JP2018145326A - Water-absorbing resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water-absorbing resin in which deterioration of water absorption and water retention performance accompanying contact with a calcium component is alleviated, a nitrogen component is not essentially contained, and an influence on an environment is small even when decomposition occurs in soil, and an agricultural water retention material having the water-absorbing resin as a constituent component.SOLUTION: A water-absorbing resin that does not substantially contain a nitrogen component, and has a ratio Xb/Xa between pure water absorption magnification Xa before contact with an aqueous solution containing calcium ions corresponding a hardness 20 gpg and pure water absorption magnification Xb after contact is 0.1 or larger.SELECTED DRAWING: None

Description

  The present invention relates to a water absorbent resin. More specifically, the present invention relates to a water-absorbing resin that retains its action in soil for a long time and has little influence on the environment, and an agricultural water-retaining material comprising the water-absorbing resin as a constituent component. More specifically, the present invention has a small degree of decrease in water absorption characteristics due to calcium ions contained in soil or irrigation water, and a decomposition product generated by decomposition by the action of fungi, enzymes and substances in the soil is a wastewater system. The present invention relates to a water retention material for agriculture that is unlikely to cause water pollution due to eutrophication and has minimal environmental impact.

With the recent depletion of water resources, attempts to use agricultural water effectively and appropriately, or to maintain or increase the yield of agricultural products with a smaller amount of irrigation water than before, are the so-called agricultural water retention. It has been studied using materials (for example, see Patent Documents 1 to 3). These agricultural water-retaining materials have a superabsorbent resin (SAP) as a main component, and for example, when compared to peat moss, which is used to improve the water retention of the entire soil, it produces an effect in a very small amount. There is an advantage that the burden on the farmer is small.
As a typical superabsorbent resin, polyacrylate gel is well known and widely used in liquid absorbents for sanitary goods such as diapers. Patent Document 1 discloses a method of preventing plant growth inhibition due to excessive adsorption of calcium by containing calcium ions from the beginning in a polyacrylate gel.

Japanese Patent No. 4346112 Special table 2013-544929 gazette US Patent No. 9,181,377

  Certainly, according to the method of Patent Document 1, since the excessive adsorption of calcium is prevented, plant growth inhibition is suppressed. On the other hand, the calcium component contained in the soil or irrigation water is a polyacrylate gel. When it contacts, there is a problem that the cross-linking of the polyacrylate gel proceeds and the water absorption or water retention properties that are the original purpose are lost. This problem is alleviated by introducing a nonionic component into the polyacrylate gel. For example, a method by introducing an acrylamide unit is known. However, when an acrylamide unit is introduced, there is a problem that the highly toxic acrylamide monomer component remains in the product. Since acrylamide contains a nitrogen component, it generates nitrogen-containing substances such as ammonia when decomposed in soil. However, this may lead to eutrophication of the wastewater system and water quality deterioration due to flowing into the wastewater.

  The object of the present invention is to reduce the water absorption / retention performance associated with the contact with the calcium component, and is substantially free of nitrogen component and has a small impact on the environment even when decomposition occurs in the soil. It is in providing the water retention material for agriculture which uses resin and this water absorbing resin as a structural component.

  In order to achieve the above object, the present inventors have conducted intensive studies. As a result, a resin introduced with a specific molecular structure, more specifically, modified polyvinyl alcohol obtained by acetalizing at least a part of polyvinyl alcohol with glyoxylic acid and / or glyoxylic acid, exhibits high water absorption and water retention properties. And, it was found that this performance is not easily lost by contact with calcium ions. Moreover, since this resin does not contain a nitrogen component substantially, even when decomposition | disassembly occurred, it discovered that a nitrogen-containing decomposition product was not produced | generated and reached this invention.

That is, the object of the present invention is achieved by providing the following [1] to [5].
[1] The ratio Xb / Xa between the pure water absorption ratio Xa before contact with an aqueous liquid substantially free of nitrogen components and containing calcium ions equivalent to a hardness of 20 gpg and the pure water absorption ratio Xb after contact is 0. A water-absorbent resin that is 1 or more.
[2] The water absorbent resin according to [1], wherein the ratio Xb / Xa is 0.25 or more.
[3] The water-absorbent resin according to [1] or [2], comprising a modified polyvinyl alcohol at least partially acetalized with glyoxylic acid and / or a glyoxylic acid derivative as a constituent component.
[4] An agricultural water retention material mainly comprising the water-absorbing resin according to any one of [1] to [3].
[5] The agricultural water retention material according to [4], further comprising at least one fertilizer component or agrochemical component.

