JP4622366B2 - Polybenzazole film and method for producing the same - Google Patents

Description

  The present invention relates to a polybenzazole-based film having high strength and high elastic modulus and having both surface smoothness and slipperiness, and a method for producing the same. In particular, the present invention relates to a polybenzazole film suitable as a base film for magnetic recording.

As a base film for magnetic tape, a biaxially oriented polyethylene terephthalate film is widely used in general. However, there is a limit in mechanical strength for the demand to increase the recording time and the recording capacity by thinning the base film. Therefore, development of films such as higher-strength wholly aromatic polyamides and wholly aromatic polyesters is underway. It is disclosed that polybenzazole, especially polybenzazole, is a rod-like polymer and its oriented film has extremely high mechanical strength (see Patent Document 1). However, when used as a base for a magnetic tape, the surface of the film must be smooth and defect-free, and at the same time, fine surface irregularities are required to obtain traveling properties.
In any of the above methods, a rigid polymer solution (referred to as a solution, not as a solution in a strict sense; hereinafter the same) is processed in a high-concentration region exhibiting optical anisotropy, that is, liquid crystallinity. It is focused on the property that the shear viscosity at that time is relatively low, but when the film is formed with a solution exhibiting optical anisotropy, the longitudinal direction of the film (MD, the flow direction of the film, machine direction) and the width direction ( It is difficult to balance mechanical properties in the direction perpendicular to TD and MD and in the transverse direction. As a method to solve the problem related to the balance of mechanical properties, a film forming method is disclosed in which the process up to die extrusion is carried out in an optically anisotropic phase, and after die extrusion, the phase is changed to the optically isotropic phase by composition change or temperature change. (See Patent Document 2). However, when a polybenzazole-based solution exhibiting optical anisotropy is die-extruded, the surface of the extruded sheet is observed to be markedly rough due to its liquid crystallinity. In addition, the polybenzazole solution having optical anisotropy is cast on a support and then changed to a state in which it is optically isotropic. A polybenzazole-based film is disclosed (see Patent Document 3).
US Pat. No. 4,973,442 JP-A-9-118758 Japanese Unexamined Patent Publication No. 2000-273214

However, in any of the above methods, it is a film formed from a liquid crystalline anisotropic solution, and because of the high orientation of the liquid crystal phase, the balance between mechanical properties in the longitudinal direction and the width direction is sufficiently solved. That's not true. Furthermore, when the liquid crystalline anisotropic solution is extruded by a T-die or the like, surface roughness is observed in the extruded sheet, but these problems have not been solved yet.
This point has not been studied at all, and the polybenzazole film disclosed heretofore has rough fibril-like projections on the surface and is difficult to use as a practical magnetic tape. Although the improvement with respect to this is also tried, the present condition is that the film with favorable surface smoothness is not obtained (refer patent document 4, 5).
JP-T 6-503521 Japanese Patent Laid-Open No. 11-171993

  The present invention is intended to solve the above-described points, and provides a laminated polybenzazole-based film having high strength, high elastic modulus, and having both surface smoothness and slipperiness, and a method for producing the same. Is an issue.

That is, the present invention contains a core layer formed from an optically anisotropic polybenzazole-based solution and fine particles on both surfaces of the core layer in an amount of 0.20 to 2.00% by mass based on the polybenzazole-based polymer. And a polybenzazole film comprising a skin layer formed from an optically isotropic polybenzazole solution and satisfying all of the following formulas (1) to (6): It is a sol-based film.
(1) Tensile elastic modulus in the longitudinal direction and the width direction ≧ 10.0 GPa,
(2) Tensile breaking strength in the longitudinal direction and the width direction ≧ 350 MPa,
(3) Tensile breaking elongation in the longitudinal direction and the width direction ≧ 10%,
(4) 0.85 ≦ (tensile modulus in the width direction) / (tensile modulus in the longitudinal direction) ≦ 1.20,
(5) 0.85 ≦ (tensile breaking strength in the width direction) / (tensile breaking strength in the longitudinal direction) ≦ 1.20,
(6) 0.85 ≦ (tensile elongation at break in the width direction) / (tensile elongation at break in the longitudinal direction) ≦ 1.20.

