Method for producing stretched film

Abstract

This method produces a stretched film that contains a poly(3-hydroxybutyrate) resin, and comprises the following steps (i) to (iii-a): (i) a step for melting a film starting material and forming same into a film configuration by extrusion onto a cast roll; (ii) a step for detaching the film from the cast roll to obtain a film having a crystallinity of 20-45%; and (iii-a) a step for stretching the film in the MD direction to obtain a film having a crystallinity of 30-55%. The following step (iii-b) and / or step (iv) may also be included: (iii-b) a step for stretching the film in the TD direction to obtain a film having a crystallinity of 30-60%; and (iv) a step for heating the film to a temperature higher than the temperature in step (iii-a) or step (iii-b) to obtain a film having a crystallinity of at least 60%.

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 combat this type of marine pollution, 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 temperatures, and therefore cannot be used to combat marine pollution.

[0006] In this situation, poly(3-hydroxybutyrate) resins are attracting attention as a material that can solve the above problems because they are biodegradable even in seawater.

[0007]

[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 a stretched film with good productivity.

[0008] However, due to its properties, poly(3-hydroxybutyrate)-based resins are known to be difficult to stretch. Patent Document 1 describes a method for stretching a film containing a poly(3-hydroxybutyrate)-based resin, in which a thermoplastic resin containing a poly(3-hydroxybutyrate)-based resin as a main component is melted and formed into a film, crystallized over a certain period of time, and then sandwiched between two rolls and roll-rolled to perform primary stretching, followed by secondary stretching at a temperature higher than the temperature during the rolling, thereby producing a unidirectionally stretched film.

[0009] Patent Document 2 describes a method for producing a unidirectionally stretched film by rapidly cooling a molten film made from a poly(3-hydroxybutyrate) resin to a temperature equal to or lower than the glass transition temperature of the resin + 10°C and solidifying it to produce an amorphous film, followed by cold-stretching the amorphous film at a temperature equal to or lower than the glass transition temperature + 20°C (specifically, 3°C), and further subjecting the film to a tension heat treatment at a temperature of 25 to 160°C (specifically, at 100°C for 2 hours).

[0010] JP 2006-168159 A JP 2003-311824 A

[0011] 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 stretch 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 requires a long time, such as 12 hours, which makes it impossible to produce the film in a continuous process and results in poor productivity. Furthermore, the method described in Patent Document 1 requires a two-stage stretching step, namely, a primary stretching step by roll rolling and a secondary stretching step at a high temperature, in order to achieve a high stretch ratio in one direction, which also results in a complicated production process.

[0012] Patent Document 2 also describes that the tension heat treatment after stretching requires a long time, such as two hours, and does not describe or suggest the production of stretched film in a continuous process, nor does it take productivity into consideration at all.

[0013] Furthermore, although the methods described in Patent Documents 1 and 2 allow stretching in one direction at a high ratio, neither document describes a stretched film stretched at a high ratio in both the MD and TD directions.

[0014] On the other hand, when the technology for continuously producing stretched films from general-purpose resins such as polypropylene is directly applied to poly(3-hydroxybutyrate)-based resins, stretching becomes difficult, and it is not possible to increase the stretch ratio and film strength.

[0015] In view of the above-described current situation, a first aspect of the present invention aims to provide a method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin at a high draw ratio in a continuous process with high productivity. A second aspect of the present invention aims to provide a stretched film containing a poly(3-hydroxybutyrate)-based resin that is stretched in both the MD and TD directions and can be produced in a continuous process with high productivity.

[0016] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors discovered that, in a technology in which a molten resin is cooled and solidified using a casting roll and then stretched, by controlling the crystallinity of the film peeled from the casting roll and the crystallinity of the stretched film so that they each fall within a specific range, a stretched film containing a poly(3-hydroxybutyrate)-based resin can be produced in a continuous process with high productivity and at a high stretch ratio, thereby completing the present invention. Furthermore, the present inventors have succeeded in producing a film stretched in both the MD and TD directions with high productivity using a poly(3-hydroxybutyrate)-based resin as a raw material using a continuous process. Such a stretched film has not been reported until now.

[0017] That is, a first aspect of the present invention is 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 formed in step (i) from the cast roll to obtain a film with a crystallinity of 20 to 45%, and (iii-a) a step of stretching the film obtained in step (ii) in the MD direction to obtain a film with a crystallinity of 30 to 55%. The production method can further comprise the following step (iii-b). (iii-b) A step of stretching the film obtained in step (iii-a) in the TD direction to obtain a film having a crystallinity of 30 to 60%. A second aspect of the present invention relates to a stretched film containing a poly(3-hydroxybutyrate)-based resin and having a breaking strength of 50 MPa or more in the MD and TD directions.

