JP7753702B2 - Biaxially oriented laminated polyester film, release film, and method for producing biaxially oriented laminated polyester film - Google Patents

Description

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.

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. However, after a ceramic sheet is formed on the release layer surface of the release film and then wound up, the back surface of the release film can transfer to the surface of the ceramic sheet, creating depressions—a problem known as back surface transfer. For this reason, various methods have been developed to achieve a release layer back surface with excellent smoothness. Patent Document 1 discloses a technology that specifies the type of particles added to the layer that forms the back surface to suppress transfer, while Patent Document 2 discloses a film with a configuration that does not contain particles on the surface.

International Publication No. 2014/061410 Japanese Patent Application Laid-Open No. 2020-11436

The technologies described in Patent Documents 1 and 2 were useful in achieving a release layer back surface with excellent smoothness, but recently there has been a demand for the development of an even 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.

The objective 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 that has excellent smoothness and can suppress blocking between the ceramic sheet and the back surface of the film when used as a film for producing an ultra-thin ceramic sheet.

As a result of extensive research in light of the above-mentioned circumstances, the inventors discovered a biaxially oriented laminated polyester film that satisfies the problem to be solved by controlling the density of protrusions with a specific protrusion height on the film surface, leading to the present invention.

That is, a preferred embodiment of the present invention is as follows.
(1) A biaxially oriented laminated polyester film having 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 using 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 ).
(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 laminated 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 sulfur content is 2 to 20 ppm by mass, the phosphorus content 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), which comprises 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).

The present invention provides 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 the ceramic sheet and the back surface of the film when used as a film for producing an ultra-thin ceramic sheet.

The present invention is described in further detail below.

The term "biaxially oriented" in the context of the biaxially oriented laminated polyester film of the present invention refers to a state in which an unstretched (unoriented) film has been stretched in two dimensions using conventional methods, and exhibits a biaxially oriented pattern in wide-angle X-ray diffraction. Stretching can be performed using either sequential biaxial stretching or simultaneous biaxial stretching. Sequential biaxial stretching can involve stretching in the longitudinal direction (length) and width direction (width), either once lengthwise and once widthwise, or twice, such as lengthwise-widthwise-lengthwise-widthwise.

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. 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 glycols, aliphatic diols such as ethylene glycol, propylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, and diethylene glycol can be used; aromatic diols such as naphthalenediol, 2,2-bis(4-hydroxydiphenyl)propane, 2,2-bis(4-hydroxyethoxyphenyl)propane, bis(4-hydroxyphenyl)sulfone, and hydroquinone can be used; and alicyclic diols such as cyclohexanedimethanol and cyclohexanediol can be used.

The above polyesters can be produced by known methods, and it is preferable to use polyesters with an intrinsic viscosity of 0.5 (lower limit) to 0.8 (upper limit). More preferably, the lower limit is 0.55 (lower limit) to 0.70 (upper limit). The intrinsic viscosity is measured using the value calculated using the following formula from the solution viscosity measured in orthochlorophenol at 25°C.

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

A preferred embodiment of the biaxially oriented laminate polyester film of the present invention is a laminate structure of two or more layers. A two-layer laminate structure consists of a polyester A layer and a polyester B layer, and a three-layer laminate structure consists of 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).

In a preferred embodiment of the biaxially oriented laminate 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 a number of protrusions having a height of 5 nm to 50 nm, as measured by the force volume method with an atomic force microscope (AFM), of 100 protrusions/50 μm square (2500 μm ² ) to 5000 protrusions/50 μm square (2500 μm ² ), and a number of protrusions having a height of more than 50 nm to 150 nm, of 1 protrusion/50 μm square (2500 μm ² ) to 100 protrusions/50 μm square (2500 μm ² ). Details of the method for evaluating this characteristic will be described later in the examples.

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 surface of the ceramic sheet and causing depressions, known as back surface transfer. In particular, to address the problem of back surface transfer of the release film, it was necessary to make the surface corresponding to the back surface of the release film as smooth as possible. Therefore, one preferred aspect of the biaxially oriented laminate polyester film of the present invention is that the back surface of the film contains substantially no particles.

