JPH0455583B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a biaxially oriented polyester film, and more specifically, the present invention relates to a biaxially oriented polyester film, and more specifically, the present invention relates to a biaxially oriented polyester film. The present invention relates to a biaxially oriented polyester film useful for the production of. <Prior Art> Magnetic tapes have been widely used in audio, computers, video, and the like. In the production of magnetic tapes, a magnetic layer is provided on one side of a non-magnetic support, such as a polyester film, with a width of several 100 to 1000 mm, and then the tape is slit to a width according to each specification, and the resulting tape is once shaped into a long sheet. Usually, the pancake is rolled up into a pancake, and then the pancake is wound to a predetermined length to form a reel or wound into a cassette. Since these pancakes are sold as they are or transported to an in-cassette location, their ability to retain their shape, such as the absence of irregular rolling or crumbling, is an important quality point in terms of product yield. It's summery. If this shape retention is poor, for example, changes in temperature and humidity during pancake storage or transportation, especially changes in temperature, may cause gaps in some parts of the pancakes, or even parts of the pancakes may This causes problems such as misalignment of the pancakes, which disturbs the shape of the pancakes, and significantly impairs the workability of the incassette. <Object of the Invention> The present inventor has arrived at the present invention as a result of intensive research to obtain a polyester film useful for producing pancakes that has good shape retention even in the face of rapid environmental changes. An object of the present invention is to provide a biaxially oriented polyester film that can produce pancakes that have good shape retention even in the face of rapid environmental changes, particularly temperature changes, and is extremely useful in the production of magnetic tapes. . <Configuration and Effects of the Invention> According to the present invention, an object of the present invention is to
It is a biaxially oriented polyester film with a diameter of 25 μm, and the center line average roughness (Ra) of the film surface is
Protrusion height (h: μm) of 0.008 to 0.0.060 μm and measured with a multiple interference reflection microscope (Tl monochromatic light)
and the number of protrusions (pieces/mm ² ) is 2.0 > h > 1.5...5 pieces/mm ² or less 1.5h > 1.0...10 pieces/mm ² or less 1.0h > 0.75...1 to 25 pieces/mm ² 0.75h ＞0.5...30 to 100 pieces/ ^mm2 0.5h＞0.25...80 pieces/mm2 Satisfies the relationship of ² or more, and the residual strain rate in the longitudinal direction of the film (from 25℃ to 40℃ under a load of 400g/mm2 ⁾ The absolute value (residual strain rate) was 0.02% as an absolute value.
This is achieved by a biaxially oriented polyester film having a stiffness of 0.8 g/mm ² or more in the longitudinal direction of the film. The polyester in the present invention is a polyester containing an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component. Such polyester is substantially linear;
It also has the ability to form films by special melt molding. Aromatic dicarboxylic acids include, for example, terephthalic acid, naphthalene dicarboxylic acid, isophthalic acid, diphenoxyethane dicarboxylic acid,
These include diphenyl dicarboxylic acid, diphenyl ether carboxylic acid, diphenyl sulfone dicarboxylic acid, diphenyl ketone dicarboxylic acid, anthracene dicarboxylic acid, and the like. Aliphatic glycols include, for example, polymethylene glycols having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, and decamethylene glycol, or alicyclic glycols such as cyclohexanedimethanol. diol etc. In the present invention, polyesters containing alkylene terephthalate and/or alkylene naphthalate as main constituents are preferably used. Among these polyesters, polyethylene terephthalate, polyethylene-2,6-
Particularly preferred are copolymers in which 80 mol% or more of the total dicarboxylic acid component is terephthalic acid and/or 2,6-naphthalene dicarboxylic acid, including naphthalate, and 80 mol% or more of the total glycol component is ethylene glycol. . In this case, up to 20 mol% of the dicarboxylic acid of the total acid component can be the above-mentioned aromatic dicarboxylic acids; for example, aliphatic dicarboxylic acids such as adipic acid and sebacic acid; fatty acids such as cyclohexane-1,4-dicarboxylic acid. dicarboxylic acids and the like. In addition, 20 mol% or less of the total glycol component can be the above glycol other than ethylene glycol,
Or for example, hydroquinone, resorcinol,
Aromatic diols such as 2,2-bis(4-hydroxyphenyl)propane; aliphatic diols having an aromatic ring such as 1,4-hydroxymethylbenzene; polyalkylenes such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. It can also be glycol (polyoxyalkylene glycol) or the like. In addition, the polyester in the present invention includes, for example, an aromatic oxyacid such as hydroxybenzoic acid;
Also included are those containing a component derived from an oxycarboxylic acid such as an aliphatic oxyacid such as -hydroxycaproic acid in an amount of 20 mol % or less based on the total amount of the dicarboxylic acid component and the oxycarboxylic acid component. Furthermore, the polyester in the present invention contains a trifunctional or higher functional polycarboxylic acid or a polyhydroxy compound, such as trimellitic acid, in an amount within a substantially linear range, for example, an amount of 2 mol % or less based on the total acid components.
