Method for producing stretched film

Abstract

Provided is a production method by which a stretched film less susceptible to heat shrinkage and including a poly(3-hydroxybutyrate)-based resin can be produced with high production efficiency.　The production method is for producing a stretched film including a poly(3-hydroxybutyrate)-based resin, and comprises a step in which a film-forming raw material including the poly(3-hydroxybutyrate)-based resin is melted with an extruder and then formed into a film, a step in which the formed film is stretched, and a step in which the stretched film is heat-treated. The heat treatment includes a treatment in which the film is made to have a temperature T1 and then have a temperature T2, the temperatures T1 and T2 satisfying all of the following requirements (1) to (3).　(1) ((Melting point of poly(3-hydroxybutyrate)-based resin)-70)°C≤T1 (2) T2≤(Melting point of poly(3-hydroxybutyrate)-based resin)-20)°C (3) T1≠T2 and T1<T2

Description

Stretched film manufacturing method

[0001] The present invention relates to a method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin.

[0002] In recent years, the separate collection and composting of food waste has been promoted, particularly in Europe, and there is a demand for plastic products that can be composted together with food waste.

[0003] On the other hand, environmental problems caused by discarded plastics have been highlighted, and it has become clear that large amounts of plastic, particularly plastics dumped in the ocean or that have entered the ocean via rivers, are drifting in the ocean on a global scale. Because such plastics retain their shape for a long period of time, they can trap and capture marine organisms, a phenomenon known as ghost fishing, and if ingested by marine organisms, they can become trapped in the digestive tract, causing feeding disorders, and other problems have been pointed out as having an impact on the ecosystem.

[0004] Furthermore, it has been pointed out that microplastics, which are plastics that have broken down and become tiny particles due to ultraviolet rays and other factors, absorb harmful compounds in seawater, and when marine organisms ingest these, the harmful substances are absorbed into the food chain.

[0005] The use of biodegradable plastics is expected to address this type of marine pollution caused by plastics, but a report compiled by the United Nations Environment Programme in 2015 pointed out that plastics that can be biodegraded through compost, such as polylactic acid, cannot be expected to decompose in a short period of time in the cold ocean, and therefore cannot be used to combat marine pollution.In this context, poly(3-hydroxybutyrate) resins are attracting attention as a material that can resolve the above issues, as they are capable of biodegrading even in seawater.

[0006]

[0003] Meanwhile, a method of stretching a film is known as a technique for producing a thin, high-strength film. For example, to produce a stretched film from a general-purpose resin such as polypropylene, a molten resin is cooled and solidified using a cast roll to form a raw sheet, and the raw sheet is then preheated to a temperature at which it can be stretched, and then stretched, thereby enabling continuous production of the stretched film with good productivity.

[0007] However, poly(3-hydroxybutyrate)-based resins are known to be difficult to stretch due to their properties. Patent Document 1 discloses a method for producing a biaxially stretched film containing a poly(3-hydroxybutyrate)-based resin with high productivity.

[0008] Japanese Patent Application Laid-Open No. 2022-062759

[0009] When a stretched film containing a poly(3-hydroxybutyrate)-based resin as a main component is used, for example, as a packaging film, it is subjected to heat bonding between stretched films to seal the contents, or heat fixation of ink applied to the stretched film for printing. However, such heating causes a problem that the stretched film shrinks, distorting the sealed portion or the printing.

[0010] In view of the above-mentioned current situation, an object of the present invention is to provide a method for producing a stretched film containing a poly(3-hydroxybutyrate) resin with low heat shrinkage, with good productivity.

[0011] As a result of intensive research aimed at solving the above problems, the present inventors have found that a stretched film containing a poly(3-hydroxybutyrate)-based resin with little heat shrinkage can be produced with good productivity by stretching the film containing a poly(3-hydroxybutyrate)-based resin and then heat-treating it under specific conditions, and have thus completed the present invention.

[0012] That is, the present invention relates to a method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin, the method comprising the steps of melting a film raw material containing the poly(3-hydroxybutyrate)-based resin in an extruder and then forming the melted film into a film, stretching the formed film, and heat-treating the stretched film, wherein the heat treatment includes a treatment of raising the film to temperature T1 and then to temperature T2, and the temperatures T1 and T2 satisfy all of the conditions of the following formulas (1) to (3): (melting point of poly(3-hydroxybutyrate)-based resin - 70) ° C. ≦ T1 (1) T2 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 20) ° C. (2) T1 ≠ T2 and T1 < T2 (3)

[0013] According to the present invention, it is possible to provide a production method capable of producing a stretched film containing a poly(3-hydroxybutyrate) resin with little heat shrinkage with good productivity.

[0014] Although an embodiment of the present invention will be described below, the present invention is not limited to the following embodiment. The present embodiment relates to a method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin, which includes the steps of melting a film raw material containing the poly(3-hydroxybutyrate)-based resin in an extruder and then forming it into a film, stretching the formed film, and heat-treating the stretched film, wherein the heat treatment includes a treatment of raising the film to temperature T1 and then to temperature T2, and the temperatures T1 and T2 satisfy specific conditions.

[0015] <Poly(3-hydroxybutyrate)-based Resin> The poly(3-hydroxybutyrate)-based resin is an aliphatic polyester resin that can be produced from a microorganism and has 3-hydroxybutyrate as a repeating unit. The poly(3-hydroxybutyrate)-based resin may be a poly(3-hydroxybutyrate) having only 3-hydroxybutyrate as a repeating unit, or may be a copolymer of 3-hydroxybutyrate and another hydroxyalkanoate. The poly(3-hydroxybutyrate)-based resin may also be a mixture of a homopolymer and one or more copolymers, or a mixture of two or more copolymers.

[0016] Specific examples of the poly(3-hydroxybutyrate)-based resin include poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [hereinafter, may be referred to as P3HB3HH], poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [hereinafter, may be referred to as P3HB3HV], poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [hereinafter, may be referred to as P3HB4HB], poly(3-hydroxybutyrate-co-3-hydroxyoctanoate), poly(3-hydroxybutyrate-co-3-hydroxyoctadecanoate), and the like. Among these, poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) are preferred because they are easy to produce industrially.

[0017] Furthermore, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) is preferred from the viewpoints that by changing the composition ratio of the repeating units, it is possible to change the melting point and degree of crystallinity, and thereby change physical properties such as Young's modulus and heat resistance, making it possible to impart physical properties between those of polypropylene and polyethylene, and that it is easy to produce industrially and is a physically useful plastic. In particular, among poly(3-hydroxybutyrate)-based resins that tend to be thermally decomposed when heated to 180°C or higher, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) is preferred from the viewpoints that it can lower the melting point and enable molding and processing at low temperatures.

[0018] Commercially available products of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) include Kaneka Biodegradable Polymer PHBH (registered trademark) manufactured by Kaneka Corporation.

