A core-shell type polyimide-based high-temperature-resistant water-based paint and a preparation method thereof

Abstract

The present application belongs to the technical field of high-temperature-resistant coatings, and particularly relates to a core-shell polyimide-based high-temperature-resistant water-based coating and a preparation method thereof. The coating comprises the following components by mass percentage: core-shell emulsion 35-38%, wherein the core layer is polyimide resin, the transition layer is organic silicon emulsion modified by 5,6-dimethylbenzimidazole and silane coupling agent KH-550, and the shell layer is polyamide-imide resin; modified high-temperature filler 28-32%; functional additive 6-7.5%; and deionized water in the rest. The present application improves the high-temperature resistance, adhesion and environmental friendliness of the coating through gradient design of the core-shell emulsion, synergistic effect of multi-scale fillers and low-temperature gradient curing process.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-temperature-resistant coatings, and particularly relates to a core-shell polyimide-based high-temperature-resistant water-based coating and a preparation method thereof. BACKGROUND

[0002] With the expansion of extreme working condition scenes such as aerospace engine hot end components, electrical equipment and industrial high-temperature pipelines, traditional organic silicon high-temperature-resistant coatings face severe challenges in long-term service temperature, interfacial bonding strength and multi-stage curing adaptability. In the prior art, although the organic silicon resin coating can improve the temperature resistance limit to 500-700℃ by introducing ceramic fillers (such as mica powder, aluminum oxide), the curing process needs to be continuously baked at 150-300℃ for 2-4h, which intensifies the risk of thermal deformation of the substrate, especially the applicability of lightweight materials such as aluminum alloy and magnesium alloy is limited. At the same time, the difference in thermal expansion coefficient between the high-temperature filler and the resin matrix easily causes the coating to crack, and the traditional single-layer structure is difficult to realize the gradient dissipation of thermal stress, which seriously restricts the adhesion stability of the coating in the thermal shock cycle environment.

[0003] In recent years, the core-shell structure design has been tried to be used in epoxy / polyurethane systems to improve the mechanical properties, but due to the limitation of the heat resistance of the shell material (<300℃), it cannot meet the high-temperature protection demand. For example, the organic silicon coating, ceramicizable organic silicon elastomer coating and preparation method disclosed in patent publication No. CN118755383A propose an organic silicon coating with MoS2 / MXene micro-nano heterostructure and cerium oxide, which can resist 1300℃ high temperature for a short time, but its melting processing temperature requirement is high (high-temperature spraying layer is required), and its long-term oxidation stability is still insufficient. In addition, the existing filler system mostly relies on a single type of oxide (such as zirconium oxide, titanium oxide), and lacks the synergistic design of radiation heat dissipation and thermal barrier effect, which causes the accumulation of heat in the coating under high-temperature heat flux density, and accelerates the thermal decomposition of the resin matrix.

[0004] In terms of curing process, such as the high-temperature-resistant waterproof coating and its preparation method disclosed in patent publication No. CN119081542A, the organic silicon coating is cured under medium and low temperature conditions by the oxygen-rich microwave plasma composite treatment of methylphenyl silicone resin and methyl MQ silicone resin, but the curing depth and uniformity of thick coating are still limited by the uneven distribution of microwave energy. For example, the water-based organic silicon anti-cracking anticorrosive coating and its preparation method disclosed in patent publication No. CN110628324A develop a water-based organic silicon anti-cracking anticorrosive coating with step-by-step crosslinking, which uses a three-functionality and four-functionality crosslinking agent to compound, but sacrifices the glass transition temperature (Tg<200℃) of the coating, resulting in a sharp drop in high-temperature mechanical strength.

[0005] Therefore, the application urgently needs to construct a new generation of waterborne organic silicon coating system with low-temperature processability, high-temperature resistance and stable thermal-mechanical properties through molecular structure design and multi-scale filler compounding. SUMMARY

[0006] The application aims to provide a core-shell type polyimide-based high-temperature-resistant waterborne coating and a preparation method thereof.

[0007] To solve the above technical problems, the application provides a core-shell type polyimide-based high-temperature-resistant waterborne coating, which is composed of the following components in mass percentage:

[0008] The core-shell emulsion: 35% to 38%; the core layer is polyimide resin; the transition layer is 5,6-dimethylbenzimidazole and silicone emulsion modified by silane coupling agent KH-550; and the shell layer is polyamide-imide resin;

[0009] Modified high-temperature filler: 28% to 32%;

[0010] Functional additives: 6% to 7.5%;

[0011] Deionized water: the balance.

