Laminated film, lid material, and packaging container

Abstract

The purpose of the present invention is to provide a laminated film which is capable of reducing the weight and volume of a packaging material, is excellent in impact resistance and puncture resistance, and is excellent in steam permeability. The present invention is a laminated film comprising a biaxially stretched polyester film and a sealant film, wherein: the biaxially stretched polyester film contains 25%-60% by mass of a polybutylene terephthalate resin and 40%-75% by mass of a polyethylene terephthalate resin with respect to 100% by mass of the constituent resin component; and the biaxially stretched polyester film and the laminated film have specific physical properties.

Description

Laminated films, lids and packaging containers

[0001] The present invention relates to a laminated film having a biaxially oriented polyester film containing a polybutylene terephthalate resin and a polyethylene terephthalate resin, and a sealant film, and to a lid material and a packaging container using the same.

[0002] Various packages have been proposed in the past, in which food or other contents are contained and sealed, and heated in a microwave oven before consumption. Since food packed in such packages is heated in an unopened state, the internal pressure increases with heating, and the package is also affected by the heat from the contents, so heat resistance and mechanical strength are required. However, when a package containing such food or other contents is heated in a microwave oven, the moisture contained in the contents evaporates as the heat is applied, increasing the internal pressure of the container, which can lead to problems such as deformation or rupture of the package, or the food contained inside being scattered or boiled over, contaminating the microwave oven.

[0003] In consideration of such problems, various types of packaging have been proposed that have vapor-releasing properties, for example, by giving a special shape to a portion of the seal where the container and lid are heat-sealed together, so that the load based on the internal pressure of the lidded container is concentrated on this specially shaped portion, thereby releasing the sealed state and allowing the internal pressure to escape. However, there is a risk that the structure will become complicated if a specially shaped portion is provided in the seal portion.

[0004] In addition, as a method of releasing steam from inside the package, a lid material is made from a laminated film with a sealant layer laminated on the side of the base film, and a laser is irradiated from the base film side of the lid material to form a weakened line for steam release that is deep enough not to penetrate the sealant layer located on the inside of the package.When heated in a microwave oven, the base film opens at the weakened line, and the sealant layer located on the inside of the package is stretched, causing it to partially tear and form a small through-hole, allowing steam to be released from inside the package.However, there is a risk that tears will occur due to external impacts or external forces during transportation.

[0005] For this reason, there is an increasing demand for a package that can stably release the internal pressure that has increased due to steam generated during heating in a microwave oven and that can release the steam in a simple manner.

[0006] Patent Document 1 discloses a lid material having, in that order, a first stretched plastic film, a second stretched plastic film, and a sealant layer. According to this technology, the lid material contains two stretched plastic films, and both the first and second stretched plastic films are biaxially oriented polyester films containing polyester as a main component. A laminate strength adjustment layer is provided between the second stretched plastic film and the sealant layer. The breaking elongation, measured in an 80°C environment after holding the laminate strength adjustment layer in an 80°C environment for one minute in an area where the laminate strength adjustment layer is not provided, falls within a specific range. This allows a lid material that can release steam when heated in a microwave oven to be obtained in a simple manner.

[0007] Although it is said that the use of biaxially oriented polyester films as the first and second stretched plastic films can provide a lid material that can easily release steam during heating in a microwave oven, the biaxially oriented polyester films used for both plastic films have issues such as low impact strength, puncture strength, and pinhole resistance, and there is room for improvement. As the volume of the package is reduced, there are issues such as a decrease in mechanical strength and puncture strength, and insufficient impact resistance, which have been problematic in terms of resource conservation and the impact on the environment.

[0008] Furthermore, it is said that by using a biaxially oriented nylon film as the first oriented plastic film and a biaxially oriented polyester film as the second oriented plastic film, it is possible to provide a lid material that can release steam when heated in a microwave oven in a simple manner. However, while biaxially oriented nylon film has excellent mechanical strength, pinhole resistance, puncture strength, and impact resistance, nylon film generally has problems in that it undergoes large dimensional changes when absorbing moisture and tends to curl during processing, and there is room for improvement as a package that can release steam when heated in a microwave oven in a simple manner.

[0009] On the other hand, biaxially oriented polyester films primarily composed of polybutylene terephthalate resin are known to have heat resistance and flexibility (Patent Document 2). According to this technology, biaxially oriented polyester films primarily composed of polybutylene terephthalate resin can be used in applications requiring toughness, such as those traditionally associated with polyamide films. Furthermore, in applications where a laminate of polyethylene terephthalate and polyamide films has traditionally been used, the polyethylene terephthalate and polyamide films can be replaced with a single layer of biaxially oriented polyester film primarily composed of polybutylene terephthalate resin, thereby reducing the volume of packaging materials. If the vapor permeability of packages using such flexible films can be improved, they are expected to be used to package a wider range of contents.

[0010] JP 2020-193009 A JP 2018-20844 A

[0011] Polybutylene terephthalate resin is known to have a faster crystallization rate and a lower final crystallinity than other crystalline polymers, making it difficult to produce biaxially oriented polyester films primarily composed of polybutylene terephthalate resin. This has made it difficult to improve the puncture strength and vapor escape properties of the film while enhancing its longitudinal and transverse orientation.

[0012] The present invention has been made in light of the above technical problems. That is, an object of the present invention is to provide a laminated film that enables weight reduction and volume reduction of packaging materials, has excellent impact resistance and puncture resistance, and has excellent vapor escape properties.

