JP2014117905A - Polyester film for coating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film for coating capable of forming good cupping when coating is formed.SOLUTION: A polyester film for coating has a layer structure with 2 or more layers, and both WA and WB of over 0 mass% and 15 mass% or less and WB-WA of over 1.3 mass% to 10 mass% or less, where WA is the content of a heat-resistant thermoplastic resin contained in a part from a center to one side of a surface, A side in a thickness direction and WB is the content of the heat-resistant thermoplastic resin contained is a part from the center to another side of the surface, B side in the thickness direction, and the cupping in a width direction has a recess shape in the surface A side when heat treated at temperature in a range of 100 to 120°C, and the amount of cupping in the width direction, d (mm) is in the range of 0.0 mm≤d≤0.5 mm with 50 mm width.

Description

  The present invention relates to a polyester film for coating.

  In recent years, magnetic recording media for data storage, digital video tape, and the like have been increasing in density and capacity. For example, linear recording magnetic recording media such as LTO (Linear Tape Open) and SDLT (Super Digital Linear Tape) have been developed that have a high capacity of 500 GB or more per roll.

  Many studies have been made to increase the capacity, such as increasing the strength of the base film by increasing the draw ratio, optimizing the temperature expansion coefficient and humidity expansion coefficient in the tape width direction, and reducing the additive particle size. It was. However, even if these techniques are used, sufficient characteristics cannot be obtained for a magnetic recording medium having a high capacity of 500 GB or more per roll.

  In these magnetic recording media, the film surface is subjected to heat treatment (Patent Document 1) or the constituent components of the magnetic layer and the back coat layer are controlled (Patent Document 2) so that the medium surface can easily hit the head. Control is performed so that the tape-like magnetic recording surface is convexly curved in the width direction. As a result, good contact between the running tape and the magnetic head can be obtained, and characteristics such as the SN ratio can be improved. Such bending in the width direction is called cupping, and when the magnetic recording surface is convexly curved in the width direction when viewed from the contact surface of the magnetic head, it is called-(minus) cupping, and the magnetic recording surface is in the width direction. Those that are concavely curved are called + (plus) cupping. As described above, in order to obtain good contact between the running tape and the magnetic head, it is preferable to use a magnetic recording medium having -cupping. The back coat layer is provided on the surface opposite to the magnetic layer, and plays a role of mainly improving the running property of the tape. This layer is a coating film formed by applying a coating material for forming a backcoat layer on the surface opposite to the surface on which the magnetic layer is provided and drying it.

  This also applies to a thermal ink ribbon used in a thermal printer. With a thermal ink ribbon, an ink coating that melts by heat is formed on one surface of a tape-shaped film, and the ink coating melts by the heat of the thermal head that comes into contact with the other surface of the film and is transferred to recording paper. Is done. This ink coating film is formed by applying a coating material for forming an ink coating film on the surface of the film and drying it, as in the case of the back coat layer. Even in this case, as described above, it is desirable to obtain good contact between the running tape and the thermal head, and as a result, the surface of the thermal ink ribbon is viewed from the contact surface of the thermal head. It is preferable to be convexly curved in the width direction.

JP 2001-325723 A JP 2005-259287 A

  In the process of forming a coating film such as a backcoat layer on a magnetic recording medium or an ink coating film on a thermal ink ribbon, it is generally applied when a coating material for forming these layers is applied and dried. The coated film shrinks as it dries. And these coating films are provided in the surface on the opposite side to the surface which a magnetic head and a thermal head contact. Therefore, the tape-like film provided with such a coating film will be convexly curved in the width direction when viewed from the contact surface of the magnetic head or the thermal head, and as described above, it will have a favorable cupping at first glance. It is. However, the shrinkage due to the drying of the coating film is large, and in many cases, the convex curve becomes extremely large, and the contact between the tape-shaped film and the head may worsen. Many.

  This invention is made | formed in view of the above condition, and it aims at providing the film for coating which can form favorable cupping when a coating film is formed.

  As a result of intensive studies to solve the above problems, the present inventors have two or more layers, and the heat-resistant heat contained in a portion from the center in the thickness direction to one surface A side. When the content of the plastic resin is WA and the content of the heat-resistant thermoplastic resin contained in the portion from the center in the thickness direction to the other surface B side is WB, both WA and WB exceed 0% by mass. When using a polyester film that is 15% by mass or less and WB-WA is more than 1.3% by mass and 10% by mass or less, the polyester film is on the surface A side due to heat given when the coating film is dried. It is curved in the width direction by showing different heat shrinkage between the layer and the layer on the surface B side, and a moderate cupping can be obtained by canceling a part of the excessive convex curve accompanying the drying shrinkage of the coating film. Heading, the following The has been completed.

