HK1139169A - Method for producing polylactic acid - Google Patents

Description

Method for preparing polylactic acid

Technical Field

The invention relates to a preparation method of polylactic acid. More particularly, it relates to a method for preparing polylactic acid containing stereocomplex crystals.

Background

Most of plastics derived from petroleum are light, tough, and excellent in durability, and can be easily and arbitrarily molded, so that they can be mass-produced and support our lives in many ways. However, these plastics are not easily decomposed and accumulated when discarded into the environment. In addition, a large amount of carbon dioxide is released during incineration, which increases global warming.

In view of the above-mentioned situation, there have been studies on a resin containing a material from which oil is removed or a biodegradable plastic which is biodegradable by microorganisms. Most of the biodegradable plastics under study at present have aliphatic carboxylic acid ester units and are easily decomposed by microorganisms. However, since they are poor in thermal stability, they are seriously deteriorated in molecular weight or hue in a molding step of exposure to high temperature such as melt spinning, injection molding, melt film formation, and the like.

Among them, polylactic acid is a plastic excellent in heat resistance and well-balanced in hue and mechanical strength, but has a problem that it has low heat resistance as compared with petrochemical polyesters represented by polyethylene terephthalate or polybutylene terephthalate, and cannot be ironed when made into a fabric, for example.

In order to solve the above-mentioned situation, various studies have been made to improve the heat resistance of polylactic acid. One of them is stereocomplex polylactic acid. The stereocomplex polylactic acid is polylactic acid containing stereocomplex crystals, and has a melting point 30 to 50 ℃ higher than that of a conventional polylactic acid containing homogeneous crystals (ホモ crystals).

However, stereocomplex crystals are not always present, especially in the high molecular weight region, and mostly appear as homogeneous crystals. In addition, even a polylactic acid containing only stereocomplex crystals may be mixed with homogeneous crystals when it is remelted and crystallized. In order to improve the above phenomenon, studies have been made on a crystal nucleating agent for growing only the stereocomplex crystal.

For example, patent document 1 states: poly-L lactic acid and poly-D lactic acid in chloroform/hexafluoro-2-propanol solution with a weight average molecular weight (hereinafter sometimes referred to as Mw) of about 12 ten thousand were mixed in the presence of an oxamide derivative, and the resulting mixture was measured by DSC and was shown to be polylactic acid containing only stereocomplex crystals.

Patent document 2 teaches that if an aromatic urea-based compound is used, polylactic acid containing only stereocomplex crystals can be obtained.

However, when a stereocomplex polylactic acid is produced by these methods, a large amount of a halogen-containing organic solvent is used, and therefore, it is necessary to perform a recovery treatment, and the environmental stress is significant. In order to avoid this problem, when preparing a stereocomplex polylactic acid by melt kneading, the oxamide derivative or the aromatic urea-based compound is a nitrogen-containing compound, and therefore, there is a problem that the molecular weight is low, and it is difficult to obtain a stereocomplex polylactic acid having an Mw of 15 ten thousand or more.

Patent document 3 discloses a method for producing a multiblock copolymer containing poly-L lactic acid and poly-D lactic acid having a short chain length with Mw of less than 10 ten thousand, and indicates that the copolymer is a polylactic acid containing only stereocomplex crystals, but reprecipitation is required for increasing the number of segments of the copolymer, and is not suitable for industrial production.

As described above, a method for producing polylactic acid having Mw exceeding 10 ten thousand and having only stereocomplex crystals grown even by repeating melting and crystallization has not yet appeared.

(patent document 1) Japanese patent laid-open No. 2005-255806

(patent document 2) Japanese patent laid-open No. 2005-187630

(patent document 3) Japanese patent laid-open publication No. 2002-356543

Disclosure of Invention

The object of the present invention is to provide: a polylactic acid having a high molecular weight with a weight average molecular weight (Mw) of more than 10 ten thousand and having only stereocomplex crystals grown even by repeated melting and crystallization.

The inventor finds that: when poly-L lactic acid and poly-D lactic acid are kneaded at a temperature equal to or higher than the melting point of both acids while shearing, the poly-L lactic acid crystallizes and becomes solid. It was also found that: the solid polylactic acid is melt-kneaded again, and the content of stereocomplex crystals is extremely high, and even if melting and crystallization are repeated, the content of stereocomplex crystals is not easily reduced, and the present invention has been completed.

