Polyester film

Abstract

The purpose of the present invention is to provide a polyester film that can reduce environmental load and reduce yellowing. This polyester film includes a material recycled polyester resin containing a terephthalate unit. In the polyester film, in a fluorescence spectrum measured with excitation light having a wavelength of 330 nm, the ratio of fluorescence emission intensity at 460 nm to fluorescence emission intensity at 395 nm (i.e., fluorescence emission intensity at 460 nm / fluorescence emission intensity at 395 nm) is 0.43 to 0.60.

Description

polyester film

[0001] This invention relates to a polyester film.

[0002] Recycling systems that collect and reuse used polyester molded products are attracting attention. In the case of polyester films, various proposals have been made to use such collected polyester resin as a raw material to create films (see, for example, Patent Documents 1 and 2).

[0003] Furthermore, various proposals have been made to recycle used polyester film by washing and removing the functional layer on the used polyester film (see, for example, Patent Documents 3-4).

[0004] However, in polyester films containing recycled polyester resin made from recovered used polyester films, it could not be said that mechanical and optical properties due to thermal degradation and other factors were sufficiently suppressed.

[0005] Japanese Patent Publication No. 2006-182858, Japanese Patent Publication No. 2019-127536, Japanese Patent Publication No. 2009-537686, Japanese Patent Publication No. 2020-090094

[0006] The objective of this invention is to provide a polyester film that can reduce environmental impact and also reduce yellowing.

[0007] To solve this problem, the present invention has the configuration of [Item 1] below. [Item 1] A polyester film comprising a material recycled polyester resin, wherein the material recycled polyester resin contains terephthalate units, and in a fluorescence spectrum measured with excitation light at a wavelength of 330 nm, the ratio of the fluorescence emission intensity at 460 nm (hereinafter sometimes referred to as "E1") to the fluorescence emission intensity at 395 nm (hereinafter sometimes referred to as "E2") (i.e., E1 / E2) is 0.43 or more and 0.60 or less.

[0008] According to [Item 1], since the polyester film contains recycled polyester resin, the environmental impact can be reduced.

[0009] According to [Item 1], since the E1 / E2 ratio of the polyester film is 0.60 or less, the degree of oxidative degradation of the polyester resin containing terephthalate units is not excessively high, and therefore, the yellowing that the polyester film may exhibit can be reduced.

[0010] Furthermore, since the E1 / E2 ratio of the polyester film is 0.43 or higher, the thermal dimensional stability can be improved. This is thought to be because the degree of oxidative degradation of the polyester resin containing terephthalate units is not excessively low, and therefore the molecular chains are broken to some extent by oxidative degradation, resulting in improved crystallinity.

[0011] The present invention preferably comprises the following configurations from item 2 onwards. [Item 2] A polyester film according to item 1, wherein the average refractive index is 1.603 or more. [Item 3] A polyester film according to item 1 or 2, wherein the total light transmittance is 85% or more. [Item 4] A polyester film according to any one of items 1 to 3, comprising a recycled material film containing a polyester resin including the material recycled polyester resin, wherein the content of the material recycled polyester resin in the recycled material film is 25% by mass or more and 100% by mass or 50% by mass or more and 100% by mass or less, based on 100% by mass of the polyester resin in the recycled material film. [Item 5] A polyester film according to any one of items 1 to 4, wherein the intrinsic viscosity is 0.50 dL / g or more and 0.60 dL / g or less. [Item 6] A polyester film according to any one of items 1 to 5, comprising a recycled material film containing the material recycled polyester resin, and including a functional layer provided on the surface of the recycled material film. [Clause 7] The polyester film according to Claim 6, wherein the functional layer comprises at least one resin selected from the group consisting of polyester resin, polyurethane resin, and acrylic resin. [Clause 8] The polyester film according to any one of Claims 1 to 7, wherein the raw material of the material recycled polyester resin is at least a film. [Clause 9] The polyester film according to Claim 8, wherein the film of the raw material comprises a base film containing a polyester resin and a functional layer provided on the base film, and the material recycled polyester resin is a material recycled polyester resin obtained by a method comprising removing the functional layer from the film of the raw material and melting the base film. [Clause 10] The polyester film according to any one of Claims 1 to 9, wherein the material recycled polyester resin is a material recycled polyethylene terephthalate resin. [Clause 11] The polyester film according to any one of Claims 1 to 10, further comprising a virgin polyester resin.[Clause 12] The polyester film according to any one of Clauses 1 to 11, wherein the virgin polyester resin contains terephthalate units. [Clause 13] The polyester film according to any one of Clauses 1 to 12, wherein the virgin polyester resin is virgin polyethylene terephthalate resin. [Clause 14] The polyester film according to any one of Clauses 1 to 13, comprising a recycled material film containing a polyester resin including the material recycled polyester resin, wherein when the repeating units of the polyester resin in the recycled material film are set to 100 mol%, the terephthalate units of the polyester resin in the recycled material film (hereinafter sometimes referred to as "total terephthalate units") are 70 mol% or more or 90 mol% or more. [Clause 15] The polyester film according to any one of Clauses 1 to 14, comprising a recycled material film containing a polyester resin including the material recycled polyester resin, wherein when the repeating units of the polyester resin in the recycled material film are set to 100 mol%, the terephthalate units of the polyester resin in the recycled material film (i.e., total terephthalate units) are 93 mol% or more or 95 mol% or more. [Clause 16] The polyester film according to any one of Clauses 1 to 15, wherein the recycled material film is a stretched recycled material film. [Clause 17] The polyester film according to any one of Clauses 1 to 16, wherein the recycled material film is a biaxially stretched recycled material film. [Clause 18] The polyester film according to any one of Clauses 1 to 17, wherein the thickness of the recycled material film is 90% or more or 95% or more of the thickness of the polyester film. [Clause 19] The polyester film according to any one of Clauses 1 to 18, wherein the content of the material recycled polyester resin in the polyester film is 25% or more by mass or 35% or more by mass. [Clause 20] The polyester film according to any one of Clauses 1 to 19, wherein the total content of the virgin polyester resin and the material recycled polyester resin in the polyester film is 80% or more by mass or 90% or more by mass.[Item 21] The polyester film according to any one of items 1 to 20, wherein the total content of the virgin polyester resin and the material recycled polyester resin in the polyester film is 95% by mass or more or 98% by mass or more.