  The highly water-absorbent resin of the present invention has a reduced water absorption / retention performance due to contact with the calcium component, is substantially free of nitrogen components, and has no impact on the environment even when decomposition occurs in the soil. Since it is small, it can be suitably used as a water retention material for agriculture. For example, it is possible to grow crops with a smaller amount of irrigation water than in the past, leading to saving water resources and reducing irrigation water costs.

Hereinafter, the present invention will be described in detail.
The water-absorbing resin of the present invention is a ratio of pure water absorption ratio Xa before contact with an aqueous liquid substantially free of nitrogen components and containing calcium ions having a hardness of 20 gpg, and pure water absorption ratio Xb after contact. Xb / Xa is 0.1 or more. Preferably, the water-absorbent resin comprises as a constituent component a modified polyvinyl alcohol in which the ratio Xb / Xa is 0.25 or more and at least a part thereof is acetalized with glyoxylic acid and / or a glyoxylic acid derivative.
First, polyvinyl alcohol which is a suitable raw material will be described.

  Polyvinyl alcohol may contain only a vinyl alcohol unit as its constituent unit, or may contain other constituent units. Examples of other structural units include structural units derived from vinyl carboxylates such as vinyl acetate and vinyl pivalate; structural units derived from olefins such as ethylene, 1-butene and isobutylene; acrylic acid and its derivatives, methacrylic acid and its Examples thereof include structural units derived from derivatives, maleic acid and derivatives thereof, maleic anhydride and derivatives thereof, and the like. Other structural units may contain one kind or plural kinds. When the content of other structural units is too large, the reaction point with glyoxylic acid and / or a derivative thereof is reduced in the acetalization reaction, so that the effects of the present invention may not be achieved. Therefore, the content of other structural units is preferably 50% by mass or less, more preferably 30% by mass or less, and further preferably 15% by mass or less.

  Although there is no restriction | limiting in particular in the molecular weight of polyvinyl alcohol, From a viewpoint of the ease of a subsequent acetalization reaction and the ease of handling, it is preferable that it is 1 million g / mol or less, and it is 500,000 g / mol or less. Is more preferable. On the other hand, from the viewpoint of the mechanical properties of the resulting modified polyvinyl alcohol, it is preferably 5000 g / mol or more, more preferably 10,000 g / mol or more, and further preferably 20,000 g / mol or more. The molecular weight of polyvinyl alcohol can be determined by a method of measuring the average degree of polymerization, for example, by a method based on JIS K 6726.

  The polyvinyl alcohol used as a raw material in the present invention may be an industrially produced and marketed one, a vinyl carboxylate monomer such as a vinyl acetate monomer, and a monomer constituting another constituent unit as necessary. In the presence of a polymerization initiator, a known polymerization reaction is performed using a known polymerization initiator, and then a saponification reaction is performed by a known method, even though it undergoes a cationic polymerization reaction of vinyl ether and a subsequent hydrolysis reaction. It may be produced, or may be produced by direct polymerization of acetaldehyde.

  Preferably, the polyvinyl alcohol is obtained by polymerizing vinyl acetate to obtain polyvinyl acetate, and then undergoing a saponification reaction. And the degree of saponification is 30 mol% or more, and preferably 60 mol% or more. From the viewpoint of performing acetalization using glyoxylic acid and / or glyoxylic acid derivatives described below to the extent that the effects of the present invention are exhibited. Is more preferably 80 mol% or more.

Glyoxylic acid and / or glyoxylic acid derivatives are used for acetalization of polyvinyl alcohol. Glyoxylic acid may be used alone, or may be used with glyoxylic acid derivatives such as glyoxylate, glyoxylic acid ester, and acetalized products of these compounds, or only glyoxylic acid derivatives may be used.
Examples of the glyoxylate include alkali metal salts of glyoxylic acid such as sodium glyoxylate, potassium glyoxylate, lithium glyoxylate, and alkaline earth metal salts of glyoxylic acid such as calcium glyoxylate and magnesium glyoxylate. Among these, potassium glyoxylate, calcium glyoxylate, and magnesium glyoxylate are preferable, and potassium glyoxylate is more preferable from the viewpoint that the effects of the present invention can be further exerted when used as a water retention material for agriculture.
Examples of glyoxylic acid esters include methyl glyoxylate, ethyl glyoxylate, propyl glyoxylate, isopropyl glyoxylate, butyl glyoxylate, isobutyl glyoxylate, sec-butyl glyoxylate, tert-butyl glyoxylate, hexyl glyoxylate, glyoxylic acid Examples include octyl and 2-ethylhexyl glyoxylate.

