JPH04331244A - Biaxially oriented polyester film - Google Patents

Abstract

PURPOSE:To provide a biaxially oriented polyester film which is excellent in slip properties, abrasion resistance, and scratching resistance and is especially useful as a base film for magnetic recording tapes. CONSTITUTION:A biaxially oriented polyester film which has the ratio of a static fictional coefficient muS8, as measured under a load of 800g, of 0.5-0.9 to its static frictional coefficient, muS4 as measured under a load of 400g being 1.3-3.0. The film has a surface hardness of 21 or more on at least one side thereof.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a biaxially oriented polyester film, and more particularly to a biaxially oriented polyester film that has excellent slip properties, abrasion resistance and scratch resistance, and is particularly useful as a base film for magnetic recording tapes.

0002

BACKGROUND OF THE INVENTION Due to its excellent properties, biaxially oriented polyester films are used in many applications such as magnetic tapes, electricity, photography, metallization, and packaging. In particular, because of its high strength and elastic modulus, it is widely used as a base film for magnetic recording media such as video tapes, audio tapes, computer tapes, floppy disks, etc.

[0003] These fields of application have recently become more popular due to high-density recording,
As the demand for higher quality increases, there is an increasing demand for the base polyester film to have a flat surface.

However, when the surface of the film becomes flat, the friction coefficient of the film becomes high in applications such as magnetic tapes, causing problems such as poor running and easy scratching. In addition, if the film surface becomes flat, the winding appearance of the film during the process of winding the film into a roll during film production will deteriorate significantly, making it difficult to obtain a film roll with a good winding appearance. As productivity increases, film winding speeds are increasingly increased,
Further, it is necessary to increase the width, but as the speed increases and the width increases, there is a problem in that it becomes increasingly difficult to obtain a film roll with a good shape.

[0005] Defects in the winding appearance of film rolls include: (1) bump-like protrusions on the roll, (2) wrinkles in the longitudinal direction of the film, and (2) misalignment of the end surface. (2) is likely to occur when the film is wound with high tension to prevent bump-like protrusions, and (2) is likely to occur when winding up a flat film and the air layer created between the films has difficulty escaping.

[0006] Therefore, the base polyester film is required to have not only flatness but also excellent slipperiness and air escape properties in order to obtain a good film winding appearance. In particular, when winding a film at higher speeds and wider widths, better air escape performance is required.

Conventionally, methods for improving the slipperiness of films include adding inorganic particles such as silicon oxide or calcium carbonate to polyester, or precipitating fine particles containing calcium, lithium or phosphorus within the polymerization system during polyester synthesis. A method has been proposed. In either method, when polyester is formed into a film, projections are formed on the surface of the film due to fine particles, thereby improving the slipperiness of the film.

However, in the above-mentioned method of improving the slipperiness of a film using protrusions made of fine particles, the slipperiness usually improves as the film surface becomes rougher, but on the other hand, due to the roughening, For example, in applications for magnetic recording media, the surface of the magnetic layer tends to become rough after the magnetic paint is applied, and the electromagnetic conversion characteristics tend to deteriorate.

[0009] One of the measures to solve these contradictory problems of flatness, slipperiness, and air escape property is to utilize composite inorganic particles in which large-sized particles and small-sized particles coexist. Many proposals have also been made. However, these means also have problems, and as they are, it is difficult to meet the demands for higher grades of magnetic recording media, such as higher density and higher quality. The reason for this is that the size of the large particles used in composite inorganic particles is too large for the quality required for high-grade grades, and the larger the particles, the higher the protrusions on the film surface. In addition, the larger the particles, the higher the protrusions on the film surface and the larger the voids surrounding the particles. Scraping occurs during the cleaning process or calendar processing process, causing dropouts (missing information that occurs during recording and playback), and furthermore, calendar stains during the processing process and dust fabric stains for cleaning the base film surface. This causes a large loss in the properties of the magnetic recording medium.

[0010] In recent years, the above-mentioned processing speed has tended to become faster and faster, and it has become even more important to improve the abrasion resistance of the film.

