Preparation method of polyimide film, high-thermal-conductivity graphite flake and application of high-thermal-conductivity graphite flake

Abstract

The invention discloses a preparation method of a polyimide film, a high-thermal-conductivity graphite flake and application of the high-thermal-conductivity graphite flake. Diamine, a dianhydride monomer, a large aromatic ring monomer and an end-capping reagent are added into a solvent in batches, and polyamide acid resin is obtained through reaction; respectively preparing an inorganic particle dispersion liquid and a chemical tape casting method accelerant; and continuously, instantly and uniformly mixing the three components in a needle type mixer on line, and immediately performing salivation and imidization to obtain the polyimide film. The plane orientation degree and the mechanical property of the thin film are improved by utilizing a large aromatic ring structure and a cross-linking type end-capping reagent, and the carbon density is improved. The problem of non-uniform dispersion caused by agglomeration and sedimentation of inorganic particles in a resin system is solved through a continuous on-line instantaneous mixing technology, the ratio of the resin, the dispersion liquid and the accelerant can be flexibly adjusted on line, and a new way is developed for continuous and stable engineering preparation of the inorganic particle functionalized polyimide film. The heat conductivity coefficient of the graphite flake prepared from the polyimide film can reach 2000W / m.K.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, and particularly relates to a polyimide film, graphite sheet, its preparation method and application. Background Technology

[0002] With the continuous iteration and upgrading of new technologies represented by AI, electronic product design is developing towards thinner, lighter, smarter, and more multifunctional directions. The rapid growth of big data, cloud computing, and parallel computing has led to increased data transmission speed and data volume, resulting in a surge in data center power consumption. Heat dissipation of components has become a bottleneck problem for electronic terminal equipment.

[0003] Depending on the specific needs of different devices, new heat dissipation solutions can be divided into two directions: first, increasing the thickness of the heat dissipation material to achieve higher heat flux in the planar direction; and second, maximizing the thermal conductivity of the material to improve heat transfer speed. High-performance polyimide (PI) films, after high-temperature carbonization and graphitization, can yield thermally conductive graphite sheets with thermal conductivity several times that of copper, making them a core material for solving heat dissipation problems in electronic products. Chinese patent application CN110423467A discloses a method for preparing ultra-thick polyimide films exceeding 90μm in thickness. The graphite sheets obtained through high-temperature sintering can reach a thickness of 45-130μm, solving the problem of insufficient heat flux in the planar direction caused by insufficient thickness in existing polyimide-based artificial graphite. However, the thermal conductivity of polyimide-based artificial graphite in the industry cannot reach 2000W / m·K, failing to meet the latest heat dissipation requirements of the electronics industry, necessitating a breakthrough.

[0004] There are two main methods in the industry to improve the thermal conductivity of artificial graphite. One method is to improve the planar orientation of the precursor polyimide film by introducing a rigid coplanar structure. Its molecular structure can form a more continuous carbon layer during graphitization, thereby providing a better heat conduction path. However, the introduction of rigid segments inevitably reduces the flexibility of the film, affecting the molding and processing yield. Moreover, the thermal conductivity of graphite sheets prepared solely by controlling the PI molecular structure is difficult to exceed 2000 W / m·K. Chinese patent application CN114014657A discloses a polyimide-based high thermal conductivity graphite film, which is obtained by polycondensing a dianhydride monomer containing 1,4,5,8-naphthalenetetracarboxylic anhydride and a diamine monomer in a polar solvent to obtain polyamic acid. Then, it is cast, stretched longitudinally and laterally, and heated to obtain a 30-50 μm polyimide film. The naphthalenetetracarboxylic anhydride with a regular molecular structure is used to induce the graphitization transformation. Unlike traditional technologies, it does not require the addition of inorganic additives, thus avoiding the appearance defects of graphite caused by uneven dispersion and excessively large particle size of inorganic additives. However, the thickness range of the precursor polyimide film prepared by this technology is too narrow, and the maximum elongation at break is only 20%. It is easy to break during winding or processing, resulting in low yield and difficulty in achieving large-scale production. The final graphite film has a planar thermal conductivity of no more than 900 W / m·K, which is far from meeting the heat dissipation requirements of electronic terminals.

