JP7620205B2 - Fluororesin molded body and its manufacturing method - Google Patents

Description

The present invention relates to a fluororesin molded body and a method for producing the same.

Conventionally, labyrinth seals are often used as shaft seals for metal rotating shafts of various rotating equipment. A labyrinth seal is one of the seal structures for the fluid that flows along the axial direction of the rotating shaft through the gap between the outer circumferential surface of the rotating shaft and the stationary part provided on the housing side facing this surface. For example, by providing fins on the stationary part side, uneven gaps are formed between the rotating shaft and the stationary part in multiple stages, and the leakage pressure is gradually reduced for each stage to reduce the leakage amount. If the rotating shaft and the stationary part that constitute the labyrinth seal are both made of metal, the metals will seize together when they come into contact with each other, causing damage to the rotating equipment. To avoid this, it has been considered to make the stationary part out of resin. For example, fluororesin is one such resin. This is because fluororesin has excellent characteristics such as heat resistance, chemical resistance, non-stickiness, and sliding properties, and is widely used for various sealing materials, sliding materials, etc. However, when fluororesin is used as a sealing material for the metal rotating shaft of a rotating device, the thermal expansion coefficient of fluororesin is greater than that of common metals such as aluminum and steel that are commonly used for rotating shafts, so as temperatures rise the gap between the rotating shaft and the sealing material becomes larger, and fluids can easily leak from this gap.

As an improvement, Patent Document 1 proposes a sealing material in which voids in a sealing material obtained by molding a fluororesin composition consisting of 10 to 60% by weight of fluororesin, 37 to 60% by weight of natural graphite powder, and 2 to 30% by weight of carbon fiber are impregnated with a thermosetting resin and cured to seal the voids. The invention described in Patent Document 1 claims that impregnating the thermosetting resin in this way can prevent fluids from penetrating and leaking from voids that occur when a large amount of graphite or carbon fiber is added to a conventional fluororesin, and can suppress the expansion of the gap between the rotating shaft and the sealing member that still occurs even when expanded graphite and carbon fiber are added to a specified modified polytetrafluoroethylene.

In fluororesin compositions used as circuit board materials, rather than as sealing or sliding materials as mentioned above, it has been proposed to use a specific filler to improve the high thermal expansion coefficient of fluororesin, in order to use a fluororesin with a low dielectric constant instead of conventional polyimide resins, etc. (for example, Patent Documents 2 to 4).

Patent Document 2 discloses a fluororesin composition containing a fluororesin exhibiting melt flowability and silica, the fluororesin having 25 or more carbonyl-containing functional groups per ¹⁰⁶ main chain carbon atoms, the silica being spherical silica, and having a linear expansion coefficient of 100 ppm/° C. or less. Such a fluororesin composition is said to have low dielectric constant, low linear expansion coefficient, and excellent adhesion, thereby making it possible to suppress warping of substrates and improve heat cycle resistance.

Patent Document 3 discloses a composition having a low dielectric constant and a low coefficient of thermal expansion, which is composed of A. a fluorinated polymer material; and B. a filler having an average particle size not greater than about 7 μm and selected from the group consisting of silica, quartz particles, hollow microspheres, and mixtures thereof, and does not contain a fibrous filler, and the amount of A is about 50 to about 90% by weight and the amount of B is about 50 to about 10% by weight, based on the total amount of A and B in the composition. And such a fluororesin composition is said to be suitable for providing a substrate containing vias therein obtained by laser drilling and/or mechanical drilling and/or punching.

Patent document 4 discloses an electrical substrate material that contains a fluoropolymer material, a ceramic filler material having a low dielectric constant, low loss and low coefficient of thermal expansion, and 2% or less by weight of a fiber reinforcement material, the filler material being in an amount of at least about 55% by weight of the total substrate material, and the ceramic filler being coated with a silane coating. Such an electrical substrate material is said to exhibit improved electrical performance over the prior art, and to exhibit relatively high compliance and a low coefficient of thermal expansion that results in improved surface mounting and plated through hole reliability.

