Polyimide varnish and its preparation method

Abstract

This invention belongs to the field of polymer materials and discloses a polyimide varnish, which is mainly obtained by an imidization polymerization reaction of a thermosetting polyamic acid precursor solution capped with a crosslinking end-capping agent, N-vinylimidazolium, a free radical initiator, and an imidization accelerator. The thermosetting polyamic acid precursor solution capped with the crosslinking end-capping agent is obtained by polymerizing aromatic diamines and aromatic dianhydrides in an aprotic solvent to obtain a thermosetting polyamic acid precursor solution, followed by adding the crosslinking end-capping agent. This polyimide varnish has moderate viscosity, good adhesion and film-forming properties, and excellent bonding with copper conductors. The preparation method of this polyimide varnish is simple to operate, low in cost, and has a high imidization film-forming rate.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials, and particularly relates to a polyimide varnish and its preparation method. Background Technology

[0002] Drive motors are the core components of new energy electric vehicles. Currently, flat-wire motors have become the mainstream development trend for new energy electric vehicle motors due to their advantages such as high power density, high integration, low cost, and high efficiency. While flat-wire motors improve the slot fill factor, they also significantly increase the temperature resistance requirements of the insulating varnish used for the enameled wire. Polyimide insulating varnish, with its excellent high-temperature resistance and flame-retardant properties, has become a technical solution for insulation materials in high-voltage flat-wire motors.

[0003] Enamel film adhesion refers to the strength of the bond between the bare copper wire and the enamel film, and it is one of the most important performance indicators of enameled wire. If there are problems with the adhesion between the enamel film and the conductor, then the insulation between the conductors will be severely compromised.

[0004] The structure of enameled copper flat wire: The inner layer is a copper flat wire conductor. Depending on the performance requirements and application fields of the enameled copper flat wire, the copper flat wire conductor is coated with two or three layers of insulating varnish. The varnish specifically includes a primer layer, a corona-resistant or low-dielectric varnish layer, and a topcoat layer. The primer layer provides a tight bond between the bare copper wire and the varnish.

[0005] Patent document CN115424770A discloses a high-voltage load wire. The high-voltage load wire includes a conductor and a double-layer structure consisting of a high-density bonding layer and a surge-resistant layer located near the conductor. The double-layer structure comprises a high-density bonding layer covering the outer periphery of the conductor and a surge-resistant layer located on the surface of the high-density bonding layer. The varnish used to form the high-density bonding layer includes one of polyesterimide varnish, polyamideimide varnish, polyimide varnish, polyamide varnish, polyvinyl alcohol formal varnish, polyurethane varnish, and polyester varnish. From the perspective of high-temperature resistance, polyimide varnish is the optimal high-density bonding layer, but this patent does not disclose the specific composition of the high-density bonding layer varnish.

[0006] Patent document CN103596360A discloses a flexible adhesive-free circuit board substrate and its manufacturing method. The flexible adhesive-free circuit board substrate comprises a vinyl imidazole compound and a thermoplastic polyimide adhesive, wherein the vinyl imidazole compound is 0.5%-2% by mass of the liquid thermoplastic polyimide adhesive. Under the action of heat or plasma, the vinyl imidazole will break the vinyl double bond and form a bond with the amide group in the thermoplastic polyimide adhesive. The bonded imidazole group forms a coordination bond with copper, ensuring good adhesion between copper and the polymer film. However, heat treatment (170-220℃, 1-2h) or plasma treatment is not suitable for enameled wire coating application processes.

[0007] Therefore, polyimide varnishes, which have a simple manufacturing process and high bonding strength with copper conductors, are still in demand. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the background art above, and to provide a polyimide varnish with a simple manufacturing process and excellent bonding with copper conductors, as well as a method for preparing the same.

[0009] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0010] A polyimide varnish is mainly obtained by mixing a thermosetting polyamic acid precursor solution capped with a crosslinking end-capping agent, N-vinylimidazolium, a free radical initiator, and an imidization accelerator evenly through stirring. The thermosetting polyamic acid precursor solution capped with the crosslinking end-capping agent is obtained by polymerizing aromatic diamine and aromatic dianhydride in an aprotic solvent to obtain a thermosetting polyamic acid precursor solution, and then adding the crosslinking end-capping agent to react.

