Preparation method of polyimide film for flexible display

Abstract

The invention relates to the technical field of polyimide, in particular to a preparation method of a polyimide film and the polyimide film prepared by the preparation method. The invention provides the preparation method of the polyimide film, two different coupling agents are compounded to modify the inorganic filler, and the prepared polyimide film has the light transmittance of more than or equal to 85%, the tensile strength of more than or equal to 165 MPa, the elongation at break of more than or equal to 30% and the coefficient of linear thermal expansion (CTE) of less than or equal to 18 ppm / K.

Description

Technical Field

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

[0002] Flexible displays have the characteristics of being lightweight, thin, foldable, bendable, wearable and portable, breaking through the design limitations of traditional displays and meeting the diverse application needs of future display terminals. Therefore, they have become the development direction of new display technologies. In the past, glass substrates were mostly used as substrates for display devices. Glass substrates are excellent in transparency, solvent resistance, gas barrier properties, and heat resistance, but the disadvantage is that they cannot be bent or deformed and are not flexible. In the past decade or so, flexible substrates have great potential to replace traditional glass substrates on a large scale, and are considered to be the key to the development of flexible displays.

[0003] Polyimide (PI) films are widely used in flexible display substrates due to their good thermal stability, good mechanical properties and chemical properties. However, compared with inorganic materials, polyimide has poor thermal stability, which is mainly reflected in the large coefficient of thermal expansion (CTE). In the process of display substrates, it will warp, crack or delaminate with the inorganic substrate. Flexible displays also have high requirements for the bending resistance of the substrate. In addition, traditional polyimides are mostly aromatic polyimides. Due to the presence of conjugated aromatic ring structures on their main chains, they are easy to form intramolecular and intermolecular charge transfer complexes, resulting in poor light transmittance of the film in the visible light region and a characteristic yellow color, which cannot meet the requirements of optoelectronic display substrate materials for colorless and high transparency of the film.

[0004] Patent application CN112375241A provides a colorless transparent polyimide film and a preparation method thereof. In the process of synthesizing the polyimide film, the molar ratio of the mixture of the dianhydride selected from the alicyclic structure and the dianhydride of the non-alicyclic structure is limited to 1:0.8-1.5, that is, by controlling the type and molar ratio of the dianhydride, the transparency and mechanical properties of the polyimide film can be improved. However, although the light transmittance of the obtained polyimide film is very high, its tensile strength and elongation at break are both low.

[0005] At present, there are many improvement methods for the problem of low light transmittance of polyimide film. However, the existing improvement methods are usually not conducive to obtaining polyimide film with high mechanical properties while improving the light transmittance of polyimide film, so that polyimide film cannot meet the requirements of flexible display screen for bending resistance and low thermal expansion coefficient. Therefore, how to achieve high light transmittance, bending resistance and low thermal expansion coefficient of polyimide film has become a hot topic in research. Summary of the invention

[0006] In order to overcome the shortcomings of the prior art, the present invention provides a method for preparing a polyimide film, which has a simple process, low production cost, and good film-forming performance. The prepared polyimide film has high light transmittance, low linear thermal expansion coefficient, and excellent bending resistance, and can be widely used in the field of flexible displays.

[0007] The object of the present invention is achieved by providing a method for preparing a polyimide film for a flexible display, which is characterized in that the method comprises the following steps:

[0008] S1. Adding an inorganic filler and a coupling agent to a solvent, ultrasonically treating at 30 to 60° C. for 1 to 16 hours to form a uniform dispersion;

[0009] S2. The diamine is added to the dispersion, stirred to completely dissolve, and then the dianhydride is added under stirring to obtain a polyamic acid solution by polycondensation reaction;

[0010] S3. Add the catalyst and the dehydrating agent to the polyamic acid solution, mix them evenly, and cast them onto a glass plate. Treat them in an oven at 30 to 100° C. for 1 to 12 hours, and then treat them at 150 to 250° C. for 10 to 40 minutes to obtain a polyimide film.

