A polyimide-epoxy resin composite material for high frequency insulation and a preparation method and application thereof

Abstract

This invention discloses a polyimide-epoxy resin composite material for high-frequency insulation, its preparation method, and its application, belonging to the technical field of high-frequency power equipment insulation materials. The preparation method provided by this invention is as follows: a polyimide film is subjected to surface carboxylation treatment to obtain a surface carboxylated film; the surface carboxylated film is immersed in a silane coupling agent hydrolysate for surface modification to obtain a surface silanized film; the silane coupling agent hydrolysate includes an amino-containing silane coupling agent, a solvent, acetic acid, and a divalent tin salt; an epoxy resin composition is cast onto the surface of the surface silanized film and cured to obtain the final product. This invention utilizes a synergistic catalytic system composed of acetic acid and divalent tin salt to construct a chemically bridged interface on the polyimide surface through silane coupling agent treatment, significantly improving the insulation performance, interfacial bonding strength, and electrothermal aging resistance of the composite material at high frequencies.

Description

Technical Field

[0001] This invention relates to the field of high-frequency power equipment insulation materials, and in particular to a polyimide-epoxy resin composite material for high-frequency insulation, its preparation method, and its application. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] For power electronic equipment operating at high frequencies (typically 1kHz and above), especially high-frequency transformers operating above 10kHz, a composite insulation system consisting of integral epoxy resin (EP) casting and polyimide (PI) film wrapping is commonly used to achieve high power density and efficient power conversion. However, polyimide and epoxy resin have inherent differences in molecular structure, surface energy, and thermodynamic properties, making them typical thermodynamically incompatible systems. When directly composited, the interface relies mainly on physical forces such as van der Waals forces to form a weak boundary layer with low cohesive strength and unstable structure. Under the long-term coupling of multiple stresses such as high frequency, high voltage, and high temperature, this physically bonded interface is prone to microscopic defects and interface debonding due to the mismatch in material expansion coefficients, forming micron-sized air gaps. These interface defects lead to severe distortion of the electric field, becoming the starting point for space charge accumulation and partial discharge. The continuous erosion of partial discharge further accelerates the degradation of the interface material, forming a vicious cycle, ultimately causing electrical tree growth and early insulation breakdown, severely restricting the reliability and service life of the equipment.

[0004] To improve the interfacial properties of polyimide-epoxy resin, existing technologies mainly focus on two directions: one is to modify the single matrix material, such as incorporating nanofillers into polyimide or epoxy resin to enhance its bulk dielectric or heat resistance properties; the other is to perform physical or simple chemical treatments on the interface, such as plasma cleaning to increase surface energy or coating with a transition coating to improve compatibility. However, the former often fails to fundamentally solve the problem of missing interfacial chemical bonds; the latter generally suffers from limitations such as unstable treatment effects, complex processes, or the introduction of new interfacial defects. Furthermore, existing methods struggle to construct a high-strength chemical bond between polyimide and epoxy resin that can maintain long-term stability under high-frequency dynamic electrical stress.

[0005] Therefore, how to establish a stable and durable chemical bonding interface between the two phases of polyimide and epoxy resin to effectively suppress interfacial discharge and insulation aging under harsh high-frequency operating conditions has become a key technical bottleneck for improving the reliability of composite insulation in high-frequency power equipment. Summary of the Invention

[0006] In view of this, the present invention provides a polyimide-epoxy resin composite material for high-frequency insulation, its preparation method, and its application. The present invention utilizes a synergistic catalytic system composed of acetic acid and divalent tin salt to construct a chemically bridged interface on the polyimide surface through silane coupling agent treatment, significantly improving the composite material's insulation performance, interfacial bonding strength, and electrothermal aging resistance at high frequencies.

[0007] In a first aspect, the present invention provides a method for preparing a polyimide-epoxy resin composite material for high-frequency insulation, comprising the following steps:

[0008] S1. The polyimide film is subjected to surface carboxylation treatment to obtain a surface carboxylated film;

[0009] S2. The surface carboxylated film is immersed in a silane coupling agent hydrolysate for surface modification to obtain a surface silanized film; the silane coupling agent hydrolysate includes an amino-containing silane coupling agent, a solvent, acetic acid, and a divalent tin salt;

[0010] S3. Cast an epoxy resin composition onto the surface of the silanized film and cure it to obtain the polyimide-epoxy resin composite material for high-frequency insulation.

[0011] Preferably, the surface carboxylation treatment includes: first treating the polyimide film in an alkaline solution, and then treating it in an acidic solution.

[0012] Furthermore, the alkaline solution is a sodium hydroxide solution with a concentration of 1.0~5.0 mol / L, and the treatment temperature in the alkaline solution is 50~70℃ for 30~60 min; the acid solution is a hydrochloric acid, sulfuric acid, or acetic acid solution with a concentration of 0.5~1.5 mol / L, and the treatment time in the acid solution is 20~40 min.

[0013] Preferably, in step S2, the immersion time in the silane coupling agent hydrolysate is 4 to 10 minutes.

[0014] Preferably, the preparation method of the silane coupling agent hydrolysate is as follows: divalent tin salt and amino-containing silane coupling agent are mixed evenly in a solvent, acetic acid is added to adjust the pH to 3-5, and hydrolysis is carried out for 15-60 minutes to obtain the solution.