[0005] Another way to improve the thermal conductivity of artificial graphite is to introduce nanoparticles into the precursor polyimide film, such as nano-hybrid films prepared by doping graphene or carbon nanotubes. During the graphitization process, the nanoparticles overlap with the artificial graphite sheets to form a more continuous thermally conductive network, reducing phonon scattering and thus improving the thermal conductivity of graphite. However, how to achieve a uniform distribution of nanoparticles in the system has always been a challenge for the industry. Chinese patent application CN118439878A discloses a high thermal conductivity polyimide composite graphite film and its preparation method. First, surface-modified carbon nanotubes are dispersed in an organic solvent. Then, diamine and dianhydride are added, and the polycondensation reaction is carried out for more than 12 hours to obtain a polyamic acid / carbon nanotube mixture. Subsequently, a pyridine / propionic anhydride mixture is added as a catalyst, stirred, and then cast onto a glass plate. After heat treatment, the film is separated from the glass plate and treated at 150-230℃ for 5-30 minutes to obtain a polyimide film. Further high-temperature sintering yields a graphite film. The highest thermal conductivity of this film is only 1115.7 W / (m·K). Moreover, the residence time of the nanoparticles in the resin system exceeds 12 hours, inevitably leading to sedimentation and secondary agglomeration, affecting product performance and consistency, and making stable engineering production impossible.

[0006] To reduce the residence time of nanoparticles in resin systems, Chinese patent application CN116375016A proposes a method for preparing high thermal conductivity polyimide graphite films by doping graphene. First, a polyamic acid resin solution is prepared. Then, graphene oxide, carbon nanotubes, and swollen starch ethers are added to an imidizing agent to obtain a dispersion. After ultrasonic dispersion for approximately 2 hours, the dispersion is mixed with the polyamic acid resin, centrifuged to remove bubbles, and cast for imidization to obtain a film. Finally, high-temperature graphitization is performed to obtain the high thermal conductivity graphite film. Although this method avoids graphene agglomeration during polyamic acid resin synthesis, the ultrasonication, centrifugation, and degassing processes take several hours. At this time, nanomaterials will still experience sedimentation and secondary agglomeration; moreover, once the dispersion is prepared, the amount of graphene and imidizing agent cannot be adjusted online independently, and the ratio of dispersion to polyamic acid resin cannot be adjusted online independently, so the product performance cannot be controlled according to the actual production situation, which is not suitable for continuous industrial production; after the dispersion and polyamic acid resin are mixed, the imidizing agent will promote the rapid dehydration and imidization of the resin, and a large amount of gel and impurities will be formed during centrifugation and degassing, which has no industrial value; the thermal conductivity of the final graphite material is only 1658 W / (m·K) at most, which is difficult to meet the latest application requirements. Summary of the Invention

[0007] To overcome the problems in the prior art, this invention provides a method for preparing polyimide films, high thermal conductivity graphite sheets, and their applications. By utilizing instantaneous online mixing technology, the residence time of nanoparticles in the resin system is reduced, solving the problem of uneven dispersion of inorganic particles in the prior art. The ratio of resin, dispersion liquid, and casting accelerator can be flexibly adjusted online to obtain a high-performance polyimide film. The thermal conductivity of the graphite sheets prepared from this film can reach 2000 W / m·K, opening up a new avenue for the continuous and stable engineering preparation of polyimide films functionalized with inorganic particles.

[0008] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows: This invention provides a method for preparing a polyimide film, comprising the following steps: S1. Prepare raw materials: polyamic acid resin solution, inorganic particle dispersion, and chemical casting accelerator; the polyamic acid resin solution is prepared by polycondensation reaction of diamine, dianhydride, macroaromatic ring monomer, and crosslinking end-capping agent; the macroaromatic ring monomer is an aromatic ring containing one or more of naphthalene ring, anthracene ring, or anthraquinone.

[0009] S2. Instantly mix the raw materials prepared in S1 in an online needle mixer until uniform.

[0010] S3. After mixing, without degassing, the mixture is immediately cast and imidized to obtain a polyimide film.

