Method for producing water absorbent resin

Abstract

The purpose of the present invention is to provide a method for producing a water absorbent resin that has high production efficiency and excellent water absorbing capability. The present invention pertains to a method for producing a water absorbent resin that forms a physical gel through water absorption, the method comprising: a step (a1) for obtaining a partial degradation product of starch by lowering the molecular weight of starch; a step (a2) for obtaining a water-soluble polymer by introducing an acidic group to the partial degradation product of starch obtained in the step (a1); and a step (a3) for drying the water-soluble polymer in the presence of water at 70-180°C so as to achieve a solid content of 90% or more.

Description

Manufacturing method of water-absorbent resin

[0001] The present invention relates to a method for producing a water-absorbent resin.

[0002] Water-absorbent resins are widely used in various fields such as sanitary products, food, agriculture and forestry, civil engineering, etc. As such water-absorbent resins, partially neutralized salts of polyacrylic acid and polymethacrylic acid are widely used, and water-absorbent resins made from polysaccharides such as starch as raw materials are also known.

[0003] Patent Document 1 discloses a method for producing a water-absorbent resin by heating and drying carboxyalkylated starch to crosslink the starch particles, and describes that it is preferable to suppress a reduction in the molecular weight of the starch during the carboxyalkylation reaction.

[0004] Patent Document 2 discloses a method for producing a water-absorbent resin by surface-treating carboxyalkylated polysaccharide particles with a non-crosslinking acid such as hydrochloric acid, and then crosslinking the polysaccharides together by heating and drying or by using a crosslinking agent.

[0005] Patent Document 3 discloses a method for producing a water-absorbing material by reacting starch with an acid anhydride of a polybasic acid in an extruder, and describes that it is preferable to suppress reduction in the molecular weight of the starch during the reaction with the acid anhydride of the polybasic acid.

[0006] U.S. Patent No. 5,079,354, JP-A-2010-504414, JP-A-2007-222704

[0007] Conventional water-absorbent resins made from polysaccharides have not been sufficient in water absorption performance. Furthermore, because they are made from high-molecular-weight polysaccharides, the viscosity of the raw materials is high, which makes it difficult to operate during production. The present invention aims to provide a method for producing a water-absorbent resin with high production efficiency and excellent water absorption performance.

[0008] The present inventors have focused on the molecular weight and crosslinking conditions of starch used as a raw material for water-absorbent resins and have completed the present invention.

[0009] That is, the present invention relates to a method for producing a water-absorbent resin that forms a physical gel upon absorbing water, the method comprising: a step (a1) of lowering the molecular weight of starch to obtain a partial hydrolysis product of starch; a step (a2) of introducing an acidic group into the partial hydrolysis product of starch obtained in the step (a1) to obtain a water-soluble polymer; and a step (a3) ​​of drying the water-soluble polymer in the presence of water at 70 to 180°C until the water-soluble polymer has a solid content of 90% or more.

[0010] It is preferable to include a step (aa) of desalting the water-soluble polymer after the step (a2).

[0011] It is preferable to include a step (ab) of partially neutralizing the acidic groups of the water-soluble polymer in the step (a2), or after the step (a2) and before the step (a3).

[0012] It is preferable that the method does not include a step of partially neutralizing the acidic groups of the water-soluble polymer after the step (a2) and before the step (a3).

[0013] In the step (a3), it is preferable to dry the mixture until the solid content reaches 90% or more and 99% or less.

[0014] The acidic group is preferably a carboxyalkyl group, a carboxyalkenyl group, or a sulfoalkyl group.

[0015] In the step (a1), it is preferable to obtain a partially hydrolyzed starch product having a weight average molecular weight (Mw) of 7.5 million or less and / or a dispersity (weight average molecular weight (Mw) / number average molecular weight (Mn)) of 5 or more.

[0016] In the step (a1), the molecular weight of the starch is preferably reduced by an enzyme treatment.

[0017] In the step (a2), it is preferable to introduce an acidic group into the mixture of the partially hydrolyzed starch obtained in the step (a1) and the starch that has not been degraded.

[0018] The water-soluble polymer obtained in the step (a2) preferably has a pullulan-equivalent weight average molecular weight (Mw) of 500,000 to 40,000,000 as determined by aqueous size exclusion chromatography.

[0019] The obtained water-absorbent resin preferably has the following characteristics: (a) a water absorption capacity of ion-exchanged water under no pressure of 100 to 500 g / g, (b) a water retention rate of ion-exchanged water of 80 to 300 g / g, (c) a water absorption capacity of physiological saline under no pressure of 10 to 70 g / g, and / or (d) a water retention rate of physiological saline of 5 to 65 g / g.

[0020] According to the method for producing a water-absorbent resin of the present invention, a water-absorbent resin having excellent water-absorbing properties can be obtained with high production efficiency.

[0021] 1 is a photograph of a water-absorbent resin in a gel state.

[0022] <<Method 1 for producing water-absorbent resin>> The method for producing a water-absorbent resin of the present invention includes a step (a1) of lowering the molecular weight of starch to obtain a partial hydrolysis product of starch, a step (a2) of introducing an acidic group into the partial hydrolysis product of starch obtained in the step (a1) to obtain a water-soluble polymer, and a step (a3) ​​of drying the water-soluble polymer in the presence of water at 70 to 180°C until the water-soluble polymer has a solid content of 90% or more, and is characterized by producing a water-absorbent resin that forms a physical gel by absorbing water.

[0023] <Step (a1) of reducing the molecular weight of starch to obtain a partial hydrolysis product of starch> In this step, the molecular weight of starch is reduced to reduce the viscosity. The starch used as the raw material is not particularly limited, and examples thereof include waxy corn starch, tapioca starch, potato starch, corn starch (including waxy corn starch and high-amylose starch), wheat starch, rice starch, and sweet potato starch.

[0024] The starch may be a derivative (also called processed starch or chemically modified starch) in which the hydrogen atoms of some of the hydroxyl groups in the glucose unit have been substituted with functional groups, or bleached starch. The position of the hydroxyl group where the hydrogen atom is substituted may be any of the 1st, 2nd, 3rd, 4th, and 6th positions of the glucose unit, with the 2nd, 3rd, and 6th positions being preferred. Examples of functional groups include hydrocarbon groups such as methyl and ethyl groups; substituents having a hydroxyl group such as hydroxypropyl and hydroxyethyl groups; substituents having a carboxyl group such as carboxymethyl groups; and acyl groups such as formyl, acetyl, and propionyl groups.

[0025] Specific examples of starch derivatives include acetylated starch, acetylated adipic acid cross-linked starch, acetylated phosphate cross-linked starch, oxidized acetylated starch, sodium octenyl succinate starch, starch acetate, oxidized starch, hydroxypropyl starch, hydroxypropyl phosphate cross-linked starch, phosphate monoesterified phosphate cross-linked starch, phosphated starch, phosphate cross-linked starch, cationized starch, and urea phosphate esterified starch, which are used in food and industrial applications and are known as so-called modified starches. Also usable are methylethylated starch and hydroxypropylmethylated starch, which are substituted with two or more functional groups. Among these, starch acetate, oxidized starch, acetylated starch, oxidized acetylated starch, hydroxypropylated starch, hydroxypropylated phosphate cross-linked starch, and sodium octenyl succinate starch, which are preferred because of the ease of controlling the properties of the water-absorbent resin.

[0026] The method for reducing the molecular weight of starch is not particularly limited, and examples thereof include enzyme treatment, acid treatment, physical crushing, etc. These methods may also be combined. It is preferable to hydrolyze part of the glucosidic bonds of the α-glucose molecules that constitute the starch using these methods, but there are no restrictions on the position or manner of decomposition. A reaction kettle, extruder, etc. can be used as the reaction apparatus.

[0027] When the molecular weight of starch is reduced by enzymatic treatment, the enzyme to be used is not particularly limited, but it is preferable to use an endo-enzyme to efficiently reduce the molecular weight. Specific examples of enzymes include α-amylase, cyclomaltodextrin glucanotransferase, 4-α-glucanotransferase, 6-α-glucanotransferase, 4,6-α-glucanotransferase, amylomaltase, neopullulanase, and amylopullulanase. These enzymes may also be used in combination. The pH during the enzymatic treatment is not particularly limited, but a pH of 5.0 to 7.0 is preferred. The pH can be adjusted by adding hydrochloric acid, acetic acid, sodium hydroxide, potassium hydroxide, or the like. The enzymatic treatment may be performed while gelatinizing the starch by heating and kneading it at 70 to 110°C. The enzymatic treatment may also be performed after or simultaneously with starch gelatinization. A method of carrying out the enzyme treatment after starch gelatinization includes first suspending the starch in water and heating it to gelatinize it, and then adding an enzyme to carry out the enzymatic reaction.A method of carrying out the enzyme treatment simultaneously with starch gelatinization includes suspending the starch in water, adding the enzyme, and heating the mixture within a temperature range in which the enzyme is not completely inactivated.

[0028] When the molecular weight of starch is reduced by acid treatment, the acid to be used is not particularly limited, but specific examples include hydrochloric acid, sulfuric acid, oxalic acid, acetic acid, formic acid, trifluoroacetic acid, etc. The temperature during the acid treatment is preferably 150 to 160°C.

[0029] When partial decomposition of starch is carried out by physical crushing, specific means include irradiation, shearing, grinding, high-pressure treatment, ultrasonic waves, thermal decomposition, photolysis, and combinations thereof.

[0030] The weight-average molecular weight of the partial starch hydrolysate obtained in step (a1) is preferably 7.5 million or less, more preferably 6 million or less. If the weight-average molecular weight exceeds 7.5 million, the viscosity increases, and the reaction during the introduction of acidic groups in step (a2) and the operability during purification tend to decrease. There are no particular restrictions on the lower limit of the weight-average molecular weight of the partial starch hydrolysate, 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, the water retention capacity of the water-absorbent resin tends to decrease. The method for measuring the weight-average molecular weight is not particularly limited, but it can be determined, for example, in aqueous size exclusion chromatography based on a calibration curve of molecular weight and elution time prepared using pullulan with a known molecular weight.

[0031] The number-average molecular weight of the partial starch hydrolysate obtained in step (a1) is not particularly limited, but is preferably 1,000,000 or less, taking viscosity into consideration. The lower limit of the number-average molecular weight of the partial starch hydrolysate is preferably 10,000 or more, more preferably 50,000 or more. The method for measuring the number-average molecular weight is not particularly limited, but for example, it can be determined by aqueous size exclusion chromatography based on a calibration curve of molecular weight vs. elution time prepared using pullulan with a known molecular weight.

[0032] The weight-average molecular weight or number-average molecular weight of the partial hydrolyzate obtained in step (a1) may be adjusted by mixing two or more types of partial hydrolyzates of starch. In this case, it is preferable that the weight-average molecular weight or number-average molecular weight of the mixture falls within the above-mentioned numerical range.

[0033] The degree of dispersion (weight average molecular weight / number average molecular weight) of the partial starch hydrolyzate obtained in step (a1) is preferably at least 5, more preferably at least 7. As long as the weight average molecular weight falls within the above-mentioned range, the upper limit of the degree of dispersion is not particularly limited, but is usually 70 or less.

[0034] In step (a1), starch that has not been degraded may remain. When starch that has not been degraded remains, the amount of the remaining starch is preferably less than 20% by weight based on the total amount of starch charged in step (a1).

[0035] <Step (a2) of Obtaining a Water-Soluble Polymer> In this step, acidic groups are introduced into the partial starch hydrolyzate obtained in step (a1) to obtain a water-soluble polymer. The water-soluble polymer is a partial starch hydrolyzate into which acidic groups have been introduced. The introduced acidic groups may be in any state of free acid, salt, or ionized, or these may coexist.

[0036] In this step, acidic groups may be introduced into a mixture of the starch partial hydrolysis product obtained in step (a1) and non-degraded starch. The non-degraded starch may be starch that was subjected to the molecular weight reduction reaction in step (a1) but was not degraded to a weight-average molecular weight (Mw) of 7.5 million or less, or may be untreated starch added after step (a1). The presence of non-degraded starch can increase the water absorption capacity when made into a water-absorbent resin. This is thought to be because the presence of a water-soluble polymer derived from non-degraded starch increases the distance between crosslinking points of the physical gel, making it easier for the crosslinked network to expand. In this step, two or more types of starch and / or partial hydrolysis products of starch with different weight-average molecular weights may be used in combination depending on the application and acceptable costs.

