Fluorine-free, high-transparency and heat-resistant polyimide flexible material as well as preparation method and application thereof

Abstract

According to the fluorine-free, high-transparency and heat-resistant polyimide flexible material and the preparation method and application thereof, through specific molecular design, on the premise that a fluorine-containing monomer is not used, the light transmittance of the prepared polyimide material in a visible light region is not lower than 85%, the yellowness index is not higher than 2.0, and the optical performance of the polyimide material is superior to that of a similar fluorine-containing system; meanwhile, the glass transition temperature is higher than 300 DEG C, the thermal decomposition temperature is higher than 450 DEG C, the carbon residue rate is higher than 50%, and excellent thermal stability and high-temperature stability are shown. The polyimide film can resist solvent erosion in a color photoresist process and the post-baking temperature of 230 DEG C, is completely compatible with a standard photoetching process, successfully realizes complete transfer of a mask pattern to a polyimide substrate, and has no warping or stripping phenomenon. And the material does not have creases or damages after being subjected to 200 thousand times of folding tests, so that the reliability requirement of the flexible display on the substrate material is met, and the application prospect is good.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of organic synthesis and polymer synthesis, in particular to a kind of fluorine-free, high-transparency, heat-resistant polyimide flexible material and its preparation method and application. BACKGROUND

[0002] Transparent polyimide material has important application value in the field of flexible display, and its high heat resistance and optical transparency make it a key material to replace traditional glass substrate. However, the transparent polyimide in the prior art still faces many challenges. Traditional polyimide usually presents a deep color due to intramolecular charge transfer effect, resulting in reduced visible light transmittance, which is difficult to meet the needs of high-end display applications. To improve transparency, the industry generally uses fluorine-containing monomers for molecular design, which uses the strong electronegativity of fluorine atoms to suppress charge transfer, thereby achieving higher optical performance. However, the widespread use of fluorine-containing polyimides is being restricted by increasingly stringent environmental regulations, especially the EU's control policy on perfluoroalkyl substances (PFAS), which limits the development prospects of fluorine-containing materials. In addition, the synthesis process of fluorine-containing monomers is complex and the cost is high, which further limits its industrialization and popularization.

[0003] In the development of fluorine-free transparent polyimide, the prior art has not effectively balanced the key properties such as transparency, thermal stability and process compatibility. Although some fluorine-free materials avoid environmental problems, their optical performance is often poor due to insufficient molecular structure design, resulting in low transmittance or obvious yellowing, which is difficult to meet the needs of high-precision displays. At the same time, the thermal stability of fluorine-free polyimides is usually weak, with a low glass transition temperature, which is prone to deformation during high-temperature processing, affecting the reliability of the device.

[0004] In addition, the actual application of flexible display substrate requires the material to have excellent process compatibility, especially the adaptability to photoresist process. In the process of manufacturing color filter, the polyimide film needs to resist the chemical solvent erosion in the steps of photoresist coating, exposure, development, etc., and can withstand the post-baking temperature (230℃) of color photoresist. The existing fluorine-free polyimides often have low glass transition temperature or insufficient solvent resistance, resulting in film warping, peeling or performance degradation during the process, affecting the yield of the final device. In summary, the existing transparent polyimide material still has significant shortcomings in environmental compliance, optical performance, thermal stability and process compatibility. Therefore, it is urgent to develop a new type of fluorine-free polyimide material that meets the requirements of high light transmittance and low yellowness, while having excellent thermal stability and process adaptability, to promote the further development of flexible display technology. SUMMARY

[0005] In order to solve the technical problems of not being able to simultaneously meet the requirements of no fluorine, high transparency and heat resistance in the prior art, the polyimide material with excellent optical performance and heat stability is unexpectedly obtained by specific molecular structure design, and has high optical transmittance and glass transition temperature.

[0006] Firstly, the application protects a fluorine-free, high-transparency and heat-resistant polyimide flexible material, and the structure is shown in the following general formula I:

[0007]

[0008] In the formula, R1 is selected from one of ODPA, 9,9-BFDA, CPDA, BTDA, CBDA, HPMDA and PMDA.

[0009] R2 is selected from one of A1-A6.

[0010]

[0011] n is an integer between 30 and 100.

