JPH0628099B2 - Magnetic recording medium - Google Patents

Description

TECHNICAL FIELD The present invention relates to a magnetic recording medium, more specifically, it has excellent electromagnetic conversion characteristics, contains spherical silica fine particles having characteristics, and has excellent abrasion resistance and slipperiness. Of the improved biaxially oriented polyester film as a substrate.

[Prior art]

As a magnetic recording medium based on polyester film, for example, a video tape, an audio tape, a computer tape, a floppy disc, etc. are known and widely used.

In recent years, demands for high-density recording and high quality have been increasing in these fields of application, and along with this, the base polyester film has a flat surface and excellent slipperiness, as well as durability and wear resistance. The demand for superiority is growing stronger.

Heretofore, as a method for improving slipperiness, there has been proposed a method of adding inorganic particles such as calcium carbonate to polyester, or a method of precipitating fine particles containing calcium, lithium or phosphorus in a polymerization system during synthesis of polyester. In either method, when the polyester is molded and stretched to form a film, protrusions are formed on the surface of the film due to the fine particles to improve the slipperiness of the film.

However, in the method of improving the slipperiness of the film by the projections by the fine particles as described above, the slipperiness is usually improved as the film surface is roughened, but on the other hand, the magnetic coating is caused by the roughening. The surface after coating has a tendency to become rough and electromagnetic conversion characteristics to deteriorate.

As one of the measures for solving these contradictory flatness and slipperiness, many means for utilizing composite inorganic particles in which large-sized particles and small-sized particles coexist are proposed. However, these means also have problems, and it is difficult to meet the demands for high-grade magnetic recording media such as high density and high quality. The reason for this is that the size of the large-sized particles used for the composite inorganic particles is coarser than the quality required for high-grade grading, and the larger the particles, the higher the protrusions on the film surface and the particle size. The voids become large, and high protrusions are scraped off with a non-woven fabric during the cleaning process or calendar processing process, which causes dropout (a missing information portion during recording / playback), and calendar stains in the processing process. This causes dust dust contamination for cleaning the surface of the base film, greatly impairing the characteristics of the magnetic recording medium.

[Object of the Invention]

The present inventors have solved the above-mentioned problems and succeeded in developing a film having flatness, slipperiness, and abrasion resistance which are applicable to high-quality magnetic recording application fields, and the film is used as a substrate. The present invention has been accomplished by finding that the magnetic recording medium that does have excellent characteristics.

An object of the present invention is to provide a magnetic recording medium capable of high-density recording, of high quality and capable of withstanding repeated use.Furthermore, in the base film, (1) there is no large protrusion on the surface and it is flat. However, there are minute protrusions that do not cause noise such as dropouts, (2)
The coefficient of friction during repeated running is small, and (3) the magnetic recording medium has excellent durability in continuous use because the base film is scarcely scraped due to the magnetic recording medium processing step and contact with the magnetic recording / reproducing device. To provide.

[Constitution / Effect of Invention]

The object of the invention is, according to the invention, that the average particle size is from 0.3 to
Spherical silica particles having a particle size ratio (major axis / minor axis) of 1.0 to 1.2 and a relative standard deviation of 0.3 or less at 4 .mu.m are used.
This is achieved by a magnetic recording medium comprising a biaxially oriented polyester film containing 0.1 to 1% by weight dispersed therein, and a magnetic layer on at least one side of the film.

Here, the major axis, the minor axis, and the area equivalent circle diameter of the silica particles are measured by a scanning electron microscope after depositing a gold thin film layer on the particle surface.
The average particle size and the particle size ratio are calculated by the following formulas from the image magnified 10,000 to 30,000 times.

Average particle size = total area circle equivalent diameter of measured particles / number of measured particles Particle size ratio = average major axis of silica particles / average minor axis of the particles The relative standard deviation is calculated by the following formula.

Here, Di: equivalent circle diameter of each particle (μm) n: represents the number of particles.

The polyester in the present invention is a polyester having an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component. Such polyesters are substantially linear and have film forming properties, especially film formation by melt molding. Examples of the aromatic dicarboxylic acid include terephthalic acid, naphthalenedicarboxylic acid, isophthalic acid, diphenylethanedicarboxylic acid,
Examples thereof include diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenyl ketone dicarboxylic acid and anthracene dicarboxylic acid. Examples of the aliphatic glycol include polymethylene glycol having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and decamethylene glycol, and alicyclic diols such as cyclohexanedimethanol. Etc. can be mentioned.

