Biaxially stretched film and protective film

Abstract

Provided are: a biaxially stretched film which is a film containing polylactic acid but has both punching workability and heat resistance; and a protective film containing the biaxially stretched film. The biaxially stretched film according to the present invention contains polylactic acid, has a film tear strength of 50 N / mm or more and less than 100 N / mm as measured in accordance with JIS-K7128-3 when torn in at least one direction, and has a heated dimensional change rate of + / -5% or less in both the MD direction and the TD direction in a 100 DEG C atmosphere. And the thermal load dimensional change rate in the MD direction in a 100 DEG C atmosphere is within + / -7%.

Description

Technical Field

[0001] The present invention relates to a biaxially stretched film and a protective film. Background Art

[0002] Resin films have excellent properties such as moisture resistance, water resistance, and oil resistance, and have good mechanical strength. Therefore, they are widely used in various applications such as packaging materials for food and medicines, protective films for displays, etc., and are functional materials with high utilization value.

[0003] On the other hand, in recent years, various issues have been raised, such as the increase in the amount of plastic material waste, and global warming caused by carbon dioxide generated when incinerating plastic materials. Therefore, biodegradable biomass plastics have attracted much attention from the perspective of the global environment and human body, and the development of various materials combining existing plastic materials and biomass plastics is being actively promoted.

[0004] Polylactic acid is a representative known biomass plastic, but its mechanical properties such as impact resistance and tear resistance, or heat resistance are inferior to those of existing plastics. Therefore, in order to solve this problem, it is usually carried out to compensate for the shortcomings of polylactic acid by combining polylactic acid with other components (for example, refer to Patent Document 1).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent No. 4699180 Summary of the invention

[0008] Problem that the invention aims to solve

[0009] Although various methods for eliminating the disadvantages of polylactic acid have been proposed as described above, there is still room for improvement in biaxially stretched films containing polylactic acid. In particular, biaxially stretched films that have mechanical properties, heat resistance, and processability, although containing a biodegradable resin such as polylactic acid, can be used for various purposes, and therefore, the development of such biaxially stretched films is an urgent matter.

[0010] The present invention has been made in view of the above, and an object of the present invention is to provide a biaxially stretched film having both punching processability and heat resistance, although being a film composed of polylactic acid, and a protective film composed of the biaxially stretched film.

[0011] Solutions for solving problems

[0012] The present inventors have repeatedly conducted in-depth studies to achieve the above-mentioned object, and as a result, have found that the above-mentioned object can be achieved by forming the film from polylactic acid resin and setting the tear strength to a specific range, thereby completing the present invention. In particular, the present inventors have successfully found for the first time that punching workability depends on tear strength, thereby completing the present invention.

[0013] That is, the present invention includes, for example, the subject matters described in the following items.

[0014] Item 1

[0015] A biaxially stretched film,

[0016] It contains polylactic acid,

[0017] The biaxially stretched film has a tear strength of 50 N / mm or more and less than 100 N / mm when the biaxially stretched film is torn in at least one direction, as measured in accordance with JIS-K7128-3.

[0018] The heating dimensional change rate of the biaxially stretched film in an atmosphere at 100° C. is within ±5% in both the MD direction and the TD direction, and the heating load dimensional change rate in the MD direction in an atmosphere at 100° C. is within ±7%.

[0019] Item 2

[0020] The biaxially stretched film according to Item 1, wherein the at least one direction is an MD direction or a TD direction.

[0021] Item 3

[0022] A protective film comprising the biaxially stretched film according to item 1 or 2.

[0023] Effects of the Invention

[0024] The biaxially stretched film of the present invention is a film composed of polylactic acid, but has both punching processability and heat resistance. DETAILED DESCRIPTION

[0025] Hereinafter, the embodiments of the present invention will be described in detail. It should be noted that in this specification, the expressions "contain" and "comprise" include the concepts of "contain", "include", "consist essentially of", and "consist only of".

[0026] 1. Biaxially stretched film

[0027] The biaxially stretched film of the present invention contains polylactic acid, and the film tear strength of the biaxially stretched film measured in accordance with JIS-K7128-3 when the biaxially stretched film is torn in at least one direction is 50N / mm or more and less than 100N / mm, the heating dimensional change rate of the biaxially stretched film in an atmosphere of 100°C is within ±5% in both the MD direction and the TD direction, and the heating load dimensional change rate in the MD direction in an atmosphere of 100°C is within ±7%.

