Environment-friendly polyamide-imide insulating paint, preparation method and application thereof

Abstract

The application discloses an environment-friendly polyamide-imide insulating paint and a preparation method and application thereof. The polyamide-imide insulating paint comprises the following components in percentage by weight: 20-30% of polyamide-imide resin, 70-80% of solvent; the solvent comprises butyrolactone, one or both of propylene carbonate and dimethyl acid dimethyl ester, and one or more of dimethylbenzene and high-boiling-point petroleum solvent; the butyrolactone accounts for 50-80% of the weight percentage of the solvent. The application uses a solvent without nitrogen element to replace an original nitrogen-containing solvent for production of the polyamide-imide insulating paint, adjusts types and proportions of raw materials for synthesizing the polyamide-imide resin, so that normal synthesis reaction is ensured, and the environment-friendly polyamide-imide insulating paint is produced, so as to be applied to the enameled wire industry and reduce emission of nitrogen oxide compounds in the enameled wire operation process.

Description

Technical Field

[0001] This invention relates to the field of polyamide-imide insulating varnish technology, specifically to an environmentally friendly polyamide-imide insulating varnish, its preparation method, and its application. Background Technology

[0002] Polyamide-imide is a modified polyimide resin that is widely used in the automotive, electronics, and aerospace industries due to its excellent heat resistance, flexibility, chemical stability, and abrasion resistance. Polyamide-imide is also used to make polyamide-imide insulating varnish, which is mainly used in the enameled wire industry.

[0003] Currently, the synthesis of polyamide-imide mainly uses nitrogen-containing solvents. The polyamide-imide resin in polyamide-imide insulating varnish is dissolved in N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), or N,N-dimethylformamide (DMF). During the production of enameled wire, the solvent decomposes into NO, NO2, or other nitrogen oxides after high-temperature baking in the enameling oven and is then emitted. Currently, when polyamide-imide insulating varnish is used in enameled wire, its nitrogen oxide emissions are approximately 1500–2000 mg / m³. 3 Nitrogen oxides can damage the ozone layer and form ozone holes, which is extremely detrimental to the ecological environment. Therefore, there is an urgent need to develop an environmentally friendly polyamide-imide insulating varnish with low nitrogen oxide emissions. Summary of the Invention

[0004] To address the aforementioned technical problems, the present invention aims to provide an environmentally friendly polyamide-imide insulating varnish, its preparation method, and its application. This invention utilizes a nitrogen-free solvent instead of the original nitrogen-containing solvent in the production of the polyamide-imide insulating varnish, thereby reducing nitrogen oxide emissions during the enameled wire processing. However, commonly used polyamide-imide resins exhibit poor solubility and reactivity in nitrogen-free solvents. Therefore, the present invention also adjusts the types and proportions of raw materials used in the synthesis of polyamide-imide resin to improve the solubility of the polyamide-imide resin in the nitrogen-free solvent, ensuring the normal progress of the synthesis reaction and thus producing an environmentally friendly polyamide-imide insulating varnish for application in the enameled wire industry.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution:

[0006] A polyamide-imide insulating varnish, comprising, by weight percentage: 20%–30% polyamide-imide resin and 70%–80% solvent;

[0007] The polyamide-imide resin is obtained by reacting an acid anhydride, an acid, and an isocyanate; the acid is adipic acid (AA) or trimellitic acid;

[0008] The solvent includes:

[0009] Butyrolactone;

[0010] One or both of propylene carbonate and dimethyl dicarboxylate; and

[0011] One or more of xylene and high-boiling-point petroleum solvents;

[0012] The butyrolactone constitutes 50% to 80% of the solvent by weight.

[0013] This invention employs different solvent combinations to improve the solubility and reactivity of nitrogen-free solvents with polyamide-imide resins. On one hand, different solvents can enhance the viscosity reduction effect of the resin. Combining different solvents can reduce the viscosity of the insulating varnish while maintaining the solid content, making subsequent coating operations easier. On the other hand, different solvents have different boiling points. During the curing process of the enameled wire, the different boiling points of the solvents can effectively control the evaporation rate of the solvents, thereby ensuring the appearance of the enameled wire.

[0014] The polyamide-imide resin of the present invention differs from existing polyamide-imide resins in that it introduces isocyanates with different structures, thereby ensuring normal reaction in a reaction system dominated by ester solvents, and achieving the goal of low nitrogen oxide emissions while ensuring the appearance and properties of the coating.