[0019] When the poly(3-hydroxybutyrate) resin contains a copolymer of 3-hydroxybutyrate units and other hydroxyalkanoate units, the average content ratio of 3-hydroxybutyrate units and other hydroxyalkanoate units to all monomer units constituting the poly(3-hydroxybutyrate) resin is preferably 3-hydroxybutyrate units / other hydroxyalkanoates = 99 / 1 to 80 / 20 (mol % / mol %), and more preferably 97 / 3 to 85 / 15 (mol % / mol %), from the viewpoint of achieving both strength and productivity of the stretched film.

[0020] The average content ratio of each monomer unit in all monomer units constituting the poly(3-hydroxybutyrate)-based resin can be determined by a method known to those skilled in the art, for example, the method described in paragraph

[0047] of WO 2013 / 147139. The average content ratio means the molar ratio of each monomer unit in all monomer units constituting the poly(3-hydroxybutyrate)-based resin, and when the poly(3-hydroxybutyrate)-based resin is a mixture of two or more poly(3-hydroxybutyrate)-based resins, it means the molar ratio of each monomer unit contained in the entire mixture.

[0021] The poly(3-hydroxybutyrate) resin may be a mixture of at least two types of poly(3-hydroxybutyrate) resins that differ from each other in the type of constituent monomer and / or the content ratio of the constituent monomer.

[0022] The weight average molecular weight of the entire poly(3-hydroxybutyrate) resin is not particularly limited, but from the viewpoint of achieving both strength and productivity of the stretched film, it is preferably 200,000 to 2,000,000 g / mol, more preferably 250,000 to 1,500,000 g / mol, and even more preferably 300,000 to 1,000,000 g / mol.

[0023] The weight-average molecular weight of the poly(3-hydroxybutyrate) resin can be measured in terms of polystyrene using gel permeation chromatography (HPLC GPC system manufactured by Shimadzu Corporation) using a chloroform solution. As the column for the gel permeation chromatography, a column appropriate for measuring the weight-average molecular weight may be used.

[0024] The method for producing poly(3-hydroxybutyrate) resins is not particularly limited, and may be a production method using chemical synthesis or a production method using a microorganism. Among these, a production method using a microorganism is preferred. Known methods can be applied to the production method using a microorganism. For example, known bacteria that produce copolymers of 3-hydroxybutyrate and other hydroxyalkanoates include Aeromonas caviae, which produces P3HB3HV and P3HB3HH, and Alcaligenes eutrophus, which produces P3HB4HB. In particular, with regard to P3HB3HH, in order to increase the productivity of P3HB3HH, Alcaligenes eutrophus AC32 strain (FERM BP-6038) (T. Fukui, Y. Doi, J. Bateriol., 179, pp. 4821-4830 (1997)) or the like into which genes encoding P3HA (poly(3-hydroxyalkanoate)) synthases have been introduced is more preferred, and microbial cells obtained by culturing these microorganisms under appropriate conditions and allowing P3HB3HH to accumulate within the cells are used. In addition to the above, genetically modified microorganisms into which various poly(3-hydroxybutyrate) resin synthesis-related genes have been introduced may also be used depending on the poly(3-hydroxybutyrate) resin to be produced, or the culture conditions, including the type of substrate, may be optimized.

[0025] The poly(3-hydroxybutyrate) resin may be an unmodified resin, or may be a resin obtained by modifying an unmodified poly(3-hydroxybutyrate) resin with a raw material that reacts with the resin, such as a peroxide (hereinafter referred to as a "modifying raw material").

[0026] When a modified resin is used as a film raw material, a film raw material containing a poly(3-hydroxybutyrate)-based resin that has been previously reacted with a modifying raw material may be molded into a film, or a film raw material containing an unmodified poly(3-hydroxybutyrate)-based resin and a modifying raw material may be molded with the modifying raw material. When reacting a resin with a modifying raw material, the entire resin may be reacted with the modifying raw material, or a portion of the resin may be reacted with the modifying raw material to obtain a modified resin, and the remaining unmodified resin may then be added to the modified resin.

[0027] The modification raw material is not particularly limited as long as it is a compound that can react with the poly(3-hydroxybutyrate)-based resin. However, an organic peroxide is preferably used in terms of ease of handling and ease of controlling the reaction with the poly(3-hydroxybutyrate)-based resin.

[0028] Examples of the organic peroxides include diisobutyl peroxide, cumyl peroxyneodecanoate, di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-sec-butyl peroxydicarbonate, 1,1,3,3-tetramethylbutyl peroxyneodecanoate, bis(4-t-butylcyclohexyl)peroxydicarbonate, bis(2-ethylhexyl)peroxydicarbonate, t-hexyl peroxyneodecanoate, t-butyl peroxyneodecanoate, t-butyl peroxyneoheptanoate, t-hexyl peroxypivalate, t-butyl peroxypivalate, di(3,5,5-trimethylhexanoyl)peroxide, dilauroyl peroxide, 1,1,3,3-tetramethylbutylperoxy-2-methyl- t-ethylhexanoate, disuccinic acid peroxide, 2,5-dimethyl-2,5-bis(2-ethylhexanoylperoxy)hexane, t-hexylperoxy-2-ethylhexanoate, di(4-methylbenzoyl)peroxide, dibenzoyl peroxide, t-butylperoxy 2-ethylhexyl carbonate, t-butylperoxy isopropyl carbonate, 1,6-bis(t-butylperoxycarbonyloxy)hexane, t-butylperoxy-3,5,5-trimethylhexanoate, t-butylperoxyacetate, t-butylperoxybenzoate, t-amylperoxy, 3,5,5-trimethylhexanoate, 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, 2,2-di-t-butylperoxybutane, and the like. Among these, t-butylperoxy 2-ethylhexyl carbonate and t-butylperoxy isopropyl carbonate are preferred. Furthermore, a combination of two or more of these organic peroxides can also be used.

[0029] The organic peroxide is used in various forms such as solid or liquid, and may be in liquid form diluted with a diluent, etc. Among these, an organic peroxide in a form that can be easily mixed with the poly(3-hydroxybutyrate)-based resin (particularly an organic peroxide that is liquid at room temperature (25°C)) is preferred because it can be more uniformly dispersed in the poly(3-hydroxybutyrate)-based resin and is likely to suppress local modification reactions in the resin composition.

[0030] The content of the poly(3-hydroxybutyrate) resin in the stretched film may be 50% by weight or more, 55% by weight or more, 60% by weight or more, 70% by weight or more, or 80% by weight or more. The upper limit of the content of the poly(3-hydroxybutyrate) resin is not limited, and may be 100% by weight or less.

