Starch-based water-absorbent resin

Abstract

To provide a biodegradable starch-based water-absorbing resin having a relatively slow biodegradation rate in soil while having sufficient water-absorbing performance.SOLUTION: A starch-based water absorbent polymer obtained by cross-linking carboxyalkyl starch, wherein a water absorption ratio of ion-exchanged water of the starch-based water absorbent polymer is 5 to 200g / g, and a viscosity of a cross-linked cut product obtained by cutting cross-links of the starch-based water absorbent polymer is 120 to 40000mPa·s.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a starch-based water-absorbent resin obtained by crosslinking carboxyalkyl starch. [Background technology]

[0002] In crop cultivation, irrigation is an important task because it can affect the yield and quality of the harvested crop. However, irrigation throughout the crop cultivation period is a significant burden on workers in terms of both time and labor.

[0003] To address the above-mentioned issues, an approach has been proposed in which a polyacrylic acid-based water-absorbent resin composition or the like capable of absorbing water at a rate of several tens to several thousands times its own weight is placed in the soil to increase the water retention capacity of the soil and reduce the frequency of irrigation work (for example, Patent Document 1). However, placing a polyacrylic acid-based water-absorbent resin composition or the like, which has poor biodegradability, in the soil places a heavy burden on the environment. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 62-273283 Summary of the Invention [Problem to be solved by the invention]

[0005] It has been considered to use a biodegradable water-absorbent resin composition instead of a polyacrylic acid-based water-absorbent resin composition which has poor biodegradability. However, conventional biodegradable water-absorbent resin compositions have had the problem that they are inferior in water-absorbing performance or have a high biodegradation rate, so that they decompose in a short period of time and are unable to maintain their water-absorbing performance until the purpose of promoting the establishment of trees and their initial growth at the time of planting is achieved.

[0006] An object of the present invention is to provide a biodegradable starch-based water-absorbent resin that has sufficient water-absorbing performance and a relatively slow rate of biodegradation in soil. [Means for solving the problem]

[0007] The present invention provides A starch-based water absorbent resin obtained by crosslinking a carboxyalkyl starch, wherein the starch-based water absorbent resin has an absorption capacity for ion-exchanged water of 5 to 200 g / g, and a crosslinked product obtained by cutting the crosslinks of the starch-based water absorbent resin using the crosslink cutting method described below has a viscosity of 120 to 40,000 mPa·s as measured using the viscosity measurement method described below. <Crosslink cutting method> 5 g of a starch-based water absorbent resin having a gel form is added to 100 g of a mixed solvent of methanol and 4% aqueous sodium hydroxide solution (methanol:4% aqueous sodium hydroxide solution is 20:80 in mass ratio), and stirred at room temperature with a stirring bar and a magnetic stirrer until the starch-based water absorbent resin in the mixed solvent no longer has a gel form. Next, the mixed solvent containing the starch-based water-absorbent resin is poured into a 1 L beaker containing 300 g of methanol to obtain a precipitate of the starch-based water-absorbent resin. Next, the precipitate in methanol is recovered by filtration under reduced pressure (vacuum degree 900±100 hPa) using filter paper (retention particle size 5 μm). The recovered precipitate is added to an 80% by mass aqueous methanol solution and washed by stirring with a stir bar and a magnetic stirrer for 10 minutes. The precipitate is then recovered by filtration under reduced pressure (vacuum degree 900±100 hPa) using filter paper (retention particle size 5 μm) after washing with the 80% by mass aqueous methanol solution. Next, the precipitate that has been recovered and washed with the 80% by mass aqueous methanol solution is immersed in methanol and stirred for 10 minutes using a stirring bar and a magnetic stirrer, and then filtered under reduced pressure (vacuum degree 900±100 hPa) using filter paper (retention particle size 5 μm) to recover the precipitate that has been washed with methanol. Next, the precipitate after washing with methanol is dried in a ventilation dryer to obtain a cross-linked cut product of the starch-based water-absorbent resin. <Viscosity measurement method> Five grams of the crosslinked fragments were allowed to stand for one hour at an atmospheric humidity of 50±10% RH, and then weighed (W1). The sample was heated at 120±5°C for 30 minutes using an infrared moisture content meter (KETT Corporation, JE400: lamp specifications 100V, 40W), and the weight of the sample was then weighed (W2). The loss on drying (%) of the crosslinked fragments was calculated using the following formula (1). Drying loss S (%) of the crosslinked cut product = ((W1 - W2) / W1) × 100 (1) The loss on drying S of the fragmented crosslinks is water, and the rest is solids. The amount (g) of the fragmented crosslinks required to obtain 50 g of an aqueous solution containing 1% by mass of the fragmented crosslinks is calculated using the following formula (2). The amount of the crosslinked fragments P (g) = 50 (g) × 0.01 × (100 / (100-S)) (2) An amount (g) of ion-exchanged water obtained by dividing the amount P (g) of ion-exchanged water by 50 (g) was charged into screw tube No. 7, and the amount P (g) of the cross-linked fragments was added to the ion-exchanged water in the screw tube while stirring with a stirrer and a magnetic stirrer.The screw tube was then capped and the mixture was stirred with a stirrer and a magnetic stirrer to dissolve the added cross-linked fragments, thereby obtaining 50 g of an aqueous solution of the cross-linked fragments with a solid content of 1% by mass. The screw tube containing 50 g of an aqueous solution containing 1% by mass of solids of the crosslinked fragments was placed in a thermostatic bath set at 25°C for at least 1 hour, and then the rotor of a B-type viscometer (Toki Sangyo Viscometer TVB-10M) was placed in the 1% by mass aqueous solution of solids in the screw tube while it was still in the thermostatic bath, and the rotor was rotated at 60 rpm for 1 minute, and the viscosity measured was taken as the viscosity of the crosslinked fragments. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a biodegradable starch-based water-absorbent resin that has sufficient water-absorbing performance and a relatively slow rate of biodegradation in soil. DETAILED DESCRIPTION OF THE INVENTION

