JPH0697500B2 - High density magnetic recording medium - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a flexible magnetic recording disk capable of avoiding a tracking error. More specifically, the present invention relates to a recording medium having a high linear recording density capable of recording a high track density, mainly a flexible disk.

(2) Prior art The magnetic disk recording / reproducing apparatus itself has a mechanism for suppressing a temperature change and a special circuit for detecting a track (track servo, etc.).
It is conventionally known to prevent a tracking error by providing the. In addition, since these means complicate the recording / reproducing apparatus, they are not versatile. Practically, as far as possible, the base film and the magnetic material have a coefficient of thermal expansion close to that of the disk drive, and a flexible magnetic disk is made by selecting a material having a small coefficient of humidity expansion. , Measures have been taken to prevent tracking mistakes.

However, even such a flexible magnetic disk causes a tracking error when used at high temperature (40 to 50 ° C.) and / or high humidity (about 80% RH). Especially low temperature (10 ℃
Flexible magnetic disk recorded under conditions of high humidity (about 20% RH) or low humidity (about 20% RH) will be tracked when reproduced in a high temperature (about 40 to 50 ° C) and high humidity (about 60 to 30% RH) atmosphere. There was a drawback that mistakes occurred. In addition, there is a drawback that a tracking mistake occurs when a flexible magnetic disk recorded at low temperature and high humidity is reproduced at high temperature and low humidity. Due to this tracking mistake, the problem that the output drops and the dropout occurs is still unsolved.

Separately, attempts have been made to improve the magnetic material and to coat the magnetic layer uniformly and thinly on the substrate in order to increase the linear recording density. However, by increasing both the track density and the linear density, high magnetic recording can be achieved. Flexible discs that have been densified are not yet known.

[Object of the Invention]

An object of the present invention is to expand the usable atmospheric condition temperature and humidity range, improve it so as not to cause a track mistake even under high temperature and high humidity conditions, and have a high linear recording density and a flexible magnetic recording medium (disk). To provide.

Furthermore, with a flexible disk, such temperature
A magnetic recording medium having high dimensional stability with respect to humidity makes it possible to increase the density of magnetic recording, and particularly to improve the track density. It is also another object of the present invention to provide such a disk. is there.

[Structure of Invention]

The present invention provides a magnetic recording medium in which a magnetic layer is coated on a non-magnetic substrate made of a flexible thermoplastic polymer and the surface of the magnetic layer is smoothed by a smoothing treatment. Has a maximum and minimum difference of 2 × 10 ^-6 / ° C or less in all circumferential directions and a coefficient of humidity expansion of 8
X 10 ^-6 /% RH or less, the maximum and minimum difference in all circumferential directions is 1 x 10 ^-6 /% RH or less, and surface roughness is 0.012
It is characterized by comprising a biaxially oriented polyethylene naphthalene dicarboxylate film having a thickness of 2 μm or less and a magnetic layer having a thickness of 2 μm or less and comprising a composition of ferromagnetic fine powder and a polymer material. It is a density magnetic recording medium.

In the case of a polyethylene naphthalene dicarboxylate film as the non-magnetic substrate, a biaxially oriented polyethylene naphthalate film having a density of 1.355 g / cm ³ or less excluding high specific gravity inert fine particles in the film is used. preferable.

Further, a magnetic recording high density flexible disk using a polymer binder containing 50% or more of an epoxy ester type or epoxy / polyamide type component having low water permeability and low water absorption is preferable as the polymer binder.

The polyethylene naphthalene dicarboxylate in the present invention means a homopolymer of polyethylene 2,6 naphthalene dicarboxylate or a copolymer or mixture containing 70% by weight or more of this polymer, and essentially polyethylene naphthalene dicarboxylate ( Hereinafter, a polyester composition which does not lose the property of PEN) is also included.

