ORGANIC SOLVENT DISPERSION OF HYDROLYSABLE POLYMER

Abstract

A high volume organic solvent polymer dispersion that is prepared by dispersing a hydrolysable polymer in a non-volatile water-soluble organic solvent, has a water content of not more than 1 mass %, and has a falling time at 50° C. of not more than 120 seconds which is measured by a Zahn cup with an orifice diameter of not more than 5 mm. Also disclosed is a method of using the high volume organic solvent polymer dispersion, which includes feeding the high volume organic solvent polymer dispersion into the ground.

Claims

1. A high volume organic solvent polymer dispersion that is prepared by dispersing a hydrolysable polymer in a non-volatile water-soluble organic solvent, has a water content of not more than 30 mass %, and has a falling time at 50° C. of not more than 180 seconds which is measured by a Zahn cup with an orifice diameter of not more than 5 mm.

2. The high volume organic solvent polymer dispersion according to claim 1 having a volume of not less than 1,000 L.

3. The high volume organic solvent. polymer dispersion according to claim 1, wherein the water-soluble organic solvent is a polyhydric alcohol.

4. The high volume organic solvent polymer dispersion according to claim 1, wherein the hydrolysable polymer is an aliphatic polyester.

5. The high volume organic solvent polymer dispersion according to claim 4, wherein the aliphatic polyester is polylactic acid or polyoxalate.

6. The high volume organic solvent polymer dispersion according to claim 1, wherein a weight average molecular weight retention rate of the hydrolysable polymer left to stand at 50° C. for 30 days is maintained at not less than 80%.

7. A method of using a high volume organic solvent polymer dispersion comprising feeding a high volume organic solvent polymer dispersion according to claim 1 into the ground.

8. The method of using according to claim 7, wherein the high volume organic solvent polymer dispersion is injected into the soil to purify contaminated groundwater.

Description

MODE FOR CARRYING OUT THE INVENTION

[0035] The high volume organic solvent polymer dispersion of the present invention is prepared by dispersing a hydrolysable polymer in a water-soluble organic solvent so as to allow a hydrolysate of the hydrolysable polymer and the organic solvent no contribute no the formation of the anaerobic environment in the ground. Further, when the present invention is applied to the preparation of a fracturing fluid, the hydrolysable polymer functions as a filler to temporality seal cracks on a well.

Hydrolysable Polymer:

[0036] The hydrolysable polymer used in the present invention is a water-insoluble resin that is hydrolyzed to a low molecular weight monomer in the presence of water. For example, an aqueous dispersion containing this resin in a concentration of 5 mg/ml is prepared and left to stand still at 25° C. After a lapse of certain days, 20 mL of this dispersion is extracted and filtered through a 0.45 μm filter. The amount of total organic carbon (TOC) measured by a total organic carbon meter (TOC meter) is not less than 0.5 ppm/day for 30 days or longer. Such a hydrolysable polymer can be injected even into the soil with high permeability without being carried by flowing water, and is gradually hydrolyzed in the soil to produce a hydrolysate, which serves as a hydrogen donor to function as a nutritive source for aerobic microorganisms. Consequently, aerobic microorganisms are activated, allowing the underground environment to be anaerobic. As a result, anaerobic microorganisms are activated to decompose contaminants such as volatile organic chlorine compounds (VOCs), nitrate nitrogen and nitrite nitrogen. Further, the hydrogen donor, i.e., the hydrolysate also has a property of decomposing VOCs by reacting with and capturing a chlorine atom possessed by the VOCs.

[0037] Typical examples of the hydrolysable polymer as described above include hydrolysable resins such as polyester and polyamide, polysaccharides, and proteins, though the present invention is not limited thereto.

[0038] The polyester is basically a polycondensation product of a polycarboxylic acid, and a polyhydric alcohol.

[0039] Typical examples of the polycarboxylic acid include dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, giutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, decanedicarboxylic acid, and cyclohexanedicarboxylic acid.

