HK1240895B - Polyester film, laminate, and method for producing polyester film - Google Patents

Description

Polyester film, laminate, and method for producing polyester film

Technical Field

The present invention relates to a novel polyester film having a stress and an elastic modulus during elongation within specific ranges, and a method for producing the same. Furthermore, the present invention relates to a laminate comprising the polyester film.

Background Art

Polyester films are used in a wide range of fields, such as packaging films, magnetic tape films, optical films, and films for electronic components, because of their excellent heat resistance, chemical resistance, and insulation properties.

In recent years, laminated lithium ion battery exterior materials, extrusion packaging, and the like can be obtained by cold forming a laminate composed of a resin film and a metal foil.

Generally speaking, the laminate used for the above-mentioned cold forming is composed of nylon film (Ny)/Al foil/unstretched polypropylene film (CPP), polyethylene terephthalate film (PET)/Ny/Al foil/CPP, etc. In order to give it ductility and enable it to be cold formed, a nylon film is stacked on the laminate containing Al foil.

However, the lamination of polyamide films in the laminate directly leads to increased costs. In addition, since polyamide films have poorer heat resistance than polyester films, there is a problem of reduced physical properties due to thermal degradation under high temperature and high humidity. In addition, since polyamide films are hygroscopic, there is a problem of dimensional changes due to moisture absorption, and the resulting packaging bags may curl.

On the other hand, polyester film is harder and more brittle than nylon film. Furthermore, it is typically manufactured using a tenter frame sequential stretching method, resulting in high anisotropy and difficulty imparting ductility to a metal foil laminated thereon. However, polyester films capable of forming laminates with excellent cold formability have been proposed. For example, Patent Documents 1 and 2 disclose polyester films for lithium battery packaging in which the stress during elongation in the longitudinal and width directions of the film is regulated within specific ranges. Furthermore, in recent years, laminates used in laminated lithium-ion battery exterior packaging materials, extrusion packaging, and the like have adopted a PET/Al/CPP structure in which polyester film is used only in the outer layer, rather than in the nylon film.

In the laminated body that comprises resin film and metal foil, when carrying out cold forming, importantly, utilize resin film to give ductility to metal foil, for this reason, need resin film to elongate in all directions uniformly.If there is deviation in the physical property of resin film in its MD, 45 °, TD, these 4 directions of 135 °, then be difficult to stretch to all directions uniformly when cold forming.That is, owing to there is the direction that easily elongates and the direction that is difficult to elongate in resin film, therefore metal foil fracture or produce delamination or pinhole in resin film when cold forming.If such problem occurs, then formed body likely can not bring into play the function as packaging body etc., causes the damage etc. of packaged body (contents).Therefore, need to reduce the deviation of the physical property of resin film in all directions as far as possible.

As the main reason that the formability during cold forming has an impact, the flexibility of the resin film can be cited. If the flexibility of the resin film is low, then it is possible to apply a strong load during the elongation of the cold forming, resulting in pinholes and delamination. On the contrary, if the flexibility of the resin film is too high, the effect of protecting the laminate comprising the metal foil as the base material is weak, and the physical property of the laminate obtained decreases. Therefore, it is important that the resin film has a flexibility that is neither too high nor too low.

Another physical property that affects formability during cold forming is the thickness of the resin film. Cold forming a laminated body layered with polyester films of varying thickness increases the likelihood of cracking the relatively thin portions of the polyester film, creating pinholes or causing delamination. Therefore, it is important to uniformly control the thickness of the polyester film used in cold forming.

In recent years, demands for thinner resin films and laminates used in lithium-ion battery exteriors have driven the pursuit of higher output, smaller size, and lower costs. Generally speaking, thicker resin films are easier to maintain in terms of thickness uniformity, while thinner films (particularly those below 25 μm) experience poorer thickness uniformity, significantly impacting moldability.

In this way, there is an urgent need for the development of a polyester film that has small deviations in the physical properties in the above four directions, has flexibility within an appropriate range, has excellent thickness uniformity, and has good cold formability even when thinner. However, the current situation is that such a film has not yet been developed.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Laid-Open No. 2004-362953

Patent Document 2: International Publication No. 2015/125806

Summary of the Invention

The object of the present invention is to provide a polyester film which has good ductility to the laminated metal foil, has excellent heat resistance, and is suitable for cold forming applications by suppressing the deviation of the physical properties in the four directions, having neither excessive nor insufficient flexibility and excellent thickness uniformity.

The present inventors conducted intensive studies to solve the above-mentioned problems and found that by adjusting the stretching ratio and the temperature during stretching within specific ranges, a polyester film with suppressed deviations in physical properties in the above-mentioned four directions, moderate softness, and excellent thickness uniformity can be obtained, thereby completing the present invention.

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

(1) A polyester film characterized by:

The stress at 5% elongation in each of the four directions (45°, 90°, and 135° clockwise) with respect to any direction of the film surface being 0° is such that the difference between the maximum and minimum values of these stresses is 50 MPa or less.

The difference between the maximum and minimum stresses at 15% elongation in each of the four directions is 70 MPa or less.

The elastic modulus in the four directions is within the range of 2.0 to 3.5 GPa in any direction.

(2) The polyester film according to (1), wherein the dry heat shrinkage rates in the four directions are all within a range of 0 to 10%.

(3) The polyester film according to (1) or (2), wherein the average value of the thickness in the four directions is 30 μm or less.

(4) The polyester film according to any one of (1) to (3), wherein the standard deviation of the thickness in the four directions is 0.4 μm or less.

(5) A laminate comprising the polyester film according to any one of (1) to (4) above and a metal foil.

(6) A laminate comprising a metal foil, an adhesive layer, and the polyester film according to any one of (1) to (4) laminated in this order.

