Biaxially oriented film

Abstract

To provide a biaxially oriented polyester film having sufficiently low dielectric properties and high general applicability, and provide a biaxially oriented polyester film compatible with flexible displays and having excellent bending resistance.SOLUTION: A biaxially oriented film contains two or more polyesters, at least one of which is a polybutylene naphthalate resin (A), with the dielectric constant at 28 GHz of 3.13 or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a biaxially stretched film.

Background Art

[0002] In recent years, with the high performance and high functionality of electric and electronic devices, high-speed communication of information has been required. For example, in smartphones, with the start of high-speed communication services of 5G (fifth-generation mobile communication system), high-speed communication services are spreading not only in the consumer field but also in the industrial field (such as factories and vehicles such as automobiles). For 5G high-speed and large-capacity data communication, radio waves in the "millimeter wave" (wavelength 1 to 10 mm, frequency 30 to 300 GHz) band are used. The advantages of millimeter waves include that a large amount of data can be transmitted at once and the obtained images can be made high-definition.

[0003] On the other hand, when a high-frequency digital signal such as the millimeter wave flows through a circuit board, so-called "transmission loss" occurs, in which part of the transmitted digital signal is consumed as heat on the wiring of the circuit board, resulting in dielectric loss, and reaches the receiving side as an attenuated digital signal. Therefore, measures to reduce transmission loss are also required for the members used. The transmission loss is the sum of dielectric loss and conductor loss, and the dielectric loss α d is calculated from the following formula (1).

[0004]

Equation

[0005] Here, f is the frequency, c is the speed of light, ε r is the relative permittivity, and tan δ is the dielectric tangent.

[0006] For example, in an FPC (Flexible Printed Circuits), which is a flexible circuit board formed of a resin film and a copper foil, reduction of the dielectric loss α d is required for the resin film to reduce transmission loss. More specifically, εr There have been attempts to lower εr and tanδ, particularly to lower tanδ.

[0007] For reducing the dielectric constant and the dielectric loss tangent of resin films, various materials have been proposed. Among them, fluororesins typified by polytetrafluoroethylene (PTFE) have both low dielectric constant and low dielectric loss tangent, and are widely used as insulating layers for various electrical components handling high-frequency signals (Non-Patent Document 1). However, fluororesins have many limitations in terms of mechanical properties, processability, cost, etc., and there is a demand for a resin film with high versatility.

[0008] As a resin film with high versatility, a polyester film can be mentioned. Polyester films are excellent in heat resistance, weather resistance, mechanical strength, transparency, etc., and are also easily available in terms of price, so they are used in various applications such as packaging materials and optical applications, but their low dielectric properties have not been much studied.

[0009] For example, Patent Document 1 discloses a laminated biaxially stretched polyester film containing 5 to 45% by volume of voids inside as a polyester film having excellent low dielectric properties. By containing voids, voids (air) can be dispersed, achieving a lower dielectric constant and a lower dielectric loss tangent.

[0010] In recent years, with the miniaturization and weight reduction of electronic devices, etc., flexible substrates and flexible printed circuits tend to be used. Along with this trend, in the display field where the demand for flexibility is increasing, a film with excellent restorability and excellent repeated bend resistance is strongly demanded.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Non-Patent Documents

[0012] [Non-Patent Document 1] "Development Trends of High-Frequency Compatible Components and Applications to 5G and Millimeter-Wave Radars", Technical Information Association, Chapter 3, Section 2, pp. 77-84 "Development Trends of High-Speed, High-Frequency Compatible FPCs and Reduction of Transmission Losses" [Summary of the Invention] [Problems to be Solved by the Invention]

[0013] The cavity-containing biaxially stretched polyester film described in the above Patent Document 1 forms cavities by mixing different materials. However, in such cases, it is difficult to control the size of the cavities or the dispersion state of the different materials. For example, when the dispersion state of the different materials is insufficient, the desired low dielectric characteristics may not be obtained.

[0014] The problem to be solved by the present invention is to solve the above problems and provide a highly versatile biaxially stretched polyester film that has excellent low dielectric characteristics even without having cavities. Another object is to provide a biaxially stretched polyester film excellent in flex resistance that can also be applied to flexible display applications. [Means for Solving the Problems]

[0015] As a result of intensive studies to achieve the above problems, the present inventors have completed the present invention. In one aspect, the present invention has the following gist in [1] to

[19] . [1] A biaxially stretched film containing two or more polyesters, at least one of which is polybutylene naphthalate resin (A), and having a dielectric constant at 28 GHz of 3.13 or less. [2] The biaxially stretched film according to [1] above, having a dielectric tangent at 28 GHz of 0.0040 or less. [3] The average value of the hysteresis loss rate during a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the width direction (TD) is 45.0% or less, the biaxially stretched film according to the above [1] or [2]. [4] The average value of the residual strain during a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the width direction (TD) is 0.900% or less, the biaxially stretched film according to any one of the above [1] to [3]. [5] The biaxially stretched film according to any one of the above [1] to [4], comprising an amorphous polyester (B). [6] The amorphous polyester (B) has a higher glass transition temperature than the polybutylene naphthalate resin (A), the biaxially stretched film according to the above [5]. [7] The biaxially stretched film according to the above [5] or [6], containing the amorphous polyester (B) in a ratio of 1 part by mass or more and 100 parts by mass or less with respect to 100 parts by mass of the polybutylene naphthalate resin (A). [8] The amorphous polyester (B) is a polyarylate, the biaxially stretched film according to any one of the above [5] to [7]. [9] The biaxially stretched film according to any one of the above [1] to [8], for use in a high-speed communication circuit.

[10] The biaxially stretched film according to any one of the above [1] to [8], for use in a flexible display.

[11] The biaxially stretched film according to any one of the above [1] to [8], for use in a flexible display equipped with a high-speed communication circuit.

[12] A film with a cured resin layer on at least one surface layer of the biaxially stretched film according to any one of the above [1] to [8], the cured resin layer being formed from a resin composition containing 70% by mass or more of a crosslinking agent with respect to the non-volatile components.

[13] A metal laminated film having a metal layer on the cured resin layer of the film with a cured resin layer according to the above

[12] .

[14] The metal laminated film according to the above

[13] , wherein the metal layer is patterned.

[15] The metal laminated film according to

[13] or

[14] above, wherein the metal layer is made of copper or silver.

[16] The film with a cured resin layer according to

[12] above, which is for high-speed communication circuits.

[17] The film with a cured resin layer according to

[16] above, which is for transparent antenna films.

[18] The metal laminated film according to any one of

[13] to

[15] above, which is for high-speed communication circuits.

[19] The metal laminated film according to

[18] above, which is for transparent antenna films. [Advantages of the Invention]

[0016] The biaxially stretched film of the present invention has excellent low dielectric characteristics. Also, the biaxially stretched film of the present invention has excellent flex resistance. Therefore, the biaxially stretched film of the present invention can be suitably used not only for high-speed communication circuits and flexible displays but also for flexible displays equipped with high-speed communication circuits. [Brief Description of the Drawings]

[0017]

Figure 1

[0018] Hereinafter, the present invention will be described in detail. However, the present invention is not limited to the embodiments described below.

[0019] [[Biaxially Stretched Film]] The biaxially stretched film of the present invention (hereinafter also referred to as "the present film") contains two or more types of polyesters, at least one of which is polybutylene naphthalate (hereinafter also referred to as "PBN") resin (A), and is a biaxially stretched film having a dielectric constant at 28 GHz of 3.13 or less. Since this film is a biaxially stretched film, it can be made into a thin film. Moreover, by using a specific mixed polyester to adjust the dielectric constant within a specific range, it has excellent low dielectric characteristics. Furthermore, since this film controls the crystallization rate by using a specific mixed polyester, it is also excellent in extrusion molding and stretching processes that are difficult for the PBN resin (A) alone.

[0020] Although it is not clear about the mechanism by which this film containing the PBN resin (A) has excellent low dielectric characteristics, it is presumed that this is because the movement of dipoles is suppressed by aromatic stacking. Generally, when a dielectric is placed in an electric field, dipoles are oriented. Then, the dipoles rotate and reverse so as to follow the phase of the alternating electric field. Friction occurs along with this rotation and reversal movement of the dipoles, and dielectric loss occurs. Therefore, it is presumed that suppressing the movement of dipoles leads to excellent low dielectric characteristics.

[0021] Also, although it is not clear about the mechanism by which this film containing the PBN resin (A) has excellent flex resistance, it is presumed that this is because a crystal transition occurs due to the conformation of the alkyl chain portion.

[0022] <Polybutylene naphthalate resin (A)> The PBN resin (A) constituting this film is a polyester containing 2,6-naphthalenedicarboxylic acid units as the dicarboxylic acid component (a-1) and 1,4-butanediol units as the diol component (a-2). Preferably, it is mainly composed of 2,6-naphthalenedicarboxylic acid and 1,4-butanediol, that is, it preferably contains 50 mol% or more of 2,6-naphthalenedicarboxylic acid units as the dicarboxylic acid component (a-1) and 50 mol% or more of 1,4-butanediol units as the diol component (a-2). In particular, the PBN resin (A) used in the present invention preferably contains 2,6-naphthalenedicarboxylic acid units in an amount of 90 mol% or more as the dicarboxylic acid component (a-1) and 1,4-butanediol units in an amount of 90 mol% or more as the diol component (a-2).