[0037] Examples of the acidic group include acidic groups having a carboxyl group, such as a carboxyalkyl group or a carboxyalkenyl group; acidic groups having a sulfo group, such as a sulfoalkyl group or a sulfoalkenyl group; and acidic groups having a phospho group, such as a phosphoalkyl group or a phosphoalkenyl group.

[0038] A carboxyalkyl group is an alkyl group substituted with a carboxyl group. The number of carbon atoms in the alkyl group substituted with a carboxyl group is preferably 1 to 8, and more preferably 1 to 5. The alkyl group may be either linear or branched. Specific examples of the alkyl group include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, an s-butyl group, a t-butyl group, an n-pentyl group, a 1-methyl-n-butyl group, a 2-methyl-n-butyl group, a 3-methyl-n-butyl group, a 1,1-dimethyl-n-propyl group, a 1,2-dimethyl-n-propyl group, a 2,2-dimethyl-n-propyl group, and a 1-ethyl-n-propyl group.

[0039] Specific examples of the carboxyalkyl group include a carboxymethyl group, a carboxyethyl group, a carboxypropyl group, a carboxybutyl group, and a carboxypentyl group.

[0040] A carboxyalkenyl group is an alkenyl group substituted with a carboxyl group. The alkenyl group substituted with a carboxyl group preferably has 2 to 8 carbon atoms, more preferably 2 to 4 carbon atoms. The alkenyl group may be either linear or branched. Specific examples of the alkenyl group include ethenyl, 1-propenyl, 2-propenyl, 1-methyl-1-ethenyl, 1-butenyl, 2-butenyl, 3-butenyl, 2-methyl-1-propenyl, 2-methyl-2-propenyl, 1-ethylethenyl, 1-methyl-1-propenyl, and 1-methyl-2-propenyl.

[0041] Specific examples of the carboxyalkenyl group include a carboxyethenyl group, a carboxypropenyl group, and a carboxybutenyl group.

[0042] A sulfoalkyl group is an alkyl group substituted with a sulfo group. Examples of the alkyl group substituted with a sulfo group include the alkyl groups mentioned in relation to the carboxyalkyl group. Specific examples of sulfoalkyl groups include a sulfomethyl group, a sulfoethyl group, and a sulfopropyl group.

[0043] The sulfoalkenyl group is an alkenyl group substituted with a sulfo group. The alkenyl group substituted with a sulfo group can be any of the alkenyl groups described above for the carboxyalkenyl group. Specific examples of the sulfoalkenyl group include a sulfoethenyl group and a sulfopropenyl group.

[0044] A phosphoalkyl group is an alkyl group substituted with a phospho group. The alkyl groups substituted with a phospho group can be the same as those mentioned for the carboxyalkyl group. Specific examples of the phosphoalkyl group include a phosphomethyl group, a phosphoethyl group, and a phosphopropyl group.

[0045] A phosphoalkenyl group is an alkenyl group substituted with a phospho group. The alkenyl group substituted with a phospho group can be any of the alkenyl groups described above for the carboxyalkenyl group. Specific examples of phosphoalkenyl groups include phosphoethenyl and phosphopropenyl groups.

[0046] Of the acidic groups, acidic groups having a carboxyl group or a sulfo group are preferred, with carboxyalkyl groups, carboxyalkenyl groups and sulfoalkyl groups being more preferred, and carboxyalkyl groups having 1 to 5 carbon atoms being even more preferred.

[0047] To introduce acidic groups, a partial hydrolysis product of starch is reacted with an acidic group-containing compound or its precursor. The acidic group-containing compound is not particularly limited as long as it can introduce the aforementioned acidic group, and examples thereof include haloalkyl compounds having an acidic group, haloalkenyl compounds having an acidic group, acid anhydrides, and salts thereof. Examples of halogen atoms constituting haloalkyl compounds and haloalkenyl compounds include chlorine and bromine.

[0048] Specific examples of the acidic group-containing compound include monochloroacetic acid, monobromoacetic acid, 3-bromopropionic acid, 6-bromohexanoic acid, succinic anhydride, maleic anhydride, vinyl sulfonic acid, phosphorus oxychloride, ethyl monochloroacetate, and sodium salts and potassium salts thereof.

[0049] Examples of the precursor of the acidic group-containing compound include acrylonitrile, etc. Examples of the method using acrylonitrile include a method in which acrylonitrile is first reacted with starch or a partial decomposition product thereof under basic conditions to introduce a cyanoethyl group, which is then converted into an amide group (Synthesis; 1989(12):949-950), and the resulting amide is then subjected to alkaline hydrolysis.

[0050] The reaction scheme for producing a water-soluble polymer by reacting a partial starch hydrolysate with monochloroacetic acid is shown in formula (I).

[0051] The reaction scheme for producing a water-soluble polymer by reacting a partial starch hydrolyzate with 3-bromopropionic acid is shown in formula (II).

[0052] The reaction scheme for producing a water-soluble polymer by reacting a partial starch hydrolyzate with 6-bromohexanoic acid is shown in formula (III).

[0053] The reaction scheme for producing a water-soluble polymer by reacting a partial starch hydrolyzate with succinic anhydride is shown in formula (IV).

[0054] The reaction scheme for producing a water-soluble polymer by reacting a partial starch hydrolyzate with maleic anhydride is shown in formula (V).

[0055] The reaction scheme for producing a water-soluble polymer by reacting a partial starch hydrolyzate with sodium vinyl sulfonate is shown in formula (VI).

[0056] Although formulas (I) to (VI) show water-soluble polymers in which sodium salts of acidic groups have been introduced into all hydroxyl groups at the 6-position of glucose units, hydroxyl groups to which no acidic group has been introduced may remain. Furthermore, acidic groups that have not been neutralized by salts may also exist. The position at which the acidic group is introduced is not limited as long as it is a hydroxyl group present in the partial hydrolyzate of starch, and may be any of the 1-, 2-, 3-, 4-, and 6-positions.

[0057] The reaction conditions for the partial starch hydrolysate and the acidic group-containing compound are not particularly limited. However, when a haloalkyl compound having a carboxyl group is used as the acidic group-containing compound, it is preferable to use 1 to 1.5 equivalents of an alkaline agent relative to the acidic group-containing compound. When a haloalkyl compound having a carboxyl group (acidic group), such as monochloroacetic acid or monobromoacetic acid, is used as the acidic group-containing compound, it is preferable to use an alkaline agent required for neutralizing the carboxyl group, i.e., an alkaline agent in the same amount as the carboxyl group. Examples of alkaline agents include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, sodium carbonate, and potassium carbonate. These alkaline agents may be used alone or in combination of two or more. The pH of the reaction between the partial starch hydrolysate and the acidic group-containing compound or its precursor is preferably 4 or higher, more preferably 8 or higher.

[0058] The bond between the partial starch hydrolysate and the acidic group is preferably a covalent bond, specifically an ether bond or an ester bond formed between a hydroxyl group on the partial starch hydrolysate and the alkyl group moiety of the acidic group.

[0059] The acidic groups introduced into the partial hydrolyzate of starch preferably form salts with sodium, potassium, lithium, ammonia, etc. derived from the alkaline agent, and for this reason, it is preferable to use the alkaline agent in an amount necessary for both the reaction between the acidic group-containing compound and the partial hydrolyzate of starch and the neutralization of the acidic groups of the acidic group-containing compound. For example, when monochloroacetic acid is used as the acidic group-containing compound, it is theoretically preferable to use the alkaline agent in an amount of 2 equivalents or more relative to the monochloroacetic acid. When sodium monochloroacetate is used, the acidic groups have been neutralized in advance, so it is preferable to use the alkaline agent in an amount of 1 equivalent or more relative to the sodium monochloroacetate.

[0060] The amount of the acidic group-containing compound used can be set arbitrarily depending on the target total acid value (degree of etherification) of the water-soluble polymer. Typically, 0.5 to 5.0 equivalents are preferred, and 0.5 to 2.0 equivalents are more preferred, per mole of hydroxyl groups in the starch partial hydrolyzate. When a haloalkyl compound such as monochloroacetic acid is used and the reaction is carried out in an aqueous solution, the acidic group introduction reaction and the hydrolysis reaction of the haloalkyl compound compete with each other, so the haloalkyl compound is required in excess of the theoretical amount. The amount of the haloalkyl compound used in the aqueous reaction is preferably set to 5 equivalents or less relative to the theoretical value.

[0061] When a haloalkyl compound or a salt thereof is used as the acidic group-containing compound, the reaction temperature with the partial hydrolysis product of starch is preferably 0 to 100°C, and in order to prevent hydrolysis by water in the reaction solution, the reaction is preferably carried out at 25 to 90°C.

[0062] The reaction time is preferably the time until the raw material haloalkyl compound is consumed, and more preferably 1 to 12 hours in terms of the stability of the haloalkyl compound and the efficiency of the process. The reaction may be carried out in water, or in a mixed solvent of water with alcohols such as methanol, ethanol, isopropanol, and butanol, or glycol ethers such as ethylene glycol dimethyl ether. Alternatively, the reaction may be carried out by dispersing a powder of the dried starch partial hydrolyzate in a hydrophilic solvent such as alcohols such as methanol, ethanol, isopropanol, and butanol, or glycol ethers such as ethylene glycol dimethyl ether. When a mixed solvent is used, the proportion of solvents other than water in the mixed solvent is preferably 50% by volume or less. A reaction kettle, extruder, or the like can be used as the reaction apparatus.

[0063] When an acid anhydride is used as the acidic group-containing compound, the reaction proceeds simply by mixing the partial hydrolysis product of starch with the acid anhydride and heating, but to promote the reaction, a catalyst such as sodium carbonate, sodium hydroxide, tertiary amines such as triethylamine, imidazoles such as 2-methylimidazole, quaternary ammonium salts such as tetrabutylammonium bromide, or phosphonium salts such as tetrabutylphosphonium bromide may also be used. The amount of these catalysts added is preferably 0.1 equivalents or less relative to the acidic group-containing compound. These catalysts may be used alone or in combination of two or more.

[0064] The reaction time is preferably the time until the acid anhydride is consumed, more preferably 1 to 12 hours. The end point of the reaction can be determined by acid value measurement or IR measurement. The reaction may be carried out in water, but to prevent hydrolysis or alcoholysis of the acid anhydride, it is preferable to use an aprotic solvent such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, or N-methylpyrrolidone as the reaction solvent. When a mixed solvent is used, the proportion of solvents other than water in the mixed solvent is preferably 50% by volume or more. When the reaction is carried out without a solvent, the acid anhydride can function 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, more preferably 70 to 90°C. A reaction kettle, extruder, or the like can be used as the reaction apparatus.

[0065] <Physical Properties of Water-Soluble Polymer> The molecular weight of the water-soluble polymer produced in step (a2) is not particularly limited, but the weight-average molecular weight, calculated as pullulan by aqueous size exclusion chromatography analysis, is preferably 500,000 to 40,000,000, and more preferably 700,000 to 35,000,000. If the weight-average molecular weight is less than 500,000, the water retention capacity of the water-absorbent resin tends to decrease, and if it exceeds 40,000,000, the water absorption performance of the water-absorbent resin tends to decrease. The weight-average molecular weight, calculated as pullulan by aqueous size exclusion chromatography analysis, can be determined based on a calibration curve of molecular weight and elution time prepared using pullulan with a known molecular weight in aqueous size exclusion chromatography.

[0066] The total acid value of the water-soluble polymer is preferably 50 to 350 mgKOH / g, more preferably 70 to 300 mgKOH / g. The total acid value indicates the amount of all acidic groups introduced into the water-soluble polymer. If the total acid value is less than 50 mgKOH / g or more than 350 mgKOH / g, the water-absorbent resin after crosslinking the water-soluble polymer tends to have reduced water absorption performance for aqueous solutions containing electrolytes, such as physiological saline.

[0067] When acidic groups on a water-soluble polymer are introduced by reacting a haloalkyl compound having a carboxyl group with hydroxyl groups of starch or its partial hydrolyzates, the amount of introduced acidic groups (carboxyalkyl groups) 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, the total acid value is detected by the introduction of carboxyalkyl groups. If the raw material starch partial hydrolyzate does not contain acidic groups such as carboxyalkyl groups, the acidic groups detected by measuring the total acid value are considered to be equal to those introduced by the etherification reaction. Therefore, if the raw material starch or partial starch hydrolyzate does not contain acidic groups, the degree of etherification can be simply calculated from the total acid value. For example, if the carboxyalkyl groups are carboxymethyl groups and all of them are neutralized as sodium salts, the degree of etherification can be calculated by the following formula: degree of etherification = (162 × total acid value) ÷ (56100 − 80 × total acid value). The unit of the total acid number at this time is mgKOH / g.