[0015] Furthermore, the molecular weight of the polyamide-imide resin is 40,000 to 50,000 Da.

[0016] Furthermore, the acid anhydride is selected from one or more of trimellitic anhydride (TMA), maleic anhydride, and hexahydrophthalic anhydride, preferably TMA.

[0017] Furthermore, the isocyanate includes 4,4-diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI), and toluene diisocyanate (TDI).

[0018] TMA and MDI are the main reactants, HDI can improve the winding properties, and TDI can improve the solubility of the resin and solvent.

[0019] Furthermore, the molar ratio of the anhydride to the acid is 1:(0.01~0.5). The polyamide-imide structure contains amide and imine groups. The imine groups are mainly obtained through the reaction of anhydrides, and the amide groups are mainly obtained through the reaction of carboxylic acids. TMA contains both anhydrides and carboxylic acids. By replacing part of the anhydride with an acid, the proportion of amide groups in the polyamide-imide resin will increase, thereby improving the winding properties.

[0020] Furthermore, the molar ratio of the acid anhydride to the isocyanate is 1:(0.5 to 1.5).

[0021] In one embodiment of the present invention, the raw materials for synthesizing polyamide-imide resin, by molar percentage, include: TMA 10%–50%, MDI 10%–50%, HDI 0.5%–15%, TDI 0.5%–15%, and AA 1%–20%.

[0022] In one embodiment of the present invention, the raw materials for synthesizing polyamide-imide resin, by molar percentage, include: TMA 40%–50%, MDI 40%–50%, HDI 0.5%–10%, TDI 0.5%–10%, and AA 1%–10%.

[0023] In one embodiment of the present invention, the raw materials for synthesizing polyamide-imide resin, by molar percentage, include: TMA 45%–50%, MDI 40%–45%, HDI 1%–5%, TDI 1%–5%, and AA 1%–5%.

[0024] Furthermore, the propylene carbonate accounts for 0% to 30% of the solvent by weight.

[0025] Furthermore, the dimethyl diacid ester accounts for 0% to 30% of the solvent by weight.

[0026] Furthermore, the xylene accounts for 0% to 30% of the solvent by weight, preferably 10% to 30%.

[0027] Furthermore, the high-boiling-point petroleum solvent accounts for 0% to 30% of the solvent by weight, preferably 10% to 20%.

[0028] Furthermore, the high-boiling-point petroleum solvent includes trimethylbenzene, 100# aromatic solvent oil, and 150# aromatic solvent oil.

[0029] In one embodiment of the present invention, the butyrolactone accounts for 50% to 80% of the solvent by weight, the propylene carbonate and / or dimethyl dicarboxylate account for 1% to 30% of the solvent by weight, and the xylene and / or high-boiling-point petroleum solvent accounts for 10% to 30% of the solvent by weight.

[0030] In one embodiment of the present invention, the butyrolactone accounts for 50% to 80% of the solvent by weight, the propylene carbonate and / or dimethyl dicarboxylate account for 10% to 30% of the solvent by weight, and the xylene and / or high-boiling-point petroleum solvent accounts for 10% to 20% of the solvent by weight.

[0031] A method for preparing a polyamide-imide insulating varnish includes the following steps:

[0032] A first solvent, acid anhydride, acid, and isocyanate are added to a reaction vessel, and the temperature is gradually increased under an inert atmosphere to carry out the reaction. After cooling to 60-80°C, a second solvent is added for dilution to obtain the polyamide-imide insulating varnish.

[0033] The first solvent includes:

[0034] Butyrolactone; and

[0035] One or both of propylene carbonate and dimethyl dicarboxylate;

[0036] The acid is adipic acid (AA) or trimellitic acid;

[0037] The second solvent includes one or more of xylene and high-boiling-point petroleum solvents;

[0038] The specific method of the staged temperature increase is as follows: raise the temperature to 60-70℃ within 1-2 hours, maintain the temperature at 60-70℃ for 1-3 hours, raise the temperature to 80-90℃ within 2-4 hours, raise the temperature to 100-120℃ within 2-4 hours, and raise the temperature to 130-140℃ within 1-3 hours.

