JPS6238766B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a biaxially stretched polyester film for magnetic tape. More specifically, the present invention relates to a biaxially oriented polyester film suitable for use as a base film for magnetic tapes, which has excellent properties including adhesion to the magnetic layer of magnetic tapes. Cellulose di-acetate film and cellulose tri-acetate film, which were originally used as base materials for magnetic tape, have smooth surfaces and have excellent sensitivity and frequency characteristics.
It gradually came to be replaced by polyester film because of its poor moisture resistance and strength when made thin. Polyester films can satisfy most of the properties required as a base material for magnetic tapes, but as the properties required for magnetic tapes have become more sophisticated in recent years, more excellent films have been sought. One of the particularly important properties of the base film for magnetic tape is its adhesion to the magnetic layer. If the adhesion between the magnetic layer and the polyester film is insufficient, the magnetic layer may peel off or fall off during use, causing serious trouble. Conventionally, attempts have been made to improve both the magnetic layer and the polyester film in order to improve the adhesion between the polyester film and the magnetic layer. Generally, methods for forming a magnetic layer on a support to form a magnetic tape include a coating type and a vapor deposition type, and the polyester film of the present invention is applicable to the former. That is, the magnetic layer coated magnetic tape is manufactured by coating a polyester film with a composition comprising magnetic powder, binder resin, antistatic agent, abrasive, lubricant, dispersant, plasticizer, etc. Among the magnetic layer components, the component that controls the adhesiveness with the polyester film is a binder resin, for example, vinyl resins, cellulose resins, urethane resins, epoxy resins, phenoxy resins, etc. are generally known. It is also well known to use mixtures of these. However, if the composition of the binder resin is changed, the abrasion resistance characteristics with the magnetic head, electromagnetic conversion characteristics, etc. will change at the same time, so it is not permissible to change these compositions solely for the purpose of improving adhesion. Therefore, improving the adhesion from the polyester film side is of great significance, and for this purpose, methods of incorporating various polymeric compounds, such as polyalkylene glycol, have been studied. However, such methods generally impair the strength and thermal dimensional stability of the film, and may not be effective at all depending on the binder composition, so there is a desire for a method to improve adhesiveness. . The present inventors have conducted intensive studies on improving such adhesion properties, and have discovered that a film with specific physical properties developed under special film-forming conditions can exhibit excellent effects. It is something that The present invention is applied to a magnetic layer-coated magnetic tape containing one or more of the above-mentioned binder resins, and in this case, the magnetic powder includes gamma iron oxide, cobalt-coated gamma iron oxide, and chromium dioxide. used. In the magnetic layer-coated magnetic tape of the present invention, there is no particular need to apply an undercoat, and the thickness of the magnetic layer is approximately 1 to 6 microns. Other problems that often arise with polyester films for magnetic tapes include running properties, winding properties,
Abrasion resistant and dropout. Furthermore, the quality of workability during slitting after the magnetic layer coating step is also an important item. One way to achieve the highly demanded high-density recording is to make magnetic tape thinner, but in this case tape running becomes unstable and irregular winding is more likely to occur, and especially in video tapes, jitter is a problem. or the tape may become deformed. Therefore, maintaining excellent running performance is an essential condition. Furthermore, since the tape speed, tape tension, etc. of magnetic tapes vary widely depending on the usage conditions at the time, irregular winding often occurs. A similar phenomenon occurs with base films, and it is necessary to control the deviation of the film and the hardness of the winding state to make the winding state uniform, but its effectiveness depends on the surface roughness of the base film. There are many things. Next, as a base film for magnetic tape, it goes without saying that wear resistance is required. This is a necessary characteristic in both steps before and after applying the magnetic layer, and it is necessary to minimize the generation of white powdery substances due to friction and wear between the polyester film and the roll or at the guide portion. Although there are many factors regarding dropout that can be attributed to the magnetic layer coating process, it is also true that the flatness of the base film surface is an important factor, and a film that is as smooth as possible is desired. Further, after the film is coated with a magnetic layer, it is slit and used as strips, but if the slitting properties are poor at this time, the ends of the film will bulge and the cross section will be uneven, causing various troubles. Conventionally, as a means of improving the various properties necessary for these base films for magnetic tapes, a method has been known in which fine particles inert to polyester are present in a polyester film to appropriately roughen the film surface. Although this method is certainly simple and effective, it did not bring about any improvements in adhesiveness or slitting properties. The inventors of the present invention have focused on this point and have carried out intensive studies, and have found that these properties can be improved by keeping the average refractive index and degree of plane orientation of the polyester film within a certain range. The inventors have discovered that if the number of interference fringes satisfies a certain formula, other necessary characteristics are also fully satisfied, and the present invention has been completed. That is, in the present invention, the average refractive index and the degree of plane orientation are within the range obtained by connecting the points A, B, C, and D shown in the table below on the orthogonal coordinates with straight lines, and The number of n-th interference fringes measured Nn (pieces/
mm ² ) satisfies the following formulas (1) to (3) at the same time: a biaxially oriented polyester film for magnetic layer-coated magnetic tape;

[Table] 50≦N ₁ ≦250 ………(1) 10≦N ₂ ≦250 ………(2) 0.5≦N ₁ /N ₂ ≦15 ………(3) The present invention will be explained in more detail below. The polyester constituting the biaxially oriented polyester film of the present invention is a polyester whose main repeating unit, i.e. at least 80 mol%, is ethylene terephthalate, with other constituents being 20% by mole.