The temperature expansion coefficient of the biaxially oriented PEN film in the present invention is preferably in the range of 15 to 25 (× 10 ^-6 / ° C). The drive device of a flexible disk, which is a magnetic recording medium, is mainly composed of aluminum having a relatively small temperature expansion coefficient (temperature expansion coefficient 25 (× 10 ⁻⁶ / ° C.)) as a metal, and the above temperature expansion coefficient is out of this range. Design is difficult if there is. A humidity expansion coefficient of 8.0 (× 10 ^-6 /% RH) or less is essential, and it is preferable that it is as small as possible.

The method of biaxially orienting the PEN film is, for example, stretching the unstretched film in the machine direction (longitudinal direction) and the width direction (transverse direction) in the range of about 2 to 5 times (preferably 2.5 to 4.5 times), Stretched area ratio is 4 to 25 times (preferably area ratio)
6.5 to 20). A known stretching device is used for this biaxial stretching, and the unstretched film is heated to a temperature range above the glass transition point (usually about 115 to 145 ° C.) to stretch in the longitudinal and transverse directions. The stretching method may be a simultaneous biaxial stretching method or a sequential biaxial stretching method, and in addition to the biaxial bistep stretching method, the stretching step number may be 3 or more. As a heat setting method for the PEN film, a known means such as a stenter or a heating roll can be used. Besides this, heat treatment may be carried out by gripping the four sides of the film in a tensioned or relaxed state, but the industrial stenter method is advantageous in industrial terms, and is also preferable for maintaining the surface roughness described later.

The surface roughness of the PEN film in the present invention (the measuring method will be described later) needs to be 0.012 μm or less, and preferably 0.005 to 0.008 μm. If the surface roughness of the PEN film exceeds 0.012μm, the coercive force is increased and the spacing loss is reduced. Therefore, if the film thickness of the magnetic layer is set to 2.0μm or less, it is extremely difficult to form a uniform magnetic layer coating film. become. Further, if the film surface roughness is 0.005 μm or less, it becomes difficult to handle the non-magnetic substrate (winding it on a roll or transporting the film by a roll in the coating step). As a method of adjusting the surface roughness of the PEN film,
Inert solid fine particles may be contained in the polymer,
Further, other surface processing treatment such as forming a coating film in which inert solid fine particles are dispersed may be performed.

In the present invention, the inert solid fine particles are preferably silicon dioxide (including hydrates, diatomaceous earth, silica sand, quartz, etc.); alumina; silicates containing 30% by weight or more of SiO ₂ ( For example, amorphous or crystalline clay minerals, aluminosilicates (including fired products and hydrates), asbestos,
Zircon, fly ash, etc.); Mg, Zn, Zr, and Ti
Oxides of Ca; sulfates of Ca and Ba; phosphates of Li, Na, and Ca (including monohydrogen salts and dihydrogen salts); benzoates of Li, Na, and K; Ca, Ba, Zn, And Mn terephthalates; Mg, Ca, Ba, Zn, Cd, Pb, Sr, Mn, Fe, Co and Ni
Examples are titanates of Ba; and chromates of Ba and Pb; carbon (for example, carbon black, graphite, etc.); glass (for example, glass powder, glass beads, etc.); Ca, and Mg carbonates; fluorite and ZnS. To be done. More preferably, silicic acid anhydride, hydrous silicic acid, aluminum oxide,
Aluminum silicates (including calcined products, hydrates, etc.), 1 lithium phosphate, 3 lithium phosphate, sodium phosphate, calcium phosphate, barium sulfate, titanium oxide, lithium benzoate, double salts of these compounds (hydrates Glass powder, clay (including kaolin, pentonite, clay etc.),
Examples are talc, diatomaceous earth, calcium carbonate and the like. Particularly preferred are silicon dioxide, titanium oxide and calcium carbonate. The average particle size of these inert solid fine particles is preferably 0.05 to 0.6 μm, more preferably 0.08 to 0.4 μm, and the addition amount is 0.01 to 1.5% by weight (relative to polyester), and further 0.03 to 1.0% by weight (same). , Particularly preferably 0.05 to 0.6% by weight (the same).