[0011]

SUMMARY OF THE INVENTION The present inventor has solved the above-mentioned problems, and has created a film that has both flatness and good film winding appearance that can be applied to high-quality magnetic recording applications, and has good durability. As a result of intensive research to develop a film that is resistant to abrasion and scratches, we found that a film whose coefficient of static friction depends on the load during measurement is within a specific range, and whose surface hardness has a specific value is resistant to abrasion. The inventors have discovered that this material has excellent properties such as durability and scratch resistance, and have arrived at the present invention.

Therefore, an object of the present invention is to provide a biaxial magnetic recording medium that has excellent slip properties, abrasion resistance, scratch resistance, etc., while maintaining flatness that is compatible with high-density recording and high-quality magnetic recording media. An object of the present invention is to provide an oriented polyester film.

[0013]

[Means for Solving the Problems] In order to achieve the above object, the present invention consists of the following configuration.

The static friction coefficient μS8 when measured with a load of 800 g is 0.5 to 0.9, and the ratio of this to the static friction coefficient μS4 when measured with a load of 400 g (μS8/μS
4) is from 1.3 to 3.0, and at least one surface of the film has a surface hardness of 21 or more.

The static friction coefficient in the present invention is the static friction coefficient of the film, and the load of the thread metal is 8.
The static friction coefficient when the load is 00g is μS8, and the static friction coefficient when the thread metal load is 400g is μS4.
Display as .

In the present invention, the biaxially oriented polyester film must have a static friction coefficient μS8 of 0.5 to 0.9. Preferably it is 0.55 to 0.85, particularly preferably 0.71 to 0.85. If the static friction coefficient μS8 is less than 0.5, end face deviation tends to occur in the film winding state, whereas if it exceeds 0.9, bump-like protrusions will occur in the film winding state, which is not preferable.

The biaxially oriented polyester film further has a ratio of the static friction coefficient μS8 to the static friction coefficient μS4 (μ
S8/μS4) needs to be 1.3 to 3.0. This ratio is preferably between 1.5 and 2.6, particularly preferably between 1.7 and 2.3. If this ratio (μS8/μS4) is less than 1.3, calendering properties will be poor, while if it exceeds 3.0, running friction will be poor even if the surface properties are good, which is not preferable. Furthermore, the static friction coefficient μS4 is 0.
It is preferable that it is 3-0.7.

In the present invention, the biaxially oriented polyester film further needs to have a surface hardness of 21 or more. If the surface hardness is less than 21, the scratch resistance will be poor, which is not preferable. From this point of view, the surface hardness is preferably 23 or more, more preferably 28 or more.

In the present invention, polyesters constituting the film are preferably those whose main constituents are alkylene terephthalate and/or alkylene naphthalate.

Among such polyesters, examples include polyethylene terephthalate and polyethylene-2,6-naphthalate, and for example, 80 mol% or more of the total dicarboxylic acid component is terephthalic acid and/or 2,6-naphthalate.
It is naphthalene dicarboxylic acid, and 8 of the total glycol components.
A copolymer in which 0 mol % or more is ethylene glycol is preferred. At that time, the dicarboxylic acid that accounts for 20 mol% or less of the total acid component is an aromatic dicarboxylic acid other than terephthalic acid and/or 2,6-naphthalene dicarboxylic acid, such as isophthalic acid, diphenyl dicarboxylic acid, diphenylether dicarboxylic acid, diphenylethanedicarboxylic acid, It can be diphenylsulfonic dicarboxylic acid, diphenylketone dicarboxylic acid, etc., and can also be aliphatic dicarboxylic acids such as adipic acid, sebacic acid, etc., alicyclic dicarboxylic acids such as cyclohexane-1,4-dicarboxylic acid, etc. . Glycols that account for 20 mol% or less of the total glycol component include glycols other than ethylene glycol, such as trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol,
decamethylene glycol, etc., alicyclic glycols such as cyclohexanedimethanol, aromatic diols such as hydroquinone, resorcinol, 2,2-bis(4-hydroxyphenyl)propane, etc., 1,4
- Aliphatic diols containing an aromatic ring such as dihydroxymethylbenzene, polyalkylene glycols (polyoxyalkylene glycols) such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol, etc. can also be used.

