JP7366397B2 - Laminated film and method for manufacturing the laminated film - Google Patents

Description

The present invention relates to a laminated film having a liquid crystal polyester layer and a method for manufacturing the laminated film.

Liquid crystal polyester film has excellent low moisture absorption, high frequency properties, flexibility, high gas barrier properties, and thin wall forming properties, so it is used for electronic substrates such as flexible printed wiring boards, rigid printed wiring boards, and module boards. Demand has been increasing in recent years because it can be suitably used as an insulating film for industrial use or as a surface protection film.

However, for example, when using a metal-clad laminate in which the liquid crystal polyester film is laminated as an insulating film to a metal foil as a wiring board on which electronic components are mounted, the liquid crystal polyester film is damaged by the driving heat generated from the electronic components. There is a problem in that the metal-clad laminate is warped or warped due to linear expansion.

This is not limited to liquid crystal polyester, but the coefficient of linear expansion of synthetic resin insulating films is generally higher than that of metal foils, and as a result, the coefficient of linear expansion of insulating films made of synthetic resins is generally higher than that of metal foils. This is because stretched laminates experience warping and distortion.

A possible solution to this problem is to add an insulating inorganic filler to the liquid crystal polyester film in order to adjust the coefficient of linear expansion of the liquid crystal polyester film.

Note that Patent Document 1 discloses a liquid crystal polyester film containing silica as a filler.

JP2011-201971A

However, since the inorganic filler absorbs water, the addition of the inorganic filler increases the water absorption (hygroscopicity) of the film, which loses the advantage of the low hygroscopic property of liquid crystalline polyester, and furthermore, the filler's surface unevenness increases. Alternatively, there is a problem that dielectric loss increases due to surface precipitation.

For example, regarding a flexible printed wiring board manufactured from a metal-clad laminate made of a liquid crystal polyester film containing an inorganic filler and metal foil, the inorganic filler absorbs water, causing electrical characteristics to deteriorate due to an increase in transmission loss and insertion loss. This results in various problems such as deterioration and corrosion of the metal foil (conductive pattern).

Furthermore, when the metal foil is partially etched to form a conductive pattern and the insulating film covered by the metal foil is exposed to the surface, the filler added to the insulating film in the exposed part falls off. However, dirt or impurities may adhere to the surface of the FPC, and separate cleaning or other operations may be required to remove this as necessary.

In addition, insulating films made of synthetic resins such as polyimide and liquid crystalline polyester have poor adhesion to metal foils such as copper foil, but if an inorganic filler is added to them, the adhesion of the insulating film to metal foils becomes even stronger. When used in applications that involve repeated bending and stretching, delamination may easily occur.

On the other hand, the method for manufacturing liquid crystalline polyester into a film is usually carried out by extruding it into a film using a T-die method while heating it above its melting point temperature, or by molding it into a film using an inflation method.

In this case, since liquid crystal polyester has the property of being crystalline in its molten state (liquid state) and has the property of instantaneously oriented from the glass transition Tg state (amorphous), the above method can be used to form a film. When a film is formed, molecular orientation occurs in the flow direction (MD: Machine Direction) of the molten resin, resulting in an anisotropic film, which makes the liquid crystal weak in the TD (Transverse Direction) direction, which is perpendicular to the MD. A metal-clad laminate, which is a polyester film and is formed by bonding the liquid crystal polyester film with metal foil as an insulating film, has a problem that it tends to tear in the MD direction when pulled in the TD direction.

In particular, when a conductor pattern whose longitudinal direction is in the MD direction is formed by etching a metal foil, the insulating film (liquid crystal polyester film) between the conductor patterns is easily torn along the conductor pattern.

Further, the anisotropy of the liquid crystal polyester film causes warping and distortion in a metal-clad laminate in which such a liquid crystal polyester film is laminated as an insulating film.

Therefore, when the insulating film of the metal-clad laminate is formed from liquid crystal polyester, it is desirable that the polyester has no orientation in either the MD or TD direction, and preferably in the Z (thickness) direction.

The present invention was made in order to solve the above-mentioned problem, and although a laminated film having a liquid crystal polyester layer contains a filler to adjust the coefficient of linear expansion of the entire film, the filler does not fall off. It is an object of the present invention to provide a laminated film that does not have any problems, maintains low water absorption, and is non-oriented.

Means for solving the problem will be described below along with the symbols used in the detailed description. This code is written to clarify the correspondence between the description of the claims and the description of the mode for carrying out the invention, and needless to say, it does not limit the interpretation of the technical scope of the invention. It is not used.

In order to achieve the above object, the laminated film 1 of the present invention has a filler-added layer made of a synthetic resin (excluding polyimide) to which a filler is added, in the laminated film 1 used by laminating metal foil. A non-oriented laminate film 1 in which liquid crystal polyester layers 11 and 12 to which no filler is added are laminated on both sides (front and back surfaces) of a film 10, the laminate film 1 having a linear expansion coefficient reduced by the addition of the filler. The present invention is characterized in that the difference between the coefficient of linear expansion of the film 1 and the coefficient of linear expansion of the metal foil is 25% or less (Claim 1: Fig. 1).

Note that the filler is preferably silica, talc, silica nitride, aluminum nitride, or fluorine powder (Claim 2 ).

Further, the synthetic resin is preferably liquid crystal polyester (Claim 3).

Further, the thickness of the laminated film 1 is preferably 3 to 100 μm (Claim 4 ).

