Polyimide film and preparation method thereof

Abstract

The present invention relates to a polyimide film and a preparation method thereof. The method comprises: step 1: dissolving a diamine in a non-protonic polar solvent, then adding a dianhydride and a diacyl halide thereto and stirring to carry out a polymerization reaction, then adding a neutralizing agent to carry out a neutralization reaction to obtain a polyamic acid solution; step 2: adding a capping agent, a dehydrating agent and a catalyst to the polyamic acid solution, reacting to obtain a polyimide solution; step 3: precipitating the polyimide solution in a solvent, then drying to obtain a polyimide resin; step 4: dissolving the polyimide resin in a non-protonic polar solvent to obtain a polyimide resin solution; step 5: degassing, filtering, heating and drying the polyimide resin solution, and then cooling. The method can improve the performance of the film and reduce the production cost. The film has high tensile strength, high Young's modulus, high hardness and low thermal expansion coefficient (CTE), and is colorless and transparent.

Description

Technical Field

[0001] The invention relates to polyimide, in particular to a flexible, colorless and transparent polyimide film and a preparation method thereof. Background Art

[0002] Polyimide is an aromatic heterocyclic polymer compound containing an imide ring in the main chain. It has excellent heat resistance, radiation resistance, and high insulation performance. It is one of the best performing insulation materials in the world and also an organic polymer material with excellent comprehensive performance.

[0003] Polyimide film is widely used and is known as the "golden film". Traditional polyimide film (yellow) is a high-temperature resistant insulating material developed in the United States in the early 1960s. It is obtained by polycondensing pyromellitic dianhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA). It has excellent heat resistance and mechanical properties such as tensile strength, but its visible light transmittance is less than 80%. In addition, these polyimide films can only be prepared by thermal imidization of polyamic acid, but cannot be prepared by chemical imidization. The chemical imidization reaction requires the use of a dehydrating agent and a catalyst.

[0004] According to market demand, the yellow polyimide film was improved and a colorless and transparent polyimide film was developed. The polyimide film has a high transmittance to visible light and can be used as a flexible substrate material for OLED lighting / display devices, thin-film solar cells, and electronic paper. It can also be applied to new touch / display modules, transparent flexible circuit boards, or as a transparent insulating layer material. In addition, with the rapid progress of optoelectronic materials, flexible electronic appliances based on organic light-emitting semiconductors (OLEDs) have made great progress, and thus flexible colorless and transparent polyimide films are also widely used in the field of components and semiconductors. However, the flexible colorless and transparent polyimide films currently sold on the market have the problems of low tensile strength, low Young's modulus, low hardness, and high coefficient of thermal expansion (CTE), which limits the development of such films.

[0005] In order to improve these properties of polyimide film, usually add rigid structure monomer (such as p-phenylenediamine (PPD), average anhydride (PMDA) or diacyl halide (such as diformyl chloride) etc.) to the polyimide molecular structure. However, the introduction of rigid structure monomer can form electron transfer complex in the process of imidization reaction of polyamic acid to prepare polyimide, resulting in reduced light transmittance of polyimide film. In addition, the introduction of diacyl halide can generate unfavorable acid substances such as hydrochloric acid, which can affect polymerization reaction, cause difficulty in controlling molecular weight, affect air environment and corrode equipment, and then significantly reduce the performance of polyimide film and increase production cost.

[0006] In addition, the prior art also discloses a method in which a diamine and a diacyl halide are first combined, and then a dianhydride is added to carry out a polymerization reaction. Although this method does not produce the above-mentioned acidic substances (such as hydrochloric acid) during the polymerization reaction, an acidic substance (such as hydrochloric acid) is produced when the diamine and the diacyl halide are initially combined. Moreover, the process of this method is complicated, which significantly increases the production cost.

[0007] Therefore, it is necessary to develop a novel polyimide film and a preparation method thereof to overcome the above problems. Summary of the invention

[0008] Technical issues

[0009] An object of the present invention is to provide a novel method for preparing a flexible, colorless and transparent polyimide film, which has the advantages of simple process, low production cost and the ability to improve film performance.

[0010] Another object of the present invention is to provide a novel flexible, colorless and transparent polyimide film, which can be prepared by the above method and has high tensile strength, high Young's modulus, high hardness and low coefficient of thermal expansion (CTE).

[0011] Technical Solution

[0012] According to a first aspect of the present invention, the present invention provides a method for preparing a polyimide film, the method comprising:

[0013] Step 1: dissolving a diamine in a non-protonic polar solvent, then adding dianhydride and diacyl halide thereto and stirring to perform a polymerization reaction, and then adding a neutralizing agent to perform a neutralization reaction to obtain a polyamic acid solution;

[0014] Step 2: adding a capping agent, a dehydrating agent and a catalyst to the polyamic acid solution to react to obtain a polyimide solution;

[0015] Step 3: precipitating the polyimide solution in a solvent, and then drying to obtain a polyimide resin;

[0016] Step 4: dissolving the polyimide resin in a non-protonic polar solvent to obtain a polyimide resin solution;

[0017] Step 5: Degassing, filtering, heating and drying the polyimide resin solution, and then cooling the solution to obtain a polyimide film.