[0068] The free acid value of the water-soluble polymer is not particularly limited, but is preferably 0 to 70 mgKOH / g, and more preferably 0 to 50 mgKOH / g. When the acidic groups on the water-soluble polymer form salts with sodium, potassium, lithium, ammonia, or the like and a step of partially neutralizing the acidic groups is not included, the free acid value is preferably 0 mgKOH / g. When the step (ab) of partially neutralizing the acidic groups after introducing acidic groups into a partially hydrolyzed starch product is included, the free acid value is preferably 5 to 70 mgKOH / g, and more preferably 7 to 50 mgKOH / g. The free acid value refers to the acid value measured for unneutralized acidic groups. When the free acid value exceeds 70 mgKOH / g, water absorption performance tends to decrease.

[0069] The dispersity (weight average molecular weight / number average molecular weight) of the water-soluble polymer is not particularly limited, but is preferably 5 to 50, and more preferably 7 to 45. If it is less than 5 or exceeds 50, the water absorption performance of the water-absorbent resin tends to decrease. The number average molecular weight of the water-soluble polymer can be determined by aqueous size exclusion chromatography analysis.

[0070] The biomass ratio of the water-soluble polymer is preferably 50% or more, more preferably 60% or more. The biomass ratio is the proportion (mass %) of elements derived from natural resources among the elements constituting the water-soluble polymer, and can be measured by the method described in the Examples.

[0071] <Desalting Step (aa)> After the step (a2), a step (aa) of desalting the water-soluble polymer may be included. Desalting can remove alkali metal halide salts by-produced in the step (a2), salts of unreacted haloalkyl compounds used in the step (a2), and salts produced in the partial neutralization step (ab) described below. Examples of desalting methods include solvent desalting methods and ultrafiltration membrane desalting methods. Examples of solvent desalting methods include a method in which a water-soluble polymer is dissolved in water to form an aqueous solution, which is then dropped into a hydrophilic solvent such as methanol, ethanol, isopropanol, acetone, or acetonitrile to reprecipitate the water-soluble polymer and recover it by filtration, and then the water-soluble polymer recovered by filtration is redispersed in aqueous methanol (water content of about 70 to 90%), stirred, and washed by a process in which the water-soluble polymer particles are recovered by filtration. Examples of methods using an ultrafiltration membrane include a method in which an aqueous solution of a hydrophilic polymer is treated with a filter having an ultrafiltration membrane. The washing liquid used in desalting may be water or a mixture of water and a hydrophilic organic solvent such as methanol, ethanol, propanol, acetone, acetonitrile, etc. Desalting is preferably carried out until the salt concentration in the water-soluble polymer becomes 1% or less.

[0072] <Step (ab) of Partially Neutralizing the Water-Soluble Polymer> The step (a2) or the method may include a step (ab) of partially neutralizing the acidic groups of the water-soluble polymer after the step (a2) and before the drying step (a3).

[0073] In the water-soluble polymer obtained in step (a2), if the carboxyl, sulfo, or phospho group moiety of the acidic groups introduced into the partial starch hydrolyzate forms a salt with sodium, potassium, lithium, ammonia, or the like derived from the alkaline agent, this salt is neutralized with an acid in step (ab). By neutralization, some of the carboxyl, sulfo, or phospho groups that formed a salt are converted to free carboxylic acid, sulfonic acid, or phosphoric acid. Also, in the water-soluble polymer obtained in step (a2), if the carboxyl, sulfo, or phospho group moiety of the acidic groups introduced into the partial starch hydrolyzate is a free acid that does not form a salt, this salt is neutralized with an alkali in step (ab). By neutralization, some of the free carboxylic acid, sulfonic acid, or phosphoric acid is converted to carboxyl, sulfo, or phospho groups that form a salt.

[0074] The partial neutralization is preferably carried out on some of the salt-forming carboxyl, sulfo, or phospho groups on the water-soluble polymer. For example, when monochloroacetic acid is used as the acidic group-containing compound and sodium hydroxide is used as the alkaline agent in step (a2), a water-soluble polymer is obtained in which sodium salts of carboxyl groups are added to the starch partial hydrolyzate. By adding an acid to this, some of the carboxyl groups are converted to free carboxylic acids.

[0075] In addition, partial neutralization is preferably carried out on a portion of the carboxyl groups, sulfo groups, or phospho groups on the water-soluble polymer that do not form salts. For example, when succinic anhydride is used as the acidic group-containing compound in step (a2), a water-soluble polymer is obtained in which free carboxylic acids are added to the starch partial hydrolyzate. By adding an alkali to this, some of the free carboxylic acids are converted to carboxyl groups that form salts.

[0076] In step (ab), the degree of neutralization of the acidic groups in the water-soluble polymer after partial neutralization is preferably 1 to 50%, more preferably 5 to 25%. By setting the degree of neutralization in this range, the production of the water-absorbent resin tends to be easier. The degree of neutralization is the proportion of neutralized acidic groups present in the water-soluble polymer, and is calculated by the following formula: Degree of neutralization (%) = ((free acid number of water-soluble polymer) / (total acid number of water-soluble polymer)) × 100 The total acid number of the water-soluble polymer and the free acid number of the water-soluble polymer are as described above with respect to the physical properties of the water-soluble polymer.

[0077] An acid is used to neutralize the carboxyl group forming the salt. While the acid is not particularly limited, an acid with a pKa equal to or lower than that of the carboxyl group is preferred, and examples thereof include hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, acetic acid, formic acid, and trichloroacetic acid. A strong acid is preferably used to neutralize the sulfo or phospho group forming the salt, and more preferably a mineral acid such as hydrochloric acid or sulfuric acid, or a strongly acidic ion exchange resin. Neutralization can be carried out using known devices such as a reaction kettle or extruder. After the addition of the acid, the mixture is preferably stirred at 0 to 50°C for 0.2 to 1 hour to allow for the neutralization reaction. The neutralization reaction is preferably carried out under conditions of pH 6.8 to 7.2.

[0078] An alkali is used to neutralize free carboxylic acids, sulfonic acids, and phosphoric acids. The alkali is not particularly limited, but examples include sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, ammonia, dimethylamine, and tetramerylammonium hydroxide. Neutralization can be carried out using known equipment such as a reaction kettle or extruder. After adding the alkali, the mixture is preferably stirred at 0 to 50°C for 0.2 to 1 hour for the neutralization reaction. The neutralization reaction is preferably carried out under conditions of pH 6.8 to 7.2.

[0079] In the neutralization reaction of the salt-forming carboxyl group, sulfo group, or phospho group, a salt may be formed between the halogen derived from the acidic group-containing compound and the metal or ammonia derived from the alkaline agent. This salt can be removed by the desalting step (aa) described above.

[0080] When the step (ab) is carried out in the step (a2), it is preferable to carry out partial neutralization after the reaction of the partial starch hydrolyzate with the acidic group-containing compound in the step (a2).

[0081] On the other hand, the step (ab) is not essential. One embodiment of the present invention also includes a method for producing a water-absorbent resin that does not include a step of partially neutralizing the acidic groups of the water-soluble polymer after the step (a2) and before the drying step (a3). When the partial neutralization step is not included, the cost required for partial neutralization can be reduced.

[0082] <Drying Step (a3)> In this step, the water-soluble polymer is dried in the presence of water at 70 to 180° C. until the solid content reaches 90% or more. The water-soluble polymer at the start of step (a3) ​​may be in the form of an aqueous solution or a wet powder containing water.

[0083] Examples of liquid components contained in the water-soluble polymer include water and mixed solvents of water and hydrophilic solvents. Examples of hydrophilic solvents include lower aliphatic alcohols such as methanol, ethanol, n-propanol, and isopropanol; ketones such as acetone; ethers such as dioxane, tetrahydrofuran, and methoxy(poly)ethylene glycol; and amides such as ε-caprolactam and N,N-dimethylformamide. Two or more of these hydrophilic solvents may be used in combination. The proportion of the hydrophilic solvent in the mixed solvent is preferably adjusted based on the boiling point of the hydrophilic solvent. When the boiling point of the hydrophilic solvent is 100°C or less, the proportion is preferably 70% by volume or more, and when the boiling point is higher than 100°C, the proportion is preferably 30% by volume or less.

[0084] When the water-soluble polymer is in the form of a wet powder at the start of step (a3), the content of the liquid component is preferably 50 to 90 wt %, more preferably 60 to 80 wt %, and even more preferably 65 to 75 wt %.

[0085] Drying is carried out until the solid content of the water-soluble polymer is 90% or more, preferably 93% or more, more preferably 95% or more, and even more preferably 97% or more. The upper limit of the solid content is preferably 99% or less. Here, the solid content means the weight ratio of components other than water and the hydrophilic solvent to the weight of the water-soluble polymer. By drying until the solid content of the water-soluble polymer is 90% or more, it is possible to obtain a water-absorbent resin that forms a physical gel by absorbing water, even when a step of partially neutralizing the acidic groups of the water-soluble polymer prior to step (a3) ​​is not included.

[0086] The drying temperature is preferably 70 to 180°C. Furthermore, it is more preferable to select the drying temperature depending on whether or not the step (ab) of partially neutralizing the acidic groups is performed. When the step (ab) of partially neutralizing the acidic groups is performed, the drying temperature is preferably 70 to 150°C, more preferably 70 to 130°C. When the step (ab) of partially neutralizing the acidic groups is not performed, the drying temperature is preferably 80 to 150°C, more preferably 100 to 130°C. The drying apparatus is not particularly limited, and a drum dryer, spray dryer, Nauta mixer, etc. can be used.

[0087] In step (a3), a crosslinked structure is formed between the water-soluble polymers. The crosslink formed here corresponds to internal crosslinking of the water-absorbent resin. The crosslink is preferably formed via the acidic group on the water-soluble polymer introduced in step (a2). Examples of the crosslinked structure include ionic bonds between acidic groups, coordinate bonds via metal ions, and hydrogen bonds formed by dimerization of acidic groups. The crosslinks may include crosslinks formed by covalent bonds, as long as they do not inhibit decomposition of the resulting water-absorbent resin. When crosslinks formed by covalent bonds are included, the proportion of the crosslinked structures is preferably 5% or less, more preferably 2% or less. Examples of crosslinks formed by covalent bonds include ester bonds, ether bonds, carbon-carbon single bonds (C-C bonds), and carbon-carbon double bonds (C=C bonds).

[0088] The crosslinked structure can be formed without using a crosslinking agent as described above, but a crosslinking agent may also be used. Examples of the crosslinking agent include epoxy compounds, polyhydric alcohol compounds, polyamine compounds, polyisocyanate compounds, alkylene carbonate compounds, haloepoxy compounds, halohydrin compounds, polyoxazoline compounds, carbodiimide compounds, silane coupling agents, and polyvalent metal compounds.

[0089] Examples of the epoxy compound include succinic acid glycidyl ester, sorbitol polyglycidyl ether, trimethylolpropane polyglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and glycidol.

[0090] Examples of the polyhydric alcohol compound 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, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanediol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, and sorbitol.

[0091] Examples of the polyvalent amine compound include ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, inorganic salts or organic salts (such as azithinium salts) of these polyvalent amine compounds, and polysaccharides having an amino group such as chitin.

[0092] Examples of the polyisocyanate compound include 2,4-tolylene diisocyanate and hexamethylene diisocyanate, and examples of the polyoxazoline compound include 1,2-ethylenebisoxazoline.

[0093] Examples of the alkylene carbonate compound include 1,3-dioxolan-2-one, 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, and 4,6-dimethyl-1,3-dioxan-2-one.

[0094] Examples of the haloepoxy compound include epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, and polyamine adducts thereof (eg, Kaimen (registered trademark) manufactured by Hercules).

[0095] Other known crosslinking agents that can be used include aqueous carbodiimide compounds (e.g., Carbodilite manufactured by Nisshinbo Chemical Inc.), silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane, and polyvalent metal compounds such as hydroxides and chlorides of zinc, calcium, magnesium, aluminum, iron, zirconium, and the like.