[0018] According to a first aspect of the present invention, there is provided a method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin at a high stretch ratio in a continuous process with good productivity. According to the first aspect of the present invention, a uniaxially stretched film stretched in the MD direction or a biaxially stretched film stretched in both the MD and TD directions can be produced, and a high stretch ratio can be achieved in each direction.

[0019] According to a second aspect of the present invention, it is possible to provide a stretched film containing a poly(3-hydroxybutyrate)-based resin, which is stretched in both the MD direction and the TD direction and can be produced with good productivity by a continuous process.

[0020] The present invention relates to an embodiment of the present invention, and is a conceptual diagram showing an example of a production line from extrusion of a film raw material to film formation, film stretching, and film winding. (A) is a top view, and (B) is a side view. (B) is a top view showing the shape of a test piece used when measuring the breaking strength of a stretched film in Examples.

[0021] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0022] A first aspect of the present invention 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.

[0023] 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), poly(3-hydroxybutyrate-co-3-hydroxyoctadecanoate), etc. 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 of ease of industrial production.

[0024] 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.

[0025] Commercially available products of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) include Kaneka Biodegradable Polymer PHBH (registered trademark) manufactured by Kaneka Corporation.

[0026] The melting point, Young's modulus, etc. of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) vary depending on the ratio of the 3-hydroxybutyrate component and the 3-hydroxyvalerate component. However, since both components co-crystallize, the degree of crystallinity is high at 50% or more, and although it is more flexible than poly(3-hydroxybutyrate), the improvement in brittleness is insufficient.

[0027] 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 %), more preferably 97 / 3 to 85 / 15 (mol % / mol %), from the viewpoint of achieving both strength and productivity of the stretched film.

[0028] 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.

[0029] The poly(3-hydroxybutyrate) resin may be a mixture of at least two poly(3-hydroxybutyrate) resins differing in the type and / or content of the constituent monomers, in which case at least one highly crystalline poly(3-hydroxybutyrate) resin and at least one lowly crystalline poly(3-hydroxybutyrate) resin may be used in combination.

[0030] In general, highly crystalline poly(3-hydroxybutyrate)-based resins have excellent productivity but poor mechanical strength, while low-crystalline poly(3-hydroxybutyrate)-based resins have poor productivity but excellent mechanical properties. It is presumed that when both resins are used in combination, the highly crystalline poly(3-hydroxybutyrate)-based resin forms fine resin crystal particles, while the low-crystalline poly(3-hydroxybutyrate)-based resin forms tie molecules that crosslink the resin crystal particles. The strength and productivity of stretched films can be improved by using these resins in combination.

[0031] The content of 3-hydroxybutyrate units 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 poly(3-hydroxybutyrate) resin mixture. When the 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 %.

[0032] The highly crystalline poly(3-hydroxybutyrate) resin is preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) or poly(3-hydroxybutyrate-co-4-hydroxybutyrate), and more preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

[0033] 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 among all monomer units constituting the poly(3-hydroxybutyrate) resin mixture. When the 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%.

[0034] The low-crystalline poly(3-hydroxybutyrate) resin is preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) or poly(3-hydroxybutyrate-co-4-hydroxybutyrate), and more preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

[0035] When a high-crystalline poly(3-hydroxybutyrate) resin and a low-crystalline poly(3-hydroxybutyrate) resin are used in combination, the proportion of each resin relative to the total amount of both resins is not particularly limited, but it is preferable that the former be from 10% by weight to 60% by weight and the latter be from 40% by weight to 90% by weight, and it is more preferable that the former be from 25% by weight to 45% by weight and the latter be from 55% by weight to 75% by weight.

[0036] According to one embodiment, in addition to the high-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.

[0037] 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.

[0038] The medium-crystalline poly(3-hydroxybutyrate) resin is preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) or poly(3-hydroxybutyrate-co-4-hydroxybutyrate), and more preferably poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).

[0039] When the medium-crystalline poly(3-hydroxybutyrate) resin is further used in combination, the proportion of the medium-crystalline poly(3-hydroxybutyrate) resin 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.

[0040] The method for obtaining a blend of two or more poly(3-hydroxybutyrate) resins is not particularly limited, and may be a method for obtaining a blend by microbial production or a method for 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 mill, or the like, or by dissolving two or more resins in a solvent, mixing, and drying the resins.

[0041] 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.

[0042] Furthermore, when the poly(3-hydroxybutyrate)-based resin is a mixture of two or more poly(3-hydroxybutyrate)-based resins, the weight-average molecular weight of each poly(3-hydroxybutyrate)-based resin constituting the mixture is not particularly limited. However, when the above-mentioned high-crystalline poly(3-hydroxybutyrate)-based resin and low-crystalline poly(3-hydroxybutyrate)-based resin are used in combination, the weight-average molecular weight of the high-crystalline poly(3-hydroxybutyrate)-based 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 strength and productivity of the stretched film. On the other hand, the weight-average molecular weight of the low-crystalline poly(3-hydroxybutyrate)-based 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. 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.

