阅读说明：本技术 聚乳酸固体组合物及其生产方法 (Polylactic acid solid composition and production method thereof ) 是由 片山传喜 吉川成志 柴田幸树 于 2020-03-30 设计创作，主要内容包括：一种固体聚(乳酸)组合物,其特征在于,包括如通过GPC测定的、以聚苯乙烯为基准的重均分子量为40,000以下的聚(乳酸),和作为低分子量化促进剂的残留物的碱金属和碱土金属的碱性化合物,并且以0.5-20质量％的量包含碱性化合物。(A solid poly (lactic acid) composition characterized by comprising a poly (lactic acid) having a weight-average molecular weight of 40,000 or less based on polystyrene as measured by GPC, and alkali metal and alkaline earth metal basic compounds as residues of a low-molecular-weight accelerator, and containing the basic compounds in an amount of 0.5 to 20 mass%.)

1. A polylactic acid solid composition, comprising: polylactic acid having a weight average molecular weight of 40,000 or less based on polystyrene measured by GPC; and a basic compound of an alkali metal or an alkaline earth metal as a residue of the low molecular weight accelerator, wherein the basic compound is contained in an amount in the range of 0.5 to 20 mass%.

2. The polylactic acid solid composition according to claim 1, wherein the weight average molecular weight of the polylactic acid is in the range of 12,000 to 40,000.

3. The polylactic acid solid composition according to claim 1, having an average particle diameter (D) measured by a laser diffraction scattering method₅₀) Has a particle shape of 10 μm or less.

4. The polylactic acid solid composition according to claim 3, wherein the sieve particle size is 10 μm or less.

5. The polylactic acid solid composition according to claim 1, wherein the basic compound is sodium carbonate.

6. The polylactic acid solid composition according to claim 1, wherein the polylactic acid is the only polymer component contained therein.

7. The polylactic acid solid composition according to claim 1, which is packed in a bag.

8. A method for producing a polylactic acid solid composition, comprising:

preparing high molecular weight polylactic acid having a weight average molecular weight of 150,000 or more based on polystyrene measured by GPC; and

the high-molecular-weight polylactic acid and an alkali compound of an alkali metal or an alkaline earth metal are melt-kneaded so as to reduce the weight average molecular weight of the high-molecular-weight polylactic acid to 40,000 or less.

9. The method for producing a polylactic acid solid composition according to claim 8, wherein the melt-kneading is carried out at a temperature in the range of 220 ℃ to 250 ℃ under normal pressure.

10. The method for producing a polylactic acid solid composition according to claim 8, wherein said melt-kneading is carried out by using an extruder.

11. The production method of a polylactic acid solid composition according to claim 8, wherein said basic compound is used in an amount in the range of 1 to 30 parts by mass relative to 100 parts by mass of said high molecular weight polylactic acid.

Technical Field

The invention relates to a polylactic acid solid composition and a production method thereof. More particularly, the present invention relates to a polylactic acid solid composition which has improved crushability, contains a low molecular weight polylactic acid as a polymer component, and can be used as a fine particulate; and a method for producing the same.

Background

Polylactic acid is known as an environmentally friendly biodegradable resin, and is used in various applications.

Recently, it has been proposed to use polylactic acid in a soil clarification method using microorganisms, which is called bioremediation (biormediation). Lactic acid produced by hydrolysis of polylactic acid is used as a nutrient source for microorganisms. Thus, by spraying polylactic acid to impregnate the soil with it, the propagation and activity of microorganisms can be promoted.

For example, patent document 1 discloses a polylactic acid resin in a solid state and having a weight average molecular weight of 12,000 or less, and proposes the use of such a low molecular weight polylactic acid resin as a nutritional agent. The polylactic acid-based resin has such a molecular weight that it does not convert into a liquid state, and thus it can be packed in a bag to achieve excellent transportability and also exhibit excellent processability. In addition, the low molecular weight contributes to achieving a high sustained release of lactic acid and to obtaining a nutrient source highly effective for enzymes.

