Method for producing polycarbosilane

Abstract

To provide a method for producing a polycarbosilane (PCS) having high stability in which a decrease in weight average molecular weight after being stored is suppressed.　The present invention is a method for producing a polycarbosilane, the method including a storage step for storing a polycarbosilane having a weight average molecular weight (Mw) of 15,000 or more in an atmosphere having an oxygen concentration of 5.0 vol% or less. An atmosphere containing an inert gas or an atmosphere formed by an antioxidant can be used as the atmosphere.

Description

Polycarbosilane manufacturing method

[0001] The present invention relates to a method for producing polycarbosilane.

[0002] Polycarbosilane (PCS) is widely used as a precursor polymer for producing ceramics. It is used to produce silicon carbide (SiC)-containing fibers and powders, silicon carbide-based composite materials, and the like. In this specification, polycarbosilane may be referred to as "PCS," and silicon carbide may be referred to as "SiC."

[0003] When PCS is used to produce SiC fibers, the SiC fibers are produced by sintering PCS yarns produced by spinning PCS. Patent Document 1 (paragraph

[0016] ) discloses that if a large number of PCS molecules with a large molecular weight are contained, it is advantageous from the viewpoint of preventing fusion of the fibers when the SiC fibers are sintered.

[0004] Furthermore, as described in Non-Patent Document 1, a polymer impregnation and firing method (PIP method) is known as a method for producing ceramic matrix composites (CMCs). The PIP method involves repeatedly impregnating and firing a prepreg laminate (preform) in which PCS is impregnated into a ceramic woven fabric. This is an effective means for densifying CMCs, but to obtain dense CMCs, PCS molecules with a large molecular weight and advanced intermolecular cross-linking are required.

[0005] Furthermore, PCS molecules with large molecular weights contribute to improving the mechanical strength of SiC fibers, so it is advantageous to have a high content of PCS molecules with large molecular weights.

[0006] Japanese Patent Application Laid-Open No. 2019-137935

[0007] "Research on a new manufacturing process to achieve densification of the matrix of ceramic matrix composites (CMC)," Ibaraki Prefectural Industrial Technology Center Research Report 2016 (No. 45)

[0008] The PCS obtained by a predetermined manufacturing method is stored for a certain period of time before being transferred to the spinning process and calcination process for producing SiC fibers. The storage condition is usually storage in air. The inventors discovered that when PCS is stored in air, oxygen in the air causes an oxidation reaction with the PCS, resulting in a decrease in the proportion of PCS molecules with large molecular weights (an increase in the proportion of PCS molecules with small molecular weights), and as a result, the weight-average molecular weight of the PCS decreases.

[0009] PCS with a reduced content of large molecular weight PCS molecules leads to deterioration in heat resistance and mechanical strength when used as a raw material to produce SiC fibers or ceramic composite materials, etc. Therefore, an object of the present invention is to provide a method for producing highly stable PCS in which the decrease in weight average molecular weight is suppressed even after storage.

[0010] The present inventors have found that, by examining the storage conditions in the storage step, the present invention makes it possible to produce PCS in which the decrease in weight-average molecular weight is suppressed.

[0011] The present inventors have discovered that when PCS having a large weight-average molecular weight is stored, decomposition of the PCS can be suppressed by storing the PCS under a specific atmosphere, and have completed the present invention. Specifically, the present invention includes the following aspects (1) to (4).

[0012] (1) A method for producing polycarbosilane, comprising a storage step of storing polycarbosilane having a weight-average molecular weight (Mw) of 15,000 or more in an atmosphere having an oxygen concentration of 5.0 vol % or less.

[0013] (2) The method for producing polycarbosilane according to (1) above, wherein the atmosphere is an atmosphere containing an inert gas or an atmosphere formed by an antioxidant.

[0014] (3) The method for producing polycarbosilane according to (1) or (2) above, wherein the polycarbosilane has a decomposition degree of 0.10 or less after storage in the storage step.

[0015] (4) The method for producing polycarbosilane according to (1) or (2) above, further comprising a preparation step of preparing the polycarbosilane, and storing the polycarbosilane obtained in the preparation step in the storage step.