[0028] The biaxially stretched film of the present invention has moderate tear strength, good punching processability, and excellent heat resistance, and therefore has high dimensional stability, although it is a film containing polylactic acid.

[0029] In the biaxially stretched film of the present invention, the type of polylactic acid is not particularly limited, and for example, known polylactic acids can be widely used, for example, known polylactic acids such as polylactic acids obtained by polycondensation of lactic acid components as raw material monomers can be widely used. It should be noted that the polylactic acid can contain only any optical isomer of L-lactic acid (L-form) and D-lactic acid (D-form), or can contain both.

[0030] The melting point of polylactic acid is preferably, for example, 110° C. to 200° C. In this case, the heat resistance of the obtained biaxially stretched film is not easily reduced, and the adjustment of the stretch ratio is also easy to become simple. The melting point of polylactic acid is preferably 120° C. to 190° C., more preferably 130° C. to 185° C., and further preferably 140° C. to 180° C.

[0031] The glass transition temperature of the polylactic acid is not particularly limited, and can be, for example, -40°C to 70°C, more preferably 0°C to 70°C.

[0032] In the present invention, the melting point and glass transition temperature of polylactic acid are values ​​measured using a differential scanning calorimeter (for example, power compensation DSC and Diamond DSC manufactured by PerkinElmer Co., Ltd.).

[0033] The melt mass flow rate of polylactic acid is not particularly limited. In order to make the fluidity of the resin into an appropriate range, and to make it easy to control the size of the aforementioned fine dispersion and to make it easy to produce the desired biaxially stretched film, the melt mass flow rate of polylactic acid is preferably 0.5 g / 10 min to 15 g / 10 min, more preferably 1 g / 10 min to 10 g / 10 min, and further preferably 2 g / 10 min to 10 g / 10 min. The melt mass flow rate in this specification refers to the value measured at 230°C and 21.18 N according to JIS K-7210 (1999).

[0034] The manufacturing method of polylactic acid is not particularly limited, for example, a known manufacturing method can be widely adopted. In addition, polylactic acid can also be obtained from commercial products, etc. As representative commercial products of polylactic acid, for example, NatureWorks "4032D" (melting point 163°C), Total Corbion "L175" (melting point 175°C), "LX175" (melting point 155°C), "LX930" (melting point 130°C), etc. can be cited.

[0035] The polylactic acid contained in the biaxially stretched film may be a single type or two or more types.

[0036] The content ratio of the resin components other than polylactic acid in the biaxially stretched film of the present invention is preferably 10% by mass or less, more preferably 5% by mass or less, further preferably 1% by mass or less, and particularly preferably 0.1% by mass or less relative to polylactic acid. The biaxially stretched film of the present invention may be composed only of polylactic acid as a resin component.

[0037] As long as the effects of the present invention are not hindered, the biaxially stretched film of the present invention may contain other additives in addition to polylactic acid. As the additives, for example, heat stabilizers, antioxidants, organic and inorganic lubricants, chlorine capture agents, antistatic agents, antifogging agents, hydrolysis inhibitors, etc. can be cited.

[0038] When the biaxially stretched film of the present invention includes an additive, the content ratio thereof is not particularly limited. In terms of the high heat resistance and good punching processability of the biaxially stretched film, the content ratio of polylactic acid relative to the biaxially stretched film of the present invention is preferably 80% by mass or more, more preferably 90% by mass or more, further preferably 95% by mass or more, and particularly preferably 99% by mass or more.

[0039] The biaxially stretched film of the present invention has a tear strength of 50 N / mm or more and less than 100 N / mm when torn in at least one direction, as measured in accordance with JIS-K7128-3. The tear strength of the film is specifically measured in accordance with JIS-K7128-3 Plastics - Test method for tear strength of film and sheet - Part 3: Right angle tear method.

[0040] When the film tear strength when torn in at least one direction is less than 50 N / mm, the mechanical strength is insufficient, the film is easily damaged, and it becomes difficult to apply the biaxially stretched film to various uses. On the other hand, if the film tear strength when torn in at least one direction exceeds 100 N / mm, the punching processability deteriorates.

