HK40000825A - Polyester film for cold forming and method for producing same - Google Patents

Description

Polyester film for cold forming and method for producing same

The present invention is a divisional application of an application having an application date of 2013, 07/23/2013, an application number of 201380034421.X, and an invention name of "polyester film for cold forming and a method for producing the same".

Technical Field

The present invention relates to a polyester film for cold forming which can be subjected to cold forming such as stretch forming ( out し forming) and draw forming (deep り forming), and a method for producing the same.

Background

Conventionally, a metal can has been used as an exterior package of a lithium ion secondary battery. However, in order to cope with weight reduction and diversification of shapes of lithium ion secondary batteries, a bag body formed by a laminate in which a plastic film is laminated on a metal foil such as an aluminum foil is used as an outer package. Such a bag body is required to have indispensable physical properties such as moisture resistance, sealability, puncture resistance, insulation properties, heat/cold resistance, moldability, etc., and therefore the laminate is formed of a polyester film/a polyamide film/a metal foil/a sealing film in this order from the outer layer.

In order to efficiently install a lithium ion secondary battery together with a printed circuit board and other members in an electronic device that is small and thin, it is necessary to form the corner portions of the outer package of the battery into sharp shapes. Accordingly, in cold forming such as stretch forming and draw forming, development of a polyamide film having excellent formability is receiving attention, and patent documents 1 and 2 propose a polyamide film having mechanical properties with little directivity, an impact strength of 30000J/m or more, and a tensile strength up to fracture in 4 directions (0 °, 45 °, 90 ° and 135 °) of 150N/mm²The elongation in 4 directions is 80% or more.

Prior art documents

Patent document

Patent document 1: japanese patent laid-open No. 2000-123800

Patent document 2: japanese laid-open patent publication No. 2005-022336

Disclosure of Invention

However, the physical properties of such a polyamide film are achieved by stretching a polyamide resin substantially by an inflation method (インフレーション method). Therefore, a polyamide film stretched by a tenter method (テンター method) for mass production cannot be used for this purpose, and a technique of cold forming a laminate in which polyamide films are laminated cannot be widely used.

In addition, the polyamide film tends to be degraded by decomposition reaction of amide groups in the presence of an acid or an electrolytic solution. Therefore, when the surface layer of the exterior package of the lithium ion secondary battery is a polyamide film, if an electrolyte solution or an acidic substance overflows in the liquid injection step and adheres to the polyamide film, the film may be broken, and therefore, a polyester film having excellent acid resistance and electrolyte solution resistance needs to be laminated as the outermost layer of the exterior package of the lithium ion secondary battery.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a polyester film which is excellent in formability in cold forming such as stretch forming and drawing and can be formed into a sharp shape. Then, by using the film, a laminate having a structure of a polyester film, a metal foil, and a sealing film can be used as an exterior package of a lithium ion secondary battery.

As a result of intensive studies to solve the above problems, the inventors of the present invention have found that a polyester film containing polybutylene terephthalate and polyethylene terephthalate at a specific ratio is excellent in formability in cold forming such as stretch forming and drawing forming and can be formed into a sharp shape, and have completed the present invention.

Namely, the gist of the present invention is as follows.

(1) A polyester film for cold forming, which is characterized by comprising polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), wherein the mass ratio of the PBT to the PET (PBT/PET) is 30/70-80/20.

(2) The polyester film for cold forming according to the item (1), wherein the thermal shrinkage percentage in the longitudinal direction (MD) and the thermal shrinkage percentage in the width direction (TD) are 5 to 20%, respectively, after the heat treatment at 160 ℃ for 15 minutes.

(3) The polyester film for cold forming according to the item (1), wherein the polyethylene terephthalate (PET) contains 0 to 15 mol% of isophthalic acid as an acid component.

(4) The polyester film for cold forming according to the item (1), wherein a transesterification index between polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) is 1 to 10%.

(5) A method for producing a polyester film for cold forming according to any one of the above (1) to (4), wherein the polyester film is stretched in the longitudinal direction (MD) and the width direction (TD) so that the surface magnification (MD stretch magnification. times. TD stretch magnification) is 6 to 20 times and the ratio of the stretch magnifications (MD stretch magnification/TD stretch magnification) is 0.4 to 1.