That is, the present invention is a method for producing polylactic acid, comprising the steps of: (i) a step of mixing poly-L lactic acid and poly-D lactic acid at a temperature of 160-225 ℃ to crystallize them to obtain a solid; and (ii) a step of melt-kneading the obtained solid.

The invention also includes polylactic acid prepared by the method. The present invention further includes molded articles such as fibers and films containing the polylactic acid.

Best Mode for Carrying Out The Invention

The present invention is described in detail below.

(method for producing polylactic acid)

(Poly-L-lactic acid, Poly-D-lactic acid)

The poly-L-lactic acid (hereinafter, sometimes referred to as PLLA) and poly-D-lactic acid (hereinafter, sometimes referred to as PDLA) used in the present invention are substantially formed of an L-lactic acid unit (D-lactic acid unit) represented by the following formula.

PLLA contains preferably 90 to 100 mol%, more preferably 95 to 100 mol%, and still more preferably 98 to 100 mol% of L-lactic acid units. Other units are: d-lactic acid unit, and a unit other than lactic acid. The content of the D-lactic acid unit or units other than lactic acid is preferably 0 to 10 mol%, more preferably 0 to 5 mol%, and still more preferably 0 to 2 mol%.

PDLA contains D-lactic acid units in an amount of preferably 90 to 100 mol%, more preferably 95 to 100 mol%, and still more preferably 98 to 100 mol%. Other units are: an L-lactic acid unit, a unit other than lactic acid. The content of the L-lactic acid unit or units other than lactic acid is preferably 0 to 10 mol%, more preferably 0 to 5 mol%, and still more preferably 0 to 2 mol%.

Examples of the units other than lactic acid include units derived from dicarboxylic acids, polyols, hydroxycarboxylic acids, lactones having 2 or more functional groups capable of forming ester bonds, and units derived from various polyesters, various polyethers, and various polycarbonates containing these various components.

The dicarboxylic acids are: succinic acid, adipic acid, azelaic acid, sebacic acid, terephthalic acid, isophthalic acid, and the like. The polyhydric alcohols are: aliphatic polyhydric alcohols such as ethylene glycol, propylene glycol, butylene glycol, pentanediol, hexanediol, octanediol, glycerin, sorbitan, neopentyl glycol, diethylene glycol, triethylene glycol, polyethylene glycol, and polypropylene glycol; or aromatic polyols such as products obtained by adding ethylene oxide to bisphenol. The hydroxycarboxylic acids are: glycolic acid, hydroxybutyric acid, and the like. The lactone includes glycolide, epsilon-caprolactone, beta-lactide, delta-butyrolactone, beta-or gamma-butyrolactone, pivalolactone, delta-valerolactone, etc.

The weight average molecular weight of both PLLA and PDLA is preferably 10 to 50 ten thousand, more preferably 10 to 35 ten thousand.

PLLA and PDLA can be prepared according to known methods. For example, L-or D-lactide can be heated in the presence of a metal catalyst to cause ring-opening polymerization. It can also be prepared by crystallizing a low molecular weight polylactic acid containing a metal catalyst and then heating it under reduced pressure or under an inert gas stream to solid-phase polymerize it. It can also be produced by a direct polymerization method of dehydrating and condensing lactic acid in the presence/absence of an organic solvent. The polymerization reaction can be carried out in a known reaction vessel, and for example, a vertical reaction vessel equipped with a high-viscosity stirring paddle such as a propeller blade can be used alone or in parallel.

Here, the metal catalyst used is a compound containing at least one metal element selected from the group consisting of alkaline earth metals, rare earth metals, transition metals of the III th cycle, aluminum, germanium, tin and antimony. The alkaline earth metals include: magnesium, calcium, strontium, and the like. The rare earth elements include scandium, yttrium, lanthanum, cerium, and the like. Transition metals of the third period are: iron, cobalt, nickel, zinc and titanium. The tin compound may be tin octylate, tin chloride, alkoxy tin, ethoxy tin, or methoxy tin. The metal catalyst may be added to the composition in the form of a carboxylate, alkoxide, aryloxide or enolate of a beta-diketone of these metals, or the like. In view of polymerization activity and hue, tin octylate, titanium tetraisopropoxide and aluminum triisopropoxide are particularly preferable.

