POLYVINYL ALCOHOL-BASED CROSSLINKED COPOLYMER

Abstract

A polyvinyl alcohol-based crosslinked copolymer may include an unsaturated monocarboxylic acid-based structural unit, wherein an amount of carboxylate-forming structural units in the crosslinked copolymer is 1% by mole or more and 35% by mole or less with respect to all structural units constituting the crosslinked copolymer, and a solubility of the crosslinked copolymer in water is 90% or less.

Claims

1. A polyvinyl alcohol-based crosslinked copolymer, comprising: an unsaturated monocarboxylic acid-based structural unit, wherein an amount of carboxylate-forming structural units in the crosslinked copolymer is in a range of from 1 to 35 mol. %, based on total structural units in the crosslinked copolymer, and wherein a solubility of the crosslinked copolymer in water is 90% or less.

2. The copolymer of claim 1, wherein an amount of vinyl alcohol units in the polyvinyl alcohol-based crosslinked copolymer is in a range of from 20 to 99 mol. %, based on the total structural units in the crosslinked copolymer.

3. The copolymer of claim 1, comprising a potassium ion as a counter cation of the carboxylate.

4. The copolymer of claim 1, wherein the amount of the carboxylate-forming structural units in the polyvinyl alcohol-based crosslinked copolymer is in a range of from 1.5 to 15 mol. %, based on the total structural units in the crosslinked copolymer.

5. The copolymer of claim 1, wherein an amount of water that can be absorbed by a plant per 1 g of the polyvinyl alcohol-based crosslinked copolymer is in a range of from 10 to 100 g.

6. The copolymer of claim 1, comprising an acetal structure as a crosslink structure.

7. The copolymer of claim 6, wherein the acetal structure is derived from at least a polyfunctional aldehyde comprising 2 to 20 carbon atoms.

8. The copolymer of claim 7, wherein the polyfunctional aldehyde is at least one selected from the group consisting of glyoxal, malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde, malealdehyde, fumaraldehyde, tartaraldehyde, citraldehyde, terephthalaldehyde, isophthalaldehyde, phthalaldehyde, 1,9-nonanedial, and ethylenediamine tetraacetaldehyde.

9. The copolymer of claim 1, wherein the unsaturated monocarboxylic acid-based structural unit is derived from acrylic acid or a derivative thereof, or methacrylic acid or a derivative thereof.

10. A method of producing a polyvinyl alcohol-based crosslinked copolymer, the method comprising: reacting a polyvinyl alcohol-based copolymer containing an unsaturated monocarboxylic acid-based structural unit with a crosslinking agent, wherein an amount of carboxylate-forming structural units in the crosslinked copolymer is in a range of from 1 to 35 mol. %, based on total structural units in the crosslinked copolymer.

11. The method of claim 10, comprising, in the presence of a solvent capable of swelling the polyvinyl alcohol-based copolymer: reacting particles of the polyvinyl alcohol-based copolymer swollen with the solvent with the crosslinking agent.

12. A water-retaining material, comprising: the copolymer of claim 1.

13. The material of claim 12, which is suitable for agriculture.

14. The copolymer of claim 7, wherein the polyfunctional aldehyde comprises glyoxal, malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde, malealdehyde, fumaraldehyde, tartaraldehyde, citraldehyde, terephthalaldehyde, isophthalaldehyde, phthalaldehyde, 1,9-nonanedial, and/or ethylenediamine tetraacetaldehyde.

Description

EXAMPLES

[0067] The present invention will now be described in more detail by way of Examples thereof; however, the present invention is not limited by the following Examples at any rate.

[Measured Items and Measurement Methods]

(1) Amount of Carboxylate-Forming Structural Units and Amount of Vinyl Alcohol Units

[0068] For each water absorbent resin (polyvinyl alcohol-based crosslinked copolymer) produced in Examples and Comparative Examples, IR measurement was performed using an infrared spectrometer “NICOLET iS10” manufactured by Thermo Fisher Scientific K.K. In the thus obtained IR spectrum, the area of peaks (2,990 to 2,560 cm.sup.−1) based on methylene groups and methine groups of vinyl alcohol units and carboxylic acid-based structural units, and the area of peaks (1,625 to 1,510 cm.sup.−1) based on carboxylate groups of carboxylate-forming structural units were determined.