[0039] Furthermore, the reaction is carried out at a constant temperature of 130–140°C, and the degree of polymerization (molecular weight) of the product is tested. When the molecular weight reaches the desired range, the reaction is stopped.

[0040] Furthermore, the reaction can also be stopped by adding a first solvent, which accounts for 5% to 10% of the solvent by weight, to lower the temperature to 60 to 80°C.

[0041] The staged heating in the preparation method of this invention is to prevent the risk of material overflow and spillage caused by violent reaction. If the temperature is directly raised to 130-140°C, it is impossible to prepare a good polyamide-imide insulating varnish, and the reaction is relatively violent in the middle, which poses a safety hazard.

[0042] Furthermore, the acid anhydride is selected from one or more of trimellitic anhydride (TMA), maleic anhydride and hexahydrophthalic anhydride, preferably TMA, and the isocyanate includes 4,4-diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI).

[0043] TMA and MDI are the main reactants, HDI can improve the winding properties, and TDI can improve the solubility of the resin and solvent.

[0044] Furthermore, the molar ratio of the anhydride to the acid is 1:(0.01~0.5), and the acid can replace part of the anhydride and play a role in improving the winding properties.

[0045] Furthermore, the molar ratio of the acid anhydride to the isocyanate is 1:(0.5 to 1.5).

[0046] In one embodiment of the present invention, the raw materials for synthesizing polyamide-imide resin, by molar percentage, include: TMA 10%–50%, MDI 10%–50%, HDI 0.5%–15%, TDI 0.5%–15%, and AA 1%–20%.

[0047] In one embodiment of the present invention, the raw materials for synthesizing polyamide-imide resin, by molar percentage, include: TMA 40%–50%, MDI 40%–50%, HDI 0.5%–10%, TDI 0.5%–10%, and AA 1%–10%.

[0048] In one embodiment of the present invention, the raw materials for synthesizing polyamide-imide resin, by molar percentage, include: TMA 45%–50%, MDI 40%–45%, HDI 1%–5%, TDI 1%–5%, and AA 1%–5%.

[0049] In one embodiment of the present invention, the butyrolactone accounts for 50% to 80% of the solvent by weight, the propylene carbonate and / or dimethyl dicarboxylate account for 1% to 30% of the solvent by weight, and the xylene and / or high-boiling-point petroleum solvent accounts for 10% to 30% of the solvent by weight.

[0050] In one embodiment of the present invention, the butyrolactone accounts for 50% to 80% of the solvent by weight, the propylene carbonate and / or dimethyl dicarboxylate account for 10% to 30% of the solvent by weight, and the xylene and / or high-boiling-point petroleum solvent accounts for 10% to 20% of the solvent by weight.

[0051] In a specific embodiment, a first solvent is added to a reaction vessel, and acid anhydride, acid, and isocyanate are added under an inert atmosphere. The temperature is gradually increased to carry out the reaction. After cooling to 60-80°C, a second solvent is added for dilution to obtain the polyamide-imide insulating varnish.

[0052] In a specific embodiment, a first solvent (accounting for 50% to 60% of the total solvent by weight, the total solvent being the first solvent and the second solvent) is added to a reaction vessel. An acid anhydride, an acid, and an isocyanate are added under an inert atmosphere. The temperature is gradually increased to carry out the reaction. The first solvent (accounting for 10% to 30% of the total solvent by weight) is added and the temperature is lowered to 60 to 80°C. The second solvent (accounting for 10% to 30% of the total solvent by weight) is added for dilution to obtain the polyamide-imide insulating varnish.

[0053] This invention also protects the application of the above-mentioned environmentally friendly polyamide-imide insulating varnish in the preparation of enameled wires.

[0054] Compared with the prior art, the beneficial effects of the present invention are:

[0055] This invention uses a nitrogen-free solvent instead of the original nitrogen-containing solvent in the production of polyamide-imide insulating varnish. The types and proportions of raw materials for synthesizing polyamide-imide resin are adjusted to improve the solubility of the nitrogen-free solvent in the polyamide-imide resin, so as to ensure the normal progress of the synthesis reaction. This produces an environmentally friendly polyamide-imide insulating varnish for application in the enameled wire industry, reducing nitrogen oxide emissions during the enameled wire processing. Detailed Implementation

[0056] Unless otherwise defined, 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. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0057] The present invention will be further described below with reference to specific embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0058] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used are commercially available.