JP 2010-156398 A International Publication No. 2020/145133 Japanese Patent Application Publication No. 6-88000 Japanese Patent Application Publication No. 63-259907

However, according to the inventor's investigation of the sealing material described in Patent Document 1, it was found that when a fluororesin that has a high melt viscosity and does not exhibit melt fluidity, such as polytetrafluoroethylene, is used as the matrix and is highly filled with inorganic filler, it is difficult for the preformed body obtained by preforming to maintain its shape. In other words, it was found that it is difficult to manufacture a molded body using the general free sintering method (corresponding to the free baking described in Patent Document 1), in which a preformed body obtained by preforming is heated and sintered to manufacture a molded body.

In addition, the composition for circuit boards described in Patent Document 2, which shows melt flowability and uses a fluororesin having adhesive properties as the matrix, has room for improvement if it is used as a sealing material or a sliding material. Even if the composition for circuit boards described in Patent Document 3 can be used for coating purposes, it is considered difficult to maintain the shape of the molded product as in Patent Document 1. The composition for circuit boards described in Patent Document 4 uses a ceramic filler coated with a silane coating as the inorganic filler, so voids originating from the silane coating are generated in the molded product when it is heated and melted during molding, and a molded product having the desired linear expansion coefficient may not be obtained.

As described above, in the case of fluororesins that have a high melt viscosity and do not exhibit melt fluidity, such as polytetrafluoroethylene, it is actually more difficult to uniformly disperse inorganic fillers at a high filling rate when heated and melted, compared to those with low melt viscosities. As a result, it is difficult to suppress the thermal expansion of the molded body. Furthermore, if inorganic fillers are highly filled into fluororesins with high melt viscosity, it is difficult to form molded bodies with practically acceptable strength for use as sealing materials or sliding materials when attempting to mold such fluororesins using the general free sintering method, which is more mass-producible.

The object of the present invention is to provide a fluororesin molded article having a linear expansion coefficient of a predetermined value or less and a strength that is practically acceptable. It is also to provide a manufacturing method that can easily manufacture such a fluororesin molded article.

The present inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have found that by using a particulate inorganic filler with a specified particle size for a specified fluororesin matrix and employing a specified manufacturing method, it is possible to easily obtain a fluororesin molded article with a linear expansion coefficient of a specified value or less and a strength that is acceptable for practical use. The gist of the present invention is as follows.

(1) A fluororesin molded product made of a fluororesin composition containing an inorganic filler, wherein the fluororesin composition has a matrix of a fluororesin having a melt viscosity of 10 ¹⁰ Pa·sec or more, the inorganic filler contains a particulate inorganic filler having a D50 particle size of 0.01 to 1 μm, the particulate inorganic filler is not coated with a silane coating, and is contained in an amount of 40 to 70 volume % of the entire fluororesin composition, and the fluororesin molded product has a linear expansion coefficient of 30 ppm (3.0 × 10 ^-5 )/K or less at 25°C to 200°C.
(2) The fluororesin molded article according to the above (1), wherein the fluororesin is polytetrafluoroethylene.
(3) The fluororesin molded article according to the above item (1) or (2), wherein the inorganic filler comprises a fibrous inorganic filler, and the fibrous inorganic filler is contained in an amount of 10 vol % or more and 30 vol % or less based on the total volume of the fluororesin composition.
(4) The fluororesin molded article according to the above (3), wherein the fibrous inorganic filler contains short fibers of pitch-based carbon fibers.
(5) The fluororesin molded article according to any one of the above (1) to (4), wherein the particulate inorganic filler contains fused silica.
(6) The fluororesin molded product according to any one of the preceding items (1) to (5), wherein the fluororesin composition contains an adhesive fluorine-containing copolymer as an additive other than the fluororesin that serves as a matrix, and the adhesive fluorine-containing copolymer is contained in an amount of 5 vol% or more and 10 vol% or less based on the total volume of the fluororesin composition.
(7) The method for producing a fluororesin molded article according to any one of (1) to (6) above, further comprising hot-press molding in which the fluororesin composition is pressurized in a molten state and cooled and solidified under pressure.

According to the present invention, it is possible to provide a fluororesin molded article having a linear expansion coefficient of a predetermined value or less and a strength that is practically acceptable. It is also possible to provide a manufacturing method that can easily manufacture such a fluororesin molded article.

