A high solid content low viscosity polyamic acid solution and a method for preparing the same

Abstract

This invention discloses a high-solids-content, low-viscosity polyamic acid solution and its preparation method. The preparation method includes the following steps: S1, adding solvent, then adding 5%–20% (by weight of total diamine monomers) of diamine monomers, stirring evenly, and then adding 40%–60% (by weight of total dianhydride monomers) of dianhydride monomers; S2, adding the remaining diamine monomers and solvent sequentially to the viscous liquid obtained in S1, stirring evenly; S3, adding the remaining dianhydride monomers and solvent in batches, continuing the reaction to obtain a high-solids-content, low-viscosity polyamic acid solution. This invention provides a high-solids-content, low-viscosity polyamic acid solution with controllable viscosity and its preparation method, which, while possessing excellent mechanical properties and thermal stability, increases the solids content of the polyamic acid solution and reduces its viscosity, thereby ensuring the stability of the final polyimide product's performance.

Description

Technical Field

[0001] This invention relates to the field of polyamic acid solutions, and in particular to a high-solids-content, low-viscosity polyamic acid solution and its preparation method. Background Technology

[0002] Typically, synthesized polyamic acid (PAA) solutions have long molecular chains and very large molecular weights, resulting in high apparent viscosity even at low solid content. Furthermore, the viscosity increases further with increasing solid content. When the solid content reaches 30%, obtaining a homogeneous and transparent PAA solution using traditional synthesis methods becomes extremely difficult. In existing technologies, regardless of the viscosity reduction method used, a decrease in polymer molecular weight is unavoidable, leading to a decline in the overall performance of the final polyimide (PI) coating compared to traditional PI films, severely limiting its application and development.

[0003] Patent CN112708133A uses monofunctional anhydrides or amines as active end-capping agents to obtain low molecular weight PAA. Research results show that the addition of end-capping agents effectively improves the flowability of PAA, i.e., reduces viscosity. Patent CN104292459B, in addition to adding end-capping agents, also added a certain proportion of tackifiers to obtain PAA solutions with a viscosity range of 500-10000 cP. However, this study did not compare the effects of adding tackifiers and end-capping agents on the final polyimide product properties.

[0004] Surfactants, as amphiphilic substances, can also be effectively used to regulate the rheological properties of solutions. Furthermore, surfactants typically possess a low critical micelle concentration, meaning that even trace amounts can significantly alter solution properties. Patent CN112409612A introduces a novel type of ammonium carboxylate gemini surfactant into the polyamic acid (PAA) synthesis system. By utilizing its hydrophobic micelle core to encapsulate aromatic diamines, the degree of polycondensation is effectively controlled, resulting in a PAA solution with high solids content and low viscosity.

[0005] Patent CN103788651B adds trimethylchlorosilane to PAA solution, which reduces the apparent viscosity of PAA by more than 90%. However, the addition of trimethylchlorosilane will affect the mechanical properties and thermal decomposition temperature of the final polyimide product.

[0006] Patent CN108753244A polymerizes dianhydride monomers with alicyclic structures with aromatic diamine monomers containing large substituents, utilizing the alicyclic structure to disrupt the strong intramolecular and intermolecular forces in the traditional fully aromatic PI molecular structure. The large substituents in the diamine monomer increase the free volume of the PI molecular chain, facilitating solvent penetration and thus improving the solubility of the PI resin. Ultimately, the synergistic effect of these groups endows the PI resin with high solids content and low viscosity.

[0007] Patent CN114181392A employs partial chemical imidization to reduce the viscosity of PAA. By adding a certain amount of acetic anhydride and pyridine as dehydrating agents to the PAA, partial chemical imidization disrupts the gel network structure of the PAA, reducing its viscosity and restoring its fluidity. However, this method, while adding dehydrating agents to partially imidize the PAA, also generates some water, which significantly reduces the stability of the PAA, especially its storage stability.