(7) A method for producing a polyester film, which is a method for producing the polyester film described in any one of (1) to (4) above, characterized in that an unstretched sheet is biaxially stretched successively or simultaneously in a manner such that the stretching ratio (DRMD) in the longitudinal direction (MD) and the stretching ratio (DRTD) in the transverse direction (TD) satisfy the following (a) and (b).

0.70≤DR _MD /DR _TD ≤0.90 (a)

12.5≤DR _MD ×DR _TD ≤15.5 (b)

(8) The method for producing a polyester film according to (7), wherein:

The stretching is a successive biaxial stretching.

The first stretching step of stretching the unstretched sheet in the machine direction (MD) at a temperature range of 65 to 105° C. to obtain a first stretched film is performed.

The second stretching is performed in a temperature range of 90 to 160° C. to stretch the first stretched film in the transverse direction (TD) to obtain a second stretched film.

(9) The method for producing a polyester film according to (7) or (8), wherein

The biaxially stretched film is heat-treated at a temperature ranging from 160 to 210°C.

The polyester film of the present invention has excellent stress balance during elongation in four directions and has appropriate flexibility. Therefore, the metal foil of the laminated body composed of the polyester film of the present invention and the metal foil has good ductility. When the metal foil is drawn by cold forming (especially deep drawing or embossing), the metal foil will not break, delaminate, or have pinholes, and a high-reliability, high-quality product (molded body) can be obtained.

Furthermore, the polyester film of the present invention has an excellent balance of stress during elongation in the four directions and excellent thickness uniformity even if it is a thin polyester film with a thickness of less than 25 μm. Therefore, the laminate formed by laminating with metal foil can be cold-formed to obtain a miniaturized product, which is also advantageous in terms of cost.

Moreover, since conventional polyester films have poor cold formability, a ductile resin film such as a polyamide film needs to be laminated when making a laminate. However, the polyester film of the present invention has very excellent cold formability even without laminating a polyamide film. Therefore, a product with a shortened lamination process and miniaturization can be obtained, and a laminate with excellent economic efficiency can be provided.

Furthermore, according to the production method of the present invention, by adjusting the stretching ratios in MD and TD and the temperature during stretching within specific ranges, a polyester film having the above-mentioned excellent properties can be produced efficiently and with good productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing positions at which samples are taken for measuring stress during elongation of a polyester film.

FIG. 2 is a diagram showing a method for measuring the thickness of a polyester film.

DETAILED DESCRIPTION

Hereinafter, the present invention will be described in detail.

Examples of the polyester resin constituting the polyester film of the present invention include polyester resins composed of a dicarboxylic acid component and a diol component and polyester resins composed of a hydroxycarboxylic acid component.

Examples of the dicarboxylic acid component include terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalene dicarboxylic 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.

Examples of the diol component include ethylene glycol, diethylene glycol, 1,3-propylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, cyclohexanedimethanol, triethylene glycol, polyethylene glycol, polypropylene glycol, polybutylene glycol, bisphenol A, and ethylene oxide adducts of bisphenol S.

Examples of the hydroxycarboxylic acid component include ε-caprolactone, lactic acid, and 4-hydroxybenzoic acid.

The polyester resin constituting the polyester film of the present invention (hereinafter sometimes referred to as "the polyester resin (R) in the present invention") may be a homopolymer composed of the above-mentioned components or a copolymer, and may further contain a small amount of trifunctional compound components such as trimellitic acid, trimesic acid, pyromellitic acid, trimethylolpropane, glycerol, and pentaerythritol.

Furthermore, the polyester resin (R) in the present invention may be a combination of two or more homopolymers or copolymers composed of the above components.

The polyester resin (R) in the present invention preferably has an intrinsic viscosity of 0.65 to 0.88, particularly preferably 0.67 to 0.84, before being used in the polyester film production method of the present invention. If the intrinsic viscosity of the polyester resin (R) is within this range, the polyester film of the present invention can be obtained by the production method of the present invention described below. If the intrinsic viscosity of the polyester resin (R) is outside this range, it may be difficult to obtain a film that satisfies the stress balance and elastic modulus during elongation in the four directions specified in the present invention.

In order to adjust the intrinsic viscosity of the polyester resin (R) in the present invention to the above range, the temperature and time during polymerization may be adjusted. In addition to melt polymerization, solid phase polymerization may also be performed.

The intrinsic viscosity of the polyester resin (R) in the present invention is measured by dissolving 0.25 g of the polyester resin in 50 ml of phenol/tetrachloroethane = 5/5 (mass ratio) and measuring the intrinsic viscosity at 25°C using an Ubbelohde viscometer.

Specifically, the polyester resin (R) in the present invention preferably contains a polybutylene terephthalate resin (A) and a polyethylene terephthalate resin (B). In the polyester resin (R) in the present invention, the ratio of the polybutylene terephthalate resin (A) to the polyethylene terephthalate resin (B) is preferably 90% by mass or more, and more preferably 95% by mass or more.

In the present invention, the polybutylene terephthalate resin (A) contains terephthalic acid and 1,4-butanediol as main polymerization components, and other components may be copolymerized therewith. As the copolymerization components, the dicarboxylic acid components and diol components exemplified above can be used.

In the present invention, when a copolymer is used as the polybutylene terephthalate resin (A), the types of copolymerized components may be appropriately selected, and the proportions of the dicarboxylic acid component and the diol component are preferably 20 mol% or less, more preferably 10 mol% or less. If the proportion of the copolymerized component in the polybutylene terephthalate resin (A) exceeds 20 mol%, the melting point may be lower than the range described below, resulting in reduced crystallinity and decreased heat resistance of the polyester film.

In the polyester film of the present invention, the melting point of the polybutylene terephthalate resin (A) is preferably 200 to 223° C., more preferably 210 to 223° C. If the melting point is lower than 200° C., the heat resistance of the polyester film decreases.

The polyethylene terephthalate resin (B) in the present invention contains terephthalic acid and ethylene glycol as main polymerization components, and other components may be copolymerized therewith. As the copolymerization components, the dicarboxylic acid components and diol components exemplified above can be used.

