Preparation method of branched polyimide-based high-thermal-conductivity graphite fiber

Abstract

A method for preparing branched polyimide-based high thermal conductivity graphite fibers belongs to the field of carbon fiber technology. A partially branched PI fiber precursor is prepared by adding a triamine monomer containing three amino groups. High thermal conductivity polyimide-based graphite fibers are then prepared from this precursor through a series of heat treatment processes, including thermal imidization, carbonization, and graphitization. The presence of the triamine monomer's own spatial branched structure significantly reduces the disordered entanglement of the long PI molecular chains, which is beneficial for forming graphite fibers with larger graphite crystallite sizes. Simultaneously, the triamine is added to the system after a portion of the dianhydride has undergone a polymerization reaction to generate low molecular weight PAA molecular chains, which then connects the short molecular chains, achieving partial cross-linking and preventing gelation of the spinning solution, thus avoiding loss of spinnability. Finally, a branched polyimide-based high thermal conductivity graphite fiber is prepared.

Description

Technical Field

[0001] This invention relates to a method for preparing graphite fibers using polyimide (PI) with a branched structure as a precursor, and more particularly to a method for preparing high thermal conductivity graphite fibers, belonging to the field of carbon fiber technology. Background Technology

[0002] As aerospace, high-end manufacturing, and other fields increasingly move towards lightweighting and integration, my country's manufacturing industry is paying more and more attention to the development of the carbon fiber industry. Whether in the defense and military sectors or in major national strategic areas such as 5G, big data, artificial intelligence, electric vehicle batteries, and Industry 4.0, there is an increasingly urgent demand for high thermal conductivity materials in thermal management. Due to its lightweight, high thermal conductivity, and axial directional thermal conduction capability, high thermal conductivity carbon fiber can replace traditional thermally conductive metals in the preparation of high thermal conductivity composite materials. Currently, the quality and quantity of high thermal conductivity carbon fiber in China are far from meeting the demand. Companies such as Toray Industries of Japan and DuPont of the United States started researching carbon fiber earlier, but they have imposed strict technological blockades on my country. Carbon fiber can be prepared using mesophase pitch (MP), polyacrylonitrile (PAN), and viscose as precursors. Currently, MP is the only precursor that has been successfully used to prepare high thermal conductivity carbon fiber, but it still faces problems such as cumbersome and complex production processes, high precursor costs, and poor mechanical properties. Therefore, developing new high thermal conductivity carbon fiber precursors has become a key issue that urgently needs to be addressed and a technological barrier that needs to be overcome in my country's carbon fiber industry.

[0003] PI refers to a class of compounds whose molecular chains contain imide rings. High thermal conductivity graphite films prepared from special polyimide films can achieve thermal conductivity up to 1800 W·m. -1 ·k -1 Therefore, theoretically, PI fibers with a preferred axial orientation and intrinsically ordered structure can be used as a novel high thermal conductivity graphite fiber precursor through appropriate heat treatment techniques. However, current research on PI-based carbon materials mostly focuses on PI-based high thermal conductivity graphite films. Due to technological limitations, PI, as a precursor for high thermal conductivity carbon fibers, still faces various problems such as low orientation, poor mechanical properties, and irregular microstructure, remaining far from meeting the target values ​​and practical applications of high thermal conductivity carbon fibers. The main reason for the low performance of PI-based graphite fibers stems from their low orientation. PAA molecular chains, being long chains, tend to entangle themselves. Concentrated heat release during heat treatment causes deorientation and damage to the ordered structure. Furthermore, the carbonization process involves pyrolysis of the PI molecular chains and a tendency towards disorder.

