CN118906594A - Polyimide composite film, preparation method thereof and flexible circuit board - Google Patents

Abstract

A polyimide composite film and a preparation method thereof and a flexible circuit board belong to the field of multilayer polyimide films. The polyimide composite film comprises a non-thermoplastic polyimide layer and a thermoplastic polyimide layer which are stacked; the thermoplastic polyimide layer contains a thermoplastic polyimide terminated with an amine group, and the non-thermoplastic polyimide layer contains a non-thermoplastic polyimide terminated with a diisocyanate, and there is a urea group generated by the reaction of the diisocyanate with the amine group between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer, and the presence of the urea group is used to effectively improve the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer, so as to obtain a polyimide composite film with high peel strength.

Description

Polyimide composite film and preparation method thereof and flexible circuit board

Technical Field

The present application relates to the field of multilayer polyimide films, and in particular to a polyimide composite film and a preparation method thereof, and a flexible circuit board.

Background Art

The polyimide composite film coating method is to coat the surface of the polyimide film with a polyamic acid solution and imidize it by heating, thereby obtaining a multilayer polyimide film, but this method has many steps and high cost. Therefore, a multilayer casting method is currently used, in which the casting layer is peeled off from the steel belt after drying, and the solvent is removed at high temperature to manufacture a multilayer polyimide film (i.e., a multilayer co-extruded polyimide film), which greatly simplifies the steps, but this method will result in a phenomenon of low peel strength between the thermoplastic polyimide layer and the non-thermoplastic polyimide layer.

Summary of the invention

The present application provides a polyimide composite film and a preparation method thereof and a flexible circuit board, which can improve the peel strength between a non-thermoplastic polyimide layer and a thermoplastic polyimide layer to obtain a polyimide composite film with high peel strength.

The embodiment of the present application is implemented as follows:

In a first aspect, the present application provides an example of a polyimide composite film, which includes a non-thermoplastic polyimide layer and a thermoplastic polyimide layer stacked;

The thermoplastic polyimide layer contains thermoplastic polyimide terminated with amine groups, the non-thermoplastic polyimide layer contains non-thermoplastic polyimide terminated with diisocyanate, and urea groups generated by the reaction of diisocyanate and amine groups exist between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

The polyimide composite film provided by the present application comprises a thermoplastic polyimide layer containing an amine-terminated thermoplastic polyimide, and a non-thermoplastic polyimide layer containing a diisocyanate-terminated non-thermoplastic polyimide. By utilizing the urea group 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, thereby obtaining a polyimide composite film with high peel strength.

In some embodiments, the first polyamic acid for forming the non-thermoplastic polyimide layer is formed by polymerization of a dianhydride monomer, a diamine monomer, and a 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.

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

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

In some embodiments, the diisocyanate monomer includes an aromatic diisocyanate monomer.

Optionally, the diisocyanate monomer includes p-phenylene diisocyanate and/or naphthalene diisocyanate.

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

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

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.

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.

In some embodiments, the polyimide composite film includes two thermoplastic polyimide layers and a non-thermoplastic polyimide layer, and the non-thermoplastic polyimide layer is located between the two thermoplastic polyimide layers.

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.

In a second aspect, the present application provides an example of a method for preparing a polyimide composite film, which comprises:

Obtaining a first solution, comprising at least one imidization catalyst, at least one dehydrating agent, a first polyamic acid, and a non-protonic polar solvent, wherein the first polyamic acid can be imidized to produce a non-thermoplastic polyimide terminated with diisocyanate;

Obtaining a second solution, the second solution comprising at least one imidization catalyst, at least one dehydrating agent, a second polyamic acid, and a non-protonic polar solvent, wherein the second polyamic acid can be imidized to produce an amine-terminated thermoplastic polyimide;

Co-extruding the first solution and the second solution, removing the solvent to obtain a co-extruded cast sheet, wherein the co-extruded cast sheet includes a first polyamic acid layer and a second polyamic acid layer stacked;

The co-extruded cast sheet is imidized to obtain a polyimide composite film.

