JP2706338B2 - Biaxially oriented polyester film and its processed product - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a biaxially oriented polyester film and a processed product using the same, particularly a leader tape and a magnetic recording medium for thermal transfer.

2. Description of the Related Art A leader tape connected to the beginning and end of a magnetic tape is known (for example, Japanese Patent Application Laid-Open No. 49-67603).
This leader tape is connected to the beginning and end of the magnetic tape, and utilizes the fact that the light transmittance is much higher than that of the magnetic tape portion, and detects the beginning and end of the magnetic tape by an optical sensor. This is for stopping the tape so that a large force is not applied to the magnetic tape.

Therefore, if the difference in light transmittance between the leader tape and the magnetic tape is small, the detection sensitivity is reduced. Also recently,
Since the amount of carbon black in the magnetic layer is reduced in order to improve the performance by increasing the filling rate of the magnetic powder in the magnetic layer, the light transmittance of the magnetic tape tends to increase, making it more and more a leader tape. Must be transparent.

On the other hand, since the leader tape runs repeatedly, scratch resistance (scratch resistance) is required. If the scratch resistance is poor, the powder generated therefrom causes dropout and the like.

As a method of dubbing (copying a record) a magnetic tape, a method using thermal transfer is known (for example, Nation).
al Technical Report Vol.20, No.1, pp.92-108, 1974 2
Month). Dubbing of this magnetic tape by thermal transfer is promising as a dubbing method for soft tapes in the future, since the S / N is not reduced by dubbing and high image quality can be maintained. At the time of thermal transfer, the magnetic layer of the tape to be copied needs to be higher than the Curie point, but recently, in order to perform efficient heating, the tape is heated with a laser beam through the opposite side of the magnetic layer, that is, through the base film. ing.

Therefore, a base film having poor transparency is not preferable because the heating efficiency is low and the S / N is largely reduced by dubbing. Also, because the above copy is made at high speed,
Scratch resistance is also essential.

Thus, a base film used for a leader tape or a magnetic recording medium for thermal transfer is required to have scratch resistance as well as transparency.

Among them, regarding the improvement of the scratch resistance, particles are contained in the film, and the particles form fine irregularities (fine projections) on the film surface, so that the scratch resistance is enhanced through the reduction of the friction coefficient of the film surface. Known techniques are known. A biaxially oriented polyester film in which a polyester contains substantially spherical silica particles caused by colloidal silica is known as a film having fine irregularities (projections) formed on such a surface (for example, see Japanese Unexamined Patent Publication No. 59-171623).

[Problems to be Solved by the Invention] However, in the conventional biaxially oriented polyester film as described above, the contained silica particles are distributed almost randomly throughout the thickness direction of the film.
There is a limit to the increase in the density of protrusions due to the contained particles on the film surface, and the heights of the protrusions also vary considerably at random. Therefore, the high portions of the projections formed on the film surface are easily shaved, and the effect of improving the scratch resistance becomes insufficient accordingly. In addition, since inert particles are mixed in the entire area in the thickness direction of the film, the transparency of the film itself is reduced, and may be a fatal defect as a base film of a leader tape or a magnetic recording medium for thermal transfer. is there.

The present invention improves the scratch resistance by achieving a high density of projections on the surface of the biaxially oriented polyester film and makes the height uniform, and secures high transparency of the entire film by using a leader tape or thermal transfer. It is an object of the present invention to provide an optimal magnetic recording medium base film.

[Means for Solving the Problems] A biaxially oriented polyester film according to the present invention, which meets this object, comprises at least a film mainly composed of polyester B by co-extrusion of a film mainly composed of polyester A and inert particles. A biaxially oriented polyester film laminated on one side, wherein the average particle size of the inert particles is 0.3 to 10 times the thickness of the polyester A laminated film containing the inert particles, and the polyester A laminated film layer surface protrusions number of 5000 / mm ² or more and the turbidity value is 1.5 to 10
%.

By using this biaxially oriented polyester film, a leader tape and a magnetic recording medium for thermal transfer having excellent scratch resistance and transparency can be obtained.