[0013] As a result of extensive research to achieve this object, the present inventors have succeeded in obtaining a biaxially oriented polyester film in which the longitudinal and transverse orientation of the film is enhanced by optimizing the blending ratio of the polybutylene terephthalate resin in the film and the stretching conditions during film production, and in which the puncture strength and the ratio of the breaking elongation in the longitudinal direction to the breaking elongation in the transverse direction are within specific ranges. Furthermore, the present inventors have found that a laminated film obtained by laminating the biaxially oriented polyester film and a sealant film has excellent vapor escape properties when the breaking strength and breaking elongation, and the absolute value of the stress difference of the film at 100% elongation in the longitudinal direction and the transverse direction, and the tear strength, measured in an environment of 120°C, are within specific ranges, leading to the completion of the present invention, as exemplified below. [1] A laminated film comprising a biaxially oriented polyester film and a sealant film, wherein the biaxially oriented polyester film contains 25 to 60% by mass of polybutylene terephthalate resin and 40 to 75% by mass of polyethylene terephthalate resin relative to 100% by mass of the constituent resin components, has a puncture strength per film thickness of 0.57 to 1.00 N / μm, and has a ratio BEa / BEb of the breaking elongation BEa in the longitudinal direction of the film to the breaking elongation BEb in the transverse direction perpendicular to the longitudinal direction of the film, of 0.65 to 1.50, and the laminated film satisfies the following conditions (a) to (d): (a) The breaking strength of the laminated film in the longitudinal and width directions at 120°C is 20 to 60 MPa. (b) The breaking elongation of the laminated film in the longitudinal and width directions at 120°C is 120 to 250%. (c) The absolute value of the stress difference of the laminated film in the longitudinal and width directions at 120°C when elongated by 100% is 0 to 12 MPa. (d) The tear strength of the laminated film in the longitudinal and width directions is 200 to 350 mN.

[0014] [2] The laminate film according to [1], further comprising an aluminum foil layer. [3] The laminate film according to [1] or [2], wherein the laminate film is used with weakening lines formed in at least the biaxially oriented polyester film. [4] A lid material comprising the laminate film according to [1] or [2]. [5] The lid material according to [4], wherein weakening lines are formed in at least the biaxially oriented polyester film of the laminate film. [6] A packaging container comprising the laminate film according to [1] or [2]. [7] The packaging container according to [6], wherein weakening lines are formed in at least the biaxially oriented polyester film of the laminate film.

[0015] The present invention makes it possible to provide a laminated film that has excellent impact resistance and puncture resistance, and also has excellent vapor escape properties, and is suitable for packaging such as easy-peel films and sealed packaging containers.

[0016] Although the details of the reason why the present invention has particularly excellent steam escape properties are not clear, it is believed that the following is the case. In other words, the lower the tear strength in the longitudinal and width directions of the laminated film and the breaking strength at 120°C, the more likely it is that through-holes will form at the weakened line. However, if the absolute value of the stress difference in the laminated film at 100% elongation at 120°C in the longitudinal and width directions is large, or if the physical properties in each direction are outside the specified range, the stress due to internal pressure will become non-uniform, making it more likely that the seal line will break or peel. In the present invention, by using a biaxially oriented polyester film with a breaking elongation ratio BEa / BEb in a specified range, and by setting the tear strength, breaking strength at 120°C, breaking elongation, and stress difference at 100% elongation of the laminated film within the specified ranges, it is believed that through-holes will be more likely to form at the weakened line, and the uniformity of the stress due to internal pressure will be increased, improving steam escape properties.

[0017] The present invention will be described in detail below. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. (Biaxially Stretched Polyester Film) The biaxially oriented polyester film of the present invention contains, as its resin components, 25 to 60% by mass of polybutylene terephthalate resin (hereinafter sometimes abbreviated as PBT resin) and 40 to 75% by mass of polyethylene terephthalate resin (hereinafter sometimes abbreviated as PET resin). By having a PBT resin content of 25 to 60% by mass, excellent dimensional stability and processability can be achieved, as well as bag tear resistance, chemical resistance, pinhole resistance at low temperatures, and steam escape properties for packaging. The PBT resin content is more preferably 35 to 55% by mass. By having a PBT resin content of 25% by mass or more, it is possible to suppress a decrease in puncture strength and mechanical properties, thereby achieving sufficient film properties. Furthermore, transparency is improved, making the film suitable for use in printing.

[0018] The PET resin content of the biaxially oriented polyester film of the present invention is more preferably 45 to 65% by mass. A PET resin content of 40% by mass or more is preferable because the biaxially oriented polyester film has good transparency, making it suitable for printing, and is also inexpensive because the relatively inexpensive PET resin is the main constituent. Furthermore, a PET resin content of 75% by mass or less is preferable because it can prevent a decrease in puncture strength and mechanical properties, thereby achieving sufficient film properties.

[0019] The intrinsic viscosity of the PBT resin used in the present invention is preferably 0.9 to 1.3 dl / g, more preferably 0.95 to 1.3 dl / g, and even more preferably 1.0 to 1.3 dl / g. By making the intrinsic viscosity of the PBT resin 0.9 dl / g or higher, the intrinsic viscosity of the film obtained by film formation is reduced, and it is possible to prevent a decrease in pin puncture strength, impact strength, bag rupture resistance, and the like. On the other hand, by making the intrinsic viscosity 1.3 dl / g or lower, it is possible to prevent excessive stress during film stretching, thereby achieving good film formability. In addition, it is possible to prevent the production of decomposition products during extrusion, which would be caused by a high melt viscosity and the need to increase the extrusion temperature.

[0020] The intrinsic viscosity of the PET resin used in the present invention is preferably 0.5 to 1.2 dl / g, more preferably 0.6 to 1.2 dl / g.

