JPH071695A - Laminated polyester film and its production - Google Patents

Abstract

PURPOSE:To enhance the running properties and resistance to scratching of a film and obtain a good image quality in use for a magnetic recording medium by a method wherein a biaxially oriented film in use is obtained by laminating a layer mainly composed of a polyester A on one surface of a layer mainly composed of a polyester B. CONSTITUTION:In a biaxially oriented film obtained by laminating a layer A mainly composed of a polyester A on one surface of a layer B mainly composed of a polyester B, a center line average surface roughness difference DELTARa between both the surfaces of the film is not less than 0.005mum, 5X10<3>/mm<2> or more projections are formed on the surface having a larger surface roughness Ra, and a projection/particle number ratio, i.e., a ratio of the number of projections to the number of particles contained in the layer A as a surface layer, is 5 or more. The kind of the polyester A is not particularly limited, but the crystallizing parameter DELTA Tcg of the polyester A is preferably 60 deg.C or lower.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laminated polyester film, and more particularly to a laminated polyester film having fine projections formed on its surface and suitable for use in magnetic recording media.

[0002]

2. Description of the Related Art Polyester films are widely used for various purposes. As the processing speed increases in the processing of polyester film, such as printing in packaging, magnetic layer coating in magnetic recording media, or thermal transfer layer coating in thermal transfer applications, or the required quality of the final product. With sophistication, the polyester film is required to have better surface properties such as running property and abrasion resistance. It is known that it is effective to uniformly form fine projections 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 surface of the film is known (for example, JP-A-59-171623). . 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, for example, in magnetic recording media applications, in order to obtain high output characteristics, it is increasingly demanded that the surface be even flat while forming fine projections on the surface. . The conventional polyester film having protrusions formed on the surface thereof by containing particles (for example, inert particles) as described above is becoming unable to meet such requirements.

That is, in the conventional polyester film, the friction coefficient is lowered and the running property in the processing step,
In order to improve handling properties and the like, particles such as inert particles different from polyester are added to the polyester to form surface protrusions, so that voids are easily generated around the particles.
When the voids are generated, the formed projections are easily broken or the contained particles are easily dropped off, and the film surface is easily scraped off by a guide pin or the like during VTR traveling or the like, which causes a problem that white powder is generated.

Further, the conventional polyester film has
Sufficiently good image quality, that is, S
There is also a problem that it is becoming difficult to obtain / N (signal / noise ratio).

An object of the present invention is to provide a laminated polyester film having desired fine projections formed on the surface thereof by utilizing the crystallization of polyester essentially without relying on the particles contained therein, and thereby the film of the film is provided. The object is to obtain better runnability, abrasion resistance, and sufficiently good image quality when used for magnetic recording medium applications.

[0008]

A polyester film of the present invention for this purpose has a biaxially oriented structure in which an A layer containing polyester A as a main component is laminated on at least one side of a B layer containing polyester B as a main component. The film having a center line average surface roughness difference ΔRa on the front and back sides of 0.005 μm or more and having a large surface roughness Ra is 5 ×.
10 ³ protrusions / mm ² or more are formed, and the number of protrusions / the number of particles (N _R ), which is the ratio of the number of protrusions to the number of particles contained in the A layer forming the surface, is 5 or more. It is characterized by being.

That is, in the laminated polyester film of the present invention, the ratio N _R of the number of projections formed on the surface to the number of particles contained in the surface layer is 5 or more, and the surface projections essentially contain particles. Instead of being formed by crystallization, it is formed by utilizing the crystallization of polyester A itself. Therefore, the problem of void generation when particles are added is substantially eliminated, and protrusions that are difficult to cut are formed. When N _R is less than 5, the ratio of protrusions formed by the contained particles increases, and the ratio of protrusions that are easily scraped due to void formation increases, so that desirable abrasion resistance cannot be obtained.