  That is, the present invention has a layer structure of two or more layers, and the content of the heat-resistant thermoplastic resin contained in a portion from the center in the thickness direction to one surface A side is defined as WA, and the center in the thickness direction. When the content of the heat-resistant thermoplastic resin contained in the portion from the other surface B to WB is WB, both WA and WB are more than 0% by mass and 15% by mass or less, and WB-WA is 1.3%. It is a polyester film for coating that is more than 10% by mass and less than 10% by mass, and the cupping in the width direction when heat-treated at a temperature in the range of 100 to 120 ° C. is concave on the surface A side. The cupping amount d (mm) is a polyester film for coating having a width of 50 mm and a range of 0.0 mm ≦ d ≦ 0.5 mm.

  In the coating polyester film, the coating surface is the surface B side, or the coating surface is both the surface A side and the surface B side, and the drying shrinkage of the coating material applied to the surface B side is large. Is preferred.

  The heat-resistant thermoplastic resin is preferably a polyetherimide.

  The coating polyester film is preferably used as a magnetic recording medium support or a thermal ink ribbon support.

  ADVANTAGE OF THE INVENTION According to this invention, the polyester film for a coating which can form favorable cupping when a coating film is formed is provided.

  Hereinafter, an embodiment of the polyester film for coating of the present invention will be described.

  The polyester film for coating of the present invention (hereinafter also simply referred to as “the film of the present invention”) has a step of forming a coating film by drying a coating material applied on a support in the production process. It is preferably used as a support for products such as magnetic recording media and thermal ink ribbons. The film of the present invention has a structure of at least two layers, and the shrinkage ratios of these layers with respect to heating are different from each other. Therefore, if the coating material is applied to a convex surface by heating among the surfaces of the film of the present invention, the drying shrinkage of the coating material is caused by the curvature of the film accompanying heating in the heating step for drying the coating material. As a result, good cupping can be obtained.

  The film of the present invention has a layer structure including at least two layers containing polyester. In the film of the present invention, the content of the heat-resistant thermoplastic resin contained in a portion from the center (center position) to one surface A side in the thickness direction of the film is WA, and the other from the center. When the content of the heat-resistant thermoplastic resin contained in the portion up to the surface B side is WB, both WA and WB are more than 0% by mass and 15% by mass or less, and WB-WA is 1.3% by mass. It is a polyester film which exceeds 10 mass%.

  In the film of the present invention, a method for adjusting the shrinkage ratio of each layer with respect to heat is to make the concentrations of heat-resistant thermoplastic resins contained in each layer different from each other. Originally, the polyester contained in the film of the present invention has large shrinkage due to heating, but as the concentration of the heat-resistant thermoplastic resin contained therein increases, the heat resistance improves and shrinkage due to heating decreases. Therefore, if the concentration of the heat-resistant thermoplastic resin contained in each layer constituting the film of the present invention is different from each other, the thermal shrinkage rate of each layer will be different, and when the film is heated, in the width direction Will be curved. In the film of the present invention, as described above, since WB-WA is positive, the concentration of the heat-resistant thermoplastic resin in the layer on the surface B side is larger than the concentration of the heat-resistant thermoplastic resin in the layer on the surface A side. That is, the heat shrinkage rate of the layer on the surface B side is smaller than the heat shrinkage rate of the layer on the surface A side. For this reason, the film of this invention will curve so that the surface A side may become concave when it receives a heating. Therefore, if the surface B side is a surface to be coated with the coating material, the coating material is dried and contracted, and even if the surface B side is concave (that is, the surface A side is convex), the curvature is the surface A side. Will be canceled by the curvature of the film itself having a concave shape, and as a result, the surface A side will be prevented from being excessively convex.

  From the above, in the film of the present invention, the coated surface on which the coating material is applied is preferably on the surface B side. As a result, during the heat treatment for drying the coating material, the drying shrinkage of the coating material that tends to make the surface A side excessively convex is counteracted somewhat by the curvature of the film itself, and as a result, the surface A side becomes a practical convex state. A good cupping can be obtained. For example, when the film of the present invention is used as a support for a thermal ink ribbon, in the thermal ink ribbon, the print head on the surface (surface A side) opposite to the coating surface (surface B side) on which the ink coating film is formed. Therefore, it is desirable that the surface A side is a practical convex state.

