CN114423601A

CN114423601A - Polyester film

Info

Publication number: CN114423601A
Application number: CN202080066434.5A
Authority: CN
Inventors: 川浪敬太; 东条好晃
Original assignee: Mitsubishi Chemical Corp
Current assignee: Mitsubishi Chemical Corp
Priority date: 2019-09-24
Filing date: 2020-09-10
Publication date: 2022-04-29
Also published as: TW202120335A; WO2021060004A1; KR20220063766A; JPWO2021060004A1

Abstract

A polyester film, comprising: the polyester film comprises a polyester layer (A) containing a black pigment and a polyester layer (B) on at least one side thereof, wherein the polyester layer (B) contains substantially no particles or particles (X) having an average particle diameter of 1.0 [ mu ] m or less, the polyester film has a transmission concentration (OD value) of 0.4 or more, and the difference in gloss (Gs60 DEG) between the front and back surfaces of the film is 3.0 or less. There may be provided: a polyester film having high light-shielding properties, excellent surface smoothness and a small difference in gloss on the front and back surfaces.

Description

Polyester film

Technical Field

The present invention relates to a polyester film, for example, a polyester film used for applications requiring light-shielding properties.

Background

In the past, a metal case has been used for small electronic devices such as smartphones and personal computers, but from the viewpoint of weight reduction and design of the electronic devices, an operation of making at least a part of the metal case glass has been attempted. If a part of the case is made of glass, it is considered that a resin film having a light-shielding property is laminated on the glass so that the inside is not visible. Further, a resin film having a light-shielding property may be used around a display or the like to make the inside of the device invisible.

Resin films are known to impart light-blocking properties by compounding pigments. For example, patent document 1 describes a film containing a black pigment, wherein the film has a surface roughness (SRa) of 100 to 150nm on at least one surface and a breaking strength in the film width direction of 200MPa or more.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2018-130950

Disclosure of Invention

Problems to be solved by the invention

As in patent document 1, when a black pigment such as carbon black is blended with a resin, a sufficient light-shielding property can be imparted to the film, but on the other hand, since carbon black is aggregated in the resin, the roughness of the film surface becomes large, and the surface smoothness of the film is impaired.

In this case, when the metal layer is formed by sputtering or the like for the purpose of imparting metallic luster to the resin film, there is a problem that unevenness is emphasized and design properties are impaired. In addition, in the case of using another pigment, the surface smoothness of the film may be impaired in the same manner.

Further, if there is a difference in surface smoothness between the front and back surfaces of the polyester film, distortion of a reflection image may occur when the metal layer is provided, and good visibility may not be obtained.

The present invention has been made in view of the above-described conventional problems, and an object thereof is to provide: a polyester film having high light-shielding properties, excellent surface smoothness and a small difference in gloss on the front and back surfaces.

Means for solving the problems

The present inventors have conducted intensive studies and, as a result, have found that: by providing a polyester layer (B) having a specific structure on the surface of the polyester layer (a) containing a pigment and setting the transmission density to a predetermined value or more, a polyester film excellent in light-shielding properties and surface smoothness can be obtained, and the following invention has been completed.

Namely, the present invention relates to the following [1] to [16 ].

[1] A polyester film, comprising: a polyester layer (A) containing a black pigment and a polyester layer (B) on at least one side thereof,

the polyester layer (B) contains substantially no particles or particles (X) having an average particle diameter of 1.0 μm or less,

the polyester film has a transmission density (OD value) of 0.4 or more and a difference in gloss (Gs60 DEG) between the front and back surfaces of the film of 3.0 or less.

[2] The polyester film according to the above [1], wherein the black pigment is carbon black, and the content of the carbon black in the polyester layer (A) is 0.1 to 5.0% by mass.

[3] The polyester film according to the above [1] or [2], wherein the content of the particles (X) in the polyester layer (B) is 0.01 to 1.0% by mass.

[4] The polyester film according to any one of the above [1] to [3], wherein the ratio of the thickness of the polyester layer (B) to the thickness of the polyester layer (A), (the thickness of B/(the thickness of A)), is 0.025 to 0.25.

[5] The polyester film according to any one of the above [1] to [4], wherein the arithmetic average height (Sa) of the surface of the polyester film on the side of the polyester layer (B) is 20.0nm or less.

[6] The polyester film according to any one of the above [1] to [5], wherein the maximum height (Sp) of the surface of the polyester film on the side of the polyester layer (B) is 1000nm or less.

[7] The polyester film according to any one of the above [1] to [6], wherein the polyester layer (A) has a polyester layer (B) on both surfaces thereof.

[8] The polyester film according to any one of the above [1] to [7], wherein a cured resin layer is provided on at least one surface of the polyester layer (B).

[9] A polyester film with a metal layer, which is laminated with a metal layer on the cured resin layer of the polyester film according to [8 ].

[10] A polyester film with a metal layer, wherein a metal layer is directly laminated on at least one surface of the polyester film according to any one of the above items [1] to [7], said surface being on the polyester layer (B) side.

[11] A film laminate comprising an optical member laminated on the surface of the metal layer of the polyester film with a metal layer according to [9] or [10] with an adhesive layer interposed therebetween.

[12] The film laminate according to [11], wherein the optical member is a resin film or a glass substrate.

[13] The thin film laminate according to [11] or [12], which has a total thickness of 150 μm or less.

[14] The polyester film according to any one of the above [1] to [8], which is used for a communication terminal.

[15] The polyester film with a metal layer according to the above [9] or [10], which is used for a communication terminal.

[16] The thin film laminate according to any one of the above [11] to [13], which is used for a communication terminal.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, there can be provided: a polyester film having high light-shielding properties and excellent surface smoothness. For example, the present invention is suitable as a member for a terminal device (a smartphone, an I-Pad, or the like) having a communication function.

Detailed Description

[ polyester film ]

The polyester film of the present invention is a polyester film comprising a polyester layer (a) containing a black pigment and a polyester layer (B) on at least one side thereof. In the polyester film, the polyester layer (B) contains substantially no particles or particles having an average particle diameter of 1.0 μm or less (hereinafter, also referred to as particles (X)), and the transmission concentration (OD value) of the polyester film is 0.4 or more.

In the polyester layer (a) of the polyester film, although the pigment may be aggregated, the polyester layer (B) containing substantially no particles or particles having a small average particle diameter is provided as a surface layer, whereby the aggregation of the pigment can prevent the surface smoothness of the polyester film from being impaired. Therefore, in the case where the metal is deposited by sputtering, for example, the polyester film of the present invention can impart excellent metallic luster without emphasizing the unevenness of the film surface.

Further, the polyester film of the present invention contains a large amount of pigment or the like and has a transmission concentration of 0.4 or more, and therefore has high light-shielding properties, and when used in a housing of an electronic device, the interior of the housing cannot be seen through, and therefore, the design is improved.

