JP2017043668A - Polyimide film and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a graphite sheet having few surface projections and a good appearance, and a polyimide film to serve as a raw material for the graphite sheet.SOLUTION: A polyimide film has inorganic particles so that the percentage of the inorganic particles becomes 0.05-0.8 wt.% per film resin weight, the inorganic particle having a maximum dispersion diameter of 15 μm or less.SELECTED DRAWING: None

Description

  TECHNICAL FIELD The present invention relates to a graphite sheet having an excellent appearance with few surface protrusions, used as a heat radiating material and a soaking material, a polyimide film used for producing the graphite sheet, and a method for producing the same.

  Polyimide film is known to have excellent properties in heat resistance, cold resistance, chemical resistance, electrical insulation, mechanical strength, and the like. Electrical insulation material for wire, heat insulating material, flexible printed circuit board (FPC) It is widely used as a base film, a carrier tape film for IC (Integrated Circuit) tape automated bonding (TAB), a lead frame fixing tape for IC, and the like.

  An important practical characteristic when a polyimide film is used in these applications is the slipperiness (slidability) of the film. This improves operability and handleability in each process by ensuring the slidability between the film support (for example, roll) and the film and the slidability between films in various film processing steps. Furthermore, it is because generation | occurrence | production of defective parts, such as a wrinkle, can be avoided on a film.

  In the conventional smoothing technology for polyimide films, an inert inorganic compound (for example, alkaline earth metal orthophosphate, dibasic calcium phosphate anhydride, calcium pyrophosphate, silica, talc, etc.) is added to polyamic acid as a filler. (See Patent Document 1).

  On the other hand, a flexible graphite sheet having flexibility and elasticity can be obtained by heat-treating (firing) the polyimide film in an inert gas at a temperature of 2400 ° C. or higher to form a graphite, followed by rolling. It is known (see Patent Documents 2 and 3). Since this graphite sheet has a higher thermal conductivity than a metal sheet such as copper or aluminum, it has recently attracted attention as a heat radiating member for electronic devices.

It is known that it is preferable to add an inorganic filler in order to obtain a good quality graphite sheet also in the use of a graphite sheet obtained by firing a polyimide film (see Patent Document 4).
The role of the inorganic particles added to the polyimide film used as a raw material for graphite sheet is to expand (swell) by the gas generated when the inorganic particles dispersed in the film are sublimated from the inside of the film when firing the polyimide film. ). Due to this swelling, it is possible to improve the properties such as flexibility and breaking strength of the graphite sheet obtained by applying the rolling treatment.
On the other hand, partial excessive swelling in the polyimide film creates a protrusion defect on the surface of the graphite sheet and affects the appearance. In particular, when a thick polyimide film is used as a raw material, the gas permeability is low, so that the influence is likely to occur.
The following two methods are known as methods for controlling the degree of surface protrusions of this graphite sheet.

One of them is a method of applying a rapid thermal history in a specific temperature range at the initial stage of baking of the polyimide film, disturbing the orientation of molecular chains of the polyimide, and improving the gas permeability. By passing through this process, internal volatile gases such as inorganic particles generated during the firing process can be removed smoothly, so that a partial expansion is suppressed, and a graphite sheet with a small surface protrusion and a good surface appearance is obtained. (See Patent Document 5).
However, in the improvement of such firing conditions, it is difficult to apply the same thermal history to the entire firing container uniformly and unevenly, and this affects the quality variation and productivity of the graphite sheet. Improvement was needed.

Another method is to improve the excessive foaming (swelling) by controlling the particle size of the inorganic particles added to the polyimide film, which is the raw material of the graphite sheet, and the ratio of inorganic particles added per film resin weight. (See Patent Document 6).
However, there is a problem that the protrusion defects generated on the surface of the graphite sheet cannot be sufficiently improved only by controlling the maximum particle diameter and the addition ratio of the inorganic particles to be added.

JP-A-62-68852 JP-A-3-75211 JP-A-4-21508 JP-A-8-267647 JP 2014-129226 A JP 2014-136721 A

The present invention has been achieved as a result of studying the solution of the problems in the prior art described above as an issue.
Accordingly, an object of the present invention is to provide a graphite sheet with few surface protrusions and excellent appearance, and a polyimide film useful for obtaining this sheet.
Another object of the present invention is to provide a graphite sheet excellent in flexibility, breaking strength, and thermal diffusion coefficient, and a polyimide film useful for obtaining this sheet.
Furthermore, another object of the present invention is to provide a method for producing the above polyimide film and graphite sheet.

In Patent Document 6, as described above, it is described that the occurrence of abnormal swelling in the graphite sheet can be suppressed by adjusting the particle size and the addition ratio of the inorganic particles dispersed in the polyimide film. It describes that the particle diameter is 0.01 μm or more and 6.0 μm or less.
That is, in Patent Document 6, by setting the particle diameter of the inorganic particles used for dispersion in a specific range, the particle diameter can be reflected in the polyimide film and further in the graphite sheet, and abnormal swelling can be suppressed.

