JP2016014083A - Polyester film for inner bag of can - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film for an inner bag of a can, having excellent acid resistance while being provided with required mechanical strength and, moreover, good lamination workability when laminating a heat seal layer for forming an inner bag, and having heat resistance.SOLUTION: The polyester film for an inner bag of a can is a film comprising a polyester (I) including polybutylene terephthalate as a main constituent and a polyester (II) including polyethylene terephthalate as a main constituent. A mass ratio, (I)/(II), of the polyester (I) and the polyester (II) is (80-40)/(20-60). Tensile moduli in both a longitudinal direction and a width direction of the film are 2.3 to 3.5 GPa. Thermal shrinkage of the film in the longitudinal direction at 150°C×15 minutes is 10.0% or less, and thermal shrinkage of the film in the width direction at 150°C×15 minutes is 2.0% or less.

Description

  The present invention relates to a polyester film for an inner bag of a can.

When a corrosive liquid such as an acidic liquid is stored inside a can such as a drum can, an inner bag is installed in the can for the purpose of preventing the corrosion of the can (Patent Document 1, 2).
Among these, Patent Document 1 describes an inner bag of this type that is formed in a plurality of layers by superposing thermoplastic resin sheet materials. As the thermoplastic resin constituting the sheet material, a flexible thermoplastic resin such as polyethylene, polypropylene, and polyvinyl chloride is used (paragraph 0011).

  Patent Document 2 describes a polyethylene inner bag sheet or a laminated inner bag sheet of polyethylene and nylon as an inner bag sheet for a paint drum.

  However, in the inner bag described in Patent Document 1, only a general thermoplastic resin, that is, a thermoplastic resin having flexibility such as polyethylene, polypropylene, and polyvinyl chloride is used. As the inner bag sheet, only an inner bag made of a general resin such as a polyethylene inner bag sheet or a laminated inner bag sheet of polyethylene and nylon is used. Therefore, when the liquid stored in the drum can is a liquid exhibiting strong acidity, such as an electrolyte of a lithium ion secondary battery, the acid resistance for preventing corrosion of the can is not sufficient. There is.

  The present applicant has previously made a mass ratio (I) / (II) of 80 to 40/20 to 60 of polyester (I) mainly composed of polybutylene terephthalate and polyester (II) mainly composed of polyethylene terephthalate. And a biaxially stretched polyester film obtained by adjusting the stretching ratio and the stretching temperature was proposed. (Japanese Patent Application No. 2013-178900). The film proposed here is excellent in acid resistance after having a predetermined mechanical strength, and also has excellent laminating properties when a heat seal layer for forming an inner bag is laminated.

JP 2001-219965 A JP 2000-309373 A

By the way, recently, due to the high performance of processing machines, a film that can withstand high temperature sealing conditions in addition to high speed is demanded.
Accordingly, the present invention solves such problems and has excellent mechanical resistance while also having the required mechanical strength, and also has a laminate processability when laminating a heat seal layer for forming an inner bag. An object is to obtain a film for an inner bag of a can which is good and has heat resistance.

The summary of the polyester film for an inner bag of the present invention for achieving this object is as follows.
(1) A film containing polyester (I) mainly composed of polybutylene terephthalate and polyester (II) mainly composed of polyethylene terephthalate, and a mass ratio (I) of polyester (I) to polyester (II) / (II) is 80 to 40/20 to 60, the tensile modulus is 2.3 to 3.5 GPa in both the longitudinal direction and the width direction of the film, and the heat in the longitudinal direction of the film at 150 ° C. for 15 minutes A polyester film for an inner bag of a can having a shrinkage ratio of 10.0% or less and a heat shrinkage ratio in the width direction of the film at 150 ° C. for 15 minutes of 2.0% or less.
(2) The polyester film for inner bags of cans according to (1) above, wherein the impact strength is 0.4 J or more in terms of a film thickness of 12 μm.
(3) The polyester film for an inner bag of the above (1) or (2), wherein the polyester film has a thickness of 10 to 25 μm.
(4) A composite film for an inner bag of a can, wherein a polyolefin film is laminated on the polyester film for the inner bag of the can of (1), (2) or (3).
(5) A composite container in which a bag formed of the composite film for an inner bag of the can according to (4) is mounted on the inner surface of the can.