[0012] According to the present invention, it is possible to provide a polyester film that can reduce environmental impact and also reduce yellowing.

[0013] The embodiments of the present invention will be described in detail below.

[0014] <1. Polyester Film> The polyester film of this embodiment includes a recycled material film and a functional layer provided on the recycled material film (hereinafter sometimes referred to as the "first functional layer"). The functional layer is provided on one of the two sides of the recycled material film.

[0015] The functional layer forms one of the two surfaces of the polyester film. The recycled material film forms the other of the two surfaces of the polyester film.

[0016] <2. Recycled Material Film> The recycled material film may be a film containing at least a material-recycled polyester resin containing terephthalate units as a recycled material. Material-recycled polyester resin containing terephthalate units will be described later.

[0017] <2.1. Polyester resin containing terephthalate units> The recycled material film contains a polyester resin containing terephthalate units. Examples of polyester resins containing terephthalate units include polyethylene terephthalate resin, polybutylene terephthalate resin, polytrimethylene terephthalate resin, polypropylene terephthalate resin, and polycyclohexanedimethylene terephthalate resin. These may also contain copolymer components.

[0018] When the repeating units of a polyester resin containing terephthalate units are set to 100 mol%, the terephthalate units are preferably 70 mol% or more, more preferably 90 mol% or more, even more preferably 93 mol% or more, and still more preferably 95 mol% or more.

[0019] As a polyester resin containing terephthalate units, a polyester resin containing alkylene terephthalate units is preferred. Examples of alkylene terephthalate units include ethylene terephthalate units, trimethylene terephthalate units, and polybutylene terephthalate units.

[0020] When the repeating units of a polyester resin containing alkylene terephthalate units are set to 100 mol%, the alkylene terephthalate units are preferably 70 mol% or more, more preferably 90 mol% or more, even more preferably 93 mol% or more, and still more preferably 95 mol% or more.

[0021] Polyethylene terephthalate resin is preferred as the polyester resin containing terephthalate units. Examples of polyethylene terephthalate resins include homopolymer polyethylene terephthalate resin and copolymerized polyethylene terephthalate resin.

[0022] In the recycled material film, the content of polyester resin containing terephthalate units is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. This may be, for example, 98% by mass or more, or 100% by mass.

[0023] <2.1.1. Material-Recycled Polyester Resin> Polyester resins containing terephthalate units include material-recycled polyester resin. In other words, recycled material films include material-recycled polyester resin. Therefore, the environmental impact can be reduced.

[0024] Examples of raw materials for materially recycled polyester resin, or mechanically recycled polyester resin, include film (hereinafter sometimes referred to as "raw material film"), bottles (PET bottles as an example), and clothing. The raw material film may be used film or waste film generated during manufacturing.

[0025] Preferably, the raw material film includes a base film containing polyester resin and a functional layer provided on at least one side of the base film.

[0026] The polyester resin of the base film preferably contains terephthalate units. The description of the polyester resin containing terephthalate units in the base film is omitted as it overlaps with the description of the polyester resin containing terephthalate units in the recycled material film. The description of the polyester resin containing terephthalate units in the recycled material film can also be treated as the description of the polyester resin containing terephthalate units in the base film.

[0027] The base film may contain particles. Examples of particles include inorganic particles such as silica, calcium carbonate, and alumina, and organic particles such as acrylic.

[0028] The explanation of the functional layer in the raw material film is omitted as it overlaps with the explanation of the functional layer in polyester film.

[0029] The functional layer of the raw material film may contain resin.

[0030] Material-recycled polyester resin may be manufactured by a method that includes removing the functional layer from the raw material film and melting the base film. Alternatively, material-recycled polyester resin may be manufactured by a method that includes melting the raw material film without removing the functional layer. The former method is preferred because it can reduce impurities in the material-recycled polyester resin. Methods for removing the functional layer include physical methods using a metal brush or a plastic brush containing an abrasive, chemical methods using an alkaline aqueous solution or an organic solvent, and methods combining these. The functional layer removal process may be carried out while the raw material film is being transported, or the raw material film may be finely cut and, if necessary, crushed before the functional layer removal process is carried out.

[0031] A preferred method for producing material-recycled polyester resin includes cutting and, if necessary, crushing the raw material film, removing the functional layer from the cut raw material film with at least an alkaline aqueous solution, and melting the raw material film after the functional layer has been removed, i.e., the base film, in an extruder. In this example, it is preferable to put the cut raw material film into an alkaline aqueous solution and stir it in order to remove the functional layer. It is preferable to wash the raw material film after the alkaline aqueous solution treatment with water and, if necessary, dry it. This allows to obtain the raw material film after the functional layer has been removed, i.e., the base film. In this example, if components of the alkaline aqueous solution remain excessively on the base film, the oxidative degradation will progress excessively when melting the base film or when melting the material-recycled polyester resin. Therefore, it is preferable to thoroughly wash the raw material film after the alkaline aqueous solution treatment with water.

[0032] The raw material film may be the polyester film of this embodiment. Therefore, the raw material film may be a used polyester film of this embodiment, a waste film generated during the manufacture of the polyester film of this embodiment, or both.

[0033] As the material recycled polyester resin, a material recycled polyester resin containing alkylene terephthalate units is preferred. When the repeating units of the material recycled polyester resin containing alkylene terephthalate units are set to 100 mol%, the alkylene terephthalate units are preferably 70 mol% or more, more preferably 90 mol% or more, even more preferably 93 mol% or more, and even more preferably 95 mol% or more.

[0034] As the material-recycled polyester resin, material-recycled polyethylene terephthalate resin is more preferred. The material-recycled polyethylene terephthalate resin may contain copolymer components.

[0035] The content of material-recycled polyester resin in the recycled material film is preferably 25% by mass or more, more preferably 35% by mass or more, and even more preferably 50% by mass or more. This may also be, for example, 70% by mass or more, or 80% by mass or more. If it is 25% by mass or more, the environmental burden can be further reduced. The content of material-recycled polyester resin in the recycled material film may also be, for example, 100% by mass or less, or 90% by mass or less, or 80% by mass or less.

