JP4864293B2 - Method for producing highly oriented graphite - Google Patents

Description

  The present invention relates to a highly oriented graphite used for a radiation optical element, a high thermal conductor, and a method for producing the same.

  As a method for producing highly oriented graphite, a method is known in which a polyimide film or a film obtained by carbonizing a polyimide film is laminated and heat-pressed (Non-Patent Document 1).

  In particular, when 30 or more polyimide films are laminated to obtain thick highly oriented graphite, it is necessary to use a thin polyimide having a thickness of 25 μm as a raw material. However, if the thickness is small, wrinkles and internal strain are likely to occur due to shrinkage and pressurization during heat treatment, so pressurization is a temperature range in which dimensional change of the film does not occur. It is necessary to perform only in the temperature range of 2 ° C. and the temperature range of 2600 ° C. or higher. The rocking characteristic of the graphite thus obtained is 0.7 (Patent Document 1).

  In order to further improve the orientation, there is known a method in which after heat-pressing once to 2600 ° C. or higher, the temperature is once lowered to a temperature range of 1600 ° C. or lower and then heat-treated again to a temperature range of 2600 ° C. or higher. The rocking characteristic of the graphite thus obtained is 0.45 (Patent Document 2).

JP-A-4-202052 JP-A-4-202055 Carbon Vol. 30, no. 2, P255-262, 1992

  The highly oriented graphite obtained by heat-pressing a polyimide film synthesized from conventional pyromellitic dianhydride and 4,4′-diaminodiphenyl ether (4,4′-oxydianiline) has a thickness of 0. 1 mm or more and 6 mm or less, rocking characteristics of 0.45 or more, thicknesses of 6 mm or more and 20 mm or less, and rocking characteristics of 1.0 or more, resulting in inferior radiation reflectivity and thermal conductivity, high performance radiation optics It was insufficient as an element and a high thermal conductor.

  Next, before clarifying the problems of the conventional method for producing highly oriented graphite by heating and pressing a polyimide film, the graphitization of the polyimide film will be described.

  As shown in FIGS. 1 and 2, the graphitization of a conventional polyimide film composed of pyromellitic dianhydride and 4,4′-diaminodiphenyl ether (4,4′-oxydianiline) is carbonized up to 1000 ° C. And proceeds in two stages of graphitization at 2000 ° C. or higher. Specifically, the carbonization is almost completed by the 1000 ° C. heat treatment, the thickness is once increased, and after the 1000 ° C. treatment, it shrinks to 90% compared to the initial stage and to 75% in the plane direction. Next, graphitization proceeds from 2000 ° C., and after heat treatment at 2800 ° C., it shrinks to 40% in the thickness direction compared to the initial stage, becomes larger than the carbonization stage in the plane direction, and recovers to 90% contraction compared to the initial stage. In addition, as the thickness of the conventional polyimide film increases, the carbon layer is less likely to be rearranged and becomes less graphitized.

  For this reason, as described in (Patent Document 1), in the method for producing highly oriented graphite by heating and pressurizing a polyimide film, the raw material is thin and pressurization does not cause dimensional change of the film. Must be done in the temperature range.

  However, as described above, the conventional polyimide film has a large dimensional change that shrinks to 75% in the direction of the carbonized surface and restores to 90% after graphitization. Cheap. In order to prevent a decrease in orientation, it is conceivable to apply a pressure treatment during carbonization. However, the heat treatment up to 1000 ° C. increases the thickness in the thickness direction from the initial stage, and the shrinkage in the length direction is large. Yes, if it is pressurized at this time, it will be damaged, so it cannot be pressurized during carbonization. Further, since the dimensional change after graphitization occurs up to 2600 ° C., the pressure treatment after graphitization must be performed after 2600 ° C., and therefore sufficient pressure treatment cannot be performed, resulting in deterioration of quality. .

  In addition, since a conventional polyimide film is graphitized only with a thin film, it is necessary to use a thin film as a raw material in order to obtain a graphite block. However, since the strength of the film is low, wrinkles easily occur during heat treatment, Defects increase. Also, in order to obtain thick graphite, a large number of films are required, the interface between the films increases, the possibility of air entrapment and the entry of dust increases, the internal strain increases, and the rocking characteristics are inferior It becomes. In addition, the work becomes complicated and the work efficiency also deteriorates.

In the method of repeatedly heat-treating described in (Patent Document 2), it takes time to produce, which causes productivity deterioration and cost increase. Moreover, by repeating the heat treatment at a graphite sublimation temperature of 2800 ° C. or higher, the life of the heater is deteriorated, resulting in an increase in cost.

  In the radiation optical element and the high thermal conductor, graphite having a rocking characteristic of 0.44 or less obtained by heating and pressing a polyimide film having high birefringence is used.

Moreover, in the manufacturing method of highly oriented graphite, a polyimide film having high birefringence is manufactured by heating and pressing. Furthermore, it is better to increase the thickness of the raw material and pressurize during carbonization and graphitization.

  Since it is excellent in locking characteristics, it is excellent as a radiation optical element and a high thermal conductor. The rocking property is a value that represents the relative total number of crystallites having axes in the direction inclined from the normal of the graphite layer surface. The smaller the value, the more the graphite layer is oriented in a plane and the better the reflection of radiation. The thermal conductivity in the surface direction is increased.

  In addition, when a highly oriented graphite film is used in the production of highly oriented graphite, the orientation expansion is disturbed because the thickness expansion in the medial direction of the graphitization and the change in length in the plane direction are small during carbonization. Decreases and the orientation increases. Furthermore, it becomes possible to apply pressure during carbonization and graphitization, and a high quality product can be obtained.