[0012] For the technical solution described above, further preferably, the polyimide flexible material is polymerized from a diamine monomer selected from 9,9-bis(4-aminophenyl) fluorene or its derivative and one monomer selected from the following acid anhydride compounds, the light transmittance of the polyimide flexible material at 450 nm wavelength is greater than or equal to 80% (further preferably greater than or equal to 88%, and more preferably greater than or equal to 90%), preferably the yellowness index YI is less than or equal to 5.0 (further preferably less than or equal to 2.0), the glass transition temperature (Tg) is greater than or equal to 230 DEG C (further preferably greater than or equal to 300 DEG C, and more preferably greater than or equal to 320 DEG C), the 5% heat loss temperature T5% is greater than or equal to 450 DEG C, and the carbon residue rate is preferably greater than 50% (further preferably greater than or equal to 60%); wherein:

[0013] The structural formula of the acid anhydride compound ODPA, 9,9-BFDA, CPDA, BTDA, CBDA, HPMDA and PMDA is as follows:

[0014]

[0015] For the technical solution described above, further preferably, the polyimide flexible material, the diamine monomer is 9,9-bis(4-amino-3-methylphenyl) fluorene.

[0016] For the technical solution described above, further preferably, the polyimide flexible material, the acid anhydride monomer is 4,4'-oxydiphthalic anhydride (ODPA).

[0017] A further preferred embodiment of the above-described technical solution is the preparation method of the polyimide flexible material, which uses 9,9-bis(4-aminophenyl)fluorene and 9,9-di(4-amino-3-tolyl)fluorene as parent structures, and prepares a series of different nitro compounds through the reaction of acyl chloride and aniline. Since these are the most reactive carboxylic acid derivatives, the reaction proceeds very easily. The acid anhydride selected in this invention is highly beneficial in reducing the conjugation effect in the polyimide backbone structure, thereby reducing the formation of polyimide electron transfer complexes. The specific steps are as follows:

[0018] (1) The diamine monomer and the acid anhydride monomer are dissolved in a solvent at a molar ratio of 1:0.99-1.02 (more preferably 1:1), and reacted under nitrogen protection and at -5-20°C (more preferably 0-5°C) to generate a polyamic acid solution; the solvent is selected from N,N-dimethylacetamide, N-methylpyrrolidone or N,N-dimethylformamide;

[0019] (2) Add a mixed solution of acetic anhydride and triethylamine to the polyamic acid solution obtained in step (1) and carry out an imidization reaction.

[0020] (3) The film is formed by solution casting and then subjected to shaping, curing and step annealing treatment in sequence.

[0021] For the technical solution described above, a further preferred embodiment is that, in the preparation method, the volume ratio of acetic anhydride to triethylamine is 1:0.8-1.2 (more preferably 1:1); and the volume addition ratio of polyamic acid solution to acetic anhydride is 20:1.

[0022] For the technical solution described above, it is further preferred that the solid content of the polyamic acid solution in step (1) of the preparation method is less than 10 wt% (more preferably less than 5 wt%) to prevent gelation.

[0023] For the technical solution described above, a further preferred embodiment is that the imidization reaction is carried out at 20-25°C for 12-24 hours.

[0024] For the technical solution described above, a further preferred embodiment is that in the preparation method, step (2) involves an imidization reaction at 15-30°C for 18-30 hours.

[0025] For the technical solution described above, a further preferred embodiment of the preparation method is that the shaping and curing steps in step (3) are as follows: the solution obtained in step (2) is precipitated with ethanol, vacuum dried at 40-60℃ for 4-6 hours, and then redissolved with a polar solvent to form a casting solution of 15-20wt%; the casting solution is then scraped onto the substrate through a gap of 200-300μm.

[0026] For the technical solution described above, a further preferred embodiment of the preparation method is the stepwise annealing described in step (3):

[0027] First stage: Curing at 40-50℃ and humidity <30% for 4-6 hours;

[0028] Second stage: Heat treatment at 120-140℃ for 1-3 hours;

[0029] The third stage: step annealing with a gradient of 15-25℃, holding each gradient for 15-25 minutes, first heating to 170-190℃, and finally 20-30℃.

[0030] For the technical solution described above, a further preferred embodiment is that the polar solvent is a mixture of γ-butyrolactone and N,N-dimethylacetamide in a volume ratio of 1:1-3.

