Polyimide and preparation method thereof

Abstract

The invention discloses polyimide and a preparation method thereof. The method for preparing polyimide comprises the following steps: mixing a sealing agent with a polyamide acid solution to obtain polyamide acid slurry; carrying out heating treatment at a first temperature to volatilize the solvent in the slurry; raising the temperature from the first temperature to a second temperature, keeping the temperature for a first time, and carrying out preliminary curing; the temperature is continuously increased to a third temperature, heat preservation is conducted for a second time, deep curing is conducted, and the third temperature is 20-40 DEG C higher than the second temperature; the sealing agent is a compound represented by formula 1, in the formula 1, R1 is an asymmetric structure, and R1 is selected from one of the following structures: alkylene, a divalent group derived from a compound containing an aromatic ring, and a divalent group derived from a compound containing an aliphatic ring, and R2 and R3 each have an m-dinitrogen five-membered heterocyclic ring. By using the method to prepare polyimide, low-temperature stepped curing and terminal crosslinking of polyamide acid can be realized, and generation of salt byproducts can be reduced or even avoided.

Description

Technical Field

[0001] This application relates to the field of polyimide technology, and more specifically, to polyimides and their preparation methods. Background Technology

[0002] Polyimide (PI) is a key matrix material for advanced electronic devices and aerospace composite materials. Its curing quality directly affects the mechanical reliability, dimensional stability and high-frequency electrical performance of the devices.

[0003] Polyimide is obtained by curing polyamic acid. The traditional curing process has the following problems: (1) The curing temperature of polyamic acid is high, which leads to substrate deformation or yellowing, thermal stress and film defects; (2) In order to reduce the curing temperature, a catalyst is usually added to polyamic acid. Some catalysts form by-product salts with polyamic acid, which will affect imidization; (3) The conventional PI chain ends with inert aniline groups, which cannot participate in crosslinking, resulting in low crosslinking density of the material; (4) Uneven shrinkage in the thickness direction during one-step curing process generates residual stress gradient, which causes substrate warping and interface delamination.

[0004] Therefore, polyimide and its preparation methods still need improvement. Summary of the Invention

[0005] This application aims to at least partially alleviate or solve at least one of the aforementioned problems. One object of this application is to provide a method for preparing polyimide that improves the curing process of polyamic acid, wherein the curing temperature is consistent with the stepwise deblocking temperature of the sealing agent, and the sealing agent can simultaneously achieve the dual functions of terminal amine crosslinking and low-temperature stepwise catalytic curing in the curing of polyamic acid.

[0006] In one aspect of this application, a method for preparing polyimide is provided. In some embodiments of this application, the method for preparing polyimide includes: mixing a blocking agent with a polyamic acid solution to obtain a polyamic acid slurry; performing a heat treatment at a first temperature to evaporate the solvent in the slurry; raising the temperature from the first temperature to a second temperature and holding it at that temperature for a first time to perform preliminary curing; continuing to raise the temperature to a third temperature and holding it at that temperature for a second time to perform deep curing, wherein the third temperature is 20°C-40°C higher than the second temperature; the blocking agent is a compound represented by Formula 1. Formula 1, R1 is an asymmetric structure and is selected from one of the following structures: alkylene, a divalent group derived from a compound containing an aromatic ring, or a divalent group derived from a compound containing an aliphatic ring. R2 and R3 both have a meta-diazine five-membered heterocycle.

[0007] Using the above method to prepare polyimide can achieve low-temperature stepwise curing and end crosslinking of polyamic acid. Furthermore, it can reduce or even avoid the formation of salt byproducts, which is beneficial to improving the imidization rate and crosslinking density of polyimide, and also helps to reduce the residual stress of polyimide.

[0008] In some embodiments of this application, R1 contains a benzene ring, a cyclohexane ring, a naphthyl ring, or an alkylene ring.

[0009] In some embodiments of this application, R1 is selected from one of the following structures: , , , , , , , , , , , , , , , , , , , , , , , C3-C6 alkylene groups; Among them, R4-R8, R 17 -R 19 Each independently is C1-C 10 Chain alkyl, C3-C 10 cycloalkyl or C6-C 12 Aryl, R9-R 16 R 20 -R 27 Each independently is hydrogen, C1-C 10 Chain alkyl, C3-C 10 cycloalkyl or C6-C 12 aryl, R 28 -R 46 Each of the following is independently hydrogen or a C1-C6 alkyl group, and x1, x2, x3, y1, and y2 are each independently an integer from 0 to 10; excluding the following cases: R9, R 10 R 11 R 12 All are the same; R9, R 11 Same, and R 10 R 12 Same; R 13R 14 R 15 R 16 All are the same; R 13 R 15 Same, and R 14 R 16 Same; R 20 R 21 R 22 R 23 All are the same; R 20 R 22 Same, and R 21 R 23 Same; R 24 R 25 R 26 R 27 All are the same; R 24 R 26 Same, and R 25 R 27 Same; R 31 R 32 R 33 R 34 All are the same; R 31 and R 33 R 34 One of them is the same, and R 32 and R 33 R 34 Another identical one in R; 35 R 36 R 37 R 38 All are the same; R 35 and R 37 R 38 One of them is the same, and R 36 and R 37 R 38 Another identical one.

[0010] In some embodiments of this application, R1 is selected from one of the following structures: , , , , , , , .

[0011] In some embodiments of this application, R2 and R3 are each independently selected from one of the following structures: , , , , , , , , , , , , , , , , , , , , ; Among them, R 47 -R 91 Each is independently hydrogen, a C1-C6 chain alkyl group, a C3-C6 cycloalkyl group, or -N(R) 92 2. C6-C 12 aryl, R 92 Selected from hydrogen and C1-C6 alkyl groups.

[0012] In some embodiments of this application, R2 and R3 are each independently selected from one of the following structures: , , , , .

[0013] In some embodiments of this application, the mass of the blocking agent in the slurry is 0.01%-5% of the mass of polyamic acid; and / or, the blocking agent, antioxidant and polyamic acid solution are mixed to obtain the polyamic acid slurry.

[0014] In some embodiments of this application, the first temperature is 20°C-60°C lower than the second temperature; and / or, the heat treatment time of the workpiece at the first temperature is 30 min-60 min.

[0015] In some embodiments of this application, the first time is 30 min-120 min; and / or, the second time is 30 min-120 min.