[0096] In step (a3), optionally, a structural unit other than the water-soluble polymer may be blended and crosslinked. Examples of structural units other than the water-soluble polymer include polyacrylic acid (salts) such as partially neutralized crosslinked polyacrylic acid, self-crosslinking partially neutralized polyacrylic acid, and starch-acrylic acid graft polymers. Examples of salts of acrylic acid include sodium salts, potassium salts, and ammonium salts. Other structural units other than the water-soluble polymer include anionic unsaturated monomers and salts thereof such as methacrylic acid, maleic acid, vinyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, and 2-(meth)acryloylpropanesulfonic acid; 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 2-hydroxypropyl(meth)acrylamide. nonionic hydrophilic group-containing unsaturated monomers such as N,N-dimethylaminoethyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, and N-acryloylpyrrolidine; cationic unsaturated monomers such as N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and quaternary salts thereof, linear cellulose, and poly(γ-glutamic acid).

[0097] In the case where the water-absorbent resin contains a constituent unit other than the water-soluble polymer, the content thereof is preferably 90% by weight or less, more preferably 50% by weight or less, and even more preferably 20% by weight or less, based on the total amount of the water-soluble polymer used as the main component.

[0098] The water-absorbent resin may contain other additives such as disinfectants, deodorants, antibacterial agents, fragrances, various inorganic powders, foaming agents, pigments, dyes, hydrophilic short fibers, fertilizers, oxidizing agents, reducing agents, water, and salts, for the purpose of imparting various functions. The method of adding these and the amount of addition will be appropriately selected by those skilled in the art.

[0099] After the drying step (a3), surface cross-linking may be performed. The strength of the water-absorbent resin can be improved by surface cross-linking. The cross-linking agent used for surface cross-linking can be the same as the cross-linking agent described above for the cross-linking in step (a3). Among these, epoxy compounds are preferred, and ethylene glycol diglycidyl ether, sorbitol polyglycidyl ether, and succinic acid glycidyl ester are more preferred.

[0100] The surface crosslinking can be formed by spraying a surface crosslinking agent onto a water absorbent resin, mixing the mixture by a known method using a cylindrical mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, a double-arm mixer, a pulverizing kneader, etc., and then crosslinking the mixture. During spraying and mixing, a surfactant can be added as necessary.

[0101] The produced water absorbent resin may be subjected to treatments such as desalting and washing in order to remove impurities and by-products. Desalting can be performed using a filter having a reverse osmosis membrane. As a washing liquid, water or a mixture of water and a hydrophilic organic solvent such as methanol, ethanol, or propanol can be used.

[0102] <<Production Method 2 for Water-Absorbent Resin>> The production method of the present invention is characterized by comprising: a step (b1) of introducing acidic groups into starch; a step (b2) of lowering the molecular weight of the starch having acidic groups to obtain a water-soluble polymer; and a step (b3) of drying the water-soluble polymer in the presence of water at 70 to 180°C until the water-soluble polymer has a solid content of 90% or more.

[0103] <Step (b1) of introducing acidic groups into starch> In this step, starch is reacted with an acidic group-containing compound. The starch, acidic groups, and acidic group-containing compound used are as described above in connection with step (a2) of introducing acidic groups into a partial hydrolyzate of starch.

[0104] The conditions for the reaction between starch and an acidic group-containing compound are not particularly limited, but when a haloalkyl compound having a carboxyl group is used as the acidic group-containing compound, it is preferable to use 1 to 1.5 equivalents of an alkaline agent relative to the acidic group-containing compound. In order to stabilize the starch, a pH of 10.5 to 12.5 is preferred, and a pH of 11 to 12 is more preferred. Examples of alkaline agents used to adjust the pH include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, sodium carbonate, and potassium carbonate.

[0105] The acidic group introduced into the starch preferably forms a salt with sodium, potassium, lithium, ammonia, or the like derived from the alkaline agent, and for this reason, it is preferable to use the alkaline agent in an amount necessary for both the reaction between the acidic group-containing compound and the starch and the neutralization of the acidic group of the acidic group-containing compound. For example, when monochloroacetic acid is used as the acidic group-containing compound, it is theoretically preferable to use the alkaline agent in an amount of 2 equivalents or more relative to the monochloroacetic acid. When sodium monochloroacetate is used, the acidic group has been neutralized in advance, so it is preferable to use the alkaline agent in an amount of 1 equivalent or more relative to the sodium chloroacetate.

[0106] The amount of the acidic group-containing compound used can be set arbitrarily depending on the desired total acid value (degree of etherification) of the water-soluble polymer. Typically, 0.5 to 5.0 equivalents per mole of hydroxyl groups on starch is preferred, and 0.5 to 2.0 equivalents is more preferred. When a haloalkyl compound such as monochloroacetic acid is used and the reaction is carried out in an aqueous solution, the acidic group introduction reaction and the hydrolysis reaction of the haloalkyl compound compete with each other, so the haloalkyl compound is required in excess of the theoretical amount. The amount of the haloalkyl compound used in the aqueous reaction is preferably set to 5 equivalents or less relative to the theoretical value.

[0107] The reaction temperature between starch and the acidic group-containing compound is not particularly limited, but is preferably 0 to 120°C, more preferably 0 to 100°C. The reaction time is not particularly limited, but is preferably 1 to 24 hours. The reaction may be carried out in water, or in a mixed solvent of water with alcohols such as methanol, ethanol, isopropanol, butanol, ethylene glycol, diethylene glycol, propylene glycol, and ethylene glycol monoethyl ether, or glycol ethers such as ethylene glycol dimethyl ether. When a mixed solvent is used, the proportion of solvents other than water in the mixed solvent is preferably 50% by volume or less. A reaction kettle, extruder, or the like can be used as the reaction apparatus.

[0108] In particular, when chloroacetic acid or its salt is used as the haloalkyl compound, the reaction is preferably carried out at 25 to 60°C to prevent hydrolysis by water in the reaction solution. The reaction time is preferably the time until the raw material haloalkyl compound is consumed, and more preferably 1 to 6 hours to ensure the stability of the haloalkyl compound and the efficiency of the process. The reaction may be carried out in water, or in a mixed solvent of water with alcohols such as methanol, ethanol, isopropanol, and butanol, or glycol ethers such as ethylene glycol dimethyl ether. Alternatively, the reaction may be carried out by dispersing dried starch powder in a hydrophilic solvent such as alcohols such as methanol, ethanol, isopropanol, and butanol, or glycol ethers such as ethylene glycol dimethyl ether. When a mixed solvent is used, the proportion of solvents other than water in the mixed solvent is preferably 50% by volume or less. A reaction vessel, extruder, or the like can be used as the reaction apparatus.

[0109] Furthermore, when an acid anhydride is used as the acidic group-containing compound, the reaction proceeds simply by mixing starch with the acid anhydride and heating, but to promote the reaction, a catalyst such as sodium carbonate, sodium hydroxide, or a tertiary amine such as triethylamine, an imidazole such as 2-methylimidazole, a quaternary ammonium salt such as tetrabutylammonium bromide, or a phosphonium salt such as tetrabutylphosphonium bromide may be used. The amount of these catalysts added is preferably 0.1 equivalents or less relative to the acidic group-containing compound. These catalysts may be used alone or in combination of two or more.

[0110] The reaction time is preferably the time until the raw material acid anhydride is consumed, more preferably 1 to 12 hours. The end point of the reaction can be determined by acid value measurement or IR measurement. The reaction may be carried out in water, but to prevent hydrolysis or alcoholysis of the acid anhydride, it is preferable to use an aprotic solvent such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, or N-methylpyrrolidone as the reaction solvent. When a mixed solvent is used, the proportion of solvents other than water in the mixed solvent is preferably 50% by volume or more. When the reaction is carried out without a solvent, the acid anhydride can function 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, more preferably 70 to 90°C. A reaction kettle, extruder, or the like can be used as the reaction apparatus.

[0111] <Step (b2) of Obtaining a Water-Soluble Polymer> In this step, the starch having acidic groups obtained in step (b1) is reduced in molecular weight to reduce the viscosity. The method for reducing the molecular weight is not particularly limited, and examples include methods of subjecting the starch to enzyme treatment, acid treatment, physical crushing, etc. These methods are as described above in relation to step (a1) of reducing the molecular weight of starch to obtain a partial starch hydrolyzate.

[0112] After step (a2), the water-soluble polymer may be desalted in the same manner as in step (aa) above, and the acidic groups of the water-soluble polymer may be partially neutralized in the same manner as in step (ab) above.

[0113] <Drying Step (b3)> In this step, the water-soluble polymer is dried in the presence of water at 70 to 180°C until the solid content reaches 90% or more. The water-soluble polymer at the start of step (b3) may be an aqueous solution or a wet powder containing water. The liquid components contained in the water-soluble polymer used and their contents, drying conditions, solid content, and crosslinked structure formed between the water-soluble polymers are as described above for the drying step (a3).

[0114] <<Crosslinked Structure of Water-Absorbent Resin>> Generally, water-absorbent resins having polyacrylic acid as a main constituent unit have a network structure formed by covalent bonds, and form a chemical gel upon absorbing water. In contrast, the water-absorbent resins obtained by Production Method 1 for Water-Absorbent Resin and Production Method 2 for Water-Absorbent Resin of the present invention are characterized by forming a physical gel upon absorbing water. In a physical gel, crosslinking points are relatively easily eliminated due to thermal motion of molecular chains or changes in pH or ionic strength, and the physical gel can be converted into a sol having fluidity. Therefore, the physical gel is easily decomposed by adding alkali or acid, heating, shaking, etc., and leads to a reduction in the environmental load at the time of disposal.

[0115] The formation of a physical gel can be confirmed by confirming a sol-gel transition due to the disappearance of the crosslinking points. For example, in the case of crosslinking by ionic bonds and / or hydrogen bonds, it can be confirmed by fluidization after alkali or acid treatment. Specifically, when a water-absorbent resin is suspended in a 1% aqueous sodium hydroxide solution to a final concentration of 5% by weight, stirred for 60 minutes, and then sieved through a sieve with an opening of 500 μm, it can be determined that the water-absorbent resin forms a physical gel by absorbing water when the dry weight of the water-absorbent resin remaining on the sieve is less than 2% by weight of the dry weight of the water-absorbent resin contained in the aqueous solution.

[0116] More specifically, 0.5 g of water-absorbent resin was suspended in 9.5 g of 1% NaOH aqueous solution and stirred for 60 minutes, then naturally filtered through a 30-mesh wire mesh (500 μm mesh size) measuring approximately 50 mm x 50 mm, and the mesh was washed with ion-exchanged water. After washing, the wire mesh was dried for 2 hours in a 120°C air dryer. When the weight of the wire mesh before filtration is Wm1 and the weight of the wire mesh after filtration and drying is Wm2, it can be determined that a physical gel has formed by water absorption if the 30-mesh pass residue calculated using the following formula is less than 2 wt%. The 500 μm mesh sieve is a 30-mesh sieve specified by JIS. Residue passing 30 mesh (%)=(Wm2-Wm1) / 0.5×100 The residue passing 30 mesh is preferably less than 2% by weight, more preferably less than 1.5% by weight, even more preferably less than 1.0% by weight, and even more preferably less than 0.5% by weight.

[0117] The formation of a physical gel by water absorption can also be confirmed by observing the properties of a water-absorbent resin after adding water. That is, 0.2 g of a water-absorbent resin placed in a nylon mesh tea bag is immersed in 1 L of water, and left to stand for 3 hours. The solubility in water and gel strength are evaluated according to the criteria described in the Examples, and it can be determined that a physical gel has been formed when the evaluation score is 2 to 6.

[0118] <<Water Absorption Performance of Water Absorbent Resin>> The water absorption capacity under no load of the water absorbent resin obtained by Production Method 1 for Water Absorbent Resin and Production Method 2 for Water Absorbent Resin of the present invention is determined by measuring the absorbency of physiological saline or ion-exchanged water when no load is applied to the water absorbent resin by the method described in the Examples. The water absorbent resin, in a solid state, preferably has a water absorption capacity under no load for ion-exchanged water of 100 to 500 g / g, more preferably 120 to 460 g / g. Furthermore, the water absorption capacity under no load for physiological saline is preferably 10 to 70 g / g, more preferably 20 to 70 g / g, and even more preferably 30 to 65 g / g.

[0119] The ratio (A / B) of the absorbency of the water-absorbent resin for ion-exchanged water without load (A) to the absorbency of physiological saline without load (B) is preferably 7 or less, more preferably 5 or less.