In the preparation method provided in the present application, on the one hand, a co-extrusion method is adopted to prepare a polyimide composite film, which is easy to operate and is conducive to improving the peel strength of the polyimide composite film compared with the coating method. On the other hand, by improving the formula of the first solution and the second solution, the prepared thermoplastic polyimide layer contains a thermoplastic polyimide terminated with an amine group, and the non-thermoplastic polyimide layer contains a non-thermoplastic polyimide terminated with a diisocyanate, so that diisocyanate and amine groups react to generate urea groups between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer, and cross-linking is carried out by the urea group, so as to effectively improve the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer. The combined effect finally obtains a polyimide composite film with high peel strength.

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

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

Optionally, the diisocyanate monomer includes p-phenylene diisocyanate and/or naphthalene diisocyanate.

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

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

In terms of mass percentage, the second solution comprises: 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.

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

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

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

In some embodiments, imidization includes heating the coextruded cast sheet.

Optionally, the heating procedure includes sequentially: keeping at 150°C for 2-3 min, keeping at 200°C for 2-5 min, keeping at 300°C for 2-5 min, keeping at 350°C for 2-5 min, and keeping at 400°C for 2-5 min.

In a second aspect, the present application provides an example of a flexible circuit board, which includes the polyimide composite film provided in the first aspect of the present application or the polyimide composite film prepared by the preparation method provided in the second aspect of the present application.

DETAILED DESCRIPTION

The embodiments of the present application will be described in detail below in conjunction with the examples, but it will be appreciated by those skilled in the art that the following examples are only used to illustrate the present application and should not be considered as limiting the scope of the present application. In the examples, if specific conditions are not specified, they are carried out according to normal conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

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

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, diphenyl ether, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 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, etc. can be used alone or in a mixture in any proportion.

Alicyclic diamine compounds include, but are not limited to, cyclobutanediamine, isophoronediamine, 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 can be used alone or in a mixture in any proportion.

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, etc. These aliphatic diamine compounds and their derivatives can be used alone or mixed in any proportion to form a mixture for use.

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).

The dianhydride monomers involved in the present 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.

The aromatic 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, 1,3-dimethyl-1,2 , 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 or any combination thereof.

The alicyclic acid dianhydride includes but is 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, 1,2-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,3-dimethyl-1,2,3,4-cyclobutanetetracarboxylic dianhydride, or 1,2,3,4-cycloheptanetetracarboxylic dianhydride.

The aliphatic acid dianhydride includes but is not limited to 1,2,3,4-butanetetracarboxylic dianhydride, 1,2,3,4-pentanetetracarboxylic dianhydride and their derivatives.

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'-benzophenonetetracarboxylic dianhydride (BTDA), and 4,4'-diphenyl ether dianhydride (ODPA).

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

The following is a detailed description of the polyimide composite film and its preparation method and the flexible circuit board according to the embodiment of the present application:

The present application provides a polyimide composite film, which includes a non-thermoplastic polyimide layer and a thermoplastic polyimide layer stacked;

The thermoplastic polyimide layer contains thermoplastic polyimide terminated with amine groups, the non-thermoplastic polyimide layer contains non-thermoplastic polyimide terminated with diisocyanate, and urea groups generated by the reaction of diisocyanate and amine groups exist between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

It is understood that non-thermoplastic polyimide is a polyimide other than thermoplastic polyimide. Thermoplastic polyimide generally refers to a polyimide having a glass transition temperature in DSC, i.e., differential scanning calorimetry. Non-thermoplastic polyimide refers to a polyimide that does not substantially have a glass transition temperature in DSC, i.e., differential scanning calorimetry.

Since the thermoplastic polyimide layer contains a thermoplastic polyimide terminated with an amine group, and the non-thermoplastic polyimide layer contains a non-thermoplastic polyimide terminated with a diisocyanate, the principle of the existence of a urea group between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer is as follows:

, wherein (1) represents a diisocyanate terminal group, (2) represents an amine terminal group, and (3) represents a urea group formed by the reaction of a terminal diisocyanate with a terminal amine group.

