JPH0670853B2 - Polyester film for magnetic recording media - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polyester film for a magnetic recording medium, which has a uniform surface and is highly improved in running property and abrasion resistance.

More specifically, the present invention relates to a polyester film having these properties improved by setting the refractive index in the thickness direction of a film containing vaterite type calcium carbonate particles to a certain range.

[Problems to be Solved by Conventional Techniques and Inventions]

The polyester film has excellent physical and chemical properties and is widely used in industry. In particular, the biaxially stretched polyethylene terephthalate film is now indispensable as a base material for a magnetic recording medium because it is excellent in flatness, mechanical strength, dimensional stability and the like as compared with other films.

On the other hand, in recent years, magnetic recording media have been rapidly improved, and along with this, the demand for a base film has become more severe.

For high density recording such as video tape, the surface of the base film needs to be flatter. However, when the film surface becomes flat,
Not only the running property is deteriorated, but also friction and wear between the film and the base material are increased, which causes various harmful effects such as scratches on the film and generation of abrasion powder. The abrasion powder causes a dropout of the recording signal, that is, causes dropout, and significantly lowers the product of the film.

In general, it has been found that the surface of the film should be appropriately roughened in order to improve the running property and abrasion property of the film.
In order to achieve this, a method in which fine particles are present in the raw material polyethylene has been adopted, and some of them have been put into practical use, but these properties have not necessarily been highly satisfied.

For example, when so-called precipitated particles are used as the fine particles from a catalyst residue during the production of polyester, the fine particles are easily broken by stretching, so that the running property and abrasion resistance are poor, and the recycling is difficult.

Also, when inert inorganic compound particles are added to polyester such as kaolin, silicon oxide, titanium dioxide, calcium phosphate, etc., the particles are not broken or deformed by stretching, and a relatively steep projection is given, so that the particles run However, since the particle size distribution is wide, deterioration of electromagnetic conversion characteristics due to coarse protrusions and frequent dropouts are often observed.

In order to overcome this trade-off, it has recently been proposed to use inorganic or organic particles having a sharp particle size distribution. For example, JP-A-62-207356 and JP-A-
59-217755 discloses monodisperse silicon oxide,
Crosslinked organic particles by emulsion polymerization are shown. However, in this case as well, the silicon oxide particles have a high hardness, so that the substrate with which the film comes into contact is likely to be damaged, and the crosslinked organic particles are difficult to be deformed by stretching and have heat resistance.

As described above, the polyester film for magnetic recording, which is highly satisfying the running property and the abrasion resistance and has other necessary properties, has not been obtained.

[Means for Solving the Problems]

The present inventors, in view of the above problems, as a result of diligent studies, it was found that a polyester film containing a specific calcium carbonate particles and having a refractive index in the thickness direction in a specific range can achieve this. The present invention has now been completed.

That is, the gist of the present invention is a polyester film containing 0.005 to 2.0% by weight of vaterite-type calcium carbonate particles having an average particle size of 0.07 to 1.5 μm and a particle size distribution value defined by the following formula of 1.60 or less, and The polyester film for a magnetic recording medium is characterized by having a refractive index in the thickness direction of the film of 1.492 or more.

(In the formula, d ₂₅ and d ₇₅ are measured by measuring the cumulative volume of the particle group from the large particle side, and the particle diameters corresponding to 25% and 75% of the total volume (μ
m) is shown. Hereinafter, the present invention will be described in more detail.

The polyester used in the present invention refers to a polyester obtained by using terephthalic acid or its ester and ethylene glycol as a main starting material, but may contain other third component. In this case, as the dicarboxylic acid component,
For example, it is possible to use one or more of isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, terephthalic acid, adipic acid, sebacic acid, and oxycarboxylic acid components such as p-oxyethoxybenzoic acid. is there. As the glycol component, one kind or two or more kinds of ethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol and the like can be used. In any case, the polyester of the present invention is preferably a polyester having 80% or more of repeating structural units having ethylene terephthalate units.