  When the birefringence of the polyimide film is increased, the strength is increased, the wrinkles are difficult to enter, and the damage during heating and pressurization is reduced.

  Furthermore, since it is easy to graphitize even if the thickness is large, it is possible to use a thick material as a raw material, and the film strength increases and soot is difficult to enter. In addition, it is possible to reduce the number of sheets used, and it is possible to reduce the distortion at the interface and the biting of air and dust, so that an excellent quality can be obtained. In addition, the complexity of the work can be reduced. During carbonization, pressure during graphitization can be applied to obtain a high quality product.

  Further, when the birefringence of the polyimide film is increased, the graphitization temperature is lowered and graphitization is facilitated, and the pressurization time after graphitization can be increased, so that a high quality film can be obtained.

By using the method of this patent, graphite with excellent orientation can be obtained by a single pressurization heat treatment, so utility costs (electric power and cooling water) used for raising and lowering the temperature can be reduced, thereby reducing costs. And heater consumption can be reduced.

  In the polyimide film used in the present invention, the birefringence Δn related to the in-plane orientation of molecules is 0.12 or more, preferably 0.14 or more, most preferably 0.16 or more in any direction in the film plane. It is. If the birefringence of the film is smaller than 0.12, it indicates that the plane orientation of the molecules of the film is poor, and during carbonization, the thickness expansion in the graphitized middle surface direction is large, and the amount of change in length in the surface direction is large. The orientation is disturbed and the orientation is lowered. Furthermore, when pressure is applied during carbonization and graphitization, the steel is liable to be damaged and easily wrinkled. Furthermore, graphitization becomes difficult as the thickness increases. Since it is necessary to use a thin material as the raw material, and the film strength is weakened, wrinkles are easily formed. In addition, it is necessary to increase the number of sheets used, which increases the distortion at the interface and the biting of air and dust, resulting in poor quality. Also, the complexity of the work increases. Furthermore, the graphitization temperature is increased, the pressurization time after graphitization is shortened, and the quality is inferior. Moreover, it is necessary to perform the pressure heat treatment many times, and utility costs (power and cooling water) used for raising and lowering the temperature increase, resulting in an increase in cost. Also, heater consumption increases.

  On the other hand, if the birefringence is 0.12 or more, especially 0.14 or more, the thickness expansion in the graphitized middle surface direction is small during carbonization, and the change in length in the surface direction is small. The orientation is improved. Furthermore, it becomes possible to apply pressure during carbonization and graphitization, and a high quality product can be obtained. Strength becomes high, so that wrinkles are difficult to enter, and damage during heating and pressurization is reduced. Furthermore, since it is easy to graphitize even if the thickness is large, it is possible to use a thick material as a raw material, and the film strength increases and soot is difficult to enter. In addition, it is possible to reduce the number of sheets used, and it is possible to reduce the distortion at the interface and the biting of air and dust, so that an excellent quality can be obtained. In addition, the complexity of the work can be reduced. During carbonization, pressure during graphitization can be applied to obtain a high quality product. Furthermore, since the graphitization temperature becomes low and graphitization becomes easy, the pressurization time after graphitization can be lengthened, so that a high quality product can be obtained. Because graphite with excellent orientation can be obtained with a single pressure heat treatment, utility costs (power and cooling water) used to raise and lower temperatures can be reduced, costs can be reduced, and heater consumption can be reduced. Can be reduced. The reason for this is not clear, but it is necessary to rearrange the molecules for graphitization, and polyimide with excellent molecular orientation requires minimal molecular rearrangement. Presumed to be high graphite.

Birefringence here means the difference between the refractive index in an arbitrary direction in the film plane and the refractive index in the thickness direction, and the birefringence Δnx in the X direction in the film plane is given by the following formula (Formula 1).

図１と図２において、複屈折の具体的な測定方法が図解されている。図１の平面図において、フィルム１から細いくさび形シート２が測定試料として切り出される。このくさび形シート２は一つの斜辺を有する細長い台形の形状を有しており、その一底角が直角である。このとき、その台形の底辺はＸ方向と平行な方向に切り出される。図２は、このようにして切り出された測定試料２を斜視図で示している。台形試料２の底辺に対応する切り出し断面に直角にナトリウム光４を照射し、台形試料２の斜辺に対応する切り出し断面側から偏光顕微鏡で観察すれば、干渉縞５が観察される。この干渉縞の数をｎとすれば、フィルム面内Ｘ方向の複屈折Δｎｘは、次式（数式２）で表される。

1 and 2, a specific method for measuring birefringence is illustrated. In the plan view of FIG. 1, a thin wedge-shaped sheet 2 is cut out from the film 1 as a measurement sample. The wedge-shaped sheet 2 has an elongated trapezoidal shape having one oblique side, and its base angle is a right angle. At this time, the base of the trapezoid is cut out in a direction parallel to the X direction. FIG. 2 is a perspective view of the measurement sample 2 cut out in this way. When sodium light 4 is irradiated at right angles to the cut-out cross section corresponding to the bottom side of the trapezoidal sample 2 and observed with a polarizing microscope from the cut-out cross section side corresponding to the oblique side of the trapezoidal sample 2, the interference fringes 5 are observed. If the number of interference fringes is n, the birefringence Δnx in the film in-plane X direction is expressed by the following formula (Formula 2).

ここで、λはナトリウムＤ線の波長５８９ｎｍであり、ｄは試料２の台形の高さに相当する試料の幅３である。

Here, λ is the wavelength 589 nm of the sodium D line, and d is the width 3 of the sample corresponding to the trapezoidal height of the sample 2.