The fluororesin molded article according to the embodiment of the present invention is made of a fluororesin composition containing an inorganic filler. This fluororesin composition has a matrix of a fluororesin having a melt viscosity of 10 ¹⁰ Pa·sec or more. The inorganic filler contains a particulate inorganic filler having a D50 particle size of 0.01 to 1 μm. This particulate inorganic filler is preferably not coated with a silane coating. The particulate inorganic filler is contained in an amount of 40 volume % to 70 volume % of the entire fluororesin composition. This content is maintained in the fluororesin molded article. Furthermore, the linear expansion coefficient of this fluororesin molded article at 25° C. to 200° C. is 30 ppm (3.0×10 ⁻⁵ )/K or less. Here, the volume % adopted as the content of a component contained in the fluororesin composition can be calculated, for example, from the density of the component used and the amount added.

By using a particulate inorganic filler having a predetermined fine particle size, even if a fluororesin having a high melt viscosity of 10 ¹⁰ Pa·sec or more is used, even if a particulate inorganic filler is filled in the matrix fluororesin at a high content during melt molding, the contact area between the fluororesin and the particulate inorganic filler in the fluororesin molded body increases, and the physical constraint of the fluororesin during thermal expansion increases, making it possible to make the linear expansion coefficient below a predetermined value and to impart a practically acceptable strength to the fluororesin molded body. In addition, it is preferable that the particulate inorganic filler is not coated with a silane coating, which can suppress the generation of voids due to gas derived from the silane coating during melt molding. From this perspective, it is also possible to impart a low linear expansion coefficient and a practically acceptable strength to the fluororesin molded body.

The fluororesin constituting the matrix has a melt viscosity of 10 ¹⁰ Pa·sec or more. Examples of such fluororesins include polytetrafluoroethylene (PTFE), etc., which is particularly preferred because it generally has excellent sliding properties, chemical resistance, etc., and does not burn even when in contact at high speed. Since PTFE has such properties, it is suitable as a constituent material of the seal member of the metal rotating shaft of a rotating device, for example. In particular, it is suitable as a matrix resin material constituting the labyrinth seal of the metal rotating shaft. PTFE can be obtained by a known manufacturing method, and examples of such PTFE include fine powder (emulsion polymerization product) produced by emulsion polymerization and molding powder (suspension polymerization product) produced by suspension polymerization.

The content of the fluororesin having a melt viscosity of 10 ¹⁰ Pa·sec or more in the entire fluororesin composition can be, for example, 10 to 60 volume %, taking into consideration other components. By setting the content of the fluororesin in this range, a fluororesin composition molded product having better strength and a lower linear expansion coefficient can be obtained. The lower limit is preferably 15 volume % or more, more preferably 20 volume % or more, and even more preferably 25 volume % or more. The upper limit is preferably 55 volume % or less, more preferably 45 volume % or less, and even more preferably 40 volume % or less.

The melt viscosity of the fluororesin that constitutes the matrix can be measured, for example, using a flow tester (manufactured by Shimadzu Corporation) in accordance with ASTM D 1238.

The inorganic filler includes a particulate inorganic filler having a D50 particle size of 0.01 to 1 μm. The particulate inorganic filler means an inorganic filler other than a fibrous one. Examples of particulate inorganic fillers include silica, graphite, mica, talc, glass spheres, metal particles, and ceramic spheres. Among these, silica is preferred from the viewpoint of lowering the linear expansion coefficient of the molded body and from the viewpoint of uniform dispersion in the fluororesin. Silica may be crystalline or non-crystalline, but since it is easy to obtain spherical particles as described later, amorphous silica is preferred, and fused silica is more preferred. The shape of the particulate inorganic filler is not particularly limited, but a spherical shape is preferred from the viewpoints of uniform dispersion in the fluororesin matrix, high filling ability, and isotropy of thermal expansion of the molded body. The spherical shape is preferably close to a true sphere. For example, the sphericity described in Patent Document 2 can be used as a standard for a true sphere. Specifically, a photograph is taken with a scanning electron microscope (SEM), and the sphericity is calculated from the area and perimeter of the observed particles by (sphericity) = {4π x (area) ÷ (perimeter) ² }. The closer the sphericity is to 1, the closer it is to a true sphere. The area and perimeter values used to calculate the sphericity can be average values measured for 100 particles using an image processing device, for example. The sphericity calculated in this way is preferably 0.80 or more, more preferably 0.85 or more, even more preferably 0.90 or more, and most preferably 0.95 or more. As described above, spherical fused silica is particularly suitable as a particulate inorganic filler with a predetermined particle size. Such spherical fused silica can be commercially available.