  The polyester film for the inner bag of the can of the present invention contains polyester (I) mainly composed of polybutylene terephthalate and polyester (II) mainly composed of polyethylene terephthalate, and includes polyester (I) and polyester (II). The mass ratio (I) / (II) is 80 to 40/20 to 60, and the tensile modulus is 2.3 to 3.5 GPa in both the longitudinal direction and the width direction of the film. It is excellent in impact resistance and bending resistance, and can be made into a film having good laminate processability when laminating a heat seal layer for forming an inner bag, and 150 ° C. in the longitudinal and width directions of the film. × The heat shrinkage rate (%) in 15 minutes is 10.0% or less and 2.0% or less, respectively. It can be a film that can suppress the occurrence of appearance defect even during Le.

  The polyester film for an inner bag of a can of the present invention contains polyester (I) mainly composed of polybutylene terephthalate (PBT) and polyester (II) mainly composed of polyethylene terephthalate (PET).

  The polybutylene terephthalate (PBT) in the present invention has terephthalic acid and 1,4-butanediol as polymerization components, and the polyester (I) mainly composed of polybutylene terephthalate (PBT) Copolymerized components may be used.

The copolymer component is not particularly limited, but as the acid component, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, 5-sodium sulfoisophthalic acid, oxalic acid, succinic acid, adipic acid, sebacic acid, azelaic acid , Dodecanedioic acid, dimer acid, maleic anhydride, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, cyclohexanedicarboxylic acid and other dicarboxylic acids, 4-hydroxybenzoic acid, ε-caprolactone, lactic acid, etc. .
Examples of alcohol components include ethylene glycol, diethylene glycol, 1,3-propanediol, neopentyl glycol, 1,6-hexanediol, cyclohexanedimethanol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, bisphenol A, Examples include ethylene oxide adducts of bisphenol S.
Further, a small amount of trifunctional compounds such as trimellitic acid, trimesic acid, pyromellitic acid, trimethylolpropane, glycerin, pentaerythritol, and the like may be used.
Two or more of these copolymer components may be used in combination.

  In the present invention, when a copolymer is used as the polyester (I), the type and ratio of the components to be copolymerized may be appropriately selected, but 1,4-butanediol is 80 mol% based on the total alcohol components. The above is preferable, and 90 mol% or more is more preferable. When 1,4-butanediol is less than 80 mol%, the melting point may be lower than the range described later. As a result, the crystallinity is lowered and the heat resistance of the film may be lowered.

  In the polyester film of the present invention, the melting point derived from polyester (I) is preferably 200 to 223 ° C, and more preferably 210 to 223 ° C. When the melting point is less than 200 ° C., polybutylene terephthalate (PBT) has low crystallinity and easily causes harmful effects.

  In the present invention, polyethylene terephthalate (PET) is composed of terephthalic acid and ethylene glycol as polymerization components, and polyester (II) mainly composed of polyethylene terephthalate (PET) is obtained by copolymerizing other components. But you can.

The copolymer component is not particularly limited, and examples thereof include the components exemplified in the above-mentioned polyester (I) mainly composed of polybutylene terephthalate (PBT).
The acid component to be copolymerized is preferably isophthalic acid because the following melting point can be easily adjusted. The content of isophthalic acid in the polyester (II) is preferably 0 to 15 mol%, more preferably 0 to 12 mol%, based on the total acid component.

  In the polyester film for an inner bag of the can of the present invention, the melting point derived from the polyester (II) is preferably 230 to 256 ° C, and more preferably 236 to 256 ° C. When the melting point is less than 230 ° C., PET has low crystallinity, and as a result, harmful effects are likely to occur.

  The polyester film for the inner bag of the can of the present invention has a mass ratio (I) / (II) of polyester (I) mainly composed of polybutylene terephthalate (PBT) and polyester (II) mainly composed of polyethylene terephthalate (PET). ) Is required to be 80 to 40/20 to 60, and in order to obtain sufficient laminate processability with a polyolefin film, which will be described later, and impact resistance of the film, 70 to 55/30 to 45 Preferably there is.

  When the mass ratio of the polyester (I) mainly composed of polybutylene terephthalate (PBT) in the polyester film exceeds 80, the characteristics of the soft polybutylene terephthalate (PBT) are remarkably exhibited, and the dry lamination with the polyolefin film On the other hand, when the mass ratio of polyester (I) mainly composed of polybutylene terephthalate (PBT) is less than 40, the characteristics of hard polyethylene terephthalate (PET) are present. Remarkably manifests too much and the impact resistance of the polyester film decreases.

