polyester film

Abstract

The present invention aims to provide a polyester film that exhibits excellent film-forming properties and suppresses the occurrence of streak defects in the film-forming direction. [Solution] A polyester film characterized in that it is mainly composed of a polyalkylene glycol copolymer polyester resin, the largest peak top in the molecular weight distribution curve obtained by gel permeation chromatography (GPC) measurement is in the range of molecular weight between 10,000 and 100,000, and the area in the range of molecular weight between 1,000 and 5,000 is less than 5.00% of the total area of ​​the region enclosed by the molecular weight distribution curve and the baseline.

Description

Technical Field

[0001] The present invention relates to a polyester film.

Background Art

[0002] As multilayer laminated films using light interference, for example, those having a metallic luster, those having a near-infrared reflection function, a polarization reflection film having polarization reflection characteristics, a dichroic filter, a monochromatic filter, etc. are known. These have an interference reflection function by alternately laminating two types of thermoplastic resins having different refractive indices. In order to increase the reflectance, it is important to select a combination of thermoplastic resins having significantly different refractive indices. For example, when polyethylene- (2,6-naphthalate) (hereinafter, PEN) is used for the layer with a high refractive index, it exhibits a very high refractive index by biaxial orientation.

[0003] On the other hand, when such a combination of thermoplastic resins is selected, there are problems such as an increase in the midpoint glass transition temperature and an increased tendency to split in the thickness direction. To address this problem, by copolymerizing a certain amount of polyalkylene glycol, the difference in the midpoint glass transition temperature with the layer having a low refractive index can be reduced, improving the film-forming property. Also, an increase in flexibility and affinity with a thermoplastic resin having a low refractive index reduces splitting in the thickness direction, which is known to work advantageously.

[0004] However, in films using a polyester resin copolymerized with polyalkylene glycol on the surface layer, such as the polyoxyalkylene glycol copolymerized polyester resin for electrostatic and easily dyeable fibers described in Patent Document 1 and the polyalkylene glycol copolymerized polyester resin having excellent optical properties described in Patent Document 2, streak defects may occur in the film-forming direction, which becomes a problem in film production.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2006-176626 [Patent Document 2] Japanese Patent Publication No. 2021-031560 [Overview of the project] [Problems that the invention aims to solve]

[0006] The object of the present invention is to provide a polyester film that exhibits excellent film-forming properties and suppresses the occurrence of streak defects in the film formation direction. [Means for solving the problem]

[0007] To solve the above problems, the present invention has the following configuration. (1) A polyester film characterized in that it is mainly composed of a polyalkylene glycol copolymer polyester resin, the largest peak top in the molecular weight distribution curve obtained by gel permeation chromatography (GPC) measurement is in the range of molecular weight 10,000 to 100,000, and the area in the range of molecular weight 1,000 to less than 5,000 of the total area enclosed by the molecular weight distribution curve and the baseline is less than 5.00%. (2) The polyester film according to (1), wherein the polyalkylene glycol copolymer polyester resin is a random copolymer containing naphthalenedicarboxylic acid units and diol units. (3) The polyester film according to (2), wherein the random copolymer has a naphthalenedicarboxylic acid unit content of more than 50 mol% of the total dicarboxylic acid units, a ethylene glycol unit content of more than 50 mol% of the total diol units, and a polyalkylene glycol unit content of 1 mol% or more and 10 mol% or less of the total diol units. (4) The polyester film according to any one of (1) to (3), wherein the number average molecular weight of the polyalkylene glycol units constituting the polyalkylene glycol copolymer polyester resin is 200 or more and 2000 or less. (5) A polyester film according to any one of (1) to (4), containing a total of 5 ppm to 100 ppm of metal atoms selected from at least one of titanium (Ti), germanium (Ge), and tin (Sn). (6) A laminated polyester film according to any one of (1) to (5), having a layer A mainly composed of the polyalkylene glycol copolymer polyester resin and a layer B mainly composed of a thermoplastic resin different from the layer A. (7) The laminated film according to (6), having a laminated unit in which the A layer and the B layer are alternately laminated in 51 or more layers. (8) A polyester film according to any one of (1) to (7), having a sea-island structure made of a plurality of thermoplastic resins including the polyalkylene glycol copolymer polyester resin. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a polyester film with excellent film-forming properties and suppression of streak defects in the film-forming direction. The polyester film of the present invention can be suitably used for optical applications such as displays, and in particular, by making a multilayer laminated film with a controlled refractive index distribution, it can be provided for applications such as transparent heat-shielding films and metallic decorative films. [Modes for carrying out the invention]

[0009] The following describes embodiments for carrying out the present invention, but the present invention is not to be interpreted as being limited to the embodiments including the following examples, and various modifications are naturally possible as long as the objective of the invention is achieved and the gist of the invention is not departed. Furthermore, in the case of embodiments in which two or more thermoplastic resin layers with different main components are regularly laminated, for the purpose of simplifying the explanation, some explanations will be given using an example in which two thermoplastic resin layers with different main components are alternately laminated, but embodiments in which three or more thermoplastic resin layers with different main components are regularly laminated should be understood in the same way.

[0010] The polyester film of the present invention is characterized in that it mainly comprises a polyalkylene glycol copolymer polyester resin, the largest peak top in the molecular weight distribution curve obtained by gel permeation chromatography (GPC) measurement is in the range of molecular weight 10,000 to 100,000, and the area in the range of molecular weight 1,000 to less than 5,000 of the total area enclosed by the molecular weight distribution curve and the baseline is less than 5.00%.