The particulate inorganic filler is preferably not coated with a silane coating as described in Patent Document 4, for example. This makes it possible to suppress the generation of voids due to gas derived from the silane coating during melt molding using a fluororesin composition, as described above. This effectively reduces the linear expansion coefficient due to the particulate inorganic filler. As described above, the significance of not being coated with a silane coating is to prevent the generation of gas derived from the coating during melt molding. Therefore, it is preferable to use a filler that is not coated with a coating containing a component that causes such gas to be generated, even if it is not a silane coating. Conversely, a coating may be formed as long as it does not contain a substance that causes gas. Note that there are cases where a particulate inorganic filler that is coated with a silane coating is acceptable.

The particle diameter of the particulate inorganic filler may be such that the D50 particle diameter (median diameter) is within the above-mentioned range. The content of such particulate inorganic filler may be such that the content of the particulate inorganic filler is within the above-mentioned range relative to the entire fluororesin composition. The D50 particle diameter of the particulate inorganic filler may be measured, for example, by a laser diffraction/scattering method.

The inorganic filler may include a fibrous inorganic filler from the viewpoint of improving the strength of the molded body and from the viewpoint of sliding properties. Examples of fibrous inorganic fillers include carbon fiber, glass fiber, metal fiber, whisker, etc., but carbon fiber is preferred from the viewpoint of uniform dispersion in the fluororesin matrix, as it has a specific gravity close to that of the fluororesin matrix. There is no particular limitation on the type of carbon fiber, and it may be pitch-based or PAN-based, but pitch-based carbon fibers are preferred from the viewpoint of further improving sliding properties. There is no particular limitation on the size of the carbon fiber, but short fibers with an average fiber length of 0.1 to 0.3 mm and an average fiber diameter of 5 to 20 μm are preferred from the viewpoint of uniform dispersion, etc. As described above, short fibers of pitch-based carbon fibers are particularly preferred as fibrous inorganic fillers.

From the viewpoint of uniform dispersion in the fluororesin matrix, it is preferable that the inorganic filler has a specific gravity close to that of the fluororesin matrix. In other words, it is preferable that the specific gravity of the particulate inorganic filler and fibrous inorganic filler is close to that of the fluororesin, and the ratio of specific gravities is preferably 0.7 to 1.5.

The content of the particulate inorganic filler is 40% by volume or more and 70% by volume or less with respect to the entire fluororesin composition. When a fibrous inorganic filler is contained, from the viewpoint of the strength of the molded body, the content is preferably 10% by volume or more and 30% by volume or less with respect to the entire fluororesin composition. In addition, the content of the inorganic filler is preferably 80% by volume or less with respect to the entire fluororesin composition. In other words, when a particulate inorganic filler and a fibrous inorganic filler are contained, the total of both is preferably 80% by volume or less. From the viewpoint of the linear expansion coefficient and strength of the molded body, the content of the inorganic filler is preferably 40% by volume or more, more preferably 50% by volume or more. When a particulate inorganic filler and a fibrous inorganic filler are contained, the total of both is preferably 50% by volume or more.

In addition to the above-mentioned components, the fluororesin composition may contain an adhesive fluorine-containing copolymer as an additive, particularly as a dispersing aid, from the viewpoint of improving the dispersibility of inorganic filler in a specific fluorine resin matrix.As the adhesive fluorine-containing copolymer, for example, there is a fluorine-containing copolymer in which an adhesive functional group is introduced into the main chain skeleton.As the fluorine-containing copolymer constituting the main chain skeleton, for example, there are tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA), copolymer having chlorotrifluoroethylene (CTFE) unit (CTFE copolymer), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylene copolymer (ECTFE), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer (THV), tetrafluoroethylene-vinylidene fluoride copolymer, etc. Examples of adhesive functional groups that can be introduced into such fluorine-containing copolymers include carbonyl group-containing functional groups. Examples of carbonyl group-containing functional groups include those that are part of a chemical structure, such as carbonate group, carboxylic acid halide group (halogenoformyl group), formyl group, carboxyl group, ester group [-C(=O)O-], acid anhydride group [-C(=O)O-C(=O)-], isocyanate group, amide group, imide group [-C(=O)-NH-C(=O)-], urethane group [-NH-C(=O)O-], carbamoyl group [NH ₂ -C(=O)-], carbamoyloxy group [NH ₂ -C(=O)O-], ureido group [NH ₂ -C(=O)-NH-], and oxamoyl group [NH ₂ -C(=O)-C(=O)-]. Examples of such adhesive fluorine-containing copolymers include those described in Patent Document 1, and PFAs (also called adhesive PFAs) having adhesive functional groups introduced therein described in Japanese Patent No. 4424246, Japanese Patent No. 5365939, and Japanese Patent No. 5263269. Among these, adhesive PFAs are preferred. Commercially available adhesive PFAs can be used.

