JP7496417B2 - Polyimide film and its manufacturing method - Google Patents

Description

This invention relates to a polyimide film and a method for manufacturing the same. More specifically, this invention relates to a polyimide film that has a low elastic modulus, a high yield point, and is less damaged even with repeated deformation, and a method for manufacturing the same.

Flexible displays, such as curved, bendable, foldable, and rollable displays, are next-generation displays that have been attracting attention from both academia and industry recently. Among the various types of materials that make up flexible displays, functional film/coating materials are important polymer substrate materials that make up flexible displays, and can be said to be core materials that are absolutely necessary for the successful realization and development of flexible displays, with polyimide attracting attention as such a material.

Polyimides are polymers that have heteroimide rings in the main chain. In addition to excellent heat resistance, they also have excellent mechanical properties, flame retardancy, chemical resistance, and low dielectric constant, and are used in a wide range of applications, including coating materials, molding materials, and composite materials.

The most important physical property required for a polymer substrate for a flexible display is flexibility. In particular, such a polymer substrate must not only not be damaged during the curving, bending, folding, rolling, and stretching processes that repeatedly cause deformation in flexible displays, but also must not lose its various initial physical properties.

The object of the present invention is to provide a polyimide film that has a low elastic modulus, a high yield point, and is less susceptible to damage even with repeated deformation.

Another object of the present invention is to provide a method for producing the aforementioned polyimide film.

1. According to one embodiment, a polyimide film is provided which is derived from the imidization of a polyamic acid formed from the reaction of a dianhydride monomer and a diamine monomer, the dianhydride monomer comprising about 10 mol % to about 90 mol % of biphenyltetracarboxylic dianhydride (BPDA) based on the total molar amount of the dianhydride monomer, and the polyimide film satisfies the following formula 1 and has a modulus of about 2 GPa to about 4.5 GPa.

<Formula 1>
About 21 MPa/%≦A/B≦about 30 MPa/%

In the above formula 1, A is the yield strength of the polyimide film in MPa, and B is the yield point of the polyimide film in %.

2. In the first embodiment, the dianhydride monomer may further include pyromellitic dianhydride (PMDA).

3. In the second embodiment, the pyromellitic dianhydride may be included in an amount of about 10 mol% to about 90 mol% based on the total molar amount of the dianhydride monomers.

4. In any of the first to third embodiments, the diamine monomer may include m-tolidine (m-TD), 4,4'-oxydianiline (ODA), 1,3-bis(4-aminophenoxy)benzene (TPE-R), 2,2-bis(4-[4-aminophenoxy]-phenyl)propane (PAPP), or a combination thereof.

5. In the fourth embodiment, the diamine monomer may include m-tolidine (m-TD) and 4,4'-oxydianiline (ODA) in a molar ratio of about 1:99 to about 20:80.

6. In any of the first to fifth embodiments, the polyimide film may have a yield strength of about 50 MPa to about 80 MPa.

7. In any of the first to sixth embodiments, the yield point of the polyimide film may be about 2.2% to about 2.9%.

8. According to another aspect, in the method for producing a polyimide film according to any one of the first to seventh embodiments, the method may include the steps of mixing and reacting a dianhydride monomer, a diamine monomer, and an organic solvent to form a polyamic acid solution, mixing a dehydrating agent and an imidization agent into the polyamic acid solution to form a polyimide precursor composition, casting the polyimide precursor composition on a support and drying to produce a gel film, and heat-treating the gel film to form a polyimide film.

9. In the eighth embodiment, the heat treatment may be performed at about 100°C to about 700°C.

The polyimide film and manufacturing method thereof of the present invention have the effect of providing a polyimide film having a low elastic modulus and a high yield point.

When describing the present invention, if it is determined that a specific description of related publicly known technology may unnecessarily obscure the gist of the present invention, that detailed description will be omitted.

When the terms "include," "have," "be made," etc. are used in this specification, other parts may be added unless "only" is used. When a component is expressed in the singular, it includes the plural unless otherwise expressly stated.

In addition, when analyzing components, it is assumed that a margin of error is included even if there is no other explicit statement.

In this specification, when a numerical range is indicated as "a~b," "~" is defined as ≧a and ≦b.