A preferred embodiment is the polybenzazole-based film described above in which the three-dimensional average roughness SRa of the film surface is 5 nm or more and 50 nm or less, and the number of coarse protrusions of 1 μm or more is 10 pieces / mm ² or less. The polybenzazole film, wherein the thickness of the core layer formed from the optically anisotropic polybenzazole solution is 25 to 75% of the total film thickness (total thickness of the laminate), The polybenzazole-based film is a polyparaphenylene benzobisoxazole.
In the present invention, a core layer is formed from an optically anisotropic polybenzazole-based solution, and fine particles on both surfaces of the core layer are 0.20 to 2.00% by mass with respect to the polybenzazole-based polymer. A method for producing a polybenzazole film, comprising a step of laminating and forming a skin layer from an optically isotropic polybenzazole-based solution, and more preferably optically anisotropic The polybenzazole polymer solution has a polymer concentration of 8.0 to 16.0% by mass, and the optically isotropic polybenzazole polymer solution has a concentration of 0.5 to 5% by mass. This is a method for producing a sol-based film.

  By adopting the configuration as described above, the polybenzazole-based film of the present invention has good slipperiness and good runnability when used as a film or a magnetic tape. Also, since the surface is smooth and does not have coarse protrusions, the magnetic recording characteristics are good. Furthermore, it is possible to correct and improve the defects of the film produced only from the anisotropic polymer solution, and the physical property balance such as strong elongation in the longitudinal direction (MD) and the width direction (TD) of the film is good, Since it has excellent physical properties such as extremely high tensile modulus, it can be used as a thin film substrate film. In particular, it is useful as a thin film substrate film for magnetic recording.

  The polybenzazole-based polymer in the present invention is a polybenzoxazole (PBO) homopolymer, a polybenzothiazole (PBT) homopolymer and a polybenzimidazole (PBI) homopolymer, or PBO, PBT and / or PBI. Random copolymer, alternating copolymer or block copolymer. Here, polybenzoxazole, polybenzothiazole and their random, alternating or block copolymer may be exemplified by US Pat. No. 4,703,103, US Pat. No. 4,533,692, US Pat. No. 4,533,724, US Pat. No. 4, U.S. Pat. No. 4,578,432 and the like.

  The structural unit contained in the polybenzazole polymer is preferably selected from lyotropic liquid crystal polymers. The structural unit is composed of structural units described in structural formulas (a) to (n), and more preferably is a structural unit selected from structural formulas (a) to (f). Particularly preferred is a PBO polymer comprising monomer units selected from structural formulas (a) to (b).

  In the present invention, the above-described polybenzazole-based polymer preferably contains 80% by mass or more, and less than 20% by mass may contain other polymers such as aromatic polyimide. In order to obtain a polybenzazole film having more excellent mechanical properties, the content of the polybenzazole polymer is desirably 90% by mass or more.

In the present invention, as described in claim 5, it is a preferred embodiment that the polybenzazole-based polymer is polyparaphenylene benzobisoxazole. By using the polymer, excellent heat resistance and mechanical properties can be obtained.
In the polybenzazole solution used for skin layer formation in the present invention, it is necessary to contain fine particles inactive to the polybenzazole polyphosphate solution. The inert fine particles should be contained in the range of 0.20 to 2.0% by mass or less with respect to polybenzazole. If the content of the inert fine particles is less than 0.20% by mass, for example, the running property when processed into a magnetic tape is lowered, which is not preferable. On the other hand, if it exceeds 2.0% by mass, the magnetic recording characteristics deteriorate, which is not preferable. As the inactive fine particles, inorganic fine particles such as silica and silica alumina, and crosslinked organic polymer fine particles are suitable.