Less than mol % of dicarboxylic acid component, glycol component, or oxycarboxylic acid component may be included. The biaxially stretched polyester film of the present invention is produced by melting and extruding polyester in a conventional manner, then sequentially or simultaneously biaxially stretching, and further stretching again in the longitudinal or transverse direction as necessary, at a temperature of 130 to 240°C. The most important feature of the film is that the relationship between the average refractive index and the degree of plane orientation is within a specific range. That is, FIG. 1 shows the relationship between the average refractive index and the degree of plane orientation of a polyester film.
Generally, the higher the average refractive index, that is, the higher the crystallinity, the higher the degree of plane orientation.
Under normal stretching conditions, the relationship between the two is almost uniquely determined, and they only move along the line X-Y.
No special effects were observed in terms of improvement in adhesion and slitting properties. However, according to what the present inventors have revealed for the first time, when using a film in which the relationship between the two falls within a certain range located at the lower right of the X-Y line, magnetic Excellent effects are exhibited in terms of adhesion to the magnetic layer and slitting properties of the film coated with the magnetic layer. Note that the effect of the present invention is exhibited even when the relationship between the average refractive index and the degree of plane orientation is located below the line BC or to the right of the line CD shown in FIG. It is better to avoid it because it will have inferior strength and will be difficult to put into practical use. Obtaining such a film of the present invention cannot be achieved using the stretching conditions normally employed in industry, but rather requires the use of somewhat special conditions. For example, in longitudinal-horizontal sequential biaxial stretching, this can be achieved by setting the longitudinal stretching temperature to about 105-115°C, which is 5-30°C higher than the normal stretching temperature, or It can also be obtained by significantly transversely relaxing after biaxial stretching and before heat treatment. Of course, not all of these methods will meet the requirements of the present invention, and the conditions of the film forming machine, such as the film forming speed and stretching width, or the conditions of the polyester raw material, such as the amount of copolymerization,
Since these values depend on the crystallization rate and the like, the requirements of the present invention must be satisfied by appropriately selecting these conditions. As described above, in the present invention, by keeping the average refractive index and degree of plane orientation of the polyester film within a certain range, we have achieved completely unexpected improvements in adhesiveness and slitting properties. Furthermore, it has been discovered that a well-balanced polyester film for magnetic tape can be obtained when the surface roughness of the film satisfies certain specific conditions. That is, in the present invention, n measured by multiple interferometry (the measurement method is described in detail before the examples)
The number Nn (pieces/mm ² ) of the following interference fringes must be within a certain range. If N ₁ is less than 50 or N ₂ is less than 10, the film will have insufficient slipperiness. On the other hand, these values
If it exceeds 250, the surface protrusions may overlap, giving an undesirable gentle spread, so these values are preferably within a range that satisfies equations (1) and (2). In particular, when the number of secondary interference fringes exceeds 250, tertiary interference fringes inevitably occur frequently and dropouts often occur. Particularly when the film is used as a base film for video, the number of third-order interference fringes is preferably 3 or less/mm ² , preferably 2/mm ² or less. The ratio of N ₁ to N ₂ is preferably 0.5 to 15, preferably 1 to 7, and more preferably 1 to 3. If this value falls outside of this range, the film surface will be relatively rough. However, the slipperiness and abrasion resistance are not significantly improved. As a method for imparting fine irregularities to the surface of a polyester film, for example, a so-called particle addition method and a particle precipitation method are known, and either of these may be employed in the present invention. The particle addition method involves pulverizing and classifying kaolin, talc, calcium carbonate, calcium phosphate, etc. to a certain particle size, and then adding it to the polyester reaction system or during molding. In addition, the particle precipitation method is a method in which metal compounds such as calcium compounds and lithium compounds that are soluble in ethylene glycol and polyester oligomers are precipitated as fine compounds during the polyester formation reaction in the presence or absence of a phosphorus compound. Of course, if these metal compounds are added at the beginning of the transesterification reaction, they can also serve as a catalyst. As described above, in the present invention, both the particle addition method and the particle precipitation method can be employed, but among these, the latter is particularly preferably used. This is because when precipitated particles obtained by a particle precipitation method are used, the required surface roughness can be relatively easily provided without impairing dropout characteristics by appropriately selecting manufacturing conditions. When a particle precipitation method is employed in the present invention, it is particularly preferable to have a phosphorus compound present in an amount substantially equivalent to or more than these metal compounds, and to precipitate the phosphoric acid or phosphite derivative as fine particles.
