Aromatic polyamide film and method for producing aromatic polyamide film

Abstract

To provide an aromatic polyamide film for magnetic tape capable of reducing generation of coating irregularities in a magnetic layer or the like.SOLUTION: An aromatic polyamide film for use as a base of a magnetic recording material tape, has at least one surface on which a ratio (P / K) of a peak intensity (P) of a fragment derived from polydimethylsiloxane to a peak intensity (K) of a fragment derived from an aromatic polyamide basic skeleton, as measured by a mass spectrometer, is 0.20 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aromatic polyamide film that is excellent in suppressing coating unevenness in the magnetic layer and / or backcoat layer and is ideal for magnetic recording media applications requiring high terabyte-level recording capacities, particularly magnetic tape applications, and to a method for producing the aromatic polyamide film. [Background technology]

[0002] In recent years, significant advances have been made in the means of transmitting terabytes of information at high speeds, enabling the transfer of images and data containing enormous amounts of information. However, advanced technologies are also required for recording, reproducing, and storing such information. Recording and reproducing media include flexible disks, magnetic drums, hard disks, and magnetic tapes (hereinafter also referred to as the magnetic recording media of the present invention). Magnetic tapes, in particular, have a large recording capacity per reel and are less expensive to manage than hard disks, making them particularly useful for data backup and archiving. Furthermore, with the handling of big data, the expanded range of uses for magnetic tapes has led to increased demands for reliability in data storage under a wide range of environmental conditions (especially high temperature and humidity conditions), reliable performance in recording and reading data over multiple runs due to repeated high-speed use, and longer and thinner magnetic tape cartridges due to the increased recording capacity per reel. Magnetic tape is generally manufactured through the steps of preparing the coating liquid for each layer, applying the resulting coating liquid to a non-magnetic support, drying, calendaring (smoothing), cutting to specified dimensions, and finally wrapping the resulting tape into a cartridge.

[0003] In the process leading to the magnetic tape, the film unwound from the original roll is processed under a certain tension depending on the processing step, which makes the film prone to longitudinal stretching, and over time after production, the magnetic tape wound into the cartridge gradually shrinks, causing the tape to become tightly wound and prone to deformation. To solve these problems, Patent Documents 1 and 2 propose using a polyamide film, which has higher strength than PET and PEN and can be made thinner and longer, as the non-magnetic support, to suppress reversible dimensional changes, such as the thermal expansion coefficient and humidity expansion coefficient, in the longitudinal, width, and thickness directions of the film. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2004-67872 A (Published March 4, 2004) [Patent Document 2] Japanese Patent Application Publication No. 9-71669 (published on March 18, 1997) [Patent Document 3] JP 2020-146889 A (Published September 17, 2020) [Patent Document 4] JP 2020-192805 A (Published December 3, 2020)

[0005] To meet the demand for ever higher capacity data storage systems, magnetic tapes are becoming thinner and thinner in order to allow for longer tape lengths within each cartridge.

[0006] As the thickness of the magnetic layer and the non-magnetic layer applied to the support became thinner, coating unevenness became apparent.

[0007] These coating irregularities were visible through transmitted light and had a circular shape with a major axis of 1 mm or more, and there was concern that they would cause wear on the drive head when used as a magnetic recording medium. The occurrence of uneven coating was attributed to polydimethylsiloxane derived from silicone rubber that had adhered from the silicone rubber-coated nip roll.

[0008] Generally, silicone rubber is composed of a silicone main skeleton made of siloxane bonds.

[0009] As a technique for controlling the amount of polydimethylsiloxane transferred from the surface of a nip roll coated with silicone rubber to the surface of a film, Patent Documents 3 and 4 propose a method of heat-treating a nip roll coated with silicone rubber in advance. Summary of the Invention [Problem to be solved by the invention]

[0010] However, Patent Documents 3 and 4 are methods specific to the melt production process, and there has been a need to prevent the transfer of polydimethylsiloxane and suppress the occurrence of coating unevenness in solution casting, which does not involve heating the nip rolls in the production process. [Means for solving the problem]

