JPH054218B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a composite biaxially oriented polyester film with excellent transparency, and more specifically, to base films that require extremely high transparency, such as for plate-making applications, transparent conductive film applications, etc. The present invention relates to a composite biaxially oriented polyester film that is suitable for excellent transparency and film winding properties. [Prior Art] Polyester films, especially biaxially oriented polyester films represented by polyethylene terephthalate, have excellent electrical properties, mechanical properties, thermal properties, as well as processability and chemical resistance. It is used as a base film in a wide range of fields such as tapes, capacitors, packaging materials, photolithography, and electrical insulation materials. Polyester films are also used in applications that require extremely high transparency, such as plate-making applications and transparent conductive film applications. It is extremely high, less than 0.8. [Problems to be Solved by the Invention] However, such a highly transparent film has the disadvantage that there are almost no protrusions on the film surface, and therefore it has no slipperiness at all. For this reason, there are problems in that the film is easily scratched during production and is extremely difficult to wind up. Therefore, conventionally, in order to improve the winding workability, methods have been adopted, such as embossing only the film edges before winding the film, mechanically creating irregularities on the film surface, and then winding the film. There is. However, even with this method, it is difficult to avoid scratches that occur during the film stretching process, and if the film width is wide, it cannot be expected to improve the winding workability, making it ineffective. It was hot. [Means for Solving the Problems] The present inventors have made extensive studies in order to solve the above-mentioned conventional problems and provide a polyester film that maintains good transparency and has excellent winding workability. As a result, three or more layers are composited by coextrusion, and an unstretched film containing particles in the two exposed layers is biaxially stretched and heat-set to satisfy specific physical properties. It was discovered that the resulting film has extremely excellent transparency and excellent winding workability, and the present invention was achieved based on this finding. That is, the present invention is a polyester film in which three or more layers are composited by coextrusion, biaxially stretched and heat-set, and has the following formula ~0.155≦ΔP≦0.165 ... 1.604≦≦1.610 ... ... Δn≦0.02 ... 7≦X _I ≦11 ... 0.55≦[η] ⁽¹⁾ _F ≦0.65 ... 0.007√[] ⁽¹⁾ _F −0.55＋0.003 ≧−1.604 ≧0.007√[] ^{(1 )} _F −0.55 ... ΔH≦0.5 ... [However, in the above formula, ΔP, Δn, X _I , [η]
⁽¹⁾ _F and ΔH are the degree of plane orientation, average refractive index, birefringence, and X of the biaxially stretched polyester film, respectively.
These are the percentage of the intensity ratio of the (110) plane to the (100) plane of the line, the intrinsic viscosity of the exposed surface of the film, and the surface haze. ] A composite biaxially stretched polyester film with excellent transparency, which satisfies the following and is characterized in that the two exposed layers are layers containing fine particles. The present invention will be explained in detail below. The composite biaxially stretched film as used in the present invention refers to a film extruded by a co-extrusion method in which all layers are co-melt extruded from a die, biaxially stretched and heat-set. Below, 3 is used as a composite biaxially stretched film.
Although a layered composite film will be described, in the present invention, the number of composite layers is not limited to three layers, and may be more than three layers. In the present invention, the raw material polyester constituting each layer to be composited includes aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, and naphthalene-2,6-dicarboxylic acid or esters thereof, ethylene glycol, diethylene glycol, tetramethylene glycol, Examples include polymers obtained by polycondensation with glycols such as neopentyl glycol. Such polyesters can be obtained by direct polycondensation of aromatic dicarboxylic acids and glycols, or by polycondensation after transesterification of aromatic dicarboxylic acid dialkyl esters and glycols, or by polycondensation of aromatic dicarboxylic acid dialkyl esters and glycols, or It can also be obtained by a method such as polycondensation of diglycol ester. Typical examples of such polymers include polyethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate, and the like. This polymer may be a non-copolymerized homopolymer, with 15 mol% of the dicarboxylic acid component.
It may be a copolymerized polyester having a diol component other than aliphatic glycol in an amount of 15 mol% or less of the following non-aromatic dicarboxylic acid component and/or diol component. Furthermore, it may be a polymer blend of these polyesters and other polymers. Examples of other polymers that can be blended include polyamide, polyolefin, and various other polyesters (including polycarbonate). Further, the polyester may contain additives such as stabilizers, colorants, antioxidants, antifoaming agents, etc., if necessary. In the present invention, such polyesters are composited by co-melt extrusion, but the three to be composited are
Two of the exposed layers, that is, the two surface layers, are coated with a lubricant to impart a lubricant to the polyester film.