  For the same reason, in the film of the present invention, when the coating surface is both the surface A side and the surface B side, the drying shrinkage of the coating material applied to the surface B side is applied to the surface A side. A coating material that is larger than the drying shrinkage of the coating material is selected. In this case as well, during the heat treatment for drying the coating material, the drying shrinkage of the coating material tends to make the surface A side excessively convex, but this action is somewhat counteracted by the curvature of the film itself, resulting in the surface A side being It is possible to obtain a good cupping that becomes a practical convex state. For example, when the film of the present invention is used as a support for a magnetic recording medium, in the magnetic recording medium, a backcoat layer having a large drying shrinkage is formed on the surface B side, and a magnetic recording layer having a smaller drying shrinkage is formed on the surface A. Formed on the side. Similarly, even when the magnetic recording layer is formed by vapor deposition instead of application of a coating material, the contraction of the magnetic recording layer is small. As a result, the surface A side in contact with the recording head is preferably in a practical convex state by the same mechanism as described above.

  WA and WB are 15 mass% or less. When WA and WB are 15 mass% or less, it is preferable at the point from which favorable film forming property is obtained. In addition, since film forming property becomes so favorable that the density | concentration of a heat resistant thermoplastic resin is small, from such a viewpoint, it is preferable that WA is 0-8 mass%, and is 0-4 mass%. It is more preferable. Moreover, it is preferable that WB is 0.5-10 mass%, and it is more preferable that it is 0.5-5 mass%.

  WB-WA is more than 1.3 mass% and 10 mass% or less. When WB-WA exceeds 1.3% by mass, the effect of canceling cupping by the coating material is sufficiently obtained, and when WB-WA is 10% by mass or less, cupping by the coating material is excessively canceled. It is suppressed that the surface A side becomes concave. WB-WA is more preferably more than 1.3% by mass and not more than 5% by mass.

  In the film of the present invention, the cupping in the width direction when heat-treated in the temperature range of 100 to 120 ° C. is concave on the surface A side, and the cupping amount d (mm) in the width direction is 50 mm width. In the range of 0.0 mm ≦ d ≦ 0.5 mm. As already described, the drying shrinkage of the coating material applied to the surface B side has an effect of making the surface A side excessively convex. Therefore, if the cupping of the film itself is a concave shape on the surface A side, this effect Is somewhat canceled out, and the surface A side becomes a practical convex state. If the cupping amount of the film itself is in the above range, the action due to drying shrinkage is excessively canceled and the surface A side becomes concave, or the action due to drying shrinkage is insufficiently canceled and the surface A side is still excessively convex. Since it is suppressed that it remains in a state, it is preferable. The cupping amount d (mm) in the width direction, which has a dent shape on the surface A side, is more preferably 0.0 mm ≦ d ≦ 0.3 mm with a width of 50 mm. The cupping amount d (mm) is the depth (or height) of the valley (or mountain) portion when the heights at both ends in the width direction are used as a reference.

  The polyester that can be used in the present invention is not particularly limited as long as it is a polyester that becomes a high-strength film by molecular orientation, but polyethylene terephthalate or polyethylene-2,6-naphthalate is preferably a constituent. Examples of the polyester copolymer component other than ethylene terephthalate include diol components such as diethylene glycol, propylene glycol, neopentyl glycol, polyethylene glycol, p-xylylene glycol, and 1,4-cyclohexanedimethanol, adipic acid, sebacic acid, Examples thereof include dicarboxylic acid components such as phthalic acid, isophthalic acid and 5-sodium sulfoisophthalic acid, polyfunctional carboxylic acid components such as trimellitic acid and pyromellitic acid, and p-oxyethoxybenzoic acid.

  The heat-resistant thermoplastic resin in the present invention refers to a thermoplastic resin having a Tg higher than the glass transition temperature (Tg) of the polyester used. Preferably, any thermoplastic resin having melt moldability and compatibility with polyester may be used. Examples of such heat-resistant thermoplastic resins include polyimide resins (including polyetherimide), polysulfone, polyethersulfone, polyamideimide, polyetheretherketone, and polyarylate. Among these, from the viewpoint of affinity with polyester and melt moldability, polyimide resins, particularly polyetherimide, are preferably exemplified.

  As already described, the film of the present invention has a laminated structure of two or more layers. As described above, this is indispensable for canceling the drying shrinkage of the coating material and controlling cupping, and when used as a support for a magnetic recording medium, it has excellent electromagnetic conversion characteristics on one surface. This is because it is easy to achieve both smoothness to obtain the appropriate roughness for imparting excellent transportability in film formation and processing steps and tape runnability to the other surface.

  As described above, when the film of the present invention is used as a support for a magnetic recording medium, the surface A is a surface on the side where the magnetic layer is provided, and the surface B on the opposite side is provided with a backcoat layer or the like. The surface is the surface side.