The content of the adhesive fluorine-containing copolymer is preferably 5 to 10% by volume of the entire fluororesin composition. The ratio of the adhesive fluorine-containing copolymer (B) to the matrix fluororesin (A) (A/B, by volume) is preferably 3/1 to 8/1.

In addition to the above-mentioned components, other additives may be added to the fluororesin composition. Examples of such additives include colorants. The content of such additives may be 5% by volume or less.

The fluororesin composition can be obtained by mixing the above-mentioned components to a desired volumetric content and stirring with a stirrer such as a Henschel mixer. The properties of the fluororesin composition are appropriately determined depending on the properties of each component and the conditions during mixing, but it is preferable that the fluororesin composition is in a powder form from the viewpoint of ensuring a good mixing state of each component and from the viewpoint of uniform dispersion of each component during melt molding. When obtaining such a powdered fluororesin composition, it is preferable to use each component in a powder form, and it is preferable to adjust the temperature so that each component can be maintained in a powder form during stirring. It is also preferable to use each component in a powder form with a similar specific gravity. The content of each component can be calculated from the mass of each component added and the density of each component as described above. The volumetric content of each component in the fluororesin composition is also maintained in the molded product.

The fluororesin molded article according to the embodiment is, for example, a molded article made using the above-mentioned fluororesin composition. By manufacturing the molded article by the method described below, for example, it is possible to obtain a molded article having a linear expansion coefficient of a predetermined value or less. Furthermore, this molded article has a strength that is practically acceptable for use in applications such as sealing materials and sliding materials. Here, the linear expansion coefficient can be measured by a method conforming to JIS K7197. Furthermore, the standard for practically acceptable strength can be, for example, a 5% compression strength of 10 MPa or more when the compression strength (compression speed 5 mm/min) is measured by JIS K7181, as described below.

The fluororesin molded article can be produced, for example, by hot press molding, which is characterized in that the above-mentioned fluororesin composition is pressurized in a molten state and then cooled and solidified under pressure as described below.
First, the desired fluororesin composition is filled into, for example, a compression molding die, and compressed at room temperature at a predetermined pressure for a predetermined time using a compression press, and then the pressure is released (premolding process). In the free sintering method, the premolded product is then released from the mold and placed in a heating furnace, but when the above-mentioned fluororesin composition is used, it is difficult or impossible to maintain the shape. Therefore, it has been found that the following process can finally maintain the shape and provide the desired linear expansion coefficient and strength.

After the preforming process, the compression molding die filled with the fluororesin composition and depressurized is placed in a heating furnace heated to a temperature equal to or higher than the melting point of the fluororesin, and heated for a predetermined period of time to melt the fluororesin in the compression die (in-mold heating and melting process). Any furnace can be used as long as it can heat the die uniformly, and for example, a hot air circulating furnace can be used.

Next, the compression molding die is removed from the heating furnace while the fluororesin is still in a molten state, and is immediately placed in a compression press. At room temperature, pressure is started at a pressure equal to or greater than that used in the preforming process. After the die temperature reaches 150°C or less, the pressure is released and the die is demolded (compression molding process). After cooling to room temperature, it can be machined into the desired shape as necessary. In this way, the specified fluororesin in a molten state is cooled under pressure and solidified, and the fluororesin sufficiently wets the inorganic filler, increasing the interfacial adhesion between the fluororesin particles and the inorganic filler, resulting in a molded product that has practically acceptable strength and can maintain its shape. When the fluororesin is PTFE, it is believed that the desired molded product can be obtained by good fusion of the PTFE particles together.

