Method for producing polylactic acid film, and polylactic acid film

Abstract

This method for producing a polylactic acid film includes: an unstretched film production step for producing an unstretched film which is formed of a resin composition that contains at least one selected from a poly-DL-lactic acid, a poly-DL-lactic acid derivative, and a copolymer with a polylactic acid; an immersion step for immersing the unstretched film in water for a preset immersion time; and a stretching step for stretching the unstretched film in a first direction that is orthogonal to the thickness direction of the unstretched film after the immersion step. In the immersion step, the temperature of the water is 36°C to 38°C inclusive and the immersion time is 24 hours to 96 hours inclusive.

Description

Method for producing polylactic acid film and polylactic acid film

[0001] The present invention relates to a method for producing a polylactic acid film and a polylactic acid film.

[0002] A method for producing a polylactic acid film has been proposed in which an unstretched film made of a resin composition containing polylactic acid is produced by a melt extrusion method and the produced unstretched film is stretched in directions orthogonal to each other (see, for example, Patent Document 1).

[0003] Japanese Patent No. 7553890

[0004] By the way, since polylactic acid is derived from biomass raw materials and has biodegradability, development has been promoted as a substitute for conventional fossil raw materials. For films mainly composed of such polylactic acid, it has been demanded to improve their toughness.

[0005] [[ID=(15)]] The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing a polylactic acid film and a polylactic acid film that can improve toughness.

[0006] The method for producing a polylactic acid film according to the present invention includes: an unstretched film production step of producing an unstretched film formed from a resin composition containing at least one selected from poly-DL-lactic acid, a poly-DL-lactic acid derivative, and a copolymer with lactic acid; a dipping step of dipping the unstretched film for a preset dipping time in water; and a stretching step of stretching the unstretched film in a first direction orthogonal to the thickness direction of the unstretched film after the dipping step.

[0007] The polylactic acid film according to the present invention viewed from another aspect has a plurality of crazes with a length of 1 mm or less formed at least on the surface along at least one of a first direction orthogonal to the thickness direction, a thickness direction, and a second direction orthogonal to the first direction.

[0008] Note: In ID=15, there seems to be a numbering error in the original Chinese text. It should probably be ID=15 instead of (15). The translation is adjusted accordingly.According to the polylactic acid film manufacturing method of the present invention, in the immersion step, an unstretched film formed from a resin composition is immersed in water for a predetermined immersion time, and then, in the stretching step, the unstretched film is stretched in a first direction perpendicular to the thickness direction of the unstretched film. As a result, in the stretching step, a plurality of crazes with a length of 1 mm or less can be formed on at least the surface along at least one of the first and second directions, thereby increasing the toughness of the manufactured polylactic acid film. Furthermore, in the polylactic acid film according to the present invention, because the plurality of crazes described above are formed, the stress generated when the polylactic acid film is deformed is relieved by the plurality of crazes, thus increasing its toughness.

[0009] This is a flowchart showing the flow of the polylactic acid film manufacturing method according to the embodiment. (A) is a figure showing the results of tensile tests of polylactic acid films according to Comparative Examples 1 to 4, (B) is a figure showing the results of tensile tests of polylactic acid films according to Comparative Example 5 and Examples 1 to 3, (C) is a figure showing the results of tensile tests of polylactic acid films according to Comparative Example 6 and Examples 4 to 6, and (D) is a figure showing the results of tensile tests of polylactic acid films according to Comparative Example 7 and Examples 7 and 8. This is a figure showing the results of differential scanning calorimetry of polylactic acid films according to Comparative Example 5 and Examples 1 to 3. (A-1) is a figure showing the results of the WAXD measurement of the unstretched film according to Comparative Example 5, (A-2) is a figure showing the results of the WAXD measurement of the polylactic acid film according to Comparative Example 5, (B-1) is a figure showing the results of the WAXD measurement of the unstretched film according to Example 1, (B-2) is a figure showing the results of the WAXD measurement of the polylactic acid film according to Example 1, (C-1) is a figure showing the results of the WAXD measurement of the unstretched film according to Example 2, (C-2) is a figure showing the results of the WAXD measurement of the polylactic acid film according to Example 2, (D-1) is a figure showing the results of the WAXD measurement of the unstretched film according to Example 3, and (D-2) is a figure showing the results of the WAXD measurement of the polylactic acid film according to Example 3. (A) is a figure showing the results of the USAXS measurement of the polylactic acid film according to Comparative Example 5, (B) is a figure showing the results of the USAXS measurement of the polylactic acid film according to Example 1, (C) is a figure showing the results of the USAXS measurement of the polylactic acid film according to Example 2, and (D) is a figure showing the results of the USAXS measurement of the polylactic acid film according to Example 3. (A) is a figure showing the results of the USAXS measurement of the polylactic acid film according to Comparative Example 6, (B) is a figure showing the results of the USAXS measurement of the polylactic acid film according to Example 4, (C) is a figure showing the results of the USAXS measurement of the polylactic acid film according to Example 5, and (D) is a figure showing the results of the USAXS measurement of the polylactic acid film according to Example 6. This is an SEM image of the surface of the polylactic acid film according to Example 2. (A) is an SEM image of the surface of the polylactic acid film according to Example 2, (B) is an enlarged SEM image of a part of (A), (C) is an enlarged SEM image of a part of (B), and (D) is an enlarged SEM image of a different part of (B) than (C).