[0016] According to the present invention, even if the PCS is stored for a long period of time, decomposition is suppressed, and the physical properties and state (properties) at the time of production are almost maintained. Therefore, the present invention can provide a PCS with a large weight-average molecular weight.

[0017] Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and can be practiced with appropriate modifications within the scope of the object of the present invention. In this specification, the expression "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less."

[0018] (Production Method) The method for producing polycarbosilane according to this embodiment includes a storage step of storing polycarbosilane having a weight average molecular weight (Mw) of 15,000 or more in an atmosphere having an oxygen concentration of 5.0 vol % or less.

[0019] (1) Polycarbosilane In the method for producing polycarbosilane according to this embodiment, it is preferable to use polycarbosilane having a weight-average molecular weight (Mw) of 15,000 or more, and to store the polycarbosilane under specified conditions in the storage step. Since the PCS obtained after storage contains many PCS molecules with large molecular weights, when the PCS is used as a raw material to produce SiC fibers, fusion between fibers can be suppressed when the PCS yarn spun from the PCS is sintered. Furthermore, when the PCS is used to produce a ceramic matrix composite (CMC), the PCS contributes to densifying the CMC.

[0020] (Stored PCS) The PCS used in the storage step of the production method according to this embodiment preferably has a weight-average molecular weight (Mw) of 15,000 or more, more preferably 16,000 or more, even more preferably 20,000 or more, and particularly preferably 24,000 or more. The type of PCS used in the storage step is not particularly limited. For example, the PCS used in the storage step of polycarbosilane according to this embodiment may be a PCS produced by a synthesis step. Alternatively, a PCS produced by a third party, such as a commercially available PCS product, may be used.

[0021] The form of the PCS subjected to the storage step may be, for example, solid PCS. The form of the solid is not particularly limited. For example, it may be a block PCS, a powder PCS obtained by pulverizing the block PCS, or a PCS obtained by dissolving PCS in a solvent and then drying to remove the solvent. Alternatively, PCS in the form of raw silk spun from the produced PCS may be used.

[0022] (Production Method) The PCS used in the storage step of the production method according to this embodiment may be one produced by a known PCS production method. Specific examples include liquid-gas phase pyrolysis condensation and pyrolysis reactions using an autoclave. Examples of PCS raw materials include linear polysilane compounds having a skeleton in which silicon atoms are linked together in a chain, and cyclic silane compounds having a skeleton in which silicon atoms are linked together in a ring. Examples of linear polysilane compounds include polydimethylsilane (PDMS), polymethylphenylsilane (PMPS), polydiphenylsilane (PDPS), and polymethylvinylsilane (PMVS). Examples of cyclic silane compounds include octamethylcyclotetrasilane, decamethylcyclopentasilane, dodecamethylcyclohexasilane, and tetradecamethylcycloheptasilane.

[0023] One or more compounds selected from the group consisting of the chain polysilane compounds and cyclic silane compounds can be used as raw materials. For example, PCS can be produced by a liquid-vapor phase pyrolysis condensation method using dodecamethylcyclohexasilane (DMCHS), a cyclic silane compound.

[0024] (Content of Molecular Weights with Log M≧4.5) The polycarbosilane used in the storage step of the production method according to this embodiment preferably has a content of polycarbosilane molecules with a molecular weight (M) with Log M≧4.5 of 8.0% or more.

[0025] The content (%) of molecular weights having Log M≧4.5 in the PCS according to this embodiment is calculated based on the cumulative molecular weight distribution obtained by analysis by gel permeation chromatography (GPC) equipped with a differential refractometer as a detector. Specifically, the content of the molecular weight is specified as a numerical value calculated by dividing the total value of signal intensities corresponding to molecular weights having Log M≧4.5 by the total value of all signal intensities in the cumulative molecular weight distribution, based on the cumulative molecular weight distribution calculated from the signal intensities of the differential refractometer.