[0043] 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.

[0044] The method for producing poly(3-hydroxybutyrate)-based 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 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 be used depending on the poly(3-hydroxybutyrate) resin to be produced, or the culture conditions, including the type of substrate, may be optimized.

[0045] 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 using a modified resin as a film raw material, the raw material obtained by reacting the resin with the modifying raw material in advance may be molded into a film, or the modifying raw material may be mixed with the resin and reacted during film molding. Furthermore, when reacting the resin with the 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] (Other Resins) The film raw material or the stretched film may contain other resins besides the poly(3-hydroxybutyrate)-based resin, 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 sebate terephthalate, and polybutylene azelate terephthalate. Only one type of other resin may be contained, or two or more types may be contained.

[0050] 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, relative to 100 parts by weight of the poly(3-hydroxybutyrate) resin. The lower limit of the content of the other resin is not particularly limited, and may be 0 parts by weight or more.

[0051] The film raw material or the stretched film may contain additives that can be used with the poly(3-hydroxybutyrate) 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. 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. Crystal nucleating agents, lubricants, fillers, and plasticizers are described in more detail below.

[0052] (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 crystal 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 poly(3-hydroxybutyrate)-based resin.

[0053] (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.

[0054] (Filler) The film raw material or the stretched film may contain a filler. The inclusion of a filler can result in a stretched film with higher strength. The filler may be either an inorganic filler or an organic filler, or a combination of both. Examples of inorganic fillers include, but are not limited to, 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 the poly(3-hydroxybutyrate) resin. However, the film raw material or the stretched film does not necessarily need to contain a filler.

[0055] (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-hydroxybutyrate) 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 or more plasticizers 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)-based resin. However, the film raw material or the stretched film does not necessarily contain a plasticizer.

[0056] 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) resin and extruding it onto a casting roll to form a film; (ii) a step of peeling the film formed in step (i) from the casting roll to obtain a film with a crystallinity of 20 to 45%; and (iii-a) a step of stretching the film obtained in step (ii) in the MD direction to obtain a film with a crystallinity of 30 to 55%. Steps (i), (ii), and (iii-a) allow a uniaxially stretched film to be obtained that is stretched in the MD direction.

[0057] The MD direction is also called the machine direction, flow direction, or longitudinal direction. The TD direction, which will be described later, is a direction perpendicular to the MD direction and is also called the perpendicular direction or width direction.

[0058] After the step (iii-a), it is preferable to carry out the following step (iv): (iv) a step of heating the film obtained in the step (iii-a) to a temperature higher than the temperature in the step (iii-a) to obtain a film having a crystallinity of 60% or more.By steps (i), (ii), (iii-a), and (iv), it is possible to obtain a uniaxially stretched film that is stretched in the MD direction and has high strength in the MD direction.

[0059] Furthermore, 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 the step (iii-a) in the TD direction to obtain a film with a crystallinity of 30 to 60% By steps (i), (ii), (iii-a), and (iii-b), a biaxially stretched film stretched in both the MD and TD directions can be obtained.

[0060] After the step (iii-b), it is preferable to carry out the following step (iv): (iv) a step of heating the film obtained in the step (iii-b) to a temperature higher than the temperature in the step (iii-b) to obtain a film having a crystallinity of 60% or more. By steps (i), (ii), (iii-a), (iii-b), and (iv), it is possible to obtain a biaxially stretched film that is stretched in both the MD and TD directions and has high strength in both the MD and TD directions.

[0061] It is not necessary to perform step (iv). The crystallinity of the film can be increased by leaving the stretched film obtained without performing step (iv) for a long time, for example, at room temperature, and the stretched film can be made to have high strength. However, since the stretched film may shrink when left standing, it is preferable to perform step (iv) to increase the film strength.

[0062] 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 preferably carried out by extruding the molten film raw material through a T-die, i.e., by extrusion molding. By extrusion molding, a film with a uniform thickness can be easily produced. In extrusion molding, a single-screw extruder, a twin-screw extruder, etc. can be used as appropriate.

[0063] The conditions for melting the film raw material may be any conditions that allow the poly(3-hydroxybutyrate) resin to melt, and the temperature of the molten film raw material may be, for example, about 140 to 210°C.

[0064] The molten film material is then extruded onto a casting roll to form a film. The molten film 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.

[0065] This step may involve extruding a melt onto one or more casting rolls, or may involve placing a touch roll opposite the casting roll and sandwiching the melt extruded onto the casting roll between the touch rolls. This step does not involve applying pressure to the film to perform roll rolling. An air knife or an air chamber may be used to ensure stable contact of the melt with the casting roll. To efficiently cool the side opposite to the contact surface with the casting roll, the casting roll may be placed in a water tank or an air chamber may be used.