  Polyester (I) mainly composed of polybutylene terephthalate (PBT) as a raw material used for producing a polyester film for an inner bag of a can of the present invention has an intrinsic viscosity of 0.75 to 1.6 dl / g. Preferably, the polyester (II) mainly composed of polyethylene terephthalate (PET) has an intrinsic viscosity of 0.65 to 1.0 dl / g. The reason for this is to achieve both long-term durability and hydrolysis resistance and melt stress during melt mixing. When the intrinsic viscosity falls below these ranges, problems are likely to occur in long-term durability and hydrolysis resistance. On the other hand, when the intrinsic viscosity exceeds these ranges, the melt viscosity of the raw material resin becomes too high, and the productivity of the film tends to decrease.

  Moreover, it is preferable that the intrinsic viscosity after melt-mixing these is 0.75-1.2 dl / g. If the intrinsic viscosity after the melt mixing is less than 0.75 dl / g, the mechanical strength of the obtained film is inferior, which may cause a disadvantage that the impact resistance is lowered. On the other hand, if the intrinsic viscosity after melt mixing exceeds 1.2 dl / g, in the film production process, the load applied to the melt extruder increases, and the production speed may have to be sacrificed. Moreover, since the melt residence time of the resin in the extruder becomes too long and the reaction between the polyester resins proceeds too much, the film characteristics may be deteriorated. In addition, the production of a raw material having a high intrinsic viscosity is a factor that increases costs because the polymerization time and the polymerization process are relatively long.

  The polymerization method of polyester (I) mainly composed of polybutylene terephthalate (PBT) and polyester (II) mainly composed of polyethylene terephthalate (PET) is not particularly limited. Polymerization can be performed by a transesterification method, a direct polymerization method, or the like. Examples of the transesterification catalyst include Mg, Mn, Zn, Ca, Li, and Ti oxides and acetates. Examples of the polycondensation catalyst include Sb, Ti, Ge oxides and acetates.

  Since the polyester after polymerization contains monomers, oligomers, by-products such as acetaldehyde and tetrahydrofuran, it is preferable to perform solid-phase polymerization at a temperature of 200 ° C. or higher under reduced pressure or through an inert gas flow.

  In the polymerization of polyester (I) mainly composed of polybutylene terephthalate (PBT) and polyester (II) mainly composed of polyethylene terephthalate (PET), additives such as antioxidants, heat stabilizers, ultraviolet Absorbers, antistatic agents and the like can be added. Examples of the antioxidant include hindered phenol compounds and hindered amine compounds, examples of the heat stabilizer include phosphorus compounds, and examples of the ultraviolet absorber include benzophenone compounds and benzotriazole compounds. Can be mentioned. Moreover, it is preferable to add a conventionally known phosphorus compound as a reaction inhibitor between polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) before, during and after polymerization.

  A small amount of inorganic lubricant such as silica, alumina, kaolin, etc. is added to the raw material polyester in order to improve the process passability during the production of the polyester film of the present invention and the lamination of a low melting point film for heat sealing to the polyester film described later. By adding and forming a film, it is preferable to give slip property to the surface of the polyester film. For this purpose, the content of the inorganic lubricant in the polyester film is preferably 0.001 to 0.5% by mass, and more preferably 0.05 to 0.3% by mass.

  The following method is mentioned as a manufacturing method of the polyester film for inner bags of the can of this invention. That is, a polyester (I) mainly composed of polybutylene terephthalate (PBT) and a polyester (II) mainly composed of polyethylene terephthalate (PET) have a mass ratio (I) / (II) of 80/20 to 40 / 60, blended in an extruder at a temperature of 250 to 280 ° C. for a residence time of 3 to 15 minutes, and then extruded into a sheet through a T-die. The sheet is brought into close contact with a cooling drum whose temperature is adjusted to room temperature or lower and cooled to obtain an unstretched film.