This is because unlike fine particles of metal salt derivatives of polyester oligomers, they generally have a high affinity with polyester, so when stretched, the voids generated around the particles are small, which is an undesirable phenomenon for use as a base film for magnetic tapes. This is because the gentle spread around the particles is reduced.
In the present invention, among these phosphoric acid or phosphite derivative fine particles, those containing lithium, calcium, and phosphorus elements each in an amount of 1% by weight or more based on the particles are particularly preferred. This is because such particles have good dispersibility, especially in polyester, and therefore hardly give rise to undesirable smooth spreading. Examples of methods for producing polyesters containing such precipitated particles that are particularly suitable for use in the present invention are described in more detail below. That is, the polyester containing such particles is, for example, subjected to a transesterification reaction in the presence of a lithium compound and a calcium compound, and after the transesterification reaction is completed, a polyester selected from the group consisting of phosphoric acid, phosphorous acid, or an alkyl or aryl ester thereof. It can be obtained by adding one or more types of compounds in an amount of 0.6 to 3 times the total amount of these metal compounds, and then performing a polycondensation reaction. The lithium compound used in this case may be one that is soluble in the esterification or transesterification reaction product, such as salts of aliphatic carboxylic acids such as acetic acid, propionic acid, and butyric acid, and aromatic compounds such as benzoic acid and p-methylbenzoic acid. Salts of carboxylic acids, as well as lithium glycolates such as ethylene glycol and propylene glycol, can be used. Among these, lithium aliphatic carboxylates, particularly lithium acetate, are preferred. The amount is 0.03 to 0.4 mol%, especially 0.1 to 0.4 mol% based on the aromatic dicarboxylic acid component.
0.3 mol% is preferably used. There are no particular restrictions on the calcium compound as long as it is soluble in the esterification or transesterification reaction product, such as salts of aliphatic carboxylic acids such as acetic acid, propionic acid, and butyric acid, benzoic acid, and p-methylbenzoic acid. Salts of aromatic carboxylic acids such as salts of aromatic carboxylic acids, as well as calcium glycolates such as ethylene glycol and propylene glycol can be used. Among these, calcium aliphatic carboxylates, particularly calcium acetate, are preferably used.
The amount of the calcium compound used is preferably 0.05 to 0.3 mol%, particularly 0.08 to 0.15 mol%, based on the aromatic dicarboxylic acid component. The phosphorus compound may be any compound as long as it reacts with the above-mentioned metal compound and converts part or all of it into a phosphate or phosphite derivative, but especially phosphoric acid and trialkyl esters of phosphoric acid. , a partial alkyl ester of phosphoric acid, phosphorous acid, a trialkyl ester of phosphorous acid, and a partial alkyl ester of phosphorous acid are preferably used. The amount of these phosphorus compounds added is 0.6 to 3 times equivalent, preferably 0.8 to 2 times equivalent, and more preferably 0.9 to 1.5 times equivalent relative to the total amount of the metal compounds. The equivalent ratio of the phosphorus compound to the metal compound is expressed by the following formula: (In the formula, P, Ca, and Li represent the number of moles of a phosphorus compound, a calcium compound, and a lithium compound, respectively.) By using a metal compound and a phosphorus compound in combination as described above, precipitated particles containing the metal element and phosphorus element used in the particles can be obtained. Of course, part or all of the lithium compound or calcium compound may be added after the transesterification reaction is completed. In order to achieve the object of the present invention, the amount of particles containing lithium, calcium and phosphorus elements precipitated in this polyester should be 0.01 to 0.6% by weight, preferably 0.03 to 0.4% by weight, more preferably 0.03 to 0.4% by weight, based on the polyester film. Selected from the range of 0.06 to 0.18% by weight. In this case, the amount of precipitated particles is determined as follows. (Quantification of precipitated particles in polyester film: Add 1.0 g of O-chlorophenol to 100 g of polyester, heat at 120°C for 3 hours, and then use a Beckman ultracentrifuge L3-50 at 30,000 rpm.