[0011] This invention ensures rapid crosslinking between the carbon-carbon unsaturated double bond end-capping agent and N-vinylimidazole by introducing a free radical initiator. Imidazole groups are incorporated into the PI structure via free radical polymerization, and these groups form coordinate bonds with copper, ensuring a tight bond between the varnish and the copper conductor. The introduction of an imidization accelerator improves the imidization film-forming rate and film-forming properties of the varnish. N-vinylimidazole is grafted onto the polyimide molecular chain via free radical polymerization, reacting with the carbon-carbon unsaturated double bond crosslinking end-capping agent. This results in the final polyimide varnish exhibiting excellent adhesion to the copper conductor.

[0012] Preferably, in the above-mentioned polyimide varnish, the thermosetting polyamic acid precursor solution capped by the cross-linking capping agent has a solid content of 25%~35% and a viscosity of 80~250 Pa•S at 23±2℃.

[0013] Preferably, the aromatic diamine monomer is selected from at least one of 4,4'-diaminodiphenyl ether (4,4-ODA), 3,4'-diaminodiphenyl ether (3,4-ODA), p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-bis(3-aminophenoxy)diphenyl sulfone, and 4,4'-diaminodiphenylmethane.

[0014] Preferably, the aromatic dianhydride monomer is selected from at least one of 1,2,4,5-pyromellitic dianhydride (PMDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxophthalic anhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride, and bisphenol A dianhydride; Preferably, the aprotic solvent is selected from at least one of N,N'-dimethylformamide, N,N'-dimethylacetamide, and N-methylpyrrolidone.

[0015] Preferably, the crosslinking end-capping agent is a carbon-carbon unsaturated double bond end-capping agent, selected from at least one of maleic anhydride and norbornene.

[0016] Preferably, the free radical initiator is a peroxide free radical initiator, selected from at least one of benzoyl peroxide, cyclohexanone peroxide, lauroyl peroxide, tert-amyl peroxybenzoate, tert-butyl hydroperoxide, dicumyl peroxide, and cumyl perhydrogen peroxide. The amount of the free radical initiator is 0.1 to 1% of the total weight of the above-mentioned crosslinking end-capping agent and N-vinylimidazole.

[0017] Preferably, the iminolation promoter is a tertiary amine catalyst, more preferably a heterocyclic tertiary amine catalyst and / or an aliphatic tertiary amine catalyst, and most preferably one or more of pyridine and its derivatives, methylpyridine and its derivatives, β-pyrrolline, dimethylpyridine, quinoline, isoquinoline, imidazole, triethylamine, triethanolamine, and N,N-dimethylethanolamine; the amount of the iminolation promoter added is 2 to 4% of the total weight of the aromatic diamine and the aromatic dianhydride.

[0018] Preferably, the molar ratio of the aromatic diamine to the aromatic dianhydride is 100:(95~99), the molar ratio of the crosslinking end-capping agent to the aromatic diamine is 100:(2~10), and the molar ratio of the crosslinking end-capping agent to N-vinylimidazole is 100:(50~100).

[0019] Based on a general inventive concept, the present invention also provides a method for preparing polyimide varnish, comprising the following steps: (1) Add solvent and aromatic diamine monomer to reaction vessel. After the aromatic diamine monomer dissolves, add aromatic dianhydride to reaction vessel in batches to carry out polymerization reaction. Then add cross-linking end-capping agent to carry out end-capping reaction to obtain thermosetting polyamic acid precursor solution end-capped by cross-linking end-capping agent. In this process, the viscosity and solid content of polyamic acid precursor solution are controlled by the amount of aromatic dianhydride added and the amount of solvent added. (2) Add a thermosetting polyamic acid precursor solution capped with a cross-linking capping agent to the reaction vessel, and then add N-vinylimidazol, peroxide free radical initiator and imidization accelerator in sequence. Stir and mix evenly to obtain the polyimide varnish.

[0020] In the above-mentioned method for preparing polyimide varnish, preferably, in step (1), the polymerization reaction temperature is 40~80℃ and the reaction time is 2~8h; the end-capping reaction temperature is 10~40℃ and the reaction time is 1~4h.