However, according to the study by the present inventors, even if the particle size of the inorganic particles used for dispersion is adjusted to 0.01 μm or more and 6.0 μm or less, the surface of the graphite sheet is not necessarily abnormally swollen. In some cases, protrusions, particularly large surface protrusions of 0.1 mm or more, were formed, and a graphite sheet with poor appearance was obtained.
Therefore, when the cause of the formation of such surface protrusions was investigated, in the polyimide film, even when the particle diameter of the inorganic particles was 0.01 μm or more and 6.0 μm or less, the mixture of inorganic particles and polyamic acid It has been found that the particle diameter of the inorganic particles may not reflect the dispersion diameter in the film due to aggregation of particles specific to the polyimide film, possibly due to the film production process.
Under such circumstances, the present inventors have conducted further intensive studies. As a result, when mixing the inorganic particles with the polyamic acid, instead of simply selecting particles having a small particle size, the slurry of the inorganic particles is previously filtered. Surprisingly, it is possible to suppress or prevent aggregation of the inorganic particles in the polyimide film (in other words, by selecting the proportion of the inorganic particles and the composition of the polyamic acid) (in other words, the particle size of the inorganic particles is reduced). The dispersion diameter can be made small, particularly 15 μm or less. Furthermore, by using a polyimide film having such a dispersion diameter of 15 μm or less, the formation of surface protrusions can be remarkably suppressed, and the appearance is excellent. The inventors have found that a graphite sheet can be obtained efficiently, and have further advanced research based on this finding, and have completed the present invention.

That is, the present invention relates to the following polyimide films and the like.
[1] A polyimide film in which inorganic particles are dispersed, wherein the proportion of inorganic particles is 0.05 to 0.8% by weight per film resin weight, and the maximum dispersion diameter of inorganic particles is 15 μm or less.
[2] The above-mentioned polyimide formed of at least a constituent comprising an aromatic diamine component containing 4,4′-diaminodiphenyl ether and an aromatic tetracarboxylic dianhydride component containing pyromellitic dianhydride [ 1].
[3] The polyimide film as described in [1] or [2], wherein the inorganic particles are contained at a ratio of 0.05 to 0.5% by weight per film resin weight.
[4] The polyimide film according to any one of [1] to [3], wherein the inorganic particles contain calcium hydrogen phosphate as a main component.
[5] The polyimide film according to any one of [1] to [4], wherein the film thickness is 25 to 80 μm.
[6] A method for producing a polyimide film according to any one of [1] to [5] above using a polyamic acid solution containing inorganic particles, wherein the slurry containing inorganic particles is filtered. Then, the manufacturing method including the process of mixing with a polyamic acid and obtaining a polyamic acid solution.
[7] A graphite sheet containing the polyimide film according to any one of [1] to [6] as a raw material and having 3 protrusion defects having a diameter of 0.1 mm or more on the surface and 50 cm ² or less.
[8] A method for producing a graphite sheet, comprising firing the polyimide film according to any one of [1] to [6].

The polyimide film of the present invention can provide a graphite sheet with few surface protrusions and an excellent appearance.
Further, according to the present invention, a graphite sheet excellent in flexibility, breaking strength, and thermal diffusion coefficient can be obtained.

It is a photograph of the surface protrusion defect by the partial excessive foaming of the graphite sheet of the comparative example 1.

Hereinafter, the present invention will be described in detail.
The polyimide film of the present invention is produced by imidizing a precursor polyamic acid (polyamic acid). The polyamic acid is obtained by addition polymerization of a diamine component and an acid dianhydride component in an organic solvent. First, the polyamic acid solution will be described.

[Polyamide acid]
In the present invention, the diamine component is preferably an aromatic diamine component, and the acid dianhydride component is preferably an aromatic tetracarboxylic dianhydride component.

  Examples of the aromatic diamine component include 4,4′-diaminodiphenyl ether, paraphenylenediamine, 3,3′-diaminodiphenyl ether, metaphenylenediamine, 4,4′-diaminodiphenylpropane, and 3,4′-diaminodiphenylpropane. 3,3'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 3,3'-diaminodiphenylmethane, benzidine, 4,4'-diaminodiphenyl sulfide, 3,4'- Diaminodiphenyl sulfide, 3,3′-diaminodiphenyl sulfide, 4,4′-diaminodiphenyl sulfone, 3,4′-diaminodiphenyl sulfone, 3,3′-diaminodiphenyl sulfone, 2,6-diaminopyridine, bis- ( 4-aminopheny L) Diethylsilane, 3,3′-dichlorobenzidine, bis- (4-aminophenyl) ethylphosphinoxide, bis- (4-aminophenyl) phenylphosphinoxide, bis- (4-aminophenyl)- N-phenylamine, bis- (4-aminophenyl) -N-methylamine, 1,5-diaminonaphthalene, 3,3′-dimethyl-4,4′-diaminobiphenyl, 3,4′-dimethyl-3 ′ , 4-diaminobiphenyl, 3,3′-dimethoxybenzidine, 2,4-bis (p-β-amino-t-butylphenyl) ether, bis (p-β-amino-t-butylphenyl) ether, p- Bis (2-methyl-4-aminopentyl) benzene, p-bis- (1,1-dimethyl-5-aminopentyl) benzene, m-xylylenediamine, p-xylylene Range amine, 2,5-diamino-1,3,4-oxadiazole, 2,2-bis (4-aminophenyl) hexafluoropropane, N- (3-aminophenyl) -4-aminobenzamide, 4- Examples thereof include aminophenyl-3-aminobenzoate, and preferably 4,4′-diaminodiphenyl ether. These can be used alone or in combination of two or more.