  Examples of acetal compounds of glyoxylic acid, glyoxylic acid salts and glyoxylic acid esters include compounds in which the aldehyde part of glyoxylic acid, glyoxylic acid salts and glyoxylic acid esters is acetalized with alcohol compounds such as methanol, ethanol, propanol and ethylene glycol. Can be mentioned.

  There is no particular limitation on the method of acetalization reaction with glyoxylic acid and / or glyoxylic acid derivatives of polyvinyl alcohol, and it may be a homogeneous reaction or a heterogeneous reaction. Specifically, for example, a method in which a polyvinyl alcohol aqueous solution and a glyoxylic acid aqueous solution are mixed and reacted can be exemplified. A catalyst may be used for the purpose of accelerating the reaction. Examples of the catalyst include inorganic acids such as hydrochloric acid, sulfuric acid and phosphoric acid; organic acids such as carboxylic acid and sulfonic acid; solid acids such as cation exchange resin and heteropoly acid. Since glyoxylic acid is also an acid that can accelerate the acetalization reaction, it can be used as an autocatalyst. These catalysts may be used independently and may be used in combination of multiple types. From the viewpoint of ease of treatment after the reaction, a method using glyoxylic acid as a reaction substrate and an autocatalyst is preferable.

  In the acetalization reaction of polyvinyl alcohol with glyoxylic acid and / or glyoxylic acid derivative, the amount of glyoxylic acid and / or glyoxylic acid derivative introduced into polyvinyl alcohol can be defined by, for example, the consumption of hydroxyl groups possessed by polyvinyl alcohol. That is, when the hydroxyl group consumption is low, the introduction amount of glyoxylic acid and / or glyoxylic acid derivative is low, and when the hydroxyl group consumption is high, the introduction amount of glyoxylic acid and / or glyoxylic acid derivative is high. If the amount of glyoxylic acid and / or glyoxylic acid derivative introduced is too low, the water absorption of the resulting modified polyvinyl alcohol tends to be inferior, while if it is too high, the resulting polyvinyl alcohol tends to have poor mechanical properties during water absorption. In consideration of the range in which the effects of the present invention can be achieved, the consumption of hydroxyl groups of polyvinyl alcohol used as a raw material is preferably in the range of 3 to 87 mol%, more preferably in the range of 5 to 70 mol%. More preferably, it is in the range of ˜50 mol%. The consumption amount of the hydroxyl group can be measured by, for example, NMR (nuclear magnetic resonance spectroscopy), FTIR (Fourier transform infrared spectroscopy) or the like.

  In addition, in order to suppress the reduction of water absorption and water retention performance due to contact with calcium components, the reaction is carried out in the presence of other aldehydes in the acetalization reaction of polyvinyl alcohol with glyoxylic acid and / or glyoxylic acid derivatives. Modified polyvinyl alcohol can also be obtained. That is, it has been found that by introducing so-called coacetalization and introducing an acetal structure derived from such other aldehydes, a decrease in water absorption and water retention performance due to contact with the calcium component is further suppressed. Examples of such other aldehydes include aliphatic aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde, i-butyraldehyde, sec-butyraldehyde, tert-butyraldehyde; benzaldehyde, anisaldehyde, 4-benzyloxybenzaldehyde. Aromatic aldehydes such as 3-benzyloxybenzaldehyde, 4-amyloxybenzaldehyde, and 3-amyloxybenzaldehyde. Among these, formaldehyde, acetaldehyde, and n-butyraldehyde are preferable from the viewpoint of ease of introduction and water absorption characteristics of the resulting modified polyvinyl alcohol. When other aldehyde is allowed to coexist, the amount is not particularly limited, but is usually within a range of 0.01-30 mol%, preferably 0.1-10 mol%, with respect to glyoxylic acid and / or glyoxylic acid derivative. Within the range, more preferably within the range of 1-5 mol%. If it is larger than this range, the water-absorbing properties of the resulting modified polyvinyl alcohol may be reduced. On the other hand, if it is smaller than this range, it is accompanied by contact with the calcium component due to the introduction of other aldehyde components into the modified polyvinyl alcohol. It tends to be difficult to obtain an effect of suppressing a decrease in water absorption and water retention performance. In addition, you may use said other aldehyde as derivatives, such as an acetal body, for example.