It is necessary that fine particles be contained. The fine particles contained include lithium fluoride,
Salts or oxides containing elements selected from Groups 1, 2, 3, and 3 of the Periodic Table of the Elements, such as kaolin, clay, calcium carbonate, silicon oxide, calcium terephthalate, aluminum oxide, calcium phosphate, titanium oxide, etc. Inert particles such as high melting point organic compounds, crosslinked polymers, etc. that are insoluble during melt film formation of polyester resin (these particles added from outside are called external particles), or during polyester synthesis. Examples include particles formed inside the polymer during polyester production (particles formed during polyester production are referred to as internal particles) depending on the metal compound catalyst used, such as an alkali metal compound or an alkaline earth metal compound. These inert external particles and internal particles can be used alone or in combination of two or more types. The average particle diameter of these particles is preferably in the range of 0.01 to 3.5 μm, and the amount of fine particles contained in the film is preferably in the range of 0.005 to 0.9% by weight. In the present invention, in order to obtain better effects, the additive particles are preferably particles that have a refractive index comparable to that of polyethylene terephthalate and that are less likely to cause voids during film stretching. As such particles, particles of amorphous silica are particularly suitable. When using amorphous silica particles, they may be used alone or in combination with other particles, but the most preferred form is a bimodal system of large particles and small particles, specifically, an average particle size of 0.01 to 0.01.
A bimodal system consisting of small particles of 0.8μ and large particles with an average particle size of 0.8 to 1.5μ, both of which are made of amorphous silica, is mentioned. In this case, the content of large particles in the film is 0.003 to 0.015% by weight, and the content of small particles is 0 to 0.015% by weight.
Preferably, it is 0.05% by weight. If the average particle diameter of the added large particles is less than 0.8μ, their contribution to the slipperiness of the film is extremely small, whereas if it exceeds 1.5μ, band-like unevenness will occur on the film surface, which is not preferable. Furthermore, when the content of large particles is less than 0.003% by weight, the resulting film has low slipperiness;
Transparency may deteriorate if this value is exceeded. In the present invention, in addition to the above-mentioned fine particles, in order to improve the slipperiness of the film, it is also a preferable method to incorporate an organic lubricant into the two exposed layers of the film. The type of organic lubricant is not particularly limited, but aliphatic compounds, fatty acid esters, alkylene bis aliphatics, aromatic amides, and the like are preferred. As the aliphatic compound, those having a large number of carbon atoms such as montanic acid are preferred. Moreover, examples of aliphatic esters include montanic acid ethylene glycol ester and the like. Examples of the alkylene bis aliphatic and/or aromatic amide include hexamethylene bis behenamide, hexamethylene bis stearyl amide, N,N'-distearyl terephthalamide, and the like. The content of these organic lubricants in the film is preferably 0 to 500 ppm, preferably 0 to 200 ppm. If these lubricants are mixed in too large a quantity, it is not preferable because the adhesion properties when the film is subjected to treatments such as vapor deposition or coating may deteriorate, and the color of the film may become too yellowish. Furthermore, for the purpose of improving the printability of the film and the adhesion during vapor deposition, it is also suitable to contain polyalkylene glycols in the two exposed layers of the film. Examples of the polyalkylene glycol include polyethylene glycol, polytetramethylene glycol, polypropylene glycol, and the like. These polyalkylene glycols can be incorporated into the film by adding them to the reaction system during transesterification or polymerization. Any method may be used, such as blending a polymer copolymerized with polyalkylene glycol or kneading the polyester during drying or extrusion. However, in order not to impair the transparency of the film of the present invention, the polyalkylene glycol preferably has a molecular weight of 10,000 or less, preferably 8,000 or less, and its content in the film is 1% by weight or less, preferably is preferably 0.5% by weight or less. As described above, among the three composite layers of the present invention, the two exposed layers are layers containing fine particles, while the inner one layer is substantially thin to ensure the transparency of the film. Preferably, the layer does not contain particles. Note that "substantially free of particles" means that when the polyester film obtained by biaxially stretching the single layer alone is dissolved in an O-chlorophenol solvent, the amount of undissolved residual particles is 0.10% by weight or less. In addition, in the present invention, the thickness of the two exposed film layers (A+A') and the thickness of one internal layer (in the case of a composite of four or more layers, the total thickness of the internal layers other than the two exposed layers) (B) ratio (A+
A')/B is preferably in the range of 1 to 0.1.