The obtained unstretched film is guided to a simultaneous biaxial stretching machine and simultaneously biaxially stretched in the longitudinal direction (MD) and the width direction (TD) at a temperature of 50 to 150 ° C. In the biaxial stretching, the surface magnification (MD stretching magnification × TD stretching magnification) is preferably 6 to 20 times, and more preferably 8.75 to 15.75. If the surface magnification exceeds 20 times, the in-plane orientation of the film proceeds and crystallization increases, and thus adverse effects are likely to occur. On the other hand, if the surface magnification is less than 6 times, the film tends to have insufficient strength.
The draw ratio (MD draw ratio / TD draw ratio) is preferably 0.4 to 1, and more preferably 0.55 to 1. If the draw ratio is out of this range, the film is inferior in orientation balance due to in-plane MD and TD orientation, and the resulting film is more prone to cracks and wrinkles than in the weak orientation direction. Become.
The MD draw ratio is preferably 2 to 4 times, and more preferably 2.5 to 3.5 times as the draw ratio in each direction for obtaining the above-described surface magnification and draw ratio. The TD stretch ratio is preferably 3 to 5 times, more preferably 3.5 to 4.5 times.
The unstretched film may be subjected to preliminary longitudinal stretching of about 1.2 times or less before being led to the simultaneous biaxial stretching machine.

The polyester film for an inner bag of the can of the present invention may be produced by a sequential biaxial stretching method. In the sequential biaxial stretching method, an unstretched film obtained by the same method as in the case of the simultaneous biaxial stretching method is heated with a roll or infrared ray, and using the peripheral speed difference of two or more rolls, The film is stretched in the longitudinal direction (MD) at 50 to 150 ° C. to obtain a longitudinally stretched film. In this longitudinal stretching, the MD stretching ratio is preferably 2 to 4 times, and more preferably 2.5 to 3.5 times.
Subsequently, the obtained longitudinally stretched film is continuously stretched in the width direction (TD) to form a biaxially stretched film. Stretching in the width direction (TD) starts at 50 to 150 ° C., and the TD stretching ratio is preferably 3 to 5 times, more preferably 3.5 to 4.5 times.
Even in the case of sequential biaxial stretching, for the same reason as in the case of simultaneous biaxial stretching, the surface magnification (MD stretching magnification × TD stretching magnification) is preferably 6 to 20 times, and 8.75. The stretch ratio (MD stretch ratio / TD stretch ratio) is preferably 0.4 to 1, more preferably 0.55 to 1, and more preferably 0.75. It is especially preferable that it is -0.85.

After simultaneous biaxial stretching or sequential biaxial stretching, it is preferable to heat-set by setting the relaxation rate of TD to 3.0 to 7.0%. The heat setting temperature is preferably 180 to 200 ° C, and more preferably 180 to 195 ° C. When the heat treatment temperature is less than 180 ° C, the resulting polyester film may have a thermal shrinkage rate at 150 ° C x 15 minutes of 10.0% or less in MD and 2.0% or less in TD, In secondary processing such as laminating and heat sealing, the operability is deteriorated and appearance defects such as wrinkles or sealing failure are caused. As a result, the inner bag may be inferior in the long-term storage stability of the contents.
The heat setting treatment after simultaneous biaxial stretching and sequential biaxial stretching is an important process for imparting dimensional stability of the film. As the method, a known method such as a method of blowing hot air, a method of irradiating infrared rays, or a method of irradiating microwaves can be used. Of these, the method of blowing hot air is optimal because it can be heated uniformly and accurately.

  The polyester film for the inner bag of the can of the present invention needs to have a tensile elastic modulus in the range of 2.3 to 3.5 GPa in both the longitudinal direction (MD) and the width direction (TD) of the film. If the tensile elastic modulus is less than 2.3 GPa, the film is too soft, and there is a problem that wrinkles are likely to be mixed in the film during secondary processing such as laminating or heat shrinkage. When the tensile elastic modulus exceeds 3.5 GPa, the film is too hard and is inferior in impact resistance and bending resistance.

  In the polyester film for the inner bag of the can according to the present invention, in order to make the tensile elastic modulus in the longitudinal direction (MD) and the width direction (TD) of the film both in the range of 2.3 to 3.5 GPa, The blending ratio of the polyester (I) mainly composed of polybutylene terephthalate (PBT) and the polyester (II) mainly composed of polyethylene terephthalate (PET) is within the above-mentioned range, and the stretching ratio and the stretching temperature are appropriately adjusted. It is necessary to.

  The polyester film for the inner bag of the can according to the present invention has a heat shrinkage rate (%) in the film longitudinal direction (MD) of 10.0% or less when treated at 150 ° C. for 15 minutes, and the film width direction (TD ) Is required to be 2.0% or less, and the heat shrinkage rate (%) in the film longitudinal direction (MD) is preferably 9.5% or less. When the heat shrinkage rate is out of the above range, heat shrinkage may occur at the time of heat sealing when forming a bag, which may promote poor sealing.