[0016] In another aspect of this application, a polyimide is provided, which is prepared using the method described above. This polyimide thus exhibits a high imidization rate, a high crosslinking density, and low residual stress. Attached Figure Description

[0017] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 The hydrogen spectrum of the blocking agent in Example 1 is shown; Figure 2 The hydrogen spectrum of the blocking agent in Example 2 is shown; Figure 3 The hydrogen spectrum of the blocking agent in Example 3 is shown; Figure 4 The hydrogen spectrum of the blocking agent in Example 4 is shown; Figure 5 The hydrogen spectrum of the blocking agent in Example 5 is shown; Figure 6 The proton NMR spectra of the blocking agents in Examples 6-11 are shown; Figure 7 The hydrogen spectrum of the blocking agent in Example 12 is shown. Detailed Implementation

[0018] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0019] In one aspect of this application, a method for preparing polyimide is provided. In some embodiments of this application, the method for preparing polyimide may include the following steps: S10: Mix the sealing agent with the polyamic acid solution to obtain a polyamic acid slurry.

[0020] In some embodiments of this application, the sealing agent is a compound represented by Formula 1. Formula 1, R1 is an asymmetric structure and is selected from one of the following structures: alkylene, a divalent group derived from a compound containing an aromatic ring, or a divalent group derived from a compound containing an aliphatic ring. R2 and R3 both have a meta-diazine five-membered heterocycle.

[0021] Therefore, the two isocyanate groups (-NCO) connected to R1 have different kinetic reactivity. Using this blocking agent in the curing process of polyamic acid (i.e., the preparation of polyimide) can improve the imidization rate and crosslinking density of the product, and reduce the residual stress of the product. Specifically, the compound shown in Formula 1 can unblock the more reactive isocyanate groups at a relatively low temperature, releasing -NCO and imidazole compounds. The imidazole compounds can catalyze the initial curing of polyamic acid. At a slightly higher temperature, it can unblock the less reactive isocyanate groups, releasing the remaining -NCO and imidazole compounds. The imidazole compounds can continue to catalyze the deep curing of polyamic acid. The two -NCO groups of the diisocyanate can crosslink with the terminal amine group (e.g., -NH2) to achieve terminal crosslinking and increase the crosslinking density of the product. The imidazole compounds released at a lower temperature catalyze the imidization of polyamic acid without forming by-product salts with polyamic acid.

[0022] In this application, R1 being an asymmetric structure means that R1 does not have a center of symmetry, a plane of symmetry, or a secondary rotation axis. A secondary rotation axis refers to an object (molecule, group, etc.) whose shape completely coincides with its original position after being rotated 180° around that axis.

[0023] The divalent group in "derived from a divalent group of a compound containing an aromatic ring" originates from a compound containing an aromatic ring. For example, a divalent group... It can come from .

[0024] The divalent group in "derived from a compound containing an aliphatic ring" originates from a compound containing an aliphatic ring, for example, a divalent group It can come from .

[0025] In some embodiments of this application, R1 has an asymmetric structure and contains a benzene ring, a cyclohexane ring, a naphthalene ring, or an alkylene ring.

[0026] In some embodiments of this application, R1 is an asymmetric structure, and R1 is selected from one of the following structures: , , , , , , , , , , , , , , , , , , , , , , , C3-C6 alkylene groups; Among them, R4-R8, R 17 -R 19 Each independently is C1-C 10 Chain alkyl, C3-C 10 cycloalkyl or C6-C 12 Aryl, R9-R 16 R 20 -R 27 Each independently is hydrogen, C1-C 10 Chain alkyl, C3-C 10 cycloalkyl or C6-C 12 aryl, R 28 -R 46 Each of the following is independently hydrogen or a C1-C6 alkyl group, and x1, x2, x3, y1, and y2 are each independently an integer from 0 to 10; excluding the following cases: R9, R 10 R 11 R 12 All are the same; R9, R 11 Same, and R 10 R 12 Same; R 13 R 14 R 15 R 16 All are the same; R 13 R 15 Same, and R 14 R 16 Same; R 20 R 21 R 22 R 23 All are the same; R 20 R 22 Same, and R 21 R 23 Same; R 24 R 25 R 26 R 27 All are the same; R 24 R 26 Same, and R 25 R 27 Same; R 31 R 32 R 33 R 34 All are the same; R31 and R 33 R 34 One of them is the same, and R 32 and R 33 R 34 Another identical one in R; 35 R 36 R 37 R 38 All are the same; R 35 and R 37 R 38 One of them is the same, and R 36 and R 37 R 38 Another identical one.

[0027] In this article, symbols It refers to a bond that is attached to another group or atom.

[0028] “C1-C 10 The "chain alkyl" can be a straight-chain alkyl or a branched alkyl, specifically, C1-C 10 The chain alkyl group can be a straight-chain alkyl group having 1-10 carbon atoms, a straight-chain alkyl group having 1-5 carbon atoms, a branched alkyl group having 3-10 carbon atoms, or a branched alkyl group having 3-6 carbon atoms. More specifically, C1-C 10 The chain alkyl group can be methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, or neopentyl, etc.

[0029] “C3-C 10 "Cycloalkyl" refers to a cyclic alkyl group having 3-10 carbon atoms, specifically C3-C4. 10 The cycloalkyl group can be a cycloalkyl group having 3-5 carbon atoms or a cycloalkyl group having 6-10 carbon atoms. More specifically, C3-C 10 The cycloalkyl group can be cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, or cyclodecyl, etc.

[0030] “C6-C 12 "Aryl" refers to a monocyclic or bicyclic aromatic hydrocarbon group containing 6-12 ring carbon atoms and at least one benzene ring or fused benzene ring unit. Specifically, C6-C 12 The aryl group can be phenyl (C6) or naphthyl (C7). 10 ).

[0031] "C1-C6 alkyl groups" can be straight-chain alkyl groups, branched alkyl groups, or cycloalkyl groups. Specifically, C1-C6 alkyl groups can be straight-chain alkyl groups with 1-6 carbon atoms, branched alkyl groups with 3-6 carbon atoms, or cycloalkyl groups with 3-6 carbon atoms. More specifically, C1-C6 alkyl groups can be methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl, cyclopropane, or cyclohexane.

[0032] "C3-C6 alkylene" refers to branched alkyl groups with 3-6 carbon atoms. Specifically, C3-C6 alkylene groups can be -CH(CH3)CH2-, -C(CH3)2CH2-, -CH(CH2CH3)CH2-, -CH(CH2CH3)(CH2)2-, -CH(CH2CH3)(CH2)3-, etc.

[0033] In some embodiments, x1, x2, x3, y1, and y2 can each be independently 0, 1, 2, 3, 5, 8, 10, etc.