According to one embodiment, a polyimide film is provided. The inventors of the present invention have discovered that in a polyimide film derived from the imidization of a polyamic acid formed by the reaction of a dianhydride monomer and a diamine monomer, when the polyimide film contains about 10 mol% to about 90 mol% of biphenyltetracarboxylic dianhydride (BPDA) as the dianhydride monomer and satisfies the following formula 1, the polyimide film has a low modulus (e.g., a modulus of about 4.5 GPa or less) and a high yield point (e.g., a yield point of about 2.2% or more), and as a result, the polyimide film is less damaged even when repeatedly deformed, and have completed the present invention.

The dianhydride monomer may contain biphenyltetracarboxylic dianhydride in an amount of about 10 mol% to about 90 mol% (e.g., 10 mol%, 15 mol%, 20 mol%, 25 mol%, 30 mol%, 35 mol%, 40 mol%, 45 mol%, 50 mol%, 55 mol%, 60 mol%, 65 mol%, 70 mol%, 75 mol%, 80 mol%, 85 mol% or 90 mol%) based on the total molar amount of the dianhydride monomer. In this range, the elastic range of the polyimide film is extended, and a polyimide film having a low elastic modulus and a high yield point can be produced. For example, the dianhydride monomer may contain biphenyltetracarboxylic dianhydride in an amount of about 15 mol% to about 80 mol%, another example, about 20 mol% to about 70 mol%, and another example, about 30 mol% to about 50 mol%, based on the total molar amount of the dianhydride monomer, but is not limited thereto.

The polyimide film may satisfy the following formula 1:

<Formula 1>
About 21 MPa/%≦A/B≦about 30 MPa/%

In the above formula 1, A is the yield strength of the polyimide film in MPa, and B is the yield point of the polyimide film in %,. If A/B is less than about 21 MPa/%, there is a problem of low yield point and low tensile strength, and if A/B exceeds about 30 MPa/%, there is a problem of not meeting the low modulus required by the present invention, and as a result, when the polyimide film is repeatedly deformed, there is a possibility of a lot of damage occurring. Here, the "yield strength" and "yield point" may be measured using a tensile tester at a tensile speed of 200 mm/min based on the ASTM D 882 standard, but are not limited thereto. According to one embodiment, the A/B value may be, but is not limited to, 21 MPa/%, 22 MPa/%, 23 MPa/%, 24 MPa/%, 25 MPa/%, 26 MPa/%, 27 MPa/%, 28 MPa/%, 29 MPa/% or 30 MPa/%, according to another embodiment, about 21 MPa/% to about 29 MPa/%, according to another embodiment, about 22 MPa/% to about 28 MPa/%.

The polyimide film may have a modulus of about 2 GPa to about 4.5 GPa (e.g., 2 GPa, 2.1 GPa, 2.2 GPa, 2.3 GPa, 2.4 GPa, 2.5 GPa, 2.6 GPa, 2.7 GPa, 2.8 GPa, 2.9 GPa, 3 GPa, 3.1 GPa, 3.2 GPa, 3.3 GPa, 3.4 GPa, 3.5 GPa, 3.6 GPa, 3.7 GPa, 3.8 GPa, 3.9 GPa, 4 GPa, 4.1 GPa, 4.2 GPa, 4.3 GPa, 4.4 GPa, or 4.5 GPa). Here, the "modulus" may be measured using a tensile tester at a tensile speed of 200 mm/min according to the ASTM D 882 standard, but is not limited thereto. For example, the modulus of the polyimide film may be, but is not limited to, about 2 GPa to about 4.2 GPa, or, in another example, about 2.5 GPa to about 4.1 GPa, or, in another example, about 2.5 GPa to about 4.0 GPa.

