CN110423467B

CN110423467B - Ultra-thick polyimide film, preparation method thereof and graphite sheet

Info

Publication number: CN110423467B
Application number: CN201910833194.6A
Authority: CN
Inventors: 张步峰; 廖波; 杨军; 钱心远; 凌建强
Original assignee: Zhuzhou Times Huaxin New Material Technology Co ltd
Current assignee: Zhuzhou Times Huaxin New Material Technology Co.,Ltd.
Priority date: 2019-09-04
Filing date: 2019-09-04
Publication date: 2021-11-16
Anticipated expiration: 2039-09-04
Also published as: CN110423467A

Abstract

The invention discloses an ultra-thick polyimide film and a preparation method thereof, wherein the preparation method comprises the following steps: dissolving the surface modified carbon nano tube and the inorganic filler in an organic solvent and uniformly stirring; adding a diamine monomer, adding a dianhydride monomer in batches after the diamine monomer is dissolved, finally adding a crosslinking type end-capping reagent, and defoaming to obtain a polyamide acid resin; adding dehydrating agent and catalyst, mixing to obtain precursor resin, casting, high-temperature imidization, cross-linking, annealing and heat-setting. The ultra-thick polyimide film is carbonized, graphitized and calendered to obtain a graphite sheet with a single-layer thickness of 45-130 microns, and the technical problem that the single-layer thick graphite sheet cannot be obtained due to excessive expansion of the ultra-thick PI film in the high-temperature graphitization process in the current industry is solved.

Description

Ultra-thick polyimide film, preparation method thereof and graphite sheet

Technical Field

The invention belongs to the technical field of polyimide, and particularly relates to an ultra-thick polyimide film, a preparation method thereof and a graphite sheet.

Background

With the rapid development of microelectronic technology, especially the communication technology is upgraded from "4G" to "5G", the introduction of high frequency, the continuous improvement of hardware integration, the continuous miniaturization of chips, and the multiplication of the number of networking devices and antennas lead to the continuous increase of power consumption of devices, the heat productivity also increases dramatically, and the heat dissipation of components has become the bottleneck problem faced by the communication terminal devices in the "5G" era.

After the high-performance PI film is carbonized and graphitized at high temperature, the heat-conducting graphite flake with the heat conductivity being several times that of copper can be obtained, and the PI film is a core material for solving the heat dissipation problem of the current electronic products. However, the current PI film manufacturing technology for graphite sheets is limited to the production of products with a thickness of less than 90 μm, and only heat conducting graphite sheets within 40 μm can be manufactured, including specifications of 17 micrometers, 25 micrometers, 32 micrometers, 40 micrometers and the like, the single-layer heat dissipation flux is low, the thickness of the graphite layer must be increased by stacking multiple layers, the heat flux is increased, and the PI film manufacturing technology is temporarily applied to electronic products in the stage of "4G". Moreover, the existing process of the heat-conducting graphite flake is complex in process and high in cost, and the use of the glue layer can form a thermal resistance effect during the overlapping process, so that the thermal diffusion capacity is obviously reduced, and the dual requirements of the 5G technology on high heat storage capacity and rapid heat transfer capacity cannot be met. Therefore, the preparation of the ultra-thick PI film with the thickness of more than 90 μm and suitable for firing the heat-conducting graphite sheet is the best scheme for solving the heat dissipation problem in the '5G' era.

Firstly, when the polymerization process of the traditional PI film precursor resin is finished, amino and acid anhydride at two ends of a molecular chain still slowly react to cause difficulty in controlling a reaction end point, the molecular weight of the resin fluctuates during storage and transportation to influence quality stability, in order to reduce molecular weight change, the resin is generally preserved at an ultralow temperature of about-20 ℃ after the polymerization reaction is finished, but the ultralow temperature causes overlarge apparent viscosity of the resin, reduces the leveling property of the resin during coating, and causes uneven thickness and more defects of a casting film; secondly, the resin in the ultralow temperature state has strict requirements on the activity and the dosage of the catalyst, and is difficult to be used for preparing an ultra-thick PI film; finally, the expansion degree, particularly the Z-axis direction, of the PI film with the thickness of more than 90 microns is difficult to control in the high-temperature graphitization process, excessive foaming is easily generated, a high-density product cannot be obtained by calendaring, the heat conduction performance is obviously reduced, and meanwhile, the excessive expansion can cause the graphite flake to have a plurality of appearance defects such as layering, powder falling, cracking and the like, so that the surface quality and the process application of the final product are influenced.