  Examples of the aromatic tetracarboxylic dianhydride component include pyromellitic dianhydride, 3,3 ′, 4,4′-biphenyltetracarboxylic dianhydride, and 2,3 ′, 3,4′-biphenyl. Tetracarboxylic dianhydride, 3,3 ′, 4,4′-benzophenonetetracarboxylic dianhydride, 2,3,6,7-naphthalenedicarboxylic dianhydride, 2,2-bis (3,4 Dicarboxyphenyl) ether, pyridine-2,3,5,6-tetracarboxylic dianhydride, naphthalene tetracarboxylic dianhydride (for example, 1,2,4,5-naphthalene tetracarboxylic dianhydride, 1 , 4,5,8-naphthalenetetracarboxylic dianhydride, 1,4,5,8-decahydronaphthalenetetracarboxylic dianhydride, 4,8-dimethyl-1,2,5,6-hexahydronaphthalene Tetra Rubonic acid dianhydride, 2,6-dichloro-1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,7-dichloro-1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-tetrachloro-1,4,5,8-naphthalenetetracarboxylic dianhydride, 1,8,9,10-phenanthrenetetracarboxylic dianhydride, 2,2-bis (2 , 3-dicarboxyphenyl) propane dianhydride, 1,1-bis (3,4-dicarboxyphenyl) ethane dianhydride, 1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride, Bis (2,3-dicarboxyphenyl) methane dianhydride, bis (3,4-dicarboxyphenyl) methane dianhydride, bis (3,4-dicarboxyphenyl) sulfone dianhydride, benzene-1,2 , 3,4 Tetracarboxylic acid dianhydride, 3,4,3 ', 4'-benzophenone tetracarboxylic dianhydride, etc.) and the like, preferably, pyromellitic dianhydride or the like. These can be used alone or in combination of two or more.

  That is, the polyamic acid preferably has two components, 4,4′-diaminodiphenyl ether and pyromellitic dianhydride as essential components, and in addition to these two components, other diamine components and other acid dianhydrides. A physical component may be contained in the range which does not prevent the effect of this invention.

In the present invention, the polyamic acid contains an aromatic diamine component (for example, a component containing 4,4′-diaminodiphenyl ether) as a main component (for example, 50 to 100 mol%, preferably 60 to 100% of all diamine components). Mol%, more preferably 70 to 100 mol%).
When the aromatic diamine component is composed of 4,4′-diaminophenyl ether, the ratio of 4,4′-diaminodiphenyl ether to the whole aromatic diamine component is, for example, 50 to 100 mol%, preferably 60 to 100. It may be about mol%, more preferably about 70 to 100 mol%.

In addition, the polyamic acid is composed of an aromatic tetracarboxylic dianhydride component (for example, a component containing pyromellitic dianhydride) as a main component (for example, 50 to 100 mol% of the total acid dianhydride component, Preferably it is included as 60 to 100 mol%, more preferably 70 to 100 mol%.
In addition, when comprising an aromatic tetracarboxylic dianhydride component with a pyromellitic dianhydride, the ratio of the pyromellitic dianhydride with respect to an aromatic tetracarboxylic dianhydride component is 50-100 mol%, for example. , Preferably it may be 60-100 mol%, More preferably, about 70-100 mol% may be sufficient.
In the present invention, as described above, another diamine component (non-aromatic diamine component) may be included as the diamine component. The acid dianhydride component may contain other acid dianhydride components (non-aromatic tetracarboxylic anhydride components such as aliphatic tetracarboxylic dianhydride components).

  In the present invention, specific examples of the organic solvent used for forming the polyamic acid solution include sulfoxide solvents such as dimethyl sulfoxide and diethyl sulfoxide, N, N-dimethylformamide, N, N-diethylformamide and the like. Formamide solvents, N, N-dimethylacetamide, acetamide solvents such as N, N-diethylacetamide, pyrrolidone solvents such as N-methyl-2-pyrrolidone and N-vinyl-2-pyrrolidone, phenol, o-, Examples thereof include phenolic solvents such as m- or p-cresol, xylenol, halogenated phenol, and catechol, and aprotic polar solvents such as hexamethylphosphoramide and γ-butyrolactone. These may be used alone or as a mixture. It is desirable to use Emissions, the use of aromatic hydrocarbons such as toluene are also possible.

The polymerization method may be performed by any known method, for example,
(1) A method in which the total amount of the diamine component is first put in a solvent, and then the acid dianhydride component is added so as to be equivalent to the total amount of the diamine component and polymerized.
(2) A method in which the total amount of the acid dianhydride component is first put in a solvent, and then the diamine component is added to be equivalent to the acid dianhydride component for polymerization.
(3) After putting one diamine component in the solvent, after mixing for the time required for the reaction at a ratio that the acid dianhydride component is 95 to 105 mol% with respect to the reaction component, the other diamine component is And then polymerizing the other acid dianhydride component so that the total diamine component and the total acid dianhydride component are approximately equivalent.
(4) After one acid dianhydride component is placed in a solvent, the diamine component is mixed with the reaction component at a ratio of 95 to 105 mol% for the time required for the reaction, and then the other acid A method in which a dianhydride component is added, and then the other aromatic diamine component is added and polymerized so that the total diamine component and the total acid dianhydride component are approximately equivalent.
(5) A polyamic acid solution (A) is prepared by reacting one of the diamine components and the acid dianhydride component in a solvent so that either one becomes excessive, and the other diamine component and the acid dianhydride in another solvent. The polyamic acid solution (B) is prepared by reacting either one of the anhydride components in excess. A method in which the polyamic acid solutions (A) and (B) thus obtained are mixed to complete the polymerization. At this time, when the polyamic acid solution (A) is adjusted, if the diamine component is excessive, the polyamic acid solution (B) has excessive acid dianhydride component, and the polyamic acid solution (A) has excessive acid dianhydride component. In the case of the polyamic acid solution (B), the diamine component is excessive, the polyamic acid solutions (A) and (B) are mixed, and the total diamine component and total acid dianhydride component used in these reactions are approximately equivalent. Adjust so that The polymerization method is not limited to these, and other known methods may be used.

  The polyamic acid solution thus obtained contains, for example, 5 to 40% by weight, preferably 10 to 30% by weight, of the solid content of the polyamic acid. The viscosity is a value measured with a Brookfield viscometer, for example, 10 to 2000 Pa · s, preferably 100 to 1000 Pa · s is preferably used for stable liquid feeding. Moreover, the polyamic acid in the organic solvent solution may be partially imidized.