[0018] In one embodiment, in step 1,

[0019] The diamine is one or more selected from N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide) (AB-TFMB), 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6FODA), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB), 2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP), 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP), 2,2-bis(4-aminophenyl)hexafluoropropane (6FBA) and 4,4'-diamino-2,2'-bistrifluoromethylbiphenyl (TFDB).

[0020] The dianhydride is one or more selected from 4,4'-diphenyl ether dianhydride (ODPA), cyclobutanetetracarboxylic dianhydride (CBDA), p-phenylene-triphenylene dianhydride (TAHQ), hexafluoro dianhydride (6FDA), bisphenol A diether dianhydride (BPADA) and triphenyl diether dianhydride (HQDPA),

[0021] The dicarboxylic acid halide is one or more selected from isophthaloyl chloride, terephthaloyl chloride, naphthalene dicarboxylic acid chloride, furan dicarboxylic acid chloride, thiophene dicarboxylic acid chloride and bromoisophthaloyl chloride.

[0022] The neutralizing agent is one or more selected from calcium hydroxide (Ca(OH)2), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), calcium carbonate (CaCO3), potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3),

[0023] The aprotic polar solvent is one or more selected from dimethylformamide (DMF), dimethylacetamide (DMAC), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF).

[0024] In one embodiment, in step 1,

[0025] The molar ratio of the diamine, the dianhydride and the diacyl halide is 2:0.6 to 1:1;

[0026] The mass ratio of the combination of the diamine, the dianhydride and the diacyl halide to the aprotic polar solvent is 7-30%:93-70%, and

[0027] The molar ratio of the neutralizing agent to the diacyl halide is 1 to 3:1.

[0028] In one embodiment, in step 1, the polymerization reaction time is 6 to 30 hours, and

[0029] The neutralizing agent is added before the polymerization reaction is completed and after the polymerization reaction has been carried out for 1 to 20 hours.

[0030] In one embodiment, in step 2,

[0031] The end-capping agent is one or more selected from phthalic anhydride (PA), maleic anhydride (MA), phenylacetylene phthalic anhydride (PEPA), tetrahydrophthalic anhydride (THPA), ethynyl aniline (APA), p-(trimethoxysilyl) aniline (APTS) and aminopropyltriethoxysilane (APTES),

[0032] The dehydrating agent is one or more selected from acetic anhydride, trifluoroacetic anhydride, acetyl chloride and thionyl chloride;

[0033] The catalyst is one or more selected from triethylamine, pyridine, NN dimethylethanolamine, tert-butylpyridine and isoquinoline;

[0034] In step 3,

[0035] The solvent is water, ethanol or a mixture thereof.

[0036] The weight average molecular weight of the polyimide resin is 20,000 to 450,000.

[0037] In one embodiment, in step 4, the aprotic polar solvent is one or more selected from dimethylformamide (DMF), dimethylacetamide (DMAC), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF).

[0038] In one embodiment, in step 4, the filtered solution is coated on a glass plate and then dried to obtain a polyimide film.

[0039] According to a second aspect of the present invention, the present invention provides a polyimide film, which is prepared by the above method.

[0040] In one embodiment, the structural unit of the polyimide is:

[0041]

[0042] Wherein R1 is a combination of one or more of the following groups:

[0043] R2 is a combination of one or more of the following groups:

[0044]

[0045]

[0046] R3 is a combination of one or more of the following groups:

[0047]

[0048] Beneficial Effects

[0049] The method for preparing a polyimide film of the present invention prepares a polyamic acid solution by simultaneously polymerizing diamine, dianhydride and diacyl halide, and removes unfavorable acid substances generated in the polymerization reaction by a neutralizer, thereby improving the performance of the final polyimide film. The method of the present invention has the advantages of simple process, low production cost, and ability to improve film performance.

[0050] In addition, through the above method, the present invention can obtain a flexible, colorless and transparent polyimide film having high tensile strength, high Young's modulus, high hardness and low thermal expansion coefficient (CTE), thereby expanding the application field of the flexible, colorless and transparent polyimide film. DETAILED DESCRIPTION

[0051] Hereinafter, the present invention will be described in detail. The terms or words used in this specification and claims should not be restrictively interpreted as common or dictionary definitions, and should be interpreted as meanings and concepts corresponding to the technical ideas of the present invention on the basis that the inventor can appropriately define the concepts of the terms to describe the principles of the invention in the best possible way.

[0052] Unless otherwise specified, "%" used herein means % by weight.