[0008] Patent CN112409612A discloses a method for preparing a high-solids-content, low-viscosity polyamic acid solution. The polyamic acid solution prepared by adding ammonium carboxylate gemini surfactant has excellent casting processing performance.

[0009] Patent CN111808285A uses a mixed solution of triethylamine and water, a mixed solution of dimethylethanolamine and water, or a mixed solution of dimethylethanolamine and methanol as an active agent. When the amount of active agent added is less than 2000 ppmw and the temperature is increased to 60°C, the prepared PAA has a viscosity of less than 10 Pa·S at 30°C and a solid content of 10% to 30%.

[0010] The most common synthesis method for polyimide is to polymerize diamine monomers and dianhydride monomers in a solvent to form polyamic acid, which is then imidized by thermal or chemical methods to obtain polyimide. In the synthesis of high-solids-content polyamic acid, because dianhydride monomers have very low solubility in commonly used solvents for polyimide synthesis (DMF, DMAc, NMP, and DMSO), when using the forward feeding method (dissolving the diamine monomer in the solvent first and then adding the dianhydride monomer), the dianhydride monomer cannot dissolve and participate in the reaction in time after addition. This easily leads to agglomeration and rapid surface reaction of the agglomerates, forming lumps that are difficult to completely dissolve and participate in the reaction, making it difficult to obtain a high-quality polyamic acid solution.

[0011] While existing technologies offer methods for preparing high-solids-content, low-viscosity polyamic acid (PAA) solutions, the methods described above still present some challenges. For instance, adding other reagents (trimethylchlorosilane, surfactants, tackifiers, dehydrating agents, and water-based active additives, etc.) can yield PAA solutions with high solids content and low viscosity, improving their subsequent processability. However, this often results in a decrease in the mechanical and thermal properties of the final polyimide product. Therefore, there is an urgent need to develop a method that, while maintaining the excellent mechanical properties and thermal stability of polyimide, increases the solids content of the polyamic acid solution and reduces its viscosity, thereby ensuring the stability of the final polyimide product's performance. Summary of the Invention

[0012] The technical problem to be solved by the present invention is to provide a polyamic acid solution with high solid content and low viscosity and its preparation method, which has excellent mechanical properties and thermal stability, while increasing the solid content of the polyamic acid solution and reducing its viscosity, thereby ensuring the stability of the final polyimide product performance.

[0013] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for preparing a polyamic acid solution with high solids content and low viscosity, comprising the following steps:

[0014] S1. Add solvent, then add 5% to 20% of the total diamine monomer by mass, stir evenly, and then add 40% to 60% of the total dianhydride monomer by mass.

[0015] S2. Add the remaining diamine monomer and solvent to the viscous liquid obtained in S1 and stir until homogeneous.

[0016] S3. Add the remaining dianhydride monomer in batches and dilute with the remaining solvent, and continue the reaction to obtain a high solids content, low viscosity polyamic acid solution.

[0017] The total molar ratio of the diamine to the dianhydride is (0.965–1.035):1.0, and the dianhydride and the diamine account for 25%–35% of the total weight of the reaction system.

[0018] The purpose of this invention is to overcome the problems of high viscosity, easy gelation and loss of fluidity, and difficult processing of high solid content polyamic acid solutions. It provides a method for preparing high solid content, low viscosity polyamic acid solutions, and can reduce the viscosity of polyamic acid solutions without changing the solid content according to actual needs. The polyimide film prepared by the polyamic acid solution prepared by this method has excellent mechanical properties and thermal stability, and its performance will not decrease as the viscosity decreases.

[0019] This invention employs a unique feeding method. First, a small amount of diamine monomer is added to the solvent and allowed to dissolve completely. Then, a portion of dianhydride monomer is added to obtain a reaction solution with a certain viscosity. The remaining diamine monomer is then added, followed by the remaining dianhydride monomer in batches. Because the reaction solution already has a certain viscosity after the addition of some dianhydride monomer, the remaining dianhydride monomer will not rapidly participate in the reaction and cause clumping. Instead, it will gradually and evenly disperse in the reaction solution and slowly participate in the reaction, ultimately yielding a high-quality polyamic acid solution.