Further, the polyester may contain any additive such as a heat stabilizer, an antiblocking agent, an antioxidant, a coloring agent, an antistatic agent, an ultraviolet absorber and the like.

One of the characteristics of the present invention is that certain specific vaterite type calcium carbonate particles are used. As a method for producing calcium carbonate particles, in addition to a method of pulverizing and classifying natural calcium carbonate, a so-called method of reacting a carbon dioxide-containing gas with a liquid of calcium hydroxide as shown in, for example, JP-A-59-69425. Precipitable calcium carbonate by a synthetic method is known. In this case, calcite-type calcium carbonate particles having a relatively uniform particle size can be obtained.
It is also known to be incorporated into polyester films as disclosed in -16856, but the particles still cannot reach the desired particle size distribution.

The present inventors have found that in such a synthesis method, vaterite-type calcium carbonate having an extremely sharp particle size distribution can be produced by selecting production conditions such as the hydrogen ion concentration and reaction rate of the system.

As a typical example of this, VAN manufactured by Maruo Calcium Co., Ltd.
You can list the brands. The vaterite-type calcium carbonate can be added to the polyester production step as it is, but it is preferable to perform a surface treatment in order to reduce the solubility in the reaction system and improve the dispersibility. Examples of the surface treatment agent which also serves as the dispersant include, for example, JP-A-59-
Surface treatment agents such as those described in JP-B No. 69426 or JP-A No. 1-256558, particularly polymer polycarboxylic acids or their sodium salts, ammonium salts and the like are preferably used. These are usually effective when added during the production stage of the particles.

Such vaterite-type calcium carbonate is obtained in the shape of a sphere or an ellipsoid, and the ratio of its major axis to its minor axis is usually about 1.8 or less. In the present invention, the particle size distribution value must be 1.60 or less, preferably 1.50 or less, more preferably 1.40 or less. If this value exceeds 1.60, the surface roughness of the finally obtained film will be non-uniform and the electromagnetic characteristics will be impaired.

The average particle size of the particles used in the present invention is 0.07 to 1.5 μm.
m, preferably 0.15 to 1.0 μm. If this value is less than 0.07 μm, the running property and wear resistance are hardly improved, while if it exceeds 1.5 μm, the roughness is uniform but the absolute value becomes too large, and the electromagnetic characteristics deteriorate.

Further, the compounding amount in the film is selected from the range of 0.005 to 2.0% by weight, preferably 0.01 to 0.8% by weight. If the blending amount is less than 0.005% by weight, the running property and wear resistance will not be improved, and conversely, if it exceeds 2.0% by weight, the electromagnetic characteristics will be deteriorated.

As described above, certain specific calcium carbonate particles are used in the present invention, but at the same time, it is essential in the present invention that the refractive index in the thickness direction of the polyester film is 1.492 or more. When the refractive index in the thickness direction of the film is less than 1.492, the effect of imparting slipperiness and abrasion resistance is insufficient even if the particles of the present invention are used, which is not preferable. In addition,
This value is preferably 1.494 or more and 1.505 or less. The film having such physical properties, for example, in the case of longitudinal-transverse sequential biaxial stretching,
Longitudinal stretching temperature is 5 to 30 ℃ higher than normal stretching temperature 105 to 1
It can be obtained at about 15 ° C. Alternatively, it can also be obtained by performing a large lateral relaxation after the biaxial stretching and before the heat treatment.

When the refractive index in the thickness direction of the film is 1.492 or more, another advantage is that the adhesiveness of the magnetic layer is improved when applied.

As described above, in the present invention, a desired effect can be obtained only by using a combination of both the specific vaterite type calcium carbonate particles and the refractive index in the thickness direction of the film within a specific range. One or more kinds of other particles may be used in combination for the purpose of further improving the film properties and the like within the range of not impairing the purpose of the above.