[0041] The biaxially stretched film of the present invention has a film tear strength when torn in at least one direction of preferably 55 N / mm or more, more preferably 60 N / mm or more, further preferably 65 N / mm or more, particularly preferably 70 N / mm or more.

[0042] The biaxially stretched film of the present invention preferably has a film tear strength of 95 N / mm or less, more preferably 90 N / mm or less, when the film is torn in at least one direction.

[0043] Here, in the aforementioned "when torn in at least one direction", the aforementioned at least one direction is preferably the MD direction and / or the TD direction, preferably the MD direction. That is, the biaxially stretched film of the present invention preferably has a film tear strength of 50 N / mm or more and less than 100 N / mm when torn in the MD direction and / or the TD direction, as measured in accordance with JIS-K7128-3, and more preferably has a film tear strength of 50 N / mm or more and less than 100 N / mm when torn in the MD direction, as measured in accordance with JIS-K7128-3. In these cases, the punching processability of the biaxially stretched film is particularly easy to improve. It should be noted that, for the sake of caution, in this specification, the MD direction refers to the moving direction of the film in the biaxial stretching, and the TD direction refers to the direction perpendicular to the MD direction and is the width direction of the biaxially stretched film.

[0044] In the biaxially stretched film of the present invention, the method for adjusting the tear strength is not particularly limited. For example, the tear strength can be adjusted according to the thickness of the biaxially stretched film, the manufacturing conditions of the biaxially stretched film, and in particular, when the stretching ratio and / or the relaxation conditions and heat treatment conditions of the stretched film described later are adjusted, the tear strength of the film can be easily adjusted to a more appropriate range.

[0045] The heating dimensional change rate of the biaxially stretched film of the present invention in an atmosphere of 100° C. is within ±5% in both the MD direction and the TD direction, and the heating load dimensional change rate in the MD direction in an atmosphere of 100° C. is within ±7%. As a result, the biaxially stretched film of the present invention has excellent dimensional stability and excellent heat resistance.

[0046] The heating dimensional change rate of the biaxially stretched film of the present invention in an atmosphere at 100° C. is more preferably within ±4% in both the MD direction and the TD direction, further preferably within ±3%, and particularly preferably within ±2.5%.

[0047] The heating load dimensional change rate of the biaxially stretched film of the present invention in the MD direction at 100°C atmosphere is preferably within ±6%, more preferably within ±5.5%, further preferably within ±5%, particularly preferably within ±4.9%.

[0048] The thickness of the biaxially stretched film of the present invention is not particularly limited and can be appropriately set according to the intended use. In terms of easy adjustment of the tear strength of the biaxially stretched film, the thickness of the biaxially stretched film is preferably 5 to 150 μm, preferably 10 to 100 μm, and more preferably 15 to 90 μm.

[0049] The biaxially stretched film of the present invention may have a single-layer structure or a multi-layer structure. When the biaxially stretched film of the present invention has a multi-layer structure, for example, other layers may be provided on one or both sides of the polylactic acid layer.

[0050] As other layers, for example, the layer formed in the known protective film can be cited, for example, a removable adhesive layer can be set on one side or both sides of the layer (base material) comprising polylactic acid. The type of the removable adhesive layer is not particularly limited, for example, it can be set to the same structure as the known removable adhesive layer. In addition, between the base material and the removable adhesive layer, other layers (intermediate layers) can be provided as needed. In addition, during the period until the protective film is used, a separator for protecting the removable adhesive layer can be set on the removable adhesive layer.

[0051] The biaxially stretched film of the present invention contains polylactic acid as an essential component and has a tear strength within a predetermined range. Thus, although it is a film containing polylactic acid, it has moderate tear strength, good punching processability, and excellent heat resistance, and therefore has high dimensional stability. In particular, it was previously believed that there was no correlation between tear strength and punching processability, but the present inventors successfully discovered for the first time that punching processability depends on tear strength, and thus completed the biaxially stretched film of the present invention.

[0052] The biaxially stretched film of the present invention can be suitably used for protective films, food packaging, pharmaceutical packaging, decoration (including fashion), labels, melt-cut seal bags, tape substrates, printing substrates, stationery, home appliances, poster paper, thermal paper substrates, recording paper substrates, interior and exterior decoration of houses, automobiles, containers, etc.