In addition, the method for manufacturing the laminated film 1 of the present invention (FIGS. 2 to 4) is as follows:
A laminated film 1 used by laminating with metal foil, in which a filler is added to both sides (front and back sides) of a filler-added layer 10 made of a synthetic resin (excluding polyimide) to which a filler is added. A method for producing a laminated film 1 having a laminated structure in which liquid crystal polyester layers 11 and 12 are laminated,
The manufacturing process of the laminated film 1 includes:
A first liquid composition 31 consisting of a precursor of liquid crystal polyester and a solvent, a second liquid composition 32 containing a filler and consisting of a precursor of the synthetic resin and a solvent, on a base material 40 which is a process paper, The precursor layer (first precursor layer) 21 of the liquid crystal polyester layer 11 and the precursor layer (first precursor layer) of the liquid crystal polyester layer 11 are formed on the base material 40 by sequentially casting and drying the third liquid composition 33 consisting of a liquid crystal polyester precursor and a solvent. A first laminate in which a laminate film precursor 20 consisting of a precursor layer (second precursor layer) 22 of the filler-added layer 10 and a precursor layer (third precursor layer) 23 of the liquid crystal polyester layer 12 is laminated. A first laminate manufacturing process (FIG. 2) for manufacturing L1;
After peeling the laminated film precursor 20 from the base material 40 of the first laminate L1, the laminated film precursor 20 is transferred to a metal base material 50 having a release layer on the surface, thereby removing the metal base. a second laminate manufacturing step (FIG. 3) of manufacturing a second laminate L2 in which the laminated film precursor 20 is laminated on the material 50;
a third laminate manufacturing step (FIG. 4) of manufacturing a third laminate L3 in which the laminate film 1 is laminated on the metal base material 50 by heat-treating the second laminate L2;
a film peeling step (FIG. 4) of peeling the metal base material 50 from the third laminate L3 , and the linear expansion coefficient of the laminated film whose linear expansion coefficient has been reduced by the addition of the filler and the metal foil. It is characterized in that the difference in linear expansion coefficients is 25% or less (Claim 5 ).

The base material 40 is preferably a glass plate, stainless steel foil, polyethylene terephthalate film, polyethylene film, polypropylene film, polymethylpentene film, or polytetrafluoroethylene sheet (Claim 6 ).

The metal base material 50 is characterized in that the material is aluminum, stainless steel, iron, or copper (Claim 7 ).

The mold release layer is characterized in that it is a rubber-like elastic layer (claim 8 ).

Note that the heat treatment is preferably performed under an inert gas atmosphere (Claim 9 ).

With the structure of the present invention explained above, the following remarkable effects could be obtained in the laminated film 1 of the present invention.

The laminated film 1 of the present invention has a filler-added layer 10 made of a synthetic resin to which a filler is added, and both sides (front and back surfaces) thereof are covered with liquid crystal polyester layers 11 and 12 to which no filler is added. Regardless of the hygroscopicity of the inorganic filler, it is possible to maintain the low water absorption of the original liquid crystal polyester film, prevent the filler from being deposited on the surface, and reduce the dielectric loss due to the unevenness of the filler's surface. It was possible to prevent the filler from falling off.

Note that the surface of the filler-added layer 10 may become a rough and matte surface due to the addition of the filler, but a laminate in which the liquid crystal polyester layers 11 and 12 to which no filler is added are laminated on both sides of the filler-added layer 10 Film 1 was able to have a smooth, glossy and beautiful surface.

Furthermore, for example, when a metal clad laminate is manufactured by laminating metal foil using the laminated film 1 of the present invention as an insulating film, the linear expansion coefficient of the entire laminated film 1 can be changed by the filler addition layer 10 to the linear expansion coefficient of the metal foil. The warpage and distortion of the metal-clad laminate could be further reduced.

Furthermore, by disposing the liquid crystal polyester layer 11 (or 12) to which no filler is added on the laminated side with the metal foil, the adhesive force between the laminated film 1 and the metal foil due to the addition of filler is reduced. We were able to prevent the decline.

Furthermore, the laminated film 1 of the present invention can be non-oriented, does not have the anisotropy that common liquid crystal polyester films have, and can improve mechanical strength such as tensile force in the width direction. For example, when a metal clad laminate is manufactured by laminating metal foil using the laminate film 1 of the present invention as an insulating film, warpage and distortion of the metal clad laminate occur due to the anisotropy, and insulation It was possible to more effectively prevent the occurrence of delamination between the film and the metal foil.

FIG. 1 is a schematic cross-sectional view of the laminated film 1 of the present invention (single-layer insulating film). FIG. 3 is a process diagram of the method for manufacturing the laminated film 1 of the present invention, and is a diagram showing the process of manufacturing the first laminate L1. FIG. 3 is a diagram showing a manufacturing process of the first laminate L1 to the second laminate L2 in FIG. 2. The figure which shows the process of manufacturing the 3rd laminated body L3 from the 2nd laminated body L2 of FIG. 3, and peeling the laminated film 1 (processing paper). The figure which shows the manufacturing process of other Examples of the 1st laminated body L1. A diagram showing a chemical formula of an example of fluorine-based powder.

Hereinafter, the laminated film 1 of the present invention and its manufacturing method will be explained with reference to the accompanying drawings.

〔overall structure〕
As shown in FIG. 1, the laminated film 1 of the present invention has a filler-added layer 10, which is a layer of synthetic resin added with a filler, and liquid crystal polyester layers 11 and 12, which do not contain filler, on both sides of the filler-added layer 10 (for convenience, In this specification, reference numeral 11 is also referred to as a first liquid crystal polyester layer, and reference numeral 12 is also referred to as a second liquid crystal polyester layer).

Furthermore, the aforementioned laminated film 1 preferably has a thickness of 3 to 100 μm, for example.

Since the laminated film 1 is equipped with a filler-added layer 10, when laminating the laminated film 1 as an insulating film to metal foil to produce a metal-clad laminate, the linear expansion coefficient of the entire laminated film 1 is determined by the linear expansion coefficient of the metal foil. By bringing the coefficient of expansion closer to the coefficient of linear expansion, it is possible to prevent warping or distortion of the metal-clad laminate caused by a difference in coefficient of linear expansion.

[Liquid crystal polyester layer]
The liquid crystal polyester layers 11 and 12 described above have a structure that does not include fillers, and are made of liquid crystal polyester, which will be described later, and have a moisture permeability of 0.5 g/m ² ·24 hours or less.

The thickness of the liquid crystal polyester layers 11 and 12 is preferably 0.3 to 20 μm.

Note that if the thickness of the liquid crystal polyester layers 11 and 12 is 1 μm or less, for example, when used in a metal-clad laminate, it may not be possible to absorb variations in the unevenness of the surface of the metal foils to be bonded, and as a result, the dielectric Since an increase in loss may occur, the preferred thickness of the liquid crystal polyester layers 11 and 12 is 2 to 15 μm from the viewpoint of maintaining bonding properties with the metal foil and dielectric properties.

Further, regarding the thickness of the liquid crystal polyester layers 11 and 12, from the necessity of reducing the coefficient of linear expansion of the entire laminated film 1, the thickness of the liquid crystal polyester layers 11 and 12 should not exceed the thickness of the filler-added layer 10, which will be described later. The thickness is preferably 50% or less, more preferably 40% or less.

