Polyimide composite film, method for manufacturing the same, and flexible circuit board

Abstract

The application relates to a polyimide composite film, a preparation method thereof and a flexible circuit board, and belongs to the polyimide film field. The polyimide composite film comprises a non-thermoplastic polyimide layer and a thermoplastic polyimide layer which are arranged in a laminated mode; the thermoplastic polyimide layer contains thermoplastic polyimide terminated by an amine group, the non-thermoplastic polyimide layer contains non-thermoplastic polyimide terminated by diisocyanate, and a urea group generated by the reaction of diisocyanate and the amine group exists between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer; the existence of the urea group effectively improves the peeling strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer, and a polyimide composite film with high peeling strength is obtained.

Description

Technical Field

[0001] This application relates to the field of multilayer polyimide films, specifically to a polyimide composite film, its preparation method, and a flexible circuit board. Background Technology

[0002] The polyimide composite film coating method involves coating a polyamic acid solution onto the surface of a polyimide film and then heating it to imidize it, thereby obtaining a multilayer polyimide film. However, this method has many steps and is costly. Therefore, the current method uses a multilayer casting process. After drying, the cast layer is peeled off from a steel strip, and then the solvent is removed at high temperature to manufacture a multilayer polyimide film (i.e., multilayer co-extruded polyimide film). This method greatly simplifies the process, but it can result in lower peel strength between the thermoplastic polyimide layer and the non-thermoplastic polyimide layer. Summary of the Invention

[0003] This application provides a polyimide composite film, its preparation method, and a flexible circuit board, which can improve the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer, thereby obtaining a polyimide composite film with high peel strength.

[0004] The embodiments of this application are implemented as follows:

[0005] In a first aspect, this application provides a polyimide composite film comprising a non-thermoplastic polyimide layer and a thermoplastic polyimide layer stacked together;

[0006] The thermoplastic polyimide layer contains amine-terminated thermoplastic polyimide, the non-thermoplastic polyimide layer contains diisocyanate-terminated non-thermoplastic polyimide, and there are urea groups formed by the reaction of diisocyanate and amine between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

[0007] The polyimide composite film provided in this application contains thermoplastic polyimide with amine-terminated thermoplastic polyimide in a thermoplastic polyimide layer and non-thermoplastic polyimide with diisocyanate-terminated non-thermoplastic polyimide in a non-thermoplastic polyimide layer. By utilizing the urea groups generated by the reaction of diisocyanate and amine groups between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer, the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer can be effectively improved, thus obtaining a polyimide composite film with high peel strength.

[0008] In some embodiments, the first polyamic acid for forming the non-thermoplastic polyimide layer is polymerized from dianhydride monomer, diamine monomer, and diisocyanate monomer; wherein the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.007, and the molar ratio of the sum of dianhydride monomer and diisocyanate monomer to diamine monomer is 1:0.998-1.002.

[0009] In some embodiments, the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.005.

[0010] In some embodiments, the molar ratio of the diisocyanate monomer to the dianhydride monomer is 0.001-0.005.

[0011] In some embodiments, the diisocyanate monomer includes an aromatic diisocyanate monomer.

[0012] Optionally, the diisocyanate monomers include terephthalic diisocyanate and / or naphthalene diisocyanate.

[0013] In some embodiments, the second polyamic acid for forming the thermoplastic polyimide layer is formed by polymerization of dianhydride monomer and diamine monomer; wherein the molar ratio of dianhydride monomer to diamine monomer is 1:1-1.005.

[0014] In some embodiments, the non-thermoplastic polyimide layer and the thermoplastic polyimide layer each contain at least one dehydrating agent and at least one imidization catalyst.

[0015] In some embodiments, the imidization catalyst includes at least one of pyridine, quinoline, isoquinoline, and triethylamine; and / or, the dehydrating agent includes at least one of acetic anhydride, propionic anhydride, and benzoic anhydride.

[0016] In some embodiments, the dehydrating agent in the non-thermoplastic polyimide layer is the same as the dehydrating agent in the thermoplastic polyimide layer; and / or, the imidization catalyst in the non-thermoplastic polyimide layer is the same as the imidization catalyst in the thermoplastic polyimide layer.

[0017] In some embodiments, the polyimide composite film includes two thermoplastic polyimide layers and one non-thermoplastic polyimide layer, with the non-thermoplastic polyimide layer located between the two thermoplastic polyimide layers.

[0018] Optionally, the thickness of the thermoplastic polyimide layer is 3-5 μm, and the thickness of the non-thermoplastic polyimide layer is 15-19 μm.

[0019] In a second aspect, this application provides a method for preparing a polyimide composite film, comprising:

[0020] A first solution is obtained, comprising at least one imidization catalyst, at least one dehydrating agent, a first polyamic acid, and an aprotic polar solvent, wherein the first polyamic acid can be used to prepare a diisocyanate-terminated non-thermoplastic polyimide by imidization;

[0021] A second solution is obtained, the second solution comprising at least one imidization catalyst, at least one dehydrating agent, a second polyamic acid, and an aprotic polar solvent, wherein the second polyamic acid can be used to prepare an amino-terminated thermoplastic polyimide by imidization;

[0022] The first and second solutions are co-extruded to remove the solvent and obtain a co-extruded cast sheet, which includes a first polyamic acid layer and a second polyamic acid layer stacked together.

[0023] The co-extruded cast sheet was imidized to obtain a polyimide composite film.

[0024] The preparation method provided in this application involves, on the one hand, preparing a polyimide composite film using a co-extrusion method, which is convenient to operate and, compared to the coating method, helps to improve the peel strength of the polyimide composite film. On the other hand, through improvements in the formulations of the first and second solutions, the prepared thermoplastic polyimide layer contains amine-terminated thermoplastic polyimide, and the non-thermoplastic polyimide layer contains diisocyanate-terminated non-thermoplastic polyimide. This results in a reaction between diisocyanate and amine groups between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer to generate urea groups, which are then used for crosslinking, effectively improving the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer. Through these combined effects, a polyimide composite film with high peel strength is finally obtained.