[0066] The set temperature of the casting roll is preferably 50°C or lower in order to achieve a crystallinity of 45% or lower in the film described below. It is more preferably 45°C or lower, and even more preferably 40°C or lower. However, if the set temperature of the casting roll is too low, the poly(3-hydroxybutyrate) resin will not solidify sufficiently, and the resin will tend to stick to the casting roll, making it difficult to peel. Therefore, the lower limit of the set temperature of the casting roll 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 set temperature of the casting roll may be a temperature exceeding the glass transition temperature (Tg) of the poly(3-hydroxybutyrate) resin + 10°C, or may be a temperature equal to or higher than Tg + 12°C, or may be a temperature equal to or higher than Tg + 14°C.

[0067] (Step (ii)) In step (ii), the film formed in step (i) is peeled off from the casting roll to obtain a film with a crystallinity of 20 to 45%. The film can be peeled off from the casting roll by conveying the film toward the next stretching step while rotating the casting roll.

[0068] The film peeled from the casting roll has a crystallinity of 20 to 45%. When the peeled film has a crystallinity of 20% or more, the film can be easily peeled from the casting roll. It is preferably 25% or more, and more preferably 30% or more. Furthermore, by controlling the crystallinity of the peeled film to be a relatively low range of 45% or less, it becomes possible to control the crystallinity of the film to a relatively low level in the next step (iii-a), and it becomes possible to stretch a film containing a poly(3-hydroxybutyrate)-based resin at a high stretching ratio. It is preferably 43% or less, more preferably 42% or less, and even more preferably 40% or less.

[0069] The crystallinity of the film when peeled from the casting roll is mainly determined 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 influence the crystallinity 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 crystallinity of the film by taking these parameters into account.

[0070] The crystallinity of a film can be determined with reference to POLYMER, 1992, Volume 33, Number 7, pp. 1563-1567. For example, it can be determined by the following procedure. The film to be measured is quickly cut into 2 cm square pieces, laminated to a thickness of 200 to 500 μm, and fixed on a glass holder. This glass holder is fixed to a sample clip next to the characteristic X-ray Cu-Kα light source in the XRD instrument, and XRD measurement is performed over a range of 5 to 40° at a scan speed of 0.02 to 0.5° / min. The XRD instrument used can be a Rigaku Rint2500. The area (integrated intensity) of the waveform obtained from this measurement, with both ends zero-corrected, is 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) is defined as Ic. The crystallinity is calculated by the formula: Ia / (Ia+Ic)×100.

[0071] The film temperature in step (ii) is preferably 50°C or lower so as to suppress crystallization of the poly(3-hydroxybutyrate) resin and achieve a crystallinity of 45% or lower. It is more preferably 45°C or lower, even more preferably 40°C or lower, and even more preferably 35°C or lower. Furthermore, in order to facilitate peeling of the film from the casting roll, the lower limit of the film temperature in step (ii) 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 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.

[0072] (Step (iii-a)) In step (iii-a), the film obtained in step (ii) is stretched in the MD direction to obtain a film with a crystallinity of 30 to 55%. Step (iii-a) is preferably carried out continuously from step (ii) in one production line. In step (iii-a), the film is preferably stretched by stretching in the MD direction. In the present application, stretching a film in the MD direction refers to pulling the film in the MD direction, and is distinguished from stretching in which pressure is applied in the thickness direction of the film, such as roll rolling in which the film is sandwiched between two rolls.

[0073] The stretching in the MD direction can be carried out by, for example, using a roll longitudinal stretching machine, by varying the rotation speed of the rolls that transport the film, and 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.

[0074] The crystallinity of the film stretched in the MD direction obtained in step (iii-a) is 30 to 55%. A film with a crystallinity of 30% or more facilitates peeling of the film from the equipment such as rolls used in the stretching step. The crystallinity is preferably 35% or more, more preferably 40% or more, and even more preferably 45% or more.

[0075] Furthermore, by controlling the crystallinity of the stretched film to a relatively low range of 55% or less, a relatively large amount of amorphous regions is contained in the film, allowing crystalline molecules to be oriented in the amorphous regions, thereby achieving a high stretch ratio. Furthermore, when step (iii-b) is subsequently performed, it is possible to control the crystallinity of the film to a relatively low level in step (iii-b) as well, making it possible to stretch the film at a high ratio in the TD direction as well. If the crystallinity of the film in step (iii-a) exceeds 55%, the film becomes brittle, increasing the possibility of the film breaking during stretching, making it difficult to achieve a high stretch ratio. The crystallinity is preferably 53% or less, more preferably 50% or less.