[0011] This invention introduces a unique large aromatic ring structure containing naphthalene rings, anthracene rings, and anthraquinones into the polyimide structure, which improves its molecular orientation and ensures good mechanical properties while exhibiting excellent dimensional stability. Graphite sheets made from this film have better thermal conductivity. The instantaneous online mixing technology using a needle mixer reduces the residence time of inorganic nanoparticles in the resin system, solving the problem of uneven dispersion caused by agglomeration and sedimentation of inorganic particles in existing technologies. This instantaneous online mixing technology also allows for flexible online adjustment of the ratio of resin, dispersion, and casting accelerator, enabling control of product performance based on actual production conditions, making it more suitable for continuous industrial production. The film prepared using this invention and sintered into graphite sheets has a thermal conductivity exceeding 2000 W / m·K.

[0012] As an optional implementation, in the preparation method provided by the present invention, the instantaneous online mixing time in the needle mixer is 10~50s, the mixing temperature is -15~10℃, and the mixer speed is 1000~4000rpm.

[0013] As an optional implementation, in the preparation method provided by the present invention, the top of the needle mixer is provided with a polyamic acid resin inlet, and the same height of the needle mixer is provided with a dispersion liquid inlet and a chemical casting accelerator inlet; the polyamic acid resin is fed first, and then the dispersion liquid and the chemical casting accelerator are fed simultaneously.

[0014] In this invention, the polyamic acid resin first fills the cavity of the needle mixer, and under the action of the high speed of the stirring paddle, it is rapidly sheared and thinned, which is more conducive to uniform mixing with the inorganic particle dispersion and the chemical casting accelerator. The addition of the chemical casting accelerator will further reduce the viscosity of the solution in the mixer, which is conducive to the uniform mixing of the inorganic particle dispersion in the resin. The simultaneous addition of the inorganic particle dispersion and the chemical casting accelerator can avoid the resin from gelling prematurely due to contact with the accelerator alone.

[0015] As an optional implementation method, in the preparation method provided by the present invention, the molar amount of the macroaromatic ring monomer accounts for 5 to 30% of the total molar amount of the diamine and dianhydride.

[0016] In this invention, the molar amount of the large aromatic ring monomer is controlled to be 5-30% of the total molar amount of the diamine and dianhydride. Reducing the amount of the large aromatic ring monomer results in only a small improvement in the orientation structure and little impact on performance. Excessive addition leads to excessive film rigidity and brittleness, making industrialization difficult.

[0017] As an optional implementation, in the preparation method provided by the present invention, the macroaromatic ring monomer is selected from one or more of 1,4,5,8-naphthocarboxylic dianhydride, 2,3,6,7-naphthocarboxylic dianhydride, 2,3,6,7-anthracitelic dianhydride, anthracene-2,6-diamine, 1,5-naphthodiamine, 1,4-diaminonaphthalene, and 1,8-dihydroxy-2,4,5,7-tetraaminoanthraquinone (4NADA).

[0018] As an optional implementation, in the preparation method provided by the present invention, the crosslinking end-capping agent is selected from one or more of 4-phenylacetylene phthalic anhydride (4-PEPA), 4-acetylene phthalic anhydride (EPA), and 4-acetylene aniline.

[0019] As an optional implementation, in the preparation method provided by the present invention, the amount of cross-linking end-capping agent added is 0.05% to 0.5% of the mass of polyamic acid resin.

[0020] As an optional implementation, in the preparation method provided by the present invention, the inorganic particle dispersion liquid has a solid content of 10-25%, and the inorganic particles are selected from one or more of silicon oxide, silicon carbide, aluminum nitride, silicon nitride, boron nitride, calcium phosphate, dicalcium phosphate, calcium pyrophosphate, calcium carbonate, calcium bicarbonate, alumina, carbon nanotubes, and graphene.

[0021] As an optional implementation, in the preparation method provided by the present invention, the particle size of the inorganic particles is 50 nm to 5 μm; the amount of inorganic particles added is 0.03% to 20% of the mass of the polyimide film.

[0022] Furthermore, the particle size of the inorganic particles is preferably 500 nm to 3 μm. The amount of inorganic particles added is preferably 0.2% to 10% of the mass of the polyimide film.

[0023] As an optional implementation method, in the preparation method provided by the present invention, in S3, the casting temperature is 60-180°C, the imidization temperature is 200-600°C, and the casting speed is 10-35 m / min.

[0024] In this invention, the online mixing time is 5-60 seconds, the casting temperature is 60-180°C, and the solvent is rapidly evaporated by heating to obtain a polyamic acid gel film, which is then peeled off from the steel strip to enter the imidization stage. The imidization temperature is 200-600°C, during which the gel film completes imidization, and the end-capping agent achieves chemical cross-linking, further improving the film performance, thereby obtaining an ultra-high planar orientation polyimide film.