[0034] In some embodiments of this application, R2 and R3 are each independently selected from one of the following structures: , , , , , , , , , , , , , , , , , , , , ; Among them, R 47 -R 93 Each is independently hydrogen, a C1-C6 chain alkyl group, a C3-C6 cycloalkyl group, or -N(R) 92 2. C6-C 12 aryl, R 92 Selected from hydrogen and C1-C6 alkyl groups.

[0035] "C1-C6 chain alkyl" can be a straight-chain alkyl or a branched-chain alkyl. Specifically, C1-C6 chain alkyl can be methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl or n-hexyl, etc.

[0036] "C3-C6 cycloalkyl" refers to cycloalkyl groups with 3-6 carbon atoms, specifically cyclopropyl, cyclobutyl, cyclopentyl, or cyclohexyl.

[0037] In some embodiments of this application, the polyamic acid solution includes polyamic acid and an aprotic polar solvent, wherein the aprotic polar solvent may include one or more of N,N-dimethylformamide (DMF), tetrahydrofuran (THF), ethyl acetate, dimethyl sulfoxide, acetonitrile, acetone, and dimethylacetamide.

[0038] Polyamic acid (PAA) is obtained by polymerization of diamine monomers and dianhydride monomers and is a precursor of polyimide (PI). In some embodiments, polyamic acid can be obtained by low-temperature ring-opening polycondensation of dianhydride monomers and diamine monomers in an aprotic polar solvent, wherein the aprotic polar solvent may include one or more of N,N-dimethylformamide, tetrahydrofuran, ethyl acetate, dimethyl sulfoxide, acetonitrile, acetone, and dimethylacetamide.

[0039] In some specific embodiments, the diamine monomer may be PDA (p-phenylenediamine, structural formula: The dianhydride monomer can be BTDA (3,3',4,4-benzophenone tetracarboxylic dianhydride, structural formula: Of course, those skilled in the art may also choose other diamine monomers and dianhydride monomers.

[0040] In some embodiments, the mass of the blocking agent in the polyamic acid slurry can be 0.01%-5% of the mass of the polyamic acid, for example, the mass of the blocking agent can be 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 5%, etc., of the polyamic acid mass. Thus, after deblocking, the blocking agent possesses crosslinking and low-temperature catalytic imidization functions, enabling it to catalyze the curing of polyamic acid into polyimide at lower temperatures, and can crosslink with terminal amine groups, increasing the crosslinking density of the polyimide.

[0041] In some specific embodiments, the sealing agent can be mixed with a polyamic acid solution, stirred for a period of time, and then vacuum degassed to obtain a polyamic acid slurry.

[0042] In other embodiments, a blocking agent, an antioxidant (e.g., antioxidant 1010), and a polyamic acid solution can be mixed to obtain a polyamic acid slurry. Antioxidants can reduce oxidative side reactions, which is beneficial for improving the purity and mechanical properties of the product. The specific content of the antioxidant in the polyamic acid slurry is not particularly limited in this application; those skilled in the art can set and adjust it as needed. In some embodiments, the mass of the antioxidant can be 0.01%-0.5% of the mass of the polyamic acid.

[0043] S20: Heat treatment is performed at the first temperature to evaporate the solvent in the slurry.

[0044] In some embodiments of this application, the first temperature can be 80°C-100°C, for example, the first temperature can be 80°C, 85°C, 90°C, 95°C, 100°C, etc. At the above temperatures, the solvent in the polyamic acid slurry will evaporate, thereby removing the solvent from the slurry.

[0045] In some embodiments, the heating treatment time at the first temperature can be 30-60 minutes, for example, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, etc. at the first temperature. This facilitates the complete removal of solvent from the slurry.

[0046] S30: The temperature is raised from the first temperature to the second temperature, and the temperature is maintained for a first time to carry out preliminary curing.

[0047] In some embodiments, the first temperature can be 20°C-60°C lower than the second temperature, that is, the second temperature can be 20°C-60°C higher than the first temperature. For example, the first temperature can be 20°C, 30°C, 40°C, 50°C, 60°C, etc., lower than the second temperature. Thus, at a lower first temperature, the solvent can evaporate, and at a higher second temperature, the raw material can undergo a certain degree of curing reaction.

[0048] In some embodiments, the second temperature can be 120°C-140°C, for example, 120°C, 125°C, 130°C, 135°C, 140°C, etc. At the above temperatures, the highly reactive isocyanate groups in the sealing agent are desealed, releasing -NCO and imidazole compounds, which can catalyze the initial curing of polyamic acid at this temperature.

[0049] In some embodiments, the first time can be 30 min to 120 min, for example, 30 min, 50 min, 70 min, 90 min, 100 min, 120 min, etc. This can promote the deblocking of highly reactive isocyanate groups, and the released imidazole compounds can also catalyze the curing of polyamic acid.

[0050] S40: Continue heating to the third temperature, hold for a second time, and perform deep curing.

[0051] In some embodiments, the third temperature can be 20°C-40°C higher than the second temperature. For example, the third temperature can be 20°C, 25°C, 30°C, 35°C, 40°C, etc., higher than the second temperature. Thus, the less reactive isocyanate groups in the blocking agent can be deblocked at the higher third temperature, releasing -NCO and imidazole compounds, thereby achieving further curing of the raw material.

[0052] In some embodiments, the third temperature can be 140℃-160℃, for example, 140℃, 145℃, 150℃, 155℃, 160℃, etc. At these temperatures, the less active isocyanate groups in the blocking agent can be deblocked, releasing the remaining -NCO and imidazole compounds. The imidazole compounds can further catalyze the curing of polyimide, achieving deep curing; the -NCO active groups can react with the amino groups at the ends of the polyimide to achieve end crosslinking, thereby improving the crosslinking density of the material. Imidazole is released at 120℃-160℃ and catalyzes the imidization of polyamic acid at these temperatures. The reaction temperature is relatively low, and imidazole is less likely to undergo side reactions with polyamic acid to form byproduct salts, which is beneficial for improving the amidation rate.

[0053] In some embodiments of this application, the second time can be 30 min to 120 min, for example, the second time can be 40 min, 60 min, 90 min, 110 min, 120 min, etc. This is beneficial for the full curing and crosslinking of polyamic acid.