Precipitated particles can be mentioned as one of such particles. The term "precipitated particles" as used herein refers to particles that are precipitated in the reaction system by polymerizing a system using an alkali metal or alkaline earth metal compound as a transesterification catalyst by a conventional method. Alternatively, terephthalic acid may be added during the transesterification reaction or polycondensation reaction to cause precipitation. In these cases, phosphoric acid, trimethyl phosphate,
One or more phosphorus compounds such as triethyl phosphate, tributyl phosphate, ethyl acid phosphate, phosphorous acid, trimethyl phosphite, triethyl phosphite and tributyl phosphite may be present.

In addition, the inert substance particles can be deposited by these methods even when the esterification step is performed. For example, before or after the completion of the esterification reaction, an alkali metal or alkaline earth metal compound is present, and the polymerization reaction is carried out in the presence or absence of the phosphorus compound.

In any case, the fine precipitation compound generated during the polyester forming reaction in the present invention includes calcium, lithium,
It contains one or more elements such as antimony and phosphorus.

Another example is so-called additive particles. The additive particles refer to particles externally added to the polyester manufacturing process, and specifically, kaolin, talc, carbon, molybdenum sulfide, gypsum, rock salt, aluminum oxide, barium sulfate, lithium fluoride, calcium fluoride, zeolite. , Calcium phosphate, silicon dioxide, titanium dioxide and the like.

As an example of such added particles, heat-resistant polymer fine powder can be cited. As a typical example in this case, a monovinyl compound having only one aliphatic unsaturated bond in the molecule, as described in, for example, JP-B-59-5216, and a divinyl compound in the molecule as a cross-linking agent. It is possible to exemplify a copolymer with a compound having one or more aliphatic unsaturated bonds, but of course not limited to these, for example, a thermosetting phenol resin, a thermosetting epoxy resin,
It is also possible to use a fine powder of a thermosetting urea resin, a benzoguanamine resin, or a fluorine-based resin such as polytetrafluoroethylene.

Naturally, examples of particles that can be combined with the vaterite-type calcium carbonate particles of the present invention include natural calcium carbonate and calcite-type calcium carbonate produced by a synthetic method.

In the present invention, the particles to be used in combination have an average particle size larger than that of the vaterite-type calcium carbonate of the present invention and the same weight or less as the calcium carbonate, preferably 0.00
It is preferable to select from the range of 5 to 0.5 times weight, more preferably 0.01 to 0.3 times weight. On the contrary, when the particle size of the particles used in combination is smaller, the weight may be the same as or more than the calcium carbonate, for example, 1 to 20 times.

In the present invention, two or more kinds of such vaterite type particles having different average particle sizes may be used.

By the way, in recent years, in addition to flatness, slipperiness, abrasion resistance and adhesiveness, a base film for magnetic recording is often required to have new scratch resistance. This is because, for example, when the tape travels at a high speed, the tape may be damaged from its contact portion, or the abrasion powder generated may easily scratch the film. Not only does it look bad if it is scratched, but it also causes process contamination and increased dropouts.

In order to highly improve the scratch resistance, aluminum oxide is particularly preferably selected as the particles to be used in combination. Above all, delta-type or gamma-type aluminum oxide particles having an average particle size of 0.5 μm or less, preferably 0.1 μm or less, particularly delta-type aluminum oxide particles are preferably used. As a method for producing these particles, for example, a thermal decomposition method, that is, a flame hydrolysis method using anhydrous aluminum chloride as a raw material, or an ammonium alum thermal decomposition method, that is, aluminum hydroxide is used as a starting material and reacted with sulfuric acid to form aluminum sulfate. Then, a method of reacting with ammonium sulfate and baking as ammonium alum can be mentioned.

The primary particle diameter of aluminum oxide obtained by these methods is usually in the range of 5 to 40 nm, but since aggregates often exceeding 0.5 μm are formed, it is desirable to appropriately crush and use. In this case, the secondary particles may be agglomerated to some extent, but the apparent average particle diameter is preferably 0.5 μm or less, and more preferably 0.1 μm or less.