The polyimide film may further contain a dianhydride monomer other than biphenyltetracarboxylic dianhydride. As such a dianhydride monomer, various dianhydride monomers may be used without limitation within the scope that does not adversely affect the effects of the present invention. An example of such a dianhydride monomer is pyromellitic dianhydride (PMDA). The dianhydride monomer other than biphenyltetracarboxylic dianhydride may be present in an amount, for example, from about 10 mol% to about 90 mol% (e.g., 10 mol%, 15 mol%, 20 mol%, 25 mol%, 30 mol%, 35 mol%, 40 mol%, 45 mol%, 50 mol%, 55 mol%, 60 mol%, 65 mol%, 70 mol%, 75 mol%, 80 mol%, 85 mol%, or 90 mol%), for example, from about 20 mol% to about 85 mol%, for example, from about 30 mol% to about 80 mol%, and for example, from about 50 mol% to about 70 mol%, based on the total molar amount of the dianhydride monomers, but is not limited thereto. For example, pyromellitic dianhydride may be present in an amount of, for example, about 10 mol% to about 90 mol%, for example, about 20 mol% to about 85 mol%, for example, about 30 mol% to about 80 mol%, or for example, about 50 mol% to about 70 mol%, based on the total molar amount of dianhydride monomers, but is not limited thereto.

As the diamine monomer, various diamine monomers may be used without limitation within the scope that does not adversely affect the effects of the present invention. Examples of such diamine monomers include m-tolidine (m-TD), 4,4'-oxydianiline (ODA), 1,3-bis(4-aminophenoxy)benzene (TPE-R), 2,2-bis(4-[4-aminophenoxy]-phenyl)propane (PAPP), etc., which may be used alone or in combination of two or more kinds, but are not limited thereto.

According to one embodiment, the diamine monomer may include 4,4'-oxydianiline, in which case it is possible to produce a polyimide film having a low elastic modulus and a high yield point. In this case, the content of 4,4'-oxydianiline may be, for example, from more than 0 mol% to about 100 mol% (e.g., 1 mol%, 5 mol%, 10 mol%, 15 mol%, 20 mol%, 25 mol%, 30 mol%, 35 mol%, 40 mol%, 45 mol%, 50 mol%, 55 mol%, 60 mol%, 65 mol%, 70 mol%, 75 mol%, 80 mol%, 85 mol%, 90 mol%, 95 mol%, or 100 mol%) based on the total number of moles of the diamine monomer, or from about 50 mol% to about 100 mol%, or from about 70 mol% to about 100 mol%, or from about 75 mol% to about 100 mol%, or from about 80 mol% to about 100 mol%, but is not limited thereto.

In another embodiment, the diamine monomer may include m-tolidine, in which case it is possible to produce a polyimide film having a low elastic modulus and a high yield point. In this case, the content of m-tolidine may be, for example, from more than 0 mol% to about 100 mol% (e.g., 1 mol%, 5 mol%, 10 mol%, 15 mol%, 20 mol%, 25 mol%, 30 mol%, 35 mol%, 40 mol%, 45 mol%, 50 mol%, 55 mol%, 60 mol%, 65 mol%, 70 mol%, 75 mol%, 80 mol%, 85 mol%, 90 mol%, 95 mol% or 100 mol%) based on the total number of moles of the diamine monomer, or from more than 0 mol% to about 50 mol%, or from more than 0 mol% to about 30 mol%, or from more than 0 mol% to about 25 mol%, or from more than 0 mol% to about 20 mol%, but is not limited thereto.

According to another embodiment, the diamine monomer may include m-tolidine and 4,4'-oxydianiline. In this case, the respective effects of m-tolidine and 4,4'-oxydianiline cause synergy, making it possible to produce a polyimide film having a low elastic modulus and a high yield point. In this case, the molar ratio of m-tolidine to 4,4'-oxydianiline is about 1:99 to about 99:1 (e.g., 1:99, 5:95, 10:90, 15:85, 20:80, 25:75, 30:70, 35:65, 40:60, 45:55, 50:50, 55:45, 60:40, 65:35, 70:30, 75:25, 80:20, 85:15, 90:10, 95 :5, or 99:1), for example, about 1:99 to about 50:50, for example, about 1:99 to about 30:70, for example, about 1:99 to about 25:75, for example, about 1:99 to about 30:70, for example, about 1:99 to about 20:80, for example, about 5:95 to about 20:80, but is not limited thereto. The total amount of m-tolidine and 4,4'-oxydianiline may be, for example, about 11 mol% to about 100 mol%, for example, about 50 mol% to about 100 mol%, for example, about 90 mol% to about 100 mol%, based on the total number of moles of diamine monomer, but is not limited thereto.