[0015] Preferably, the amino-containing silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane (KH-550), γ-aminopropyltrimethoxysilane, and N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane (KH-792); the solvent is a mixture of ethanol and water.

[0016] Preferably, in the silane coupling agent hydrolysate, the concentration of the amino-containing silane coupling agent is 1-5 wt%; and the concentration of the divalent tin salt is 0.05-0.5 wt%.

[0017] Preferably, the epoxy resin composition comprises epoxy resin, curing agent and accelerator, wherein the mass ratio of epoxy resin, curing agent and accelerator is 100 : (80~90) : (0.8~1.2).

[0018] Secondly, the present invention provides a polyimide-epoxy resin composite material for high-frequency insulation, which is prepared by the above-described preparation method.

[0019] Thirdly, the present invention provides the application of the above-mentioned polyimide-epoxy resin composite material for high-frequency insulation, for the preparation of insulating components for high-frequency power electronic devices.

[0020] Preferably, the operating frequency of the high-frequency power electronic device is 10kHz or higher.

[0021] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0022] (1) This invention treats surface-carboxylated polyimide (PI) films using a silane coupling agent hydrolysate containing acetic acid and divalent tin salts. The weakly acidic environment provided by acetic acid significantly optimizes the hydrolysis and condensation process of the silane coupling agent on the film surface. Simultaneously, the synergistic catalytic system of acetic acid and divalent tin salts effectively promotes the formation of denser and more stable Si-OC covalent bonds between the silane coupling agent and the carboxyl groups on the polyimide surface, creating an ideal chemical bridging interface for subsequent strong bonding with epoxy resin (EP).

[0023] (2) The PI-EP composite material prepared by this invention fundamentally improves the interfacial bonding strength and density between the polyimide and epoxy resin phases. This directly leads to a significant improvement in the insulation performance of the composite material under a high-frequency alternating electric field, specifically manifested as an increase in the partial discharge initiation voltage, a significant reduction in the discharge amplitude and number of discharges, and a significant enhancement in the interfacial breakdown field strength, thereby effectively suppressing early insulation failure caused by interfacial defects.

[0024] (3) Due to the excellent structural stability of the chemically bonded interface, the durability of this composite material under long-term high-frequency electrical stress and thermal stress coupling is significantly improved. Its resistance to electrothermal aging is significantly better than that of the physically bonded interface, which is manifested in a significant reduction in the decay rate of insulation performance under high temperature and high field strength conditions, and the service life is effectively extended, thus meeting the stringent requirements of high-frequency power electronic equipment for the long-term reliability of insulation materials. Attached Figure Description

[0025] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0026] Figure 1 This is a schematic diagram illustrating the preparation principle of the PI-EP composite material in Example 1 of the present invention;

[0027] Figure 2 This is an X-ray photoelectron spectroscopy (XPS) analysis of the PI thin film, PI-COOH thin film and PI-COOH-Si thin film of Example 1 of the present invention; wherein, A is the XPS full spectrum, B is the XPS C1s spectrum, C is the XPS N1s spectrum and D is the XPS Si2p spectrum.

[0028] Figure 3 XPS analysis of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2 of this invention; wherein, A is the full XPS spectrum of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2, and B is the XPS Si2p spectrum of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2.

[0029] Figure 4 These are cross-sectional scanning electron microscope (SEM) images of the PI-EP composite materials prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention; wherein, a is the PI-EP interface of Comparative Example 1, b is the PI interface and air gap of Comparative Example 1, c is the EP interface and air gap of Comparative Example 1, d and e are the PI-EP interfaces of Example 1, f is the PI-EP interface of Comparative Example 2, and g is the PI-EP interface of Example 1.

[0030] Figure 5 This is an elemental distribution imaging (mapping) analysis of the PI-EP composite materials prepared in Example 1 and Comparative Example 2 of the present invention; wherein, A is the elemental distribution imaging analysis of the PI-EP interface in Comparative Example 2, and B is the elemental distribution imaging analysis of the PI-EP interface in Example 1.

[0031] Figure 6 These are phase-resolved partial discharge (PRPD) spectra of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2 of this invention; wherein, A-F are the PRPD spectra of Comparative Example 2, Example 2, Example 3, Example 1, Example 4, and Example 5, respectively.

[0032] Figure 7These are partial discharge data statistics of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2 of the present invention; wherein, A is a statistical chart of total discharge amplitude and average discharge amplitude data, and B is a statistical chart of total discharge number and average discharge number data.

[0033] Figure 8 These are partial discharge data statistics charts for Embodiment 1 and Comparative Examples 3-5 of the present invention; wherein, A is a statistical chart of total discharge amplitude and average discharge amplitude data, and B is a statistical chart of total discharge number and average discharge number data.

[0034] Figure 9 These are the interfacial breakdown voltages of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2 of this invention at 20°C, 60°C, and 100°C.

[0035] Figure 10 This refers to the interfacial breakdown voltages of the PI-EP composite materials prepared in Examples 1 and Comparative Examples 3-5 of this invention at 20°C, 60°C, and 100°C.

[0036] Figure 11 These are the test results of the heat aging resistance of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2 of this invention;

[0037] Figure 12 These are the results of the combined electro-thermal aging test of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2 of this invention. Detailed Implementation

[0038] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0039] This invention provides a method for preparing a polyimide-epoxy resin composite material for high-frequency insulation, comprising the following steps:

[0040] Step S1: Perform surface carboxylation treatment on the polyimide (PI) film to obtain a surface carboxylated film.