As described above, in the present invention, the combination of vaterite-type calcium carbonate particles and the refractive index in the thickness direction of the film is further combined with other particles such as fine aluminum oxide particles, if necessary, to obtain extremely excellent magnetic recording. Although it has been possible to obtain a polyester film for use, the effect is maximized when some of the following requirements in the film forming step are added to the combination of the former two.

That is, in the present invention, when the specific resistance of the polyester composition used for film formation containing such vaterite-type calcium carbonate during melting falls within a specific range, a highly planar film of high value can be obtained.

That is, when obtaining the film of the present invention, usually 280 ~ 3
The polyester is extruded from the extruder through the lip portion into a sheet at a temperature in the range of 10 ° C., cooled to 40 to 70 ° C. to obtain a substantially amorphous sheet, and then the sheet is longitudinally and laterally at least area-wise. A biaxially oriented film is obtained by stretching the film to a draw ratio of 4 times, and then heat-treated at a temperature in the range of 150 to 240 ° C. At this time, in obtaining an amorphous sheet,
The so-called electrostatic applied cooling method is adopted, and the specific resistance of the polyester composition at the time of melting is 6 × 10 ^{6 to} 5 × 10 ⁸ Ω-cm.
Then, it was found that a film having more excellent flatness can be obtained.

The electrostatic applied cooling method is, for example, as described in JP-B-37-6142, when an amorphous sheet is obtained from a molten polymer, an electrostatic charge is applied to the sheet to electrostatically rotate and cool the sheet. It is a method of pressing strongly against a drum and rapidly cooling, but when the specific resistance at the time of melting the polyester containing vaterite type calcium carbonate is in a range where it can be applied particularly effectively, it is possible to further improve the flatness of the obtained film. it can. The more preferable range of the specific resistance is 8 × 10 ⁶ to 1
× 10 ⁸ Ω-cm.

The following method may be adopted to adjust the specific resistance during melting to a desired value.

That is, in order to reduce the specific resistance, a metal component may be solubilized in polyester. For this purpose, for example, the metal element used as a transesterification reaction catalyst or, if necessary, added after the transesterification reaction or the esterification reaction is added. A means for adding a relatively small amount, for example, an equivalent amount or less of a phosphorus compound to the metal element is preferably adopted.

On the other hand, in order to increase the specific resistance, the amount of the metal compound dissolved in the polyester is reduced.

In the present invention, either a transesterification method or an esterification reaction method can be adopted, but the former is preferable,
Specifically, for example, a magnesium catalyst, a manganese catalyst, a calcium catalyst or the like is used to increase the degree of polymerization of the polyester containing the particles in the presence of a phosphorus compound in an amount of about 0.3 to 1.0 times equivalent.

The average refractive index () of the film of the present invention is 1.598-
A range of 1.605 is preferable. When the value is less than 1.598, the dimensional stability is deteriorated when the film is heated to some extent, and the skew property is deteriorated. On the other hand, when the value exceeds 1.605, the film surface becomes brittle and white powder is remarkably generated. The value of is preferably in the range 1.600 to 1.603.

In these cases, when aluminum oxide particles are used in combination as particles, a high degree of scratch resistance is also imparted, so that a more excellent film can be obtained.

By the way, in the production of the polyester containing the vaterite-type calcium carbonate particles of the present invention, the particles are preferably added during the polyester synthesis reaction. In particular, it is preferable to add after the completion of the transesterification reaction or the esterification reaction and before the start of the polycondensation reaction.

The particles are usually added as a slurry having a particle concentration of 3 to 50% by weight in a solvent such as ethylene glycol. If the particle concentration of the slurry is less than 3% by weight, the amount of ethylene glycol used increases and the basic unit of ethylene glycol increases, which is not preferable. Moreover, when a slurry having a particle concentration of more than 50% by weight is added, the dispersibility of particles is often deteriorated.