  The fine particles are preferably inactivated with respect to the solution by surface treatment. The fine particles are added to a polybenzazole solution or its raw material and kneaded and dispersed with a biaxial kneader or the like. In this case, it is not preferable that a side reaction occurs, so it is necessary to minimize the side reaction. The weight average particle diameter of the fine particles is preferably 50 to 2000 nm. Particularly preferred are those having a weight average particle diameter of 70 to 700 nm and a narrow particle size distribution. Particularly preferred are those which do not substantially contain fine particles having a particle diameter of 5000 nm or more. When the weight average particle diameter of the fine particles is 50 nm or less, protrusions effective for film running properties cannot be formed on the film surface. When the weight average particle diameter is larger than 1000 nm, the protrusions formed on the surface are large, and the magnetic recording characteristics when the magnetic tape is formed deteriorate. When particles having a weight average particle diameter of 5000 nm or more are present, coarse protrusions are formed on the film surface, and the magnetic conversion characteristics are deteriorated when the magnetic tape is formed. Appropriate kneading and dispersion of inert fine particles is important in obtaining polybenzazole having the surface characteristics of this patent. It is necessary to knead the fine particles with a twin screw extruder of an appropriate screw, and further disperse them by passing through a filter, and to remove aggregated fine particles and coarse particles derived from foreign matters entering from outside the system.

The laminated polybenzazole film of the present invention is a polybenzazole film containing 0.20 to 2.00% by mass of fine particles in at least one skin layer, preferably the following formulas (1) to (6 ) Is satisfied.
(1) Tensile elastic modulus in the longitudinal direction and the width direction ≧ 10.0 GPa,
(2) Tensile breaking strength in the longitudinal direction and the width direction ≧ 350 MPa,
(3) Tensile breaking elongation in the longitudinal direction and the width direction ≧ 10%,
(4) 0.85 ≦ (tensile modulus in the width direction) / (tensile modulus in the longitudinal direction) ≦ 1.20,
(5) 0.85 ≦ (tensile breaking strength in the width direction) / (tensile breaking strength in the longitudinal direction) ≦ 1.20,
(6) 0.85 ≦ (tensile elongation at break in the width direction) / (tensile elongation at break in the longitudinal direction) ≦ 1.20.

  If the tensile modulus is less than 10.0 GPa, the post-processing speed cannot be increased, or film deformation occurs due to high tension and tension during high-speed winding at the time of use. It becomes difficult. The tensile modulus of the polyobenzazole film of the present invention is more preferably 12.0 GPa or more. Further, if the tensile strength at break is less than 350 MPa, it is not preferable that the film is broken when post-processing is performed at a high speed and high tension. The tensile elongation at break is preferably 10% or more. If the elongation is less than this, not only the handling during processing becomes difficult, but also the film becomes brittle during use, which is not preferable.

  Further, the polybenzazole-based film of the present invention has the relationship between the tensile modulus in the longitudinal direction, the tensile breaking strength and the tensile breaking elongation and the tensile modulus in the width direction, the tensile breaking strength and the tensile breaking elongation as described in (1). It is necessary to satisfy the expressions (4) to (6) simultaneously with ~ (3). By satisfying the above expressions (4) to (6) at the same time, the mechanical properties of the film in the longitudinal direction and the width direction can be balanced, so that the film can be obtained by a conventionally known production method that is easy to tear in a specific direction. Can be improved and can be suitably developed for various uses.

The three-dimensional average surface roughness (SRa) of the polybenzazole-based film containing the fine particles of the present invention is preferably 5 nm to 50 nm. Also, the number of protrusions of 1 μm or more present on the film surface needs to be 10 pieces / mm ² or less. When SRa is smaller than 3 nm, the workability of winding the film, the running property when used as a magnetic tape, and the guide roll Insufficient wear resistance. On the other hand, when SRa is 50 nm or more, electromagnetic conversion characteristics are insufficient when a magnetic tape or a flexible disk is used. When used as a magnetic recording substrate for high-density recording, the SRa of a film (referred to as a substrate film) is preferably 20 nm or less. Further, when the number of protrusions of 1 μm or more on the film surface is 10 / mm ² or more, a signal defect occurs when a magnetic tape or a flexible disk is used. In particular, when used as a magnetic recording base material for high recording density, it is necessary to be 1 piece / mm ² or less. In order to make the three-dimensional average surface roughness (SRa) and the number of coarse protrusions of 1 μm or more within the scope of the present invention, the particle size distribution, addition amount, and dispersion method of the inert fine particles are appropriately used and stretched. It is also necessary to select appropriate conditions for solidification. In addition, it is desirable to take measures to prevent the entry of foreign substances and to remove the mixed foreign substances.