An alcohol may be further used as a polymerization initiator. The alcohol preferably does not inhibit the polymerization of the polylactic acid and is nonvolatile, and for example, decanol, dodecanol, tetradecanol, hexadecanol, octadecanol and the like can be preferably used.

In the solid-phase polymerization method, a lactic acid polyester having a relatively low molecular weight, which is obtained by the above ring-opening polymerization method or direct polymerization method of lactic acid, is used as a prepolymer. It is preferable to crystallize the prepolymer in advance in a temperature range of not lower than the glass transition temperature (Tg) and lower than the melting point (Tm) from the viewpoint of preventing melt adhesion. The crystallized prepolymer is charged into a fixed vertical reaction vessel or a reaction vessel such as a drum or kiln which is rotatable, and heated to a temperature range of not lower than the glass transition temperature (Tg) of the prepolymer but lower than the melting point (Tm). The polymerization temperature may be increased in stages as the polymerization proceeds. In order to efficiently remove water produced in the solid-phase polymerization, it is preferable to use a method of reducing the pressure inside the reaction vessels or introducing heated inert gas.

In the present invention, it is preferable to remove the lactide contained in excess in the raw material PLLA and PDLA. The removal of the excess lactide can be carried out by, for example, reducing the pressure in the reaction system or washing with an organic solvent, and is preferably carried out by reducing the pressure in the reaction system from the viewpoint of the easiness of the operation.

In the present invention, it is also preferable to reduce or remove the catalyst component in the raw material PLLA and PDLA. As a method for reducing and removing the catalyst component, a method of contacting with an acidic washing solution and washing and removing the catalyst component may be employed in addition to previously reducing the amount of the polymerization catalyst to be added. For example, an aqueous acetone solution containing hydrochloric acid can be used as the washing solution. For example, when a tin catalyst is used, the amount of the catalyst component after the reduction is preferably less than 1 ppm.

Therefore, washed poly-L-lactic acid and poly-D-lactic acid are preferably used in the present invention.

PLLA and PDLA used in the present invention preferably contain a component which deactivates the catalyst, because this can stabilize the performance and quality of the resin. As the deactivator, a phosphoric acid-based compound, a phosphorous acid-based compound, a hypophosphorous acid-based compound, a condensed phosphoric acid, a condensed phosphorous acid-based compound, an organic chelate compound, an alkylsulfonate, or the like can be preferably used.

PLLA or PDLA may be used in the form of chips, powder, flakes or blocks, or may be directly supplied to the subsequent step in a molten state. In this case, it is preferable that the moisture content of PLLA or PDLA is as low as possible because the decrease in molecular weight during processing can be reduced. The water content is preferably 200ppm or less, more preferably 100ppm or less.

(step 1)

The production method of the present invention comprises a step (i) (hereinafter, sometimes referred to as step 1) of kneading PLLA and PDLA while shearing them; and (ii) kneading and melting the obtained crystallized solid (hereinafter, sometimes referred to as "step 2").

Step 1 is a step of kneading PLLA and PDLA at a predetermined temperature and crystallizing them to obtain a solid. The kneading temperature is a temperature at which PLLA and PDLA melt while crystals are generated in kneading. Therefore, the mixing temperature is 160-.

The kneading is preferably carried out under conditions in which shear is applied to the resin component. The kneading may be carried out by a twin-screw extruder, various kneaders, or the like. PLLA and PDLA are in a molten state at the initial stage of kneading, and a high-melting-point stereocomplex crystal is formed by shearing by kneading to become a powdery solid.

The thus obtained solid intermediate mixture may be processed into a powder, a flake, a pulverized chip, or the like, or may be directly supplied to step 2 in the obtained solid state.

(step 2)

Step 2 is a step of melt-kneading the obtained solid again. In step 2, the intermediate mixture obtained in step 1 is melt-kneaded to obtain polylactic acid having a high stereocomplex crystal content.

The melt-kneading may be carried out by a single-screw extruder, a twin-screw extruder, various kneaders, a reactor with a stirring paddle, a horizontal reactor, or the like. The temperature for melt kneading is a temperature higher than the melting point of the intermediate mixture, preferably a temperature higher by 5 ℃ or more, more preferably a temperature higher by 10 to 30 ℃. That is, the mixing temperature is preferably 230-270 ℃ and more preferably 235-250 ℃. The melt-kneading is preferably carried out under an inert atmosphere or under reduced pressure. In the above atmosphere, deterioration of the resin can be suppressed and the quality can be stabilized.