[0053] For example, when the biaxially stretched film of the present invention is used as a protective film, various electronic devices can be used as the target. Since the protective film includes the biaxially stretched film of the present invention, it is excellent in processability and heat resistance.

[0054] 2. Method for producing biaxially stretched film

[0055] The method for producing the biaxially stretched film of the present invention is not particularly limited, and for example, a widely known method can be used. For example, a resin raw material containing at least polylactic acid is extruded to obtain a resin sheet, and the resin sheet is stretched to produce the stretched film of the present invention. The above-mentioned method is referred to as "manufacturing method A".

[0056] The resin raw material used in the production method A may contain various additives as necessary.

[0057] The method for preparing the resin raw material may be the same as the known preparation method, and the following methods may be mentioned: a method of dry-mixing resin pellets, powders, etc. using an intermittent mixing device such as a drum or a mixer, or a continuous metering mixing device; or a method of supplying the resin pellets, powders, etc. together with other resin pellets, powders, and / or additives as required to a kneader, and melt-kneading to obtain a melt-blended resin composition; etc. Among them, it is preferred to prepare the resin raw material by melt-kneading.

[0058] As the mixer used in melt kneading, a known mixer can be used, and a single screw type, a twin screw type, or a multi-screw type or more thereof can be used, and further, in the case of a screw type or more than a twin screw type, any mixer type of co-rotation or counter-rotation can be used. In terms of easy kneading of polylactic acid, a twin screw type mixer rotating in the same direction is preferred.

[0059] The kneading temperature of melt kneading is preferably in the range of 200° C. to 300° C., more preferably 220° C. to 280° C. In order to prevent degradation of the resin during melt kneading, an inert gas such as nitrogen may be purged. The resin after melt kneading is usually pelletized into a suitable size using a known pelletizer, thereby obtaining melt-blended resin composition pellets.

[0060] In the production method A, a resin sheet can be obtained using the resin raw material obtained as described above. Specifically, the resin raw material is supplied to an extruder and heated and melted, and after removing fine foreign matter and the like using a filter or the like as needed, the resin sheet can be obtained by melt-extruding from a T-die into a sheet.

[0061] For example, a well-known extruder can be widely used as an extruder for obtaining a resin sheet. There is no limitation on the screw type of the extruder, and a single screw type, a twin screw type, or a multi-screw type thereof can be used. When the resin raw material is prepared by the aforementioned dry mixing, if a twin screw type or a multi-screw type thereof is used, the mixing and dispersibility are easy to be excellent. The extrusion temperature is preferably in the range of 200°C to 300°C, more preferably 220°C to 280°C. In order to prevent thermal degradation of the resin during extrusion, an inert gas purge such as nitrogen can be performed.

[0062] The melt-extruded resin sheet is formed into a sheet by a known method such as an air knife, other rollers, or static electricity, and is closely attached to at least one metal roll set at a temperature of 25 to 120° C., thereby obtaining a resin sheet in the form of a so-called green sheet. The temperature of the metal roll is more preferably 30 to 80° C.

[0063] The production method A may further include a lamination step as required. For example, when it is desired to obtain a stretched film in which the layer b is formed on one or both sides of the layer a, the production method A preferably includes a lamination step.

[0064] In the lamination step, for example, a wide range of conventional lamination methods can be used, and examples thereof include films obtained by lamination using a co-extrusion method, a lamination method, a heat sealing method, and the like.

[0065] Specifically, in the case of manufacturing the stretch film with a laminated structure, the dry blending and / or melt blending resin compositions (the composition of each resin composition may be different or the same) of more than 2 can be coextruded to obtain the resin sheet with a laminated structure, and the resin sheet with a laminated structure is stretched. Alternatively, by the stretch film and other films of the laminated monolayer structure, the stretch film with a laminated structure can also be manufactured. In addition, the non-stretched film extruded as a monolayer can also be laminated to each other more than 2 layers to obtain a multilayer non-stretched film (the composition of the resin composition constituting each layer may be different or the same), by stretching the multilayer non-stretched film, the stretch film with a laminated structure can be manufactured.