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 that occur when the polyester film is oriented or when poor contact with rollers occurs during transport. Defects also occur when unintentional foreign matter is introduced into the film, intentionally added particles aggregate, or unintentionally introduced particles cause 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.

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 objectives of the invention can be achieved by optimizing the compatibility between the surface on which the release layer is formed and the layer behind the release layer. Specifically, by using a substantially particle-free surface layer A as the layer behind the release layer, favorable blocking properties between the ceramic sheet and the backside of the release layer can be maintained when the ceramic sheet is fabricated. Furthermore, by including 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 fabricated, improving contact with the surface of layer A and maintaining favorable 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. Particles with a volume-average particle size (d) exceeding 1.0 μm can cause serious defects during use and are also undesirable because they can adversely affect surface properties when the film is manufactured using recycled raw materials. With regard to particles, it is desirable that the content of particles contained in the surface layer B be 0.20 to 0.80 mass% relative 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 relative to the entire surface layer B, the peel 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 relative to the entire surface layer B, the effect of the surface condition of the surface layer B on the unevenness of the green sheet surface can be reduced.

In addition, in the biaxially oriented laminated polyester film of the present invention, a preferred embodiment is one in which the film surface of the surface layer A, which is substantially free of particles, has a number of protrusions with a height of 5 nm or more and 50 nm or less, measured by the force volume method using an atomic force microscope (AFM), of 100/50 μm□ (2500 μm ² ) to 5000/50 μm□ (2500 μm ² ), and a number of protrusions with a height of more than 50 nm and 150 nm or less, of 1/50 μm□ (2500 μm ² ) to 100/50 μm□ (2500 μm ² ), 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 the surface layer A is the layer that corresponds to the back surface of the release layer, and the surface layer B is the surface for forming the release layer.

If the number of protrusions with a protrusion height of 5 nm or more and 50 nm or less is less than 100/50 μm (2500 μm ² ), blocking with the release layer is likely to occur, and if the number of protrusions with a height of 5 nm or more and 50 nm or less in the protrusion height distribution exceeds 5000/50 μm (2500 μm ² ), the aforementioned back surface transfer is likely to occur. It is more preferable that the number of protrusions with a protrusion height of 5 nm or more and 50 nm or less be 100/50 μm (2500 μm ² ) to 3000/50 μm (2500 μm ² ), and even more preferably be 100/50 μm (2500 μm ² ) to 1500/50 μm (2500 μm ² ).

In addition, in a preferred embodiment of the biaxially oriented laminate polyester film of the present invention, the number of protrusions having a height of more than 50 nm and less than 150 nm is 1/50 μm (2500 μm ² ) to 100/50 μm (2500 μm ² ). It is necessary to control the number of protrusions in this protrusion height range to prevent blocking and improve winding performance after forming a release layer or ceramic sheet. If the number of protrusions having a height of more than 50 nm and less than 150 nm is less than 1/50 μm (2500 μm ² ), winding performance after forming a release layer or ceramic sheet is significantly impaired, which is undesirable. On the other hand, if the number of protrusions having a height of more than 50 nm and less than 150 nm exceeds 100/50 μm (2500 μm ² ), this can cause back transfer marks to form during the production of ultrathin ceramic sheets, and can also cause holes (pinholes) in the ceramic sheet, which is undesirable. The number of protrusions having a protrusion height of more than 50 nm and not more than 150 nm is more preferably 1/50 μm square (2500 μm ² ) or more and 75/50 μm square (2500 μm ² ) or less.