  In the polyester film for the inner bag of the can of the present invention, in order to satisfy the heat shrinkage rate, polyester (I) mainly composed of aromatic polybutylene terephthalate (PBT) and polyester mainly composed of polyethylene terephthalate (PET). It is necessary to adjust the draw ratio, the draw temperature, and the heat setting temperature as appropriate after the blending ratio with (II) is in the above range.

  The polyester film for an inner bag of a can according to the present invention preferably has an impact strength of 0.4 J or more in terms of a film thickness of 12 μm. If the impact strength is less than 0.4 J, the film is prone to cracking when it is impacted or scratched during transportation with the liquid contained as an inner bag of the can. There is a possibility of leakage. In order to set the impact strength to 0.4 J or more in terms of the film thickness of 12 μm, the blending ratio of the polyester (I) and the polyester (II) is within the above range, and the draw ratio, stretch temperature, heat It is necessary to adjust the fixing temperature appropriately.

  The polyester film for an inner bag of the can according to the present invention preferably has a film thickness of 10 to 25 μm. When the thickness is less than 10 μm, the strength of the film alone is lowered, the film is inferior in impact resistance and puncture resistance, and the film loses its elasticity. Wrinkles are easily mixed. When the thickness exceeds 25 μm, the film strength increases, but the stiffness of the film becomes too strong and the bending resistance tends to be poor.

  The polyester film for an inner bag of a can according to the present invention is used as an inner bag of a can after being formed into a bag body. In order to obtain a bag body, a general heat sealing method can be applied. For this purpose, a low melting point film for heat sealing is laminated on the polyester film. A polyolefin film is suitable as the low melting point film, and the composite film for an inner bag of the can of the present invention is constituted by laminating the polyolefin film on the polyester film for the inner bag. When the polyolefin film is laminated on the inner bag polyester film, the surface of the inner bag polyester film is subjected to surface treatment such as corona discharge treatment, plasma treatment, ozone treatment, frame treatment, etc., if necessary. be able to.

  A composite body of the present invention is configured by mounting a bag formed of a composite film for an inner bag of a can in a can such as a drum can. When this bag body is mounted in the can, the strong acid liquid contained in the bag body, for example, a corrosive liquid such as an electrolytic solution directly contacts the inner surface of the can, and the can is corroded. It can be prevented from occurring. It is also possible to adopt a configuration in which the corrosive liquid is accommodated in another bag and further accommodated in the bag formed of the composite film of the present invention.

Next, the present invention will be specifically described by way of examples.
The raw materials of the film and the method for measuring the characteristic values in the examples and comparative examples are as follows.

[material]
(Polyester (I))
Polybutylene terephthalate (PBT) subjected to solid phase polymerization, intrinsic viscosity 1.08 dl / g, melting point (Tm) 223 ° C., Ti catalyst 40 ppm contained.
(Polyester (II))
Polyethylene terephthalate (PET) subjected to solid phase polymerization, intrinsic viscosity 0.75 dl / g, melting point (Tm) 255 ° C., containing Ge catalyst 40 ppm.

[Measurement method of characteristic values] (heat shrinkage)
A sample film was cut into a strip of 10 mm width × 150 mm, and a test piece was prepared in which two marked lines were placed so that the distance was 100 mm.
The obtained test piece was heat-treated in an oven at 150 ° C. for 15 minutes under no load, and then the test piece was taken out and returned to room temperature to measure the distance between marked lines. The thermal contraction rate was calculated according to the following formula, and the average value of the measured values for the number of samples N = 5 for each of MD and TD was calculated.
Thermal contraction rate (%) = (A−B) / A × 100
A: Distance between marked lines before heat treatment (mm) B: Distance between marked lines after heat treatment (mm)
Further, the heat shrinkage rate after heat treatment at 130 ° C. for 30 minutes was measured by the same method.

(Tensile modulus (GPa), Tensile strength at break (MPa), Tensile elongation at break (%))
In accordance with the method described in JIS K-7127, in a 23 ° C. × 50% RH atmosphere, a sample film having a 10 mm width × 150 mm strip is used as a sample, and an autograph (tensile tester) manufactured by Shimadzu Corporation It measured using AG-IS. The average value of the measured values for the number of samples N = 5 for each of MD and TD was calculated. The tensile modulus (GPa) was acceptable within the range of 2.3 to 3.5 GPa, the tensile strength at break (MPa) was 200 MPa or higher, and the tensile elongation at break (%) was 90% or higher.