Patent CN201610825198.6 discloses a method for preparing a water-based carbon black modified polyimide film by thermal imidization; patent CN201810206525.9 provides a device for preparing polyimide thick film or ultra-thick film by thermal imidization process; patent CN102093715A discloses a thermal imidization preparation method of carbon nanotube reinforced PI film and fiber, and the adopted carbon nanotube with the length of 10-10000 microns is too long, so that a large amount of winding can be generated, and the comprehensive performance of the film is influenced. The PI films prepared by the hot imidization method cannot obtain films with uniform structures and high in-plane orientation, and are not suitable for preparing heat-conducting graphite sheets, particularly thick graphite sheets.

Patents CN201510409013.9 and CN201610726723.9 use blending method to disperse inorganic particles such as oxide and inorganic acid salt in resin, and prepare polyimide film by chemical imidization, and finally obtain graphite flake by high temperature treatment, and the PI film produced by this method cannot control the normal over expansion in the graphitization process, so the film thickness is generally not more than 90 μm, and thick graphite flake with excellent performance cannot be obtained.

Disclosure of Invention

The invention aims to solve the technical problem of overcoming the defects and shortcomings in the background technology and provides an ultra-thick polyimide film, a preparation method thereof and a graphite sheet.

In order to solve the technical problems, the technical scheme provided by the invention is as follows:

an ultra-thick polyimide film with a thickness of 90-250 μm.

As a general inventive concept, the present invention also provides a method for preparing the ultra-thick polyimide film, including the steps of:

(1) dissolving the surface modified carbon nano tube and the inorganic filler in an organic solvent and uniformly stirring;

(2) adding a diamine monomer into the uniformly stirred solution obtained in the step (1), adding a dianhydride monomer in batches after the diamine monomer is dissolved, adding a capping agent, defoaming to obtain polyamic acid resin, and storing at 0-20 ℃;

(3) and (3) uniformly mixing the polyamic acid resin obtained in the step (2) with a dehydrating agent and a catalyst, and carrying out salivation, imidization and high-temperature heat setting to prepare the ultra-thick polyimide film.

The preparation method of the invention introduces the cross-linking type end capping agent in the process, reduces the active points at the molecular chain ends, has controllable reaction end point, ensures the stable resin viscosity, does not need to be stored at ultralow temperature, reduces the apparent viscosity during flowing, improves the leveling property of thick films, improves the apparent quality, reduces the requirement on catalytic activity, can reduce the catalyst dosage or adopt milder catalyst, and improves the selection surface of a catalytic system. In addition, the end capping agent can generate a crosslinking reaction at high temperature, can lock the molecular chain tail end of polyimide, reduce the motion capability of the molecular chain, improve the regularity of the molecular structure, and is more suitable for preparing the heat-conducting graphite sheet.

In the above production method, in the step (1), the surface-modified carbon nanotube preferably refers to a carbon nanotube having a surface treated by any one of carboxylation, amination and fluorination. The carbon nano tube can form a good interface effect with the PI substrate due to chemical bonding or hydrogen bond action, a moderate cross-linking effect is generated, the acting force between molecular chains is effectively increased, the expansion of a graphite film is effectively inhibited in the high-temperature graphitization process, particularly the excessive foaming in the Z-axis direction is effectively inhibited, the carbon density of the calendered graphite flake is improved, and the heat conductivity of the graphite flake is improved. The carbon nano tube after carboxylation, amination and fluoration treatment has better compatibility with a resin system and stronger binding force, and can better inhibit over expansion.

In the preparation method, preferably, in the step (1), the diameter of the surface-modified Carbon Nanotubes (CNTs) is 10-200 nm, and the length is 0.5-50 μm; the addition amount of the surface modified carbon nano tube is 0.1-1% of the mass of the polyimide film.

In the above preparation method, preferably, in the step (1), the inorganic filler is one or more selected from silicon oxide, silicon carbide, silicon nitride, boron nitride, calcium phosphate, calcium hydrogen phosphate, calcium pyrophosphate, calcium carbonate and calcium bicarbonate; the particle size of the inorganic filler is 50 nm-5 mu m, and the addition amount of the inorganic filler is 0.1-1% of the mass of the polyimide film.

