JP7807897B2 - Stretched film manufacturing method - Google Patents

Description

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

In recent years, the separate collection and composting of food waste has become more common, particularly in Europe, and there is a demand for plastic products that can be composted along with food waste.

On the other hand, environmental problems caused by discarded plastics have been brought into the spotlight, and it has become clear that large amounts of plastic, particularly plastic that has been dumped in the ocean or has entered the sea via rivers, are drifting in the oceans on a global scale. Because such plastics retain their shape for long periods of time, they have been pointed out to have an impact on the ecosystem, such as trapping and capturing marine life (a practice known as ghost fishing), and, if ingested by marine life, becoming trapped in their digestive tracts and causing feeding disorders.

Furthermore, it has been pointed out that microplastics, which are plastics that break down and atomize due to ultraviolet rays and other factors, absorb harmful compounds in seawater, and when these are ingested by marine organisms, harmful substances are introduced into the food chain.

The use of biodegradable plastics is expected to be a solution to 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 the short term in the cold ocean temperatures, 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.

A method of stretching film is known as a technique for producing thin, high-strength films. For example, to produce stretched film from a general-purpose resin such as polypropylene, the molten resin is cooled and solidified using a cast roll to form a raw sheet, which is then preheated to a temperature at which it can be stretched and then stretched, allowing for continuous, highly productive production of stretched film.

However, poly(3-hydroxybutyrate) resins are known to be difficult to stretch due to their properties.
Patent Document 1 describes a method for producing a stretched film by melting a thermoplastic resin containing a poly(3-hydroxybutyrate)-based resin as a main component, forming it into a film, crystallizing it over a certain period of time, sandwiching it between two rolls and rolling it to perform a primary stretching, and then performing a secondary stretching at a temperature higher than the temperature during the rolling.

Patent Document 2 describes a method for producing a stretched film by rapidly cooling a molten film made from a poly(3-hydroxybutyrate) resin to a temperature below the glass transition temperature of the resin + 10°C and solidifying it to produce an amorphous film, then cold-stretching the amorphous film at a temperature below the glass transition temperature + 20°C (specifically, 3°C), and further subjecting it to a tension heat treatment at a temperature of 25 to 160°C (specifically, at 100°C for 2 hours).

Japanese Patent Application Laid-Open No. 2006-168159 Japanese Patent Application Laid-Open No. 2003-311824

According to the method described in Patent Document 1, a stretched film containing a poly(3-hydroxybutyrate)-based resin as a main component can be produced and a high stretching ratio can be achieved, but an annealing step for crystallizing the poly(3-hydroxybutyrate)-based resin must be carried out before stretching. It is described that this annealing step takes a long time, such as 12 hours, and therefore the film cannot be produced in a continuous process, resulting in poor productivity.
Furthermore, the method described in Patent Document 1 requires two-stage stretching steps, namely, primary stretching by roll rolling and secondary stretching at a high temperature, in order to achieve a high stretch ratio, which also poses a problem of making the production process complicated.

The method described in Patent Document 2 also allows for high stretching ratios to be achieved in stretched films primarily composed of poly(3-hydroxybutyrate) resin. However, this document does not describe or suggest the continuous production of stretched films, and does not take productivity into consideration at all. It also describes that tension heat treatment after stretching requires a long time, on the order of two hours.

On the other hand, when the technology for continuously producing stretched film from general-purpose resins such as polypropylene was directly applied to poly(3-hydroxybutyrate)-based resins, stretching became difficult and it was not possible to increase the stretch ratio.

In light of the above-mentioned current situation, the present invention aims to provide a method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin at a high stretch ratio in a continuous process with high productivity.

As a result of extensive research to solve the above-mentioned problems, the inventors discovered that in a technology in which molten resin is cooled and solidified using a casting roll before being stretched, by controlling the film temperature in the process of peeling the film from the casting roll and the process of stretching the peeled film to fall within specific ranges, stretched films containing poly(3-hydroxybutyrate) resins can be produced in a continuous process with good productivity and at high stretch ratios, leading to the completion of the present invention.

That is, the present invention provides a method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin, the method comprising the following steps (i) to (iii-a):
(i) A step of melting a film raw material containing the poly(3-hydroxybutyrate)-based resin and extruding it onto a cast roll to form a film; (ii) A step of peeling the film from the cast roll under conditions in which the temperature of the film formed in step (i) is in the range of 0 to 50°C; (iii-a) A step of stretching the film in the MD direction under conditions in which the temperature of the film obtained in step (ii) is in the range of 10 to 65°C. The production method can further include the following step (iii-b).
(iii-b) A step of stretching the film obtained in the step (iii-a) in the TD direction under conditions in which the temperature of the film is in the range of 10 to 70°C. The production method can further include the following step (iv).
(iv) a step of heating the film obtained in step (iii-a) or (iii-b) so that the temperature of the film is at least 10°C higher than the temperature of the film in step (iii-a) or (iii-b) and is 60°C or higher. Preferably, the temperature of the film formed in step (i) is a temperature that exceeds the glass transition temperature of the poly(3-hydroxybutyrate)-based resin + 10°C.
Preferably, the stretching ratio in the steps (iii-a) and/or (iii-b) is 2 to 8 times.
Preferably, steps (i) through the final step are carried out while the film is being transported continuously.
Preferably, the stretching in the MD direction in the step (iii-a) is carried out by varying the rotation speed of the rolls that transport the film.
Preferably, the poly(3-hydroxybutyrate)-based resin includes poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

According to the present invention, it is possible to provide a method for producing a stretched film containing a poly(3-hydroxybutyrate) resin at a high stretch ratio in a continuous process with good productivity.
According to the present invention, it is possible to produce a uniaxially stretched film stretched in the MD direction, or a biaxially stretched film stretched in both the MD and TD directions, and it is possible to achieve a high stretch ratio in each direction.