[0026] In the method for producing polycarbosilane according to the present embodiment, the content of PCS molecules having a molecular weight of LogM ≥ 4.5 in the PCS used in the storage step is preferably 8.0% or more, more preferably 11.0% or more, even more preferably 12.0% or more, and particularly preferably 14.0% or more, 17.0% or more. If the content of PCS molecules is less than 8.0%, the produced PCS will be dominated by low-molecular-weight PCS molecules. This may result in problems such as increased fusion between fibers during firing of the PCS yarn when used as a raw material for SiC fibers, or reduced density of the CMC when used as a raw material for CMC (ceramic matrix composite).

[0027] The method for producing polycarbosilane according to the present embodiment may include a preparation step of preparing a PCS to be subjected to the storage step. In the preparation step, the PCS may be prepared by obtaining a commercially available PCS product, or by producing the PCS through a synthesis step.

[0028] (Synthesis Step) The method for producing polycarbosilane according to this embodiment may include a synthesis step using the production method exemplified above in order to produce PCS to be subjected to the storage step.

[0029] (Molecular weight adjustment treatment) In the preparation process, a molecular weight adjustment treatment may be performed on PCS manufactured by another party or PCS obtained by the synthesis process to remove low molecular weight PCS molecules as shown in Patent Document 1 in order to increase the content of PCS molecules with large molecular weights.

[0030] The PCS prepared in the preparation step preferably has a weight average molecular weight (Mw) of 15,000 or more, more preferably 16,000 or more, even more preferably 20,000 or more, and particularly preferably 24,000 or more.

[0031] (2) Storage Step The polycarbosilane manufacturing method according to this embodiment includes a storage step of storing polycarbosilane in an atmosphere with an oxygen concentration of 5.0 vol% or less. When PCS is used as a raw material for ceramic products, it is necessary to store the PCS until it is supplied to the manufacturing process of the ceramic product. The manufacturing method according to this embodiment suppresses decomposition of the PCS even when the PCS is stored for a certain period of time, thereby allowing the physical properties and state (characteristics) of the produced PCS to be well maintained. As a result, it is possible to provide PCS with a high weight-average molecular weight.

[0032] "Storage" in this embodiment refers to a process of storing a PCS under a specific atmosphere. The storage treatment in the storage step has the same meaning as a preservation treatment that preserves the functions and properties of the PCS. The "storage step" in this embodiment refers to a process of storing a PCS so that the quality and physical properties of the PCS are maintained well even after storage. Specifically, it refers to a process of producing a PCS in which the "decomposition degree" of the weight-average molecular weight of the PCS is small before and after storage, and which maintains the weight-average molecular weight, physical properties, and properties before storage.

[0033] As will be described later, the degree of decomposition is determined by dividing the difference between the weight average molecular weight before storage (Mw before storage) and the weight average molecular weight after the storage period (Mw after storage) by the Mw before storage, and calculated using the formula "degree of decomposition = (Mw before storage - Mw after storage) / Mw before storage." The degree of decomposition of the PCS according to this embodiment is preferably 0.10 or less, more preferably 0.07 or less, even more preferably 0.05 or less, and most preferably 0.00.

[0034] (Oxygen Concentration of the Atmosphere) In the storage step according to this embodiment, it is preferable to store the polycarbosilane in an atmosphere with an oxygen concentration of 5 vol% or less. This storage treatment sufficiently prevents contact between the PCS and oxygen, thereby suppressing decomposition of the PCS due to oxidation reactions. Therefore, the oxygen concentration of the atmosphere is preferably 5.0 vol% or less, more preferably 4.0 vol% or less, and even more preferably 3.0 vol% or less, 2.0 vol% or less, or 1.0 vol% or less. An oxygen-free atmosphere (oxygen concentration of 0.0 vol%) is most preferable. In this specification, an atmosphere with an oxygen concentration of 0.0 to 5.0 vol% is referred to as a "low-oxygen atmosphere" or "low-oxygen atmosphere."

[0035] (Type of Atmosphere) In the storage step according to this embodiment, it is necessary to provide a low-oxygen atmosphere. Examples of the low-oxygen atmosphere include an atmosphere containing an inert gas, and an atmosphere formed by an antioxidant. When the atmosphere contains an inert gas, the low-oxygen atmosphere may be formed in the container by, for example, replacing the air in the container storing the PCS with the inert gas. As the inert gas, it is preferable to select argon gas, nitrogen gas, or the like.