[0011] Here, "main component" refers to a component whose proportion of the total components of the polyester film exceeds 50% by mass but is 100% by mass or less. Furthermore, when referring to the "main component" of individual layers below, the same interpretation can be applied, except that it refers to the total resin components of that layer.

[0012] Here, "molecular weight distribution curve obtained by gel permeation chromatography (GPC) measurement" refers to the molecular weight distribution curve obtained using a UV detector at a measurement wavelength of 360 nm in gel permeation chromatography (GPC) measurement (hereinafter, it may simply be referred to as the molecular weight distribution curve, and details of its acquisition method will be described later). Furthermore, the maximum peak top in the molecular weight distribution curve obtained by GPC measurement refers to the highest peak among the multiple peaks in the molecular weight distribution curve. In the following, "the maximum peak top in the molecular weight distribution curve obtained by GPC measurement" may be referred to as "the maximum peak top in the molecular weight distribution curve" or simply "the maximum peak top".

[0013] In this invention, the "area in the range of molecular weights between 1,000 and 5,000" within the total area enclosed by the molecular weight distribution curve and the baseline is determined by the following method (hereinafter, the "area in the range of molecular weights between 1,000 and 5,000" within the total area enclosed by the molecular weight distribution curve and the baseline may be referred to as the "area ratio of the range of molecular weights between 1,000 and 5,000"). First, the area between the molecular weight distribution curve and the baseline is calculated for each molecular weight, and the sum of these areas for all molecular weights is taken as the "total area". Next, the "sum of areas in the range of molecular weights between 1,000 and 5,000" is calculated in the same manner. Furthermore, the ratio of the "sum of areas in the range of molecular weights between 1,000 and 5,000" to the "total area" is calculated, and this is taken as the area (%) of the range of molecular weights between 1,000 and 5,000 within the total area. Details of the measurement method for this area will be described later.

[0014] The polyester film of the present invention must have a maximum peak top in the molecular weight distribution curve within the range of 10,000 to 100,000 molecular weight. Furthermore, considering the reduction of flow marks, it is preferable that the molecular weight at the maximum peak top be within the range of 20,000 to 80,000 molecular weight. If the maximum peak top is less than 10,000, the resin constituting the polyester film becomes mechanically brittle, making film formation difficult, and even if film formation is possible, streaks in the film formation direction are likely to occur. On the other hand, if the maximum peak top exceeds 100,000, the viscosity of the resin becomes too high, requiring a large pressure for extrusion, making film formation difficult. The maximum peak top can be controlled by manipulating the degree of polymerization in the polymerization reaction of the polyester.

[0015] In the polyester film of the present invention, from the viewpoint of suppressing streak defects in the film formation direction, the area ratio of molecules with a molecular weight in the range of 1000 to less than 5000 must be less than 5.00%, preferably 3.00% or less, and more preferably 1.00% or less. Theoretically, the lower limit of this area ratio is 0.00%, which means that the molecular weight of all thermoplastic resins constituting the polyester film is outside the range of 1000 to less than 5000.

[0016] Components in the range of having a molecular weight of more than 1,000 and less than 5,000 are mostly derived from unreacted substances in which the copolymerization of polyalkylene glycol did not proceed sufficiently in the process of obtaining the polyalkylene glycol copolymerized polyester resin. Therefore, by sufficiently promoting the copolymerization of polyalkylene glycol, it is possible to achieve an area ratio in the range of having a molecular weight of more than 1,000 and less than 5,000 of less than 5.00% or the above-mentioned preferred range.

[0017] In the prior art, it has been difficult to control the molecular weight distribution in the range of having a molecular weight of more than 1,000 and less than 5,000 of a polyester film using a polyalkylene glycol copolymerized polyester resin having a maximum peak top of from 10,000 to 100,000 in molecular weight. As a result of intensive studies in the present invention, by controlling the area ratio in the range of having a molecular weight of more than 1,000 and less than 5,000 to less than 5.00%, unreacted substances in which the copolymerization did not proceed sufficiently are reduced, and thermal decomposition is suppressed when forming a polyester film. Therefore, it has been found that streak defects in the film forming direction can be suppressed.

[0018] In order to sufficiently promote the copolymerization of polyalkylene glycol, it is preferable to add polyalkylene glycol at the start of the transesterification reaction or the esterification reaction to increase the time for the copolymerization reaction, to increase the temperature raising and pressure reducing time in the polymerization reaction to increase the time for the reaction, to increase the degree of polymerization, and to use a catalyst having high reaction activity or to adjust its amount. These may be carried out individually or a plurality of them may be carried out simultaneously. However, none of these methods is essential, and as long as the copolymerization reaction of polyalkylene glycol proceeds sufficiently and the area ratio in the range of having a molecular weight of more than 1,000 and less than 5,000 is less than 5.00%, it is not limited to the above methods.

