Folding-resistant polyimide film and preparation method thereof

Abstract

The invention belongs to the technical field of high polymer material polymer films, and particularly relates to a folding-resistant polyimide film and a preparation method thereof. According to the preparation method, a polyamine monomer containing a plurality of amino groups (more than 3) is introduced at the same time, Sr salt is used as a filler, Sr ions can perform weak coordination with a plurality of N atoms in polyimide in different directions in space by utilizing the large radius and completely empty 4d orbits of the Sr ions, and more than three amino groups can form weak coordination in more than three directions, so that the stability of the polyimide is improved, and the stability of the polyimide is improved. The multi-point geometric energy dissipation unit serves as a multi-point geometric energy dissipation unit. Meanwhile, Sr ions and aromatic rings form cation-pi interaction, the unique point-to-face geometrical configuration can also form a stress buffer area larger than a hydrogen bond and become another important energy dissipation unit, and finally the folding resistance of the polyimide film is greatly enhanced.

Description

Technical Field

[0001] This invention belongs to the field of polymer film technology, specifically relating to a foldable polyimide film and its preparation method. Background Technology

[0002] Polyimide (PI) film, with its unique electrical properties, high-temperature stability, chemical corrosion resistance, and high strength and toughness, has become one of the key materials for flexible display devices, flexible solar cells, flexible printed circuit boards, and flexible touch panels. When used as a substrate material for flexible displays, polyimide film needs to withstand hundreds of thousands of folds. However, the energy during folding of polyimide film can only be dissipated through irreversible molecular chain slippage, leading to the accumulation of plastic deformation. Therefore, after tens of thousands of folds, irreversible creases will form on the film surface and inside, eventually forming visible creases and causing mechanical failure. This results in problems such as cracking and delamination between different layers of the flexible display, directly affecting the service life of the device.

[0003] CN113667304A discloses a process for preparing a light-colored, transparent, and bend-resistant semi-aromatic polyimide film. This process introduces nano-SiO2 and crown ethers into the semi-aromatic polyimide to improve its bend resistance. The patent utilizes crown ethers containing heterocyclic or aromatic ring substituents to encapsulate the semi-aromatic polyamic acid, and then introduces nano-silica particles using a solution blending method. This achieves simultaneous reinforcement, toughening, and flexibility of the semi-aromatic polyimide, thereby overcoming the problems of brittleness and poor bend resistance in semi-aromatic polyimide films.

[0004] However, in the aforementioned patents, there is competition or interference between the inclusion effect of the crown ether and the polymer chain, and between the physical / chemical interaction of SiO2 and the polymer (or crown ether). An improper balance may lead to: 1) the crown ether primarily adsorbing on the SiO2 surface, losing its plasticizing / flexible effect on the polymer; 2) excessive encapsulation of SiO2 by the crown ether, resulting in a loss of effective stress transfer with the matrix. Furthermore, the rigid backbone of aromatic polyimides is fundamental to their high heat resistance, high modulus, low coefficient of thermal expansion, and excellent dielectric properties. Excessive flexibility (plasticization) may lead to a significant decrease in the glass transition temperature (Tg), an increase in the coefficient of thermal expansion, a decrease in modulus, and an increase in the dielectric constant (the crown ether may have high polarity), which is unacceptable in high-end electronic packaging and other fields. Summary of the Invention

[0005] This invention provides a fold-resistant polyimide film, solving the aforementioned technical problems existing in the prior art.

[0006] Therefore, the technical solution provided by the present invention is as follows:

[0007] A foldable polyimide film is obtained by polycondensation reaction of organic amine monomer and dianhydride monomer, followed by the addition of filler to obtain polyimide powder. The polyimide powder is dissolved in an organic solution to form a polyimide coating liquid, which is then coated on a substrate and baked in stages.

[0008] The organic amine monomer comprises three or more amino groups.

[0009] Furthermore, the mass percentages of the polyamine monomer, dianhydride monomer, and filler are 30-58%, 36-63%, and 6-9%, respectively.

[0010] Furthermore, the polyamine monomer is 3,3'-diaminobenzidine, 1,4-bis(3,4-diaminophenoxy)benzene, or 3,3',4,4'-tetraaminodiphenyl ether.

