65 0.65 0.66 0.23 0.76 0.76 0.61 0.48 0.50 0.57 0.32 (*1) 0.57 (*2) 0.64 - - Free acid value (mgKOH / g) 17 twenty two 11 11 twenty two 17 8 8 11 11 11 17 8 17 11 17 11 17 37 11 - - viscosity ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Δ Δ Water absorption performance FSC (Follicular Unit Extraction) 179 140 216 227 156 212 268 331 185 289 152 171 251 228 216 233 143 40 160 62 69 292 CRC (Ion Exchange Water) 147 112 131 160 127 154 172 186 152 184 122 130 145 182 160 140 112 twenty one 63 39 twenty one 81 FSC (physiological saline) 43 36 34 51 41 42 63 64 44 58 twenty three 29 53 48 52 47 27 11 25 20 15 29 CRC (Rectal Coronary Reproductive Technology) 39 33 29 44 38 36 55 50 41 51 twenty one 27 42 44 46 39 twenty four 7 twenty two 14 5 26 Alkali decomposition Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Dissolve Insoluble Insoluble *1: The degree of esterification of maleic anhydride is calculated as (162 * total acid value (mgKOH / g)) / (56100 - 120 * total acid value (mgKOH / g)). *2: The degree of esterification of succinic anhydride is calculated as (162 * total acid value (mgKOH / g)) / (56100 - 122 * total acid value (mgKOH / g))

[0157] As shown in Table 4, the water-absorbing resins of Examples 1 to 20 have superior water absorption properties compared with Comparative Examples 1 to 2.

[0158] Furthermore, the water-soluble polymers used as raw materials in Examples 1-20 have lower viscosity than those in Comparative Examples 1-2, resulting in excellent operability when manufacturing water-absorbing resins. Higher viscosity makes it more difficult to mix uniformly, which may limit the choice of manufacturing equipment or lead to inconsistent quality. In addition, in order to mix uniformly, it is necessary to increase the degree of dilution, which may reduce manufacturing efficiency. [Simplified Explanation of the Diagram]

[0026] None

Claims

1. A water-absorbing resin comprising a crosslinked polymer containing a water-soluble polymer having an acidic group having a carboxyl alkyl group having 1 to 5 carbon atoms or a carboxyl alkenyl group having 2 to 4 carbon atoms, wherein the starch partial decomposition product having a weight average molecular weight of 7.5 million or less is introduced.

2. The absorbent resin as described in claim 1, wherein, The weight average molecular weight of the partially decomposed starch products is over 50,000.

3. The absorbent resin as described in claim 1 or 2, wherein, The dispersion (weight average molecular weight / number average molecular weight) of the starch partial decomposition products is 5 or higher.

4. The absorbent resin as described in claim 1 or 2, wherein, The weight-average molecular weight of pullulan obtained from aqueous size exclusion chromatography analysis of water-soluble polymers ranges from 500,000 to 50 million.

5. The absorbent resin of claim 1 or 2 has the following characteristics: (a) the water absorption ratio of ion-exchanged water under no pressure is 100 to 400 g / g; (b) the water retention ratio of ion-exchanged water is 80 to 300 g / g; (c) the water absorption ratio of physiological saline under no pressure is 20 to 70 g / g; and / or (d) the water retention ratio of physiological saline is 7 to 60 g / g.

6. For the absorbent resin of claim 1 or 2, the ratio (A / B) of its absorbency of ion-exchanged water under no pressure (A) to its absorbency of physiological saline under no pressure (B) is 7 or less.

7. The absorbent resin as claimed in claim 1 or 2, wherein, The content of water and / or hydrophilic solvents is 0.1% to 10%.

8. The absorbent resin of claim 1 or 2 does not have an internal cross-linked structure formed by covalent bonds.

9. A method for decomposing a water-absorbing resin, comprising the step of alkali treatment of the water-absorbing resin as claimed in any one of claims 1 to 8.

10. An absorbent article comprising an absorbent resin as claimed in any one of claims 1 to 8.

11. A water-soluble polymer, which is formed by introducing an acidic group having a carboxyl alkyl group having 1 to 5 carbon atoms or a carboxyl alkenyl group having 2 to 4 carbon atoms into a starch partial decomposition product having a weight average molecular weight of less than 7.5 million.

12. The water-soluble polymer as claimed in claim 11, wherein, The dispersion (weight average molecular weight / number average molecular weight) of the starch partial decomposition products is 5 or higher.

13. The water-soluble polymer as claimed in claim 11 or 12, wherein, The weight-average molecular weight (Mw) obtained by pullulan conversion using aqueous size exclusion chromatography is 500,000 to 50,000,000.

14. A resin composition for manufacturing a water-absorbing resin, comprising a water-soluble polymer as claimed in any one of claims 11 to 13.