In the above preparation method, preferably, in the step (1), the organic solvent is any one of Dimethylformamide (DMF), dimethylacetamide (DMAc) and N-methylpyrrolidone (NMP);

in the step (2), the diamine monomer is one or more of 4,4 '-diaminodiphenyl ether (4, 4' -ODA), 3,4 '-diaminodiphenyl ether (3, 4' -ODA), p-phenylenediamine (1,4-PDA) and 2, 2-bis (4-aminophenoxy) Benzene (BAPP); the dianhydride monomer is one or more of pyromellitic dianhydride (PMDA), 3,3 ', 4, 4' -biphenyl tetracarboxylic dianhydride (s-BPDA), 2,3,3 ', 4-biphenyl tetracarboxylic dianhydride (alpha-BPDA), 3, 3', 4, 4-Benzophenone Tetracarboxylic Dianhydride (BTDA) and 4,4 '-diphenyl ether dianhydride (4, 4' -ODPA); the molar total amount ratio of the diamine to the dianhydride is 1: (0.85-1.15).

In the preparation method, preferably, in the step (2), the crosslinking type end-capping reagent is one or more of 4-phenylacetylene phthalic anhydride (4-PEPA), 4-Ethynyl Phthalic Anhydride (EPA) and 4-ethynyl aniline, and the addition amount of the crosslinking type end-capping reagent is 0.1-1% of the mass of the polyamic acid resin.

In the step (3), the dehydrating agent is at least one of acetyl chloride, acetic anhydride, propionic anhydride and benzoic anhydride, and the adding amount of the dehydrating agent is 10-30% of the mass of the polyamic acid resin; further preferably 20-30%;

the catalyst is selected from at least one of pyridine and derivatives thereof, imidazole, quinoline and isoquinoline, and the adding amount of the catalyst is 0.05-1% of the mass of the polyamic acid resin.

In the above preparation method, preferably, in the step (2), the solid content of the prepared polyamic acid resin is 15% to 35%.

In the preparation method, preferably, in the step (3), the casting temperature is 100-200 ℃, and the imidization temperature is 200-450 ℃; the high-temperature heat setting refers to annealing heat setting at 250-350 ℃. Heating at 100-200 ℃ to rapidly volatilize the solvent in casting to obtain a polyamide acid gel film, then passing through an imine furnace at 200-450 ℃ to realize imidization, simultaneously, chemically crosslinking is realized by the end capping agent at 400-450 ℃ to further improve the performance of the film, and then annealing is carried out at 250-350 ℃ to stretch molecular chains and eliminate the phenomenon of stress concentration, thereby obtaining the polyimide film which is uniformly oriented and is suitable for firing high-heat-conductivity graphite sheets.

As a general inventive concept, the present invention also provides a graphite sheet, which is obtained by carbonizing, graphitizing and calendaring the ultra-thick polyimide film or the ultra-thick polyimide film prepared by the preparation method.

The graphite sheet is preferably 45-130 μm in single-layer thickness.

Compared with the prior art, the invention has the advantages that:

(1) according to the invention, the cross-linking type end capping agent is introduced into the polyamide acid resin raw material, the cross-linking type end capping agent can reduce the active site at the molecular chain end, so that the reaction end point is controllable, the resin viscosity is stable, compared with the traditional chemical imidization film-making process, ultralow-temperature storage is not required, and the resin storage temperature is increased; meanwhile, the defects of high surface viscosity and difficult leveling of the ultralow-temperature resin are avoided, the leveling property of the resin during thick film preparation is improved, and the surface quality is improved; on the other hand, the chemical imine method adopted in the invention also reduces the requirement of the resin on the catalytic activity, can reduce the dosage of the catalyst or adopt a milder catalyst, and improves the selection surface of a catalytic system; in addition, the cross-linking type end capping agent can generate cross-linking reaction at high temperature, can lock the molecular chain tail end of polyimide, reduce the motion capability of the molecular chain, improve the regularity of the molecular structure and is more suitable for preparing the heat-conducting graphite sheet.

(2) According to the invention, the surface modified Carbon Nanotubes (CNTs) with the diameter of 10-200 nm and the length of 0.5-50 μm are used as one of the modified fillers of the PI film, and form a good interface effect with the PI matrix due to chemical bonding or hydrogen bond effect, so that a proper crosslinking effect is generated, the acting force between molecular chains is effectively increased, the expansion of the graphite film, especially the excessive foaming in the Z-axis direction, is effectively inhibited in the high-temperature graphitization process, the carbon density of the rolled graphite sheet is increased, and the heat conductivity of the graphite sheet is improved.