[0023] The dicarboxylic acid component (a-1) constituting the PBN resin (A) contains 2,6-naphthalenedicarboxylic acid units. Among the dicarboxylic acid components (a-1), it is more preferable that the 2,6-naphthalenedicarboxylic acid units are 92 mol% or more, further preferably 94 mol% or more, particularly preferably 96 mol% or more, and most preferably 98 mol% or more. It is most preferable that all (100 mol%) of the dicarboxylic acid component (a-1) is 2,6-naphthalenedicarboxylic acid. By setting the 2,6-naphthalenedicarboxylic acid units to 90 mol% or more as the dicarboxylic acid component (a-1), the glass transition temperature and crystallinity of the PBN resin (A) are improved, and consequently, the heat resistance of this film is improved.

[0024] For the purpose of improving moldability and heat resistance, the PBN resin (A) may copolymerize an acid component other than 2,6-naphthalenedicarboxylic acid. Specifically, aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid; aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, etc. can be mentioned. Among these, from the viewpoint of moldability, isophthalic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, and 3,4-furandicarboxylic acid are preferable. These acid components can be used alone or in combination of two or more. In addition, the content of the acid component other than the 2,6-naphthalenedicarboxylic acid is preferably 10 mol% or less in all acid components including the 2,6-naphthalenedicarboxylic acid.

[0025] The diol component (a-2) constituting the PBN resin (A) contains a 1,4-butanediol unit. Among the diol components (a-2), it is more preferable that the 1,4-butanediol unit is 92 mol% or more, further preferably 94 mol% or more, particularly preferably 96 mol% or more, and most preferably 98 mol% or more. It is most preferable that all (100 mol%) of the diol component (a-2) is 1,4-butanediol. By setting the 1,4-butanediol unit to 90 mol% or more as the diol component (a-2), the compatibility with the polyester to be mixed is improved, and further, the glass transition temperature and crystallinity of the PBN resin (A) are improved, and thus the heat resistance of this film is improved.

[0026] The PBN resin (A) may copolymerize a diol component other than 1,4-butanediol for the purpose of improving moldability and heat resistance. Specifically, 1,2-propanediol, 1,3-propanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, ethylene glycol, diethylene glycol, triethylene glycol, polyalkylene glycol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, hydroquinone, bisphenol, spiroglycol, 2,2,4,4-tetramethylcyclobutane-1,3-diol, isosorbide, etc. may be mentioned. Among these, from the viewpoint of moldability, ethylene glycol, diethylene glycol, 1,3-propanediol, 1,3-cyclohexanedimethanol, and 1,4-cyclohexanedimethanol are preferable. These diol components can be used alone or in combination of two or more. In addition, the content of the diol component other than the 1,4-butanediol is preferably 10 mol% or less in all diol components including the 1,4-butanediol.

[0027] The glass transition temperature (Tg(A)) of the PBN resin (A) is preferably 50°C or higher and 130°C or lower, more preferably 58°C or higher and 125°C or lower, and even more preferably 65°C or higher and 120°C or lower. If the glass transition temperature (Tg(A)) is within such a range, the balance between heat resistance and extrusion moldability is excellent. Incidentally, the glass transition temperature (Tg(A)) can be measured by the method described in the examples.

[0028] <At least one kind of polyester> This film is composed of a mixed polyester containing at least one kind of polyester in addition to the PBN resin (A). The at least one kind of polyester is not particularly limited, but is preferably an amorphous polyester (B). Furthermore, it is more preferable that the glass transition temperature of the amorphous polyester (B) is higher than that of the PBN resin (A).

[0029] By mixing the amorphous polyester (B) with the PBN resin (A), the crystallization rate of the PBN resin (A) can be controlled, and a film excellent in extrusion moldability and stretching processability can be obtained. In addition, by adding the amorphous polyester (B) which is amorphous, the crystallinity of the PBN resin (A) itself can be relaxed, breakage during stretching can be suppressed, and the handling property during processing can be improved. Furthermore, since the glass transition temperature of the amorphous polyester (B) is higher than that of the PBN resin (A), a resin composition having a higher glass transition temperature than the PBN resin (A) alone can be obtained, and the heat resistance is good.

[0030] The amorphous polyester (B) preferably has a structure obtained by polycondensing at least three or more kinds of dicarboxylic acid components and diol components together. Examples of the dicarboxylic acid component include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,5-furandicarboxylic acid, 2,4-furandicarboxylic acid, 3,4-furandicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid; aliphatic dicarboxylic acids such as cyclohexanedicarboxylic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dimer acid; and oxycarboxylic acids such as p-oxybenzoic acid, etc. Examples of the diol component include ethylene glycol, diethylene glycol, propylene glycol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, 1,4-cyclohexanedimethanol, 1,5-pentanediol, neopentyl glycol, polytetramethylene ether glycol, dimer diol, polyalkylene glycol, 2,2-bis(4'-β-hydroxyethoxyphenyl)propane, isosorbide, spiroglycol, bisphenols (bisphenol compounds such as bisphenol A, bisphenol F, bisphenol S, bisphenol TMC, or their derivatives or their ethylene oxide adducts), etc. Among the amorphous polyesters (B), from the viewpoint of flex resistance, an amorphous copolyester containing at least one diol component selected from divalent phenols such as 1,4-cyclohexanedimethanol, polytetramethylene ether glycol, dimer diol, and bisphenols is preferred. Furthermore, from the viewpoints of heat resistance and flex resistance, wholly aromatic polyesters are more preferred, and among them, polyarylate (hereinafter sometimes referred to as "PAR") is even more preferred.

[0031] The above-mentioned PAR is a polycondensate of a dicarboxylic acid component (b-1) and a dihydric phenol component (b-2). The dicarboxylic acid component (b-1) constituting the PAR is not particularly limited as long as it is a divalent aromatic carboxylic acid, but among them, a mixture of a terephthalic acid component and an isophthalic acid component is preferably used. The mixing ratio (molar ratio) of the terephthalic acid component and the isophthalic acid component is preferably terephthalic acid / isophthalic acid = 99 / 1 to 1 / 99, more preferably 90 / 10 to 10 / 90, still more preferably 80 / 20 to 20 / 80, particularly preferably 70 / 30 to 30 / 70, and especially preferably 60 / 40 to 40 / 60. When the mixing ratio of terephthalic acid and isophthalic acid as the dicarboxylic acid component (b-1) is within the above range, the PAR is excellent in heat resistance and extrusion moldability.

[0032] The PAR may copolymerize an acid component other than terephthalic acid and isophthalic acid as the dicarboxylic acid component (b-1). Specifically, aromatic dicarboxylic acids such as phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, benzophenonedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 3,3'-diphenyldicarboxylic acid, and 4,4'-diphenyletherdicarboxylic acid are preferred. Also, in order not to impair the heat resistance of the PAR, the copolymerization ratio of the acid component other than terephthalic acid and isophthalic acid is preferably less than 10 mol%.

[0033] The dihydric phenol component (b-2) constituting the PAR is not particularly limited as long as it is a dihydric phenol, but it preferably contains either a bisphenol A (2,2-bis(4-hydroxyphenyl)propane) component, a bisphenol TMC (1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane) component, or both a bisphenol A and a bisphenol TMC.

[0034] Generally, by containing a bisphenol A component, the PAR becomes excellent in extrusion moldability (fluidity). On the other hand, by containing a bisphenol TMC component, it becomes a PAR having an improved glass transition temperature and excellent heat resistance. When it is desired to balance the extrusion moldability and heat resistance, it is preferable to use both a bisphenol A component and a bisphenol TMC component. In this case, the ratio (mol%) of the bisphenol A component to the bisphenol TMC component is preferably bisphenol A / bisphenol TMC = 99 / 1 to 1 / 99, more preferably 90 / 10 to 10 / 90, still more preferably 80 / 20 to 20 / 80, particularly preferably 70 / 30 to 30 / 70, and especially preferably 60 / 40 to 40 / 60. By setting the ratio of the bisphenol A component and the bisphenol TMC component within such a range, a PAR having an excellent balance between heat resistance and extrusion moldability is obtained.

[0035] The PAR may copolymerize bisphenols other than bisphenol A and bisphenol TMC as the divalent phenol component (b-2). Specifically, bisphenol AP (1,1-bis(4-hydroxyphenyl)-1-phenylethane), bisphenol AF (2,2-bis(4-hydroxyphenyl)hexafluoropropane), bisphenol B (2,2-bis(4-hydroxyphenyl)butane), bisphenol BP (bis(4-hydroxyphenyl)diphenylmethane), bisphenol C (2,2-bis(3-methyl-4-hydroxyphenyl)propane), bisphenol E (1,1-bis(4-hydroxyphenyl)ethane), bisphenol F (bis(4-hydroxyphenyl)methane), bisphenol G (2,2-bis(4-hydroxy-3-isopropylphenyl)propane), bisphenol M (1,3-bis(2-(4-hydroxyphenyl)-2-propyl)benzene), bisphenol S (bis(4-hydroxyphenyl)sulfone), bisphenol P (1,4-bis(2-(4-hydroxyphenyl)-2-propyl)benzene), bisphenol PH (5,5'-(1-methylethylidene)-bis[1,1'-(biphenyl)-2-ol]propane), bisphenol Z (1,1-bis(4-hydroxyphenyl)cyclohexane), etc. can be mentioned. The copolymerization ratio of the above compound is preferably less than 10 mol% so as not to impair the heat resistance of PAR.

