JPS59207022A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium with no curl and superior planeness by using a biaxially oriented poly-p-phenylene sulfide film having specified characteristics as a principal constituent layer. CONSTITUTION:A biaxially oriented poly-p-phenylene sulfide (PPS) film contg. units represented by structural formula I by >=90mol% of the repeating units is used as the principal constituent layer of a base film, and a ferromagnetic layer is formed on at least one side of the PPS film. The characteristics of the base film are allowed to satisfy an equation 2X10<-7=AXC<=5X10<-5> in all directions in the surface. In the equation, A is the coefft. ( deg.C<-1>) of thermal expansion at 120 deg.C, and B is the rate (%) of thermal shrinkage at 120 deg.C. PPS used is required to contain >=90mol% repeating units represented by structural formula II. In case of <90mol%, the crystallizability of the polymer is reduced, so the mechanical characteristics of the resulting film are unsatisfactory. Units of m- phenylene sulfide, ether or the like account for the remaining <=10mol%. PPS is melted and molded into a sheet, and this sheet is biaxially stretched and heat treated to obtain said biaxially oriented PPS film. The preferred thickness is 5-100mum.

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]

With the spread of office computers and microcomputers, flexible magnetic disks, commonly known as floppy disks, have come into widespread use, but they are cheaper than conventional hard magnetic disks and are extremely difficult to replace. On the other hand, it has great advantages such as being easy to use.

The recording capacity is small, and since the base film is a glass sheet, it has drawbacks such as somewhat unreliable shape stability such as curling and bending.

The recording capacity of a floppy disk is determined by the linear recording density within the track (a thin annular recording area arranged concentrically) and the track density (generally TPI = the number of tracks per unit length in the radial direction of the floppy disk). /inch). An increase in linear recording density is achieved by improving the magnetic layer and magnetic recording method, and an increase in track density is achieved by minimizing reversible and irreversible dimensional changes in the disk's radial direction due to changes in temperature and humidity, and by minimizing these changes. The key point is that the dimensional change occurs equidirectionally in each direction.

On the other hand, it goes without saying that the most important thing is that the disk be completely flat and have excellent planarity. If the magnetic layer is curled or uneven, the contact between the recording/reproducing head and the magnetic layer becomes unstable, which significantly reduces the reliability of recording/reproducing. The floppy disk itself is generally protected by a cover, generally called a "jacket", made of paper or plastic sheet, and the rigidity of the cover is designed to prevent any slight curling from causing any real inconvenience; However, it is preferable to have little curl, and this requirement becomes even stronger when the recording density is increased to 2% or when the device is miniaturized.

Conventionally, two-phase oriented polyethylene terephthalate film has been most widely used as a base film for floppy disks, and this is largely due to its excellent mechanical properties and surface smoothness. However, the limited humidity expansion coefficient using this base film is approximately 10 x 10"
/RH%, which is large.

Since accurate track tracking performance is required over a wide range of humidity environment changes from 80 to 90 RH% when the disk is used, a certain limit automatically exists for the track density. For this reason, various efforts have been made to lower the actual humidity expansion coefficient by modifying polyethylene terephthalate or combining it with other low moisture absorption materials, but other thermal and mechanical properties deteriorate. Therefore, favorable results have not yet been obtained. Therefore, JP-A-55-4
A floppy disk using a biaxially oriented borylphenylene sulfide film as a base film, which has essentially excellent humidity characteristics, as proposed in 8613, has the potential for high recording density, but conventional biaxially oriented Discs made with borylphenylene sulfide films have included several problems in terms of creating flat shapes, such as the aforementioned curls. In particular, when a ferromagnetic thin film layer is formed directly on a base film by vapor deposition or the like to increase the density, the shape of the disk is often unsatisfactory.

In these vacuum thin film forming processes, the following thermal and mechanical loads are applied to the base film, so it is generally necessary to maintain the shape of the magnetic disc with sufficiently good flatness. It is difficult.

11) Generation of cohesive force due to deposition of metal thin film (2) Transfer of energy possessed by the metal atoms themselves to the base film (3) Effect of axial heat of heat during metal melting or heat generated by plasma, etc. (4) ) After the film and thin metal film have the same dimensions under high temperature, the bimetallic effect is due to the difference in thermal expansion coefficient between the two when they return to room temperature. Avoid flatness defects.

This often results in unsatisfactory results.