[Inorganic particles]
It is preferable that the inorganic particles added to the polyimide film of the present invention are insoluble in all chemical substances that are contacted in the polyimide film manufacturing process.

Examples of inorganic particles that can be used in the present invention include oxides {eg, SiO ₂ (silica), TiO ₂ (titanium oxide (IV)), etc.}, inorganic acid salts {eg, CaHPO ₄ (calcium hydrogen phosphate) And phosphoric acid (hydrogen) salts such as CaPO ₄ (calcium phosphate), CaCO ₃ (calcium carbonate), and Ca ₂ P ₂ O ₇ (calcium diphosphate). Above all in the case of using a CaHPO ₄ containing phosphoric acid, for good foaming by gas generated when the sublimed from within a polyimide film (swelling) occurs, good graphite sheet having excellent thermal conductivity is obtained, CaHPO ₄ Is a main component (including, for example, 50 to 100% by weight, preferably 70 to 100% by weight of all inorganic particles).

  Use inorganic particles as a slurry (inorganic particle slurry) dispersed in a solvent (for example, a polar solvent such as N, N-dimethylformamide, N, N-dimethylacetamide, dimethylsulfoxide, N-methylpyrrolidone). Is preferable because aggregation can be prevented. Since this slurry has a very small particle size, the sedimentation rate is low and stable. Even if it settles, it can be easily redispersed by re-stirring.

  The manufacturing method of an inorganic particle slurry is not specifically limited, You may follow a conventionally well-known method. As a manufacturing method of an inorganic particle slurry, the method of mixing an inorganic particle and a solvent using a mixer etc. are mentioned, for example. As the mixer, it is preferable to use a mixer having a high shearing force such as a high-speed disper, a homomixer, a ball mill, a coreless mixer, and a stirring disper. Further, wet pulverization may be performed to reduce the average particle size. For example, a bead mill or a sand mill can be used for the wet pulverization treatment.

As the inorganic particle slurry, a commercially available product in which inorganic particles are dispersed in a solvent in advance may be used.
Moreover, the inorganic particle slurry may contain other organic solvents, compounding agents, and the like as necessary.

  The concentration of the inorganic particles in the inorganic particle slurry is not particularly limited, but is, for example, 1 to 80% by weight, preferably 1 to 60% by weight, and more preferably 1 to 40% by weight with respect to 100% by weight of the inorganic particle slurry. It is.

  The average particle diameter of the inorganic particles (inorganic particles used for dispersion) is preferably from the standpoint that the aggregation of the inorganic particles in the polyimide film can be reduced, and the foaming of the inorganic particles by the sublimation gas when the polyimide film is fired is uniform. It is 0.5-1.5 micrometers, More preferably, it is 0.5-1.3 micrometers, More preferably, it is 0.5-1.0 micrometers. The thickness of 0.5 μm or more is preferable from the viewpoints of improving the slipperiness of the polyimide film, the flexibility of the graphite sheet, the breaking strength, and the like. Moreover, aggregation of inorganic particles in the polyimide film can be suppressed when it is 1.5 μm or less, and as a result, partial excessive foaming due to the sublimation gas of the inorganic particles at the time of firing can be suppressed. This is preferable from the viewpoint of reducing surface protrusion defects. In addition, the measuring method of the average particle diameter of an inorganic particle is not specifically limited, For example, it can measure using the method as described in the below-mentioned Example.

  As for the particle size distribution of the inorganic particles, it is better that the particle size distribution is narrow, that is, the proportion of the inorganic particles having a similar size in the total inorganic particles is high. Specifically, the particle size is 0.5 to 2.5 μm. It is preferable that the inorganic particles occupy a proportion of 80% by volume or more (for example, 80 to 100% by volume) in all the inorganic particles. If it is this range, it is preferable from a viewpoint that the characteristics, such as the slipperiness of a film, the softness | flexibility of a graphite sheet, and breaking strength, become favorable. In addition, the measuring method of the particle size distribution of an inorganic particle is not specifically limited, For example, it can measure using the method as described in the below-mentioned Example.

[Production method of polyimide film]
Next, the manufacturing method of the polyimide film of this invention is demonstrated.
In the production of a polyimide film, first, polyamic acid and inorganic particles are mixed.
In the present invention, an inorganic particle slurry (preferably an inorganic particle slurry in which inorganic particles are dispersed in the same solvent as the organic solvent used for polyamic acid polymerization) is added to the polyamic acid solution, and then decyclized and desolvated. It is preferable to obtain a polyimide film, but after adding an inorganic particle slurry to an organic solvent before polyamic acid polymerization, a polyimide film is obtained through polyamic acid polymerization and decyclization desolvation. The inorganic particle slurry may be added in any step as long as it is a step before the solvent.

  In particular, the inorganic particle is not a powder but an inorganic particle slurry, and the inorganic particle slurry is filtered (for example, a pore diameter of 15 μm or less, preferably a pore diameter of 13 μm or less, more preferably a pore diameter of 11 μm or less, more preferably a pore diameter of 5 μm or less, particularly From the viewpoint of being able to suppress aggregation of inorganic particles and removing inorganic particles of 15 μm or more in the polyimide film by applying (filter treatment) (preferably a cut filter having a pore size of 3 μm or less). ,preferable.

  The material of the filter is not particularly limited, and examples thereof include polymer materials (for example, polyethylene, polypropylene, polytetrafluoroethylene), metals (for example, stainless steel), and the like.