[0012] Preferably, the preparation method of the core-shell emulsion comprises the following steps:

[0013] Step S1: 5,6-dimethylbenzimidazole is added to the preheated silicone emulsion to 70 DEG C, stirred at a speed of 600 rpm, then 5wt% silane coupling agent KH-550 ethanol solution is added dropwise at a rate of 0.5 mL / min, and the reaction is kept for 1.5 h to form a modified silicone emulsion;

[0014] Step S2: polyimide resin is dissolved in pre-cooled N-methylpyrrolidone, then non-ionic emulsifier Tween-80 is added, and ultrasonic emulsification is carried out at an alternating frequency of 25 kHz and 40 kHz for 20 min to form a PI dispersion;

[0015] Step S3: the above PI dispersion is added to the modified silicone emulsion at a rate of 2 mL / min, the first stage is sheared at a speed of 10000 rpm for 15 min, the second stage is sheared at a speed of 6000 rpm for 25 min, and the system temperature is kept ≤ 35 DEG C during the shearing process to form a homogeneous composite emulsion;

[0016] Step S4: The polyamide-imide resin is premixed with N,N dimethylacetamide under nitrogen protection for 40 min, dissolved at 80-90℃, then emulsifier Span-80 is added, and the PAI dispersion is formed by defoaming treatment under a vacuum degree of -0.1 MPa;

[0017] Step S5: The PAI dispersion is injected into the composite emulsion of step S3 in a laminar flow mode after being passed through a 300-mesh sieve, the pH is adjusted to 8-8.5, and stirring is first carried out at a speed of 800 rpm for 30 min at a temperature of 70℃, and then at a speed of 500 rpm for 1.5-2 h.

[0018] Preferably, in the step S1, the mass ratio of the silicone emulsion, 5,6-dimethylbenzimidazole and silane coupling agent KH-550 is 85-95: 1.5-2.5: 0.8-1.2;

[0019] In the step S2, the mass ratio of the polyimide resin, N-methylpyrrolidone and nonionic emulsifier Tween-80 is 1:3.8-4.2:0.6-0.7;

[0020] In the step S3, the mass ratio of the PI dispersion and modified silicone emulsion is 1:0.7-0.8;

[0021] In the step S4, the mass ratio of the polyamide-imide resin, N,N dimethylacetamide and emulsifier Span-80 is 1:0.18-0.2:5.2-5.8;

[0022] In the step S5, the mass ratio of the PAI dispersion and composite emulsion is 1:0.35-0.45.

[0023] Preferably, the preparation method of the modified high-temperature filler comprises the following steps:

[0024] The hollow glass beads, yttrium oxide and silicon powder are mixed with 8wt% silane coupling agent KH-550 ethanol solution, stirred at a speed of 400 rpm at a temperature of 80℃ for 1 h, vacuum dried and then passed through a 200-mesh sieve for standby use;

[0025] The cerium oxide is premixed with a dispersant, ball milled by a sand mill at a speed of 300 rpm for 2 h to a particle size D50 <0.8 μm for standby use.

[0026] Preferably, the modified high-temperature filler is composed of the following components in terms of mass percentage: hollow glass beads: 60%-70%; yttrium oxide: 15%-20%; cerium oxide: 10%-20%; silicon powder: 5%-8%.

[0027] Preferably, the molecular weight of the polyimide resin is selected from 20000 g / mol to 25000 g / mol.

[0028] Preferably, the hollow glass microsphere has a particle size of 10 μm to 80 μm and a thermal conductivity of <0.15 W / (m·K); the yttrium oxide has a particle size D50 of <500 nm; the cerium oxide has a particle size D50 of <1 μm; and the silicon powder has a particle size D50 of <5 μm.

[0029] Preferably, the functional additive is composed of the following components: dispersant 1% to 1.1%; anti-settling agent 0.6% to 0.7%; defoaming agent 0.15% to 0.2%; wetting agent 0.3% to 0.4%; pigment 2.5% to 3%; film-forming aid 1% to 1.1%; and thickening agent 0.7% to 0.8%.