1A and 1B are schematic diagrams illustrating an example of a production line according to one embodiment of the present invention, from extrusion of a film raw material to film formation and film stretching, followed by film winding.

The following describes an embodiment of the present invention, but 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.
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.

Specific examples of the poly(3-hydroxybutyrate)-based resin include poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), poly(3-hydroxybutyrate-co-4-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyoctanoate), and poly(3-hydroxybutyrate-co-3-hydroxyoctadecanoate). 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 due to their ease of industrial production.

Furthermore, by changing the composition ratio of the repeating units, it is possible to change the melting point and degree of crystallinity, thereby changing physical properties such as Young's modulus and heat resistance, and it is possible to impart physical properties between those of polypropylene and polyethylene. Furthermore, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) is preferred from the viewpoints of ease of industrial production and useful physical properties. In particular, among poly(3-hydroxybutyrate)-based resins, which tend to thermally decompose when heated to 180°C or above, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) is preferred from the viewpoints of its ability to lower the melting point and enable molding and processing at low temperatures.

Commercially available poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) products include Kaneka Corporation's "Kaneka Biodegradable Polymer PHBH" (registered trademark).

The melting point and Young's modulus of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) vary depending on the ratio of the 3-hydroxybutyrate and 3-hydroxyvalerate components. However, because the two components co-crystallize, the degree of crystallinity is high at 50% or more, making it more flexible than poly(3-hydroxybutyrate), but its brittleness is not sufficiently improved.

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.

The average content ratio of each monomer unit to all monomer units constituting the poly(3-hydroxybutyrate)-based resin can be determined by methods known to those skilled in the art, such as the method described in paragraph [0047] of WO 2013/147139. The average content ratio refers to the molar ratio of each monomer unit to all monomer units constituting the poly(3-hydroxybutyrate)-based resin. When the poly(3-hydroxybutyrate)-based resin is a mixture of two or more poly(3-hydroxybutyrate)-based resins, the average content ratio refers to the molar ratio of each monomer unit contained in the entire mixture.

The poly(3-hydroxybutyrate) resin may be a mixture of at least two poly(3-hydroxybutyrate) resins that differ in the type and/or content ratio of the constituent monomers. In this case, at least one highly crystalline poly(3-hydroxybutyrate) resin and at least one low-crystalline poly(3-hydroxybutyrate) resin can be used in combination.

Generally, highly crystalline poly(3-hydroxybutyrate) resins have excellent productivity but poor mechanical strength, while low-crystalline poly(3-hydroxybutyrate) resins have poor productivity but excellent mechanical properties. It is believed that when both resins are used in combination, the highly crystalline poly(3-hydroxybutyrate) resin forms fine resin crystal particles, while the low-crystalline poly(3-hydroxybutyrate) resin forms tie molecules that crosslink the resin crystal particles. Using these resins in combination can improve the strength and productivity of stretched films.

The content of 3-hydroxybutyrate units contained in the highly crystalline poly(3-hydroxybutyrate) resin is preferably higher than the average content of 3-hydroxybutyrate units in all monomer units constituting the mixture of poly(3-hydroxybutyrate) resins.
When a highly crystalline poly(3-hydroxybutyrate) resin contains 3-hydroxybutyrate units and other hydroxyalkanoate units, the content of the other hydroxyalkanoate units in the highly crystalline resin is preferably 1 to 5 mol %, more preferably 2 to 4 mol %.

The highly crystalline poly(3-hydroxybutyrate) resin is preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) or poly(3-hydroxybutyrate-co-4-hydroxybutyrate), with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) being more preferred.

Furthermore, the content of 3-hydroxybutyrate units in the low-crystalline poly(3-hydroxybutyrate) resin is preferably lower than the average content of 3-hydroxybutyrate units in all monomer units constituting the mixture of poly(3-hydroxybutyrate) resins.
When a low-crystalline poly(3-hydroxybutyrate) resin contains 3-hydroxybutyrate units and other hydroxyalkanoate units, the content of the other hydroxyalkanoate units in the low-crystalline resin is preferably 24 to 99 mol%, more preferably 24 to 50 mol%, even more preferably 24 to 35 mol%, and particularly preferably 24 to 30 mol%.

The low-crystalline poly(3-hydroxybutyrate) resin is preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) or poly(3-hydroxybutyrate-co-4-hydroxybutyrate), with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) being more preferred.

When a highly crystalline poly(3-hydroxybutyrate) resin and a low crystalline poly(3-hydroxybutyrate) resin are used in combination, there are no particular restrictions on the proportion of each resin relative to the total amount of both resins, but it is preferable that the former be 10% by weight or more and 60% by weight or less, and the latter be 40% by weight or more and 90% by weight or less, and it is even more preferable that the former be 25% by weight or more and 45% by weight or less, and the latter be 55% by weight or more and 75% by weight or less.