[0019] The polyester film of the present invention mainly comprises a polyalkylene glycol copolymer polyester resin, and it is preferable that the polyalkylene glycol copolymer polyester resin is a random copolymer containing naphthalenedicarboxylic acid units and diol units. Random copolymers containing naphthalenedicarboxylic acid units and diol units have a high refractive index, low birefringence, excellent transparency, and are less prone to whitening even in high-temperature environments, making them suitable for optical applications and applications requiring heat resistance. Furthermore, copolymerization of polyalkylene glycol makes the polyester resin mechanically flexible, which helps suppress crack defects in film formation. In addition, when the polyester film is a laminated film, there are effects such as improved adhesion between the layer mainly composed of the resin and other thermoplastic resin layers, and improved film formation by controlling the intermediate glass transition temperature. The intermediate glass transition temperature in the present invention can be calculated from differential scanning calorimeter (DSC) measurement results, and details of the measurement method and calculation method will be described later.

[0020] Suitable naphthalenedicarboxylic acid units for the polyalkylene glycol copolymer polyester resin constituting the polyester film of the present invention include, for example, 2,6-naphthalenedicarboxylic acid units, 2,3-naphthalenedicarboxylic acid units, 1,4-naphthalenedicarboxylic acid units, and 1,8-naphthalenedicarboxylic acid units, but among these, 2,6-naphthalenedicarboxylic acid units are preferred from the viewpoint of reactivity and crystalline properties. Furthermore, the polyalkylene glycol copolymer polyester resin may contain naphthalenedicarboxylic acid units other than those mentioned above and other dicarboxylic acid units to the extent that they do not hinder the effects of the present invention, and may include, for example, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, 4,4′-diphenyletherdicarboxylic acid, and 4,4′-diphenylsulfondicarboxylic acid.

[0021] Further, in the random copolymer containing naphthalenedicarboxylic acid units and diol units, the naphthalenedicarboxylic acid units account for more than 50 mol% of all dicarboxylic acid units, the ethylene glycol units account for more than 50 mol% of all diol units, and the polyalkylene glycol units account for 1 mol% or more and 10 mol% or less of all diol units, which is preferable. The upper limit of the amount of naphthalenedicarboxylic acid units is theoretically 100 mol%, and the upper limit of the amount of ethylene glycol units is preferably in the range of 90 mol% or more and 99 mol% or less according to the amount of polyalkylene glycol units.

[0022] The random copolymer containing naphthalenedicarboxylic acid units has a high refractive index, low birefringence, and excellent transparency, and thus can be suitably used for optical applications. Therefore, the naphthalenedicarboxylic acid units preferably account for more than 50 mol% of all dicarboxylic acid units, and more preferably 90 mol% or more. By setting the naphthalenedicarboxylic acid units to more than 50 mol%, a random copolymer excellent in heat resistance, high refractive index, and excellent transparency can be obtained. In particular, when a multilayer laminated film containing a layer mainly composed of this is used, whitening under a high-temperature environment can be suppressed, the refractive index difference from a layer mainly composed of other thermoplastic resins can be increased, and the interference reflection function can be improved. Furthermore, by setting the ratio of naphthalenedicarboxylic acid units to 90 mol% or more, a random copolymer with a higher refractive index can be obtained, and the above characteristics can be further improved.

[0023] In addition, the random copolymer containing ethylene glycol units has good properties and film-forming properties in an optical film from the viewpoints of refractive index, crystallinity, etc. Therefore, the ethylene glycol units preferably account for more than 50 mol% and 99 mass% or less of all diol units of the random copolymer, more preferably in the range of 90 mol% or more and 99 mol% or less, and even more preferably in the range of 90 mol% or more and 98 mol% or less. Also, from the viewpoint of controlling the midpoint glass transition temperature, the polyalkylene glycol units preferably account for 1 mol% or more and 10 mol% or less of all diol units of the random copolymer, more preferably 2 mol% or more and 10 mol% or less.

[0024] By copolymerizing polyalkylene glycol units at a concentration of 1 mol% or more, the random copolymer becomes mechanically flexible, suppressing cracking and streaking defects in film formation. Furthermore, when used as the main component in a multilayer laminated film, it improves adhesion to layers primarily composed of other thermoplastic resins and enhances film-forming properties by controlling the intermediate glass transition temperature. On the other hand, by limiting the copolymerization amount of polyalkylene glycol units to 10 mol% or less, the decrease in reactivity with polyester is suppressed, making it easier to maintain a small area ratio in the molecular weight range of 1000 to less than 5000 and to keep the intermediate glass transition temperature within the intended range. The copolymerization amount of polyalkylene glycol can be determined by nuclear magnetic resonance (NMR), and details of the measurement method will be described later.

[0025] In the polyester film of the present invention, from the viewpoint of reducing streaks and other defects in the film formation direction, the number-average molecular weight of the polyalkylene glycol units constituting the polyalkylene glycol copolymer polyester resin is preferably 200 to 2000, and more preferably 200 to 1000. The number-average molecular weight of the polyalkylene glycol is closely related to the intermediate glass transition temperature of the resulting polyalkylene glycol copolymer polyester resin (random copolymer) or polyester film. The smaller the number-average molecular weight, the greater the amount of copolymer required to control the intermediate glass transition temperature. On the other hand, the larger the number-average molecular weight, the less copolymer is needed to control the intermediate glass transition temperature. In other words, from the above viewpoint, within the range of the above number-average molecular weight, it is preferable to reduce the amount of copolymer as the number-average molecular weight increases, and to increase the amount of copolymer as the number-average molecular weight decreases.