[0030] Preferably, the dispersant is BYK-190; the anti-settling agent is Disparlon AQ-633E;

[0031] The defoaming agent is BYK-028; the wetting agent is Degussa TEGO-4100; the pigment is one or more of titanium dioxide, black iron oxide, red iron oxide, and chrome yellow; the film-forming aid is one of DPNB, DB, and PPH; and the thickening agent is hydroxyethyl cellulose or LA-8108 benzylstyrene emulsion.

[0032] The present application also provides a preparation method of the core-shell polyimide-based high-temperature-resistant water-based paint.

[0033] Step one: mix and stir the deionized water, dispersant, wetting agent, and defoaming agent for 15 min;

[0034] Step two: add the modified high-temperature filler, then disperse at 1500 rpm for 1.5 h, and then ultrasonically treat at 40 kHz for 0.3 h to 0.5 h;

[0035] Step three: sequentially add the core-shell emulsion and pigment, and then stir at 500 rpm for 20 min;

[0036] Step four: sequentially add the anti-settling agent, film-forming aid, and thickening agent, and then adjust the viscosity to 3500 cP to 4500 cP;

[0037] Step five: coat the high-temperature-resistant water-based organic silicon paint after adjusting the viscosity on the surface of a metal substrate, then air dry at room temperature for 30 min, then preliminarily solidify by heating at 80℃ for 30 min, then secondarily solidify by heating at 150℃ for 1 h, and then completely solidify by heating at 280℃ for 2 h.

[0038] Compared with the prior art, the present application has the following beneficial effects:

[0039] 1、The core-shell structure enhances the interface bonding force: the PI in the core layer and the modified silicone in the transition layer KH-550 form physical entanglement through the π-π stacking effect of the benzimidazole group, the PAI in the shell layer is chemically bonded to the surface hydroxyl group through an amide bond, the thermal stress gradient is dissipated, and the high temperature resistance of the coating is improved.

[0040] 2、The multi-scale filler cooperates heat dissipation and crack resistance: hollow glass microbeads (low thermal conductivity) and yttrium oxide (thermal barrier effect) cooperatively reduce the heat conduction rate, cerium oxide (high infrared emissivity) accelerates heat dissipation through radiation, and silicon powder fills the interface microcracks.

[0041] 3、The low temperature gradient curing and high temperature stability are cooperated: through the gradient crosslinking design of the core-shell emulsion combined with the staged curing process (80℃→150℃→280℃), the shell layer crosslinking is preferentially completed at the low temperature stage (80℃), and the thermal stress of the substrate is reduced; at the high temperature stage (280℃), the core layer polycondensation reaction is activated, a dense Si-O-Si network is formed, and the VOC emission is less than 50g / L. DETAILED DESCRIPTION

[0042] The present application will be further described in detail below in conjunction with specific embodiments. The advantages and features of the present application will be clearer according to the following description.

[0043] Example 1

[0044] The formula composition of this embodiment includes: core-shell emulsion: 370g; modified high-temperature filler (containing hollow glass microbeads 195g, yttrium oxide 54g, cerium oxide 36g, and silicon powder 15g): 300g; functional additives (containing BYK-190, AQ-633E, BYK-028, TEGO-4100, titanium white, DPNB, and hydroxyethyl cellulose): 70g; deionized water: 260g.

[0045] According to the above formula, the raw materials of each component are weighed:

[0046] (1) Mix and stir the deionized water, dispersant, wetting agent and defoaming agent for 15min;

[0047] (2) After adding the modified high-temperature filler, then disperse at a speed of 1500rpm for 1.5h, and then ultrasonic treatment at 40kHz for 0.5h;

[0048] (3) Add the core-shell emulsion and pigment in turn, and then stir at a speed of 500rpm for 20min;

[0049] (4) sequentially adding the anti-settling agent, the film-forming aid, and the thickening agent, and then adjusting the viscosity to 4000 cP;

[0050] (5) coating the high-temperature resistant waterborne silicone coating with the adjusted viscosity on the surface of the metal substrate, then surface drying at room temperature for 30 min, then preliminary curing at a temperature of 80 °C for 30 min, then secondary curing at a temperature of 150 °C for 1 h, and complete curing at a temperature of 280 °C for 2 h.

[0051] Example 2

[0052] The formula composition of this example includes: core-shell emulsion: 380 g, modified high-temperature filler: (hollow glass microbead 224 g, yttrium oxide 48 g, cerium oxide 32 g, silicon powder 16 g) 320 g; functional aid (containing BYK-190, AQ-633E, BYK-028, TEGO-4100, titanium white, DPNB, hydroxyethyl cellulose): 70 g; deionized water: 225 g.

