A method for preparing colored polyimide film

Abstract

The invention belongs to the technical field of film preparation, and discloses a method for preparing a colored polyimide film, wherein the colors include but are not limited to blue, blue-purple, purple-blue, blue-green, green-blue, green, green-yellow, yellow-green, purple-red, red, orange-red, and orange-yellow, and the preparation method is to mix a soluble yellow polyimide film with an intrinsic cyan polyimide film derived from a 1,4-BDDA monomer structural unit or an intrinsic magenta polyimide film derived from a 1,8-BDDA monomer structural unit in proportion. The method can obtain a PI film with the same color and performance as that prepared by a copolymerization method by mixing, and realizes PI colorization more efficiently and conveniently, and has high designability and customizability.

Description

Technical Field

[0001] The invention relates to the technical field of film preparation, and in particular to a method for preparing a colored polyimide film. Background Art

[0002] Polyimide is a polymer material with both insulation and heat resistance. Since its application in the 1960s, polyimide film has been widely used in insulation materials, heat-resistant film materials and other fields due to its outstanding high temperature resistance, high strength, low dielectric and other excellent properties. With the rapid development of electronic technology, various industries have higher and higher requirements for the performance of polyimide films. In some fields, PI films of various special colors are required, such as colored tapes, colored high-temperature labels and colored PCB protective films, while the traditional color of polyimide film is yellow.

[0003] At present, the preparation of colored PI films is mainly achieved by adding inorganic fillers or organic pigments. Although the introduction of inorganic fillers can improve the mechanical and thermal properties of PI films to a certain extent, it will destroy the insulation and breakdown strength of PI films, which will have an adverse effect on its application in the electronics industry. At the same time, inorganic pigments are not environmentally friendly, and are prone to agglomeration, shedding, and sedimentation, etc., and show poor tinting power in polyimide systems. Although the introduction of organic pigments avoids affecting the electrical properties of PI films, their low thermal decomposition temperature hinders the application of PI films in high-temperature fields, and they are easily affected by environmental factors and decompose, and have poor weather resistance. In addition, the method of adding inorganic fillers or organic pigments does not have systematic customization, and often requires a needle-in-a-haystack search for suitable fillers or pigments after the demand target appears, and requires a large number of screening attempts.

[0004] Some researchers have tried to modify organic pigments. For example, Chinese patent application CN110724287A discloses a method for preparing a high-temperature resistant red polyimide film. Before using the organic pigment, the organic pigment is modified with a coupling agent and a heat stabilizer in a solvent, which solves the problem of the temperature difference resistance of the organic red pigment and ensures the coloring effect of the film. However, most of the red polyimide films Td prepared by this patented technology 5％ It is only 400℃, and the high temperature resistance needs to be improved. In addition, the preparation process is complicated, and the filler needs to be modified in advance to disperse it more evenly in the PI matrix. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the above-mentioned background technology and provide a method for preparing a colored polyimide film, wherein the colored polyimide film includes but is not limited to an intrinsic yellow polyimide film, an intrinsic blue polyimide film, an intrinsic purple polyimide film, etc. The method prepares high-temperature resistant colored polyimide films of various colors by mixing polyimide materials of three intrinsic colors, namely, intrinsic cyan, intrinsic yellow and intrinsic blue.

[0006] To achieve the purpose of the present invention, the present invention provides a method for preparing a colored polyimide film, wherein the colors include but are not limited to blue, blue-purple, purple-blue, blue-green, green-blue, green, green-yellow, yellow-green, purple-red, red, orange-red, and orange-yellow. The preparation method is to mix a soluble yellow polyimide film with an intrinsic cyan polyimide film derived from a 1,4-BDDA monomer structural unit or an intrinsic magenta polyimide film derived from a 1,8-BDDA monomer structural unit in proportion.

[0007] Further, in some embodiments of the present invention, the ratio is obtained by combining the film thickness and the following absorbance curve formula:

[0008]

[0009] Wherein α(λ) is the absorbance of the target polyimide film at the wavelength λ, δ is the film thickness, ω is the mass fraction of the polyimide film used for mixing, and A, B, C... are the polyimide films involved in the mixing.