Further, the thickness of the first liquid crystal polyester layer 11 and the thickness of the second liquid crystal polyester layer 12 may not be the same.

Next, the liquid crystal polyester constituting the liquid crystal polyester layers 11 and 12 will be explained.

This liquid crystal polyester is a polyester that exhibits optical anisotropy when melted and forms an anisotropic melt at a temperature of 450° C. or lower.

This liquid crystal polyester includes a structural unit represented by the following formula 1 (hereinafter referred to as "Formula 1 structural unit"), a structural unit represented by the following formula 2 (hereinafter referred to as "Formula 2 structural unit"), and It has a structural unit represented by the following formula 3 (hereinafter referred to as "formula 3 structural unit"), and the content of the structural unit represented by formula 1 is 30 to 80% of the total content of all structural units. It is preferable to use a liquid crystal polyester having a content of structural units represented by formula 2 in mol% of 10 to 35 mol%, and a content of structural units represented by formula 3 in a range of 10 to 35 mol%.
-O-Ar ¹ -CO- (Formula 1)
-CO-Ar ² -CO- (Formula 2)
-X-Ar ³ -Y- (Formula 3)
(In formulas 1 to 3, Ar ¹ represents a phenylene group or a naphthylene group, Ar ² represents a phenylene group, a naphthylene group, or a group represented by the following formula 4, and Ar ³ represents a phenylene group or a group represented by the following formula 4. X and Y each independently represent O or NH.The hydrogen atoms bonded to the aromatic rings of Ar ¹ , Ar ² and Ar ³ are halogen atoms, alkyl groups or aryl groups. ).
-Ar ¹¹ -Z-Ar ¹² - (Formula 4)
(In the formula, Ar ¹¹ and Ar ¹² each independently represent a phenylene group or a naphthylene group, and Z represents O, CO or SO _2. )

The structural unit of formula 1 is a structural unit derived from an aromatic hydroxycarboxylic acid, and examples of the aromatic hydroxycarboxylic acid include p-hydroxybenzoic acid, m-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, Examples include 3-hydroxy-2-naphthoic acid and 4-hydroxy-1-naphthoic acid.

The structural unit of formula 2 is a structural unit derived from an aromatic dicarboxylic acid, and examples of the aromatic dicarboxylic acid include terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, and diphenyl ether. -4,4'-dicarboxylic acid, diphenylsulfone-4,4'-dicarboxylic acid, diphenylketone-4,4'-dicarboxylic acid, and the like.

The structural unit of formula 3 is a structural unit derived from an aromatic diol, an aromatic amine having a phenolic hydroxyl group (phenolic hydroxyl group), or an aromatic diamine. Examples of the aromatic diol include hydroquinone, resorcinol, 2,2-bis(4-hydroxy-3,5-dimethylphenyl)propane, bis(4-hydroxyphenyl)ether, and bis-(4-hydroxyphenyl)ketone. , bis-(4-hydroxyphenyl)sulfone, and the like.

Further, examples of aromatic amines having this phenolic hydroxyl group include 4-aminophenol (p-aminophenol), 3-aminophenol (m-aminophenol), etc., and examples of this aromatic diamine include 1, Examples include 4-phenylenediamine and 1,3-phenylenediamine.

The liquid crystalline polyester used in the present invention is soluble in a solvent, and soluble in a solvent means that it is dissolved in a solvent at a concentration of 1% by mass or more at a temperature of 50°C. The solvent in this case is any one of the suitable solvents used for producing the liquid composition described below.

Such a liquid crystalline polyester having solvent solubility preferably contains a structural unit derived from an aromatic amine having a phenolic hydroxyl group and/or a structural unit derived from an aromatic diamine as the structural unit of formula 3. That is, as a formula 3 structural unit, a structural unit represented by a structural unit (formula 3') in which at least one of X and Y is NH is hereinafter referred to as a "formula 3' structural unit." ) is preferable because it tends to have excellent solvent solubility in suitable solvents (aprotic polar solvents) described below. In particular, it is preferred that substantially all of the formula 3 structural units are formula 3' structural units. Further, this formula 3' structural unit is advantageous in that it not only makes the liquid crystal polyester sufficiently soluble in solvents, but also makes the liquid crystal polyester lower in water absorption.
-X-Ar ³ -NH- (formula 3')
(In the formula, Ar ³ and X have the same meanings as above.)

It is more preferable that the structural unit of formula 3 is contained in a range of 25 to 33 mol % based on the total content of all structural units, and by doing so, the solvent solubility is further improved. As described above, the liquid crystal polyester having the formula 3' structural unit as the formula 3 structural unit has the advantage that it has better solubility in a solvent and a liquid crystal polyester film with low water absorption can be obtained.

The structural unit of formula 1 is preferably contained in an amount of 30 to 80 mol%, more preferably 35 to 50 mol%, based on the total content of all structural units. Liquid crystalline polyesters containing structural units of Formula 1 in such a molar fraction tend to have better heat resistance while maintaining sufficient liquid crystallinity. Furthermore, considering the availability of the aromatic hydroxycarboxylic acid that induces the structural unit of Formula 1, p-hydroxybenzoic acid and/or 6-hydroxy-2-naphthoic acid are preferable as the aromatic hydroxycarboxylic acid. It is.

The structural unit of formula 2 is preferably contained in an amount of 10 to 35 mol%, more preferably 25 to 33 mol%, based on the total content of all structural units. Liquid crystalline polyesters containing structural units of formula 2 in such a molar fraction tend to have better heat resistance while maintaining sufficient liquid crystallinity. Furthermore, considering the availability of the aromatic dicarboxylic acid that induces the structural unit of formula 2, the aromatic dicarboxylic acid should be at least one selected from the group consisting of terephthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid. It is preferable that there is also one type.

In addition, in terms of the obtained liquid crystalline polyester exhibiting a higher degree of liquid crystallinity, the molar fraction of the formula 2 structural unit and the formula 3 structural unit is expressed as [formula 2 structural unit]/[formula 3 structural unit]. , 0.9/1 to 1/0.9 is suitable.

Next, a method for manufacturing liquid crystal polyester will be explained.

