A low temperature-cured polyamide-imide varnish, its preparation method and use

Abstract

The application provides a low-temperature cured polyamide-imide varnish as well as a preparation method and application thereof, raw materials of the polyamide-imide varnish include aromatic tricarboxylic anhydride, aromatic diisocyanate and a compounded organic solvent, the compounded organic solvent includes a first type of organic solvent and a second type of organic solvent, the first type of organic solvent includes any one or a combination of at least two of dimethyl sulfoxide, N-dimethylacetamide, N-methyl pyrrolidone or gamma-butyrolactone, and the second type of organic solvent includes aliphatic polyesters and / or aliphatic cyclic ketones; by selecting the specific parts of the above-mentioned compounded organic solvent, the obtained polyamide-imide varnish has long-term storage stability and a relatively low curing temperature, and meanwhile, the varnish film formed after curing also has excellent dielectric properties and mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of coating technology, specifically relating to a low-temperature curing polyamide-imide varnish, its preparation method, and its application. Background Technology

[0002] Polyamide-imide is widely used as a protective layer for various substrates due to its excellent thermodynamic, electrical and chemical stability properties, such as high-temperature resistant insulating coatings for white goods, high-temperature resistant varnishes for enameled wires and insulating electrical tapes.

[0003] Currently, conventional polyamide-imide is prepared by high-temperature condensation of triphenyl benzoic anhydride and isocyanate in a polar solvent. Commonly used polar solvents include organic solvents such as NMP and DMAc. For example, CN101397477A discloses a wire coating varnish, particularly relating to a wire coating varnish with high-temperature resistance. This invention uses a method to synthesize polyamide-imide wire coating varnish by reacting trimellitic anhydride and isocyanate in a polar solvent. The ratio of MDI to TMA used in this invention can be MDI / TMA = 1:0.85 to 1.05, the amount of solvent added is MDI / solvent = 1:5.0 to 10.0, and the ratio of diluent added can be MDI / diluent = 1:1.5 to 4.5. The reaction process of this invention does not require the addition of a catalyst, and the ring can be closed without a curing agent during the high-temperature oven baking process. This invention has a short process flow, simple operation, and the synthesized PAI solution has the advantages of low viscosity and stable storage.

[0004] However, the solvents such as NMP and DMAc used in the above invention have high boiling points, requiring high processing temperatures during curing, which is not economical. Although the curing temperature can be lowered by adding aromatic solvents, the amount of aromatics added is very limited. At the same time, the addition of aromatics will greatly increase the viscosity and storage stability of the slurry, which is not conducive to subsequent processability. In addition, these solvents contain nitrogen, and during high-temperature processing, the solvent will decompose or partially decompose while evaporating, producing a large amount of nitrogen oxides, which is not environmentally friendly.

[0005] Therefore, developing a polyamide-imide varnish with a low curing temperature and excellent storage performance remains a pressing technical problem in this field. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the present invention aims to provide a low-temperature curing polyamide-imide varnish, its preparation method and application, wherein the polyamide-imide varnish has long-term storage stability and a low curing temperature, and the varnish film formed after curing has excellent insulation and mechanical properties.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a low-temperature curing polyamide-imide varnish, wherein the raw materials for preparing the polyamide-imide varnish include aromatic tricarboxylic anhydride, aromatic diisocyanate and compounded organic solvent;

[0009] The compounded organic solvent includes a first type of organic solvent and a second type of organic solvent;

[0010] The first type of organic solvent includes any one or a combination of at least two of dimethyl sulfoxide (DMSO), N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), or γ-butyrolactone;

[0011] The second type of organic solvent includes aliphatic polyesters and / or aliphatic cyclic ketones.

[0012] This invention selects a specific compounded organic solvent as the reaction medium for aromatic tricarboxylic anhydride and aromatic diisocyanate, resulting in a polyamide-imide varnish with long-term storage stability and a low curing temperature. The cured varnish film has excellent insulation and mechanical properties. At the same time, the organic solvent volatilized during the curing process has a lower or no nitrogen content in the exhaust gas after incineration, which is more environmentally friendly.