The polyimide composite film provided by the present application comprises a thermoplastic polyimide layer containing an amine-terminated thermoplastic polyimide, and a non-thermoplastic polyimide layer containing a diisocyanate-terminated non-thermoplastic polyimide. The diisocyanate between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer is reacted with the amine group to generate a urea group for cross-linking, thereby effectively improving the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer, thereby obtaining a polyimide composite film with high peel strength.

The amine-terminated thermoplastic polyimide and the diisocyanate-terminated non-thermoplastic polyimide are obtained by imidizing the corresponding polyamic acid to form the corresponding thermoplastic polyimide layer and non-thermoplastic polyimide layer.

In some embodiments, the first polyamic acid for forming the non-thermoplastic polyimide layer is formed by polymerization of a dianhydride monomer, a diamine monomer, and a 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.

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

Illustratively, the molar ratio of the diisocyanate monomer to the 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 the dianhydride monomer and the diisocyanate monomer to the diamine monomer is any one of 1:0.998, 1:0.999, 1:1, 1:1.001, 1.002 or between any two values.

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

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

By controlling the molar ratio of the dianhydride monomer, the diamine monomer and the diisocyanate monomer within the above range, it is beneficial to reduce the production cost while further improving the peel strength of the polyimide composite film.

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

By controlling the molar ratio of the two within the above range, the peel strength of the polyimide composite film is further optimized so that the peel strength can reach 0.8 kgf/cm or above.

The diisocyanate monomers include aromatic diisocyanate monomers and/or aliphatic diisocyanate monomers. The 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.

In some embodiments, the diisocyanate monomer is an aromatic diisocyanate monomer.

The use of aromatic diisocyanate monomers is beneficial to improving the peel strength and tensile strength of the polyimide composite film.

The aromatic diisocyanate monomers include but are not limited to 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, 2,2'-methylene diphenyl diisocyanate, 2,4'-methylene diphenyl diisocyanate, 4,4'-methylene diphenyl diisocyanate, m-xylene diisocyanate, naphthalene diisocyanate or any combination thereof.

In some embodiments, the diisocyanate monomer is p-phenylene diisocyanate and/or naphthalene diisocyanate.

The above-mentioned diisocyanate monomer has a good coordination effect with the dianhydride monomer and the diamine monomer, and is conducive to the formation of a non-thermoplastic polyimide terminated with diisocyanate.

In some embodiments, the second polyamic acid for forming the thermoplastic polyimide layer is formed by polymerizing a dianhydride monomer and a diamine monomer;

Wherein, the molar ratio of the dianhydride monomer to the diamine monomer is 1:1-1.005.

By controlling the molar ratio of the dianhydride monomer to the diamine monomer within the above range, it is advantageous to form a thermoplastic polyimide terminated with an amine group through imidization.

Illustratively, the molar ratio of the dianhydride monomer to the 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 values.

In some embodiments, the non-thermoplastic polyimide layer and the thermoplastic polyimide layer respectively include at least one dehydrating agent and at least one imidization catalyst.

The dehydrating agent has the function of a dehydrating and ring-closing agent for the polyamic acid, and any component can be used as the imidization catalyst as long as it has the effect of promoting the dehydrating and ring-closing action of the dehydrating agent.

The imidization catalyst includes but is 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 closure of the dehydrating agent.

Illustratively, the imidization catalyst includes but is not limited to at least one of the group consisting of free quinoline, isoquinoline, β-picoline, pyridine, imidazole, 2-imidazole, 1,2-dimethylimidazole, 2-phenylimidazole and benzimidazole.

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

The dehydrating agent includes, but is not limited to, aliphatic anhydrides, aromatic anhydrides, N,N'-dialkylcarbodiimide, lower aliphatic halides, halogenated lower aliphatic anhydrides, arylsulfonic acid dihalides, thionyl halides and the like.