[0041] The purpose of this step is to introduce highly reactive polar functional groups such as carboxyl groups (-COOH) onto the surface of the inert PI film, creating anchoring sites for subsequent chemical bonding with silane coupling agents.

[0042] In this invention, the surface carboxylation treatment is a stepwise chemical process: first, the polyimide film is treated in an alkaline solution, and then in an acidic solution. Under a strongly alkaline hydrothermal environment, the imide rings in the polyimide molecular chain undergo a hydrolytic ring-opening reaction, generating soluble polyamic acid salts. This process destroys the chemical inertness of the PI surface. Subsequently, under acidic conditions, these polyamic acid salts rapidly undergo protonation, transforming into a polyamic acid structure containing a large number of carboxyl groups and a small number of amino groups, thus completing the surface "carboxylation" activation. Through this treatment, the PI surface can change from hydrophobic to hydrophilic, significantly increasing surface energy and greatly improving the wettability of subsequent water-based treatment solutions.

[0043] In this invention, the alkaline solution is preferably an aqueous solution of sodium hydroxide (NaOH) with a concentration of 1.0~5.0 mol / L, more preferably 2.0~3.0 mol / L. Too low a concentration may lead to incomplete ring-opening reaction and insufficient carboxyl group density; too high a concentration may cause excessive etching of the PI matrix, damaging its mechanical and electrical properties. The treatment temperature is 50~70℃, more preferably 55~65℃, and the time is 30~60 min, more preferably 40~50 min. These mild hydrothermal conditions ensure uniform and complete reaction.

[0044] In this invention, the acid solution is a hydrochloric acid, sulfuric acid, or acetic acid solution with a concentration of 0.5~1.5 mol / L. More preferably, the acid solution is an aqueous solution of hydrochloric acid (HCl) with a concentration of 0.8~1.2 mol / L. Hydrochloric acid can provide sufficient H+. + It achieves efficient protonation, is highly volatile, and leaves minimal residue. The treatment time in acid solution is 20–40 min, more preferably 25–35 min, to ensure complete protonation.

[0045] In this invention, the film treated with acid and alkali is preferably rinsed repeatedly with plenty of water until the washing solution is neutral to completely remove residual ions. Subsequently, the film should be placed in a forced-air drying oven at 70~90°C for 10~20 minutes to remove surface moisture and prevent it from affecting subsequent steps.

[0046] Step S2: Immerse the surface carboxylated film in a silane coupling agent hydrolysate for surface modification to obtain a surface silanized film; the silane coupling agent hydrolysate includes an amino-containing silane coupling agent, a solvent, acetic acid, and a divalent tin salt.

[0047] This step involves preparing a specific silane coupling agent hydrolysate to construct a layer of oriented silane molecules on the activated PI surface, with one end firmly anchored to the PI by covalent bonds and the other end reserved with active amino groups.

[0048] Amino-containing silane coupling agents first hydrolyze in the solvent to generate highly active silanols (-SiOH). Acetic acid, acting as a weak acid buffer and reaction environment regulator, stabilizes the system pH at a weakly acidic environment of 3-5. This environment effectively inhibits excessive self-condensation between silanol molecules (forming a Si-O-Si network), forcing the reaction towards condensation with carboxyl groups on the PI surface. This ensures that the silanols in solution exist as highly active monomers or oligomers, which is beneficial for forming a dense, ordered monolayer on the PI surface. Divalent tin salts act as Lewis acid catalysts, with their Sn... 2+ Ions can specifically coordinate with and activate carboxyl groups or residual carbonyl groups on the PI surface, greatly accelerating their condensation and dehydration reaction with silanols and promoting the formation of strong Si-OC covalent bonds. Acetic acid and divalent tin salts together ensure the construction of a high-quality interface layer.

[0049] In this invention, the immersion time in the silane coupling agent hydrolysate is 4-10 min, more preferably 5-7 min, and most preferably 6 min. If the time is too short, the reaction will be insufficient, resulting in low coverage; if the time is too long, the anchored silane molecules may undergo excessive lateral cross-linking, or unreacted silane in the solution may physically adsorb onto the surface, forming multiple layers. All of these factors reduce the uniformity of the interface layer and impair high-frequency insulation performance.

[0050] In this invention, the preparation method of the silane coupling agent hydrolysate is as follows: a divalent tin salt and an amino-containing silane coupling agent are mixed evenly in a solvent, acetic acid is added to adjust the pH to 3-5, and hydrolysis is performed for 15-60 minutes to obtain the hydrolysate. The divalent tin salt and the amino-containing silane coupling agent can be pre-dissolved separately in a small amount of solvent and then mixed evenly in solution form.

[0051] In this invention, the divalent tin salt is selected from at least one of stannous chloride, stannous iodide, and stannous bromide, or a hydrated salt of the above-mentioned divalent tin salt, more preferably stannous chloride or its hydrated salt.

[0052] In this invention, the amino-containing silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane (KH-550), γ-aminopropyltrimethoxysilane, and N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane (KH-792), more preferably KH-550. The solvent is a mixture of ethanol and water, preferably with a volume ratio of ethanol to water of 80:20 to 95:5. Sufficient ethanol content contributes to the uniformity of dissolution and hydrolysis of the silane coupling agent.