[0059] In the following examples, gel permeation chromatography was used to test the degree of polymerization (molecular weight).

[0060] Example 1

[0061] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 66% butyrolactone, 20% dimethyl dicarboxylate, and 14% xylene.

[0062] The polyamide-imide insulating varnish of Example 1 was prepared according to the following method:

[0063] Butyrolactone and dimethyl diacid were added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 48%, MDI 42%, HDI 4%, TDI 4%, AA 2%) were added while stirring. The temperature was raised to 65°C after 1 hour and maintained at 65°C for 2 hours. After 3 hours, the temperature was raised to 85°C. Within 3 hours, the temperature was raised to 110°C. Within 2 hours, the temperature was raised to 130°C and the reaction was carried out at a constant temperature of 130°C. The degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 41500 Da, butyrolactone and dimethyl diacid were added and the temperature was lowered to 80°C to stop the reaction. Xylene was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0064] Example 2

[0065] A polyamide-imide insulating varnish comprises, by weight percentage: 20% polyamide-imide resin and 80% solvent; wherein the solvent comprises, by weight percentage: 50% butyrolactone, 30% propylene carbonate, and 20% xylene.

[0066] The polyamide-imide insulating varnish of Example 2 was prepared according to the following method:

[0067] Butyrolactone and propylene carbonate were added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 42%, MDI 43%, HDI 5%, TDI 5%, AA 5%) were added while stirring. The temperature was raised to 65°C after 1 hour and maintained at 65°C for 2 hours. After 3 hours, the temperature was raised to 85°C. Within 3 hours, the temperature was raised to 110°C. Within 2 hours, the temperature was raised to 130°C and the reaction was carried out at a constant temperature of 130°C. The degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 47890 Da, butyrolactone and propylene carbonate were added and the temperature was lowered to 80°C to stop the reaction. Xylene was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0068] Example 3

[0069] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 75% butyrolactone, 15% dimethyl dicarboxylate, and 10% 100# aromatic solvent oil.

[0070] The polyamide-imide insulating varnish of Example 3 was prepared according to the following method:

[0071] Butyrolactone and dimethyl diacid were added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 50%, MDI 40%, HDI 4%, TDI 4%, AA 2%) were added while stirring. The temperature was raised to 65℃ after 1 hour and maintained at 65℃ for 2 hours. After 3 hours, the temperature was raised to 85℃. Within 3 hours, the temperature was raised to 110℃. Within 2 hours, the temperature was raised to 130℃ and the reaction was carried out at a constant temperature of 130℃. The degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 40500 Da, butyrolactone and dimethyl diacid were added and the temperature was lowered to 80℃ to stop the reaction. 100# aromatic solvent oil was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into barrels.

[0072] Example 4

[0073] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent, by weight percentage, comprises: 70% butyrolactone, 8% propylene carbonate, 7% dimethyl dicarboxylate, 10% xylene, and 5% 100# aromatic solvent oil.

[0074] The polyamide-imide insulating varnish of Example 4 was prepared according to the following method:

[0075] Butyrolactone, propylene carbonate, and dimethyl diacid were added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI, and AA (by molar percentage: TMA 48%, MDI 42%, HDI 4%, TDI 4%, AA 2%) were added while stirring. The temperature was raised to 65°C after 1 hour and maintained at 65°C for 2 hours. After 3 hours, the temperature was raised to 85°C, and within 3 hours, the temperature was raised to 110°C. Within 2 hours, the temperature was raised to 130°C and the reaction was carried out at a constant temperature of 130°C. The degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 42000 Da, butyrolactone, propylene carbonate, and dimethyl diacid were added, and the temperature was lowered to 80°C to stop the reaction. Xylene and 100# aromatic solvent oil were added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into drums.

[0076] Comparative Example 1

[0077] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 80% butyrolactone and 20% xylene.

[0078] The polyamide-imide insulating varnish of Comparative Example 1 was prepared according to the following method:

[0079] Butyrolactone was added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 48%, MDI 42%, HDI 4%, TDI 4%, AA 2%) were added while stirring. The temperature was raised to 65℃ after 1 hour and maintained at 65℃ for 2 hours. After 3 hours, the temperature was raised to 85℃. Within 3 hours, the temperature was raised to 110℃. Within 2 hours, the temperature was raised to 130℃ and the reaction was carried out at a constant temperature of 130℃. The degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 42800 Da, butyrolactone was added and the temperature was lowered to 80℃ to stop the reaction. Xylene was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0080] Comparative Example 2

[0081] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 80% butyrolactone and 20% 100# aromatic solvent oil.