[0021] Furthermore, the polyester in the present invention includes:
For example, aromatic oxyacids such as hydroxybenzoic acid, ω-
Also included are those in which a component derived from an oxycarboxylic acid such as an aliphatic oxyacid such as hydroxycaproic acid is copolymerized or combined 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 of the present invention contains a trifunctional or higher functional polycarboxylic acid or a polyhydroxy compound, such as a trimellitic acid, in an amount within a substantially linear range, for example, an amount of 2 mol % or less based on the total acid component. Copolymers of acids, pentaerythritol, etc. are also included.

The above polyester is known per se and can be produced by a method known per se.

The above-mentioned polyester preferably has an intrinsic viscosity of about 0.4 to 0.8, preferably 0.5 to 0.8, measured as a solution in o-chlorophenol at 35°C.
7 is more preferred, and 0.55 to 0.65 is particularly preferred.

The biaxially oriented polyester film of the present invention has many fine protrusions on its surface. According to the present invention, these many fine protrusions originate from a large number of inert fine particles dispersed and contained in the polyester.

[0026] The polyester containing dispersed inert fine particles is usually prepared at any time during the reaction to form the polyester, for example, during the transesterification reaction when using the transesterification method, or during the polycondensation reaction, or when using the direct polymerization method. can be prepared by adding inert microparticles (preferably as a slurry in glycol) into the reaction system at any time. 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. And 2
It is preferable to contain at least one kind of inert fine particles.

[0027] In the present invention, the inert fine particles are contained in polyester and have a specific surface area of 50 to 120 m2/g,
Inert particles A with a total pore volume of 0.5-1.0 ml/g and an average particle size dA of 0.02-0.3 μm
, the Mohs hardness is less than 3, and the average particle size is dB.
is within the range of 0.2 to 1.5 μm and is preferably used in combination with inert particles B having an average particle diameter dA larger than the average particle diameter dA of the inert particles A.

The inert particles A in the present invention have a specific surface area of 50 to 120 m2/g, more preferably 60 to 100 m2/g.
It is preferable that it is m2/g. If this specific surface area is too small, the primary particle size will become too large, and although scratch resistance is good, it will easily generate shavings, and there will be more damage to the metal guide with a good surface finish.On the other hand, if the specific surface area is too large, the primary particle size will This is not preferable because it becomes too small and can no longer form sufficient protrusions, which reduces the generation of scraping powder but worsens scratches.

[0029] The inert particles A further have a total pore volume of 0.5 to 1.0 ml/g, more preferably 0.6 to 0.9.
Preferably it is ml/g. If the total pore volume is too small, the particles will no longer be porous, resulting in poor affinity with polyester and the generation of abrasion powder. On the other hand, if the total pore volume is too large, the particles will become brittle and lose their shape during stretching. , which is not preferable because the scratch resistance deteriorates.

The inert particles A preferably have an average particle size (average secondary particle size: dA) of 0.02 to 0.3 μm. If this average particle size is too small, the particles will become too fine and will not be able to form the fine protrusions necessary for scratch resistance and abrasion resistance, while if it is too large, agglomeration will be too large and abrasion powder will easily occur. Undesirable. In addition, the amount of inert particles A added is 0.05 to 1.5% by weight,
Furthermore 0.1-1.0% by weight, especially 0.23-0.58
Preferably, it is % by weight. If the amount added is too small, the effect of adding fine particles will be small, while if it is too large, particles will overlap and scraping powder will be likely to be generated, which is not preferable.

As the inert particles A in the present invention, aluminum oxide, silicon oxide, and titanium oxide are particularly useful, and among them, aluminum oxide is preferred because it has a high effect of improving abrasion resistance and scratch resistance. The crystal form of this aluminum oxide is preferably selected from α, θ, and γ types, and it is more preferable that the α type crystal form is included in the θ type or γ type crystal form, especially the θ type. Preferably.

The Mohs hardness of the inert particles B in the present invention is preferably less than 3, more preferably 2 or less, particularly 1 or less. If the Mohs hardness is too large, the inert particles B may fall off, probably because the stress concentration on the inert particles B that forms protrusions increases when high stress (high shear force) is applied during the calendering process during the processing process. This is not preferable because it becomes easy to scrape and generates powder. When the Mohs hardness decreases to less than 3, it is presumed that this is because the inert particles B are deformed when stress is concentrated on the surface protrusions during the calendering process, but fewer particles fall off and less shavings are produced.