[0004] To address this issue, existing technologies have mentioned some methods for preparing high-performance PI-based graphite fibers. For example, patent CN105696116A discloses a novel method for preparing high thermal conductivity carbon fibers, using a two-step method to prepare PI fibers, followed by carbonization and graphitization. While this method yields carbon fibers with high crystallinity and high thermal conductivity, it still suffers from poor spinnability and unstable spinning. Patent CN110644075A discloses a method for preparing carbon fibers by doping planar aromatic macromolecules with PI. The planar aromatic macromolecules have high planarity, and the PI molecular chains are oriented and aligned through π-π conjugation; however, the axial orientation of the fibers still does not meet expectations. Some technologies also mention methods to improve the thermal conductivity of PI high thermal conductivity graphite films. Patent CN202211090037.9 uses aminated fillers as nucleation sites during the graphitization process of PI films, which is beneficial for the formation of graphite nuclei in the PI film. It can also "graft" some PI molecular chains onto the diamond surface, bridging the upper and lower graphite layers and constructing thermally conductive pathways between the graphite film layers, thus improving interlayer thermal conductivity. However, this method is not suitable for aggregated fiber structures. Patent CN202110515783.7 uses naphthalene-containing dianhydrides as raw materials, undergoing polymerization and chemical imidization reactions to obtain polyimide films, which are further processed through carbonization and graphitization to prepare high thermal conductivity polyimide-based graphite films. High-performance graphite films are prepared by controlling the molecular structure of the PI precursor. Summary of the Invention

[0005] To address this issue, this patent describes a process involving the addition of a triamine monomer containing three amino groups to prepare partially branched PI fibers. These fibers are then subjected to a series of heat treatments to obtain high thermal conductivity graphite fibers. The presence of the triamine monomer's own spatial branched structure significantly improves the disordered entanglement of the long PI molecular chains, providing space for carbon atom rearrangement and carbon network formation during heat treatment, thus facilitating the formation of graphite fibers with larger graphite crystallites. Simultaneously, the addition of a portion of the triamine to the system after the partial addition of dianhydride (i.e., after a polymerization reaction between the triamine and dianhydride to generate low-molecular-weight PAA molecular chains) serves to connect the various short molecular chains, achieving partial cross-linking. This avoids the formation of an overly cross-linked three-dimensional structure, which would cause the spinning solution to gel and lose spinnability. The cross-linked structure also increases fiber strength and, to some extent, prevents the concentrated release of latent heat during low-temperature heat treatment from damaging the ordered molecular chain structure. This invention not only provides a new method for improving the thermal conductivity of graphite fibers but also offers a general approach for improving fiber axial orientation and microstructure. Currently, no methods or processes have been reported for preparing branched polyimide precursors to improve fiber axial orientation by using a third monomer with trifunctional groups to achieve partial crosslinking of PI molecular chains and preparing high thermal conductivity PI-based graphite fibers.

[0006] The present invention provides a method for preparing branched polyimide-based high thermal conductivity graphite fibers, which mainly includes the following steps: (1) Disperse the diamine (containing two amino functional groups) monomer in a polar solvent, and then add dianhydride (the ratio of amino and carboxyl functional groups is 1:1.01) in batches. When 30 mol%-70 mol% of dianhydride has been added, add triamine (containing three amino functional groups) monomer in a ratio of 1:10000-1:100 with the diamine monomer. Then continue to add the remaining dianhydride until the reaction is complete. Condensate polymerize at -15~30 ℃ for 0.5~10 h to obtain a PAA spinning solution with a solid content of 10~50%. (2) Slowly add a chemical imidizing agent composed of a dehydrating agent and a catalyst mixed in a certain proportion to the above PAA solution, and stir thoroughly for 0.5~5 h to obtain a partially imidized PAA-PI spinning solution; use wet or dry-spray wet spinning process to spin the above spinning solution into PAA-PI nascent fibers. (3) The above fibers are heated in a gradient temperature to perform thermal imidization, while stretching tension is applied to both ends of the fibers to obtain PI fibers; (4) Apply stretching tension along the axial direction to the PI fiber obtained in (3), and raise the temperature in a programmed manner under nitrogen protection to obtain PI-based carbon fiber; (5) The PI-based carbon fiber obtained in step (4) is heated in a high-temperature graphitization furnace under argon protection to obtain high thermal conductivity PI-based graphite fiber.

[0007] The diamine mentioned above is one or more of 4,4'-diaminodiphenyl ether (ODA), 3,3'-diaminobenzophenone (DABP), and p-phenylenediamine (PPD); the polar solvent may be one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO); the dianhydride mentioned above is one or more of pyromellitic dianhydride (PMDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), and 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA); the triamine mentioned above is one or more of 2,4,6-tris(4-aminophenyl)triazine (TT), 2,4,6-tris(4-aminophenyl)benzene (TAPB), tris(4-aminophenyl)amine (TAPA), and 3,4,4'-triaminodiphenyl ether. The molar ratio of triamine to diamine is 1:10000-1:100.