The melt-kneading is preferably carried out in the presence of a transesterification catalyst. The ester exchange catalyst may, for example, be an alkali metal compound, an alkaline earth metal compound, a tin compound, a zinc compound or a titanium compound.

The alkali metal compound includes lithium compounds, sodium compounds and potassium compounds. The alkaline earth metal compound includes a magnesium compound and a calcium compound. The tin compound may be tin octylate, tin chloride, alkoxy tin, ethoxy tin, methoxy tin, or tin oxide. Among them, calcium compounds, particularly calcium carbonate, are preferable.

In the present invention, the metal catalyst such as tin compound used in the preparation of PLLA and PDLA and the transesterification catalyst are repeated. Therefore, in the present invention, the transesterification catalyst is preferably an alkaline earth metal compound, a tin compound, or a mixture thereof.

These transesterification catalysts are preferably used as finely divided as possible. Particularly, when PLLA or PDLA containing a deactivator is used as a raw material, it is very preferable to allow a transesterification catalyst to coexist. The amount of the transesterification catalyst is preferably 0.000001 to 0.005 part by weight, more preferably 0.00005 to 0.001 part by weight, relative to 100 parts by weight of PLLA and PDLA in total.

(polylactic acid)

The weight average molecular weight (Mw) of the polylactic acid obtained by the production method of the present invention is preferably 10 ten thousand or more and less than 30 ten thousand, more preferably 18 ten thousand or more and less than 25 ten thousand. The Mw of PLLA or PDLA contained in polylactic acid is preferably 10 to less than 50 ten thousand, more preferably 12 to less than 25 ten thousand. When Mw of the polylactic acid is less than the above range, mechanical strength of the polylactic acid is insufficient, and when it is more than the above range, melt viscosity is extremely increased, and the processing step of molding or melt spinning is difficult. The weight average molecular weight (Mw) is a value of the weight average molecular weight in terms of standard polystyrene, which is measured by Gel Permeation Chromatography (GPC) using chloroform on an eluate.

The polylactic acid obtained by the present invention forms a stereocomplex crystal. The stereocomplex crystal content (Rs) of the polylactic acid represented by the following formula is preferably 95 to 100%, more preferably 98 to 100%, still more preferably 99 to 100%, and particularly preferably 100%.

Rs＝{ΔHb/(ΔHa+ΔHb)}×100(％)

In the formula, Δ Ha and Δ Hb represent the enthalpy of fusion (Δ Ha) of the crystal melting point appearing at 150 ℃ or higher and lower than 190 ℃ and the enthalpy of fusion (Δ Hb) of the crystal melting point appearing at 190 ℃ or higher and lower than 250 ℃ during the temperature rise in the Differential Scanning Calorimeter (DSC) measurement of the sample. Here, as the sample, a sample that is increased in temperature from room temperature to the melting point of the stereocomplex crystal or higher and then quenched to be amorphized is used. The temperature increase rate was 20 ℃ per minute.

The melting point of the polylactic acid obtained by the present invention is preferably 190-250 ℃, more preferably 200-220 ℃. The enthalpy of fusion (. DELTA.Ha) of the crystal melting point at 150 ℃ or higher but lower than 190 ℃ is preferably 0 to 10J/g, more preferably 0 to 5J/g, and still more preferably 0 to 2.5J/g. The enthalpy of fusion (Δ Hb) of the crystal melting point appearing at 190 ℃ or higher and less than 250 ℃ is preferably 20J/g or higher, more preferably 30J/g or higher, and still more preferably 40J/g or higher.

In order to impart excellent heat resistance to polylactic acid, the stereocomplex crystal content, the crystal melting point, and the enthalpy of fusion are preferably within the above numerical ranges.

The molar ratio (L/D) of the L lactic acid unit and the D lactic acid unit in the polylactic acid is preferably 30/70-70/30, more preferably 40/60-60/40. When the L/D is less than the above range or has an optical purity of not less than the above range, the crystallinity of the polylactic acid is significantly reduced, which is not preferable.