[0076] The crystallinity of the film stretched in the MD direction obtained in step (iii-a) is preferably equal to or greater than the crystallinity of the film peeled from the cast roll obtained in step (ii), and more preferably higher than the crystallinity of the film.

[0077] Conventionally, when a stretched film is produced from a general-purpose resin such as polypropylene, the molten resin is generally cooled and solidified, and then preheated to a high temperature such as 145° C. to increase the crystallinity of the film before stretching. On the other hand, the present embodiment is clearly different from the production of a stretched film from a general-purpose resin such as polypropylene in that the film is stretched while controlling the crystallinity of the film to a relatively low range.

[0078] The crystallinity of the film stretched in the MD direction is determined mainly depending on the crystallinity of the film peeled from the cast roll in the previous step (ii), and the temperature conditions and time required in this step (iii-a). Those skilled in the art can easily control the crystallinity of the film by taking these parameters into consideration.

[0079] The film temperature in step (iii-a) is preferably 65°C or lower so as to suppress crystallization of the poly(3-hydroxybutyrate) resin and achieve a crystallinity of 55% or lower. It is more preferably 60°C or lower, even more preferably 55°C or lower, still more preferably 45°C or lower, and particularly preferably 35°C or lower. In addition, the film temperature is preferably 10°C or higher so as to facilitate peeling of the film from a tool such as a roll used in the stretching step. It is more preferably 15°C or higher.

[0080] The means for controlling the film temperature in step (iii-a) is not particularly limited, and examples thereof include a method of exposing the film to an air current adjusted to a predetermined temperature, a method of controlling the film temperature by setting a roll to a predetermined temperature, a method of heating the film using an auxiliary heating means such as an IR heater to control the film temperature to a predetermined temperature, a method of passing the film through an oven adjusted to a predetermined temperature, etc. These methods may be used alone or in combination.

[0081] 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 crystallinity of the film 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.

[0082] (Step (iii-b)) In step (iii-b), the stretched film obtained in step (iii-a) is further stretched in the TD direction to obtain a film with a crystallinity of 30 to 60%. Step (iii-b) is preferably carried out continuously from step (iii-a) in one production line. In step (iii-b), the film is preferably stretched by stretching in the TD direction. As 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.

[0083] The stretching in the TD direction is not particularly limited, but can be carried out, for example, 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. The stretching 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.

[0084] The crystallinity of the film stretched in the TD direction obtained in step (iii-b) is 30 to 60%. A film with a crystallinity of 30% or more facilitates peeling of the film from the equipment such as rolls used in the stretching step. The crystallinity is preferably 35% or more, more preferably 40% or more, even more preferably 45% or more, and particularly preferably 50% or more.

[0085] Furthermore, by controlling the crystallinity of the stretched film to a relatively low range of 60% or less, a relatively large amount of amorphous regions is contained in the film, allowing crystalline molecules to be oriented in the amorphous regions, thereby achieving a high stretch ratio. If the crystallinity of the film exceeds 60% in this process, the film becomes brittle, increasing the possibility of the film breaking during stretching, making it difficult to achieve a high stretch ratio. The crystallinity is preferably 59% or less, more preferably 56% or less.

[0086] The crystallinity of the film stretched in the TD direction is determined mainly by the crystallinity of the film obtained in the previous step (iii-a) and the temperature conditions and time required in this step (iii-b). Those skilled in the art can easily control the crystallinity of the film by taking these parameters into consideration.

[0087] The crystallinity of the film stretched in the TD direction obtained in step (iii-b) is preferably equal to or greater than the crystallinity of the film stretched in the MD direction obtained in step (iii-a), and more preferably higher than the crystallinity of the film stretched in the MD direction.

[0088] The film temperature in step (iii-b) is preferably 70°C or lower so as to suppress crystallization of the poly(3-hydroxybutyrate) resin and achieve a crystallinity of 60% or lower. It is more preferably 60°C or lower, even more preferably 50°C or lower, still more preferably 40°C or lower, and particularly preferably 35°C or lower. In addition, it is preferably 10°C or higher so as to facilitate peeling of the film from a tool such as a roll used in the stretching step. It is more preferably 15°C or higher, and even more preferably 20°C or higher.

[0089] The means for controlling the film temperature in step (iii-b) is not particularly limited, and the methods described above in step (iii-a) can be appropriately adopted. These methods may be used alone or in combination.

[0090] The stretching ratio in step (iii-b) is not particularly limited, but is preferably 2 times or more. It is more preferably 3 times or more, and even more preferably 4 times or more. According to this embodiment, such a high stretching ratio can be achieved by controlling the crystallinity of the film 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, 8 times or less.