[0010] Among them, the intrinsic cyan polyimide film comprising a monomer structural unit derived from 1,4-BDDA refers to patent application CN202210612142.8, and the intrinsic magenta polyimide film comprising a monomer structural unit derived from 1,8-BDDA refers to patent application CN202210614316.4.

[0011] To achieve the purpose of the present invention, the present invention provides a method for preparing a soluble yellow polyimide film, the method comprising the following steps:

[0012] (1) under the conditions of dehydration, deoxygenation and inert gas protection, p-diaminoazobenzene, TFMB and DMAc were added to a reaction container, stirred until dissolved to form a homogeneous solution, and 6FDA was added to react to obtain a yellow polyamic acid solution with a certain viscosity;

[0013] (2) After the yellow polyamic acid solution is vacuum degassed, it is coated on a substrate and subjected to thermal imidization to obtain a yellow intrinsic polyimide, or an acetic anhydride / pyridine solution is added and stirred overnight to precipitate, and the product is vacuum dried to obtain a solution after dissolution, which is then coated on a carrier and the solvent is removed to obtain a yellow intrinsic polyimide.

[0014] Furthermore, in some embodiments of the present invention, the molar volume ratio of p-diaminoazobenzene, TFMB and DMAc in step (1) is 0.5-0.6 mmol: 8-9 mmol: 10-30 ml.

[0015] Furthermore, in some embodiments of the present invention, 6FDA in step (1) is added in 2-5 portions.

[0016] Furthermore, in some embodiments of the present invention, after adding 6FDA in step (1), the reaction is stirred in a cold water bath; preferably, in some embodiments of the present invention, the reaction time is 12-16 hours.

[0017] Furthermore, in some embodiments of the present invention, the solid content of the solution after dissolution in step (2) is 7-13%.

[0018] Furthermore, the present invention also provides a method for preparing a purple polyimide film, the method comprising the following steps:

[0019] (1) Under the conditions of dehydration and deoxygenation and inert gas protection, 1,4-BDDA, 1,8-BDDA, TFMB and DMAc are added to a reaction container, stirred until dissolved to form a homogeneous solution, and 6FDA is added to react to obtain a purple polyamic acid solution with a certain viscosity;

[0020] (2) After the purple polyamic acid solution is vacuum degassed, it is coated on a substrate and subjected to thermal imidization to obtain purple intrinsic polyimide, or acetic anhydride / pyridine solution is added and stirred overnight to precipitate, and the product is vacuum dried to obtain a solution after dissolution, which is then coated on a carrier and the solvent is removed to obtain purple intrinsic polyimide.

[0021] Furthermore, in some embodiments of the present invention, the molar volume ratio of 1,4-BDDA, 1,8-BDDA, TFMB and DMAc in step (1) is 0.04-0.05 mmol: 0.04-0.05 mmol: 8.2-9.5 mmol: 10-30 ml.

[0022] Furthermore, in some embodiments of the present invention, 6FDA in step (1) is added in 2-5 portions.

[0023] Furthermore, in some embodiments of the present invention, after adding 6FDA in step (1), the reaction is stirred in a cold water bath; preferably, in some embodiments of the present invention, the reaction time is 12-16 hours.

[0024] Furthermore, in some embodiments of the present invention, the solid content of the solution after dissolution in step (2) is 7-13%.

[0025] Compared with the prior art, the advantages of the present invention are as follows:

[0026] (1) The preparation method of the present invention is different from the currently commonly used additive colored PI doped with inorganic fillers or organic pigments. The colored monomer is introduced into the PI main chain by chemical synthesis to prepare an intrinsic colored PI. The problems of uneven dispersion and poor thermal stability are effectively solved through chemical bonding, while the color of the introduced colored body is retained. It will not deteriorate other properties of the PI film. On the contrary, the thermal properties, elongation at break and tensile strength of some colored PI films are improved to varying degrees.

[0027] (2) The present invention can obtain a PI film with the same color and performance as that prepared by copolymerization by mixing, which can realize PI colorization more efficiently and conveniently, and has high designability and customizability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the ultraviolet absorption curve of the polyimide obtained in Examples 1-3 of the present invention;

[0029] Figure 2-6 These are the ultraviolet absorption curves and fitting curves of some embodiments of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. Additional aspects and advantages of the present invention will be partially given in the following description, partially become apparent from the following description, or be understood through the practice of the present invention. It should be understood that the following description is only used to explain the present invention and is not intended to limit the present invention.