Hereinafter, the structure of the present invention will be described in detail.

< polyester >

In the present invention, the polyester which is a raw material of the polyester layer (a) and the polyester layer (B) is not particularly limited, and a polycondensation polymer of a dicarboxylic acid and a diol is preferable, and an aromatic dicarboxylic acid is preferable as the dicarboxylic acid, and an aliphatic diol is preferable as the diol.

Examples of the aromatic dicarboxylic acid include terephthalic acid, isophthalic acid, phthalic acid, diphenyldicarboxylic acid, diphenyl ether-dicarboxylic acid, diphenylsulfone-dicarboxylic acid, diphenylketone-dicarboxylic acid, 2, 6-naphthalenedicarboxylic acid, 1, 4-naphthalenedicarboxylic acid, and 2, 7-naphthalenedicarboxylic acid. Among them, terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, and 4, 4' -biphenyldicarboxylic acid are preferable, and terephthalic acid is more preferable.

Examples of the aliphatic diol include aliphatic diols having a linear or branched structure such as ethylene glycol, 2-butene-1, 4-diol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, methylpentanediol, and diethylene glycol; alicyclic diols such as cyclohexanedimethanol, isosorbide, spiroglycol, 2,4, 4-tetramethyl-1, 3-cyclobutanediol, norbornene dimethanol, and tricyclodecane dimethanol. Among them, ethylene glycol, neopentyl glycol and cyclohexanedimethanol are preferable, and ethylene glycol is more preferable.

As the polyester used in the present invention, a polyester resin in which 50 mol% or more of the dicarboxylic acid units are structural units derived from terephthalic acid and 50 mol% or more of the diol units are structural units derived from ethylene glycol, that is, polyethylene terephthalate is preferably used. When the polyester resin is polyethylene terephthalate, the polyester resin is less likely to be in an amorphous state, and the transparency and light resistance are excellent.

In the polyethylene terephthalate, preferably 70 mol% or more, more preferably 90 mol% or more of the dicarboxylic acid units are structural units derived from terephthalic acid, and preferably 70 mol% or more, more preferably 90 mol% or more of the diol units are structural units derived from ethylene glycol.

In the case of using polyethylene terephthalate as the polyester resin, the polyester resin may be composed of polyethylene terephthalate alone, and may contain polyester resins other than polyethylene terephthalate in addition to polyethylene terephthalate.

In the present invention, the amount of polyethylene terephthalate in the total amount of the polyester resin is preferably 80 to 100% by mass, and more preferably 90 to 100% by mass.

The polyethylene terephthalate used in the present invention is preferably composed only of structural units derived from terephthalic acid and ethylene glycol, but may also contain structural units derived from a bifunctional compound other than terephthalic acid and ethylene glycol.

Examples of the bifunctional compound include the aromatic dicarboxylic acid and the aliphatic diol other than terephthalic acid and ethylene glycol, and bifunctional compounds other than aromatic dicarboxylic acid and aliphatic diol.

Examples of the bifunctional compound other than the aromatic dicarboxylic acid and the aliphatic diol include a linear or branched aliphatic bifunctional compound, and specifically include aliphatic dicarboxylic acids such as malonic acid, succinic acid, adipic acid, azelaic acid, and sebacic acid; aliphatic hydroxycarboxylic acids such as 10-hydroxyoctadecanoic acid, lactic acid, hydroxyacrylic acid, 2-hydroxy-2-methylpropanoic acid, and hydroxybutyric acid.

Examples of the alicyclic dicarboxylic acid include cyclohexane dicarboxylic acid, norbornene dicarboxylic acid, tricyclodecane dicarboxylic acid, and the like; alicyclic difunctional compounds such as alicyclic hydroxycarboxylic acids including hydroxymethylcyclohexane carboxylic acid, hydroxymethylnorbornene carboxylic acid and hydroxymethyltricyclodecane carboxylic acid.

Further, for example, aromatic hydroxycarboxylic acids such as hydroxybenzoic acid, hydroxytoluene carboxylic acid, hydroxynaphthoic acid, 3- (hydroxyphenyl) propionic acid, hydroxyphenylacetic acid, and 3-hydroxy-3-phenylpropionic acid; and aromatic diols such as bisphenol compounds and hydroquinone compounds.

The structural unit derived from the bifunctional compound is preferably 20 mol% or less, more preferably 10 mol% or less, relative to the total mol of all the structural units constituting the polyester resin.

When the polyethylene terephthalate used in the present invention contains a structural unit derived from an aromatic dicarboxylic acid other than terephthalic acid, the aromatic dicarboxylic acid is preferably 1 or 2 or more selected from isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, and 4, 4' -biphenyldicarboxylic acid. They are low in cost, and a copolyester resin containing 1 of them can be easily produced.

When the polyethylene terephthalate contains a structural unit derived from these aromatic dicarboxylic acids, the proportion of the constituent derived from the aromatic dicarboxylic acid is preferably 1 to 20 mol%, more preferably 1 to 10 mol%, of the dicarboxylic acid unit.

When the polyethylene terephthalate used in the present invention contains a structural unit derived from an aliphatic diol other than ethylene glycol, the aliphatic diol is preferably diethylene glycol. The proportion of the constituent derived from diethylene glycol is preferably 0.1 to 10 mol%, more preferably 0.4 to 5 mol%, based on the glycol unit.

The intrinsic viscosity of the polyester resin used in the present invention is not particularly limited, but is preferably 0.45 to 1.0dl/g, more preferably 0.5 to 0.9dl/g, from the viewpoint of film-forming properties, productivity, and the like.

The amount of the polyester in the polyester layer (a) and the polyester layer (B) is preferably 70% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more, respectively. If the amount of the polyester is not less than the lower limit, the flexibility, strength and the like of the polyester film can be secured.

< polyester layer (A) >

The polyester film of the present invention has a polyester layer (a) containing a black pigment. The polyester film has light-shielding properties by containing a black pigment in the polyester layer (a).

[ pigment ]

As the black pigment used in the polyester layer (a), a black pigment having a high hiding power is preferable from the viewpoint of light-shielding properties and surface smoothness of the polyester film. By using a black pigment having a high hiding power, the light-shielding property, that is, the transmission density (OD value) of the polyester film can be improved even in a small amount, and if it is in a small amount, the roughness of the surface of the polyester film can be suppressed.

Examples of the black pigment include carbon black (furnace black, channel black, acetylene black, thermal black, lamp black, etc.), carbon nanotubes, graphite, copper oxide, manganese dioxide, titanium black, activated carbon, ferrite, magnetite, chromium oxide, iron oxide, molybdenum disulfide, and other inorganic black pigments, aniline black, perylene black, cyanine black, chromium complexes, complex oxide-based black pigments, anthraquinone-based organic black pigments, and other organic black pigments, and from the viewpoint of hiding properties, inorganic black pigments are preferable. These black pigments may be used alone in 1 kind, or may be used in combination in 2 or more kinds.