If (A+A')/B is larger than 1, the transparency of the film will decrease, which is not preferable. Also, (A+
If A')/B is smaller than 0.1, it is difficult to control the film thickness on the exposed surface and is unsuitable. The three-layer composite unstretched film obtained by co-melting extrusion of such three layers through a die is then biaxially stretched and heat-set, and as a biaxially stretched and heat-set film, it has excellent transparency and good properties. In order to obtain a film that has excellent slipperiness and excellent winding properties, it must satisfy various physical properties as described below. That is, in the present invention, the degree of plane orientation (ΔP) of the film, the average refractive index ( ) of the film, the birefringence index (Δn) of the film, and the ( ₁₁₀ ) surface strength
Percentage of intensity ratio of X ₁₁₀ (X _I = X ₁₁₀ / X ₁₀₀ × 100
(%)), intrinsic viscosity of the film [η] ⁽¹⁾ By setting _F and surface haze (ΔH) to specific values, it is possible to obtain a film with excellent transparency and slipperiness. . The reason for limiting each physical property value will be explained below. Note that the average refractive index of the film is, if the refractive index in the thickness direction n〓, the refractive index n〓 in the main orientation direction, and the refractive index n in the direction perpendicular to the main orientation direction, then = 1/3 (n〓 + n〓 + n 〓) is given by. is the film density ratio (d-
da)/(dc-da) (dc: density of crystalline part, da: density of amorphous part, d: density of film), and is generally controlled by the heat-setting temperature. It increases with growth. On the other hand, the degree of plane orientation ΔP of the film is the above n〓,
Using n〓 and n〓, it is given by ΔP=n〓+n〓/2−n〓...(). As can be seen from the above definition, the degree of plane orientation ΔP is an index indicating how oriented within the plane of the film, and is generally a numerical value representing the degree of parallelism of the benzene rings to the plane of the film. Therefore, in a biaxially stretched film, the higher the stretching ratio in the longitudinal and/or transverse directions at lower temperatures, the higher the heat setting temperature during heat setting, and the greater the strength to resist the shrinkage stress received. The larger the value, the larger the value of the degree of plane orientation ΔP. The birefringence Δn in the film plane is given by Δn=n〓−n〓 using n〓 and n〓. Δn is a numerical value representing the anisotropy of the refractive index within the plane of the film, and the main orientation direction differs between the center and the edges of the film, but in any case, the larger the anisotropy of the film, the larger Δn becomes. In the present invention, the average refractive index of the film is 1.604≦≦1.610. A preferable value of is 1.605 or more and 1.609 or less. If the average refractive index is lower than 1.604,
This is undesirable because the dimensions of the film change during the application of solvents and adhesives in subsequent steps. On the other hand, the average refractive index
If it exceeds 1.610, the film tends to have poor mechanical strength. The film plane orientation degree ΔP and the percentage X _I of the intensity ratio of the (110) plane to the (100) plane of X-rays are respectively 0.155≦ΔP≦0.165…7≦X _I ≦11…preferably 0.159≦ΔP≦0.162 8 It is necessary to satisfy ≦X _I ≦11. The numerical limitations of ΔP and X _I are the most important features of the invention. Lower ΔP so that it falls within the range of the above formula,
It is possible to increase _XI by lowering the birefringence index (Δn) after longitudinal stretching, as in Japanese Patent Application No. 59-227785, but this method has drawbacks in various ways. In the present invention, without lowering the birefringence (Δn) after longitudinal stretching, a longitudinally stretched film with Δn exceeding 0.080 after longitudinal stretching is used, and ΔP is in the range of 0.155 to 0.165 and X _I is in the range of 7 to 11. It is preferable to process so that Such a film can be obtained by lowering the refractive index in the lateral direction (direction perpendicular to the main orientation direction), as can be seen from the above formula (). However, it is a well-known fact that if the lateral stretching ratio is simply lowered in order to lower the refractive index in the lateral direction, the thickness unevenness of the film will be extremely worsened. As a result of intensive study to solve this problem, after horizontal stretching at a normal magnification,
It was found that by performing lateral relaxation during heat setting, ΔP could be lowered without worsening film thickness unevenness. The film thus obtained has a small refractive index in the lateral direction, and as a result, the in-plane birefringence (Δn) of the film becomes small. For this reason, it is essential that the birefringence of the film of the present invention satisfies Δn≦0.02. Here, preferably Δn≦0.015, more preferably Δn≦0.01. Such a film has little in-plane anisotropy and has extremely excellent flatness. On the other hand, as a result of further studies on film properties, it was found that even some films that satisfy the formula (-) have poor slipperiness. After careful consideration of the cause, we found that when the intrinsic viscosity of the film on the exposed surface [η] ⁽¹⁾ _F exceeds 0.65,
It was found that a slippery film could not be obtained. On the other hand, the film's intrinsic viscosity [η] ⁽¹⁾ _F is too low.