[0053] According to the above formula, each component raw material is weighed:

[0054] (1) mixing and stirring the deionized water, the dispersant, the wetting agent, and the defoaming agent for 15 min;

[0055] (2) after adding the modified high-temperature filler, sanding the filler for 3 h (D50 < 0.5 μm), then dispersing at a speed of 1500 rpm for 1.5 h, then ultrasonic treatment at 40 kHz for 0.5 h;

[0056] (3) sequentially adding the core-shell emulsion and the pigment, then stirring at a speed of 500 rpm for 20 min;

[0057] (4) sequentially adding the anti-settling agent, the film-forming aid, and the thickening agent, and then adjusting the viscosity to 4000 cP;

[0058] (5) coating the high-temperature resistant waterborne silicone coating with the adjusted viscosity on the surface of the metal substrate, then surface drying at room temperature for 30 min, then preliminary curing at a temperature of 80 °C for 30 min, then secondary curing at a temperature of 150 °C for 1 h, and complete curing at a temperature of 280 °C for 2 h.

[0059] Example 3

[0060] The formula composition of this example includes: core-shell emulsion: 350 g, modified high-temperature filler: (hollow glass microbead 168 g, yttrium oxide 56 g, cerium oxide 42 g, silicon powder 14 g) 280 g; functional aid (containing BYK-190, AQ-633E, BYK-028, TEGO-4100, titanium white, DPNB, hydroxyethyl cellulose): 55 g; deionized water: 305 g.

[0061] The raw materials of each component were weighed according to the above formula:

[0062] (1) The deionized water, dispersant, wetting agent and defoaming agent were mixed and stirred for 15 min;

[0063] (2) After adding the modified high-temperature filler, it was then dispersed at a speed of 1500 rpm for 1.5 h, and then ultrasonically treated at 40 kHz for 0.5 h;

[0064] (3) The core-shell emulsion and pigment were added in turn, and then stirred at a speed of 500 rpm for 20 min;

[0065] (4) The anti-settling agent, film-forming aid and thickening agent were added in turn, and then the viscosity was adjusted to 4000 cP;

[0066] (5) The high-temperature resistant waterborne silicone coating after adjusting the viscosity was coated on the surface of the metal substrate, and then air dried at room temperature for 30 min, then heated at 80°C for 30 min for preliminary curing, then heated at 150°C for 1 h for secondary curing, and heated at 280°C for 2 h for complete curing.

[0067] Example 4

[0068] The formula composition of this example includes: core-shell emulsion: 360 g, modified high-temperature filler: (hollow glass microbeads 180 g, yttrium oxide 45 g, cerium oxide 60 g, silicon powder 15 g) 280 g; functional aid (containing BYK-190, AQ-633E, BYK-028, TEGO-4100, titanium white, DPNB, hydroxyethyl cellulose): 70 g; deionized water: 270 g.

[0069] The raw materials of each component were weighed according to the above formula:

[0070] (1) The deionized water, dispersant, wetting agent and defoaming agent were mixed and stirred for 15 min;

[0071] (2) After adding the modified high-temperature filler (after the cerium oxide was co-modified with KH-550 and then pre-mixed with the dispersant), it was then dispersed at a speed of 1500 rpm for 1.5 h, and then ultrasonically treated at 40 kHz for 0.5 h;

[0072] (3) The core-shell emulsion and pigment were added in turn, and then stirred at a speed of 500 rpm for 20 min;

[0073] (4) The anti-settling agent, film-forming aid and thickening agent were added in turn, and then the viscosity was adjusted to 4000 cP;

[0074] (5) The high-temperature-resistant waterborne silicone coating with adjusted viscosity is coated on the surface of the metal substrate, then surface-dried at room temperature for 30 min, then preliminarily cured by heating at 80°C for 30 min, then secondarily cured by heating at 150°C for 1 h, and completely cured by heating at 280°C for 2 h.

[0075] Example 5

[0076] The formula composition of this example includes: core-shell emulsion: 370 g; modified high-temperature filler (containing hollow glass beads 204 g, yttrium oxide 54 g, cerium oxide 36 g, and silicon powder 8 g): 302 g; functional additives (containing BYK-190, AQ-633E, BYK-028, TEGO-4100, titanium white, DPNB, and hydroxyethyl cellulose): 70 g; deionized water: 260 g.