In the present invention, as the polyester, for example, one having alkylene terephthalate and / or alkylene naphthalate as a main constituent component is preferably used.

Among such polyesters, not only polyethylene terephthalate and polyethylene-2,6-naphthalate but also, for example, 80 mol% or more of all dicarboxylic acid components are terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and all glycols are used. A copolymer in which 80 mol% or more of the component is ethylene glycol is preferable. 20 mol% or less of the total acid component can be the above aromatic dicarboxylic acids other than terephthalic acid and / or 2,6-naphthalenedicarboxylic acid, and aliphatic dicarboxylic acids such as adipic acid and sebacic acid; Cyclohexane-
It can be an alicyclic dicarboxylic acid such as 1,4-dicarboxylic acid. Also, 20 mol% of all glycol components
The following can be the above glycols other than ethylene glycol, or aromatic diols such as hydroquinone, resorcin, 2,2-bis (4-hydroxyphenyl) propane; fragrances such as 1,4-dihydroxymethylbenzene. A ring-containing aliphatic diol; polyalkylene glycol (polyoxyalkylene glycol) such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol may be used.

In the polyester used in the present invention, a component derived from an oxycarboxylic acid such as an aromatic oxyacid such as hydroxybenzoic acid; an aliphatic oxyacid such as ω-hydroxycaproic acid, a dicarboxylic acid component and an oxycarboxylic acid. Those which are copolymerized or bonded at 20 mol% or less based on the total amount of the components are also included.

Further, in the polyester of the present invention, a polycarboxylic acid or polyhydroxy compound having a functionality of 3 or more, such as drimellitic acid or pentaerythritol, is used in an amount in a substantially linear range, for example, an amount of 2 mol% or less based on all acid components. It also includes a copolymer of the above.

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

The polyester has an intrinsic viscosity of about 0.4 when measured at 35 ° C. as a solution in o-chlorophenol.
Of about 0.9 are preferred.

The biaxially stretched polyester film of the present invention has many fine projections on the surface of the film. The large number of fine projections derives from the large number of spherical silica particles contained according to the invention dispersed in the polyester.

The polyester containing spherical silica particles dispersed therein is usually used at any time during the reaction for forming the polyester, for example, during the transesterification reaction or the polycondensation reaction in the case of the transesterification method, or in the case of the direct polymerization method. It can be produced by adding spherical silica particles (preferably as a slurry in glycol) to the reaction system at a certain time. Preferably, spherical silica particles are added to the reaction system at the initial stage of the polycondensation reaction, for example, until the intrinsic viscosity reaches about 0.3.

The spherical silica particles to be dispersed and contained in the polyester in the present invention are silica particles having an average particle diameter of 0.3 to 4 μm and a particle diameter ratio (major axis / minor axis) of 1.0 to 1.2. Each of the spherical silica particles has a spherical shape that is extremely close to a true sphere, and the silica particles conventionally known as a lubricant are ultrafine aggregated particles of about 10 mμ or aggregated particles of about 0.5 μm. It is characterized in that it is significantly different from the formation of aggregates (aggregated particles). The average particle size of the spherical silica particles is preferably 0.3 to 3 μm, more preferably 0.3 to 2 μm. This average particle size is 0.3 μm
If it is less than the above range, the effect of improving the slipperiness and the abrasion resistance is insufficient, which is not preferable. If the average particle size is 4 μm, the film surface becomes too rough, which is not preferable. The particle size ratio of the spherical silica particles is preferably 1.0 to 1.15, more preferably 1.0 to 1.1.

Further, the spherical silica particles preferably have a sharp particle size distribution, and the relative standard deviation representing the steepness of the distribution is 0.3.
It is the following. Further, it is preferably 0.2 or less, and particularly preferably 0.12 or less.

This relative standard deviation is expressed by the following equation.

Here, Di: equivalent circle diameter of each particle (μm) Represents

When spherical silica particles having a relative standard deviation of 0.3 or less, more preferably 0.2 or less are used, the particles have a spherical shape and the particle size distribution is extremely steep, so that the height of the film surface protrusions becomes extremely uniform. Further, the individual projections on the film surface have a very sharp projection shape because the voids around the lubricant are small, so that even with the same number of projections, the slipperiness is extremely good.

The spherical silica particles are not limited to the production method and the like as long as the above-mentioned conditions are satisfied. For example, spherical silica particles are produced by hydrolyzing ethyl orthosilicate [Si (OC ₂ H ₅ ) ₄ ] to form hydrous silica [Si (OH) ₄ ] monodisperse spheres, which are further dehydrated. Silica bond [≡
It can be produced by three-dimensionally growing Si—O—Si≡] (Journal of the Chemical Society of Japan '81, No. 9, P. 1503).

