JPH0322651B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to magnetic recording media, and more particularly to flexible magnetic disks. [Prior Art] Flexible magnetic disks (typified by what is commonly called a ``floppy disk,'' hereinafter simply referred to as a floppy disk) that are often used in auxiliary recording devices of small computers, etc. Normally, information is recorded in the form of a large number of thin circular recording areas called "tracks" arranged concentrically on the disk surface, but recently, in order to increase the recording capacity, track density (disk (number of tracks per unit length in the radial direction)
There is a strong need for an increase in track width (i.e., a decrease in track width). Conventional floppy disks use biaxially oriented polyethylene terephthalate film (hereinafter referred to as PET film) as the base film.
Those with a magnetic layer provided on one or both sides are widely used. Further, the use of a biaxially oriented poly p-phenylene sulfide film (hereinafter referred to as PPS film) as a base film for magnetic recording media has been proposed in Japanese Patent Application Laid-Open No. 55-38613. However, conventional floppy disks using PET film undergo extremely large dimensional changes due to changes in humidity. The coefficient of thermal expansion has a large in-plane anisotropy, and when the temperature changes, the dimensional change differs depending on the direction of the disk. If left under high temperature and high humidity for a long period of time, permanent shrinkage will be significant. If the track width is narrowed for such reasons, the recording/playback device (hereinafter referred to as drive) may
The relative position of the recording/reproducing head (hereinafter referred to as the head) of the disk and the track on the disk shifts, making it impossible to reproduce the signal recorded on the track (causing so-called "off-track"), making it difficult to increase the density of the track. It was possible. On the other hand, compared to PET film, PPS film has
Although it has extremely desirable properties in that both dimensional change due to humidity and permanent shrinkage under high temperature and high humidity are small, floppy disks using conventional PPS film have the following characteristics and have a track record comparable to PET film base. Even density could not be achieved, and it had no commercial value.In other words, the anisotropy of the coefficient of thermal expansion was greater than that of PET film, and even a track density comparable to that of a PET film-based floppy disk would off-track. Because the average coefficient of thermal expansion of the disk is greater than that of typical metal materials, the difference between the effective coefficient of thermal expansion of the drive's head support system is large, and off-track is likely to occur when the temperature changes. [Object of the Invention] The object of the present invention is to overcome the drawbacks of the above-mentioned prior art, that is, to improve the drawbacks regarding dimensional stability, achieve a remarkable increase in track density, and, as a result, create a magnetic recording medium with a large recording capacity. , preferably a floppy disk. [Structure of the Invention] The present invention provides that 70 mol% or more of the repeating units have the structural formula

In a flexible magnetic recording medium comprising a biaxially oriented polyp-phenylene sulfide film consisting of the following formula as the main constituent layer of a base film, and a magnetic recording layer provided on at least one side of the base film, the consistency degree C _p is 8×10 ^-6 ℃ ^-1
and the dimensional stability index D _s of the base film is 5.2×10 ⁻⁴ or less. Polyp-phenylene sulfide (hereinafter referred to as "PPS") in the present invention has the structural formula

Refers to a compound containing 70 mol% or more, preferably 90 mol% or more of repeating units represented by the formula. If the content of para-bonded phenylene sulfide units is less than 70 mol%, the crystallinity of the polymer will decrease. This is undesirable because it tends to cause permanent deformation due to heat shrinkage during storage. The remaining 30 mol% or less of repeating units in the polymer are metaphenylene sulfide units.

[Formula] Ether unit

[Formula] Sulfone unit

[Formula] Biphenyl Uni Tsuto

[Formula] Naphthyl Uni Tsuto

[Formula] Substituted phenyl sulfur feed unit

[Formula] (where R represents an alkyl group having 1 to 10 carbon atoms, a nitro group, a phenyl group, or an alkoxy group having 1 to 10 carbon atoms), trifunctional phenyl sulfide unit