[0031] The stretched film may contain additives that can be used with the poly(3-hydroxybutyrate)-based resin, provided that the effects of the invention are not impaired. Examples of such additives include colorants such as pigments and dyes, odor absorbers such as activated carbon and zeolite, fragrances such as vanillin and dextrin, fillers, plasticizers, antioxidants, weather resistance improvers, UV absorbers, crystal nucleating agents, lubricants, release agents, water repellents, antibacterial agents, and sliding properties improvers. The film may contain only one type of additive, or two or more types. The content of these additives can be appropriately determined by those skilled in the art depending on the intended use. Even when the poly(3-hydroxybutyrate)-based resin contains these additives, its melting point is approximately the same as that of the poly(3-hydroxybutyrate)-based resin.

[0032] The crystal nucleating agent, lubricant, filler, and plasticizer will be described in more detail below. (Crystal Nucleating Agent) Examples of crystal nucleating agents include polyhydric alcohols such as pentaerythritol, galactitol, and mannitol; orotic acid, aspartame, cyanuric acid, glycine, zinc phenylphosphonate, and boron nitride. Among these, pentaerythritol is preferred because of its particularly excellent effect of promoting the crystallization of poly(3-hydroxybutyrate) resins. One or more crystal nucleating agents may be used, and the ratio of their use can be appropriately adjusted depending on the purpose.

[0033] The amount of the nucleating agent used is not particularly limited, but is preferably 0.1 to 5 parts by weight, more preferably 0.5 to 3 parts by weight, and even more preferably 0.7 to 1.5 parts by weight, per 100 parts by weight of the total amount of the poly(3-hydroxybutyrate) resin.

[0034] (Lubricant) Examples of lubricants include behenamide, oleamide, erucamide, stearamide, palmitamide, N-stearylbehenamide, N-stearylerucamide, ethylenebisstearamide, ethylenebisoleamide, ethylenebiserucamide, ethylenebislauricamide, ethylenebiscapricamide, p-phenylenebisstearamide, and polycondensates of ethylenediamine, stearic acid, and sebacic acid. Among these, behenamide or erucamide is preferred because of its particularly excellent lubricating effect on poly(3-hydroxybutyrate)-based resins. One or more types of lubricants may be used, and the ratio of use can be appropriately adjusted depending on the purpose.

[0035] The amount of the lubricant used is not particularly limited, but is preferably 0.01 to 5 parts by weight, more preferably 0.05 to 3 parts by weight, and even more preferably 0.1 to 1.5 parts by weight, per 100 parts by weight of the total amount of the poly(3-hydroxybutyrate) resin.

[0036] (Filler) By including a filler, a stretched film with higher strength can be obtained. The filler may be either an inorganic filler or an organic filler, or both may be used in combination. The inorganic filler is not particularly limited, but examples thereof include silicates, carbonates, sulfates, phosphates, oxides, hydroxides, nitrides, and carbon black. Only one type of inorganic filler may be used, or two or more types may be used in combination.

[0037] The content of the filler is not particularly limited, but is preferably 1 to 100 parts by weight, more preferably 3 to 80 parts by weight, even more preferably 5 to 70 parts by weight, and even more preferably 10 to 60 parts by weight, relative to 100 parts by weight of the total amount of the poly(3-hydroxybutyrate) resin. However, the stretched film does not have to contain a filler.

[0038] (Plasticizers) Examples of plasticizers include glycerin ester compounds, citrate ester compounds, sebacate compounds, adipate compounds, polyether ester compounds, benzoate ester compounds, phthalate ester compounds, isosorbide ester compounds, polycaprolactone compounds, and dibasic acid ester compounds. Among these, glycerin ester compounds, citrate ester compounds, sebacate compounds, and dibasic acid ester compounds are preferred because of their particularly excellent plasticizing effect on poly(3-hydroxyalkanoate) resins. Examples of glycerin ester compounds include glycerin diacetomonolaurate. Examples of citrate ester compounds include acetyl tributyl citrate. Examples of sebacate ester compounds include dibutyl sebacate. Examples of dibasic acid ester compounds include benzyl methyl diethylene glycol adipate. One type of plasticizer may be used, or two or more types may be used, and the ratio of use can be adjusted appropriately depending on the purpose.

[0039] The amount of the plasticizer used is not particularly limited, but is preferably 1 to 20 parts by weight, more preferably 2 to 15 parts by weight, and even more preferably 3 to 10 parts by weight, per 100 parts by weight of the total amount of the poly(3-hydroxybutyrate) resin. However, the stretched film does not necessarily need to contain a plasticizer.

[0040] (Other Resins) The stretched film may contain other resins besides the poly(3-hydroxybutyrate)-based resin, provided that the effects of the invention are not impaired. Examples of such other resins include aliphatic polyester-based resins such as poly(3-hydroxypropionate), poly(4-hydroxybutyrate), polybutylene succinate adipate, polybutylene succinate, polycaprolactone, and polylactic acid, and aliphatic aromatic polyester-based resins such as polybutylene adipate terephthalate (hereinafter sometimes referred to as PBAT), polybutylene sebate terephthalate, and polybutylene azelate terephthalate. Only one type of other resin may be contained, or two or more types may be contained.

[0041] The content of the other resin is not particularly limited, but may be 100 parts by weight or less, 80 parts by weight or less, 70 parts by weight or less, 50 parts by weight or less, 30 parts by weight or less, 20 parts by weight or less, 10 parts by weight or less, 5 parts by weight or less, or 1 part by weight or less, relative to 100 parts by weight of the poly(3-hydroxybutyrate)-based resin. The lower limit of the content of the other resin is not particularly limited, and may be 0 parts by weight or more.

[0042] Furthermore, when the other resin has a lower melting point than the poly(3-hydroxybutyrate)-based resin, the lower limit may be 10 parts by weight or more, 20 parts by weight or more, 50 parts by weight or more, or 65 parts by weight or more relative to 100 parts by weight of the poly(3-hydroxybutyrate)-based resin, and the upper limit may be less than 100 parts by weight relative to 100 parts by weight of the poly(3-hydroxybutyrate)-based resin.

[0043] <Production of Stretched Film> A stretched film containing the poly(3-hydroxybutyrate)-based resin of the present disclosure can be produced by the following production method. This production method includes the steps of melting a film raw material containing the poly(3-hydroxybutyrate)-based resin in an extruder and then forming the film into a film, stretching the formed film, and heat-treating the stretched film, wherein the heat treatment includes a treatment of raising the film to temperature T1 and then to temperature T2, and the temperatures T1 and T2 satisfy all of the conditions of the following formulas (1) to (3): (melting point of poly(3-hydroxybutyrate)-based resin - 70) ° C. ≦ T1 (1) T2 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 20) ° C. (2) T1 ≠ T2 and T1 < T2 (3)

[0044] (Molding Step) In the step of melting a film raw material containing a poly(3-hydroxybutyrate) resin in an extruder and then molding it into a film, the method for molding it into a film is not particularly limited, and known manufacturing methods can be used as appropriate. Specific examples include inflation molding, T-die extrusion molding using an extruder equipped with a T-die, calendar molding, and rolling. Among these, inflation molding and T-die extrusion molding are preferred because they allow for the production of strip-shaped films with good productivity. Furthermore, the extruder can be a single-screw extruder (also called a single-screw extruder), a twin-screw extruder, or the like as appropriate.