Copolymers of pentaerythritol and the like are also included. The above polyester is known per se, and can be produced by a method known per se. The polyester preferably has an intrinsic viscosity of about 0.4 to about 0.9 as measured as a solution in O-chlorophenol at 35°C. Moreover, the above-mentioned polyester may contain additives such as stabilizers, colorants, and antioxidants, as required. The biaxially oriented polyester film in the present invention is a biaxially oriented film manufactured from the above-mentioned polyester. This biaxially oriented film was first heated from 25°C to 40°C at a rate of 2°C per minute under a load of 400 g/ ^mm2 , then held at that temperature for 1 hour, and then left to cool. The residual strain rate in the longitudinal direction of the film determined after returning it to 25℃ is the absolute value.
Must be 0.020% or less. A preferable residual strain rate is 0.010% or less. If this characteristic is satisfied, the pancake will not unroll due to changes in temperature and humidity, especially changes in temperature, during storage or transportation, and the tape can be inserted into the cassette smoothly. It is possible to increase the commercial value of pancakes. In the film of the present invention, the longitudinal direction is the extrusion direction during film formation, and refers to the longitudinal direction of the tape, ie, the longitudinal direction of the film. This biaxially oriented film further needs to have a longitudinal stiffness of 0.8 g/mm ² or more. The preferred stiffness is 1.0 g/mm ² or more. When these characteristics are satisfied, the tape has increased rigidity and can be slit into a tape width from a web coated with a magnetic layer and rolled up into a good shape when wound into pancakes. If the stiffness is smaller than the above value, the tape will be weak and will "sag", making the running performance of the tape unstable, and the end surface of the tape will easily stretch or bend. It is difficult to roll the pancake into a pancake with good winding tension for both the inner layer and the outer layer, and therefore the shape retention of the pancake may be lost due to changes in temperature and humidity, especially changes in temperature. Furthermore, this biaxially oriented film has a center line average roughness (Ra) of 0.008 to 0.060 μm on the film surface, and a protrusion height (h: μm) and a protrusion height measured with a multiple interference reflection microscope (Tl monochromatic light). Number (pcs/mm ² ) is 2.0 > h > 1.5...5 pcs/mm ² or less 1.5h > 1.0...10 pcs/mm ² or less 1.0h > 0.75...1 to 25 pcs/mm ² 0.75h > 0.5 ...30 to 100 pieces/mm ² 0.5h>0.25...80 pieces/mm ² It is necessary to satisfy the following relationship. Center line average roughness
Ra is preferably 0.008 to 0.050 μm, more preferably 0.008 to 0.045 μm. When these characteristics are satisfied, pancakes with excellent shape retention against changes in environmental conditions, ie, changes in temperature and humidity, especially changes in temperature, can be obtained. According to the research results of the present inventors, the shape retention of pancakes due to changes in temperature is largely related to surface roughness as well as the influence of residual distortion of the film or tape due to temperature history. In other words, as the temperature rises and falls, a film or tape expands and contracts, resulting in permanent deformation and residual strain.On the other hand, in the case of a rolled object such as a pancake, the level of surface roughness causes the film or tape to expand and contract. Due to the effect of the slipperiness between the layers, when wound under the same winding conditions, a material with a very large surface roughness will be wound with gaps between the layers compared to a material with a small surface roughness (soft winding). Therefore, the expansion and contraction of the film or tape caused by the above-mentioned temperature change is absorbed by the gaps between the layers, and the overall shape of the pancake remains unchanged without any particular changes. Po can be held. From these points, it is necessary to satisfy the above film surface characteristics. If the film surface is too flat and does not satisfy the above properties, the tape will be wound tightly with no gaps between the layers, and the winding tension will be uneven between the layers, or the film may become loose due to changes in humidity, etc. Rolls (pancakes) are formed when the tape expands and contracts.