The proportion of the copolymerization component is preferably 20 mol% or less for both the acid component and the alcohol component, and more preferably 10 mol% or less.

The melting point of the polyethylene terephthalate resin (B) is preferably 225 to 260° C., more preferably 240 to 260° C. If the melting point is lower than 225° C., the heat resistance of the polyester film decreases.

The polyester resin in the present invention preferably has a mass ratio (A/B) of polybutylene terephthalate resin (A) to polyethylene terephthalate resin (B) of 5/95 to 40/60, more preferably 5/95 to 30/70, and even more preferably 5/95 to 25/75.

Compared with polyethylene terephthalate resin (B), polybutylene terephthalate resin (A) has two more carbon atoms in the aliphatic chain contained in the unit skeleton, so the mobility of the molecular chain is high and the flexibility is high. By mixing polybutylene terephthalate resin (A) with polyethylene terephthalate resin (B), the flexibility of the resulting polyester film is increased. In short, the higher the mass ratio of polybutylene terephthalate resin (A) within the above range, the more the flexibility of the polyester film is improved. On the other hand, if the mass ratio of polybutylene terephthalate resin (A) is lower than the above range, the resulting polyester film lacks flexibility and the elastic modulus becomes higher. In addition, if the mass ratio of polybutylene terephthalate resin (A) is higher than the above range, the resulting polyester film strongly exhibits the characteristics of polybutylene terephthalate resin (A), becomes too soft, the elastic modulus becomes low, and sometimes the heat resistance decreases.

The polyester resin (R) in the present invention preferably contains two types of polyethylene terephthalate resins (B), namely, a polyethylene terephthalate resin (Bc) containing a copolymerization component and a polyethylene terephthalate resin (Bh) containing substantially no copolymerization component, in addition to the polyester resin containing the polybutylene terephthalate resin (A) and the polyethylene terephthalate resin (B). In the polyester resin (R) in the present invention, the ratio of the polyethylene terephthalate resins (Bc) to (Bh) is preferably 90% by mass or more, and more preferably 95% by mass or more.

The polyethylene terephthalate resin (Bc) containing a copolymer component is preferably polyethylene terephthalate copolymerized with isophthalic acid, and the melting point of the polyethylene terephthalate resin (Bc) containing a copolymer component is preferably 200 to 225° C., more preferably 210 to 225° C. If the melting point is lower than 200° C., the heat resistance of the polyester film decreases.

When the polyester resin (R) in the present invention contains polyethylene terephthalate resins (Bc) and (Bh), the mass ratio (Bc/Bh) of (Bc) to (Bh) is preferably 5/95 to 40/60, more preferably 5/95 to 30/70, and even more preferably 5/95 to 25/75.

The method for polymerizing the polyester resins such as the polybutylene terephthalate resin (A) and the polyethylene terephthalate resin (B) is not particularly limited, and examples thereof include transesterification and direct polymerization. Examples of transesterification catalysts include oxides and acetates of Mg, Mn, Zn, Ca, Li, and Ti. Examples of polycondensation catalysts include oxides and acetates of Sb, Ti, and Ge.

Since the polyester after polymerization contains monomers or oligomers, and by-products such as acetaldehyde and tetrahydrofuran, solid phase polymerization may be carried out at a temperature of 200° C. or higher under reduced pressure or in an inert gas flow.

During the polymerization of the polyester resin, additives such as antioxidants, heat stabilizers, ultraviolet absorbers, and antistatic agents may be added as needed. Examples of antioxidants include hindered phenol compounds and hindered amine compounds, examples of heat stabilizers include phosphorus compounds, and examples of ultraviolet absorbers include benzophenone compounds and benzotriazole compounds. Furthermore, when the polyester resin contains two or more resins, such as polybutylene terephthalate resin (A) and polyethylene terephthalate resin (B), it is preferable to add a phosphorus compound as a reaction inhibitor to suppress the reaction between them.

Next, the characteristic values of the polyester film of the present invention will be described. As an indicator indicating that the stress balance during elongation during secondary processing is very good, the polyester film of the present invention must simultaneously satisfy the following (1) and (2). That is, the polyester film of the present invention must (1) have a stress at 5% elongation in each of four directions (45°, 90°, and 135° clockwise relative to the direction of the film surface) with respect to any direction being 0°, and the difference between the maximum and minimum values of these stresses must be 50 MPa or less; and (2) have a stress at 15% elongation in each of the four directions mentioned above with respect to the direction of elongation being 70 MPa or less.

If the difference (ΔF5) between the maximum and minimum stress values at 5% elongation (F5) in the four directions and the difference (ΔF15) between the maximum and minimum stress values at 15% elongation (F15) in the four directions are greater than the above ranges, the stress balance in all directions of the polyester film is poor, making it difficult to achieve uniform formability. For example, when a laminated body laminated with a metal foil is cold-formed, a polyester film with uneven formability cannot impart sufficient ductility to the metal foil (i.e., the polyester film has difficulty following the metal foil), resulting in breakage of the metal foil and the development of defects such as delamination and pinholes.

The ΔF5 needs to be 50 MPa or less, preferably 35 MPa or less, more preferably 25 MPa or less, and further preferably 15 MPa or less. The ΔF15 needs to be 70 MPa or less, preferably 60 MPa or less, more preferably 50 MPa or less, and further preferably 35 MPa or less.

Generally, when film is produced using the tenter-frame sequential stretching method, the film is obtained as a roll wound into a cylindrical shape, typically with a roll width of approximately 2 to 8 meters. The resulting roll is then slotted and shipped as a finished product with a roll width of approximately 1 to 3 meters. In the tenter-frame sequential stretching method, the film is stretched while being clamped at both ends. Therefore, a difference in stress during elongation tends to occur near the center of the roll width and at the ends.

