JPH0564594B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a base film for high-density recording media. [Prior Art] Metal thin film type magnetic recording media, in which Co, Ni, Cr, Fe, or alloys thereof are formed on a film substrate by methods such as vacuum evaporation, sputtering, and ion plating, have a high recording density. Polyester film is often used as a substrate. However, polyester films do not have sufficient heat resistance, which limits the conditions for forming metal thin films, and the use of aromatic polyimide films and aromatic polyamide films as substrates for perpendicular magnetic recording media is being considered. Regarding the surface forming method of these heat-resistant films, please refer to JP-A-60-127523 and JP-A-62-
Various proposals have been made in 116637. [Problem to be solved by the invention] However, in JP-A-60-127523, the vapor-deposited tape may have a narrow tape width.
Runnability in a VCR is not a problem, but in the manufacturing process for making wide tapes, the surface is too smooth and wrinkles easily appear when running or winding up, resulting in low production yields and the inability to increase production speed. A problem existed. In order to meet the demand for improving runnability while maintaining the surface properties of the side on which the magnetic layer is provided to be extremely smooth to withstand use in vapor-deposited tapes, it is no longer possible to have the same surface properties on both sides. This cannot be done, and we have no choice but to resort to a method of changing the surface properties of the front and back sides of the film. JP-A-62-116637 is a film with such surface properties, but one side is too rough, so when the film is wound into a roll, the opposite side (the side on which the magnetic layer is provided) becomes uneven. There was a problem in that the surface properties of the magnetic layer side were impaired due to the occurrence of a show-through phenomenon in which the magnetic layer was transferred. On the other hand, the coextrusion method used in the production of polyester films and cellulose acetate films (e.g., JP-A-162617) is a good method for changing the surface properties between the front and back sides, but the film thickness is extremely high. When making a thin film using the solution casting method, there is a problem that the two liquids may partially or completely mix during the drying of the solvent, and the surface of one side may be kept very smooth while the other side may be mixed. A thin film with a rough surface has never been obtained by solution casting. It is an object of the present invention to solve these problems and provide a film that has both surface properties and runnability. [Means for Solving the Problems] The present invention is a film made of aromatic polyamide or aromatic polyimide, and the film consists of at least two layers, one layer (A layer) having a surface _RPA (center Line depth) is 10 to 200 Å, and the other layer (B layer) has a surface R _PB (center line depth) of 10 to 200 Å.
It is within the range of formula (1), and R _aB (center line average roughness) is 20
This is a base film for high-density recording media characterized by having 30 or more protrusions with a height of ~200 Å and 50 Å or more per mm. R _PA +50≦R _PB ≦1500 (Å) ……(1) The aromatic polyamide of the present invention contains 50 mol% of repeating units represented by the general formula (-HN-Ar ₁ -NHOC-Ar ₂ -CO)- Polymers containing 70% or more are preferred, and more preferably 70% or more.
It is mol% or more. Here, Ar ₁ and Ar ₂ contain at least one aromatic ring and may be the same or different, and representative examples thereof include the following.

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[Formula] Also, some of the hydrogens on the aromatic rings are
It also includes those substituted with substituents such as halogen groups (especially chlorine), nitro groups, _C1 - _C3 alkyl groups (especially methyl groups), and _C1 - _C3 alkoxy groups. Moreover, X is -O-, _-CH2- , _-SO2- , -S-, -CO-, etc. These may be included alone or in copolymerized form. Particularly for applications such as thin magnetic tapes, from the viewpoint of mechanical properties and dimensional stability against environmental changes (temperature, humidity), it is preferable to use a material mainly consisting of para bonds, and a material with chlorine introduced into the aromatic ring is preferable. More preferred. for example, (where p and q are 0 to 3). In the present invention, the aromatic polyimide is defined as having one aromatic ring and one imide ring in the repeating unit of the polymer.
A general formula that contains one or more or Those containing repeating units represented by 50 mol % or more are preferred, and more preferably 70 mol % or more. Here, Ar ₃ and Ar ₅ contain at least one aromatic ring, and the two carbonyl groups forming the imide ring are bonded to adjacent carbon atoms on the aromatic ring. This Ar ₃ is derived from aromatic tetracarboxylic acid or its anhydride. Representative examples include the following:

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[Formula] Here, Y is -O-, -CO-, -CH ₂ -, -S
−, −SO ₂ −, etc. Furthermore, Ar ₅ is derived from tricarboxylic anhydride or its halide. Ar ₄ and Ar ₆ contain at least one aromatic ring,
Derived from aromatic diamines and aromatic diisocyanates. Further, it may contain an amide bond, a urethane bond, or the like. Representative examples of Ar ₄ and Ar ₆ include the following.

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[Formula] Here, some of the hydrogen atoms on the aromatic rings are substituted with substituents such as halogen groups, nitro groups, C ₁ to _{C 3} alkyl groups, and C ₁ to _{C 3} alkoxy groups. Including things that are. Z is -O-, -CH ₂ -, -S
−, −SO ₂ −, −CO−, etc. These may be included alone or in copolymerized form. Furthermore, the aromatic polyamide and aromatic polyimide of the present invention may be blended with lubricants, antioxidants, other additives, and other polymers to the extent that the physical properties of the film are not impaired. The film of the present invention is composed of at least two layers, layer A and layer B, and these two layers are preferably made of polymers having the same structure, but may be different. In the film of the present invention, the magnetic layer is provided on the surface of the A layer, and the surface property of this A layer (the surface is referred to as the A side) needs to have an R _PA of 10 to 200 Å. Preferably
It is 20-150 Å. Here, R _PA is called the centerline depth, and is the distance from the highest peak of the sampled portion to the centerline by extracting a reference length from the roughness curve.
If R _PA is less than 10 Å, the surface will be too smooth, the coefficient of friction will increase, and scratches will easily occur during the manufacture of magnetic recording media. On the other hand, if it is larger than 200 Å, the spacing loss between the magnetic layer and the head becomes large, and the electromagnetic conversion characteristics deteriorate. Furthermore, the surface properties of the B layer on the side on which no magnetic layer is provided (the surface is referred to as the B surface) need to have _RPB in the range of formula (1), preferably from 220 to 1000 Å.
If P _PA is less than +50, the B side will be too smooth and will not contribute to improving the running properties of the film. On the other hand, if the film is larger than 1,500 Å, the running properties are good, but if the film is wound into a roll or a film with a magnetic layer formed is stored in a roll, the transfer of unevenness due to show-through becomes large, resulting in high The smoothness of the film or magnetic layer necessary for density recording is impaired, making it unusable. Also, R _aB (center line average roughness) of this B side is 20~
200Å is required. Preferably it is 25-150 Å. If it is smaller than 20Å, running performance cannot be improved;
If it is larger than Å, the film will be scratched. Furthermore, there is one protrusion with a height of 50 Å or more on this B surface.
It is necessary that there are 30 or more pieces per mm. The number of protrusions is preferably 50 or more, and more preferably 5 or more with a height of 100 Å or more. Here, the height of the protrusion is defined as the standard length extracted from the roughness curve.
It shows the distance from the center line of the cut out part to the top of the protrusion, and the roughness curve is drawn on a chart and the number of protrusions from the roughness curve corresponding to 1 mm of the film is determined. If the height of the protrusions is less than 50 Å or if the number of protrusions with a height of 50 Å or more is less than 30, no improvement in running performance can be expected. In order to produce a film in which the surface of the B side is rougher than that of the A side according to the present invention, particles must first be present on the B side. Organic particles,
Although there are inorganic materials, inorganic materials are more preferable from the viewpoint of heat resistance. For example, SiO ₂ , TiO ₂ , ZnO, Al ₂
Examples include O ₃ , CaSO ₄ , BaSO ₄ , CaCO ₃ , carbon black, zeolite, and other fine metal powders. Particle size is 0.02~2μm, addition amount is 0.1 per polymer
~10wt% is preferred. More preferably 0.05-1μm
It is recommended to use particles with a particle size of . Moreover, two or more types may be used. Layer A does not need to contain particles, but layer B
Particles smaller than the layer may be added, or small amounts may be added, or small amounts of small particles may be added. The particle size is 0.005 to 0.3 μm, and the amount added is 0 per polymer.
~5wt% is preferred. Colloidal particles with uniform particle sizes are particularly preferred, and silica sol, alumina sol, titanium oxide sol, etc. are particularly preferred. The thickness of the film of the present invention is preferably 1 to 15 μm, more preferably 2 to 10 μm.
The thinner the film, the greater the effect of the present invention. Regarding the thickness of layer A and layer B, first layer A is _RPB.
It is preferable to make it 10 times or more of (center line depth of surface B). For example, if _PPB is 500 Å, the thickness of layer A is 0.5 μm or more. In solution casting, it is necessary to dry the solvent when forming a film, but at this time the layers mix at the interface between the two layers. The surface of layer A (A side) must be sufficiently smooth even if the two layers are mixed, so B
The minimum value of the thickness is determined by the relationship between the layers. On the other hand, B tier is not as demanding as A tier.
The thickness is preferably 0.1 μm, more preferably 0.5 μm or more. In this way, the A layer and B layer maintain the minimum thickness, and the thickness ratio is A layer/B layer = 0.1 ~
It is preferable to adjust within a range of 10. More preferably it is 0.2-5. Further, it is preferable that the center line average roughness R _aA of the A side of the film of the present invention is 2 to 20 Å. Also
The presence of 10 ³ to ^{10 8} microprotrusions/mm ² with a diameter of 0.01 μm or more is effective in improving the durability of the magnetic layer and is preferable, but the presence of microprotrusions may be omitted. Moreover, in the B side of the present invention, the ratio R _PB /R _aB of R _PB and R _aB is preferably 3 to 20, more preferably 4 to 15. The film of the present invention preferably has a Young's modulus in at least one direction of 400 Kg/mm ² or more, more preferably 800 Kg/mm ² or more. Further, the humidity expansion coefficient in at least one direction is preferably 30×10 ^-6 1/%RH or less, more preferably 20×10 ^-6 1/%RH or less. Furthermore, the coefficient of dynamic friction between the A side and the B side is preferably 1 or less, more preferably 0.8 or less. This friction coefficient is a value measured in an atmosphere of 25°C and 55% RH. Next, the manufacturing method of the present invention will be explained, but it is not limited thereto. First, aromatic polyamides are made from acid chloride and diamine, such as N-methylpyrrolidone (NMP), dimethylacetamide (DMAC),
It is synthesized by solution polymerization in an aprotic organic polar solvent such as dimethylformamide (DMF) or interfacial polymerization using an aqueous medium. Hydrogen chloride is produced as a by-product when acid chloride and diamine are used as monomers in the polymer solution, so in order to neutralize this, inorganic neutralizing agents such as calcium hydroxide, calcium carbonate, lithium carbonate, or ethylene oxide are used. Add an organic neutralizing agent such as , ammonia or triethylamine. Further, the reaction between isocyanate and carboxylic acid is carried out in an aprotic organic polar solvent in the presence of a catalyst. These polymer solutions may be used directly as a film-forming stock solution to form a film, or the polymer may be isolated once and then redissolved in the above-mentioned solvent to prepare a film-forming stock solution. Inorganic salts such as calcium chloride, magnesium chloride, lithium chloride, etc. may be added to the membrane forming stock solution as a solubilizing agent. The polymer concentration in the membrane forming stock solution is preferably about 2 to 40 wt%. On the other hand, a solution of aromatic polyimide or polyamic acid can be obtained as follows. That is, polyamic acid is prepared by reacting a tetracarboxylic dianhydride with an aromatic diamine in an aprotic organic polar solvent such as N-methylpyrrolidone (NMP), dimethylacetamide (DMAC), or dimethylformamide (DMF). It can be prepared. Also,
Aromatic polyimide can be prepared by heating a solution containing the polyamic acid described above or adding an imidizing agent such as pyridine to obtain a polyimide powder, which is then redissolved in a solvent. The polymer concentration in the membrane forming stock solution is preferably about 5 to 40 wt%. The film-forming stock solution prepared as described above is formed into a film by a so-called solution film-forming method. Solution film forming methods include a wet-dry method, a dry method, a wet method, etc., and the wet-dry method and dry method are preferred in order to obtain a film with good surface properties. When forming a film by a wet method, the stock solution is directly extruded from a die into a film-forming bath, or it is once extruded onto a support such as a drum and then introduced into a wet bath together with the support. This bath generally consists of an aqueous medium, and may contain an organic solvent, an inorganic salt, etc. in addition to water. Salts, organic solvents, imidizing agents, etc. contained in the film are extracted by passing it through a wet bath, but the time it takes to pass through the entire wet bath is 10 seconds or more, depending on the thickness of the film.
It is 30 minutes. Furthermore, stretching is performed in the longitudinal direction of the film. Next, drying, lateral stretching, and heat treatment are performed, and these treatments are generally performed at 100 to 500°C for a total time of 1 second to 30 minutes. When forming a film by a dry-wet method, the stock solution is extruded from a die onto a support such as a drum or an endless belt to form a thin film, and then the solvent is scattered from the thin film layer and the film is dried until it has self-retaining properties. Drying conditions range from room temperature to 300°C for up to 60 minutes. After the dry process, the film is peeled off from the support and introduced into the wet process, where it is subjected to desalting, solvent removal, etc. in the same way as in the wet process described above, and then stretched, dried, and
Heat treatment is performed to form a film. If a drying process is used, a drum,
Alternatively, a self-retaining film dried on an endless belt or the like is peeled off from these supports and stretched in the longitudinal direction of the film. Furthermore, drying, stretching, and heat treatment are performed to remove residual solvent, but these treatments are carried out at temperatures of 100 to 500℃.
It takes 1 second to 30 minutes. The film formed as described above is stretched during the film forming process, and the stretching ratio is the area ratio.
0.8 to 5.0 (area magnification is defined as the value obtained by dividing the area of the film after stretching by the area of the film before stretching.1
The following means relaxation. ), more preferably 1.1 to 3.0. In order to form the laminated film of the present invention, two types of film forming stock solution, a film forming stock solution corresponding to the A layer side and a film forming stock solution corresponding to the B layer side, are prepared using a known method such as JP-A-56
It can be formed by laminating in a confluence pipe like 162617, or by laminating in a mouthpiece. In casting, it is preferable to place the B layer on the support surface side, since this allows the surface properties of the A side to be kept smooth. Alternatively, a self-retentive film may be formed using one of the membrane-forming stock solutions, and then the other film-forming stock solution may be cast onto the film to remove the solvent, thereby forming a laminated film. In particular, when laminating in a confluence pipe or a mouthpiece, it is preferable to adjust the viscosity of the stock solution to 100 to 10,000 poise. If it is smaller than this range, the two liquids will easily mix before the stock liquid comes out of the nozzle, and in the case of a thin film, the A side will become rough even with very slight mixing. Conversely, if it is larger than this range, 2
Mixing of liquids becomes less likely to occur, but melt fracture occurs and the film surface tends to become rough, which is not preferable. Further, it is preferable that the viscosity of the two liquids be the same, but there may be some viscosity difference, and there is no problem as long as the viscosity of the high viscosity side is within 100% of the low viscosity side. Furthermore, when a dry method or a wet-dry method is employed, the two liquids may mix during the drying process. The viscosity of the stock solution cast onto the support decreases once when it is heated, and then increases again as the solvent evaporates, but when the viscosity drops below 10 poise, the two solutions become easier to mix. It is necessary to adjust the drying conditions sufficiently so that the viscosity does not fall below 10 poise. For example, a method can be adopted in which the drying temperature is increased in at least two stages. The film of the present invention is produced as described above, but if a glow discharge treatment or a corona discharge treatment is performed as a pretreatment before attaching the magnetic layer etc., the adhesion and durability of the magnetic layer will be improved. preferable. [Effects of the Invention] In the film of the present invention, the characteristics required for each of the front and back surfaces are separated. Therefore, although the surface properties of the A side required for base films for high-density magnetic recording such as vapor-deposited tapes are kept very smooth, if the B side is within the range of the surface properties of the present invention, There is no show-through phenomenon, and it has good runnability and winding performance even at wide widths for manufacturing vapor-deposited tapes, etc., and can improve productivity and quality. Furthermore, since the film of the present invention has good heat resistance, a magnetic layer can be provided at high temperatures, and the magnetic properties can be improved. The magnetic recording method does not matter whether horizontal magnetization or perpendicular magnetization is used. The film of the present invention can also be used in the continuous production of optical recording or magneto-optical recording media in the same way as in the case of magnetic recording. [Method for Measuring Characteristics and Evaluating Effects] Methods for measuring characteristic values and evaluating effects of the present invention are as follows. (1) Ra (center line average roughness), Rp (center line depth), number of protrusions Using a thin film step measuring instrument (ET-10) manufactured by Kosaka Laboratory, stylus tip radius 0.5 μm, stylus load 5 mg , cutoff value of 0.008 mm, measurement length of 0.5 mm, and the average values were used to express Ra and Rp. The number of protrusions was determined by drawing a chart under the above conditions, counting the number of protrusions with a distance of 50 Å or more from the center line of the roughness curve to the top of the protrusion, and expressing the number of protrusions per mm of film. The definitions of Ra and Rp can be found, for example, in Jiro Nara's "Measurement and Evaluation Method of Surface Roughness" (General Technology Center,
1983). (2) Runnability The film was slit to a width of 30 cm and loaded into a vapor deposition machine, and the A side was glow-treated in an Ar atmosphere of 10 ^-2 Torr, then vacuumed to 10 ^-5 Torr and heated at 100°C.
A Co-Ni alloy (80% Co, 20% Ni) was deposited on the A side by electron beam evaporation along the heated drum.
A thickness of 1000 Å was deposited. The runnability was evaluated by observing the appearance of wrinkles on the drum and guide roll during these treatments and the winding appearance of the film roll after the treatments. (3) Electromagnetic conversion characteristics The vapor-deposited film is slit into 1/2 inch width,
It was incorporated into a VTR cassette and used as a VTR tape. This tape was 100% tested using Shibasoku's TV test waveform generator (TG7/U706) using a home VTR.
The chroma signal was recorded, and the chroma S/N was measured from the reproduced signal using a Shibasoku color video noise measuring device (925D/1). Evaluation was made using a commercially available tape as a standard. "Examples" The present invention will be explained below based on Examples. However, the present invention is not limited to these examples. Example 1 70 mol% of 2-chloroparaphenylenediamine and 30 mol% of 4,4'-diaminodiphenylsulfone were polymerized in NMP as amine components and 100 mol% of terephthalic acid chloride as acid component to form lithium hydroxide. to obtain a polymer solution. Intrinsic viscosity is 2.5
It is. This was divided into two parts, and 0.08 μm spherical silica, which had been previously dispersed in NMP, was added to the polymer solution forming the A layer at 0.1 wt% per polymer.
In addition, the average particle size dispersed in NMP
Add 0.2μm silica to the polymer solution that will become layer B at 1wt% per polymer, and then add both polymer solutions.
The solution was adjusted to 1000 poise at 30°C and used as a film forming stock solution. Add these stock solutions to the final film in the nozzle and form the A layer.
5μm, layer B is 3μm, and then casted on a metal belt at 30℃, dried at 80℃ for 2 minutes, and then gradually raised to 120℃ and 150℃ until the total Ten
After drying for several minutes, a self-retentive gel film was obtained. Note that the film was formed with layer B facing the belt side. This gel film was continuously peeled off from the belt and introduced into a water bath, where it was desolventized and desalted, and then water was dried and heat treated in a tenter to obtain a final film of 8 μm. During this time, the film was stretched 1.3 times in the longitudinal direction in a water bath and 1.3 times in the width direction in a tenter, and the heat treatment conditions were 300° C. for 5 minutes. The properties of the obtained film are as shown in Table 1.
The surface was very smooth, and the B side was rougher than the A side. This film also has excellent mechanical properties with a Young's modulus of 1000 Kg/mm ² in both the MD and TD directions, a strength of 53 Kg/mm ² and an elongation of 55%, and a humidity expansion coefficient of 6×10 ^-6 It was as small as 1/%RH. Next, Co-Ni was applied to the A side of this film to a thickness of 1000Å.
Although the film was vapor-deposited, there were no wrinkles on the drum or guide roll during vapor-deposition, and the wound appearance of the film roll was very good. This was then slit into a magnetic tape, and its electromagnetic conversion characteristics were evaluated, and it was found to be very excellent. Example 2 The same polymer as in Example 1 was used, and the A layer side was
0.5wt% of 0.08μm silica per polymer, 2wt% of 0.2μm silica per polymer on the B layer side,
In the final film, both layers A and B were laminated to have a thickness of 4 μm, and the film was formed in the same manner as in Example 1 to obtain a film of 8 μm. A magnetic tape was prepared by depositing a magnetic layer on this film and evaluated, and the running properties during the manufacturing process and the electromagnetic conversion characteristics of the tape were very good. Example 3 2-chloroparaphenylenediamine 80 mol%
was polymerized in NMP with 20 mol% of 4,4'-diaminodiphenyl ether as the amine component and 100 mol% of 2-chloroterephthalic acid chloride as the acid component, and neutralized with calcium carbonate to obtain a polymer solution. . This was divided into two parts, and 0.05 μm silica was added to the A layer side at 0.2 wt% per polymer, and 0.2 μm silica was added at 2.5 wt% per polymer to the B layer side. were laminated in a confluence tube to a thickness of 1 μm and cast onto a metal belt at 30°C. The viscosity of the polymer solution is 3000 poise at 30°C. This was dried in a stepwise manner as in Example 1 until it had self-retention properties, and then passed through a water tank and a tenter to obtain a 4 μm film. The stretching ratio is the same as in Example 1. The obtained film had a Young's modulus of 1200 Kg/mm ² in both the MD and TD directions, a strength of 58 Kg/mm ² and an elongation of 50%, and the surface properties are as shown in Table 1. This film was then vapor-deposited, and although it was very thin, there were no problems with runnability, and the film had a good winding appearance. Furthermore, this was evaluated as a magnetic tape,
The electromagnetic conversion characteristics were very good. Example 4 4,4'-diaminodiphenyl ether 50 mol%
and 50 mol% of paraphenylenediamine as the amine component and 100 mol% of pyromellitic anhydride.
A polyamic acid solution was obtained by polymerization in DMAC.
This was divided into two parts, and 0.05 μm silica was added at 0.05 wt% per polymer on the A layer side, and the average particle size was added on the B layer side.
Add 0.8μm silica powder at 1wt% per polymer,
The final film was laminated on a metal belt at 30°C using two ferrules so that both layers A and B had a thickness of 5 μm. The viscosity of each polymer solution is 500 poise at 30°C. This was dried at elevated temperatures of 60°C, 100°C, and 130°C until it had self-retention properties, and then peeled off from the belt and heat-treated in a tenter at 420°C. The stretching ratio is 1.0 times in both the MD and TD directions, the thickness is 10 μm, the Young's modulus is 420 Kg/mm ² , and the surface quality is 1st.
As shown in the table, the A side was very smooth. Next, this film was subjected to vapor deposition, and although the A side was very smooth, it had good runnability and good winding appearance. Furthermore, when it was evaluated as a magnetic tape, it showed good electromagnetic conversion characteristics. Comparative Examples 1 to 5 The same polymer as in Example 1 was used, and the particles added to the A layer side were 0.08 μm silica as in Example 1.
Films were formed in the same manner as in Example 1, with various silica particles added to the B layer side as shown in Table 2, to obtain 8 μm films. The thickness of layer A is 5 μm, and the thickness of layer B is
3μm, and the surface properties, running properties,
The electromagnetic conversion characteristics are shown in Table 1. In Comparative Examples 1 to 3, even when attempting to deposit a film in a width of 30 cm, wrinkles appeared on the deposition drum and heat loss of the film was observed. Furthermore, wrinkles appeared on the guide roll, making it difficult to wind the film smoothly, and the winding appearance of the film roll was very poor. Therefore, when we made the film 5 cm wide and deposited it in the same way, we were able to deposit it without any problems. A magnetic tape was prepared with this, and its electromagnetic conversion characteristics were evaluated and found to be good. As can be seen from this, in Comparative Examples 1 to 3, the surface of the B side was too smooth, and the running performance over a wide width was extremely poor. In Comparative Example 4, running properties were good at the start of vapor deposition, but as time went on, scraped powder from the film began to adhere to the drum and guide roll, and the film slipped on the drum, causing wrinkles in the MD direction. It was recognized that film had lost its enthusiasm. In addition, since the film easily slipped on the guide roll, the film began to meander and could not be wound smoothly. Furthermore, the electromagnetic conversion characteristics were also poor due to the adhesion of scraping powder on the film. Comparative Example 5 had good runnability due to its large R _PB , but show-through was large, the surface of the produced magnetic tape was undulated in an orange-skin shape, and the electromagnetic conversion characteristics were very poor. It was hot. Comparative Example 6 A film of 8 μm was produced in the same manner as in Example 1, except that the temperature of the belt during casting and the drying temperature were 190° C. As shown in Table 1, almost the same surface properties were obtained on both sides A and B, and the electromagnetic conversion properties were poor. When the viscosity behavior of the stock solution cast onto the belt in the early stage of drying was investigated, the viscosity had decreased to 8 poise, and it was found that the A and B layers were mixed by convection and did not form two layers.

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Claims (1)

[Scope of Claims] 1. A film made of aromatic polyamide or aromatic polyimide, which consists of at least two layers, one layer (layer A) on the surface.
R _PA (center line depth) is 10 to 200 Å, and the surface R _PB (center line depth) of the other layer (B layer) is within the range of formula (1) below, and R _aB (center line average roughness) is 10 to 200 Å. 20~200
A base film for high-density recording media, characterized by having 30 or more protrusions with a height of 50 Å or more per mm. R _PA +50≦R _PB ≦1500 (Å) ……(1)