(6) The method for producing a polyester film for cold forming according to (5), wherein the polyester film is stretched at an MD stretch ratio of 2 to 4 times and at a TD stretch ratio of 3 to 5 times.

(7) A laminate film, which is obtained by laminating a metal foil and a sealing film in this order on the polyester film for cold forming described in any one of the above (1) to (4).

The present invention can provide a polyester film which is excellent in moldability in cold forming such as stretch forming and drawing and can be molded into a sharp shape. The polyester film of the present invention has good acid resistance to hydrofluoric acid and good resistance to an electrolytic solution, and therefore, by using the polyester film, a laminated film having a structure of polyester film/metal foil/sealing film can be used as an exterior package of a lithium ion secondary battery.

Drawings

FIG. 1 is an NMR chart of a membrane of the present invention in which peaks (Sab, Sba, Saa, and Sbb) due to transesterification are partially enlarged.

Detailed Description

The present invention will be described in detail below.

The polyester film of the present invention contains polybutylene terephthalate (PBT) and polyethylene terephthalate (PET).

The polybutylene terephthalate (PBT) of the present invention is obtained by copolymerizing terephthalic acid and 1, 4-butanediol as polymerization components with other components.

The copolymerization component is not particularly limited, and examples of the acid component include dicarboxylic acids such as isophthalic acid, phthalic acid, 2, 6-naphthalenedicarboxylic acid, 5-sodium sulfoisophthalate, 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, and cyclohexanedicarboxylic acid; 4-hydroxybenzoic acid, epsilon-caprolactone, lactic acid and the like.

Examples of the alcohol component include ethylene glycol, diethylene glycol, 1, 3-propanediol, neopentyl glycol, 1, 6-hexanediol, cyclohexanedimethanol, triethylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and ethylene oxide adducts of bisphenol a and bisphenol S.

In addition, a small amount of a trifunctional compound such as trimellitic acid, trimesic acid, pyromellitic acid, trimethylolpropane, glycerol, pentaerythritol, or the like can be used.

These copolymerization components may be used in combination of 2 or more.

In the present invention, when polybutylene terephthalate (PBT) is used as the copolymer, the kind and ratio of the component to be copolymerized may be appropriately selected, but 1, 4-butanediol is preferably 80 mol% or more, more preferably 90 mol% or more, based on the entire alcohol component. When the 1, 4-butanediol content is less than 80 mol%, the melting point may be less than the range described later, and as a result, the crystallinity is lowered, and the heat resistance of the film is lowered.

In the polyester film of the present invention, the melting point of polybutylene terephthalate (PBT) is preferably 200 to 223 ℃, more preferably 210 to 223 ℃. When the melting point is less than 200 ℃, the crystallinity of polybutylene terephthalate (PBT) is low, and as a result, the heat resistance of the film is lowered.

The polyethylene terephthalate (PET) in the present invention may be obtained by copolymerizing terephthalic acid and ethylene glycol as polymerization components with other components.

The copolymerization component is not particularly limited, and examples thereof include those exemplified above for polybutylene terephthalate (PBT).

The acid component to be copolymerized is preferably isophthalic acid from the viewpoint of easy adjustment of the melting point described below. The content of isophthalic acid in the polyethylene terephthalate (PET) is preferably 0 to 15 mol%, more preferably 0 to 12 mol%, based on the total acid components.

In the polyester film of the present invention, the melting point of polyethylene terephthalate (PET) is preferably 230 to 256 ℃, more preferably 236 to 256 ℃. When the melting point is less than 230 ℃, the crystallinity of polyethylene terephthalate (PET) is low, and as a result, the heat resistance of the film is lowered.

In the polyester film of the present invention, the mass ratio (PBT/PET) of polybutylene terephthalate (PBT) to polyethylene terephthalate (PET) must be 30/70 to 80/20, and in order to sufficiently obtain moldability in cold molding, it is preferably 40/60 to 70/30.

When the ratio of polybutylene terephthalate (PBT) in the polyester film exceeds 80 mass%, the characteristics of highly crystalline polybutylene terephthalate (PBT) are remarkably exhibited, and a laminated film obtained by laminating a metal foil and a sealing film on the polyester film has reduced moldability and impact resistance, and also has reduced adhesiveness to the metal foil. On the other hand, if the ratio of polybutylene terephthalate (PBT) is less than 30 mass%, the tensile stress increases, and the moldability of the laminated film deteriorates.