Centrifuge for 40 minutes and dry the resulting particles under vacuum at 100°C. When the particles are measured using a scanning differential calorimeter, if a melting peak corresponding to the polymer is observed, O-chlorophenol is added to the particles, and after heating and cooling, centrifugation is performed again. When the melting peak is no longer observed, the particles are treated as precipitated particles and the weight ratio to the polyester is calculated. ) Of course, in the present invention, additive particles can be used in combination with these precipitated particles, but the amount thereof is preferably kept to a relatively small amount. This is because when additive particles become the main component, third-order interference fringes often occur due to the coarse particles contained therein. In any case, in the present invention, it is necessary to maintain a certain surface roughness in order to improve slipperiness and wear resistance without impairing dropout characteristics, but according to the knowledge of the inventors, Such a specific surface roughness also gives good results with respect to adhesion. That is, even if the relationship between the average refractive index and the degree of plane orientation of a polyester film is within a certain range shown in FIG.
When the following conditions are satisfied, the effects of the present invention are maximized. The reason for this is not clear, but it is probably because the so-called anchoring effect is effectively exerted when a moderate amount of surface unevenness exists. As detailed above, the present invention focused on the relationship between the average refractive index of a film and the degree of plane orientation, which had never been looked at, and as a result of intensive studies, it was found that when both are within a certain range, the film It has been discovered that the adhesion to the magnetic layer is improved and the slitting properties are also improved. Furthermore, we have discovered that when the surface roughness of such a film satisfies certain specific conditions, a well-balanced base film for magnetic tapes with improved adhesion, excellent slipperiness and abrasion resistance can be obtained. be.
The film obtained by the method of the present invention is effective in improving characteristics in areas with relatively small surface roughness, and is particularly suitable as a base film for video. EXAMPLES The present invention will be explained in more detail with reference to examples below, but the present invention is not limited to the following examples unless it exceeds the spirit thereof. In the examples and comparative examples, "parts" indicate "parts by weight." The method for measuring the physical properties and characteristics of the film is as follows. 1 Measurement of average refractive index and degree of plane orientation The refractive index of the film was measured using an Atsupé refractometer manufactured by Atago Co., Ltd. and a sodium lamp as the light source. The maximum refractive index nγ in the film plane, the refractive index Nβ in the direction perpendicular to it, and the refractive index nα in the thickness direction were determined, and the average refractive index and degree of plane orientation were calculated according to the following formula. Average refractive index = nγ + nβ + nα / 3 Degree of plane orientation = nγ + nβ / 2 - nα 2 Measurement of surface roughness by multiple interference method After applying aluminum vapor deposition to the film surface,
Interference fringes are produced at a measurement wavelength of 0.54μ using multiple interference method, the interference fringes are photographed, and the number of n-th order interference fringes is counted and converted to 1 mm ² . The measuring instrument used was a Surf Eighth Fish Microscope manufactured by Nippon Kogaku Co., Ltd., and the mirror reflectance was 65.
%, and the microscope magnification was 200x. 3 Adhesion First, after cleaning the surface of the polyester film, a magnetic layer consisting of the following composition was applied to a layer with a dry thickness of 6.