In DSC measurement, the melting point of the crystal observed in the heating process is 190 ℃ or higher but less than 250 ℃ even if the procedure comprising the heating process of 20-250 ℃ and the cooling process of 250-20 ℃ is repeated 3 times or more. That is, the stereocomplex crystal grows even if melting and crystallization are repeated.

The polylactic acid obtained according to the present invention may contain, if necessary, conventional additives such as plasticizers, antioxidants, light stabilizers, ultraviolet absorbers, heat stabilizers, lubricants, mold release agents, various fillers, antistatic agents, flame retardants, foaming agents, fillers, antibacterial and antifungal agents, nucleating agents, dyes, pigment-containing colorants, and the like, and compatibilizers, within a range not to impair the object of the present invention.

The polylactic acid obtained by the present invention can be used to obtain injection molded articles, extrusion molded articles, vacuum-pressure molded articles, blow molded articles, films, sheet nonwoven fabrics, fibers, fabrics, composites with other materials, agricultural materials, fishery materials, civil engineering and construction materials, stationery, medical supplies, or other molded articles. The molding may be carried out by a conventional method.

Examples

The present invention will be described in further detail with reference to examples. The present invention is not limited to the following examples. In the examples, the physical properties of the compositions were measured by the following methods.

(1) Weight average molecular weight (Mw)

Weight average molecular weight (Mw) 50mg of the sample was dissolved in 5mL of chloroform using シヨ - デツクス GmbH GPC-11, and developed with chloroform at 40 ℃. The weight average molecular weight (Mw) is calculated as a polystyrene equivalent.

(2) Three replicate determination of DSC

5mg of the test piece was put into a special aluminum pan and measured by a differential scanning calorimeter (DSC2920) manufactured by TA インスツルメンツ. The measurement conditions are as follows, and the crystal melting enthalpy is calculated from the area of the region surrounded by the crystal melting peak and the base line appearing in the DSC chart.

(a) Heating to 20-250 deg.c at 20 deg.c/min,

(b) cooling to 250 deg.C with dry ice to 20 deg.C,

(c) the above (a) and (b) were repeated 3 times in total.

(3) Crystal melting point (Tm) and stereocomplex crystal content (Rs)

The crystal melting point (Tm) was determined by DSC measurement of a sample. The stereocomplex crystal content (Rs) is obtained by DSC measurement of a sample and is calculated from the enthalpy of crystal fusion DeltaHa occurring at 150 ℃ or higher and lower than 190 ℃ and the enthalpy of crystal fusion DeltaHb occurring at 190 ℃ or higher and lower than 250 ℃ in accordance with the following formula. Here, as the sample, a sample which is heated from room temperature to a temperature not lower than the melting point of the stereocomplex crystal and then quenched to be amorphized is used. The temperature increase rate was 20 ℃ per minute.

Rs＝{ΔHb/(ΔHa+ΔHb)}×100(％)

(4) Measurement of the ratio of L-lactic acid Unit to D-lactic acid Unit (L/D)

L/D was determined by using the optical rotation [ α ] measured at 25 ℃ in 95/5(v/v) solution of chloroform/hexafluoro-2-propanol, and was determined by the following formula.

L/D＝([α]/320+0.5)/(0.5+[α]/(-320))

[ wherein 320 is the optical rotation of pure L-lactic acid and-320 is the optical rotation of pure D-lactic acid ]

(5) Tensile strength, modulus of elasticity, elongation

The mechanical properties of the fibers were measured by an テンシロン tensile tester (RTC-1225A) manufactured by A & D under the conditions of a gauge pitch of 20cm and a tensile speed of 100 cm/min.

Preparation example 1 preparation of PLLA-1

The amount of the lactide to be added was determined based on 100 parts by weight of L-lactide (prepared by chemical research in Wucang wild, K.K.,optical purity 100%), 0.005 part by weight of tin octylate was added, and the mixture was reacted at 180 ℃ for 2 hours in a reactor equipped with a stirring paddle under nitrogen atmosphere, then the residual lactide was removed under reduced pressure, and crushed to give PLLA-1. The weight-average molecular weight (Mw) of the obtained PLLA-1 was 19.7X 10⁴The glass transition temperature (Tg) was 63 ℃ and the melting point was 180 ℃.