Among these black pigments, inorganic black pigments are more preferable from the viewpoint of light-shielding effect, and among them, carbon black is particularly preferable.

When carbon black is used, the average primary particle diameter is preferably 5 to 100nm, more preferably 10 to 50nm, and still more preferably 15 to 40 nm.

When the average primary particle diameter is not more than the upper limit, the dispersibility in the polyester layer (A) is improved and the surface smoothness of the polyester film is improved. On the other hand, carbon black may exist as aggregates of fine primary particles, but when the aggregates are present in a polyester and biaxial stretching is performed, a tensile stress applied to a film also acts on the aggregates, and a phenomenon of dispersion is observed. When the average primary particle diameter is not less than the lower limit, the aggregation force between the primary particles is not excessively increased, and aggregates are easily dispersed by the tensile stress at the time of stretching the film, thereby preventing the aggregates of carbon black from becoming large.

The average primary particle diameter in the present invention is a particle diameter measured by observing the carbon black particles alone or in the polyester with an electron microscope, and when the particles are present as aggregates, it means the particle diameter of the primary particles constituting the particles.

The content of the black pigment in the polyester layer (a) is preferably 0.1 to 25.0 mass%, more preferably 0.5 to 20.0 mass%, even more preferably 1.0 to 10.0 mass%, and even more preferably 1.5 to 5.0 mass%. When the content of the black pigment is not less than the lower limit, sufficient light-shielding properties can be imparted to the polyester film, and the OD value can be easily increased. On the other hand, if the content of the black pigment is not more than the above upper limit, the black pigment is easily dispersed in the polyester layer, and therefore, the surface smoothness is improved. If the content of the black pigment is increased, the black pigment is easily aggregated, but even if the black pigment is aggregated, the polyester layer (B) can be provided to ensure surface smoothness.

In particular, when the black pigment is carbon black, the content of the carbon black is preferably 0.1 to 5.0% by mass, more preferably 0.5 to 4.0% by mass, and still more preferably 1.2 to 3.0% by mass. When the content of carbon black is within the above range, sufficient light-shielding properties can be obtained while suppressing the amount of pigment used to be low. Although carbon black may aggregate, if the content is within the above range, excellent surface smoothness can be ensured without impairing surface smoothness due to aggregation by providing the polyester layer (B).

The polyester layer (a) may contain, in addition to the above pigments, conventionally known antioxidants, ultraviolet absorbers, heat stabilizers, lubricants and the like as needed.

< polyester layer (B) >

The polyester layer (B) is a polyester layer provided on at least one surface of the polyester layer (A), and is a layer containing substantially no particles or particles (X) having an average particle diameter of 1.0 μm or less. In the present invention, by providing the polyester layer (B) containing substantially no particles or particles having a small average particle diameter on the surface of the polyester layer (a) having a pigment, excellent surface smoothness can be obtained, and thus excellent visual recognition can be obtained.

In the present invention, the polyester layer (B) may be provided on at least one side of the polyester layer (a), or may be provided on both sides of the polyester layer (a). By providing the polyester layer (B) on both sides of the polyester layer (a), the surface smoothness of both sides can be improved. Further, by providing the polyester layers (B) on both surfaces, the difference in glossiness can be easily reduced, and it is not necessary to confirm in advance every time the film surface provided with the metal layer is processed, which is advantageous in that the work efficiency can be improved.

[ particle (X) ])

The average particle diameter of the particles (X) in the polyester layer (B) is 1.0 μm or less as described above. When the average particle diameter exceeds 1.0. mu.m, the surface smoothness of the polyester film is lowered. The average particle diameter of the particles (X) in the polyester layer (B) is preferably 0.01 to 0.8. mu.m, more preferably 0.02 to 0.4. mu.m, and most preferably 0.02 to 0.15. mu.m. When the average particle diameter of the particles is not more than the upper limit, the smoothness of the surface can be improved. Further, if the lower limit value is set to the above value, the surface of the polyester film is easily provided with a slip property and the like.

In the present invention, when the particles (X) have an average particle diameter of a powder, the particle diameter (d50) at which the cumulative volume fraction in the equivalent spherical distribution of the powder is 50% can be measured using a centrifugal sedimentation type particle size distribution measuring apparatus (SA-CP3 type).

In addition, as for the average particle diameter of the particles (X) in the polyester film in the present invention, for example, the film is observed by using a scanning electron microscope (manufactured by Hitachi High-Technologies Corporation, S3400N "), the size of 1 particle is determined from the obtained image data, and the average value of 10 points (10 particles) is taken as the average particle diameter.

Examples of the particles (X) contained in the polyester layer (B) include inorganic particles such as silica, alumina, calcium carbonate, and kaolin, and organic polymer particles. From the viewpoint of effectively improving the surface smoothness and the manufacturing cost, inorganic particles are preferable, and alumina such as spherical alumina, calcium carbonate, and silica are more preferable. Further, alumina is more preferable from the viewpoint of reducing the average particle diameter and surface smoothness and from the viewpoint of improving the sliding property.

In the polyester layer (B), the content of the particles (X) is preferably 0.01 to 1.0 mass%. By setting the content to 0.01 mass% or more, the effect of containing particles, for example, improvement of the slidability of the surface of the polyester film can be exhibited. Further, the smoothness of the surface can be improved by setting the content to 1.0% by mass or less, and for example, if a metal layer is formed on the surface of a polyester film, a film having excellent metallic luster can be obtained. From the above viewpoint, the content of the particles (X) is preferably 0.05 to 0.5 mass%, more preferably 0.1 to 0.4 mass%.

The polyester layer (B) is also preferably substantially free of particles. Here, the substantially no particles means that the content of particles in the polyester layer (B) is less than 100 mass ppm.

The polyester layer (B) may contain a minute amount of a polymerization catalyst (e.g., antimony trioxide) or particles such as impurities inevitably mixed in the production process, in addition to the particles (X), as described above, but even if the polyester layer contains a minute amount of particles, the effect on smoothness is slight, and the surface smoothness can be maintained satisfactorily. In the embodiment containing substantially no particles, the particles in the polyester layer (B) contained in a small amount preferably have an average particle diameter of 1 μm or less, but the average particle diameter may exceed 1 μm.

In order to minimize the influence of smoothness due to the particles, the content of the particles in the polyester layer (B) is preferably 50 mass ppm or less, more preferably 30 mass ppm or less, and still more preferably 1 mass ppm or less.

The polyester layer (B) may contain, in addition to the above particles, an antioxidant, an ultraviolet absorber, a heat stabilizer, a lubricant, and the like, which are conventionally known, as necessary.