[0036] Furthermore, when the atmosphere is formed by an antioxidant, for example, the PCS and the antioxidant may be packaged together, i.e., the antioxidant may be placed inside the container storing the PCS and then sealed to form a low-oxygen atmosphere inside the container. Since the oxygen inside the container is absorbed by the antioxidant within about one day after sealing, the PCS is kept in a low-oxygen atmosphere for most of the storage period. The antioxidant is not particularly limited as long as it is a product capable of absorbing oxygen. For example, an oxygen scavenger whose main component is iron powder can be used. The antioxidant may also be placed in an atmosphere containing an inert gas.

[0037] Alternatively, a low-oxygen atmosphere may be formed by reducing the pressure inside the container using a pressure reducing means such as a vacuum pump.

[0038] The PCS may also be stored in a liquid in a low-oxygen atmosphere. For example, by maintaining the PCS in a state where it is dissolved in a solution or dispersed in a solvent such as a slurry in a low-oxygen atmosphere, decomposition of the PCS can be suppressed.

[0039] In the method for producing polycarbosilane according to this embodiment, various treatments such as pretreatment, processing, and post-treatment may be optionally performed on the PCS before and after the storage step, and are not particularly limited as long as the PCS according to this embodiment is stable.

[0040] Examples of the present invention will be described below, but the present invention is not limited to the following description.

[0041] A storage test was conducted in which PCS was stored under a specific atmosphere for a certain period of time. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the PCS before and after storage were measured by gel permeation chromatography (GPC), the degree of decomposition was calculated, and the stability of the PCS was evaluated. The PCS used in the test may be referred to as the "PCS sample" or "sample" hereinafter. Room temperature and high temperature were selected as storage temperatures. The test at room temperature is referred to as the "room temperature test," and the test at high temperature is referred to as the "high temperature test."

[0042] (Synthesis of PCS) The PCS used in the storage test was produced by a liquid-gas phase pyrolysis condensation method at atmospheric pressure using a liquid-gas phase pyrolysis apparatus, using polydimethylsilane (PDMS) or dodecamethylcyclohexasilane (DMCHS) as a raw material.

[0043] (Measurement of number average molecular weight and weight average molecular weight, and calculation of the content of molecular weights with LogM≧4.5) The number average molecular weight (Mn) and weight average molecular weight (Mw) of the PCS sample were measured by GPC using the following method. PCS was dissolved in toluene to prepare an approximately 0.5 wt % solution, and 20 μl of this solution was used for analysis. The analytical equipment and analytical conditions were as follows: - Apparatus name: HPLC manufactured by Shimadzu Corporation - Column: KF-604, KF-602, KF-601, one each, connected and used so that the liquid passed through from the pump side in that order (all manufactured by Resonaq Holdings Inc.) - Measurement solvent: toluene - Flow rate: analytical section and reference: 0.40 mL / min - Oven: 40°C - Detector: differential refractometer (RID-20A manufactured by Shimadzu Corporation) - Sample solution: 0.5 wt% Furthermore, the content of molecular weights where LogM ≧ 4.5 in the PCS was calculated by dividing the total signal intensity corresponding to LogM ≧ 4.5 by the total signal intensity, based on the cumulative molecular weight distribution calculated from the signal intensity during measurement with the differential refractometer used as the GPC detector.

[0044] (Measurement of oxygen concentration) The oxygen concentration during the storage process was measured using an oxygen concentration meter with a syringe needle attached to the tip of the detection unit. The oxygen concentration in the atmosphere measured when the oxygen concentration displayed on the oxygen concentration meter stopped changing for 10 seconds was adopted as the oxygen concentration during the storage process. The model of the oxygen concentration meter used is as follows: Model: XP-3180 (manufactured by New Cosmos Electric Co., Ltd.)