[0011] Furthermore, the dianhydride monomer is 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride or 1,2,3,4-cyclobutanetetracarboxylic dianhydride.

[0012] Furthermore, the filler is strontium chloride, strontium sulfate, or strontium carbonate.

[0013] This invention also provides a method for preparing a fold-resistant polyimide film, comprising the following steps:

[0014] Step 1) Mix the organic amine monomer and dianhydride monomer with an organic solvent until fully dissolved at room temperature, then heat to 175~185℃ for polycondensation reaction for 11~13h to obtain polycondensation reaction solution. Then add seasoning to disperse it in the polycondensation reaction solution.

[0015] Step 2) Add methanol to the polycondensation reaction solution to precipitate solid, filter, wash the solid product three or more times, and then vacuum dry at 150~160℃ for 20~24h to obtain polyimide powder.

[0016] Step 3) Dissolve the obtained polyimide powder in N,N-dimethylformamide to obtain a polyimide coating solution. After filtration and vacuum degassing, apply the solution to a glass or metal substrate with a doctor blade. After segmented baking, the total baking time is 200 min.

[0017] The organic solvent mentioned in step 1) is a mixture of isoquinoline, m-cresol, toluene and N-methylpyrrolidone, wherein the volume ratio of isoquinoline, m-cresol, toluene and N-methylpyrrolidone is 0.05:3:0.5:10.

[0018] The segmented baking temperatures mentioned in step 3) are 60~65℃, 115~120℃, 175~180℃, and 200-210℃, and the baking time for each segment is 50 minutes.

[0019] The polyimide film can withstand more than 250,000 folds at a fold radius of 0.5 mm.

[0020] The beneficial effects of this invention are as follows:

[0021] The fold-resistant polyimide film provided by this invention introduces multiple amine groups (more than three) into a polyamine monomer simultaneously, using Sr salt as a filler. Leveraging the large radius and completely empty 4d orbitals of Sr ions, Sr ions can weakly coordinate with multiple N atoms in the polyimide in different spatial directions. Three or more amine groups can form weak coordination interactions in more than three directions, resulting in more complete energy dissipation and serving as a multi-point geometric energy dissipation unit. Simultaneously, Sr ions form cation-π interactions with aromatic rings. This unique "point-to-surface" geometric configuration can also form a stress buffer region larger than hydrogen bonds, becoming another important energy dissipation unit, ultimately greatly enhancing the fold resistance of the polyimide film. Detailed Implementation

[0022] The following specific embodiments illustrate the implementation of the invention. Those skilled in the art can easily understand other advantages and effects of the invention from the content disclosed in this specification.

[0023] Exemplary embodiments of the invention are now described; however, the invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments is not intended to limit the invention.

[0024] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0025] Example 1

[0026] This embodiment provides a foldable polyimide film, which is obtained by polycondensation reaction of organic amine monomer and dianhydride monomer, followed by the addition of filler to obtain polyimide powder. The polyimide powder is dissolved in an organic solution to form a polyimide coating liquid, which is then coated on a substrate and baked in stages.

[0027] The organic amine monomer comprises three or more amino groups.

[0028] In this embodiment, the mass percentages of the polyamine monomer, dianhydride monomer, and filler are 30%, 63%, and 7%, respectively.

[0029] The polyamine monomer is 3,3'-diaminobenzidine, with the following structural formula:

[0030]

[0031] 3,3'-Diaminobenzidine contains four amino groups, forming a three-dimensional network structure that can prevent the plastic slippage of macromolecular chains under cyclic bending, thereby avoiding the early formation of creases.

[0032] The dianhydride monomer is 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride, and the filler is strontium chloride. As a filler, strontium chloride, due to its large Sr ion radius and completely empty 4d orbitals, allows Sr ions to weakly coordinate with multiple N atoms in polyimide in different spatial directions. Three or more amine groups can form weak coordination interactions in more than three directions, resulting in more complete energy dissipation, thus serving as a multi-point geometric energy dissipation unit. Simultaneously, Sr ions form cation-π interactions with aromatic rings. This unique "point-to-surface" geometric configuration can also form a stress buffer region larger than hydrogen bonds, becoming another important energy dissipation unit, ultimately greatly enhancing the folding resistance of the polyimide film.