< Transmission concentration (OD value) >

The polyester film of the present invention has a transmission concentration (OD value) of 0.4 or more. If the transmission density is less than the lower limit, the polyester film does not have sufficient light-shielding properties, and therefore, when used in an electronic device, the polyester film is in a state in which internal circuits, substrates, and the like can be visually recognized. From the viewpoint of sufficient light-shielding properties and making it impossible to visually recognize the inside of the small electronic device, the transmission density (OD value) is preferably 1.0 or more, more preferably 3.5 or more, and still more preferably 5.0 or more.

When the pigment of the polyester layer (a) is added to adjust the transmission concentration (OD value) of the polyester film of the present invention to be equal to or higher than the above value, and a pigment having a high hiding power is used as the pigment, the transmission concentration (OD value) can be equal to or higher than the above value even if added in a small amount, and the influence on the surface roughness of the polyester film can be reduced by adding in a small amount.

< arithmetic mean height (Sa) >

In the present invention, the arithmetic mean height (Sa) of the polyester layer (B) side surface of the polyester film is preferably 20.0nm or less. When the arithmetic mean height (Sa) is not more than the upper limit, the smoothness is excellent, and unevenness is less likely to occur on the surface of the polyester film after sputtering, for example, and therefore, a polyester film having excellent metallic luster can be obtained. From this viewpoint, the arithmetic average height (Sa) is more preferably 15.0nm or less, and still more preferably 7.0nm or less. From the viewpoint of smoothness, the arithmetic average height (Sa) is preferably as low as possible, but from the viewpoint of imparting a constant slip property, it is, for example, 1.0nm or more, preferably 2.0nm or more.

The arithmetic mean height (Sa) in the present specification can be measured by the method described in examples.

In order to make the arithmetic mean height (Sa) of the surface of the polyester film of the present invention on the polyester layer (B) side equal to or less than the above value, the kind and content of the pigment in the polyester layer (a), the kind and content of the particles (X) in the polyester layer (B), the thickness of the polyester layer (B), the thickness ratio of the polyester layer (a) to the polyester layer (B), and the like can be adjusted.

The same applies to the maximum height (Sp) described later.

In the present specification, when the polyester layer (B) is provided only on one side of the polyester layer (a), the surface of the polyester film on the polyester layer (B) side means the surface on the polyester layer (B) side provided on the one side. When the polyester layer (B) is provided on both sides of the polyester layer (a), the surface of the polyester film on the polyester layer (B) side means the surface of any polyester layer (B) side, provided that the arithmetic average height (Sa) of the surface of any polyester layer (B) side falls within the above range, and the arithmetic average height (Sa) of both surfaces of the polyester film may fall within the above range. In addition, as described later, in the case where a layer such as an easy-adhesion layer or an easy-slip layer is further provided on the surface of the polyester layer (B) of the polyester film, the surface of the polyester film on the polyester layer (B) side means the surface of the layer (the easy-adhesion layer, the easy-slip layer, or the like), provided that the arithmetic average height (Sa) of the surface of the layer falls within the above range. The same applies to the maximum height (Sp) described later.

< maximum height (Sp) >)

In the present invention, the maximum height (Sp) of the surface of the polyester film on the polyester layer (B) side is preferably 1000nm or less, more preferably 500nm or less, still more preferably 200nm or less, and still more preferably 150nm or less. If the value of the maximum height (Sp) is large, a grainy feeling is imparted to the surface of the film, but if the value is not more than the upper limit, the grainy feeling is reduced, and an excellent metallic gloss can be imparted to the polyester film. The lower the maximum height (Sp) is, the better from the viewpoint of reducing the graininess, but from the viewpoint of imparting a constant sliding property, it is, for example, 10.0nm or more, preferably 20.0nm or more. The maximum height (Sp) in the present specification can be measured by the method described in examples.

< gloss (Gs (60 degree) Difference >)

The difference in gloss between the front and back surfaces of the polyester film of the present invention is 3.0 or less, preferably 2.0 or less, and more preferably 1.5 or less.

By satisfying the foregoing range, for example, when a metal layer is provided, good visual recognition can be ensured without causing distortion of a reflected image and graininess from a film. Further, when the metal layer is provided on the film surface, if both surfaces of the film are not designed to have the same level of glossiness, the visibility after the metal layer is provided is not greatly different regardless of which film surface is used, and the work efficiency is improved. If there is a difference in the gloss on the front and back surfaces of the film, there is an inconvenience that the film surface must be reversed every time to be processed.

< thickness >

From the viewpoint of sufficiently improving the light-shielding property of the polyester film of the present invention, the thickness of the polyester layer (A) is preferably 20 to 70 μm, more preferably 25 to 60 μm, and still more preferably 30 to 50 μm.

The thickness of the polyester layer (B) is preferably 0.5 to 20 μm. By setting the thickness to 20 μm or less, the thickness of the polyester film can be reduced, and by setting the thickness to 0.5 μm or more, it becomes easy to maintain good surface smoothness. From these viewpoints, the thickness of the polyester layer (B) is more preferably 1.5 to 15 μm, and still more preferably 4.0 to 10 μm. Further, if the thickness of the polyester layer (B) is set to 4.0 μm or more, it becomes easy to further reduce the unevenness derived from the particles (X) having an average particle diameter of 1 μm or less, and the Sa and Sp can be easily reduced while the particles (X) are contained.

In the present invention, the ratio of the thickness of the polyester layer (B) to the thickness of the polyester layer (A) [ (thickness of B/(thickness of A) ] is preferably 0.025 to 0.25. When the thickness ratio is within the above range, the light-shielding property and the surface smoothness of the polyester film can be easily ensured. From this viewpoint, the thickness ratio [ (B)/(a) ] is more preferably 0.04 to 0.20, and still more preferably 0.09 to 0.18.

It is generally considered that a highly smooth surface can be obtained without adding particles to the polyester layer of the surface layer, but this is not always the case when a black pigment is contained.

For example, in order to obtain a deep black color, in a polyester film having a laminate structure, if a large amount of a black pigment is added to the polyester layer of the base layer, unevenness is easily formed on the surface of the base layer. Further, the unevenness affects the polyester layer in contact with the base layer, and the polyester layer on the surface portion is also likely to form unevenness. This problem is inherent in the black polyester film, and is not found out because the colorless and transparent polyester film is hardly seen in appearance. Therefore, from the viewpoint of a countermeasure against this problem, it is effective to adjust the thickness ratio of the polyester layers in the film having a laminated structure as described above.

< cured resin layer >

The polyester film of the present invention may have a cured resin layer on the outermost surface. By providing the cured resin layer, it is possible to easily reduce thermal damage to the base film or to easily improve adhesion to the metal layer in the process of forming the metal layer or the like formed by sputtering or the like. For example, when the cured resin layer is an easy-adhesion layer, it may be provided on the surface of the polyester layer (B). When the polyester layer (B) is provided on both surfaces of the polyester layer (a), the easy-adhesion layer may be provided on the surfaces of both the polyester layers (B) or may be provided on the surface of one polyester layer (B).