Thus, on the surface of the film,
Most or all of the surface protrusions are formed by utilizing the crystallization of polyester A, and the number of surface protrusions is set to 5 × 10 ³ / mm ² or more, so that minute protrusions that are not easily broken are formed on the film surface. Formed uniformly and densely. As a result, a laminated polyester film having a good surface running property and a high abrasion resistance can be obtained. The number of surface protrusions is 5 × 10
^If it is less than ³ / mm ² , the effect of reducing the friction coefficient due to the formation of protrusions becomes small, and it is difficult to obtain good running performance.

Further, in the laminated polyester film of the present invention, the difference ΔRa between the center line average surface roughnesses of the front and back sides is set to 0.005 μm or more, and 5 × 10 ³ pieces are formed on the surface having the larger surface roughness Ra. / Mm ² or more projections are formed. By setting ΔRa to be 0.005 μm or more,
The surface with the higher Ra is a surface with a low coefficient of friction, extremely good runnability, and extremely high abrasion resistance due to the protrusions of 5 × 10 ³ pieces / mm ² or more formed by utilizing crystallization. Become. On the other hand, the surface with the smaller Ra has a surface roughness smaller than that of the surface on which the fine protrusions are formed by utilizing the above-mentioned crystallization, and thus has an extremely flat surface. That is, one surface shows extremely good running property and extremely high abrasion resistance, and the other surface shows excellent flatness, which exactly matches the high quality requirements of the magnetic tape, that is, good running property and abrasion resistance. , An ideal film that can satisfy both the image quality. Here, regarding whether or not the surface protrusions are made of fine crystals of polyester A, etching is performed below the target protrusion in the film thickness direction with an appropriate solvent, and the cause of the formation of the protrusions is When it remains as an insoluble substance, it is a particle added from the outside or a particle deposited inside (I). When there is substantially nothing that remains as an insoluble matter, it can be presumed that the substance that forms the protrusion is fine crystals (II). As the above-mentioned solvent, for example, a mixed solvent of phenol / carbon tetrachloride (weight ratio: 6/4) is preferably used.
The frequency of I when the field of view was set to about 1 mm ² by this method, II
It is preferable that the value of II / (I + II) is 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 7
0 ° C or lower, preferably 65 ° C or lower, more preferably 6
It is preferably 0 ° C or lower. Crystallization parameter ΔT
When cg is larger than 70 ° C., it is difficult to obtain the surface protrusions targeted by the present invention. Even if it is obtained, the runnability and abrasion resistance of the film surface are poor.

The polyester A is not particularly limited as long as it satisfies the above conditions, but ethylene terephthalate, ethylene α, β-bis (2-chlorophenoxy) ethane-4,4'-dicarboxylate, ethylene 2, It is particularly preferable when at least one structural unit selected from 6-naphthalate units is the main constituent.
Of these, polyesters containing ethylene terephthalate as a main constituent are particularly preferable. Two or more kinds of polyesters may be mixed, or a copolymerized polymer may be used, as long as the object of the present invention is not impaired.

The laminated polyester film of the present invention is a laminated film in which an A layer containing polyester A as a main component is laminated on at least one side of a B layer containing polyester B as a main component. / A3 layer structure, and further may have a structure of 4 or more layers. The type of this polyester B is not particularly limited. The polyester B preferably does not contain particles, but may contain it.

Next, the method for producing the polyester film of the present invention will be described. In the present invention, the layers consisting of polyester A and polyester B are laminated at the stage of polymer tube or die and discharged into a sheet to form an unstretched film. Then, at least one surface of the unstretched film is heat-treated (that is, the surface of the layer A on which surface protrusions are to be formed), and then biaxially stretched.
Here, the unstretched film refers to a state immediately after being extruded from the die and before being cooled and solidified, to a slightly stretched (up to about 2 times) slightly 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, a temperature (Tm + 1) higher than the glass transition temperature Tg of polyester A and 100 ° C. higher than the melting temperature Tm is used.
It is preferable that the unstretched film is heat-treated from the surface opposite to the surface in contact with the cooling roll at a temperature of not more than 00 ° C.), and then the unstretched film is biaxially stretched to form a desired surface protrusion. More preferably 20 ° C from Tg
High temperature (Tg + 20 ° C.) or higher and temperature higher than Tm by 80 ° C. (Tm + 80 ° C.) or lower, more preferably Tg
The temperature is 40 ° C. higher (Tg + 40 ° C.) or more and Tm or less. As a method of heat-treating the unstretched film from the surface opposite to the surface in contact with the cooling roll, hot air or radiant heat from an infrared heater can be used, but the method is not limited to this.