The thickness of the polybenzazole-based film used in the present invention is not particularly limited. For example, when used as a high-density magnetic tape, 0.5 to 10 μm is preferable. When the film is thick, it is often unnecessary to use the high-strength film of the present invention. In general, a thickness of 0.5 μm or more is suitable because it is necessary to withstand the tension during the processing step and tape running.
A polybenzazole-based film layer not containing fine particles can be laminated on the polybenzazole-based film containing fine particles of the present invention.

  In the present invention, as described in claims 3 and 6, in order to exhibit the above-described characteristics, optically and the like are formed on both surfaces of the core layer formed from the optically anisotropic polybenzazole-based solution. It is necessary to form and form a skin layer formed from a polybenzazole-based solution that is isotropic. By forming the film by this method, the polymer molecular chain is oriented in the flow direction generated when the film is formed using only the optically anisotropic polybenzazole polymer solution, and is expressed by the orientation of this molecular chain. The disadvantage of the prior art that the balance between the mechanical properties of the film in the longitudinal direction and the width direction of the film is lost and the roughness of the film surface that occurs during die extrusion are remarkably improved.

  Whether or not the polybenzazole-based solution is optically isotropic is confirmed by observing a preparation prepared by thinly extending the solution on a slide glass and sandwiching the solution with a cover glass under a crossed Nicol. In other words, if the liquid crystal state is optically anisotropic, when the sample stage is rotated, a domain structure derived from the liquid crystal structure is observed and a bright field is obtained. For example, even if the sample stage is rotated, the dark field remains, and the difference between the two can be distinguished.

  In the present invention, the laminated polybenzazole film is formed from the above optically isotropic solution and the optically anisotropic solution. An optically anisotropic solution and an optically isotropic solution may be laminated by coextrusion to form a thin film. The skin layer may be formed by thinning and then applying an optically isotropic solution to both surfaces of the core layer. A laminated polybenzazole-based film can be obtained by coagulating, washing, drying and the like for the obtained thin film laminate. In the process of forming into a thin film, it can be formed by a known method such as a T-die method or a circular die method. However, in terms of control of mechanical properties, stretching treatment in the longitudinal direction and / or the width direction is performed before solidification. May be applied.

  The following method can be illustrated when performing a extending | stretching process. That is, a co-extrusion method is used to form a laminate comprising a polybenzazole solution exhibiting optical anisotropy in the core layer and a polybenzazole solution exhibiting optical isotropy in both skin layers, followed by a counter roll, rolling A composite laminated structure of a support and a laminated polybenzazole film precursor is formed by sandwiching the laminate between at least two supports via an apparatus or a press apparatus. The obtained sandwich composite laminate is guided to a coagulation bath, and the support is peeled off and coagulated in the coagulation bath. The configuration and arrangement of the facing roll, the rolling device, the pressing device, and the like can be variously combined. Desirably, the laminated polybenzazole film precursor sandwiched between at least two supports is sandwiched between at least a pair of opposing rolls, and the opposed rolls are rotated in the opposite direction to form a support / laminated polybenzazole film precursor. This is a method of forming a composite laminate of a body / support sandwich. The obtained support / laminated polybenzazole film precursor / support sandwich composite laminate is stretched by a longitudinal stretching machine and / or a transverse stretching machine such as a tenter.

  Thereafter, the polybenzazole-based solution is solidified by being guided to a coagulation bath and peeling the support in the coagulation bath. As the support, a plastic film such as a polyester film typified by a PET film or a polyolefin film such as a polypropylene (also referred to as PP) film can be used. Further, a porous film, fabric or nonwoven fabric having moisture permeability as a support may be used as it is or in combination with the plastic film support. When the plastic film is used as a support, the solidification time after the support is peeled off. When the porous film is used as a support alone, the time until the support is peeled off after the sandwich composite laminate is immersed in the coagulation bath and This is important because it affects the film structure and physical properties of the coagulated wet film that provides the time for immersion in the coagulation bath after peeling off the support.

