JP2020100092A - Biaxially oriented film - Google Patents

Abstract

To provide a biaxially oriented film that has both low heat shrinkage and high flatness.SOLUTION: A biaxially oriented film has an integrated value of 4,000% nm or more and 80,000% nm or less in a light absorption spectrum in a wavelength range of 900 to 2,600 nm, and a maximum value of a heat shrinkage rate of 0.5% or less when heated in an atmosphere of 150°C for 30 minutes.SELECTED DRAWING: None

Description

The present invention relates to a biaxially oriented film that has both low heat shrinkage and high flatness.

A biaxially oriented film obtained by stretching a sheet containing a thermoplastic resin as a main component in a biaxial direction using a roll-type stretching machine, a tenter, or the like has mechanical properties due to the oriented crystallization of the thermoplastic resin component. It has excellent electrical properties, dimensional stability, transparency, and chemical resistance, and is used in many applications.

On the other hand, since the strain due to stretching remains in the molecular chain of the thermoplastic resin forming the biaxially oriented film, the biaxially oriented film contracts due to the strain of the molecular chain of the thermoplastic resin being released by heating. Has characteristics. Therefore, when the biaxially oriented film is exposed to a high temperature environment in a processing step, an environmental test, etc., the biaxially oriented film causes many troubles due to shrinkage. As an example, when the biaxially oriented film is used for indium tin oxide (hereinafter sometimes referred to as ITO) substrate application, the ITO layer is deformed due to the dimensional change of the biaxially oriented film caused by aging after sputtering. There are things to do. Under such circumstances, when the biaxially oriented film is used for an ITO base material, an annealing treatment may be performed before processing at a high temperature in order to reduce the influence of deformation caused by shrinkage.

Further, when a biaxially oriented film is used as the base material layer of the laminated hard coat film, the difference between the shrinkage rate of the hard coat layer and the heat shrinkage rate of the base material layer becomes large during thermal processing, and the laminated hard coat film is curled. There was a problem that occurs. Furthermore, even when a biaxially oriented film is used as the FPC reinforcing film in the manufacturing process of a flexible printed circuit board (hereinafter, sometimes referred to as FPC), the heat shrinkage rate of the FPC reinforcing film and the heat of the FPC film base material are reduced. If the difference from the shrinkage ratio is large, there is a problem that curling occurs after hot pressing.

In order to reduce the heat shrinkage of the biaxially oriented film and improve the thermal dimensional stability, heat treatment (also called heat setting) is performed subsequently to the transverse stretching in a tenter mainly used for the transverse stretching. A method of releasing the distortion of the molecular chain of the thermoplastic resin is used. In this method, the heat shrinkage generally decreases depending on the temperature of the heat treatment, but the heat treatment alone may be insufficient to remove the strain. A method of removing this residual strain by narrowing the rail width of the tenter and shrinking it slightly in the width direction (called toe-in, relaxing, etc.) is also used. Can be reduced, but it is difficult to reduce the heat shrinkage in the longitudinal direction. From such a background, various methods have been studied as a method for reducing the heat shrinkage in the longitudinal direction.

As a method of reducing the thermal contraction in the longitudinal direction, for example, Patent Document 1 discloses a method of performing a relaxation treatment in the longitudinal direction by gradually narrowing the clip interval of the tenter. Patent Document 2 discloses a method in which a biaxially oriented film once wound into a roll is heat-treated in an oven while gradually being unwound, and a relaxation treatment is performed by making a speed difference in the longitudinal direction at that time. There is. Patent Document 3 discloses a method of providing a relaxation treatment device in a longitudinal direction by an oven during a film forming process of a biaxially oriented film. Further, Patent Document 4 discloses a method in which a biaxially oriented film before a heat treatment step is subjected to a rapid heat treatment with infrared rays to relax in the longitudinal direction to perform bowing or low heat shrinkage.

Japanese Patent Publication No. 4-28218 WO2016/084568 JP, 2008-265298, A JP, 6-262675, A

However, in the method of Patent Document 1, there is an upper limit to the relaxation rate due to a problem on the device, and if the relaxation rate is excessively large, the clip interval before the relaxation process becomes wide, and the clip gripping portion and the non-gripping portion are separated. There arises a problem that the unevenness of the physical properties becomes large. Further, in this method, since the clip has a mechanism capable of sliding on the rail, there is a big problem that metal powder due to abrasion of the clip adheres to the product and becomes a defect. In the method of Patent Document 2, since the biaxially oriented film is heat-treated over a long section in a state where the film is not fixed in the width direction, heat shrinkage in the width direction remarkably occurs, resulting in a problem that the flatness thereof deteriorates. Another problem is that it is expensive. In the method of Patent Document 3, if the treatment temperature is raised in consideration of the film forming speed, the flatness of the biaxially oriented film is deteriorated, so that it is difficult to raise the temperature sufficiently, and as a result, the heat shrinkage is sufficiently reduced. There is a problem that is not done. Therefore, both low heat shrinkage and high flatness cannot be achieved at the same time. Furthermore, the methods of Patent Documents 2 and 3 perform relaxation treatment by heating with hot air, and therefore, in the case of a thin film or a flexible film, deformation due to wind pressure occurs, which makes it difficult to use. Become. It is also difficult for the method of Patent Document 4 to obtain a sufficient low heat shrinkage effect.

In view of the background of the related art, it is an object of the present invention to provide a biaxially oriented film having both a low heat shrinkage rate and a high flatness.

In order to solve the above problems, the present invention has the following configurations.
(1) The integrated value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum is 4,000%·nm or more and 80,000%·nm or less, and the heat shrinkage when heated for 30 minutes in a 150° C. atmosphere. The maximum value of the ratio is 0.5% or less, a biaxially oriented film.
(2) When the direction in which the degree of molecular orientation is maximum is the X direction and the direction orthogonal to the X direction in the plane is the Y direction, the thickness unevenness in both the X direction and the Y direction is 5.0% or less. The biaxially oriented film as described in (1), wherein
(3) The biaxially oriented film as described in (1) or (2), which has a total light transmittance of 70% or more and a haze of 3.0% or less.
(4) The biaxially oriented film according to any one of (1) to (3), which contains an infrared absorber.
(5) The biaxially oriented film as described in (4), wherein the infrared absorbent is a carbon material.
(6) The biaxially oriented film as described in (5), wherein the carbon material is carbon black.
(7) In (5) or (6), the content of the carbon material is 0.001% by mass or more and 0.400% by mass or less when all the constituent components are 100% by mass. The biaxially oriented film described.
(8) The infrared absorber is at least one metal compound selected from tin-based metal oxides, tungsten-based metal oxides, and hexaborides, according to (4) Biaxially oriented film.
(9) The biaxial according to (8), characterized in that the content of the metal compound is 0.01% by mass or more and 1.90% by mass or less when all the constituent components are 100% by mass. Oriented film.
(10) The biaxially oriented film as described in any one of (1) to (9), which contains a polyester resin as a main component.
(11) The method for producing a biaxially oriented film according to any one of (1) to (10), which comprises a heat treatment step of heating at a temperature not lower than the stretching temperature and not higher than the melting point of the thermoplastic resin as the main component, and A longitudinal relaxation step of heating the entire width in the width direction to relax the film in the longitudinal direction is provided in this order, and the traveling speed of the film in the longitudinal relaxation step is 10 m/min or more and 70 m/min or less, Film manufacturing method.