〔Effect of the invention〕

By having the magnetic recording medium of the present invention configured as described above, the positional stability of the track against changes in temperature and humidity is significantly improved, and the decrease in output due to poor flatness is reduced. High track density recording has become possible, and as a result, the capacity can be increased, making it suitable for magnetic recording media for various uses, especially floppy disks. Next, a method for manufacturing the magnetic recording medium of the present invention, particularly a floppy disk, will be described. First, as a method for polymerizing PPS used in the present invention, a method is used in which alkali sulfide and P dihalobenzene are reacted in a polar solvent at high temperature and high pressure. In particular, it is preferable to react sodium sulfide and P dichlorobenzene in an amide high boiling point polar solvent such as N-methyl-pyrrolidone. In this case, in order to adjust the degree of polymerization, a so-called polymerization aid such as a caustic alkali or an alkali metal carboxylic acid salt is added.
Most preferably, the reaction is carried out at 230°C to 280°C. The pressure within the polymerization system and the polymerization time are appropriately determined depending on the type and amount of the auxiliary agent used, the desired degree of polymerization, and the like. The PPS thus obtained is supplied to a well-known melt extrusion device such as an extruder and melted. The molten resin is continuously extruded from a so-called T-die, cast onto a cooled metal drum, and rapidly solidified to form an unoriented amorphous sheet. Next, the sheet thus obtained is biaxially stretched. Conventionally, various methods such as sequential biaxial stretching and simultaneous biaxial stretching are known as methods for stretching plastic films, but in order to stably obtain the PPS film used as the main constituent layer of the base film of the present invention, In this method, a so-called vertical and horizontal sequential biaxial stretching method is used, in which the sheet is rapidly stretched in the longitudinal direction by a group of rolls, and then stretched in the width direction by a tenter.
Like the simultaneous biaxial stretching method and the horizontal and vertical sequential biaxial stretching method,
In the first stretching method using a tenter, not only the physical properties of the film become different and non-uniform between the center and the edges, but even in the center, the above-mentioned properties required for the PPS film used in the product of the present invention are not achieved. I can't satisfy you. Stretching in the longitudinal direction (longitudinal stretching) is performed by rapid stretching between rolls with different circumferential speeds, and the film temperature immediately before stretching is in the range of 95 to 105°C.
In addition, the temperature difference in the width direction (difference between the highest temperature and the lowest temperature in the portion that will later be effectively stretched transversely) is set to be 3° C. or less. Further, the stretching temperature is set to 30000%/min or higher. If the temperature difference in the width direction is large or the stretching speed is slow, not only will the obtained biaxially stretched film lack uniformity in thickness, but also in-plane isotropy of the coefficient of thermal expansion will not be obtained. The stretching ratio is determined as follows. In other words, the degree of optical orientation immediately before lateral stretching is
The magnification is chosen to be in the range of 0.145 to 0.165. "Immediately before horizontal stretching" here does not necessarily mean immediately after the end of longitudinal stretching. For example, if the tenter has a preheating section for horizontal stretching,
This means immediately after preheating. In any case, it refers to the state immediately before the film is stretched in the width direction. The actual magnification varies depending on the stretching temperature and the degree of polymerization of the polymer, but is approximately in the range of 3.9 to 4.9 times. In addition, the density of the film just before horizontal stretching was 1.317
The longitudinal stretching temperature, magnification, and preheating conditions for transverse stretching are selected to be in the range of ~1.330. If the density is too low, it will be difficult to stretch to a uniform thickness;
If it is too large, the film will have a large anisotropy in thermal expansion coefficient. The longitudinally uniaxially stretched film whose degree of orientation has been accurately controlled in this way is laterally stretched using a tenter. The stretching temperature is 95 to 110°C, and the stretching speed V is
When the stretching temperature is T°C, at least V=40T
-3200 [unit: %/min] or higher is required.
If the stretching temperature is too low or too high, the film will tear during stretching. The same applies when the stretching speed is slow. The stretching ratio is selected in the range of 3.5 to 3.9 times. Next, the stretched film thus obtained is subjected to fixed length heat treatment. Such heat treatment is usually carried out by providing a heat treatment chamber after the stretching chamber in the same tenter as that used for stretching.
