Biaxially oriented laminated polyester film, release film, and method for producing biaxially oriented laminated polyester film

Abstract

To provide a biaxially oriented laminated polyester film excellent in smoothness and capable of suppressing blocking between an ultrathin ceramic sheet and the rear surface of a film when used as the film for manufacturing the ceramic sheet; a release film; and a method for manufacturing the biaxially oriented laminated polyester film.SOLUTION: A laminated polyester film includes two or more layers, where at least one surface layer (a surface layer A) does not substantially include particles, the other surface layer (a surface layer B) includes particles having the average particle size of 1.0 μm or less, and the surface of the surface layer A has the number of projections, of which a projection height measured by the force volume method of an atomic force microscope (AFM) is 5 nm or more and 50 nm or less, of 100 pieces / 50 μm square (2500 μm2) or more and 5000 pieces / 50 μm square (2500 μm2) or less, and the number of projections, of which a projection height is more than 50 nm and 150 nm or less, of one piece / 50 μm square (2500 μm2) or more and 100 pieces / 50 μm square (2500 μm2) or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented laminated polyester film, a release film, and a method for producing a biaxially oriented laminated polyester film. [Background technology]

[0002] Polyester film is a substrate that offers an excellent balance of dimensional stability, rigidity, and cost-effectiveness. In recent years, there has been particularly strong demand for it as a support for release films used in the production of ultra-thin ceramic sheets. In this case, after a ceramic sheet is formed on the release layer surface of the release film and wound up, the back surface of the release film is transferred to the surface of the ceramic sheet, creating a depression—a problem known as back surface transfer. For this reason, various methods have been developed to achieve a release layer with excellent smoothness. Patent Document 1 discloses a technique for suppressing transfer by specifying the type of particles added to the layer that forms the back surface, and Patent Document 2 discloses a film with a configuration that does not contain particles on the surface. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2014 / 061410 [Patent Document 2] Japanese Patent Publication No. 2020-11436 Summary of the Invention [Problem to be solved by the invention]

[0004] The techniques of Patent Documents 1 and 2 were useful in realizing a release layer back surface with excellent smoothness, but recently there has been a demand for the development of a more useful film that can suppress blocking between the ceramic sheet and the release layer back surface after winding, thereby reducing peeling electrification and breakage of the ceramic sheet.

[0005] An object of the present invention is to provide a biaxially oriented laminated polyester film, a release film, and a method for producing a biaxially oriented laminated polyester film, which has excellent smoothness and can suppress blocking between a ceramic sheet and the back surface of the film when used as a film for producing an ultrathin ceramic sheet. [Means for solving the problem]

[0006] As a result of intensive research in light of the above-mentioned circumstances, the inventors have discovered a biaxially oriented laminated polyester film that satisfies the problem to be solved by the invention by controlling the density of protrusions having a specific protrusion height on the film surface, and have arrived at the present invention.

[0007] That is, a preferred embodiment of the present invention is as follows. (1) A laminated polyester film having two or more layers, at least one of the surface layers (surface layer A) is substantially free of particles, and the other surface layer (surface layer B) contains particles with a volume average particle diameter of 1.0 μm or less, and the film surface of surface layer A has 100 protrusions per 50 μm square (2500 μm square) with a protrusion height of 5 nm or more and 50 nm or less, as measured by the force volume method of an atomic force microscope (AFM). 2 ) or more 5000 pieces / 50μm□(2500μm 2 ) or less, and the number of protrusions with a height of more than 50 nm and less than 150 nm is 1 / 50 μm□(2500 μm 2 ) or more 100 pieces / 50μm□(2500μm 2 ) or less biaxially oriented laminated polyester film. (2) The biaxially oriented laminated polyester film according to (1), wherein the film surface of the surface layer A has a center line roughness SRa of 1 nm or more and 10 nm or less and a maximum peak height SRp of 100 nm or less. (3) The biaxially oriented laminate polyester film according to (1) or (2), wherein the content of the particles contained in the surface layer B is 0.20 to 0.80% by mass based on the entire surface layer B. (4) The biaxially oriented laminate polyester film according to any one of (1) to (3), wherein the biaxially oriented laminate polyester film contains sulfur and phosphorus, and in any layer of the biaxially oriented laminate polyester film, the content of sulfur is 2 to 20 ppm by mass, the content of phosphorus is 2 to 20 ppm by mass, and the total content of calcium, magnesium, and manganese is 5 ppm by mass or less, relative to the total mass of the layer. (5) A method for producing a biaxially oriented laminated polyester film according to any one of (1) to (4), comprising the steps of melt-extruding a polyester resin into a sheet, imparting a microcrystalline structure to the surface of the unstretched film extruded into a sheet, stretching the film in the longitudinal direction, and stretching the film in the width direction, in that order. (6) The method for producing a biaxially oriented laminated polyester film according to (5), wherein the step of imparting the microcrystalline structure is at least one treatment selected from etching treatment, printing treatment, laser treatment, embossing treatment, plasma treatment, corona treatment, and gamma ray treatment. (7) A release film having a release layer on the surface layer A and / or the surface layer B of the biaxially oriented laminated polyester film according to any one of (1) to (4). [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a biaxially oriented laminated polyester film, a release film, and a method for producing a biaxially oriented laminated polyester film that has excellent smoothness and can suppress blocking between a ceramic sheet and the back surface of the film when used as a film for producing an ultrathin ceramic sheet. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be described in further detail below.

[0010] The biaxially oriented laminated polyester film of the present invention refers to a state in which an unstretched (unoriented) film is stretched in two dimensions by a conventional method, and shows a biaxially oriented pattern in wide-angle X-ray diffraction. Stretching can be performed by either sequential biaxial stretching or simultaneous biaxial stretching. Sequential biaxial stretching can involve stretching in the longitudinal direction (longitudinal) and the width direction (transverse), either once longitudinally and once transversely, or twice, such as longitudinal-transverse-longitudinal-transverse.