[0053] In this invention, the concentration of the amino-containing silane coupling agent in the silane coupling agent hydrolysate is 1-5 wt%, more preferably 2-4 wt%. The concentration of the divalent tin salt is 0.05-0.5 wt%, more preferably 0.1-0.2 wt%.

[0054] After the surface modification in step S2 is completed, the film is taken out and gently rinsed with anhydrous ethanol to remove loosely adsorbed molecules. Then it is placed in an oven at 80~100℃ for 10~30 minutes to cure, so as to promote the complete formation of Si-OC bonds and remove residual solvent, and obtain a surface silanized film.

[0055] S3. Cast an epoxy resin composition onto the surface of the silanized film and cure it to obtain the polyimide-epoxy resin composite material for high-frequency insulation.

[0056] This step completes the final molding of the composite material. The amino group at the other end of the silane layer undergoes a ring-opening addition reaction with the epoxy resin to achieve covalent bonding.

[0057] In this invention, the epoxy resin composition comprises epoxy resin, curing agent and accelerator, wherein the mass ratio of epoxy resin, curing agent and accelerator is 100 : (80~90) : (0.8~1.2).

[0058] In an optional embodiment of the present invention, the epoxy resin (EP) is preferably a bisphenol A type epoxy resin (such as E-51); the curing agent is preferably methyl hexahydrophthalic anhydride (MTHPA); and the accelerator is preferably 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30).

[0059] The preferred preparation process of the epoxy resin composition of the present invention is as follows: The epoxy resin and curing agent are preheated and mixed at 60-65°C, and stirred for 20-30 minutes. Then, an accelerator is added, and stirring continues for 10-15 minutes. After uniform mixing, the mixture is placed in a vacuum drying oven and degassed at 60°C for 15-20 minutes to remove air bubbles introduced by stirring, thereby obtaining the epoxy resin composition.

[0060] The preferred casting process of this invention is as follows: The epoxy resin composition is poured onto a preheated surface-silanized film. A stepped curing procedure is then employed to reduce internal stress: first, curing at 70-90°C for 0.5-2 hours; then, curing at 95-110°C for 1-3 hours; next, curing at 140-160°C for 2-4 hours; and finally, curing at 120-140°C for 0.5-2 hours. During this process, the amino groups on the silane react with the epoxy groups to form strong CN bonds, ultimately yielding a PI-EP composite material with a chemically bonded interface.

[0061] This invention also provides a polyimide-epoxy resin composite material for high-frequency insulation, prepared by the above-described method. The PI and EP phases are connected by continuous chemical bonds consisting of a Si-OC covalent bond, a silane bridge, and a CN covalent bond. This interface layer is dense and defect-free, effectively blocking charge injection and accumulation, and dispersing the interfacial electric field. Therefore, this material exhibits outstanding characteristics such as high partial discharge initiation voltage, low discharge quantity, strong breakdown field, and long resistance to electrothermal aging at high frequencies above 10 kHz.

[0062] This invention also provides applications of the above-mentioned polyimide-epoxy resin composite material for high-frequency insulation, which is particularly suitable for preparing insulating components for high-frequency power electronic devices with operating frequencies above 10kHz, for example:

[0063] Inter-turn insulation, inter-layer insulation, and main insulation in high-frequency transformers;

[0064] Power module packaging and insulation in electric drive systems for new energy vehicles;

[0065] Key insulating components in aerospace power supplies and high-end communication equipment power supplies, etc.

[0066] In these applications, the polyimide-epoxy resin composite material for high-frequency insulation of the present invention can significantly improve the long-term operational reliability and power density of equipment under harsh operating conditions.

[0067] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0068] Example 1

[0069] This embodiment provides a polyimide-epoxy resin (PI-EP) composite material for high-frequency insulation, and its preparation method is as follows:

[0070] (1) Surface carboxylation treatment of polyimide film:

[0071] A 50 μm thick polyimide (PI) film was ultrasonically cleaned sequentially with deionized water and anhydrous ethanol for 15 minutes each, and then air-dried. The cleaned PI film was then immersed in a 2.5 mol / L sodium hydroxide (NaOH) aqueous solution and treated in a 60°C constant temperature water bath for 45 minutes. After removal, it was rinsed with deionized water until neutral, and then immersed in a 1.0 mol / L hydrochloric acid (HCl) aqueous solution for protonation treatment at room temperature for 30 minutes. After removal, it was rinsed with a large amount of deionized water until the conductivity of the washing solution remained unchanged, and then dried in an 80°C forced-air oven for 15 minutes to obtain a surface carboxylated PI film, denoted as PI-COOH film.