[0025] In some embodiments, the first polyamic acid is obtained by polymerizing dianhydride monomer, diamine monomer, and diisocyanate monomer; wherein the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.007, and the molar ratio of the sum of dianhydride monomer and diisocyanate monomer to diamine monomer is 1:0.998-1.002.

[0026] Optionally, the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.005.

[0027] Optionally, the diisocyanate monomers include terephthalic diisocyanate and / or naphthalene diisocyanate.

[0028] In some embodiments, the second polyamic acid is obtained by polymerization of dianhydride monomer and diamine monomer; wherein the molar ratio of dianhydride monomer to diamine monomer is 1:1-1.005.

[0029] In some embodiments, the first solution comprises, by mass percentage: 60-80 parts of an aprotic polar solvent, 10-30 parts of a first polyamic acid, 5-20 parts of an imidization catalyst, and 5-30 parts of a dehydrating agent; and / or,

[0030] The second solution comprises, by mass percentage: 60-80 parts of aprotic polar solvent, 10-30 parts of second polyamic acid, 5-20 parts of imidization catalyst, and 5-30 parts of dehydrating agent.

[0031] In some embodiments, the aprotic polar solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone;

[0032] Optionally, the imidization catalyst includes at least one of pyridine, quinoline, isoquinoline, and triethylamine;

[0033] Optionally, the dehydrating agent includes at least one of acetic anhydride, propionic anhydride, and benzoic anhydride.

[0034] In some embodiments, imidization includes: heated co-extrusion casting.

[0035] Optionally, the heating process includes the following steps performed sequentially: holding at 150°C for 2-3 minutes, holding at 200°C for 2-5 minutes, holding at 300°C for 2-5 minutes, holding at 350°C for 2-5 minutes, and holding at 400°C for 2-5 minutes.

[0036] In a second aspect, this application provides a flexible circuit board, which includes the polyimide composite film provided in the first aspect of this application or the polyimide composite film prepared by the preparation method provided in the second aspect of this application. Detailed Implementation

[0037] The embodiments of this application will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of this application. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0038] It should be noted that the diamine monomers involved in this application include, but are not limited to, aromatic diamine compounds, alicyclic diamine compounds, or aliphatic diamine compounds. Aromatic diamine compounds, alicyclic diamine compounds, and aliphatic diamine compounds can be used alone or in combination of two or more.

[0039] The aromatic diamine compounds include, but are not limited to, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, p-phenylenediamine, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, benzidine, 3,3'-dichlorobenzidine, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, etc. Diphenyl ethers, 3,3'-diaminodiphenyl ethers, 3,4'-diaminodiphenyl ethers, 1,5-diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane, 4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine oxide, 4,4'-diaminodiphenyl-N-methylamine, 4,4'-diaminodiphenyl-N-phenylamine, 1,4-diaminobenzene (p-phenylenediamine), 1,3-diaminobenzene, 1,2-diaminobenzene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and derivatives of these aromatic diamines, etc. These aromatic diamine compounds and their derivatives can be used alone or mixed in any proportion.

[0040] Alicyclic diamine compounds include, but are not limited to, cyclobutane diamine, isophorone diamine, bicyclo[2,2,1]heptanedimethylamine, tricyclo[3,3,1,13,7]decane-1,3-diamine, 1,2-cyclohexyldiamine, 1,3-cyclohexyldiamine, cis-1,4-cyclohexyldiamine, 4,4'-diaminodicyclohexylmethane, 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane, 3,3'-diethyl-4,4'-diaminodicyclohexylmethane, and derivatives of these alicyclic diamine compounds. These alicyclic diamine compounds and their derivatives may be used alone or mixed in any proportion.

[0041] Aliphatic diamine compounds include, but are not limited to, ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, 1,7-diaminoheptane, 1,8-diaminooctane, and derivatives of these aliphatic diamine compounds. These aliphatic diamine compounds and their derivatives may be used alone or mixed in any proportion.

[0042] In some optional embodiments, the diamine monomer is one or any combination of p-phenylenediamine (PDA), 4,4'-diaminodiphenyl ether (ODA), 3,4'-diaminodiphenyl ether (3,4'-ODA), 4,4'-diaminodiphenyl sulfone, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP).

[0043] The dianhydride monomers involved in this application include aromatic acid dianhydrides, alicyclic acid dianhydrides, or aliphatic acid dianhydrides, wherein aromatic acid dianhydrides, alicyclic acid dianhydrides, and aliphatic acid dianhydrides can be used alone or in combination of two or more.

[0044] Among them, aromatic acid dianhydrides include, but are not limited to, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, and 1,3-dimethyl-1,2... One or any combination of the following: 3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cycloheptanetetracarboxylic dianhydride, 2,3,4,5-tetrahydrofurantetracarboxylic dianhydride, 3,4-dicarboxy-1-cyclohexylsuccinic dianhydride, 2,3,5-tricarboxycyclopentylacetic dianhydride, 3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalenesuccinic dianhydride, or bicyclo[3,3,0]octane-2,4,6,8-tetracarboxylic dianhydride.

[0045] Alicyclic dianhydrides include, but are not limited to, one or any combination of 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,3,4-tetramethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, or 1,2,3,4-cycloheptanetetracarboxylic dianhydride.

[0046] Aliphatic acid dianhydrides include, but are not limited to, 1,2,3,4-butanetetracarboxylic dianhydride, 1,2,3,4-pentanetetracarboxylic dianhydride, and their derivatives.

[0047] In some embodiments, the dianhydride monomer is one or any combination of pyromellitic dianhydride (PMDA), 3,3,4,4'-biphenyltetracarboxylic dianhydride (BPDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), and 4,4'-diphenyl ether dianhydride (ODPA).