The surface roughness of the cooling roll surface is 0.2S.
Above, and 10S or less is preferable because the crystallinity of the film surface before stretching can be increased to a desired crystallinity. 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 solidified by cooling, the temperature (Tcc-20 ° C.) of the surface (or surface layer) of at least one surface thereof is 20 ° C. lower than the cold crystallization temperature Tcc of polyester A. Above, the temperature (Tmc +
40 ° C.) or less so that it is kept for 0.5 to 100 seconds, and then the glass transition temperature Tg of polyester A or higher and a temperature 20 ° C. higher than Tcc (Tcc + 20 ° C.).
By biaxially stretching below, a desired surface protrusion is formed, which is preferable. More preferably, Tcc or more,
And Tmc or less, 0.5 to 50 seconds, more preferably,
The heat treatment is such that it is maintained at Tcc or more and Tmc or less for 0.5 to 20 seconds.

Further, in the present invention, the unstretched film is finely stretched in the uniaxial direction to give a birefringence of 0.5 × 10 ⁻³ to 50 ×.
10 ^-3, and then the temperature of at least one surface (or surface layer) of the slightly stretched film is 20 ° C lower than the cold crystallization temperature Tcc of polyester A (Tcc-20 ° C) or more, and the cooling crystallization temperature is 40 ° C higher temperature than Tmc (T
mc + 40 ° C.) or less, heat treatment is performed for 0.3 to 50 seconds, and then the glass transition temperature T of polyester A
g or more and a temperature 20 ° C higher than Tcc (Tcc + 20
By biaxially stretching at (° C.) or less, a desired surface protrusion is formed, which is preferable. More preferably, Tcc or more and Tmc or less, 0.5 to 20 seconds, and further preferably, a temperature 10 ° C higher than Tcc (Tcc + 10 ° C) or higher and a temperature 20 ° C lower than Tmc (Tmc-20 ° C).
In the following, the heat treatment is maintained for 0.5 to 15 seconds.

The heat treatment method includes a method of heat treatment by winding on a heating roll, a method of hot air treatment from the surface opposite to the surface in contact with the roll in the state of being wound on the roll, or a surface in contact with the roll in the state of being wound on the roll. There is a method of heat treatment with an infrared heater from the opposite side, a method of heat treatment with an infrared heater between rolls, a method of heating with a stenter, and the like, but the method is not particularly limited to these.

Further, in the present invention, the temperature of at least one surface (or surface layer) of the melt-extruded film containing polyester as a main component is 70 ° C. lower than the cooling crystallization temperature Tmc of polyester A (Tmc-70 ° C.). ) Or more and at a temperature falling crystallization temperature Tmc of the polyester A or less, for 0.5 to 20 seconds, then cooled to a glass transition temperature Tg or less of the polyester A, and then biaxially stretching the unstretched film. It is preferable because a desired surface protrusion is formed.

As described above, the treatment method is a method in which a film having a high temperature immediately after extrusion is gradually cooled to be crystallized, or a film which has been once cooled and solidified is reheated to be crystallized, or There is a method of heat treatment in the 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.

Polyethylene terephthalate (PET) is preferably used as the polyester A according to the present invention. It is desirable that the polyester A contains substantially no particles. In the polymerization of polyester A, it is preferable to use antimony trioxide as a polymerization catalyst, and to use a acetate as a metal compound as a transesterification catalyst in order to reduce ΔTcg and enhance the crystal nucleating agent effect.
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 phosphonate as the phosphorus compound added during the polymerization of PET. However, the method for producing the polyester A is not limited to the above. To enhance the effect of nucleating agents,
Increasing the amount of catalyst added is not preferable because it causes precipitation of internal particles and increases haze.