In the present invention, it is necessary that the PEN film, which is a non-magnetic substrate, does not exhibit significant anisotropy in temperature expansion coefficient and humidity expansion coefficient. For the biaxially oriented PEN film, the density of the film (a polymer in which inert solid fine particles having a high specific gravity are dispersed is excluded as a condition for providing substantially isotropic properties). It is desirable that the value) is 1.359 g / cm ³ or less. When the PEN film is heat-fixed after being biaxially stretched, both side edges of the film gripped by a clip or the like tend to run ahead of the central part of the film (called a bowing phenomenon), which is caused by this. Since anisotropy is likely to occur, it is practical to suppress the crystallization (densification) of the PEN film after heat setting to minimize the anisotropy. More specifically, it was extremely difficult to make the physical properties of the product film uniform in the width direction in the usual sequential biaxial stretching method, that is, the method of performing longitudinal stretching and then transverse stretching with a tenter. This is because the both ends of the film are gripped in the tenter for the following reasons, so the contraction stress in the longitudinal direction due to the lateral stretching is restricted by clips, but the central part of the film has a relatively weak restriction force. . As a result, the central portion of the running film advances with a positional delay due to the above-mentioned contraction stress, and a straight line in the width direction is formed on the film surface before transverse stretching, for example, a film that is run by applying an oil-based ink to the yarn is run. When drawn by contacting with each other instantaneously, this straight line becomes a concave curve in the film advancing direction during the transverse stretching and the subsequent tension heat treatment. This phenomenon is called bowing, and it causes a difference in physical properties (especially non-uniformity of coefficient of thermal expansion and coefficient of humidity expansion) between the center and both ends in the width direction due to bowing. There is. When the physical properties of the central portion of the film are balanced in the vertical and horizontal directions, oriented spindles further inclined in the vertical direction with respect to the bowing line are formed at both ends of the film, and the temperature expansion coefficient and humidity expansion coefficient in the main axis direction become small, Each value in the direction perpendicular to the main axis becomes large.

From these, it can be seen that the isotropic properties of the temperature expansion coefficient and the humidity expansion coefficient deteriorate in the portions near both sides of the tenter. Therefore, even if the coefficient of temperature expansion and the coefficient of humidity expansion are good in the central part (center), the isotropy may be poor in both sides (edges). Further, it is known that the difference in isotropicity between the central portion (center) and both side portions (edge) is smaller when the density is lower. In this way, from the viewpoint of suppressing bowing, the density of PEN film
It should be 1.359 g / cm ³ or less, preferably 1.353 g / cm ³ or less.

In this way, the coefficient of thermal expansion, which was practically difficult with polyethylene terephthalate, is 2.0 in all directions for discs, and the difference between the maximum and minimum for PEN film is 2.0.
(× 10 ^-6 / ° C) or less, and the similar difference in humidity expansion coefficient is 2.0 (× 10 ^-6 / ° C) or less.

The thickness of the magnetic layer in the present invention is 2.0 μm or less, preferably 1.5 μm or less. As the ferromagnetic fine powder, for example, γ-Fe ₂ O ₃ , Co-containing γ-Fe ₂ O ₃ , Fe ₃ O ₄ , Co-containing Fe ₃ O ₄ ,
CrO ₂ , Co-Ni-P alloy, Co-Ni-Fe alloy, Co-Cr alloy, Co-
Ferromagnetic materials such as Ni alloy and barium ferrite can be exemplified.

Examples of the polymer material used together with the ferromagnetic fine powder in the present invention include a thermoplastic resin, a thermosetting resin, a reactive resin, or a mixture thereof. Examples thereof include polymers of epoxy / ester type, epoxy / amine type, epoxy / polyamide type, vinyl type, phenolic type, urethane type and the like, or a mixture thereof.

Among the above examples, a single body or composition containing 50% or more of an epoxy ester-based or epoxy / polyamide-based component having low water permeability and low water absorption is preferable.

When a magnetic layer is formed by selecting a polymer material having low water permeability and low water absorption, it is difficult to bring the magnetic layer close to 0 in the case where the plastic film is used as a substrate because of the humidity expansion coefficient of the magnetic recording medium. The change can be substantially zero in a short time environmental change.