[0082] The polyamide-imide insulating varnish of Comparative Example 2 was prepared according to the following method:

[0083] Butyrolactone was added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 48%, MDI 45%, HDI 2%, TDI 3%, AA 2%) were added while stirring. The temperature was raised to 65℃ after 1 hour and maintained at 65℃ for 2 hours. The temperature was raised to 85℃ after 3 hours, then raised to 110℃ within 3 hours, and then raised to 130℃ within 2 hours. The reaction was carried out at a constant temperature of 130℃, and the degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 42560 Da, butyrolactone was added and the temperature was lowered to 80℃ to stop the reaction. 100# solvent oil was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0084] Comparative Example 3

[0085] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 80% dimethyl diacid and 20% xylene.

[0086] The polyamide-imide insulating varnish of Comparative Example 3 was prepared according to the following method:

[0087] Dimethyl diacid was added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 48%, MDI 42%, HDI 4%, TDI 4%, AA 2%) were added while stirring. The temperature was raised to 65°C after 1 hour and maintained at 65°C for 2 hours. The temperature was raised to 85°C after 3 hours, then raised to 110°C within 3 hours, and then raised to 130°C within 2 hours. The reaction was carried out at a constant temperature of 130°C, and the degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 43000 Da, dimethyl diacid was added and the temperature was lowered to 80°C to stop the reaction. Xylene was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0088] Comparative Example 4

[0089] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 20% butyrolactone, 66% dimethyl dicarboxylate, and 14% xylene.

[0090] The polyamide-imide insulating varnish of Comparative Example 4 was prepared according to the following method:

[0091] Butyrolactone and dimethyl dicarboxylate were added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 48%, MDI 42%, HDI 4%, TDI 4%, AA 2%) were added while stirring. The temperature was raised to 65°C after 1 hour and maintained at 65°C for 2 hours. The temperature was raised to 85°C after 3 hours, then raised to 110°C within 3 hours, and then raised to 130°C within 2 hours. The reaction was carried out at a constant temperature of 130°C, and the degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 41980 Da, butyrolactone was added and the temperature was lowered to 80°C to stop the reaction. Xylene was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0092] Comparative Example 5

[0093] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 66% butyrolactone, 20% dimethyl dicarboxylate, and 14% xylene.

[0094] The polyamide-imide insulating varnish of Comparative Example 5 was prepared according to the following method:

[0095] Butyrolactone and dimethyl diacid were added to the reactor. Under a nitrogen atmosphere, TMA, MDI and AA (by molar percentage: TMA 48%, MDI 50%, AA 2%) were added while stirring. The temperature was raised to 65°C after 1 hour and maintained at 65°C for 2 hours. The temperature was raised to 85°C after 3 hours, then raised to 110°C within 3 hours, and then raised to 130°C within 2 hours. The reaction was carried out at a constant temperature of 130°C, and the degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 44500 Da, butyrolactone and dimethyl diacid were added, and the temperature was lowered to 80°C to stop the reaction. Xylene was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0096] Comparative Example 6

[0097] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 66% butyrolactone, 20% dimethyl dicarboxylate, and 14% xylene.

[0098] The polyamide-imide insulating varnish of Comparative Example 6 was prepared according to the following method:

[0099] Butyrolactone and dimethyl diacid were added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI and TDI (by molar percentage: TMA 50%, MDI 42%, HDI 4%, TDI 4%) were added while stirring. The temperature was raised to 65°C after 1 hour and maintained at 65°C for 2 hours. After 3 hours, the temperature was raised to 85°C. Within 3 hours, the temperature was raised to 110°C. Within 2 hours, the temperature was raised to 130°C and the reaction was carried out at a constant temperature of 130°C. The degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 57800 Da, butyrolactone and dimethyl diacid were added and the temperature was lowered to 80°C to stop the reaction. Xylene was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0100] Comparative Example 7

[0101] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 66% butyrolactone, 20% dimethyl diacid, and 20% diphenol.