[0008] The order of addition of diamine, dianhydride, and triamine in the solvent is: diamine → 30%-70% of dianhydride → triamine → remaining dianhydride.

[0009] The dehydrating agent is one or a mixture of acetic anhydride, propionic anhydride, and butyric anhydride, and the addition amount is 1~80 mol%; the catalyst is one or a mixture of pyridine, triethylamine, imidazole, isoquinoline, 2-methylpyridine, and 3-methylpyridine; the ratio of dehydrating agent to catalyst is 5:1~1:5.

[0010] The above spinning process is either wet or dry-spray wet; the coagulation bath is V water:V ethanol = 9:1~1:9; the coagulation bath temperature is 0~80 ℃; the draw ratio is 0.01~5; and the spinning speed is 5~300 m / min.

[0011] During the above-mentioned thermal imidization process, the heating rate is 1~10 ℃ / min, the temperature is raised to 100~400 ℃ and held for 0.5~3 h, and the tensile tension on the fiber is 0.01~80 MPa.

[0012] During the carbonization process described above, the heating rate is 1~15 ℃ / min, the temperature is raised to 800~1600 ℃ and held for 0.5~3 h, and the tensile tension on the fiber is 0.01~20 MPa.

[0013] The temperature during the graphitization process is 2400~3200 ℃, and the holding time is more than 10 min.

[0014] The present invention has the following advantages: (1) Using PI as a precursor for graphite fiber preparation is simple to synthesize, has a variety of types, and does not require a complex pre-oxidation or non-melting process, thus reducing energy consumption.

[0015] (2) This method incorporates a triamine monomer with three amino groups to prepare a PI fiber precursor with a partially branched structure.

[0016] (3) In this method, due to the presence of the spatial branching structure of the triamine monomer itself, the disordered entanglement of the long chain of PI molecules is significantly improved, which provides a space for carbon atom rearrangement and carbon network formation during heat treatment, and is conducive to the formation of graphite fibers with larger graphite crystallite size.

[0017] (4) In this method, after a portion of the dianhydride is added to the system, that is, after a portion of the diamine reacts with the dianhydride to generate low molecular weight PAA molecular chains, it is added to connect the short chains of each molecule and achieve partial cross-linking. At the same time, it avoids the formation of an over-cross-linked three-dimensional structure, which would cause the spinning solution to gel and lose its spinnability. The cross-linked structure makes the fiber stronger and can, to a certain extent, avoid the destruction of the ordered structure of the molecular chain by the concentrated release of a large amount of latent heat during the low-temperature heat treatment.

[0018] (5) The chemical imidization method used in this method improves the reactivity.

[0019] (6) This method applies a large stretching tension along the axial direction during the thermal imidization and carbonization processes, which improves the size and order of graphite microcrystals.

[0020] (7) The branched PI-based graphite fibers prepared by this method have excellent mechanical and thermal conductivity properties. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in this embodiment, the accompanying drawings used in the embodiment description will be briefly introduced below. The accompanying drawings in the description are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the molecular chain structure of PI or PAA segments after the addition of triamine.

[0023] Figure 2 The image shows the cross-sectional microstructure of the PI-based graphite fiber prepared in Example 1. Detailed Implementation

[0024] To make the objectives and technical solutions of this invention clearer, the invention will be further described below with reference to specific embodiments, but this does not constitute a limitation on the invention.

[0025] Example 1: Equimolar amounts of PMDA and ODA were weighed and polymerized under a nitrogen atmosphere at 5 °C. ODA was first added to DMF, followed by batches of PMDA. After adding 50 mol% PMDA, 3,4,4'-triamine diphenyl ether (molar ratio: 3,4,4'-triamine diphenyl ether: ODA = 1:1000) was added. After reacting for 30 min, the remaining PMDA was added, and the reaction continued for 2 h to obtain a PAA solution with a solid content of 10%. A certain amount of acetic anhydride (30 mol% of PAA) and pyridine (molar ratio of acetic anhydride to pyridine 4:1) were mixed thoroughly and slowly added dropwise to the above PAA solution for chemical imidization while stirring. After the addition was complete, the reaction was stirred for 1 h, and then allowed to stand for 24 h to remove bubbles.