[0013] It should be noted that the "low-temperature curing polyamide-imide varnish" mentioned in this invention refers to a polyamide-imide varnish with a curing temperature not exceeding 400°C.

[0014] Preferably, the aromatic tricarboxylic anhydride includes any one or a combination of at least two of trimellitic anhydride, benzophenone tricarboxylic anhydride, or diphenylmethane tricarboxylic anhydride.

[0015] Preferably, the aromatic diisocyanate includes any one or a combination of at least two of 4,4'-diphenylmethane diisocyanate, toluene diisocyanate, naphthalene diisocyanate, phenyl dimethylene diisocyanate, biphenyl diisocyanate, diphenyl sulfone diisocyanate, or diphenyl ether diisocyanate.

[0016] Preferably, the molar ratio of acidic groups to isocyanate groups in the raw materials is (0.95 to 1.05):1, for example, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1:1, 1.01:1, 1.02:1, 1.03:1 or 1.04:1, etc.

[0017] Preferably, the mass ratio of the first type of organic solvent to the second type of organic solvent is (1-9):(9-1), for example, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1 or 9:1, etc.

[0018] Preferably, the compounded organic solvent includes a combination of a first type of organic solvent, an aliphatic polyester, and an aliphatic cyclic ketone. Selecting the above three organic solvents for combination can better dissolve polyamide-imide, giving it better storage stability, and thus better insulation properties after curing.

[0019] Preferably, the mass ratio of the aliphatic polyester to the aliphatic cyclic ketone is 1:(0.1 to 10), for example, 1:0.1, 1:0.5, 1:1, 1:2, 1:4, 1:6, 1:8 or 1:10.

[0020] Preferably, the aliphatic polyester comprises any one or a combination of at least two of the following: propylene glycol monomethyl ether acetate (PGMEA), propylene glycol methyl ether propionate, methyl glycol ethyl ether acetate, propylene glycol ethyl ether acetate, ethylene glycol ethyl ether, ethyl ethoxylate, 1,2-propanediol diacetate, ethylene glycol diacetate, 2,3-butanediol diacetate, ethyl acetate diethoxylate, methyl dimethoxyacetate, methyl 4-methoxybutyrate, methyl methoxyacetate, diethyl ethyl malonate, tert-butyl ethyl malonate, or dimethyl malonate.

[0021] Preferably, the aliphatic cyclic ketone includes any one or a combination of at least two of the following: cyclopentanone, cyclohexanone (CYC), 2-methylcyclopentanone, 1,3-cyclopentanedione, 2-methyl-1,3-cyclopentanedione, 2-ethylcyclopentanone, 2,2-dimethylcyclopentanone, 4-methylcyclohexanone, 1,3-cyclohexanedione, 4-propylcyclohexyl ketone, 4-tert-butylcyclohexanone, 4-ethylcyclohexanone, 2-methyl-1,3-cyclohexanedione, 1,4-cyclohexanedione, p-methoxycyclohexanone, 4,4-dimethylcyclohexanone, 2,2-dimethylcyclohexanone, 5,5-dimethyl-1,3-cyclohexanedione, or 5-methylcyclohexane-1,3-dione.

[0022] Preferably, the raw materials used in the preparation also include aromatic diacids.

[0023] Preferably, the aromatic diacid includes any one or at least a combination of two of terephthalic acid, isophthalic acid, biphenyl acid, or 4,4'-dicarboxylic acid diphenyl ether.

[0024] Preferably, the solid content of the polyamide-imide varnish is 20-45%, such as 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, or 44%.

[0025] In a second aspect, the present invention provides a method for preparing polyamide-imide varnish as described in the first aspect, the method comprising: reacting an aromatic diisocyanate, an aromatic tricarboxylic anhydride and optionally an aromatic diacid in a compounded organic solvent to obtain the polyamide-imide varnish.

[0026] Preferably, the reaction is carried out under gradient heating conditions.

[0027] Preferably, the gradient heating method includes: first heating the system to 40-100℃ (e.g., 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃ or 90℃, etc.), holding the reaction at that temperature for 1-5 hours (e.g., 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours or 4.5 hours, etc.), then heating it to 110-170℃ (e.g.), holding the reaction at that temperature for 3-15 hours, thus completing the gradient heating.