The easy adhesion layer is formed of, for example, an easy adhesion layer composition containing a binder resin and a crosslinking agent. Examples of the binder resin include a polyvinyl resin such as a polyester resin, an acrylic resin, a polyurethane resin, or polyvinyl alcohol, a polyalkylene glycol, a polyalkyleneimine, a methyl cellulose, a hydroxy cellulose, and a starch. Among them, polyester resins, acrylic resins, and urethane resins are preferably used from the viewpoint of improving adhesion to a metal layer or the like. The content of the binder resin in the easy-adhesion layer composition is, for example, 20 to 90% by mass, preferably 30 to 80% by mass, based on the solid content.

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, silane coupling compounds, and the like. Among them, oxazoline compounds are preferably used from the viewpoint of improving the durable adhesion. In addition, from the viewpoint of improving the durability and coatability of the easy-adhesion layer, a melamine compound is preferably used. The content of the crosslinking agent in the easy-adhesion layer composition is, for example, 5 to 50% by mass, preferably 10 to 40% by mass, based on the solid content.

The easy-adhesive layer composition may contain particles in a range that does not impair the adhesiveness of the easy-adhesive layer and the smoothness of the surface of the polyester film. As the particles, those described below as the slipping-easily layer can be suitably used. The content of the particles in the easy-adhesion layer composition is, for example, 1 to 20% by mass, preferably 3 to 15% by mass, based on the solid content.

In addition, the easy-adhesive layer composition may contain a component for promoting crosslinking, for example, a crosslinking catalyst. Further, a defoaming agent, a coating property improving agent, a thickener, an organic lubricant, an antistatic agent, an ultraviolet absorber, an antioxidant, a foaming agent, a dye, a pigment, and the like may be used in combination.

The easy-adhesion layer composition is preferably diluted with water, an organic solvent, or a mixture thereof, and the easy-adhesion layer can be formed as follows: the easy adhesion layer composition can be formed by applying a diluted solution of the easy adhesion layer composition as a coating liquid to the outermost surface of the polyester film and drying the coating liquid. The coating can be carried out by a conventionally known method.

The thickness of the easy-adhesion layer is usually in the range of 0.003 to 1 μm, preferably 0.005 to 0.6 μm, and more preferably 0.01 to 0.4. mu.m. Sufficient adhesiveness can be ensured by making the thickness 0.003 μm or more. Further, by setting the thickness to 1 μm or less, deterioration in appearance, blocking, and the like are less likely to occur.

Easy sliding layer

The polyester film of the present invention may have an easy-slip layer on the outermost surface. By providing the slipping-facilitating layer, the slipping property is improved, and the roll taking-up property and handling property of the polyester film are improved. The slipping layer is preferably provided on the surface of the polyester layer (B). When the polyester layer (B) is provided on both surfaces of the polyester layer (a), the slipping-easily layer may be provided on the surfaces of both the polyester layers (B) or may be provided on the surface of one polyester layer (B).

In the polyester film of the present invention, it is preferable that the easy-adhesion layer is provided on one surface and the easy-slip layer is provided on the other surface. Therefore, when the polyester layer (B) is provided on both surfaces of the polyester layer (a), it is preferable to provide an easy-adhesion layer on the surface of one polyester layer (B) and an easy-slip layer on the surface of the other polyester layer (B). When the easy-adhesion layer and the easy-slip layer are provided on the respective surfaces, the easy-slip layer improves the workability by improving the conveyance between the rollers, and facilitates the lamination of other layers such as a metal layer on the surface of the easy-adhesion layer.

The slip-susceptible layer is formed from, for example, a slip-susceptible layer composition comprising a binder resin, a cross-linking agent, and particles. The compounds usable in the binder resin and the crosslinking agent are those described in the binder resin and the crosslinking agent used in the above easy-adhesion layer.

The content of the binder resin in the easy-slip layer composition is, for example, 20 to 90 mass%, preferably 30 to 80 mass% based on the solid content. The content of the crosslinking agent in the slip-susceptible layer composition is, for example, 5 to 50% by mass, preferably 10 to 40% by mass, based on the solid content.

Specific examples of the particles used for the slipping-off facilitating layer include silica, alumina, kaolin, calcium carbonate, organic polymer particles, and the like. Among them, silica is preferable from the viewpoint of transparency. From the viewpoint of improving the slidability without impairing the surface smoothness of the polyester film, the average particle diameter of the particles is preferably in the range of 0.005 to 1.0. mu.m, more preferably 0.01 to 0.8. mu.m, and still more preferably 0.01 to 0.6. mu.m. The average particle diameter can be measured by the same method as that for the particles (X). The content of the particles in the slip-susceptible layer composition is, for example, 1 to 20 mass%, preferably 3 to 15 mass% based on the solid content.

The number of particles used in the slipping layer may be 1, but it is also preferable to use 2 types of particles having different average particle diameters in combination.

The slip-susceptible layer composition is preferably diluted with water, an organic solvent, or a mixture thereof, and the slip-susceptible layer may be formed as follows: the easy adhesion layer composition can be formed by applying a diluted solution of the easy adhesion layer composition as a coating liquid to the outermost surface of the polyester film and drying the coating liquid. The coating can be carried out by a conventionally known method.

The thickness of the slip-facilitating layer is usually in the range of 0.003 to 1 μm, preferably 0.005 to 0.6 μm, and more preferably 0.01 to 0.4. mu.m. By making the thickness 0.003 μm or more, the particles contained in the slipping-prone layer can be sufficiently retained, and the slipping property can be imparted. Further, by setting the thickness to 1 μm or less, deterioration in appearance, blocking, and the like are less likely to occur.

The polyester film of the present invention is preferably used for applications requiring light-shielding properties and surface smoothness, for example, electronic equipment applications, particularly communication terminals. In the electronic device, if the electronic device is used in a housing, a display, or the like, it is possible to prevent an electronic substrate or the like in the housing from being seen through. Further, by having surface smoothness, even if a metal layer described later is laminated, for example, smoothness of the metal layer can be secured, and design properties can be improved.

< polyester film with Metal layer >

The polyester film of the present invention is preferably used as a polyester film with a metal layer laminated on the surface thereof. The metal layer may be laminated on the surface of the polyester film on the side of the polyester layer (B). The polyester film with a metal layer has a metallic luster due to the metal layer, and can improve design properties.

The surface on the side of the polyester layer (B) is smooth as described above, and therefore, the metal layer provided on the surface is also smooth, and the grainy feel is also less likely to be seen. In addition, in the case where the polyester layer (B) is provided on both sides of the polyester layer (a), the metal layer may be provided on the surface of two polyester layers (B) or may be provided on the surface of one polyester layer (B).