[0026] When the number-average molecular weight of the polyalkylene glycol units constituting the polyalkylene glycol copolymer polyester resin is 200 or higher, the amount of copolymerization required to control the intermediate glass transition temperature is reduced, and the increase in the area ratio in the molecular weight range of 1000 to less than 5000, which is associated with a decrease in reactivity, is mitigated. As a result, the increase in streaks and defects in the film formation direction is suppressed. On the other hand, when the number-average molecular weight is 2000 or lower, reactivity with the polyester is maintained, which reduces deterioration of transparency, a decrease in the intermediate glass transition temperature below the intended usage range, and elution into hot water. The number-average molecular weight of polyalkylene glycol can be measured by GPC, and details of the measurement method will be described later.

[0027] The polyester film of the present invention preferably contains a total of 5 ppm to 100 ppm of metal atoms selected from at least one of titanium (Ti), germanium (Ge), and tin (Sn), and more preferably contains 5 ppm to 80 ppm.

[0028] Ti, Ge, and Sn are metal atoms contained in catalysts used in the polymerization of polyester resins. Catalysts containing these metal atoms exhibit high activity in esterification, transesterification, and polycondensation reactions in the polyester manufacturing process, and show good reaction characteristics even in small amounts. Therefore, in the present invention, catalysts containing these metal atoms can be used to sufficiently advance the copolymerization reaction to obtain polyalkylene glycol copolymer polyester resin, making it easy to reduce the area ratio of the molecular weight distribution in the range of 1000 to less than 5000.

[0029] When the total content of these metal atoms is 5 ppm or more, the amount of catalyst is sufficient, allowing the copolymerization reaction of polyalkylene glycol to proceed sufficiently, and reducing the area ratio of molecules with molecular weights between 1000 and 5000. As a result, the occurrence of streaks in the film formation direction can be reduced. On the other hand, when the total content of these metal atoms is 100 ppm or less, the generation of metal-induced foreign matter and deterioration of heat resistance during film formation can be reduced. The total content of these metal atoms can be adjusted by selecting the type and amount of catalyst added during the production of the polyalkylene glycol copolymer polyester resin.

[0030] The polyester film of the present invention preferably has an A layer mainly composed of a polyalkylene glycol copolymer polyester resin and a B layer mainly composed of a thermoplastic resin different from that of the A layer, and more preferably the A layer is the surface layer on both sides from the viewpoint of film-forming properties. By having an A layer and a B layer mainly composed of a thermoplastic resin different from that of the A layer, it is possible to improve durability and impart functionality by laminating thermoplastic resin layers with different properties. In particular, by using a transparent thermoplastic resin with a low refractive index for the B layer, while the A layer mainly composed of a polyalkylene glycol copolymer polyester resin is a transparent crystalline resin with a high refractive index, it is possible to impart interference reflection functionality and good interlayer adhesion by using an amorphous thermoplastic resin. For this reason, cyclohexanedimethanol copolymer polyethylene terephthalate, which is transparent, has a lower refractive index than the polyalkylene glycol copolymer polyester resin of the A layer, and is amorphous, can preferably be used as the main component of the B layer. In this invention, amorphous means that the heat of fusion is 5 J / g or less (crystallinity means that the heat of fusion exceeds the same level).

[0031] The polyester film of the present invention preferably has a laminated unit in which 51 or more layers of A and B layers are alternately laminated, more preferably 101 or more layers, and even more preferably 201 or more layers in order to control the wavelength range of reflected light. By laminating 51 or more layers, a reflection characteristic of a certain wavelength can be obtained by interference reflection, and by further increasing the number of layers, optical properties that reflect wavelengths within a desired range can be imparted. There is no particular upper limit on the number of layers in the laminated unit, but from the viewpoint of preventing deterioration of the handling properties of the resulting polyester film and the need for larger manufacturing equipment, it is practical to have 1001 layers or less.

[0032] The polyester film of the present invention may also preferably have a sea-island structure made of multiple types of thermoplastic resins, including a polyalkylene glycol copolymer polyester resin. Having such a sea-island structure can be expected to impart flexibility and functionality to the polyester film. In the present invention, a sea-island structure refers to a structure consisting of sea components that exist continuously within a layer or film and island components that exist discontinuously dispersed within it. The ratio of sea components to island components, the size of the island components, and the miscibility state are not particularly limited, and any structure in which island components are dispersed within continuous sea components is acceptable. Furthermore, the polyalkylene glycol copolymer polyester resin may be either a sea component or an island component and is not particularly limited, but it is more preferable to be a sea component from the viewpoint of durability and crystalline properties.

[0033] The polyester film of the present invention can be manufactured by conventionally known methods. Specific manufacturing examples are shown below, but are not limited thereto. Furthermore, for the purpose of simplifying the explanation, the following description uses a multilayer laminated film in which two different thermoplastic resin layers are alternately laminated as an example. However, even a multilayer laminated film in which, for example, three or more thermoplastic resin layers are regularly laminated should be understood similarly, provided that the lamination apparatus is configured appropriately.

[0034] A laminated unit of 51 or more layers in the polyester film of the present invention can be formed by the following method. Different polyester resins or polyester resin compositions (hereinafter referred to as thermoplastic resins A and B) are supplied from two extruders, Extruder A corresponding to layer A and Extruder B corresponding to layer B. The polymers from each flow path are laminated using a known lamination apparatus such as a multi-manifold type feed block and a square mixer, or a comb type feed block. The molten material is then formed into a sheet using a T-type die and melt-extruded, and then cooled and solidified on a casting drum to obtain an unstretched film. As a method to improve the lamination accuracy of layers A and B, the methods described in Japanese Patent Publication No. 2007-307893, Japanese Patent No. 4691910, Japanese Patent No. 4816419, etc. are preferred. If necessary, it is also preferable to pre-dry the polyester resin used for layer A and the polyester resin used for layer B.