[0036] In order to enhance the compatibility with the PBN resin (A), PAR preferably selects a mixture of a terephthalic acid component and an isophthalic acid component as the dicarboxylic acid component (b-1), and either a bisphenol A component or a bisphenol TMC component as the dihydric phenol component (b-2), or a mixture of bisphenol A and bisphenol TMC.

[0037] The PAR used in the present invention may be mixed with a polycarbonate resin for the purpose of improving the extrusion moldability. Since PAR and the polycarbonate resin are compatible, by mixing the polycarbonate resin with PAR, the glass transition temperature of PAR can be lowered while maintaining transparency and mechanical properties, and as a result, the extrusion moldability can be improved. When mixing PAR and polycarbonate, the mixing ratio (mass ratio) is preferably PAR / polycarbonate = 99 / 1 to 50 / 50, more preferably 98 / 2 to 60 / 40, still more preferably 97 / 3 to 70 / 30, and particularly preferably 96 / 4 to 80 / 20. If the mixing ratio of PAR and polycarbonate is within such a range, the melt moldability can be improved while maintaining the heat resistance of PAR.

[0038] The amorphous polyester (B) such as PAR used in the present invention has a higher glass transition temperature than the PBN resin (A), and the difference in their glass transition temperatures is preferably 60°C or more, more preferably 70°C or more, still more preferably 80°C or more, particularly preferably 90°C or more, and especially preferably 100°C or more. When the difference in the glass transition temperatures of the PBN resin (A) and the amorphous polyester (B) is within the above range, the glass transition temperature of the film becomes a suitable range, and a film excellent in heat resistance and extrusion moldability can be obtained. The upper limit of the difference in glass transition temperature between the PBN resin (A) and the amorphous polyester (B) is not particularly limited, but is usually 220°C or lower, preferably 190°C or lower.

[0039] The glass transition temperature (Tg(B)) of the amorphous polyester (B) such as PAR is preferably 130°C or higher and 280°C or lower, more preferably 140°C or higher and 260°C or lower, and even more preferably 150°C or higher and 240°C or lower. When the glass transition temperature (Tg(B)) of the amorphous polyester (B) is within such a range, the glass transition temperature of this film becomes a suitable range, and a film excellent in heat resistance and extrusion moldability can be obtained. The glass transition temperature (Tg(B)) can be measured by the method described in the examples.

[0040] This film preferably contains the amorphous polyester (B) such as PAR in a proportion of 1 part by mass or more and 100 parts by mass or less with respect to 100 parts by mass of the PBN resin (A). If the content ratio of the amorphous polyester (B) in this film is 1 part by mass or more, the crystallization rate can be slowed down, so that the extrusion moldability is improved. Also, if the content ratio of the amorphous polyester (B) is 1 part by mass or more, the heat resistance is improved. On the other hand, if the content ratio of the amorphous polyester (B) is 100 parts by mass or less, the low dielectric property and flex resistance of this film will be sufficient. From the above viewpoints, the content ratio of the amorphous polyester (B) is more preferably 5 parts by mass or more and 90 parts by mass or less, even more preferably 10 parts by mass or more and 80 parts by mass or less, and particularly preferably 20 parts by mass or more and 70 parts by mass or less with respect to 100 parts by mass of the PBN resin (A).

[0041] This film can be allowed to contain other resins other than the PBN resin (A) and at least one polyester within a range that does not impair the effects of the present invention. Examples of other resins include polystyrene resins, polyvinyl chloride resins, polyvinylidene chloride resins, chlorinated polyethylene resins, polycarbonate resins other than those described above, polyamide resins, polyacetal resins, acrylic resins, ethylene vinyl acetate copolymers, polymethylpentene resins, polyvinyl alcohol resins, cyclic olefin resins, polylactic acid resins, polybutylene succinate resins, polyacrylonitrile resins, polyethylene oxide resins, cellulose resins, polyimide resins, polyurethane resins, polyphenylene sulfide resins, polyphenylene ether resins, polyvinyl acetal resins, polybutadiene resins, polybutene resins, polyamideimide resins, polyamide bismaleimide resins, polyetherimide resins, polyetheretherketone resins, polyetherketone resins, polyethersulfone resins, polyketone resins, polysulfone resins, aramid resins, and fluorine-based resins, among others.

[0042] This film may be blended with particles for the main purposes of imparting slipperiness and preventing the occurrence of scratches in each process. The type of particles to be blended is not particularly limited as long as they can impart slipperiness. Specific 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, and organic particles such as acrylic resins, styrene resins, urea resins, phenolic resins, epoxy resins, and benzoguanamine resins. Furthermore, during the polymer production process of polyester or the like, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst can also be used.

[0043] In addition, the film can appropriately contain commonly blended additives. Examples of the additives include recycled resins generated from trimming losses such as ears, which are added for the purpose of improving and adjusting the moldability, productivity, and various physical properties of the porous film, pigments such as titanium oxide and carbon black, flame retardants, weather resistance stabilizers, heat resistance stabilizers, antistatic agents, melt viscosity improvers, crosslinking agents, lubricants, nucleating agents, plasticizers, anti-aging agents, antioxidants, light stabilizers, ultraviolet absorbers, neutralizing agents, anti-fogging agents, antiblocking agents, slip agents, and colorants.

[0044] <Method for manufacturing the film> The method for manufacturing the biaxially stretched film of the present invention will be described. However, the following description is an example of the method for manufacturing the present resin composition, and the film is not limited to the film manufactured by such a manufacturing method.

[0045] The method for manufacturing the film according to an example of the embodiment of the present invention is a manufacturing method in which a resin composition containing the PBN resin (A) and at least one polyester is formed into a film shape and biaxially stretched.

[0046] The method for kneading the PBN resin (A), at least one polyester, other resins, and additives to obtain a resin composition is not particularly limited. However, in order to obtain the resin composition as simply as possible, it is preferably manufactured by melt kneading using an extruder. In order to uniformly mix the raw materials constituting the resin composition, it is preferable to perform melt kneading using a co-rotating twin-screw extruder. The kneading temperature is preferably equal to or higher than the glass transition temperature of all polymers used, and for crystalline resins, it is preferably equal to or higher than the crystal melting temperature of the polymer. With respect to the glass transition temperature and crystal melting temperature of the polymers used, the higher the kneading temperature, the more likely it is that a partial transesterification reaction of the polymer will occur and the compatibility will be more easily improved. However, if the kneading temperature is too high, resin decomposition will occur, which is not preferable. Therefore, the kneading temperature is preferably 250°C or higher and 330°C or lower, more preferably 255°C or higher and 325°C or lower, still more preferably 260°C or higher and 320°C or lower, and particularly preferably 265°C or higher and 315°C or lower. Within this range of kneading temperature, compatibility and melt moldability can be improved without causing polymer decomposition.

[0047] The obtained resin composition can be formed by general molding methods, such as extrusion molding, injection molding, blow molding, vacuum molding, pressure-air molding, press molding, etc., to produce a biaxially stretched film. In each molding method, the apparatus and processing conditions are not particularly limited. This film is preferably manufactured, for example, by the following method.

[0048] A film that is substantially amorphous and not oriented (hereinafter also referred to as "unstretched film") is produced from the resin composition obtained by mixing by an extrusion method. The production of this unstretched film can adopt, for example, an extrusion method in which the above raw materials are melted by an extruder, extruded from a flat die or a circular die, and then rapidly cooled to form a flat or circular unstretched film. In this case, a laminated structure using a plurality of extruders may be used depending on the situation.

[0049] Next, the above unstretched film is stretched in at least one direction, usually 1.1 to 6.0 times, preferably 1.1 to 6.0 times in both the longitudinal direction (MD) and the width direction (TD) perpendicular thereto, of the film, from the viewpoints of stretching effect, film strength, etc.

[0050] As the biaxial stretching method, any of the conventionally known stretching methods such as tenter type sequential biaxial stretching, tenter type simultaneous biaxial stretching, tubular type simultaneous biaxial stretching, etc. can be adopted. For example, in the case of the tenter type sequential biaxial stretching method, an unstretched film is heated to a temperature range of Tg to Tg + 50 °C, where Tg is the glass transition temperature of the resin composition, and stretched 1.1 to 6.0 times in the longitudinal direction by a roll type longitudinal stretching machine, and then stretched 1.1 to 6.0 times in the transverse direction within the temperature range of Tg to Tg + 50 °C by a tenter type transverse stretching machine to produce it. Also, in the case of the tenter type simultaneous biaxial stretching or tubular type simultaneous biaxial stretching method, for example, it can be produced by simultaneously stretching 1.1 to 6.0 times in each axial direction longitudinally and transversely in the temperature range of Tg to Tg + 50 °C.