[0011] The polyester in the biaxially oriented laminate polyester film of the present invention is a polyester composed of a dibasic acid and a glycol as constituent components, and examples of aromatic dibasic acids that can be used include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenyletherdicarboxylic acid, diphenylketonedicarboxylic acid, phenylindanedicarboxylic acid, sodium sulfoisophthalic acid, and dibromoterephthalic acid. Examples of alicyclic dibasic acids that can be used include oxalic acid, succinic acid, adipic acid, azelaic acid, sebacic acid, and dimer acid. As the glycol, ethylene glycol, propylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, diethylene glycol, etc. can be used as the aliphatic diol; naphthalenediol, 2,2-bis(4-hydroxydiphenyl)propane, 2,2-bis(4-hydroxyethoxyphenyl)propane, bis(4-hydroxyphenyl)sulfone, hydroquinone, etc. can be used as the aromatic diol; and cyclohexanedimethanol, cyclohexanediol, etc. can be used as the alicyclic diol.

[0012] The polyester can be produced by a known method, and the intrinsic viscosity is preferably 0.5 (lower limit) to 0.8 (upper limit). More preferably, the intrinsic viscosity is 0.55 (lower limit) to 0.70 (upper limit). The intrinsic viscosity is calculated from the solution viscosity measured in orthochlorophenol at 25°C using the following formula:

[0013] ηsp / C=[η]+K[η]2·C Here, ηsp = (solution viscosity / solvent viscosity)-1, C is the mass of dissolved polymer per 100 ml of solvent (g / 100 ml, usually 1.2), and K is the Huggins constant (0.343). The solution viscosity and solvent viscosity are measured using an Ostwald viscometer. The unit is [dL / g].

[0014] A preferred embodiment of the biaxially oriented laminated polyester film of the present invention has a laminate structure of two or more layers. In the case of a two-layer laminate structure, it is composed of a polyester A layer and a polyester B layer, and in the case of a three-layer laminate structure, it is a laminate film composed of three layers: a polyester A layer, a polyester B layer, and a polyester C layer (three types, three layers) or a polyester A layer, a polyester B layer, and a polyester A layer (two types, three layers).

[0015] In a preferred embodiment of the biaxially oriented laminated polyester film of the present invention, at least one surface layer (surface layer A) is substantially free of particles, and the other surface layer (surface layer B) contains particles having a volume average particle diameter of 1.0 μm or less, and the film surface of surface layer A has 100 protrusions / 50 μm square (2500 μm square) with a protrusion height of 5 nm or more and 50 nm or less as measured by the force volume method of an atomic force microscope (AFM). 2 ) or more 5000 pieces / 50μm□(2500μm 2 ) or less, and the number of protrusions with a height of more than 50 nm and less than 150 nm is 1 / 50 μm□(2500 μm 2 ) or more 100 pieces / 50μm□(2500μm 2 The method for evaluating this property will be described in detail in the Examples below.

[0016] In the biaxially oriented laminate polyester film of the present invention, it is necessary to improve the smoothness of the release film and maintain good blocking properties between the ceramic sheet and the back surface of the release layer, while simultaneously solving the problem of the back surface of the release film transferring to the ceramic sheet surface and causing depressions, i.e., the problem of back surface transfer. In particular, to address the problem of back surface transfer of the release film, it is necessary to make the surface corresponding to the back surface of the release film as smooth as possible. Therefore, one preferred embodiment of the biaxially oriented laminate polyester film of the present invention is that the back surface of the film is substantially free of particles.

[0017] The biaxially oriented laminate polyester film of the present invention is deemed to be substantially particle-free if, when measuring the volume average particle diameter of particles, no particles are detected in 10 fields of view at 5,000x magnification when observed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). It is desirable for the biaxially oriented laminate polyester film of the present invention to have uniform surface properties and minimize defects. Defects here include scratches caused by poor contact with rollers during the process of orienting the polyester film or during transport. Furthermore, defects can occur due to unintentional contamination of the film, aggregations of intentionally added particles, or unintentional addition of particles, resulting in coarse protrusions or large protrusions or other internal foreign matter. In recent years, polyester films have often been required to achieve "zero defects," making the suppression of internal foreign matter particularly important.

[0018] The height and number of protrusions forming the surface of the biaxially oriented laminate polyester film of the present invention must be well-balanced. The inventors discovered that the object of the invention can be achieved by optimizing the compatibility of the surface properties of the surface for forming the release layer and the layer behind the release layer. Specifically, by using a substantially particle-free surface layer A as the layer behind the release layer, good blocking properties can be maintained between the ceramic sheet and the backside of the release layer when the ceramic sheet is produced. Furthermore, by containing particles with a volume-average particle size of 1.0 μm or less in layer B, smooth irregularities can be formed on the surface of the ceramic sheet when the ceramic sheet is produced, improving contact with the surface of layer A and maintaining good blocking properties. Therefore, if the other surface layer (surface layer B) contains particles, the particles must have a volume-average particle size (d) of 1.0 μm or less. In this case, particles with a volume-average particle size (d) exceeding 1.0 μm can cause serious defects in use and are also undesirable because the particles can affect the surface properties when the film is produced using recycled raw materials. With regard to particles, the content of particles contained in the surface layer B is desirably 0.20 to 0.80 mass% with respect to the entire surface layer B. When forming a thin green sheet, if the content of particles contained in the surface layer B is 0.20 mass% or more with respect to the entire surface layer B, the peeling point of the green sheet is easily determined, improving productivity during green sheet production. If the content of particles contained in the surface layer B is 0.80 mass% or less with respect to the entire surface layer B, the influence of the surface state of the surface layer B on the unevenness of the green sheet surface can be reduced.

[0019] In the biaxially oriented laminated polyester film of the present invention, the film surface of the surface layer A, which is substantially free of particles, has a protrusion height of 5 nm or more and 50 nm or less, measured by the force volume method of an atomic force microscope (AFM), of 100 protrusions / 50 μm (2500 μm 2 ) or more 5000 pieces / 50μm□(2500μm 2) or less, and the number of protrusions with a height of more than 50 nm and less than 150 nm is 1 / 50 μm□(2500 μm 2 ) or more 100 pieces / 50μm□(2500μm 2 ) or less, and the other surface layer B is a biaxially oriented laminated polyester film containing particles with a volume average particle size of less than 1.0 μm, and such surface layer A is the layer that faces the back of the release layer, and surface layer B is the surface on which the release layer is formed, as a preferred embodiment.

