polyester film

Abstract

To provide an eco-friendly polyester film serving as a release film substrate, designed to protect an adhesive layer, which enables inspection while bonded to the adhesive layer and facilitates easy identification and determination.SOLUTION: Within the absolute reflectance spectrum at wavelengths from 300 to 800 nm, a polyester film demonstrates a positive peak between 310 and 410 nm. The polyester film satisfies following (1) and (2) requirements. (1) The total light transmittance is 85.0% or more. (2) The haze is 5.0% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a polyester film. [Background technology]

[0002] Polyester films offer excellent transparency, dimensional stability, mechanical properties, heat resistance, and electrical properties. For example, release films made from biaxially oriented polyester film with a release layer primarily composed of silicone resin are used in many fields.

[0003] Common applications of release films include, for example, the bonding of a release film to an adhesive layer, such as an adhesive film formed by laminating an adhesive layer onto a substrate, or to an adhesive layer without a substrate, in order to protect the adhesive layer. The adhesive film is stored with a release film laminated on top of the adhesive layer to prevent the adhesive layer from becoming contaminated and losing its adhesive strength. Furthermore, by peeling off the release film, the adhesive layer is exposed, allowing it to be bonded to other substrates or components.

[0004] For example, in optical applications such as touch panels and displays, it is common practice to manufacture composite components by bonding optical film components with an adhesive. In such applications, these optical films and composite components are inspected with a release film attached to the adhesive layer (for example, Patent Document 1).

[0005] Furthermore, in the case of release films used for protecting display components as described above, identifiability and discriminability may be required (for example, Patent Document 2). For example, an OCA (Optical Clear Adhesive) substrate-less adhesive sheet, which consists of a laminated structure of release film (light release type) / adhesive layer / release film (heavy release type), and where only the adhesive layer remains after peeling off the release films on both sides, is typically used by peeling off the light release film, adhering one side of the exposed adhesive layer to an object surface, then peeling off the heavy release film, and adhering the other side of the exposed adhesive layer to another object surface.

[0006] Furthermore, the removed release film is unnecessary and is discarded, but with the recent rise of environmental concerns, recycling release film has become an urgent issue, and there is a need to consider ways to utilize it. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2000-141568 [Patent Document 2] Japanese Patent Publication No. 2013-67743 [Overview of the project] [Problems that the invention aims to solve]

[0008] As described in Patent Document 1 above, in the case of a release film in which the base film is a colorless, transparent polyester film, there was a problem in that it was difficult to distinguish which adhesive layer surface had a light-peel type release film attached to it. Furthermore, it was difficult to determine the presence or absence of the release film by visual inspection in the first place, and there were cases where people forgot to peel off the release film.

[0009] On the other hand, while the film described in Patent Document 2 allows for identification and distinction due to the coloring of the polyester film base material, it also presents a problem in inspection processes such as foreign object inspection, as the coloring of the film makes it easier to overlook foreign objects mixed into the product.

[0010] Furthermore, the films disclosed in Patent Documents 1 and 2 have not undergone any consideration for reducing environmental impact.

[0011] Therefore, the present invention has been made in view of the above circumstances, and its problem to be solved is to provide an environmentally friendly polyester film that, when used as a base material for a release film protecting the adhesive layer, can be inspected while bonded to the adhesive layer, and also possesses identifiability and discriminability. [Means for solving the problem]

[0012] As a result of diligent research, the inventors have found that the above problems can be solved by having the following configuration. The present invention has the following aspects.

[0013] [1] A polyester film having a positive maximum value at a wavelength of 310 to 410 nm in the spectrum of absolute reflectance at wavelengths of 300 to 800 nm, and satisfying the following requirements (1) and (2). (1) The total light transmittance must be 85.0% or higher. (2) The haze level is 5.0% or less. [2] The polyester film described in [1] above, wherein the silicon content contained in the film is 700 ppm or more. [3] The polyester film according to [1] or [2] above, wherein the y value by the color reflection method when multiple films are stacked so that the total thickness at the time of measurement is equivalent to 500 μm is 0.3166 or higher. [4]L * The value is 96.8 or less, and b * A polyester film as described in any of the above [1] to [3], having a value of 0.18 or higher. [5] A polyester film according to any of [1] to [4] above, wherein the average absolute reflectance at wavelengths of 300 to 800 nm is 6.5% or more. [6] A polyester film according to any of [1] to [5] above, which contains recycled raw materials. [7] The polyester film according to [6] above, wherein the recycled raw material contains a silicone compound. [8] The polyester film according to [6] or [7] above, wherein the recycled raw material is derived from a laminated polyester film having a silicone layer. [9] The polyester film according to [1] to [8] above, which has a laminated structure composed of at least three layers having a surface layer, an intermediate layer, and a surface layer in this order.

[10] The polyester film according to [9] above, wherein the laminated structure is composed of three layers having a surface layer, an intermediate layer, and a surface layer in this order, and the intermediate layer contains a recycled raw material.

[11] A release film having a release layer on at least one surface of the polyester film according to any one of [1] to

[10] above.

[12] A film laminate in which the release film according to

[11] above is bonded to an optical member via an adhesive layer.

[13] The film laminate according to

[12] above, wherein the optical member is a resin film or a glass substrate.

[14] The film laminate according to

[13] above, wherein the resin film is selected from any one of a polyester film, a polyimide film, and a cyclic polyolefin film.

[15] The film laminate according to

[13] or

[14] above, which is a laminated film provided with a functional layer on the surface in contact with the adhesive layer of the resin film.

[16] The film laminate according to

[15] above, wherein the functional layer is another release layer.