[0011] In order to solve this problem, the present invention comprises the following configuration. (1) An aromatic polyamide film used as the base of a magnetic recording material tape, in which at least one surface has a ratio (P / K) of the peak intensity (P) of fragments derived from polydimethylsiloxane to the peak intensity (K) of fragments derived from the aromatic polyamide basic skeleton measured by mass spectrometry of 0.20 or less. (2) A method for producing an aromatic polyamide film, comprising a step of stretching a film in the running direction of the film by using the difference in peripheral speed between a plurality of stretching rolls in solution casting, wherein the stretching rolls comprise nip rolls having a core metal coated with silicone rubber that presses the film, and the nip roll is a nip roll from which polydimethylsiloxane on the nip roll surface has been removed after an ambient temperature T of 150°C or more and less than 300°C has been reached by the average temperature in the thickness direction of the silicone rubber laminated on the roll surface, measured at the center of the roll width direction, for a treatment time t of 10 hours or more and 20 hours or less at temperature T. (3) The method for producing an aromatic polyamide film according to (2), wherein the removal of polydimethylsiloxane from the surface of the nip roll comprises a step of heat-treating the nip roll and then polishing the surface of the nip roll to remove polydimethylsiloxane deposited on the surface of the nip roll. [Effects of the Invention]

[0012] The aromatic polyamide film of the present invention can provide an aromatic polyamide film that is ideal for magnetic tapes that use the aromatic polyamide film as a support, as it is excellent at suppressing coating unevenness even when the total thickness of the magnetic layer and non-magnetic layer applied to the support is 1 μm or less. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below with reference to preferred embodiments, but the present invention is not limited thereto.

[0014] The aromatic polyamide film of the present invention preferably has, for example, repeating units represented by the following chemical formula (1) and / or chemical formula (2).

[0015] [ka]

[0016] [ka]

[0017] Here, examples of groups represented by Ar1, Ar2, and Ar3 include those represented by the following chemical formula (3).

[0018] [ka]

[0019] The X and Y groups are selected from, but are not limited to, -O-, -CH2-, -CO-, -CO2-, -S-, -SO2-, and -C(CH3)2-. Furthermore, those in which some of the hydrogen atoms on these aromatic rings are substituted with substituents such as halogen groups (especially chlorine) such as fluorine, bromine, and chlorine, nitro groups, alkyl groups (especially methyl groups) such as methyl, ethyl, and propyl, and alkoxy groups (especially methoxy, ethoxy, and propoxy) are preferred because they reduce moisture absorption and dimensional change due to humidity changes. Hydrogen in the amide bonds constituting the polymer may also be substituted with other substituents. The aromatic polyamide used in the present invention preferably has para-oriented aromatic rings that account for 80 mol % or more, more preferably 90 mol % or more, of all aromatic rings. Para-orientation here refers to a state in which the divalent bonds constituting the main chain on the aromatic rings are coaxial or parallel to each other. If this para-orientation is less than 80 mol %, the film may have insufficient rigidity and heat resistance. Furthermore, when the aromatic polyamide contains 60 mol % or more of repeating units represented by the following chemical formula (4), stretchability and film properties are particularly excellent, which is preferable.

[0020] [ka]

[0021] Furthermore, the film of the present invention has an average light transmittance of 70% or more and 100% or less in the wavelength range of 400 nm to 750 nm, which reduces the yellowish tint of the film, making it preferable as a glass replacement.

[0022] In order to achieve an average light transmittance of 70% or more and 100% or less in the wavelength range of 400 nm to 750 nm, it is preferable to have a structural unit represented by any one of the following chemical formulas (5) to (9).

[0023] [ka]

[0024] R1 and R2 are -H, an aliphatic group having 1 to 5 carbon atoms, -CF3, -CCl3, -OH, -F, -Cl, -Br, -OCH3, a silyl group, or a group containing an aromatic ring, preferably -CF3, -F, -Cl, or a group containing an aromatic ring.

[0025] [ka]

[0026] R3 is a group containing Si, a group containing P, a group containing S, a halogenated hydrocarbon group, a group containing an aromatic ring, or a group containing an ether bond (however, structural units having these groups may be present in the molecule), and is preferably a group containing Si, a halogenated hydrocarbon group, a group containing an aromatic ring, or a group containing an ether bond.