[0031] For the technical solutions described above, a further preferred application of the polyimide flexible material in flexible display substrates includes using it as a plastic transparent substrate for flexible displays of LCDs and OLEDs, a TFT substrate, a flexible printed circuit board substrate, a flexible OLED flat lighting substrate, a flexible color filter, or a flexible touch sensor, etc.

[0032] Another aspect of the present invention protects a flexible display device comprising a polyimide flexible material as described in any one of claims 1-3 as a substrate layer, wherein a color filter pattern is formed on the surface of the substrate layer by a photolithography process.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] This invention innovatively proposes a class of fluorine-free transparent polyimide molecular structures. Through specific molecular design, it successfully achieves a balance between high light transmittance and excellent heat resistance without using fluorine-containing monomers, overcoming the contradiction between optical performance and thermal stability inherent in traditional fluorine-free polyimides. This polyimide material exhibits a visible light transmittance of no less than 85% and a yellowness index of no more than 2.0, demonstrating superior optical performance compared to similar fluorine-containing systems. Simultaneously, it possesses a glass transition temperature above 300℃, a thermal decomposition temperature exceeding 450℃, and a char residue greater than 50%, exhibiting excellent thermal stability and high-temperature dimensional stability. The polyimide film can withstand solvent erosion in color photoresist processes and a post-baking temperature of 230℃, is fully compatible with standard photolithography processes, and successfully achieves complete transfer of mask patterns to the polyimide substrate without warping or peeling. Furthermore, the material showed no creases or damage after 200,000 folding tests, demonstrating excellent mechanical folding resistance and meeting the reliability requirements of flexible display substrate materials. This fluorine-free system not only significantly reduces potential harm to health and the environment, but also has a high brightness index and good overall performance. Attached Figure Description

[0035] Figure 1 To load a color photoresist pattern onto Example 5, the surface was relatively clean, with no residual photoresist left.

[0036] Figure 2 The image shows the bending effect after photoresist loading. Loading photoresist has virtually no adverse effect on the bending properties of the film.

[0037] Figure 3 Thermogravimetric analysis curves for Examples 7, 9, and 10 were used to test the thermal decomposition temperature of the three examples. The thermal decomposition temperature (T5%) of the examples was above 450°C, demonstrating the excellent heat resistance of polyimide.

[0038] Figure 4 The thermogravimetric analysis diagrams for Examples 5 and 11 are shown below. Figure 4 It can be concluded that the char residue of both polyimides is very high, above 50%. The high char residue endows the polyimides with excellent thermal stability, proving that the polyimides in this invention can maintain good structural stability.

[0039] Figure 5 For the thermogravimetric analysis diagrams of Comparative Examples 1 and 2, from Figure 5 The results show that although the comparative example has a higher thermal decomposition temperature (T5%), its char residue is lower, much lower than that of the embodiments in this invention, and its structural stability is not as good as that of the embodiments in this invention.

[0040] Figure 6 The thermogravimetric analysis diagram of Example 12 is shown below. Figure 6It can be seen that this embodiment also has a high char residue rate, proving that the design strategy of the all-aromatic polyimide in this invention is effective and endows the polyimide with excellent structural stability.

[0041] Figure 7 The differential scanning calorimeter plots for Examples 5, 7, and 8, and Comparative Examples 1 and 2 are shown below. Figure 7 Analysis shows that Examples 5, 7, 8 and the comparative examples have high glass transition temperatures, while the operating temperature of polyimide often depends on its glass transition temperature, which is much higher than the post-baking temperature (230°C) of industrial color photoresists. This proves that the polyimide in this invention can withstand all processes of color photoresists and can work in harsh environments.

[0042] Figure 8 The light transmittance diagrams for Examples 5, 7, and 9 are shown below. Figure 8 It can be seen that although the examples do not contain fluorine, they have high light transmittance. The transmittance of Example 5 at a wavelength of 450nm is 92.8%, and the transmittance of Examples 7 and 9 is similar, both around 88%, which are all very high.

[0043] Figure 9 The light transmittance diagrams for Examples 10 and 15 are shown. These examples are characterized by the introduction of an aliphatic structure, but their light transmittance is not significantly improved and may even decrease.