  The water-absorbent resin of the present invention is such that the modified polyvinyl alcohol suitably obtained by acetalization reaction with glyoxylic acid and / or glyoxylic acid derivative of polyvinyl alcohol is further crosslinked from the viewpoint of preventing elution during use, That is, it is preferably in a so-called gel state at the time of use. There is no restriction | limiting in particular in the form of bridge | crosslinking, For example, the crosslinked structure by an ester bond, an ether bond, an acetal bond, a carbon-carbon bond etc. is mentioned.

  As an example of an ester bond, the ester bond formed between the hydroxyl group derived from polyvinyl alcohol and the carboxyl group derived from glyoxylic acid (derivative) can be mentioned. As an example of an ether bond, the ether bond formed by the dehydration condensation between the hydroxyl groups derived from polyvinyl alcohol can be mentioned. As an example of an acetal bond, an acetal bond formed by the acetalization reaction of a hydroxyl group between polyvinyl alcohol molecules with glyoxylic acid can be exemplified. As an example of a carbon-carbon bond, the carbon-carbon bond formed by the coupling between the carbon radicals of modified polyvinyl alcohol, which is generated, for example, by irradiation with active energy rays, can be mentioned. These cross-linked structures may be contained singly or in plural. Among these, a crosslinked structure by an ester bond or an acetal bond is preferable, and a crosslinked structure by an ester bond is more preferable. Such a form of cross-linking may be formed simultaneously during the acetalization reaction of glyoxylic acid and / or glyoxylic acid derivative of polyvinyl alcohol, or may be formed separately.

  The carboxyl group introduced by the acetalization reaction with glyoxylic acid and / or glyoxylic acid derivative of polyvinyl alcohol may be partially or completely treated to be in the form of carboxylate. Examples of carboxylate counter cations include alkali metal ions such as sodium ion, potassium ion, rubidium ion and cesium ion; alkaline earth metal ions such as magnesium ion, calcium ion, strontium ion and barium ion; aluminum ion, And other metal ions such as zinc ions; onium cations such as ammonium ions, imidazoliums, pyridiniums, and phosphonium ions; Of these, potassium ions, calcium ions, and ammonium ions are preferred, and potassium ions are more preferred, especially when used as a water retention material for agriculture. These counter cations may be contained alone or in combination of two or more.

  The water-absorbent resin of the present invention is characterized by having a small change in water absorption before and after contact with an aqueous solution containing substantially no nitrogen component and containing calcium ions. Here, the phrase “substantially free of nitrogen component” means that the amount of nitrogen element contained in the water-absorbent resin in a dry state is at most 5% by mass, preferably 3% by mass or less. Preferably, it means 1% by mass or less. The content of nitrogen element can be measured by methods known to those skilled in the art, such as infrared spectroscopy, ultraviolet-visible spectroscopy, nuclear magnetic resonance spectroscopy, and organic elemental analysis. Among them, it is preferable to employ an analysis method based on an organic elemental analysis method from the viewpoint of ease of measurement and quantitativeness.

  As changes in the water absorption characteristics of the water-absorbent resin due to contact and action with an aqueous solution containing calcium ions, a decrease in water absorption capacity is observed in most cases, but the smaller the degree, the better. Irrigation water generally contains calcium components, although the amount varies depending on the region and season.Although there are calcium components in the soil even though they vary depending on the region and fertilizer application conditions, water-absorbing resin is used for agriculture. When used as a water retaining material, contact with calcium components is inevitable. Therefore, when the performance of the water absorbent resin is lowered by contact with the calcium component, particularly when the degree of reduction in water absorption is large, the water absorbent resin easily loses its water absorption / water retention performance. When considering use as an actual agricultural water retention material, it is difficult to define the performance degradation due to calcium components because the content of calcium components can vary greatly depending on the region and season, but an aqueous solution containing calcium ions The resistance to the calcium component during actual use can be predicted by the water absorption characteristics of the water absorbent resin before and after soaking, for example, the change in the water absorption magnification.