[0048] It should be noted that the dianhydride monomer in the second polyamic acid and the dianhydride monomer in the first polyamic acid may be the same or different; the diamine monomer in the second polyamic acid and the diamine monomer in the first polyamic acid may be the same or different, and can be selected according to actual needs.

[0049] The following provides a detailed description of the polyimide composite film, its preparation method, and the flexible circuit board according to embodiments of this application:

[0050] This application provides a polyimide composite film, which includes a non-thermoplastic polyimide layer and a thermoplastic polyimide layer stacked together;

[0051] The thermoplastic polyimide layer contains amine-terminated thermoplastic polyimide, the non-thermoplastic polyimide layer contains diisocyanate-terminated non-thermoplastic polyimide, and there are urea groups formed by the reaction of diisocyanate and amine between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

[0052] It is understood that non-thermoplastic polyimides are polyimides other than thermoplastic polyimides. Thermoplastic polyimides generally refer to polyimides that have a glass transition temperature in DSC (Differential Scanning Calorimetry). Non-thermoplastic polyimides refer to polyimides that do not substantially have a glass transition temperature in DSC.

[0053] Since the thermoplastic polyimide layer contains amine-terminated thermoplastic polyimide and the non-thermoplastic polyimide layer contains diisocyanate-terminated non-thermoplastic polyimide, the principle behind the existence of urea groups between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer is as follows:

[0054]

[0055] , where (1) represents a diisocyanate end group, (2) represents an amino end group, and (3) represents a diisocyanate end group reacting with an amino end group to form a urea group.

[0056] The polyimide composite film provided in this application contains thermoplastic polyimide with amine-terminated thermoplastic polyimide in a thermoplastic polyimide layer and non-thermoplastic polyimide with diisocyanate-terminated non-thermoplastic polyimide in a non-thermoplastic polyimide layer. The diisocyanate between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer reacts with the amine to generate urea groups for crosslinking, effectively improving the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer, thus obtaining a polyimide composite film with high peel strength.

[0057] The thermoplastic polyimide with amine end-capping and the non-thermoplastic polyimide with diisocyanate end-capping are both obtained by imidizing their respective polyamic acids, thereby forming their respective thermoplastic polyimide layers and non-thermoplastic polyimide layers.

[0058] In some embodiments, the first polyamic acid for forming the non-thermoplastic polyimide layer is polymerized from dianhydride monomer, diamine monomer, and diisocyanate monomer; wherein the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.007, and the molar ratio of the sum of dianhydride monomer and diisocyanate monomer to diamine monomer is 1:0.998-1.002.

[0059] By controlling the molar ratio of dianhydride monomer, diamine monomer, and diisocyanate monomer within the above range, it is beneficial to form a non-thermoplastic polyimide with diisocyanate end-capping through imidization, and it is also beneficial to improve the peel strength of the polyimide composite film.

[0060] For example, the molar ratio of diisocyanate monomer to dianhydride monomer is any one of 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007 or between any two values, and the molar ratio of the sum of dianhydride monomer and diisocyanate monomer to diamine monomer is any one of 1:0.998, 1:0.999, 1:1, 1:1.001, 1.002 or between any two values.

[0061] The peel strength first increases and then decreases with the increase of the molar ratio of diisocyanate monomer to dianhydride monomer, and the use of excessive diisocyanate monomer increases the cost.

[0062] In some embodiments, the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.005.

[0063] By controlling the molar ratio of dianhydride monomer, diamine monomer, and diisocyanate monomer within the above-mentioned range, it is beneficial to reduce manufacturing costs while further improving the peel strength of polyimide composite films.

[0064] In some embodiments, the molar ratio of diisocyanate monomer to dianhydride monomer is 0.001-0.005.

[0065] By controlling the molar ratio of the two components within the above range, the peel strength of the polyimide composite film can be further optimized to achieve a peel strength of 0.8 kgf / cm or higher.

[0066] Diisocyanate monomers include aromatic diisocyanate monomers and / or aliphatic diisocyanate monomers, wherein aliphatic diisocyanate monomers include one or any combination of hexamethylene diisocyanate, 1,4-cyclohexane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, isophorone diisocyanate and hydrogenated xylene diisocyanate.

[0067] In some embodiments, the diisocyanate monomer is an aromatic diisocyanate monomer.

[0068] Using aromatic diisocyanate monomers is beneficial for improving the peel strength and tensile strength of polyimide composite films.

[0069] The aromatic diisocyanate monomers include, but are not limited to, one or any combination of 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, terephthalic diisocyanate, 2,2'-methylene diphenyl diisocyanate, 2,4'-methylene diphenyl diisocyanate, 4,4'-methylene diphenyl diisocyanate, m-xylene diisocyanate, and naphthalene diisocyanate.

[0070] In some embodiments, the diisocyanate monomer is terephthalic diisocyanate and / or naphthalene diisocyanate.

[0071] The above-mentioned diisocyanate monomers have good synergistic effects with dianhydride monomers and diamine monomers, which is beneficial for forming non-thermoplastic polyimides with diisocyanate end capping.

[0072] In some embodiments, the second polyamic acid used to form the thermoplastic polyimide layer is formed by polymerization of a dianhydride monomer and a diamine monomer;

[0073] The molar ratio of dianhydride monomer to diamine monomer is 1:1-1.005.

[0074] By controlling the molar ratio of dianhydride monomer and diamine monomer within the above range, it is beneficial to form amine-terminated thermoplastic polyimides through imidization.

[0075] For example, the molar ratio of dianhydride monomer to diamine monomer is any one of 1:1, 1:1.001, 1:1.002, 1:1.003, 1:1.004, 1.005 or between any two of these values.