Meanwhile, a low molecular weight polylactic acid-based resin used for the above intended use is produced by heating a high molecular weight polylactic acid under high pressure using an autoclave for the following reason. That is, it involves high cost for producing low molecular weight polylactic acid by polymerization of monomers, and in addition, the method also results in production of polylactic acid of lower molecular weight than necessary. It is true that such polylactic acid having an extremely low molecular weight can be excluded by, for example, fractionating the obtained polylactic acid, thereby producing polylactic acid having a molecular weight within a desired range. However, this further increases the cost.

The low molecular weight polylactic acid used in patent document 1 has a problem in fragmentability. More specifically, the polylactic acid has a pellet form with a particle diameter of the order of mm, and cannot be broken into fine particles. This seems to be because low molecular weight polylactic acid is obtained by heating under high pressure, and therefore it contains a large amount of low molecular weight components causing stickiness (tack) and the like.

Further, patent document 2 discloses a biodegradable resin composition comprising a difficultly hydrolyzable biodegradable resin (a), an ester decomposition promoter (B) formed from a easily hydrolyzable polymer, and an ester decomposition promoter (C). Patent document 2 describes the use of polylactic acid as the non-hydrolyzable biodegradable resin (a), an acid-releasing polyester such as polyoxalate as the ester decomposition accelerator (B), and a basic inorganic compound such as calcium carbonate or sodium carbonate as the ester decomposition accelerator (C).

The technique of patent document 2 has achieved improved hydrolyzability of a hardly hydrolyzable resin such as polylactic acid. However, it is necessary to use a high-cost easily hydrolyzable polymer such as polyoxalate as the ester decomposition accelerator (B). Thus, modifying polylactic acid by, for example, reducing the molecular weight using this technique becomes a problem in terms of cost. Further, since the ester decomposition promoting assistant (C) in the technique of patent document 2 is used to promote hydrolysis of the ester decomposition promoting agent (B), patent document 2 does not give consideration to whether or not the assistant (C) contributes to reduction of the molecular weight of the resin difficult to hydrolyze.

Further, the patent applicant previously proposed a method of reducing the molecular weight of a high-molecular weight polylactic acid, in which a polylactic acid (aliphatic polyester) is put into a solution containing an organic acid, followed by heating (JP 2017-. According to this method, low-molecular weight polylactic acid can be obtained at low cost. However, low molecular weight polylactic acid is obtained in a state of being dissolved or dispersed in a liquid, which leads to a need for improvement in workability.

Prior art documents:

patent documents:

patent document 1: JP 2011-104551A

Patent document 2: JP 5633291B 2

Disclosure of Invention

Problems to be solved by the invention

Accordingly, it is an object of the present invention to provide a polylactic acid solid composition which comprises a low-molecular weight polylactic acid as a polymer component, is produced at low cost without using a liquid such as an organic solvent and an expensive ester decomposition accelerator such as an easily hydrolyzable polymer, and exhibits excellent disintegratability and lactic acid-releasing property; and a method for producing the same.

Means for solving the problems

The present invention provides a polylactic acid solid composition, comprising: polylactic acid having a weight average molecular weight of 40,000 or less based on polystyrene measured by GPC; and an alkali metal or alkaline earth metal basic compound as a residue of the low molecular weight accelerator. The basic compound is contained in an amount within the range of 0.5 to 20 mass%.

In the polylactic acid solid composition of the present invention, suitable are:

(1) the weight average molecular weight of the polylactic acid is in the range of 12,000 to 40,000;

(2) the polylactic acid solid composition has an average particle diameter (D) measured by a laser diffraction scattering method₅₀) Is in a granular form of 10 μm or less;

(3) the sieving particle size of the polylactic acid solid composition is less than 10 mu m;

(4) the alkaline compound is sodium carbonate;

(5) polylactic acid is the only polymer component contained therein; and

(6) the polylactic acid solid composition is packed into a bag.