[0066] As the aforementioned co-extrusion method, the following methods can be cited: a die-front lamination method in which the molten resin is brought into contact in a feed block in front of the die; a die-in-die lamination method in which the molten resin is brought into contact in a path inside a die, such as a multi-manifold die; a die-out-die lamination method in which the molten resin is discharged from a plurality of concentric lip openings and brought into contact, etc. For example, in the case of the die-in-die lamination method, a multi-layer die such as a 3-layer multi-manifold die can be used to form a 3-layer layer structure of layer b / layer a / layer b including a skin layer (skin layer: layer b) and a core layer (intermediate layer: layer a).

[0067] Examples of the lamination method include an extrusion lamination method in which a thin film of a molten resin is directly extruded onto another film using an apparatus for melt extrusion molding used in the T-die method to form a laminated film.

[0068] As heat sealing methods, the following methods can be cited: an external heating method in which a heated metal body is pressed onto a plurality of films bonded together from the outside of the film, and the conducted heat causes the films to melt and bond; and an internal heat release method in which high-frequency radio waves or ultrasonic waves are used to generate heat in the films and cause them to bond; etc.

[0069] In the production method A, the above-mentioned lamination methods can be used alone or in combination of two or more.

[0070] As described above, when the manufacturing method A includes a lamination step, the resin sheet may have a laminated structure, and when the manufacturing method A does not include a lamination step, the resin sheet may have a single-layer structure. By using a resin sheet having a single-layer structure and performing the stretching described below, a biaxially stretched film consisting of layer a can be obtained. When a resin sheet having a laminated structure is used and performed the stretching described below, a multi-layer biaxially stretched film having at least layer a can be obtained.

[0071] In manufacturing method A, the resin sheet (blank sheet) having a monolayer or laminated structure is stretched. As a stretching method, known methods such as a method of stretching between rollers with a circumferential speed difference, a tentering method, and a tubular method can be used. As a stretching direction, it can be uniaxial stretching, biaxial stretching, biaxial stretching along an inclined direction, etc., and any of sequential stretching and simultaneous stretching can be applied in the stretching of more than two axes. Among these, from the aspect of easily obtaining the desired biaxially stretched film, it is preferred to use a simultaneous biaxial stretching method based on a tentering method, a sequential biaxial stretching method based on a tentering method, and a sequential biaxial stretching method in which longitudinal (movement, MD) stretching is performed between rollers with a circumferential speed difference and then transverse (width, TD) stretching is performed by a tentering method. Below, the method for obtaining the biaxially stretched film of the present invention by a sequential biaxial stretching method is described, but is not limited thereto.

[0072] In the sequential biaxial stretching method, it is preferred to adjust the stretching temperature and stretching ratio according to the melting point and glass transition temperature of the resin to be used. First, the resin sheet (blank sheet) is maintained at a temperature of preferably 50 to 130°C, more preferably 60 to 100°C, and is stretched longitudinally to preferably 2 to 6 times, more preferably 2.5 to 5 times, and further preferably 3 to 4 times by a tentering method through rollers with a circumferential speed difference. Next, the stretched film is stretched transversely to preferably 2 to 6 times, more preferably 2.5 to 5 times, and further preferably 3 to 4 times at a temperature of preferably 50 to 130°C, more preferably 60 to 100°C, and then a relaxation treatment and a heat setting treatment as a heat treatment of the film are performed, and then the film is wound up. After the wound film is subjected to an aging treatment in an atmosphere preferably at about 20 to 45°C, it can be cut into the desired product width.

[0073] The conditions of the relaxation treatment are preferably such that the transversely stretched film is relaxed at about 60°C to 70°C. In this case, the tear strength of the film can be easily adjusted to a specified range of values. In addition, in the relaxation treatment, the relaxation rate is preferably 0.3 times / second to 0.7 times / second. In this case, the tear strength of the film can be easily adjusted to a specified range of values. The conditions of the heat setting treatment are preferably at a temperature of 100°C to 130°C for 10 seconds to 80 seconds (more preferably 15 seconds to 60 seconds, and further preferably 20 to 40 seconds). In this case, the tear strength of the film can be easily adjusted to a specified range of values. By adjusting the conditions of at least one of the stretching ratio, relaxation treatment, and heat setting treatment, it becomes easy to adjust the tear strength to a specified range of values.