[17] The film laminate according to any one of

[12] to

[16] above, having a total thickness of 200 μm or less. [Effect of the Invention]

[0014] According to the present invention, when used as a base material of a release film for protecting an adhesive layer, it can be inspected in a state of being bonded to the adhesive layer, and an environmentally friendly polyester film having identification and discrimination properties is provided. [Brief Description of the Drawings]

[0015] [Figure 1] These are the absolute reflectance spectra of the polyester films of the examples and comparative examples at wavelengths of 300 to 800 nm. [Figure 2] Figure 1 shows the absolute reflectance spectrum, magnified for the wavelength range of 300-420 nm. [Modes for carrying out the invention]

[0016] Next, an example of an embodiment of the present invention will be described. However, the present invention is not limited to the embodiment described below.

[0017] <<Polyester film>> The polyester film of the present invention (hereinafter also referred to as "this film") is a polyester film having a positive maximum value at a wavelength of 310 to 410 nm in the spectrum of absolute reflectance at wavelengths of 300 to 800 nm, and satisfies the following requirements (1) and (2). (1) The total light transmittance must be 85.0% or higher. (2) The haze level is 5.0% or less.

[0018] The film is not particularly limited as long as it satisfies the above requirements, and may have a single-layer structure or a laminated (multilayer) structure. If the film has a laminated structure, it may have a two-layer structure, a three-layer structure, or more, and may have four or more layers, as long as it does not depart from the gist of the present invention. The number of layers to be laminated is not particularly limited, but it is preferably 10 layers or less. With 10 layers or less, the thickness of each layer is sufficient, so the lamination during film formation is sufficient, flow marks and the like are less likely to occur, and the quality of the film is well maintained. Furthermore, if the film has a laminated structure of two or more layers, a 2-type 3-layer or 3-type 3-layer structure is preferred, and a 2-type 3-layer structure is more preferred.

[0019] Furthermore, this film may be an unstretched film (sheet) or a stretched film. In particular, a stretched film stretched in one or two axes is preferred. Among these, a biaxially stretched film is more preferred in terms of superior balance of mechanical properties and flatness.

[0020] <Polyester> The polyester used as the raw material for this film refers to a polymer compound having ester bonds continuously in its main chain, and may be either a homopolyester or a copolymerized polyester. Specifically, polyesters obtained by polycondensation reaction of a dicarboxylic acid component and a diol component can be mentioned. Furthermore, it is preferable to use a polyester that contains more than 50 mol% of aromatic dicarboxylic acid or aliphatic dicarboxylic acid when the dicarboxylic acid component is 100 mol%.

[0021] Examples of the dicarboxylic acid component include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, and 4,4'-diphenylsulfondicarboxylic acid, as well as aliphatic dicarboxylic acids such as adipic acid, suberic acid, sebacic acid, dimer acid, dodecanedionic acid, cyclohexanedicarboxylic acid, and their ester derivatives.

[0022] Examples of the diol component include ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-hexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, 2,2-bis(4-hydroxyethoxyphenyl)propane, isosorbate, and spiroglycol.

[0023] When the above polyester is a homopolyester, it is preferable to obtain one obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic diol. Preferred aromatic dicarboxylic acids include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and preferred aliphatic diols include ethylene glycol, diethylene glycol, and 1,4-cyclohexanedimethanol. Representative homopolyesters include polyethylene terephthalate (PET) and polyethylene-2,6-naphthalenedicarboxylate (PEN), with polyethylene terephthalate being preferred.

[0024] On the other hand, the copolymerized polyester is preferably a polycondensation polymer of a dicarboxylic acid component and an aliphatic diol. The dicarboxylic acid component is preferably one or more of isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid, and oxycarboxylic acids (e.g., p-oxybenzoic acid). The aliphatic diol is preferably one or more of ethylene glycol, diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, and neopentyl glycol. The copolymerized polyester preferably contains terephthalic acid as the dicarboxylic acid component and ethylene glycol as the aliphatic diol.

[0025] If the above polyester is a copolymerized polyester, it is preferable that it is a copolymer containing 30 mol% or less of a third component. The third component is a component other than the compound that is the main component (i.e., the component with the highest content) of the dicarboxylic acid component constituting the polyester and the compound that is the main component of the diol component. For example, in copolymerized polyethylene terephthalate, it is a component other than terephthalic acid and ethylene glycol.

[0026] Furthermore, the copolymerized polyester may also contain structural units derived from difunctional compounds other than dicarboxylic acid components and aliphatic diols. The structural units derived from difunctional compounds other than dicarboxylic acid components and aliphatic diols are preferably 20 mol% or less, more preferably 10 mol% or less, relative to the total moles of all structural units constituting the polyester. Examples of difunctional compounds include various hydroxycarboxylic acids and aromatic diols.

[0027] The content of terephthalic acid in the total dicarboxylic acid components constituting this film is preferably 50 mol% or more, more preferably 70 mol% or more, and even more preferably 90 mol% or more. Furthermore, the ethylene glycol content in the total diol components constituting this film is preferably 50 mol% or more, more preferably 70 mol% or more, and even more preferably 90 mol% or more. The upper limit for the content of terephthalic acid and ethylene glycol is 100 mol%.

[0028] Normally, when polyester is produced (polycondensed) using ethylene glycol as one of the raw materials, diethylene glycol is produced as a by-product from ethylene glycol. In this specification, this diethylene glycol is referred to as by-product diethylene glycol. The amount of diethylene glycol produced as a by-product from ethylene glycol varies depending on the type of polycondensation, but it is approximately 5 mol% or less of the ethylene glycol. In this invention, diethylene glycol of 5 mol% or less is considered by-product diethylene glycol, and this by-product diethylene glycol is also included in ethylene glycol and distinguished from copolymer components. On the other hand, depending on the content of diethylene glycol, more specifically, if the content of diethylene glycol exceeds 5 mol%, the diethylene glycol is treated as a copolymer component rather than as by-product diethylene glycol.

[0029] <Recycled raw materials> The film preferably contains recycled materials. Recycled materials refer to materials obtained by recovering used products and waste generated from the manufacturing process and making them available as raw materials for new products. The products and waste referred to here also include polyester films. The recycled materials preferably contain silicone compounds, and more preferably are derived from laminated polyester films having a silicone layer.