[0021] The biaxially stretched polyester film of the present invention may contain a polyester resin other than PBT resin and PET resin. Examples of PET resin and polyester resin other than PET resin include polyester resins such as polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), and polypropylene terephthalate (PPT), as well as polyester resins copolymerized with dicarboxylic acids such as isophthalic acid, orthophthalic acid, naphthalenedicarboxylic acid, biphenyldicarboxylic acid, cyclohexanedicarboxylic acid, adipic acid, azelaic acid, and sebacic acid, and polyester resins copolymerized with diol components such as ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, diethylene glycol, cyclohexanediol, polyethylene glycol, polytetramethylene glycol, and polycarbonate diol. Furthermore, the film may contain a copolymer formed by a transesterification reaction produced by melt-kneading PBT resin and PET resin.

[0022] The biaxially stretched polyester film of the present invention may contain lubricants, stabilizers, colorants, antioxidants, antistatic agents, ultraviolet absorbers, etc., as required.

[0023] As the lubricant, inorganic lubricants such as silica, calcium carbonate, and alumina are preferred, as well as organic lubricants, with silica and calcium carbonate being more preferred, and silica being particularly preferred in terms of reducing haze, which can provide transparency and lubricity.

[0024] The lower limit of the lubricant content is preferably 100 to 10,000 ppm by mass, more preferably 500 to 6,000 ppm by mass, and even more preferably 800 to 1,800 ppm by mass. When the content is 100 ppm by mass or more, the film exhibits slipperiness, and when the content is 10,000 ppm by mass or less, the film has good transparency.

[0025] The manufacturing method for obtaining the biaxially stretched polyester film of the present invention will be specifically described below, but is not limited thereto.

[0026] The method for obtaining the biaxially stretched polyester film of the present invention is not particularly limited, but the T-die method is preferred from the viewpoint of thickness accuracy. In the inflation method, the stretch ratio is difficult to increase due to the manufacturing method, and thickness defects in the width direction may occur. Here, a typical manufacturing process of the T-die method is described. The T-die method includes: (1) a step of melt-extruding a polyester resin composition into a sheet using an extruder and a T-die and cooling it on a cooling roll to form an unstretched sheet; (2) a step of stretching the formed unstretched sheet in the MD direction and the TD direction perpendicular to the MD direction; (3) a heat-setting step of heating and crystallizing the film after the stretching; (4) a heat-relaxing step (sometimes referred to as a relaxation step) to remove residual strain in the heat-set film; and (5) a cooling step of cooling the film after the heat-relaxing step.

[0027] The resin melt temperature in the extruder is preferably 230 to 310°C, more preferably 240 to 300°C. By setting the temperature at 230°C or higher, the resin can be extruded, and the extrusion is stable, resulting in good thickness accuracy. By setting the temperature at 310°C or lower, decomposition of the resin can be suppressed, preventing the film from becoming brittle and preventing deterioration of the film quality due to heat degradation.

[0028] The chill roll temperature is preferably 0 to 40°C, and more preferably 0 to 20°C or less. By setting the temperature to 0°C or higher, the crystallization suppression effect when the molten polyester resin composition is cooled and solidified can be sufficiently exhibited. Furthermore, when the chill roll temperature is set within the above range, it is preferable to reduce the humidity of the environment near the chill roll to prevent condensation. On the other hand, by setting the temperature to 40°C or less, the crystallinity of the molten polyester resin composition when cooled and solidified does not become too high, making stretching easier and also suppressing a decrease in transparency due to crystallization.

[0029] The thickness of the unstretched sheet is preferably in the range of 15 to 2500 μm, more preferably 500 μm or less, and most preferably 300 μm or less.

[0030] The stretching method can be either simultaneous biaxial stretching or sequential biaxial stretching. However, in order to increase the puncture strength, it is necessary to increase the degree of planar orientation. In addition, sequential biaxial stretching is preferred in terms of fast film production speed and high productivity.

[0031] The stretching temperature in the longitudinal direction (also referred to as the MD direction) is preferably 55 to 130°C, more preferably 60 to 120°C. By setting the temperature at 55°C or higher, not only can breakage be made less likely, but stretching at low temperatures strengthens the longitudinal orientation, which can prevent the shrinkage stress during heat setting from increasing, thereby preventing the distortion of the molecular orientation in the width direction from increasing. By setting the temperature at 130°C or lower, the planar orientation can be increased, thereby increasing the puncture strength. Furthermore, it is possible to prevent the mechanical properties from decreasing due to the lack of orientation.

[0032] The stretching ratio in the MD direction is preferably 3.0 to 4.5 times, and particularly preferably 3.4 to 4.4 times. By making it 3.0 times or more, the degree of planar orientation can be increased, and the puncture strength can be increased. In addition, deterioration of mechanical properties and thickness unevenness can be suppressed, and slack in the film roll can be prevented. By making it 4.5 times or less, the effects of improving mechanical strength and thickness unevenness can be sufficiently obtained. In addition, since the orientation in the MD direction is strong, the shrinkage stress during heat setting treatment increases, and it is possible to suppress an increase in distortion of the molecular orientation in the width direction.

[0033] The lower limit of the stretching temperature in the width direction (also referred to as the TD direction) is preferably 80 to 130°C, more preferably 85 to 125°C, and particularly preferably 90 to 120°C. By setting the temperature to 80°C or higher, breakage can be made less likely to occur. By setting the temperature to 130°C or lower, the degree of planar orientation can be increased, and the puncture strength can be increased. In addition, a decrease in mechanical properties due to a lack of orientation can be suppressed.

