JP2020063354A - Polyester film for lithium ion battery member - Google Patents

Abstract

To provide a polyester film for a lithium ion battery member excellent in flex resistance, and crack resistance.SOLUTION: The polyester film for a lithium ion battery member has a difference Δn between a refractive index in a longitudinal direction and a refractive index in a width direction orthogonal thereof of 0.05 or more and 0.3 or less, a flexure hysteresis 2HB in at least either one direction of 0.01 gf cm/cm or more and 0.03 gf cm/cm or less, and a breaking strength in at least either one direction of 250 MPa or more and 500 MPa or less.SELECTED DRAWING: None

Description

  The present invention relates to a polyester film for a lithium-ion battery member having excellent flex resistance and crack resistance, which relates to a polyester film for a lithium-ion battery member to be used in bendable or curved wearable applications that require flexibility. is there.

  In recent years, with regard to lithium-ion batteries, a laminate type flexible lithium-ion battery with flexibility that can be bent and twisted has been put into practical use, and the exterior material is required to have flexibility such as bending resistance that can be repeatedly bent. Has been.

  Conventionally, a laminate type lithium-ion battery exterior material is composed of a film such as PET, nylon, aluminum or CPP, and in order to provide deep drawability, the outermost PET has a method of increasing strength. (For example, refer to Patent Document 1).

International publication number WO2012 / 086501

However, the film used for the laminate type lithium-ion battery exterior material described in Patent Document 1 takes into consideration the flex resistance due to the increased strength, but does not consider the crack resistance, Since the tension of the molecular chain has advanced, it has been difficult to apply it for wearable applications and flexibility when it is applied with external force such as bending when it is applied. Further, when the orientation of the film is lowered in order to improve the crack resistance of the film, the crack resistance is improved but the flex resistance is lowered.
Then, the subject of this invention is providing the polyester film for lithium ion battery members which can make crack resistance and bending resistance compatible.

The polyester film for a lithium-ion battery member of the present invention employs the following means in order to solve such problems.
(1) The difference Δn in the refractive index between the longitudinal direction and the width direction which is the direction orthogonal to the longitudinal direction is 0.05 or more and 0.3 or less, and the bending hysteresis 2HB in at least one direction is 0.01 gf · cm / cm or more. 0.03 gf · cm / cm or less, and breaking strength in at least one direction of 250 MPa or more and 500 MPa or less.
(2) at least one direction of bending stiffness is less than 0.06gf · ^cm 2 · ^cm or more 0.1gf · ^cm 2 · ^cm.
(3) The heat shrinkage ratio at 100 ° C. in the longitudinal direction and the width direction orthogonal to the longitudinal direction is 15% or less.
(4) The film thickness is 5 μm or more and 40 μm or less.
(5) A lithium ion battery using the polyester film for a lithium ion battery member according to (1) to (4).

  According to the present invention, a film having excellent bending resistance and crack resistance can be provided, and it can be suitably used as a lithium-ion battery member that particularly needs flexibility and crack resistance.