[0077] The raw materials of each component are weighed according to the above formula:

[0078] (1) The deionized water, dispersant, wetting agent, and defoaming agent are mixed and stirred for 15 min;

[0079] (2) After adding the modified high-temperature filler, then dispersed at a speed of 1500 rpm for 1.5 h, and then ultrasonically treated at 40 kHz for 0.3 h;

[0080] (3) The core-shell emulsion and pigment are added in sequence, and then stirred at a speed of 500 rpm for 20 min;

[0081] (4) The anti-settling agent, film-forming aid, and thickening agent are added in sequence, and then the viscosity is adjusted to 3500 cP;

[0082] (5) The high-temperature-resistant waterborne silicone coating with adjusted viscosity is coated on the surface of the metal substrate, then surface-dried at room temperature for 30 min, then preliminarily cured by heating at 80°C for 30 min, then secondarily cured by heating at 150°C for 1 h, and completely cured by heating at 280°C for 2 h.

[0083] Example 6

[0084] The formula composition of this example includes: core-shell emulsion: 370 g; modified high-temperature filler (containing hollow glass beads 204 g, yttrium oxide 54 g, cerium oxide 36 g, and silicon powder 8 g): 302 g; functional additives (containing BYK-190, AQ-633E, BYK-028, TEGO-4100, titanium white, DPNB, and hydroxyethyl cellulose): 70 g; deionized water: 260 g.

[0085] The raw materials of each component are weighed according to the above formula:

[0086] (1) mixing and stirring deionized water, dispersant, wetting agent and defoaming agent for 15 min;

[0087] (2) adding modified high-temperature filler, then dispersing at 1500 rpm for 1.5 h, and then ultrasonic treatment at 40 kHz for 0.5 h;

[0088] (3) sequentially adding core-shell emulsion and pigment, and then stirring at 500 rpm for 20 min;

[0089] (4) sequentially adding anti-settling agent, film-forming aid and thickening agent, and then adjusting the viscosity to 3500 cP-4500 cP;

[0090] (5) coating the high-temperature resistant waterborne organic silicon coating with adjusted viscosity on the surface of the metal substrate, then air drying at room temperature for 30 min, then preliminary curing at 80℃ for 30 min, then secondary curing at 150℃ for 1 h, and then complete curing at 280℃ for 2 h.

[0091] Comparative Example 1

[0092] The formula composition of the present comparative example includes: silicone resin: 400 g, alumina: 350 g, xylene: 200 g, and traditional auxiliary agent: 50 g.

[0093] Comparative Example 2

[0094] The formula composition of the present comparative example includes: core-shell emulsion: 370 g, alumina: 300 g, functional auxiliary agent: 70 g, and deionized water: 260 g.

[0095] The specific formula of the above-mentioned Examples 1-6 and Comparative Examples 1 and 2 of the present application is shown in Table 1 below:

[0096] Table 1

[0097]

[0098] The specific formula of the modified high-temperature filler in the above-mentioned Examples 1-6 of the present application is shown in Table 2 below:

[0099] Table 2

[0100]

[0101]

[0102] The specific formula of the core-shell emulsion in the above-mentioned Examples 1-6 of the present application is shown in Table 3 below:

[0103] Table 3

[0104]

[0105] The comprehensive performance tests of the above-mentioned embodiments 1 to 6 of the present application and comparative examples 1 and 2 are shown in Table 4 as follows:

[0106] Table 4

[0107]

[0108]

[0109] The present application has a significant higher infrared emissivity of 0.95 than that of embodiment 1 (0.92) and comparative examples (0.75-0.78) by adding cerium oxide 20% in the above-mentioned embodiment 4, and the heat dissipation efficiency is improved to reduce the substrate temperature by 50℃. That is, the addition of 20% cerium oxide and the modified hollow glass microbeads of KH-550 form a "radiation-thermal insulation" dual-mode barrier. The present application has an adhesion of 20.5 MPa, which is increased by 12.6% than that of embodiment 1 (18.2 MPa) and far exceeds that of comparative example 2 (9.2 MPa) by adding KH-550 1.5% in the above-mentioned embodiment 6. The solvent resistance is zero swelling, because the 1.5% KH-550 forms a dense coupling layer at the interface to prevent solvent penetration (swelling rate of comparative example 1 is 18%). The above-mentioned embodiment 1 has balanced comprehensive performance: the temperature resistance and the thermal conductivity coefficient are better than those of comparative examples 1 and 2, which verifies the synergistic effect of the core-shell structure and the filler compounding.