[0031] As used herein, the terms "comprises," "including," "having," "containing," or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0032] When amount, concentration or other value or parameter is expressed as range, preferred range or a series of upper preferred value and lower preferred value limit range, this should be understood as specifically disclosing all ranges formed by any pairing of any range upper limit or preferred value and any range lower limit or preferred value, regardless of whether the range is disclosed separately. For example, when disclosing range "1 to 5", described range should be interpreted as including range "1 to 4", "1 to 3", "1 to 2", "1 to 2 and 4 to 5", "1 to 3 and 5" etc. When numerical range is described in this article, unless otherwise stated, the range is intended to include its end value and all integers and fractions within the range.

[0033] The indefinite articles "a" and "an" before the elements or components of the present invention have no limitation on the quantity requirements (i.e. the number of occurrences) of the elements or components. Therefore, "a" or "an" should be interpreted as including one or at least one, and the elements or components in the singular form also include the plural form, unless the quantity obviously refers to the singular form only.

[0034] In addition, the descriptions of the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" described below mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0035] According to the Beer-Lambert law, the present invention deduces the absorbance curve formula suitable for the film based on the film thickness as follows:

[0036]

[0037] Wherein α(λ) is the absorbance of the target polyimide film at the wavelength λ, δ is the film thickness, ω is the mass fraction of the polyimide film used for mixing, A, B, C... are the polyimide films involved in the mixing. It can be seen from the drawings and embodiments of the present invention that the present invention can perfectly fit the color curve by using this formula, R 2 All of them are greater than 0.997, thus predicting the formula color and replacing repeated manual experiments.

[0038] In the following examples, the characterization methods used are as follows.

[0039] 1. Thermal performance evaluation method:

[0040] 1.1 Thermomechanical Analysis (TMA) The prepared polyimide film was tested in a thermal analyzer (TA Company, USA, Q400 series) with a heating rate of 10°C / min.

[0041] 1.2 Dynamic Mechanical Analysis (DMA) The prepared polyimide film was tested on a dynamic mechanical analyzer (TA Company, USA, Q800 series) with a heating rate of 5°C / min and a frequency of 1 Hz.

[0042] Thermogravimetric analysis (TGA): The prepared polyimide film was tested on a dynamic thermomechanical analyzer (TA Company, USA, Q800 series) with a heating rate of 5°C / min and a frequency of 1 Hz.

[0043] 2. Optical performance evaluation method:

[0044] The prepared polyimide film was tested by ultraviolet visible spectroscopy (UV-Vis) with an ultraviolet spectrophotometer (Shimadzu Corporation, UV-2600) in a test wavelength range of 200-800 nm.

[0045] 3. Chromaticity parameter evaluation method:

[0046] The prepared polyimide film was tested by a spectrophotometer (Shenzhen Sanenshi Company, YS6060) using CIE1976 color space parameters (LAB), with a D65 standard light source and a viewing field of 10°.

[0047] 4.Mechanical properties evaluation method:

[0048] The prepared polyimide film was characterized by tensile strength, Young's modulus and elongation at break using a universal tensile testing machine (Zhuhai Sansi Taijie Electrical Equipment Co., Ltd., CMT-1104) according to the "Standard Test Method for Tensile Properties of Plastics" (ASTM-D638). Tensile speed: 5 mm / min.

[0049] Example 1

[0050] The intrinsic cyan polyimide film was prepared from monomers 1,4-BDDA, TFMB and 6FDA in a ratio of 1:99 (molar ratio), and the specific method is as follows:

[0051] (1) 1,4-dihydroxyanthraquinone leuco (0.3 mmol), 1,4-dihydroxyanthraquinone (1 mmol), p-phenylenediamine (2.2 mmol), sodium bisulfite (0.1 mmol), anhydrous sodium sulfate (1 mmol), and 25 ml of ethanol were added to a three-necked flask, and the temperature was raised to 85°C in a nitrogen atmosphere. The reaction was stopped after 12 hours, the reaction liquid was filtered, and the solid matter was washed with water and placed in a vacuum oven for vacuum drying to obtain a cyan crude product. The crude product was purified by silica gel chromatography and eluted with an eluent (ethyl acetate) to obtain a cyan product 1,4-bis((4-aminophenyl)amino)-9,10-anthraquinone (hereinafter referred to as 1,4-BDDA), with a yield of 90.7%.