[0033] Preparation process:

[0034] (1) Mix 6.4 g (30 mmol) of 3,3'-diaminobenzidine, 13.33 g (30 mmol) of 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride with 1 ml isoquinoline, 60 ml m-cresol, 10 ml toluene and 200 ml N-methylpyrrolidone. Stir at room temperature for 2 h to dissolve completely, then heat to 175 °C for polycondensation reaction. React for 12 h to obtain polycondensation reaction solution. Add 1.5 g of strontium chloride to disperse it in the polycondensation reaction solution.

[0035] (2) Add 800 ml of methanol to the polycondensation reaction solution obtained in step (1), precipitate solid, filter, wash the solid product three times with deionized water, and finally dry it under vacuum at 150°C for 24 hours to obtain polyimide powder.

[0036] (3) Dissolve the polyimide powder obtained in step (2) in N,N-dimethylformamide to obtain a polyimide coating solution. After filtration and vacuum degassing, apply the solution to a glass or metal substrate with a doctor blade. Bake at four temperature ranges: 60℃ (50 minutes), 120℃ (50 minutes), 180℃ (50 minutes), and 210℃ (50 minutes), for a total baking time of 200 minutes, and finally obtain a polyimide film.

[0037] Example 2

[0038] This embodiment provides a foldable polyimide film, which is obtained by polycondensation reaction of organic amine monomer and dianhydride monomer, followed by the addition of filler to obtain polyimide powder. The polyimide powder is dissolved in an organic solution to form a polyimide coating liquid, which is then coated on a substrate and baked in stages.

[0039] The organic amine monomer comprises three or more amino groups.

[0040] In this embodiment, the mass percentages of the polyamine monomer, dianhydride monomer, and filler are 58%, 36%, and 6%, respectively.

[0041] The polyamine monomer is 1,4-bis(3,4-diaminophenoxy)benzene, the dianhydride monomer is 1,2,3,4-cyclobutanetetracarboxylic dianhydride, and the filler is strontium sulfate.

[0042] In this embodiment, the 3,4-diaminobenzene structural unit of 1,4-bis(3,4-diaminophenoxy)benzene facilitates the formation of a rigid imide ring, ensuring the material's high glass transition temperature, high thermal stability, and excellent mechanical strength. The intermediate ether bond and benzene ring together form a flexible "hinge." The ether bond (-O-) provides rotational freedom of bond angles, while the introduction of the benzene ring avoids the introduction of excessively long aliphatic chains, thus improving heat resistance. This structure gives the polymer backbone a macroscopic "rigid-flexible block" characteristic. The rigid portion provides strength, while the flexible hinge absorbs and disperses bending stress, thereby greatly improving the film's flexibility, tear resistance, and resistance to repeated folding fatigue.

[0043] Preparation process:

[0044] (1) Mix 9.66g of 1,4-bis(3,4-diaminophenoxy)benzene, 5.88g of 1,2,3,4-cyclobutanetetracarboxylic dianhydride with 1 ml of isoquinoline, 60 ml of m-cresol, 10 ml of toluene and 200 ml of N-methylpyrrolidone. Stir at room temperature for 2 hours to dissolve completely, then heat to 180°C for polycondensation reaction. React for 12 hours to obtain polycondensation reaction solution. Add 1g of strontium chloride to disperse it in the polycondensation reaction solution.

[0045] (2) Add 800 ml of methanol to the polycondensation reaction solution obtained in step (1), precipitate solid, filter, wash the solid product three times with deionized water, and finally dry it under vacuum at 160°C for 24 hours to obtain polyimide powder.

[0046] (3) Dissolve the polyimide powder obtained in step (2) in N,N-dimethylformamide to obtain a polyimide coating solution. After filtration and vacuum degassing, apply it to a glass or metal substrate with a doctor blade. Bake at four temperature ranges: 65℃ (50 minutes), 115℃ (50 minutes), 180℃ (50 minutes), and 210℃ (50 minutes), for a total baking time of 200 minutes, and finally obtain a polyimide film.