The polyester A in the present invention is not particularly limited, but is preferably ethylene terephthalate, ethylene α, β-bis (2-chlorophenoxy) ethane-4,4 ′.
-Dicarboxylate, ethylene When forming at least one structural unit selected from 2,6-naphthalate units as a main component, the formation of surface projections, transparency as a base film, and even better scratch resistance. Is desirable. Further, when the polyester constituting the present invention is crystalline, transparency and scratch resistance are further improved, which is very desirable. The crystallinity here indicates that it is not so-called amorphous, and quantitatively the cold crystallization temperature Tcc in the crystallization parameters is detected,
And the crystallization parameter ΔTcg is 150 ° C. or less.
Further, when the heat of fusion (change in enthalpy of fusion) measured by a differential scanning calorimeter indicates a crystallinity of 7.5 cal / g or more, transparency and scratch resistance are further improved, which is extremely desirable. Further, in the case of polyester containing ethylene terephthalate as a main component, scratch resistance on the film surface is further improved, which is particularly desirable. In addition, two or more types of polyesters may be mixed or a copolymer may be used within a range not to impair the present invention.

The shape of the inert particles in the polyester A of the present invention is not particularly limited, but the particles having a particle diameter ratio (particle major axis / minor axis) in the film of 1.0 to 1.3, particularly spherical particles, are preferred. Since the height of the projections formed on the surface can be easily made uniform and the density can be easily made high, the scratch resistance can be greatly improved.

Further, when the single particle index in the film of the inert particles in the polyester A of the present invention is 0.7 or more, preferably 0.9 or more, projections having a uniform height are easily formed, and the scratch resistance of the film surface is improved. Is particularly desirable since it becomes even better.

Although the kind of the inert particles of the polyester A of the present invention is not particularly limited, as the preferable particles for satisfying the above preferable particle characteristics, substantially spherical silica particles due to colloidal silica, particles of a crosslinked polymer ( For example, cross-linked polystyrene). In particular, crosslinked polymer particles whose degree of crosslinking has been increased until the temperature at the time of weight loss by 10% by weight (measured using a thermogravimetric analyzer Shimadzu TG-30M in nitrogen. desirable. In the case of spherical silica originating from colloidal silica, substantially spherical silica having a low sodium content and produced by the alkoxide method is particularly desirable. However, other particles, such as particles of calcium carbonate, titanium dioxide, alumina, etc., can be sufficiently used by appropriately controlling the film thickness and the average particle size.

The size of the inert particles is such that the average particle size in the laminated film containing the inert particles is 0.3 to 1 of the thickness of the laminated film.
0 times, preferably 0.5 to 5 times, more preferably 1.1 to 3 times
Double the range. When the average particle / thickness film thickness ratio is smaller than the above range, the height of the projections formed on the surface becomes uneven, and the transparency and scratch resistance become poor. It is not preferable because the surface projections are easily shaved and the scratch resistance becomes poor. The average particle size (diameter) of the inert particles in the film of the polyester A is 0.007 to 3 μm, preferably 0.07 to 3 μm.
When the thickness is in the range of 2 to 0.8 μm, the transparency of the whole film is further improved, which is desirable.

That is, the laminated film layer of the biaxially oriented polyester film in the present invention contains inert particles having an average particle size near or greater than the thickness of the film layer. In other words, the ultra-thin laminated film contains fine inert particles having an average particle diameter near or greater than the film thickness. Therefore, the inert particles can be distributed in the thickness direction of the entire biaxially oriented polyester film, substantially concentrated only on the laminated film layer. As a result, the particle density in the laminated film can be easily increased, and the density of projections on the film surface formed by the particles can be easily increased.
In addition, the inert particles are contained in the laminated film, so that the position is regulated in the thickness direction with respect to the entire biaxially oriented polyester film. Is as described above, the height of the surface projections formed by the particles becomes extremely uniform. Therefore, the scratch resistance of the surface of the laminated film is kept high. Since the inert particles are substantially present only in the surface layer, the transparency of the film as a whole is kept extremely high.