(Bending resistance)
The sample film was cut into a size of MD 300 mm × TD 200 mm to prepare a sample. Using this sample, in accordance with ASTM F392, the number of holes (number of pinholes) was counted after repeating bending 1000 times in a gel bot tester manufactured by Tester Sangyo Co., Ltd. in an atmosphere of 20 ° C. × 65% RH. The average value of the number of samples N = 3 was defined as the number of holes, and judged according to the following criteria.
◎: Number of perforations is less than 10 ○: Number of perforations is less than 10 to 30 ×: Number of perforations is less than 30 to 50 × ×: Number of perforations is 50 or more

(Impact strength)
Using an impact tester (manufactured by Toyo Seiki Seisakusho Co., Ltd.) in an atmosphere of 20 ° C. × 65% RH and using an impact head with a pendulum capacity of 30 kg · cm and a 1/2 inch φ, the impact head penetrates the film. The strength of was measured. The measured value was converted to a thickness of 12 μm, and 0.4 J or more was regarded as acceptable.

(Lamination processability)
In a state where a sample film subjected to corona discharge treatment on the surface in the manufacturing process is run and a tension of 10 kg / m is applied to the sample film, the dry coating amount is 3. with a gravure roll on the corona treatment surface of the sample film. A dry laminate adhesive (Dick Graphics LX-401A / SP-60) was applied so as to be 0 g / m ² , followed by heat treatment at 80 ° C. And the adhesive application surface in the film in which the above-mentioned tension was maintained after heat treatment, and a corona-treated surface of linear low density polyethylene (LLDPE) (manufactured by Mitsui Chemicals, Inc., TUX-FCS, thickness 70 μm) Bonding was performed at a nip pressure of 490 kPa on a metal roll heated to ° C. Further, an adhesive recommended aging was applied to obtain a laminate film.
At this time, the running state of the film after application of the adhesive was evaluated according to the following criteria, and ○ was set as acceptable.
○: No heat shrinkage / wrinkle was observed on the running film and the appearance of the laminate film was good. Δ: Some heat shrinkage / wrinkle was found on the running film, and some wrinkles were mixed in the laminate film. Heat shrinkage and wrinkles of the film inside is large, and many wrinkles are mixed in the laminate film

(Heat sealability)
Laminate films (MD × TD: 300 mm × 210 mm) created in the evaluation of the laminate processability are overlapped after the sealant layers (LLDPE) are half-folded, and are heated 160 ° C. at the top and 130 ° C. at the bottom. Under a temperature condition, the sheet was pressed at a seal pressure of 9.8 N (1 kg / cm ² ) for 1 second to obtain a three-sided bag (MD × TD: 150 mm × 210 mm, seal width: 10 mm). Then, the state of the seal part was evaluated according to the following criteria, and ○ was set as acceptable.
○: No heat loss or width shrinkage is observed in the film appearance after sealing △: Heat loss or width shrinkage slightly occurs in the film appearance after sealing ×: Heat loss / width shrinkage in the film appearance after sealing Is noticeable

(Acid resistance)
After filling the three-sided bag prepared in the evaluation of the heat sealability with hydrochloric acid (Kanto Chemical Co., Ltd., concentration: 36%), it was sealed under the same conditions, and the four-sided bag (MD × TD: 150 mm × 210 mm, seal width: 10 mm) ). After the contents were stored for one month in an environment of 23 ° C. × 50%, the contents were evaluated as “O” (pass) if no leakage of the contents had occurred, and “X” if they had occurred.

Examples 1 and 6 and Comparative Examples 1 and 5
0.08% by mass of agglomerated silica having an average particle size of 2.5 μm was added to polyesters (I) and (II) having a mass ratio shown in Table 1, and 275 ° C. (Example 1, Comparative Example 1) or 270 ° C. It was melted at the temperature of (Example 6, Comparative Example 5), extruded from the T-die outlet, and rapidly cooled and solidified to obtain an unstretched film.
Next, the end of the unstretched film was held by a clip of a tenter type simultaneous biaxial stretching machine, and after running through a preheating zone at 60 ° C., the temperature was 80 ° C., 3.0 times the MD, and 3. Simultaneous biaxial stretching was performed at 3 times. Thereafter, the relaxation rate of TD was set to 5%, and after heat treatment for 4 seconds at the heat fixing peak temperature shown in Table 1, the film was cooled to room temperature, subjected to corona discharge treatment on one side of the film, and wound to a thickness of 12 μm. A biaxially stretched film was obtained.
Table 1 shows the results of evaluating the thermal shrinkage rate, tensile elastic modulus, tensile breaking strength, tensile breaking elongation, bending resistance, impact strength, laminate processability, heat sealability, and acid resistance using the obtained film. Show.