[0072] (2) Preparation of silane coupling agent hydrolysate and surface modification:

[0073] 0.15 g of stannous chloride dihydrate (SnCl2·2H2O) was dissolved in 5 mL of anhydrous ethanol to prepare a stannous chloride solution. 90 mL of anhydrous ethanol and 10 mL of deionized water were added to a beaker, and 3.0 g of 3-aminopropyltriethoxysilane (KH-550) was slowly added under magnetic stirring for 10 minutes. Then, the above stannous chloride solution was added and stirred until homogeneous. Subsequently, glacial acetic acid was added dropwise to adjust the pH of the solution to 4.0, and stirring was continued for 30 minutes to allow hydrolysis. The above hydrolysate was added under stirring and mixed thoroughly to obtain a silane coupling agent hydrolysate. The carboxylated PI film obtained in step (1) was immersed in the silane coupling agent hydrolysate and treated at room temperature for 6 minutes. After removal, it was rinsed three times with anhydrous ethanol and heat-treated in a 90℃ oven for 15 minutes to obtain a surface-silanized PI film, denoted as PI-COOH-Si film.

[0074] (3) Epoxy resin casting and curing:

[0075] Bisphenol A type epoxy resin (E-51), methyl hexahydrophthalic anhydride (MTHPA) curing agent, and 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) accelerator were weighed at a mass ratio of 100:85:1. The epoxy resin and curing agent were mixed at 60°C and magnetically stirred for 30 minutes, then the accelerator was added and stirring continued for 15 minutes. The mixture was then degassed under vacuum at 60°C for 20 minutes. A surface-silanized PI film was laid flat on the bottom of a mold preheated at 60°C. The degassed epoxy resin composition was poured onto the film, and the mixture was degassed under vacuum again for 30 minutes to remove interfacial bubbles. Finally, the mold was placed in an oven for stepped curing: first cured at 80°C for 1 hour, then at 100°C for 2 hours, then at 150°C for 3 hours, and finally at 130°C for 1 hour. After the process was completed, the mixture was allowed to cool naturally to room temperature and demolded to obtain the PI-EP composite material, denoted as PI-EP-SI-6, with a thickness of 1 mm.

[0076] Figure 1 This is a schematic diagram illustrating the preparation principle of the PI-EP composite material in this embodiment. Carboxyl groups are introduced onto the PI film through sequential treatment with NaOH and HCl. Then, the PI film is modified with amino groups using a silane coupling agent hydrolysate. Finally, a ring-opening addition reaction occurs with epoxy resin to obtain the PI-EP composite material.

[0077] Figure 2 This describes the X-ray photoelectron spectroscopy (XPS) analysis of the PI thin film, PI-COOH thin film, and PI-COOH-Si thin film in this embodiment. Figure 2As shown in Figure A, compared to the PI film and PI-COOH film, the PI-COOH-Si film shows a new characteristic peak for Si, indicating that the silane was successfully coupled to the polyimide film. Furthermore, the significant increase in O content in PI-COOH-Si is due to the presence of three hydroxyl groups in the monomolecular silane obtained from KH550 hydrolysis, further confirming the successful connection of the silane coupling agent. Figure 2 China B and Figure 2 As shown in Figure C, compared to the PI film, the decreased CN / O and C=O area ratio of the PI-COOH film indicates successful hydrolysis of the CN bond in the polyimide and successful ring-opening of the five-membered ring. Furthermore, compared to the PI film, the wider and slightly right-shifted C=O bond in the PI-COOH film indicates an increased C=O binding energy, proving the successful introduction of carboxyl groups. Figure 10 As shown in D, compared to the PI film and the PI-COOH film, the PI-COOH-Si film shows a new Si element peak. The appearance of Si-OC in the film proves that the silane coupling agent was successfully coupled to the polyimide film, and the appearance of Si-O-Si indicates that the silane has undergone a certain condensation reaction.

[0078] Example 2

[0079] The difference between this embodiment and Embodiment 1 is that in step (2) of this embodiment, the carboxylated PI film obtained in step (1) is immersed in a silane coupling agent hydrolysate and treated at room temperature for 2 minutes. The remaining steps and parameters are exactly the same. The resulting PI-EP composite material is denoted as PI-EP-SI-2.

[0080] Example 3

[0081] The difference between this embodiment and Embodiment 1 is that in step (2) of this embodiment, the carboxylated PI film obtained in step (1) is immersed in a silane coupling agent hydrolysate and treated at room temperature for 4 minutes. The remaining steps and parameters are exactly the same. The resulting PI-EP composite material is designated as PI-EP-SI-4.

[0082] Example 4

[0083] The difference between this embodiment and Embodiment 1 is that in step (2) of this embodiment, the carboxylated PI film obtained in step (1) is immersed in a silane coupling agent hydrolysate and treated at room temperature for 8 minutes. The remaining steps and parameters are exactly the same. The resulting PI-EP composite material is designated as PI-EP-SI-8.

[0084] Example 5

[0085] The difference between this embodiment and Embodiment 1 is that in step (2) of this embodiment, the carboxylated PI film obtained in step (1) is immersed in a silane coupling agent hydrolysate and treated at room temperature for 10 minutes. The remaining steps and parameters are exactly the same. The resulting PI-EP composite material is designated as PI-EP-SI-10.