[0036] The content of material-recycled polyester resin in the recycled material film is preferably 25% by mass or more, more preferably 35% by mass or more, and even more preferably 50% by mass or more, based on 100% by mass of polyester resin in the recycled material film. This may be, for example, 70% by mass or more, or 80% by mass or more. This may be, for example, 100% by mass, 90% by mass or less, or 80% by mass or less.

[0037] The content of the material recycled polyester resin in the polyester film is preferably 25% by mass or more, more preferably 35% by mass or more, and still more preferably 50% by mass or more. For example, it may be 70% by mass or more, or may be 80% by mass or more. When this is 25% by mass or more, the environmental load can be further reduced. The content of the material recycled polyester resin in the polyester film may be, for example, 100% by mass, may be 90% by mass or less, or may be 80% by mass or less.

[0038] <2.1.2. Virgin Polyester Resin> The polyester resin containing terephthalate units may contain virgin polyester resin.

[0039] As the virgin polyester resin, a virgin polyester resin containing alkylene terephthalate units is preferable. When the repeating unit of the virgin polyester resin containing alkylene terephthalate units is 100 mol%, the alkylene terephthalate unit is preferably 70 mol% or more, more preferably 90 mol% or more, still more preferably 93 mol% or more, and still more preferably 95 mol% or more.

[0040] As the virgin polyester resin, virgin polyethylene terephthalate resin is more preferable. The virgin polyethylene terephthalate resin may contain a copolymerization component.

[0041] The content of the virgin polyester resin in the recycled material film may be, for example, 10% by mass or more, or may be 20% by mass or more. The content of the virgin polyester resin in the recycled material film may be, for example, 75% by mass or less, may be 65% by mass or less, or may be 50% by mass or less.

[0042] The virgin polyester resin content in the recycled material film may be, for example, 10% by mass or more, or 20% by mass or more, based on 100% by mass of the polyester resin in the recycled material film. This may also be, for example, 75% by mass or less, 65% by mass or less, or 50% by mass or less.

[0043] In polyester films, the total content of virgin polyester resin and material recycled polyester resin is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. This may be, for example, 98% by mass or more, or 100% by mass.

[0044] <2.1.3. Biomass Polyester Resins> Polyester resins containing terephthalate units may also contain biomass polyester resins. An example of a biomass polyester resin is a polyester resin in which at least one of ethylene glycol, an ester-forming derivative of ethylene glycol, terephthalic acid, and an ester-forming derivative of terephthalic acid is derived from biomass resources.

[0045] <2.2. Other Components> The recycled material film may further contain polyester resin that does not contain terephthalate units.

[0046] Recycled material films may have particles added to them. Examples of particles include inorganic particles such as silica, calcium carbonate, and alumina, and heat-resistant organic particles.

[0047] Recycled material films may have additives such as antistatic agents, UV absorbers, and stabilizers added to them.

[0048] <2.3. Physical properties, thickness, etc. of recycled material film> For recycled material film, it is preferable that the ratio of the fluorescence emission intensity at 460 nm (i.e., E1) to the fluorescence emission intensity at 395 nm (i.e., E2) (i.e., E1 / E2) in the fluorescence spectrum measured with excitation light at a wavelength of 330 nm is 0.43 or more and 0.60 or less. The explanation of E1 / E2 in recycled material film is omitted as it overlaps with the explanation of E1 / E2 in polyester film. The explanation of E1 / E2 in polyester film can also be used as an explanation of E1 / E2 in recycled material film.

[0049] Average refractive index, haze, total light transmittance, and color b per 1 μm of thickness of recycled material film. ＊ The explanation of the values ​​and intrinsic viscosity is omitted as it overlaps with their explanations in polyester films. Average refractive index, haze, total light transmittance, and color b per 1 μm thickness in polyester films. ＊ The values ​​and descriptions of intrinsic viscosity can also be treated as descriptions of those in recycled material films.

[0050] The thickness of the recycled material film is preferably 38 μm to 200 μm, and more preferably 50 μm to 190 μm. A thickness of 38 μm or more can improve the elasticity of the recycled material film and improve the shape retention ability of the polyester film. A thickness of 200 μm or less is advantageous for weight reduction and may also be advantageous in terms of flexibility, processability, and handling. The recycled material film may be a single-layer structure or a structure of two or more layers. For example, the recycled material film may include a first skin layer, a core layer, and a second skin layer.

[0051] The thickness of the recycled material film is preferably 90% or more, and more preferably 95% or more, of the total thickness of the polyester film. This could be, for example, 98% or more, or even 100%.

[0052] The recycled material film may be an unoriented film, a uniaxially oriented film, or a biaxially oriented film.

[0053] The recycled material film may be surface-treated. Plasma treatment is one example of such a surface treatment.

[0054] <3. Functional Layer> The functional layer is preferably a layer formed of a coating agent, as described below. In other words, the functional layer is mainly a coating layer.

[0055] The functional layer may be an easy-adhesion layer, a hard coat layer, an antistatic layer, an adhesive layer, a gas barrier layer, or a release layer. The functional layer may also have multiple functions.

[0056] The functional layer contains a resin. Examples of resins include polyester resin, polyurethane resin, acrylic resin, and polyether resin. Among these, polyester resin, polyurethane resin, and acrylic resin are preferred. The functional layer may also contain a crosslinking agent. Examples of crosslinking agents include epoxy-based, melamine-based, isocyanate-based, and carbodiimide-based agents.

[0057] The functional layer may contain particles. Examples of particles include the inorganic particles and organic particles mentioned above.

[0058] The average particle size is preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more. When the average particle size is 10 nm or more, the particles tend not to aggregate easily. The average particle size is preferably 1000 nm or less, more preferably 800 nm or less, and even more preferably 600 nm or less. When the average particle size is 1000 nm or less, it is possible to avoid excessively low transparency. It is also possible to reduce particle shedding.

[0059] <4. Method for Manufacturing Polyester Film> When the recycled material film is a biaxially oriented film, the recycled material film can be manufactured, for example, by the following procedure. A material for forming the recycled material film (for example, a material-recycled polyester resin containing terephthalate units) is supplied to an extruder, melt-extruded from a die, then solidified in a casting drum, biaxially stretched, heat-fixed, and loosened as necessary to produce the recycled material film. The biaxial stretching may be simultaneous biaxial stretching or sequential biaxial stretching. In sequential biaxial stretching, for example, it is preferable to stretch the unstretched film that has passed through the casting drum in the longitudinal direction, i.e., the machine direction, and then stretch the film after longitudinal stretching in the width direction, i.e., the transverse direction. The longitudinal stretching temperature, longitudinal stretching ratio, width direction stretching temperature, and width direction stretching ratio can be set as appropriate.