In particular, when the ratio of polybutylene terephthalate (PBT) in the polyester film is 40 to 70 mass%, the obtained laminate film has good conformability to cold forming, does not cause whitening due to voids caused by deformation of the film, does not cause microcracks, has excellent adhesion to the metal foil, and can be deformed largely without breaking the metal foil. As a result, the laminated film can be molded into a sharp shape, and a lithium ion secondary battery having a high degree of freedom in shape can be obtained.

The intrinsic viscosity of the polybutylene terephthalate (PBT) used as a raw material for producing the polyester film of the present invention is preferably 0.75 to 1.6dl/g, and the intrinsic viscosity of the polyethylene terephthalate (PET) as a raw material is preferably 0.65 to 1.0 dl/g.

The limiting viscosity after melt-mixing is preferably 0.75 to 1.2 dl/g. When the limiting viscosity after melt-mixing is less than 0.75dl/g, the resulting laminated film may be broken at the time of cold molding, and productivity may be significantly reduced. On the other hand, if the limiting viscosity after melt-mixing exceeds 1.2dl/g, the load of the melt extruder increases in the film production process, and the production rate must be sacrificed, and the melt residence time of the resin in the extruder becomes too long, and the reaction between the polyester resins excessively proceeds, thereby deteriorating the properties of the film, and as a result, the physical properties of the laminated film are lowered. In addition, since the production of a raw material having a high limiting viscosity requires a relatively long polymerization time and a relatively long polymerization step, it is an important factor to increase the cost.

The polymerization method of polybutylene terephthalate (PBT) or polyethylene terephthalate (PET) as a raw material is not particularly limited, and for example, polymerization can be carried out by a transesterification method, a direct polymerization method, or the like. Examples of the transesterification catalyst include oxides and acetates of Mg, Mn, Zn, Ca, Li, and Ti. Further, the polycondensation catalyst includes oxides, acetates and the like of Sb, Ti and Ge.

The polyester after polymerization contains monomers, oligomers, by-products such as acetaldehyde and tetrahydrofuran, and is preferably solid-phase polymerized at a temperature of 200 ℃ or higher under reduced pressure or by inert gas flow.

In the polymerization of polybutylene terephthalate (PBT) or polyethylene terephthalate (PET), additives such as an antioxidant, a heat stabilizer, an ultraviolet absorber, an antistatic agent, and the like may be added as necessary. 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. In addition, as the reaction inhibitor between polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), it is preferable to add a conventionally known phosphorus compound before, during, or after polymerization.

The polyester film of the present invention preferably has a heat shrinkage ratio in the film longitudinal direction (MD) and the film Transverse Direction (TD), that is, MD heat shrinkage ratio and TD heat shrinkage ratio, of 5 to 20%, more preferably 5 to 15%, respectively, after treatment at 160 ℃ for 15 minutes.

When the thermal shrinkage ratio of the polyester film is less than 5%, the obtained laminated film may have poor adhesion to the metal foil. On the other hand, when the heat shrinkage rate exceeds 20%, shrinkage wrinkles may occur in the film during lamination, or the obtained laminated film may curl.

The thermal shrinkage of the polyester film can be adjusted by changing the thermal setting temperature in the stretching step. By setting the heat setting temperature to be high, the crystallinity of the polyester film increases, and the heat shrinkage rate can be reduced. Conversely, by setting the heat setting temperature to be low, the crystallinity of the polyester film is reduced, and the heat shrinkage rate can be increased.

The polyester film of the present invention preferably has a transesterification index of polybutylene terephthalate (PBT) to polyethylene terephthalate (PET) of 1 to 10%, more preferably 3 to 7%. The laminated film using the polyester film having the ester interchange index within the above range has good deformation following properties in cold forming, does not adhere to a processing jig for cold forming, and has less friction, so that the uniformity of the surface of the obtained molded article is improved, and the breakage of the metal foil during cold forming is also reduced.