Preparation example 2 preparation of PDLA-2

0.005 part by weight of tin octylate was added to 100 parts by weight of D-lactide (optical purity 100% prepared by Kyowa chemical Co., Ltd.) to react at 180 ℃ for 2 hours in a reactor equipped with a stirring paddle under a nitrogen atmosphere, and then residual lactide was removed under reduced pressure to obtain PDLA-2. The weight-average molecular weight of the obtained PDLA-2 was 17.7X 10⁴The glass transition temperature (Tg) was 63 ℃ and the melting point was 180 ℃.

Preparation example 3 preparation of PLLA-3

To 100 parts by weight of L-lactide (prepared by Kyowa Kagaku K.K., having an optical purity of 100%) was added 0.005 part by weight of tin octylate, and the mixture was reacted at 180 ℃ for 2 hours in a reactor equipped with a stirring paddle under a nitrogen atmosphere, and then 0.005 part by weight of phosphorous acid was added (to remove water), and then the remaining lactide was removed under reduced pressure to obtain PLLA-3. The weight-average molecular weight (Mw) of the obtained PLLA-3 was 14X 10⁴The glass transition temperature (Tg) was 63 ℃ and the melting point was 180 ℃.

Preparation example 4 preparation of PDLA-4

To 100 parts by weight of D lactide (prepared by Kyowa Kagaku K.K., having an optical purity of 100%) was added 0.005 part by weight of tin octylate, and the mixture was reacted at 180 ℃ for 2 hours in a reactor equipped with a stirring paddle under a nitrogen atmosphere, and then 0.005 part by weight of phosphorous acid was added (to remove water), and then the remaining lactide was removed under reduced pressure to obtain PDLA-4. The weight-average molecular weight of the obtained PDLA-4 was 14.5X 10⁴The glass transition temperature (Tg) was 63 ℃ and the melting point was 180 ℃.

Example 1

(washing)

Washing of the fractions of PLLA-1 and PDLA-2 obtained in preparation examples 1 and 2 was carried out as follows. 200mL of an acetone solution containing 7% by weight of 3N hydrochloric acid was added to 100g of each tablet, and the mixture was stirred for 1 hour, and then the chips were collected. Then washed 3 times with acetone. This operation was repeated 2 times. Immediately before melt kneading, PLLA-1 and PDLA-2 were dried under vacuum at 80 ℃ for 2 hours and 130 ℃ for 12 hours.

(step 1)

PLLA-1 and PDLA-2 in the form of chips were mixed at a weight ratio of 1: 1 to prepare a kneading sample. Kneading was performed using a small-sized kneading extruder (PPK) manufactured by hogel fabrication. The melting temperature was 190 ℃. The kneading sample was charged at a revolution of 120rpm, and the residence time was about 10 seconds, whereby kneading and extrusion were carried out. The kneaded resin was solidified to obtain white powder.

(step 2)

Subsequently, the obtained powder was melt-kneaded at 250 ℃ using a small single-screw extruder (Φ 10mm, L/D10, 12 rpm). The residence time was 5 minutes. The properties of the obtained resin are shown in tables 1 and 2.

Example 2

(step 1)

The pieces of PLLA-3 and PDLA-4 obtained in preparation examples 3 and 4 were mixed at a ratio of 1: 1 (by weight) to prepare a kneading sample. Melt kneading was performed using a small kneading extruder (PKK) manufactured by wellmaker. The melting temperature was 190 ℃. The kneading sample was charged at a revolution of 120rpm, and the residence time was about 10 seconds, whereby kneading and extrusion were carried out. The kneaded resin was solidified to obtain white powder.

(step 2)

Then, 0.005 wt% of calcium carbonate (obtained by pulverizing the obtained powder in methanol using a wet ball mill) was added to the obtained powder, and the mixture was extruded at 250 ℃ using a small single-screw extruder (Φ 10mm, L/D10, rotation speed 12 rpm). The residence time was about 5 minutes, adding the previous powder. The properties of the obtained resin are shown in tables 1 and 2.

Example 3 fibers

The resin obtained in example 1 was extruded at a nozzle temperature of 170 ℃ by a small spinning device to obtain an undrawn yarn. Subsequently, the resultant was stretched 4-fold by a batch stretcher at 80 ℃ and then heat-set at 140 ℃. The physical properties of the obtained fiber are shown in table 3.