If it is less than 0.55, the film satisfying the above formulas will have too high a film haze, making it unsuitable. From this, [η] ⁽¹⁾ _F should be 0.55≦[η] ⁽¹⁾ _F ≦0.65...preferably 0.58≦[η] ⁽¹⁾ _F ≦0.62. Furthermore, [η] ⁽¹⁾ When _F is high, the higher the heat setting temperature, the more slippery the film becomes;
⁽¹⁾ It was found that when _F is low, transparency deteriorates if the heat setting temperature is too high. Therefore,
[η] ⁽¹⁾ The correspondence between _F and the average refractive index, which is proportional to the heat fixation temperature, was found to be 0.007√[] ⁽¹⁾ _F −0.550.003 ≧−1.604≧0.007√[] ^{( 1)} It was found that it was necessary to satisfy _F −0.55…. By satisfying the above formulas, a film with excellent transparency, slipperiness, flatness, surface properties, etc. can be obtained, but the surface haze ΔH of the film must also satisfy ΔH≦0.5. When ΔH exceeds 0.5, even if the total haze of the film is low, the film has good transparency when viewed from the right angle, but
When viewed from an angle, diffuse reflection on the film surface becomes large and transparency is impaired, which is not preferable.
Preferably ΔH≦0.4, more preferably ΔH≦0.3. The composite biaxially stretched polyester film of the present invention has a film-to-film dynamic friction coefficient lower than a static friction coefficient, and a static friction coefficient of 1.1 or less.
It is preferably 0.9 or less. If the coefficient of static friction exceeds 1.1, blocking will occur during film manufacturing and film use processes, which is inappropriate. Further, when transparency is particularly required for the base film, the total haze of the polyester film of the present invention is preferably 1.0% or less, particularly 0.8 or less, in terms of 75μ. The polyester film of the present invention may have any thickness as long as it is formed as a film, for example, it can be made into a film with a thickness of 3 to 500 μm, but the film of the present invention has excellent properties especially when made into a film of about 20 to 150 μm. be effective. Next, a method for forming a composite biaxially stretched polyester film of the present invention will be specifically explained, but the film of the present invention is not limited to the following production examples. Polyester raw material A is a mixture of fine particles such as kaolin and silica with additives such as stabilizers, colorants, antifoaming agents, organic lubricants, polyalkylene glycols, etc., and raw material B is substantially free of particles. are dried separately, extruded using separate extruders, and combined using a coextrusion method so that raw material A is exposed in two layers and raw material B is in one inner layer,
The mixture is cooled and solidified on a casting drum to form an unstretched polyester sheet consisting of three layers.
At this time, it is preferable to use a conventional electrostatic application cooling method. The sheet thus obtained is then stretched in the first axial direction, usually in the longitudinal direction, so that its birefringence Δn exceeds 0.080. At that time, the stretching temperature is 75℃ or higher.