The biaxially oriented laminate polyester film of the present invention is required to have a smooth surface. It is preferable that the film surface of the surface layer A 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. In addition to MLCC (multilayer ceramic capacitor) release applications, demand for surface smoothness is also increasing in other fields, including applications where release agents or adhesives are applied and then attached to protect components, such as polarizing plates and organic electroluminescent devices, release applications (non-carrier sheets) used as substrates for transferring adhesives, applications where 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 where the film is used as a substrate for laminating, exposing, and transferring resist agents used to form circuit boards. These applications include fields where the thickness of components molded on the substrate is becoming thinner and where packaging density per unit area is becoming denser. Substrates requiring high visibility, such as display devices, are also examples of applications where finer surface smoothness is required. In this case, the more preferred ranges for center line roughness SRa and maximum peak height SRp are SRa of 1 nm or more and 5 nm or less, and SRp of 70 nm or less, and the even more preferred ranges for center line roughness SRa and maximum peak height SRp are SRa of 1 nm or more and 3 nm or less, and SRp of 30 nm or less.

A more preferred embodiment of the biaxially oriented laminate polyester film of the present invention is produced by a manufacturing method comprising, in order, 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. 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, the process for imparting a fine crystalline structure includes, in order, a step 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) into a sheet; a step of imparting a fine crystalline 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.

In this 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 treatments in a regular pattern to crystallize the film surface. Specific treatment methods include chemical treatments such as etching, which locally erodes the surface of the unstretched film, and printing protrusions on the surface of the unstretched film. Mechanical treatments include creating minute indentations 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 methods 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 impart regular marks to the film surface. Surface modification can also be achieved by irradiating the film with radiation such as gamma rays. When gamma rays are irradiated onto polymeric materials, reactive species are formed, which can cause polymer chains to break, cross-link polymer chains to form bonds, or bond with other highly reactive compounds, creating new branches on the original polymer chains. These become the seeds (seeds) that form the uneven shape when stretched.

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 thinner than 1 μm, tearing may be more likely to occur during the stretching process after the process of imparting a regular microcrystalline structure to the surface of the unstretched film. If the thickness exceeds 350 μm, the environmental impact at the time of disposal may be greater.

When particles are used in the biaxially oriented laminate polyester film of the present invention, inorganic particles such as spherical silica, agglomerated silica, calcium carbonate, aluminum oxide, barium titanate, and titanium oxide, as well as 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.

When adding particles, as mentioned above, it is preferable that the particle shape and particle size distribution are uniform, 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:

f = V/D m ³
Here, V is the particle volume (μm ³ ), and Dm is the maximum diameter (μm) of the particle on the projected surface.

The volume shape factor f reaches a maximum of π/6 (= 0.52) when the particles are spherical. It is also preferable to remove aggregated particles and coarse particles, if necessary, by filtration or other methods. Among these, cross-linked polystyrene resin particles, cross-linked silicone resin particles, and cross-linked acrylic resin particles synthesized by methods such as emulsion polymerization are particularly suitable. However, cross-linked polystyrene particles, cross-linked silicone particles, and spherical silica particles have a volume shape factor close to that of a perfect sphere, an extremely uniform particle size distribution, and are therefore preferred from the perspective of uniformly forming protrusions on the film surface.

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 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 maintaining 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 a sulfur and phosphorus content of 2 ppm by mass or more each 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.

Next, we will explain the method for producing the biaxially oriented laminated polyester film of the present invention, but the present invention should not be interpreted as being limited to this example.

One method for incorporating inert particles into polyester is, for example, to disperse the inert particles in a specified ratio in the form of a slurry in ethylene glycol, a diol component, and then add 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, as this improves particle dispersibility and prevents the formation of coarse protrusions. Another effective method for producing the present invention is to directly mix the aqueous slurry of particles with the specified polyester pellets, feed the mixture into a vented twin-screw kneading extruder, and knead it into the polyester.

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 step. For layers with high extrusion rates, such as 1000 kg/Hr or more, a tandem extruder can be used, in which the functions of melting the pellets and maintaining the molten pellets at a constant temperature are shared between the extruders. Tandem extruders stabilize the polymer temperature during high extrusion rates, thereby reducing viscosity variations in the polymer, making them preferable for processes that reduce thickness unevenness.