  The added amount of the inorganic particles is usually 0.05 to 0.8% by weight, preferably 0.05 to 0.55% by weight, more preferably 0.00% per 1 weight of the polyimide resin solid content when the polyimide is formed. The amount is preferably from 0.5 to 0.52% by weight, particularly preferably from 0.05 to 0.5% by weight. When the amount of the inorganic particles is 0.05% by weight or more, a large amount of gas is generated when the inorganic particles dispersed in the polyimide film sublimate from the inside of the film, and the graphite sheet obtained by applying a rolling treatment is flexible, From the viewpoint of excellent properties such as breaking strength, it is preferable. Moreover, since aggregation of inorganic particles in the polyimide film can be suppressed when it is 0.8% by weight or less, partial excessive foaming due to sublimation gas of inorganic particles during polyimide film baking can be suppressed. This is preferable from the viewpoint of reducing surface protrusion defects.

  As a method for forming a polyimide film, a polyamic acid solution containing inorganic particles is cast into a film and thermally decyclized and desolvated to obtain a polyimide film, and a cyclization catalyst and a dehydrating agent are added to the polyamic acid solution. A method of obtaining a polyimide film by mixing and chemically decyclizing a gel film and heating and desolvating it is mentioned, but the latter method keeps the thermal expansion coefficient of the resulting polyimide film low Since the height distribution in the film surface direction can be improved, the graphite sheet is preferable because it has excellent thermal conductivity and can be thickened.

  In the method of chemically decyclizing, first, the polyamic acid solution is prepared. The polyamic acid solution can contain a cyclization catalyst (imidization catalyst), a dehydrating agent, a gelation retarder, and the like.

  Specific examples of the cyclization catalyst used in the present invention include aliphatic tertiary amines such as trimethylamine and triethylenediamine, aromatic tertiary amines such as dimethylaniline, and complex such as isoquinoline, pyridine and β-picoline. A ring tertiary amine etc. are mentioned, These can be used individually or in combination of 2 or more types. Among these, an embodiment in which at least one heterocyclic tertiary amine is used is preferable.

  Specific examples of the dehydrating agent used in the present invention include aliphatic carboxylic acid anhydrides such as acetic anhydride, propionic anhydride, and butyric anhydride, and aromatic carboxylic acid anhydrides such as benzoic anhydride, Of these, acetic anhydride and / or benzoic anhydride are preferred.

  As a method for producing a polyimide film from a polyamic acid solution, a polyamic acid solution containing a cyclization catalyst and a dehydrating agent is cast on a support from a base with a slit and formed into a film, and an imide is formed on the support. The gel film is partially advanced to form a gel film having self-supporting properties, and then peeled off from the support, heat-dried / imidized, and subjected to heat treatment.

  The polyamic acid solution is formed into a film shape through a slit-shaped die, cast on a heated support, undergoes a thermal ring-closing reaction on the support, and is supported as a gel film having self-supporting properties. It is peeled from the body.

  The support is a metal rotating drum or an endless belt, and its temperature is controlled by radiant heat from a liquid or gaseous heat medium and / or an electric heater.

  The gel film is heated to 30 to 200 ° C., preferably 40 to 150 ° C. by receiving heat from a support and / or receiving heat from a heat source such as hot air or an electric heater, and causes a ring-closing reaction, and a free organic solvent or the like. By drying the volatile matter, it becomes self-supporting and is peeled off from the support.

  The gel film peeled off from the support is usually stretched in the running direction while regulating the running speed with a rotating roll. Stretching is carried out at a temperature of 140 ° C. or lower at a magnification of 1.01-1.90 times, preferably 1.05-1.60 times, more preferably 1.10-1.50 times. The gel film stretched in the running direction is introduced into the tenter device, and both ends in the width direction are held by the tenter clip, and stretched in the width method while running with the tenter clip.

  The film dried in the drying zone is heated for 15 seconds to 30 minutes with hot air, an infrared heater or the like. Next, heat treatment is performed for 15 seconds to 30 minutes at a temperature of 250 to 500 using hot air and / or an electric heater. The thickness of the polyimide film is adjusted while adjusting the draw ratio in the running direction and the draw ratio in the width direction.

[Polyimide film]
The content of inorganic particles (that is, inorganic particles dispersed in the polyimide film of the present invention) in the polyimide film of the present invention is usually per polyimide film resin weight (that is, 1 weight of solid content of polyimide resin in the polyimide film). Is a proportion of 0.05 to 0.8% by weight, preferably 0.05 to 0.55% by weight, more preferably 0.05 to 0.52% by weight, particularly preferably 0.05 to 0.3% by weight. It is. When the amount of the inorganic particles is 0.05% by weight or more, a large amount of gas is generated when the inorganic particles dispersed in the polyimide film sublimate from the inside of the film, and the graphite sheet obtained by applying a rolling treatment is flexible, From the viewpoint of excellent properties such as breaking strength, it is preferable. Moreover, since aggregation of inorganic particles in the polyimide film can be suppressed when it is 0.8% by weight or less, partial excessive foaming due to sublimation gas of inorganic particles during polyimide film baking can be suppressed. This is preferable from the viewpoint of reducing surface protrusion defects.

In the polyimide film of the present invention, the maximum dispersion diameter (maximum dispersion particle diameter) of the inorganic particles is preferably 15 μm or less (for example, 3 to 15 μm, preferably 5 to 15 μm), more preferably 13 μm or less (for example, 3 ˜13 μm, preferably 5 to 13 μm), more preferably 11 μm or less (for example, 3 to 11 μm, preferably 5 to 11 μm), that is, no dispersed particles having a particle diameter of more than 15 μm are present. When the thickness is in the range of 15 μm or less, the polyimide film has good swelling during graphitization without causing partial excessive foaming (blowing) due to the sublimation gas of the inorganic particles, and the surface protrusion defect of the graphitized sheet This is preferable from the standpoint of less appearance and excellent appearance.
In the present invention, the dispersed particle size of the inorganic particles is usually the particle size of the dispersed inorganic particles in the polyimide film, and not only the primary particle size but also the secondary particle size (aggregated particle particles). Diameter). That is, the maximum dispersion diameter of the inorganic particles refers to the maximum particle diameter of the inorganic particles in the polyimide film.