The method for adjusting the transesterification index within the above range is not particularly limited, but may be a method of adjusting the melting temperature of polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) in an extruder, adjusting the kneading degree in an extruder, adjusting the residence time in an extruder, or the like. The melt-mixing method is not particularly limited, and a method of mixing and melting the mixed raw material pieces in the same extruder, a method of melting and mixing the raw material pieces in separate extruders, and the like may be mentioned.

The transesterification index is also greatly influenced by the type and amount of the polymerization catalyst for the polyester and the residual activity thereof. Therefore, techniques such as selection of a catalyst, rationalization of the amount, and addition of a catalyst activity inhibitor such as a phosphorus compound can be used in combination.

The polyester film of the present invention can be produced by the following method.

Specifically, polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) are mixed so that the mass ratio (PBT/PET) is 30/70 to 80/20, and the mixture is melted and mixed in an extruder at a temperature of 250 to 280 ℃ for a residence time of 3 to 15 minutes, and then extruded through a T-die into a sheet. The sheet was cooled by being closely adhered to a cooling drum adjusted to a temperature of room temperature or lower, thereby obtaining an unstretched film.

The obtained unstretched film is introduced into 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 ℃.

In the biaxial stretching, the area magnification (MD stretch magnification. times. TD stretch magnification) is preferably 6 to 20 times, and more preferably 8.75 to 15.75. If the surface magnification exceeds 20 times, in-plane orientation of the film is promoted and crystallization becomes high, so that the obtained laminated film cannot be formed into a molded article by cold molding, and cracks and interlayer peeling are generated. On the other hand, if the surface magnification is less than 6 times, the film strength becomes insufficient, and therefore the obtained laminated film is cracked during cold forming.

The ratio of stretch ratios (MD stretch ratio/TD stretch ratio) is preferably 0.4 to 1, and more preferably 0.55 to 1. If the stretch ratio is outside this range, the film tends to have strong and weak orientation in the MD and TD within the plane, and the orientation balance is deteriorated, so that the resulting laminated film tends to have a problem that cracks and wrinkles are more likely to occur in the direction of weak orientation during cold forming.

In order to obtain the above-mentioned surface magnification ratio and stretch magnification ratio, the MD stretch magnification is preferably 2 to 4 times, more preferably 2.5 to 3.5 times, and the TD stretch magnification is preferably 3 to 5 times, more preferably 3.5 to 4.5 times, as the stretch magnification in each direction.

After the simultaneous biaxial stretching, the TD is preferably heat-set so that the relaxation rate (relaxation rate) is 3.0 to 7.0%. The heat setting temperature is preferably 140 to 185 ℃, and more preferably 155 to 180 ℃.

The unstretched film may be subjected to preliminary longitudinal stretching by a factor of about 1.2 or less before being introduced into the simultaneous biaxial stretching machine.

The polyester film of the present invention can be produced by a sequential biaxial stretching method.

In the sequential biaxial stretching method, the unstretched film obtained in the same manner as described above is heated with a roll, infrared rays, or the like, and stretched in the longitudinal direction (MD) at 50 to 150 ℃ by a difference of 2 or more roll peripheral speeds to obtain a longitudinally stretched film. In the longitudinal stretching, the MD stretching magnification is preferably 2 to 4 times, and more preferably 2.5 to 3.5 times.

The obtained longitudinally stretched film is then continuously stretched in the width direction (TD) to be a biaxially oriented film. The Transverse Direction (TD) stretching is started at 50 to 150 ℃, 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, the area magnification (MD stretch magnification × TD stretch magnification) is preferably 6 to 20 times, more preferably 8.75 to 15.75 times, and the stretch magnification ratio (MD stretch magnification/TD stretch magnification) is preferably 0.4 to 1, more preferably 0.55 to 1, and particularly preferably 0.75 to 0.85, for the same reason as in the case of simultaneous biaxial stretching.

After the successive biaxial stretching, the heat-setting is preferably performed so that the relaxation rate of TD is 3.0 to 7.0%. The heat setting temperature is preferably 140 to 185 ℃, and more preferably 155 to 180 ℃.

The heat-setting treatment after the simultaneous biaxial stretching and the sequential biaxial stretching is an important step for imparting dimensional stability to the film. As this method, a known method such as a method of blowing hot air, a method of irradiating infrared rays, and a method of irradiating microwaves can be used. Among them, a method of blowing hot air is the most preferable method in terms of uniform and accurate heating.