As described above, in order to increase the scratch resistance of the film surface, especially in order to withstand severe repetitive conditions and high-speed running conditions as a leader tape or a magnetic recording medium for thermal transfer, the number of surface protrusions of the polyester A layer is 5,000. / m
m ² or more is required, if smaller than the above, the desired scratch resistance is hardly obtained.

On the other hand, from the viewpoint of transparency, the particle content of the entire film can be suppressed. As a base film for a leader tape or a magnetic recording medium for thermal transfer, the biaxially oriented polyester film of the present invention can satisfy a turbidity value (haze) of 10% or less. If the turbidity value is higher than this, the transparency desired for a leader tape or a magnetic recording medium for thermal transfer cannot be obtained, and in the case of a leader tape, the ability to detect the first end of the magnetic tape is reduced. In the case of a recording medium, there is a possibility that poor dubbing properties may occur.

Further, as described below, in the present invention, in order to ensure scratch resistance, an inert particle is contained in the laminated film of polyester A, so that the turbidity value is 0%, that is, it is completely transparent. There is no such thing, but it is preferable that the transparency is as high as possible. The lower limit of the turbidity value for ensuring the transparency while securing the necessary scratch resistance is 1.5% as shown in the examples described later. Therefore, the turbidity value for satisfying both the required scratch resistance and transparency is in the range of 1.5 to 10%.

In order to achieve both the scratch resistance and transparency, in the biaxially oriented polyester film of the present invention, the content of the inert particles in the polyester A laminated film is as follows:
It is in the range of 0.1 to 20% by weight. If the amount is less than this range, the density of the formed projections is low, so that the intended effect of improving scratch resistance cannot be obtained. Conversely, if it is more than the upper limit, the inert particles are likely to aggregate, and the aggregated particles are more likely to form projections having an uneven height. This is not preferred because the overall transparency is reduced.

Such a film mainly composed of polyester A and inert particles is laminated on a film mainly composed of polyester B.

Polyester B is the same as polyester A described above, and polyester B and polyester A may be the same or different. The polyester A film layer is laminated on both sides or one side of the polyester B film layer. In other words, the lamination structure is A / B /
The case of A, A / B, but of course, A / B / C in which a C layer having a surface state different from A is provided on the surface opposite to A, or a multilayer structure of more than that may be used. (Where A, B, C
The type of each resin may be the same or different. Also, at least one surface needs to be the A layer. When the magnetic layer is provided, the B layer and the A / B
In the case of the / C configuration, it is preferable to provide it on the smooth surface. Also, as the polyester B, a crystalline polymer is desirable.
In particular, when the crystallinity parameter ΔTcg is in the range of 20 to 100 ° C., the scratch resistance of the film surface of the polyester B is further improved, which is desirable. As a specific example,
Ethylene terephthalate, ethylene α, β-bis (2-
When at least one structural unit selected from chlorphenoxy) ethane-4,4'-dicarboxylate and ethylene 2,6-naphthalate units is used as a main component, scratch resistance is particularly improved, which is desirable. However, other components may be copolymerized within a range that does not impair the present invention and within a range that does not impair desired crystallinity, preferably within 5 mol%.

Polyester B of the present invention may be blended with other polymers as long as the object of the present invention is not hindered, and organic additives such as antioxidants, lubricants and ultraviolet absorbers are usually added. It may be added to some extent.

It is not particularly necessary for the film containing polyester B as a main component to contain inert particles. However, when this film layer forms one surface of the base film, the average particle size is 0.000.
When the inert particles of 7 to 2 μm, particularly 0.02 to 0.45 μm are contained in a very small amount, for example, 0.001 to 0.2% by weight, particularly 0.005 to 0.15% by weight, and even 0.005 to 0.12% by weight, the processing step and the use are difficult. In this case, not only the coefficient of friction and the scratch resistance are further improved, but also the winding appearance of the film is improved. As the kind of the inert particles to be contained, it is desirable to use those desirably used for the polyester A. The types and sizes of the particles contained in the polyesters A and B may be the same or different.