[0026] The above PAA-PI spinning solution was subjected to wet spinning. The coagulation bath ratio was V. 水 V 乙醇The spinning solution was prepared at a ratio of 9:1 and a temperature of 20 °C. The spinning solution was coagulated in a coagulation bath through a spinneret. After multi-stage drawing, it was taken in by a take-up roller to obtain nascent fibers with a draw ratio of 1.5 and a drawing speed of 50 m / min. The fibers were then thermally imidized in a muffle furnace under a constant axial drawing tension of 30 MPa. The temperature was increased to 100, 200, and 300 °C at a heating rate of 5 °C / min and held for 1 h each to obtain PI fibers. The PI fibers were then carbonized in a nitrogen atmosphere under a constant axial drawing tension of 5 MPa. The temperature was increased to 1000 °C at a heating rate of 5 °C / min and held for 1 h to obtain PI-based carbon fibers. Finally, the carbon fibers were held in a high-purity argon atmosphere at 2800 °C for 1 h to obtain fibers with an average diameter of 8 μm and a thermal conductivity of 833 W·m. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2600 MPa.

[0027] Example 1 By changing the addition ratio of 3,4,4'-triamine diphenyl ether to 1:100, and maintaining the same conditions as in Example 1, an average diameter of 14 μm and a thermal conductivity of 660 W·m were obtained. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2000 MPa.

[0028] Example 2 By changing the addition ratio of 3,4,4'-triamine diphenyl ether to 1:10000, and keeping other conditions the same as in Example 1, an average diameter of 9 μm and a thermal conductivity of 720 W·m were obtained. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2400 MPa.

[0029] Example 3 By replacing 3,4,4'-triamine diphenyl ether with an equal amount of TT, and with other conditions remaining the same as in Example 1, an average diameter of 10 μm and a thermal conductivity of 720 W·m were obtained. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2500 MPa.

[0030] Example 4 By replacing 3,4,4'-triamine diphenyl ether with an equal amount of TAPB, and with other conditions remaining the same as in Example 1, an average diameter of 9 μm and a thermal conductivity of 750 W·m were obtained. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2400 MPa.

[0031] Example 5 The order of addition of diamine, dianhydride, and triamine in the solvent was changed to diamine and triamine → dianhydride, with other conditions remaining the same as in Example 1. An average diameter of 15 μm and a thermal conductivity of 620 W·m were obtained. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2200 MPa.

[0032] Example 6 The order of adding diamine, dianhydride, and triamine to the solvent was changed to diamine → 70% of dianhydride → triamine → the remaining dianhydride. Other conditions remained the same as in Example 1, resulting in an average diameter of 11 μm and a thermal conductivity of 720 W·m. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2300 MPa.

[0033] Example 7 By changing the molar ratio of acetic anhydride to pyridine to 5:1, and keeping other conditions the same as in Example 1, an average diameter of 12 μm and a thermal conductivity of 660 W·m were obtained. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2200 MPa.

[0034] Example 8 The tensile stress experienced by the PI fiber during the thermal imidization process was reduced to 15 MPa, while other conditions remained the same as in Example 1, resulting in an average diameter of 9 μm and a thermal conductivity of 720 W·m. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2300 MPa.

[0035] Example 9 The tensile stress experienced by the PI fiber during carbonization was reduced to 3 MPa, while other conditions remained the same as in Example 1, resulting in an average diameter of 9 μm and a thermal conductivity of 770 W·m. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2400 MPa.

[0036] Example 10 The tensile stress experienced by the PI fiber during carbonization was reduced to 0 MPa, while other conditions remained the same as in Example 1, resulting in an average diameter of 10 μm and a thermal conductivity of 650 W·m. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2000 MPa.

[0037] Example 11 The carbonization temperature was increased to 1500 °C, and other conditions remained the same as in Example 1, resulting in an average diameter of 8 μm and a thermal conductivity of 826 W·m. -1 ·K-1 High-performance graphite fiber with a tensile strength of 2600 MPa.