[0069] Subsequently, for each water absorbent resin produced in Examples and Comparative Examples, the amount (unit: % by mole) of carboxylate-forming structural units and the amount (unit: % by mole) of vinyl alcohol units in the crosslinked copolymer were calculated based on the following equations.

Amount of carboxylate(acrylate)-forming structural units=13.07×(Area of peaks based on carboxylate groups)/(Area of peaks based on methylene groups and methine groups)

Amount of carboxylate(maleate)-forming structural units=13.07×{(Area of peaks based on carboxylate groups)/(Area of peaks based on methylene groups and methine groups)}/2

Amount of vinyl alcohol units=100−(Amount of carboxylate-forming structural units+Amount of crosslinked structures)

[0070] In the above equations, “13.07” is a coefficient for calculating the amount of carboxylate-forming structural units using the respective equations with regard to each water absorbent resin produced in Examples and Comparative Examples. This coefficient was determined from a calibration curve prepared in advance.

(2) Solubility in Water

[0071] To 100 mL of 25° C. pure water, 0.1 g of each crosslinked copolymer produced in Examples and Comparative Examples was added, and the resulting mixture was left to stand for 6 hours. Thereafter, this mixture was naturally filtered through a TETORON mesh (280-mesh), the thus recovered gel was vacuum-dried at 40° C. for 12 hours, and the solubility S in water (unit: %) of the crosslinked copolymer was calculated based on the following equation:

S=(Sample mass after drying/Sample mass before absorption of pure water)×100

(3) Amount of Water that can be Absorbed by Plant

[0072] First, in accordance with JIS K7223, the pure water absorption amount of each crosslinked copolymer produced in Examples and Comparative Examples was measured (number of samples: n=3), and the average value and the standard deviation of the pure water absorption amount W.sub.1 (unit: g/g) per 1 g of each crosslinked copolymer were calculated based on the following equation:

W.sub.1=[(Sample mass after absorption of pure water)−(Sample mass before absorption of pure water)]/(Sample mass before absorption of pure water)

[0073] In the present invention, the “ratio of water that can be absorbed by a plant” [W.sub.2 (unit: %)] means a ratio of water that can be absorbed by a plant with respect to the saturated water absorption amount of the crosslinked copolymer. This ratio W.sub.2 can be simply determined by a centrifugation method. In the present invention, the ratio W.sub.2 was determined by the following method.

[0074] A crosslinked copolymer was allowed to absorb 50 times by mass of water with respect to the mass of the crosslinked copolymer, and therefrom 2.4 g of the crosslinked copolymer was taken out and introduced to a syringe as a sample. This syringe was immobilized in a centrifuge tube at a position of 10.2 cm from the center of a small-sized centrifuge “H-36” manufactured by KOKUSAN Co., Ltd. The centrifuge was operated at 2,200 rpm for 60 minutes, and the ratio W.sub.2 of water that can be absorbed by a plant and the ratio W.sub.3 (unit: g/g) of water that can be absorbed by a plant per 1 g of the crosslinked copolymer were calculated based on the following equations. It is noted here that, when the crosslinked copolymer was not able to absorb 50 times by mass of water, the resin was allowed to absorb water to saturation, and therefrom 2.4 g of the resin was taken out and introduced to the syringe.