[0030] More specifically, the recycled raw material used in the present invention is preferably a laminated polyester film in which a layer containing a silicone compound (silicone layer) is provided on at least one side of the polyester film, which is then crushed into flakes and / or chips. Among these, it is most preferable to use a laminated polyester film in which a silicone release layer is provided on at least one side of the polyester film. By using a laminated polyester film with a silicone release layer as the recycled raw material, the release film that would otherwise be discarded after being peeled off can be utilized. In this case, it is preferable to use the laminated polyester film with the silicone layer still attached as the recycled raw material, without peeling or removing the silicone layer.

[0031] Regarding the method for obtaining flakes, conventionally known methods can be used, including a method of obtaining flakes by crushing with pulverized material. Furthermore, regarding the method for turning the flakes into chips, conventionally known methods can also be used, for example, a method of obtaining chipped raw material by melt-extruding the flakes and then cutting the strand-shaped melt-extruded material.

[0032] The aforementioned silicone release layer is a layer mainly composed of silicone having release properties, and curable silicone is preferred as the silicone. The curable silicone may be a type mainly composed of curable silicone resin, or a modified silicone type produced by graft polymerization with organic resins such as urethane resin, epoxy resin, or alkyd resin.

[0033] As for the type of curable silicone resin mentioned above, any curing reaction type can be used, including addition reaction, condensation reaction, ultraviolet curing, electron beam curing, and solvent-free types. Examples of commercially available products include, for example, KS-774, KS-775, KS-778, KS-779H, KS-847H, KS-856, X-62-2422, X-62-2461, X-62-1387, X-62-5039, X-62-5040, KNS-3051, X-62-1496, KNS320A, KNS316, X-62-1574A / B, X-62-7052, X-62-7028A / B, X-62-7619, X-62-7213, and SRX357, SRX211, SD7220, SD7292, LTC750A, LTC760A, LTC303E from Dow Toray Industries, Inc. , LTC856, LTC761, SP7259, BY24-468C, SP7248S, BY24-452, DKQ3-202, DKQ3-203, DKQ3-204, DKQ3-205, DKQ3-210, manufactured by Momentive Performance Materials, YSR-3022, TPR-6700, TPR-6720, TPR-6721, TPR6500, TPR6501, UV9300, UV9425, XS56-A2775, XS56-A2982, UV9430, TPR6600, TPR6604, TPR6605, and from the DEHESIVE series manufactured by Asahi Kasei Wacker Silicone Co., Ltd., DEHESIVE Examples include 636, 919, 920, 921, 924, 929, etc.

[0034] The silicone release layer is preferably provided by a conventionally known coating method such as reverse gravure coating, direct gravure coating, roll coating, die coating, bar coating, or curtain coating, and may be provided by either offline coating or inline coating.

[0035] Furthermore, recycled materials obtained from (A) to (C) below may also be used as recycled materials. (A) Film that does not become part of the final product during the manufacturing process (for example, film edges that have been cut off from the final product) (B) Film that broke during film formation (C) Film that did not meet product standards due to quality defects, etc. The method for producing the recycled raw material obtained from (A) to (C) above is not particularly limited, but it is preferable to crush the film to obtain flakes and / or chipped flakes as the raw material.

[0036] <Particularly preferred form> A preferred form of this film is a laminated structure consisting of at least three layers having a surface layer, an intermediate layer, and another surface layer in that order. More preferably, the laminated structure consists of three layers having a surface layer, an intermediate layer, and another surface layer in that order.

[0037] The thickness of at least one of the surface layers is preferably 4.5 μm or less, more preferably 4.0 μm or less, and even more preferably 3.5 μm or less. On the other hand, the lower limit of this thickness is 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more. If the thickness is within this range, the film can have a positive maximum value at a wavelength of 310-410 nm while achieving the desired total light transmittance and haze. Furthermore, if the thickness is 3.5 μm or less, the thickness of the intermediate layer can be ensured, and if recycled raw materials are included in the intermediate layer, the recycled raw material content of this film can be increased. On the other hand, if the thickness is 2.0 μm or more, the film can be made easier to handle when particles are included in the surface layer, and the shedding of particles can be suppressed.

[0038] From a similar viewpoint, the ratio of the sum of the thicknesses of the surface layers to the sum of the thicknesses of the intermediate layers is preferably 1:3 to 1:18, more preferably 2:7 to 1:12, and even more preferably 1:4 to 1:8. In this context, "intermediate layer" refers to any layer other than the two surface layers.

[0039] If the film has a laminated structure consisting of three layers in the order of a surface layer, an intermediate layer, and another surface layer, it is preferable that the intermediate layer contains the recycled material described above. Most preferably, the intermediate layer contains a recycled material containing a silicone compound, while neither surface layer contains a recycled material containing a silicone compound. By having such a laminated structure, the film can have a positive maximum value at a wavelength of 310 to 410 nm while achieving desired values ​​for total light transmittance and haze.

[0040] Furthermore, the content of recycled raw materials containing silicone compounds in the intermediate layer, preferably recycled raw materials derived from laminated polyester films having a silicone layer (hereinafter also referred to as "silicone films"), is preferably 10 to 70% by mass. If the content is 10% by mass or more, it will have a positive maximum value at a wavelength of 310-410 nm, and can be considered an environmentally friendly film. In addition, the silicone makes the film appear whitish, which gives it identifiable and distinguishable features when used as a base material for a release film that protects the adhesive layer. On the other hand, if the content is 70% by mass or less, the total light transmittance can be set to the desired value, making it applicable to optical applications where light is transmitted throughout. Furthermore, the haze can be set to the desired value, and since the film is transparent when viewed up close, it can be inspected while bonded to an adhesive layer. From this viewpoint, the content is more preferably 20 to 60% by mass, and even more preferably 30 to 50% by mass.

[0041] The layer structure and thickness of each layer of this film are determined by observing the cross-section, obtained by cryogenic fracture using an ultramicrotome, at a magnification of 3,000 to 200,000 times using a transmission electron microscope, and taking cross-sectional photographs.