[0009] <Method for producing polysaccharide-based water-absorbent resin> The starch-based water absorbent resin of the present embodiment is a starch-based water absorbent resin obtained by crosslinking carboxyalkyl starch, wherein the starch-based water absorbent resin has an absorption capacity for ion-exchanged water of 5 to 200 g / g, and a crosslink cut product obtained by cutting crosslinks of the starch-based water absorbent resin by the crosslink cutting method described below has a viscosity of 120 to 40,000 mPa s, as measured by the viscosity measurement method described below. [Crosslink cutting method] 5 g of a starch-based water absorbent resin having a gel form is added to 100 g of a mixed solvent of methanol and 4% aqueous sodium hydroxide solution (methanol:4% aqueous sodium hydroxide solution is 20:80 in mass ratio), and stirred at room temperature with a stirring bar and a magnetic stirrer until the starch-based water absorbent resin in the mixed solvent no longer has a gel form. Next, the mixed solvent containing the starch-based water-absorbent resin is poured into a 1 L beaker containing 300 g of methanol to obtain a precipitate of the starch-based water-absorbent resin. Next, the precipitate in methanol is recovered by filtration under reduced pressure (vacuum degree 900±100 hPa) using filter paper (No. 2, manufactured by Advantec Toyo Co., Ltd., retention particle size 5 μm). The recovered precipitate is added to an 80% by mass aqueous methanol solution and washed by stirring with a stir bar and a magnetic stirrer for 10 minutes. The precipitate is then recovered by filtration under reduced pressure (vacuum degree 900±100 hPa) using filter paper (No. 2, manufactured by Advantec Toyo Co., Ltd., retention particle size 5 μm) after washing with the 80% by mass aqueous methanol solution. Next, the precipitate that has been recovered and washed with the 80% by mass aqueous methanol solution is immersed in methanol and stirred for 10 minutes using a stirring bar and a magnetic stirrer, and then filtered under reduced pressure (vacuum degree 900±100 hPa) using filter paper (No. 2, manufactured by Advantec Toyo Co., Ltd., retention particle size 5 μm) to recover the precipitate that has been washed with methanol. Next, the precipitate after washing with methanol is dried in a ventilation dryer to obtain a cross-linked cut product of the starch-based water-absorbent resin. [Viscosity measurement method] Five grams of the crosslinked fragments were allowed to stand for one hour at an atmospheric humidity of 50±10% RH, and then weighed (W1). The sample was heated at 120±5°C for 30 minutes using an infrared moisture content meter (KETT Corporation, JE400: lamp specifications 100V, 40W), and the weight of the sample was then weighed (W2). The loss on drying (%) of the crosslinked fragments was calculated using the following formula (1). Drying loss S (%) of the crosslinked cut product = ((W1 - W2) / W1) × 100 (1) The loss on drying S of the fragmented crosslinks is water, and the rest is solids. The amount (g) of the fragmented crosslinks required to obtain 50 g of an aqueous solution containing 1% by mass of the fragmented crosslinks is calculated using the following formula (2). The amount of the crosslinked fragments P (g) = 50 (g) × 0.01 × (100 / (100-S)) (2) An amount (g) of ion-exchanged water obtained by dividing the amount P (g) of ion-exchanged water by 50 (g) was charged into screw tube No. 7, and the amount P (g) of the cross-linked fragments was added to the ion-exchanged water in the screw tube while stirring with a stirrer and a magnetic stirrer.The screw tube was then capped and the mixture was stirred with a stirrer and a magnetic stirrer to dissolve the added cross-linked fragments, thereby obtaining 50 g of an aqueous solution of the cross-linked fragments with a solid content of 1% by mass. The screw tube containing 50 g of an aqueous solution containing 1% by mass of solids of the crosslinked fragments was placed in a thermostatic bath set at 25°C for at least 1 hour, and then the rotor of a B-type viscometer (Toki Sangyo Viscometer TVB-10M) was placed in the 1% by mass aqueous solution of solids in the screw tube while it was still in the thermostatic bath, and the rotor was rotated at 60 rpm for 1 minute, and the viscosity measured was taken as the viscosity of the crosslinked fragments.