  When the magnetic layer is provided on the surface A side, the polyester film preferably has a surface roughness SRa of 0.5 to 10 nm. If the SRa of the surface A is 0.5 nm or more, it is preferable because process troubles associated with an increase in the coefficient of friction with a transport roll or the like are suppressed in film production, processing steps, and the like. Further, SRa of 10 nm or less is preferable because it can maintain appropriate electromagnetic conversion characteristics when used as a magnetic tape for high-density recording. When the magnetic layer is provided on the surface A side, the lower limit of SRa is more preferably 1 nm, still more preferably 2 nm, and the upper limit of SRa is more preferably 9 nm, still more preferably 8 nm.

  On the other hand, when the back coat layer is provided on the surface B side, the surface roughness SRa is preferably 2 to 30 nm. If the SRa of the surface B is 2 nm or more, the friction coefficient with the transport roll or the like is reduced, and the occurrence of process trouble is suppressed, which is preferable. Moreover, if SRa is 30 nm or less, when storing as a film roll or a pancake, it is preferable because a decrease in electromagnetic conversion characteristics accompanying transfer of surface protrusions to the opposite surface can be suppressed. When the backcoat layer is provided on the surface B side, the lower limit of SRa is more preferably 3 nm, still more preferably 4 nm, and the upper limit of SRa is more preferably 20 nm, still more preferably 15 nm.

  In order to make SRa within the above range, it is preferable to add inert particles in the layer. In the present invention, when the inert particles I are used in the layer A constituting the surface A, the average particle diameter dI is 0. 0.04 to 0.30 μm is preferable, 0.05 to 0.15 μm is more preferable, and the content is preferably 0.001 to 0.30 mass%, and 0.01 to More preferably, it is 0.25 mass%. In the magnetic recording medium support, it is preferable to use particles having an average particle size of 0.30 μm or less because deterioration of electromagnetic characteristics can be suppressed. In general, the smaller the average particle size and the added amount of the inert particles added, the smaller the SRa, and the larger the average particle size and added amount of the added inert particles, the larger the SRa.

  When the layer B constituting the surface B contains particles, the particles may be one type or two or more types. When the inert particle II having the largest average particle diameter contained in the layer B is defined as the inert particle II, the average particle diameter dII is preferably 0.1 to 1.0 μm, and 0.4 to 0.9 μm. More preferably, the content is 0.002 to 0.10% by mass, and more preferably 0.005 to 0.05% by mass. When the layer B further contains the inert particles III, the average particle diameter dIII is smaller than dII, and dIII is preferably 0.05 to 0.5 μm, and preferably 0.2 to 0.4 μm. Is more preferable, and the content is preferably 0.1 to 1.0% by mass, and more preferably 0.2 to 0.4% by mass.

  In the film of the present invention, layer A and layer B are the outermost layers constituting surface A and surface B, respectively, but the film of the present invention has layer C, layer D, etc. between these two layers. Three, four, or more configurations may be used.

  The inert particles added to the film of the present invention include inorganic particles such as spherical silica, aluminum silicate, titanium dioxide, and calcium carbonate, crosslinked polystyrene particles, crosslinked silicone resin particles, crosslinked acrylic resin particles, and crosslinked styrene-acrylic resin. Preferable examples include organic polymer particles such as particles, crosslinked polyester particles, polyimide particles, and melamine resin particles. These can be used alone or in combination of two or more.

  The inert particles added to the film of the present invention preferably have a uniform particle size and particle distribution, and the volume shape factor f is preferably 0.3 to π / 6. More preferably, it is 4 to π / 6. The volume shape factor f is expressed by the following equation.

f = V / Dm ³
Here, V is the particle volume (μm ³ ), and Dm is the maximum diameter (μm) on the projection plane of the particles.

  The volume shape factor f is the maximum π / 6 (= 0.52) when the particle is a sphere. The inert particles are preferably subjected to filtration or the like to remove coarse particles and inclusions as necessary. Among the inert particles, spherical silica is excellent in monodispersibility, so that the formation of surface protrusions on the coating film can be easily controlled, and is preferably used in the present invention. Further, from the viewpoint of reinforcing the background, α-type alumina, γ-type alumina, δ-type alumina, and θ-type alumina having a primary particle size of 0.005 to 0.10 μm, preferably 0.01 to 0.05 μm, as necessary. In addition, inert particles selected from zirconia, silica, titanium particles, and the like may be added to the polyester film layer as long as surface protrusions do not occur.

  In the film of the present invention, the Young's modulus in the longitudinal direction is preferably 4 to 13 GPa. If the Young's modulus in the longitudinal direction is 4 GPa or more, the film is stretched in the longitudinal direction due to the tension in the longitudinal direction in the tape drive, and the recording track shift due to the shrinking of the film in the width direction due to this elongation deformation can be suppressed. preferable. The lower limit of the Young's modulus in the longitudinal direction is more preferably 4.3 GPa, and even more preferably 4.5 GPa. Moreover, it is preferable if the Young's modulus in the longitudinal direction is 13 GPa or less because edge damage can be suppressed by obtaining a sufficient Young's modulus. The upper limit of the Young's modulus in the longitudinal direction is more preferably 10 GPa, and even more preferably 8 GPa.