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

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

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.

It should be noted that the number of the non-thermoplastic polyimide layer is one or more layers, and the number of the thermoplastic polyimide layer is one or more layers, and the multiple layers here refer to two layers or more.

Exemplarily, the polyimide composite film is obtained by laminating a non-thermoplastic polyimide layer and a thermoplastic polyimide layer.

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

In some embodiments, the polyimide composite film includes two thermoplastic polyimide layers and a non-thermoplastic polyimide layer, wherein the non-thermoplastic polyimide layer is located between the two thermoplastic polyimide layers. The three-layer polyimide composite film can be used to manufacture a double-sided metal-clad laminate, and can also achieve lightweight, miniaturization, and high density of a flexible printed circuit board.

It should be noted that the thickness of the polyimide composite film can be set according to actual needs, for example, the thickness is 5 μm-125 μm, etc., and those skilled in the art can design it according to actual needs.

It should also be noted that the thicknesses of the non-thermoplastic polyimide layer and the thermoplastic polyimide layer may be the same or different, and the thicknesses of the two layers may be adjusted according to actual needs and are not limited here.

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.

Within the above thickness range, the polyimide composite film has better mechanical properties and mechanical properties, meeting the use requirements of flexible circuit boards.

Exemplarily, 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.

Exemplarily, the thickness of the thermoplastic polyimide layer is 3.5 μm, and the thickness of the non-thermoplastic polyimide layer is 18 μm.

It should be noted that there are many methods for preparing the polyimide composite film, including but not limited to any one of a coating method and a co-extrusion method.

The present application also provides a method for preparing a polyimide composite film, which comprises:

S1, obtaining a first solution, comprising at least one imidization catalyst, at least one dehydrating agent, a first polyamic acid, and a non-protonic polar solvent, wherein the first polyamic acid can be imidized to produce a non-thermoplastic polyimide terminated with diisocyanate;

A second solution is obtained, wherein the second solution comprises at least one imidization catalyst, at least one dehydrating agent, a second polyamic acid and a non-protonic polar solvent, and the second polyamic acid can be used to prepare an amine-terminated thermoplastic polyimide through imidization.

S2, co-extruding the first solution and the second solution, removing the solvent to obtain a co-extruded cast sheet, wherein the co-extruded cast sheet includes a first polyamic acid layer and a second polyamic acid layer stacked;

S3, imidizing the co-extruded cast sheet to obtain a polyimide composite film.

Among them, co-extrusion refers to extruding and casting the first solution and the second solution in a co-extrusion mold onto a template, such as a steel plate, to obtain multiple layers of liquid films of different materials stacked and composited, and forming a co-extruded cast sheet after removing the solvent.

It can be understood that the polyimide composite film finally obtained after the co-extruded cast sheet is imidized includes a non-thermoplastic polyimide layer and a thermoplastic polyimide layer which are stacked, wherein the first polyamic acid forms a non-thermoplastic polyimide terminated with diisocyanate, and the second polyamic acid forms a thermoplastic polyimide terminated with amine groups, that is, the thermoplastic polyimide layer contains a thermoplastic polyimide terminated with amine groups, and the non-thermoplastic polyimide layer contains a non-thermoplastic polyimide terminated with diisocyanate. From the beginning of coextrusion to the imidization stage in the die section of the coextrusion mold, diisocyanate and amine groups will react to form urea groups, and finally, urea groups generated by the reaction of diisocyanate and amine groups exist between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

In summary, in the preparation method provided in the present application, on the one hand, a co-extrusion method is adopted to prepare a polyimide composite film, which is easy to operate and is beneficial to improving the peel strength of the polyimide composite film compared with the coating method. On the other hand, by improving the formula of the first solution and the second solution, the prepared thermoplastic polyimide layer contains a thermoplastic polyimide terminated with an amine group, and the non-thermoplastic polyimide layer contains a non-thermoplastic polyimide terminated with a diisocyanate. Moreover, the diisocyanate between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer reacts with the amine group to generate a urea group for cross-linking, thereby effectively improving the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer. The combined effect finally obtains a polyimide composite film with high peel strength.