[0027] [ka]

[0028] R4 is any group, and is not particularly limited, but is preferably -H, -Cl, or -F.

[0029] [ka]

[0030] R5 is any aromatic group or any alicyclic group, and is not particularly limited, but is more preferably phenyl, biphenyl, cyclohexane, or decalin.

[0031] Furthermore, among the above, when the structural unit represented by chemical formula (5) or (6) is present, a thin film having excellent rigidity is easily obtained, and an intermediate layer is easily formed, so that excellent surface hardness, impact resistance, and flex resistance can be easily achieved after the formation of a cured layer. Among the above, it is particularly preferable to have a structural unit represented by the following chemical formula (9).

[0032] [ka]

[0033] R6 is any group, and is not particularly limited, but is preferably -H, -Cl, or -F. The structural unit represented by the above chemical formula (9) preferably accounts for 20 to 100 mol %, more preferably 50 to 100 mol %, and even more preferably 80 to 100 mol % of the aromatic nitrogen-containing polymer constituting the film of the present invention.

[0034] Regarding the method for producing a film of the present invention, an aromatic polyamide film will be exemplified below, but the present invention is not limited thereto.

[0035] For example, aromatic polyamides can be synthesized from acid chlorides and diamines by solution polymerization in aprotic organic polar solvents such as N-methylpyrrolidone (NMP), dimethylacetamide (DMAc), and dimethylformamide (DMF), or by interfacial polymerization using an aqueous medium. When acid chlorides and diamines are used as monomers in a polymer solution, hydrogen chloride is by-produced. To neutralize this, inorganic neutralizers such as calcium hydroxide, calcium carbonate, and lithium carbonate, or organic neutralizers such as ethylene oxide, propylene oxide, ammonia, triethylamine, triethanolamine, and diethanolamine, are used. Furthermore, aromatic polyamides can be synthesized by reacting isocyanates with carboxylic acids in aprotic organic polar solvents in the presence of a catalyst.

[0036] These polymer solutions may be used as they are as a membrane-forming solution, or the polymer may be isolated and then redissolved in the organic solvents mentioned above or an inorganic solvent such as sulfuric acid to prepare a membrane-forming solution.

[0037] To obtain the aromatic polyamide film of the present invention, the intrinsic viscosity ηjnh of the polymer (measured at 30°C by dissolving 0.5 g of the polymer in 100 ml of 98% sulfuric acid) is preferably 0.5 (dl / g) or more.

[0038] The particles can be added by first forming a sufficient slurry in a solvent and then using the resulting slurry as a polymerization solvent or dilution solvent, or by preparing a membrane-forming solution and then adding the slurry directly to the solution. Inorganic salts such as calcium chloride, magnesium chloride, lithium chloride, and lithium nitrate may be added to the membrane-forming solution as a dissolution aid. The neutralized polymer solution may be used as is, or the polymer may be isolated and then redissolved in a solvent. As the solvent, organic polar solvents such as N-methyl-2-pyrrolidone, dimethylacetamide, and dimethylformamide are preferred because of their ease of handling, but strong acidic solvents such as concentrated sulfuric acid, concentrated nitric acid, and polyphosphoric acid may also be used. The polymer concentration in the membrane-forming solution is preferably about 2 to 40% by mass.

[0039] The film-forming solution prepared as described above is formed into a film by a so-called solution film-forming method, which includes a dry-wet method, a dry method, and a wet method. When inorganic salts are contained in the polymer solution, a wet process is required to extract the salts, and therefore the dry-wet method is used.

[0040] When forming a membrane using a dry / wet process, the raw solution is extruded from a die onto an endless belt support to form a sheet. The solvent is then evaporated from the sheet, and the sheet is dried until it reaches a polymer concentration (PC) of 35 to 60% by mass, making the sheet self-supporting. After the dry process, the sheet is cooled and then peeled off from the support and introduced into a wet bath for the subsequent wet process, where it is desalted and desolvated. The wet bath composition is not particularly limited as long as it is a poor solvent for the polymer, but water or a mixture of organic solvent and water can be used. In this case, to reduce impurities in the sheet, the organic solvent / water mixture composition ratio (the following values ​​are based on mass) is organic solvent / water = 70 / 30 to 0 / 100, preferably 60 / 40 to 30 / 70, and the bath temperature is preferably 40°C or higher. The wet bath may contain inorganic salts, but it is preferable to ultimately extract the solvent and inorganic salts contained in the sheet with a large amount of water.