[0052] (2) Under the conditions of sufficient dehydration and deoxygenation and nitrogen protection, 0.09mmol 1,4-BDDA, 8.91mmol TFMB and 20ml DMAc were added to a 100mL three-necked round-bottom flask equipped with a nitrogen inlet, a mechanical stirrer and a cold water bath, and stirred until dissolved to form a homogeneous solution; a total of 9mmol 6FDA was added to the above solution in three times, and stirred for 14 hours under a cold water bath to obtain a cyan polyamic acid solution with a certain viscosity. 3ml acetic anhydride / pyridine solution was added and stirred overnight, precipitated in an ethanol solution, and placed in a vacuum oven for vacuum drying.

[0053] (3) The polyimide precipitate obtained in step (2) was re-formed into a solution with a solid content of 10%, and then evenly coated on a dry and clean glass plate by an automatic coating machine, and the solvent was removed to obtain a 20 μm film. The temperature curing program was: 80°C / 3h, 100°C / 1h, 200°C / 2h.

[0054] You can also refer to the method of patent application CN202210612142.8 to prepare intrinsic cyan polyimide film.

[0055] Example 2

[0056] The method of reference patent application CN202210614316.4 prepares an intrinsic magenta polyimide film by mixing 1,4-BDDA and TFMB with 6FDA in a ratio of 1:99 (molar ratio).

[0057] Example 3

[0058] The polyimide film was prepared from commercial monomers of p-diaminoazobenzene (cas: 538-41-0) and TFMB and 6FDA in a ratio of 6:94 (molar ratio). The specific method is as follows:

[0059] (1) Under the conditions of sufficient dehydration and deoxygenation and nitrogen protection, 0.54mmol of the target monomer, 8.46mmol of TFMB and 20ml of DMAc were added to a 100mL three-necked round-bottom flask equipped with a nitrogen inlet, a mechanical stirrer and a cold water bath, and stirred until dissolved to form a homogeneous solution; a total of 9mmol of 6FDA was added to the above solution in three times, and stirred for 14 hours under a cold water bath to obtain a yellow polyamic acid solution with a certain viscosity. 3ml of acetic anhydride / pyridine solution was added and stirred overnight, precipitated in an ethanol solution, and placed in a vacuum oven for vacuum drying.

[0060] (2) The polyimide precipitate obtained in step (1) was re-formed into a solution with a solid content of 10%, and then evenly coated on a dry and clean glass plate by an automatic coating machine, and the solvent was removed to obtain a 20 μm film. The temperature curing program was: 80°C / 3h, 100°C / 1h, 200°C / 2h, 300°C / 2h.

[0061] Example 4

[0062] The polyimide precipitates obtained in Example 1 and Example 2 were mixed in a ratio of 3:1 (by mass) to prepare an intrinsic blue polyimide film.

[0063] 3 g of the polyimide precipitate obtained in Example 1 and 1 g of Example 2 were weighed, dissolved in 20 ml of DMAc, and then evenly coated on a dry and clean glass plate by an automatic coating machine, and the solvent was removed to obtain a 20 μm film. The temperature curing program was: 80°C / 3h, 100°C / 1h, 200°C / 2h, 300°C / 2h.

[0064] Example 5

[0065] The intrinsic blue polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 2 were mixed in a ratio of 2:1 (by mass).

[0066] Example 6

[0067] The intrinsic blue polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 2 were mixed in a ratio of 1:1 (by mass).

[0068] Example 7

[0069] The intrinsic blue-purple polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 2 were mixed in a ratio of 1:2 (by mass).

[0070] Example 8

[0071] The intrinsic purple-blue polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 2 were mixed in a ratio of 1:3 (mass).

[0072] Example 9

[0073] The intrinsic blue-green polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 3 were mixed in a ratio of 3:1 (by mass).

[0074] Example 10

[0075] The intrinsic green-blue polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 3 were mixed in a ratio of 2:1 (mass).