A polyester A containing inert particles as described above,
Polyester B is laminated by coextrusion, formed into a sheet, and then biaxially stretched to obtain a biaxially oriented polyester film. Lamination by co-extrusion in the present invention means that polyester A and polyester B containing inert particles are extruded by different extrusion devices, respectively, and these are laminated before discharging a laminated sheet from a die. This lamination may be performed in a die (for example, a manifold) for forming and discharging into a sheet shape. However, since the laminated film layer is extremely thin as described above, the lamination is performed in a polymer tube before being introduced into the die. Is preferred. In particular, it is particularly preferable to form the laminated portion in the polymer tube in a rectangular shape, since the laminated portion can be uniformly laminated in the width direction. The molten polymer laminated at the rectangular laminated portion in the polymer tube is widened to a predetermined width in the sheet width direction by the manifold in the die, discharged from the die in a sheet shape, and then biaxially stretched. Therefore, for example, even when the laminated film layer after biaxial orientation is extremely thin, the polyester polymer containing inert particles is laminated in a considerable thickness in the rectangular laminated portion in the polymer tube, so that it is easy and accurate. Can be laminated well.

Further, in the biaxially oriented polyester film of the present invention, the particle concentration ratio of the inert particles in the surface layer on the side of the laminated film containing the inert particles is preferably 0.1 or less. This surface layer particle concentration ratio, as shown in the measurement method described below,
It shows how the inactive particles forming the film surface projections are covered with a thin film of polyester A on the film surface, and the higher the degree of the particles being substantially directly exposed on the film surface, the higher the surface particle concentration. The ratio is high, and surface protrusions are formed, but the higher the degree of coverage with the polyester A thin film, the lower the surface particle concentration ratio. Since the inactive particles forming the projections are covered with the polyester A thin film, even when the inactive particles are densely distributed in the ultra-thin laminated film layer, the particles remain in the laminated film layer. ,
As a result, it is firmly held by the base film layer of polyester B. Therefore, by making the surface layer particle concentration ratio equal to or less than the above value, the particles are prevented from falling off, and the scratch resistance of the film surface is kept high. Such a surface particle concentration ratio can be achieved by performing lamination by coextrusion. By the way, even with a coating method, a film having surface protrusions having the same density as that of the present invention, that is, a resin layer is coated with an extremely thin film on a film layer of polyester B, and inert particles are contained in the resin layer. Although it is possible to do so, the surface layer particle concentration ratio becomes extremely high (that is, the degree of substantially direct exposure of the particles to the surface becomes extremely high), and only a very brittle surface is obtained as compared with the film of the present invention.

In the biaxially oriented polyester film of the present invention, there is some correlation between an increase in turbidity and a decrease in the uniformity of the height of the surface protrusions of the polyester A layer.
In other words, even when the average particle size of the particles and the thickness ratio of the laminated polyester A layer, the number of protrusions, etc., satisfy the above conditions, the dispersion in the particle size distribution is large, or the thickness of the laminated film layer is large. If the spots are large, the uniformity of the height of the projections decreases and the turbidity increases. From this viewpoint, the relative standard deviation in the particle size distribution is preferably 0.6 or less, and more preferably 0.5 or less. The thickness unevenness of the laminated polyester A layer is preferably 50% or less, and 25% or less.
The following is more preferred, and the content is particularly preferably 20% or less.

Next, a method for producing the biaxially oriented polyester film of the present invention will be described.