[0060] In sequential biaxial stretching, for example, an unstretched film can be stretched 2.5 to 5.0 times in the longitudinal direction on a roll heated to 80°C to 120°C. After longitudinal stretching, the film can be guided into a hot air zone heated to 80°C to 180°C for preheating, and then stretched 2.5 to 5.0 times in the width direction. Next, it can be guided into a heat treatment zone at 160°C to 240°C and heat-treated for 1 to 60 seconds. This completes the crystal orientation. During this heat treatment process, a relaxation treatment of 1 to 12% in the width direction or longitudinal direction can be applied as needed.

[0061] Furthermore, it is preferable to thoroughly vacuum-dry the polyester resin (for example, a material-recycled polyester resin containing terephthalate units) before supplying it to the extruder.

[0062] During melt extrusion, it is preferable to perform high-precision filtration to remove foreign matter from the molten resin at a location where the molten resin is maintained at approximately 280°C.

[0063] The filtration particle size (initial filtration efficiency of 95%) is preferably 50 μm or less, and more preferably 20 μm or less. When it is 50 μm or less, foreign matter larger than 50 μm can be effectively removed. Although productivity may decrease when performing high-precision filtration of molten resin using a filter material with a filtration particle size (initial filtration efficiency of 95%) of 50 μm or less, it is possible to manufacture recycled material films with fewer protrusions caused by coarse particles.

[0064] The functional layer can be formed by coating the surface of the recycled material film with a coating agent, preferably a coating liquid. The solid content concentration of the coating liquid is preferably 2% to 35% by mass, and more preferably 4% to 15% by mass.

[0065] <5. Physical Properties, Thickness, and Applications of Polyester Films> In the fluorescence spectrum of polyester films measured with excitation light at a wavelength of 330 nm, the ratio of the fluorescence emission intensity at 460 nm (i.e., E1) to the fluorescence emission intensity at 395 nm (i.e., E2) (i.e., E1 / E2) is between 0.43 and 0.60. E1 / E2 can be positioned as an indicator of the degree of oxidative degradation of polyester resins containing terephthalate units. This will be explained below. In the fluorescence spectrum measured with excitation light at a wavelength of 330 nm, the fluorescence emission intensity at 460 nm, i.e., E1, is proportional to the amount of monohydroxyterephthalate. Monohydroxyterephthalate is produced when polyester resins containing terephthalate units undergo oxidative degradation. For example, if the polyester resin is polyethylene terephthalate resin, monohydroxyterephthalate is produced in the main chain of polyethylene terephthalate when the polyethylene terephthalate resin undergoes oxidative degradation. Therefore, the more the oxidative degradation of the polyester resin containing terephthalate units progresses, the greater the amount of monohydroxyterephthalate. On the other hand, the fluorescence emission intensity at 395 nm in the fluorescence spectrum measured with excitation light at a wavelength of 330 nm, i.e., E2, is proportional to the amount of polyester aggregates or aggregates. The amount of aggregates or aggregates is less affected by the progression of oxidative degradation of polyester resins containing terephthalate units compared to the amount of monohydroxyterephthalate. In light of the above, E1 / E2 can be positioned as an indicator of the degree of oxidative degradation of polyester resins containing terephthalate units.

[0066] Since the E1 / E2 ratio of the polyester film is 0.60 or less, the degree of oxidative degradation of the polyester resin containing terephthalate units is not excessively high, and therefore, the yellowish tint that the polyester film may exhibit can be reduced. In other words, the color b per unit thickness ＊ The value can be reduced. The E1 / E2 ratio of the polyester film is preferably 0.55 or less, and more preferably 0.50 or less.

[0067] Since the E1 / E2 ratio of the polyester film is 0.43 or higher, the thermal dimensional stability can be improved. This is thought to be because the degree of oxidative degradation of the polyester resin containing terephthalate units is not excessively low, and therefore the molecular chains are broken to some extent by oxidative degradation, resulting in improved crystallinity. The E1 / E2 ratio of the polyester film may be, for example, 0.45 or higher, or 0.47 or higher. The E1 / E2 ratio can be controlled, for example, by the method of producing the material recycled polyester resin, the content of the material recycled polyester resin, etc.

[0068] The average refractive index of the polyester film is preferably 1.603 or higher, and more preferably 1.604 or higher. When the average refractive index is 1.603 or higher, the degree of crystallinity is not excessively low, and therefore, thermal dimensional stability can be improved. The average refractive index of the polyester film may be, for example, 1.609 or lower, 1.608 or lower, or 1.607 or lower. The average refractive index of the polyester film is a value that can be calculated using the following formula: Average refractive index = (Nx + Ny + Nz) / 3 Nx is the refractive index in the longitudinal direction of the polyester film, Ny is the refractive index in the width direction of the polyester film, and Nz is the refractive index in the thickness direction of the polyester film.

[0069] The haze of the polyester film is preferably 5% or less, more preferably 3% or less, and even more preferably 1% or less. When the haze is 5% or less, the polyester film can be suitably used in applications where transparency is required. The haze of the polyester film may be, for example, 0.1% or more, 0.3% or more, 0.5% or more, or 0.7% or more.

[0070] The total light transmittance of the polyester film is preferably 85% or higher, and more preferably 90% or higher. When the polyester film is used as an optical film for displays, for example, the image visibility is excellent if the transmittance is 85% or higher. While a higher total light transmittance of the polyester film is preferable, it is acceptable even if it is 99% or lower, or even 97% or lower.

[0071] Color b per 1 μm of polyester film thickness ＊ The value is preferably 0.010 or less, and more preferably 0.050 or less. Color b ＊ If the value is 0.010 or less, when the polyester film is used, for example, as an optical film for displays, color calibration problems of the display can be avoided or improved. Color b per 1 μm of polyester film thickness ＊ The value may be, for example, 0.002 or greater, or 0.003 or greater.