  In measuring the water absorption ratio Xa of the water-absorbent resin before contact with the aqueous solution containing calcium ions and the water absorption ratio Xb of pure water after contact, the aqueous solution containing calcium ions is a so-called hard water calcium ion. Use the one containing. For example, gpg (grains per gallon) is adopted as an index of the hardness of water, and 1 gpg means that 64.8 mg of calcium carbonate is contained in 1 gallon (3.875 L) of water and less than 1 gpg of water. Is soft water, the range of 1.0-3.5 is slightly hard water, the range of 3.5-7.0 is moderate hard water, the range of 7.0-10.5 is hard water, and the water of 10.5 or more Sometimes called super hard water. In the present invention, the aqueous solution containing calcium ions refers to hard water having a hardness of 10.5-50 gpg, specifically 15-35 gpg, and more specifically 20 gpg.

  As described above, gpg, which is an index of water hardness, is defined using the amount of calcium carbonate in an aqueous solution, but in order to evaluate a more accurate material, aqueous solutions of different salts containing calcium are used. In this case, the equivalent amount of calcium carbonate can be calculated from the amount of calcium ions, and gpg which is an index of hardness can be obtained. There is no particular limitation on the calcium salt that can be used, such as calcium chloride, calcium bromide, calcium iodide, calcium sulfate, calcium hydrogen sulfate, calcium nitrate, calcium phosphate, calcium monohydrogen phosphate, calcium dihydrogen phosphate, calcium chlorate, Calcium bromate, calcium formate, calcium acetate, calcium lactate, calcium citrate, etc. are mentioned. Among them, calcium chloride, calcium bromide, and calcium iodide are preferred from the viewpoint of ease of handling and aqueous solution adjustment. Preferably it is calcium chloride.

In this specification, the pure water absorption magnification of the water-absorbing resin can be represented by the ratio of the mass of the resin and the mass of pure water that can be absorbed by the resin, as shown in the following formula 1.

Pure water absorption ratio = [mass of pure water that can be absorbed by resin] / [dry mass of resin] (Formula 1)

The water absorption magnification Xa of the water absorbent resin of the present invention can be measured according to JIS K 7223. Further, a resin obtained by immersing the water-absorbing resin of the present invention in an aqueous solution containing calcium ions (for example, a calcium chloride aqueous solution corresponding to 20 gpg) at room temperature for a predetermined time (for example, at 20 ° C. for 1 day) and then vacuum drying the resin. The water absorption magnification Xb is measured according to JIS K 7223 described above. The ratio Xb / Xa between the water absorption ratios Xa and Xb thus obtained can be used as an indicator of the resistance to the calcium component of the water absorbent resin. That is, it can be said that the larger the Xb / Xa, the stronger the resistance to the calcium component. Therefore, this value is preferably higher, for example, 0.1 or more, preferably 0.25 or more, more preferably 0.3 or more, and even more preferably. Is 0.4 or more, more preferably 0.5 or more.

  The water-absorbent resin of the present invention is substantially free of nitrogen component and has no limitation except that Xb / Xa, which is an index of resistance to calcium component, is 0.1 or more, preferably polyvinyl alcohol It is a modified polyvinyl alcohol obtained by acetalizing at least a part of glyoxylic acid and / or glyoxylic acid derivative, and it is preferable that a crosslinked structure is introduced from the viewpoint of not eluting into water during actual use.

  The water-absorbent resin of the present invention may be in the form of a composition further containing other additives as long as the effects of the present invention are not impaired. Examples of such other additives include polysaccharides such as starch, modified starch, sodium alginate, cellulose and derivatives thereof, polyethylene, polypropylene, ethylene-propylene copolymer, polystyrene, polyvinyl chloride, polycarbonate resin, polyethylene terephthalate, Polybutylene terephthalate, polylactic acid, polysuccinic acid, polyamide 6, polyamide 6-6, polyphenylene ether, polyoxymethylene, polyethylene glycol, polypropylene glycol, polytrimethylene glycol, polytetramethylene glycol, polyvinyl alcohol, ethylene-vinyl alcohol copolymer Polymer, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyacrylic acid, polyacrylate, polyacrylate, polymethacrylate Rylic acid, polymethacrylic acid ester, polymethacrylic acid salt, ethylene-acrylic acid copolymer, ethylene-acrylic acid ester copolymer, ethylene-acrylic acid salt copolymer, ethylene-methacrylic acid copolymer, ethylene-methacrylic acid Resins such as acid ester copolymer, ethylene-methacrylate copolymer, natural rubber, synthetic isoprene rubber, chloroprene rubber, silicone rubber, fluoro rubber, urethane rubber, acrylic rubber, styrene thermoplastic elastomer, olefin heat Examples include rubber and elastomers such as plastic elastomers and ester thermoplastic elastomers, kaolinite, smectite, montmorillonite, sericite, chlorite, glowconite, talc, natural zeolite, synthetic minerals such as synthetic zeolite, sand, etc. Can . These may be used alone or in combination.