Hereinafter, the polyester film for a lithium ion battery member according to the present invention will be described in detail together with embodiments.
In the polyester film for a lithium ion battery member of the present invention, the bending hysteresis 2HB in at least one direction is 0.01 gf · cm / cm or more and 0.03 gf · cm / cm or less. When the bending hysteresis 2HB is in the above range, the crack resistance after repeated bending is excellent. Here, the crack resistance is obtained by evaluation by the method described in Evaluation method (7) Crack resistance. If the bending hysteresis 2HB exceeds 0.03 gf · cm / cm, the crack resistance deteriorates, and when controlling to less than 0.01 gf / cm / cm, it is necessary to set the film thickness to 3 μm or less, and the extreme strength reduction occurs. Since it is necessary, it is inferior in handleability. The bending hysteresis 2HB can be controlled to 0.03 gf · cm / cm or less by setting the stretching ratio in the film manufacturing process to 2.3 times or less in the desired direction. In the case of a resin structure mainly composed of polyethylene terephthalate, it is a preferable control method to set the refractive index in the thickness direction to 1.5 or more and the refractive index in the surface direction to 1.46 or more and 1.62 or less. The refractive index of the film can be adjusted by the stretching temperature under the film forming conditions, the area stretching ratio, the heat treatment temperature after stretching, and the crystallinity of the resin.
The polyester film for a lithium ion battery member of the present invention has a breaking strength in at least one direction of 250 MPa or more and 500 MPa or less. Within this range, the bending resistance is excellent. Here, the flex resistance means that the number of flex breaks measured by an MIT flex test machine is a specific number or more, and is as described in Evaluation method (9) Flex resistance. The most preferable control method is to increase the orientation of the film so that the breaking strength is 250 MPa or more and 500 MPa or less. Specifically, it can be controlled by stretching the film 3.5 times or more in the direction in which the breaking strength is desired to be increased. is there. Among the polyester films, in the case of a resin constituent film mainly composed of polyethylene terephthalate, the breaking strength can be set to 250 MPa or more by setting the refractive index to 1.65 or more. If the breaking strength is 250 MPa or less, the flex resistance is poor, and it is difficult to control the polyester-based resin to 500 MPa or more only by improving the draw ratio.
In the polyester film for a lithium ion battery member of the present invention, the difference Δn in refractive index between the longitudinal direction and the width direction which is the direction orthogonal to the longitudinal direction is 0.05 or more and 0.3 or less. In the present invention, in order to obtain crack resistance, the orientation is low, and in order to obtain bending resistance, the orientation is increased. Therefore, in order to obtain both the characteristics, it is necessary that the longitudinal direction and the direction orthogonal to the longitudinal direction have low orientation and high orientation. Specifically, the difference Δn in refractive index is 0.05 or more and 0.3 or less. ing. If Δn is less than 0.05, the flex resistance is poor, and if it exceeds 0.3, the crack resistance is poor. In order to make Δn 0.05 or more and 0.3 or less, a preferred method is to perform stretching so that the absolute value of the difference between the stretching ratio in the longitudinal direction and the stretching ratio in the width direction is 2 times or more and 6 times or less.
The polyester film for lithium-ion batteries member of the present invention, the bending stiffness of the longitudinal and transverse direction either one direction from the viewpoint of impact resistance 0.06gf · ^cm 2 · ^cm or more 0.1gf · ^cm 2 · ^cm or less Is preferred. It is preferably at most 0.08gf · ^cm 2 · ^cm or more 0.1gf · ^cm 2 · ^cm. In order to make the content within the above range, the bending hardness can be increased by increasing the film crystallinity by setting the heat treatment temperature in the film manufacturing process to 120 ° C. or higher. The crystallinity of the film may be adjusted by the heat treatment temperature according to the required characteristics.
The polyester film for a lithium-ion battery member of the present invention preferably has a heat shrinkage rate of 100 ° C. of 15% or less in both the longitudinal direction and the width direction from the viewpoint of wrinkles and curls during lamination. Here, the heat shrinkage in the longitudinal direction and the width direction at 100 ° C. is performed by the method described in Evaluation method (10) 100 ° C. heat shrinkage.
The polyester film for a lithium-ion battery member of the present invention preferably has a film thickness of 5 μm or more and 40 μm or less, and more preferably 5 μm or more and 35 μm or less, from the viewpoint of handleability and bending resistance. Further, when the thickness is 40 μm or more, the bending hysteresis 2HB tends to be extremely high regardless of the material, so that it is particularly important that the thickness is 40 μm or less.
The polyester film for a lithium-ion battery member of the present invention contains polyester as a main component, and glycols or derivatives thereof that give polyesters include ethylene glycol, 1,2-propanediol, 1,3-propanediol, and 1,3. -Aliphatic dihydroxy compounds such as butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, polyethylene such as diethylene glycol, polyethylene glycol, polypropylene glycol and polytetramethylene glycol Alicyclic dihydroxy compounds such as oxyalkylene glycol, 1,4-cyclohexanedimethanol and spiroglycol, aromatic dihydroxy compounds such as bisphenol A and bisphenol S As well as derivatives thereof.

Examples of the dicarboxylic acid or its derivative which gives the polyester used in the present invention include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenoxyethanedicarboxylic acid, 5 -Aromatic dicarboxylic acids such as sodium sulfone dicarboxylic acid, aliphatic carboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid and fumaric acid, and alicyclic rings such as 1,4-cyclohexanedicarboxylic acid Oxycarboxylic acids such as group dicarboxylic acids, paraoxybenzoic acids, and derivatives thereof can be mentioned. Examples of the dicarboxylic acid derivative include dimethyl terephthalate, diethyl terephthalate, 2-hydroxyethyl methyl terephthalate, dimethyl 2,6-naphthalene dicarboxylate, dimethyl isophthalate, dimethyl adipate, diethyl maleate, dimethyl dimerate and the like. An ester compound can be mentioned.
As the composition of the polyester in the present invention, 80 mol% or more of the glycol units are preferably structural units derived from ethylene glycol, more preferably 85 mol% or more, and most preferably 90 mol% or more. Further, 80 mol% or more of the dicarboxylic acid unit is preferably a structural unit derived from terephthalic acid, more preferably 85 mol% or more, and most preferably 90 mol% or more.
The polyester film for a lithium ion battery member of the present invention is preferably used for applications requiring flexibility, bending resistance, and crack resistance, and is particularly preferably used for a lithium ion battery exterior material, a wearable terminal requiring a lithium ion battery, and the like. be able to.