In one embodiment, in addition to the highly crystalline poly(3-hydroxybutyrate)-based resin and the low crystalline poly(3-hydroxybutyrate)-based resin, a medium crystalline poly(3-hydroxybutyrate)-based resin, whose crystallinity is intermediate between the two resins, can be used in combination.

When a medium-crystalline poly(3-hydroxybutyrate) resin contains 3-hydroxybutyrate units and other hydroxyalkanoate units, the content of other hydroxyalkanoate units in the medium-crystalline resin is preferably 6 mol% or more and less than 24 mol%, more preferably 6 mol% or more and 22 mol% or less, even more preferably 6 mol% or more and 20 mol% or less, and preferably 6 mol% or more and 18 mol% or less.

The medium-crystalline poly(3-hydroxybutyrate) resin is preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) or poly(3-hydroxybutyrate-co-4-hydroxybutyrate), with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) being more preferred.

When the medium-crystalline poly(3-hydroxybutyrate) resin is further used in combination, the proportion of the medium-crystalline poly(3-hydroxybutyrate) resin relative to the total of the high-crystalline poly(3-hydroxybutyrate) resin, the low-crystalline poly(3-hydroxybutyrate) resin, and the medium-crystalline poly(3-hydroxybutyrate) resin is preferably 1% by weight or more and 99% by weight or less, more preferably 5% by weight or more and 90% by weight or less, and even more preferably 8% by weight or more and 85% by weight or less.

The method for obtaining a blend of two or more poly(3-hydroxybutyrate) resins is not particularly limited, and may be a method of obtaining a blend by microbial production or a method of obtaining a blend by chemical synthesis. Alternatively, a blend may be obtained by melt-kneading two or more resins using an extruder, kneader, Banbury mixer, roll, etc., or by dissolving two or more resins in a solvent, mixing, and drying them.

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, more preferably 250,000 to 1,500,000, and even more preferably 300,000 to 1,000,000.

Furthermore, when the poly(3-hydroxybutyrate) resin is a mixture of two or more poly(3-hydroxybutyrate) resins, the weight-average molecular weight of each poly(3-hydroxybutyrate) resin constituting the mixture is not particularly limited. However, when the aforementioned high-crystalline poly(3-hydroxybutyrate) resin and low-crystalline poly(3-hydroxybutyrate) resin are used in combination, the weight-average molecular weight of the high-crystalline poly(3-hydroxybutyrate) resin is preferably 200,000 to 1,000,000, more preferably 220,000 to 800,000, and even more preferably 250,000 to 600,000, from the viewpoint of achieving both the strength and productivity of the stretched film. On the other hand, the weight-average molecular weight of the low-crystalline poly(3-hydroxybutyrate) resin is preferably 200,000 to 2,500,000, more preferably 250,000 to 2,300,000, and even more preferably 300,000 to 2,000,000, from the viewpoint of achieving both the strength and productivity of the stretched film. Furthermore, when the aforementioned medium-crystalline poly(3-hydroxybutyrate) resin is further used, the weight-average molecular weight of the medium-crystalline poly(3-hydroxybutyrate) resin is preferably 200,000 to 2,500,000, more preferably 250,000 to 2,300,000, and even more preferably 300,000 to 2,000,000, from the viewpoint of achieving both strength and productivity of the stretched film.

The weight-average molecular weight of poly(3-hydroxybutyrate) resins can be measured in polystyrene equivalent terms using gel permeation chromatography (HPLC GPC system manufactured by Shimadzu Corporation) with a chloroform solution. A column suitable for measuring weight-average molecular weights can be used for the gel permeation chromatography.

The method for producing poly(3-hydroxybutyrate) resins is not particularly limited, and may be a chemical synthesis method or a microbial production method. Microbial production methods are preferred. Known methods can be applied to microbial production methods. 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 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 can be 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.

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").
When a modified resin is used as a film raw material, the resin may be reacted with the modifying raw material in advance and the resulting raw material may be molded into a film, or the modifying raw material may be mixed with the resin and reacted during film molding. When reacting the resin with the modifying raw material, the entire resin may be reacted with the modifying raw material, or a part 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.

The modifying raw material is not particularly limited as long as it is a compound that can react with the poly(3-hydroxybutyrate) resin. However, organic peroxides are preferred because of their ease of handling and the ease of controlling the reaction with the poly(3-hydroxybutyrate) resin.

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 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. Of these, t-butylperoxy 2-ethylhexyl carbonate and t-butylperoxy isopropyl carbonate are preferred. Furthermore, combinations of two or more of these organic peroxides can also be used.

The organic peroxide may be used in various forms, such as solid or liquid, and may be in liquid form diluted with a diluent or the like. Among these, organic peroxides in a form that can be easily mixed with the poly(3-hydroxybutyrate)-based resin (particularly organic peroxides that are liquid at room temperature (25°C)) are preferred because they can be more uniformly dispersed in the poly(3-hydroxybutyrate)-based resin and are more likely to suppress localized modification reactions within the resin composition.

(Other resins)
The film raw material or the stretched film may contain other resins besides poly(3-hydroxybutyrate)-based resins, as long as the effects of the invention are not impaired. Examples of such other resins include aliphatic polyester-based resins such as polybutylene succinate adipate, polybutylene succinate, polycaprolactone, and polylactic acid, and aliphatic aromatic polyester-based resins such as polybutylene adipate terephthalate, polybutylene sebatate terephthalate, and polybutylene azelate terephthalate. Only one type of other resin may be contained, or two or more types may be contained.