[0086] Comparative Example 1

[0087] The difference between this comparative example and Example 1 is that this comparative example does not involve alkali treatment, acid treatment, or silane coupling agent surface modification of the polyimide film. The specific steps of this comparative example are as follows:

[0088] A 50 μm thick PI film was ultrasonically cleaned sequentially with deionized water and anhydrous ethanol for 15 minutes each, and then air-dried. Bisphenol A type epoxy resin (E-51), methyl hexahydrophthalic anhydride (MTHPA) curing agent, and 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) accelerator were weighed at a mass ratio of 100:85:1. The epoxy resin and curing agent were mixed at 60°C and magnetically stirred for 30 minutes, then the accelerator was added and stirring continued for 15 minutes. The mixture was then degassed under vacuum at 60°C for 20 minutes. The cleaned and dried PI film was laid flat on the bottom of a mold preheated at 60°C. The degassed epoxy resin composition was poured onto the film, and the mixture was degassed under vacuum again for 30 minutes to remove interfacial bubbles. Finally, the mold was placed in an oven for stepped curing: first, curing at 80°C for 1 hour, then increasing the temperature to 100°C for 2 hours, then curing at 150°C for 3 hours, and finally curing at 130°C for 1 hour. After the process is completed, the material is allowed to cool naturally to room temperature, then demolded to obtain the PI-EP composite material, denoted as PI-EP.

[0089] Comparative Example 2

[0090] The difference between this comparative example and Example 1 is that step (2) is omitted in this comparative example. The specific steps of this comparative example are as follows:

[0091] A 50 μm thick PI film was ultrasonically cleaned sequentially with deionized water and anhydrous ethanol for 15 minutes each, and then air-dried. The cleaned PI film was then immersed in a 2.5 mol / L sodium hydroxide (NaOH) aqueous solution and treated in a 60℃ constant temperature water bath for 45 minutes. After removal, it was rinsed with deionized water until neutral, and then immersed in a 1.0 mol / L hydrochloric acid (HCl) aqueous solution for protonation treatment at room temperature for 30 minutes. After removal, it was rinsed with a large amount of deionized water until the conductivity of the washing solution remained unchanged, and then dried in an 80℃ forced-air oven for 15 minutes to obtain a surface carboxylated PI film.

[0092] Bisphenol A type epoxy resin (E-51), methyl hexahydrophthalic anhydride (MTHPA) curing agent, and 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) accelerator were weighed at a mass ratio of 100:85:1. The epoxy resin and curing agent were mixed at 60°C and magnetically stirred for 30 minutes, then the accelerator was added and stirring continued for 15 minutes. The mixture was then degassed under vacuum at 60°C for 20 minutes. The cleaned and dried surface-carboxylated PI film was laid flat on the bottom of a mold preheated at 60°C. The degassed epoxy resin composition was poured onto the film, and the mixture was degassed under vacuum again for 30 minutes to remove interfacial bubbles. Finally, the mold was placed in an oven for stepped curing: first cured at 80°C for 1 hour, then at 100°C for 2 hours, then at 150°C for 3 hours, and finally at 130°C for 1 hour. After the process was completed, the mixture was allowed to cool naturally to room temperature and demolded to obtain the PI-EP composite material, denoted as PI-EP-SI-0.

[0093] Comparative Example 3

[0094] The difference between this comparative example and Example 3 is that glacial acetic acid is not added in step (2) of this comparative example to adjust the pH, while the other steps and parameters are exactly the same.

[0095] Comparative Example 4

[0096] The difference between this comparative example and Example 3 is that stannous chloride dihydrate is not added in step (2) of this comparative example, while the other steps and parameters are exactly the same.

[0097] Comparative Example 5

[0098] The difference between this comparative example and Example 3 is that in step (2) of this comparative example, 1 mol / L hydrochloric acid (HCl) is used instead of glacial acetic acid to adjust the pH of the solution to 4.0.

[0099] Test case

[0100] 1. Characterization of interfacial chemical structure and morphology

[0101] (1) X-ray photoelectron spectroscopy (XPS) analysis:

[0102] like Figure 3 As shown in Figure A, as the immersion time of the silane coupling agent hydrolysate increased from 0 minutes to 10 minutes, the silicon content on the PI film surface gradually increased, indicating that the longer the silane coupling agent treatment time, the more silane molecules react with the PI surface. Figure 3 As shown in Figure B, from 0 to 6 minutes, the Si-OC covalent bond content gradually increases, while the Si-O-Si bond content remains almost unchanged. From 6 to 10 minutes, the Si-OC covalent bond content remains almost unchanged, while the Si-O-Si bond content gradually increases.

[0103] (2) Cross-sectional scanning electron microscopy (SEM) and elemental distribution imaging (mapping) analysis:

[0104] like Figure 4 As shown in a, the PI-EP composite material prepared in Comparative Example 1 has a significant air gap between PI and EP. Figure 4 b and c in the text are Figure 4 The image shows a further magnified view of the PI interface and air gap, as well as the EP interface and air gap. This demonstrates that due to the differences in the physicochemical properties of PI and EP, directly casting EP onto the PI film surface will prevent the interfaces from forming a stable bond.

[0105] Figure 4 In the figure, f represents the PI-EP interface of the PI-EP composite material prepared in Comparative Example 2. Compared with Comparative Example 1, the distance between the interfaces of PI and EP is significantly reduced, and obvious delamination of PI, PI-EP interface and EP is observed. There is structural connection between the interfaces of PI and EP.

[0106] Figure 4 In the image, d, e, and g are SEM images of the PI-EP composite material prepared in Example 1. It can be seen that there is a stable interface connection between PI and EP, and the interface is dense and defect-free. The interface between PI and EP has almost no obvious boundary and exhibits an interlocking structure. Figure 5 In the figure, A and B are the elemental distribution imaging (mapping) analyses of the PI-EP composite materials prepared in Comparative Example 2 and Example 1, respectively. The significant increase in Si element in Example 1 confirms the successful introduction of the silane coupling agent.