[0020] The number of protrusions with a height of 5 nm or more and 50 nm or less is 100 / 50 μm (2500 μm 2 ), blocking with the release layer is likely to occur, and the number of protrusions with a height of 5 nm to 50 nm in the protrusion height distribution is 5,000 / 50 μm□ (2,500 μm 2 ), the above-mentioned back surface transfer is likely to occur. The number of protrusions with a height of 5 nm or more and 50 nm or less is 100 / 50 μm square (2500 μm 2 ) or more 3000 pieces / 50μm□(2500μm 2 ) or less, and 100 pieces / 50 μm square (2500 μm 2 ) or more 1500 pieces / 50μm□(2500μm 2 ) or less is even more preferable.

[0021] In a preferred embodiment of the biaxially oriented laminated polyester film of the present invention, the number of protrusions having a height of more than 50 nm and not more than 150 nm is 1 / 50 μm (2500 μm 2 ) or more 100 pieces / 50μm□(2500μm 2 ) or less. It is required to control the number of protrusions in this protrusion height range in order to prevent blocking and to improve winding performance after forming a release layer or ceramic sheet. The number of protrusions with a protrusion height of more than 50 nm and 150 nm or less is 1 / 50 μm (2500 μm 2 ), it is not preferable because the winding performance after forming the release layer or ceramic sheet is significantly deteriorated. On the other hand, if the number of protrusions having a height of more than 50 nm and not more than 150 nm is 100 / 50 μm□ (2500 μm 2), it is not preferable because it can cause transfer marks on the back surface when manufacturing an ultra-thin ceramic sheet, and can also cause holes (pinholes) in the ceramic sheet. The number of protrusions with a height of more than 50 nm and less than 150 nm is 1 / 50 μm (2500 μm 2 ) or more 75 pieces / 50μm□(2500μm 2 ) or less is more preferable.

[0022] The biaxially oriented laminate polyester film of the present invention is required to have a smooth surface. The film surface of the surface layer A preferably has a centerline roughness SRa of 1 nm or more and 10 nm or less, and a maximum peak height SRp of 100 nm or less. Demand for a smooth surface is increasing in applications other than MLCC (multilayer ceramic capacitor) release applications. Examples include applications in which a release agent or adhesive is applied and then attached to protect components, such as polarizing plates and organic electroluminescent (EL) panels, as well as release applications (non-carrier sheets) used as a substrate for transferring adhesives, applications in which the film is used as a substrate for molding and transferring interlayer insulating resins in electrical circuit substrates, and so-called DFR (dry film resist) applications in which the film is used as a substrate for laminating, exposing, and transferring resist agents used to form circuit boards. These applications include fields in which the thickness of components molded on the substrate is becoming thinner and fields in which the packaging density per unit area is becoming denser. Substrates that require high visibility, such as display devices, are also examples of applications in which finer surface smoothing is required. In this case, the more preferable ranges for the center line roughness SRa and maximum peak height SRp are 1 nm or more and 5 nm or less for SRa, and 70 nm or less for SRp, and the even more preferable ranges for the center line roughness SRa and maximum peak height SRp are 1 nm or more and 3 nm or less for SRa, and 30 nm or less for SRp.

[0023] A more preferred embodiment of the biaxially oriented laminate polyester film of the present invention is a production method comprising the steps of melt-extruding a polyester resin into a sheet, imparting a microcrystalline structure to the surface of the unstretched sheet, stretching the film in the longitudinal direction, and stretching the film in the width direction, in that order. The step of melt-extruding a polyester resin into a sheet will be described in detail when describing the embodiments of the present invention. The sheet obtained in this step is in an unoriented state, and by imparting a microcrystalline structure to this unoriented sheet, more uniform processing can be achieved. Furthermore, after imparting a microcrystalline structure to the unoriented sheet, a micro-convex / concave structure can be formed by stretching the film in the longitudinal direction and then stretching the film in the width direction, in that order. In particular, a fine crystalline structure can be imparted by the process comprising the steps of melt-extruding a polyester resin that is substantially particle-free into one surface layer (surface layer A) and a polyester resin that contains particles with a volume average particle diameter of 1.0 μm or less into the other surface layer (surface layer B) to form a sheet, imparting a fine crystalline structure to the surface of the unstretched film extruded into a sheet, stretching the film in the longitudinal direction, and stretching the film in the width direction, in that order.

[0024] In the present invention, the process of imparting a microcrystalline structure refers to a process in which an unstretched (unoriented) film is subjected to chemical, mechanical, electrical, or optical treatment in a regular manner to crystallize the film surface. Specific treatment methods include chemical treatments such as etching, which locally erodes the surface of the unstretched film, or printing protrusions on the surface of the unstretched film. Mechanical treatments include creating fine depressions on the surface of the unstretched film or creating extremely small, regular scratches. Electrical treatments include glow discharge or arc discharge, which create random marks on the unstretched film, such as plasma treatment and corona treatment. These treatments form crystal nuclei on the surface of the unstretched film, resulting in the formation of protrusions during the stretching process. Optical treatments include laser treatment, which can create regular marks on the film surface. Furthermore, surface modification can also be achieved by irradiating the film with radiation such as gamma rays. When polymeric materials are irradiated with gamma rays, reactive species are formed, which can cause the polymer chains to break, cross-link between polymer chains, or bond with other highly reactive compounds, creating new branches on the original polymer chains. These become seeds that form the uneven shape when stretched.

[0025] The thickness of the biaxially oriented laminated polyester film of the present invention is preferably 1 μm or more, more preferably 20 μm or more, and even more preferably 25 μm or more. It is also preferably 350 μm or less, more preferably 100 μm or less, and even more preferably 40 μm or less. If the thickness is less than 1 μm, tearing may occur more easily during the stretching process after the step of imparting a regular microcrystalline structure to the surface of the unstretched film. If the thickness exceeds 350 μm, the environmental impact during disposal may be increased.