[0034] The stretching ratio in the TD direction is preferably 3.6 to 5.0 times, more preferably 3.8 to 4.8 times, and particularly preferably 4.0 to 4.5 times. By setting it to 3.6 times or more, the degree of planar orientation can be increased, and the puncture strength can be increased. In addition, deterioration of mechanical properties and thickness unevenness due to lack of orientation can be suppressed. By setting it to 5.0 times or less, the effects of improving mechanical strength and thickness unevenness can be sufficiently obtained.

[0035] The heat setting temperature in the TD direction is preferably 180 to 240°C, more preferably 190 to 225°C, and particularly preferably 200 to 210°C. By setting the temperature to 180°C or higher, the heat shrinkage rate can be further reduced, and the occurrence of displacement and shrinkage during processing can be suppressed. By setting the temperature to 240°C or lower, melting of the film can be suppressed, and the film can be prevented from becoming significantly brittle. In addition, the degree of planar orientation can be increased, and the puncture strength can be increased.

[0036] The relaxation rate in the TD direction is preferably 1 to 12%, more preferably 2 to 11%, and particularly preferably 3 to 10%. By setting it to 1% or more, the thermal shrinkage rate in the TD direction can be kept small, preventing breakage during heat setting. By setting it to 12% or less, it is possible to prevent thickness unevenness caused by slack, etc., and improve flatness.

[0037] The thickness of the biaxially oriented polyester film of the present invention is preferably 8 to 30 μm, more preferably 10 to 20 μm. By making the thickness 8 μm or more, the strength of the film can be maintained. By making the thickness 30 μm or less, the object of the present invention, volume reduction, can be achieved. One of the characteristics of the biaxially oriented polyester film of the present invention is that it can be made thinner than conventional polyester films.

[0038] The haze of the biaxially stretched polyester film of the present invention is preferably 10% or less, more preferably 9% or less, and even more preferably 7% or less. A haze of 10% or less is preferable because the contents can be clearly seen and the print can be clearly seen when the film is used as a package. The lower limit is not particularly limited, but may be 1% or more, or 2% or more.

[0039] The puncture strength per thickness of the biaxially oriented polyester film of the present invention is preferably 0.57 to 0.90 N / μm, more preferably 0.58 to 0.85 N / μm, and particularly preferably 0.60 to 0.80 N / μm. By setting the puncture strength to 0.57 N / μm or more, insufficient strength when used as a package can be prevented, and the film can be suitably used, for example, for packaging relatively heavy foods or packaging for sharp contents. By setting the puncture strength to 0.90 N / μm or less, thermal shrinkage can be suppressed, thereby reducing processing problems caused by film shrinkage during processes such as vapor deposition and printing.

[0040] The breaking strength of the biaxially stretched polyester film in the present invention is preferably 140 to 240 MPa, more preferably 150 to 230 MPa, and particularly preferably 160 to 220 MPa in the longitudinal direction, and is preferably 160 to 270 MPa, more preferably 170 to 260 MPa, and particularly preferably 180 to 250 MPa in the transverse direction.

[0041] The breaking elongation of the biaxially stretched polyester film in the present invention is preferably 70 to 180%, more preferably 80 to 170%, and particularly preferably 90 to 160% in the longitudinal direction (BEa), and preferably 60 to 160%, more preferably 70 to 150%, and particularly preferably 80 to 140% in the transverse direction (BEb).

[0042] The biaxially stretched polyester film of the present invention preferably has a ratio BEa / BEb of the breaking elongation BEa in the longitudinal direction of the film to the breaking elongation BEb in the transverse direction, which is orthogonal to the longitudinal direction, of 0.65 to 1.50, more preferably 0.75 to 1.45, and particularly preferably 0.85 to 1.40. By setting the ratio within the range of 0.65 to 1.50, when the film is used as an easy-peel film for a lid of a sealed packaging container, for example, the film can have good steam escape properties and prevent the contents from scattering or boiling over when the lidded container containing food or the like is heated in a microwave oven.

[0043] The static and dynamic friction coefficients of the biaxially oriented polyester film of the present invention are preferably 0.25 to 0.50, more preferably 0.30 to 0.48, and particularly preferably 0.34 to 0.45. By setting the coefficients within the range of 0.25 to 0.50, the film has appropriate slip properties and the winding quality of the film roll can be improved.

[0044] The heat shrinkage rate in the longitudinal direction of the biaxially stretched polyester film of the present invention after heating at 150°C for 15 minutes is preferably 1.0 to 5.0%, more preferably 1.2 to 4.5%, and even more preferably 1.4 to 4.0%. By setting the shrinkage rate in the range of 1.0 to 5.0%, the puncture strength can be increased. On the other hand, the heat shrinkage rate in the width direction after heating at 150°C for 15 minutes is preferably -1.0 to 1.5%, more preferably -0.8 to 1.2%, and even more preferably -0.6 to 1.0%. By setting the shrinkage rate in the range of -1.0 to 1.5%, processing problems such as pitch misalignment due to dimensional changes during processing such as printing can be reduced.

[0045] A printed layer may be laminated on the biaxially stretched polyester film of the present invention. Aqueous and solvent-based resin-containing printing inks are preferably used as the printing ink for forming the printed layer. Examples of resins used in printing inks include acrylic resins, urethane resins, polyester resins, vinyl chloride resins, vinyl acetate copolymer resins, and mixtures thereof. The printing ink may contain known additives such as antistatic agents, light-blocking agents, ultraviolet absorbers, plasticizers, lubricants, fillers, colorants, stabilizers, lubricants, antifoaming agents, crosslinking agents, antiblocking agents, and antioxidants.