Si (OC ₂ H ₅ ) ₄ + 4H ₂ O → Si (OH) ₄ + 4C ₂ H ₅ OH ≡Si-OH + HO-Si≡ → ≡Si-O-Si≡ + H ₂ O Spherical silica in the present invention The amount of particles added should be 0.01 to 1.0% by weight with respect to the polyester,
It is preferably 0.01 to 0.5% by weight, more preferably 0.05 to 0.3% by weight. If the addition amount is less than 0.01% by weight, the effect of improving the sliding property and the abrasion resistance becomes insufficient, while if it exceeds 1.0% by weight, the surface flatness decreases.
Not preferable.

The biaxially oriented polyester film of the present invention can be manufactured according to the conventionally accumulated manufacturing method of biaxially stretched film.
For example, polyester containing spherical silica particles may be melt-cast into an amorphous unstretched film, and then the unstretched film may be biaxially stretched, heat-fixed, and if necessary subjected to relaxation heat treatment. It is manufactured. At that time, the film surface characteristics are changed depending on the particle size, amount and the like of the spherical silica particles, and also depending on the stretching conditions, so that they are appropriately selected from the conventional stretching conditions. The density, heat shrinkage, etc. also change depending on the temperature, magnification, speed, etc. during drawing and heat treatment, so the conditions for simultaneously satisfying these characteristics are determined. For example, the stretching temperature is such that the first stage stretching temperature (for example, the longitudinal stretching temperature: T ₁ ) is in the range of (Tg-10) to (Tg + 45) ° C. (where Tg: the glass transition temperature of polyester) and the second stage stretching temperature. Temperature (for example, transverse stretching temperature: T ₂ ) is (T ₁ +15) to (T ₁ +4)
It is recommended to select from the range of 0) ° C. The stretching ratio may be selected from the range in which the uniaxial stretching ratio is 2.5 or more, particularly 3 or more, and the area ratio is 8 or more, particularly 10 or more. Furthermore, the heat setting temperature is 180 to 250.
C., and more preferably from 200 to 230.degree. The film thickness is preferably 1 to 100 μm.

The biaxially oriented polyester film of the present invention is a film having extremely small voids as compared with the conventional ones. The reason for this small void is that the spherical silica particles have good affinity to polyester, and the particles themselves are Since it is extremely close to a true sphere, it is speculated that the stress around the lubricant is evenly propagated during stretching and the stress is not concentrated on a part of the interface between the polyester and the lubricant.

The biaxially oriented polyester film of the present invention is characterized in that it has uniform uneven surface characteristics, excellent slipperiness and abrasion resistance, and that the generation of scratches, white powder, etc. is extremely small. In the present invention, utilizing these characteristics of the biaxially oriented polyester film, magnetic recording media such as video tapes, audio tapes, computer tapes,
Since the floppy disk is used, the magnetic recording medium has excellent electromagnetic conversion characteristics, slipperiness, running durability, and the like.

In the present invention, the magnetic layer provided on at least one side of the biaxially oriented polyester film, that is, one side or both sides may be a coating type magnetic layer or a metal thin film type magnetic layer.

The magnetic layer and the method for providing the magnetic layer on the base film are known per se, and the known magnetic layer and the method for providing the magnetic layer can be adopted in the invention.

For example, when the magnetic layer is provided by a method of coating a base film with a magnetic coating material, the ferromagnetic powder used in the magnetic layer is γ-Fe ₂ O ₃ , Co-containing γ-Fe ₂ O ₃ , Fe
Known ferromagnetic materials such as ₃ O ₄ , Co-containing Fe ₃ O ₄ , CrO ₂ , and barium ferrite can be used. The binder used together with the magnetic powder is a known thermoplastic resin, thermosetting resin, reactive resin or a mixture thereof. Examples of these resins include vinyl chloride-vinyl acetate copolymer,
Examples include polyurethane elastomers.

The magnetic coating material further includes an abrasive such as α-Al ₂ O _{3 and the} like, a conductive agent such as carbon black and the like, a dispersant such as lecithin and the like,
Lubricants such as n-butyl stearate, lecithin acid, etc.
It may contain a curing agent such as an epoxy resin, a solvent such as methyl ethyl ketone, methyl isobutyl ketone, toluene and the like.