The winding pressure is concentrated in one part, and as a reaction, gaps are created in other parts.As a result, the shape of the pancakes is not maintained properly, the steps are shifted in some parts, and the workability when placing the pancake in the in-cassette is affected. This is undesirable because it will make the pancakes worse and lower the commercial value of the pancakes themselves. On the other hand, if the film surface is too rough and does not satisfy the above characteristics, it may not be possible to stably control the running properties of the tape, it may rub against the guides in the running path, causing scratches, or the surface properties of the magnetic layer may become flat. This is not preferable because it causes quality problems such as not being able to obtain desired electromagnetic conversion characteristics. The above-mentioned surface roughness of the biaxially oriented polyester film in the present invention is due to the presence of inert inorganic materials in the film.
It is preferable to form by adding organic fine particles or the like. When using these inert fine particles, 0.01 to 5% by weight of particles with an average particle size of 0.01 to 10 μm, and further 0.01% of particles with an average particle size of 0.03 to 4 μm.
It is preferable to add up to 1.5% by weight. On this occasion,
The inert inorganic or organic fine particles added may be a single component, or two or more components may be used simultaneously. In the present invention, the above-mentioned inert fine particles are preferably silicon dioxide (including hydrates, diatomaceous earth, silica sand, quartz, etc.); alumina;
Silicates containing 30% by weight or more of _SiO2 (e.g. amorphous or crystalline clay minerals, aluminosilicate (including calcined products and hydrates), hot asbestos, zircon, fly ash, etc.), Mg, Zn, Zr and Ti
oxides; sulfates of Ca and Ba; Li, Na and
Ca phosphate (including monohydrogen and dihydrogen salts);
Benzoates of Li, Na and K; Terephthalates of Ca, Ba, Zn and Mn; Mg, Ca, Ba, Zn,
Titanates of Cd, Pb, Sr, Mn, Fe, Co and Ni; Chromates of Ba and Pb; Carbon (e.g. carbon black, graphite, etc.); Glass (e.g. glass powder, glass beads, etc.); Ca and
Examples include Mg carbonate; fluorite; and ZnS. More preferably, anhydrous silicic acid, hydrated silicic acid, aluminum oxide, aluminum silicate (including calcined products, hydrates, etc.), monolithium phosphate, trilithium phosphate, sodium phosphate, barium phosphate, titanium oxide, and lithium benzoate. , double salts (including hydrates) of these compounds, glass powder, clay (including kaolin, bentonite, clay, etc.), talc, diatomaceous earth, calcium carbonate, and the like. Polyesters containing these inert particulates are usually reacted to form polyesters by
For example, inert fine particles (preferably as a slurry of glycol) are added to the reaction system at any time during the transesterification reaction or polycondensation reaction when using the transesterification method, or at any time when using the direct polymerization method. It can be manufactured by Preferably, inert fine particles are added to the reaction system at the beginning of the polycondensation reaction, for example, until the intrinsic viscosity reaches about 0.3. The biaxially oriented polyester film in the present invention is not particularly limited by its manufacturing method, but
Usually, polyester containing a predetermined proportion of fine particles is melted, extruded into a sheet through a slit die, cooled and solidified in a casting drum to form an unstretched sheet, and then the unstretched sheet is stretched biaxially. A product (film) is produced by heating the product (heat setting), adjusting the heat shrinkage rate in the transverse direction, and then subjecting it to longitudinal relaxation treatment. In this case, in order to suitably satisfy the requirements of the present invention,