[0047] Example 3

[0048] This embodiment provides a foldable polyimide film, which is obtained by polycondensation reaction of organic amine monomer and dianhydride monomer, followed by the addition of filler to obtain polyimide powder. The polyimide powder is dissolved in an organic solution to form a polyimide coating liquid, which is then coated on a substrate and baked in stages.

[0049] The organic amine monomer comprises three or more amino groups.

[0050] In this embodiment, the mass percentages of the polyamine monomer, dianhydride monomer, and filler are 31%, 60%, and 9%, respectively.

[0051] The polyamine monomer is 3,3',4,4'-tetraaminodiphenyl ether, the dianhydride monomer is 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride, and the filler is strontium chloride.

[0052] In this embodiment, the COC bond angle (approximately 120°) in the ether bond of the 3,3',4,4'-tetraaminodiphenyl ether is more flexible than the C / C bond, and the oxygen atom provides a certain degree of rotational freedom for the chain segments. When this structure is introduced into the polyimide backbone, it acts as a built-in "flexible joint" or "hinge." During repeated bending of the film, this flexible hinge can undergo minute twists and bends, effectively absorbing and dispersing mechanical stress, preventing excessive stress concentration in localized areas that could lead to crack initiation and propagation.

[0053] Preparation process:

[0054] (1) Mix 6.9g of 3,3',4,4'-tetraaminodiphenyl ether, 13.33g of 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride with 1ml isoquinoline, 60ml m-cresol, 10ml toluene and 200ml N-methylpyrrolidone. Stir at room temperature for 2 hours to dissolve completely, then heat to 180℃ for polycondensation reaction. React for 12 hours to obtain polycondensation reaction solution. Add 2g of strontium chloride to disperse it in the polycondensation reaction solution.

[0055] (2) Add 800 ml of methanol to the polycondensation reaction solution obtained in step (1), precipitate solid, filter, wash the solid product three times with deionized water, and finally dry it under vacuum at 160°C for 24 hours to obtain polyimide powder.

[0056] (3) Dissolve the polyimide powder obtained in step (2) in N,N-dimethylformamide to obtain a polyimide coating solution. After filtration and vacuum degassing, coat it onto a glass or metal substrate with a doctor blade. Bake at four temperature ranges: 62℃ (50 minutes), 118℃ (50 minutes), 175℃ (50 minutes), and 200℃ (50 minutes), for a total baking time of 200 minutes, and finally obtain a polyimide film.

[0057] Comparative Example 1

[0058] Compared with Example 1, the organic amine in Comparative Example 1 was a diamine monomer, specifically 5.24 g of p-phenylenediamine, equimolar with 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride, and the remaining steps were the same.

[0059] Comparative Example 2

[0060] Comparative Example 2 does not contain strontium chloride, but the remaining steps are the same as in Example 1.

[0061] To further illustrate the effects of the present invention, the polyimide films prepared in Examples 1-3 and Comparative Example 1 were subjected to the following performance tests.

[0062] The folding properties of polyimide films were determined using GB / T 13542.6-2006, and the results are shown in Table 1.

[0063] Table 1 Performance Tests

[0064]

[0065] Experimental results show that the polyimide films prepared in Examples 1-3 of this invention, due to the introduction of polyamine monomers with multiple amine groups (more than 3) and the use of Sr salts as fillers, utilize the large radius of Sr ions and their completely empty 4d orbitals. Sr ions can weakly coordinate with multiple N atoms in polyimide in different spatial directions. The presence of more than three amine groups allows for weak coordination in more than three directions, resulting in more complete energy dissipation and serving as a multi-point geometric energy dissipation unit. Simultaneously, Sr ions form cation-π interactions with aromatic rings. This unique "point-to-surface" geometric configuration can also form a stress buffer region larger than hydrogen bonds, becoming another important energy dissipation unit. Ultimately, this significantly enhances the folding resistance of the polyimide film.