However, according to the manufacturing method of the present invention, it is difficult to produce a difference in stress during elongation of the wound film near the ends and the center of the obtained film roll, and even for the polyester film wound at the ends of the film roll, the values of ΔF5 and ΔF15 are within the above range.

Furthermore, according to the production method of the present invention, in the polyester film obtained, ΔF5 of the polyester film near the center of the film roll can be 15 MPa or less, and ΔF15 can be 35 MPa or less.

Furthermore, the stress at 5% elongation (F5) in all four directions of the polyester film is preferably 80 to 130 MPa, more preferably 85 to 125 MPa, and even more preferably 90 to 120 MPa, from the perspective of cold formability when formed into a laminate. Furthermore, the stress at 15% elongation (F15) is preferably 80 to 160 MPa, more preferably 90 to 155 MPa, and even more preferably 95 to 150 MPa, also from the perspective of cold formability when formed into a laminate.

If the stresses in the four directions of the polyester film of the present invention at 5% and 15% elongation do not satisfy the above ranges, sufficient cold formability cannot be obtained.

The stress in the four directions of the film of the present invention is measured as follows. First, after the polyester film is humidified at 23°C × 50% RH for 2 hours, as shown in Figure 1, the reference direction (0° direction) of the film is arbitrarily determined with any point A on the film as the center point. The four directions of 45° direction (b), 90° direction (c) and 135° direction (d) rotated clockwise from the reference direction (a) are used as measurement directions. The film is cut into strips with a length of 100 mm in each measurement direction from the center point A and a length of 15 mm in the perpendicular direction relative to the measurement direction. The obtained film is used as a sample. For example, as shown in Figure 1, in the 0° direction, a sample is cut in the range of 30 mm to 130 mm from the center point A as sample X (100 mm vertical × 15 mm horizontal). Samples are cut in the same way for other directions. For these samples, a tensile testing machine (AG-1S, manufactured by Shimadzu Corporation) equipped with a 1 kN load cell and sample chuck was used to measure the stress at 5% elongation (F5) and the stress at 15% elongation (F15) at a tensile speed of 500 mm/min. Five samples were measured in each direction, and the average value was calculated as the stress value in each direction. The difference between the maximum and minimum stress values in each of the four directions was then determined.

In addition, regarding the above-mentioned reference direction (0°), when determining the MD in the stretching step during film production, it is preferable to use the MD as the reference direction.

Next, as an indicator of flexibility suitable for cold forming, the polyester film of the present invention needs to have an elastic modulus in the above four directions in any direction within the range of 2.0 to 3.5 GPa, preferably within the range of 2.2 to 3.4 GPa, and more preferably within the range of 2.4 to 3.3 GPa.

The polyester film of the present invention can achieve the effects of the present invention when the stress balance during elongation in the four directions satisfies (1) and (2) at the same time and the elastic modulus in the four directions is within the above range. In short, the metal foil of the laminated body in which the polyester film of the present invention is laminated with a metal foil has good ductility. When the metal foil is drawn (particularly deep drawing or embossing) by cold forming, no breakage, delamination, pinholes, etc. occur in the metal foil, and a high-reliability, high-quality product (molded body) can be obtained. If the elastic modulus of the polyester film in the four directions is less than 2.0 GPa, the flexibility becomes too high. On the other hand, if the elastic modulus of the polyester film in the four directions is greater than 3.5 GPa, the flexibility decreases. Moreover, if the elastic modulus of the polyester film in the four directions is outside the range of the present invention, even if the stress balance during elongation in the four directions satisfies (1) and (2) at the same time, the metal foil cannot be given good ductility, and the cold formability decreases.

The elastic modulus in the four directions of the polyester film of the present invention was measured using a tensile testing machine (AG-1S manufactured by Shimadzu Corporation) when measuring the stress in the four directions.

Furthermore, as an indicator of flexibility suitable for cold formability, the polyester film of the present invention preferably has a dry heat shrinkage rate in each of the four directions preferably within a range of 0 to 10%, more preferably within a range of 2 to 9.5%, and even more preferably within a range of 2.5 to 9.0%. When the dry heat shrinkage rates in the four directions are all within the above ranges, crystallization is optimized and the polyester film has flexibility suitable for cold formability.

If the dry heat shrinkage of the polyester film in the four directions mentioned above exceeds 10%, crystallization may not proceed sufficiently, resulting in excessive flexibility. On the other hand, if the dry heat shrinkage of the polyester film in the four directions mentioned above is less than 0%, crystallization may proceed excessively, resulting in reduced flexibility. In either case, cold formability may also be reduced.

The dry heat shrinkage ratios in the four directions of the polyester film of the present invention are measured as follows.

According to the sampling method for measuring stress during elongation, the polyester film was cut into strips so that the distance from the center point A in each measuring direction was 100 mm and the distance in the direction perpendicular to the measuring direction was 10 mm.

After conditioning the sample at 23°C and 50% RH for 2 hours (conditioning 1), expose it to dry air at 160°C for 15 minutes. Then, condition it again at 23°C and 50% RH for 2 hours (conditioning 2). The sample lengths after conditioning 1 and 2 were measured, and the dry heat shrinkage ratio was calculated using the following formula. Five samples were measured, and the average value was used as the dry heat shrinkage ratio.

Dry heat shrinkage (%) = {(sample length after humidity conditioning 1 - sample length after humidity conditioning 2) / sample length after humidity conditioning 1} × 100

For the polyester film of the present invention, as an indicator showing that the thickness accuracy (uniformity of thickness) is very high, it is preferred to set a reference point on the film, and set multiple measurement points along the above-mentioned four directions from the reference point, and the standard deviation of the measured values when measuring the thickness at each measurement point is less than 0.4 μm, more preferably less than 0.3 μm, and even more preferably less than 0.28 μm.