[Purpose of the invention]

An object of the present invention is to create a floppy disk with substantially no curl and excellent flatness by using a biaxially oriented polyphenylene sulfide film as the main base film, thereby achieving stable and high recording density. The present invention aims to provide a recordable magnetic recording medium, a portable magnetic disk (floppy disk).

[Structure of the invention]

In order to achieve the above object, the present invention uses a biaxially oriented p-phenylene polysulfide film containing 90 or more moles of repeating units of the structural formula ÷S+ as the main constituent layer of a base film, and a ferromagnetic layer on at least one side of the base film. The present invention is characterized by a flexible magnetic recording medium in which the characteristics of the base film satisfy the following characteristics in all in-plane directions.

2 x 10""≦AXO≦5 x I D-"where A: Coefficient of thermal expansion at 120°C (°C-') B: 12
Heat shrinkage rate (%) at 0°C Borylphenylene sulfide (hereinafter referred to as PP) used in the present invention
S) must contain 90 or more moles, preferably 95 or more moles of repeating units represented by the structural formula Mizuta s+. If the number of such repeating units is less than 90 molar units, the crystallinity of the polymer decreases, resulting in poor mechanical properties of the film and making it brittle as a base film for floppy disks, or in severe cases, it becomes difficult to provide biaxial orientation. Even axial stretching is impossible, and the glass transition point is 65.
℃ or below, which causes a serious defect in the high-temperature dimensional stability of the disk, which is not preferable. The remaining 10 moles or less of the repeating units of the polymer are selected from alkyl groups having 1 to 10 carbon atoms, alkoxy groups, nitro groups, phenyl groups, and sulfonic acid groups), trifunctional phenyl sulfides are block copolymerized. It may be contained in a random copolymerized state. Furthermore, the fact that a structural unit other than ÷S0, which is the main structural unit of the polymer, is arranged at or near the end of the polymer does not change the essence of the present invention. Furthermore, the PPS and other components 1, such as polymers, inorganic fillers, etc. mixed for other purposes, may coexist in the base film as long as they do not seriously affect the object of the present invention.

The melt viscosity of the PPS used in the present invention is 200 to 20.00 Dpoise, preferably 500 to 1, under the conditions of a temperature of 300°C and an apparent shear rate of 200 sec.
A void of 2,000 is desirable from the viewpoint of film formability and mechanical properties.

The PPIE biaxially oriented film is a film obtained by melt-molding the above-mentioned PPS to form a sheet, biaxially stretching, and heat-treating the film, and its thickness is preferably in the range of 5 to 100 μm.

The base film is a flexible support on which a magnetic recording layer is provided, and its thickness is at least 50 inches or more comprised of the above-mentioned PPS biaxially oriented film.

The thickness of the base film is preferably in the range of 10 to 100 .mu.m (more preferably 20 to 80 .mu.m) in terms of shape retention as a floppy disk, head touch, etc.

The part that is less than 50% of the base film thickness.

It can be composed of flexible sheets other than pps biaxially oriented films. Examples include polyethylene terephthalate film, polyethylene 2.6 f phthalate film, polyethylene alpha, beta bis(2-chlorophenoxy)ethane 4,4' dicarboxylate film, polypropylene film, aromatic polyamide film, aromatic polyimide film. , aluminum foil, etc.

It is not limited to this.

Further, in order to especially improve the surface smoothness of the PPS film, a coating layer of an organic polymer may be provided. This is because the thickness of the magnetic layer tends to become thinner as the magnetic layer has a higher magnetic recording density, and improvements such as surface roughness and surface protrusions of the base film are required, so an extremely thin surface smoothing layer is provided. However, the fact that the PP S film is the main constituent layer of the base film remains unchanged. Here, the requirements for surface roughness, especially the maximum permissible protrusion height, vary depending on the type of magnetic layer, but it is 01 to 2μ for a general coating type, and o, o i to 0.2μ for a thin film metal type that directly evaporates magnetic metal. Therefore, the thickness of the smoothing layer is also somewhat thicker than this.

The present invention is constructed by providing a magnetic layer on one or both sides of a base film, and the base film is subjected to various treatments in order to improve the adhesion between the magnetic layer and the base film. This is because if the adhesion is weak, the magnetic layer may be damaged by repeated contact with the head, or if it is severe, it may peel off. The most typical treatment is corona treatment. This is generally air or nitrogen.

The PPS film is treated by corona discharge in an atmosphere of carbon dioxide gas, and the surface tension of the untreated PPS film is about 32 dyn/an, which increases to 35-60 dyn/c+n by corona discharge treatment in air. As can be inferred from this, it is effective as a treatment for stably providing a magnetic layer. As will be described later, especially when providing a ferromagnetic metal layer, glow discharge treatment (
Plasma group/electric treatment) is effective. Glow discharge treatment is performed under a vacuum of about 10-5 to 10 Torr using a power source with a wide range of frequencies from DC to high frequency.
It is processed for seconds, and the gases used are oxygen, nitrogen, and hydrogen.

Argon, helium, carbon dioxide, - carbon oxide.

Ammonia, sulfur dioxide gas, hydrogen sulfide, and mixed gases thereof are used.

The magnetic layer in the present invention is Fe2O3, Fe, 04+C
Fine ferromagnetic powder such as rO2, Fe, Co, Ni, etc. was coated on the base film together with an organic binder.

So-called "coating type" and vacuum deposition of two ferromagnetic metals.

It was deposited on the base film by methods such as sputtering and plating. It is called a "metal thin film type".

Such a magnetic layer has a thickness in the range of 0.3 to 5 μm in the case of a coating type, and 0.05 to 2 μm in the case of a metal thin film type, and the linear density of magnetic recording is generally higher in the latter. In the case of a metal thin film, an extremely thin protective layer is often provided to prevent mechanical deterioration of the magnetic layer due to direct contact with the head. An example of the protective layer is a silicone-based organic ultra-thin layer (thickness of several tens to hundreds of angstroms), but the present invention is of course not limited to this.

Furthermore, the magnetization form of the magnetic layer is either horizontal magnetization or perpendicular magnetization, and the present invention is applicable to both of them.

The magnetic recording medium of the present invention can be applied to various uses such as tapes and floppy disks, but it is preferably used in 9% floppy disks (magnetic disks).

Next, a method for manufacturing a magnetic recording medium, particularly a magnetic disk, according to the present invention will be described.

First, the PPS used in the present invention is basically prepared using N-methylpyrrolidone as a solvent, p=; It is obtained by high-temperature reaction of sulfur-containing compounds represented by a mixture of Before starting the polymerization reaction, first add sodium sulfide (or a compound capable of synthesizing sodium sulfide) to N-methylpyrrolidone, and then add a polymerization auxiliary agent in a range of 10 to 100 moles based on the sodium sulfide. Dehydrate by heating to 100°C or higher. As a polymerization aid, an alkali metal salt of carboxylic acid such as lithium benzoate is used. Next, a dihalogenated aromatic compound having a 9B to 105 molar width (90 molar or more of which is p-dichlorobenzene) was added to sodium sulfide as a monomer, and after nitrogen blanketing, the reaction system was sealed and heated at 260 to 280°C. Polymerize for about 2 to 6 hours.

After the reaction is completed, the cooled system is diluted with a sufficient amount of water and washed. The polymer is obtained in the form of powder or granules, washed several times with water and then dried to provide the polymer used in the present invention.

Forming the polymer into a biaxially oriented film by known methods. That is, the resin melted by the extruder is quantitatively cast from nine nozzles onto a cooling drum, and rapidly cooled to form a non-oriented, amorphous sort. Next, this notebook is sequentially biaxially or simultaneously biaxially stretched, but generally, sequential biaxial stretching is performed in the longitudinal direction (longitudinal stretching) between rolls with different circumferential speeds, and then transverse stretching is performed in a tenter. . Next, the obtained biaxially stretched oriented film is heat-treated for a fixed length in the same tenter, and then relaxed as required. Relaxation is carried out in the lateral direction by reducing the distance between the rails at the rear of the tenter by a certain ratio, and in the longitudinal direction by reducing the peripheral speed of the roll by a certain ratio after exiting the tenter using a suitable device.

In each case this ratio is within a few percent.

In order to produce the film used in the present invention using a gloss film for producing biaxially oriented films, the longitudinal and lateral stretching conditions, heat treatment conditions, and relaxation conditions are particularly important. That is, in the longitudinal stretching step, conditions must be selected so that the degree of optical orientation after stretching and immediately before transverse stretching is in the range of 0165 to 0165. The actual stretching temperature and stretching ratio vary depending on the polymer, but generally the stretching force is within the range of 95 to 105°C and 69 to 49 times. Next, in the transverse stretching step, it is desirable to select conditions such that the degree of optical orientation after stretching is 0.05 or less in the longitudinal direction or 0.05 or less in the transverse direction.

The actual conditions are approximately a stretching temperature of 95 to 110°C and a stretching ratio of 65 to 40. Also, the heat treatment temperature is 200℃
to 270℃, relaxation is still 0 to 5 both vertically and horizontally.
A range of 1 is common. The film of the present invention is produced by an appropriate combination of these conditions, but as mentioned above, there are large differences depending on the polymer, and within this condition range, the film with the product of the coefficient of thermal expansion and the coefficient of thermal contraction of the present invention can be produced. Select the conditions that apply.

As already mentioned, the present invention requires a process for forming a magnetic layer on the biaxially oriented PPS film obtained by the above process, and this process includes a coating type and a metal thin film type. The effect of the present invention is particularly noticeable in the latter case, and the latter will be explained here.

Co, Ni,? Ferromagnetic metal represented by θ.

Furthermore, Co-Ni, Co-0r, Co-Fe
, Co-V, Co-Rh, Co0-R
u, Coo-W, C0-3n, Ni
-Fe, Co-N1-A7? Components such as ferromagnetic metal alloys with iron oxide, ferromagnetic metal oxides such as Co-doped iron oxide, chromium oxide, and barium ferrite, as well as non-ferromagnetic metals contained in these in the range of 20 yen or less. There are the following methods for forming a thin metal film layer of a low melting point metal or the like on a film.

Namely, vacuum evaporation methods using resistance heating, electron beam heating, induction heating, laser beam heating, etc.

Furthermore, sputtering methods such as DC dipole method, high frequency dipole method, DC red tron, high frequency magnetron, and opposed dipole method, ion brating method using cathode heating method, high frequency excitation method, cluster ion beam method, etc. etc.

The following nine were used in the description of the present invention. Polymer.

Definition of film and floppy disk characteristic values,
The measurement method and evaluation method will be explained.

(1) Characteristic melt viscosity (μ) When polymer is extruded at temperature T and pressure P using a Koka-type float tester having a capillary die with length and radius R, the volumetric discharge amount is Q. Then, the apparent shear stress τ, the apparent shear rate n, and the apparent viscosity μ are defined as follows, τ−(RP)/(2L)(4Q)/(πR3>μ−τ/ In this case, the curve μmf(n) obtained by plotting μ at that time for two different n is 2-200(sec)-1
Characteristic melt viscosity μ with the value of . Define.

In the present invention, L=10mm, R=0.5
The value measured at 00° C. using a T-rotation die was used.

(2) Optical orientation (nr nβ) Set the sample film on a polarizing microscope equipped with crossed Nicols so that the film surface is perpendicular to the optical axis, and then set the sample film at an azimuth angle of 45° with respect to the analyzer axis. Insert as shown. Next, the sample is rotated around the optical axis and placed at a phase reduction position (a position where the phase difference generated by the sample is reduced by the compensator) located in the direction of plus or minus 45 degrees from the extinction position. Optical path difference F caused by birefringence of the sample. is calculated from the compensation value of the compensator, and Fo/
The degree of optical orientation n, -nβ is defined by do (here d represents the thickness of the sample).

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.

Measurement was performed using monochromatic light of the sodium D line (wavelength 05896 μm) using a polarizing microscope POH type manufactured by Nippon Kogaku, universal stage L manufactured by Leitz, and a compensator manufactured by former TZ.

(3) Film density (ρ) Measured at 20°C using a density gradient tube using an aqueous lithium bromide solution.

(4) Polymer glass transition point (T8) and melting point (Tm
) and melting point (Tmf) of the film after biaxial stretching DSC
Measured according to the law.

(Thermal expansion coefficient (A) at 5112D°C) Cut a section with a length of 3' Omm and a width of 5 mm from the base film.
3000) and record the expansion and contraction of the sample when a 9th-order thermal history is applied under a constant tensile stress of 10 g/mm2.

a. Raise the temperature from 25℃ to 120℃ at a rate of 1℃/min.
Hold at 120°C for 5 minutes and increase temperature at 1°C/ml using CO1 filter and O°C. 11 immediately
At this time, the temperature is lowered at 1°C/min at 0°C, during the process of d (1
ΔL is the dimensional change when the temperature decreases from 30℃ to 110℃
The thermal expansion coefficient A at 120°C in the longitudinal direction of the sample is defined by A-ΔL/(20X Lo) (where L is the length when the sample is set in TMA).

(6) Heat yield at 120°C (C) After placing the sample at 23°C and 50% RH for 24 hours, the ρ distance between two previously marked points is measured in that atmosphere and set to +'0. -Subsequently, after 30 minutes in hot air set at 120℃,
After leaving it for 24 hours at 50° C. and 50 lRH, the distance between the two points previously measured was measured again under that atmosphere, and the distance was determined to be 11.

At this time, c=100x Bo-z, )/lo
Define the heat yield C at 120°C.

(Direction of 71AXC The measurements of A and B above are
8 directions at 25° intervals (0° to 1515°, remaining 18 directions
The eight directions from 0° to 63Z5° can be considered to be the same as the previous eight directions due to the symmetry of 9 circles), and AXO is calculated for each direction. AxC for these 8 directions
All of the O values are between 2 x 10'' and 5 x 1 as mentioned above.
When it is within the range of 0-5, AxC is considered to be within the scope of the present invention for all directions within plane 1, and when it is not, it is considered to be outside the scope of the present invention.

(8) Playback characteristics of floppy disk A commercially available 8-inch standard disk drive (manufactured by NEC Corporation) was used.

◎Test R...Characteristics against humidity changes Insert the floppy disk to be tested into a drive placed in a humidity tank with a gutter temperature of 4J and leave it for 24 hours. Below, a 125 KHz sine wave signal is recorded and the minimum signal amplitude when immediately reproduced is Ro. After that, the atmosphere was changed to 25℃ and 70%, and the minimum signal amplitude when playing again 24 hours later was R.
1, and R4/Ro is determined and used as an index of humidity change resistance.

◎Test S: Insert the floppy disk to be tested for permanent deformation under high temperature and high humidity into the drive, and heat it at 26℃ and 50%.
A 12°5 KHz sine signal is recorded on the outermost track under RH, and the minimum signal amplitude when immediately played back is S.
. shall be. next.

The disc alone is taken out, and after being placed in the drive at 70° C. and 90% RH for 2 hours, it is reinserted into the drive at 23° C. and 50% RH, and the minimum signal amplitude when played back 24 hours later is defined as Sl. S and /So are used as an index of permanent deformation characteristics.

(9) Curl After the step of forming a magnetic layer on the base film.

In order to store it in the jacket, a 52.5-inch disk was cut out and placed naturally with one side facing down on a flat, horizontal table.

Measure the maximum height of the disc. Do this for the other side, and use the larger value to curl (unit knee)
shall be.

〔Effect of the invention〕

As described above, the present invention uses a biaxially oriented borylphenylene sulfene film having specific properties as the main constituent layer, so there is no curling.

It was possible to obtain a magnetic recording medium, especially a floppy disk, which has excellent flatness and a high recording density.

〔Example〕

Next, the present invention will be explained in more detail by giving examples.

Example 1 +1) Preparation of PPS polymer 32.6 kg (25 kg) of sodium sulfide was placed in an autoclave.
0 mol, water of crystallization 4 D wt), 100 g of sodium hydroxide, 361 kg of sodium benzoate (25
0 mol), and N,,z-thyl-2-pyrrolidone (
Hereinafter, it will be abbreviated as NMP. 7.92 bags were charged and the temperature was gradually raised to 205°C while stirring, and a distillate containing 69 cans of water was prepared.
0 J was removed. To the remaining mixture, 375 g (255 mol) of 1,4-dichlorobenzene (hereinafter abbreviated as DCB) and 0 kg of NM'P2O were added, and the mixture was heated at 265°C for 4 hours. The reaction product was washed 8 times with hot water and dried at 80°C for 24 hours using a vacuum dryer to obtain μ. 2900 poise 1. N1.17. 21.1 kg (yield: 78) of high polymerization degree P'PS having Tg of 91°C and T of 285°C were obtained.

(Let it be pps-yu).

(2) Melt molding 0.1'wt% of calcium stearate powder was added as a lubricant to the composition obtained in (1) above, and after stirring and mixing with a mixer, the mixture was poured into the hopper of an extruder of 40 [[Imφ]. Then, the air in the hopper is replaced with nitrogen gas. The composition melted at 310° C. was passed through a filter (metal fiber R material used, maximum pore diameter 10 μmφ) equipped with 9 at the tip of the extruder, and the length was 250 mm with a gap of 0.
It was extruded from a T-die having an 8-stroke linear lip, cast onto a metal drum whose surface temperature was kept at 60°C, and cooled and solidified.

At this time, a stainless steel wire with a thickness of O,i + nmφ is stretched parallel to the drum axis at a position approximately 5 mm away in the radial direction of the drum from the point where the extruded melt contacts the drum. Casting (electrostatic application casting) was performed while applying a DC voltage of about 5 kV between the drum and the drum. The resulting film has a width of 260 mm and a thickness of 800 mm.
It was an unstretched film with a density of 1.3201 μm and a density of 1.3201 (
film U).

(3) Biaxially stretched, heat-treated film U is stretched in the longitudinal direction of the film at a stretching temperature of 96° C. and a stretching speed of 30.0 using a longitudinal stretching device consisting of a group of rolls.
It was stretched 4.1 times at a rate of 00/min.

Subsequently, the film was supplied to a tenter and stretched at a stretching temperature of 99°C.
The film was stretched 375 times in the width direction at a stretching speed of 10 DD%/min. At this time, the degree of optical orientation immediately before stretching is 0155
Met. Further, in a subsequent heat treatment chamber in the same tenter, the sample was heat treated at 220° C. for 10 seconds, and then relaxed by 2% in the width direction while slowly cooling. Next, the film is heated to 200°C in the longitudinal direction by a longitudinal relaxing device consisting of a group of rolls.
The film was then subjected to 2 chill laxation to obtain a PPS biaxially stretched film with a thickness of 50 μm (referred to as base film U).

(4) Characteristics of base film When the characteristics of the base film thus obtained were measured, AXO was found to be.

Minimum value + 7.2xlO""(A=6.5x10"", B=0.11) Maximum value:
-8.5×10-' (A=6.IXl0-5.B=0.14), which satisfied the characteristics required for the base film used in the present invention.

(5) Formation of magnetic layer If one side of the base film was subjected to corona discharge treatment in air to increase the surface tension by 50 d, yne/an i. Next, a target consisting of 80% by weight Oo and 80% by weight Ni2O was created, and this target
A Co--Ni based magnetic thin film having a thickness of about 3000''A was formed on the corona-treated surface of the base film by RF sputtering.

At this time, the distance between the target and the film is 60 plates,
The plate voltage was 2 kV, the plate current was 150 mA, and the argon pressure was 1.2 x 10'' mmHg.

Next, this was punched into a disk having an outer diameter of the 5.25-inch mini-floppy disk standard, and seven tral holes of the same standard were punched out to create a magnetic disk (
(Let's call it disk U).

On the other hand, for comparison, a Co-Ni magnetic layer of about 3000X was formed on a polyethylene terephthalate film with a thickness of 50 μm under the same conditions as above, and this was also used as a mini- and floppy disk-sized magnetic disk (Disk V and do).

(6) Evaluation Table 1 shows the results of evaluation by inserting the prepared disk into the jacket of a commercially available mini floppy disk.

From Table 1, it can be seen that the magnetic disk of the present invention has excellent reproduction characteristics when exposed to stones with varying humidity, high temperature and high humidity, and is suitable for high track density recording.

Table 1 Example 2 Preparation of tllPps polymer In the same manner as in Example 1, μ. 6500 months is is.

N'1.25. A high degree of polymerization PPS having T691°C and Tm 285°C was obtained (referred to as p p s -V/).

(2) Melt molding 9 width 240mm, thickness 1200μ as in Example 1
An unstretched film of m was obtained (referred to as film-W).

(3) Biaxial stretching, heat treatment The longitudinal and lateral stretching ratios were determined in the same manner as in Example 1 (3).

Five types of biaxially stretched PP with different thermal expansion coefficients and heat yields at 120°C were prepared by adjusting the heat treatment temperature and relaxation rate.
An S film (about 75 μm thick) was obtained (referred to as base films W-1 to W-5).

(4) Characteristics of base film The properties of the obtained base film are shown in Table 2.

(5) Creation of floppy disks Floppy disks were created in exactly the same manner as in Example 1 (disks w-1~
w-5).

(6) Evaluation results Table 2 shows the evaluation results of the created discs.

From this result, it can be seen that even with the same (PPS biaxially stretched film) the product of the present invention can be expected to record stable curling in small packages, whereas the coefficient of thermal expansion and heat loss of the penis film are outside the range of the present invention. It can be seen that the curl is large and the recording becomes unstable.

Table 2

Claims (1)

[Scope of Claims] The main constituent layer of the base film is a biaxially oriented borylphenylene sulfide film containing 90 or more moles of repeating units of the structural formula -+Q-8→-, and at least one side of the base film is ferromagnetic. In a magnetic recording medium having a layer,
A flexible magnetic recording medium in which the characteristics of the base film satisfy the following formula in all in-plane directions. 12 x 10-7≦AxC<5x10-5 where A:
Thermal expansion coefficient at 120°C (°C-”) a: Thermal contraction rate at 120°C (%)