JP2024022704A - Water retention material for agriculture, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water retention material for agriculture characterized by a high rate of water absorption, a high capacity for water absorption even when calcium salt is present, and the ability to retain this capacity for water absorption over an extended period.

SOLUTION: The present invention pertains to a water retention material for agriculture that contains a water-absorbent resin. The water-absorbent resin has 0.1-50 mol% of an ionic group with respect to all constituent units of the water-absorbent resin. The change rate of the volume-based 10% particle size D₁₀ of the water retention material before and after application of ultrasonic waves as measured in a water retention material-swelling test in which a 20-mass% aqueous sodium chloride solution is allowed to be absorbed by the water retention material by means of application of ultrasonic waves, i.e., the change rate of D₁₀=(D₁₀ of the water retention material after application of ultrasonic waves)/(D₁₀ of the water retention material before application of ultrasonic waves), is at most 1.

SELECTED DRAWING: None

COPYRIGHT: (C)2024,JPO&INPIT

Description

The present invention relates to an agricultural water retaining material and a method for producing the same.

In recent years, with the chronic depletion of water resources, attempts have been made to use agricultural water effectively and appropriately, and to maintain or increase the yield of agricultural products even with a smaller amount of irrigation water than in the past. (For example, see Patent Documents 1 and 2). These agricultural water retention materials have super absorbent polymers (SAP) as their main component, and compared to, for example, peat moss, which is used to improve the water retention capacity of the soil as a whole, they exhibit water retention effects in extremely small amounts. It has the advantage of being less burdensome for farmers to use.

Patent Documents 1 and 2 disclose that a water-absorbing resin containing polyacrylate gel as a main component can be used as an agricultural water retaining material. Patent Document 3 discloses polyvinyl alcohol-based water-absorbing resin particles having complex surface shape characteristics. Patent Document 4 discloses an agglomerated polyacrylic acid-based water absorbent resin obtained by a reverse phase suspension polymerization method, and it is disclosed that this resin has high liquid permeability and can be suitably used for sanitary material applications. ing. Patent Document 5 discloses a polyvinyl alcohol-based water absorbent resin having specific pores, and discloses that this resin has excellent water and liquid fertilizer absorption properties and can be suitably used for agricultural purposes.

International Publication No. 1998/005196 pamphlet Special Publication No. 2013-544929 Special Publication No. 2013-540164 International Publication No. 2012/023433 pamphlet Japanese Patent Application Publication No. 2019-119891

However, according to the studies of the present inventors, the polyacrylic acid-based water absorbent resins disclosed in Patent Documents 1, 2, and 4 have a significantly lower water absorption performance due to calcium salts contained in soil, When used for commercial purposes, water absorption is insufficient and it is difficult to maintain stable water absorption over a long period of time. Further, in the vinyl alcohol-based water absorbent resins disclosed in Patent Documents 3 and 5, there are cases where a further improved water absorption rate is required.
The problem to be solved by the present invention is to solve the above-mentioned problems. The purpose is to provide agricultural water retention materials.

In order to solve the above-mentioned problems, the present inventors have conducted detailed studies on agricultural water retaining materials and have completed the present invention.
That is, the present invention includes the following preferred embodiments.
[1] An agricultural water retaining material comprising a water-absorbing resin,
The water-absorbing resin has an ionic group of 0.1 mol% or more and 50 mol% or less based on the total constituent units of the water-absorbing resin,
In a water-retaining material swelling test in which a 20% by mass aqueous sodium chloride solution is absorbed into a water-retaining material by applying ultrasound, the rate of change in the volume-based 10% particle diameter _D10 of the water-retaining material before and after applying ultrasound:
Rate of change in volume-based 10% particle diameter _D10 of water retaining material before and after applying ultrasound = ( _D10 of water retaining material after applying ultrasound)/( _D10 of water retaining material before applying ultrasound) )
is 1 or less, a water retaining material.
[2] The water-retaining material according to [1], wherein the water-retaining material has a particle size that passes through a sieve with a nominal opening of 3000 μm but does not pass through a sieve with a nominal opening of 10 μm.
[3] The water-retaining material according to [1] or [2] above, wherein the maximum particle size on the smallest particle size side after being subjected to ultrasonic waves in the water-retaining material swelling test is 1 μm or more and 1000 μm or less. .
[4] The water-absorbing resin according to any one of [1] to [3] above, has one or more selected from the group consisting of a carboxyl group, a sulfonic acid group, and an ammonium group as the ionic group. Water retention material.
[5] The water-absorbing resin includes one or more selected from the group consisting of a vinyl alcohol polymer, an acrylic acid polymer, an acrylamide polymer, and a methacrylic acid polymer, [1] to [4] above. ] The water retaining material described in any of the above.
[6] The vinyl alcohol polymer according to [5] above, wherein the vinyl alcohol polymer contains one or more monomer constituent units selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, itaconic acid, and derivatives thereof. Water retention material.
[7] The water retaining material according to any one of [1] to [6], wherein the water absorbent resin has a crosslinked structure.
[8] The water retaining material according to any one of [1] to [7] above, which is used for raising seedlings.
[9] A coagulation step in which the water-absorbing resin present as primary particles is brought into contact with each other in a swollen state to aggregate, and a drying step in which the aggregated water-absorbing resin is dried under a pressure of 0.2 MPa or less. The method for producing a water retaining material according to any one of [1] to [8] above.
[10] The method according to [9], further comprising a crosslinking step of crosslinking the water absorbent resin before the aggregation step, at the same time as the aggregation step, or after the aggregation step.

According to the present invention, it is possible to provide an agricultural water retaining material which has an excellent water absorption rate, has a sufficiently high water absorption amount even in the presence of calcium salts, and whose water absorption amount does not easily decrease over a long period of time.

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments.

が１以下であることも特徴とする。ここで、体積基準１０％粒子径とは、これ以下の粒子の比率が体積基準で１０％である粒子径を意味する。 [Agricultural water retention material]
The agricultural water-retaining material (hereinafter also simply referred to as "water-retaining material") of the present invention contains a water-absorbing resin. The present invention is characterized in that the water-absorbing resin has an ionic group of 0.1 mol % or more and 50 mol % or less based on the total structural units of the water-absorbing resin. In addition, the present invention provides a rate of change in the volume-based 10% particle diameter _D10 of the water-retaining material before and after applying ultrasonic waves in a water-retaining material swelling test:

is 1 or less. Here, the 10% particle diameter on a volume basis means a particle diameter at which the proportion of particles smaller than this is 10% on a volume basis.

A D ₁₀ change rate of 1 or less can be achieved, for example, by the presence of the water-absorbing resin as aggregates in the water-retaining material. In a preferred embodiment of the present invention, in the water-retaining material, the water-absorbing resin is present as an aggregate of particles, preferably as an aggregate of primary particles. As used herein, primary particles refer to particles that are not aggregated.

Surprisingly, the present inventors found that because the water-retaining material has the above-mentioned characteristics, the water-retaining material exhibits an excellent water absorption rate, exhibits a sufficiently high water absorption amount even in the presence of calcium salts, and shows that this water absorption amount It was found that the value does not tend to decrease over a long period of time.
Although the reason for this is not clear, the following reason is presumed as a non-limiting mechanism of action. Since the water retaining material of the present invention contains a water-absorbing resin, it swells when it absorbs water. On the other hand, since the cohesive force of the water-absorbing resin in the water-retaining material is moderately weak, the water-retaining material disintegrates as it expands. When the water-retaining material collapses, the surface area of the water-absorbing resin that comes into contact with water increases, so it is presumed that the water absorption rate increases. In addition, the surface of the water-absorbing resin that is in contact with water can be improved by ensuring that the water-absorbing resin has an ionic group content that is not too high, that is, an ionic group content of 50 mol% or less based on the total constituent units of the water-absorbing resin. Water does not only absorb water rapidly, but also enters the voids in the water-absorbing resin inside the water-retaining material, and as a result, the surface area of the water-absorbing resin, which has expanded due to the collapse of the water-retaining material, comes into contact with water. It is estimated that the water absorption rate is significantly faster because the water can be used more effectively. Furthermore, because the water-absorbing resin has the above-mentioned specific ionic group content, ionic crosslinking between calcium salts and ionic groups is less likely to occur, so even in the presence of calcium salts, the water-retaining material can absorb sufficient water. It is estimated that this amount of water absorption will be difficult to decrease over a long period of time.

本発明者らは、保水材を形成している吸水性樹脂粒子のうち、比較的小さい粒子径を有する粒子が、保水材全体の吸水速度を決める因子の１つであることを見出した。そのため、本発明では、優れた吸水速度をもたらすための構成要件に、Ｄ_１０を用いている。 In the water-retaining material swelling test, the state of the water-retaining material at the initial stage of water absorption is reproduced by dispersing the water-retaining material in a medium that does not absorb much liquid, rather than in a medium that is quickly absorbed by the water-retaining material (e.g., water). Can be done. In the present invention, a 20% by mass aqueous sodium chloride solution is used as such a medium that does not absorb much liquid. Thereafter, by applying ultrasonic waves to the water-retaining material in the aqueous solution, the aqueous solution can be absorbed by the water-retaining material, thereby making it possible to reproduce the swelling and collapse of the water-retaining material.
The water retaining material swelling test can be performed using the following procedure. First, a water-retaining material as a sample is dispersed in a dispersion medium, and using, for example, a laser diffraction/scattering particle size distribution measuring device, the volume-based 10% particle diameter D ₁₀ (D ₁₀ of the water-retaining material before applying ultrasonic waves) is measured. (equivalent to). Next, the water-retaining material is swollen by absorbing the dispersion medium into the water-retaining material by ultrasonic irradiation for ₅ minutes. _D10 ) is measured. The measurement conditions are shown below.
Device name: Horiba Laser diffraction/scattering particle size distribution measuring device LA-950V2
Measurement method: Wet type (circulation type)
Sample refractive index: 1.51
Dispersion medium: 20% by mass aqueous sodium chloride solution (refractive index: 1.368)
Circulation speed: 10
Stirring speed: 10
Ultrasonic strength: 7
Transmittance: 80-90%
Circulating fluid volume: 280mL
Next, the rate of change in the volume-based 10% particle diameter D ₁₀ of the water retaining material before and after applying ultrasonic waves is determined using the following formula.

The present inventors have discovered that among the water-absorbing resin particles forming the water-retaining material, particles having a relatively small particle size are one of the factors that determine the water absorption rate of the entire water-retaining material. Therefore, in the present invention, _D10 is used as a component for providing an excellent water absorption rate.

In the water-retaining material swelling test, the rate of change in the volume-based 10% particle diameter D ₁₀ of the water-retaining material before and after applying ultrasonic waves is 1 or less. If the rate of change in _D10 is greater than 1, it is difficult to obtain the desired water absorption rate. The rate of change in _D10 is preferably 0.95 or less, more preferably 0.94 or less, more preferably 0.93 or less, more preferably 0.92 or less, more preferably 0.91 or less, and even more preferably 0. .90 or less, even more preferably 0.85 or less, particularly preferably 0.80 or less, particularly more preferably 0.75 or less. When the rate of change in D ₁₀ is below the upper limit value, expansion and collapse of the water retaining material proceed in a well-balanced manner during water absorption, making it easier to obtain a more preferable water absorption rate.
The lower limit of the rate of change of _D10 is not particularly limited. The rate of change of _D10 is, for example, 0.30 or more. When the rate of change in _D10 is equal to or greater than the lower limit value, it is easy to prevent the water retaining material from collapsing before use and causing dust generation.
The rate of change in _D10 can be determined by, for example, making the water-absorbing resin exist as an aggregate in the water-retaining material, adjusting the amount of binder, pressure, temperature, or plasticizer in the process of agglomerating the water-absorbing resin, or controlling the amount of the aggregated water-absorbing resin. By adjusting the pressure, temperature, or amount of plasticizer in the drying step, or by adjusting the particle size or shape of the water-absorbing resin particles before aggregation, it can be adjusted to be equal to or higher than the lower limit and lower than or equal to the upper limit.