If the standard deviation of the thickness accuracy (thickness uniformity) of the polyester film is less than 0.4 μm, the thickness deviation is very small. For example, even if the thickness is less than 15 μm, the laminate bonded with the metal foil will not produce delamination, pinholes and other defects during deep drawing and deep cold forming, and good formability can be obtained.

A polyester film with low thickness accuracy, with a standard deviation exceeding 0.4 μm, may fail to impart sufficient ductility to the metal foil when laminated to the metal foil, particularly when the thickness is small, resulting in significant delamination and pinhole formation, and failure to achieve good formability.

The evaluation method of the thickness accuracy is as follows. After the polyester film is humidified at 23°C × 50% RH for 2 hours, as shown in Figure 2, with an arbitrary position on the film as the center point A, the reference direction of the film (0° direction) is arbitrarily determined, and a total of four straight lines L1 to L4 of 100 mm are drawn in the four directions of 45° direction (b), 90° direction (c), and 135° direction (d) rotated clockwise from the reference direction (a). The thickness of 10 points is measured at intervals of 10 mm from the center point on each straight line using a length meter (HEIDENHAIN-METRO MT1287 manufactured by HEIDENHAIN). Then, the average value of the thickness of 40 points obtained by measuring in the four straight lines is calculated and used as the thickness. In addition, the standard deviation is calculated using the measured values of the thickness at 40 points. It should be noted that the reference direction is preferably MD as the reference direction when the MD in the stretching process during film manufacturing is determined.

In the present invention, the average thickness and standard deviation can be based on any point (point A) on the polyester film. According to the manufacturing method of the present invention, even if the polyester film is wound near the end and center of the obtained film roll, a polyester film with an average thickness and standard deviation within the above range can be formed.

The average thickness of the polyester film of the present invention is preferably 30 μm or less, more preferably 26 μm or less, and even more preferably 16 μm or less. The polyester film of the present invention is suitable for forming a laminated body laminated with a metal foil and is suitable for cold forming applications. However, by performing biaxial stretching using a tenter as described below under stretching conditions that meet specific conditions, even a thin film can be obtained that has excellent stress balance during elongation in the four directions and has very high thickness accuracy (thickness uniformity, etc.) in the four directions.

When the average thickness of the polyester film exceeds 30 μm, the moldability may be reduced, making it difficult to use the film for a small battery exterior material. In addition, there is a possibility of being disadvantageous in terms of cost.

The thinner the polyester film, the more difficult it is to impart sufficient ductility to the metal foil. Simply put, the thinner the film, the lower the thickness accuracy of the resulting film, and the more likely it is to experience deviations in stress during elongation. Consequently, the indentation force during cold forming can significantly cause breakage of the polyester film or metal foil. In contrast, the polyester film of the present invention, by employing a specific manufacturing method described below, successfully provides a polyester film that exhibits excellent stress balance during elongation in the four aforementioned directions and high thickness uniformity, particularly even when the thickness is 26 μm or less. The lower limit of the average thickness of the polyester film of the present invention is not particularly limited; generally, it is sufficient to be approximately 2 μm. When the average thickness is less than 2 μm, the ductility imparted to the metal foil during lamination to the metal foil is likely to be insufficient.

The polyester film of the present invention may contain particles to improve the windability of the resulting polyester film during the production method of the present invention described below. The particles incorporated into the polyester film are not particularly limited as long as they impart lubricity. Examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide. Heat-resistant organic particles such as thermosetting urea resins, thermosetting phenolic resins, thermosetting epoxy resins, and benzoguanamine resins may also be used. Furthermore, during the polyester resin production process, particles formed by precipitating or microdispersing a portion of a metal compound such as a catalyst may also be used.

The shape of the particles used is not particularly limited, and any one of spherical, blocky, rod-shaped, flat, etc. can be used. In addition, its hardness, specific gravity, color, etc. are not particularly limited either. These particles can be used in combination with more than two kinds as needed.

One or more coating layers may be laminated on at least one side of the polyester film of the present invention depending on the purpose. Examples of such coating layers include those that impart electrolyte resistance, acid resistance, alcohol resistance, abrasion resistance, antistatic properties, printability, and adhesiveness.

Furthermore, as an adhesion-facilitating treatment for improving the adhesion between the substrate and the aluminum foil, the polyester film may be subjected to a surface treatment to express an adhesion-facilitating effect.

The polyester film of the present invention preferably has a primer layer on at least one surface as a coating layer for improving adhesion. The presence of the primer layer improves adhesion between the polyester film and the metal foil in a laminate comprising the polyester film of the present invention and a metal foil. This allows for more effective ductility to be imparted to the metal foil during cold forming, thereby making the metal foil less susceptible to breakage and also effectively suppressing delamination.

The main component of the primer layer includes water-soluble or water-dispersible polyurethane compounds, acrylic compounds, and polyester compounds, preferably an anionic water-dispersible polyurethane resin. The curing agent of the primer layer includes melamine compounds, isocyanate compounds, and oxazoline compounds.

The thickness of the primer layer is preferably 0.01 to 0.5 μm. If the primer layer is thinner than 0.01 μm, adhesion is reduced. If the primer layer is thicker than 0.5 μm, no significant improvement in adhesion is observed. Instead, it may cause brushing and sticking on the film roll, primer layer print-through, damage to the primer layer during film unwinding, and film breakage, which is also disadvantageous in terms of cost.

As a method for applying the aqueous solution or aqueous dispersion of the compound to form a primer layer, any known method may be selected, and for example, bar coating, air knife coating, reverse roll coating, or gravure roll coating can be used.

A lubricating agent for preventing blocking and a wetting agent for improving coating properties may be added to the primer layer as needed within a range that does not affect the adhesiveness.

The polyester film of the present invention that satisfies the above-mentioned property values can be obtained by the production method of the present invention.

The following describes a method for producing a polyester film composed of a polyester resin (R) containing a polybutylene terephthalate resin (A) and a polyethylene terephthalate resin (B). The polyester film of the present invention can be produced through a sheet forming step and a subsequent stretching step.