The average particle diameter (dB) of the inert particles B is 0.2 to 1.5 μm, more preferably 0.35 to 1.0 μm, particularly 0.45 to 0.9 μm, especially 0.65 to 0.0 μm. It is preferably 8 μm, which is larger than the average particle size of the inert particles A. If this average particle diameter (dB) is too small, even if the static friction coefficient μS8 is set to 0.9 or less, end face deviation tends to occur in the film wound state, which is not preferable. On the other hand, if the average particle size (dB) is too large, the abrasion resistance will deteriorate even if the Mohs hardness of the particles is less than 3, which is not preferable. In addition, inert particles B are 0.0005 to 0.02
9% by weight, furthermore 0.0005-0.02% by weight, especially 0.0005-0.01% by weight, especially 0.0005-0
．． 0.005% by weight is preferred. If the amount added is too small, the coefficient of static friction will increase and bump-like protrusions will occur, which is not preferable. On the other hand, if the amount added is too large, end face misalignment tends to occur, which is not preferable. The amount of inert particles B added is preferably 10% by weight or less, more preferably 5% by weight or less, particularly 4% by weight or less, especially 2.5% by weight or less, based on the total amount of inert particles. If this ratio is too high, the scratch resistance will not be high, which is not preferable.

The inert particles B in the present invention are not particularly limited as long as they satisfy the above characteristics, but organic particles such as particles made of crosslinked polymers such as silicone resin, crosslinked polystyrene resin, and crosslinked acrylic resin are used. This is preferable because the particles can easily obtain the above characteristics.

In the present invention, silicone resin particles have the following formula (A) RxSiO2 -x/2...
(A) Here, R is a hydrocarbon group having 1 to 7 carbon atoms, and x is a number of 1 to 1.2.

R in the above formula (A) is a hydrocarbon group having 1 to 7 carbon atoms, such as an alkyl group having 1 to 7 carbon atoms,
Phenyl group or tolyl group is preferred. Carbon number 1-7
The alkyl group may be linear or branched, such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, n-pentyl, n- -Heptyl, etc. can be mentioned.

Among these, R is preferably methyl and phenyl, with methyl being particularly preferred.

In the above formula (A), x is a number from 1 to 1.2. When x is 1 in the above formula (A), the above formula (A) is the following formula (A)-1 RSiO1.5...
...(A)-1 Here, the definition of R is the same as above.

The composition of the above formula (A)-1 includes the following structural part in the three-dimensional polymer chain structure of the silicone resin;

00
40]

[Chemical formula 1] It originates from.

In addition, when x in the above formula (A) is 1.2, the above formula (A) becomes the following formula (A)-2R1.2
SiO1.4...(A)-
2 Here, the definition of R is the same as above.

The composition of the above formula (A)-2 is the above formula (A)-
0.8 mol of the structure of 1 and the following formula (A)' R2 SiO...
...(A)' Here, the definition of R is the same as above and can be understood to consist of 0.2 moles of the structure represented by.

The above formula (A)' represents the following structural part in the three-dimensional polymer chain of silicone resin;

[0044]

[Chemical 2] It originates from

As understood from the above explanation, the composition of the above formula (A) in the present invention is, for example, the composition of the above formula (A) -
1, or the above formula (A
It can be seen that the structure consists of a structure in which the structure of )-1 and the structure of the above formula (A)-2 coexist in a state where they are randomly bonded in an appropriate ratio.

[0046] The silicone resin particles in the present invention are
Preferably, in the above formula (A), x has a value between 1 and 1.1.

In the present invention, the crosslinked polystyrene resin particles include styrene derivative monomers such as styrene monomer, methylstyrene monomer, α-methylstyrene monomer, and dichlorostyrene monomer, as well as conjugated diene monomers such as butadiene and acrylonitrile. unsaturated nitrile monomers, monomers such as methacrylic esters such as methyl methacrylate, functional monomers such as unsaturated carboxylic acids, monomers with hydroxyl such as hydroxyethyl methacrylate, and epoxide groups such as glycidyl methacrylate. and one or more monomers selected from unsaturated sulfonic acids, etc., and a polyfunctional vinyl compound such as divinylbenzene, ethylene glycol dimethacrylate, etc. as a crosslinking agent to give the polymer particles a three-dimensional structure. Trimethylolpropane triacrylate, diallyl phthalate, etc. are emulsion polymerized in an aqueous medium in which a water-soluble polymer is dissolved as a protective colloid to prepare an emulsion of polymer particles, which is then crushed using a jet mill. , and then by classification.