As the polycondensation reaction catalyst for polyester synthesis, S
Commonly used catalysts such as b, Ge, Ti, Sn and Si compounds are used.

The film of the present invention suitable for a magnetic recording medium is first obtained by a combination of specific particles and specific film physical properties, but the film surface roughness is usually 0.005 to 0.1 μm in center line average roughness, Preferably 0.007-0.08μ
m, more preferably 0.01 to 0.03 μm.

The film of the present invention can be particularly effective when used as a base film for a video tape and also as an audio film. In other words, in the field, there are two types of conventional radio boomboxes, compo stereos, etc.
As a result of the widespread use of models equipped with a double or more high-speed dubbing function, the tape and the base material come into contact with each other at a higher speed during the dubbing process, fast-forwarding, and rewinding. The effect of is effectively exhibited.

Further, the base film for magnetic disks, especially for high-density floppy disks, is required to reduce coarse protrusions due to its nature, and the film of the present invention can be effectively used in combination with the requirements for workability and wear resistance.

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples as long as the gist thereof is not exceeded.

The properties of the present invention were measured by the following methods.

In the examples, "parts" and "%" are "parts by weight", respectively.
And "% by weight".

(1) Average particle size and particle size distribution value The particle size was measured by a photographic method using an electron microscope and converted into equivalent spheres. Regarding the particle size distribution, the particle size of about 1000 particles was measured, and the volume was integrated from the large particle side. The particle size at 25% of the total volume
and d _25, the ratio of the particle size when 75% as d ₇₅ [d ₂₅ / d _75]
The value of indicates the sharpness of the particle size distribution. The closer this value is to 1, the sharper. The average particle size is represented by d ₅₀ (μm).

(2) Specific resistance during melting Britz J.Appl.Phya, Volume 17, Pages 1149-1154 (1966)
Year). However, in this case, the melting temperature of the polymer is 295 ° C., and the value immediately after applying a direct current of 1,000 V is the specific resistance during melting.

(3) The refractive index in the thickness direction of the film and the average refractive index are values with respect to the sodium D line measured at 23 ° C. using a refractometer. The average refractive index is the refractive index n in the thickness direction.
Let α be the refractive index in the main alignment direction be nγ, and the refractive index in the direction perpendicular to the main alignment direction be nβ, then = 1/3 (nα + nβ + nγ).

(4) Sliding property of film Using the device shown in Fig. 1, the film is brought into contact with a fixed hard chrome-plated metal roll (diameter 6 mm) at a winding angle of 135 ° (θ), and a load of 53 g (T ₂ ) is applied to one end, This was run at a speed of 1 m / min, the resistance force (T ₁ (g)) at the other end was measured, and the friction coefficient (μd) during running was determined by the following formula.

(5) Surface roughness of film Center line average roughness (Ra): It was determined as follows using a surface roughness measuring device (SE-3F) manufactured by Kosaka Laboratory Ltd. That is, a portion having a reference length L (2.5 mm) is extracted from the film cross-section curve in the direction of the center line, and the center line of the extracted portion is taken as the x-axis, and the longitudinal magnification direction is taken as the y-axis. When represented by x), the value given by the following equation is represented by [μm]. The center line average roughness was represented by the average value of the center line average roughness of the extracted portions obtained from 10 cross-section curves obtained from the sample film surface. The tip radius of the stylus is 2 μm,
The load was 30 mg and the cutoff value was 0.08 mm.

Maximum height (Rt); A part of the cross-section curve obtained by the surface roughness measuring device (SE-3F) manufactured by Kosaka Laboratory Ltd., which is extracted by the reference length (2.5 mm) (hereinafter, the extracted part) When the extracted part is sandwiched by two straight lines parallel to the average line, the sensation of these two straight lines is measured in the direction of the longitudinal magnification of the sectional curve, and the value is expressed in units of micrometers (μm). The maximum height of the extracted part is set. The maximum height was obtained by calculating 10 sectional curves from the surface of the sample film and expressing the average value of the maximum heights of the extracted portions obtained from these sectional curves. The radius of the stylus used at this time was 2.0 μm, the load was 30 mg, and the cutoff value was 0.08 mm. The smaller the Rt / Ra, the more uniform the surface roughness of the film.