In the sheet forming step, the polyester resin (R) is formed into a sheet shape to obtain an unstretched sheet.

The polyester resin (R) can be prepared according to a known method. For example, it can be obtained by introducing raw materials containing polybutylene terephthalate resin (A) and polyethylene terephthalate resin (B) into an extruder equipped with a heating device and melt-kneading them at 270-300°C for 3-15 minutes. The melt-kneaded resin composition is extruded through a T-die and cooled and solidified using a casting drum adjusted to a temperature of 50°C or less to obtain an unstretched sheet as a sheet-like molded body.

The average thickness of the unstretched sheet is not particularly limited, but is generally preferably about 15 to 250 μm, more preferably 50 to 235 μm. When the average thickness of the unstretched sheet is within the above range, stretching can be performed more efficiently.

In the stretching step, the unstretched sheet is biaxially stretched in the machine direction (MD) and the transverse direction (TD) sequentially or simultaneously to obtain a stretched film.

Examples of simultaneous biaxial stretching include a method of simultaneously performing biaxial stretching in MD and TD by gripping both ends of an unstretched film with a tenter and stretching it in MD and TD simultaneously.

On the other hand, in the case of successive biaxial stretching, it is preferred to use a tenter to stretch in at least one direction of MD and TD, thereby obtaining a more uniform film thickness. The following methods are available for successive biaxial stretching using a tenter: (1) a method in which an unstretched sheet is stretched in MD by passing it through a plurality of rollers having different rotation speeds, and then the stretched film is stretched in TD using a tenter; (2) a method in which an unstretched sheet is stretched in MD using a tenter, and then the stretched film is stretched in TD using a tenter. In view of the physical properties and productivity of the obtained film, the method (1) is particularly preferred. Successive biaxial stretching using a tenter is advantageous in terms of productivity and equipment because it uses rollers to stretch in MD, and it is advantageous in terms of controlling the film thickness because it uses a tenter to stretch in TD.

In the production method of the present invention, it is important to sequentially or simultaneously biaxially stretch the unstretched sheet in the stretching step so that the stretch ratio in MD (DR _MD ) and the stretch ratio in TD (DR _TD ) simultaneously satisfy the following (a) and (b).

0.70≤DR _MD /DR _TD ≤0.90 (a)

12.5≤DR _MD ×DR _TD ≤15.5 (b)

If any one of the above-mentioned (a) and (b) is not satisfied, the stress balance in the four directions of the obtained polyester film will be poor, and it will be difficult to obtain the polyester film of the present invention.

In short, when the stretch ratio ratio (DR _MD /DR _TD ) is less than 0.70, the TD ratio becomes higher than the MD ratio, resulting in high stress values in the TD stress-strain curve of the polyester film and low elongation. On the other hand, when the stretch ratio ratio (DR _MD /DR _TD ) exceeds 0.90, the MD ratio becomes higher than the TD ratio, resulting in high stress values in the MD stress-strain curve of the polyester film and low elongation. Furthermore, this affects the stress-strain curves in the 45° and 135° directions, making it difficult to obtain a polyester film that simultaneously satisfies the requirements for the difference between the maximum and minimum stress values during elongation (ΔF5 and ΔF15) specified in the present invention.

When the area ratio (DR _MD × DR _TD ) is less than 12.5, the area ratio is too low and stretching becomes insufficient, so the polyester film cannot obtain sufficient molecular orientation. On the other hand, when the area ratio (DR _MD × DR _TD ) exceeds 15.5, the area ratio is too high, so the polyester film cannot be stretched uniformly in all directions during stretching. As a result, it is difficult to simultaneously meet the conditions for the difference between the maximum and minimum stress values during elongation (ΔF5, ΔF15) specified in the present invention.

The stretch ratio ratio (DR _MD /DR _TD ) and the area ratio (DR _MD ×DR _TD ) need to satisfy (a) and (b) as described above. Among them, DR _MD is preferably 3.0 to 3.7, more preferably 3.1 to 3.6.

When performing sequential biaxial stretching, the first stretching step, in which the unstretched sheet is stretched in the machine direction (MD) to obtain a first stretched film, is preferably performed at a temperature range of 65 to 105°C, preferably 70 to 100°C. Subsequently, the second stretching step, in which the first stretched film is stretched in the transverse direction (TD) to obtain a second stretched film, is preferably performed at a temperature range of 90 to 160°C, preferably 100 to 150°C. The temperature during the stretching step can be set and controlled during preheating, for example, using preheating rollers or a preheating zone of a tenter.

By setting the temperature range of the first stretching and the temperature range of the second stretching within the above range, the polyester film of the present invention can be reliably obtained. In addition, it is preferred that the temperature of both the first stretching and the second stretching be gradually increased along the film retrieving direction within the above temperature range.

In both simultaneous biaxial stretching and sequential biaxial stretching using a tenter, a relaxation heat treatment is preferably performed after stretching. The temperature during the relaxation heat treatment is preferably 160-210°C, more preferably 170-210°C. The temperature during the relaxation heat treatment can be set and controlled in the relaxation heat treatment zone of the tenter. The relaxation rate during the relaxation heat treatment is preferably 2-9%, with 3-7% being particularly preferred.

Methods for adjusting the temperature during stretching and relaxation heat treatment to the above range include blowing hot air onto the film surface, using a far-infrared or near-infrared heater, and a combination thereof. In the present invention, the method including blowing hot air is preferred.

In addition, when obtaining the polyester film of the present invention having an easy-adhesive layer on at least one side of the film surface, it is also preferably carried out using the same stretching method and stretching conditions as described above. It should be noted that in order to form an easy-adhesive layer on the film surface, in the above-mentioned production method, it is preferred that an aqueous coating agent for forming an easy-adhesive layer be applied to the polyester film after being stretched in the MD. Then, it is preferred that the film is subsequently stretched in the TD together with the aqueous coating agent under the same stretching conditions as described above (in-mold coating). The amount of the aqueous coating agent applied is preferably adjusted so that the thickness of the easy-adhesive layer formed on the film surface after stretching is 0.01 to 0.10 μm.