[0072] The intrinsic viscosity of the polyester film is preferably 0.50 dL / g or higher, more preferably 0.52 dL / g or higher, and even more preferably 0.55 dL / g or higher. An intrinsic viscosity of 0.50 dL / g or higher avoids excessively low elastic modulus of the polyester film. The intrinsic viscosity of the polyester film is preferably 0.60 dL / g or lower. An intrinsic viscosity of 0.60 dL / g or lower facilitates the manufacture of the polyester film.

[0073] The thickness of the polyester film is preferably 38 μm to 200 μm, and more preferably 50 μm to 190 μm. A thickness of 38 μm or more can improve the elasticity of the polyester film and improve its shape retention ability. A thickness of 200 μm or less is advantageous for weight reduction and may also be advantageous in terms of flexibility, processability, and handling.

[0074] Polyester film can be used without restriction for a wide range of applications. For example, it can be used as a base film for prisms and lens sheets, hard coat film, electrode base film for touch panels, shatterproof film, anti-reflective film, polarizer protective film, polarizer protective film, surface protective film for display devices and circuits, release film for ceramic green sheets, polarizer release film, transfer film, in-mold transfer film, in-mold molded film, barrier film for aluminum and inorganic oxide vapor deposition, solar cell backsheet, circuit base film, flat cable base film, magnetic recording medium base film, ink ribbon, image receiving film, film for labels, tags, and cards, film for packaging bags, heat shrinkable film, trays, and cover tape.

[0075] <6. Various modifications can be made to the above embodiments> Various modifications can be made to the above embodiments. For example, the polyester film may consist only of recycled material film.

[0076] Furthermore, the polyester film may also include a recycled material film, a first resin layer provided on the surface (first surface) of the recycled material film, and a second resin layer provided on the back surface (second surface) of the recycled material film. The explanation of the second resin layer is omitted as it overlaps with the explanation of the first resin layer.

[0077] Next, the present invention will be described using examples and comparative examples. First, the evaluation method for characteristic values ​​and the like will be explained.

[0078] <Evaluation Method> (1) Total Light Transmittance, Haze Measured in accordance with JIS K 7136:2000 "Method for Determining Haze of Plastic Transparent Materials". A haze meter NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd. was used as the measuring instrument. At that time, the total light transmittance was measured for each of the two surfaces of the biaxially oriented polyester film, and the average value (arithmetic mean) was obtained. That is, the light of the light source was incident on one surface of the biaxially oriented polyester film to measure the total light transmittance, and the light of the light source was incident on the other surface of the biaxially oriented polyester film to measure the total light transmittance, and the arithmetic mean was obtained as the total light transmittance of the biaxially oriented polyester film. The haze was also measured in accordance with JIS K 7136:2000 "Method for Determining Haze of Plastic Transparent Materials". A haze meter NDH5000 manufactured by Nippon Denshoku Industries Co., Ltd. was used as the measuring instrument. At that time, the haze was measured by irradiating the light of the light source onto one surface of the biaxially oriented polyester film.

[0079] (2) Color b ＊ value Ten biaxially oriented polyester films were stacked and set on a color difference meter (J E2000 manufactured by Nippon Denshoku Industries Co., Ltd.) in the reflection mode to obtain the Color b ＊ value. The Color b ＊ value per 1 μm thickness was calculated by the following formula. Color b ＊ value per 1 μm thickness = (Color b ＊ value of ten stacked biaxially oriented polyester films) / (10 × thickness of the biaxially oriented polyester film)

[0080] (3) A fluorescence emission intensity sample (specifically, a biaxially oriented polyester film) was weighed and a 1.0% by mass trifluoroacetic acid solution was prepared (hereinafter referred to as the "prepared solution"). The prepared solution was placed in a sample measurement cell (10 mm inner diameter x 10 mm, 45 mm height), covered with a quartz glass plate, and mounted in the sample holder of a spectrofluorometer (Hitachi High-Tech, F-7000 model). The fluorescence emitted by the incident excitation light was extracted in a perpendicular direction and introduced into the spectrometer to measure the fluorescence spectrum under the following conditions. Measurement conditions: SCAN SPEED: 1200 nm / min; EXCITATION SLIT: 5 nm; EMISSION SLIT: 5 nm; EXCITATION WAVELENGTH: 330 nm; EMISSION START WAVELENGTH: 350 nm; EMISSION END WAVELENGTH: 800 nm

[0081] In the fluorescence emission intensity obtained by the above method, the fluorescence emission intensity at 395 nm (E2) can be treated as a peak intensity originating from polyester aggregates or aggregates. On the other hand, the fluorescence emission intensity at 460 nm (E1) can be treated as a peak originating from the oxidized structure in the polyester molecular chain (Reference: Norman S. Allen et al. Polymer Degradation and Stability, G000, vol. 67, no. 2, p325-334).

[0082] (4) Intrinsic viscosity (IV) Approximately 3 g of the sample was freeze-dried and dried at 140°C for 15 minutes, and then 0.20 g was weighed out. This was completely dissolved in 20 mL of a mixed solvent of 1,1,2,2-tetrachloroethane and p-chlorophenol in a 1:3 (mass ratio) ratio at 100°C for 60 minutes under stirring, cooled to room temperature, and passed through a glass filter. The settling time of the obtained sample solution and the settling time of the mixed solvent were measured using an Ubbelohde viscometer (manufactured by Rigosha Co., Ltd.) heated to 30°C, and the intrinsic viscosity [η] was determined by the following formula. [η] = (-1 + √(1 + 4K'ηSp)) / 2K'C ηSp = (τ - τ0)τ0 Where, [η]: Intrinsic viscosity (dL / g) ηSp: Specific viscosity (-) K': Huggins constant (= 0.33) C: Concentration (= 1 g / dL) τ: Sample settling time (sec) τ0: Solvent settling time (sec)