The maximum particle size on the smallest particle size side in the state after applying ultrasonic waves in the water retention material swelling test is preferably 1 μm or more, more preferably 5 μm or more, even more preferably 10 μm or more, and particularly preferably 30 μm or more. , preferably 1000 μm or less, more preferably 500 μm or less, even more preferably 200 μm or less, particularly preferably 100 μm or less. The maximum particle size is 0.011 μm, 0.013 μm, 0.015 μm, 0.017 μm, 0.02 μm, 0.023 μm, 0.026 μm, 0.03 μm, 0.034 μm, 0.039 μm, 0.044 μm, 0.051 μm, 0.058 μm, 0.067 μm, 0.076 μm, 0.087 μm, 0.1 μm, 0.115 μm, 0.131 μm, 0.15 μm, 0.172 μm, 0.197 μm, 0.226 μm, 0. 259 μm, 0.296 μm, 0.339 μm, 0.389 μm, 0.445 μm, 0.51 μm, 0.584 μm, 0.669 μm, 0.766 μm, 0.877 μm, 1.005 μm, 1.151 μm, 1.318 μm, 1.51 μm, 1.729 μm, 1.981 μm, 2.269 μm, 2.599 μm, 2.976 μm, 3.409 μm, 3.905 μm, 4.472 μm, 5.122 μm, 5.867 μm, 6.72 μm, 7. 697 μm, 8.816 μm, 10.097 μm, 11.565 μm, 13.246 μm, 15.172 μm, 17.377 μm, 19.904 μm, 22.797 μm, 26.111 μm, 29.907 μm, 34.255 μm, 39.234 μm, 44.938 μm, 51.471 μm, 58.953 μm, 67.523 μm, 77.34 μm, 88.583 μm, 101.46 μm, 116.21 μm, 133.103 μm, 152.453 μm, 174.616 μm, 200 μm, 229.075 μm, 133 7. This value is determined from the frequency at 481 μm, 1531.914 μm, 1754.613 μm, 2009.687 μm, 2301.841 μm, 2636.467 μm, or 3000 μm. When the maximum particle size is greater than or equal to the lower limit and less than or equal to the upper limit, a more preferable water absorption rate can be easily obtained. The maximum particle size can be adjusted to be greater than or equal to the lower limit value and less than or equal to the upper limit value, for example, by adjusting the particle size of the water-absorbing resin particles before aggregation.

In a preferred embodiment of the present invention, the water-absorbing resin contained in the water-retaining material exists as an aggregate of primary particles. The average particle diameter of the primary particles of the water absorbent resin contained in the water retaining material is preferably 1 μm or more, more preferably 10 μm or more, even more preferably 30 μm or more, particularly preferably 50 μm or more, and preferably 2000 μm or less, more preferably It is 1000 μm or less, more preferably 700 μm or less, particularly preferably 150 μm or less. When the average particle diameter of the primary particles is greater than or equal to the lower limit and less than or equal to the upper limit, it is easier to obtain a more preferable water absorption rate. Moreover, when the average particle diameter of the primary particles is equal to or larger than the lower limit value, dust generation can be easily suppressed during production of the water retaining material. The average particle diameter of the primary particles can be measured using, for example, an electron microscope or a sieve with a specific opening.

The water-absorbing resin contained in the water-retaining material is 0.1 mol% or more, preferably 0.5 mol% or more, more preferably 1.0 mol% or more, and even more preferably 2 mol% or more, based on the total constituent units of the water-absorbing resin. It has an ionic group content of .0 mol % or more, particularly preferably 3.0 mol % or more. Here, the term "constituent unit" means a repeating unit that constitutes the water-absorbing resin. When the ionic group content is equal to or higher than the lower limit, the water absorbent resin tends to have improved water absorption amount or water absorption rate. The water-absorbing resin is 50 mol% or less, preferably 40 mol% or less, more preferably 30 mol% or less, even more preferably 20 mol% or less, particularly preferably 10 mol% or less, based on the total constituent units of the water-absorbing resin. It has an ionic group of When the ionic group content is below the upper limit, the water-absorbing resin easily maintains an excellent liquid absorption amount or liquid absorption rate even in the presence of divalent ions (e.g. calcium ions) contained in soil, This liquid absorption amount or liquid absorption rate is unlikely to decrease over a long period of time, and the water absorbent resin is unlikely to be decomposed by ultraviolet rays.
The content of ionic groups in the water-absorbing resin and the content of various structural units described below can be determined by, for example, solid ¹³ C-NMR (nuclear magnetic resonance spectroscopy), FTIR (Fourier transform infrared spectroscopy) or acid-base spectroscopy. It can be measured by titration etc.
In addition, the content of ionic groups in the water-absorbing resin and the content of various structural units described below are determined by, for example, the blending ratio of monomers that provide ionic groups and monomers that form various structural units, and their ratio during reaction. It can be adjusted by adjusting the consumption rate or reactivity ratio, reaction temperature, solvent, etc.

The ionic group may be present as an ionic group or a derivative thereof. The ionic group is preferably a carboxyl group, a sulfonic acid group, an ammonium group, or a salt thereof, more preferably a carboxyl group, an ammonium group, or a salt thereof, and particularly preferably a carboxyl group or a salt thereof. Therefore, in a preferred embodiment of the present invention, the water-absorbing resin has one or more ionic groups selected from the group consisting of carboxyl groups, sulfonic acid groups, and ammonium groups.
In addition, when some or all of the ionic groups contained in the water-absorbing resin are in the form of their derivatives (for example, salts), the above-mentioned ionic group content refers to the content of the ionic groups and their derivatives or This is the content of derivatives of ionic groups.

The water retaining material preferably passes through a sieve with a nominal opening of 3000 μm, more preferably through a sieve with a nominal opening of 2000 μm, even more preferably through a sieve with a nominal opening of 1500 μm, particularly preferably through a sieve with a nominal opening of 1000 μm. It has a particle size that allows it to pass through a sieve, preferably does not pass through a sieve with a nominal opening of 10 μm, more preferably does not pass through a sieve with a nominal opening of 50 μm, even more preferably does not pass through a sieve with a nominal opening of 100 μm, particularly preferably It has a particle size that does not pass through a sieve with a nominal opening of 200 μm. In one preferred embodiment, the water retaining material has a particle size that passes through a sieve with a nominal opening of 3000 μm but does not pass through a sieve with a nominal opening of 10 μm. When the water retaining material has such a particle size, it is easy to obtain better handling properties and a better water absorption rate.

The water-absorbing resin preferably has a crosslinked structure from the viewpoint of suppressing elution of the water-absorbing resin due to irrigation. When the water absorbent resin has a crosslinked structure, the water absorbent resin becomes a gel state when absorbing water. There is no particular restriction on the form of the crosslinked structure, and examples include crosslinked structures with ester bonds, ether bonds, acetal bonds, carbon-carbon bonds, and the like.

The water absorbent resin contained in the water retaining material is not particularly limited. From the viewpoint of ease of manufacture and water retention, the water-absorbing resin preferably contains one or more selected from the group consisting of vinyl alcohol polymers, acrylic acid polymers, acrylamide polymers, and methacrylic acid polymers. more preferably one or more selected from the group consisting of a vinyl alcohol polymer and an acrylamide polymer, and still more preferably a vinyl alcohol polymer or an acrylamide polymer.
As used herein, the vinyl alcohol polymer refers to a polymer having the highest content of constitutional units derived from vinyl alcohol (hereinafter referred to as "vinyl alcohol units") among all constitutional units. This also applies to acrylic acid polymers, acrylamide polymers, and methacrylic acid polymers.

<Vinyl alcohol polymer>
Examples of vinyl alcohol polymers [hereinafter sometimes referred to as vinyl alcohol polymers (A)] include polyvinyl alcohol, ethylene-vinyl alcohol copolymers, and their vinyl alcohol units are acetalized by an acetalizing agent. I can list some things that have been transformed. The content of the vinyl alcohol polymer (A) in the water-absorbing resin is preferably 70% by mass or more, more preferably 80% by mass or more, still more preferably 90% by mass or more, particularly preferably 95% by mass or more. , 100% by mass.

The water-absorbing resin in the present invention has an ionic group. Accordingly, in a preferred embodiment of the present invention, the vinyl alcohol polymer (A) consists of a copolymer of vinyl alcohol units and monomer constituent units having an ionic group or a derivative thereof. The ionic group or derivative thereof that the vinyl alcohol polymer (A) has is preferably a carboxyl group, a sulfonic acid group, an ammonium group, or a salt thereof, and more preferably a carboxyl group, an ammonium group, or a salt thereof. and particularly preferably a carboxyl group or a salt thereof. The content of the copolymer in the vinyl alcohol polymer (A) is preferably 70% by mass or more, more preferably 80% by mass or more, still more preferably 90% by mass or more, even more preferably 95% by mass or more. The content is particularly preferably 100% by mass.

When the vinyl alcohol polymer (A) has a carboxyl group, a sulfonic acid group, and an ammonium group as ionic groups, the vinyl alcohol polymer (A) includes, for example, (i-1) a carboxyl group, a sulfonic acid group, and an ammonium group. and saponified products of copolymers of vinyl esters and one or more selected from the group consisting of monomers having ammonium groups and derivatives of the monomers; and the like.

In (i-1) above, the monomer having a carboxyl group is not particularly limited, and examples thereof include acrylic acid, methacrylic acid, maleic acid, and itaconic acid. Examples of derivatives of the monomer having a carboxyl group include anhydrides, esters, and neutralized products of the monomer, such as methyl acrylate, methyl methacrylate, monomethyl maleate, and itacon. Dimethyl acid, maleic anhydride, and the like can be used. Therefore, in one embodiment of the present invention, the vinyl alcohol polymer (A) contains one or more monomer constituent units selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, itaconic acid and derivatives thereof. include.

In (i-1) above, the monomer having a sulfonic acid group is not particularly limited, and examples thereof include vinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, and p-styrenesulfonic acid. be able to. Further, examples of derivatives of the monomer having a sulfonic acid group include esterified products and neutralized products of the monomer, such as sodium vinyl sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, etc. Sodium, sodium p-styrene sulfonate, and the like can be used.

In (i-1) above, the monomer having an ammonium group is not particularly limited, and examples include diallyldimethylammonium chloride, vinyltrimethylammonium chloride, allyltrimethylammonium chloride, p-vinylbenzyltrimethylammonium chloride, and -(methacrylamido)propyltrimethylammonium chloride and the like. Examples of derivatives of the monomer having an ammonium group include amines of the monomer, such as diallylmethylamine, vinylamine, allylamine, p-vinylbenzyldimethylamine, and 3-(methacrylamide). Propyldimethylamine and the like can be used.

In (i-1) above, the vinyl ester is not particularly limited, and examples thereof include vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, vinyl stearate, vinyl benzoate, vinyl trifluoroacetate, and Examples include vinyl pivalate, and vinyl acetate is preferred.

There are no particular limitations on the method for producing the saponified product (i-1). The saponified product of (i-1) above can be obtained by, for example, polymerizing a vinyl ester with one or more selected from the group consisting of monomers having a carboxyl group and derivatives of the monomers using a known polymerization initiator. It can be produced by carrying out a reaction and then carrying out a saponification reaction by a known method.

In one embodiment of the present invention, part or all of the ionic group (for example, carboxyl group) possessed by the vinyl alcohol polymer (A) is in the form of a salt (carboxylate when the ionic group is a carboxyl group). It may be. Examples of countercations of salts include alkali metal ions such as lithium, sodium, potassium, rubidium, and cesium ions; alkaline earth metal ions such as magnesium, calcium, strontium, and barium ions; Examples include other metal ions such as aluminum ions and zinc ions; onium cations such as ammonium ions, imidazoliums, pyridiniums, and phosphonium ions; and the like. Among these, potassium ions, ammonium ions, calcium ions, and magnesium ions are preferred from the viewpoint of easily obtaining a more preferable water absorption amount or water absorption rate. Potassium ions and ammonium ions are more preferred from the viewpoint of plant growth, and calcium ions are more preferred from the viewpoint of easily maintaining the liquid absorption amount or liquid absorption rate even when in contact with divalent ions contained in soil.

When the ionic group is a carboxyl group, the method for producing the vinyl alcohol polymer (A) in which some or all of the carboxyl groups are carboxylates includes, for example, Method (I) using a neutralized monomer; method (III) in which the vinyl alcohol polymer (A) having a carboxyl group is produced in the above (i-1) and then neutralized; etc. Among them, the above method (III) is preferable.

The content of ionic groups in the vinyl alcohol polymer (A) is 0.1 mol% or more, preferably 1 mol% or more, more preferably 3 mol% or more, more preferably 4 mol% or more, particularly preferably 5 mol% or more, and 50 mol% or less, preferably 40 mol% or less, more preferably 30 mol% or less, even more preferably 25 mol% or less. , even more preferably 20 mol % or less, particularly preferably 15 mol % or less, even more preferably 10 mol % or less. When the content of the ionic group is at least the lower limit, the vinyl alcohol polymer (A) tends to have improved water absorption or water absorption rate. When the content of the ionic group is below the upper limit, the vinyl alcohol polymer (A) maintains an excellent liquid absorption amount or liquid absorption rate even when it comes into contact with divalent ions contained in soil. The amount of liquid absorbed or the rate of liquid absorption is unlikely to decrease over a long period of time, and the vinyl alcohol polymer (A) is unlikely to be decomposed by ultraviolet rays.