[0054] Polyimide was prepared using the above method. R1 has an asymmetric structure, and the two isocyanate groups connected to R1 have different kinetic reactivity. At the second temperature, the more reactive isocyanate group is unblocked, releasing -NCO and imidazole compounds. The imidazole compounds can catalyze the initial curing of polyamic acid. Upon heating to the third temperature, the less reactive isocyanate group is unblocked, releasing the remaining -NCO and imidazole compounds. The imidazole compounds can further catalyze the curing of polyamic acid, achieving deep curing and increasing the imidization rate of the cured product. Diisocyanate can be used as a crosslinking agent; -NCO reacts with terminal amine groups (e.g., -NH2) to achieve terminal crosslinking, increasing the crosslinking density of the cured product. Matching the unblocking temperature of the blocking agent with the imidization window (curing temperature of polyamic acid) allows for low-temperature stepped curing at 120℃-160℃, avoiding catalyst salt formation.

[0055] In some embodiments, the blocking agent described above can be obtained by adding the compound shown in Formula 2 to the compounds shown in Formula 3 and Formula 4.

[0056] Equation 2, Formula 3, Formula 4.

[0057] R1, R2, and R3 have been explained in detail above, and will not be repeated here.

[0058] Formula 2 is a diisocyanate. The R1 group contains a sterically hindered or electronically asymmetric structure, and its two isocyanate groups (-NCO) have different kinetic reactivity, such that k1 >> k2. Here, k1 is the reaction rate of the first -NCO with imidazole when the diisocyanate is unreacted, and k2 is the reaction rate of the remaining -NCO after the diisocyanate is blocked.

[0059] The imidazoles shown in Formulas 3 and 4 simultaneously possess the functions of catalyzing the curing of polyamic acid and blocking isocyanates. Their structure contains an NH group (i.e., a 1H-imidazolium structure), ensuring that the imidazole reacts with isocyanates (-NCO) to form a stable adduct (blocking). Furthermore, the substituents in the imidazole should maintain or enhance basicity / nucleophilicity. Introducing electron-donating groups onto the C atom of the imidazole can enhance the basicity and nucleophilicity of the other nitrogen atom on the ring (usually the N atom at the 3-position), thereby improving its catalytic activity.

[0060] In some embodiments, the compound represented by Formula 2 may include one or more of the following compounds: , , , , , , , .

[0061] In some embodiments, the compounds shown in Formulas 3 and 4 may each independently include one or more of the following compounds: , , , , .

[0062] In some embodiments, the compound shown in Formula 3 and the compound shown in Formula 4 may be the same; for example, the compounds shown in Formula 3 and Formula 4 may both be 2E4MZ, 2E4MBI, or 4,5,6,7-tetrahydro-1H-benzimidazole. In other embodiments, the compound shown in Formula 3 and the compound shown in Formula 4 may be different.

[0063] In some embodiments, in the reaction system for preparing the blocking agent, the ratio of the molar number of -NCO to the sum of the molar numbers of the compounds shown in Formula 3 and Formula 4 can be from 1:0.8 to 1:1.5. For example, the ratio of the molar number of -NCO to the sum of the molar numbers of the compounds shown in Formula 3 and Formula 4 can be 1:0.8, 1:0.9, 1:1, 1:1.05, 1:1.08, 1:1.1, 1:1.15, 1:1.2, 1:1.3, 1:1.4, 1:1.5, etc. A ratio of raw materials within the above range is beneficial for a more complete chemical reaction to form the blocking agent shown in Formula 1.

[0064] In some embodiments, in the reaction system for preparing the blocking agent, the ratio of the molar number of -NCO to the sum of the molar numbers of the compounds shown in Formula 3 and Formula 4 can be from 1:1 to 1:1.2. This facilitates the full reaction of the diisocyanate to form a blocked isocyanate group.

[0065] In some embodiments, in the reaction system for preparing the blocking agent, the ratio of the molar number of -NCO to the sum of the molar numbers of the compounds shown in Formula 3 and Formula 4 can be from 1:1.05 to 1:1.1. This is beneficial for the complete reaction of the diisocyanate and can further improve the utilization rate of the raw materials.

[0066] In some embodiments, the reaction temperature of the addition reaction can be 40℃-70℃, for example, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, etc. At these temperatures, the addition reaction is relatively mild, easy to control, and less prone to side reactions, which is beneficial for improving the purity of the blocking agent.

[0067] In some embodiments, the reaction time of the addition reaction can be 3-7 hours, for example, 3 hours, 3.2 hours, 3.5 hours, 3.7 hours, 4 hours, 5 hours, 6 hours, 7 hours, etc. This facilitates the complete reaction of the raw materials, thereby improving the utilization rate of the raw materials.

[0068] In some embodiments, the addition reaction is carried out in an aprotic polar solvent. The starting material can be dissolved in the aprotic polar solvent, and the aprotic polar solvent does not react with the starting material, which helps to reduce side reactions.

[0069] In some embodiments, the aprotic polar solvent may include one or more of N,N-dimethylformamide (DMF), tetrahydrofuran, ethyl acetate, dimethyl sulfoxide, acetonitrile, acetone, and dimethylacetamide. These solvents can dissolve diisocyanates and imidazole compounds, serving as a medium for the addition reaction of diisocyanates and imidazole compounds; furthermore, after the reaction is complete, the solvents are relatively easy to remove, facilitating the acquisition of a high-purity blocking agent.

[0070] In some embodiments, when the addition reaction is carried out in a system containing a solvent, the solid content of the reaction system for preparing the blocking agent can be ≥15% and less than 100%. For example, the solid content of the reaction system can be 15%, 18%, 20%, 22%, 25%, etc. A solid content within the above range indicates that the reaction system contains an appropriate amount of raw material, which is beneficial for the addition reaction to proceed.

[0071] In some embodiments, diisocyanate (the compound shown in Formula 2) can be dissolved in DMF under a protective atmosphere, and then a DMF solution of imidazole compounds (the compounds shown in Formulas 3 and 4) can be slowly added dropwise to obtain a mixed solution. The solution is then heated to allow the raw materials to undergo a heating reaction. In this application, the solid content of the reaction system refers to the mass percentage of the raw materials in the reaction system. Taking this embodiment as an example, the solid content of the reaction system refers to the proportion of the sum of the masses of diisocyanate and imidazole compounds to the total mass of the mixed solution.

[0072] In some embodiments, when the addition reaction is carried out in a system containing a solvent, after the addition reaction is completed, the solvent is removed to obtain a solid product, which is then purified. This yields a blocking agent with high purity.

[0073] In some specific embodiments, after the addition reaction is completed, the solvent can be removed by rotary evaporation. For example, rotary evaporation can be carried out at 60-80°C to remove the solvent and obtain a solid product.

[0074] In other specific embodiments, the addition reaction can be carried out in a solvent-free system with a solid content of 100%. After the addition reaction is complete, the solid product can be directly purified.