The above-mentioned “arbitrary direction X in the film plane” means, for example, the X direction is the 0 ° direction, 45 ° direction, 90 ° direction, 135 ° direction in the plane with reference to the direction of material flow during film formation. It means in any direction.

Moreover, the polyimide film used as the raw material for graphite used in the present invention preferably has an average linear expansion coefficient of less than 2.5 × 10 ⁻⁵ / ° C. in the range of 100 to 200 ° C. By using such a polyimide film as a raw material, conversion to graphite starts at 2400 ° C., and conversion can occur in graphite having sufficiently high crystallinity at 2700 ° C. In addition, compared with the case where a polyimide film having a linear expansion coefficient of 2.5 × 10 ⁻⁵ / ° C. or higher, which is conventionally known as a raw material for graphite, is used, a wire of less than 2.5 × 10 ⁻⁵ / ° Polyimide films having an expansion coefficient can be converted to graphite at a lower temperature even if they have the same thickness. That is, graphitization can easily proceed even if a thicker film than the conventional one is used as a raw material. The linear expansion coefficient is more preferably 2.0 × 10 ⁻⁵ / ° C. or less.

If the linear expansion coefficient of the film is larger than 2.5 × 10 ⁻⁵ / ° C., the change during the heat treatment becomes large, so that graphitization is disturbed and brittle, the resulting graphite crystals tend to be inferior, and the rocking characteristics tend to be inferior. . On the other hand, if the linear expansion coefficient is less than 2.5 × 10 ⁻⁵ / ° C., the elongation during the heat treatment is small, graphitization proceeds smoothly and is not brittle, and graphite excellent in various properties can be obtained.

  The linear expansion coefficient of the film was determined by using a TMA (thermomechanical analyzer) to first heat the sample to 350 ° C. at a temperature rising rate of 10 ° C./min, then air-cooled to room temperature, and again 10 ° C. / It is obtained by raising the temperature to 350 ° C. at a temperature rise rate of minutes and measuring the average linear expansion coefficient at 100 ° C. to 200 ° C. at the time of the second temperature rise. Specifically, using a thermomechanical analyzer (TMA: Seiko Electronics SSC / 5200H; TMA120C), a 3 mm wide × 20 mm long film sample is set in a predetermined jig, and a load of 3 g is applied in a tensile mode. Is measured in a nitrogen atmosphere.

In addition, the polyimide film used in the present invention is preferable if its elastic modulus is 350 kgf / mm ² because it can be graphitized more easily. That is, if the elastic modulus is 350 kgf / mm ² or more, the film can be prevented from being damaged due to the shrinkage of the film during the heat treatment, and graphite excellent in various characteristics can be obtained.

In addition, the elasticity modulus of a film can be measured based on ASTM-D-882. More preferred elastic modulus of the polyimide film at 400 kgf / mm ² or more, further preferably 500 kgf / mm ² or more. If the elastic modulus of the film is smaller than 350 kgf / mm ² , the film tends to be broken and deformed due to the shrinkage of the film during heat treatment, and the resulting graphite tends to be inferior in crystallinity and inferior in rocking characteristics.

  The rocking characteristics of the film are measured using, for example, an X-ray diffractometer manufactured by Rigaku Corporation (Rotor Flex RU-200B type X-ray commentator, etc.), and the rocking characteristics are measured at the peak position of the graphite (002) diffraction line. It can be evaluated with the half width of the diffraction line.

The water absorption rate of the film was measured as follows. In order to dry the film completely, the film was dried at 100 ° C. for 30 minutes to prepare a 25 μm thick 10 cm square sample. This weight is measured and designated as A1. A 25 μm thick 10 cm square sample was immersed in distilled water at 23 ° C. for 24 hours, and the surface water was wiped away and the weight was immediately measured. This weight is A2. The water absorption was determined from the following formula.

Water absorption rate (%) = (A2−A1) ÷ A1 × 100

The polyimide film used in the present invention is prepared by mixing an organic solution of polyamic acid, which is a polyimide precursor, with an imidization accelerator, and then casting it on a support such as an endless belt or a stainless drum, followed by drying and baking. Can be produced by imidization.

  As a method for producing the polyamic acid used in the present invention, a known method can be used. Usually, at least one kind of aromatic dianhydride and at least one kind of diamine are substantially equimolar amounts in an organic solvent. Dissolved in. The obtained organic solution is stirred under controlled temperature conditions until the polymerization of the acid dianhydride and the diamine is completed, whereby a polyamic acid can be produced. Such a polyamic acid solution is usually obtained at a concentration of 5 to 35 wt%, preferably 10 to 30 wt%. When the concentration is in this range, an appropriate molecular weight and solution viscosity can be obtained.

  Any known method can be used as the polymerization method. For example, the following polymerization methods (1) to (5) are preferable.

  (1) A method in which an aromatic diamine is dissolved in an organic polar solvent, and this is reacted with a substantially equimolar aromatic tetracarboxylic dianhydride for polymerization.

  (2) An aromatic tetracarboxylic dianhydride is reacted with a small molar amount of an aromatic diamine compound in an organic polar solvent to obtain a prepolymer having acid anhydride groups at both ends. Then, the method of superposing | polymerizing using an aromatic diamine compound so that it may become substantially equimolar with respect to aromatic tetracarboxylic dianhydride.