[Effect of the present invention]

In the present invention, the difference in the coefficient of thermal expansion of the non-magnetic substrate is 2.0
(× 10 ^-6 / ℃) or less and the humidity expansion coefficient of 8.0 (× 10 ^-6 /
% RH) or less and the difference is 1.0 (× 10 ^-6 /% RH) or less, it is possible to prevent a tracking error due to a change in temperature and humidity, and to obtain a magnetic recording medium having an increased track density.

Further, by using a polymer material having low water permeability and low water absorption in the composition of the magnetic layer, expansion or contraction of the magnetic recording medium due to a change in humidity can be substantially eliminated.

Furthermore, by setting the surface roughness of the non-magnetic substrate to 0.012 μm or less, the magnetic layer composed of ferromagnetic fine powder and polymer material can be 2.
Since a thin coating film having a thickness of 0 μm or less can be obtained without causing a spacing loss and having high coercive force and reproduction output, the magnetic recording medium (flexible disk) has a high linear density.

〔Example〕

 Hereinafter, the present invention will be described with reference to examples.

The method of measuring the characteristic value in the present invention is as follows.

(1) Temperature expansion coefficient Place a constant load and testing machine ITL2 manufactured by Japan Automatic Control Co., Ltd. in the constant temperature and humidity chamber and perform measurement. The measurement sample is heat-treated in advance under predetermined conditions (for example, 70 ° C for 30 minutes), and this sample is mounted on a tester between a temperature of 20 ° C and a humidity of 60% RH (relative humidity) and a temperature of 40 ° C and a humidity of 60% RH. The coefficient of thermal expansion is measured by reading the dimensional change at. At this time, the original sample length is 505 mm and the sample width is 1/4 inch. The weight applied at the time of measurement was constant per 5 g / 1/4 inch width. When a long sample cannot be obtained, it can be measured using a thermomechanical analyzer TM-3000 manufactured by Vacuum Riko Co., Ltd. Use TM-3000 to determine the difference between the maximum and minimum values of the coefficient of thermal expansion. The dimensions of the sample are 15 mm in length and 5 mm in width.
By reading the dimensional changes at 10 ° C / humidity 0% RH and temperature 40 ° C / relative humidity 0%, the difference between the maximum and minimum of the thermal expansion coefficient can be known. Since the values obtained by both measuring methods are completely the same, either measuring method may be used.

(2) Humidity expansion rate As in the case of obtaining the temperature expansion rate, using a constant load seed elongation tester manufactured by Japan Automatic Control Co., attach a sample that has been pre-treated under the conditions of temperature 40 ° C and relative humidity 90%, The coefficient of humidity expansion is obtained by reading the dimensional change between a temperature of 20 ° C, relative humidity of 30% and a humidity of 20 ° C and 70% RH. When the sample could not be taken for a long time, the thermomechanical analyzer manufactured by Vacuum Riko Co., Ltd. was placed in a thermostatic chamber as in the measurement of temperature expansion, and the measurement was performed under the above conditions. Also in this case, the values obtained by either method are completely the same.

(3) Density Using a mixed solution of heptane and carbon tetrachloride, measurement was carried out at 25 ° C by a density gradient tube method. The unit is [g / cm ³ ]. However, when containing high-density inert fine particles such as titanium oxide,
The composition is obtained and corrected.

(4) Tracking deviation test (temperature change) 5 1/4 inch Flotspie disk drive JU-581 (for 1.6MB) made by Matsushita Communication Industrial Co., Ltd. is put in a constant temperature and humidity chamber and passed through an interface. The mixing pulse was measured by connecting to a drop-in / drop-out counter SK-444B manufactured by Tokyo Engineering Co., Ltd.

Recorded after stabilizing at a temperature of 15 ℃ and humidity of 60% RH, then recording at a temperature of 40 ℃.
Reproduced and output after 3 hours under the condition of humidity 60% RH
The presence or absence of a mixing pulse of 50% or less or 30% or less was measured.