[0044] Figure 10 The figures show the light transmittance of Comparative Example 3 and Example 8. Although Comparative Example 3 has a high transmittance (T450nm = 87.15), it is still lower than that of Example 5, and its thermal performance is also far inferior to that of Example 5, indicating that the fluorine-free structure of Example 5 has unexpected advantages.

[0045] Figure 11 The figures show the light transmittance of Comparative Examples 1 and 4. While Comparative Examples 1 and 4 also exhibit transmittance exceeding 80% at 450 nm, they are not as high as those of Examples 5-10 of this invention. This demonstrates that the embodiments of this invention possess high optical transparency, and their fluorine-free structure achieves an unexpected breakthrough in light transmittance.

[0046] Figure 12 The diagram and results of Example 5 are shown in the figure. The film was subjected to 200,000 repeated folding / unfolding experiments at a bending angle of 180°. After repeated unfolding / folding, no obvious creases appeared on the film and there was no damage on the surface. In the field of flexible display, its high folding life can meet the needs of most flexible display devices. Detailed Implementation

[0047] The present invention will be further described below with reference to embodiments. The embodiments of the present invention are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. The raw material specifications, process equipment and testing methods involved in the embodiments are all common knowledge in the art. Any adaptive adjustments made by those skilled in the art according to actual production conditions should be considered as not departing from the protection scope of the core process of the present invention.

[0048] The test methods involved in the following embodiments are uniformly described as follows:

[0049] 1. Thermogravimetric analysis of polyimide

[0050] Thermogravimetric analysis was performed using a Netzsch analytical instrument with nitrogen as the protective and purging gas. The heating rate was 10 °C / min, from 50 °C to 800 °C, to obtain thermogravimetric analysis curves for different polymers.

[0051] 2. Glass transition temperature test of polymers

[0052] The glass transition temperature of polymers was analyzed using differential scanning calorimetry (DSC), with a test range of 25℃-420℃. Argon was used as the purge gas at a flow rate of 50 ml / min to analyze the glass transition temperature of different types of polymers.

[0053] 3. Polymer molecular weight determination: The relative molecular weight of different polyimides was determined using gel permeation chromatography. The internal standard was polystyrene, and the test solvent was THF or DMF.

[0054] 4. Testing of polymer films

[0055] The light transmittance was measured using the Otsuka Electronics multichannel spectrophotometer (MCPD-6800) in the group. On a 5*5cm glass substrate stand, with the same glass substrate as the background, the signal intensity of the background light source was controlled between 0.6 and 0.8, and the mode was selected as relative transmission. Then, the glass substrate loaded with polymer film was placed in the stand, and the transmission spectrum was measured.

[0056] 5. Use MCPD to analyze the color data of the polymer, including its Lab and XYZ values, and select light source C to calculate the yellowness index.

[0057] The haze test of the polymer was commissioned to Shenyang Research and Testing Technology Co., Ltd.

[0058] The thickness of the thin film was measured using a Bruker profilometer.

[0059] The yellowness index of the thin film was calculated based on the C light source;

[0060] Glossary: ​​DMF: N,N-dimethylformamide, NMP: N-methylpyrrolidone, DMAc: N,N-dimethylacetamide, THF: tetrahydrofuran.

[0061] Example 1: Monomer Synthesis and Preparation Example – Preparation of Compound A1:

[0062] Taking the reaction of 9,9-bis(4-amino-3-tolyl)fluorene and 4-nitrobenzoyl chloride as an example, 2 g (5.31 mmol) of 9,9-bis(4-amino-3-tolyl)fluorene was added to a clean three-necked flask, and 2.5 equivalents of 4-nitrobenzoyl chloride (13.275 mol) were weighed. Under a nitrogen atmosphere, 2 ml of pyridine was added to the system. Both raw materials were dissolved in 20 ml of dichloromethane, and then the reaction was carried out using a constant-pressure dropping funnel to add the amino compound. A solution of acyl chloride in dichloromethane was continuously added dropwise, followed by the addition of methanol to precipitate a pale yellow nitro compound solid of 3.384 g (5.02 mmol) with a yield of 94.3%. The precipitate was filtered, washed with ether, dried, and then placed in a clean three-necked flask. 4 g of activated carbon and 2 g of ferric chloride hexahydrate were weighed out, and 40 ml of ethanol was added as a solvent. The mixture was refluxed for 24 h. After the reaction was completed, the solution was filtered, and water was added to the filtrate to precipitate a white solid of compound A1 of 2.47 g.