  (3) An aromatic tetracarboxylic dianhydride and an excess molar amount of the aromatic diamine compound are reacted in an organic polar solvent to obtain a prepolymer having amino groups at both ends. Subsequently, after adding an aromatic diamine compound to the prepolymer, polymerization was performed using the aromatic tetracarboxylic dianhydride so that the aromatic tetracarboxylic dianhydride and the aromatic diamine compound are substantially equimolar. how to.

  (4) After dissolving and / or dispersing aromatic tetracarboxylic dianhydride in an organic polar solvent, an aromatic diamine compound is used so as to be substantially equimolar with respect to the acid dianhydride. Polymerization method.

  (5) A method of polymerizing by reacting a substantially equimolar mixture of aromatic tetracarboxylic dianhydride and aromatic diamine in an organic polar solvent.

  Among these, a method of polymerizing by sequential control (combination of block polymers / control of connection between block polymer molecules) via the prepolymer shown in (2) and (3) is preferable. This is because it is easy to obtain a polyimide film having a large birefringence and a low linear expansion coefficient by using this method. By heat treating this polyimide film, a graphite having high crystallinity and excellent rocking characteristics can be obtained. This is because it becomes easier. Further, it is presumed that, when controlled regularly, the aromatic rings overlap, and graphitization is likely to proceed even at low temperature heat treatment. Also, increasing the imide group content to increase birefringence decreases the carbon ratio in the resin and decreases the carbonization yield after graphite treatment, but the polyimide film synthesized with sequential control is This is preferable because the birefringence can be increased without reducing the carbon ratio.

  Acid dianhydrides that can be used for the synthesis of polyimide in the present invention are pyromellitic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 3,3 ′, 4,4′-biphenyl. Tetracarboxylic dianhydride, 1,2,5,6-naphthalene tetracarboxylic dianhydride, 2,2 ′, 3,3′-biphenyltetracarboxylic dianhydride, 3,3 ′, 4,4 ′ -Benzophenonetetracarboxylic dianhydride, 2,2-bis (3,4-dicarboxyphenyl) propane dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, bis (3,4- Dicarboxyphenyl) propane dianhydride, 1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride, 1,1-bis (3,4-dicarboxyphenyl) ethane dianhydride, bis (2 , 3-Zical Xylphenyl) methane dianhydride, bis (3,4-dicarboxyphenyl) ethane dianhydride, oxydiphthalic dianhydride, bis (3,4-dicarboxyphenyl) sulfone dianhydride, p-phenylenebis (trimerit Acid monoester acid anhydride), ethylene bis (trimellitic acid monoester acid anhydride), bisphenol A bis (trimellitic acid monoester acid anhydride), and the like, which may be used alone or in any It can be used in a mixture of proportions.

  Examples of the diamine that can be used in the synthesis of the polyimide in the present invention include 4,4′-oxydianiline, p-phenylenediamine, 4,4′-diaminodiphenylpropane, 4,4′-diaminodiphenylmethane, benzidine, 3,3. '-Dichlorobenzidine, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl ether (4,4'-oxydianiline), 3,3'-diaminodiphenyl ether (3,3'-oxydianiline), 3,4'-diaminodiphenyl ether (3,4'-oxydianiline), 1,5-diaminonaphthalene, 4,4'-diaminodiphenyl Diethylsilane, 4,4'-diaminodiphenylsilane, 4,4 ' Diaminodiphenylethylphosphine oxide, 4,4′-diaminodiphenyl N-methylamine, 4,4′-diaminodiphenyl N-phenylamine, 1,4-diaminobenzene (p-phenylenediamine), 1,3-diaminobenzene, Including 1,2-diaminobenzene and the like, which can be used alone or in any proportion of the mixture.

  In particular, from the viewpoint of increasing the elastic modulus and increasing the birefringence by decreasing the linear expansion coefficient, the acid dianhydride represented by the following formula (1) is used as a raw material in the production of the polyimide film in the present invention. It is preferable.

ここで、Ｒ₁は、下記の式（２）〜式（１３）に含まれる２価の有機基の群から選択されるいずれかであって、

Here, R ₁ is any one selected from the group of divalent organic groups included in the following formulas (2) to (13),

ここで、Ｒ₂、Ｒ₃、Ｒ₄、およびＲ₅の各々は−ＣＨ₃、−Ｃｌ、−Ｂｒ、−Ｆ、または−ＣＨ₃Ｏの群から選択されるいずれかであり得る。

Here, each of R ₂ , R ₃ , R ₄ , and R ₅ can be any selected from the group of —CH ₃ , —Cl, —Br, —F, or —CH ₃ O.

  By using the above acid dianhydride, a polyimide film having a relatively low water absorption can be obtained, which is also preferable from the viewpoint of preventing foaming due to moisture in the graphitization process.

In particular, when an organic group containing a benzene nucleus as shown in Chemical Formula 2 is used as R ₁ in the acid dianhydride, the molecular orientation of the resulting polyimide film is increased, the linear expansion coefficient is small, and the elastic modulus Is preferable from the viewpoints of large, high birefringence, and low water absorption.

  In order to further reduce the linear expansion coefficient, increase the elastic modulus, increase the birefringence, and decrease the water absorption, an acid dianhydride represented by the following formula (14) is used as a raw material for the synthesis of the polyimide in the present invention. That's fine.