[0053] 1. Method for preparing polyimide film

[0054] The present invention provides a method for preparing a polyimide film, the method comprising:

[0055] Step 1: dissolving a diamine in a non-protonic polar solvent, then adding dianhydride and diacyl halide thereto and stirring to perform a polymerization reaction, and then adding a neutralizing agent to perform a neutralization reaction to obtain a polyamic acid solution;

[0056] Step 2: adding a capping agent, a dehydrating agent and a catalyst to the polyamic acid solution to react to obtain a polyimide solution;

[0057] Step 3: precipitating the polyimide solution in a solvent, and then drying to obtain a polyimide resin;

[0058] Step 4: dissolving the polyimide resin in a non-protonic polar solvent to obtain a polyimide resin solution;

[0059] Step 5: Degassing, filtering, heating and drying the polyimide resin solution, and then cooling the solution to obtain a polyimide film.

[0060] Each step is described in detail below.

[0061] Step 1: Prepare polyamic acid solution

[0062] A diamine is dissolved in an aprotic polar solvent, and then a dianhydride and a diacyl halide are added thereto and stirred to perform a polymerization reaction, and then a neutralizing agent is added to perform a neutralization reaction to obtain a polyamic acid solution.

[0063] Diamine is a compound containing two amino groups and is a common component used to prepare polyamic acid solution.

[0064] The diamine may be one or more selected from N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide) (AB-TFMB), 2,2'-bis(trifluoromethyl)-4,4'-diaminophenyl ether (6FODA), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB), 2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP), 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP), 2,2-bis(4-aminophenyl)hexafluoropropane (6FBA) and 4,4'-diamino-2,2'-bistrifluoromethylbiphenyl (TFDB).

[0065] Aprotic polar solvents are solvents that do not contain protons in their molecules. Compared with water, they are almost non-acidic, have weak alkalinity and the ability to form weak hydrogen bonds, and have extremely strong solubility. Aprotic polar solvents can provide a reaction environment for the reaction of raw monomers (diamines, dianhydrides, and diacyl halides). Other types of solvents cannot provide the above reaction environment.

[0066] The aprotic polar solvent may be one or more selected from dimethylformamide (DMF), dimethylacetamide (DMAC), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF).

[0067] Dianhydrides are compounds containing two carboxylic acid anhydride groups and are common components used to prepare polyamic acid solutions.

[0068] The dianhydride may be one or more selected from 4,4'-diphenyl ether dianhydride (ODPA), cyclobutanetetracarboxylic dianhydride (CBDA), p-phenylene-triphenylene dianhydride (TAHQ), hexafluoro dianhydride (6FDA), bisphenol A diether dianhydride (BPADA) and triphenyl diether dianhydride (HQDPA),

[0069] A diacyl halide is a compound containing two acyl halide groups. An acyl halide group is a group formed when the hydroxyl group in the carboxyl group is replaced by a halogen atom. The general formula is -COX, and it can be divided into acyl fluoride, acyl chloride, acyl bromide, and acyl iodide. Among them, acyl chloride is the most commonly used acyl halide group.

[0070] The dicarboxylic acid halide may be one or more selected from the group consisting of isophthaloyl dichloride, terephthaloyl dichloride, naphthalene dichloride, furan dichloride, thiophene dichloride and bromoisophthaloyl dichloride.

[0071] In the present invention, the neutralizing agent is a substance that adjusts the pH value by reacting with an acid (including an acid salt). It has a great influence on emulsion polymerization, acetalization reaction, resin curing, storage of latex, etc. In the present invention, any organic or inorganic alkaline substance that can form a salt with -COOH (carboxyl) or -OH group can be used as a neutralizing agent. Different alkaline substances have a large difference in the neutralizing effect.

[0072] The neutralizing agent may be one or more selected from calcium hydroxide (Ca(OH)2), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), calcium carbonate (CaCO3), potassium carbonate (K2CO3) and potassium bicarbonate (KHCO3).

[0073] The mass ratio of the combination of the diamine, the dianhydride and the diacyl halide to the aprotic polar solvent may be 7-30%:93-70%, preferably 10-25%:90-75%, more preferably 12-20%:88-80%.

[0074] When diacyl halides participate in polyreaction, unfavorable acid substances can be formed. For example, when diacyl chlorides participate in polyreaction, hydrochloric acid (HCl) can be formed. In polyreaction, the acidic environment produced by these acid substances is unfavorable for the carrying out of positive reaction (that is, the reaction forming polymer), and the molecular weight of polymer can be reduced thus. In addition, these acid substances such as hydrochloric acid can corrode production equipment and pollute the environment, causing difficulties to the recovery of solvent. The present invention neutralizes the acid substances by adding a neutralizing agent (such as sodium bicarbonate), to solve the above problem.

[0075] Polyamic acid can be obtained by the polymerization reaction of diamine (such as AB-TFDB), dianhydride (such as ODPA) and diacid halide (such as terephthaloyl chloride).