When the magnetic layer is provided by a method of forming a metal thin film on the base film, a vacuum deposition method, a sputtering method, an ion plating method, C.I. V.
D. (Chemical Vapor Deposition) method, electroless plating method and the like can be adopted. As a metal,
Iron, cobalt, nickel and their alloys (eg Co
-Ni-P alloy, Co-Ni-Fe alloy, Co-Cr alloy or Co-Ni
Alloy, etc.) and the like.

〔Example〕

Hereinafter, the present invention will be further described with reference to examples. Various physical properties and characteristics in the present invention are measured as follows.

(1) Particle size The particle size can be measured in the following states.

1) Obtaining the average particle size, particle size ratio, etc. from the silica particles 2) Obtaining the average particle size, particle size ratio, etc. of the silica particles in the film 1) From the silica particle: Electron microscope sample table Silica powder is scattered on the top so that individual particles do not overlap as much as possible, and a gold thin film vapor deposition layer is formed on this surface with a thickness of 200 Å to 300 Å by a gold sputter device. Observed, Luzex 50 manufactured by Nippon Regulator Co., Ltd.
At 0, the major axis (Dli), the minor axis (Dsi) and the area circle equivalent diameter (Di) of at least 100 particles are obtained. And
The number average value represented by the following equation is used to express the major axis (Dl), the minor axis (Ds), and the average particle diameter () of the silica particles.

2) Silica particles in the film: A small piece of the sample film is fixed on a sample stand for a scanning electron microscope, and a spattering device (JFC-11 manufactured by JEOL Ltd.) is fixed.
The film surface was subjected to an ion etching treatment under the following conditions using a 00 type ion sputtering device. The condition is that the sample is placed in a bell jar, the degree of vacuum is raised to a vacuum state of about 10 ^-3 Torr, the voltage is 0.25 kV, and the current is 12.5.
Ion etching was performed at mA for about 10 minutes. Further, a gold spatter is applied to the surface of the film by the same apparatus, and observed with a scanning electron microscope at a magnification of 10,000 to 30,000, and a long diameter (Dli) and a short diameter of at least 100 particles are measured by Luzex 500 manufactured by Nippon Regulator Co., Ltd. (Dsi) and area circle equivalent diameter (Di) are calculated. Thereafter, the same procedure as 1) above is performed.

(2) Film surface roughness (Ra) This is a value defined by JIS-B0601 as the center line average roughness (Ra), and in the present invention, a stylus surface roughness meter (SURFCORDER SE of Kosaka Laboratory Ltd.) is used. -30C). The measurement conditions are as follows.

(a) Stylus tip radius: 2 μm (b) Measuring pressure: 30 mg (c) Cut-off: 0.25 mm (d) Measuring length: 0.5 mm (e) Method of collecting data Repeated measurement 5 times for the same sample , The largest value is 1
Except for one, the average value of the remaining four data is rounded off to the fourth decimal place and displayed up to the third decimal place.

(3) Void ratio According to the method of (1) -2) above, the area around the lubricant in the film (surface) is exposed, and the major axis of at least 50 solid fine particles and the major axis of the void are measured. It is expressed by the number average value of the void ratio obtained in step 1.

(4) Film abrasion coefficient (μk) The temperature was 20 ° C and the humidity was 60%, and the film cut into 1/2 inch width was attached to a fixed rod (surface roughness 0.3 μm) at an angle θ = 152 / 180π. It is brought into contact with radians (152 °) and moved (rubbed) at a speed of 200 cm per minute. When the tension controller is adjusted so that the entrance tension T ₁ is 35 g, the exit tension (T ₂ : g) is sent by the film for 90 m and then detected by the exit tension detector. To calculate.

μk = (2.303 / θ) log (T 2 / T 1) = 0.8681log (T 2/35) (5) scraping of the base film scraping of the running surface of using a 5-stage mini-super calender of evaluation did. The calender is a five-stage calender of nylon roll and steel roll, the processing temperature is 80 ° C, the linear pressure applied to the film is 200 kg / cm,
The film speed was 50 m / min. The running film was evaluated for the shaving property of the base film by the dirt attached to the top roller of the calendar when the running film was run for 2000 m.

<Four-level judgment> ◎ Nylon roll is not soiled at all 〇 Nylon roll is barely soiled × Nylon roll is very soiled × × Nylon roll is heavily soiled (6) Electromagnetic conversion (chroma S / N) Commercial household VTR 50% white level signal (10
The 0% white level signal has a peak: two: peak voltage of 0.
714 volts), a signal obtained by weighting the 100% chroma level signal is recorded, and the reproduced signal is measured using a Shibasoku noise meter Type 925R. Chroma S /
The definition of N is as follows according to the definition of Shibasoku.