[0041] The sheet that has passed through the wet process is then dried and heat-treated in a tenter. It is preferable to dry the sheet until the solvent content in the sheet is less than 5% by mass before heat-treating it in the tenter. If the sheet enters the heat-treating process with a solvent content of 5% by mass or more, uneven heat treatment is likely to occur, resulting in uneven physical properties in the film width direction and making the sheet more susceptible to breakage during the stretching process. A lower solvent content is more preferable, and it is desirable to completely dry the sheet to nearly 0% by mass.

[0042] The sheet formed as described above is stretched during the wet process to improve the mechanical properties and dimensional stability of the sheet during the heat treatment step. The stretching can be performed by first stretching the sheet in the longitudinal direction and then in the width direction, or by sequential biaxial stretching, in which stretching is performed first in the width direction and then in the longitudinal direction, or by simultaneous biaxial stretching, in which stretching is performed simultaneously in the longitudinal and width directions. These stretching methods are well known as stretching methods in melt film-forming, which is used to produce films of polyethylene, polypropylene, polyester, etc. However, in the case of films obtained by solution film-forming as in the present invention, the sheet contains solvents and wet bath components, and the process involves migration of these components outside the sheet. Therefore, it is preferable to use the method described below to obtain the desired film.

[0043] As a stretching method, sequential biaxial stretching is preferred from the viewpoints of equipment and operability. While appropriate stretching conditions must be selected depending on factors such as the polymer composition, a stretch ratio of 1.0 to 1.5 times in the longitudinal direction of the sheet and 1.2 to 2.0 times in the transverse direction is preferred to achieve the desired dimensional stability. Furthermore, transverse stretching is preferably performed at a temperature of 270°C or higher for 3 seconds or longer, and more preferably at a temperature of 270 to 320°C for 3 seconds or longer. If the transverse stretching temperature is lower than 270°C, crystallization may be insufficient, and sufficient dimensional properties such as moisture absorption and tensile strength may not be achieved. If the stretching temperature exceeds 320°C, the degree of crystallization increases too much, making the film brittle and prone to tearing. Furthermore, the solvent content in the sheet during stretching is preferably less than 5% by mass. It is effective to slowly cool the film after stretching or heat treatment, and cooling at a rate of 50° C. / second or less is effective in reducing the thermal shrinkage rate.

[0044] The aromatic polyamide film of the present invention may be a single-layer film or a laminated film. For example, in the case of a two-layer film, one example is to divide the polymerized aromatic polyamide solution into two parts, add different particles to each part, and then laminate them. The same applies to three or more layers. Examples of these lamination methods include lamination in a die, lamination in a composite pipe, and a method in which one layer is first formed and then another layer is formed on top of it.

[0045] It is important that at least one surface of the aromatic polyamide film of the present invention has a ratio (P / K) of the peak intensity (P) of fragments derived from polydimethylsiloxane to the peak intensity (K) of fragments derived from the aromatic polyamide basic skeleton, as measured by mass spectrometry, of 0.20 or less, more preferably 0.15 or less.

[0046] If the ratio (P / K) exceeds 0.20, when the coating liquid is applied to an aromatic polyamide film, which is a non-magnetic support, during the process of manufacturing a magnetic tape, circular coating irregularities with a major axis of 1 mm or more that can be seen by light transmission may occur.

[0047] These circular coating irregularities with a major axis of 1 mm or more may cause wear on the drive head when used as a magnetic recording medium.

[0048] A method for making at least one surface of the aromatic polyamide film of the present invention such that the ratio (P / K) of the peak intensity (P) of fragments derived from polydimethylsiloxane to the peak intensity (K) of fragments derived from the aromatic polyamide basic skeleton as measured by a mass spectrometer is 0.20 or less is preferably performed by stretching the film in the running direction of the film using the difference in peripheral speed between multiple stretching rolls, the stretching rolls comprising nip rolls with a silicone rubber coating on a core metal that presses the film, and the nip rolls are rolls from which polydimethylsiloxane has been removed.