This liquid crystal polyester can be produced by various known methods. When producing a suitable liquid-crystalline polyester, that is, a liquid-crystalline polyester consisting of structural units of formula 1, structural units of formula 2 and structural units of formula 3, after converting the monomers deriving these structural units into ester-forming and amide-forming derivatives. , a method of producing a liquid crystalline polyester by polymerization is preferable because it is easy to operate.

The ester-forming and amide-forming derivatives will be explained by giving examples.

Ester-forming and amide-forming derivatives of monomers having carboxyl groups, such as aromatic hydroxycarboxylic acids and aromatic dicarboxylic acids, are acid-forming derivatives in which the carboxyl group promotes the reaction that produces polyesters and polyamides. Highly reactive groups such as chlorides and acid anhydrides, and carboxyl groups form esters with alcohols, ethylene glycol, etc. to produce polyesters and polyamides through transesterification and transamidation reactions. Examples include things that are done.

As ester-forming and amide-forming derivatives of monomers having phenolic hydroxyl groups, such as aromatic hydroxycarboxylic acids and aromatic diols, phenolic hydroxyl groups can be used to form polyesters and polyamides through transesterification reactions. Examples include those in which ester forms an ester with a carboxylic acid.

In addition, examples of amide-forming derivatives of monomers having an amino group, such as aromatic diamines, include those in which the amino group forms an amide with carboxylic acids, such as those that produce polyamide through an amidation exchange reaction. Can be mentioned.

Among these, aromatic hydroxycarboxylic acids, aromatic diols, aromatic amines with phenolic hydroxyl groups, and aromatic diamines having phenolic hydroxyl groups and/or amino groups are recommended for easier production of liquid crystalline polyesters. After the monomer is acylated with a fatty acid anhydride to form an ester-forming/amide-forming derivative (acylated product), the acyl group of the acylated product and the carboxyl group of the monomer having a carboxyl group undergo transesterification/amidation. Particularly preferred is a method in which liquid crystalline polyester is produced by polymerizing the liquid crystalline polyester.

A method for producing such a liquid crystal polyester is described, for example, in JP-A No. 2002-220444 or JP-A No. 2002-146003.

In acylation, the amount of fatty acid anhydride added is preferably 1 to 1.2 times equivalent, and preferably 1.05 to 1.1 times equivalent, relative to the total of phenolic hydroxyl groups and amino groups. and more preferable. If the amount of fatty acid anhydride added is less than 1 equivalent, the acylated product and raw material monomers tend to sublime during polymerization and the reaction system tends to be clogged, while if it exceeds 1.2 times equivalent, the resulting liquid crystal Polyester tends to become noticeably colored.

The acylation is preferably carried out at 130 to 180°C for 5 minutes to 10 hours, more preferably at 140 to 160°C for 10 minutes to 3 hours.

The fatty acid anhydride used for acylation is preferably acetic anhydride, propionic anhydride, butyric anhydride, isobutyric anhydride, or a mixture of two or more selected from these, particularly preferably anhydrous It is acetic acid.

Polymerization following acylation is preferably carried out at a rate of 0.1-50°C/min at 130-400°C, and at a rate of 0.3-5°C/min at 150-350°C. It is more preferable to do this while

In addition, in the polymerization, it is preferable that the acyl group of the acylated product has an equivalent weight of 0.8 to 1.2 times that of the carboxyl group.

During acylation and/or polymerization, in order to shift the equilibrium according to Le Chatelier-Brown's law (principle of equilibrium movement), by-product fatty acids and unreacted fatty acid anhydrides are evaporated, etc. It is preferable to distill it off to the outside.

Note that acylation and polymerization may be carried out in the presence of a catalyst. As this catalyst, those conventionally known as polyester polymerization catalysts can be used, such as magnesium acetate, stannous acetate, tetrabutyl titanate, lead acetate, sodium acetate, potassium acetate, antimony trioxide, etc. metal salt catalysts, N,N-dimethylaminopyridine, N-methylimidazole and other organic compound catalysts.

Among these catalysts, heterocyclic compounds containing two or more nitrogen atoms, such as N,N-dimethylaminopyridine and N-methylimidazole, are preferably used (see JP-A-2002-146003).

This catalyst is usually added together with the monomers, and it is not necessarily necessary to remove it after acylation. If this catalyst is not removed, the acylation can proceed directly to polymerization.

The liquid crystalline polyester obtained by such polymerization can be used as is in the present invention, but in order to further improve the properties such as heat resistance and liquid crystallinity, it is preferable to increase the molecular weight. For this purpose, it is preferable to carry out solid phase polymerization. A series of operations related to this solid phase polymerization will be explained. The relatively low molecular weight liquid crystal polyester obtained by the above polymerization is taken out and ground into powder or flakes. Subsequently, solid phase polymerization can be carried out by heat-treating the pulverized liquid crystal polyester in a solid phase state at 20 to 350° C. for 1 to 30 hours in an atmosphere of an inert gas such as nitrogen. This solid phase polymerization may be performed while stirring or may be performed while standing still without stirring. In addition, from the viewpoint of obtaining a liquid crystal polyester having a suitable flow start temperature, which will be described later, the preferred conditions for this solid phase polymerization are detailed: The reaction temperature is preferably over 210°C, and even more preferably 220°C to 350°C. is within the range of Preferably, the reaction time is selected from 1 to 10 hours.

The liquid crystal polyester used in the present invention preferably has a flow initiation temperature of 250° C. or higher. If the flow start temperature of this liquid crystal polyester is within this range, when a conductive layer (electrode) is formed on a layer containing this liquid crystal polyester, there will be a higher degree of resistance between the layer containing this liquid crystal polyester and this conductive layer. adhesion tends to be obtained. The flow start temperature herein refers to the temperature at which the melt viscosity of the liquid crystalline polyester becomes 4800 Pa·s or less under a pressure of 9.8 MPa in evaluating the melt viscosity using a flow tester. This flow start temperature is well known to those skilled in the art as a guideline for the molecular weight of liquid crystal polyester (for example, Naoyuki Koide, ed., "Liquid Crystal Synthesis, Molding, and Applications", pp. 95-105, CMC, 1987). (Refer to issue published June 5, 2017).

The upper limit of the flow start temperature of the liquid crystal polyester is determined within the range in which the liquid crystal polyester is soluble in the solvent, and is preferably 350° C. or lower. If the upper limit of the flow start temperature is within this range, not only will the solubility of the liquid crystalline polyester in the solvent be better, but also the viscosity will not increase significantly when the liquid composition described below is obtained. There is a tendency for the handling properties to be good. In order to control the flow start temperature of the liquid crystal polyester within such a suitable range, the polymerization conditions for the solid phase polymerization may be appropriately optimized.