The content of the other resin is not particularly limited, but is preferably 100 parts by weight or less, more preferably 50 parts by weight or less, even more preferably 30 parts by weight or less, even more preferably 10 parts by weight or less, and particularly preferably 5 parts by weight or less, per 100 parts by weight of the poly(3-hydroxybutyrate) resin. There is no particular lower limit for the content of the other resin, and it may be 0 parts by weight or more.

The film raw material or the stretched film may contain additives that can be used with the poly(3-hydroxybutyrate) resin, as long as they do not impair the effects of the invention. 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. Only one type of additive may be contained, or two or more types may be contained. The content of these additives can be appropriately determined by those skilled in the art depending on the intended use.
The nucleating agent, lubricant, filler, and plasticizer will be described in more detail below.

(Crystal nucleating agent)
The film raw material or the stretched film may also contain a 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)-based resins. One or more crystal nucleating agents may be used, and the ratio of their use can be appropriately adjusted depending on the purpose.
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.

(lubricant)
The film raw material or the stretched film may also contain a 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 lubricants may be used, and the ratio of use can be appropriately adjusted depending on the purpose.
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, relative to 100 parts by weight of the total amount of the poly(3-hydroxybutyrate)-based resin.

(filling material)
The film raw material or the stretched film may contain a filler. By including a filler, the stretched film can have higher strength. 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 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.

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, per 100 parts by weight of poly(3-hydroxybutyrate) resin. However, the film raw material or the stretched film does not have to contain a filler.

(Plasticizer)
The film raw material or the stretched film may contain a plasticizer. 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.
The amount of 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 poly(3-hydroxybutyrate) resin. However, the film raw material or the stretched film does not necessarily need to contain a plasticizer.

Next, a method for producing a stretched film according to this embodiment will be described.
The method for producing a stretched film according to this embodiment includes at least the following steps.
(i) a step of melting a film raw material containing the poly(3-hydroxybutyrate)-based resin and extruding it onto a cast roll to form a film; (ii) a step of peeling the film formed in step (i) from the cast roll under conditions in which the temperature of the film is in the range of 0 to 50°C; and (iii-a) a step of stretching the film obtained in step (ii) in the MD direction under conditions in which the temperature of the film is in the range of 10 to 65°C. By steps (i), (ii), and (iii-a), a uniaxially stretched film stretched in the MD direction can be obtained.

The MD direction is also called the machine direction, flow direction, or longitudinal direction. The TD direction, described below, is the direction perpendicular to the MD direction and is also called the vertical direction or width direction.

After the step (iii-a), it is preferable to carry out the following step (iv).
(iv) a step of heating the film obtained in step (iii-a) so that the temperature of the film becomes 10°C or more higher than the temperature of the film in step (iii-a) and becomes 60°C or higher. By steps (i), (ii), (iii-a), and (iv), a uniaxially stretched film that is stretched in the MD direction and has high strength in the MD direction can be obtained.

After the step (iii-a), the following step (iii-b) can also be carried out.
(iii-b) A step of stretching the film obtained in step (iii-a) in the TD direction under conditions in which the temperature of the film is in the range of 10 to 70°C. By steps (i), (ii), (iii-a), and (iii-b), a biaxially stretched film stretched in both the MD and TD directions can be obtained.

After the step (iii-b), it is preferable to carry out the following step (iv).
(iv) a step of heating the film obtained in step (iii-b) so that the temperature of the film is at least 10°C higher than the temperature of the film in step (iii-b) and is 60°C or higher. By steps (i), (ii), (iii-a), (iii-b) and (iv), a biaxially stretched film is stretched in both the MD and TD directions, and high strength can be obtained in both the MD and TD directions.

Each step will be described below.
(Step (i))
In step (i), the film raw material is first melted. The melting method is not particularly limited, but it is preferable to extrude the molten film raw material through a T-die, i.e., to carry out the extrusion molding method. The extrusion molding method makes it easy to produce a film with a uniform thickness. For extrusion molding, a single-screw extruder, a twin-screw extruder, or the like can be used as appropriate.

The conditions for melting the film raw material may be any conditions that melt the poly(3-hydroxybutyrate) resin, and the temperature of the molten film raw material may be, for example, approximately 140 to 210°C.

The molten film raw material is then extruded onto a casting roll to form a film. The molten film raw material comes into contact with the casting roll and is cooled while moving along the surface of the casting roll. This causes a portion of the poly(3-hydroxybutyrate) resin to crystallize.

This step may be a step of extruding a melt onto one or more casting rolls, or a step of placing a touch roll opposite a casting roll and sandwiching the melt extruded onto the casting roll between the touch rolls. Note that this step is not a step of applying pressure to the film to perform roll rolling.
An air knife or an air chamber may be used to stably bring the melt into contact with the casting roll. To efficiently cool the opposite side of the contact surface with the casting roll, the casting roll may be placed in a water tank or an air chamber may be used.

The set temperature of the casting roll is preferably 50°C or less in order to control the film temperature in step (ii) described below. It is more preferably 45°C or less, and even more preferably 40°C or less.