[0063] The synthetic route of Example 1 is as follows, and it is named compound A1.

[0064]

[0065] Examples 2-4

[0066] The synthesis of Examples 2-4 followed the same scheme as in Example 1, and they were named compounds A2-A4.

[0067] The synthesis method for compound A2 is the same as that for compound A1 (yield 80%).

[0068] A3 and A4 replace only the acyl chloride reagent with the following structure:

[0069] The acyl chloride reagent was replaced with 3-nitrobenzoyl chloride; the product was compound A2 (yield 73%).

[0070] The acyl chloride reagent was replaced with 4-nitrobenzoyl chloride; the product was compound A3 (yield 75%).

[0071] Structural formula of Example 2

[0072]

[0073] The structural diagrams of Examples 3 and 4 are shown below, and they are named A3 and A4:

[0074]

[0075] The structural formulas of A5 and A6 are shown below. Both A5 and A6 are commercially available compounds.

[0076]

[0077] Example 5 Polyimide Synthesis and Film Preparation - Preparation of ODPA / A1 polyimide system:

[0078] First, the vacuum-dried compound A1 (diamine) and ODPA (acid anhydride) were dissolved in DMAc solvent at a molar ratio of 1:1. Under nitrogen protection and ice-water bath conditions, the acid anhydride solution was slowly added dropwise to the diamine solution, and the mixture was stirred continuously for 12 hours to complete the synthesis of polyamic acid.

[0079] The synthetic route for polyamic acid is as follows:

[0080]

[0081] Subsequently, equal volumes of acetic anhydride and triethylamine (1:1, v / v) were added to the polyamic acid solution to carry out a closed-ring imine reaction for 24 hours.

[0082] The synthesis process of polyimide is as follows:

[0083]

[0084] Finally, the film forming operation is carried out: the solid product is precipitated by ethanol precipitation, dried under vacuum at 40°C for 4 hours and under vacuum at 120°C for 10 hours, and then redissolved in DMAc solution with a solid content of 15wt% to prepare the corresponding polyimide film by solution casting.

[0085] The specific steps of the solution casting method described above are as follows:

[0086] The above polyimide solution was left to stand for one day, filtered, defoamed, and then quickly coated onto a glass substrate using a 200μm four-sided preparation tool. The unshaped film was placed in a hot stage with the temperature set at 40°C and the humidity below 20% and baked for 5 hours to set the shape. Then it was baked at 180°C for 2 hours to remove all solvent.

[0087] The polyimide film is immersed in water and sonicated for 5-10 minutes to obtain the polyimide film. The polyimide film is then placed in an oven and heated to 180°C. Subsequently, the temperature is maintained for 20 minutes at each temperature gradient to eliminate the internal stress of the film.

[0088] The structural formula of Example 5 is as follows:

[0089]

[0090] The structural formula of Example 8 is as follows:

[0091]

[0092] The structure of Example 9 is as follows:

[0093]

[0094] The structural formulas of Examples 5-22 are shown in Table 1 below.

[0095] Table 1

[0096] Anhydride / Diamine A1 A2 A3 A4 A5 A6 ODPA Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 CTDA Example 11 Example 12 Example 13 Example 14 Example 15 Example 16 CBDA Example 17 Example 18 Example 19 Example 20 Example 21 Example 22

[0097] The polyimide solution is placed in a constant temperature and humidity hot stage. Before this, all polyimide solutions should be filtered and defoamed to minimize defects on the film surface. The film is quickly scraped onto the glass substrate using a 200-micron four-sided fabricator. The temperature of the hot stage is set to 40 degrees Celsius and the humidity to less than 20%. The film is baked in the hot stage for more than 5 hours to set the shape. Then it is transferred to a constant temperature precision oven and dried at 180 degrees Celsius for 10 hours to remove all solvents.

[0098] The polyimide film is obtained by immersing it in water and sonicating it for 5-10 minutes.

[0099] The polyimide film was placed in a precision oven at a constant temperature and heated to 180°C. Then, the temperature was maintained for 20 minutes at each temperature gradient to eliminate the internal stress of the film.