[0010] The following describes a polylactic acid film manufacturing method and a polylactic acid film according to an embodiment of the present invention. The polylactic acid film manufacturing method according to this embodiment includes, as shown in Figure 1, an unstretched film manufacturing step (S1) for producing an unstretched film formed from a resin composition containing poly-DL-lactic acid (hereinafter referred to as "PDLLA") and at least one selected from copolymers of PDLLA derivatives and various lactic acids; an immersion step (S2) for immersing the unstretched film in water for a predetermined immersion time; a drying step (S3) for drying the unstretched film after the immersion step; and a stretching step (S4) for stretching the unstretched film in one direction perpendicular to the thickness direction of the unstretched film after the drying step.

[0011] In the process of producing an unstretched film, first, pellets are prepared from a resin composition containing at least one selected from, for example, PDLLA, PDLLA derivatives, and copolymers with various lactic acids. Here, when the resin composition contains PDLLA or a PDLLA derivative, the ratio of either the L-form or D-form contained in the PDLLA or PDLLA derivative in the resin composition to the other is not particularly limited. However, it is preferable that the ratio of either the L-form or D-form contained in the PDLLA or PDLLA derivative in the resin composition to the other is greater than 0 and 1 / 9 or less. In this case, the elasticity of the polylactic acid film can be increased. It is also preferable that the ratio of the L-form to the D-form contained in the PDLLA or PDLLA derivative in the resin composition is greater than 0 and 1 / 9 or less. In this case, the affinity of the polylactic acid film to biological tissue, particularly the biological tissue of infants, can be increased. Examples of PDLLA derivatives include PDLLA derivatives having structures represented by the following formulas (1) to (4).

[0012] ...Formula (1) In formula (1), one of X1 to X5 is an aldehyde group, another is an alkoxy group, and the remaining three are hydrogen atoms. Also, n in formula (1) represents the number of repeating units.

[0013] ...Formula (2) In formula (2), one of R1 to R3 is an aldehyde group, an alkoxy group, an alkyl group, or an ester, and the remaining 2 are chlorine, fluorine, an aldehyde group, an alkoxy group, an alkyl group, an ester, or hydrogen. Also, n in formula (2) represents the number of repeating units.

[0014] ...Equation (3) In equation (3), n represents the number of repeating units.

[0015] ...Equation (4) In equation (4), n represents the number of repeating units.

[0016] Furthermore, examples of copolymers with various lactic acids include glycolic acid / L-lactic acid copolymer (PGLLA), glycolic acid / DL-lactic acid copolymer (PGDLLA), and LLA / ε-caprolactone copolymer. The resin composition may also contain various additives such as inorganic particles, heat-resistant polymer particles, crosslinked polymer particles, and other inert particles, as well as fluorescent whitening agents, UV inhibitors, infrared absorbing dyes, heat stabilizers, surfactants, and antioxidants.

[0017] Next, the pellets formed from the resin composition are molded into a film by pressing them while heating. Here, the resin composition is heated to a temperature (e.g., 200°C) above the melting point of at least one copolymer selected from PDLLA, PDLLA derivatives, and various lactic acids contained in the resin composition. After that, the molded product made of the resin composition is rapidly cooled with a liquid with a relatively large heat capacity (e.g., water). This produces an amorphous, unstretched film.