[0042] <Polycondensation catalyst> There are no particular restrictions on the polycondensation catalyst used when polycondensing the above polyesters; conventionally known compounds can be used, such as titanium compounds, germanium compounds, antimony compounds, manganese compounds, aluminum compounds, magnesium compounds, and calcium compounds.

[0043] <Intrinsic viscosity> The intrinsic viscosity (IV) of this film is preferably 0.50 dL / g or higher, more preferably 0.52 dL / g or higher, and even more preferably 0.54 dL / g or higher. Within this range, there are advantages such as increased particle dispersion due to increased shear stress during kneading. Alternatively, the intrinsic viscosity (IV) may be, for example, 1.00 dL / g or lower.

[0044] <particle> This film can also contain particles. By containing particles, the polyester film is given slipperiness and prevents scratches during each process, resulting in improved handling. The type of particles to be contained in this film is not particularly limited as long as they can impart slipperiness. Specific examples include inorganic particles such as silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, magnesium phosphate, kaolin, aluminum oxide, and titanium oxide, as well as crosslinked polymers such as crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked styrene-acrylic resin particles, and crosslinked polyester particles, and organic particles such as calcium oxalate and ion exchange resins. Among these, silica is preferred from the viewpoint of optical properties such as transparency. Furthermore, during the polyester manufacturing process, precipitated particles obtained by precipitating and finely dispersing a portion of metal compounds such as catalysts can also be used.

[0045] There are no particular restrictions on the shape of the particles used; spherical, lumpy, rod-shaped, flattened, or any other shape may be used. Furthermore, there are no particular restrictions on their hardness, specific gravity, color, etc. Two or more types of these particles may be used in combination as needed.

[0046] Furthermore, the average particle size of the particles used is usually 5 μm or less, preferably 0.01 to 3 μm, more preferably 0.02 to 1.5 μm, and even more preferably 0.03 to 1 μm. Within this range of average particle size, both the handling properties and surface smoothness of the film can be achieved. Furthermore, if the particles are in powder form, the average particle size can be determined by using a centrifugal sedimentation particle size distribution analyzer (e.g., Shimadzu Corporation's "SA-CP3" model) to measure the particle size distribution at 50% of the cumulative volume fraction (d50). For particles in films, layers, or resins, the average particle size can be determined by observing 10 or more particles with a scanning electron microscope (SEM), measuring the diameter of each particle, and taking the average value. In the case of non-spherical particles, the average of the longest and shortest diameters can be used as the diameter of each particle.

[0047] When incorporating particles into this film, it is preferable to provide a surface layer and an intermediate layer, with the particles being incorporated into the surface layer. Furthermore, in the case of a three-layer structure with different designs on the front and back, it is also possible to incorporate particles into at least one of the surface layers. Therefore, it is preferable to incorporate particles into at least one of the surface layers, and as mentioned above, silica is more preferably used as the particles. The particle content, although it also depends on the average particle size, is preferably 5,000 ppm or less by mass in the particle-containing layer, more preferably 4,000 ppm or less, and even more preferably 3,000 ppm or less. Within this range, the transparency of the film can be made good. If no particles are present, or if the particle content is low, the slipperiness may be insufficient, so the particle content is preferably 50 ppm or more, and more preferably 100 ppm or more.

[0048] The method for adding particles to this film is not particularly limited, and conventionally known methods can be employed. For example, in the case of a laminated polyester film, the particles can be added at any stage in the production of the polyester constituting each layer, but it is preferable to add them after the esterification or transesterification reaction is completed.

[0049] <Other> To suppress the precipitation of oligomer components, the film may be manufactured using polyester with a low oligomer content as the raw material. Various known methods can be used to manufacture polyester with a low oligomer content, such as a method of solid-phase polymerization after polyester production. Furthermore, the film may be constructed with three or more layers, and the surface layer of the film may be made of a polyester raw material with a low oligomer content to suppress the amount of oligomer component precipitated. Alternatively, polyester may be obtained by esterification or transesterification, followed by further increasing the reaction temperature and melt polycondensation under reduced pressure.

[0050] In addition to the particles mentioned above, conventionally known ultraviolet absorbers, antioxidants, antistatic agents, heat stabilizers, lubricants, dyes, pigments, etc., may be added to this film as needed.

[0051] The total thickness of this film is not particularly limited as long as it is within the range that can be formed as a film, but from the viewpoint of mechanical strength, handling and productivity, it is preferably 5 μm or more, more preferably 10 μm or more, even more preferably 15 μm or more, particularly preferably 20 μm or more, and preferably 200 μm or less, more preferably 125 μm or less, even more preferably 80 μm or less, particularly preferably 50 μm or less.

[0052] <<Method for manufacturing polyester film>> Next, we will specifically describe examples of the manufacturing of this film, but the manufacturing examples are not limited to those listed below. For example, when manufacturing a biaxially oriented film, it is preferable to extrude the dried pellets of the polyester raw material (including recycled raw material) mentioned above as a molten sheet from a die using a melt extrusion device such as an extruder, and then cool and solidify them on a cooling roll such as a rotating cooling drum to obtain an unstretched sheet. Here, cooling is carried out to a temperature below the glass transition point of the polymer, for example, to obtain a substantially amorphous, unoriented sheet (unstretched sheet). Furthermore, in order to improve the flatness of the sheet, it is preferable to increase the adhesion between the sheet and the cooling roll, and electrostatic application adhesion and / or liquid coating adhesion methods are preferably employed.

[0053] Next, the obtained unstretched sheet is stretched in two axial directions. In this case, first, the unstretched sheet is stretched in one direction using a roll or tenter type stretcher. The stretching temperature is usually 70 to 120°C, preferably 80 to 110°C, and the stretching ratio is usually 2.5 to 7.0 times, preferably 3.0 to 6.0 times.