A separate tenter or heated roll may be used. In either case, the heat treatment must be performed with a constant length in both the longitudinal and width directions. The constant length mentioned here means that the change in width and length during heat treatment is 1% or less. Heat treatment temperature is 200~250℃
(more preferably 210 to 240℃), and
Do this for ~50 seconds. After fixed length heat treatment, at a temperature below the heat treatment temperature,
It is permissible to perform relaxation of a few percent or less in the longitudinal direction and/or width direction. The biaxially oriented PPS film obtained as described above may be used as it is as a base film, or at least 50% of the thickness of the base film may be used as a base film.
As long as the dimensional stability index D s of the PPS film is occupied and the dimensional stability index D _s of the base film as a whole falls within the above-mentioned range, it may be laminated with other suitable films to form the base film. As already mentioned, in the present invention, the magnetic recording layer formed on the base film is of a coating type,
Although any type of metal thin film type may be used, a method for manufacturing a coated type disk will be explained here as an example. Coating type discs use magnetic paint, which is made by dispersing magnetic powder in a polymer binder solution.
It is manufactured by applying it to a base film and drying it, then punching it out into a disc shape. The composition of the binder used is not particularly limited, but a thermosetting type such as a urethane type binder is preferable. Such a binder is dissolved in an organic solvent such as methyl isobutyl ketone, ethyl acetate, toluene, or the like. Furthermore, magnetic powder is added to the solution and mixed and dispersed. The type of magnetic powder is not limited, but generally, γ-Fe ₂ O ₃ , CrO ₂ ,
Fine powder of Fe, Ni, Co, etc. is used. Also suitable is a composite magnetic powder in which cobalt is epitaxially grown on the surface of fine iron powder. The composition of the paint is approximately 30 to 70 parts by weight of magnetic powder and 15 to 40 parts by weight of binder to 100 parts by weight of solvent. Also,
It is also effective to add a lubricant such as stearic acid, a hardness modifier such as fine alumina powder, etc. to the paint. The paint thus obtained is applied to one or both sides of the base film and dried to a thickness of 0.3 to 5 μm.
m (preferably 1 to 3.0 μm) of the magnetic recording layer is formed. Thereafter, if necessary, the surface is smoothed by calendering, polishing, etc., and then heated to crosslink the coating film. Next, the sheet-like product thus obtained is punched out into a disk shape, and polished as necessary to give a mirror finish to the surface. Furthermore, if necessary, the above-mentioned disk is inserted into a protective case called a jacket to obtain a floppy disk. Next, the definition, measurement method, and evaluation method of the characteristic values of the polymer, film, and floppy disk used in the description of the present invention will be explained. (1) Characteristic melt viscosity (μ) Using a Koka type float tester with a capillary die of length L and radius R, the pressure P at temperature T was measured.
When the volume discharge amount when extruding the polymer is Q, the apparent shear stress τ, the apparent shear rate γ〓, and the apparent viscosity μ are defined as follows. τ = (RP) / (2L) γ = (4Q) / (πR ³ ) μ = τ / γ At this time, the curve μ = f obtained by plotting μ at that time for various γ = (γ〓), γ〓
= 200 (seconds) Define the characteristic melt viscosity μ ₀ with the value at ^-1 . In the present invention, a die with L=10 mm and R=0.5 mm was used, and values measured at T=300° C. were used. (2) Optical orientation n〓−n〓 Set the sample film in a polarizing microscope equipped with crossed Nicols so that the film surface is perpendicular to the optical axis, and then set the compensator at an azimuth angle of 45° with respect to the analyzer axis. Insert it so that it has.
Next, the sample is rotated around the optical axis and placed at a phase reduction position (a position where the phase difference produced by the sample is reduced by the compensator) in the direction of plus or minus 45° from the extinction position. At this time, the optical path difference Γ ₀ caused by the birefringence of the sample
is obtained from the compensation value of the compensator, and the degree of optical orientation n〓−n〓 is defined by Γ ₀ /d ₀ . (Here d ₀ represents the thickness of the sample). Furthermore, at this time,
On the sample, the direction perpendicular to the rotation axis of the compensator is the γ direction of the sample, and the parallel direction is the β direction. In the present invention, measurements were performed using a polarizing microscope POH type manufactured by Nippon Kogaku, a universal stage manufactured by Leitz, and a compensator manufactured by Leitz, using monochromatic light of the sodium D line (wavelength 0.5893 μm). (3) Film density (ρ) Measured at 20°C using a density gradient tube using an aqueous lithium bromide solution. (4) Polymer glass transition point (T _g ), melting point (T _n ),