  In the film of the present invention, the Young's modulus in the width direction is preferably 4 to 13 GPa. A Young's modulus in the width direction of 4 GPa or more is preferable because edge damage can be suppressed. The lower limit of the Young's modulus in the width direction is more preferably 5 GPa, and even more preferably 6 GPa. Further, it is preferable that the Young's modulus in the width direction is 13 GPa or less because a sufficient Young's modulus in the longitudinal direction can be obtained. The upper limit of the Young's modulus in the width direction is preferably 11 GPa, more preferably 10 GPa.

  In addition, in this invention, a longitudinal direction is a direction generally called MD direction, Comprising: The same direction as the longitudinal direction at the time of a polyester film manufacturing process is pointed out. Moreover, in this invention, a width direction is a direction generally called a TD direction, Comprising: The same direction as the width direction at the time of a polyester film manufacturing process (direction orthogonal to MD direction) is pointed out.

  Moreover, it is preferable that the thickness of the film of this invention is 2-10 micrometers. It is preferable that the thickness of the film is 2 μm or more because the necessary strain as a magnetic recording medium can be obtained. The lower limit of the thickness of the film is preferably 3 μm, and more preferably 4 μm. In addition, since the film thickness is 10 μm or less, the tape length per tape can be sufficiently secured, which is preferable because the magnetic tape can be reduced in size and increased in capacity. The upper limit of the thickness of the film is preferably 8 μm, and more preferably 6 μm.

  In order to provide strength and dimensional stability required for the magnetic recording medium, metal deposition such as aluminum oxide may be performed on at least one surface of the film of the present invention. In a vapor deposition film, it is common to cause cupping due to a difference in expansion and contraction of the vapor deposition film when it is deposited and when it is cooled after vapor deposition. In addition, when a metal oxide such as aluminum oxide is deposited, the deposited film may change in size due to an oxidation reaction or a hydroxylation reaction that proceeds after the deposition, thereby causing cupping. Furthermore, when oxide vapor deposition is performed on both sides, it is necessary to control the film thickness and the degree of oxidation on the front and back sides. Since the film of the present invention shows cupping of the film itself by heat treatment after the coating material is applied, it is also useful for correcting cupping in such a metal-deposited polyester film.

  The film of the present invention is produced, for example, as follows. Hereinafter, examples in which polyethylene terephthalate (PET) is used as the polyester and polyether imide is used as the heat-resistant thermoplastic resin will be described as representative examples, but the present invention is not particularly limited thereto.

  As a method for incorporating the inert particles into the polyester, for example, the inert particles I are dispersed in a predetermined ratio in the form of a slurry in ethylene glycol, which is a diol component, and this ethylene glycol slurry is dispersed at any stage before the completion of polyester polymerization. It may be added. Here, when the particles are added, for example, when the water sol or alcohol sol obtained when the particles are synthesized is added at any stage before the completion of polyester polymerization without drying, the dispersibility of the particles is good. The slipperiness and electromagnetic conversion characteristics of the film can be improved. Another effective technique is to mix a water slurry of particles with a predetermined polyester pellet, supply the mixture to a vent type twin-screw kneading extruder, and knead it into the polyester. By these methods, polyester pellets containing particles are prepared.

  Polyester pellets substantially free of particles and polyetherimide pellets are mixed at a predetermined ratio, supplied to a vented twin-screw kneading extruder heated at 270 to 300 ° C., melt-extruded, Polyetherimide-containing polyester pellets are prepared.

  Prepared as described above, polyester pellets containing particles, polyetherimide-containing polyester pellets, and polyester pellets substantially free of particles, etc. are mixed at a predetermined ratio, dried, and then used for known melt lamination Feed to the extruder and filter the polymer through a filter.

  Also, in high-density magnetic recording medium applications where a very thin magnetic layer is applied, very small foreign matter can cause DO (dropout), which is a magnetic recording defect. It is effective to use a high-accuracy fiber sintered stainless steel filter that can collect 95% or more. Subsequently, the polymer is extruded into a sheet form from a slit-shaped slit die, and this polymer is cooled and solidified on a casting roll to obtain an unstretched film. That is, a plurality of extruders, a plurality of manifolds, or a merge block (for example, a merge block having a rectangular merge section) is used to stack the required number of layers, the sheet is extruded from the die, cooled by a casting roll, and unstretched film Get. In this case, it is effective to install a static mixer or a gear pump in the polymer flow channel from the viewpoint of stabilizing the back pressure and suppressing the thickness variation.