However, if the casting roll temperature is set too low, the poly(3-hydroxybutyrate) resin will not solidify sufficiently, and will tend to stick to the casting roll, making it difficult to remove. Therefore, the lower limit of the casting roll temperature is preferably 0°C or higher, more preferably 5°C or higher, even more preferably 10°C or higher, even more preferably 12°C or higher, and particularly preferably 15°C or higher. Furthermore, the lower limit of the casting roll temperature may be a temperature above the glass transition temperature (Tg) of the poly(3-hydroxybutyrate) resin + 10°C, or may be a temperature of Tg + 12°C or higher, or may be a temperature of Tg + 14°C or higher.

(Step (ii))
In step (ii), the film formed in step (i) is peeled from the casting roll under conditions in which the temperature of the film is within a range of 0 to 50° C. While the casting roll is rotating, the film is transported toward the next stretching step, whereby the film can be peeled from the casting roll.

The film temperature in step (ii) is controlled to be 50°C or less. This makes it possible to control the crystallinity of the film to a relatively low level in the subsequent step (iii-a), making it possible to stretch the film containing poly(3-hydroxybutyrate) resin at a high ratio. The film temperature is preferably 45°C or less, more preferably 40°C or less, and even more preferably 35°C or less.

In order to facilitate peeling of the film from the casting roll, the lower limit of the film temperature in step (ii) is 0°C or higher. 5°C or higher is preferred, 10°C or higher is more preferred, 12°C or higher is even more preferred, and 15°C or higher is particularly preferred. In addition, the lower limit of the film temperature in step (ii) may be a temperature exceeding the glass transition temperature (Tg) of the poly(3-hydroxybutyrate) resin + 10°C, or may be a temperature of Tg + 12°C or higher, or may be a temperature of Tg + 14°C or higher.

The film temperature in step (ii) is determined primarily by the temperature of the film raw material melted in step (i) and the set temperature of the casting roll described above. Other factors that also have an effect include the ambient temperature around the casting roll and the contact time between the casting roll and the film. Those skilled in the art can easily control the film temperature by taking these parameters into account.

(Step (iii-a))
In step (iii-a), the film obtained in step (ii) is stretched in the MD direction under conditions in which the temperature of the film is in the range of 10 to 65° C. Step (iii-a) is preferably carried out continuously from step (ii) in one production line.
In this step (iii-a), the film is preferably stretched by stretching in the MD direction. In the present application, stretching the film in the MD direction refers to pulling the film in the MD direction, and is distinguished from stretching by applying pressure in the thickness direction of the film, such as roll rolling in which the film is sandwiched between two rolls.

Stretching in the MD direction is not particularly limited, but can be carried out, for example, using a roll longitudinal stretching machine by varying the rotation speed of the multiple rolls that transport the film. The stretching ratio in the MD direction can be determined by the ratio of the rotation speed of the rolls before stretching to the rotation speed of the rolls after stretching.

Conventionally, when producing stretched films from general-purpose resins such as polypropylene, the molten resin is cooled and solidified, and then preheated to a high temperature, such as 145°C, to increase the crystallinity of the film before stretching. In contrast, this embodiment is clearly different from the production of stretched films from general-purpose resins such as polypropylene in that the film is stretched at a temperature of 65°C or less in order to control the crystallinity of the film to a relatively low range.

The film temperature in step (iii-a) is controlled to 65°C or lower. This suppresses the progression of crystallization of the poly(3-hydroxybutyrate) resin during this step, resulting in a relatively large amount of amorphous regions in the film. This allows crystalline molecules to be oriented in the amorphous regions, enabling a high stretch ratio. Furthermore, when step (iii-b) is subsequently performed, the film's crystallinity can be controlled to a relatively low level in step (iii-b) as well, enabling high stretch ratios in the TD direction. If the film temperature exceeds 65°C during this step, the film becomes brittle, increasing the likelihood of the film breaking during stretching and making it difficult to achieve a high stretch ratio. The film temperature is preferably 55°C or lower, more preferably 45°C or lower, and even more preferably 35°C or lower.

In addition, to facilitate peeling of the film from the rolls and other equipment used in the stretching process, the lower limit of the film temperature in step (iii-a) is 10°C or higher, preferably 15°C or higher.

Furthermore, it is preferable that the film temperature in step (iii-a) be equal to or higher than the film temperature in step (ii).

The film temperature in step (iii-a) is determined primarily by the film temperature in the previous step (ii), the temperature conditions in this step (iii-a), the time required, etc. Those skilled in the art can easily control the film temperature by taking these parameters into account.

The means for controlling the film temperature in step (iii-a) is not particularly limited, but examples include a method of directing airflow adjusted to a predetermined temperature onto the film, a method of controlling the film temperature by setting a roll to a predetermined temperature, a method of heating the film using auxiliary heating means such as an IR heater to control the film temperature to a predetermined temperature, and a method of passing the film through an oven adjusted to a predetermined temperature. These methods may be used alone or in combination.

The stretching ratio in step (iii-a) is not particularly limited, but is preferably 2 times or more. It is more preferably 2.5 times or more, and even more preferably 3 times or more. According to this embodiment, such a high stretching ratio can be achieved by controlling the film temperature in steps (ii) and (iii-a). The upper limit of the stretching ratio is not particularly limited and may be determined appropriately, but may be, for example, 8 times or less.