First, the method of incorporating inert particles into the polyester A may be any of after polymerization, during polymerization, and before polymerization, but the method of kneading the polymer with a vented twin-screw extruder is within the scope of the present invention. It is effective for obtaining a film in a form. Also, as a method of adjusting the content of particles,
A method of preparing a high-concentration master by the above method and diluting it with a polyester substantially free of inert particles during film formation to adjust the content of the particles provides a film having a surface morphology within the scope of the present invention. It is effective for Further, a method of adjusting the melt viscosity of the high-concentration master polymer of the particles, the copolymerization component, and the like to keep the crystallization parameter ΔTc g in the range of 30 to 80 ° C. does not cause stretching breakage, and the film having the surface morphology within the range of the present invention. It is effective to get

Thus, after the pellet A containing the inert particles is sufficiently dried, the pellet A is supplied to a known melt extruder and melted at a concentration not lower than the melting point of the polyester and not higher than the decomposition point, and does not substantially contain the other inert particles. Polyester B (the type may be the same as or different from the polyester containing inert particles) is supplied to the laminating apparatus as described above, extruded from a slit die into a sheet, and cooled on a casting roll. It is solidified to make an unstretched film. That is, these polyesters are laminated by using two or three extruders, a merging block for two or three layers or a die. When the confluent block system is used, the laminated portion is made rectangular as described above, and the difference (absolute value) between the melt viscosities of both polyesters is 0 to 2000 poise, preferably 0 to 10 poise.
It is effective to keep the film having the surface morphology within the range of the present invention stably, without unevenness in the width direction, and to industrially produce the film in the range of 00 poise.

Next, the multilayer unstretched film is biaxially stretched and biaxially oriented. The method of biaxial stretching may be any of simultaneous biaxial stretching and sequential biaxial stretching, but in the case of sequential biaxial stretching in which the film is stretched in the longitudinal direction and width direction in order to stabilize the film having the surface form within the scope of the present invention. Therefore, it is effective for industrial production without unevenness in the width direction. In the case of sequential biaxial stretching, the stretching in the longitudinal direction is divided into three stages, especially four or more stages, at a stretching temperature of 40 to 150 ° C. and a stretching speed of 1000 to 50,000% / min.
The 6-fold method is effective for obtaining a film having a surface morphology within the range of the present invention. The stretching temperature and speed in the width direction are
A range of 80 to 170 ° C. and 1000 to 20000% / min is suitable. The stretching ratio is preferably 3 to 10 times. Further, if necessary, the film can be further stretched in at least one of the longitudinal direction and the width direction. In any case, after providing a very thin layer containing inert particles, the area stretching ratio (longitudinal magnification ×
The point of the present invention is to perform stretching of 9 times or more (widthwise magnification). Next, this stretched film is heat-treated. In this case, the heat treatment conditions are as follows: relaxation in the width direction, fine stretching, 140 to 280 ° C in any state under constant length, preferably 0.5 to 60 seconds in the range of 160 to 220 ° C. It is desirable that the film having the surface morphology within the range of the present invention be easily obtained by using microwave heating together.

A feature of the method for producing the film of the present invention is that, using a high-concentration particle polymer having a specific range of thermal characteristics prepared by a special method, biaxially stretching the film after providing a very thin layer containing inert particles. In the film-forming process, after uniaxially stretching the film, a method of further stretching by applying a coating or the like, or a laminated film made by coating a biaxially stretched film far reaches the performance of the film of the present invention, The film of the present invention is also excellent in cost.

The biaxially oriented polyester film obtained as described above is connected to the start and end of a magnetic tape and used as a leader tape. As the leader tape, the above transparency, a biaxially oriented polyester film having excellent scratch resistance may be used as it is, and as usual, a predetermined pigment or the like is used in a state applied to the diamagnetic surface side. You can also.

When the biaxially oriented polyester film is used as a base film of a magnetic recording medium for thermal transfer, it may be manufactured by the same method as in the manufacture of a normal magnetic tape. However, it is desirable to use chromium oxide as the magnetic material. Since the transparency of the base film is excellent, laser light from the opposite side of the magnetic layer surface is efficiently transmitted, the heating efficiency is enhanced, and excellent S / N is obtained.

[Method for Measuring Physical Properties and Method for Evaluating Effect] The method for measuring characteristic values and the method for evaluating effect according to the present invention are as follows.