[0100] Comparative Example 1: Fluorinated polyimide system (fluorinated polyimide film)

[0101] The preparation process followed the ODPA / A1 system, using hexafluoroisopropylphthalic anhydride and 4,4'-diaminodiphenyl ether as monomers: a monomer molar ratio of 1:1.01, polyamic acid was synthesized under nitrogen / ice-water bath (stirring for 12 hours); acetic anhydride / triethylamine (1:1, v / v) ring-closing reaction was carried out for 24 hours; ethanol precipitation, gradient drying (40℃→120℃ vacuum for 10 hours), 15wt% DMAc solution coating (200μm), step curing (40℃ / humidity <20%→180℃), and annealing were performed.

[0102] Comparative Example 2 used the same synthetic scheme as Comparative Example 1, but replaced the diamine monomer with compound A7 synthesized using fluorescein as the parent compound.

[0103] Comparative Example 3 used the same synthetic scheme as Comparative Example 1, but replaced the diamine monomer with compound A8, which was synthesized using p-phenylenediamine as the parent compound. The synthesis of A8 followed the synthetic scheme of A1.

[0104] The specific synthesis method of A7 is as follows: Weigh 4g (12.04mmol) of fluorescein into a clean three-necked flask, dissolve it thoroughly in 40ml of DMSO, then add 2.5 equivalents of p-fluoronitrobenzene and 4 equivalents of potassium carbonate to the solution, react at 120℃ for 12h, precipitate the nitrate with water, then purify the nitrate by column chromatography using ethyl acetate:petroleum ether 1:3 as eluent, then transfer the purified nitrate to another clean three-necked flask, add 60mg of palladium on carbon and 15ml of hydrazine hydrate, reflux in ethanol to obtain pure compound A7 (2g, yield 30.2%).

[0105] Comparative Example 4 uses a polyimide optical film (26μm thick) directly provided by Kingcrown New Materials Co., Ltd., without secondary processing, as a performance reference benchmark.

[0106] The structural formulas of comparative examples 2 and 3 are as follows:

[0107]

[0108] The performance test data of various polyimide films prepared in the above embodiments and comparative examples are shown in Table 2:

[0109] Table 2

[0110]

[0111]

[0112] As shown in Table 2, Example 5 (ODPA / A1 system) achieved a transmittance (T450nm) of 93.15% at 450nm, significantly higher than the 82.88% of Comparative Example 1 (fluorine-containing system). Simultaneously, its yellowness index (YI) was only 1.49, far lower than the 16.7 of Comparative Example 1. This data comparison demonstrates that the present invention, through a specific fluorine-free molecular structure design (the combination of 9,9-bis(4-aminophenyl)fluorene and ODPA), not only solves the problem of insufficient transmittance (typically <85%) of traditional fluorine-free polyimides without using any fluorine-containing monomers, but also unexpectedly achieves superior optical performance compared to fluorine-containing materials. Of particular note is that, while achieving high light transmittance, Example 5 exhibits thermal stability (T5% = 465°C) comparable to that of the fluorine-containing comparative example 1 (T5% = 503°C), and its glass transition temperature (Tg = 320°C) fully meets the requirements of the color photoresist post-baking process (230°C). This successfully achieves a balance between optical performance and thermal stability, a technical achievement not reported in existing technologies.

[0113] The molecular weights of the comparative examples and each embodiment were determined by gel permeation chromatography. The test results are shown in Table 3 below. The solvent was N-methylpyrrolidone.

[0114] Table 3 Molecular weight tests for comparative examples and embodiments

[0115]

[0116]

[0117] Application Example: Photoresist Compatibility Verification of Polyimide Thin Films