The temperature is preferably 130°C or lower. In addition, as a role,
Hard chrome roll, ceramic roll,
Although it is possible to use an elastomer roll such as Teflon as appropriate, it is best to use a hard chrome plated roll in particular, and to stretch in one step at a stretching temperature of 80 to 90°C. The uniaxially stretched film thus stretched in the first axial direction is then stretched in the second axial direction. Stretching in the second axial direction is carried out by first cooling the uniaxially stretched film below the glass transition point, or by preheating it to a temperature of, for example, 90 to 150°C without cooling, and then stretching it in the second axial direction at approximately the same temperature. This is done by stretching 3.5 to 5.0 times in the direction. The stretching ratio in the second axis direction is
If it is less than 3.5 times, the thickness unevenness in the second axis direction will be amplified, which is not preferable. The stretching ratio in the second axis direction is preferably 3.8 to 5.0 times, more preferably 4.0 to 5.0 times. Then, the obtained biaxially stretched film is
The film is heat-set at 250°C for 1 second to 10 minutes, but in the present invention, it is important that the film relaxes by 1-15% in the heat-setting zone at 200°C or higher in the film width direction. It is particularly preferred that the relaxation treatment is carried out in the width direction in the maximum heat setting temperature range of the heat setting zone. In the present invention, in order to further improve the transparency and slipperiness of the obtained film,
It is preferable to adopt so-called two-stage heat setting, in which the material is heat-set at 200°C or higher, then cooled once to 120°C or lower, and then heat-set again at 200°C or higher. [Function] It is almost only the film surface layer that is involved in the slipperiness of the film. Therefore, as in the present invention, by making two of the exposed layers of the composite layer contain fine particles, the slipperiness of the film can be improved. In addition, a film whose ΔP, Δn, X _I , [η] ⁽¹⁾ _F and ΔH satisfy the formula ~ above has a lower ΔH and is easier to use than a film that does not satisfy the formula ~ even if the raw materials are the same. Extremely smooth. Although the details of the reason for this are not clear, in order to satisfy the formula ~, by employing relaxation treatment during heat fixation, no haze appears other than the fine particles contained in the exposed two layers, and fine particles due to crystals. It is thought that because the film was formed on the surface of the film, good slipperiness was imparted while maintaining transparency. The film of the present invention is a completely new transparent film with improved slipperiness and almost no surface haze on the exposed surface layer. [Examples] The present invention will be explained in more detail with reference to Examples and Comparative Examples below, but the present invention is not limited to these Examples. In addition, the evaluation method of film characteristics in Examples and Comparative Examples is as shown below. Film Haze Surface haze and internal haze were determined using an integrating sphere turbidity meter NDH-20D manufactured by Nippon Denshoku Kogyo Co., Ltd. in accordance with JIS-K6714. If the thickness of the film is d (μ), the measured value of film haze is H ₁ , and the measured value of haze after applying liquid paraffin to the film surface is H ₀ , then ΔH and the film haze with a thickness of 75 μ are H ⁷⁵ ₁ is defined by the following formula. ΔH=H ₁ −H ₀ H ⁷⁵ ₁ = H ₀ ×75/d+(H ₁ −H ₀ ) Average normal refractive index, birefringence, and degree of plane orientation The refractive index of the film can be measured using Atsupe manufactured by Atago Co., Ltd. A sodium lamp was used as the light source. Find the maximum refractive index n〓 in the film plane, the refractive index n〓 in the direction perpendicular to it, and the refractive index n〓 in the thickness direction, and calculate the average refractive index, birefringence Δn, and degree of plane orientation ΔP according to the following formula. Calculated. n=n〓+n〓+n〓/3 Δn=n〓−n〓 ΔP=n〓+n〓/2−n〓 Birefringence (longitudinally stretched film) Retardation R was measured using a Carl Zeiss polarizing microscope. Then, the birefringence Δn was determined from the film thickness d (μm) using the following formula. Δn=R/d Slip property Layer two films on a smooth glass plate,
A rubber plate was further placed on top of the rubber plate, and a load was placed on it, and the frictional force was measured by sliding the films against each other at 20 mm/min with a contact pressure of 2 g/cm ² . The coefficient of friction at the point where it slid 5 mm was defined as the coefficient of dynamic friction. X-ray diffraction measurement Measure the sample on a film using an automatic X-ray diffractometer, and measure the peak value of the (100) plane near 2θ = 26° and the 2θ
The peak value of the (100) plane near =23° was read and the ratio was calculated. X-ray output was 30KV and 15mA. Intrinsic viscosity of film [η] Dissolve 1 g of film in 100 ml of mixed solvent of phenol/tetrachloroethane = 50/50 (weight ratio) and
Measured at °C. Example 1 <Production of polyester chips> 100 parts by weight of dimethyl terephthalate, 70 parts by weight of ethylene glycol, and calcium acetate monohydrate
0.07 parts by weight was placed in a reactor, heated to raise the temperature, and methanol was distilled off to carry out the transesterification reaction.
It took about 4 and a half hours after the start of the reaction to reach 230°C.
The transesterification reaction was substantially completed. Next, 0.04 parts by weight of phosphoric acid and antimony trioxide
0.035 parts by weight was added, and polymerization was carried out according to a conventional method.