[0025] As an optional implementation, in the preparation method provided by the present invention, the organic solvent is selected from one or more of dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP).

[0026] As an optional implementation, in the preparation method provided by the present invention, the diamine is an aromatic diamine and the dianhydride is an aromatic dianhydride.

[0027] As an optional implementation, in the preparation method provided by the present invention, the diamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether (ODA), 1,3-bis(4'-aminophenoxy)benzene (TPE-R), 1,4-bis(4'-aminophenoxy)benzene (TPE-Q), p-phenylenediamine (PDA), and 2,2-bis(4-aminophenoxy)benzene (BAPP).

[0028] As an optional implementation, in the preparation method provided by the present invention, the dianhydride monomer is selected from one or more of pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA), 2,3,3',4-biphenyltetracarboxylic dianhydride (α-BPDA), 3,3',4,4-benzophenone tetracarboxylic dianhydride (BTDA), and 4,4'-diphenyl ether dianhydride (ODPA).

[0029] As an optional implementation method, in the preparation method provided by the present invention, the solid content of the prepared polyamic acid resin is 12% to 30%, the reaction temperature is 20 to 55°C, and the viscosity is 1200P to 5000P.

[0030] Furthermore, the viscosity is preferably 1800–2500 P.

[0031] As an optional implementation, in the preparation method provided by the present invention, the chemical casting accelerator includes a catalyst and a dehydrating agent.

[0032] As an optional implementation, in the preparation method provided by the present invention, the dehydrating agent is selected from at least one of acetic anhydride, propionic anhydride, and benzoic anhydride, and the amount added is 15-35% of the mass of polyamic acid resin; further, the amount added is 20-30% of the mass of polyamic acid resin.

[0033] As an optional implementation, in the preparation method provided by the present invention, the catalyst is selected from at least one of pyridine and its derivatives, imidazole, quinoline, and isoquinoline, and the amount of catalyst added is 1 to 8% of the mass of polyamic acid resin.

[0034] Based on the same technical concept, the present invention also provides a high thermal conductivity graphite sheet, which is obtained by carbonization, graphitization and calendering of the polyimide film prepared by the above preparation method.

[0035] As an optional implementation, in the high thermal conductivity graphite sheet provided by the present invention, the single-layer thickness of the graphite sheet is 17-130 μm.

[0036] As an optional implementation, in the high thermal conductivity graphite sheet provided by the present invention, the thermal conductivity of the graphite sheet is ≥2000W / m·K.

[0037] Based on the same technical concept, the present invention also provides the application of the polyimide film prepared by the above preparation method or the graphite sheet prepared by the above preparation method in heat dissipation materials for electronic devices.

[0038] Based on the same technical concept, the present invention also provides applications of polyimide thin films prepared by the above-described preparation method in the fields of electronic information, flexible display, aerospace, chip semiconductor or defense.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention designs a continuous online instantaneous mixing technology, in which the nanoparticle dispersion and the catalytic system are simultaneously and continuously added online to the resin solution during the film casting stage, and the film can be directly cast without degassing. On the one hand, the diffusion efficiency is improved by ultra-high speed and high flow rate shear dispersion, realizing instantaneous uniform mixing of resin and catalytic system, solving the problem of local gelation defects caused by uneven mixing of catalytic system and resin, and enabling large-scale, stable and efficient production of thin films.

[0040] On the other hand, since the mixing time between nanoparticles and resin is only a few seconds to tens of seconds, the nanoparticles are quickly cast into a film on a steel strip. Under the action of the catalytic system, they are rapidly imidized, thereby locking the distribution state of the nanoparticles. Compared with traditional doping methods, the residence time of nanoparticles in the resin solution system is shortened from more than ten hours to a few seconds. This not only effectively avoids secondary agglomeration and sedimentation, but also improves the mechanical properties, application performance and consistency of functional films. Moreover, the proportion and type of polyamic acid resin, accelerator and nanoparticles can be controlled online according to the actual production situation. It is a more flexible common technology with huge development potential and industrialization value, opening up a new way for the continuous and stable engineering preparation of inorganic particle functionalized polyimide films.