(1) Number of particles, average particle size, relative standard deviation of particle size distribution The polyester is removed from the film by a plasma insulative incineration method (for example, Model PR-503 manufactured by Yamato Scientific Co., Ltd.) to expose the particles. The processing conditions are selected such that the polyester is ashed but the particles are not damaged. This is observed with a scanning electron microscope (SEM), and the image of the particles (shading of light generated by the particles) is linked to an image analyzer (for example, QTM900 manufactured by Cambridge Instrument) to measure the number of particles and change the observation location. Number of particles
The following numerical processing is performed for 5000 or more pieces, and the average diameter D obtained thereby is defined as the average particle diameter.

D = ΣD _i / N where, D _i is a circle equivalent diameter of the particles, N is the is the number.

The particle size distribution was obtained from the above observation, and the relative standard deviation was obtained.

(2) Content of Particles A solvent in which the polyester is dissolved but the particles are not dissolved is selected, the particles are centrifuged from the polyester, and the ratio (% by weight) to the total weight of the particles is defined as the particle content.
In some cases, the combined use of infrared spectroscopy is also effective.

(3) Glass transition point Tg, cold crystallization temperature Tcc, crystallization parameter ΔTc g, melting point Measured using a DSC (differential scanning calorimeter) type II manufactured by Perx Elmer. The measurement conditions for DSC are as follows.
That is, 10 mg of a sample is set in a DSC apparatus, melted at a temperature of 300 ° C. for 5 minutes, and then rapidly cooled in liquid nitrogen. The quenched sample is heated at a rate of 10 ° C./min, and the glass transition point Tg is detected.
The temperature was further increased, and the crystallization exothermic peak temperature from the glassy state was defined as the cold crystallization temperature Tcc. The temperature was further raised, and the melting peak temperature was taken as the melting point. The difference between Tcc and Tg (Tcc−Tg) is defined as a crystallization parameter ΔTc g.

(4) Average height of surface protrusions The flat surface of the film surface using a two-detector scanning electron microscope [ESM-3200, manufactured by Elionix Inc.] and a cross-section measuring device [PMS-1, manufactured by Elionix Inc.] The height measurement value of the protrusion when scanning is performed with the height of 0 being set to 0 is sent to an image processing device [IBAS2000, manufactured by Carl Zeiss Co., Ltd.], and the film surface protrusion image is reconstructed on the image processing device. Next, the highest value among the individual projections obtained by binarizing the projections in this surface projection image is defined as the height of the projection, and this is determined for each individual projection. This measurement was repeated 500 times at different locations to determine the number of protrusions, and the average value of the heights of all the measured protrusions was defined as the average height. The magnification of the scanning electron microscope is selected to be a value between 1000 and 8000 times. In some cases, a high-precision optical interference type three-dimensional surface analyzer (TOPO-3D manufactured by WYKO, objective lens: 40 to 200
The height information obtained by using (2, the use of a high-resolution camera is effective) may be replaced with the value of the SEM and used.

(5) Surface layer particle concentration ratio Using a secondary ion mass spectrum (SIMS), the concentration ratio between the highest concentration element among the elements caused by the particles in the film and the carbon element of the polyester is defined as the particle concentration, and the thickness is determined. Perform direction analysis. The ratio between the particle concentration A at the outermost surface particle concentration (point at depth 0) measured by SIMS and the highest concentration B obtained by further analyzing the depth direction, A / B was defined as the surface particle concentration ratio. The measuring device and conditions are as follows.

Measurement device Secondary ion mass spectrometer (SIMS) A-DIDA3000 manufactured by ATOMIKA, West Germany Measurement conditions Primary ion species: O ₂ ⁺ Primary ion acceleration voltage: 12 KV Primary ion current: 200 nA Raster area: 400 μm □ Analysis area: Gate 30% Measurement degree of vacuum: 6.0 × 10 ⁹ Torr E-GUN: 0.5 KV-3.0 A (6) Single particle index The cross section of the film is observed by a photograph with a transmission electron microscope (TEM) to detect particles. When the observation magnification is set to about 100,000, one particle that cannot be further divided can be observed. When the total area occupied by the particles is A, and the area occupied by the aggregate in which two or more particles are agglomerated is B, (A
-B) / A is defined as a single particle index. The TEM conditions are as follows. One visual field area: 2 μm ² , measurement was performed at 500 visual fields at different locations.