For example, the stretching temperature is such that the first-stage stretching temperature (e.g. longitudinal stretching temperature: T ₁ ) is in the range of (Tg-10) to (Tg+45)°C (where Tg is the glass transition temperature of polyester), and the second-stage stretching temperature ( For example, the transverse direction stretching temperature: T ₂ ) is in the range of (T ₁ +15) to (T ₁ +40)°C, and the stretching ratio is 2.5 to 6.0 times in the first stage stretching, especially
It is preferably 3.5 to 5.5 times, and 2.5 to 4.0 times, particularly 2.8 to 3.7 times, in the second stage stretching. Further, the obtained biaxially stretched film is preferably heat set at a temperature in the range of 150 to 245°C, more preferably 170 to 240°C, for about 1 to 200 seconds. Furthermore, if necessary, the heat treatment conditions in the tenter are adjusted to adjust the heat shrinkage rate in the transverse direction, and then longitudinal relaxation treatment is performed. Adjustment of the heat shrinkage rate in the transverse direction is usually performed before the longitudinal relaxation treatment. It is usually adjusted during heat treatment in a tenter. For example, if the heat shrinkage rate in the lateral direction is insufficient, it is recommended to stretch the film in the width direction during the above heat treatment, and if the heat shrinkage rate is too large, it is recommended to relax the film in the width direction during the above heat treatment. good. More specifically, when the heat treatment temperature is 160°C, it is recommended to relax the entire width by 9 to 13%, and at 170°C
When the temperature is 180℃, it is recommended to relax 5 to 11%, when the temperature is 180℃, it is recommended to relax 1 to 8%, and when the temperature is 200℃, it is recommended to tense or relax 0 to 5%.
At 205℃, it is best to stretch or relax by 3 to -2%, and at 220℃, it is recommended to stretch or relax by 1 to -6%.
It is best to stretch or relax. Methods for loosening in the longitudinal direction include, for example, a floating treatment method using air force, heating under low tension, and relaxing in a non-contact state; relaxing by giving a speed difference between a heating roll and a cooling roll, each having a nip roll. There is a method in which the film is loosened in the vertical direction by sequentially pinching the advancing speed of the clip holding the film in the tenter, but any method can be used as long as it can be relaxed in the vertical direction. . The temperature when relaxing in the longitudinal direction is (Tg + 20) °C or higher (heat treatment temperature - 30) °C or lower, preferably (Tg
+30)℃ or more (heat treatment temperature -40)℃ or less.
At temperatures lower than (Tg + 20) °C, the thermal shrinkage rate near Tg cannot be sufficiently lowered, and at temperatures higher than (heat treatment temperature - 30) °C, although the amount of relaxation in the longitudinal direction is large, the shrinkage in the lateral direction is also large. This not only makes it impossible to satisfy the heat shrinkage rate in the transverse direction, but also deteriorates the mechanical properties in the transverse direction, further worsening uneven thickness, and the method in which relaxation is performed by the speed difference between two rolls. In this case, the width shrinkage on the heating roll causes scratches in the lateral direction on the film surface, which is undesirable. The amount of relaxation in the longitudinal direction varies depending on the heat treatment temperature, but the film tension at the time of relaxation is 10 kg/cm ² or more and 80 kg/cm ² or less, preferably
For example, in the case of a method in which relaxation is performed by a speed difference between two rolls, it is preferable to adjust the speed of the cooling roll relative to the heating roll so that the relaxation is 20 Kg/cm ² or ^more and 60 Kg/cm 2 or less. If the film tension is less than 10Kg/ ^cm2 , the film will sag and wrinkles will occur.