[0076] The polymerization temperature can generally be -5 to 35° C., preferably 0 to 25° C., and more preferably 10 to 20° C. If the reaction temperature is higher than 35° C., the hydrolysis reaction will be accelerated and the reverse reaction rate will be increased, affecting the preparation of the polyamic acid solution; and if the reaction temperature is lower than -5° C., the reaction rate will be reduced and the reaction time will be prolonged.

[0077] The polymerization reaction time may generally be 6 to 30 hours, preferably 7 to 25 hours, more preferably 8 to 24 hours.

[0078] The polymerization reaction is usually carried out in a nitrogen atmosphere at normal pressure. If the polymerization reaction is carried out in air, the dianhydride will absorb water to form a tetraacid, which may prevent the polymerization reaction from proceeding.

[0079] The molar ratio of diamine, dianhydride and diacyl halide can be 2:0.6 to 1:1, preferably 2:0.7 to 1:1, and more preferably 2:0.8 to 1:1. Through the polymerization reaction of diamine, dianhydride and diacyl halide, a polyamic acid with a longer chain segment and a higher relative molecular mass can be obtained, which is conducive to obtaining a polyimide film with higher mechanical properties. Among them, the apparent viscosity of the polyamic acid solution is usually 1,000 to 400,000 cp, preferably 5,000 to 350,000 cp, and more preferably 10,000 to 300,000 cp.

[0080] If the molar amount of one or two of the diamines, dianhydrides, and diacyl halides used as monomers is too large or too small, the chain length of the polyamic acid will be reduced, the relative molecular weight will be reduced, and the apparent viscosity of the polyamic acid and polyimide resin solution will be reduced, which may lead to the subsequent inability to prepare polyimide films, or the performance of the prepared films will be poor.

[0081] The molar ratio of the neutralizing agent to the diacyl halide can be 1 to 3: 1, preferably 1.5 to 2.5: 1. If the amount of the neutralizing agent is too large, it may react with the carboxyl group formed by the polymerization reaction; and if the amount of the neutralizing agent is too small, it cannot completely neutralize the acidic substances formed, which is not conducive to obtaining a high molecular weight polymer.

[0082] The neutralizing agent is usually added at an appropriate time before the end of the polymerization reaction, for example, at a time point when the polymerization reaction has been carried out for 1 to 20 hours, preferably at a time point when the polymerization reaction has been carried out for 1.5 to 15 hours, and more preferably at a time point when the polymerization reaction has been carried out for 2 to 10 hours. For different monomer raw materials, the appropriate time point for adding the neutralizing agent can be determined through a series of control tests.

[0083] If the neutralizer is added too early, the polymerization reaction does not generate enough acidic substances such as hydrochloric acid, so the carboxyl groups at the ends of the polymer molecular chains will react with the neutralizer, inhibiting the production of high molecular weight polymers. If the neutralizer is added too late, the polymerization reaction will proceed in an acidic environment, which is not conducive to the positive reaction (i.e., the formation of polymers), and the production of high molecular weight polymers will also be inhibited, which may result in the inability to prepare a polyimide film later or the poor performance of the resulting film.

[0084] After the neutralizing agent is added, the neutralization reaction will proceed for a period of time simultaneously with the polymerization reaction. In addition, after the polymerization reaction is completed, the neutralization reaction will also end soon.

[0085] The structural formula of N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide) (AB-TFDB) as a diamine is as follows:

[0086]

[0087] The structural formula of 4,4'-biphenyl ether dianhydride (ODPA) as dianhydride is as follows:

[0088]

[0089] The structural formula of terephthaloyl chloride as a diacyl halide is as follows:

[0090]

[0091] In one embodiment, the structural units of the polyamic acid obtained by the above three monomers are as follows:

[0092]

[0093] Step 2: Preparation of polyimide solution by chemical imidization reaction

[0094] A capping agent, a dehydrating agent and a catalyst are added to the polyamic acid solution to react and obtain a polyimide solution.

[0095] End-capping agent refers to a monofunctional molecular substance added in the polymerization reaction, and its role is to make the functional groups at both ends of the polymer inactive, thereby terminating the polymerization reaction. In the polymerization reaction of step (1), active functional groups are usually present at both ends of the formed polymer (i.e., polyamic acid). Due to the presence of these functional groups, the ends of the polymer molecule chains can continue to participate in the reaction, allowing the chain to grow. In order to eliminate the activity of the end group, a monofunctional compound can be added to make the active functional group disappear, which is called end-capping. These monofunctional compounds are customarily called end-group blocking agents, i.e., end-capping agents.

[0096] The end-capping agent may be one or more selected from phthalic anhydride (PA), maleic anhydride (MA), phenylacetylene phthalic anhydride (PEPA), tetrahydrophthalic anhydride (THPA), ethynyl aniline (APA), p-(trimethoxysilyl) aniline (APTS) and aminopropyltriethoxysilane (APTES).