特に、２つ以上のエステル結合でベンゼン環が直線状に結合された構造を有する酸二無水物を原料に用いて得られるポリイミドフィルムは、屈曲鎖を含むけれども全体として非常に直線的なコンフォメーションをとりやすく、比較的剛直な性質を有する。その結果、この原料を用いることによってポリイミドフィルムの線膨張係数を小さくすることができ、例えば１．５×１０^-5／℃以下にすることができる。また、弾性率は５００ｋｇｆ／ｍｍ²以上に大きくすることができ、吸水率は１．５％以下に小さくすることができる。

In particular, a polyimide film obtained by using an acid dianhydride having a structure in which a benzene ring is linearly bonded by two or more ester bonds as a raw material has a very linear conformation as a whole although it contains a bent chain. It has a relatively rigid property. As a result, the linear expansion coefficient of the polyimide film can be reduced by using this raw material, for example, 1.5 × 10 ⁻⁵ / ° C. or less. Further, the elastic modulus can be increased to 500 kgf / mm ² or more, and the water absorption can be decreased to 1.5% or less.

  In order to further reduce the linear expansion coefficient, increase the elastic modulus, and increase the birefringence, the polyimide in the present invention is preferably synthesized using p-phenylenediamine as a raw material.

  In the present invention, the most suitable acid dianhydride used for the synthesis of the polyimide film is pyromellitic dianhydride and / or p-phenylenebis represented by the formula (14) (trimellitic acid monoester dianhydride). And the total mole of these alone or the two is 40 mole% or more, further 50 mole% or more, further 70 mole% or more, or even 80 mole% or more based on the total acid dianhydride. Is preferred. If the amount of these acid dianhydrides used is less than 40 mol%, the resulting polyimide film tends to have a large linear expansion coefficient, a small elastic modulus, and a small birefringence.

  In the present invention, the most suitable diamines used for the synthesis of polyimide are 4,4′-oxydianiline and p-phenylenediamine, and the total moles of these alone or the two are 40% by mole based on the total diamines. More preferably, it is 50 mol% or more, more preferably 70 mol% or more, and even more preferably 80 mol% or more. Furthermore, it is preferable that p-phenylenediamine contains 10 mol% or more, further 20 mol% or more, further 30 mol% or more, and further 40 mol% or more. If the content of these diamines is less than the lower limit of these mol% ranges, the resulting polyimide film tends to have a high linear expansion coefficient, a low elastic modulus, and a low birefringence. However, if the total amount of diamine is p-phenylenediamine, it is difficult to obtain a thick polyimide film with less foaming, so 4,4'-oxydianiline is preferably used.

Preferred solvents for synthesizing the polyamic acid are amide solvents N, N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone and the like, N, N-dimethylformamide, N, N-dimethylacetamide can be used particularly preferably.

Next, the polyimide production method includes a thermal curing method in which the precursor polyamic acid is converted to imide by heating, or a polyhydric acid such as acetic anhydride and other dehydrating agents, picoline, quinoline, isoquinoline, Any of the chemical curing methods in which imide conversion is performed using a tertiary amine such as pyridine as an imidization accelerator may be used. Among them, a higher boiling point such as isoquinoline is preferable. This is preferable because it does not evaporate in the initial stage of film formation, and the catalytic effect is easily exhibited until the last step of drying. In particular, the resulting film has a low coefficient of linear expansion, high modulus of elasticity, high birefringence, and can be rapidly graphitized at a relatively low temperature to obtain high-quality graphite. Cure is preferred. In particular, the combined use of a dehydrating agent and an imidization accelerator is preferable because the resulting film has a small linear expansion coefficient, a large elastic modulus, and a large birefringence. In addition, the chemical cure method is an industrially advantageous method that is excellent in productivity because the imidization reaction proceeds faster and the imidization reaction can be completed in a short time in the heat treatment.

In the production of a film by a specific chemical cure, first, an imidization accelerator comprising a dehydrating agent and a catalyst of a stoichiometric amount or more is added to a polyamic acid solution, an organic film such as a support plate, PET, a drum, or an endless belt. A film having self-supporting properties is obtained by casting or coating on a support such as a film to evaporate the organic solvent. Next, the self-supporting film is further heated and dried to be imidized to obtain a polyimide film. The temperature during the heating is preferably in the range of 150 ° C to 550 ° C. There is no particular limitation on the rate of temperature increase at the time of heating, but it is preferable to gradually heat in a continuous or stepwise manner so that the maximum temperature falls within the predetermined temperature range. Although the heating time varies depending on the film thickness and the maximum temperature, it is generally preferably in the range of 10 seconds to 10 minutes after reaching the maximum temperature. Furthermore, if the process of fixing or stretching the film to prevent shrinkage is included in the polyimide film manufacturing process, the resulting film has a low coefficient of linear expansion, a high elastic modulus, and a high birefringence. Since it tends to be easy, it is preferable.

The thickness of the polyimide film of the present invention is 30 μm or more, preferably 60 μm or more, and more preferably 100 μm or more. As the thickness of the polyimide film increases, the film strength increases and wrinkles are less likely to enter. In addition, it is possible to reduce the number of sheets used, and it is possible to reduce the distortion at the interface and the biting of air and dust, so that an excellent quality can be obtained. In addition, the complexity of the work can be reduced. It is preferable because a high quality product can be obtained by applying a pressure during graphitization during carbonization.

In the process of laminating a polyimide film and graphitizing, the laminate of the polyimide film is carbonized under reduced pressure or in nitrogen gas using a hot press or an electric furnace. This carbonization is usually performed at a temperature of about 1000 ° C. In order to obtain highly oriented graphite, it is preferable to apply pressure in the vertical direction using a hot press. During carbonization, it is preferable to apply a pressure of 5 kg / cm ² or more, preferably 15 kg / cm ² or more, more preferably 25 kg / cm ² or more.