In some embodiments, the first polyamic acid is obtained by polymerizing a dianhydride monomer, a diamine monomer, and a diisocyanate monomer;

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.

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

Illustratively, the molar ratio of the diisocyanate monomer to the 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 the dianhydride monomer and the diisocyanate monomer to the diamine monomer is any one of 1:0.998, 1:0.999, 1:1, 1:1.001, 1.002 or between any two values.

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

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

When the molar ratio of the two is controlled within the above range, the polyimide composite film has better peel strength.

The diisocyanate monomers include aromatic diisocyanate monomers and/or aliphatic diisocyanate monomers. The 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.

In some embodiments, the diisocyanate monomer is an aromatic diisocyanate monomer.

The aromatic diisocyanate monomers include but are not limited to 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, 2,2'-methylene diphenyl diisocyanate, 2,4'-methylene diphenyl diisocyanate, 4,4'-methylene diphenyl diisocyanate, m-xylene diisocyanate, naphthalene diisocyanate or any combination thereof.

In some embodiments, the diisocyanate monomer is p-phenylene diisocyanate and/or naphthalene diisocyanate.

The above-mentioned diisocyanate monomer has a good coordination effect with the dianhydride monomer and the diamine monomer, and is conducive to the formation of thermoplastic polyimide terminated with diisocyanate.

In some embodiments, the second polyamic acid is obtained by polymerizing a dianhydride monomer and a diamine monomer;

Wherein, the molar ratio of the dianhydride monomer to the diamine monomer is 1:1-1.005.

By controlling the molar ratio of the dianhydride monomer to the diamine monomer within the above range, it is advantageous to form a thermoplastic polyimide terminated with an amine group through imidization.

Illustratively, the molar ratio of the dianhydride monomer to the 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 values.

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

In terms of mass percentage, the second solution comprises: 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.

When the dehydrating agent and the imidization catalyst are less than the range, the imidization is insufficient, which affects the mechanical strength of the polyimide composite film. When the dehydrating agent and the imidization catalyst are greater than the above range, the imidization proceeds rapidly and it is difficult to form a film.

Therefore, by controlling the mass ratio of the components in the first solution and the second solution, it is beneficial to smoothly prepare the polyimide composite film by co-extrusion and imidization, and it is beneficial to improve the quality of the polyimide composite film.

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.

The above arrangement is helpful to control the amidation rates of the first polyamic acid layer and the second polyamic acid layer to be substantially the same, which is helpful to improve the quality of the polyimide composite film.

Any aprotic polar solvent can be used as long as it can dissolve the polyamic acid.

The aprotic polar solvent may be an amide solvent. For example, the aprotic polar solvent includes but is not limited to at least one of N,N'-dimethylformamide (DMF), N,N'-dimethylacetamide, N-methyl-pyrrolidone (NMP), γ-butyrolactone (GBL), and diglyme (Diglyme), but is not limited thereto, and can be used alone or in combination of two or more as needed.

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

The dehydrating agent has the function of a dehydrating and ring-closing agent for the polyamic acid, and any component can be used as the imidization catalyst as long as it has the effect of promoting the dehydrating and ring-closing action of the dehydrating agent.

The imidization catalyst includes but is 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 closure of the dehydrating agent.

Illustratively, the imidization catalyst includes but is not limited to at least one of the group consisting of free quinoline, isoquinoline, β-picoline, pyridine, imidazole, 2-imidazole, 1,2-dimethylimidazole, 2-phenylimidazole and benzimidazole.

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

The dehydrating agent includes, but is not limited to, aliphatic anhydrides, aromatic anhydrides, N,N'-dialkylcarbodiimide, lower aliphatic halides, halogenated lower aliphatic anhydrides, arylsulfonic acid dihalides, thionyl halides and the like.

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

The method of removing the solvent is drying.