[0102] The polyamide-imide insulating varnish of Comparative Example 7 was prepared according to the following method:

[0103] Butyrolactone and dimethyl diacid were added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 48%, MDI 42%, HDI 4%, TDI 4%, AA 2%) were added while stirring. The temperature was raised to 65℃ after 1 hour and maintained at 65℃ for 2 hours. After 3 hours, the temperature was raised to 85℃. Within 3 hours, the temperature was raised to 110℃. Within 2 hours, the temperature was raised to 130℃ and the reaction was carried out at a constant temperature of 130℃. The degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 43500 Da, butyrolactone and dimethyl diacid were added and the temperature was lowered to 80℃ to stop the reaction. Diphenol was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0104] Comparative Example 8

[0105] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 66% butyrolactone, 20% dimethyl dicarboxylate, and 14% xylene.

[0106] The polyamide-imide insulating varnish of Comparative Example 8 was prepared according to the following method:

[0107] Butyrolactone and dimethyl diacid were added to the reactor. Under a nitrogen atmosphere, TMA, MDI, HDI, TDI and AA (by molar percentage: TMA 48%, MDI 42%, HDI 4%, TDI 4%, AA 2%) were added while stirring. The temperature was raised to 130℃ within 11 hours and the reaction was carried out at a constant temperature of 130℃. The degree of polymerization (molecular weight) was tested. When the molecular weight of the polyamide-imide resin reached 47600 Da, butyrolactone and dimethyl diacid were added and the temperature was lowered to 80℃ to stop the reaction. Xylene was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0108] Comparative example: Commercial polyamide-imide insulating varnish

[0109] A polyamide-imide insulating varnish comprises, by weight percentage: 30% polyamide-imide resin and 70% solvent; wherein the solvent comprises, by weight percentage: 60% NMP, 15% DMAC, 10% DMF, and 15% xylene.

[0110] The polyamide-imide insulating varnish of the comparative example was prepared according to the following method:

[0111] NMP and DMAC were added to the reactor. TMA and MDI (50% TMA, 50% MDI by molar percentage) were added while stirring under a nitrogen atmosphere. The temperature was raised to 65°C after 1 hour and maintained at 65°C for 2 hours. The temperature was raised to 85°C after 3 hours, then raised to 110°C within 3 hours, and then raised to 130°C within 2 hours. The reaction was carried out at a constant temperature of 130°C to test the degree of polymerization (molecular weight). When the molecular weight of the polyamide-imide resin reached 41500 Da, NMP was added and the temperature was lowered to 80°C to stop the reaction. DMF was added for dilution and final property adjustment. After adjustment, the mixture was filtered and packaged into containers.

[0112] Test Example 1

[0113] The reaction system state, coating appearance, and state after 3 months of storage at room temperature of the polyamide-imide insulating varnishes of Examples 1-4, Comparative Examples 1-8, and Control Examples are observed and the results are shown in Table 1:

[0114] Table 1

[0115]

[0116]

[0117] Since the polyamide-imide insulating varnishes prepared in Comparative Examples 3-5 and Comparative Example 7 had a cloudy appearance, their viscosity over time was not tested.

[0118] The solvent in Comparative Example 3 did not contain butyrolactone, and the solvent in Comparative Example 4 had a reduced butyrolactone content. The resulting coating was cloudy, demonstrating that dimethyl diacidate has poorer solubility for polyamide-imide resin than butyrolactone. The raw materials for synthesizing polyamide-imide resin in Comparative Example 5 did not contain HDI and TDI, resulting in poorer solubility between the resin and solvent, and both the reaction system and the coating were cloudy. The raw materials for synthesizing polyamide-imide resin in Comparative Example 6 did not contain AA, resulting in a higher molecular weight of the resin and increased viscosity after 3 months of storage at room temperature. This is because AA has an aliphatic structure with a long linear chain, which can effectively regulate reactivity and subsequent coating preservation during the reaction with NCO groups. Comparative Example 7 used diphenol instead of xylene, resulting in a cloudy coating, indicating that diphenol has poor solubility for polyamide-imide. Comparative Example 8 did not use a staged heating method for the reaction, resulting in a more vigorous reaction, severe liquid level rise, and a risk of overflow. Furthermore, it was difficult to control the desired molecular weight during degree of polymerization testing.