[0091] (Step (iv)) In step (iv), the film obtained in step (iii-a) or the film obtained in step (iii-b) is heated to a temperature higher than the temperature in step (iii-a) or (iii-b) to obtain a film having a crystallinity of 60% or more. Step (iv) is preferably carried out continuously from step (iii-a) or (iii-b) in one production line.

[0092] By carrying out step (iv), the strength of the stretched film can be increased and the physical properties of the stretched film can be stabilized. The crystallinity of the stretched film achieved in step (iv) may be a value higher than the crystallinity achieved in step (iii-a) or step (iii-b). Specifically, the crystallinity may be 60% or more, preferably a value exceeding 60%, more preferably 63% or more, even more preferably 65% ​​or more, and particularly preferably 70% or more.

[0093] The film temperature in step (iv) is higher than the film temperature in steps (iii-a) or (iii-b), and is preferably 60° C. or higher, more preferably 70° C. or higher, and even more preferably 80° C. or higher. The upper limit of the temperature is sufficient as long as it is equal to or lower than 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.

[0094] From the viewpoint of sufficiently promoting the crystallization of the poly(3-hydroxybutyrate) resin in the step (iv), the film temperature in the step (iv) is preferably at least 10°C higher than the film temperature in the step (iii-a) or (iii-b), more preferably at least 20°C higher, even more preferably at least 30°C higher, still more preferably at least 40°C higher, and particularly preferably at least 50°C higher.

[0095] 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 appropriately adopted. The above-mentioned methods may be used alone or in combination.

[0096] 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, step (iv) does not substantially stretch the film. "Without substantially stretching the film" means that no operation intended to stretch the film is carried out in step (iv).

[0097] In the method for producing a stretched film according to this embodiment, the processes from melt extrusion of the film raw material to formation of the stretched film can be carried out in a continuous process. Here, the continuous process refers to carrying out the stretching process after forming into a film without carrying out the crystallization process (specifically, the process of quenching in ice water and then annealing at 40°C for 12 hours) which requires a long time as described in Patent Document 1.

[0098] In the method for producing a stretched film according to this embodiment, steps (i) through the final step are preferably carried out while continuously transporting the film. This enables the stretched film to be produced with good productivity through an industrially simple process. This mode can be carried out while the produced stretched film is wound up on a winding roll. The final step refers to step (iii-a) when steps (iii-a) and (iii-b) are carried out, and to step (iv) when steps (iii-b) and (iv) are carried out.

[0099] When the film is continuously transported, the transport speed is not particularly limited, but from the viewpoint of film productivity, it is preferably 5 m / min or more at the stage before the start of stretching, and from the viewpoint of production stability, it is preferably 50 m / min or less at the stage before the start of stretching.

[0100] Figure 1 shows an example of a production line in which steps (i) to (iv) are performed while continuously transporting a film. The right-pointing arrow in the figure indicates the film transport direction. First, a film raw material containing a poly(3-hydroxybutyrate) resin is melted in an extruder 11. The molten 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)).

[0101] The molten film material is cooled while moving along the surface of the casting roll 12. During this cooling, a part 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)).

[0102] The film 22 is then guided to stretching rolls 13, 13', which are arranged at the front and rear of the film transport direction. The rear roll 13' is set to rotate at a higher 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.

[0103] 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 the film is stretched in the TD direction by being pulled in the TD direction (step (iii-b)). At this time, the film temperature during TD stretching is controlled by applying an airflow adjusted to a predetermined temperature to the film inside the transverse stretching machine.

[0104] After the desired stretching ratio in the TD direction is achieved, the internal temperature of the transverse stretching machine 14 is raised while both widthwise ends of the film are clamped, thereby heating the film and promoting crystallization of the poly(3-hydroxybutyrate)-based resin (step (iv)).

[0105] The film 22 is then taken up by the take-up roll 15. This results in a high-strength biaxially stretched film that is stretched in both the MD and TD directions. In the above process, the film is continuously transported while being subjected to the processes from extrusion of the film raw material to film forming, film stretching, and winding.

[0106] The length, width, length / width, and thickness of the stretched film to be produced are not particularly limited, but are as described below for the second embodiment.

[0107] The stretched film to be produced may also be laminated with other layers as will be described later in the second embodiment.

[0108] The stretched film produced is thin yet strong, and can be suitably used as a packaging film, a heat-sealable film, a twist film, etc.

[0109] A second aspect of the present invention relates to a stretched film containing a poly(3-hydroxybutyrate)-based resin. The stretched film according to the second aspect contains a poly(3-hydroxybutyrate)-based resin and exhibits a breaking strength of 50 MPa or more in both the MD and TD directions.