[0051] The biaxially stretched film stretched by the above method is subsequently heat-set. By heat-setting, dimensional stability at room temperature can be imparted. In this case, the treatment temperature is preferably selected in the range of the crystal melting temperature Tm - 1 to Tm - 50 °C of the resin composition. If the heat-setting temperature is within the above range, heat-setting is sufficiently performed, the stress during stretching is relaxed, excellent heat resistance and mechanical properties are obtained, and an excellent film without troubles such as breakage or whitening of the film surface can be obtained.

[0052] In the present invention, in order to relax the stress of crystallization shrinkage due to heat-setting, it is preferable to perform relaxation in the range of 0 to 15%, preferably 1 to 10% in the width direction during heat-setting. When relaxation is sufficiently performed and uniformly relaxed in the width direction of the film, the shrinkage rate in the width direction becomes uniform, and a film excellent in room temperature dimensional stability can be obtained. Also, since relaxation following the shrinkage of the film is performed, there is no sagging or fluttering of the film in the tenter, and no breakage of the film.

[0053] <Physical properties of this film> This film has a dielectric constant at 28 GHz of 3.13 or less, preferably 3.10 or less, more preferably 3.08 or less, and even more preferably 3.05 or less. If the dielectric constant at 28 GHz is 3.13 or less, it can be said that the film has excellent low dielectric characteristics. The dielectric constant can be adjusted by the type and content of the polyester to be mixed, the stretching conditions, and the like. The lower limit value is not particularly limited, but it may be 2.00 or more. Also, the dielectric constant was measured by the method described in the examples.

[0054] In addition, the dielectric tangent (tanδ) of this film at 28 GHz is preferably 0.0040 or less, more preferably 0.0038 or less, even more preferably 0.0036 or less, particularly preferably 0.0034 or less, and especially preferably 0.0032 or less. The lower limit value is not particularly limited, but it is about 0.0010. If the dielectric tangent (tanδ) at 28 GHz is 0.0040 or less, it can be said that the film has excellent low dielectric characteristics and can be suitably used for high-speed communication circuits. The dielectric tangent can be adjusted by the type and content of the polyester to be mixed, the stretching conditions, and the like. Note that the dielectric tangent was measured by the method described in the examples.

[0055] When this film is subjected to a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the width direction (TD) at 23°C, the average value of the hysteresis loss rate is preferably 45.0% or less, more preferably 43.0% or less, and even more preferably 41.0% or less. The lower limit value is not particularly limited, but it is 0.100% or more. By setting the hysteresis loss rate to 45.0% or less, the restoring force of the film increases, and the bending resistance (flexural resistance) of the film is maintained within a practical range.

[0056] Also, by taking the average value of the hysteresis loss rate in each of the longitudinal direction (MD) and the width direction (TD), it can be used as a characteristic index for the entire film. Note that the hysteresis loss rate can be adjusted according to the type of resin used, its content, stretching conditions, etc. The hysteresis loss rate of this film can be measured by the method described in the examples in accordance with JIS K 7312:1996. More specifically, when a graph of a stress-strain curve as shown in Fig. 1 is obtained by a tensile cycle test, the ratio of the area enclosed by abcef to the area of the whole (abcda) is defined as the hysteresis loss rate.

[0057] Furthermore, the difference in the hysteresis loss rate between the machine direction (MD) and the transverse direction (TD) of this film at 23°C is preferably 15.0% or less, more preferably 14.0% or less, still more preferably 13.0% or less, and particularly preferably 12.0% or less. When the difference in the hysteresis loss rate between the machine direction (MD) and the transverse direction (TD) is within the above numerical range, the bending resistance of the film, and thus the anisotropy of various properties of the film, becomes small, so that the film is not restricted by the direction. Therefore, when using the film as a member or in a manufacturing process such as secondary processing, there is no need to select a specific direction, which has the advantage of avoiding an increase in the workload of the operator. In addition, since defects only in a specific direction due to anisotropy are less likely to occur, the film also has excellent handling properties. Note that as a method for reducing the difference in the hysteresis loss rate between the machine direction (MD) and the transverse direction (TD), for example, by controlling the draw ratio in each direction and the crystallinity of the film, a desired value can be obtained.

[0058] In the present invention, the machine direction (MD) of the film refers to the direction in which the film advances in the film manufacturing process, that is, the winding direction of the film roll. Also, the transverse direction (TD) of the film refers to the direction parallel to the film surface and orthogonal to the machine direction, that is, the direction parallel to the central axis of the roll when the film is in a roll shape.

[0059] When a tensile cycle test is performed up to 5% tensile strain in the longitudinal direction (MD) and the transverse direction (TD) of this film at 23°C, the average value of the residual strain is preferably 0.900% or less, more preferably 0.850% or less, still more preferably 0.800% or less, and particularly preferably 0.750% or less. The lower limit is not particularly limited, but is 0.100% or more. By setting the residual strain to 0.900% or less, the restoring force of the film increases, and the bending resistance (flexural resistance) of the film is maintained within a practical range.

[0060] Also, by taking the average value of the residual strain in the longitudinal direction (MD) and the transverse direction (TD) respectively, it can be used as a characteristic index for the whole film. Note that the residual strain can be adjusted according to stretching conditions and the like. The residual strain of this film can be measured by the method described in the examples in accordance with JIS K 7312:1996. More specifically, when a graph of a stress-strain curve as shown in FIG. 1 is obtained by a tensile cycle test, the value of f is defined as the residual strain.

[0061] Furthermore, the difference in the residual strain in the longitudinal direction (MD) and the transverse direction (TD) of this film at 23°C is preferably 0.300% or less, more preferably 0.298% or less, and still more preferably 0.296% or less. When the difference in the residual strain in the longitudinal direction (MD) and the transverse direction (TD) is within the above numerical range, the bending resistance of the film, and thus the anisotropy of various properties of the film, becomes smaller, so that the film is not restricted by the direction. Therefore, when the film is used as a member or in a manufacturing process such as secondary processing, there is no need to select a specific direction, which has the advantage of avoiding an increase in the working load of the operator. Also, since defects only in a specific direction due to anisotropy are less likely to occur, the film has excellent handling properties.

[0062] The glass transition temperature (Tg) of this film is preferably 80°C or higher and 150°C or lower, more preferably 82°C or higher and 140°C or lower, and even more preferably 84°C or higher and 130°C or lower. If the glass transition temperature (Tg) of this film is 80°C or higher, it can be said to have good heat resistance. On the other hand, if the glass transition temperature (Tg) is 150°C or lower, it is also suitable for extrusion moldability and stretching processability. Note that the glass transition temperature (Tg) can be measured by the method described in the examples.

[0063] The crystal melting temperature (Tm) of this film is preferably 200°C or higher and 300°C or lower, more preferably 210°C or higher and 290°C or lower, and even more preferably 220°C or higher and 280°C or lower. If the crystal melting temperature (Tm) of this film is within such a range, this film has an excellent balance between heat resistance and extrusion moldability. Note that the crystal melting temperature (Tm) can be measured by the method described in the examples.

[0064] Regarding this film, when heat-treated at 150°C for 30 minutes, the heat shrinkage rate in both the longitudinal direction (MD) and the width direction (TD) is preferably 5.0% or less, more preferably 3.5% or less, and even more preferably 2.0% or less. If the heat shrinkage rate of this film is within such a range, it has sufficient heat resistance for use as a film. Note that the lower limit is not particularly limited, but it is 0.01% or more.

[0065] The haze of this film is preferably 3.0% or less, more preferably 2.0% or less, even more preferably 1.0% or less, and particularly preferably 0.5% or less. If the haze of this film is below the above upper limit value, it has good transparency. Note that the lower limit is not particularly limited, but it is 0.01% or more.

[0066] The thickness of this film is preferably 1 to 250 μm, more preferably 5 to 200 μm, and even more preferably 10 to 150 μm. By setting it to 1 μm or more, the film strength can be maintained within a practical range. By setting it to 250 μm or less, it can be suitably used for high-speed communication circuits and optical applications. Note that the thickness can be adjusted according to stretching conditions and the like.

[0067] The density of this film is preferably 3 1.320 g / cm or less, more preferably 3 1.310 g / cm or less, and even more preferably 3 1.300 g / cm or less. The lower limit is not particularly limited, but it is 3 1.100 g / cm or more. When the film density is within such a range, the dielectric tangent becomes good, and the dielectric loss proportional to the dielectric tangent can be effectively reduced. The relationship between the film density and the dielectric tangent is estimated as follows. The dielectric tangent is a parameter indicating the degree of energy loss generated by the vibration of dipoles when an alternating voltage is applied. When the film density is high, it is estimated that the vibrations of the dipoles cancel each other out, resulting in a small energy loss, that is, a small dielectric tangent.

[0068] <Use> Since the biaxially stretched film of the present invention has excellent low dielectric properties, it can be suitably used for high-speed communication circuits. Examples of high-speed communication circuit applications include FPC (Flexible Printed Circuits), which is a flexible circuit board formed of a resin film and a copper foil, and a transparent antenna film in which ultra-fine metal mesh wiring that is not visible is formed on a transparent film.