  Moreover, the thickness of the polyimide film of this invention is 10-150 micrometers, for example, Preferably it is 25-90 micrometers, More preferably, it is 25 micrometers-80 micrometers. In particular, if it is 25 μm or more, it is preferable from the viewpoint that the graphite sheet after baking the polyimide film is rounded, or that the graphite sheet is shattered and cannot be formed when the graphite sheet is made flat. Also, particularly if it is 80 μm or less, gas permeability is high, so that good quality foaming occurs on the surface of the graphite sheet, and there are few surface protrusion defects on the graphitized sheet, which is preferable from the viewpoint of excellent appearance and the like. .

[Production method of graphite sheet]
The graphite sheet of the present invention can be obtained by baking and graphitizing the polyimide film of the present invention.
Although the manufacturing method of the graphite sheet of this invention is not specifically limited, For example, the method demonstrated below can be used.

  In obtaining the graphite sheet of the present invention, first, the polyimide film is cut into a predetermined size, and the film surface of the polyimide film is placed horizontally or vertically (for example, vertically) and placed in a graphite holding container.

Next, the holding container containing the polyimide film is heated, and the polyimide film is baked to perform graphitization.
The temperature of the graphitization firing (hereinafter also referred to as the main heat treatment) is usually 2000 ° C. or higher (for example, 2000 to 3500 ° C.), preferably 2400 ° C. or higher (for example, 2400 to 3500 ° C.), and 2600 ° C. The above (for example, 2600 to 3500 ° C.) is more preferable.
Moreover, 2700 degreeC or more (for example, 2700-3500 degreeC) is preferable, as for final calcination temperature, 2800 degreeC or more (for example, 2800-3500 degreeC) is more preferable, and 3000 degreeC vicinity (for example, 3000-3500 degreeC) is further more preferable. .
When the firing temperature is 3500 ° C. or lower, the heat-resistant deterioration of the firing furnace is small and long-time production is possible. If the maximum firing temperature is 2000 ° C. or higher, the resulting graphite sheet tends to be flexible and strong.
Although the temperature increase rate at the time of baking is not specifically limited, For example, it is performed at about 1-10 degreeC / min. A known heating means can be used for firing. Further, the firing time is not particularly limited.

  Firing is usually performed in an inert gas. The inert gas is not particularly limited, and examples thereof include helium, argon, nitrogen, etc., but it is preferable to use argon. Moreover, the pressure at the time of baking may be a normal pressure.

  Prior to the firing (main heat treatment), a pre-heat treatment may be performed as necessary. The temperature of the preliminary heat treatment is preferably lower than the temperature of the main heat treatment. Specifically, about 900 ° C. or higher and 1500 ° C. or lower is preferable. The temperature increase rate of the preheating treatment is not particularly limited, but is, for example, about 1 to 15 ° C./min. The preheating treatment is also usually performed in an inert gas. As the inert gas, the same ones as described above can be used. The time for the preheating treatment is not particularly limited.

  It is preferable that the graphite sheet obtained by firing the polyimide film is further sandwiched between rolling rollers and rolled. By the rolling treatment, the thickness unevenness due to the swelling of the graphite sheet caused by the firing of the polyimide film can be reduced. Moreover, the density of a graphite sheet can be enlarged and thermal conductivity can be improved by a rolling process. In addition, the rolling process method is not specifically limited, You may follow a conventionally well-known method.

[Graphite sheet]
Since the graphite sheet of the present invention produced as described above has a large thermal diffusion coefficient, it has excellent thermal conductivity. Although the measuring method of a thermal diffusion coefficient is not specifically limited, It can measure with the method as described in the below-mentioned Example.
The thermal diffusion coefficient of the graphite sheet of the present invention is not particularly limited, but is preferably 7 cm ² / s or more (for example, 7 to 15 cm ² / s).

  The breaking strength of the graphite sheet of the present invention is usually 10 MPa or more (for example, 10 to 200 MPa), preferably 15 MPa or more (for example, 15 to 200 MPa), more preferably 20 MPa or more (for example, 30 to 30 MPa). 200 MPa). Although the measuring method of breaking strength is not specifically limited, It can measure by the method as described in the below-mentioned Example.

The number of protrusion defects having a diameter of 0.1 mm or more on the surface of the graphite sheet of the present invention is preferably 3 pieces / 50 cm ² or less (for example, 0 pieces / 50 cm ^{2 to} 3 pieces / 50 cm ² ). The number of protrusion defects on the surface can be evaluated by the method described in Examples described later.

  The graphite sheet of this invention can be used for uses, such as a heat radiating member [For example, the heat radiating member of small portable electronic devices (for example, a smart phone etc.)].

  EXAMPLES Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples at all, and many variations are within the technical idea of the present invention. This is possible by those with ordinary knowledge.

A method for measuring various physical properties in the present invention will be described below.
[Evaluation of dispersion diameter of inorganic particles in polyimide film]
Using a microscope VHX-2000 manufactured by Keyence Corporation, the maximum dispersed particle size of inorganic particles present in a 50 cm square polyimide film was observed in a transmission mode, and the number of particles having a particle size of 15 μm or more was read. .