According to the present invention, it is possible to provide a biaxially oriented film that has both low heat shrinkage and high flatness. Further, the biaxially oriented film of the present invention is suitable for applications requiring a high level of thermal dimensional stability and flatness, for example, optical applications such as release of polarizing plates, coating substrates, and ITO sputter substrates. Can be used.

Hereinafter, the biaxially oriented film of the present invention will be described in detail. The biaxially oriented film of the present invention has an integrated value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum of 4,000%·nm or more and 80,000%·nm or less, and for 30 minutes in an atmosphere of 150°C. It is characterized in that the maximum value of the heat shrinkage rate when heated is 0.5% or less. By adopting such an embodiment, the biaxially oriented film of the present invention has both low heat shrinkage and high flatness.

In the biaxially oriented film of the present invention, it is important that the maximum value of the heat shrinkage rate when heated in an atmosphere of 150° C. for 30 minutes is 0.5% or less. The maximum value of the heat shrinkage ratio means the value of the heat shrinkage ratio in the direction in which the heat shrinkage ratio is the largest in the film. In the case where the film is a biaxially oriented film, the direction in which the maximum value of the heat shrinkage is given is usually either the direction in which the degree of orientation of the molecular chains of the thermoplastic resin in the film is the highest or the direction orthogonal to the direction. .. Further, in most cases, the biaxially oriented film is obtained by stretching the thermoplastic resin sheet in two orthogonal directions, and therefore, the direction in which the maximum value of the heat shrinkage rate is given is usually any one of the stretching directions.

From the above, when the stretching direction is known in advance, the heat shrinkage rates in the two stretching directions can be measured, and the larger value can be set as the maximum value of the heat shrinkage rate. However, if the stretching direction cannot be specified, the direction in which the molecular chain orientation of the thermoplastic resin has the highest degree of orientation can be specified with a molecular orientation meter, and the specified direction and the direction orthogonal to the direction can be regarded as the stretching direction.

The biaxially oriented film of the present invention has a maximum heat shrinkage of 0.5% or less when heated in an atmosphere of 150° C. for 30 minutes, so that it is required to have thermal dimensional stability under high temperature conditions. Can be suitably used. From the above viewpoint, the maximum value of the heat shrinkage rate when heated for 30 minutes in an atmosphere of 150° C. is preferably 0.3% or less, and more preferably 0.1% or less. In addition, the smaller the maximum value of the thermal contraction rate when heated in an atmosphere of 150° C. for 30 minutes, the better, so the lower limit is not particularly limited, but it is 0.01% from the viewpoint of feasibility.

The method for setting the maximum value of the heat shrinkage ratio at the time of heating for 30 minutes in a 150° C. atmosphere to 0.5% or less or the above preferable range is not particularly limited as long as the effects of the present invention are not impaired. There is a method in which the integral value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum is adjusted within a preferable range described later, and the film is heated by an infrared heater to perform a relaxation treatment in the longitudinal direction. More specifically, with respect to the film that has been subjected to heat treatment after biaxial stretching, the traveling speed on the winder side is slightly relaxed, and the film tension is 10 N/m or more and 50 N/m or less, more preferably 15 N/m or more and 35 N or more. /M or less, a method of rapidly performing heat treatment with an infrared heater so that the film temperature is in the range of 120° C. or higher and 180° C. or lower can be mentioned. By setting the film temperature during heating to 120° C. or higher, it becomes easy to set the heat shrinkage rate of the obtained biaxially oriented film to 0.5% or lower, while setting the film temperature during heating to 180° C. or lower. As a result, the deterioration of the quality of the obtained biaxially oriented film can be reduced. In addition, in order to raise the film temperature to 120° C. or more and 180° C. or less in a short time, it is preferable to set the peak top of the infrared wavelength to be irradiated to 1,000 to 1,600 nm.

The measurement of the thermal contraction rate when heating for 30 minutes in a 150° C. atmosphere can be performed by the following procedure. First, a biaxially oriented film is sampled in a rectangular shape of 250 mm (measurement direction)×10 mm, and two reference points are attached at intervals of about 200 mm. The distance between these two gauge points is accurately measured, and this is designated as To (mm). After that, this sample was left in a hot air oven at 150° C. for 30 minutes under a load of 3 g, then sufficiently cooled at room temperature, the gauge length was measured again, and the obtained value is defined as T (mm). From the thus obtained To (mm) and T (mm), the heat shrinkage rate (%) is calculated by the following formula 1.
Formula 1: Thermal shrinkage rate (%)=((To-T)/To)×100.

Thus, from the viewpoint of reducing the heat shrinkage rate when heated for 30 minutes in a 150° C. atmosphere, the biaxially oriented film of the present invention has an integrated value of 4 in the wavelength range of 900 to 2,600 nm of the light absorption spectrum. It is important that it is 2,000%·nm or more and 80,000%·nm or less. When the integral value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum is in the above range, the absorption efficiency in the infrared region of the film is improved and the temperature raising effect by infrared irradiation is increased. From the above viewpoint, the preferable range of the integrated value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum is 6,000%·nm or more and 49,000%·nm or less, and the more preferable range is 6,000%·nm or more. 000%·nm or less.

When the integral value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum is less than 4,000%·nm, the temperature raising effect by infrared rays is hardly recognized. On the other hand, when the integrated value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum is larger than 80,000%·nm, the temperature raising effect becomes excessively high, the film running speed is increased, and the infrared irradiation time is shortened. May also melt the film.