[0049] A method for removing polydimethylsiloxane from the surface of the nip roll preferably includes a step of performing a heat treatment so that most of the polydimethylsiloxane contained inside the coated silicone rubber precipitates on the surface, and then removing the polydimethylsiloxane precipitated on the surface of the nip roll.

[0050] A preferred method for heat-treating a nip roll coated with silicone rubber is to keep the ambient temperature T at 150°C or higher and lower than 300°C, and to continue heating at temperature T for a treatment time t of 10 hours or higher and 20 hours or lower after the average temperature in the thickness direction of the silicone rubber at the center of the width direction of the nip roll reaches temperature T.

[0051] If the temperature T is less than 150°C, it will take a long time for the polydimethylsiloxane inside the silicone rubber to precipitate on the surface, which is not economically preferable.

[0052] Furthermore, if the temperature T exceeds 300°C, the silicone rubber may thermally deteriorate, making the film more susceptible to scratches.

[0053] The treatment time t is preferably 10 hours or more and 20 hours or less. If the treatment time t is less than 10 hours, the polydimethylsiloxane inside the silicone rubber will not be able to completely precipitate on the surface, and when used as a stretching nip roll, the polydimethylsiloxane may adhere to the surface of the aromatic polyamide film, causing the ratio (P / K) to exceed 0.20.

[0054] If the time t exceeds 20 hours, the silicone rubber may be thermally deteriorated, and the film may become more susceptible to scratches.

[0055] The method for removing polydimethylsiloxane precipitated on the surface of the nip roll is not particularly limited, and examples thereof include a method of polishing the surface with sandpaper, a method of wiping the surface of the nip roll with a cloth such as nonwoven fabric or gauze, and a method of washing the surface of the nip roll with water or a chemical solution such as ethanol. [Example]

[0056] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0057] (Peak intensity (K) of fragments derived from the aromatic polyamide basic skeleton, Peak intensity (P) of fragments derived from polydimethylsiloxane) The aromatic polyamide film was cut into A4 size pieces and used as samples. The polydimethylsiloxane peak intensity on the sample surface was measured using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The fragment peak (C6H3Cl) representing the basic structure of the aromatic polyamide film was + (Mass 110), C7H3OCl + (Mass138), C 14 H6O2Cl2 + (Mass 276)) and the fragment Si(CH3)3 indicating the presence of polydimethylsiloxane. + The ratio (P / K) of the peak intensity (P) of (Mass73) was measured at 10 random points, and the average value was evaluated as the polydimethylsiloxane peak intensity. Equipment: Physical Electronics TFS-2000 Primary ion: 69Ga+ Measurement area (raster size): 180 μm square Measurement time: 3 minutes Measurement vacuum degree; 4×10 -7 Pa Polydimethylsiloxane peak intensity (P / K) = Polydimethylsiloxane (Si(CH3)3 + (Mass73)) peak intensity (counts) / peak intensity (counts) representing the skeleton of aromatic polyamide film.

[0058] (Uneven coating) A non-magnetic coating material having the following composition was applied to an aromatic polyamide film to a dry thickness of 0.8 μm and dried at 100°C. Immediately thereafter, a magnetic coating material having the following composition was applied to a dry thickness of 0.08 μm and dried at 100°C. While the magnetic layer was still wet, it was passed through an AC magnetic field generator for orientation treatment. The backcoat coating material described above was applied to the side of the support opposite the non-magnetic underlayer and magnetic layer to a dry thickness of 0.5 μm and dried to obtain a magnetic recording laminate raw roll. The resulting magnetic recording laminate raw roll was run through a heat treatment zone at 130°C under a tension of 3.0 kg / m (residence time in the heat treatment zone: 15 seconds) to produce a tape. Coating unevenness of the tape was evaluated according to the following criteria. ×: 30 or more circular coating irregularities with a major diameter of 1 mm or more per 10 cm square that can be seen by light transmission. ○: The number of circular coating irregularities visible through light transmission, with a major axis of 1 mm or more, is less than 30 per 10 cm square.