[0097] The dehydrating agent is used to reduce the moisture in the material or remove the crystal water in the compound molecule. The dehydrating agent of the present invention can be used for intramolecular dehydration during the imidization reaction. The dehydrating agent can be one or more selected from acetic anhydride, trifluoroacetic anhydride, acetyl chloride and thionyl chloride.

[0098] A catalyst (solid catalyst is also called a catalyst) refers to a substance that can change the chemical reaction rate of reactants in a chemical reaction (either increase or decrease) without changing the chemical equilibrium, and its own quality and chemical properties do not change before and after the chemical reaction. The catalyst of the present invention is used to improve the reaction efficiency of chemical imidization reaction. The catalyst can be one or more selected from triethylamine, pyridine, NN dimethylethanolamine, tert-butylpyridine and isoquinoline.

[0099] In one embodiment, a capping agent (such as phthalic anhydride) is added to the polyamic acid solution for capping, and the mixture is stirred for 1 to 4 hours, preferably 2 to 3 hours, and then a dehydrating agent (such as acetic anhydride) and a catalyst (such as triethylamine) are added simultaneously to carry out a chemical imidization reaction. The reaction can be carried out for 2 to 20 hours, preferably 5 to 15 hours, to obtain a polyimide solution.

[0100] The apparent viscosity of the polyamic acid solution obtained in step 1 can be controlled by adding an end-capping agent. The molar ratio of the end-capping agent to the dianhydride can be 0.1 to 2.5:1, preferably 0.3 to 2.2:1, and more preferably 0.5 to 2.0:1.

[0101] The structural formula of the phthalic anhydride is as follows:

[0102]

[0103] The structural unit of the polyimide is as follows:

[0104]

[0105] Step 3: Preparation of polyimide resin

[0106] The polyimide solution is precipitated in a solvent and then dried to obtain a polyimide resin.

[0107] The solvent may be water, ethanol or a mixture thereof. The polyimide solution precipitates a polyimide resin solid in the solvent, and then the solid is dried by heating to obtain a pure polyimide resin without a solvent.

[0108] The precipitated polyimide resin solid may be in the form of powder, flocculent, etc., and may be regular or irregular.

[0109] When the solvent is a mixture of water and ethanol, the mass ratio of water to ethanol may be 9-5:1-5, preferably 8-6:2-4.

[0110] The drying temperature may be 120 to 220°C, preferably 130 to 210°C, and more preferably 140 to 200°C.

[0111] Step 4: Prepare polyimide resin solution

[0112] The polyimide resin is dissolved in an aprotic polar solvent to obtain a polyimide resin solution.

[0113] In one embodiment, the polyimide resin may be added to an aprotic solvent under nitrogen protection and mechanically stirred until the polyimide resin is completely dissolved.

[0114] The aprotic polar solvent is as described in step 1 above, and can be one or more selected from dimethylformamide (DMF), dimethylacetamide (DMAC), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF).

[0115] The mechanical stirring time may be 4 to 20 hours, preferably 5 to 18 hours, and more preferably 6 to 16 hours.

[0116] Step 5: Prepare polyimide film

[0117] The polyimide resin solution is degassed, filtered, heated and dried, and then cooled to obtain a polyimide film.

[0118] In the preparation process of the polyimide resin solution, air is inevitably mixed in to form bubbles. The presence of bubbles will lead to the formation of breakpoints during the cast film, affecting the quality of the film, and thus a degassing process is required to remove these bubbles. The degassing process can be carried out by vacuum degassing. Vacuum degassing refers to the discharge of bubbles present in the polyimide resin solution by negative pressure. The vacuum degassing time can be 1 to 10 hours, preferably 3 to 7 hours.

[0119] The filtration process is used to remove solid impurities in the polyimide resin solution to improve the final film quality. Usually, a G4 sand core funnel can be used for filtration. The filtered polyimide resin solution can be cast on a glass plate for subsequent heating and drying processes.

[0120] The heating and drying process is used to remove the solvent of the polyimide resin solution. The heating and drying process generally includes a three-step heating process and a high-temperature drying process.

[0121] The three-step heating process may include:

[0122] The first heating step: heating to 40-100°C and maintaining for 20-60 min, preferably heating to 50-80°C and maintaining for 30-50 min;

[0123] The second heating step is heating to 100-200°C and maintaining for 20-60 min, preferably heating to 120-180°C and maintaining for 30-50 min; and

[0124] The third heating step is heating to 200-400° C. and maintaining for 20-60 min, preferably heating to 220-280° C. and maintaining for 30-50 min.

[0125] A low-temperature cured film can be obtained through the above three-step heating process.

[0126] The high temperature drying process can be performed in a high temperature oven at 300-500° C. to further remove the solvent in the film.

[0127] Finally, the polyimide film is obtained by cooling. The cooling step can be performed by naturally cooling the film after heating and drying to room temperature to obtain the final polyimide film as a product.