[0107] 2. Peel strength test

[0108] The interfacial peel strength of the composite material was tested using an Instron 5967 electronic universal testing machine according to ASTM D903 standard. Samples were cut into 25 mm wide strips and tested at a peel angle of 180° and a peel speed of 50 mm / min. The average force during the stable peel phase was used to calculate the peel strength. Specific data are summarized in Table 1.

[0109] Table 1. Peel strength data for Examples 1-5 and Comparative Examples 1-5

[0110]

[0111] Compared to the untreated Comparative Example 1, Comparative Example 2, which only underwent carboxylation, showed increased strength due to enhanced physical adsorption. However, all examples modified with silane coupling agents exhibited strengths exceeding those of Comparative Example 2, confirming the decisive role of chemical bonding bridging. In the acetate-stannous chloride synergistic system, the peel strength initially increased and then decreased with treatment time. Example 1 (6 minutes) reached a peak of 3.92 N / cm, indicating the formation of a dense and complete monomolecular chemically bonded layer. Excessive treatment time (Example 5) resulted in a decrease in strength due to excessive silane self-polymerization or the formation of a weak interfacial layer through physical adsorption.

[0112] The strengths of Comparative Examples 3 and 4 were both lower than that of Example 1, demonstrating that both acetic acid and stannous chloride are indispensable. The strength of Comparative Example 5 was lower than that of Example 1, but comparable to that of Comparative Example 4. This demonstrates that the weak acid buffering effect of acetic acid is unique, creating a mild environment to inhibit silane self-condensation. The above comparative examples demonstrate the synergistic effect of acetic acid and stannous chloride.

[0113] 3. High-frequency insulation performance test

[0114] (1) Partial discharge characteristics test

[0115] The samples from the examples and comparative examples were placed on a high-frequency discharge test platform and tested for 10 minutes under a constant sinusoidal voltage of 30 kHz and 9 kV. Partial discharge signals were collected using the pulse current method. Each group of samples was tested 10 times and the statistical values ​​were collected.

[0116] Figure 6 The images show the phase-resolved partial discharge (PRPD) spectra of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2, where A-F are the PRPD spectra of Comparative Example 2 (0 min), Example 2 (2 min), Example 3 (4 min), Example 1 (6 min), Example 4 (8 min), and Example 5 (10 min), respectively. Figure 6 The scattering points indicate the occurrence of discharge, with their density reflecting the number of discharges and their intensity reflecting the severity of the discharge. From 0 to 6 min, the insulation performance of the sample gradually increased with the extension of modification time, reaching a peak at 6 min. As the modification time increased, silane molecules gradually coupled to the PI film. After EP curing, silane molecules were coupled to the PI-EP interface, and the insulation performance improved with the increase in the number of coupled silane molecules. When the modification time increased from 6 min to 10 min, the insulation performance of the sample gradually decreased. At this point, the silane molecules were completely coupled to the PI interface, and excessive silane molecules underwent hydroxyl dehydration condensation, thus affecting the coupling with EP. Therefore, the insulation performance is best when the silane coupling agent impregnation modification time is 6 min.

[0117] The above partial discharge data were statistically analyzed and summarized as follows: Figure 7 .from Figure 7 As shown in A and B, after 6 minutes of silane modification, the total discharge amplitude, average discharge amplitude, and number of discharges all decreased significantly. Compared to Comparative Example 2, Example 1 showed a decrease of 81.7% in the average number of discharges, 83.3% in the total number of discharges, 64.1% in the total discharge amplitude, and 84.8% in the average discharge amplitude.

[0118] Between 0 and 6 minutes, silanol molecules have the opportunity to diffuse and contact the polar sites on the polyimide film surface, undergoing a condensation reaction to form stable Si-OC covalent bonds. Furthermore, silanol molecules also form a stable Si-O-Si network structure through self-condensation. This process significantly enhances the chemical bonding strength between the PI and the subsequent EP, reducing interfacial defects caused by physical adsorption and incomplete bonding. Therefore, the insulation performance of the sample gradually improves. At 6 minutes, the system reaches a kinetic equilibrium state. At this point, the chemical adsorption and bonding of silane molecules on the PI surface tend to saturate, forming an optimal near-monolayer coverage. The interfacial phase in this state possesses both the highest chemical bonding density and optimal physical compactness, thereby maximizing the interfacial breakdown strength and achieving peak insulation performance. Between 6 and 10 minutes, excessively long processing times can lead to excessive self-condensation of already bonded and silane molecules in the solution, consuming the organic functional groups that should have reacted with the EP. This results in the formation of an excessively thick, physically cross-linked, rigid silanol layer or multilayer at the interface, reducing the overall stability of the interface.

[0119] Partial discharge data from Examples 1 and Comparative Examples 3-5 are summarized in Figure 8 ,from Figure 8 As can be seen from A and B, Example 1 has the lowest total discharge amplitude, average discharge amplitude, and number of discharges, indicating that the introduction of acetic acid and stannous chloride is beneficial to obtaining a PI-EP interface with better insulation performance.