[0110] Analysis of the test results of the above-mentioned embodiments of the present application:

[0111] Temperature resistance: all embodiments pass the 700℃ / 500h test, while comparative example 1 (traditional organic silicon) fails at 300h, and comparative example 2 (single filler) fails at 400h, highlighting the high-temperature resistance advantage of the core-shell structure and the multi-scale filler.

[0112] Adhesion: the adhesion of embodiment 6 reaches 20.5 MPa due to excessive coupling of KH-550, which is increased by 123% than that of comparative example 2 (9.2 MPa), verifying the strengthening effect of the interface chemical bonding.

[0113] Thermal conductivity coefficient: the thermal conductivity coefficient of embodiment 4 is only 0.18 W / m·K due to the radiation heat dissipation of cerium oxide and the thermal insulation of hollow microbeads, which is decreased by 73.5% than that of comparative example 1 (0.68).

[0114] Environmental protection: the VOC emission of embodiment is less than 50g / L, which meets the environmental protection standard (GB 30981-2020), while the VOC emission of comparative example 1 is as high as 280g / L, which has a significant environmental protection defect.

[0115] In summary, the core-shell type polyimide-based high-temperature-resistant water-based paint realizes excellent thermal stability at 700 DEG C extreme high temperature by constructing a multi-layer thermal barrier through core-shell structure design (polyimide rigid core layer, benzimidazole / KH-550 transition layer and polyamide-imide protective shell layer), combining the gradient thermal resistance effect of modified high-temperature fillers (hollow glass microsphere heat insulation, yttrium / cerium synergistic oxidation resistance, silicon powder densification) and the stable crosslinking network formed by the step curing process. The core-shell interface hydrogen bond strengthening, filler-resin synergistic oxidation resistance and molecular level dispersion process make the coating have low thermal conductivity, strong adhesion, anti-cracking and thermal shock resistance. The present application significantly improves the high-temperature resistance, adhesion and environmental friendliness of the coating through the gradient design of core-shell emulsion, multi-scale filler synergism and low-temperature gradient curing process. The unexpected effects of the above-mentioned example 4 and example 6 further verify the innovativeness of the formula design, providing a technical breakthrough for extreme working condition protection.

[0116] The above description is only a description of the preferred embodiments of the present application, and is not any limitation on the scope of the present application, and any modification or modification made by a person skilled in the art according to the above disclosure is within the protection scope of the claims.

Claims

1. A core-shell type polyimide-based high-temperature-resistant water-based paint, characterized by, Consists of components by mass percentage: Core-shell emulsion: 35%~38%; the core layer is polyimide resin; the transition layer is organic silicone emulsion modified by 5,6-dimethyl benzimidazole and silane coupling agent KH-550; the shell layer is polyamide-imide resin; Modified high-temperature filler: 28%~32%; Functional auxiliary agent: 6%~7.5%; Deionized water: the balance.

2. The core-shell type polyimide-based high-temperature resistant water-based paint according to claim 1, characterized by, The preparation method of the core-shell emulsion comprises the following steps: Step S1: 5,6-dimethyl benzimidazole is added to the preheated silicone emulsion to 70℃, stirred at a speed of 600 rpm, then 5wt% silane coupling agent KH-550 ethanol solution is added dropwise at a rate of 0.5mL / min, and the reaction is kept for 1.5h to form a modified silicone emulsion; Step S2: polyimide resin is dissolved in pre-cooled N-methyl pyrrolidone, then non-ionic emulsifier Tween-80 is added, and ultrasonic emulsification is carried out at an alternating frequency of 25kHz and 40kHz for 20min to form a PI dispersion; Step S3: the above PI dispersion is added dropwise to the modified silicone emulsion at a rate of 2mL / min, the first stage is sheared at a speed of 10000rpm for 15min, the second stage is sheared at a speed of 6000rpm for 25min, and the system temperature is kept ≤35℃ during shearing to form a homogeneous composite emulsion; Step S4: polyamide-imide resin is premixed with N,N dimethylacetamide under nitrogen protection for 40min, dissolved at a temperature of 80℃~90℃, then emulsifier Span-80 is added, and defoaming treatment is carried out under a vacuum degree of-0.1MPa to form a PAI dispersion; Step S5: the above PAI dispersion is sieved through a 300 mesh sieve, then injected into the composite emulsion of step S3 in a laminar flow mode, the pH is adjusted to 8~8.5, and stirring is carried out at a speed of 800rpm for 30min at a temperature of 70℃, then stirring is carried out at a speed of 500rpm for 1.5h~2h.