W.sub.2=[{(Sample mass before centrifugation)−(Sample mass after centrifugation)}/Sample mass before centrifugation]×100

W.sub.3=W.sub.1×W.sub.2/100

(4) CaCl.sub.2 Solution Absorption Amount W.sub.4 Per 1 g of Crosslinked Copolymer

[0075] In accordance with the sample setting method of JIS K7223, each crosslinked copolymer produced in Examples and Comparative Examples was immersed in a 1.44 g/L calcium chloride solution for 6 hours. The crosslinked copolymer thus allowed to absorb the calcium chloride solution and the calcium chloride solution that was not absorbed by the crosslinked copolymer were separated using a TETRON 280-mesh, and the CaCl.sub.2 solution absorption amount W.sub.4 (unit: g/g) per 1 g of the crosslinked copolymer was calculated using the following equation:

W.sub.4=[(Sample mass after absorption of calcium chloride solution)−(Sample mass before absorption of calcium chloride solution)]/(Sample mass before absorption of calcium chloride solution)

(4) Average Particle Diameter

[0076] Using a laser diffraction/scattering-type particle diameter distribution analyzer (LA-950V2, manufactured by HORIBA, Ltd.), the average particle diameter was measured for each of the polyvinyl alcohol-based copolymers produced in Preparation Examples and the crosslinked copolymers produced in Examples and Comparative Examples.

Preparation Example 1: Preparation of Polyvinyl Alcohol-Based Copolymer Containing Unsaturated Monocarboxylic Acid-Based Structural Unit (Methyl Acrylate-Derived Structural Unit

[0077] To a reactor equipped with a stirrer, a reflux condenser, a nitrogen introduction tube and an initiator addition port, 602 g of vinyl acetate (VAc), 1.21 g of methyl acrylate (MA), and 255 g of methanol were introduced, and the reactor was purged with an inert gas for 30 minutes under nitrogen bubbling. The temperature inside the reactor was increased by heating in a water bath and, once the temperature reached 60° C., 0.16 g of azobisisobutyronitrile (AIBN) was added as an initiator to start polymerization. Sampling was performed as appropriate to confirm the progress of the polymerization based on the solid concentration, and the consumption rate (Conv.), which is a ratio of a total mass of vinyl acetate and methyl acrylate that were consumed by the polymerization with respect to a total mass of vinyl acetate and methyl acrylate that were introduced, was determined. Once the consumption rate reached 4%, the polymerization was terminated by cooling the inside of the reactor to 30° C. The reactor was connected to a vacuum line, and the residual vinyl acetate was removed by vacuum distillation at 30° C. along with methanol. While visually checking the inside of the reactor, the distillation was continued with an addition of methanol as appropriate in response to an increase in the viscosity, whereby a polyvinyl acetate containing 5.2% by mole of a methyl acrylate-derived structural unit (PVAc-PMA) was obtained. It is noted here that the amount of the methyl acrylate-derived structural unit (MA modification amount) was determined by solid-state NMR.

[0078] Next, 1 g of the thus obtained PVAc-PMA and 18.2 g of methanol were introduced to the same reactor as described above, and the PVAc-PMA was dissolved in methanol. By heating in a water bath, the content in the reactor was heated with stirring until the temperature inside the reactor reached 70° C. Subsequently, 0.78 g of a methanol solution of sodium hydroxide (concentration: 15% by mass) was added to perform saponification at 70° C. for 2 hours. The resulting solution was filtered to obtain a polyvinyl alcohol-based copolymer containing 5.2% by mole of a methyl acrylate-derived structural unit. The thus obtained polyvinyl alcohol-based copolymer was in the form of particles.

Preparation Examples 2 to 7: Preparation of Polyvinyl Alcohol-Based Copolymers Containing Unsaturated Monocarboxylic Acid-Based Structural Unit

[0079] Polyvinyl alcohol-based copolymers containing a methyl acrylate-derived structural unit were obtained in the same manner as in Preparation Example 1, except the amount of each component, the consumption rate, and the MA modification amount were changed from those of Preparation Examples 1 as shown in Table 1. All of the thus obtained polyvinyl alcohol-based copolymers were in the form of particles.