-Apparatus: JEM-1200EX manufactured by JEOL-Observation magnification: 100,000 times-Section thickness: about 1000 angstroms (7) Particle size ratio In the measurement of (1) above, average value of major axis / average value of minor axis Is the ratio of

 That is, it is obtained by the following equation.

Major axis = ΣD1 _i / N long diameter short diameter _{= ΣD2 i / N D1 i,} D2 i each individual particle (maximum diameter), short diameter (the shortest diameter), N is the total number.

(8) Thickness of thermoplastic resin A layer in laminated film Using a secondary ion mass spectrometer (SIMS), the element resulting from the highest concentration of particles in the film and the carbon element of polyester The concentration ratio of (M ⁺ / C ⁺ ) is the particle concentration,
Analysis in the depth (thickness) direction from the surface of the polyester A layer is performed. In the surface layer, the particle concentration is low due to the interface of the surface, and the particle concentration increases as the distance from the surface increases. In the case of the film of the present invention, the particle concentration which once reached a maximum at the depth [I] starts to decrease again. Based on this concentration distribution curve, the depth [II] (here, I
I> I) was taken as the lamination thickness. The conditions are the same as in the measurement method (5).

In addition, when the particles most contained in the film are organic polymer particles, it is difficult to measure by SIMS. Therefore, XPS (X-ray photoelectron spectroscopy), IR
(Infrared spectroscopy) or a confocal microscope
The depth profile of the particle concentration may be measured, and the layer thickness may be obtained by the same method as described above.

(9) Variation in the thickness of the laminated polyester film A layer The laminated thickness of the polyester A layer was measured over 1 m,
The difference between the maximum value and the minimum value is a, and the average value of the lamination thickness is b.
Then, the thickness variation is defined as (a / b) × 100 (%).

(10) Transparency of base film Film haze was measured according to ASTM-D-1003-52. A haze of less than 10% was determined to be excellent, 10% to 40% was determined to be good, and a haze of more than 40% was determined to be poor.

(11) Scratch resistance Using a tape running tester, a film slit into a 1/2 inch wide tape is run on guide pins (surface roughness: 100 nm with Ra) (running speed 1000m / Min, 10 passes, winding angle: 60 °, running tension: 20g). At this time, observe the wound in the film with a microscope,
Excellent if 2 or less scratches per tape width are less than 2
Less than 10 pieces were judged as good, and 10 or more pieces were judged as bad. Although excellent is desirable, even good is practically usable.

(12) Detection sensitivity at the beginning and end of the magnetic tape by the leader tape After passing the video tape 500 times in the VTR, the detection sensitivity at the end at the time of fast-forward and the beginning at the rewind is tested 50 times each.
The sample was judged as good when it stopped completely 100 times, and was judged as poor when it did not decrease even once.

(13) Resistance to dubbing of magnetic recording media for thermal transfer A video tape for thermal transfer was prepared, and the chroma S / N drop when the thermal transfer was performed by laser light heating from the base film side was less than 2 dB. The case was regarded as bad.

[Examples] The present invention will be described based on examples.