[0076] Embodiment 11

[0077] The intrinsic green polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 3 were mixed in a ratio of 1:1 (mass).

[0078] Example 12

[0079] The intrinsic green-yellow polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 3 were mixed in a ratio of 1:2 (mass).

[0080] Example 13

[0081] The intrinsic yellow-green polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 1 and Example 3 were mixed in a ratio of 1:3 (by mass).

[0082] Embodiment 14

[0083] The intrinsic purple-red polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 2 and Example 3 were mixed in a ratio of 3:1 (by mass).

[0084] Embodiment 15

[0085] The intrinsic red polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 2 and Example 3 were mixed in a ratio of 2:1 (mass).

[0086] Example 16

[0087] The intrinsic red polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 2 and Example 3 were mixed in a ratio of 1:1 (by mass).

[0088] Embodiment 17

[0089] The intrinsic orange-red polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 2 and Example 3 were mixed in a ratio of 1:2 (by mass).

[0090] Embodiment 18

[0091] The intrinsic orange-yellow polyimide film was prepared. Different from Example 4, the polyimide precipitates obtained in Example 2 and Example 3 were mixed in a ratio of 1:3 (mass).

[0092] Embodiment 19

[0093] The intrinsic purple polyimide film was prepared from monomers 1,4-BDDA, 1,8-BDDA and TFMB, 6FDA in a ratio of 0.5:0.5:99 (molar ratio), and the specific method is as follows:

[0094] (1) Under the conditions of sufficient dehydration and deoxygenation and nitrogen protection, 0.045mmol 1,4-BDDA, 0.045mmol 1,8-BDDA, 8.91mmol TFMB and 20ml DMAc were added to a 100mL three-necked round-bottom flask equipped with a nitrogen inlet, a mechanical stirrer and a cold water bath, and stirred until dissolved to form a homogeneous solution; a total of 9mmol 6FDA was added to the above solution in three times, and stirred for 14 hours under a cold water bath to obtain a purple polyamic acid solution with a certain viscosity. 3ml acetic anhydride / pyridine solution was added and stirred overnight, precipitated in an ethanol solution, and placed in a vacuum oven for vacuum drying.

[0095] (2) The polyimide precipitate obtained in step (1) is re-formed into a solution with a solid content of 10%, and then evenly coated on a dry and clean glass plate by an automatic coating machine, and the solvent is removed to obtain a 20 μm film. The temperature curing program is: 80°C / 3h, 100°C / 1h, 200°C / 2h, 300°C / 2h.

[0096] Comparative Example 1

[0097] Colorless and transparent polyimide film was prepared from TFMB and 6FDA.

[0098] Performance Characterization

[0099] The LAB values ​​of the polyimides obtained in Examples 1-19 and Comparative Example 1 were measured, and the test results are shown in Table 1 below.

[0100] Table 1 Polyimide LAB values ​​of Examples 1-6 and Comparative Examples

[0101] PI <![CDATA[L * ]]> <![CDATA[a * ]]> <![CDATA[b * ]]> Example 1 75.37 22.97 -14.38 Example 2 77.43 -20.26 -9.10 Example 3 92.50 -0.94 29.93 Example 4 70.09 16.83 -16.99 Example 5 75.06 10.30 -13.56 Example 6 71.64 3.80 -14.79 Example 7 74.84 -4.39 -12.24 Example 8 71.66 -9.98 -11.72 Example 9 80.24 15.23 -3.86 Example 10 81.06 14.17 -0.56 Embodiment 11 81.85 12.38 4.39 Example 12 85.04 7.00 13.38 Example 13 85.95 5.95 19.32 Embodiment 14 81.44 -15.58 0.41 Embodiment 15 78.78 -17.22 3.80 Example 16 82.54 -12.92 11.06 Embodiment 17 84.82 -9.73 19.36 Embodiment 18 89.25 -7.65 20.29 Embodiment 19 71.85 3.60 -15.62 Comparative Example 1 90 0.3 -0.6

[0102] The thermal properties of the polyimide obtained in some examples and comparative example 1 were measured, and the test results are shown in Table 2 below.