and melting point (T _nf ) of the film after biaxial stretching DSC
It was measured by the method. (5) Coefficient of thermal expansion at 25℃ Cut a section 70 to 200 mm long and 10 mm wide from the measurement sample (base film or floppy disk), place it in a constant temperature and humidity chamber without any load, and adjust the humidity.
At a constant 35%RH, the temperature is raised from 18℃ to 35℃ at a rate of 1℃/min, and then returned to 18℃. Next, this sample piece was set at a tension of 10 g/mm ² and an initial length of l ₀ in an expansion/contraction measuring device equipped with a constant tension micro-displacement meter (manufactured by Japan Automatic Control Co., Ltd.) in a constant temperature and humidity chamber. The temperature is raised from 18℃ to 35℃ at constant humidity (35%RH). At this time, 20℃
Read the displacement meter outputs x ₁ and x ₂ at 30°C and convert (x ₂ − x ₁ ) into the change in length of the sample, and let it be Δl〓. Then, α=Δl〓/(10×l ₀ ) The coefficient of thermal expansion at 25℃ in the longitudinal direction of the sample is defined by the formula: (6) Floppy disc (or base film)
Average coefficient of thermal expansion _d (or _b ) of the floppy disk (or base film)
Let's assume that any one direction is 0°, and measure the azimuth θ clockwise from 0°. The coefficient of thermal expansion α _di ~ α _d8 (or the coefficient of thermal expansion α _bi ~ α b8 of the base film) of the floppy disk was measured in each of the eight directions at 22.5° intervals from 0° to 157.5° using the method described in ( ₅ ) above. , _d = ( ₈ 〓 ⁱ⁼¹ α _di ) / 8, _b = ( ₈ 〓 ^{i = 1} α _bi ) / 8 Define the average thermal expansion coefficient _d (or _b ) of the floppy disk (or base film). do. (7) Thermal expansion anisotropy Δα _b of the base film The difference between the maximum and minimum values of α _b1 to _{α b8} measured for the base film in (6) above is defined as the thermal expansion anisotropy Δα _b of the base film. (8) Humidity expansion coefficient Cut a section 70 to 200 mm long and 10 mm wide from the sample, and measure the tension using the extensometer used in (5) above.
10g/mm ² , initial length l ₀ , set at 20℃, 35%
Keep at RH for several hours until the displacement meter output stabilizes.
Let the displacement meter output at this time be x ₃ . Next, keep the tension constant and the temperature constant and increase the humidity to 85%.
Change to RH and keep it for several hours until the displacement meter output stabilizes. Let the displacement meter output at this time be x ₄ . The humidity expansion coefficient β is defined by the formula β=Δl/(50×l ₀ ), where (x ₄ −x ₃ ) is converted into a change in length and Δl is used. (9) Maximum humidity expansion coefficient β _b.nax of the base film Measure the humidity expansion coefficients β _b1 to β _b8 of the base film in the same eight directions as in (6) above using the method described in (8) above. The largest value among β _b1 to β _b8 is β _b.nax
shall be. (10) Evaluation of playback characteristics of floppy disk Commercially available 8-inch floppy disk drive (48 tracks/inch, track width approximately 320 μm)
The test was conducted using a modified test equipment, with only the head section and drive section placed inside a constant temperature and humidity chamber, and the control circuit etc. placed outside. Test R Positional stability of the track against changes in temperature and humidity Set the floppy disk to be tested in the drive section of the above test equipment, and test at 10℃ and 8%RH.
After leaving it for 12 hours, record a 125KHz sine wave signal on the outermost track (track 00) under the same conditions, and let the minimum signal amplitude when immediately played back be R ₀ . After that, the atmosphere was changed to 51.7℃ and 80%RH.
Let _R1 be the minimum signal amplitude when the outermost track is reproduced under the same conditions after being kept for 12 hours. R _d =R ₁ /R ₀ is used as an index of the positional stability of the track against changes in temperature and humidity (needless to say, the closer R _d is to 1, the better). Test S Permanent deformation under high temperature and high humidity The floppy disk to be tested was set in the above test equipment, left for 24 hours at 23℃ and 50%RH, and then placed on the outermost track under the same conditions.
Let S ₀ be the minimum signal amplitude when a 125KHz sine wave signal is recorded and immediately played back. Next, the atmosphere was changed to 70°C and 90% RH, held for 2 hours, returned to 23°C and 50% RH, and held for 24 hours. Thereafter, the minimum signal amplitude when the outermost track is reproduced under the same conditions is set as _S1 . S _d = S ₁ /S ₀ is used as an index of permanent deformation characteristics under high temperature and high humidity. (The closer S _d is to 1, the better). Test T Initial output stability Set the floppy disk to be tested in the drive section of the above test equipment, and set it at 23℃, 50%RH.
After leaving the disc for 12 hours, record a 250KHz sine wave signal on the outermost track under the same conditions, and immediately play it back. Measure the output fluctuation during one revolution of the disc. , and ranked them as follows:

[Table] (11) Curling of disk When the floppy disk to be tested is taken out of the jacket and placed on a horizontal table, the degree of curling is determined by the size of the gap between the disk and the table. I judged it. The evaluation criteria are as follows.

〔Example〕

Next, the present invention will be explained in more detail by giving examples. Example 1 (1) Preparation of PPS polymer 32.6 kg of sodium sulfide in an autoclave
(250 mol, including 40 wt% crystallization water), 100 g of sodium hydroxide, 36.1 kg of sodium benzoate (250
mol) and 79.2 kg of N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) were charged and the temperature was gradually raised to 205°C while stirring, and 7.0 kg of distillate containing 6.9 kg of water was removed. 1,4- in the residual mixture
Dichlorobenzene (hereinafter abbreviated as DCB) 37.5
Kg (255 mol) and NMP20.0Kg were added, 265℃
It was heated for 4 hours. The reaction product was washed 8 times with boiling water and dried for 24 hours at 80°C using a vacuum dryer, μ, 2900 poise, N1.17, T _g 91°C, T _n 285.
21.1 kg (yield: 78%) of high polymerization degree PPS having a temperature of 21.1 kg (yield: 78%) was obtained (referred to as PPS-A). (2) Melt molding 0.1wt% of calcium stearate powder is added as a lubricant to the composition obtained in (1) above, and after stirring and mixing with a mixer, it is poured into the hopper of a 40mmφ extruder, and the mixture is poured into the hopper of a 40mmφ extruder. Replace the air with nitrogen gas. The composition melted at 310°C is attached to the extruder tip using a filter (using metal fiber material, maximum pore diameter 10 μmφ).
It was extruded through a T-die having a linear lip with a length of 250 mm and a gap of 1.5 mm, and was cast onto a metal drum whose surface temperature was maintained at 30° C. and solidified by cooling. At this time, at a position approximately 5 mm away from the point where the extruded melt contacts the drum in the radial direction of the drum, a diameter of 0.1 mmφ is placed parallel to the drum axis.
A stainless steel wire was stretched, and casting was performed while applying a DC voltage of about 5 kV between the wire and the drum (electrostatic casting). The obtained film has a width of 230 mm, a thickness of 1100 μm, and a density of
1.315 was an unstretched film (film A
). (3) Biaxial stretching and heat treatment Film A was stretched in the longitudinal direction of the film using a longitudinal stretching device consisting of a group of rolls at a stretching temperature of 95°C.
The film was stretched 4.1 times at a stretching speed of 100,000%/min, then fed to a tenter and stretched at a stretching temperature of 98%.
The film was stretched 3.7 times in the width direction at a stretching speed of 1000%/min at 1000%/min, and then heat treated at 230°C for 10 seconds in a subsequent heat treatment chamber in the same tenter to obtain a biaxially oriented PPS film with a thickness of 75 μm. (4) Base film The dimensional stability index D _s of the above film was measured and found to be 2.5×10 ^-4 , which satisfies the requirements for a base film used in the present invention, so this can be used as it is as a base film. I made it. (5) Preparation of floppy disk The base film described above was subjected to low-temperature plasma treatment on both sides in an oxygen atmosphere. γ-Fe ₂ O ₃ magnetic powder 45 parts by weight VAGH (vinyl chloride-vinyl acetate copolymer manufactured by UCC) 17 parts by weight N 1432J (acrylonitrile manufactured by Nippon Zeon)
Butadiene copolymer) 3.5 parts by weight Coronate L (polyisocyanate manufactured by Nippon Polyurethane Co., Ltd.) 1.5 parts by weight Methyl isobutyl ketone 50 parts by weight Toluene 50 parts by weight Carbon black 4 parts by weight A magnetic paint consisting of the above composition was prepared and Coat the above base film on both sides to a thickness of 3.0 μm (after drying), and after drying, punch out an 8-inch diameter disc along with a central hole and index hole to form a commercially available 8-inch floppy disk jacket. Insert it into
It was made into a floppy disk (referred to as disk A). The degree of consistency C _p of this floppy disk was measured and was found to be 1.0×10 ^-6 . Therefore, this disk is the floppy disk of the present invention. (6) Evaluation The evaluation results of the floppy disk prepared in Table 1 were compared with a commercially available floppy disk (an 8-inch single-sided single-density type, referred to as disk B) whose base film is polyethylene terephthalate film. Compare and show. From Table 1, the floppy disk of the present invention is as follows:
Compared to conventional products, both the positional stability of the track against changes in temperature and humidity and the permanent deformation characteristics under high temperature and high humidity conditions are significantly superior, and it is clear that it is possible to increase the density of the track.