When the easy adhesion layer is provided on the surface of the polyester layer (B) as described above, a metal layer is preferably laminated on the easy adhesion layer. However, the metal layer may be directly laminated on the surface of the polyester layer (B) without interposing the easy adhesion layer.

The thickness of the metal layer is not particularly limited, but is preferably 5 to 900nm, more preferably 10 to 300 nm. If the thickness of the metal layer is not less than the lower limit, cracks are less likely to enter the metal layer, and the metal layer becomes strong. On the other hand, if the thickness of the metal layer is not more than the upper limit value, the metal layer can be formed in a short time. In addition, when the content is within the above range, sufficient metallic luster can be provided.

As a method for forming the metal layer, a conventionally known method can be used. Specifically, the metal oxide film can be formed by 1 or more methods selected from a vapor deposition method, a sputtering method, and an ion plating method, but is particularly preferably formed by a sputtering method from the viewpoint of ease of production.

As the sputtering method, it is preferable to place the polyester film in a vacuum vessel, introduce an inert gas such as argon gas, apply a direct current voltage, collide the ionized inert gas with a target metal, and form a metal layer on the surface of the polyester film by the sputtered metal.

Since the base film of the polyester film with a metal layer of the present invention is black, the metal layer can be inspected at the same time in the step of forming the metal layer by sputtering or in the step of winding the film. In general, when a colorless and transparent base film is used, light is reflected from the back surface side of the film on which no metal layer is provided, so-called back surface reflection occurs, and therefore, excessive light returns may disturb inspection. Therefore, conventionally, a process of providing a metal layer by sputtering and an inspection process are often separated, and the work efficiency is sometimes reduced as the number of processes increases.

In the polyester film with a metal layer using the black base film of the present invention, the back reflection caused by the base film as described above does not occur, and the inspection of the metal layer with high accuracy can be realized. Further, since the surface of the base material film on the side where the metal layer is provided is highly smooth, light irradiated to the surface of the metal layer during inspection is reflected in the same direction and returned to the detector, and therefore, there is an advantage that inspection accuracy is further improved.

< thin film laminate >

The polyester film with a metal layer of the present invention is suitably used as a film laminate in which an optical member is laminated on the surface of a metal layer with an adhesive layer interposed therebetween. The optical member includes resin films such as a polarizing plate film, a protective film, a retardation film, and a protective film, and a glass substrate of a liquid crystal cell is preferably used.

Further, if it is assumed that the glass substrate is replaced with a resin film, when a resin film such as a polyimide film, a polyester film, or a cyclic polyolefin film is bonded to a polyester film with a metal layer with an adhesive layer interposed therebetween, it is expected that the film will be further thinned from a technical viewpoint. Therefore, the total thickness of the thin film laminate is 150 μm or less, more preferably 130 μm or less, and particularly 100 μm or less.

Further, in the film laminate, by appropriately selecting the resin films to be combined with each other, the folding property can be imparted. The film laminate can be folded, bent, and rolled for various display applications, and the application range of the film laminate is expected to be further expanded.

< method for producing polyester film >

Next, a method for producing a polyester film of the present invention will be specifically described, but the present invention is not limited to the following production method.

First, the polyester is added with the additives such as the pigment and the particles to prepare a raw material for the polyester layer (a) or the polyester layer (B). In the production of the raw materials, a twin-screw extruder is particularly preferably used in order to perform the mixing while dispersing the raw materials in the polyester satisfactorily.

Next, the raw materials for the polyester layer (a) and the polyester layer (B) were fed into a plurality of extruders, and the respective polyesters were laminated by a multi-layer multi-manifold die or a feed block, and a multi-layer molten sheet was extruded from a nozzle and cooled and solidified on a cooling roll to obtain an unstretched sheet. In this case, in order to improve the planarity of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum, and it is preferable to use an electrostatic application method and/or a liquid coating method.

Next, the obtained unstretched film is biaxially stretched to be biaxially oriented. That is, the unstretched sheet is stretched in the longitudinal direction by a roll stretcher. The stretching temperature is usually 70 to 120 ℃, preferably 75 to 110 ℃, and the stretching ratio is usually 2.5 to 7.0 times, preferably 3.0 to 6.0 times.

Thereafter, stretching is performed in the transverse direction. The stretching temperature is usually 70 to 125 ℃, preferably 80 to 120 ℃, and the stretching ratio is usually 3.0 to 7.0 times, preferably 3.5 to 6.0 times. Then, the film is heat-treated at a temperature of 170 to 250 ℃ under tension or under relaxation of 30% or less to obtain a biaxially stretched film.

In the above stretching, a method of performing unidirectional stretching in 2 stages or more may be used. In this case, it is preferable to perform the biaxial stretching ratios so that the respective ratios finally fall within the above ranges. Further, the unstretched sheet may be simultaneously biaxially stretched so that the area ratio is 10 to 40. Further, stretching may be performed again in the longitudinal and/or transverse directions before or after the heat treatment, as necessary.

The surface of the polyester film obtained by the above-mentioned method may be coated as necessary, and the above-mentioned easy-adhesion layer and easy-slipping layer may be formed by coating. The coating can be carried out on-line or off-line or a combination of both, but is preferably carried out on-line. In the coating in-line, a coating solution mainly diluted with water is applied at the stage of completion of longitudinal stretching, and then drying, preheating, transverse stretching, and further heat setting are performed in a tenter.

Further, the polyester film of the present invention is continuously produced and wound into a roll, whereby a roll of the polyester film can be obtained.

< description of the terms, etc. >)

The term "sheet" is generally defined in JIS as a small flat product having a small thickness over the length and width, and the term "film" is generally defined as a thin flat product having an extremely small thickness as compared with the length and width and having a maximum thickness arbitrarily defined, and is generally supplied in the form of a roll (japanese industrial standards; JIS K6900). However, since the boundary between the sheet and the film is not clear, and the two are not distinguished from each other in the present invention, the "sheet" is included in the case of being referred to as "film" in the present invention, and the "film" is included in the case of being referred to as "sheet".

In the present specification, when "X to Y" (X, Y is an arbitrary number), unless otherwise specified, the meaning of "X or more and Y or less" and the meaning of "preferably more than X" or "preferably less than Y" are included.

In addition, when the term "X" or more (X is an arbitrary number) "includes a meaning of" preferably more than X "unless otherwise specified, and when the term" Y "or less (Y is an arbitrary number)" includes a meaning of "preferably less than Y" unless otherwise specified.

Examples

The present invention will be described in further detail with reference to examples, but the present invention is not limited to these examples.

[ evaluation method ]

In the present example and comparative example, evaluation was performed as follows.

(1) Method for measuring intrinsic viscosity of polyester

1g of polyester from which other polymer components and pigments incompatible with the polyester were removed was precisely weighed, and 100ml of a mixed solvent of phenol/tetrachloroethane (mass ratio) 50/50 was added to dissolve the polyester, and the solution was measured at 30 ℃.