[0074] When the relaxation treatment and heat setting treatment are performed, the thickness of the biaxially stretched film is preferably less than 50 μm, more preferably 45 μm or less, and further preferably 40 μm or less. When the relaxation treatment and heat setting treatment are not performed, the thickness of the biaxially stretched film is preferably 40 μm or more, more preferably 50 μm or more, and preferably 100 μm or less, more preferably 90 μm or less, and further preferably 80 μm or less.

[0075] When determining the invention included in the present disclosure, each configuration (property, structure, function, etc.) described in each embodiment of the present disclosure can be arbitrarily combined. That is, the subject matter including all combinations of each configuration that can be combined described in this specification is included in the present disclosure.

[0076] Example

[0077] Hereinafter, the present invention will be described in more detail based on Examples, but the present invention is not limited to the embodiments of these Examples.

[0078] (Example 1)

[0079] Luminy (registered trademark) LX175 (manufactured by Total Corbion PLA, polylactic acid) was prepared as polylactic acid, fed from a hopper into a single screw extruder a and melted, and extruded from a single-layer die. The extruded resin layer was pressed onto a cooling drum controlled at 45° C. under air pressure with an air knife, and cooled and solidified to obtain a green sheet.

[0080] The obtained blank sheet was stretched using an intermittent biaxial stretching machine KARO manufactured by Bruckner. The stretching method was implemented by a sequential biaxial stretching method of stretching in the longitudinal direction and then in the transverse direction. After preheating in an oven set at a temperature of 70°C until the film temperature (Ts) reached 65°C, it was stretched in the longitudinal direction at a stretching speed of 6 times / second until it reached 3 times. Next, preheating in an oven set at a temperature of 70°C until the film temperature reached 65°C, and stretching in the transverse direction at a stretching speed of 1 times / second until it reached 3.5 times. Next, in the same oven, as a heat treatment, a relaxation treatment was performed in the transverse direction at a relaxation speed of 0.5 times / second until it reached 3.0 times, and then, after a heat setting treatment was performed at 120°C for 30 seconds, it was taken out of the oven and cooled to room temperature to obtain a biaxially stretched film with a thickness of 20μm.

[0081] (Example 2)

[0082] A biaxially stretched film was obtained in the same manner as in Example 1 except that the thickness was changed to 30 μm.

[0083] (Example 3)

[0084] A biaxially stretched film was obtained in the same manner as in Example 1 except that the thickness was changed to 40 μm.

[0085] (Example 4)

[0086] A biaxially stretched film having a thickness of 20 μm was obtained in the same manner as in Example 1 except that the heat setting treatment time was changed to 60 seconds.

[0087] (Example 5)

[0088] A biaxially stretched film was obtained in the same manner as in Example 4 except that the thickness was changed to 30 μm.

[0089] (Example 6)

[0090] A biaxially stretched film was obtained in the same manner as in Example 1 except that the relaxation treatment and the heat setting treatment were not carried out and the thickness was changed to 50 μm.

[0091] (Comparative Example 1)

[0092] A biaxially stretched film having a thickness of 20 μm was obtained in the same manner as in Example 1 except that the heat setting treatment time was changed to 90 seconds.

[0093] (Comparative Example 2)

[0094] A biaxially stretched film was obtained in the same manner as in Comparative Example 1 except that the thickness was changed to 30 μm.

[0095] (Comparative Example 3)

[0096] A biaxially stretched film was obtained in the same manner as in Example 1 except that the thickness was changed to 50 μm.

[0097] (Comparative Example 4)

[0098] A biaxially stretched film was obtained in the same manner as in Example 1 except that the heat setting treatment was not performed.

[0099] [Thickness of film]

[0100] The thickness of the biaxially stretched film was measured using a paper thickness measuring instrument MEI-11 manufactured by CITIZEN SEIMITSU CO., LTD. in accordance with JIS-C2330.