The integrated value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum can be measured by the following method. First, with a spectrophotometer, the relative reflectance (%) and the relative transmittance (%) in the wavelength range of 300 to 2,600 nm of the biaxially oriented film when the reflection of the aluminum oxide standard white plate is set to 100%. , Scanning speed 600 nm/min, sampling pitch 1 nm, and continuously measured, subtracting the values of relative reflectance (%) and relative transmittance (%) from 100%, the light absorption rate (%) at each wavelength Ask for. After that, the light absorption rate (%) at each wavelength is plotted with the light absorption rate (%) on the vertical axis and the wavelength (nm) on the horizontal axis, and a line is drawn by connecting adjacent points with a straight line. Calculate the area (% nm) of the part surrounded by the drawn polygonal line, the horizontal axis, the straight line parallel to the vertical axis indicating wavelength = 900 nm, and the straight line parallel to the vertical axis indicating wavelength = 2,600 nm, and calculate this as the wavelength It is an integrated value (%·nm) of the absorptance in the range of 900 to 2,600 nm. The spectrophotometer and the integrated value calculation software are not particularly limited as long as they can measure, and can be appropriately selected from known ones. Examples of spectrophotometers that can be used include spectrophotometer U-4100 manufactured by Hitachi, and examples of integration value calculation software include spreadsheet software “EXCEL” (registered trademark) manufactured by Microsoft Corporation.

As long as the integrated value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum is 4,000%·nm or more and 80,000%·nm or less or the above preferable range, the effect of the present invention is not impaired. There is no particular limitation. Specific examples thereof include, for example, a method in which the biaxially oriented film contains an infrared absorbent described later, a method in which the traveling speed of the film is adjusted while keeping the infrared irradiation range constant, and the like. More specifically, by increasing the amount of the infrared absorbent in the biaxially oriented film, or decreasing the traveling speed of the film while keeping the infrared irradiation range constant, the wavelength of the light absorption spectrum of 900 to 2,600 nm. The integrated value in the range can be increased.

In the biaxially oriented film of the present invention, when the direction in which the degree of molecular orientation is maximum is the X direction and the direction orthogonal to the X direction in the plane is the Y direction, the thickness unevenness in both the X direction and the Y direction is 5 It is preferably at most 0.0%. The thickness unevenness in the X and Y directions is more preferably 4.5% or less, and further preferably 3.5% or less. Here, the degree of molecular orientation refers to the orientation of the molecular chains of the thermoplastic resin forming the biaxially oriented film. This degree of molecular orientation can be measured by a known molecular orientation meter, for example, a molecular orientation meter "MOA-2001" manufactured by Oji Paper Instruments Co., Ltd.

The thickness unevenness can be measured by the following procedure, for example. First, the biaxially oriented film is sampled in a rectangular shape of 30 mm×1 m (measurement direction), and the biaxially oriented film is conveyed at a constant speed in parallel with the measuring direction using a known film thickest tester and an electronic micrometer. The thickness of the biaxially oriented film at a plurality of points is continuously measured at regular intervals. The maximum value T _max (μm), the minimum value T _min (μm), and the average value T _ave (μm) are obtained from the thickness data obtained by the measurement, and the thickness unevenness is calculated by the following formula 2. The measurement conditions are as follows: the transport speed of the biaxially oriented film is 0.5 m/min, the measurement interval is 0.1 s, and the number of measurement points is 2,056.
Formula 2: Thickness unevenness (%)=(T _max −T _min )×100/T _ave .

The film thickness tester and the electronic micrometer are not particularly limited as long as they can be measured, and can be appropriately selected from known ones. For example, the film thickness tester “KG601A” and the electronic micrometer “K306C” manufactured by Anritsu Corporation. Can be used.

In general, the degree of molecular orientation is increased by stretching, so that the X direction in a biaxially oriented film is usually the longitudinal direction or the width direction. The longitudinal direction means the running direction of the film, and the width direction means the direction orthogonal to the longitudinal direction in the plane. That is, when the biaxially oriented film is wound around a core to form a film roll, the winding direction corresponds to the longitudinal direction and the direction parallel to the central axis of the core corresponds to the width direction. By setting the thickness unevenness in both the X and Y directions to 5.0% or less, the flatness of the biaxially oriented film is improved. Therefore, the biaxially oriented film can be more suitably used for optical applications such as release of a polarizing plate, a coating substrate, and an ITO sputter substrate.

Means for setting the thickness unevenness in both the X and Y directions to 5.0% or less or the above preferable range is not particularly limited as long as the effects of the present invention are not impaired, but it is likely to occur due to reduction of heat shrinkage in the longitudinal direction. It is preferable to use a method of reducing uneven thickness in the width direction. As a specific example, for example, a method of rapidly increasing the film temperature with an infrared heater or the like to reduce thickness unevenness in the width direction can be preferably used.

As described above, usually, when the longitudinal direction is relaxed in a situation where the position of the biaxially oriented film in the width direction is not fixed, heat shrinkage in the width direction may occur and uneven thickness in the width direction may occur. This tendency becomes more remarkable as the section in which the relaxation process is performed becomes longer. By using an infrared heater or the like to heat the biaxially oriented film, it is easy to raise the temperature of the biaxially oriented film in a short time, so that the section in which the relaxation treatment is performed is shortened and, as a result, uneven thickness in the width direction is also caused. Will be reduced. Further, in that case, the width direction of the biaxially oriented film may be uniformly heated in all, but heating so that the film temperature near the width direction end part is lower than the width direction center part, It is preferable in that the thickness unevenness in the direction is reduced. It is considered that this is because the widthwise end portion having a relatively low temperature functions as a fixed end, and thus the shrinkage in the widthwise direction of the entire biaxially oriented film is reduced.

The biaxially oriented film of the present invention has a total light transmittance of 70% or more and a haze of 3. from the viewpoint that it can be suitably used for applications in which transparency is increased and visibility is required. It is preferably 0% or less, the total light transmittance is 80% or more, and the haze is more preferably 2.0% or less, the total light transmittance is 85% or more, and the haze is 1. It is more preferably 3% or less. The total light transmittance and the haze can be measured using a known measuring device such as HGM-2DP manufactured by Suga Test Instruments Co., Ltd.

The biaxially oriented film has a total light transmittance of 70% or more or the above preferable range, and a haze of 3.0% or less or the above preferable range, in a visible light region of a wavelength of 350 to 800 nm. A method of adjusting the content of the compound having absorption can be mentioned. More specifically, by reducing the content of such a compound, the total light transmittance can be increased and the haze can be reduced. The content of such a compound can be appropriately adjusted depending on the presence or absence of an absorption band in a region of the compound having a wavelength of less than 800 nm and the degree thereof.