[0045] (1) Room Temperature Test (Table 1) As shown in Table 1, Examples 1 to 3 are test examples in which the samples were stored at room temperature in an atmosphere containing argon gas and water vapor. The atmospheric condition containing argon gas and water vapor is hereinafter referred to as "Argon No. 1." Comparative Examples 1 to 8 are test examples in which the samples were stored in air at room temperature, and were performed in air containing water vapor and in dry air without water vapor. The atmospheric condition of air containing water vapor is hereinafter referred to as "Air No. 1," and the atmospheric condition of dry air is hereinafter referred to as "Air No. 2." In the "Water Vapor" column of the "Atmosphere" column in Table 1, "Yes" indicates an atmosphere containing water vapor, and "-" indicates an atmosphere without water vapor.

[0046] As shown in Table 1, (i) Example 1, Comparative Examples 1 and 2, (ii) Example 2, Comparative Examples 3 and 4, and (iii) Example 3, Comparative Examples 5 and 6 each used samples derived from the same PCS. In (i) and (ii) above, the sample was a PCS prepared using DMCHS as the raw material. In (iii) above, the sample was a PCS prepared using PDMS as the raw material. In (i) and (ii) above, different synthesis conditions were used to prepare PCSs having the number-average molecular weight and weight-average molecular weight shown in Table 1. In Comparative Examples 7 and 8, PCSs were prepared using DMCHS as the raw material, and different synthesis conditions from those in (i) and (ii) above were used to prepare PCSs having the number-average molecular weight and weight-average molecular weight shown in Table 1. Below, Examples 1 to 3 and Comparative Examples 1 to 8 are described.

[0047] (1-1) Room Temperature Test Using Argon No. 1 (Argon Gas + Water Vapor) (Example 1) 0.5 g of PCS powder having the physical properties shown in Table 1, such as the weight-average molecular weight, was placed in a vial, and each vial was placed in an unsealed aluminum storage bag (hereinafter referred to as an "aluminum pack") along with another vial containing water-soaked absorbent cotton. The air in the aluminum pack was then replaced with argon gas, and the aluminum pack was then heat-sealed. The syringe needle of an oxygen concentration meter was then pierced through the aluminum pack, and the oxygen concentration inside the aluminum pack containing argon gas was measured. The oxygen concentration displayed on the oxygen concentration meter was held for 10 seconds until it no longer changed, confirming that the oxygen concentration was 0.1 vol%. The PCS sample sealed in the aluminum pack was then left to stand at room temperature (24.0°C) and stored for 6 months.

[0048] Example 2 Storage treatment was carried out in the same manner as in Example 1, except that the PCS had the physical properties such as the weight average molecular weight shown in Table 1.

[0049] Example 3 Storage treatment was carried out in the same manner as in Example 1, except that the PCS had the physical properties such as the weight average molecular weight shown in Table 1.

[0050] (1-2) Room Temperature Test Using Air No. 1 (Air + Water Vapor) (Comparative Example 1) 0.5 g of PCS powder made of the same PCS as in Example 1 was placed in a vial. Next, the vial was left standing for one day in a test room in air at room temperature (24.0°C), relative humidity of 32.7%, and oxygen concentration of 21.0 vol%. The PCS sample in the vial was then placed in an aluminum pack in air at room temperature, and the aluminum pack was then heat-sealed. The sample enclosed in the aluminum pack was then left standing at room temperature and stored for six months.

[0051] Comparative Example 3 Storage treatment was carried out in the same manner as in Comparative Example 1, except that the same PCS as in Example 2 was used.

[0052] Comparative Example 5 Storage treatment was carried out in the same manner as in Comparative Example 1, except that the same PCS as in Example 3 was used.

[0053] Comparative Example 7 Storage treatment was carried out in the same manner as in Comparative Example 1, except that PCS having the molecular weight shown in Table 1 was used.

[0054] Comparative Example 8 Storage treatment was carried out in the same manner as in Comparative Example 1, except that PCS having the physical properties such as weight-average molecular weight shown in Table 1 was used. The PCS used in Comparative Example 8 was subjected to the following molecular weight adjustment treatment. PCS was synthesized using DMCHS as a raw material by a liquid-vapor phase pyrolysis condensation method. The PCS was washed four times at 50°C with ethyl acetate in an amount five times by weight of the raw material, and separated into a soluble component and an insoluble component. The insoluble component was then dissolved in toluene at room temperature. The insoluble component remaining after dissolution in toluene was then removed by filtration, and a molecular weight adjustment treatment was carried out. The solvent was then removed, and the resulting PCS was used as the PCS of Comparative Example 8.