[Physical property measuring method and effect evaluating method] The characteristic value measuring method and effect evaluating method of the present invention are as follows. (1) Number of protrusions on the film surface A flat surface of the film surface was measured by a scanning electron microscope [ESM-3200, manufactured by Elionix Co., Ltd.] using a two-detector method and a cross-section measurement device [PMS-1, manufactured by Elionix Co., Ltd. The height measurement value of the protrusions when the height is 0 is sent to an image processing apparatus [IBAS2000, manufactured by Carl Zeiss Co., Ltd.] to reconstruct the film surface protrusion image on the image processing apparatus. Next, the highest value among the individual projections obtained by binarizing the projections in this surface projection image is taken as the height of the projection, and this is calculated for each projection. This measurement was repeated 500 times at different places, and those having a height of 20 nm or more were used as protrusions to determine the number of protrusions. The magnification of the scanning electron microscope is selected to be 1000 to 8000 times. In some cases, a high-precision optical interference type three-dimensional surface analyzer (WY
KO TOPO-3D, objective lens: 40-200
It is also possible to read the number information such as the peak count obtained by double use of the high resolution camera (effective) and replace it with the value of the SEM. In order to capture the projections 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 times, and the number of projections was calculated from the average value of 100 visual field measurements. It may be converted 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 cross section of the film is observed with a transmission electron microscope (TEM), and the number of particles existing in a portion from the surface to a depth of 3D is magnified from 3000 to 10000.
Observation is performed twice at 500 times, and the average number of particles converted per 1 mm ² is obtained.

(3) Average Particle Diameter of Particles in Film The polymer is removed from the film by a low temperature 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 the 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 from the particle diameter and its volume fraction with the number of particles of 5000 or more by changing the observation location. d = Σd _i · Nv _i where d _i is the particle size and Nv _i 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. Using a transmission electron microscope (TEM), the film cross section is 30
Observe at 00-100,000 times. The section thickness of the TEM is set to about 1000Å, the location is changed and 500 or more visual fields are measured, and the volume average diameter d is obtained from the above formula.

(4) Crystallization parameter ΔTcg DSC (Differential Scanning Calorimeter) II manufactured by Perkin Elmer II
It was measured using a mold. The measurement conditions of DSC are as follows. That is, 10 mg of the sample is set in the DSC device,
After melting at a temperature of 300 ° C. for 5 minutes, it is rapidly cooled 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 crystallization exothermic peak temperature from the glass state is applied to the cold crystallization temperature Tcc,
The endothermic peak temperature based on crystal melting was taken as the melting temperature Tm, and similarly the crystallization exothermic peak temperature at the time of cooling was taken as the lowered crystallization temperature Tmc. The difference between Tcc and Tg (Tcc-Tg) is defined as the crystallization parameter ΔTcg.

(5) Center line average surface roughness Ra High precision thin film step measuring instrument ET-10 manufactured by Kosaka Laboratory Ltd.
Was measured using. The conditions are as follows, and the average value of 20 measurements was used as the value.・ Stylus tip radius: 0.5 μm ・ Stylus load: 5 mg ・ Measurement length: 1 mm ・ Cutoff value: 0.08 mm Ra is defined, for example, by Jiro Nara, “Measurement / Evaluation Method of Surface Roughness” ( General Technical Center, 1983).

(6) Birefringence An Abbe refractometer is used to measure the longitudinal direction refractive index n _MD and the width direction refractive index n _TD of the uniaxially oriented film, and the difference between these values, that is, | n _MD −n _TD Defined with |. The light source was sodium D line (wavelength: 589 nm), and the mount solution was methylene iodide at 25 ° C. and 65% RH.