The crosslinked polystyrene resin particles in the present invention do not dissolve or melt during the polymerization of the aromatic polyester, and do not melt when the polymer is melted during film molding.

Furthermore, in the present invention, the crosslinked acrylic resin particles include unsaturated nonionic monomers such as (meth)acrylic acid esters (eg, methyl methacrylate, butyl acrylate, etc.), unsaturated monomers such as maleic acid dialkyl esters, etc. Dicarboxylic acid esters, unsaturated vinyl compounds such as styrene, unsaturated nitriles such as acrylonitrile, functional monomers such as unsaturated carboxylic acids, hydroxyl-containing monomers such as hydroxyethyl methacrylate, epoxide groups such as glycidyl methacrylate. and one or more monomers selected from unsaturated sulfonic acids, etc., and a polyfunctional vinyl compound such as divinylbenzene, ethylene glycol dimethacrylate, etc. as a crosslinking agent to give the polymer particles a three-dimensional structure. An emulsion of polymer particles is prepared by emulsion polymerization of trimethylolpropane triacrate, diallyl phthalate, etc. in an aqueous medium in which a water-soluble polymer is dissolved as a protective colloid, and the polymer particles are recovered from this emulsion and dried. Then, it is obtained by crushing it in a jet mill and then classifying it.

[0050] The crosslinked acrylic resin particles in the present invention are
It does not dissolve or melt during polymerization of polyester,
Moreover, it does not melt when the polymer is melted during film molding.

The biaxially oriented polyester film of the present invention can be produced in accordance with conventional methods for producing biaxially oriented films. For example, polyester containing inert particles is melt-extruded and rapidly cooled to obtain an unstretched film.
The unstretched film can then be biaxially stretched, heat-set, and, if necessary, subjected to relaxation heat treatment. At this time, the surface characteristics, density, thermal shrinkage rate, and other properties of the film vary depending on the stretching conditions and other manufacturing conditions, so they are appropriately selected as necessary.

For example, the unstretched film is uniaxially (vertical or transverse) [Tg-10] to [Tg+60]
℃ (where Tg is the glass transition temperature of polyester) and stretched at a ratio of 2.5 times or more, preferably 3 times or more, and then stretched in the direction perpendicular to the stretching direction from Tg to [Tg+7
It is preferable to stretch the film at a temperature of 0]° C. at a ratio of 2.5 times or more, preferably 3 times or more. Furthermore, it may be stretched again in the longitudinal direction and/or the transverse direction, if necessary. In this way, the total stretching ratio is preferably 9 times or more as an area stretching ratio, more preferably 12 to 35 times, and 15 to 25 times.
Double is particularly preferred. Furthermore, the biaxially oriented film is
Temperature from [Tg+70]℃ to [Tm-10]℃ (however,
Tm: melting point of polyester) can be heat-fixed,
For example, 180°C to 250°C is preferable. Heat setting time is 1
~60 seconds is preferred.

[0053] Various physical property values and characteristics in the present invention were measured and defined as follows.

(1) Average particle size (d) Centrifugal Particle Size Analyzer Model CP-50 manufactured by Shimadzu Corporation (Centrifugal Pa
ticle Size Analyzer). From the integrated curve of particles of each particle size and their abundance calculated based on the obtained centrifugal sedimentation curve, the particle size corresponding to 50 mass percent is read, and this value is taken as the above average particle size (Book "Particle Size Measurement "Technology" published by Nikkan Kogyo Shimbun,
1975, pp. 242-247).

(2) Film running friction coefficient (μk) Measured as follows using the apparatus shown in FIG. 1. In FIG. 1, 1 is an unwinding reel, 2 is a tension controller, 3, 5, 6, 8, 9 and 11 are free rollers, 4
is tension detector (inlet), 7 is stainless steel SUS
Fixing rod made of 304 (outer diameter 5 mmφ. Surface roughness Ra = 0.0
2 μm), 10 is a tension detector (outlet), 12 is a guide roller, and 13 is a take-up reel.