  Subsequently, the unstretched film is stretched biaxially in the longitudinal direction and the width direction and then heat treated. Although a extending process is not specifically limited, It is preferable to extend | stretch in 2 steps or more in each direction. That is, it is preferable to perform re-longitudinal and re-lateral stretching because it is easy to obtain a high-strength film optimum as a magnetic tape for high-density recording.

  The stretching method may be simultaneous biaxial stretching or sequential biaxial stretching. Since simultaneous biaxial stretching does not involve stretching by a roll, local heating of the film surface does not occur, the surface property is easy to control, and the stretching method is more preferable. In simultaneous biaxial stretching, an unstretched film is first stretched simultaneously in the longitudinal direction and the width direction at a stretching temperature of, for example, 80 to 160 ° C, preferably 85 to 130 ° C, more preferably 90 to 110 ° C. Setting the stretching temperature to 80 ° C. or higher is preferable because breakage of the film can be suppressed, and setting the stretching temperature to 160 ° C. or lower is preferable because sufficient strength can be obtained when used as a magnetic recording medium. Moreover, from the viewpoint of preventing stretching unevenness, the total stretching ratio in the longitudinal direction and the width direction is, for example, 8 to 30 times, preferably 9 to 25 times, and more preferably 10 to 20 times. It is preferable to set the draw ratio to 8 times or more because sufficient strength can be obtained for a high-density magnetic recording medium. Moreover, it is preferable to set the draw ratio to 30 times or less because the film can be prevented from being broken during the production process. From the viewpoint of obtaining the necessary strength for the high-density magnetic recording medium, the temperature is preferably 140 to 210 ° C., more preferably 160 to 200 ° C., and again in the longitudinal direction and / or the width direction, if necessary. The stretching is preferably performed at 1.05 to 1.8 times, more preferably 1.2 to 1.6 times. A stretching ratio of 1.05 times or more is preferable because sufficient strength can be obtained, and a stretching ratio of 1.8 times or less is preferable because the film can be prevented from being broken during the manufacturing process. Thereafter, heat setting is performed at 180 to 235 ° C., preferably 190 to 220 ° C., for example, for 0.5 to 20 seconds, preferably 1 to 15 seconds. It is preferable that the heat setting temperature is 180 ° C. or higher because the crystallization of the film proceeds and the structure can be stabilized. Further, by setting the heat setting temperature to 235 ° C. or lower, it is possible to suppress a decrease in Young's modulus accompanying the progress of relaxation of the polyester amorphous chain portion, which is preferable from the viewpoint of obtaining sufficient strength as a magnetic recording medium application. . Thereafter, a relaxation treatment of 0.5 to 7.0% is performed in the longitudinal direction and / or the width direction.

  In addition, since the film of this invention has a difference in content of the heat resistant thermoplastic resin in each layer of front and back, there is a difference in heat resistance of front and back. For this reason, the glass transition point of the layer containing a large amount of the heat-resistant thermoplastic resin is high, and this layer is apparently stretched at a low temperature, and the amorphous chain is likely to be tensed. For this reason, in a relaxation process, the layer containing many heat-resistant thermoplastic resins shrinks more, and as a result, the film after a relaxation process may have a cupping. However, this cupping is different from the cupping which is the subject of the present invention in that it is not the cupping generated by the heat treatment in the drying process of the coating material, and the cupping which is the subject of the present invention is the opposite direction. Of course, the film of the present invention may be a film that has been cupped after the relaxation treatment as described above, but the cupping produced by the heat treatment in the drying process of the coating material is within a predetermined range. I will add that.

  When the film of the present invention is used as a support for a coating film that causes drying shrinkage when a coating material is applied and thermally dried, such as a backcoat layer in a magnetic recording medium or an ink coating film in a thermal ink ribbon. By the cupping (curling in the width direction) generated by the film itself, it is possible to cancel the cupping excessively generated due to the drying shrinkage and to obtain an appropriate cupping. Therefore, the film of the present invention can be preferably used as a support such as a magnetic recording medium or a thermal ink ribbon.

[Methods for measuring physical properties and methods for evaluating effects]
The characteristic value measurement method and effect evaluation method in the present invention are as follows.

(1) Heat-resistant thermoplastic resin concentration WA and WB evaluation method Heat-resistant thermoplastic resin concentrations WA and WB are determined as mass average values according to the following formula. The following formula is for the case of a two-layer structure, but the same applies to the case of a multilayer stack such as a three-layer or four-layer structure.