Here, ES (pp) is the peak-to-peak voltage difference (pp) of the reproduced signal of the white level signal.

ES (pp) = 0.714V (pp) Further, EN (rmp) is the square root value of the peak voltage of the reproduction signal of the chroma level signal.

EN (rms) = AM noise effective value voltage (V) (7) Dropout Commercial dropout counter (eg Shibasoku VH01)
A dropout of 5 μsec × 10 dB was counted with a BZ type), and the count number per minute was calculated.

(8) Scratch judgment The base film is slit to 1/2 inch width and
Simultaneously with the measurement of the friction coefficient in (4), the fixed rod is coated with a film up to an angle of 152 °, and the film speed is 10 cm for 10 cm.
After being run for m times and repeated 50 times, the thickness, depth and number of the scratches on the surface of the 1/2 inch width base film were comprehensively evaluated, and the judgment was made in the following 5 steps.

<5-level judgment> ◎ No scratches are found on the 1/2 inch width base film. 〇 Scratches are hardly found on the 1/2 inch width base film. △ Scratches are found on the 1/2 inch width base film. (Several) x Some thick scratches are found on the 1/2 inch width base film.

XX A large number of thick and deep scratches are observed on the entire surface of the 1/2 inch wide base film.

Comparative Example 1 Dimethyl terephthalate and ethylene glycol, manganese acetate as a transesterification catalyst, antimony trioxide as a polymerization catalyst, phosphorous acid as a stabilizer, and a lubricant having an average particle size of 0.7 μm and a particle size ratio of 10.0. Polymerization was carried out by a conventional method using kaolin to obtain polyethylene terephthalate having an intrinsic viscosity of 0.62 (orthochlorophenol, 35 ° C.).

170 of this polyethylene terephthalate pellet
After drying at ℃ for 3 hours, it is fed to the extruder hopper and melt temperature 2
It was melted at 80 to 300 ° C., and formed and extruded on a rotating cooling drum having a surface finish of about 0.3 S and a surface temperature of 20 ° C. through a slit die of 1 mm of this molten polymer to obtain an unstretched film of 200 μm.

The unstretched film thus obtained is preheated at 80 ° C., and 90 mm from above 15 mm between low speed and high speed rolls.
3.5 by heating with 1R heater with surface temperature of 0 ℃
Stretched twice and quenched, then fed to a stenter 105
It was stretched 3.7 times in the transverse direction at ℃. The biaxially oriented film obtained was heat-set at a temperature of 205 ° C. for 5 seconds to give a thickness of 15
A heat-fixed biaxially oriented film of μm was obtained.

The obtained film had a void ratio of 1.7 and was good on a calendar, but was poor in scratching and unsatisfactory. The characteristics of this film are shown in Table 1.

Further, 100 parts by weight of iron oxide powder having the following composition Co was used on this film: Esretk A (vinyl chloride-vinyl acetate copolymer manufactured by Sekisui Chemical Co., Ltd. 10 〃 Nitporan 2304 (polyurethane elastomer manufactured by Nippon Polyurethane) 10 〃 Coronate L (Japan Polyurethane) Made polyisocyanate) 5〃 lecithin 1〃 methyl ethyl ketone 75〃 methyl isobutyl ketone 75〃 toluene 75〃 additive (lubricant, silicone resin) 0.15〃 magnetic powder paint with gravure roll, magnetic paint with doctor knife The layer was smoothed, magnetically oriented by a conventional method during the still drying period of the magnetic paint, then introduced into an oven and dried and cured, and then calendered to make the coated surface uniform and slit to form a magnetic layer with a thickness of about 5μ. 1/2 inch layered A magnetic tape having a width was prepared, and the characteristics of this magnetic tape are shown in Table 1.

Comparative Example 2 The procedure of Comparative Example 1 was repeated except that calcium carbonate having an average particle size of 0.6 μm and a particle size ratio of 1.5 was used instead of kaolin.
A polyethylene terephthalate pellet was obtained.

Using this pellet, a thickness of 15 was obtained in the same manner as in Comparative Example 1.
A biaxially oriented film of μm was obtained. This film had a void ratio of 2.0 and had good runnability, but white powder was generated in the calendar process. The characteristics of this film are shown in Table 1.

