JPH08142180A - Biaxially oriented polyester film - Google Patents

Abstract

PURPOSE: To improve the scuff resistance and to suppress the drop-out when used as a magnetic recording medium by setting the surface roughness parameter relative standard deviation value at least on one surface of a biaxially oriented polyester film to a specific value or less, and hence uniformly forming surface protrusions having high strength. CONSTITUTION: A biaxially oriented polyester film contains polyester as a main ingredient. The relative standard deviation of the surface roughness parameter Ra of four sides of 5μm of at least one surface is set to 0.30 or preferably 0.25 or less. The number of protrusions on at least one surface is 5,000 pieces/mm<2> or more or preferably 10,000 pieces/mm<2> or more. Further, the protrusions of the surface are desired to be protrusions caused mainly by a crystal. The number of the protrusions/number of particles(NR) of the ratio of the number of the protrusions of the surface to the number of the particles contained is desirably 5 or more, or preferably 10 or more.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyester film, and more particularly to a polyester film having fine projections formed on its surface.

[0002]

2. Description of the Related Art Polyester films are widely used for various purposes. As the processing speed of polyester film processing, such as printing in packaging applications, magnetic layer coating in magnetic recording media applications, and thermal transfer layer coating in thermal transfer applications increases, polyester film is becoming more Surface properties such as good runnability and wear resistance are being demanded. It is known that it is effective to uniformly form fine protrusions on the film surface in order to obtain good runnability.

A polyester film containing substantially spherical silica particles derived from colloidal silica in order to form fine projections on the film surface is known (for example, JP-A-59-171623). . Also,
A polyester film in which a thin layer containing particles for forming surface protrusions is laminated on a base layer is also known (for example, JP-A-2-77431).

[0004]

However, the conventional polyester film having particles (for example, inert particles) contained in at least the surface layer and having projections formed on the surface thereof has the following major problems. .

Since particles such as inert particles, which are different from polyester, are present immediately below the protrusions, voids are often generated. Therefore, in the process of processing the film or in the stage of using the final product, guide rolls and When traveling on a guide means such as a guide pin, the particles existing under the projection are easily scraped off due to the unevenness of the surface of the contact partner. The scraped particles adhere to the surface of the contacting partner and cause not only a problem of stains but also the problem that the particles act as a blade to damage the film surface or deteriorate the handling property by increasing the friction coefficient. In particular, with regard to scratches that occur in high-speed processes (hereinafter referred to as scratches), as a result of examination, a mechanism has been clarified that scraped particles accumulate on the microscopic projections of the contact partner, which cause wide and deep scratches on the film. There is.

On the other hand, if the surface protrusion density of the film is too low, the contact surface pressure applied to one protrusion becomes high, which may cause a problem that the abrasion resistance of the film surface is lowered.

Further, recent films are often required to have an ultra-smooth surface having a center line average roughness (Ra) of 10 nm or less. In such a case, the film surface has a high height. It is not possible to provide many protrusions. The concentration of the particles contained in the film is limited in terms of high dispersion, and therefore the prior art (method of forming surface protrusions by adding particles) has insufficient surface uniformity.

An object of the present invention is to provide a polyester film in which desired fine projections are densely and uniformly formed on the surface by utilizing the crystallization of polyester essentially without depending on the contained particles. In particular, it is possible to provide a polyester film in which high-strength surface protrusions are uniformly formed, which has good scratch resistance, and which can suppress the occurrence of dropout particularly when used as a magnetic recording medium. is there.

[0009]

The polyester film of the present invention for this purpose is a biaxially oriented polyester film containing polyester A as a main component and having a surface roughness parameter Ra of 5 μm square on at least one surface thereof. Relative standard deviation of 0.30 or less, preferably 0.
It is characterized by being 25 or less, more preferably 0.20 or less. By setting the relative standard deviation of the surface roughness parameter Ra to 0.30 or less, the projections on the surface are formed densely and uniformly in the scratch scratch generation mechanism, so that shavings are less likely to occur, Further, even when shavings are generated, the effect of trapping the shavings on the surface (storage effect) can be exhibited. As a result, it is possible to prevent shavings from accumulating on the guide pins and causing dropouts. However, when the value of the relative standard deviation of Ra is smaller than 0.01, the friction on the film surface may increase.