[0069] In addition, since the biaxially stretched film of the present invention is excellent in flexural resistance, it can also be suitably used for flexible displays (bendable, rollable, stretchable, foldable). Specifically, it is preferably used as a display component such as a front panel, a base film for a touch sensor, and a lower protection film. Note that the lower protection film is a film that protects the back side of the display device. The display may be used in a mobile phone, smartphone, digital camera, personal computer, etc. The type of the display is not particularly limited, and any of a liquid crystal display, a plasma display, an organic EL display, etc. may be used, and a touch panel type display may also be used. As the display, an organic EL display is preferable.

[0070] Furthermore, since the biaxially stretched film of the present invention is excellent in low dielectric characteristics and flexural resistance, it can be suitably used for a flexible display equipped with a high-speed communication circuit.

[0071] <<Film with a cured resin layer>> For the purpose of improving the adhesion to the metal layer, the biaxially stretched film of the present invention may have a cured resin layer on at least one surface layer of the biaxially stretched film as necessary, and the cured resin layer is preferably formed from a resin composition containing 70% by mass or more of a crosslinking agent with respect to the non-volatile component. Among them, it is more preferable to provide cured resin layers on both surface layers of the biaxially stretched film. Here, the biaxially stretched film having a cured resin layer is referred to as a film with a cured resin layer and is distinguished from the biaxially stretched film. Note that other layers may be provided between the biaxially stretched film and the cured resin layer.

[0072] As the crosslinking agent, various known crosslinking agents can be used, and examples thereof include oxazoline compounds, melamine compounds, epoxy compounds, isocyanate compounds, carbodiimide compounds, and silane coupling compounds. Among these, when a metal layer is provided on the cured resin layer, an oxazoline compound is preferably used from the viewpoint of improving the durable adhesion. Also, from the viewpoints of preventing the precipitation of oligomers on the film surface by heating and improving the durability of the cured resin layer, a melamine compound is preferably used.

[0073] (Oxazoline compound) An oxazoline compound is a compound having an oxazoline group in the molecule, and a polymer containing an oxazoline group is particularly preferable and can be prepared by polymerization of an addition-polymerizable oxazoline group-containing monomer alone or with other monomers. Examples of the addition-polymerizable oxazoline group-containing monomer include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline, etc., and a mixture of one or more of these can be used. Among these, 2-isopropenyl-2-oxazoline is industrially easily available and suitable. Other monomers are not limited as long as they are copolymerizable with the addition-polymerizable oxazoline group-containing monomer. For example, (meth)acrylic acid esters such as alkyl (meth)acrylates (the alkyl group includes methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group, etc.); unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, styrenesulfonic acid and its salts (sodium salt, potassium salt, ammonium salt, tertiary amine salt, etc.); unsaturated nitriles such as acrylonitrile, methacrylonitrile; unsaturated amides such as (meth)acrylamide, N-alkyl (meth)acrylamide and N,N-dialkyl (meth)acrylamide (the alkyl group includes methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, 2-ethylhexyl group, cyclohexyl group, etc.); vinyl esters such as vinyl acetate, vinyl propionate; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether; α-olefins such as ethylene, propylene; halogen-containing α,β-unsaturated monomers such as vinyl chloride, vinylidene chloride; α,β-unsaturated aromatic monomers such as styrene, α-methylstyrene, etc. One or more of these monomers can be used. From the viewpoint of improving the durability of the coating film, the amount of oxazoline groups in the oxazoline compound is preferably in the range of 0.5 to 10 mmol / g, more preferably 3 to 9 mmol / g, and even more preferably 5 to 8 mmol / g.

[0074] (Melamine compound) The melamine compound refers to a compound having a melamine skeleton in the compound. For example, an alkylolated melamine derivative, a compound obtained by reacting an alkylolated melamine derivative with an alcohol to be partially or completely etherified, and a mixture thereof can be used. As the alcohol used for etherification, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, isobutanol, etc. are preferably used. Moreover, as the melamine compound, it may be either a monomer or a multimer of dimer or higher, or a mixture thereof may be used. Furthermore, those obtained by co - condensing urea or the like with a part of melamine can also be used, and it is also possible to use a catalyst to increase the reactivity of the melamine compound.

[0075] (Epoxy compound) An epoxy compound is a compound having an epoxy group in the molecule. For example, condensates with hydroxyl groups or amino groups such as epichlorohydrin, ethylene glycol, polyethylene glycol, glycerin, polyglycerin, and bisphenol A, polyepoxy compounds, diepoxy compounds, monoepoxy compounds, glycidylamine compounds, etc. are available. Examples of polyepoxy compounds include sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, diglycerol polyglycidyl ether, triglycidyl tris(2 - hydroxyethyl) isocyanate, glycerol polyglycidyl ether, and trimethylolpropane polyglycidyl ether, etc. Examples of diepoxy compounds include neopentyl glycol diglycidyl ether, 1,6 - hexanediol diglycidyl ether, resorcin diglycidyl ether, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and polytetramethylene glycol diglycidyl ether, etc. Examples of monoepoxy compounds include allyl glycidyl ether, 2 - ethylhexyl glycidyl ether, and phenyl glycidyl ether, etc. Examples of glycidylamine compounds include N,N,N’,N’ - tetraglycidyl - m - xylylenediamine, 1,3 - bis(N,N - diglycidylamino) cyclohexane, etc.

[0076] (Isocyanate compound) An isocyanate compound refers to a compound having an isocyanate or an isocyanate derivative structure represented by blocked isocyanate. Examples of isocyanates include aromatic isocyanates such as tolylene diisocyanate, xylylene diisocyanate, methylene diphenyl diisocyanate, phenylene diisocyanate, and naphthalene diisocyanate; aliphatic isocyanates having an aromatic ring such as α,α,α’,α’-tetramethylxylylene diisocyanate; aliphatic isocyanates such as methylene diisocyanate, propylene diisocyanate, lysine diisocyanate, trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate; alicyclic isocyanates such as cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, methylene bis(4-cyclohexyl isocyanate), and isopropylidene dicyclohexyl diisocyanate. Also included are polymers and derivatives such as biuret compounds, isocyanurate compounds, uretdione compounds, and carbodiimide-modified products of these isocyanates. These may be used alone or in combination of multiple types. Among the above isocyanates, aliphatic isocyanates or alicyclic isocyanates are more preferable than aromatic isocyanates in order to avoid yellowing due to ultraviolet rays.

[0077] When used in the state of blocked isocyanate, examples of the blocking agent include phenolic compounds such as bisulfites, phenol, cresol and ethylphenol; alcohol compounds such as propylene glycol monomethyl ether, ethylene glycol, benzyl alcohol, methanol and ethanol; active methylene compounds such as methyl isobutanoyl acetate, dimethyl malonate, diethyl malonate, methyl acetoacetate, ethyl acetoacetate and acetylacetone; mercaptan compounds such as butyl mercaptan and dodecyl mercaptan; lactam compounds such as ε-caprolactam and δ-valerolactam; amine compounds such as diphenylaniline, aniline and ethyleneimine; acid amide compounds such as acetanilide and acetic acid amide; oxime compounds such as formaldehyde, acetaldoxime, acetone oxime, methyl ethyl ketone oxime and cyclohexanone oxime. These may be used alone or in combination of two or more kinds.

[0078] Also, the isocyanate compound may be used alone, or may be used as a mixture or a conjugate with various polymers. In the sense of improving the dispersibility and crosslinkability of the isocyanate compound, it is preferable to use a mixture or a conjugate with a polyester resin or a urethane resin.

[0079] (carbodiimide compound) The carbodiimide compound refers to a compound having a carbodiimide structure, which is a compound having one or more carbodiimide structures in the molecule. However, for better adhesion and the like, a polycarbodiimide compound having two or more carbodiimide structures in the molecule is more preferable.

[0080] The carbodiimide compound can be synthesized by a conventionally known technique. Generally, a condensation reaction of a diisocyanate compound is used. The diisocyanate compound is not particularly limited, and either an aromatic type or an aliphatic type can be used. Specifically, tolylene diisocyanate, xylene diisocyanate, diphenylmethane diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, trimethylhexamethylene diisocyanate, cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, dicyclohexyl diisocyanate, dicyclohexylmethane diisocyanate, etc. can be mentioned.

[0081] The content of the carbodiimide group contained in the carbodiimide compound is in terms of carbodiimide equivalent (the weight [g] of the carbodiimide compound for providing 1 mol of the carbodiimide group), usually in the range of 100 to 1000, preferably 250 to 700, more preferably 300 to 500. By using within the above range, the durability of the coating film is improved.

[0082] Furthermore, within a range not impairing the gist of the present invention, in order to improve the water solubility and water dispersibility of the polycarbodiimide compound, a surfactant may be added, or a hydrophilic monomer such as a polyalkylene oxide, a quaternary ammonium salt of a dialkylamino alcohol, and a hydroxyalkyl sulfonate may be added and used.