[0010] The starch-based water-absorbent resin of this embodiment has sufficient water-absorbing performance and also has a relatively slow rate of biodegradation in soil.

[0011] The starch-based water-absorbent resin of this embodiment is a water-absorbent resin obtained by crosslinking one or more selected from the group consisting of carboxyalkyl starch and its salts. Carboxyalkyl starch is a starch having a carboxyalkyl group introduced therein. Examples of salts of carboxyalkyl starch include alkali metal salts such as sodium salts and potassium salts.

[0012] In this embodiment, the starch species used as raw material may be refined or unrefined. Examples of refined starch species include tapioca starch, potato starch, cornstarch (including waxy cornstarch and high-amylose starch), wheat starch, rice starch, sweet potato starch, cassava starch, arrowroot starch, and mung bean starch. Examples of unrefined starch species include rice flour (including non-glutinous rice, glutinous rice, high-amylose rice, long-grain varieties, and their brown rice varieties) and wheat flour. These may be used in combination of one or more types, or a starch species in which refined and unrefined starches are mixed in any ratio may be used.

[0013] Examples of carboxyalkyl starch include carboxymethyl starch, carboxyethyl starch, and carboxypropyl starch, and among these, carboxymethyl starch is preferred from the viewpoint of water absorbency.

[0014] The number of carboxyalkyl groups in the carboxyalkyl starch before crosslinking is preferably 0.1 to 1.5 per monosaccharide unit constituting the starch, more preferably 0.3 to 0.8. Within this range, sufficient water absorption performance can be obtained. The number of carboxyalkyl groups in the carboxyalkyl starch can be measured by the method described in the Examples.

[0015] The mode of crosslinking of the starch-based water absorbent resin of the present embodiment is not particularly limited, and a crosslinking agent may cause a carboxyalkyl group and a hydroxyl group of the carboxyalkyl starch to form an ester bond, or the crosslinking agent may react with a carboxyalkyl group or a hydroxyl group of the carboxyalkyl starch to form an ester bond.

[0016] Examples of the crosslinking agent include one or more selected from the group consisting of malic acid, succinic acid, maleic acid, citric acid, tartaric acid, oxalic acid, itaconic acid, acetic acid, hydrochloric acid, phosphoric acid, sulfuric acid, boric acid, potassium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium chloride, ammonium dihydrogen phosphate, and ammonium sulfate. Among these, from the viewpoint of reducing the environmental load after biodegradation, one or more selected from the group consisting of malic acid, succinic acid, tartaric acid, phosphoric acid, sodium dihydrogen phosphate, ammonium chloride, and ammonium dihydrogen phosphate are preferred.

[0017] The amount of the crosslinking agent added is preferably 0.02 to 2.0 parts by weight, more preferably 0.05 to 1.50 parts by weight, and even more preferably 0.05 to 1.0 part by weight, relative to 100 parts by weight of carboxyalkyl starch. When the amount of the crosslinking agent added is within the above range, a good balance between the decomposition rate in soil and water absorbency is achieved.

[0018] The starch-based water absorbent resin of this embodiment can be produced by mixing carboxyalkyl starch and the crosslinking agent and heating the mixture. Examples of a method for mixing carboxyalkyl starch and the crosslinking agent include a method for uniformly mixing carboxyalkyl starch and the crosslinking agent using a mixing device such as a cylindrical mixer, a screw mixer, a screw extruder, a turbulizer, a Nauta mixer, a double-arm kneader, a fluid mixer, a V-mixer, a mincing mixer, a ribbon mixer, an airflow mixer, a rotating disk mixer, a conical blender, and a roll mixer.

[0019] The temperature at which the carboxyalkyl starch and the crosslinking agent are mixed is not particularly limited, but is preferably 10 to 150°C, and more preferably 20 to 100°C.

[0020] After mixing the carboxyalkyl starch with the crosslinking agent, it is preferable to carry out a heat treatment. From the viewpoint of improving water absorption performance, the heating temperature in the heat treatment is 100 to 180°C, more preferably 110 to 170°C, and particularly preferably 120 to 160°C. The heating time in the heat treatment can be appropriately set depending on the heating temperature, but from the viewpoint of absorption performance, it is preferably 5 to 180 minutes, more preferably 15 to 90 minutes.

[0021] The water absorption capacity of the starch-based water absorbent resin for ion-exchanged water is 5 to 200 g / g, preferably 10 to 150 g / g, from the viewpoint of the growth of plants such as crops, etc. In this specification, the water absorption capacity of the starch-based water absorbent resin for ion-exchanged water is measured by the method described in the Examples.

[0022] From the viewpoint of controlling biodegradability, the starch-based water absorbent resin has a viscosity of 120 to 40,000 mPa·s, preferably 150 to 10,000 mPa·s, as measured by the viscosity measurement method, of the crosslinked fragments obtained by cutting the crosslinks of the starch-based water absorbent resin using the crosslink cutting method. If the viscosity of the crosslinked fragments measured by the viscosity measurement method is less than 120 Pa·s, the starch-based water absorbent resin will decompose too quickly in the soil, and will not be able to maintain sufficient water for the initial growth of plants for the required period (about 30 days) before decomposing. On the other hand, if the viscosity of the crosslinked fragments measured by the viscosity measurement method exceeds 40,000 Pa·s, the starch-based water absorbent resin will decompose too slowly in the soil, and may remain in the soil even after application the following year, raising concerns about increased environmental impact.