In addition, when the vinyl alcohol polymer (A) has a carboxyl group as an ionic group, the amount of carboxyl groups derived from acrylic acid or its salt among the carboxyl groups is determined based on the total structural units of the vinyl alcohol polymer. On the other hand, it is preferably 20 mol% or less, more preferably 15 mol% or less, particularly preferably 10 mol% or less, and may be 0 mol%. When the amount of carboxyl groups derived from acrylic acid or its salt among the above carboxyl groups is below the above upper limit, it is easy to obtain better weather resistance (especially ultraviolet resistance).
In a preferred embodiment, half or more of the ionic groups contained in the vinyl alcohol polymer (A) are in the form of derivatives, and in a more preferred embodiment, the ionic groups contained in the vinyl alcohol polymer (A) are in the form of derivatives. Most of the groups are in the form of derivatives, and in one particularly preferred embodiment, all of the ionic groups contained in the vinyl alcohol polymer (A) are in the form of derivatives.
In this specification, as mentioned above, the term "constituent unit" means a repeating unit that constitutes a polymer. The structure will be counted as "2 units".

The content of vinyl alcohol units in the vinyl alcohol polymer (A) is preferably more than 20 mol%, more preferably 50 mol% or more, and even more preferably is 60 mol% or more, preferably 98 mol% or less, more preferably 95 mol% or less, even more preferably 90 mol% or less. The content of the vinyl alcohol unit can be measured by FTIR or solid ¹³ C-NMR as described above, but it can also be calculated from the amount of acetic anhydride consumed when reacting with a certain amount of acetic anhydride.

The vinyl alcohol polymer (A) contains structural units other than vinyl alcohol units. Examples of the other structural units mentioned above 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, and methacrylic acid. Examples include structural units derived from acids and derivatives thereof, acrylamide and derivatives thereof, methacrylamide and derivatives thereof, maleic acid and derivatives thereof, maleimide derivatives, and the like. The above-mentioned other structural units may contain one type or a plurality of types. The content of the other structural units is preferably 50 mol% or less, more preferably 30 mol% or less, even more preferably 20 mol% or less, based on the total structural units of the vinyl alcohol polymer (A). More preferably 10 mol% or less, particularly preferably 5 mol% or less, preferably 0.1 mol% or more, more preferably 0.5 mol% or more, still more preferably 1 mol% or more, even more preferably 2 It is mol% or more, particularly preferably 3 mol% or more. When the content of the other structural units is greater than or equal to the lower limit and less than or equal to the upper limit, it is easy to obtain superior water absorption amount or water absorption rate of the water retaining material of the present invention.

The viscosity average degree of polymerization of the vinyl alcohol polymer (A) is not particularly limited, but from the viewpoint of ease of production, it is preferably 20,000 or less, more preferably 10,000 or less, still more preferably 4,000 or less, and particularly preferably 3,000 or less. be. On the other hand, from the viewpoint of the mechanical properties and water elution resistance of the vinyl alcohol polymer (A), the viscosity average degree of polymerization is preferably 100 or more, more preferably 200 or more, and even more preferably 400 or more. The viscosity average degree of polymerization of the vinyl alcohol polymer (A) can be measured by a method based on JIS K 6726. When the vinyl alcohol polymer (A) has a crosslinked structure as described below, for example, when the vinyl alcohol polymer (A) has an acetal structure or an ester structure as the crosslinked structure, the measurement of the viscosity average degree of polymerization It can be done after cutting the structure. The cleavage can be performed by a common method (for example, hydrolysis using an acid or an alkali).

The vinyl alcohol polymer (A) preferably contains a crosslinked structure from the viewpoint of suppressing elution of the vinyl alcohol polymer due to irrigation. There is no particular restriction on the form of the crosslinked structure, and examples include crosslinked structures with ester bonds, ether bonds, acetal bonds, carbon-carbon bonds, and the like. The presence or absence of a crosslinked structure in the vinyl alcohol polymer (A) can be determined, for example, by the elution rate in 100° C. hot water or dimethyl sulfoxide. Specifically, the elution rate, which is expressed as the ratio of the mass of the eluted sample to the mass of the sample (ratio of the mass of the eluted sample x 100/mass of the sample), is below a certain value (for example, 90% by mass or less). The existence of a crosslinked structure can be confirmed by the presence of the crosslinked structure.

As an example of the above ester bond, when the vinyl alcohol polymer (A) has a carboxyl group as an ionic group, an ester formed between a hydroxyl group and a carboxyl group that the vinyl alcohol polymer (A) has. A combination can be mentioned. An example of the ether bond is an ether bond formed by dehydration condensation between hydroxyl groups of the vinyl alcohol polymer (A). Another example of the above-mentioned ether bond is an ether bond formed when a polyvalent epoxy compound having multiple epoxy groups in one molecule is used in the production of the vinyl alcohol polymer (A). can. As an example of the above acetal bond, when an aldehyde having a carboxyl group is used in the production of the vinyl alcohol polymer (A), the hydroxyl groups of the two vinyl alcohol polymers (A) are acetalized with the aldehyde. Examples include acetal bonds formed by reaction. Another example of the above acetal bond is the acetal bond formed when a polyvalent aldehyde compound having multiple aldehyde groups in one molecule is used in the production of the vinyl alcohol polymer (A). can. The above-mentioned carbon-carbon bonds include carbon-carbon bonds formed by coupling between carbon radicals of the vinyl alcohol polymer (A), which occurs when the vinyl alcohol polymer (A) is irradiated with active energy rays, for example. A combination can be mentioned. One type of these crosslinked structures may be included, or multiple types may be included. Among these, a crosslinked structure with an ester bond or an acetal bond is preferred from the viewpoint of ease of production, and a crosslinked structure with an acetal bond is more preferred from the viewpoint of maintaining water retention during seedling raising and UV resistance.

Such a crosslinked structure may be formed simultaneously with the acetalization reaction in the step of acetalizing at least some vinyl alcohol units with one or more selected from polyhydric aldehyde compounds, or may be formed simultaneously with the acetalization reaction step. It may be formed before the aggregation process by further adding a crosslinking agent before the aggregation process described below, or it may be formed simultaneously with the aggregation process by further adding a crosslinking agent in the aggregation process. or may be formed after the aggregation step by further adding a crosslinking agent after the aggregation step. In the present invention, it is preferable to form a crosslinked structure by further adding a crosslinking agent.

Examples of crosslinking agents include polyvalent epoxy compounds and polyvalent aldehyde compounds, and among them, polyvalent aldehyde compounds are preferred.
Examples of the polyvalent epoxy compound include bifunctional epoxy compounds such as ethylene glycol diglycidyl ether.
Examples of polyvalent aldehyde compounds include glyoxal, malonaldehyde, succinaldehyde, glutaraldehyde, 1,9-nonanedial, adipaldehyde, malealdehyde, tartaraldehyde, citrualdehyde, phthalaldehyde, isophthalaldehyde, and terephthalaldehyde. Mention may be made of difunctional aldehydes. When a polyvalent aldehyde compound is used as a crosslinking agent, the hydroxyl group of one of the two vinyl alcohol polymers (A) undergoes an acetalization reaction with one aldehyde group of the polyvalent aldehyde compound, and Two acetal bonds can be introduced by carrying out an acetalization reaction between the hydroxyl group of the other of the two vinyl alcohol polymers (A) and another aldehyde group of the polyvalent aldehyde compound. .

When adding a crosslinking agent, the amount of crosslinking agent in the vinyl alcohol polymer (A) is preferably 0.001 mol% or more, more preferably 0.001% by mole or more, from the viewpoint of easily maintaining water retention in soil. 005 mol% or more, more preferably 0.01 mol% or more, even more preferably 0.03 mol% or more, preferably 0.5 mol% or less, more preferably 0.4 mol% or less, even more preferably It is 0.3 mol% or less.

<Acrylic acid polymer>
The acrylic acid-based polymer that may be included in the water-absorbing resin of the present invention contains the most structural units derived from acrylic acid or acrylic acid derivatives (hereinafter referred to as "acrylic acid-derived structural units") among all structural units. Contains high content. Examples of raw materials for acrylic acid polymers include acrylic acid, sodium acrylate, potassium acrylate, calcium acrylate, methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, and sec acrylate. -butyl, 2-ethylhexyl acrylate, cyclohexyl acrylate, phenyl acrylate, and 2-hydroxyethyl acrylate. Examples of acrylic acid polymers include those obtained by copolymerizing one or more of these monomers and at least one other copolymerizable monomer using a known method. Examples of other copolymerizable monomers include monomers that provide structural units other than acrylic acid-derived structural units, which will be described later.
The content of the acrylic acid polymer in the water absorbent resin is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and 100% by mass. It may be.

In one embodiment of the present invention, part or all of the ionic groups (for example, carboxyl groups) possessed by the acrylic acid polymer are in the form of salts (carboxylate salts when the ionic groups are carboxyl groups). Good too. Examples of countercations of salts include alkali metal ions such as lithium, sodium, potassium, rubidium, and cesium ions; alkaline earth metal ions such as magnesium, calcium, strontium, and barium ions; Examples include other metal ions such as aluminum ions and zinc ions; onium cations such as ammonium ions, imidazoliums, pyridiniums, and phosphonium ions; and the like. Among these, potassium ions, ammonium ions, calcium ions, and magnesium ions are preferred from the viewpoint of easily obtaining a more preferable water absorption amount or water absorption rate. Potassium ions and ammonium ions are more preferred from the viewpoint of plant growth, and calcium ions are more preferred from the viewpoint of easily maintaining the liquid absorption amount or liquid absorption rate even when in contact with divalent ions contained in soil.

The content of ionic groups in the acrylic acid polymer is 0.1 mol% or more, preferably 1 mol% or more, more preferably 3 mol% or more, based on the total constitutional units of the acrylic acid polymer. Preferably 4 mol% or more, particularly preferably 5 mol% or more, and 50 mol% or less, preferably 40 mol% or less, more preferably 30 mol% or less, still more preferably 25 mol% or less, and even more preferably 20 mol% or less. It is less than mol %, particularly preferably less than 15 mol %, even more preferably less than 10 mol %. When the content of the ionic group is at least the lower limit, the acrylic acid polymer tends to have improved water absorption or water absorption rate. When the content of the ionic group is below the upper limit, the acrylic acid polymer can easily maintain an excellent liquid absorption amount or liquid absorption rate even when it comes into contact with divalent ions contained in soil, This liquid absorption amount or liquid absorption rate is unlikely to decrease over a long period of time, and decomposition of the acrylic acid polymer by ultraviolet rays is unlikely to occur.

In addition, when the acrylic acid polymer has a carboxyl group as an ionic group, the amount of carboxyl groups derived from acrylic acid or its salt among the carboxyl groups is based on the total constitutional units of the acrylic acid polymer. It is preferably 20 mol% or less, more preferably 15 mol% or less, particularly preferably 10 mol% or less, and may be 0 mol%. When the amount of carboxyl groups derived from acrylic acid or its salt among the above carboxyl groups is below the above upper limit, it is easy to obtain better weather resistance (especially ultraviolet resistance).
In a preferred embodiment, more than half of the ionic groups contained in the acrylic acid polymer are in the form of derivatives, and in a more preferred embodiment, most of the ionic groups contained in the acrylic acid polymer are in the form of derivatives. In one particularly preferred embodiment, all of the ionic groups contained in the acrylic acid polymer are in the form of derivatives.

The content of the acrylic acid-derived structural units in the acrylic acid polymer is preferably more than 20 mol%, more preferably 50 mol% or more, and even more preferably 60 mol%, based on the total structural units of the acrylic acid polymer. The content is preferably 98 mol% or less, more preferably 95 mol% or less, and even more preferably 90 mol% or less.

The acrylic acid-based polymer contains structural units other than acrylic acid-derived structural units. Examples of the other structural units mentioned above include vinyl alcohol units; 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; methacrylic acid and Structural units derived from derivatives thereof, acrylamide and derivatives thereof, methacrylamide and derivatives thereof, maleic acid and derivatives thereof, maleimide derivatives, etc. can be mentioned. The above-mentioned other structural units may contain one type or a plurality of types. The content of the above-mentioned other structural units is preferably 50 mol% or less, more preferably 30 mol% or less, still more preferably 20 mol% or less, and even more preferably 10 mol% or less, particularly preferably 5 mol% or less, preferably 0.1 mol% or more, more preferably 0.5 mol% or more, still more preferably 1 mol% or more, even more preferably 2 mol% or more. , particularly preferably 3 mol% or more. When the content of the other structural units is greater than or equal to the lower limit and less than or equal to the upper limit, it is easy to obtain superior water absorption amount or water absorption rate of the water retaining material of the present invention.