[0011] Preferably, the coupling agent is a mixture of a first coupling agent and a second coupling agent, wherein the first coupling agent is a coupling agent containing an epoxy group; the second coupling agent is a silane coupling agent containing a long-chain alkyl group, and the mass ratio thereof is 1:(0.5-3);

[0012] Preferably, the diamine is one of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 2,6-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 2,2'-difluoro-4,4'-diaminobiphenyl and 2,6-difluoro-4,4'-diaminobiphenyl.

[0013] Preferably, the dianhydride is an aromatic dianhydride monomer, preferably at least one of 1,2,4,5-pyromellitic dianhydride, 3,3'4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ketone tetracarboxylic dianhydride, 2,3',3,4'-biphenyltetracarboxylic dianhydride and 4-p-phenylene-diphenyltrimethylol dianhydride.

[0014] Compared with the prior art, the present invention has the following significant features and positive effects:

[0015] 1. The present invention utilizes the spatial effect of the large-volume fluorine-containing groups in the fluorine-containing aromatic diamine to increase the distance between molecular chains, inhibit the charge transfer within and between molecules, and improve the light transmittance of the polyimide film. In addition, the polyimide film is prepared by using fluorine-containing aromatic diamine and aromatic dianhydride monomers, and its molecular chains are locally tightly stacked and regularly arranged, which can effectively regulate the aggregation structure between polyimide resins, thereby increasing the interaction between molecular chains and improving the tensile properties of the polyimide film.

[0016] 2. The present invention modifies the inorganic filler by compounding two different coupling agents, so that during the process of film preparation, the modified inorganic filler can be effectively dispersed and adsorbed by the polyamic acid having carboxyl and amino groups, which not only helps the filler to be stably dispersed in the resin, but also allows the inorganic filler to be combined with the resin molecules through chemical bonds, thereby increasing the interfacial tension between the resin and the inorganic filler, and making the polyimide film have excellent mechanical properties and a low linear expansion coefficient.

[0017] 3. The light transmittance of the polyimide film prepared by the present invention is ≥85%, the tensile strength is ≥165MPa, the elongation at break is ≥30%, and the linear thermal expansion coefficient (CTE) is ≤18ppm / K.

[0018] 4. The preparation method of the polyimide film of the present invention is simple, the production cost is low, the film-forming performance is good, and large-scale production can be achieved. DETAILED DESCRIPTION

[0019] The specific implementation methods of the present invention are further described in detail below in conjunction with the examples. In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise expressly stated, throughout the specification and claims, the term "including" or its variations such as "comprising" or "including" will be understood to include the stated components and steps, without excluding the presence of other material components or steps.

[0020] In addition, in order to better illustrate the present invention, numerous specific details are provided in the specific embodiments below. It should be understood by those skilled in the art that the present invention can be implemented without certain specific details. In certain embodiments, raw materials, methods, means, etc. well known to those skilled in the art are not described in detail to highlight the gist of the present invention.

[0021] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with embodiments.

[0022] All the raw materials in the examples can be purchased from the market.

[0023] Embodiment 1, a method for preparing a polyimide film for a flexible display, the steps comprising:

[0024] S1. 4 g of 80 nm alumina, 2 g of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and 2 g of dodecyltrimethoxysilane were added to 160 g of N-methylpyrrolidone and ultrasonicated at 50 ° C for 3 h to form a uniform dispersion;

[0025] S2. 16.11 g (0.05 mol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was added to the dispersion under stirring until it was completely dissolved, and then 16.11 g (0.05 mol) of 3,3',4,4'-dibenzophenone tetracarboxylic dianhydride was added under stirring, and the mixture was stirred and reacted at 5° C. for 6 h to obtain a polyamic acid solution;

[0026] S3. Add 0.1611 g of pyridine and 0.534 g of acetic anhydride to the polyamic acid solution, mix well, and cast onto a glass plate. Treat in an oven at 40° C. for 4 h, and then treat at 200° C. for 30 min to obtain a polyimide film.