In some embodiments, the method of removing the solvent includes heating at 140-160° C., and the specific heating time can be selected according to the film thickness. For example, the heating time is 120-170 seconds. For example, the method of removing the solvent includes heating at 150° C. for 150 seconds.

In some embodiments, imidization includes heating the coextruded cast sheet.

In some embodiments, during the process of heating the co-extruded cast sheet for imidization, the heating procedure includes sequentially: keeping warm at 150°C for 2-3 minutes, keeping warm at 200°C for 2-5 minutes, keeping warm at 300°C for 2-5 minutes, keeping warm at 350°C for 2-5 minutes, and keeping warm at 400°C for 2-5 minutes.

The above-mentioned step-wise temperature increase method is beneficial to the adequacy of imidization in combination with a dehydrating agent.

The present application also provides a flexible circuit board, which comprises the above-mentioned polyimide composite film or the polyimide composite film prepared by the above-mentioned preparation method.

The polyimide composite film and its preparation method and the flexible circuit board of the present application are further described in detail below in conjunction with the embodiments.

In the following examples and comparative example 2,

Example 1

The polyimide composite film includes two thermoplastic polyimide layers and one non-thermoplastic polyimide layer, wherein the non-thermoplastic polyimide layer is located between the two thermoplastic polyimide layers. The thermoplastic polyimide layer contains a thermoplastic polyimide terminated with an amine group, and the non-thermoplastic polyimide layer contains a non-thermoplastic polyimide terminated with a diisocyanate, and a urea group generated by the reaction of the diisocyanate with the amine group exists between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

The polyimide composite film is prepared by the following preparation method:

[Obtaining the first solution]

At -5-0°C, N,N-dimethylformamide and 4,4,-diaminodiphenyl ether (ODA) were added to a reactor filled with nitrogen. After dissolving, an equimolar amount of pyromellitic dianhydride (PMDA) was added to carry out polymerization reaction for 3 hours. The polyamic acid precursor solution was obtained by cooling to room temperature. The dissolved p-phenylene diisocyanate solution was added to react to obtain a first polyamic acid solution with p-phenylene diisocyanate as the terminal group.

Triethylamine and acetic anhydride are added to the first polyamic acid solution, mixed evenly and then degassed to obtain a first solution.

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

[Obtaining the second solution]

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

Triethylamine and acetic anhydride are added to the second polyamic acid precursor solution, mixed evenly and then degassed to obtain a second solution.

Among them, the ratio of ODPA:BTDA:BAPP is 0.5:0.5:1, and the mass ratio of the sum of ODPA, BTDA and BAPP:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

【Film forming】

The first solution and the second solution are cast onto a steel belt in a multi-layer manner by co-extrusion, heated at 150°C for 150s to remove the solvent, and then kept at 200°C for 250s, 300°C for 250s, 350°C for 250s, and 400°C for 250s in sequence for imidization to obtain a composite polyimide film with a thickness of 25 μm, wherein the thickness of the upper and lower thermoplastic polyimide layers are 3.5 μm respectively; the thickness of the non-thermoplastic polyimide layer is 18 μm.

Example 2

The difference between it and Example 1 is only that:

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

Example 3

The difference between it and Example 1 is only that:

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

Example 4

The difference between it and Example 1 is only that:

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

Example 5

The difference between it and Example 1 is only that:

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 p-phenylene diisocyanate, PMDA and ODA:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

Example 6

The difference between it and Example 1 is only that:

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 p-phenylene diisocyanate, PMDA and ODA:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

Example 7

The difference between it and Example 1 is only that:

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:N,N-dimethylformamide:triethylamine:acetic anhydride is 20:80:10:30.

Comparative Example 1

The difference between it and Example 1 is only that:

[Obtaining the first solution]

At -5-0°C, N,N-dimethylformamide and 4,4,-diaminodiphenyl ether (ODA) were added to a reactor filled with nitrogen, and after dissolving, an equal mole of pyromellitic dianhydride (PMDA) was added to polymerize for 3 hours, and then cooled to room temperature to obtain a polyamic acid precursor solution.