  The water-absorbent resin or composition of the present invention can be suitably used as an agricultural water retention material. There are various ways to use it as a water retention material for agriculture, a method of spraying directly on the field, a method of applying together with seeds when sowing crop seeds, a method of coating seeds once and seeding the coated seeds Etc. Moreover, you may use the mixture of the superabsorbent resin or composition of this invention, and water as a culture medium. There is no restriction | limiting in particular in the crop made into application object, For example, various vegetables, root vegetables, fruits, cereals, potatoes, beans, ornamental plants, flowering plants, turf, trees etc. can be mentioned.

  When the water-absorbent resin of the present invention is used as an agricultural water retention material, it may be in the form of a composition containing a fertilizer component and / or an agrochemical component as long as the effects of the present invention are not impaired. Examples of fertilizers are three major fertilizers such as nitrogen fertilizer, phosphorus fertilizer, potassium fertilizer, calcium fertilizer; plant such as calcium, magnesium, sulfur, iron, copper, manganese, zinc, boron, molybdenum, chlorine, nickel Examples include fertilizers that contain essential elements. Examples of the agrochemical component include insecticides, fungicides, insecticide fungicides, herbicides, rodenticides, plant growth regulators and the like. These may be used alone or in combination.

  Since the water-absorbent resin of the present invention exhibits high water absorption characteristics and excellent mechanical properties, for example, as an absorbent in hygiene products such as baby diapers, infant diapers, children's diapers, adult diapers, sanitary products, protective underwear, etc. It is preferably used for sanitary materials. Since the water-absorbent resin of the present invention can be decomposed after use, it is a preferable material from the viewpoint of environmental impact.

  The water-absorbent resin of the present invention is used for commonly known water-absorbent resin applications such as prevention of water penetration in underground power cables or communication cables; carriers in drug delivery systems; absorption of spilled and discharged aqueous liquids. Materials; Paints; Inks; Absorbent coatings for colorant compositions; Carriers for controlled release of insecticides, herbicides, fragrances and drugs; Flame retardant gels; Funeral pads; Surgical pads; Materials; Solid waste materials for medical use; Absorbent pads and packaging materials for foodstuffs; Cosmetic gelling agents; Sealing composites; Filtration applications; Fuel monitoring systems for aircraft and automobiles; Water supply for cage animals; It can also be used for beds, toys that grow in water, drilling fluid additives, artificial snow, and the like.

Hereinafter, the present invention will be described more specifically with reference to examples and the like, but the present invention is not limited to the examples and the like. The raw materials used and the method of evaluation analysis are shown below.
(1) Sodium polyacrylate gel:
Wako Pure Chemical Industries, Ltd. Superabsorbent polymer (acrylic acid salt)
(2) Polyacrylamide gel:
Water Storing Crystals from Miracle-Gro
(3) Polyvinyl alcohol:
Kuraray Co., Ltd. PVA117S Saponification degree 98 mol%, polymerization degree 1700
(4) 50% glyoxylic acid aqueous solution: manufactured by Tokyo Chemical Industry Co., Ltd. (5) potassium hydroxide: manufactured by Wako Pure Chemical Industries, Ltd. Special grade

[2] Characteristic evaluation of modified polyvinyl alcohol The modified polyvinyl alcohol obtained in Experimental Example 1 described below was evaluated as follows.
(1) Water absorption ratio (1-a) According to JIS K 7223, pure water absorption of the obtained modified polyvinyl alcohol is measured, and contact with an aqueous solution containing calcium ions based on the following formula (2) The previous water absorption ratio Xa (g / g) was calculated.
Xa (g / g) =
[Sample weight after water absorption-Sample mass before water absorption] / [Mass before water absorption] Formula (2)