(3) The thickness of a single layer of the graphite flake prepared by the invention is 45-130 mu m, when the total thickness of the graphite flake is the same, the heat flux of the single-layer thick graphite flake is higher than that of the multilayer stacked graphite flake, because the heat resistance of the latter is increased due to the influence of double-sided adhesive used during stacking, for example, the heat conductivity coefficient and the heat flux of the single-layer thick graphite flake are better than those of the double-layer 25 mu m stacked graphite flake, and the heat conductivity of the single-layer 65 mu m thick graphite flake is also higher than that of the double-layer 32 mu m stacked graphite flake, so the invention solves the technical problem that the single-layer thick graphite flake cannot be obtained due to excessive expansion of the ultra-thick PI film in the high-temperature graphitization process in the prior industry.

(4) The invention provides a 90-250 mu m ultra-thick PI film and a chemical imidization preparation method, which can be used for manufacturing thick heat-conducting graphite sheets and are more suitable for being applied to communication terminals with high heat dissipation requirements in the 5G era.

Detailed Description

In order to facilitate an understanding of the present invention, the present invention will be described more fully and in detail with reference to the preferred embodiments, but the scope of the present invention is not limited to the specific embodiments below.

Unless otherwise defined, all terms of art used hereinafter have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention.

Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.

Comparative example 1:

the method for producing a graphite sheet of this comparative example, comprising:

(1) preparing resin: 0.39kg of Ca with a particle size of 1 μm₃(PO₄)₂Dissolving in 280.14kg DMAc, adding 30.3kg 4, 4' -ODA and 5.2kg 1,4-PDA after stirring uniformly, adding 43.51kg PMDA in batches after the diamine is dissolved, the resin viscosity reaches 1560 poise, defoaming and storing at-20 ℃ for later use;

(2) preparing a PI film: uniformly mixing the polyamic acid resin prepared in the step (1) with 71.83kg of acetic anhydride and 25.77kg of pyridine, carrying out gradient heating and casting at 100-190 ℃ to form a film, and carrying out high-temperature imidization at 210-400 ℃ to prepare a polyimide film with the thickness of 90 microns;

(3) preparing graphite flakes: and sequentially putting the polyimide film with the thickness of 90 mu m into a carbonization furnace and a graphitization furnace for high-temperature treatment, and then calendering to obtain the high-heat-conductivity graphite sheet.

Comparative example 2:

(1) preparing resin: 0.4kg of CNTs with the diameter of 30nm and the length of 1 mu m, the surface of which is carboxylated and modified, and 0.6kg of CaCO with the grain diameter of 0.6 mu m₃Dissolving in 453.55kg DMF, stirring, adding 35.5kg 3,4 '-ODA and 10.0kg 4, 4' -ODA, adding 54.06kg PMDA in batches after the diamine is dissolved, the resin viscosity reaches 1780 poise, defoaming and storing at-18 ℃ for later use;

(2) preparing a PI film: uniformly mixing the polyamic acid resin prepared in the step (1) with 138.27kg of propionic anhydride and 50.83kg of quinoline, carrying out gradient heating and salivation at 100-180 ℃ to form a film, carrying out high-temperature imidization at 220-380 ℃, and finally carrying out annealing and heat setting at 270 ℃ to prepare a polyimide film with the thickness of 90 mu m;

(3) preparing graphite flakes: and sequentially putting the polyimide film into a carbonization furnace and a graphitization furnace for high-temperature treatment, and then calendering to obtain the high-heat-conductivity graphite sheet.

Comparative example 3:

(1) preparing resin: firstly, 0.19kg of SiC with the grain diameter of 1.4 mu m is dissolved in 510.3kg of DMF, 45kg of 4, 4' -ODA is added after the stirring is uniform, 40.06kg of PMDA and 12.14kg of s-BPDA are added in batches after the diamine is dissolved, finally 3.04kg of aniline is added for sealing, the resin viscosity reaches 1600 poise, and the mixture is defoamed and stored at 3 ℃ for standby application.

(2) Preparing a PI film: the resin is uniformly mixed with 72.9kg of benzoic anhydride and 1.82kg of triethylamine, then the mixture is subjected to gradient temperature rise and salivation at the temperature of 130-160 ℃ to form a film, the film is imidized at the high temperature of 200-410 ℃, and finally, the film is annealed and heat-set at the temperature of 290 ℃ to prepare a polyimide film with the thickness of 90 mu m.