Further, the present invention provides a method for producing a polylactic acid solid composition, comprising:

preparing high molecular weight polylactic acid having a weight average molecular weight of 150,000 or more based on polystyrene measured by GPC; and

the high-molecular-weight polylactic acid and an alkali compound of an alkali metal or an alkaline earth metal are melt-kneaded so as to reduce the weight average molecular weight of the high-molecular-weight polylactic acid to 40,000 or less.

In the production method of the present invention, suitable are:

(1) melt-kneading is carried out at a temperature in the range of 220 ℃ to 250 ℃ under normal pressure.

(2) Melt-kneading is performed by using an extruder; and

(3) the basic compound is used in an amount in the range of 1 to 30 parts by mass relative to 100 parts by mass of the polylactic acid.

In the present invention, a solid composition means a material which is neither in a liquid form nor in a form dispersed in a liquid, but exists as a solid, exhibiting no fluidity and viscosity at least at room temperature (23 ℃).

ADVANTAGEOUS EFFECTS OF INVENTION

The polylactic acid solid composition of the present invention is obtained by reducing the molecular weight of a high molecular weight polylactic acid without melt-kneading water and an organic solvent, and contains a low molecular weight polylactic acid having a weight average molecular weight of 40,000 or less, particularly 35,000 or less, based on polystyrene as measured by GPC, as a polymer component. Further, the polylactic acid solid composition contains 0.5 to 20 mass% of an alkali metal or alkaline earth metal basic compound (low molecular weight accelerator) for reducing the molecular weight. That is, since the molecular weight is reduced by melt-kneading in the presence of an alkaline compound without using a liquid, the polylactic acid solid composition is caused to contain a certain amount of the alkaline compound as an essential component.

The melt viscosity of the low molecular weight polylactic acid obtained by melt kneading is too low to be measured by a melt indexer. That is, since the weight average molecular weight of the low molecular weight polylactic acid is extremely low, the polylactic acid solid composition containing the polylactic acid as a polymer component is excellent in mechanical crushability, so that it can be crushed into an average particle diameter (D) measured by a laser diffraction scattering method₅₀) Is in the form of fine particles of 10 μm or less, and is further crushed into fine particles having a mesh size of 10 μm or less. For example, in the case where the solid composition contains polylactic acid having a weight average molecular weight higher than the above range, such solid composition is difficult to finely pulverize and can be broken only to a particle size of about 125 μm. Needless to say, a fine particulate matter having a sieve particle size of 10 μm or less cannot be obtained.

Furthermore, the low molecular weight polylactic acid in the solid composition of the present invention contains only a very small amount of a very low molecular weight component, although the melt viscosity is too low to be measured by a melt index meter. For example, as shown in examples described later, the weight average molecular weight is higher than 12,000. Further, as a result of containing very little component having an extremely low molecular weight, the molecular weight distribution of the low-molecular weight polylactic acid has an extremely sharp monodispersion peak (see a molecular weight distribution curve), indicating that the component having a molecular weight of 8,000 or less is not substantially contained.

Furthermore, the solid composition according to the present invention comprising the above-mentioned low molecular weight polylactic acid is extremely inexpensive because it does not contain a high-cost easily hydrolyzable polyester such as polyoxalate or the like. Further, since the solid composition of the present invention is not obtained in a state of being dispersed in a liquid, it can be obtained immediately by mechanical crushing or the like, and is easy to produce and excellent in handling and the like.