[0083] (Silane coupling compound) A silane coupling compound is an organosilicon compound having an organic functional group and a hydrolyzable group such as an alkoxy group in one molecule. For example, epoxy group-containing compounds such as 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; vinyl group-containing compounds such as vinyltrimethoxysilane, vinyltriethoxysilane; styryl group-containing compounds such as p-styryltrimethoxysilane, p-styryltriethoxysilane; (meth)acrylic group-containing compounds such as 3-(meth)acryloxypropyltrimethoxysilane, 3-(meth)acryloxypropyltriethoxysilane, 3-(meth)acryloxypropylmethyldimethoxysilane, 3-(meth)acryloxypropylmethyldiethoxysilane; amino group-containing compounds such as 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldiethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, N-phenyl-3-aminopropyltriethoxysilane; isocyanurate group-containing compounds such as tris(trimethoxysilylpropyl)isocyanurate, tris(triethoxysilylpropyl)isocyanurate; mercapto group-containing compounds such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldimethoxysilane, 3-mercaptopropylmethyldiethoxysilane, etc. can be mentioned.

[0084] These crosslinking agents may be used alone or in combination of two or more. However, by using two or more in combination, the adhesion to the metal layer provided on the cured resin layer and the prevention of oligomer precipitation after heating can be improved. Among them, in particular, a combination of an oxazoline compound that can improve the adhesion to the metal layer on the cured resin layer and a melamine compound that has good oligomer precipitation prevention properties after heating is preferable.

[0085] Also, in order to further improve the adhesion to the metal layer on the cured resin layer, it is more preferable to combine three or more crosslinking agents. As a combination of three or more crosslinking agents, it is suitable to select a melamine compound as one of the crosslinking agents. As the crosslinking agent to be combined with the melamine compound, an oxazoline compound and an epoxy compound, and a carbodiimide compound and an epoxy compound are more preferable.

[0086] When containing such a crosslinking agent, components for promoting crosslinking at the same time, such as a crosslinking catalyst, can be used in combination.

[0087] As a ratio with respect to all non-volatile components in the resin composition for forming the cured resin layer according to the present invention, the crosslinking agent is preferably 70% by mass or more, more preferably 80% by mass or more, and still more preferably 90% by mass or more. If it is 70% by mass or more, the adhesion to the metal layer provided on the cured resin layer and the prevention of oligomer precipitation after heating will be good.

[0088] The resin composition may contain a binder resin within a range not impairing the gist of the present invention in order to improve the appearance of the cured resin layer and the adhesion to the metal layer provided on the cured resin layer. As the binder resin, conventionally known ones can be used. However, from the viewpoint of improving the adhesion to the layer provided on the cured resin layer, it is preferable to use a polyester resin, an acrylic resin, or a urethane resin.

[0089] In addition, the resin composition may contain particles for the purpose of improving blocking and slipperiness. From the viewpoint of the transparency of the film, the average particle size is preferably 1.0 μm or less, more preferably 0.5 μm or less, and even more preferably 0.2 μm or less. On the other hand, in order to more effectively improve the slipperiness, it is preferably 0.01 μm or more, more preferably 0.03 μm or more, and particularly preferably in a range larger than the film thickness of the cured resin layer. Specific examples of the particles include silica, alumina, kaolin, calcium carbonate, organic particles, and the like.

[0090] Furthermore, within a range not impairing the gist of the present invention, a crosslinking catalyst, an antifoaming agent, a coating property improver, a thickener, an organic lubricant, an antistatic agent, an ultraviolet absorber, an antioxidant, a foaming agent, a dye, a pigment, etc. can be used in combination with the resin composition as needed.

[0091] The film thickness (after drying) of the cured resin layer is preferably 0.003 to 1.0 μm, more preferably 0.005 to 0.5 μm, and even more preferably 0.01 to 0.2 μm. If the film thickness is 1.0 μm or less, the appearance and blocking resistance of the cured resin layer are sufficient. On the other hand, if the film thickness is 0.003 μm or more, the amount of oligomer precipitation from the film is small, resulting in good performance.

[0092] The resin composition is generally preferably diluted with water, an organic solvent, or a mixture thereof, and the cured resin layer may be formed by coating a diluted solution of the resin composition on the surface of the film as a coating solution and then drying. As a method for applying the coating solution to the film, for example, conventionally known coating methods such as air doctor coating, blade coating, rod coating, bar coating, knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, kiss roll coating, cast coating, spray coating, curtain coating, calendar coating, extrusion coating, etc. can be used. In addition, in order to improve the coatability and adhesion of the coating agent to the film, chemical treatment, corona discharge treatment, plasma treatment, etc. may be performed on the film before coating.

[0093] The film surface can be coated as needed, and the cured resin layer may be formed by the coating. As the method for forming the cured resin layer, there are in-line coating and off-line coating, but it is preferably performed by in-line coating. In-line coating is a method of performing coating within the process of film production. Specifically, it is a method of performing coating at any stage from melting and extruding the raw material polyester to heat setting after stretching and then winding up. Usually, coating is performed on any of the unstretched sheet obtained by melting and quenching, the uniaxially stretched film, the biaxially stretched film before heat setting, and the biaxially stretched film after heat setting and before winding up. In particular, a method of coating a uniaxially stretched film stretched in the longitudinal direction (vertical direction) and then stretching it in the width direction (horizontal direction) is preferable. When providing the cured resin layer by in-line coating, it is preferable to use a coating solution prepared by adjusting the solid content concentration to about 0.1 to 50% by mass with the above-mentioned series of compounds as an aqueous solution or an aqueous dispersion. Also, within the range not impairing the gist of the present invention, a small amount of one or more organic solvents may be contained in the coating solution for the purpose of improving dispersibility in water, improving film-forming properties, etc.

[0094] Regarding the drying and curing conditions when forming the cured resin layer on the film, there is no particular limitation. For example, when providing the cured resin layer by off-line coating, it is preferably heat-treated at 80 to 200°C for 3 to 40 seconds, more preferably at 100 to 180°C for 3 to 40 seconds as a guide. On the other hand, when providing the cured resin layer by in-line coating, it is preferably heat-treated at 70 to 280°C for 3 to 200 seconds as a guide.

[0095] Further, regardless of offline coating or inline coating, heat treatment and irradiation with active energy rays such as ultraviolet rays may be used in combination as necessary.

[0096] In addition, the analysis of various compounds (components) in the cured resin layer can be performed by, for example, TOF-SIMS, ESCA, fluorescent X-rays, etc.

[0097] <Physical properties of the film with a cured resin layer> The cured resin layer according to the present invention is effective not only in improving the adhesion to the metal layer but also in preventing the precipitation of oligomers on the film surface due to heating. By reducing the precipitation of oligomers, it is possible to suppress the reduction in visibility due to the whitening of the film appearance caused by the precipitation and whitening of oligomers. In the film with a cured resin layer of the present invention, in the aspect where the cured resin layers are provided on both surface layers of the biaxially stretched film, the precipitation amount of oligomers (ester cyclic trimer) on the surface of at least one of the cured resin layers is preferably 2 0.50 mg / m or less, more preferably 2 0.45 mg / m or less, and even more preferably 2 0.40 mg / m or less. If the oligomer precipitation amount is 2 0.50 mg / m or less, it is preferable because there is no reduction in visibility due to the whitening of the film appearance caused by the precipitation and crystallization of oligomers on the surface, no occurrence of defects in post-processing, and no contamination in the process or of components. The lower limit is not particularly limited, but it is 2 0.01 mg / m or more. The oligomer precipitation amount is a value obtained by the method described in the examples.

[0098] <<Metal laminated film>> In the metal laminated film of the present invention, a metal layer may be provided on the cured resin layer. In addition, other layers may be provided between the cured resin layer and the metal layer. The metal layer is a layer containing a metal as a main component. Here, the main component means that the metal occupies 50% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more, and particularly preferably 90% by mass or more of the metal layer. Examples of the metal to be used include copper, copper alloy, silver, stainless steel, nickel, nickel alloy, aluminum, aluminum alloy, titanium, titanium alloy, etc. From the viewpoint of electromagnetic wave shielding characteristics, copper and silver are preferable, and from the viewpoint of flexibility, copper is more preferable.

[0099] From the viewpoint of maintaining the transparency of the biaxially stretched film and the film with a cured resin layer, the metal layer is preferably patterned, for example, in a mesh shape or a wire shape.

[0100] The thickness of the metal layer is preferably 2 to 30 μm, and more preferably 3 to 25 μm. When the thickness of the metal layer is equal to or greater than the above lower limit value, sufficient conductivity is ensured, and when it is equal to or less than the above upper limit value, the visibility can be reduced when the metal layer is provided. Incidentally, the thickness of the metal layer can be measured by a method of observing the cross section of the sample with an electron microscope.

[0101] <Use> The film with a cured resin layer and the metal laminated film of the present invention have excellent low dielectric characteristics without impairing transparency. Therefore, it can be suitably used for high-speed communication circuits. Examples of high-speed communication circuit applications include FPC (Flexible Printed Circuits), which is a flexible circuit board formed of a resin film and a copper foil, and a transparent antenna film in which ultrafine metal mesh wiring that is not visible is formed on a transparent film. Among them, it can be suitably used for a transparent antenna film that requires high transparency.