Furthermore, the laminate of the present invention comprises a polyester film and a metal foil. As a representative example of the laminate of the present invention, a laminate comprising the polyester film of the present invention and a metal foil laminated on the film can be cited. In this case, the polyester film of the present invention and the metal foil can be laminated in direct contact, or can be laminated with other layers such as an adhesive layer interposed therebetween. In particular, the present invention preferably comprises a laminate laminated in the order of the film of the present invention/metal foil/sealant film. In this case, an adhesive layer may or may not be interposed between the layers.

In addition, since the polyester film of the present invention can impart good ductility to the metal foil, there is no need to laminate another resin film having ductility such as a polyamide film.

Examples of the metal foil include metal foils (including alloy foils) containing various metal elements (aluminum, iron, copper, nickel, etc.), with pure aluminum foil or aluminum alloy foil being particularly preferred. Aluminum alloy foil preferably contains iron (such as aluminum-iron alloys). Other components may be present as long as they do not impair the formability of the laminate and are within known content ranges specified in JIS, etc.

The thickness of the metal foil is not particularly limited, but is preferably 15 to 80 μm, and more preferably 20 to 60 μm, from the viewpoint of formability and the like.

The sealant film constituting the laminate of the present invention is preferably a heat-sealable thermoplastic resin such as polyethylene, polypropylene, olefin copolymers, or polyvinyl chloride. The thickness of the sealant film is not limited, but is generally preferably 20 to 80 μm, and more preferably 30 to 60 μm.

Example

The present invention will be described in more detail below with reference to examples. However, the present invention is not limited by the following examples. The properties of the polyester film and the laminate were measured by the following methods.

The resulting film roll was divided into three equal parts in the width direction. The central film roll was designated "a," the film roll on the right side viewed from the upstream side of the film flow direction was designated "b," and the film roll on the left side viewed from the upstream side of the film flow direction was designated "c."

(1) Stress, elastic modulus, and dry heat shrinkage of polyester film in four directions at 5% and 15% elongation

The stress, elastic modulus, and dry heat shrinkage of the polyester film in four directions at 5% and 15% elongation were measured and calculated by the above-described method with the reference direction (0° direction) being defined as MD.

At this time, samples were taken from film rolls "a" and "b" for measurement. Films taken from a position corresponding to half the roll length were used, with the center point in the width direction being the center point A shown in FIG1 .

These values in the film roll "b" were measured only in the Examples and Comparative Examples shown in Table 7.

(2) Average thickness and standard deviation of polyester film

The average thickness and standard deviation of the polyester film were measured and calculated by the above-mentioned methods.

At this time, samples were taken from film rolls "a," "b," and "c" for measurement. For film rolls "a" and "b," samples were taken at a point halfway through the roll length, with the center point in the width direction being designated as center point A in Figure 2. For film roll "c," samples were taken near the end of the roll, with the center point being designated as center point A in Figure 2, 20 cm from the left end as viewed from the upstream side in the film flow direction.

(3) Cold formability

The resulting laminate was humidified at 23°C and 50% RH for at least one hour. Then, using an Erichsen tester (No. 5755, manufactured by Yasuda Seiki Co., Ltd.) in accordance with JIS Z2247, a steel ball punch was pressed into the laminate to a predetermined depth at 23°C and 50% RH to determine the Erichsen value. The sample laminate had a dimension of 10 cm in length and 10 cm in width. The Erichsen value was measured at 0.5 mm intervals for 10 samples, and the average value was calculated.

When the Erichsen value is 6.5 mm or more, and in particular, when it is 7 mm or more, it is determined to be suitable for deep drawing.

As the polyester resin (R), the following polyester resins were used.

A-1: Polybutylene terephthalate (NOVADURAN 5010S manufactured by Mitsubishi Engineering-Plastics Corporation, intrinsic viscosity: 1.10)

A-2: Polybutylene terephthalate (NOVADURAN 5505S manufactured by Mitsubishi Engineering-Plastics Corporation, intrinsic viscosity: 0.92)

Bh-1: Polyethylene terephthalate (UT-CBR manufactured by Nippon Ester Co., Ltd., intrinsic viscosity: 0.67)

Bh-2: polyethylene terephthalate (NEH2050 manufactured by Nippon Ester Co., Ltd., intrinsic viscosity: 0.78)

Bc-3: Polyethylene terephthalate copolymerized with isophthalic acid (MA-1342 manufactured by Nippon Ester Co., Ltd., intrinsic viscosity: 0.63)

Bc-4: Polyethylene terephthalate copolymerized with isophthalic acid (SA-1345 manufactured by Nippon Ester Co., Ltd., intrinsic viscosity: 0.78)

Example 1

(Production of Polyester Film)

The above-mentioned A-1 and Bh-1 as polyester resins (R) are mixed in a mass ratio of (A-1/Bh-1) 5/95, and a condensed silica masterbatch (GS-BR-MG manufactured by Nippon Ester Co., Ltd.) is added in a manner such that the silica content is 0.05% by mass. The mixture is melted at 280°C, extruded from a T-die outlet with a residence time of 5 minutes, and rapidly cooled and solidified to obtain an unstretched film with a thickness of 25 μm after stretching.

Next, the unstretched film was sequentially stretched. First, it was heated to 85°C using a heated roller in a longitudinal stretching machine and stretched to 3.4 times in the MD. Next, transverse stretching was initiated at 120°C and stretched to 4.25 times in the TD. During this stretching, the draw ratio (DR _MD /DR _TD ) was 0.80, and the area ratio (DR _MD × DR _TD ) was 14.5.