Example 4 film

The resin obtained in example 1 was pressed at 220 ℃ in a molding press to obtain a film-shaped molded article. The DSC of the resulting molded article is shown in Table 4.

Comparative example 1 (step 2 only)

(washing)

Washing of the fragments of PLLA-1 and PDLA-2 was performed as follows. 200mL of an acetone solution containing 7% by weight of 3N hydrochloric acid was added to 100g of each tablet, and the mixture was stirred for 1 hour, and then the chips were collected. Then washed 3 times with acetone. This operation was repeated 2 times. Immediately before melt kneading, PLLA-1 and PDLA-2 were vacuum-dried at 80 ℃ for 2 hours and 130 ℃ for 12 hours, respectively.

(step 2)

PLLA-1 and PDLA-2 in the form of pellets were mixed at a weight ratio of 1: 1 to prepare a kneading sample. Melt kneading was carried out at 250 ℃ using a small single-screw extruder (. PHI.10 mm, L/D. 10, 12 rpm). The residence time was approximately 5 minutes. The properties of the obtained resin are shown in tables 1 and 2.

Comparative example 2 (step 1 only)

(washing)

Washing of the fragments of PLLA-3 and PDLA-4 was performed as follows. 200mL of an acetone solution containing 7% by weight of 3N hydrochloric acid was added to 100g of each tablet, and the mixture was stirred for 1 hour, and then the chips were collected. Then washed 3 times with acetone. This operation was repeated 2 times. Immediately before melt kneading, PLLA-3 and PDLA-4 were vacuum-dried at 80 ℃ for 2 hours and 130 ℃ for 12 hours, respectively.

(step 1)

PLLA-3 and PDLA-4 in the form of pellets were mixed at a weight ratio of 1: 1 to prepare a kneading sample. The kneading was carried out using a small kneading extruder (PKK) manufactured by Yuan Ministry of China. The melting temperature was 190 ℃. The kneading sample was added at 120rpm, and kneading and extrusion were carried out for a residence time of about 10 seconds. The kneaded resin was solidified to obtain white powder. The properties of the obtained resin are shown in tables 1 and 2.

TABLE 1

	Tm(℃)	Rs(％)	Tm (. degree.C.) in DSC measurement 3 time	Rs (%) at 3 rd DSC measurement
					Example 1	215	100	210	100
Example 2	220	100	215	100
					Comparative example 1	233.5	100	222	25
Comparative example 2	223	41	220	55

Rs: crystal content of stereocomplex

Tm: melting Point

TABLE 2

	Mw of PLLA	Mw of PDLA	Mw of the resin
				Example 1	20 ten thousand	18 ten thousand	16 ten thousand
Example 2	14 ten thousand	14.5 ten thousand	12 ten thousand
				Comparative example 1	20 ten thousand	18 ten thousand	18 ten thousand
Comparative example 2	14 ten thousand	14.5 ten thousand	12 ten thousand

TABLE 3

	Tm(℃)	Rs(％)	Tensile Strength (MPa)	Tensile modulus of elasticity (GPa)	Elongation (%)
						Example 3	210	100	580	5.8	25

Rs: crystal content of stereocomplex

TABLE 4

	Tm(℃)	Rs(％)	Appearance of the product
				Example 4	213	100	Semi-transparent

Rs: crystal content of stereocomplex

Example 5

(washing)

Washing of the fractions of PLLA-1 and PDLA-2 obtained in preparation examples 1 and 2 was carried out as follows. 200mL of an acetone solution containing 7% by weight of 3N hydrochloric acid was added to 100g of each tablet, and the mixture was stirred for 1 hour, and then the chips were collected. Then washed 3 times with acetone. This operation was repeated 2 times. Immediately before melt kneading, PLLA-1 and PDLA-2 were dried under vacuum at 80 ℃ for 2 hours and 130 ℃ for 12 hours.

(step 1)

PLLA-1 and PDLA-2 in the form of chips were mixed at a weight ratio of 1: 1 to prepare a kneading sample. Kneading was performed using a small-sized kneading extruder (PPK) manufactured by hogel fabrication. The melting temperature was 190 ℃. The kneading sample was added at 120rpm, and kneading and extrusion were carried out for a residence time of about 10 seconds. The kneaded resin was solidified to obtain white powder.