Preparation Example 8: Preparation of Polyvinyl Alcohol-Based Copolymer Containing Unsaturated Dicarboxylic Acid-Based Structural Unit (Dimethyl Maleate-Derived Structural Unit

[0080] A polyvinyl acetate containing a dimethyl maleate-derived structural unit (PVAc-PMM) was obtained by performing polymerization in the same manner as in Preparation Example 1, except that dimethyl maleate (MM) was used in place of methyl acrylate (MA). Subsequently, a polyvinyl alcohol-based copolymer containing a dimethyl maleate-derived structural unit was obtained in the same manner as in Preparation Example 1, except that the PVAc-PMM was used in place of the PVAc-PMA. The amount of each component, the consumption rate, and the MM modification amount were as shown in Table 1. The thus obtained polyvinyl alcohol-based copolymer was in the form of particles.

TABLE-US-00001 TABLE 1 Polymerization step Saponification step Average particle MA or MM Methanol diameter of Consumption modification PVAc-PMA or solution of polyvinyl alcohol- Preparation VAc MA or MM methanol AIBN rate amount PVAc-PMM Methanol NaOH based copolymer Example [g] [g] [g] [g] Conv. [%] [% by mole] [g] [g] [g] [μm] 1 602 1.21 255 0.16 4 5.2 1 18.2 0.78 125 2 602 0.30 254 0.16 2 2.0 1 18.3 0.68 128 3 602 3.01 255 0.16 4 14.8 1 17.9 1.08 112 4 602 7.53 257 0.16 6 20.2 1 17.8 1.25 135 5 602 15.07 260 0.16 8 29.7 1 17.5 0.54 142 6 602 0.12 254 0.16 4 0.5 1 18.4 0.64 115 7 602 30.13 267 0.17 12 40.0 1 17.1 1.86 128 8 602 0.14 262 0.17 4 15.2 1 17.6 0.50 132

Example 1

[0081] To a three-necked separable flask equipped with a reflux condenser and a stirring blade, 58.9 g of acetonitrile, 6.28 g of ion-exchanged water, 0.171 g of a 25%-by-mass aqueous glutaraldehyde solution, and 20 g of the polyvinyl alcohol-based copolymer obtained in Preparation Example 1 were introduced, and the polyvinyl alcohol-based copolymer was dispersed by stirring at 23° C. Subsequently, 12.38 g of a 16.9%-by-mass aqueous sulfuric acid solution was added dropwise over a period of 15 minutes, and the resulting mixture was heated to 65° C. and allowed to react for 6 hours. The polyvinyl alcohol-based copolymer obtained after the reaction was recovered by filtration, and washed with 160 g of methanol six times. Thereafter, the washed copolymer was introduced to a three-necked separable flask equipped with a reflux condenser and a stirring blade, and 71 g of methanol, 13.3 g of ion-exchanged water and 5.7 g of potassium hydroxide were added and allowed to react at 65° C. for 2 hours. The copolymer obtained after the reaction was recovered by filtration, washed with 160 g of methanol six times, and then vacuum-dried at 40° C. for 12 hours, whereby a polyvinyl alcohol-based crosslinked copolymer containing a desired acrylic acid-based structural unit was obtained. The above-described measurements (1) to (3) were performed for the thus obtained crosslinked copolymer. The results thereof are shown in Table 2.

Example 2

[0082] A polyvinyl alcohol-based crosslinked copolymer containing a desired acrylic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the added amount of the 25%-by-mass aqueous glutaraldehyde solution was changed from 0.171 g to 0.341 g. The results thereof are shown in Table 2.

Example 3

[0083] A polyvinyl alcohol-based crosslinked copolymer containing a desired acrylic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the added amount of the 25%-by-mass aqueous glutaraldehyde solution was changed from 0.171 g to 0.512 g. The results thereof are shown in Table 2.

Example 4

[0084] A polyvinyl alcohol-based crosslinked copolymer containing a desired acrylic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the polyvinyl alcohol-based copolymer obtained in Preparation Example 2 was used in place of the polyvinyl alcohol-based copolymer obtained in Preparation Example 1, 12.42 g of a 17.2%-by-mass aqueous sulfuric acid solution was used in place of 12.38 g of the 16.9%-by-mass aqueous sulfuric acid solution, and the added amount of the 25%-by-mass aqueous glutaraldehyde solution and that of potassium hydroxide were changed from 0.171 g and 5.7 g to 0.174 g and 2.4 g, respectively. The results thereof are shown in Table 2.