In order to improve the process-passing property during the production of the polyester film of the present invention and the cold-forming of the obtained laminate film, it is preferable to add a small amount of an inorganic lubricant such as silica, alumina, kaolin, or the like to the raw polyester to form a film so as to impart a slip property (スリップ property) to the surface of the polyester film. The content of the inorganic lubricant in the polyester film is preferably 0.001 to 0.5 mass%, more preferably 0.05 to 0.3 mass%.

In addition, the polyester film may contain, for example, a silicone compound for the purpose of improving appearance and printability.

The polyester film of the present invention preferably contains 2000 to 6000ppm of an incompatible low molecular weight polyethylene in the polyester constituting the film. When the content of the low-molecular-weight polyethylene is less than 2000ppm, the effect of improving the slip property (slip り property) is insufficient, while when the content exceeds 6000ppm, the quality of the film surface is excessive in slip property, and the polyester film is brittle due to excessive incompatible resins.

In the present invention, the number average molecular weight of the low molecular weight polyethylene is preferably 1000 to 8000, more preferably 2000 to 6000. When the low-molecular-weight polyethylene having a molecular weight within the above range is contained in the incompatible polyester, the film surface can be roughened to improve the smoothness, and the obtained laminated film can also maintain the roughened film surface. When the number average molecular weight of the low molecular weight polyethylene is less than 1000, the molecular weight is too low, and the low molecular weight polyethylene precipitates on the film surface and peels off at the time of film processing or film lamination, and sometimes contaminates a jig in a cold forming step, and conversely damages the film itself. On the other hand, if the number average molecular weight of the low molecular weight polyethylene exceeds 8000, the effect of roughening the surface of the obtained laminated film is insufficient, and the slipperiness during cold molding is poor.

The method of adding the low-molecular weight polyethylene to polybutylene terephthalate (PBT) or polyethylene terephthalate (PET) is not particularly limited, but a method of preparing a master piece containing about 0.5 to 5 mass% of the low-molecular weight polyethylene in advance and adding the master piece to polybutylene terephthalate (PBT), polyethylene terephthalate (PET) or a mixture thereof is preferable.

The laminate film of the present invention is a film obtained by laminating a metal foil and a sealing film in this order on the polyester film of the present invention.

Examples of the metal foil include aluminum foil, copper foil, tin foil, gold foil, silver foil, zinc foil, brass foil, nickel foil, stainless steel foil, iron foil, and titanium foil.

The metal foil may be subjected to chemical formation processes such as chromic acid treatment, phosphoric acid treatment, electrolytic chromic acid treatment, and chromate (クロメート) treatment, or may be plated with nickel, tin, zinc, aluminum, copper-tin alloy, brass, or other various plating processes.

The thickness of the metal foil is preferably 9 to 60 μm, and more preferably 20 to 50 μm. If the thickness of the metal foil is less than 9 μm, the probability of pinholes in the metal foil is high, and if a laminated film having the metal foil is used as an exterior package of a lithium ion secondary battery, the defective fraction increases. On the other hand, if the thickness exceeds 60 μm, the stress during cold forming becomes high, and the metal foil and the polyester film are likely to be cracked in the laminated film.

As a method of laminating the polyester film on the metal foil, a method of laminating through an adhesive layer is exemplified.

The resin used as the adhesive is not particularly limited, and examples thereof include resins such as polyester-based resins, epoxy-based resins, polyurethane-based resins, and polyester-epoxy copolymer resins, which are used as the main component, melamine resins, isocyanate resins, and epoxy resins,1 or 2 or more of oxazoline resin, phenol resin and the like as a curing agent. The adhesive layer provided by such an adhesive is suitable for cold forming.

The adhesive layer may be provided by a coextrusion method, a lamination process, or a coating process.

The thickness of the adhesive layer is preferably 0.5 to 5.0 μm. If the thickness of the adhesive layer is less than 0.5 μm, the adhesive strength is insufficient, and if the thickness exceeds 5.0 μm, the adhesiveness, workability, and cohesion after processing of the adhesive are reduced, and in either case, the film may peel off during the molding of the obtained laminated film.