[0046] The printing method for providing the printed layer is not particularly limited, and known printing methods such as offset printing, gravure printing, screen printing, etc. can be used. To dry the solvent after printing, known drying methods such as hot air drying, hot roll drying, and infrared drying can be used.

[0047] The biaxially stretched polyester film of the present invention may be subjected to corona discharge treatment, glow discharge treatment, flame treatment, or surface roughening treatment, and may also be subjected to known anchor coating treatment, printing, decoration, etc., as long as the treatment does not impair the object of the present invention.

[0048] (Laminated Film) The laminated film of the present invention is characterized by comprising at least the biaxially oriented polyester film and a sealant film. The sealant film may be a heat-sealable resin layer. The sealant film is usually formed by extrusion lamination or dry lamination. The thermoplastic copolymer forming the sealant film may be any copolymer that can sufficiently exhibit sealant adhesiveness, and examples of such copolymers include polyethylene resins such as HDPE, LDPE, and LLDPE, polypropylene resins, ethylene-vinyl acetate copolymers, ethylene-α-olefin random copolymers, and ionomer resins.

[0049] The sealant film may be a single-layer film or a multilayer film, and may be selected depending on the required function. For example, to provide moisture resistance, a multilayer film containing a resin such as an ethylene-cyclic olefin copolymer or polymethylpentene may be used. The sealant layer may also contain various additives such as a flame retardant, a slip agent, an antiblocking agent, an antioxidant, a light stabilizer, and a tackifier.

[0050] The thickness of the sealant film is preferably 10 to 100 μm, more preferably 20 to 60 μm.

[0051] The laminated film of the present invention may further include an aluminum foil laminated thereon in addition to the biaxially oriented polyester film and the sealant film, for example, a biaxially oriented polyester film / aluminum foil / sealant film structure.

[0052] The breaking strength of the laminated film of the present invention is preferably 40 to 120 MPa in both the longitudinal direction and the width direction, more preferably 45 to 110 MPa, and particularly preferably 50 to 110 MPa. By making the breaking strength 40 MPa or more, when used as an easy-peel film for a lid of a sealed packaging container, for example, the contents can be protected from external impacts and forces during transportation.

[0053] The breaking strength of the laminated film of the present invention at 120°C is preferably 20 to 60 MPa in both the longitudinal and transverse directions, more preferably 21 to 58 MPa, and particularly preferably 22 to 55 MPa. By making the breaking strength 20 MPa or more, when used as an easy-peel film for a lid of a sealed packaging container, for example, the lid breaks easily when heated in a microwave oven, and steam escapes well, preventing the contents from scattering or boiling over.

[0054] The breaking elongation of the laminated film of the present invention is preferably 120 to 230% in both the longitudinal direction and the width direction, more preferably 125 to 220%, and particularly preferably 130 to 210%. By making it 120% or more, when it is used as an easy-peel film for a lid material of a sealed packaging container, for example, the lid material stretches at room temperature, and therefore the contents can be protected from external impacts and forces during transportation.

[0055] The breaking elongation of the laminated film of the present invention at 120°C is preferably 120 to 250%, more preferably 130 to 250%, and particularly preferably 140 to 250%, in both the longitudinal and transverse directions. By making the breaking elongation 250% or less, when used as an easy-peel film for a lid of a sealed packaging container, for example, the lid is less likely to stretch when heated in a microwave oven, and the lid is more likely to break due to the internal pressure increased by steam, resulting in good steam escape properties and preventing the contents from scattering or boiling over.

[0056] The stress of the laminated film of the present invention at 100% elongation at 120° C. is preferably 10 to 80 MPa, more preferably 15 to 70 MPa, in both the longitudinal and transverse directions. By making the stress 10 MPa or more, when used as an easy-peel film for a lid of a sealed packaging container, for example, the internal pressure increased by steam during heating in a microwave oven is less likely to be applied unevenly, making the lid easier to break and improving steam escape properties, preventing the contents from scattering or boiling over.

[0057] The absolute value of the stress difference of the laminated film of the present invention when elongated 100% at 120°C in the longitudinal direction and the orthogonal width direction is preferably 12% or less, more preferably 11% or less, and particularly preferably 10% or less. By setting the stress difference to 12% or less, when the film is used as an easy-peel film for a lid of a sealed packaging container, for example, the internal pressure increased by steam during heating in a microwave oven is easily applied uniformly not only in one direction within the film plane but also in at least two directions, or even in various directions within the film plane, making the lid easily rupturable and improving steam escape, preventing the contents from scattering or boiling over. The lower limit is preferably 0% or more, and even 1% or more is preferable.

[0058] The edge tear resistance of the laminated film of the present invention is preferably 100 to 200 N in both the longitudinal and transverse directions, more preferably 105 to 190 N, and particularly preferably 110 to 180 N. By making the edge tear resistance 100 N or more, when used as an easy-peel film for a lid of a sealed packaging container, for example, the contents can be protected from external impacts and forces during transportation. By making the edge tear resistance 200 N or less, the seal line formed by the heat seal between the peripheral flange portion of the sealed packaging container and the lid can be prevented from breaking or peeling.

[0059] The tear strength of the laminated film of the present invention is preferably 200 to 350 mN in both the longitudinal and transverse directions, more preferably 210 to 340 mN, and particularly preferably 220 to 330 mN. Having a tear strength of 200 mN or more allows, for example, when used as an easy-peel film for the lid of a sealed packaging container to protect the contents from external impacts and forces during transportation. Furthermore, a laminated film having a tear strength of 350 mN or less is preferred because the lid is easily ruptured by the internal pressure increased by steam during heating in a microwave oven, allowing for good steam escape, preventing the contents from scattering or boiling over.