3. The core-shell type polyimide-based high-temperature resistant water-based paint according to claim 2, characterized by, In the step S1, the mass ratio of the silicone emulsion, 5,6-dimethyl benzimidazole and silane coupling agent KH-550 is 85~95:1.5~2.5:0.8~1.2; In the step S2, the mass ratio of the polyimide resin, N-methyl pyrrolidone and non-ionic emulsifier Tween-80 is 1:3.8~4.2:0.6~0.7; In the step S3, the mass ratio of the PI dispersion and modified silicone emulsion is 1:0.7~0.8; In the step S4, the mass ratio of the polyamide-imide resin, N,N dimethylacetamide and emulsifier Span-80 is 1:0.18~0.2:5.2~5.8; In the step S5, the mass ratio of the PAI dispersion and composite emulsion is 1:0.35~0.

45.

4. The core-shell type polyimide-based high-temperature resistant water-based paint according to claim 1, characterized by, The preparation method of the modified high-temperature filler comprises the following steps: The hollow glass microbeads, yttrium oxide and silicon powder are mixed with 8wt% silane coupling agent KH-550 ethanol solution, stirred at a speed of 400rpm at a temperature of 80℃ for 1h, sieved through a 200 mesh sieve after vacuum drying, and reserved for use; The cerium oxide is premixed with dispersant, ball milled with sand mill at 300 rpm for 2h to particle size D50 <0.8μm, ready for use.

5. The core-shell type polyimide-based high-temperature resistant water-based paint according to claim 1, characterized by, The molecular weight of the polyimide resin is selected from 20000g / mol to 25000g / mol.

6. The core-shell type polyimide-based high-temperature resistant water-based paint according to claim 4, characterized by, The hollow glass microsphere is selected from particle size of 10μm to 80μm and thermal conductivity coefficient <0.15W / (m·K); the particle size D50 of the yttrium oxide <500nm; the particle size D50 of the cerium oxide <1μm; the particle size D50 of the silicon powder <5μm.

7. The core-shell type polyimide-based high-temperature resistant water-based paint according to claim 1, characterized by, The functional additives are composed of dispersant: 1%~1.1%; anti-settling agent: 0.6%~0.7%; defoaming agent: 0.15%~0.2%; wetting agent: 0.3%~0.4%; pigment: 2.5%~3%; film forming aid: 1%~1.1% and thickening agent: 0.7%~0.8%. The dispersant is selected from BYK-190; the anti-settling agent is selected from Disparlon AQ-633E; the defoaming agent is selected from BYK-028; the wetting agent is selected from Degussa TEGO-4100; the pigment is selected from one or several of titanium dioxide, iron oxide black, iron oxide red and chrome yellow; the film forming aid is selected from one of DPNB, DB and PPH; the thickening agent is selected from hydroxyethyl cellulose.

8. The core-shell type polyimide-based high-temperature resistant water-based paint according to claim 7, characterized by, The method comprises the following steps:

9. A method for producing a coating layer from the core-shell polyimide-based high-temperature-resistant water-based paint according to any one of claims 1 to 8, characterized by, Step one: mix and stir the deionized water, dispersant, wetting agent and defoaming agent for 15min; Step two: add the modified high temperature filler, then disperse at 1500rpm for 1.5h, then ultrasonic treatment at 40kHz for 0.3h~0.5h; Step three: add the core-shell emulsion and pigment in sequence, then stir at 500rpm for 20min; Step four: add the anti-settling agent, film forming aid and thickening agent in sequence, then adjust the viscosity to 3500cP~4500cP; Step five: coat the core-shell type polyimide based high temperature resistant water-based paint with adjusted viscosity on the surface of metal substrate, then air dry at room temperature for 30min, then preliminary curing at 80℃ for 30min, then secondary curing at 150℃ for 1h, and complete curing at 280℃ for 2h. ​

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