(6) Evaluation of abrasion resistance Using the tape abrasion evaluation machine shown in FIG. 2, a polyester film having a width of 10 mm was run over a length of 200 m and the fixing pin (diameter 6 mm, material SUS420-J2) shown in (I) in the figure. ,
The amount of abrasion powder adhering to the finishing 0.2S) was visually evaluated and divided into 4 ranks. The running speed of the film is 11.4 m / min, the tension is detected by the tension pickup shown in (V), the initial tension is 300 g, and the winding angle θ with the film is 135.
It was ゜.

Rank A: No adhesion is observed.

Rank B: Adhesion is very slight, but practically no problem.

Rank C: The amount of adhesion is rather large and may be a problem when used for a long time.

Rank D: Large amount of adhesion makes it practically difficult to use.

(7) Magnetic Tape Properties First, the following magnetic paint was applied to a polyester film to form a magnetic layer so that the expanded thickness after drying was 2 μm. That is, 200 parts of magnetic fine powder, 30 parts of polyurethane resin, 10 parts of nitrocellulose, 10 parts of vinyl chloride / vinyl acetate copolymer, 5 parts of lecithin.
Part, cyclohexanone 100 parts, methyl isobutyl ketone 1 part
48 parts of 00 parts and 300 parts of methyl ethyl ketone in a ball mill
After mixing and dispersing for 5 hours, 5 parts of a polyisocyanate compound was added to form a magnetic paint, which was applied to a polyester film, magnetically oriented before the paint was sufficiently dried and solidified, and then dried. Further, this coated film was surface-treated with a super calendar and slit into a 1/2 inch width to obtain a video tape. This video tape is manufactured by Matsushita Electric Co., Ltd.
The following magnetic tape characteristics were evaluated at normal speed using an NV-3700 VCR.

VTR head output: The VTR head output at a measurement frequency of 4 MHz was measured by a synchroscope, and the relative value to the reference sample was displayed in decibel (dB).

Number of dropouts: Play back a videotape recording a signal of 4,4 MHz, and use a dropout counter manufactured by Okura Industry Co., Ltd. to reduce the number of dropouts at 15 μsec-20 dB to about 2
Measured for 0 minutes and the number of dropouts per minute (pieces /
Minutes).

(8) Scratch resistance A magnetic tape slit to a width of 1/2 inch is applied to a hard chrome-plated metal pin (diameter mm, finishing 3S) with a tension of 50 g, a winding angle of 135 °, and a running speed of 3 m / sec. Rubbing once.

Next, aluminum was vacuum-deposited on the scraped surface in an amount of about 1000Å, and the amount of scratches was visually evaluated and divided into the following 5 ranks.

Rank 1: The amount of scratches is extremely large.

Rank 2: A lot of scratches.

Rank 3: The amount of scratches is between 2,4.

Rank 4: The amount of scratches is small.

Rank 5: Not scratched.

Example 1 100 parts of dimethyl terephthalate and 60 of ethylene glycol
Parts and 0.09 parts of magnesium acetate / tetrahydrate in a reactor, the temperature is raised by heating, methanol is distilled off, and a transesterification reaction is carried out. It takes 4 hours from the start of the reaction to 230 ° C.
The temperature was raised to 0 and the transesterification reaction was substantially completed.

Next, 0.5 part of vaterite-type calcium carbonate (VAN-030 manufactured by Maruo Calcium Co., Ltd.) surface-treated with a high molecular weight polycarboxylic acid having an average particle size of 0.25 μm and a particle size distribution value of 1.29 was added as an ethylene glycol slurry, and ethylene was further added. Acid phosphate 0.04 part and antimony trioxide 0.04 part were added and polycondensation reaction was carried out for 4 hours to obtain polyethylene terephthalate.