[Evaluation of amount of inorganic particles in polyimide film]
Using Fluorescent X-ray S2Ranger manufactured by Bruker AXS Co., Ltd., the amount of CAP (calcium hydrogen phosphate) added was evaluated from the energy amount of phosphorus Kα rays contained in the inorganic particles.

[Evaluation of inorganic particles added to polyimide film]
Using a laser diffraction / scattering particle size distribution measuring apparatus LA-910 manufactured by HORIBA, Ltd., a sample dispersed in a polar solvent was measured and analyzed, and the particle size range, average particle size, and particle size 0.5 were obtained. The occupancy for all particles of ~ 2.5 μm was read.

[Evaluation of polyimide film thickness and graphite sheet thickness]
It measured using the lightmatic (Series 318) by Mitutoyo.

[Evaluation of breaking strength of graphite sheet]
Using an autograph AGS-X manufactured by Shimadzu Corporation, evaluation was performed under the conditions of a measurement temperature: 25 ° C., a distance between chucks: 50 mm, a tensile speed: 25 mm / min, and a test piece: a width of 10 mm.
Good (O): 10 MPa or more Poor (x): Less than 10 MPa

[Surface protrusion defects due to local excessive foaming (blowing) of graphite sheet]
Using a microscope VHX-2000 manufactured by Keyence Corporation, the surface of a 50 cm ² graphite sheet was observed, and protrusion defects having a size of Φ (diameter) of 0.1 mm or more were evaluated according to the following evaluation criteria. The defects due to the surface protrusions of the graphite sheet are preferably 0 pieces / 50 cm ² , but if 3 pieces / 50 cm ² or less, they were evaluated as good.
A photograph of this protrusion defect is shown in FIG.
Good (◯): 3 pieces / 50 cm ² or less Poor (x): 3 pieces / 50 cm ² or more

[Evaluation of flexibility of graphite sheet]
A graphite sheet of 100 mm (vertical) × 100 mm (width) was folded so that the ends in the vertical direction or the ends in the width direction were exactly overlapped, and then a load of 100 g was applied to the center of the fold of the sheet for 3 seconds. The sheet after pressing and unloading was returned to its original state and visually evaluated according to the following evaluation criteria. In this evaluation method, it was evaluated that the sheet returned to its original state as having good flexibility.
Good (O): The sheet returns almost to its original state (X): The sheet is partially deformed

[Evaluation of thermal diffusion coefficient of graphite sheet]
Heat in the sheet surface direction under the conditions of a xenon flash method, measurement temperature: 25 ° C., light source: xenon flash lamp, IR detector: InSb detector (liquid nitrogen cooling) using a thermal conductivity measuring device LFA447 manufactured by NETZSCH Co., Ltd. The diffusion coefficient was evaluated.
Good (◯): 7 cm ² / s or more Defect (x): Less than 7 cm ² / s

[Examples of polyamic acid synthesis]
Pyromellitic dianhydride (molecular weight 218.12) / 4,4′-diaminodiphenyl ether (molecular weight 200.24) was prepared at a molar ratio of 1: 1, and 23 in DMAc (N, N-dimethylacetamide). The solution was polymerized to a 7 wt% solution to obtain a 4000 poise polyamic acid solution.

[Example 1]
An N, N-dimethylacetamide slurry of calcium hydrogen phosphate having an average particle size of 0.87 μm and a particle size of 0.5 to 2.5 μm of 81.5% by volume in all particles (6 calcium calcium phosphate in the slurry). Per weight of polyimide resin when the polyimide is formed in the polyamic acid solution obtained in the synthesis example after passing through a cut filter (Nihon Seisen made metal fiber sintered filter) with a pore diameter of 5 μm. The slurry was added so that the calcium hydrogen phosphate was 0.10 wt%, and the mixture was stirred at room temperature for 30 minutes to disperse the inorganic particles in the polyamic acid solution. The polyamic acid solution was mixed with a conversion agent composed of acetic anhydride (molecular weight 102.09) and β-picoline at a ratio of 2.0 molar equivalents with respect to the polyamic acid, and stirred. The obtained mixture was cast on a stainless steel drum at 65 ° C. rotating from a die to obtain a gel film having self-supporting properties. This gel film was peeled off from the drum, both ends thereof were gripped, and treated in a heating furnace at 250 ° C. × 30 seconds, 400 ° C. × 30 seconds, 550 ° C. × 30 seconds to obtain a polyimide film having a thickness of 60 μm.

  The polyimide film obtained as described above was cut into a size of 250 mm in width and 600 mm in length, and the film surface was placed vertically in a graphite low and high holding container made of graphite. Subsequently, the temperature was raised to 1000 ° C. at 3 ° C./min in argon gas and held for 1 hour, further heated to 3000 ° C. at 3 ° C./min and held for 1 hour to baked the polyimide film, and graphitized. went. The obtained graphite sheet was sandwiched between two rolling rolls, rolled by rolling, and a graphite sheet having a thickness of 32 μm was produced.

[Example 2]
A polyimide film was obtained in the same manner as in Example 1 except that the addition amount of calcium hydrogen phosphate per weight of the polyimide resin added to the polyamic acid solution was 0.05% by weight, and the polyimide film was baked to obtain graphite. A sheet was produced.

[Example 3]
A polyimide film was obtained in the same manner as in Example 1 except that the addition amount of calcium hydrogen phosphate per weight of the polyimide resin added to the polyamic acid solution was 0.50% by weight. A sheet was produced.

[Example 4]
Except that the average particle size of calcium hydrogen phosphate added to the polyamic acid solution is 0.54 μm, a polyimide film was obtained in the same manner as in Example 1, and the graphite film was produced by firing the polyimide film. .