(3) Preparing graphite flakes: and sequentially putting the PI film into a carbonization furnace and a graphitization furnace for high-temperature treatment, and then calendering to obtain the high-heat-conductivity graphite flake.

Comparative example 4:

and (3) superposing two layers of commercially available heat-conducting graphite sheets with the thickness of 25 mu m, and adhering the two layers of commercially available heat-conducting graphite sheets with the thickness of 5 mu m by using a double-sided adhesive tape to obtain the composite heat-conducting graphite sheet.

Comparative example 5:

and (3) superposing two layers of commercially available heat-conducting graphite sheets with the thickness of 32 mu m, and adhering the two layers of commercially available heat-conducting graphite sheets with the thickness of 5 mu m by using a double-sided adhesive tape in the middle to obtain the composite heat-conducting graphite sheet.

Comparative example 6:

a commercially available PI film (prepared by a traditional chemical imine method) with the thickness of 125 μm is used, and the PI film and the graphite film are sequentially put into a carbonization furnace and a graphitization furnace for high-temperature treatment and then are rolled to obtain the heat-conducting graphite sheet.

Comparative example 7:

a commercially available PI film (prepared by a thermal imine method) with the thickness of 125 μm is sequentially put into a carbonization furnace and a graphitization furnace for high-temperature treatment, and then is rolled to obtain the heat-conducting graphite sheet.

Example 1:

a method of making the graphite sheet of the present invention comprises:

(1) preparing resin: 0.88kg of carboxylated modified CNTs with the diameter of 100nm and the length of 0.6 mu m and 0.19kg of Ca with the particle size of 0.2 mu m₂P₂O₇Dissolving in 486.05kg of DMF, stirring uniformly, adding 21.6kg of 1,4-PDA and 15.6kg of BAPP, adding 43.6kg of PMDA and 11.78kg of 4,4-ODPA in batches after the diamine is dissolved, finally adding 5.2kg of 4-PEPA for end capping, defoaming when the resin viscosity reaches 1920 poise, and storing at 4 ℃ for later use (the solid content is about 17%);

(2) preparing a PI film: uniformly mixing the resin prepared in the step (1) with 163.44kg of propionic anhydride and 1.75kg of pyridine, carrying out gradient temperature rise and casting at 120-190 ℃ to form a film, carrying out high-temperature imidization at 225-440 ℃, and finally carrying out annealing and heat setting treatment at 310 ℃ to prepare a polyimide film with the thickness of 90 mu m;

(3) preparing graphite flakes: and sequentially putting the polyimide film into a carbonization furnace and a graphitization furnace for high-temperature treatment, and then calendering to obtain the high-heat-conductivity graphite flake.

Example 2:

a method of making the graphite sheet of the present invention comprises:

(1) preparing resin: 0.25kg of CNTs which are aminated and have the diameter of 15nm and the length of 15 mu m and 0.52kg of SiO with the grain diameter of 4 mu m₂Dissolving in 313.2kg of NMP, stirring uniformly, adding 43.2kg of 4,4 '-ODA, adding 54.4kg of PMDA in batches after the diamine is dissolved completely, then adding 6.8kg of 3, 4' -ODA for continuous reaction, finally adding 2.92kg of EPA for end capping, defoaming when the resin viscosity reaches 1820 poise, and storing at 12 ℃ for later use;

(2) preparing a PI film: uniformly mixing the resin prepared in the step (1) with 116.52kg of acetic anhydride and 4kg of pyridine, carrying out gradient heating and casting at 105-185 ℃ to form a film, carrying out high-temperature imidization at 230-435 ℃, and finally carrying out annealing and heat setting at 340 ℃ to prepare a polyimide film with the thickness of 125 microns;

Example 3:

a method of making the graphite sheet of the present invention comprises:

(1) preparing resin: 0.23kg of carboxylation modified CNTs with the diameter of 150nm and the length of 2 mu m and 0.94kg of CaHPO with the grain diameter of 4.2 mu m₄Dissolving in 369.3kg of DMAc, stirring uniformly, adding 36.2kg of 4, 4' -ODA and 5.8kg of PDA, adding 57.11kg of PMDA in batches after diamine is dissolved completely, then adding 11.2kg of BAPP to adjust viscosity, finally adding 0.96kg of 4PEPA to terminate the end, defoaming when the resin viscosity reaches 1950 poise, and storing at 2 ℃ for later use;