[Table] Example 2 (1) Preparation of base film Same as Example 1, μ ₀ : 3100 poise,
A high polymerization degree PPS having N: 1.20, T _g : 91°C, and T _n : 286°C was obtained. This was melt-molded in the same manner as in Example 1,
An unstretched film having a width of 240 mm and a thickness of 1100 μm was obtained. This unstretched film was biaxially stretched in the same manner as in Example 1 by variously changing the longitudinal and transverse stretching ratios and the preheating conditions before transverse stretching, resulting in four types of biaxial orientation.
A PPS film (about 75 μm thick) was obtained. These will be used as base films, and will be referred to as base films C-1 to C-4. Table 2 shows the dimensional stability index of each base film. (2) Preparation of floppy disks Floppy disks were prepared for each of the above base films C-1 to C-4 in the same manner as in (5) of Example 1 (referred to as disks C-1 to C-4). . Table 2 shows the consistency C _p values of these floppy disks. (3) Evaluation Table 2 shows the evaluation results of the created floppy disk. From these results, it is clear that all of the floppy disks of the present invention exhibit excellent playback characteristics and are capable of increasing track density, and that even if a PPS film is used as the base film, the floppy disks do not meet the requirements of the present invention. has poor playback characteristics,
It can be seen that high track density is not possible.

[Table] Example 3 (1) Preparation of base film Same as Example 1, μ ₀ : 2300 poise,
A high polymerization degree PPS having N: 1.15, T _g : 91°C, and T _n : 284°C was obtained. This was melt-molded in the same manner as in Example 1, and the width
An unstretched film having a length of 240 mm and a thickness of 480 μm was obtained. This unstretched film was treated in the same manner as in Example 1, and was heated 4.7 times in the longitudinal direction at 100°C and at 103°C in the transverse direction.
The film was stretched 3.8 times and further heat treated at 220° C. for 5 seconds to obtain a biaxially oriented PPS film with a thickness of about 30 μm. The dimensional stability index of this film is 3.4×10 ^-4
It was hot. This film will be used as a base film (referred to as base film E). (2) Creating a floppy disk An alloy target consisting of 90 wt% Co and 10 wt% Ni was made, and using this target, a thin Co-Ni magnetic layer was formed on both sides of the base film E by the RF sputtering method. did. At this time, argon was used as the carrier gas, the pressure was 0.1 Torr, the plate voltage was 2 kV, and the current was 150 mA. Under the above conditions, the thickness of the thin film was 0.2 to 2.5 μm/
Four types of samples were prepared with different film thicknesses on one side. Example 1 from these samples, respectively.
Floppy disks were prepared in the same manner as (Disks E-1 to E-4). The values of the degree of consistency C _p of these disks were as shown in Table 3. (3) Evaluation Table 3 shows the evaluation results of the disks obtained. These results show that the floppy disk of the present invention has excellent initial playback characteristics and little curling, whereas the disks with a large C _p value have large curling and poor initial playback characteristics.

【table】

Claims (1)

[Scope of Claims] 1. A biaxially oriented polyp-phenylene sulfide film in which 70 mol% or more of the repeating units have the structural formula [Formula] as the main constituent layer of a base film, and at least one side of the base film A flexible magnetic recording medium having a magnetic recording layer provided thereon, the degree of consistency C _p is 8×10 ^-6 °C ^-1 or less, and the dimensional stability index of the base film is
A magnetic recording medium characterized in that D _s is 5.2×10 ^-4 or less.