The laminated film of the present invention is a film obtained by further laminating a sealing film on a metal foil which has been laminated on a polyester film. By having the sealing film, the laminated film can be processed into a bag body in a sealable manner.

The sealing film is preferably an unstretched film made of polyethylene, polypropylene, maleic acid-modified polyethylene, ethylene-acrylic acid ester copolymer, ionomer resin, polyvinyl chloride, or the like, from the viewpoint of excellent sealing properties and chemical resistance.

As a method for laminating the sealing film on the metal foil, the same method as the above-described method for laminating the polyester film on the metal foil can be cited.

Examples

The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples.

The following resins were used for the production of the polyester film.

PBT-1: PBT subjected to solid-phase polymerization had an ultimate viscosity of 1.08dl/g, a Tm of 223 ℃ and a Ti catalyst content of 40 ppm.

PET-1: the PET subjected to solid-phase polymerization had an intrinsic viscosity of 0.75dl/g, a Tm of 255 ℃ and a Ge catalyst content of 40 ppm.

PET-2: isophthalic acid subjected to solid-phase polymerization was copolymerized with PET at 6 mol%, having an intrinsic viscosity of 0.75dl/g, a Tm of 233 ℃ and a Ge content of 40 ppm.

PET-3: isophthalic acid subjected to solid-phase polymerization was copolymerized with PET at 15 mol%, having an intrinsic viscosity of 0.75dl/g, a Tm of 215 ℃ and containing 40ppm of a Ge catalyst.

The properties of the raw material resin, polyester film and laminated film were measured or evaluated by the following methods.

A. Limiting viscosity of resin

0.25g of the resin was dissolved in 50ml of phenol/tetrachloroethane (mass ratio) 5/5, and the solution was measured at 25 ℃ using an Ubbelohde viscometer.

B. Melting Point (Tm) of resin and polyester film

The melting point at 20 ℃ per minute was measured by DSC using Perkin Elmer.

The polyester film is a sample that is melted and then rapidly cooled at a rate of 100 ℃/min or more to be in an amorphous state. In each of the examples and comparative examples, 2 melting points were observed, but the lower one (Tm1) was derived from polybutylene terephthalate (PBT) and the higher one (Tm2) was derived from polyethylene terephthalate (PET).

C. Thermal shrinkage of polyester film

As a test piece for measuring the heat shrinkage rate in the film longitudinal direction (MD), 5 test pieces were prepared in which a polyester film was cut into TD10mm × MD150mm and 2 markings were marked thereon at an interval of 100 mm.

Similarly, 5 test pieces were prepared by cutting a polyester film into MD10mm × TD150mm and marking the cut pieces with 2 marks at intervals of 100mm, as test pieces for measuring the heat shrinkage ratio in the film width direction (TD).

The test piece thus obtained was heat-treated in an oven at 160 ℃ for 15 minutes under no load, and then the test piece was taken out and returned to room temperature, and the inter-reticle distance was measured. The heat shrinkage was determined by the following formula, and the average of 5 sheets was taken as the MD heat shrinkage and the TD heat shrinkage, respectively.

Heat shrinkage (%) - (A-B)/A.times.100

A: distance between standard lines before heat treatment (mm), B: distance between standard lines (mm) after heat treatment

D. Ester interchange index (Ex)

The magnetic field was measured by a GEMINI2000/300 nuclear magnetic resonance apparatus (magnetic field strength: 7.05T) manufactured by Varian corporation¹³Measurement of CNMR. The sample is measured by dissolving 60-100 mg of the film in CF₃The transesterification index (Ex) of a 0.7ml COOD solvent solution was determined from the integrated value of the peak (FIG. 1) derived from transesterification according to the following formula.

Ex(％)＝(Sab+Sba)/(Saa+Sbb+Sab+Sba)×100

E. Resistance of polyester film to electrolyte and resistance to hydrofluoric acid

1 drop of an electrolyte solution (an electrolyte solution obtained by dissolving lithium tetrafluoroborate in an ethyl carbonate/diethyl carbonate mixed solvent (mass ratio 1/1)) and hydrofluoric acid (47%) was dropped on the surface of the polyester film, and the surface state of the film was observed, and the resistance to the electrolyte solution and the acid resistance to hydrofluoric acid were evaluated as ○ and x when no hole was formed in the film until 10 minutes after dropping.