[0059] (Preparation of upper layer magnetic coating liquid) Ferromagnetic plate-like hexagonal ferrite powder 100 parts by mass Polyurethane resin 15 parts by mass Phenylphosphonic acid 3 parts by mass α-Al2O3 (particle size 0.15μm) 5 parts by mass Plate-shaped alumina powder (average particle size: 50 nm) 1 part by mass Carbon black (particle size 20 nm) 2 parts by mass Cyclohexanone 110 parts by mass Methyl ethyl ketone 100 parts by mass Toluene 100 parts by mass Butyl stearate 2 parts by mass 1 part by mass of stearic acid.

[0060] (Preparation of non-magnetic coating liquid for lower layer) Non-magnetic inorganic powder 85 parts by mass α-iron oxide, surface treatment agent: Al2O3, SiO2 Carbon black 20 parts by mass Polyurethane resin 15 parts by mass Phenylphosphonic acid 3 parts by mass α-Al2O3 (average particle size 0.2μm) 5 parts by mass Cyclohexanone 140 parts by mass Methyl ethyl ketone 170 parts by mass Butyl stearate 2 parts by mass Stearic acid 1 part by mass.

[0061] (Adjustment of backcoat coating solution) Carbon black (average particle size: 25 nm) 40.5 parts by mass Carbon black (average particle size: 370 nm) 0.5 parts by mass Barium sulfate 4.05 parts by mass Nitrocellulose 28 parts by weight Polyurethane resin (containing SO3Na groups) 20 parts by mass Cyclohexanone 100 parts by mass Toluene 100 parts by mass Methyl ethyl ketone 100 parts by mass.

[0062] (Aromatic polyamide solution A) 85 mol % of 2-chloroparaphenylenediamine (CPA) and 15 mol % of diphenyl ether (DPE) were dissolved in N-methyl-2-pyrrolidone (NMP). The solution was filtered through a polypropylene filter with a filtration accuracy of 1.0 μm and then transferred to a polymerization tank. 98.5 mol % of chloroterephthalic acid chloride (CTPC), which had also been filtered through a polypropylene filter with a filtration accuracy of 1.0 μm, was added to the solution. Prior to polymerization, silica particles with an average particle size of 80 nm and organic particles with an average particle size of 100 nm were added so that the total amount was 0.02 mass % relative to the aromatic diamine component, and 6.0 mass % relative to the aromatic diamine component. The mixture was stirred at 30°C or below for 2 hours to obtain a polymer.

[0063] Next, 98.5 mol% of lithium carbonate was added relative to the hydrogen chloride in the polymer, and neutralization was carried out for 4 hours. Then, 10 mol% of triethanolamine relative to the hydrogen chloride in the polymer was added, and the mixture was stirred for 1 hour, to obtain an aromatic polyamide solution A with a polymer concentration of 10.8% by mass.

[0064] (Aromatic polyamide solution B) CPA equivalent to 85 mol % and DPE equivalent to 15 mol % were dissolved in NMP, and the solution was filtered through a polypropylene filter with a filtration accuracy of 1.0 μm and then transferred to a polymerization tank. CTPC equivalent to 98.5 mol % that had been passed through a polypropylene filter with a filtration accuracy of 1.0 μm was added to the solution, and the mixture was stirred at 30°C or below for 2 hours to obtain a polymer.

[0065] Next, 98.5 mol% of lithium carbonate was added to the hydrogen chloride in the polymer, and neutralization was carried out for 4 hours. Then, 1.6 mass% of silica having an average primary particle size of 16 nm ("AEROSIL" R972 type manufactured by Nippon Aerosil Co., Ltd.) was added to the polymer, and the mixture was stirred for 1 hour. After that, 10 mol% of triethanolamine was added to the hydrogen chloride in the polymer, and the mixture was stirred for 1 hour, to obtain aromatic polyamide solution B having a polymer concentration of 10.8 mass%.

[0066] (Heat treatment of silicone rubber coated nip roll) A nip roll with a core metal coated with silicone rubber is placed in a device that can heat the roll at an ambient temperature T for a specified time.