[0056] In an environment of temperature 20°C and humidity 60%, width 1/
A film cut into 2-inch pieces is attached to the fixing rod 7 at an angle θ=(
152/181) π radians (152°) and move (friction) at a speed of 200 cm/min. Outlet tension (T2: g) when tension controller 2 is adjusted so that inlet tension T1 is 40g
is detected by the outlet tension detector after the film has traveled 90 m, and the running friction coefficient μk is calculated using the following formula.

[0057]

[Math 1]

(3) Film surface roughness (Ra) This is the value defined by JIS-B0601 as center line average roughness (Ra), and in the present invention, a stylus type surface roughness meter manufactured by Kosaka Institute Co., Ltd. (SURFCORDER SE-30C). The measurement conditions are as follows: (a) Stylus tip radius: 2 μm (b) Measurement pressure: 30 mg (c) Cutoff: 0.08 mm (d) Measurement length: 1.0 mm (e) Summary of data Method: Measure the same sample 5 times, remove the largest value, and calculate the average value of the remaining 4 data to 5 decimal points.
Round off the digits and display up to the fourth digit after the decimal point.

(4) Static friction coefficient (μs) A fixed glass is placed on the underside of two overlapping films, and the lower side of the overlapping films (the film in contact with the glass plate) is rolled at a constant speed. (approximately 10 cm/min), fix the detector to one end of the upper film (opposite to the lower film pulling direction), and measure the tensile force between the films (approximately 10 cm/min).
F) is detected. The thread used at that time has a lower surface area of 50 cm2 (80 mm x 62.5 mm),
The surface in contact with the film is 80° neoprene rubber,
Its weight (P) is 400g or 800g.

[0060] The static friction coefficient is calculated by the following formula,

006
1]

[Equation 2] When the load is 800g, μS8, when the load is 400g, μ
S4 is displayed.

(5) Calendar abrasion resistance The abrasion resistance of the running surface of the base film was evaluated using a three-stage mini super calender. The calender is a three-stage calender with nylon rolls and steel rolls, the processing temperature is 80℃, and the linear pressure applied to the film is 200kg/c.
m, film speed is 100 m/min. After running the running film for a total length of 4,000 m, the abrasion resistance of the base film is evaluated based on the dirt that adheres to the top roller of the calendar: <5-level evaluation> Grade 1: No stains on the nylon roll Grade 2: Almost no stains on the nylon roll None Grade 3: There is some dirt on the nylon roll, but it can be easily removed with a dry cloth.Grade 4: The nylon roll is dirty and difficult to remove with a dry cloth.
Grade 5: Can be wiped off with a solvent such as acetone: The nylon roll is extremely dirty and is difficult to remove even with a solvent.

(6) Specific surface area and total pore volume of powder The specific surface area and total pore volume are measured by the BET method using Autosorb-1 manufactured by Quantachrome.

(7) Scratch Judgment Slit the base film into 1/2 inch width and
) The film was placed on the same fixed rod at an angle of 30° and run for 200 m at a film speed of 4 m/sec to measure the thickness, depth, and number of scratches on the surface of the 1/2 inch wide base film. Judgment is made in the following five stages: <Five-stage evaluation> Grade 1: No scratches are observed on the 1/2 inch wide base film. Grade 2: Almost no scratches are observed on the 1/2 inch wide base film. None (1 to 2 scratches) 3rd grade: Scratches are observed on the 1/2 inch wide base film (3 to 8 scratches) 4th grade: Several thick scratches are observed on the 1/2 inch wide base film (9 to 8 scratches) 19) Grade 5: Many thick and deep scratches are observed on the entire surface of the 1/2 inch wide base film (20 or more).

(8) The wound film is wound onto a roll having a width of 1200 mm and a length of 12000 m at a speed of 250 m/min, and the appearance of this wound roll is inspected in detail and graded into grades 1 to 5.

The end face deviation is graded as follows based on the distance of the end face deviation in the width direction.

Regarding the lump-like protrusion, it is a lump-like protrusion with a major diameter of 2.
The number of pieces larger than mm is counted and graded as follows.