Next, graphitization of the carbonized film is performed using a hot press. Graphitization is carried out in an inert gas. Argon is suitable as the inert gas, and it is more preferable to add a small amount of helium to argon. The heat treatment temperature is required to be at least 2400 ° C. or higher, preferably 2800 ° C. or higher, and finally 3000 ° C. During graphitization, it is preferable to apply a pressure of 100 kg / cm ² or more, preferably 200 kg / cm ² or more, more preferably 300 kg / cm ² or more. Carbonization and graphitization may be performed continuously.

The higher the heat treatment temperature is, the higher the quality can be converted to graphite, but from the viewpoint of economy, it is preferable that the conversion to high quality graphite is possible at the lowest possible temperature. In order to obtain an ultra-high temperature of 2500 ° C. or higher, usually, a current is directly applied to the graphite heater, and heating using the Joule heat is performed. The consumption of the graphite heater proceeds at 2700 ° C. or more, and the consumption rate becomes about 10 times at 2800 ° C., and further about 10 times at 2900 ° C. Therefore, if the heat treatment is not repeated due to the improvement of the raw material polymer film, a great economic effect is produced. In a generally available industrial furnace, the maximum heat-treatable temperature is 3000 ° C.

In the graphitization treatment, the carbonized film produced by the preliminary heat treatment is converted into a graphite structure, and in that case, the carbon-carbon bond must be cleaved and recombined. In order to obtain good quality graphitization, it is necessary to ensure that its cleavage and recombination occur with minimal energy. The molecular orientation of the starting polyimide film affects the arrangement of carbon atoms in the carbonized film, which can have the effect of reducing the energy of carbon-carbon bond cleavage and recombination during graphitization. Therefore, high-quality graphitization can be achieved by designing a molecule so that a high degree of molecular orientation is likely to occur. The effect of this molecular orientation becomes more prominent by adopting a two-dimensional molecular orientation parallel to the film surface.

  In the polyimide film of the present invention, since graphitization occurs even in a thick film, it is considered that graphitization proceeds almost simultaneously in the surface layer and inside, and the graphite structure formed in the surface layer due to gas generated from the inside It helps to avoid the situation of being destroyed, and you can get higher quality graphite. The polyimide film produced in the present invention is considered to have an optimal molecular orientation for producing such an effect.

  As described above, highly oriented graphite obtained by heating and pressurizing a laminate of polyimide films having high birefringence produced in the present invention is excellent as a radiation optical element and a high thermal conductor because of excellent rocking characteristics. .

  Moreover, when the birefringence of a polyimide film becomes high, intensity | strength will become high, a wrinkle will become difficult to enter, and the damage at the time of heat pressurization will also decrease. Since it is easy to graphitize even if the thickness is large, it is possible to use a thick material as the raw material, the film strength is increased, and soot is difficult to enter. In addition, it is possible to reduce the number of sheets used, and it is possible to reduce the distortion at the interface and the biting of air and dust, so that an excellent quality can be obtained. In addition, the complexity of the work can be reduced. During carbonization, pressure during graphitization can be applied to obtain a high quality product. Furthermore, since the graphitization temperature becomes low and graphitization becomes easy, the pressurization time after graphitization can be lengthened, so that a high quality product can be obtained. In addition, graphite with excellent orientation can be obtained with a single heat and pressure treatment, so utility costs (power and cooling water) used to raise and lower the temperature can be reduced, and costs can be reduced. Heater consumption can also be reduced.

  In the following, various embodiments of the present invention will be described along with some comparative examples.

Example 1
After 4 equivalents of pyromellitic dianhydride is dissolved in a DMF solution in which 3 equivalents of 4,4′-oxydianiline is dissolved, a prepolymer having acid anhydrides at both ends is synthesized, and then the prepolymer is contained. By dissolving 1 equivalent of p-phenylenediamine in the solution, a solution containing 18.5 wt% polyamic acid was obtained.

  While this solution was cooled, an imidation catalyst containing 1 equivalent of acetic anhydride, 1 equivalent of isoquinoline, and DMF was added to the carboxylic acid group contained in the polyamic acid to degas. Next, this mixed solution was applied onto an aluminum foil so as to have a predetermined thickness after drying. The mixed solution layer on the aluminum foil was dried using a hot air oven and a far infrared heater.

  The drying conditions when the finished thickness is 75 μm are shown. The mixed solution layer on the aluminum foil was dried in a hot air oven at 120 ° C. for 240 seconds to form a self-supporting gel film. The gel film was peeled off from the aluminum foil and fixed to the frame. Furthermore, the gel film is heated stepwise in a hot air oven at 120 ° C. for 30 seconds, 275 ° C. for 40 seconds, 400 ° C. for 43 seconds, 450 ° C. for 50 seconds, and a far infrared heater at 460 ° C. for 23 seconds. Has been dried. For other thicknesses, the firing time was adjusted in proportion to the thickness. For example, in the case of a film having a thickness of 25 μm, the firing time was set to be 1/3 shorter than that in the case of 75 μm. Further, when the thickness is large, it is necessary to take a sufficient baking time at a low temperature in order to prevent foaming due to evaporation of the solvent or imidization catalyst of the polyimide film.

As described above, four types of polyimide films having a thickness of 25 μm, 50 μm, 75 μm and 125 μm (sample A: elastic modulus 420 kgf / mm ² , water absorption 2.2%, birefringence 0.14, linear expansion coefficient 1. 6 × 10 ⁻⁵ / ° C.) was produced.