[0027] Embodiment 2, a method for preparing a polyimide film for a flexible display, the steps comprising:

[0028] S1. 8 g of 80 nm silicon nitride, 2 g of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and 2 g of dodecyltrimethoxysilane were added to 200 g of N-methylpyrrolidone and ultrasonicated at 50 ° C for 3 h to form a uniform dispersion;

[0029] S2. 32.02 g (0.1 mol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was added to the dispersion under stirring until it was completely dissolved, and then 32.22 g (0.1 mol) of 3,3',4,4'-dibenzophenone tetracarboxylic dianhydride was added under stirring, and the mixture was stirred and reacted at 5° C. for 6 h to obtain a polyamic acid solution;

[0030] S3. 0.6404 g of pyridine and 1.054 g of acetic anhydride were added to the polyamic acid solution, mixed evenly, and cast onto a glass plate. The mixture was treated in an oven at 40° C. for 4 h, and then treated at 200° C. for 30 min to obtain a polyimide film.

[0031] Embodiment 3, a method for preparing a polyimide film for a flexible display, the steps comprising:

[0032] S1. 4 g of 100 nm alumina, 1 g of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane and 1 g of dodecyltrimethoxysilane were added to 80 g of N-methylpyrrolidone and ultrasonicated at 50 ° C for 3 h to form a uniform dispersion;

[0033] S2. 16.11 g (0.05 mol) of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl was added to the dispersion under stirring until it was completely dissolved, and then 16.11 g (0.05 mol) of 3,3',4,4'-dibenzophenone tetracarboxylic dianhydride was added under stirring, and the mixture was stirred and reacted at 5° C. for 6 h to obtain a polyamic acid solution;

[0034] S3. Add 0.1611 g of pyridine and 0.534 g of acetic anhydride to the polyamic acid solution, mix well, and cast onto a glass plate. Treat in an oven at 40° C. for 4 h, and then treat at 200° C. for 30 min to obtain a polyimide film.

[0035] Example 4, a method for preparing a polyimide film for a flexible display, the preparation method of the polyimide film is the same as that of Example 1, except that in step S1, the mass of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane is replaced with 3g, and the mass of dodecyltrimethoxysilane is replaced with 1.5g.

[0036] Example 5, a method for preparing a polyimide film for a flexible display, the preparation method of the polyimide film is the same as that of Example 1, except that in step S1, the mass of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane is replaced with 1g, and the mass of dodecyltrimethoxysilane is replaced with 3g.

[0037] Example 6, a method for preparing a polyimide film for a flexible display, the method for preparing the polyimide film is the same as that of Example 1, except that in step S1, the average particle size of aluminum oxide is 100 nm.

[0038] Comparative Example 1:

[0039] This embodiment provides a method for preparing a polyimide film. The method for preparing a polyimide film is the same as that of Embodiment 1, except that in step S1, dodecyltrimethoxysilane is replaced with an equal mass of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

[0040] Comparative Example 2:

[0041] This embodiment provides a method for preparing a polyimide film, which is the same as that of Example 1, except that in step S1, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane in Comparative Example 2 is replaced with an equal mass of dodecyltrimethoxysilane.

[0042] Comparative Example 3:

[0043] This embodiment provides a method for preparing a polyimide film, which is the same as that of Embodiment 1, except that in step S2, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl is replaced with a certain amount of 2,2'-bis(trimethyl)-4,4'-diaminobiphenyl.

[0044] Comparative Example 4:

[0045] This embodiment provides a method for preparing a polyimide film. The method for preparing a polyimide film is the same as that of Embodiment 1, except that in step S1, dodecyltrimethoxysilane is replaced by 3-aminopropyltrihydroxysilane.

[0046] Performance Testing:

[0047] 1. Light transmittance: The light transmittance of the polyimide film in the visible light range of 400nm to 700nm was measured by an ultraviolet spectrophotometer.