The biaxially oriented film of the present invention preferably contains an infrared absorbing agent from the viewpoint of increasing the temperature raising effect by infrared irradiation. Examples of the infrared absorber that can be preferably used for the biaxially oriented film of the present invention include, for example, cyanine dyes, phthalocyanine dyes, polymethine dyes, squarylium dyes, porphyrin dyes, metal dithiol complex dyes, diimonium dyes. Examples thereof include infrared absorbing dyes such as dyes, infrared absorbing agents based on carbon materials and inorganic compounds described later, which mainly selectively and strongly absorb light having a wavelength of around 800 to 1,200 nm.

Since substances that selectively absorb in the infrared region are generally expensive, if the use of the biaxially oriented film does not place importance on transparency, the absorption band in the wavelength range of less than 800 nm can be used as an infrared absorber. Compounds with may be used. Examples of such a compound include magnesium carbonate, magnesium silicate, silicon dioxide, aluminum oxide, barium sulfate, calcium sulfate, magnesium hydroxide, aluminum hydroxide, calcium hydroxide, phosphate, silicate, and hydrosilicate. Inorganic compounds such as talcites, and carbon materials such as carbon black having a graphite structure, graphene, fullerene, and carbon nanotubes can be used. Among them, in the case where the biaxially oriented film of the present invention does not place importance on transparency, the infrared absorber is preferably a carbon material from the viewpoint of production cost, dispersibility in a thermoplastic resin, and absorption capacity. More preferably, it is carbon black.

When the biaxially oriented film of the present invention contains a carbon material, the biaxially oriented film of the present invention has a carbon material content of 0.001% by mass or more and 0.400% when all the constituent components are 100% by mass. It is preferably not more than mass%, more preferably not less than 0.001 mass% and not more than 0.200 mass%, even more preferably not less than 0.001 mass% and not more than 0.070 mass%. When the content of the carbon material is 0.001% by mass or more, a sufficient temperature raising effect can be obtained at the time of infrared irradiation, so that the heat flux reducing effect is also large. On the other hand, when the content of the carbon material is less than 0.400% by mass, the absorption and heat generation at the time of infrared irradiation does not become excessive, the deformation of the film is suppressed, and the total light transmittance is excessively decreased or haze is reduced. Excessive rise can be reduced. When the biaxially oriented film contains a plurality of kinds of carbon materials, the content is calculated by adding all the carbon materials. This also applies to the metal oxide described later.

On the other hand, when the use of the biaxially oriented film is such that transparency is important, it is preferable to select an infrared absorbing agent that absorbs less in the wavelength region of less than 800 nm. Considering that such an infrared absorber generally has low heat resistance and often decomposes during extrusion molding of a thermoplastic resin, it is preferable to use an inorganic compound-based infrared absorber. Furthermore, when transparency is required for the biaxially oriented film of the present invention, the infrared absorber is at least one metal compound selected from tin-based metal oxides, tungsten-based metal oxides, and hexaborides. Is preferred. The tin-based metal oxide, the tungsten-based metal oxide, and the hexaboride may be used together within a range that does not impair the effects of the present invention, and a plurality of compounds corresponding to the tin-based metal oxide may be used in combination. May be. The latter point is the same for tungsten-based metal oxides and hexaborides.

The type of the tin-based metal oxide is not particularly limited as long as the effect of the present invention is not impaired, but examples thereof that can be preferably used include indium tin oxide, antimony tin oxide, and a mixture of antimony oxide and tin oxide. Are listed.

The tungsten-based metal oxide is a metal compound represented by a chemical formula of MWO _n (M is a metal element, n is an oxidation number). At this time, M is one or more elements selected from the elements Cs, Rb, K, TI, In, Li, Ca, Sr, Fe, and Sn.

The hexaboride is a metal compound represented by the chemical formula of MB ₆ (M is a metal element). At this time, M is one kind selected from each element of La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Y, Sm, Eu, Er, Tm, Yb, Lu, Sr, and Ca. These are the above elements. The type of hexaboride is not particularly limited as long as the effect of the present invention is not impaired, but lanthanum hexaboride (LaB ₆ ) is preferably used because of its high infrared absorptivity.

The biaxially oriented film of the present invention contains at least one metal compound selected from tin-based metal oxides, tungsten-based metal oxides, and hexaborides when all the constituents are 100% by mass. The amount is preferably 0.010% by mass or more and 1.900% by mass or less, and more preferably 0.010% by mass or more and 0.600% by mass or less in consideration of transparency. When the content of these metal oxides is 0.010% by mass or more, a sufficient temperature increasing effect can be obtained at the time of irradiation with infrared rays, and thus the effect of reducing heat heat is also increased. On the other hand, if the content of these metal oxides is 1.900% by mass or less, the absorption and heat generation at the time of infrared irradiation are not excessive, the deformation of the film is suppressed, and the decrease in transparency is also reduced. it can.

The biaxially oriented film of the present invention is mainly composed of a thermoplastic resin. The type of thermoplastic resin is not particularly limited as long as the effects of the present invention are not impaired, and examples thereof include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, polyester elastomers, polyamide resins such as nylon 6 and nylon 66, polyamide elastomers. Polyolefin resins such as polypropylene and polyethylene, polyolefin-based elastomers, polyurethane, and polylactic acid can be used alone or in combination of two or more kinds. Further, these thermoplastic resins may contain a copolymerization component within a range that does not impair the effects of the present invention.

The biaxially oriented film of the present invention preferably contains a polyester resin as a main component from the viewpoints of transparency, heat resistance, and cold resistance. The polyester resin means a polymer having a dicarboxylic acid unit and a glycol unit as main constituent units. The polymer having a dicarboxylic acid unit and a glycol unit as main constituent units includes a total of 60 mol% or more and 100 mol% or less of the dicarboxylic acid unit and the glycol unit, when the total constituent units constituting the polymer is 100 mol %. A polymer. The polyester resin is not particularly limited as long as it contains 60 mol% or more and 100 mol% or less in total of dicarboxylic acid units and glycol units, but may contain 80 mol% or more and 100 mol% or less in total of dicarboxylic acid units and glycol units. preferable. The term “polyester resin as a main component” means that the polyester resin is contained in an amount of 60% by mass or more and 100% by mass or less, when all the constituent components are 100% by mass. When there are a plurality of types of polyester resins in the biaxially oriented film, the content is calculated by adding all the polyester resins.