Triethylamine and acetic anhydride are added to the polyamic acid precursor solution, mixed evenly and then degassed to obtain a first solution.

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

Comparative Example 2

The difference between it and Example 3 is only that:

【Film forming】

The first solution was cast onto a steel strip, and the steel strip was firstly peeled off after being heated at 150° C. for 150 seconds to remove the solvent, thereby obtaining a first polyamic acid layer.

The second solution was applied to the upper and lower surfaces of the first polyamic acid layer after solvent removal, and then heated at 150°C for 150s to remove the solvent, and then kept at 200°C for 250s, 300°C for 250s, 350°C for 250s, and 400°C for 250s in sequence for imidization to obtain a composite polyimide film with a thickness of 25 μm.

Test Example 1

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

The tensile strength and elongation at break were tested using GB/T 1040.1-2018, and the peel strength was tested using the national standard GB/T 2792-2014 "180° Peel Strength Test Method Standard".

The results are shown in Table 1.

Table 1 Test results

Sample No. 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

It can be seen from Table 1 that 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 group generated by the reaction of diisocyanate and amine between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer.

And according to Examples 1-4, the molar ratio of diisocyanate monomer to the dianhydride monomer is greater than 0 and less than or equal to 0.007, which can improve the peel strength between the layers of the polyimide composite film, wherein the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.005, as the proportion increases, the tensile strength and peel strength both show an upward trend. When the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0.005, the peel strength decreases, so it is preferred that the molar ratio of diisocyanate monomer to dianhydride monomer is greater than 0 and less than or equal to 0.005, and it is further preferred that the molar ratio of diisocyanate monomer to dianhydride monomer is 0.001-0.005.

According to Example 3 and Comparative Example 2, the preparation method will also significantly affect 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 embodiment is significantly better than that by the coating method shown in Comparative Example 2.

According to Examples 5 and 6, compared with Example 1, there is no significant change in tensile strength, elongation at break and peel strength when using aromatic naphthalene diisocyanate and paraphenylene diisocyanate; while the peel strength and tensile strength are significantly reduced when using aliphatic hexamethylene diisocyanate.

According to Example 7 compared with Example 1, after the proportion of BTDA increases, the peel strength and elongation at break decrease, and the tensile strength increases.

In summary, the polyimide composite film provided in the present application effectively improves the peel strength between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer by cross-linking the non-thermoplastic polyimide layer and the thermoplastic polyimide layer through the reaction of diisocyanate and amine group to generate urea group, and the co-extrusion method is adopted to prepare the polyimide composite film, which is beneficial to further improve its peel strength.

The above description is only a specific embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims (17)