In the film of the present invention, the number of protrusions on at least one surface is 5000 / mm.
^It is preferably ² or more, more preferably 10,000 / mm ² or more. By forming a large number of protrusions in this way, it is possible to reduce the friction between the film and the contact partner,
This suppresses wear.

Further, in the polyester film of the present invention, it is desirable that the projections on the surface are projections mainly caused by crystals. That is, the protrusions on the surface of the polyester A
It is formed by utilizing its own crystallization. Therefore, the problem of void generation when particles are added is substantially eliminated, and protrusions with high strength that are difficult to break are formed.

Particles may be supplementarily added for the purpose of further reducing friction, facilitating the cutting of the film, and the like. In this case as well, the number of projections on the surface and the number of particles contained are It is desirable that the ratio of the number of protrusions / the number of particles (NR), which is the ratio, is 5 or more, more preferably 10 or more. If the NR is less than 5, the proportion of protrusions formed by the contained particles increases and the proportion of protrusions that are easily broken due to void formation increases, so that desirable scratch resistance cannot be obtained. That is, in the film of the present invention, it is possible to suppress the dropout due to the storage effect of the shavings and to suppress the generation of the shavings themselves by forming the high-strength fine projections.

The formation of surface protrusions utilizing the crystallization of polyester A as described above is carried out as follows.

In producing a biaxially oriented film containing polyester A as a main component, at least one surface of the unstretched film is heat-treated, and then the unstretched film is biaxially stretched to form a desired surface protrusion. It

By first subjecting the unstretched film to heat treatment, crystallization of the surface of the unstretched film proceeds,
A large number of fine crystals are formed. This unstretched film is biaxially stretched, the film is biaxially oriented to achieve the target strength of the film itself, and due to the difference in hardness between the crystal and the other part, the uniform crystal resulting from the fine crystals is obtained. Fine surface protrusions are formed.

Whether the surface protrusions are made of fine crystals of polyester A is determined by etching under the subject protrusions in the film thickness direction with an appropriate solvent to form the protrusions. When the substance remains as an insoluble substance, it is treated as particles added from the outside or particles precipitated internally (I). When there is substantially nothing that remains as an insoluble matter, it can be inferred that the substance that forms the protrusion is a fine crystal (II). Examples of the solvent include phenol / carbon tetrachloride (weight ratio: 6 /
The mixed solvent of 4) is preferably used. The frequency of I and the frequency of II when the field of view is set to about 1 mm ² are obtained by this method, and the value of II / (I + II) is preferably 70% or more. However, the method for determining whether or not the surface protrusions are made of fine crystals of polyester A is not limited to the above method, and an appropriate method can be selected.

In the present invention, the type of polyester A is not particularly limited, but the crystallization parameter ΔTcg is 70.
℃ or less, preferably 65 ℃ or less, more preferably 60 ℃
The following is desirable. If the crystallization parameter ΔTcg is larger than 70 ° C., it is difficult to obtain the surface protrusions targeted by the present invention. Even if it is obtained, the scratch resistance of the film surface is poor.

In the present invention, the type of polyester A is not particularly limited, but ethylene terephthalate, ethylene α, β-bis (2-chlorophenoxy) ethane-
It is particularly preferable when at least one structural unit selected from 4,4'-dicarboxylate and ethylene naphthalate units is the main constituent. Of these, polyesters containing ethylene terephthalate as a main constituent are particularly preferable. Two or more types of polyesters may be mixed or a copolymerized polymer may be used as long as the object of the present invention is not impaired. Further, it is desirable that the polyester A contains substantially no particles.

The polyester film of the present invention may be used as a single layer of a biaxially oriented film containing polyester A as a main component, or as a laminated film laminated on at least one side of a film containing polyester B as a main component. You may be asked.

The type of polyester B is not particularly limited. Polyester B preferably contains no particles, but may contain them.

Next, the method for producing the polyester film of the present invention will be described more specifically. In the present invention, at least one surface of the unstretched film is heat-treated and then biaxially stretched. Here, the unstretched film refers to a state immediately after being extruded from the die and before being solidified by cooling, to a film slightly stretched (up to about 2 times) in the uniaxial direction. The purpose of this heat treatment is to enhance the crystallinity of the film surface before stretching to a preferable crystallinity.