[0066] As shown in Table 1, although both examples contain strontium salt fillers, the polyimide film prepared in Example 2 exhibits better bending resistance than that in Examples 1 and 3. This is because the p-phenylenedioxy group in 1,4-bis(3,4-diaminophenoxy)benzene forms a super-strong flexible hinge. Compared to the single ether bond in Example 3, this "benzene ring-ether bond-benzene ring" hinge is longer and more flexible, allowing for greater torsional deformation when the polymer chain is bent. This effectively absorbs and disperses mechanical stress, preventing microcrack formation. Example 1, containing unstable N-N bonds, shows significantly improved bending resistance through the addition of strontium chloride. In Comparative Document 1, the two amino groups of p-phenylenediamine are located at the para position on the benzene ring, resulting in a linear molecular chain structure. Chain segment rotation is difficult, meaning that bending energy cannot be dissipated through internal rotation. Stress is highly concentrated locally, leading to a very brittle film with extremely poor bending and fatigue resistance. Comparative Example 2, lacking strontium salt, cannot dissipate energy through weak coordination, thus exhibiting poor folding resistance.

[0067] As shown in Table 1, the polyimide film prepared by this invention can withstand more than 250,000 folds at a fold radius of 0.5 mm, meeting the needs of flexible display devices, flexible solar cells, flexible printed circuit boards, and flexible touch panels.

[0068] The examples above are merely illustrative of the invention and do not constitute a limitation on the scope of protection of the invention. Any design that is the same as or similar to the invention falls within the scope of protection of the invention.

Claims

1. A fold-resistant polyimide film, characterized in that: Polyimide powder is obtained by polycondensation reaction of organic amine monomer and dianhydride monomer, followed by the addition of filler. The polyimide powder is dissolved in an organic solution to form a polyimide coating solution, which is then coated on a substrate and baked in stages. The organic amine monomer comprises three or more amino groups.

2. The fold-resistant polyimide film according to claim 1, characterized in that: The mass percentages of the polyamine monomer, dianhydride monomer, and filler are 30-58%, 36-63%, and 6-9%, respectively.

3. The fold-resistant polyimide film according to claim 1, characterized in that: The polyamine monomer is 3,3'-diaminobenzidine, 1,4-bis(3,4-diaminophenoxy)benzene, or 3,3',4,4'-tetraaminodiphenyl ether.

4. The fold-resistant polyimide film according to claim 1, characterized in that: The dianhydride monomer is 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride or 1,2,3,4-cyclobutanetetracarboxylic dianhydride.

5. A fold-resistant polyimide film according to any one of claims 1-4, characterized in that: The filler is strontium chloride, strontium sulfate, or strontium carbonate.

6. A method for preparing a fold-resistant polyimide film as described in any one of claims 1-5, characterized in that: Includes the following steps: Step 1) Mix the organic amine monomer and dianhydride monomer with an organic solvent until fully dissolved at room temperature, then heat to 175~185℃ for polycondensation reaction for 11~13h to obtain polycondensation reaction solution. Then add seasoning to disperse it in the polycondensation reaction solution. Step 2) Add methanol to the polycondensation reaction solution to precipitate solid, filter, wash the solid product three or more times, and then vacuum dry at 150~160℃ for 20~24h to obtain polyimide powder. Step 3) Dissolve the obtained polyimide powder in N,N-dimethylformamide to obtain a polyimide coating solution. After filtration and vacuum degassing, apply the solution to a glass or metal substrate with a doctor blade. After segmented baking, the total baking time is 200 min.

7. The method for preparing a fold-resistant polyimide film according to claim 6, characterized in that: The organic solvent mentioned in step 1) is a mixture of isoquinoline, m-cresol, toluene and N-methylpyrrolidone, wherein the volume ratio of isoquinoline, m-cresol, toluene and N-methylpyrrolidone is 0.05:3:0.5:

10.

8. The method for preparing a fold-resistant polyimide film according to claim 6, characterized in that: The segmented baking temperatures mentioned in step 3) are 60~65℃, 115~120℃, 175~180℃, and 200-210℃, and the baking time for each segment is 50 minutes.

9. The method for preparing a fold-resistant polyimide film according to claim 6, characterized in that: The polyimide film can withstand more than 250,000 folds at a fold radius of 0.5 mm.

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