[0076] In some embodiments, the non-thermoplastic polyimide layer and the thermoplastic polyimide layer each contain at least one dehydrating agent and at least one imidization catalyst.

[0077] The dehydrating agent acts as a dehydrating and ring-closing agent for polyamic acid. The imidization catalyst can be any ingredient, as long as it promotes the dehydrating and ring-closing effect of the dehydrating agent.

[0078] Imidization catalysts include, but are not limited to, aliphatic tertiary amines, aromatic tertiary amines, heterocyclic tertiary amines, etc. Any component can be used as long as it has the effect of promoting the dehydration and ring-closing of the dehydrating agent.

[0079] By way of example, the imidization catalyst includes, but is not limited to, at least one of the group consisting of free quinoline, isoquinoline, β-methylpyridine, pyridine, imidazole, 2-imidazole, 1,2-dimethylimidazole, 2-phenylimidazole and benzimidazole.

[0080] In some embodiments, the imidization catalyst is selected from at least one of pyridine, quinoline, isoquinoline, and triethylamine.

[0081] Dehydrating agents include, but are not limited to, compounds such as aliphatic anhydrides, aromatic anhydrides, N,N'-dialkylcarbodiimides, lower aliphatic halides, halogenated lower aliphatic anhydrides, aryl sulfonic acid dihalides, and thionyl halides.

[0082] In some embodiments, the dehydrating agent includes at least one of acetic anhydride, propionic anhydride, and benzoic anhydride.

[0083] It should be noted that the dehydrating agent in the non-thermoplastic polyimide layer can be the same as or different from that in the thermoplastic polyimide layer; the imidization catalyst in the non-thermoplastic polyimide layer can be the same as or different from that in the thermoplastic polyimide layer.

[0084] In some embodiments, the dehydrating agent in the non-thermoplastic polyimide layer is the same as the dehydrating agent in the thermoplastic polyimide layer; and / or, the imidization catalyst in the non-thermoplastic polyimide layer is the same as the imidization catalyst in the thermoplastic polyimide layer.

[0085] It should be noted that the number of non-thermoplastic polyimide layers is one or more, and the number of thermoplastic polyimide layers is one or more, where "multiple layers" refers to two or more layers.

[0086] For example, the polyimide composite film is obtained by stacking a non-thermoplastic polyimide layer and a thermoplastic polyimide layer.

[0087] For example, the polyimide composite film contains n layers of non-thermoplastic polyimide (n is greater than or equal to 1) and n+1 layers of thermoplastic polyimide, wherein the thermoplastic polyimide layers and non-thermoplastic polyimide layers are stacked alternately in sequence, so that both sides of the polyimide composite film along the stacking direction are thermoplastic polyimide layers.

[0088] In some embodiments, the polyimide composite film includes two thermoplastic polyimide layers and one non-thermoplastic polyimide layer, with the non-thermoplastic polyimide layer located between the two thermoplastic polyimide layers. The aforementioned three-layer polyimide composite film can be used to manufacture double-sided metal-coated laminates, and can also achieve lightweighting, miniaturization, and high-density flexible printed circuit boards.

[0089] It should be noted that the thickness of the polyimide composite film can be set according to actual needs, such as 5μm-125μm, and those skilled in the art can design it according to actual needs.

[0090] It should also be noted that the thickness of the non-thermoplastic polyimide layer and the thermoplastic polyimide layer can be the same or different. The thickness of both can be adjusted according to actual needs and is not limited here.

[0091] In some embodiments, the thickness of the thermoplastic polyimide layer is 3-5 μm, and the thickness of the non-thermoplastic polyimide layer is 15-19 μm.

[0092] Within the aforementioned thickness range, polyimide composite films exhibit superior mechanical and mechanical properties, meeting the requirements for flexible circuit board applications.

[0093] For example, the thickness of the thermoplastic polyimide layer is any value of 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm or between any two values, and the thickness of the non-thermoplastic polyimide layer is any value of 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or between any two values.

[0094] For example, the thickness of the thermoplastic polyimide layer is 3.5 μm, and the thickness of the non-thermoplastic polyimide layer is 18 μm.

[0095] It should be noted that there are various methods for preparing polyimide composite films, including but not limited to coating and co-extrusion methods.

[0096] This application also provides a method for preparing a polyimide composite film, comprising:

[0097] S1. Obtain a first solution comprising at least one imidization catalyst, at least one dehydrating agent, a first polyamic acid, and an aprotic polar solvent, wherein the first polyamic acid can be used to prepare a diisocyanate-terminated non-thermoplastic polyimide by imidization.

[0098] A second solution is obtained, comprising at least one imidization catalyst, at least one dehydrating agent, a second polyamic acid, and an aprotic polar solvent, wherein the second polyamic acid can be used to prepare an amine-terminated thermoplastic polyimide by imidization.

[0099] S2. Co-extruding the first and second solutions to remove the solvent to obtain a co-extruded cast sheet, the co-extruded cast sheet comprising a first polyamic acid layer and a second polyamic acid layer stacked together;

[0100] S3. Imidize the co-extruded cast sheet to obtain a polyimide composite film.

[0101] Co-extrusion refers to the simultaneous extrusion and casting of a first solution and a second solution into a template, such as a steel plate, in a co-extrusion die to obtain multiple layers of liquid film of different materials stacked together. After removing the solvent, a co-extruded cast sheet is formed.