[0128] 2. Polyimide film

[0129] The present invention also provides a polyimide film, which can be prepared by the above preparation method. The polyimide film has high tensile strength, high Young's modulus, high hardness and low thermal expansion coefficient (CTE), and is colorless and transparent.

[0130] In one embodiment, the optical performance of the polyimide film can be tested using a multifunctional X-ray diffractometer, and the visible light transmittance of the film is above 89%, such as 90%-91%.

[0131] In one embodiment, the mechanical properties of the polyimide film are measured according to IEC674-3-4 to 6 (first edition, 1993). The results show that the tensile strength is 165-175 MPa and the Young's modulus is 4.5-5.5 GPa.

[0132] In one embodiment, the hardness of the polyimide film can be tested using an electric pencil hardness tester, and the result is 3-4H.

[0133] In one embodiment, the thermal stability of the polyimide film is measured using a dynamic thermomechanical analyzer (TMA), model Q400. The test conditions may be: a heating rate of 10°C / min, a heating temperature of 50-200°C, and a thermal expansion coefficient (CTE) of 10-12ppm.

[0134] In one embodiment, the structural unit of the polyimide may be:

[0135]

[0136] Wherein R1 is a combination of one or more of the following groups:

[0137] R2 is a combination of one or more of the following groups:

[0138]

[0139]

[0140] R3 is a combination of one or more of the following groups:

[0141]

[0142] Example

[0143] Hereinafter, the present invention will be described in detail with reference to embodiments to specifically describe the present invention. However, embodiments of the present invention may be modified into various other forms and the scope of the present invention should not be construed as being limited to the embodiments described below. Embodiments of the present invention are provided to more completely describe the present invention to those of ordinary skill in the art.

[0144] The experimental methods in the following examples, unless otherwise specified, are generally conventional conditions in the art or conditions recommended by the manufacturer; the raw materials and equipment used are all commercially available from conventional markets, etc., unless otherwise specified.

[0145] Example 1 (polymerization reaction was carried out for a total of 16 hours, wherein the neutralizing agent was added when the polymerization reaction was carried out for 4 hours)

[0146] Step 1: nitrogen was introduced into a double-layer glass reactor for 5 minutes, the circulating water temperature was controlled at 20° C., and 1065.73 g of N,N-dimethylacetamide (DMAC) as an aprotic polar solvent was added thereto.

[0147] Then, 125.38 g of AB-TFMB as a diamine was dissolved in the N,N-dimethylacetamide (DMAC) and mechanically stirred. After AB-TFMB was completely dissolved, 41.37 g of ODPA as a dianhydride and 17.93 g of terephthaloyl chloride as a diacyl halide were added to the above solution under stirring and stirred for 4 hours.

[0148] Then, 23.54 g of sodium bicarbonate as a neutralizing agent was added, and stirring was continued for 12 hours to obtain a polyamic acid solution with a solid content of 15 wt %.

[0149] Step 2: Add 0.49 g of phthalic anhydride as a capping agent to the polyamic acid solution and stir for 2 hours. Then, add 90.40 g of acetic anhydride as a dehydrating agent and 41.14 g of triethylamine as a catalyst to the polyamic acid solution and continue stirring for 12 hours to obtain a polyimide solution.

[0150] Step 3: The obtained polyimide solution was precipitated in water and ethanol (mass ratio was 4:1), and dried at 170° C. for 5 hours to obtain a polyimide resin.

[0151] Step 4: nitrogen was introduced into the three-necked flask for 5 minutes, and 262.50 g of DMAc as a non-protonic polar solvent was added. 50.00 g of polyimide resin was added to DMAc, and mechanically stirred for 8 hours to obtain a polyimide resin solution with a solid content of 16%.

[0152] Step 5: The polyimide resin solution was vacuum degassed for 4 hours, and then filtered using a G4 sand core funnel, and then cast on a clean glass plate to obtain a glass plate with a polyimide resin solution.

[0153] Then, the glass plate with the polyimide resin solution was placed on a heating table and kept at 50° C. for 20 min, at 70° C. for 20 min, and at 100° C. for 20 min to obtain a low-temperature cured film.

[0154] Next, the glass plate with the low-temperature cured film was moved into a high-temperature oven, and the temperature of the oven was raised from room temperature to 340° C. at a heating rate of 3° C. / min and maintained for 20 minutes.

[0155] Then, the high-temperature oven is turned off, the temperature is allowed to naturally drop to room temperature, and the glass plate is taken out of the high-temperature oven to obtain a polyimide film as a product.

[0156] Comparative Example 1 (without addition of diacyl halide and neutralizing agent)

[0157] Step 1: nitrogen was introduced into a double-layer glass reactor for 5 minutes, the circulating water temperature was controlled at 20° C., and 1075.81 g of N,N-dimethylacetamide (DMAC) was added thereto.