Examples of the dicarboxylic acid unit include an isophthalic acid unit, a terephthalic acid unit, a diphenyl-4,4′-dicarboxylic acid unit, a 2,6-naphthalenedicarboxylic acid unit, a naphthalene-2,7-dicarboxylic acid unit, and a naphthalene-1. ,5-dicarboxylic acid unit, diphenoxyethane-4,4'-dicarboxylic acid unit, diphenylsulfone-4,4'-dicarboxylic acid unit, diphenylether-4,4'-dicarboxylic acid unit, malonic acid unit, 1,1 -A dimethylmalonic acid unit, a succinic acid unit, a glutaric acid unit, an adipic acid unit, a sebacic acid unit, a decamethylenedicarboxylic acid unit and the like can be used.

As such a glycol unit, for example, an aliphatic glycol unit, an alicyclic glycol unit, an aromatic glycol unit or the like can be used. Examples of the aliphatic glycol unit include an ethylene glycol unit, a tetramethylene glycol unit, a hexamethylene glycol unit, a neopentyl glycol unit, a 1,3-propanediol unit and a spiroglycol unit. Examples of the alicyclic glycol unit include a cyclohexanedimethanol unit. Further, examples of the aromatic glycol unit include a bisphenol-A unit and a bisphenol-S unit. In addition, the polyester resin may be a copolymer containing a polyethylene glycol unit, a polypropylene glycol unit, a polytetramethylene glycol unit, an ethylene glycol-propylene glycol unit, etc. within a range that does not impair the effects of the present invention.

Specific examples of the polyester resin that can be used in the biaxially oriented film of the present invention include polyethylene terephthalate (PET), isophthalate copolymerized PET (PET/I), and 1,4-cyclohexanedimethanol copolymerized PET (PETG). ), spiroglycol copolymerized PET, polypropylene terephthalate (PPT), polybutylene terephthalate (PBT), isophthalate copolymerized PBT (PBT/I), polyhexamethylene terephthalate (PHT), polyethylene naphthalate (PEN), polypropylene naphthalate (PPN), polybutylene naphthalate (PBN), polycyclohexane dimethylene terephthalate (PCT), polyhydroxybenzoate (PHB), and the like. These polyesters may be used alone or in combination of two or more kinds as long as the effects of the present invention are not impaired.

Hereinafter, the method for producing the biaxially oriented film of the present invention will be described. The manufacturing method of the biaxially oriented film of the present invention is a heat treatment step of heating at a temperature not lower than the stretching temperature and not higher than the melting point of the thermoplastic resin which is the main component, and a length to relax the film in the longitudinal direction by heating the entire width in the width direction. It is characterized in that the film has a direction relaxing step in this order, and the traveling speed of the film in the longitudinal relaxing step is 10 m/min or more and 70 m/min or less. Here, having a heat treatment step and a longitudinal direction relaxation step in this order refers to all aspects having a longitudinal direction relaxation step downstream of the heat treatment step, and other steps between both steps do not matter. With such a mode, it is possible to achieve both low heat shrinkage and high flatness without impairing production efficiency.

It is important that the method for producing a biaxially oriented film of the present invention has a heat treatment step of heating at a temperature not lower than the stretching temperature and not higher than the melting point of the thermoplastic resin which is the main component. A known method can be used for stretching in the longitudinal direction and the width direction in the method for producing a biaxially oriented film of the present invention. Unless the effects of the present invention are impaired, the stretching in the longitudinal direction and the width direction is performed sequentially. May be performed simultaneously or simultaneously (hereinafter, the former may be referred to as sequential biaxial stretching, the latter may be referred to as simultaneous biaxial stretching. In addition, stretching in the longitudinal direction is referred to as longitudinal stretching, and stretching in the width direction is referred to as transverse stretching. Sometimes.). Here, the stretching temperature refers to the temperature during transverse stretching in the case of sequential biaxial stretching in which transverse stretching is performed after longitudinal stretching, and the stretching temperature in the case of simultaneous biaxial stretching refers to the temperature during stretching.

Since the method for producing a biaxially oriented film of the present invention has a heat treatment step, the dimensional stability of the finally obtained biaxially oriented film during heating can be improved. A known method or apparatus can be used for this heat treatment, and for example, it can be performed by heating in a section after stretching of a tenter device that performs transverse stretching (stretching in both longitudinal and transverse directions in the case of simultaneous biaxial stretching). it can. When the temperature in the heat treatment step (hereinafter sometimes referred to as heat treatment temperature) is not less than the stretching temperature, the dimensional stability improving effect is sufficiently obtained, and when the temperature is not more than the melting point of the thermoplastic resin, the film at the time of heating Can be prevented from melting. The heat treatment temperature can be appropriately set depending on the resin composition or the like constituting the biaxially oriented film as long as it is a temperature not lower than the stretching temperature and not higher than the melting point of the thermoplastic resin which is the main component. For example, the main component is PET. In such a case, it is preferable to set the glass transition temperature of PET to the glass transition temperature of PET+120° C. or less from the viewpoint of simultaneously improving the dimensional stability and preventing the film from melting.

The manufacturing method of the biaxially oriented film of the present invention is, from the viewpoint of improving the planarity of the finally obtained biaxially oriented film, downstream of the heat treatment step, heating the entire width in the width direction to relax the film in the longitudinal direction. It is important to have a directional relaxation step. By adopting such an embodiment, it is possible to achieve both low thermal shrinkage and high flatness of the finally obtained biaxially oriented film. Here, heating the entire width in the width direction means heating the film after the heat treatment step over the entire width in the width direction. This heating may be performed at the same timing over the entire width in the width direction, or may be partially performed at another timing. As an example of performing a part at another timing, there is a mode in which only the widthwise central portion is heated first, and then only the widthwise both ends are heated. The heating means can be appropriately selected from known ones as long as the effect of the present invention is not impaired, but an infrared heater is preferable from the viewpoint of easy installation and temperature rise in a short time, and further irradiation is performed. More preferably, the peak top of the infrared wavelength is 1,000 to 1,600 nm.

The method of longitudinal relaxation treatment is not particularly limited as long as the effect of the present invention is not impaired, and can be appropriately selected. As a specific method, for example, with respect to the film after the heat treatment step, the traveling speed on the winding machine side is slightly relaxed, and the film tension is 10 N/m or more and 50 N/m or less, more preferably 15 N/m or more and 35 N/m or less. A method in which heat treatment is rapidly performed with an infrared heater so that the film temperature is in the range of 120° C. or higher and 180° C. or lower in a state of being adjusted to m or less. At this time, by setting the film temperature during heating to 120° C. or higher, it becomes easy to reduce the heat shrinkage rate of the obtained biaxially oriented film, while setting the film temperature during heating to 180° C. or lower The flatness of the obtained biaxially oriented film can be improved.