According to one embodiment, the polyimide film may have a yield strength of about 50 MPa to about 80 MPa (e.g., 50 MPa, 55 MPa, 60 MPa, 65 MPa, 70 MPa, 75 MPa, or 80 MPa). For example, the yield strength of the polyimide film may be, but is not limited to, about 50 MPa to about 75 MPa, for example, about 50 MPa to about 70 MPa, and for another example, about 60 MPa to about 70 MPa.

According to one embodiment, the polyimide film may have a yield point of about 2.2% to about 2.9% (e.g., 2.2%, 2.25%, 2.3%, 2.35%, 2.4%, 2.45%, 2.5%, 2.55%, 2.6%, 2.65%, 2.7%, 2.75%, 2.8%, 2.85%, or 2.9%). For example, the yield point of the polyimide film may be about 2.2% to about 2.7%, or another example, about 2.2% to about 2.65%, but is not limited thereto.

The thickness of the polyimide film may be appropriately selected taking into consideration the application, usage environment, physical properties, etc. of the polyimide film. For example, the thickness of the polyimide film may be about 10 μm to about 500 μm, as another example, about 20 μm to about 50 μm, as another example, about 40 μm to about 50 μm, but is not limited thereto.

The polyimide film may be manufactured by various methods commonly used in the field of polyimide film manufacturing. For example, the polyimide film may be manufactured by the steps of mixing and reacting a dianhydride monomer, a diamine monomer, and an organic solvent to form a polyamic acid solution, mixing a dehydrating agent and an imidizing agent into the polyamic acid solution to form a polyimide precursor composition, casting the polyimide precursor composition on a support and drying it to form a gel film, and heat-treating the gel film to form a polyimide film. The dianhydride monomer and the diamine monomer have been described above, so their description will be omitted.

First, a dianhydride monomer and a diamine monomer are reacted to produce a polyamic acid. More specifically, a dianhydride monomer and a diamine monomer are polymerized in an organic solvent to produce a polyamic acid solution. In this case, all the monomers may be added at once, or each monomer may be added sequentially, in which case partial polymerization between the monomers may occur.

The organic solvent is not particularly limited as long as it is a solvent in which polyamic acid can be dissolved, and may be, for example, an aprotic polar organic solvent. Non-limiting examples of the aprotic polar organic solvent include amide-based solvents such as N,N'-dimethylformamide (DMF) and N,N'-dimethylacetamide (DMAc), phenol-based solvents such as p-chlorophenol and o-chlorophenol, N-methylpyrrolidone (NMP), gamma-butyrolactone (GBL), diglyme, etc., which may be used alone or in combination of two or more. In some cases, the solubility of the polyamic acid may be adjusted using an auxiliary solvent such as toluene, tetrahydrofuran, acetone, methyl ethyl ketone, methanol, ethanol, water, etc. In one embodiment, the organic solvent may be an amide-based solvent, for example, N,N-dimethylformamide or N,N-dimethylacetamide, but is not limited thereto.

Then, a dehydrating agent and an imidizing agent can be mixed into the polyamic acid solution to form a polyimide precursor composition.

The dehydrating agent promotes the ring-closing reaction by dehydrating the polyamic acid. Examples of the dehydrating agent include aliphatic acid anhydrides, aromatic acid anhydrides, N,N'-dialkylcarbodiimides, lower aliphatic halides, halogenated lower fatty acid anhydrides, arylphosphonic acid dihalides, and thionyl halides. These may be used alone or in combination of two or more. Among these, from the viewpoints of availability and cost, aliphatic acid anhydrides such as acetic anhydride, propionic anhydride, and lactic anhydride may be used alone or in combination of two or more.

The imidizing agent means a component that has the effect of promoting the ring-closing reaction of polyamic acid, and examples of the imidizing agent include aliphatic tertiary amines, aromatic tertiary amines, and heterocyclic tertiary amines. Among these, heterocyclic tertiary amines may be used from the viewpoint of reactivity as a catalyst. Examples of the imidizing agent include quinoline, isoquinoline, β-picoline, and pyridine, and these may be used alone or in combination of two or more kinds.