In the present invention, in the process of cooling the melt-extruded film containing polyester as the main component on the surface of the cooling roll, the temperature (Tm + 1) which is higher than the glass transition temperature Tg of polyester A and 100 ° C. higher than the melting temperature Tm.
It is preferable that the desired surface projections are formed by heat-treating the unstretched film from the opposite surface in contact with the cooling roll at a temperature of not more than 00 ° C.) and thereafter biaxially stretching the unstretched film. The temperature is more preferably 20 ° C. higher than Tg (Tg + 20 ° C.) or higher and 80 ° C. higher than Tm (Tm + 80 ° C.) or lower, further preferably 40 ° C. higher than Tg (Tg + 40 ° C.) or higher and Tm or lower. As a method of heat-treating the unstretched film from the surface opposite to the one in contact with the cooling roll, hot air or radiant heat from an infrared heater can be used, but it is not limited to this method.

The surface roughness of the cooling roll surface is 0.2.
When it is S or more and 10 S or less, the crystallinity of the film surface before stretching can be increased to a desired crystallinity, which is preferable. More preferably, the surface roughness of the surface of the cooling roll is 0.3 S or more and 8 S or less. When the surface roughness of the roll surface is less than 0.2 S, the unstretched film adheres to the cooling roll, which is not preferable. Further, if the surface roughness exceeds 10 S, desired surface protrusions may not be formed, or the film may slip on the cooling roll, which is not preferable.

In the present invention, when heat-treating an unstretched film which has been cooled and solidified, the surface (or surface layer) temperature of at least one surface thereof is 20 ° C. lower than the cold crystallization temperature Tcc of polyester A (Tcc-20 ° C.) or more. And 40 ° C. higher than the temperature falling crystallization temperature Tmc (Tmc + 40 ° C.)
In the following, heat treatment is performed so as to be maintained for 0.5 to 100 seconds, and thereafter, Tg or more and 20 ° C higher than Tcc (Tcc +
By biaxially stretching at a temperature of 20 ° C. or less, desired surface protrusions are formed, which is preferable. More preferably,
0.5 to 50 seconds at Tcc or more and Tmc or less, more preferably 0.5 to 2 at Tcc or more and Tmc or less.
The heat treatment is such that it is held for 0 seconds.

The following manufacturing conditions are also possible. Unstretched film is finely stretched uniaxially and has birefringence of 0.5 × 1
0 ^-3 to 50 x 10 ^-3, and then the surface (or surface layer) temperature of at least one side of the slightly stretched film is 20 ° C lower than the cold crystallization temperature Tcc of polyester A (Tcc-20.
C.) and a temperature lower than the crystallization temperature Tmc by 40.degree. C. or less (Tmc + 40.degree. C.) or less so as to be maintained for 0.3 to 50 seconds, and then Tg or more and 20.degree. C. from Tcc.
By biaxially stretching at a high (Tcc + 20 ° C.) or lower temperature, desired surface protrusions are formed, which is preferable. More preferably, Tcc or more and Tmc or less, 0.5
˜20 seconds, more preferably, a heat treatment that is maintained at 10 ° C. higher than Tcc (Tcc + 10 ° C.) or higher and 20 ° C. lower than Tmc (Tmc−20 ° C.) or lower for 0.5 to 15 seconds.

As for the heat treatment method, a method of heat treatment by winding on a heating roll, a method of hot air treatment from the side opposite to the roll which is in contact with the roll in the state of being wound around the roll, or a side opposite to the side of contact with the roll in the state of being wound around the roll There is a method of heat treatment with an infrared heater, a method of heat treatment with an infrared heater between rolls, a method of heating with a stenter, etc., but the method is not particularly limited to these.

In the present invention, the temperature of at least one surface (or surface layer) of the melt-extruded film containing polyester as the main component is set to the temperature-lowering crystallization temperature Tm of polyester A.
When the temperature is 70 ° C. lower than C (Tmc-70 ° C.) or higher and the temperature-lowering crystallization temperature Tmc of the polyester A is lower,
It is preferable to hold for 20 seconds, then cool to a glass transition temperature Tg or lower, and then biaxially stretch the unstretched film to form a desired surface protrusion.