[0102] It is understandable that the polyimide composite film obtained after imidizing the co-extruded cast sheet includes a non-thermoplastic polyimide layer and a thermoplastic polyimide layer stacked together. The first polyamic acid forms a non-thermoplastic polyimide with diisocyanate end-capping, and the second polyamic acid forms a thermoplastic polyimide with amine end-capping. That is, the thermoplastic polyimide layer contains thermoplastic polyimide with amine end-capping, and the non-thermoplastic polyimide layer contains non-thermoplastic polyimide with diisocyanate end-capping. From the start of co-extrusion to the imidization stage in the die head section of the co-extrusion die, both diisocyanate and amine groups will react to generate urea groups. Finally, there are urea groups generated by the reaction of diisocyanate and amine groups between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

[0103] In summary, the preparation method provided in this application, on the one hand, uses co-extrusion to prepare polyimide composite films, which is convenient to operate and, compared with coating methods, is beneficial to improving the peel strength of polyimide composite films. On the other hand, through the improvement of the formulation of the first solution and the second solution, the prepared thermoplastic polyimide layer contains thermoplastic polyimide with amine groups at the end, and the non-thermoplastic polyimide layer contains non-thermoplastic polyimide with diisocyanate at the end. Furthermore, the diisocyanate between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer reacts with the amine groups to generate urea groups for crosslinking, effectively improving the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer. Through the combined effect, a polyimide composite film with high peel strength is finally obtained.

[0104] In some embodiments, the first polyamic acid is obtained by polymerization of a dianhydride monomer, a diamine monomer, and a diisocyanate monomer;

[0105] The molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.007, and the molar ratio of the sum of dianhydride monomer and diisocyanate monomer to diamine monomer is 1:0.998-1.002.

[0106] By controlling the molar ratio of dianhydride monomer, diamine monomer, and diisocyanate monomer within the above range, it is beneficial to form a non-thermoplastic polyimide with diisocyanate end-capping through imidization, and it is also beneficial to improve the peel strength of the polyimide composite film.

[0107] For example, the molar ratio of diisocyanate monomer to dianhydride monomer is any one of 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007 or between any two values, and the molar ratio of the sum of dianhydride monomer and diisocyanate monomer to diamine monomer is any one of 1:0.998, 1:0.999, 1:1, 1:1.001, 1.002 or between any two values.

[0108] Optionally, the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.005.

[0109] Optionally, the molar ratio of diisocyanate monomer to dianhydride monomer is 0.001-0.005.

[0110] When the molar ratio of the two is controlled within the above range, the polyimide composite film exhibits better peel strength.

[0111] Diisocyanate monomers include aromatic diisocyanate monomers and / or aliphatic diisocyanate monomers, wherein aliphatic diisocyanate monomers include one or any combination of hexamethylene diisocyanate, 1,4-cyclohexane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, isophorone diisocyanate and hydrogenated xylene diisocyanate.

[0112] In some embodiments, the diisocyanate monomer is an aromatic diisocyanate monomer.

[0113] The aromatic diisocyanate monomers include, but are not limited to, one or any combination of 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, terephthalic diisocyanate, 2,2'-methylene diphenyl diisocyanate, 2,4'-methylene diphenyl diisocyanate, 4,4'-methylene diphenyl diisocyanate, m-xylene diisocyanate, and naphthalene diisocyanate.

[0114] In some embodiments, the diisocyanate monomer is terephthalic diisocyanate and / or naphthalene diisocyanate.

[0115] The above-mentioned diisocyanate monomers have good synergistic effects with dianhydride monomers and diamine monomers, which is beneficial for the formation of thermoplastic polyimides with diisocyanate end capping.

[0116] In some embodiments, the second polyamic acid is obtained by polymerization of a dianhydride monomer and a diamine monomer;

[0117] The molar ratio of dianhydride monomer to diamine monomer is 1:1-1.005.

[0118] By controlling the molar ratio of dianhydride monomer and diamine monomer within the above range, it is beneficial to form amine-terminated thermoplastic polyimides through imidization.

[0119] For example, the molar ratio of dianhydride monomer to diamine monomer is any one of 1:1, 1:1.001, 1:1.002, 1:1.003, 1:1.004, 1.005 or between any two of these values.

[0120] In some embodiments, the first solution comprises, by mass percentage: 60-80 parts of an aprotic polar solvent, 10-30 parts of a first polyamic acid, 5-20 parts of an imidization catalyst, and 5-30 parts of a dehydrating agent; and / or,

[0121] The second solution comprises, by mass percentage: 60-80 parts of aprotic polar solvent, 10-30 parts of second polyamic acid, 5-20 parts of imidization catalyst, and 5-30 parts of dehydrating agent.

[0122] When the amount of dehydrating agent and imidization catalyst is less than the specified range, imidization is insufficient, affecting the mechanical strength of the polyimide composite film. When the amount of dehydrating agent and imidization catalyst is greater than the specified range, imidization proceeds rapidly, making it difficult to form a film.

[0123] Therefore, by controlling the mass ratio of each component in the first and second solutions, it is beneficial to successfully prepare polyimide composite films by co-extrusion and imidization, and also to improve the quality of polyimide composite films.

[0124] In some embodiments, the content of each component in the first solution is the same as the content of the corresponding component in the second solution.

[0125] The above settings help to control the amidation rates of the first and second polyamic acid layers to be basically the same, which is beneficial to improving the quality of the polyimide composite film.

[0126] Any non-protic polar solvent that can dissolve polyamic acid can be used.

[0127] Aprotic polar solvents can be amide solvents. For example, aprotic polar solvents include, but are not limited to, at least one of N,N'-dimethylformamide (DMF), N,N'-dimethylacetamide, N-methylpyrrolidone (NMP), γ-butyrolactone (GBL), and diethylene glycol dimethyl ether (Diglyme), but are not limited thereto. They can be used alone or in combination of two or more, as needed.

[0128] In some embodiments, the aprotic polar solvent is at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0129] The dehydrating agent acts as a dehydrating and ring-closing agent for polyamic acid. The imidization catalyst can be any ingredient, as long as it promotes the dehydrating and ring-closing effect of the dehydrating agent.

[0130] Imidization catalysts include, but are not limited to, aliphatic tertiary amines, aromatic tertiary amines, heterocyclic tertiary amines, etc. Any component can be used as long as it has the effect of promoting the dehydration and ring-closing of the dehydrating agent.