[0040] Examples of the polyhydric alcohol include ethylene glycol, propylene glycol, butanediol, octanediol, dodecanediol, neopentyl glycol, glycerin, pentaerythritol, sorbitan, bisphenol A, and polyethylene glycol.

[0041] The polyester may be obtained by the polycondensation of a hydroxycarboxylic acid or the ring-opening polymerization of a lactone or the like.

[0042] Examples of the hydroxycarboxylic acid include glycolic acid, lactic acid, malic acid, hydroxypropionic acid, hydroxybutyric acid, hydroxyvaleric acid, hydroxycaproic acid, and hydroxybeozoic acid. Examples of the lactone include glycolide, caprolactone, butyrolactone, valerolactone, propiolactone, and undecalactone.

[0043] The polyamide is a polycondensation product of a polycarboxylic acid and a polyamine or is obtained by the ring-opening polymerization of a lactam.

[0044] Typical examples of the polycarboxylic acid include dicarboxylic acids such as succinic acid, adipic acid, sebacic acid, giutaric acid, decanedicarboxylic acid, cyclohexanedicarboxylic acid, terephthalic acid, isophthalic acid, and anthracenedicarboxilic acid.

[0045] Examples of the polyamine include diamines such as hexamethylenediamine, nonanediamine, methylpentadiamine, and phenylenedimine.

[0046] Examples of the lactam include caprolactam, undecanelactam, and lauryllactam.

[0047] Examples of the polysaccharides and proteins include starch, modified starch, cellulose, chitin, chitosan, gluten, gelatin, soy protein, collagen, and keratin.

[0048] In the present invention, each of the above-described hydrolysable polymers maybe used alone or in combination with one or more of the other hydrolysable polymers.

[0049] Among the above-described hydrolysable polymers, aliphatic polyesters are suitable from, the view point of moderate hydrolyzability. For example, poly(α-hydroxy acid), poly(β-hydroxyalkanoate), poly(ω)-hydroxyalkanoate), polyalkylene dicarboxylate and the like are preferable. In particular, polyglycolic acid, polylactic acid, poly-β-hydroxybutyric acid), poly(β-hydroxyvaleric acid), poly-β-propiolactone, poly-ε-caprolactone, polyethylene succinate, polybutylene succinate, polyethylene oxalate, polybutylene oxalate and the like are suitable. Above all, polylactic acid and polyoxalate are most suitable.

[0050] Polylactic acid is hydrolyzed to form lactic acid, which is particularly useful as a nutritive source and a hydrogen donor for microorganisms.

[0051] Polyoxalate, which is typically polyethylene oxalate or polybutyl oxalate, is a polymer obtained by the polycondensation of an oxalic acid and a diol such as ethylene glycol or butylene glycol, and has as an extremely high hydrolysis rate as compared with polylactic acid.

[0052] Thus, depending on the type of the soil in which contaminated groundwater is present, the degree of pollution, the type of contaminants, and the like, either polylactic acid or polyoxalate may be selected for use as the hydrolysable polymer, so that the properties of each of these polymers can be optimized.

[0053] Further, in a case where polylactic acid and polyoxalate are used in combination, the anaerobic environment can be formed by the hydrolysis of polyoxalate during an early stage. Thereafter, when most of the polyoxalate is hydrolyzed, the hydrolysis of polylactic acid can then form the anaerobic environment. As a result, the anaerobic environment can be formed stably for a long term. Further, polylactic acid has a higher hydrolysis rate when used in combination with polyoxalate, because polyoxalate is hydrolyzed to form an acid (oxalic acid) that accelerates the hydrolysis of polylactic acid.

[0054] In light of the above, it is preferable in the present invention to use polylactic acid and polyoxalate in combination. For example, it is most suitable to use polyoxalate in an amount of 1 to 30 parts by mass per 100 parts by mass of polylactic acid.