In the method for producing a biaxially oriented film of the present invention, the traveling speed of the film in the longitudinal relaxation step is 10 m/min or more and 70 m/min from the viewpoint of improving the flatness of the biaxially oriented film obtained without impairing the productivity. It is important to be less than or equal to min. By setting the traveling speed of the film in the longitudinal relaxation step to 10 m/min or more, the decrease in the production speed is reduced. On the other hand, by setting the traveling speed of the film in the longitudinal relaxation step to 70 m/min or less, air entrapment in casting caused by an excessive increase in production speed is reduced, resulting in the occurrence of defects and flatness. Can be suppressed. From the above viewpoint, the traveling speed of the film in the longitudinal relaxation step is preferably 10 m/min or more and 50 m/min or less.

Next, a preferred method for producing the biaxially oriented film of the present invention will be described by taking a biaxially oriented PET film as an example. However, the present invention should not be construed as being limited to such examples.

First, PET is prepared in the form of pellets, dried in hot air or under reduced pressure, if necessary, and then supplied to an extruder together with an infrared absorbent. After the PET is heated and melted at a temperature equal to or higher than the melting point in an extruder and the extrusion amount is made uniform by a gear pump or the like and extruded, foreign matters and modified substances are removed from the molten PET composition through a filter or the like. Then, the molten PET composition is formed into a sheet by a die, discharged, and cooled and solidified on a cooling body such as a casting drum to obtain a casting film. At this time, it is preferable to use a wire-shaped, tape-shaped, needle-shaped, or knife-shaped electrode to bring the sheet-shaped material into close contact with a cooling body such as a casting drum by electrostatic force for rapid cooling and solidification. Further, as a means for bringing the sheet-like material into close contact with the cooling body such as the casting drum, a method of blowing air in a slit shape, a spot shape, or a plane shape, or a method using a nip roll may be adopted.

Then, the casting film is biaxially stretched in the longitudinal direction and the width direction. The biaxial stretching may be performed in the longitudinal direction and the width direction sequentially or simultaneously.

First, the case of sequential biaxial stretching will be described. In the case of sequential biaxial stretching, the casting film is first longitudinally stretched to give the molecular orientation in the longitudinal direction. In general, longitudinal stretching can be performed by the difference in peripheral speed of rolls, and can be performed in one stage or in multiple stages by a plurality of roll pairs. The longitudinal stretching ratio can be appropriately selected depending on the desired degree of orientation, thickness, etc., but when the resin is PET, it is preferably 2 to 7 times. The longitudinal stretching temperature is preferably not lower than the glass transition temperature of PET and not higher than the glass transition temperature of PET +100°C.

The uniaxially oriented film thus obtained is subjected to surface treatments such as corona treatment, flame treatment, and plasma treatment, if necessary, and then inline functions such as slipperiness, easy adhesion, and antistatic property are provided. It may be applied by coating.

Subsequently, the obtained uniaxially oriented film is transversely stretched to give a molecular orientation in the width direction. In general, the transverse stretching can be performed by extending the interval between the opposing clips while running the clips holding both ends of the uniaxially oriented film in the width direction with a tenter in the transport direction. The lateral stretching ratio can be appropriately selected depending on the desired degree of orientation, thickness, etc., but is particularly preferably 2 to 7 times when the resin is PET. The transverse stretching temperature is preferably not lower than the glass transition temperature of PET and not higher than the glass transition temperature of PET +120°C.

Next, the case of simultaneous biaxial stretching will be described. In the case of simultaneous biaxial stretching, the obtained casting film is subjected to surface treatment such as corona treatment, flame treatment, plasma treatment, etc., if necessary, and then slipperiness, easy adhesion, antistatic property, etc. The function of may be provided by in-line coating.

Next, the casting film is guided to a tenter, and is conveyed while gripping both ends thereof with clips, and is stretched in the width direction by widening the gap between the opposing clips and in the longitudinal direction by adjusting the running speed of the clips. Simultaneous biaxial stretching machines include pantograph type, screw type, drive motor type, and linear motor type, but the stretching ratio can be changed arbitrarily and the relaxation process can be performed at any place. A motor type or a linear motor type is preferable. The stretching ratio can be appropriately selected depending on the desired degree of orientation, thickness, etc., but when the resin is PET, the area ratio is preferably 8 to 30 times. Further, the stretching temperature is preferably the glass transition temperature of PET or higher and the glass transition temperature of PET +120° C. or lower.

In order to impart flatness and dimensional stability, it is preferable that the film sequentially or simultaneously biaxially stretched is subsequently subjected to a heat treatment in a tenter at a temperature not lower than the stretching temperature and not higher than the melting point. In addition, after the heat treatment, a relaxation treatment in the width direction may be performed together with slow cooling, if necessary.

Further, after exiting the tenter, the widthwise end portion held by the clip is cut and removed, and the traveling speed on the winder side is slightly slowed to perform a relaxation process in the longitudinal direction. The heating at this time is preferably performed by an infrared heater having a peak top at a wavelength of 1,000 to 1,600 nm so that the film temperature rapidly becomes 120° C. or more and 180° C. or less in terms of high irradiation energy. .. In addition, the relaxation treatment time is preferably as short as possible from the viewpoint of flatness, and is preferably 3.00 seconds or less, more preferably 1.00 seconds or less. In order to finish the heating within this processing time, the traveling speed of the film, the output of the infrared heater, the number of heaters arranged in the traveling direction, etc. can be adjusted according to the absorption coefficient and thickness of the film.

The running speed of the film in the longitudinal relaxation step is 10 m/min or more and 70 m/min or less from the viewpoint of improving the flatness of the biaxially oriented film obtained without impairing the productivity. The running speed of this film is preferably high from the viewpoint of production efficiency per unit time, but considering the occurrence of air entrapment in casting, for example, 10 m/min or more and 50 m/min or more. The following are preferred.

Usually, the effect of raising the temperature during infrared irradiation increases in proportion to the absorption coefficient of the film and the infrared irradiation time. Therefore, when increasing the running speed of the film, it is preferable to increase the absorption coefficient of the film by adjusting the raw material composition and to reduce the shortening of the infrared irradiation time. Examples of means for increasing the absorption coefficient of the film include increasing the content of the above-mentioned carbon material or metal compound within the appropriate range described above. Further, as a means for reducing the shortening of the infrared irradiation time, for example, a large number of infrared heaters may be arranged in the longitudinal direction.