As the heat treatment method, as described above, a film having a high temperature immediately after extrusion is gradually cooled to be crystallized, or a film once cooled and solidified is reheated to be crystallized. There is a method of heat treatment in a state of being slightly stretched in the uniaxial direction, and one of these methods can be carried out in the film forming process of the film to obtain a target surface morphology. Two or more of the above may be used in combination in the film forming process of the film.

As the polyester A according to the present invention,
Polyethylene terephthalate (PET) is preferably used. It is preferable that the polyester A contains substantially no particles. Polymerization of polyester A
It is preferable to use antimony trioxide as a polymerization catalyst, or acetate as a metal compound as a transesterification catalyst in order to reduce ΔTcg and enhance the effect of a crystal nucleating agent.
The acetate salt is not particularly limited, but it is particularly preferable to use a magnesium compound in order to achieve the object of the present invention. Further, it is preferable to use a phosphonic acid derivative as the phosphorus compound added during the polymerization of PET. In the case of the present invention, it is desirable to increase the amount of the catalyst added in order to enhance the effect of the nucleating agent, but if it is added too much, it causes the precipitation of internal particles and further increases the haze, which is not preferable. When magnesium acetate is used as the magnesium compound and phenyl phosphonate is used as the phosphonic acid derivative, magnesium acetate 6 is added to the weight of the film formed using polyester A.
0 to 200 ppm, dimethyl phenylphosphonate 20 to
It is good to add 400 ppm. However, the method for producing the polyester A is not limited to the above.

The biaxially oriented polyester film in the present invention is particularly preferably used as a support for magnetic recording, and is a support for versatile video tapes, a support for high-grade metal tapes such as digital video or digital audio, It is particularly effective when used as a support for a video tape for high-speed dubbing which is subjected to a strong abrasion load in the dubbing process. Also, since the film has very few voids, it is also suitable for capacitors and packaging. Further, since the surface is dense and uniformly roughened, it is suitable for a drawable film and a light diffusion film used for a liquid crystal screen. [Method of measuring physical properties and method of evaluating effects] The method of measuring characteristic values and the method of evaluating effects of the present invention are as follows.

(1) Number of protrusions on film surface, protrusion height, relative standard deviation of protrusion height 2 Scanning electron microscope [E]
SM-3200, manufactured by Elionix Co., Ltd. and a cross-section measurement device [PMS-1, manufactured by Elionix Co., Ltd.] are used to perform image processing on the measured values of the height of protrusions when the height of the flat surface of the film is 0. It is sent to an apparatus [IBAS2000, manufactured by Carl Zeiss Co., Ltd.] and a film surface projection image is reconstructed on the image processing apparatus. Next, the highest value among the individual projections obtained by binarizing the projections in this surface projection image is set as the projection height of the projection, and this is calculated for each projection. This measurement was repeated 500 times at different places, and those having a diameter of 20 nm or more were regarded as protrusions, and the number of protrusions and the average protrusion height were determined. Moreover, the standard deviation of the height distribution was calculated based on the individual projection height data. The value obtained by dividing the obtained standard deviation by the average value of the above heights was taken as the relative standard deviation. The magnification of the scanning electron microscope is selected to be 1000 to 8000 times. In some cases, the high-precision optical interference type 3
Even if the number information such as height information and peak count obtained by a three-dimensional surface analyzer (TOPO-3D manufactured by WYKO, objective lens: 40 to 200 times, use of a high precision camera is effective) is read into the value of the SEM. good. Regarding the number of protrusions, in order to capture the protrusions three-dimensionally, the film was tilted by 82.5 °, and a photograph was taken with an electron microscope (SEM) at a magnification of 10,000 to 500,000, and 100 fields of view were measured. The number of protrusions may be converted from the value per 1 mm ² .