[0075] In some embodiments, after obtaining the solid product, the solid product can be dissolved in chloroform (CHCl3) to obtain a solution, and then excess petroleum ether (PE) can be added repeatedly while stirring to precipitate the product. The centrifugation and washing steps are repeated, and finally the product is dried in an oven to obtain the sealing agent. In some embodiments, drying can be carried out in a vacuum oven at around 40°C. The drying time is not particularly limited in this application, and those skilled in the art can set and adjust it according to the actual situation.

[0076] In this application, a high-performance blocking agent can be obtained by using the diisocyanate shown in Formula 2 to undergo an addition reaction with the imidazole compounds shown in Formulas 3 and 4. Furthermore, this method is simple to operate and easy to control.

[0077] In another aspect of this application, a polyimide is proposed, which is prepared using the method described above. Thus, the polyimide exhibits a high imidization rate and low residual stress.

[0078] The present application will be described below through specific embodiments. Those skilled in the art will understand that the specific embodiments below are merely illustrative and do not limit the scope of the present application in any way. Furthermore, in the following embodiments, unless otherwise specified, the materials and equipment used are commercially available. If specific processing conditions and methods are not explicitly described in the later embodiments, conditions and methods known in the art can be used for processing.

[0079] Example 1: Preparation of blocked MDI-2E4MZ: Weigh 10.00 g of 2,4'-MDI (2,4'-diphenylmethane diisocyanate), 9.24 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (molar ratio of -NCO to imidazolium = 1:1.05), and 81.44 mL of N,N-dimethylformamide (DMF) (solid content 20%, i.e., the sum of the masses of 2,4'-MDI and 2E4MZ accounts for 20% of the total mass of the solution). Under nitrogen protection, dissolve 2,4'-MDI in DMF and stir at 25°C until completely dissolved; slowly add the DMF solution of 2E4MZ dropwise, controlling the temperature at 60°C, and react for 3 h. FT-IR was used to detect the reaction at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed successful blocking, yielding a blocked MDI-2E4MZ solution. The solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain blocked MDI-2E4MZ. The 1H NMR spectrum is shown below. Figure 1 As shown, the horizontal axis in the proton spectrum represents the chemical shift, with units of ppm.

[0080] Preparation of polyamic acid slurry: The raw materials are 100 g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of which is 15%, that is, the sum of the mass contents of BTDA and PDA in the mixed solution is 15%, and the solvent is DMF), 0.15 g of blocked MDI-2E4MZ (accounting for 1% of the mass of PAA by solids), and 0.05 g of antioxidant (1010). The blocked MDI-2E4MZ is added to the polyamic acid solution, stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0081] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 120℃ for 30 min to deblock the -NCO at the 4-position of 2,4'-MDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 90 min to deblock the -NCO at the 2-position of 2,4'-MDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥90%.

[0082] Example 2: Preparation of blocked TDI-2E4MZ: 6.96 g of 2,4-toluene diisocyanate (2,4-TDI), 9.24 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (-NCO to imidazolium molar ratio = 1:1.05), and 68.57 mL of N,N-dimethylformamide (DMF) (solid content 20%) were weighed. Under nitrogen protection, 2,4-TDI was dissolved in DMF and stirred at 25°C until completely dissolved. The DMF solution of 2E4MZ was slowly added dropwise, and the reaction was carried out at 60°C for 3 h. The reaction was analyzed by FT-IR at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed successful blocking, yielding a blocked TDI-2E4MZ solution. Solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain blocked TDI-2E4MZ. The 1H NMR spectrum is shown below. Figure 2 As shown.

[0083] Preparation of polyamic acid slurry: The raw materials are 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g of blocked TDI-2E4MZ (accounting for 1% of PAA mass by solids), and 0.05g of antioxidant (1010). The blocked TDI-2E4MZ is added to the polyamic acid solution, stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0084] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 130℃ for 30 min to deblock the -NCO at position 4 of 2,4-TDI, releasing -NCO and imidazole, and initiating cross-linking and curing; 3) Hold at 150℃ for 90 min to deblock the -NCO at position 2 of 2,4-TDI, releasing the remaining -NCO and imidazole, completing curing. The imidization rate after curing is ≥92%.

[0085] Example 3: Preparation of blocked IPDI-2E4MZ: 8.89 g of IPDI (3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate), 9.24 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (-NCO to imidazole molar ratio = 1:1.05), and 76.74 mL of N,N-dimethylformamide (DMF) (solid content 20%) were weighed. Under nitrogen protection, IPDI was dissolved in DMF and stirred at 25°C until completely dissolved. The DMF solution of 2E4MZ was slowly added dropwise, and the reaction was carried out at 70°C for 3 hours. The reaction was analyzed by FT-IR at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed successful blocking, yielding a blocked IPDI-2E4MZ solution. The solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked IPDI-2E4MZ. The 1H NMR spectrum is shown below. Figure 3 As shown.

[0086] Preparation of polyamic acid slurry: The raw materials are 100g polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g blocked IPDI-2E4MZ (1% of PAA mass by solids), and 0.05g antioxidant (1010). The blocked IPDI-2E4MZ is added to the polyamic acid solution, stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0087] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 30 min to deblock the -NCO at position 1 in IPDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 60 min to deblock the -NCO at position 3 in IPDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥92%.

[0088] Example 4: Preparation of blocked IPDI-2E4MBI: 8.89 g of IPDI (3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate), 13.44 g of 2-ethyl-4-methyl-1H-benzimidazole (2E4MBI) (purity ≥99%) (-NCO to imidazole molar ratio = 1:1.05), and 94.52 mL of N,N-dimethylformamide (DMF) (solid content 20%) were weighed. Under nitrogen protection, IPDI was dissolved in DMF and stirred at 25°C until completely dissolved. The DMF solution of 2E4MBI was slowly added dropwise, and the reaction was carried out at 70°C for 4 hours. The reaction was then analyzed by FT-IR at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed successful blocking, yielding a blocked IPDI-2E4MBI solution. Solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked IPDI-2E4MBI. The 1H NMR spectrum is shown below. Figure 4 As shown.

[0089] Preparation of polyamic acid slurry: The raw materials are 100g polyamic acid solution (BTDA / PDA, solid content of 15%), 0.15g blocked IPDI-2E4MBI (1% of PAA mass by solids) and 0.05g antioxidant (1010). The blocked IPDI-BIM is added to the polyamic acid solution and stirred at 25°C for 2 hours. After vacuum degassing, the polyamic acid slurry is obtained.

[0090] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 90 min to deblock the -NCO at position 1 in IPDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 90 min to deblock the -NCO at position 3 in IPDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥80%.