(step 2)

Then, 0.005 wt% of calcium carbonate (obtained by pulverizing the obtained powder in methanol using a wet ball mill) was added to the obtained powder, and the mixture was extruded at 250 ℃ using a small single-screw extruder (Φ 10mm, L/D10, rotation speed 12 rpm). The residence time was about 5 minutes, adding the previous powder. The properties of the obtained resin are shown in tables 5 and 6.

Example 6

(step 1)

The pieces of PLLA-3 and PDLA-4 obtained in preparation examples 3 and 4 were mixed at a ratio of 1: 1 (by weight) to prepare a kneading sample. Melt kneading was performed using a small kneading extruder (PKK) manufactured by wellmaker. The melting temperature was 190 ℃. The kneading sample was added at 120rpm, and kneading and extrusion were carried out for a residence time of about 10 seconds. The kneaded resin was solidified to obtain white powder.

(step 2)

Subsequently, the obtained powder was melt-kneaded at 250 ℃ using a small-sized single-screw extruder (Φ 10mm, L/D10, 12 rpm). The residence time was about 5 minutes. The properties of the obtained resin are shown in tables 5 and 6.

TABLE 5

	Tm(℃)	Rs(％)	Tm (. degree.C.) in DSC measurement 3 time	Rs (%) at 3 rd DSC measurement
					Example 5	215	100	210	100
Example 6	219	100	210	100

Rs: crystal content of stereocomplex

Tm: melting Point

TABLE 6

	Mw of PLLA	Mw of PDLA	Mw of the resin
				Example 5	18 ten thousand	18 ten thousand	12 ten thousand
Example 6	14 ten thousand	14.5 ten thousand	11 ten thousand

According to the production method of the present invention, a polylactic acid having an Mw of more than 10 ten thousand and in which stereocomplex crystals grow even when melting and crystallization are repeated can be obtained.

Industrial applicability

The polylactic acid obtained by the present invention is excellent in heat resistance, and therefore can be melt-molded to produce various molded articles such as fibers and films.

Claims (12)

1. A method for preparing polylactic acid, comprising the steps of: (i) mixing poly-L lactic acid and poly-D lactic acid at the temperature of 160-225 ℃ to crystallize the poly-L lactic acid and the poly-D lactic acid to obtain a solid; and (ii) a step of melt-kneading the obtained solid.

2. The method of claim 1, wherein the method uses washed poly-L-lactic acid and poly-D-lactic acid.

3. The process according to claim 1 or 2, which comprises melt-kneading in the step (ii) in the presence of a transesterification catalyst.

4. The process according to claim 3, wherein the transesterification catalyst is an alkaline earth metal compound, a tin compound or a mixture thereof.

5. The process according to claim 4, wherein the alkaline earth metal compound is a calcium compound.

6. Polylactic acid produced by the method of any one of claims 1 to 5.

7. The polylactic acid according to claim 6, wherein the stereocomplex crystal content Rs represented by the following formula is 95 to 100%:

Rs＝{ΔHb/(ΔHa+ΔHb)}×100(％)

wherein Δ Ha and Δ Hb represent melting enthalpies of crystal melting points appearing at 150 ℃ or higher and lower than 190 ℃ and crystal melting points appearing at 190 ℃ or higher and lower than 250 ℃ during temperature rise in Differential Scanning Calorimeter (DSC) measurement of a sample; as the sample, a sample which is heated from room temperature to a temperature higher than the melting point of the stereocomplex crystal and then quenched to be amorphized; the temperature increase rate was 20 ℃ per minute.

8. The polylactic acid according to claim 6, wherein the weight average molecular weight Mw is 10 to less than 30 ten thousand.

9. The polylactic acid according to claim 6, wherein the molar ratio L/D of the L lactic acid unit to the D lactic acid unit is from 30/70 to 70/30.

10. The polylactic acid according to claim 6, wherein the melting point of the crystals observed in the DSC during the temperature rise is 190 ℃ or higher but less than 250 ℃ even when the DSC is repeated 3 times or more with the temperature rise of 20 to 250 ℃ and the cooling of 250 to 20 ℃.

11. A molded article comprising the polylactic acid according to any one of claims 6 to 10.

12. The molded article of claim 11, wherein the molded article is a fiber or a film.