If the tension is greater than 80 kg/cm ² , the heat shrinkage rate cannot be sufficiently lowered. The biaxially oriented polyester film thus obtained usually exhibits the same surface characteristics on both the front and back sides.
In this case, the magnetic layer may be on the front or back side of the film. Such a magnetic layer may be of a coating type or a metal thin film type. This coated magnetic layer may be made of, for example, r-Fe ₂ O ₃ as magnetic powder, Co-containing r-
Metal powders such as Fe ₂ O ₃ , CrO ₂ , Fe-Co or Fe-Co-Ni, etc., and vinyl chloride resin, vinyl acetate resin, polyurethane, nitrocellulose, copolymers or mixtures thereof, or other resins. This includes those coated with a magnetic paint consisting of a binder mainly composed of and a small amount of lubricant. The metal thin film type magnetic layer includes, for example, one in which Co, Co--Cr, or other metal is deposited by a vacuum deposition method such as vacuum deposition, sputtering, or ion plating. <Examples> The present invention will be further explained below with reference to Examples.
Note that various physical property values and characteristics in the present invention were measured and defined as follows. (1) Film surface roughness (Ra) Measured according to JIS B 0601. Tokyo Precision Co., Ltd.
Stylus type surface roughness meter (SURFCOM 3B) manufactured by Co., Ltd.
A chart (film surface roughness curve) was drawn using a needle with a radius of 2μ and a load of 0.07g. A portion of measurement length L is extracted from the film surface roughness curve in the direction of its center line, the center line of this extracted portion is the X axis, and the direction of vertical magnification is Y.
When the roughness curve is expressed as Y=f(x) as the axis, the value (Ra: μm) given by the following equation is defined as the film surface roughness. Ra=1/L∫ ^L ₀ |f(x)|dx In the present invention, eight measurements were taken using a reference length of 0.25 mm, and Ra was expressed as the average value of the five measurements, excluding three from the largest value. (2) Number of surface projections Aluminum is uniformly vacuum-deposited on the surface of the film to a thickness of 400 to 500 Å or less, and collodion is sewn and pasted on the opposite non-vapor-deposited surface (film surface), and dried. Observe an arbitrary 100 ^cm2 of the aluminum deposited surface at 100x magnification using a Tl monochromatic light multiple interference reflection microscope (e.g., manufactured by Cayl Zeiss JENA), and measure the height of the protrusions in the microscope field. The number of protrusions each having interference fringes produced is counted. (3) Stiffness Using a loop stiffness tester “Type D” manufactured by Toyo Seiki Co., Ltd., the loop length was 50 mm.
Measured with a crushing amount of 10 mm and a load of 10 gr, and the average value of n = 5 is expressed in units of g/mm ² . (4) Residual strain rate The thermomechanical tester “type” manufactured by Shinku Riko Co., Ltd.
TM-3000", hold both ends of a specified test piece with chucks, raise the temperature from 25°C to 40°C at a rate of 2°C per minute under a specified load, and then leave it for 1 hour.
After holding for a while, let it cool and return to 25℃. Read the dimensional changes before and after temperature treatment from the chart and calculate the residual strain rate using the formula below. In addition, measurements were taken for 5 samples, and the average value is expressed as an absolute value. Residual strain rate (%) = (Original length) - (Length after treatment) / (Original length) x 100 However, test piece length (between chucks): 15.0 mm Test piece width: 3.8 mm Load: 400 g/mm ² , 900g/ ^mm2 . (5) Average particle size of lubricant particles Shimadzu CP-50 Centrifuge Particle Size Analyzer
Measured using fugal particle size analyzer). From the cumulative curve of particles of each particle size and their abundance calculated based on the obtained stretching sedimentation curve, 50
Read the particle size that corresponds to the mass percent and take this value as the average particle size. (6) Pancake rolling format Place the pancake horizontally on a flat plate that is hard and difficult to conduct heat, then place the pancake in a constant temperature oven that can change the temperature between 40℃ and -10℃.