[0091] Example 5: Preparation of blocked IPDI-tetrahydrobenzimidazole: Weigh 8.89 g of IPDI (3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate), 10.25 g of 4,5,6,7-tetrahydro-1H-benzimidazole (purity ≥99%) (molar ratio of -NCO to imidazole = 1:1.05), and 81.02 mL of N,N-dimethylformamide (DMF) (solid content 20%). Under nitrogen protection, dissolve IPDI in DMF and stir at 25°C until completely dissolved; slowly add the DMF solution of 4,5,6,7-tetrahydro-1H-benzimidazole, controlling the temperature at 70°C, and react for 4 h. Detect the reaction at 2270 cm⁻¹ using FT-IR. -1 The disappearance of the -NCO characteristic peak confirmed successful blocking, yielding a blocked IPDI-tetrahydrobenzimidazole solution. Solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked IPDI-tetrahydrobenzimidazole. The 1H NMR spectrum is shown below. Figure 5 As shown.

[0092] Preparation of polyamic acid slurry: The raw materials are 100g polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g blocked IPDI-tetrahydrobenzimidazole (1% of PAA mass by solids), and 0.05g antioxidant (1010). The blocked IPDI-tetrahydrobenzimidazole is added to the polyamic acid solution and stirred and mixed at 25°C for 2 hours. After vacuum degassing, the polyamic acid slurry is obtained.

[0093] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 60 min to deblock the -NCO at position 1 in IPDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 60 min to deblock the -NCO at position 3 in IPDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥85%.

[0094] Example 6: Preparation of the blocked HTDI-2E4MZ: 7.20 g of 2,4-diisocyanate-1-methylcyclohexane (2,4-HTDI), 9.24 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (molar ratio of -NCO to imidazolium = 1:1.05), and 69.60 mL of N,N-dimethylformamide (DMF) (solid content 20%) were weighed. Under nitrogen protection, 2,4-HTDI was dissolved in DMF and stirred at 25°C until completely dissolved. The DMF solution of 2E4MZ was slowly added dropwise, and the reaction was carried out at 40°C for 7 h. The reaction was analyzed by FT-IR at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed the completion of the blocking process, yielding a blocked HTDI-2E4MZ solution. The solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked HTDI-2E4MZ.

[0095] Preparation of polyamic acid slurry: The raw materials are 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g of blocked HTDI-2E4MZ (accounting for 1% of PAA mass by solids), and 0.05g of antioxidant (1010). The blocked HTDI-2E4MZ is added to the polyamic acid solution, stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0096] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 90 min to deblock the -NCO at position 4 in HTDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 90 min to deblock the -NCO at position 2 in HTDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥88%.

[0097] Example 7: Preparation of the blocked HTDI-2E4MZ: Weigh 7.20 g of 2,4-diisocyanate-1-methylcyclohexane (2,4-HTDI), 9.68 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (molar ratio of -NCO to imidazolium = 1:1.1), and 71.46 mL of N,N-dimethylformamide (DMF) (solid content 20%). Under nitrogen protection, dissolve 2,4-HTDI in DMF and stir at 25°C until completely dissolved; slowly add the DMF solution of 2E4MZ dropwise, controlling the temperature at 40°C, and react for 7 h. FT-IR was used to detect the reaction at 2270 cm⁻¹. -1The disappearance of the -NCO characteristic peak confirmed the completion of the blocking process, yielding a blocked HTDI-2E4MZ solution. The solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked HTDI-2E4MZ.

[0098] Preparation of polyamic acid slurry: The raw materials are 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g of blocked HTDI-2E4MZ (accounting for 1% of PAA mass by solids), and 0.05g of antioxidant (1010). The blocked HTDI-2E4MZ is added to the polyamic acid solution, stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0099] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 90 min to deblock the -NCO at position 4 in HTDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 90 min to deblock the -NCO at position 2 in HTDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥90%.

[0100] Example 8: Preparation of the blocked HTDI-2E4MZ: 7.20 g of 2,4-diisocyanate-1-methylcyclohexane (2,4-HTDI), 8.8 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (molar ratio of -NCO to imidazolium = 1:1), and 67.73 mL of N,N-dimethylformamide (DMF) (solid content 20%) were weighed. Under nitrogen protection, 2,4-HTDI was dissolved in DMF and stirred at 25°C until completely dissolved; the DMF solution of 2E4MZ was slowly added dropwise, and the reaction was carried out at 40°C for 7 h. The reaction was measured by FT-IR at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed the completion of the blocking process, yielding a blocked HTDI-2E4MZ solution. The solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked HTDI-2E4MZ.

[0101] Preparation of polyamic acid slurry: The raw materials are 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g of blocked HTDI-2E4MZ (accounting for 1% of PAA mass by solids), and 0.05g of antioxidant (1010). The blocked HTDI-2E4MZ is added to the polyamic acid solution, stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0102] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 90 min to deblock the -NCO at position 4 in HTDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 90 min to deblock the -NCO at position 2 in HTDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥85%.

[0103] Example 9: Preparation of the blocked HTDI-2E4MZ: 7.20 g of 2,4-diisocyanate-1-methylcyclohexane (2,4-HTDI), 10.56 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (molar ratio of -NCO to imidazolium = 1:1.2), and 75.19 mL of N,N-dimethylformamide (DMF) (solid content 20%) were weighed. Under nitrogen protection, 2,4-HTDI was dissolved in DMF and stirred at 25°C until completely dissolved; the DMF solution of 2E4MZ was slowly added dropwise, and the reaction was carried out at 40°C for 7 h. The reaction was analyzed by FT-IR at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed the completion of the blocking process, yielding a blocked HTDI-2E4MZ solution. The solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked HTDI-2E4MZ.

[0104] Preparation of polyamic acid slurry: The raw materials are 100g polyamic acid solution (BTDA / PDA, solid content of 15%), 0.15g blocked HTDI-2E4MZ (1% of PAA mass by solids) and 0.05g antioxidant (1010). The blocked HTDI-2E4MZ is added to the polyamic acid solution and stirred at 25°C for 2 hours. After vacuum degassing, the polyamic acid slurry is obtained.

[0105] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 90 min to deblock the -NCO at position 4 in HTDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 90 min to deblock the -NCO at position 2 in HTDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥92%.