[0055] (1-3) Room Temperature Test Using Air No. 2 (Dry Air) (Comparative Example 2) 0.5 g of PCS powder made of the same PCS as in Example 1 was placed in a vial. Next, the vial was left standing for one day in a drying chamber in air at room temperature (24.0°C), a relative humidity of 0.0%, a dew point of less than -50°C, and an oxygen concentration of 21.0 vol%. After that, in the drying chamber, the PCS sample in the vial was placed in an aluminum pack, and the aluminum pack was heat-sealed with dry air inside. The sample sealed in the aluminum pack was then left standing at room temperature and stored for six months.

[0056] Comparative Example 4 Storage treatment was carried out in the same manner as in Comparative Example 2, except that the same PCS as in Example 2 was used.

[0057] Comparative Example 6 Storage treatment was carried out in the same manner as in Comparative Example 2, except that the same PCS as in Example 3 was used.

[0058] (2) High-Temperature Test (Table 2) It was found that the decomposition reaction of PCS is greatly accelerated by storing it in air at a high temperature. That is, the decomposition degree of PCS stored in air at room temperature for 6 months was 0.38. In contrast, the decomposition degree of PCS stored in air at 60°C reached 0.34 in 1 week. From this, it can be said that the decomposition reaction corresponding to the storage condition at room temperature for 6 months can be reproduced by storing it in air at 60°C for about 1 week.

[0059] On the other hand, when PCS was stored for two weeks in an oxygen-free argon gas atmosphere, no decomposition behavior of PCS was observed at any of the storage temperatures of 40°C, 50°C, and 60°C, confirming that no side reactions not involving oxygen occur. The above test results indicate that the decomposition behavior of PCS can be evaluated even when the storage temperature is increased. Therefore, in this example, in addition to room temperature testing, a high-temperature test at 60°C was also conducted.

[0060] As shown in Table 2, the PCS samples used in Examples 4 to 13 and Comparative Examples 9 to 12 were those prepared using DMCHS as the raw material. The PCS samples in Examples 4 to 12 and Comparative Example 11, Comparative Examples 9 and 10, and Example 13 and Comparative Example 12 were prepared under different preparation conditions, and as shown in Table 2, have different number average molecular weights and weight average molecular weights.

[0061] As shown in Table 2, Examples 4, 5, and 13 are test examples in which the samples were stored at 60°C in an atmosphere containing argon gas but not containing water vapor. The atmospheric conditions containing argon gas but not containing water vapor are hereinafter sometimes referred to as "Argon No. 2." Examples 6 and 7 are test examples in which the samples were stored at 60°C in an atmosphere containing nitrogen gas but not containing water vapor. The atmospheric conditions containing nitrogen gas but not containing water vapor are hereinafter sometimes referred to as "Nitrogen." Examples 8 and 9 are test examples in which the samples were stored at 60°C in an atmosphere containing water vapor formed by an antioxidant. Since an oxygen scavenger was used as the antioxidant in Examples 8 and 9, the atmospheric conditions containing water vapor formed by an antioxidant are hereinafter sometimes referred to as "Oxygen Desorption." In Table 2, in the "Water Vapor" column of the "Atmosphere" column, "Yes" indicates an atmosphere containing water vapor, and "-" indicates an atmosphere not containing water vapor.

[0062] Comparative Examples 9, 10, and 12 are test examples in which the samples were stored at 60°C in the same atmosphere as Air No. 1 in the room temperature test. Examples 10 to 12 and Comparative Example 11 are test examples in which the samples were stored at 60°C in an atmosphere containing argon gas and in which the oxygen concentration was varied. The atmospheric condition containing argon gas and in which the oxygen concentration was varied may hereinafter be referred to as a "mixed atmosphere."