The case where there is no mixing pulse of 50% or less is good, the case where there is no mixing pulse of 30% or less but there is a mixing pulse of 50% or less is possible, and the case where there is a mixing pulse of 30% or less is not possible .

(5) Tracking deviation test (temperature change) Using the same equipment as in (4), record at a temperature of 25 ° C and a humidity of 20% RH.
The temperature was 25 ° C. and the humidity was 70% RH, and the same judgment was made.

(6) Tracking deviation test temperature / humidity change) (4) Using the same device as in (5), recording was performed at a temperature of 15 ° C and humidity of 20% RH, and reproduction was performed at a temperature of 40 ° C and humidity of 70% RH. I made a decision.

(7) Resolution test Matsushita Communication Industrial Co., Ltd.'s 5 1 / 4-inch PROTOPS disk drive JU-581 (for 1.6MB) Floppy disk test by Tokyo Engineering Co., Ltd. The apparatus [SK-403C] was used to measure the resolution [2F playback output value / 1F playback output value x 100 (%)] of the innermost track. 0 for standard disk resolution.
A value of 95 or more was rated as good, a value of 0.90 or more but less than 0.95 was acceptable, and a value of less than 0.90 was not acceptable.

(8) Film surface roughness Measured according to JIS B0601. Using a stylus type surface roughness meter (SURFOOM 3B) made by Tokyo Seimitsu Co., Ltd., the radius of the needle is 2 μm, and the load is
A chart (film surface roughness curve) was applied under the condition of 0.07 g. A portion of the measurement length L is extracted from the film surface roughness 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, and the roughness curve is taken as Y = f (x). Is given by (R
a: μm) is defined as the film surface roughness.

In the present invention, 8 pieces were measured with a reference length of 0.25 mm, and Ra was expressed as an average value of 5 pieces excluding 3 pieces having the larger values.

Example 1, Comparative Examples 1 to 6 PEN homopolymer pellets containing 0.3% by weight of titanium oxide having an average particle diameter of 0.25 μm and an intrinsic viscosity of 0.60 were placed at 180 ° C.
And dried for 4 hours. This polymer is melt extruded by a T-die according to a conventional method to prepare an unstretched film having a thickness of 1080 μm, which is 3.7 times at 120 ° C in the longitudinal direction and 130 ° C at the lateral direction.
The film was sequentially biaxially stretched 3.9 times and further heat-set at 235 ° C. for 30 seconds to prepare a film having a surface roughness of 0.008 μm, a thickness of 75 μm and a width of 3 m.

0.5m from the widthwise center of this film
After measuring the densities of the middle part and the edge part that are 1.0m apart, measure the temperature expansion coefficient and humidity expansion coefficient from 0 to 150 degrees every 30 degrees, and determine the difference between the average value and the maximum value and the minimum value. I asked. The results are shown in Table 1 together with the heat setting conditions.

The center portion, middle portion and edge portion of the film thus obtained were slit into a film having a width of 250 mm, and a magnetic layer having the following composition was formed to a thickness of 1.5 μm.

(Magnetic coating liquid) γ-Fe ₂ O ₃ 200 parts by weight Vinyl chloride-vinyl acetate copolymer resin (VAGH manufactured by UCC) 30
20 parts by weight Polyurethane (PP-88 manufactured by Nippon Polyurethane Industry) 20 parts by weight Isocyanate compound (Coronate HL manufactured by Nippon Polyurethane Industry) 40 parts by weight Carbon (average size 0.5 μm diameter) 20 parts by weight Dimethylsiloxane 2 parts by weight Toluene 70 parts by weight Methyl ethyl ketone 70 Parts by weight Cyclohexanone 70 parts by weight The above coating composition was thoroughly mixed and stirred to be applied. Then, the surface of the magnetic layer was subjected to calendar roll treatment. After that, it was punched into a 5 1/4 inch disk. The results of the off-track test and the resolution are shown in Table 1 as Example-1 and Comparative Examples-1 and 2. The films of Example 1, Comparative Examples 1 and 2 were subjected to heat treatment at a relatively high temperature of 235 ° C., and their density was as high as 1.359 g / cm ³ , so that the center part (center) of the 3000 mm wide film was ) Is only isotropic, and the middle part (500 mm from the center)
There is anisotropy in the distant position) and the edge portion (the position 1 m away from the center), and Δαt and Δαh are high, so that a good quality flexible disk cannot be obtained. Further, as Comparative Example 3, instead of the pellet of Example 1, the average particle size is 0.4 μm.
PE with an intrinsic viscosity of 0.60 containing 0.13% by weight of kaolin
N pellets were used. The surface roughness of the obtained film is
It was 0.015 μm. The evaluation result of the center portion is shown in Table 1 as Comparative Example 3.