[0045] The molding temperature is not particularly limited as long as it is a temperature at which the resin can be properly melted, but for example, 130 to 200°C is preferable. The molding temperature here refers to the resin temperature from the extruder to the time when the resin is discharged from the die. The resin temperature can generally be measured, for example, by a thermometer installed in the adapter.

[0046] (Blown-film extrusion method) The blown-film extrusion method is a molding method in which a molten resin is extruded into a tube from an extruder equipped with a cylindrical die at the tip, and immediately thereafter, a gas is blown into the tube to inflate it into a balloon shape to form a film. The blown-film extrusion method is not particularly limited, but can be carried out using, for example, a general blown-film extrusion machine used for forming a thermoplastic resin into a film.

[0047] A typical inflation molding machine is a single-screw extruder equipped with a cylindrical die. The single-screw extruder may be any extruder that melts and kneads the introduced raw resin and maintains the desired temperature while achieving a constant discharge. The screw shape of the single-screw extruder is not particularly limited, but extruders equipped with mixing elements are preferred from the viewpoint of kneading performance. The structure of the cylindrical die is also not particularly limited, but a spiral mandrel die is preferred because it reduces the occurrence of welds and facilitates thickness uniformity.

[0048] The take-up speed in inflation molding is determined by the film thickness, width, and resin discharge rate, but can be adjusted within a range that maintains bubble stability. Generally, a speed of 1 to 100 m / min is preferred.

[0049] In inflation molding, air rings blown from outside the bubble can be used to solidify the discharged molten resin and stabilize the bubble. A suitable air ring blowing structure is a slit type with multiple annular slits for blowing air out and chambers between each slit to promote bubble stabilization.

[0050] The blow-up ratio (hereinafter sometimes referred to as BUR) in inflation molding is the circumferential length of the bubble cross section divided by the die diameter. From the viewpoint of improving film strength, the lower limit of BUR is preferably 1.5 times or more, more preferably 1.7 times or more, even more preferably 1.9 times or more, and particularly preferably 2 times or more. From the viewpoint of molding stability, the upper limit of BUR is preferably 5.5 times or less, more preferably 4.5 times or less, even more preferably 4.0 times or less, and particularly preferably 3.5 times or less.

[0051] (T-die extrusion molding method) The T-die extrusion molding method refers to a molding method in which a resin molten by an extruder is extruded into a film shape from a slit-shaped outlet onto a cast roll to form a film. The T-die is not particularly limited, and any known T-die can be used as appropriate. For example, the T-die preferably has an outlet shaped to be able to extrude a film-shaped raw material, but the shape is not particularly limited. The shape of the outlet is also not particularly limited.

[0052] In the T-die extrusion molding method, a film-shaped raw material is extruded from the discharge port of a T-die. The shape of the raw material may be a film, and the thickness and width are not particularly limited. The thickness is preferably about 20 μm to 600 μm, since this results in less thickness variation and allows for easy cooling after extrusion.

[0053] The melt viscosity of the raw material extruded from the discharge port of the T-die is not particularly limited, but is preferably 1500 Pa sec or less in order to minimize thickness unevenness and prevent the occurrence of die lines. The melt viscosity can be measured according to a known method as appropriate.

[0054] The thickness of the film before stretching is not particularly limited and may be appropriately set in consideration of the desired thickness of the stretched film, the stretching ratio, strength, etc. For example, the thickness is preferably 20 to 600 μm, more preferably 40 to 500 μm, and even more preferably 50 to 300 μm. The thickness of the film can be measured using a vernier caliper.

[0055] (Stretching Step) In the step of stretching the formed film, the method is not particularly limited as long as it is possible to stretch the film, and known manufacturing methods can be appropriately used.

[0056] The stretching direction in the stretching step is not particularly limited, and the film can be stretched in any direction in the plane direction. When the stretched film of the present disclosure is a strip-shaped film, the stretching direction may be either the MD direction or the TD direction of the film, or both the MD direction and the TD direction. Stretching in either the MD direction or the TD direction is called uniaxial stretching, and stretching in both the MD direction and the TD direction is called biaxial stretching. Here, the MD direction is also called the machine direction, flow direction, or longitudinal direction. The TD direction is the direction perpendicular to the MD direction, and is also called the perpendicular direction or width direction.

[0057] Although the specific stretching method is not particularly limited, a method of stretching the film by stretching it in the stretching direction is preferred. Stretching the film in the stretching direction means pulling the film in the stretching direction. On the other hand, when stretching is performed by applying pressure in the thickness direction of the film, such as roll rolling in which the film is sandwiched between two rolls, the film tends to adhere to the rolling rolls, which may reduce the productivity of the stretched film.

[0058] The method for stretching the film in the stretching direction is not particularly limited. When stretching is performed by a batch method, the film may be stretched in the stretching direction by gripping the edges of the film.

[0059] When the film is stretched in the MD direction while being continuously transported, for example, a roll longitudinal stretching machine can be used to stretch the film in the MD direction by varying the rotation speed of the rolls transporting the film. In this case, the stretching ratio in the MD direction can be determined by the ratio of the rotation speed of the rolls after stretching to the rotation speed of the rolls before stretching.

[0060] When stretching a film in the TD direction while continuously transporting it, the film can be stretched in the TD direction by clamping both widthwise ends of the film using a transverse stretching machine such as a clip-type tenter and pulling it in the TD direction. In this case, the stretching ratio in the TD direction can be determined by the ratio of the distance between both widthwise ends of the clamped film after stretching to the distance between both widthwise ends of the clamped film before stretching.

[0061] The stretching ratio achieved in the step of stretching the formed film is not particularly limited, but is preferably 1.1 times or more, more preferably 1.3 times or more, even more preferably 1.5 times or more, and particularly preferably 2 times or more. The upper limit is not particularly limited and may be determined appropriately, but may be, for example, 8 times or less, 7 times or less, 5 times or less, or 3 times or less.

[0062] The stretching temperature is not particularly limited as long as the film can be stretched appropriately, and may be changed depending on the mechanical strength, surface properties, thickness accuracy, etc. required for the stretched film to be produced.