(2) Number of Particles Contained in Surface Layer In the present invention, the surface layer means a portion from the film surface to a depth of 3D. Here, 3D means the average particle diameter D × 3 of the particles contained in the film. The term "particles" as used herein means that when the cross section of a film is observed with a TEM, the particles observed in the thickness direction are those having an average protrusion height greater than or equal to the average protrusion height obtained by the above method. But particles smaller than that are not considered as particles.

A transmission electron microscope (TE
It is also possible to observe the number of particles existing in a portion from the surface to a depth of 3D at a magnification of 3000 to 10000 in a 500 field of view and convert it per 1 mm ² .

(3) Average Particle Size of Particles in Film The polyester is removed from the film by a plasma ashing method to expose the particles. The processing conditions are such that the polymer is incinerated but the particles are not damaged as much as possible. The particles are observed with a scanning electron microscope (SEM), and the particle image is processed with an image analyzer. The magnification of SEM is approximately 2000 to 10,000 times, and the visual field in one measurement is appropriately selected from approximately 10 to 50 μm on each side. The volume average diameter d is obtained by the following formula from the volume fraction with the particle diameter when the number of particles is 5000 or more by changing the observation location.

D = Σdi · Nvi Here, di is the particle size, and Nvi is the volume fraction thereof.

When the particles are organic particles or the like and are significantly damaged by the plasma low temperature ashing method, the following method may be used.

The cross section of the film was examined with a transmission electron microscope,
Observe at 3,000 to 100,000 times. The TEM section thickness was set to about 1000 angstroms, and the location was changed to 5
The volume average diameter d is calculated from the above formula by measuring over 100 fields of view.

(4) Crystallization parameter ΔTcg This was measured using a Perkin-Elmer DSC (differential scanning calorimeter) type II. The measurement conditions of DSC are as follows. That is, 10 mg of the sample was set in the DSC device, and 3
After melting for 5 minutes at a temperature of 00 ° C., it is quenched in liquid nitrogen. The temperature of this quenched sample is raised at 10 ° C./min, and the glass transition point Tg is detected. The temperature is further raised, and the difference (Tcc-Tg) between Tcc and Tg, which is the cold crystallization temperature Tcc based on the crystallization exothermic peak temperature from the glass state, is used as the crystallization parameter ΔTc.
Define as g.

(5) High-speed sharpness and friction coefficient A film slit into a tape having a width of 1/2 inch was used to guide a guide pin (surface roughness: Ra) using a tape running tester. 100nm) running (travel speed 25
0 m / min, 1 pass, wrapping angle: 60 °, running tension: 90 g). At this time, visually observing the guide pins after the film has finished running, those with no white powder adhesion are excellent (○), those with slight white powder adhesion are good (△), and many white powder Those that adhered were judged to be defective (x). Good is desirable, but good is practically usable.

In this measurement, the tension on the entry side and the exit side of the guide pin was measured, and the coefficient of friction was determined according to the following formula.

Μk = 2.20 log (T1 / T2) where T1 is the inlet tension and T2 is the outlet tension. The guide diameter is 6 mmφ, the guide material is SUS27 (surface roughness 0.2S), the winding angle is 180 °, and the running speed is 3.3 cm / sec. Μk obtained by this measurement
Is 0.35 or less, the coefficient of friction is good (○), 0.35
When it exceeded, it was determined that the friction coefficient was poor (x). This μ
The value of k is a critical point when processing magnetic recording media, capacitors, packaging films and the like.

(6) Evaluation of Dropout A magnetic coating composition having the following composition is applied to the film by a gravure roll, magnetically oriented, and dried. In addition, a small test calendar device (steel roll / nylon roll,
(5 stages), after calendering at a temperature of 70 ° C. and a linear pressure of 200 kg / cm, curing is performed at 70 ° C. for 48 hours. The above tape raw fabric was slit into 1/2 inch to prepare a pancake. A length of 250 m from this pancake was incorporated into a VTR cassette to obtain a VTR cassette tape.