Examples 2-5, Comparative Examples 2-4, 6-9
Using the same method as in Example 1, after obtaining an unstretched film under the polyester composition and extrusion temperature conditions described in Table 1, in the longitudinal direction and then in the lateral direction under the conditions of temperature and stretch ratio shown in Table 1. Sequential biaxial stretching was performed. Next, after heat treatment for 4 seconds under the heat setting conditions shown in Table 1, the film was cooled to room temperature, subjected to corona discharge treatment on one side of the film, and wound to obtain a biaxially stretched film having the thickness described in Table 1.
Table 1 shows the results of evaluating the thermal shrinkage rate, tensile elastic modulus, tensile breaking strength, tensile breaking elongation, bending resistance, impact strength, laminate processability, heat sealability, and acid resistance using the obtained film. Show.

  The polyester film for the inner bag of the cans of Examples 1 to 6 has sufficient mechanical strength and is excellent in heat resistance. Therefore, even in the secondary processing such as laminating property and heat sealing property, the appearance defect is not good. It did not occur, and was excellent in acid resistance and long-term storage stability.

Since the films of Comparative Examples 1 and 3 have large TD and MD heat shrinkage, appearance defects such as wrinkles occur at the time of laminating and heat sealing, and liquid leakage of contents occurs from the defective sealing portion. It was inferior in long-term storage.
Since the films of Comparative Examples 2 and 4 had a large thermal shrinkage ratio of TD, appearance defects such as wrinkles occurred at the time of heat sealing, and the heat sealing properties were inferior to the films of the examples.

Since the film of Comparative Example 5 was formed only of polyester (I) mainly composed of polybutylene terephthalate (PBT), some mechanical properties were inferior to those of the Example film, and TD Since the thermal contraction rate of MD is large, appearance defects such as wrinkles occur at the time of laminating or heat sealing, and liquid leakage of the contents from the defective sealing portion causes poor long-term storage stability.
Since the film of Comparative Example 6 was formed only from polyester (II) mainly composed of polyethylene terephthalate (PET), the tensile elastic modulus was too high compared to the film of Example. Thus, not only the bending resistance was remarkably inferior, but also the impact strength was low.
The film of Comparative Example 7 had a lower impact strength than the film of Example because the mass ratio of polyester (I) mainly composed of polybutylene terephthalate (PBT) was less than the range of the present invention. .

The polyester film for inner bags of Comparative Example 8 was inferior in impact strength because of the thin film thickness, and wrinkled during lamination or heat sealing, and was inferior in quality as compared with the Examples.
The film of Comparative Example 9 was inferior in bending resistance as compared with the film of Example, because the film thickness was large.

Claims (5)

  A film containing polyester (I) mainly composed of polybutylene terephthalate and polyester (II) mainly composed of polyethylene terephthalate, and a mass ratio (I) / (II) of polyester (I) to polyester (II) ) Is 80 to 40/20 to 60, the tensile elastic modulus is 2.3 to 3.5 GPa in both the longitudinal direction and the width direction of the film, and the thermal contraction rate in the longitudinal direction of the film at 150 ° C. for 15 minutes. A polyester film for an inner bag of a can, characterized by being 10.0% or less and having a heat shrinkage in the width direction of the film at 150 ° C. for 15 minutes of 2.0% or less.   The polyester film for an inner bag of a can according to claim 1, wherein the impact strength is 0.4 J or more in terms of a film thickness of 12 µm.   The polyester film for an inner bag of a can according to claim 1 or 2, wherein the polyester film has a thickness of 10 to 25 µm.   A composite film for an inner bag of a can, wherein a polyolefin film is laminated on the polyester film for an inner bag of a can according to claim 1, 2 or 3. 5. A composite container, wherein a bag formed of the composite film for an inner bag of a can according to claim 4 is mounted on the inner surface of the can.