The weight average molecular weight of the acrylic acid polymer is not particularly limited, but from the viewpoint of ease of production, it is preferably 10,000,000 or less, more preferably 5,000,000 or less, and even more preferably 3,000,000. It is particularly preferably 1,000,000 or less. On the other hand, from the viewpoint of the mechanical properties of the acrylic acid polymer and its resistance to elution into water, the weight average molecular weight is preferably 1,000 or more, more preferably 5,000 or more, and still more preferably 10,000 or more. The weight average molecular weight of the acrylic acid polymer can be measured, for example, by GPC. When the acrylic acid polymer has a crosslinked structure as described below, for example, when the acrylic acid polymer has an acetal structure or an ester structure as the crosslinked structure, the weight average molecular weight should be measured after cutting the crosslinked structure. Can be done. The cleavage can be performed by a common method (for example, hydrolysis using an acid or an alkali).

The acrylic acid polymer preferably contains a crosslinked structure from the viewpoint of suppressing elution of the acrylic acid polymer due to irrigation. There is no particular restriction on the form of the crosslinked structure, and an example thereof is a crosslinked structure using a general crosslinking agent. As mentioned above, the presence or absence of a crosslinked structure in the acrylic acid polymer can be determined by the elution rate in hot water at 100° C. or dimethyl sulfoxide, for example.

For example, the crosslinked structure may be formed simultaneously with the copolymerization reaction in the copolymerization process of a monomer that provides an acrylic acid-derived structural unit and a monomer that provides other structural units other than the acrylic acid-derived structural unit, or may be formed simultaneously with the copolymerization reaction. It may be formed before the aggregation step by further adding a crosslinking agent before the aggregation step, which will be described later, or it may be formed before the aggregation step by further adding a crosslinking agent in the aggregation step. They may be formed simultaneously or after the aggregation step by further adding a crosslinking agent after the aggregation step. In the present invention, it is preferable to form a crosslinked structure by further adding a crosslinking agent.

Examples of crosslinking agents include N,N'-methylenebisacrylamide, divinylbenzene, ethylene glycol diglycidyl ether, pentaerythritol triallyl ether, pentaerythritol triacrylate, pentaerythritol tetraacrylate, 1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate, neopentyl glycol diacrylate, 2-hydroxy-3-methacrylpropyl acrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, trimethylolpropane triacrylate and Tris-(2-acryloxyethyl)isocyanurate and the like can be mentioned.

When adding a crosslinking agent, the amount of crosslinking agent in the acrylic acid polymer is preferably 0.001 mol% or more, more preferably 0.01 mol%, from the viewpoint of easily maintaining water retention in soil. Above, the content is more preferably 1.0 mol% or more, even more preferably 2.0 mol% or more, preferably 10 mol% or less, more preferably 5 mol% or less, even more preferably 3 mol% or less.

Specific examples of acrylic acid polymers include crosslinked acrylic acid-sodium acrylate copolymers. Commercially available products include superabsorbent polymers (acrylate-based, manufactured by Wako Pure Chemical Industries, Ltd.), Acryhope (registered trademark) (manufactured by Nippon Shokubai Co., Ltd.), and Sunwet (registered trademark) (Sanyo Chemical Industries, Ltd.). (manufactured by), etc.

<Acrylamide polymer>
The acrylamide-based polymer that may be included in the water-absorbing resin of the present invention has the highest content of structural units derived from acrylamide or acrylamide derivatives (hereinafter referred to as "acrylamide-derived structural units") among all structural units. include. Examples of raw materials for acrylamide polymers include acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, N-alkylacrylamide, and trimethyl[3-(acryloylamino)propyl]aminium chloride. Examples of acrylamide polymers include those obtained by copolymerizing one or more of these monomers and at least one other copolymerizable monomer using a known method. Examples of other copolymerizable monomers include monomers that provide structural units other than acrylamide-derived structural units, which will be described later.
The content of the acrylamide polymer in the water absorbent resin is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and 100% by mass. There may be.

In one embodiment of the present invention, some or all of the ionic groups (for example, carboxyl groups) possessed by the acrylamide polymer may be in the form of a salt (carboxylate when the ionic group is a carboxyl group). good. Examples of countercations of salts include alkali metal ions such as lithium, sodium, potassium, rubidium, and cesium ions; alkaline earth metal ions such as magnesium, calcium, strontium, and barium ions; Examples include other metal ions such as aluminum ions and zinc ions; onium cations such as ammonium ions, imidazoliums, pyridiniums, and phosphonium ions; and the like. Among these, potassium ions, ammonium ions, calcium ions, and magnesium ions are preferred from the viewpoint of easily obtaining a more preferable water absorption amount or water absorption rate. Potassium ions and ammonium ions are more preferred from the viewpoint of plant growth, and calcium ions are more preferred from the viewpoint of easily maintaining the liquid absorption amount or liquid absorption rate even when in contact with divalent ions contained in soil.

The content of ionic groups in the acrylamide polymer is 0.1 mol% or more, preferably 1 mol% or more, more preferably 3 mol% or more, and even more preferably 4 mol% or more, particularly preferably 5 mol% or more, and 50 mol% or less, preferably 40 mol% or less, more preferably 30 mol% or less, still more preferably 25 mol% or less, and even more preferably 20 mol%. The content is particularly preferably 15 mol% or less, and even more preferably 10 mol% or less. When the content of the ionic group is at least the lower limit, the acrylamide polymer tends to have improved water absorption or water absorption rate. When the content of the ionic group is below the upper limit, the acrylamide polymer can easily maintain an excellent liquid absorption amount or absorption rate even when it comes into contact with divalent ions contained in soil. The liquid absorption amount or liquid absorption rate is unlikely to decrease over a long period of time, and the acrylamide polymer is unlikely to be decomposed by ultraviolet rays.

Further, when the acrylamide polymer has a carboxyl group as an ionic group, the amount of carboxyl groups derived from acrylamide or its salt among the carboxyl groups is preferably 20 It is mol% or less, more preferably 15 mol% or less, particularly preferably 10 mol% or less, and may be 0 mol%. When the amount of carboxyl groups derived from acrylamide or its salt among the above carboxyl groups is below the above upper limit, it is easy to obtain better weather resistance (particularly ultraviolet resistance).
In one preferred embodiment, more than half of the ionic groups contained in the acrylamide-based polymer are in the form of derivatives, and in a more preferred embodiment, most of the ionic groups contained in the acrylamide-based polymer are in the form of derivatives. In one particularly preferred embodiment, all of the ionic groups contained in the acrylamide polymer are in the form of derivatives.

The content of acrylamide-derived structural units in the acrylamide-based polymer is preferably more than 20 mol%, more preferably 50 mol% or more, and still more preferably 60 mol% or more, based on the total structural units of the acrylamide-based polymer. , preferably 98 mol% or less, more preferably 95 mol% or less, still more preferably 90 mol% or less.

The acrylamide-based polymer contains structural units other than acrylamide-derived structural units. Examples of the other structural units mentioned above include vinyl alcohol units; 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 Structural units derived from derivatives thereof, methacrylic acid and derivatives thereof, methacrylamide and derivatives thereof, maleic acid and derivatives thereof, maleimide derivatives, etc. can be mentioned. The above-mentioned other structural units may contain one type or a plurality of types. The content of the above-mentioned other structural units is preferably 50 mol% or less, more preferably 30 mol% or less, still more preferably 20 mol% or less, and even more preferably 10 mol% or less, based on the total structural units of the acrylamide polymer. mol% or less, particularly preferably 5 mol% or less, preferably 0.1 mol% or more, more preferably 0.5 mol% or more, still more preferably 1 mol% or more, even more preferably 2 mol% or more, Particularly preferably, it is 3 mol% or more. When the content of the other structural units is greater than or equal to the lower limit and less than or equal to the upper limit, it is easy to obtain superior water absorption amount or water absorption rate of the water retaining material of the present invention.

There is no particular restriction on the weight average molecular weight of the acrylamide polymer, but from the viewpoint of ease of production, it is preferably 10,000,000 or less, more preferably 5,000,000 or less, and even more preferably 3,000,000 or less. , particularly preferably 1,000,000 or less. On the other hand, from the viewpoint of mechanical properties and water elution resistance of the acrylamide polymer, the weight average molecular weight is preferably 1,000 or more, more preferably 5,000 or more, and still more preferably 10,000 or more. The weight average molecular weight of the acrylamide polymer can be measured, for example, by GPC. When the acrylamide-based polymer has a crosslinked structure as described below, for example, when the acrylamide-based polymer has an acetal structure or an ester structure as the crosslinked structure, the weight average molecular weight can be measured after cutting the crosslinked structure. . The cleavage can be performed by a common method (for example, hydrolysis using an acid or an alkali).

The acrylamide polymer preferably includes a crosslinked structure from the viewpoint of suppressing elution of the acrylamide polymer due to irrigation. There is no particular restriction on the form of the crosslinked structure, and examples include crosslinked structures using common crosslinking agents. As mentioned above, the presence or absence of a crosslinked structure in the acrylamide polymer can be determined by the elution rate in hot water at 100° C. or dimethyl sulfoxide, for example.

For example, the crosslinked structure may be formed simultaneously with a copolymerization reaction in a copolymerization process of a monomer that provides an acrylamide-derived structural unit and a monomer that provides a structural unit other than an acrylamide-derived structural unit, or may be formed simultaneously with a copolymerization reaction process other than the copolymerization reaction process. It may be formed before the aggregation process by further adding a crosslinking agent before the aggregation process described later, or it may be formed simultaneously with the aggregation process by further adding a crosslinking agent in the aggregation process. or may be formed after the aggregation step by further adding a crosslinking agent after the aggregation step. In the present invention, it is preferable to form a crosslinked structure by further adding a crosslinking agent.

Examples of crosslinking agents include N,N'-methylenebisacrylamide, divinylbenzene, ethylene glycol diglycidyl ether, pentaerythritol triallyl ether, pentaerythritol triacrylate, pentaerythritol tetraacrylate, 1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate, neopentyl glycol diacrylate, 2-hydroxy-3-methacrylpropyl acrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, trimethylolpropane triacrylate and Tris-(2-acryloxyethyl)isocyanurate and the like can be mentioned.

When adding a crosslinking agent, the amount of crosslinking agent in the acrylamide polymer is preferably 0.001 mol% or more, more preferably 0.005 mol% or more, from the viewpoint of easily maintaining water retention in soil. , more preferably 0.01 mol% or more, even more preferably 0.03 mol% or more, preferably 0.5 mol% or less, more preferably 0.4 mol% or less, even more preferably 0.3 mol%. % or less.

Specific examples of acrylamide-based polymers include crosslinked products of acrylamide-acrylic acid-sodium acrylate copolymer and crosslinked products of acrylamide-acrylic acid-potassium acrylate copolymer. Commercially available products include Miracle-Gro (registered trademark) water storing crystal (manufactured by Scotts Miracle-Gro) and Aquasorb (registered trademark) (manufactured by SNF Holding Company, Inc.).

<Methacrylic acid polymer>
The methacrylic acid-based polymer that may be included in the water-absorbing resin of the present invention contains the most structural units derived from methacrylic acid or methacrylic acid derivatives (hereinafter referred to as "methacrylic acid-derived structural units") among all structural units. Contains high content. Examples of raw materials for methacrylic acid polymers include methacrylic acid, sodium methacrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, isopropyl methacrylate, cyclohexyl methacrylate, phenyl methacrylate, and methacrylic acid ( Mention may be made of methacrylic acid (t-butylcyclohexyl), benzyl methacrylate and methacrylic acid (2,2,2-trifluoroethyl) and trimethyl[3-(methacryloylamino)propyl]aminium chloride. . Examples of methacrylic acid polymers include those obtained by copolymerizing one or more of these monomers and at least one other copolymerizable monomer using a known method. Examples of other copolymerizable monomers include monomers that provide structural units other than methacrylic acid-derived structural units, which will be described later.
The content of the methacrylic acid polymer in the water absorbent resin is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and 100% by mass. It may be.

In one embodiment of the present invention, part or all of the ionic groups (for example, carboxyl groups) possessed by the methacrylic acid polymer are in the form of salts (carboxylate salts when the ionic groups are carboxyl groups). Good too. Examples of countercations of salts include alkali metal ions such as lithium, sodium, potassium, rubidium, and cesium ions; alkaline earth metal ions such as magnesium, calcium, strontium, and barium ions; Examples include other metal ions such as aluminum ions and zinc ions; onium cations such as ammonium ions, imidazoliums, pyridiniums, and phosphonium ions; and the like. Among these, potassium ions, ammonium ions, calcium ions, and magnesium ions are preferred from the viewpoint of easily obtaining a more preferable water absorption amount or water absorption rate. Potassium ions and ammonium ions are more preferred from the viewpoint of plant growth, and calcium ions are more preferred from the viewpoint of easily maintaining the liquid absorption amount or liquid absorption rate even when in contact with divalent ions contained in soil.