[0063] (2-1) High-Temperature Test Using Argon No. 2 (Argon Gas) (Example 4) 1.0 g of PCS powder having the physical properties such as weight-average molecular weight shown in Table 2 was placed in a vial. The air in the vial was then replaced with argon gas, after which the vial was sealed and placed in an aluminum pack. Argon gas was then filled into the aluminum pack, which was then sealed by heat sealing. The oxygen concentration in the aluminum pack was then measured and confirmed to be 0.1 vol%. The PCS sample sealed in the aluminum pack was then placed in a thermostatic chamber set at 60°C and stored for one week.

[0064] Example 5 Storage treatment was carried out in the same manner as in Example 4, except that the storage period was 2 weeks.

[0065] (Example 13) Storage treatment was carried out in the same manner as in Example 4, except that the PCS had the physical properties, such as the weight-average molecular weight, shown in Table 2. The PCS used in Example 13 was subjected to the following molecular weight adjustment treatment. PCS was synthesized using DMCHS as a raw material by a liquid-vapor phase pyrolysis condensation method. It was washed once at 25°C with 20 times the weight of hexane to separate it into a soluble component and an insoluble component. The insoluble component was then dissolved in toluene at room temperature. The insoluble component remaining after dissolution in toluene was removed by filtration, and a molecular weight adjustment treatment was carried out. The solvent was then removed, and the resulting PCS was used as the PCS of Example 13.

[0066] (2-2) High-Temperature Test Using Nitrogen (Nitrogen Gas) (Example 6) 1.0 g of PCS powder made of the same PCS as in Example 4 was placed in a vial. Next, the air in the vial was replaced with nitrogen gas, and the vial was then sealed and placed in an aluminum pack. Next, nitrogen gas was sealed in the aluminum pack, and the pack was then heat-sealed. Thereafter, the oxygen concentration in the aluminum pack was measured and confirmed to be 0.1 vol%. The PCS sample sealed in the aluminum pack was then placed in a thermostatic chamber set to 60°C and stored for one week.

[0067] Example 7 Storage treatment was carried out in the same manner as in Example 6, except that the storage period was 2 weeks.

[0068] (2-3) High-Temperature Test by Deoxidation (Example 8) 1.0 g of PCS powder made of the same PCS as in Example 4 was placed in a vial. Then, using an antioxidant, a deoxidizer "Everfresh QJ-200 (manufactured by Torishige Sangyo Co., Ltd.)," the sample in the vial and the deoxidizer were placed in an aluminum pack in air at room temperature (24.0°C), relative humidity 32.7%, and oxygen concentration 21.0 vol%. The vial was not sealed, and the aluminum pack was filled with air and then heat-sealed. The PCS sample sealed in the aluminum pack was then left to stand at room temperature for one day. The oxygen concentration in the aluminum pack was then measured and confirmed to be 0.0 vol%. The PCS sample sealed in the aluminum pack was then placed in a thermostatic chamber set to 60°C and stored for one week.

[0069] Example 9 Storage treatment was carried out in the same manner as in Example 8, except that the storage period was 2 weeks.

[0070] (2-4) High-Temperature Test Using Air No. 1 (Air + Water Vapor) (Comparative Example 9) 1.0 g of PCS powder made of the same PCS as in Example 4 was placed in a vial. The sample was then left to stand for 1 hour in a test room at room temperature (24.0°C), a relative humidity of 32.7%, and an oxygen concentration of 21.0 vol%. The sample in the vial was then placed in an aluminum pack in air at room temperature, and the aluminum pack was heat-sealed while filled with air. The PCS sample enclosed in the aluminum pack was then placed in a thermostatic chamber set to 60°C and stored for 1 week.

[0071] Comparative Example 10 Storage treatment was carried out in the same manner as in Comparative Example 9, except that the storage period was 2 weeks.

[0072] Comparative Example 12 Storage treatment was carried out in the same manner as in Comparative Example 9, except that the PCS had the physical properties such as the weight average molecular weight shown in Table 2.