[0120] (2) Interface breakdown test

[0121] The interface breakdown test was conducted at a frequency of 30 kHz using a uniform voltage ramp method, gradually increasing the voltage amplitude at a rate of 0.5 kV / s until the sample broke down.

[0122] The interfacial breakdown voltages of the PI-EP composite materials prepared in Examples 1-5 and Comparative Example 2 at 20°C, 60°C, and 100°C are as follows: Figure 9 As shown, the breakdown voltage of each sample is the final value based on the average of 10 test results. Figure 9It can be seen that the breakdown voltage gradually increases from 0 to 6 minutes and gradually decreases from 6 to 10 minutes, reaching a peak at 6 minutes of silane coupling agent impregnation modification. Compared with the sample without silane coupling agent impregnation modification (Comparative Example 2), the breakdown voltage of the sample after silane coupling agent impregnation modification is significantly improved. For PI-EP composite materials treated at different temperatures, the breakdown voltage gradually decreases with increasing temperature, indicating that high temperature promotes breakdown, thereby reducing the insulation performance of the material.

[0123] Example 1, Comparative Examples 3-5: Interfacial breakdown voltages of PI-EP composite materials prepared at 20°C, 60°C, and 100°C. Figure 10 As shown, Example 1 has the highest breakdown voltage, which also indicates that the introduction of acetic acid and stannous chloride is beneficial to obtaining a PI-EP interface with better insulation performance.

[0124] (3) Heat aging resistance test

[0125] The samples were subjected to a single thermal aging treatment in a 180℃ constant temperature oven, and samples were taken at aging times of 0, 24, 48, 96, 144, and 192 hours. Interfacial breakdown tests were then conducted at 30 kHz to evaluate the effect of different thermal aging times on the breakdown performance of the samples. The average value of 10 measurements was calculated. The test results of the PI-EP composite materials in Examples 1-5 and Comparative Example 2 are as follows: Figure 11 As shown, the breakdown voltage of each sample gradually decreases with increasing thermal aging time. This indicates that thermal aging weakens the interfacial insulation properties of the samples. PI-EP-SI-6 from Example 1 exhibits the best thermal aging resistance.

[0126] (4) Combined electro-thermal aging test

[0127] The PI-EP composite material samples of Examples 1-5 and Comparative Example 2 were simultaneously subjected to a high-frequency voltage of 9kV and 3kHz and a high temperature of 180℃ until the sample interface broke down. The breakdown time was recorded, and the average value was calculated after 10 tests to characterize the durability of the material under electro-thermal synergistic stress. The results are as follows: Figure 12 As shown.

[0128] Compared with the untreated sample (Comparative Example 2), the electrothermal aging tolerance time of the silane-coupled PI-EP composite sample was significantly increased. The electrothermal aging tolerance time increased with the silane treatment time increasing from 0 min to 6 min. The electrothermal aging tolerance time decreased with the silane treatment time increasing from 6 min to 10 min. The longest electrothermal aging tolerance time was observed at a silane treatment time of 6 min.

[0129] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide-epoxy resin composite material for high-frequency insulation, characterized in that, Includes the following steps: S1. The polyimide film is subjected to surface carboxylation treatment to obtain a surface carboxylated film; S2. The surface carboxylated film is immersed in a silane coupling agent hydrolysate for surface modification to obtain a surface silanized film; the silane coupling agent hydrolysate includes an amino-containing silane coupling agent, a solvent, acetic acid, and a divalent tin salt; S3. Cast an epoxy resin composition onto the surface of the silanized film and cure it to obtain the polyimide-epoxy resin composite material for high-frequency insulation. In step S1, the surface carboxylation treatment includes: first treating the polyimide film in an alkaline solution, and then treating it in an acidic solution; the alkaline solution is an aqueous sodium hydroxide solution with a concentration of 1.0~5.0 mol / L. In step S2, the immersion time in the silane coupling agent hydrolysate is 4 to 10 minutes.

2. The preparation method according to claim 1, characterized in that, The alkaline solution is treated at a temperature of 50-70°C for 30-60 minutes; the acid solution is hydrochloric acid, sulfuric acid, or acetic acid, with a concentration of 0.5-1.5 mol / L, and is placed in the acid solution for 20-40 minutes.

3. The preparation method according to claim 1, characterized in that, The preparation method of the silane coupling agent hydrolysate is as follows: divalent tin salt and amino-containing silane coupling agent are mixed evenly in a solvent, acetic acid is added to adjust the pH to 3-5, and hydrolysis is carried out for 15-60 minutes to obtain the solution.

4. The preparation method according to claim 1, characterized in that, The amino-containing silane coupling agent is selected from at least one of 3-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane, and N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane; the solvent is a mixture of ethanol and water.

5. The preparation method according to claim 1, characterized in that, In the silane coupling agent hydrolysate, the concentration of the amino-containing silane coupling agent is 1~5wt%; the concentration of the divalent tin salt is 0.05~0.5wt%.

6. The preparation method according to claim 1, characterized in that, The epoxy resin composition includes epoxy resin, curing agent and accelerator, and the mass ratio of epoxy resin, curing agent and accelerator is 100 : (80~90) : (0.8~1.2).

7. A polyimide-epoxy resin composite material for high-frequency insulation, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the polyimide-epoxy resin composite material for high-frequency insulation as described in claim 7, characterized in that, Insulating components used in the manufacture of high-frequency power electronic devices.

Images