Further, the timing of performing the relaxation treatment in the longitudinal direction on the film after biaxial stretching can be appropriately selected after the exit of the tenter, as long as the effect of the present invention is not impaired. In general, from the viewpoint that the temperature is high and the energy required for heating can be suppressed, it is preferable to perform a relaxation treatment in the longitudinal direction immediately after leaving the tenter, but after cooling to room temperature, the longitudinal direction is applied to the film gripped between the rolls in the longitudinal direction. Relaxation treatment may be performed.

Further, the heating by the infrared heater for the relaxation treatment in the longitudinal direction may be performed under the same condition for the entire width direction at a time, but for example, after heating the center portion in the width direction, both end portions in the width direction are heated. , You may go in stages. In order to perform the heating stepwise, an infrared heater may be arranged on the upstream side for the portion to be heated first and on the downstream side for the portion to be heated later. When performing heating stepwise, the output of the infrared heater may be varied within a range that does not impair the effects of the present invention.

The biaxially oriented film obtained as described above is trimmed to a required width through a winding device and wound into a roll so that winding wrinkles do not occur. In addition, in order to improve the winding appearance, embossing treatment may be performed near both ends of the biaxially oriented film before winding.

Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples. Each property was measured by the following methods.

(Characteristic measuring method and effect evaluating method)
(1) Integrated value of light absorption spectrum in wavelength range of 900 to 2,600 nm A spectrophotometer U-4100 manufactured by Hitachi was used. First, the relative reflectance (%) and the relative transmittance in the wavelength range of 300 to 2,600 nm of the biaxially oriented film when the reflection of the aluminum oxide standard white plate (attached to the main body) was set to 100% by attaching an integrating sphere. (%) is continuously measured under the conditions of a scan speed of 600 nm/min and a sampling pitch of 1 nm, and the relative reflectance (%) and relative transmittance (%) values are subtracted from 100% to obtain the optical absorption at each wavelength. The rate (%) was calculated. Then, plotting the light absorption rate (%) at each wavelength with the light absorption rate (%) on the vertical axis and the wavelength (nm) on the horizontal axis, and drawing a polygonal line by connecting adjacent points with a straight line. , A horizontal axis, a straight line parallel to the vertical axis indicating wavelength=900 nm, and an area (%·nm) surrounded by a straight line parallel to the vertical axis indicating wavelength=2,600 nm is obtained. , The integrated value (%·nm) of the absorptance in the range of 600 nm. Note that the spreadsheet software "EXCEL" (registered trademark) manufactured by Microsoft Corporation was used to calculate the integral value (%·nm) of the absorptance in the range of 900 to 2,600 nm.

(2) Maximum value of heat shrinkage rate Since the stretching directions of the biaxially oriented films in all Examples and all Comparative Examples are known, the heat shrinkage rate (%) in the longitudinal direction and the heat shrinkage rate in the width direction are shown. (%) was compared, and the larger value was taken as the maximum value (%) of heat shrinkage. Hereinafter, a method of measuring the heat shrinkage ratio (%) in the longitudinal direction will be described. First, the biaxially oriented film is sampled in a rectangular shape of 10 mm (width direction) x 250 mm (longitudinal direction), and at a distance of about 200 mm, the straight lines connecting each other are centered in the width direction so that they are parallel to the longitudinal direction. Two marks were attached to the part, and the distance To (mm) between these two marks was accurately measured. Next, the obtained sample was allowed to stand in a hot air oven at 150° C. for 30 minutes under a load of 3 g and sufficiently cooled at room temperature, and then the gauge length T (mm) was similarly measured. Using the obtained To (mm) and T (mm), the heat shrinkage rate (%) in the longitudinal direction was calculated by the following formula 1.
Formula 1: Thermal contraction rate (%)=((To-T)/To)×100
The thermal shrinkage ratio (%) in the width direction was measured in the same manner by obtaining a rectangular sample of 10 mm (longitudinal direction)×250 mm (width direction) such that the width direction was the measuring direction.

(3) Thickness unevenness in the direction (width direction) in which the degree of molecular orientation becomes maximum It is clear that in the biaxially oriented films of Examples and Comparative Examples, the degree of molecular orientation becomes maximum in the width direction depending on the stretching conditions. Therefore, the thickness unevenness in the width direction was measured, and this was used as the thickness unevenness in the direction in which the degree of molecular orientation was maximum. Hereinafter, a method of measuring the thickness unevenness in the width direction will be specifically described. The biaxially oriented film was sampled in a size of 30 mm (longitudinal direction)×1 m (width direction). Then, the film thickness tester "KG601A" manufactured by Anritsu Corporation and the electronic micrometer "K306C" were used to continuously measure the thickness of the biaxially oriented film while conveying it in parallel with the width direction. At this time, the transport speed of the biaxially oriented film was 0.5 m/min, the measurement speed was 0.1 s, and the number of measurement points was 2,056 points. The maximum value T _max (μm), the minimum value T _min (μm), and the average value T _ave (μm) were obtained from the thickness data obtained by the measurement, and the thickness unevenness was calculated by the following formula 2.
Formula 2: Thickness unevenness (%)=(T _max −T _min )×100/T _ave
The same measurement was performed 10 times, and the obtained value was defined as the thickness unevenness in the width direction.

(4) Total Light Transmittance/Haze Measured using HGM-2DP manufactured by Suga Test Instruments Co., Ltd.

(5) Film Temperature Immediately after Irradiation of Infrared Light The film temperature at a position immediately after the irradiation of infrared radiation (a position 10 mm downstream from the outlet of the heater module on the most downstream side) was measured with a radiation thermometer FT-H50 manufactured by Keyence. A metal shielding plate was installed at the outlet of the heater module at the most downstream side so that the reflected light of the infrared heater did not enter the radiation thermometer.

(Example 1)
Polyethylene terephthalate (PET) having a melting point of 258° C. and carbon black were mixed at a mass ratio of 99.98:0.02 and charged into a single-screw extruder, and PET was melted at 280° C. to extrude the PET mixture. Then, the molten PET mixture was formed into a sheet with a T die and discharged, and was rapidly cooled and solidified on a casting drum having a surface temperature of 25° C. while applying an electrostatic applied voltage of 8 kV with a wire to obtain a cast sheet.