Examples 1 to 7, Comparative Examples 1 to 6 Crosslinked polystyrene particles having different average particle sizes, an ethylene glycol slurry containing silica particles derived from colloidal silica was prepared, and the ethylene glycol slurry was heat-treated at 190 ° C. for 1.5 hours. , After transesterification with dimethyl terephthalate, polycondensation, the particles 0.3 to 55
Polyethylene terephthalate (hereinafter referred to as PET)
Pellets). A polyester A was prepared using the pellets, and a PET substantially free of inert particles was produced by a conventional method to obtain a polyester B. Each of these polymers was dried under reduced pressure (3 Torr) at 180 ° C. for 3 hours. The polyester A is supplied to the extruder 1 and melted at 310 ° C., and the polyester B is supplied to the extruder 2 and melted at 280 ° C. These polymers are merged and laminated by a merging block provided with a rectangular laminating section. It was wound around a casting drum having a surface temperature of 30 ° C. by cooling using an electrification casting method, and solidified by cooling to produce a two-layer unstretched film having a polyester A layer on one side. At this time, the total thickness of each extruder and the thickness of the polyester A layer were adjusted. (However, Comparative Examples 5 and 6 have a single layer A). This unstretched film is heated at a temperature of 80 ° C in the longitudinal direction for 4.
Stretched 5 times. This stretching was performed in four stages with a difference in peripheral speed between two sets of rolls. This uniaxially stretched film is stretched 4.0 times in the width direction at 100 ° C. at a stretching speed of 2000% / min using a stenter, and is heat-treated at 200 ° C. for 5 seconds under a constant length to give a total thickness of 1
A biaxially oriented laminated film having a thickness of 5 μm and a polyester A layer thickness of 0.1 to 4 μm was obtained. The transparency of this film was measured by haze, and the scratch resistance of the polyester A layer side was evaluated.

Next, the initial and end detection sensitivity of the magnetic tape when these films were used as a leader tape was evaluated.

Each parameter as the base film is as shown in Table 1, and when these are within the scope of the present invention,
Transparency, scratch resistance, initial and end detection sensitivities showed good values as shown in Table 1, but otherwise, these characteristics could not be combined.

Further, a magnetic layer made of chromium oxide was provided on the surface of the layer B using the above base film, a video tape for thermal transfer was prepared, and thermal transfer by laser light was performed to evaluate the dubbing resistance. The results are also shown in Table 1. As a result, good anti-dubbing characteristics were obtained with the film in the range of the present invention.

[Effects of the Invention] As described above, when the biaxially oriented polyester film of the present invention is used, the base film is a laminated biaxially oriented polyester film by co-extrusion, and the polyester A layer containing inert particles of a specific size. A uniform number of projections with a uniform height are formed on the surface of the film at a high density and a predetermined number or more, and the turbidity of the entire film is also suppressed to a predetermined value or less, so that excellent scratch resistance can be obtained by uniform projection formation. In addition, by making inert particles for forming projections substantially exist only in the ultra-thin polyester A layer, the transparency of the entire film can be maintained extremely high. It is possible to obtain an excellent film having both scratch resistance and transparency, which is optimal for use as a medium.

Further, the biaxially oriented polyester film of the present invention is a film made by directly composite laminating by coextrusion without an operation such as coating in the film forming process, and is made during or after the film forming process. Compared with the laminated film, the molecules of the outermost layer are also biaxially oriented, so that besides the above-mentioned characteristics, for example, the surface has much better abrasion resistance, and is advantageous in terms of cost, quality stability, and the like.

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shoji Nakajima 1-1-1, Sonoyama, Otsu City, Shiga Prefecture Toray Co., Ltd. Shiga Works (56) References JP-A-1-2066042 (JP, A) 62-225345 (JP, A) JP-A-2-77431 (JP, A)

Claims (4)

(57) [Claims] 1. A biaxially oriented polyester film in which a film mainly composed of polyester A and inert particles is laminated on at least one surface of a film mainly composed of polyester B by co-extrusion. The average particle size is 0.3 to 10 times the thickness of the laminated film of the polyester A containing the inert particles, the number of surface projections of the laminated film layer of the polyester A is 5000 / mm ² or more, and the turbidity value is 1 .
A biaxially oriented polyester film characterized by being in the range of 5 to 10%. 2. The biaxially oriented polyester film according to claim 1, wherein the particle concentration ratio of the inert particles in the surface layer of the laminated film of the polyester A containing the inert particles is 0.1 or less. 3. A leader tape using the biaxially oriented polyester film according to claim 1. 4. A magnetic recording medium for thermal transfer, wherein a magnetic layer is provided on one side of the biaxially oriented polyester film according to claim 1.