[0018] In the immersion process, the unstretched film is immersed in distilled water. The temperature of the distilled water into which the unstretched film is immersed (hereinafter referred to as the "immersion temperature") is below the crystallization temperature of at least one copolymer selected from PDLLA, PDLLA derivatives, and various lactic acids contained in the resin composition. The immersion time is set appropriately according to the immersion temperature. There is a correlation between the immersion temperature and the immersion time, and it is necessary to set it appropriately within an industrially reasonable range. For example, if the temperature of the distilled water is 36°C or higher and 38°C or lower, the immersion time is set to 24 hours or higher and 96 hours or lower. In the immersion process, by immersing unstretched films in distilled water in which the ratio of at least one of the L-form and D-form differs, a crystalline region can be formed substantially uniformly throughout the unstretched film. The proportion of the crystalline region in the unstretched film is less than 90%.

[0019] In the drying process, the unstretched film after the immersion process is dried under reduced pressure. Here, it is preferable to set the temperature to be below at least one glass transition temperature selected from copolymers of PDLLA, PDLLA derivatives, and various lactic acids contained in the resin composition, and to set the atmospheric pressure to 100 Pa or less.

[0020] In the stretching process, the unstretched film is stretched while being heated to a temperature above the glass transition temperature of at least one copolymer selected from the PDLLA and PDLLA derivatives contained in the resin composition and various lactic acids. The temperature in this stretching process (hereinafter referred to as the "stretching temperature") is set, for example, to 50°C or more and less than 170°C. The stretching ratio at this time can be any ratio, but is preferably 5 times or more. By performing this stretching process and plastically deforming the unstretched film, so-called strain hardening is generated. Strain hardening refers to the phenomenon in which, when an external force is applied to the unstretched film and it is plastically deformed, a change occurs in the crystal structure or molecular chain arrangement of the crystalline region of the unstretched film, resulting in a state where a larger external force is required to further deform it. In addition, the ratio of the L-form to the D-form of the PDLLA or PDLLA derivative contained in the resin composition of the unstretched film is different for at least one of them. This ratio is preferably greater than 0 and less than or equal to 1 / 9. In this case, the unstretched film becomes a so-called semi-crystalline film with crystalline and amorphous regions. In this case, the stretching process causes stress concentration at the boundary between the crystalline and amorphous regions in the film, resulting in the formation of crazes on the outer periphery of the crystalline regions. Furthermore, in this case, the stretching process causes oriented crystallization within the film, improving the elastic modulus of the film. On the other hand, for example, if the ratio of the L-form to the D-form in the PDLLA or PDLLA derivative contained in the aforementioned resin composition is approximately equal, the entire unstretched film becomes a so-called amorphous film. In the immersion process, the hydrophilic and hydrophobic parts of the PDLLA absorb water, and the unstretched film is thought to have a fine domain structure formed by local separation. In this case, the stretching process causes phase separation in this domain structure, generating fine crazes in the film. Furthermore, in this case, even if the stretching process is performed, the aforementioned oriented crystallization does not occur within the film, resulting in a lower elastic modulus compared to the aforementioned so-called semi-crystalline film.

[0021] The polylactic acid film according to this embodiment has a plurality of crazes with a length of 1 mm or less formed on its surface, at least along one of a first direction perpendicular to the thickness direction and a second direction perpendicular to both the thickness direction and the first direction. Furthermore, the crystallinity of the polylactic acid film is 90% or less. In addition, the polylactic acid film has a lip location portion formed between two adjacent crazes in the aforementioned second direction. This lip location portion is, for example, a curved portion that protrudes in the thickness direction of the polylactic acid film in the portion between two adjacent crazes.

[0022] As described above, according to the polylactic acid film manufacturing method of this embodiment, the unstretched film that has undergone the immersion step is immersed in water for a predetermined immersion time, and then, in the stretching step, the unstretched film is stretched in one direction perpendicular to the thickness direction of the unstretched film. As a result, in the stretching step, multiple crazes with a length of 1 mm or less can be formed on the surface in at least one of a first direction perpendicular to the thickness direction and a second direction perpendicular to both the thickness direction and the first direction, thereby increasing the toughness of the manufactured polylactic acid film. Furthermore, in the polylactic acid film of this embodiment, since multiple crazes with a length of 1 mm or less are formed on the surface in at least one of a first direction perpendicular to the thickness direction and a second direction perpendicular to both the thickness direction and the first direction, the stress generated when the polylactic acid film is deformed is more easily relieved by the multiple crazes, thus increasing its toughness.