[Example 5]
Except that the average particle diameter of calcium hydrogen phosphate added to the polyamic acid solution is 1.47 μm, a polyimide film was obtained in the same manner as in Example 1, and the graphite film was produced by firing the polyimide film. .

[Examples 6 and 7]
A polyimide film was obtained in the same manner as in Example 1 except that the thickness of the polyimide film was adjusted to 25 μm and 80 μm by adjusting the drum rotation speed and the gel film conveyance speed (film formation speed), and the polyimide film was baked. Thus, a graphite sheet was produced.

[Example 8]
A graphite sheet was produced by obtaining and baking a polyimide film in the same manner as in Example 1 except that the amount of calcium hydrogen phosphate added per weight of the polyimide resin added to the polyamic acid solution was 0.55% by weight.

[Comparative Example 1]
A polyimide film was obtained in the same manner as in Example 1 except that the N, N-dimethylacetamide slurry of calcium hydrogen phosphate was added to the polyamic acid solution obtained in the synthesis example without passing through a 5 μm cut filter. Was fired to produce a graphite sheet.

[Comparative Example 2]
A polyimide film was obtained and fired to produce a graphite sheet in the same manner as in Example 1 except that the addition amount of calcium hydrogen phosphate per weight of the polyimide resin added to the polyamic acid solution was 0.02% by weight. .

[Comparative Example 3]
A polyimide film was obtained in the same manner as in Example 2 except that N, N-dimethylacetamide slurry of calcium hydrogen phosphate was added to the polyamic acid solution obtained in the synthesis example without passing through a 5 μm cut filter. Was fired to produce a graphite sheet.

  Table 1 shows the characteristics of the polyimide films obtained in Examples 1 to 8 and Comparative Examples 1 to 3 and the number of inorganic particles having a particle diameter (dispersion diameter) of 10 μm or more. The number is shown in Table 2.

As shown in Table 2, the graphite sheets obtained in Examples 1 to 8 were excellent in flexibility, breaking strength, and thermal diffusion coefficient. Further, the protrusion defects on the surface of the graphite sheet were excellent in appearance at 0 / cm ² .

On the other hand, in Comparative Examples 1 and 3, protrusion defects occurred on the surface of the graphite sheet due to partial excessive foaming due to the influence of inorganic particles having a particle diameter of 15 μm or more contained in the film, which is excellent in appearance. There wasn't.
Further, from these results, the inorganic particles are filtered using a cut filter and then added to the polyamic acid, whereby aggregation of the inorganic particles can be suppressed, and the resulting polyimide film has a particle diameter of 15 μm or more. It was confirmed that the inorganic particles were not included. And it has confirmed that the graphite sheet excellent in the external appearance like a title was obtained by using such a polyimide film.

  Comparative Example 2 is poor in slipperiness of the film and has few inorganic particles, so the foamability of the film at the time of firing is poor, and when rolled after firing, it becomes shattered due to inferior flexibility and breaking strength, and a graphite sheet is formed. The projection on the sheet surface could not be evaluated.

  As is apparent from the results of Table 2, the main component is 4,4′-diaminodiphenyl ether as the diamine component, and pyromellitic dianhydride as the acid dianhydride component, and 0.05 to 0.8 weight per film resin weight. %. The polyimide film of the present invention in which inorganic particles are dispersed at a ratio of 15% and the dispersed diameter of the inorganic particles is 15 μm or less has good foamability when graphitized by heat treatment, and is manufactured by heat-treating the polyimide film. The resulting graphite sheet has excellent thermal conductivity, flexibility, and breaking strength, has very few surface protrusions generated during firing, and is excellent in appearance.

In addition, as in Examples 1 to 8, even when several inorganic particles having a maximum particle size of 10 to 15 μm are contained in the polyimide film, the graphite sheet obtained by firing the polyimide film has surface protrusions. It has been found that it has very little appearance and excellent appearance.
On the other hand, as in Comparative Examples 1 and 3, when the polyimide film contains inorganic particles having a maximum particle size of 15 μm or more, the graphite sheet obtained by firing the polyimide film has a marked increase in surface protrusions. It turned out that the appearance was bad.

  The graphite sheet obtained by firing the polyimide film of the present invention has excellent thermal conductivity, flexibility and breaking strength, has very few surface protrusions generated during firing, and is excellent in appearance. It is suitable as a heat radiating member.

Claims (8)

  A polyimide film in which inorganic particles are dispersed, wherein the proportion of inorganic particles is 0.05 to 0.8% by weight per film resin weight, and the maximum dispersion diameter of inorganic particles is 15 μm or less.   The aromatic diamine component containing 4,4'- diaminodiphenyl ether and the aromatic tetracarboxylic dianhydride component containing pyromellitic dianhydride are formed with the polyimide which consists at least as a component. Polyimide film.   3. The polyimide film according to claim 1, wherein the inorganic particles are contained at a ratio of 0.05 to 0.5% by weight per film resin weight.   The polyimide film according to any one of claims 1 to 3, wherein the inorganic particles contain calcium hydrogen phosphate as a main component.   Film thickness is 25-80 micrometers, The polyimide film of any one of Claims 1-4 characterized by the above-mentioned.   A method for producing a polyimide film according to any one of claims 1 to 5 using a polyamic acid solution containing inorganic particles, wherein the slurry containing inorganic particles is filtered and then mixed with the polyamic acid. And a production method including a step of obtaining a polyamic acid solution. A graphite sheet comprising the polyimide film according to any one of claims 1 to 6 as a raw material and having 3 protrusion defects having a diameter of 0.1 mm or more on the surface and 50 cm ² or less.   A method for producing a graphite sheet, comprising firing the polyimide film according to any one of claims 1 to 6.