[0083] (5) Refractive Index The refractive index was measured using an Abbe refractometer manufactured by Atago Co., Ltd. at a wavelength of 589.3 nm and 23°C. This will be explained below. An intermediate solution was applied to the prism surface of the Abbe refractometer. Then, with one side of the biaxially oriented polyester film facing the prism surface, the biaxially oriented polyester film and the prism were brought into close contact. At this time, the biaxially oriented polyester film was lightly pressed to push out excess intermediate solution. An intermediate solution was dropped onto the biaxially oriented polyester film in close contact with the prism, and a glass piece was placed on top. The refractive index was measured in this state. In this state, the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction were measured. An intermediate solution was applied to the prism surface of the Abbe refractometer. Then, with the other side of the biaxially oriented polyester film facing the prism surface, the biaxially oriented polyester film and the prism were brought into close contact. At this time, the biaxially oriented polyester film was lightly pressed to push out excess intermediate solution. An intermediate solution was dropped onto the biaxially oriented polyester film in close contact with the prism, and a glass piece was placed on top. The refractive index was measured in this state. In this state, the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction were measured. The average value (arithmetic mean) of the refractive index in the longitudinal direction obtained by the above two methods was calculated as Nx. The average value (arithmetic mean) of the refractive index in the width direction obtained by the above two methods was calculated as Ny. The average value (arithmetic mean) of the refractive index in the thickness direction obtained by the above two methods was calculated as Nz. Then, the average refractive index of the biaxially oriented polyester film was calculated using the following formula: Average refractive index = (Nx + Ny + Nz) / 3

[0084] (6) If the amount of material-recycled polyester resin was 20% by mass or more of the 100% by mass of polyester resin used to produce the recycled film layer, it was judged as A. If the amount of material-recycled polyester resin was less than 20% by mass, it was judged as B.

[0085] (7) Optical properties Color b per 1 μm thickness ＊ If the value was 0.010 or less, it was determined to be A. Otherwise, it was determined to be B.

[0086] <Coating Solution (D)> (Preparation of Water-Dispersible Urethane Resin Solution (A)) In a four-necked flask equipped with a stirrer, a Liebig condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer, 27.5 parts by mass of hydrogenated m-xylylene diisocyanate, 6.5 parts by mass of dimethylolpropanoic acid, 61 parts by mass of polyhexamethylene carbonate diol with a number average molecular weight of 1800, 5 parts by mass of neopentyl glycol, and 84.00 parts by mass of acetone as a solvent were added and stirred at 75°C under a nitrogen atmosphere for 3 hours, and it was confirmed that the reaction solution reached the predetermined amine equivalent. Next, 2.2 parts by mass of trimethylolpropane was added and stirred at 75°C under a nitrogen atmosphere for 1 hour, and it was confirmed that the reaction solution reached the predetermined amine equivalent. After the reaction solution was cooled to 40°C, 5.17 parts by mass of triethylamine was added to obtain a polyurethane prepolymer solution. Next, 450 g of water is added to a reaction vessel equipped with a homodisperser capable of high-speed stirring, and the temperature is adjusted to 25°C for 2000 min. －１ While stirring and mixing, the polyurethane prepolymer solution was added and dispersed in water. Then, under reduced pressure, a portion of the acetone and water was removed to prepare a water-dispersible urethane resin solution (A) with a solid content of 34% by mass.

[0087] (Preparation of Block Polyisocyanate Aqueous Dispersion (B)) In a flask equipped with a stirrer, thermometer, and reflux condenser, 66.04 parts by mass of a polyisocyanate compound having an isocyanurate structure derived from hexamethylene diisocyanate (manufactured by Asahi Kasei Chemicals, Duranate TPA), 17.50 parts by mass of N-methylpyrrolidone, and 23.27 parts by mass of 3,5-dimethylpyrazole (dissociation temperature: 120°C, boiling point: 218°C) were added dropwise, and the mixture was held at 70°C for 1 hour under a nitrogen atmosphere. Then, 8.3 parts by mass of dimethylolpropanoic acid were added dropwise. The infrared spectrum of the reaction solution was measured to confirm that the absorption of the isocyanate group had disappeared. Then, 5.59 parts by mass of N,N-dimethylethanolamine and 132.5 parts by mass of water were added to obtain a Block Polyisocyanate Aqueous Dispersion (B) with a solid content of 40% by mass. The blocked isocyanate crosslinking agent has 4 functional groups and an NCO equivalent of 280.

[0088] (Polymerization of copolymer polyester resin (C)) In a stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser, 194.2 parts by mass of dimethyl terephthalate, 184.5 parts by mass of dimethyl isophthalate, 14.8 parts by mass of dimethyl-5-sodium sulfoisophthalate, 233.5 parts by mass of diethylene glycol, 136.6 parts by mass of ethylene glycol, and 0.2 parts by mass of tetra-n-butyl titanate were charged, and a transesterification reaction was carried out at a temperature of 160°C to 220°C for 4 hours. The temperature was then raised to 255°C, the reaction system was gradually depressurized, and the reaction was carried out under a reduced pressure of 30 Pa for 1 hour and 30 minutes to obtain copolymer polyester resin (C). The obtained copolymer polyester resin (C) was pale yellow and transparent. The reduced viscosity of copolymer polyester resin (C) was measured to be 0.70 dL / g. The glass transition temperature determined by DSC was 40°C.

[0089] (Preparation of Polyester Aqueous Dispersion (Cw)) In a reaction vessel equipped with a stirrer, thermometer, and reflux device, 15 parts by mass of copolymerized polyester resin (C) and 15 parts by mass of ethylene glycol n-butyl ether were placed and heated and stirred at 110°C until dissolved. After the copolymerized polyester resin (C) was completely dissolved, 70 parts by mass of water were gradually added while stirring. This liquid was cooled to room temperature while stirring. This produced a milky white polyester aqueous dispersion (Cw) with a solid content of 15% by mass.