[0038] Example 12 The graphitization temperature was increased to 3000℃, and other conditions remained the same as in Example 1, resulting in an average diameter of 8 μm and a thermal conductivity of 820 W·m. -1 ·K -1 High-performance graphite fiber with a tensile strength of 2560 MPa.

[0039] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the embodiments described. Those skilled in the art can make equivalent modifications or substitutions without departing from the spirit of the present invention, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A method for preparing branched polyimide (PI)-based high thermal conductivity graphite fibers, characterized in that, Includes the following steps: (1) Disperse the diamine monomer (containing two amino functional groups) in a polar solvent, and then add dianhydride (the ratio of amino and carboxyl functional groups is 1:1.01) in batches. When 30 mol%-70 mol% of dianhydride has been added, add triamine monomer (containing three amino functional groups) in a ratio of 1:10000-1:100 with the diamine monomer. Then continue to add the remaining dianhydride until the reaction is complete. Condensate polymerize at -15~30 ℃ for 0.5~10 h to obtain a polyamic acid (PAA) spinning solution with a solid content of 10~50%. (2) Slowly add a chemical imidizing agent composed of a dehydrating agent and a catalyst mixed in a certain proportion to the above PAA solution, and stir thoroughly for 0.5~5 h to obtain a partially imidized PAA-PI spinning solution; use wet or dry-spray wet spinning process to spin the above spinning solution into PAA-PI nascent fibers. (3) The above fibers are heated in a gradient temperature to perform thermal imidization, while stretching tension is applied to both ends of the fibers to obtain PI fibers; (4) Apply stretching tension along the axial direction to the PI fiber obtained in (3), and raise the temperature in a programmed manner under nitrogen protection to obtain PI-based carbon fiber; (5) The PI-based carbon fiber obtained in step (4) is heated in a high-temperature graphitization furnace under argon protection to obtain high thermal conductivity PI-based graphite fiber.

2. The preparation method according to claim 1, characterized in that, The diamine is one or more of 4,4'-diaminodiphenyl ether (ODA), 3,3'-diaminobenzophenone (DABP), and p-phenylenediamine (PPD); the polar solvent can be one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO); the dianhydride is one or more of pyromellitic dianhydride (PMDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), and 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA); the triamine is one or more of 2,4,6-tris(4-aminophenyl)triazine (TT), 2,4,6-tris(4-aminophenyl)benzene (TAPB), tris(4-aminophenyl)amine (TAPA), and 3,4,4'-triaminodiphenyl ether.

3. The preparation method according to claim 1, characterized in that, The order of addition of diamine, dianhydride, and triamine to the solvent is: diamine → 30%-70% of dianhydride → triamine → remaining dianhydride.

4. The preparation method according to claim 1, characterized in that, The dehydrating agent in step (2) is one or a mixture of acetic anhydride, propionic anhydride, and butyric anhydride, with an addition amount of 1 to 80 mol%; the catalyst is one or a mixture of pyridine, triethylamine, imidazole, isoquinoline, 2-methylpyridine, and 3-methylpyridine; the ratio of dehydrating agent to catalyst is 5:1 to 1:

5.

5. The preparation method according to claim 1, characterized in that, Wet or dry-jet wet spinning processes are employed; the coagulation bath has a composition of V. 水 V 乙醇 =9:1~1:9; coagulation bath temperature is 0~80 ℃; draw ratio is 0.01~5; spinning speed is 5~300m / min.

6. The preparation method according to claim 1, characterized in that, During the thermal imidization process, the heating rate is 1~10℃ / min, and the temperature is raised to 100~400℃ and held for 0.5~3 h. The tensile tension on the fiber is 0.01~80 MPa.

7. The preparation method according to claim 1, characterized in that, During carbonization, the heating rate is 1~15 ℃ / min, and the temperature is raised to 800~1600 ℃ and held for 0.5~3 h. The tensile tension on the fiber is 0.01~20 MPa.

8. The preparation method according to claim 1, characterized in that, The temperature during graphitization is 2400~3200 ℃, and the temperature is maintained for more than 10 minutes.

9. The preparation method according to claim 1, characterized in that, The PI-based graphite fibers described herein have a tensile strength greater than 1.0 GPa and a thermal conductivity greater than 400 W·m. -1 ·k -1 .

10. PI-based graphite fibers obtained by any of the preparation methods according to claims 1 to 8.

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