[Filler added layer]
The laminated film 1 of the present invention is provided with a filler-added layer 10 made of a synthetic resin to which a filler is added, thereby making it possible to lower the linear expansion coefficient of the laminated film 1. Thereby, for example, when manufacturing a metal-clad laminate using the laminated film 1 of the present invention, the coefficient of linear expansion can be made close to that of the metal foils to be bonded together.

As the synthetic resin constituting this filler-added layer 10, various synthetic resins known as materials for insulating films can be used, but polyimide or liquid crystal polyester is preferably used from the viewpoint of heat resistance, and low hygroscopicity. Therefore, it is preferable to use the same liquid crystal polyester as the liquid crystal polyester layers 11 and 12 described above.

The filler added to the filler-added layer 10 is added to adjust the linear expansion coefficient of the entire laminated film 1 (mainly for the purpose of lowering it), and is preferably an insulating inorganic filler, such as silica, talc, Silica nitride, aluminum nitride, etc. can be used. Further, in addition to the inorganic filler, a fluorine-based powder, which will be described later, may be added. Below, the inorganic filler will be explained first.

The blending ratio of the inorganic filler is in the range of 0.5 to 30 parts by mass per 100 parts by mass of the solid content of the liquid crystal polyester, preferably 1 to 15 parts by mass in order to maintain the toughness of the finished film, and more preferably in the range of low dielectric constant. It is 0.5 to 7 parts by mass.

The average particle size of the inorganic filler used is 0.001 to 15 μm, preferably 0.05 to 3 μm, more preferably 0.05 to 1 μm based on the thickness of the filler addition layer 10, and the particle shape may be spherical or acicular. , it may be of any other shape, or may be of an amorphous shape.

As such an inorganic filler, in order to make the obtained laminated film 1 have a low dielectric constant, it is preferable to use water-free heated silica or the like obtained by a combustion method.

Note that it is also possible to use as a filler an insulating inorganic powder produced by a method other than the above, such as a wet pulverization method or a normal pulverization method, but in this case, in order to obtain a filler with a low moisture content, It is desirable to dry the powder by heating at a temperature of 100°C to 400°C or higher for 30 minutes to 12 hours.

Further, in the present invention, in addition to the above-mentioned inorganic filler, fluorine-based powder may be added to the filler-added layer 10 as a linear expansion modifier or an extender.

The fluorine-based powder is a powder of a composition containing fluorine, and is preferably hydrophobic.

As the fluorine-based powder, for example, powders such as PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxyalkane), and FEP (perfluoroethylene propene copolymer) can be used. Furthermore, for example, a copolymer of tetrafluoroethylene (component A) and a monomer containing an unsaturated hydrocarbon as a main component (component B) (see Figure 6. In addition, R in the figure is a hydrogen atom, It is also possible to use a fluorine-based powder consisting of a hydroxyl group, an organic group having a hydroxyl group, or an alkyl group (preferably an alkyl group).

Note that the monomer containing an unsaturated hydrocarbon as a main component may have a hydroxyl group. Regarding the hydroxyl group, for example, the hydroxyl group participates in an ester bond accompanying the polymerization of the liquid crystal polyester forming the filler-added layer 10, and the hydroxyl group drops and becomes hydrophobic.

The fluorine-based powder preferably has a particle size of 0.01 to 5 microns (μm), but is not limited thereto. Further, the average particle diameter of the fluorine-based powder is particularly preferably 0.2 to 0.25 (μm), but is not limited to this.

Further, the blending ratio of the fluorine-based powder is in the range of 1 to 10 parts by mass, preferably 2 to 4 parts by mass, based on 100 parts of solid content of the liquid crystal polymer forming the filler-added layer 10.

As described above, the coefficient of linear expansion of the entire laminated film 1 can be adjusted by adding the filler in an amount within the range mentioned above and adjusting the thickness of the liquid crystal polyester layers 11 and 12 and the filler-added layer 10. For example, the linear expansion coefficient is adjusted to approach the linear expansion coefficient of the metal foil bonded to the laminated film 1. For example, the difference between the coefficient of linear expansion of the metal foil and the coefficient of linear expansion of the laminated film 1 is adjusted so that the value of the coefficient of linear expansion of the laminated film 1 is 25% or less of the coefficient of linear expansion of the metal foil. It's good as well.

As an example, when the coefficient of linear expansion of the copper foil that is the metal foil is 19 ppm, 25% thereof is 4.75, and in this case, the coefficient of linear expansion of the laminated film 1 of the present invention is 23.75 ppm or less. Adjust accordingly.

Although FIG. 1 shows an example in which the filler-added layer 10 has a single-phase structure, the filler-added layer 10 can be further formed into a multilayer structure, and the amount of filler added in each layer can be changed to add filler in the thickness direction. The amount may have a gradient.

In this case, in the configuration shown in FIG. 1, it is preferable to adopt a gradient structure in which the amount of filler added decreases toward the first liquid crystal polyester layer 11 side and the second liquid crystal polyester layer 12 side.

Further, regarding the laminated film 1 of the present invention, for example, a resin layer, a metal layer, etc. may be further laminated on both sides of the three-layered laminated film 1 shown in FIG. 1.

[Method for manufacturing laminated film 1]
Next, as shown in FIG. 1, a laminated structure in which liquid crystal polyester layers 11 and 12 to which no filler is added are laminated on both sides (front and back surfaces) of a filler-added layer 10 made of a synthetic resin to which a filler is added is formed. An example of a method for manufacturing the laminated film 1 having the following will be described with reference to FIGS. 2 to 4.

The method for manufacturing the laminated film 1 includes a step (first step) of manufacturing a first laminate L1 in which a laminated film precursor 20 is laminated on a base material 40, and a step of manufacturing the first laminate L1 from the base material 40 of the first laminate L1. After peeling off the laminated film precursor 20, the laminated film precursor 20 is transferred onto the surface of a metal base material 50 to form a second laminated body L2 in which the laminated film precursor 20 is laminated on the metal base material 50. (second step); and a step (third step) of heat-treating the second laminate L2 to produce a third laminate L3 in which the laminate film 1 is laminated on the metal base material 50. , includes a film peeling step (fourth step) of peeling the laminated film 1 from the metal base material 50 of the third laminate L3.