Example 5

[0085] A polyvinyl alcohol-based crosslinked copolymer containing a desired acrylic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the polyvinyl alcohol-based copolymer obtained in Preparation Example 3 was used in place of the polyvinyl alcohol-based copolymer obtained in Preparation Example 1, 11.92 g of a 13.5%-by-mass aqueous sulfuric acid solution was used in place of 12.38 g of the 16.9%-by-mass aqueous sulfuric acid solution, and the added amount of the 25%-by-mass aqueous glutaraldehyde solution and that of potassium hydroxide were changed from 0.171 g and 5.7 g to 0.131 g and 15 g, respectively. The results thereof are shown in Table 2.

Example 6

[0086] A polyvinyl alcohol-based crosslinked copolymer containing a desired acrylic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the polyvinyl alcohol-based copolymer obtained in Preparation Example 4 was used in place of the polyvinyl alcohol-based copolymer obtained in Preparation Example 1, 11.67 g of a 12.1%-by-mass aqueous sulfuric acid solution was used in place of 12.38 g of the 16.9%-by-mass aqueous sulfuric acid solution, and the added amount of the 25%-by-mass aqueous glutaraldehyde solution and that of potassium hydroxide were changed from 0.171 g and 5.7 g to 0.31 g and 24 g, respectively. The results thereof are shown in Table 2.

Example 7

[0087] A polyvinyl alcohol-based crosslinked copolymer containing a desired acrylic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the polyvinyl alcohol-based copolymer obtained in Preparation Example 5 was used in place of the polyvinyl alcohol-based copolymer obtained in Preparation Example 1, 11.35 g of a 10.0%-by-mass aqueous sulfuric acid solution was used in place of 12.38 g of the 16.9%-by-mass aqueous sulfuric acid solution, and the added amount of the 25%-by-mass aqueous glutaraldehyde solution and that of potassium hydroxide were changed from 0.171 g and 5.7 g to 0.375 g and 26.2 g, respectively. The results thereof are shown in Table 2.

Comparative Example 1

[0088] A polyvinyl alcohol-based crosslinked copolymer containing an acrylic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the polyvinyl alcohol-based copolymer obtained in Preparation Example 6 was used in place of the polyvinyl alcohol-based copolymer obtained in Preparation Example 1, 12.48 g of a 17.7%-by-mass aqueous sulfuric acid solution was used in place of 12.38 g of the 16.9%-by-mass aqueous sulfuric acid solution, and the added amount of the 25%-by-mass aqueous glutaraldehyde solution and that of potassium hydroxide were changed from 0.171 g and 5.7 g to 0.18 g and 0.6 g, respectively. The results thereof are shown in Table 2.

Comparative Example 2

[0089] A polyvinyl alcohol-based crosslinked copolymer containing an acrylic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the polyvinyl alcohol-based copolymer obtained in Preparation Example 7 was used in place of the polyvinyl alcohol-based copolymer obtained in Preparation Example 1, 11.15 g of a 8.0%-by-mass aqueous sulfuric acid solution was used in place of 12.38 g of the 16.9%-by-mass aqueous sulfuric acid solution, and the added amount of the 25%-by-mass aqueous glutaraldehyde solution and that of potassium hydroxide were changed from 0.171 g and 5.7 g to 0.37 g and 32.2 g, respectively. The results thereof are shown in Table 2.