In the PEN film of Comparative Example-3, the surface of the film was roughened due to the addition of kaolin having a slightly rough particle size, and the surface roughness of the magnetic layer surface affected the resolution.

Next, in place of the pellets of Examples 1 and 2, polyethylene terephthalate containing 0.3% by weight of titanium oxide having an average particle size of 0.25 μm was used in the same manner except for the melting temperature and the stretching temperature, and magnetic recording flexibility was obtained. A disk was created and the same evaluation was performed. The results of the center portion, middle portion and edge portion are shown in Table 1 as Comparative Examples 4, 5 and 6.

Examples 2 and 3 and Comparative Example 7 In Example 1, instead of heat setting at 235 ° C. for 30 seconds, heat setting was performed at 210 ° C., and other conditions were evaluated in the same manner. The results are shown in Table 2 as Examples 2 and 3 and Comparative Example 7.

The PEN films of Examples 2 and 3 have a heat treatment temperature of 210.
C., and the density was 1.353 g / cm ³ , which was lower than those of Example 1 and Comparative Examples 1 and 2. As a result, the 3 m wide film becomes isotropic (weak anisotropy) not only in the center part but also in the middle part (position 0.5 m away from the center), and it is homogeneous and has a low coefficient of thermal expansion and humidity expansion. Is to be presented.

Example 4, Comparative Example 8 For the magnetic coating liquid of Example 3, vinyl chloride
The composition ratio of the vinyl acetate copolymer and the polyurethane was changed.
The water permeability of the former was 0.261 g / day / cm ³ , and the equilibrium moisture content at 20 ° C / 65% R was 1.32%, but the water permeability of the latter was 0.399 g / day / cm 3.
^{At 3} , the equilibrium moisture content was 2.34%. Otherwise, magnetic recording disks were prepared in the same manner, and changes in temperature and humidity in a tracking test were evaluated. The results are shown in Table 3 as Example 4 and Comparative Example 8.

Comparative Examples 9 and 10 1 mol of Na ₂ S.9H ₂ O, 0.05 mol of NaOH, 0.6 mol of sodium benzoate and 3.5 mol of N-methylpyrrolidone were placed in an autoclave and heated to 210 ° C. for dehydration. After the completion of dehydration, the temperature of the system was lowered to 160 ° C., 1 mol of P-dichlorobenzene was added, and polymerization was carried out at 280 ° C. with stirring. After completion of the polymerization, the temperature was lowered to room temperature and washing with water and acetone was repeated to obtain a poly-P-phenylene sulfide polymer.

These polymers were melt extruded at 300 ° C. to obtain an unstretched film. Then, the film was stretched at 105 ° C. in the longitudinal direction by 3.6 times and at 115 ° C. in the lateral direction by 3.7 times, and further heat-set at 240 ° C. for 30 seconds to obtain a biaxially oriented film having a thickness of 75 μm. Separately, a test was conducted in which the stretching temperature was 115 ° C (3.6 times) in the longitudinal direction, 125 ° C (3.8 times) in the transverse direction, and the heat setting temperature was 260 ° C (30 seconds). The former is referred to as Comparative Example 9 and the latter is referred to as Comparative Example 10. The results are shown in Table-4.