[0026] When particles are used in the biaxially oriented laminate polyester film of the present invention, inorganic particles such as spherical silica, aggregated silica, calcium carbonate, aluminum oxide, barium titanate, and titanium oxide, and organic particles such as crosslinked polystyrene resin particles, crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked styrene-acrylic resin particles, crosslinked polyester particles, polyimide particles, and melamine resin particles can be used. However, particles tend to become internal foreign matter during recycling. Because they can clog filters when melted and filtered, or become foreign matter when re-agglomerated, it is desirable to select the type of particle as well as the particle size. Particles with a uniform particle size distribution are preferred, and organic particles selected from crosslinked polystyrene resin particles, crosslinked silicone resin particles, crosslinked acrylic resin particles, crosslinked styrene-acrylic resin particles, and crosslinked polyester particles are particularly preferred. Among inorganic particles, spherical silica and aluminum oxide are particularly preferred.

[0027] When particles are added, it is preferable that the particle shape and particle size distribution are uniform as described above, and it is particularly preferable that the particle shape is close to spherical. The volume shape factor f is preferably 0.3 to π / 6, and more preferably 0.4 to π / 6. The volume shape factor f is expressed by the following formula:

[0028] f=V / Dm 3 where V is the particle volume (μm 3 ), and Dm is the maximum diameter (μm) of the particle on its projected surface.

[0029] The volume shape factor f is a maximum of π / 6 (=0.52) when the particles are spherical. It is also preferable to remove aggregated particles and coarse particles by filtration or the like, as necessary. Among these, crosslinked polystyrene resin particles, crosslinked silicone resin particles, and crosslinked acrylic resin particles synthesized by emulsion polymerization or the like are preferably used, but crosslinked polystyrene particles, crosslinked silicone particles, and spherical silica particles are particularly preferred from the viewpoint of forming uniform protrusions on the film surface, as they have a volume shape factor close to that of a perfect sphere and an extremely uniform particle size distribution.

[0030] The biaxially oriented laminate polyester film of the present invention contains elemental sulfur and elemental phosphorus. In any layer of the biaxially oriented laminate polyester film, the content of elemental sulfur is preferably 2 to 20 ppm by mass, the content of elemental phosphorus is preferably 2 to 20 ppm by mass, and the total content of elemental calcium, magnesium, and manganese is preferably 5 ppm by mass or less, relative to the total mass of the layer. By ensuring appropriate contents of these elements, it is possible to suppress the generation of coarse foreign matter while ensuring good adhesion, or so-called electrostatic applicability, in the casting process described below, i.e., the process of adhering the cast using electrostatic force using a pinning device. Having the contents of elemental sulfur and elemental phosphorus each at 2 ppm by mass or more ensures good electrostatic applicability. On the other hand, having a sulfur content of 20 ppm by mass or less, a phosphorus content of 20 ppm by mass or less, and a total content of elemental calcium, magnesium, and manganese of 5 ppm by mass or less is preferable because it suppresses the generation of coarse foreign matter.

[0031] Next, a method for producing the biaxially oriented laminated polyester film of the present invention will be described, but the present invention should not be construed as being limited to such an example.

[0032] For example, inert particles can be incorporated into polyester by dispersing them in a slurry in a predetermined ratio in ethylene glycol, a diol component, and adding this ethylene glycol slurry at any stage before the completion of polyester polymerization. When adding the particles, for example, adding the aqueous sol or alcohol sol obtained during particle synthesis without first drying it is preferable because this improves particle dispersibility and prevents the formation of coarse protrusions. Another effective method for producing the polyester of the present invention is to directly mix the aqueous slurry of particles with the desired polyester pellets, feed the mixture into a vented twin-screw kneading extruder, and knead the mixture into the polyester.

[0033] The particle-containing master pellets and pellets substantially free of particles are mixed in a predetermined ratio, dried, and then fed into a known melt lamination extruder. A single-screw or twin-screw extruder can be used as the extruder for producing the biaxially oriented polyester film of the present invention. A vented extruder equipped with a vacuum line can also be used to eliminate the pellet drying process. For layers with a high extrusion rate, such as 1000 kg / hr or more, a tandem extruder can be used, in which each extruder is responsible for melting the pellets and maintaining the molten pellets at a constant temperature. Tandem extruders stabilize the polymer temperature during high extrusion rates, thereby reducing viscosity variations, making them preferable for reducing thickness unevenness.

[0034] The polymer melted and extruded in the extruder is filtered through a filter. Even the smallest foreign particles can become large protrusions in the film, so it is effective to use a filter with a high-precision collection efficiency, capturing, for example, 95% or more of foreign particles 5 μm or larger. On the other hand, a filter with too high a collection efficiency can result in a high degree of pressure buildup. Therefore, a filter with even higher collection efficiency, such as capturing 95% or more of foreign particles less than 3 μm, can accelerate the increase in filter pressure due to the trapped foreign particles and may condense inside the filter before being discharged, making this an undesirable embodiment. The polymer is then extruded into a sheet through a slit die and cooled and solidified on a casting roll to produce an unstretched film. For example, in the case of a three-layer laminate, three extruders and a three-layer manifold or confluence block (e.g., a confluence block with a rectangular confluence section) are used to laminate the three layers, and the sheet is extruded through a die. It is desirable for the die to be able to automatically adjust the die gap using a heater. The sheet extruded from the die is cooled by a casting roll to produce an unstretched film. The unstretched film that lands on the casting roll is pinned by a pinning device using electrostatic force to adhere to the cast.

[0035] The film that has been cooled while in close contact with the casting roll is peeled off from the casting roll using a peeling roll, which may be passed through with water for film cooling or may be driven.