(Step (iii-b))
In step (iii-b), the stretched film obtained in step (iii-a) is stretched in the TD direction under conditions in which the temperature of the stretched film is in the range of 10 to 70° C. Step (iii-b) is preferably carried out continuously from step (iii-a) in one production line.
In this step (iii-b), the film is preferably stretched by stretching in the TD direction. As described above, stretching the film in the TD direction refers to pulling the film in the TD direction, and is distinguished from stretching by applying pressure in the thickness direction of the film, such as roll rolling in which the film is sandwiched between two rolls.

Stretching in the TD direction is not particularly limited, but can be carried out, for example, by using a transverse stretching machine such as a clip-type tenter to clamp both widthwise ends of the film and pull it in the TD direction. The stretch ratio in the TD direction can be determined by the ratio of the clamped width of the film before stretching to the clamped width of the film after stretching.

The film temperature in step (iii-b) is controlled to be 70°C or lower. This suppresses the progression of crystallization of the poly(3-hydroxybutyrate) resin during this step, resulting in a relatively large amount of amorphous regions in the film. This allows crystalline molecules to be oriented in the amorphous regions, enabling a high stretch ratio to be achieved. If the film temperature exceeds 70°C during this step, the film becomes brittle, increasing the likelihood of the film breaking during stretching and making it difficult to achieve a high stretch ratio. The film temperature is preferably 60°C or lower, more preferably 50°C or lower, even more preferably 40°C or lower, and particularly preferably 35°C or lower.

In addition, to facilitate peeling of the film from the roll or other equipment used in the stretching step, the lower limit of the film temperature in step (iii-b) is 10°C or higher. It is preferably 15°C or higher, and more preferably 20°C or higher.

Furthermore, it is preferable that the film temperature in step (iii-b) be equal to or higher than the film temperature in step (iii-a).

The film temperature in step (iii-b) is determined primarily by the film temperature in the previous step (iii-a) and the temperature conditions and time required in step (iii-b). Those skilled in the art can easily control the film temperature by taking these parameters into account.

The means for controlling the film temperature in step (iii-b) is not particularly limited, but the methods described above in step (iii-a) can be used as appropriate. These methods may be used alone or in combination.

The stretching ratio in step (iii-b) is not particularly limited, but is preferably 2x or greater. It is more preferably 3x or greater, and even more preferably 4x or greater. According to this embodiment, such a high stretching ratio can be achieved by controlling the film temperature in steps (ii), (iii-a), and (iii-b). The upper limit of the stretching ratio is not particularly limited and may be determined appropriately, but may be, for example, 8x or less.

(Step (iv))
In step (iv), the film obtained in step (iii-a) or step (iii-b) is heated to a temperature that is at least 10° C. higher than the temperature of the film in step (iii-a) or (iii-b) and is at least 60° C. Step (iv) is preferably carried out continuously from step (iii-a) or (iii-b) in one production line.

By carrying out this step, the crystallinity of the film, which was controlled to a relatively low level in step (iii-a) or step (iii-b), can be increased, resulting in increased strength of the stretched film. It can also stabilize the physical properties of the stretched film.

From the perspective of increasing the crystallinity of the film, which was suppressed in the previous step, in this step, the film temperature in this step is at least 10°C higher than the film temperature in step (iii-a) or (iii-b). It is preferably at least 20°C higher, more preferably at least 30°C higher, even more preferably at least 40°C higher, and particularly preferably at least 50°C higher.

In addition, from the viewpoint of sufficiently promoting crystallization of the poly(3-hydroxybutyrate) resin in this process, the film temperature in this process is 60°C or higher, preferably 70°C or higher, and more preferably 80°C or higher. The upper limit should be below the melting temperature of the resin, and is preferably 150°C or lower, more preferably 145°C or lower, and even more preferably 140°C or lower.

The means for controlling the film temperature in step (iv) is not particularly limited, and the methods described above in step (iii-a) can be used as appropriate. The methods described above may be used alone or in combination.

This step (iv) is preferably carried out while applying tension to the film in the stretched direction. This makes it possible to avoid thermal shrinkage of the film. That is, when step (iv) is carried out after step (iii-a), it is preferably carried out while applying tension to the film in the MD direction. When step (iv) is carried out after step (iii-b), it is preferably carried out while applying tension to the film in both the MD and TD directions. When tension is applied in the MD direction, for example, the rotation speeds of the multiple rolls that transport the film may be individually controlled. When tension is applied in the TD direction, for example, step (iv) may be carried out while clamping both widthwise ends of the film in a transverse stretching machine and pulling it in the TD direction.
However, the step (iv) does not substantially involve stretching the film. The phrase "the film is not substantially stretched" means that no operation intended to stretch the film is performed in the step (iv).

The stretched film manufacturing method according to this embodiment can be carried out in a continuous process, from melt extrusion of the film raw material to the formation of the stretched film. Here, a continuous process refers to carrying out the stretching process after forming the film, without carrying out the crystallization process, which requires a long time as described in Patent Document 1 (specifically, the process of quenching in ice water and then annealing at 40°C for 12 hours).

In the stretched film manufacturing method according to this embodiment, steps (i) through the final step are preferably carried out while the film is continuously transported. This enables the stretched film to be manufactured with high productivity through an industrially simple process. This mode can be carried out while the manufactured stretched film is wound on a winding roll. Note that the final step refers to step (iii-a) when steps (iii-a) and (iii-b) are carried out, and step (iv) when steps (iii-iv) and (iv) are carried out.