[0055] Further, it is preferable in the present invention to use the hydrolysable polymer in the form of a granular substance. For example, the granular substance preferably has an average particle diameter D50 in a range of 1 to 100 μm, which is measured in terms of volume by a laser diffraction scattering method. When in into the soil, a hydrolysable polymer particle having an appropriate particle diameter tends to remain in the same place, because it is less likely to be carried even by fast flowing groundwater, and easily penetrates into a space between soil particles in the ground. As a result, the hydrolysable polymer particle is gradually hydrolyzed on the spot, contributing. to the formation of the anaerobic environment. Further, even if groundwater contains oxygen, nitrate ion, sulfate ion and the like, they just have a limited effect, such as decreased hydrolyzability caused by oxidation and the like, on the surface of the particle, while the inside of the particle can be effectively free from such an adverse effect.

[0056] Such a granular substance of the hydrolysable polymer can be obtained by producing a hydrolysable polymer by, for example, emulsion polymerization or suspension polymerization, or alternatively by subjecting a hydrolysable resin pellet or the like obtained by melt extrusion to mechanical crushing, followed by appropriate sieve classification or the like.

[0057] Further, the hydrolysable polymer used in the present invention suitably has a weight average molecular weight (Mw) of not less than 12,000, particularly not less than 20,000. When the weight average molecular weight is too low, the hydrolysable polymer is entirely hydrolyzed and disappears in a short time, which is infavorable for the anaerobic environment to be maintained for a long term. Also, such a low weight average molecular weight hydrolysable polymer tends to be unfavorable in light of granulation by mechanical crushing.

[0058] The weight average molecular weight (Mw) is measured by GPC in terms of polystyrene as a standard substance.

Water-Soluble Organic Solvent:

[0059] The water-soluble organic solvent used in the present invention itself is a hydrogen donor, which serves as a nutritive source for aerobic microorganisms and anaerobic microorganisms to contribute to the form of the anaerobic environment. This, the organic solvent needs so be a non-volatile liquid with a boiling point of higher than 100° C., for example. In the case of a volatile liquid such as methanol or ethanol, it is easily volatilized without contributing to the formation of the anaerobic environment. In addition, such a volatile liquid is unsuitable for long-term storage in the form of a dispersion before being injected into the soil, because the organic solvent is volatilized from the system.

[0060] Further, the organic solvent of the present invention has a high affinity for the hydrolysable polymer so as to allow particles of the hydrolysable polymer to be uniformly dispersed, and is soluble in water to easily penetrate into groundwater. In the case of a water-insoluble organic solvent (with a solubility in water of not more than 1 g/100 mL, for example), it is less likely to penetrate into the ground and, thus, cannot contribute to the formation of the anaerobic environment.

[0061] Furthermore, the water-soluble organic solvent of the present invention needs to be capable of maintaining the water content of the polymer dispersion at not higher than 30 mass %, preferably not higher than 10 mass %, and more preferably not higher than 1 mass %. When the water content is higher, the hydrolysis of the hydrolysable polymer dispersed in the organic solvent proceeds. As a result, most of the hydrolysable polymer has already been hydrolyzed and disappeared before it is injected into the ground. For the same reason, it is impossible to use, as the solvent of the present invention, a low carbon primary alcohol such as ethanol or methanol, or its anhydride even if it is capable of maintaining the water content at the suitable range, because such an alcohol itself is likely to allow the hydrolysable polymer to be hydrolyzed.

[0062] In the present invention, the water-soluble organic solvent is mixed with the hydrolysable polymer particle so that the hydrolysable polymer in the form of a granular substance is dispersed. The thus-obtained dispersion needs to have a viscosity (50° C.) such that the falling time measured by a Zahn cup with an orifice diameter of not more than 5 mm is 180 seconds or less, particularly 120 seconds or less. When the viscosity is too high, the dispersion adheres to the wall of a mixer in a large amount, and is difficult to extract from the mixer. As a result, it becomes difficult to obtain the dispersion in high volume.

[0063] In light of the foregoing, typical examples of the water-soluble organic solvent used in the present invention include polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butandiol, and triethylene glycol, though the present invention is not limited thereto. An appropriate selection is made from among these alcohols in terms of, for example, the affinity for the hydrolysable polymer to be used. A plurality of the alcohols may be selected to prepare a mixed solvent.