The content of ionic groups in the methacrylic acid polymer is 0.1 mol% or more, preferably 1 mol% or more, more preferably 3 mol% or more, based on the total constitutional units of the methacrylic acid polymer. Preferably 4 mol% or more, particularly preferably 5 mol% or more, and 50 mol% or less, preferably 40 mol% or less, more preferably 30 mol% or less, still more preferably 25 mol% or less, and even more preferably 20 mol% or less. It is less than mol %, particularly preferably less than 15 mol %, even more preferably less than 10 mol %. When the content of the ionic group is at least the lower limit, the methacrylic acid polymer tends to have improved water absorption or water absorption rate. When the content of the ionic group is below the upper limit, the methacrylic acid polymer can easily maintain an excellent liquid absorption amount or liquid absorption rate even when it comes into contact with divalent ions contained in soil, This liquid absorption amount or liquid absorption rate is unlikely to decrease over a long period of time, and the methacrylic acid polymer is unlikely to be decomposed by ultraviolet rays.

In addition, when the methacrylic acid polymer has a carboxyl group as an ionic group, the amount of carboxyl groups derived from methacrylic acid or its salt among the carboxyl groups is based on the total constitutional units of the methacrylic acid polymer. It is preferably 20 mol% or less, more preferably 15 mol% or less, particularly preferably 10 mol% or less, and may be 0 mol%. When the amount of carboxyl groups derived from methacrylic acid or its salt among the above carboxyl groups is below the above upper limit, better weather resistance (particularly ultraviolet resistance) can be easily obtained.
In a preferred embodiment, more than half of the ionic groups contained in the methacrylic acid polymer are in the form of derivatives, and in a more preferred embodiment, most of the ionic groups contained in the methacrylic acid polymer are in the form of derivatives. In one particularly preferred embodiment, all of the ionic groups contained in the methacrylic acid polymer are in the form of derivatives.

The content of the methacrylic acid-derived structural units in the methacrylic acid polymer is preferably more than 20 mol%, more preferably 50 mol% or more, and even more preferably 60 mol%, based on the total structural units of the methacrylic acid polymer. The content is preferably 98 mol% or less, more preferably 95 mol% or less, and even more preferably 90 mol% or less.

The methacrylic acid polymer contains structural units other than methacrylic acid-derived structural units. Examples of the other structural units mentioned above include vinyl alcohol units; 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 Structural units derived from derivatives thereof, acrylamide and derivatives thereof, methacrylamide and derivatives thereof, maleic acid and derivatives thereof, maleimide derivatives, etc. can be mentioned. The above-mentioned other structural units may contain one type or a plurality of types. The content of the above-mentioned other structural units is preferably 50 mol% or less, more preferably 40 mol% or less, still more preferably 30 mol% or less, and even more preferably 20 mol% or less, particularly preferably 10 mol% or less, preferably 0.1 mol% or more, more preferably 0.5 mol% or more, still more preferably 1 mol% or more, even more preferably 2 mol% or more. , particularly preferably 3 mol% or more. When the content of the other structural units is greater than or equal to the lower limit and less than or equal to the upper limit, it is easy to obtain superior water absorption amount or water absorption rate of the water retaining material of the present invention.

There is no particular restriction on the weight average molecular weight of the methacrylic acid polymer, but from the viewpoint of ease of production, it is preferably 10,000,000 or less, more preferably 5,000,000 or less, and even more preferably 3,000,000. It is particularly preferably 1,000,000 or less. On the other hand, from the viewpoint of the mechanical properties of the methacrylic acid polymer and its resistance to elution into water, the weight average molecular weight is preferably 1,000 or more, more preferably 5,000 or more, and still more preferably 10,000 or more. The weight average molecular weight of the methacrylic acid polymer can be measured, for example, by GPC. When the methacrylic acid polymer has a crosslinked structure as described below, for example, when the methacrylic acid polymer has an acetal structure or an ester structure as the crosslinked structure, the weight average molecular weight should be measured after cutting the crosslinked structure. Can be done. The cleavage can be performed by a common method (for example, hydrolysis using an acid or an alkali).

The methacrylic acid polymer preferably includes a crosslinked structure from the viewpoint of suppressing elution of the methacrylic acid polymer due to irrigation. There is no particular restriction on the form of the crosslinked structure, and examples include crosslinked structures using common crosslinking agents. As mentioned above, the presence or absence of a crosslinked structure in a methacrylic acid-based polymer can be determined by, for example, the elution rate in 100° C. hot water or dimethyl sulfoxide.

For example, the crosslinked structure may be formed simultaneously with the copolymerization reaction in the copolymerization process of a monomer that provides a methacrylic acid-derived structural unit and a monomer that provides a structural unit other than the methacrylic acid-derived structural unit, or may be formed simultaneously with the copolymerization reaction. It may be formed before the aggregation step by further adding a crosslinking agent before the aggregation step, which will be described later, or it may be formed before the aggregation step by further adding a crosslinking agent in the aggregation step. They may be formed simultaneously or after the aggregation step by further adding a crosslinking agent after the aggregation step. In the present invention, it is preferable to form a crosslinked structure by further adding a crosslinking agent.

Examples of crosslinking agents include N,N'-methylenebisacrylamide, divinylbenzene, ethylene glycol diglycidyl ether, pentaerythritol triallyl ether, pentaerythritol tri- and tetraacrylate, 1,6-hexanediol diacrylate, 1,9 -nonanediol diacrylate, 1,10-decanediol diacrylate, neopentyl glycol diacrylate, 2-hydroxy-3-methacrylpropyl acrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, trimethylolpropane triacrylate and tris-( Examples include 2-acryloxyethyl) isocyanurate.

When adding a crosslinking agent, the amount of crosslinking agent in the methacrylic acid polymer is preferably 0.001 mol% or more, more preferably 0.005 mol% from the viewpoint of easily maintaining water retention in soil. Above, more preferably 0.01 mol% or more, even more preferably 0.03 mol% or more, preferably 0.5 mol% or less, more preferably 0.4 mol% or less, still more preferably 0.3 mol%. It is less than mol%.

Specific examples of methacrylic acid-based polymers include crosslinked products of methacrylic acid-sodium methacrylate copolymers, and crosslinked products of methacrylic acid-sodium methacrylate-acrylic acid copolymers.

In a preferred embodiment of the present invention, the water content of the water retaining material of the present invention is preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 33% by mass or less, even more preferably 30% by mass or less. It is.
In one embodiment of the present invention, the water retaining material of the present invention contains water in the water retaining material before being used as an agricultural water retaining material, that is, during storage (for example, in a storage container) and immediately before being used in combination with a culture medium etc. described below. It is preferable that you do so. In the above embodiment in which the water-retaining material contains water, the water content of the water-retaining material of the present invention before being used as an agricultural water-retaining material is preferably 11% by mass or more, more preferably 15% by mass or more, and even more preferably It is 16% by mass or more, and even more preferably 20% by mass or more.
Moisture content can be measured using a halogen moisture meter.

In the above embodiment in which the water retaining material contains water, the bulk density of the water retaining material of the present invention is preferably 0.20 g/mL or more, more preferably 0.30 g/mL or more, and even more preferably 0.35 g/mL. That's all. The bulk density of the water retaining material of the present invention is preferably 1.25 g/mL or less, more preferably 0.95 g/mL or less, still more preferably 0.70 g/mL or less. Bulk density can be measured by the following method.
Pour 10 g of the water retaining material into a 50 mL beaker all at once, then let it stand without tapping, etc., and mark the height at which the top surface of the water retaining material is located. Remove the water retaining material, fill it with water up to the mark, and measure the mass of the water. Using the fact that water is 1 g/mL, the bulk density is calculated from the following formula.
Bulk density (g/mL) = 10 (g)/volume of water added (mL)

<Optional additives>
The water retaining material of the present invention may optionally contain additives in addition to the water absorbent resin. Examples of such additives include starch, modified starch, sodium alginate, chitin, chitosan, polysaccharides such as cellulose and its derivatives; ethylene-propylene copolymers, polystyrene, acrylonitrile-styrene copolymers, acrylonitrile-butadiene. -Styrene copolymer, polyvinyl chloride, polycarbonate resin, polyethylene terephthalate, polybutylene terephthalate, polylactic acid, polysuccinic acid, polyamide 6, polyamide 6/6, polyamide 6/10, polyamide 11, polyamide 12, polyamide 6/12, Polyhexamethylene diamine terephthalamide, polyhexamethylene diamine isophthalamide, polynonamethylene diamine terephthalamide, polyphenylene ether, polyoxymethylene, polypropylene glycol, polytrimethylene glycol, polytetramethylene glycol, polyvinyl acetate, ethylene-vinyl acetate co- Polymer, polyacrylic acid, polyacrylic ester, polyacrylate, polyacrylamide, polymethacrylic acid, polymethacrylic ester, polymethacrylate, ethylene-acrylic acid copolymer, ethylene-acrylic ester copolymer Resins such as , ethylene-acrylate copolymer, ethylene-methacrylic acid copolymer, ethylene-methacrylate ester copolymer, and ethylene-methacrylate copolymer; natural rubber, synthetic isoprene rubber, chloroprene rubber Rubber elastomers such as silicone rubber, fluororubber, urethane rubber, acrylic rubber, styrene thermoplastic elastomer, olefin thermoplastic elastomer, ester thermoplastic elastomer, urethane thermoplastic elastomer, and amide thermoplastic elastomer; Examples include ultraviolet absorbers, antioxidants, light stabilizers, plasticizers, organic solvents, antifoaming agents, thickeners, surfactants, lubricants, antifungal agents, and antistatic agents. These additives can be used alone or in combination of two or more. Furthermore, all the resins listed as examples of additives are different from the water-absorbing resin of the present invention.
When the water-retaining material contains additives, the total content may be within a range that does not impair the effects of the present invention, and is usually 20% by mass or less, preferably 15% by mass or less, based on the total mass of the water-retaining material. More preferably it is 10% by mass or less, for example 5% by mass or less.
In addition, components used in the water-retaining material or the culture medium described below (i.e., water-absorbing resin, any of the above-mentioned additives, or any component (Z) described below), or components used in the manufacturing process described below (for example, water-absorbing resin, etc.) ) When referring to the content or mass, the content or mass is based on the mass in a dry state. In the present invention, "dry state" refers to a state in which the constituent components do not contain volatile components such as water or organic solvents. For example, by performing vacuum drying at 40° C. until the mass of each of the constituent components becomes constant, they can be brought into a dry state.

[Method for manufacturing water retaining material]
The water retaining material of the present invention is, for example,
Manufactured by a manufacturing method that includes an aggregation step in which the water-absorbing resin present as primary particles is aggregated by contacting them in a swollen state, and a drying step in which the aggregated water-absorbing resin is dried under a pressure of 0.2 MPa or less. can. This manufacturing method may hereinafter be referred to as the manufacturing method of the first embodiment of the present invention.

<Agglomeration process>
In the aggregation step, the water-absorbing resin present as primary particles is brought into contact in a swollen state to be agglomerated. As the water-absorbing resin, a water-absorbing resin whose primary particle average particle size has been adjusted by pulverization and/or sieving as necessary may be used. Further, if necessary, any of the above-mentioned additives may be mixed with the water-absorbing resin before the aggregation step, at the same time as the aggregation step, or after the aggregation step.
In one embodiment of the present invention, before the water-absorbing resin is swollen, the water content of the water-absorbing resin present as primary particles is preferably 15% by mass or less, more preferably 13% by mass or less, and even more preferably 10% by mass. % or less, particularly preferably 5% by mass or less. When the moisture content is below the upper limit, good handling properties can be easily obtained. The lower limit of the water content is not particularly limited, and is 0% by mass or more. The water content can be calculated using the mass of the water absorbent resin and the mass after drying the water absorbent resin until it becomes dry.

In the aggregation step of the production method of the first embodiment, the water-absorbing resin present as primary particles is swollen with a swelling solvent (e.g., water, methanol, ethanol, propanol, dimethyl sulfoxide, N-methylpyrrolidone or acetic acid, preferably water). At this time, the water-absorbing resin can be uniformly swollen by spraying the swelling solvent in a mist state using a sprayer or the like. The amount of the swelling solvent to be sprayed is preferably 1% by mass to 500% by mass, more preferably 5% by mass, based on the mass of the water-absorbing resin, from the viewpoint of easily obtaining the preferred particle size and/or cohesive force of the water-retaining material. ~100% by weight, more preferably 10% by weight ~ 50% by weight.