[0106] Example 10: Preparation of the blocked HTDI-2E4MZ: Weigh 7.20 g of 2,4-diisocyanate-1-methylcyclohexane (2,4-HTDI), 7.04 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (molar ratio of -NCO to imidazolium = 1:0.8), and 60.28 mL of N,N-dimethylformamide (DMF) (solid content 20%). Under nitrogen protection, dissolve 2,4-HTDI in DMF and stir at 25°C until completely dissolved; slowly add the DMF solution of 2E4MZ dropwise, controlling the temperature at 40°C, and react for 7 h. FT-IR was used to detect the reaction at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed the completion of the blocking process, yielding a blocked HTDI-2E4MZ solution. The solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked HTDI-2E4MZ.

[0107] Preparation of polyamic acid slurry: The raw materials are 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g of blocked HTDI-2E4MZ (accounting for 1% of PAA mass by solids), and 0.05g of antioxidant (1010). The blocked HTDI-2E4MZ is added to the polyamic acid solution, stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0108] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 90 min to deblock the -NCO at position 4 in HTDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 90 min to deblock the -NCO at position 2 in HTDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥80%.

[0109] Example 11: Preparation of the blocked HTDI-2E4MZ: Weigh 7.20 g of 2,4-diisocyanate-1-methylcyclohexane (2,4-HTDI), 13.2 g of 2-ethyl-4-methylimidazolium (2E4MZ) (purity ≥99%) (molar ratio of -NCO to imidazolium = 1:1.5), and 86.36 mL of N,N-dimethylformamide (DMF) (solid content 20%). Under nitrogen protection, dissolve 2,4-HTDI in DMF and stir at 25°C until completely dissolved; slowly add the DMF solution of 2E4MZ dropwise, controlling the temperature at 40°C, and react for 7 h. FT-IR was used to detect the reaction at 2270 cm⁻¹. -1 The disappearance of the -NCO characteristic peak confirmed the completion of the blocking process, yielding a blocked HTDI-2E4MZ solution. The solvent was removed by rotary evaporation at 60-80℃, yielding a white powder solid. This powder was dissolved in chloroform to obtain a colorless, transparent solution. Excess petroleum ether (PE) was added repeatedly with stirring to precipitate the product. The centrifugation and washing steps were repeated. Finally, the product was placed in a vacuum oven at 40℃ for 24 hours to obtain the blocked HTDI-2E4MZ.

[0110] Preparation of polyamic acid slurry: The raw materials are 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g of blocked HTDI-2E4MZ (accounting for 1% of PAA mass by solids), and 0.05g of antioxidant (1010). The blocked HTDI-2E4MZ is added to the polyamic acid solution, stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0111] Low-temperature curing process: 1) Hold at 80℃ for 30 min to allow solvent evaporation; 2) Hold at 140℃ for 90 min to deblock the -NCO at position 4 in HTDI, releasing -NCO and imidazole for initial cross-linking and curing; 3) Hold at 160℃ for 90 min to deblock the -NCO at position 2 in HTDI, releasing the remaining -NCO and imidazole to complete curing. The imidization rate after curing is ≥92%.

[0112] The proton NMR spectra of the blocking agents prepared in Examples 6-11 are as follows: Figure 6 As shown.

[0113] Example 12: The only difference between Example 12 and Example 1 is that 2-ethyl-4-methylimidazolium is replaced with imidazolinone, while maintaining the molar ratio of -NCO to imidazolium = 1:1.05. The proton NMR spectrum of the blocking agent in Example 12 is shown below. Figure 7 As shown.

[0114] Comparative Example 1: Preparation of polyamic acid slurry: The raw material is 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), without isocyanate-imidazolium blocking agent.

[0115] Curing process: 1) Hold at 80℃ for 30 minutes to allow solvent evaporation; 2) Hold at 230℃ for 30 minutes for high-temperature curing. The imidization rate after curing is ≥78%.

[0116] Comparative Example 2: Preparation of polyamic acid slurry: The raw materials are 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g of 4,4'-MDI (4,4'-diphenylmethane diisocyanate), and 0.05g of antioxidant (1010). The mixture is stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0117] Curing process: 1) Hold at 80℃ for 30 minutes to allow solvent evaporation; 2) Hold at 230℃ for 30 minutes for high-temperature curing. The imidization rate after curing is ≥79%.

[0118] Comparative Example 3: Preparation of polyamic acid slurry: The raw materials are 100g of polyamic acid solution (the polyamic acid solution is obtained by reacting a mixed solution of BTDA and PDA, the solid content of the mixed solution is 15%, and the solvent is DMF), 0.15g of 2-ethyl-4-methylimidazolium (2E4MZ), and 0.05g of antioxidant (1010). The mixture is stirred and mixed at 25°C for 2 hours, and the polyamic acid slurry is obtained after vacuum degassing.

[0119] Curing process: 1) Hold at 80℃ for 30 minutes to allow solvent evaporation; 2) Hold at 150℃ for 90 minutes for high-temperature curing. The imidization rate after curing is ≥90%.

[0120] Methods for testing the degree of closure reaction: In this application, the degree of blocking reaction was determined using Jasco's Fourier transform infrared spectroscopy (FTIR-8X FV), by monitoring the characteristic peak (2270 cm⁻¹) of the free isocyanate group (-NCO). -1 To determine if the blocking reaction is complete, samples are taken before and after the blocking reaction, coated onto a KBr salt plate, and compared at 2270 cm⁻¹. -1 The intensity change of the -NCO peak. Judgment criterion: If the -NCO peak disappears or significantly weakens, it indicates that the blocking reaction is complete.

[0121] The imidization rate and residual stress of the products of each example and comparative example were tested, and the test results are recorded in Table 1.

[0122] DOI (%) of imidization rate: In this application, the degree of blocking reaction was determined by Jasco's Fourier transform infrared spectroscopy FTIR-8X FV, and the imidization rate was calculated by monitoring characteristic peaks.

[0123] Characteristic peak of polyamic acid (PAA): C=O stretching vibration of carboxyl group (-COOH): ~1720 cm⁻¹ -1 (Broad peak); C=O stretching vibration of amide (-CONH-): ~1660 cm⁻¹ -1 .

[0124] Characteristic peaks of polyimide (PI): Imine C=O asymmetric stretching vibration: ~1780 cm⁻¹ -1 (Sharp); Imidine C=O symmetric stretching vibration: ~1720 cm⁻¹ -1 imide ring CN stretching vibration: ~1380 cm -1 .

[0125] Method: PAA solution was coated onto a KBr salt plate or silicon wafer to form a uniform thin film, followed by stepwise temperature increase for curing. After curing, samples were taken for Fourier transform infrared spectroscopy (FTIR) analysis, with a scanning range of 4000-400 cm⁻¹. -1 4 cm resolution -1 .