[0131] By way of example, the imidization catalyst includes, but is not limited to, at least one of the group consisting of free quinoline, isoquinoline, β-methylpyridine, pyridine, imidazole, 2-imidazole, 1,2-dimethylimidazole, 2-phenylimidazole and benzimidazole.

[0132] In some embodiments, the imidization catalyst is selected from at least one of pyridine, quinoline, isoquinoline, and triethylamine.

[0133] Dehydrating agents include, but are not limited to, compounds such as aliphatic anhydrides, aromatic anhydrides, N,N'-dialkylcarbodiimides, lower aliphatic halides, halogenated lower aliphatic anhydrides, aryl sulfonic acid dihalides, and thionyl halides.

[0134] In some embodiments, the dehydrating agent includes at least one of acetic anhydride, propionic anhydride, and benzoic anhydride.

[0135] The solvent is removed by drying.

[0136] In some embodiments, solvent removal includes heating at 140-160°C for a specific time, which can be selected according to the film thickness. For example, the heating time is 120-170 seconds. For example, solvent removal includes heating at 150°C for 150 seconds.

[0137] In some embodiments, imidization includes: heated co-extrusion casting.

[0138] In some embodiments, during the imidization process of heating the co-extruded cast sheet, the heating procedure includes the following sequential steps: holding at 150°C for 2-3 min, holding at 200°C for 2-5 min, holding at 300°C for 2-5 min, holding at 350°C for 2-5 min, and holding at 400°C for 2-5 min.

[0139] The above-mentioned staged heating method, combined with a dehydrating agent, is beneficial to the completeness of imidization.

[0140] This application also provides a flexible circuit board, which includes the above-described polyimide composite film or the polyimide composite film prepared by the above-described preparation method.

[0141] The following detailed description of the polyimide composite film, its preparation method, and the flexible circuit board of this application, in conjunction with the embodiments, provides further insight.

[0142] In the following embodiments and Comparative Example 2,

[0143] Example 1

[0144] The polyimide composite film comprises two thermoplastic polyimide layers and one non-thermoplastic polyimide layer, with the non-thermoplastic polyimide layer located between the two thermoplastic polyimide layers. The thermoplastic polyimide layer contains amine-terminated thermoplastic polyimide, and the non-thermoplastic polyimide layer contains diisocyanate-terminated non-thermoplastic polyimide. Urea groups, formed by the reaction of diisocyanate and amine groups, exist between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

[0145] The polyimide composite film was prepared by the following method:

[0146] [Obtain the first solution]

[0147] At -5 to 0°C, N,N-dimethylformamide and 4,4,-diaminodiphenyl ether (ODA) were added to a nitrogen-filled reactor and dissolved. Then, an equimolar amount of pyromellitic dianhydride (PMDA) was added and the polymerization reaction was carried out for 3 hours. After cooling to room temperature, a polyamic acid precursor solution was obtained. The dissolved terephthalic diisocyanate solution was added and reacted to obtain a first polyamic acid solution with terephthalic diisocyanate end groups.

[0148] Triethylamine and acetic anhydride were added to the first polyamic acid solution, and after mixing and degassing, the first solution was obtained.

[0149] The molar ratio of terephthalic diisocyanate:PMDA:ODA is 0.001:0.999:1, and the mass ratio of the sum of terephthalic diisocyanate, PMDA and ODA:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

[0150] [Obtain the second solution]

[0151] At 25°C, N,N-dimethylformamide and 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) were added to a nitrogen-filled reactor and dissolved. Then, 4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA) and 4,4'-diphenyl ether dianhydride (ODPA) were added and polymerized for 3 hours. The mixture was then cooled to room temperature to obtain a second polyamic acid precursor solution.

[0152] Triethylamine and acetic anhydride were added to the second polyamic acid precursor solution, and after mixing and degassing, the second solution was obtained.

[0153] The ratio of ODPA:BTDA:BAPP is 0.5:0.5:1, and the mass ratio of the sum of ODPA, BTDA and BAPP to N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

[0154] [Film Formation]

[0155] The first and second solutions were co-extruded into a multilayer steel strip. The solvent was removed by heating at 150°C for 150 seconds. Then, the solutions were sequentially heated at 200°C for 250 seconds, 300°C for 250 seconds, 350°C for 250 seconds, and 400°C for 250 seconds to perform imidization, resulting in a composite polyimide film with a thickness of 25 μm. The thickness of the upper and lower thermoplastic polyimide layers is 3.5 μm, and the thickness of the non-thermoplastic polyimide layer is 18 μm.

[0156] Example 2

[0157] Its only difference from Example 1 is:

[0158] In the first solvent, the molar ratio of terephthalic diisocyanate:PMDA:ODA is 0.003:0.997:1, and the mass ratio of the sum of terephthalic diisocyanate, PMDA and ODA:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

[0159] Example 3

[0160] Its only difference from Example 1 is:

[0161] In the first solvent, the molar ratio of terephthalic diisocyanate:PMDA:ODA is 0.005:0.995:1, and the mass ratio of the sum of terephthalic diisocyanate, PMDA and ODA:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

[0162] Example 4

[0163] Its only difference from Example 1 is:

[0164] In the first solvent, the molar ratio of terephthalic diisocyanate:PMDA:ODA is 0.007:0.993:1, and the mass ratio of the sum of terephthalic diisocyanate, PMDA and ODA:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

[0165] Example 5

[0166] Its only difference from Example 1 is:

[0167] In the first solvent, the molar ratio of naphthalene diisocyanate:PMDA:ODA is 0.001:0.999:1, and the mass ratio of the sum of terephthalene diisocyanate, PMDA and ODA:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

[0168] Example 6

[0169] Its only difference from Example 1 is:

[0170] In the first solvent, the molar ratio of hexamethylene diisocyanate:PMDA:ODA is 0.001:0.999:1, and the mass ratio of the sum of terephthalic diisocyanate, PMDA and ODA:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

[0171] Example 7

[0172] Its only difference from Example 1 is:

[0173] In the second solvent, the molar ratio of ODPA:BTDA:BAPP is 0.3:0.7:1, and the mass ratio of the sum of ODPA, BTDA and BAPP to N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

[0174] Comparative Example 1

[0175] Its only difference from Example 1 is:

[0176] [Obtain the first solution]

[0177] At -5 to 0°C, N,N-dimethylformamide and 4,4,-diaminodiphenyl ether (ODA) were added to a nitrogen-filled reactor. After dissolution, an equimolar amount of pyromellitic dianhydride (PMDA) was added, and the polymerization reaction was carried out for 3 hours. The mixture was then cooled to room temperature to obtain a polyamic acid precursor solution.

[0178] Triethylamine and acetic anhydride were added to the polyamic acid precursor solution, and after mixing and degassing, the first solution was obtained.

[0179] The molar ratio of PMDA to ODA is 1:1, and the mass ratio of the sum of PMDA and ODA to N,N-dimethylformamide to triethylamine to acetic anhydride is 20:80:10:30.

[0180] Comparative Example 2

[0181] Its only difference from Example 3 is:

[0182] [Film Formation]

[0183] The first solution is cast onto a steel strip, heated at 150°C for 150 seconds to remove the solvent, and then the steel strip is peeled off to obtain the first polyamic acid layer.

[0184] The second solution was coated on the upper and lower surfaces of the first polyamic acid layer after solvent removal. Then, the solvent was removed by heating at 150°C for 150s. Then, the solution was sequentially heated at 200°C for 250s, 300°C for 250s, 350°C for 250s, and 400°C for 250s to perform imidization, resulting in a composite polyimide film with a thickness of 25μm.

[0185] Experimental Example 1

[0186] The performance of the composite polyimide films prepared in Examples 1-4 and Comparative Examples 1-2 was tested.

[0187] Tensile strength and elongation at break were tested according to GB / T 1040.1-2018, and peel strength was tested according to the national standard GB / T 2792-2014 "Standard for 180° Peel Strength Test Method".

[0188] The results are shown in Table 1.

[0189] Table 1 Test Results

[0190] Sample number Tensile strength / MPa Elongation at break / % Peel strength (kgf / cm) Example 1 238 74 0.8 Example 2 240 74 0.95 Example 3 240 73 1.0 Example 4 240 74 0.7 Example 5 238 74 0.8 Example 6 235 75 0.7 Example 7 245 72 0.7 Comparative Example 1 235 75 0.6 Comparative Example 2 238 75 0.4

[0191] As can be seen from Table 1, compared with Comparative Example 1, under the premise of the same preparation process, Examples 1-7 can effectively improve the peel strength between the layers of the polyimide composite film by utilizing the urea groups generated by the reaction of diisocyanate and amine groups between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

[0192] Furthermore, as shown in Examples 1-4, a molar ratio of diisocyanate monomer to dianhydride monomer greater than 0 and less than or equal to 0.007 can improve the peel strength between the layers of the polyimide composite film. Specifically, when the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.005, both tensile strength and peel strength increase with increasing proportion. When the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0.005, the peel strength decreases. Therefore, it is preferable that the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.005, and more preferably, the molar ratio of diisocyanate monomer to dianhydride monomer is 0.001-0.005.

[0193] As can be seen from Example 3 and Comparative Example 2, the preparation method also significantly affects the peel strength. Under the premise of the same raw materials and formula, the peel strength of the polyimide composite film prepared by the co-extrusion method shown in the Example is significantly better than that of the coating method shown in Comparative Example 2.

[0194] Compared with Example 1, using Aromatic Naphthalene Diisocyanate and Terephthalic Diisocyanate showed no significant changes in tensile strength, elongation at break, and peel strength; however, using Aliphatic Hexamethylene Diisocyanate resulted in a significant decrease in peel strength and tensile strength.

[0195] Compared with Example 1, according to Example 7, after the BTDA content increased, the peel strength and elongation at break decreased, while the tensile strength increased.

[0196] In summary, the polyimide composite film provided in this application crosslinks between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer by reacting diisocyanate with amine groups to generate urea groups, which effectively improves the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer. Furthermore, the polyimide composite film is prepared by co-extrusion, which is beneficial to further improve its peel strength.

[0197] The above description is merely a specific embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A polyimide composite film, characterized in that, It includes a non-thermoplastic polyimide layer and a thermoplastic polyimide layer, which are stacked and prepared by a co-extrusion process; The thermoplastic polyimide layer contains an amine-terminated thermoplastic polyimide, the non-thermoplastic polyimide layer contains a diisocyanate-terminated non-thermoplastic polyimide, and there are urea groups formed by the reaction of diisocyanate and amine groups between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer. The first polyamic acid used to form the non-thermoplastic polyimide layer is polymerized from dianhydride monomers, diamine monomers, and diisocyanate monomers, wherein the molar ratio of the diisocyanate monomer to the dianhydride monomer is greater than 0 and less than or equal to 0.007, and the molar ratio of the sum of the dianhydride monomer and the diisocyanate monomer to the diamine monomer is 1:0.998-1.002; the diisocyanate monomer includes aromatic diisocyanate monomers. The second polyamic acid used to form the thermoplastic polyimide layer is formed by polymerization of dianhydride monomers and diamine monomers; The polyimide composite film has n layers of non-thermoplastic polyimide and n+1 layers of thermoplastic polyimide. The thermoplastic polyimide and non-thermoplastic polyimide layers are stacked alternately in sequence, and the thermoplastic polyimide layers are located on both sides of the polyimide composite film along the stacking direction.