  In the laminated polybenzazole film of the present invention, the thickness of the core layer formed from an optically anisotropic polybenzazole-based solution is important from 25 to 75% of the total film thickness (total thickness of the laminated body). When the thickness of the core layer is less than 25% with respect to the total film thickness, the mechanical property balance in the longitudinal direction and the width direction is excellent, but the mechanical strength is reduced. On the other hand, when the thickness of the core layer is greater than 75% with respect to the total thickness of the film, the mechanical properties in the longitudinal direction are improved, but the balance between the mechanical properties in the longitudinal direction and the width direction is lost. Arise.

  In the present invention, in order to obtain the above optically anisotropic solution, the polymer concentration is desirably 8.0 to 16.0% by mass. When the content is less than 8% by mass, optical anisotropy is hardly exhibited, and when the content exceeds 16% by mass, the viscosity of the solution increases and molding processing becomes difficult. In order to obtain an optically isotropic solution, the concentration of the polybenzazole-based polymer is preferably 0.5 to 5% by mass. More preferably, it is 3 mass% or less. On the other hand, when the concentration is less than 0.5% by mass, the viscosity of the polymer solution becomes small, and the applicable film forming methods are limited. In addition, the resulting polybenzazole-based film has a tensile elastic modulus, tensile breaking strength, and tensile breaking elongation. It is not preferable because mechanical properties such as the above become small. Since it becomes economically disadvantageous, 0.5 mass% or more is preferable. If the solution concentration exceeds 5% by mass, the optical anisotropy of the solution tends to be developed, and the solution viscosity increases and the moldability becomes difficult.

  In order to adjust the concentration of the polybenzazole polymer solution to the range shown above, the following method can be used. That is, a method in which a polymer solid is once separated from a polymerized polybenzazole-based polymer solution, and the concentration is adjusted by adding a solvent again and dissolving. Furthermore, a method of adjusting the concentration by adding a solvent to the polymer solution and diluting the polymer solution without separating the polymer solid from the polymer solution that has been subjected to condensation polymerization in polyphosphoric acid. Furthermore, there is a method of directly obtaining a polymer solution in the above concentration range by adjusting the polymerization composition of the polymer.

  Preferred solvents for use in adjusting the concentration of the polymer solution include methanesulfonic acid, dimethylsulfuric acid, polyphosphoric acid, sulfuric acid, trifluoroacetic acid, and the like, or a mixed solvent combining these solvents can also be used. Of these, methanesulfonic acid and polyphosphoric acid are particularly preferable.

  The degree of polymerization of the polymer in the polybenzazole-based polymer solution used for obtaining the polybenzazole-based film of the present invention is represented by an intrinsic viscosity. The intrinsic viscosity is preferably 15 dl / g or more and 35 dl / g or less, and more preferably 20 dl / g or more and 30 dl / g or less. If it is less than this range, the mechanical properties of the resulting polybenzazole-based film will decrease, and if it exceeds this range, the viscosity of the solution will be high and processing will be difficult.

  The coagulating liquid is not limited as long as it is compatible with the solvent forming the above solution and is a poor solvent for the polymer. For example, water, aqueous metal salt solution, polyhydric alcohol, Examples include protic polar solvents and protic polar solvents.

  Since the coagulated film contains an acid that is a solvent of the polymer solution, it is necessary to wash and remove the acid. Water is usually used as the cleaning liquid, but it may be washed with warm water as necessary, or may be washed with water after neutralizing and washing with an aqueous alkali solution. Cleaning includes, for example, a method of running the film in the cleaning liquid or spraying the cleaning liquid.