[0110] A breaking strength of 50 MPa or more can be achieved by stretching the film at a high stretching ratio and increasing the crystallinity of the film. Specifically, as described above for the first embodiment, steps (i), (ii), (iii-a), and (iii-b) are carried out to stretch the film to a stretching ratio of 2 or more in each of the MD and TD directions, and then step (iv) is preferably carried out to increase the crystallinity of the film, thereby producing a stretched film according to the second embodiment.

[0111] The breaking strength is preferably 60 MPa or more in both the MD and TD directions, more preferably 70 MPa or more. The upper limit is not particularly limited, but may be 300 MPa or less, or may be 200 MPa or less. The breaking strength in the MD direction and the breaking strength in the TD direction may be the same or different. The breaking strength can be measured based on the description in the Examples section.

[0112] The details of the poly(3-hydroxybutyrate) resin and other components contained in the stretched film according to the second embodiment are the same as those in the first embodiment, and therefore will not be described here.

[0113] The stretched film is preferably in a long, strip-like shape because it can be produced while being continuously transported. In this case, the stretched film is preferably wound in a roll because it is easy to handle. The stretched film wound in a roll may be wound around a rod-shaped member.

[0114] The ratio of the length of the stretched film to the width of the stretched film (length / width) is not particularly limited, but may be, for example, 10 or more. It may also be 50 or more, or 100 or more. The upper limit of the ratio is also not particularly limited, but may be, for example, 10,000 or less. It may also be 5,000 or less, or 3,000 or less.

[0115] The length of the stretched film is not particularly limited, but may be, for example, 1 m or more. It may also be 5 m or more, or 10 m or more. The upper limit of the length is also not particularly limited, but may be, for example, 1,000 m or less. It may also be 500 m or less, 300 m or less, or 100 m or less.

[0116] The width of the stretched film is not particularly limited, but may be, for example, 10 mm or more. It may also be 50 mm or more, 100 mm or more, or 200 mm or more. The upper limit of the width is also not particularly limited, but may be, for example, 2000 mm or less, 1000 mm or less, or 500 mm or less.

[0117] The thickness of the stretched film is not particularly limited and can be appropriately determined by a person skilled in the art. From the viewpoints of the 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.

[0118] The stretched film may be laminated with other layers. Examples of such other layers include a resin layer, an inorganic layer, a metal layer, a metal oxide layer, and a printed layer. These other layers may be laminate layers, coating layers, or vapor-deposited layers.

[0119] The stretched film according to the second embodiment is thin yet has high strength, and therefore can be suitably used as a packaging film, a heat-sealable film, a twist film, etc.

[0120] 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, 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 casting roll to form a film; (ii) a step of peeling the film formed in step (i) from the casting roll to obtain a film with a crystallinity of 20 to 45%; and (iii-a) a step of stretching the film obtained in step (ii) in the MD direction to obtain a film with a crystallinity of 30 to 55%. [Item 2] The production method according to item 1, further comprising the following step (iii-b): (iii-b) A step of stretching the film obtained in step (iii-a) in the TD direction to obtain a film having a crystallinity of 30 to 60%. [Item 3] The manufacturing method according to item 1 or 2, further comprising the following step (iv): (iv) A step of heating the film obtained in step (iii-a) or (iii-b) to a temperature higher than the temperature in step (iii-a) or (iii-b) to obtain a film having a crystallinity of 60% or more. [Item 4] The manufacturing method according to any of items 1 to 3, wherein the stretching ratio in step (iii-a) and / or (iii-b) is 2 to 8 times. [Item 5] The manufacturing method according to any of items 1 to 4, wherein steps (i) to the final step are carried out while continuously conveying the film. [Item 6] The manufacturing method according to any one of items 1 to 5, wherein the stretching in the MD direction in step (iii-a) is carried out by varying the rotation speeds of the rolls that transport the film. [Item 7] The manufacturing method according to any one of items 1 to 6, wherein the poly(3-hydroxybutyrate)-based resin comprises poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). [Item 8] A stretched film comprising a poly(3-hydroxybutyrate)-based resin, and having breaking strengths of 50 MPa or more in the MD direction and the TD direction. [Item 9] The stretched film according to item 8, wherein the thickness of the stretched film is 10 to 200 μm.[Item 10] The stretched film according to item 8 or 9, wherein the poly(3-hydroxybutyrate)-based resin comprises poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). [Item 11] The stretched film according to any one of items 8 to 10, which is a strip-shaped film wound into a roll.

[0121] 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.

[0122] The following raw materials were used in the examples and comparative examples. (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.

[0123] (Evaluation of Film Thickness) The thickness of the film 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.

[0124] (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, approximately 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 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).