[0035] Next, the unstretched film is subjected to biaxial stretching and heat treatment. The preferred stretching method is either sequential biaxial stretching or simultaneous biaxial stretching. Sequential biaxial stretching involves stretching the film longitudinally followed by stretching in the width direction, or stretching in the width direction followed by stretching in the longitudinal direction. Multiple stretching in the longitudinal and width directions may be combined. The longitudinal direction refers to the direction in which the film travels during the manufacturing process (the winding direction in the case of a film roll), and the width direction refers to the direction perpendicular to the longitudinal direction within the film plane.

[0036] Generally, biaxial stretching is performed at a temperature higher than the midpoint glass transition temperature of either layer A or layer B, and up to 50°C higher than that midpoint glass transition temperature. Heat treatment is performed at a temperature higher than the stretching temperature and lower than the melting point of the higher thermoplastic resin A or B.

[0037] The following describes a case in which sequential biaxial stretching or simultaneous biaxial stretching and heat treatment are performed using polyethylene naphthalate copolymerized with polyethylene glycol in layer A (thermoplastic resin A) and polyethylene terephthalate copolymerized with cyclohexanedimethanol in layer B (thermoplastic resin B). However, this is just one example, and the present invention is not limited to what can be obtained by this example. When sequential biaxial stretching is performed, the unstretched film cast on a cooling roll temperature-controlled to 10°C to 50°C is stretched in a longitudinal stretcher that utilizes the speed difference between the rolls at a temperature of 90°C to 170°C, preferably 100°C to 150°C from the viewpoint of thickness uniformity, at a thickness of 2.0 to 6.0 times, preferably 3.0 to 4.0 times.

[0038] The uniaxially oriented film obtained in this manner may be subjected to surface treatments such as corona treatment, flame treatment, or plasma treatment as needed, and then given functions such as slipperiness, adhesion, and antistatic properties by in-line coating.

[0039] Next, the uniaxially oriented film is stretched laterally (stretched in the width direction) in a transverse stretching machine at a temperature of 100°C to 170°C, preferably 110°C to 150°C, to a length of 2.0 to 6.0 times, preferably 3.0 to 4.0 times. The known tenter method is used for stretching in the width direction. That is, the uniaxially oriented film is conveyed while gripping both ends in the width direction with multiple clips, and the film is stretched in the width direction by widening the distance between opposing clips.

[0040] Next, we will explain the case of simultaneous biaxial stretching. As described above, the unstretched film cast on the cooling roll is guided to a simultaneous biaxial tenter, and while gripping both ends in the width direction with multiple clips, it is transported and stretched simultaneously and / or in stages in the longitudinal and width directions. Longitudinal stretching is achieved by increasing the distance between the front and rear clips, and widthwise stretching is achieved by increasing the spacing between the rails on which opposing clips run. The clips of the tenter used for biaxial stretching are preferably driven by a linear motor system. Other systems include pantograph and screw systems, but the linear motor system is superior in that it allows for free adjustment of the stretching ratio due to the high degree of freedom of each clip. The stretching temperature and stretching ratio are similar to those for sequential biaxial stretching. Specifically, the stretching temperature is 100°C to 150°C, and the stretching ratio is 4 to 36 times as an area ratio, preferably 9 to 16 times.

[0041] Next, the film obtained by sequential biaxial stretching or simultaneous biaxial stretching is subjected to heat treatment in a heat treatment machine. Heat treatment is generally performed inside a transverse stretching machine (tenter). After transverse stretching, heat treatment is performed at a temperature of 160°C to 240°C, preferably 180°C to 240°C, with a relaxation rate of more than 0% and 10% or less, preferably more than 0% and 5% or less. Relaxation may be performed only in the width direction, only in the longitudinal direction, or in both the width and longitudinal directions. Furthermore, the upper limit of the heat treatment temperature is set below the melting point of the thermoplastic resin A and B with the higher melting point if it is 240°C or less, and relaxation may be performed during rapid cooling after heat treatment.

[0042] In the production of the polyester film of the present invention, it is preferable that the heat treatment temperature after stretching be below the melting point of at least one thermoplastic resin and above the melting point of at least one of the remaining thermoplastic resins. In this case, one thermoplastic resin maintains a highly oriented state, while the orientation of the other thermoplastic resin is relaxed, making it easy to create a refractive index difference between these resins.

[0043] The biaxially oriented film obtained in this way can be cut at both ends in the width direction using a known slitter, and then wound up using a winding machine to obtain a roll of the polyester film of the present invention. [Examples]

[0044] The polyester film of the present invention will be described below with reference to specific examples. Even when using polyester resins other than those specifically illustrated below, the polyester film of the present invention can be obtained in the same manner by referring to the description in this specification, including the examples below.

[0045] [Methods for measuring physical properties and evaluating their effects] The methods for evaluating physical properties and effects are as follows.