[0041] (2) This invention improves molecular orientation by introducing a large aromatic ring structure containing naphthalene ring, anthracene ring and anthraquinone, and regulates the soft and hard segment structure to ensure good mechanical properties while having excellent dimensional stability. When applied in the field of heat dissipation, it can not only increase the carbon density of graphite, but also reduce the difficulty of disordered carbon atoms forming ordered graphite crystals during graphitization, forming a more uniform graphite crystal structure, providing a better heat conduction path, and forming a synergistic effect with uniformly dispersed inorganic particles to better induce graphitization transformation. The thermal conductivity of the prepared graphite sheet can exceed 2000 W / m·K. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0043] Figure 1 This is a schematic diagram illustrating the needle mixer and continuous online doping process used in an embodiment of the present invention. Detailed Implementation

[0044] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0045] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0046] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0047] Example 1 A method for preparing a high thermal conductivity graphite sheet includes the following steps: (1) Resin preparation: 70.64 kg of ODA, 4.239 kg of PDA, 72.67 kg of PMDA and 12.61 kg of 1,4,5,8-naphthalenetetracarboxylic dianhydride were added in batches to 840 kg of DMAc and polycondensation reaction was carried out at 45 °C. After completion, 1 kg of 4-PEPA was added for end capping. After defoaming, a polyamic acid resin with a viscosity of 2100 P was obtained.

[0048] (2) Preparation of dispersion: First, add 0.32 kg of calcium carbonate with a particle size of 1 μm to 1.28 kg of DMAc, disperse and stir at high speed, and then set aside for use.

[0049] (3) Preparation of chemical casting accelerator: Dissolve 280 kg of acetic anhydride and 20 kg of imidazole in 100 kg of DMAc, stir evenly and set aside for use.

[0050] (4) PI film preparation: The above three components are mixed online in a needle mixer for 50s, the mixing temperature is -10℃, the mixer speed is 1200rpm, and the casting temperature is 100℃ to obtain a polyamic acid gel film. Then, it is peeled off from the steel belt and imidized at 200-600℃. The casting speed is 33m / min to obtain a PI film with a thickness of 43μm.

[0051] (5) Graphite sheet preparation: PI film is placed in carbonization furnace and graphitization furnace for high temperature treatment, and then rolled to obtain high thermal conductivity graphite sheet.

[0052] Example 2 A method for preparing a high thermal conductivity graphite sheet includes the following steps: (1) Resin preparation: 92.91 kg of ODA, 75.90 kg of PMDA and 31.10 kg of 2,3,6,7-naphthalenetetracarboxylic dianhydride were added in batches to 800 kg of DMF and polycondensation was carried out at 30 °C. After completion, 4 kg of EPA was added for end capping. After defoaming, a polyamic acid resin with a viscosity of 2450 P was obtained.

[0053] (2) Preparation of dispersion: First, add 2 kg of carbon nanotubes with a particle size of 2 μm and 0.4 kg of calcium bicarbonate with a particle size of 0.8 μm to 11.2 kg of DMF, disperse and stir at high speed, and then set aside for use.

[0054] (3) Preparation of chemical casting accelerator: Dissolve 240 kg of propionic anhydride and 30 kg of quinoline in 130 kg of DMF, stir evenly and set aside for use.

[0055] (4) PI film preparation: The above three components are mixed online in a needle mixer for 40s, the mixing temperature is -5℃, the mixer speed is 1800rpm, and the casting temperature is 90℃ to obtain a polyamic acid gel film. Then, it is peeled off from the steel belt and imidized at 200-600℃. The casting speed is 28m / min to obtain a PI film with a thickness of 90μm.

[0056] (5) Graphite sheet preparation: PI film is placed in carbonization furnace and graphitization furnace for high temperature treatment, and then rolled to obtain high thermal conductivity graphite sheet.

[0057] Example 3 A method for preparing a high thermal conductivity graphite sheet includes the following steps: (1) Resin preparation: 79.29 kg of ODA, 10.16 kg of BAPP, 8.02 kg of PDA, 91.77 kg of PMDA, 14.56 kg of BPDA and 10.40 kg of 4NADA were added in batches to 760 kg of NMP and polycondensation reaction was carried out at 35 °C. After completion, 2 kg of 4-ethynylaniline was added for end capping. After defoaming, a polyamic acid resin with a viscosity of 2350 P was obtained.