[0064] For example, when a carboxylate salt such as sodium lactate is mixed with the hydrolysable polymer, the resultant mixture becomes extremely highly viscous, which makes it impossible to prepare the dispersion in high volume using a mixer. In order to reduce the viscosity, the mixture needs to be diluted with water, which then makes it impossible to suppress the hydrolysable polymer from being hydrolyzed during storage or transportation.

High Volume Organic Solvent Polymer Dispersion:

[0065] In order for the polymer dispersion of the present invention formed of the hydrolysable polymer and the water-soluble organic solvent as described above to be prepared in high volume by homogeneous mixing using a mixer, the hydrolysable resin and the water-soluble organic solvent are used at an amount ratio that is set so as to allow the dispersion so have a viscosity (50° C.) such that the falling time measured by a Zahn cup with an orifice diameter of not more than 5 mm is 120 seconds or less. From the viewpoint of high volume preparation, the dispersion preferably has a lower viscosity. However, when the dispersion has a lower than necessary viscosity, it contains an excessively large amount of the organic solvent, which impairs the effect of the hydrolysable polymer, i.e., maintaining the anaerobic environment for a long term. On this account, it is desirable in the present invention to set the amount ratio between the hydrolysable resin and the water soluble organic solvent so that the hydrolysable polymer and the organic solvent are contained in a well-balanced manner. A specific amount ratio cannot be defined definitely because it varies depending on the molecular weight and type of the hydrolysable polymer to be used as well as the type of the water-soluble organic solvent. It is usually preferable that a non-hygroscopic water-soluble organic solvent is contained in an amount of not less than 50 parts by mass, particularly not less than 100 parts by mass per 100 parts by mass of the hydrolysable polymer.

[0066] In view of industrial application, the above-described high volume organic solvent polymer dispersion has a high volume of not less than 500 L, preferably not less than 1,000 L. When the volume is not less than this, it becomes difficult so extract the dispersion from the top of the mixer with a ladle or the like or to extract the dispersion by inverting the mixer itself. Accordingly, it is particularly useful chat the viscosity is controlled within the low viscosity range.

[0067] Further, the high volume organic solvent polymer dispersion of the present invention needs to have a water content that is controlled to be not more than 30 mass %, preferably not more than 10 mass %, and more preferably not more than 1 mass % in order to prevent the hydrolysable polymer from being hydrolyzed before being injected into the ground. For this purpose, it is basically most suitable that the hydrophilic organic solvent is an anhydride. However, the organic solvent may contain a certain amount of moisture due to its hydrophilicity as long as the water content of the dispersion is within the aforementioned range. Further, the dispersion may atmospherically exposed as long as the water content is maintained within the aforementioned range.

[0068] Since the high volume organic solvent polymer dispersion of the present invention contains the limited amount of water as described above, the hydrolysable polymer is suppressed from being hydrolyzed. For example, the retention rate of the weight average molecular weight (Mw) of the hydrolysable polymer left to stand at 50° C. for 30 days is not less than 70%, particularly not less than 80%.

Use of High Volume Organic Solvent Polymer Dispersion

[0069] The above-described high volume organic solvent polymer dispersion of the present invention is particularly suitable for use in the purification of contaminated groundwater and is injected in situ into the soil in which contaminated groundwater is present.

[0070] The dispersion may be directly injected into the soil. Usually, however, the dispersion is diluted with water before being injected. This allows de dispersion to be injected into the ground immediately. The dispersion of the present invention is immediately and homogeneously diluted with water without being whipped, for example.

[0071] In the present invention, contaminants with which ground water to be purified is contaminated are not limited particularly. However, the present invention is effectively applied to volatile organic chlorine compounds (VOCs), nitrate nitrogen, nitrite nitrogen and the like which are regulated in line with the environmental quality standards for groundwater pollution.

[0072] Here, VOCs are chemical substances that are widely used for industrial purposes as a solvent and a detergent. Examples thereof include tetrachloroethylene, trichloroethylene, cis-1,2-dichloroethylene, 1,1-dichloroethylene, vinyl chloride, 1,1,1-trichloroethane, 1,1,2-trichloroethane, 1,2-dichloroethane, ethane tetrachloride, carbon tetrachloride, chloroform, and dichloromethane.