[0046] (1) Gel permeation chromatography (GPC) analysis of polyester film A GPC developing solvent was prepared by adding 420 mg / kg of sodium trifluoroacetate to hexafluoroisopropanol (HFIP). Then, 2.5 mg of the sample was taken, and 3.0 mL of the developing solvent was added to prepare a polymer solution for analysis. This solution was filtered through a 0.45 μm filter, and the filtrate was used for GPC measurement. The GPC analysis was performed under the following analytical conditions. [Analysis conditions] Detector: Waters 2998 PDA Detector Columns: Shodex GPCHFIP-806M x 2 (in series) Flow rate: 1.0mL / min Column temperature: 30℃ UV wavelength: 360nm Standard material: Polymethylene methacrylate (Shodex Mw2.8 x 10) 3 ~9.5×10 5 ) In the molecular weight distribution curve obtained by GPC analysis under the above analytical conditions, the highest peak was identified as the maximum peak top. Furthermore, the area between the molecular weight distribution curve and the baseline for each molecular weight was calculated, and these were summed to determine the total area for all molecular weights. Subsequently, the sum of the areas in the molecular weight range of 1000 to less than 5000 was calculated, and the ratio of the sum of the areas in the molecular weight range of 1000 to less than 5000 to the total area was calculated.

[0047] (2) Streak defects in the direction of film formation A polyester film was cut to a length of 10 cm in the longitudinal direction. A 15 cm sample was taken from the center of the film in the width direction, excluding the ends, for a total length of 30 cm if the film was unstretched. If the film was stretched in the width direction, a length equivalent to the 30 cm sample for the unstretched film was taken by multiplying the width direction stretching ratio by 30 cm. The sampled polyester film was suspended above the floor, and light from a three-wavelength fluorescent lamp was transmitted from above. The image projected onto the floor was visually inspected for streaks in the film formation direction. The number of streaky defects observed in the film formation direction was counted, and the results were judged as follows: 0 to 3 streaks = ◎, 4 to 6 streaks = ○, 7 to 9 streaks = △, and 10 or more streaks = ×. In this invention, ◎, ○, and △ were considered pass, and × was considered fail.

[0048] (3) Melting point, heat of fusion, and intermediate glass transition temperature of the polyester resin composition Approximately 10 mg of the sample to be measured was weighed, sealed in an aluminum pan and pan cover, and measured using a differential scanning calorimeter (Q2000 model, TA Instruments). In the measurement, the first heating was performed in a nitrogen atmosphere to 300°C and held for 5 minutes, then rapidly cooled with liquid nitrogen. A second heating was performed again in a nitrogen atmosphere, from 20°C to 300°C at a rate of 16°C / min, held for 5 minutes, and then cooled at a rate of 16°C / min to obtain a DSC curve. The peak temperature of the endothermic peak in the DSC curve obtained during the second heating was defined as the melting point (Tm), and the integral value of the DSC curve including the baseline and the melting point peak within a range of ±20°C of the melting point peak was defined as the heat of fusion (ΔHm). Furthermore, the temperature at the point where the line extending from each baseline of the DSC curve during the second heating intersects with the curve of the stepwise transition portion of the glass transition was defined as the intermediate glass transition temperature (Tg).

[0049] (4) Film forming properties The obtained laminated film was visually inspected for the presence or absence of flow marks to determine its film-forming properties. For the evaluation of film-forming properties, ◎ indicated no flow marks, ○ indicated slight flow marks, and × indicated numerous flow marks. In this invention, ◎ and ○ were considered pass, and × was considered fail.

[0050] (5) Molecular weight of polyalkylene glycol contained in polyester film Polyalkylene glycol was extracted from the polyester film using the following procedure, and the molecular weight of the polyalkylene glycol was measured by GPC. [Extraction Procedure] 0.05 g of the obtained polyester film was taken and dissolved by heating in 1 mL of 28% by mass ammonia water at 120°C for 5 hours. After cooling, 1 mL of purified water and 1.5 mL of 6 M hydrochloric acid were added, and the volume was adjusted to 5 mL with purified water. Subsequently, the solution was centrifuged and filtered through a 0.45 μm filter, and the molecular weight of the polyalkylene glycol was analyzed by GPC measurement of the filtrate under the following conditions. [Measurement conditions for GPC] Detector: Waters 2410 differential refractive index detector, sensitivity 128x Column: Tosoh TSKgel G3000PWXLI Solvent: 0.1M sodium chloride aqueous solution Flow rate: 0.8mL / min Injection volume: 200μL Column temperature: 40℃ Standard substance: Polyethylene glycol (Mw106~10,100, manufactured by AML Co., Ltd.).

[0051] (6) Amount of copolymerization of polyalkylene glycol contained in the copolymerized polyester composition The copolymerization amount of polyalkylene glycol in the copolymerized polyester composition was analyzed by dissolving the film sample in a 1 / 1 (volume ratio) mixed solvent of deuterated hexafluoroisopropanol / deuterated chloroform and performing the analysis using nuclear magnetic resonance (NMR) spectroscopy. Equipment: AL-400 manufactured by JEOL Ltd. Deuterated solvent: Deuterated 1,1,1,3,3,3-hexafluoro-2-isopropanol (HFIP) Total number of times: 128 Sample concentration: 0.05 g of sample / 1 mL of deuterated solvent.

[0052] (7) Types and content of metal elements contained in polyester film The intensity of X-ray fluorescence from polymers was measured using a Horiba MESA-500W X-ray fluorescence spectrometer. The types of metal elements present were identified from these values, and the metal content was analyzed using a calibration curve previously created with samples whose content was known.