It was applied so that it was μ. Composition (weight) Ferromagnetic material mainly composed of γ-Fe ₂ O ₃ 250 Polyurethane resin 50 Vinyl chloride/vinyl acetate copolymer 30 Nitrocellulose 20 Methyl ethyl ketone 900 Polyisocyanate compound 15 24 at 80℃ after applying the magnetic layer After drying for an hour, it was supercalendered and slit into 1/2 inch width tape. Next, the adhesiveness is measured by the peel strength when the film is peeled off at 180°C using double-sided tape so that the coating surface and the stainless steel plate are in contact with each other. 4. Slitting property When the polyester film coated with the magnetic layer was slit, the slits were classified into three ranks by observing the degree of swelling at the slits. A is one in which there is almost no bulge and the rolled shape is good, C is one in which the bulge is large and the end bulges when rolled, and B is in the middle between the two. 5 Sliding property The slipping property of the film was evaluated by measuring the coefficient of friction according to the method of ASTM D 1894-63. 6 Abrasion Resistance The film was run over a length of 500 m using the running system shown in Figure 2, and the amount of wear on the 6mmφ hard chrome fixing pin shown in parentheses was visually evaluated and ranked according to the rankings shown below. The film speed was 10 m/min, the tension was about 200 g, and θ=130°C. Rank A: Almost no adhesion. Rank B: Slight adhesion. Rank C: Large amount of adhesion. 7. Dropout The 1/2-inch wide tape obtained during the adhesive evaluation in step 3 was measured using a dropout counter manufactured by Japan Automatic Control Co., Ltd., and the signal output was 50% or less, which was caused by the base film. I counted. Example 1 100 parts of dimethyl terephthalate, 70 parts of ethylene glycol, 0.10 parts of calcium acetate hydrate, and 0.17 parts of lithium acetate dihydrate were placed in a reactor and heated to raise the temperature, and methanol was distilled off to carry out transesterification reaction to start the reaction. It takes about 4 hours to reach 230℃.
The transesterification was substantially completed. Next, this reaction product was heated to 230° C., and then 0.35 part of triethyl phosphate was added, and furthermore, 0.05 part of antimony trioxide was added as a polycondensation catalyst, and then polymerized according to a conventional method to obtain a polyester. A large number of uniform fine precipitated particles having a particle size of about 0.5 to 1 μm were observed in the polyester, and the amount thereof was 0.38% by weight based on the polyester. Further, the precipitated particles contained calcium, lithium, and phosphorus elements in amounts of 3.6% by weight, 2.0% by weight, and 7.0% by weight, respectively, based on the precipitated particles. Next, the polyester was dried and melt-extruded at 285°C to obtain an amorphous film having a thickness of 185μ. Then, it was heated 3.5 times in the vertical direction at 107℃, and then in the horizontal direction.
The film was stretched 3.5 times at 107°C, heat treated at 215°C for 3 seconds, and then cooled to obtain a biaxially stretched film with a thickness of 15μ. When the refractive index of the film was measured, nγ, n
β and nα were 1.6658, 1.6487, and 1.4972, respectively. Therefore, the average refractive index of the film is 1.6039,
The degree of plane orientation is 0.1601. On the other hand, when the surface roughness of the film was measured by multiple interference method, the number of first-order and second-order interference fringes was
It was 150 and 155. The evaluation results for this film are shown in Table 1. Comparative Example 1 A polyester film was obtained in the same manner as in Example 1, except that the same polyester raw material as in Example 1 was used and the stretching temperature in the longitudinal direction was changed to 85°C. Table 1 shows the physical properties and evaluation results of the obtained film. Comparative Example 2 A polyester film was obtained in the same manner as in Comparative Example 1 except that the heat treatment temperature was changed to 155°C. Comparative Example 3 In the production of the polyester of Example 1, 0.27 parts of triethyl phosphate and ethyl acid phosphate were used instead of 0.35 parts of triethyl phosphate.
A polyester was obtained in the same manner as in Example 1, except that 0.06 part was used. A large number of uniform fine particles with a particle size of approximately 0.5 μm were found in the polyester, and the amount thereof was 0.2% by weight based on the polyester. Further, the precipitated particles contained calcium, lithium, and phosphorus elements in amounts of 3.5% by weight, 2.1% by weight, and 7.5% by weight, respectively, based on the precipitated particles. Next, this polyester and a polyester containing almost no particles were blended at a ratio of 1:2, and a polyester film was obtained in the same manner as in Example 1. Table 1 shows the physical properties and evaluation results of the obtained film. Examples 2 to 14 Using the same polyester raw material as in Example 1, by selecting the longitudinal stretching temperature from the range of 106 to 112°C and the heat treatment temperature from the range of 160 to 220°C, the average refractive index and degree of plane orientation shown in Table 1 were obtained. A biaxially stretched film having a surface roughness of 1 and 2 was obtained, and then the polyester film was formed into a tape in the same manner as in Example 1.