The polylactic acid solid composition of the present invention exhibits excellent sustained-release properties of lactic acid due to low molecular weight polylactic acid. In particular, the solid composition of polylactic acid mechanically crushed into fine particles has significant advantages in terms of packability, transportability, processability, and the like, and is extremely suitable for use as a soil conditioner. When the solid composition is sprayed in soil, an alkaline compound such as sodium carbonate contained in the solid composition dissolves in soil moisture to become alkaline, which promotes hydrolysis of polylactic acid contained in the solid composition. As a result, the amount of lactic acid used as a nutrient source for the enzyme increases, so that the soil is favorably activated. Alternatively, the granulated solid composition may be put into water to prepare an aqueous dispersion suitable for use in underground mining.

Drawings

[ FIG. 1 ]: a graph showing a weight average molecular weight distribution curve of polylactic acid in the polylactic acid solid composition obtained in example 1;

[ FIG. 2 ]: SEM photograph of sodium carbonate particles contained in the polylactic acid solid composition obtained in example 1;

[ FIG. 3 ]: SEM photograph of sodium carbonate, a reagent used for producing the polylactic acid solid composition in example 1;

[ FIG. 4 ]: a graph showing a weight average molecular weight distribution curve of polylactic acid in the polylactic acid solid composition obtained in comparative example 1;

[ FIG. 5 ]: a graph showing a weight average molecular weight distribution curve of polylactic acid in the polylactic acid solid composition obtained in comparative example 2;

[ FIG. 6 ]: a graph showing the decomposition rate of the polylactic acid solid composition obtained in each of example 1 and comparative examples 1 and 2 in 70 ℃ water; and

[ FIG. 7 ]: a graph showing the decomposition rate of the polylactic acid solid composition obtained in each of example 1 and comparative examples 1 and 2 in water at 90 ℃.

Detailed Description

< production of polylactic acid solid composition >

The polylactic acid solid composition of the present invention is produced by melt-kneading a high molecular weight polylactic acid and an alkaline compound. More specifically, the molecular weight of the high-molecular weight polylactic acid is reduced in the solid phase in the presence of a basic compound, thereby obtaining a desired low-molecular weight polylactic acid. That is, melt kneading does not require a hydrolysis-susceptible polyester such as polyoxalate, but uses only a basic compound as a decomposition accelerator. As a result, the molecular weight of the high molecular weight polylactic acid can be moderately reduced, resulting in a polylactic acid solid composition containing a low molecular weight polylactic acid as a polymer component, which does not contain a very low molecular weight component.

High molecular weight polylactic acid:

the weight average molecular weight (Mw) of the high molecular weight polylactic acid to be subjected to low molecular weight quantification, as measured by GPC (gel permeation chromatography), based on polystyrene, is 150,000 or more, particularly 170,000 or more. When the weight average molecular weight is below this range, such polylactic acid contains a large amount of low molecular weight component, which is further subjected to low molecular weight quantization. Thus, it is not appropriate to use such polylactic acid to obtain low molecular weight polylactic acid containing no extremely low molecular weight component. On the other hand, in the case of using polylactic acid having an extremely high molecular weight, it takes a long time to reduce the molecular weight, or in some cases, it may be impossible to reduce the molecular weight. Therefore, the weight average molecular weight is preferably 200,000 or less.

The polylactic acid may be 100% poly-L-lactic acid or 100% poly-D-lactic acid, a melt blend of poly-L-lactic acid and poly-D-lactic acid, or a random or block copolymer of L-lactic acid and D-lactic acid.

Further, the high molecular weight polylactic acid may be copolymerized with a small amount (for example, 10 parts by mass or less with respect to 100 parts by mass of polylactic acid) of various aliphatic polyhydric alcohols, aliphatic polybasic acids, hydroxycarboxylic acids, lactones or the like as long as the molecular weight is within the above range so that the low molecular weight is not hindered and the properties such as mechanical crushability, hydrolyzability, and lactic acid slow release property are not impaired.

Examples of polyols include ethylene glycol, propylene glycol, butylene glycol, octanediol, dodecanediol, neopentyl glycol, glycerol, pentaerythritol, sorbitan, and polyethylene glycol.