[0126] Quantitative analysis: Select internal standard peaks (e.g., C=C stretching vibration of benzene ring ~1510 cm⁻¹). -1 (Unaffected by the reaction), the imidization rate (%) is calculated according to the following formula: Calculate the imidization rate (DOI): DOI = (A1 / A2) / (A3 / A4) × 100%, A1: The area of ​​the CN stretching vibration peak of the imide ring in the polyimide samples that have been cured in each example and comparative example; A2: Area of ​​the C=C stretching vibration peak of the benzene ring in the polyimide samples that have been cured in each example and comparative example; A3: The peak area of ​​the CN stretching vibration of the imide ring after the film is fully cured; A4: Area of ​​the C=C stretching vibration peak of the benzene ring after the film is fully cured.

[0127] "Complete curing of the film" means that the polyamic acid slurry of the corresponding embodiment or comparative example is cured at 300°C for the same time as the corresponding embodiment or comparative example.

[0128] Residual stress testing methods: In this application, the residual stress was tested using a Frontier Semiconductor FSM500TC residual stress tester.

[0129] Test Procedure: Using glass as the substrate, polyamic acid slurry was spin-coated onto the substrate and cured using a low-temperature stepped heating method to ensure the final film thickness was controlled within 10±1 μm. The uncoated substrate was placed on an FSM 500TC measurement platform, and its initial radius of curvature R0 was measured and the data recorded. The cured sample was placed on the stage, and laser scanning was initiated to measure the change in the radius of curvature of the substrate before and after spin-coating. The scanning path needed to cover the center and edge areas of the sample to obtain stress results. The measured substrate and film parameters (E) were then analyzed. s t s t f V s Substituting R0 and R into the Stoney formula, the residual stress σ of the thin film is calculated. The data processing and analysis are performed using the software that comes with the FSM 500TC.

[0130] Stoney formula:

[0131] Where Es: elastic modulus of the substrate; t s t f : Substrate and film thickness (unit: μm); V s : Poisson's ratio of the substrate; R0, R: radii of curvature before and after coating.

[0132] Table 1. Curing process of polyamic acid slurry and properties of polyimide products

[0133] As shown in Table 1, in Comparative Example 1, no isocyanate or imidazole was used during the polyamic acid curing process. Curing was carried out at a lower temperature, resulting in a lower imidization rate and higher residual stress. In Comparative Example 2, unblocked diisocyanate was added, and one-step curing was performed at a lower temperature, resulting in a product with higher residual stress and a lower imidization rate. In Comparative Example 3, unblocked imidazole was used as a catalyst, and the resulting product still exhibited high residual stress. Examples 1-12 all added isocyanate-imidazole blocking agents to the curing system, enabling low-temperature stepwise curing of polyamic acid, terminal amine crosslinking, and the absence of salt byproducts. The products exhibited higher imidization rates and lower residual stress.

[0134] In the description of this specification, references to terms such as "one embodiment," "another embodiment," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment is included in at least one embodiment of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples, without contradiction. Additionally, it should be noted that in this specification, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. In the description of this application, unless otherwise stated, "multiple" means two or more.

[0135] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for preparing polyimide, characterized in that, include: The sealing agent is mixed with a polyamic acid solution to obtain a polyamic acid slurry; The slurry is heated at a first temperature to evaporate the solvent. The temperature is raised from the first temperature to the second temperature and held at that temperature for a short period of time to allow for initial curing. Continue heating to the third temperature, hold for a second time, and perform deep curing. The third temperature is 20℃-40℃ higher than the second temperature. The sealing agent is a compound represented by Formula 1. Formula 1, R1 is an asymmetric structure and is selected from one of the following structures: alkylene, a divalent group derived from a compound containing an aromatic ring, or a divalent group derived from a compound containing an aliphatic ring. R2 and R3 both have a meta-diazine five-membered heterocycle.

2. The method according to claim 1, characterized in that, R1 contains a benzene ring, a cyclohexane ring, a naphthyl ring, or an alkylene ring.

3. The method according to claim 1, wherein R1 is selected from one of the following structures: , , , , , , , , , , , , , , , , , , , , , , , C3-C6 alkylene groups; in, R4-R8, R 17 -R 19 Each independently is C1-C 10 Chain alkyl, C3-C 10 cycloalkyl or C6-C 12 Aryl, R9-R 16 R 20 -R 27 Each independently is hydrogen, C1-C 10 Chain alkyl, C3-C 10 cycloalkyl or C6-C 12 aryl, R 28 -R 46 Each of the following is independently hydrogen or a C1-C6 alkyl group, and x1, x2, x3, y1, and y2 are each independently an integer from 0 to 10; excluding the following cases: R9, R 10 R 11 R 12 All are the same; R9, R 11 Same, and R 10 R 12 Same; R 13 R 14 R 15 R 16 All are the same; R 13 R 15 Same, and R 14 R 16 Same; R 20 R 21 R 22 R 23 All are the same; R 20 R 22 Same, and R 21 R 23 Same; R 24 R 25 R 26 R 27 All are the same; R 24 R 26 Same, and R 25 R 27 Same; R 31 R 32 R 33 R 34 All are the same; R 31 and R 33 R 34 One of them is the same, and R 32 and R 33 R 34 Another identical one in R; 35 R 36 R 37 R 38 All are the same; R 35 and R 37 R 38 One of them is the same, and R 36 and R 37 R 38 Another identical one.

4. The method according to claim 1, characterized in that, R1 is selected from one of the following structures: 、 、 、 、 、 、 、 。 5. The method according to claim 1, characterized in that, R2 and R3 are each independently selected from one of the following structures: 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 ； Among them, R 47 -R 91 Each is independently hydrogen, a C1-C6 chain alkyl group, a C3-C6 cycloalkyl group, or -N(R) 92 2. C6-C 12 aryl, R 92 Selected from hydrogen and C1-C6 alkyl groups.

6. The method according to claim 1, characterized in that, R2 and R3 are each independently selected from one of the following structures: 、 、 、 、 。 7. The method according to any one of claims 1-6, characterized in that, In the slurry, the mass of the sealing agent is 0.01%-5% of the mass of polyamic acid; And / or, the sealing agent, antioxidant and polyamic acid solution are mixed to obtain the polyamic acid slurry.

8. The method according to any one of claims 1-6, characterized in that, The first temperature is 20°C-60°C lower than the second temperature; And / or, the heat treatment time of the workpiece at the first temperature is 30 min to 60 min.

9. The method according to any one of claims 1-6, characterized in that, The first time is 30min-120min; And / or, the second time is 30 min to 120 min.

10. A polyimide, characterized in that, The polyimide is prepared using the method described in any one of claims 1-9.

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