  The thicknesses of the A layer and the B layer are TA and TB, and the heat-resistant thermoplastic resin concentrations in the A layer and the B layer are CA and CB. When the thickness of the A layer is the same as or larger than the thickness of the B layer (TA ≧ T / 2 ≧ TB), if the total thickness of the film is T,

  When the thickness of the B layer is larger than the thickness of the A layer (TB> T / 2> TA),

It is.

  The CA and CB evaluation methods are performed as follows after ashing and removing unnecessary layers by the plasma low-temperature ashing method.

Pretreatment: Freeze crushing, vacuum drying (room temperature, 2 hours)
Measurement method: Total nitrogen analysis by oxidative decomposition and reduced pressure chemiluminescence method
Vaporization and oxidation in a reaction furnace, and measurement of the generated nitric oxide was performed by chemiluminescence method. The quantification was performed by calculating the concentration using a calibration curve prepared in advance. In quantification, the measurement was performed three times, and the average value thereof was obtained. When quantification by total nitrogen analysis was difficult, quantification was performed by NMR (nuclear magnetic resonance method) of ¹ H nucleus.

(2) Cupping A strip having a width direction of 50 mm and a length direction of 300 mm was cut out, heated at 110 ° C. for 30 minutes, and stored for 10 minutes under the following conditions. The distance between the strip that is held vertically with no tension in the lengthwise direction and that connects both ends of the 50 mm width and a line that passes through the center of the strip (25 mm from both ends) and is parallel to the above line Was evaluated as cupping. The case where the A side was convex was evaluated as-(minus), and the case where the B side was convex was evaluated as + (plus).

Storage / environmental measurement: temperature 23 ° C, humidity 65% RH
Number of measurements: Measured 5 times and calculated from the average value Note that, as already mentioned, the film of the present invention self-capping by heat treatment, and mitigating -capping due to drying shrinkage of the applied coating material It is characterized by. In order to reduce the cupping due to the drying shrinkage of the coating material, it is desirable to have a cupping of 0.0 to +0.5 mm (that is, a cupping of 0.0 to 0.5 mm in which the A side is concave). Therefore, cupping was evaluated according to the following criteria.

A: Cupping is 0.0 mm or more and 0.3 mm or less B: Cupping is larger than 0.3 mm and 0.5 mm or less X: Cupping is smaller than 0.0 mm or larger than 0.5 mm (3) Three-dimensional surface Roughness (SRa)
Three-dimensional surface roughness SRa (central surface average roughness) using a three-dimensional fine shape measuring instrument (model ET-350K) and a three-dimensional surface roughness analysis system (model TDA-22) manufactured by Kosaka Laboratory. It was measured. Conditions were as follows, and an average value of five measurements was used as an evaluation value.

Stylus diameter: 2 μm
Load of stylus: 0.04 mN
Vertical magnification: 50,000 times Cutoff: 0.08mm
Feed pitch: 5μm
Measurement length: 0.5 mm
Measurement area: 0.2 mm ²
Measurement speed: 0.1 mm / second (4) Evaluation of Young's modulus Young's modulus was measured according to JIS-K7161 (1994). In the measurement, an Instron type tensile tester was used, and the conditions were as follows. The average value of five measurement results was defined as the Young's modulus in the present invention.

Sample size: width 10 mm x sample length 100 mm
Tensile strength: 200 mm / min Measurement environment: Temperature 23 ° C., humidity 65% RH
Number of measurements: Measured five times and calculated from the average value (5) Average particle size of particles The average particle size of the particles contained in the film is obtained by removing the polymer from the film by the plasma low-temperature ashing method and exposing the particles. It was measured. The treatment conditions were selected such that the polymer was ashed but the particles were not damaged as much as possible. The particles were observed with a scanning electron microscope (SEM), and the particle images were processed with an image analyzer. The SEM magnification was 20,000 times, and the volume average diameter d was obtained from the particle size and its volume fraction with the number of particles of 100 or more while changing the observation location by the following formula. When two or more kinds of particles having different particle sizes were included, the same measurement was performed for each particle to obtain the particle size.

d = Σ (di · Nvi)
Here, di is the particle size, and Nvi is the volume fraction. When the particles were significantly damaged by the plasma low-temperature ashing method, the film cross section was observed at 20,000 times with a transmission electron microscope (TEM). The section thickness of TEM was about 100 nm, the observation location was changed, and the number of particles of 100 or more was measured, and the volume average diameter d was determined from the above formula.

(6) Volumetric shape factor of particles With a scanning electron microscope, 10 views of the particles were photographed at a magnification of 20,000. Furthermore, the maximum diameter of the projection plane and the average volume of the particles were calculated using an image analysis processing apparatus, and a volume shape factor was obtained by the following formula.

f = V / Dm ³
Here, V is the average volume (μm ³ ) of the particles, and Dm is the maximum diameter (μm) of the projection plane.