  In the present invention, the drying step is desirably performed under tension and by restricting shrinkage of the film under constant tension. When the film is dried by free shrinkage, partial shrinkage occurs, resulting in uneven thickness, and the flatness of the film may be impaired. In order to dry the film while restricting the shrinkage, for example, a tenter dryer or a metal frame can be used for drying. Other conditions related to drying are not particularly limited, and examples include a drying method using a heated gas such as air or nitrogen or a room temperature gas, a drying method using a heater or an infrared lamp, and the like.

  The film forming step, the coagulation step, the washing step, the drying step, and the like may be performed continuously or in a batch manner. Furthermore, you may add another special process between each process.

  The polybenzazole-based film used in the present invention may be applied with an anchor agent or subjected to chemical etching treatment, corona treatment, plasma treatment, etc. in order to increase the adhesive strength with a magnetic recording layer or a metal vapor deposition layer. Good.

The film of the present invention may contain known additives such as ultraviolet absorbers, heat stabilizers, stretching aids, lubricants and the like.
In the present invention, the same polybenzazole polymer may be used for the core layer and the skin layer, or different polybenzazole polymers may be used. In the polybenzazole-based polymer, the core layer is formed from an anisotropic solution of one kind of polybenzazole-based polymer selected from polyoxazole-based polymer, polythiazole-based polymer, and polyimidazole-based polymer. A skin layer is laminated from an isotropic solution of a kind of polybenzazole polymer selected from polyoxazole polymer, polythiazole polymer, and polyimidazole polymer to form the polybenzazole film of the present invention. I can do it.

Hereinafter, the contents and effects of the present invention will be described with reference to examples, but the present invention is not limited to the following examples. In addition, the measurement / evaluation method in an Example is as follows.
1. Intrinsic viscosity The viscosity of a polymer solution adjusted to a concentration of 0.5 g / l using methanesulfonic acid as a solvent was measured in an oven at 30 ° C. using an Ostwald viscometer and calculated.

2. Evaluation of optical isotropy of polymer solution The solution was placed on a slide glass, sandwiched between cover glasses and stretched thinly, and observed under crossed Nicols using a polarizing microscope. Even when the sample stage was rotated, it was a dark field, and when the domain structure was not observed, it was judged as an optically isotropic solution. When a domain structure changing from a dark field to a bright field was observed by rotating the sample stage, the solution was judged to be an optically anisotropic solution.

3. Average particle size of fine particles The fine particles were determined by calculating the weight average particle size using a centrifugal sedimentation type particle size distribution analyzer SA-CP4 manufactured by Shimadzu Corporation.
4). Tensile modulus, tensile strength at break, and tensile elongation at break The dried film was cut into strips of length 100 mm and width 10 mm in the longitudinal direction (MD direction) and width direction (TD direction), respectively, and used as a test piece. Using a machine (Shimadzu Corporation Autograph (R), model name AG-5000A), measured at a tensile speed of 50 mm / min and a distance between chucks of 40 mm, the tensile modulus, tensile breaking strength and tensile breaking elongation were measured. Asked.

5. 3D surface roughness of film Using a stylus type 3D surface roughness meter (SE-3AK, manufactured by Kosaka Laboratories), the film surface was measured under the conditions of a needle radius of 2 μm and a load of 30 mgf. A cut-off value of 0.25 mm in the direction was measured over a measurement length of 1 mm, divided into 500 points at a pitch of 2 μm, and the height of each point was taken into a three-dimensional roughness analyzer (SPA-11). The same operation was performed 150 times continuously at intervals of 2 μm in the width direction of the film, that is, over 0.3 mm in the width direction of the film, and the data was taken into the analyzer. Then, the analysis apparatus was made to calculate the three-dimensional average surface roughness SRa.

6). The number of coarse protrusions of 1 μm or more on the film surface After thinly depositing aluminum on the film surface, using a ^two- beam interference microscope manufactured by Nache, observe an area of 1 mm ² at 400 times, and create interference fringes that can be made corresponding to the protrusion height Counted the protrusions.
7). Coefficient of dynamic friction of film The coefficient of dynamic friction at 20 ° C. and 65 RH% was measured according to ASTM-D1894. A film having a dynamic friction coefficient of 0.8 or less has good slipperiness and excellent workability.