[0120] The water retention rate of a water-absorbent resin is determined by measuring the absorbency of physiological saline or ion-exchanged water when a load of 150 G is applied to the water-absorbent resin using the method described in the Examples. The water-absorbent resin, in a solid state, preferably has a water retention rate for ion-exchanged water of 80 to 300 g / g, more preferably 100 to 300 g / g. Furthermore, the water retention rate for physiological saline is preferably 5 to 65 g / g, more preferably 10 to 60 g / g, and even more preferably 20 to 60 g / g.

[0121] The biomass degree of the water-absorbent resin is preferably 50% or more, more preferably 60% or more. The biomass degree is the proportion (mass%) of elements derived from natural resources among the elements constituting the water-absorbent resin, and can be calculated, for example, from the biomass degree of the water-soluble polymer measured by the method described in the Examples, taking into consideration fluctuations in molecular weight due to partial neutralization and crosslinking.

[0122] <<Method for Decomposing Water-Absorbent Resin>> The water-absorbent resin obtained by the method for producing a water-absorbent resin 1 and the method for producing a water-absorbent resin 2 of the present invention can be decomposed by alkali treatment. During the alkali treatment, the water-absorbent resin is placed under conditions of preferably pH 9 or higher, more preferably pH 10 or higher. By the alkali treatment, the crosslinked structure and glycoside bond of the water-absorbent resin are cleaved, and the resin is decomposed into water-soluble polymers, thereby reducing the environmental load at the time of disposal.

[0123] The alkaline agent used in the alkaline treatment is not particularly limited, but examples thereof include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, sodium carbonate, potassium carbonate, etc. The temperature during the alkaline treatment is not particularly limited, but the treatment can be carried out, for example, under conditions of 5 to 50°C.

[0124] <<Articles Comprising Water-Absorbent Resins>> A variety of articles can be manufactured using the water-absorbent resins obtained by the water-absorbent resin manufacturing method 1 and the water-absorbent resin manufacturing method 2 of the present invention. Examples of articles include sanitary and medical products such as disposable diapers, sanitary products, incontinence pads, portable toilets, waste disposal bags, animal excrement treatment agents, medical dressings, surgical sheets, dental waste fluid treatment agents, medical waste blood coagulants, wound dressings, and moisturizers; agricultural and horticultural products such as soil water retention agents, seedling raising sheets, seed coatings, fertilizer slow-release agents, pesticide and fertilizer disintegration aids, desert greening materials, agricultural films, and freshness-preserving agents; civil engineering materials such as soil conditioners, sludge solidification agents, and water-stopping materials; and articles such as ice packs, thickeners, deodorizers, food drip absorbents, pet sheets, disposable body warmers, battery gelling agents, anti-condensation sheets, packing agents, and artificial snow. Examples of sanitary products include laminates in which a back sheet, an absorbent, and a top sheet are laminated in this order. The absorbent body contains the water-absorbent resin of the present invention, and may further contain water-absorbent paper or pulp, if necessary. The specific manufacturing method of the article is not particularly limited, and each article can be manufactured by a well-known method.

[0125] These articles are characterized by their ease of decomposition due to the inclusion of the water-absorbent resin described above, which can be decomposed under alkaline treatment conditions, thereby reducing the environmental impact when disposed of.

[0126] The present invention will be described below with reference to examples, but is not limited to the following examples. Hereinafter, "parts" and "%" mean "parts by weight" and "% by weight", respectively, unless otherwise specified.

[0127] (1) Materials used (1-1) Starch raw materials Corn starch Tapioca starch Acetate tapioca starch Oxidized acetylated tapioca starch Acetylated tapioca starch Hydroxypropylated starch Hydroxypropylated phosphate cross-linked starch

[0128] (1-2) Hydrolases α-amylase (Spitase (registered trademark) HK / R, manufactured by Nagase ChemteX Corporation), 12,200 units / g amylomaltase: Thermus thermophilus was aerobically cultured, and the collected cells were disrupted and the extract was centrifuged, and the supernatant was used as a crude enzyme solution. The crude enzyme solution was subjected to column chromatography in a conventional manner, and a sample purified to an electrophoretically homogeneous state was used as a purified enzyme solution.

[0129] The activity of amylomaltase was measured as follows: 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. The reaction was then stopped by heating at 100°C for 10 minutes. The amount of glucose in the reaction solution was measured by the glucose oxidase method. The unit amount of amylomaltase was defined as the amylomaltase activity that produces 1 μmol of glucose per minute.

[0130] (2) Production of Partial Starch Hydrolysates (Production Examples 1 to 7) Partial starch hydrolysates were produced by the following method. The weight-average molecular weights of the obtained partial starch hydrolysates were determined by aqueous size exclusion chromatography based on a calibration curve of molecular weight and elution time prepared using pullulan with a known molecular weight. The weight-average molecular weights and dispersities of the partial starch hydrolysates are shown in Table 1.

[0131] (Production Example 1) Cornstarch-Derived Partial Starch Hydrolysate Cornstarch was suspended in city water to a concentration of 15% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. To this starch milk, an amylomaltase crude enzyme solution was added at 0.4 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by reaction at 80°C for 6 hours with stirring to prepare liquefied starch. The resulting partial starch hydrolysate had a weight-average molecular weight of 1,930,000 and a dispersity of 9.6.

[0132] (Production Example 2) Cornstarch was suspended in city water to a concentration of 30% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain a starch milk. To this starch milk, 0.15 units of amylomaltase crude enzyme solution per gram of starch solids and 0.04 units of α-amylase per gram of starch solids were added, and the mixture was stirred at room temperature for 30 minutes, followed by a reaction at 80°C for 6 hours with stirring to prepare a liquefied starch. The resulting partial starch hydrolyzate had a weight-average molecular weight of 1,340,000 and a dispersity of 10.6.

[0133] (Production Example 3) Liquefied starch was prepared in the same manner as in Production Example 1, except that the cornstarch concentration was 30% (w / w) and the amylomaltase crude enzyme solution was 0.2 units per gram of starch solids. The weight-average molecular weight of the resulting partial starch hydrolysate was 1,970,000 and the dispersity was 10.5.

[0134] (Production Example 4) Except for using a different lot of cornstarch, liquefied starch was prepared by the same procedure as in Production Example 3. The resulting partial starch hydrolyzate had a weight-average molecular weight of 1,910,000 and a dispersity of 10.1.

[0135] (Production Example 5) Liquefied starch was prepared by the same procedure as in Production Example 3, except that the corn starch and enzyme concentrations were maintained while the process was scaled up 14 times. The resulting partial starch hydrolysate had a weight-average molecular weight of 5,160,000 and a dispersity of 26.3. It was presumed that the reaction efficiency changed due to the scale-up, resulting in an increase in molecular weight and dispersity compared to Production Example 3.

[0136] (Production Example 6) The aqueous solution of the partial starch hydrolysate obtained in Production Example 2 was vacuum dried at 80°C, and the resulting dried product was pulverized to recover a dry powder product with a 1 mm pass size. The moisture content of the dry powder product was 2.7%. The molecular weight and dispersity of the resulting partial starch hydrolysate were in accordance with Production Example 2.

[0137] (Production Example 7) Cornstarch was suspended in city water to a concentration of 15% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. α-Amylase was added to this starch milk at 0.5 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by a reaction at 100°C for 20 minutes with stirring to prepare liquefied starch. The resulting partial starch hydrolyzate had a weight-average molecular weight of 1,370,000 and a dispersity of 65.6. Although the cornstarch concentration was the same as in Production Example 1, it was presumed that the reaction efficiency changed due to the scale-up and the difference in the enzyme used, resulting in an increased dispersity compared to Production Example 1.

[0138]

[0139] (3) Production of Water-Soluble Polymer (Production Example 8) 200 g of the 15 wt % aqueous solution of the starch partial hydrolysate produced in Production Example 1 (0.56 mol of hydroxyl groups in the starch partial hydrolysate) was charged into a 500 ml separable flask equipped with a stirrer, thermometer, and condenser. Next, 76.5 g of a 48.8% aqueous solution of sodium hydroxide (0.93 mol, 1.68 equivalents relative to the hydroxyl groups in the starch partial hydrolysate) was charged, and the solution was stirred at 60°C or below until it became completely homogeneous. After confirming that the solution was homogeneous, an aqueous solution prepared by dissolving 42.0 g of monochloroacetic acid (0.44 mol, 0.8 equivalents relative to the hydroxyl groups in the starch partial hydrolysate) in 10.5 g of ion-exchanged water was added dropwise at 50-60°C over 30 minutes. After charging the aqueous monochloroacetic acid solution, the temperature was adjusted to 45-50°C, and the mixture was stirred for 10 hours. The reaction was terminated when the chloride ion content in the reaction solution was measured by potentiometric titration using a 0.01 N silver nitrate solution, and reached 98% or more of the calculated chloride ion content of 4.8% when all the monochloroacetic acid had reacted. In this production example, the chloride content was 4.8%.

[0140] After the reaction was completed, the reaction solution was diluted with 200 g of ion-exchanged water. The diluted reaction solution was cooled to room temperature and added to 1.7 L of methanol over approximately 30 minutes to precipitate and reprecipitate the water-soluble polymer. After the entire reaction solution was added, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the methanol was subjected to solid-liquid separation by vacuum filtration.

[0141] Subsequently, in order to remove sodium chloride contained in the water-soluble polymer, the recovered water-soluble polymer was redispersed in 0.9 L of aqueous methanol (methanol / water 80 / 20 (volume ratio)), stirred at room temperature for 30 minutes, washed, and then subjected to solid-liquid separation by vacuum filtration, and the water-soluble polymer was recovered again. The chlorine content of the recovered water-soluble polymer was measured by potentiometric titration using a 0.01 N silver nitrate solution, and the washing process was repeated until the chlorine content was less than 1%. The total acid number of the obtained water-soluble polymer was 166 mg KOH / g, and the degree of etherification calculated from the total acid number was 0.63.

[0142] (Production Examples 9 to 14) The reaction was carried out in the same manner as in Production Example 8, except that the raw materials, amounts charged, and reaction temperature were changed as shown in Table 2, to obtain water-soluble polymers.

[0143] (Production Example 15) 200 g of the 30 wt % aqueous solution of the partial starch hydrolysate produced in Production Example 4 (1.11 mol of hydroxyl groups in the partial starch hydrolysate) was charged into a 500 ml separable flask equipped with a stirrer, thermometer, and condenser. Next, 79.4 g of a 48.8% aqueous solution of sodium hydroxide (0.96 mol, 0.86 equivalents relative to the hydroxyl groups in the partial starch hydrolysate) was charged, and the solution was stirred at 60°C or below until it became completely homogeneous. After confirming that the solution was homogeneous, 89.9 g of 6-bromohexanoic acid crystals (0.46 mol, 0.4 equivalents relative to the hydroxyl groups in the partial starch hydrolysate) were added in small portions over 30 minutes at 50-60°C. After the addition of 6-bromohexanoic acid, the temperature was adjusted to 45-50°C, and the mixture was stirred for 10 hours. The reaction was terminated when the bromide ion content in the reaction solution was measured by potentiometric titration using a 0.01 N silver nitrate solution, and reached 98% or more of the calculated bromide ion content of 9.8% when all 6-bromohexanoic acid had reacted. In this production example, the bromine content was 10.1%.

[0144] After the reaction was completed, the reaction solution was diluted with 250 g of ion-exchanged water. The diluted reaction solution was cooled to room temperature and added to 1.6 L of ethanol over approximately 30 minutes to precipitate and reprecipitate the water-soluble polymer. After the entire reaction solution was added, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the ethanol was subjected to solid-liquid separation by vacuum filtration.

[0145] 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 (ethanol / water 90 / 10 (volume ratio)), stirred at room temperature for 30 minutes, washed, and then subjected to solid-liquid separation by vacuum filtration, and the water-soluble polymer was recovered again. The bromine content of the recovered water-soluble polymer was measured by potentiometric titration using a 0.01 N silver nitrate solution, and the washing process was repeated until the bromide ion content was less than 1%. The total acid number of the obtained water-soluble polymer was 145 mg KOH / g, and the degree of etherification calculated from the total acid number was 0.64.

[0146]

[0147] (Production Example 16) 27.2 g of the powder of the partial starch hydrolysate produced in Production Example 7 (0.47 mol of hydroxyl groups in the partial starch hydrolysate) and 58.1 g of dimethyl sulfoxide (DMSO) were placed in 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 partial starch hydrolysate) was placed in the flask, and the mixture was stirred and reacted at 70 to 75°C for 1 hour. After stirring, the reaction solution was sampled and subjected to neutralization titration with 0.1 N NaOH. The acid value of the reaction solution was 85 mg KOH / g (theoretical end-point acid value: 80.1 mg KOH / g).