Comparative Example 3

[0090] A polyvinyl alcohol-based crosslinked copolymer containing a maleic acid-based structural unit was produced and measured in the same manner as in Example 1, except that the polyvinyl alcohol-based copolymer obtained in Preparation Example 8 was used in place of the polyvinyl alcohol-based copolymer obtained in Preparation Example 1, 10.96 g of a 5.3%-by-mass aqueous sulfuric acid solution was used in place of 12.38 g of the 16.9%-by-mass aqueous sulfuric acid solution, the amount of the 25%-by-mass aqueous glutaraldehyde solution was changed from 0.171 g to 0.343 g, and the amount of potassium hydroxide was changed from 5.7 g to 14.2 g. The results thereof are shown in Table 2.

TABLE-US-00002 TABLE 2 Pure water Amount of absorption Standard Amount of carboxylate- Amount amount W.sub.1 deviation of pure crosslinked forming of vinyl per 1 g of water absorption structures structural alcohol Solubility crosslinked amount W.sub.1 per [% by units units [% in water copolymer 1 g of crosslinked mole] [% by mole] by mole] S [%] [g/g] copolymer Example 1 0.1 5.2 94.7 45 199 10 Example 2 0.2 5.2 94.6 13 94 3 Example 3 0.3 5.2 94.5 9 43 2 Example 4 0.1 2.0 97.9 40 73 2 Example 5 0.1 14.8 85.1 61 221 12 Example 6 0.3 20.2 79.5 32 140 11 Example 7 0.3 29.7 70.0 48 264 12 Comparative 0.1 0.5 99.4 38 9 2 Example 1 Comparative 0.3 40.0 59.7 64 294 13 Example 2 Comparative 0.3 15.2 67.7 52 272 32 Example 3 Average Amount W.sub.3 of particle Ratio W.sub.2 of water that can be CaCl.sub.2 solution diameter of water that absorbed by absorption polyvinyl can be plant per 1 g of amount W.sub.4 per alcohol-based absorbed by crosslinked 1 g of crosslinked crosslinked plant copolymer copolymer copolymer [%] [g/g] [g/g] [μm] Example 1 26 51.74 20 128 Example 2 52 48.88 14 134 Example 3 66 28.38 10 118 Example 4 29 21.17 12 140 Example 5 7 15.47 18 150 Example 6 10 14.00 3 123 Example 7 5 13.20 2 134 Comparative 31 2.75 5 131 Example 1 Comparative 2 5.88 1 137 Example 2 Comparative — — — — Example 3

[0091] As seen from Table 2, all of the crosslinked copolymers according to the present invention were water absorbent resins capable of absorbing a greater amount of water that can be absorbed by a plant. On the other hand, due to a notably small amount of pure water absorption (Comparative Example 1) or a notably low ratio of water that can be absorbed by a plant (Comparative Example 2), the crosslinked copolymers of these Comparative Examples absorbed only a smaller amount of water that can be absorbed by a plant. In addition, the crosslinked copolymer of Comparative Example 3 had a large standard deviation of the pure water absorption amount W.sub.1 per 1 g of the crosslinked copolymer, and was inferior in the stability of its quality.

[0092] Moreover, the crosslinked copolymers according to the present invention were modified with an unsaturated monocarboxylic acid or a derivative thereof. Therefore, a crosslinking reaction between a carboxylic acid-derived carboxyl group and a hydroxy group of a vinyl alcohol unit, which easily occurs in a crosslinked copolymer modified with a polyfunctional unsaturated carboxylic acid or a derivative thereof, did not easily occur in the post-modification drying step, and the crosslinked copolymers according to the present invention thus had a superior stability of its quality.

INDUSTRIAL APPLICABILITY

[0093] The polyvinyl alcohol-based crosslinked copolymer of the present invention has a superior stability of its quality. Further, the polyvinyl alcohol-based crosslinked copolymer of the present invention has a high water release capacity in addition to a high water absorption capacity. Therefore, the crosslinked copolymer can be used as a water absorbent resin. Moreover, plants can absorb a greater amount of water available for their growth from the polyvinyl alcohol-based crosslinked copolymer of the present invention; therefore, the polyvinyl alcohol-based crosslinked copolymer of the present invention can be suitably used as, for example, a water-retaining material for agriculture.