Further, a magnetic tape was prepared in the same manner as in Comparative Example 1 except that this film was used. The characteristics of this magnetic tape are shown in Table 1.

Comparative Example 3 The procedure of Comparative Example 1 was repeated except that calcium carbonate having an average particle size of 0.6 μm and a particle size ratio of 1.5 was used instead of kaolin.
A polyethylene terephthalate pellet was obtained.

After drying at 170 ° C. for 3 hours using this pellet, it is supplied to an extruder hopper and melted at a melting temperature of 280 ° C. to 300 ° C., and this molten polymer is extruded from a slit die of 1 mm, and a surface finish of about 0.3S, It was formed and extruded on a rotary cooling drum having a surface temperature of 20 ° C. to obtain an unstretched film of 200 μm. The unstretched film thus obtained is
Preheated to 5 ° C, and effectively 1.5 between low speed roll and high speed roll.
Double stretching, then cooling to about 40 ° C., again contacting 110 ° C. roll rows between roll rows for about 3.0 seconds, and subsequently cooling water to 80 ° C. low speed rolls with controlled surface temperature and inside. Was circulated, and three infrared heaters were installed between the high speed rolls at a distance of 15 mm on the film surface, and the film was stretched to 2.6 times. The stretched film is further fed to a stenter and stretched 3.5 times in the transverse direction at 105 ° C to obtain a thickness of 15
To obtain a biaxially stretched film. This film does not completely eliminate voids and does not have very good calendering properties.

Further, a magnetic tape was prepared in the same manner as in Comparative Example 1 except that this film was used. The characteristics of this magnetic tape are shown in Table 1.

Comparative Example 4 An unstretched film was obtained in the same manner as in Comparative Example 2, but the unstretched film was preheated with a low-speed roll whose surface temperature was controlled to 70 ° C. to further increase the strength in the machine direction, and a non-stretched film was prepared. Three infrared (IR) heaters are installed between them, and the film is drawn at a draw ratio of 4.5 times and then passed through a stenter.
After preheating to ℃, it was stretched 3.5 times in the transverse direction. This film had a large void ratio of 2.5 and was extremely vulnerable to calendering, and was not satisfactory.

Further, a magnetic tape was prepared in the same manner as in Comparative Example 1 except that this film was used. The characteristics of this magnetic tape are shown in Table 1.

Comparative Example 5 A polyethylene terephthalate biaxially stretched film having a thickness of 15 μm and a magnetic tape were obtained in the same manner as in Comparative Example 1 except that titanium oxide having an average particle size of 0.4 μm and a particle size ratio of 1.7 was used instead of kaolin. .

The void ratio of this film was 1.7, and although the characteristics were improved due to the flat surface, the friction coefficient was high and the calendering was poor, so that it was not satisfactory as a magnetic recording medium.

Examples 1 to 3 Polyethylene terephthalate pellets were obtained in the same manner as in Comparative Example 1 except that spherical silica fine particles adjusted to the average particle size and particle size ratio shown in Table 1 were used instead of kaolin.

A heat-fixable biaxially oriented polyester film having a thickness of 15 μm was obtained in the same manner as in Comparative Example 1 except that this pellet was used. The characteristics of this film are shown in Table 1.

Further, a magnetic tape was prepared in the same manner as in Comparative Example 1 except that this film was used. The characteristics of this magnetic tape are shown in Table 1.

The magnetic tapes obtained in the examples show excellent slipperiness even though the base film has a flat surface, and since the surface can be made flat, the electromagnetic conversion characteristics are significantly improved, and the calendar is strong. Almost no scraping was observed, and accordingly, D / O generated from scraping of the tape was also reduced, which is extremely good.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hideo Kato 3-37-19 Oyama, Sagamihara City, Kanagawa Prefecture, Teijin Ltd. Plastics Laboratory (56) Reference JP-A-59-171623 (JP, A) JP JP-A-55-108926 (JP, A) JP-A-63-112631 (JP, A) JP-A-62-252522 (JP, A)

Claims (1)

[Claims] 1. Spherical silica particles having an average particle size of 0.3 to 4 .mu.m, a particle size ratio (major axis / minor axis) of 1.0 to 1.2 and a relative standard deviation of 0.3 or less represented by the following formula: 0.01 to 1
A magnetic recording medium comprising a biaxially oriented polyester film which is dispersed and contained in a weight percentage, and a magnetic layer on at least one side of the film. Here, Di: equivalent circle diameter of each particle (μm) n: represents the number of particles.