[0148] After the reaction was completed, 125 g of ion-exchanged water was added to the reaction solution to dilute it. Furthermore, 10.8 g (0.13 mol) of 48% NaOH aqueous solution was added to neutralize 93% of the theoretical amount of carboxylic acid introduced by the reaction with succinic anhydride, and the carboxylic acid was converted into a sodium salt. Subsequently, the resulting solution was added to 750 ml of methanol over about 30 minutes, and the water-soluble polymer was precipitated and reprecipitated. After all the reaction solution was added, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the methanol was subjected to solid-liquid separation by vacuum filtration and recovered.

[0149] Subsequently, in order to remove unreacted succinic anhydride contained in the water-soluble polymer and succinic acid produced by hydrolysis, the recovered water-soluble polymer was redispersed in 500 mL of methanol, stirred at room temperature for 30 minutes, washed, and then subjected to solid-liquid separation by vacuum filtration, and the water-soluble polymer was recovered again. The total acid value and free acid value of the obtained water-soluble polymer were measured, and it was confirmed that the total acid value was 138 mg KOH / g and the free acid value was 37 mg KOH / g, which was a partially neutralized polymer. The weight-average molecular weight of the obtained water-soluble polymer was 3.7 × 10 ６ The dispersity was 13.8.

[0150] (Production Example 17) 20.0 g of the powder of the partial starch hydrolysate produced in Production Example 6 (0.36 mol of hydroxyl groups in the partial starch hydrolysate) and 110 g of dimethyl sulfoxide (DMSO) were placed in a 300 ml separable flask equipped with a stirrer, thermometer, and condenser and dissolved. Next, 10.6 g of maleic anhydride (0.11 mol, 0.30 equivalents relative to the hydroxyl groups in the partial starch hydrolysate) was placed in the flask, and the mixture was stirred and reacted at 90 to 95°C for 3 hours. After stirring, the reaction solution was sampled and subjected to neutralization titration with 0.1 N NaOH. The acid value of the reaction solution was 70 mg KOH / g (theoretical end-point acid value: 43 mg KOH / g).

[0151] After the reaction was completed, the reaction mixture was added to 2.5 L of acetone over about 30 minutes to precipitate and reprecipitate the water-soluble polymer. After the entire reaction mixture was added, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the acetone was subjected to solid-liquid separation by filtration under reduced pressure and recovered.

[0152] Subsequently, in order to remove unreacted maleic anhydride contained in the water-soluble polymer and maleic acid produced by hydrolysis, the recovered water-soluble polymer was redispersed in 500 mL of acetone, stirred at room temperature for 30 minutes, washed, and then subjected to solid-liquid separation by vacuum filtration, and the water-soluble polymer was recovered again. The total acid value of the obtained water-soluble polymer was 90 mg KOH / g. The weight-average molecular weight of the obtained water-soluble polymer was 3.1 × 10 ６ The dispersity was 39.6.

[0153]

[0154] (4) Production of Water-Absorbent Resin (4-1) Method Without Partial Neutralization (Example 1) 25 g (wet crystals) of the water-soluble polymer obtained in Production Example 9 (wet product with hydrous methanol, wet ratio 67%) was transferred to a petri dish, placed in a blower dryer set at 100°C, and dried for 20 hours to carry out a crosslinking treatment. After the treatment, the obtained solid was pulverized in a mortar and sieved using sieves with openings of 150 μm and 850 μm, and particles with particle sizes of 150 to 850 μm were collected.

[0155] Examples 2 to 9 and 13 and Comparative Examples 2 and 3 The same operations as in Example 1 were carried out except that the raw materials and amounts thereof were changed as shown in Table 4, to obtain water-absorbent resins.

[0156] (4-2) Method of Partial Neutralization (Example 10) 25 g of the water-soluble polymer obtained in Production Example 12 (wet product with hydrous methanol, wet ratio 71%) was placed in a 300 ml beaker, and 100 ml of hydrous methanol (methanol / water = 80 / 20 (volume ratio)) was added to disperse the water-soluble polymer. While stirring with a magnetic stirrer, 1.1 ml of 1 N hydrochloric acid was gradually added using a measuring pipette. After the addition of hydrochloric acid, the mixture was stirred for 15 minutes and subjected to vacuum suction filtration to recover the water-soluble polymer that had been partially neutralized to free acid. The recovered water-soluble polymer was wet crystals containing hydrous alcohol. The wet crystals were transferred to a petri dish, placed in a blower dryer set at 70°C, and dried for 20 hours to undergo a crosslinking treatment. The solid obtained after the treatment was pulverized in a mortar and mortar, and sieved using sieves with openings of 150 μm and 850 μm to recover particles with particle sizes of 150 to 850 μm.

[0157] Examples 11 and 12 and Comparative Example 1 Except for changing the raw materials and the amounts thereof as shown in Table 4, the same operation as in Example 10 was carried out to obtain water-absorbent resins.

[0158] Example 14 25 g of the water-soluble polymer obtained in Production Example 15 (wet product with aqueous ethanol, wet ratio 61%) was placed in a 300 ml beaker, and 80 ml of aqueous ethanol (ethanol / water = 90 / 10 (volume ratio)) was added to disperse the water-soluble polymer. While stirring with a magnetic stirrer, 12.1 ml of 1N hydrochloric acid was gradually added using a measuring pipette. After the addition of hydrochloric acid, the mixture was stirred for 15 minutes and subjected to vacuum suction filtration to recover the water-soluble polymer that had been partially neutralized to free acid. The recovered water-soluble polymer was wet crystals containing aqueous alcohol. The wet crystals were transferred to a petri dish, placed in a blower dryer set at 150°C, dried for 1 hour, and subjected to a crosslinking treatment. The solid obtained after the treatment was pulverized in a mortar and mortar, and sieved using sieves with 150 μm and 850 μm mesh sizes to recover particles with particle sizes of 150 to 850 μm.

[0159] Example 16: 20 g of the water-soluble polymer obtained in Production Example 17 (wet product with hydrous acetone, wet ratio 61%) was placed in a 300 ml beaker, and 60 ml of hydrous acetone (acetone / water = 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 gradually added using a measuring pipette. After the addition of the aqueous NaOH solution, the mixture was stirred for 15 minutes and subjected to vacuum suction filtration to recover a water-soluble polymer in which a portion of the carboxylic acid introduced by maleic anhydride had been neutralized to its sodium salt. The recovered water-soluble polymer was wet crystals containing hydrous acetone. The wet crystals were transferred to a petri dish, placed in a blower dryer set at 70°C, and dried for 12 hours to undergo a crosslinking treatment. The solid obtained after the treatment was pulverized in a mortar and sieved using sieves with 150 μm and 850 μm mesh sizes to recover particles with particle sizes of 150 to 850 μm.

[0160] (4-3) Method of Partial Neutralization During Synthesis of Water-Soluble Polymer (Example 15) 25 g (wet crystals) of the water-soluble polymer obtained in Production Example 16 (wet product with hydrous methanol, wet ratio 63%) was transferred to a petri dish, placed in a blower dryer set at 70° C., and dried for 12 hours to carry out a crosslinking treatment. After the treatment, the obtained solid was pulverized in a mortar and sieved using sieves with openings of 150 μm and 850 μm, and particles with particle sizes of 150 to 850 μm were collected.

[0161] (5) Evaluation Method of Water-Soluble Polymers (5-1) Total Acid Number of Water-Soluble Polymers Approximately 0.3 g of water-soluble polymer was weighed into a 100 ml beaker and dissolved in 40 ml of ion-exchanged water. This aqueous solution was placed in a potentiometric titrator (AT-610, manufactured by Kyoto Electronics Manufacturing Co., Ltd.) equipped with a glass electrode (C-171, manufactured by Kyoto Electronics Manufacturing Co., Ltd.). If the sample was entirely sodium salt, the potential at this stage would be approximately 30 mV or less. 1N hydrochloric acid was added until the potential reached 320 mV or more, converting all carboxylic acid groups in the water-soluble polymer into a free acid state (resulting in an excess of hydrochloric acid). After confirming that the potential was 320 mV or more, neutralization titration was performed with a 0.1 N NaOH aqueous solution. In this titration, two inflection points were detected: the first inflection point was detected near 220 mV, and the second inflection 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 number is calculated by the following formula 1: Total acid number (mg KOH / g) = [{(Vb - Va) x 0.1 x fa x 56.11} / Sa] / (1 - wr) (Formula 1) where Va is the volume (ml) of 0.1 N NaOH consumed up to the first inflection point, Vb is the volume (ml) of 0.1 N NaOH consumed up to the second inflection point, fa is the titer of 0.1 N NaOH, and Sa is the amount of sample collected. wr is the wet ratio of the water-soluble polymer measured by the method described below.

[0162] (5-2) Free Acid Value of Water-Soluble Polymer in Case of Producing Crosslinked Polymer Subsequent to Acid Treatment or Alkali Treatment (Examples 10 to 12, Example 14, Example 16, Comparative Example 1) Since a crosslinked polymer is produced subsequent to an acid treatment or alkali treatment, direct quantification by titration is difficult. Therefore, the value calculated by the following calculation is defined as the free acid value.

[0163] When the acid treatment is performed with an acid having a pKa lower than that of the carboxylic acid of the water-soluble polymer, the amount of acid added during the acid treatment is essentially equal to the free acid value. Therefore, the free acid value is calculated using the following formula 2: Free acid value (mg KOH / g) = (Vc × N × fb × 56.11) ÷ (Sb - Vc × N × fb × 22) (Formula 2) where Vc is the volume (ml) of the aqueous acid solution used in the acid treatment, N is the normality of the aqueous acid solution, fb is the titer of the aqueous acid solution, and Sb is the weight (pure content) of the water-soluble polymer charged in the acid treatment.

[0164] When alkali treatment using sodium hydroxide is performed, free acid is neutralized by the alkali, and therefore the free acid value is calculated using the following formula 3: Free acid value (mg KOH / g) = {(Tav ÷ 56.11) × Sc - Vd × N × fc) ÷ (Sc + Vd × N × fc × 22)} × 56.11 (Formula 3) Here, Tav is the total acid value (mg KOH / g) of the water-soluble polymer, Vd is the volume (ml) of the aqueous sodium hydroxide solution used in the alkali treatment, N is the normality of the aqueous sodium hydroxide solution, fc is the titer of the aqueous sodium hydroxide solution, and Sc is the weight (net content) of the water-soluble polymer charged in the alkali treatment.

[0165] (5-3) Free Acid Value of Water-Soluble Polymer When Partial Neutralization is Performed During Synthesis of Water-Soluble Polymer (Example 15) In the method described in (5-1) Total Acid Value of Water-Soluble Polymer above, neutralization titration is performed without adding 1N hydrochloric acid before titration. In this case, one inflection point is detected and calculated using the following equation 4: Free Acid Value (mg KOH / g) = [Va × 0.1 × fa × 56.11 ÷ Sa] / (1 - wr) (Equation 4) Here, Va is the volume (ml) of 0.1N NaOH consumed up to the inflection point, fa is the titer of 0.1N NaOH, and Sa is the amount of sample collected. wr is the wet ratio of the water-soluble polymer.

[0166] (5-4) Wet Ratio of Water-Soluble Polymer The wet ratio refers to the percentage of weight loss relative to the initial weight of a sample when the sample is dried at a drying temperature of 130°C using a halogen moisture meter. In this example, 0.5 to 1.0 g of the water-soluble polymer was set in a halogen moisture meter HC103 manufactured by Mettler Toledo K.K., and measurement was performed at a drying temperature of 130°C, a switch-off standard of 1 mg / 50 seconds, and in % MC mode (a mode in which the MC value = (initial sample weight - dry weight) ÷ initial sample weight × 100) . The displayed MC value was the wet ratio.

[0167] (5-5) Degree of Etherification of Water-Absorbent Polymer Degree of Etherification = (162 x TAV) / (56100 - 80 x TAV) Here, TAV is the total acid value (unit: mgKOH / g) of the water-soluble polymer.