[0068]

[Table 1]

(9) Film surface hardness Insert a diamond square pyramid indenter (the tip is pyramid-shaped, and the facing angle is 136°) on the base film on the glass plate.
, the opposite helix angle is 148°7') and the load (P) is 25g.
The hardness is defined as the value obtained by dividing the load by the area of the depression created after pressing for 10 seconds.

[0070]

[Math 3]

[0071]

[Examples 1 to 6, Comparative Examples 1 to 3] Dimethyl terephthalate and ethylene glycol, manganese acetate as a transesterification catalyst, antimony trioxide as a polymerization catalyst,
Phosphorous acid was used as a stabilizer, and as a lubricant in Example 1, 0.0.
4% by weight, and 0.002% by weight of fine particles made of silicone resin having a Mohs hardness of 2 and an average particle size of 0.9 μm. The particles were polymerized in a conventional manner to obtain polyethylene terephthalate having an intrinsic viscosity (orthochlorophenol, 35° C.) of 0.62.

[0072] After drying the polyethylene terephthalate pellets at 170°C for 3 hours, they were fed to an extruder hopper.
The molten polymer was melted at a melting temperature of 280 to 300°C, extruded through a 1 mm slit die onto a rotating cooling drum with a surface temperature of 20°C, and rapidly cooled to obtain an unstretched film.

The unstretched film thus obtained was preheated at 75°C, and further rolled for 10 m between low speed and high speed rolls.
The film was heated from above with an IR heater with a surface temperature of 850°C, stretched to 3.1 times, rapidly cooled, then fed to a stenter, and stretched 4.0 times in the transverse direction at 100°C.

[0074] Further, successively, preheating at 110°C,
The film was stretched 1.8 times in the machine direction between low speed and high speed rolls. The obtained biaxially stretched film was heat set with hot air at 215° C. for 4 seconds to obtain a biaxially oriented polyester film with a thickness of 9 μm.

These properties are shown in Tables 1 and 2.

It can be seen that the film according to the present invention has excellent scratch resistance and abrasion resistance, and also has a good winding appearance as a film. The F-5 value of these films is 19 kg/mm2 in the vertical direction and 12 kg/mm2 in the horizontal direction.
Met.

[0077]

[Table 2]

[0078]

[Table 3]

[0079]

According to the present invention, it is possible to provide a biaxially oriented polyester film which has excellent slip properties, abrasion resistance and scratch resistance and is particularly useful as a base film for magnetic recording tapes.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an apparatus for measuring the running friction coefficient of a film.

[Explanation of symbols]

1: Unwinding reel 2: Tension controllers 3, 5, 6, 8, 9 and 11: Free roller 4: Tension detector (inlet), 7: Fixed rod made of stainless steel SUS304 (outer diameter 5 mm
φ, surface roughness Ra = 0.02 μm) 10: Tension detector (outlet) 12: Guide roller 13: Take-up reel

Claims (5)

[Claims] Claim 1: The coefficient of static friction μS8 when measured with a load of 800 g is 0.5 to 0.9, and the ratio of this to the coefficient of static friction μS4 when measured with a load of 400 g (μS8/μS
4) is from 1.3 to 3.0, and at least one surface of the film has a surface hardness of 21 or more. Claim 2: A biaxially oriented polyester film having a specific surface area of 50 to 120 m2/g and a total pore volume of 0.
．． 5 to 1.0 ml/g and the average particle size dA is 0.
．． Inert particles A with a diameter of 0.02 to 0.3 μm to 0.05 to 1
．． Contains 5% by weight, and has a Mohs hardness of less than 3,
Then, inert particles B whose average particle diameter dB is within the range of 0.2 to 1.5 μm and larger than the average particle diameter dA of the inert particles A are contained in a content of 0.0005 to 0.0.
29% by weight and 10% of the total inert particle content
The biaxially oriented polyester film according to claim 1, wherein the biaxially oriented polyester film is contained in a proportion of not more than % by weight. 3. The biaxially oriented polyester film according to claim 2, wherein the inert particles B are particles made of an organic polymer. 4. The biaxially oriented polyester film according to claim 3, wherein the inert particles B are particles made of silicone resin. 5. The biaxially oriented polyester film according to claim 3, wherein the inert particles B are particles made of crosslinked acrylic resin.