When left at room temperature (25°C) after being kept at a temperature of 40°C or -10°C for a certain period of time (24 hours), the shape of the pancakes collapses as the temperature changes (partial shifts occur). Visually judge the condition. At this time, the temperature of the pancake is measured using a non-contact temperature detector (pyroelectric infrared radiation thermometer "ER2008" manufactured by Matsushita Electric Co., Ltd.) at a distance of about 1 cm from the surface of the pancake. The visual judgment criteria are as follows. ○: The shape of the pancakes does not change appreciably due to temperature changes, and even if the pancakes are subsequently placed in a cassette at room temperature, no trouble occurs in running the tape. △: The form of the pancake shows some streak-like steps shifting due to temperature changes, but when the pancake is then placed in a cassette at room temperature, there is hardly any trouble in running the tape. ×...The shape of the pancake shows clear steps shifting due to temperature changes, making it impossible to place the pancake in a cassette. Example 1 Ethylene glycol (hereinafter abbreviated as EG) 90
After adding 10 parts by weight of calcium carbonate (average particle size 1.5 μm) to the parts by weight, mixing and stirring were performed to obtain a slurry. Next, 100 parts by weight of dimethyl terephthalate and
After transesterifying 70 parts by weight of EG in a conventional manner using 0.035 parts by weight of manganese acetate tetrahydrate as a catalyst, the calcium carbonate obtained above (concentration 0.4% by weight) was transesterified.
(copolymer) was added under stirring. Subsequently, 0.03 parts by weight of trimethyl phosphate and 0.03 parts by weight of antimony trioxide were added, and then a polycondensation reaction was carried out in a conventional manner under high temperature vacuum to obtain polyethylene terephthalate pellets having an intrinsic viscosity of 0.620. Furthermore, after drying the polyethylene terephthalate pellets at 170°C for 3 hours, the polyethylene terephthalate pellets were fed to an extruder hopper.
The molten polymer is melted at a melting temperature of 280-300℃ and passed through a 1mm slit die to give a surface finish of 0.3S.
The film was formed and extruded on a rotating cooling drum with a surface temperature of 20° C. to obtain an unstretched film with a thickness of about 170 μm. The unstretched film thus obtained was heated at 75°C.
Preheat to 15mm between low speed and high speed rolls.
Heated from above with one IR heater with a surface temperature of 900℃, and the low and high roll surface speed caused the temperature to rise to 1.6
Double stretching, then quenching, heating again to the above temperature conditions, stretching 1.5 times, and then quenching again.
The film was stretched in the longitudinal direction by repeating heating stretching (total re-stretching ratio: 3.6 times). This longitudinally stretched film is
Next, it was stretched 3.9 times in the transverse direction at a temperature of 105°C in hot air, and then heat-treated at 230°C for 7 seconds to a thickness of 12 μm.
A biaxially oriented film with a thickness of . Note that the stretching speed at this time was 20 m/min. Next, this biaxially oriented film is heated with a heated roll.
After heating to 120°C, relaxation treatment was applied between a cooling roll and a tension corresponding to contraction according to the heat treatment temperature. The properties of the obtained film are shown in Table 1. Furthermore, the film obtained in this way has
A paint consisting of 70 wt% r-Fe ₂ O ₂ and 30 wt% binder was applied. This binder is 5wt%
urethane rubber, 3.5wt% nitrocellulose,
It contains 1.5wt% of vinyl chloride resin, 90wt% of methyl ethyl ketone, and 15wt% of isocyanate compound added as a hardening agent to the resin.