F. Formability of laminated film

Moldability was evaluated by cold molding the laminated film from the side of the sealing film (unstretched polypropylene film) using a molding die of 115 mm. times.115 mm. times.6 mm in depth using a 1-ton bench press (SBN-1000 manufactured by Ioka manufacturing company).

In the laminated film after molding, the film was evaluated as ◎ for good appearance without cracks and delamination, without wrinkles at the molding site and the periphery thereof, whitening of the film, etc., ○ for no cracks and delamination, △ for cracks not penetrating the aluminum foil but delamination, and x for cracks penetrating the aluminum foil and delamination.

Example 1

To 60 parts by mass of polybutylene terephthalate (PBT-1) and 40 parts by mass of polyethylene terephthalate (PET-1), coagulated silica having an average particle diameter of 2.5 μm was added so that the content became 0.08% by mass, and the mixture was melted at 275 ℃ and extruded from the outlet of a T die for a residence time of 5 minutes, and rapidly cooled and solidified to obtain an unstretched film.

Next, the unstretched film was stretched using a tenter-type sequential stretcher. First, an unstretched film was subjected to roll heating by a longitudinal stretcher to stretch 3.39 times in the MD, followed by starting transverse stretching at 80 ℃ and stretching 3.84 times in the TD. In this stretching, the area magnification (MD stretching magnification × TD stretching magnification) was 13, and the stretching magnification ratio (MD stretching magnification/TD stretching magnification) was 0.88.

The polyester film for cold forming was obtained by subjecting the polyester film to a heat treatment at a heat setting temperature of 167 ℃ and a TD relaxation rate of 5% for 4 seconds, cooling the film to room temperature, and winding the film to a thickness of 25 μm.

On the polyester film for cold forming obtained, an aluminum foil (AA 8079, thickness 40 μm, manufactured by Lightal foil Co., Ltd.) as a metal foil and an unstretched polypropylene film (Mitsui chemical s Tohcello Co., Ltd.) as a sealing film were laminated in this order by dry laminationGHC, thickness 40 μm) was prepared, and aging treatment was performed at 60 ℃ for 72 hours to prepare a laminated film. The adhesive was prepared in an amount of 4.0g/m using TM-K55/CAT-10L (compounding ratio 100/10) manufactured by Toyo-Morton Co., Ltd²。

Examples 2 to 23 and comparative examples 1 to 5

A polyester film for cold forming and a laminated film were obtained in the same manner as in example 1, except that the blending ratio of polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), the type of polyethylene terephthalate (PET) (the ratio of isophthalic acid copolymerized), the MD stretching ratio, the TD stretching ratio, and the heat setting temperature were changed as shown in table 1.

The properties of the polyester films and laminated films obtained in examples 1 to 23 and comparative examples 1 to 5 are shown in table 1.

[ Table 1]

Example 24

An unstretched film was obtained in the same manner as in example 1.

Then, the end portions of the unstretched film were nipped by a nip of a tenter simultaneous biaxial stretcher, passed through a preheating zone of 60 ℃, and then simultaneously biaxially stretched at a temperature of 80 ℃ at a MD of 3.0 times and a TD of 3.3 times. In this stretching, the area magnification (MD stretching magnification × TD stretching magnification) was 9.9, and the stretching magnification ratio (MD stretching magnification/TD stretching magnification) was 0.91.

Thereafter, the relaxation rate of TD was set to 5%, and the polyester film was heat-treated at a heat fixation temperature of 165 ℃ for 4 seconds, cooled to room temperature, and then wound up to obtain a polyester film for cold forming having a thickness of 25 μm.

The obtained polyester film for cold forming was dry-laminated with a metal foil and a sealing film in the same manner as in example 1 to prepare a laminated film.

Examples 25 to 29 and comparative examples 6 to 7

A polyester film for cold forming and a laminated film were obtained in the same manner as in example 24, except that the blending ratio of polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), the kind of polyethylene terephthalate (PET), and the heat setting temperature were changed as shown in table 2.

Example 30

A polyester film for cold forming and a laminated film were obtained in the same manner as in example 24, except that an unstretched film which was melted at 285 ℃ and had a residence time of 15 minutes was used.