  Next, an example of a method for producing the polyester film for a lithium ion battery member of the present invention will be described. Here, polyethylene terephthalate is used as an example of the resin forming the film, but the present invention is not construed as being limited to such an example.

  First, as a resin used for the film, a polyethylene terephthalate resin is dried and pre-crystallized, and then supplied to a single-screw extruder and melt-extruded. At this time, the resin temperature is preferably controlled to 265 to 295 ° C. Then, the foreign matter is removed and the extrusion amount is balanced through a filter or a gear pump, and the sheet is discharged from the T die onto the cooling drum. At that time, an electrostatic application method in which a cooling drum and a resin are electrostatically adhered to each other by using an electrode applied with a high voltage, a casting method in which a water film is provided between a casting drum and an extruded polymer sheet, and a casting drum temperature is set to that of a polyester resin. The sheet polymer is brought into close contact with the casting drum by a method of adhering the extruded polymer at a glass transition point to (glass transition point-20 ° C) or a method of combining a plurality of these methods, solidified by cooling, and unstretched. Get the film. Among these casting methods, when polyester is used, a method of electrostatic application is preferably used from the viewpoint of productivity and flatness.

  After stretching the unstretched film obtained in the casting step in the longitudinal direction, stretching in the width direction, or after stretching in the width direction, by a sequential biaxial stretching method of stretching in the longitudinal direction, or the longitudinal direction of the film. It can be obtained by performing stretching by a simultaneous biaxial stretching method in which the width direction is stretched almost simultaneously. The stretching ratio in such a stretching method is 3.5 to 9 times in the direction of controlling the breaking strength to 250 MPa or more and 500 MPa or less, and the bending hysteresis 2HB is 0.01 gf · cm / cm or more and 0.03 gf · cm / cm. In the control direction below, 1 to 2.3 times is adopted. From the viewpoint of controlling the thickness unevenness and Δn to be 0.05 or more and 0.3 or less, the area magnification is preferably stretched to 5 times or more and 10 times or less, and the plane orientation coefficient is preferably 0.05 to 0.12. . Here, the plane orientation coefficient is a value indicating the film orientation state obtained by the method described in Evaluation Method (4). Further, the stretching temperature is preferably such that unevenness in stretching does not occur, and from the viewpoint of setting the refractive index in the thickness direction to 1.5 or more, for example, a sequential biaxial stretching method of stretching in the longitudinal direction and then in the width direction When adopting, the preheating temperature in the longitudinal direction is preferably the glass transition temperature of the resin −20 ° C. or higher and + 0 ° C. or lower, the stretching temperature is preferably the glass transition temperature of the resin or higher + 20 ° C. or lower, and the preheating temperature in the width direction is the resin. It is preferable that the glass transition temperature is + 20 ° C or lower and the stretching temperature is the glass transition temperature of the resin + 10 ° C or higher and + 60 ° C or lower. Further, the stretching may be performed plural times in each direction.

  Further, the polyester film for a lithium ion battery member of the present invention is preferably subjected to heat treatment after being biaxially stretched. The heat treatment can be performed by any conventionally known method such as heating in an oven or on a heated roll. The heat treatment is preferably performed at 80 ° C. to 180 ° C., and the heat treatment is performed by gradually increasing or decreasing the temperature in a plurality of zones, or in the width direction in the heat treatment process by 1.01 to 1.2 times. A method of slightly stretching can also be used. Further, the heat treatment time can be arbitrarily set within a range not deteriorating the characteristics, and is preferably 10 to 60 seconds, more preferably 15 to 30 seconds. Further, the heat treatment may be performed by relaxing the film in the longitudinal direction and / or the width direction.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to these Examples. Various properties were measured by the following methods.
[Evaluation methods]
(1) Film Thickness When measuring the total thickness of the film, a dial gauge was used to measure the thickness of the sample cut out from the film at five arbitrary locations, and the average value was obtained.