[0023] Furthermore, the polylactic acid film according to this embodiment has multiple crazes, which are regions having fine voids or fibril-like structures. As a result, when an external force is applied to the polylactic acid film in the tensile direction, the stress generated within the polylactic acid film by the external force is locally dispersed in each of the multiple crazes, resulting in a so-called stress dispersion effect that delays the progression of fracture, thereby increasing the overall resistance of the polylactic acid film. This stress dispersion effect delays the fracture of the polylactic acid film when an external force is applied in the tensile direction. In addition, when an external force is applied to the polylactic acid film according to this embodiment, new crazes are formed. Therefore, as a portion of the energy applied to the polylactic acid film is consumed as energy to form the crazes, a large amount of energy can be absorbed before the polylactic acid film is fractured, resulting in an energy absorption effect. This increases the toughness of the film. Furthermore, multiple filamentous fine fibers are formed in each of the multiple crazes formed in the polylactic acid film according to this embodiment. Furthermore, when a tensile load is applied to the polylactic acid film, each of the multiple fine fibers shares the load applied to the polylactic acid film by the external force, thereby increasing the tensile strength of the polylactic acid film.

[0024] Furthermore, in the polylactic acid film according to this embodiment, a lip location portion is formed between two adjacent crazes. As a result, when tensile stress is applied to the polylactic acid film, this lip location portion functions as a stress absorption mechanism, thereby increasing the toughness of the polylactic acid film.

[0025] Although embodiments of the present invention have been described above, the present invention is not limited to the configuration of the embodiments described above. For example, in the unstretched film manufacturing process, the unstretched film may be manufactured by other methods such as the solution casting method or the melt extrusion method.

[0026] Although embodiments and modifications of the present invention have been described above, the present invention can be implemented in various forms and modifications without departing from the broad spirit and scope of the invention. Furthermore, the embodiments described above are for illustrative purposes only and do not limit the scope of the invention. In other words, the scope of the present invention is indicated not by the embodiments, but by the claims. Various modifications made within the scope of the claims and the equivalent scope of the meaning of the invention are considered to be within the scope of this invention.

[0027] This application is based on Japanese Patent Application No. 2024-191442, filed on 31 October 2024. The entire specification, claims, and drawings of Japanese Patent Application No. 2024-191442 are incorporated herein by reference.

[0028] The method for producing polylactic acid film and the polylactic acid film according to the present invention will be described based on examples. However, the present invention is not limited to the examples described below.

[0029] Samples according to Comparative Examples 1 to 7 and Examples 1 to 8 were prepared using the polylactic acid production method described in the embodiment. In the preparation of the samples according to Comparative Examples 1 to 4, first, pellets formed from poly-L-lactic acid with a weight-average molecular weight of 240,000 were prepared. The poly-L-lactic acid pellets were dried by leaving them in an environment of 0.1 kPa pressure and 60°C for 12 hours, and then an unstretched film with a thickness of 100 μm was produced by pressing it at a pressure of 10 MPa while heated to 200°C. The unstretched film was then rapidly cooled to -4°C.

[0030] Next, the unstretched films according to Comparative Examples 2 to 4 were immersed in distilled water. The temperature of the distilled water was maintained at 37°C. The unstretched film according to Comparative Example 1 was not immersed in distilled water. The immersion times for the unstretched films according to Comparative Examples 2 to 4 in distilled water were 24 hours, 48 ​​hours, and 96 hours, respectively. Subsequently, the unstretched films according to Comparative Examples 1 to 4 were subjected to so-called vacuum drying by being left in an environment of 0.1 kPa and a temperature of 25°C for 24 hours. After that, the unstretched films according to Comparative Examples 1 to 4 were stretched in one direction perpendicular to the thickness direction to obtain the samples according to Comparative Examples 1 to 4. Here, the stretching ratio was 7 times.

[0031] Furthermore, in the preparation of the samples for Comparative Examples 5 to 7 and Examples 1 to 8, first, pellets formed from poly-DL-lactic acid with a weight-average molecular weight of 240,000 were prepared. For Comparative Example 5 and Examples 1 to 3, poly-DL-lactic acid with a ratio of L-isomer to D-isomer of 94:6 was used. For Comparative Example 6 and Examples 4 to 6, poly-DL-lactic acid with a ratio of L-isomer to D-isomer of 90:10 was used. For Comparative Example 7 and Examples 7 and 8, poly-DL-lactic acid with a ratio of L-isomer to D-isomer of 50:50 was used. The poly-DL-lactic acid pellets were then dried by leaving them in an environment of 0.1 kPa pressure and 60°C for 12 hours, and then an unstretched film with a thickness of 100 μm was produced by pressing it at a pressure of 10 MPa while heated to 200°C. The unstretched film was then rapidly cooled to -4°C.