Comparative Examples 11 and 12 As a dibasic acid component, terephthalic acid was 85 mol%, isophthalic acid was 15 mol%, 1,4-cyclohexanedimethanol was used as a glycol component, and titanium oxide was used as a catalyst.
05 mol% was put into an autoclave, heated under agitation to undergo transesterification, and then polycondensed to obtain poly-1,4-cyclohexylene, dimethylene and 1,4-cyclohexanedimethanol and terephthalic acid and isophthalic acid. A terephthalate polyester was obtained.

This polyester is melt extruded at 300 ° C. to obtain an unstretched film, and then stretched at 95 ° C. in the machine direction by a factor of 3.6,
It was stretched 3.7 times in the transverse direction at ℃ and heat set at 220 ℃ for 30 seconds. This is Comparative Example 11. The stretching conditions are longitudinal stretching 120 ° C (3.6 times), horizontal stretching 130 ° C (3.7 times), and 235 ° C.
Sample heat-fixed for 30 seconds (film thickness 75 μm)
Was designated as Comparative Example 12. Each of the samples of Comparative Examples 9 to 12 was slit into a film having a width of 250 mm in the same manner as in Example 1, and the magnetic coating solution used in Example 1 was applied to form a magnetic layer having a thickness of 1.5 mm. Next, the surfaces of these magnetic layers were subjected to calender roll treatment and punched into a 5 1/4 inch disk. After that, a track off test was conducted and the results are shown in Table-4.
It was shown to.

Examples 5 to 7 and Comparative Examples 13 to 15 In Example 1, instead of heat fixing at 235 ° C for 30 seconds, heat fixing was performed at 198 ° C, and other conditions were evaluated in the same manner. The results are shown in Table-5 as Examples-5, -6 and -7. Also, the results were evaluated by carrying out heat setting at 242 ° C. and under the same conditions other than Comparative conditions -13 and -14.
And -15 are also shown in Table-5.

[Examples 5 to 7], [Example 1, Comparative Example 1]
2], [Examples-2 and -3, Comparative Example 7], [Comparative Example 13 to
Comparing 15], when the density of the film increases due to the high temperature heat treatment, the position in the width direction of the film (effective width), which exhibits good temperature expansion coefficient and humidity expansion coefficient, is narrow, and the density increases further. It was found that if it is too much, the isotropic portion disappears, and if the density becomes low, the position (width interval) in the film width direction showing good isotropic property becomes wider.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junji Kobayashi 3-1937 Koyama, Sagamihara City, Kanagawa Prefecture Teijin Ltd. Plastics Research Institute (72) Inventor Tomoyuki Nakamura 3-1937 Koyama, Sagamihara City, Kanagawa Prefecture Teijin Ltd. Plastics Laboratory (56) References JP-A-59-185029 (JP, A) JP-A-59-186120 (JP, A)

Claims (4)

[Claims] 1. A magnetic recording medium comprising a flexible non-magnetic substrate coated with a smooth magnetic layer, wherein the non-magnetic substrate has a maximum value and a minimum value of a coefficient of thermal expansion in all directions in its plane. The difference is 2.0 × 10 ^-6 / ° C. or less, the maximum value of the humidity expansion coefficient in all directions in the plane is 8 × 10 ^-6 /% RH or less, and the minimum value of the temperature expansion coefficient of the maximum value. Difference is 1.0 × 10 ^-6 /% RH or less, and the surface roughness is 0.012μ
a biaxially oriented film of polyethylene naphthalene dicarboxylate having a thickness of m or less, and a magnetic layer made of a composition of ferromagnetic fine powder and a polymer material, and having a thickness of 2.0 μm or less. Characteristic high-density magnetic recording medium. 2. The high density magnetic recording medium according to claim 1, wherein the density of the polyethylene naphthalene dicarboxylate film as the non-magnetic substrate is 1.359 g / cm ³ or less. 3. The polymer material for forming the magnetic layer is a polymer containing an epoxy-ester type or epoxy-amide type component having low water permeability and water absorption. High density magnetic recording medium. 4. The high-density magnetic recording medium according to claim 1, wherein the magnetic recording medium has a disk shape.