[0103] Table 2 Thermal properties of polyimides obtained in some examples and comparative example 1

[0104]

[0105] The elastic modulus, tensile strength, elongation at break, thermal expansion coefficient and other properties of the polyimide obtained in some examples and comparative example 1 were measured, and the test results are shown in Table 3 below.

[0106] Table 3 Other properties of polyimide obtained in some examples and comparative example 1

[0107]

[0108] The molecular weight and distribution of the polyimide obtained in some examples and comparative example 1 were measured, and the test results are shown in Table 4 below.

[0109] Table 4 Molecular weight and distribution of polyimide obtained in some examples and comparative example 1

[0110]

[0111] From the above data, it can be seen that the performance of the colored polyimide prepared by the present invention is not deteriorated due to the influence of color, and the thermal properties, elongation at break and tensile strength of some colored polyimides are improved to varying degrees compared with the polyimide prepared in Comparative Example 1.

[0112] Those skilled in the art will easily understand that the above descriptions are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a colored polyimide film, characterized in that: The colors include but are not limited to blue, blue-purple, purple-blue, blue-green, green-blue, green, green-yellow, yellow-green, purple-red, red, orange-red, and orange-yellow. The preparation method is to mix a soluble yellow polyimide film with an intrinsic cyan polyimide film derived from a 1,4-BDDA monomer structural unit or an intrinsic magenta polyimide film derived from a 1,8-BDDA monomer structural unit in a certain proportion; the proportion is obtained by combining the film thickness and the following absorbance curve formula: Wherein α(λ) is the absorbance of the target polyimide film at the wavelength λ, δ is the film thickness, ω is the mass fraction of the polyimide film used for mixing, and A, B, C... are the polyimide films involved in the mixing.

2. A method for preparing a soluble yellow polyimide film, characterized in that: The method comprises the following steps: (1) Under the conditions of dehydration, deoxygenation and inert gas protection, p-diaminoazobenzene, TFMB and DMAc are added to a reaction container, stirred until dissolved to form a homogeneous solution, 6FDA is added and stirred in a cold water bath for reaction for 12-16 hours to obtain a yellow polyamic acid solution with a certain viscosity, wherein the molar volume ratio of p-diaminoazobenzene, TFMB and DMAc is 0.5-0.6mmol:8-9mmol:10-30ml; (2) After the yellow polyamic acid solution is vacuum degassed, it is coated on a substrate and subjected to thermal imidization to obtain a yellow intrinsic polyimide, or an acetic anhydride / pyridine solution is added and stirred overnight to precipitate, and the product is vacuum dried to obtain a solution after dissolution, which is then coated on a carrier and the solvent is removed to obtain a yellow intrinsic polyimide.

3. The method for preparing the soluble yellow polyimide film according to claim 2, characterized in that: In the step (1), 6FDA is added in 2-5 portions.

4. The method for preparing a soluble yellow polyimide film according to claim 2, characterized in that: The solid content of the solution after dissolution in step (2) is 7-13%.

5. A method for preparing a purple polyimide film, characterized in that: The method comprises the following steps: (1) Under the conditions of dehydration, deoxygenation and inert gas protection, 1,4-BDDA, 1,8-BDDA, TFMB and DMAc were added to a reaction container, stirred until dissolved to form a homogeneous solution, 6FDA was added and stirred in a cold water bath for reaction for 12-16 hours to obtain a purple polyamic acid solution with a certain viscosity, wherein the molar volume ratio of 1,4-BDDA, 1,8-BDDA, TFMB and DMAc was 0.04-0.05mmol: 0.04-0.05mmol: 8.2-9.5mmol: 10-30ml; (2) After the purple polyamic acid solution is vacuum degassed, it is coated on a substrate and subjected to thermal imidization to obtain purple intrinsic polyimide, or acetic anhydride / pyridine solution is added and stirred overnight to precipitate, and the product is vacuum dried to obtain a solution after dissolution, which is then coated on a carrier and the solvent is removed to obtain purple intrinsic polyimide.

6. The method for preparing the purple polyimide film according to claim 5, characterized in that: In the step (1), 6FDA is added in 2-5 portions.

7. The method for preparing the purple polyimide film according to claim 5, characterized in that: The solid content of the solution after dissolution in step (2) is 7-13%.

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