[0032] Next, the unstretched films according to Examples 1 to 8 were immersed in distilled water. The temperature of the distilled water was maintained at 37°C. The unstretched films according to Comparative Examples 5 to 7 were not immersed in distilled water. The immersion time in distilled water for the unstretched films according to Examples 1, 4, and 7 was 24 hours, for the unstretched films according to Examples 2, 5, and 8 it was 48 hours, and for the unstretched films according to Examples 3 and 6 it was 96 hours. Subsequently, the unstretched films according to Comparative Examples 1 to 7 and Examples 1 to 8 were subjected to so-called vacuum drying by being left in an environment with a pressure of 0.1 kPa and a temperature of 25°C for 24 hours. After that, the unstretched films according to Comparative Examples 1 to 7 and Examples 1 to 8 were stretched in one direction perpendicular to the thickness direction to obtain the samples according to Comparative Examples 1 to 7 and Examples 1 to 8. Here, the stretching ratio was 7 times. The preparation conditions for the samples according to Comparative Examples 1 to 7 and Examples 1 to 8 are summarized in Table 1.

[0033]

[0034] Next, the results of tensile tests performed on the samples related to Comparative Examples 1 to 7 and Examples 1 to 8 will be described. A tensile testing machine (STA-1150, manufactured by Orientec Co., Ltd.) was used for the tensile tests. As shown in Figures 2(B) to (D), it was found that the tensile strength of the samples related to Examples 1, 2, 4, 5, 7, and 8 was increased compared to the samples related to Comparative Examples 5, 6, and 7, respectively. In other words, it was found that the tensile strength of the samples immersed in water for 24 to 48 hours in the unstretched film state was increased compared to the samples that were not immersed in water in the unstretched film state. In particular, as shown in Figure 2(B), the maximum point stress of the sample of Example 2 was approximately 300 MPa. This was more than three times the maximum point stress of the samples related to Comparative Examples 1 to 4 shown in Figure 2(A). On the other hand, as shown in Figures 2(B) and (C), it was found that the maximum point stress of the samples of Examples 3 and 6 was lower compared to the samples of Examples 2 and 5, respectively. Furthermore, as shown in Figures 2(B) and (C), the elongation at the breaking point of the samples according to Examples 1 to 6 is higher than that of Comparative Examples 5 and 6, respectively. In other words, it was found that the samples immersed in water for 24 to 48 hours in their unstretched film state had a higher elongation at the breaking point after stretching compared to the samples that were not immersed in water in their unstretched film state.

[0035] Next, the results of differential scanning calorimetry (DSC3100SA) measurements performed on the samples of Comparative Example 4 and Examples 1 to 3 will be described. A differential scanning calorimetry meter (Bruker AXS, DSC3100SA) was used for differential scanning calorimetry. The temperature scanning speed in differential operating calorimetry was set to 10°C / min. As shown in Figure 3, the DSC (Differential Scanning Calorimetry) thermograms of the samples of Examples 1 to 3 showed a peak associated with crystallization around 140°C to 150°C, whereas no such peak was observed in the sample of Comparative Example 4. Furthermore, the degree of crystallinity of each sample, estimated from the area of ​​the peak portion in the DSC thermograms of each sample of Examples 1 to 3, was 22.0%, 24.4%, and 32.5%, respectively. From this, it was found that immersing the unstretched film in water promotes the crystallization of the unstretched film due to the plasticizing effect of water molecules. In this type of crystallization of unstretched film, the microcrystals generated by immersing the unstretched film in water serve as a starting point for the molecular chains of polylactic acid contained in the unstretched film to orient with a high degree of orientation, thereby increasing the strength of the unstretched film.

[0036] Next, we will describe the results of wide-angle X-ray diffraction (WAXD) measurements performed on the samples related to Comparative Example 4 and Examples 1 to 3. For these measurements, a wide-angle X-ray diffractometer (CN4037A1, manufactured by Rigaku Corporation) using CuKα rays (wavelength: 0.154956 nm) was used, and measurements were performed under the conditions of an applied voltage of 40 kV and an applied current of 15 mA. As shown in Figures 4 (A-1), (B-1), (C-1), and (D-1), in the unstretched film related to Comparative Example 1, a ring-shaped pattern caused by the polycrystalline portion with a random crystal axis direction could not be observed, but in the unstretched films related to Examples 1 to 3, a ring-shaped pattern caused by the polycrystalline portion was observed. This also shows that crystallization is promoted by immersing the unstretched film in water. As shown in Figures 4(A-2), (B-2), (C-2), and (D-2), spot patterns caused by highly oriented crystalline portions were observed in all samples after stretching treatment according to Comparative Example 1 and Examples 1 to 3.