[0060] The laminated film of the present invention is suitable for use as an easy-peel film used as a lid material for packaging containers for prepared foods, dessert foods, dairy products, etc. Furthermore, it can be suitably used for packaging bags and containers for foods, beverages, and pharmaceuticals, various label materials, laminated tubes, etc.

[0061] The laminated film of the present invention is particularly effective when used in a state in which at least a weakened line has been formed in the biaxially oriented polyester film. That is, as a package that can stably release the internal pressure increased by water vapor generated during heating in a microwave oven and release steam in a simple manner, there is a package in which a weakened line of a depth that does not penetrate the lid material is formed by laser irradiation or the like, and when heated, the base film opens at the weakened line, stretching the sealant layer located on the inside of the package and forming a through-hole through which steam inside the package is released, and the laminated film of the present invention is particularly excellent in terms of steam release properties in such applications.

[0062] Therefore, the laminated film of the present invention is particularly effective for lid materials or packaging containers containing the laminated film of the present invention when weakened lines are formed in at least the biaxially oriented polyester film of the laminated film.

[0063] The weakened processed line may be a continuous or discontinuous processed line, and may be any line that is more easily broken than before processing, such as laser processing, half-cutting, perforation processing, groove processing, etc. The weakened processed line may be formed at least in the biaxially oriented polyester film, and may also be formed in other layers. However, in order to maintain internal pressure, it is preferable that no through holes penetrating the lid material are formed.

[0064] The film was evaluated by the following measurement methods. Unless otherwise specified, the measurements were carried out in a measurement room at an environment of 23° C. and a relative humidity of 65%.

[0065] [Film Thickness] The thickness was measured using a dial gauge in accordance with JIS K7130:1999 Method A.

[0066] [Film Haze] According to JIS K7361-1:1997, the film was cut into a square shape with a side of 10 cm, and the haze was measured using a haze meter NDH2000 manufactured by Nippon Denshoku Industries Co., Ltd. Measurement was performed at three locations, and the average value was taken as the actual haze measurement value.

[0067] [Dynamic and static friction coefficients of film] A sample film was prepared by cutting out a piece of film measuring 400 mm in length and 100 mm in width from the obtained film. This was aged for 12 hours in an atmosphere of 23°C and 65% RH, and then divided into a test piece measuring 300 mm in length and 100 mm in width for the test table and a test piece measuring 100 mm in length and 100 mm in width for the sliding piece.

[0068] The test piece for the test table was set on the test table, and the test piece for the sliding piece was the bottom surface (area size: 39.7 mm) of a metal sliding piece with a load of 1.5 kg. ２ The test piece was attached to a square sheet (square, 100 mm square) with the opposite surfaces facing each other. In accordance with JIS K-7125:1999, the dynamic friction coefficient and static friction coefficient of the test piece were measured using a tensile tester (Tensilon RTG-1210 manufactured by A&D Co., Ltd.) under conditions of a sliding speed of 200 mm / min, 23°C, and 65% RH, and the average of three measurements was used.

[0069] [Heat Shrinkage of Film] The heat shrinkage was measured by a dimensional change test method in accordance with JIS C2318:2020, except that the test temperature was 150°C and the heating time was 15 minutes.

[0070] [Impact Strength of Film] In accordance with JIS K7160:1996, the impact strength of the film against punching was measured in an atmosphere of 23°C using a film impact tester manufactured by Toyo Seiki Seisakusho Co., Ltd. The impact spherical surface used had a diameter of 1 / 2 inch. The unit is J / µm.

[0071] [Film Breaking Strength] A sample measuring 15 mm in width and 180 mm in length was cut out from the film. The cut-out sample was left in an atmosphere of 23°C and 65% RH for 12 hours, and then measured under conditions of 23°C and 65% RH, with a chuck distance of 100 mm and a pulling speed of 200 mm / min. The breaking strength (MPa) in each direction was determined from the obtained load-strain curve, and the average value of five measurement results was used. The measuring device used was an Autograph AG-1 manufactured by Shimadzu Corporation.

[0072] [Film Breaking Elongation] A sample measuring 15 mm in width and 180 mm in length was cut out from the film. The cut-out sample was left in an atmosphere of 23°C and 65% RH for 12 hours, and then measured under conditions of 23°C and 65% RH, with a chuck distance of 100 mm and a pulling speed of 200 mm / min. The average value of five measurement results was used. The measurement device used was an Autograph AG-1 manufactured by Shimadzu Corporation.

[0073] [Film Puncture Strength] The value measured by a test method in accordance with JIS-Z1707:2019 was calculated in terms of 1 μm using the following formula.

[0074] Puncture strength (N / μm) = measured puncture strength / film thickness

[0075] [Breaking strength of laminated film] A sample measuring 30 mm in width and 100 mm in length was cut out from the film. The cut sample was left in an atmosphere of 23°C and 65% RH for 12 hours, and then measured under conditions of 23°C, 65% RH, a chuck distance of 20 mm, and a pulling speed of 200 mm / min. The breaking strength (MPa) in each direction was determined from the obtained load-strain curve, and the average value of five measurement results was used. The measuring device used was an Instron 68TM-5 tabletop universal material testing machine.

[0076] [Elongation at break of laminated film] A sample measuring 30 mm in width and 100 mm in length was cut out from the film. The cut-out sample was left in an atmosphere of 23°C and 65% RH for 12 hours, and then measured under the conditions of 23°C, 65% RH, a chuck distance of 20 mm, and a tensile speed of 200 mm / min. The average value of five measurement results was used. The measurement device used was an Instron 68TM-5 tabletop universal material testing machine.