[0021] Preferably, in step (2), the stirring temperature is 0~5℃ and the stirring time is 0.5~2h.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The polyimide varnish provided by this invention has moderate viscosity, good adhesion and film-forming properties, and excellent bonding with copper conductors.

[0023] 2. The polyimide varnish provided by the present invention has a high solid content (25-35%) and a suitable viscosity (100-250 Pa•S), which can reduce the number of coating layers for copper flat wires and reduce production costs.

[0024] 3. The preparation method of the present invention is simple to operate, low in cost, and has a high imidization film formation rate. Detailed Implementation

[0025] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0026] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0027] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0028] Example 1: A method for preparing a polyimide varnish includes the following steps: (1) Preparation of maleic anhydride-terminated thermosetting polyamic acid precursor solution: 6007.20g of ODA (4,4'-diaminodiphenyl ether) was dissolved in DMAc (N,N'-dimethylacetamide) in a reaction vessel, and 6478.16g of PMDA (pyromellitic dianhydride) was added in batches to carry out the polymerization reaction. The reaction was carried out at 80℃ for 8h, and then 58.83g of maleic anhydride was added. The reaction was then carried out at 35℃ for 4h. The total amount of solvent DMAc added was 37632.59g, and the solid content of the polyamic acid precursor solution was adjusted to 25%. (2) Preparation of polyimide varnish: Add 56.472g of N-vinylimidazolium, 1.15g of dicumyl peroxide and 125g of pyridine to the reaction vessel, stir at 5℃ for 1h for imidization polymerization reaction, and mix evenly.

[0029] Examples 2-3: The preparation process of the thermosetting polyamic acid precursor solution and the formulation process of the polyimide varnish in Examples 2 and 3 were carried out in accordance with the preparation example 1. The specific feeding ratio and reaction process are shown in Table 1.

[0030] Comparative Examples 1-4: The preparation process of the thermosetting polyamic acid precursor solution and the formulation process of the polyimide varnish in Comparative Examples 1-4 were carried out in accordance with Preparation Example 1. The specific feeding ratio and reaction process are shown in Table 1.

[0031] Table 1: Feed ratios and reaction processes of Examples 1-3 and Comparative Examples 1-3

[0032] The following properties of Examples 1-3 and Comparative Examples 1-4 were measured and evaluated. The results are shown in Table 2.

[0033] Solid content (wt%): Take a clean weighing dish and record its weight as m0. Pour approximately 3.0g of sample solution onto the weighing dish and record its weight as m1. Then, place the weighing dish in a 180℃ oven and heat for 60 minutes. Measure the weight after heat treatment and record it as m2. The solid content (wt%) can be calculated using the formula (m2-m0) / (m1-m0)×100%. Take 3 samples and calculate their average value.

[0034] Viscosity (Pa•S): Place the sample in a 250ml plastic container and incubate it in a constant temperature water bath at 23℃. Measure the sample viscosity using a rotational viscometer when the sample temperature reaches 23±2℃ using a digital thermometer. The unit is Pa•S. Perform the test three times and calculate the average value.

[0035] Storage stability, viscosity change (η%): The sample was placed in a 250ml plastic container and stored in a cold storage below 5℃ for 3 months. The initial viscosity was recorded as η0, and the viscosity after three months was recorded as η3. Viscosity change = |1-η3 / η0| ×100%. If the viscosity change is less than 15%, the storage stability is considered good.

[0036] Film formation performance: The sample was coated onto a glass substrate using a coating applicator, and then the substrate was placed in a thermal cycling oven for imidization film formation. The state of the formed film was visually evaluated. Films without powdering, cracks, or swelling were rated ○, otherwise rated ×.

[0037] Fabrication of enameled flat wire: Enameled flat wire is fabricated using the aforementioned polyimide varnish. The specific method involves casting, stretching, drawing, and softening copper to obtain 2.50... A 4.00mm copper flat wire is coated with polyimide varnish prepared according to the above method on the outer circumference of the copper flat wire through a mold. It is then baked into a film under the conditions of an oven inlet temperature of 100℃ and an outlet temperature of 450℃. After multiple coats of paint, a paint film with a total thickness of 0.12-0.13mm is formed.