Comparative example 8

A laminated film was produced in the same manner as in example 1, except that a polyamide film (product of japan, BonylRX, thickness 25 μm) stretched in an inflation manner was used instead of the polyester film.

The properties of the polyester films and laminated films obtained in examples 24 to 30 and comparative examples 6 to 8 are shown in Table 2.

[ Table 2]

In examples 1 to 30, the polyester films obtained were excellent in acid resistance and electrolyte resistance, and the laminated films obtained were excellent in moldability. Among them, in examples 2, 3, 5, and 7, the polyester film has a particularly preferable stretch ratio range, so that the orientation balance is particularly excellent, and in examples 4 and 6, the polyethylene terephthalate contains isophthalic acid as an acid component, so that crystallization can be suppressed to improve moldability, and a laminated film having excellent moldability and good appearance after molding can be obtained.

In examples 10 and 13, the surface magnification ratio of the polyester film was outside the preferred range, in examples 14 and 17, the stretching magnification ratio of the polyester film was outside the preferred range, and therefore, the alignment balance was poor, and stress concentration was generated, and in examples 18 to 19, the thermosetting temperature was set to be slightly low in order to suppress crystallization of the film and improve moldability, and as a result, the residual stress in the film was increased, and in examples 20 to 21, the thermosetting temperature was set to be slightly high in order to reduce the residual stress in the film, and as a result, crystallization of the film was performed, and cracks that did not penetrate through the laminated film after molding but peeling between layers was not generated, and these were not problematic in practice.

In comparative examples 1 and 6, the content of polybutylene terephthalate (PBT) in the polyester film was high, the characteristics of polybutylene terephthalate (PBT) having high crystallinity were remarkably exhibited, and the obtained laminated film was not excellent in moldability. In comparative example 2, the heat setting temperature was high, the polyester film crystallized, the stress of the film during molding increased, the polybutylene terephthalate (PBT) content was high, the characteristics of polybutylene terephthalate (PBT) having high crystallinity were remarkably exhibited, and the obtained laminated film was broken in aluminum foil, and the moldability was not excellent.

In comparative examples 3 to 5 and 7, the content of polybutylene terephthalate (PBT) in the polyester film was small, and the film stress during molding was high, and in comparative examples 4 to 5, the heat fixing temperature was high, and the polyester film was crystallized, and the obtained laminated films all had aluminum foil breakage, and the moldability was not excellent.

In comparative example 8, although a laminated film having excellent moldability was obtained, the polyamide film constituting the laminated film did not have acid resistance and electrolyte resistance.

Claims (7)

1. A polyester film for cold forming, which is characterized by comprising PBT (polybutylene terephthalate) and PET (polyethylene terephthalate), wherein the PBT/PET (polybutylene terephthalate/polyethylene terephthalate) ratio by mass is 30/70-80/20.

2. The polyester film according to claim 1, wherein the heat shrinkage rate in the MD, which is the longitudinal direction, and the heat shrinkage rate in the TD, which is the width direction, are 5 to 20%, respectively, after the heat treatment at 160 ℃ for 15 minutes.

3. The polyester film for cold forming according to claim 1, wherein the PET which is polyethylene terephthalate contains 0 to 15 mol% of isophthalic acid as an acid component.

4. The polyester film for cold forming according to claim 1, wherein a transesterification index between PBT that is polybutylene terephthalate and PET that is polyethylene terephthalate is 1 to 10%.

5. A method for producing a polyester film for cold forming according to any one of claims 1 to 4, wherein the polyester film is stretched in the MD direction, which is the longitudinal direction, and in the TD direction, which is the width direction, so that the surface magnification is 6 to 20 times and the stretch magnification ratio is 0.4 to 1, wherein the surface magnification is the MD stretch magnification × TD stretch magnification and the stretch magnification ratio is the MD stretch magnification/TD stretch magnification.

6. A method for producing a polyester film for cold forming according to claim 5, wherein the polyester film is stretched at an MD stretch ratio of 2 to 4 times and at a TD stretch ratio of 3 to 5 times.

7. A laminated film obtained by laminating the polyester film for cold forming according to any one of claims 1 to 4 with a metal foil and a sealing film in this order.