When the film is continuously transported, the transport speed is not particularly limited, but from the perspective of film productivity, it is preferable that the transport speed be 5 m/min or more before the start of stretching. Furthermore, from the perspective of production stability, it is preferable that the transport speed be 50 m/min or less before the start of stretching.

An example of a production line in which steps (i) to (iv) are carried out while continuously transporting a film is shown in Figure 1. The right-pointing arrow in the figure indicates the transport direction of the film.
First, a film raw material containing a poly(3-hydroxybutyrate) resin is melted in an extruder 11. The melted film raw material 21 is extruded onto a casting roll 12 from a T-die connected to the tip of the extruder, and is formed into a film on the surface of the roll (step (i)).

The molten film material is cooled while moving along the surface of the casting roll 12. During this process, some of the resin contained in the film material crystallizes. The solidified film 22 is peeled off from the casting roll 12 along the film transport path (step (ii)).

The film 22 is then guided to stretching rolls 13, 13', which are located at the front and rear of the film's transport direction. The rear roll 13' is set to rotate at a faster speed than the front roll 13. This speed difference causes the film 22 to be pulled in the MD direction, resulting in stretching in the MD direction (step (iii-a)). At this time, the temperature of the roll 13 is set to a predetermined value using a heat medium, thereby controlling the film temperature during MD stretching.

The film 22 stretched in the MD direction is introduced into the transverse stretching machine 14, which is a clip-type tenter. Here, both widthwise ends of the film are clamped and pulled in the TD direction, thereby stretching it in the TD direction (step (iii-b)). At this time, the film temperature during TD stretching is controlled by applying airflow adjusted to a predetermined temperature to the film inside the transverse stretching machine.

After achieving the desired stretch ratio in the TD direction, the internal temperature inside the transverse stretching machine 14 is raised while both widthwise ends of the film are clamped. This heats the film and promotes crystallization of the poly(3-hydroxybutyrate) resin (step (iv)).

The film 22 is then taken up by the take-up roll 15. This results in a biaxially stretched film that is stretched in both the MD and TD directions. In the above process, the film is continuously transported from the extrusion of the film raw material to film formation, film stretching, and winding.

The thickness of the stretched film produced is not particularly limited and can be set as appropriate by those skilled in the art. However, from the standpoint of the uniform thickness, appearance, strength, lightness, etc. of the film, it is preferably 10 to 200 μm, more preferably 15 to 150 μm, and even more preferably 20 to 100 μm.

The stretched film may have other layers laminated thereon. Examples of such other layers include resin layers, inorganic layers, metal layers, metal oxide layers, and printed layers. These other layers may be laminate layers, coating layers, or vapor-deposited layers.

The stretched film is thin yet highly strong, making it suitable for use as packaging film, heat-sealable film, twist film, etc.

The present invention will be explained in more detail below using examples and comparative examples, but the present invention is not limited to these examples in any way.

In the examples and comparative examples, the following raw materials were used.
(Poly(3-hydroxybutyrate)-based resin)
A-1: P3HB3HH (average content ratio 3HB/3HH = 94/6 (mol%/mol%), weight average molecular weight 600,000 g/mol, glass transition temperature 6°C) Produced in accordance with the method described in Example 1 of WO 2019/142845.

(Evaluation of 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 as the film thickness.

(Method for measuring glass transition temperature)
The glass transition temperature (Tg) of each resin was determined by differential scanning calorimetry in accordance with JIS K-7121.
Specifically, first, about 5 mg of the resin 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 was drawn equidistant from these two straight lines in the vertical direction. The temperature at the point where this center line intersects with the curve of the stepwise change due to the glass transition was determined to be the glass transition temperature (Tg).

(Method for measuring crystallinity)
The crystallinity was measured for each of the film immediately after peeling from the casting roll, immediately after MD stretching, immediately after TD stretching, and immediately after heat treatment.
The film to be measured was quickly cut into a 2 cm square, laminated to a thickness of 200 to 500 μm, and fixed on a glass holder.
This glass holder was fixed to a sample clip next to a characteristic X-ray Cu-Kα light source in an XRD device (Rigaku Rint2500), and XRD measurements were carried out in the range of 5 to 40° at a scan speed of 0.02 to 0.5°/min.
The area (integrated intensity) of the waveform obtained from this measurement, with both ends zero-corrected, was defined as Ia + Ic (area of the halo derived from the amorphous portion + area of the peak derived from the crystal). From this, the area of the waveform obtained by subtracting the halo derived from the amorphous portion (so as to maintain the symmetry of the scattering peak intensity) was defined as Ic. The crystallinity was calculated using the formula: Ia/(Ia + Ic) × 100.

[Production of poly(3-hydroxybutyrate)-based resin pellets P-1]
100 parts by weight of poly(3-hydroxybutyrate) resin A-1 was dry-blended with 0.5 parts by weight of behenic acid amide (Nippon Fine Chemicals Co., Ltd.: BNT-22H) as a lubricant and 0.5 parts by weight of pentaerythritol as a crystal nucleating agent. The resulting resin material was extruded into a φ26 mm co-rotating twin-screw extruder with cylinder and die temperatures set to 150°C, and the extruded strands were solidified by passing them through a water bath filled with hot water at 45°C. Resin pellets P-1 were obtained by cutting them with a pelletizer. The glass transition temperature of the resin pellets P-1 was 6°C.