[0054] Next, the material is stretched in a direction perpendicular to the stretching direction of the first stage. In this case, the stretching temperature is usually 70 to 170°C, and the stretching ratio is usually 3.0 to 7.0 times, preferably 3.5 to 6.0 times.

[0055] Next, a heat treatment is performed at a temperature of typically 180-270°C, under tension or with a relaxation of up to 30%, to obtain a biaxially oriented film. This heat treatment is also called the heat setting process. The heat treatment may be carried out in two or more stages at different temperatures. Furthermore, cooling may be performed in a cooling zone after the heat treatment. The cooling temperature is preferably higher than the glass transition temperature (Tg) of the polyester constituting the film, and more specifically, it is preferably in the range of 100 to 160°C. This cooling may be carried out in two or more stages at different temperatures. In the stretching described above, a method can be adopted in which stretching is performed in two or more stages in one direction. In that case, it is preferable to perform the stretching so that the final stretching ratios in both directions fall within the above ranges.

[0056] Furthermore, simultaneous biaxial stretching can also be used in the manufacture of this film. Simultaneous biaxial stretching is a method of simultaneously stretching and oriented the aforementioned unstretched sheet in the machine direction (longitudinal direction) and width direction (transverse direction) while the temperature is controlled, usually at 70 to 120°C, preferably 80 to 110°C. The stretching ratio is preferably 4 to 50 times, more preferably 7 to 35 times, and even more preferably 10 to 25 times in terms of area. Then, heat treatment is carried out at a temperature of typically 170-250°C under tension or under relaxation of 30% or less to obtain a stretched and oriented film. For the simultaneous biaxial stretching apparatus employing the above stretching method, conventional known stretching methods such as screw type, pantograph type, and linear drive type can be used.

[0057] The longitudinal direction (MD) of the film refers to the direction in which the film progresses during the film manufacturing process, i.e., the winding direction of the film roll, and is also called the machine direction or vertical direction. The width direction (TD) of the film refers to the direction parallel to the film surface and perpendicular to the longitudinal direction; in other words, when the film is in a roll, it refers to the direction parallel to the central axis of the roll, and is also called the transverse direction.

[0058] <Particularly preferred form> A preferred form of the method for manufacturing this film is a manufacturing method having the following steps (I) to (IV). (I) A step of preparing a laminated polyester film having a silicone layer, preferably a laminated polyester film having a silicone release layer. (II) A step of preparing recycled raw materials by crushing the laminated polyester film, with the silicone layer still attached, to form flakes and / or chips from the flakes. (III) A process in which the recycled raw material is used as the raw material for at least a portion of the intermediate layer, the raw materials for each layer are fed into separate extruders, and the process involves melt-extrusion and cooling to form a film. (IV) A process of stretching in the longitudinal direction (MD) and / or the width direction (TD).

[0059] <<Physical Properties of Polyester Film>> This film has a positive maximum value in the absolute reflectance spectrum at wavelengths of 300 to 800 nm at wavelengths of 310 to 410 nm, more preferably at wavelengths of 310 to 380 nm, and even more preferably at wavelengths of 310 to 360 nm. Here, the positive maximum value may have a peak shape, but it also includes values ​​that are flat from a predetermined point. Preferably, the peak height after subtracting the baseline is 0.1% or more, more preferably 0.2% or more, and particularly preferably 0.3% or more. There is no particular upper limit, but 2.0% or less is practical. The aforementioned maximum value is thought to be a peak originating from the silicone compound contained in the film. Although the mechanism by which this silicone compound has a positive maximum value at wavelengths of 310-410 nm is not clear, it is thought that the silicone compound in the film has a specific reflectance peak due to a certain degree of uniform aggregation. This uniform aggregation morphology of the silicone makes the film appear whitish, which contributes to the film's identifiability and discriminability. Furthermore, as mentioned above, the silicone compound in the film is derived from recycled raw materials, most preferably from recycled raw materials derived from laminated polyester film with a silicone release layer. To be precise, the silicone compound is not limited to being derived from recycled raw materials, but since silicone compounds are not usually intentionally included, if the film has the aforementioned maximum value, it can be said that this film contains recycled raw materials and is an environmentally friendly film.

[0060] The average absolute reflectance of this film at wavelengths of 300 to 800 nm is preferably 6.5% or higher, more preferably 6.6% or higher, and even more preferably 6.7% or higher. If the average absolute reflectance is 6.5% or higher, it can be said that an increase in reflectance is observed due to a positive maximum value at wavelengths of 310 to 410 nm, and that this film contains recycled raw materials including a silicone compound. The average absolute reflectance is not particularly limited, but it is preferably 8.0% or lower, more preferably 7.5% or lower, and even more preferably 7.0% or lower. When the average absolute reflectance is above the above predetermined value, it contributes to the identifiability and discriminability of the film. On the other hand, if the average absolute reflectance exceeds the above upper limit, it means that the film becomes too whitish, resulting in poor inspection suitability. Furthermore, the average absolute reflectance can be adjusted depending on the type and content of the polyester used in this film, particularly the recycled material.

[0061] The total light transmittance of this film is 85.0% or higher. If the total light transmittance is less than 85.0%, it may be difficult to apply it to optical applications where light needs to be transmitted overall. From this viewpoint, a total light transmittance of 86.0% or higher is preferable, and more preferably 87.0% or higher. While a higher total light transmittance is generally better, from the standpoint of stable production, it may be 99.0% or lower, 95.0% or lower, or 90.0% or lower. Furthermore, the average light transmittance of this film at wavelengths of 300 to 800 nm is preferably 70% or higher, more preferably 75% or higher, and even more preferably 80% or higher. Within this range, it can be used without any general problems. There is no particular upper limit, but it is practical to keep it at 99% or lower. Furthermore, the total light transmittance can be adjusted depending on the type and content of the polyester used in this film, particularly the recycled material.