Note that the above-mentioned laminated film precursor 20 is a stage before the final product, the laminated film 1, in the manufacturing process of the laminated film 1 of the present invention, and is transformed into the laminated film 1 of the present invention by heat treatment. The precursor layer (first precursor layer) 21 of the liquid crystal polyester layer 11, the precursor layer (second precursor layer) 22 of the filler-added layer 10, and the precursor of the liquid crystal polyester layer It consists of a body layer (third precursor layer) 23.

The first to fourth steps described above will be explained below.

[Process of manufacturing the first laminate L1]
In the step (first step) of manufacturing the first laminate L1, a liquid composition consisting of a synthetic resin precursor and a solvent to form each layer of the laminate film 1 is applied from a coating machine onto the base material 40, which is processing paper. 31 to 33 were cast in order. More specifically, a first liquid composition 31 consisting of a precursor and a solvent for the first liquid crystal polyester layer 11, and a second liquid composition 32 containing the filler and consisting of a synthetic resin precursor and a solvent. , a casting step of sequentially casting a third liquid composition 33 consisting of a precursor of the second liquid crystal polyester layer 12 and a solvent, and a drying step of drying the liquid compositions 31 to 33 at a predetermined temperature for a predetermined time. Through these steps, the first laminate L1 in which the above-described laminate film precursor 20 is formed on the base material 40 is obtained.

First, regarding the liquid compositions 31 to 33, as described above, the liquid compositions are composed of two components, a precursor of the synthetic resin forming each layer and a solvent, and hereinafter, the synthetic resin is the liquid crystal composition described above. The liquid composition in the case of polyester will be explained in detail.

Regarding the liquid composition comprising the liquid crystal polyester and a solvent, the solvent used for dissolving the liquid crystal polyester is not particularly limited as long as it can dissolve the liquid crystal polyester, and examples thereof include N,N-dimethylacetamide, N-methyl -2-pyrrolidone, N-methylcaprolactam, N,N-dimethylformamide, N,N-diethylformamide, N,N-diethylacetamide, N-methylpropionamide, dimethyl sulfoxide, γ-butyrolactone, dimethylimidazolidinone, tetra Examples include methylphosphoric amide and ethyl cellosolve acetate, and halogenated phenols such as p-fluorophenol, p-chlorophenol, and perfluorophenol. These solvents may be used alone or in combination of two or more.

Among such solvents, from the viewpoint of handling, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, N-methylcaprolactam, N,N-dimethylformamide, N,N-diethylformamide, N,N-diethylacetamide , N-methylpropionamide, dimethylsulfoxide, γ-butyrolactone, dimethylimidazolidinone, tetramethylphosphoricamide and ethyl cellosolve acetate are preferred.

The amount of this solvent to be used can be selected as appropriate depending on the type of solvent to be applied, as long as it produces a liquid composition containing 0.1% by mass or more of liquid crystalline polyester. The amount is preferably 0.5 to 50 parts by weight, more preferably 10 to 30 parts by weight, per part of the liquid crystal polyester.

When the amount of liquid crystal polyester is less than 0.5 parts by mass, the viscosity of the liquid composition 31 tends to be too low to be uniformly coated, and when it exceeds 50 parts by mass, the viscosity tends to increase.

In the liquid composition thus obtained, the liquid crystal polyester exists in the state of a precursor, and this liquid composition is further diluted with the organic solvent to produce a 0.5 g/dl solution of the liquid crystal polyester. The intrinsic viscosity at 25° C. is 0.1 to 10.

Next, the casting process of casting the liquid compositions 31 to 33 onto the base material 40 will be described.

The base material 40 is not particularly limited as long as the laminated film precursor 20 can be peeled off, but glass plates, stainless steel foils, polyethylene terephthalate films, polyethylene films, polypropylene films, polymethylpentene films, polytetra Fluoroethylene sheets and the like are preferred.

In addition, various known methods can be used to cast the liquid composition onto the base material 40, such as a roller coating method, a gravure coating method, a knife coating method, a blade coating method, a rod coating method, Dip coating method, spray coating method, curtain coating method, slot coating method, screen printing method, etc. can be mentioned. Among these methods, knife coating method is preferred because it is easy to control and can make the film thickness uniform with high precision. Alternatively, a slot coating method is preferred.

Next, a drying process of forming the laminated film precursor 20 by drying the liquid compositions 31 to 33 cast onto the base material 40 in the casting process will be described.

In this drying process, if the drying temperature is too high, the precursor will start polymerizing and become a polymer before the heat treatment described below, and defects may occur on the coating surface. If the temperature is too low, it will take a long time to dry and the productivity will decrease, so the temperature is 60°C or higher and 160°C or lower, preferably 150°C or lower, more preferably 140°C or lower.

Note that in this drying step, it is preferable that the drying is completed while the solvent contained in the liquid composition described above remains, but the solvent may be completely removed.

Regarding the amount of remaining solvent, the amount of remaining solvent in the laminated film precursor 20 is preferably 18 to 2% by mass, more preferably 15 to 5% by mass. If the amount of residual solvent is 18% by mass or less, it is possible to suppress the occurrence of stickiness on the surface of the laminated film precursor 20, and it is possible to prevent the films from sticking to each other. In addition, if the amount of residual solvent is 2% by mass or more, it becomes possible to maintain the film strength of the laminated film precursor 20, and the laminated film can be used in the manufacturing process (second step) of the second laminate L2 described below. Breaking of the laminated film 1 can be prevented when the precursor 20 is peeled from the base material 40 or during heat treatment in the manufacturing process of the third laminate L3, which will be described later.

Next, to give an example of the step (first step) of actually manufacturing the first laminate L1 by the above-mentioned casting step and drying step, as shown in FIG. 2, each layer to be formed will be described below. , by repeating the first to third precursor layer forming steps consisting of the casting step and the drying step three times, a first laminate L1 in which the laminate film precursor 20 is laminated on the base material 40 is obtained. can be obtained.

In the first precursor layer forming step, a liquid composition 31 containing no filler and consisting of a liquid crystal polyester precursor and a solvent is cast onto the base material 40, and is dried to form a liquid composition 31 on the base material 40. , a precursor layer (first precursor layer) 21 of the first liquid crystal polyester layer 11 containing no filler is formed. Note that the solvent used, the casting method, the temperature conditions during drying, etc. are as described above.