Examples of the polybasic acid include oxalic acid, succinic acid, adipic acid, sebacic acid, glutaric acid, dodecanedioic acid, cyclohexanedicarboxylic acid, and terephthalic acid. Examples of hydroxycarboxylic acids include glycolic acid, hydroxypropionic acid, hydroxypentanoic acid, hydroxycaproic acid, and mandelic acid.

Examples of lactones include caprolactone, butyrolactone, valerolactone, propiolactone, undecalactone, glycolide, and mandelonitrile.

Basic compound (b):

the present invention uses a basic compound to promote the reduction of molecular weight by melt kneading described later. That is, by melt kneading in the presence of a basic compound, the ester unit of the high molecular weight polylactic acid is decomposed, so that the molecular weight of the polylactic acid can be reduced.

The basic compound comprises an alkali metal or an alkaline earth metal. Examples thereof include sodium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, sodium bicarbonate, potassium bicarbonate, sodium silicate, potassium silicate, calcium silicate, magnesium silicate, sodium phosphate, calcium hydroxide, and magnesium hydroxide. These compounds may be used alone or in combination of two or more.

The basic compound is finally contained as a residue in the solid composition containing the low molecular weight polylactic acid. Therefore, in view of the influence on the environment and the like, among the above-mentioned basic compounds, a basic compound of calcium or sodium is most suitable.

The alkali compound is uniformly contacted with the high molecular weight polylactic acid during melt-kneading. In view of this, it is advantageous that the particle diameter (circle equivalent diameter) of the basic compound as measured by SEM observation is 10 μm or less, particularly in the range of 0.01 to 7 μm.

In the present invention, it is advantageous to use the basic compound in an amount of 1 to 30 parts by mass, particularly 1 to 10 parts by mass, relative to 100 parts by mass of the aforementioned high molecular weight polylactic acid. When the basic compound is used in an excessive amount, it is eventually contained in a large amount in the resulting solid composition, which may deteriorate mechanical crushability and the like required for the low-molecular weight polylactic acid. On the other hand, when the basic compound is used in an excessively small amount, the molecular weight of the high-molecular weight polylactic acid cannot be sufficiently reduced, making it difficult to obtain a desired low-molecular weight polylactic acid.

Melting and mixing:

in the present invention, the high-molecular weight polylactic acid and the basic compound are melt-kneaded, thereby reducing the molecular weight of the high-molecular weight polylactic acid. As a result, a solid composition containing a desired low-molecular weight polylactic acid as a polymer component can be obtained.

It is important here that the molecular weight of the high molecular weight polylactic acid is not reduced by using a hydrolysis-susceptible polyester such as polyoxalate. When a readily hydrolyzable polyester or the like is used in combination with a basic compound as a decomposition accelerator, the molecular weight reduction proceeds more than necessary, and a component having an extremely low molecular weight is produced. As a result, the intended low molecular weight polylactic acid cannot be obtained. In addition, the use of such a decomposition accelerator not only increases the cost but also changes the characteristics of the resulting polylactic acid by copolymerization with the polylactic acid through transesterification or the like. In the present invention, only the basic compound is used as the decomposition accelerator. Thereby, it is possible to obtain low molecular weight polylactic acid containing no component having an extremely low molecular weight, and effectively avoid an increase in cost and deterioration of the polylactic acid.

The melt kneading can be easily performed by a melt kneading section such as an extruder. In particular, it is carried out at a temperature in the range of, for example, 220 ℃ to 250 ℃ at which polylactic acid does not thermally decompose. The melt-kneading is carried out for at least 1 minute or more, particularly about 1 to 5 minutes, thereby obtaining a desired low-molecular weight polylactic acid.

Melt-kneading must be carried out under normal pressure. For example, heat treatment under high pressure using an autoclave or the like generates a component having an extremely low molecular weight, resulting in a decrease in mechanical crushing characteristics. Further, in the case of using an autoclave in which heated water vapor is used, the alkali compound is dissolved and removed. As a result, the resulting solid composition does not contain an alkaline compound, which would impair the benefits of blending the alkaline compound (e.g., the hydrolyzability of polylactic acid when it is sprayed in soil).