[0158] Then, 122.49 g of AB-TFMB was dissolved in N,N-dimethylacetamide (DMAC) in a double-layer glass reactor and mechanically stirred. After AB-TFMB was completely dissolved, 67.36 g of ODPA was added to the above solution under stirring and stirred for 16 hours to obtain a polyamic acid solution with a solid content of 15 wt%.

[0159] Step 2: Add 0.64 g of phthalic anhydride to the polyamic acid solution and stir for 3 hours. Then, add 89.49 g of acetic anhydride and 44.74 g of triethylamine to the polyamic acid solution and continue stirring for 12 hours to obtain a polyimide solution.

[0160] Steps 3 to 5 are the same as steps 3 to 5 in Example 1, and a polyimide film is obtained as a product.

[0161] Comparative Example 2 (adding dianhydride and diacyl halide simultaneously, but without adding neutralizing agent)

[0162] Step 1: nitrogen was introduced into a double-layer glass reactor for 5 minutes, the circulating water temperature was controlled at 20° C., and 1065.73 g of N,N-dimethylacetamide (DMAC) was added thereto.

[0163] Then, 131.77 g of AB-TFMB was dissolved in the N,N-dimethylacetamide (DMAC) and mechanically stirred. After AB-TFMB was completely dissolved, 41.54 g of ODPA and 14.37 g of terephthaloyl chloride were added to the above solution under stirring and stirred for 16 hours to generate a polyamic acid solution with a solid content of 15 wt%.

[0164] Step 2: Add 0.49 g of phthalic anhydride to the polyamic acid solution and stir for 2 hours. Then add 97.73 g of acetic anhydride (dehydrating agent) and 48.87 g of triethylamine (catalyst) to the polyamic acid solution and continue stirring for 12 hours to obtain a polyimide solution.

[0165] Steps 3 to 5 are the same as steps 3 to 5 in Example 1, and a polyimide film is obtained as a product.

[0166] Comparative Example 3 (adding dianhydride and diacyl halide separately, but not adding neutralizing agent)

[0167] Step 1: nitrogen was introduced into the first double-layer glass reactor for 5 minutes, the circulating water temperature was controlled at 20° C., and 565.73 g of N,N-dimethylacetamide (DMAC) was added thereto.

[0168] Then, 122.47 g of AB-TFMB was dissolved in the N,N-dimethylacetamide (DMAC) and mechanically stirred. After AB-TFMB was completely dissolved, 36.73 g of ODPA was added to the above solution under stirring and stirred for 4 hours to obtain a first solution a.

[0169] In addition, nitrogen was introduced into the second double-layer glass reactor for 5 minutes, the circulating water temperature was controlled at 20° C., and 500 g of DMAC was added thereto. Then, 17.37 g of terephthaloyl chloride was added, and mechanical stirring was performed for 4 hours to obtain a second solution b.

[0170] Then, the first solution a and the second solution b were mixed and mechanically stirred for 12 hours to obtain a polyamic acid solution with a solid content of 15 wt %.

[0171] Step 2: Add 0.64 g of phthalic anhydride to the polyamic acid solution and stir for 2 hours. Then, add 89.42 g of acetic anhydride (dehydrating agent) and 34.64 g of triethylamine (catalyst) to the polyamic acid solution and continue stirring for 12 hours to obtain a polyimide solution.

[0172] Steps 3 to 5 are the same as steps 3 to 5 in Example 1, and a polyimide film is obtained as a product.

[0173] Comparative Example 4 (adding dianhydride, diacyl halide and neutralizing agent simultaneously, so that the polymerization reaction and the neutralization reaction start almost simultaneously, i.e., adding the neutralizing agent too early)

[0174] Step 1: nitrogen was introduced into a double-layer glass reactor for 5 minutes, the circulating water temperature was controlled at 20° C., and 1065.73 g of N,N-dimethylacetamide (DMAC) was added thereto.

[0175] Then, 126.24 g of AB-TFMB was dissolved in the N,N-dimethylacetamide (DMAC) and mechanically stirred. After AB-TFMB was completely dissolved, 49.71 g of ODPA, 22.95 g of terephthaloyl chloride and 26.92 g of sodium bicarbonate were added to the above solution under stirring and stirred for 16 hours to obtain a polyamic acid solution with a solid content of 15 wt%.

[0176] Step 2: Add 0.49 g of phthalic anhydride to the polyamic acid solution and stir for 2 hours, then add 92.23 g of acetic anhydride and 46.11 g of triethylamine to the polyamic acid solution and stir for 12 hours to generate a polyimide solution.

[0177] Steps 3 to 5 are the same as steps 3 to 5 in Example 1, and a polyimide film is obtained as a product.

[0178] Comparative Example 5 (the polymerization reaction was carried out for a total of 16 hours, wherein the neutralizing agent was added after the polymerization reaction was completed, i.e., the neutralizing agent was added too late)

[0179] Step 1: nitrogen was introduced into a double-layer glass reactor for 5 minutes, the circulating water temperature was controlled at 20° C., and 1065.73 g of N,N-dimethylacetamide (DMAC) was added thereto.