(2) Average particle diameter of the particles

The equivalent spherical distribution of the particles made of the powder was measured by a centrifugal sedimentation type particle size distribution measuring apparatus (model SA-CP 3) manufactured by shimadzu corporation, to determine the cumulative volume fraction of each particle size range. The average particle diameter was determined as the particle diameter at which the cumulative volume fraction in the equivalent spherical distribution of the measured particles was 50%.

(3) Film thickness and layer thickness

The small piece of the film was embedded in epoxy resin, and a section was cut out with a microtome so that a cross section in the thickness direction could be observed, and the section was observed with a transmission electron microscope photograph. The cross section is substantially parallel to the film surface, and the laminated interface is observed by light and shade. The thickness was calculated by averaging the distances between the interface and the film surface in 1 transmission electron micrograph.

(4) Average primary particle diameter of pigment

After the small piece of the polyester film was fixed and molded with an epoxy resin, the piece was cut with a slicer, and the cross section of the polyester film was observed with a transmission electron microscope. The maximum diameter (a) of the pigment observed in the cross section of the polyester film and the diameter (b) perpendicular thereto were measured, and the primary particle diameter was determined by the following formula. The same measurement was performed for 500 randomly extracted pigments, and the arithmetic mean was taken as the average primary particle diameter of the pigment.

Average primary particle diameter of (a + b)/2

(5) Transmission concentration (OD value)

The transmission concentration was measured using a portable white-black transmission concentration meter (Ihac-T5, manufactured by Iyoto electronics industries, Ltd.). The measurement was performed at 5 points, and the average value was defined as the OD value. The larger the value, the lower the light transmittance.

(6) Arithmetic mean height (Sa) and maximum height (Sp)

The surface of the polyester film on the side of the polyester layer (B) was measured using a non-contact surface/layer cross-sectional shape measuring system VertScan (registered trademark) R550GML manufactured by mitsubishi system, ltd, a CCD camera: SONY HR-501/3', objective: 20 times, lens barrel: 1X Body, zoom lens: no Relay, wavelength filter: 530white, measurement mode: at Wave, a region of 640 μm × 480 μm was measured, and the arithmetic mean height (Sa) and the maximum height (Sp) were calculated using the power corrected by the polynomial of degree 4. The arithmetic average height (Sa) and the maximum height (Sp) of the film surface were measured at 10 points and the average values were defined as the arithmetic average height (Sa) and the maximum height (Sp) of the film surface.

(7) Evaluation of difference in gloss (60 ℃ C.)

The surface of the polyester film on the side of the polyester layer (B) (front and back surfaces) was measured for its gloss at 60 ℃ to the surface of the sample to be measured, using a gloss meter (VG-2000, manufactured by Nippon Denshoku K.K.).

(8) Evaluation of surface smoothness (1)

An aluminum layer of about 100nm was deposited on the surface of the polyester film on the polyester layer (B) side so as to cover the entire surface of the polyester film using a vacuum deposition apparatus. The film on which the aluminum layer was formed was inverted, and a brown bottle was placed so as to face the aluminum surface, and a reflection image of the brown bottle reflected on the film surface was visually recognized, and smoothness was evaluated by the following criteria.

[ evaluation criteria ]

A: the reflected image was clearly observed, and the film surface was particularly excellent in smoothness.

B: the reflected image was slightly white and turbid, but practically, there was no problem in the smoothness of the film surface.

C: the reflected image was white and turbid, and the smoothness of the film surface was poor.

(9) Evaluation of surface smoothness (2)

An aluminum layer of about 100nm was deposited on the surface of the polyester film on the polyester layer (B) side so as to cover the entire surface of the polyester film using a vacuum deposition apparatus. The aluminum layer was set to be an upper side, the film was left on a stand, and a fluorescent lamp was disposed directly above the film. The light reflected from the fluorescent lamp on the surface of the film was visually recognized, and smoothness was evaluated according to the following criteria.

[ evaluation standards ]

A: granular unevenness was not visually recognized, and the film surface was particularly excellent in smoothness.

B: the granular unevenness was slightly visually recognized, but there was no problem in the smoothness of the film surface in practical use.

C: the granular unevenness was clearly visually recognized, and the smoothness of the film surface was poor.

[ Material ]

Materials used in examples and comparative examples are shown below.

< polyester A >

A polyethylene terephthalate homopolypolymer having an intrinsic viscosity of 0.65 dl/g.

< polyester B >

80 parts by mass of the polyester A and 20 parts by mass of carbon black particles having an average primary particle diameter of 20nm were dry-blended, and extruded by a biaxial kneading extruder to obtain a polyester B. The intrinsic viscosity of the resulting polyester B was 0.58 dl/g.

< polyester C >

99.8 parts by mass of polyester A and 0.2 part by mass of silica particles having an average particle diameter of 3.2 μm were dry-blended and extruded by a twin-screw kneading extruder to obtain polyester C. The intrinsic viscosity of the resulting polyester C was 0.64 dl/g.

< polyester D >

Polyester A was precrystallized at 160 ℃ in advance, and then subjected to solid-phase polymerization in a nitrogen atmosphere at 220 ℃ to obtain polyethylene terephthalate having an intrinsic viscosity of 0.78dL/g, 99% by mass of ester units of ethylene terephthalate, and the balance of ester units of diethylene glycol and terephthalic acid.

< polyester E >

Polyester A was precrystallized at 160 ℃ in advance, and then subjected to solid-phase polymerization at 220 ℃ in a nitrogen atmosphere, and the polyester A had an intrinsic viscosity of 0.70dL/g, 99% by mass of ester units of ethylene terephthalate, and the remainder of ester units of polyethylene terephthalate obtained by polymerizing diethylene glycol and terephthalic acid.

< polyester F >

Antimony trioxide was used as a polymerization catalyst, and the content of the polymerization catalyst was 330 ppm by mass in terms of antimony atom, and 2% by mass of polyethylene terephthalate containing calcium carbonate particles having an average particle diameter of 0.7 μm was contained. The intrinsic viscosity was 0.61 dl/g.

< polyester G >

Antimony trioxide was used as a polymerization catalyst, and the polymerization catalyst contained polyethylene terephthalate in an amount of 330 ppm by mass in terms of antimony atom and containing 20% by mass of spherical alumina particles having an average particle diameter of 0.05 μm. The intrinsic viscosity was 0.61 dl/g.

[ examples and comparative examples ]

< example 1 >

Polyester A, B was mixed at a ratio of 90 mass% to 10 mass% to form a raw material for the polyester layer (a). Further, the polyester D, F was mixed at a ratio of 92.5 mass% and 7.5 mass% respectively to form a raw material for the polyester layer (B) as a surface layer, 2 kinds of 3-layer laminated polyester resins formed by the composition of layer B/layer a/layer B were coextruded in a film form by a melt extruder, and the sheet was coextruded on a casting drum cooled to 20 ℃ and cooled to solidify to obtain a non-oriented sheet.