[0102] <<Explanation of terms, etc.>> In the present invention, when referring to a "film", it includes a "sheet", and when referring to a "sheet", it includes a "film". In the present invention, when described as "X to Y" (X and Y are arbitrary numbers), unless otherwise specified, it includes the meaning of "X or more and Y or less", as well as the meaning of "preferably greater than X" or "preferably less than Y". Also, when described as "X or more" (X is an arbitrary number), unless otherwise specified, it includes the meaning of "preferably greater than X", and when described as "Y or less" (Y is an arbitrary number), unless otherwise specified, it also includes the meaning of "preferably less than Y".

Example

[0103] Examples are shown below, but the present invention is not limited by these.

[0104] <Evaluation method> (1) Dielectric constant and dielectric tangent For the obtained film, in accordance with JIS R1641, the dielectric constant and dielectric tangent at frequencies of 10 GHz, 28 GHz, and 40 GHz were measured using a dielectric constant measurement system (cavity resonator (TE mode), control software, vector network analyzer MS46122B (manufactured by Anritsu Corporation)) manufactured by A&T Co., Ltd.

[0105] (2) Hysteresis loss rate In accordance with JIS K 7312:1996, the average value of the hysteresis loss rate at 23°C was determined by the following method. The measuring device used was a tensile testing machine (AG-1kNXplus tensile testing machine manufactured by Shimadzu Corporation). As the test piece, a rectangle with a length of 100 mm and a width of 10 mm in the measurement direction was cut out from this film. Both ends in the length direction of the test piece were chucked with a chuck distance of 50 mm, and after raising the strain to 5% at a crosshead speed of 0.5 mm / min, a stress-strain curve was obtained from a single-cycle tensile cycle test in which it was lowered to the initial position at the same speed. The stress-strain curve took a profile as shown in Fig. 1, and the hysteresis loss rate was calculated using the following formula (2) with the area A1 (abcda) of the curve obtained in the ascending operation and the area A2 (abcef), which is the difference in the area of the curve obtained in the descending operation, from the obtained stress-strain curve. The test was measured three times and the average value was obtained. The above tensile cycle test was carried out in the longitudinal direction (MD) and the width direction (TD) of the film respectively, and the average value was obtained. Hysteresis loss rate = (A2 / A1) × 100 ··· Formula (2)

[0106] (3) Residual strain In accordance with JIS K 7312:1996, the average value of the residual strain at 23°C was obtained by the following method. The measuring device used was a tensile testing machine (AG-1kNXplus tensile testing machine manufactured by Shimadzu Corporation). As the test piece, a rectangle with a length of 100 mm and a width of 10 mm in the measurement direction was cut out from this film. Both ends in the length direction of the test piece were chucked with a chuck distance of 50 mm, and after raising the strain to 5% at a crosshead speed of 0.5 mm / min, in the stress-strain curve obtained from a single-cycle tensile cycle test in which it was lowered to the initial position at the same speed, the strain at the point where the stress disappeared was taken as the residual strain. The test was measured three times and the average value was obtained. The above tensile cycle test was carried out in the longitudinal direction (MD) and the width direction (TD) of the film respectively, and the average value was obtained.

[0107] (4) Glass transition temperature For the obtained film, using DSC8000 (manufactured by PerkinElmer Japan), in accordance with JIS K7121 (2012), after heating to the melting temperature at a heating rate of 10 °C / min once, it was cooled at a cooling rate of 10 °C / min, and then the glass transition temperature in the heating process at a heating rate of 10 °C / min was measured.

[0108] (5) Crystallization melting temperature For the obtained film, using DSC8000 (manufactured by PerkinElmer Japan), in accordance with JIS K7121 (2012), the crystallization melting temperature in the heating process at a heating rate of 10 °C / min was measured.

[0109] (6) Formability In extrusion molding, when obtaining a pre-stretched sheet by cooling and solidifying with a casting roll, if a transparent film was obtained without crystallization and whitening, it was evaluated as ○, and if a film that crystallized and whitened was obtained, it was evaluated as ×.

[0110] (7) Thermal shrinkage rate This film was cut into a rectangle with a length of 120 mm and a width of 10 mm in the measurement direction, and the one with a mark made at a position 100 mm from the end was used. The ends of these test pieces were clamped with clips and suspended, and heated at 150 °C for 30 minutes. After cooling, the length from the end of the test piece to the mark was measured to obtain the thermal shrinkage rate. In addition, the measurement was performed in both the longitudinal direction (MD) and the width direction (TD).

[0111] (8) Haze Using a haze meter NDH-7000II (manufactured by Nippon Denshoku Industries Co., Ltd.), based on JIS K7136 (2000), the total light transmittance and diffuse transmittance were measured, and the haze was calculated by the following formula. [Haze] = ([Diffuse transmittance] / [Total light transmittance]) × 100

[0112] (9) Thickness Regarding the thickness of this film, it was measured at 5 unspecified points in the plane with a dial gauge of 1 / 1000 mm, and the average was taken as the thickness.

[0113] (10) Film density This film was cut into a square with a length of 100 mm and a width of 100 mm, the thickness of the film was measured, and the film volume was calculated by the following formula. Also, the weight of the film was measured with an electronic balance, and the film density was calculated from the film volume and weight by the following formula. [Film volume] = [Film area] × [Film thickness] [Film density] = [Film weight] / [Film volume]

[0114] (11) Amount of oligomer (ester cyclic trimer) deposited on the surface of the cured resin layer by heating Regarding the film with a cured resin layer obtained in the example, as a sample with a size of 300 mm in length and 225 mm in width, heat treatment was performed for 120 minutes in a hot air oven maintained at a predetermined temperature (180 °C). After the heat treatment, with the measurement surface as the inner surface, a box-shaped shape with an open upper part of 200 mm in length and 125 mm in width was fabricated. Next, 10 mL of DMF (dimethylformamide) was placed in the above box-shaped shape and left for 3 minutes, then the DMF was recovered and supplied to liquid chromatography (manufactured by Shimadzu Corporation: LC-7A, mobile phase A: acetonitrile, mobile phase B: 2% acetic acid aqueous solution, column: "MCI GEL ODS 1HU" manufactured by Mitsubishi Chemical Corporation, column temperature: 40 °C, flow rate: 1 mL / min, detection wavelength: 254 nm) to determine the amount of ester cyclic trimer in the DMF. This value was divided by the film area in contact with the DMF to obtain the amount of oligomer (ester cyclic trimer) on the surface of the cured resin layer (mg / m 2 ). The ester cyclic trimer in the DMF was determined from the peak area ratio of the standard sample peak area and the measurement sample peak area (absolute calibration curve method). Note that the standard sample was prepared by accurately weighing the pre-fractionated ester cyclic trimer and dissolving it in accurately weighed DMF.

[0115] [Polybutylene naphthalate resin (A)] As the PBN resin (A), a homopolymer PBN with a dicarboxylic acid component (a-1): 2,6-naphthalenedicarboxylic acid = 100 mol% and a diol component (a-2): 1,4-butanediol = 100 mol% was used. The glass transition temperature (Tg(A)) of the PBN resin was 77°C.

[0116] [Polyarylate resin] As at least one kind of polyester, an amorphous polyester (B), a PAR with a dicarboxylic acid component (b-1): terephthalic acid / isophthalic acid (molar ratio) = 50 / 50 and a dihydric phenol component (b-2): bisphenol A 100 mol% was used. The glass transition temperature (Tg(B)) of the PAR was 193°C. Hereinafter, PAR is referred to as (B).

[0117] [Polyethylene terephthalate (PET) film] As the PET film, a biaxially stretched PET film with a thickness of 50 μm ("Diafoil T100-50" manufactured by Mitsubishi Chemical Corporation) was used.

[0118] [Polyethylene naphthalate (PEN) resin] As the PEN resin, a homopolymer PEN with a dicarboxylic acid component: 2,6-naphthalenedicarboxylic acid = 100 mol% and a diol component: ethylene glycol 100 mol% was used.

[0119] [Cured resin layer] The following was used as the resin composition for forming the cured resin layer. (A1): Hexamethoxymethylol melamine (A2): Epocros (manufactured by Nippon Shokubai Co., Ltd.), an oxazoline compound, with an oxazoline group content of 7.7 mmol / g (A3): Polyglycerol polyglycidyl ether (B1): Silica particles with an average particle size of 0.07 μm Note that the composition of the coating solution used in the examples is as shown in Table 1.

[0120] [Table 1]

[0121] (Example 1) Pellet-shaped (B) was added at a ratio of 20% by mass to pellet-shaped (A) (25 parts by mass of (B) with respect to 100 parts by mass of (A)), and after dry blending, it was melt-kneaded using a Φ40 mm twin-screw extruder set at 285 °C, extruded as a film from a T-die with a gap of 1.0 mm, taken up by a casting roll at 65 °C, cooled and solidified to obtain a film-like material (cast film) with a thickness of approximately 450 μm. Subsequently, the obtained cast film was passed through a longitudinal stretching machine and heated with an infrared heater (temperature near the heater: 133 °C) to perform 2.8-fold stretching in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film was passed through a transverse stretching machine (tenter), and 4.3-fold stretching was performed in the width direction (TD) at a preheating temperature of 110 °C, a stretching temperature of 115 °C, and a heat setting temperature of 190 °C. Then, while heat setting in the tenter, 5% relaxation treatment of the film was performed in the width direction (TD). Table 2 shows the results of measurements on the obtained film.