[0058] (2) Preparation of dispersion: First, add 21.42 kg of graphene with a particle size of 3 μm and 4.28 kg of dicalcium phosphate with a particle size of 1.2 μm to 77.2 kg of NMP, disperse and stir at high speed, and then set aside for use.

[0059] (3) Preparation of chemical casting accelerator: Dissolve 200 kg of propionic anhydride and 40 kg of pyridine in 160 kg of NMP, stir evenly and set aside for use.

[0060] (4) PI film preparation: The above three components are mixed online in a needle mixer for 30s, the mixing temperature is 0℃, the mixer speed is 2500rpm, and the casting temperature is 110℃ to obtain a polyamic acid gel film. Then, it is peeled off from the steel belt and imidized at 200-600℃. The casting speed is 23m / min to obtain a PI film with a thickness of 115μm.

[0061] (5) Graphite sheet preparation: PI film is placed in carbonization furnace and graphitization furnace for high temperature treatment, and then rolled to obtain high thermal conductivity graphite sheet.

[0062] Example 4 A method for preparing a high thermal conductivity graphite sheet includes the following steps: (1) Resin preparation: 98.90 kg of ODA, 12.01 kg of TPE-R, 12.60 kg of anthracene-2,6-diamine, 115.42 kg of PMDA, 9.12 kg of ODPA and 18.71 kg of 2,3,6,7-anthracene tetracarboxylic dianhydride were added in batches to 720 kg of NMP and polycondensation reaction was carried out at 25 °C. After completion, 3 kg of 4-PEPA was added for end capping. After defoaming, a polyamic acid resin with a viscosity of 2050 P was obtained.

[0063] (2) Preparation of dispersion: 13.33 kg of carbon nanotubes with a particle size of 0.6 μm were added to 77.2 kg of DMF and dispersed and stirred at high speed for later use.

[0064] (3) Preparation of chemical casting accelerator: Dissolve 260 kg of propionic anhydride and 50 kg of 2-methylpyridine in 90 kg of NMP, stir evenly and set aside for use.

[0065] (4) PI film preparation: The above three components are mixed online in a needle mixer for 20s, the mixing temperature is 5℃, the mixer speed is 3200rpm, and the casting temperature is 130℃ to obtain a polyamic acid gel film. Then, it is peeled off from the steel belt and imidized at 200-600℃. The casting speed is 16m / min to obtain a PI film with a thickness of 170μm.

[0066] (5) Graphite sheet preparation: PI film is placed in carbonization furnace and graphitization furnace for high temperature treatment, and then rolled to obtain high thermal conductivity graphite sheet.

[0067] Example 5 A method for preparing a high thermal conductivity graphite sheet includes the following steps: (1) Resin preparation: 85.78 kg of ODA, 11.96 kg of 1,5-naphthyldiamine, 87.94 kg of PMDA, 9.74 kg of BTDA and 20.27 kg of 1,4,5,8-naphthyltetracarboxylic dianhydride were added in batches to 780 kg of DMF and polycondensation reaction was carried out at 28 °C. After completion, 2.5 kg of EPA was added for end capping. After defoaming, a polyamic acid resin with a viscosity of 1850 P was obtained.

[0068] (2) Preparation of dispersion: First, add 4.31 kg of boron nitride with a particle size of 2.5 μm to 11.2 kg of DMF, disperse and stir at high speed, and then set aside for use.

[0069] (3) Preparation of chemical casting accelerator: Dissolve 300 kg of acetic anhydride and 60 kg of isoquinoline in 40 kg of NMP, stir evenly and set aside for use.

[0070] (4) PI film preparation: The above three components are mixed online in a needle mixer for 10s, the mixing temperature is 6℃, the mixer speed is 3800rpm, and the casting temperature is 150℃ to obtain a polyamic acid gel film. Then, it is peeled off from the steel belt and imidized at 200-600℃. The casting speed is 12m / min to obtain a PI film with a thickness of 190μm.

[0071] (5) Graphite sheet preparation: PI film is placed in carbonization furnace and graphitization furnace for high temperature treatment, and then rolled to obtain high thermal conductivity graphite sheet.

[0072] The schematic diagram of the needle mixer and continuous online doping process used in the embodiment is shown below. Figure 1 As shown.