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

(8) Image Quality (Chroma S / N) 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), temperature: 70 ° C., linear pressure: 200 kg / cm, followed by calendaring, and curing at 70 ° C. for 48 hours.
The above tape raw material is slit into 1/2 inch and the length is 25
A length of 0 m was incorporated into a VTR cassette to obtain a VTR cassette tape. (Composition of magnetic paint) -Co-containing iron oxide (BET value 50 m ² / g): 100 parts by weight-ESREC A (Sekisui Chemical's vinyl chloride / vinyl acetate copolymer): 10 parts by weight-Nipporan 2304 (Nippon Polyurethane) Polyurethane Elastomer): 10 parts by weight-Coronate L (Polyisocyanate made by Nippon Polyurethane): 5 parts by weight-Lecithin: 1 part 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 a VTR for home use on this tape, a 100% chroma signal was recorded by a TV test waveform generator (TG7 / U706) made by Shibasoku, and the reproduced signal was used as shibasoku. The chroma S / N was measured with a color video noise measuring instrument (925 D / 1) manufactured by K.K.

(9) Runnability (friction coefficient μk) A film slitted into a tape having a width of 1/2 inch was used with a tape runnability tester SFT-700 (manufactured by Yokohama System Research Co., Ltd.). The film was run in an atmosphere of 20 ° C. and 60% RH, and the initial coefficient of friction was determined by the following formula (film width was 1/2 inch). μk = 2 / πln (T ₂ / T ₁ ) where T ₁ is the inlet tension and T ₂ is the outlet tension. The guide diameter is 6 mmφ, the guide material is SUS27 (surface roughness 0.2S), the winding angle is 90 °, and the running speed is 3.
It is 3 cm / sec. When μk obtained by this measurement was 0.3 or less, it was determined that the coefficient of friction was good, and when it exceeded 0.3, it was determined that the coefficient of friction was poor. This μk is a critical point that affects the handling property when processing the film for magnetic recording media, capacitors, packaging, and the like.

(10) Scraping resistance The video tape 250m assembled in the cassette as described in (6) above is fast forwarded and rewound 100 times with a household VTR, and the pins in the VTR that come into contact with the tape running surface are repeated 100 times. The amount of shavings adhering to the pins in the cassette,
The amount of shavings adhering to the tape running surface was observed and judged as follows. No adhesion of shavings: "Excellent", Slight adhesion of shavings occurs slightly, but there is no problem in using it for video applications: "Good", a lot of shavings are generated, making it impossible to use for video applications : Judged as "poor".

[0034]

EXAMPLES Next, the present invention will be explained based on examples. Example As polyester A, polyethylene terephthalate polymerized by a conventional method (polymerization catalyst: magnesium acetate 0.1
0% by weight, antimony trioxide 0.03% by weight, dimethyl phenylphosphonate as a phosphorus compound 0.35% by weight
Was used (intrinsic viscosity: 0.60, melting point: 25)
8 ° 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. : 0.62, melting point: 259 ° C).

Examples 1 and 2 A laminated film having a two-layer structure of polyester A / polyester B was prepared. 180 ° C for pellets of polyester A and B
After drying for 3 hours, feed each to two extruders,
The mixture was melted at 0 ° C., and was joined and laminated by a rectangular joining block (feed block) for two layers. Using an electrostatically applied casting method, the film was wound on a casting drum having a surface temperature of 30 ° C., cooled and solidified to prepare an unstretched film. The surface of the unstretched film on the layer A side was heat-treated using a known radiation heater under the conditions such that the film surface had the following temperature. The heat treatment conditions were 160 ° C. and 10 seconds in both Examples 1 and 2. After the heat treatment, the 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 95 ° C. at a stretching rate of 2000% / min using a stenter. Heat treatment at 210 ℃ for 5 seconds under constant length, total thickness 15μm
A biaxially oriented laminated film (A layer thickness: 1.0 μm) was obtained.

Examples 3, 4 A / B / A three-layered laminated film was prepared. Pellets of polyesters A and B were dried at 180 ° C. for 3 hours, then supplied to two extruders, melted at 290 ° C., and combined and laminated by a rectangular confluent block (feed block) for three layers. Then, a total thickness of 15 is obtained by the same process as in Examples 1 and 2.
A biaxially oriented laminated film of μm was obtained. However, the heat-treated surface was the surface not in contact with the drum, and the heat treatment conditions for the unstretched film were 190 ° C. for 1.5 seconds in Example 3 and Example 4
In the above, the temperature was 180 ° C. for 2 seconds.