(2) Longitudinal direction and width direction Basically, the flow direction in the film production line is taken as the longitudinal direction, but if it is unknown, a sample is cut out in an arbitrary direction of the film with a size of 100 mm × 100 mm and made by KS Systems (currently Oji Using a microwave molecular orientation meter MOA-2001A (frequency: 4 GHz) (measurement instrument company), the main orientation axis in the film plane was determined, the main orientation axis was defined as the width direction, and the direction orthogonal to the width direction was defined as the longitudinal direction. .

(3) The composition film of the resin constituting the film is dissolved in hexafluoroisopropanol (HFIP), and the content of each monomer residue and by-product diethylene glycol is quantified using ¹ H-NMR and ¹³ C-NMR.

(4) Film Refractive Index and Plane Orientation Coefficient A sodium D line (wavelength 589 nm) is used as a light source and an Abbe refractometer is used to measure the longitudinal refractive index (n _MD ) and widthwise refractive index (n _TD ) of the film. The refractive index (n _ZD ) in the thickness direction was measured, and the plane orientation coefficient (fn) was calculated from the following formula.
fn = (n _MD + n _TD ) / 2-n _ZD
(5) Bending hysteresis 2HB
Using a multi-purpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., the longitudinal direction and the width direction were measured under the following conditions to obtain bending hysteresis 2HB. For the bending hysteresis 2HB, the data obtained at the fifth cycle (the number of repetitions: 5 times) was adopted with the curvature of −2.5 to +2.5 cm ⁻¹ as one cycle.
Number of repetitions: 5 times (5th cycle of recruitment data)
SENS: 2 x 5
Curvature: -2.5 to +2.5 cm ^-1
Deformation rate: 0.50 cm ^-1 / sec
Sample size: longitudinal direction x width direction = 20 cm x 20 cm
(6) Bending hardness Using a multipurpose bending tester (KES-FB2) manufactured by KATO TECH, under the following conditions, the longitudinal direction and the width direction were measured to obtain the bending hardness. As the bending hardness, the data obtained at the fifth cycle (the number of repetitions: 5 times) was adopted with the curvature of −2.5 to +2.5 cm ⁻¹ as one cycle.
Number of repetitions: 5 times (5th cycle of recruitment data)
SENS: 2 x 5
Curvature: -2.5 to +2.5 cm ^-1
Deformation rate: 0.50 cm ^-1 / sec
Sample size: longitudinal direction x width direction = 20 cm x 20 cm
(7) Using a crack resistance U-shaped expansion / contraction tester (DLDMLH-FS manufactured by Yuasa System Instruments Co., Ltd.), a sample cut out to a length of 112 mm and a width of 108 mm was made longitudinal with the direction in which the bending hysteresis 2HB in the film plane was the smallest. With the tilt clamp horizontal, attach it to the end of the tilt clamp so that the stroke direction is the sample length direction, bend 10,000 times at a test speed of 60 r / min, a test stroke of 60 mm, and a face-to-face distance of 3 mm. With respect to the sample (1), the following judgment was made from the reflected light of the fluorescent lamp on the side and the appearance.
A: No change in appearance and no distortion of reflected light was observed.
B: No change in appearance, but distortion of reflected light was observed.
C: A bending line was clearly observed in the appearance.

(8) Number of MIT flex breaks Using a MIT folding endurance tester (Tester No. 702 manufactured by Mize Co., Ltd.), a sample cut into a size of 110 mm in length (test direction) and 15 mm in width is loaded according to JIS-P8115. Bending test was performed at 1000 g, bending angle left and right 135 ° (R: + 135 °, L: −135 °), bending speed 175 times / min, chuck tip R: 0.38 mm, and the number of times of bending when the film was broken was determined. It was defined as the number of MIT flexural breaks. The test was carried out three times, and the average value was adopted.

(9) Bending resistance A sample was cut into a rectangle of 110 mm (test direction) x 15 mm in width with the direction of the highest breaking strength of the film being the longitudinal direction, and used as a sample (8) The number of MIT bending breaks was measured according to the number of MIT bending breaks. The number of breaks was determined as follows.