The above-mentioned fluororesin molded body has a low linear expansion coefficient of less than a predetermined value, and thus can suppress dimensional changes due to heating, and can be applied to various applications in which it is used in combination with materials with a low linear expansion coefficient. In particular, since it contains a predetermined fluororesin as a matrix, it is suitable as a sealing material or sliding material for various rotating devices. In particular, it is suitable as a sealing material for metal rotating shafts of rotating devices, and in particular as a sealing material constituting a labyrinth seal or an abradable seal. An abradable seal is a seal having a seal material to be cut in the stationary part, and the seal material to be cut is cut by the rotation of the fins formed on the rotating shaft, thereby ensuring an optimal distance. A labyrinth seal is one that does not use a seal material to be cut, such as an abradable seal, and as described above, for example, a fin is provided on the stationary part side, and uneven gaps are formed between the rotating shaft and the stationary part in multiple stages. The above-mentioned labyrinth seal for rotating devices is particularly suitable as a sealing material for turbo compressors.

The following describes the embodiment of the present invention in more detail based on the examples.

Example 1
<Preparation of Fluororesin Composition>
PTFE (fine) (manufactured by Mitsui Chemours Fluoroproducts Co., Ltd., Teflon (registered trademark) PTFE Fine Powder 641-J, emulsion polymerized product, average particle size: 400 μm, specific gravity: 2.17, melting point: 327° C., melt viscosity: 10 ¹² Pa·sec) 35% by volume, particulate inorganic filler (manufactured by Denka Co., Ltd., UFP-35HH, fused silica, D50 particle size: 0.1 μm, specific gravity: 2.2, shape: spherical (spherical)) 40% by volume, fibrous inorganic filler (manufactured by Kureha Corporation, Kureka Chop KGF-200 M-201S, carbon fiber, pitch-based, average fiber length 0.15 mm, average fiber diameter 14.5 μm, specific gravity: 1.60) 20% by volume, adhesive fluorine-containing copolymer (manufactured by AGC Inc., Fluon+ EA-2000 Powder, specific gravity: 2.13, melting point: 310° C., melt viscosity: 10 ⁴ Pa·sec, adhesive functional group-introduced PFA (5% by volume) was mixed and stirred for 2 minutes in a small laboratory blender to obtain a fluororesin composition.

<Production of fluororesin molded article>
The obtained fluororesin composition was used to obtain a fluororesin molded product by hot press molding as follows. First, the obtained fluororesin composition was put into a compression molding die capable of forming a cylindrical space of φ40 mm, and premolded in a compression press at a molding pressure of 10 MPa and a die temperature of 25 ° C., and then the die with the composition in it was heated in a heating furnace at 360 ° C. to melt PTFE. Next, the die was removed from the heating furnace and placed in a compression press so that PTFE remained in a molten state, and naturally cooled while being pressurized at 30 MPa, and then demolded to obtain a fluororesin molded product of φ40 mm x 70 mm in height. The obtained fluororesin molded product was used for the evaluation described below.

Example 2
A fluororesin composition was prepared and a fluororesin molded article was produced in the same manner as in Example 1, except that PTFE (fine) was changed to PTFE (molding) (manufactured by Mitsui-Chemours Fluoroproducts Co., Ltd., Teflon (registered trademark) PTFE molding powder 7-J, suspension polymerization product, average particle size 30 μm, specific gravity 2.17, melting point: 327° C., melt viscosity: 10 ¹² Pa·sec). The obtained fluororesin molded article was used for the evaluation described below.

Example 3
A fluororesin composition was prepared and a fluororesin molded article was produced in the same manner as in Example 1, except that the PTFE content was set to the content shown in Table 1, and SFP-30M (fused silica, D50 particle size: 0.7 μm, specific gravity: 2.2, shape: spherical (spherical shape)) manufactured by Denka Company Ltd. was used as the particulate inorganic filler at the content shown in Table 1. The obtained fluororesin molded article was used for the evaluation described later.