The amounts of the dehydrating agent and the imidizing agent to be added are not particularly limited, but the dehydrating agent may be added in an amount of about 0.5 mol to about 5 mol (e.g., 0.5 mol, 1 mol, 1.5 mol, 2 mol, 2.5 mol, 3 mol, 3.5 mol, 4 mol, 4.5 mol, or 5 mol) per mol of amic acid groups in the polyamic acid, for example, about 1.0 mol to about 4 mol, and the imidizing agent may be added in an amount of about 0.05 mol to about 3 mol (e.g., 0.05 mol, 0.1 mol, 0.5 mol, 1 mol, 1.5 mol, 2 mol, 2.5 mol, or 3 mol) per mol of amic acid groups in the polyamic acid, for example, about 0.2 mol to about 2 mol, and within the above ranges, imidization is sufficient and it is easy to cast into a film.

According to one embodiment, the polyamic acid may be present in an amount of about 5 wt % to about 35 wt % (e.g., 5 wt %, 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, or 35 wt %) based on the total weight of the polyimide precursor composition. Within this range, the precursor composition has a molecular weight and solution viscosity suitable for forming a film. Based on the total weight of the precursor composition, the polyamic acid may be present in an amount of, for example, about 10 wt % to about 30 wt %, and in another example, about 15 wt % to about 20 wt %, but is not limited thereto.

According to one embodiment, the polyimide precursor composition may have a viscosity of about 100,000 cP to about 500,000 cP (e.g., 100,000 cP, 150,000 cP, 200,000 cP, 250,000 cP, 300,000 cP, 350,000 cP, 400,000 cP, 450,000 cP, or 500,000 cP) at 25°C. In this range, the polyamic acid may have a predetermined weight average molecular weight while exhibiting excellent processability during the formation of a polyimide film. Here, the "viscosity" may be measured using a Brookfield viscometer. The precursor composition may have a viscosity at 25°C of, for example, but not limited to, about 150,000 cP to about 450,000 cP, in other examples, about 200,000 cP to about 400,000 cP, and in other examples, about 250,000 cP to about 350,000 cP.

The polyimide precursor composition can then be cast onto a substrate and dried to produce a gel film.

The support may be any support commonly used in the art without limitation, and examples of such supports include glass plates, aluminum foil, endless stainless steel belts, stainless steel drums, etc.

Drying may be performed at a temperature of, for example, about 40°C to about 300°C, or, for example, about 80°C to about 200°C, or, for example, about 100°C to about 180°C, or, for example, about 100°C to about 130°C, which may activate the dehydrating agent and imidizing agent and partially cure and/or dry to form a gel film. The gel film is in an intermediate stage of curing from polyamic acid to polyimide and is self-supporting.

In some cases, the method may further include a step of stretching the gel film to adjust the thickness and size of the final polyimide film and improve orientation, and the stretching may be performed in at least one of the machine direction (MD) and the transverse direction (TD) to the machine direction.

The volatile content of the gel film may be, but is not limited to, about 5% to about 500% by weight, for example, about 5% to about 200% by weight, or another example, about 5% to about 150% by weight. In this range, defects such as film breakage, color unevenness, and characteristic fluctuations can be avoided during the subsequent heat treatment process to obtain a polyimide film. Here, the volatile content of the gel film can be calculated using the following formula 2, in which C is the weight of the gel film and D is the weight of the gel film after heating at 450°C for 20 minutes.

<Formula 2>
(C-D) x 100/D

According to one embodiment, in the step of heat treating the gel film, the gel film is heat treated at a variable temperature ranging from about 50°C to about 700°C, for example, from about 150°C to about 600°C, for example, from about 200°C to about 600°C, to remove the solvent remaining in the gel film and imidize most of the remaining amic acid groups to obtain a polyimide film.

In some cases, the polyimide film obtained as described above can be finished by heating it at a temperature of about 400°C to about 650°C for about 5 seconds to about 400 seconds to further harden the polyimide film, and this can also be done under a predetermined tension to relieve internal stress that may remain in the obtained polyimide film.

The configuration and operation of the present invention will be described in more detail below through preferred embodiments of the present invention. However, these are presented as preferred examples of the present invention and should not be construed as limiting the present invention in any way.