Example 5 A laminated film having an A / B two-layer structure was prepared. Pellets of polyester A and polyester B were dried at 180 ° C for 3 hours, then fed to two extruders each and melted at 290 ° C to form a rectangular confluent block (feed block) for two layers.
Then, extrusion was carried out after confluent lamination, and the film was wound around a cooling roll having a surface temperature of 30 ° C. so that the surface of the polyester B layer was in contact with it by an electrostatic applied casting method.
After blowing hot air of 0 ° C., it was cooled and solidified to prepare an unstretched film. This unstretched film was stretched 3.5 times in the longitudinal direction at a temperature of 90 ° C., and further stretched 4.0 times in the width direction at 95 ° C. at a stretching rate of 2000% / min using a stenter.
Further, it was heat-treated at 210 ° C. for 5 seconds under a fixed length to obtain a biaxially oriented film having a total thickness of 15 μm.

Example 6 A laminated film having an A / B two-layer structure was prepared. Pellets of polyester A and polyester B were dried at 180 ° C for 3 hours, then fed to two extruders each and melted at 290 ° C to form a rectangular confluent block (feed block) for two layers.
Then, it was extruded after being combined and laminated, and was wound around a casting drum having a surface temperature of 30 ° C. and cooled and solidified by using an electrostatic applied casting method to prepare an unstretched film. This unstretched film was stretched 1.8 times in the longitudinal direction at a temperature of 98 ° C., heat-treated for 4 seconds between rolls using an infrared heater at 230 ° C., and further in the longitudinal direction at a temperature of 90 ° C.
At a stretching speed of 2000% / min, and then stretched 4.5 times in the width direction at 95 ° C. using a stenter.
Further, it was heat-treated at 210 ° C. for 5 seconds under a fixed length to obtain a biaxially oriented film having a total thickness of 15 μm.

Comparative Examples 1 to 4 In Comparative Examples 1 and 2, a laminated film having an A / B two-layer structure was used, and in Comparative Examples 3 and 4, an laminated film having an A / B / A three-layer structure was used. A biaxially oriented laminated film was prepared by the same process as in Examples 1 to 4 above.
ΔTcg, ΔRa, the number of protrusions, N _R, etc. were changed respectively.

Comparative Example 5 A laminated film having a total A / B2 structure was prepared. Pellets of polyester A and polyester B were dried at 180 ° C for 3 hours, then fed to two extruders each and melted at 290 ° C to form a rectangular confluent block (feed block) for two layers.
Then, it is extruded after being combined and laminated, and is wound around a cooling roll having a surface temperature of 30 ° C. so that the surface of the polyester B layer is in contact with it by an electrostatic applied casting method.
After blowing hot air at ℃, it was cooled and solidified to prepare an unstretched film. This unstretched film was stretched 3.5 times in the longitudinal direction at a temperature of 90 ° C. and further stretched at a stretching speed of 2 using a stenter.
The film was stretched at a rate of 000% / min at 95 ° C in the width direction by 4.0 times, and further heat-treated at 210 ° C for 5 seconds under a constant length to obtain a biaxially oriented film having a total thickness of 15 µm.

Comparative Example 6 A laminated film having an A / B two-layer structure was prepared. Pellets of polyester A and polyester B were dried at 180 ° C for 3 hours, then fed to two extruders each and melted at 290 ° C to form a rectangular confluent block (feed block) for two layers.
Then, it was extruded after being combined and laminated, and was wound around a casting drum having a surface temperature of 30 ° C. and cooled and solidified by using an electrostatic applied casting method to prepare an unstretched film. This unstretched film was stretched 2.8 times in the longitudinal direction at a temperature of 98 ° C., heat-treated for 4 seconds between rolls using an infrared heater at 230 ° C., and further in the longitudinal direction at a temperature of 90 ° C.
At a stretching speed of 2000% / min, and then stretched 4.5 times in the width direction at 95 ° C. using a stenter.
Further, it was heat-treated at 210 ° C. for 5 seconds under a fixed length to obtain a biaxially oriented film having a total thickness of 15 μm.