[0053] (8) Heat resistance of polyester film The obtained polyester film was left in a 100°C atmosphere for 500 hours, and then visually inspected for whitening compared to before the test. For heat resistance evaluation, ◎ indicated no whitening, ○ indicated slight whitening, △ indicated clear whitening in some areas, and × indicated clear whitening across the entire surface. In this invention, ◎, ○, and △ were considered pass, and × was considered fail.

[0054] (9) Number of layers The layer thickness and number of layers of each film in the polyester film were determined by transmission electron microscopy (TEM) observation of samples obtained by cutting cross-sections using a microtome. Using a transmission electron microscope H-7100FA (manufactured by Hitachi, Ltd.), the cross-section of the film was observed at magnifications of 4,000 to 100,000 times under an acceleration voltage of 100 kV, and cross-sectional photographs were taken to measure the layer structure and the thickness of each layer. In order to obtain high contrast, known staining techniques using RuO4 or OsO4 were used during observation. The observation and image analysis procedures are described below. First, TEM photographic image files taken at magnifications ranging from 4,000 to 100,000 times, which is suitable for observing layer thickness, were opened with the image processing software Image-Pro Plus ver.4 (distributed by Planetron Co., Ltd.), and the relationship between the average brightness of the region between the thickness direction position and two lines in the width direction was read as numerical data in vertical thick profile mode. The obtained numerical data was analyzed using spreadsheet software (Microsoft Excel® for Microsoft 365). After selecting data for position (nm) and brightness in sampling step 2 (decimation 2), a 4-point moving average was applied. Furthermore, the obtained data with periodically changing brightness was differentiated, and the maximum and minimum values ​​of the derivative curve were read using a VBA (Visual Basic for Applications) program. The layer thickness was calculated by taking the interval between adjacent values ​​as the layer thickness. This operation was performed for each photograph to calculate the layer thickness for all layers.

[0055] (10) Adhesion of laminated polyester film 1mm on the surface 2 Place 100 cross-cut pieces inside, stick Nichiban Co., Ltd.'s "Sellotape" (registered trademark) "No. 405" on top, and roll it with a rubber roller at 1.5 kg / cm². 2After applying a load and pressing down, force was applied in a 90° direction perpendicular to the sample surface within 5 minutes to peel it off. Peeling was performed within 0.5 seconds from the start of applying force to the end of peeling, and was done three times each in the MD (longitudinal direction) and TD (width direction). The average number of squares where the film remained without peeling was evaluated as the number of remaining squares. The peeling force at this time was 3.5 N / mm. For adhesion evaluation, a score of ◎ was given if the number of squares remaining without peeling was 100 (no peeling), ○ was given if it was 95 or more but 99 or less, and × was given if it was 94 or less. In this invention, ◎ and ○ were considered pass, and × was considered fail.

[0056] [Example 1] (Manufacturing of polyester resin A) 94.3 parts by mass (100 mol%) of dimethyl naphthalenedicarboxylate, 45.5 parts by mass (95 mol%) of ethylene glycol, and 7.7 parts by mass (5 mol%) of polyethylene glycol (number average molecular weight 400) were charged into a reaction vessel and dissolved at 180°C. Then, while stirring, 0.06 parts by mass of manganese acetate tetrahydrate and 0.0045 parts by mass of germanium dioxide as a polycondensation catalyst were added to start the transesterification reaction. Subsequently, the temperature was raised to 235°C over 3.5 hours while methanol was distilled off to complete the transesterification reaction. Furthermore, 0.02 parts by mass of phosphoric acid, 0.03 parts by mass of sodium dihydrogen phosphate dihydrate, and 0.1 parts by mass of "IRGANOX" (registered trademark) 1010 (manufactured by BASF) were added, and the excess ethylene glycol was removed by distillation. The obtained reaction product was transferred to a polycondensation reaction vessel, and the temperature was increased from 240°C while the pressure was reduced to below 133 Pa, allowing the excess ethylene glycol to be distilled off while the temperature was increased to 290°C. When the desired melt viscosity was reached, the mixture was discharged into a water bath and chipped using a strand cutter to obtain a random copolymer containing naphthalenedicarboxylic acid units and diol units, which was designated as polyester resin A. The intermediate glass transition temperature (Tg) of the obtained polyester resin A was 96°C, the melting point was 255°C, and the heat of fusion was 11 J / g.

[0057] (Manufacturing of polyester film) The obtained polyester resin A and copolymerized polyethylene terephthalate resin B, obtained by copolymerizing cyclohexanedimethanol at a rate of 33 mol%, were melt-extruded in an extruder in a ratio of polyester resin A to copolymerized polyethylene terephthalate resin B of 51:49. These resins were then combined in a lamination device with 201 slits, and extruded in a sheet form such that the outermost layers on both sides consisted of polyester resin A. The sheets were then solidified on a cooling roll at 25°C, and a laminated cast film was obtained with 201 layers alternately laminated in the thickness direction. The obtained laminated cast film was heated in a group of rolls set to 60°C, then stretched 3.0 times in the longitudinal direction on a roll at 100°C, and then cooled. The uniaxially oriented film thus obtained was guided to a tenter, preheated with hot air at 90°C, and then stretched 3.0 times in the width direction at 110°C. The stretched film was then heat-treated in a tenter with hot air at 220°C, followed by a 2% relaxation treatment in the width direction at the same temperature, and then a further 5% relaxation treatment in the width direction after rapid cooling to 100°C. Finally, it was wound into a roll to obtain a polyester film. The properties of the obtained polyester film are shown in Table 2.