This evaluation result, together with other examples and comparative examples, was
Shown in the table. Comparative Example 4 After blending the polyester used in Example 1 with a polyester containing almost no particles at a ratio of 1:5, a film was obtained in the same manner as in Example 7, and then formed into a tape. Comparative Example 5 A polyester film was obtained in the same manner as in Example 1, except that the same polyester raw material as in Example 1 was used, the longitudinal stretching temperature was 86°C, and the heat treatment temperature was 210°C.
Next, it was made into a tape. Comparative Example 6 Using the same polyester raw material as in Example 1, a film was obtained in the same manner as in Example 1, except that the longitudinal stretching temperature was 113°C, the heat treatment temperature was 225°C, and 10% transverse relaxation was performed before heat treatment. Next, it was made into a tape. Comparative Example 7 A film was obtained in the same manner as in Example 1, except that the same polyester raw material as in Example 1 was used, the longitudinal stretching temperature was 110°C, and the heat treatment temperature was 238°C, and then it was formed into a tape. Comparative Example 8 After blending the polyester used in Comparative Example 4 with a polyester containing almost no particles at a ratio of 2:1, the longitudinal stretching temperature was 108°C and the heat treatment temperature was
A film was obtained in the same manner as in Example 1 except that the temperature was 220°C, and then it was made into a tape. All of the examples satisfy the relationship between the average refractive index and the degree of plane orientation and the surface roughness required in the present invention,
As a result, the films of the comparative examples were inferior in several respects, whereas they highly satisfied the various properties necessary for a base film for magnetic tapes, including adhesiveness. That is, although the surface roughness of Comparative Examples 1 and 2 is substantially the same as that of Example 1, the relationship between the average refractive index and the degree of plane orientation is not within the range specified by the present invention, so the adhesiveness and It is inferior in slitting properties. Comparative Example 3 is an example in which even though the relationship between the average refractive index and the degree of plane orientation is within the range of the present invention, the surface roughness is inappropriate and the slipperiness and abrasion resistance of the film are poor. Comparative Example 4, like Comparative Example 3, satisfies the requirements of the present invention in the relationship between the average refractive index and the degree of plane orientation;
This is an example in which the slipperiness and abrasion resistance of the film are poor because Equation (1) regarding surface roughness is not satisfied. Comparative Example 5, like Comparative Examples 1 and 2, is an example in which the commonly used longitudinal stretching temperature and heat treatment temperature are used, and is particularly insufficient in terms of adhesion and slitting properties. In Comparative Examples 6 and 7, the relationship between the average refractive index and the degree of plane orientation is below or below the BC line shown in Figure 1.
This is an example of a case where the position is to the right of the CD line. In this case, although the properties shown in Table 1 are comparable to those of the examples, the mechanical strength of the obtained film is inferior and tape elongation is likely to occur. It was difficult to use. That is, while the initial elastic modulus of the films of Examples 1 to 14 was 450 to 470 Kg/mm ² , that of Comparative Examples 6 and 7 was 400 to 420 Kg/mm ² , which was considerably inferior. Comparative Example 8 is an example in which only equation (3) regarding surface roughness is not satisfied, but in this case, the wear resistance was somewhat inferior and the dropout increased significantly, making it difficult to put it to practical use.

【table】 [Brief explanation of the drawing]

FIG. 1 shows the relationship between the average refractive index and the degree of plane orientation of a polyester film. Figure 2 shows the running system for evaluating wear resistance.
() indicates a hard chrome fixing pin with a diameter of 6 mm, () indicates a tension meter, and θ is 130°C.

Claims (1)

[Scope of Claims] 1. The average refractive index and degree of plane orientation are within the range obtained by connecting the points A, B, C, and D shown in the table below with straight lines on orthogonal coordinates, and multiple interference The number Nn (pieces/mm ² ) of n-th order interference fringes measured by the method is given by the following formula:
A biaxially stretched polyester film for a magnetic layer-coated magnetic tape, which satisfies (1) to (3) at the same time. [Table] 50≦N ₁ ≦250 ………(1) 10≦N ₂ ≦250 ………(2) 0.5≦N ₁ /N ₂ ≦15 ………(3) 2 Contains precipitated particles A biaxially stretched polyester film according to claim 1. 3. The biaxially stretched polyester film according to claim 2, wherein the precipitated particles contain calcium, lithium, and phosphorus elements.