[0023] The viscosity of the crosslinked fragments measured by the viscosity measurement method can be adjusted by the weight average molecular weight (Mw) of the starch-based water absorbent resin. There is no particular limitation on the method for adjusting the Mw of the starch-based water absorbent resin, and it can be adjusted by the type of solvent used in producing the starch-based water absorbent resin, the amount of crosslinking agent added, the degree of shear when mixing the crosslinking agent, etc. Note that the method for measuring the Mw of the starch-based water absorbent resin is not particularly limited, and for example, it can be determined in aqueous size exclusion chromatography based on a calibration curve of molecular weight and elution time prepared using pullulan with a known molecular weight. [Example]

[0024] The present invention will be further described below with reference to Examples and Comparative Examples, but the present invention is not limited thereto. The water retention capacity and other properties of the polysaccharide-based water-absorbent resin were measured in a room at 25±2°C and 50±5% RH by the following method.

[0025] <Measurement method> [Method for measuring absorption capacity] 1.00 g of a water-absorbent resin composition as a measurement sample was placed in a tea bag (20 cm long, 10 cm wide) made of nylon netting with a mesh size of 63 μm (JIS Z8801-1:2006), and the bag was immersed in 1,000 ml of ion-exchanged water for 1 hour without stirring, then pulled out and hung for 15 minutes to drain. Thereafter, the weight (w1) including the tea bag was measured. The weight (w2) of the tea bag without the measurement sample was measured in the same manner as above. The water retention capacity was calculated using the measured (w1) and (w2) and the following formula. Ion exchange water absorption capacity (g / g) = (w1) - (w2)

[0026] [Method for calculating the number of carboxyl groups per monosaccharide unit of carboxyalkyl starch] 100 mL of 0.1 mol / L hydrochloric acid was added to a 100 mL glass beaker, and 5 g of carboxyalkyl starch was added. After stirring for 1 hour, 400 g of methanol was added. The precipitate was collected by vacuum filtration and added to 100 g of a mixed solvent of methanol / water = 80 / 20 (w / w) and stirred for 10 minutes. The precipitate was collected again by vacuum filtration and washed with the mixed solvent, a process repeated five times. The final precipitate was washed with methanol and then dried at 40 °C to obtain approximately 3 g of dried carboxyalkyl starch (CMS-H). Ion-exchange water and a stir bar were placed in a 100 mL beaker. While stirring with a magnetic stirrer, approximately 0.5 g of CMS-H was added, followed by approximately 0.1 g of 48% aqueous sodium hydroxide solution. The exact amounts of CMS-H and 48% aqueous sodium hydroxide solution added were determined by the m CMS-H [g],m NaOH After adding the sodium hydroxide solution, the mixture was stirred for 1 hour to obtain CMS-Na solution-1. CMS-Na solution-1 was titrated with 0.1 mol / L hydrochloric acid using an automatic titrator, and the titration volume at the inflection point was defined as V [mL]. In addition, a blank was prepared by adding 48% sodium hydroxide solution m blank [g] was added to approximately 50 mL of ion-exchanged water and titrated with 0.1 mol / L hydrochloric acid in the same manner, and the titer at the inflection point was designated V0 [mL]. The loss on drying of the resulting CMS-H was measured using a heat-drying moisture meter MS-70 (set temperature: 125°C, heating time: 15 minutes, measurement mode: standard mode, heating pattern: standard heating, measurement unit: %MOIST / W), and the obtained value was designated w [%].

[0027] [Method for measuring viscosity of carboxyalkyl starch] Five grams of carboxyalkyl starch was left to stand for one hour at an atmospheric humidity of 50±10% RH, then weighed (W3). After heating at 120±5°C for 30 minutes using an infrared moisture content meter (KETT Corporation, JE400: lamp specifications 100V, 40W), the sample weight was then weighed (W4). The loss on drying (%) of the carboxyalkyl starch was calculated using the following formula (3): Loss on drying of carboxyalkyl starch S' (%) = ((W3 - W4) / W3) × 100 (3) The loss on drying S' of the carboxyalkyl starch was taken as water content, and the rest was taken as solid content. The amount (g) of carboxyalkyl starch needed to obtain 50 g of an aqueous solution with a solid content of 1% by mass of carboxyalkyl starch was calculated using the following formula (4). Amount of carboxyalkyl starch added: P'(g) = 50(g) × 0.01 × (100 / (100-S')) (4) An amount (g) of ion-exchanged water obtained by subtracting the amount P' (g) from 50 (g) was charged into screw tube No. 7, and while stirring with a stirrer and a magnetic stirrer, the amount P' (g) of carboxyalkyl starch was added to the ion-exchanged water in the screw tube.The screw tube was then capped and the mixture was stirred with a stirrer and a magnetic stirrer to dissolve the added carboxyalkyl starch, thereby obtaining 50 g of an aqueous solution of carboxyalkyl starch with a solids content of 1% by mass. The screw tube containing 50 g of a 1% by mass solids aqueous solution of carboxyalkyl starch was placed in a thermostatic bath set at 25°C for at least 1 hour, and then the rotor of a B-type viscometer (Toki Sangyo Viscometer TVB-10M) was placed in the 1% by mass solids aqueous solution in the screw tube while it was still in the thermostatic bath, and the rotor was rotated at 60 rpm for 1 minute, and the measured viscosity was taken as the viscosity of the carboxyalkyl starch.

[0028] [Method for measuring viscosity of starch-based water-absorbent resin after crosslinking is broken] 5 g of a starch-based water absorbent resin having a gel form was added to 100 g of a mixed solvent of methanol and 4% aqueous sodium hydroxide solution (methanol:4% aqueous sodium hydroxide solution in a mass ratio of 20:80), and the mixture was stirred at room temperature with a stirring bar and a magnetic stirrer until the starch-based water absorbent resin in the mixed solvent no longer had a gel form. Next, the mixed solvent containing the starch-based water-absorbent resin was poured into a 1 L beaker containing 300 g of methanol, to obtain a precipitate of the starch-based water-absorbent resin. Next, the precipitate in methanol was recovered by filtration under reduced pressure (vacuum degree 900±100 hPa) using filter paper (No. 2, manufactured by Advantec Toyo Co., Ltd., retention particle size 5 μm). The recovered precipitate was added to an 80% by mass aqueous methanol solution and washed by stirring with a stir bar and a magnetic stirrer for 10 minutes. The precipitate was then recovered by filtration under reduced pressure (vacuum degree 900±100 hPa) using filter paper (No. 2, manufactured by Advantec Toyo Co., Ltd., retention particle size 5 μm) after washing with the 80% by mass aqueous methanol solution. Next, the precipitate that had been recovered and washed with the 80% by mass aqueous methanol solution was immersed in methanol in its entirety and stirred for 10 minutes using a stirring bar and a magnetic stirrer. The precipitate was then filtered under reduced pressure (vacuum degree 900±100 hPa) using filter paper (No. 2, manufactured by Advantec Toyo Co., Ltd., retention particle size 5 μm) to recover the precipitate that had been washed with methanol. Next, the precipitate was washed with methanol and then dried in a ventilated dryer to obtain a cross-linked cut product of the starch-based water-absorbent resin. Five grams of the crosslinked fragments were allowed to stand for one hour at an atmospheric humidity of 50±10% RH, and then weighed (W1). After heating at 120±5°C for 30 minutes using an infrared moisture content meter (KETT Corporation, JE400: lamp specifications 100V, 40W), the sample weight was then weighed (W2). The loss on drying (%) of the crosslinked fragments was calculated using the following formula (1): Drying loss S (%) of the crosslinked cut product = ((W1 - W2) / W1) × 100 (1) The loss on drying S of the fragmented crosslinks was taken as water content, and the rest was taken as solid content. The amount (g) of the fragmented crosslinks required to obtain 50 g of an aqueous solution containing 1% by mass of the fragmented crosslinks as solid content was calculated using the following formula (2). The amount of the crosslinked fragments P (g) = 50 (g) × 0.01 × (100 / (100-S)) (2) An amount (g) of ion-exchanged water obtained by dividing the charge amount P (g) from 50 (g) was charged into screw tube No. 7, and the charge amount P (g) of the crosslinked fragments was added to the ion-exchanged water in the screw tube while stirring with a stirrer and a magnetic stirrer.The screw tube was then capped and the mixture was stirred with a stirrer and a magnetic stirrer to dissolve the added crosslinked fragments, thereby obtaining 50 g of an aqueous solution of the crosslinked fragments with a solid content of 1% by mass. The screw tube containing 50 g of an aqueous solution containing 1% by mass of solids of the crosslinked fragments was placed in a thermostatic bath set at 25°C for at least 1 hour, and then, while the screw tube was still in the thermostatic bath, the rotor of a B-type viscometer (Toki Sangyo Viscometer TVB-10M) was placed in the aqueous solution containing 1% by mass of solids, and the rotor was rotated at 60 rpm for 1 minute, and the measured viscosity was taken as the viscosity of the crosslinked fragments.

[0029] [Number of days for soil decomposition] The number of days for soil decomposition was measured in accordance with JIS K 6954:2008. However, while JIS K 6954:2008 requires a film-shaped measurement sample, since it is difficult to mold starch-based water-absorbent resin into a film, 0.5 g of granular starch-based water-absorbent resin (product passed through a 150 μm mesh) was placed in a polyethylene mesh with a 144 μm mesh and sealed with a heat sealer to prevent leakage. The weight change of the starch-based water-absorbent resin within the mesh was observed, and the number of days until the starch-based water-absorbent resin was completely eluted from the mesh was recorded as the number of days for soil decomposition. Furthermore, instead of the synthetic solid waste used in JIS K 6954:2008, the compost components listed in Table 1 below were used.

[0030] [Table 1]

[0031] <Example> [Synthesis example of carboxyalkyl starch] [Synthesis Example 1 of Carboxymethyl Starch] A 1 L flask equipped with a Three-One Motor (Shinto Scientific Co., Ltd., Model No. 600G) was placed in a 40°C bath to regulate the temperature. 100 parts of rice flour (water content 12% by mass) and 800 parts of isopropanol were added to the flask. 82.3 parts of 48% aqueous sodium hydroxide and 50.3 parts of sodium chloroacetate were then added and stirred at 100 rpm for 3 hours. The entire contents of the flask were then transferred to a 2 L flask containing 500 mL of methanol. The contents of the 3 L flask were then transferred to a Nutsche tube containing filter paper (Toyo Roshi Kaisha, Ltd., Qualitative Filter Paper No. 2, 110 mm) and filtered under reduced pressure to separate the insoluble matter (residue on the filter paper) and the solution. The residue on the filter paper and 500 parts of 80% aqueous methanol were added to a 1 L flask and stirred for 10 minutes using a magnetic stirrer. Then, vacuum filtration was performed under the same conditions as above. The precipitate on the filter paper was repeatedly washed with 80% aqueous methanol until no precipitate formed when a 1% aqueous silver nitrate solution (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the filtrate obtained by vacuum filtration. The washed precipitate was spread evenly on a stainless steel tray (32 cm long, 20 cm wide, 3 cm high) and placed in a ventilated, forward-flow dryer at 40°C for 18 hours at a wind speed of 2 m / s to obtain a dried carboxymethyl starch (CMS-1). The number of carboxymethyl groups in CMS-1 was 0.60 per monosaccharide unit constituting the starch.

[0032] [Synthesis Example 2 of Carboxymethyl Starch] A 1 L separable flask equipped with a thermometer, condenser, and Three-One Motor (Shinto Scientific Co., Ltd., Model No. 600G) was placed in a 40 °C bath to regulate the temperature. 100 parts of rice flour (water content 12% by mass) and 800 parts of ethanol were added to the flask. 82.3 parts of 48% aqueous sodium hydroxide and 50.3 parts of sodium chloroacetate were then added and stirred at 100 rpm for 3 hours. The entire contents of the flask were then transferred to a 2 L flask containing 500 mL of methanol. The contents of the 3 L flask were then transferred to a Nutsche tube containing filter paper (Toyo Roshi Kaisha, Ltd., Qualitative Filter Paper No. 2, 110 mm) and filtered under reduced pressure to separate the insoluble matter (residue on the filter paper) and the solution. The residue on the filter paper and 500 parts of 80% aqueous methanol were added to a 1 L flask and stirred for 10 minutes using a magnetic stirrer. Then, vacuum filtration was performed under the same conditions as above. The precipitate on the filter paper was repeatedly washed with 80% aqueous methanol until no precipitate formed when a 1% aqueous silver nitrate solution (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the filtrate obtained by vacuum filtration. The washed precipitate was spread evenly on a stainless steel tray (32 cm long, 20 cm wide, 3 cm high) and placed in a ventilated, forward-flow dryer at 40°C for 18 hours at a wind speed of 2 m / s to obtain a dried carboxymethyl starch product (CMS-2). The number of carboxymethyl groups in CMS-2 was 0.62 per monosaccharide unit constituting the starch.

[0033] [Synthesis Example 3 of Carboxymethyl Starch] A 1-L separable flask equipped with a thermometer, condenser, and Three-One Motor (Shinto Scientific Co., Ltd., Model No. 600G) was charged with 37.5 parts of cornstarch and 212.5 parts of 20 mM acetate buffer (pH 6.0) and suspended to prepare starch milk. To this starch milk, 0.8 units of amylomaltase crude enzyme solution per gram of starch solids and 20 units of branching enzyme per gram of starch solids were added, stirred at room temperature for 30 minutes, and then incubated at 80°C for 6 hours with stirring to prepare liquefied starch. Next, 87.7 parts of a 47.5% aqueous sodium hydroxide solution was added and stirred at 60°C or below until the solution became completely homogeneous. After confirming that the solution was homogeneous, an aqueous solution of 121.7 parts of sodium chloroacetate in 156.8 parts of ion-exchanged water was added dropwise over 30 minutes at 50-60°C. After adding the aqueous sodium chloroacetate solution, the temperature was adjusted to 80 to 85°C and the mixture was stirred for 1 hour. After the reaction was completed, the reaction solution was diluted with 56 parts of ion-exchanged water. The diluted reaction solution was cooled to room temperature and added to 1 L of methanol over approximately 30 minutes to precipitate and reprecipitate the water-soluble polymer. After all the reaction solution was added, the mixture was stirred for 30 minutes, and the solid was collected by vacuum filtration. The resulting solid was then redispersed in 600 parts of aqueous methanol (80 / 20 by weight), stirred at room temperature for 30 minutes, washed, and then transferred to a funnel containing filter paper (Toyo Roshi Kaisha, Ltd., Qualitative Filter Paper No. 2, 110 mm) and filtered under reduced pressure to separate the insoluble matter (residue on the filter paper) from the solution. The residue on the filter paper and 500 parts of 80% aqueous methanol were placed in a 1-L flask, stirred for 10 minutes using a magnetic stirrer, and then filtered under reduced pressure under the same conditions as above. The precipitate was repeatedly washed with 80% aqueous methanol from the residue on the filter paper until no precipitation occurred when a 1% aqueous silver nitrate solution (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the filtrate obtained by vacuum filtration. The washed precipitate was spread evenly on a stainless steel tray (32 cm long, 20 cm wide, 3 cm high) and left to stand in a ventilated, forward-flow dryer set at 40°C with an air speed of 2 m / s for 18 hours to obtain a dried carboxymethyl starch (CMS-3). The number of carboxymethyl groups in CMS-3 was 0.71 per monosaccharide unit constituting the starch.

[0034] [Synthesis Example 4 of Carboxymethyl Starch] A 1 L separable flask equipped with a thermometer, condenser, and Three-One Motor (Shinto Scientific Co., Ltd., Model No. 600G) was placed in a 40 °C bath to regulate the temperature. 100 parts of rice flour (water content 12% by mass) and 800 parts of ethanol were added to the flask. 115.4 parts of 48% aqueous potassium hydroxide solution and 40.8 parts of chloroacetic acid were then added and stirred at 100 rpm for 3 hours. The entire contents of the flask were then transferred to a 2 L flask containing 500 mL of methanol. The contents of the 3 L flask were then transferred to a Nutsche tube containing filter paper (Toyo Roshi Kaisha, Ltd., Qualitative Filter Paper No. 2, 110 mm) and filtered under reduced pressure to separate the insoluble matter (residue on the filter paper) and the solution. The residue on the filter paper and 500 parts of 80% aqueous methanol solution were added to a 1 L flask and stirred for 10 minutes using a magnetic stirrer. Then, vacuum filtration was performed under the same conditions as above. The precipitate on the filter paper was repeatedly washed with 80% aqueous methanol until no precipitate formed when a 1% aqueous silver nitrate solution (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the filtrate obtained by vacuum filtration. The washed precipitate was spread evenly on a stainless steel tray (32 cm long, 20 cm wide, 3 cm high) and placed in a ventilated, forward-flow dryer at 40°C for 18 hours at a wind speed of 2 m / s to obtain a dried carboxymethyl starch (CMS-4). The number of carboxymethyl groups in CMS-4 was 0.57 per monosaccharide unit constituting the starch.

[0035] <Examples and Comparative Examples> Comparative Example 1 95 parts by weight of water was placed in a 300 mL beaker and stirred at 500 rpm with a magnetic stirrer. 5 parts by weight of CMS-1 obtained in Synthesis Example 1 and 0.25 parts by weight of malic acid (Fuso Chemical Co., Ltd.) were then placed in the beaker and stirred until homogenous. The resulting aqueous solution was poured into a metal tray (bottom 20 cm × 30 cm, height 2.5 cm), dried at 140°C for 2 hours, and pulverized to obtain a starch-based water-absorbent resin according to Comparative Example 1.

[0036] Comparative Examples 2 to 4 Starch-based water-absorbing resins according to Comparative Examples 2 to 4 were obtained in the same manner as in Comparative Example 1, except that the type of carboxymethyl starch and the amount of crosslinking agent were changed as shown in Table 2.

[0037] [Examples 1 and 3 to 8] Starch-based water-absorbing resins according to Examples 1 and 3 to 8 were obtained in the same manner as in Comparative Example 1, except that the type of carboxymethyl starch, the type of crosslinking agent and the amount of crosslinking agent were changed as shown in Table 2.

[0038] Example 2 The rotation speed of the twin-screw extrusion kneader (L / D=40, φ=11 mm) was set to 200 rpm, the barrel temperature was set to 95°C, and CMS-2 was fed from the powder feeder at a rate of 0.56 kg / h, water from the tube pump at a rate of 0.17 kg / h, and a 40% aqueous malic acid solution at a rate of 0.07 kg / h, respectively, to obtain a kneaded product of CMS, malic acid, and water. At this time, the temperature of the kneaded product near the outlet was 92°C. The obtained kneaded product was dried at 140°C for 2 hours and pulverized to obtain a starch-based water absorbent resin according to Example 2.

[0039] <Measurement of germination rate> 5 kg of sandy soil (river sand, manufactured by Sun and Hope Co., Ltd.) was placed in a plastic planter measuring 30 cm in length, 30 cm in width, and 20 cm in depth. 0.05 kg each of starch-based water-absorbent resin and 0.3 kg of chemical fertilizer (nitrogen:phosphate:potassium = 1:1:1, manufactured by Akagi Engei Co., Ltd.) were added to 8.3 kg of sandy soil, and the resulting mixture was thoroughly mixed. The resulting soil was layered on top of the planter containing the sandy soil, and 1000 g of tap water was added for irrigation to create test medium A. Komatsuna (variety: Nanako, manufactured by Takii Seed Co., Ltd.) seeds were sown at 2 cm intervals on test medium A and then placed in an artificial climate chamber [LPH-411SPC (manufactured by Nippon Medical and Chemical Instruments Manufacturing Co., Ltd.)] (with cultivation conditions set at 25°C, 70% humidity, 6000 lux, 16 hours light, 8 hours dark). 100 g of tap water was irrigated every 3 days, and the germination rate was investigated. The germination rate was calculated using the following formula. The higher the germination rate, the more suitable the medium for growing crops. Germination rate = number of germinated seeds / number of seeds sown x 100 (%)

[0040] The evaluation results are shown in Table 2 below.