[0073] Nitrate nitrogen and nitrite nitrogen are mainly derived from nitrogen fertilizer, livestock excretion and domestic wastewater. Apart of them is converted via ammoniacal nitrogen and then nitrite nitrogen to nitrate nitrogen in the end by the actions of microorganisms in the soil. Nitrate nitrogen and nitrite nitrogen are easily dissolved in water and less likely to be held in the soil, and thus are easily eluted in groundwater.

[0074] The polymer dispersion for purifying groundwater containing the aforementioned contaminants can be injected into the ground by a method known per se, such as the method disclosed in JP 2018-143918 A (Patent Document 2).

[0075] The high volume polymer dispersion of the present invention is not only suitable for being consumed in a massive amount by being fed underground but also available for use in the preparation of a fracturing fluid and the like in the field of drilling because the hydrolysable polymer exhibits moderate hydrolyzability.

EXAMPLES

[0076] Excellent effects of the present invention will be described by way of the following experimental examples.

<Raw Materials>

<Organic Solvent>

Ethylene Glycol (EG)

[0077] Ethylene glycol (Guaranteed Reagent) manufactured by FUJIFILM Wako Pure Chemical Corporation (purity: at least 99.5% (w/w) or more)

Triethylene Glycol (TEG):

[0078] Triethylene glycol manufactured by Tokyo Chemical Industry Co., Ltd. (purity: more than 99.0%)

Propylene Glycol (PC):

[0079] Propylene glycol (Guaranteed Reagent) manufactured by FUJIFILM Wako Pure Chemical Corporation (purity: at least 99.0% (w/w) or more)

Methanol:

[0080] Methanol manufactured by FUJIFILM Wako Pure Chemical Corporation (methanol content: 99.7%, for high-performance liquid chromatograph)

50% Lactic Acid Aqueous Solution:

[0081] Musashino Lactic Acid 50F manufactured by Musashino Chemical Laboratory, Ltd.

Sodium Lactate (LaNa):

[0082] Sodium L-lactate solution (about 70%) manufactured by FUJIFILM Wako Pure Chemical Corporation; 16.7 g of pure water was added per 100 g of this solution.

<Hydrolysable Polymer>

[0083] The hydrolysable polymer used was a low molecular weight product obtained by melt-kneading polylactic acid (PLA) and polyethylene oxalate (PEOx).

[0084] The PLA used was REVODE 101 manufactured by Zheijiang Hisun Biomaterials Co., LTD. It had a molecular weight in a range of 120,000<Mw<170,000 when used as a raw material.

[0085] The PEOx used was a product obtained by polymerization which will be described in the following section. It had a reduced viscosity of 0.84 dL/g. A production method thereof will be described below.

<Synthesis of PEOx>

[0086] 40 kg (339 mol) of dimethyl oxalate, 23.2 kg (374 mol) of ethylene glycol, 2.9 kg (32.2 mol) of 1,4-butandiol, and 8.4 g of dibutyltin oxide were introduced into a 150 L capacity reactor that could be heated by a heating medium, which were then warmed in a current of nitrogen so that the liquid in the reactor reached 110° C., followed by normal pressure polymerization.

[0087] After methanol started being distilled away, the liquid was kept warm for one and a half hour so that a reaction proceeds. After a lapse of one and a half hour, the temperature of the liquid was increased to 130° C. at a temperature increase rate of 10° C./hour and further to 190° C. at a temperature increase rate of 20° C./hour. The liquid was recovered in an amount of 21.2

[0088] Thereafter, the liquid in a flask was subjected to reduced pressure polymerization at a temperature of 190° C. and a pressure reduction degree of 0.1 to 0.8 kPa, and the thus-obtained polymer was extracted. This polymer was subjected to a heat treatment at 90° C. for 2 hours and at 120° C. for 2 hours.