[0063] The stretching temperature is preferably 40°C or higher, more preferably 50°C or higher, and even more preferably 60°C or higher. The upper limit is sufficient as long as it is equal to or lower than the melting point of the poly(3-hydroxybutyrate) resin, and is preferably 150°C or lower, more preferably 145°C or lower, and even more preferably 140°C or lower. When the stretching temperature is within the above temperature range, thickness unevenness of the resulting stretched film can be reduced, and further, mechanical properties such as elongation, tear propagation strength, and flexural fatigue resistance can be improved. Furthermore, problems such as the film sticking to the roll can be prevented.

[0064] The stretching temperature here refers to the temperature of the film during stretching. The stretching temperature can generally be measured by measuring the temperature of the film itself or the ambient temperature near the film using an infrared thermometer, thermolabel, thermocouple, etc.

[0065] The means for adjusting the film temperature during stretching is not particularly limited, and examples thereof include non-contact heating methods such as a method of applying hot air heated to within the above temperature range to the film during stretching, a method of heating the film during stretching using an auxiliary heating means such as an infrared heater, and a method of stretching the film in a heating furnace whose temperature is adjusted to within the above temperature range; and contact heating methods such as a method of bringing the film into contact with a roll heated to within the above temperature range. These methods may be used alone or in combination.

[0066] In the method of bringing the film into contact with rolls heated to within the above temperature range, hot air may be applied to the film between the upstream stretching roll and the downstream stretching roll in the MD direction.

[0067] From the viewpoint of heating efficiency, it is preferable to use a floating heating method as a method for applying hot air heated to within the above temperature range to the film during stretching. Floating heating is a method in which hot air is blown onto both sides of the film from upper and lower nozzles. Multiple upper nozzles and multiple lower nozzles are arranged alternately toward the film surface, and the film can be heated by the hot air blown from each of the upper and lower nozzles without the film coming into contact with either the upper or lower nozzle.

[0068] In the method of using an auxiliary heating means such as an infrared heater to heat the film during stretching, the temperature of the film surface and inside can be raised to the same temperature in a short period of time, enabling uniform stretching throughout the entire film.

[0069] The infrared rays to be irradiated can be electromagnetic waves in the general infrared region, and may be any of near infrared rays: wavelength 0.74 μm to 1.5 μm, mid infrared rays: wavelength 1.5 μm to 3.0 μm, and far infrared rays: wavelength 3.0 μm to 1 mm.

[0070] In the method of bringing the film into contact with rolls heated to the above temperature range, when the film is stretched using two adjacent stretching rolls while being continuously transported, it is sufficient to heat the upstream stretching roll in the MD direction of the two adjacent stretching rolls to the above temperature range. In this case, the stretching temperature, i.e., the film temperature during stretching, can be controlled by setting the roll temperature to the desired stretching temperature.

[0071] In the production method of the present disclosure, a method of adjusting the film temperature during stretching is preferably a method of bringing the film into contact with rolls heated to the above temperature range, from the viewpoint of excellent productivity and easy heating particularly in mass production. This method is suitable for uniaxial stretching, particularly when stretching in the MD direction using multiple rolls of a roll longitudinal stretching machine.

[0072] From the viewpoint of avoiding the problem of the film sticking to the heating tool, a non-contact heating method in which the heating tool heated to the above temperature range does not come into contact with the film is preferred.

[0073] (Heat Treatment Step) The heat treatment step of the stretched film includes a step of heating the film to a temperature T1 and then to a temperature T2, and the temperatures T1 and T2 satisfy all of the conditions of the following formulas (1) to (3): (melting point of poly(3-hydroxybutyrate)-based resin - 70) ° C. ≦ T1 (1) T2 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 20) ° C. (2) T1 ≠ T2 and T1 < T2 (3)

[0074] If the temperature T2 is higher than (the melting point of the poly(3-hydroxybutyrate)-based resin - 20)°C, the molecular orientation obtained by stretching will be lost, and the mechanical strength of the resulting stretched film may decrease, or the film may stick to the heating tool, or if multiple films are stacked, the films may stick to each other.

[0075] If temperature T1 is lower than (melting point of poly(3-hydroxybutyrate)-based resin - 70)°C and temperature T2 is higher than (melting point of poly(3-hydroxybutyrate)-based resin - 20)°C, the temperature difference between temperature T1 and temperature T2 will be too large, resulting in a rapid temperature rise, which may cause the film to stick to the heating tool or, if multiple films are stacked on top of each other, the films to stick to each other.

[0076] It is also preferable that the temperatures T1 and T2 satisfy the conditions of the following formulas (4) and (5): (melting point of poly(3-hydroxybutyrate)-based resin - 70) ° C. ≦ T1 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 30) ° C. (4) (melting point of poly(3-hydroxybutyrate)-based resin - 60) ° C. ≦ T2 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 20) ° C. (5)

[0077] This can further reduce heat shrinkage in the MD and TD directions, and in particular, can significantly reduce heat shrinkage in the stretched direction in the film stretching step.

[0078] The melting point of a poly(3-hydroxybutyrate) resin refers to the temperature at the top of the melting point peak in a DSC curve obtained by differential scanning calorimetry. Details of differential scanning calorimetry will be described in the Examples section.

[0079] Furthermore, temperatures T1 and T2 preferably satisfy the relationship 0°C<T2-T1≦40°C, and more preferably satisfy the relationship 10°C≦T2-T1≦30°C.

[0080] The heat treatment may include a treatment in which the temperature is raised to T2 and then to T3, and it is preferable that the temperatures T1, T2, and T3 satisfy the conditions of the following formulas (6) and (7): (melting point of poly(3-hydroxybutyrate)-based resin - 40) ° C. ≦ T3 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 15) ° C. (6) T1 < T2 < T3 (7)

[0081] This can further reduce heat shrinkage in the MD and TD directions, and in particular, can significantly reduce heat shrinkage in the stretched direction in the film stretching step.

[0082] Furthermore, temperatures T2 and T3 preferably satisfy the relationship 0°C<T3-T2≦40°C, and more preferably satisfy the relationship 10°C≦T3-T2≦20°C.

[0083] When the stretched film produced by the production method of the present disclosure is heated at 110°C for 10 minutes, the heat shrinkage in the stretching direction is preferably 15% or less, more preferably 10% or less, even more preferably 8% or less, and particularly preferably 6% or less. The smaller the heat shrinkage, the better, but it may be 0% or more, or 1% or more. Details of the method for measuring the heat shrinkage are as described in the Examples section.

[0084] In the step of heat-treating the stretched film, the film may be relaxed when it is brought to the temperatures T1, T2, and T3. Of these, relaxing the film when it is first brought to temperature T1 is preferred because it can effectively prevent the stretched film from rapidly shrinking on heating and from breaking during production and processing. The amount of relaxation is preferably 0 to 10%, more preferably 5 to 10%. In particular, by relaxing the film in the stretching direction in the step of stretching, heat shrinkage can be further reduced.