[0036] After passing through the peeling rolls, the unstretched film undergoes a process to impart a regular microcrystalline structure to the film surface. Specifically, for example, the unstretched film is subjected to etching, printing, laser treatment, embossing, radiation treatment, etc. to impart a regular microcrystalline structure to the film surface. This process may involve imparting fine irregularities to the film surface, or crystallization alone may be sufficient without imparting irregularities. The etching process involves eroding the film surface by forming a dot pattern (dot pattern) similar to that achieved when a minute amount of chemical is dropped onto the film in the form of droplets. These eroded areas then serve as nuclei for forming protrusions during stretching. The dot pattern referred to here refers to a state in which the dots are arranged regularly. Examples of chemicals that can be used include concentrated sulfuric acid, concentrated nitric acid, sodium hydroxide solution, concentrated aqueous ammonia, ethanol, acetone, and alkaline solutions. The printing process involves printing protrusions on the film surface to impart microcrystalline material to the printed areas. The material used for printing is preferably at least one resin selected from the group consisting of polyester resins, acrylic resins, polyurethane resins, polystyrene resins, polyamide resins, melamine resins, epoxy resins, and silicone resins. Each of these resins may be used alone or in a mixture of two or more. Laser treatment is a method of forming protrusions by irradiating a laser beam in a pattern onto an unstretched film. Embossing is a process of imparting marks or minute scratches to the film surface, such as pressing a metal roll with a regular uneven pattern against the unstretched film to impart marks or minute scratches. Thermal treatment imparts regular heat to the surface of the unstretched film to form a microcrystalline structure. This process may be combined with embossing, in which the aforementioned metal roll is heated and brought into contact with the unstretched film.

[0037] The stretching method may be simultaneous biaxial stretching or sequential biaxial stretching. In simultaneous biaxial stretching, when longitudinal and transverse stretching are performed simultaneously, uneven wind speed and air currents flowing along the film (accompanying air currents) affect the stretching in the longitudinal direction as well as the width direction, making it difficult to produce regular surface texture, and therefore sequential biaxial stretching is the preferred mode of application.

[0038] When the biaxially oriented laminated polyester film of the present invention is produced by sequential stretching, the preheating temperature for the first longitudinal stretching is preferably 80°C to 110°C. The stretching temperature is 90°C to 130°C, preferably 100°C to 120°C. A stretching temperature lower than 90°C is prone to film breakage, while a stretching temperature higher than 130°C is undesirable because the film surface is susceptible to thermal damage, uniform stretching is not achieved, and protrusions originating from crystallized nuclei are not formed. In the stretching section, in order to prevent uneven stretching and scratches, it is preferable to perform stretching in two or more stages, and the total stretching ratio is 2.8 to 5.0 times, preferably 3.3 to 4.0 times, in the longitudinal direction, and 3.5 to 5 times, preferably 4.0 to 4.5 times, in the transverse direction. Furthermore, during the longitudinal stretching process, the film comes into contact with the rolls, and the difference in the peripheral speed between the rolls and the film can easily cause scratches when the film slips, which can also cause uneven thickness in the longitudinal direction. Therefore, a drive system that allows the peripheral speed of the rolls to be set individually for each roll is preferred.

[0039] The unstretched film is then transported to the stretching zone while maintained at a temperature below its glass transition point. When the film is heated all at once during stretching, it is preferable to use metal rolls in the preheating zone that have been surface-treated with hard chrome or tungsten carbide and have a surface roughness Ra of 0.2 μm to 0.6 μm in order to suppress adhesion, which can cause thermal wrinkles and uneven thickness in the longitudinal direction. The uniaxially stretched film thus stretched in the longitudinal direction is then heated to 90°C or higher but lower than 120°C in a transverse stretching machine, and then stretched in the width direction at a magnification of 3 to 6 times to produce a biaxially stretched (biaxially oriented) film.

[0040] The biaxially oriented laminated polyester film of the present invention may be further re-stretched one or more times in each direction, or may be re-stretched biaxially simultaneously. Furthermore, the film may be subjected to a longitudinal re-stretching process without applying a stretching ratio. After the longitudinal re-stretching, further transverse stretching is performed. After stretching, the film is heat-treated. This heat treatment can be performed by any conventional method or at any conventional location, such as in an oven or on a heated roll. The heat treatment temperature can be typically any temperature between 150°C and 245°C, and the heat treatment time is typically preferably between 1 second and 60 seconds. The heat treatment temperature not only fixes the structure of the polyester film but also effectively adjusts the surface roughness. This is because the polyester surface structure is roughened depending on the degree of crystallization, resulting in the formation of microprotrusions. A heat treatment temperature of 235°C to 245°C can form preferred surface protrusions. Furthermore, a heat treatment temperature of 240°C to 243°C can form even more preferred surface protrusions. The heat treatment may be carried out while relaxing the film in the longitudinal and / or transverse directions. After the heat treatment, the film may be relaxed in the transverse direction by 0% to 10% at a temperature 0°C to 150°C lower than the heat treatment temperature.

[0041] The dimensional change rate and flatness of the heat-treated film can be adjusted, for example, by providing an intermediate cooling zone or a slow cooling zone. In particular, to impart specific heat shrinkability, the film may be relaxed in the longitudinal and / or transverse directions during or after heat treatment in an intermediate cooling zone or a slow cooling zone. In this case, it is preferable to control the temperature difference in the width direction inside the oven to within 5°C in order to suppress variations in roughness in the width direction.

[0042] The biaxially stretched film is cooled in a conveying process, then the edges are cut and wound up to obtain an intermediate product. During this conveying process, the film thickness in the width direction is measured, and the data is used as feedback to adjust the film thickness by adjusting the die thickness, etc., and foreign matter can also be detected using a defect detector.

[0043] The intermediate product is slit to the appropriate width and length through a slitting process and wound onto a core to obtain a roll of biaxially oriented laminated polyester film.

[0044] The biaxially oriented laminated polyester film obtained by the above-mentioned method can prevent surface defects of the film caused by aggregation of lubricants and catalysts added to the film, allow for highly flexible surface design regardless of the properties of the recycled raw materials, and reduce the inclusion of foreign matter when the film is recycled after use.

[0045] A preferred embodiment of the release film of the present invention is a release film having a release layer on the surface layer A and / or the surface layer B of the biaxially oriented laminate polyester film. Surface layer A of the biaxially oriented laminate polyester film has few large irregularities but many small irregularities, and surface layer B contains small particles. Therefore, even when the biaxially oriented laminate polyester film is wound up after forming an object to be released, shape transfer to the surface of the non-release object can be suppressed and blocking can also be prevented. In particular, a release film having a release layer on surface layer B of the biaxially oriented laminate polyester film of the present invention is particularly excellent in blocking resistance after winding and can therefore be preferably used.