[0028] Thirdly, a low-temperature curing enameled wire, the low-temperature curing enameled wire comprising a conductor and a polyamide-imide varnish as described in the first aspect, which is cured and then covers the conductor.

[0029] It should be noted that the "low-temperature curing enameled wire" mentioned in this invention refers to an enameled wire whose polyamide-imide varnish covering the conductor has a curing temperature of no more than 450°C.

[0030] Preferably, the curing temperature is not higher than 400℃, such as 400℃, 390℃, 380℃, 370℃, 360℃ or 350℃.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) The raw materials for preparing the polyamide-imide varnish provided by the present invention include aromatic tricarboxylic anhydride, aromatic diisocyanate and compounded organic solvent. The compounded solvent includes a first type of organic solvent and a second type of organic solvent. The first type of organic solvent includes any one or a combination of at least two of dimethyl sulfoxide, N-dimethylacetamide, N-methylpyrrolidone or γ-butyrolactone. The second type of organic solvent includes aliphatic polyester and / or aliphatic cyclic ketone. By selecting the above specific compounded organic solvent, the obtained polyamide-imide varnish has good compatibility before curing, and thus has long-term storage stability. At the same time, it also has a low curing temperature, which can effectively reduce the processing temperature of enameled wire without reducing the excellent mechanical properties and insulation properties of the polyamide-imide varnish itself.

[0033] (2) The polyamide-imide varnish provided by the present invention contains far less nitrogen oxides when the volatilized organic solvent is burned during the preparation of insulating wires than existing products, which is environmentally friendly.

[0034] (3) The low-temperature curing polyamide-imide paint provided by the present invention can be used as a topcoat or primer, and can be used alone or in combination with conventional polyimide, polyamide-imide and polyester-imide paints. Detailed Implementation

[0035] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0036] Unless otherwise specified, the raw materials involved in the following specific embodiments of the present invention are all conventional materials in the art and can be purchased commercially; at the same time, the test methods and test instruments used are also common test methods and commonly used test instruments in the art.

[0037] Example 1

[0038] A low-temperature curing polyamide-imide varnish is prepared by adding 12 kg NMP, 1 kg PGMEA and 0.5 kg CYC to a 30 L reactor, followed by adding 5.091 kg diphenylmethane diisocyanate. When the mixture is stirred and heated to 80 °C, 3.909 kg trimellitic anhydride is added to the reactor. The system temperature is maintained at 80 °C, and the reaction is carried out for 3 h.

[0039] Add 4 kg of PGMEA to the reactor, heat to 90°C and react for 3 hours, then heat the system to 150°C and react for 2 hours to obtain the low-temperature curing polyamide-imide varnish.

[0040] Example 2

[0041] A low-temperature curing polyamide-imide varnish is prepared by adding 8 kg NMP, 3.5 kg PGMEA and 0.5 kg CYC to a 30 L reactor, followed by adding 5.091 kg diphenylmethane diisocyanate. When the mixture is stirred and heated to 80 °C, 3.909 kg trimellitic anhydride is added to the reactor. The system temperature is maintained at 80 °C, and the reaction is carried out for 3 h.

[0042] Add 4 kg of PGMEA to the reactor, heat to 90°C and react for 3 hours, then heat the system to 150°C and react for 2 hours to obtain the low-temperature curing polyamide-imide varnish.

[0043] Example 3

[0044] A low-temperature curing polyamide-imide varnish is prepared by adding 6 kg NMP, 5.5 kg PGMEA and 0.5 kg CYC to a 30 L reactor, followed by adding 5.091 kg diphenylmethane diisocyanate. When the mixture is stirred and heated to 80 °C, 3.909 kg trimellitic anhydride is added to the reactor. The system temperature is maintained at 80 °C, and the reaction is carried out for 3 h.

[0045] Add 4 kg of PGMEA to the reactor, heat to 90°C and react for 3 hours, then heat the system to 150°C and react for 2 hours to obtain the low-temperature curing polyamide-imide varnish.