[0085] The relaxation amount can be calculated by the following formula: Relaxation amount [%] = {(film dimension immediately before temperature Tn) - (film dimension when temperature Tn is reached)} / (film dimension immediately before temperature Tn) × 100, where n is 1, 2, or 3.

[0086] Here, relaxation refers to reducing the film dimensions in the stretched direction to remove stress present in the film in the stretched direction. Furthermore, the film dimensions refer to the distance between two arbitrarily specified points in the film plane, and may be the distance from one end of the film to the other. When the stretched film of the present disclosure is a strip-shaped film, the film dimensions in the MD direction may be the distance between two arbitrarily specified points in the MD direction in the film plane, and the film dimensions in the TD direction may be the distance between both ends of the film in the width direction. When slack occurs in the film during heat treatment, the film dimensions are the linear distance between two arbitrarily specified points in the film plane, and may be the linear distance from one end of the film to the other end. In particular, the film dimensions in the TD direction may be the linear distance between both ends of the film in the width direction.

[0087] When the film is relaxed using two adjacent rolls while being continuously transported, the film dimension in the MD direction can be adjusted by varying the rotation speed of the two adjacent rolls, while the film dimension in the TD direction can be adjusted by clamping both widthwise ends of the film using a transverse stretching machine such as a clip-type tenter and changing the distance between the clamps.

[0088] Specifically, in the heat treatment step, the amount of relaxation in the MD direction [%] can be calculated by the following formula (i-i), and the amount of relaxation in the TD direction [%] can be calculated by the following formula (i-ii): MD relaxation amount [%] = {(Roll rotation speed immediately before the roll that brings the film to temperature Tn) - (Roll rotation speed of the roll that brings the film to the temperature Tn)} / (Roll rotation speed immediately before the roll that brings the film to temperature Tn) x 100, where n is 1, 2, or 3 (i-i) TD relaxation amount [%] = {(Distance between both end points in the width direction of the film immediately before bringing the film to temperature Tn) - (Distance between both end points in the width direction of the film when the film is brought to temperature Tn)} / (Distance between both end points in the width direction of the film immediately before bringing the film to temperature Tn) x 100, where n is 1, 2, or 3 (i-ii)

[0089] The time for bringing the film to temperatures T1, T2 and T3 is not particularly limited, but from the viewpoint of productivity, it is preferably 0.5 to 30 seconds, more preferably 0.5 to 10 seconds, and even more preferably 0.5 to 5 seconds.

[0090] The means for adjusting the film temperature in the heat treatment is not particularly limited, and examples thereof include non-contact heating methods such as a method of applying hot air heated to within the above-mentioned temperature range to the film, a method of heating the film using an auxiliary heating means such as an infrared heater, and a method of heating the film by placing it in a heating furnace whose temperature is adjusted to within the above-mentioned temperature range; and contact heating methods such as a method of bringing the film into contact with a roll heated to within the above-mentioned temperature range. These methods may be used alone or in combination.

[0091] The means for adjusting the film temperature during heat treatment can be the same as the means for adjusting the film temperature during stretching, so a description of each method will be omitted.

[0092] According to the production method of the present disclosure, the film does not stick to the heating equipment, particularly the rolls, and therefore, as a means for adjusting the film temperature during heat treatment, a method in which the film is brought into contact with a roll heated to within the above temperature range is preferred from the viewpoints of excellent productivity and easy heating, particularly in mass production. This method is suitable for uniaxial stretching, particularly when stretching in the MD direction of the film using multiple rolls of a roll longitudinal stretching machine, because it allows the film to be continuously transported and is therefore excellent in productivity.

[0093] Furthermore, when inflation molding is used in the process of melting a film raw material containing a poly(3-hydroxybutyrate) resin in an extruder and then molding it into a film, the molten resin is extruded from the extruder into a tubular shape, so that the films are in contact with each other after molding. Therefore, depending on the heat treatment conditions, the films may stick to each other. However, according to the production method of the present disclosure, this sticking of the films to each other can be prevented, resulting in excellent productivity.

[0094] The process of sequentially bringing the film to temperatures T1, T2, and T3 may involve contacting the film with rolls. Specifically, the film may be contacted sequentially with roll R1 at temperature T1, roll R2 at temperature T2, and roll R3 at temperature T3.

[0095] When the method of contacting the film with rolls heated to within the above temperature ranges is used as a means for adjusting the film temperature, the times for bringing the film to temperatures T1, T2, and T3 refer to the times for which the film is in contact with roll R1 heated to temperature T1, roll R2 heated to temperature T2, and roll R3 heated to temperature T3, respectively. Roll R1 heated to temperature T1 may consist of not only one roll, but also two or more rolls. Similarly, roll R2 heated to temperature T2 and roll R3 heated to temperature T3 may each consist of one or two or more rolls. The times for bringing the film to temperatures T1, T2, and T3 can be adjusted by changing the rotation speeds of rolls R1, R2, and R3 or by increasing or decreasing the number of rolls.

[0096] The rotational speeds of roll R1, which brings the film to temperature T1, roll R2, which brings the film to temperature T2, and roll R3, which brings the film to temperature T3, are not particularly limited. From the viewpoint of adjusting the aforementioned relaxation amount to 0 to 10% or 5 to 10% and further reducing heat shrinkage after stretching, the ratio of the rotational speed of roll R1 to the rotational speed of the roll immediately preceding roll R1, the ratio of the rotational speed of roll R2 to the rotational speed of roll R1, and the ratio of the rotational speed of roll R3 to the rotational speed of roll R2 are each preferably 90 to 100%, more preferably 90 to 95%. Among these, adjusting the ratio of the rotational speed of roll R1 to the rotational speed of the roll immediately preceding roll R1 within the above range is particularly preferred, since it can effectively suppress rapid heat shrinkage of the stretched film and breakage of the stretched film during production and processing.

[0097] The manufacturing method of the present disclosure may include a step of cooling the film after the step of heat-treating the film. The film temperature in the step of cooling the film may be 60°C or lower, preferably 40°C or lower. As long as the film temperature can be lowered to a temperature lower than the heat treatment temperature, there are no particular limitations on the means for doing so, and examples include a method of bringing the film into contact with rolls cooled to 60°C or lower, preferably 40°C or lower. More specifically, the film may be cooled on one or more rolls, or by sandwiching the film between two rolls.

[0098] In the production method of the present disclosure, from the viewpoint of productivity, it is preferable to carry out the steps from melting the film raw material in an extruder and then forming it into a film to heat-treating it, particularly up to obtaining a stretched film, as a continuous process. Here, the continuous process refers to obtaining a stretched film by sequentially carrying out the steps from melting the film raw material in an extruder and then forming it into a film, stretching the formed film, heat-treating the stretched film, and, if necessary, cooling the film.