[0073] (2-5) High-Temperature Test in an Atmosphere with Varying Oxygen Concentration (Mixed Atmosphere) (Example 10) 1.0 g of PCS powder made of the same PCS as in Example 4 was placed in a vial. Next, the vial was placed in an aluminum pack without sealing it, and argon gas was sealed in the aluminum pack, which was then heat-sealed. A predetermined amount of air was injected into the aluminum pack using a syringe and allowed to stand for 10 minutes. Thereafter, the oxygen concentration in the aluminum pack was measured, and it was confirmed that the oxygen concentration was 0.5 vol% (mixed atmosphere No. 1). Next, the gap in the aluminum pack that had occurred when the air was injected was again heat-sealed. The PCS sample sealed in the aluminum pack was then placed in a thermostatic chamber set to 60°C and stored for one week.

[0074] Example 11 Storage treatment was carried out in the same manner as in Example 10, except that the oxygen concentration in the aluminum pack for storage was changed to 1.1 vol % (mixed atmosphere No. 2).

[0075] Example 12 Storage treatment was carried out in the same manner as in Example 10, except that the oxygen concentration in the aluminum pack for storage was changed to 3.3 vol % (mixed atmosphere No. 3).

[0076] Comparative Example 11 Storage treatment was carried out in the same manner as in Example 10, except that the oxygen concentration in the aluminum pack for storage was changed to 5.8 vol % (mixed atmosphere No. 4).

[0077] (3) Calculation of Decomposition Degree In order to quantitatively evaluate the decomposition behavior of PCS, the difference between the weight average molecular weight of PCS before storage (Mw before storage) obtained by GPC measurement and the weight average molecular weight of PCS after the storage period (Mw after storage) was determined, and the "decomposition degree" was calculated using the following formula (1).

[0078] Degree of decomposition = (Mw before storage - Mw after storage) / Mw before storage Formula (1)

[0079] The results of calculating the decomposition degree for each PCS sample in the Examples and Comparative Examples are shown in Tables 1 and 2. When the molecular weight after storage increased compared to the molecular weight before storage and the decomposition degree was displayed as a negative value, the decomposition degree was displayed as zero. The reason for the increase in molecular weight after storage is unknown.

[0080] As a criterion for evaluating stability, a PCS with a decomposition rate of 0.10 or less can be evaluated as having high stability.

[0081]

[0082]

[0083] (4) Evaluation (4-1) Room Temperature Test As shown in Table 1, in Examples 1 to 3 within the scope of the present invention, the PCS was not substantially decomposed and the stability was good. In contrast, in Comparative Examples 1 to 6, the degree of decomposition of PCS was large. Because the samples were stored in air, the PCS was decomposed by the oxygen in the air, resulting in inferior stability compared to Examples 1 to 3. Note that, because Examples 1 to 3 were all stored in an argon gas atmosphere containing water vapor, the presence of water vapor in the storage atmosphere did not substantially contribute to the decomposition of PCS. Furthermore, the PCS in Comparative Examples 7 and 8 contained a small amount of PCS molecules with large molecular weights, so no significant decomposition of PCS due to oxygen in the air was observed.

[0084] (4-2) High-Temperature Test As shown in Table 2, in Examples 4 to 13 within the scope of the present invention, the PCS was not substantially decomposed after storage, demonstrating high stability. Because the PCS of Examples 4 to 13 were stored in an atmosphere with a low oxygen concentration, they were not decomposed even after storage for the specified period, or the degree of decomposition was small.

[0085] In contrast, in Comparative Examples 9 to 12, the degree of decomposition of PCS after storage was large, and the stability was inferior to that of Examples 4 to 13. Storing PCS in an atmosphere with an oxygen concentration exceeding 5.0 vol% promoted the decomposition of PCS.

Claims

1. A method for producing polycarbosilane, comprising a storage step of storing polycarbosilane having a weight average molecular weight (Mw) of 15,000 or more in an atmosphere having an oxygen concentration of 5.0 vol% or less.

2. The method for producing polycarbosilane according to claim 1, wherein the atmosphere is an atmosphere containing an inert gas or an atmosphere formed by an antioxidant.

3. The method for producing polycarbosilane according to claim 1 or 2, wherein the decomposition degree of the polycarbosilane after storage in the storage step is 0.10 or less.

4. The method for producing polycarbosilane according to claim 1 or 2, further comprising a preparation step of preparing the polycarbosilane, and storing the polycarbosilane prepared in the preparation step in the storage step.

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