<Melt-Kneading of PLA and PEOx>

[0089] 90 parts by mass of the PEA as a raw material and 10 parts by mass of the synthesized PEOx were quantitatively fed to a continuous twin screw extruder by respective quantitative feeders, followed by melt-kneading. The extruder had a temperature of 200° C. The melt-kneaded resin was formed into a spherical shape by an underwater cutter.

<Molecular Weight Reduction and Crushing of Hydrolysable Polymer>

[0090] 200 kg of the above-described melt-kneaded resin and 200 kg of the 50% lactic acid aqueous solution were introduced into a 1,000 L capacity reactor that could be heated by a heating medium, which were then stirred at 100° C. for 2 hours to react with each other. After the reaction, the product was cooled to room temperature, followed by separating the resin from the solution by a centrifugal separator. The separated resin was washed with water and then dried under vacuum at 70° C. to 90° C. by a 300 L capacity conical dryer, thereby obtaining a low molecular weight hydrolysable polymer.

[0091] The thus-obtained resin was mechanically crushed into a fine powder (D.sub.50=7 μm) by a jet mill.

<Evaluation Methods>

<Measurement of Reduced Viscosity of PEOx>

[0092] Apparatus: Cannon-Fenske viscometer

[0093] Solvent: 1,1,1,2,2,2-hexafluoro2-propanol

[0094] Temperature: 25° C.

[0095] Sample preparation: 10 mL of the solvent was added to 40 mg of the sample, which were stirred gently at room temperature. After a visual confirmation that the sample was dissolved in the solvent, it was filtered through a 0.45 μm filter to prepare a measurement sample.

<Molecular Weight Measurement of Hydrolysable Polymer>

[0096] The molecular weight of the hydrolysable polymer was measured under the following conditions. [0097] Apparatus: High-speed GPC apparatus HLC-8320 manufactured by Tosoh Corporation [0098] Detector: Differential refractometer RI [0099] Column: SuperMultipore HZ-M (2 columns) [0100] Solvent: Chloroform [0101] Flow rate: 0.5 mL/min [0102] Column temperature: 40° C. [0103] Sample preparation: 3 mL of the solvent was added to about 0 mg of the powder sample. After a visual confirmation that the sample was dissolved in the solvent, it was filtered through a 0.45 μm filter to prepare a measurement sample. Polystyrene was used as a standard.

<Measurement of Molecular Weight Retention Rate>

[0104] About 10 g of the hydrolysable polymer dispersion prepared in each Example was put in a 20 mL capacity glass vial container, which was then closed with a PP stopper and left to stand still in an oven set at 50° C. In 30 days, 2 mL of the dispersion was extracted and centrifuged at 10,000 rpm for 2 minutes, thereby separating the fine powder from the solvent. The fine powder was centrifuged in a like manner with pure water to be washed and then dried under vacuum at 40° C. for 4 hours, thereby obtaining a powder sample. The thus-obtained powder sample was subjected to GPC measurement, thereby evaluating the molecular weight.

[0105] The molecular weight retention rate X.sub.keep was calculated by Equation (1) below using the molecular weight obtained by The GPC measurement.

X.sub.keep=M.sub.Xfinish/M.sub.initial

[0106] X.sub.keep: Molecular weight retention rate

[0107] M.sub.Xfinish: Weight average molecular weight of hydrolysable polymer left. to stand. at 50° C. for 30 days

[0108] M.sub.Xinitial: Weight average molecular weight of freshly prepared hydro usable polymer

<Measurement of Water Content Ratio>

[0109] The water content ratio W of the organic solvent polymer dispersion was calculated by Equation (2) below using the water content ratios of the organic solvent and the powder which were measured respectively.

W=X.sub.solvent×W.sub.solvent+X.sub.powder×W.sub.powder   (2)

[0110] W: Water content ratio (%) of organic solvent polymer dispersion

[0111] X.sub.solvent: Mass fraction of solvent in organic solvent polymer dispersion

[0112] W.sub.solvent: Water content ratio (%) of solvent

[0113] X.sub.powder: Mass fraction of powder in organic solvent polymer dispersion

[0114] W.sub.powder: Water content ratio (%) of powder

[0115] The water content ratio of the solvent was measured under the following conditions, and the water content ratio of the hydrolysable polymer was measured.