[0125] (Method for measuring crystallinity) The crystallinity was measured for each of the film immediately after peeling from the cast roll, the film immediately after MD stretching, the film immediately after TD stretching, and the film immediately after heat treatment. The film to be measured was quickly cut into 2 cm square pieces, laminated to a thickness of 200 to 500 μm, and fixed on a glass holder. This glass holder was fixed to the sample clip next to the characteristic X-ray Cu-Kα light source in an XRD device (Rigaku Rint2500), and XRD measurements were performed in the range of 5 to 40° at a scan speed of 0.02 to 0.5° / min. For the waveform obtained from this measurement, the area (integrated intensity) of the waveform 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 that the symmetry of the scattering peak intensity is maintained) was defined as Ic. The crystallinity was calculated by the formula: Ia / (Ia+Ic)×100.

[0126] (Method for measuring breaking strength) The breaking strength of a stretched film was measured by preparing a test piece (a test piece conforming to old JIS K7113-2 1 / 3) by cutting it into the shape shown in FIG. 2 from the stretched film, and conducting a tensile test at a tension speed of 100 mm / min in the stretching direction of the stretched film using a tensile tester (manufactured by Shimadzu Corporation: EZ-LX 1kN) in accordance with JIS K7127, to determine the stress (breaking strength) at which the test piece broke.

[0127] [Production of Poly(3-hydroxybutyrate)-Based Resin Pellets P-1] 100 parts by weight of poly(3-hydroxybutyrate)-based resin A-1 were 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, 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. The glass transition temperature of the resin pellets P-1 was 6°C.

[0128] Example 1 The cylinder temperature and die temperature of a φ40 mm single-screw extruder connected to a 350 mm wide T-die were each 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 cooled to a film temperature of 20 ° C., after which the film was peeled off from the casting roll. The peeled film was taken up by a take-off roll and continuously stretched in a roll longitudinal stretching machine to a stretch ratio of 6 in the machine direction (MD) at a film temperature of 20 ° C. at the time of stretching. 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 a clip-type tenter transverse stretching machine to a stretch ratio of 6 in the transverse direction (TD) at a film temperature of 25 ° C. at the time of stretching. The film temperature at this time was controlled by applying an airflow of the same temperature (25°C) to the film in the transverse stretching machine. Subsequently, heat treatment was performed 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 airflow 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%. The widthwise end of the heat-treated film was slit to obtain a biaxially stretched film with a width of 1000 mm and a thickness of 20 μm. The above process was performed while the film was continuously transported. The MD breaking strength of the obtained film was 63 MPa, and the TD breaking strength was 122 MPa.

[0129] 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 cast 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 breaking strength of the obtained film in the MD direction was 60 MPa, and the breaking strength in the TD direction was 116 MPa.

[0130] Comparative Example 1: An attempt was made to produce a stretched film in the same manner as in Example 1, except that the film temperature cooled by the casting roll in Example 1 was changed to 55°C. However, the film broke during MD stretching, and a stretched film could not be produced. In this comparative example, the crystallinity of the film when peeled from the casting roll was 55%.

[0131] 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 65°C. However, the film broke during MD stretching, and a stretched film could not be produced. In this comparative example, the crystallinity of the film when peeled from the cast roll was 35%, and the crystallinity of the film after MD stretching was 60%.

[0132] REFERENCE SIGNS LIST 11 Extruder 12 Cast roll 13, 13' Stretching roll 14 Transverse stretching machine 15 Winding roll 21 Molten film raw material 22 Film

Claims

1. A method for producing a stretched film containing a poly(3-hydroxybutyrate)-based resin, comprising the following steps (i) to (iii-a): 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 formed in step (i) from the casting roll to obtain a film having a crystallinity of 20 to 45%. (iii-a) A step of stretching the film obtained in step (ii) in the MD direction to obtain a film having a crystallinity of 30 to 55%.

2. The method according to claim 1, further comprising the following step (iii-b): (iii-b) A step of stretching the film obtained in step (iii-a) in the TD direction to obtain a film having a crystallinity of 30 to 60%.

3. 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) to a temperature higher than the temperature in step (iii-a) or (iii-b) to obtain a film having a crystallinity of 60% or more.

4. The method according to claim 1 or 2, wherein the stretching ratio in the step (iii-a) and / or (iii-b) is 2 to 8 times.

5. 3. The method according to claim 1, wherein steps (i) through the final step are carried out while the film is being continuously transported.

6. The method according to claim 1 or 2, wherein the stretching in the MD direction in the step (iii-a) is carried out by varying the rotation speeds of the rolls that transport the film.

7. Contains a poly(3-hydroxybutyrate)-based resin, the poly(3-hydroxybutyrate)-based resin contains poly(3-hydroxybutyrate-co-3-hydroxyhexanoate); A stretched film having a breaking strength of 50 MPa or more in the MD direction and the TD direction.

8. The stretched film according to claim 7, wherein the thickness of the stretched film is 10 to 200 μm.

9. 9. The stretched film according to claim 7 or 8, which is a strip-shaped film wound into a roll.