[0046] Example 4

[0047] A low-temperature curing polyamide-imide varnish differs from Example 2 in that an equal amount of DMAc is used to replace NMP, while other substances, amounts, and steps are the same as in Example 2.

[0048] Example 5

[0049] A low-temperature curing polyamide-imide varnish is prepared by: adding 8 kg DMAc, 3.5 kg PGMEA and 0.5 kg CYC to a 30 L reactor, followed by adding 5.115 kg diphenylmethane diisocyanate, stirring and heating to 80 °C, then adding 3.545 kg trimellitic anhydride and 0.34 kg isophthalic acid to the reactor, maintaining the system temperature at 80 °C, and reacting for 3 h;

[0050] Add 4 kg of PGMEA to the reactor, heat to 90°C and react for 3 hours, then heat the system to 150°C and react for 2 hours to obtain the low-temperature curing polyamide-imide varnish.

[0051] Example 6

[0052] A low-temperature curing polyamide-imide varnish is prepared by adding 8 kg NMP and 2 kg PGMEA to a 30 L reactor, followed by adding 5.091 kg diphenylmethane diisocyanate, stirring and heating to 80 °C, then adding 3.909 kg trimellitic anhydride to the reactor, maintaining the system temperature at 80 °C, and reacting for 3 h.

[0053] Add 4 kg of PGMEA to the reactor, heat to 90°C and react for 3 hours, then heat the system to 150°C and react for 2 hours to obtain the low-temperature curing polyamide-imide varnish.

[0054] Example 7

[0055] A low-temperature curing polyamide-imide varnish differs from Example 6 in that DMAc is used instead of NMP to obtain the low-temperature curing polyamide-imide varnish, while other substances, amounts, and steps are the same as in Example 6.

[0056] Example 8

[0057] A low-temperature curing polyamide-imide varnish is prepared by adding 8.5 kg NMP and 4.25 kg CYC to a 30 L reactor, followed by adding 5.091 kg diphenylmethane diisocyanate. While stirring and heating to 80 °C, 3.909 kg trimellitic anhydride is added to the reactor. The system temperature is maintained at 80 °C, and the reaction is carried out for 3 h.

[0058] Add 4.25 kg of CYC to the reactor, heat to 90°C and react for 3 hours, then heat the system to 150°C and react for 2 hours to obtain the low-temperature curing polyamide-imide varnish.

[0059] Example 9

[0060] A low-temperature curing polyamide-imide varnish differs from Example 8 in that DMAc is used instead of NMP, while the other substances, amounts, and steps are the same as in Example 8.

[0061] Comparative Example 1

[0062] A low-temperature curing polyamide-imide varnish is prepared by means of: adding 12 kg NMP to a 30 L reactor, followed by adding 5.091 kg diphenylmethane diisocyanate, stirring and heating to 80 °C, then adding 3.909 kg trimellitic anhydride to the reactor, maintaining the system temperature at 80 °C, reacting for 3 h, then adding 4 kg NMP to the reactor, raising the temperature to 90 °C and reacting for 3 h, and then raising the system temperature to 150 °C and reacting for 2 h to obtain the low-temperature curing polyamide-imide varnish.

[0063] Comparative Example 2

[0064] A low-temperature curing polyamide-imide varnish is prepared by means of: adding 12 kg of DMAc to a 30 L reactor, followed by adding 5.091 kg of diphenylmethane diisocyanate, stirring and heating to 80 °C, then adding 3.909 kg of trimellitic anhydride to the reactor, maintaining the system temperature at 80 °C, reacting for 3 h, then adding 4 kg of DMAc to the reactor, raising the temperature to 90 °C and reacting for 3 h, and then raising the system temperature to 150 °C and reacting for 2 h to obtain the low-temperature curing polyamide-imide varnish.

[0065] Comparative Example 3

[0066] A low-temperature curing polyamide-imide varnish is prepared by means of: adding 8 kg NMP and 4 kg DMAc to a 30 L reactor, then adding 5.091 kg diphenylmethane diisocyanate, stirring and heating to 80 °C, then adding 3.909 kg trimellitic anhydride to the reactor, maintaining the system temperature at 80 °C, reacting for 3 h, then adding 4 kg DMAc to the reactor, raising the temperature to 90 °C and reacting for 3 h, then raising the system temperature to 150 °C and reacting for 2 h to obtain the low-temperature curing polyamide-imide varnish.