(2) preparing a PI film: uniformly mixing the resin prepared in the step (1) with 57.55kg of benzoic anhydride and 2.55kg of imidazole, carrying out gradient temperature rise and casting at 110-175 ℃ to form a film, carrying out high-temperature imidization at 235-415 ℃, and finally carrying out annealing and heat setting at 300 ℃ to prepare a polyimide film with the thickness of 150 microns;

(3) preparing graphite flakes: and sequentially putting the PI film into a carbonization furnace and a graphitization furnace for high-temperature treatment, and then calendering to obtain the high-heat-conductivity graphite sheet.

Example 4:

a method of making the graphite sheet of the present invention comprises:

(1) preparing resin: 0.98kg of fluorinated modified CNTs with diameter of 70nm and length of 35 μm and 0.91kg of CaCO with particle size of 1.5 μm₃Dissolving in 477.94kg of DMAc, stirring uniformly, adding 51.5kg of 4, 4' -ODA, adding 42.16kg of PMDA and 18.45kg of s-BPDA in batches after diamine is dissolved completely, adding 3.54kg of 4-ethynylaniline for end capping, defoaming and storing at 10 ℃ for later use when the resin viscosity reaches 1690 poise;

(2) preparing a PI film: uniformly mixing the resin prepared in the step (1) with 88.5kg of acetic anhydride and 1.77kg of quinoline, carrying out gradient heating and salivation at 125-190 ℃ to form a film, carrying out high-temperature imidization at 240-420 ℃, and finally carrying out annealing and heat setting at 265 ℃ to prepare a polyimide film with the thickness of 175 mu m;

Example 5:

a method of making the graphite sheet of the present invention comprises:

(1) preparing resin: firstly, 0.18kg of aminated CNTs with the diameter of 30nm and the length of 1 mu m and 0.19kg of boron nitride with the particle size of 0.8 mu m are dissolved in 372.11kg of DMAc, 46.5kg of 4, 4' -ODA is added after uniform stirring, 42kg of PMDA and 24.69kg of ODPA are added in batches after diamine is dissolved, 4.3kg of 1,4-PDA is added to adjust the viscosity, 2.45kg of EPA is added to terminate the end, when the resin viscosity reaches 1790 poise, defoaming is carried out, and the resin is stored at 5 ℃ for standby;

(2) preparing a PI film: uniformly mixing the resin prepared in the step (1) with 132.68kg of acetic anhydride and 2.84kg of quinoline, carrying out gradient temperature rise and casting at 130-185 ℃ to form a film, carrying out high-temperature imidization at 205-405 ℃, and finally carrying out annealing and heat setting at 305 ℃ to prepare a polyimide film with the thickness of 200 mu m;

Example 6:

a method of making the graphite sheet of the present invention comprises:

(1) preparing resin: 0.77kg of aminated CNTs with a diameter of 175nm and a length of 45 μm and 0.25kg of Ca (HCO) with a particle size of 2 μm₃)₂Dissolving in 404.6kg DMF, stirring, adding 48.6kg 4, 4' -ODA, adding 51.8kg PMDA in batches after the diamine is dissolved, finally adding 3.2kg 4-PEPA for capping, defoaming when the resin viscosity reaches 1850 poise, and storing at 15 ℃ for later use;

(2) preparing a PI film: uniformly mixing the resin with 75.67kg of propionic anhydride and 0.75kg of isoquinoline, carrying out gradient temperature rise and salivation at 115-165 ℃ to form a film, carrying out high-temperature imidization at 240-445 ℃, and finally carrying out annealing and heat setting at 325 ℃ to prepare a polyimide film with the thickness of 250 micrometers;

Properties of the polyamic acid resins prepared in examples 1 to 6 and comparative examples 1 to 3 are shown in Table 1.

TABLE 1 comparison of the Properties of the resins prepared in the examples and comparative examples

From the data in Table 1, it can be seen that the resin using the crosslinking type end capping agent has less viscosity fluctuation at the end of the reaction and at the time of storage, and the storage temperature of the resin is relatively high, so that the apparent viscosity of the resin at the time of coating is low and the leveling property is good.

The properties of the polyimide films and graphite sheets prepared in examples 1 to 6 and comparative examples 1 to 3 are shown in Table 2.