[0099] <Thickness of stretched film> The thickness of the stretched film is not particularly limited and may be appropriately set to the desired thickness. From the viewpoints of uniform thickness, appearance, strength, lightness, etc. of the film, the thickness is preferably 10 to 200 μm, more preferably 15 to 150 μm, and even more preferably 20 to 100 μm. The thickness of the film can be measured using a vernier caliper.

[0100] The stretched film of the present disclosure is thin yet has high strength, and therefore can be suitably used as a packaging film, for example, a packaging film for food and the like that requires heat sealability.

[0101] The following items list preferred aspects of the present disclosure, but the present invention is not limited to them. [Item 1] A method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin, comprising the steps of melting a film raw material containing the poly(3-hydroxybutyrate)-based resin in an extruder and then forming it into a film, stretching the formed film, and heat-treating the stretched film, wherein the heat treatment includes a process of raising the film to temperature T1 and then to temperature T2, and the temperatures T1 and T2 satisfy all of the conditions of the following formulas (1) to (3): (melting point of poly(3-hydroxybutyrate)-based resin - 70) ° C. ≦ T1 (1) T2 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 20) ° C. (2) T1 ≠ T2 and T1 < T2 (3) [Item 2] The production method according to Item 1, wherein the temperature T1 and the temperature T2 satisfy the conditions of the following formulas (4) and (5). (melting point of poly(3-hydroxybutyrate)-based resin - 70) ° C. ≦ T1 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 30) ° C. (4) (melting point of poly(3-hydroxybutyrate)-based resin - 60) ° C. ≦ T2 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 20) ° C. (5) [Item 3] The production method according to Item 1 or 2, wherein the heat treatment includes a treatment of raising the temperature to T2 and then raising the temperature to T3, and the temperatures T1, T2, and T3 satisfy the conditions of the following formulas (6) and (7). (melting point of poly(3-hydroxybutyrate) resin - 40) °C ≦ T3 ≦ (melting point of poly(3-hydroxybutyrate) resin - 15) °C (6) T1 < T2 < T3 (7) [Item 4] The manufacturing method according to any one of items 1 to 3, wherein the treatment of bringing the film to temperature T1 is a treatment of bringing the film into contact with roll R1 that has been set to temperature T1, and the treatment of bringing the film to temperature T2 is a treatment of bringing the film into contact with roll R2 that has been set to temperature T2. [Item 5] The manufacturing method according to any one of items 1 to 4, wherein the film is brought to temperature T1 with a relaxation amount of 0 to 10% in the stretching direction. [Item 6] The manufacturing method according to any one of items 1 to 5, wherein the time for bringing the film to temperature T1 is 0.5 to 30 seconds.[Item 7] The manufacturing method according to any one of items 1 to 6, wherein the time for bringing the film to temperature T2 is 0.5 to 30 seconds. [Item 8] The manufacturing method according to any one of items 1 to 7, wherein the poly(3-hydroxybutyrate)-based resin is poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). [Item 9] The manufacturing method according to any one of items 1 to 8, wherein the stretching is uniaxial stretching.

[0102] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples in any way.

[0103] In the examples and comparative examples, the following raw materials were used. Production of poly(3-hydroxybutyrate) resin A-1 and resin pellets P-1 Poly(3-hydroxybutyrate) resin A-1: ​​P3HB3HH (average content ratio: 3-hydroxybutyrate unit / 3-hydroxyhexanoate unit [3HB / 3HH] = 94 / 6 (mol% / mol%), weight average molecular weight 600,000 g / mol, glass transition temperature 6°C) was produced in accordance with the method described in Example 1 of WO 2019 / 142845. The melting point of resin A-1 was 146°C.

[0104] 100 parts by weight of poly(3-hydroxybutyrate) resin A-1 was dry-blended with 0.5 parts by weight of behenic acid amide (BNT-22H, manufactured by Nippon Fine Chemicals Co., Ltd.) as a lubricant. The dry blend was fed into a φ26 mm co-rotating twin-screw extruder with cylinder and die temperatures set to 150°C, extruded, passed through a water bath filled with hot water at 45°C to solidify the strands, and cut with a pelletizer to obtain resin pellets P-1.

[0105] (Weight Average Molecular Weight) The weight average molecular weight of the resin was measured in terms of polystyrene using the aforementioned gel permeation chromatography (HPLC GPC system manufactured by Shimadzu Corporation).

[0106] (Glass Transition Temperature) The glass transition temperature (Tg) of the resin was determined by differential scanning calorimetry in accordance with JIS K-7121. Specifically, first, about 5 mg of the sample to be measured was precisely weighed, and the temperature was raised from -20°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter (Seiko Instruments Inc., SSC5200), to obtain a DSC curve. Next, in the obtained DSC curve, the baseline before and after the stepwise change due to the glass transition was extended, and a center line equidistant from these two straight lines in the vertical direction was drawn. The temperature at the point where this center line intersects with the curve of the stepwise change due to the glass transition was taken as the glass transition temperature (Tg).

[0107] (Melting Point) The melting point was determined by differential scanning calorimetry in accordance with JIS K-7121. Specifically, first, about 4 to 5 mg of the sample to be measured was precisely weighed, and the sample was heated from 0°C to 180°C at a heating rate of 10°C / min using a differential scanning calorimeter (Seiko Instruments Inc., SSC5200) to obtain a DSC curve. The apex temperature of the melting point peak in the obtained DSC curve was taken as the melting point.

[0108] (Film Thickness) The thickness was measured at 10 points at 10 cm intervals along the TD direction of the film using a vernier caliper, and the arithmetic mean value of the thicknesses at the 10 points was calculated to be the film thickness.

[0109] (Stickness to Roll) The film produced in each Example and Comparative Example was brought into contact with a roll (having a stainless steel surface) heated to the highest temperature among the heat treatment conditions in each Example and Comparative Example, and the presence or absence of sticking to the roll was confirmed.

[0110] (Stickness of films to each other) Two films produced in each Example and Comparative Example were stacked together and brought into contact with a roll (with a stainless steel surface) heated to the highest temperature among the heat treatment conditions in each Example and Comparative Example, and the presence or absence of sticking of the films to each other was confirmed.

[0111] (Film Heat Shrinkage) The film to be measured was cut into a square measuring 5 cm in the MD direction x 5 cm in the TD direction, and heated for 10 minutes in an oven set to 110°C. Furthermore, the dimensions of the film in the MD direction and the TD direction after heating were measured, and the heat shrinkage in the MD direction and the TD direction were calculated based on the dimensions in the MD direction and the TD direction, respectively, using the following formula: Heat shrinkage [%] = (1 - (dimension after heating) / (dimension before heating)) x 100

[0112] Example 1 Using an inflation molding machine having a φ50 mm single-screw extruder and an inflation molding die (die diameter 100 mm, lip clearance 1.0 mm), the resin pellets P-1 and PBAT (manufactured by BASF: ecoflex (registered trademark) F Blend C1200) were dry-blended in a weight ratio of P-1:PBAT = 10:7. The resin was fed into the extruder as a film raw material and molded into a film at a discharge rate of 29 kg / h, a resin temperature of 165 ° C, a film folding width of 390 mm (blow-up ratio 2.5), and a take-up speed of 5 m / min. After being molded into a film, the raw film was made into a raw film. The raw film was taken up by a take-up roll and continuously stretched in the machine direction (MD) at a stretching temperature of 64 to 65 ° C. to a stretching ratio of 3 times.