< Low molecular weight polylactic acid >

The thus obtained solid composition of the present invention contains a low molecular weight polylactic acid as a polymer component. The low-molecular weight polylactic acid has a weight average molecular weight (Mw) of 40,000 or less, particularly in the range of 12,000 to 40,000, and more preferably more than 12,000 and 38,000 or less, as measured by GPC, based on polystyrene. Although such low molecular weight polylactic acid shows a melt viscosity too low to be measured by a melt index meter, it does not contain a component having an extremely low molecular weight, and has a molecular weight distribution as shown in fig. 1, which has an extremely sharp monodispersion peak.

Further, the solid composition of the present invention contains a residue of a basic compound for reducing the molecular weight of polylactic acid as an inevitable component. As shown in the examples described later, such residues are contained in the solid composition in an amount of 0.5 to 20 mass%, particularly 1 to 10 mass%, measured by the total organic carbon. The remainder is low molecular weight polylactic acid contained as a polymer component.

The basic compound partially reacts with the polylactic acid to form a salt. Therefore, the amount and particle diameter of the basic compound contained as the residue are smaller than those of the basic compound initially added to and blended with the high-molecular weight polylactic acid. Specifically, the particle size is reduced to 3% or less of the particle size of the alkali compound initially added. In addition, a part of the polylactic acid in the solid composition forms a salt with an alkali metal or an alkaline earth metal.

The solid composition containing the low molecular weight polylactic acid can be mechanically pulverized into an extremely fine particulate matter by a jet mill or the like. For example, the solid composition may be broken down into an average particle size (D) as measured by laser diffraction scattering₅₀) Is in the form of fine particles of 10 μm or less, particularly 7 μm or less. Further, these fine particles hardly show stickiness with a very small sieving particle size of 10 μm or less, so that aggregation of particles with each other is effectively suppressed.

Since the polylactic acid solid composition is produced by melt-kneading without using a liquid, there is no need to separate the liquid or to take out the product from the liquid. Therefore, the polylactic acid solid composition thus obtained can be used immediately.

Further, although the molecular weight of polylactic acid is low, it does not contain a very low molecular weight component that exhibits stickiness. Thereby, the polylactic acid solid composition can be broken into fine particles, which are very suitable for transport in bags and easy to use.

Examples

The present invention will be described by way of the following experimental examples.

< materials used >

High molecular weight polylactic acid:

the high molecular weight polylactic acid (PLA) used as the starting material was revolute 101(120,000< Mw <170,000) manufactured by Zheijiang Hisun Biomaterials co.

Sodium carbonate:

as the low molecular weight accelerator, sodium carbonate (purity: 99.8%) manufactured by Wako Pure Chemical Industries, Ltd. The average particle size was about 900 μm.

< measurement of molecular weight of PLA having a reduced molecular weight >

The weight average molecular weight (Mw) of the low molecular weight PLA contained in the solid composition was measured under the following conditions.

Equipment: HLC-8320, high speed GPC device manufactured by Tosoh Corporation

A detector: differential refractive index RI

Column: SuperMultipore HZ-M (2 pieces)

Solvent: chloroform

Flow rate: 0.5 mL/min

Column temperature: 40 deg.C

Sample preparation: 3mL of solvent was added to about 10mg of sample, which was then left at room temperature. After visual confirmation that the sample was dissolved in the solvent, it was filtered through a 0.45 μm filter. Polystyrene was used as a standard.

< measurement of sodium carbonate content >

The content of sodium carbonate in the obtained solid composition was measured by the following method.