A 600 mm wide magnetic material coated original fabric was obtained by coating using a conventional method. This coating material was subjected to 3/20 inch microslits using a shear type slitter [RT model manufactured by Nishimura Seisakusho Co., Ltd.]. The characteristics of the obtained magnetic tape were measured by the following method, and the results are shown in Table 1. Measuring method and evaluation of audio characteristics: Input/output sensitivity and S/N ratio (signal -
Noise) was measured, and the in-house standard tape was used as a comparison, and each was evaluated as follows. Γ Input/output sensitivity ○…Output sensitivity is not lower than input sensitivity. Δ...The output sensitivity is slightly lower than the input sensitivity, but there is no problem in practical use. ×... Output sensitivity is low compared to input sensitivity and cannot be used. ΓS/N ratio ○: Same as the standard tape for comparison, no problem. △...Inferior to the comparative tape, but no problem in practical use. ×...Inferior to the standard tape for comparison and cannot be used. As is clear from Table 1, the tape of Example 1 had good tape quality in terms of pancake winding form and audio characteristics. Examples 2 to 7 Films were obtained in the same manner as in Example 1, except that the type, average particle size, and amount of the added lubricant were as shown in Table 1. It was coated and slit to 3/20 inch to obtain a magnetic tape. The various evaluation results are shown in Table 1, and the obtained base film had good tape quality in both pancake winding form and audio characteristics.

[Table] Comparative Example 1 A 3/20-inch magnetic tape was prepared in the same manner as in Example 1, except that the step of subjecting the biaxially oriented film to a relaxation treatment was omitted. The results are shown in Table 2. This product had a high residual distortion rate among the quality of the base film, and as a result, the tape quality was poor in pancake winding form, causing trouble in running during in-casing, and was unusable. Comparative Examples 2 and 3 The same procedure as Comparative Example 1 was carried out except that the type, average particle size, and amount of added lubricant were changed as shown in Table 2. The results are shown in Table 2. This tape had a large residual strain rate, and as a result, the tape quality was poor in pancake winding form, causing trouble in running the in-cassette, and was unusable. Comparative Example 4 A tape was prepared in exactly the same manner as in Example 1, except that the type, average particle size, and amount of the added lubricant were changed as shown in Table 2. The results are shown in Table 2. The surface roughness (Ra) of this base film was too flat, and the height and number of protrusions were too low, causing the pancakes to crumble due to temperature changes and having poor shape retention as pancakes. It was hot. Comparative Example 5 A tape was prepared in exactly the same manner as in Example 1, except that the type, average particle size, and amount of the added lubricant were changed as shown in Table 2. The results are shown in Table 2. The quality of this base film is that the surface roughness (Ra) is rough, the height and number of protrusions are large, it is slippery, and the shape of the pancake is poor.
Moreover, the audio characteristics were also poor.

【table】

Claims (1)

[Scope of Claims] 1. A biaxially oriented polyester film having a thickness of 3 to 25 μm, the center line average roughness (Ra) of the film surface being 0.008 to 0.060 μm, and a multi-interference reflection microscope (Tl Protrusion height (h: μm) and number of protrusions (pcs/mm ² ) measured using monochromatic light (monochromatic light) are 2.0>h>1.5...5 pieces/ ^mm2 or less 1.5h>1.0...10 pieces/ ^mm2 or less 1.0 h＞0.75...1 to 25 pieces/mm ² 0.75h＞0.5...30 to 100 pieces/mm ² 0.5h＞0.25...80 pieces/mm ² Satisfies the relationship above, and the residual strain rate in the longitudinal direction of the film (Residual strain rate determined after heating from 25℃ to 40℃ under a load of 400g/mm ² , holding at this temperature for 1 hour, and returning to 25℃ by cooling again) is 0.02% as an absolute value.
A biaxially oriented polyester film having a stiffness of 0.8 g/mm ² or more in the longitudinal direction of the film.