(Composition of magnetic paint) Co-containing iron oxide: 100 parts by weight Vinyl chloride / vinyl acetate copolymer: 10 parts by weight Polyurethane elastomer: 10 parts by weight Polyisocyanate): 5 parts by weight Lecithin: 1 Parts by weight-methyl ethyl ketone: 75 parts by weight-methyl isobutyl ketone: 75 parts by weight-toluene: 75 parts by weight-carbon black: 2 parts by weight-lauric acid: 1.5 parts by weight Using the resulting VTR for household use, An evaluation was made. A television test signal generator provided a 4.4 MHz signal and measurements were performed using a dropout counter. For the dropout, a reproduction signal attenuation of −16 dB or more and a length of 15 μsec or more was obtained. 25 ° C, 6
The number of dropouts after 100 times of reproduction / rewinding for 3 minutes under 5% RH was converted to the number of dropouts per minute, and 0 to 15 pieces / minute were: Excellent (○) 16 to 30 pieces Those of / min were: good (△) 31 items / min: were bad (×).

(7) Film temperature A radiation thermometer, a contact type surface thermometer, or a thermolabel was attached to the film for measurement. The film temperature in the molten state was measured by using a radiation thermometer or a thermocouple inserted in the molten film.

(8) Laminate Thickness Using a transmission electron microscope (H-600 type manufactured by Hitachi), the film cross section was subjected to an ultrathin section method (R) at an acceleration voltage of 100 kV.
uO ₄ staining), the interface is captured, and the laminated thickness is determined. The magnification is usually selected according to the laminated thickness to be judged, and is not particularly limited, but 10,000 to 100,000 times is suitable.

(9) Surface roughness Ra and its relative standard deviation Atomic force microscope (NanoScope II manufactured by Digital Instruments)
The measurement was performed using AFM). The measurement conditions are as follows.

Tip: SiN cantilever integrated needle “OMCL” scanning mode: constant force mode (topographic mode) scanning range: 50 μm □ scanning speed: 0.35 Hz Measurement environment: room temperature, 50 μm in the atmosphere AFM Area of 5 μm square in the image is 9
Visual field selection The Ra value and its relative standard deviation were measured for each visual field. This measurement was performed for 10 visual fields of 50 μm square, and the average Ra and relative standard deviation for 50 μm 1 visual field were determined.

[0048]

EXAMPLES Next, the present invention will be explained based on examples.

Example Polyester A, which was polymerized by a conventional method, was used as polyester A (polymerization catalyst: magnesium acetate 0.2
% By weight, 0.03% by weight of antimony trioxide, and 0.70% by weight of dimethylphenylphosphonate as a phosphorus compound. ) Was used (intrinsic viscosity: 0.60, melting point: 25
8 ° C, ΔTcg: 50 ° C).

As the polyester B, polyethylene terephthalate polymerized by a conventional method using 0.06% by weight of magnesium acetate, 0.008% by weight of antimony trioxide and 0.02% by weight of trimethyl phosphate was used (intrinsic viscosity : 0.62, melting point: 259 ° C, ΔTc
g: 84 ° C). Neither polyester A nor B contains inert particles.

Examples 1 and 2 Monolayer films of polyester A were prepared. Polyester A
Pellets were dried at 180 ° C. for 3 hours, melt-extruded at 290 ° C. using a known extruder, and then wound on a casting drum having a surface temperature of 30 ° C. using an electrostatic cast method, and cooled. Solidified to make an unstretched film. The surface of this unstretched film that was not in contact with the drum was subjected to heat treatment using a known radiation heater under the conditions such that the film surface had the following temperature.

Example 1: 160 ° C., 15 seconds Example 2: 180 ° C., 7 seconds The heat-treated film was heated at 90 ° C. in the longitudinal direction of 3.
Stretched 4 times, further stretched 3.5 times in the width direction at 95 ° C. at a stretching rate of 2000% / min using a known stenter, and further heat-treated at 210 ° C. for 5 seconds under a constant length,
A biaxially oriented film having a thickness of 15 μm was obtained.

Examples 3 and 4 A / B / A three-layer laminated film was prepared. Pellets of polyesters A and B dried at 180 ° C. for 3 hours were respectively fed to two extruders, melted at 290 ° C., and combined and laminated in a rectangular confluent block (feed block) for three layers. Then, a biaxially oriented laminated film having a total thickness of 15 μm (A layer thickness of 1 μm) was obtained by the same process as in Examples 1 and 2. However, the heat treatment condition for the unstretched film is 1
The temperature was 80 ° C. for 7 seconds. Further, in Example 4, the stretching ratio was 4.0 times in the longitudinal direction and 4.5 times in the width direction.