The polymer melted and extruded in the extruder is filtered through a filter. Because even the smallest foreign particles can become large protrusions in the film, it is effective to use a filter with a high capture efficiency, capturing, for example, 95% or more of foreign particles 5 μm or larger. However, a filter with too high a capture efficiency can result in a high degree of pressure buildup. Therefore, filters with even higher capture efficiency, such as those capturing 95% or more of foreign particles smaller than 3 μm, are undesirable because they accelerate the increase in filter pressure due to trapped foreign particles and may condense inside the filter before being discharged. 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 then 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 on a casting roll to create an unstretched film. Once the unstretched film lands on the casting roll, a pinning device is used to adhere it to the cast using electrostatic force.

Once the film has been cooled and adhered to the casting roll, it is peeled off from the casting roll using a peel-off roll. Water may be passed through the peel-off roll to cool the film, or the peel-off roll may be driven.

After passing through the peeling rollers, the unstretched film undergoes a process to impart a regular microcrystalline structure to the film surface. Specifically, 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 also impart fine irregularities to the film surface, or it may be crystallized alone without imparting irregularities. Etching involves eroding the film surface by creating a dot pattern, similar to the pattern created when minute amounts of chemicals are dropped onto the film in the form of droplets. These eroded areas then serve as nuclei for the formation of protrusions during stretching. The dot pattern referred to here refers to a state in which the dots are arranged in a regular pattern. Chemicals that can be used include concentrated sulfuric acid, concentrated nitric acid, sodium hydroxide solution, concentrated aqueous ammonia, ethanol, acetone, and alkaline solutions. Printing involves printing protrusions on the film surface to impart microcrystalline structures 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. These resins may be used singly or in combination. Laser treatment involves irradiating an unstretched film with a laser beam in a pattern to form protrusions. Embossing involves creating marks or microscopic scratches on the film surface, such as by pressing a metal roll with a regular concave-convex pattern against the unstretched film. Thermal treatment involves applying regular heat to the unstretched film surface to form a microcrystalline structure. This may be combined with the embossing process described above, in which a heated metal roll is brought into contact with the unstretched film.

The stretching method may be simultaneous biaxial stretching or sequential biaxial stretching. In simultaneous biaxial stretching, when stretching is performed simultaneously in the machine and cross directions, uneven wind speeds and air currents flowing along the film (accompanying air currents) affect the stretching in the machine direction as well as the width direction, making it difficult to create regular surface texture. Therefore, sequential biaxial stretching is the preferred method.

When producing the biaxially oriented laminate polyester film of the present invention using sequential stretching, the preheating temperature for the first longitudinal stretching is preferably 80°C or higher and 110°C or lower. The stretching temperature is 90°C or higher and 130°C or lower, preferably 100°C or higher and 120°C or lower. 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. To prevent uneven stretching and scratches, it is preferable to perform stretching in two or more stages during the stretching process. 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.

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 prevent adhesion, which can cause thermal wrinkles and uneven thickness in the longitudinal direction. The uniaxially stretched film stretched in the longitudinal direction is then heated to 90°C to 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.

The biaxially oriented laminate 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, it may be subjected to a longitudinal re-stretching process without applying a stretching ratio. After the longitudinal re-stretching, a transverse stretching is further performed. After stretching, the film is heat-treated. This heat treatment can be performed by any conventional method or location, such as in an oven or on a heated roll. The heat treatment temperature can typically be any temperature between 150°C and 245°C, and the heat treatment time is typically between 1 second and 60 seconds. The heat treatment temperature not only fixes the polyester film structure but also effectively adjusts the surface roughness. This is because the polyester surface structure roughens depending on the degree of crystallization, resulting in the formation of microprotrusions. A heat treatment temperature of between 235°C and 245°C can form preferred surface protrusions. Furthermore, a heat treatment temperature of between 240°C and 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 by 0% to 10% in the transverse direction at a temperature between 0°C and 150°C lower than the heat treatment temperature.

The dimensional change rate and flatness of the heat-treated film can be adjusted, for example, by providing an intermediate cooling zone or 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 slow cooling zone. In this case, it is preferred 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.

After biaxial stretching, the film is cooled in the 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 this data is used as feedback to adjust the film thickness by adjusting the die thickness, etc., and foreign objects can also be detected using a defect detector.