Subsequently, the film was stretched 3.3 times in the machine direction (longitudinal direction) at 80 ℃ and then the ends of the film were fixed with clips, introduced into a tenter, and stretched 4.3 times in the direction perpendicular to the machine direction (transverse direction) at 115 ℃ through a preheating step. After biaxial stretching, heat treatment was carried out at 234 ℃ for 3 seconds to obtain a polyester film having a thickness of 50 μm (4.0 μm/42.0 μm/4.0 μm).

< example 2 >

Polyester films having a thickness of 50 μm were obtained in the same manner as in example 1, except that the layer configuration and the thickness were changed as shown in Table 1.

< example 3 >

Polyester films having a thickness of 50 μm were obtained in the same manner as in example 1, except that the polyester E, G was mixed at a ratio of 98.5 mass% and 1.5 mass%, respectively, to form the raw material of the polyester layer (B), and the layer configuration and the thickness were changed as shown in table 1.

< example 4 >

Polyester films having a thickness of 50.2 μm were obtained in the same manner as in example 3, except that the polyester A, B was mixed at a ratio of 89% by mass and 11% by mass, respectively, to obtain the raw materials for forming the polyester layer (a) and the layer composition and the thickness were changed as shown in table 1.

< example 5 >

A polyester film having a thickness of 50 μm was obtained in the same manner as in example 1, except that the layer structure and the thickness were changed as shown in Table 1, using the polyester E as a raw material for the polyester layer (B). The polyester layer (B) has particles of less than 100 mass ppm and contains substantially no particles.

< example 6 >

Polyester films having a thickness of 50 μm were obtained in the same manner as in example 5, except that the layer configuration and the thickness were changed as shown in Table 1. The polyester layer (B) has particles of less than 100 mass ppm and contains substantially no particles.

< comparative example 1 >

The polyester A, B was mixed at a ratio of 95% by mass and 5% by mass, the resulting mixture was melt-extruded by a melt extruder, and the sheet was co-extruded on a casting drum cooled to 20 ℃ and cooled to solidify, to obtain a single-layer non-oriented sheet.

Subsequently, the film was stretched 3.2 times in the machine direction (longitudinal direction) at 80 ℃ and then the ends of the film were fixed with clips, introduced into a tenter, and subjected to a preheating step to stretch 4.0 times in the direction perpendicular to the machine direction (transverse direction) at 115 ℃. After biaxial stretching, the film was heat-treated at 234 ℃ for 3 seconds to obtain a polyester film having a thickness of 50 μm.

< comparative example 2 >

A polyester film having a thickness of 50 μm was obtained in the same manner as in comparative example 1, except that the polyester A, B was mixed in a ratio of 90% by mass to 10% by mass.

< comparative example 3 >

The polyester B, C, D was mixed at a ratio of 2 mass%, 3 mass% and 95 mass%, respectively, the obtained mixture was melt-extruded by a melt extruder, and the sheet was co-extruded on a casting drum cooled to 20 ℃ and cooled to solidify, to obtain a single-layer non-oriented sheet.

Subsequently, the film was stretched 2.8 times in the machine direction (longitudinal direction) at 87 ℃, then the ends of the film were fixed with clips, introduced into a tenter, and subjected to a preheating step to stretch 3.4 times in the direction perpendicular to the machine direction (transverse direction) at 115 ℃. After biaxial stretching, the film was heat-treated at 228 ℃ for 3 seconds to obtain a 188 μm thick polyester film.

[ Table 1]

[ Table 2]

TABLE 2

From the above examples, it is understood that both the light-shielding property and the surface smoothness are improved by providing the polyester layer (B) containing substantially no particles or particles (X) having an average particle diameter of 1.0 μm or less on the surface of the polyester layer (a) having a black pigment. Further, since the difference in glossiness can be reduced, the visibility is improved regardless of the surface on which the metal layer is provided.

As is clear from the results in table 1, examples 3 and 4 in which the polyester layer (B) was thick and alumina having a small particle size was used, and examples 5 and 6 in which the polyester layer (B) contained no particles were particularly excellent in light-shielding properties and surface smoothness. Among them, examples 3, 4 and 6 in which the polyester layer (B) was thick were suitable from the viewpoint of surface smoothness, and examples 3 and 4 in which the slidability was improved by using alumina particles were suitable from the viewpoint of workability.

In comparative example 3, it is found that the reflected image is distorted due to the difference in glossiness between the front and back surfaces, and the rough surface side has a sense of discomfort due to the grainy feeling of the film, and the visibility is lowered. Therefore, the film is not suitable for the surface provided with the metal layer, and therefore, there is an inconvenience that the film surface must be reversed when the film is used.

Claims

1. A polyester film, comprising: a polyester layer (A) containing a black pigment and a polyester layer (B) on at least one side thereof,

2. The polyester film according to claim 1, wherein the black pigment is carbon black, and the content of carbon black in the polyester layer (A) is 0.1 to 5.0% by mass.

3. The polyester film according to claim 1 or 2, wherein the content of the particles (X) in the polyester layer (B) is 0.01 to 1.0 mass%.

4. The polyester film according to any one of claims 1 to 3, wherein a ratio of a thickness of the polyester layer (B) to a thickness of the polyester layer (A), (thickness of B/(thickness of A)), is 0.025 to 0.25.

5. The polyester film according to any one of claims 1 to 4, wherein an arithmetic average height (Sa) of a surface of the polyester film on the side of the polyester layer (B) is 20.0nm or less.

6. The polyester film according to any one of claims 1 to 5, wherein the maximum height (Sp) of the surface of the polyester film on the side of the polyester layer (B) is 1000nm or less.

7. The polyester film according to any one of claims 1 to 6, wherein a polyester layer (B) is provided on both surfaces of the polyester layer (A).

8. The polyester film according to any one of claims 1 to 7, wherein a cured resin layer is provided on at least one surface of the polyester layer (B).

9. A polyester film with a metal layer, which is laminated with the metal layer sandwiched between the cured resin layers of the polyester film according to claim 8.

10. A polyester film with a metal layer, wherein a metal layer is directly laminated on at least one surface of the polyester film according to any one of claims 1 to 7 on the side of the polyester layer (B).

11. A film laminate comprising an optical member laminated on the surface of the metal layer of the polyester film with a metal layer according to claim 9 or claim 10 with an adhesive layer interposed therebetween.

12. The film laminate according to claim 11, wherein the optical member is a resin film or a glass substrate.

13. The thin film laminate according to claim 11 or 12, which has a total thickness of 150 μm or less.

14. The polyester film according to any one of claims 1 to 8, which is used for a communication terminal.

15. The polyester film with a metal layer according to claim 9 or 10, which is used for a communication terminal.

16. A film laminate according to any one of claims 11 to 13, for use in a communications terminal.