[0058] [Examples 2, 6-8] A polyester film was obtained in the same manner as in Example 1, except that the types and amounts of diol and dicarboxylic acid units in polyester resin A were as shown in Table 1. The properties of the obtained polyester film are shown in Table 2.

[0059] [Examples 3 and 4, Comparative Examples 1 and 2] A polyester film was obtained in the same manner as in Example 1, except that the degree of polymerization was adjusted by changing the predetermined melt viscosity during the production of polyester resin A, and the maximum peak top in GPC measurement was manipulated as shown in Table 2. The properties of the obtained polyester film are shown in Table 2. In Comparative Example 1, the increase in filtration pressure when extruding polyester resin A was large, making film formation impossible.

[0060] [Example 5, Comparative Example 3] In Example 5, a polyester film was obtained in the same manner as in Example 1, except that the timing of the addition of polyethylene glycol during the polycondensation of polyester resin A was changed. In Example 5, polyester resin A was obtained by adding polyethylene glycol after the completion of the transesterification reaction. The properties of the obtained polyester film are shown in Table 2. In Comparative Example 3, polyester resin A was obtained by adding polyethylene glycol at the time when the viscosity increase in the polycondensation reaction began. The properties of the obtained polyester film are shown in Table 2.

[0061] [Examples 9, 10, Comparative Example 4] A polyester film was obtained in the same manner as in Example 1, except that the number-average molecular weight and copolymerization amount of polyethylene glycol units of polyester resin A were as shown in Table 1. The properties of the obtained polyester film are shown in Table 2.

[0062] [Examples 11-15] A polyester film was obtained in the same manner as in Example 1, except that the type and amount of polycondensation catalyst used for the polycondensation of polyester resin A were as shown in Table 1. The physical properties of the obtained polyester film are shown in Table 2.

[0063] [Example 16] A polyester film was obtained in the same manner as in Example 1, except that polyester resin B was polyethylene terephthalate as shown in Table 1. The properties of the obtained polyester film are shown in Table 2.

[0064] [Example 17] A polyester film was obtained in the same manner as in Example 1, except that the number of layers was set to 51 by adjusting the number of slits in the feed block, as shown in Table 1. The properties of the obtained polyester film are shown in Table 2.

[0065] [Example 18] A polyester film was obtained in the same manner as in Example 1, except that polyester resins A and B were melt-kneaded in a mass ratio of 60:40 to form a sea-island structure, and a single-layer polyester film was obtained without lamination. The properties of the obtained polyester film are shown in Table 2.

[0066] [Comparative Example 5] As shown in Table 1, a polyester film was obtained in the same manner as in Example 1, except that some of the naphthalenedicarboxylic acid units were replaced with isophthalic acid units and the polycondensation catalyst was changed to 0.050 parts by mass of antimony trioxide. The properties of the obtained polyester film are shown in Table 2.

[0067] [Comparative Example 6] As shown in Table 1, a polyester film was obtained in the same manner as in Example 1, except that the naphthalenedicarboxylic acid units were changed to terephthalic acid units and the polycondensation catalyst was changed to 0.140 parts by weight of tetrabutoxytitanium. The properties of the obtained polyester film are shown in Table 2.

[0068] [Table 1]

[0069] [Table 2] [Industrial applicability]

[0070] According to the present invention, it is possible to provide a polyester film that has excellent film-forming properties and suppresses the occurrence of streak defects in the film-forming direction. Due to the above properties, the polyester film of the present invention can be suitably used for optical applications such as displays, and in particular, by making a multilayer laminated film with a controlled refractive index distribution, it can be suitably used for applications such as transparent heat-shielding films and metallic decorative films.

Claims

1. A polyester film characterized in that it is mainly composed of a polyalkylene glycol copolymer polyester resin, the largest peak top in the molecular weight distribution curve obtained by gel permeation chromatography (GPC) measurement is in the range of molecular weight 10,000 to 100,000, and the area in the range of molecular weight 1,000 to less than 5,000 of the total area enclosed by the molecular weight distribution curve and the baseline is less than 5.00%.

2. The polyester film according to claim 1, wherein the polyalkylene glycol copolymer polyester resin is a random copolymer containing naphthalenedicarboxylic acid units and diol units.

3. The polyester film according to claim 2, wherein the random copolymer has a naphthalenedicarboxylic acid unit content of more than 50 mol% of the total dicarboxylic acid units, a ethylene glycol unit content of more than 50 mol% of the total diol units, and a polyalkylene glycol unit content of 1 mol% to 10 mol% of the total diol units.

4. The polyester film according to claim 1 or 2, wherein the number average molecular weight of the polyalkylene glycol units constituting the polyalkylene glycol copolymer polyester resin is 200 or more and 2000 or less.

5. The polyester film according to claim 1, which contains a total of 5 ppm to 100 ppm of metal atoms selected from at least one of titanium (Ti), germanium (Ge), and tin (Sn).

6. The laminated polyester film according to claim 1 or 2, having a layer A mainly composed of the polyalkylene glycol copolymer polyester resin and a layer B mainly composed of a thermoplastic resin different from the layer A.

7. The laminated film according to claim 6, having a laminated unit in which the A layer and the B layer are alternately laminated in 51 or more layers.

8. The polyester film according to claim 1 or 2, having a sea-island structure made of a plurality of thermoplastic resins including the polyalkylene glycol copolymer polyester resin.

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