TABLE 2 comparison of PI film and graphite flake Properties prepared in examples and comparative examples

As can be analyzed from the results in Table 2, the resin without end capping had poor leveling property, resulting in poor film surface quality, as in comparative example 2; the PI film without annealing treatment has concentrated stress in the plane, and the thermal conductivity coefficient is low after the PI film is made into a graphite sheet, as in comparative example 1; the film without the surface modified carbon nano tube has obviously increased thickness of the graphite flake after carbonization and graphitization, poor surface quality and poor heat conductivity after calendering. The film added with the reactive end-capping reagent and the modified carbon nano tube has good surface quality, fully inhibits excessive foaming after graphitization, and has low thickness of the graphite flake and good heat-conducting property.

Note:

(1) the viscosity of the polyamic acid resin was measured using a rotational viscometer;

(2) the surface quality of the PI film is as follows:

PI films with a width of 1.5m and a length of 10m were evaluated visually according to the following criteria:

good (√): the thickness and the color are uniform, the surface is smooth and flat, and the defects of pinholes, bubbles and the like are avoided;

poor (x): uneven thickness and color, rough and uneven surface, pinhole, bubble and other defects.

(3) The surface quality of the rolled graphite sheet is as follows:

the thermally conductive graphite sheet was evaluated visually for a width of 0.25m and a length of 2m according to the following criteria:

good (√): uniform thickness and color, and no layering, powder falling, cracking and other defects.

Poor (x): the thickness and the color are not uniform, and the defects of layering, powder falling, cracking and the like exist.

Claims

1. A preparation method of an ultra-thick polyimide film is characterized in that the thickness of the ultra-thick polyimide film is 90-250 micrometers, and the preparation method comprises the following steps:

(2) adding a diamine monomer into the uniformly stirred solution obtained in the step (1), adding a dianhydride monomer in batches after the diamine monomer is dissolved, and finally adding a crosslinking type end capping agent for defoaming to obtain a polyamide acid resin; the cross-linking type end-capping reagent is one or more of 4-phenylacetylene phthalic anhydride (4-PEPA), 4-Ethynyl Phthalic Anhydride (EPA) and 4-ethynyl aniline, and the addition amount of the cross-linking type end-capping reagent is 0.1-1% of the mass of the polyamic acid resin;

(3) and (3) uniformly mixing the polyamic acid resin obtained in the step (2) with a dehydrating agent and a catalyst to obtain a precursor resin, and carrying out salivation, imidization and high-temperature heat setting to obtain the ultra-thick polyimide film, wherein the high-temperature heat setting refers to annealing heat setting at 250-350 ℃.

2. The method of claim 1, wherein in the step (1), the surface-modified carbon nanotubes are carbon nanotubes whose surfaces have been treated with any one of carboxylation, amination and fluorination.

3. The method according to claim 1, wherein in the step (1), the surface-modified carbon nanotubes have a diameter of 10 to 200nm and a length of 0.5 to 50 μm; the addition amount of the surface modified carbon nano tube is 0.1-1% of the mass of the polyimide film.

4. The preparation method according to claim 1, wherein in the step (1), the inorganic filler is one or more selected from the group consisting of silicon oxide, silicon carbide, silicon nitride, boron nitride, calcium phosphate, calcium hydrogen phosphate, calcium pyrophosphate, calcium carbonate and calcium bicarbonate; the particle size of the inorganic filler is 50 nm-5 mu m, and the addition amount of the inorganic filler is 0.1-1% of the mass of the polyimide film.

5. The method according to claim 1, wherein in the step (1), the organic solvent is any one of dimethylformamide, dimethylacetamide and N-methylpyrrolidone;

6. The method according to claim 1, wherein the reaction mixture,

in the step (3), the dehydrating agent is at least one of acetyl chloride, acetic anhydride, propionic anhydride and benzoic anhydride, and the adding amount of the dehydrating agent is 10-30% of the mass of the polyamic acid resin;

7. The method according to claim 1, wherein the polyamic acid resin prepared in the step (2) has a solid content of 15 to 35%.

8. A graphite sheet, characterized in that it is obtained by carbonizing, graphitizing and calendering the ultra-thick polyimide film prepared by any one of the preparation methods of claims 1 to 7.

9. The graphite sheet of claim 8, wherein the graphite sheet has a monolayer thickness of 45 to 130 μ ι η.