[0119] Test Example 2

[0120] The polyamide-imide insulating varnishes of Examples 1, 1-2, and the control example were used to prepare enameled wires. The preparation method was as follows:

[0121] A heat exchange enameling furnace is used, with a furnace length of 3.8 meters, furnace inlet and outlet temperatures of 440℃ / 480℃, wire gauge of 0.4mm, wire speed of 12m / min, and 7 coating cycles.

[0122] The prepared enameled wires were tested for relevant wire properties (enameled wire testing standard: GB / T6109.14-2008 / IEC60317-26:1990). A third-party testing agency was commissioned to place nitrogen oxide detection equipment at the enameling furnace exhaust port during the enameling process to detect the nitrogen oxide content. The test results are shown in Table 2.

[0123] Table 2

[0124]

[0125]

[0126] As shown in Table 2, the nitrogen oxide emissions of Example 1 and Comparative Examples 1-2 were significantly lower than those of the control example, meeting the requirements of the Integrated Emission Standard for Air Pollutants DB32 / 4041—2021 (nitrogen oxide emission standard is 200 mg / m³). 3 The enameled wires prepared in Comparative Examples 1 and 2 have particles on their surface because they only have two solvents, and the proportion of high-boiling-point solvent is relatively high. During the enameling process, the amount of solvent volatilization is large when the coating passes through the evaporation section, which easily leads to particles on the surface of the wire. However, Example 1 uses three solvents. By using the different boiling points of the three solvents, the solvent evaporation rate when entering the evaporation section can be effectively controlled, thereby ensuring that the enameled wire achieves a better appearance. Therefore, the appearance of the enameled wire prepared in Example 1 is comparable to that of the control example. In terms of properties, the enameled wire prepared in Example 1 has better tensile strength (adhesion) and softening resistance (heat resistance) than the control example.

[0127] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A polyamide-imide insulating varnish, characterized in that, By weight percentage, the polyamide-imide insulating varnish comprises the following components: 20%~30% polyamide-imide resin and 70%~80% solvent; The polyamide-imide resin is obtained by reacting anhydride, acid, and isocyanate; the acid is adipic acid or trimellitic acid; the isocyanate includes 4,4-diphenylmethane diisocyanate, hexamethylene diisocyanate, and toluene diisocyanate; the molar ratio of the anhydride to the acid is 1:(0.01~0.5). The solvent includes: Butyrolactone; One or both of propylene carbonate and dimethyl dicarboxylate; and One or more of xylene and high-boiling-point petroleum solvents; The butyrolactone accounts for 50% to 80% of the solvent by weight, the propylene carbonate and / or dimethyl dicarboxylate account for 1% to 30% of the solvent by weight, and the xylene and / or high-boiling-point petroleum solvent account for 10% to 30% of the solvent by weight.

2. A method for preparing a polyamide-imide insulating varnish, characterized in that, Includes the following steps: A first solvent, acid anhydride, acid, and isocyanate are added to a reaction vessel, and the temperature is gradually increased under an inert atmosphere to carry out the reaction. After cooling to 60-80 °C, a second solvent is added for dilution to obtain the polyamide-imide insulating varnish. The first solvent includes: Butyrolactone; and One or both of propylene carbonate and dimethyl dicarboxylate; The acid is adipic acid or trimellitic acid; The second solvent includes one or more of xylene and high-boiling-point petroleum solvents; The specific method of the staged temperature increase is as follows: raise the temperature to 60-70 ℃ within 1-2 hours, maintain the temperature at 60-70 ℃ for 1-3 hours, raise the temperature to 80-90 ℃ within 2-4 hours, raise the temperature to 100-120 ℃ within 2-4 hours, and raise the temperature to 130-140 ℃ within 1-3 hours.

3. The preparation method according to claim 2, characterized in that, The acid anhydride is selected from one or more of trimellitic anhydride, maleic anhydride and hexahydrophthalic anhydride, and the isocyanate includes 4,4-diphenylmethane diisocyanate, hexamethylene diisocyanate and toluene diisocyanate.

4. The preparation method according to claim 2 or 3, characterized in that, The molar ratio of the anhydride to the acid is 1:(0.01~0.5).

5. The preparation method according to claim 2 or 3, characterized in that, The molar ratio of the acid anhydride to the isocyanate is 1:(0.5~1.5).

6. The application of the polyamide-imide insulating varnish according to claim 1 or the polyamide-imide insulating varnish prepared by the method according to any one of claims 2 to 5 in the preparation of enameled wire.

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