1. A polyimide composite film, characterized in that it comprises a non-thermoplastic polyimide layer and a thermoplastic polyimide layer stacked; The thermoplastic polyimide layer contains thermoplastic polyimide terminated with amine groups, the non-thermoplastic polyimide layer contains non-thermoplastic polyimide terminated with diisocyanate, and urea groups generated by the reaction of diisocyanate and amine groups exist between the non-thermoplastic polyimide layer and the thermoplastic polyimide layer. 2. The polyimide composite film according to claim 1, characterized in that the first polyamic acid for forming the non-thermoplastic polyimide layer is formed by polymerization of a dianhydride monomer, a diamine monomer and a diisocyanate monomer; 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. 3 . The polyimide composite film according to claim 2 , wherein a molar ratio of the diisocyanate monomer to the dianhydride monomer is greater than 0 and less than or equal to 0.005. 4 . The polyimide composite film according to claim 2 , wherein the molar ratio of the diisocyanate monomer to the dianhydride monomer is 0.001-0.005. 5. The polyimide composite film according to claim 2, wherein the diisocyanate monomer comprises an aromatic diisocyanate monomer; Optionally, the diisocyanate monomer includes p-phenylene diisocyanate and/or naphthalene diisocyanate. 6. The polyimide composite film according to claim 1, characterized in that the second polyamic acid for forming the thermoplastic polyimide layer is formed by polymerization of a dianhydride monomer and a diamine monomer; Wherein, the molar ratio of the dianhydride monomer to the diamine monomer is 1:1-1.005. 7 . The polyimide composite film according to claim 1 , wherein the non-thermoplastic polyimide layer and the thermoplastic polyimide layer respectively contain at least one dehydrating agent and at least one imidization catalyst. 8. The polyimide composite film according to claim 7, characterized in that the imidization catalyst comprises 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. 9. The polyimide composite film according to claim 7, 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. 10. The polyimide composite film according to any one of claims 1 to 9, characterized in that the polyimide composite film comprises two layers of the thermoplastic polyimide layers and one layer of the non-thermoplastic polyimide layer, and the non-thermoplastic polyimide layer is located between the two layers of the thermoplastic polyimide layers; 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. 11. A method for preparing a polyimide composite film, comprising: Obtaining a first solution comprising at least one imidization catalyst, at least one dehydrating agent, a first polyamic acid, and a non-protonic polar solvent, wherein the first polyamic acid can be imidized to produce a non-thermoplastic polyimide terminated with diisocyanate; Obtaining a second solution, wherein the second solution comprises at least one imidization catalyst, at least one dehydrating agent, a second polyamic acid, and a non-protonic polar solvent, wherein the second polyamic acid can be imidized to produce an amine-terminated thermoplastic polyimide; Co-extruding the first solution and the second solution, removing the solvent to obtain a co-extruded cast sheet, wherein the co-extruded cast sheet includes a first polyamic acid layer and a second polyamic acid layer stacked; The co-extruded cast sheet is imidized to obtain the polyimide composite film. 12. The preparation method according to claim 11, characterized in that the first polyamic acid is obtained by polymerizing a dianhydride monomer, a diamine monomer and a diisocyanate monomer; 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; Optionally, the molar ratio of the diisocyanate monomer to the dianhydride monomer is greater than 0 and less than or equal to 0.005; Optionally, the molar ratio of the diisocyanate monomer to the dianhydride monomer is 0.001-0.005; Optionally, the diisocyanate monomer includes p-phenylene diisocyanate and/or naphthalene diisocyanate. 13. The preparation method according to claim 11, characterized in that the second polyamic acid is obtained by polymerizing a dianhydride monomer and a diamine monomer; Wherein, the molar ratio of the dianhydride monomer to the diamine monomer is 1:1-1.005. 14. The preparation method according to claim 11, characterized in that: In terms of mass percentage, the first solution comprises: 60-80 parts of aprotic polar solvent, 10-30 parts of first polyamic acid, 5-20 parts of imidization catalyst, and 5-30 parts of dehydrating agent; and/or, In terms of mass percentage, the second solution comprises: 60-80 parts of aprotic polar solvent, 10-30 parts of a second polyamic acid, 5-20 parts of an imidization catalyst, and 5-30 parts of a dehydrating agent. 15. The preparation method according to claim 11, characterized in that the aprotic polar solvent comprises at least one of N,N-dimethylformamide, N,N-dimethylacetamide and N-methylpyrrolidone; Optionally, the imidization catalyst includes at least one of pyridine, quinoline, isoquinoline and triethylamine; Optionally, the dehydrating agent includes at least one of acetic anhydride, propionic anhydride and benzoic anhydride. 16. The preparation method according to any one of claims 11 to 15, characterized in that the imidization comprises: heating the co-extruded cast sheet; Optionally, the heating procedure includes sequentially: keeping at 150°C for 2-3 min, keeping at 200°C for 2-5 min, keeping at 300°C for 2-5 min, keeping at 350°C for 2-5 min, and keeping at 400°C for 2-5 min. 17. A flexible circuit board, characterized in that the flexible circuit board comprises the polyimide composite film according to any one of claims 1 to 10 or the polyimide composite film prepared by the preparation method according to any one of claims 11 to 16.