In the second precursor layer forming step, a synthetic resin precursor containing the filler (for example, an insulating inorganic filler) is further formed on the first precursor layer 21 formed on the base material 40. A liquid composition 32 consisting of a solvent, in this embodiment, a liquid composition 32 consisting of a liquid crystal polyester precursor and a solvent is cast and dried to form a first layer containing the filler on the first precursor layer 21. 2 precursor layer 22 is formed. In this second casting and drying step, the solvent used, the casting method, the temperature conditions during drying, etc. are as described above.

In the third precursor layer forming step, a liquid composition consisting of a liquid crystalline polyester precursor and a solvent, which does not contain a filler, is added on the second precursor layer 22 formed in the second precursor layer forming step. This is a step of further forming a third precursor layer 23 on the second precursor layer 22 by casting a material 33 and drying it. In addition, also in this third precursor layer forming step, the solvent used, the casting method, the temperature conditions during drying, etc. are as described above.

Through the first to third precursor layer forming steps described above, a precursor layer (first precursor layer) 21 of the liquid crystal polyester layer 11 and a precursor layer (first precursor layer) of the filler-added layer 10 are formed on the base material 40. A first laminate L1 is obtained in which a laminate film precursor 20 consisting of a second precursor layer) 22 and a precursor layer (third precursor layer) 23 of the liquid crystal polyester layer is laminated. Note that the liquid crystal polyester precursor includes a low molecular weight liquid crystal polyester such as a liquid crystal polyester oligomer.

Thereafter, as shown in FIG. 2, this first laminate L1 is wound up into a roll.

In addition, as another example of the step (first step) of manufacturing the first laminate L1, as shown in FIG. 5, the above-mentioned The first laminate L1 may be manufactured by simultaneously stacking and casting three layers of the three liquid compositions 31 to 33 and drying the three layers simultaneously.

[Step of manufacturing the second laminate L2]
When the first laminate L1 in which the laminated film precursor 20 is laminated on the base material 40 is manufactured in the step of manufacturing the first laminate L1 described above (first step), the manufacturing process of the second laminate L2 is performed. (Second step), as shown in FIG. 3 as an example, the roll-shaped first laminate L1 is unrolled, the laminate film precursor 20 is peeled off from the base material 40, and then a rubber-like elastic layer is formed on the surface. The laminated film precursor 20 is transferred onto the surface of the metal base material 50 having the above-mentioned structure. Then, a second laminate L2 is obtained in which the metal base material 50 is laminated on the back surface of the laminate film precursor 20. Thereafter, this second laminate L2 is wound up into a roll.

At this time, the metal base material 50 is laminated on the back surface of the laminated film precursor 20 (that is, the surface that was in contact with the base material 40), but as long as the surface of the base material 40 is smooth, the laminated film precursor The back surface of the body 20 also becomes smooth, and the adhesion between the laminated film precursor 20 and the metal base material 50 can be ensured.

Here, examples of the material of the metal base material 50 include aluminum, stainless steel, iron, copper, and the like. Among these, stainless steel is particularly preferred from the viewpoint of strength and corrosion resistance.

Further, the thickness of the metal base material 50 is preferably within the range of 20 to 200 μm. When the thickness of the metal base material 50 is 20 μm or more, the metal base material 50 has high resistance to dents and is excellent in recyclability. If the thickness of the metal base material 50 is 200 μm or less, it will be easy to wind it up into a roll.

Further, the surface of the metal base material 50 may be subjected to any surface treatment as long as adhesion with the laminated film precursor 20 and peelability of the laminated film 1 in the film peeling step (fourth step) described later can be ensured. can be applied. For example, in order to make it difficult for the rubber-like elastic layer on the surface of the metal base material 50 to peel off from the metal base material 50, the surface of the metal base material 50 may be embossed.

Examples of the rubbery elastic layer include a silicone rubber elastic layer, a fluorine rubber elastic layer, and an acrylic rubber elastic layer. Among these, silicone rubber elastic layers are particularly preferred. A silicone-based rubber elastic layer has good adhesion to the metal base material 50, and at the same time has good peelability of the laminated film 1 in the film peeling step (fourth step) described later.

Further, the thickness of the rubber-like elastic layer is preferably within the range of 5 to 100 μm. If the thickness of the rubber-like elastic layer is 5 μm or more, the difference in elastic modulus of the metal base material 50 can be sufficiently alleviated. If the thickness of the rubber-like elastic layer is 100 μm or less, chipping of the rubber-like elastic layer can be prevented when the metal base material 50 is handled.

The method for transferring the laminated film precursor 20 is not particularly limited, but as shown in FIG. Preferable from the viewpoint of improving productivity.

Furthermore, the transfer temperature of the laminated film precursor 20 is not particularly limited, but is preferably within the range of 10 to 200°C. If this transfer temperature is 10° C. or higher, the adhesion to the metal base material 50 is good. If this transfer temperature is 200° C. or less, the peelability of the laminated film 1 in the film peeling step (fourth step) described later is good.

[Step of manufacturing the third laminate L3]
After the second laminate L2 in which the laminated film precursor 20 is laminated on the metal base material 50 is manufactured in the step (third step) of manufacturing the second laminate L2 described above, the third laminate L3 is manufactured. Moving on to the step (third step), as shown in FIG. 4 as an example, the roll-shaped second laminate L2 is unrolled, and the second laminate L2 is heated in a heating furnace at a predetermined temperature for a predetermined time in a nitrogen atmosphere. Continuously heat-treated at 65°C. Then, a third laminate L3 consisting of the laminate film 1 and the metal base material 50, which substantially does not contain a solvent, is obtained.

At this time, since the heat treatment of the second laminate L2 is performed in a nitrogen atmosphere, deterioration of the laminate film 1 due to oxidation of the liquid crystal polyester can be prevented.

Note that the heat treatment temperature of the second laminate L2 is preferably within the range of 200 to 350°C. If the heat treatment temperature is 200° C. or higher, the molecular weight of the liquid crystal polyester is increased by the heat treatment, and the laminated film precursor 20 can exhibit characteristics as the laminated film 1. If this heat treatment temperature is 350° C. or lower, thermal decomposition of the laminated film 1 (liquid crystal polyester film) can be suppressed.