[0168] (5-6) Biomass Degree of Water-Soluble Polymer The biomass degree is the proportion (mass %) of elements derived from natural resources among the elements constituting a water-soluble polymer. The biomass degree was calculated using the degree of etherification according to the following formula. Note that in a water-absorbent resin obtained by partially neutralizing a water-soluble polymer and then drying it, the biomass degree is slightly higher because sodium salts are removed from acidic groups by partial neutralization. ((Molecular weight of glucose unit) - degree of etherification x 1) / ((Molecular weight of glucose unit) + (degree of etherification x (molecular weight of acidic group - 1))) x 100 In the numerator of the above formula, the molecular weight of the starch monomer (glucose unit) is 162. Because a hydrogen atom is removed upon introduction of an acidic group, the degree of etherification x 1 is subtracted. In the denominator of the above formula, when the acidic group is a carboxymethyl group (sodium salt), the molecular weight is 81. When the acidic group is a carboxyhexyl group, the molecular weight is 137. Because a hydrogen atom is removed upon introduction of an acidic group, a value obtained by subtracting 1 from the molecular weight is used.

[0169] (6) Evaluation Method for Water-Absorbent Resins (6-1) Absorption Capacity Without Load (Saline Solution) 1.0 g of a measurement sample was placed in a tea bag (20 cm long, 10 cm wide) made of nylon mesh with a mesh size of 63 μm (JIS Z8801-1:2006), and the bag was immersed in 1,000 ml of saline (salt concentration: 0.9 wt%) for 3 hours without stirring, and then hung for 10 minutes to drain. The weight (h1) including the tea bag was measured, and the water retention capacity was calculated using the following formula. The saline solution used and the temperature of the measurement atmosphere were 25°C ± 2°C. FSC (g / g) = (h1) - (h2). Note that (h2) is the weight of the tea bag measured using the same procedure as above, but without a measurement sample. Here, FSC is an abbreviation for Free Swell Capacity, which means free swelling capacity and refers to the absorption capacity without load.

[0170] (6-2) Water absorption capacity without pressure (ion-exchanged water) 0.2 g of a measurement sample was placed in a tea bag, and the weight (h1') including the tea bag after immersion was measured in the same manner as in the water absorption capacity without pressure (physiological saline), except that ion-exchanged water was used instead of physiological saline. The water retention capacity was calculated using the following formula. Note that (h2') is the weight of the tea bag measured in the same manner as above, but without a measurement sample. FSC (g / g) = {(h1') - (h2')} / 0.2

[0171] (6-3) Water Retention Capacity (Saline Solution) After measuring the water absorption capacity without pressure as described above, the tea bag was placed in a centrifuge and centrifuged at 150 G for 90 seconds to remove excess liquid components. The weight (h3) including the tea bag was measured and the water retention capacity was calculated using the following formula: CRC (g / g) = (h3) - (h4) where (h4) is the weight of the tea bag measured using the same procedure as above but without a measurement sample. Here, CRC is an abbreviation for Centrifuge Retention Capacity, which means centrifuge retention capacity and refers to water retention capacity.

[0172] (6-4) Water Retention Capacity (Ion-Exchanged Water) After measuring the water absorption capacity without pressure as described above, the tea bag was placed in a centrifuge and centrifuged at 150 G for 90 seconds to remove excess liquid components, and the weight (h3') including the tea bag was measured and the water retention capacity was calculated using the following formula: CRC (g / g) = {(h3') - (h4')} / 0.2 Note that (h4') is the weight of the tea bag measured using the same procedure as above but without a measurement sample.

[0173] (6-5) Gel properties of water-absorbent resin After the water retention test for physiological saline solution described in (6-3) was conducted, the gel was removed from the tea bag, and the appearance of the gel was visually evaluated according to the following six levels. A photograph of the gel state is shown in Figure 1. 1: Soluble in water 2: Semi-soluble in water 3: Insoluble in water and watery 4: Insoluble in water and slightly watery 5: Insoluble in water and soft 6: Insoluble in water and firm

[0174] (6-6) Alkali Decomposability 0.5 g of water-absorbent resin and 9.5 g of 1% NaOH aqueous solution were weighed into a 13.5 ml screw cap bottle and dissolved by stirring for 60 minutes using a mix rotor. Next, a 30-mesh wire mesh (mesh opening: 500 μm) measuring approximately 50 mm x 50 mm was prepared, and the weight (Wm1) of the wire mesh was measured. The above mixture was naturally filtered using the weighed wire mesh, and after filtration, the mesh was washed with approximately 5 ml of ion-exchanged water. After washing, the wire mesh was dried in a 120 °C air dryer for 2 hours, and the weight (Wm2) of the wire mesh after drying was measured, and the 30-mesh pass residue was calculated using the following formula. If the water-absorbent resin is completely dissolved, the 30-mesh pass residue will be 0%, and if it is not dissolved and a gel remains, the 30-mesh pass residue will exceed 0%. 30-mesh pass residue (%) = (Wm2 - Wm1) / 0.5 x 100

[0175] (7) Evaluation of Water-Absorbent Resin The water-absorbent resins obtained in Examples 1 to 16 and Comparative Examples 1 to 3 were evaluated for water absorption performance and gel properties. The results are shown in Table 4.

[0176]

[0177] In Comparative Examples 1 to 3, the solid content of the water-soluble polymer when dried was less than 90%, so gelation did not occur and water absorption performance was not observed. In Examples 1 to 16, the solid content of the water-soluble polymer when dried was 90% or more, so that a water-insoluble water-absorbent resin was obtained and sufficient water absorption performance was achieved.

[0178] (8) Production of Partially Hydrolyzed Products of Processed Starch (Production Examples 18 to 22) Partially hydrolyzed products of processed starch were produced by the following method. The weight average molecular weights and dispersities of the partial hydrolyzed products are shown in Table 5.

[0179] (Production Example 18) Partial starch hydrolyzate derived from tapioca starch acetate Tapioca starch acetate was suspended in city water to a concentration of 45% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. To this starch milk, an amylomaltase crude enzyme solution was added at 3 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by reaction at 90°C for 5 hours with stirring.

[0180] (Production Example 19) Partial starch hydrolysate derived from oxidized acetylated tapioca starch Oxidized acetylated tapioca starch was suspended in city water to a concentration of 45% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. To this starch milk, an amylomaltase crude enzyme solution was added at 3 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by reaction at 90°C for 5 hours with stirring.

[0181] (Production Example 20) Partial starch hydrolyzate derived from acetylated tapioca starch Oxidized acetylated tapioca starch was suspended in city water to a concentration of 45% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. To this starch milk, an amylomaltase crude enzyme solution was added at 5 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by reaction at 90°C for 5 hours with stirring.

[0182] (Production Example 21) Partial starch hydrolysate derived from hydroxypropylated starch Hydroxypropylated starch was suspended in city water to a concentration of 45% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. To this starch milk, an amylomaltase crude enzyme solution was added at 3 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by reaction at 90°C for 5 hours with stirring.

[0183] (Production Example 22) Partial starch hydrolysate derived from hydroxypropylated phosphate cross-linked starch Hydroxypropylated phosphate cross-linked starch was suspended in city water to a concentration of 30% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. To this starch milk, an amylomaltase crude enzyme solution was added at 3 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by reaction at 90°C for 5 hours with stirring.

[0184]

[0185] (9) Production of Water-Soluble Polymer (Production Example 23) 100 g of a 45 wt % aqueous solution of the starch partial hydrolysate produced in Production Example 18 (0.83 mol of hydroxyl groups in the starch partial hydrolysate) was charged into a 500 ml separable flask equipped with a stirrer, thermometer, and condenser. Next, 34.4 g of a 48.8% aqueous solution of sodium hydroxide (0.42 mol, 0.5 equivalents relative to the hydroxyl groups in the starch partial hydrolysate) was charged, and the solution was stirred at 60°C or below until it became completely homogeneous. After confirming that the solution was homogeneous, an aqueous solution prepared by dissolving 18.9 g of monochloroacetic acid (0.2 mol, 0.2 equivalents relative to the hydroxyl groups in the starch partial hydrolysate) in 25 g of ion-exchanged water was added dropwise at 50-60°C over 30 minutes. After charging the aqueous monochloroacetic acid solution, the temperature was adjusted to 45-50°C, and the mixture was stirred for 10 hours. The reaction was terminated when the chloride ion content in the reaction solution was measured by potentiometric titration using a 0.01 N silver nitrate solution, and reached 98% or more of the calculated chloride ion content of 4.0% when all the monochloroacetic acid had reacted. In this production example, the chloride content was 4.0%.

[0186] After the reaction was completed, the reaction solution was diluted with 400 g of ion-exchanged water. The diluted reaction solution was cooled to room temperature and added to 5 L of methanol over approximately 30 minutes to precipitate and reprecipitate the water-soluble polymer. After the entire reaction solution was added, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the methanol was subjected to solid-liquid separation by filtration under reduced pressure.

[0187] Subsequently, in order to remove sodium chloride contained in the water-soluble polymer, the recovered water-soluble polymer was redispersed in 1 L of aqueous methanol (methanol / water 80 / 20 (volume ratio)), stirred at room temperature for 30 minutes, washed, and then subjected to solid-liquid separation by vacuum filtration, and the water-soluble polymer was recovered again. The chlorine content of the recovered water-soluble polymer was measured by potentiometric titration using a 0.01 N silver nitrate solution, and the washing process was repeated until the chlorine content was less than 1%. The total acid number of the obtained water-soluble polymer was 146 mg KOH / g, and the degree of etherification calculated from the total acid number was 0.53. The weight-average molecular weight was 18.6 × 10 ６ The dispersity was 19.7.

[0188] (Production Examples 24 to 27) Water-soluble polymers were produced by the same procedure as in Production Example 23, except that the raw materials, amounts charged, and reaction conditions were changed as shown in Table 6. The total acid number, degree of etherification, weight-average molecular weight, and dispersity of the obtained water-soluble polymers are shown in Table 6.

[0189]

[0190] (10) Production of Water-Absorbent Resin (Example 17) 25 g of the water-soluble polymer obtained in Production Example 23 (wet product with hydrous methanol, wet ratio 65%) was placed in a 300 ml beaker, and 120 ml of hydrous 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, 0.8 ml of 1 N hydrochloric acid was gradually added using a measuring pipette. After adding the hydrochloric acid, the mixture was stirred for 15 minutes and subjected to vacuum suction filtration to recover a water-soluble polymer that had been partially neutralized to free acid. The recovered water-soluble polymer was wet crystals containing hydrous alcohol. The wet crystals were transferred to a petri dish, placed in a blower dryer set at 70 °C, and dried for 12 hours to undergo a crosslinking treatment. The solid obtained after the treatment was pulverized in a mortar and sieved using sieves with openings of 150 μm and 850 μm, and particles with particle sizes of 150 to 850 μm were recovered.

[0191] (Examples 18 to 25) Water-absorbent resins were obtained by the same operation as in Example 17, except that the raw materials, charged amounts, and reaction conditions were changed to those shown in Table 7. Table 7 shows the free acid value, water absorption performance, gel properties, and alkali decomposition properties of each water-absorbent resin.

[0192]

[0193] Even in Examples 17 to 25 in which processed starch was used as a raw material, by making the solid content of the water-soluble polymer at the time of drying 90% or more, a water-insoluble water-absorbent resin was obtained, and sufficient water absorption performance was achieved.

[0194] (Production Example 28) Partial starch hydrolysate (1) derived from tapioca starch: Tapioca starch was suspended in city water to a concentration of 45% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. To this starch milk, an amylomaltase crude enzyme solution was added at 8.0 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by a reaction at 95°C for 6 hours with stirring to prepare liquefied starch. The weight-average molecular weight of the resulting partial starch hydrolysate was 2,610,000, and the dispersity was 7.9.

[0195] (Production Example 29) Production of Tapioca Starch-Derived Water-Soluble Polymer (1) 100 g of a 45 wt % aqueous solution of the tapioca starch-derived starch partial hydrolysate obtained in Production Example 28 (0.83 mol of hydroxyl groups in the partial starch hydrolysate) was charged into a 500 ml separable flask equipped with a stirrer, thermometer, and condenser. Next, 34.0 g of a 48.8% aqueous solution of sodium hydroxide (0.42 mol, 0.50 molar equivalents per mole of hydroxyl groups in the partial starch hydrolysate) was charged, and the solution was stirred at 60°C or below until it became completely homogeneous. After confirming that the solution was homogeneous, an aqueous solution prepared by dissolving 19.8 g of monochloroacetic acid (0.21 mol, 0.3 molar equivalents per mole of hydroxyl groups in the partial starch hydrolysate) in 50 g of ion-exchanged water was added dropwise at 50-60°C over 30 minutes. After charging the aqueous monochloroacetic acid solution, the temperature was adjusted to 45-50°C, and the mixture was stirred for 12 hours. The reaction was terminated when the chloride ion content in the reaction solution was measured by potentiometric titration using a 0.01 N silver nitrate solution, and reached 98% or more of the calculated chloride ion content of 3.7% when all the monochloroacetic acid had reacted. In this production example, the chloride content was 3.7%.