[0118] Example 5 exhibits the highest light transmittance, and its glass transition temperature is significantly higher than the post-baking temperature of the color photoresist. Furthermore, it can withstand up to 200,000 repeated folding and unfolding cycles. Based on these properties, it is expected to meet the process requirements of photolithography. Combining light transmittance and the polymer's glass transition temperature (an important indicator for evaluating the polymer's operating temperature), this example verifies the compatibility of Example 5 with the color photoresist using a standard photolithography process. Three sets of color photoresists were first prepared: the red photoresist was prepared by mixing 0.4g of anthraquinone dye mother liquor (0.1g dye / 1ml LDMF) with 0.8g of TOK P-1000 photoresist mother liquor; the green photoresist was prepared by mixing 0.5g of phthalocyanine dye mother liquor with 0.7g of photoresist mother liquor; and the blue photoresist was prepared by mixing 0.55g of phthalocyanine dye mother liquor with 0.65g of photoresist mother liquor. The 30.2 μm thick polyimide film prepared in Example 5 was laminated onto a glass substrate cleaned with ethanol. After preheating at 80°C for 2 minutes, it was coated using a spin coater speed of 800 rpm (red / green) or 750 rpm (blue). The coated film was pre-baked at 90°C for 5 minutes, followed by UV exposure at 254 nm wavelength (8 s for red, 15 s for green, and 30 s for blue), and finally patterned using PGMEA for 30 seconds. Test results showed that the prepared pattern was regular and residue-free, and there was no warping or peeling after baking at 230°C, and the properties of the polyimide remained essentially unchanged. This fully demonstrates that the polyimide film of the present invention is fully compatible with standard photolithography processes and can meet the stringent requirements for substrate materials in the manufacture of flexible display devices.

[0119] The specific embodiments described in this invention are merely preferred examples to facilitate understanding and implementation of the invention, and do not constitute a limitation on the technical solution of the invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the protection scope of this invention.

Claims

1. A fluorine-free, highly transparent, and heat-resistant flexible polyimide material, characterized in that, The structure is shown in the following general formula I: R1 is selected from one of ODPA, 9,9-BFDA, CPDA, BTDA, CBDA, HPMDA, and PMDA; R2 is selected from A1-A6: n is an integer between 30 and 100.

2. The polyimide flexible material according to claim 1, characterized in that, The polyimide flexible material has a light transmittance of ≥80% at a wavelength of 450nm, a yellowness index of ≤5.0, a glass transition temperature of ≥230℃, a 5% thermal weight loss temperature (T5%) of ≥450℃, and preferably a char residue of >50%.

3. The method for preparing the polyimide flexible material according to claim 1, characterized in that, The specific steps are as follows: (1) The diamine monomer and the acid anhydride monomer are dissolved in a solvent at a molar ratio of 1:0.99-1.02 and reacted under nitrogen protection and at -5-20℃ to generate a polyamic acid solution; the solvent is selected from N,N-dimethylacetamide, N-methylpyrrolidone or N,N-dimethylformamide; (2) Add a mixed solution of acetic anhydride and triethylamine to the polyamic acid solution obtained in step (1) and carry out an imidization reaction. (3) The film is formed by solution casting and then subjected to shaping, curing and step annealing treatment in sequence.

4. The preparation method according to claim 3, characterized in that, In step (2), the volume ratio of acetic anhydride to triethylamine is 1:0.8-1.2; the volume ratio of polyamic acid solution to acetic anhydride is 20:

1.

5. The preparation method according to claim 3, characterized in that, The solid content of the polyamic acid solution in step (2) is less than 10 wt%.

6. The preparation method according to claim 3, characterized in that, The shaping and curing steps in step (3) include: precipitating the solution obtained in step (2) with ethanol, drying it under vacuum at 40-60°C for 4-6 hours, and then redissolving it with a polar solvent to form a 15-20wt% casting solution; and applying the casting solution to the substrate through a 200-300μm gap.

7. The preparation method according to claim 6, characterized in that, The polar solvent is a mixture of γ-butyrolactone and N,N-dimethylacetamide in a volume ratio of 1:1-3.

8. The preparation method according to claim 3, characterized in that, Step (3) of the stepped annealing includes the following steps: First stage: Curing at 40-50℃ and humidity <30% for 4-6 hours; Second stage: Heat treatment at 120-140℃ for 1-3 hours; The third stage: step annealing with a gradient of 15-25℃, holding each gradient for 15-25 minutes, first heating to 170-190℃, and finally 20-30℃.

9. The application of the polyimide flexible material as described in claim 1 in a flexible display substrate, characterized in that, This includes plastic transparent substrates for flexible displays such as LCDs and OLEDs, TFT substrates, flexible printed circuit boards, flexible OLED flat lighting substrates, flexible color filters, or flexible touch sensors.

10. A flexible display device, characterized in that, The substrate layer comprises the polyimide flexible material as described in claim 1, and the surface of the substrate layer is formed with a color filter pattern by photolithography.

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