On the other hand, the heat treatment time for the second laminate L2 is not particularly limited, but usually the temperature is raised to the heat treatment temperature at a temperature increase rate of 10° C./min or less and then held at the same temperature for 0 to 10 hours.

Further, the form of heat treatment of the second laminate L2 is not particularly limited, but may be a form in which the second laminate L2 is continuously passed through the heating furnace 65 in a roll-to-roll manner (a method in which raw materials are supplied with a roll and the product is wound with a roll). Besides, for example, the form described in Japanese Patent Application Laid-open No. 2008-207537 can be adopted.

[Film peeling process]
As described above, after the third laminate L3 has been manufactured, the process moves to the film peeling step (fourth step), and the laminated film 1 is peeled from the metal base material 50, as shown in FIG. At this time, since the rubber-like elastic layer is provided on the surface of the metal base material 50, that is, the surface on the side of the laminated film 1, the laminated film 1 is easily peeled off from the metal base material 50.

Here, the method for peeling the laminated film 1 is not particularly limited, but as shown in FIG. is preferred.

After peeling off the laminated film 1, contaminants (silicone, fluorine-containing substances, etc.) may be removed from the metal base material 50 by solvent cleaning, UV treatment, corona treatment, plasma treatment, flame treatment, or other methods, if necessary. May be removed.

When the laminated film 1 is peeled off from the metal base material 50 in this way, the manufacturing process of the laminated film 1 is completed.

As described above, the laminated film 1 of the present invention includes a structure in which a coating film formed by casting and drying a liquid composition comprising a liquid crystal polyester precursor and a solvent is transferred onto the metal substrate 50 and then heated. Since the manufacturing method does not involve forming a film by flowing the liquid crystal polyester in a molten state in a fixed direction as in the hot melt casting method, the resulting laminated film 1 can be made entirely non-oriented, and tearing in the orientation direction does not occur. It is possible to solve the problem of general liquid crystalline polyester, such as easy liquid crystal polyester, and furthermore, it is possible to obtain a laminated film 1 in which warpage, distortion, etc. are less likely to occur because the anisotropy is relaxed.

In addition, in the laminated film 1 manufactured by the above method, not only the liquid crystal polyester layers 11 and 12 to which no filler is added, but also the filler-added layer 10 are non-oriented, and it has excellent mechanical strength in any direction. It was confirmed that it was.

Further, by providing the filler-added layer 10 on the laminated film 1, the linear expansion coefficient of the entire film can be brought close to that of, for example, a metal foil, and the laminated film 1 and the metal foil can be bonded together to form a metal-clad laminate. However, since the laminated surface with the metal foil is the liquid crystal polyester layer 11 (or 12) that does not contain filler, the addition of filler can prevent the laminated film 1 from warping or distorting. The advantageous properties of liquid crystal polyester, which are exhibited by its adhesiveness with metal foils not being impaired and its low moisture absorption, enable the metal-clad laminate to be highly functional.

1 Laminated film 10 Filler addition layer 11 Liquid crystal polyester layer (first liquid crystal polyester layer)
12 Liquid crystal polyester layer (second liquid crystal polyester layer)
20 Laminated film precursor 21 First precursor layer (precursor of liquid crystal polyester layer)
22 Second precursor layer (precursor of filler addition layer)
23 Third precursor layer (precursor of liquid crystal polyester layer)
31 First liquid composition 32 Second liquid composition 33 Third liquid composition 40 Base material 50 Metal base material 63 Roller 65 Heating furnace 67 Peeling roller L1 First laminate L2 Second laminate L3 Third laminate

Claims (9)

In a laminated film used in lamination with metal foil, a liquid crystalline polyester layer to which no filler is added is laminated on both sides of a filler-added layer made of a synthetic resin (excluding polyimide) to which a filler is added. A non-oriented laminated film ,
A laminated film characterized in that the difference between the linear expansion coefficient of the laminated film whose linear expansion coefficient has been reduced by addition of the filler and the linear expansion coefficient of the metal foil is 25% or less. 2. The laminated film according to claim 1, wherein the filler is silica, talc, silica nitride, aluminum nitride, or fluorine-based powder. 3. The laminated film according to claim 1 , wherein the synthetic resin is liquid crystal polyester. The laminated film according to any one of claims 1 to 3 , having a thickness of 3 to 100 μm. A laminated film used by laminating with metal foil, in which liquid crystalline polyester layers to which no filler is added are laminated on both sides of a filler-added layer made of synthetic resin (excluding polyimide) to which filler is added. A method for producing a laminated film having a laminated structure, the method comprising :
The manufacturing process of the laminated film includes:
A first liquid composition comprising a precursor of liquid crystal polyester and a solvent as a base material, a second liquid composition containing a filler and comprising a precursor of the synthetic resin and a solvent, and a precursor of liquid crystal polyester and a solvent. By sequentially casting and drying a third liquid composition consisting of a precursor layer of the liquid crystal polyester layer, a precursor layer of the filler-added layer, and a precursor layer of the liquid crystal polyester layer on the base material, a first laminate manufacturing step of manufacturing a first laminate in which a laminate film precursor is laminated;
After peeling the laminated film precursor from the base material of the first laminate, the laminated film precursor is transferred to a metal base material having a release layer on the surface, thereby forming a laminated film on the metal base material. a second laminate manufacturing step of manufacturing a second laminate in which the precursor is laminated;
a third laminate manufacturing step of manufacturing a third laminate in which a laminate film is laminated on the metal base material by heat-treating the second laminate;
and a film peeling step of peeling the metal base material from the third laminate, and determining the difference between the linear expansion coefficient of the laminated film whose linear expansion coefficient has been reduced by the addition of the filler and the linear expansion coefficient of the metal foil. A method for producing a laminated film, characterized in that the content is 25% or less . 6. The method for producing a laminated film according to claim 5 , wherein the base material is a glass plate, stainless steel foil, polyethylene terephthalate film, polyethylene film, polypropylene film, polymethylpentene film, or polytetrafluoroethylene sheet. 7. The method for manufacturing a laminated film according to claim 5 , wherein the material of the metal base material is aluminum, stainless steel, iron, or copper. The method for producing a laminated film according to any one of claims 5 to 7 , wherein the release layer is a rubber-like elastic layer. The method for producing a laminated film according to any one of claims 5 to 8 , wherein the heat treatment is performed under an inert gas atmosphere.

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