[0101] [Stretchability]

[0102] The stretchability is evaluated using an intermittent biaxial stretching machine KARO manufactured by Bruckner. The stretching method used is a sequential biaxial stretching method of stretching in the longitudinal direction and then in the transverse direction. In an oven set at a temperature of 70°C, the blank sheet is preheated. After the film temperature (Ts) reaches 65°C, it is stretched in the longitudinal direction at a stretching speed of 6 times / second to a stretching ratio of 3 times. Then, in the same oven, after the film temperature (Ts) reaches 65°C, it is stretched in the transverse direction at a stretching speed of 1 times / second to a stretching ratio of 3.5 times. The stretchability of the obtained stretched film is evaluated using the following judgment criteria.

[0103] <Criteria for judging stretchability>

[0104] ○: The film is stretched uniformly with little variation in thickness, and the stretchability is particularly excellent.

[0105] ×: Unable to stretch and broke.

[0106] [Film tear strength]

[0107] The film tear strength was measured in accordance with JIS-K7128-3 Plastics - Test method for tear strength of film and sheet - Part 3: Right angle tear method. Specifically, the film tear strength was measured when a biaxially stretched film was torn in the MD direction.

[0108] [Dimensional change rate after heating]

[0109] The biaxially stretched film obtained in the examples and comparative examples was cut into 100 mm × 100 mm, and a 50 mm mark line was written in parallel to the MD direction and the TD direction in the moving direction (hereinafter referred to as the MD direction) and the width direction (hereinafter referred to as the TD direction) of the film during film formation, and then the biaxially stretched film was placed in an oven heated to 100°C for 1 minute. After that, the film was taken out, and the length L1 of the mark line applied in the MD direction before the test and the length L2 of the mark line after the test were measured, and the heating dimensional change rate in the MD direction was calculated using the following formula (similarly, the length L1 of the mark line applied in the TD direction before the test and the length L2 of the mark line after the test were measured, and the heating dimensional change rate in the MD direction was calculated using the following formula).

[0110] Heating dimensional change rate (%) = {(L2-L1) / L1}×100

[0111] [Dimensional change rate under heating load]

[0112] The biaxially stretched films obtained in the examples and comparative examples were cut into pieces of 100 mm (MD direction) × 20 mm (TD direction), and a 50 mm marking line was written in parallel with the MD direction. 2 The film was placed in an oven heated to 100°C for 1 minute while being subjected to a load of 300 g. Afterwards, the film was taken out, and the length of the marking line L3 applied along the MD direction before the test and the length of the marking line L4 after the test were measured. The heating load dimensional change rate in the MD direction was calculated using the following formula.

[0113] Heating load dimensional change rate in MD direction (%) = {(L4-L3) / L3}×100

[0114] [Blanking processability]

[0115] The biaxially stretched film obtained in the examples and comparative examples was placed on a punching die of 60 mm in width and 120 mm in length, and a PET film of 188 μm in thickness and a polycarbonate plate of 5 mm in thickness were stacked thereon in order to obtain a laminate. The laminate was punched out using an electric driven press (PAC-HD manufactured by Dietecs Co., Ltd.) at a pressure of 5 t and a speed of 10 mm / sec. The film end faces of the punched laminate were visually inspected for roughness, cracks, etc., and the punching process was evaluated according to the following criteria.

[0116] ○: There is no roughness on the end surface, and there is no problem in use.

[0117] △: The end surface is rough, but there is no problem in use.

[0118] ×: The end surface is noticeably rough, which is a level that is problematic in use.

[0119] [Table 1]

[0120]

[0121] Evaluation results of the biaxially stretched films obtained in each example and comparative example are shown in Table 1. Since the biaxially stretched films obtained in the examples have a specific tear strength, they have both punching workability and heat resistance despite being films containing polylactic acid.

Claims

1. A biaxially stretched film, It contains polylactic acid, The biaxially stretched film has a tear strength of 50 N / mm or more and less than 100 N / mm when the biaxially stretched film is torn in at least one direction, as measured in accordance with JIS-K7128-3. The heating dimensional change rate of the biaxially stretched film in an atmosphere at 100° C. is within ±5% in both the MD direction and the TD direction, and the heating load dimensional change rate in the MD direction in an atmosphere at 100° C. is within ±7%.

2. The biaxially stretched film according to claim 1, wherein The at least one direction is the MD direction or the TD direction. 3 . A protective film comprising the biaxially stretched film according to claim 1 .

Images