[0020] Low viscosity refers to a viscosity of less than 20,000 mPa·s, and high solid content refers to the dianhydride and diamine accounting for more than or equal to 25% of the total weight of the reaction system.

[0021] In a preferred embodiment of the present invention, the solvent comprises solvent A with a mass percentage of 0% to 100% and solvent B with a mass percentage of 0% to 100%, wherein solvent A is at least one of acetonitrile or propylene carbonate, and solvent B is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0022] This invention employs a special feeding method to reduce the viscosity of the polyamic acid solution. To further reduce viscosity while maintaining the overall performance of the final polyimide, this invention abandons the existing method of reducing viscosity by adding other reagents. Instead, it achieves viscosity reduction by adding solvent A to the solvent system. Solvent A is at least one of acetonitrile or propylene carbonate.

[0023] 1. Propylene carbonate

[0024] Calculations of the Hansen solubility parameter (HSP) of polyamic acid revealed that the HSP distance (or Ra) between the solute polyamic acid and the HSPs of solvents such as DMF, DMAc, NMP, and DMSO does not match the interaction radius (R0) between the solute and solvent. This indicates poor solubility of the solute polyamic acid in the solvent, resulting in higher viscosity when the solid content of the polyamic acid solution is high. Calculations showed that the Ra and R0 match between polyamic acid and propylene carbonate is much better than that between DMF, DMAc, NMP, and DMSO. Therefore, polyamic acid exhibits better solubility in propylene carbonate, resulting in lower viscosity. Furthermore, the Ra and R0 match between dianhydride monomers and propylene carbonate is even better, better preventing agglomeration and gelation during the reaction.

[0025] 2. Acetonitrile

[0026] Unlike N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and propylene carbonate, acetonitrile has a smaller molecular size, thus easily penetrating the interior of polyamic acid polymers. This significantly improves the solubility of polyamic acid, resulting in lower viscosity solutions. Furthermore, the viscosity of polyamic acid solutions can be easily controlled by adjusting the proportion of acetonitrile in the solvent system.

[0027] In addition, the addition of propylene carbonate and acetonitrile has a relatively small impact on the structure and molecular weight of polyamic acid polymers, and therefore will not have a significant impact on the performance of the final polyimide products.

[0028] By changing the proportion of solvent A in the solvent system, high-solids-content, low-viscosity polyamic acid solutions with different viscosities can be obtained, thus achieving controllable viscosity of the polyamic acid solution.

[0029] In a preferred embodiment of the present invention, the solvent A has a mass percentage content of 0% to 19%. The apparent viscosity of the obtained polyamic acid solution is 13000 to 20000 mPa·s.

[0030] In a preferred embodiment of the present invention, the solvent A has a mass percentage content of 20% to 60%. The apparent viscosity of the obtained polyamic acid solution is 8560 to 12900 mPa·s.

[0031] In a preferred embodiment of the present invention, the solvent A has a mass percentage content of 61% to 100%. The apparent viscosity of the obtained polyamic acid solution is 4260 to 8600 mPa·s.

[0032] In a preferred embodiment of the present invention, the remaining dianhydride monomer and solvent in S3 are added in three batches in equal proportions.

[0033] In a preferred embodiment of the present invention, the volume of solvent added in S1: the volume of solvent added in S2: the volume of solvent added in S3 = 6-12:1-4:5-9. A high-solids-content polyamic acid solution is synthesized using existing feeding methods and then diluted to obtain a low viscosity. If this ratio is not used, the resulting polyamic acid solution has a relatively high viscosity.

[0034] Propylene carbonate and acetonitrile may be added in S1 and / or S2 and / or S3.

[0035] In a preferred embodiment of the present invention, the stirring time in S2 is 0.5-2 hours.

[0036] In a preferred embodiment of the present invention, the reaction time in S3 is 3 to 8 hours.