[0037] Next, we will describe the results of measurements performed using the Ultra Small Angle X-ray Scattering (USAXS) method on the samples related to Comparative Examples 4 and 5 and Examples 1 to 6. The USAXS measurements were performed at the large synchrotron radiation facility Spring 8. As shown in Figures 5(A) and (B), no clear streak patterns were observed in the samples related to Comparative Example 5 and Example 1. On the other hand, as shown in Figures 5(C) and (D), relatively clear streak patterns were observed along the stretching direction indicated by the arrows in the samples related to Examples 2 and 3. Also, as shown in Figures 6(A) and (B), no clear streak patterns were observed in the samples related to Comparative Example 6 and Example 4. On the other hand, as shown in Figure 6(C), a faint streak pattern was observed along the stretching direction indicated by the arrow in the sample related to Example 5, and as shown in Figure 6(C), a relatively clear streak pattern was observed along the stretching direction indicated by the arrow in the sample related to Example 6. From this, it was found that in the samples of Examples 2, 3, 5, and 6, a periodic structure on the order of μm was formed along the stretching direction. It was also found that a periodic structure was formed in a direction perpendicular to the stretching direction.

[0038] Next, we will describe the results of observing the morphology of the surface of the sample according to Example 2. The morphology was observed using a scanning electron microscope (SEM) (Hitachi High-Technologies Corporation, S-3400N) by setting the electron acceleration voltage to 10 kV and acquiring SEM (scanning electron microscopy) images. As shown in Figures 7 and 8(A) to (D), multiple crazes extending along the stretching direction were observed on the surface of the sample according to Example 2. Fine crazes extending along a direction perpendicular to the stretching direction were also observed. It is thought that such crazes are formed when the unstretched film is immersed in water. Furthermore, the crazes formed near the surface of the sample are relatively narrow in width and are formed intermittently at equal intervals across the entire surface of the sample in a direction perpendicular to the stretching direction.

[0039] Furthermore, as shown in the dashed lines in Figures 8(C) and (D), lip location portions formed between two adjacent crazes in a direction perpendicular to the extension direction were also observed.

[0040] The present invention is suitable for manufacturing films for food packaging, medical devices requiring bioabsorbability and biodegradability, sutures, and other industrial applications.

Claims

1. A method for producing a polylactic acid film, comprising: an unstretched film production step of producing an unstretched film formed from a resin composition containing at least one selected from poly-DL-lactic acid, a poly-DL-lactic acid derivative, and a copolymer with lactic acid; an immersion step of immersing the unstretched film in water for a predetermined immersion time; and a stretching step of stretching the unstretched film in a first direction perpendicular to the thickness direction of the unstretched film after the immersion step.

2. The method for producing a polylactic acid film according to claim 1, wherein the water temperature in the immersion step is 36°C or higher and 38°C or lower, and the immersion time is 24 hours or higher and 96 hours or lower.

3. The method for producing a polylactic acid film according to claim 1 or 2, wherein in the stretching step, the unstretched film is stretched while being heated to a temperature at or above the glass transition temperature of at least one selected from poly-DL-lactic acid, poly-DL-lactic acid derivatives, and copolymers with lactic acid contained in the resin composition.

4. The stretching ratio in the stretching step is 5 times or more, the method for producing a polylactic acid film according to claim 3.

5. The method for producing a polylactic acid film according to claim 1 or 2, wherein in the unstretched film production step, pellets formed from the resin composition are pressed while being heated to a temperature above the melting point of at least one selected from poly-DL-lactic acid, poly-DL-lactic acid derivatives, and copolymers with lactic acid contained in the resin composition, and then the unstretched film is produced by cooling the molded product to a temperature of 0°C or lower.

6. A method for producing a polylactic acid film according to claim 1 or 2, further comprising a drying step of drying the unstretched film under reduced pressure after the immersion step and before the stretching step.

7. A polylactic acid film having a plurality of crazes, each 1 mm in length or less, formed on at least the surface along at least one of a first direction perpendicular to the thickness direction and a second direction perpendicular to both the thickness direction and the first direction.

8. The polylactic acid film according to claim 7, wherein the degree of crystallinity is 90% or less.

9. The polylactic acid film according to claim 7 or 8, having a lip location portion formed between two adjacent crazes in the second direction.

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