[0090] (Preparation of coating solution (D)) The following coating agents were mixed with a mixed solvent of water and isopropanol to prepare coating solution (D) in which the solid content mass ratio of the water-dispersible urethane resin solution (A) / block polyisocyanate aqueous dispersion (B) / polyester aqueous dispersion (Cw) was 25 / 26 / 49. Water-dispersible urethane resin solution (A) 3.55 parts by mass Block polyisocyanate aqueous dispersion (B) 3.16 parts by mass Polyester aqueous dispersion (Cw) 16.05 parts by mass Particles 0.47 parts by mass (Dry-processed silica with an average particle size of 200 nm, solid content concentration 3.5% by mass) Particles 1.85 parts by mass (Silica sol with an average particle size of 40-50 nm, solid content concentration 30% by mass) Surfactant 0.30 parts by mass (Silicone-based, solid content concentration 10% by mass)

[0091] <Virgin Polyester Resin> (Preparation of Polyester Resin (X)) The esterification reaction vessel was heated to 200°C, at which point 86.4 parts by mass of terephthalic acid and 64.6 parts by mass of ethylene glycol were charged in. While stirring, 0.017 parts by mass of antimony trioxide, 0.064 parts by mass of magnesium acetate tetrahydrate, and 0.16 parts by mass of triethylamine were charged in as catalysts. Next, the temperature was raised under pressure and the esterification reaction was carried out under conditions of a gauge pressure of 0.34 MPa and 240°C. After that, the esterification reaction vessel was returned to atmospheric pressure and 0.014 parts by mass of phosphoric acid was added. Furthermore, the temperature was raised to 260°C over 15 minutes and 0.012 parts by mass of trimethyl phosphate was added. After 15 minutes, the mixture was dispersed using a high-pressure disperser, and after another 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reaction vessel and a polycondensation reaction was carried out under reduced pressure at 280°C. After the polycondensation reaction was complete, the mixture was filtered through a Naslon filter with a 95% cut diameter of 5 μm. It was then extruded from a nozzle into strands, cooled and solidified using pre-filtered cooling water (pore size: 1 μm or less), and cut into pellets. The resulting polyethylene terephthalate resin had an intrinsic viscosity of 0.62 dL / g and an oligomer content of 0.96% by mass. The polyethylene terephthalate resin contained virtually no inert particles or internally precipitated particles. (Hereafter, this may be referred to as polyester resin (X).)

[0092] <Material-Recycled Polyester Resins> Each material-recycled polyester resin is approximately as follows: Here, roll P is the polyester laminated film roll (P) described later. Roll Q is the polyester film roll (Q) described later. Direct recycling means material recycling without alkaline washing. The method for preparing the material-recycled polyester resin will be explained in detail below.

[0093] (Preparation of polyester resin (Y1)) Polyester resin (X) pellets were dried under reduced pressure at 150°C for 8 hours (3 Torr), then supplied to an extruder and melted at 285°C in a vacuum barrel. This polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut of 10 μm particles), extruded in sheet form through a die, and then cooled and solidified using an electrostatic casting method in contact with a casting drum with a surface temperature of 30°C to produce an unstretched film. This unstretched film was uniformly heated to 75°C using a heating roll, and then heated to 100°C with a non-contact heater to perform 3.3 times roll stretching (stretching in the machine direction). Next, coating liquid (D) was applied to the casting drum contact surface side of the uniaxially oriented film using the reverse kiss coat method so that the thickness of the resin solids after drying was 0.3 μm. Next, the uniaxially oriented film was guided to a tenter while drying, heated to 140°C and transversely stretched to 4.0 times its original size, then heat-treated at 240°C for 5 seconds with the width fixed, and further relaxed by 4% in the width direction at 210°C. This yielded a polyester laminated film roll (P) with a thickness of 100 μm. The polyester laminated film roll (P) was used industrially as a carrier film for sheet molding and then recovered as a resource in roll form. The recovered polyester laminated film roll (P) was finely cut into pieces with sides of approximately 5 mm to 30 mm. To remove the functional layer, a metal pressurized washing device was used to add 25% by mass of film fragments to a mixed aqueous solution of 1% by mass of sodium hydroxide and 5% by mass of polyethylene glycol ether (Emulgen 106, manufactured by Kao Chemical Co., Ltd.), a higher alcohol (hereinafter sometimes referred to as "alkaline washing solution"), and the mixture was stirred at 120°C for 40 minutes at a rotation speed of 200 rpm. After discharging the processing solution, the material was thoroughly drained. Next, water was added while stirring, and the mixture was washed for 15 minutes. After thoroughly draining the water, the film fragments were removed and dried at 100°C for 30 minutes. After vacuum drying, they were placed in a twin-screw molten extruder at 280°C and chipped using a conventional method. This yielded polyester resin (Y1).

[0094] (Preparation of Polyester Resin (G)) The above-mentioned polyester laminated film roll (P), recovered as a resource, was finely cut into pieces with sides of approximately 5 mm to 30 mm. To remove the functional layer, a metal pressurized washing device was used to add 25% by mass of film fragments to a mixed aqueous solution of 1% by mass of sodium hydroxide and 5% by mass of polyethylene glycol ether (Emulgen 106, manufactured by Kao Chemical Co., Ltd.), a higher alcohol (i.e., an alkaline washing solution), and the mixture was stirred at 120°C for 40 minutes at a rotational speed of 200 rpm. After draining the processing solution, the material was thoroughly drained. Next, water was added while stirring and the material was washed for 5 minutes. After thoroughly draining the water, the film fragments were removed and dried at 60°C for 30 minutes. The film fragments were put into a twin-screw molten extruder at 280°C and chipped using a conventional method. This yielded polyester resin (G). The chips were quite yellowish, possibly due to insufficient rinsing of the alkaline washing solution and insufficient drying of the chips.

[0095] (Preparation of polyester resin (H)) A coating solution with the following composition (hereinafter sometimes referred to as "melamine-based release layer forming coating solution") was applied to one side of a polyester film roll (Q) with a thickness of 100 μm as described below using reverse gravure so that the release layer thickness after drying was 50 nm, and a release film was obtained by drying at 140°C for 15 seconds. Methyl ethyl ketone 49.45 parts by mass Toluene 49.45 parts by mass Melamine compound 1.00 part by mass (manufactured by Sanwa Chemical Co., Ltd., trade name MW-30M, solids content 100%) Release agent 0.05 parts by mass (manufactured by Shin-Etsu Chemical Co., Ltd., X22-3710, solids content 100%) p-toluenesulfonic acid 0.05 parts by mass The release film was used industrially as a carrier film for sheet molding, and then recovered as a resource in roll form. The release film recovered as a resource was finely cut into pieces with sides of about 5 mm to 10 mm. The film fragments were dried and then placed in a twin-screw molten extruder at 280°C to chip them using a conventional method. This yielded polyester resin (H).