[0062] The haze of this film is 5.0% or less. If the haze exceeds 5.0%, the transparency is insufficient, and for example, when used as a base material for a release film protecting the adhesive layer, it may be difficult to inspect the film in the state where it is bonded to the adhesive layer. From this viewpoint, the haze is preferably 4.5% or less, and more preferably 4.0% or less. The lower limit is not particularly limited, but is approximately 0.01%. The internal haze of this film is preferably 1.3% or more, more preferably 1.4% or more, and even more preferably 1.5% or more. If the internal haze is 1.3% or more, the increase in internal haze will cause the film to appear whitish when viewed from a distance, providing identifiable and distinguishable features when used as a base material for a release film protecting the adhesive layer. There is no particular upper limit, but it is approximately 3.7%. Furthermore, the haze can be adjusted depending on the type and content of the polyester used in this film, particularly the type of recycled material.

[0063] The silicon content in this film is preferably 700 ppm or more. If the silicon content is 700 ppm or more, the film will have a positive maximum value at a wavelength of 310 to 410 nm due to the peak derived from the silicone compound contained in the film. Although this silicon content includes silicon such as silica particles included for the purpose of providing slipperiness, if the silicon content is within this range while having a positive maximum value at a wavelength of 310 to 410 nm, it can be said that the film contains sufficient recycled raw materials including the silicone compound. From this viewpoint, the silicon content is more preferably 800 ppm or more, even more preferably 900 ppm or more, and particularly preferably 1,000 ppm or more. On the other hand, from the viewpoint of film transparency, the silicon content is preferably 3,000 ppm or less, more preferably 2,500 ppm or less, and even more preferably 2,000 ppm or less. The silicon content can be adjusted depending on the type and amount of recycled materials used in the polyester film.

[0064] When multiple films are stacked so that the total thickness during measurement of the film is equivalent to 500 μm, the y value by the colorimetric reflection method is preferably 0.3166 or more. The fact that the y value is 0.3166 or more indicates that the film has a strong yellow tint, meaning that the film uses recycled raw materials. That is, a film with a y value of 0.3166 or more can be said to be environmentally considerate. In particular, considering the recent requirements for reducing environmental impact, it is considered important to be able to distinguish whether it is environmentally considerate or not. From such a perspective, the y value is preferably 0.3167 or more, more preferably 0.3168 or more, and even more preferably 0.3170 or more. On the other hand, from the perspective of clearly distinguishing coloring due to deterioration, the upper limit value of the y value is preferably 0.3300 or less, more preferably 0.3200 or less. Incidentally, the y value can be adjusted according to the type of polyester used in the film, particularly the type and content of recycled raw materials.

[0065] The L value of this film * is preferably 96.8 or less, and the b * value is preferably 0.18 or more. The L * value represents brightness, and the larger the numerical value from 0 to 100, the brighter it is. The b * value represents yellow - blue tone, and the more positive the direction, the stronger the yellow tint. Therefore, if the L * value and the b * value are within such ranges, it is presumed that the film uses recycled raw materials and can be said to be an environmentally considerate film. From such a perspective, the L * value is more preferably 96.7 or less, even more preferably 96.6 or less, and particularly preferably 96.5 or less. Also, the b * value is more preferably 0.20 or more, even more preferably 0.25 or more, and particularly preferably 0.30 or more. Incidentally, without particular limitation, the lower limit value of the L * value is about 95.0, and the upper limit value of the b * value is about 1.0. Incidentally, the L * value and the b* The value can be adjusted depending on the type and content of the polyester used in this film, particularly the recycled material.

[0066] <<Release film>> The release film of the present invention (hereinafter also referred to as "this release film") has a release layer on at least one surface of this film. The release layer is a layer provided to impart release properties to the polyester film. There are no particular restrictions on the materials that make up the release layer, and conventionally known materials can be used. Examples include those mainly composed of curable silicone resin, or modified silicone resins obtained by graft polymerization with urethane resin, epoxy resin, etc., long-chain alkyl group-containing compounds, fluorine compounds, hydrocarbon waxes, etc.

[0067] The method for forming the release layer is not particularly limited, and conventionally known coating methods such as reverse gravure coating, direct gravure coating, roll coating, die coating, bar coating, and curtain coating can be used. Examples of coating methods can be found in "Coating Methods," by Yuji Harasaki, published by Maki Shoten in 1979.

[0068] Furthermore, methods for forming the release layer include in-line coating and off-line coating. The drying and curing conditions are not particularly limited; for example, when providing a release layer by off-line coating, it is generally recommended to perform heat treatment at 80-200°C for 3-40 seconds, preferably at 120-180°C for 3-40 seconds. On the other hand, when providing a release layer by in-line coating, it is generally recommended to perform heat treatment at 70-280°C for 3-200 seconds.

[0069] Furthermore, regardless of whether it is offline coating or in-line coating, heat treatment and active energy ray irradiation such as ultraviolet irradiation may be used in combination as needed.

[0070] Furthermore, the polyester film that constitutes this release film may be subjected to surface treatments such as corona treatment or plasma treatment beforehand.

[0071] <<Film Laminate>> The film laminate of the present invention (hereinafter also referred to as "this laminate") is in a form in which the release film is bonded to an optical member via an adhesive layer.

[0072] As the adhesive for forming the adhesive layer, acrylic adhesives, urethane adhesives, synthetic rubber adhesives, natural rubber adhesives, silicone adhesives, polyester adhesives, vinyl alkyl ether adhesives, epoxy adhesives, etc., can be used. Among these, acrylic adhesives, urethane adhesives, and silicone adhesives are preferred, and silicone adhesives are more preferred from the viewpoint of heat resistance and transparency. In addition, as long as it does not impair the optical properties, two or more materials can be mixed and used, or two or more layers can be used. Furthermore, the adhesive may be of emulsion type, solvent type, or solvent-free type, and may be of crosslinked type or non-crosslinked type. Furthermore, particles such as fillers or additives can be added to the adhesive to form the adhesive layer.