The obtained cast sheet was heated by a group of rolls set at 90° C., then rapidly heated by a radiation heater from both sides in a 100 mm stretch section, stretched 3.3 times in the longitudinal direction and cooled once. Both sides of the uniaxially oriented film thus obtained were subjected to corona discharge treatment in air, and an aqueous coating agent containing vinyl acetate/acrylic resin containing 3% by mass of colloidal silica having a particle size of 100 nm with #4 metabar on both sides. Was applied to form a functional layer. At this time, the corona treatment conditions were an applied voltage of 6 kV, a power supply frequency of 42 kHz, and a discharge degree of 2.5 W/cm ² . The uniaxially oriented film after forming the functional layer is guided to a tenter while gripping both widthwise end portions with a plurality of clips, preheated with hot air at 90° C., and then 3 times in the widthwise direction at a constant stretching speed in an atmosphere at a temperature of 120° C. It was stretched 3 times. After that, heat treatment with hot air at 235° C. in the tenter and 2% relaxation treatment in the width direction under the same temperature conditions were performed. After that, the mixture was passed through a heat setting zone of 150° C., a region of 150 mm from each end in the width direction was cut off, and both ends in the width direction were opened, and a twin-tube short wavelength infrared heater (Model: ZKC13500 manufactured by Heraeus Co., Ltd.) /900, peak wavelength: 1,200 nm) with a heater module (two units are arranged in the longitudinal direction) to irradiate infrared rays over the entire width to perform relaxation treatment in the longitudinal direction to obtain a biaxially oriented film. At this time, the distance between the infrared heater and the film was 30 mm, the running speed of the film was 15 m/min, the heating length in the longitudinal direction was 0.21 m, the heating time with the heater was 0.84 seconds, and the tension was 25 N/m. .. Then, after cooling to room temperature, it wound up and it was set as the film roll. A biaxially oriented film was sampled from the obtained film roll and each item was evaluated. The evaluation results are shown in Table 1.

(Examples 2-11, 13-15, Comparative Examples 1-3)
Production of a biaxially oriented film and evaluation of each item were performed in the same manner as in Example 1 except that the content and type of the infrared absorbent and the relaxation treatment conditions in the longitudinal direction were as shown in Tables 1 and 2. The evaluation results are shown in Tables 1 and 2.

(Example 12)
Four heater modules were continuously arranged in the running direction, and only the widthwise central portion of the first and second heaters was heated and only the widthwise both end portions of the third to fourth heaters were heated. A biaxially oriented film was manufactured and each item was evaluated. The evaluation results are shown in Table 2. The 1st to 2nd heaters had an irradiation width of 70% of the total width, and the 3rd to 4th heaters had an irradiation width of 30% (15% from each end in the width direction) of the entire width. did.

(Comparative Example 4)
Production of a biaxially oriented film and evaluation of each item were performed in the same manner as in Comparative Example 2 except that four heater modules were continuously arranged in the running direction. The evaluation results are shown in Table 2.

(Comparative example 5)
A film biaxially stretched under the same conditions as in Comparative Example 2 was wound up without being irradiated with infrared rays. Then, this film was unrolled, conveyed in roll-to-roll with tension of 25 N/m into an oven of preheating zone 120° C. and heating zone 150° C., and heated for 18 seconds. Table 2 shows the evaluation results of the obtained biaxially oriented film.

In the table, the content of the infrared absorbing agent was calculated assuming that all the constituent components were 100% by mass (the same applies to each Example and Comparative Examples 1 and 3 in Table 2). Further, lanthanum hexaboride Example 6 to 9 (LaB ₆₎ includes a master chip containing 20% by weight of lanthanum boride (LaB ₆₎ and 80 wt% of PET, the PET 5:95 (mass ratio ), the mixture was put into a co-rotating twin-screw kneading extruder (TEM-35B manufactured by Toshiba Machine Co., Ltd.), and kneaded under the conditions of an extrusion temperature of 290° C. and a residence time of 3.5 minutes, and then discharged and water-cooled. The obtained chips (chips containing 1% by mass of LaB ₆ ) were used (the same applies to Examples 11, 12, and 15 in Table 2).

In Comparative Example 3, since the film was melted by infrared irradiation, physical property evaluation after irradiation was not performed.

According to the present invention, it is possible to provide a biaxially oriented film that has both low heat shrinkage and high flatness. Further, the biaxially oriented film of the present invention is suitably used for applications requiring a high level of thermal dimensional stability and flatness, for example, optical applications such as release of polarizing plate, coating substrate, and ITO sputter substrate. be able to.

Claims (11)

The integrated value in the wavelength range of 900 to 2,600 nm of the light absorption spectrum is 4,000%·nm or more and 80,000%·nm or less, and the maximum heat shrinkage rate when heated for 30 minutes in an atmosphere of 150°C. A biaxially oriented film having a value of 0.5% or less. When the direction in which the degree of molecular orientation is maximum is the X direction and the direction orthogonal to the X direction in the plane is the Y direction, the thickness unevenness in the X direction and the Y direction are both 5.0% or less. The biaxially oriented film according to claim 1, which is characterized in that: The biaxially oriented film according to claim 1 or 2, which has a total light transmittance of 70% or more and a haze of 3.0% or less. The biaxially oriented film according to any one of claims 1 to 3, comprising an infrared absorber. The biaxially oriented film according to claim 4, wherein the infrared absorber is a carbon material. The biaxially oriented film according to claim 5, wherein the carbon material is carbon black. Content of the said carbon material is 0.001 mass% or more and 0.400 mass% or less when all the components are 100 mass %, The biaxial orientation of Claim 5 or 6 characterized by the above-mentioned. the film. The biaxial orientation according to claim 4, wherein the infrared absorber is at least one metal compound selected from tin-based metal oxides, tungsten-based metal oxides, and hexaborides. the film. The biaxially oriented film according to claim 8, wherein the content of the metal compound is 0.01% by mass or more and 1.90% by mass or less when all the constituent components are 100% by mass. The biaxially oriented film according to any one of claims 1 to 9, which comprises a polyester resin as a main component. It is a manufacturing method of the biaxially oriented film in any one of Claims 1-10, Comprising:
A heat treatment step of heating at a temperature equal to or higher than the stretching temperature and not higher than the melting point of the thermoplastic resin which is the main component, and a longitudinal relaxation step of heating the width direction full width to relax the film in the longitudinal direction in this order,
A method for producing a film, characterized in that the traveling speed of the film in the longitudinal relaxation step is 10 m/min or more and 70 m/min or less.