[0113] A 100 mm x 210 mm test piece was cut from the stretched film (cut with the short side in the MD direction), and while holding all four sides (relaxation amount in the MD direction: 0%), it was placed in a hot air oven and held at 90°C for 10 seconds, after which it was removed to obtain a first-stage heat-treated stretched film. Then, while holding all four sides (relaxation amount in the MD direction: 0%), it was placed in a hot air oven and held at 120°C for 10 seconds, after which it was removed to obtain a second-stage heat-treated stretched film. Furthermore, while holding all four sides (relaxation amount in the MD direction: 0%), it was placed in a hot air oven and held at 130°C for 10 seconds, after which it was removed to obtain a third-stage heat-treated stretched film. The resulting stretched film showed no sticking to the rolls or to each other. Furthermore, the heat shrinkage was 4% in the MD direction and 0% in the TD direction. The evaluation results of the stretched film are shown in Table 1.

[0114]

[0115] Example 2 A stretched film was obtained in the same manner as in Example 1, except that the first heat treatment conditions were a film temperature of 90°C and a relaxation amount in the MD direction of 10%, the second heat treatment conditions were a film temperature of 110°C, and the third heat treatment conditions were a film temperature of 120°C. The obtained stretched film did not stick to the rolls or to other films. Furthermore, the heat shrinkage was 4% in the MD direction and 0% in the TD direction. The evaluation results of the stretched film are shown in Table 1.

[0116] Example 3 A stretched film was obtained in the same manner as in Example 1, except that the first heat treatment conditions were a film temperature of 80°C and a relaxation amount in the MD direction of 5%, the second heat treatment conditions were a film temperature of 90°C, and the third heat treatment conditions were a film temperature of 110°C and a relaxation amount in the MD direction of 10%. The stretched film obtained did not stick to the roll or to each other. Furthermore, the heat shrinkage was 3% in the MD direction and 1% in the TD direction. The evaluation results of the stretched film are shown in Table 1.

[0117] Example 4 A stretched film was obtained in the same manner as in Example 1, except that the third heat treatment step was omitted. The obtained stretched film did not stick to the rolls or to other films. Furthermore, the amount of heat shrinkage was 6% in the MD direction and 0% in the TD direction. The evaluation results of the stretched film are shown in Table 1.

[0118] <Comparative Example 1> A stretched film was obtained in the same manner as in Example 1, except that the heat treatment was not performed. However, sticking to the roll and sticking between films occurred. The amount of heat shrinkage was 20% in the MD direction and 3% in the TD direction. The evaluation results of the stretched film are shown in Table 1.

[0119] Comparative Example 2 A film was obtained in the same manner as in Example 1, except that the film temperature was set to 130°C as the first heat treatment condition, and the second and third heat treatments were not performed. However, sticking to the roll and sticking between films occurred. The evaluation results of the stretched film are shown in Table 1.

[0120] Comparative Example 3 An attempt was made to produce a film in the same manner as in Example 1, except that the first heat treatment conditions were a film temperature of 90°C, the second heat treatment conditions were a film temperature of 130°C, and the third heat treatment was not performed. However, sticking to the rolls and sticking between films occurred. The evaluation results of the stretched film are shown in Table 1.

[0121] Comparative Example 4: An attempt was made to produce a film in the same manner as in Example 1, except that the film temperature was changed to 90°C in all heat treatment conditions from the first to third stages in Example 1. As a result, there was no sticking to the rolls or to each other, but the amount of heat shrinkage was 16% in the MD direction and 2% in the TD direction. The evaluation results of the stretched film are shown in Table 1.

[0122] The heat treatment conditions included a treatment in which the film was heated to temperature T1 and then to temperature T2, and in Examples 1 to 4, in which the temperatures T1 and T2 satisfied all of the conditions of the above formulas (1) to (3), the amount of heat shrinkage in the TD direction was 1% or less, and the amount of heat shrinkage in the MD direction (stretching direction) was also small, being less than 10%, and there was no sticking to the rolls or between films, which indicates that production could be carried out with good productivity.

Claims

1. A method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin, comprising: a step of melting a film raw material containing the poly(3-hydroxybutyrate)-based resin in an extruder and then forming it into a film; stretching the formed film; heat-treating the stretched film; the heat treatment includes a process of raising the film to temperature T1 and then to temperature T2, wherein the temperatures T1 and T2 satisfy all of the conditions of the following formulas (1) to (3), and the heat treatment includes a process of raising the film to temperature T2 and then to temperature T3, wherein the temperatures T1, T2, and T3 satisfy the conditions of the following formulas (6) and (7). (melting point of poly(3-hydroxybutyrate)-based resin - 70) ° C. ≦ T1 (1) T2≦(melting point of poly(3-hydroxybutyrate)-based resin−20)°C (2) T1≠T2 and T1<T2 (3) (melting point of poly(3-hydroxybutyrate)-based resin - 40) ° C. ≦ T3 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 15) ° C. (6) T1<T2<T3 (7)

2. The manufacturing method according to claim 1 , wherein the temperature T1 and the temperature T2 satisfy the conditions of the following formulas (4) and (5). (melting point of poly(3-hydroxybutyrate)-based resin - 70) ° C. ≦ T1 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 30) ° C. (4) (melting point of poly(3-hydroxybutyrate)-based resin - 60) ° C. ≦ T2 ≦ (melting point of poly(3-hydroxybutyrate)-based resin - 20) ° C. (5)

3. 3. The manufacturing method according to claim 1 or 2, wherein the process of bringing the film to temperature T1 is a process of bringing the film into contact with a roll R1 brought to temperature T1, and the process of bringing the film to temperature T2 is a process of bringing the film into contact with a roll R2 brought to temperature T2.

4. The method according to claim 1 or 2, wherein the film is brought to a temperature T1 with a relaxation amount of 0 to 10% in the stretching direction.

5. The method according to claim 1 or 2, wherein the time for bringing the film to the temperature T1 is 0.5 to 30 seconds.

6. The method according to claim 1 or 2, wherein the time for raising the film to the temperature T2 is 0.5 to 30 seconds.

7. 3. The method according to claim 1, wherein the poly(3-hydroxybutyrate)-based resin is poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

8. The method according to claim 1 or 2, wherein the stretching is uniaxial stretching.