[0073] Comparative Example 1 A method for preparing graphite sheets includes the following steps: (1) Resin preparation: 69.74 kg of ODA, 4.19 kg of PDA and 86.1 kg of PMDA were added in batches to 840 kg of DMAc and polycondensation reaction was carried out at 45°C. After completion, 1 kg of 4-PEPA was added for end capping. After defoaming, a polyamic acid resin with a viscosity of 2100 P was obtained.

[0074] The rest is the same as in Example 1.

[0075] Comparative Example 2 A method for preparing graphite sheets includes the following steps: (1) Resin preparation: 64.33 kg of ODA, 3.86 kg of PDA, 58.40 kg of PMDA and 33.51 kg of 1,4,5,8-naphthalenetetracarboxylic dianhydride were added in batches to 840 kg of DMAc, and polycondensation reaction was carried out at 45 °C. After completion, 1 kg of [unspecified substance] was added. 4-PEPA end capping and defoaming yields a polyamic acid resin with a viscosity of 2100P.

[0076] The rest is the same as in Example 1.

[0077] Comparative Example 3 A method for preparing graphite sheets includes the following steps: (1) Resin preparation: 70.65 kg of ODA, 4.24 kg of PDA, 72.67 kg of PMDA and 12.61 kg of 1,4,5,8-naphthalenetetracarboxylic dianhydride were added in batches to 840 kg of DMAc and polycondensation reaction was carried out at 45 °C. After defoaming, a polyamic acid resin with a viscosity of 2100 P was obtained.

[0078] The rest is the same as in Example 1.

[0079] Comparative Example 4 A method for preparing graphite sheets, wherein no dispersion liquid is used in the preparation process, and the rest is the same as in Example 1.

[0080] Comparative Example 5 A method for preparing graphite sheets, comprising the following steps: (1) Preparation of hybrid resin: First, 0.32 kg of calcium carbonate with a particle size of 1 μm was dissolved in 1.28 kg of DMAc and dispersed and stirred at high speed to obtain a dispersion. Then, the above dispersion, 70.65 kg of ODA, 4.24 kg of PDA, 72.67 kg of PMDA and 12.61 kg of 1,4,5,8-naphthalenetetracarboxylic dianhydride were added in batches to 840 kg of DMAc. Polycondensation reaction was carried out at 45 °C. After completion, 1 kg of 4-PEPA was added for end capping. After defoaming, a polyamic acid resin with a viscosity of 2100 P was obtained.

[0081] The preparation of the chemical casting accelerator, PI film, and graphite sheet is the same as in Example 1.

[0082] Comparative Example 6 A method for preparing graphite sheets, wherein the preparation process involves online mixing in a needle mixer for 75 seconds, and the rest is the same as in Example 1.

[0083] Comparative Example 7 A method for preparing graphite sheets, wherein step (4) involves online mixing in a needle mixer at a temperature of 15°C, and the rest is the same as in Example 1.

[0084] Performance testing This invention characterizes the mechanical properties of PI films by tensile strength and elongation at break, the planar orientation of PI films by birefringence, and the thermal conductivity of graphite by carbon density and thermal conductivity. The properties of the polyimide films and graphite sheets prepared in Examples 1-5 and Comparative Examples 1-7 are shown in Table 1.

[0085] Table 1: Performance Comparison of PI Films and Graphite Sheets Prepared in Examples and Comparative Examples

[0086] *Note: The " / " indicates that the PI film has low mechanical properties and is too brittle to be produced continuously.

[0087] The results in Table 1 show that, in Comparative Example 1, the PI film lacking large aromatic ring structure inorganic particles failed to achieve high thermal conductivity due to low carbon density or poor orientation induction. In Comparative Example 2, the excessive content of large aromatic ring structure resulted in excessively rigid molecular chains, significantly reducing the mechanical properties of the PI film and preventing the formation of graphite sheets with good appearance. In Comparative Example 3, the PI film without a cross-linking end-capping agent showed a decreased degree of orientation, and its thermal conductivity could not reach 2000 W / m·K. In Comparative Example 4, the absence of specific fillers hindered the formation of a continuous thermally conductive network during graphitization, affecting the thermal conductivity of the graphite. In Comparative Example 5, the hybrid resin prepared through in-situ polymerization resulted in secondary agglomeration and sedimentation of inorganic particles within the resin system due to their long residence time, leading to uneven distribution within the film and affecting the performance of both the PI film and its graphite sheets. In Comparative Examples 6 and 7, excessively long online mixing times or excessively high temperatures of the imidization accelerator, resin, and inorganic particle dispersions led to the formation of a large amount of gel impurities within the system, causing a sharp decline in the mechanical properties of the PI film and making it impossible to obtain a PI film with a good appearance through smooth winding. In this invention, by introducing a certain amount of naphthalene rings, anthracene rings, and anthraquinone macrocyclic aromatic ring structures, along with a crosslinking end-capping agent, into the PI molecular structure, the planar orientation of the PI film can be improved, and the carbon density increased. Furthermore, this synergistic effect with the inorganic particles introduced through online hybridization technology induces a graphitization transformation during high-temperature sintering, resulting in graphite sheets with a thermal conductivity reaching 2000 W / m·K.