The intrinsic viscosity of the polyester is 0.63, and the specific resistance when melted is 1.
It was 5 × 10 ⁷ Ω-cm, and when the inside was observed with a microscope, it was confirmed that the particles were uniformly dispersed.

Then, a biaxially stretched polyester film was produced using the polyester.

That is, the polyester was extruded into a sheet form from an extruder at a temperature of 295 ° C., and an amorphous sheet having a thickness of 200 μm was obtained by the electrostatic cooling method.

Then, the amorphous sheet is placed at 110 ° C. in the sheet flow direction.
3.5 times, 3.5 at 110 ℃ in the direction orthogonal to the sheet flow
The film was double-stretched, heat-treated at 220 ° C. for 3 seconds and then cooled to obtain a biaxially stretched film having a thickness of 15 μm.

Examples 2-5 and Comparative Examples 1-3 As shown in Table 1, as in Example 1 except that the particles contained in the film and the refractive index of the film were changed, 2
An axially stretched film was obtained, and finally a magnetic tape was manufactured.

The particle size distribution value of the vaterite type calcium carbonate used in Examples 2, 3, 4 and Comparative Example 3 is 1.29, and that of Example 5 is 1.25. Each of these is surface-treated with a high molecular weight polycarboxylic acid. The calcite-type calcium carbonate used in Comparative Example 1 was obtained by a synthetic method, and its particle size distribution value is 1.72. Comparative Example 2
Kaolin and spherical silica of Example 4 have particle size distribution values of 2.65 and 1.44, respectively.

The aluminum oxide used in Example 3 has a delta type crystal form obtained by a thermal decomposition method, and the deposited particles of Example 5 are calcium element, lithium element,
And particles each containing 1 wt% or more of phosphorus element.

The results obtained above are summarized in Table 1 below.

The films of Examples satisfying the requirements of the invention are all runnable,
It can be seen that the film of Example 3 which is excellent in abrasion resistance, particularly fine aluminum oxide is also used is excellent in scratch resistance and is suitable as a base film for a magnetic recording medium. Comparative Examples 1 and 2 are examples in which particles outside the range of the present invention are used, but the film surface is non-uniform and the magnetic tape characteristics deteriorate. Further, Comparative Example 3 shows that even if the same vaterite-type calcium carbonate as in the present invention is used, the film forming conditions of the film are unsatisfactory and the slidability and wear resistance are deteriorated, and desired electromagnetic characteristics cannot be obtained.

〔The invention's effect〕

The film of the present invention contains a specific calcium carbonate particle, and further has a refractive index in the thickness direction of the film within a specific range to provide a film for a magnetic recording medium excellent in slipperiness and abrasion resistance. And its industrial value is high.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a running system of an apparatus for evaluating the running property of a film. In the figure, (I) is 6mmφ, SUS-420-J2 fixing pin, (II) is inlet tension meter, (III) is outlet tension meter, and wrap angle (θ) is 135.
It is ゜. FIG. 2 is a schematic diagram of a running system of an apparatus for evaluating the abrasion resistance of a film. In the figure, (IV) is 6 mmφ, SUS
-420-J2 fixing pin, (V) indicates a tension meter, and the winding angle (θ) is 135 °.

Claims (1)

[Claims] 1. A polyester film containing 0.005 to 2.0% by weight of vaterite-type calcium carbonate particles having an average particle size of 0.07 to 1.5 μm and a particle size distribution value defined by the following formula of 1.60 or less, and The refractive index in the thickness direction
A polyester film for a magnetic recording medium, which is 1.492 or more. (In the formula, d ₂₅ and d ₇₅ are measured by measuring the cumulative volume of the particle group from the large particle side, and the particle diameters corresponding to 25% and 75% of the total volume (μ
m) is shown. )