[0037] The diamine monomer used is at least one of 4,4'-(naphthalene-2,7-diyl)diphenylamine, 3,3'-(naphthalene-2,7-diyl)diphenylamine, 4,4'-diaminobenzoylaniline, 1,4-diaminobenzene, 4,4'-diaminobiphenyl, 4,4'-diaminodiphenyl sulfone, and 4,4'-diaminodiphenyl ether.

[0038] The dianhydride monomer used is at least one of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, 4,4'-oxobisphthalic anhydride, and pyromellitic dianhydride.

[0039] The present invention also discloses a polyamic acid solution with high solids content and low viscosity prepared by the aforementioned preparation method.

[0040] Existing technologies for preparing high-solids-content, low-viscosity polyimide materials primarily involve adding other reagents (trimethylchlorosilane, surfactants, tackifiers, activators, dehydrating agents, and catalysts, etc.) to reduce the viscosity of the polyamic acid solution, thereby improving its processing performance. However, this process simultaneously leads to a decrease in the mechanical and thermal properties of the final polyimide product.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] This invention successfully reduces the viscosity of high-solids-content polyamic acid solutions through a special feeding method without adding other reagents. By adding a specific solvent A (acetonitrile or propylene carbonate), the viscosity of the high-solids-content polyamic acid solution is further reduced. Furthermore, by controlling the proportion of the specific solvent, a low-viscosity polyamic acid solution with a precise viscosity range can be obtained. While ensuring the processing performance of the polyamic acid solution, this invention effectively avoids the adverse effects of existing technologies on the mechanical properties and thermal stability of the final polyimide products. Attached Figure Description

[0043] Figure 1 This is the FT-IR image of the polyimide prepared in an embodiment of the present invention.

[0044] Figure 2 This is a glass transition temperature diagram of the polyimide prepared according to an embodiment of the present invention.

[0045] Figure 3 This is a thermogravimetric temperature diagram of the polyimide prepared according to an embodiment of the present invention.

[0046] Figure 4 The graph shows the coefficient of thermal expansion of the polyimide prepared according to an embodiment of the present invention. Detailed Implementation

[0047] Example 1

[0048] (1) Dissolve 300.36 g (1.5 mol) of diamine ODA in 7000 mL of N,N-dimethylformamide DMAc solvent. After complete dissolution, first add 1635.9 g (7.5 mol) of dianhydride PMDA, then add 2703.24 g (13.5 mol) of diamine ODA and 1000 mL of N,N-dimethylformamide DMAc. Stir at low speed for 1 h at room temperature. Then add 1635.9 g (7.5 mol) of dianhydride PMDA in three portions and dilute with 7633 mL of N,N-dimethylformamide DMAc. Continue the reaction for 6 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 19560 mPa·s.

[0049] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0050] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0051] Example 2

[0052] (1) Dissolve 300.36 g (1.5 mol) ODA (diamine) in 7000 mL of DMAc solvent. After complete dissolution, first add 1635.9 g (7.5 mol) PMDA (dianhydride), then add 2703.24 g (13.5 mol) ODA and 1000 mL of DMAc. Stir at low speed for 1 h at room temperature. Then add 1635.9 g (7.5 mol) PMDA in three portions, and dilute with 6063 mL of DMAc and 1884 mL of acetonitrile. Continue the reaction for 6 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 14700 mPa·s.

[0053] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0054] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4.

[0055] Example 3

[0056] (1) Dissolve 300.36 g (1.5 mol) ODA in 7000 mL of DMAc solvent. After complete dissolution, first add 1635.9 g (7.5 mol) PMDA, then add 2703.24 g (13.5 mol) ODA and 1000 mL of DMAc. Stir at low speed for 1 h at room temperature. Then add 1635.9 g (7.5 mol) PMDA in three portions, and dilute with 4502 mL of DMAc and 3768 mL of acetonitrile. Continue the reaction for 6 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 12900 mPa·s.