[0116] Apparatus: Moisture meter (volumetric titration method) KF-31 manufactured by Mitsubishi Chemical Analytech, Co., Ltd.

[0117] Titration solvent: AQUAMICRON SS-Z 3 mg

[0118] Sample preparation: A commercially available reagent was subjected to measurement immediately after opening.

[0119] The water content ratio of the powder was measured by using the following apparatus, and the water content ratio of the hydrolysable polymer was measured.

[0120] Apparatus: Trace moisture meter CA-200 manufactured by Mitsubishi Chemical Analytech, Co., Ltd.

<Measurement of Viscosity>

[0121] Apparatus: Zahn cup #4 (orifice diameter: 4.4 mm) manufactured by BEVS Industrial Co., Ltd

[0122] Sample preparation: The prepared hydrolysable polymer dispersion was left to stand overnight in a room kept at 50° C., and then the falling time was measured by using the Zahn cup in the same room.

[0123] Measuring method: The Zahn cup was dipped into the hydrolysable polymer dispersion and then lifted to a height where the bottom is about 5 cm away from the liquid surface. The time from when the Zahn cup was lifted until when the entire hydrolysable polymer dispersion in the Zahn cup fell and the liquid streaming from the bottom broke up was measured as an evaluation value.

[0124] Evaluation method: When the evaluation value of this measurement is less than 120 seconds, the hydrolysable polymer dispersion can be extracted suitably through a discharge pipe even when produced by a high volume mixer.

Example 1

[0125] 100 g of the hydrolysable polymer and 200 g of the EG were put in a 600 mL capacity plastic cup container, followed by stirring at 20,000 rpm for 3 minutes by a high speed stirrer (HSIANGTAI ST-200 manufactured by AS ONE Corporation), thereby obtaining a hydrolysable polymer dispersion.

Example 2

[0126] A hydrolysable polymer dispersion was obtained in the same manner as in Example 1 except that the EG was replaced with the TEG.

Example 3

[0127] A hydrolysable polymer dispersion was obtained in the same manner as in Example 1 except that the EG was replaced with the PG.

Example 4

[0128] 30 g of the hydrolysable polymer and 300 g of the PG were put in a 600 mL capacity plastic cup container, followed by the same treatment as in Example 1, thereby obtaining a hydrolysable polymer dispersion.

Comparative Example 1

[0129] A hydrolysable polymer dispersion was obtained in the same manner as in Example 1 except that the EG was replaced with water.

Comparative Example 2

[0130] A hydrolysable polymer dispersion was obtained in the same manner as in Example 1 except that the EG was replaced with methanol.

Comparative Example 3

[0131] A hydrolysable polymer dispersion was obtained in the same manner as in Example 1 except that the EG was replaced with the LaNa.

<Physical Properties of Hydrolysable Polymer Dispersion>

[0132] Physical properties of each of the hydrolysable polymer dispersions prepared as described above were measured; the results are shown in Table 1.

[0133] In Examples 1 to 4, the falling time at 50° C. measured by a Zahn cup was suitable, and the molecular weight retention rate was as high as not less than 80%. This proves that the polymer dispersion obtained in the present invention is easy to extract from a mixer and pack even when prepared in high volume, and the hydrolysable polymer can be effectively prevented from being deteriorated even in long term storage.

TABLE-US-00001 TABLE 1 Zahn cup Molecular Water content Powder:solvent falling time weight Solvent ratio (%) ratio (sec.) retention rate Example 1 EG 0.066 1:2 37.0 96% Example 2 TEG 0.022 1:2 21.3 94% Example 3 PG 0.047 1:2 49.2 85% Example 4 PG 0.057  1:10  6.5 95% Comparative Water Not measured 1:2 No falling 20% Example 1 Comparative Methanol 0.014 1:2  5.8 60% Example 2 Comparative LaNa 26.387  1:2 No falling 27% Example 3