(7) Film Lamination Thickness The film cross section was observed with a transmission electron microscope (TEM) at 20,000 times. The section thickness of TEM was about 100 nm, and the thickness of each layer was evaluated from the observation result of the interface based on the contained particle diameter and particle concentration. Moreover, when observation by the above was difficult, it evaluated using SIMS (secondary ion mass spectrometer). While etching from the surface, the depth profile of the element concentration resulting from the particles or the heat-resistant thermoplastic resin was measured, and the thickness of each layer was evaluated.

  Next, embodiments of the present invention will be described based on the following examples.

[Example 1]
Polyethylene terephthalate pellets (50 parts by mass) substantially free of particles and “Ultem” 1010 (hereinafter abbreviated as PEI) (50 parts by mass) manufactured by General Electric (GE) Co., heated in the same direction at 290 ° C. The pellets containing 50% by mass of PEI were prepared by feeding into a rotary type vented twin-screw kneading extruder.

  A polyethylene terephthalate pellet containing spherical silica particles having an average particle diameter of 0.10 μm and a volume shape factor f = 0.51, a pellet containing 50% by mass of PEI, and a polyethylene terephthalate pellet containing substantially no particles. The thermoplastic resin A was prepared by mixing so that the content of spherical silica particles was 0.2% by mass and the PEI concentration was 0.5% by mass.

  Further, polyethylene terephthalate containing divinylbenzene / styrene copolymer crosslinked particles having an average particle size of 0.3 μm and a volume shape factor f = 0.52, divinylbenzene having an average particle size of 0.8 μm and a volume shape factor f = 0.52 / Polyethylene terephthalate containing styrene copolymer crosslinked particles, pellets containing 50% by mass of PEI, and pellets of polyethylene terephthalate containing substantially no particles, the particle content of 0.3 μm is 0.26% by mass, and. A thermoplastic resin B was obtained by mixing so that the particle content of 8 μm was 0.01% by mass and the PEI amount was 6.1% by mass.

  Thermoplastic resin A and thermoplastic resin B are each dried under reduced pressure at 160 ° C. for 8 hours, then supplied to separate extruders, melt-extruded at 275 ° C. and filtered with high precision, and then merged at a rectangular two-layer merge block Stacked to form a two-layer stack. Thereafter, the film was wound around a casting drum having a surface temperature of 25 ° C. by using an electrostatic application casting method on a cooling roll through a slit die kept at 295 ° C., and solidified by cooling to obtain an unstretched laminated film. This unstretched laminated film was stretched 3.5 times in the longitudinal direction and the width direction at 95 ° C. by a linear motor type coaxial biaxial stretching machine, and 12.3 times in total, and then 1.2 times in the longitudinal direction at 190 ° C. The film was stretched 1.4 times in the width direction and heat treated at 205 ° C. for 3 seconds under a constant length. Thereafter, a relaxation treatment of 2% in the width direction was performed to obtain a polyester film having a total thickness of 4.3 μm, a thickness of layer B of 0.5 μm, WA = 0.5 mass%, and WB = 1.8 mass%. At this time, the Young's modulus in the longitudinal direction was 5 GPa, the Young's modulus in the width direction was 7 GPa, and the surface roughness SRa was 5 nm on the layer A side and 8 nm on the layer B side.

[Examples 2 to 5, Comparative Examples 1 to 3]
The polyester films of Examples 2 to 4 and Comparative Examples 1 and 2 were the same as Example 1 except that the thicknesses of Layer A and Layer B and the amount of heat-resistant thermoplastic resin added to each layer were changed. Got.

Claims (4)

  It has two or more layers, and the content of the heat-resistant thermoplastic resin contained in the portion from the center in the thickness direction to one surface A side is WA, and the other surface B side from the center in the thickness direction When the content of the heat-resistant thermoplastic resin contained in the parts up to WB is WB, both WA and WB are more than 0% by mass and 15% by mass or less, and WB-WA is more than 1.3% by mass and 10% by mass. %, The cupping in the width direction when heat-treated in the temperature range of 100 to 120 ° C. is concave on the surface A side, and the cupping amount in the width direction d (mm) Is a polyester film for coating, characterized by having a width of 50 mm and a range of 0.0 mm ≦ d ≦ 0.5 mm.   The coating surface is the surface B side, or the coating surface is both the surface A side and the surface B side, and the drying shrinkage of the coating material applied to the surface B side is large. The polyester film for coating according to 1.   The polyester film for coating according to claim 1 or 2, wherein the heat-resistant thermoplastic resin is a polyetherimide.   The polyester film for coating according to any one of claims 1 to 3, which is used as a support for a magnetic recording medium or a support for a thermal ink ribbon.