  Prior to producing highly oriented graphite from this polyimide film, changes in length and thickness in the plane direction during the graphitization process of this film alone were confirmed. 5 and 6 show the measurement results. Comparing FIG. 1 and FIG. 5, unlike the conventional polyimide film, the polyimide film of Example 1 has a length change of 78% or more in the plane direction, which is longer than 75% of the conventional product. It was confirmed that the change was small. When FIG. 2 and FIG. 6 were compared, it was confirmed that the polyimide film of Example 1 had a small thickness expansion at 1000 ° C. or less, and the thickness smoothly decreased, unlike the conventional polyimide film. Moreover, it has confirmed that the polyimide film of Example 1 had thickness reduction with the same tendency irrespective of raw material thickness.

Cut out 7cm length and 7cm width polyimide films of various thicknesses, 500 sheets for a thickness of 25μm, 250 sheets for a thickness of 50μm, 167 sheets for a thickness of 75μm, and a film with a thickness of 125μ. In this case, 100 sheets were stacked and set on a graphite jig, and the temperature was raised to 1200 ° C. During this time, only 100 g / cm ² based on the jig weight was added to the film. After reaching 1200 ° C., a pressure of 20 kg / cm ² was applied until the temperature reached 1400 ° C. while maintaining the same heating rate as described above. Thereafter, the pressure was decreased, and only the pressure corresponding to the weight of the jig was applied until the temperature was raised to 2400 ° C. After the temperature reached 2400 ° C., pressure was applied again. The applied pressure was 200 kg / cm ² . While maintaining this pressure, the temperature was raised to 3000 ° C. to produce highly oriented graphite.
(Example 2)
The polyimide films of various thicknesses used in Example 1 were sandwiched between graphite plates, heated to 1000 ° C. in a nitrogen atmosphere, and then heat treated by holding at 1000 ° C. for 1 hour to obtain a carbonaceous film. Using this film, highly oriented graphite was prepared in the same manner as in Example 1.
(Example 3)
The polyimide films of various thicknesses were set on a jig following Example 1, and the temperature was raised to 2400 ° C. During this period, when the thickness of 25μm is 10 kg / cm ^2, in the case of a thickness of ^{50μm 20kg / cm 2, 30kg /} cm 2 in the case of thickness 75 [mu] m, it is 50 kg / cm in the case of a thickness of 125μ film A pressure of ² was applied. After the temperature reached 2400 ° C., a pressure of 200 kg / cm ² was applied, and while maintaining this pressure, the temperature was raised to 3000 ° C. to produce highly oriented graphite.
( Reference Example 4)
It was obtained by dissolving 2 equivalents of pyromellitic dianhydride in a DMF (dimethylformamide) solution in which 1 equivalent of 4,4′-oxydianiline and 1 equivalent of p-phenylenediamine were dissolved in polyamic acid. Four types of polyimide films having a thickness of 25 μm, 50 μm, 75 μm, and 125 μm (sample B: elastic modulus 500 kgf / mm ² , water absorption 3.0%, birefringence 0) except that polyamic acid was used in the same manner as in Example 1. .14, linear expansion coefficient 1.5 × 10 ⁻⁵ / ° C.). Using this film, highly oriented graphite was prepared in the same manner as in Example 1.
(Comparative Example 1)
Example 1 except that a polyamic acid obtained by dissolving 1 equivalent of pyromellitic dianhydride in a DMF (dimethylformamide) solution in which 1 equivalent of 4,4′-oxydianiline was dissolved was used. 4 types of polyimide films having a thickness of 25 μm, 50 μm, 75 μm and 125 μm (sample C: elastic modulus 320 kgf / mm ² , water absorption 3.0%, birefringence 0.10, linear expansion coefficient 3.0 × 10 ⁻⁵ / ° C.) was produced.

Using this film, highly oriented graphite was prepared in the same manner as in Example 1.

(Comparative Example 2)
Using the polyamic acid solution of Comparative Example 1, a polyamic acid solution to which no catalyst was added was applied on the aluminum foil so as to have a predetermined thickness after drying. The mixed solution layer on the aluminum foil was dried using a hot air oven and a far infrared heater.

The mixed solution layer on the aluminum foil was dried in a hot air oven at 120 ° C. for 10 minutes to form a self-supporting gel film. The gel film was peeled off from the aluminum foil and fixed to the frame. Further, the gel film was dried by heating from 120 ° C. to 400 ° C. over 1 hour in a hot air oven. Four types of polyimide films with thicknesses of 25 μm, 50 μm, 75 μm and 125 μm (Sample D: elastic modulus 300 kgf / mm ² , water absorption> 2.0%, birefringence 0.08, linear expansion coefficient 3.5 × 10 ⁻⁵ / ° C) was produced. Further, when the thickness is large, it is necessary to take a sufficient baking time at a low temperature in order to prevent foaming due to evaporation of the solvent of the polyimide film.

  Using this film, highly oriented graphite was prepared in the same manner as in Example 1.

(Comparative Example 3)
Using the polyimide film of Comparative Example 1, graphite was produced in the same manner as in Example 4. However, cracks occurred and highly oriented graphite was not obtained.

The rocking characteristics of the graphites obtained in Examples 1 to 4 and Comparative Examples 1 and 2 were measured using an X-ray diffractometer manufactured by Rigaku Corporation, and the rocking characteristics were measured at the peak position of the graphite (002) diffraction line. Evaluation was made with the half-width of the diffraction line. The measurement results are shown in Table 1.