[0041] [Table 2]

Claims

1. A starch-based water-absorbing resin obtained by crosslinking carboxyalkyl starch, The starch-based water absorbent resin has an ion-exchange water absorption capacity of 5 to 200 g / g, The starch-based water-absorbent resin has a viscosity of 120 to 40,000 mPa·s as measured by the viscosity measurement method described below of a crosslinked cut product obtained by cutting crosslinks of the starch-based water-absorbent resin by the crosslinking cutting method described below. <Crosslink cutting method> 5 g of a starch-based water absorbent resin having a gel form is added to 100 g of a mixed solvent of methanol and a 4% aqueous sodium hydroxide solution (methanol:4% aqueous sodium hydroxide solution is 20:80 in mass ratio), and the mixture is stirred at room temperature with a stirrer and a magnetic stirrer until the starch-based water absorbent resin in the mixed solvent no longer has a gel form. Next, the mixed solvent containing the starch-based water-absorbent resin is poured into a 1 L beaker containing 300 g of methanol to obtain a precipitate of the starch-based water-absorbent resin. Next, the precipitate in methanol is recovered by filtration under reduced pressure (vacuum degree 900±100 hPa) using filter paper (retention particle size 5 μm), the recovered precipitate is added to an 80% by mass aqueous methanol solution and washed by stirring with a stirrer and a magnetic stirrer for 10 minutes, and the precipitate is recovered after washing with an 80% by mass aqueous methanol solution by filtration under reduced pressure (vacuum degree 900±100 hPa) using filter paper (retention particle size 5 μm). Next, the entire precipitate that has been recovered after washing with the 80% by mass aqueous methanol solution is immersed in methanol and stirred for 10 minutes with a stirring bar and a magnetic stirrer, and then filtered under reduced pressure (vacuum degree 900±100 hPa) using filter paper (retention particle size 5 μm) to recover the precipitate that has been washed with methanol. Next, the precipitate after washing with methanol is dried in a ventilated dryer to obtain a cross-linked cut product of the starch-based water-absorbent resin. <Viscosity measurement method> Five grams of the crosslinked fragments were allowed to stand for one hour in an atmospheric humidity of 50±10% RH, and then weighed (W1). The sample was heated at 120±5°C for 30 minutes using an infrared moisture content meter (JE400 manufactured by KETT Corporation: lamp specifications 100V, 40W), and the weight of the sample was then weighed (W2). The loss on drying (%) of the crosslinked fragments was calculated using the following formula (1): Drying loss of the crosslinked cut product S (%) = ((W1 - W2) / W1) × 100 (1) The loss on drying S of the crosslinked fragments is water, and the rest is solids. The amount (g) of the crosslinked fragments required to obtain 50 g of an aqueous solution containing 1% by mass of the solids of the crosslinked fragments is calculated using the following formula (2). Amount of the crosslinked fragments P (g) = 50 (g) × 0.01 × (100 / (100−S)) (2) A screw tube No. 7 was charged with ion-exchanged water in an amount (g) obtained by subtracting the charged amount P (g) from 50 (g), and the charged amount P (g) of the crosslink fragments was added to the ion-exchanged water in the screw tube while stirring with a stirrer and a magnetic stirrer.The screw tube was then closed and the added crosslink fragments were stirred with a stirrer and a magnetic stirrer to dissolve the added crosslink fragments, thereby obtaining 50 g of an aqueous solution of the crosslink fragments having a solids content of 1% by mass. The screw tube containing 50 g of an aqueous solution containing 1% by mass of solids of the crosslinked fragments is placed in a thermostatic bath set at 25°C for at least 1 hour, and then the rotor of a B-type viscometer (Toki Sangyo Viscometer TVB-10M) is placed in the aqueous solution containing 1% by mass of solids in the screw tube while it is still in the thermostatic bath, and the rotor is rotated at 60 rpm for 1 minute, and the viscosity measured is taken as the viscosity of the crosslinked fragments.

2. The starch-based water absorbent resin according to claim 1, wherein the viscosity of a crosslinked cut product obtained by cutting crosslinks of the starch-based water absorbent resin using the following crosslink cutting method is 150 to 10,000 mPa s, as measured using the viscosity measurement method.

3. The starch-based water-absorbing resin is a starch-based water-absorbing resin obtained by crosslinking carboxyalkyl starch with a crosslinking agent, 3. The starch-based water-absorbent resin according to claim 1, wherein the amount of the crosslinking agent added is 0.02 to 2.0 parts by weight per 100 parts by weight of the carboxyalkyl starch.

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