That is, the reaction temperature is gradually raised until finally
280℃, while the pressure gradually decreases to 0.5
mmHg. After 4 hours, the reaction was completed and the mixture was chipped according to a conventional method to obtain polyester (2). On the other hand, amorphous silica-containing polyester () was produced in the same manner as in the production of polyester (), except that 0.10 parts by weight of amorphous silica with an average particle size of 1.3μ was added after the transesterification was completed. Obtained. Furthermore, in the production of polyester (),
Amorphous silica-containing polyester () was prepared in the same manner except that the amount of amorphous silica added was 0.05 parts by weight.
I got it. <Production of polyester film> The above polyester (), polyester (), and polyester () were mixed in a ratio of 70:10:20, respectively.
Blend and use as raw material A. On the other hand, raw material B is a raw material for polyester (). Raw materials A and B are dried separately, and a separate melt extruder with a diameter of 90 mm is used to form two layers (layer A) in which raw material A is exposed.
Raw material B is in one layer (B layer) inside, thickness ratio A/B=
A three-layer composite was formed using 2/8, and the resultant was cooled and solidified on a smooth casting drum to produce an unstretched sheet. This sheet was first stretched 3.6 times in the longitudinal direction at 85℃ using an IR heater with a mirror-finished hard chrome roll to set Δn to 0.090, and then stretched at 120℃.
It was stretched 4.0 times in the transverse direction. Next, heat setting was performed at 240°C, and during heat setting, relaxation treatment was performed in the width direction at a relaxation rate of 10% to obtain a biaxially stretched film with a thickness of 75 μm. Table 1 shows various properties of the obtained film. Comparative Examples 1 and 2 Unstretched sheets were made using only raw material A (comparative example 1) or only raw material B (comparative example 2), and biaxially stretched and heat-set in the same manner as in example 1. thickness
A 75μ film was obtained. Table 1 shows various properties of the obtained film.

【table】

[Table] As is clear from Table 1, the film of Example 1 was more slippery than the film of Comparative Example 1, which is considered to be the film with the best slipperiness among the obtained films. is about the same level, and the transparency is much better. Therefore, the film of the present invention can be effectively used in applications requiring high transparency, where the film of Comparative Example 1 could not be used.
On the other hand, although the film of Comparative Example 2 has excellent transparency, it has poor slipperiness and cannot be wound up as it is. Furthermore, even when the film was embossed and wound, many scratches occurred during stretching, making it unsuitable for high transparency applications. In contrast,
The film of Example 1 according to the present invention was excellent in both transparency and slipperiness. [Effects of the Invention] The composite biaxially stretched polyester film of the present invention has excellent transparency required for transparent conductive films, plate-making applications, etc., as well as excellent slipperiness and good winding workability. It is. Moreover, it has good flatness, and has no thickness unevenness, scratches, adhesive spots, etc., and has excellent surface properties. Therefore, the industrial utility of the composite biaxially stretched polyester film of the present invention is extremely high.

Claims (1)

[Scope of Claims] 1. A polyester film in which three or more layers are composited by coextrusion, biaxially stretched and heat-set, which satisfies the following formula ~, and the exposed two layers contain fine particles. A composite biaxially oriented polyester film with excellent transparency characterized by a layer containing: 0.155≦ΔP≦0.165 … 1.604≦≦1.610 … Δn≦0.02 … 7≦X _I ≦11 … 0.55≦［η］ ⁽¹⁾ _F ≦0.65 … 0.007√［］ ⁽¹⁾ _F −0.55＋0 .003 ≧−1.604 ≧0.007√[] ⁽¹⁾ _F −0.55 … ΔH≦0.5 … [However, in the above formula, ΔP, Δn, X _I , [η]
⁽¹⁾ _F and ΔH are the degree of plane orientation, average refractive index, birefringence, and X of the biaxially stretched polyester film, respectively.
These are the percentage of the intensity ratio of the (110) plane to the (100) plane of the line, the intrinsic viscosity of the exposed surface of the film, and the surface haze. 2. The composite biaxially stretched polyester film according to claim 1, wherein among the composite layers, layers other than the two exposed layers do not substantially contain fine particles. 3 Among the composite layers, the ratio of the total thickness of the two exposed layers (A+A') to the total thickness of the layers other than the two exposed layers (B) is (A+A')/B=1 to 0.1. A composite biaxially stretched polyester film according to claim 1 or 2, characterized in that: