Magnetic recording medium

Abstract: A magnetic recording medium having a magnetic layer on at least one side of a base film which is a stretched film of styrene polymer having a high degree of syndiotactic configuration or a composition thereof or a laminate film containing the stretched film, wherein linear expansion coefficient is not more than 5 x 10⁻⁵/°C and static friction coefficient is not less than 0.3 and not more than 1.0 is disclosed. The magnetic recording medium has good sliding properties, smoothness, heat resistance and moisture resistance, is excellent in dimensional stability to temperature and moisture, and has good coersive force, thus can be highly densified.

Description: Field of the invention The present invention relates to magnetic recording medium, magnetic tape, and magnetic disk, more specifically, to magnetic recording medium which is excellent in physical properties such as sliding properties, smoothness, heat resistance, moisture resistance, dimensional stability, electrical insulating properties and further capable of densification, and magnetic tape and magnetic disk, and magnetic card which have these properties and high whiteness degree. Background of the Invention Hitherto, polyethylene terephthalate (PET) film substrate coated with magnetic layer has been widely used as a magnetic recording medium. However, PET film is inferior in water resistance, and sometimes it can not be used because it may be hydrolyzed at high temperature under high humidity. Glass transition temperature of PET is 60 to 80°C. Elastic modulus may be sharply decreased at a temperature higher than the above range, and elongation or the like may readily occur. Accordingly, reliability is not always sufficient. Magnetic tape using such PET film as a base film has problem, that is, bad reproduction due to elongation of the base film at high temperature, for example, in a car. As a base film for magnetic disk such as floppy disk demand for which has been increasing, the above PET film has been used. Recently, as miniaturization and densification of floppy disk, further improvement of physical properties such as heat resistance, moisture resistance, dimensional stability and the like has been required. However, a magnetic disk using the above PET film as a base material has problems in dimensional change with change of moisture, anisotropism of thermal expansion coefficient in face, eternal shrinkage when left at high temperature under high moisture over a long period of time. Polyphenylene sulfide (PPS) film used as a base film for this magnetic disk may overcome the above problems, but it is too expensive for practical use. Satisfactory base materials for magnetic disk have not been proposed, yet. On the other hand, magnetic recording medium has been further densified, and demand for its reliability has been increasing. One of the means for densification is a method wherein a magnetic layer is laminated on a base film by a vapor deposition method. This method, however, has problems, for example, insufficient heat resistance, separation of oligomer when PET film is used as a base film. As described above, the magnetic recording medium obtained from the conventional base film have various problems for practical application, for example, insufficient sliding properties, smoothness, heat resistance, moisture resistance, dimensional stability and the like. The document EP 0 337 316 A1, which is effective under Article 54(3) EPC, discloses a resin laminate which may be used for magnetic tapes comprising a layer of styrene-based polymer having mainly syndiotactic configuration and a thermoplastic resin layer, a metallized laminate comprising a layer of the styrene-based polymer described above and a metal layer, and a process for producing these laminates are disclosed. The thermoplastic resin layer may be biaxially stretched. A process to produce such stretched film is specified in example 12, however, according to applicant's experimental repetition of this example, the stretched film has a linear expansion coefficient of 5.6 x 10 -5 /°C. The document EP 0 376 021 A2, which is also effective under Article 54(3) EPC discloses a styrene-based resin composite material in which a layer composed mainly of a curable phosphazene compound is formed on the surface of a molding material of a styrene-based polymer having a mainly syndiotactic configuration. Disclosure of the Invention The present inventors have studied to develop a magnetic recording medium which overcomes the above problems. Previously, the group of the present inventors have succeeded to develop a styrene polymer having a high degree of syndiotactic configuration. This polymer, which is superior to PET in heat resistance, water resistance, dimensional stability and the like, is a polymer with less oligomer and expected to be widely applied. The present inventors have studied intensively to develop a highly-densified magnetic recording medium with high reliability even under severe conditions such as high temperature, high humidity using this styrene polymer having a high degree of syndiotactic configuration. The object of the present invention is to provide a magnetic recording medium excellent in physical properties such as heat resistance, moisture resistance, smoothness, dimensional stability, and high density magnetic recording medium which has the above properties and is further excellent in, for example, coercive force. Another object of the present invention is to provide heat resistant magnetic tape excellent in heat resistance, moisture resistance, smoothness, dimensional stability, mechanical strength, and a magnetic disk which is especially excellent in heat resistance, moisture resistance, smoothness, dimensional stability and the like. As a result of study from such point of view, it has been found that stretched film of the above styrene polymer or a composition thereof, or laminated film containing said stretched film, which has a magnetic layer in a base film, the physical properties of the film, for example, linear expansion coefficient, static friction coefficient, surface roughness, elastic modulus, heat deformation temperature being in a certain range, may provide magnetic recording medium, magnetic tape, magnetic disk which meet the above object. The present invention has been attained based on such findings, and is defined in claims 1, 3 and 4. Best Mode to Conduct the Invention In the present invention, as a base film of a magnetic recording medium, a stretched film which comprises a monolayer film comprising the above stretched film of the styrene polymer having a high degree of syndiotactic configuration or a laminate film containing said stretched film is used. This base film has linear expansion coefficient of not more than 5 x 10 -5 /°C, preferably not more than 4 x 10 -5 /°C, and static friction coefficient, µs, of 0.3 to 1.0, preferably 0.3 to 0.9. This static friction coefficient, µs, determines the surface condition of the base film. Especially, static friction coefficient µs of the opposite side to the side, on which magnetic layer is formed, of the base film is desirably in the above range. Surface roughness Ra of the surface of this base film is not particularly limited, but surface roughness Ra of at least one side thereof is preferably 0.001 to 0.05 µm, most preferably, 0.001 to 0.04 µm. For production of high densified magnetic recording medium, surface roughness Ra is preferably 0.001 to 0.03, particularly, 0.005 to 0.02 µm. Especially, surface roughness Ra of the base film on the magnetic layer-formed side is desirably in the above range. When a magnetic layer is, for example, vapor deposited for densification of the aforementioned base film, the film having heat deformation temperature of not less than 230°C, particularly not less than 235°C is the most preferable. Moisture expansion coefficient of the above base film is not particularly limited, but preferably not more than 5 x 10 -5 /%RH. As the base film for the magnetic tape of the present invention, a monolayer film consisting of the aforementioned stretched film or a laminate film containing said stretched film is used. The physical properties of this base film may be as follows: elastic modulus at room temperature (about 0°C to 40°C) in MD and TD is not less than 30,000 kg/cm 2 , preferably not less than 33,000 kg/cm 2 , the ratio of elastic modulus in MD to elastic modulus in TD (MD/TD) is not less than 0.8, preferably, 0.8 to 6, and elastic modulus in MD at 90°C is not less than 10,000 kg/cm 2 , preferably, not less than 15,000 kg/cm 2 , linear expansion coefficient at 0 to 90°C is not more than 5 x 10 -5 /°C, preferably, not more than 4 x 10 -5 /°C, and surface roughness Ra is 0.001 to 0.03, preferably, 0.003 to 0.03. When elastic modulus in MD at room temperature is less than 30,000 kg/cm 2 , the tape may be stretched with a little load on running and good reproduction may not be obtained. When elastic modulus in TD at room temperature is less than 30,000 kg/cm 2 , the tape may readily folded, sometimes resulting in bad reproduction. When the ratio of elastic modulus in MD to that in TD, i.e., MD/TD is less than 0.8, the tape may be readily torn off. When elastic modulus in MD at 90°C is less than 10,000 kg/cm 2 , the resulting tape can not be used at the high temperature, i.e. about 90°C. When linear expansion coefficient is more than 5 x 10 -5 /°C, the resulting tape can not be used at the high temperature, i.e. about 90°C. When surface roughness Ra is outside of the range of 0.001 to 0.3, good running conditions as magnetic tape may not be obtained. A monolayer film comprising the aforementioned stretched film or a laminate film containing said stretched film may be used as a base film of the magnetic disk of the present invention. The physical properties of this base film are as follows: Linear expansion coefficient in MD (α MD ) and that in TD (α TD ) are not more than 5 x 10 -5 /°C, preferably, not more than 4 x 10 -5 /°C, and the ratio thereof (α MD /α TD ) is 0.3 to 3, preferably, 0.5 to 2, and surface roughness is 0.001 to 0.03 µm, preferably, 0.005 to 0.03 µm, and thickness is 20 to 200 µm, preferably, 50 to 150 µm. The film with α MD or α TD over 5 x 10 -5 /°C is not practical because it is deficient in dimensional stability. When the film with α MD /α TD below 0.3 or over 3 is used as a magnetic disk, the disk may have isotropic dimensional stability in the surface upon environmental change, and good reproduction can not be obtained. Surface roughness Ra over 0.03 µm produces problems such as abrasion of the magnetic head. Ra below 0.001 µm produces problems in sliding properties, and tape with good running condition can not be readily obtained. Too thin base film provides magnetic disk deficient in resilience and self-supporting property which may be necessary for a magnetic disk, and the product can not be practically used, on the other hand too thick film is deficient in flexibility. For production of a base film of such magnetic recording medium, magnetic tape and magnetic disk, various methods can be employed. For example, three methods shown below can be employed. (1) A method wherein a composition comprising a styrene polymer having a high degree of syndiotactic configuration compounded with an inorganic filler, particularly, 0.001 to 1% by weight of an inorganic filler with an average particle diameter of 0.01 to 3 µm, is heat-melted, extruded, cooled and solidified, heated, stretched and heat-treated to form a stretched film, thereby providing a readily slidable monolayer film comprising this stretched film alone and whose both sides are smooth and roughened. (2) A method wherein the composition used in the above (1) and a styrene polymer having a high degree of syndiotactic configuration, particularly, a styrene polymer of high purity with residual aluminum content of not more than 3,000 ppm, residual titanium content of not more than 10 ppm and residual styrene monomer content of not more than 7,000 ppm or a composition containing this styrene polymer as a main ingredient (containing no inorganic filler) are heat melted, co-extruded, cooled and solidified, heated, stretched to form a laminate of two or more stretched films. Accordingly, a readily slidable multilayer (laminate) film whose one side is extraordinary smooth and the other face is roughened may be obtained. (3) A method wherein a styrene polymer having a high degree of syndiotactic configuration, particularly, a styrene polymer of high purity with residual aluminum content of not more than 3,000 ppm, residual titanium content of not more than 10 ppm and residual styrene monomer content of not more than 7,000 ppm, or a composition containing this styrene polymer as a main ingredient (containing no inorganic filler) is heat-melted, extruded, cooled and solidified, heated, stretched and heat-treated, while a layer comprising resin composition containing resin which can roughen the surface or an extremely fine inorganic filler is formed onto at least one face thereof by lamination, coating, vapor deposition or the like to give a readily slidable multilayer film wherein one face is extremely smooth and the other face is roughened. In this case, the layer comprising the above resin composition which may be stretched as needed can be used. As the resin used for lamination, a styrene polymer having a high degree of syndiotactic configuration or various blend resins may be used. Among them, resins with high melting point or softening point are preferred. As for type, amount and particle diameter of the inorganic filler contained in the other resins, those described below is preferably used. The method for lamination is not particularly limited, but, in addition to the co-extrusion method, a process for production of stretched film involving a method for dissolving and coating the other resin and resin composition thereof provides good productivity. Particularly in tape, the above layer having a rough surface is used as a back coat layer, and a magnetic layer is mainly formed on the opposite side, i.e., extremely smooth surface. Thus obtained stretched film is 2 to 500 µm thick with linear expansion coefficient of not more than 5 x 10 -5 /°C and static friction coefficient µs of 0.3 to 1.0. The surface roughness Ra of the both sides according to the method (1) is 0.005 to 0.05 µm, and according to the methods (2) and (3) the layer have both readily slidable rough surface and smooth surface, and surface roughness of said surfaces is 0.005 to 0.05 µm and 0.001 to 0.04 µm, respectively. Selecting conditions for stretching and additives and the like, a film with surface roughness of 0.001 to 0.03 µm, elastic modulus at room temperature in MD and TD of at least 30,000 kg/cm 2 , the ratio (MD/TD) of at least 0.8, elastic modulus at 90°C in MD of at least 10,000 kg/cm 2 , linear expansion coefficient in MD (α MD ) and in TD (α TD ) of not more than 5 x 10 -5 /°C, and the ratio α MD /α TD of 0.3 to 3 may be readily obtained. Generally, heat deformation temperature of the resulting stretched film is at least 230°C, and more preferably, moisture expansion coefficient is not more than 5 x 10 -5 /%RH. As for the thickness of the film, films of 2 to 20 µm thick, 20 to 150 µm thick and 100 to 500 µm thick are suitably used as a magnetic tape, a magnetic disk and a magnetic card, respectively. Magnetic recording medium may be prepared using these films. Particularly in the production of magnetic disk, film whose surface adhesion is improved by corona treatment or the like may be used as needed. Further, an adhesive layer may be provided on the side on which a magnetic layer is to be provided. Here, a styrene polymer having a high degree of syndiotactic configuration used as a raw material for the above film means a styrene polymer wherein stereochemical structure is a high degree of syndiotactic configuration, that is, the stereostructure in which phenyl groups or substituted phenyl groups as side chains are located alternately in opposite directions relative to the main chain consisting of carbon-carbon bonds. Tacticity is quantitatively determined by the nuclear magnetic resonance method ( 13 C-NMR method) using carbon isotope. The tacticity measured by the 13 C-NMR method can be indicated in terms of proportions of structural units continuously connected to each other, i.e., a diad in which two structural units are connected to each other, a triad in which three structural units are connected to each other and a pentad in which five structural units are connected to each other. The styrene polymer having a high degree of syndiotactic configuration in the present invention means styrene polymer, poly(alkylstyrene), poly(halogenated styrene), poly(alkoxystyrene), poly(vinyl benzoate), hydrogenated polymers thereof and a mixture thereof, or copolymers containing these structural units each having such syndiotacticity that the proportion of racemic diad is at least 75%, preferably at least 85%, or proportions of racemic pentad is at least 30% and preferably at least 50%. The poly(alkylstyrene) includes poly(methylstyrene), poly(ethylstyrene), poly(propylstyrene), poly(butylstyrene), poly(phenylstyrene), poly(vinylnaphthalene), poly(vinylstyrene), poly(acenaphthylene); and the poly(halogenated styrene) includes poly(chlorostyrene), poly(bromostyrene) and poly(fluorostyrene). The poly(alkoxystyrene) includes poly(methoxystyrene), poly(ethoxystyrene). Of these, a particularly preferred styrene polymer includes polystyrene, poly(p-methylstyrene), poly(m-methylstyrene), poly(p-tert-butylstyrene), poly(p-chlorostyrene), poly(m-chlorostyrene), poly(p-fluorostyrene) and further a copolymer of styrene and p-methylstyrene (see Japanese Patent Application Laid-Open No. 187708/1987). Comonomer of the styrene copolymer includes, in addition to the above-described monomer of styrene polymer, olefin monomer such as ethylene, propylene, butene, hexene, octene; diene monomer such as butadiene, isoprene; cyclic diene monomer or polar vinyl monomer such as methyl methacrylate, maleic anhydride, acrylonitrile. Molecular weight of the styrene polymer is not particularly limited, but the styrene polymers having weight average molecular weight of 10,000 to 3,000,000, especially, 50,000 to 1,500,000 are most suitable. When weight average molecular weight is less than 10,000, the product may not be sufficiently stretched. Further, the range of molecular-weight distribution is not limited and various ranges can be used. The value, weight average molecular weight (Mw)/number average molecular weight (Mn) is preferably 1.5 to 8. The styrene polymer having a high degree of syndiotactic configuration is much superior in heat resistance to the conventional styrene polymer having an atactic configuration. The styrene polymer having a high degree of syndiotactic configuration which constitutes the aforementioned stretched film (especially, a film having an extremely smooth surface) is as described above. Especially, the styrene polymer, wherein the residual aluminum content in the styrene polymer is not more than 3,000 ppm, the residual titanium content is not more than 10 ppm and the residual styrene monomer content is not more than 7,000 ppm, is preferred. Particularly, the styrene polymer wherein the residual aluminum content is not more than 1,000 ppm, the residual titanium content is not more than 7 ppm and the residual styrene monomer content is not more than 5,000 ppm is most preferable. The methods for production of such styrene polymer of high purity include various ones as shown below. Firstly, for control of the residual aluminum content and the residual titanium content within the above range, (1) a method in which a highly active catalyst is used to produce a styrene polymer (see, Japanese Patent Application Laid-Open No. 294705/1989) or (2) a method which comprises deashing and washing, that is, a method wherein a styrene monomer is polymerized using a conventional organometallic compound of group IVA described in Japanese Patent Application laid-Open No. 187708/1987 and the like and alkylaluminoxane such as methylaluminoxane as the catalyst components, then the resulting styrene polymer having a syndiotactic configuration is deashed with a solution of acid or alkali in a suitable solvent, and washed with a suitable solvent. As mentioned above, a styrene polymer having a syndiotactic configuration with less residual aluminum and residual titanium content can be obtained by the method (1) or (2). Further, the product is treated by the following method (3) or (4) to control the residual styrene monomer content below 7,000 ppm. (3) A method wherein the above styrene polymer is dried under reduced pressure. For drying under reduced pressure, it is efficient to set the drying temperature at the glass transition temperature of the polymer or higher. (4) A method wherein the above styrene polymer is degassed by an extruder. The above styrene polymer or the styrene polymer dried under reduced pressure by the method (3) is degassed by an extruder and simultaneously formed into a material for molding (pellet). An extruder used in this step is preferably equipped with a vent. Either a uniaxial or biaxial extruder may be used. Such treatment provides a styrene polymer of high purity which contains less residual aluminum, residual titanium and residual styrene monomer and has a syndiotactic configuration. The other resin includes various kinds, for example, styrene polymer having an atactic configuration, styrene polymer having an isotactic configuration, polyphenylene ether and the like. Such resins may be readily compatibilized with the above styrene polymer having a syndiotactic configuration and effective to control crystallization when pre-molded product for stretching is prepared, thereby providing a film with enhanced stretching properties and excellent mechanical properties, whose stretching conditions may be readily controlled. Among them, when styrene polymer having an atactic and/or isotactic configuration is compounded, it is preferably composed of the same monomers as those of the styrene polymer having a syndiotactic configuration. The content of the compatible resin component is 70 to 1% by weight, preferably, 50 to 2% by weight. When the content of the compatible resin component exceeds 70% by weight, heat resistance, which is an advantage of the styrene polymer having a syndiotactic configuration, may be undesirably spoiled. The other non-compatible resins which can be added to the polymer of the present invention include a polyolefin such as polyethylene, polypropylene, polybutene, polypentene; a polyester such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate; a polyamide such as nylon-6, nylon-6,6; a polythioether such as polyphenylene sulfide, a polycarbonate, a polyarylate, a polysulfone, a polyether ether ketone, a polyethersulfone, a polyimide, a halogenated vinyl polymer such as Teflon, an acrylic polymer such as polymethyl methacrylate, a polyvinyl alcohol, and all but the aforementioned compatible resins. There are also cross linked resins containing the aforementioned compatible resins. When the styrene polymer of the present invention having a syndiotactic configuration contains a small amount of the resin, such resin, which is incompatible with the above styrene polymer, can be dispersed like islands in the styrene polymer having a syndiotactic configuration. Accordingly, it is effective to provide proper gloss and to improve sliding property of the surface after stretching. The content of these non-compatible resin components is 50 to 2% by weight for the purpose of providing gloss, and 0.001 to 5% by weight for the purpose of controlling the surface properties. When the temperature at which the product is used is high, non-compatible resin with considerable heat resistance is preferably used. Base film of the magnetic recording medium, magnetic tape, magnetic disk of the present invention can be formed according to the aforementioned methods. The operations from heat melting to heat treatment (annealing) will be explained in detail. Firstly, the above material for molding is usually extruded (or coextruded) to give a pre-molded product for stretching (film, sheet or tube). In this molding, the aforementioned heat-melted material for molding is generally molding into a desired form by an extruder. Alternatively, the material for molding may be molded without heat melting while it is softened. An extruder used in this case may be either a uniaxial extruder or a biaxial extruder, with or without vent. A uniaxial tandem type is preferred. By using an extruder with a suitable mesh, impurities and contaminants can be removed. Preferably, for production of a stretched film having a smooth surface, the mesh is preferably at least 100 mesh, most preferably, at least 400 mesh. Considering pressure resistance and strength of the mesh itself, mesh with gauge lower than the above may be placed before or behind thereof. As for the shape of the mesh, for example, plane or cylinder mesh may be properly selected and used. The extrusion conditions are not particularly limited and properly selected depending on the various circumstances. Preferably, the temperature is selected in the range from melting point to the temperature 50°C higher than decomposition temperature of the material for molding, and shear stress is not more than 5 x 10 6 dyne/cm 2 . The die used is a T-die, a ring die or the like. After the above extrusion, the resulting pre-molded product is cooled and solidified. As a refrigerant used in this step, for example, gas, liquid, metal roller and the like may be used. When a metal roller is used, a method using air knife, air chamber, touch roll, electrostatic application and the like is effective to prevent uneven thickness and surge. The temperature of cool solidification is generally 0°C to 30°C higher than glass transition temperature of the premolded product for stretching, preferably from 20°C to glass transition temperature. The cooling rate is properly selected within the range from 200 to 3 °C/sec. In the present invention, the cooled and solidified pre-molded product is preferably uni- or bi-axially stretched. For biaxial stretching, transverse direction (TD) stretching and machine direction (MD) stretching may be simultaneously conducted, or successively conducted in suitable order. Alternatively, stretching may be conducted in one step, or in multiple steps. Methods for stretching include various methods such as a method using a tenter, a method wherein the product is stretched between rollers, a method by bubbling using a pressure of a gas, a method by rolling and the like. These methods may be applied singly or in combination. Especially, a method comprising MD stretching between rollers and subsequent TD stretching by a tenter, optionally followed by re-stretching, is preferable to reduce surface roughness, Ra. In the successive stretching, the absolute value of birefringence |Δn| of MD stretching during the primary step should be 3 x 10 3 to 70 x 10 -3 . Subsequently, TD stretching, or in some case re-stretching is conducted, and after heat treatment, the absolute value of birefringence |Δn| of the resulting film is kept below 4 x 10 -3 to form a film with well-balanced strength and linear expansion coefficient. The temperature for stretching is generally set between glass transition temperature of the pre-molded product and melting point of the pre-molded product. The stretching rate is generally 1 x 10 to 1 x 10 5 %/min., preferably, 1 x 10 3 to 1 x 10 5 %/min. The stretching ratio is preferably at least 2.5 (MD) and at least 2.0 (TD). Aerial stretching ratio is preferably at least 6 to improve elastic modulus and the like. Too low aerial stretching ratio disadvantageously results in low dimensional stability. It is preferable to further conduct heat treatment (annealing or heat setting) for the stretched film obtained by stretching under the aforementioned conditions when dimensional stability, heat resistance, strength balance of the surface of the film at high temperature are further required. Heat treatment may be conducted by the usual method. It can be conducted by maintaining the stretched film in the temperature range from glass transition temperature to melting point of the film, preferably, 100°C lower than melting point to a little lower than melting point for 0.5 to 120 seconds under a state of tension, a relaxed state or a state of controlling shrinkage. Such heat setting may be conducted twice or more changing the conditions within the above range. If the temperature during the heat setting is too low, sufficient heat resistance may not be undesirably obtained during the formation of a layer of magnetic substance by vapor deposition. The heat setting may be conducted in an atmosphere of inert gas such as argon gas, nitrogen gas or the like. Without the aforementioned heat setting, linear expansion coefficient at 0 to 90°C may become more than 5 x 10 -5 , and particularly deformation around glass transition temperature may become greater. Thus obtained base film for magnetic recording medium, magnetic tape or magnetic disk of the present invention has linear expansion coefficient, static friction coefficient µs, elastic modulus, surface roughness Ra and heat deformation temperature within the above range. This base film may be formed into various forms such as tape, disk, card. In the present invention, stabilizer, antioxidant, lubricant, brightening agent, UV absorbent, matting agent, or other additives may be added to the above composition as needed. By forming at least one magnetic layer is formed onto thus obtained base film, the magnetic recording medium, magnetic tape or magnetic disk of the present invention can be prepared. Further, undercoating layer, back coating layer or topcoating layer may be formed. These layers may be totally or partly formed on both or either side of the base film. The magnetic materials which is a raw material of the magnetic layer includes various materials, for example, Co, Co-O, Co-Cr, Co-V, Co-Ni, Co-P, Co-γFe 2 O 3 , Co-Ni-P, Co-Ni-N, Co-Ni-W, Co-Ni-Pt, CoNi(Cr)/Cr, Fe, Fe-O, Fe-Ag, γFe 2 O 3 , Fe- Co, BaO.γFe 2 O 3 , Ni, CrO 2 . Particularly, for production of densified magnetic recording medium, it is preferable to use a vertically magnetizable material such as Co, Co-O, Co-Cr, Co-V, Co-Ni, Co-P, Co-N-O, Co-Ni-P, Co-Ni-N, Co-Ni-W, CoNi(Cr)/Cr, BaO.γFe 2 O 3 and the like. A magnetic layer is formed on a base film using this magnetic material according to various methods such as coating, vapor deposition, sputtering, metal plating method or the like. The operation conditions and the like may be optionally selected according to the conventional methods. It is particularly preferable to form Co and BaO.γFe 2 O 3 magnetic materials by vapor deposition or sputtering, and coating, respectively, from the viewpoint of formation of vertically magnetic layer. Thickness of the magnetic layer is not particularly limited, but generally 0.01 to 10 µm, and particularly 0.5 to 10 µm in the case of coating, 0.01 to 1 µm in the case of vapor deposition and sputtering, and 0.1 to 5 µm in the case of metal plating. The resin for binder used for coating includes, for example, vinyl copolymer resin such as vinyl chloride - vinyl acetate copolymer, vinyl-chloride - vinyl acetate partly saponified copolymer, vinyl chloride - vinylidene chloride copolymer, vinyl chloride - acrylonitrile, vinyl butyral, vinyl formal; fiber resin such as nitrocellulose, cellulose acetobutyrate; condensation polymerization resin such as saturated polyester, polyurethane polyamide, epoxy; synthetic rubber resin such as butadiene acrylonitrile copolymer; inorganic macromolecular resin such as phosphazene. A crosslinking agent such as isocyanate compound may be used. The surface of thus obtained magnetic recording medium may be polished to prevent abrasion of a magnetic head. The present invention is described in greater detail with reference to examples, comparative examples and reference examples. Reference Example 1 (Preparation of a material for molding of styrene polymer containing inorganic filler (a polymer composition) (1) In a 500-milliliter glass vessel in which air had been replaced with argon, 17.8 g (71 mmol) of copper sulfate pentahydrate (CuSO 4 ·5H 2 O), 200 ml of toluene and 24 ml (250 mmol) of trimethylaluminum were placed and reacted at 40°C for 8 hours. And then, from the solution obtained by removing the solid, toluene was further distilled away under reduced pressure at room temperature to obtain 6.7 g of a catalytic product. The molecular weight of the product as determined by the freezing point depression method was 610. The aforementioned high magnetic component (i.e., -0.1 to -0.5 ppm) as observed by 1 H-NMR was 43%. Separately, 0.4 parts of dry method silica (Aerosil TT-600 (diameter of primary particle, 40 µm), manufactured by Degussa) was added to 100 parts of pure styrene monomer, and the resultant was mixed and stirred in a cylindrical container using T. K. Homomixer type L (manufacture by Tokushukika Kogyo) to prepare a styrene mixture. In this step, 0.05 parts by weight of calcium stearate was added.In a 2-liter reactor, 1 L (L=liter) of styrene mixture obtained by the above method, 5 mmol of the catalytic product obtained by the above method as aluminum atom, 5 mmol of triisobutyl aluminum and 0.025 mmol of pentamethylcyclopentadienyltitanium trimethoxide were placed and polymerized at 90°C for 5 hours. After completion of the reaction, a catalyst component of the product was decomposed with sodium hydroxide in methanol, then the resultant was washed with methanol repeatedly and dried to obtain 308 g of a polymer.The weight average molecular weight of said polymer as determined by gel permeation chromatography at 135°C using 1,2,4-trichlorobenzene as a solvent was 389,000, and weight average molecular weight/number average molecular weight was 2.64. The melting point and 13 C-NMR measurements confirmed that said polymer was polystyrene having a syndiotactic configuration.The polymer was dissolved in 1,2,4-trichlorobenzene at 130°C, then filtered and the silica content in the polymer was determined to 0.4% by weight. This solution was dropped onto a slide glass, and observed by a microscope. As the result, an average particle diameter of silica was 0.08 µm.Further, the styrene polymer was dried in a vacuum at 150°C for 2 hours. The resulting powder was extruded at 300°C by a vented biaxial extruder equipped with a die containing several capillaries at the tip thereof, then cooled and cut to prepare pellet. The pellet was crystallized and dried in a hot air with stirring. The pellet had crystallinity of 35% and contained 700 ppm of styrene monomer. (2) Preparation of styrene polymer without containing inorganic filler In the same manner as that in the above (1) using styrene polymer without containing dry method silica, styrene polymer was prepared. The resulting polymer had weight average molecular weight of 417,000, weight average molecular weight/number average molecular weight of 2.54, and contained 75 ppm of A1 and 2 ppm of Ti.The styrene polymer was formed into pellet in the same manner as that in the above (1). Crystallinity and styrene monomer content of the pellet were 30% and 800 ppm, respectively. Reference Example 2 (Production of stretched film of styrene polymer) (1) The material for molding obtained in the same manner as that in Reference Example 1 (1) was used and melt extruded by a serial tandem type extruder with a T-die at the tip thereof at 330°C. Shear stress was 1.5 x 10 5 dyne/cm 2 . The melt extruded sheet was contacted and cooled onto a metal cooling roller at 63°C by electrostatic application to cool and solidify. In this step, the cooling rate was 55 °C/sec in an average and 130 µm thick sheet for stretching was obtained. This sheet was stretched between rollers in MD (by three times) at 110°C and stretching rate of 6,000 %/min while circumferential speed of each roller was changed. Subsequently, the sheet was stretched by three times in TD at 120°C and stretching rate of 6,000 %/min using a tenter. Further, the sheet was re-stretched in MD by 1.5 times at 130°C and 2,000 %/min while fixed in TD using a tenter. This film was fixed on a tenter, slightly relaxed and heat treated at 255°C for 10 seconds.Thickness of the film thus obtained was 12 µm. Linear expansion coefficient of the film was measured at 0°C to 90°C. Moreover, surface roughness was measured according to JIS B-0601 (cut-off: 0.08 mm), and static friction coefficient was measured according to ASTM D-1984B. The properties of the resulting film are shown in the table. (2) The operation of Reference Example 2 (1) was repeated, except that lip opening of the T-die and re-stretching ratio were increased by four times and 1.3 times, respectively. The properties of the resulting film are shown in the table. (3) The operation of Reference Example 2 (1) was repeated, except that the crystallized pellet of styrene polymers of Reference Example 1 (1) and (2) were melt coextruded by an extruder with a T-die at the tip thereof at 330°C. The properties of the resulting film are shown in the table. In this case, pellet of styrene polymers of Reference Example 1 (2) and (1) were melt-extruded by a serial tandem type uniaxial extruder with a main extruder containing 50/150/400/150/50 mesh and a double extruder, respectively. (4) The operation of Reference Example 2 (1) was repeated, except that pellet of styrene polymer of Reference Example 1 (2) was used and 50/150/400/150/50 mesh was placed in an extruder. The film was corona treated. Subsequently, a 0.5 wt% solution of styrene having a syndiotactic configuration - divinyl benzene copolymer obtained in Example 1 of Japanese Patent Application Laid-Open Publication No. 95113/1989 (wherein divinyl benzene unit, 9.4 mol%; ethyl benzene unit, 5.0 mol%; weight average molecular weight, 360,000) in chloroform was prepared. Dry method silica (Aerosil TT-600 (diameter of primary particle, 40 µm) manufactured by Degussa) (0.5 wt% based on styrene - divinyl benzene copolymer) was added to the above solution and homogeneously mixed in a cylindrical container using a homomixer type L (manufactured by Tokushukika Kogyo) to prepare a slurry solution. The slurry solution was coated on the above film using a bar coater, and dried at 250°C for 10 seconds. The properties of the resulting film are shown in the table. (5) The operation of Reference Example 2 (1) was repeated, except that heat treatment was not carried out. (6) The operation of Reference Example 1 (1) was repeated, except that the inorganic filler used was silica having an average diameter of 4 µm (Silton AMT-40 manufactured by Mizusawa Kagaku Kogyo), and after a styrene polymer was prepared, the operation of Reference Example 2 (1) was repeated. (7) In the same manner as that in Reference Example 2 (1), a film was prepared from a styrene polymer of Reference Example 1 (2). Example 1 Corona treatment was carried out on one side of the base film obtained in the same manner as that in Reference Example 2 (1). Subsequently, a target comprising 80 wt% Co and 20 wt% Ni was prepared, and a Co-Ni magnetic thin layer about 3,000 A thick was formed using this target by RF sputter method. The distance between the target and the film was 60 mm, plate voltage was 1.9 kV, plate current was 160 mA and argon pressure was 1.1 x 10 -2 mmHg. The tape was carefully slitted to the same width as that of VHS video tape. The commercially available tape was dismantled, only tape was replaced and video cassette was prepared. Sliding properties were estimated by static friction coefficient of this tape. Static friction coefficient of this tape was as good as that of the base film. The prepared VHS video cassette tape was used for record and reproduction by a home video recorder. As an index of reliability, the conditions of record and reproduction were observed under room temperature conditions and under high temperature and high humidity conditions (85°C, 75 %RH). Little difference was observed between both case. Both cases were good, presenting no disturbed picture. The results are shown in Table 1. Example 2 One side of the base film obtained in the same manner as that in Reference Example 2 (1) was corona treated and coated with magnetic coating composition. The constitution of this magnetic coating composition was as follows: γ-FeO 3 magnetic powder, 45 parts by weight; vinyl chloride - vinyl acetate copolymer (VAGH manufactured by U.C.C.), 17 parts by weight; acrylonitrile - butadiene copolymer (N 1432 J manufactured by Nippon Zeon), 3.5 parts by weight; polyisocyanate (coronate L manufactured by Nippon Polyurethane), 1.5 parts by weight, methyl isobutyl ketone, 50 parts by weight; toluene, 50 parts by weight and carbon black, 4 parts by weight. Thickness of the magnetic layer after drying was 3 µm. The procedure of Example 1 was repeated using this tape. The results are shown in Table 1. Example 3 After corona discharge treatment was carried out on the both sides of the base film obtained in the same manner as that in Reference Example 2 (2), magnetic layers were formed on the both sides in the same manner as that in Example 1. The both sides of this film were properly polished, then cut into the same size as the commercially available floppy (5-inch). Further, this disc was enclosed in a jacket of the commercially available floppy to prepare a floppy disc. Static friction coefficient of the film with a magnetic layer was determined in the same manner as that in Example 1. The conditions of SAVE and LOAD were observed using the prepared floppy and NEC PC-9801F under room temperature conditions and high temperature and high humidity conditions. Both case was good. The results are shown in Table 1. Example 4 The procedure of Example 1 was repeated, except that a magnetic layer was formed on the side of smooth surface of the film obtained in the same manner as that in Reference Example 2 (3). The results are shown in Table 1. Example 5 The procedure of Example 1 was repeated, except that a magnetic layer was formed on the side of smooth surface of the film obtained in the same manner as that in Reference Example 2 (4). The results are shown in Table 1. Comparative Example 1 The procedure of Example 1 was repeated, except that a magnetic layer was formed on one side of the film obtained in the same manner as that in Reference Example 2 (5). The results are shown in Table 1. Comparative Example 2 The procedure of Example 1 was repeated, except that a magnetic layer was formed on one side of the film obtained in the same manner as that in Reference Example 2 (6). The results are shown in Table 1. Comparative Example 3 The procedure of Example 1 was repeated, except that a magnetic layer was formed on one side of the film obtained in the same manner as that in Reference Example 2 (7). The results are shown in Table 1. Comparative Example 4 The procedure of example 1 was repeated, except that polyethylene terephthalate (PET) film (Tetron NS 12 µm, manufactured by Teijin) was used. The results are shown in Table 1. Reference Example 3 (Production of stretched film of styrene polymer) (1) The material for molding obtained in the same manner as that in Reference Example 1 (1) was used and melt extruded by a serial tandem type extruder with a T-die at the tip thereof at 330°C. Shear stress was 1.5 x 10 5 dyne/cm 2 . The melt extruded sheet was contacted and cooled onto a metal cooling roller at 63°C by electrostatic application to cool and solidify. In this step, the cooling rate was 55°C/sec in an average and 130 µm thick sheet for stretching was obtained. This sheet was stretched between rollers in MD (by three times) at 110°C and stretching rate of 6,000 %/min while circumferential speed of each roller was changed. Subsequently, the sheet was stretched in TD at 120°C and 6,000 %/min by 3 times using a tenter. Further, the sheet was re-streched in MD at 130°C and 2,000 %/min by 1.5 times while fixed in TD using a tenter. This film was fixed on a tenter, slightly relaxed and heat treated at 255°C for 10 seconds. Thickness of the film thus obtained was 12 µm. Moisture expansion coefficient of the film was measured in the range from 20 to 80 %RH. Expansion coefficient was measured at 0°C to 90°C. Moreover, surface roughness was measured according to JIS B-0602 (cut-off: 0.08 mm), and static friction coefficient was measured according to ASTM D-1984B. Heat deformation temperature of this film was measured by thermal mechanical analysis (TMA). Heat deformation temperature means the temperature at which 2% change of the specimen length was observed. The properties of the resulting film are shown in Table 2. (2) The procedure of Reference Example 3 (1) was repeated, except that lip opening of the T-die and re-stretching ratio were increased by 4 times and 1.3 times, respectively. (3) The procedure of Reference Example 3 (1) was repeated, except that the crystallized pellet of styrene polymers of Reference Example 1 (1) and (2) were melt coextruded by an extruder with a T-die at the tip thereof at 330°C. The properties of the resulting film are shown in Table 2. In this case, pellet of styrene polymers of Reference Example 1 (2) and (1) were melt extruded by a serial tandem type uniaxial extruder with a main extruder containing 50/150/400/150/50 mesh and a double extruder, respectively. (4) The procedure of Reference Example 3 (1) was repeated except that pellet of styrene polymer of Reference Example 1 (2) was used and 50/150/400/150/50 mesh was placed in an extruder. The film was corona treated. Subsequently, a 0.5 wt% solution of styrene having a syndiotactic configuration-divinyl benzene copolymer obtained in Example 1 of Japanese Patent Application Laid-Open Publication No. 95113/1989 (wherein divinyl benzene unit, 9.4 mol%; ethyl benzene unit, 5.0 mol%; weight average molecular weight, 360,000) in chloroform was prepared. Dry method silica (Aerosil TT-600 (diameter of primary particle, 40 µm) manufactured by Degussa) (0.5 wt% based on styrene - divinyl benzene copolymer) was added to the above solution and homogeneously mixed in a cylindrical container using a homomixer type L (manufactured by Tokushukika Kogyo) to prepare a slurry solution. The slurry solution was coated on the above film using a bar coater, and dried at 250°C for 10 seconds. The properties of the resulting film are shown in Table 2. (5) The procedure of Reference Example 3 (1) was repeated, except that heat treatment was not carried out. (6) In the same manner as that in Reference Example 3 (1), a film was prepared from a styrene polymer of Reference Example 1 (2). Example 6 Corona treatment was carried out on one side of the base film obtained in the same manner as that in reference Example 3 (1). Subsequently, a 0.1 µm thick Co-Cr membrane (Cr: 20 wt%) was formed by a vacuum vapor deposition. In this case the temperature of the base was 250°C, and curling of the tape or the like was not observed. Static friction coefficient of the tape after such vapor deposition was determined according to ASTM D-1984 and was 0.55, and the tape presented good sliding properties. Coersive force Hc after vapor deposition was 900 Oe, as determined from a magnetizing characteristic curve using a vibration sample magnetometer VSM. The results are shown in Table 2. Example 7 One side of the film of Reference Example 3 (1) was corona treated and coated with magnetic coating composition. The constitution of this magnetic coating composition was as follows: barium ferrite magnetic powder, 100 parts by weight; vinyl chloride - vinyl acetate copolymer (VAGH manufactured by U.C.C.), 15 parts by weight; polyurethane, 20 parts by weight; lecitin, 4 parts by weight; stearic acid, 3 parts by weight; methyl isobutyl ketone, 100 parts by weight; methyl ethyl ketone, 100 parts by weight; cyclohexanone, 100 parts by weight; and isocyanate, 3 parts by weight. Thickness of the magnetic layer after drying was 3 µm. This magnetic tape was examined in the same manner as in Example 6. The results are shown in Table 2. Example 8 The procedure of Example 6 was repeated, except that both sides of the film of Reference Example 3 (2) were corona treated and magnetic materials were coated thereon, and the film was formed into a disc. The results are shown in Table 2. Example 9 The one side of the film of Reference Example 3 (3) was corona treated and on which a 0.8 µm thick Co-O membrane (0.21 wt%) was formed. During the formation of this membrane, electron beam was used for evaporation of Co, O 2 gas was introduced and separation speed was about 900 Å/sec. O 2 composition in the membrane was analyzed using EPMA. The vapor-deposited tape was investigated in the same manner as that in Example 6. The results are shown in Table 2. Example 10 The procedure of Example 9 was repeated, except that Co-Ni-O membrane (0.22 wt%) was formed using CoNi (Ni, 0.1 wt%). The results are shown in Table 2. Comparative Example 5 The procedure of Example 6 was repeated, except that a film of Reference Example 3 (5) was used and temperature of the base was set at 100°C. The results are shown in the table. Formation of a magnetic layer at the base temperature of 250°C was difficult due to shrinkage and curling of the film. The results are shown in Table 2. Comparative Example 6 The procedure of Example 6 was repeated, except that a film of Reference Example 3 (6) was used. The results are shown in Table 2. Comparative Example 4 The procedure of Example 6 was repeated, wherein PET film (Tetron NS 12 µm, manufactured by Teijin) was used and the temperature of the base was 200°C. The results are shown in the table. Formation of the magnetic layer at temperature of the base of 250°C was difficult because film was partly melted, stretched or the like. The results are shown in Table 2. Reference Example 4 (Production of stretched film of styrene polymer) (1) The material for molding obtained in the same manner as that in Reference Example 1 (1) was used and melt extruded by a serial tandem type extruder with a T-die at the tip thereof at 330°C. Shear stress was 1.5 x 10 5 dyne/cm 2 . The melt extruded sheet was contacted onto a cooling roller at 70°C by electrostatic application to cool and solidify. In this step, the cooling rate was 60 °C/sec in an average and 110 µm thick sheet for stretching was obtained. This sheet was stretched between rollers in MD (by three times) at 110°C and stretching rate of 6,000 %/min while circumferential speed of each roller was changed. Subsequently, the sheet was stretched in TD by three times at 120°C and stretching rate of 6,000 %/min. Further, the sheet was re-stretched in Md at 130°C and 2,000 %/min by 1.5 times while fixed in TD using a tenter. This film was fixed on a tenter, slightly relaxed and heat treated at 255°C for 10 seconds. Thickness of the film thus obtained was 8 µm. Elastic moduli of the film in MD and TD at room temperature and elastic modulus in MD at 90°C were measured. Linear expansion coefficient of the film at 0°C to 90°C was measured by thermal mechanical analysis. Moreover, surface roughness was measured according to JIS B-0601 (cut-off: 0.08 mm), and static friction coefficient was measured according to ASTM D-1984B. The properties of the resulting film are shown in Table 3. (2) The procedure of Reference Example 4 (1) was repeated, except that stretching ratio of the film in MD and in TD was increased by 3.5 times and 2.7 times, respectively. The properties of the resulting film are shown in Table 3. (3) The procedure of Reference Example 4 (1) was repeated, except that the crystallized pellets of styrene polymers of Reference Example 1 (1) and (2) were melt coextruded by an extruder with a T-die at the tip thereof at 330°C. The properties of the resulting film are shown in Table 3. In this case, pellets of styrene polymers of Reference Example 1 (2) and (1) were melt-extruded by a serial tandem type uniaxial extruder with a main extruder containing 50/150/400/150/50 mesh and a double extruder, respectively. (4) The procedure of Reference Example 4 (1) was repeated, except that pellet of styrene polymer of Reference Example 1 (2) was used and 50/150/400/150/50 mesh was placed in an extruder. The film was corona treated. Subsequently, a 0.5 wt% solution of styrene having a syndiotactic configuration - divinyl benzene copolymer obtained in Example 1 of Japanese Patent Application Laid-Open Publication NO. 95113/1989 (wherein divinyl benzene unit, 9.4 mol%; ethyl benzene unit, 5.0 mol%; weight average molecular weight, 360,000) in chloroform was prepared. Dry method silica (Aerogil TT-600 (diameter of primary particle, 40 µm) manufactured by Degussa) (0.5 wt% based on styrene - divinyl benzene copolymer) was added to the above solution and mixed homogeneously in a cylindrical container using a homomixer type L (manufactured by Tokushukika Kogyo) to prepare a slurry solution. The slurring solution was coated on the above film using a bar coater, and dried at 250°C for 10 seconds. The properties of the resulting film are shown in Table 3. (5) The procedure of Reference Example 4 (1) was repeated, except that the stretching ratio in MD and that in TD were doubled and restretching was not carried out. The properties of the resulting film are shown in Table 3. (6) The procedure of Reference Example 4 (1) was repeated, except that the stretching ratio in MD was increased by 4 times and restretching was not carried out. The properties of the resulting film are shown in Table 3. (7) The procedure of Reference Example 4 (1) was repeated, except that the stretching ratio in MD and that in TD were increased by 4 times and twice, respectively and restretching was not carried out. The properties of the resulting film are shown in Table 3. (8) The procedure of Reference Example 4 (1) was repeated, except that heat treatment was not carried out. The properties of the resulting film are shown in Table 3. (9) The procedure of Reference Example 1 (1) was repeated, except that 4 µm silica (Silton AMT-40 manufactured by Mizusawa Kagaku Kogyo) was used to prepare a styrene polymer, and subsequent operation was carried out in the same manner as that in Reference Example 4 (2). The properties of the resulting film are shown in Table 3. Example 11 Corona treatment was carried out on one side of the base film obtained in Reference Example 4 (1). Subsequently, a target comprising 80 wt% Co and 20 wt% Ni was prepared, and a Co-Ni magnetic thin layer about 3,000 Å thick was formed using this target by RF sputter method. The distance between the target and the film was 60 mm, plate voltage was 1.9 kV, plate current was 160 mA and argon pressure was 1.1 x 10 -2 mmHg. The tape was carefully slitted to the same width as that of audio tape. The commercially available cassette tape was dismantled, only tape was replaced and audio cassette was prepared. Sliding properties of this tape were evaluated by static friction coefficient according to ASTM D-1984, which showed a good result of 0.55. This tape was recorded and set in a car stereo, then the tape was repeatedly played back for 2 hours while the car temperature was maintained at 90°C, and change in musical intervals was checked. As the result, the tape was good, showing little change. Further, after this tape was heated an cooled between room temperature and 90°C at the rate of 10 °C/min under load of 6 kg/cm 2 , displacement as measured by thermal mechanical analysis (TMA) was good, i.e. lower than 0.1 %. The results are shown in Table 3. Example 12 One side of the base film obtained in the same manner as that in Reference Example 4 (1) was corona treated and coated with magnetic coating composition. The constitution of this magnetic coating composition was as follows: γ-FeO 3 magnetic powder, 45 parts by weight; vinyl chloride - vinyl acetate copolymer (VAGH manufactured by U.C.C.), 17 parts by weight; acrylonitrile - butadiene copolymer (N 1432 J manufactured by Nippon Zeon), 3.5 parts by weight; polyisocyanate (coronate L manufactured by Nippon Polyurethane), 1.5 parts by weight, methyl isobutyl ketone, 50 parts by weight; toluene, 50 parts by weight and carbon black, 4 parts by weight. Thickness of the magnetic layer after drying was 3 µm. The procedure of Example 11 was repeated using this tape. The results are shown in Table 3. Example 13 After corona discharge treatment was carried out on one side of the film obtained in Reference Example 4 (2), 0.1 µm thick Co-Cr membrane (Cr, 20 wt%) was formed on the side. In this step, temperature of the base was 250°C. The results are shown in Table 3. Example 14 The side of smooth surface of the film of Reference Example 4 (3) was corona treated and a magnetic coating composition was applied on the surface. The constitution of this magnetic coating composition was as follows: barium ferrite magnetic powder, 100 parts by weight; vinyl chloride - vinyl acetate copolymer (VAGH manufactured by U.C.C.), 15 parts by weight; polyurethane, 20 parts by weight; lecitin, 4 parts by weight; stearic acid, 3 parts by weight; methyl isobutyl ketone, 100 parts by weight; methyl ethyl ketone, 100 parts by weight; cyclohexanone, 100 parts by weight; and isocyanate, 3 parts by weight. Thickness of the magnetic layer after drying was 3 µm. This tape was investigated in the same manner as that in Example 11. The results are shown in Table 3. Example 15 The procedure of Example 11 was repeated, except that a magnetic layer was formed on the side of smooth surface of the film obtained in Reference Example 4 (4). The results are shown in Table 3. Comparative Example 8 The procedure of Example 11 was repeated using the film obtained in Reference Example 4 (5). The results are shown in Table 3. Comparative Example 9 The procedure of Example 11 was repeated using the film obtained in the same manner as that in Reference Example 4 (6). The results are shown in Table 3. Comparative Example 10 The procedure of Example 11 was repeated using the film obtained in Reference Example 4 (7). The results are shown in Table 3. Comparative example 11 The procedure of Example 11 was repeated using the film obtained in Reference Example 4 (8). The results are shown in Table 3. Comparative example 12 The procedure of Example 11 was repeated using the film obtained in Reference Example 4 (9). The results are shown in Table 3. Comparative example 13 The procedure of Example 11 was repeated, except that PET film (Tetron Film, 6 µm, manufactured by Teijin) was used. The results are shown in Table 3. Reference Example 5 (Production of stretched film of styrene polymer) (1) The material for molding obtained in the same manner as that in Reference Example 1 (1) was used and melt-extruded by a serial tandem type extruder with a T-die at the tip thereof at 330°C. Shear stress was 1.5 x 10 5 dyne/cm 2 . The melt extruded sheet was contacted onto a metal cooling roller at 63°C by electrostatic application to cool and solidify. In this step, the cooling rate was 70 °C/sec on an average and 1,500 µm thick sheet for stretching was obtained. This sheet was stretched between rollers in MD by three times at 110°C and stretching rate of 6,000 %/min while circumferential speed of each roller was changed. Absolute value of birefringence |Δn| was 35 x 10 -3 . Subsequently, the sheet was stretched by three times in TD at 120°C and stretching rate of 6,000 %/min using a tenter. Further, the sheet was re-stretched simultaneously in MD and TD (1.5 times, each) at 130°C and 2,000 %/min while fixed in TD using a tenter. This film was fixed on a tenter, slightly relaxed and heat treated at 255°C for 10 seconds.Thickness of the film thus obtained was 75 µm. Absolute value of birefringence |Δn| was 14 x 10 -3 . Linear expansion coefficient of the film at 0°C to 90°C was measured by thermal mechanical analysis. Moreover, surfaces roughness was measured according to JIS B-0601 (cut-off: 0.08 mm). The properties of the resulting film are shown in Table 4. (2) The procedure in Reference Example 5 (1) was repeated, except that pellets of crystallized styrene polymers of Reference Example 1 (1) and (2) were used and melt-coextruded at 330°C by an extruder with a T-die at the tip thereof into three layers of two kinds of polymers. The properties of the resulting film are shown in Table 4. In this case, the pellets of styrene polymers of Reference Example 1 (2) and (1) were melt-extruded by a serial tandem type uniaxial extruder with a main extruder containing 50/150/400/150/50 mesh and a double extruder, respectively, to give a constitution of polymer of Reference Example 1 (1)/polymer of Reference Example 1 (2)/polymer of Reference Example 1 (1). Absolute value of birefringence |Δn| of this film was 10 x 10 -3 . (3) The procedure in Reference Example 5 (1) was repeated, except that pellet of styrene polymer of Reference Example 1 (2) was used and 50/150/400/150/50 mesh was placed in an extruder. Absolute value of birefringence |Δn| of this film was 9 x 10 -3 . The both sides of the film were corona treated. Subsequently, a 0.5 wt% solution of styrene having a syndiotactic configuration - divinyl benzene copolymer obtained in Example 1 of Japanese Patent Application Laid-Open Publication No. 95113/1989 (wherein divinyl benzene unit, 9.4 mol%; ethyl benzene unit, 5.0 mol%; weight average molecular weight, 360,000) in chloroform was prepared. Dry method silica (Aerogil TT-600 (diameter of primary particle, 40 µm) manufactured by Degussa Co.) (0.5 wt% based on styrene - divinyl benzene copolymer) was added to the above solution and homogeneously mixed in a cylindrical container using a homomixer type L (manufactured by Tokushukika Kogyo) to prepare a slurry solution. The slurry solution was coated on the both sides of the above film using a bar coater, and dried at 250°C for 10 seconds. The properties of the resulting film are shown in Table 4. (4) The procedure in Reference Example 5 (1) was repeated, except that heat treatment was not carried out. Absolute value of birefringence |Δn| of this film was 18 x 10 -3 . The properties of the resulting film are shown in Table 4. (5) The procedure in Reference Example 5 (1) was repeated, except that re-stretching ratio in MD was changed to 2. Absolute value of birefringence |Δn| of this film was 53 x 10 -3 . The properties of the resulting film are shown in Table 4. (6) The procedure in Reference Example 1 (1) was repeated, except that the inorganic filler used was silica having an average diameter of 4 µm (Silton AMT-40 manufactured by Mizusawa Kagaku Kogyo), and after a styrene polymer was prepared, the procedure in Reference Example 5 (1) was repeated. Absolute value of birefringence |Δn| of this film was 8 x 10 -3 . The properties of the resulting film are shown in Table 4. Example 16 Both sides of the base film obtained in the same manner as that in Reference Example 2 (1) were corona treated and coated with magnetic coating composition. The constitution of this magnetic coating composition was as follows: γ-Fe 2 O 3 magnetic powder, 45 parts by weight; vinyl chloride - vinyl acetate copolymer (VAGH manufactured by U.C.C.), 17 parts by weight; acrylonitrile - butadiene copolymer (N 1432 J manufactured by Nippon Zeon), 3,5 parts by weight; polyisocyanate (coronate L manufactured by Nippon Polyurethane), 1.5 parts by weight, methyl isobutyl ketone, 50 parts by weight; toluene, 50 parts by weight and carbon black, 4 parts by weight. Thickness of the magnetic layer after drying was 3 µm. Subsequently, the both sides of this film were properly polished, then cut into the same size as the commercially available floppy (5-inch). Further, this disk was enclosed in a jacket of the commercially available floppy disk to prepare a floppy disk. A program was fed to this floppy using NEC PC-9801F. Then, the floppy was left at 80°C under 70 %RH (relative humidity) for 50 hours. After the floppy was brought to the normal condition and LOAD and RUN of the program were carried out. The conditions were good. This fact showed that dimensional changes in all directions under high temperature and high humidity conditions were very small and well balanced. LOAD and RUN were continuously repeated (x100), resulting in no special abnormality but good sliding properties and good performance traverse. The results are shown in Table 4. Example 17 Corona treatment was carried out on both sides of the base film obtained in the same manner as that in Reference Example 5 (1). Subsequently, a target comprising 80 wt% Co and 20 wt% Ni was prepared, and a Co-Ni magnetic thin films of about 3,000 Å thickness were formed on both sides using this target by RF sputter method, which was subjected to anodic oxidation in a solution containing an electrolyte to form passive state membrane on the surface. As condition for sputtering, the distance between the target and the film was 60 mm, plate voltage was 1.9 kV, plate current was 160 mA and argon pressure was 1.1 x 10 -2 mmHg. The procedure in Example 16 was repeated using this film. the results are shown in Table 4. Example 18 The procedure in Example 16 was repeated, except that the film obtained in Reference Example 5 (2) was used. The results are shown in Table 4. Example 19 The procedure in Example 16 was repeated, except that the film obtained in Reference Example 5 (3) was used. The results are shown in Table 4. Comparative Example 14 The procedure in Example 16 was repeated, except that the film obtained in Reference Example 5 (4) was used. The results are shown in Table 4. Comparative Example 15 The procedure in Example 16 was repeated, except that the film obtained in Reference Example 5 (5) was used. The reults are shown in Table 4. Comparative Example 16 The procedure in Example 16 was repeated, except that the film obtained in Reference Example 5 (6) was used. The results are shown in Table 4. Comparative Example 17 The procedure in Example 16 was repeated, except that polyethylene terephthalate (PET) film (Tetron V, 75 µm, manufactured by Teijin) was used. The results are shown in Table 4. Example 20 Based on 100 parts by weight of the styrene polymer obtained in Reference Example 1 (1) which had been sufficiently dried under reduced pressure, 0.03 parts by weight of fluorescent whitening agent (OB-1, manufactured by Eastmann) and 12 parts by weight of fine-grain calcium carbonate (average particle diameter, 0.9 µm; natural product, surface treated with stearic acid) were mixed using a super mixer for a minute, then extruded by a biaxial extruder with capillary die at the tip thereof and cut into pellet. This pellet was stirred in a dry hot air at 120°C. This pellet was extruded at 320°C by a uniaxial extruder with a T-die at the tip thereof. The extruded molten sheet was taken up via cooling metal rollers of touch roll type. The temperature of the metallic rollers was 70°C and the average cooling rate was about 50 °C/sec. The resulting 1,000 µm thick pre-molded product was stretched between nip rolls with different circumferential speeds by 3 times while heated at 115°C. The stretching rate was 6,000 %/min. Subsequently, this film was stretched by 3 times at 120°C and 3,000 %/min. using a tenter, then heat treated at 250°C for 20 seconds under a state of tension. Thickness of the film thus obtained was 120 µm. Whiteness measured according to JIS L-1074 was 85. Linear expansion coefficient of the film at O°C to 90°C as measured by thermal mechanical analysis was 5 x 10 -5 /°C, and heat shrinkage after 30 minutes at 200°C was 0.5%. Elastic modulus was 45,000 kg/cm 2 . One side of the base film thus obtained was corona treated and coated with magnetic coating composition. The constitution of this magnetic coating composition was as follows: γ-Fe 3 O 3 magnetic powder, 45 parts by weight; vinyl chloride - vinyl acetate copolymer (VAGH manufactured by U.C.C.), 17 parts by weight; acrylonitrile - butadiene copolymer (N 1432 J manufactured by Nippon Zeon), 3.5 parts by weight; polyisocyanate (coronate L manufactured by Nippon Polyurethane), 1.5 parts by weight, methyl isobutyl ketone, 50 parts by weight; toluene, 50 parts by weight and carbon black, 4 parts by weight. Thickness of the magnetic layer after drying was 3 µm. Picture was printed on the opposite side to the magnetic layer and input and output of the information was attempted, providing good result. The surface did not glare, and the printed surface was presented vivid and preferable appearance. The card had sufficient resiliency and its appearance and performance were not affected by temperature change from 0 to 100°C. The results are shown in Table 5. Example 21 The procedure in Example 20 was repeated, except that 42 parts by weight of titanium oxide (tipake A-100; an average particle diameter, 0.2 µm) was used as white inorganic filler and a magnetic card was obtained. Elastic modulus of this card was 52,000 kg/cm 2 . The results are shown in Table 5. Comparative Example 18 The procedure in Example 20 was repeated, except tht white inorganic filler was not used. the resulting magnetic card had same performance as that of Example 20, but inferior in sliding properties. That is, after repeated usage, printed image was partially peeled off. The appearance glared, and the card was sticky, providing bad touch. That is, the appearance was not preferable. The results are shown in Table 5. Comparative Example 19 The procedure in Example 20 was repeated by including 70% by weight of white inorganic filler. As the result, the base film was not obtained because it was broken during stretching. Comparative Example 20 A 100 µm thick vinyl chloride film containing 30% by weight of TiO 2 was obtained. The procedure in Example 20 was repeated using this film. The resulting magnetic card had insufficient heat resistance and dimensional stability. The results are shown in Table 5. Comparative Example 21 The procedure in Example 20 was repeated, except that polyethylene terephthalate having intrinsic viscosity of 0.61 (measured in o-chlorophenol solution at 35°C) was used. The resulting magnetic card had low heat resistance and bad appearance, and particularly the surface glared, which was not preferred from the viewpoint of appearance. Elastic modulus of this card was 40,000 kg/cm 2 . The results are shown in Table 5. INDUSTRIAL AVAILABILITY The above obtained magnetic recording medium of the present invention is excellent in sliding properties and smoothness, has high heat resistance, moisture resistance and excellent dimentional stability to temperature and moisture, and further has good coersive force and can be densified. In addition to the aforementioned characteristics, the magnetic tape of the present invention has good dimensional stability at high temperature, and can provide good reproduction free from elongation of tape upon usage at high temperature of 90°C. In addition to the above characteristics, the magnetic disk of the present invention has flexibility and less dimensional changes under high temperature and high humidity conditions, while dimentional changes in any direction may be well balanced. Moreover, sliding properties of the surface is extremely excellent. The magnetic card of the present invention, which has, in addition to the above characteristics, sufficient whiteness, is a resilient card with excellent appearance and durable upon usage for a long period of time. Accordingly, the magnetic recording medium of the present invention can be suitably used widely, for example, as various kinds of magnetic tapes, magnetic disks, magnetic drums, magnetic cards, etc. Further, according to the present invention, the magnetic tape can be efficiently used as heat resistant magnetic tape which is used for on board audio apparatus such as car stereo or video; the magnetic disk as flexible magnetic disk such as floppy disk; and the magnetic card as various kinds of magnetic cards such as prepaid card, ticket, card key and the like.

Claims: A magnetic recording medium wherein at least one side of a base film has a magnetic layer, said base film being a stretched film comprising a styrene polymer having a high degree of syndiotactic configuration or a composition thereof, excluding inorganic filler, or a laminate film containing said stretched film, having a linear expansion coefficient of not more than 5 x 10 -5 /°C and a static friction coefficient of not less than 0.3 and not more than 1.0; the magnetic layer contains no curable phosphazene compound. The magnetic recording medium according to claim 1, wherein at least one side of the base film has a magnetic layer, said base film being the base film according to claim 1 wherein heat deformation temperature is not less than 230°C, and surface roughness Ra of at least one side is 0.001 to 0.02 µm. A heat resistant magnetic tape which comprises a magnetic layer formed on at least one side of a base film which is a stretched film comprising a styrene polymer having a high degree of syndiotactic configuration or a composition thereof, excluding inorganic filler, or a laminate film containing said stretched film wherein elastic moduli in MD and TD at room temperature are not less than 30,000 kg/cm 2 , the ratio (MD/TD) is not less than 0.8, elastic modulus at 90°C in MD is not less than 10,000 kg/cm 2 , linear expansion coefficient at 0 to 90 °C is not more than 5 x 10 -5 /°C and surface roughness is 0.001 to 0.03 µm, the magnetic layer contains no curable phosphazene compound. A magnetic disk which comprises a magnetic layer formed on at least one side of a base film which is a 20 to 200 µm thick stretched film comprising a styrene polymer having a high degree of syndiotactic configuration or a composition thereof, exluding inorganic filler, or a laminate film containing said stretched film wherein linear expansion coefficient in MD (α MD ) and that in TD (α TD ) are not more than 5 x 10 -5 /°C, and the ratio (α MD /α TD ) is 0.3 to 3 and surface roughness Ra is 0.001 to 0.03 µm, the magnetic layer contains no curable phosphazene compound.

Inventors: FUNAKI KEISUKE, OHKI YUICHI, FUNAKI, KEISUKE, OHKI, YUICHI
Applicants: IDEMITSU KOSAN CO, IDEMITSU KOSAN COMPANY LIMITED
Application year: 1990
Main classification: G11B5
IPC codes: G11B5