Example
Examples 1 to 13 and Comparative Examples 1 to 3
A polyamic acid solution having a solid content of 18.5 wt % was prepared by mixing dianhydride monomers, 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride, and diamine monomers, m-tolidine (m-TD) and 4,4'-oxydianiline (ODA), in dimethylformamide (DMF) at the molar ratios shown in Table 1 below, and then polymerizing the mixture. The molar amounts of the dianhydride monomer and the diamine monomer were substantially equimolar.

To the polyamic acid solution thus prepared, 3.5 molar ratios of acetic anhydride and 1.1 molar ratios of isoquinoline per mole of amic acid group were added to obtain a composition for preparing a polyimide film, which was then cast onto a SUS plate (100SA, Sandvik) using a doctor blade and dried at 90°C for 4 minutes to prepare a gel film. The gel film was separated from the SUS plate and then heat-treated at 250-380°C for 14 minutes to prepare a polyimide film with an average thickness of 50 μm.

However, in the case of Comparative Example 3, no polyimide film was produced.

Evaluation example: Measurement of modulus (unit: GPa), yield point (unit: %), and yield strength (unit: MPa)

The polyimide film thus produced was cut into test pieces measuring 15 mm x 50 mm, and the modulus, yield point, and yield strength were measured at room temperature (room temp.) using a tensile tester (Instron 5564, Instron) at a tensile speed of 200 mm/min according to the ASTM D 882 standard, and the results are shown in Table 1 below.

As can be seen from Table 1, Examples 1 to 13, in which the BPDA content and the value according to Formula 1 fall within the range of the present invention, have a low modulus within the range of the present invention and a high yield point, and as a result, it is easily predicted that the polyimide film will be less damaged by repeated deformation.

On the other hand, the polyimide film of Comparative Example 1, in which the BPDA content was outside the range of the present invention and the value according to Formula 1 was outside the range of the present invention, had a low yield point of 2.16%. The polyimide film of Comparative Example 2, in which the BPDA content was within the range of the present invention but the value according to Formula 1 was outside the range of the present invention, had a high modulus of 4.6 GPa. Therefore, it is predicted that the polyimide films of Comparative Examples 1 and 2 will be significantly damaged when repeatedly deformed.

On the other hand, in the case of Comparative Example 3, where the BPDA content exceeded the range of the present invention, it was impossible to produce a polyimide film when applying the above-mentioned manufacturing method.

Simple variations or modifications of the present invention can be easily implemented by a person having ordinary skill in the art, and all such variations or modifications are deemed to be within the scope of the present invention.

Claims (5)

1. A polyimide film derived from the imidization of a polyamic acid formed from the reaction of a dianhydride monomer and a diamine monomer,
The dianhydride monomers include biphenyltetracarboxylic dianhydride (BPDA) and pyromellitic dianhydride (PMDA);
The dianhydride monomer contains the biphenyltetracarboxylic dianhydride in an amount of 10 mol % to 90 mol % based on the total molar amount of the dianhydride monomer;
The diamine monomer comprises m-tolidine (m-TD) and 4,4'-oxydianiline (ODA);
the molar ratio of the m-tolidine to the 4,4'-oxydianiline is 1:99 to 20:80;
The polyimide film has a yield strength of 50 MPa to 80 MPa;
The polyimide film satisfies the following formula 1 and has a modulus of 2 GPa to 4.5 GPa.
<Formula 1>
21 MPa/%≦A/B≦30 MPa/%
In the above formula 1, A is the yield strength of the polyimide film in MPa, and B is the yield point of the polyimide film in %. 2. The polyimide film according to claim 1 , wherein the pyromellitic dianhydride is contained in an amount of 10 mol % to 90 mol % based on the total molar amount of the dianhydride monomers. 3. The polyimide film according to claim 1 , wherein the polyimide film has a yield point of 2.2% to 2.9%. A method for producing the polyimide film according to any one of claims 1 to 3 ,
The method comprises:
mixing and reacting a dianhydride monomer, a diamine monomer, and an organic solvent to form a polyamic acid solution;
a dehydrating agent and an imidizing agent are mixed with the polyamic acid solution to form a polyimide precursor composition;
casting the polyimide precursor composition onto a support and drying to produce a gel film; and heat treating the gel film to form a polyimide film.
A method for producing a polyimide film, comprising the steps of: The method for producing a polyimide film according to claim 4 , wherein the heat treatment is carried out at 100°C to 700°C.