[0038] Adhesion: Referring to GB / T 4074.3-2008, cut a 400mm enameled flat wire around the middle of the sample until the copper conductor is reached. After stretching by 15%, test the distance at which the insulation loses adhesion.

[0039] Table 2. Thin film properties of Examples 1-3 and Comparative Examples 1-4

[0040] As can be seen from Table 2, Examples 1-3 and Comparative Examples 1-4 exhibited uniform and good film-forming properties and storage stability, with Examples 1-3 showing better adhesion compared to Comparative Examples 1-4.

Claims

1. A polyimide varnish, characterized in that, It is obtained by uniformly mixing a thermosetting polyamic acid precursor solution capped with a crosslinking end-capping agent, N-vinylimidazole, a peroxide radical initiator, and an imidization accelerator through stirring; the molar ratio of the crosslinking end-capping agent to N-vinylimidazole is 100:(50~100); the thermosetting polyamic acid precursor solution capped with the crosslinking end-capping agent is obtained by polymerizing aromatic diamine and aromatic dianhydride in an aprotic solvent to obtain a thermosetting polyamic acid precursor solution, and then adding the crosslinking end-capping agent to react; the solid content of the thermosetting polyamic acid precursor solution capped with the crosslinking end-capping agent is 25%~35%; the crosslinking end-capping agent is selected from at least one of maleic anhydride and norbornene adiene anhydride.

2. The polyimide varnish according to claim 1, characterized in that, The aromatic diamine is selected from at least one of 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-bis(3-aminophenoxy)diphenyl sulfone, and 4,4'-diaminodiphenylmethane; The aromatic dianhydride is selected from at least one of 1,2,4,5-pyromellitic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4'-oxobisphthalic anhydride, 3,3',4,4'-diphenylsulfone tetracarboxylic dianhydride and bisphenol A dianhydride; The aprotic solvent is selected from at least one of N,N'-dimethylformamide, N,N'-dimethylacetamide, and N-methylpyrrolidone.

3. The polyimide varnish according to claim 1, characterized in that, The peroxide radical initiator is selected from at least one of benzoyl peroxide, cyclohexanone peroxide, lauroyl peroxide, tert-amyl peroxybenzoate, tert-butyl hydroperoxide, dicumyl peroxide, and cumyl perhydrogen peroxide. The amount of the peroxide radical initiator is 0.1 to 1% of the total weight of the crosslinking end-capping agent and N-vinylimidazole.

4. The polyimide varnish according to claim 1, characterized in that, The iminolation accelerator is one or more of pyridine and its derivatives, methylpyridine and its derivatives, β-pyrrolline, dimethylpyridine, quinoline, isoquinoline, imidazole, triethylamine, triethanolamine, and N,N-dimethylethanolamine; the amount of the iminolation accelerator added is 2 to 4% of the total weight of the aromatic diamine and aromatic dianhydride.

5. The polyimide varnish according to any one of claims 1 to 4, characterized in that, The molar ratio of the aromatic diamine to the aromatic dianhydride is 100:(95~99), and the molar ratio of the crosslinking end-capping agent to the aromatic diamine is 100:(2~10).

6. A method for preparing a polyimide varnish as described in any one of claims 1 to 5, characterized in that, Includes the following steps: (1) Add an aprotic solvent and an aromatic diamine to the reaction vessel. After the aromatic diamine dissolves, add aromatic dianhydride to the reaction vessel in batches to carry out the polymerization reaction. Then add a cross-linking end-capping agent to carry out the end-capping reaction to obtain a thermosetting polyamic acid precursor solution end-capped by the cross-linking end-capping agent. (2) Add a thermosetting polyamic acid precursor solution capped with a cross-linking capping agent to the reaction vessel, and then add N-vinylimidazol, peroxide free radical initiator and imidization accelerator in sequence. Stir and mix evenly to obtain the polyimide varnish.

7. The method for preparing polyimide varnish according to claim 6, characterized in that, In step (1), the polymerization reaction temperature is 40~80℃ and the reaction time is 2~8h; the end-capping reaction temperature is 10~40℃ and the reaction time is 1~4h.

8. The method for preparing polyimide varnish according to claim 6, characterized in that, In step (2), the stirring temperature is 0~5℃ and the stirring time is 0.5~2h.

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