8). Scratch resistance of the film After connecting the lead film of both ends of the polybenzazole film cut to 10mm width and 1m length, and running it while rubbing the plastic pin at a tension of 100gf, winding angle of 90 degrees, running speed of 150m / min Then, aluminum was vapor-deposited on the friction surface of the polybenzazole film portion, and the amount of scratches on the surface was observed with a stereomicroscope, and ranked as follows.
Excellent: No scratches are observed.
Good: Slightly scratched but very small amount.
Good: A small amount of scratches is observed.
Impossible: A lot of scratches are observed.

(Preparation of polybenzazole solution)
(1) Production of fine particle-containing polymer solution (solution (A)) After adding 14.49 kg of diphosphorus pentoxide in a nitrogen stream to 43.86 kg of polyphosphoric acid of 116% by mass per batch, 4,6-diaminoresorcinol 9.10 kg of dihydrochloride, 7.10 kg of terephthalic acid pulverized to an average particle diameter of 2 μm, and 250 g of monodispersed spherical silica fine particles having an average particle diameter of 200 nm manufactured by Nippon Shokubai Chemical Industry Co., Ltd. Stir and mix in the reactor. The mixture was further heated and mixed at 150 ° C. for 10 hours and then polymerized using a biaxial extruder heated to 200 ° C. to obtain a 14% by mass solution of polyparaphenylenebenzoxazole (PBO) through a nominal aperture 50 μm filter. The color of the polymer solution was yellow, and the intrinsic viscosity measured with a methanesulfonic acid solution was 27 dl / g.

(2) Polymer solution not containing fine particles (solution (B))
A fine particle-free PBO solution was obtained in the same manner as (1) except that no monodispersed spherical silica was added. The intrinsic viscosity of the obtained solution was 27 dl / g.
(3) Optically isotropic polymer solution Solution (A) and solution (B) are mixed at a mass ratio of 1/3, diluted by adding polyphosphoric acid, and used for film formation with a PBO concentration of 1.0% by mass. A polymer solution (solution (C)) was prepared. Separately, the solution (A) and the solution (B) were mixed at a mass ratio of 1/1 and diluted by adding polyphosphoric acid to prepare a polymer solution for film formation (solution (D)) having a PBO concentration of 1.0% by mass. . Solution (C) and solution (D) were both optically isotropic.

Example 1
Using a multi-manifold type two-layer three-layer laminated T-die extruder, a laminated PBO film precursor having a solution (B) for the core layer and a solution (C) for the skin layer was prepared, and the speed was 0.30 m / min. Then, it was introduced into a 20 ° C. coagulating water tank. The residence time in the coagulation water tank was about 2 minutes, and the solidified laminated PBO film was wound up at 0.31 m / min. The coagulated film was sufficiently washed with water, and then heat fixed at 150 ° C. for 30 seconds while holding both ends with a tenter to obtain a 5.0 μm thick PBO film. At this time, the discharge amounts of various solutions and the lip gap were adjusted so that the total thickness of the completed laminated film was about 5.0 μm and the thickness of the core layer was 2.5 μm. The solution temperature at the time of extrusion was set to 170 ° C.

(Example 2)
A laminated PBO film having a thickness of 4.9 μm was obtained in the same manner as in Example 1 except that the thickness of the core layer was 1.3 μm.

(Example 3)
A laminated PBO film having a thickness of 5.0 μm and a core layer thickness of 2.5 μm was obtained in the same manner as in Example 1 except that the polymer solution of the skin layer was changed to the solution (D).

(Comparative Example 1)
The solution (B) was diluted with polyphosphoric acid to prepare a PBO concentration 1.0 mass% solution (solution (E)). This solution was optically isotropic. A laminated PBO film having a thickness of 5.0 μm and a core layer thickness of 2.5 μm was obtained in the same manner as in Example 1 except that this solution (E) was used as a skin layer.

(Comparative Example 2)
The solution (A) was diluted with polyphosphoric acid to prepare a PBO concentration 1.0 mass% solution (solution (F)). This solution was optically isotropic. A laminated PBO film having a thickness of 5.0 μm and a core layer thickness of 2.5 μm was obtained in the same manner as in Example 1 except that this solution (F) was used as a skin layer.