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.

The biaxially oriented laminated polyester film obtained using the above-mentioned method prevents surface defects caused by aggregation of lubricants and catalysts added to the film, allows for a high degree of freedom in surface design regardless of the nature of the recycled raw materials, and reduces the inclusion of foreign matter when the film is recycled after use.

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 release target is formed and then wound up, shape transfer to the surface of the non-release target can be suppressed, and blocking can also be prevented. In particular, a release film having a release layer on the 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 used preferably.

Furthermore, because the release film of the present invention has the above-mentioned properties, it can be particularly well suited for use in the production of ceramic sheets.

The present invention will be explained in detail below through examples.

The measurement and evaluation methods used in this invention are as follows:

(1) Number of protrusions Images obtained using an atomic force microscope (AFM) under the following measurement conditions were measured using threshold values for protrusion heights 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 for 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 determined 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 determined 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 made 20 times at different locations, and the average value was converted to 50 μm x 50 μm (50 μm square (2500 μm ² )) to obtain the number of protrusions for each location.
Apparatus: NanoScope III AFM
(Digital Instruments)
Cantilever: Silicon single crystal Scanning mode: Tapping mode Scanning rate: 0.8 Hz
Measurement field of view: 5 μm square Sample lines: 256
Sample preparation: 23°C, 65% RH, left stationary for 24 hours. AFM measurement environment: 23°C, 65% RH, 24 hours.

(2) Volume-average particle diameter of particles: A sample of polymer is removed from the film using a plasma low-temperature ashing method to expose the particles. The processing conditions are selected so that the polymer is ashed but the particles are damaged as little as possible. The particles are observed using a scanning electron microscope (SEM; Hitachi, Ltd., Model S-4000), and the particle images are imported into an image analyzer (Nireco Corporation, Model "LUZEX" (registered trademark)_AP). The equivalent circle diameter is measured to determine the volume-average particle diameter of the particles. The SEM magnification is appropriately selected from 5,000 to 20,000 times depending on the particle size. The equivalent circle diameters of the particle diameters of at least 5,000 particles are measured at various observation locations, and the volume-average particle diameter is determined from the average value. If the particles are significantly damaged by the plasma low-temperature ashing method, 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 about 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 the particles, if no particles are found even after observing 10 fields of view at 5,000x magnification when observing with SEM and TEM, it is determined that the particles are not substantially contained.

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

[Example 1]
(1) Preparation of polyester pellets (preparation of polyester A)
An esterification reaction was carried out between 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, and the mixture was subsequently heated to 290° C. under reduced pressure to carry out a polycondensation reaction, thereby obtaining polyester pellets A having an intrinsic viscosity of 0.63 dl/g.

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

(Preparation of Polyester C)
An aqueous 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.

(Preparation 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%. The slurry was added to an ester exchange reaction product prepared in the same manner as for Polyester A, and subsequently antimony trioxide was added thereto, followed by a polycondensation reaction, to obtain master pellets containing 1.5% by mass of agglomerated alumina (Polyester D).

(Preparation 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 and 90 parts by mass of ethylene glycol were wet-ground to obtain a calcium carbonate/ethylene glycol dispersion slurry. The volume average particle diameter of this calcium carbonate was 1.1 μm. On the other hand, 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.

(2) Preparation of Polyester Pellets The polyester pellets to be fed into the extruders for Layers A and B were prepared in the following ratios, which are mass ratios (unit: mass%) relative to the polyester pellets constituting each layer.

A layer Polyester A: 100.0
B layer Polyester A: 70.0
Polyester B: 20.0
Polyester C: 10.0.