[0057] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0058] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0059] Example 4

[0060] (1) Dissolve 300.36 g (1.5 mol) ODA in 7000 mL of DMAc solvent. After complete dissolution, first add 1635.9 g (7.5 mol) PMDA, then add 2703.24 g (13.5 mol) ODA and 1000 mL of DMAc. Stir at low speed for 1 h at room temperature. Then add 1635.9 g (7.5 mol) PMDA in three portions, and dilute with 1376 mL of DMAc and 7537 mL of acetonitrile. Continue the reaction for 6 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 11070 mPa·s.

[0061] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0062] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0063] Example 5

[0064] (1) Dissolve 300.36 g (1.5 mol) ODA in a mixed solvent of 6250 mL DMAc and 1500 mL acetonitrile. After complete dissolution, first add 1635.9 g (7.5 mol) PMDA, then add 2703.24 g (13.5 mol) ODA and 2000 mL acetonitrile. Stir at low speed for 1 h at room temperature. Then add 1635.9 g (7.5 mol) PMDA in three portions and dilute with 7804 mL acetonitrile. Continue the reaction for 6 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 8560 mPa·s.

[0065] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0066] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0067] Example 6

[0068] (1) Dissolve 300.36 g (1.5 mol) ODA in a mixed solvent of 3125 mL DMAc and 5000 mL acetonitrile. After complete dissolution, first add 1635.9 g (7.5 mol) PMDA, then add 2703.24 g (13.5 mol) ODA and 2000 mL acetonitrile. Stir at low speed for 1 h at room temperature. Then add 1635.9 g (7.5 mol) PMDA in three portions and dilute with 8072 mL acetonitrile. Continue the reaction for 6 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 6650 mPa·s.

[0069] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0070] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0071] Example 7

[0072] (1) Dissolve 300.36 g (1.5 mol) ODA in 9000 mL of acetonitrile solvent. After complete dissolution, first add 1635.9 g (7.5 mol) PMDA, then add 2703.24 g (13.5 mol) ODA and 2000 mL of acetonitrile. Stir at low speed for 1 h at room temperature. Then add 1635.9 g (7.5 mol) PMDA in three portions and dilute with 7849 mL of acetonitrile. Continue the reaction for 8 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 4260 mPa·s.

[0073] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0074] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0075] Comparative Example 1

[0076] (1) 3003.6 g (15 mol) of ODA was dissolved in 9633 mL of DMAc solvent. After complete dissolution, 3271.8 g (15 mol) of PMDA was added in three portions, with 2000 mL of DMAc added after each addition of PMDA. During the reaction, agglomeration and local gelation occurred. The reaction was stirred continuously for 24 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 165000 mPa·s.

[0077] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0078] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0079] Comparative Example 2

[0080] (1) 3003.6 g (15 mol) of ODA was dissolved in a mixed solvent of 9380 mL of DMAc and 1540 mL of acetonitrile. After complete dissolution, 3271.8 g (15 mol) of PMDA was added in three portions, with 2000 mL of acetonitrile added after each addition of PMDA. Slight agglomeration occurred during the reaction. The reaction was stirred continuously for 12 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 76000 mPa·s.

[0081] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0082] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0083] Example 8

[0084] (1) Dissolve 300.36 g (1.5 mol) ODA in 7000 mL of DMAc solvent. After complete dissolution, first add 1635.9 g (7.5 mol) PMDA, then add 2703.24 g (13.5 mol) ODA and 1000 mL of DMAc. Stir at low speed for 1 h at room temperature. Then add 1635.9 g (7.5 mol) PMDA in three portions, and add 1376 mL of DMAc and 4870 mL of propylene carbonate for dilution. Continue the reaction for 6 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 10860 mPa·s.

[0085] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0086] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0087] Comparative Example 3

[0088] (1) 3003.6 g (15 mol) of ODA was dissolved in a mixed solvent of 9380 mL of DMAc and 1000 mL of propylene carbonate. After complete dissolution, 3271.8 g (15 mol) of PMDA was added in three portions, with 1290 mL of propylene carbonate added after each addition of PMDA. Slight agglomeration occurred during the reaction. The reaction was stirred continuously for 12 h to obtain a light yellow homogeneous transparent polyamic acid solution with a solid content of 30 wt% and a viscosity of 55600 mPa·s.