[0067] Comparative Example 4

[0068] A low-temperature curing polyamide-imide varnish is prepared by means of: adding 12 kg PGMEA to a 30 L reactor, followed by adding 5.091 kg diphenylmethane diisocyanate, stirring and heating to 80 °C, then adding 3.909 kg trimellitic anhydride to the reactor, maintaining the system temperature at 80 °C, reacting for 3 h, then adding 4 kg PGMEA to the reactor, raising the temperature to 90 °C and reacting for 3 h, then raising the system temperature to 150 °C and reacting for 2 h to obtain the low-temperature curing polyamide-imide varnish.

[0069] Comparative Example 5

[0070] A method for preparing an adhesive mixture includes: adding 12.75 kg of CYC to a 30 L reactor, followed by adding 5.091 kg of diphenylmethane diisocyanate, stirring and heating to 80 °C, then adding 3.909 kg of trimellitic anhydride to the reactor, maintaining the system temperature at 80 °C, reacting for 3 h, then adding 4.25 kg of CYC to the reactor, raising the temperature to 90 °C and reacting for 3 h, and then raising the system temperature to 150 °C and reacting for 2 h to obtain the adhesive mixture.

[0071] Comparative Example 6

[0072] A low-temperature curing polyamide-imide varnish is prepared by means of: adding 8.5 kg PGMEA and 4.25 kg CYC to a 30 L reactor, followed by adding 5.091 kg diphenylmethane diisocyanate, stirring and heating to 80 °C, then adding 3.909 kg trimellitic anhydride to the reactor, maintaining the system temperature at 80 °C, reacting for 3 h, then adding 4.25 kg CYC to the reactor, raising the temperature to 90 °C and reacting for 3 h, then raising the system temperature to 150 °C and reacting for 2 h to obtain the low-temperature curing polyamide-imide varnish.

[0073] Application Example 1

[0074] A low-temperature curing insulated wire includes a copper conductor with a diameter of 0.65 mm and a primer layer and a topcoat layer sequentially wrapped around the copper conductor.

[0075] The primer is a commercially available polyamide-imide paint (BOYA AIP 36UN); the topcoat is a low-temperature curing polyamide-imide varnish provided in Example 1.

[0076] The low-temperature curing insulation wire preparation process provided in this application example is as follows: The process uses a Sun H5000-1T-20D enameled wire machine with the following parameters: oven inlet: 300℃, oven middle: 350℃, oven outlet: 380℃, and wire speed: 18m / min.

[0077] Application Examples 2-9

[0078] A low-temperature curing insulated wire, which differs from Application Example 1 in that the low-temperature curing polyamide-imide varnish provided in Examples 2 to 9 is used instead of the low-temperature curing polyamide-imide varnish provided in Example 1, while other processes, conditions and parameters are the same as in Application Example 1.

[0079] Comparative Application Examples 1-6

[0080] A low-temperature curing insulated wire, which differs from Application Example 1 in that the low-temperature curing polyamide-imide varnish provided in Comparative Examples 1 to 6 is replaced with the polyamide-imide varnish provided in Example 1, while the other processes, conditions and parameters are the same as in Application Example 1.

[0081] Performance testing:

[0082] (1) Testing of polyamide-imide varnish

[0083] ① Solid content: Take an aluminum foil tray with a diameter of 60mm and weigh it as m0; take 1±0.1g of paint sample and place it in the aluminum tray, weigh the aluminum tray plus paint as m1; place the aluminum tray containing paint in a forced-air drying oven at 180±5℃ for 1 hour, then take it out and weigh the aluminum tray containing paint as m2; Solid content = (m2-m0) / (m1-m0)×100%;

[0084] ② Viscosity: The viscosity of the paint was tested at 25°C using a Boller Feld cone-plate viscometer.

[0085] ③ Appearance: Visual inspection;

[0086] ④ Storage stability: Observe whether the paint separates after being placed at room temperature for 30 days.