[0046] Furthermore, since the release film of the present invention has the above-mentioned properties, it can be particularly suitably used for producing ceramic sheets. [Example]

[0047] The present invention will be described in detail below with reference to examples.

[0048] The measurement and evaluation methods for the present invention are as follows.

[0049] (1) Number of protrusions Images obtained using an atomic force microscope (AFM) under the following measurement conditions were counted under a threshold of protrusion height in 5 nm increments (5 nm, 10 nm, 15 nm, etc.) up to 150 nm, and the number of protrusions up to 150 nm was counted, such as protrusion heights of 5 nm or more, 10 nm or more, and 15 nm or more. The number of protrusions with a protrusion height of 5 nm or more but less than 50 nm was calculated by subtracting the number of protrusions with a protrusion height of 50 nm or more from the number of protrusions with a protrusion height of 5 nm or more. The number of protrusions with a protrusion height of 50 nm or more but less than 150 nm was calculated by subtracting the number of protrusions with a protrusion height of 150 nm or more from the number of protrusions with a protrusion height of 50 nm or more. Measurements were taken 20 times at different locations, and the average value was calculated as a 50 μm x 50 μm (50 μm square (2500 μm 2 )) and used as the number of protrusions for each. Equipment: NanoScope III AFM (Digital Instruments) Cantilever: silicon single crystal Scanning mode: Tapping mode Scanning speed: 0.8Hz Measurement field of view: 5 μm square Sample lines: 256 Sample preparation: 23℃, 65% RH, left to stand for 24 hours AFM measurement environment: 23°C, 65% RH, 24 hours.

[0050] (2) Volume average particle size of particles The polymer sampled from the film is removed using low-temperature plasma ashing to expose the particles. The processing conditions are selected to ashed the polymer but minimize damage to the particles. The particles are observed using a scanning electron microscope (SEM; Hitachi, Ltd., Model S-4000). The particle images are imported into an image analyzer (Nireco Corporation, Model "LUZEX" (registered trademark)_AP). The equivalent circle diameter is measured and the volume-average particle diameter of the particles is determined. The SEM magnification is appropriately selected from 5,000 to 20,000 times depending on the particle size. The equivalent circle diameter of at least 5,000 particles is measured at various observation locations, and the volume-average particle diameter is calculated from the average value. If the particles are significantly damaged by the low-temperature plasma ashing, the cross section of the film is observed using a transmission electron microscope (TEM; Hitachi, Ltd., Model H-600) at 3,000 to 20,000 times magnification depending on the particle size. The thickness of the TEM section is approximately 100 nm, and the equivalent circle diameters of at least 100 particles are measured at different locations, and the volume average particle diameter is calculated from the average value.When measuring the volume average particle diameter of particles, if no particles are found even after checking 10 fields of view at 5,000x magnification when observing with SEM and TEM, it is determined that the particles do not substantially contain any particles.

[0051] (3) Center line roughness and maximum peak height of film surface (SRa, SRp values) Measurements are taken using a three-dimensional micro surface profiler (Kosaka Manufacturing Co., Ltd. ET-4000A), and the arithmetic mean roughness (SRa) and maximum peak height (SRp) are calculated from the obtained surface profile curve in accordance with JIS·B0601 (2001). The measurement conditions are as follows: X-direction measurement length: 0.5 mm, X-direction feed rate: 0.1 mm / sec. Y-direction feed pitch: 5 μm, number of Y-direction lines: 40 Cutoff: 0.25mm. Stylus pressure: 0.02mN. Height (Z direction) magnification: 50,000 times.

[0052] [Example 1] (1) Preparation of polyester pellets (Preparation of Polyester A) Esterification reaction was carried out on 86.5 parts by mass of terephthalic acid and 37.1 parts by mass of ethylene glycol at 255° C. while distilling off water. After completion of the esterification reaction, 0.02 parts by mass of trimethyl phosphate, 0.06 parts by mass of magnesium acetate, 0.01 parts by mass of lithium acetate, and 0.0085 parts by mass of antimony trioxide were added, followed by heating to 290° C. under reduced pressure and carrying out a polycondensation reaction, yielding polyester pellets A having an intrinsic viscosity of 0.63 dl / g.

[0053] (Creating Polyester B) An aqueous slurry of divinylbenzene / styrene copolymer crosslinked particles (crosslinking degree 80%) consisting of 80% by mass of divinylbenzene, 15% by mass of ethylvinylbenzene, and 5% by mass of styrene prepared by the seed method was added to the above-mentioned homopolyester pellets containing substantially no particles using a vented twin-screw kneader, to obtain master pellets containing 1% by mass of divinylbenzene / styrene copolymer crosslinked particles with a volume average particle size of 0.3 μm relative to the polyester (Polyester B).

[0054] (Creating Polyester C) A water slurry of silica particles having an average particle size of 0.06 μm was mixed with the above polyester A containing substantially no particles using a vented twin-screw kneader to obtain master pellets containing 1% by mass of 0.06 μm silica particles relative to the polyester.

[0055] (Creation of Polyester D) An ethylene glycol slurry containing 10 parts by mass of δ-type alumina as agglomerated alumina was prepared, and the slurry was subjected to pulverization and dispersion treatment using a sand grinder, and further filtered using a 3 μm filter with a collection efficiency of 95%. This was added to an ester exchange reaction product prepared in the same manner as for Polyester A, followed by addition of antimony trioxide, and a polycondensation reaction was carried out to obtain master pellets containing 1.5% by mass of agglomerated alumina (Polyester D).

[0056] (Creation of Polyester E) To produce Polyester A, after transesterification, 10 parts by mass of calcium carbonate (volume average particle diameter: 1.1 μm, Mohs hardness: 3) prepared by a carbon dioxide gas method was wet-ground with 90 parts by mass of ethylene glycol to obtain a calcium carbonate / ethylene glycol dispersion slurry. The volume average particle diameter of this calcium carbonate was 1.1 μm. Separately, 100 parts by mass of dimethyl terephthalate and 64 parts by mass of ethylene glycol were added with 0.04 parts by mass of manganese acetate and 0.03 parts by mass of antimony trioxide as catalysts to carry out a transesterification reaction. Then, 0.04 parts by mass of trimethyl phosphate as a phosphorus compound was added to the reaction product, and then 1 part by mass of the previously prepared slurry was added to carry out a polycondensation reaction to obtain master pellets (Polyester E) containing 1% by mass of calcium carbonate relative to the polyester.