50,000 times or more: ◎ Very excellent 30,000 times or more: ○ No practical problem less than 30,000 times: × Practical problem
(10) Heat shrinkage at 100 ° C. The film was cut into a rectangle having a length of 150 mm and a width of 10 mm in each of the longitudinal direction and the width direction to obtain a sample. The samples were marked at 100 mm intervals (50 mm from the center to both ends) and placed in a hot air oven heated to 100 ° C. for 30 minutes by hanging a 3 g weight and heat treatment was performed. The distance between the marked lines after the heat treatment was measured, and the heat shrinkage rate was calculated from the change in the distance between the marked lines before and after heating by the following formula.
Thermal contraction rate (%) = {(mark distance before heat treatment)-(mark distance after heat treatment)} / (mark distance before heat treatment) × 100.

(Manufacture of polyester)
The polyester resin used for film formation was prepared as follows.

(Polyester A)
A polyethylene terephthalate resin having an terephthalic acid component of 100 mol% as a dicarboxylic acid component and an ethylene glycol component of 100 mol% as a glycol component (intrinsic viscosity 0.65).

(Polyester B)
Polyester resin having 100 mol% of terephthalic acid component as dicarboxylic acid component, 20 mol% of 1-4, cyclohexanedimethanol component as glycol component, and 80 mol% of ethylene glycol component (intrinsic viscosity 0.70)
(Polyester C)
Polyester resin containing 17 mol% of isophthalic acid as the dicarboxylic acid component, 83 mol% of the terephthalic acid component, and 100 mol% of the ethylene glycol component as the glycol component (intrinsic viscosity 0.70)
(Particle master A)
A particle master (intrinsic viscosity 0.62) containing calcium carbonate particles having an average particle diameter of 1.2 μm in polyester A at a particle concentration of 1% by mass.

(Particle master B)
A particle master (intrinsic viscosity 0.65) containing calcium carbonate particles having an average particle diameter of 1.2 μm in polyester B at a particle concentration of 1% by mass.

(Particle master C)
A particle master (intrinsic viscosity 0.65) containing calcium carbonate particles having an average particle diameter of 1.2 μm in polyester C at a particle concentration of 1% by mass.

(Examples 1 to 14, Comparative Examples 1 to 3)
As shown in Table 1, the resin type and the particle master type were mixed at a content ratio of 98% by mass: 2% by mass, charged into an extruder, and then melted at an extruder temperature set to 280 ° C. to give a T-die. The sheet was discharged on a cooling drum controlled at 25 ° C. At that time, static electricity was applied using a wire-shaped electrode having a diameter of 0.1 mm, and it was brought into close contact with the cooling drum to obtain an unstretched sheet. Then, it is stretched in the longitudinal direction and the width direction at the stretching temperature and the stretching ratio shown in Table 1, respectively, and finally subjected to a heat treatment at the temperature shown in Table 1 in the two transverse stretching steps to obtain a 20 μm lithium ion battery. A member polyester film was obtained. The physical properties of the obtained film are as shown in Tables 2 and 3, and in Examples, the bending hysteresis in at least one direction is 0.01 gf · cm ² / cm or more and 0.03 gf · cm ² / cm or less, and at least Since the breaking strength in any one direction was 250 MPa or more and 500 MPa or less, it was excellent in crack resistance and bending resistance.
On the other hand, in Comparative Example, the bending resistance or the breaking strength was out of the range, and therefore the crack resistance or the bending resistance was poor.

  Since the polyester film for a lithium ion battery member of the present invention has a hysteresis 2HB and a breaking strength in a specific range, it can be particularly suitably used as a lithium ion battery member application in which flexibility and wearability are required.

Claims (5)

The difference Δn in the refractive index between the longitudinal direction and the width direction orthogonal to the longitudinal direction is 0.05 or more and 0.3 or less, and the bending hysteresis 2HB in at least one direction is 0.01 gf · cm / cm or more and 0.03 gf. A polyester film for a lithium-ion battery member, which has a breaking strength of 250 MPa or more and 500 MPa or less in cm / cm or less and in at least one direction. Lithium ion battery materials for polyester film of claim 1, wherein the at least one direction of bending stiffness is less than 0.06gf · ^cm 2 · ^cm or more 0.1gf · ^cm 2 · ^cm. The polyester film for a lithium ion battery member according to claim 1, which has a heat shrinkage ratio of 100 ° C. of 15% or less in a longitudinal direction and a width direction orthogonal to the longitudinal direction. The polyester film for a lithium ion battery member according to claim 1, which has a thickness of 5 μm or more and 40 μm or less. A lithium ion battery using the polyester film for a lithium ion battery member according to claim 1.