[0087] The varnishes provided in Examples 1-9 and Comparative Examples 1-6 were tested according to the above test methods, and the test results are shown in Table 1:

[0088] Table 1

[0089]

[0090]

[0091] According to the data in Table 1:

[0092] (1) The solid content of the polyamide-imide varnishes provided in Examples 1 to 9 is 27.3 to 32.4%, the viscosity is 1530 to 2580 cp, and the appearance is brownish-black transparent liquid. The storage stability is very uniform. According to the viscosity comparison of Examples 1 to 3, it can be seen that the viscosity of the obtained polyamide-imide varnish will gradually increase as the amount of the second type of solvent increases.

[0093] (2) The polyamide-imide varnishes provided in Comparative Examples 1 to 3 only added the first type of solvent, resulting in a lower overall viscosity. In contrast, the polyamide-imide varnishes provided in Comparative Examples 4 to 6 only added the second type of solvent, resulting in a higher viscosity and poorer storage stability. After being placed at room temperature for 30 days, the varnishes showed stratification and sedimentation.

[0094] (2) Testing of insulated wires

[0095] ① Elongation: The test was conducted according to the test method provided in the national standard GB / T 4074.3-2008;

[0096] ② Coating film continuity: The leakage current test was conducted according to the test method provided in the national standard GB / T4074.5-2008;

[0097] ③ Pressure resistance: The test is conducted according to the test method provided in the national standard GB / T 4074.21-2018, to test whether it will soften and break down at 400℃;

[0098] ④ Adhesion: The test shall be conducted in accordance with the test method provided in standard JB / T 4279.6;

[0099] ⑤ Dielectric loss: The test shall be conducted in accordance with the test method provided in standard JB / T 4074.3-2008;

[0100] ⑥ 15% stretching and 1d winding: The test shall be conducted in accordance with the test method provided in standard GB / T6109;

[0101] ⑦ Voltage: Test according to the test method provided in standard GB / T6109.

[0102] The low-temperature curing insulated wires provided in test cases 1-9 and comparative application examples 1-6 were tested according to the above test methods. The test results are shown in Table 2.

[0103] Table 2

[0104]

[0105] In Table 2, " / " indicates that the test cannot be performed.

[0106] According to the data in Table 2:

[0107] (1) The voltage value of the insulated wires provided in Application Examples 1 to 5 is 7.4 to 7.6 cp, the elongation is as high as 37 to 39%, no pores were found in the varnish continuity test, no cracks were found in the 15% 1d winding, no breakdown was found in the withstand voltage test, the adhesion is as high as 7.3 to 7.4 N, and the dielectric loss is 282.7 to 296.4 °C. Under the same process conditions, the combination of the first type of solvent and the second type of solvent is beneficial to reducing the curing temperature of polyamide-imide varnish, thereby increasing the dielectric loss of the obtained insulated wire and reducing its energy consumption. At the same time, the addition of the second type of solvent can also effectively reduce the amount of nitrogen-containing solvent, ensuring performance while reducing the generation of nitrogen oxides, which is beneficial to environmental protection.

[0108] (2) Compared with Application Example 1, the insulation wires provided in Application Examples 1 to 3 all cracked during the 15% 1d tensile winding test and were all broken down during the withstand voltage test, indicating that the winding performance and withstand voltage performance were poor.

[0109] (3) Compared with Application Example 1, the polyamide-imide varnish provided in Comparative Examples 4 to 6 was too high, making it impossible to make insulated wires for further testing.

[0110] (4) Compared with Application Example 1, the dielectric loss of the insulated wires provided in Application Examples 6 to 9 also deteriorated, indicating that the combination of the first type of organic solvent, aliphatic polyester and aliphatic cyclic ketone is more effective.

[0111] The applicant declares that this invention illustrates a low-temperature curing polyamide-imide varnish, its preparation method, and its application through the above embodiments. However, this invention is not limited to the above embodiments, meaning that this invention does not necessarily rely on the above embodiments for implementation. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection and disclosure scope of this invention.