Example 1
The cylinder temperature and die temperature of a single-screw extruder having a diameter of 40 mm and connected to a T-die having a width of 350 mm were set to 165°C.
The resin pellets P-1 were charged into the single-screw extruder and melted, and the molten resin at a temperature of 165°C was extruded into a film using a T-die. The film-like molten resin was extruded onto a casting roll set at 20°C and molded. After cooling to a film temperature of 20°C, the film was peeled off from the casting roll.
The peeled film was taken up by a take-up roll and continuously stretched in the machine direction (MD) by 6 times in a roll longitudinal stretching machine at a film temperature of 20° C. The film temperature at this time was controlled by adjusting the roll temperature in the roll longitudinal stretching machine to the same temperature (20° C.).
Subsequently, the film was continuously stretched in the transverse (TD) direction by a stretching ratio of 6 times using a clip-type tenter transverse stretching machine, with the film temperature during stretching being 25° C. The film temperature at this time was controlled by applying an air flow of the same temperature (25° C.) to the film inside the transverse stretching machine.
Subsequently, the film was heat-treated in a clip-type tenter transverse stretching machine so that the film temperature reached 90° C. The film temperature at this time was also controlled by applying an air current of the same temperature (90° C.) to the film.
In the above, the crystallinity of the film when peeled from the cast roll was 35%, the crystallinity of the film after MD stretching was 49%, the crystallinity of the film after TD stretching was 52%, and the crystallinity of the film after heat treatment was 72%.
After the heat treatment, the film was slit at its widthwise edge to obtain a biaxially stretched film having a width of 1000 mm and a thickness of 20 μm. The above process was carried out while the film was continuously transported.
The resulting film had a breaking strength of 63 MPa in the MD direction and 122 MPa in the TD direction.

Example 2
A biaxially stretched film was obtained in the same manner as in Example 1, except that the film temperature after cooling with the cast roll, the film temperature during MD stretching, and the film temperature during TD stretching were each changed to 30°C.
In this example, the crystallinity of the film when peeled from the casting roll was 42%, the crystallinity of the film after MD stretching was 47%, the crystallinity of the film after TD stretching was 59%, and the crystallinity of the film after heat treatment was 67%.
The resulting film had a breaking strength of 60 MPa in the MD direction and a breaking strength of 116 MPa in the TD direction.

<Comparative Example 1>
An attempt was made to produce a stretched film in the same manner as in Example 1, except that the temperature of the film cooled by the cast roll in Example 1 was changed to 55°C.
However, the film broke during MD stretching, and it was not possible to produce a stretched film.
In this comparative example, the film had a crystallinity of 55% when peeled from the casting roll.

<Comparative Example 2>
An attempt was made to produce a stretched film in the same manner as in Example 1, except that the film temperature during MD stretching was changed to 68°C.
However, the film broke during MD stretching, and it was not possible to produce a stretched film.
In this comparative example, the crystallinity of the film when peeled from the casting roll was 35%, and the crystallinity of the film after MD stretching was 60%.

11 Extruder 12 Cast rolls 13, 13' Stretching rolls 14 Transverse stretching machine 15 Winding roll 21 Molten film raw material 22 Film

Claims (8)

A method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin, comprising the following steps (i) to (iii-a):
the content of the resin other than the poly(3-hydroxybutyrate)-based resin in the stretched film is 0 to 100 parts by weight relative to 100 parts by weight of the poly(3-hydroxybutyrate)-based resin;
The poly(3-hydroxybutyrate)-based resin comprises poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) .
(i) A step of melting a film raw material containing the poly(3-hydroxybutyrate)-based resin and extruding it onto a cast roll to form a film. (ii) A step of peeling the film from the cast roll under conditions in which the temperature of the film formed in step (i) is in the range of 0 to 50°C. (iii-a) A step of stretching the film in the MD direction under conditions in which the temperature of the film obtained in step (ii) is in the range of 10 to 65°C. The method according to claim 1, further comprising the following step (iii-b):
(iii-b) A step of stretching the film obtained in the step (iii-a) in the TD direction under conditions in which the temperature of the film is in the range of 10 to 70°C. The method according to claim 1 or 2, further comprising the following step (iv):
(iv) a step of heating the film obtained in step (iii-a) or (iii-b) so that the temperature of the film is at least 10°C higher than the temperature of the film in step (iii-a) or (iii-b) and is at least 60°C. The manufacturing method described in any one of claims 1 to 3, wherein the temperature of the film formed in step (i) is a temperature exceeding the glass transition temperature of the poly(3-hydroxybutyrate) resin + 10°C. The manufacturing method described in any one of claims 1 to 4, wherein the stretching ratio in steps (iii-a) and/or (iii-b) is 2 to 8 times. The manufacturing method described in any one of claims 1 to 5, wherein steps (i) through the final step are carried out while the film is continuously transported. The manufacturing method described in any one of claims 1 to 6, wherein the MD stretching in step (iii-a) is carried out by varying the rotation speeds of the rolls that transport the film. The method according to any one of claims 1 to 7 , wherein the stretching in the MD direction in step (iii-a) is carried out under conditions in which the temperature of the film obtained in step (iii-a) is in the range of 10 to 45 ° C.