(Examples 4 and 5, Comparative Examples 1 to 3)
A fluororesin composition was prepared and a fluororesin molded article was produced in the same manner as in Example 1, except that the component composition was as shown in Table 1. The obtained fluororesin molded article was used for the evaluations described below. In Comparative Example 2, mica (manufactured by Yamaguchi Mica Co., Ltd., SJ-005, D50 particle size: 5 μm) was used as the particulate inorganic filler.

(Reference Example 1)
A fluororesin composition was prepared in the same manner as in Example 1, except that the component composition was as shown in Table 1. Using this fluororesin composition, an attempt was made to manufacture a fluororesin molded body by the free sinter molding method as described below. First, the obtained fluororesin composition was placed in a compression molding die capable of forming a cylindrical space of φ40 mm, and premolded by a compression press at a molding pressure of 50 MPa and a die temperature of 25°C. When the premolded body was subsequently released from the die, it was found to be very brittle and unable to retain its shape, and therefore could not be sintered by heating. In other words, a molded body could not be obtained by the free sinter method.

(evaluation)
<Measurement of linear expansion coefficient>
The linear expansion coefficient was measured according to JIS K7197. First, test pieces of φ5×10 mm were prepared by machining from the fluororesin molded bodies obtained in Examples 1 to 5 and Comparative Examples 1 to 3. Next, the average linear expansion coefficient was measured for each of the obtained test pieces by increasing the temperature at 5° C./min in the temperature range of 25° C. to 200° C. using a thermomechanical measuring device (TMA/SS6100, manufactured by Hitachi High-Tech Science Corporation). The evaluation results are shown in Table 1.

<Compressive strength measurement>
Compression properties were measured in accordance with JIS K7181. First, test pieces of φ8×20 mm were prepared by machining from the fluororesin molded articles obtained in Examples 1 to 5 and Comparative Examples 1 to 3. Next, the compression strength of each of the obtained test pieces was measured at a test speed of 5 mm/min using an Autograph AG-X plus manufactured by Shimadzu Corporation, and the 5% compression strength was calculated. The evaluation results are shown in Table 1.

Table 1 shows the component compositions, molding methods, moldability, and evaluation results for Examples 1 to 5, Comparative Examples 1 to 3, and Reference Example 1. The "Moldability" item in Table 1 indicates whether molding was possible using each molding method employed, with "○" indicating that a molded body was produced without any problems and "×" indicating that a preform could not be formed.

As shown in Table 1, in the case of fluororesin compositions (Examples 1 to 5) that have a matrix of fluororesin with a specified melt viscosity and contain a specific particulate inorganic filler at a specified content, the fluororesin molded product obtained by hot press molding has a specified linear expansion coefficient and deformation strength that is practically acceptable.

Claims (6)

A fluororesin molded article made of a fluororesin composition containing an inorganic filler,
The fluororesin composition has a matrix of a fluororesin having a melt viscosity of 10 ¹⁰ Pa·sec or more,
The inorganic filler comprises a particulate inorganic filler having a D50 particle size of 0.01 to 1 μm,
the particulate inorganic filler is not coated with a silane coating, and is included in the fluororesin composition in an amount of 40% by volume or more and 70% by volume or less based on the entire fluororesin composition;
The fluororesin composition contains an adhesive fluorine-containing copolymer as an additive in addition to the fluororesin that serves as a matrix,
the adhesive fluorine-containing copolymer is contained in an amount of 5% by volume or more and 10% by volume or less based on the entire fluororesin composition,
The fluororesin molded article has a linear expansion coefficient of 30 ppm (3.0×10 ⁻⁵ )/K or less at 25° C. to 200° C. The fluororesin molded article according to claim 1, wherein the fluororesin is polytetrafluoroethylene. The fluororesin molded article according to claim 1 or 2, wherein the inorganic filler includes a fibrous inorganic filler, and the fibrous inorganic filler is contained in an amount of 10% by volume or more and 30% by volume or less relative to the entire fluororesin composition. The fluororesin molded article according to claim 3, wherein the fibrous inorganic filler contains short fibers of pitch-based carbon fibers. The fluororesin molded article according to any one of claims 1 to 4, wherein the particulate inorganic filler contains fused silica. The method for producing a fluororesin molded article according to any one of claims 1 to 5 , further comprising hot-press molding in which the fluororesin composition is pressurized in a molten state and cooled and solidified under pressure.