[0077] [Breaking strength of laminated film at 120°C] A sample 30 mm wide and 100 mm long was cut out from the film. The cut out sample was left in an atmosphere of 23°C and 65% RH for 12 hours, then held in an atmosphere of 120°C for 30 seconds, and then the atmosphere containing the sample was maintained at 120°C. Measurements were performed under conditions of a chuck distance of 20 mm and a tensile speed of 200 mm / min. The breaking strength (MPa) in each direction was determined from the obtained load-strain curve, and the average value of five measurement results was used. The measurement device used was an Instron 68TM-5 tabletop universal material testing machine.

[0078] [Elongation at break of laminated film at 120°C] A sample of 30 mm width and 100 mm length was cut out from the film. The cut out sample was left in an atmosphere of 23°C and 65% RH for 12 hours, then held in an atmosphere of 120°C for 30 seconds, and then the temperature in the atmosphere where the sample was present was maintained at 120°C. Measurement was performed under conditions of a chuck distance of 20 mm and a tensile speed of 200 mm / min, and the average value of five measurement results was used. The measurement device used was an Instron tabletop universal testing machine 68TM-5. [Stress at 100% elongation of laminated film at 120°C] A sample of 30 mm width and 100 mm length was cut out from the film. The cut-out sample was left in an atmosphere of 23°C and 65% RH for 12 hours, then held in an atmosphere of 120°C for 30 seconds, and then the atmosphere in which the sample was present was maintained at 120°C, and measurements were carried out under conditions of a chuck distance of 20 mm and a tensile speed of 200 mm / min. The load at 100% elongation in the obtained load-strain curve was divided by the cross-sectional area of ​​the sample before tension to determine the stress at 100% elongation (MPa) in each direction, and the average value of five measurement results was used. The measuring device used was an Instron 68TM-5 benchtop universal material testing machine.

[0079] [Edge tear resistance of laminated film] The test was carried out according to JIS C2151: 2019. Measurements were made in the longitudinal direction and width direction of the laminated film with N=3, and the average value was calculated.

[0080] [Tear Strength of Laminated Film] The tear strength of the laminated film was measured in accordance with JIS K7128-1:1998. The laminated film was evaluated. Measurements were made in the longitudinal direction and width direction, with N=3, and the average value was calculated.

[0081] [Evaluation of vapor escape properties of lid material] A urethane-based two-component curing adhesive ("Takelac (registered trademark) A525S" and "Takenate (registered trademark) A50" manufactured by Mitsui Chemicals, Inc.) was blended in a ratio of 13.5:1.0 (mass ratio) on the biaxially stretched film obtained in the following Examples and Comparative Examples, and a 50 μm thick unstretched co-extruded multilayer film ("9501E" manufactured by Toray Film Processing Co., Ltd.) was bonded as a heat-sealable resin by the dry lamination method, and aging was carried out at 40° C. for 2 days to obtain a laminate. The thickness of the adhesive layer formed with the urethane-based two-component curing adhesive after drying was approximately 4 μm in all cases.

[0082] The laminate was irradiated with a carbon dioxide laser to form a linear weakening line 30 mm long. While the weakening line penetrated the biaxially oriented film, the unstretched coextruded multilayer film, which is a heat-sealable resin that does not absorb carbon dioxide laser light, remained intact. This laminate was cut into a 12 cm square. A heat-resistant PP tray ("Super Range K Square 111" manufactured by Sumi Corporation) was used. A tissue soaked in 10 g of tap water was placed in the tray, which was then placed over the opening of the container. A manual pack sealer (manufactured by Eishin Pack Kogyo Co., Ltd.) was used to heat seal the tray at 180°C for 2 seconds to obtain a sealed packaging container sample. The resulting sealed packaging container sample was heated in a microwave oven at 500 W to evaluate its suitability. As the internal pressure inside the sealed packaging container increased, many tiny through-holes were formed at the position of the weakened processing line, and water vapor escaped, which was marked as ○.If no through-holes were formed at the position of the weakened processing line, and the seal line formed by the heat seal between the peripheral flange portion of the sealed packaging container and the lid material broke, or the seal line peeled off and retreated from the inside to the outside of the sealed packaging container, it was marked as ×.

[0083] Example 1 Using a single-screw extruder, a resin composition containing 40 parts by mass of PBT resin (intrinsic viscosity 1.28 dL / g), 60 parts by mass of PET resin (intrinsic viscosity 0.62 dL / g), and 900 ppm of porous silica particles (average particle size 2.4 μm) was melted, and then cast from a T-die at 280°C and adhered to a cooling roll at 20°C by electrostatic adhesion to obtain an unstretched sheet.

[0084] The unstretched sheet was then roll-stretched 3.8 times in the MD direction at a temperature of 80°C, and then passed through a tenter to stretch 4.5 times in the TD direction at 110°C, followed by a tension heat treatment at 200°C for 3 seconds and a 9% relaxation treatment for 1 second to obtain a biaxially stretched polyester film with a thickness of 15 μm.

[0085] A dry laminating adhesive (Toyo-Morton Co., Ltd., TM569, CAT-10L) was applied to the obtained biaxially stretched film at a solid content of 3 g / m ２ The solvent was evaporated and removed in an oven at 80°C, and then a 40 µm thick unstretched polypropylene film (L4202 manufactured by Toyobo Co., Ltd.) was bonded as a sealant film by nipping it between temperature-controlled rolls at 60°C, and aging was carried out at 40°C for 2 days to obtain a laminated film.

[0086] The physical properties and evaluation results of the obtained film and laminated film are shown in Table 1.