[0122] (Example 2) Pellet-shaped (B) was added at a ratio of 30% by mass to pellet-shaped (A) (42.9 parts by mass of (B) with respect to 100 parts by mass of (A)), and after dry blending, it was melt-kneaded using a Φ40 mm twin-screw extruder set at 285 °C, extruded as a film from a T-die with a gap of 1.0 mm, taken up by a casting roll at 90 °C, cooled and solidified to obtain a film-like material (cast film) with a thickness of approximately 450 μm. Subsequently, the obtained cast film was passed through a longitudinal stretching machine and heated with an infrared heater (temperature near the heater: 133 °C) to perform 2.8-fold stretching in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film was passed through a transverse stretching machine (tenter), and 4.3-fold stretching was performed in the width direction (TD) at a preheating temperature of 110 °C, a stretching temperature of 115 °C, and a heat setting temperature of 190 °C. Then, while heat setting in the tenter, 5% relaxation treatment of the film was performed in the width direction (TD). Table 2 shows the results of measurements on the obtained film.

[0123] (Example 3) Pelletized (A) was added at a ratio of 40% by mass of pelletized (B) with respect to 60% by mass ((B) was 66.7 parts by mass with respect to 100 parts by mass of (A)). After dry blending, it was melt-kneaded using a Φ40 mm twin-screw extruder set at 285°C, extruded as a film from within a T-die with a gap of 1.0 mm, taken up by a casting roll at 95°C, cooled and solidified to obtain a film-like material (cast film) with a thickness of approximately 450 μm. Subsequently, the obtained cast film was passed through a longitudinal stretching machine and heated with an infrared heater (temperature near the heater: 145°C) to perform 2.8-fold stretching in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film was passed through a transverse stretching machine (tenter), and 4.1-fold stretching was performed in the width direction (TD) at a preheating temperature of 120°C, a stretching temperature of 125°C, and a heat setting temperature of 190°C. Thereafter, while heat setting within the tenter, 5% relaxation treatment of the film was performed in the width direction (TD). Table 2 shows the results of measurements conducted on the obtained film.

[0124] (Comparative Example 1) When attempting to form a cast film in the same manner as in Example 1 except that 100% by mass of pelletized (A) was used and the temperature of the casting roll was set at 75°C, whitening thought to be crystallization occurred, resulting in poor adhesion to the casting roll, a film with a poor appearance and uneven thickness, and a stretchable amorphous film could not be obtained. Therefore, the formability in Table 2 was marked as ×.

[0125] (Comparative Example 2) Table 2 shows the results of evaluations conducted on a biaxially stretched PET film.

[0126] (Comparative Example 3) 100% by mass of pelletized PEN resin was used, melt-kneaded using a Φ40 mm twin-screw extruder set at 285°C, extruded as a film from within a T-die with a gap of 1.0 mm, taken up by a casting roll at 110°C, cooled and solidified to obtain a film-like material (cast film) with a thickness of approximately 450 μm. Subsequently, the obtained cast film was passed through a longitudinal stretching machine and heated with an infrared heater (temperature near the heater: 175°C) to perform 2.6-fold stretching in the longitudinal direction (MD). Subsequently, the obtained longitudinally stretched film was passed through a transverse stretching machine (tenter), and 3.8-fold stretching was performed in the width direction (TD) at a preheating temperature of 125°C, a stretching temperature of 130°C, and a heat setting temperature of 190°C. Then, while heat setting in the tenter, 5% relaxation treatment of the film was performed in the width direction (TD). Table 2 shows the results of measurements conducted on the obtained film.

[0127]

Table 2

[0128] Furthermore, Table 3 below shows the dielectric tangent and dielectric constant at 10 GHz and 40 GHz for the films of Examples 1 to 3, Comparative Examples 2 and 3.

[0129]

Table 3

[0130] As is clear from Examples 1 to 3 above, the biaxially stretched film of the present invention has low dielectric characteristics in the high-frequency band, so it has low transmission loss and is applicable to high-speed large-capacity data communication of 5G. Also, since the hysteresis loss rate and residual strain are small, it has excellent restorability and repeated bending resistance, and can be suitably used for applications that require flexibility. Furthermore, since the haze is small and the transparency is excellent, it is also useful as an optical application. In addition, the biaxially stretched film of the present invention also has sufficient heat resistance. In addition to these characteristics, since it is a highly versatile polyester film, it is also advantageous in terms of cost and is easy to manufacture, so it can be applied to various applications.

[0131] (Example 4) After the longitudinal direction (MD) stretching of Example 2 and before the transverse direction (TD) stretching, the coating liquid was applied to both sides of the uniaxially stretched film to obtain a biaxially stretched film (film with a cured resin layer) having a cured resin layer with a film thickness (after drying) of 0.04 μm. In Example 4, the film was formed so that the thickness of the biaxially stretched film was 125 μm. The evaluation results of the amount of oligomer precipitation on the surface of the cured resin layer by heating for the obtained film with a cured resin layer are shown in Table 4 below.

[0132]

Table 4

[0133] In Table 4, 0.38 / 0.45 means that the amount of oligomer precipitation on one side is 0.38 mg / m 2 , and the amount of oligomer precipitation on the other side is 0.45 mg / m 2 .

[0134] In the film with a cured resin layer of the present invention, from the results of Example 4, by having a cured resin layer on at least one surface layer of the biaxially stretched film, the amount of oligomer precipitation on the surface of the cured resin layer can be reduced. Therefore, the film with a cured resin layer of the present invention has good visibility and can be suitably used as a transparent antenna that requires particularly high transparency.

Industrial Applicability

[0135] Since the biaxially stretched film of the present invention has excellent low dielectric characteristics, it can be suitably used for high-speed communication circuits, particularly for FPCs, transparent antenna films, etc. Further, since it has excellent flex resistance, it can also be suitably used for flexible displays. Specifically, it is useful for liquid crystal displays, plasma displays, organic EL displays, etc. of mobile phones, smartphones, digital cameras, personal computers, etc., and also for touch panel type displays. In particular, since it is excellent in low dielectric characteristics and flex resistance, it can be suitably used for flexible displays equipped with high-speed communication circuits.

Claims

1. comprising two or more polyesters, wherein at least one of them is polybutylene naphthalate resin (A), containing amorphous polyester (B) in a proportion of 1 part by mass or more with respect to 100 parts by mass of the polybutylene naphthalate resin (A), a biaxially stretched film having a dielectric constant at 28 GHz of 3.13 or less.

2. The biaxially stretched film according to claim 1, having a dielectric tangent at 28 GHz of 0.0040 or less.

3. The biaxially stretched film according to claim 1 or 2, wherein the average value of the hysteresis loss rate in a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the width direction (TD) is 45.0% or less.

4. The biaxially stretched film according to any one of claims 1 to 3, wherein the average value of the residual strain in a tensile cycle test up to 5% tensile strain in each of the longitudinal direction (MD) and the width direction (TD) is 0.900% or less.

5. The biaxially stretched film according to any one of claims 1 to 4, having a crystal melting temperature of 200°C or higher and 300°C or lower.

6. The biaxially stretched film according to any one of claims 1 to 5, wherein the amorphous polyester (B) has a glass transition temperature higher than that of the polybutylene naphthalate resin (A).

7. The biaxially stretched film according to any one of claims 1 to 6, containing the amorphous polyester (B) in a proportion of 1 part by mass or more and 100 parts by mass or less with respect to 100 parts by mass of the polybutylene naphthalate resin (A).

8. The biaxially stretched film according to any one of claims 1 to 7, wherein the amorphous polyester (B) is polyarylate.

9. The biaxially stretched film according to any one of claims 1 to 8, having a density of 1.100 g / cm3 or more and 1.320 g / cm3 or less.

10. The biaxially stretched film according to any one of claims 1 to 9, for use in a high-speed communication circuit.

11. The biaxially stretched film according to any one of claims 1 to 9, for use in a flexible display.

12. The biaxially stretched film according to any one of claims 1 to 9, for use in a flexible display equipped with a high-speed communication circuit.

13. A film with a cured resin layer, having the cured resin layer on at least one surface layer of the biaxially stretched film according to any one of claims 1 to 9, wherein the cured resin layer is formed from a resin composition containing 70% by mass or more of a crosslinking agent based on the non-volatile components.

14. A metal laminated film comprising a metal layer on the cured resin layer of the film with a cured resin layer according to claim 13.

15. The metal laminated film according to claim 14, wherein the metal layer is patterned.

16. The metal laminated film according to claim 14 or 15, wherein the metal layer is made of copper or silver.

17. The film with a cured resin layer according to claim 13, which is for a high-speed communication circuit.

18. The film with a cured resin layer according to claim 17, which is for a transparent antenna film.

19. The metal laminated film according to any one of claims 14 to 16, which is for a high-speed communication circuit.

20. The metal laminated film according to claim 19, which is for a transparent antenna film.

Images