[0057] (2) Blending polyester pellets The polyester pellets fed to the extruders for Layers A and B were blended in the following ratios, which are mass ratios (unit: mass%) relative to the polyester pellets constituting each layer.

[0058] A layer Polyester A: 100.0 B layer Polyester A: 70.0 Polyester B: 20.0 Polyester C:10.0.

[0059] (3) Manufacturing of biaxially oriented laminated polyester film The raw materials for each layer were mixed in a blender. The mixed raw materials for Layer A were then fed into a vented twin-screw extruder for Layer A, while the raw materials for Layer B were dried under reduced pressure at 160°C for 8 hours and then fed into a single-screw extruder for Layer B. The mixture was melt-extruded at 275°C and filtered through a high-precision filter capturing over 95% of foreign matter 3μm or larger. The mixture was then joined and laminated in a rectangular two-layer joining block to form a two-layer laminate consisting of Layers A and B. The mixture was then passed through a slit die maintained at 285°C onto a chilled roll, which was then wrapped around a casting drum with a surface temperature of 25°C using an electrostatic casting method, allowing it to cool and solidify, yielding an unstretched laminate film.

[0060] This unstretched laminated film was sequentially stretched (longitudinal and transverse directions). First, longitudinal stretching was performed, and then transverse stretching was performed. For longitudinal stretching, the film was preheated to 105°C and transported using a Teflon (registered trademark) roll, and then stretched 4.0 times at 120°C using the difference in peripheral speed of the roll to obtain a uniaxially stretched film.

[0061] This uniaxially stretched film was stretched 4 times in the transverse direction at 115°C in a stenter, then heat-set at 240°C, during which it was relaxed 5% in the width direction and cooled in the conveying process. After cutting the edges, it was wound up to obtain a 25μm thick biaxially stretched film intermediate product. This intermediate product was slit using a slitter to obtain a 25μm thick biaxially stretched film roll. The roll appearance was good, with no misalignment, mayonnaise, or surface deformation.

[0062] A film sample was taken from the roll of this biaxially stretched film and the projection height distribution was measured. As a result, as shown in Table 1, the surface was excellent.

[0063] (4) Coating of release layer Next, 100 parts by weight of melamine resin [manufactured by Mitsui Cytec Co., Ltd., product name "Cymel" (registered trademark) 303, hexamethoxymethylmelamine, mass average molecular weight 390, solids content 100% by weight], 5 parts by weight of p-toluenesulfonic acid as an acid catalyst, and a mixed solvent of isopropyl alcohol and isobutyl alcohol (mass ratio 4 / 1) were mixed with the B layer of this biaxially stretched film roll to obtain a smoothing layer-forming composition with a solids content of 15% by weight. The obtained smoothing layer-forming composition was then applied with a bar coater to obtain a coating layer. Next, this coating layer was heated at 120°C for 1 minute to harden the coating layer, thereby forming a smoothing layer (thickness: 0.55 μm). Next, 95 parts by weight of melamine resin [manufactured by Mitsui Cytec Co., Ltd., product name "Cymel" (registered trademark) 303, hexamethoxymethylmelamine, mass average molecular weight 390, solids content 100% by weight], 5 parts by weight of silanol-terminated polydimethylsiloxane as a silicone compound [manufactured by Shin-Etsu Chemical Co., Ltd., KF-9701, solids content 100% by weight], 5 parts by weight of p-toluenesulfonic acid as an acid catalyst, and a mixed solvent of isopropyl alcohol and isobutyl alcohol (mass ratio 4 / 1) were mixed to obtain a release agent layer-forming composition with a solids content of 15% by weight. The resulting release agent layer-forming composition was then applied to the smoothing layer with a bar coater to obtain a coating layer. Next, this coating layer was cured by heating at 120°C for 1 minute to form a release agent layer (thickness: 0.50 μm). That is, a release layer having a thickness of 1.05 μm and consisting of two layers, a smoothing layer and a release material layer, was formed.

[0064] (5) Forming the green sheet (applying ceramic slurry) Glass beads with a number average particle size of 2 mm were added to 100 parts by weight of barium titanate (manufactured by Fuji Titanium Industries Co., Ltd., product name HPBT-1), 10 parts by weight of polyvinyl butyral (manufactured by Sekisui Chemical Co., Ltd., product name BL-1), 5 parts by weight of dibutyl phthalate, and 60 parts by weight of toluene-ethanol (30:30 mass ratio), and the mixture was mixed and dispersed using a jet mill for 20 hours, followed by filtration to prepare a paste-like ceramic slurry. The resulting ceramic slurry was applied to a release film using a die coater to a dry thickness of 1.0 μm, dried, and wound up to obtain a green sheet.

[0065] (6) Evaluation of the green sheet coating condition: presence or absence of pinholes and dents The green sheet wound up as described above is unwound by hand and inspected visually without removing it from the release film to check for pinholes and the state of the coating on the surface and edges of the sheet. The area to be inspected is 300 mm wide and 500 mm long.

[0066] The green sheet molded on the release film is illuminated from behind with a 1000 lux backlight unit and observed for pinholes caused by coating voids or dents caused by surface transfer from the back of the release film. A: There are no pinholes or dents. B: No pinholes, and up to 3 dents C: There are pinholes and four or more dents.

[0067] Regarding the presence or absence of pinholes and dents, grades A and B are considered to be good, with A being the best.

[0068] In Example 1, there were no pinholes or dents, and therefore the evaluation was A.