After spraying, the resin is stirred to improve the uniformity of swelling, but stirring with high shear force, for example with a stirrer equipped with high-speed rotating stirring blades, can lead to the pulverization of once aggregated particles or the collapse of the aggregates. There is a risk. For this reason, stirring with a low shear force, for example, stirring by shaking the mixture in a bag, stirring by a conical mixer, etc. is preferable. The shearing force is preferably 10000 MPa or less, more preferably 100 MPa or less, still more preferably 1 MPa or less. The contact time is preferably 0.001 seconds or more, more preferably 0.1 seconds or more, and still more preferably 1 second or more.

Any additive may be added before or simultaneously with the aggregation step, and by adding a binder, the strength of the aggregation of the water-absorbing resin can be adjusted. Examples of binders include polymer components other than water-absorbing resins, such as polyvinyl alcohol, ethylene-vinyl alcohol copolymer, polyvinyl butyral, starch, cellulose, polyethylene oxide, polyethylene glycol, carboxymethyl cellulose, polyacrylic acid and its salts. , polyacrylamide, a copolymer of acrylic acid and its salt with acrylamide, and styrene-butadiene rubber. One type of binder may be used alone, or two or more types may be used in combination. The larger the amount of binder added, the stronger the aggregation becomes. The weaker the aggregation, the more likely the water retaining material will collapse upon water absorption, and the more likely it will be able to exhibit a high water absorption rate. The amount of the binder added is usually 10% by mass or less, preferably 5% by mass or less, more preferably It is 1% by mass or less, more preferably 0.5% by mass or less, and may be 0% by mass.

Primary particles are particles that are not aggregated, and include, for example, particles obtained by melting or dissolving a water-absorbing resin in a solvent, drying and pulverizing, or particles obtained by melting or dissolving a monomer in a solvent. It refers to particles obtained by polymerizing, drying, and pulverizing, or commercially available water-absorbing resin (not aggregates), or particles obtained by pulverizing it. The primary particles may be not only spherical particles but also irregularly shaped particles such as those obtained by pulverization. The particle diameter of the primary particles is as described in the explanation of "agricultural water retaining material" above.

<Drying process>
In the drying step, the aggregated water absorbent resin is dried using a dryer under a pressure of 0.2 MPa or less, preferably 0.15 MPa or less, more preferably 0.1 MPa or less, for example under vacuum. Common dryers can be used, examples of which include conical dryers, hot air dryers, and the like. The drying temperature depends on the water absorbent resin used, but is usually 20 to 150°C, preferably 40 to 100°C. The drying time may be appropriately selected so that the obtained water retaining material is in a dry state, and is usually 1 to 1440 minutes, preferably 5 to 720 minutes. Further, if necessary, any of the above-mentioned additives may be mixed with the aggregated water-absorbing resin before the drying process, simultaneously with the drying process, or after the drying process.

In a dry state, drying while stirring with high shear force, for example, in a dryer equipped with stirring blades rotating at high speed, may lead to pulverization of once aggregated particles or collapse of the aggregates. For this reason, drying with a low shear force, for example, drying in a stationary state, or drying with a conical dryer, etc. is preferable. In this step, the swelling solvent may be completely removed, but from the viewpoint of suppressing disintegration, a portion of the swelling solvent may remain. When a part of the swelling solvent remains, the amount of the swelling solvent left is preferably 50% by mass or less based on the mass of the water-absorbing resin.

<Crosslinking process>
The manufacturing method may further include a crosslinking step of crosslinking the water absorbent resin before the aggregation step, simultaneously with the aggregation step, or after the aggregation step.
From the viewpoint of easily obtaining an excellent water absorption rate, it is preferable to include a crosslinking step of crosslinking the water absorbent resin before the aggregation step. In this case, the water absorbent resin after crosslinking is brought into contact with it in a swollen state to cause aggregation, but the cohesive force is not excessively strong. Therefore, expansion and collapse of the water retaining material proceed in a well-balanced manner during water absorption, and a more preferable water absorption rate can be obtained.
In addition, from the viewpoint of easily increasing the proportion of the crosslinked structure on the surface of the water-absorbing resin, and as a result, obtaining a low elution rate of the water-absorbing resin from the water-retaining material and/or from the viewpoint of obtaining uniform crosslinking, crosslinking It is preferable that the particle size of the water-absorbing resin is below a specific value. The particle size of the water absorbent resin to be crosslinked can be adjusted to a desired value by sieving. When carrying out the crosslinking step before the aggregation step, the particle size of the water absorbent resin to be crosslinked is preferably 2000 μm or less, more preferably 1000 μm or less, still more preferably 700 μm or less, particularly preferably 150 μm or less. When the crosslinking step is carried out simultaneously with the aggregation step or after the aggregation step, the particle size of the water absorbent resin to be crosslinked is preferably 5000 μm or less, more preferably 3000 μm or less, particularly preferably 2000 μm or less. The particle diameter in the case where the crosslinking step is carried out simultaneously with the aggregation step refers to the particle diameter at the time when the crosslinking step and the aggregation step are completed.

The water retaining material of the present invention can also be produced, for example, by a production method that includes an aggregation step in which water-absorbing resins present as particles are brought into contact with each other in a swollen state to agglomerate them. This manufacturing method may hereinafter be referred to as the manufacturing method of the second embodiment of the present invention.

<Agglomeration process>
In the aggregation step, the water-absorbing resin present as particles is brought into contact with the particles in a swollen state to agglomerate them. As the water-absorbing resin, a water-absorbing resin whose average particle size has been adjusted by crushing and/or sieving as necessary may be used. Further, if necessary, any of the above-mentioned additives may be mixed with the water-absorbing resin before the aggregation step, at the same time as the aggregation step, or after the aggregation step.
In one embodiment of the present invention, before the water-absorbing resin is swollen, the water content of the water-absorbing resin present as particles is preferably 15% by mass or less, more preferably 13% by mass or less, particularly preferably 10% by mass or less. It is. When the moisture content is less than or equal to the upper limit, good handling properties can be easily obtained. The lower limit of the moisture content is not particularly limited, and is 0% by mass or more. The water content can be calculated using the mass of the water absorbent resin and the mass after drying the water absorbent resin until it becomes dry.

The swelling solvent used in the aggregation step of the production method of the second embodiment, the swelling method, the amount of swelling solvent with respect to the mass of the water-absorbing resin, the stirring method, the stirrer, and the shear force during stirring are the production method of the first embodiment. A similar agglomeration process can be adopted.

Any additive may be added before or simultaneously with the aggregation step, and by adding a binder, the strength of the aggregation of the water-absorbing resin can be adjusted. The type and amount of binder that can be used can be the same as in the manufacturing method of the first embodiment.

Particles include, for example, particles obtained by polymerization in a suspended or emulsified state, particles obtained by spray drying a resin solution, commercially available particulate water absorbent resin, or particles obtained by crushing them. , or primary particles as mentioned above. The particles may be not only spherical particles, such as those obtained by polymerization in suspension or emulsion, but also irregularly shaped particles, such as those obtained by grinding. The particle diameter of the particles is preferably 1 μm or more, more preferably 10 μm or more, even more preferably 30 μm or more, particularly preferably 50 μm or more, and preferably 3000 μm or less, more preferably 1000 μm or less, still more preferably 600 μm or less, and particularly preferably is 300 μm or less. When the average particle diameter of the particles is not less than the lower limit value and not more than the upper limit value, it is easy to obtain a more preferable water absorption rate. Moreover, when the average particle diameter of the particles is equal to or larger than the lower limit value, dust generation can be easily suppressed during production of the water retaining material. The average particle diameter of the particles can be measured using, for example, an electron microscope or a sieve with a specific opening.

<Crosslinking process>
The manufacturing method may further include a crosslinking step of crosslinking the water absorbent resin before the aggregation step, simultaneously with the aggregation step, or after the aggregation step.
From the viewpoint of easily obtaining an excellent water absorption rate, it is preferable to include a crosslinking step of crosslinking the water absorbent resin before the aggregation step. In this case, the water absorbent resin after crosslinking is brought into contact with it in a swollen state to cause aggregation, but the cohesive force is not excessively strong. Therefore, expansion and collapse of the water retaining material proceed in a well-balanced manner during water absorption, and a more preferable water absorption rate can be obtained.
In addition, from the viewpoint of easily increasing the proportion of the crosslinked structure on the surface of the water-absorbing resin, and as a result, obtaining a low elution rate of the water-absorbing resin from the water-retaining material and/or from the viewpoint of obtaining uniform crosslinking, crosslinking It is preferable that the particle size of the water-absorbing resin is below a specific value. The particle size of the water absorbent resin to be crosslinked can be adjusted to a desired value by sieving. When carrying out the crosslinking step before the aggregation step, the particle size of the water absorbent resin to be crosslinked is preferably 3000 μm or less, more preferably 1000 μm or less, still more preferably 600 μm or less, particularly preferably 300 μm or less. When the crosslinking step is carried out simultaneously with the aggregation step or after the aggregation step, the particle size of the water absorbent resin to be crosslinked is preferably 5000 μm or less, more preferably 3000 μm or less, particularly preferably 2000 μm or less. The particle diameter in the case where the crosslinking step is carried out simultaneously with the aggregation step refers to the particle diameter at the time when the crosslinking step and the aggregation step are completed.

The agricultural water retaining material of the present invention can be used in a culture medium in combination with any component (Z) if necessary, and among these, it can be used, for example, in a culture medium for raising seedlings. Therefore, in one embodiment, the water retaining material of the present invention is for raising seedlings. Since the water retaining material of the present invention can exhibit an excellent water absorption rate, when used in a culture medium, it suppresses a decrease in water use efficiency caused by water flowing out before the water retaining material absorbs water during irrigation. be able to. In addition, in the production of paddy rice seedling boxes, which are often manufactured in assembly lines using belt conveyors, etc., when a water-retaining material is used as a component of the culture medium for sowing seed rice, the culture medium may not exhibit sufficient water absorption speed. can.

[Arbitrary component (Z)]
Examples of such an arbitrary component (Z) include resins other than the water-absorbing resin contained in the water retaining material, soil, other arbitrary components described below, and combinations thereof.

<Resin other than water absorbent resin>
Examples of resins other than water-absorbing resins include polyethylene, polypropylene, alkyd resins, phenolic resins, polyethylene glycols, and polyurethanes. These resins can be used alone or in combination of two or more. When the medium contains the resin, the total content thereof is preferably 20% by mass or less, more preferably 10% by mass or less, particularly preferably 5% by mass or less, based on the total mass of the medium.

<Cultivating soil>
When the culture medium contains culture soil, roots grow in the gaps in the culture soil, making it easier for the roots to intertwine with each other, and also making it easier to obtain excellent drainage and aeration properties of the culture medium. The culturing soil is not particularly limited, and for example, one type of commercially available cultivating soil may be used alone or two or more types may be used in combination. Further, other optional components described below can be attached to the soil using a conventional method (for example, a method in which a solution or dispersion of the other optional components is sprayed onto the soil and then dried).

From the viewpoint of easily obtaining better drainage and air permeability, the soil is preferably granular. The particle size of the granular soil is preferably 0.2 to 20 mm, more preferably 0.5 to 10 mm, particularly preferably 1 to 5 mm. In order to adjust the particle size of the granular soil within the above range, commercially available granular soil may be sieved and used. For producing granular soil, granulation methods such as compression granulation, extrusion granulation, rolling granulation, and fluidized bed granulation can be used. The particle size of granular soil can be measured by the following method. Thirty particles are randomly selected from the granular soil, the diameter of each particle is measured using a caliper, and the average value is taken as the particle size of the granular soil. If the particle is not spherical, the diameter of the particle is the average value of the longest side and the shortest side.

When the culture medium contains soil, the content of the soil is preferably 20 to 99.9999% by mass, more preferably 70 to 99.95% by mass, particularly preferably 80 to 99.9% by mass, based on the total mass of the medium. % by weight, most preferably 90-99.8% by weight.

<Other optional ingredients>
Examples of other optional ingredients include peat, grass charcoal, peat, peat moss, coco peat, rice husks, humic ferment materials, charcoal, calcined diatomaceous earth particles, shell fossil powder, shell powder, crab shells, VA mycorrhizal fungi, microbial materials, etc. plant and animal materials; minerals such as vermiculite, perlite, bentonite, natural zeolite, synthetic zeolite, gypsum, fly ash, rock wool, kaolinite, smectite, montmorillonite, sericite, chlorite, gluconite and talc; fertilizers and A combination of these can be mentioned. These may be used after being disinfected or sterilized as necessary, and may be used together with a pH adjuster or an agricultural chemical. If the medium contains other optional components, the total content may be within a range that does not impair the effects of the present invention, and is usually 50% by mass or less, preferably 30% by mass or less based on the total mass of the medium. be.