(1-b) Calcium carbonate aqueous solution (1.b) prepared by adjusting the tea bag containing the obtained modified polyvinyl alcohol sample to a hardness of 20 gpg in accordance with the tea bag specified in JIS K 7223 and the sample setting method. 296 g CaCO ₃ / L) at 20 ° C. for 1 day. After vacuum drying this sample to a constant weight at 40 ° C. or lower, the sample mass B (g) was measured, and then immersed in pure water according to JIS K 7223. Based on the following formula (3), calcium ions were The water absorption magnification Xb (g / g) after contact with the aqueous solution was calculated.
Xb (g / g) = [sample mass after water absorption test with pure water−B] / B Formula (3)

(2) Nitrogen element content of modified polyvinyl alcohol: The nitrogen content was measured by Perkin Elmer 2400II.

(3) Ammonium ion amount Xc contained in the decomposition product: The following operation was performed as a simulated decomposition test. That is, 0.1 g of a superabsorbent resin sample was immersed in 10 mL of 1N hydrochloric acid aqueous solution and stirred at 70 ° C. for 3 hours to prepare a simulated decomposition test. After performing the decomposition test, the filtrate obtained by removing solids by filtration was diluted 100 times with pure water, and ammonium ions were removed using an ion chromatograph (Thermo Fisher ICS-1600, column IonPac CS12A). Quantified. The formula for calculating the ammonium ion amount Xc (mass%) is shown in the following formula (4).
Ammonium ion content Xc (mass%) in decomposition product =
[Ammonium ions generated by decomposition (g)] / [0.1 (g)] × 100
... Formula (4)
In this test, when the degradation product in the filtrate contains nitrogen, when the water retention material for agriculture containing the modified polyvinyl alcohol used for the degradation test as a superabsorbent resin is decomposed in the soil, it produces nitrogen compounds. It is thought that there is a strong tendency to adversely affect the surrounding soil and water system.

Experimental example 1
In a 500 mL four-necked separable flask equipped with a reflux condenser and a stirring blade, 200.0 g of ion-exchanged water and 40.0 g of polyvinyl alcohol were charged and dissolved by heating to 90 ° C. with stirring. The system was cooled to 80 ° C., and 19.1 g of a 50% aqueous solution of glyoxylic acid was added dropwise over 5 minutes to react for 1 hour. The contents of the flask were taken out and dried at 105 ° C. for 3 hours with a hot air dryer (DKM400 manufactured by Yamato Scientific Co., Ltd.). The obtained resin was cooled to room temperature, immersed in 300 mL of 50% aqueous potassium hydroxide solution for 3 hours, and then washed with ion-exchanged water to obtain a modified polyvinyl alcohol. About the obtained modified polyvinyl alcohol, the pure water absorption ratio Xa before contact with the aqueous solution containing calcium ions, the pure water absorption ratio Xb after contact with the aqueous solution containing calcium ions, the nitrogen element content, and the nitrogen element of the decomposition product The content was determined. The results are shown in Table 1.

Experimental Examples 2-3
Evaluation tests similar to Experimental Example 1 were performed on commercially available sodium polyacrylate gel and polyacrylamide gel. The results are shown in Table 1.

  Since the water-absorbent resin of the present invention has little functional deterioration due to contact with calcium ions, and since the water-absorbent resin or a decomposition product thereof does not contain a nitrogen element, the environmental impact is small. Can be suitably used.

Claims (5)

  The ratio Xb / Xa of pure water absorption ratio Xa before contact with an aqueous liquid substantially free of nitrogen components and containing calcium ions equivalent to a hardness of 20 gpg and pure water absorption ratio Xb after contact is 0.1 or more A water absorbent resin.   The water absorbent resin according to claim 1, wherein the ratio Xb / Xa is 0.25 or more.   The water absorbent resin according to claim 1 or 2, comprising a modified polyvinyl alcohol at least partially acetalized with glyoxylic acid and / or a glyoxylic acid derivative as a constituent component.   An agricultural water retention material comprising the water-absorbent resin according to any one of claims 1 to 3 as a main component. Furthermore, the water retention material for agriculture of Claim 4 containing at least 1 or more types of a fertilizer component or an agrochemical component.