Example 5 A laminated film having an A / B / A three-layer structure was prepared. Example 1,
Melt extrusion was carried out in the same manner as in 2, and the mixture was cooled and solidified to prepare an unstretched film. This unstretched film was heat-treated on a silicone roll heated to 140 ° C. for 10 seconds and then biaxially stretched in the same manner as in Examples 1 and 2 to obtain a biaxially oriented laminated film.

Comparative Examples 1 and 2 Inactive particles were added to Polyester A (colloidal silica particles having an average particle size of 0.3 μm dispersed in ethylene glycol and added at the time of polymerization) to form pellets (particle content: 0.5). %) To obtain a biaxially oriented monolayer film having a total thickness of 15 μm in the same process as in Examples 1 and 2. However, the heat treatment conditions for the unstretched film were 190 ° C. and 5 seconds in Comparative Examples 1 and 2. Further, in Comparative Example 2, the stretching ratio was 4.0 times in the longitudinal direction and 5.0 times in the width direction.

Comparative Example 3 Using only polyester B, a laminated biaxially oriented film having a total thickness of 15 μm was obtained by the same process as above.

Comparative Example 4 Polyester B was prepared by adding inactive particles (colloidal silica particles having an average particle size of 0.3 μm dispersed in ethylene glycol and added at the time of polymerization) to pellets (particle content 2% by weight) A. As a layer polymer, a film having a three-layer structure of A / B / A was obtained without heat treatment before stretching.

Comparative Example 5 Using polyester B, an unstretched film was prepared in the same manner as in Examples 1 and 2. This film was stretched 3.4 times in the longitudinal direction at a temperature of 90 ° C., and further stretched 3.5 times in the width direction at a stretching rate of 2000% / min at 95 ° C. using a stenter, and further stretched to a constant tension. Then, heat treatment was performed at 210 ° C. for 5 seconds to obtain a biaxially oriented film having a total thickness of 15 μm.

This film was wrapped in a silicone roll heated to 140 ° C., and a biaxially oriented PET film containing 0.3 wt% of spherical silica having a particle size of 0.3 micron was wound around it beforehand. Linear pressure of 500 kg
/ Cm and pressure treatment. The film made using the polyester B has a depression on the surface by this treatment,
The Ra value was 4.6 nm and the relative standard deviation was 0.70.

The results of evaluating the films prepared in the above Examples and Comparative Examples are shown in Table 1. It can be seen that all the samples within the scope of the present invention are superior to the comparative example in high-speed sharpness, dropout, and friction coefficient.

[0061]

[Table 1]

[0062]

According to the polyester film and the method for producing the same of the present invention, crystallization of polyester A is utilized without depending on the contained particles to form a specific number or more of fine projections on the film surface, and the surface roughness is also improved. Parameter R
Since the value of the relative standard deviation of a is set to a specific value or more, it is possible to suppress the generation of voids, form protrusions that are not easily broken, and significantly improve the scratch resistance of the film surface, and use it as a magnetic recording medium. The effect of being able to suppress the occurrence of dropout when

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display area B29L 7:00 9:00 C08L 67:02

Claims (6)

[Claims] 1. A biaxially oriented polyester film containing polyester A as a main component, wherein the value of the relative standard deviation of the surface roughness parameter Ra of 5 μm square on at least one surface thereof is 0.30 or less. Characteristic biaxially oriented polyester film. 2. The number of protrusions on the film surface is 500.
The polyester film according to claim 1, wherein the number is 0 / mm ² or more. 3. The polyester film according to claim 1, wherein the projections on the film surface are projections mainly caused by crystals. 4. The number of projections / the number of particles (NR), which is the ratio of the number of projections on the surface of the film to the number of particles contained in the film, is 5 or more. Polyester film. 5. The polyester film according to claim 1, wherein the crystallization parameter ΔTcg of the polyester A is 70 ° C. or lower. 6. A laminated polyester film, characterized in that the polyester film according to any one of claims 1 to 5 is laminated on at least one side of a film containing polyester B as a main component.