[0073] The optical component is a resin film or a glass substrate. The resin film is preferably a resin film selected from polyester film, polyimide film, or cyclic polyolefin film. The glass substrate is preferably ultrathin glass (UTG). Furthermore, it is preferable that the laminated film has a functional layer provided on the surface of the resin film that is in contact with the adhesive layer. A release layer is preferred as the functional layer. This functional layer is a different layer from the release layer in this release film, and will be referred to as "another release layer" for convenience below.

[0074] The aforementioned "other release layer" preferably contains a curable silicone resin from the viewpoint of release properties from the adhesive layer. An example of the aforementioned "other release layer" is a structure comprising, in sequence, a first layer formed from a silicone composition mainly containing a curable silicone resin that does not contain a fluorine substituent, and a second layer containing a component having a fluorine substituent.

[0075] Another example of the "other release layer" is a layer formed from a silicone composition mainly containing a curable silicone resin containing a fluorine substituent.

[0076] Furthermore, another example of the "other release layer" is a layer formed from a silicone composition mainly containing a curable silicone resin that does not contain fluorine substituents.

[0077] In this context, "principal component" refers to the component that has the largest mass proportion among the constituent components.

[0078] From the viewpoint of ease of handling, the total thickness of this laminate is preferably 200 μm or less, more preferably 9 μm to 125 μm, even more preferably 12 μm to 125 μm, and particularly preferably 25 μm to 125 μm.

[0079] The method for manufacturing this laminate is not particularly limited, but for example, the laminate can be manufactured by applying an adhesive layer forming liquid to the release layer of the release film with an applicator to form an adhesive layer, and then laminating another resin film or glass substrate onto the adhesive layer. The method for applying the adhesive layer forming liquid is not particularly limited and can be carried out by conventionally known methods. Furthermore, as mentioned above, other resin films may have a functional layer provided on the surface that comes into contact with the adhesive layer.

[0080] <<Application>> This film can be suitably used as a polyester base film in a release film that protects the adhesive layer. When used as such a release film, it can be inspected while bonded to the adhesive layer, while also possessing identifiability and discriminability. Furthermore, this film can be suitably used for optical applications such as touch panels and displays. However, the present invention is not limited to such uses.

[0081] This film, this release film, and this laminate can be suitably used in the following forms, but are not limited to such uses. In this laminate, which comprises a release film having a release layer on at least one surface of the film, laminated on one side of an adhesive layer, and a release film (heavy-release type) with higher peel strength than the release film laminated on the opposite side of the adhesive layer, the laminate can be used by peeling off the release film, attaching the exposed adhesive layer surface to an object such as an optical component, curing the adhesive layer, and then peeling off the release film (heavy-release type) with higher peel strength. However, the method of use is not limited to this, and the release film can also be used as a release film (heavy-release type) with higher peel strength.

[0082] Examples of adherends include optical components. As mentioned above, examples of optical components include resin films selected from polyester films, polyimide films, and cyclic polyolefin films, or glass substrates such as ultrathin glass (UTG).

[0083] <<Explanation of terms>> In this invention, the term "film" includes "sheets," and the term "sheet" includes "film." In this invention, when "X~Y" (where X and Y are any numbers) is written, unless otherwise specified, it means "X or greater and Y or less," and also includes the meaning of "preferably greater than X" or "preferably less than Y." Furthermore, when "X or greater" (where X is any number) is written, unless otherwise specified, it includes the meaning of "preferably greater than X," and when "Y or less" (where Y is any number) is written, unless otherwise specified, it also includes the meaning of "preferably less than Y." [Examples]

[0084] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following embodiments, unless it exceeds the gist of the invention.

[0085] <Evaluation Method> (1) Intrinsic viscosity (IV) One g of polyester, from which incompatible components had been removed, was accurately weighed, and 100 mL of a phenol / tetrachloroethane mixed solvent (50 / 50 by mass ratio) was added to dissolve it. The viscosity was then measured at 30°C using a viscosity measuring device "VMS-022UPC·F10" (manufactured by Rigosha Co., Ltd.).

[0086] (2) Average particle size For the average particle size of the particles contained in the polyester, the particle size at which the cumulative volume fraction of 50% of the equivalent spherical distribution was measured using a centrifugal sedimentation particle size distribution analyzer (SA-CP3 type) manufactured by Shimadzu Corporation was defined as the average particle size d50.

[0087] (3) Total light transmittance, haze, internal haze and surface haze Measurements were taken using a haze meter (HZ-2) manufactured by Suga Test Instruments Co., Ltd., in accordance with JIS K 7136:2000. The measurement was performed by sandwiching the film between two jigs, each with a 50 mm diameter circular cutout in the center, and inserting them into a glass cell. Ethanol was then filled into the glass cell as a surface compensation solvent for the film.

[0088] (4)Light transmittance Using a spectrophotometer (UV-Vis spectrophotometer "V-670" manufactured by JASCO Corporation), the light transmittance in the wavelength range of 300-800 nm was measured with the air layer as a standard, and the average light transmittance in the wavelength range of 300-800 nm was calculated.

[0089] (5) Absolute reflectance Using a spectrophotometer (UV-Vis spectrophotometer "V-670" and automatic absolute reflectance analyzer "ARMN-735" manufactured by JASCO Corporation), the absolute reflectance in the wavelength range of 300-800 nm was measured in synchronous mode, with an incident angle of 5°, N polarization, fast response, data interval of 1.0 nm, bandwidth of 10 nm, and scanning speed of 1000 m / min. The presence or absence of a positive maximum value at wavelengths of 310-410 nm was confirmed. The average absolute reflectance at wavelengths of 300-800 nm was also calculated. The results of the absolute reflectance measurements for each sample in Examples 1 and 2 and Comparative Examples 1 and 2 are shown in the graphs in Figures 1 and 2. Figure 1 is the spectrum of absolute reflectance at wavelengths of 300-800 nm, and Figure 2 is an enlarged view of the spectrum from Figure 1, showing the portion at wavelengths of 300-420 nm.