[0088] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. However, it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide film, characterized in that, Includes the following steps: S1. Prepare raw materials: polyamic acid resin solution, inorganic particle dispersion, and chemical casting accelerator; the polyamic acid resin solution is prepared by polycondensation reaction of diamine, dianhydride, macroaromatic ring monomer and crosslinking end-capping agent, wherein the macroaromatic ring monomer is an aromatic ring containing one or more of naphthalene ring, anthracene ring or anthraquinone. S2. Instantly mix the raw materials prepared in S1 online in a needle mixer until uniform; S3. After mixing, without degassing, the mixture is immediately cast and imidized to obtain a polyimide film.

2. The method for preparing the polyimide film according to claim 1, characterized in that, In S2, the raw materials are instantaneously mixed online in the needle mixer for 10~50s, the mixing temperature is -15~10℃, and the mixer speed is 1000~4000rpm.

3. The method for preparing the polyimide film according to claim 2, characterized in that, The top of the needle mixer is provided with a polyamic acid resin inlet, and at the same height of the needle mixer are a dispersion liquid inlet and a chemical casting accelerator inlet; the polyamic acid resin is fed first, and then the dispersion liquid and the chemical casting accelerator are fed simultaneously.

4. The method for preparing the polyimide film according to claim 1, characterized in that, In S1, the molar amount of the macroaromatic ring monomer accounts for 5 to 30% of the total molar amount of the diamine and dianhydride.

5. The method for preparing the polyimide film according to any one of claims 1-4, characterized in that, The macroaromatic ring monomer is selected from one or more of 1,4,5,8-naphthocarboxylic dianhydride, 2,3,6,7-naphthocarboxylic dianhydride, 2,3,6,7-anthracitelic dianhydride, anthracene-2,6-diamine, 1,5-naphthodiamine, 1,4-diaminonaphthalene, and 1,8-dihydroxy-2,4,5,7-tetraaminoanthraquinone.

6. The method for preparing the polyimide film according to any one of claims 1-4, characterized in that, The cross-linking end-capping agent is selected from one or more of 4-phenylacetylene phthalic anhydride, 4-acetylene phthalic anhydride, and 4-acetylene aniline; the amount of the cross-linking end-capping agent added is 0.05% to 0.5% of the mass of the polyamic acid resin.

7. The method for preparing the polyimide film according to claim 1, characterized in that, The inorganic particle dispersion has a solid content of 10-25%, and the inorganic particles are selected from one or more of silicon oxide, silicon carbide, aluminum nitride, silicon nitride, boron nitride, calcium phosphate, dicalcium phosphate, calcium pyrophosphate, calcium carbonate, calcium bicarbonate, alumina, carbon nanotubes, and graphene; the particle size of the inorganic particles is 50 nm to 5 μm; and the amount of inorganic particles added is 0.03% to 20% of the mass of the polyimide film.

8. The method for preparing the polyimide film according to claim 1, characterized in that, In S3, the casting temperature is 60–180℃, the imidization temperature is 200–600℃, and the casting speed is 10–35 m / min.

9. A high thermal conductivity graphite sheet, characterized in that, The polyimide film prepared by any one of claims 1-8 is obtained by carbonization, graphitization and calendering, wherein the thermal conductivity of the graphite sheet is ≥2000W / m·K.

10. The application of a polyimide film prepared by any one of claims 1-8 or the graphite sheet of claim 9 in a heat dissipation material for electronic devices.

11. An application of a polyimide film prepared by any one of claims 1-8 in the fields of electronic information, flexible display, aerospace, semiconductor chips, or defense.

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