[0089] (2) In a clean room, the prepared polyamic acid solution was degassed and then uniformly coated onto the surface of a clean glass plate using an automatic coating machine. Then it was placed in a high-temperature precision constant temperature chamber for thermal imidization. The heating program was as follows: heating from room temperature to 100°C and holding for 1 hour, heating from 100°C to 200°C and holding for 1 hour, heating from 200°C to 300°C and holding for 1 hour, heating from 300°C to 350°C and holding for 0.5 hours, and then cooling to room temperature to obtain a polyimide film.

[0090] (3) The obtained polyimide was characterized by FT-IR, such as... Figure 1 As shown; the mechanical properties of the polyimide film were tested according to standard GB / T 1040.3-2006; the heat resistance and thermal stability of the polyimide were tested according to standards GB / T 40396-2021, GB / T 27761-2011 and GB / T36800.2-2018, and detailed data are listed in Tables 1-2 and 2. Figures 2-4 .

[0091] Table 1. Basic properties of polyamic acid solution and polyimide film

[0092]

[0093] Examples 4 and 8 are the best, as their viscosities are suitable for the product (insulating coating for automotive drive motors) and they exhibit good coating uniformity.

[0094] Comparative Examples 1-3 all used a conventional forward feeding method. The difference was that Comparative Example 1 only added DMAc solvent, Comparative Example 2 used a mixed solvent of DMAc and acetonitrile, and Comparative Example 3 used a mixed solvent of DMAc and propylene carbonate. Comparative Examples 1-3 all exhibited agglomeration and clumping, and their viscosity was greater than 55600 mPa·s, which was excessively high.

[0095] Table 2 Thermal stability of polyimide films

[0096]

Claims

1. A method for preparing a high solid content, low viscosity polyamic acid solution, characterized by Includes the following steps: S1. Add solvent, then add 5%~20% of the total diamine monomer by mass of diamine monomer, stir evenly, and then add 40%~60% of the total dianhydride monomer by mass of dianhydride monomer; S2. Add the remaining diamine monomer and solvent to the viscous liquid obtained in S1 and stir until homogeneous. The stirring time in S2 is 0.5-2 hours. S3. Add the remaining dianhydride monomer and solvent in batches and continue the reaction to obtain a high solids content and low viscosity polyamic acid solution. The reaction time in S3 is 3-8 hours. The remaining dianhydride monomer and solvent in S3 are added in three batches in sequence. The total molar ratio of the diamine to the dianhydride is (0.965–1.035):1.0, and the dianhydride and the diamine account for 25%–35% of the total weight of the reaction system.

2. The method for preparing a high solid content polyamic acid solution with low viscosity according to claim 1, characterized by, The solvent comprises solvent A (0% to 100% by mass) and solvent B (0% to 100% by mass), wherein solvent A is at least one of acetonitrile or propylene carbonate, and solvent B is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

3. The method for preparing a high-solids-content, low-viscosity polyamic acid solution according to claim 2, characterized in that, The mass percentage of solvent A is 0% to 19%.

4. The method for preparing a high-solids-content, low-viscosity polyamic acid solution according to claim 2, characterized in that, The mass percentage of solvent A is 20% to 60%.

5. The method for preparing a high-solids-content, low-viscosity polyamic acid solution according to claim 2, characterized in that, The mass percentage of solvent A is 61% to 100%.

6. The method for preparing a high-solids-content, low-viscosity polyamic acid solution according to any one of claims 1-5, characterized in that, The ratio of the volume of solvent added to S1: the volume of solvent added to S2: the volume of solvent added to S3 is 6-12:1-4:5-9.

7. A polyamic acid solution with high solids content and low viscosity prepared by the preparation method according to any one of claims 1-6.

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