2. The polyimide composite film according to claim 1, characterized in that, In the raw material for the first polyamic acid used to form the non-thermoplastic polyimide layer, the molar ratio of the diisocyanate monomer to the dianhydride monomer is greater than 0 and less than or equal to 0.

005.

3. The polyimide composite film according to claim 1, characterized in that, In the raw material for the first polyamic acid used to form the non-thermoplastic polyimide layer, the molar ratio of the diisocyanate monomer to the dianhydride monomer is 0.001-0.

005.

4. The polyimide composite film according to claim 1, characterized in that, The diisocyanate monomers include terephthalic diisocyanate and / or naphthalene diisocyanate.

5. The polyimide composite film according to claim 1, characterized in that, In the raw materials for the second polyamic acid used to form the thermoplastic polyimide layer, the molar ratio of the dianhydride monomer to the diamine monomer is 1:1-1.

005.

6. The polyimide composite film according to claim 1, characterized in that, The non-thermoplastic polyimide layer and the thermoplastic polyimide layer each contain at least one dehydrating agent and at least one imidization catalyst.

7. The polyimide composite film according to claim 6, characterized in that, The imidization catalyst includes at least one selected from pyridine, quinoline, isoquinoline, and triethylamine; and / or, The dehydrating agent includes at least one of acetic anhydride, propionic anhydride, and benzoic anhydride.

8. The polyimide composite film according to claim 6, characterized in that, The dehydrating agent in the non-thermoplastic polyimide layer is the same as the dehydrating agent in the thermoplastic polyimide layer; and / or, The imidization catalyst in the non-thermoplastic polyimide layer is the same as the imidization catalyst in the thermoplastic polyimide layer.

9. The polyimide composite film according to any one of claims 1-8, characterized in that, The polyimide composite film comprises two thermoplastic polyimide layers and one non-thermoplastic polyimide layer, wherein the non-thermoplastic polyimide layer is located between the two thermoplastic polyimide layers.

10. The polyimide composite film according to any one of claims 1-8, characterized in that, The thickness of the thermoplastic polyimide layer is 3-5 μm, and the thickness of the non-thermoplastic polyimide layer is 15-19 μm.

11. A method for preparing a polyimide composite film, characterized in that, include: A first solution is obtained, comprising at least one imidization catalyst, at least one dehydrating agent, a first polyamic acid, and an aprotic polar solvent, wherein the first polyamic acid can be imidized to form a diisocyanate-terminated non-thermoplastic polyimide for forming a non-thermoplastic polyimide layer. A second solution is obtained, the second solution comprising at least one imidization catalyst, at least one dehydrating agent, a second polyamic acid and an aprotic polar solvent, wherein the second polyamic acid can be imidized to form an amine-terminated thermoplastic polyimide for forming a thermoplastic polyimide layer; The first solution and the second solution are co-extruded to remove the solvent and obtain a co-extruded cast sheet, the co-extruded cast sheet comprising a first polyamic acid layer and a second polyamic acid layer stacked together; The co-extruded cast sheet is imidized to obtain the polyimide composite film; The polyimide composite film has n layers of non-thermoplastic polyimide and n+1 layers of thermoplastic polyimide. The thermoplastic polyimide and non-thermoplastic polyimide layers are stacked alternately in sequence, and the thermoplastic polyimide layers are located on both sides of the polyimide composite film along the stacking direction. The first polyamic acid is formed by polymerization of dianhydride monomer, diamine monomer, and diisocyanate monomer, wherein the molar ratio of the diisocyanate monomer to the dianhydride monomer is greater than 0 and less than or equal to 0.007, and the molar ratio of the sum of the dianhydride monomer and the diisocyanate monomer to the diamine monomer is 1:0.998-1.002; the diisocyanate monomer includes aromatic diisocyanate monomer; the second polyamic acid is formed by polymerization of dianhydride monomer and diamine monomer.

12. The preparation method according to claim 11, characterized in that, The molar ratio of the diisocyanate monomer to the dianhydride monomer is greater than 0 and less than or equal to 0.

005.

13. The preparation method according to claim 11, characterized in that, The molar ratio of the diisocyanate monomer to the dianhydride monomer is 0.001-0.

005.

14. The preparation method according to claim 11, characterized in that, The diisocyanate monomers include terephthalic diisocyanate and / or naphthalene diisocyanate.

15. The preparation method according to claim 11, characterized in that, In the raw materials of the second polyamic acid, the molar ratio of the dianhydride monomer and the diamine monomer is 1:1-1.

005.

16. The preparation method according to claim 11, characterized in that, By mass percentage, the first solution comprises: 60-80 parts of an aprotic polar solvent, 10-30 parts of a first polyamic acid, 5-20 parts of an imidization catalyst, and 5-30 parts of a dehydrating agent; and / or, The second solution comprises, by mass percentage: 60-80 parts of aprotic polar solvent, 10-30 parts of second polyamic acid, 5-20 parts of imidization catalyst, and 5-30 parts of dehydrating agent.

17. The preparation method according to claim 11, characterized in that, The aprotic polar solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone; and / or; The imidization catalyst includes at least one selected from pyridine, quinoline, isoquinoline, and triethylamine; and / or; The dehydrating agent includes at least one of acetic anhydride, propionic anhydride, and benzoic anhydride.

18. The preparation method according to any one of claims 11-17, characterized in that, The imidization includes heating the co-extruded cast sheet.

19. The preparation method according to claim 18, characterized in that, The heating process includes the following steps performed sequentially: holding at 150°C for 2-3 minutes, holding at 200°C for 2-5 minutes, holding at 300°C for 2-5 minutes, holding at 350°C for 2-5 minutes, and holding at 400°C for 2-5 minutes.

20. A flexible circuit board, characterized in that, The flexible circuit board includes the polyimide composite film according to any one of claims 1-10 or the polyimide composite film prepared by the preparation method according to any one of claims 11-19.

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