[0196] After the reaction was completed, the reaction solution was diluted with 900 g of ion-exchanged water. The diluted reaction solution was cooled to room temperature and added to 6000 ml of methanol over approximately 30 minutes to precipitate and reprecipitate the water-soluble polymer. After the entire reaction solution was added, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the methanol was subjected to solid-liquid separation by vacuum filtration.

[0197] Subsequently, to remove the sodium chloride contained in the water-soluble polymer, the recovered water-soluble polymer was redispersed in 1000 ml of aqueous methanol (methanol / water 80 / 20 (volume ratio)), stirred at room temperature for 30 minutes, washed, and then subjected to solid-liquid separation by vacuum filtration, and the water-soluble polymer was recovered again. The chlorine content of the recovered water-soluble polymer was measured by potentiometric titration using a 0.01 N aqueous silver nitrate solution, and the washing process was repeated until the chlorine content was less than 1%. The total acid number of the obtained water-soluble polymer was 130 mg KOH / g, the degree of etherification calculated from the total acid number was 0.46, the weight-average molecular weight was 8.12 million, the degree of dispersity was 13.7, and the biomass degree was 81%.

[0198] (Production Example 30) Production of Tapioca Starch-Derived Water-Soluble Polymer (2) 95 g of a 45 wt % aqueous solution of the tapioca starch-derived partial starch hydrolysate obtained in Production Example 28 (0.79 mol of hydroxyl groups in the partial starch hydrolysate) and 2.5 g of tapioca starch (2.2 g of pure tapioca starch, 0.04 mol of hydroxyl groups in the tapioca starch) were placed in a 500 ml separable flask equipped with a stirrer, thermometer, and condenser. Next, 34.0 g of a 48.8% aqueous sodium hydroxide solution (0.42 mol, 0.50 molar equivalents per mole of hydroxyl groups in the partial starch hydrolysate) was added, and the mixture was stirred at 60° C. or below until the solution became completely homogeneous. After confirming that the solution was homogeneous, an aqueous solution prepared by dissolving 19.8 g of monochloroacetic acid (0.21 mol, 0.3 molar equivalents per mole of hydroxyl groups in the starch partial hydrolyzate) in 50 g of ion-exchanged water was added dropwise over 30 minutes at 50-60°C. After the addition of the monochloroacetic acid aqueous solution, the temperature was adjusted to 45-50°C and the mixture was stirred for 12 hours. The reaction was terminated when the chloride ion content in the reaction solution was sampled and measured by potentiometric titration using a 0.01 N silver nitrate aqueous solution. The end point of the reaction was determined by the condition that the chloride ion content had reached 98% or more of the calculated chloride ion content of 3.7% when all the monochloroacetic acid had reacted. In this production example, the chlorine content was 3.7%.

[0199] After the reaction was completed, the reaction solution was diluted with 900 g of ion-exchanged water. The diluted reaction solution was cooled to room temperature and added to 6000 ml of methanol over approximately 30 minutes to precipitate and reprecipitate the water-soluble polymer. After the entire reaction solution was added, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the methanol was subjected to solid-liquid separation by vacuum filtration.

[0200] Subsequently, to remove the sodium chloride contained in the water-soluble polymer, the recovered water-soluble polymer was redispersed in 1000 ml of aqueous methanol (methanol / water 80 / 20 (volume ratio)), stirred at room temperature for 30 minutes, washed, and then subjected to solid-liquid separation by vacuum filtration, and the water-soluble polymer was recovered again. The chlorine content of the recovered water-soluble polymer was measured by potentiometric titration using a 0.01 N aqueous silver nitrate solution, and the washing process was repeated until the chlorine content was less than 1%. The total acid number of the obtained water-soluble polymer was 132 mg KOH / g, the degree of etherification calculated from the total acid number was 0.47, the weight-average molecular weight was 8,050,000, the degree of dispersity was 14.7, and the biomass degree was 81%.

[0201] (Production Example 31) Partial starch hydrolysate (2) derived from tapioca starch: Tapioca starch was suspended in city water to a concentration of 45% (w / w), and then 1N sodium hydroxide was added to adjust the pH to 6.0 to obtain starch milk. To this starch milk, an amylomaltase crude enzyme solution was added at 4.0 units per gram of starch solids, and the mixture was stirred at room temperature for 30 minutes, followed by a reaction at 85°C for 5 hours with stirring to prepare liquefied starch. The weight-average molecular weight of the resulting partial starch hydrolysate was 4,010,000, and the dispersity was 14.1.

[0202] (Production Example 32) Production of Tapioca Starch-Derived Water-Soluble Polymer (3) 100 g of a 45 wt % aqueous solution of the tapioca starch-derived starch partial hydrolysate obtained in Production Example 31 (0.83 mol of hydroxyl groups in the partial starch hydrolysate) was charged into a 500 ml separable flask equipped with a stirrer, thermometer, and condenser. Next, 59.5 g of a 48.8% aqueous solution of sodium hydroxide (0.73 mol, 0.87 molar equivalents per mole of hydroxyl groups in the partial starch hydrolysate) was charged, and the solution was stirred at 60°C or below until it became completely homogeneous. After confirming that the solution was homogeneous, an aqueous solution prepared by dissolving 32.7 g of monochloroacetic acid (0.35 mol, 0.4 molar equivalents per mole of hydroxyl groups in the partial starch hydrolysate) in 80 g of ion-exchanged water was added dropwise at 50-60°C over 30 minutes. After charging the aqueous monochloroacetic acid solution, the temperature was adjusted to 45-50°C, and the mixture was stirred for 12 hours. The reaction was terminated when the chloride ion content in the reaction solution was measured by potentiometric titration using a 0.01 N silver nitrate solution, and reached 98% or more of the calculated chloride ion content of 4.6% when all the monochloroacetic acid had reacted. In this production example, the chloride content was 4.6%.

[0203] After the reaction was completed, the reaction solution was diluted with 1,000 g of ion-exchanged water. The diluted reaction solution was cooled to room temperature and added to 7,000 ml of methanol over approximately 30 minutes to precipitate and reprecipitate the water-soluble polymer. After the entire reaction solution was added, the mixture was stirred for 30 minutes, and the water-soluble polymer dispersed in the methanol was subjected to solid-liquid separation by vacuum filtration.

[0204] Subsequently, in order to remove sodium chloride contained in the water-soluble polymer, the recovered water-soluble polymer was redispersed in 1000 ml of aqueous methanol (methanol / water 80 / 20 (volume ratio)), stirred at room temperature for 30 minutes, washed, and then subjected to solid-liquid separation by vacuum filtration, and the water-soluble polymer was recovered again. The chlorine content of the recovered water-soluble polymer was measured by potentiometric titration using a 0.01 N silver nitrate aqueous solution, and the washing process was repeated until the chlorine content was less than 1%. The total acid number of the obtained water-soluble polymer was 179 mg KOH / g, the degree of etherification calculated from the total acid number was 0.69, the weight average molecular weight was 9.6 million, the degree of dispersity was 36.7, and the biomass degree was 74%.

[0205] Example 26: Production of absorbent resin 25 g of the water-soluble polymer obtained in Production Example 29 (wet product with hydrous methanol, wet ratio 65%) was placed in a 500 ml beaker, and 200 ml of hydrous 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, 1.7 ml of a 1N aqueous hydrochloric acid solution was gradually added using a measuring pipette. After the addition of hydrochloric acid, the mixture was stirred for 1 hour and subjected to vacuum suction filtration to recover a water-soluble polymer that had been partially neutralized to free acid. The recovered water-soluble polymer was wet crystals containing hydrous alcohol. The wet crystals were transferred to a petri dish, placed in a blower dryer set at 70 °C, and dried for 20 hours to undergo a crosslinking treatment. The solid obtained after the treatment was pulverized in a mortar and sieved using sieves with openings of 150 μm and 850 μm to recover absorbent resin particles with particle sizes of 150 to 850 μm.

[0206] (Example 27) Production of liquid-absorbent resin The same operation as in Example 26 was carried out except that the 1N hydrochloric acid aqueous solution was changed to 0.23 g of DL-malic acid, and water-absorbent resin particles were collected.

[0207] (Example 28) Production of liquid-absorbent resin

[0222] The same operation as in Example 26 was performed except that the water-soluble polymer was changed to 25 g of the water-soluble polymer obtained in Production Example 30 (wet product with hydrous methanol, wet rate 66%), so that water-absorbent resin particles were recovered.

[0208] (Example 29) Production of liquid-absorbent resin

[0123] The same operation as in Example 26 was carried out except that the water-soluble polymer was changed to 20 g of the water-soluble polymer obtained in Production Example 32 (a wet product of hydrous methanol, wet rate 69%), the amount of 1 N hydrochloric acid aqueous solution was changed to 0.7 ml, the mixture was placed in an air-blowing dryer set at 85°C, dried for 10 hours, and subjected to a crosslinking treatment, so as to recover water-absorbent resin particles.

[0209]

[0210] In Examples 26 to 27 and 29, which used tapioca starch as a raw material, a water-insoluble water-absorbent resin was obtained by increasing the solid content of the water-soluble polymer to 90% or more when dried, and sufficient water absorption performance was achieved. In Example 28, a tapioca starch-derived starch partial hydrolyzate (Production Example 28) was mixed with unhydrolyzed tapioca starch, and then acidic groups were introduced to produce a water-soluble polymer (Production Example 30). As a result, the water-absorbent resin exhibited a high free-flow capacity (FSC) for ion-exchanged water.

Claims

1. A step (a1) to obtain a partially hydrolyzed starch product by reducing the molecular weight of the starch, Step (a2) involves introducing an acidic group into the partially decomposed starch product obtained in step (a1) to obtain a water-soluble polymer, and (a3) A step of drying a water-soluble polymer in the presence of water at 70-180°C until the solids content is 90% or more. A method for producing a water-absorbent resin that forms a physical gel upon water absorption.

2. The process includes a step (aa) of desalting the water-soluble polymer after the above step (a2), A method for producing a water-absorbent resin according to claim 1.

3. In the above step (a2), or, After step (a2) and before step (a3), The process includes a step (a) of partially neutralizing the acidic groups of a water-soluble polymer. A method for producing a water-absorbent resin according to claim 1 or 2.

4. After step (a2) and before step (a3), This process does not involve a step to partially neutralize the acidic groups of the water-soluble polymer. A method for producing a water-absorbent resin according to claim 1 or 2.

5. In step (a3) ​​above, the mixture is dried until the solid content is 90% or more and 99% or less. A method for producing a water-absorbent resin according to claim 1 or 2.

6. The acidic group is a carboxyalkyl group, a carboxyalkenyl group, or a sulfoalkyl group. A method for producing a water-absorbent resin according to claim 1 or 2.

7. In step (a1) above, a partially decomposed starch product is obtained having a weight-average molecular weight (Mw) of 7.5 million or less and / or a degree of dispersion (weight-average molecular weight (Mw) / number-average molecular weight (Mn)) of 5 or more. A method for producing a water-absorbent resin according to claim 1 or 2.

8. In step (a1) above, the molecular weight of the starch is reduced by enzymatic treatment. A method for producing a water-absorbent resin according to claim 1 or 2.

9. In step (a2), an acidic group is introduced into a mixture of the partially decomposed starch obtained in step (a1) and starch that has not been reduced in molecular weight. A method for producing a water-absorbent resin according to claim 1 or 2.

10. The water-soluble polymer obtained in step (a2) has a pullulan-based weight-average molecular weight (Mw) of 500,000 to 40,000,000, determined by aqueous size exclusion chromatography analysis. A method for producing a water-absorbent resin according to claim 1 or 2.

11. A method for producing a water-absorbent resin according to claim 1 or 2, wherein the water-absorbent resin obtained has the following characteristics: (a) The water absorption rate of ion-exchanged water under no pressure is 100 to 500 g / g, (b) The water retention rate of ion-exchanged water is 80 to 300 g / g. (c) The unpressurized water absorption rate of physiological saline is 10 to 70 g / g, and / or (d) The water retention rate of physiological saline is 5 to 65 g / g.