Examples of fertilizers include the three major fertilizers: nitrogen, phosphorus, and potassium; essential plant elements such as calcium, magnesium, sulfur, iron, copper, manganese, zinc, boron, molybdenum, chlorine, and nickel. Fertilizers containing: Bark compost, cow manure, pig manure, chicken manure, composts such as food waste and pruning waste, etc. can be mentioned. Examples of nitrogen-based fertilizers include ammonium sulfate, ammonium chloride, ammonium nitrate, sodium nitrate, lime nitrate, ammonium humic acid fertilizer, urea, lime nitrogen, ammonium nitrate lime, sodium ammonium nitrate, and magnesium nitrate fertilizer; Examples of fertilizers include superphosphate lime, heavy superphosphate lime, meltable phosphorus fertilizer, humic acid phosphate fertilizer, burnt phosphorus, heavy burnt phosphorus, linster, magnesia superphosphate, mixed phosphate fertilizer, Examples of potassium-based fertilizers include potassium sulfate, potassium chloride, potassium sulfate, potassium carbonate, potassium bicarbonate, and potassium silicate. Can be done. These fertilizers may be used in the form of solid, paste, liquid or solution, or may be used as coated fertilizers.
Examples of agricultural chemicals include insecticides, fungicides, insecticides, herbicides, rodenticides, preservatives, and plant growth regulators.

When producing a culture medium by combining the water retaining material of the present invention with fertilizer, in one preferred embodiment, the fertilizer is used as a coated fertilizer. Coated fertilizer is fertilizer coated with resin. This resin may be, for example, a polyolefin. When using coated fertilizer, fertilizer can be supplied to the culture medium over time as the resin decomposes. Furthermore, when mat seedlings are produced using a granular coated fertilizer, the strength of the resulting mat seedlings tends to be high. The particle size of the coated fertilizer is preferably 1 mm to 10 mm, more preferably 3 mm to 6 mm. When using a coated fertilizer, the content of the coated fertilizer in the medium is preferably 10 to 99.99% by mass, more preferably 15 to 90% by mass, particularly preferably 20 to 80% by mass, and most preferably 30 to 60% by mass. %.

When the water-retaining material of the present invention is used in combination with any component (Z), the water-retaining material and component (Z) are used in combination, or the water-retaining material contains component (Z) but does not contain the water-retaining material of the present invention. After sprouting in a medium, the water retaining material of the present invention can be sprinkled on the medium for use. When the water retaining material and component (Z) are used in combination, the mixing method is not particularly limited. The water retaining material and component (Z) may be mixed using a general method. When using the water retaining material of the present invention by spreading it after germination, the time of spreading is not particularly limited, but it is preferably during the seedling-raising period (approximately one month after sowing) when plants are sensitive to dryness.

When the water-retaining material of the present invention is used in combination with component (Z), the amount of water-retaining material varies depending on the type of component (Z) to be combined, but is usually 0.0001% by mass to 0.0001% by mass based on the total mass of the entire medium. 20% by weight, preferably 0.05% to 15% by weight, more preferably 0.1% to 10% by weight.

Seed rice is often sown in paddy rice seedling boxes into which a paddy rice seedling growing medium has been introduced. Usually, the amount of seed rice is 100 to 500 g per paddy rice seedling box (28 cm long x 58 cm wide).
When the water-retaining material of the present invention is used for raising paddy rice seedlings, the water-retaining material of the present invention can be optionally mixed with component (Z) and used in a medium for sowing rice seeds. This medium may be used for either bed soil (soil that is introduced into the paddy rice nursery box before sowing seed rice) or covering soil (soil that is covered from above after sowing seed rice), or it can be used for both. Good too. When used for both, the composition of the culture medium may be the same or different for the bedding soil and the covering soil. Furthermore, the water retaining material of the present invention or a mixture of the water retaining material of the present invention and component (Z) may be sprinkled on the medium in which the seed rice has sprouted.

In the present invention, the water absorption time (T1) of the water retaining material is preferably 200 seconds or less, more preferably 30 seconds or less, even more preferably 20 seconds or less. The water absorption time (T1) is the water absorption time measured by the measuring method described in Examples below.

In the present invention, the water absorption amount of the water retaining material in the presence of a calcium salt is preferably 4 g/g or more, more preferably 6 g/g or more, and still more preferably 8 g/g or more. The amount of water absorbed by the water retaining material in the presence of a calcium salt can be measured by a liquid absorption test using an aqueous calcium chloride solution, as described in Examples below.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples in any way.

なお、上述した以外の構成単位を含む場合、その構成単位も全構成単位のモル数に含める。 [Evaluation items and evaluation method]
<Content of ionic groups in water absorbent resin>
Solid ¹³ C-NMR measurements were performed on the resins used in Examples and Comparative Examples (manufactured by JEOL Ltd., model name: ECZ-500R, 500 MHz). In the obtained ¹³ C-NMR spectrum, there are a peak derived from the ionic group (usually observed at 160 to 180 ppm) and a peak derived from the methine carbon to which the hydroxyl group of the vinyl alcohol unit is bonded (usually observed at 60 to 80 ppm). ), a peak derived from the methyl carbon of the vinyl ester group of vinyl acetate units (usually observed at 10 to 30 ppm), a peak derived from the methylene carbon of ethylene units (usually observed at 30 to 50 ppm), acrylic acid ions contained in the resin from the peak derived from the carbonyl carbon of the methacrylic acid unit (usually observed at 170 to 180 ppm) and the peak derived from the carbonyl carbon of the acrylamide unit (usually observed at 160 to 180 ppm). Determine the number of moles of functional groups, the number of moles of vinyl alcohol units, the number of moles of vinyl acetate units, the number of moles of ethylene units, the number of moles of acrylic acid and methacrylic acid units, and the number of moles of acrylamide units, and calculate the ionicity according to the following formula. The content of groups was calculated.

In addition, when a structural unit other than those mentioned above is included, that structural unit is also included in the number of moles of all structural units.

上述したいずれの測定も１つの試料について２回ずつ行い、得られた測定値およびＤ_１０変化率それぞれの平均値を、その試料についての測定値およびＤ_１０変化率として採用した。 <Water retention material swelling test>
Disperse the water retention material sample in a dispersion medium, and measure the average particle diameter and volume-based 10% particle diameter _D10 (by ultrasonic wave) using a laser diffraction/scattering particle size distribution analyzer LA-950V2 (manufactured by Horiba, Ltd.). (corresponding to _D10 ) of the water retaining material before application was measured. Next, the water-retaining material is swollen by absorbing the dispersion medium into the water-retaining material by ultrasonic irradiation for ₅ minutes. _D10 ) and the maximum particle size on the smallest particle size side were measured. The measurement conditions are shown below.
Measurement method: Wet type (circulation type)
Sample refractive index: 1.51
Dispersion medium: 20% by mass aqueous sodium chloride solution (refractive index: 1.368)
Circulation speed: 10
Stirring speed: 10
Ultrasonic strength: 7
Transmittance: 80-90%
Circulating fluid volume: 280mL
In addition, the said maximum particle diameter is 0.011 micrometer, 0.013 micrometer, 0.015 micrometer, 0.017 micrometer, 0.02 micrometer, 0.023 micrometer, 0.026 micrometer, 0.03 micrometer, 0.034 micrometer, 0.039 micrometer, 0.03 micrometer. 044 μm, 0.051 μm, 0.058 μm, 0.067 μm, 0.076 μm, 0.087 μm, 0.1 μm, 0.115 μm, 0.131 μm, 0.15 μm, 0.172 μm, 0.197 μm, 0.226 μm, 0.259 μm, 0.296 μm, 0.339 μm, 0.389 μm, 0.445 μm, 0.51 μm, 0.584 μm, 0.669 μm, 0.766 μm, 0.877 μm, 1.005 μm, 1.151 μm, 1. 318 μm, 1.51 μm, 1.729 μm, 1.981 μm, 2.269 μm, 2.599 μm, 2.976 μm, 3.409 μm, 3.905 μm, 4.472 μm, 5.122 μm, 5.867 μm, 6.72 μm, 7.697 μm, 8.816 μm, 10.097 μm, 11.565 μm, 13.246 μm, 15.172 μm, 17.377 μm, 19.904 μm, 22.797 μm, 26.111 μm, 29.907 μm, 34.255 μm, 39. 234 μm, 44.938 μm, 51.471 μm, 58.953 μm, 67.523 μm, 77.34 μm, 88.583 μm, 101.46 μm, 116.21 μm, 133.103 μm, 152.453 μm, 174.616 μm, 200 μm, 229. 075 μm, 262.376 μm, 300.518 μm, 344.206 μm, 394.244 μm, 451.556 μm, 517.2 μm, 592.387 μm, 678.504 μm, 777.141 μm, 890.116 μm, 1019.515 μm, 1167.725 μm, It was determined by the frequency at 1337.481 μm, 1531.914 μm, 1754.613 μm, 2009.687 μm, 2301.841 μm, 2636.467 μm, or 3000 μm.
Subsequently, the rate of change in the volume-based 10% particle diameter D ₁₀ of the water retaining material before and after applying ultrasound was calculated using the following formula.

All of the above-mentioned measurements were performed twice for each sample, and the average values of the obtained measured values and _D10 change rates were employed as the measured values and _D10 change rates for the sample.

<Water absorption time (T1), water absorption speed improvement>
0.12 g of water retaining material was placed in a petri dish with a diameter of 3.5 cm, and 3.0 g of pure water was poured into the petri dish over 0.5 seconds using a pipette. The time from when the water retaining material absorbed water until the water surface disappeared was measured. The shorter this time, the faster the water absorption rate of the water retaining material.
In addition, in order to evaluate the degree of improvement in water absorption speed due to the _D10 change rate of the water retaining material being 1 or less, as shown in the formula below, the _D10 change rate of the water retaining material is 1 or less as the water absorption rate improvement. The ratio of the water absorption time (T1) of water retaining materials that differ only in terms of whether or not the water absorbing material has the water retention material was determined. At that time, the water absorption time (T1) of a water retaining material that did not have a _D10 change rate of 1 or less was measured using a water retaining material prepared in a comparative example described below. Specifically, for example, the water retention materials of Examples 1 and 4 were compared with the water retention material of Comparative Example 1 to calculate the degree of improvement in water absorption rate.

<Amount of pure water absorbed per 1g of water retaining material>
The amount of pure water absorbed by the water retaining materials in Examples and Comparative Examples was measured according to JIS K 7223, and the amount of pure water absorbed per 1 g of the water retaining material [g/g] was calculated based on the following formula.

<Amount of calcium chloride aqueous solution absorbed per 1 g of water retaining material>
The water retaining materials in Examples and Comparative Examples were immersed in a 1.44 g/L calcium chloride aqueous solution for 6 hours according to the sample setting method of JIS K 7223. Using Tetron 280 mesh, filter the water retaining material that has absorbed the calcium chloride aqueous solution and the calcium chloride aqueous solution that has not been absorbed by the water retaining material, and calculate the amount of calcium chloride aqueous solution absorbed per 1 g of the water retaining material [g] using the following formula. /g] was calculated.

[Water absorbent resin]
As the water absorbent resin (A1), Aquasorb 3005KB manufactured by SNF Holding Company (acrylamide polymer, non-agglomerated particles, average particle size 644 μm, crosslinked structure) was used. The content of ionic groups based on all the constituent units of the water absorbent resin (A1) was 22 mol%.
An acrylic acid polymer (particulate) was used as the water absorbent resin (C1). The content of ionic groups based on all the constituent units of the water absorbent resin (C1) was 54 mol%.

A vinyl alcohol polymer [water absorbent resin (B1)] was synthesized by the following procedure.
9,030 g of vinyl acetate, 18.15 g of methyl acrylate, and 3,810 g of methanol were introduced into a reactor equipped with a stirrer, a reflux condenser, a nitrogen inlet tube, and an initiator addition port, and the mixture was reacted for 30 minutes while bubbling with nitrogen. The inside of the vessel was replaced with inert gas. The temperature of the reactor was started to rise using a water bath, and when the internal temperature of the reactor reached 60°C, 2.40 g of azobisisobutyronitrile (AIBN) was added as an initiator to initiate polymerization. . Sampling is carried out as appropriate, and the progress of polymerization is confirmed from the solid content concentration, and the consumption rate, which is the total mass of vinyl acetate and methyl acrylate consumed by polymerization relative to the total mass of introduced vinyl acetate and methyl acrylate, is calculated. I asked for it. When the consumption rate reached 4% by mass, the internal temperature of the reactor was cooled to 30°C to stop polymerization. It was connected to a vacuum line, and the remaining vinyl acetate was distilled off under reduced pressure at 30°C together with methanol. While visually checking the interior of the reactor, when the viscosity increased, distillation was continued while adding methanol as appropriate to obtain polyvinyl acetate containing 5.2 mol % of acrylic acid-derived structural units. The content of acrylic acid-derived structural units was measured using solid-state ¹³ C-NMR.
Next, 360 g of the obtained polyvinyl acetate containing structural units derived from acrylic acid and 6552 g of methanol were added to the same reactor as above, and the polyvinyl acetate containing structural units derived from acrylic acid was dissolved. The reactor was heated using a water bath and heated with stirring until the internal temperature of the reactor reached 70°C. 280.8 g of a methanol solution of sodium hydroxide (meth caustic, concentration 15% by mass) was added thereto, and saponification was performed at 70° C. for 2 hours.