[0087] [Example 2] A biaxially stretched polyester film having a thickness of 15 μm was obtained by film production in the same manner as in Example 1, except that the raw material composition and stretching conditions in Example 1 were changed to those shown in Table 1. The physical properties and evaluation results of the obtained film are shown in Table 1.

[0088] [Example 3] A biaxially stretched polyester film having a thickness of 15 μm was obtained by film production in the same manner as in Example 1, except that the raw material composition and stretching conditions in Example 1 were changed to those shown in Table 1. The physical properties and evaluation results of the obtained film are shown in Table 1.

[0089] [Example 4] A biaxially stretched polyester film having a thickness of 20 µm was produced by the same film production method as in Example 1. The physical properties and evaluation results of the obtained film are shown in Table 1.

[0090] Comparative Example 1 A biaxially oriented polyester film having a thickness of 15 μm was produced in the same manner as in Example 1, except that the raw material composition and stretching conditions in Example 1 were changed to those shown in Table 1. The breaking elongation of the obtained film had a high ratio of the MD direction to the TD direction, and the stress difference at 100% elongation at 120°C was large, resulting in non-uniform stress due to internal pressure, easy peeling of the seal line, and difficulty in forming through holes at the position of the weakened processing line due to the high tear strength in the TD direction, resulting in insufficient steam escape properties.

[0091] Comparative Example 2 A biaxially oriented polyester film having a thickness of 15 μm was produced in the same manner as in Example 1, except that the raw material composition and stretching conditions were changed to those shown in Table 1. The resulting film had a low puncture strength, which was disadvantageous in terms of resistance to external impacts and forces applied during transport of the package and in terms of reducing the volume of the package. Furthermore, the resulting laminated film had a low breaking strength at 120°C and a large stress difference at 100% elongation at 120°C, which resulted in non-uniform stress due to internal pressure, causing breakage and peeling of the seal line and insufficient steam escape properties.

[0092] Comparative Example 3: A biaxially oriented polyester film having a thickness of 15 μm was produced in the same manner as in Example 1, except that the raw material composition and stretching conditions were changed to those shown in Table 1. The ratio of the MD to TD elongation of the resulting film at break was high, and the resulting laminated film had low MD breaking strength at 120°C, low MD breaking elongation at 120°C, a large stress difference at 100% elongation at 120°C, and high TD tear strength. This resulted in non-uniform stress due to internal pressure, leading to breakage and peeling of the seal line and insufficient steam escape properties. Furthermore, the resulting film had low puncture strength, which was unfavorable for resistance to external impacts and forces applied from the outside during transport of the package and was also unfavorable when attempting to reduce the volume of the package.

[0093] [Comparative Example 4] A biaxially oriented polyester film having a thickness of 15 μm was obtained by film production in the same manner as in Example 1, except that the raw material composition and stretching conditions in Example 1 were changed to those shown in Table 1. Because the tear strength of the obtained laminated film was high, it was difficult to form through holes at the positions of the weakened processing lines, and the vapor escape property was insufficient.

[0094] Comparative Example 5 A biaxially oriented polyester film having a thickness of 15 μm was obtained by film production in the same manner as in Example 1, except that the raw material composition and stretching conditions in Example 1 were changed to those shown in Table 1. The resulting laminated film had a high MD breaking strength at 120°C, a high MD breaking elongation at 120°C, a large stress difference at 100% elongation at 120°C, non-uniform stress due to internal pressure, and a high tear strength, which made it difficult to form through holes at the positions of the weakened processing lines, resulting in insufficient steam escape properties.

[0095] [Comparative Example 6] A biaxially oriented polyester film was obtained by film production in the same manner as in Example 1, except that the raw material composition and stretching conditions in Example 1 were changed to those shown in Table 1 and the thickness was changed to 12 μm. The resulting laminated film had a high MD breaking strength at 120°C, a high MD breaking elongation at 120°C, a large stress difference at 100% elongation at 120°C, and non-uniform stress due to internal pressure. Because the MD tear strength was high, it was difficult to form through-holes at the positions of the weakened processing lines, and steam escape properties were insufficient.

[0096]

Claims

1. A laminated film comprising a biaxially oriented polyester film and a sealant film, wherein the biaxially oriented polyester film contains 25 to 60% by mass of polybutylene terephthalate resin and 40 to 75% by mass of polyethylene terephthalate resin relative to 100% by mass of the constituent resin components, has a puncture strength per film thickness of 0.57 to 1.00 N / μm, and has a ratio BEa / BEb of the breaking elongation BEa in the longitudinal direction of the film to the breaking elongation BEb in the transverse direction, which is orthogonal to the longitudinal direction, of 0.65 to 1.50, and the laminated film satisfies the following conditions (a) to (d): (a) The breaking strength of the laminated film in the longitudinal and width directions at 120°C is 20 to 60 MPa. (b) The breaking elongation of the laminated film in the longitudinal and width directions at 120°C is 120 to 250%. (c) The absolute value of the stress difference of the laminated film in the longitudinal and width directions at 120°C when elongated by 100% is 0 to 12 MPa. (d) The tear strength of the laminated film in the longitudinal and width directions is 200 to 350 mN.

2. The laminate film according to claim 1, wherein said laminate film further comprises an aluminum foil layer.

3. The laminated film according to claim 1, wherein the laminated film is used in a state where at least a weakening line is formed in the biaxially oriented polyester film.

4. A lid material comprising the laminated film according to claim 1 or 2.

5. The covering material according to claim 4, wherein weakened lines are formed in at least the biaxially oriented polyester film of the laminated film.

6. A packaging container comprising the laminated film according to claim 1 or 2.

7. The packaging container according to claim 6, wherein weakened lines are formed in at least the biaxially oriented polyester film of the laminated film.

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