(Comparative Example 3)
A laminated PBO film having a thickness of 5.0 μm was produced in the same manner as in Example 1 except that the thickness of the core layer was 4.5 μm. At this time, remarkable surface roughness was observed on the surface of the PBO film precursor extruded from the T-die. For this reason, the three-dimensional surface roughness is not measured.

(Comparative Example 4)
A laminated PBO film having a thickness of 4.9 μm was produced in the same manner as in Example 1 except that the thickness of the core layer was 1.0 μm.

(Comparative Example 5)
A solution (solution (C ')) in which the mass ratio of the solution (A) and the solution (B) was 1/3 was prepared. A PBO film having a thickness of 5.2 μm was produced using only the solution (C ′). Although the surface roughness of the film was improved as compared with Comparative Example 3, it was determined that the three-dimensional surface roughness was not at a level for evaluation, and SRa was not measured.
Table 1 shows the results of evaluating the films obtained in each of the above Examples and Comparative Examples.

  The laminated polybenzazole-based film of the present invention adopts the configuration as described above, so that the slipperiness is good, so that the handling property of the film and the running property when using a magnetic tape are good, and the surface is smooth. Since it has no coarse protrusions, it has good magnetic recording characteristics and is useful as a thin film substrate film for magnetic recording. Further, since it has a very high tensile modulus and can be used as a thin film, it is particularly useful as a base film for a high-capacity magnetic recording medium.

Claims (5)

A core layer formed from an optically anisotropic polybenzazole-based solution in which a domain structure changing from a dark field to a bright field is observed by rotating the solution sample by observation under orthogonal crossed Nicols using a polarizing microscope Even if the solution sample is rotated by observation under a crossed cross Nicol using a polarizing microscope containing fine particles of 0.20 to 2.00% by mass with respect to the polybenzazole polymer on both surfaces of the core layer. And a skin layer formed from an optically isotropic polybenzazole-based solution in which the domain structure is not observed, and the thickness of the core layer is 25 to 75% of the total film thickness polybenzazole film to be. The polybenzazole film according to claim 1 , wherein the polybenzazole is polyparaphenylenebenzobisoxazole and satisfies all of the following formulas (1) to (6).
(1) Tensile elastic modulus in the longitudinal direction and the width direction ≧ 10.0 GPa,
(2) Tensile breaking strength in the longitudinal direction and the width direction ≧ 350 MPa,
(3) Tensile breaking elongation in the longitudinal direction and the width direction ≧ 10%,
(4) 0.85 ≦ (tensile modulus in the width direction) / (tensile modulus in the longitudinal direction) ≦ 1.20,
(5) 0.85 ≦ (tensile breaking strength in the width direction) / (tensile breaking strength in the longitudinal direction) ≦ 1.20,
(6) 0.85 ≦ (tensile elongation at break in the width direction) / (tensile elongation at break in the longitudinal direction) ≦ 1.20. 3. The polybenzazole according to claim 1, wherein the three-dimensional average roughness SRa of at least one surface of the film surface is 5 nm or more and 50 nm or less, and the number of coarse protrusions of 1 μm or more is 10 / mm 2 or less. Film. A core layer is formed from an optically anisotropic polybenzazole-based solution in which a domain structure changing from a dark field to a bright field is observed by rotating the solution sample by observation under crossed crossed Nicols using a polarizing microscope Even if the solution sample is rotated by observation under orthogonal crossed Nicol using a polarizing microscope containing fine particles of 0.20 to 2.00% by mass with respect to the polybenzazole polymer on both surfaces of the core layer. A method for producing a polybenzazole-based film comprising a step of laminating and forming a skin layer from a polybenzazole-based solution that is dark field and has no domain structure observed and is optically isotropic. The polymer concentration of the optically anisotropic polybenzazole polymer solution is 8.0 to 16.0% by mass, and the concentration of the optically isotropic polybenzazole polymer solution is 0.5 to 5% by mass. A method for producing a laminated polybenzazole film according to claim 4 .