The solution after saponification was filtered and vacuum dried at 40°C. The obtained polymer was dissolved in pure water so that the solid content concentration was 15% by mass, and then poured into a Teflon tray and dried with hot air at 90°C. The obtained polymer was pulverized to obtain 165 g of polyvinyl alcohol (hereinafter referred to as "polyvinyl alcohol (b1)") containing 5.2 mol% of acrylic acid-derived structural units.

Into a three-necked separable flask equipped with a reflux condenser and a stirring blade, 445.5 g of methanol, 47.4 g of pure water, 1.206 g of a 25% by mass glutaraldehyde aqueous solution, and 150 g of polyvinyl alcohol (b1) were introduced. The mixture was stirred at ℃ to disperse polyvinyl alcohol (b1). A mixed solution of 5.66 g of a 47% by mass sulfuric acid aqueous solution and 3 g of pure water was added dropwise over 10 minutes, and the temperature was raised to 65° C. to cause a crosslinking reaction for 6 hours. After the reaction, the polymer was taken out by filtration, and washed by dispersing the filtered polymer in 220 g of methanol, stirring for 30 minutes, and filtering. Washing was repeated twice.

The washed polymer was introduced into a three-neck separable flask equipped with a reflux condenser and a stirring blade, and 245 g of methanol, 40.8 g of pure water, and 24.35 g of 50% by mass potassium hydroxide were added, and the mixture was heated at 65°C. The reaction was allowed to proceed for 2 hours. After the reaction, the polymer was taken out by filtration, and then washed by dispersing the filtered polymer in 330 g of methanol, stirring for 30 minutes, and filtering. Washing was repeated twice. The washed polymer was vacuum dried at 40° C. for 12 hours to obtain the desired water absorbent resin (B1). The content of ionic groups based on all the constituent units of the water absorbent resin (B1) was 5 mol%.

Examples 1 to 3
The sieved particles were collected from the water absorbent resin (A1) using a sieve with a nominal opening of 53 μm. In addition, the particle diameter of the collected particles is described as a particle diameter of "<53" for convenience. In the following, the particle diameter of the sieved particles will be similarly described using the value of the nominal opening of the sieve.
28 g of the collected particles were placed in a bag measuring 28 cm long and 20 cm wide, and 9 g of pure water was sprayed little by little, and the bag was shaken and mixed each time to obtain an aggregate. At this time, instead of kneading the resins by putting pressure on them, they filled the bag with air and rotated it to mix it. All of the pure water in the bag was absorbed by the water absorbent resin (A1). After vacuum drying at 40°C for 12 hours, the particle size was determined as in Example 1 using a sieve with a nominal opening of 300 μm, a sieve with a nominal opening of 600 μm, a sieve with a nominal opening of 1000 μm, and a sieve with a nominal opening of 1400 μm. Particles with a particle size of 300 μm to 600 μm, particles with a particle size of 600 μm to 1000 μm as Example 2, and particles with a particle size of 1000 μm to 1400 μm as Example 3 were obtained, respectively.

Examples 4-6
Particles with a particle size of 53 μm to 106 μm were collected from the water absorbent resin (A1) using a sieve with a nominal opening of 53 μm and a sieve with a nominal opening of 106 μm. Aggregates were obtained in the same manner as in Examples 1 to 3, except that 28 g of the collected particles and 12 g of pure water were used. At this time, all the pure water in the bag was absorbed by the water absorbent resin (A1). After vacuum drying at 40°C for 12 hours, the particle size was determined as Example 4 using a sieve with a nominal opening of 300 μm, a sieve with a nominal opening of 600 μm, a sieve with a nominal opening of 1000 μm, and a sieve with a nominal opening of 1400 μm. Particles with a particle size of 300 μm to 600 μm, particles with a particle size of 600 μm to 1000 μm as Example 5, and particles with a particle size of 1000 μm to 1400 μm as Example 6 were obtained.

Examples 7-8
Particles having a particle size of 212 μm or more were collected from the water absorbent resin (A1) using a sieve with a nominal opening of 212 μm. Aggregates were obtained in the same manner as in Examples 1 to 3, except that 28 g of the collected particles and 12 g of pure water were used. At this time, all the pure water in the bag was absorbed by the water absorbent resin (A1). After vacuum drying at 40° C. for 12 hours, particles with a particle size of 600 μm to 1000 μm were prepared as Example 7 using a sieve with a nominal opening of 600 μm, a sieve with a nominal opening of 1000 μm, and a sieve with a nominal opening of 1400 μm. As Example 8, particles having particle diameters of 1000 μm to 1400 μm were obtained.

Examples 9-10
Particles with a particle size of 53 μm to 106 μm were collected from the water absorbent resin (B1) using a sieve with a nominal opening of 53 μm and a sieve with a nominal opening of 106 μm. Aggregates were obtained in the same manner as in Examples 1 to 3, except that 28 g of the collected particles and 12 g of pure water were used. At this time, all the pure water in the bag was absorbed by the water absorbent resin (A1). After vacuum drying at 40° C. for 12 hours, particles with a particle size of 300 μm to 600 μm were prepared as Example 9 using a sieve with a nominal opening of 300 μm, a sieve with a nominal opening of 600 μm, and a sieve with a nominal opening of 1000 μm. As Example 10, particles having particle diameters of 600 μm to 1000 μm were obtained.

Example 11
Particles with a particle size of 300 μm to 600 μm were collected from the water absorbent resin (B1) using a sieve with a nominal opening of 300 μm and a sieve with a nominal opening of 600 μm. Aggregates were obtained in the same manner as in Examples 1 to 3, except that 28 g of the collected particles and 12 g of pure water were used. At this time, all the pure water in the bag was absorbed by the water absorbent resin (A1). After vacuum drying at 40° C. for 12 hours, particles with a particle size of 600 μm to 1000 μm were obtained using a sieve with a nominal opening of 600 μm and a sieve with a nominal opening of 1000 μm.

Comparative example 1
Particles with a particle diameter of 300 μm to 600 μm were obtained from the water absorbent resin (A1) using a sieve with a nominal opening of 300 μm and a sieve with a nominal opening of 600 μm.

Comparative example 2
Particles with a particle size of 600 μm to 1000 μm were obtained from the water absorbent resin (A1) using a sieve with a nominal opening of 600 μm and a sieve with a nominal opening of 1000 μm.

Comparative example 3
Particles with a particle size of 300 μm to 600 μm were obtained from the water absorbent resin (B1) using a sieve with a nominal opening of 300 μm and a sieve with a nominal opening of 600 μm.

Comparative example 4
Particles with a particle diameter of 600 μm to 1000 μm were obtained from the water absorbent resin (B1) using a sieve with a nominal opening of 600 μm and a sieve with a nominal opening of 1000 μm.

Comparative example 5
Particles with a particle size of 106 μm to 212 μm were collected from the water absorbent resin (C1) using a sieve with a nominal opening of 106 μm and a sieve with a nominal opening of 212 μm. Aggregates were obtained in the same manner as in Examples 1 to 3, except that 28 g of the collected particles and 12 g of pure water were used. At this time, all the pure water in the bag was absorbed by the water absorbent resin (A1). After vacuum drying at 40° C. for 12 hours, particles with a particle size of 300 μm to 600 μm were obtained using a sieve with a nominal opening of 300 μm and a sieve with a nominal opening of 600 μm.

Comparative example 6
Particles with a particle diameter of 300 μm to 600 μm were obtained from the water absorbent resin (C1) using a sieve with a nominal opening of 300 μm and a sieve with a nominal opening of 600 μm.

The particles obtained in Examples and Comparative Examples were evaluated as water retaining materials. The results are shown in Table 1.

As shown in Table 1, all of the water-retaining materials of the examples exhibited a water absorption rate superior to that of the water-retaining materials of the comparative examples having similar particle sizes. Achieving such an excellent water absorption rate is of great significance because the particle size of the water-retaining material is sometimes limited by the desire to prevent dust in the water-retaining material or the structure of the hopper for feeding the water-retaining material. .
In addition, in the water-retaining material, the water-absorbing resin having an ionic group of 0.1 mol% or more and 50 mol% or less with respect to the total constituent units of the water-absorbing resin has a _D10 change rate of 1 or less. The water absorption rate was significantly improved by the water absorption rate [specifically, the ratio of the water absorption time (T1) of Example 1 or 4 to the water absorption time (T1) of Comparative Example 1 (Example Water absorption speed improvement rate described in row 1 or 4), ratio of water absorption time (T1) of Example 2 or 5 or 7 to water absorption time (T1) of Comparative example 2 (row of Example 2 or 5 or 7) water absorption speed improvement degree described in the row of Example 9), ratio of water absorption time (T1) of Example 9 to water absorption time (T1) of Comparative Example 3 (water absorption speed improvement degree described in the row of Example 9), Example 10 or This can be seen from the ratio of the water absorption time (T1) of No. 11 to the water absorption time (T1) of Comparative Example 4 (degree of improvement in water absorption speed described in the row of Example 10 or 11). On the other hand, even if the _D10 change rate of the water-retaining material is 1 or less, the unexpected result is that the water absorption rate does not improve much unless the water-absorbing resin has the above-mentioned specific ionic group content. This can be seen from the ratio between the water absorption time (T1) of Comparative Example 5 and the water absorption time (T1) of Comparative Example 6 (the degree of improvement in water absorption speed described in the row of Comparative Example 5).
Furthermore, as shown in Table 1, the water retaining material of the example also showed a higher absorption amount of calcium chloride aqueous solution than the water retaining material of the comparative example. This indicates that the water retaining material of the present invention has a sufficiently high water absorption amount even in the presence of calcium salts. Further, the water absorption amount of the water retaining material of the present invention in the presence of calcium salt was difficult to decrease over a long period of time.

The water retaining material of the present invention has an excellent water absorption rate, exhibits a sufficiently high water absorption amount even in the presence of calcium salts, and this water absorption amount does not easily decrease over a long period of time, so it can be suitably used as an agricultural water retaining material.

Claims (10)

An agricultural water retaining material comprising a water-absorbing resin,
The water-absorbing resin has an ionic group of 0.1 mol% or more and 50 mol% or less based on the total constituent units of the water-absorbing resin,
In a water-retaining material swelling test in which a 20% by mass aqueous sodium chloride solution is absorbed into a water-retaining material by applying ultrasound, the rate of change in the volume-based 10% particle diameter _D10 of the water-retaining material before and after applying ultrasound:
Rate of change in _D10 = ( _D10 of water retaining material after applying ultrasound)/( _D10 of water retaining material before applying ultrasound)
is 1 or less, a water retaining material. The water-retaining material according to claim 1, wherein the water-retaining material has a particle size that passes through a sieve with a nominal opening of 3000 μm but does not pass through a sieve with a nominal opening of 10 μm. The water retaining material according to claim 1 or 2, wherein the maximum particle diameter on the smallest particle diameter side in the state after applying ultrasonic waves in the water retaining material swelling test is 1 μm or more and 1000 μm or less. The water-retaining material according to any one of claims 1 to 3, wherein the water-absorbing resin has one or more selected from the group consisting of a carboxyl group, a sulfonic acid group, and an ammonium group as the ionic group. According to any one of claims 1 to 4, the water-absorbing resin contains one or more selected from the group consisting of a vinyl alcohol polymer, an acrylic acid polymer, an acrylamide polymer, and a methacrylic acid polymer. water retention material. The water retaining material according to claim 5, wherein the vinyl alcohol polymer contains one or more monomer constituent units selected from the group consisting of acrylic acid, methacrylic acid, maleic acid, itaconic acid, and derivatives thereof. The water retaining material according to any one of claims 1 to 6, wherein the water absorbent resin has a crosslinked structure. The water retaining material according to any one of claims 1 to 7, which is used for raising seedlings. Claim 1, comprising: an aggregation step of aggregating the water-absorbing resin present as primary particles by contacting them in a swollen state; and a drying step of drying the agglomerated water-absorbent resin under a pressure of 0.2 MPa or less. A method for producing a water retaining material according to any one of items 8 to 8. The method according to claim 9, further comprising a crosslinking step of crosslinking the water absorbent resin before the aggregation step, at the same time as the aggregation step, or after the aggregation step.