[0069] (7) Evaluation of blocking characteristics of green sheets When the wound green sheet was unwound by the winding machine, the peeling state of the green sheet laminated surface on the unwound side was observed and the amount of charge was measured. Based on this state, four levels of evaluation were made. A: The absolute value of the charge is 1 kV or less, and the unwinding condition is good. B: The absolute value of the charge is 5 kV or less, and the unwinding condition is good. C: The absolute value of the charge is 5 kV or less, and there is slight fluttering when unwound. D: The absolute value of the charge exceeds 5 kV, and there is fluttering when unwound. E: The absolute value of the charge exceeds 5 kV and the green sheet is torn. The blocking properties of the green sheet are evaluated as good from A to C, with A being the best.

[0070] In Example 1, the absolute value of the charge was 1 kV or less, there was no chattering during unwinding, and the film was judged as A.

[0071] [Examples 2 to 5] A film was obtained in the same manner as in Example 1, except that the type and particle size of the particles were changed as shown in the table.

[0072] [Example 6] The thickness and composition were as shown in the table, and the film was wound around a casting drum with a surface temperature of 25°C and cooled and solidified to obtain an unstretched laminated film. The unstretched laminated film was then introduced between the opposing electrodes and the earth roll, and nitrogen gas was introduced into the device, and the E value was set to 160 W·min / m 2 A laminated polyester film was obtained in the same manner as in Example 1, except that the atmospheric pressure glow discharge treatment was carried out under the conditions of:

[0073] [Examples 7 to 9] After changing the thickness ratio and the composition of Layer A and Layer B in the same embodiment as in Example 1, unstretched laminated films were obtained and then subjected to radiation treatment using a gamma ray irradiation facility at absorbed doses of 100 kV, 30 kGy (Example 7), 100 kV, 100 kGy (Example 8), and 100 kV, 20 kGy (Example 9). There were no pinholes or dents, and the unwinding condition was good.

[0074] [Comparative Example 1] A film was obtained in the same manner as in Example 1, except that the composition of layer B was changed and the heat setting temperature was changed to 240°C.

[0075] Comparative Example 2 A film was obtained in the same manner as in Example 1, except that particles were contained in the A layer and the B layer as shown in Table 1.

[0076] Comparative Example 3 A film was obtained in the same manner as in Example 6, except that the particle contents in Layer A and Layer B were changed as shown in Table 1.

[0077] Comparative Example 4 In the same embodiment as in Example 4, the heat setting temperature was adjusted to 210°C.

[0078] Comparative Example 5 In the same embodiment as in Example 1, the amount of particles added to layers A and B was changed as shown in Table 1.

[0079] [Table 1] [Industrial Applicability]

[0080] According to the present invention, a biaxially oriented polyester film can be provided that, when used as a support for sheet molding, minimizes the transfer of the shape of the back surface of the support to the surface of the sheet after winding and the occurrence of blocking, and can be particularly suitably used as a support for release films for producing ultra-thin ceramic sheets.

Claims

1. A laminated polyester film having two or more layers, at least one of the surface layers (surface layer A) is substantially free of particles, and the other surface layer (surface layer B) contains particles having a volume average particle diameter of 1.0 μm or less, and the film surface of surface layer A has 100 protrusions / 50 μm square (2500 μm square) with a protrusion height of 5 nm or more and 50 nm or less as measured by a force volume method using an atomic force microscope (AFM). ２ ) or more 5000 pieces / 50μm□(2500μm ２ ) or less, and the number of protrusions having a height of more than 50 nm and not more than 150 nm is 1 / 50 μm (2500 μm ２ ) or more 100 pieces / 50μm□(2500μm ２ ) or less, and the content of particles contained in the surface layer B is 0.20 to 0.80 mass % based on the entire surface layer B.

2. A laminated polyester film having two or more layers, at least one surface layer (surface layer A) is substantially free of particles, and the other surface layer (surface layer B) contains particles having a volume average particle diameter of 1.0 μm or less, and the film surface of surface layer A has a number of protrusions having a height of 5 nm or more and 50 nm or less, measured by the force volume method of an atomic force microscope (AFM), of 100 protrusions / 50 μm□ (2500 μm 2 ) to 5000 protrusions / 50 μm□ (2500 μm 2 ), and the number of protrusions having a height of more than 50 nm and 150 nm or less is 1 protrusion / 50 μm□ (2500 μm 2 ) to 100 protrusions / 50 μm□ (2500 μm 2 ). ) or less, the laminated polyester film contains elemental sulfur and elemental phosphorus, and in any one of the layers of the laminated polyester film, the content of elemental sulfur is 2 to 20 ppm by mass, the content of elemental phosphorus is 2 to 20 ppm by mass, and the total content of elemental calcium, elemental magnesium, and elemental manganese is 5 ppm by mass or less, relative to the total mass of the layer.

3. 3. The biaxially oriented laminate polyester film according to claim 1, wherein the film surface of the surface layer A has a center line roughness SRa of 1 nm or more and 10 nm or less and a maximum peak height SRp of 100 nm or less.

4. 4. The biaxially oriented laminate polyester film according to claim 1, comprising, in that order, a step of melt-extruding a polyester resin into a sheet, a step of imparting a microcrystalline structure to the surface of the unstretched film extruded into a sheet, a step of stretching the film in the longitudinal direction, and a step of stretching the film in the width direction.

5. 5. The method for producing a biaxially oriented laminated polyester film according to claim 4, wherein the step of imparting a microcrystalline structure is at least one treatment selected from the group consisting of etching, printing, laser treatment, embossing, plasma treatment, corona treatment, and gamma ray treatment.

6. A laminated polyester film of two or more layers, wherein at least one surface layer (surface layer A) is substantially free of particles and the other surface layer (surface layer B) contains particles having a volume average particle diameter of 1.0 μm or less, and the film surface of surface layer A has a number of protrusions having a height of 5 nm or more and 50 nm or less, measured by the force volume method with an atomic force microscope (AFM), of 100 protrusions per 50 μm square (2500 μm 2 ) to 5000 protrusions per 50 μm square (2500 μm 2 ), and a number of protrusions having a height of more than 50 nm and 150 nm or less, of 1 protrusion per 50 μm square (2500 μm 2 ) to 100 protrusions per 50 μm square (2500 μm 2 ), and a release film having a release layer on surface layer A and / or surface layer B.

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