[0090] (6) Color (y value) The color (y-value) of the polyester film was determined using the Konica Minolta Japan Co., Ltd. spectrophotometer "CM-3700d" as follows. Samples were obtained by punching out a sample from a designated location using a round holder blade with a diameter of approximately 60 mm. The number of test sheets was determined to be the number of sheets that resulted in a stacking thickness (total thickness at the time of measurement) closest to 500 μm. For example, for a 25 μm film, 20 films should be stacked, and for a 38 μm film, 13 films should be stacked. The measurement conditions were reflective conditions. The color (y value) was measured in an environment of 23°C.

[0091] (7)L * Value and b * value Polyester film L * Value and b * The values ​​were determined using the Konica Minolta Japan Co., Ltd. spectrophotometer "CM-3700d" as follows. Samples were obtained by punching out a sample from a designated location using a round holder blade with a diameter of approximately 60 mm. The number of test sheets was determined to be the number of sheets that resulted in a stack thickness (total thickness at the time of measurement) closest to 500 μm. For example, for a 25 μm film, 20 films should be stacked, and for a 38 μm film, 13 films should be stacked. The measurement conditions were reflective conditions. Note that L * Value and b * The values ​​were measured in an environment of 23°C.

[0092] (8) Silicon content A polyester film was subjected to quantitative analysis of the Si element using an X-ray fluorescence analyzer (XRF, Shimadzu Corporation "XRF-1800").

[0093] <Materials used> Raw material A: Homopolyethylene terephthalate (intrinsic viscosity = 0.64 dL / g) Raw material B: A masterbatch containing 0.6% by mass of silica particles with an average particle size of 2.7 μm, made from homopolyethylene terephthalate (intrinsic viscosity = 0.61 dL / g). Raw material C: A masterbatch containing 0.7% by mass of silica particles with an average particle size of 2.7 μm, made from homopolyethylene terephthalate (intrinsic viscosity = 0.59 dL / g). Raw material D: Laminated polyester film having a silicone release layer on one side of homopolyethylene terephthalate film; used commercially available product (Mitsubishi Chemical Corporation's "MRF38"; also referred to as "silicone film" in Table 2) crushed into flakes (intrinsic viscosity = 0.58 dL / g) Raw material E: Homopolyethylene terephthalate (intrinsic viscosity = 0.59 dL / g) Raw material F: Recycled homopolyethylene terephthalate (intrinsic viscosity = 0.58 dL / g) made from polyester film without a coating layer.

[0094] Raw materials A, B, C, and E are virgin raw materials, while raw materials D and F are recycled raw materials.

[0095] (Example 1) Mixed raw materials A and B were prepared in proportions of 75% by mass and 25% by mass, respectively, to form the raw materials for both surface layers. Mixed raw materials A and D were prepared in proportions of 70% by mass and 30% by mass, respectively, to form the raw material for the intermediate layer. Each of the raw materials for the surface and intermediate layers was supplied to two extruders, melted at 280°C, and then co-extruded onto a cooling roll set at 25°C in a layer configuration of 2 types and 3 layers (surface layer / intermediate layer / surface layer = discharge volume 1 / 10 / 1) and cooled and solidified to obtain an unstretched sheet. Next, the obtained unstretched sheet was stretched 3.2 times in the longitudinal direction (MD) at 86°C using a roll stretcher. Furthermore, after preheating in a tenter at 100°C, it was stretched 4.3 times in the width direction (TD) at 115°C. Finally, it was heat-treated at 230°C to obtain a biaxially oriented polyester film with a thickness of 38 μm (each surface layer: 3.2 μm, intermediate layer: 31.6 μm). The evaluation results are shown in Table 2.

[0096] (Example 2) The procedure was the same as in Example 1, except that the composition and film formation conditions were as described in Table 1 below. The evaluation results are shown in Table 2.

[0097] (Comparative Examples 1-2) The procedure was the same as in Example 1, except that the composition and film formation conditions were as described in Table 1 below. The evaluation results are shown in Table 2.

[0098] [Table 1]

[0099] [Table 2]

[0100] In the films of Examples 1 and 2, the total light transmittance is 85.0% or higher, and the haze is 5.0% or lower. Since the total light transmittance is sufficiently high and the haze is sufficiently low, the polyester film is suitable for inspection when used as a base material for a release film that protects the adhesive layer, and when the adherend such as an optical component is inspected in the state where it is bonded via the adhesive layer. On the other hand, as in Examples 1 and 2, the presence of a positive maximum value at a wavelength of 310-410 nm in the absolute reflectance spectrum (see Figures 1 and 2) suggests the presence of a silicone compound. As a result, the appearance of the film is slightly whitish, and the film has excellent identifiability and discriminability. This is also reflected in the average absolute reflectance, which is larger in the example films compared to the comparative example films, indicating that they have sufficient identifiability and discriminability. In Comparative Example 2, although a considerable amount of Si was detected, it originated from silica particles and not from the silicone compound. Therefore, the predetermined peak (positive maximum value) in the absolute reflectance did not appear, the average absolute reflectance value was low, and it is presumed that the desired effect was not obtained. [Industrial applicability]

[0101] The polyester film of the present invention, when used as a base material for a release film protecting an adhesive layer, can be inspected while bonded to the adhesive layer, and yet possesses identifiability and discriminability. As an environmentally friendly polyester film, it is a film that can be applied to current environmental issues. Furthermore, the present invention provides a novel film as a way to utilize release films that would otherwise be discarded after being peeled off, and therefore has high industrial value.

Claims

[Claim 1] A method for using a polyester film, comprising a polyester film containing recycled raw materials obtained by crushing laminated polyester film, having a positive maximum value at a wavelength of 310 to 410 nm in the spectrum of absolute reflectance at wavelengths of 300 to 800 nm, and satisfying the following requirements (1) and (2), as a base material for a release film, for inspection of the release film and adhesive layer in a bonded state. (1) The total light transmittance must be 85.0% or higher. (2) The haze is 5.0% or less.

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