[0043]

[Table 1]

[0044]

According to the laminated polyester film of the present invention, the 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 at least a specific value or more on the front and back sides. Difference in surface roughness ΔR
Since it has a, it is possible to form a protrusion that is hard to be shaved by suppressing the generation of voids, and it is possible to make the front and back surfaces have surface morphologies that can achieve desired runnability, abrasion resistance, image quality, etc., respectively. Especially, a film suitable for use in a magnetic recording medium can be obtained.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuya Okamoto 1-1-1, Sonoyama, Otsu City, Shiga Prefecture Toray Co., Ltd. Shiga Plant

Claims (8)

[Claims] 1. A biaxially oriented film obtained by laminating an A layer containing polyester A as a main component on at least one surface of a B layer containing polyester B as a main component, wherein the difference in the center line average surface roughness between the front and back surfaces. On the surface having ΔRa of 0.005 μm or more and a large surface roughness Ra, 5 × 10 ³ pieces /
mm ² or more protrusions are formed, and the number of protrusions / the number of particles (N _R ), which is the ratio of the number of protrusions to the number of particles contained in the layer A forming the surface, is 5 or more. Characteristic laminated polyester film. 2. A crystallization parameter ΔTcg of polyester A in at least the number of protrusions of the A layers of the A layers.
Is 70 ° C. or less, the laminated polyester film according to claim 1. 3. The method for producing a polyester film according to claim 1, wherein at least one surface of the unstretched film is heat-treated, and then the unstretched film is biaxially stretched. 4. In the process of cooling the melt-extruded film containing polyester as the main component on the surface of a chill roll, at a temperature not lower than the glass transition temperature Tg of polyester A and not higher than the melting temperature Tm by 100 ° C. (Tm + 100 ° C.),
The method for producing a polyester film according to claim 1, wherein the unstretched film is heat-treated from the surface opposite to the surface in contact with the cooling roll, and then the unstretched film is biaxially stretched. 5. The temperature of at least one surface (or surface layer) of the unstretched film which has been cooled and solidified is polyester A
20 ° C lower than the cold crystallization temperature Tcc (Tcc-2
0 ° C.) and a temperature 40 ° C. higher than the temperature-lowering crystallization temperature Tmc (Tmc + 40 ° C.) or less so as to be maintained for 0.5 to 100 seconds, and then the glass transition temperature Tg or more of the polyester A and Tcc. 20 ° C higher temperature (T
cc + 20 ° C.) or less, and biaxially stretched,
The method for producing the polyester film according to claim 1 or 2. 6. An unstretched film is finely stretched in a uniaxial direction,
The birefringence is set to 0.5 × 10 ⁻³ to 50 × 10 ^−3, and then the temperature of at least one surface (or surface layer) of the slightly stretched film is 20 from the cold crystallization temperature Tcc of polyester A.
℃ lower temperature (Tcc-20 ℃) or more, and a temperature lower than the crystallization temperature Tmc 40 ℃ higher (Tmc + 40 ℃) or less, heat treatment so as to be maintained for 0.3 to 50 seconds, then the glass transition of polyester A Temperature Tg or higher and Tcc
The method for producing a polyester film according to claim 1, wherein the polyester film is biaxially stretched at a temperature 20 ° C. higher (Tcc + 20 ° C.) or less. 7. The temperature of at least one surface (or surface layer) of the melt-extruded film containing polyester as a main component is 70 ° C. from the temperature-lowering crystallization temperature Tmc of polyester A.
Lower temperature (Tmc-70 ° C) or higher, and polyester A
0.5 to 20 seconds below the falling crystallization temperature Tmc of
Next, the polyester film A is cooled to a glass transition temperature Tg or lower, and then the unstretched film is biaxially stretched, and the method for producing a polyester film according to claim 1 or 2, wherein 8. The method for producing a polyester film according to claim 4, wherein the surface roughness of the surface of the cooling roll according to claim 4 is 0.2 S or more and 10 S or less.