(3) Production of Biaxially Oriented Laminated Polyester Film The raw materials prepared for each layer were mixed in a blender, and the mixed raw materials for Layer A were fed into a vented twin-screw extruder for Layer A. The raw materials for Layer B were dried under reduced pressure at 160°C for 8 hours and fed into a single-screw extruder for Layer B. The mixture was melt-extruded at 275°C, filtered through a high-precision filter capturing 95% or more of foreign matter 3 μm or larger, and then joined and laminated in a rectangular two-layer joining block to form a two-layer laminate consisting of Layer A and Layer B. The mixture was then passed through a slit die maintained at 285°C onto a cooling roll, and then wrapped around a casting drum with a surface temperature of 25°C using an electrostatic casting method, followed by cooling and solidification to obtain an unstretched laminate film.

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

This uniaxially stretched film was stretched 4 times laterally 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 an intermediate biaxially stretched film product with a thickness of 25 μm. This intermediate product was slit using a slitter to obtain a roll of biaxially stretched film with a thickness of 25 μm. The roll had a good winding appearance, with no misalignment, mayonnaise, or surface deformation.

A film sample was taken from the roll of this biaxially stretched film and the protrusion height distribution was measured. The results, as shown in Table 1, showed a good surface.

(4) Application of Release Layer Next, 100 parts by mass 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 mass], 5 parts by mass 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 Layer B of this biaxially stretched film roll to obtain a composition for forming a smoothing layer with a solids content of 15% by mass. Next, the obtained composition for forming a smoothing layer was 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 a 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 a 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 obtained release agent layer-forming composition was then applied onto 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). In other words, 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.

(5) Forming the green sheet (applying ceramic slurry)
To 100 parts by mass of barium titanate (manufactured by Fuji Titanium Industries Co., Ltd., trade name HPBT-1), 10 parts by mass of polyvinyl butyral (manufactured by Sekisui Chemical Co., Ltd., trade name BL-1), 5 parts by mass of dibutyl phthalate, and 60 parts by mass of toluene-ethanol (mass ratio 30:30), glass beads with a number average particle size of 2 mm were added, mixed and dispersed in a jet mill for 20 hours, and then filtered to prepare a paste-like ceramic slurry. The obtained ceramic slurry was applied to a release film with a die coater so that the thickness after drying was 1.0 μm, dried, and wound up to obtain a green sheet.

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

The green sheet formed on the release film is illuminated from behind with a 1000 lux backlight unit and observed for pinholes due to coating voids or depressions due to surface transfer from the back of the release film.
A: No pinholes or dents.
B: No pinholes, and up to three dents were observed. C: Pinholes were observed, and four or more dents were observed.

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

In Example 1, there were no pinholes or dents, so the evaluation was given an A.

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

In Example 1, the absolute value of the charge was 1 kV or less, there was no rattle during unwinding, and the result was rated A.

[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.

[Example 6]
A laminated polyester film was obtained in the same manner as in Example 1, except that the thickness and composition were as shown in the table, the film was wound around a casting drum having a surface temperature of 25°C, and cooled ^and solidified to obtain an unstretched laminate film. The unstretched laminate film was then introduced between a facing electrode and an earth roll, nitrogen gas was introduced into the device, and atmospheric pressure glow discharge treatment was performed under conditions such that the E value became 160 W min/m2.

[Examples 7 to 9]
After changing the thickness ratio and the compositions of Layer A and Layer B in the same embodiment as in Example 1, unstretched laminate 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.

[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.

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

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

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

[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.

The present invention provides a biaxially oriented polyester film that, when used as a support for sheet molding, minimizes the transfer of the shape of the back surface of the support to the sheet surface after winding and the occurrence of blocking. This film is particularly suitable for use as a support for release films used in the production of ultra-thin ceramic sheets.

Claims (6)

A biaxially oriented laminated polyester film having 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 a force volume method using an atomic force microscope (AFM), of 100 protrusions/50 μm square (2500 μm ² ) to 5000 protrusions/50 μm square (2500 μm ² ), and the number of protrusions having a height of more than 50 nm and 150 nm or less, of 1 protrusion/50 μm square (2500 μm ² ) to 100 protrusions/50 μm square (2500 μm ² ) , and the particle content of surface layer B is 0.20 to 0.80 mass % of the entire surface layer B. 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, 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. 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. 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. 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. 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 having a release layer on the surface layer A and/or the surface layer B.