Equipment: total organic carbon TOC-L manufactured by Shimadzu Corporation

Carrier gas: high purity oxygen

Carrier gas flow: 500 mL/min

Measurement items: IC (inorganic carbon)

Calibration material: sodium bicarbonate

Combustion temperature: 200 deg.C

< measurement of particle size of sodium carbonate contained >

Pellets of 1g of the obtained polylactic acid solid composition were dissolved in 50mL of chloroform. After visual confirmation of PLA dissolution and sodium carbonate precipitation, the supernatant was removed and repeated 3 times. Then, the average particle diameter (circle equivalent diameter) of the sodium carbonate was measured by SEM observation.

< example 1>

High molecular weight PLA and sodium carbonate were metered into a continuous twin-screw extruder through respective quantitative feeders so that the mass ratio (PLA: sodium carbonate) was 9:1, followed by melt-kneading at 220 ℃ for 3 minutes. Then, the melt-kneaded product was extruded to obtain pellets of the polylactic acid solid composition.

The weight average molecular weight (Mw) of the PLA contained in the pellet is measured by the method described previously. The weight average molecular weight (Mw) was 35,000.

The weight average molecular weight distribution curve is shown in fig. 1.

Further, the content of sodium carbonate particles in the pellets was 7.4 mass%. The sodium carbonate particles had an average particle size of 1.5 μm.

Fig. 2 shows an SEM photograph of sodium carbonate, and fig. 3 shows an SEM photograph of sodium carbonate as a reagent used as a raw material.

The pellets obtained were broken up by a jet mill maintained by MATERIS co. Average particle diameter D of the resulting pellets₅₀6.492 μm and a sieve particle size of 10 μm or less.

< comparative example 1>

High molecular weight PLA and sodium carbonate were metered into a continuous twin-screw extruder through respective quantitative feeders so that the mass ratio (PLA: sodium carbonate) was 9:1, followed by melt-kneading at 170 ℃ for 3 minutes. Then, the melt-kneaded product was extruded to obtain pellets of the polylactic acid solid composition. Thereafter, the pellets were subjected to hydrothermal treatment at 110 ℃ for 70 minutes by means of an autoclave, thereby obtaining a solid composition.

The weight average molecular weight of the polylactic acid contained in the solid composition was 42,000. The weight average molecular weight distribution curve is shown in fig. 4.

The solid composition was crushed by a jet mill in the same manner as in example 1. The resulting particles had a particle size of 125 μm or more and could not be broken finer.

< comparative example 2>

High molecular weight PLA and sodium carbonate (mass ratio: 9:1) were metered into a continuous twin-screw extruder through respective quantitative feeders in exactly the same manner as in comparative example 1, followed by melt-kneading at 170 ℃ for 3 minutes. Then, the melt-kneaded product was extruded to obtain pellets of the polylactic acid solid composition.

Subsequently, 200g of the pellets thus obtained were mixed with 400mL of 50% lactic acid and stirred at 90 ℃ for 4 hours.

The weight average molecular weight of the polylactic acid contained in the stirred mixture was 38,000. The weight average molecular weight distribution curve is shown in fig. 5.

The solid composition was crushed by a jet mill in the same manner as in example 1. Average particle diameter D of the resulting pellets₅₀The particle size of the nano-particles is 6.895 mu m,but the composition has low hydrolyzability.

The results obtained in example 1 and comparative examples 1 and 2 are shown in table 1.

Further, 120mg of the low molecular weight PLA obtained in each of example 1 and comparative examples 1 and 2 was weighed and immersed in 10mL of water, which was allowed to stand at 70 ℃ and 90 ℃. The low molecular weight PLA immersed in water was weighed every 3 days. The weight remaining rate was obtained by using the weight on day 0 as 100%, and a graph of the decomposition rate in water was obtained.

Fig. 6 shows a graph of the decomposition rate in water at 70 c, and fig. 7 shows a graph of the decomposition rate in water at 90 c.

As can be understood from fig. 6 and 7, example 1 is superior to comparative examples 1 and 2 in terms of hydrolyzability.

[ Table 1]