Claims

1. A low-temperature curing polyamide-imide varnish, characterized in that, The raw materials for preparing the polyamide-imide varnish include aromatic tricarboxylic anhydride, aromatic diisocyanate, and a compounded organic solvent; The compounded organic solvent includes a first type of organic solvent and a second type of organic solvent; The mass ratio of the first type of organic solvent to the second type of organic solvent is (1~9):(9~1); The first type of organic solvent is any one or a combination of at least two of dimethyl sulfoxide, N-dimethylacetamide or N-methylpyrrolidone; The second category of organic solvents includes aliphatic polyesters and aliphatic cyclic ketones; The mass ratio of the aliphatic polyester to the aliphatic cyclic ketone is 1:(0.1~10); The aliphatic polyesters include any one or a combination of at least two of the following: propylene glycol monomethyl ether acetate, propylene glycol methyl ether propionate, methyl glycol ethyl ether acetate, propylene glycol ethyl ether acetate, ethylene glycol ethyl ether, ethoxyethyl acetate, 1,2-propanediol diacetate, ethylene glycol diacetate, 2,3-butanediol diacetate, ethyl acetate, dimethoxyethyl acetate, methyl 4-methoxybutyrate, methyl methoxyacetate, diethyl ethyl malonate, tert-butyl ethyl malonate, or dimethyl malonate. The aliphatic cyclic ketones include any one or a combination of at least two of the following: cyclopentanone, cyclohexanone, 2-methylcyclopentanone, 1,3-cyclopentanedione, 2-methyl-1,3-cyclopentanedione, 2-ethylcyclopentanone, 2,2-dimethylcyclopentanone, 4-methylcyclohexanone, 1,3-cyclohexanedione, 4-propylcyclohexylone, 4-tert-butylcyclohexanone, 4-ethylcyclohexanone, 2-methyl-1,3-cyclohexanedione, 1,4-cyclohexanedione, p-methoxycyclohexanone, 4,4-dimethylcyclohexanone, 2,2-dimethylcyclohexanone, 5,5-dimethyl-1,3-cyclohexanedione, or 5-methylcyclohexane-1,3-dione.

2. The polyamide-imide varnish according to claim 1, characterized in that, The aromatic tricarboxylic anhydrides include trimellitic anhydride.

3. The polyamide-imide varnish according to claim 1, characterized in that, The aromatic diisocyanate includes any one or a combination of at least two of 4,4'-diphenylmethane diisocyanate, toluene diisocyanate, naphthalene diisocyanate, phenylmethylene diisocyanate, or biphenyl diisocyanate.

4. The polyamide-imide varnish according to claim 1, characterized in that, The molar ratio of acidic groups to isocyanate groups in the raw materials used for preparation is (0.95~1.05):

1.

5. The polyamide-imide varnish according to claim 1, characterized in that, The raw materials used in the preparation also include aromatic diacids.

6. The polyamide-imide varnish according to claim 5, characterized in that, The aromatic diacid includes any one or at least a combination of two of terephthalic acid, isophthalic acid, biphenyl acid, or 4,4'-dicarboxylic acid diphenyl ether.

7. The polyamide-imide varnish according to claim 1, characterized in that, The solid content of the polyamide-imide varnish is 20-45%.

8. A method for preparing a polyamide-imide varnish as described in any one of claims 1 to 7, characterized in that, The preparation method includes: reacting aromatic diisocyanate, aromatic tricarboxylic anhydride and optionally aromatic diacid in a compounded organic solvent to obtain the polyamide-imide varnish.

9. The preparation method according to claim 8, characterized in that, The reaction is carried out under gradient heating conditions.

10. The preparation method according to claim 9, characterized in that, The gradient heating method includes: first heating the system to 40~100℃ and holding the temperature for 1~5 h, then heating it to 110~170℃ and holding the temperature for 3~15 h to complete the gradient heating.

11. A low-temperature curing enameled wire, characterized in that, The low-temperature curing enameled wire includes a conductor and a polyamide-imide varnish as described in any one of claims 1 to 7, which is cured and then covers the conductor.

12. The low-temperature curing enameled wire according to claim 11, characterized in that, The curing temperature should not exceed 400℃.

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