[0096] (Preparation of polyester resin (J)) Polyester resin (X) pellets were dried under reduced pressure at 150°C for 8 hours (3 Torr), then supplied to an extruder and melted at 285°C in a vacuum barrel. This polymer was filtered through a stainless steel sintered filter medium (nominal filtration accuracy: 95% cut of 10 μm particles), extruded in sheet form through a die, and then cooled and solidified using an electrostatic casting method by contacting it with a casting drum at a surface temperature of 30°C to produce an unstretched film. This unstretched film was uniformly heated to 75°C using a heating roll, then heated to 100°C with a non-contact heater to perform roll stretching (stretching in the machine direction) by 3.3 times. Next, the uniaxially oriented film was guided to a tenter while drying, heated to 140°C to stretch laterally by 4.0 times, the width was fixed and heat treated at 240°C for 5 seconds, and then relaxed by 4% in the width direction at 210°C. This obtained a polyester film roll (Q) with a thickness of 100 μm. In the manufacturing of polyester film rolls (Q), the portion of the uniaxially oriented film held by clips during transverse stretching was cut by a slitting blade during the film-forming process, resulting in waste. Additionally, when the polyester film rolls (Q) were slit to a predetermined width to produce narrow film rolls, excess material was generated at the ends in the roll width direction, resulting in waste rolls. These (specifically, waste rolls and waste generated during transverse stretching), i.e., waste from the film manufacturing process, were recovered as resources. The waste from the film manufacturing process was finely shredded to approximately 5 mm to 30 mm on each side. To remove the functional layer, a metal pressurized washing device was used. A mixed aqueous solution of 1% by mass of sodium hydroxide and 5% by mass of polyethylene glycol ether (Emulgen 106, manufactured by Kao Chemical Co., Ltd.), a higher alcohol (i.e., alkaline washing solution), was added to 25% by mass of film scraps, and the process was carried out at 120°C for 40 minutes at a rotation speed of 200 rpm. After draining the processing solution, the liquid was thoroughly drained. Then, water was added while stirring, and the mixture was washed with water for 15 minutes. After thoroughly draining the water, the film fragments were removed and dried at 100°C for 30 minutes. Next, they were vacuum-dried and then placed in a twin-screw molten extruder at 280°C to be chipped using a conventional method. This yielded polyester resin (J).

[0097] (Preparation of polyester resin (K)) Polyester resin (K) was prepared in the same manner as polyester resin (Y1), except that the stirring time after adding the film fragments to the alkaline washing solution was changed from 40 minutes to 10 minutes.

[0098] <Preparation of Biaxially Oriented Polyester Film> (Example 1) Pellets of polyester resin (Y1) and pellets of polyester resin (J) were dried under reduced pressure at 150°C for 8 hours (3 Torr), and then supplied to an extruder according to the resin composition ratio shown in Table 2 and melted at 285°C. This polymer was filtered through a stainless steel sintered filter material (nominal filtration accuracy, 95% cut of 10 μm particles), extruded in sheet form from a die, and then cooled and solidified using an electrostatic casting method in contact with a casting drum with a surface temperature of 30°C to produce an unstretched film. This unstretched film was uniformly heated to 75°C using a heating roll, and then heated to 100°C with a non-contact heater to perform roll stretching (stretching in the machine direction) by 3.3 times. Next, coating liquid (D) was applied to the casting drum contact surface side of the uniaxially oriented film by reverse kiss-coat method so that the thickness of the resin solids after drying was 0.3 μm in both cases. Next, the uniaxially oriented film was guided to a tenter while drying, heated to 140°C and transversely stretched to 4.0 times its original size, then the width was fixed and heat-treated at 240°C for 5 seconds, followed by further relaxation in the width direction by 4% at 210°C. This yielded a biaxially oriented polyester film with a thickness of 100 μm.

[0099] (Examples 2-4) A biaxially oriented polyester film with a thickness of 100 μm was obtained in the same manner as in Example 1, except that the resin composition ratio was changed according to Table 2.

[0100] (Comparative Examples 1-3, 5) A biaxially oriented polyester film with a thickness of 100 μm was obtained in the same manner as in Example 1, except that the resin composition ratio was changed according to Table 2.

[0101] (Comparative Example 4) Although film formation was carried out under the same conditions as in Example 1, except that the resin composition ratio was changed according to Table 2, the film broke during film formation due to the influence of foreign matter contained in the polyester resin (H), and a biaxially oriented polyester film could not be obtained.

[0102] (Example 5) A biaxially oriented polyester film with a thickness of 99.7 μm was obtained in the same manner as in Example 1, except that the coating solution (D) was not applied to the uniaxially oriented film. In other words, a biaxially oriented polyester film was obtained in the same manner as in Example 1, except that a functional layer was not formed.

[0103] (Example 6) A biaxially oriented polyester film with a thickness of 100.3 μm was obtained in the same manner as in Example 1, except that the coating solution (D) was applied to both sides of the uniaxially oriented film.

[0104]

[0105]

[0106] Since this invention relates to a polyester film, it has potential for industrial applications.

Claims

1. A polyester film comprising a material-recycled polyester resin, wherein the material-recycled polyester resin contains terephthalate units, and in a fluorescence spectrum measured with excitation light at a wavelength of 330 nm, the ratio of the fluorescence emission intensity at 460 nm to the fluorescence emission intensity at 395 nm is 0.43 or more and 0.60 or less.

2. The polyester film according to claim 1, wherein the average refractive index is 1.603 or higher.

3. The polyester film according to claim 1, wherein the total light transmittance is 85% or more.

4. The polyester film according to claim 1, comprising a recycled material film containing a polyester resin including the material recycled polyester resin, wherein the content of the material recycled polyester resin in the recycled material film is 25% by mass or more and 100% by mass or less of the polyester resin in the recycled material film by mass.

5. The polyester film according to claim 1, wherein the intrinsic viscosity is 0.50 dL / g or more and 0.60 dL / g or less.

6. The polyester film according to claim 1, comprising a recycled material film containing the material recycled polyester resin, and comprising a functional layer provided on the surface of the recycled material film.

7. The polyester film according to claim 6, wherein the functional layer comprises at least one resin selected from the group consisting of polyester resin, polyurethane resin, and acrylic resin.

8. The polyester film according to claim 1, wherein the raw material of the material recycled polyester resin is at least a film.

9. The polyester film according to claim 8, wherein the film of the raw material comprises a base film containing a polyester resin and a functional layer provided on the base film, and the material recycled polyester resin is a material recycled polyester resin obtained by a method comprising removing the functional layer from the film of the raw material and melting the base film.

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