Next, a relaxation heat treatment was performed for 4 seconds at a relaxation rate of 6.0% at a temperature of 190° C., and then cooled to room temperature to obtain a polyester film having a thickness of 25 μm. The obtained polyester film was wound into a roll.

(Production of Laminated Body)

Next, a two-component polyurethane adhesive (TM-K55/CAT-10L, manufactured by ToyoMorton) was applied to the resulting polyester film at a coating weight of 5 g/ ^m² and dried at 80°C for 10 seconds. Aluminum foil (AA standard 8079P, 50 μm thick) was laminated to the adhesive-coated surface. Next, the same adhesive was applied to the aluminum foil side of the aluminum foil laminated to the polyester film under the same conditions. An unstretched polypropylene film (GHC, 50 μm thick, manufactured by Mitsui Chemicals Tohcello) was laminated and aged for 72 hours at 40°C to produce a laminate.

Examples 2 to 67, Comparative Examples 1 to 34

A polyester film was obtained by the same method as in Example 1, except that the type and mass ratio of the polyester resin used as the polyester resin (R), the stretch ratio in MD and TD, the stretching temperature, the relaxation heat treatment temperature, the relaxation rate, and the thickness after stretching were changed as described in Tables 1 to 7. When the thickness after stretching was changed, the feed rate of the polyester resin (R) extruded from the T-die outlet was changed.

Using the obtained polyester film, a laminate was obtained in the same manner as in Example 1.

Tables 1 to 7 show the configurations, production conditions, and property values of the polyester films obtained in Examples 1 to 67 and Comparative Examples 1 to 34, and the cold formability of the obtained laminates.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5]

[Table 6]

[Table 7]

These results clearly show that in Examples 1 to 67, the draw ratio ratio (DR _MD /DR _TD ) and the area ratio (DR _MD × DR _TD ) were within the ranges specified in the present invention. Consequently, the difference between the maximum and minimum stress values in the four directions of the resulting polyester films at 5% elongation was 50 MPa or less, and the difference between the maximum and minimum stress values at 15% elongation was 70 MPa or less. Furthermore, the elastic modulus in all four directions was within the range of 2.0 to 3.5 GPa. Furthermore, the standard deviation of the thickness in the four directions was 0.4 μm or less, indicating excellent thickness uniformity.

Furthermore, as is clear from the examples shown in Table 7, the aforementioned characteristic values are also satisfied at the ends of the wound width of the wound film roll, and a polyester film satisfying the characteristic values of the present invention can be obtained over the entire width of the film roll.

Furthermore, laminates obtained using polyester films meeting the characteristic values specified in the present invention exhibit high Erichsen values and uniform ductility in all directions during cold forming. In short, the polyester films of each example exhibit excellent cold formability, without causing aluminum foil breakage, delamination, or pinholes during cold forming.

On the other hand, in Comparative Examples 1 to 33, the stretch ratio (DR _MD /DR _TD ) and the surface ratio (DR _MD ×DR _TD ) when the polyester films were obtained were not within the ranges specified in the present invention, and therefore, the obtained polyester films did not satisfy the aforementioned characteristic values of the present invention. In addition, in Comparative Example 34, the intrinsic viscosity of the polyester resin constituting the polyester film was high, and the relaxation heat treatment temperature was too low, and therefore, the obtained polyester film did not satisfy the aforementioned characteristic values of the present invention. Therefore, the Erichsen value of the laminate obtained using the polyester films of these Comparative Examples 1 to 34 was low, and it did not have uniform ductility in all directions during cold forming. Therefore, during cold forming, aluminum foil breakage, delamination, pinholes, etc. occurred, and cold formability was poor.

Explanation of symbols

A Center Point

X Specimen for measuring stress during elongation in the reference direction (0° direction) of polyester film

Claims (8)

1. A polyester film, characterized in that, This refers to polyester films used for cold forming. For the stresses at which a film surface is set to 0° and then subjected to 5% elongation in four directions (45°, 90°, and 135° clockwise relative to that direction), the difference between the maximum and minimum stresses is less than 50 MPa. For the stress at 15% elongation in each of the four directions, the difference between the maximum and minimum values of these stresses is less than 70 MPa. The elastic modulus in the four directions is in the range of 2.0 to 3.5 GPa in any one direction. The average thickness in the four directions is less than 30 μm. 2. The polyester film according to claim 1, wherein the dry heat shrinkage rate in all four directions is within the range of 0 to 10%. 3. The polyester film as described in claim 1 or 2, characterized in that the standard deviation of the thickness in the four directions is less than 0.4 μm. 4. A laminate for cold forming, comprising the polyester film and metal foil as described in any one of claims 1 to 3. 5. The laminate as described in claim 4, wherein the thickness of the metal foil is 15–80 μm. 6. The laminate as described in claim 4 or 5 is formed by sequentially laminating a metal foil, an adhesive layer, and a polyester film as described in any one of claims 1 to 3. 7. A method for manufacturing a polyester film, which is a method for manufacturing the polyester film according to any one of claims 1 to 3, characterized in that an unstretched sheet is subjected to successive biaxial stretching in such a manner that the stretching ratio DR MD in the longitudinal direction _MD and the stretching ratio DR _TD in the transverse direction TD satisfy the following (a) and (b). The first stretching is performed in the longitudinal direction (MD) stretching of the unstretched sheet to obtain the first stretched film within a temperature range of 65–105°C. The second stretching is performed by stretching the first stretching film laterally in a TD manner within a temperature range of 100–150°C to obtain the second stretching film. The temperature is increased sequentially along the direction of membrane retrieval. 0.70≤DR _MD /DR _TD ≤0.90 (a), 12.5≤DR _MD ×DR _TD ≤15.5 (b). 8. The method for manufacturing a polyester film as described in claim 7, characterized in that the biaxially stretched film is heat-treated in a temperature range of 160 to 210°C.