[0180] Then, 124.24 g of AB-TFMB was dissolved in the N,N-dimethylacetamide (DMAC) and mechanically stirred. After AB-TFMB was completely dissolved, 40.99 g of ODPA and 17.79 g of terephthaloyl chloride were added to the above solution under stirring and stirred for 16 hours.

[0181] Then, 21.06 g of sodium bicarbonate was added and stirring was continued for 3 hours to obtain a polyamic acid solution with a solid content of 15 wt %.

[0182] Step 2: Add 0.65 g of phthalic anhydride to the polyamic acid solution and stir for 2 hours. Then, add 90.76 g of acetic anhydride and 40.19 g of triethylamine to the polyamic acid solution and continue stirring for 12 hours to obtain a polyimide solution.

[0183] Steps 3 to 5 are the same as steps 3 to 5 in Example 1, and a polyimide film is obtained as a product.

[0184] Experimental Example 1

[0185] The measuring instruments and methods are as follows:

[0186] The light transmittance of the film was tested using a multifunctional X-ray diffractometer;

[0187] The tensile strength and Young's modulus of the film were tested using an electronic universal testing machine;

[0188] Use an electric pencil hardness tester to test the hardness of the film.

[0189] The CTE (coefficient of thermal expansion) of the films was measured using a dynamic thermomechanical analyzer (TMA) (model Q400).

[0190] The optical properties, mechanical properties and hardness of the films obtained in Example 1 and Comparative Examples 1 to 5 were tested, and the results are shown in Table 1.

[0191] Table 1 Film performance table

[0192]

[0193] As can be seen from Table 1, the film obtained in Example 1 has significantly improved performance compared to Comparative Example 1 in which no diacyl halide and neutralizer were added, Comparative Examples 2 and 3 in which diacyl halide was added but no neutralizer was added, Comparative Example 4 in which the neutralizer was added too early, and Comparative Example 5 in which the neutralizer was added too late.

[0194] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

1. A method for preparing a colorless and transparent polyimide film, the method comprising: Step 1: dissolving a diamine in a non-protonic polar solvent, then adding dianhydride and diacyl halide thereto and stirring to perform a polymerization reaction, and then adding a neutralizing agent to perform a neutralization reaction to obtain a polyamic acid solution, Wherein, the diamine is N,N'-(2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diyl)bis(4-aminobenzamide), The dianhydride is 4,4'-biphenyl ether dianhydride, The diacyl halide is terephthaloyl chloride, The neutralizing agent is one or more selected from sodium bicarbonate, sodium carbonate, calcium carbonate, potassium carbonate and potassium bicarbonate. The molar ratio of the diamine, the dianhydride and the diacyl halide is 2:0.6 to 1:1, The molar ratio of the neutralizing agent to the diacyl halide is 1 to 3:1, The neutralizing agent is added before the polymerization reaction is completed, and the polymerization reaction is carried out for 4 hours; Step 2: adding a capping agent, a dehydrating agent and a catalyst to the polyamic acid solution to react to obtain a polyimide solution; Step 3: precipitating the polyimide solution in a solvent, and then drying to obtain a polyimide resin; Step 4: dissolving the polyimide resin in a non-protonic polar solvent to obtain a polyimide resin solution; Step 5: Degassing, filtering, heating and drying the polyimide resin solution, and then cooling the solution to obtain a polyimide film.

2. The method according to claim 1, wherein: In step 1, The aprotic polar solvent is one or more selected from dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide and tetrahydrofuran.

3. The method according to claim 1, wherein: In step 1, The mass ratio of the combination of the diamine, the dianhydride and the diacyl halide to the aprotic polar solvent is 7-30%:93-70%.

4. The method according to claim 1, wherein: In the step 1, the polymerization reaction time is 6 to 30 hours.

5. The method according to claim 1, wherein: In step 2, The end-capping agent is one or more selected from phthalic anhydride, maleic anhydride, phenylacetylene phthalic anhydride, tetrahydrophthalic anhydride, ethynyl aniline, p-(trimethoxysilyl)aniline and aminopropyltriethoxysilane. The dehydrating agent is one or more selected from acetic anhydride, trifluoroacetic anhydride, acetyl chloride and thionyl chloride; The catalyst is one or more selected from triethylamine, pyridine, NN dimethylethanolamine, tert-butylpyridine and isoquinoline; In step 3, The solvent is water, ethanol or a mixture thereof. The weight average molecular weight of the polyimide resin is 20,000 to 450,000.

6. The method according to claim 1, wherein: In step 4, the aprotic polar solvent is one or more selected from dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide and tetrahydrofuran.

7. The method according to claim 1, wherein: In the step 4, the filtered solution is coated on a glass plate and then dried to obtain a polyimide film.

8. A polyimide film prepared by the method according to claim 1.

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