The locking characteristics of Examples 1 to 3 and Reference Example 4 were all 0.44 or less, indicating excellent characteristics. On the other hand, all of the locking characteristics of Comparative Examples 1 and 2 were 0.44 or more, and the characteristics were inferior. In Examples 1 to 3 and Reference Example 4 , a graphite having excellent rocking characteristics was obtained as the thickness of the raw material was increased. In Comparative Examples 1 and 2, the quality was inferior as the raw material thickness was increased. became. In Example 3, sufficient pressure was applied during carbonization, so that a product superior in quality to Example 1 was obtained. However, in Comparative Example 3, damage was caused by the same treatment, and graphite was obtained. I couldn't. Reference Example 4 was obtained in the same quality as Example 1, but the thickness of Examples 1 to 3 was about 5 mm, and Reference Example 4 was about 5% thinner than Examples 1 to 3. there were.

（実施例５）
実施例２の２５μｍのポリイミドフィルムを５０枚、５００枚、１０００枚、１５００枚用いて、参考例４と同様にして高配向グラファイトが作成された。
（比較例４）
比較例１の２５μｍのポリイミドフィルムを５０枚、５００枚、１０００枚、１５００枚用いて、比較例１と同様にして高配向グラファイトが作成された。
（実施例６）
実施例２の７５μｍのポリイミドフィルムを１７枚、１６７枚、３３３枚、５００枚用いて、参考例４と同様にして高配向グラファイトが作成された。
（比較例５）
比較例１の７５μｍのポリイミドフィルムを１７枚、１６７枚、３３３枚、５００枚用いて、比較例１と同様にして高配向グラファイトが作成された。
実施例５〜６、比較例４〜５も同様にしてロッキング特性の測定を行った。その測定結果は表２の通りである。

(Example 5)
Highly oriented graphite was prepared in the same manner as in Reference Example 4 using 50, 500, 1000, and 1500 25 μm polyimide films of Example 2.
(Comparative Example 4)
Highly oriented graphite was prepared in the same manner as in Comparative Example 1 using 50, 500, 1000, and 1500 25 μm polyimide films of Comparative Example 1.
(Example 6)
Highly oriented graphite was prepared in the same manner as in Reference Example 4 using 17 sheets, 167 sheets, 333 sheets, and 500 sheets of the 75 μm polyimide film of Example 2.
(Comparative Example 5)
Highly oriented graphite was prepared in the same manner as Comparative Example 1 using 17, 75, 333 and 500 75 μm polyimide films of Comparative Example 1.
In Examples 5-6 and Comparative Examples 4-5, the rocking characteristics were measured in the same manner. The measurement results are shown in Table 2.

  In Examples 5 and 6, when 50 or 500 raw material films were used, the finished thickness was 0.1 to 6 mm and the rocking characteristic was 0.44 or less, which showed excellent rocking characteristics. When the number of used raw material films was 1000 and 1500, the finished thickness was 6 to 20 mm and the rocking characteristics were 0.99 or less. On the other hand, Comparative Examples 4 and 5 were inferior in rocking characteristics.

Relationship between heat treatment temperature and length retention ratio of polyimide film composed of pyromellitic dianhydride with thickness of 25 μm, 50 μm, 75 μm, 125 μm and 4,4′-diaminodiphenyl ether (4,4′-oxydianiline) Relationship between heat treatment temperature and thickness retention ratio of polyimide film composed of pyromellitic dianhydride with thickness of 25μm, 50μm, 75μm and 125μm and 4,4'-diaminodiphenyl ether (4,4'-oxydianiline) Polyimide film and wedge-shaped sheet Perspective view of wedge-shaped sheet Heat treatment of polyimide film obtained by sequential polymerization of pyromellitic dianhydride with thickness of 25 μm, 50 μm, 75 μm, 125 μm and 4,4′-diaminodiphenyl ether (4,4′-oxydianiline) / p-phenylenediamine Relationship between temperature and length retention in surface direction Heat treatment of polyimide film obtained by sequential polymerization of pyromellitic dianhydride with thickness of 25 μm, 50 μm, 75 μm, 125 μm and 4,4′-diaminodiphenyl ether (4,4′-oxydianiline) / p-phenylenediamine Relationship between temperature and thickness retention

Explanation of symbols

1 Polyimide film 2 Wedge-shaped sheet 3 Wedge-shaped sheet width 4 Sodium light 5 Interference fringes

Claims (5)

Having a step of laminating 50 or more carbonaceous films obtained from a polyimide film having a birefringence of 0.12 or more or a polyimide film having a birefringence of 0.12 or more, and performing a heat and pressure treatment,
The polyimide film uses a first type diamine and an acid dianhydride to synthesize a prepolymer having the acid dianhydride at both ends, and reacts the prepolymer with a second type diamine to synthesize a polyamic acid. , A polyimide film produced by imidizing the polyamic acid ,
The method for producing highly oriented graphite, wherein the first type diamine and the second type diamine are different diamines .   The method for producing highly oriented graphite according to claim 1, wherein the polyimide film is a polyimide film having a thickness of 30 μm or more. Polyimide film, using the 4,4'-oxydianiline as the first one diamine, before using pyromellitic dianhydride as hexane dianhydride, a p- phenylenediamine as the second type of diamine The method for producing highly oriented graphite according to claim 1, wherein the polyamic acid obtained by use is a polyimide film produced by imidization using a dehydrating agent and an imidization accelerator. Method for producing a highly oriented graphite of any one of claims 1 to 3, characterized in applying a 5 kg / cm ² or more pressure during carbonization. The thickness of the graphite obtained is 6 mm or more and 20 mm or less, The manufacturing method of the highly oriented graphite of any one of Claims 1-4 characterized by the above-mentioned.

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