[0048] 2. Linear thermal expansion coefficient: The coefficient was measured using a thermomechanical analyzer in a nitrogen atmosphere with a load of 50 mN applied and a heating rate of 10°C / min to obtain the average value.

[0049] 3. Mechanical properties: Use electronic universal testing machine (Shimadzu AG-X plus) to test tensile strength and elongation at break. Test conditions: test speed 5mm / min, sample size 10mm*15mm*20μm, test gauge length: 50mm, extensometer gauge length 20mm.

[0050] The test results of polyimide film properties are shown in Table 1.

[0051] Table 1

[0052]

[0053]

Claims

1. A method for preparing a polyimide film for a flexible display, characterized in that: The method comprises the following steps: S1. Adding an inorganic filler and a coupling agent to a solvent, ultrasonicating at 30 to 60° C. for 1 to 16 hours to form a uniform dispersion; S2. The diamine is added to the dispersion, stirred to completely dissolve, and then the dianhydride is added under stirring to obtain a polyamic acid solution by polycondensation reaction; S3. The catalyst and the dehydrating agent are added to the polyamic acid solution, mixed and evenly cast onto a glass plate, treated in an oven at 30 to 100 ° C for 1 to 12 h, and then treated at 150 to 250 ° C for 10 to 40 min to obtain a polyimide film; The coupling agent is a mixture of a first coupling agent and a second coupling agent, and the mass ratio thereof is 1:(0.5-3). The first coupling agent is a coupling agent containing an epoxy group, and the second coupling agent is a silane coupling agent containing a long-chain alkyl group.

2. The method for preparing a polyimide film for a flexible display according to claim 1, characterized in that: The first coupling agent is selected from at least one of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidyloxypropyltrimethoxysilane, 3-glycidyloxypropylmethyldiethoxysilane, and 3-glycidyloxypropyltriethoxysilane; the second coupling agent is selected from one of dodecyltrimethoxysilane, tridecyltrimethoxysilane, tetradecyltrimethoxysilane, pentadecyltrimethoxysilane, hexadecyltrimethoxysilane, heptadecyltrimethoxysilane, and octadecyltrimethoxysilane.

3. The method for preparing a polyimide film for a flexible display according to claim 1, characterized in that The average particle size of the inorganic filler is 20 to 100 nm.

4. The method for preparing a polyimide film for a flexible display according to claim 1, characterized in that: The inorganic filler is at least one of an oxide or a nitride.

5. The method for preparing a polyimide film for a flexible display according to claim 1, characterized in that: The solvent is at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc).

6. The method for preparing a polyimide film for a flexible display according to claim 1, characterized in that: The mass ratio of the inorganic filler, the coupling agent and the solvent is (0.2-2):1:(20-60).

7. The method for preparing a polyimide film for a flexible display according to claim 1, characterized in that: The diamine is one of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 2,6-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 2,2'-difluoro-4,4'-diaminobiphenyl and 2,6-difluoro-4,4'-diaminobiphenyl.

8. The method for preparing a polyimide film for a flexible display according to claim 1, characterized in that: The dianhydride is an aromatic dianhydride monomer, preferably at least one of 1,2,4,5-pyromellitic acid dianhydride, 3,3'4,4'-biphenyltetracarboxylic acid dianhydride, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, 2,3',3,4'-biphenyltetracarboxylic acid dianhydride and 4-p-phenylene-diphenyltrimethylol dianhydride.

9. The method for preparing a polyimide film for a flexible display according to claim 1, characterized in that: The mass ratio of the sum of the mass of the diamine and the dianhydride to the mass of the dispersion is (1-3):(4-8).

10. The method for preparing a polyimide film for a flexible display according to any one of claims 1 to 9, characterized in that: The obtained polyimide film has a light transmittance of ≥85%, a tensile strength of ≥165 MPa, an elongation at break of ≥30%, and a linear thermal expansion coefficient (CTE) of ≤18 ppm / K.

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