[0067] The temperature inside the device and the nip rolls are raised to atmospheric temperature T with hot air, and after the average temperature in the thickness direction of the silicone rubber at the center of the width of the nip rolls reaches temperature T, heating is continued for a treatment time t.

[0068] Here, when measuring the average temperature in the thickness direction of the silicone rubber at the center of the width direction of the nip roll, it was calculated from the surface temperatures of the core metal and silicone rubber of the nip roll, the thickness of the silicone rubber, the size of the core metal, and the thermal conductivity, density, and specific heat of each of the core metal and silicone rubber.

[0069] Example 1 Aromatic polyamide solution A was passed through a stainless steel metal fiber filter with a filtration accuracy of 1.2 μm, and aromatic polyamide solution B was passed through a stainless steel metal fiber filter with a filtration accuracy of 5.0 μm, and then the two were laminated inside the die. The lamination ratio (thickness ratio) of aromatic polyamide solutions A and B was 50% each. Next, the laminated polymer solution was cast onto a stainless steel belt with a mirror-finish surface, and heated with hot air at 180°C for 2 minutes on the belt downstream of the die in the running direction to evaporate the solvent, and the self-supporting film was continuously peeled off from the belt. The lamination was performed so that aromatic polyamide solution B was the layer in contact with the stainless steel belt.

[0070] Next, the membrane was introduced into the tank of a solvent extraction treatment device by pressing and stretching the membrane with a nip roll coated with silicone rubber that had been heat-treated at a temperature of T 200°C for a specified time of t 20 hours, and stretched to 1.15 times in the MD direction while water-extracting the remaining solvent and inorganic salts produced by neutralization.

[0071] The obtained film was then held by the clips of a transverse stretching machine and stretched transversely by 1.70 times while being stretched with hot air for 3 seconds so that the stretching temperature reached 280°C, and then further heat-treated with hot air for 3 seconds at a relaxation rate of 3% so that the heat treatment temperature reached 200°C.

[0072] The film was cooled slowly at a rate of 20°C / sec, the film edges were removed, and the film was wound around a core with an oscillation of 60 mm to obtain an aromatic polyamide film roll with an average thickness of 3.0 µm.

[0073] The results for the aromatic polyamide film roll are shown in Table 1.

[0074] (Examples 2 to 5, Comparative Example 1) An aromatic polyamide film roll having an average thickness of 3.0 μm was obtained in the same manner as in Example 1, except that the temperature T of the silicone rubber-coated nip roll and the specified time t were changed to the conditions shown in Table 1. The results of the aromatic polyamide film roll are shown in Table 1.

[0075] In Comparative Examples 2 and 3, the P / K was 0.20 or less and the coating unevenness was rated as "good", but scratches were caused by the silicone rubber nip roll. This was due to the silicone rubber deteriorating due to the heat treatment.

[0076] [Table 1]

Claims

1. An aromatic polyamide film used as a base for a magnetic recording material tape, in which at least one surface has a ratio (P / K) of the peak intensity (P) of fragments derived from polydimethylsiloxane to the peak intensity (K) of fragments derived from the aromatic polyamide basic skeleton, as measured by mass spectrometry, of 0.20 or less.

2. A method for producing an aromatic polyamide film, comprising a step of stretching a film in the running direction of the film by using the difference in peripheral speed between a plurality of stretching rolls in solution casting, wherein the stretching rolls comprise nip rolls having a core metal coated with silicone rubber that presses the film, and the nip rolls are nip rolls from which polydimethylsiloxane on the nip roll surface has been removed after an ambient temperature T of 150°C or more and less than 300°C has been reached, and a treatment time t of 10 hours or more and 20 hours or less has been reached after the average temperature in the thickness direction of the silicone rubber laminated on the roll surface, measured at the center of the nip roll in the roll width direction, has reached temperature T.

3. 3. The method for producing an aromatic polyamide film according to claim 2, wherein the removal of polydimethylsiloxane from the surface of the nip roll comprises a step of heat-treating the nip roll and then polishing the surface of the nip roll to remove polydimethylsiloxane precipitated on the surface of the nip roll.

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